Evidence for Mitochondrial UPR Gene Activation in Familial and Sporadic Alzheimer's Disease

Structural and Functional Alterations in Mitochondria-Associated Membranes (MAMs) and in Mitochondria Activate Stress Response Mechanisms in an In Vitro Model of Alzheimer's Disease

Alzheimer’s disease (AD) is characterized by the accumulation of extracellular plaques composed by amyloid-β (Aβ) and intracellular neurofibrillary tangles of hyperphosphorylated tau. AD-related neurodegenerative mechanisms involve early changes of mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs) and impairment of cellular events modulated by these subcellular domains. In this study, we characterized the structural and functional alterations at MAM, mitochondria, and ER/microsomes in a mouse neuroblastoma cell line (N2A) overexpressing the human amyloid precursor protein (APP) with the familial Swedish mutation (APPswe). Proteins levels were determined by Western blot, ER-mitochondria contacts were quantified by transmission electron microscopy, and Ca²⁺ homeostasis and mitochondria function were analyzed using fluorescent probes and Seahorse assays. In this in vitro AD model, we found APP accumulated in MAM and mitochondria, and altered levels of proteins implicated in ER-mitochondria tethering, Ca²⁺ signaling, mitochondrial dynamics, biogenesis and protein import, as well as in the stress response. Moreover, we observed a decreased number of close ER-mitochondria contacts, activation of the ER unfolded protein response, reduced Ca²⁺ transfer from ER to mitochondria, and impaired mitochondrial function. Together, these results demonstrate that several subcellular alterations occur in AD-like neuronal cells, which supports that the defective ER-mitochondria crosstalk is an important player in AD physiopathology.

Role of PITRM1 in Mitochondrial Dysfunction and Neurodegeneration

Mounting evidence shows a link between mitochondrial dysfunction and neurodegenerative disorders, including Alzheimer Disease. Increased oxidative stress, defective mitodynamics, and impaired oxidative phosphorylation leading to decreased ATP production, can determine synaptic dysfunction, apoptosis, and neurodegeneration. Furthermore, mitochondrial proteostasis and the protease-mediated quality control system, carrying out degradation of potentially toxic peptides and misfolded or damaged proteins inside mitochondria, are emerging as potential pathogenetic mechanisms. The enzyme pitrilysin metallopeptidase 1 (PITRM1) is a key player in these processes; it is responsible for degrading mitochondrial targeting sequences that are cleaved off from the imported precursor proteins and for digesting a mitochondrial fraction of amyloid beta (Aβ). In this review, we present current evidence obtained from patients with PITRM1 mutations, as well as the different cellular and animal models of PITRM1 deficiency, which points toward PITRM1 as a possible driving factor of several neurodegenerative conditions. Finally, we point out the prospect of new diagnostic and therapeutic approaches.

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.

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 function in development and disease

Mitochondria are organelles with vital functions in almost all eukaryotic cells. Often described as the cellular 'powerhouses' due to their essential role in aerobic oxidative phosphorylation, mitochondria perform many other essential functions beyond energy production. As signaling organelles, mitochondria communicate with the nucleus and other organelles to help maintain cellular homeostasis, allow cellular adaptation to diverse stresses, and help steer cell fate decisions during development. Mitochondria have taken center stage in the research of normal and pathological processes, including normal tissue homeostasis and metabolism, neurodegeneration, immunity and infectious diseases. The central role that mitochondria assume within cells is evidenced by the broad impact of mitochondrial diseases, caused by defects in either mitochondrial or nuclear genes encoding for mitochondrial proteins, on different organ systems. In this Review, we will provide the reader with a foundation of the mitochondrial 'hardware', the mitochondrion itself, with its specific dynamics, quality control mechanisms and cross-organelle communication, including its roles as a driver of an innate immune response, all with a focus on development, disease and aging. We will further discuss how mitochondrial DNA is inherited, how its mutation affects cell and organismal fitness, and current therapeutic approaches for mitochondrial diseases in both model organisms and humans.

Interplay between Mitochondrial Protein Import and Respiratory Complexes Assembly in Neuronal Health and Degeneration

The fact that >99% of mitochondrial proteins are encoded by the nuclear genome and synthesised in the cytosol renders the process of mitochondrial protein import fundamental for normal organelle physiology. In addition to this, the nuclear genome comprises most of the proteins required for respiratory complex assembly and function. This means that without fully functional protein import, mitochondrial respiration will be defective, and the major cellular ATP source depleted. When mitochondrial protein import is impaired, a number of stress response pathways are activated in order to overcome the dysfunction and restore mitochondrial and cellular proteostasis. However, prolonged impaired mitochondrial protein import and subsequent defective respiratory chain function contributes to a number of diseases including primary mitochondrial diseases and neurodegeneration. This review focuses on how the processes of mitochondrial protein translocation and respiratory complex assembly and function are interlinked, how they are regulated, and their importance in health and disease.

Mitochondrial Dysfunction as a Driver of Cognitive Impairment in Alzheimer's Disease

Alzheimer’s disease (AD) is the most frequent cause of age-related neurodegeneration and cognitive impairment, and there are currently no broadly effective therapies. The underlying pathogenesis is complex, but a growing body of evidence implicates mitochondrial dysfunction as a common pathomechanism involved in many of the hallmark features of the AD brain, such as formation of amyloid-beta (Aβ) aggregates (amyloid plaques), neurofibrillary tangles, cholinergic system dysfunction, impaired synaptic transmission and plasticity, oxidative stress, and neuroinflammation, that lead to neurodegeneration and cognitive dysfunction. Indeed, mitochondrial dysfunction concomitant with progressive accumulation of mitochondrial Aβ is an early event in AD pathogenesis. Healthy mitochondria are critical for providing sufficient energy to maintain endogenous neuroprotective and reparative mechanisms, while disturbances in mitochondrial function, motility, fission, and fusion lead to neuronal malfunction and degeneration associated with excess free radical production and reduced intracellular calcium buffering. In addition, mitochondrial dysfunction can contribute to amyloid-β precursor protein (APP) expression and misprocessing to produce pathogenic fragments (e.g., Aβ1-40). Given this background, we present an overview of the importance of mitochondria for maintenance of neuronal function and how mitochondrial dysfunction acts as a driver of cognitive impairment in AD. Additionally, we provide a brief summary of possible treatments targeting mitochondrial dysfunction as therapeutic approaches for AD.

Generation of a humanized Aβ expressing mouse demonstrating aspects of Alzheimer's disease-like pathology

The majority of Alzheimer's disease (AD) cases are late-onset and occur sporadically, however most mouse models of the disease harbor pathogenic mutations, rendering them better representations of familial autosomal-dominant forms of the disease. Here, we generated knock-in mice that express wildtype human Aβ under control of the mouse App locus. Remarkably, changing 3 amino acids in the mouse Aβ sequence to its wild-type human counterpart leads to age-dependent impairments in cognition and synaptic plasticity, brain volumetric changes, inflammatory alterations, the appearance of Periodic Acid-Schiff (PAS) granules and changes in gene expression. In addition, when exon 14 encoding the Aβ sequence was flanked by loxP sites we show that Cre-mediated excision of exon 14 ablates hAβ expression, rescues cognition and reduces the formation of PAS granules.

Mitochondrial Dysfunction and Mitophagy Closely Cooperate in Neurological Deficits Associated with Alzheimer's Disease and Type 2 Diabetes

Alzheimer's disease (AD) and type 2 diabetes (T2D) are known to be correlated in terms of their epidemiology, histopathology, and molecular and biochemical characteristics. The prevalence of T2D leading to AD is approximately 50-70%. Moreover, AD is often considered type III diabetes because of the common risk factors. Uncontrolled T2D may affect the brain, leading to memory and learning deficits in patients. In addition, metabolic disorders and impaired oxidative phosphorylation in AD and T2D patients suggest that mitochondrial dysfunction is involved in both diseases. The dysregulation of pathways involved in maintaining mitochondrial dynamics, biogenesis and mitophagy are responsible for exacerbating the impact of hyperglycemia on the brain and neurodegeneration under T2D conditions. The first section of this review describes the recent views on mitochondrial dysfunction that connect these two disease conditions, as the pathways are observed to overlap. The second section of the review highlights the importance of different mitochondrial miRNAs (mitomiRs) involved in the regulation of mitochondrial dynamics and their association with the pathogenesis of T2D and AD. Therefore, targeting mitochondrial biogenesis and mitophagy pathways, along with the use of mitomiRs, could be a potent therapeutic strategy for T2D-related AD. The last section of the review highlights the known drugs targeting mitochondrial function for the treatment of both disease conditions.

Mitochondria-Targeted Therapeutics for Alzheimer's Disease: The Good, the Bad, the Potential

Significance: Alzheimer's disease (AD) is the leading cause of dementia. Thus far, 99.6% of clinical trials, including those targeting energy metabolism, have failed to exert disease-modifying efficacy. Altered mitochondrial function and disruption to the brain bioenergetic system have long-been documented as early events during the pathological progression of AD. Recent Advances: While therapeutic approaches that directly promote mitochondrial bioenergetic machinery or eliminate reactive oxygen species have exhibited limited translatability, emerging strategies targeting nonenergetic aspects of mitochondria provide novel therapeutic targets with the potential to modify AD risk and progression. Growing evidence also reveals a critical link between mitochondrial phenotype and neuroinflammation via metabolic reprogramming of glial cells. Critical Issues: Herein, we summarize major classes of mitochondrion-centered AD therapeutic strategies. In addition, the discrepancy in their efficacy when translated from preclinical models to clinical trials is addressed. Key factors that differentiate the responsiveness to bioenergetic interventions, including sex, apolipoprotein E genotype, and cellular diversity in the brain, are discussed. Future Directions: We propose that the future development of mitochondria-targeted AD therapeutics should consider the interactions between bioenergetics and other disease mechanisms, which may require cell-type-specific targeting to distinguish neurons and non-neuronal cells. Moreover, a successful strategy will likely include stratification by metabolic phenotype, which varies by sex and genetic risk profile and dynamically changes throughout the course of disease. As the network of mitochondrial integration expands across intracellular and systems level biology, assessment of intended, the good, versus unintended consequences, the bad, will be required to reach the potential of mitochondrial therapeutics.

Mind the Gap: Mitochondria and the Endoplasmic Reticulum in Neurodegenerative Diseases

The way organelles are viewed by cell biologists is quickly changing. For many years, these cellular entities were thought to be unique and singular structures that performed specific roles. However, in recent decades, researchers have discovered that organelles are dynamic and form physical contacts. In addition, organelle interactions modulate several vital biological functions, and the dysregulation of these contacts is involved in cell dysfunction and different pathologies, including neurodegenerative diseases. Mitochondria–ER contact sites (MERCS) are among the most extensively studied and understood juxtapositioned interorganelle structures. In this review, we summarise the major biological and ultrastructural dysfunctions of MERCS in neurodegeneration, with a particular focus on Alzheimer’s disease as well as Parkinson’s disease, amyotrophic lateral sclerosis and frontotemporal dementia. We also propose an updated version of the MERCS hypothesis in Alzheimer’s disease based on new findings. Finally, we discuss the possibility of MERCS being used as possible drug targets to halt cell death and neurodegeneration.

Mitochondrial unfolded protein response: An emerging pathway in human diseases

Mitochondrial unfolded protein response (UPR^mt) is a mitochondria stress response, which the transcriptional activation programs of mitochondrial chaperone proteins and proteases are initiated to maintain proteostasis in mitochondria. Additionally, the activation of UPR^mt delays aging and extends lifespan by maintaining mitochondrial proteostasis. Growing evidences suggests that UPR^mt plays an important role in diverse human diseases, especially ageing-related diseases. Therefore, this review focuses on the role of UPR^mt in ageing and ageing-related neurodegenerative diseases such as Alzheimer's disease, Huntington's disease and Parkinson's disease. The activation of UPR^mt and the high expression of UPR^mt components contribute to longevity extension. The activation of UPR^mt may ameliorate Alzheimer's disease, Parkinson's disease and Huntington's disease. Besides, UPR^mt is also involved in the occurrence and development of cancers and heart diseases. UPR^mt contributes to the growth, invasive and metastasis of cancers. UPR^mt has paradoxical roles in heart diseases. UPR^mt not only protects against heart damage, but may sometimes aggravates the development of heart diseases. Considering the pleiotropic actions of UPR^mt system, targeting UPR^mt pathway may be a potent therapeutic avenue for neurodegenerative diseases, cancers and heart diseases.

The Heart of the Alzheimer's: A Mindful View of Heart Disease

Purpose of Review: This review summarizes the current evidence for the involvement of proteotoxicity and protein quality control systems defects in diseases of the central nervous and cardiovascular systems. Specifically, it presents the commonalities between the pathophysiology of protein misfolding diseases in the heart and the brain. Recent Findings: The involvement of protein homeostasis dysfunction has been for long time investigated and accepted as one of the leading pathophysiological causes of neurodegenerative diseases. In cardiovascular diseases instead the mechanistic focus had been on the primary role of Ca2+ dishomeostasis, myofilament dysfunction as well as extracellular fibrosis, whereas no attention was given to misfolding of proteins as a pathogenetic mechanism. Instead, in the recent years, several contributions have shown protein aggregates in failing hearts similar to the ones found in the brain and increasing evidence have highlighted the crucial importance that proteotoxicity exerts via pre-amyloidogenic species in cardiovascular diseases as well as the prominent role of the cellular response to misfolded protein accumulation. As a result, proteotoxicity, unfolding protein response (UPR), and ubiquitin-proteasome system (UPS) have recently been investigated as potential key pathogenic pathways and therapeutic targets for heart disease. Summary: Overall, the current knowledge summarized in this review describes how the misfolding process in the brain parallels in the heart. Understanding the folding and unfolding mechanisms involved early through studies in the heart will provide new knowledge for neurodegenerative proteinopathies and may prepare the stage for targeted and personalized interventions.

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.

Modifiable, Non-Modifiable, and Clinical Factors Associated with Progression of Alzheimer's Disease

There is an extensive literature relating to factors associated with the development of Alzheimer's disease (AD), but less is known about factors which may contribute to its progression. This review examined the literature with regard to 15 factors which were suggested by PubMed search to be positively associated with the cognitive and/or neuropathological progression of AD. The factors were grouped as potentially modifiable (vascular risk factors, comorbidities, malnutrition, educational level, inflammation, and oxidative stress), non-modifiable (age at clinical onset, family history of dementia, gender, Apolipoprotein E ɛ4, genetic variants, and altered gene regulation), and clinical (baseline cognitive level, neuropsychiatric symptoms, and extrapyramidal signs). Although conflicting results were found for the majority of factors, a positive association was found in nearly all studies which investigated the relationship of six factors to AD progression: malnutrition, genetic variants, altered gene regulation, baseline cognitive level, neuropsychiatric symptoms, and extrapyramidal signs. Whether these or other factors which have been suggested to be associated with AD progression actually influence the rate of decline of AD patients is unclear. Therapeutic approaches which include addressing of modifiable factors associated with AD progression should be considered.

Loss of function of the mitochondrial peptidase PITRM1 induces proteotoxic stress and Alzheimer's disease-like pathology in human cerebral organoids

Mutations in pitrilysin metallopeptidase 1 (PITRM1), a mitochondrial protease involved in mitochondrial precursor processing and degradation, result in a slow-progressing syndrome characterized by cerebellar ataxia, psychotic episodes, and obsessive behavior, as well as cognitive decline. To investigate the pathogenetic mechanisms of mitochondrial presequence processing, we employed cortical neurons and cerebral organoids generated from PITRM1-knockout human induced pluripotent stem cells (iPSCs). PITRM1 deficiency strongly induced mitochondrial unfolded protein response (UPR^mt) and enhanced mitochondrial clearance in iPSC-derived neurons. Furthermore, we observed increased levels of amyloid precursor protein and amyloid β in PITRM1-knockout neurons. However, neither cell death nor protein aggregates were observed in 2D iPSC-derived cortical neuronal cultures. On the other hand, over time, cerebral organoids generated from PITRM1-knockout iPSCs spontaneously developed pathological features of Alzheimer's disease (AD), including the accumulation of protein aggregates, tau pathology, and neuronal cell death. Single-cell RNA sequencing revealed a perturbation of mitochondrial function in all cell types in PITRM1-knockout cerebral organoids, whereas immune transcriptional signatures were substantially dysregulated in astrocytes. Importantly, we provide evidence of a protective role of UPR^mt and mitochondrial clearance against impaired mitochondrial presequence processing and proteotoxic stress. Here, we propose a novel concept of PITRM1-linked neurological syndrome whereby defects of mitochondrial presequence processing induce an early activation of UPR^mt that, in turn, modulates cytosolic quality control pathways. Thus, our work supports a mechanistic link between mitochondrial function and common neurodegenerative proteinopathies.

Network Simulations Reveal Molecular Signatures of Vulnerability to Age-Dependent Stress and Tau Accumulation

Alzheimer’s disease (AD) is the leading cause of dementia and one of the most common causes of death worldwide. As an age-dependent multifactorial disease, the causative triggers of AD are rooted in spontaneous declines in cellular function and metabolic capacity with increases in protein stressors such as the tau protein. This multitude of age-related processes that cause neurons to change from healthy states to ones vulnerable to the damage seen in AD are difficult to simultaneously investigate and even more difficult to quantify. Here we aimed to diminish these gaps in our understanding of neuronal vulnerability in AD development by using simulation methods to theoretically quantify an array of cellular stress responses and signaling molecules. This temporally-descriptive molecular signature was produced using a novel multimethod simulation approach pioneered by our laboratory for biological research; this methodology combines hierarchical agent-based processes and continuous equation-based modeling in the same interface, all while maintaining intrinsic distributions that emulate natural biological stochasticity. The molecular signature was validated for a normal organismal aging trajectory using experimental longitudinal data from Caenorhabditis elegans and rodent studies. In addition, we have further predicted this aging molecular signature for cells impacted by the pathogenic tau protein, giving rise to distinct stress response conditions needed for cytoprotective aging. Interestingly, our simulation experiments showed that oxidative stress signaling (via daf-16 and skn-1 activities) does not substantially protect cells from all the early stressors of aging, but that it is essential in preventing a late-life degenerative cellular phenotype. Together, our simulation experiments aid in elucidating neurodegenerative triggers in the onset of AD for different genetic conditions. The long-term goal of this work is to provide more detailed diagnostic and prognostic tools for AD development and progression, and to provide more comprehensive preventative measures for this disease.

GSK3-ARC/Arg3.1 and GSK3-Wnt signaling axes trigger amyloid-β accumulation and neuroinflammation in middle-aged Shugoshin 1 mice

The cerebral amyloid‐β accumulation that begins in middle age is considered the critical triggering event in the pathogenesis of late‐onset Alzheimer's disease (LOAD). However, the molecular mechanism remains elusive. The Shugoshin 1 (Sgo1^−/+) mouse model, a model for mitotic cohesinopathy‐genomic instability that is observed in human AD at a higher rate, showed spontaneous accumulation of amyloid‐β in the brain at old age. With the model, novel insights into the molecular mechanism of LOAD development are anticipated. In this study, the initial appearance of cerebral amyloid‐β accumulation was determined as 15‐18 months of age (late middle age) in the Sgo1^−/+ model. The amyloid‐β accumulation was associated with unexpected GSK3α/β inactivation, Wnt signaling activation, and ARC/Arg3.1 accumulation, suggesting involvement of both the GSK3‐Arc/Arg3.1 axis and the GSK3‐Wnt axis. As observed in human AD brains, neuroinflammation with IFN‐γ expression occurred with amyloid‐β accumulation and was pronounced in the aged (24‐month‐old) Sgo1^−/+ model mice. AD‐relevant protein panels (oxidative stress defense, mitochondrial energy metabolism, and β‐oxidation and peroxisome) analysis indicated (a) early increases in Pdk1 and Phb in middle‐aged Sgo1^−/+ brains, and (b) misregulations in 32 proteins among 130 proteins tested in old age. Thus, initial amyloid‐β accumulation in the Sgo1^−/+ model is suggested to be triggered by GSK3 inactivation and the resulting Wnt activation and ARC/Arg3.1 accumulation. The model displayed characteristics and affected pathways similar to those of human LOAD including neuroinflammation, demonstrating its potential as a study tool for the LOAD development mechanism and for preclinical AD drug research and development.

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.

Does proteostasis get lost in translation? Implications for protein aggregation across the lifespan

Protein aggregation is a phenomenon of major relevance in neurodegenerative and neuromuscular disorders, cataracts, diabetes and many other diseases. Research has unveiled that proteins also aggregate in multiple tissues during healthy aging yet, the biological and biomedical relevance of this apparently asymptomatic phenomenon remains to be understood. It is known that proteome homeostasis (proteostasis) is maintained by a balanced protein synthesis rate, high protein synthesis accuracy, efficient protein folding and continual tagging of damaged proteins for degradation, suggesting that protein aggregation during healthy aging may be associated with alterations in both protein synthesis and the proteostasis network (PN) pathways. In particular, dysregulation of protein synthesis and alterations in translation fidelity are hypothesized to lead to the production of misfolded proteins which could explain the occurrence of age-related protein aggregation. Nevertheless, some data on this topic is controversial and the biological mechanisms that lead to widespread protein aggregation remain to be elucidated. We review the recent literature about the age-related decline of proteostasis, highlighting the need to build an integrated view of protein synthesis rate, fidelity and quality control pathways in order to better understand the proteome alterations that occur during aging and in age-related diseases.

Endoplasmic reticulum stress-mediated mitochondrial dysfunction in aged hearts

Aging impairs the mitochondrial electron transport chain (ETC), especially in interfibrillar mitochondria (IFM). Mitochondria are in close contact with the endoplasmic reticulum (ER). Induction of ER stress leads to ETC injury in adult heart mitochondria. We asked if ER stress contributes to the mitochondrial dysfunction during aging. Subsarcolemmal mitochondria (SSM) and IFM were isolated from 3, 18, and 24 mo. C57Bl/6 mouse hearts. ER stress progressively increased with age, especially in 24 mo. mice that manifest mitochondrial dysfunction. OXPHOS was decreased in 24 mo. IFM oxidizing complex I and complex IV substrates. Proteomic analysis showed that the content of multiple complex I subunits was decreased in IFM from 24 mo. hearts, but remained unchanged in in 18 mo. IFM without a decrease in OXPHOS. Feeding 24 mo. old mice with 4-phenylbutyrate (4-PBA) for two weeks attenuated the ER stress and improved mitochondrial function. These results indicate that ER stress contributes to the mitochondrial dysfunction in aged hearts. Attenuation of ER stress is a potential approach to improve mitochondrial function in aged hearts.

Mitochondria dysfunction in the pathogenesis of Alzheimer's disease: recent advances

Alzheimer's disease (AD) is one of the most prevalent neurodegenerative diseases, characterized by impaired cognitive function due to progressive loss of neurons in the brain. Under the microscope, neuronal accumulation of abnormal tau proteins and amyloid plaques are two pathological hallmarks in affected brain regions. Although the detailed mechanism of the pathogenesis of AD is still elusive, a large body of evidence suggests that damaged mitochondria likely play fundamental roles in the pathogenesis of AD. It is believed that a healthy pool of mitochondria not only supports neuronal activity by providing enough energy supply and other related mitochondrial functions to neurons, but also guards neurons by minimizing mitochondrial related oxidative damage. In this regard, exploration of the multitude of mitochondrial mechanisms altered in the pathogenesis of AD constitutes novel promising therapeutic targets for the disease. In this review, we will summarize recent progress that underscores the essential role of mitochondria dysfunction in the pathogenesis of AD and discuss mechanisms underlying mitochondrial dysfunction with a focus on the loss of mitochondrial structural and functional integrity in AD including mitochondrial biogenesis and dynamics, axonal transport, ER-mitochondria interaction, mitophagy and mitochondrial proteostasis.

The Role of Mitochondria in Neurodegenerative Diseases: the Lesson from Alzheimer's Disease and Parkinson's Disease

Although the pathogenesis of neurodegenerative diseases is still widely unclear, various mechanisms have been proposed and several pieces of evidence are supportive for an important role of mitochondrial dysfunction. The present review provides a comprehensive and up-to-date overview about the role of mitochondria in the two most common neurodegenerative disorders: Alzheimer's disease (AD) and Parkinson's disease (PD). Mitochondrial involvement in AD is supported by clinical features like reduced glucose and oxygen brain metabolism and by numerous microscopic and molecular findings, including altered mitochondrial morphology, impaired respiratory chain function, and altered mitochondrial DNA. Furthermore, amyloid pathology and mitochondrial dysfunction seem to be bi-directionally correlated. Mitochondria have an even more remarkable role in PD. Several hints show that respiratory chain activity, in particular complex I, is impaired in the disease. Mitochondrial DNA alterations, involving deletions, point mutations, depletion, and altered maintenance, have been described. Mutations in genes directly implicated in mitochondrial functioning (like Parkin and PINK1) are responsible for rare genetic forms of the disease. A close connection between alpha-synuclein accumulation and mitochondrial dysfunction has been observed. Finally, mitochondria are involved also in atypical parkinsonisms, in particular multiple system atrophy. The available knowledge is still not sufficient to clearly state whether mitochondrial dysfunction plays a primary role in the very initial stages of these diseases or is secondary to other phenomena. However, the presented data strongly support the hypothesis that whatever the initial cause of neurodegeneration is, mitochondrial impairment has a critical role in maintaining and fostering the neurodegenerative process.

Mitochondrial retrograde signalling in neurological disease

Neuronal mitochondrial dysfunction causes primary mitochondrial diseases and likely contributes to neurodegenerative diseases including Parkinson's and Alzheimer's disease. Mitochondrial dysfunction has also been documented in neurodevelopmental disorders such as tuberous sclerosis complex and autism spectrum disorder. Only symptomatic treatments exist for neurodevelopmental disorders, while neurodegenerative diseases are largely untreatable. Altered mitochondrial function activates mitochondrial retrograde signalling pathways, which enable signalling to the nucleus to reprogramme nuclear gene expression. In this review, we discuss the role of mitochondrial retrograde signalling in neurological diseases. We summarize how mitochondrial dysfunction contributes to neurodegenerative disease and neurodevelopmental disorders. Mitochondrial signalling mechanisms that have relevance to neurological disease are discussed. We then describe studies documenting retrograde signalling pathways in neurons and glia, and in animal models of neuronal mitochondrial dysfunction and neurological disease. Finally, we suggest how specific retrograde signalling pathways can be targeted to develop novel treatments for neurological diseases. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.

Mitochondrial cross-compartmental signalling to maintain proteostasis and longevity

Lifespan in eukaryotic species can be prolonged by shifting from cellular states favouring growth to those favouring maintenance and stress resistance. For instance, perturbations in mitochondrial oxidative phosphorylation (OXPHOS) can shift cells into this latter state and extend lifespan. Because mitochondria rely on proteins synthesized from nuclear as well as mitochondrial DNA, they need to constantly send and receive messages from other compartments of the cell in order to function properly and maintain homeostasis, and lifespan extension is often dependent on this cross-compartmental signalling. Here, we describe the mechanisms of bi-directional mitochondrial cross-compartmental signalling resulting in proteostasis and longevity. These proteostasis mechanisms are highly context-dependent, governed by the origin and extent of stress. Furthermore, we discuss the translatability of these mechanisms and explore therapeutic developments, such as the antibiotic studies targeting mitochondria or mitochondria-derived peptides as therapies for age-related diseases such as neurodegeneration and cancer. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.

Activation of Mitochondrial Unfolded Protein Response in SHSY5Y Expressing APP Cells and APP/PS1 Mice

Alzheimer disease (AD) is the most common form of dementia. Amyloid β-peptide (Aβ) deposition is a major neuropathologic feature of AD. When unfolded or misfolded proteins accumulate in mitochondria, the unfolded protein responses (UPRmt) is initiated. Numerous lines of evidence show that AD pathogenesis involves mitochondrial dysfunction. However little is known about whether the UPRmt is engaged in the process of AD development. In this study, we investigated the UPRmt in mouse and cell models of AD. We found that UPRmt was activated in the brain of 3 and 9 months old APP/PS1 mice, and in the SHSY5Y cells after exposure to Aβ_25–35, Aβ_25–35 triggered UPRmt in SHSY5Y cells could be attenuated upon administration of simvastatin or siRNA for HMGCS-1 to inhibit the mevalonate pathway, and or upon knocking down Serine palmitoyltransferase long chain subunit 1 (SPTLC-1) to lower sphingolipid biosynthesis. We observed that inhibition of UPRmt aggravated cytotoxic effects of Aβ_25–35 in SHSY5Y cells. Our research suggests that the UPRmt activation and two pathways necessary for this response, and further provides evidence for the cytoprotective effect of UPRmt during the AD process.

Metformin attenuates the D‑galactose‑induced aging process via the UPR through the AMPK/ERK1/2 signaling pathways

Age‑related hearing loss, also termed central presbycusis, is a progressive neurodegenerative disease; it is a devastating disorder that severely affects the quality of life of elderly individuals. Substantial evidence has indicated that oxidative stress and associated protein folding dysfunction have a marked influence on neurodegenerative diseases. In this study, we aimed to cells to investigate whether metformin protects against age‑related pathologies and to elucidate the underlying mechanisms; specifically, we focused on the role of unfolded protein response (UPR) via the AMPK/ERK1/2 signaling pathways. For this purpose, the biguanide compound, metformin, a medication widely used in the treatment of type 2 diabetes, was administered to rats in a model of mimetic aging. In addition, senescent PC12 were treated with metformin. Although it has been well established that UPR signaling is activated in response to cellular stress and is associated with the pathogenesis of neuronal deterioration, the detailed functions of the UPR in the auditory cortex remain unclear. We found that metformin treatment markedly affected the UPR and the AMPK/ERK1/2 signaling pathway, and maintained the auditory brainstem response (ABR) threshold during the aging process. The results indicated that the regulation of the UPR and AMPK/ERK1/2 signaling pathway by metformin significantly attenuated hearing loss, cell apoptosis and age‑related neurodegeneration. Reversing these harmful effects through the use of metformin suggests its involvement in restoring the antioxidant status and protein homeostasis related to the underlying pathology of presbycusis. The findings of this study may provide a better approach for the treatment of age‑related neurodegeneration diseases.

Dysfunctional Mitochondria and Mitophagy as Drivers of Alzheimer's Disease Pathogenesis

Neurons are highly specialized post-mitotic cells that are inherently dependent on mitochondria owing to their high bioenergetic demand. Mitochondrial dysfunction is therefore associated with various age-related neurodegenerative disorders such as Alzheimer's disease (AD), wherein accumulation of damaged and dysfunctional mitochondria has been reported as an early symptom further contributing to disease progression. In AD, impairment of mitochondrial function causes bioenergetic deficiency, intracellular calcium imbalance and oxidative stress, thereby aggravating the effect of Aβ and tau pathologies, leading to synaptic dysfunction, cognitive impairment and memory loss. Although there are reports suggesting intricate parallelism between mitochondrial dysfunction and AD pathologies such as Aβ aggregation and hyperphosphorylated tau accumulation, the factors that drive the pathogenesis of either are unclear. In addition, emerging evidence suggest that mitochondrial quality control (QC) mechanisms such as mitophagy are impaired in AD. As an important mitochondrial QC mechanism, mitophagy plays a critical role in maintaining neuronal health and function. Studies show that various proteins involved in mitophagy, mitochondrial dynamics, and mitochondrial biogenesis are affected in AD. Compromised mitophagy may also be attributed to impairment in autophagosome-lysosome fusion and defects in lysosomal acidification. Therapeutic interventions aiming to restore mitophagy functions can be used as a strategy for ameliorating AD pathogenesis. Recent evidence implicates the role of microglial activation via mitophagy induction in reducing amyloid plaque load. This review summarizes the current developments in the field of mitophagy and mitochondrial dysfunction in AD.

TDP-43 induces mitochondrial damage and activates the mitochondrial unfolded protein response

Mutations in or dys-regulation of the TDP-43 gene have been associated with TDP-43 proteinopathy, a spectrum of neurodegenerative diseases including Frontotemporal Lobar Degeneration (FTLD) and Amyotrophic Lateral Sclerosis (ALS). The underlying molecular and cellular defects, however, remain unclear. Here, we report a systematic study combining analyses of patient brain samples with cellular and animal models for TDP-43 proteinopathy. Electron microscopy (EM) analyses of patient samples revealed prominent mitochondrial impairment, including abnormal cristae and a loss of cristae; these ultrastructural changes were consistently observed in both cellular and animal models of TDP-43 proteinopathy. In these models, increased TDP-43 expression induced mitochondrial dysfunction, including decreased mitochondrial membrane potential and elevated production of reactive oxygen species (ROS). TDP-43 expression suppressed mitochondrial complex I activity and reduced mitochondrial ATP synthesis. Importantly, TDP-43 activated the mitochondrial unfolded protein response (UPR^mt) in both cellular and animal models. Down-regulating mitochondrial protease LonP1 increased mitochondrial TDP-43 levels and exacerbated TDP-43-induced mitochondrial damage as well as neurodegeneration. Together, our results demonstrate that TDP-43 induced mitochondrial impairment is a critical aspect in TDP-43 proteinopathy. Our work has not only uncovered a previously unknown role of LonP1 in regulating mitochondrial TDP-43 levels, but also advanced our understanding of the pathogenic mechanisms for TDP-43 proteinopathy. Our study suggests that blocking or reversing mitochondrial damage may provide a potential therapeutic approach to these devastating diseases.

TDP-43 proteinopathy is a group of fatal neurological diseases. Here, we report a systematic examination of the role of mitochondrial damage in TDP-43 proteinopathy using patient brain tissues, as well as cellular and animal models. Our data show that TDP-43 induces severe mitochondrial damage, accompanied by activation of UPR^mt in both cellular and animal models of TDP-43 proteinopathy. LonP1, one of the key mitochondrial proteases in UPR^mt, protects against TDP-43 induced cytotoxicity and neurodegeneration. Our study uncovers LonP1 as a modifier gene for TDP-43 proteinopathy and suggests protecting against or reversing mitochondrial damage as a potential therapeutic approach to these neurodegenerative disorders.

Mutant huntingtin inhibits the mitochondrial unfolded protein response by impairing ABCB10 mRNA stability

Numerous studies have shown that mitochondrial dysfunction contributes to consequential phenotypes of Huntington's disease (HD), a fatal and inherited neurodegenerative disease caused by the expanded CAG repeats in the N-terminus of the huntingtin (Htt) gene. To maintain proper function, mitochondria develop a dedicated protein quality control mechanism by activating a stress response termed the mitochondrial unfolded protein response (UPR^mt). Defects in the UPR^mt have been linked to aging and are also associated with neurodegenerative diseases. However, little is known about the role of the UPR^mt in HD. In this study, we find that ABCB10, a mitochondrial transporter involved in the UPR^mt pathway, is downregulated in HD mouse striatal cells, HD patient fibroblasts, and HD R6/2 mice. Deletion of ABCB10 causes increased mitochondrial reactive oxygen species (ROS) production and cell death, whereas overexpression of ABCB10 reduces these aberrant events. Moreover, the mitochondrial chaperone HSP60 and mitochondrial protease Clpp, two well-established markers of the UPR^mt, are reduced in the in vitro ABCB10-deficienct HD models. CHOP, a key transcription factor of HSP60 and Clpp, is regulated by ABCB10 in HD mouse striatal cells. Furthermore, we find that mutant huntingtin (mtHtt) inhibits the mtUPR by impairing ABCB10 mRNA stability. These findings demonstrate a suppression of the UPR^mt by mtHtt, suggesting that disturbance of mitochondrial protein quality control may contribute to the pathogenesis of HD.

UPRmt regulation and output: a stress response mediated by mitochondrial-nuclear communication

The mitochondrial network is not only required for the production of energy, essential cofactors and amino acids, but also serves as a signaling hub for innate immune and apoptotic pathways. Multiple mechanisms have evolved to identify and combat mitochondrial dysfunction to maintain the health of the organism. One such pathway is the mitochondrial unfolded protein response (UPRmt), which is regulated by the mitochondrial import efficiency of the transcription factor ATFS-1 in C. elegans and potentially orthologous transcription factors in mammals (ATF4, ATF5, CHOP). Upon mitochondrial dysfunction, import of ATFS-1 into mitochondria is reduced, allowing it to be trafficked to the nucleus where it promotes the expression of genes that promote survival and recovery of the mitochondrial network. Here, we discuss recent findings underlying UPRmt signal transduction and how this adaptive transcriptional response may interact with other mitochondrial stress response pathways.

Enhancing mitochondrial proteostasis reduces amyloid-β proteotoxicity

Alzheimer's disease is a common and devastating disease characterized by aggregation of the amyloid-β peptide. However, we know relatively little about the underlying molecular mechanisms or how to treat patients with Alzheimer's disease. Here we provide bioinformatic and experimental evidence of a conserved mitochondrial stress response signature present in diseases involving amyloid-β proteotoxicity in human, mouse and Caenorhabditis elegans that involves the mitochondrial unfolded protein response and mitophagy pathways. Using a worm model of amyloid-β proteotoxicity, GMC101, we recapitulated mitochondrial features and confirmed that the induction of this mitochondrial stress response was essential for the maintenance of mitochondrial proteostasis and health. Notably, increasing mitochondrial proteostasis by pharmacologically and genetically targeting mitochondrial translation and mitophagy increases the fitness and lifespan of GMC101 worms and reduces amyloid aggregation in cells, worms and in transgenic mouse models of Alzheimer's disease. Our data support the relevance of enhancing mitochondrial proteostasis to delay amyloid-β proteotoxic diseases, such as Alzheimer's disease.

Repairing Mitochondrial Dysfunction in Disease

Mitochondria are essential organelles for many aspects of cellular homeostasis, including energy harvesting through oxidative phosphorylation. Alterations of mitochondrial function not only impact on cellular metabolism but also critically influence whole-body metabolism, health, and life span. Diseases defined by mitochondrial dysfunction have expanded from rare monogenic disorders in a strict sense to now also include many common polygenic diseases, including metabolic, cardiovascular, neurodegenerative, and neuromuscular diseases. This has led to an intensive search for new therapeutic and preventive strategies aimed at invigorating mitochondrial function by exploiting key components of mitochondrial biogenesis, redox metabolism, dynamics, mitophagy, and the mitochondrial unfolded protein response. As such, new findings linking mitochondrial function to the progression or outcome of this ever-increasing list of diseases has stimulated the discovery and development of the first true mitochondrial drugs, which are now entering the clinic and are discussed in this review.

The burden of trisomy 21 disrupts the proteostasis network in Down syndrome

Down syndrome (DS) is a genetic disorder caused by trisomy of chromosome 21. Abnormalities in chromosome number have the potential to lead to disruption of the proteostasis network (PN) and accumulation of misfolded proteins. DS individuals suffer from several comorbidities, and we hypothesized that disruption of proteostasis could contribute to the observed pathology and decreased cell viability in DS. Our results confirm the presence of a disrupted PN in DS, as several of its elements, including the unfolded protein response, chaperone system, and proteasomal degradation exhibited significant alterations compared to euploid controls in both cell and mouse models. Additionally, when cell models were treated with compounds that promote disrupted proteostasis, we observed diminished levels of cell viability in DS compared to controls. Collectively our findings provide a cellular-level characterization of PN dysfunction in DS and an improved understanding of the potential pathogenic mechanisms contributing to disrupted cellular physiology in DS. Lastly, this study highlights the future potential of designing therapeutic strategies that mitigate protein quality control dysfunction.

Locus coeruleus cellular and molecular pathology during the progression of Alzheimer's disease

A major feature of Alzheimer’s disease (AD) is the loss of noradrenergic locus coeruleus (LC) projection neurons that mediate attention, memory, and arousal. However, the extent to which the LC projection system degenerates during the initial stages of AD is still under investigation. To address this question, we performed tyrosine hydroxylase (TH) immunohistochemistry and unbiased stereology of noradrenergic LC neurons in tissue harvested postmortem from subjects who died with a clinical diagnosis of no cognitive impairment (NCI), amnestic mild cognitive impairment (aMCI, a putative prodromal AD stage), or mild/moderate AD. Stereologic estimates of total LC neuron number revealed a 30% loss during the transition from NCI to aMCI, with an additional 25% loss of LC neurons in AD. Decreases in noradrenergic LC neuron number were significantly associated with worsening antemortem global cognitive function as well as poorer performance on neuropsychological tests of episodic memory, semantic memory, working memory, perceptual speed, and visuospatial ability. Reduced LC neuron numbers were also associated with increased postmortem neuropathology. To examine the cellular and molecular pathogenic processes underlying LC neurodegeneration in aMCI, we performed single population microarray analysis. These studies revealed significant reductions in select functional classes of mRNAs regulating mitochondrial respiration, redox homeostasis, and neuritic structural plasticity in neurons accessed from both aMCI and AD subjects compared to NCI. Specific gene expression levels within these functional classes were also associated with global cognitive deterioration and neuropathological burden. Taken together, these observations suggest that noradrenergic LC cellular and molecular pathology is a prominent feature of prodromal disease that contributes to cognitive dysfunction. Moreover, they lend support to a rational basis for targeting LC neuroprotection as a disease modifying strategy.

Regulator of Cell Cycle (RGCC) Expression During the Progression of Alzheimer's Disease

Unscheduled cell cycle reentry of postmitotic neurons has been described in cases of mild cognitive impairment (MCI) and Alzheimer's disease (AD) and may form a basis for selective neuronal vulnerability during disease progression. In this regard, the multifunctional protein regulator of cell cycle (RGCC) has been implicated in driving G1/S and G2/M phase transitions through its interactions with cdc/cyclin-dependent kinase 1 (cdk1) and is induced by p53, which mediates apoptosis in neurons. We tested whether RGCC levels were dysregulated in frontal cortex samples obtained postmortem from subjects who died with a clinical diagnosis of no cognitive impairment (NCI), MCI, or AD. RGCC mRNA and protein levels were upregulated by ∼50%-60% in MCI and AD compared to NCI, and RGCC protein levels were associated with poorer antemortem global cognitive performance in the subjects examined. To test whether RGCC might regulate neuronal cell cycle reentry and apoptosis, we differentiated neuronotypic PC12 cultures with nerve growth factor (NGF) followed by NGF withdrawal to induce abortive cell cycle activation and cell death. Experimental reduction of RGCC levels increased cell survival and reduced levels of the cdk1 target cyclin B1. RGCC may be a candidate cell cycle target for neuroprotection during the onset of AD.