Alpha-synuclein suppresses mitochondrial protease ClpP to trigger mitochondrial oxidative damage and neurotoxicity

The role of SIRT3 in homeostasis and cellular health

Mitochondria are responsible for maintaining cellular energy levels, and play a major role in regulating homeostasis, which ensures physiological function from the molecular to whole animal. Sirtuin 3 (SIRT3) is the major protein deacetylase of mitochondria. SIRT3 serves as a nutrient sensor; under conditions of mild metabolic stress, SIRT3 activity is increased. Within the mitochondria, SIRT3 regulates every complex of the electron transport chain, the tricarboxylic acid (TCA) and urea cycles, as well as the mitochondria membrane potential, and other free radical scavengers. This article reviews the role of SIRT3 in regulating homeostasis, and thus physiological function. We discuss the role of SIRT3 in regulating reactive oxygen species (ROS), ATP, immunological function and mitochondria dynamics.

Diastolic dysfunction in Alzheimer's disease model mice is associated with Aβ-amyloid aggregate formation and mitochondrial dysfunction

Alzheimer's Disease (AD) is a progressive neurodegenerative disease caused by the deposition of Aβ aggregates or neurofibrillary tangles. AD patients are primarily diagnosed with the concurrent development of several cardiovascular dysfunctions. While few studies have indicated the presence of intramyocardial Aβ aggregates, none of the studies have performed detailed analyses for pathomechanism of cardiac dysfunction in AD patients. This manuscript used aged APPSWE/PS1 Tg and littermate age-matched wildtype (Wt) mice to characterize cardiac dysfunction and analyze associated pathophysiology. Detailed assessment of cardiac functional parameters demonstrated the development of diastolic dysfunction in APPSWE/PS1 Tg hearts compared to Wt hearts. Muscle function evaluation showed functional impairment (decreased exercise tolerance and muscle strength) in APPSWE/PS1 Tg mice. Biochemical and histochemical analysis revealed Aβ aggregate accumulation in APPSWE/PS1 Tg mice myocardium. APPSWE/PS1 Tg mice hearts also demonstrated histopathological remodeling (increased collagen deposition and myocyte cross-sectional area). Additionally, APPSWE/PS1 Tg hearts showed altered mitochondrial dynamics, reduced antioxidant protein levels, and impaired mitochondrial proteostasis compared to Wt mice. APPSWE/PS1 Tg hearts also developed mitochondrial dysfunction with decreased OXPHOS and PDH protein complex expressions, altered ETC complex dynamics, decreased complex activities, and reduced mitochondrial respiration. Our results indicated that Aβ aggregates in APPSWE/PS1 Tg hearts are associated with defects in mitochondrial respiration and complex activities, which may collectively lead to cardiac diastolic dysfunction and myocardial pathological remodeling.

Mitochondrial proteases modulate mitochondrial stress signalling and cellular homeostasis in health and disease

Maintenance of mitochondrial homeostasis requires a plethora of coordinated quality control and adaptations' mechanisms in which mitochondrial proteases play a key role. Their activation or loss of function reverberate beyond local mitochondrial biochemical and metabolic remodelling into coordinated cellular pathways and stress responses that feedback onto the mitochondrial functionality and adaptability. Mitochondrial proteolysis modulates molecular and organellar quality control, metabolic adaptations, lipid homeostasis and regulates transcriptional stress responses. Defective mitochondrial proteolysis results in disease conditions most notably, mitochondrial diseases, neurodegeneration and cancer. Here, it will be discussed how mitochondrial proteases and mitochondria stress signalling impact cellular homeostasis and determine the cellular decision to survive or die, how these processes may impact disease etiopathology, and how modulation of proteolysis may offer novel therapeutic strategies.

A Plant Model of α-Synucleinopathy: Expression of α-Synuclein A53T Variant in Hairy Root Cultures Leads to Proteostatic Stress and Dysregulation of Iron Metabolism

Synucleinopathies, typified by Parkinson's disease (PD), entail the accumulation of α-synuclein (αSyn) aggregates in nerve cells. Various αSyn mutants, including the αSyn A53T variant linked to early-onset PD, increase the propensity for αSyn aggregate formation. In addition to disrupting protein homeostasis and inducing proteostatic stress, the aggregation of αSyn in PD is associated with an imbalance in iron metabolism, which increases the generation of reactive oxygen species and causes oxidative stress. This study explored the impact of αSyn A53T expression in transgenic hairy roots of four medicinal plants (Lobelia cardinalis, Artemisia annua, Salvia miltiorrhiza, and Polygonum multiflorum). In all tested plants, αSyn A53T expression triggered proteotoxic stress and perturbed iron homeostasis, mirroring the molecular profile observed in human and animal nerve cells. In addition to the common eukaryotic defense mechanisms against proteostatic and oxidative stresses, a plant stress response generally includes the biosynthesis of a diverse set of protective secondary metabolites. Therefore, the hairy root cultures expressing αSyn A53T offer a platform for identifying secondary metabolites that can ameliorate the effects of αSyn, thereby aiding in the development of possible PD treatments and/or treatments of synucleinopathies.

Gypenoside XLIX alleviates acute liver injury: Emphasis on NF-κB/PPAR-α/NLRP3 pathways

Liver is one of the vital organs in the human body and liver injury will have a very serious impact on human damage. Gypenoside XLIX is a PPAR-α activator that inhibits the activation of the NF-κB signaling pathway. The components of XLIX have pharmacological effects such as cardiovascular protection, antihypoxia, anti-tumor and anti-aging. In this study, we used cecum ligation and puncture (CLP) was used to induce in vivo mice hepatic injury, and lipopolysaccharide (LPS)-induced inflammation in RAW264.7 cells, evaluated whether Gypenoside XLIX could have a palliative effect on sepsis-induced acute liver injury via NF-κB/PPAR-α/NLRP3. In order to gain insight into these mechanisms, six groups were created in vivo: the Contol group, the Sham group, the CLP group, the CLP + XLIX group (40 mg/kg) and the Sham + XLIX (40 mg/kg) group, and the CLP + DEX (2 mg/kg) group. Three groups were created in vitro: Control, LPS, LPS + XLIX (40 μM). The analytical methods used included H&E staining, qPCR, reactive oxygen species (ROS), oil red O staining, and Western Blot. The results showed that XLIX attenuated hepatic inflammatory injury in mice with toxic liver disease through inhibition of the TLR4-mediated NF-κB pathway, attenuated lipid accumulation through activation of PPAR-α, and attenuated hepatic pyroptosis by inhibiting NLRP3 production. Regarding the imbalance between oxidative and antioxidant defenses due to septic liver injury, XLIX reduced liver oxidative stress-related biomarkers (ALT, AST), reduced ROS accumulation, decreased the amount of malondialdehyde (MDA) produced by lipid peroxidation, and increased the levels of antioxidant enzymes such as glutathione (GSH) and catalase (CAT). Our results demonstrate that XLIX can indeed attenuate septic liver injury. This is extremely important for future studies on XLIX and sepsis, and provides a potential pathway for the treatment of acute liver injury.

Dual roles of UPRer and UPRmt in neurodegenerative diseases

The unfolded protein response (UPR) is a cellular stress response mechanism induced by the accumulation of unfolded or misfolded proteins. Within the endoplasmic reticulum and mitochondria, a dynamic balance exists between protein folding mechanisms and unfolded protein levels under normal conditions. Disruption of this balance or an accumulation of unfolded proteins in these organelles can result in stress responses and UPR. The UPR restores organelle homeostasis and promotes cell survival by increasing the expression of chaperone proteins, regulating protein quality control systems, and enhancing the protein degradation pathway. However, prolonged or abnormal UPR can also have negative effects, including cell death. Therefore, many diseases, especially neurodegenerative diseases, are associated with UPR dysfunction. Neurodegenerative diseases are characterized by misfolded proteins accumulating and aggregating, and neuronal cells are particularly sensitive to misfolded proteins and are prone to degeneration. Many studies have shown that the UPR plays an important role in the pathogenesis of neurodegenerative diseases. Here, we will discuss the possible contributions of the endoplasmic reticulum unfolded protein response (UPRer) and the mitochondrial unfolded protein response (UPRmt) in the development of several neurodegenerative diseases.

Artemisinin Confers Neuroprotection against 6-OHDA-Induced Neuronal Injury In Vitro and In Vivo through Activation of the ERK1/2 Pathway

Parkinson's disease (PD) is an age-related, progressive neurodegenerative disease characterized by the gradual and massive loss of dopaminergic neurons in the substantia nigra pars compacta (SNc). We have recently reported that artemisinin, an FDA-approved first-line antimalarial drug, possesses a neuroprotective effect. However, the effects and underlying mechanisms of artemisinin on Parkinson's disease remain to be elucidated. In this study, we investigated the neuroprotective effects of artemisinin on 6-OHDA and MPP⁺ in neuronal cells and animal models, as well as the underlying mechanisms. Our results showed that artemisinin significantly attenuated the loss of cell viability, LDH release, elevated levels of reactive oxygen species (ROS), the collapse of the mitochondria trans-membrane potential and cell apoptosis in PC12 cells. Western blot results showed that artemisinin stimulated the phosphorylation of ERK1/2, its upstream signaling proteins c-Raf and MEK and its downstream target CREB in PC12 cells in a time- and concentration-dependent manner. In addition, the protective effect of artemisinin was significantly reduced when the ERK pathway was blocked using the ERK pathway inhibitor PD98059 or when the expression of ERK was knocked down using sgRNA. These results indicate the essential role of ERK in the protective effect of artemisinin. Similar results were obtained in SH-SY5Y cells and primary cultured neurons treated with 6-OHDA, as well as in cellular models of MPP⁺ injury. More interestingly, artemisinin attenuated PD-like behavior deficit in mice injected with 6-OHDA evaluated by behavioral tests including swimming test, pole-test, open field exploration and rotarod tests. Moreover, artemisinin also stimulated the phosphorylation of ERK1/2, inhibited apoptosis, and rescued dopaminergic neurons in SNc of these animals. Application of ERK pathway inhibitor PD98059 blocked the protective effect of artemisinin in mice during testing. Taking these results together, it was indicated that artemisinin preserves neuroprotective effects against 6-OHDA and MPP⁺ induced injury both in vitro and in vivo by the stimulation of the ERK1/2 signaling pathway. Our findings support the potential therapeutic effect of artemisinin in the prevention and treatment of Parkinson's disease.

Mitochondrial dysfunction in aging

Aging is a complex process that features a functional decline in many organelles. Although mitochondrial dysfunction is suggested as one of the determining factors of aging, the role of mitochondrial quality control (MQC) in aging is still poorly understood. A growing body of evidence points out that reactive oxygen species (ROS) stimulates mitochondrial dynamic changes and accelerates the accumulation of oxidized by-products through mitochondrial proteases and mitochondrial unfolded protein response (UPRmt). Mitochondrial-derived vesicles (MDVs) are the frontline of MQC to dispose of oxidized derivatives. Besides, mitophagy helps remove partially damaged mitochondria to ensure that mitochondria are healthy and functional. Although abundant interventions on MQC have been explored, over-activation or inhibition of any type of MQC may even accelerate abnormal energy metabolism and mitochondrial dysfunction-induced senescence. This review summarizes mechanisms essential for maintaining mitochondrial homeostasis and emphasizes that imbalanced MQC may accelerate cellular senescence and aging. Thus, appropriate interventions on MQC may delay the aging process and extend lifespan.

Mitochondrion: A bridge linking aging and degenerative diseases

Aging is a natural process, characterized by progressive loss of physiological integrity, impaired function, and increased vulnerability to death. For centuries, people have been trying hard to understand the process of aging and find effective ways to delay it. However, limited breakthroughs have been made in anti-aging area. Since the hallmarks of aging were summarized in 2013, increasing studies focus on the role of mitochondrial dysfunction in aging and aging-related degenerative diseases, such as neurodegenerative diseases, osteoarthritis, metabolic diseases, and cardiovascular diseases. Accumulating evidence indicates that restoring mitochondrial function and biogenesis exerts beneficial effects in extending lifespan and promoting healthy aging. In this paper, we provide an overview of mitochondrial changes during aging and summarize the advanced studies in mitochondrial therapies for the treatment of degenerative diseases. Current challenges and future perspectives are proposed to provide novel and promising directions for future research.

Parkinson's Disease, Parkinsonisms, and Mitochondria: the Role of Nuclear and Mitochondrial DNA

PURPOSE OF REVIEW

Overwhelming evidence indicates that mitochondrial dysfunction is a central factor in Parkinson's disease (PD) pathophysiology. This paper aims to review the latest literature published, focusing on genetic defects and expression alterations affecting mitochondria-associated genes, in support of their key role in PD pathogenesis.

RECENT FINDINGS

Thanks to the use of new omics approaches, a growing number of studies are discovering alterations affecting genes with mitochondrial functions in patients with PD and parkinsonisms. These genetic alterations include pathogenic single-nucleotide variants, polymorphisms acting as risk factors, and transcriptome modifications, affecting both nuclear and mitochondrial genes. We will focus on alterations of mitochondria-associated genes described by studies conducted on patients or on animal/cellular models of PD or parkinsonisms. We will comment how these findings can be taken into consideration for improving the diagnostic procedures or for deepening our knowledge on the role of mitochondrial dysfunctions in PD.

Role of rodent models in advancing precision medicine for Parkinson's disease

With a current lack of disease-modifying treatments, an initiative toward implementing a precision medicine approach for treating Parkinson's disease (PD) has emerged. However, challenges remain in how to define and apply precision medicine in PD. To accomplish the goal of optimally targeted and timed treatment for each patient, preclinical research in a diverse population of rodent models will continue to be an essential part of the translational path to identify novel biomarkers for patient diagnosis and subgrouping, understand PD disease mechanisms, identify new therapeutic targets, and screen therapeutics prior to clinical testing. This review highlights the most common rodent models of PD and discusses how these models can contribute to defining and implementing precision medicine for the treatment of PD.

ATF5 Attenuates the Secretion of Pro-Inflammatory Cytokines in Activated Microglia

The highly dynamic changes in microglia necessary to achieve a rapid neuroinflammatory response require a supply of energy from mitochondrial respiration, which leads to the accumulation of unfolded mitochondrial proteins. We previously reported that microglial activation is correlated with the mitochondrial unfolded protein response (UPRmt) in a kaolin-induced hydrocephalus model, but we still do not know the extent to which these changes in microglia are involved in cytokine release. Here, we investigated the activation of BV-2 cells and found that treatment with lipopolysaccharide (LPS) for 48 h increased the secretion of pro-inflammatory cytokines. This increase was accompanied by a concurrent decrease in oxygen consumption rate (OCR) and mitochondrial membrane potential (MMP), in association with the up-regulation of the UPRmt. Inhibition of the UPRmt by knockdown of ATF5, a key upstream regulator of the UPRmt, using small-interfering RNA against ATF5 (siATF5) not only increased production of the pro-inflammatory cytokines, interleukin-6 (IL-6), IL-1β and tumor necrosis factor-α (TNF-α), but also decreased MMP. Our results suggest that ATF5-dependent induction of the UPRmt in microglia acts as a protective mechanism during neuroinflammation and may be a potential therapeutic target for reducing neuroinflammation.

Common Mechanisms Underlying α-Synuclein-Induced Mitochondrial Dysfunction in Parkinson's Disease

Parkinson's disease (PD) is the most common neurological movement disorder characterized by the selective and irreversible loss of dopaminergic neurons in substantia nigra pars compacta resulting in dopamine deficiency in the striatum. While most cases are sporadic or environmental, about 10% of patients have a positive family history with a genetic cause. The misfolding and aggregation of α-synuclein (α-syn) as a casual factor in the pathogenesis of PD has been supported by a great deal of literature. Extensive studies of mechanisms underpinning degeneration of the dopaminergic neurons induced by α-syn dysfunction suggest a complex process that involves multiple pathways, including mitochondrial dysfunction and increased oxidative stress, impaired calcium homeostasis through membrane permeabilization, synaptic dysfunction, impairment of quality control systems, disruption of microtubule dynamics and axonal transport, endoplasmic reticulum/Golgi dysfunction, nucleus malfunction, and microglia activation leading to neuroinflammation. Among them mitochondrial dysfunction has been considered as the most primary target of α-syn-induced toxicity, leading to neuronal cell death in both sporadic and familial forms of PD. Despite reviewing many aspects of PD pathogenesis related to mitochondrial dysfunction, a systemic study on how α-syn malfunction/aggregation damages mitochondrial functionality and leads to neurodegeneration is missing in the literature. In this review, we give a detailed molecular overview of the proposed mechanisms by which α-syn, directly or indirectly, contributes to mitochondrial dysfunction. This may provide valuable insights for development of new therapeutic approaches in relation to PD. Antioxidant-based therapy as a potential strategy to protect mitochondria against oxidative damage, its challenges, and recent developments in the field are discussed.

AAA+ proteases: the first line of defense against mitochondrial damage

Mitochondria play essential cellular roles in Adenosine triphosphate (ATP) synthesis, calcium homeostasis, and metabolism, but these vital processes have potentially deadly side effects. The production of the reactive oxygen species (ROS) and the aggregation of misfolded mitochondrial proteins can lead to severe mitochondrial damage and even cell death. The accumulation of mitochondrial damage is strongly implicated in aging and several incurable diseases, including neurodegenerative disorders and cancer. To oppose this, metazoans utilize a variety of quality control strategies, including the degradation of the damaged mitochondrial proteins by the mitochondrial-resident proteases of the ATPase Associated with the diverse cellular Activities (AAA+) family. This mini-review focuses on the quality control mediated by the mitochondrial-resident proteases of the AAA+ family used to combat the accumulation of damaged mitochondria and on how the failure of this mitochondrial quality control contributes to diseases.

The mitochondrial unfolded protein response: A multitasking giant in the fight against human diseases

Mitochondria, which serve as the energy factories of cells, are involved in cell differentiation, calcium homeostasis, amino acid and fatty acid metabolism and apoptosis. In response to environmental stresses, mitochondrial homeostasis is regulated at both the organelle and molecular levels to effectively maintain the number and function of mitochondria. The mitochondrial unfolded protein response (UPR^mt) is an adaptive intracellular stress mechanism that responds to stress signals by promoting the transcription of genes encoding mitochondrial chaperones and proteases. The mechanism of the UPR^mt in Caenorhabditis elegans (C. elegans) has been clarified over time, and the main regulatory factors include ATFS-1, UBL-5 and DVE-1. In mammals, the activation of the UPR^mt involves eIF2α phosphorylation and the uORF-regulated expression of CHOP, ATF4 and ATF5. Several additional factors, such as SIRT3 and HSF1, are also involved in regulating the UPR^mt. A deep and comprehensive exploration of the UPR^mt can provide new directions and strategies for the treatment of human diseases, including aging, neurodegenerative diseases, cardiovascular diseases and diabetes. In this review, we mainly discuss the function of UPR^mt, describe the regulatory mechanisms of UPR^mt in C. elegans and mammals, and summarize the relationship between UPR^mt and various human diseases.

Mitochondrial protease ClpP supplementation ameliorates diet-induced NASH in mice

BACKGROUND & AIMS

Mitochondrial dysfunction is considered a pathogenic linker in the development of non-alcoholic steatohepatitis (NASH). Inappropriate mitochondrial protein-quality control, possibly induced by insufficiency of the mitochondrial matrix caseinolytic protease P (ClpP), can potentially cause mitochondrial dysfunction. Herein, we aimed to investigate hepatic ClpP levels in a diet-induced model of NASH and determine whether supplementation of ClpP can ameliorate diet-induced NASH.

METHODS

NASH was induced by a high-fat/high-fructose (HF/HFr) diet in C57BL/6J mice. Stress/inflammatory signals were induced in mouse primary hepatocytes (MPHs) by treatment with palmitate/oleate (PA/OA). ClpP levels in hepatocytes were reduced using the RNAi-mediated gene knockdown technique but increased through the viral transduction of ClpP. ClpP activation was induced by administering a chemical activator of ClpP.

RESULTS

Hepatic ClpP protein levels in C57BL/6J mice fed a HF/HFr diet were lower than the levels in those fed a normal chow diet. PA/OA treatment also decreased the ClpP protein levels in MPHs. Overexpression or activation of ClpP reversed PA/OA-induced mitochondrial dysfunction and stress/inflammatory signal activation in MPHs, whereas ClpP knockdown induced mitochondrial dysfunction and stress/inflammatory signals in these cells. On the other hand, ClpP overexpression or activation improved HF/HFr-induced NASH characteristics such as hepatic steatosis, inflammation, fibrosis, and injury in the C57BL/6J mice, whereas ClpP knockdown further augmented steatohepatitis in mice fed a HF/HFr diet.

CONCLUSIONS

Reduced ClpP expression and subsequent mitochondrial dysfunction are key to the development of diet-induced NASH. ClpP supplementation through viral transduction or chemical activation represents a potential therapeutic strategy to prevent diet-induced NASH.

LAY SUMMARY

Western diets, containing high fat and high fructose, often induce non-alcoholic steatohepatitis (NASH). Mitochondrial dysfunction is considered pathogenically linked to diet-induced NASH. We observed that the mitochondrial protease ClpP decreased in the livers of mice fed a western diet and supplementation of ClpP ameliorated western diet-induced NASH.

Pathological structural conversion of α-synuclein at the mitochondria induces neuronal toxicity

Aggregation of alpha-synuclein (α-Syn) drives Parkinson's disease (PD), although the initial stages of self-assembly and structural conversion have not been directly observed inside neurons. In this study, we tracked the intracellular conformational states of α-Syn using a single-molecule Förster resonance energy transfer (smFRET) biosensor, and we show here that α-Syn converts from a monomeric state into two distinct oligomeric states in neurons in a concentration-dependent and sequence-specific manner. Three-dimensional FRET-correlative light and electron microscopy (FRET-CLEM) revealed that intracellular seeding events occur preferentially on membrane surfaces, especially at mitochondrial membranes. The mitochondrial lipid cardiolipin triggers rapid oligomerization of A53T α-Syn, and cardiolipin is sequestered within aggregating lipid-protein complexes. Mitochondrial aggregates impair complex I activity and increase mitochondrial reactive oxygen species (ROS) generation, which accelerates the oligomerization of A53T α-Syn and causes permeabilization of mitochondrial membranes and cell death. These processes were also observed in induced pluripotent stem cell (iPSC)-derived neurons harboring A53T mutations from patients with PD. Our study highlights a mechanism of de novo α-Syn oligomerization at mitochondrial membranes and subsequent neuronal toxicity.

KPNB1 modulates the Machado-Joseph disease protein ataxin-3 through activation of the mitochondrial protease CLPP

Machado–Joseph disease (MJD) is characterized by a pathological expansion of the polyglutamine (polyQ) tract within the ataxin-3 protein. Despite its primarily cytoplasmic localization, polyQ-expanded ataxin-3 accumulates in the nucleus and forms intranuclear aggregates in the affected neurons. Due to these histopathological hallmarks, the nucleocytoplasmic transport machinery has garnered attention as an important disease relevant mechanism. Here, we report on MJD cell model-based analysis of the nuclear transport receptor karyopherin subunit beta-1 (KPNB1) and its implications in the molecular pathogenesis of MJD. Although directly interacting with both wild-type and polyQ-expanded ataxin-3, modulating KPNB1 did not alter the intracellular localization of ataxin-3. Instead, overexpression of KPNB1 reduced ataxin-3 protein levels and the aggregate load, thereby improving cell viability. On the other hand, its knockdown and inhibition resulted in the accumulation of soluble and insoluble ataxin-3. Interestingly, the reduction of ataxin-3 was apparently based on protein fragmentation independent of the classical MJD-associated proteolytic pathways. Label-free quantitative proteomics and knockdown experiments identified mitochondrial protease CLPP as a potential mediator of the ataxin-3-degrading effect induced by KPNB1. We confirmed reduction of KPNB1 protein levels in MJD by analyzing two MJD transgenic mouse models and induced pluripotent stem cells (iPSCs) derived from MJD patients. Our results reveal a yet undescribed regulatory function of KPNB1 in controlling the turnover of ataxin-3, thereby highlighting a new potential target of therapeutic value for MJD.

Quiescent Elongation of α-Synuclein Pre-form Fibrils Under Different Solution Conditions

The intracellular aggregation of α-synuclein in neurons/glia is considered to be a key step in the pathogenesis of synucleinopathy [including Parkinson’s disease (PD), dementia with Lewy body (DLB), multiple system atrophy (MSA), etc.]. Increasing evidence indicates that the initial pathological α-synuclein aggregates can replicate themselves and propagate in a “seeding” manner to multiple areas of the brain and even to peripheral tissue, which makes it the most important biomarker for the diagnosis of synucleinopathies in recent years. The amplification and propagation capabilities of α-synuclein aggregates are very similar to those of prion-like diseases, which are based on the inherent self-recruitment capabilities of existing misfolded proteins. In vitro, the rapid recruitment process can be reproduced in a simplified model by adding a small amount of α-synuclein pre-formed fibrils to the monomer solution as fibril seeds, which may partially reveal the properties of α-synuclein aggregates. In this study, we explored the elongation rate of α-synuclein pre-formed fibrils under a quiescent incubation condition (rather than shaking/agitating). By using the ThT fluorescence assay, we compared and quantified the elongation fluorescence curves to explore the factors that affect fibril elongation. These factors include proteins’ concentration, temperature, NaCl strength, SDS, temperature pretreatment, and so on. Our work further describes the elongation of α-synuclein fibrils under quiescent incubation conditions. This may have important implications for the in vitro amplification and preservation of α-synuclein aggregates to further understand the prion-like transmission mechanism of PD.

Protein interaction networks in neurodegenerative diseases: from physiological function to aggregation

The accumulation of protein inclusions is linked to many neurodegenerative diseases that typically develop in older individuals, due to a combination of genetic and environmental factors. In rare familial neurodegenerative disorders, genes encoding for aggregation-prone proteins are often mutated. While the underlying mechanism leading to these diseases still remains to be fully elucidated, efforts in the past 20 years revealed a vast network of protein–protein interactions that play a major role in regulating the aggregation of key proteins associated with neurodegeneration. Misfolded proteins that can oligomerize and form insoluble aggregates associate with molecular chaperones and other elements of the proteolytic machineries that are the frontline workers attempting to protect the cells by promoting clearance and preventing aggregation. Proteins that are normally bound to aggregation-prone proteins can become sequestered and mislocalized in protein inclusions, leading to their loss of function. In contrast, mutations, posttranslational modifications, or misfolding of aggregation-prone proteins can lead to gain of function by inducing novel or altered protein interactions, which in turn can impact numerous essential cellular processes and organelles, such as vesicle trafficking and the mitochondria. This review examines our current knowledge of protein–protein interactions involving several key aggregation-prone proteins that are associated with Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, or amyotrophic lateral sclerosis. We aim to provide an overview of the protein interaction networks that play a central role in driving or mitigating inclusion formation, while highlighting some of the key proteomic studies that helped to uncover the extent of these networks.

Rosmarinic Acid Inhibits Mitochondrial Damage by Alleviating Unfolded Protein Response

Mitochondria are essential organelles that perform important roles in cell biologies such as ATP synthesis, metabolic regulation, immunomodulatory, and apoptosis. Parkinson’s disease (PD) is connected with mitochondrial neuronal damage related to mitochondrial unfolded protein response (mtUPR). Rosmarinic acid (RA) is a naturally occurring hydroxylated polyphenolic chemical found in the Boraginaceae and the Labiatae subfamily Nepetoideae. This study looked into RA’s protective effect against mitochondrial loss in the substantia nigra (SN) caused by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), the underlying mechanism associated with the mtUPR. Pretreatment with RA reduced motor impairments and dopaminergic neuronal degeneration in the SN of a mouse model injected with MPTP. Pretreatment of SH-SY5Y cells from cell viability loss, morphological damage, and oxidative stress. Furthermore, RA pre-injection suppressed MPTP-induced mtUPR, lowered the expression of HSPA9, HSPE1, CLPP, LONP1, and SIRT 4, and protected the MPTP-mice and SH-SY5Y cells from mitochondrial failure. These findings imply that RA can prevent Parkinson’s disease by preventing mitochondrial damage in dopaminergic neurons in Parkinson’s disease via alleviating mitochondrial unfolded protein response.

Altered Mitochondrial Protein Homeostasis and Proteinopathies

Increasing evidence implicates mitochondrial dysfunction as key in the development and progression of various forms of neurodegeneration. The multitude of functions carried out by mitochondria necessitates a tight regulation of protein import, dynamics, and turnover; this regulation is achieved via several, often overlapping pathways that function at different levels. The development of several major neurodegenerative diseases is associated with dysregulation of these pathways, and growing evidence suggests direct interactions between some pathogenic proteins and mitochondria. When these pathways are compromised, so is mitochondrial function, and the resulting deficits in bioenergetics, trafficking, and mitophagy can exacerbate pathogenic processes. In this review, we provide an overview of the regulatory mechanisms employed by mitochondria to maintain protein homeostasis and discuss the failure of these mechanisms in the context of several major proteinopathies.

The Unfolded Protein Responses in Health, Aging, and Neurodegeneration: Recent Advances and Future Considerations

Aging and age-related neurodegeneration are both associated with the accumulation of unfolded and abnormally folded proteins, highlighting the importance of protein homeostasis (termed proteostasis) in maintaining organismal health. To this end, two cellular compartments with essential protein folding functions, the endoplasmic reticulum (ER) and the mitochondria, are equipped with unique protein stress responses, known as the ER unfolded protein response (UPR ER ) and the mitochondrial UPR (UPR mt ), respectively. These organellar UPRs play roles in shaping the cellular responses to proteostatic stress that occurs in aging and age-related neurodegeneration. The loss of adaptive UPR ER and UPR mt signaling potency with age contributes to a feed-forward cycle of increasing protein stress and cellular dysfunction. Likewise, UPR ER and UPR mt signaling is often altered in age-related neurodegenerative diseases; however, whether these changes counteract or contribute to the disease pathology appears to be context dependent. Intriguingly, altering organellar UPR signaling in animal models can reduce the pathological consequences of aging and neurodegeneration which has prompted clinical investigations of UPR signaling modulators as therapeutics. Here, we review the physiology of both the UPR ER and the UPR mt , discuss how UPR ER and UPR mt signaling changes in the context of aging and neurodegeneration, and highlight therapeutic strategies targeting the UPR ER and UPR mt that may improve human health.

Measuring Cysteine Exposure in Unfolded Proteins with Tetraphenylethene Maleimide and its Analogs

When proteostasis is challenged and becomes unbalanced, unfolded proteins can accumulate in the cells. Protein unfolding causes conformational changes and subsequent differentials in side-chain solvent accessibility and reactivity. In particular, when protein unfolds, non-disulfide-bonded cysteines that are usually buried in the native state can become surface exposed and thus accessible. A series of fluorogenic dyes including tetraphenylethene maleimide (TPE-MI) and its analogs were developed to capture cysteine exposure in unfolded proteins as a measure of unfolded protein load and proteostasis capacity in cells. These dyes are inherently non-fluorescent but show fluorescence turn-on effect when conjugated to unfolded proteins via reacting with exposed cysteines on the protein. Reacting with small biothiols such as glutathione does not induce fluorescence of these dyes. Here we describe the routine workflow to characterize unfolded proteins in vitro or unfolded proteomes in cells by TPE-MIs.

Genetic modulators associated with regulatory surveillance of mitochondrial quality control, play a key role in regulating stress pathways and longevity in C. elegans

The multi-factorial Parkinson's disease (PD) is known to be associated with mitochondrial dysfunction, endoplasmic reticulum stress, alpha synuclein aggregation and dopaminergic neuronal death, with oxidative stress being a common denominator to these underlying processes. The perception of mitochondria being 'just ATP producing compartments' have been counterpoised as studies, particularly related to PD, have underlined their strong role in cause and progression of the disease. During PD pathogenesis, neurons encounter chronic stress conditions mainly due to failure of Mitochondrial Quality Control (MQC) machinery. To dissect the regulatory understanding of mitochondrial dysfunction during neurological disease progression, we endeavored to identify key regulatory endpoints that control multiple facets of MQC machinery. Our studies, employing transgenic C. elegans strain expressing human α-synuclein, led us to identification of mitochondrial genes nuo-5 (involved in oxidative phosphorylation), F25B4.7 (exhibits ATP transmembrane transporter activity) and C05D11.9 (having ribonuclease activity), which form predicted downstream targets of most elevated and down-regulated mi-RNA molecules. RNAi mediated silencing, gene ontology and functional genomics analysis studies demonstrated their role in modulating major MQC pathways. The attenuated MQC pathways mainly affected clearance of misfolded and aggregated proteins, redox homeostasis and longevity with compromised dopaminergic functions. Overexpression of the mitochondrial genes by 3 beta-hydroxyl steroid, Tomatidine, was found to curtail the redox imbalance thus leading to amelioration of effects associated with PD and an increase in the lifespan of treated nematodes. Therefore, this study unveils the regulatory role of mitochondrial genes as critical modulators of stress control involved in effects associated with PD pathogenesis.

Mitochondrial Quality Control Strategies: Potential Therapeutic Targets for Neurodegenerative Diseases?

Many lines of evidence have indicated the therapeutic potential of rescuing mitochondrial integrity by targeting specific mitochondrial quality control pathways in neurodegenerative diseases, such as Parkinson's disease, Huntington's disease, and Alzheimer's disease. In addition to ATP synthesis, mitochondria are critical regulators of ROS production, lipid metabolism, calcium buffering, and cell death. The mitochondrial unfolded protein response, mitochondrial dynamics, and mitophagy are the three main quality control mechanisms responsible for maintaining mitochondrial proteostasis and bioenergetics. The proper functioning of these complex processes is necessary to surveil and restore mitochondrial homeostasis and the healthy pool of mitochondria in cells. Mitochondrial dysfunction occurs early and causally in disease pathogenesis. A significant accumulation of mitochondrial damage resulting from compromised quality control pathways leads to the development of neuropathology. Moreover, genetic or pharmaceutical manipulation targeting the mitochondrial quality control mechanisms can sufficiently rescue mitochondrial integrity and ameliorate disease progression. Thus, therapies that can improve mitochondrial quality control have great promise for the treatment of neurodegenerative diseases. In this review, we summarize recent progress in the field that underscores the essential role of impaired mitochondrial quality control pathways in the pathogenesis of neurodegenerative diseases. We also discuss the translational approaches targeting mitochondrial function, with a focus on the restoration of mitochondrial integrity, including mitochondrial dynamics, mitophagy, and mitochondrial proteostasis.

Analysis of Brain Protein Stability Changes in Mouse Models of Normal Aging and α-Synucleinopathy Reveals Age- and Disease-Related Differences

Here, we utilize the stability of proteins from rates of oxidation (SPROX) technique, to profile the thermodynamic stabilities of proteins in brain tissue cell lysates from Huα-Syn(A53T) transgenic mice at three time points including at 1 month (n = 9), at 6 months (n = 7), and at the time (between 9 and 16 months) a mouse became symptomatic (n = 8). The thermodynamic stability profiles generated here on 332 proteins were compared to thermodynamic stability profiles generated on the same proteins from similarly aged wild-type mice using a two-way unbalanced analysis of variance (ANOVA) analysis. This analysis identified a group of 22 proteins with age-related protein stability changes and a group of 11 proteins that were differentially stabilized in the Huα-Syn(A53T) transgenic mouse model. A total of 9 of the 11 proteins identified here with disease-related stability changes have been previously detected in human cerebral spinal fluid and thus have potential utility as biomarkers of Parkinson's disease (PD). The differential stability observed for one protein, glutamate decarboxylase 2 (Gad2), with an age-related change in stability, was consistent with the differential presence of a known, age-related truncation product of this protein, which is shown here to have a higher folding stability than full-length Gad2. Mass spectrometry data were deposited at ProteomeXchange (PXD016985).

Targeting α-synuclein aggregation and its role in mitochondrial dysfunction in Parkinson's disease

Lewy bodies that contain aggregated α-synuclein (α-syn) in the dopaminergic (DA) neuron are the main culprit behind neurodegeneration in Parkinson's disease (PD). Besides, mitochondrial dysfunction has a well established and prominent role in the pathogenesis of PD. However, the exact mechanism by which α-syn causes dopaminergic neuronal loss was unclear. Recent evidence suggests that aggregated α-syn localises in the mitochondria and contributes to oxidative stress-mediated apoptosis in neurons. Therefore, the involvement of aggregated α-syn in mitochondrial dysfunction-mediated neuronal loss has made it an emerging drug target for the treatment of PD. However, the exact mechanism by which α-syn permeabilises through the mitochondrial membrane and affects the electron transport chain remains under investigation. In the present study, we describe mitochondria-α-syn interactions and how α-syn aggregation modulates mitochondrial homeostasis in PD pathogenesis. We also discuss recent therapeutic interventions targeting α-syn aggregation that may help researchers to design novel therapeutic treatments for PD.

Fatal attraction - The role of hypoxia when alpha-synuclein gets intimate with mitochondria

Alpha-synuclein aggregation and mitochondrial dysfunction are main pathological hallmarks of Parkinson's disease (PD) and several other neurodegenerative diseases, collectively known as synucleinopathies. However, increasing evidence suggests that they may not be sufficient to cause PD. Here we propose the role of hypoxia as a missing link that connects the complex interplay between alpha-synuclein biochemistry and pathology, mitochondrial dysfunctions and neurodegeneration in PD. We review the partly conflicting literature on alpha-synuclein binding to membranes and mitochondria and its impact on mitochondrial functions. From there, we focus on adverse changes in cellular environments, revolving around hypoxic stress, that may trigger or facilitate PD progression. Inter-dependent structural re-arrangements of mitochondrial membranes, including increased cytoplasmic exposure of mitochondrial cardiolipins and changes in alpha-synuclein localization and conformation are discussed consequences of such conditions. Enhancing cellular resilience could be an integral part of future combination-based therapies of PD. This may be achieved by boosting the capacity of cellular and specifically mitochondrial processes to regulate and adapt to altered proteostasis, redox, and inflammatory conditions and by inducing protective molecular and tissue re-modelling.

Roles of α‑synuclein in gastrointestinal microbiome dysbiosis‑related Parkinson's disease progression (Review)

Parkinson's disease (PD) is the second most common neurodegenerative disease amongst the middle-aged and elderly populations. Several studies have confirmed that the microbiota-gut-brain axis (MGBA) serves a key role in the pathogenesis of PD. Changes to the gastrointestinal microbiome (GM) cause misfolding and abnormal aggregation of α-synuclein (α-syn) in the intestine. Abnormal α-syn is not eliminated via physiological mechanisms and is transported into the central nervous system (CNS) via the vagus nerve. The abnormal levels of α-syn aggregate in the substantia nigra pars compacta, not only leading to the formation of eosinophilic Lewis Bodies in the cytoplasm and mitochondrial dysfunction in dopaminergic (DA) neurons, but also leading to the stimulation of an inflammatory response in the microglia. These pathological changes result in an increase in oxidative stress (OS), which triggers nerve cell apoptosis, a characteristic of PD. This increase in OS further oxidizes and intensifies abnormal aggregation of α-syn, eventually forming a positive feedback loop. The present review discusses the abnormal accumulation of α-syn in the intestine caused by the GM changes and the increased levels of α-syn transport to the CNS via the MGBA, resulting in the loss of DA neurons and an increase in the inflammatory response of microglial cells in the brain of patients with PD. In addition, relevant clinical therapeutic strategies for improving the GM and reducing α-syn accumulation to relieve the symptoms and progression of PD are described.

Roles of mitochondrial unfolded protein response in mammalian stem cells

The mitochondrial unfolded protein response (UPR^mt) is an evolutionarily conserved adaptive mechanism for improving cell survival under mitochondrial stress. Under physiological and pathological conditions, the UPR^mt is the key to maintaining intracellular homeostasis and proteostasis. Important roles of the UPR^mt have been demonstrated in a variety of cell types and in cell development, metabolism, and immune processes. UPR^mt dysfunction leads to a variety of pathologies, including cancer, inflammation, neurodegenerative disease, metabolic disease, and immune disease. Stem cells have a special ability to self-renew and differentiate into a variety of somatic cells and have been shown to exist in a variety of tissues. These cells are involved in development, tissue renewal, and some disease processes. Although the roles and regulatory mechanisms of the UPR^mt in somatic cells have been widely reported, the roles of the UPR^mt in stem cells are not fully understood. The roles and functions of the UPR^mt depend on stem cell type. Therefore, this paper summarizes the potential significance of the UPR^mt in embryonic stem cells, tissue stem cells, tumor stem cells, and induced pluripotent stem cells. The purpose of this review is to provide new insights into stem cell differentiation and tumor pathogenesis.

Reassessing neurodegenerative disease: immune protection pathways and antagonistic pleiotropy

The antagonistic pleiotropy (AP) theory posits that adaptive evolutionary changes, which facilitate reproduction and individual fitness early in life, can enhance detrimental aging-related processes. Several genes associated with human brain diseases play a protective role in infection, suggesting the relevance of AP in the context of brain aging and neurodegeneration. Relatedly, genetic variants that confer immune protection against pathogens may lead to uncontrolled brain inflammation later in life. Here, we propose a conceptual framework suggesting that the pleiotropic roles of genes in infections and host-pathogen interactions should be considered when studying neurological illnesses. We reinterpret recent findings regarding the impact of neurological disease-associated genetic traits on infections and chronic inflammatory diseases. Identifying the AP pathways shared among these seemingly unrelated conditions might provide further insights into the detrimental role of the immune system in brain disease as well as the mechanisms involved in chronic infections.

Reappraisal of metabolic dysfunction in neurodegeneration: Focus on mitochondrial function and calcium signaling

The cellular and molecular mechanisms that drive neurodegeneration remain poorly defined. Recent clinical trial failures, difficult diagnosis, uncertain etiology, and lack of curative therapies prompted us to re-examine other hypotheses of neurodegenerative pathogenesis. Recent reports establish that mitochondrial and calcium dysregulation occur early in many neurodegenerative diseases (NDDs), including Alzheimer's disease, Parkinson's disease, Huntington's disease, and others. However, causal molecular evidence of mitochondrial and metabolic contributions to pathogenesis remains insufficient. Here we summarize the data supporting the hypothesis that mitochondrial and metabolic dysfunction result from diverse etiologies of neuropathology. We provide a current and comprehensive review of the literature and interpret that defective mitochondrial metabolism is upstream and primary to protein aggregation and other dogmatic hypotheses of NDDs. Finally, we identify gaps in knowledge and propose therapeutic modulation of mCa2+ exchange and mitochondrial function to alleviate metabolic impairments and treat NDDs.

Selective Neuron Vulnerability in Common and Rare Diseases-Mitochondria in the Focus

Mitochondrial dysfunction is a central feature of neurodegeneration within the central and peripheral nervous system, highlighting a strong dependence on proper mitochondrial function of neurons with especially high energy consumptions. The fitness of mitochondria critically depends on preservation of distinct processes, including the maintenance of their own genome, mitochondrial dynamics, quality control, and Ca²⁺ handling. These processes appear to be differently affected in common neurodegenerative diseases, such as Alzheimer’s and Parkinson’s disease, as well as in rare neurological disorders, including Huntington’s disease, Amyotrophic Lateral Sclerosis and peripheral neuropathies. Strikingly, particular neuron populations of different morphology and function perish in these diseases, suggesting that cell-type specific factors contribute to the vulnerability to distinct mitochondrial defects. Here we review the disruption of mitochondrial processes in common as well as in rare neurological disorders and its impact on selective neurodegeneration. Understanding discrepancies and commonalities regarding mitochondrial dysfunction as well as individual neuronal demands will help to design new targets and to make use of already established treatments in order to improve treatment of these diseases.

UPRmt activation protects against MPP+-induced toxicity in a cell culture model of Parkinson's disease

The pathogenesis of Parkinson's disease (PD) remains elusive, but mitochondrial dysfunction is believed to be one crucial step in its pathogenesis. The mitochondrial unfolded protein response (UPR^mt) is an important mitochondrial quality control strategy that maintains mitochondrial function in response to disturbances of mitochondrial protein homeostasis. Activation of the UPR^mt and the beneficial effect of rescuing mitochondrial proteostasis have been reported in several genetic models of PD. However, the pathogenic relevance of the UPR^mt in idiopathic PD is unknown. The present study examined the link between the UPR^mt and mitochondrial dysfunction in 1-methyl-4-phenylpyridinium (MPP⁺)-treated SH-SY5Y cells. Treatment with MPP ⁺ induced activation of the UPR^mt, reflected by an increase in the expression of UPR^mt-related chaperones, proteases, and transcription mediators. UPR^mt activation that was induced by overexpressing mutant ornithine transcarbamylase significantly reduced the production of mitochondrial reactive oxygen species (ROS) and improved cell survival in SH-SY5Y cells following MPP⁺ treatment. Moreover, the overexpression of activating transcription factor 5 (mammalian UPR^mt transcription factor) conferred protection against MPP⁺-induced ROS production and against cell death in SH-SY5Y cells. Overall, our results demonstrate the beneficial effect of UPR^mt activation in MPP ⁺ -treated cells, shedding new light on the mechanism of mitochondrial dysfunction in the pathogenesis of PD.

Mitochondrial Caseinolytic Protease P: A Possible Novel Prognostic Marker and Therapeutic Target in Cancer

Caseinolytic protease P (ClpP) is a mitochondrial serine protease. In mammalian cells, the heterodimerization of ClpP and its AAA+ ClpX chaperone results in a complex called ClpXP, which has a relevant role in protein homeostasis and in maintaining mitochondrial functionality through the degradation of mitochondrial misfolded or damaged proteins. Recent studies demonstrate that ClpP is upregulated in primary and metastatic human tumors, supports tumor cell proliferation, and its overexpression desensitizes cells to cisplatin. Interestingly, small modulators of ClpP activity, both activators and inhibitors, are able to impair oxidative phosphorylation in cancer cells and to induce apoptosis. This review provides an overview of the role of ClpP in regulating mitochondrial functionality, in supporting tumor cell proliferation and cisplatin resistance; finally, we discuss whether this protease could represent a new prognostic marker and therapeutic target for the treatment of cancer.

Progresses in both basic research and clinical trials of NAD+ in Parkinson's disease

The decline of nicotinamide adenine dinucleotide (NAD+) levels is a hallmark of aging in multiple organisms and tissues, including the human brain. Hence, agents that increase intracellular NAD+ could have beneficial effects in aging and age-related neurodegenerative diseases. Disturbances in NAD+ metabolism have also been observed in Parkinson's disease (PD), supporting a link between neuronal bioenergetics failure and disease pathogenesis. Here, we review emerging findings revealing key roles for NAD+ and related metabolites in experimental models of dopaminergic neurodegeneration and in PD patients. We discuss how increased NAD+ levels might ameliorate disease phenotypes by restoring neuronal mitochondrial energy metabolism, promoting cellular proteostasis, and modulating the immune system. Finally, we describe ongoing clinical trials targeting NAD+ in PD and highlight the need for further investigations to better delineate the association between NAD+, brain aging and disease, and optimal strategies for efficiently and safely raising NAD+ levels. A more comprehensive understanding of the basic mechanisms linking NAD+, energy metabolism, and PD, and of the impact of life-long NAD+ targeting strategies, are critical to inform future clinical applications.

Neurons and Glia Interplay in α-Synucleinopathies

Accumulation of the neuronal presynaptic protein alpha-synuclein within proteinaceous inclusions represents the key histophathological hallmark of a spectrum of neurodegenerative disorders, referred to by the umbrella term a-synucleinopathies. Even though alpha-synuclein is expressed predominantly in neurons, pathological aggregates of the protein are also found in the glial cells of the brain. In Parkinson's disease and dementia with Lewy bodies, alpha-synuclein accumulates mainly in neurons forming the Lewy bodies and Lewy neurites, whereas in multiple system atrophy, the protein aggregates mostly in the glial cytoplasmic inclusions within oligodendrocytes. In addition, astrogliosis and microgliosis are found in the synucleinopathy brains, whereas both astrocytes and microglia internalize alpha-synuclein and contribute to the spread of pathology. The mechanisms underlying the pathological accumulation of alpha-synuclein in glial cells that under physiological conditions express low to non-detectable levels of the protein are an area of intense research. Undoubtedly, the presence of aggregated alpha-synuclein can disrupt glial function in general and can contribute to neurodegeneration through numerous pathways. Herein, we summarize the current knowledge on the role of alpha-synuclein in both neurons and glia, highlighting the contribution of the neuron-glia connectome in the disease initiation and progression, which may represent potential therapeutic target for a-synucleinopathies.

Mitochondrial unfolded protein response: A novel pathway in metabolism and immunity

Mitochondrial unfolded protein response (mitoUPR) is a mitochondria stress response to maintain mitochondrial proteostasis during stress. Increasing evidence suggests that mitoUPR participates in diverse physiological processes especially metabolism and immunity. Although mitoUPR regulates metabolism in many aspects, it is mainly reflected in the regulation of energy metabolism. During stress, mitoUPR alters energy metabolism via suppressing oxidative phosphorylation (OXPHOS) or increasing glycolysis. MitoUPR also alters energy metabolism and regulates diverse metabolic diseases such as diabetes, cancers, fatty liver and obesity. In addition, mitoUPR also participates in immune process during stress. MitoUPR can induce innate immune response during various infections and may regulate inflammatory response during diverse inflammations. Considering the pleiotropic actions of mitoUPR, mitoUPR may supply diverse therapeutic targets for metabolic diseases and immune diseases.

Human ClpP protease, a promising therapy target for diseases of mitochondrial dysfunction

Human caseinolytic protease P (HsClpP), an ATP-dependent unfolding peptidase protein in the mitochondrial matrix, controls protein quality, regulates mitochondrial metabolism, and maintains the integrity and enzyme activity of the mitochondrial respiratory chain (RC). Studies show that abnormalities in HsClpP lead to mitochondrial dysfunction and various human diseases. In this review, we provide a comprehensive overview of the structure and biological function of HsClpP, and the involvement of its dysexpression or mutation in mitochondria for a panel of important human diseases. We also summarize the structural types and binding modes of known HsClpP modulators. Finally, we discuss the challenges and future directions of HsClpP targeting as promising approach for the treatment of human diseases of mitochondrial origin.

Recent advances in bioanalytical methods to measure proteome stability in cells

This review summarizes recent bioanalytical methods for measuring and profiling protein stability in cells on a proteome-wide scale, which can provide insights for proteostasis and associated diseases.

Keeping α-Synuclein at Bay: A More Active Role of Molecular Chaperones in Preventing Mitochondrial Interactions and Transition to Pathological States?

The property of molecular chaperones to dissolve protein aggregates of Parkinson-related α-synuclein has been known for some time. Recent findings point to an even more active role of molecular chaperones preventing the transformation of α-synuclein into pathological states subsequently leading to the formation of Lewy bodies, intracellular inclusions containing protein aggregates as well as broken organelles found in the brains of Parkinson's patients. In parallel, a short motif around Tyr39 was identified as being crucial for the aggregation of α-synuclein. Interestingly, this region is also one of the main segments in contact with a diverse pool of molecular chaperones. Further, it could be shown that the inhibition of the chaperone:α-synuclein interaction leads to a binding of α-synuclein to mitochondria, which could also be shown to lead to mitochondrial membrane disruption as well as the possible proteolytic processing of α-synuclein by mitochondrial proteases. Here, we will review the current knowledge on the role of molecular chaperones in the regulation of physiological functions as well as the direct consequences of impairing these interactions-i.e., leading to enhanced mitochondrial interaction and consequential mitochondrial breakage, which might mark the initial stages of the structural transition of α-synuclein towards its pathological states.

Intracellular A53T Mutant α-Synuclein Impairs Adult Hippocampal Newborn Neuron Integration

Dendritic dysfunction is an early event in α-synuclein (α-syn) mediated neurodegeneration. Altered postsynaptic potential and loss of dendritic spines have been observed in different in vitro and in vivo models of synucleinopathies. The integration of newborn neurons into the hippocampus offers the possibility to study dendrite and spine formation in an adult environment. Specifically, survival of hippocampal adult newborn neurons is regulated by synaptic input and was reduced in a mouse model transgenic for human A53T mutant α-syn. We thus hypothesized that dendritic integration of newborn neurons is impaired in the adult hippocampus of A53T mice. We analyzed dendritic morphology of adult hippocampal neurons 1 month after retroviral labeling. Dendrite length was unchanged in the dentate gyrus of A53T transgenic mice. However, spine density and mushroom spine density of newborn neurons were severely decreased. In this mouse model, transgenic α-syn was expressed both within newborn neurons and within their environment. To specifically determine the cell autonomous effects, we analyzed cell-intrinsic overexpression of A53T α-syn using a retrovirus. Since A53T α-syn overexpressing newborn neurons exhibited decreased spine density 1 month after labeling, we conclude that cell-intrinsic A53T α-syn impairs postsynaptic integration of adult hippocampal newborn neurons. Our findings further support the role of postsynaptic degeneration as an early feature in synucleinopathies and provide a model system to study underlying mechanisms.

The Mitochondrial Unfolded Protein Response: A Hinge Between Healthy and Pathological Aging

Aging is the time-dependent functional decline that increases the vulnerability to different forms of stress, constituting the major risk factor for the development of neurodegenerative diseases. Dysfunctional mitochondria significantly contribute to aging phenotypes, accumulating particularly in post-mitotic cells, including neurons. To cope with deleterious effects, mitochondria feature different mechanisms for quality control. One such mechanism is the mitochondrial unfolded protein response (UPRMT), which corresponds to the transcriptional activation of mitochondrial chaperones, proteases, and antioxidant enzymes to repair defective mitochondria. Transcription of target UPRMT genes is epigenetically regulated by Histone 3-specific methylation. Age-dependency of this regulation could explain a differential UPRMT activity in early developmental stages or aged organisms. At the same time, precise tuning of mitochondrial stress responses is crucial for maintaining neuronal homeostasis. However, compared to other mitochondrial and stress response programs, the role of UPRMT in neurodegenerative disease is barely understood and studies in this topic are just emerging. In this review, we document the reported evidence characterizing the evolutionarily conserved regulation of the UPRMT and summarize the recent advances in understanding the role of the pathway in neurodegenerative diseases and aging.

Neuroprotection Against Parkinson's Disease Through the Activation of Akt/GSK3β Signaling Pathway by Tovophyllin A

Parkinson’s disease (PD) is one of the most prevalent and life-threatening neurodegenerative disease and mainly characterized by lack of sufficient dopaminergic neurons in the substantia nigra pars compacta (SNc). Although current treatments help to alleviate clinical symptoms, effective therapies preventing neuronal loss remain scarce. Tovophyllin A (TA), one of the xanthones extracted from Garcinia mangostana L. (GM), has recently been reported to play a beneficial role in the therapy of neurodegenerative diseases. In our research, we explored whether TA has protective effects on dopaminergic neurons in PD models. We found that TA significantly reduced apoptotic cell death in primary cortical neurons treated with 1-methyl-4-phenyl pyridinium (MPP⁺) or paraquat (PQ) in the in vitro PD model. In an in vivo acute PD model induced by 1-methyl4-phenyl-1,2,3,5-tetrahydropyridine (MPTP) treatment, TA also attenuated the resulting behavioral dysfunctions and dopaminergic neuron loss. In the collected brain tissues, TA increased the phosphorylation of Akt and GSK-3β, which may be related to TA-mediated dopaminergic neuronal protective effects. In summary, our results illustrated that TA is a powerful cytoprotective agent for dopaminergic neurons in the MPTP-induced PD model, suggesting TA as a possible therapeutic candidate for PD.

Overexpression of PGC-1α influences the mitochondrial unfolded protein response (mtUPR) induced by MPP+ in human SH-SY5Y neuroblastoma cells

Parkinson’s disease (PD) is a common dyskinesia disease, the mitochondrial unfolded protein response (mtUPR) may be directly or indirectly involved in the occurrence and development of PD, although the exact mechanism is unclear. We established a dopaminergic neuronal-like cell model of PD, by overexpression of PGC-1α to detect evaluate the expression of proteases and molecular chaperones of involved in the mtUPR, as well as the expression of PGC-1α and LRPPRC, illustrated the distribution of LRPPRC. Remarkably, the mtUPR activation reached maximal at 24 h after MPP⁺ treatment in SH-SY5Y cells, which the protein and transcription levels of the proteases and molecular chaperones reached maximal. The proteases and molecular chaperones were significantly increased when overexpressed PGC-1α, which indicated that PGC-1α overexpression activated the mtUPR, and PGC-1α had a protective effect on SH-SY5Y cells. The expression levels of PGC-1α and LRPPRC were significantly improved in the PGC-1α overexpression groups. LRPPRC was markedly reduced in the nucleus, suggesting that PGC-1α overexpression may play a protective role to the mitochondria through LRPPRC. Our finding indicates that overexpression of PGC-1α may activate mtUPR, reducing the oxidative stress injury induced by MPP⁺ through LRPPRC signaling, thus maintain mitochondrial homeostasis.

Protein trapping leads to altered synaptic proteostasis in synucleinopathies

Parkinson's disease (PD) is associated with the accumulation of alpha-synuclein (aSyn) in intracellular inclusions known as Lewy bodies and Lewy neurites. Under physiological conditions, aSyn is found at the presynaptic terminal and exists in a dynamic equilibrium between soluble, membrane-associated and aggregated forms. Emerging evidence suggests that, under pathological conditions, aSyn begins to accumulate and acquire a toxic function at the synapse, impairing their normal function and connectivity. However, the precise molecular mechanisms linking aSyn accumulation and synaptic dysfunction are still elusive. Here, we provide an overview of our current findings and discuss the hypothesis that certain aSyn aggregates may interact with proteins with whom aSyn normally does not interact with, thereby trapping them and preventing them from performing their normal functions in the cell. We posit that such abnormal interactions start to occur during the prodromal stages of PD, eventually resulting in the overt manifestation of clinical features. Therefore, understanding the nature and behaviour of toxic aSyn species and their contribution to aSyn-mediated toxicity is crucial for the development of therapeutic strategies capable of modifying disease progression in PD and other synucleinopathies.

Mitochondrial dysfunction plays a key role in the development of neurodegenerative diseases in diabetes

Mitochondria have an essential function in cell survival due to their role in bioenergetics, reactive oxygen species generation, calcium buffering, and other metabolic activities. Mitochondrial dysfunctions are commonly found in neurodegenerative diseases (NDs), and diabetes is a risk factor for NDs. However, the role of mitochondria in diabetic neurodegeneration is still unclear. In the present study, we review the latest evidence on the role of mitochondrial dysfunctions in the development of diabetes-related NDs and the underlying molecular mechanisms. Hypoglycemic agents, especially metformin, have been proven to have neuroprotective effects in the treatment of diabetes, in which mitochondria could act as one of the underlying mechanisms. Other hypoglycemic agents, including thiazolidinediones (TZDs), dipeptidyl peptidase 4 (DPP-4) inhibitors, and glucagon-like peptide 1 (GLP-1) receptor agonists, have gained more attention because of their beneficial effects on NDs, presumably by improving mitochondrial function. Our review highlights the notion that mitochondria could be a promising therapeutic target in the treatment of NDs in patients with diabetes.

Alpha-Synuclein deficiency ameliorates chronic methamphetamine induced neurodegeneration in mice

The α-Synuclein (α-syn) and tau have synergistic effects on neurodegenerative diseases induced by environmental factors or genetic mutation. Thus, we investigated the role of α-syn and tau in neurodegeneration induced by chronic methamphetamine (METH) exposure (1.0∼20.0 mg/kg/d body weight, for 14 consecutive days). Here, we present a mice model with evidences of α-syn and tau participating in toxicology in chronic METH. METH increased α-syn level in the stratum oriens, pyramidal layer, stratum radiatum and stratum moleculare of hippocampal CA1, CA2 and CA3, polymorph layer of hippocampal dentate gyrus (DG), and substantia nigra (SN). The subcellular locations of the upregulated α-syn were mainly found in mitochondria and axons. The METH upregulated α-syn may directly induce mitochondrial damage, myelin sheath destruction, and synaptic failure. Also, the excess α-syn might indirectly promote tau phosphorylation through tau kinase GSK3β and CDK5, leading to microtubule depolymerization and eventually fusion deficit of autophagosome and lysosome. In the in vitro experiment, the autophagic vacuoles failed to fuse with the lysosome. The neuropathology induced by both the direct and indirect effects of α-syn could be alleviated by α-syn knockout. Taking together, these results indicate that the α-syn mediates the neurodegenerative process induced by chronic METH and that reducing α-syn might be a potential approach to protect the toxic effects of METH and also be, to a broader view, of therapeutic value in neurodegenerative diseases.