Re: Autophagy in healthy aging and disease 2021년 nature review

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• Review Article
• Published: 12 August 2021

Autophagy in healthy aging and disease

• Yahyah Aman, 
• Tomas Schmauck-Medina, 
• Malene Hansen, 
• Richard I. Morimoto, 
• Anna Katharina Simon, 
• Ivana Bjedov, 
• Konstantinos Palikaras, 
• Anne Simonsen, 
• Terje Johansen, 
• Nektarios Tavernarakis, 
• David C. Rubinsztein, 
• Linda Partridge, 
• Guido Kroemer, 
• John Labbadia & 
• Evandro F. Fang 

Nature Aging volume 1, pages634–650 (2021)Cite this article

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Abstract

Autophagy is a fundamental cellular process that eliminates molecules and subcellular elements, including nucleic acids, proteins, lipids and organelles, via lysosome-mediated degradation to promote homeostasis, differentiation, development and survival. While autophagy is intimately linked to health, the intricate relationship among autophagy, aging and disease remains unclear. This Review examines several emerging features of autophagy and postulates how they may be linked to aging as well as to the development and progression of disease. In addition, we discuss current preclinical evidence arguing for the use of autophagy modulators as suppressors of age-related pathologies such as neurodegenerative diseases. Finally, we highlight key questions and propose novel research avenues that will likely reveal new links between autophagy and the hallmarks of aging. Understanding the precise interplay between autophagy and the risk of age-related pathologies across organisms will eventually facilitate the development of clinical applications that promote long-term health.

자가포식은 

리소좀 매개 분해를 통해 

핵산, 단백질, 지질 및 세포 소기관을 포함한 분자와 

세포 소기관을 제거하여 

항상성, 분화, 발달 및 생존을 촉진하는 근본적인 세포 과정입니다. 

오토파지는 건강과 밀접한 관련이 있지만, 오토파지와 노화 및 질병 사이의 복잡한 관계는 아직 명확하게 밝혀지지 않았습니다. 

이 리뷰에서는 

오토파지의 몇 가지 새로운 특징을 살펴보고 

이러한 특징이 노화와 질병의 발생 및 

진행에 어떻게 연관될 수 있는지에 대해 가정합니다. 

또한, 

신경 퇴행성 질환과 같은 노화 관련 병리의 억제제로서 

자가포식 조절제의 사용을 주장하는 

현재의 전임상 증거에 대해 논의합니다. 

마지막으로, 

주요 질문을 강조하고 

오토파지와 노화의 특징 사이의 새로운 연관성을 밝힐 수 있는 

새로운 연구 방향을 제안합니다. 

유기체 전반에서 

오토파지와 노화 관련 병리의 위험 사이의 정확한 상호 작용을 이해하면 

장기적인 건강을 증진하는 임상 응용 프로그램의 개발이 촉진될 것입니다.

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Main

Aging is a biological process that is characterized by time-dependent cellular and functional decline, resulting in reduced quality of life for the organism1. In line with this, aging is the primary risk factor for the development of many disorders, including cardiovascular disease (for example, stroke), cancer and neurodegenerative disease (for example, Alzheimer’s disease (AD)). Collectively, age-related ailments represent a formidable global socioeconomic burden and a significant healthcare challenge2,3. Therefore, identifying therapeutic interventions that promote ‘healthy aging’ (that is, the maintenance of functional ability in old age, enabling older individuals to independently carry out daily tasks) and simultaneously halt the progression of multiple age-related pathological conditions is of paramount importance2.

노화는 

시간에 따른 세포 및 기능 저하를 특징으로 하는 생물학적 과정으로, 

유기체의 삶의 질이 저하됩니다1. 

이에 따라 

노화는 

심혈관 질환(예: 뇌졸중), 암, 신경 퇴행성 질환(예: 알츠하이머병(AD)) 등 

많은 질환의 주요 위험 요인입니다. 

이러한 노화 관련 질환은 

전 세계적으로 막대한 사회경제적 부담과 심각한 의료 문제를 야기합니다2,3. 

따라서 

'건강한 노화'(즉, 노년기의 기능적 능력을 유지하여 노인이 독립적으로 일상 업무를 수행할 수 있도록 하는 것)를 

촉진하는 동시에 여러 노화 관련 병리적 상태의 진행을 중단시키는 치료적 개입을 식별하는 것이 

무엇보다 중요합니다2.
.

Among the many molecular changes associated with old age, altered autophagy has emerged as a feature of aging across diverse species. However, recent advances in understanding the numerous substrates of autophagy and the temporal and spatial effects of impaired autophagy regulation on tissue homeostasis have revealed a complex and multifactorial relationship between autophagy and aging. Here we examine the relationship among autophagy, aging and disease and propose novel links between specific autophagic processes and long-term tissue health, as well as possible implications for anti-aging therapeutic interventions.

노화와 관련된 여러 가지 분자적 변화 중 

자가포식의 변화는 

다양한 종에서 노화의 특징으로 나타나고 있습니다. 

그러나 

최근 자가포식의 수많은 기질과 

자가포식 조절 장애가 

조직 항상성에 미치는 

시간적, 공간적 영향에 대한 이해가 발전하면서 

자가포식과 노화 사이의 

복잡하고 다인자적인 관계가 밝혀졌습니다. 

이 글에서는 

자가포식, 

노화, 

질병의 관계를 살펴보고 

특정 자가포식 과정과 장기적인 조직 건강 사이의 새로운 연관성과 

항노화 치료 개입에 대한 시사점을 제안합니다

Compromised autophagy is a hallmark of aging

Research over the last decade has revealed that the process of autophagy can take many different forms. Autophagy (from the Greek words auto, meaning ‘self’, and phagein, meaning ‘to eat’) is a highly conserved pathway that degrades cellular components, such as defective organelles and aggregates of misfolded protein4, through lysosomes. The process of autophagy was first described in the 1960s, but it was the identification of autophagy-related genes (ATG genes) in the 1990s that propelled major breakthroughs in unravelling the mechanistic complexities of autophagy5,6,7,8,9,10,11,12. There are three major types of autophagy: macroautophagy, microautophagy and chaperone-mediated autophagy (CMA) (Fig. 1a–c), all of which involve delivery of substrates to the lysosome for degradation (see detailed reviews in refs. 13,14). Macroautophagy (hereafter referred to as autophagy) was originally thought of as a nxxxxonselective bulk degradation process (Fig. 1a, pathway (1)).

지난 10년간의 연구를 통해 

자가포식 과정이 다양한 형태로 나타날 수 있다는 사실이 밝혀졌습니다. 

오토파지('스스로'라는 뜻의 그리스어 오토와 '먹다'라는 뜻의 파지인에서 유래)는 

리소좀을 통해 결함이 있는 세포 소기관이나 

잘못 접힌 단백질 응집체4와 같은 

세포 구성 요소를 분해하는 

고도로 보존된 경로입니다. 

오토파지 과정은 

1960년대에 처음 설명되었지만, 

1990년대에 오토파지 관련 유전자(ATG 유전자)가 확인되면서 

오토파지의 기계적인 복잡성을 밝히는 데 

큰 돌파구가 마련되었습니다5,6,7,8,9,10,11,12. 

자가포식에는 

거시적 자가포식, 미세 자가포식, 샤페론 매개 자가포식(CMA)의 

세 가지 주요 유형이 있으며(그림 1a-c), 

모두 분해를 위해 기질을 리소좀으로 전달합니다(참고 문헌 13,14에서 자세한 리뷰 참조). 

거대 오토파지(이하 오토파지라고 함)는 원래 비선택적 대량 분해 과정으로 생각되었습니다(그림 1a, 경로 (1)).

However, the discovery of selective autophagy receptors, among which p62/SQSTM1 was the first, changed this notion15,16. Today, autophagy is recognized as a highly selective cellular clearance pathway that is associated with the maintenance of cellular and tissue homeostasis17,18. Selective autophagy can be further divided into many subtypes on the basis of the specific cargos involved.

These subtypes target various macromolecules (glycophagy and lipophagy) (Fig. 1a, pathways (2)–(5)), mitochondria (mitophagy) (Fig. 1a, pathway (6)), the endoplasmic reticulum (ER) (ER-phagy) (Fig. 1a, pathway (7)), parts of the nucleus (nucleophagy) (Fig. 1a, pathway (8)), pathogens (xenophagy) (Fig. 1a, pathway (9)) and lysosomes themselves (lysophagy) (Fig. 1a, pathway (10)). Below we will discuss the links among these selective autophagy pathways, aging and disease. The core process of autophagy has been described in detail elsewhere14,19.

그러나 

선택적 오토파지 수용체, 

그중에서도 p62/SQSTM1이 최초로 발견되면서 

이러한 개념이 바뀌었습니다15,16. 

오늘날 자가포식은 

세포 및 조직 항상성 유지와 관련된 

매우 선택적인 세포 제거 경로로 인식되고 있습니다17,18. 

선택적 오토파지는 

관련된 특정화물에 따라 여러 하위 유형으로 더 나눌 수 있습니다. 

이러한 아형은 

다양한 거대 분자(글리코파지 및 리포파지)(그림 1a, 경로 (2)-(5)), 

미토콘드리아(미토파지)(그림 1a, 경로 (6)), 

소포체(ER)(ER-파지)(그림 1a, 경로 (7)을 표적으로 합니다. 

1a, 경로 (7)), 핵의 일부 (뉴클레오파지) (그림 1a, 경로 

(8)), 병원균 (이종포식) (그림 1a, 경로 

(9)) 및 리소좀 자체 (리소파지) (그림 1a, 경로 (10)). 

아래에서는 이러한 선택적 자가포식 경로와 노화 및 질병 사이의 연관성에 대해 설명합니다. 

오토파지의 핵심 과정은 다른 곳에서 자세히 설명했습니다14,19.

However, in brief, the core process is initiated following inhibition of mechanistic target of rapamycin (mTOR) or activation of 5′ AMP-activated protein kinase (AMPK), both of which are canonical inducers of autophagy in response to stress (for example, starvation or elevated temperatures) and physical exercise. In addition, transcription factor EB (TFEB) is an important positive regulator of autophagy and lysosomal biogenesis whose nuclear translocation is coupled to the activity of both mTOR (via phosphorylation) and AMPK (via folliculin (FLCN))20,21,22,23. Upon activation of autophagy, the process is initiated by membrane nucleation and phagophore formation followed by elongation and maturation before autophagosome fusion with the lysosome for cargo degradation and recycling. The key proteins involved in each step are presented in Fig. 2.

그러나 간단히 요약하면, 

스트레스(예: 굶주림 또는 온도 상승)와 

신체 운동에 대한 반응으로 

오토파지의 대표적인 유도인자인 라파마이신 기계화 표적(mTOR)의 억제 또는 

5′ AMP 활성화 단백질 키나아제(AMPK)의 활성화에 따라 

핵심 과정이 시작됩니다. 

또한 

전사인자 EB(TFEB)는 

오토파지 및 리소좀 생성과정에 중요한 긍정적 조절자로서, 

핵 전위가 인산화를 통해 mTOR 및 AMPK의 활성과 결합되어 있습니다.20,21,22,23)20,21,23). 

자가포식이 활성화되면 

세포막 핵형성 및 

식세포 형성으로 프로세스가 시작되고, 

이후 연장 및 성숙을 거쳐 

화물 분해 및 재활용을 위해 리소좀과 자가포식체가 융합됩니다.

 각 단계에 관여하는 주요 단백질은 그림 2에 나와 있습니다.

Fig. 1: Different mechanisms of autophagy.

a, Macroautophagy (referred to herein as autophagy)

(1) is a nxxxxonselective process that targets macromolecules or subcellular organelles in bulk. Cytoplasmic material is sequestered into an autophagosome and delivered to the lysosome (or endolysosome) for degradation). Selective autophagy involves recognition of specific cytoplasmic cargo via autophagy receptors that also interact with LC3 in the autophagic membrane, leading to cargo sequestration into autophagosomes that are delivered to a lysosome (or endolysosome) for degradation. This includes aggrephagy (2), where aggregated proteins are ubiquitinated and targeted by ubiquitin-binding autophagy receptors such as p62 (or NBR1); glycophagy (3), where STBD1 (genethonin-1) binds to glycogen and GABARAP, facilitating lysosomal glycogen breakdown into non-phosphorylated glucose by enzymes such as GAA; lipophagy (4), in which lysosomal lipids are degraded into free fatty acids, which are then converted into ATP; the identity of the receptor(s) (yellow) involved in sequestration of lipid droplets is unknown;; granulophagy (5), where sequestration of stress granules (RNA + protein) is mediated by Cdc48/VCP, allowing the stress granule to be delivered to the lysosome for degradation; mitophagy (6), where damaged mitochondria are bound by soluble or membrane-bound mitophagy receptors (mReceptors) that can also bind LC3, leading to engulfment of the mitochondrion into an autophagosome and subsequent delivery to a lysosome for degradation (left); in piecemeal mitophagy, degradation of parts of mitochondria occurs via binding of the outer mitochondrial membrane protein metaxin-1 (MTX1, in the extruded fraction) to LC3C, resulting in the recruitment of p62 and autophagosome formation (right); ER-phagy (7), which in mammals uses the specific receptors FAM134B, RTN3L, ATL3, SEC62, CCPG1 and TEX264, which are located in different parts of the ER; these receptors bind to LC3, leading to sequestration of the ER into an autophagosome and lysosomal degradation of the ER; nucleophagy (8), which, when triggered in mammals, results in nuclear LC3 binding to lamin B1, leading to formation of a bulge that is pinched off to the cytoplasm where degradation by autophagy occurs; xenophagy (type A, 9), where a bacterium’s DNA is detected by cGAS, a sensor that triggers a process of ubiquitination via Smurf1; this is followed by attachment of the NBR1 receptor to the ubiquitin chains and LC3 to continue the autophagy process for degradation of the bacterium; xenophagy (type B, 10), where a bacterium damages the membrane of the phagosome, exposing interior glycans that recruit galectin-8 (Gal-8), which is then recognized by NDP52 to recruit TBK1, LC3C, Nap and Sintbad; the optineurin, p62 and NDP52 receptors interact with ubiquitin on the pathogen and recruit the autophagic engulfment system, and the engulfed pathogen is then brought for degradation; and lysophagy (11), which occurs upon lysosomal membrane permeabilization and can be achieved with or without ubiquitination: recruitment of galectin-3 (Gal-3) to damaged lysosomes further recruits TRIM16 and autophagic proteins such as ULK1 and ATG16L1, and ubiquitination on the lysosome results in the recruitment of p62, which binds to LC3 to facilitate the autophagic process (left); in a parallel ubiquitin-independent process, galectin-8 is recruited to damaged lysosomes and is capable of directly binding to the NDP52 receptor that interacts with LC3 to continue the autophagic process (right). 

a, Macroautophagy (referred to herein as autophagy)

(1)은 거대 분자 또는 세포 소기관을 일괄적으로 표적으로 하는 비선택적 과정입니다. 세포질 물질은 오토파지에 격리되어 리소좀(또는 엔도리소좀)으로 전달되어 분해됩니다). 선택적 자가포식은 자가포식 막의 LC3와도 상호 작용하는 자가포식 수용체를 통해 특정 세포질화물을 인식하여 자가포식체 내로 격리시켜 리소좀(또는 엔도리소좀)으로 전달하여 분해하는 것을 포함합니다. 여기에는 응집된 단백질이 유비퀴틴화되어 p62(또는 NBR1)와 같은 유비퀴틴 결합 오토파지 수용체에 의해 표적화되는 응집포식(2), STBD1(제네토닌-1)이 글리코겐 및 GABARAP에 결합하여 GAA와 같은 효소에 의해 리소좀의 글리코겐이 비인산화 포도당으로 분해되는 것을 촉진하는 글리코파지(3) 등이 있습니다; 

리소좀 지질이 유리 지방산으로 분해되어 ATP로 전환되는 리포파지(4); 지질 방울의 격리에 관여하는 수용체(노란색)의 정체는 알려져 있지 않습니다; 

과립포식(5): 스트레스 과립(RNA + 단백질)의 격리가 Cdc48/VCP에 의해 매개되어 스트레스 과립이 리소좀으로 전달되어 분해될 수 있도록 합니다; 

미토파지(6), 손상된 미토콘드리아가 LC3와 결합할 수 있는 수용성 또는 막 결합 미토파지 수용체(mReceptor)에 결합하여 미토콘드리온이 오토파지에 포획된 후 분해를 위해 리소좀으로 전달되는 과정(왼쪽); 단편적 미토파지에서 미토콘드리아 일부의 분해는 외부 미토콘드리아 막 단백질인 메탁신-1(MTX1, 압출 분획에서)이 LC3C에 결합하여 p62가 모집되고 오토파지가 형성됨으로써 발생합니다(오른쪽); 

포유류에서 ER의 다른 부분에 위치한 특정 수용체 FAM134B, RTN3L, ATL3, SEC62, CCPG1 및 TEX264를 사용하는 ER-파지(7); 이러한 수용체는 LC3에 결합하여 ER을 오토파지솜으로 격리하고 ER의 리소좀 분해를 유도합니다; 

포유류에서 유발되면 핵 LC3가 라미네이트 B1에 결합하여 세포질로 튀어나오는 돌기를 형성하여 자가포식에 의한 분해가 일어나는 핵포식(8); 

박테리아의 DNA가 스머프1을 통해 유비퀴틴화 과정을 촉발하는 센서인 cGAS에 의해 감지되는 이종포식(A형, 9)이 있습니다; 박테리아의 분해를 위한 자가포식 과정을 계속하기 위해 유비퀴틴 사슬과 LC3에 NBR1 수용체가 부착되고, 박테리아가 파고솜의 막을 손상시켜 내부 글리칸을 노출시켜 갈렉틴-8(Gal-8)을 모집한 다음 NDP52가 이를 인식하여 TBK1, LC3C, Nap 및 신트바드를 모집하는 이종 포식(유형 B, 10)이 이어집니다; 

옵티뉴린, p62 및 NDP52 수용체는 병원체의 유비퀴틴과 상호 작용하여 자가포식 포식 시스템을 모집하고 포식된 병원체는 분해를 위해 운반되며, 리소좀 막 투과 시 발생하며 유비퀴틴화 유무에 관계없이 이루어질 수 있는 리소파지(11)가 있습니다: 

손상된 리소좀에 갈렉틴-3(Gal-3)이 결합하면 TRIM16과 ULK1 및 ATG16L1과 같은 자가포식 단백질이 추가로 결합하고, 리소좀에 유비퀴틴이 결합하면 LC3에 결합하여 자가포식 과정을 촉진하는 p62가 결합합니다(왼쪽); 유비퀴틴과 독립적인 병행 과정에서 갈렉틴-8은 손상된 리소좀에 모집되고 LC3와 상호 작용하는 NDP52 수용체에 직접 결합하여 자가포식 과정을 계속할 수 있습니다(오른쪽). 

b, Microautophagy involves capture of cytoplasmic components through direct invagination of endolysosome membranes and can be nonspecific (bulk) (12) or highly specific (13,14). Examples of selective microautophagy in mammalian cells include micro-ER-phagy (13), which uses the SEC62 receptor and involves ER capture and degradation by invagination of the lysosome/endolysosome, and endosomal microautophagy of proteins with the KFERQ pentapeptide motif (14) in a process requiring the chaperone HSC70. 

b, 마이크로 오토파지는 

엔도리소좀 막의 직접 침입을 통한 세포질 성분 포집을 포함하며 비특이적(대량)(12) 또는 매우 특이적(13,14)일 수 있습니다. 

포유류 세포에서 

선택적 마이크로오토파지의 예로는 SEC62 수용체를 사용하고 

리소좀/엔돌솜의 침범에 의한 ER 포획 및 분해를 포함하는 

마이크로-ER-파지(13)와 샤프론 HSC70이 필요한 과정에서 KFERQ 펜타펩타이드 모티브(14)를 가진 단백질의 엔도좀 마이크로오토파지가 있습니다. 

c, CMA (15) also involves targeting of proteins containing a KFERQ pentapeptide-related motif by HSC70 and other co-chaperones such as HSP40. The substrate is then imported into the lysosome through the LAMP2A receptor for further degradation. The LAMP2A receptor is modulated by the glial fibrillary acidic protein (GFAP). Finally, in a CMA-like manner, DNAutophagy/RNAutophagy (16) can occur: nucleic acids (DNA or RNA) bind to the LAMP2C receptor (orange), which also binds to lysosomes. This process allows nucleic acids to be taken up by the lysosome. It has been proposed that a transporter called SIDT2 (green) might have a role in direct uptake of nucleic acids by the lysosome.

c, CMA(15)는 또한 HSC70 및 HSP40과 같은 다른 공동 샤프론에 의해 KFERQ 펜타펩타이드 관련 모티프를 포함하는 단백질을 표적으로 삼는 것을 포함합니다. 그런 다음 기질은 추가 분해를 위해 LAMP2A 수용체를 통해 리소좀으로 옮겨집니다. LAMP2A 수용체는 아교 섬유소 산성 단백질(GFAP)에 의해 조절됩니다. 

마지막으로, CMA와 유사한 방식으로 DNA유토파지/RNA유토파지(16)가 발생할 수 있습니다. 핵산(DNA 또는 RNA)이 LAMP2C 수용체(주황색)에 결합하면 리소좀과도 결합합니다. 이 과정을 통해 핵산은 리소좀에 의해 흡수됩니다. SIDT2(녹색)라는 수송체가 리소좀에 의해 핵산이 직접 흡수되는 데 중요한 역할을 할 수 있다고 제안되었습니다.

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Fig. 2: Core machinery of autophagy.

Initiation of autophagy requires the ULK1 kinase complex, which is tightly regulated by AMPK and mTOR, which act as an activator and inhibitor, respectively. AMPK activates ULK1 through phosphorylation. The ULK1 complex, composed of FIP200, ATG13 and ATG101, stimulates the class III phosphatidylinositol 3-kinase (PIK3C3) complex, which is composed of BECN1 (which can be inhibited by BCL-2), AMBRA1, ATG14L, VPS15 and VPS34. This complex then produces a pool of phosphatidylinositol 3-phosphate (PtdIns3P), which leads to the recruitment of WIPI proteins, which recover ATG9-positive vesicles from previous membranes, as well as recruiting the ATG5–ATG12–ATG16L1 (E3) complex. LC3 is first cleaved by the ATG4 protease to form cytosolic LC3-I, which is further recognized by E1 (ATG7), E2 (ATG3) and E3 components, leading to its conjugation to phosphatidylethanolamine (PE). After this process, LC3-I is referred to as LC3-II. LC3-II binds to LIR-containing autophagy receptors (AR; such as p62) bound to cargo targeted for degradation. Fusion of autophagosomes with lysosomes is mainly mediated by the assistance of RAB proteins, SNARE proteins and a HOPS complex. After fusion, the cargo is degraded by lysosomal hydrolases and the degradation products can be reused by the cell. LC3-II bound to the outer membrane is cleaved by ATG4 to be reused for a new round of lipidation.

자가포식의 시작에는 

각각 활성화제와 억제제 역할을 하는 

AMPK와 mTOR에 의해 엄격하게 조절되는 ULK1 키나아제 복합체가 필요합니다. 

AMPK는 인산화를 통해 ULK1을 활성화합니다. 

FIP200, ATG13 및 ATG101로 구성된 ULK1 복합체는 

BECN1(BCL-2에 의해 억제될 수 있음), 

AMBRA1, ATG14L, VPS15 및 VPS34로 구성된 클래스 III 포스파티딜이노시톨 3키나제(PIK3C3) 복합체를 자극합니다. 

그런 다음 이 복합체는 

포스파티딜이노시톨 3-인산(PtdIns3P) 풀을 생성하여 

이전 막에서 ATG9 양성 소포를 회수하고 

ATG5-ATG12-ATG16L1(E3) 복합체를 모집하는 WIPI 단백질의 모집을 유도합니다. 

LC3는 먼저 ATG4 프로테아제에 의해 절단되어 세포질 LC3-I를 형성하고, 이는 E1(ATG7), E2(ATG3) 및 E3 성분에 의해 추가로 인식되어 포스파티딜에탄올아민(PE)에 접합됩니다. 

이 과정을 거친 LC3-I을 LC3-II라고 합니다. 

LC3-II는 분해 대상화물에 결합된 LIR 함유 자가포식 수용체(AR; 예: p62)에 결합합니다. 

오토파지와 리소좀의 융합은 

주로 RAB 단백질, SNARE 단백질 및 HOPS 복합체의 도움으로 이루어집니다. 

융합 후 화물은 리소좀 가수분해효소에 의해 분해되고

 분해 산물은 세포에서 재사용될 수 있습니다. 

외막에 결합된 LC3-II는 

ATG4에 의해 절단되어 새로운 지질화를 위해 재사용됩니다.

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A growing body of evidence suggests that autophagic activity declines with age in diverse organisms1. Studies in Caenorhabditis elegans, rodents and human cells have demonstrated an age-dependent reduction in lysosomal proteolytic function that thereby impairs autophagic flux24,25,26,27, exacerbating cellular impairment and contributing to the development of age-related diseases1,28,29. Further evidence stemming from Drosophila has demonstrated that aging is associated with reduced expression‎ of several Atg genes (Atg2, Atg8a and bchs (encoding blue cheese)), which are pivotal for both autophagy initiation and activity30. In aged wild-type mice, autophagy is diminished in neuronal cells, as evidenced by decreased rates of autophagolysosomal fusion and impaired delivery of autophagy substrates to lysosomes in the hypothalamus31. Moreover, a decrease in autophagic processes was observed in brain tissue from 18- to 25-month-old mice, as demonstrated by a reduction in the levels of Atg5–Atg12 and Becn1, elevated mTOR activity and increased levels of ferritin H (ferritin H is mainly removed from cells by the autophagy–lysosome pathway)32. In addition, emerging evidence in aged rats has highlighted an age-associated decline in expression‎ of the autophagy-related protein beclin 1 (BECN1) in whole brain tissue, as well as in the hippocampus of naked mole rats and Wistar rats33,34. In line with observations in rodent models, findings in humans have suggested that the expression‎ of autophagy-related genes, such as ATG5, ATG7 and BECN1, declines with age35. Moreover, the development and progression of several human pathologies is highly associated with age-dependent autophagy deficits19,36,37. Collectively, these studies demonstrate that a gradual decline in the abundance of autophagy-related proteins and reduced delivery of cargo to lysosomes occur with age, implicating compromised autophagy as a cardinal feature of organismal aging.

다양한 유기체에서 나이가 들어감에 따라 

자가포식 활동이 감소한다는 증거가 점점 더 많이 제시되고 있습니다1.  

초파리에서 비롯된 추가 증거에 따르면 

노화는 자가포식 개시와 활동 모두에 중추적인 역할을 하는 

여러 Atg 유전자(Atg2, Atg8a 및 bchs(블루치즈 코딩))의 발현 감소와 관련이 있는 것으로 나타났습니다30. 

노화된 야생형 마우스의 경우, 

자가포식 리소좀 융합 속도가 감소하고 

시상하부의 리소좀으로 자가포식 기질이 전달되지 않는 것으로 입증된 바와 같이 

신경세포에서 자가포식이 감소합니다31. 

또한, 18개월에서 25개월 된 생쥐의 뇌 조직에서 

자가포식 과정의 감소가 관찰되었는데, 

이는 Atg5-Atg12 및 Becn1의 수준 감소, 

mTOR 활성 증가, 

페리틴 H의 수준 증가(페리틴 H는 주로 자가포식-리소좀 경로를 통해 세포에서 제거됨)32에 의해 입증되었습니다. 

또한, 노화된 쥐를 대상으로 한 새로운 증거에 따르면 전체 뇌 조직과 벌거벗은 두더지 쥐와 위스타 쥐의 해마에서 자가포식 관련 단백질인 베클린1(BECN1)의 발현이 노화와 연관되어 감소하는 것으로 밝혀졌습니다33,34. 설치류 모델에서 관찰한 결과와 마찬가지로 인간을 대상으로 한 연구에서도 ATG5, ATG7, BECN1과 같은 자가포식 관련 유전자의 발현이 나이가 들면서 감소하는 것으로 나타났습니다35. 또한, 여러 인간 병리의 발생과 진행은 연령에 따른 오토파지 결핍과 밀접한 관련이 있습니다19,36,37. 

이러한 연구 결과를 종합하면, 

나이가 들면서 자가포식 관련 단백질의 양이 점진적으로 감소하고 

리소좀으로의 화물 전달이 감소하며, 

이는 자가포식 기능이 손상되는 것이 

유기체 노화의 주요 특징임을 시사합니다.

In line with a causal role for autophagy in the aging process14, genetically impairing nxxxxonselective or selective autophagy results in accelerated tissue functional decline and disease in a range of experimental models. Transcriptomic profiling in Saccharomyces cerevisiae has provided evidence of defective autophagy among short-lived as compared to long-lived mutants38. In addition, selective mutation(s) and/or knockdown of genes encoding components of the autophagic machinery in C. elegans (lgg-1 (ortholog of ATG8), unc-51 (ortholog of ATG1), bec-1, atg-7, lgg-3 (also known as atg-12) and atg-18), Drosophila (Atg3 and Atg8a) and mice (Atg5, Atg7 and Becn1) shorten lifespan and healthspan1,14,30,39. In line with these observations, systemic genetic knockout of autophagy components (Becn1, Atg5, Atg9 and Atg13) is lethal in mice, highlighting the importance of autophagy in development40. Furthermore, knockdown of genes encoding transcription factors that regulate autophagy, such as TFEB (ortholog in C. elegans, hlh-30) and FOXO (encoding forkhead box O; ortholog in C. elegans, daf-16) shortened lifespan in both wild-type worms and long-lived daf-2 (insulin/insulin-like growth factor-1 (IGF-1) receptor) mutants41.

노화 과정에서 오토파지의 인과적 역할에 따라14, 

유전적으로 비선택적 또는 선택적 오토파지를 손상시키면 

다양한 실험 모델에서 조직 기능 저하와 질병이 가속화됩니다.  

사카로미세스 세레비지애의 전사체 프로파일링은 수명이 짧은 돌연변이와 수명이 긴 돌연변이 사이에서 자가포식에 결함이 있다는 증거를 제공했습니다38.또한, C. elegans(lgg-1(ATG8의 직교 유전자), unc-51(ATG1의 직교 유전자), bec-1, atg-7, lgg-3(atg-12라고도 함) 및 atg-18), 초파리(Atg3 및 Atg8a) 및 마우스(Atg5, Atg7 및 Becn1)에서 오토파지 기계의 구성 요소를 암호화하는 유전자의 선택적 변이 및/또는 녹다운은 수명과 건강수명을 단축시킵니다1,14,30,39. 이러한 관찰에 따라, 오토파지 구성 요소(Becn1, Atg5, Atg9, Atg13)의 전신 유전적 녹아웃은 생쥐에서 치명적이며, 이는 발달에서 오토파지의 중요성을 강조합니다40. 또한, 오토파지를 조절하는 전사 인자를 코딩하는 유전자, 예를 들어 TFEB(C. elegans의 상동 유전자, hlh-30) 및 FOXO(포크헤드 박스 O 코딩, C. elegans의 상동 유전자, daf-16)를 녹다운하면 야생형 벌레와 수명이 긴 daf-2(인슐린/인슐린 유사 성장 인자-1(IGF-1) 수용체) 돌연변이 모두에서 수명이 짧아집니다41.

In contrast, studies in long-lived mutant animals have shown that increased autophagy is associated with delayed aging. In particular, the extended lifespan of C. elegans daf-2 loss-of-function mutants is dependent on autophagic genes, such as bec-1, lgg-1, atg-7 and atg-12 (refs. 1,14,42). Furthermore, HLH-30 is required for the long lifespan of multiple longevity mutants, including not only daf-2 mutants with reduced insulin/insulin-like signaling, but also germline-less glp-1(e2141) mutants, dietary-restricted eat-2(ad1116) mutants, mitochondrial respiration-defective clk-1(e2519) mutants and mRNA translation-impaired rsks-1(sv31) mutants43. These findings coincide with impaired induction of autophagosome formation and lysosomal degradation upon loss of hlh-30, suggesting that HLH-30 promotes longevity by regulating the autophagy process downstream of multiple lifespan extension paradigms43. In addition, formation of long-lived dauer worms, corresponding to a larval hibernation stage, is also associated with increased autophagy and is dependent on the autophagy genes atg-1, atg-7, lgg-1 and atg-18, underlining the essential role of autophagy in organismal adaptation during challenging conditions42.

In line with observations from long-lived mutants, genetic or pharmacological upregulation of autophagy promotes longevity in animals. Autophagy induction by overexpression‎ of Atg genes in Drosophila (Atg1 and Atg8a) and mice (Atg5) extends lifespan30,44,45. Similarly, Bcl2 mutations that disrupt the BECN1–BCL-2 complex increase basal autophagic flux, which results in long-lived male and female mice with improved healthspan46. Overexpression‎ of autophagic regulators in C. elegans and Drosophila, such as AMPK, further facilitates autophagy in diverse tissues and in turn extends longevity14,45. Additionally, hlh-30 overexpression‎ enhances autophagy and promotes lifespan extension in C. elegans43, and silencing of the nuclear export protein exportin-1 (XPO-1) enhances autophagy by enrichment of HLH-30 in the nucleus, which is accompanied by proteostatic benefits and improved longevity47. Moreover, rapamycin, an inhibitor of the mTOR pathway, has been shown to extend the median and maximum lifespan of both female and male mice when fed to them late in life48.

Accumulating evidence in aged mice, as well as in rodent models recapitulating characteristic features of human diseases, has shown that compromised autophagy is among the most common factors contributing to the collapse of tissue homeostasis. In particular, age-associated dysregulation of autophagy (demonstrated by the accumulation of autophagosomes), possibly due to impaired lysosomal fusion and/or degradation, is associated with cellular dysfunction and/or death, which contribute to neurodegeneration, as well as cardiac and skeletal muscle aging49,50,51,52,53. In hematopoietic stem cells (HSCs), autophagy has been shown to delay aging via activation of downstream sirtuin-3 (SIRT3), a key mitochondrial protein capable of rejuvenating blood and protecting against oxidative stress in mice and human HSC-enriched cells54.

Moreover, autophagy appears to be a critical mechanism to maintain immune memory in mice, and levels of the endogenous autophagy-inducing metabolite spermidine fall in human T cells with age. In fact, supplementation of T cells from older donors with spermidine restores autophagy levels to those observed in younger donors via the eIF5A translation factor and TFEB transcription factor55. Furthermore, spermidine administration in a mouse model of mild cognitive impairment, a transitional phase between healthy aging and AD, led to an improvement in degradation of misfolded proteins and an accompanying delay in age-related memory deficits, thereby implicating autophagy as a pathophysiological mechanism of action56.

While dysregulation of autophagy underlies aging and disease phenotypes, excessive autophagy may also contribute to the deterioration of cellular function in some contexts. Recent evidence has demonstrated that an age-dependent decline in the levels of Rubicon, a negative regulator of autophagy, exacerbates metabolic disorders in adipocytes57. While strongly upregulated autophagy may exacerbate metabolic disorders, this finding may also be attributed to autophagy-independent changes in metabolism. Furthermore, elevated autophagy has been found to shorten lifespan in C. elegans mutants lacking sgk-1 (encoding serum/glucocorticoid-regulated kinase-1). Loss of this kinase results in increased mitochondrial permeability, leading to excessive autophagy and reduced organismal fitness in worms and mice58. Conversely, reducing the levels of autophagy in sgk-1 mutants or suppressing the opening of the mitochondrial permeability transition pore restores normal lifespan58. Similarly, suppressing autophagy exclusively in the intestine of post-reproductive adults at higher temperatures has been proposed to prevent the emergence of age-related pathologies in C. elegans59. However, it should be noted that this is in direct contrast to findings in long-lived mutants, where intestinal autophagy is enhanced60,61.

Another study in C. elegans showed that short interfering RNA (siRNA)-based reduction in the abundance of the VPS-34–BEC-1–EPG-8 autophagic nucleation complex in aged post-reproductive worms extended lifespan and improved neuronal integrity29. However, detailed data on knockdown efficiency in aged worms, as well as an understanding of the remaining levels of neuronal autophagy, are necessary to ensure accurate in-depth data interpretation. Collectively, these observations suggest that maintenance of functional autophagy is essential for healthy cellular and organismal aging and that dysregulation of autophagy in either direction, whether insufficient or excessive, contributes to cellular deficits and functional organismal decline.

A summary of autophagy-related genes linked to longevity and disease is provided in Table 1 and Supplementary Table 1. In addition, several interventions known to promote lifespan, including dietary restriction and treatment with pharmacological agents, such as rapamycin, spermidine and NAD+ precursors, require intact autophagic machinery. In totality, these findings reinforce the notion that autophagy stimulation is necessary and sufficient to sustain organismal homeostasis and extend longevity in multiple model organisms (discussed in detail below)1. An overview of autophagy inducers linked to enhanced longevity and improved health is presented in Table 2.

Table 1 Summary of autophagy factors that can promote longevity

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Table 2 Summary of autophagy inducers that extend healthspan and increase lifespan in laboratory animals

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Together, numerous studies have provided evidence that (1) autophagy is compromised during the process of aging; (2) dysfunction of autophagy shortens lifespan in various experimental animal models; and (3) promotion or restoration of autophagy contributes to lifespan and healthspan extension in diverse organisms. This suggests that autophagy is a central regulator of aging. However, an important and fundamental question remains unanswered: how does autophagy facilitate long-term cell and tissue health?

The multifaceted role of autophagy in health and aging

Autophagy and protein homeostasis

Protein homeostasis (proteostasis) collapse is a central hallmark of aging and disease that is characterized by the appearance of misfolded, mislocalized and aggregated proteins. While age-related loss of proteostasis has been documented in numerous tissues, age-dependent protein aggregation is strongly linked to neurodegenerative pathologies such as AD, Parkinson’s disease, Huntington’s disease and amyotrophic lateral sclerosis (ALS)62,63.

Along with molecular chaperones and the ubiquitin–proteasome system (UPS), autophagy is a central regulator of cellular proteostasis that operates to (1) degrade soluble misfolded or oligomeric proteins via CMA (the selective degradation of ubiquitin-tagged protein aggregates by chaperone-assisted selective autophagy) and (2) remove bulk protein aggregates13,19,64. In line with this, genetic perturbation of core components or regulators of the autophagy machinery accelerates age-related protein aggregation, shortens lifespan and exacerbates pathological features in worm, fly and mouse models of disease. Conversely, increasing autophagy, genetically or pharmacologically, suppresses protein aggregation and promotes health and longevity48,65,66 (also reviewed in refs. 67,68,69).

In C. elegans, loss-of-function mutations in bec-1 or atg-18 or RNA interference (RNAi) against bec-1, atg-9 or lgg-1 increases susceptibility to protein aggregation, accelerates the onset of age-related paralysis and shortens lifespan39,70. Similarly, mutations in the core autophagy components Atg8a or Atg7 in Drosophila increase the levels of insoluble protein aggregates and reduce longevity30,71. Finally, knockout of Atg5 or Atg7 in mouse neurons leads to the appearance of cytoplasmic inclusion bodies in the brain and early-onset neurodegeneration72,73,74, while knockout of Lamp2 (the primary receptor for CMA) in the liver results in altered proteostasis and hepatic dysfunction with age75.

Conversely, enhanced proteostasis and extended lifespan in C. elegans occur when the lysosome–autophagy transcription factor HLH-30 or the selective autophagy receptor p62/SQSTM-1 is upregulated. Likewise, in Drosophila, overexpression‎ of Ref(2)P (p62 ortholog) or the autophagy activator FOXO reduces protein aggregation in various tissues and extends lifespan43,76,77,78,79. Pharmacological (for example, via clonidine, rilmenidine or rapamycin) or genetic (for example, atg5) upregulation of autophagy in zebrafish harboring the rare p.Ala152Thr variant of tau ameliorates tau pathology80. Increased autophagy is also associated with enhanced clearance of protein aggregates in mammals, as systemic overexpression‎ of Atg5 or Becn1 genes with mutations that disrupt BECN1–BCL-2 binding improves proteostasis and promotes longevity in mice44,81, while overexpression‎ of the selective autophagy mediator BAG3 suppresses tau accumulation in neurons82.

As a complement to the genetic modulation of autophagy, treatment of Drosophila with the mTOR inhibitor rapamycin suppresses age-related protein aggregation and extends lifespan in an autophagy-dependent manner83. Furthermore, in cell culture and fly models, rapamycin suppresses toxicity from neurodegenerative disease-associated proteins, including mutant huntingtin, polyalanine-expansion-containing proteins and tau84. Several other pharmacological autophagy inducers, such as spermidine and nicotinamide, have also been reported to protect against proteostasis collapse and proteotoxicity in various models of Huntington’s disease, AD, Parkinson’s disease and ALS67.

Autophagy has also been linked to stem cell function, with the autophagy-mediated clearance of protein aggregates central to the activation of quiescent neuronal stem cells. Activating autophagy by overexpression‎ of TFEB or rapamycin supplementation inhibits age-related protein aggregation and enhances neuronal and muscle stem cell function in aged mice85,86,87. Given the fact that stem cell exhaustion is intimately linked to age-related tissue dysfunction, these findings suggest that enhancing proteostasis specifically in stem cells may preserve many aspects of healthy tissue function during aging. Collectively, these observations strongly support the notion that autophagy promotes healthy aging by protecting cells against toxic misfolded and aggregated proteins.

Autophagy regulation of macromolecule availability

Another important role for autophagy in cellular homeostasis and organismal aging is to ensure the availability of metabolites, including amino acids, lipids, carbohydrates and nucleic acids, especially during states of stress, such as nutrient starvation (Fig. 1a). Under challenging conditions, autophagy promotes cellular metabolism and survival by recycling amino acids, which are generated from the degradation of cytosolic substrates, to replenish nutrients, produce energy and promote protein synthesis. An inability to properly recycle amino acids through autophagy is linked to growth and developmental defects in Atg5-deficient mice and impaired growth during nitrogen starvation in several atg-deficient yeast cell lines (including atg1Δ, atg2Δ, atg7Δ, atg11Δ, atg15Δ and atg32Δ mutants) 88,89,90. Autophagy can also be tailored to mediate the availability of carbohydrates, lipids and nucleic acids through three main cellular processes: glycophagy, lipophagy and RNAutophagy or DNAutophagy, respectively.

Glycophagy

Glucose is the primary energy source for cellular metabolism. It is stored as glycogen, and metabolism of glucose is tightly regulated in a tissue-dependent manner (that is, the liver maintains blood glucose levels, while muscles are the source of cellular energy). However, various conditions resulting in metabolic stress, such as starvation, stimulate glycogen breakdown to augment cellular glucose levels and promote metabolic activity91. Glycogen can be degraded in the cytosol through the activity of glycogen phosphorylase and glycogen debranching enzymes (detailed in ref. 92) or in the lysosome via autophagy. The selective clearance of glycogen via autophagy, referred to as glycophagy, has a crucial role in glucose homeostasis. In response to nutrient deficiency, the energy sensor AMPK is activated, which in turn inhibits mTOR complex 1 (mTORC1), leading to activation of the ULK1 kinase, which is important for induction of autophagy (AMPK–mTORC1–ULK1 triad)91. Recent findings in yeast have demonstrated that Atg11 is necessary to facilitate interaction between the AMPK homolog Snf1 and the ULK1 homolog Atg1 upon glucose starvation to promote autophagy93. The LC3-interacting region (LIR) motif, also known in yeast as the Atg8-family-interacting motif (AIM), in starch-binding domain-containing protein 1 (STBD1) may allow cells to physically link glycogen to GABARAPL1, facilitating the transport of glycogen to lysosomes for degradation (Fig. 1a, pathway (3))94. In parallel to glycophagy, other pathways such as β-oxidation may maintain cellular bioenergetics to compensate for glucose deprivation95. Autophagy also has a pivotal role in maintaining cell function, not only in glucose starvation but also in conditions of excess glucose. High glucose levels were associated with mitochondrial dysfunction, generation of reactive oxygen species and induction of autophagy in endothelial progenitor cells96.

Under conditions of impaired autophagy, accumulation of glycogen contributes to the pathogenesis of age-related diseases. In Pompe disease, a lysosomal storage disorder, the ability of lysosomes to degrade glycogen is impaired, owing to a deficiency in the lysosomal hydrolytic enzyme acid α-glucosidase (GAA). This results in accumulation of lysosomal glycogen in many tissues, predominantly in skeletal and cardiac muscle, leading to progressive lethal skeletal myopathy and respiratory and cardiac defects97. Impaired tissue function from the inability of lysosomes to degrade glycogen also leads to energy deficiency in skeletal muscle. For infantile-onset Pompe disease98, a promising therapeutic intervention is administration of recombinant human GAA. Furthermore, dysfunctional autophagy-mediated accumulation of glycogen has been demonstrated to be the cause of neurodegeneration in a mouse model of Lafora disease, with this accumulation suppressed when glycogen synthase is deleted99. These findings indicate that glycogen accumulation might be a cause, rather than a consequence, of impaired autophagy, resulting in impaired cellular function and disease. Glycophagy, therefore, is essential for cellular function and survival, suggesting that levels of glycophagy could determine organismal health and possibly longevity.

Lipophagy

Intracellular storage and use of lipids is critical to maintain cellular energy homeostasis. In response to starvation, triglycerides stored in lipid droplets are hydrolyzed by specific lipases into free fatty acids for energy metabolism. Lipid droplets can also undergo selective degradation by autophagy, termed lipophagy, as an alternative mechanism for regulating lipid homeostasis (Fig. 1a, pathway (4))100. Thus far, a specific receptor coupling lipid droplets to autophagosomes and trafficking to lysosomes has not yet been identified, although LC3-mediated engulfment of lipid droplets has been observed101. Moreover, CMA has been implicated in degradation of the lipid droplet-associated proteins perilipin 2 (PLIN2) and perilipin 3 (PLIN3)102. Lysosomal acid lipases are involved in the degradation of lipid droplets; in particular, lipolysis is conducted primarily by adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL), and selective knockdown of ATGL and HSL in mice results in selective inhibition of lipid droplet degradation, while other autophagy processes (that is, degradation of proteins and organelles) serve as a compensatory mechanism to replenish the reduced availability of energy substrates103. An age-dependent decline in basal autophagy in the liver may underlie the accumulation of hepatic lipids, which in turn has been proposed to contribute to metabolic conditions as well as impairing autophagy, a vicious cycle promoting aging100. For example, age-dependent reduction in CMA is likely due to alterations in the lipid composition of discrete microdomains at the lysosomal membrane, including altered dynamics and stability of the CMA receptor LAMP2A in the lysosome104. Additional mechanisms by which age-related alterations in lipid composition and/or levels may impair autophagy remain unknown. Further, age-dependent accrual of lipid droplets and ectopic fat deposition are highly interconnected with the age-dependent decline in autophagy and/or autophagic defects105,106. Autophagy and LIPL-4-dependent lipolysis are both upregulated in germline-less C. elegans and work interdependently to prolong lifespan107. The mammalian homolog of worm LIPL-4 is lysosomal acid lipase (LIPA), a key enzyme involved in the hydrolysis of cholesterol via autophagy108,109. Cellular supplementation with NAD+, which stimulates autophagy and subtypes of autophagy, including mitophagy, and stimulates the activity of the NAD+-dependent sirtuin-1 (SIRT1) and SIRT3 pathways, reduced fat accumulation and increased lifespan in progeroid animals fed a high-fat diet110,111,112, highlighting the importance of autophagic degradation of lipids in healthspan and lifespan.

In pathological conditions such as alcoholic fatty liver disease (AFLD), impaired lipophagy has been shown to be the basis of lipid peroxidation and cellular damage. AFLD results from excessive consumption of alcohol, leading to damage to the liver in the form of oxidative stress, excessive lipid droplet accumulation in the cytoplasm of hepatocytes (steatosis), mitochondrial damage and cell death. Acute exposure to ethanol triggers lipophagy, which acts as a defense mechanism against lipid peroxidation, thereby protecting hepatocytes. However, chronic exposure to ethanol leads to mTOR-mediated inhibition of lipophagy, which in turn contributes to lipid peroxidation and cell death22,113,114. In fact, inhibition of mTOR-mediated suppression of TFEB, using torin-1, resulted in enrichment of TFEB levels in the liver and protection against steatosis and ethanol-induced liver injury115. Genetic overexpression‎ of TFEB in the liver was shown to increase lysosomal biogenesis and enhance mitochondrial bioenergetics, which served as a protective mechanism against ethanol-induced liver injury in mice. In line with these findings, knockdown of TFEB in the liver of mice resulted in more severe liver injury in response to increased ethanol consumption115. In addition, lipophagy is key for the differentiation of several cell types, including hepatocytes116 and neutrophils117. Knocking out ATG7 in HSCs leads to an accumulation of immature neutrophils resembling the myeloid bias of an aging hematopoietic system. Differentiation can be rescued by supplementation with exogenous free fatty acids used for β-oxidation, further demonstrating that lipophagy usually provides these during the energy-intensive process of differentiation. Further studies on the molecular mechanisms of lipophagy, including identification of lipid-specific autophagy receptors and their impact on cellular homeostasis, will shed light on the relationship among autophagy, metabolism and aging.

Autophagic degradation of nucleic acids: RNAutophagy and DNAutophagy

Nucleic acids are degraded via multiple mechanisms (a complete description of which is beyond the scope of this Review; see details in refs. 118,119), including by autophagy. RNA and DNA are targeted for lysosomal degradation via several pathways, including LC3-dependent autophagic degradation of stress granules (condensates of protein and RNA)120, p62- and NDP52-dependent autophagic degradation of retrotransposon RNA121, lysosomal membrane protein LAMP2C-dependent direct binding to RNA (also DNA122) for lysosomal degradation123 and a lysosomal putative RNA/DNA transporter, SID1 transmembrane family, member 2 (SIDT2), that mediates direct uptake of RNA (and DNA124) for lysosomal degradation125. At present, little is known about whether or how RNAutophagy (also known as RNAphagy) and DNAutophagy (also known as DNAphagy) affect health and aging. However, it is reasonable to suggest that nucleic acid turnover is essential for health, as accumulation of damaged or unnecessary DNA and RNA in the cytosol promotes inflammation, cancer and even accelerated aging68,126,127. DNA damage triggers autophagy and subtypes of autophagy that are considered to be cell survival responses128; in contrast, genetic or age-dependent impairment of DNA repair leads to genomic instability, cellular dysfunction, cell death and accelerated aging68. Exogenous DNA or RNA (for example, microbial) or endogenous nuclear or mitochondrial DNA in the cytoplasm may trigger autophagy. Nuclear DNA (including extranuclear chromatin) could be aberrantly released into the cytoplasm as a result of impaired nuclear envelope integrity, nuclear envelope blebbing or nuclear export processes129, while mitochondrial DNA could leak into the cytoplasm as a result of mitochondrial damage and inefficient elimination of damaged mitochondria via mitophagy126,127. The cyclic GMP-AMP (cGAS)–stimulator of interferon genes (STING), or RIG-I–MAVS, signaling axis detects these nucleic acid fragments to initiate an innate immune reaction, linking it to autoimmunity, inflammation, senescence and autophagy129. Collectively, genomic instability, accumulation of mitochondrial DNA leakage in the cytoplasm and increased levels of cellular stress granules are linked to inflammation, accelerated aging and a broad range of neurodegenerative diseases120,121,126. Although maintenance of DNA and RNA homeostasis is critical for healthy aging, the contribution of RNAutophagy and DNAutophagy to long-term tissue health and pathology requires further exploration.

Autophagy of subcellular organelles: mitophagy, ER-phagy, nucleophagy and lysophagy

Aging is associated with an accumulation of damage to subcellular organelles. Timely and efficient disposal and recycling of dysfunctional organelles is necessary to maintain cellular function and viability. Selective autophagy is the common mechanism underlying the clearance of damaged and/or superfluous subcellular organelles such as mitochondria (mitophagy), the ER (reticulophagy or ER-phagy), the nucleus (nucleophagy) and lysosomes (lysophagy)17. Both membrane-bound and soluble selective autophagy receptors are involved in the selective degradation of organelles18,130.

Among the different types of autophagy targeting subcellular organelles, the most investigated is mitophagy. Mitophagy is the selective autophagic elimination of defective or surplus mitochondria. The PINK1- and parkin-mediated pathway for degradation of heavily depolarized mitochondria is best understood and involves attraction by Ser65-phosphorylated ubiquitin of the soluble selective autophagy receptors NDP52, optineurin and p62, which then recruit the core autophagy machinery for autophagosome formation on the damaged mitochondria131. Other basal, developmental and stress-induced mitophagy pathways involve binding of LC3 to a series of LIR-containing mitochondrial outer membrane proteins, such as NIX (BNIP3L), BNIP3, FKBP8, FUNDC1, BCL2L13, PHB2 and AMBRA1, as well as LC3-binding mitochondrial lipids such as cardiolipin37 (Fig. 1a, pathway (6), left). While whole mitochondria can be degraded via mitophagy, it appears that organelles with minor damage can be ‘repaired’ by other quality-control mechanisms such as the piecemeal mitophagy pathway, which is a basal housekeeping mitophagy pathway that involves degradation of mitochondrial proteins in an LC3C- and p62-dependent manner132 (Fig. 1a, pathway (6), right). Other mitochondrial degradation pathways include the mitochondria-derived vesicle (MDV) pathway, where damaged cargo (for example, impaired mitochondrial proteins) is delivered to the lysosome for degradation in a process dependent on syntaxin-17, PINK1 and parkin133. A recent study in C. elegans showed that damaged subcellular components, including mitochondria, can be budded off from certain neurons via membrane-bound vesicles (termed ‘exophers’)134. Once in the extracellular space, these damaged organelles can be engulfed and digested by surrounding cells134. This cellular release of exophers is conserved in mammals, as cardiomyocytes release exophers (containing mitochondria) to be received and eliminated by adjacent macrophages135.

Accumulating evidence has highlighted that mitophagy is a critical contributor to cellular physiology and organ homeostasis. First, there is an increase in mitophagy from juvenile stages to adulthood, followed by a dramatic reduction in aged animals. For example, there is an increase in basal mitophagy levels in fly flight muscles from the ages of 1 week to 4 weeks136; in mice, mitophagy in the dentate gyrus (DG), a region of the brain that is essential for memory, was reduced by approximately 70% between the ages of 3 and 21 months137. Mitophagy is also impaired under high-fat feeding conditions137 and in neurodegenerative diseases (reviewed in ref. 37). Indeed, mitophagy is reduced in mice with AD (by approximately 50% in the hippocampus in comparison to healthy controls)138, Parkinson’s disease (reviewed in ref. 139) and Huntington’s disease (by over 70% in the DG of huntingtin-expressing mice versus wild-type controls)137. Second, intact mitophagic machinery is required for longevity. Because there are several redundant mitophagy pathways, dysfunction of isolated individual mitophagy pathways may not affect lifespan140,141. However, mitophagy is essential for longevity under conditions of low insulin/IGF-1 signaling (C. elegans daf-2 mutants) and dietary restriction (C. elegans eat-2 mutants)140,142, as well as for the maintenance of neuronal functions in response to stressful conditions126. Third, mitophagy induction is sufficient to improve healthspan and extends lifespan in several model organisms, rescues age-associated neurodegenerative phenotypes in AD138,143 and prolongs lifespan in nematode and fly models of accelerated aging66,111,144. Moreover, functional mitophagy is essential for restraining innate immunity, as mitochondrial stress can lead to the release of damage-associated molecular patterns (DAMPs) that activate innate immunity. Inflammation resulting from excessive exercise in Pink1- and Parkin-knockout mice has been shown to be suppressed by loss of STING, a central regulator of the type I interferon response to cytosolic DNA126.

Other autophagic pathways that target subcellular organelles include ER-phagy, nucleophagy and lysophagy. In yeast, Atg39 regulates perinuclear ER-phagy and nucleophagy, while Atg40 is necessary for cortical and cytoplasmic ER-phagy145 (Fig. 1a, pathway (7)). ER-phagy is conserved in mammalian cells through specific ER-phagy receptors, such as FAM134B, SEC62, RTN3L, CCPG1, ATL3 and TEX264 (reviewed in ref. 146). Nucleophagy is conserved in mammalian cells147 and involves nuclear LC3B–lamin B1 interaction-based nuclear-to-cytoplasmic degradation, which may be a defense mechanism protecting cells from tumorigenesis148 (Fig. 1a, pathway (8)). Lysophagy is regulated by both ubiquitin-dependent (galectin-3–TRIM16–ULK1–autophagy receptor–LC3, the F-box protein FBXO27 and UBE2QL1) and ubiquitin-independent (galectin-8–autophagy receptor–LC3) pathways (reviewed in ref. 149) (Fig. 1a, pathway (11)). Maintenance of functional and effective lysosomes, via timely and efficient lysophagy, is essential for cell survival. In particular, dysfunction in lysosomal membrane proteins such as SCAV-3, the C. elegans homolog of human LIMP-2, has been linked to reduced lifespan, implicating lysosome integrity as a defining factor in longevity150,151,25. Moreover, dysfunctional lysosomal membrane proteins coupled to leakage of proteolytic enzymes (that is, cathepsin D) into the cytosol have been associated with aging and pathological aging in a broad range of neurodegenerative diseases152. Thus, maintaining physiological lysophagy is critical for many cellular processes and is presumably important for health and longevity, as lysosomal rupture triggers endolysosomal damage responses and even lysosomal cell death, which is linked to aging and diseases152,153.

Collectively, an imbalanced quality surveillance system for subcellular organelles, such as mitochondria, the ER, small nuclear fractions and lysosomes, might be a causative factor for age-related pathologies as well as premature aging. Further studies on mitophagy, ER-phagy, nucleophagy and lysophagy to decipher their multilayer regulatory network and association with aging and health are necessary. In particular, studies to address how these processes change with age and how they influence age-related tissue function will lead to critical insights with broad relevance to human health and quality of life.

Xenophagy

Xenophagy (from the Greek meaning ‘to eat foreign matter’) is the process by which autophagy targets pathogens154. Many pathogens are known to be degraded by autophagy, while others take over core autophagy components for their own benefit155 (Fig. 1a, pathway (9)). Indeed, several studies have demonstrated that autophagy can target bacteria such as Rickettsia conorii156, Listeria monocytogenes157, Streptococcus pyogenes158 and Mycobacterium tuberculosis159,160. Xenophagy may also protect the body against invasion by viruses and parasites.

Upon their intake by inhalation, M. tuberculosis bacteria are captured by alveolar macrophages. However, they have evolved the ability to impair phagosome maturation (which under normal conditions would lead to phagocytosis) and end up hijacking macrophages161. Later on, using secretions from ESX-1 (6-kDa early secretory antigenic target (ESAT-6) secretion system 1), the bacteria are able to break free from the phagosome and enter the cytosol. Here xenophagy comes into action. cGAS detects the bacterial DNA162, which results in ubiquitination of the invading bacteria by Smurf1 (or parkin)163. NBR1 (or p62) attaches to these ubiquitin chains, resulting in the recruitment of LC3B and, ultimately, autophagic degradation164. Indeed, an absence of autophagic machinery components, in particular, ULK1 (ref. 165), BECN1 (ref. 166), p62 (ref. 167), ATG7 (ref. 168) and TBK1 (ref. 167), may promote proliferation of the bacteria. The mechanism is similar for specific viruses. BECN1 and p62 in selective autophagy of viral capsids can be protective against Sindbis virus169,170. However, other viruses, such as herpes simplex virus type 1, have evolved to inhibit autophagy by targeting BECN1 (ref. 171). In addition, several studies have highlighted the importance and possible therapeutic relevance of autophagy for controlling severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes coronavirus disease 2019 (COVID-19)172,173,174. With respect to parasites, autophagy can control Toxoplasma gondii, while knockout of ATG5, ATG7 or ATG16L1 renders mice more likely to succumb to parasites175. A detailed review of the relationship between parasites and autophagy is available176.

Although there are not many data available on a direct link between xenophagy and aging or lifespan, it is conceivable that blocking infection by exogenous intruders is required for maintenance of a healthy state and reduced inflammation151,177. Further work to investigate the molecular mechanisms of xenophagy and their association with aging and longevity is required.

Tissue-specific autophagy in aging

As aging is associated with functional decline at both the tissue and organismal level, it is important to understand how aging within individual tissues affects, and is affected by, aging across the entire organism. Evidence from nematodes, flies and mice has revealed that autophagy may have tissue-specific roles in regulating aging14. Inhibition of lgg-1 and atg-18 specifically in the body wall muscle of adult worms is sufficient to shorten the lifespan of the long-lived dietary-restricted eat-2 and insulin/IGF-1 receptor-deficient daf-2 mutants28,178. In addition, the shortened lifespan in atg-18 mutants (ATG-18 is a member of the WIPI protein family, homologous to mammalian WIPI-1 and WIPI-2) can be suppressed by tissue-specific restoration of ATG-18 function: pan-neuronal or intestine-specific expression‎ of atg-18 fully restored the lifespan of atg-18 mutants to that of wild-type worms, while muscle- or hypodermis-specific rescue of ATG-18 had little to no ability to restore lifespan179. In flies, promotion of autophagy in muscle tissue via overexpression‎ of Atg8a or the transcription factor FOXO was sufficient to extend lifespan78,180, while, in mice, inhibition of autophagy through muscle-specific ATG7 deficiency resulted in impaired muscle function (possibly via mitochondrial dysfunction) and decreased lifespan181. Furthermore, enhancing autophagy specifically in the intestine results in maintenance of intestinal barrier function and promotes longevity and healthspan in worms and flies45,178. Given that tissues age unevenly, with some tissues presenting with faster degeneration than others182, it will be interesting to determine how closely rates of aging and autophagy are correlated in different tissues throughout life.

Defective autophagy in diseases associated with accelerated aging, neurodegenerative diseases and inflammaging

Accumulating evidence from studies using laboratory animals and human samples supports an essential role for autophagy in embryonic development, tissue health and lifespan through the suppression of age-associated inflammation (inflammaging), maintenance of genomic integrity, preservation of cellular and tissue homeostasis, and rejuvenation of stem cells (Fig. 3a; refs. 13,14,68,69). While autophagy is tightly regulated by multiple molecular pathways involving central modulators of energy metabolism, such as AMPK, mTORC1, sirtuins and calcineurin (Fig. 3b), several interventions such as dietary restriction, exercise and supplementation with small chemical compounds (detailed below) stimulate autophagy69. Recent preclinical studies have linked impairment of general autophagy or subtypes of autophagy (in some diseases, while a subtype of selective autophagy is impaired, there may be no change, or even an increase, in general autophagy) to pathological states such as progeria and a series of accelerated aging diseases68 (Fig. 3c), neurodegenerative diseases19,37 (Fig. 3d) and other disorders13,14,68,69. For example, maintenance of CMA in aged cells sustains HSC function183 and prevents collapse of the neuronal metastable proteome184. Similarly, mitophagy, which is reduced in both normal aging and AD, extends healthspan140 and suppresses amyloid β- and phosphorylated tau-induced memory loss when stimulated in aged tissues138.

Fig. 3: Autophagy in health and disease.

a, Autophagy participates in multiple processes that are essential for longevity. b, A brief summary of some of the major known mechanisms that regulate autophagy in multiple organisms and their influence on the process. c, A list summarizing premature aging diseases with impaired mitophagy as a cause of mitochondrial dysfunction, which contributes to short lifespan (LS) and healthspan (HS). These premature aging diseases are ataxia telangiectasia (AT), Cockayne syndrome (CS), Fanconi anemia (FA), Hutchinson–Gilford syndrome (HG), Werner syndrome (WS) and xeroderma pigmentosum (XP; especially group A). Changes in autophagy and mitophagy in Hutchinson–Gilford syndrome are elusive. d, Autophagy (including subtypes of selective autophagy, such as mitophagy) is impaired in broad neurodegenerative diseases, where impairment may drive or exacerbate disease progression. These diseases include AD, Parkinson’s disease (PD), Huntington’s disease, ALS and frontotemporal dementia (FTD). We emphasize that these are not the only drivers of the diseases and other processes may have roles leading to pathology and symptomatology.

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Understanding the relationship between compromised autophagy and other hallmarks of aging will provide a better understanding of the molecular events that promote aging and disease14,68,69. Among the many age-related changes previously described, inflammation is linked to autophagy, as impaired autophagy results in inflammation, and has emerged as a major driver of age-related tissue damage63,69,185,186. Inflammation is an evolutionarily conserved protective mechanism designed to maintain organismal homeostasis in the face of acute and local perturbations and serves as an adaptive response to infection or injury187. Chronic, systemic inflammation develops progressively with age and contributes to organismal deterioration through the process termed inflammaging186.

Autophagy has been identified as one of the pivotal mechanisms orchestrating the differentiation and metabolic state of innate immune cells. In particular, the balance between mTOR and AMPK activation has a central role in immune cell maintenance and function. Upon mTOR activation, autophagic flux is reduced, accompanied by increased cellular glycolytic activity, giving rise to a proliferative and pro-inflammatory phenotype in macrophages. In contrast, AMPK activation drives autophagy and promotes the OXPHOS-dependent function of non- or anti-inflammatory macrophages188.

Autophagy also regulates the NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome, which is an intracellular protein complex that activates caspase-1, which in turn catalyzes the cleavage, activation and subsequent release of pro-inflammatory cytokines (for example, interleukin (IL)-1β), which can induce neurodegeneration186,189. The NLRP3 inflammasome has been identified as a critical component of the innate immune response (that is, the response to microbial motifs, endogenous danger signals and environmental irritants) and orchestrates host immune homeostasis189. Defective autophagy, for example, in models of selective knockout or knockdown of genes encoding components of the autophagic core machinery (for example, ATG5, ATG7, BECN1 and MAP1LC3B), results in unrestricted inflammasome activation and consequent inflammation. Likewise, promotion of autophagy through starvation or with pharmacological agents (for example, rapamycin) inhibits the inflammasome190. In addition, evidence stemming from an APP/PS1 mouse model of AD demonstrated mitophagy-induced inhibition of the NLRP3 inflammasome, resulting in reduced neuroinflammation138. These findings imply an important role for autophagy in the regulation of inflammation and, in turn, aging and neurodegenerative diseases.

Anti-aging effects of autophagy modulators

The mounting evidence that an imbalance of autophagy is an important age-associated characteristic has driven extensive research into the development of compounds that can promote autophagy1. Pharmacological agents promoting autophagy can be classified on the basis of their effect on the mTOR pathway191. mTOR inhibition by rapamycin has been shown to reduce protein synthesis and promote autophagy, both of which contribute to extended lifespan in yeast, nematodes, flies and mice (Table 2). In addition, rapamycin has been demonstrated to protect against neurodegenerative diseases, including AD, via promotion of autophagy; however, rapamycin treatment was observed to be detrimental in the case of models of ALS, possibly owing to non-autophagy-related side effects191. Other pharmacological agents reported to promote autophagy via direct interaction with mTOR include torin-1 and PP242 (ref. 192). mTOR-independent promoters of autophagy mainly act via the AMPK pathway. Examples include metformin and trehalose, which have been demonstrated to be effective in enhancing autophagy, extending lifespan and protecting against neurodegeneration in experimental models191.

Compounds such as resveratrol and spermidine modulate the acetylation state of proteins to regulate autophagy and promote longevity. Resveratrol is a natural polyphenol that reportedly promotes lifespan in C. elegans and healthspan in mice via activation of the NAD+-dependent deacetylase SIRT1 (refs. 112,193,194). Spermidine is a polyamine that extends the lifespan of yeast, worms, flies and mice by enhancing autophagy through inhibition of the EP300 acetyltransferase195, among other mechanisms55,196,197,198. The longevity-extending effects of spermidine are abolished upon depletion or deletion of essential autophagy genes such as bec-1 in C. elegans and Atg7 in yeast and flies197,199. Furthermore, pharmacological inhibition of XPO-1 results in enhanced autophagy (as evidenced by an increase in the frequency of autophagosomes and autolysosomes) and increased lifespan in C. elegans. These effects were mediated by nuclear enrichment of HLH-30, which occurred in an mTOR-independent manner47. Additional modulators of TFEB homologs that regulate autophagy and have also been demonstrated to protect against pathophysiological aging include ouabain and fisetin. Ouabain is a cardiac glycoside that enhances activation of TFEB through inhibition of the mTOR pathway and induces downstream autophagy–lysosomal gene expression‎ and cellular restorative properties200. Ouabain has been shown to reduce the accumulation of abnormal toxic tau both in vitro and in vivo200. Fisetin is a flavonol and was shown to facilitate the clearance of endogenous tau via TFEB (through inhibition of mTOR kinases) and Nrf2 activation20.

Other small molecules that induce subtypes of autophagy, especially mitophagy, also enhance longevity and suppress age-associated diseases. These include NAD+, a fundamental metabolite in energy metabolism, redox homeostasis, mitochondrial function, and the arbitration of cell survival and death185. NAD+-activated sirtuins stimulate autophagy via mTOR inhibition and deacetylation of several key autophagy proteins (ATG5, ATG7 and ATG8)201,202. In addition, the NAD+–SIRT axis activates mitophagy by increasing the activity of a series of mitophagy-related proteins, such as PINK1, parkin, NIX (DCT-1 in C. elegans) and BNIP3 (refs. 66,203). Supplementation with NAD+ precursors, such as nicotinamide (NAM), nicotinamide riboside (NR) or nicotinamide mononucleotide (NMN), can increase lifespan and/or improve healthspan in worms, flies and mice111,204,205,206. NAD+ augmentation also prevents memory loss in both amyloid β and tau C. elegans and mouse models of AD, in a mitophagy-dependent manner (requiring pink-1, pdr-1 or dct-1)138. Over seven human clinical trials have shown the safety and bioavailability of NR (1–2 g per day for up to 3 months); there are more than 30 ongoing clinical trials on the use of NR to treat premature aging and other age-related diseases (see a review in ref. 185). Another clinically promising mitophagy inducer is urolithin A, a metabolite of ellagitannins from the gut microflora. Urolithin A extends healthspan and lifespan in C. elegans, with lifespan extension depending on genes involved in autophagy (that is, bec-1, sqst-1 and vps-34) and mitophagy (pink-1, dct-1 and the nonspecific skn-1)207. Intriguingly, urolithin A inhibits memory loss in both amyloid β and tau C. elegans and mouse models of AD in a mitophagy-dependent manner (dependent on pink-1, pdr-1 or dct-1)138. Urolithin A (500 mg and 1,000 mg per day for 4 weeks) was also shown to be safe in a phase 1 clinical trial208. A summary of different lifespan/health-benefit mitophagy inducers can be found in Table 2. Encouraged by the clinical safety of NR and urolithin A, their effects on healthspan and lifespan in older individuals deserve further investigation. Despite recent progress in the identification of novel as well as well-known autophagy-inducing compounds, it is also of great importance to highlight the pleiotropic effects of these pharmacological interventions and to completely understand the full complement of targets with which they interact to use them safely for therapeutic intervention.

While experimental/empirical evidence indicates that autophagy is defective in older individuals, it is conceivable that exposing individuals to autophagy inducers, dietary restriction and exercise late in life could boost autophagy and result in benefits to tissue function209,210 (Fig. 4a). On the basis of preclinical data, it is presumed that autophagy stimulation (ideally to increase autophagy to the levels observed early in adulthood) may be sufficient to provide benefits (Fig. 4b).

Fig. 4: Maintaining autophagy through lifestyle and medical interventions prolongs longevity.

a, Potential interventions to stimulate autophagy: autophagy inducers, dietary restriction, exercise and genetic approaches. b, Autophagy induction could positively impact human health.

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Conclusions and future perspectives

Mounting evidence from studies using laboratory animals, human tissues and related clinical trials supports the concept that (1) there is an age-dependent decline in autophagy, (2) autophagy is a crucial determinant of cellular health and organismal longevity and (3) impairment or imbalance in autophagy promotes pathological aging and disease. Given the broad spectrum of unique properties associated with autophagy, we propose that ‘compromised autophagy’ is a central feature of normal aging. Although the relationship between autophagy and aging is often described as ‘decreased autophagy is detrimental’ and ‘increased autophagy is beneficial’, this may be too simplistic a picture. Instead, long-term health benefits will likely arise from achieving the right balance of autophagy, which itself will depend on tissue and organismal age. For example, in C. elegans, impairing autophagy early in life has a negative effect on longevity, whereas knockdown of a specific subset of autophagy genes in adulthood may have beneficial effects on lifespan59. Similarly, increased autophagy through a hypermorphic allele of atg-5 has differential effects on polyglutamine aggregation in the muscles and neurons of C. elegans211. In flies, a mild increase in autophagy extends lifespan, whereas strongly increasing autophagy shortens lifespan212. It should also be noted that autophagy induction may also result in unwanted effects, such as multiple senescence pathologies59 and resistance to cancer therapy (reviewed in ref. 213). Collectively, these observations suggest that the level of and balance among the different forms of autophagy in each tissue are highly specified for each stage of life, and an understanding of this will be crucial for healthy aging. Thus, while different types of autophagy may influence aging to different extents, a central goal for promoting health will be to find approaches that can fine-tune autophagy to the right levels, at the right time and in the right tissues, to enhance health (Fig. 4). To achieve this, it will be critical to develop novel interventions that allow for controlled delivery of autophagy modulators into specific tissues or cell types at precise stages of life. Such therapeutic strategies could then be administered chronically, acutely or in a pulsed fashion as and when required. Additionally, it may be necessary to specifically induce either general or selective autophagy to provide overall long-term health benefits66. For example, premature aging diseases such as ataxia telangiectasia, xeroderma pigmentosum group A and Cockayne syndrome exhibit increased general autophagy but impaired mitophagy; therefore, specifically stimulating mitophagy, rather than general autophagy, would be the most efficient way to counteract pathological features while avoiding detrimental side effects66.

There are many outstanding questions related to autophagy in aging that need to be addressed. For example, what are the intricate mechanisms that orchestrate distinct autophagic pathways? How is autophagy spatially and temporally regulated, and how does disruption of this regulation suppress or promote disease? Are some aspects of autophagy more important in an age- and/or tissue-dependent manner? What are the determining factors that dictate the route of degradation, via the UPS or autophagy? How are clearance mechanisms balanced with synthesis and folding through the proteostasis network? What are the thresholds of life-beneficial and life-detrimental autophagy? In line with the traditional Chinese yin–yang philosophy, autophagy must be balanced, as diminished autophagy results in the accumulation of toxic subcellular components while excessive autophagy can lead to organ atrophy and other detrimental effects14,37,59,69,212. Furthermore, compensatory responses between proteolytic systems (for example, between autophagy and the UPS214) have a critical role in determining the onset and rate of age-related tissue deterioration and should be considered in future experimental designs and data interpretation. Finally, are there any conditions or diseases where we should be cautious about inducing autophagy, in that protection against one form of pathology may increase the risk for another? For example, pancreatic cancer cells may hijack autophagy processes to obtain nutrients for growth; hence, in this condition, autophagy inhibition in combination with cancer chemotherapies may inhibit pancreatic cancer growth215,216. Addressing these questions will facilitate understanding of the aging process and, more importantly, enable identification of novel targets that may be manipulated for therapeutic intervention in age-associated diseases.

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