Mitochondrial mechanisms of estrogen neuroprotection

Mitochondria have become a primary focus in our search not only for the mechanism(s) of neuronal death but also for neuroprotective drugs and therapies that can delay or prevent Alzheimer's disease and other chronic neurodegenerative conditions. This is because mitochrondria play a central role in regulating viability and death of neurons, and mitochondrial dysfunction has been shown to contribute to neuronal death seen in neurodegenerative diseases. In this article, we review the evidence for the role of mitochondria in cell death and neurodegeneration and provide evidence that estrogens have multiple effects on mitochondria that enhance or preserve mitochondrial function during pathologic circumstances such as excitotoxicity, oxidative stress, and others. As such, estrogens and novel non-hormonal analogs have come to figure prominently in our efforts to protect neurons against both acute brain injury and chronic neurodegeneration.     

线粒体在神经元程序性死亡中的作用新进展

神经元的死亡是许多神经系统疾病如阿尔茨海默病、帕金森病、急性青光眼等发生发展过程中的重要事件,传统认为,细胞死亡有凋亡、自噬、坏死三种方式,凋亡和自噬为程序性的细胞死亡,坏死为非程序性的死亡途径。而近年来的研究发现了一种名为程序性坏死(necroptosis)的可调控的坏死,因此,对这些可调控的细胞死亡的研究对治疗这类神经系统疾病有重要的意义。大量研究发现,在能量代谢和自由基代谢中占据着重要地位的线粒体在细胞死亡过程中也发挥重要作用。本文对线粒体在神经元凋亡、自噬和程序性坏死中的生物学作用的最新进展做一综述。     

Mitochondrial autophagy as a compensatory response to PINK1 deficiency

Macroautophagy (hereafter, autophagy) plays a critical role in maintaining cellular homeostasis by degrading protein aggregates and dysfunctional/damaged organelles. We recently reported that silencing the recessive familial Parkinson disease gene encoding PTEN-induced kinase 1 (PINK1) leads to neuronal cell death accompanied by mitochondrial dysfunction and Drp1-dependent fragmentation. In this model, mitochondrial fission and Beclin 1-dependent autophagy play protective roles, cooperating to sequester and eliminate damaged mitochondria. We discuss the role of superoxide and other reactive oxygen species upstream of mitochondrial depolarization, fission, and autophagy in PINK1 knockdown lines. PINK1 deficiency appears to trigger several compensatory responses that together facilitate clearance of depolarized mitochondria, through a mechanism that is further enhanced by increased expression of parkin. These data offer additional insights that broaden the spectrum of potential interactions between PINK1 and parkin with respect to the regulation of mitochondrial homeostasis and mitophagy.     

Dynamin-related protein 1: A critical protein in the pathogenesis of neural system dysfunctions and neurodegenerative diseases

Mitochondria play a key role in the maintenance of neuronal function by continuously providing energy. Here, we will give a detailed review about the recent developments in regards to dynamin-related protein 1 (Drp1) induced unbalanced mitochondrial dynamics, excessive mitochondrial division, and neuronal injury in neural system dysfunctions and neurodegenerative diseases, including the Drp1 knockout induced mice embryonic death, the dysfunction of the Drp1-dependent mitochondrial division induced neuronal cell apoptosis and impaired neuronal axonal transportation, the abnormal interaction between Drp1 and amyloid β (Aβ) in Alzheimer's disease (AD), the mutant Huntingtin (Htt) in Huntington's disease (HD), and the Drp1-associated pathogenesis of other neurodegenerative diseases such as Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS). Drp1 is required for mitochondrial division determining the size, shape, distribution, and remodeling as well as maintaining of mitochondrial integrity in mammalian cells. In addition, increasing reports indicate that the Drp1 is involved in some cellular events of neuronal cells causing some neural system dysfunctions and neurodegenerative diseases, including impaired mitochondrial dynamics, apoptosis, and several posttranslational modification induced increased mitochondrial divisions. Recent studies also revealed that the Drp1 can interact with Aβ, phosphorylated τ, and mutant Htt affecting the mitochondrial shape, size, distribution, axonal transportation, and energy production in the AD and HD neuronal cells. These changes can affect the health of mitochondria and the function of synapses causing neuronal injury and eventually leading to the dysfunction of memory, cognitive impairment, resting tremor, posture instability, involuntary movements, and progressive muscle atrophy and paralysis in patients.     

The emerging roles of protein homeostasis‐governing pathways in Alzheimer's disease

Pathways governing protein homeostasis are involved in maintaining the structural, quantitative, and functional stability of intracellular proteins and involve the ubiquitin–proteasome system, autophagy, endoplasmic reticulum, and mTOR pathway. Due to the broad physiological implications of protein homeostasis pathways, dysregulation of proteostasis is often involved in the development of multiple pathological conditions, including Alzheimer's disease (AD). Similar to other neurodegenerative diseases that feature pathogenic accumulation of misfolded proteins, Alzheimer's disease is characterized by two pathological hallmarks, amyloid‐β (Aβ) plaques and tau aggregates. Knockout or transgenic overexpression of various proteostatic components in mice results in AD‐like phenotypes. While both Aβ plaques and tau aggregates could in turn enhance the dysfunction of these proteostatic pathways, eventually leading to apoptotic or necrotic neuronal death and pathogenesis of Alzheimer's disease. Therefore, targeting the components of proteostasis pathways may be a promising therapeutic strategy against Alzheimer's disease.     

Bioenergetic adaptation in response to autophagy regulators during rotenone exposure

Parkinson's disease is the second most common neurodegenerative disorder with both mitochondrial dysfunction and insufficient autophagy playing a key role in its pathogenesis. Among the risk factors, exposure to the environmental neurotoxin rotenone increases the probability of developing Parkinson's disease. We previously reported that in differentiated SH‐SY5Y cells, rotenone‐induced cell death is directly related to inhibition of mitochondrial function. How rotenone at nM concentrations inhibits mitochondrial function, and whether it can engage the autophagy pathway necessary to remove damaged proteins and organelles, is unknown. We tested the hypothesis that autophagy plays a protective role against rotenone toxicity in primary neurons. We found that rotenone (10–100 nM) immediately inhibited cellular bioenergetics. Concentrations that decreased mitochondrial function at 2 h, caused cell death at 24 h with an LD50 of 10 nM. Overall, autophagic flux was decreased by 10 nM rotenone at both 2 and 24 h, but surprisingly mitophagy, or autophagy of the mitochondria, was increased at 24 h, suggesting that a mitochondrial‐specific lysosomal degradation pathway may be activated. Up‐regulation of autophagy by rapamycin protected against cell death while inhibition of autophagy by 3‐methyladenine exacerbated cell death. Interestingly, while 3‐methyladenine exacerbated the rotenone‐dependent effects on bioenergetics, rapamycin did not prevent rotenone‐induced mitochondrial dysfunction, but caused reprogramming of mitochondrial substrate usage associated with both complex I and complex II activities. Taken together, these data demonstrate that autophagy can play a protective role in primary neuron survival in response to rotenone; moreover, surviving neurons exhibit bioenergetic adaptations to this metabolic stressor. Exposure to the neurotoxin rotenone is a risk factor for Parkinson's disease. We tested the hypothesis that autophagy is protective against rotenone toxicity in primary neurons. Exposure to nanomolar concentrations of rotenone caused immediate mitochondrial dysfunction, associated with a suppression of macroautophagy. However, mitophagy occurred that was independent of LC3II accumulation, and the surviving neurons exhibited adaptations to their cellular bioenergetic profiles. Cotreatment with the autophagy enhancer rapamycin was protective, whereas further inhibition of autophagy with 3‐methyladenine (3‐MA) exacerbated cell death, resulting in additional bioenergetic adaptations in the surviving neurons.

     

Autophagy, mitochondria and oxidative stress: cross-talk and redox signalling

Reactive oxygen and nitrogen species change cellular responses through diverse mechanisms that are now being defined. At low levels, they are signalling molecules, and at high levels, they damage organelles, particularly the mitochondria. Oxidative damage and the associated mitochondrial dysfunction may result in energy depletion, accumulation of cytotoxic mediators and cell death. Understanding the interface between stress adaptation and cell death then is important for understanding redox biology and disease pathogenesis. Recent studies have found that one major sensor of redox signalling at this switch in cellular responses is autophagy. Autophagic activities are mediated by a complex molecular machinery including more than 30 Atg (AuTophaGy-related) proteins and 50 lysosomal hydrolases. Autophagosomes form membrane structures, sequester damaged, oxidized or dysfunctional intracellular components and organelles, and direct them to the lysosomes for degradation. This autophagic process is the sole known mechanism for mitochondrial turnover. It has been speculated that dysfunction of autophagy may result in abnormal mitochondrial function and oxidative or nitrative stress. Emerging investigations have provided new understanding of how autophagy of mitochondria (also known as mitophagy) is controlled, and the impact of autophagic dysfunction on cellular oxidative stress. The present review highlights recent studies on redox signalling in the regulation of autophagy, in the context of the basic mechanisms of mitophagy. Furthermore, we discuss the impact of autophagy on mitochondrial function and accumulation of reactive species. This is particularly relevant to degenerative diseases in which oxidative stress occurs over time, and dysfunction in both the mitochondrial and autophagic pathways play a role.     

After the banquet

Mitochondria are highly dynamic organelles of crucial importance to the proper functioning of neuronal, cardiac and other cell types dependent upon aerobic efficiency. Mitochondrial dysfunction has been implicated in numerous human conditions, to include cancer, metabolic diseases, neurodegeneration, diabetes, and aging. In recent years, mitochondrial turnover by macroautophagy (mitophagy) has captured the limelight, due in part to discoveries that genes linked to Parkinson disease regulate this quality control process. A rapidly growing literature is clarifying effector mechanisms that underlie the process of mitophagy; however, factors that regulate positive or negative cellular outcomes have been less studied. Here, we review the literature on two major pathways that together may determine cellular adaptation vs. cell death in response to mitochondrial dysfunction. Mitochondrial biogenesis and mitophagy represent two opposing, but coordinated processes that determine mitochondrial content, structure, and function. Recent data indicate that the capacity to undergo mitochondrial biogenesis, which is dysregulated in disease states, may play a key role in determining cell survival following mitophagy-inducing injuries. The current literature on major pathways that regulate mitophagy and mitochondrial biogenesis is summarized, and mechanisms by which the interplay of these two processes may determine cell fate are discussed. We conclude that in primary neurons and other mitochondrially dependent cells, disruptions in any phase of the mitochondrial recycling process can contribute to cellular dysfunction and disease. Given the emerging importance of crosstalk among regulators of mitochondrial function, autophagy, and biogenesis, signaling pathways that coordinate these processes may contribute to therapeutic strategies that target or regulate mitochondrial turnover and regeneration.     

All-you-can-eat: autophagy in neurodegeneration and neuroprotection

Autophagy is the major pathway involved in the degradation of proteins and organelles, cellular remodeling, and survival during nutrient starvation. Autophagosomal dysfunction has been implicated in an increasing number of diseases from cancer to bacterial and viral infections and more recently in neurodegeneration. While a decrease in autophagic activity appears to interfere with protein degradation and possibly organelle turnover, increased autophagy has been shown to facilitate the clearance of aggregation-prone proteins and promote neuronal survival in a number of disease models. On the other hand, too much autophagic activity can be detrimental as well and lead to cell death, suggesting the regulation of autophagy has an important role in cell fate decisions. An increasing number of model systems are now available to study the role of autophagy in the central nervous system and how it might be exploited to treat disease. We will review here the current knowledge of autophagy in the central nervous system and provide an overview of the various models that have been used to study acute and chronic neurodegeneration.     

Amyloid beta-induced glycogen synthase kinase 3β phosphorylated VDAC1 in Alzheimer's disease: Implications for synaptic dysfunction and neuronal damage

Glycogen synthase kinase 3 (GSK3) is a serine/threonine protein kinase that is involved in the multiple signaling processes of a cell. Increasing evidence suggests that GSK3β plays a key role in multiple cellular processes in the progression of diabetes, obesity, Alzheimer's disease (AD), Parkinson's disease (PD), inflammatory diseases, schizophrenia, bipolar and several mood disorders, and mitochondrial diseases. Recent research has found that increased GSK3β activity is linked to the pathogenesis of AD through amyloid beta (Aβ), phosphorylated tau and mitochondrial dysfunction. Recent research has also revealed that GSK3β is elevated in AD-affected tissues and is critically involved in dissociating the voltage-dependent anion channel 1 (VDAC1) protein from hexokinases, and causing disrupted glucose metabolism, mitochondrial dysfunction and activating apoptotic cell death. The purpose of this article is to review recent research that is elucidating the role of GSK3β in AD pathogenesis. We discuss the involvement of GSK3β in the phosphorylation of VDAC1 and dissociation of VADC1 with hexokinases in AD neurons.     

Myricetin slows liquid–liquid phase separation of Tau and activates ATG5-dependent autophagy to suppress Tau toxicity

Intraneuronal neurofibrillary tangles composed of Tau aggregates have been widely accepted as an important pathological hallmark of Alzheimer''s disease. A current therapeutic avenue for treating Alzheimer''s disease is aimed at inhibiting Tau accumulation with small molecules such as natural flavonoids. Liquid–liquid phase separation (LLPS) of Tau can lead to its aggregation, and Tau aggregates can then be degraded by autophagy. However, it is unclear whether natural flavonoids modulate the formation of phase-separated Tau droplets or promote autophagy and Tau clearance. Here, using confocal microscopy and fluorescence recovery after photobleaching assays, we report that a natural antioxidant flavonoid compound myricetin slows LLPS of full-length human Tau, shifting the equilibrium phase boundary to a higher protein concentration. This natural flavonoid also significantly inhibits pathological phosphorylation and abnormal aggregation of Tau in neuronal cells and blocks mitochondrial damage and apoptosis induced by Tau aggregation. Importantly, using coimmunoprecipitation and Western blotting, we show that treatment of cells with myricetin stabilizes the interaction between Tau and autophagy-related protein 5 (ATG5) to promote clearance of phosphorylated Tau to indirectly limit its aggregation. Consistently, this natural flavonoid inhibits mTOR pathway, activates ATG5-dependent Tau autophagy, and almost completely suppresses Tau toxicity in neuronal cells. Collectively, these results demonstrate how LLPS and abnormal aggregation of Tau are inhibited by natural flavonoids, bridging the gap between Tau LLPS and aggregation in neuronal cells, and also establish that myricetin could act as an ATG5-dependent autophagic activator to ameliorate the pathogenesis of Alzheimer''s disease.     

Mechanisms of mitochondria and autophagy crosstalk

Autophagy is a cellular survival pathway that recycles intracellular components to compensate for nutrient depletion and ensures the appropriate degradation of organelles. Mitochondrial number and health are regulated by mitophagy, a process by which excessive or damaged mitochondria are subjected to autophagic degradation. Autophagy is thus a key determinant for mitochondrial health and proper cell function. Mitophagic malfunction has been recently proposed to contribute to progressive neuronal loss in Parkinson disease. In addition to autophagy''s significance in mitochondrial integrity, several lines of evidence suggest that mitochondria can also substantially influence the autophagic process. The mitochondria''s ability to influence and be influenced by autophagy places both elements (mitochondria and autophagy) in a unique position where defects in one or the other system could increase the risk to various metabolic and autophagic related diseases.Key words: autophagy, mitochondria, fission, fusion, apoptosis     

Mitochondrial permeability transition pore in Alzheimer's disease: Cyclophilin D and amyloid beta

Amyloid beta (Aβ) plays a critical role in the pathophysiology of Alzheimer's disease. Increasing evidence indicates mitochondria as an important target of Aβ toxicity; however, the effects of Aβ toxicity on mitochondria have not yet been fully elucidated. Recent biochemical studies in vivo and in vitro implicate mitochondrial permeability transition pore (mPTP) formation involvement in Aβ-mediated mitochondrial dysfunction. mPTP formation results in severe mitochondrial dysfunction such as reactive oxygen species (ROS) generation, mitochondrial membrane potential dissipation, intracellular calcium perturbation, decrease in mitochondrial respiration, release of pro-apoptotic factors and eventually cell death. Cyclophilin D (CypD) is one of the more well-known mPTP components and recent findings reveal that Aβ has significant impact on CypD-mediated mPTP formation. In this review, the role of Aβ in the formation of mPTP and the potential of mPTP inhibition as a therapeutic strategy in AD treatment are examined.     

Mechanisms of mitochondria and autophagy crosstalk

Autophagy is a cellular survival pathway that recycles intracellular components to compensate for nutrient depletion and ensures the appropriate degradation of organelles. Mitochondrial number and health are regulated by mitophagy, a process by which excessive or damaged mitochondria are subjected to autophagic degradation. Autophagy is thus a key determinant for mitochondrial health and proper cell function. Mitophagic malfunction has been recently proposed to contribute to progressive neuronal loss in Parkinson's disease. In addition to autophagy's significance in mitochondrial integrity, several lines of evidence suggest that mitochondria can also substantially influence the autophagic process. The mitochondria's ability to influence and be influenced by autophagy places both elements (mitochondria and autophagy) in a unique position where defects in one or the other system could increase the risk to various metabolic and autophagic related diseases.     

青少年型神经元蜡样脂褐质沉积病(JNCL)的发病机制

神经元蜡样脂褐质沉积病(Neuronal ceroid lipofuscinosis, NCLs)是一组儿科神经退行性变疾病, 青少年型神经元蜡样脂褐质沉积病(Juvenile neuronal ceroid lipofuscinosis, JNCL)是其中一型。其临床表现包括视网膜退化进而失明、癫痫以及进行性的认知和运动能力的减退。本文综述了其发病机制, 包括凋亡、自噬、质膜相关的功能障碍、氧化应激与NO转导通路受阻、线粒体和溶酶体功能障碍、胞内pH失衡等。其中研究最为清楚的是细胞凋亡和自噬两种方式。在凋亡中, CLN3基因正常功能的缺陷导致神经酰胺的积累, 导致线粒体膜通透性增加(MMP), 并最终引发依赖胱酰蛋白酶(Caspase)以及非依赖胱酰蛋白酶的凋亡。自噬既有发生又有被破坏, 其破坏的主要原因是自噬小泡的不成熟导致自噬不能有效循环。本文对发病机制, 尤其是其细胞死亡的途径的阐述, 将有助于对JNCL等神经退行性病变一类疾病的认识。     

感染在神经退行性疾病中的调控机制

神经退行性疾病以突触丢失和神经元死亡为特征,表现为认知功能下降、痴呆和运动功能丧失。流行病学和实验证据提示:慢性细菌、病毒和真菌感染可能是导致神经退行性疾病如阿尔兹海默症(AD)、帕金森病(PD)、肌萎缩性侧索硬化症(ALS)和多发性硬化症(MS)等的危险因素。病原体在中枢神经系统的持续感染可导致一系列细胞生物学功能的异常,如诱导蛋白质的错误折叠和聚集、导致氧化应激损伤、细胞自噬异常以及神经元凋亡和坏死等;感染还会触发炎症介质释放并激活宿主免疫应答;此外,感染还可引起慢性神经炎症并导致能量代谢障碍等。本文就感染在神经退行性疾病中的作用机制及其研究进展作一综述,从而为科研人员开发新的药物和治疗方法提供新思路。     

Geraniol Protects Against the Protein and Oxidative Stress Induced by Rotenone in an In Vitro Model of Parkinson’s Disease

Dysfunction of autophagy, mitochondrial dynamics and endoplasmic reticulum (ER) stress are currently considered as major contributing factors in the pathogenesis of Parkinson’s disease (PD). Accumulation of oxidatively damaged cytoplasmic organelles and unfolded proteins in the lumen of the ER causes ER stress and it is associated with dopaminergic cell death in PD. Rotenone is a pesticide that selectively kills dopaminergic neurons by a variety of mechanism, has been implicated in PD. Geraniol (GE; 3,7-dimethylocta-trans-2,6-dien-1-ol) is an acyclic monoterpene alcohol occurring in the essential oils of several aromatic plants. In this study, we investigated the protective effect of GE on rotenone-induced mitochondrial dysfunction dependent oxidative stress leads to cell death in SK-N-SH cells. In addition, we assessed the involvement of GE on rotenone-induced dysfunction in autophagy machinery via α-synuclein accumulation induced ER stress. We found that pre-treatment of GE enhanced cell viability, ameliorated intracellular redox, preserved mitochondrial membrane potential and improves the level of mitochondrial complex-1 in rotenone treated SK-N-SH cells. Furthermore, GE diminishes autophagy flux by reduced autophagy markers, and decreases ER stress by reducing α-synuclein expression in SK-N-SH cells. Our results demonstrate that GE possess its neuroprotective effect via reduced rotenone-induced oxidative stress by enhanced antioxidant status and maintain mitochondrial function. Furthermore, GE reduced ER stress and improved autophagy flux in the neuroblastomal SK-N-SH cells. The present study could suggest that GE a novel therapeutic avenue for clinical intervention in neurodegenerative diseases especially for PD.     

Essential control of mitochondrial morphology and function by chaperone-mediated autophagy through degradation of PARK7

As a selective degradation system, chaperone-mediated autophagy (CMA) is essential for maintaining cellular homeostasis and survival under stress conditions. Increasing evidence points to an important role for the dysfunction of CMA in the pathogenesis of Parkinson disease (PD). However, the mechanisms by which CMA regulates neuronal survival under stress and its role in neurodegenerative diseases are not fully understood. PARK7/DJ-1 is an autosomal recessive familial PD gene. PARK7 plays a critical role in antioxidative response and its dysfunction leads to mitochondrial defects. In the current study, we showed that CMA mediated the lysosome-dependent degradation of PARK7. Importantly, CMA preferentially removed the oxidatively damaged nonfunctional PARK7 protein. Furthermore, CMA protected cells from mitochondrial toxin MPP⁺-induced changes in mitochondrial morphology and function, and increased cell viability. These protective effects were lost under PARK7-deficiency conditions. Conversely, overexpression of PARK7 significantly attenuated the mitochondrial dysfunction and cell death exacerbated by blocking CMA under oxidative stress. Thus, our findings reveal a mechanism by which CMA protects mitochondrial function by degrading nonfunctional PARK7 and maintaining its homeostasis, and dysregulation of this pathway may contribute to the neuronal stress and death in PD pathogenesis.     

Neuropathological Aspects of Mitochondrial DNA Disease

Mitochondrial DNA disorders are an important cause of neurological disease, yet despite our awareness of the importance of these conditions, relatively little is known about the neuropathology of these disorders and even less about the mechanisms involved in neuronal dysfunction and death. In this review we detail important features from neuropathological studies available and highlight deficiencies that are currently limiting our understanding of mitochondrial DNA disease. We also discuss possible future approaches that might resolve some of these outstanding issues. Further study of these disorders is critical because mitochondria play a central role in neuronal survival and it is likely that an understanding of the mechanisms involved in neuronal dysfunction and cell death in mitochondrial DNA disease may have implications for other neurodegenerative diseases.     

The role of mitochondria in inherited neurodegenerative diseases

In the past decade, the genetic causes underlying familial forms of many neurodegenerative disorders, such as Huntington's disease, Parkinson's disease, Alzheimer's disease, amyotrophic lateral sclerosis, Friedreich ataxia, hereditary spastic paraplegia, dominant optic atrophy, Charcot-Marie-Tooth type 2A, neuropathy ataxia and retinitis pigmentosa, and Leber's hereditary optic atrophy have been elucidated. However, the common pathogenic mechanisms of neuronal death are still largely unknown. Recently, mitochondrial dysfunction has emerged as a potential 'lowest common denominator' linking these disorders. In this review, we discuss the body of evidence supporting the role of mitochondria in the pathogenesis of hereditary neurodegenerative diseases. We summarize the principal features of genetic diseases caused by abnormalities of mitochondrial proteins encoded by the mitochondrial or the nuclear genomes. We then address genetic diseases where mutant proteins are localized in multiple cell compartments, including mitochondria and where mitochondrial defects are likely to be directly caused by the mutant proteins. Finally, we describe examples of neurodegenerative disorders where mitochondrial dysfunction may be 'secondary' and probably concomitant with degenerative events in other cell organelles, but may still play an important role in the neuronal decay. Understanding the contribution of mitochondrial dysfunction to neurodegeneration and its pathophysiological basis will significantly impact our ability to develop more effective therapies for neurodegenerative diseases.