Mitochondrial integrity in neurodegeneration

The mitochondrion is a unique organelle with a diverse range of functions. Mitochondrial dysfunction is a key pathological process in several neurodegenerative diseases. Mitochondria are mostly important for energy production; however, they also have roles in Ca²⁺ homeostasis, ROS production, and apoptosis. There are two major systems in place, which regulate mitochondrial integrity, mitochondrial dynamics, and mitophagy. These two processes remove damaged mitochondria from cells and protect the functional mitochondrial population. These quality control systems often become dysfunctional during neurodegenerative diseases, such as Parkinson's and Alzheimer's disease, causing mitochondrial dysfunction and severe neurological symptoms.     

Mitochondrial dysfunction in neurodegenerative diseases and the potential countermeasure

Mitochondria not only supply the energy for cell function, but also take part in cell signaling. This review describes the dysfunctions of mitochondria in aging and neurodegenerative diseases, and the signaling pathways leading to mitochondrial biogenesis (including PGC‐1 family proteins, SIRT1, AMPK) and mitophagy (parkin‐Pink1 pathway). Understanding the regulation of these mitochondrial pathways may be beneficial in finding pharmacological approaches or lifestyle changes (caloric restrict or exercise) to modulate mitochondrial biogenesis and/or to activate mitophagy for the removal of damaged mitochondria, thus reducing the onset and/or severity of neurodegenerative diseases.     

线粒体自噬在蛛网膜下腔出血中的研究进展

线粒体自噬为选择性自噬的一种,在生理状态下、各种急性应激及慢性疾病中,主要参与了线粒体稳态的调节、受损线粒体的清除及避免细胞死亡等多种细胞内生理性反应过程.在多种神经疾病过程中可见到线粒体自噬的参与,但关于线粒体自噬在蛛网膜下腔出血中的研究较少,本文从线粒体自噬的经典通路、在蛛网膜下腔出血中的作用机制及对预后的影响等方...     

Mitophagy links oxidative stress conditions and neurodegenerative diseases

Mitophagy is activated by a number of stimuli, including hypoxia, energy stress, and increased oxidative phosphorylation activity. Mitophagy is associated with oxidative stress conditions and central neurodegenerative diseases. Proper regulation of mitophagy is crucial for maintaining homeostasis; conversely, inadequate removal of mitochondria through mitophagy leads to the generation of oxidative species, including reactive oxygen species and reactive nitrogen species, resulting in various neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. These diseases are most prevalent in older adults whose bodies fail to maintain proper mitophagic functions to combat oxidative species. As mitophagy is essential for normal body function, by targeting mitophagic pathways we can improve these disease conditions. The search for effective remedies to treat these disease conditions is an ongoing process, which is why more studies are needed. Additionally, more relevant studies could help establish therapeutic conditions, which are currently in high demand. In this review, we discuss how mitophagy plays a significant role in homeostasis and how its dysregulation causes neurodegeneration. We also discuss how combating oxidative species and targeting mitophagy can help treat these neurodegenerative diseases.     

Mitochondrial autophagy in neural function, neurodegenerative disease, neuron cell death, and aging

Macroautophagy is a cellular process by which cytosolic components and organelles are degraded in double-membrane bound structures upon fusion with lysosomes. A pathway for selective degradation of mitochondria by autophagy, known as mitophagy, has been described, and is of particular importance to neurons, because of the constant need for high levels of energy production in this cell type. Although much remains to be learned about mitophagy, it appears that the regulation of mitophagy shares key steps with the macroautophagy pathway, while exhibiting distinct regulatory steps specific for mitochondrial autophagic turnover. Mitophagy is emerging as an important pathway in neurodegenerative disease, and has been linked to the pathogenesis of Parkinson's disease through the study of recessively inherited forms of this disorder, involving PINK1 and Parkin. Recent work indicates that PINK1 and Parkin together maintain mitochondrial quality control by regulating mitophagy. In the Purkinje cell degeneration (pcd) mouse, altered mitophagy may contribute to the dramatic neuron cell death observed in the cerebellum, suggesting that over-active mitophagy or insufficient mitophagy can both be deleterious. Finally, mitophagy has been linked to aging, as impaired macroautophagy over time promotes mitochondrial dysfunction associated with the aging process. Understanding the role of mitophagy in neural function, neurodegenerative disease, and aging represents an essential goal for future research in the autophagy field. This article is part of a Special Issue entitled "Autophagy and protein degradation in neurological diseases."     

Regulation of mitophagy in ischemic brain injury

Neuroscience Bulletin - The selective degradation of damaged or excessive mitochondria by autophagy is termed mitophagy. Mitophagy is crucial for mitochondrial quality control and has been...     

线粒体自噬和帕金森病

帕金森病(PD)是中老年人常见的中枢神经系统退行性疾病,目前认为此病与多个因素有关。近年的研究表明线粒体自噬是PD发病机制之一,本文就线粒体自噬及其与PD的关联作一综述。     

Deregulated mitochondrial quality control,the heel of Achilles in elucidating the role of autophagy in SARM1-mediated axon degeneration

Autophagic dysfunction in neurodegenerative diseases is being extensively studied, yet the exact mechanism of macroautophagy/autophagy in axon degeneration is still elusive. A recent study by Kim et al. links autophagic stress to the sterile α and toll/interleukin 1 receptor motif containing protein 1 (SARM1)-dependent core axonal degeneration program, providing a new insight into the role of autophagy in axon degeneration. In the classical Wallerian axon degeneration model of axotomy, disruption of axonal transport destroys the coordinated activity of pro-survival and pro-degenerative factors in the axoplasm and activates the NADase activity of SARM1, thus triggering the axonal self-destruction program. However, the mechanism for SARM1 activation in the chronic neurodegenerative disorders is more complex. Mitochondrial defects and oxidative stress contribute to the activation of SARM1, while mitophagy can inhibit mitochondrial dysfunction and promote the clearance of SARM1 on mitochondria, thus protecting against neuronal degeneration. Therefore, in-depth elucidation of the underlying mechanisms of mitophagy during axonal degeneration can help develop promising strategies for the prevention and treatment of various neurodegenerative disorders.     

Perspectives of drug-based neuroprotection targeting mitochondria

Mitochondrial dysfunction has been reported in most neurodegenerative diseases. These anomalies include bioenergetic defect, respiratory chain-induced oxidative stress, defects of mitochondrial dynamics, increase sensitivity to apoptosis, and accumulation of damaged mitochondria with instable mitochondrial DNA. Significant progress has been made in our understanding of the pathophysiology of inherited mitochondrial disorders but most have no effective therapies. The development of new metabolic treatments will be useful not only for rare mitochondrial disorders but also for the wide spectrum of common age-related neurodegenerative diseases shown to be associated with mitochondrial dysfunction. A better understanding of the mitochondrial regulating pathways raised several promising perspectives of neuroprotection. This review focuses on the pharmacological approaches to modulate mitochondrial biogenesis, the removal of damaged mitochondria through mitophagy, scavenging free radicals and also dietary measures such as ketogenic diet.     

Precision Neurology for Parkinson’s Disease: Coupling Miro1-Based Diagnosis With Drug Discovery

Parkinson’s disease (PD) is a debilitating movement disorder, significantly afflicting the aging population. Efforts to develop an effective treatment have been challenged by the lack of understanding of the pathological mechanisms underlying neurodegeneration. We have shown that Miro1, an outer mitochondrial membrane protein, situates at the intersection of the complex genetic and functional network of PD. Removing Miro1 from the surface of damaged mitochondria is a prerequisite for mitochondrial clearance via mitophagy. Parkinson’s proteins PINK1, Parkin, and LRRK2 are the molecular helpers to remove Miro1 from dysfunctional mitochondria destined for mitophagy. We have found a delay in clearing Miro1 and initiating mitophagy in postmortem brains and induced pluripotent stem cell–derived neurons from PD patients harboring mutations in LRRK2, PINK1, or Parkin, or from sporadic PD patients with no known mutations. In addition, we have shown that reducing Miro1 by both genetic and pharmacological approaches can correct this Miro1 phenotype and rescue Parkinson’s-relevant phenotypes in human neurons and fly PD models. These results suggest that the Miro1 defect may be a common denominator for PD, and compounds that reduce Miro1 promise a new class of drugs to battle PD. We propose to couple this Miro1 phenotype with Miro1-based drug discovery in future therapeutic studies, which could significantly improve the success of clinical trials. © 2020 International Parkinson and Movement Disorder Society     

重症肌无力外周血调节性T细胞线粒体自噬异常的研究

目的探讨线粒体自噬对重症肌无力(myasthenia Gravis,MG)患者外周血调节性T细胞(CD4~+CD25~+regulatory T cell,Treg)功能的影响及关系。方法选择首次确诊、尚未接受治疗的15例MG患者,以15名健康人作为对照,分别采集外周血,以磁珠分选法获得Treg细胞,经流式细胞仪鉴定纯度后,分别通过透射电镜检测Treg细胞线粒体自噬情况、Western Blot法检测微管相关蛋白轻链3-Ⅱ(microtubule associated protein light chain 3-Ⅱ,LC3-Ⅱ)的表达水平、JC-10荧光探针检测线粒体膜电位(以红色荧光细胞数/绿色荧光细胞数比值表示)变化、羧基荧光素琥珀酰亚胺酯(CFDA-SE,CFSE)检测Treg对正常CD4~+T细胞增殖抑制能力(荧光强度越强,增殖抑制能力越强)。结果与对照组比较,MG组电镜下线粒体自噬明显减少[(25.60±7.81)个、(19.20±5.49)个,t=-2.596,P0.05],且变形线粒体较多;MG组LC3-Ⅱ表达明显减少(0.450±0.137、0.334±0.124,t=-2.413,P0.05);MG组Treg细胞线粒体膜电位下降(2.153±0.537,1.726±0.494,t=2.126,P0.05);MG组CFSE检测平均荧光强度减弱(34.82±10.64、26.49±5.94,t=2.646,P0.05)。结论 MG患者有Treg细胞的线粒体自噬功能降低,这可能与其增殖抑制功能降低相关。     

线粒体自噬在缺血性卒中的机制研究与展望

线粒体自噬是细胞通过自噬清除破坏的线粒体的过程,是线粒体质量控制的重要机制。 近年来研究表明,缺血性卒中后存在明显的线粒体自噬。因此在卒中过程当中,调节控制线粒体数量 和质量的线粒体自噬过程,可能对治疗卒中、延缓细胞死亡、保护神经元有十分关键的意义。本文主 要对线粒体自噬在缺血性卒中病理过程中的作用与机制,以及其目前的研究进展进行综述,并对线 粒体自噬对缺血性卒中的治疗作用进行展望。     

The dynamics of the mitochondrial organelle as a potential therapeutic target

Mitochondria play a central role in cell fate after stressors such as ischemic brain injury. The convergence of intracellular signaling pathways on mitochondria and their release of critical factors are now recognized as a default conduit to cell death or survival. Besides the individual processes that converge on or emanate from mitochondria, a mitochondrial organellar response to changes in the cellular environment has recently been described. Whereas mitochondria have previously been perceived as a major center for cellular signaling, one can postulate that the organelle''s dynamics themselves affect cell survival. This brief perspective review puts forward the concept that disruptions in mitochondrial dynamics—biogenesis, clearance, and fission/fusion events—may underlie neural diseases and thus could be targeted as neuroprotective strategies in the context of ischemic injury. To do so, we present a general overview of the current understanding of mitochondrial dynamics and regulation. We then review emerging studies that correlate mitochondrial biogenesis, mitophagy, and fission/fusion events with neurologic disease and recovery. An overview of the system as it is currently understood is presented, and current assessment strategies and their limitations are discussed.     

Role of mitophagy in hereditary Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease. The majority of PD cases are sporadic; however, the discovery of genes linked to rare familial forms of the disease has provided crucial insight into the molecular mechanisms of disease pathogenesis. PINK1 and PARKIN are causal genes for hereditary (i. e., autosomal recessive) early-onset PD. In 2010, intense efforts by our laboratory and several other groups have revealed the mechanism by which PINK1 and Parkin maintain mitochondrial integrity. The essence of the model is that PINK1 is rapidly and constitutively degraded under steady-state conditions in a mitochondrial membrane potential (ΔΨm)-dependent manner and that a loss in ΔΨm stabilizes PINK1 on damaged mitochondria, and then recruits Parkin from the cytosol to the mitochondria for proteasomal and autophagic degradation. Recently, a pharmacological approach using various chemical reagents such as valinomycin, nigericin, 2-deoxy-D-glucose, and oligomycin demonstrated that Parkin recruitment is voltage-dependent and independent of changes in ATP or pH. Moreover, F1-ATPase inhibitor azide recruited Parkin to the mitochondria only in ρ⁰ cells, which lack mtDNA and a functional electron transport chain. These results confirm that ΔΨm is the most important factor for the discrimination of damaged mitochondria from their healthy counterparts. Here we provide an overview of how PINK1 and Parkin identify, label and clear damaged/depolarized mitochondria, focusing on the role of mitochondrial autophagy (mitophagy).     

Mitochondrial quality control in amyotrophic lateral sclerosis:towards a common pathway?

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disorder characterized by loss of upper and lower motor neurons. Different mechanisms contribute to the disease initiation and progression, including mitochondrial dysfunction which has been proposed to be a central determinant in ALS patho-genesis. Indeed, while mitochondrial defects have been mainly described in ALS-linked SOD1 mutants, it is now well established that mitochondria become also dysfunctional in other ALS conditions. In such con-text, the mitochondrial quality control system allows to restore normal functioning of mitochondria and to prevent cell death, by both eliminating and replacing damaged mitochondrial components or by degrading the entire organelle through mitophagy. Recent evidence shows that ALS-related genes interfere with the mitochondrial quality control system. This review highlights how ineffective mitochondrial quality control may render motor neurons defenseless towards the accumulating mitochondrial damage in ALS.     

Reciprocal interaction between mitochondrial fission and mitophagy in postoperative delayed neurocognitive recovery in aged rats

Introduction

Emerging evidence suggests that mitochondrial dysfunction plays a crucial role in the pathogenesis of postoperative delayed neurocognitive recovery (dNCR). Mitochondria exist in a dynamic equilibrium that involves fission and fusion to regulate morphology and maintains normal cell function via the removal of damaged mitochondria through mitophagy. Nonetheless, the relationship between mitochondrial morphology and mitophagy, and how they influence mitochondrial function in the development of postoperative dNCR, remains poorly understood. Here, we observed morphological alterations of mitochondria and mitophagy activity in hippocampal neurons and assessed the involvement of their interaction in dNCR following general anesthesia and surgical stress in aged rats.

Methods

Firstly, we evaluated the spatial learning and memory ability of the aged rats after anesthesia/surgery. Hippocampal mitochondrial function and mitochondrial morphology were detected. Afterwards, mitochondrial fission was inhibited by Mdivi-1 and siDrp1 in vivo and in vitro separately. We then detected mitophagy and mitochondrial function. Finally, we used rapamycin to activate mitophagy and observed mitochondrial morphology and mitochondrial function.

Results

Surgery impaired hippocampal-dependent spatial learning and memory ability and caused mitochondrial dysfunction. It also increased mitochondrial fission and inhibited mitophagy in hippocampal neurons. Mdivi-1 improved mitophagy and learning and memory ability of aged rats by inhibiting mitochondrial fission. Knocking down Drp1 by siDrp1 also improved mitophagy and mitochondrial function. Meanwhile, rapamycin inhibited excessive mitochondrial fission and improved mitochondrial function.

Conclusion

Surgery simultaneously increases mitochondrial fission and inhibits mitophagy activity. Mechanistically, mitochondrial fission/fusion and mitophagy activity interact reciprocally with each other and are both involved in postoperative dNCR. These mitochondrial events after surgical stress may provide novel targets and modalities for therapeutic intervention in postoperative dNCR.     

Review: Autophagy and neurodegeneration: survival at a cost?

S. J. Cherra III, R. K. Dagda and C. T. Chu (2010) Neuropathology and Applied Neurobiology 36, 125–132
Autophagy and neurodegeneration: survival at a cost? Protein aggregation, mitochondrial impairment and oxidative stress are common to multiple neurodegenerative diseases. Homeostasis is regulated by a balanced set of anabolic and catabolic responses, which govern removal and repair of damaged proteins and organelles. Macroautophagy is an evolutionarily conserved pathway for the degradation of long‐lived proteins, effete organelles and protein aggregates. Aberrations in macroautophagy have been observed in Alzheimer, Huntington, Parkinson, motor neuron and prion diseases. In this review, we will discuss the divergent roles of macroautophagy in neurodegenerative diseases and suggest a potential regulatory mechanism that could determine cell death or survival outcomes. We also highlight emerging data on neurite morphology and synaptic remodelling that indicate the possibility of detrimental functional trade‐offs in the face of neuronal cell survival, particularly if the need for elevated macroautophagy is sustained.     

Neuroglial transmitophagy and Parkinson's disease

Mitophagy is essential for the health of dopaminergic neurons because mitochondrial damage is a keystone of Parkinson's disease. The aim of the present work was to study the degradation of mitochondria in the degenerating dopaminergic synapse. Adult Sprague–Dawley rats and YFP-Mito-DAn mice with fluorescent mitochondria in dopaminergic neurons were injected in the lateral ventricles with 6-hydroxydopamine, a toxic that inhibits the mitochondrial chain of dopaminergic neurons and blockades the axonal transport. Dopaminergic terminals closest to the lateral ventricle showed an axonal fragmentation and an accumulation of damaged mitochondria in 2–9 μ saccular structures (spheroids). Damaged mitochondria accumulated in spheroids initiated (showing high Pink1, parkin, ubiquitin, p-S65-Ubi, AMBRA1, and BCL2L13 immunoreactivity and developing autophagosomes) but did not complete (mitochondria were not polyubiquitinated, autophagosomes had no STX17, and no lysosomes were found in spheroids) the mitophagy process. Then, spheroids were penetrated by astrocytic processes and DAergic mitochondria were transferred to astrocytes where they were polyubiquitinated (UbiK63+) and linked to mature autophagosomes (STX17+) which became autophagolysosomes (Lamp1/Lamp2 which co-localized with LC3). Present data provide evidence that the mitophagy of degenerating dopaminergic terminals starts in the dopaminergic spheroids and finishes in the surrounding astrocytes (spheroid-mediated transmitophagy). The neuron-astrocyte transmitophagy could be critical for preventing the release of damaged mitochondria to the extracellular medium and the neuro-inflammatory activity which characterizes Parkinson's disease.     

Mitochondrial failures in Alzheimer's disease

Mitochondrial dysfunction and free radical-induced oxidative damage have been implicated in the pathogenesis of several different neurodegenerative diseases such as Parkinson disease (PD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), and Alzheimer's disease (AD). The defective adenosine triphosphate (ATP) production and increased oxygen radicals may induce mitochondria-dependent cell death because damaged mitochondria are unable to maintain the energy demands of the cell. The role of vascular hypoperfusion-induced mitochondria failure in the pathogenesis of AD now has been widely accepted. However, the exact cellular mechanisms behind vascular lesions and their relation to oxidative stress markers identified by RNA oxidation, lipid peroxidation, or mitochondrial DNA (mtDNA) deletion remain unknown. Future studies comparing the spectrum of mitochondrial damage and the relationship to oxidative stress-induced damage during the aging process or, more importantly, during the maturation of AD pathology are warranted.