Mitochondrial 기능부전 치료 영양제, 표적치료 2024 Nature

[문형철]

1. nature  
2. signal transduction and targeted therapy  
3. review articles  
4. article

Mitochondrial dysfunction: mechanisms and advances in therapy

Download PDF

• Review Article
• Open access
• Published: 15 May 2024

Mitochondrial dysfunction: mechanisms and advances in therapy

• Yao Zong, 
• Hao Li, 
• Peng Liao, 
• Long Chen, 
• Yao Pan, 
• Yongqiang Zheng, 
• Changqing Zhang, 
• Delin Liu, 
• Minghao Zheng & 
• Junjie Gao 

Signal Transduction and Targeted Therapy volume 9, Article number: 124 (2024) Cite this article

•  
•  
•  

Abstract

Mitochondria, with their intricate networks of functions and information processing, are pivotal in both health regulation and disease progression. Particularly, mitochondrial dysfunctions are identified in many common pathologies, including cardiovascular diseases, neurodegeneration, metabolic syndrome, and cancer. However, the multifaceted nature and elusive phenotypic threshold of mitochondrial dysfunction complicate our understanding of their contributions to diseases. Nonetheless, these complexities do not prevent mitochondria from being among the most important therapeutic targets. In recent years, strategies targeting mitochondrial dysfunction have continuously emerged and transitioned to clinical trials. Advanced intervention such as using healthy mitochondria to replenish or replace damaged mitochondria, has shown promise in preclinical trials of various diseases.

Mitochondrial components, including mtDNA, mitochondria-located microRNA, and associated proteins can be potential therapeutic agents to augment mitochondrial function in immunometabolic diseases and tissue injuries. Here, we review current knowledge of mitochondrial pathophysiology in concrete examples of common diseases. We also summarize current strategies to treat mitochondrial dysfunction from the perspective of dietary supplements and targeted therapies, as well as the clinical translational situation of related pharmacology agents. Finally, this review discusses the innovations and potential applications of mitochondrial transplantation as an advanced and promising treatment.

요약

미토콘드리아는 

복잡한 기능과 정보 처리 네트워크를 통해 

건강 조절과 질병 진행에 중요한 역할을 합니다. 

특히, 미토콘드리아 기능 장애는 

심혈관 질환, 신경 퇴행, 대사 증후군, 암 등 

많은 일반적인 병리학에서 확인됩니다. 

그러나, 미토콘드리아 기능 장애의 다면적 특성과 파악하기 어려운 표현형 임계값은 질병에 대한 미토콘드리아의 기여도를 이해하는 것을 어렵게 만듭니다. 그럼에도 불구하고, 이러한 복잡성 때문에 미토콘드리아가 가장 중요한 치료 표적 중 하나가 되는 것을 막지는 못합니다. 

최근 몇 년 동안 미토콘드리아 기능 장애를 대상으로 하는 전략이 

지속적으로 등장하여 임상 시험으로 전환되고 있습니다. 

건강한 미토콘드리아를 사용하여 

손상된 미토콘드리아를 보충하거나 대체하는 것과 같은 고급 개입은 

다양한 질병의 전임상 시험에서 가능성을 보여주었습니다. 

mtDNA, 

미토콘드리아에 위치한 마이크로RNA 및 관련 단백질을 포함한 미토콘드리아 구성 요소는 

면역 대사 질환 및 조직 손상에서 미토콘드리아 기능을 강화하는

 잠재적 치료제가 될 수 있습니다. 

여기에서는 

일반적인 질병의 구체적인 예에 대한 

미토콘드리아 병리 생리학에 대한 현재의 지식을 검토합니다. 

또한, 

미토콘드리아 기능 장애를 치료하기 위한 현재의 전략을 

식이 보조제와 표적 치료의 관점에서 요약하고, 

관련 약리학 약물의 임상적 적용 현황을 살펴봅니다. 

마지막으로, 

이 리뷰에서는 미토콘드리아 이식의 혁신과 잠재적 응용에 대해 논의합니다.

Similar content being viewed by others

Mitochondria as secretory organelles and therapeutic cargos

Article Open access04 January 2024

The mitophagy pathway and its implications in human diseases

Article Open access16 August 2023

Mitochondrial diseases: from molecular mechanisms to therapeutic advances

Article Open access10 January 2025

Introduction

Over the past three decades, mitochondria have emerged as promising therapeutic targets for common diseases, driven by substantial advancements in both mitochondrial biology and clinical research.1,2,3 (See Fig. 1) Evolving from their bacterial ancestor, mitochondria have retained a limited yet unique genomic material known as the mitochondrial DNA (mtDNA), capable of self-replication and encoding indispensable constituents of respiratory complexes located on the inner mitochondrial membrane.4,5,6 Thus, as essential endosymbionts within eukaryotic cells, mitochondria are recognized as the powerhouses orchestrating cellular activities.7,8 This central function is well-established on the electrochemical gradient generated by the respiratory chain.6,9,10 Mitochondria also play critical roles in mediating lipid metabolism, Ca2+ homeostasis, and apoptosis.2,11 The concept of mitochondrial dysfunction originates from bioenergetics terminology, which is now extended to a broader relationship corresponding to their cellular environment.12 Inherited mitochondrial dysfunction resulting from deficiencies of mtDNA maintenance, and translation, stand as determinants in defining primary mitochondrial diseases (PMD), such as Leigh syndrome, mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS), and chronic progressive external ophthalmoplegia (CPEO).13,14,15 The coexistence of mutant and wild-type mtDNA, referred to as mtDNA heterogeneity, accumulates during aging and age-associated pathogenesis, and is associated with worsened clinical performance.16 On the other hand, many common pathologies are convergent to mitochondria, causing secondary mitochondrial dysfunction (SMD), such as heart failure, neurodegeneration, and metabolic syndrome.17,18,19 Abnormal dynamics and quality control, dysfunctional proteostasis, inhibited ATP production, calcium dyshomeostasis, and metabolic reprogramming often simultaneously occur and interplay within pathological conditions, thereby impacting mitochondria in their capacity as biosynthetic and signaling hubs.1,20 Consequently, it is unsurprising that mitochondrial dysfunction is frequently implicated in the pathophysiology of various diseases and the aging process.21,22,23,24,25

소개

지난 30년 동안 미토콘드리아는

미토콘드리아 생물학과 임상 연구의 상당한 발전에 힘입어 일

반적인 질병에 대한 유망한 치료 표적으로 부상했습니다. 1,2,3 (그림 1 참조)

박테리아의 조상으로부터 진화한 미토콘드리아는

미토콘드리아 DNA(mtDNA)로 알려진

제한적이지만 독특한 게놈 물질을 보유하고 있으며,

이 물질은 자가 복제 능력이 있고

미토콘드리아 내막에 위치한 호흡 복합체의 필수 구성 요소를 암호화할 수 있습니다.4,5,6

따라서

진핵 세포 내의 필수적인 내공생체로서 미토콘드리아는

세포 활동을 조율하는 동력원으로 인식되고 있습니다. 7,8

이 중심 기능은 호흡 사슬에 의해 생성된 전기화학적 구배에 의해 잘 확립되어 있습니다.6,9,10

미토콘드리아는 또한

지질 대사,

Ca2+ 항상성,

세포자멸사(apoptosis)를 매개하는 데 중요한 역할을 합니다.2,11

미토콘드리아 기능 장애의 개념은

현재 세포 환경에 해당하는 더 넓은 관계로 확장된 생체에너지학 용어에서 유래되었습니다. 12

미토콘드리아 DNA 유지 및 번역의 결핍으로 인한

유전성 미토콘드리아 기능 장애는

리 증후군, 미토콘드리아 근병증, 뇌병증, 젖산증 및 뇌졸중 유사 증후군(MELAS),

만성 진행성 외안근마비(CPEO)와 같은

원발성 미토콘드리아 질환(PMD)을 정의하는 결정 요인입니다. 13,14,15

돌연변이형과 야생형 mtDNA의 공존은

mtDNA 이질성이라고 불리며,

노화와 노화 관련 병리 발생 과정에서 축적되며,

임상적 성능의 악화와 관련이 있습니다.16

한편,

많은 일반적인 병리학이 미토콘드리아에 집중되어,

심부전, 신경 퇴행, 대사 증후군과 같은

이차적 미토콘드리아 기능 장애(SMD)를 유발합니다. 17,18,19

비정상적인 역학 및 품질 관리,

기능 장애가 있는 단백질 안정성,

억제된 ATP 생산, 칼슘 불균형, 그리고

대사 재프로그래밍은 병리학적 조건에서 종종 동시에 발생하고 상호 작용하여

생합성 및 신호 전달 허브로서의 미토콘드리아의 기능에 영향을 미칩니다.1,20

결과적으로,

미토콘드리아 기능 장애가

다양한 질병의 병태 생리학 및 노화 과정에 자주 관여한다는 것은 놀라운 일이 아닙니다.21,22,23,24,25

Fig. 1

The number of growing published articles or studies from 1980 to Aug 2023, based on mitochondrial medicine (pink), therapies (blue), and clinical trials (green). The publications of therapeutic targets increase following the pathfinding of mitochondria-related molecular mechanisms in many pathologies, including cardiac ischemia/reperfusion (IR) injury, stroke, and nonalcoholic steatohepatitis (NASH). Interventional clinical trials investigating the therapeutic potential of targeting mitochondrial dysfunction are also witnessing a yearly increment discernible in the volume. Data for this figure was extracted from PubMed by searching the term “mitochondri*” in combination with either “medicine”, “transplantation”, “transfer”, “administration”, “delivery”, “restore”, “rescue”, “treatment”, or “therap*”. Data of active clinical trials (recruiting, not yet recruiting, active, not recruiting, completed, enrolling by invitation, unknown status) were acquired from ClinicalTrials.gov

Full size image

In response to these challenges, advances in mitochondrial biology have spurred the development of mitochondria-targeted therapeutic strategies. There are abundant preclinical studies targeting mitochondrial defects, but clinical studies in humans are still scarce, showing great potential. These range from dietary interventions aimed at counteracting nutritional deficiencies to pharmacological approaches designed to directly modulate mitochondrial dynamics, enhance biogenesis, and mitigate oxidative stress. Notable interventions include: exercise protocols to promote the expression‎ of peroxisome proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1α), dietary supplements to target primary nutrient deficiency, nicotinamide riboside (NR) to augment nicotinamide adenine dinucleotide (NAD) biosynthesis26,27, MitoQ for neutralizing mitochondria-derived reactive oxygen species (ROS)28,29, the global antioxidant Coenzyme Q10 (CoQ10)30, N-acetyl cysteine (NAC)31, and the mitochondrial inhibitor ME-344 (known for its anti-tumor properties).32. These small molecular drugs can either exert their effects directly on the mitochondria or influence the organelle indirectly by binding to their cytosolic or nuclear regulators.1 The strategies of gene replacement therapies and gene editing technologies have also provided hope for addressing the severe prognosis of genetic mitochondrial defect, although the unique mitochondrial genome still entails a more comprehensive mechanistic understanding.33

Molecular components from mitochondria, owing to the similarity to their bacterial origins, have long been recognized as potent extracellular signals that elicit immune responses.34,35,36 Recent research into intercellular communication indicated that mitochondria could shuttle across cells horizontally via various transfer mechanisms, a process independent of the vertical inheritance seen during cell division.37 Importantly, this paradigm has revealed that mitochondrial components from healthy cells can precipitate a protective response to reconstitute mitochondrial bioenergetics in recipient cells that are metabolically compromised.38,39,40 Additionally, studies also demonstrated that damaged mitochondria were outsourced, as a new form of quality control, to tissue-resided macrophages for clearance in response to oxidative stresses and thus regulating tissue homeostasis.41,42 These findings have sparked interest in leveraging transplantation of functionally intact mitochondria and mitochondrial components as therapies for common diseases. Endeavors across various animal models and, even pediatric patients who received central extracorporeal membrane oxygenation (ECMO) support have substantiated the beneficial outcomes of these interventions.43,44,45,46 However, the clinical translation is still limited, including unstable mitochondrial vitality, inefficient cellular internalization, and transient therapeutic effects, which collectively hinder the medical practice.47

Here, as the therapeutic landscape for mitochondrial dysfunction continues to evolve, this review aims to provide a comprehensive view of mitochondrial signaling that controls cell fate, as well as the pathophysiology of mitochondrial dysfunction in common diseases. We also highlight the innovative approaches being pursued to improve clinical management of mitochondrial dysfunction through a critical examination of dietary supplements, pharmacological agents, and mitochondrial transplantation. A deeper understanding of mitochondrial contributions to common pathologies will further elucidate their roles in disease and may reveal co-dependent therapeutic targets.

이러한 도전과제에 대응하기 위해 미토콘드리아 생물학의 발전은

미토콘드리아를 표적으로 하는 치료 전략의 개발을 촉진했습니다.

미토콘드리아 결함을 표적으로 하는 전임상 연구는 풍부하지만,

인간을 대상으로 한 임상 연구는 아직 부족하여

큰 잠재력을 보여주고 있습니다.

영양 결핍을 막기 위한 식이요법부터 미토콘드리아의 역학을 직접 조절하고,

생합성을 강화하며, 산화 스트레스를 완화하기 위한 약리학적 접근에 이르기까지 다양합니다.

주목할 만한 중재에는 다음이 포함됩니다:

퍼옥시좀 증식자 활성화 수용체 감마 코액티베이터-1 알파(PGC-1α)의 발현을 촉진하는 운동 프로토콜,

1차 영양소 결핍을 목표로 하는 식이 보조제,

니코틴아미드 리보사이드(NR)를 통해 니코틴아미드 아데닌 디뉴클레오티드(NAD) 생합성을 강화26,27,

미토콘드리아에서 생성된 활성산소(ROS)를 중화시키는 미토큐(MitoQ)28,29,

전신 항산화제인 코엔자임 Q10(CoQ10)30,

N-아세틸 시스테인(NAC)31,

그리고 항종양 특성으로 알려진 미토콘드리아 억제제인 ME-344(ME-344)를 사용합니다. 32.

Notable interventions include:

exercise protocols to promote the expression‎ of peroxisome proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1α),

dietary supplements to target primary nutrient deficiency,

nicotinamide riboside (NR) to augment nicotinamide adenine dinucleotide (NAD) biosynthesis26,27,

MitoQ for neutralizing mitochondria-derived reactive oxygen species (ROS)28,29,

the global antioxidant Coenzyme Q10 (CoQ10)30,

N-acetyl cysteine (NAC)31, and the

mitochondrial inhibitor ME-344 (known for its anti-tumor properties

https://www.nature.com/articles/s41392-024-01756-w

PGC-1α, PGC-1β, PGC-1 관련 코액티베이터(PRC)를 포함하는 3개의 구성원으로 이루어진 퍼옥시좀 증식자 활성화 수용체 감마 코액티베이터-1(PGC-1s) 계열은 25년 전에 발견되었습니다. PGC-1s는 미토콘드리아 기능, 산화 스트레스, 소포체 항상성, 염증 등 많은 중요한 세포 활동에 필수적인 협응력을 발휘합니다. PGC-1s가 암, 심장병, 심혈관 질환, 신경계 질환, 신장 질환, 운동계 질환, 대사 장애 등 많은 질병과 관련이 있다는 증거가 축적되고 있습니다. PGC-1의 상류 조절 인자와 공동 활성화 파트너를 조사하고 PGC-1의 하류 효과기에 의해 조절되는 중요한 생물학적 사건을 확인하면 PGC-1의 정교한 네트워크를 제시하는 데 도움이 됩니다. 또한, PGC-1과 질병 간의 상관 관계에 대해 논의하고 PGC-1을 표적으로 하는 치료법을 요약하면 개별화되고 정확한 개입 방법을 만드는 데 도움이 됩니다. 이 리뷰에서는 PGC-1s 계열에 관한 기본 지식과 분자 조절 네트워크를 요약하고, 인간 질병에서 PGC-1s의 생리학적-병리학적 역할에 대해 논의하며, PGC-1s의 진단 및 예후적 가치와 전임상 연구에서의 여러 치료법을 포함한 PGC-1s의 적용을 검토하고, 향후 연구에 대한 몇 가지 방향을 제안합니다. 이 리뷰는 질병 치료에 PGC-1을 활용하는 데 있어 그 엄청난 잠재력을 보여주고, 새로운 치료 표적으로서의 PGC-1의 홍보를 촉진할 수 있기를 희망합니다.

생물학적 기능에서 PGC-1의 중요한 조절 역할에 대한 도식적 표현. PGC-1s, 특히 PGC-1α는 미토콘드리아 생합성, 분열, 융합, 미토파지를 포함한 미토콘드리아 생명 주기의 전체 과정을 조율합니다. 이 과정에서 NRF, Mfn1/2, Opa1, Drp1, Parkin과 같은 코액티베이터와 다운스트림 이펙터를 조절합니다. PGC-1α는 SOD, GPX, CAT, PRDX3를 포함한 항산화 단백질과 관련된 광범위한 유전자 발현을 상향 조절함으로써 산화적 손상에 맞서 싸웁니다. 또한, PGC-1α와 PGC-1β는 전염증성 인자를 억제함으로써 항염증 효과를 발휘합니다. 또한, PGC-1α는 CHOP, ATF3, GRP94를 상향 조절함으로써 ER 스트레스를 개선할 수 있습니다.

https://pmc.ncbi.nlm.nih.gov/articles/PMC5842119/

https://www.mdpi.com/2218-273X/10/2/320

https://pmc.ncbi.nlm.nih.gov/articles/PMC10959587/

이러한 작은 분자 약물은

미토콘드리아에 직접 작용하거나 세포질 또는 핵 조절 인자에 결합하여

세포 기관에 간접적으로 영향을 미칠 수 있습니다.1

유전자 대체 요법과 유전자 편집 기술의 전략은 유전적 미토콘드리아 결함의 심각한 예후를 해결할 수 있는 희망을 제공했지만, 독특한 미토콘드리아 게놈은 여전히 더 포괄적인 기계적 이해를 필요로 합니다.33

미토콘드리아의 분자 구성 요소는

박테리아 기원과 유사하기 때문에 면역 반응을 유도하는

강력한 세포 외 신호로 오랫동안 인식되어 왔습니다.34,35,36

최근 세포 간 통신에 대한 연구에 따르면

미토콘드리아는

다양한 전달 메커니즘을 통해 세포를 가로질러 이동할 수 있으며,

이는 세포 분열 과정에서 나타나는 수직 상속과는 무관하게 일어나는 과정입니다. 37

중요한 것은, 이 패러다임은

건강한 세포의 미토콘드리아 구성 요소가

대사적으로 손상된 수용 세포에서 미토콘드리아 생체 에너지를 재구성하기 위한

보호 반응을 촉발할 수 있다는 것을 밝혀냈습니다.38,39,40

또한, 손상된 미토콘드리아가 새로운 형태의 품질 관리로서 조직에 상주하는 대식세포에 의해 산화 스트레스에 대한 반응으로 제거되어 조직 항상성을 조절한다는 사실도 연구에서 밝혀졌습니다. 41,42 이러한 발견은 기능적으로 온전한 미토콘드리아와 미토콘드리아 구성 요소를 이식하여 일반적인 질병을 치료하는 방법에 대한 관심을 불러일으켰습니다. 다양한 동물 모델에 대한 노력과 심지어 중앙 체외막 산소화(ECMO) 지원을 받은 소아 환자들에 대한 노력도 이러한 개입의 유익한 결과를 입증했습니다.43,44,45,46 그러나 임상적 적용은 여전히 제한적이며, 불안정한 미토콘드리아 활력, 비효율적인 세포 내재화, 일시적인 치료 효과 등 여러 가지 요인으로 인해 의료 행위가 저해되고 있습니다.47

미토콘드리아 기능 장애에 대한 치료적 관점이 계속 진화함에 따라,

이 리뷰는 세포의 운명을 조절하는 미토콘드리아 신호 전달과 일반적인 질병에서 나타나는

미토콘드리아 기능 장애의 병태 생리학에 대한 포괄적인 관점을 제공하는 것을 목표로 합니다.

또한, 식이 보조제, 약리 작용제, 미토콘드리아 이식을 비판적으로 검토하여

미토콘드리아 기능 장애의 임상 관리를 개선하기 위해 추구되고 있는 혁신적인 접근 방식을 강조합니다.

일반적인 병리에 대한 미토콘드리아의 기여에 대한 심층적인 이해는

질병에서 미토콘드리아의 역할을 더 명확하게 하고,

상호 의존적인 치료 표적을 밝혀낼 수 있습니다

Overview of mitochondrial functions and behaviors

Mitochondria, the cell’s proverbial powerhouses, present a multifaceted biological landscape in biosynthesis and intra-/inter-cellular signaling pathways. Within these pivotal roles lie a harmonized regulation of static integrity and dynamic processes, such as membrane potential homeostasis and mitophagy, mtDNA maintenance and protein synthesis, inter-organelle contacts and Ca2+ regulation, enzymatic activity, and metabolic signaling. Here, we summarize the mitochondrial overview of cell-dependent behaviors, structure basis, and fuel metabolism (See Fig. 2).

Fig. 2

Schematic overview of mitochondrial activities. Mitochondria play a critical role in maintaining cell homeostasis by quality control, energy production, and metabolic regulation. Mitochondrial dynamics, including the processes of fission and fusion, are crucial for shaping mitochondrial structure, ensuring proper distribution across the cell, and facilitating selective clearance of damaged or dysfunctional mitochondria via degradative or secretory pathways, which plays a critical role in mitochondrial quality control. The core of mitochondrial energy production lies in the respiratory chain, fueled by the Krebs cycle and electron transport. The metabolism of fatty acids and glutamine into acetyl-CoA and alpha-ketoglutarate (α-kG), respectively, feeds into the Krebs cycle, culminating in ATP synthesis, illustrating the mitochondria’s central role in cellular energy and metabolic regulation

미토콘드리아 활동의 개략적 개요. 

미토콘드리아는 품질 관리, 에너지 생산, 대사 조절을 통해 세포 항상성을 유지하는 데 중요한 역할을 합니다. 분열과 융합 과정을 포함한 미토콘드리아의 역동성은 미토콘드리아 구조를 형성하고, 세포 전체에 적절하게 분포되도록 하며, 손상되거나 기능 장애가 있는 미토콘드리아를 선택적으로 제거하는 데 중요한 역할을 합니다. 미토콘드리아 에너지 생산의 핵심은 크렙스 회로와 전자 수송에 의해 연료가 공급되는 호흡 사슬에 있습니다. 지방산과 글루타민이 각각 아세틸-CoA와 알파-케토글루타레이트(α-kG)로 대사되면 크렙스 회로에 공급되어 ATP 합성을 촉진함으로써 세포 에너지와 대사 조절에 있어 미토콘드리아의 중심적인 역할을 보여줍니다.

Full size image

In cellular architecture, the mitochondrial network is influenced by quality control systems (biogenesis, dynamics, mitophagy, and proteolysis).48,49 The transcriptional activation of mitochondrial biogenesis is governed by synergistic interactions with nucleus. The upregulation of the proliferator-activated receptor-γ co-activator 1α (PGC1α) initiates the transcription cascade, activating transcription factors like nuclear respiratory factors (NRF-1, NRF-2) to the final expression‎ of mitochondrial transcription factor A (TFAM), which promotes mtDNA transcription and replication.50,51 These regulatory proteins are often subject to regulation via post-translational modifications (PTMs), exemplified by the action of AMP-activated kinase (AMPK). As an energy sensor, AMPK is allosterically activated by fluctuations in the ATP:ADP or ATP:AMP ratios, which downregulates energy-intensive anabolic pathways while concurrently upregulating energy-generating catabolic processes to restore energetic homeostasis.52 In addition, 99% of mitochondrial proteins are encoded by nuclear genes and imported into the mitochondrial compartments through different sorting and import machineries, which are under surveillance by cytosolic ribosomes, the ubiquitin–proteasome system, chaperones, and intramitochondrial proteases.48,53 Once synthesized in the cytosol, these precursor proteins, bearing distinctive amino-terminal cleavable targeting motifs, are recognized and translocated by the translocase of the outer/inner membrane (TOM/TIM) complex, such as TOM20, TOM22, TOM40, and TIM23.54,55,56,57 These channels and transporters are modulated by various PTMs, including phosphorylation, oxidative modifications, ions, and metabolites binding, glycosylation, acetylation, and others. On the other hand, mitochondrial fission and fusion define mitochondrial morphology, distribution, and turnover, which are regulated by highly conserved proteins and organelles to ensure optimal mitochondrial function. Dynamin-related protein 1 (Drp1) facilitates mitochondrial fission by translocating from the cytosol to the outer membrane of mitochondria, where it binds to the adaptors (including MID49, MID51, mitochondrial fission factor) and oligomerizes in a GTPase-dependent fashion to form the fission foci and rings.58,59,60 The depletion of Drp1 and the adaptor proteins leads to the elongation of mitochondria.61,62 Preceding the recruitment and assembly of Drp1, the endoplasmic reticulum-mitochondria membrane contact sites were marked in close proximity to future fission sites, contributing to pre-constriction, nucleoid replication, and deep regulation of fission fates.61,63,64,65 Mitophagy is a specialized form of autophagy, dedicated to the selective clearance of aged or damaged mitochondria. This process is tightly regulated by a host of signaling pathways and proteins, most notably the PINK1-Parkin pathway. Upon mitochondrial damage or stress, PINK1 accumulates on the mitochondrial outer membrane, recruiting the E3 ubiquitin ligase Parkin.66,67 This subsequently triggers the ubiquitination of mitochondrial proteins, effectively marking these damaged components for removal. These ubiquitinated mitochondria are subsequently engulfed by autophagosomes, which coalesce with lysosomes, facilitating the systematic degradation and recycling of mitochondrial contents. In addition, the mitochondria-derived vesicle represented another form of selective degradation, which might be directed to lysosomes, peroxisomes, or extracellular milieus.68,69,70,71

Within mitochondria, the folded mitochondrial inner membrane (IMM) provides an extensive surface area of protein import and effective oxidative phosphorylation.72,73 Embedded in the IMM, the respiratory chain consists of a series of protein complexes (I to IV) and ATP synthase, collectively forming the core components of mitochondrial energy production. These complexes function sequentially to transfer electrons derived from NADH and FADH2 to form an electrochemical gradient. This electron transport culminates in the reduction of molecular oxygen to water. The reduction potential difference driving electron movement through complexes I, III, and IV is used to pump protons, which builds up a protonmotive force (Δp) across the IMM. The resulting electrochemical gradient, or proton motive force, drives ATP synthesis as protons flow back into the mitochondrial matrix through ATP synthase, thereby coupling electron transport with oxidative phosphorylation.74 Integral to this process are key metabolic substrates: pyruvate, fatty acid, and glutamine are metabolized to form acetyl-CoA and alpha-ketoglutarate (α-kG), respectively.75 These feed into the tricarboxylic acid (TCA) cycle, further fueling electron transport and ATP production.

세포 구조에서 미토콘드리아 네트워크는 품질 관리 시스템(생성, 역학, 미토파지, 단백질 분해)의 영향을 받습니다.48,49 미토콘드리아 생성의 전사 활성화는 핵과의 시너지 상호 작용에 의해 조절됩니다.

증식 활성화 수용체-γ 공동 활성화 인자 1α(PGC1α)의 상향 조절은

전사 캐스케이드를 시작하여 미토콘드리아 전사 인자 A(TFAM)의 최종 발현에

핵 호흡 인자(NRF-1, NRF-2)와 같은 전사 인자를 활성화하여

mtDNA 전사와 복제를 촉진합니다. 50,51

이러한 조절 단백질은 AMP-활성화 키나아제(AMPK)의 작용을 예로 들 수 있는 번역 후 변형(PTM)을 통해 조절되는 경우가 많습니다. 에너지 센서인 AMPK는 ATP:ADP 또는 ATP:AMP 비율의 변동에 의해 알로스테릭 방식으로 활성화되어, 에너지 집약적인 동화 작용 경로를 하향 조절하는 동시에 에너지 생성 이화 작용 과정을 상향 조절하여 에너지 항상성을 회복합니다. 52

또한, 미토콘드리아 단백질의 99%는 핵 유전자에 의해 암호화되어 다양한 분류 및 수입 기계를 통해 미토콘드리아 구획으로 수입되는데, 이 과정은 세포질 리보솜, 유비퀴틴-프로테아제 시스템, 샤페론, 미토콘드리아 내 프로테아제의 감시를 받습니다. 48,53 일단 세포질에서 합성된 후, 독특한 아미노 말단 절단 가능한 표적화 모티프를 가진 이러한 전구체 단백질은 외부/내부 막(TOM/TIM) 복합체의 전좌 효소(예: TOM20, TOM22, TOM40, TIM23)에 의해 인식되고 전좌됩니다. 54,55,56,57 이러한 채널과 수송체는 인산화, 산화적 변형, 이온 및 대사 산물 결합, 당화, 아세틸화 등을 포함한 다양한 PTM에 의해 조절됩니다.

한편, 미토콘드리아의 분열과 융합은 미토콘드리아의 형태, 분포, 회전율을 결정하며, 이는 최적의 미토콘드리아 기능을 보장하기 위해 고도로 보존된 단백질과 세포기관에 의해 조절됩니다.

Dynamin-related protein 1(Drp1)은 세포질에서 미토콘드리아의 외막으로 이동하여 adaptor(MID49, MID51, mitochondrial fission factor 포함)에 결합하고 GTPase에 의존하는 방식으로 올리고머화되어 핵분열의 초점과 고리를 형성함으로써 미토콘드리아 분열을 촉진합니다. 58,59,60 Drp1과 어댑터 단백질의 고갈은 미토콘드리아의 신장을 초래합니다.61,62 Drp1의 모집과 조립에 앞서, 소포체-미토콘드리아 막 접촉 부위는 미래의 분열 부위와 가까운 근접 거리에 표시되어, 사전 수축, 핵형 복제, 분열 운명의 심층 조절에 기여합니다. 61,63,64,65

미토파지는 노화되거나 손상된 미토콘드리아를 선택적으로 제거하는 오토파지의 특수한 형태입니다. 이 과정은 수많은 신호 전달 경로와 단백질, 특히 PINK1-Parkin 경로에 의해 엄격하게 조절됩니다. 미토콘드리아가 손상되거나 스트레스를 받으면, PINK1이 미토콘드리아 외막에 축적되어 E3 유비퀴틴 리가제인 파킨을 모집합니다.66,67 이로 인해 미토콘드리아 단백질의 유비퀴틴화가 촉발되어, 손상된 구성 요소를 효과적으로 제거할 수 있도록 표시합니다. 이러한 유비퀴틴화된 미토콘드리아는 이후 자가포식체(autophagosome)에 의해 포획되어 리소좀과 합쳐지면서 미토콘드리아 내용물의 체계적인 분해와 재활용을 촉진합니다. 또한, 미토콘드리아에서 유래된 소포체는 또 다른 형태의 선택적 분해를 나타내는데, 이는 리소좀, 퍼옥시좀 또는 세포외 환경으로 향할 수 있습니다.68,69,70,71

미토콘드리아 내에서, 접힌 미토콘드리아 내막(IMM)은 단백질 수입과 효과적인 산화적 인산화반응을 위한 광범위한 표면적을 제공합니다.72,73 IMM에 내장된 호흡 연쇄는 일련의 단백질 복합체(I~IV)와 ATP 신타제로 구성되어 있으며, 미토콘드리아 에너지 생산의 핵심 구성 요소를 형성합니다. 이 복합체는 NADH와 FADH2에서 파생된 전자를 순차적으로 전달하여 전기화학적 구배를 형성합니다. 이 전자 수송은 분자 산소를 물로 환원하는 것으로 마무리됩니다. 복합체 I, III, IV를 통해 전자의 이동을 유도하는 환원 전위차는 양성자를 펌핑하는 데 사용되며, 이로 인해 IMM에 양성자 운동력(Δp)이 형성됩니다. 그 결과 생성되는 전기화학적 구배 또는 양성자 운동력은 ATP 합성을 촉진합니다. ATP 합성을 통해 양성자가 미토콘드리아 매트릭스로 다시 유입되면서 전자 수송과 산화적 인산화반응이 결합됩니다. 74 이 과정의 핵심은 피루브산, 지방산, 글루타민과 같은 대사 기질입니다. 이들은 각각 아세틸-CoA와 알파-케토글루타레이트(α-kG)로 대사됩니다.75 이들은 트리카르복실산(TCA) 순환계로 들어가 전자 수송과 ATP 생성을 촉진합니다.

Mitochondrial as signaling hubs in defining cell fate

Mitochondria epitomize an example of endosymbiotic integration, coordinating the cellular and organelle signal transduction critical for adaptive responses and evolutionary processes. Central to this coordination is the Mitochondrial Information Processing System (MIPS), a sophisticated mechanism that decodes complex amalgamations of ions, proteins, nutrients, and energetic states into targeted genetic programs.20 These programs, in turn, initiate a strategic reorganization of metabolic pathways, thereby catalyzing cell proliferation, differentiation, contractility, secretion, and apoptosis, foundational for organismal survival and evolutionary fitness. However, the mechanism by which these pathways intersect within mitochondria to sustain homeostasis is yet to be elucidated. Subsequent discussions will delve into the convergent roles of mitochondria in calcium homeostasis, the intricate networks of energy and nutrient sensing orchestrated by AMPK, mTOR, and Sirtuins, as well as the influence of endogenous mtDNA on innate immune pathways through the cyclic GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING), inflammasomes, and membrane-bound Toll-like receptors (TLRs) signaling. (See Fig. 3) These topics collectively underscore the central role of mitochondria as arbiters of cellular function and integrity, highlighting their potential as therapeutic targets for modulating health and disease states.

세포의 운명을 결정하는 신호 전달 허브로서의 미토콘드리아

미토콘드리아는 적응 반응과 진화 과정에 중요한 세포 및 세포 기관의 신호 전달을 조정하는 내공생 통합의 전형적인 예입니다. 이러한 협응력의 핵심은 미토콘드리아 정보 처리 시스템(Mitochondrial Information Processing System, MIPS)입니다. 이 시스템은 이온, 단백질, 영양소, 에너지 상태의 복잡한 혼합을 표적 유전자 프로그램으로 해독하는 정교한 메커니즘입니다.20 이러한 프로그램은 차례로 대사 경로의 전략적 재구성을 시작하여, 유기체의 생존과 진화적 적합성의 기초가 되는 세포 증식, 분화, 수축성, 분비, 세포자멸사를 촉진합니다. 그러나 이러한 경로가 미토콘드리아 내에서 교차하여 항상성을 유지하는 메커니즘은 아직 밝혀지지 않았습니다. 이후의 토론에서는 칼슘 항상성에서 미토콘드리아의 집중적인 역할, AMPK, mTOR, 시르투인(Sirtuins)에 의해 조율되는 에너지와 영양소 감지의 복잡한 네트워크, 주기적 GMP-AMP 합성효소(cGAS)와 인터페론 유전자 자극기(STING), 염증성 세포내소포체(inflammasomes), 막에 결합된 Toll-like 수용체(TLRs) 신호 전달을 통한 선천성 면역 경로에 대한 내인성 mtDNA의 영향에 대해 자세히 살펴볼 것입니다. (그림 3 참조) 이러한 주제는 세포 기능과 완전성의 중재자로서 미토콘드리아의 중심적인 역할을 강조하고, 건강과 질병 상태를 조절하는 치료 표적으로서의 잠재력을 강조합니다.

Fig. 3

Convergence of cell signaling pathways to mitochondria.

This figure delineates the integration of diverse cell signaling pathways converging on mitochondria, illustrating their central role in cellular homeostasis and stress responses. Highlighted pathways include calcium homeostasis, crucial for mitochondrial function and energy production; energy and nutrient sensing through AMPK and mTOR signaling; the innate immune response mediated by cGAS-STING, inflammasomes, and TLR9 endosomal pathways; apoptosis regulation; the unfolded protein response (UPR) as a key element of mitochondrial stress response; and the induction of cellular senescence. Together, these pathways underscore the mitochondria’s pivotal role in orchestrating cellular adaptation and survival mechanisms

세포 신호 전달 경로의 미토콘드리아로의 집중.
이 그림은 미토콘드리아에 집중되는 다양한 세포 신호 전달 경로의 통합을 묘사하고, 세포 항상성과 스트레스 반응에서 그들의 중심적인 역할을 보여줍니다. 강조된 경로에는 미토콘드리아 기능과 에너지 생산에 중요한 칼슘 항상성, AMPK와 mTOR 신호를 통한 에너지와 영양소 감지, cGAS-STING, 염증성 소포체, TLR9 엔도솜 경로를 통한 선천성 면역 반응, 세포자멸사 조절, 미토콘드리아 스트레스 반응의 핵심 요소인 미개봉 단백질 반응(UPR), 세포 노화 유도 등이 포함됩니다. 이러한 경로는 세포 적응과 생존 메커니즘을 조율하는 데 있어 미토콘드리아의 중추적 역할을 강조합니다.

Full size image

Calcium homeostasis

Calcium ions (Ca2+) are critical intracellular second messengers that orchestrate a wide array of cellular functions. Central to this role are mitochondria, which modulate Ca2+ signals through sophisticated mechanisms of uptake, storage, and efflux, particularly in response to changes in energy demand, metabolic shifts, and apoptotic signals.76,77 This intricate management of calcium allows mitochondria to control the activation of Ca2+-sensitive dehydrogenases within the Krebs cycle, subsequently enhancing ATP production to meet cellular energy demands.78 Increased mitochondrial Ca2+ is beneficial up to a point; however, chronic mitochondrial Ca2+ overload triggered Bcl-2-mediated regulated cell death, which is primarily mediated through the induction of the mitochondrial permeability transition pore (mPTP).79 In addition, although mitochondrial Ca2+ plays a minor role in reducing cytosolic Ca2+ concentrations in cardiomyocytes, their function is essential in buffering intracellular calcium levels in non-muscle cells with limited SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase) and NCX (Na+/Ca2+ exchanger) activity.80 In some spatially constrained domains, such as the proximal ER-mitochondria contact sites (also termed ‘mitochondria-associated membranes, MAMs’), calcium release is concentrated, and regulated by PTMs.81,82 The structural basis of the calcium release-associated MAMs is a series of tethering complexes that contain both subdomains from ER and mitochondria, such as vesicle-associated membrane protein-associated protein B (VAPB) on the ER and protein tyrosine phosphatase interacting protein 51 (PTPIP51) on the mitochondria (i.e., VAPB-PTPIP51); the inositol triphosphate receptor (IP3R)/glucose-regulated protein 75 (Grp75)/voltage-dependent anion (VDAC1) and sporadic PD-related protein DJ1 complex; mitofusin-2 (Mfn2) on the ER with Mfn1/Mfn2 on the OMM; Rho GTPases Miro1 and Miro2; and others.83,84,85,86,87 In aged endothelial cells, the enhancement of MAMs is associated with increased mitochondrial Ca2+ uptake but also cell death.88

The OMM is highly permeable to Ca2+ ions, primarily due to the expression‎ of VDACs which facilitate the direct and indirect calcium flow from the ER, SR, and lysosome.89,90 VDACs are differentiated into three isoforms—VDAC1, VDAC2, and VDAC3—each displaying tissue-specific distributions and capable of adopting various voltage-dependent conformations.91 Importantly, while VDACs enable Ca2+ flux across their open and closed states, the permeability for Ca2+ is significantly enhanced in the closed state, where there is concurrently reduced permeability to metabolites.92 Furthermore, advanced single cell imaging data indicate that Ca2+ may facilitate the VDAC-mediated cation and ATP transport across the OMM.93 Additionally, mitochondrial Ca2+ uptake is driven by the membrane potential difference (ΔΨ) generated by the activity of the respiratory chain, which is critical for driving the ingress of positively charged ions into the mitochondrial matrix.94 To reach the mitochondrial matrix, cytosolic Ca2+ has to cross the IMM, mainly through the mitochondrial Ca2+ uniporter (MCU) complex. The MCU complex is composed of the channel-forming MCU, the integral scaffolding protein, the essential mitochondrial response element (EMRE), and the mitochondrial calcium uptake (MICU) that senses matrix Ca2+ to inhibit or sensitize MCU-mediated Ca2+ uptake.95,96,97,98 Patch-clamp electrophysiology indicated that this inwardly rectifying channel bound Ca2+ with extremely high affinity (dissociation constant <2 nM), despite the relatively low cytoplasmic Ca2+ concentrations.99 However, Ca2+ does not remain inside mitochondria but instead is rapidly extruded into the cytoplasm through a complex system of Ca2+ antiporters, restoring the basal state. The Na+/Ca2+ exchanger NCLX localized to the IMM is an established NCX family protein mediating the efflux of matrix mitochondrial calcium.100 Deletion of Slc8b1 (encoding NCLX) in adult mouse hearts causes heart failure and sudden death, correlated with Ca2+ overload induced by mPTP.101 In addition, the H+/Ca2+ exchanger, such as leucine zipper, EF hand-containing transmembrane protein 1 (LETM1), leads to mitochondrial Ca2+ release in mammalian cells.102 The Letm1-deficient mice showed reduced Ca2+ mitochondrial uptake at a low cytosolic Ca2+ level, with impaired mitochondrial ATP generation capacity, disrupted early embryonic development, altered glucose metabolism, and increased susceptibility to seizure.103

칼슘 항상성

칼슘 이온(Ca2+)은 세포 내의 다양한 세포 기능을 조율하는 중요한 세포 내 2차 메신저입니다. 이 역할의 중심에는 미토콘드리아가 있는데, 미토콘드리아는 특히 에너지 수요의 변화, 대사 변화, 세포 자멸 신호에 반응하여 흡수, 저장, 유출의 정교한 메커니즘을 통해 Ca2+ 신호를 조절합니다. 76,77 이렇게 복잡한 칼슘 관리 덕분에 미토콘드리아는 크렙스 사이클 내에서 Ca2+에 민감한 탈수소효소의 활성화를 조절할 수 있으며, 그 결과 세포 에너지 수요를 충족시키기 위해 ATP 생산을 향상시킬 수 있습니다.78

미토콘드리아 Ca2+의 증가는 어느 정도까지는 유익하지만, 만성적인 미토콘드리아 Ca2+ 과부하는 Bcl-2 매개 조절 세포 사멸을 유발하는데, 이는 주로 미토콘드리아 투과성 전이 기공(mPTP)의 유도를 통해 매개됩니다. 79

또한, 미토콘드리아 Ca2+는 심근세포에서 세포질 Ca2+ 농도를 감소시키는 데 있어 작은 역할을 하지만, SERCA(근질/소포체 Ca2+-ATPase)와 NCX(Na+/Ca2+ 교환기) 활동이 제한적인 비근육 세포에서 세포 내 칼슘 수준을 완충하는 데 있어 그 기능은 필수적입니다. 80 근위부 ER-미토콘드리아 접촉 부위('미토콘드리아 관련 막, MAMs'라고도 함)와 같이 공간적으로 제약된 일부 영역에서는 칼슘 방출이 집중되어 있으며, PTM에 의해 조절됩니다. 81,82 칼슘 방출 관련 MAM의 구조적 기반은 소포체와 미토콘드리아의 서브도메인을 모두 포함하는 일련의 테더링 복합체입니다. 예를 들어, 소포체에는 소포체 관련 막 단백질-관련 단백질 B(VAPB)가 있고, 미토콘드리아에는 단백질 티로신 포스파타제 상호작용 단백질 51(PTPIP51)이 있습니다(즉, VAPB-PTPIP51). 이노시톨 삼인산 수용체(IP3R)/포도당 조절 단백질 75(Grp75)/전압 의존성 음이온(VDAC1) 및 산발적 PD 관련 단백질 DJ1 복합체; 미토푸신-2(Mfn2)는 ER에, Mfn1/Mfn2는 OMM에; Rho GTPase Miro1과 Miro2; 그리고 기타. 83,84,85,86,87 노화된 내피 세포에서 MAM의 증가는 미토콘드리아의 Ca2+ 흡수 증가와 관련이 있지만, 세포 사멸과도 관련이 있습니다.88

OMM은 ER, SR, 리소좀에서 직접적, 간접적으로 칼슘이 흐르도록 하는 VDAC의 발현으로 인해 Ca2+ 이온의 침투가 매우 용이합니다.89,90 VDAC은 VDAC1, VDAC2, VDAC3의 세 가지 이소폼으로 구분되며, 각 이소폼은 조직 특이적 분포를 나타내며 다양한 전압 의존적 형태를 취할 수 있습니다. 91 중요한 것은, VDAC가 개방 상태와 폐쇄 상태 모두에서 Ca2+의 흐름을 가능하게 하는 반면, Ca2+의 투과성은 대사물질에 대한 투과성이 감소하는 동시에 폐쇄 상태에서 상당히 향상된다는 것입니다.92 또한, 첨단 단일 세포 이미징 데이터에 따르면, Ca2+가 OMM을 통한 VDAC 매개 양이온과 ATP 수송을 촉진할 수 있다고 합니다. 93 또한, 미토콘드리아 Ca2+ 흡수는 호흡 사슬의 활동에 의해 생성된 막 전위차(ΔΨ)에 의해 주도되는데, 이는 양전하를 띤 이온이 미토콘드리아 매트릭스로 유입되는 것을 유도하는 데 매우 중요합니다.94 세포질 Ca2+가 미토콘드리아 매트릭스에 도달하려면 주로 미토콘드리아 Ca2+ 유니포터(MCU) 복합체를 통해 IMM을 통과해야 합니다. MCU 복합체는 채널 형성 MCU, 통합 비계 단백질, 필수 미토콘드리아 반응 요소(EMRE), 그리고 매트릭스 Ca2+를 감지하여 MCU 매개 Ca2+ 흡수를 억제하거나 민감하게 하는 미토콘드리아 칼슘 흡수(MICU)로 구성되어 있습니다. 95,96,97,98 패치 클램프 전기 생리학은 상대적으로 낮은 세포질 Ca2+ 농도에도 불구하고 이 내향 정류 채널이 Ca2+와 매우 높은 친화력(해리 상수 2 nM 미만)을 가지고 있음을 나타냅니다.99 그러나 Ca2+는 미토콘드리아 내부에 남아 있지 않고, 복잡한 Ca2+ 항포터 시스템을 통해 세포질로 빠르게 배출되어 기저 상태를 회복합니다. IMM에 국한된 Na+/Ca2+ 교환기 NCLX는 매트릭스 미토콘드리아 칼슘의 유출을 매개하는 확립된 NCX 패밀리 단백질입니다.100 성인 생쥐 심장에서 Slc8b1(NCLX를 암호화)의 결핍은 mPTP에 의해 유발된 Ca2+ 과부하와 상관관계가 있는 심부전과 돌연사를 유발합니다. 101 또한, 류신 지퍼(leucine zipper), EF 손잡이 함유 막관통 단백질 1(LETM1)과 같은 H+/Ca2+ 교환기는 포유류 세포에서 미토콘드리아 Ca2+ 방출을 유도합니다. 102 Letm1 결핍 생쥐는 낮은 세포질 Ca2+ 수준에서 Ca2+ 미토콘드리아 흡수 감소, 미토콘드리아 ATP 생성 능력 손상, 초기 배아 발달 장애, 포도당 대사 변화, 발작에 대한 민감성 증가를 보였습니다.103

Energy and nutrient sensing AMPK

AMPK is considered the primary sensor of decreased cellular AMP to ATP and ADP to ATP and subsequently acts to increase catabolic reactions and decrease anabolic reactions. Thus, AMPK activity is tightly intertwining its function with mitochondrial health, including biogenesis and dynamics. AMPK relies on PGC-1α to significantly influence GLUT4 and mitochondrial gene expression‎ in skeletal muscle, and it activates PGC-1α through direct phosphorylation at threonine-177 and serine-538.104 In addition, AMPK subunit β1 and β2 isoforms knock out mice are physically inactive and have a drastically impaired capacity for treadmill running that is associated with reductions in skeletal muscle mitochondrial content.105 Treatment with the AMPK agonist AICAR led to partial correction of COX deficiency in the defective cytochrome c-oxidase (COX) mouse models, and, importantly, significant motor improvement up to normal in knockout/knockin mice for Sco2.106 Disruptions in adiponectin and adiponectin receptor 1-induced AMPK-PGC-1α pathway contributed to mitochondrial dysfunction and insulin resistance observed in diabetes.107 The underlying mechanism by which AMPK promotes mitochondrial biogenesis is through activation of nuclear respiratory factor-1 (NRF-1).108 In subarachnoid hemorrhage (SAH)-induced early brain injury, AMPK and subsequent PGC-1α were activated by exogenous hsp22, thereby maintaining neurological function via the regulation of NRF1/TFAM-dependent mitochondrial biogenesis and Drp1-mediated.109 Additionally, mitochondrial quality control is also under the control of AMPK. Direct pharmacological activation of AMPK was sufficient to rapidly promote mitochondrial fission through the mitochondrial fission factor (MFF) phosphorylation, even in the absence of mitochondrial stress.110 The AMPK signaling pathway could also activate OPA1‐related mitochondrial fusion and mitophagy, thus attenuating cardiac I/R injury and relieving mitochondrial stress.111 Acute exercise-induced mitochondrial oxidative stress initiates AMPK activation, leading to phosphorylated unc-51 like autophagy activating kinase 1 (Ulk1)-dependent mitophagy in skeletal muscle.112 This was possibly due to the phosphorylation event of Parkin by ULK1 at Ser108 in its nine amino acid (“ACT”) domain at the early phase of AMPK activation.113 Recently, under energy stress conditions, AMPK could translocate to MAMs and directly interact with MFN2 to initiate mitochondrial fission and autophagy.114 In addition, in response to the low energy status in mitosis, the energy sensor AMPK phosphorylates and activates MCU, allowing Ca2+ entry into mitochondria to boost mitochondrial respiration.115 Thus, AMPK emerges as a central regulator of cellular energy balance, directly influencing mitochondrial function and dynamics, with implications for treating metabolic disorders and enhancing mitochondrial resilience against stress and disease.

에너지와 영양소 감지 AMPK

AMPK는 세포 내 AMP 대 ATP, ADP 대 ATP의 감소를 감지하는 주요 센서로 간주되며, 이후 이화 작용을 증가시키고 동화 작용을 감소시키는 역할을 합니다. 따라서 AMPK의 활동은 생성과 역학을 포함한 미토콘드리아 건강과 밀접하게 관련되어 있습니다. AMPK는 골격근의 GLUT4와 미토콘드리아 유전자 발현에 상당한 영향을 미치기 위해 PGC-1α에 의존하며, 트레오닌-177과 세린-538에서 직접적인 인산화를 통해 PGC-1α를 활성화합니다.104 또한, AMPK 서브유닛 β1과 β2 이소폼 노크아웃 마우스는 신체적으로 활동적이지 않으며, 골격근 미토콘드리아 함량의 감소와 관련된 러닝머신 달리기 능력이 현저하게 손상되어 있습니다. 105 AMPK 작용제 AICAR를 사용한 치료는 결함이 있는 사이토크롬 c-옥시다제(COX) 마우스 모델에서 COX 결핍의 부분적인 교정을 이끌어냈고, 중요한 것은 Sco2의 경우, 노크아웃/노크인 마우스에서 정상적인 수준으로 운동 능력이 현저하게 개선되었다는 점입니다.106 아디포넥틴과 아디포넥틴 수용체 1에 의한 AMPK-PGC-1α 경로의 교란은 당뇨병에서 관찰되는 미토콘드리아 기능 장애와 인슐린 저항성에 기여했습니다. 107 AMPK가 미토콘드리아 생성을 촉진하는 기전은 핵 호흡 인자-1(NRF-1)의 활성화를 통해 이루어집니다.108 지주막하 출혈(SAH)로 인한 초기 뇌 손상 시, AMPK와 그 후속 PGC-1α는 외인성 hsp22에 의해 활성화되어 NRF1/TFAM 의존적 미토콘드리아 생성과 Drp1 매개에 의한 조절을 통해 신경 기능을 유지합니다. 109 또한, 미토콘드리아의 품질 관리도 AMPK의 통제하에 있습니다. AMPK의 직접적인 약리학적 활성화는 미토콘드리아 스트레스가 없는 경우에도 미토콘드리아 분열 인자(MFF) 인산화를 통해 미토콘드리아 분열을 신속하게 촉진하기에 충분했습니다.110 AMPK 신호 전달 경로는 또한 OPA1 관련 미토콘드리아 융합과 미토파지를 활성화하여 심장 I/R 손상을 완화하고 미토콘드리아 스트레스를 완화할 수 있습니다. 111 급격한 운동으로 인한 미토콘드리아 산화 스트레스는 AMPK 활성화를 유발하여 골격근에서 인산화된 unc-51과 같은 autophagy activating kinase 1(Ulk1) 의존적인 미토파지(mitophagy)를 유도합니다.112 이는 아마도 AMPK 활성화의 초기 단계에서 ULK1에 의한 Parkin의 Ser108에서의 인산화 사건 때문일 것입니다. 113 최근, 에너지 스트레스 조건 하에서 AMPK는 MAM으로 이동하여 MFN2와 직접 상호 작용하여 미토콘드리아 분열과 자가포식을 시작할 수 있습니다.114 또한, 유사분열의 낮은 에너지 상태에 반응하여 에너지 센서 AMPK가 인산화되어 MCU를 활성화함으로써 Ca2+가 미토콘드리아로 유입되어 미토콘드리아 호흡을 촉진합니다. 115 따라서 AMPK는 세포 에너지 균형의 중심 조절자로 등장하여 미토콘드리아 기능과 역학에 직접적인 영향을 미치며, 대사 장애 치료와 스트레스 및 질병에 대한 미토콘드리아 회복력 강화에 영향을 미칩니다.

mTOR

Mammalian TOR (target of rapamycin) is a serine/threonine protein kinase in the PI3K-related kinase (PIKK) family that coordinates eukaryotic cell growth, metabolism, and autophagy with environmental inputs, including nutrients and growth factors.116 Transcriptionally, mTOR controls the expression‎ of mitochondrial genes (such as Cytochrome c, Atp5g, and Cox5a) by regulating the coactivation of yin-yang 1 (YY1) and PGC-1α.117 In addition, mTORC1 controls mitochondrial activity and biogenesis by selectively promoting translation of nucleus-encoded mitochondria-related mRNAs via inhibition of the eukaryotic translation initiation factor 4E (eIF4E)-binding proteins (4EBPs).118 On the other hand, mTOR activity is closely related to mitochondrial metabolism.119 Metabolomics profiling showed that mTOR independently regulated mitochondrial metabolism in regulatory T cells.120 The inhibition of rapamycin-sensitive mTOR complex 1 (mTORC1) also leads to a metabolic shift of mitochondrial respiration to aerobic glycolysis in leukemic cells, possibly in a Bcl-xl-VDAC1-dependent manner.121 A recent study reported that FOXK1 suppressed mitochondrial fatty acid oxidation in an mTORC1-dependent manner during the pathogenesis of nonalcoholic fatty liver disease (NAFLD) to non-alcoholic steatohepatitis (NASH).122 Additionally, mTORC1 is a regulator of mitochondrial dynamics and cell survival. A study revealed that the nutrient-sensing mTORC1 pathway facilitates DRP1’s pro-fission activity of mitochondria and apoptosis via promoting the translation of mitochondrial fission process 1 (MTFP).123 Inhibition of active mTOR sites leads to mitochondrial hyperfusion by reducing MTFP1 translation through the eIF4E-binding protein pathway, showcasing MTFP1’s pivotal role in mTORC1’s modulation of mitochondrial dynamics. In addition to mTORC1, mTORC2 could translocate to MAM and control mitochondrial metabolism via Akt-mediated phosphorylation of the IP3R, Hexokinase 2, and phosphofurin acidic cluster sorting protein 2 (PACS-2).124 Conclusively, mTOR orchestrates a complex regulatory network influencing cell growth, metabolism, and autophagy through its impact on mitochondrial gene expression‎, biogenesis, metabolism, and dynamics.

Sirtuin

Sirtuins serve as crucial sensors of the NAD+ to NADH ratio, closely linking them to cellular metabolism, stress response, and longevity. Mitochondrial sirtuins (SIRT3–5) are part of the sirtuin family of NAD+-dependent deacylases and ADP-ribosyl transferases, containing an N-terminal mitochondrial signal sequence that dictates the mitochondrial localization.125,126 SIRT3 interacts with ATP synthase and is indispensable in mitochondrial membrane potential restoration under mitochondrial homeostasis.127 SIRT3 knockout caused decreased levels of superoxide dismutase, catalase, and mitochondrial complex enzymes I/II/III/IV, which exacerbated mitochondrial damage in acute kidney injuries.128 Cisplatin-induced decline of SIRT3 also contributed to DRP1-dependent mitochondrial fission in cultured human tubular cells.129 However, under mitochondrial stress during cardiac pathology, SIRT3 promoted mitochondrial fusion through the direct deacetylation of OPA1 at the Lys926 and Lys931 residues and thus replenishing the GTPase activity of OPA1 in a NAD+-dependent manner.130 In addition, an adiponectin receptor agonist promoted AMPK/SIRT3 and increased the protein levels of Mfn2 and OPA1 with no effect on the expression‎ of Drp1, thereby promoting mitochondrial fusion and memory deficits in P301S mice.131 SIRT4 could specifically hydrolyze lipoamide cofactors from the DLAT E2 component of the pyruvate dehydrogenase (PDH) complex, thereby inhibiting PDH activity regulation.132 SIRT5 supports NADPH homeostasis and antioxidant defense via promoting the desuccinylation and deglutarylation of IDH2 and G6PD, respectively.133 Collectively, these insights highlight the intricate roles of mitochondrial sirtuins in modulating mitochondrial function and integrity.

Innate immunity

Innate immune cells express distinct germ-line-encoded pattern recognition receptors (PRRs) that detect conserved pathogen-associated molecular patterns (PAMPs) unique to microbes.134 Alternatively, PRRs could also be activated by the endogenous molecules as dangerous signals called damage-associated molecular patterns (DAMPs) in inflammatory diseases.135,136 These molecules include mtDNA, mitochondrial ROS, adenosine triphosphate (ATP), TFAM, cardiolipin, cytochrome c, mitochondrial Ca2+, and iron.137 Exemplified as mtDNA, mtDNA itself can be recognized by three important PRRs of the innate immune system, cGAS-STING, cytosolic inflammasomes, and TLR9, which triggers type 1 interferon (IFN) responses and/or NF-κB and other signaling pathways to induce proinflammatory cytokines.135 Thus, these DAMP-sensing receptors may be valuable targets for therapeutic approaches.

선천적 면역력

선천적 면역세포는

미생물에 고유한 보존된 병원체 관련 분자 패턴(PAMP)을 감지하는

뚜렷한 생식계 암호화된 패턴 인식 수용체(PRR)를 발현합니다.134

또는,

PRR은

염증성 질환에서 손상 관련 분자 패턴(DAMP)이라고 불리는 위험 신호로서

내인성 분자에 의해 활성화될 수도 있습니다. 135,136

이러한 분자에는

mtDNA,

미토콘드리아 ROS,

아데노신 삼인산(ATP),

TFAM,

카디오리핀,

시토크롬 c,

미토콘드리아 Ca2+,

철 등이 포함됩니다. 137

예를 들어,

mtDNA는

선천성 면역체계의 세 가지 중요한 PRR(cGAS-STING, cytosolic inflammasomes, TLR9)에 의해 인식될 수 있으며,

이 PRR은 1형 인터페론(IFN) 반응 및/또는 NF-κB 및 기타 신호 전달 경로를 유발하여

전염증성 사이토카인을 유도합니다.135

따라서,

이러한 DAMP 감지 수용체는

치료적 접근을 위한 가치 있는 표적이 될 수 있습니다.

cGAS-STING

cGAS (also known as male abnormal 21 domain containing 1 (MB21D1)) is a cytosolic DNA sensor mediating innate immunity. It catalyzes the synthesis of a noncanonical cyclic dinucleotide, 2’,5’ cGAMP, that binds to STING.138 Subsequently, cGAS mediates the activation of TANK-binding kinase 1 (TBK1) in Golgi and phosphorylates the transcription factor interferon response factor 3 (IRF3) which then translocates to the nucleus and initiates the transcription of the IFN and ISG.139 These have been comprehensively reviewed elsewhere.140,141,142

Compelling evidence underscored the critical role of mtDNA in activating cGAS-STING signaling. Moderate mitochondrial stress, precipitated by TFAM insufficiency, facilitated the translocation of mtDNA into the cytosol, subsequently activating the cGAS-STING-IRF3 signaling cascade.143 Such activation is instrumental in engendering a comprehensive antiviral defense mechanism via enhanced expression‎ of a subset of interferon-stimulated genes (ISG). In addition, during the pathogenesis of dengue virus infection, IL-1R-mediated signaling is implicated in the inducement of endogenous mtDNA release, thereby initiating the transcription of IFNB1 and IFNL1 in a paradigm that is unequivocally dependent on the functional integrity of STING and IRF3 pathways.144 On the other hand, mtDNA-release-mediated cGAS–STING signaling is involved in the inflammatory activation. STING-deficient mice showed improved inflammation and acute kidney injury (AKI) progression, evidenced by the deactivated mtDNA-cGAS-STING axis and the decreased phosphorylated TBK1 and p65.145 Aged microglia affirmed prominent cytosolic accumulation of mtDNA abundance and nucleoids adjacent to the mitochondria outer membrane, which triggered VDAC-dependent cGAS and several type I IFN and proinflammatory genes.146 

To date, the mechanism of mtDNA release in these settings is not clear. Mechanistically, several studies have reported MOMP is regulated by several pore-forming proteins, such as BAK and BAX, and VDAC. The BAK/BAX is able to facilitate the macropore formation, resulting in MOMP and mtDNA release. This process, however, was directly evidenced by IMM herniation and mitochondrial inner membrane permeabilization (MIMP) rather than mPTP, as the pharmacological inhibitor cyclosporin A (CsA) failed to block the mtDNA and nucleoid release.147,148 However, despite the mPTP typically only allows passage of components smaller than 1.5 KDa, mtDNA fragments still can pass through the VDAC oligomer pores and mPTP to activate cGAS-STING.149,150 Indeed, the inhibition of mPTP opening with CsA reduced the cytosolic pools of mtDNA by 30%–40%, and VDAC oligomerization inhibition with VBIT-4 reduced cytosolic mtDNA by 50–60%.151 Alternatively, the MOMP that mediates mtDNA release and type I IFN response may depend on the level of mitochondrial stress. In moderately stressed mitochondria, mtDNA fragments interacted with the positively charged residues in the N-terminal domain of VDAC1, promoting VDAC1 oligomerization to form pores on the OMM and release mtDNA.149 During this procedure, immune recognition of cGAS-STING to mtDNA may be also enhanced by ROS exposure due to the decreased susceptibility to TREX1-mediated degradation.152 In addition, a recent study reported that urolithin A-induced mitophagy attenuated cGAS/STING activation by free cytosolic mtDNA, and ameliorated deterioration of neurological function, although the mechanism is unclear.153

Inflammasome

NLRP3 (Nucleotide-binding domain, Leucine-Rich Repeat, and Pyrin domain-containing protein 3) serves as a multifaceted intracellular sensor, adept at recognizing an extensive spectrum of microbial epitopes, endogenous danger signals, and environmental provocateurs. This recognition precipitates the assembly and activation of a pro-inflammatory signaling platform known as inflammasome. The NLRP3 inflammasome includes an adaptor protein characterized by a caspase activation and recruitment domain (ASC) that harbors two protein interaction domains: an amino-terminal pyrin domain (PYD) and a carboxy-terminal caspase recruitment domain (CARD).154 This assembly facilitates the proteolytic cleavage and caspase-1-dependent mobilization of pro-inflammatory cytokines (pro-IL-1β and pro-IL-18), as well as gasdermin D-mediated pyroptotic cellular demise.154 Mitochondria are implicated as critical regulators within the NLRP3 inflammasome activation pathway. As an important intracellular signal, ROS mediated the activation of caspase-1 through NLRP3, and thus coactivating the translocation of mtDNA and cytochrome c into the cytosol subject to LPS and ATP stimuli.155 Besides, inhibited mitophagy/autophagy leads to mitochondrial ROS generation, prompting the translocation of NLRP3 and its adaptor ASC to mitochondria.156 Upon activation by NLRP3 activators, TLR leads to the production of oxidized mtDNA fragments through the IRF1-dependent transcription of CMPK2, a key enzyme in mtDNA synthesis.157 This mechanism establishes a feedforward loop that augments inflammation by intertwining NLRP3 activation, ROS generation, and mtDNA release.135 In addition to ROS-induced activation, recent metabolomics profiling has shown that the NLRP3 inflammasome activation benefited from ETC integrity through phosphocreatine (PCr)-dependent generation of ATP, despite the existence of unidentified targets within this pathway.158 Intriguingly, mitochondrial cardiolipin could bind to NLRP3 and activate inflammasome in a ROS-independent manner.159 Additionally, The interrelation of inflammatory pathways with mitochondrial functionality suggests a layered complexity; for example, mtDNA release induced by degenerative retinal pigmented epithelium activated caspase-4- (caspase-11 in mice) and caspase-1-dependent inflammasome, which required cGAS-dependent IFNβ production and gasdermin D–dependent IL-18 secretion.160 In a counter-regulatory manner, the antiapoptotic protein Bcl-2 is demonstrated to inversely modulate mitochondrial dysfunction and NLRP3 inflammasome activation, highlighting the intricate balance of mitochondrial engagement in the regulation of inflammatory responses.161

인플라마솜

NLRP3(뉴클레오티드 결합 도메인, 류신-풍부 반복, 피린 도메인 함유 단백질 3)은

다양한 미생물 항원, 내인성 위험 신호, 환경 자극을 인식하는 데 능숙한

다기능성 세포 내 센서 역할을 합니다.

이 인식은 프로인플루엔자신호전달체(inflammasome)로 알려진

프로인플루엔자 신호 전달 플랫폼의 집합과 활성화를 촉진합니다.

NLRP3 프로인플루엔자신호전달체는

두 개의 단백질 상호작용 도메인, 즉

아미노 말단 피린 도메인(PYD)과 카르복시 말단 카스파제 모집 도메인(CARD)을 포함하는

카스파제 활성화 및 모집 도메인(ASC)을 특징으로 하는 어댑터 단백질을 포함합니다. 154

이 어셈블리는 프로테올리틱 절단과 카스파제-1 의존성 전염증성 사이토카인(pro-IL-1β와 pro-IL-18)의 이동, 그리고 가스더민 D 매개 파이로톳틱 세포 사멸을 촉진합니다.154 미토콘드리아는 NLRP3 인플라미소스 활성화 경로 내에서 중요한 조절자로 관여합니다. 중요한 세포 내 신호로서, ROS는 NLRP3를 통해 카스파제-1의 활성화를 매개함으로써, LPS와 ATP 자극에 따라 mtDNA와 사이토크롬 c가 세포질로 이동하는 것을 활성화합니다.155 또한, 억제된 미토파지/오토파지는 미토콘드리아 ROS 생성을 유발하여 NLRP3와 그 어댑터 ASC가 미토콘드리아로 이동하도록 합니다. 156 NLRP3 활성화제에 의해 활성화되면, TLR은 mtDNA 합성의 핵심 효소인 CMPK2의 IRF1 의존적 전사를 통해 산화된 mtDNA 단편의 생성을 유도합니다.157 이 메커니즘은 NLRP3 활성화, ROS 생성, mtDNA 방출을 서로 연결함으로써 염증을 증폭시키는 피드포워드 루프를 형성합니다. 135 ROS에 의한 활성화 외에도, 최근의 대사체학 프로파일링에 따르면, 미확인 표적의 존재에도 불구하고, NLRP3 염증성 세포내소기관(inflammasome) 활성화는 크레아틴포스페이트(PCr) 의존적 ATP 생성을 통한 ETC의 완전성에 의해 촉진되는 것으로 나타났습니다.158 흥미롭게도, 미토콘드리아 카디오리핀은 NLRP3에 결합하여 ROS와 무관하게 염증성 세포내소기관을 활성화할 수 있습니다. 159 또한, 염증 경로와 미토콘드리아 기능의 상호 관계는 복잡성을 더해줍니다. 예를 들어, 퇴행성 망막 색소 상피에 의해 유도된 mtDNA 방출은 caspase-4-(마우스에서는 caspase-11)와 caspase-1 의존성 염증성 복합체를 활성화시켰고, 이로 인해 cGAS 의존성 IFN-β 생산과 gasdermin D 의존성 IL-18 분비가 필요했습니다. 160 반규제 방식으로, 항사멸 단백질 Bcl-2는 미토콘드리아 기능 장애와 NLRP3 염증성 사슬 활성화의 역조 조절을 통해 염증 반응 조절에 관여하는 미토콘드리아의 복잡한 균형을 강조하는 것으로 밝혀졌습니다.161

TLR9

The TLRs constitute a class of type I integral membrane receptors characterized by a conserved structural architecture that encompasses an N-terminal ligand-binding domain, a singular transmembrane helix, and a C-terminal intracellular signaling domain.162 These receptors are distinguished by their extracellular domains’ horseshoe-shaped conformation, pivotal for the ligand-induced dimerization that initiates innate immune responses.163 TLRs are synthesized within the ER, proceeding through the Golgi apparatus before their distribution to intracellular loci, where they are poised to recognize unmethylated CpG-DNA motifs, notably including mtDNA. The release of mitochondrial DAMPs into the circulation upon cellular injuries activates human polymorphonuclear neutrophils (PMNs) via TLR9, precipitating a sepsis-like systemic inflammatory response syndrome (SIRS).34 Additionally, the engagement of the circulating free mtDNA (cf-mtDNA)-TLR9 axis is implicated in the inflammatory progression of NASH, myositis, septic AKI, and chronic restraint stress.164,165,166,167 Interestingly, mtDNA could shuttle to recipient cells in a PINK1-dependent EV packaging mechanism to promote endosomal trafficking and invasiveness.168 The downstream signaling of TLR9 involves the adaptor protein myeloid differentiation primary response protein 88 (MYD88), which activates mitogen-activated protein kinases (MAPKs) and nuclear factor-kappa B (NF-κB), orchestrating an inflammatory cascade.169,170 Moreover, mtDNA facilitates endogenous TLR9 activation by translocation into the lysosomal compartment. Intriguingly, mtDNA that escapes autophagy-mediated degradation in cardiomyocytes can initiate TLR9-mediated inflammatory pathways, resulting in myocarditis and dilated cardiomyopathy independent of extracellular mtDNA release.171 In the context of podocyte injury within glomerular diseases, TLR9 engagement with endogenously accumulated mtDNA within endolysosomes mediated apoptosis via the p38 MAPK and NFκB signaling pathways, elucidating a complex interplay between mitochondrial components and innate immune activation mechanisms.172

Apoptosis

Mitochondria are indispensable regulators of cell death signaling, playing a central role in intrinsic apoptosis through the dissemination of critical soluble proteins including cytochrome c, Htra2, and Smac after MOMP.173,174,175 The binding of cytochrome c with apoptotic peptidase activating factor 1 (APAF1) engenders the assembly of a pivotal multi-protein complex known as the apoptosome, heralding the initiation of the apoptotic cascade.176,177 This apoptotic induction is predicated upon the activation and subsequent oligomerization of pro-apoptotic proteins BAK and BAX, facilitating pore formation and cytochrome c release. Contrastingly, the extrinsic apoptotic pathway intersects with mitochondrial-mediated apoptosis through the caspase-8-facilitated cleavage of BID, a pro-apoptotic BH3-only constituent of the BCL-2 protein family. This enzymatic processing yields a truncated form of BID that robustly instigates MOMP, thereby irreversibly committing the cell to apoptosis. Regulation against MOMP, and consequently apoptosis, is chiefly exerted by the BCL-2 protein family’s anti-apoptotic members, including BCL-2, BCL-W, BCL-X_L, A1, and MCL1, which serve to preserve cell survival by inhibiting pro-apoptotic mechanisms.178,179,180

Intriguingly, it has been observed that the apoptotic machinery renders mitochondrial cGAS immunologically quiescent during apoptosis and subsequent apoptotic clearance.181,182 Nonetheless, this regulatory schema is not absolute. Recent empirical findings elucidate that a partial induction of MOMP, orchestrated by BAK/BAX under sublethal apoptotic stress in an APAF1-dependent manner, fails to completely abrogate cGAS–STING signaling.183 Instead, it permits the activation of the senescence-associated secretory phenotype (SASP) in senescent cells, indicating a refined interplay between apoptotic signaling pathways, mitochondrial integrity, and immune response modulation. This nuanced interaction underscores the intricate balance maintained by cellular mechanisms between death signaling and immune regulation, providing a fertile ground for further investigative endeavors into the implications of such mechanisms in cellular senescence and disease pathogenesis.

세포 자멸사

미토콘드리아는 세포 자멸사 신호 전달의 필수 조절자로서,

세포 자멸사 후 사이토크롬 c,

Htra2,

Smac 등의

중요한 수용성 단백질의 분산을 통해

내인성 세포 자멸사의 중심 역할을 합니다. 173,174,175

사이토크롬 c와 세포자멸사 펩티다제 활성화 인자 1(apoptosis-associated protein kinase 1, APAF1)의 결합은

세포자멸사 캐스케이드의 시작을 알리는 중요한 다단백질 복합체인

아파토솜의 조립을 유발합니다. 176,177

이 세포자멸유도 작용은

세포자멸을 촉진하는 단백질인 BAK와 BAX의 활성화와

그에 따른 올리고머화에 의해 기인하며,

기공 형성 및 시토크롬 c 방출을 촉진합니다.

반대로,

외인성 세포자멸사 경로는

BID의 카스파제-8에 의한 절단을 통해

미토콘드리아 매개 세포자멸사와 교차합니다.

이 효소 처리는

MOMP를 강력하게 유발하는 BID의 절단된 형태를 생성하여

세포를 돌이킬 수 없는 세포자멸사로 유도합니다.

MOMP에 대한 조절과 그에 따른 세포 사멸은

주로 BCL-2, BCL-W, BCL-X_L, A1, MCL1을 포함한

BCL-2 단백질 계열의 항세포사멸성 구성원에 의해 이루어지며,

이들은 세포 사멸을 촉진하는 메커니즘을 억제함으로써

세포 생존을 유지하는 역할을 합니다.178,179,180

흥미롭게도,

세포자멸사 기전은

세포자멸사 과정과 그 이후의 세포자멸사 제거 과정에서

미토콘드리아 cGAS를 면역학적으로 정지시키는 것으로 관찰되었습니다.181,182

그럼에도 불구하고, 이 조절 체계가 절대적인 것은 아닙니다.

최근의 경험적 연구 결과에 따르면,

APAF1 의존적 방식으로 치사량 미만의 세포자멸사 스트레스 하에서

BAK/BAX에 의해 부분적으로 유도된 MOMP는

cGAS-STING 신호를 완전히 제거하지 못합니다.183

대신,

노화 세포에서 노화 관련 분비 표현형(SASP)의 활성화를 허용하여,

세포자멸사 신호 경로,

미토콘드리아 완전성,

면역 반응 조절 간의 정교한 상호 작용을 나타냅니다.

이러한 미묘한 상호작용은

세포의 사멸 신호 전달과 면역 조절 사이의 복잡한 균형을 강조하며,

세포 노화와 질병 발병에 대한 이러한 메커니즘의 함의를 더 깊이 연구할 수 있는 토대를 제공합니다.

Mitochondrial dysfunction and related diseases

Milestone of research history for mitochondrial dysfunction and therapeutics

The historical narrative of mitochondrial exploration has roots extending to the 19th century, a period characterized by the biological paradigm of ‘bioblasts’ that described a cytoplasmic, bacteria-like structure of ubiquitous occurrence, functioning as ‘elementary organisms’. In 1898, Benda termed these structures as ‘mitochondrion’, which evolved from the Greek words, ‘mitos’ (thread) and ‘chondros’ (granules).184 Yet, the elucidation of the biochemistry of mitochondria remained silent, that must ‘find a way to make direct chemical analyses of mitochondria’.185 A significant breakthrough was achieved in 1934 through Bensley’s pioneering approach of employing differential centrifugation to isolate mitochondria from guinea pig liver.186 Subsequent decades were characterized by an elaborate delineation of the morphological and biochemical attributes of isolated mitochondria.187,188,189,190,191 An abundance of empirical evidence has demonstrated a compelling linkage between mitochondria and the process of electron transport and oxidative phosphorylation. Importantly, Kennedy’s seminal research revealed the mitochondrial localization of tricarboxylic acid (TCA) cycle intermediates, aerobic oxidation mechanisms of fatty acids, and enzymes implicated in oxidative phosphorylation.192 Nevertheless, it was not until 1961 that the elusive electrochemical nature of oxidative phosphorylation was expounded by Peter Mitchell’s ‘chemiosmotic coupling’ theory.193 After this revelation, mitochondria regained focus in the late 1990s due to the regulation of cellular apoptosis, particularly through the cytochrome c and Bcl-2 protein family.194

The early trajectory of mitochondrial medicine was anchored on the correlation of clinical symptoms with aberrant mitochondrial biochemical signatures and morphological anomalies, as demonstrated in the foundational reports on Luft and Leigh disease.195,196 The sequence of the mitochondrial genome in 1981 was a pivotal moment that authenticated the contribution of mtDNA mutations to human pathology.4 In the wake of this milestone, the first mitochondrial disorders were characterized, such as Leber’s Hereditary Optic Neuropathy (LHON) due to missense mutations, and Kearns-Sayre Syndrome (KSS) as a result of considerable mtDNA deletions.197,198 Subsequent investigations continued to uncover a strong correlation of mitochondrial genetic anomalies with degenerative diseases, aging, and cancer.199 This genetic complexity is fueled by diverse mtDNA mutations, intricate phenotypic manifestations, threshold effects, and widespread exceptions. Still, specific mtDNA mutations have been successfully correlated to their clinical phenotypes, including diseases like MELAS, CPEO, and maternally inherited diabetes and deafness (MIDD).200 Additionally, nuclear DNA (nDNA) has been elucidated to function directly on the maintenance, translation, and protein synthesis of mtDNA, further convoluting the inheritance patterns of mitochondria.201 In 2008, the introduction of a comprehensive mitochondrial proteome inventory - MitoCarta, marked another milestone, and it has now evolved to its 3.0 version.202,203

미토콘드리아 기능 장애 및 관련 질병

미토콘드리아 기능 장애 및 치료에 관한 연구 역사의 이정표

미토콘드리아 탐험의 역사적 서술은 19세기로 거슬러 올라가는데, 이 시기는 '바이오블라스트'라는 생물학적 패러다임으로 특징지어지는 시기였습니다. 이 패러다임은 어디에나 존재하는 세포질, 박테리아와 유사한 구조를 묘사하고, '원소 유기체'로 기능하는 것을 의미합니다.

1898년 벤다는 이러한 구조를 그리스어 '미토스(실)'와 '콘드로스(알갱이)'에서 유래된 '미토콘드리아'라고 명명했습니다.184 그러나 미토콘드리아의 생화학에 대한 설명은 여전히 침묵 속에 남아 있으며, '미토콘드리아를 직접 화학적으로 분석할 수 있는 방법을 찾아야 한다'. 185 1934년, 벤슬리의 미토콘드리아를 분리하기 위한 차별적 원심분리법을 이용한 선구적인 접근 방식을 통해 중요한 돌파구가 마련되었습니다.186 그 후 수십 년 동안 분리된 미토콘드리아의 형태적, 생화학적 특성을 정교하게 묘사하는 것이 특징이었습니다.187,188,189,190,191 미토콘드리아와 전자 수송 및 산화적 인산화 과정 사이의 강력한 연관성을 입증하는 풍부한 경험적 증거가 있습니다. 중요한 것은, 케네디의 중요한 연구가 트리카르복실산(TCA) 순환 중간체의 미토콘드리아 국소화, 지방산의 호기성 산화 메커니즘, 그리고 산화적 인산화반응에 관여하는 효소를 밝혀냈다는 것입니다.192 그럼에도 불구하고, 산화적 인산화반응의 애매한 전기화학적 성질이 피터 미첼의 '화학성 결합' 이론에 의해 설명되기까지는 1961년이 되어서야 이루어졌습니다. 193 이 발견 이후, 미토콘드리아는 1990년대 후반에 세포의 세포자멸사 조절, 특히 시토크롬 c와 Bcl-2 단백질 군을 통해 다시 주목을 받게 되었습니다.194

미토콘드리아 의학의 초기 궤적은 루프트병과 리병에 대한 기초 연구 보고서에서 입증된 바와 같이, 임상 증상과 비정상적인 미토콘드리아 생화학적 특징 및 형태적 변이와의 상관관계에 기반을 두고 있습니다. 195,196

1981년 미토콘드리아 게놈의 순서는

인간 병리학에 대한 미토콘드리아 DNA 돌연변이의 기여를 입증하는

중추적인 순간이었습니다.4

이 이정표의 여파로,

미스센스 돌연변이로 인한 레버 유전성 시신경병증(LHON)과 상당한 미토콘드리아 DNA 결실의 결과로 인한

킨스-세이어 증후군(KSS)과 같은 최초의 미토콘드리아 질환이 특징지어졌습니다. 197,198

이후의 연구에서

미토콘드리아 유전적 변이가

퇴행성 질환, 노화, 암과 밀접한 관련이 있다는 사실이 계속 밝혀졌습니다.199

이러한 유전적 복잡성은

다양한 미토콘드리아 DNA 돌연변이, 복잡한 표현형, 한계 효과,

그리고 광범위한 예외에 의해 촉진됩니다.

그럼에도 불구하고,

특정 미토콘드리아 DNA 돌연변이는 MELAS, CPEO, 모계 유전성 당뇨병 및 난청(MIDD)과 같은 질병을 포함한 임상 표현형과 성공적으로 연관되어 있습니다. 200

또한, 핵 DNA(nDNA)가

미토콘드리아 DNA(mtDNA)의 유지, 번역, 단백질 합성에 직접적으로 관여한다는 사실이 밝혀지면서

미토콘드리아의 유전 패턴이 더욱 복잡해졌습니다.201

2008년, 종합적인 미토콘드리아 단백질 목록인 미토카르타(MitoCarta)가 소개되면서

또 다른 이정표가 세워졌고,

현재 3.0 버전으로 발전했습니다.202,203

Pathophysiology of mitochondrial dysfunction in common diseases

While the precise molecular mechanisms at play may differ depending on the disease context, common mitochondrial functions and behaviors include compromised bioenergetics, augmented oxidative stress, dysregulated calcium homeostasis, and modifications in mitochondrial dynamics along with quality control mechanisms. (See Fig. 4) In the following sections, we will elaborate on concrete disease examples.

일반적인 질병에서 미토콘드리아 기능 장애의 병태 생리학

정확한 분자 메커니즘은 질병의 맥락에 따라 다를 수 있지만, 일반적인 미토콘드리아 기능과 행동에는 손상된 생체 에너지, 증가된 산화 스트레스, 조절되지 않는 칼슘 항상성, 미토콘드리아 역학에 대한 수정과 품질 관리 메커니즘이 포함됩니다. (그림 4 참조) 다음 섹션에서는 구체적인 질병 사례를 자세히 설명합니다.

Fig. 4

Mechanisms of mitochondrial dysfunction in disease pathogenesis. This figure encapsulates the diverse mechanisms through which mitochondrial dysfunction contributes to common diseases. It illustrates six key dysfunctional processes: 1) Reverse Electron Transport: high Δp fuels the excessive ROS production during ischemia/reperfusion, leading to potential mtDNA damage; the accumulation of misfolded proteins and mistranslated respiratory complexes during AD and PD; imbalanced mitochondrial dynamics, mediated by ROS, Ca2+, and Aβ, affect mitochondrial morphology and function; various permeability transition pores lead to mitochondrial cargo release and initiate cell death; disrupted metabolism leading to energy imbalance and exacerbate insulin resistance; and mitochondrial genome instability resulting in altered gene expression‎ and mitochondrial failure. Together, these mechanisms underscore the central role of mitochondria in cellular health and the etiology of various diseases, highlighting potential therapeutic targets for mitigating mitochondrial dysfunction

질병 발병 기전에서 미토콘드리아 기능 장애의 메커니즘. 

이 그림은 미토콘드리아 기능 장애가 일반적인 질병에 기여하는 다양한 메커니즘을 요약합니다. 

6가지 주요 기능 장애 과정을 보여줍니다: 

1) 역전자수송: 높은 Δp는 허혈/재관류 동안 과도한 ROS 생성을 촉진하여 잠재적인 mtDNA 손상을 유발합니다. 

AD와 PD 동안 오접힘 단백질과 오역된 호흡기 복합체의 축적; 

미토콘드리아의 불균형적인 동역학은 ROS, Ca2+, Aβ에 의해 조절되며, 미토콘드리아의 형태와 기능에 영향을 미칩니다. 

다양한 투과성 전이 기공은 미토콘드리아 화물 방출을 유발하고 세포 사멸을 시작합니다. 

대사 장애는 에너지 불균형으로 이어지고 인슐린 저항성을 악화시킵니다. 

미토콘드리아 게놈 불안정성은 유전자 발현의 변화와 미토콘드리아 기능 장애를 초래합니다. 

이러한 메커니즘은 

세포 건강과 다양한 질병의 병인에서 미토콘드리아의 중심적인 역할을 강조하며, 

미토콘드리아 기능 장애를 완화하기 위한 잠재적인 치료 표적을 강조합니다.

Full size image

Ischemic/reperfusion injuries

Clinically, merely modulating energy balance by decreasing ventricular strain or heart rate might not rectify existing ventricular remodeling.22 Therefore, it’s imperative to target mitochondrial dysfunction, given its central role in cardio-cerebrovascular energy depletion and its influence on phenotype enhancement. Ischemia-reperfusion (IR) injury triggers a multifaceted series of metabolic and immune responses that ultimately lead to cell death and tissue remodeling. In the neural context, acute deprivation of oxygen and glucose swiftly impairs neuronal sodium (Na+) ion channels, precipitating cell swelling, degeneration, and necrosis.204 Concurrently, ischemic conditions activate the hypoxia-inducible factor (HIF), which engages in dual protective and damaging roles. By interacting with the von Hippel-Lindau (VHL) gene product, HIF inhibits proteases, while simultaneously initiating pro-inflammatory and apoptotic pathways via nuclear factor κB (NF-κB). This inflammatory response could exacerbate the hypoxic state by increasing metabolic demand.205 Moreover, hypoxia triggers HIF-1-dependent mitophagy, leading to a decrease in metabolic substrates, accumulation of metabolic waste, and dysregulation of calcium homeostasis.206 These changes collectively disrupt OXPHOS, diminishing ATP production and compromising mitochondrial permeability transition (MPT).17,207,208

Upon reperfusion, the abrupt restoration of blood flow introduces a surge of ROS, the key mediators of acute metabolic disturbance and the subsequent sterile inflammatory response.204,209 ROS contributes to cell and tissue impairment through lipid peroxidation within cell membranes and organelles, DNA oxidation, and the activation of matrix metalloproteinases and calpains.210,211 In addition, ROS could interact with nitric oxide (NO), fatty acids, or free iron, giving rise to highly reactive molecules such as peroxynitrite, peroxyl radicals, and hydroxyl radicals, which amplify cell death. Given the mitochondria’s role as the primary source of ROS in early reperfusion, their involvement is central to the pathophysiological mechanisms driving tissue injury in IR events, underscoring the critical role of mitochondrial dynamics in the etiology of reperfusion injuries.

Reverse electron transport (RET)

RET is a fundamental mechanism in ROS production during IR, involving the backward flow of electrons driven by an elevated proton motive force (Δp) and/or a reduced ubiquinone pool. This reverse flow leads to the incomplete reduction of molecular oxygen, resulting in the formation of reactive intermediates such as superoxide anion (O2−), hydrogen peroxide (H2O2), and the hydroxyl radical (HO•).212 In the mitochondrial electron transport chain, 11 potential sites have been delineated where electron leakage may occur, generating superoxide (O2-) and/or hydrogen peroxide (H2O2).213 Notably, the IQ site in Complex I and the IIIQo site in Complex III are significantly active, with Complex I identified as the predominant source of superoxide during the reperfusion phase.17,213 In the ischemic state, there is a reversal in the function of the succinate dehydrogenase complex (Complex II), which, by utilizing aspartate as a carbon source, increases the biosynthesis of succinate.17 This accumulated succinate is then swiftly oxidized within 5 min of reperfusion, sustaining a reduced ubiquinone pool and a high Δp. This scenario sets the stage for electrons to be driven back to Complex I in the reverse direction, thereby facilitating the reduction of NAD+ to NADH and thus promoting superoxide formation. Small molecules targeting suppressors of site IQ electron leak (S1QELs) during RET could reduce ROS production in primary astrocytes, caspase signaling in vivo, and IR injuries in mouse hearts.214 The complex I was thus established as the therapeutic target in the early phase of reperfusion, which provided insights to integrate the molecular mechanisms and pathways contributing to IR injury.215

Ca2+ overload and MPT

Under physiological conditions, mitochondria orchestrate calcium dynamics through the interaction with the sarcoplasmic/endoplasmic reticulum (SR/ER), which is crucial for maintaining cardiomyocyte bioenergetics and calcium homeostasis.216 During the onset of coronary artery occlusion, regional hypoxia switches mitochondrial OXPHOS to anaerobic metabolism, leading to rapid ATP exhaustion and cardiac contractile failure. This metabolic alteration, accompanied by the decreased function of SERCA and NCLX, perturbates mitochondrial Ca2+ homeostasis in the failing heart.217,218,219 When proceeding to the reperfusion phase, SR Ca2+ leak via type 2 ryanodine receptor (RyR2) channels triggers mitochondrial Ca2+ overload and exacerbates mitochondrial dysfunction and ROS production.220 Consequently, intracellular and mitochondrial Ca2+ overload activated the mitochondrial chaperone cyclophilin D (CypD), provoking cardiomyocyte death through mitochondrial permeability transition pore (MPTP) opening and excessive cardiac contraction.221 Overexpression‎ of the mitochondrial Na+/Ca2+ exchanger (NCLX) could enhance Ca2+ clearance, reduce permeability transition, and prevent myocardial cell necrosis and heart failure due to ischemia. This approach was validated in vivo where ischemic preconditioning coupled with increased NCLX expression‎ reduced infarct size by 43%, highlighting the therapeutic potential of modulating mitochondrial calcium for cardiac protection.101

Imbalanced mitochondrial dynamics

Mitochondrial fission is integrated by the translocation and binding of fission proteins, which are critical for stress resistance or apoptotic promotion. In adult cardiomyocytes, mitochondria exist as abundant and dispersed round monomers and exhibit low kinetics.222 However, their functional integrity is indispensable for cardiac activity, given the heart’s energy-intensive nature. Proper regulation of Drp1 is essential for cell health, illustrating the balance that exists between physiological resistance and apoptosis.223 For instance, while the physiological downregulation of Drp1 might be adaptive under specific conditions, its overexpression‎, especially in pathological states like glucose deprivation, can trigger apoptotic events in cardiomyocytes.224 In addition, genetic anomalies in mitochondrial fission and fusion processes can be detrimental during IR, leading to compromised mitophagy. This can, in turn, exacerbate cardiac dysfunction. Notably, there’s a noted association between dysfunctional mitochondrial dynamics and increased vulnerability of cardiomyocytes to IR.224 In studies utilizing the ex vivo Langendorff model and the in vivo LAD occlusion model of IR, inhibiting the interaction between Drp1 and Fis1, a key mediator of mitochondrial fission, yielded insightful results. These included decreased mitochondrial fragmentation and reduced release of cytochrome c (cyt c) - a marker of mitochondrial health and initiator of the intrinsic apoptotic pathway. Furthermore, a decline in ROS production was observed, which is crucial given the role of ROS in exacerbating IR injury. The pharmacological inhibition of Drp1-Fis1 interaction not only improved cardiac function but also effectively attenuated apoptosis and autophagy, as evidenced by decreased interactions with caspases and reduced LC3-II levels.225

Neurodegenerative diseases

Neurodegenerative diseases manifest region- and disease-specific alterations in brain energy metabolism that progress over time.226 These metabolic changes are characterized by decreased glucose uptake, impaired tricarboxylic acid (TCA) cycle function, deficiencies in oxidative phosphorylation (OXPHOS), and reduced energetic support provided to neurons by astrocytes and oligodendrocytes. Notably, these perturbations are parallel to mitochondrial dysfunctions, such as genetic defects, kinetics imbalcances, calcium dysregulation, and proteinopathies.1,227,228 These mitochondrial pathologies are preval‎ent across a range of neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), amyotrophic lateral sclerosis (ALS), Huntington’s disease (HD), and frontotemporal dementia (FTD).229 Due to the complex mitochondrial dysfunction and multifaced phenotypes of neurodegenerative diseases, it remains challenging to establish whether mitochondrial dysfunctions act as primary causes, consequences, or merely contributing factors in the progression of these diseases. Recent advancements in this field, however, are shedding light on potential underlying mechanisms.

Impaired mitochondrial proteostasis

Alzheimer’s Disease (AD), a prototypical neurodegenerative disorder characterized by initial synaptic failure and progressive loss of cholinergic neurons, manifests as memory and cognitive decline.230,231,232 As the hallmarks of AD’s pathology, amyloid-β (Aβ) accumulation and tau aggregation are associated with multifaceted mitochondrial disruption in respiration and dynamics.233,234 Aβ protein triggered mitochondrial fission, synaptic loss, and neuronal damage, at least partially via S-nitrosylation of Drp1 to activate GTPase activity.235 The binding of Aβ with mitochondria-localized Aβ-binding alcohol dehydrogenase underlies the neurotoxic mechanism, which induces mitochondrial ROS production, cytochrome c release, and ultimate cell death.236 On the other hand, the study revealed that Aβ peptides disrupted the turnover of peptides by inhibiting Cym1/PreP, a key mitochondrial metallopeptidase responsible for degrading mitochondrial targeting presequence peptides, resulting in enhanced protein degradation and widespread dysproteostasis in AD.237 Concurrently, amyloid precursor protein-derived C-terminal fragments (APP-CTFs) independently impaired mitochondrial morphology and were associated with a Parkin signature of mitophagy failure in human post-mortem AD brains.238

The mitochondrial cascade hypothesis, particularly relevant to sporadic and late-onset forms of AD, posits that accumulated oxidative stress reconfigures the cellular response to toxic proteins.239 This reconfiguration manifests as age-associated phenotypes driven by deficient cellular clearance mechanisms and disrupted cell cycle processes, highlighting the centrality of mitochondrial integrity in the progression of AD. Experimental models, including transgenic C. elegans models expressing human Aβ and tau proteins, have pinpointed defective mitophagy as a key pathological event in AD, suggesting it as a viable target for therapeutic interventions aimed at memory deficits.240 Furthermore, mitochondria may offer a protective response against Aβ-induced proteotoxic stress through the mitochondrial unfolded protein response (UPRmt) and enhanced mitophagy, slow the progression of AD, and improve lifespan by stabilizing mitochondrial proteostasis.241 Collectively, these insights not only illuminate the multifaceted roles of mitochondria in AD pathology but also suggest novel therapeutic strategies focused on restoring mitochondrial function and enhancing proteostasis.

Parkinson’s Disease (PD) is a progressive neurodegenerative disorder marked by the loss of specific neurons, including dopaminergic cells, where mitochondrial dysfunction frequently emerges as a contributing factor.242 This dysfunction is observed across genetic aberrations of the disease, including autosomal dominant cases with SNCA (alpha-synuclein) mutations and copy number variations, and autosomal recessive early-onset forms involving mutations in PARK2 and PARK6 (parkin) genes.243,244,245,246 Moreover, elevated mtDNA mutations linked to respiratory chain deficiencies are preval‎ent in neurons particularly vulnerable in PD and in the general process of brain aging, suggesting a possible etiological role of nucleotide pool imbalances and alterations in one-carbon metabolism pathways.228,247 The aggregation of alpha-synuclein (α-Syn), a defining feature of PD’s neuropathology through Lewy body formation, further compounds these mitochondrial challenges.248,249 Familial and sporadic PD cases often exhibit a dose-dependent mitochondrial accumulation of α-Syn aggregates, leading to the formation of toxic soluble oligomers.250,251 These oligomers provoke excessive ROS production and neuronal cell death, with the rate of these deleterious reactions accelerating due to increased oligomer formation and a breakdown in mitochondrial defensive mechanisms.252 Though this structural turnover is remarkedly slow due to the kinetic barrier, the reaction speed would sharply increase with enhanced aggregation, formation of β sheet oligomers, and dysregulation of compensatory mechanisms.252,253

At the molecular level, mitochondrial membrane constituents such as cardiolipin, ATP synthase, TOM20, and VDAC are known to interact with aggregated α-Syn and thus exacerbating mitochondrial dysfunction.254,255,256,257 This interaction notably impairs Complex I activity and leads to an overproduction of mitochondrial ROS, which in turn results in reduced ATP production and premature opening of MPTP, leading to neuronal apoptosis.257 This apoptotic cascade could potentially be attributed to a compromised protein import machinery caused by disrupted interaction with TOM22.256 However, the role of impaired Complex I function as a primary contributor to Parkinsonism is contentious. Conditional knockout of Ndufs4 (subunit of mitochondrial complex I) failed to recapitulate the loss of dopaminergic neurons or the motor deficits throughout the lifespan of mice, suggesting that Complex I dysfunction may not universally lead to parkinsonian symptoms.258 Conversely, recent findings from studies using Ndufs2-deficient mice indicate that Complex I dysfunction alone can precipitate a progressive, human-like parkinsonism phenotype, likely due to an early decline in axonal function driven by a shift in mitochondrial metabolic programming.259 This nuanced view underscores the complexity of mitochondrial involvement in PD and highlights the potential for therapeutic strategies targeting mitochondrial proteostasis to mitigate disease progression.

Aging

Aging is a complex universe that involves organismal ageing and cell senescence. The spectrum of mitochondrial-related aging signatures includes mtDNA mutation and enhanced methylation, genomic instability, decreased mitophagy, and compromised mitochondrial biogenesis.25,260 Both mtDNA point mutations and deletions in somatic cells have been shown to accumulate with age, thereby driving energy exhaustion and oxidative damage. In mice with mutated mtDNA polymerase gamma (PolgD257A/D257A), decreased life span, mitochondrial content, and activity of ETC complexes, as well as apoptotic activation were observed.261 Meanwhile, oxidants generated by mitochondria are considered the major source of the oxidative lesions that accumulate with age.262,263 In the past, the ‘free radical theory of ageing’ posits that biological aging results from the production of ROS.264 A case in point is the effective function of antioxidant enzymes in aging. Superoxide dismutase (SOD2) within the mitochondrial matrix is a critical antioxidant enzyme, mitigating oxidative stress by catalyzing the conversion of superoxide anions (O2-) into hydrogen peroxide (H2O2). In mice model of MnSOD+/− mice and ALDH-2−/− mice (aldehyde dehydrogenase), the assessment of acetylcholine-dependent vascular relaxation reveals a pronounced induction of vascular oxidative stress associated with aging, directly attributable to the compromised antioxidant defense due to partial SOD2 deficiency.265 Recent identification of five biomarkers in human plasma that were associated with the senescence-associated secretory phenotype (SASP) and an increased risk of clinical death, involving GDF15、RAGE、VEGFA, PARC, and MMP2.266 These hallmarks of biological and metabolic activity decline are often intertwined with mitochondrial dysfunction.267 Besides, ROS is also a crucial factor triggering senescence.268 In Sod2−/− mice, accumulated senescent cells and impaired complex II activity were observed.265 A reduction in mitochondrial ADP sensitivity during aging also increases mitochondrial H2O2 emission and contributes to age-associated redox stress.269 However, studies associated with age-dependent mitochondrial dysfunction engender controversial evidence in identifying the deleterious effect on longevity.270 Heterozygous Sod2+/− mice showed increased oxidative stress, as indicated by inactivation of ROS-sensitive enzymes, higher sensitivity to oxidative stress, impaired mitochondrial respiration, and higher levels of DNA oxidative damage in both the nucleus and mitochondria.271 Despite this, Sod2+/−mice appeared a wild-type lifespan. Recent advancement in understanding the aging process involves the role of apoptosis-induced changes in mitochondrial outer membrane permeability (MOMP), which have been found to facilitate the SASP through the release of pro-inflammatory mtDNA.183 This process is initiated by the activation of the pro-apoptotic proteins BAK and BAX, leading to IMM herniation and the subsequent release of mitochondrial matrix constituents, notably mtDNA and TFAM.147 Using the small-molecule BAX inhibitor BAI1 to inhibit MOMP could counteract age-associated sterile inflammation and improve lifespan.183 Furthermore, the relationship between mitochondrial dysfunction and lifespan is complex, possibly subject to a phenotype threshold or point of irreversibility. Intriguingly, increased oxidative stress resulting from the deletion of sod genes in C. elegans does not necessarily shorten lifespan; in some cases, it has been observed to extend it by altering mitochondrial function.272 This finding challenges the conventional understanding of oxidative stress and aging. Severe mitochondrial defects and reduced lifespan caused by the global deletion of ubiquinone (UQ) could also be reversed by restoring UQ levels.273 Together, these point to an uncoupled link between mitochondrial dysfunction and lifespan.

Metabolic syndrome

The indispensable involvement of mitochondria permeates numerous aspects of cellular metabolism, including the processing of diverse substrates like glucose, lipids, and amino acids.274 Metabolic syndrome refers to a cluster of metabolic disturbances including obesity, insulin resistance, dyslipidemia, and hypertension, and is associated with complications such as thrombosis, inflammation, cardiovascular disease, and NAFLD.275,276,277 The current evidence base has suggested that aberrations in mitochondrial biogenesis, lipid metabolism, and OXPHOS were involved in the pathogenesis of metabolic syndrome.277,278,279,280,281 Furthermore, alterations in mitochondrial dynamics and quality control mechanisms, as well as increased ROS production, contribute to a vicious cycle that exacerbates both mitochondrial dysfunction and metabolic syndrome.282,283 Mitochondrial impairments in key tissues such as adipose, skeletal muscle, and liver lead to systemic consequences, including insulin resistance and disrupted lipid homeostasis, which can manifest as components of metabolic syndrome.284,285,286 In light of these findings, targeting mitochondrial pathways may represent a promising therapeutic strategy to mitigate the pathological consequences of metabolic syndrome.

Insulin resistance

Insulin-resistant individuals exhibit impaired metabolism or tolerance to glucose challenge through obesity, sedentary lifestyle, high-fat diet, or genetic factors.276 Mitochondrial dysfunctions have been intricately linked to the development of insulin resistance, which is a key feature of metabolic syndrome. Metabolomic studies have provided detailed insights into the molecular mechanisms involved in this process. In skeletal muscle, decreased mitochondrial content and/or mitochondrial dysfunction led to a reduction in mitochondrial fatty acid oxidation, along with increased levels of intracellular diacylglycerol (DAG) and acyl CoA.287 These fatty acid derivatives activate a serine kinase cascade that augments the phosphorylation of Ser/Thr residues in insulin receptor substrate-1 (IRS-1), resulting in the inhibition of translocation of glucose transporter type 4 and glucose uptake.288,289,290 In the liver, mitochondrial dysfunction exacerbates hepatic insulin resistance and promotes the development of NAFLD by disrupting lipid metabolism, increasing mitochondrial uncoupling, and inducing pro-inflammatory cytokines.291 In skeletal muscle, mice lacking malonyl-CoA decarboxylase (MCD) showed reduced fat catabolism and resistance to diet-induced glucose intolerance, indicating a strong connection between skeletal muscle insulin resistance and lipid-induced mitochondrial stress.292 Accumulated intermediates in mitochondria from human muscle with insulin resistance, such as acylcarnitine and ceramide, also suggested the insulin resistance-associated stress pathway either by a deficiency in or an overload of mitochondrial oxidative capabilities.292,293 Acute and chronic consumption of a high-fat diet significantly enhanced the release of mitochondrial ROS, shifting to a more oxidized state and leading to insulin resistance within skeletal muscle.294 In addition, ROS signals could be secondary to respiratory alterations-induced insulin sensitivity when subjected to DAG accumulation, PKCθ activation, and impaired muscle insulin signaling.295 These mechanisms suggested that decreased mitochondrial function is regulated by damaging effects caused by nutrient excess and signal impairment.296 Moreover, it was also demonstrated that the impact of impaired mitochondrial dynamics and quality control mechanisms, such as mitophagy, on insulin resistance and metabolic syndrome.283 Increased Drp1 expression‎ and mitochondrial fission were associated with impaired mitochondrial function, characterized by reduced ATP production and increased ROS generation, which contributed to impaired insulin signaling in skeletal muscle cells.297 Conversely, inhibiting Drp1 activity and reducing mitochondrial fission improved mitochondrial function and restored insulin signaling. Collectively, these findings underscore the complex interplay between mitochondrial dysfunction, lipid and amino acid metabolism, and insulin signaling, emphasizing the need for a multifaceted therapeutic approach to address the diverse aspects of metabolic syndrome.

Non-alcoholic fatty liver disease (NAFLD)

NAFLD is the most preval‎ent form of chronic liver disease globally and is closely associated with metabolic syndrome.298 This metabolic perturbation in NAFLD is characterized by the abnormal accumulation of carbohydrates and fats in the liver, often driven by processes such as the transformation of excessive fructose intake into triglycerides through de novo lipogenesis.299 Additionally, insulin resistance in skeletal muscle and liver also contributes to this metabolic shift towards lipogenesis.300,301 As NAFLD progresses to nonalcoholic steatohepatitis (NASH), mitochondria evidence a distinct shift to metabolic maladaptation, indicated by ultrastructural damage, compromised ETC activity, upregulated uncoupling protein content, and increased ROS production.302 The uncoupling of OXPHOS and increased ROS generation further compromise mitochondrial ETC function, thereby exacerbating ROS formation. In addition, the hepatocyte mitochondria’s capacity to sense overnutrition may precipitate a reverse cascade of proinflammatory signaling through the release of plasma mtDNA and subsequent activation of the intracellular Toll-like receptor 9 (TLR9) pathway.164 Interestingly, in obese rodent models, a decrease in mitochondrial functions, including carnitine palmitoyl-CoA transferase-1 (CPT1) activity, fatty acid oxidation, and cytochrome c protein content, were observed preceding the onset of insulin resistance and NAFLD.303 In contrast, in mice fed with an 8-week high-trans-fat high-fructose diet (TFD), changes in mitochondrial energetics and TCA flux lagged behind hepatic insulin resistance.304 High-resolution respirometry studies showed that obese human individuals, both with and without NAFLD, exhibit 4.3- to 5.0-fold higher maximal respiration rates of isolated mitochondria compared to lean individuals, yet these rates were 31–40% decreased in NASH individuals.291 Recent research also highlighted the role of autophagy-related gene 3 (ATG3) in steatosis during NAFLD progression, with studies indicating that knock-down of hepatic ATG3 could improve fatty acid metabolism and enhance mitochondrial complex activity, partly by reducing c-Jun N-terminal protein kinase 1 activity.305

Therapeutics for mitochondrial dysfunction

Dietary supplements and diet adjustments

Patients suffering from PMD confront an elevated risk of malnutrition, which is considered as a consequence of metabolic challenges.306,307 Similarly, several cellular dysfunctions in SMD associated with mitochondrial pathophysiology-such as escalated catabolic demands, bioenergetic depletion, calcium overload, and oxidative stresses-are known to result in deficiencies in both macro and micro-nutrients.308,309 Instances might lead to diminished active nutrient absorption where the disease pathology or therapeutic management involves gastrointestinal obstructions, thereby exacerbating the risk of malnutrition.310,311 It is well-established that mitochondrial activity relies heavily on a variety of essential nutrients, including cofactors and antioxidants that maintain bioenergetic production.308 Consequently, dietary supplementation strategies devised to counter mitochondrial dysfunction offer diverse nutritional options, including carnitine, CoQ, creatine, and vitamin B2. Carnitine is involved in the transport of long-chain fatty acids to mitochondria, and offers cellular protection by restricting the accretion of these fatty acids through binding with acyl-CoA.312,313 Ubiquinone or CoQ facilitates the electron transport from Complexes I and II to Complex III, while concurrently providing antioxidant protection to cell membranes and lipoproteins by suppressing lipid peroxidation.314 Creatine, by forming a complex with phosphoric acid, enables the replenishment of ATP, thus serving as an energy buffer reservoir in skeletal muscle.315 Riboflavin or Vitamin B2, acting as a cofactor for the electron transport chain, demonstrates the capacity to improve mitochondrial Complex I and II, and enzymatic activity upon supplementation.316

Human metabolism harnesses ketone bodies to provide alternative energy to various organs during periods of limited glucose availability.226,317,318 These ketone bodies - acetoacetate, β-hydroxybutyrate (βOHB), and acetone - are predominantly synthesized in the liver’s mitochondrial matrix through fatty acid oxidation. The ketogenic diet (KD), characterized by high fat and low carbohydrate content, simulates the metabolic state akin to prolonged fasting. Preclinical studies have highlighted the protective role of both ketogenic diets and exogenous supplementation of ketone body, showing benefits in conditions such as intractable childhood epilepsy, ischemic stroke, AD, PD, MELAS, advanced heart failure, and spinal cord injury.318,319,320,321,322,323,324,325 The potential mechanism involves increased ketolysis leading to oxidative stress in mitochondria, which triggers an adaptive cellular response.317 This hormetic response is characterized by the activation of key cell-defensive regulators like Nrf2, sirtuins 1 and 3, and AMPK, leading to upregulation of genes and pathways associated with antioxidative and anti-inflammatory responses, enhanced mitochondrial function and biogenesis, DNA repair, autophagy, and reduced anabolic energy expenditure. Notably, decanoic acid, a component of the ketogenic diet, acts as a peroxisome proliferator activator receptor γ (PPARγ) agonist, significantly enhancing mitochondrial citrate synthase and complex I activities in neuronal cell lines (SH‐SY5Y).326 However, extended exposure to KD has been linked to cardiac fibrosis in rats, a process involving the activation of Sirt7, which in turn inhibited the transcription of genes encoding mitochondrial ribosomes, subsequently impeding mitochondrial biogenesis.327

Despite the overall low risk, only a few studies exploring the use of dietary supplementation and structure adjustments have thus far adopted a randomized controlled trial (RCT) design, with most remaining as open-label studies. This is understandable considering the challenges associated with subject recruitment and financial costs due to the heterogeneity of PMD and SMD. Consensus on the clinical management, care, and refined administration of empirical interventions based on dietary supplements is accessible elsewhere.328

Targeted treatment using pharmacological agents

The transition of mitochondria from adaption to a dysfunctional state involves their dual roles: providing energy for cellular and organ function and acting as signal hubs to coordinate global cellular feedback. As the landscape of mitochondrial pathophysiology continues to evolve in common diseases, the spectrum of pharmacological targets for the treatment correspondingly expanded. Several intervention strategies are currently in focus: targeting the redox state of mitochondria, stimulating mitochondrial biogenesis, and modulating mitochondrial dynamics. Pharmacological agents corresponding to these targets are currently under investigation in active and randomized clinical trials (See Table 1). Regardless of the specificities of interventions in patients with PMD and SMD, current therapeutic approaches predominantly favor symptomatic treatment. Small molecule therapies have also progressed significantly, bringing these potential treatments closer to the bedside than ever before. Owing to the clinical heterogeneity of PMDs and SMDs, a panacea remains elusive; thus, the main objective of clinical care continues to be the reduction of morbidity and mortality.

Table 1 Current randomized trials of pharmacological agents targeting mitochondrial dysfunction

Table 1 Current randomized trials of pharmacological agents targeting mitochondrial dysfunction

From: Mitochondrial dysfunction: mechanisms and advances in therapy

DrugDiseaseMode of actionDoseEnrolment & ageResultStatusPeriodIdentifier

CoQ10	PMDa)	Antioxidant	10 mg/kg/d (400 mg/d)	24 (12 m– 17 y)	No significant difference in McMaster gross motor function and pediatric quality of life scale	Phase III/Completed	6 months	NCT00432744
	CKDb)	Antioxidant	1000 mg/d	26 (30 y–79 y)	Increased free fatty acids and decreased complex medium- and long-chain triglycerides. No significant improvement in VO2 peak or total work efficiency	Phase II/Completed	6 weeks	NCT03579693
	ALSc)	Antioxidant	1800-2700 mg/d	185 (21 y–85 y)	No significant difference in the ALS Functional Rating Scale-revised (ALSFRSr) Score	Phase II/Completed	9 months	NCT00243932
Ubiquinol MitoQ (MTA)e)	PDd)	Antioxidant	600 mg/d	11 (40 y–75 y)	No results reported	Phase II/Completed	24 weeks	NCT03061513
	CVDf)	Antioxidant	20 mg/d	60 (45 y–75 y)	-	NA/Recruiting	8 weeks	NCT05561556
	CVD	Antioxidant	20 mg/d	112 (>60 y)	-	Phase II/Recruiting	3 months	NCT04851288
	COPD	Antioxidant	NA	24 (>40 y)	-	NA/Recruiting	6 weeks	NCT05605548
	Dilated cardiomyopathy	Antioxidant	40 mg/d	106 (>16 y)	-	Phase II/Recruiting	3 months	NCT05410873
	Heart failure	Antioxidant	40 mg/d	60 (50 y–75 y)	-	NA/Not recruiting	28 days	NCT03586414
	Multiple Sclerosis	Antioxidant	20–40 mg/d	60 (18 y– 70 y)	-	Phase I & II/Recruiting	12 weeks	NCT04267926
SS-31 (MTA)	Friedreich Ataxia	Antioxidant	20–30, 40–60 mg/d	18 (>16 y)	-	Active, not recruiting	52 weeks	NCT05168774
SKQ1 (MTA)	Dry Eye Syndrome	Antioxidant	0.155–1.55 µg/mL	90 ( > 18 y)	Significant improvements in corneal fluorescein staining and lissamine green staining in the central region and lid margin redness, also in the dry eye symptoms	Phase II/Completed	28 days	NCT02121301
	Dry Eye Syndrome	Antioxidant	1.55 µg/mL	452 (>18 y)	Significant impact on clearing of Central Corneal Fluorescein Staining (CCFS) and improvement of Best Corrected Visual Acuity (BCVA) at day 28	Phase III (VISTA-1)/Completed	28 days	NCT03764735
	Dry Eye Syndrome	Antioxidant	1.55 µg/mL	610 (>18 y)	Significant impact on clearing of Central Corneal Fluorescein Staining (CCFS) and improvement of Best Corrected Visual Acuity (BCVA) at day 28	Phase III (VISTA-2)/Completed	28 days	NCT04206020
KH176	PMD	Antioxidant	200 mg/d	20 (>18 y)	No results available	Phase II/Completed	28 days	NCT02909400
	PMD	Antioxidant	NA	24 (0 month - 17 y)	-	Phase II/Recruiting	26 weeks	NCT04846036
	PMD	Antioxidant	100–200 mg/d	27 (>18 y)	No results available	Phase II/Completed	28 days	NCT04165239
	PMD	Antioxidant	800–2000 mg	32 (18 y–55 y)	Well tolerated up to single doses of 800 mg and multiple doses of 400 mg b.i.d. and has a pharmacokinetic profile supportive for a twice daily dosing.	Phase I/Completed	7 days	NCT02544217
Acipimox	Sarcopenia	NAD+ boosting	750 mg/d	16 (65 y–75 y)	No results available	NA/Completed	14 days	NCT02792621
	Type 2 Diabetes	NAD+ boosting	750 mg/d	31 (40 y–70 y)	Improved insulin sensitivity of type 2 diabetes patients by ∼27% and reduced H2O2 production by ∼45%, but did not improve basal or insulin-stimulated mitochondrial oxidative capacity	NA/Completed	14 days	NCT00943059
	Obesity	NAD+ boosting	750 mg/d	39 (18 y–55 y)	Reduced free fatty acids, improved fasting measures of glucose homeostasis, lipids, and adiponectin. No effect on mitochondrial function, mitochondrial density, or muscle insulin sensitivity	Phase II/Completed	6 months	NCT01488409
	Type 1 Diabetes	NAD+ boosting	1000 mg/d	28 (25 y–59 y)	No results available	Phase III/Completed	16 weeks	NCT01816165
NRg)	CKD	NAD+ boosting	1200 mg/d	26 (30 y–79 y)	Significantly altered TCA cycle intermediates and glutamate and decreased a broad range of lipid groups including triglycerides and ceramides. No significant improvement in VO2 peak or total work efficiency	Phase II/Completed	6 weeks	NCT03579693
Metformin	Type 1 Diabetes	Unknown	500–2000 mg/d	23 (20 y–59 y)	Results not available	Phase IV/Completed	6 weeks	NCT01813929
	PMD	Complex I inhibitor	500 mg	61 (>18 y)	Results not available	Phase II/Completed	2 hours	NCT02500628
Idebenone	MELAS Syndrome	Antioxidant, ETC substrate	900–2250 mg/day	27 (8 y–65 y)	Results not available	Phase II/Completed	1 month	NCT00887562
	PPMSh)	ETC substrate	10–50 mg/kg/d (2250 mg/d)	85 (18–65 y)	No significant improvement in CSF GDF15 levels	Phase I & II/Completed	3 years	NCT00950248
	Friedreich’s Ataxia	Antioxidant, ETC substrate	900–2250 mg/day	70 (8 y–17 y)	No significant change in the International Cooperative Ataxia Rating Scale (ICARS)	Phase III/Completed	6 months	NCT00537680
Rosiglitazone	HIV Infections	PPAR agonist	8 mg/d	71 (>18 y)	Results not available	Phase II/Completed	48 weeks	NCT00367744
MIN-102	Friedreich Ataxia	PPAR agonist	15 mg/ml	39 (12 y–60 y)	No results available	Phase II/Completed	48 weeks	NCT03917225
REN001	Mitochondrial myopathy	PPAR agonist	NA	213 (>18 y)	-	Phase II/Completed	24 weeks	NCT04535609
Resveratrol	Mitochondrial myopathy	STACs	1000 mg/d	20 (18 y–80 y)	No results available	NA/Completed	20 weeks	NCT03728777
	Aging	STACs	1000–1500 mg/d	59 (>65 y)	Results not available	Phase II/Completed	3 months	NCT02123121
	Type 1 Diabetes	STACs	1000 mg/d	24 (>18 y)	-	Phase I/Recruiting	12 weeks	NCT04449198
	Friedreich Ataxia	STACs	2000 mg/d	25 (>16 y)	-	Active, not recruiting	24 weeks	NCT03933163
Urolithin A	Aging	Fission induction	1000 mg/d	50 (45 y–70 y)	-	NA/Recruiting	28 days	NCT05735886
HU6	Type 2 Diabetes Obesity	Mitochondrial uncoupler	450–600 mg/d	48 (18 y–70 y)	-	Phase II/Not yet recruiting	6 months	NCT06104358

Full size table

Preventing mitochondrial stress

As highlighted previously, mitochondria constitute a principal source of ROS. Endogenous enzymatic sources, including transmembrane NADPH oxidases (NOXs) and the mitochondrial electron transport chain (ETC), principally responsible for the production of O2− and H2O2329,330. Accordingly, the levels of mitochondrial ROS serve as stress surveillance system for assessing mitochondrial dysfunction and cellular health.331 Meanwhile, mitochondria themselves are also targets of persistent oxidative stress, leading to lipid and mtDNA damage or protein posttranslational modifications.332,333,334 Oxidants are also involved in the bidirectional regulation of mitochondrial dynamics, such as mitochondrial cristae and network remodeling. High levels of oxidants can also lead to MPT, further triggering “ROS-induced ROS release (RIRR)”.335 Upon exposure to reactive oxygen or nitrogen species, [4Fe-4S]2+ clusters in mitochondrial respiratory proteins are oxidized and may result in reversible aconitase inactivation, which impairs the inhibition of ROS production though affecting mitochondrial metabolic flux.336 Antioxidants have long been thought to protect mitochondria from damage. Certain compounds specifically target the complex I site, including S1QELs and OP2113, inhibiting RET and ROS production without disturbing normal OXPHOS.214,337,338,339 Quinone-based antioxidants, encompassing CoQ analogs idebenone and EPI-743, alongside a water-soluble Vitamin E derivative sonlicromanol (also known as KH176), are employed as a frontline defense against mitochondrial oxidative damage. These antioxidants exert their protective roles by neutralizing reactive oxygen species and safeguarding the activity of glutathione, a crucial antioxidant within cells340,341,342 Some drugs target MPTP opening and thus reducing RIRR and cell death events, such as classic cyclophilin D (CypD) inhibitor CsA, which occlude the peptidyl-prolyl cis/trans isomerase (PPIase) active site of CypD.343 Despite their promising roles, the efficacy of these global antioxidants is constrained by their suboptimal localization within mitochondria.344 To overcome this therapeutic hurdle, researchers have turned to lipophilic cations, molecules that inherently possess a low transmembrane activation energy. This characteristic enables them to penetrate the phospholipid bilayer autonomously without necessitating an uptake mechanism, facilitating their accumulation within the mitochondria which boast a high membrane potential (−150 to −170 mV).345,346 The advent of this strategy has given rise to mitochondria-targeted antioxidants such as MitoQ and SKQ1.347,348 Szeto-Schiller (SS) peptide family (also known as SS-31, MTP-131 or Bendavia) also could directly target cardiolipin peroxidation, independent to mitochondria membrane potential, and inhibit cytochrome c peroxidase and prevent mitochondrial damage.349

Inducing mitochondrial biogenesis

Mitochondrial biogenesis represents a cellular mechanism of generating new mitochondria, which is instrumental in ensuring tissue homeostasis. Its pharmacological activation emerges as a potent strategy to mitigate diseases characterized by mitochondrial dysfunction.207 This multifaceted process comprises a network of therapeutic targets, including mitochondrial energy and nutrient sensors, and downstream effectors such as transcription factors, cofactors, and nuclear receptors (NRs).344 Among the molecular entities involved in mitochondrial biogenesis, the PGC-1α acts as a nodal regulator in this intricate network, integrating upstream signals to launch a downstream mitochondrial gene programs facilitating mitochondrial biogenesis.350 However, the transcriptional regulation and post-translational modifications of PGC-1α represent intricate molecular events, which introduces substantial challenges in considering PGC-1α as a direct target for pharmacological interventions.351

Upstream sensors respond to perturbations in cellular energy status and nutrient availability. For instance, AMPK, as an essential metabolic regulator of glucose and fatty acid, is activated under conditions of energy deprivation, characterized by a high AMP/ATP ratio. This activation triggers mitochondrial biogenesis through the promotion of PGC-1α activity and its translocation into the nucleus.352 The NAD+-dependent deacetylase sirtuin family, notably SIRT1, plays a significant role in steering mitochondrial biogenesis by deacetylating and activating PGC-1α, in addition to exerting a direct influence on mitochondrial gene expression‎.353 Conversely, the mTORC1, a sensor of nutrient and growth factor availability, exerts a suppressive effect on mitochondrial biogenesis under conditions rich in nutrients.

At the downstream spectrum, an assortment of transcription factors and nuclear receptors react to these upstream signals, orchestrating a concert of nuclear and mitochondrial gene expression‎ to regulate mitochondrial biogenesis. Among these are nuclear respiratory factors 1 and 2 (NRF1 and NRF2), which dictate the transcription of a plethora of nuclear-encoded mitochondrial genes, and peroxisome proliferator-activated receptors (PPARs), governing lipid metabolism and mitochondrial functionality. The estrogen-related receptors (ERRs) also play a crucial role in the synchronization of gene expression‎ necessary for oxidative phosphorylation. Other key players include the cAMP response element-binding protein 1 (CREB1), and forkhead box O (FOXO) transcription factors, which are known to induce PGC-1α transcription. NRs, serving as transcription factors, are also vital constituents in the signaling cascade governing mitochondrial biogenesis. They mediate gene expression‎ in response to lipophilic hormones, vitamins, and dietary lipids, thus playing a role in shaping the mitochondrial biogenic response.

Several pharmacological agents have been identified to exert modulatory effects on these regulatory nodes, thereby enhancing mitochondrial biogenesis. For example, 5-Aminoimidazole-4-carboxamide ribonucleotide (AICAR) and PXL770 act as AMPK activators. AICAR, by mimicking the effects of AMP, stimulates AMPK which subsequently promotes the transcriptional activity of the coactivator PGC-1α, culminating in enhanced mitochondrial biogenesis.106,354 Similarly, PXL770 instigates mitochondrial biogenesis by acting as a direct stimulant of AMPK. In a preclinical investigation focusing on Autosomal dominant polycystic kidney disease (ADPKD), PXL770 treatment activates AMPK and restored mitochondrial DNA copy number and PGC-1α mRNA expression‎ in mice with ADPKD.355 Another compound, Metformin, widely known for its role in diabetes management, also bolsters mitochondrial biogenesis by activating AMPK, leading to an upsurge in PGC-1α expression‎ and activation.356 Furthermore, sirtuin-activating compounds (STACs), such as Resveratrol, enhance activity of the SIRT1-PGC-1α axis, thereby promoting muscle remodeling in Duchenne muscular dystrophy (DMD) mice.357 PPAR agonists, such as Bezafibrate and Thiazolidinediones, augment mitochondrial biogenesis by binding to and activating PPARs, which heterodimerize with Retinoid X Receptor (RXR) and bind to PPAR response elements in the DNA, enhancing the transcription of genes involved in mitochondrial biogenesis.358,359 Finally, REN001, a synthetic agonist of PPAR-δ, works by preferentially activating PPAR-δ, thereby promoting mitochondrial biogenesis and improving muscle function. These compounds, through their action on specific targets, highlight the potential for strategic pharmacological intervention to enhance mitochondrial biogenesis in the context of mitochondrial dysfunction.

Targeting mitochondrial dynamics

Impaired dynamics of mitochondrial fusion and fission have been implicated in numerous preval‎ent diseases, particularly those featuring neurodegeneration.360 The facilitation of mitochondrial fusion is orchestrated by GTPases, notably Mfn1, Mfn2, and Opa1, while mitochondrial fission is mediated by the DRP1.361 Mdivi-1, a derivative of quinazolinone, inhibits DRP1-dependent mitochondrial fragmentation and has been demonstrated to confer protection from cardiac IR injury and neurotoxicity.362,363 Recently, in vitro experimentation showed that Mdivi-1 partially ameliorated the aberrations in mitochondrial dynamics within human aortic smooth muscle cells (HAoSMCs) derived from bicuspid aortic valve (BAV) patients.364 Additionally, in vivo studies on mice with end-stage dilated cardiomyopathy (DCM) revealed that Mdivi-1 inhibited the activation of DRP1 signaling, partially mitigated autophagy inhibition and fatty acid metabolic disorder resultant from low-density lipoprotein receptor-related protein 6 (LRP6) deficiency, and ameliorated cardiac dysfunction.365

A critical facet of mitochondrial dynamics is the fragmentation and dynamic clearance of damaged mitochondria. Interestingly, recent findings have suggested that the physiological fragmentation of mitochondria under exercise conditions is an adaptation to energy demands.366 Physiological disruption mediated by DRP1 preserves mitochondrial membrane potential and attenuates the expression‎ of factors related to mitophagy. It’s worth noting that inhibiting fission using P110 and Mdivi-1 significantly impairs exercise capacity. Furthermore, emerging evidence suggests that Mdivi-1 may exert effects that are independent of mediating fission, including the reversible inhibition of complex I and RET-mediated ROS production.367

Uncoupling mitochondrial respiration

OXPHOS is inherently inefficient due to proton leakage across the inner mitochondrial membrane, which facilitates the return of protons to the mitochondrial matrix, bypassing ATP synthase and ATP synthesis. This futile proton cycling, driven by specific respiratory chain complexes and uncontrolled proton permeability, is a crucial modulator of both ROS production and energy expenditure. Pertinently, such dynamics are accentuated in obesity, insulin resistance, and NAFLD. Intriguingly, mild mitochondrial uncoupling can yield therapeutic benefits. The mitochondrial uncoupler 2,4-dinitrophenol (DNP) was initially noted as having potential therapeutic benefits in obesity and metabolic diseases. However, it was ultimately discontinued in clinical practice due to neurological complications and fatalities resulting from overdose.368 This led to the development of safer alternatives with higher therapeutic indexes, such as DNP derivatives and controlled-release formulations like DNP-methyl ether (DNP-ME) and controlled-release mitochondrial protonophore (CRMP).369,370 The therapeutic mechanism involves reversing insulin resistance in the liver and muscle, associated with reductions in diacylglycerol content and decreased activity of protein kinase C epsilon (PKCε) and PKCθ. Recently, a phase 2a trial eval‎uated the safety and efficacy of HU6, a metabolic accelerator and mitochondrial uncoupler, in adults with NAFLD and high BMI.371 Participants received varying doses from 100 to 400 mg of HU6 or placebo. HU6 showed a significant reduction in liver fat content compared to placebo, with mild to moderate side effects like flushing, diarrhea, and palpitations. This trial highlights the potential of modulating mitochondrial uncoupling as a viable therapeutic approach in metabolic disorders.

Mitochondrial transplantation-a strategy of organelle replacement

Mitochondrial transplantation focusing on functional tissue repair has raised great interest in recent years. Mitochondrial transplantation refers to the artificial complementation or replacement of a dysfunctional mitochondrial network with viable respiratory-competent mitochondria. The basic process involves isolating mitochondrial preparations from healthy tissue and introducing them into the vicinity of the tissue area with mitochondrial dysfunction, either by regional or circulatory injection. Until now, the application list of mitochondrial transplantation has been broadened in various disease models, including IR injuries47,372,373,374,375,376,377, neurodegenerative diseases378,379,380,381,382,383, renal injury40, ARDS384,385, and cancer386,387 (See Table 2 and Fig. 5). Conducting mitochondrial component transplantation not only delineates molecular mechanisms of tissue revitalization but provides possible nodes of intervention in the future.

Table 2 Current application of mitochondrial and its component transplantation

Full size table

Fig. 5

Therapeutic applications of mitochondrial and its component transplantation. This figure concludes therapeutic effects of mitochondria and associated components from different tissues and cells, to the mitochondria level (marked as grey arrows). The increased alterations in boxes are shown as up arrows while decreased alterations in boxes are shown as down arrows. Notably, though tissue level changes can vary, mitochondrial transplantation typically restores ATP production ability and reduces ROS production of damaged mitochondria. Besides, mitochondrial transplantation inhibited the intracellular try-IDO-kyn pathway, thereby improving the cognitive performance of psychiatric disorders and aging. In carbon tetrachloride (CCl4)-induced liver injury, transplanted mitochondria can increase a series of anti-oxidative enzymes to improve OXPHOS functions by triggering the UPRmt pathway. On the other hand, MSCs also repress TLR-signaling of macrophages via microRNA transfers to reduce inflammatory responses

Ischemia/reperfusion

Intervention targeting disrupted mitochondria at the early stage of reperfusion was proven to be effective in reducing subsequent cell necrosis.47,374,388 This was possibly due to prevention of the ROS burst derived from RET that induced reversible modification of Cys39 on the ND3 subunit of complex I.215,389 To reverse mitochondrial damage induced by ischemia, pioneering work of mitochondrial transplantation was conducted by McCully, J. D. et al. who directly applied autologous mitochondria into the infarction zone on hearts of New Zealand White rabbits instantly before reperfusion. Besides enhanced postischemic myocardial function and decreased ROS, data from proteomics also revealed that receiving exogenous mitochondria compensated for the impaired respiration and energy generation capability of myocytes.372 In addition, higher expression‎ of non-mitochondrial cargos from donor induced pluripotent stem cell-derived cardiomyocytes (iCMs), such as PGC-1α and ERRγ mRNA, also facilitated mitochondrial biogenesis in cardiac IR injuries.390 The transplanted components upregulated mitochondrial ETC protein expression‎ in a PGC-1α-dependent manner. In the rat livers and brains, mitochondrial transplantation also attenuated cell deaths and oxidative response caused by IR.375,376 Notably, it was evidenced that improvements in ventricular function could be achieved by mitochondrial transplantation in pediatric patients, though clinical trials in humans require further investigation.46

Neurodegenerative diseases and aging

Mitochondrial energetic disruption, oxidative injuries, and dynamic abnormalities are vital features of aging-related neurodegenerative diseases and traumatic injuries.18,226,391,392 Recently, mitochondrial-targeting therapy has revealed its potential in postponing the pathological course of PD. In a rat PD model induced by 6-hydroxydopamine (6-OHDA), both allogeneic and xenogeneic mitochondrial injection to the medial forebrain bundle (MFB) reversed the movement disorder and attenuated the deterioration of dopaminergic neurons.379 A similar strategy showed improvement of behavioral symptoms in PD mice after intravenous administration of HepG2 cell-derived mitochondria.380 Intranasal delivery of mitochondria to 6-OHDA-lesioned rats showed significant improvement in rotational and locomotor behavior, dopaminergic neuron survival, and oxidative damage, demonstrating potential for bypassing the blood-brain barrier and promoting neuronal recovery.393 In addition, hippocampal mitochondria showed neuroprotective and antidepressant effects in neuroinflammatory astrocytes and microglia.381 Transfer of active intact mitochondria into AD mice also significantly improved cognitive performance, decreased neuronal loss, ameliorated mitochondrial dysfunction in the brain and liver, without observed toxicity, suggesting a novel therapeutic approach for AD.394,395 In aging models, injecting mitochondria from young animals into aged ones has been shown to improve cognitive, motor, and depression-like behaviors, enhance mitochondrial function, and modulate metabolic pathways, presenting a promising approach for combating aging and age-related mood disorders.396,397

Psychiatric disorders

Multiple mechanisms of psychiatric disorders involve cerebral oxygen metabolic changes. Due to their important role in redox balance, it is possible for mitochondria to play a role in energy metabolism in psychiatric disorders.398,399 Attenuation of mitochondrial oxidative stress and structural damage were favorable in improving psychiatric symptoms, including depression, anxiety, and other cognitive deficits.381,397,400 In these models, ROS level and mitochondrial morphology were normalized via mitochondrial transplantation. In addition, BDNF signal transduction is vital to the regulation of depressive-like behaviors.401 Exogenous mitochondria promote BDNF-related neurogenesis in major depressive disorder (MDD), improving anti-depressant effects. Neuronal protection-related gene expression‎, such as that of Nfe2l1, was also upregulated after mitochondrial administration. Additionally, it was reported that dysregulated metabolites of tryptophan and activated indoleamine 2,3-dioxygenase (IDO) by inflammation were crucial parts of depression and aging regulated by the gut-brain axis.402 Supplementing mitochondria decreased the IDO activity and attenuated anxiety- and depression-like behaviors, highlighting the potential to treat psychiatric disorders.397

NAFLD and hepatic injuries

Failure of mitochondrial adaptation, particularly respiratory and metabolic deficiency, is one of the core features in the progression to insulin resistance and NAFLD.24,403,404 However, fat deposits gradually vanished after mitochondrial transplantation, along with decreased serum transaminase activity. Accordingly, it was observed that levels of ROS and malondialdehyde (MDA) were significantly reduced after mitochondrial transplantation, whereas Glutathione (GSH) content and superoxide dismutase (SOD) activity increased.405 These results suggested that mitochondrial transplantation could improve serum lipid metabolism and reduce hepatocyte injury from HFD-induced oxidation. Mitochondrial transplantation was also proven to have positive effects on hepatic injuries induced by toxic chemicals. In carbon tetrachloride (CCl4) induced hepatic injury, free radicals are formed and initiate lipid peroxidation to disrupt membrane integrity, resulting in mitochondrial damage and mass loss.406 However, the administration of liver mitochondria from healthy mice eliminated free radicals, upregulated GSH metabolism, and restored OXPHOS function in CCl4-induced chronic hepatic injury and fibrosis.407 Interestingly, the molecular mechanism of mitochondrial protection seemed to be correlated with the unfolded protein response (UPRmt) pathway, as it activated a series of anti-oxidative enzymes and increased mitophagy to rebuild mitochondrial homeostasis.407 Consistent results were observed in acetaminophen (APAP) induced liver injury where administration of exogenous mitochondria reversed mitochondrial structure disruption, GSH loss, and serum levels of alanine aminotransferase (ALT) and aspartate aminotransferase (AST).408 These provide novel therapeutic strategies to enhance resistance in response to hepatic injuries.

Acute respiratory distress syndrome/acute lung injury (ARDS/ALI)

With decades of foreshadowing of cell-based therapy, considerable potential has been shown to rescue the disrupted alveolar-capillary barrier in ARDS/ALI.409,410,411,412 As a protective mechanism of cell-based therapy, cell-based mitochondrial transfer has been reported to increase ATP content, promote an anti-inflammatory phenotype, and restore mitochondrial respiration in ARDS/ALI.413,414 Moreover, mitochondrial restoration which benefited from MSCs also protected epithelial cells from apoptosis in asthma.415,416 In cigarette smoke and LPS-induced chronic obstructive pulmonary disease (COPD), mitochondrial administration inhibited ROS production and thus decreased cholinergic sensitivity of the epithelium, resulting in altered inflammatory progression. However, direct protection of injured epithelial cells is not the only mechanism of mitochondrial transplantation against various tissue fates. It was reported that phagocytosis of macrophages was enhanced by mitochondria derived from MSCs, which partially involved the mechanism of antibacterial effects in an E. coli Pneumonia-induced ARDS model.417 In addition, an adoptive transfer model promoted M2 phenotype alteration and the phagocytic capacity of macrophages to resist LPS-induced ARDS. This was potentially achieved by increased OXPHOS capacity of macrophages via mitochondrial transfer induced by MSC-derived EVs.418 These studies highlighted cell-dependent mitochondrial transfer as an extension of cell-based therapy.

Spinal cord injury (SCI)

Mitochondrial dysfunction is increasingly recognized as a pivotal factor in the neuronal death cascades characteristic of central nervous system injuries.419 Empirical evidence has been garnered from a study using a Sprague-Dawley rat model, wherein local mitochondrial transplantation into the distal end of the injured nerve led to notable enhancements in neurobehaviors, nerve electrophysiology, and muscle activities.420 The administrated mitochondria mitigated oxidative stress and upregulated neurotrophic factor expression‎ in both the injured nerves and denervated muscles, concurrently augmenting the pool of muscular progenitor cells, and increasing total muscle weight. In a parallel investigation, intraparenchymal infusion of allogeneic soleus muscles mitochondria into injured spinal cords resulted in the recovery of locomotor and sensory functions in rats with a reduction in SCI-induced cellular apoptosis and inflammation.421 Additionally, bone marrow stromal cells (BMSCs) have been observed to confer protection against SCI, attributed to their capacity to transfer mitochondria to injured neurons via intercellular gap junctions.422 Further advancements include the development of an engineered mitochondrial compound specifically designed to target macrophages within the SCI region.423 This approach demonstrated efficacy in attenuating pro-inflammatory profiles in macrophages, both in vitro and in vivo, thereby enhancing tissue regeneration and bolstering functional recovery in SCI models.

Other diseases

Studies on other diseases also illustrated the protective effect of mitochondrial transplantation, ranging from wound healing capacity to multiple sclerosis, tendinopathy, and osteoarthritis.424,425,426,427 Mitochondria from platelets were reported to stimulate the angiogenic potential of MSCs, thus promoting wound healing and tissue repair in chronic muscle injury.424 The directly administered mitochondria enhanced the TCA cycle and fatty acid synthesis, which was previously reported to favor epithelial cell proliferation for angiogenesis.428 In an autoimmune encephalomyelitis (EAE) model, intracerebroventricular induction of neural stem cells (NSCs) actively transferred mitochondria into mononuclear phagocytes and astrocytes, thereby ameliorating EAE disability in mice.425 In addition, mitochondria from L6 cells restored the protein levels of Tenascin C (TNC) in collagenase-damaged Achilles tendons, and were involved in the tendon protection process, along with the inhibition of apoptotic proteins (BID, Bax, and Bcl-2) and inflammatory cytokines (TNF-α, IL-1β, and IL-6).426 Similarly, the progression of osteoarthritis induced by monosodium iodoacetate was significantly inhibited by mitochondria from the skeletal (L6) myoblast cell line, which was also supported by improved pain phenotypes.427 Intravenous administration of mtDNA and TFAM mRNA-containing MSC-EVs was effective in reducing mitochondrial damage and inflammatory response in acute renal injury. This was possibly due to the protection of mitochondria in stressed HK-2 cells by TFAM-induced mtDNA maintenance.40 Additionally, it was discovered that mitochondria-located circular RNA (circRNA) and steatohepatitis-associated circRNA ATP5B regulator (circRNA SCAR) could reduce ROS production and avoid fibroblast activation via mitochondrial permeability transition during NAFLD.429 Through binding with ATP5B of the mPTP complex, the circRNA SCAR could block cyclophilin D-mPTP interactions, thereby inhibiting the opening of mPTP and reducing ROS production. Importantly, a decrease in ROS level and cytokine secretion was observed in circRNA SCAR-treated mice. The intravenous injection of mitochondria-targeting circRNA, which contained circRNA SCAR overexpression‎ vectors, also evidenced the improvement of glucose and insulin resistance 2, along with weight loss and macrophage infiltration. These preclinical studies have broadened the applications of mitochondrial transplantation in non-ischemic diseases.

Conclusion and perspective

Mitochondrial medicine has unfolded a remarkable trajectory of advancement across the span of several decades. Historically, mitochondrial diseases were predominantly ascribed to genetic aberrations, however, the current understanding has broadened to incorporate many common pathologies, as critical contributors.1 This paradigm shift intensified the appeal of developing therapeutic strategies targeting mitochondria. This review underscores the significance of tailored dietary supplements and adjustments, particularly the administration of essential nutrients such as carnitine, CoQ, creatine, and vitamin B2, alongside the potential benefits of ketogenic diets in leveraging alternative energy sources through ketone bodies, aiming at mitigating the metabolic intricacies inherent in mitochondrial pathologies. In addition, conventional pharmacological targets - inclusive of those implicated in regulating mitochondrial stress responses, metabolite pools, biogenesis, and dynamics- are transitioning to rigorous eval‎uation in randomized clinical trials, which represent a strategic pivot towards the dual roles of mitochondria in energy production and cellular signaling. Concurrently, the necessity for the conception and implementation of highly precise therapies becomes increasingly evident. Small molecules that are intricately designed to target membrane potential and cardiolipin of mitochondria were included at the inception stages of drug design, with the objective of augmenting mitochondrial delivery.430 Nonetheless, the journey from bench to bedside is fraught with challenges, including the clinical heterogeneity of mitochondrial disorders and the potential side effects of long-term dietary and pharmacological interventions. For instance, the protective role of ketogenic diets, while beneficial in simulating a fasting-like metabolic state, raises concerns regarding nutritional balance and sustainability, not to mention the observed risk of cardiac fibrosis in animal models. Similarly, the clinical efficacy of antioxidants and compounds targeting mitochondrial dynamics necessitates a careful balance, avoiding interference with physiological ROS signaling mechanisms essential for cellular homeostasis.

Mitochondrial transplantation is another strategy burgeoning on the horizon of therapeutic innovation. This involves the isolation of active mitochondria from individuals devoid of mitochondrial defects, followed by their introduction into compromised cells. Recently, Hayashida, K. et al. conducted a systematic review of pertinent evidence from both animal and human studies concerning mitochondrial transplantation in IR injury, offering a practical assessment of the categorization, concentration, and accessibility of data within this field.431 Twenty animal and two human studies were included, with 14 studies (approximately 70%) focusing on IR models in heart and brain tissues with high energy demand and metabolism. The collective findings from animal studies endorsed the ability of mitochondrial transplantation to mitigate IR injury, although these results were inconsistent concerning specific biomarkers and pathological alterations.377,393,414,431 These exogenous mitochondria are postulated to integrate with the existing functional networks within the recipient cells.432 However, symbiosis was hardly observed between exogenous and endogenous mitochondria, which might explain why some studies consider exogenous mitochondrial function as a substitution for endogenous damaged mitochondria.376 While most studies reported favorable results after mitochondrial transplantation, elucidation of the underlying mechanisms pertinent to its clinical translation remains an open challenge, including risks such as susceptibility during mitochondrial isolation, immune adaptation, endocytosis, and lysosome escape.431,433 The potential for xenotransplantation, deliberation over the most suitable source of mitochondria-whether autologous, allogenic, or xenogeneic, is still governed by a matrix of factors encompassing immunological compatibility, ethical implications, and the nuanced demands of clinical application. Autologous mitochondria, procured from the patient’s own tissues, emerge as the frontrunner in terms of biocompatibility, significantly reducing the risk of immune rejection and sidestepping the ethical complexities tethered to xenogeneic sources. However, this approach is not without its limitations, particularly in the context of systemic or congenital mitochondrial disorders where the availability of functionally intact mitochondria is often compromised.434 Allogenic mitochondria, sourced from human donors, represent a pragmatic alternative, particularly in acute settings where the requirement for fresh mitochondria may be increased due to the limited intervention window of opportunity.388 Xenogeneic mitochondria, while presenting an abundant and diverse reservoir, are currently confined to the experimental realm, predominantly due to heightened immunogenic risks and profound ethical considerations. Recently, of great concern to researchers, a nanothylakoid unit (NTU) based on the controllable photosynthetic system was developed as cross-species ATP and NADPH providers to treat osteoarthritis.435 Plant-derived NTUs were extruded and encapsulated by the purified chondrocyte membrane (CM-NTUs) to avoid immune rejection. According to proteomic analysis, these chondrocyte membrane proteins were highly correlated with vesicle targeting and membrane fusion, which might result in the efficient uptake by chondrocytes rather than being eliminated by macrophages or intracellular lysosomes. In addition, proteomic profiling demonstrated that mitochondrial diversity reveals heterogeneity in morphology, state, function, and diseases, leaving a large mitochondrial pool to be discussed.436 For example, in brown adipocytes, PDMs are inactive in fusion-fission dynamics but favored with increased ATP synthesis and lipid expansion capacity, compared to cytoplasmic mitochondria.437 Stem cells might also be universal mitochondrial donors, as mitochondrial transfer partially participated in the therapeutic effect of MSCs.438 Viable transplanted mitochondria seemed to be indispensable to the postischemic restoration whereas deactivated mitochondria, such as frozen mitochondria, mitochondrial genomes, and ATP-containing vesicles, did not achieve the same results.43,372 To shorten the preparation time, McCully, J. D. et al. proposed a tissue pool to obtain fresh mitochondria, including pectoralis major muscle, rectus abdominis or even sternocleidomastoid muscle.439 Comprehensive mechanistic investigations and more extensive preclinical studies remain essential.

The mitochondrial transplantation process largely involves the internalization of transplanted mitochondria at the cell membrane level to mitigate the biological information in the cell. However, the molecular mechanism of how the mitochondria can be integrated into endogenous mitochondrial network remain heterogenous and even largely unknown. Several studies have suggested versatile mechanisms may be involved in mitochondrial horizontal transfer between cells. In vitro data showed that the autologous driving force of mitochondrial uptake included endocytosis of recipient cells, such as macropinocytosis, clathrin-mediated endocytosis, or actin polymerization, which vary among different mitochondrial recipients.440,441 Non-specific macropinocytosis was also involved in some cases, evidenced by the specific inhibitor of micropinocytosis, ethyl isopropyl amiloride (EIPA).85 Endocytosis is a cellular process whereby a vesicle engulfs extracellular material, and it was suggested that transplanted mitochondria can be internalized through this process.387,424,441 In addition, receptor, actin, and ATP-dependent phagocytosis was validated to participate in engulfment of extracellular mitochondria via membrane invagination.442 Gap junctions, which are intercellular channels that allow for the exchange of small molecules, have also been implicated in mitochondrial internalization.413,422,443 Tunneling nanotubes (TNT) are highly sensitive nanotube structures that allow for direct communication between cells and have long been implicated in mitochondrial transfer.417,444 Partial cell fusion is another mechanism that has been proposed, involving the direct fusion between cells and mitochondria-sharing after TNT was inhibited.445 Recent studies also suggested that fusogenic syncytins on mitochondria, along with their receptors, might facilitate the mitochondrial internalization process under cancerous circumstances.446 However, current evidence points out single forms of internalization, as transplantation efficiency appears to be hyperresponsive to single inhibitors.415,422,424,441 Additionally, the employment of mitochondrial dyes in assessing mitochondrial incorporation during transplantation, while a common practice in both in vitro and in vivo studies, is hindered by the propensity of these dyes to leak.447 This leakage can lead to non-specific staining and false-positive results, as these mitochondrial dyes may escape from both donor and recipient cells, inadvertently staining neighboring mitochondria. The optimal approach for studying mitochondrial transfer in both in vitro and in vivo settings involves the use of stable transgenes encoding mitochondrially localized tags or fluorescent proteins. This mitochondrial reporter system, adaptable for lineage-specific mitochondrial reporter mice or for broad cell labeling, offers stable and precise tracking of mitochondria in recipient cells. However, a major concern is to avoid misinterpretation due to unintended Cre activity in other cell types when establishing stringent cell-type-specific transgene expression‎ based on Cre recombinase systems.447 Collectively, a better understanding of these internalization mechanisms will help optimize the efficacy and safety of mitochondrial transplantation for various clinical applications.

The future direction of mitochondrial dysfunction therapeutics likely resides in personalized medicine, wherein genetic and metabolic profiling could tailor interventions to individual patient needs, enhancing both efficacy and safety. As the therapeutic landscape for mitochondrial dysfunction continues to mature, the integration of dietary, pharmacological, and preventive strategies holds the promise of a more comprehensive and effective approach to managing these complex diseases. The endeavor to translate these advances into clinical practice underscores the need for a multidisciplinary effort, bridging the gap between molecular insights and therapeutic innovation to forge a path towards improved patient outcomes in the realm of mitochondrial pathology.

References

1.  
2.  
3.  
4.  
5.  
6.  
7.  
8.  
9.  
10.  
11.  
12.  
13.  
14.  
15.  
16.  
17.  
18.  
19.  
20.  
21.