Huntington's disease: underlying molecular mechanisms and emerging concepts

Huntingtin and Its Role in Mechanisms of RNA-Mediated Toxicity

Huntington’s disease (HD) is caused by a CAG-repeat expansion mutation in the Huntingtin (HTT) gene. It is characterized by progressive psychiatric and neurological symptoms in combination with a progressive movement disorder. Despite the ubiquitous expression of HTT, pathological changes occur quite selectively in the central nervous system. Since the discovery of HD more than 150 years ago, a lot of research on molecular mechanisms contributing to neurotoxicity has remained the focal point. While traditionally, the protein encoded by the HTT gene remained the cynosure for researchers and was extensively reviewed elsewhere, several studies in the last few years clearly indicated the contribution of the mutant RNA transcript to cellular dysfunction as well. In this review, we outline recent studies on RNA-mediated molecular mechanisms that are linked to cellular dysfunction in HD models. These mechanisms include mis-splicing, aberrant translation, deregulation of the miRNA machinery, deregulated RNA transport and abnormal regulation of mitochondrial RNA. Furthermore, we summarize recent therapeutical approaches targeting the mutant HTT transcript. While currently available treatments are of a palliative nature only and do not halt the disease progression, recent clinical studies provide hope that these novel RNA-targeting strategies will lead to better therapeutic approaches.

Roles of non-canonical structures of nucleic acids in cancer and neurodegenerative diseases

Cancer and neurodegenerative diseases are caused by genetic and environmental factors. Expression of tumour suppressor genes is suppressed by mutations or epigenetic silencing, whereas for neurodegenerative disease-related genes, nucleic acid-based effects may be presented through loss of protein function due to erroneous protein sequences or gain of toxic function from extended repeat transcripts or toxic peptide production. These diseases are triggered by damaged genes and proteins due to lifestyle and exposure to radiation. Recent studies have indicated that transient, non-canonical structural changes in nucleic acids in response to the environment can regulate the expression of disease-related genes. Non-canonical structures are involved in many cellular functions, such as regulation of gene expression through transcription and translation, epigenetic regulation of chromatin, and DNA recombination. Transcripts generated from repeat sequences of neurodegenerative disease-related genes form non-canonical structures that are involved in protein transport and toxic aggregate formation. Intracellular phase separation promotes transcription and protein assembly, which are controlled by the nucleic acid structure and can influence cancer and neurodegenerative disease progression. These findings may aid in elucidating the underlying disease mechanisms. Here, we review the influence of non-canonical nucleic acid structures in disease-related genes on disease onset and progression.

Stress granules, RNA-binding proteins and polyglutamine diseases: too much aggregation?

Stress granules (SGs) are membraneless cell compartments formed in response to different stress stimuli, wherein translation factors, mRNAs, RNA-binding proteins (RBPs) and other proteins coalesce together. SGs assembly is crucial for cell survival, since SGs are implicated in the regulation of translation, mRNA storage and stabilization and cell signalling, during stress. One defining feature of SGs is their dynamism, as they are quickly assembled upon stress and then rapidly dispersed after the stress source is no longer present. Recently, SGs dynamics, their components and their functions have begun to be studied in the context of human diseases. Interestingly, the regulated protein self-assembly that mediates SG formation contrasts with the pathological protein aggregation that is a feature of several neurodegenerative diseases. In particular, aberrant protein coalescence is a key feature of polyglutamine (PolyQ) diseases, a group of nine disorders that are caused by an abnormal expansion of PolyQ tract-bearing proteins, which increases the propensity of those proteins to aggregate. Available data concerning the abnormal properties of the mutant PolyQ disease-causing proteins and their involvement in stress response dysregulation strongly suggests an important role for SGs in the pathogenesis of PolyQ disorders. This review aims at discussing the evidence supporting the existence of a link between SGs functionality and PolyQ disorders, by focusing on the biology of SGs and on the way it can be altered in a PolyQ disease context.

When Good Kinases Go Rogue: GSK3, p38 MAPK and CDKs as Therapeutic Targets for Alzheimer's and Huntington's Disease

Alzheimer's disease (AD) is a mostly sporadic brain disorder characterized by cognitive decline resulting from selective neurodegeneration in the hippocampus and cerebral cortex whereas Huntington's disease (HD) is a monogenic inherited disorder characterized by motor abnormalities and psychiatric disturbances resulting from selective neurodegeneration in the striatum. Although there have been numerous clinical trials for these diseases, they have been unsuccessful. Research conducted over the past three decades by a large number of laboratories has demonstrated that abnormal actions of common kinases play a key role in the pathogenesis of both AD and HD as well as several other neurodegenerative diseases. Prominent among these kinases are glycogen synthase kinase (GSK3), p38 mitogen-activated protein kinase (MAPK) and some of the cyclin-dependent kinases (CDKs). After a brief summary of the molecular and cell biology of AD and HD this review covers what is known about the role of these three groups of kinases in the brain and in the pathogenesis of the two neurodegenerative disorders. The potential of targeting GSK3, p38 MAPK and CDKS as effective therapeutics is also discussed as is a brief discussion on the utilization of recently developed drugs that simultaneously target two or all three of these groups of kinases. Multi-kinase inhibitors either by themselves or in combination with strategies currently being used such as immunotherapy or secretase inhibitors for AD and knockdown for HD could represent a more effective therapeutic approach for these fatal neurodegenerative diseases.

Epigenetic regulation in Huntington's disease

Huntington's disease (HD) is a devastating and fatal monogenic neurodegenerative disorder characterized by progressive loss of selective neurons in the brain and is caused by an abnormal expansion of CAG trinucleotide repeats in a coding exon of the huntingtin (HTT) gene. Progressive gene expression changes that begin at premanifest stages are a prominent feature of HD and are thought to contribute to disease progression. Increasing evidence suggests the critical involvement of epigenetic mechanisms in abnormal transcription in HD. Genome-wide alterations of a number of epigenetic modifications, including DNA methylation and multiple histone modifications, are associated with HD, suggesting that mutant HTT causes complex epigenetic abnormalities and chromatin structural changes, which may represent an underlying pathogenic mechanism. The causal relationship of specific epigenetic changes to early transcriptional alterations and to disease pathogenesis require further investigation. In this article, we review recent studies on epigenetic regulation in HD with a focus on DNA and histone modifications. We also discuss the contribution of epigenetic modifications to HD pathogenesis as well as potential mechanisms linking mutant HTT and epigenetic alterations. Finally, we discuss the therapeutic potential of epigenetic-based treatments.

Knock out of specific maternal vitellogenins in zebrafish (Danio rerio) evokes vital changes in egg proteomic profiles that resemble the phenotype of poor quality eggs

BACKGROUND

We previously reported the results of CRISPR/Cas9 knock-out (KO) of type-I and type-III vitellogenins (Vtgs) in zebrafish, which provided the first experimental evidence on essentiality and disparate functioning of Vtgs at different stages during early development. However, the specific contributions of different types of Vtg to major cellular processes remained to be investigated. The present study employed liquid chromatography and tandem mass spectrometry (LC-MS/MS) to meet this deficit. Proteomic profiles of zebrafish eggs lacking three type-I Vtgs simultaneously (vtg1-KO), or lacking only type III Vtg (vtg3-KO) were compared to those of wild type (Wt) eggs. Obtained spectra were searched against a zebrafish proteome database and identified proteins were quantified based on normalized spectral counts.

RESULTS

The vtg-KO caused severe changes in the proteome of 1-cell stage zebrafish eggs. These changes were disclosed by molecular signatures that highly resembled the proteomic phenotype of poor quality zebrafish eggs reported in our prior studies. Proteomic profiles of vtg-KO eggs and perturbations in abundances of hundreds of proteins revealed unique, noncompensable contributions of multiple Vtgs to protein and in energy homeostasis. The lack of this contribution appears to have a significant impact on endoplasmic reticulum and mitochondrial functions, and thus embryonic development, even after zygotic genome activation. Increased endoplasmic reticulum stress, Redox/Detox activities, glycolysis/gluconeogenesis, enrichment in cellular proliferation and in human neurodegenerative disease related activities in both vtg1- and vtg3-KO eggs were found to be indicators of the aforementioned conditions. Distinctive increase in apoptosis and Parkinson disease pathways, as well as the decrease in lipid metabolism related activities in vtg3-KO eggs implies compelling roles of Vtg3, the least abundant form of Vtgs in vertebrate eggs, in mitochondrial activities. Several differentially abundant proteins representing the altered molecular mechanisms have been identified as strong candidate markers for studying the details of these mechanisms during early embryonic development in zebrafish and possibly other vertebrates.

CONCLUSIONS

These findings indicate that the global egg proteome is subject to extensive modification depending on the presence or absence of specific Vtgs and that these modifications can have a major impact on developmental competence.

The heat shock response, determined by QuantiGene multiplex, is impaired in HD mouse models and not caused by HSF1 reduction

Huntington's disease (HD) is a devastating neurodegenerative disorder, caused by a CAG/polyglutamine repeat expansion, that results in the aggregation of the huntingtin protein, culminating in the deposition of inclusion bodies in HD patient brains. We have previously shown that the heat shock response becomes impaired with disease progression in mouse models of HD. The disruption of this inducible arm of the proteostasis network is likely to exacerbate the pathogenesis of this protein-folding disease. To allow a rapid and more comprehensive analysis of the heat shock response, we have developed, and validated, a 16-plex QuantiGene assay that allows the expression of Hsf1 and nine heat shock genes, to be measured directly, and simultaneously, from mouse tissue. We used this QuantiGene assay to show that, following pharmacological activation in vivo, the heat shock response impairment in tibialis anterior, brain hemispheres and striatum was comparable between zQ175 and R6/2 mice. In contrast, although a heat shock impairment could be detected in R6/2 cortex, this was not apparent in the cortex from zQ175 mice. Whilst the mechanism underlying this impairment remains unknown, our data indicated that it is not caused by a reduction in HSF1 levels, as had been reported.

Modulation of Huntington's disease in Drosophila

Huntington's disease (HD) is a progressive neurodegenerative disorder which deteriorates the physical and mental abilities of the patients. It is an autosomal dominant disorder and is mainly caused by the expansion of a repeating CAG triplet. A number of animal models ranging from worms, fruit flies, mice and rats to pigs, sheep and monkeys are available which have been helpful in understanding various pathways involved during the progression of the disease. Drosophila is one of the most commonly used model organisms for biomedical science, due to low cost maintenance, short life span and easily implications of genetic tools. The present review provides brief description of HD and the studies carried out for HD to date taking Drosophila as a model.

Huntington's disease brain-derived small RNAs recapitulate associated neuropathology in mice

Progressive motor alterations and selective death of striatal medium spiny neurons (MSNs) are key pathological hallmarks of Huntington's disease (HD), a neurodegenerative condition caused by a CAG trinucleotide repeat expansion in the coding region of the huntingtin (HTT) gene. Most research has focused on the pathogenic effects of the resultant protein product(s); however, growing evidence indicates that expanded CAG repeats within mutant HTT mRNA and derived small CAG repeat RNAs (sCAG) participate in HD pathophysiology. The individual contribution of protein versus RNA toxicity to HD pathophysiology remains largely uncharacterized and the role of other classes of small RNAs (sRNA) that are strongly perturbed in HD is uncertain. Here, we demonstrate that sRNA produced in the putamen of HD patients (HD-sRNA-PT) are sufficient to induce HD pathology in vivo. Mice injected with HD-sRNA-PT show motor abnormalities, decreased levels of striatal HD-related proteins, disruption of the indirect pathway, and strong transcriptional abnormalities, paralleling human HD pathology. Importantly, we show that the specific blockage of sCAG mitigates HD-sRNA-PT neurotoxicity only to a limited extent. This observation prompted us to identify other sRNA species enriched in HD putamen with neurotoxic potential. We detected high levels of tRNA fragments (tRFs) in HD putamen, and we validated the neurotoxic potential of an Alanine derived tRF in vitro. These results highlight that HD-sRNA-PT are neurotoxic, and suggest that multiple sRNA species contribute to striatal dysfunction and general transcriptomic changes, favoring therapeutic strategies based on the blockage of sRNA-mediated toxicity.

Protective Effect of Natural Products against Huntington's Disease: An Overview of Scientific Evidence and Understanding Their Mechanism of Action

Huntington's disease (HD), a neurodegenerative disease, normally starts in the prime of adult life, followed by a gradual occurrence of characteristic psychiatric disturbances and cognitive and motor dysfunction. To the best of our knowledge, there is no treatment available to completely mitigate the progression of HD. Among various therapeutic approaches, exhaustive literature reports have confirmed the medicinal benefits of natural products in HD experimental models. Building on this information, this review presents a brief overview of the neuroprotective mechanism(s) of natural products against in vitro/in vivo models of HD. Relevant studies were identified from several scientific databases, including PubMed, ScienceDirect, Scopus, and Google Scholar. After screening through literature from 2005 to the present, a total of 14 medicinal plant species and 30 naturally isolated compounds investigated against HD based on either in vitro or in vivo models were included in the present review. Behavioral outcomes in the HD in vivo model showed that natural compounds significantly attenuated 3-nitropropionic acid (3-NP) induced memory loss and motor incoordination. The biochemical alteration has been markedly alleviated with reduced lipid peroxidation, increased endogenous enzymatic antioxidants, reduced acetylcholinesterase activity, and increased mitochondrial energy production. Interestingly, following treatment with certain natural products, 3-NP-induced damage in the striatum was ameliorated, as seen histologically. Overall, natural products afforded varying degrees of neuroprotection in preclinical studies of HD via antioxidant and anti-inflammatory properties, preservation of mitochondrial function, inhibition of apoptosis, and induction of autophagy.

Cortical morphometry and neural dysfunction in Huntington's disease: a review

Numerous neuroimaging techniques have been used to identify biomarkers of disease progression in Huntington's disease (HD). To date, the earliest and most sensitive of these is caudate volume; however, it is becoming increasingly evident that numerous changes to cortical structures, and their interconnected networks, occur throughout the course of the disease. The mechanisms by which atrophy spreads from the caudate to these cortical regions remains unknown. In this review, the neuroimaging literature specific to T1-weighted and diffusion-weighted magnetic resonance imaging is summarized and new strategies for the investigation of cortical morphometry and the network spread of degeneration in HD are proposed. This new avenue of research may enable further characterization of disease pathology and could add to a suite of biomarker/s of disease progression for patient stratification that will help guide future clinical trials.

GPR52 Antagonist Reduces Huntingtin Levels and Ameliorates Huntington's Disease-Related Phenotypes

GPR52 is an orphan G protein-coupled receptor (GPCR) that has been recently implicated as a potential drug target of Huntington's disease (HD), an incurable monogenic neurodegenerative disorder. In this research, we found that striatal knockdown of GPR52 reduces mHTT levels in adult HdhQ140 mice, validating GPR52 as an HD target. In addition, we discovered a highly potent and specific GPR52 antagonist Comp-43 with an IC₅₀ value of 0.63 μM by a structure-activity relationship (SAR) study. Further studies showed that Comp-43 reduces mHTT levels by targeting GPR52 and promotes survival of mouse primary striatal neurons. Moreover, in vivo study showed that Comp-43 not only reduces mHTT levels but also rescues HD-related phenotypes in HdhQ140 mice. Taken together, our study confirms that inhibition of GPR52 is a promising strategy for HD therapy, and the GPR52 antagonist Comp-43 might serve as a lead compound for further investigation.

Global N6-methyladenosine profiling of cobalt-exposed cortex and human neuroblastoma H4 cells presents epitranscriptomics alterations in neurodegenerative disease-associated genes

Excessive exposure to cobalt (Co) is known to make adverse impact on the nervous system, but its detailed mechanisms of neurotoxicity have yet to be determined. In this study, C57BL/6 mice (0, 4, 8, 16 mg/kg CoCl₂, 30 days) and human neuroblastoma H4 cells (0, 100, 400, 600 μM CoCl₂) were used as in vivo and in vitro models. Our results revealed that CoCl₂ intraperitoneal injection caused significant impairments in learning and memory, as well as pathological damage in the nervous system. We further certificated the alteration of m⁶A methylation induced by CoCl₂ exposure. Our findings demonstrate for the first time, significant differences in the degree of m⁶A modification, the biological function of m⁶A-modified transcripts between cortex and H4 cell samples. Specifically, MeRIP-seq and RNA-seq elucidate that CoCl₂ exposure results in differentially m⁶A-modified and expressed genes, which were enriched in pathways involving synaptic transmission, and central nervous system (CNS) development. Mechanistic analyses revealed that CoCl₂ remarkably changed m⁶A modification level by affecting the expression of m⁶A methyltransferase and demethylase, and decreasing the activity of demethylase. We observed variation of m⁶A modification in neurodegenerative disease-associated genes upon CoCl₂ exposure and identified regulatory strategy between m⁶A and potential targets mRNA. Our novel findings provide novel insight into the functional roles of m⁶A modification in neurodegenerative damage caused by environmental neurotoxicants and identify Co-mediated specific RNA regulatory strategy for broadening the epigenetic regulatory mechanism of RNA induced by heavy metals.

Diffusion Tensor Imaging in Preclinical and Human Studies of Huntington's Disease: What Have we Learned so Far?

BACKGROUND

Huntington's Disease is an irreversible neurodegenerative disease characterized by the progressive deterioration of specific brain nerve cells. The current evaluation of cellular and physiological events in patients with HD relies on the development of transgenic animal models. To explore such events in vivo, diffusion tensor imaging has been developed to examine the early macro and microstructural changes in brain tissue. However, the gap in diffusion tensor imaging findings between animal models and clinical studies and the lack of microstructural confirmation by histological methods has questioned the validity of this method.

OBJECTIVE

This review explores white and grey matter ultrastructural changes associated to diffusion tensor imaging, as well as similarities and differences between preclinical and clinical Huntington's Disease studies.

METHODS

A comprehensive review of the literature using online-resources was performed (Pub- Med search).

RESULTS

Similar changes in fractional anisotropy as well as axial, radial and mean diffusivities were observed in white matter tracts across clinical and animal studies. However, comparative diffusion alterations in different grey matter structures were inconsistent between clinical and animal studies.

CONCLUSION

Diffusion tensor imaging can be related to specific structural anomalies in specific cellular populations. However, some differences between animal and clinical studies could derive from the contrasting neuroanatomy or connectivity across species. Such differences should be considered before generalizing preclinical results into the clinical practice. Moreover, current limitations of this technique to accurately represent complex multicellular events at the single micro scale are real. Future work applying complex diffusion models should be considered.

Bile Acids: A Communication Channel in the Gut-Brain Axis

Bile acids are signalling hormones involved in the regulation of several metabolic pathways. The ability of bile acids to bind and signal through their receptors is modulated by the gut microbiome, since the microbiome contributes to the regulation and synthesis of bile acids as well to their physiochemical properties. From the gut, bacteria have been shown to send signals to the central nervous system via their metabolites, thus affecting the behaviour and brain function of the host organism. In the last years it has become increasingly evident that bile acids affect brain function, during normal physiological and pathological conditions. Although bile acids may be synthesized locally in the brain, the majority of brain bile acids are taken up from the systemic circulation. Since the composition of the brain bile acid pool may be regulated by the action of intestinal bacteria, it is possible that bile acids function as a communication bridge between the gut microbiome and the brain. However, little is known about the molecular mechanisms and the physiological roles of bile acids in the central nervous system. The possibility that bile acids may be a direct link between the intestinal microbiome and the brain is also an understudied subject. Here we review the influence of gut bacteria on the bile acid pool composition and properties, as well as striking evidence showing the role of bile acids as neuroactive molecules.

Mediators of mitophagy that regulate mitochondrial quality control play crucial role in diverse pathophysiology

Mitochondria are double membrane-bound cellular work-horses constantly functioning to regulate vital aspects of cellular metabolism, bioenergetics, proliferation and death. Biogenesis, homeostasis and regulated turnover of mitochondria are stringently regulated to meet the bioenergetic requirements. Diverse external and internal stimuli including oxidative stress, diseases, xenobiotics and even age profoundly affect mitochondrial integrity. Damaged mitochondria need immediate segregation and selective culling to maintain physiological homeostasis. Mitophagy is a specialised form of macroautophagy that constantly checks mitochondrial quality followed by elimination of rogue mitochondria by lysosomal targeting through multiple pathways tightly regulated and activated in context-specific manners. Mitophagy is implicated in diverse oxidative stress-associated metabolic, proliferating and degenerative disorders owing to the centrality of mitopathology in diseases as well as the common mandate to eliminate damaged mitochondria for restoring physiological homeostasis. With improved health care and growing demand for precision medicine, specifically targeting the keystone factors in pathogenesis, more exploratory studies are focused on mitochondrial quality control as underlying guardian of cellular pathophysiology. In this context, mitophagy emerged as a promising area to focus biomedical research for identifying novel therapeutic targets against diseases linked with physiological redox perturbation. The present review provides a comprehensive account of the recent developments on mitophagy along with precise discussion on its impact on major diseases and possibilities of therapeutic modulation.

Bioinformatic gene analysis for potential therapeutic targets of Huntington's disease in pre-symptomatic and symptomatic stage

Background

Huntington’s disease (HD) is a neurodegenerative disorder characterized by psychiatric symptoms, serious motor and cognitive deficits. Certain pathological changes can already be observed in pre-symptomatic HD (pre-HD) patients; however, the underlying molecular pathogenesis is still uncertain and no effective treatments are available until now. Here, we reanalyzed HD-related differentially expressed genes from the GEO database between symptomatic HD patients, pre-HD individuals, and healthy controls using bioinformatics analysis, hoping to get more insight in the pathogenesis of both pre-HD and HD, and shed a light in the potential therapeutic targets of the disease.

Methods

Pre-HD and symptomatic HD differentially expressed genes (DEGs) were screened by bioinformatics analysis Gene Expression Omnibus (GEO) dataset GSE1751. A protein–protein interaction (PPI) network was used to select hub genes. Subsequently, Gene Ontology (GO) enrichment analysis of DEGs and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis of hub genes were applied. Dataset GSE24250 was downloaded to verify our hub genes by the Kaplan–Meier method using Graphpad Prism 5.0. Finally, target miRNAs of intersected hub genes involved in pre-HD and symptomatic HD were predicted.

Results

A total of 37 and 985 DEGs were identified in pre-HD and symptomatic HD, respectively. The hub genes, SIRT1, SUZ12, and PSMC6, may be implicated in pre-HD, and the hub genes, FIS1, SIRT1, CCNH, SUZ12, and 10 others, may be implicated in symptomatic HD. The intersected hub genes, SIRT1 and SUZ12, and their predicted target miRNAs, in particular miR-22-3p and miR-19b, may be significantly associated with pre-HD.

Conclusion

The PSMC6, SIRT1, and SUZ12 genes and their related ubiquitin-mediated proteolysis, transcriptional dysregulation, and histone metabolism are significantly associated with pre-HD. FIS1, CCNH, and their related mitochondrial disruption and transcriptional dysregulation processes are related to symptomatic HD, which might shed a light on the elucidation of potential therapeutic targets in HD.

Cell cycle re-entry of neurons and reactive neuroblastosis in Huntington's disease: Possibilities for neural-glial transition in the brain

Huntington's disease (HD) is an autosomal dominant pathogenic condition that causes progressive degeneration of GABAergic neurons in the brain. The abnormal expansion of the CAG repeats in the exon 1 of the Huntingtin gene (HTT gene) has been associated with the onset and progression of movement disorders, psychiatric disturbance and cognitive decline in HD. Microglial activation and reactive astrogliosis have been recognized as the key pathogenic cellular events in the brains of HD subjects. Besides, HD has been characterized by induced quiescence of neural stem cells (NSCs), reactive neuroblastosis and reduced survival of newborn neurons in the brain. Strikingly, the expression of the mutant HTT gene has been reported to induce the cell cycle re-entry of neurons in HD brains. However, the underlying basis for the induction of cell cycle in neurons and the fate of dedifferentiating neurons in the pathological brain remain largely unknown. Thus, this review article revisits the reports on the regulation of key signaling pathways responsible for altered cell cycle events in diseased brains, with special reference to HD and postulates the occurrence of reactive neuroblastosis as a consequential cellular event of dedifferentiation of neurons. Meanwhile, a substantial number of studies indicate that many neuropathogenic events are associated with the expression of potential glial cell markers by neuroblasts. Taken together, this article represents a hypothesis that transdifferentiation of neurons into glial cells might be highly possible through the transient generation of reactive neuroblasts in the brain upon certain pathological conditions.

Mesenchymal Stromal Cells' Therapy for Polyglutamine Disorders: Where Do We Stand and Where Should We Go?

Polyglutamine (polyQ) diseases are a group of inherited neurodegenerative disorders caused by the expansion of the cytosine-adenine-guanine (CAG) repeat. This mutation encodes extended glutamine (Q) tract in the disease protein, resulting in the alteration of its conformation/physiological role and in the formation of toxic fragments/aggregates of the protein. This group of heterogeneous disorders shares common molecular mechanisms, which opens the possibility to develop a pan therapeutic approach. Vast efforts have been made to develop strategies to alleviate disease symptoms. Nonetheless, there is still no therapy that can cure or effectively delay disease progression of any of these disorders. Mesenchymal stromal cells (MSC) are promising tools for the treatment of polyQ disorders, promoting protection, tissue regeneration, and/or modulation of the immune system in animal models. Accordingly, data collected from clinical trials have so far demonstrated that transplantation of MSC is safe and delays the progression of some polyQ disorders for some time. However, to achieve sustained phenotypic amelioration in clinics, several treatments may be necessary. Therefore, efforts to develop new strategies to improve MSC's therapeutic outcomes have been emerging. In this review article, we discuss the current treatments and strategies used to reduce polyQ symptoms and major pre-clinical and clinical achievements obtained with MSC transplantation as well as remaining flaws that need to be overcome. The requirement to cross the blood-brain-barrier (BBB), together with a short rate of cell engraftment in the lesioned area and low survival of MSC in a pathophysiological context upon transplantation may contribute to the transient therapeutic effects. We also review methods like pre-conditioning or genetic engineering of MSC that can be used to increase MSC survival in vivo, cellular-free approaches-i.e., MSC-conditioned medium (CM) or MSC-derived extracellular vesicles (EVs) as a way of possibly replacing the use of MSC and methods required to standardize the potential of MSC/MSC-derived products. These are fundamental questions that need to be addressed to obtain maximum MSC performance in polyQ diseases and therefore increase clinical benefits.

AggFluor: Fluorogenic Toolbox Enables Direct Visualization of the Multi-Step Protein Aggregation Process in Live Cells

Aberrantly processed or mutant proteins misfold and assemble into a variety of soluble oligomers and insoluble aggregates, a process that is associated with an increasing number of diseases that are not curable or manageable. Herein, we present a chemical toolbox, AggFluor, that allows for live cell imaging and differentiation of complex aggregated conformations in live cells. Based on the chromophore core of green fluorescent proteins, AggFluor is comprised of a series of molecular rotor fluorophores that span a wide range of viscosity sensitivity. As a result, these compounds exhibit differential turn-on fluorescence when incorporated in either soluble oligomers or insoluble aggregates. This feature allows us to develop, for the first time, a dual-color imaging strategy to distinguish unfolded protein oligomers from insoluble aggregates in live cells. Furthermore, we have demonstrated how small molecule proteostasis regulators can drive formation and disassembly of protein aggregates in both conformational states. In summary, AggFluor is the first set of rationally designed molecular rotor fluorophores that evenly cover a wide range of viscosity sensitivities. This set of fluorescent probes not only change the status quo of current imaging methods to visualize protein aggregation in live cells but also can be generally applied to study other biological processes that involve local viscosity changes with temporal and spatial resolutions.

The proteasome as a druggable target with multiple therapeutic potentialities: Cutting and non-cutting edges

Ubiquitin Proteasome System (UPS) is an adaptable and finely tuned system that sustains proteostasis network under a large variety of physiopathological conditions. Its dysregulation is often associated with the onset and progression of human diseases; hence, UPS modulation has emerged as a promising new avenue for the development of treatments of several relevant pathologies, such as cancer and neurodegeneration. The clinical interest in proteasome inhibition has considerably increased after the FDA approval in 2003 of bortezomib for relapsed/refractory multiple myeloma, which is now used in the front-line setting. Thereafter, two other proteasome inhibitors (carfilzomib and ixazomib), designed to overcome resistance to bortezomib, have been approved for treatment-experienced patients, and a variety of novel inhibitors are currently under preclinical and clinical investigation not only for haematological malignancies but also for solid tumours. However, since UPS collapse leads to toxic misfolded proteins accumulation, proteasome is attracting even more interest as a target for the care of neurodegenerative diseases, which are sustained by UPS impairment. Thus, conceptually, proteasome activation represents an innovative and largely unexplored target for drug development. According to a multidisciplinary approach, spanning from chemistry, biochemistry, molecular biology to pharmacology, this review will summarize the most recent available literature regarding different aspects of proteasome biology, focusing on structure, function and regulation of proteasome in physiological and pathological processes, mostly cancer and neurodegenerative diseases, connecting biochemical features and clinical studies of proteasome targeting drugs.

Genetic cooperativity in multi-layer networks implicates cell survival and senescence in the striatum of Huntington's disease mice synchronous to symptoms

Motivation

Huntington’s disease (HD) may evolve through gene deregulation. However, the impact of gene deregulation on the dynamics of genetic cooperativity in HD remains poorly understood. Here, we built a multi-layer network model of temporal dynamics of genetic cooperativity in the brain of HD knock-in mice (allelic series of Hdh mice). To enhance biological precision and gene prioritization, we integrated three complementary families of source networks, all inferred from the same RNA-seq time series data in Hdh mice, into weighted-edge networks where an edge recapitulates path-length variation across source-networks and age-points.

Results

Weighted edge networks identify two consecutive waves of tight genetic cooperativity enriched in deregulated genes (critical phases), pre-symptomatically in the cortex, implicating neurotransmission, and symptomatically in the striatum, implicating cell survival (e.g. Hipk4) intertwined with cell proliferation (e.g. Scn4b) and cellular senescence (e.g. Cdkn2a products) responses. Top striatal weighted edges are enriched in modulators of defective behavior in invertebrate models of HD pathogenesis, validating their relevance to neuronal dysfunction in vivo. Collectively, these findings reveal highly dynamic temporal features of genetic cooperativity in the brain of Hdh mice where a 2-step logic highlights the importance of cellular maintenance and senescence in the striatum of symptomatic mice, providing highly prioritized targets.

Availability and implementation

Weighted edge network analysis (WENA) data and source codes for performing spectral decomposition of the signal (SDS) and WENA analysis, both written using Python, are available at http://www.broca.inserm.fr/HD-WENA/.

Supplementary information

Supplementary data are available at Bioinformatics online.

Microglial depletion prevents extracellular matrix changes and striatal volume reduction in a model of Huntington's disease

Huntington's disease is associated with a reactive microglial response and consequent inflammation. To address the role of these cells in disease pathogenesis, we depleted microglia from R6/2 mice, a rapidly progressing model of Huntington's disease marked by behavioural impairment, mutant huntingtin (mHTT) accumulation, and early death, through colony-stimulating factor 1 receptor inhibition (CSF1Ri) with pexidartinib (PLX3397) for the duration of disease. Although we observed an interferon gene signature in addition to downregulated neuritogenic and synaptic gene pathways with disease, overt inflammation was not evident by microglial morphology or cytokine transcript levels in R6/2 mice. Nonetheless, CSF1Ri-induced microglial elimination reduced or prevented disease-related grip strength and object recognition deficits, mHTT accumulation, astrogliosis, and striatal volume loss, the latter of which was not associated with reductions in cell number but with the extracellular accumulation of chondroitin sulphate proteoglycans (CSPGs)-a primary component of glial scars. A concurrent loss of proteoglycan-containing perineuronal nets was also evident in R6/2 mice, and microglial elimination not only prevented this but also strikingly increased perineuronal nets in the brains of naïve littermates, suggesting a new role for microglia as homeostatic regulators of perineuronal net formation and integrity.

Manganese-induced Mitochondrial Dysfunction Is Not Detectable at Exposures Below the Acute Cytotoxic Threshold in Neuronal Cell Types

Manganese (Mn) is an essential metal, but excessive exposures have been well-documented to culminate in neurotoxicity. Curiously, the precise mechanisms of Mn neurotoxicity are still unknown. One hypothesis suggests that Mn exerts its toxicity by inhibiting mitochondrial function, which then (if exposure levels are high and long enough) leads to cell death. Here, we used a Huntington's disease cell model with known differential sensitivities to manganese-STHdhQ7/Q7 and STHdhQ111/Q111 cells-to examine the effects of acute Mn exposure on mitochondrial function. We determined toxicity thresholds for each cell line using both changes in cell number and caspase-3/7 activation. We used a range of acute Mn exposures (0-300 µM), both above and below the cytotoxic threshold, to evaluate mitochondria-associated metabolic balance, mitochondrial respiration, and substrate dependence. In both cell lines, we observed no effect on markers of mitochondrial function at subtoxic Mn exposures (below detectable levels of cell death), yet at supratoxic exposures (above detectable levels of cell death) mitochondrial function significantly declined. We validated these findings in primary striatal neurons. In cell lines, we further observed that subtoxic Mn concentrations do not affect glycolytic function or major intracellular metabolite quantities. These data suggest that in this system, Mn exposure impairs mitochondrial function only at concentrations coincident with or above the initiation of cell death and is not consistent with the hypothesis that mitochondrial dysfunction precedes or induces Mn cytotoxicity.

Organismal Protein Homeostasis Mechanisms

Sustaining a healthy proteome is a lifelong challenge for each individual cell of an organism. However, protein homeostasis or proteostasis is constantly jeopardized since damaged proteins accumulate under proteotoxic stress that originates from ever-changing metabolic, environmental, and pathological conditions. Proteostasis is achieved via a conserved network of quality control pathways that orchestrate the biogenesis of correctly folded proteins, prevent proteins from misfolding, and remove potentially harmful proteins by selective degradation. Nevertheless, the proteostasis network has a limited capacity and its collapse deteriorates cellular functionality and organismal viability, causing metabolic, oncological, or neurodegenerative disorders. While cell-autonomous quality control mechanisms have been described intensely, recent work on Caenorhabditis elegans has demonstrated the systemic coordination of proteostasis between distinct tissues of an organism. These findings indicate the existence of intricately balanced proteostasis networks important for integration and maintenance of the organismal proteome, opening a new door to define novel therapeutic targets for protein aggregation diseases. Here, we provide an overview of individual protein quality control pathways and the systemic coordination between central proteostatic nodes. We further provide insights into the dynamic regulation of cellular and organismal proteostasis mechanisms that integrate environmental and metabolic changes. The use of C. elegans as a model has pioneered our understanding of conserved quality control mechanisms important to safeguard the organismal proteome in health and disease.

Bioenergetic deficits in Huntington's disease iPSC-derived neural cells and rescue with glycolytic metabolites

Altered cellular metabolism is believed to be an important contributor to pathogenesis of the neurodegenerative disorder Huntington's disease (HD). Research has primarily focused on mitochondrial toxicity, which can cause death of the vulnerable striatal neurons, but other aspects of metabolism have also been implicated. Most previous studies have been carried out using postmortem human brain or non-human cells. Here, we studied bioenergetics in an induced pluripotent stem cell-based model of the disease. We found decreased adenosine triphosphate (ATP) levels in HD cells compared to controls across differentiation stages and protocols. Proteomics data and multiomics network analysis revealed normal or increased levels of mitochondrial messages and proteins, but lowered expression of glycolytic enzymes. Metabolic experiments showed decreased spare glycolytic capacity in HD neurons, while maximal and spare respiratory capacities driven by oxidative phosphorylation were largely unchanged. ATP levels in HD neurons could be rescued with addition of pyruvate or late glycolytic metabolites, but not earlier glycolytic metabolites, suggesting a role for glycolytic deficits as part of the metabolic disturbance in HD neurons. Pyruvate or other related metabolic supplements could have therapeutic benefit in HD.

Site-specific ubiquitination of pathogenic huntingtin attenuates its deleterious effects

Huntington's disease (HD) is a progressive incurable neurodegenerative disorder characterized by motor and neuropsychiatric symptoms. It is caused by expansion of a cytosine-adenine-guanine triplet in the N-terminal domain of exon 1 in the huntingtin (HTT) gene that codes for an expanded polyglutamine stretch in the protein product which becomes aggregation prone. The mutant Htt (mHtt) aggregates are associated with components of the ubiquitin-proteasome system, suggesting that mHtt is marked for proteasomal degradation and that, for reasons still debated, are not properly degraded. We used a novel HD rat model, proteomic analysis, and long-term live neuronal imaging to characterize the effects of ubiquitination on aggregation of mHtt and subsequent cellular responses. We identified two lysine residues, 6 and 9, in the first exon of mHtt that are specifically ubiquitinated in striatal and cortical brain tissues of mHtt-transgenic animals. Expression of mHtt exon 1 lacking these ubiquitination sites in cortical neurons and cultured cells was found to slow aggregate appearance rates and reduce their size but at the same time increase the number of much smaller and less visible ones. Importantly, expression of this form of mHtt was associated with elevated death rates. Proteomic analysis indicated that cellular reactions to mHtt expression were weaker in cells expressing the lysineless protein, possibly implying a reduced capacity to cope with the proteotoxic stress. Taken together, the findings suggest a novel role for ubiquitination-attenuation of the pathogenic effect of mHtt.

Understanding HD Psychosis: An Analysis from the ENROLL-HD Database

Background:

Psychosis is considered rare in Huntington’s Disease, with an estimated prevalence of 3–11%. However, it has a profound impact on quality of life and disease burden. This study uses the Enroll-HD database to determine the prevalence, onset, and severity of psychosis in Huntington’s Disease and to determine demographic and disease characteristics associated with psychosis.

Methods:

Data were obtained from Enroll-HD. Adults with manifest Huntington’s Disease were included. Descriptive statistics were calculated. Simple logistic regression was used to calculate the odds ratio with 95% confidence interval for association with each characteristic.

Results:

7,966 manifest Huntington’s Disease participants were analyzed, and 12.95% had a history of psychosis. Mean age of psychosis onset (48.34 years, SD 13.26) mirrored Huntington’s Disease onset. Family history of psychosis in a first degree relative was documented in 23.6% of participants with psychosis. Variables significantly (p < 0.05) associated with presence of psychosis in manifest HD included lower education level, unemployment, single marital status, depression, decreased verbal fluency score, and decreased total functional capacity & functional assessment score.

Discussion:

Psychosis in Huntington’s Disease is more prevalent than many prior studies have reported. It is associated with several demographic & psychiatric features, decreased cognitive capacity, and worse functional outcomes.

Highlights:

Psychosis in HD is more prevalent than prior studies have reported. It is associated with a range of demographic and psychiatric variables, worse cognition, and worse functional outcomes suggesting several features that may be used to predict onset of psychosis and improve understanding and management of psychosis in HD.

The Nuclear Lamina: Protein Accumulation and Disease

Cellular health is reliant on proteostasis-the maintenance of protein levels regulated through multiple pathways modulating protein synthesis, degradation and clearance. Loss of proteostasis results in serious disease and is associated with aging. One proteinaceous structure underlying the nuclear envelope-the nuclear lamina-coordinates essential processes including DNA repair, genome organization and epigenetic and transcriptional regulation. Loss of proteostasis within the nuclear lamina results in the accumulation of proteins, disrupting these essential functions, either via direct interactions of protein aggregates within the lamina or by altering systems that maintain lamina structure. Here we discuss the links between proteostasis and disease of the nuclear lamina, as well as how manipulating specific proteostatic pathways involved in protein clearance could improve cellular health and prevent/reverse disease.

Increased translation as a novel pathogenic mechanism in Huntington's disease

Huntington's disease is a neurodegenerative disorder caused by a CAG repeat expansion in exon 1 of the huntingtin gene. Striatal projection neurons are mainly affected, leading to motor symptoms, but molecular mechanisms involved in their vulnerability are not fully characterized. Here, we show that eIF4E binding protein (4E-BP), a protein that inhibits translation, is inactivated in Huntington's disease striatum by increased phosphorylation. Accordingly, we detected aberrant de novo protein synthesis. Proteomic characterization indicates that translation specifically affects sets of proteins as we observed upregulation of ribosomal and oxidative phosphorylation proteins and downregulation of proteins related to neuronal structure and function. Interestingly, treatment with the translation inhibitor 4EGI-1 prevented R6/1 mice motor deficits, although corticostriatal long-term depression was not markedly changed in behaving animals. At the molecular level, injection of 4EGI-1 normalized protein synthesis and ribosomal content in R6/1 mouse striatum. In conclusion, our results indicate that dysregulation of protein synthesis is involved in mutant huntingtin-induced striatal neuron dysfunction.

Why Woody got the blues: The neurobiology of depression in Huntington's disease

Huntington's disease (HD) is an extraordinary disorder that usually strikes when individuals are in the prime of their lives, as was the case for the influential 20th century musician Woody Guthrie. HD demonstrates the exceptionally fine line between life and death in such 'genetic diseases', as the only difference between those who suffer horribly and die slowly of this disease is often just a handful of extra tandem repeats (beyond the normal polymorphic range) in a genome that constitutes over 3 billion paired nucleotides of DNA. Furthermore, HD presents as a complex and heterogenous combination of psychiatric, cognitive and motor symptoms, so can appear as an unholy trinity of 'three disorders in one'. The autosomal dominant nature of the disorder is also extremely challenging for affected families, as a 'flip of a coin' dictates which children inherit the mutation from their affected parent, and the gene-negative family members bear the burden of caring for the other half of the family that is affected. In this review, we will focus on one of the earliest, and most devastating, symptoms associated with HD, depression, which has been reported to affect approximately half of gene-positive HD family members. We will discuss the pathogenesis of HD, and depressive symptoms in particular, including molecular and cellular mechanisms, and potential genetic and environmental modifiers. This expanding understanding of HD pathogenesis may not only lead to novel therapeutic options for HD families, but may also provide insights into depression in the wider population, which has the greatest burden of disease of any disorder and an enormous unmet need for new therapies.

Alterations of Calcium Channels in a Mouse Model of Huntington's Disease and Neuroprotection by Blockage of CaV1 Channels

Huntington’s disease (HD) is a neurodegenerative autosomal dominant disorder, characterized by symptoms of involuntary movement of the body, loss of cognitive function, psychiatric disorder, leading inevitably to death. It has been previously described that higher levels of brain expression of Cav1 channels are involved in major neurodegenerative disorders, such as Alzheimer’s disease and Parkinson’s disease. Our results demonstrate that a bacterial artificial chromosome (BAC)-mediated transgenic mouse model (BACHD mice) at the age of 3 and 12 months exhibits significantly increased Cav1.2 protein levels in the cortex, as compared with wild-type littermates. Importantly, electrophysiological analyses confirm a significant increase in L-type Ca2+ currents and total Ca2+ current density in cortical neurons from BACHD mice. By using an in vitro assay to measure neuronal cell death, we were able to observe neuronal protection against glutamate toxicity after treatment with Cav1 blockers, in wild-type and, more importantly, in BACHD neurons. According to our data, Cav1 blockers may offer an interesting strategy for the treatment of HD. Altogether, our results show that mutant huntingtin (mHtt) expression may cause a dysregulation of Cav1.2 channels and we hypothesize that this contributes to neurodegeneration during HD.

Huntington disease: Advances in the understanding of its mechanisms

Huntington disease (HD) is a devastating monogenic autosomal dominant disorder. HD is caused by a CAG expansion in exon 1 of the gene coding for huntingtin, placed in the short arm of chromosome 4. Despite its well-defined genetic origin, the molecular and cellular mechanisms underlying the disease are unclear and complex. Here, we review some of the currently known functions of the wild-type huntingtin protein and discuss the deleterious effects that arise from the expansion of the CAG repeats, which are translated into an abnormally long polyglutamine tract.

Also, we present a modern view on the molecular biology of HD as a representative of the group of polyglutamine diseases, with an emphasis on conformational changes of mutant huntingtin, disturbances in its cellular processing, and proteolytic stress in degenerating neurons. The main pathogenetic mechanisms of neurodegeneration in HD are discussed in detail, such as autophagy, impaired mitochondrial biogenesis, lysosomal dysfunction, organelle and protein transport, inflammation, oxidative stress, and transcription factor modulation. However, other unraveling mechanisms are still unknown. This practical and brief review summarizes some of the currently known functions of the wild-type huntingtin protein and the recent findings related to the mechanisms involved in HD pathogenesis.

Could metformin be therapeutically useful in Huntington's disease?

Emerging evidence suggest that dimethylbiguanide (metformin), a first-line drug for type 2 diabetes mellitus, could be neuroprotective in a range of brain pathologies, which include neurodegenerative diseases and brain injury. However, there are also contraindications that associate metformin treatment with cognitive impairment as well as adverse outcomes in Alzheimer's disease and Parkinson's disease animal models. Recently, a beneficial effect of metformin in animal models of Huntington's disease (HD) has been strengthened by multiple reports. In this brief review, the findings associated with the effects of metformin in attenuating neurodegenerative diseases are discussed, focusing on HD-associated pathology and the potential underlying mechanisms highlighted by these studies. The mechanism of action of metformin is complex, and its therapeutic efficacy is therefore expected to be dependent on the disease context. The key metabolic pathways that are effectively affected by metformin, such as AMP-activated protein kinase activation, may be altered in the later decades of the human lifespan. In this regard, metformin may nonetheless be therapeutically useful for neurological diseases with early pathological onsets, such as HD.

Rab11-mediated recycling endosome role in nervous system development and neurodegenerative diseases

STUDY

Membrane trafficking process is significant for the complex and precise regulatory of the nervous system. Rab11, as a small GTPase of the Rab superfamily, controls endocytic vesicular trafficking to the cell surface after sorting in recycling endosome (RE), coordinating with its receptors to maintain neurological function.

MATERIALS AND METHODS

This article reviewed the literature of Rab11 in nervous system.

RESULTS

Rab11-positive vesicles targeted transport growth-related molecules, such as integrins, protrudin, tropomyosin receptor kinase (Trk) A/B receptor and AMPA receptor (AMPAR) to membrane surface to promote the regeneration capacity of axon and dendrites and maintain synaptic plasticity. In addition, many studies have shown that the expression of Rab11 is decreased in multiple neurodegenerative diseases, which is highly correlated with the process of production, transport and clearance of disease-related pathological proteins. And overexpression or increased activity of Rab11 can greatly improve the defect of membrane trafficking and slow down the disease process.

CONCLUSION

With increasing research efforts on Rab11-dependent membrane trafficking mechanisms, a potential target for nerve regeneration and neurodegenerative diseases will be provided.

The therapeutic potential of galectin-1 and galectin-3 in the treatment of neurodegenerative diseases

Introduction: Neuroinflammation has been proposed as a common factor and one of the main inducers of neuronal degeneration. Galectins are a group of β-galactoside-binding lectins, that play an important role in the immune response, adhesion, proliferation, differentiation, migration and cell growth. Up to 15 members of the galectin's family have been identified; however, the expression of galectin-1 and galectin-3 has been considered a key factor in neuronal regeneration and modulation of the inflammatory response. Galectin-1 is necessary to stimulate the secretion of neurotrophic factors in astrocytes and promoting neuronal regeneration. In contrast, galectin-3 fosters the proliferation of microglial cells and modulates cellular apoptosis, therefore these proteins are considered a useful alternative for the treatment of degenerative diseases.Areas covered: This review describes the roles of galectin-1 and galectin-3 in the modulation of neuroinflammation and their potential as therapeutic targets in the treatment for neurodegenerative diseases.Expert opinion: Although data in the literature vary, the effects of galectin-1 and galectin-3 on the activation and modulation of astrocytes and microglia has been described. Due to its anti-inflammatory effects, galectin-1 is proposed as a molecule with therapeutic potential, whereas the inhibition of galectin-3 could contribute to reduce the neuroinflammatory response in neurodegenerative diseases.

Systematic microsatellite repeat expansion cloning and validation

Approximately 3% of the human genome is composed of short tandem repeat (STR) DNA sequence known as microsatellites, which can be found in both coding and non-coding regions. When associated with genic regions, expansion of microsatellite repeats beyond a critical threshold causes dozens of neurological repeat expansion disorders. To better understand the molecular pathology of repeat expansion disorders, precise cloning of microsatellite repeat sequence and expansion size is highly valuable. Unfortunately, cloning repeat expansions is often challenging and presents a significant bottleneck to practical investigation. Here, we describe a clear method for seamless and systematic cloning of practically any microsatellite repeat expansion. We use cloning and expansion of GGGGCC repeats, which are the leading genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), as an example. We employ a recursive directional ligation (RDL) technique to build multiple GGGGCC repeat-containing vectors. We describe methods to validate repeat expansion cloning, including diagnostic restriction digestion, PCR across the repeat, and next-generation long-read MinION nanopore sequencing. Validated cloning of microsatellite repeats beyond the critical expansion threshold can facilitate step-by-step characterization of disease mechanisms at the cellular and molecular level.

DNAJB6, a Key Factor in Neuronal Sensitivity to Amyloidogenesis

CAG-repeat expansions in at least eight different genes cause neurodegeneration. The length of the extended polyglutamine stretches in the corresponding proteins is proportionally related to their aggregation propensity. Although these proteins are ubiquitously expressed, they predominantly cause toxicity to neurons. To understand this neuronal hypersensitivity, we generated induced pluripotent stem cell (iPSC) lines of spinocerebellar ataxia type 3 and Huntington's disease patients. iPSC generation and neuronal differentiation are unaffected by polyglutamine proteins and show no spontaneous aggregate formation. However, upon glutamate treatment, aggregates form in neurons but not in patient-derived neural progenitors. During differentiation, the chaperone network is drastically rewired, including loss of expression of the anti-amyloidogenic chaperone DNAJB6. Upregulation of DNAJB6 in neurons antagonizes glutamate-induced aggregation, while knockdown of DNAJB6 in progenitors results in spontaneous polyglutamine aggregation. Loss of DNAJB6 expression upon differentiation is confirmed in vivo, explaining why stem cells are intrinsically protected against amyloidogenesis and protein aggregates are dominantly present in neurons.

Diagnostic and Therapeutic Potential of TSPO Studies Regarding Neurodegenerative Diseases, Psychiatric Disorders, Alcohol Use Disorders, Traumatic Brain Injury, and Stroke: An Update

Neuroinflammation and cell death are among the common symptoms of many central nervous system diseases and injuries. Neuroinflammation and programmed cell death of the various cell types in the brain appear to be part of these disorders, and characteristic for each cell type, including neurons and glia cells. Concerning the effects of 18-kDa translocator protein (TSPO) on glial activation, as well as being associated with neuronal cell death, as a response mechanism to oxidative stress, the changes of its expression assayed with the aid of TSPO-specific positron emission tomography (PET) tracers' uptake could also offer evidence for following the pathogenesis of these disorders. This could potentially increase the number of diagnostic tests to accurately establish the stadium and development of the disease in question. Nonetheless, the differences in results regarding TSPO PET signals of first and second generations of tracers measured in patients with neurological disorders versus healthy controls indicate that we still have to understand more regarding TSPO characteristics. Expanding on investigations regarding the neuroprotective and healing effects of TSPO ligands could also contribute to a better understanding of the therapeutic potential of TSPO activity for brain damage due to brain injury and disease. Studies so far have directed attention to the effects on neurons and glia, and processes, such as death, inflammation, and regeneration. It is definitely worthwhile to drive such studies forward. From recent research it also appears that TSPO ligands, such as PK11195, Etifoxine, Emapunil, and 2-Cl-MGV-1, demonstrate the potential of targeting TSPO for treatments of brain diseases and disorders.

Targeting Hsp70 facilitated protein quality control for treatment of polyglutamine diseases

The polyglutamine (polyQ) diseases are a group of nine fatal, adult-onset neurodegenerative disorders characterized by the misfolding and aggregation of mutant proteins containing toxic expansions of CAG/polyQ tracts. The heat shock protein 90 and 70 (Hsp90/Hsp70) chaperone machinery is a key component of cellular protein quality control, playing a role in the regulation of folding, aggregation, and degradation of polyQ proteins. The ability of Hsp70 to facilitate disaggregation and degradation of misfolded proteins makes it an attractive therapeutic target in polyQ diseases. Genetic studies have demonstrated that manipulation of Hsp70 and related co-chaperones can enhance the disaggregation and/or degradation of misfolded proteins in models of polyQ disease. Therefore, the development of small molecules that enhance Hsp70 activity is of great interest. However, it is still unclear if currently available Hsp70 modulators can selectively enhance disaggregation or degradation of misfolded proteins without perturbing other Hsp70 functions essential for cellular homeostasis. This review discusses the multifaceted role of Hsp70 in protein quality control and the opportunities and challenges Hsp70 poses as a potential therapeutic target in polyQ disease.

Prion-like mechanisms in neurodegenerative disease: Implications for Huntington's disease therapy

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by a CAG repeat expansions in the huntingtin gene resulting in the synthesis of a misfolded form of the huntingtin protein (mHTT) which is toxic. The current treatments for HD are only palliative. Some of the potential therapies for HD include gene therapy (using antisense oligonucleotides and clustered regularly interspaced short palindromic repeats-Cas9 system) and stem-cell-based therapies. Various types of stem cell transplants, such as mesenchymal stem cells, neural stem cells, and reprogrammed stem cells, have the potential to either replace the lost neurons or support the existing neurons by releasing trophic factors. Most of the transplants are xenografts and allografts; however, recent reports on HD patients who received grafts suggest that the mHTT aggregates are transferred from the host neurons to the grafted cells as well as to the surrounding areas of the graft by a "prion-like" mechanism. This observation seems to be true for autotransplantation paradigms, as well. This article reviews the different types of stem cells that have been transplanted into HD patients and their therapeutic efficacy, focusing on the transfer of mHTT from the host cells to the graft. Autotransplants of reprogramed stem cells in HD patients are a promising therapeutic option. However, this needs further attention to ensure a better understanding of the transfer of mHTT aggregates following transplantation of the gene-corrected cells back into the patient.

Poly (ADP-ribose) (PAR)-dependent cell death in neurodegenerative diseases

Disruption of cellular functions with aging-induced accumulation of neuronal stressors causes cell death which is a common feature of neurodegenerative diseases. Studies in a variety of neurodegenerative disease models demonstrate that poly (ADP-ribose) (PAR)-dependent cell death, also named parthanatos, is responsible for neuronal loss in neurological diseases, such as Parkinson's disease (PD), Alzheimer's disease (AD), Huntington's disease (HD) and amyotrophic lateral sclerosis (ALS). Parthanatos has distinct features that differ from caspase-dependent apoptosis, necrosis or autophagic cell death. Parthanatos can be triggered by the accumulation of PAR due to overactivation of PAR polymerase-1 (PARP-1). Excess PAR, induces the mitochondrial release apoptosis-inducing factor (AIF), which binds to macrophage migration inhibitory factor (MIF) carrying MIF into the nucleus where it cleaves genomic DNA into large fragments. In this review, we will discuss the molecular mechanisms of parthanatos and their role in neurodegenerative diseases. Furthermore, we will discuss promising therapeutic interventions within the pathological PAR signaling cascade that could be designed to halt the progression of neurodegeneration.

The Expanding Therapeutic Potential of Neuronal KCC2

Dysfunctions in GABAergic inhibitory neural transmission occur in neuronal injuries and neurological disorders. The potassium-chloride cotransporter 2 (KCC2, SLC12A5) is a key modulator of inhibitory GABAergic inputs in healthy adult neurons, as its chloride (Cl-) extruding activity underlies the hyperpolarizing reversal potential for GABAA receptor Cl- currents (EGABA). Manipulation of KCC2 levels or activity improve symptoms associated with epilepsy and neuropathy. Recent works have now indicated that pharmacological enhancement of KCC2 function could reactivate dormant relay circuits in an injured mouse's spinal cord, leading to functional recovery and the attenuation of neuronal abnormality and disease phenotype associated with a mouse model of Rett syndrome (RTT). KCC2 interacts with Huntingtin and is downregulated in Huntington's disease (HD), which contributed to GABAergic excitation and memory deficits in the R6/2 mouse HD model. Here, these recent advances are highlighted, which attest to KCC2's growing potential as a therapeutic target for neuropathological conditions resulting from dysfunctional inhibitory input.

Therapeutic Advances for Huntington's Disease

Huntington’s disease (HD) is a progressive neurological disease that is inherited in an autosomal fashion. The cause of disease pathology is an expansion of cytosine-adenine-guanine (CAG) repeats within the huntingtin gene (HTT) on chromosome 4 (4p16.3), which codes the huntingtin protein (mHTT). The common symptoms of HD include motor and cognitive impairment of psychiatric functions. Patients exhibit a representative phenotype of involuntary movement (chorea) of limbs, impaired cognition, and severe psychiatric disturbances (mood swings, depression, and personality changes). A variety of symptomatic treatments (which target glutamate and dopamine pathways, caspases, inhibition of aggregation, mitochondrial dysfunction, transcriptional dysregulation, and fetal neural transplants, etc.) are available and some are in the pipeline. Advancement in novel therapeutic approaches include targeting the mutant huntingtin (mHTT) protein and the HTT gene. New gene editing techniques will reduce the CAG repeats. More appropriate and readily tractable treatment goals, coupled with advances in analytical tools will help to assess the clinical outcomes of HD treatments. This will not only improve the quality of life and life span of HD patients, but it will also provide a beneficial role in other inherited and neurological disorders. In this review, we aim to discuss current therapeutic research approaches and their possible uses for HD.

Cotranslational Folding of Proteins on the Ribosome

Many proteins in the cell fold cotranslationally within the restricted space of the polypeptide exit tunnel or at the surface of the ribosome. A growing body of evidence suggests that the ribosome can alter the folding trajectory in many different ways. In this review, we summarize the recent examples of how translation affects folding of single-domain, multiple-domain and oligomeric proteins. The vectorial nature of translation, the spatial constraints of the exit tunnel, and the electrostatic properties of the ribosome-nascent peptide complex define the onset of early folding events. The ribosome can facilitate protein compaction, induce the formation of intermediates that are not observed in solution, or delay the onset of folding. Examples of single-domain proteins suggest that early compaction events can define the folding pathway for some types of domain structures. Folding of multi-domain proteins proceeds in a domain-wise fashion, with each domain having its role in stabilizing or destabilizing neighboring domains. Finally, the assembly of protein complexes can also begin cotranslationally. In all these cases, the ribosome helps the nascent protein to attain a native fold and avoid the kinetic traps of misfolding.

Trace elements profile in the blood of Huntington' disease patients

Huntington' disease (HD) is an autosomal dominant neurodegenerative disease characterized by progressive motor, psychiatric, and cognitive deterioration. HD is, together with spinocerebellar ataxias, spinobulbar muscular atrophy and dentatorubral-pallido- luysian atrophy, one of the nine disorders caused by an expansion of glutamine residues in the causative protein where the polyglutamine expansion cause aberrant protein folding. Since an excessive metal's accumulation in organs may induce protein misfolding and oxidative stress, we have studied the blood concentration of essential (Cr, Co, Cu, Fe, Mn, Mo, Ni, Se, Zn) and nonessential (As, Cd, Sb, Sn, V) trace elements in HD patients. We found increased levels of the essential elements iron, chromium, selenium and zinc and of the nonessential element arsenic in the blood of HD patients. Since alteration in metals homeostasis may contribute to the pathogenesis of neurodegenerative disease and could eventually constitute a target for therapy, we may suggest the utilize of the blood metal profile as a further in vivo tool to study and characterize Huntington disease.

Late-onset retinal degeneration pathology due to mutations in CTRP5 is mediated through HTRA1

Late-onset retinal degeneration (L-ORD) is an autosomal dominant macular degeneration characterized by the formation of sub-retinal pigment epithelium (RPE) deposits and neuroretinal atrophy. L-ORD results from mutations in the C1q-tumor necrosis factor-5 protein (CTRP5), encoded by the CTRP5/C1QTNF5 gene. To understand the mechanism underlying L-ORD pathology, we used a human cDNA library yeast two-hybrid screen to identify interacting partners of CTRP5. Additionally, we analyzed the Bruch's membrane/choroid (BM-Ch) from wild-type (Wt), heterozygous S163R Ctrp5 mutation knock-in (Ctrp5S163R/wt ), and homozygous knock-in (Ctrp5S163R/S163R ) mice using mass spectrometry. Both approaches showed an association between CTRP5 and HTRA1 via its C-terminal PDZ-binding motif, stimulation of the HTRA1 protease activity by CTRP5, and CTRP5 serving as an HTRA1 substrate. The S163R-CTRP5 protein also binds to HTRA1 but is resistant to HTRA1-mediated cleavage. Immunohistochemistry and proteomic analysis showed significant accumulation of CTRP5 and HTRA1 in BM-Ch of Ctrp5S163R/S163R and Ctrp5S163R/wt mice compared with Wt. Additional extracellular matrix (ECM) components that are HTRA1 substrates also accumulated in these mice. These results implicate HTRA1 and its interaction with CTRP5 in L-ORD pathology.