Re: 파킨슨 증후군 2. multiple system atrophy. 2019 nature

[문형철]

파킨슨병과 유사한 

다계통 위축증.. 

1. nature  
2. experimental & molecular medicine  
3. review articles  
4. article

Models of multiple system atrophy

• Review Article
• Open access
• Published: 18 November 2019

Models of multiple system atrophy

• He-Jin Lee, 
• Diadem Ricarte, 
• Darlene Ortiz & 
• Seung-Jae Lee 

Experimental & Molecular Medicine volume 51, pages1–10 (2019)Cite this article

•  
•  
•  
•  

Abstract

Multiple system atrophy (MSA) is a neurodegenerative disease with diverse clinical manifestations, including parkinsonism, cerebellar syndrome, and autonomic failure. Pathologically, MSA is characterized by glial cytoplasmic inclusions in oligodendrocytes, which contain fibrillary forms of α-synuclein. MSA is categorized as one of the α-synucleinopathy, and α-synuclein aggregation is thought to be the culprit of the disease pathogenesis. Studies on MSA pathogenesis are scarce relative to studies on the pathogenesis of other synucleinopathies, such as Parkinson’s disease and dementia with Lewy bodies. However, recent developments in cellular and animal models of MSA, especially α-synuclein transgenic models, have driven advancements in research on this disease. Here, we review the currently available models of MSA, which include toxicant-induced animal models, α-synuclein-overexpressing cellular models, and mouse models that express α-synuclein specifically in oligodendrocytes through cell type-specific promoters. We will also discuss the results of studies in recently developed transmission mouse models, into which MSA brain extracts were intracerebrally injected. By reviewing the findings obtained from these model systems, we will discuss what we have learned about the disease and describe the strengths and limitations of the models, thereby ultimately providing direction for the design of better models and future research.

요약

다계통 위축증(MSA)은 

파킨슨병, 소뇌 증후군, 자율신경실조 등 

다양한 임상 증상을 보이는 신경 퇴행성 질환입니다. 

병리학적으로 MSA는 

섬유소 형태의 α-시누클레인을 포함하는 

희소돌기아교세포의 아교 세포질 내포물이 특징입니다. 

MSA는 

α-시누클레인 병증 중 하나로 분류되며 

α-시누클레인 응집이 질병 발병의 원인으로 생각됩니다. 

파킨슨병이나 루이체 치매와 같은 

다른 시누클레인 병증의 발병 기전에 대한 연구에 비해 

MSA 발병 기전에 대한 연구는 상대적으로 부족합니다. 

그러나 최근 MSA의 세포 및 동물 모델, 특히 α-시누클레인 형질전환 모델의 개발로 인해 이 질환에 대한 연구가 발전하고 있습니다. 여기에서는 독성 물질로 유도된 동물 모델, α-시누클레인 과발현 세포 모델, 세포 유형별 프로모터를 통해 희소교세포에서 α-시누클레인을 특이적으로 발현하는 마우스 모델 등 현재 이용 가능한 MSA 모델을 검토합니다. 또한 최근 개발된 전달 마우스 모델에서 MSA 뇌 추출물을 대뇌에 주입한 연구 결과에 대해서도 논의할 예정입니다. 이러한 모델 시스템에서 얻은 결과를 검토하여 질병에 대해 배운 내용을 논의하고 모델의 강점과 한계를 설명함으로써 궁극적으로 더 나은 모델 설계와 향후 연구에 대한 방향을 제시할 것입니다.

Similar content being viewed by others

Multiple system atrophy: at the crossroads of cellular, molecular and genetic mechanisms

Article 21 April 2023

Brain regions susceptible to alpha-synuclein spreading

Article 24 September 2021

Reverse engineering Lewy bodies: how far have we come and how far can we go?

Article 11 January 2021

Introduction

Multiple system atrophy (MSA) is a rapidly progressive sporadic adult-onset neurodegenerative disorder. It was first termed to describe neuronal atrophy found in various diseases, including striatonigral degeneration, olivopontocerebellar atrophy, and Shy-Drager syndrome1. Epidemiologic studies of MSA have shown a preval‎ence range of 3.4 to 4.9 per 100,000 people, increasing to 7.8 per 100,000 among people older than 40 years of age2. Moreover, it affects men and women equally and has an average age onset of approximately 55–60 years3,4. The mean life expectancy of MSA is 6–10 years following diagnosis5,6.

MSA is characterized by clinical symptoms that are subdivided into extrapyramidal, pyramidal, cerebellar, and autonomic symptoms. Extrapyramidal symptoms include bradykinesia, rigidity, and postural instability, which are similar to the symptoms of Parkinson’s disease (PD). The autonomic symptoms include common autonomic dysfunctions, such as urogenital, gastrointestinal, and cardiovascular failure. Nonmotor symptoms, such as sleep and cognitive disorders, respiratory problems, and emotional/behavioral symptoms, might also occur during disease development7,8. The different symptoms of MSA can be used to categorize the disease into two subtypes: the parkinsonian subtype (MSA-P) and the cerebellar type (MSA-C). MSΑ−P patients exhibit more PD symptoms, whereas MSA-C patients commonly display cerebellar ataxia. More than two-thirds of MSA patients in Western countries are MSA-P patients, while MSA-C is more common in Japan9,10,11 However, MSA-P is more common than MSA-C in Korea, indicating that the subtypes of MSA vary among Asian countries12,13.

MSA is pathologically distinguished by a widespread neuronal loss that is accompanied by gliosis in the basal ganglia, cerebellum, pons, inferior olivary nuclei, and spinal cord. The symptoms of MSA are similar to those of PD, which makes it difficult to distinguish the two diseases. Clinical differential diagnosis is practically possible, although neuropathological confirmation is still required for a definitive diagnosis of MSA (Fig. 1).

소개

다계통위축증(MSA)은

빠르게 진행되는

산발적인 성인 발병 신경 퇴행성 질환입니다.

다계통위축증은

선조성 퇴행,

올리브소뇌위축증,

샤이-드래거 증후군1 등

다양한 질환에서 발견되는

신경세포 위축을 설명하기 위해 처음 사용되었습니다.

MSA의 역학 연구에 따르면

유병률은 10만 명당 3.4~4.9명이며,

40세 이상에서는 10만 명당 7.8명까지 증가합니다2.

또한

남성과 여성에게 동등하게 영향을 미치며

평균 발병 연령은 약 55~60세입니다3,4.

MSA의 평균 기대 수명은

진단 후 6~10년입니다5,6.

MSA는

추체외로,

피라미드,

소뇌,

자율 증상 등으로

세분화되는 임상 증상이 특징입니다.

추체외로 증상으로는

서동증, 경직, 자세 불안정성 등이 있으며,

이는 파킨슨병(PD)의 증상과 유사합니다.

자율 증상에는

비뇨생식기, 위장, 심혈관 기능 부전과 같은

일반적인 자율 기능 장애가 포함됩니다.

수면 및 인지 장애,

호흡기 문제,

정서/행동 증상과 같은

비운동 증상도 질병이 진행되는 동안 발생할 수 있습니다7,8.

MSA의 다양한 증상은

이 질환을 파킨슨병 아형(MSA-P)과

소뇌형(MSA-C)의 두 가지 하위 유형으로 분류하는 데

사용될 수 있습니다.

MSΑ-P 환자는 더 많은 PD 증상을 보이는 반면,

MSA-C 환자는 일반적으로 소뇌 운동 실조증을 보입니다.

서구 국가에서는 MSA 환자의 3분의 2 이상이 MSA-P 환자인 반면,

일본에서는 MSA-C가 더 흔합니다9,10,11

그러나

한국에서는

MSA-P가 MSA-C보다 더 흔하여

아시아 국가마다 MSA의 하위 유형이 다르다는 것을 알 수 있습니다12,13.

MSA는

병리학적으로 기저핵, 소뇌, 폰, 하부 난소핵 및 척수에서

신경교 교원증을 동반하는 광범위한 신경세포 손실로 구별됩니다.

MSA의 증상은

PD의 증상과 유사하여 두 질환을 구별하기 어렵습니다.

임상적 감별 진단은

실질적으로 가능하지만,

MSA의 확실한 진단을 위해서는 신경 병리학적인 확인이 필요합니다(그림 1).

Fig. 1: Clinical characteristics of MSA.

The symptoms are subdivided into parkinsonian, cerebellar, and autonomic. Parkinsonian symptoms include motor symptoms, such as bradykinesia, tremor, and postural instability. Patients with more evident parkinsonism are considered to have the parkinsonian subtype of the disease (MSA-P). Patients with cerebellar symptoms, such as ataxia and cerebellar oculomotor dysfunction, are considered to have the cerebellar subtype of the disease (MSA-C). Both subtypes share common autonomic dysfunctions described above.

증상은 

파킨슨병, 

소뇌 및 자율신경계로 세분화됩니다. 

파킨슨병 증상에는 

서동증, 떨림, 자세 불안정과 같은 운동 증상이 포함됩니다. 

보다 명백한 파킨슨병 증상이 있는 환자는 

파킨슨병 아형(MSA-P)으로 간주됩니다. 

운동 실조 및 소뇌 안구 운동 기능 장애와 같은 소뇌 증상이 있는 환자는 

소뇌 아형(MSA-C) 질환으로 간주됩니다. 

두 하위 유형 모두 위에서 설명한 

공통적인 자율 기능 장애를 공유합니다.

Full size image

Multiple system atrophy and glial cytoplasmic inclusions

The important neuropathological hallmark of MSA is the presence of argyrophilic filamentous glial cytoplasmic inclusions (GCIs), predominantly in oligodendrocytes14. GCIs are spherical protein aggregates located near nuclei with a diameter of 5–20 μm and various morphologies. GCIs in oligodendrocytes are usually larger and paler than nonoligodendrocyte-derived GCIs. They are primarily composed of loosely packed filaments of α-synuclein protein that is phosphorylated at residue Ser129 and ubiquitinated15,16. Immunohistochemical studies have identified other proteins that colocalize with α-synuclein. These include p25α/TPPP (tubulin polymerization-promoting protein), α,β-crystallin, tau, LRRK2, cyclin-dependent kinase 5 (cdk5), microtubule-associated protein 5, ubiquitin, and tubulin (reviewed in ref. 17). p25α/TPPP has a vital role in the stabilization of microtubules, the projection of mature oligodendrocytes, and ciliary structures18. It is essential for the differentiation and maturation of oligodendrocytes19. p25α/TPPP is commonly found in myelin sheaths, but during the first stages of MSA, it relocates to the oligodendrocyte soma, resulting in early myelin dysfunction20. The redistribution of p25α in oligodendrocytes causes an increase in the volume of cell bodies, which is a typical characteristic of cells with GCIs. Ultimately, the presence of p25α in the cell body enhances the aggregation of α-synuclein, which may lead to oligodendroglial dysfunction and neuronal degeneration18,21.

다발성 시스템 위축 및 아교세포질 내포물

MSA의 중요한 신경 병리학적인 특징은

주로 희소돌기아교세포에서 나타나는

친수성 필라멘트성 아교세포질 내포물(GCI)의 존재입니다14.

GCI는 직경 5~20 μm의 다양한 형태를 가진 핵 근처에 위치한 구형 단백질 응집체입니다. 희소돌기아교세포의 GCI는 일반적으로 비희소돌기아교세포 유래 GCI보다 더 크고 옅은 색을 띠고 있습니다. 이들은 주로 잔기 Ser129에서 인산화되고 유비퀴틴화된 α-시누클레인 단백질의 느슨하게 포장된 필라멘트로 구성됩니다15,16. 면역조직화학적 연구를 통해 α-시누클레인과 공동 위치하는 다른 단백질이 확인되었습니다. 여기에는 p25α/TPPP(튜불린 중합 촉진 단백질), α,β-크리스탈린, 타우, LRRK2, 사이클린 의존성 키나제 5(cdk5), 미세소관 관련 단백질 5, 유비퀴틴 및 튜불린(참조 17)이 포함됩니다. p25α/TPPP는 미세소관의 안정화, 성숙한 희소돌기세포의 돌출 및 섬모 구조18에 중요한 역할을 합니다. 희돌기아교세포의 분화와 성숙에 필수적입니다19. p25α/TPPP는 일반적으로 미엘린 피막에서 발견되지만, MSA의 첫 단계에서는 희돌기아교세포 체질로 이동하여 초기 미엘린 기능 장애를 초래합니다20. 희돌기아교세포에서 p25α의 재분배는 세포체의 부피를 증가시키는데, 이는 GCI를 가진 세포의 전형적인 특징입니다. 궁극적으로 세포체에 p25α가 존재하면 α-시누클레인의 응집이 강화되어 희돌기아교세포 기능 장애와 신경세포 퇴행으로 이어질 수 있습니다18,21.

Multiple system atrophy and α-synuclein

MSA belongs to a diverse group of neurodegenerative disorders described as α-synucleinopathies, which are similar to PD and dementia with Lewy bodies (DLB). These disorders are characterized by the abnormal accumulation of α-synuclein protein aggregates22,23. α-Synuclein is a predominantly neuronal presynaptic protein present in the brain and is expressed in other tissues at various levels. It is encoded by the SNCA gene, which is linked to PD and has also been associated with an increased risk of PD, DLB, and MSA24.

The presence of GCIs and the excessive accumulation of α-synuclein in the oligodendrocytes are accompanied by neuronal degeneration, brain atrophy, demyelination, and mutation of nerve cells in MSA patients25,26. A study by Peng et al. showed differences between GCI-α-synuclein and LB-α-synuclein27. These inclusions are conformationally and biologically distinct. GCI-α-synuclein is 1000-fold more potent than LB-α-synuclein in seeding the aggregation of monomeric α-synuclein, which may explain the highly aggressive and rapidly progressive nature of MSA symptoms.

Several studies have reported that there is little to no α-synuclein expression‎ in mature oligodendrocytes in the human brain11,28,29. There is no evidence of increased α-synuclein expression‎ in MSA oligodendrocytes. However, a recent study by Asi et al. showed a three-fold increase in SNCA mRNA levels in MSA oligodendrocytes postmortem, although the change did not reach statistical significance; it is still questionable whether the increase in mRNA levels can significantly change the levels of the α-synuclein protein in oligodendrocytes30. In vitro cultures of MSA patient-derived induced pluripotent stem cells (iPSCs) showed that α-synuclein is only expressed in the early stages of oligodendrocyte maturation but not in the premyelination period31.

The mechanisms of the accumulation of α-synuclein in oligodendrocytes are still unknown. Several hypotheses have provided possible explanations as to how GCIs form32. One possibility is that they form through the induced expression‎ and aggregation of α-synuclein in oligodendrocytes and other glial cells under disease conditions, but there is little evidence to support this cell-autonomous mechanism. An alternative explanation is that they form through the uptake of α-synuclein secreted from neurons by oligodendrocytes. Studies have shown the transfer of neuronal α-synuclein, both in co-cultures and through exogenous addition, into oligodendrocytes in vitro and in vivo33,34,35. In MSA, oligodendrocytes might be more prone than neuronal cells to the accumulation of neuron-derived α-synuclein, possibly because the clearance mechanism might not be as efficient as that in neurons. In support of this explanation, Peng et al. suggested that the cellular milieu determines different synucleinopathies, such as LBD and MSA27.

A failure to discard proteins through cellular degradation pathways may lead to the production of toxic aggregates that may incorporate into GCIs and cause oligodendrocyte dysfunction. Oligodendrocytes become enlarged, and nuclei turn pale when myelin degeneration occurs. The propagation of α-synuclein aggregates to other adjacent cells may lead to inflammatory responses by microglia and neurodegeneration (Fig. 2).

다계통위축증과 α-시누클레인

MSA는

α-시누클레인 병증으로 설명되는

다양한 신경 퇴행성 질환 그룹에 속하며,

이는 루이소체 치매(DLB)와 유사합니다.

이러한 장애는

α-시누클레인 단백질 응집체가

비정상적으로 축적되는 것이 특징입니다22,23.

α-시누클레인은

주로 뇌에 존재하는 신경세포 시냅스 전 단백질로,

다른 조직에서도 다양한 수준으로 발현됩니다.

이 단백질은

PD와 관련이 있는 SNCA 유전자에 의해 암호화되며

PD, DLB 및 MSA의 위험 증가와도 관련이 있습니다24.

GCI의 존재와 희소돌기아교세포에 α-시누클레인이 과도하게 축적되면 신경세포의 퇴화, 뇌 위축, 탈수초화, 돌연변이가 동반되며, MSA 환자에서 신경세포의 돌연변이가 발생합니다25,26. Peng 등의 연구에 따르면 GCI-α-시누클레인과 LB-α-시누클레인 사이에는 차이가 있는 것으로 나타났습니다27. 이러한 내포물은 형태적으로나 생물학적으로 구별됩니다. GCI-α-시누클레인은 단량체 α-시누클레인의 응집에 있어 LB-α-시누클레인보다 1000배 더 강력하며, 이는 MSA 증상의 매우 공격적이고 빠르게 진행되는 특성을 설명할 수 있습니다.

여러 연구에 따르면 인간 뇌의 성숙한 희소돌기아교세포에서는 α-시누클레인 발현이 거의 또는 전혀 없다고 보고되었습니다11,28,29. MSA 희돌기아교세포에서 α-시누클레인 발현이 증가한다는 증거는 없습니다. 그러나 Asi 등의 최근 연구에 따르면 사후 MSA 희돌기아교세포에서 SNCA mRNA 수준이 3배 증가했지만 통계적 유의성에 도달하지는 않았으며, mRNA 수준의 증가가 희돌기아교세포에서 α-시누클린 단백질의 수준을 크게 변화시킬 수 있는지 여부는 여전히 의문입니다30. MSA 환자 유래 유도만능줄기세포(iPSC)의 시험관 내 배양에서 α-시누클레인은 희돌기아교세포 성숙의 초기 단계에서만 발현되고 골수화 전 기간에는 발현되지 않는 것으로 나타났습니다31.

희소돌기아교세포에서 α-시누클레인이 축적되는 메커니즘은 아직 밝혀지지 않았습니다. 몇 가지 가설이 GCI가 어떻게 형성되는지에 대한 가능한 설명을 제공했습니다32. 한 가지 가능성은 질병 상태에서 희돌기아교세포 및 기타 신경교세포에서 α-시누클레인의 발현과 응집을 유도하여 형성된다는 것이지만, 이러한 세포 자율 메커니즘을 뒷받침하는 증거는 거의 없습니다. 다른 설명으로는 신경세포에서 분비된 α-시누클레인이 희돌기아교세포에 의해 흡수되어 형성된다는 설명이 있습니다. 연구에 따르면 공동 배양과 외인성 첨가를 통해 신경세포 α-시누클레인이 시험관 내 및 생체 내에서 올리고덴드로세포로 이동하는 것으로 나타났습니다33,34,35. MSA에서 희소돌기아교세포는 신경세포보다 뉴런 유래 α-시누클레인이 축적되기 쉬운데, 이는 아마도 제거 메커니즘이 뉴런에서만큼 효율적이지 않을 수 있기 때문일 수 있습니다. 이러한 설명을 뒷받침하기 위해 펭 등은 세포 환경이 LBD 및 MSA27와 같은 다양한 시누클레인 병증을 결정한다고 제안했습니다.

세포 분해 경로를 통해 단백질을 폐기하지 못하면 독성 응집체가 생성되어 GCI에 통합되어 희돌기아교세포 기능 장애를 일으킬 수 있습니다. 미엘린 변성이 발생하면 희돌기아교세포가 비대해지고 핵이 창백해집니다. α-시누클레인 응집체가 다른 인접 세포로 전파되면 미세아교세포에 의한 염증 반응과 신경 퇴행이 일어날 수 있습니다(그림 2).

Fig. 2: The transmission model of α-synuclein in the brain.

Neuron-derived α-synuclein aggregates in the extracellular space are taken up by neighboring glial cells. Microglia and astrocytes, which secrete inflammatory molecules, are activated. Oligodendrocytes undergo demyelination, exposing neuronal axons that may retract or be degenerated by the hostile external environment, causing neurodegeneration.

세포 외 공간에 있는 뉴런 유래 α-시누클레인 응집체는 

인접한 신경아교세포에 의해 흡수됩니다. 

염증 분자를 분비하는 

미세아교세포와 성상교세포가 활성화됩니다. 

희돌기아교세포는 

탈수초화를 겪으며 

적대적인 외부 환경에 의해 수축하거나 퇴화될 수 있는 

신경 축삭을 노출시켜 신경 퇴화를 일으킵니다.

Astrogliosis and microgliosis in MSA

The activation of astrocytes and microglia has been observed in the brains of MSA patients, as well as in those of transgenic models of MSA11,36,37. Studies have revealed the potential role of the neuron-to-glia transmission of α-synuclein in glial activation in both cell and animal models. Extracellular α-synuclein leads to inflammatory responses in astrocytes and microglia38,39,40. Activated Iba-1-positive microglia and GFAP-positive astrocytes have been shown to localize in the proximity of GCIs41,42. Astrogliosis is an important pathological characteristic of MSA. Treating astrocytes with extracellular α-synuclein induces ERK/MAPKK-dependent astrogliosis42. Activated astrocytes can secrete cytokines, which may trigger microgliosis. Therefore, the proinflammatory function of extracellular α-synuclein in astrocytes may have a crucial role in spreading MSA neuropathology41,43.

Microglia are the primary immunophagocytic cells in the brain. An increased number of activated microglia is found in α-synucleinopathies44. The injection of GCI extract into the mouse brain causes localized microgliosis, as well as astrogliosis42. Toll-like receptors (TLRs), such as TLR2 and TLR4, have been shown to interact with extracellular α-synuclein in microglia39,45. Microglia activated by extracellular α-synuclein then secrete toxic factors that can trigger further neurodegeneration and gliosis46.

MSA의 성상교세포증 및 미세아교세포증

성상교세포와 미세아교세포의 활성화는

MSA 환자의 뇌와 형질전환 모델의 뇌에서 관찰되었습니다11,36,37.

연구에 따르면 세포 및 동물 모델 모두에서 신경교세포 활성화에서 α-시누클레인의 신경세포 간 전달의 잠재적 역할이 밝혀졌습니다. 세포 외 α-시누클레인은 성상교세포와 미세아교세포에서 염증 반응을 일으킵니다38,39,40. 활성화된 Iba-1 양성 미세아교세포와 GFAP 양성 성상교세포는 GCI41,42에 근접하여 국한되는 것으로 나타났습니다. 성상교세포증은 MSA의 중요한 병리학적 특징입니다. 성상교세포를 세포 외 α-시누클레인으로 처리하면 ERK/MAPKK 의존성 성상교아증이 유도됩니다42. 활성화된 성상교세포는 사이토카인을 분비하여 미세아교세포증을 유발할 수 있습니다. 따라서 성상교세포에서 세포 외 α-시누클레인의 염증성 기능은 MSA 신경 병리를 확산시키는 데 중요한 역할을 할 수 있습니다41,43.

미세아교세포는 뇌의 주요 면역 포식 세포입니다. α-시누클레인 병증에서 활성화된 미세아교세포의 수가 증가합니다44. 쥐의 뇌에 GCI 추출물을 주입하면 국소 미세아교세포증과 성상교세포증이 유발됩니다42. TLR2 및 TLR4와 같은 톨 유사 수용체(TLR)는 미세아교세포에서 세포 외 α-시누클레인과 상호 작용하는 것으로 나타났습니다39,45. 세포 외 α-시누클레인에 의해 활성화된 미세아교세포는 독성 인자를 분비하여 추가적인 신경 퇴화 및 신경교세포 증식을 유발할 수 있습니다46.

Models of multiple system atrophy

Animal toxin models

In vitro and in vivo models have been developed to obtain a better understanding of MSA pathophysiology. The systemic administration and local stereotaxic injection of toxins induced lesions in specific anatomical areas of models to reproduce MSA symptoms, particularly L-DOPA-unresponsive parkinsonism47. The stereotaxic injection of two toxins, namely, 6-hydroxydopamine (6-OHDA) and quinolinic acid (QA), into different regions of the rat brain in sequence had distinct pathological and behavioral outcomes48. 6-OHDA caused striatal dopamine depletion and decreased the number of dopaminergic cells in the substantia nigra pars compacta (SNpc)49. The injection of QA induced the loss of spiny striatal neurons. Behavioral deficits induced by both single and combined lesions also imitated the symptoms of MSA-P. However, there was no significant difference in performance on the rotarod test or drug-induced rotation test50. The inoculation of brain regions with a single toxin also created lesions. The injection of 3-nitropropionic acid (3-NP), a succinate dehydrogenase inhibitor, or 1-methyl-4-phenylpyridinium ion (MPP+), a mitochondrial complex I inhibitor, into the striatum induced combined degeneration of nigral and striatal neurons51,52. The simultaneous or sequential systemic administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 3-NP caused motor symptoms and dyskinesia in mice and nonhuman primates that responded poorly to L-DOPA treatment53,54,55,56.

These approaches partially mimicked MSA neuropathology in the nigrostriatal system, which arises in the early to late period of the disease. The lesions only resulted in symptoms within the area of administration and failed to spread outside the basal ganglia47. In addition, toxin-based approaches did not induce GCI pathology, which is one of the essential hallmarks of MSA47.

In vitro genetic models

In vivo toxin models cannot precisely depict disease advancement or the mechanism. MSA pathophysiology has been addressed at the cellular level through modified gene expression‎. Because one of the main components of GCIs in MSA is α-synuclein, many studies have used in vitro expression‎ of α-synuclein to investigate the disease mechanism32.

U-373 MG cell line and primary mixed rat glial cultures

A study conducted by Stefanova et al. showed that the overexpression‎ of α-synuclein induced cell death in a U-373 MG human glioblastoma astrocytoma cell line and primary oligodendrocytes from mixed rat glial cultures32,57. Glial cells expressing high levels of α-synuclein were highly prone to oxidative stress. Upon treatment with TNFα, a proinflammatory cytokine released by microglia in MSA, significant cytotoxic changes were observed in α-synuclein-expressing cells. This suggested that a toxic environment, along with high levels of α-synuclein in glia, might represent a severe risk for the development of MSA.

OLN-93 cell line

OLN-93 cells, primary oligodendrocytic cells derived from Wistar rat brain cultures, have been used to study MSA pathology32,58. Kragh and colleagues co-expressed α-synuclein and p25α in the OLN-93 cell line. The co-expression‎ resulted in the enhanced expression‎ of IkBα, which sequestered the NF-κB transcription factor p65 in the cytoplasm. The inhibition of NF-κB signaling impeded its cytoprotective effects while causing the retraction of microtubules and triggering the activation of the apoptotic protein caspase-358,59. The phosphorylation of Ser129 in α-synuclein protein was necessary for the process. Human brain tissues from MSA patients also exhibit increased expression‎ of IkBα and NFκB p65 in some oligodendrocytes with GCIs.

CG4 cell line

The central glia 4 (CG4) cell line, a rodent oligodendroglial cell line, has been used to stably express α-synuclein. The overexpression‎ of the protein impaired the maturation of cells into oligodendrocytes. BDNF partially rescued this impairment32,60. α-Synuclein delayed the maturation of oligodendroglia by inhibiting the expression‎ of myelin basic protein (MBP) and myelin gene regulatory factor while increasing the levels of the oligodendroglial differentiation repressor Hes5. The lentiviral transduction of α-synuclein in CG4 cells impaired autophagy and this impairment was reversed by the inhibition of miR-101, a microRNA that regulates autophagy genes such as RAB5A, MTOR, ATG4D, and STMN161.

iPSCs from human MSA patients

The advancement of stem cell technologies has allowed the culturing of patient-derived cells and their differentiation into various cell types, including oligodendrocytes. The expression‎ of α-synuclein was examined by real-time PCR and immunocytochemistry. Oligodendrocyte precursor cells (OPCs) expressed α-synuclein, but the expression‎ levels were decreased as the cells differentiated into mature oligodendrocytes. There was no difference between patient-derived and healthy control iPSCs31.

In vivo genetic models

The transgenic expression‎ of the α-synuclein gene under oligodendrocyte-specific promoters has been used to create mouse models of MSA. Through the use of proteolipid protein (PLP), MBP, and the cyclic nucleotide 3’-phosphodiesterase (CNP) promoters, α-synuclein was exclusively expressed in oligodendrocytes62,63,64. However, transgenic mouse lines overexpressing α-synuclein displayed different pathological features of MSA under different oligodendroglial promoters. In these models, motor and nonmotor symptoms developed to varying degrees. Furthermore, these models exhibited accumulation of α-synuclein and its aggregates in oligodendrocytes similar to GCIs in MSA patients.

In addition, adeno-associated viruses (AAVs) were genetically modified to express α-synuclein, specifically in oligodendrocytes. Using these recombinant viruses, more diverse animal models, such as nonhuman primates, can be established65,66. However, the various symptoms described in each of these models and the pattern and degree of degeneration in the nervous system did not precisely correlate with disease pathology47,67.

PLP-hαSyn transgenic mice

PLP promoter-driven α-synuclein (PLP-hαSyn) transgenic mice bred on a C57/BL6 background exhibited phosphorylation of α-synuclein at Ser129 and aggregation of α-synuclein along with GCI-like inclusions62. Mitochondrial inhibition by 3-NP in these mice induced the degeneration of the striatonigral system and gliosis68. The loss of dopaminergic neurons in the SNpc, Purkinje cells, and neurons in the pons and medulla oblongata, as in the human MSA-C subtype, was also observed. Treatment with 3-NP augmented motor deficits in tg mice compared to non-tg controls at ~12 months. Autonomic symptoms, however, appeared earlier, as early as 2 months of age69. Urinary bladder dysfunction with morphological changes in the bladder wall and increased postvoid residual volumes was detected at 2 months of age70. Heart rate variability, an indication of changed sympathovagal balance, as observed in the human disease, was reduced at 5 months of age71. At the age of 13 months, respiratory failure was observed72. Progressive motor deficits emerged at 6 months and progressed until 18 months of follow-up. The loss of dopaminergic neurons in the SNpc was found during the initial stages. The loss of striatal dopaminergic terminals and DARPP32-positive projection neurons was observed at 12 months73. Gliosis and increased levels of cytokines were also reported.

MBP-hαSyn transgenic mice

Mice with MBP promoter-driven α-synuclein expression‎ in oligodendrocytes exhibited somewhat different pathological features of MSA that those exhibited by PLP-hαsyn and CNP-hαsyn tg mice63. These mice displayed demyelination along with axonal degeneration in the cerebellum, basal ganglia, brain stem, corpus callosum, and neocortex. The high expressor line (line 29) died prematurely at 6 months. The moderate expressor line (line 1) exhibited a mild phenotype with disease onset after 6 months of age. α-Synuclein accumulation was also evident, along with elevated astrogliosis. 3-NP administration increased nitrated and oxidized α-synuclein levels but not the levels of the phosphorylated or total protein74. Neurological deficits were augmented and accompanied by widespread neurodegeneration and behavioral problems. Increases in neuroinflammation were detected in regions of high α-synuclein expression‎, such as the corpus callosum and the striatum, even before symptoms were evident75. Inflammatory responses were restricted to myeloid cells, and severe astrogliosis was only detected in gray matter regions. RNA sequencing of α-synuclein-expressing primary oligodendrocytes demonstrated upregulation of cytokines and genes important for inflammatory responses, suggesting that neuroinflammation may be critical for disease development. MBP-hαsyn tg mice displayed GDNF deficiency, IκBα and miR-96 upregulation, and a delay in oligodendrocyte maturation59,69,76,77.

CNP-hαSyn transgenic mice

CNP promoter-hαSyn mice exhibited the accumulation of fibrillary human α-synuclein in oligodendrocytes, a loss and demyelination of these cells, and severe gliosis in the brain and spinal cord64,69. The aggregation of endogenous mouse α-synuclein, which is associated with neuronal loss, was also detected in the axons and axon terminals of spinal cord neurons. Motor deficits started at the age of 7 to 9 months, but the phenotypes seem to be different from typical MSA symptoms78,79,80.

In general, the accumulation of α-synuclein in oligodendroglia has been found in all three oligodendrocyte-specific promoter-driven human α-synuclein tg mice. However, each of these mice displayed distinct patterns of pathological changes that do not precisely replicate human MSA pathology47,81.

Viral-mediated oligodendroglial α-synuclein expression‎ models

Bassil and colleagues utilized AAVs to express human α-synuclein under the control of the MBP promoter in adult Sprague-Dawley (SD) rats and nonhuman primates (macaques)65. α-Synuclein expression‎ was observed in ~80% of oligodendrocytes in the injected area of SD rats. The injected rats displayed L-DOPA-unresponsive motor deficits by 6 months of age, while significant dopaminergic cell loss occurred at 3 months of age. Increased amounts of the insoluble and phosphorylated forms of human α-synuclein were detected in the striatum and SNpc of injected rats. The injected macaques showed expression‎ of human α-synuclein in ~60% of oligodendrocytes and ~40% of neurons. It has yet to be determined whether the neuronal α-synuclein came from the direct expression‎ of α-synuclein in neurons or from the oligodendrocyte-to-neuronal transmission of α-synuclein. Longitudinal studies are also required to determine if viral-mediated oligodendroglial α-synuclein expression‎ is useful as an MSA model.

A study by Mandel et al. utilized the oligotrophic AAV vector Olig001, which specifically transduces oligodendrocytes, to express human α-synuclein in nonhuman primates66. The injection of the recombinant vector into the striatum of rhesus macaques induced widespread expression‎ of α-synuclein in more than 90% of oligodendrocytes throughout the striatum by 3 months of age. Phosphorylated α-synuclein was detected, and the GCI-like inclusions in oligodendrocytes were resistant to proteinase K (PK) digestion. Demyelination in the white matter tracts of the corpus callosum and microgliosis in the striatum was also detected.

The viral expression‎ of α-synuclein in animals has advantages over genetic models in that expression‎ can be manipulated temporally. The recombinant virus can be injected at an early stage or later in development. The cell specificity of viral infection has significantly improved over the years. However, more studies of such models in rodents and nonhuman primates are still needed for the development of MSA models.

Transmission models

Numerous studies have reported little to no α-synuclein expression‎ in mature oligodendrocytes in vitro and in vivo11. The mechanisms of the accumulation of α-synuclein aggregates in the oligodendrocytes and astrocytes of MSA patients are still largely unknown. However, recent reports of the transfer of α-synuclein from one cell to another, particularly from neurons to glia, have led to the hypothesis that toxic α-synuclein aggregates secreted from neurons are taken up by oligodendrocytes and disrupts their function35,38. Based on this hypothesis, several groups have developed transmission models.

In vitro culture models of exogenously added α-synuclein aggregates

Pukas et al. utilized primary rat brain oligodendrocytes and an oligodendroglial cell line, OLN-93, and added soluble or preaggregated forms of human recombinant α-synuclein82. Both forms of exogenously added α-synuclein were internalized and formed small intracellular aggregates in oligodendroglial cells. Under oxidative stress, the levels of aggregates increased, and cell viability was reduced. Human α-synuclein preformed fibrils (PFFs) triggered the expression‎ and aggregation of endogenous α-synuclein in rat primary OPCs83. However, when using PFFs, one should consider that the PFFs may differ from neuron-derived α-synuclein aggregates in structure and composition. In addition, there may be essential cofactors secreted from surrounding cells that influence the transmission of the protein.

In vitro co-culture models of α-synuclein-expressing donor cells and naïve recipient cells have been established. Neuron-to-neuron, neuron-to-astrocyte, and neuron-to-microglia transmission of α-synuclein have recently been reported38,39,84. The transfer of α-synuclein to recipient cells caused various cellular changes, including cell toxicity and inflammatory responses. The advantage of using such models is that it is easy to manipulate the expression‎ of specific genes and to detect cellular changes by biochemical methods

The use of co-cultures of neurons and oligodendrocytes was reported recently. Mouse embryonic stem cell (ESC)-derived neural progenitor cells (NPCs) differentiated into OPCs and then into mature oligodendrocytes. These cells were co-cultured with differentiated cortical neurons from mouse ESCs on the same plate. However, unlike in other studies in which neurons overexpressed α-synuclein and were co-cultured with other naïve recipient cells, this study utilized direct lentiviral overexpression‎ of α-synuclein in oligodendrocytes, which induced myelination deficits upon co-culture with naïve neurons85.

Neuron-to-oligodendrocyte transmission and its effects remain to be studied.

Stereotaxic injection of recombinant proteins or MSA brain extracts into animal models

Brain extracts from 14 MSA patients were inoculated into the brains of TgM83+/− hemizygous mice expressing the A53T α-synuclein mutant under the prion promoter86. All mice showed neurodegeneration and accumulation of α-synuclein aggregates in neuronal cell bodies and axons at 4 months postinjection, whereas control injections did not induce neurodegeneration at the same age. Interestingly, PD brain extracts did not promote the aggregation of α-synuclein in TgM83 mice, indicating that pathogenic α-synuclein species may differ in MSA and PD. The same group performed comparison experiments with different transgenic animal models expressing wild-type, A30P, and A53T α-synuclein on a mouse α-synuclein knockout background87.

In contrast to the results obtained in TgM83+/− mice, these mice, when injected with MSA brain extracts, did not show any motor deficits, even after 330–400 days. Only A53T-expressing (Tg(SNCA*A53T+/+)) mice, but not others, displayed α-synuclein pathology in neurons and astrocytes in the limbic system. Interestingly, these injected mice retained infectivity after the initial inoculation of human MSA patient samples. Mouse brain extracts from MSA patient extract-injected tg mice were inoculated for the second time into both TgM83+/− and Tg(SNCA*A53T+/+) mice. These second-round synucleinopathy models also developed pathology, and the transmission of α-synuclein was confirmed.

These injection models support the α-synuclein transmission hypothesis, as pathological α-synuclein from diseased patients was transferred to nonsymptomatic mice and induced pathology.

Conclusion

MSA is a neurodegenerative disease with clinical symptoms similar to those of PD and cerebellar ataxia. The complexity and rapid progression of the disease, as well as its unresponsiveness to drugs, such as L-DOPA for parkinsonian symptoms, make MSA a challenging disease to treat. Various models have been generated over the years to study the mechanism of MSA development and progression. In this review, we summarized toxin-induced models, in vitro and in vivo genetic models, and transmission models for studying α-synuclein pathology and behavioral symptoms in MSA (Fig. 3).

결론

MSA는

파킨슨병 및

소뇌 운동 실조증과 유사한 임상 증상을 보이는

신경 퇴행성 질환입니다.

질병의 복잡성과 빠른 진행,

그리고 파킨슨 증상에 대한 L-DOPA와 같은 약물에 반응하지 않는 특성으로 인해

MSA는 치료하기 어려운 질환입니다.

수년 동안 MSA의 발병과 진행 메커니즘을 연구하기 위해 다양한 모델이 만들어졌습니다. 이 리뷰에서는 MSA의 α-시누클레인 병리 및 행동 증상을 연구하기 위한 독소 유발 모델, 체외 및 생체 내 유전자 모델, 전달 모델에 대해 요약했습니다(그림 3).

Fig. 3: Summary of MSA models.

In vitro cell models have been used to express α-synuclein in oligodendrocytes. The effects of the addition of exogenous α-synuclein aggregates to oligodendrocytes have been studied in in vitro transmission models. Recent developments in stem cell technologies have allowed the growth of MSA-derived iPSCs and their differentiation into oligodendrocytes. Comparison studies of RNA, protein, and epigenetic changes in normal and patient-derived cells are ongoing. Animal models induced by the injection of toxins exhibit MSA-like symptoms, and oligodendrocyte-specific expression‎ of α-synuclein has been achieved through the generation of tg mice and viral-mediated expression‎.

Full size image

The main goal of the genetic models is to generate oligodendrocytes with GCI-like α-synuclein pathology, thereby allowing us to study the roles of α-synuclein in MSA pathogenesis. Two methods have been applied to generate models with oligodendrocytic α-synuclein accumulation. The first method was to ectopically express α-synuclein in oligodendrocytes. α-Synuclein was overexpressed in oligodendroglial cells in culture. Transgenic animal models expressing α-synuclein specifically in oligodendrocytes were created. These direct expression‎ models displayed glial α-synuclein pathology and various CNS phenotypes akin to those of MSA. However, because mature oligodendrocytes have little to no α-synuclein expression‎, it is not clear whether these ectopic expression‎ models are valid representatives of human MSA.

More recent efforts to develop MSA models have utilized the fact that α-synuclein can be transferred between cells. The exogenous addition of recombinant α-synuclein aggregates to oligodendrocytes resulted in the uptake and induction of endogenous protein aggregate formation. The stereotaxic injection of human MSA brain extracts into the brains of α-synuclein tg mice induced extensive spreading of α-synuclein pathology, neuroinflammation, and neurodegeneration. However, these injected mouse models displayed neuronal α-synuclein pathology instead of oligodendroglial pathology.

Although researchers have been able to recapitulate some aspects of MSA with current models, none of these models present bona fide MSA pathology, which is represented by oligodendroglial GCIs generated from endogenous α-synuclein proteins. Recent studies have suggested that GCIs are generated through the transfer of α-synuclein from neurons to oligodendrocytes. To test this hypothesis, we will have to understand more about the biology of oligodendrocytes in MSA. Critical questions include how oligodendrocytes respond to and process neuron-derived α-synuclein and how normal oligodendrocytes and MSA oligodendrocytes respond differently to α-synuclein. We anticipate that the technical advancement of single-cell analysis and bioinformatics will contribute to answering these questions.

References

1.