A toxic mutant huntingtin species is resistant to selective autophagy

Novel prospects in targeting neurodegenerative disorders via autophagy

Protein aggregation occurs as a consequence of dysfunction in the normal cellular proteostasis, which leads to the accumulation of toxic fibrillar aggregates of certain proteins in the cell. Enhancing the activity of proteolytic pathways may serve as a way of clearing these aggregates in a cell, and consequently, autophagy has surfaced as a promising target for the treatment of neurodegenerative disorders. Several strategies involving small molecule compounds that stimulate autophagic pathway of cell have been discovered. However, despite many compounds having demonstrated favorable outcomes in experimental disease models, the translation of these findings into clinical benefits for patient's remains limited. Consequently, alternative strategies are actively being explored to effectively target neurodegeneration via autophagy modulation. Recently, newer approaches such as modulation of expression of autophagic genes have emerged as novel and interesting areas of research in this field, which hold promising potential in neuroprotection. Similarly, as discussed for the first time in this review, the use of autophagy-inducing nanoparticles by utilizing their physicochemical properties to stimulate the autophagic process, rather than relying on their role as drug carriers, offers a completely fresh avenue for targeting neurodegeneration without the risk of drug-associated adverse effects. This review provides fresh perspectives on developing autophagy-targeted therapies for neurodegenerative disorders. Additionally, it discusses the challenges and impediments of implementing these strategies to alleviate the pathogenesis of neurodegenerative disorders in clinical settings and highlights the prospects and directions of future research in this context.

MicroRNAs regulating autophagy: opportunities in treating neurodegenerative diseases

Neurodegenerative diseases (NDs) are increasingly prevalent in our aging population, imposing significant social and economic burdens. Currently, most ND patients receive only symptomatic treatment due to limited understanding of their underlying causes. Consequently, there is a pressing need for comprehensive research into the pathological mechanisms of NDs by both researchers and clinicians. Autophagy, a cellular mechanism responsible for maintaining cellular equilibrium by removing dysfunctional organelles and misfolded proteins, plays a vital role in cell health and is implicated in various diseases. MicroRNAs (miRNAs) exert influence on autophagy and hold promise for treating these diseases. These small oligonucleotides bind to the 3'-untranslated region (UTR) of target mRNAs, leading to mRNA silencing, degradation, or translation blockade. This review explores recent findings on the regulation of autophagy and autophagy-related genes by different miRNAs in various pathological conditions, including neurodegeneration and inflammation-related diseases. The recognition of miRNAs as key regulators of autophagy in human diseases has spurred investigations into pharmacological compounds and traditional medicines targeting these miRNAs in disease models. This has catalyzed a new wave of therapeutic interventions aimed at modulating autophagy.

The role of autophagy in Parkinson's disease: a gender difference overview

Recent studies have demonstrated dysregulation of the autophagy pathway in patients with Parkinson's disease (PD) and in animal models of PD, highlighting its emerging role in disease. In particular, several studies indicate that autophagy, which is an essential degradative process for the damaged protein homeostasis and the management of cell balance, can manifest significant variations according to gender. While some evidence suggests increased autophagic activation in men with PD, women may have distinct regulatory patterns. In this review, we examined the existing literature on gender differences in PD-associated autophagic processes, focusing on the autophagy related proteins (ATGs) and leucine rich repeat kinase 2 (LRRK2) genes. Also, this review would suggest that an in-depth understanding of these gender differences in autophagic processes could open new perspectives for personalized therapeutic strategies, promoting more effective and targeted management of PD.

Polyglutamine disorders: Pathogenesis and potential drug interventions

Polyglutamine/poly(Q) diseases are a group nine hereditary neurodegenerative disorders caused due to abnormally expanded stretches of CAG trinucleotide in functionally distinct genes. All human poly(Q) diseases are characterized by the formation of microscopically discernable poly(Q) positive aggregates, the inclusion bodies. These toxic inclusion bodies are responsible for the impairment of several cellular pathways such as autophagy, transcription, cell death, etc., that culminate in disease manifestation. Although, these diseases remain largely without treatment, extensive research has generated mounting evidences that various events of poly(Q) pathogenesis can be developed as potential drug targets. The present review article briefly discusses the key events of disease pathogenesis, model system-based investigations that support the development of effective therapeutic interventions against pathogenesis of human poly(Q) disorders, and a comprehensive list of pharmacological and bioactive compounds that have been experimentally shown to alleviate poly(Q)-mediated neurotoxicity. Interestingly, due to the common cause of pathogenesis, all poly(Q) diseases share etiology, thus, findings from one disease can be potentially extrapolated to other poly(Q) diseases as well.

Gut microbiome-based therapies for alleviating cognitive impairment: state of the field, limitations, and future perspectives

Cognitive impairment (CI) is a multifaceted neurological condition that can trigger negative emotions and a range of concurrent symptoms, imposing significant public health and economic burdens on society. Therefore, it is imperative to discover a remedy for CI. Nevertheless, the mechanisms behind the onset of this disease are multifactorial, which makes the search for effective amelioration difficult and complex, hindering the search for effective measures. Intriguingly, preclinical research indicates that gut microbiota by influencing brain function, plays an important role in the progression of CI. Furthermore, numerous preclinical studies have highlighted the potential of probiotics, prebiotics, fecal microbiota transplantation (FMT), and diet in modulating the gut microbiota, thereby ameliorating CI symptoms. This review provides a comprehensive evaluation of CI pathogenesis, emphasizing the contribution of gut microbiota disorders to CI development. It also summarizes and discusses current strategies and mechanisms centered on the synergistic role of gut microbiota modulation in the microbiota-gut-brain axis in CI development. Finally, problems with existing approaches are contemplated and the development of microbial modulation strategies as therapeutic approaches to promote and restore brain cognition is discussed. Further research considerations and directions are highlighted to provide ideas for future CI prevention and treatment strategies.

Redefining the Scope of Targeted Protein Degradation: Translational Opportunities in Hijacking the Autophagy-Lysosome Pathway

The advent of multi-specific targeted protein degradation (TPD) therapies has made it possible to drug targets that have long been considered to be inaccessible. For this reason, the foremost TPD modalities - molecular glues and proteolysis targeting chimeras (PROTACs) -have been widely adopted and developed in therapeutic programs across the pharmaceutical and biotechnology industries. While there are many clear advantages to these two approaches, there are also blind spots. Specifically, PROTACs and molecular glues are inherently mechanistically analogous in that targets of both are degraded via the 26s proteasome; however, not all disease-relevant targets are suitable for ubiquitin proteasome system (UPS)-mediated degradation. The alternative mammalian protein degradation pathway, the autophagy-lysosome system (or ALS), is capable of degrading targets that elude the UPS such as long-lived proteins, insoluble protein aggregates, and even abnormal organelles. Emerging TPD strategies- such as ATTEC, AUTAC, and LYTAC- take advantage of the substrate diversity of the ALS to greatly expand the clinical utility of TPD. In this Perspective, we will discuss the array of current TPD modalities, with a focus on critical evaluation of these novel ALS-mediated degradation techniques.

Emerging degrader technologies engaging lysosomal pathways

Targeted protein degradation (TPD) provides unprecedented opportunities for drug discovery. While the proteolysis-targeting chimera (PROTAC) technology has already entered clinical trials and changed the landscape of small-molecule drugs, new degrader technologies harnessing alternative degradation machineries, especially lysosomal pathways, have emerged and broadened the spectrum of degradable targets. We have recently proposed the concept of autophagy-tethering compounds (ATTECs) that hijack the autophagy protein microtubule-associated protein 1A/1B light chain 3 (LC3) for targeted degradation. Other groups also reported degrader technologies engaging lysosomal pathways through different mechanisms including AUTACs, AUTOTACs, LYTACs and MoDE-As. In this review, we analyse and discuss ATTECs along with other lysosomal-relevant degrader technologies. Finally, we will briefly summarize the current status of these degrader technologies and envision possible future studies.

Autophagy in health and disease: From molecular mechanisms to therapeutic target

Macroautophagy/autophagy is an evolutionally conserved catabolic process in which cytosolic contents, such as aggregated proteins, dysfunctional organelle, or invading pathogens, are sequestered by the double-membrane structure termed autophagosome and delivered to lysosome for degradation. Over the past two decades, autophagy has been extensively studied, from the molecular mechanisms, biological functions, implications in various human diseases, to development of autophagy-related therapeutics. This review will focus on the latest development of autophagy research, covering molecular mechanisms in control of autophagosome biogenesis and autophagosome-lysosome fusion, and the upstream regulatory pathways including the AMPK and MTORC1 pathways. We will also provide a systematic discussion on the implication of autophagy in various human diseases, including cancer, neurodegenerative disorders (Alzheimer disease, Parkinson disease, Huntington's disease, and Amyotrophic lateral sclerosis), metabolic diseases (obesity and diabetes), viral infection especially SARS-Cov-2 and COVID-19, cardiovascular diseases (cardiac ischemia/reperfusion and cardiomyopathy), and aging. Finally, we will also summarize the development of pharmacological agents that have therapeutic potential for clinical applications via targeting the autophagy pathway. It is believed that decades of hard work on autophagy research is eventually to bring real and tangible benefits for improvement of human health and control of human diseases.

Direct Analysis of Mitochondrial Damage Caused by Misfolded/Destabilized Proteins

Protein quality control is essential for cellular homeostasis. In this study, we examined the effect of improperly folded proteins that do not form amyloid fibrils on mitochondria, which play important roles in ATP production and cell death. First, we prepared domain 3 of the dengue envelope protein in wild type and four mutants with widely different biophysical properties in misfolded/aggregated or destabilized states. The effects of the different proteins were detected using fluorescence microscopy and Western blotting, which revealed that three of the five proteins disrupted both inner and outer membrane integrity, while the other two proteins, including the wild type, did not. Next, we examined the common characteristics of the proteins that displayed toxicity against mitochondria by measuring oligomer size, molten globule-like properties, and thermal stability. The common feature of all three toxic proteins was thermal instability. Therefore, our data strongly suggest that thermally unstable proteins generated in the cytosol can cause cellular damage by coming into direct contact with mitochondria. More importantly, we revealed that this damage is not amyloid-specific.

Therapeutic Strategies in Huntington’s Disease: From Genetic Defect to Gene Therapy

Despite the identification of an expanded CAG repeat on exon 1 of the huntingtin gene located on chromosome 1 as the genetic defect causing Huntington’s disease almost 30 years ago, currently approved therapies provide only limited symptomatic relief and do not influence the age of onset or disease progression rate. Research has identified various intricate pathogenic cascades which lead to neuronal degeneration, but therapies interfering with these mechanisms have been marked by many failures and remain to be validated. Exciting new opportunities are opened by the emerging techniques which target the mutant protein DNA and RNA, allowing for “gene editing”. Although some issues relating to “off-target” effects or immune-mediated side effects need to be solved, these strategies, combined with stem cell therapies and more traditional approaches targeting specific pathogenic cascades, such as excitotoxicity and bioavailability of neurotrophic factors, could lead to significant improvement of the outcomes of treated Huntington’s disease patients.

GRASP55 regulates the unconventional secretion and aggregation of mutant huntingtin

Recent studies demonstrated that the Golgi reassembly stacking proteins (GRASPs), especially GRASP55, regulate Golgi-independent unconventional secretion of certain cytosolic and transmembrane cargoes; however, the underlying mechanism remains unknown. Here, we surveyed several neurodegenerative disease-related proteins, including mutant huntingtin (Htt-Q74), superoxide dismutase 1 (SOD1), tau, and TAR DNA-binding protein 43 (TDP-43), for unconventional secretion; our results show that Htt-Q74 is most robustly secreted in a GRASP55-dependent manner. Using Htt-Q74 as a model system, we demonstrate that unconventional secretion of Htt is GRASP55 and autophagy dependent and is enhanced under stress conditions such as starvation and endoplasmic reticulum stress. Mechanistically, we show that GRASP55 facilitates Htt secretion by tethering autophagosomes to lysosomes to promote autophagosome maturation and subsequent lysosome secretion and by stabilizing p23/TMED10, a channel for translocation of cytoplasmic proteins into the lumen of the endoplasmic reticulum-Golgi intermediate compartment. Moreover, we found that GRASP55 levels are upregulated by various stresses to facilitate unconventional secretion, whereas inhibition of Htt-Q74 secretion by GRASP55 KO enhances Htt aggregation and toxicity. Finally, comprehensive secretomic analysis identified novel cytosolic cargoes secreted by the same unconventional pathway, including transgelin (TAGLN), multifunctional protein ADE2 (PAICS), and peroxiredoxin-1 (PRDX1). In conclusion, this study defines the pathway of GRASP55-mediated unconventional protein secretion and provides important insights into the progression of Huntington's disease.

Molecular Pathophysiological Mechanisms in Huntington's Disease

Huntington’s disease is an inherited neurodegenerative disease described 150 years ago by George Huntington. The genetic defect was identified in 1993 to be an expanded CAG repeat on exon 1 of the huntingtin gene located on chromosome 4. In the following almost 30 years, a considerable amount of research, using mainly animal models or in vitro experiments, has tried to unravel the complex molecular cascades through which the transcription of the mutant protein leads to neuronal loss, especially in the medium spiny neurons of the striatum, and identified excitotoxicity, transcriptional dysregulation, mitochondrial dysfunction, oxidative stress, impaired proteostasis, altered axonal trafficking and reduced availability of trophic factors to be crucial contributors. This review discusses the pathogenic cascades described in the literature through which mutant huntingtin leads to neuronal demise. However, due to the ubiquitous presence of huntingtin, astrocytes are also dysfunctional, and neuroinflammation may additionally contribute to Huntington’s disease pathology. The quest for therapies to delay the onset and reduce the rate of Huntington’s disease progression is ongoing, but is based on findings from basic research.

Development of mAb-based polyglutamine-dependent and polyglutamine length-independent huntingtin quantification assays with cross-site validation

Huntington's disease (HD) is caused by an expansion of the CAG trinucleotide repeat domain in the huntingtin gene that results in expression of a mutant huntingtin protein (mHTT) containing an expanded polyglutamine tract in the amino terminus. A number of therapeutic approaches that aim to reduce mHTT expression either locally in the CNS or systemically are in clinical development. We have previously described sensitive and selective assays that measure human HTT proteins either in a polyglutamine-independent (detecting both mutant expanded and non-expanded proteins) or in a polyglutamine length-dependent manner (detecting the disease-causing polyglutamine repeats) on the electrochemiluminescence Meso Scale Discovery detection platform. These original assays relied upon polyclonal antibodies. To ensure an accessible and sustainable resource for the HD field, we developed similar assays employing monoclonal antibodies. We demonstrate that these assays have equivalent sensitivity compared to our previous assays through the evaluation of cellular and animal model systems, as well as HD patient biosamples. We also demonstrate cross-site validation of these assays, allowing direct comparison of studies performed in geographically distinct laboratories.

Autophagy system as a potential therapeutic target for neurodegenerative diseases

Autophagy is an evolutionally conserved process by which cytoplasmic contents including protein aggregates and damaged organelles such as mitochondria and lysosomes, are sequestered by double-membrane structure, autophagosomes, and delivered to the lysosomes for degradation. Recently, considerable efforts have been made to reveal the role of autophagy in neurodegenerative diseases like Alzheimer's disease, Parkinson's disease and Huntington's disease. Impairment of autophagy aggravates the accumulation of misfolded protein and damaged organelles in neurons, while sufficient autophagic activity reduces such accumulation in nervous system and ameliorates the pathology. Here we summarize recent progress regarding the role of autophagy in several neurodegenerative diseases and the potential autophagy-associated therapies for them.

The Emerging Roles of Autophagy in Human Diseases

Autophagy, a process of cellular self-digestion, delivers intracellular components including superfluous and dysfunctional proteins and organelles to the lysosome for degradation and recycling and is important to maintain cellular homeostasis. In recent decades, autophagy has been found to help fight against a variety of human diseases, but, at the same time, autophagy can also promote the procession of certain pathologies, which makes the connection between autophagy and diseases complex but interesting. In this review, we summarize the advances in understanding the roles of autophagy in human diseases and the therapeutic methods targeting autophagy and discuss some of the remaining questions in this field, focusing on cancer, neurodegenerative diseases, infectious diseases and metabolic disorders.

Recent advances in understanding the role of proteostasis

Maintenance of a functional proteome is achieved through the mechanism of proteostasis that involves precise coordination between molecular machineries assisting a protein from its conception to demise. Although each organelle within a cell has its own set of proteostasis machinery, inter-organellar communication and cell non-autonomous signaling bring forth the multidimensional nature of the proteostasis network. Exposure to extrinsic and intrinsic stressors can challenge the proteostasis network, leading to the accumulation of aberrant proteins or a decline in the proteostasis components, as seen during aging and in several diseases. Here, we summarize recent advances in understanding the role of proteostasis and its regulation in aging and disease, including monogenetic and infectious diseases. We highlight some of the emerging as well as unresolved questions in proteostasis that need to be addressed to overcome pathologies associated with damaged proteins and to promote healthy aging.

RAB39B's role in membrane traffic, autophagy, and associated neuropathology

Neuropathological disorders are increasingly associated with dysfunctions in neuronal membrane traffic and autophagy, with defects among members of the Rab family of small GTPases implicated. Mutations in the human Xq28 localized gene RAB39B have been associated with X-linked neurodevelopmental defects including macrocephaly, intellectual disability, autism spectrum disorder (ASD), as well as rare cases of early-onset Parkinson's disease (PD). Despite the finding that RAB39B regulates GluA2 trafficking and could thus influence synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit composition, reasons for the wide-ranging neuropathological consequences associated with RAB39B defects have been unclear. Recent studies have now unraveled possible mechanisms underlying the neuropathological roles of this brain-enriched small GTPase. Studies in RAB39B knockout mice showed that RAB39B interacts with components of Class I phosphatidylinositol-3-kinase (PI3K) signaling. In its absence, the PI3K-AKT-mechanistic target of rapamycin signaling pathway in neural progenitor cells (NPCs) is hyperactivated, which promotes NPC proliferation, leading to macrocephaly and ASD. Pertaining to early-onset PD, a complex of C9orf72, Smith-Magenis syndrome chromosome region candidate 8 and WD repeat domain 41 that functions in autophagy has been identified as a guanine nucleotide exchange factor of RAB39B. Here, recent findings that have shed light on our mechanistic understanding of RAB39B's role in neurodevelopmental and neurodegenerative pathologies are reviewed. Caveats and unanswered questions are also discussed, and future perspectives outlined.

Phase Transition of Huntingtin: Factors and Pathological Relevance

Formation of intracellular mutant Huntingtin (mHtt) aggregates is a hallmark of Huntington’s disease (HD). The mechanisms underlying mHtt aggregation, however, are still not fully understood. A few recent studies indicated mHtt undergoes phase transition, bringing new clues to understand how mHtt aggregates assemble. Here in this mini review, we will summarize these findings with a focus on the factors that affect mHtt phase transition. We will also discuss the possible pathological roles of mHtt phase separation in HD.

Restoration of CTSD (cathepsin D) and lysosomal function in stroke is neuroprotective

Stroke is a leading cause of death and disability. The pathophysiological mechanisms associated with stroke are very complex and not fully understood. Lysosomal function has a vital physiological function in the maintenance of cellular homeostasis. In neurons, CTSD (cathepsin D) is an essential protease involved in the regulation of proteolytic activity of the lysosomes. Loss of CTSD leads to lysosomal dysfunction and accumulation of different cellular proteins implicated in neurodegenerative diseases. In cerebral ischemia, the role of CTSD and lysosomal function is not clearly defined. We used oxygen-glucose deprivation (OGD) in mouse cortical neurons and the middle cerebral artery occlusion (MCAO) model of stroke to assess the role of CTSD in stroke pathophysiology. Our results show a time-dependent decrease in CTSD protein levels and activity in the mouse brain after stroke and neurons following OGD, with concurrent defects in lysosomal function. We found that shRNA-mediated knockdown of CTSD in neurons is sufficient to cause lysosomal dysfunction. CTSD knockdown further aggravates lysosomal dysfunction and cell death in OGD-exposed neurons. Restoration of CTSD protein levels via lentiviral transduction increases CTSD activity in neurons and, thus, renders resistance to OGD-mediated defects in lysosomal function and cell death. This study indicates that CTSD-dependent lysosomal function is critical for maintaining neuronal survival in cerebral ischemia; thus, strategies focused on maintaining CTSD function in neurons are potentially novel therapeutic approaches to prevent neuronal death in stroke.Abbreviations: 3-MA: 3-methyladenine; ACTB: actin beta; AD: Alzheimer disease; ALS: amyotrophic lateral sclerosis; CQ: chloroquine; CTSB: cathepsin B; CTSD: cathepsin D; CTSL: cathepsin L; FTD: frontotemporal dementia, HD: Huntington disease; LAMP1: lysosomal associated membrane protein 1; LSD: lysosomal storage disease; MCAO: middle cerebral artery occlusion; OGD: oxygen glucose deprivation; OGR: oxygen glucose resupply; PD: Parkinson disease; SQSMT1: sequestosome 1; TCA: trichloroacetic acid; TTC: triphenyl tetrazolium chloride.

Autophagy in Neurodegenerative Diseases: A Hunter for Aggregates

Cells have developed elaborate quality-control mechanisms for proteins and organelles to maintain cellular homeostasis. Such quality-control mechanisms are maintained by conformational folding via molecular chaperones and by degradation through the ubiquitin-proteasome or autophagy-lysosome system. Accumulating evidence suggests that impaired autophagy contributes to the accumulation of intracellular inclusion bodies consisting of misfolded proteins, which is a hallmark of most neurodegenerative diseases. In addition, genetic mutations in core autophagy-related genes have been reported to be linked to neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease. Conversely, the pathogenic proteins, such as amyloid β and α-synuclein, are detrimental to the autophagy pathway. Here, we review the recent advances in understanding the relationship between autophagic defects and the pathogenesis of neurodegenerative diseases and suggest autophagy induction as a promising strategy for the treatment of these conditions.

Emerging New Concepts of Degrader Technologies

Traditional drug discovery focuses on identifying direct inhibitors of target proteins. This typically relies on a measurable biochemical readout and accessible binding sites whose occupancy influences the function of the target protein. These requirements preclude many disease-causing proteins from being 'druggable' targets, and these proteins are categorized as 'undruggable'. The proteolysis-targeting chimera (PROTAC) technology provides powerful tools to degrade these undruggable targets and has become a promising approach for drug discovery. However, the PROTAC technology has some limitations, and emerging new degrader technologies may greatly broaden the spectrum of targets that could be selectively degraded by harnessing a second major degradation pathway in cells. We review key emerging technologies that exploit the lysosomal degradation pathway and discuss their potential applications and limitations.

Human Induced Pluripotent Stem Cell Models of Neurodegenerative Disorders for Studying the Biomedical Implications of Autophagy

Autophagy is an intracellular degradation process that is essential for cellular survival, tissue homeostasis, and human health. The housekeeping functions of autophagy in mediating the clearance of aggregation-prone proteins and damaged organelles are vital for post-mitotic neurons. Improper functioning of this process contributes to the pathology of myriad human diseases, including neurodegeneration. Impairment in autophagy has been reported in several neurodegenerative diseases where pharmacological induction of autophagy has therapeutic benefits in cellular and transgenic animal models. However, emerging studies suggest that the efficacy of autophagy inducers, as well as the nature of the autophagy defects, may be context-dependent, and therefore, studies in disease-relevant experimental systems may provide more insights for clinical translation to patients. With the advancements in human stem cell technology, it is now possible to establish disease-affected cellular platforms from patients for investigating disease mechanisms and identifying candidate drugs in the appropriate cell types, such as neurons that are otherwise not accessible. Towards this, patient-derived human induced pluripotent stem cells (hiPSCs) have demonstrated considerable promise in constituting a platform for effective disease modeling and drug discovery. Multiple studies have utilized hiPSC models of neurodegenerative diseases to study autophagy and evaluate the therapeutic efficacy of autophagy inducers in neuronal cells. This review provides an overview of the regulation of autophagy, generation of hiPSCs via cellular reprogramming, and neuronal differentiation. It outlines the findings in various neurodegenerative disorders where autophagy has been studied using hiPSC models.

The role of sequestosome 1/p62 protein in amyotrophic lateral sclerosis and frontotemporal dementia pathogenesis

Amyotrophic lateral sclerosis and frontotemporal lobar degeneration are multifaceted diseases with genotypic, pathological and clinical overlap. One such overlap is the presence of SQSTM1/p62 mutations. While traditionally mutations manifesting in the ubiquitin-associated domain of p62 were associated with Paget’s disease of bone, mutations affecting all functional domains of p62 have now been identified in amyotrophic lateral sclerosis and frontotemporal lobar degeneration patients. p62 is a multifunctional protein that facilitates protein degradation through autophagy and the ubiquitin-proteasome system, and also regulates cell survival via the Nrf2 antioxidant response pathway, the nuclear factor-kappa B signaling pathway and apoptosis. Dysfunction in these signaling and protein degradation pathways have been observed in amyotrophic lateral sclerosis and frontotemporal lobar degeneration, and mutations that affect the role of p62 in these pathways may contribute to disease pathogenesis. In this review we discuss the role of p62 in these pathways, the effects of p62 mutations and the effect of mutations in the p62 modulator TANK-binding kinase 1, in relation to amyotrophic lateral sclerosis-frontotemporal lobar degeneration pathogenesis.

Lowering Mutant Huntingtin Levels and Toxicity: Autophagy-Endolysosome Pathways in Huntington's Disease

Huntington's disease (HD) is a monogenetic neurodegenerative disease, which serves as a model of neurodegeneration with protein aggregation. Autophagy has been suggested to possess a great value to tackle protein aggregation toxicity and neurodegenerative diseases. Current studies suggest that autophagy-endolysosomal pathways are critical for HD pathology. Here we review recent advancement in the studies of autophagy and selective autophagy relating HD. Restoration of autophagy flux and enhancement of selective removal of mutant huntingtin/disease-causing protein would be effective approaches towards tackling HD as well as other similar neurodegenerative disorders.

The pathobiology of perturbed mutant huntingtin protein-protein interactions in Huntington's disease

Mutations are at the root of many human diseases. Still, we largely do not exactly understand how they trigger pathogenesis. One, more recent, hypothesis has been that they comprehensively perturb protein-protein interaction (PPI) networks and significantly alter key biological processes. Under this premise, many rare genetic disorders with Mendelian inheritance, like Huntington's disease and several spinocerebellar ataxias, are likely to be caused by complex genotype-phenotype relationships involving abnormal PPIs. These altered PPI networks and their effects on cellular pathways are poorly understood at the molecular level. In this review, we focus on PPIs that are perturbed by the expanded pathogenic polyglutamine tract in huntingtin (HTT), the protein which, in its mutated form, leads to the autosomal dominant, neurodegenerative Huntington's disease. One aspect of perturbed mutant HTT interactions is the formation of abnormal protein species such as fibrils or large neuronal inclusions as a result of homotypic and heterotypic aberrant molecular interactions. This review focuses on abnormal PPIs that are associated with the assembly of mutant HTT aggregates in cells and their potential relevance in disease. Furthermore, the mechanisms and pathobiological processes that may contribute to phenotype development, neuronal dysfunction and toxicity in Huntington's disease brains are also discussed. This article is part of the Special Issue "Proteomics".

Biomedical Implications of Autophagy in Macromolecule Storage Disorders

An imbalance between the production and clearance of macromolecules such as proteins, lipids and carbohydrates can lead to a category of diseases broadly known as macromolecule storage disorders. These include, but not limited to, neurodegenerative diseases such as Alzheimer’s, Parkinson’s and Huntington’s disease associated with accumulation of aggregation-prone proteins, Lafora and Pompe disease associated with glycogen accumulation, whilst lipid accumulation is characteristic to Niemann-Pick disease and Gaucher disease. One of the underlying factors contributing to the build-up of macromolecules in these storage disorders is the intracellular degradation pathway called autophagy. This process is the primary clearance route for unwanted macromolecules, either via bulk non-selective degradation, or selectively via aggrephagy, glycophagy and lipophagy. Since autophagy plays a vital role in maintaining cellular homeostasis, cell viability and human health, malfunction of this process could be detrimental. Indeed, defective autophagy has been reported in a number of macromolecule storage disorders where autophagy is impaired at distinct stages, such as at the level of autophagosome formation, autophagosome maturation or improper lysosomal degradation of the autophagic cargo. Of biomedical relevance, autophagy is regulated by multiple signaling pathways that are amenable to chemical perturbations by small molecules. Induction of autophagy has been shown to improve cell viability and exert beneficial effects in experimental models of various macromolecule storage disorders where the lysosomal functionality is not overtly compromised. In this review, we will discuss the role of autophagy in certain macromolecule storage disorders and highlight the potential therapeutic benefits of autophagy enhancers in these pathological conditions.

Covalent targeting of the vacuolar H+-ATPase activates autophagy via mTORC1 inhibition

Autophagy is a lysosomal degradation pathway that eliminates aggregated proteins and damaged organelles to maintain cellular homeostasis. A major route for activating autophagy involves inhibition of the mTORC1 kinase, but current mTORC1-targeting compounds do not allow complete and selective mTORC1 blockade. Here, we have coupled screening of a covalent ligand library with activity-based protein profiling to discover EN6, a small-molecule in vivo activator of autophagy that covalently targets cysteine 277 in the ATP6V1A subunit of the lysosomal v-ATPase, which activates mTORC1 via the Rag guanosine triphosphatases. EN6-mediated ATP6V1A modification decouples the v-ATPase from the Rags, leading to inhibition of mTORC1 signaling, increased lysosomal acidification and activation of autophagy. Consistently, EN6 clears TDP-43 aggregates, a causative agent in frontotemporal dementia, in a lysosome-dependent manner. Our results provide insight into how the v-ATPase regulates mTORC1, and reveal a unique approach for enhancing cellular clearance based on covalent inhibition of lysosomal mTORC1 signaling.

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.

Evaluation of the photo-degradation of Alzheimer's amyloid fibrils with a label-free approach

Degradation of amyloid-β (Aβ) aggregates has been considered as an attractive therapeutic and preventive strategy against Alzheimer's disease (AD). However, an in situ, real-time, and label-free technique is still lacking to understand the degradation process of Aβ aggregates. In this work, we developed a novel method to quantitatively evaluate the degradation of Aβ fibrils by photoactive meso-tetra(4-sulfonatophenyl)porphyrin under UV irradiation with quartz crystal microbalance (QCM).

Mechanisms of protein toxicity in neurodegenerative diseases

Protein toxicity can be defined as all the pathological changes that ensue from accumulation, mis-localization, and/or multimerization of disease-specific proteins. Most neurodegenerative diseases manifest protein toxicity as one of their key pathogenic mechanisms, the details of which remain unclear. By systematically deconstructing the nature of toxic proteins, we aim to elucidate and illuminate some of the key mechanisms of protein toxicity from which therapeutic insights may be drawn. In this review, we focus specifically on protein toxicity from the point of view of various cellular compartments such as the nucleus and the mitochondria. We also discuss the cell-to-cell propagation of toxic disease proteins that complicates the mechanistic understanding of the disease progression as well as the spatiotemporal point at which to therapeutically intervene. Finally, we discuss selective neuronal vulnerability, which still remains largely enigmatic.

Unconventional Secretion and Intercellular Transfer of Mutant Huntingtin

The mechanism of intercellular transmission of pathological agents in neurodegenerative diseases has received much recent attention. Huntington’s disease (HD) is caused by a monogenic mutation in the gene encoding Huntingtin (HTT). Mutant HTT (mHTT) harbors a CAG repeat extension which encodes an abnormally long polyglutamine (polyQ) repeat at HTT’s N-terminus. Neuronal pathology in HD is largely due to the toxic gain-of-function by mHTT and its proteolytic products, which forms both nuclear and cytoplasmic aggregates that perturb nuclear gene transcription, RNA splicing and transport as well cellular membrane dynamics. The neuropathological effects of mHTT have been conventionally thought to be cell-autonomous in nature. Recent findings have, however, indicated that mHTT could be secreted by neurons, or transmitted from one neuronal cell to another via different modes of unconventional secretion, as well as via tunneling nanotubes (TNTs). These modes of transmission allow the intercellular spread of mHTT and its aggregates, thus plausibly promoting neuropathology within proximal neuronal populations and between neurons that are connected within neural circuits. Here, the various possible modes for mHTT’s neuronal cell exit and intercellular transmission are discussed.

Association Between Autophagy and Neurodegenerative Diseases

Autophagy is a phylogenetically conserved mechanism that controls the degradation of subcellular constituents, including misfolded proteins, and damaged organelles. The progression of many neurodegenerative diseases is thought to be driven by the aggregation of misfolded proteins; therefore, autophagic activity is thought to affect disease severity to some extent. In some neurodegenerative diseases, the suppression of autophagic activity accelerates disease progression. Given that the induction of autophagy can potentially mitigate disease severity, various autophagy-inducing compounds have been developed and their efficacy has been evaluated in several rodent models of neurodegenerative diseases.

Autophagy and lysosomal pathways in nervous system disorders

Autophagy is an evolutionarily conserved pathway for delivering cytoplasmic cargo to lysosomes for degradation. In its classically studied form, autophagy is a stress response induced by starvation to recycle building blocks for essential cellular processes. In addition, autophagy maintains basal cellular homeostasis by degrading endogenous substrates such as cytoplasmic proteins, protein aggregates, damaged organelles, as well as exogenous substrates such as bacteria and viruses. Given their important role in homeostasis, autophagy and lysosomal machinery are genetically linked to multiple human disorders such as chronic inflammatory diseases, cardiomyopathies, cancer, and neurodegenerative diseases. Multiple targets within the autophagy and lysosomal pathways offer therapeutic opportunities to benefit patients with these disorders. Here, I will summarize the mechanisms of autophagy pathways, the evidence supporting a pathogenic role for disturbed autophagy and lysosomal degradation in nervous system disorders, and the therapeutic potential of autophagy modulators in the clinic.

Proteostasis in Huntington's disease: disease mechanisms and therapeutic opportunities

Many neurodegenerative diseases are characterized by impairment of protein quality control mechanisms in neuronal cells. Ineffective clearance of misfolded proteins by the proteasome, autophagy pathways and exocytosis leads to accumulation of toxic protein oligomers and aggregates in neurons. Toxic protein species affect various cellular functions resulting in the development of a spectrum of different neurodegenerative proteinopathies, including Huntington's disease (HD). Playing an integral role in proteostasis, dysfunction of the ubiquitylation system in HD is progressive and multi-faceted with numerous biochemical pathways affected, in particular, the ubiquitin-proteasome system and autophagy routes for protein aggregate degradation. Unravelling the molecular mechanisms involved in HD pathogenesis of proteostasis provides new insight in disease progression in HD as well as possible therapeutic avenues. Recent developments of potential therapeutics are discussed in this review.

Conformation Polymorphism of Polyglutamine Proteins

Expanded polyglutamine (polyQ) stretches within endogenous proteins cause at least nine human diseases. The structural basis of polyQ pathogenesis is the key to understanding fundamental mechanisms of these diseases, but it remains unclear and controversial due to a lack of polyQ protein structures at the single-atom level. Various hypotheses have been proposed to explain the structure-cytotoxicity relationship of pathogenic proteins with polyQ expansion, largely based on indirect evidence. Here we review these hypotheses and their supporting evidence, along with additional insights from recent structural biology and chemical biology studies, with a focus on Huntingtin (HTT), the most extensively studied polyQ disease protein. Lastly, we propose potential novel strategies that may further clarify the conformation-cytotoxicity relationship of polyQ proteins.