Proteolysis of mutant huntingtin produces an exon 1 fragment that accumulates as an aggregated protein in neuronal nuclei in Huntington disease

The Role of Liquid-Liquid Phase Separation in the Accumulation of Pathological Proteins: New Perspectives on the Mechanism of Neurodegenerative Diseases

It is widely accepted that living organisms form highly dynamic membrane-less organelles (MLOS) with various functions through phase separation, and the indispensable role that phase separation plays in the mechanisms of normal physiological functions and pathogenesis is gradually becoming clearer. Pathological aggregates, regarded as hallmarks of neurodegenerative diseases, have been revealed to be closely related to aberrant phase separation. Specific proteins are assembled into condensates and transform into insoluble inclusions through aberrant phase separation, contributing to the development of diseases. In this review, we present an overview of the progress of phase separation research, involving its biological mechanisms and the status of research in neurodegenerative diseases, focusing on five main disease-specific proteins, tau, TDP-43, FUS, α-Syn and HTT, and how exactly these proteins reside within dynamic liquid-like compartments and thus turn into solid deposits. Further studies will yield new perspectives for understanding the aggregation mechanisms and potential therapeutic strategies, and future research directions are anticipated.

A CAG repeat threshold for therapeutics targeting somatic instability in Huntington's disease

The Huntington's disease mutation is a CAG repeat expansion in the huntingtin gene that results in an expanded polyglutamine tract in the huntingtin protein. The CAG repeat is unstable and expansions of hundreds of CAGs have been detected in Huntington's disease post-mortem brains. The age of disease onset can be predicted partially from the length of the CAG repeat as measured in blood. Onset age is also determined by genetic modifiers, which in six cases involve variation in DNA mismatch repair pathways genes. Knocking-out specific mismatch repair genes in mouse models of Huntington's disease prevents somatic CAG repeat expansion. Taken together, these results have led to the hypothesis that somatic CAG repeat expansion in Huntington's disease brains is required for pathogenesis. Therefore, the pathogenic repeat threshold in brain is longer than (CAG)40, as measured in blood, and is currently unknown. The mismatch repair gene MSH3 has become a major focus for therapeutic development, as unlike other mismatch repair genes, nullizygosity for MSH3 does not cause malignancies associated with mismatch repair deficiency. Potential treatments targeting MSH3 currently under development include gene therapy, biologics and small molecules, which will be assessed for efficacy in mouse models of Huntington's disease. The zQ175 knock-in model carries a mutation of approximately (CAG)185 and develops early molecular and pathological phenotypes that have been extensively characterized. Therefore, we crossed the mutant huntingtin allele onto heterozygous and homozygous Msh3 knockout backgrounds to determine the maximum benefit of targeting Msh3 in this model. Ablation of Msh3 prevented somatic expansion throughout the brain and periphery, and reduction of Msh3 by 50% decreased the rate of expansion. This had no effect on the deposition of huntingtin aggregation in the nuclei of striatal neurons, nor on the dysregulated striatal transcriptional profile. This contrasts with ablating Msh3 in knock-in models with shorter CAG repeat expansions. Therefore, further expansion of a (CAG)185 repeat in striatal neurons does not accelerate the onset of molecular and neuropathological phenotypes. It is striking that highly expanded CAG repeats of a similar size in humans cause disease onset before 2 years of age, indicating that somatic CAG repeat expansion in the brain is not required for pathogenesis. Given that the trajectory for somatic CAG expansion in the brains of Huntington's disease mutation carriers is unknown, our study underlines the importance of administering treatments targeting somatic instability as early as possible.

Solid-state nuclear magnetic resonance in the structural study of polyglutamine aggregation

The aggregation of proteins into amyloid-like fibrils is seen in many neurodegenerative diseases. Recent years have seen much progress in our understanding of these misfolded protein inclusions, thanks to advances in techniques such as solid-state nuclear magnetic resonance (ssNMR) spectroscopy and cryogenic electron microscopy (cryo-EM). However, multiple repeat-expansion-related disorders have presented special challenges to structural elucidation. This review discusses the special role of ssNMR analysis in the study of protein aggregates associated with CAG repeat expansion disorders. In these diseases, the misfolding and aggregation affect mutant proteins with expanded polyglutamine segments. The most common disorder, Huntington's disease (HD), is connected to the mutation of the huntingtin protein. Since the discovery of the genetic causes for HD in the 1990s, steady progress in our understanding of the role of protein aggregation has depended on the integrative and interdisciplinary use of multiple types of structural techniques. The heterogeneous and dynamic features of polyQ protein fibrils, and in particular those formed by huntingtin N-terminal fragments, have made these aggregates into challenging targets for structural analysis. ssNMR has offered unique insights into many aspects of these amyloid-like aggregates. These include the atomic-level structure of the polyglutamine core, but also measurements of dynamics and solvent accessibility of the non-core flanking domains of these fibrils' fuzzy coats. The obtained structural insights shed new light on pathogenic mechanisms behind this and other protein misfolding diseases.

Insulin-Degrading Enzyme Efficiently Degrades polyQ Peptides but not Expanded polyQ Huntingtin Fragments

Background

Huntington's disease is an inheritable autosomal dominant disorder caused by an expanded CAG trinucleotide repeat within the Huntingtin gene, leading to a polyglutamine (polyQ) expansion in the mutant protein.

Objective

A potential therapeutic approach for delaying or preventing the onset of the disease involves enhancing the degradation of the aggregation-prone polyQ-expanded N-terminal mutant huntingtin (mHTT) exon1 fragment. A few proteases and peptidases have been identified that are able to cleave polyQ fragments with low efficiency. This study aims to identify a potent polyQ-degrading endopeptidase.

Methods

Here we used quenched polyQ peptides to identify a polyQ-degrading endopeptidase. Next we investigated its role on HTT turnover, using purified polyQ-expanded HTT fragments and striatal cells expressing mHTT exon1 peptides.

Results

We identified insulin-degrading enzyme (IDE) as a novel endopeptidase for degrading polyQ peptides. IDE was, however, ineffective in reducing purified polyQ-expanded HTT fragments. Similarly, in striatal cells expressing mHTT exon1 peptides, IDE did not enhance mHTT turnover.

Conclusions

This study shows that despite IDE's efficiency in degrading polyQ peptides, it does not contribute to the direct degradation of polyQ-expanded mHTT fragments.

Viral-based animal models in polyglutamine disorders

Polyglutamine disorders are a complex group of incurable neurodegenerative disorders caused by an abnormal expansion in the trinucleotide cytosine-adenine-guanine tract of the affected gene. To better understand these disorders, our dependence on animal models persists, primarily relying on transgenic models. In an effort to complement and deepen our knowledge, researchers have also developed animal models of polyglutamine disorders employing viral vectors. Viral vectors have been extensively used to deliver genes to the brain, not only for therapeutic purposes but also for the development of animal models, given their remarkable flexibility. In a time- and cost-effective manner, it is possible to use different transgenes, at varying doses, in diverse targeted tissues, at different ages, and in different species, to recreate polyglutamine pathology. This paper aims to showcase the utility of viral vectors in disease modelling, share essential considerations for developing animal models with viral vectors, and provide a comprehensive review of existing viral-based animal models for polyglutamine disorders.

Dietary fasting and time-restricted eating in Huntington's disease: therapeutic potential and underlying mechanisms

Huntington's disease (HD) is a devastating neurodegenerative disorder caused by aggregation of the mutant huntingtin (mHTT) protein, resulting from a CAG repeat expansion in the huntingtin gene HTT. HD is characterized by a variety of debilitating symptoms including involuntary movements, cognitive impairment, and psychiatric disturbances. Despite considerable efforts, effective disease-modifying treatments for HD remain elusive, necessitating exploration of novel therapeutic approaches, including lifestyle modifications that could delay symptom onset and disease progression. Recent studies suggest that time-restricted eating (TRE), a form of intermittent fasting involving daily caloric intake within a limited time window, may hold promise in the treatment of neurodegenerative diseases, including HD. TRE has been shown to improve mitochondrial function, upregulate autophagy, reduce oxidative stress, regulate the sleep-wake cycle, and enhance cognitive function. In this review, we explore the potential therapeutic role of TRE in HD, focusing on its underlying physiological mechanisms. We discuss how TRE might enhance the clearance of mHTT, recover striatal brain-derived neurotrophic factor levels, improve mitochondrial function and stress-response pathways, and synchronize circadian rhythm activity. Understanding these mechanisms is critical for the development of targeted lifestyle interventions to mitigate HD pathology and improve patient outcomes. While the potential benefits of TRE in HD animal models are encouraging, future comprehensive clinical trials will be necessary to evaluate its safety, feasibility, and efficacy in persons with HD.

Latest advances on new promising molecular-based therapeutic approaches for Huntington's disease

Huntington's disease (HD) is a devastating, autosomal-dominant inherited, neurodegenerative disorder characterized by progressive motor deficits, cognitive impairments, and neuropsychiatric symptoms. It is caused by excessive cytosine-adenine-guanine (CAG) trinucleotide repeats within the huntingtin gene (HTT). Presently, therapeutic interventions capable of altering the trajectory of HD are lacking, while medications for abnormal movement and psychiatric symptoms are limited. Numerous pre-clinical and clinical studies have been conducted and are currently underway to test the efficacy of therapeutic approaches targeting some of these mechanisms with varying degrees of success. In this review, we update the latest advances on new promising molecular-based therapeutic strategies for this disorder, including DNA-targeting techniques such as zinc-finger proteins, transcription activator-like effector nucleases, and CRISPR/Cas9; post-transcriptional huntingtin-lowering approaches such as RNAi, antisense oligonucleotides, and small-molecule splicing modulators; and novel methods to clear the mHTT protein, such as proteolysis-targeting chimeras. We mainly focus on the ongoing clinical trials and the latest pre-clinical studies to explore the progress of emerging potential HD therapeutics.

Large animal models for Huntington's disease research

Huntington's disease (HD) is a hereditary neurodegenerative disorder for which there is currently no effective treatment available. Consequently, the development of appropriate disease models is critical to thoroughly investigate disease progression. The genetic basis of HD involves the abnormal expansion of CAG repeats in the huntingtin ( HTT) gene, leading to the expansion of a polyglutamine repeat in the HTT protein. Mutant HTT carrying the expanded polyglutamine repeat undergoes misfolding and forms aggregates in the brain, which precipitate selective neuronal loss in specific brain regions. Animal models play an important role in elucidating the pathogenesis of neurodegenerative disorders such as HD and in identifying potential therapeutic targets. Due to the marked species differences between rodents and larger animals, substantial efforts have been directed toward establishing large animal models for HD research. These models are pivotal for advancing the discovery of novel therapeutic targets, enhancing effective drug delivery methods, and improving treatment outcomes. We have explored the advantages of utilizing large animal models, particularly pigs, in previous reviews. Since then, however, significant progress has been made in developing more sophisticated animal models that faithfully replicate the typical pathology of HD. In the current review, we provide a comprehensive overview of large animal models of HD, incorporating recent findings regarding the establishment of HD knock-in (KI) pigs and their genetic therapy. We also explore the utilization of large animal models in HD research, with a focus on sheep, non-human primates (NHPs), and pigs. Our objective is to provide valuable insights into the application of these large animal models for the investigation and treatment of neurodegenerative disorders.

Antisense oligonucleotide-mediated disruption of HTT caspase-6 cleavage site ameliorates the phenotype of YAC128 Huntington disease mice

In Huntington disease, cellular toxicity is particularly caused by toxic protein fragments generated from the mutant huntingtin (HTT) protein. By modifying the HTT protein, we aim to reduce proteolytic cleavage and ameliorate the consequences of mutant HTT without lowering total HTT levels. To that end, we use an antisense oligonucleotide (AON) that targets HTT pre-mRNA and induces partial skipping of exon 12, which contains the critical caspase-6 cleavage site. Here, we show that AON-treatment can partially restore the phenotype of YAC128 mice, a mouse model expressing the full-length human HTT gene including 128 CAG-repeats. Wild-type and YAC128 mice were treated intracerebroventricularly with AON12.1, scrambled AON or vehicle starting at 6 months of age and followed up to 12 months of age, when MRI was performed and mice were sacrificed. AON12.1 treatment induced around 40% exon skip and protein modification. The phenotype on body weight and activity, but not rotarod, was restored by AON treatment. Genes differentially expressed in YAC128 striatum changed toward wild-type levels and striatal volume was preserved upon AON12.1 treatment. However, scrambled AON also showed a restorative effect on gene expression and appeared to generally increase brain volume.

Interaction between huntingtin exon 1 and HEAT repeat structure probed by chimeric model proteins

Huntington disease (HD) is associated with aggregation of huntingtin (HTT) protein containing over 35 continuous Q residues within the N-terminal exon 1 encoded region. The C-terminal of the HTT protein consists mainly of HEAT repeat structure which serves as a scaffold for multiple cellular activities. Structural and biochemical analysis of the intact HTT protein has been hampered by its huge size (~300 kDa) and most in vitro studies to date have focused on the properties of the exon 1 region. To explore the interaction between HTT exon 1 and the HEAT repeat structure, we constructed chimeric proteins containing the N-terminal HTT exon 1 region and the HEAT repeat protein PR65/A. The results indicate that HTT exon 1 slightly destabilizes the downstream HEAT repeat structure and endows the HEAT repeat structure with more conformational flexibility. Wild-type and pathological lengths of polyQ did not show differences in the interaction between HTT exon 1 and the HEAT repeats. With the C-terminal fusion of PR65/A, HTT exon 1 containing pathological lengths of polyQ could still form amyloid fibrils, but the higher-order architecture of fibrils and kinetics of fibril formation were affected by the C-terminal fusion of HEAT repeats. This indicates that interaction between HTT exon 1 and HEAT repeat structure is compatible with both normal function of HTT protein and the pathogenesis of HD, and this study provides a potential model for further exploration.

Interactions of amyloidogenic proteins with mitochondrial protein import machinery in aging-related neurodegenerative diseases

Most mitochondrial proteins are targeted to the organelle by N-terminal mitochondrial targeting sequences (MTSs, or "presequences") that are recognized by the import machinery and subsequently cleaved to yield the mature protein. MTSs do not have conserved amino acid compositions, but share common physicochemical properties, including the ability to form amphipathic α-helical structures enriched with basic and hydrophobic residues on alternating faces. The lack of strict sequence conservation implies that some polypeptides can be mistargeted to mitochondria, especially under cellular stress. The pathogenic accumulation of proteins within mitochondria is implicated in many aging-related neurodegenerative diseases, including Alzheimer's, Parkinson's, and Huntington's diseases. Mechanistically, these diseases may originate in part from mitochondrial interactions with amyloid-β precursor protein (APP) or its cleavage product amyloid-β (Aβ), α-synuclein (α-syn), and mutant forms of huntingtin (mHtt), respectively, that are mediated in part through their associations with the mitochondrial protein import machinery. Emerging evidence suggests that these amyloidogenic proteins may present cryptic targeting signals that act as MTS mimetics and can be recognized by mitochondrial import receptors and transported into different mitochondrial compartments. Accumulation of these mistargeted proteins could overwhelm the import machinery and its associated quality control mechanisms, thereby contributing to neurological disease progression. Alternatively, the uptake of amyloidogenic proteins into mitochondria may be part of a protein quality control mechanism for clearance of cytotoxic proteins. Here we review the pathomechanisms of these diseases as they relate to mitochondrial protein import and effects on mitochondrial function, what features of APP/Aβ, α-syn and mHtt make them suitable substrates for the import machinery, and how this information can be leveraged for the development of therapeutic interventions.

Early whole-body mutant huntingtin lowering averts changes in proteins and lipids important for synapse function and white matter maintenance in the LacQ140 mouse model

Expansion of a triplet repeat tract in exon 1 of the HTT gene causes Huntington's disease (HD). The mutant HTT protein (mHTT) has numerous aberrant interactions with diverse, pleiomorphic effects. Lowering mHTT is a promising approach to treat HD, but it is unclear when lowering should be initiated, how much is necessary, and what duration should occur to achieve benefits. Furthermore, the effects of mHTT lowering on brain lipids have not been assessed. Using a mHtt-inducible mouse model, we analyzed mHtt lowering initiated at different ages and sustained for different time-periods. mHTT protein in cytoplasmic and synaptic compartments of the striatum was reduced 38-52%; however, there was minimal lowering of mHTT in nuclear and perinuclear regions where aggregates formed at 12 months of age. Total striatal lipids were reduced in 9-month-old LacQ140 mice and preserved by mHtt lowering. Subclasses important for white matter structure and function including ceramide (Cer), sphingomyelin (SM), and monogalactosyldiacylglycerol (MGDG), contributed to the reduction in total lipids. Phosphatidylinositol (PI), phosphatidylserine (PS), and bismethyl phosphatidic acid (BisMePA) were also changed in LacQ140 mice. Levels of all subclasses except ceramide were preserved by mHtt lowering. mRNA expression profiling indicated that a transcriptional mechanism contributes to changes in myelin lipids, and some but not all changes can be prevented by mHtt lowering. Our findings suggest that early and sustained reduction in mHtt can prevent changes in levels of select striatal proteins and most lipids, but a misfolded, degradation-resistant form of mHTT hampers some benefits in the long term.

IKBKB reduces huntingtin aggregation by phosphorylating serine 13 via a non-canonical IKK pathway

N-terminal phosphorylation at residues T3 and S13 is believed to have important beneficial implications for the biological and pathological properties of mutant huntingtin, where inhibitor of nuclear factor kappa B kinase subunit beta (IKBKB) was identified as a candidate regulator of huntingtin N-terminal phosphorylation. The paucity of mechanistic information on IKK pathways, together with the lack of sensitive methods to quantify endogenous huntingtin phosphorylation, prevented detailed study of the role of IKBKB in Huntington's disease. Using novel ultrasensitive assays, we demonstrate that IKBKB can regulate endogenous S13 huntingtin phosphorylation in a manner, dependent on its kinase activity and known regulators. We found that the ability of IKBKB to phosphorylate endogenous huntingtin S13 is mediated through a non-canonical interferon regulatory factor3-mediated IKK pathway, distinct from the established involvement of IKBKB in mutant huntingtin's pathological mechanisms mediated via the canonical pathway. Furthermore, increased huntingtin S13 phosphorylation by IKBKB resulted in decreased aggregation of mutant huntingtin in cells, again dependent on its kinase activity. These findings point to a non-canonical IKK pathway linking S13 huntingtin phosphorylation to the pathological properties of mutant huntingtin aggregation, thought to be significant to Huntington's disease.

Huntingtin Interacting Proteins and Pathological Implications

Huntington's disease (HD) is caused by an expansion of a CAG repeat in the gene that encodes the huntingtin protein (HTT). The exact function of HTT is still not fully understood, and previous studies have mainly focused on identifying proteins that interact with HTT to gain insights into its function. Numerous HTT-interacting proteins have been discovered, shedding light on the functions and structure of HTT. Most of these proteins interact with the N-terminal region of HTT. Among the various HTT-interacting proteins, huntingtin-associated protein 1 (HAP1) and HTT-interacting protein 1 (HIP1) have been extensively studied. Recent research has uncovered differences in the distribution of HAP1 in monkey and human brains compared with mice. This finding suggests that there may be species-specific variations in the regulation and function of HTT-interacting proteins. Understanding these differences could provide crucial insights into the development of HD. In this review, we will focus on the recent advancements in the study of HTT-interacting proteins, with particular attention to the differential distributions of HTT and HAP1 in larger animal models.

Rhes depletion promotes striatal accumulation and aggregation of mutant huntingtin in a presymptomatic HD mouse model

Introduction

Huntington's disease (HD) is caused by CAG trinucleotide repeats in the HTT gene. Selective neurodegeneration in the striatum is prominent in HD, despite widespread expression of mutant HTT (mHTT). Ras homolog enriched in the striatum (Rhes) is a GTP-binding protein enriched in the striatum, involved in dopamine-related behaviors and autophagy regulation. Growing evidence suggests Rhes plays a critical role in the selective striatal degeneration in HD, but its specific function in this context remains complex and controversial.

Methods

In this study, we utilized CRISPR/Cas9 to knockdown Rhes at different disease stages through adeno-associated virus (AAV) transduction in HD knock-in (KI) mice. Immunoblotting and immunofluorescence were employed to assess the impact of Rhes depletion on mHTT levels, neuronal loss, astrogliosis and autophagy activity.

Results

Rhes depletion in 22-week-old HD KI mice (representing the presymptomatic stage) led to mHTT accumulation, reduced neuronal cell staining, and increased astrogliosis. However, no such effects were observed in 36-week-old HD KI mice (representing the symptomatic stage). Additionally, Rhes deletion in 22-week-old HD KI mice resulted in increased P62 levels, reduced LC3-II levels, and unchanged phosphorylation of mTOR and beclin-1, unchanged mTOR protein level, except for a decrease in beclin-1.

Discussion

Our findings suggest that knockdown Rhes promotes striatal aggregation of mutant huntingtin by reducing autophagy activity in a mTOR-independent manner. Rhes plays a protective role during the presymptomatic stage of HD KI mice.

Early whole-body mutant huntingtin lowering averts changes in proteins and lipids important for synapse function and white matter maintenance in the LacQ140 mouse model

Expansion of a triplet repeat tract in exon1 of the HTT gene causes Huntington's disease (HD). The mutant HTT protein (mHTT) has numerous aberrant interactions with diverse, pleiomorphic effects. No disease modifying treatments exist but lowering mutant huntingtin (mHTT) by gene therapy is a promising approach to treat Huntington's disease (HD). It is not clear when lowering should be initiated, how much lowering is necessary and for what duration lowering should occur to achieve benefits. Furthermore, the effects of mHTT lowering on brain lipids have not been assessed. Using a mHtt-inducible mouse model we analyzed whole body mHtt lowering initiated at different ages and sustained for different time-periods. Subcellular fractionation (density gradient ultracentrifugation), protein chemistry (gel filtration, western blot, and capillary electrophoresis immunoassay), liquid chromatography and mass spectrometry of lipids, and bioinformatic approaches were used to test effects of mHTT transcriptional lowering. mHTT protein in cytoplasmic and synaptic compartments of the caudate putamen, which is most affected in HD, was reduced 38-52%. Little or no lowering of mHTT occurred in nuclear and perinuclear regions where aggregates formed at 12 months of age. mHtt transcript repression partially or fully preserved select striatal proteins (SCN4B, PDE10A). Total lipids in striatum were reduced in LacQ140 mice at 9 months and preserved by early partial mHtt lowering. The reduction in total lipids was due in part to reductions in subclasses of ceramide (Cer), sphingomyelin (SM), and monogalactosyldiacylglycerol (MGDG), which are known to be important for white matter structure and function. Lipid subclasses phosphatidylinositol (PI), phosphatidylserine (PS), and bismethyl phosphatidic acid (BisMePA) were also changed in LacQ140 mice. Levels of all subclasses other than ceramide were preserved by early mHtt lowering. Pathway enrichment analysis of RNAseq data imply a transcriptional mechanism is responsible in part for changes in myelin lipids, and some but not all changes can be rescued by mHTT lowering. Our findings suggest that early and sustained reduction in mHtt can prevent changes in levels of select striatal proteins and most lipids but a misfolded, degradation-resistant form of mHTT hampers some benefits in the long term.

Unraveling Molecular and Genetic Insights into Neurodegenerative Diseases: Advances in Understanding Alzheimer's, Parkinson's, and Huntington's Diseases and Amyotrophic Lateral Sclerosis

Neurodegenerative diseases are, according to recent studies, one of the main causes of disability and death worldwide. Interest in molecular genetics has started to experience exponential growth thanks to numerous advancements in technology, shifts in the understanding of the disease as a phenomenon, and the change in the perspective regarding gene editing and the advantages of this action. The aim of this paper is to analyze the newest approaches in genetics and molecular sciences regarding four of the most important neurodegenerative disorders: Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. We intend through this review to focus on the newest treatment, diagnosis, and predictions regarding this large group of diseases, in order to obtain a more accurate analysis and to identify the emerging signs that could lead to a better outcome in order to increase both the quality and the life span of the patient. Moreover, this review could provide evidence of future possible novel therapies that target the specific genes and that could be useful to be taken into consideration when the classical approaches fail to shed light.

Coaggregation of polyglutamine (polyQ) proteins is mediated by polyQ-tract interactions and impairs cellular proteostasis

Nine polyglutamine (polyQ) proteins have already been identified that are considered to be associated with the pathologies of neurodegenerative disorders called polyQ diseases, but whether these polyQ proteins mutually interact and synergize in proteinopathies remains to be elucidated. In this study, 4 polyQ-containing proteins, androgen receptor (AR), ataxin-7 (Atx7), huntingtin (Htt) and ataxin-3 (Atx3), are used as model molecules to investigate their heterologous coaggregation and consequent impact on cellular proteostasis. Our data indicate that the N-terminal fragment of polyQ-expanded (PQE) Atx7 or Htt can coaggregate with and sequester AR and Atx3 into insoluble aggregates or inclusions through their respective polyQ tracts. In vitro coprecipitation and NMR titration experiments suggest that this specific coaggregation depends on polyQ lengths and is probably mediated by polyQ-tract interactions. Luciferase reporter assay shows that these coaggregation and sequestration effects can deplete the cellular availability of AR and consequently impair its transactivation function. This study provides valid evidence supporting the viewpoint that coaggregation of polyQ proteins is mediated by polyQ-tract interactions and benefits our understanding of the molecular mechanism underlying the accumulation of different polyQ proteins in inclusions and their copathological causes of polyQ diseases.

Early detection of exon 1 huntingtin aggregation in zQ175 brains by molecular and histological approaches

Huntingtin-lowering approaches that target huntingtin expression are a major focus for therapeutic intervention for Huntington's disease. When the cytosine, adenine and guanine repeat is expanded, the huntingtin pre-mRNA is alternatively processed to generate the full-length huntingtin and HTT1a transcripts. HTT1a encodes the aggregation-prone and highly pathogenic exon 1 huntingtin protein. In evaluating huntingtin-lowering approaches, understanding how the targeting strategy modulates levels of both transcripts and the huntingtin protein isoforms that they encode will be essential. Given the aggregation-propensity of exon 1 huntingtin, the impact of a given strategy on the levels and subcellular location of aggregated huntingtin will need to be determined. We have developed and applied sensitive molecular approaches to monitor the levels of aggregated and soluble huntingtin isoforms in tissue lysates. We have used these, in combination with immunohistochemistry, to map the appearance and accumulation of aggregated huntingtin throughout the CNS of zQ175 mice, a model of Huntington's disease frequently chosen for preclinical studies. Aggregation analyses were performed on tissues from zQ175 and wild-type mice at monthly intervals from 1 to 6 months of age. We developed three homogeneous time-resolved fluorescence assays to track the accumulation of aggregated huntingtin and showed that two of these were specific for the exon 1 huntingtin protein. Collectively, the homogeneous time-resolved fluorescence assays detected huntingtin aggregation in the 10 zQ175 CNS regions by 1-2 months of age. Immunohistochemistry with the polyclonal S830 anti-huntingtin antibody showed that nuclear huntingtin aggregation, in the form of a diffuse nuclear immunostain, could be visualized in the striatum, hippocampal CA1 region and layer IV of the somatosensory cortex by 2 months. That this diffuse nuclear immunostain represented aggregated huntingtin was confirmed by immunohistochemistry with a polyglutamine-specific antibody, which required formic acid antigen retrieval to expose its epitope. By 6 months of age, nuclear and cytoplasmic inclusions were widely distributed throughout the brain. Homogeneous time-resolved fluorescence analysis showed that the comparative levels of soluble exon 1 huntingtin between CNS regions correlated with those for huntingtin aggregation. We found that soluble exon 1 huntingtin levels decreased over the 6-month period, whilst those of soluble full-length mutant huntingtin remained unchanged, data that were confirmed for the cortex by immunoprecipitation and western blotting. These data support the hypothesis that exon 1 huntingtin initiates the aggregation process in knock-in mouse models and pave the way for a detailed analysis of huntingtin aggregation in response to huntingtin-lowering treatments.

Salivary Huntingtin protein is uniquely associated with clinical features of Huntington's disease

Measuring Huntingtin (HTT) protein in peripheral cells represents an essential step in biomarker discovery for Huntington's Disease (HD), however to date, investigations into the salivary expression of HTT has been lacking. In the current study, we quantified total HTT (tHTT) and mutant HTT (mHTT) protein in matched blood and saliva samples using single molecule counting (SMC) immunoassays: 2B7-D7F7 (tHTT) and 2B7-MW1 (mHTT). Matched samples, and clinical data, were collected from 95 subjects: n = 19 manifest HD, n = 34 premanifest HD (PM), and n = 42 normal controls (NC). Total HTT and mHTT levels were not correlated in blood and saliva. Plasma tHTT was significantly associated with age, and participant sex; whereas salivary mHTT was significantly correlated with age, CAG repeat length and CAP score. Plasma and salivary tHTT did not differ across cohorts. Salivary and plasma mHTT were significantly increased in PM compared to NC; salivary mHTT was also significantly increased in HD compared to NC. Only salivary tHTT and mHTT were significantly correlated with clinical measures. Salivary HTT is uniquely associated with clinical measures of HD and offers significant promise as a relevant, non-invasive HD biomarker. Its use could be immediately implemented into both translational and clinical research applications.

Ubiquitin-modifying enzymes in Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder caused by a CAG repeat expansion in the N-terminus of the HTT gene. The CAG repeat expansion translates into a polyglutamine expansion in the mutant HTT (mHTT) protein, resulting in intracellular aggregation and neurotoxicity. Lowering the mHTT protein by reducing synthesis or improving degradation would delay or prevent the onset of HD, and the ubiquitin-proteasome system (UPS) could be an important pathway to clear the mHTT proteins prior to aggregation. The UPS is not impaired in HD, and proteasomes can degrade mHTT entirely when HTT is targeted for degradation. However, the mHTT protein is differently ubiquitinated when compared to wild-type HTT (wtHTT), suggesting that the polyQ expansion affects interaction with (de) ubiquitinating enzymes and subsequent targeting for degradation. The soluble mHTT protein is associated with several ubiquitin-modifying enzymes, and various ubiquitin-modifying enzymes have been identified that are linked to Huntington's disease, either by improving mHTT turnover or affecting overall homeostasis. Here we describe their potential mechanism of action toward improved mHTT targeting towards the proteostasis machinery.

Alternative processing of human HTT mRNA with implications for Huntington's disease therapeutics

Huntington disease is caused by a CAG repeat expansion in exon 1 of the huntingtin gene (HTT) that is translated into a polyglutamine stretch in the huntingtin protein (HTT). We previously showed that HTT mRNA carrying an expanded CAG repeat was incompletely spliced to generate HTT1a, an exon 1 only transcript, which was translated to produce the highly aggregation-prone and pathogenic exon 1 HTT protein. This occurred in all knock-in mouse models of Huntington's disease and could be detected in patient cell lines and post-mortem brains. To extend these findings to a model system expressing human HTT, we took advantage of YAC128 mice that are transgenic for a yeast artificial chromosome carrying human HTT with an expanded CAG repeat. We discovered that the HTT1a transcript could be detected throughout the brains of YAC128 mice. We implemented RNAscope to visualize HTT transcripts at the single molecule level and found that full-length HTT and HTT1a were retained together in large nuclear RNA clusters, as well as being present as single transcripts in the cytoplasm. Homogeneous time-resolved fluorescence analysis demonstrated that the HTT1a transcript had been translated to produce the exon 1 HTT protein. The levels of exon 1 HTT in YAC128 mice, correlated with HTT aggregation, supportive of the hypothesis that exon 1 HTT initiates the aggregation process. Huntingtin-lowering strategies are a major focus of therapeutic development for Huntington's disease. These approaches often target full-length HTT alone and would not be expected to reduce pathogenic exon 1 HTT levels. We have established YAC128 mouse embryonic fibroblast lines and shown that, together with our QuantiGene multiplex assay, these provide an effective screening tool for agents that target HTT transcripts. The effects of current targeting strategies on nuclear RNA clusters are unknown, structures that may have a pathogenic role or alternatively could be protective by retaining HTT1a in the nucleus and preventing it from being translated. In light of recently halted antisense oligonucleotide trials, it is vital that agents targeting HTT1a are developed, and that the effects of HTT-lowering strategies on the subcellular levels of all HTT transcripts and their various HTT protein isoforms are understood.

Calpain-mediated proteolysis as driver and modulator of polyglutamine toxicity

Among posttranslational modifications, directed proteolytic processes have the strongest impact on protein integrity. They are executed by a variety of cellular machineries and lead to a wide range of molecular consequences. Compared to other forms of proteolytic enzymes, the class of calcium-activated calpains is considered as modulator proteases due to their limited proteolytic activity, which changes the structure and function of their target substrates. In the context of neurodegeneration and - in particular - polyglutamine disorders, proteolytic events have been linked to modulatory effects on the molecular pathogenesis by generating harmful breakdown products of disease proteins. These findings led to the formulation of the toxic fragment hypothesis, and calpains appeared to be one of the key players and auspicious therapeutic targets in Huntington disease and Machado Joseph disease. This review provides a current survey of the role of calpains in proteolytic processes found in polyglutamine disorders. Together with insights into general concepts behind toxic fragments and findings in polyglutamine disorders, this work aims to inspire researchers to broaden and deepen the knowledge in this field, which will help to evaluate calpain-mediated proteolysis as a unifying and therapeutically targetable posttranslational mechanism in neurodegeneration.

Emerging Therapies for Huntington's Disease - Focus on N-Terminal Huntingtin and Huntingtin Exon 1

Huntington's disease is a devastating heritable neurodegenerative disorder that is caused by the presence of a trinucleotide CAG repeat expansion in the Huntingtin gene, leading to a polyglutamine tract in the protein. Various mechanisms lead to the production of N-terminal Huntingtin protein fragments, which are reportedly more toxic than the full-length protein. In this review, we summarize the current knowledge on the production and toxicity of N-terminal Huntingtin protein fragments. Further, we expand on various therapeutic strategies targeting N-terminal Huntingtin on the protein, RNA and DNA level. Finally, we compare the therapeutic approaches that are clinically most advanced, including those that do not target N-terminal Huntingtin, discussing differences in mode of action and translational applicability.

Hunting for the cause: Evidence for prion-like mechanisms in Huntington’s disease

The hypothesis that pathogenic protein aggregates associated with neurodegenerative diseases spread from cell-to-cell in the brain in a manner akin to infectious prions has gained substantial momentum due to an explosion of research in the past 10–15 years. Here, we review current evidence supporting the existence of prion-like mechanisms in Huntington’s disease (HD), an autosomal dominant neurodegenerative disease caused by expansion of a CAG repeat tract in exon 1 of the huntingtin (HTT) gene. We summarize information gained from human studies and in vivo and in vitro models of HD that strongly support prion-like features of the mutant HTT (mHTT) protein, including potential involvement of molecular features of mHTT seeds, synaptic structures and connectivity, endocytic and exocytic mechanisms, tunneling nanotubes, and nonneuronal cells in mHTT propagation in the brain. We discuss mechanisms by which mHTT aggregate spreading and neurotoxicity could be causally linked and the potential benefits of targeting prion-like mechanisms in the search for new disease-modifying therapies for HD and other fatal neurodegenerative diseases.

Quantitative NMR analysis of the kinetics of prenucleation oligomerization and aggregation of pathogenic huntingtin exon-1 protein

The N-terminal region of the huntingtin protein, encoded by exon-1 (htt^ex1) and containing an expanded polyglutamine tract, forms fibrils that accumulate in neuronal inclusion bodies, resulting in Huntington's disease. We previously showed that reversible formation of a sparsely populated tetramer of the N-terminal amphiphilic domain, comprising a dimer of dimers in a four-helix bundle configuration, occurs on the microsecond timescale and is an essential prerequisite for subsequent nucleation and fibril formation that takes place orders of magnitude slower on a timescale of hours. For pathogenic htt^ex1, such as htt^ex1Q₃₅ with 35 glutamines, NMR signals decay too rapidly to permit measurement of time-intensive exchange-based experiments. Here, we show that quantitative analysis of both the kinetics and mechanism of prenucleation tetramerization and aggregation can be obtained simultaneously from a series of ¹H-¹⁵N band-selective optimized flip-angle short-transient heteronuclear multiple quantum coherence (SOFAST-HMQC) correlation spectra. The equilibria and kinetics of tetramerization are derived from the time dependence of the ¹⁵N chemical shifts and ¹H-¹⁵N cross-peak volume/intensity ratios, while the kinetics of irreversible fibril formation are afforded by the decay curves of ¹H-¹⁵N cross-peak intensities and volumes. Analysis of data on htt^ex1Q₃₅ over a series of concentrations ranging from 200 to 750 μM and containing variable (7 to 20%) amounts of the Met⁷O sulfoxide species, which does not tetramerize, shows that aggregation of native htt^ex1Q₃₅ proceeds via fourth-order primary nucleation, consistent with the critical role of prenucleation tetramerization, coupled with first-order secondary nucleation. The Met⁷O sulfoxide species does not nucleate but is still incorporated into fibrils by elongation.

Huntingtin and Its Partner Huntingtin-Associated Protein 40: Structural and Functional Considerations in Health and Disease

Since the discovery of the mutation causing Huntington’s disease (HD) in 1993, it has been debated whether an expanded polyglutamine (polyQ) stretch affects the properties of the huntingtin (HTT) protein and thus contributes to the pathological mechanisms responsible for HD. Here we review the current knowledge about the structure of HTT, alone (apo-HTT) or in a complex with Huntingtin-Associated Protein 40 (HAP40), the influence of polyQ-length variation on apo-HTT and the HTT-HAP40 complex, and the biology of HAP40. Phylogenetic analyses suggest that HAP40 performs essential functions. Highlighting the relevance of its interaction with HTT, HAP40 is one of the most abundant partners copurifying with HTT and is rapidly degraded, when HTT levels are reduced. As the levels of both proteins decrease during disease progression, HAP40 could also be a biomarker for HD. Whether declining HAP40 levels contribute to disease etiology is an open question. Structural studies have shown that the conformation of apo-HTT is less constrained but resembles that adopted in the HTT-HAP40 complex, which is exceptionally stable because of extensive interactions between HAP40 and the three domains of HTT. The complex— and to some extent apo-HTT— resists fragmentation after limited proteolysis. Unresolved regions of apo-HTT, constituting about 25% of the protein, are the main sites of post-translational modifications and likely have major regulatory functions. PolyQ elongation does not substantially alter the structure of HTT, alone or when associated with HAP40. Particularly, polyQ above the disease length threshold does not induce drastic conformational changes in full-length HTT. Therefore, models of HD pathogenesis stating that polyQ expansion drastically alters HTT properties should be reconsidered.

Mutant Huntingtin Protein Interaction Map Implicates Dysregulation of Multiple Cellular Pathways in Neurodegeneration of Huntington's Disease

BACKGROUND

Huntington's disease (HD) is a genetic neurodegenerative disease caused by trinucleotide repeat (CAG) expansions in the human HTT gene encoding the huntingtin protein (Htt) with an expanded polyglutamine tract.

OBJECTIVE

HD models from yeast to transgenic mice have investigated proteins interacting with mutant Htt that may initiate molecular pathways of cell death. There is a paucity of datasets of published Htt protein interactions that include the criteria of 1) defining fragments or full-length Htt forms, 2) indicating the number of poly-glutamines of the mutant and wild-type Htt forms, and 3) evaluating native Htt interaction complexes. This research evaluated such interactor data to gain understanding of Htt dysregulation of cellular pathways.

METHODS

Htt interacting proteins were compiled from the literature that meet our criteria and were subjected to network analysis via clustering, gene ontology, and KEGG pathways using rigorous statistical methods.

RESULTS

The compiled data of Htt interactors found that both mutant and wild-type Htt interact with more than 2,971 proteins. Application of a community detection algorithm to all known Htt interactors identified significant signal transduction, membrane trafficking, chromatin, and mitochondrial clusters, among others. Binomial analyses of a subset of reported protein interactor information determined that chromatin organization, signal transduction and endocytosis were diminished, while mitochondria, translation and membrane trafficking had enriched overall edge effects.

CONCLUSION

The data support the hypothesis that mutant Htt disrupts multiple cellular processes causing toxicity. This dataset is an open resource to aid researchers in formulating hypotheses of HD mechanisms of pathogenesis.

NMR spectroscopy, excited states and relevance to problems in cell biology - transient pre-nucleation tetramerization of huntingtin and insights into Huntington's disease

Solution nuclear magnetic resonance (NMR) spectroscopy is a powerful technique for analyzing three-dimensional structure and dynamics of macromolecules at atomic resolution. Recent advances have exploited the unique properties of NMR in exchanging systems to detect, characterize and visualize excited sparsely populated states of biological macromolecules and their complexes, which are only transient. These states are invisible to conventional biophysical techniques, and play a key role in many processes, including molecular recognition, protein folding, enzyme catalysis, assembly and fibril formation. All the NMR techniques make use of exchange between sparsely populated NMR-invisible and highly populated NMR-visible states to transfer a magnetization property from the invisible state to the visible one where it can be easily detected and quantified. There are three classes of NMR experiments that rely on differences in distance, chemical shift or transverse relaxation (molecular mass) between the NMR-visible and -invisible species. Here, I illustrate the application of these methods to unravel the complex mechanism of sub-millisecond pre-nucleation oligomerization of the N-terminal region of huntingtin, encoded by exon-1 of the huntingtin gene, where CAG expansion leads to Huntington's disease, a fatal autosomal-dominant neurodegenerative condition. I also discuss how inhibition of tetramerization blocks the much slower (by many orders of magnitude) process of fibril formation.

The Common Cellular Events in the Neurodegenerative Diseases and the Associated Role of Endoplasmic Reticulum Stress

Neurodegenerative diseases are inseparably linked with aging and increase as life expectancy extends. There are common dysfunctions in various cellular events shared among neurogenerative diseases, such as calcium dyshomeostasis, neuroinflammation, and age-associated decline in the autophagy-lysosome system. However, most of all, the prominent pathological feature of neurodegenerative diseases is the toxic buildup of misfolded protein aggregates and inclusion bodies accompanied by an impairment in proteostasis. Recent studies have suggested a close association between endoplasmic reticulum (ER) stress and neurodegenerative pathology in cellular and animal models as well as in human patients. The contribution of mutant or misfolded protein-triggered ER stress and its associated signaling events, such as unfolded protein response (UPR), to the pathophysiology of various neurodegenerative disorders, including Alzheimer's, Parkinson's, and Huntington's disease, amyotrophic lateral sclerosis, and prion disease, is described here. Impaired UPR action is commonly attributed to exacerbated ER stress, pathogenic protein aggregate accumulation, and deteriorating neurodegenerative pathologies. Thus, activating certain UPR components has been shown to alleviate ER stress and its associated neurodegeneration. However, uncontrolled activation of some UPR factors has also been demonstrated to worsen neurodegenerative phenotypes, suggesting that detailed molecular mechanisms around ER stress and its related neurodegenerations should be understood to develop effective therapeutics against aging-associated neurological syndromes. We also discuss current therapeutic endeavors, such as the development of small molecules that selectively target individual UPR components and address ER stress in general.

The Role of Extracellular Matrix Components in the Spreading of Pathological Protein Aggregates

Several neurodegenerative diseases are characterized by the accumulation of aggregated misfolded proteins. These pathological agents have been suggested to propagate in the brain via mechanisms similar to that observed for the prion protein, where a misfolded variant is transferred from an affected brain region to a healthy one, thereby inducing the misfolding and/or aggregation of correctly folded copies. This process has been characterized for several proteins, such as α-synuclein, tau, amyloid beta (Aβ) and less extensively for huntingtin and TDP-43. α-synuclein, tau, TDP-43 and huntingtin are intracellular proteins, and their aggregates are located in the cytosol or nucleus of neurons. They have been shown to spread between cells and this event occurs, at least partially, via secretion of these protein aggregates in the extracellular space followed by re-uptake. Conversely, Aβ aggregates are found mainly extracellularly, and their spreading occurs in the extracellular space between brain regions. Due to the inherent nature of their spreading modalities, these proteins are exposed to components of the extracellular matrix (ECM), including glycans, proteases and core matrix proteins. These ECM components can interact with or process pathological misfolded proteins, potentially changing their properties and thus regulating their spreading capabilities. Here, we present an overview of the documented roles of ECM components in the spreading of pathological protein aggregates in neurodegenerative diseases with the objective of identifying the current gaps in knowledge and stimulating further research in the field. This could potentially lead to the identification of druggable targets to slow down the spreading and/or progression of these pathologies.

Calpains as novel players in the molecular pathogenesis of spinocerebellar ataxia type 17

Spinocerebellar ataxia type 17 (SCA17) is a neurodegenerative disease caused by a polyglutamine-encoding trinucleotide repeat expansion in the gene of transcription factor TATA box-binding protein (TBP). While its underlying pathomechanism is elusive, polyglutamine-expanded TBP fragments of unknown origin mediate the mutant protein’s toxicity. Calcium-dependent calpain proteases are protagonists in neurodegenerative disorders. Here, we demonstrate that calpains cleave TBP, and emerging C-terminal fragments mislocalize to the cytoplasm. SCA17 cell and rat models exhibited calpain overactivation, leading to excessive fragmentation and depletion of neuronal proteins in vivo. Transcriptome analysis of SCA17 cells revealed synaptogenesis and calcium signaling perturbations, indicating the potential cause of elevated calpain activity. Pharmacological or genetic calpain inhibition reduced TBP cleavage and aggregation, consequently improving cell viability. Our work underlines the general significance of calpains and their activating pathways in neurodegenerative disorders and presents these proteases as novel players in the molecular pathogenesis of SCA17.

The distribution and density of Huntingtin inclusions across the Huntington disease neocortex: regional correlations with Huntingtin repeat expansion independent of pathologic grade

Huntington disease is characterized by progressive neurodegeneration, especially of the striatum, and the presence of polyglutamine huntingtin (HTT) inclusions. Although HTT inclusions are most abundant in the neocortex, their neocortical distribution and density in relation to the extent of CAG repeat expansion in the HTT gene and striatal pathologic grade have yet to be formally established. We immunohistochemically studied 65 brains with a pathologic diagnosis of Huntington disease to investigate the cortical distributions and densities of HTT inclusions within the calcarine (BA17), precuneus (BA7), motor (BA4) and prefrontal (BA9) cortices; in 39 of these brains, a p62 immunostain was used for comparison. HTT inclusions predominate in the infragranular cortical layers (layers V-VI) and layer III, however, the densities of HTT inclusions across the human cerebral cortex are not uniform but are instead regionally contingent. The density of HTT and p62 inclusions (intranuclear and extranuclear) in layers V-VI increases caudally to rostrally (BA17 < BA7 < BA4 < BA9) with the median burden of HTT inclusions being 38-fold greater in the prefrontal cortex (BA9) than in the calcarine cortex (BA17). Conversely, intranuclear HTT inclusions prevail in the calcarine cortex irrespective of HTT CAG length. Neocortical HTT inclusion density correlates with CAG repeat expansion, but not with the neuropathologic grade of striatal degeneration (Vonsattel grade) or with the duration of clinical disease since motor onset. Extrapolation of these findings suggest that HTT inclusions are at a regionally-contingent, CAG-dependent, density during the advanced stages of HD. The distribution and density of HTT inclusions in HD therefore does not provide a measure of pathologic disease stage but rather infers the degree of pathogenic HTT expansion.

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.

Innovative Therapeutic Approaches for Huntington's Disease: From Nucleic Acids to GPCR-Targeting Small Molecules

Huntington’s disease (HD) is a fatal neurodegenerative disorder due to an extraordinarily expanded CAG repeat in the huntingtin gene that confers a gain-of-toxic function in the mutant protein. There is currently no effective cure that attenuates progression and severity of the disease. Since HD is an inherited monogenic disorder, lowering the mutant huntingtin (mHTT) represents a promising therapeutic strategy. Huntingtin lowering strategies mostly focus on nucleic acid approaches, such as small interfering RNAs (siRNAs) and antisense oligonucleotides (ASOs). While these approaches seem to be effective, the drug delivery to the brain poses a great challenge and requires direct injection into the central nervous system (CNS) that results in substantial burden for patients. This review discusses the topics on Huntingtin lowering strategies with clinical trials in patients already underway and introduce an innovative approach that has the potential to deter the disease progression through the inhibition of GPR52, a striatal-enriched class A orphan G protein-coupled receptor (GPCR) that represents a promising therapeutic target for psychiatric disorders. Chemically simple, potent, and selective GPR52 antagonists have been discovered through high-throughput screening and subsequent structure-activity relationship studies. These small molecule antagonists not only diminish both soluble and aggregated mHTT in the striatum, but also ameliorate HD-like defects in HD mice. This therapeutic approach offers great promise as a novel strategy for HD therapy, while nucleic acid delivery still faces considerable challenges.

A C-terminal ataxin-2 disordered region promotes Huntingtin protein aggregation and neurodegeneration in Drosophila models of Huntington’s disease

The Ataxin-2 (Atx2) protein contributes to the progression of neurodegenerative phenotypes in animal models of amyotrophic lateral sclerosis (ALS), type 2 spinocerebellar ataxia (SCA-2), Parkinson’s disease, and Huntington’s disease (HD). However, because the Atx2 protein contains multiple separable activities, deeper understanding requires experiments to address the exact mechanisms by which Atx2 modulates neurodegeneration (ND) progression. Recent work on two ALS models, C9ORF72 and FUS, in Drosophila has shown that a C-terminal intrinsically disordered region (cIDR) of Atx2 protein, required for assembly of ribonucleoprotein (RNP) granules, is essential for the progression of neurodegenerative phenotypes as well as for accumulation of protein inclusions associated with these ALS models. Here, we show that the Atx2-cIDR also similarly contributes to the progression of degenerative phenotypes and accumulation of Huntingtin protein aggregates in Drosophila models of HD. Because Huntingtin is not an established component of RNP granules, these observations support a recently hypothesized, unexpected protein-handling function for RNP granules, which could contribute to the progression of Huntington’s disease and, potentially, other proteinopathies.

Huntington disease update: new insights into the role of repeat instability in disease pathogenesis

The causative mutation for Huntington disease (HD), an expanded trinucleotide repeat sequence in the first exon of the huntingtin gene (HTT) is naturally polymorphic and inevitably associated with disease symptoms above 39 CAG repeats. Although symptomatic medical therapies for HD can improve the motor and non-motor symptoms for affected patients, these drugs do not stop the ongoing neurodegeneration and progression of the disease, which results in severe motor and cognitive disability and death. To date, there is still an urgent need for the development of effective disease-modifying therapies to slow or even stop the progression of HD. The increasing ability to intervene directly at the roots of the disease, namely HTT transcription and translation of its mRNA, makes it necessary to understand the pathogenesis of HD as precisely as possible. In addition to the long-postulated toxicity of the polyglutamine-expanded mutant HTT protein, there is increasing evidence that the CAG repeat-containing RNA might also be directly involved in toxicity. Recent studies have identified cis- (DNA repair genes) and trans- (loss/duplication of CAA interruption) acting variants as major modifiers of age at onset (AO) and disease progression. More and more extensive data indicate that somatic instability functions as a driver for AO as well as disease progression and severity, not only in HD but also in other polyglutamine diseases. Thus, somatic expansions of repetitive DNA sequences may be essential to promote respective repeat lengths to reach a threshold leading to the overt neurodegenerative symptoms of trinucleotide diseases. These findings support somatic expansion as a potential therapeutic target in HD and related repeat expansion disorders.

Molecular Strategies to Target Protein Aggregation in Huntington's Disease

Huntington's disease (HD) is a neurodegenerative disorder caused by the aggregation of the mutant huntingtin (mHTT) protein in nerve cells. mHTT self-aggregates to form soluble oligomers and insoluble fibrils, which interfere in a number of key cellular functions. This leads to cell quiescence and ultimately cell death. There are currently still no treatments available for HD, but approaches targeting the HTT levels offer systematic, mechanism-driven routes towards curing HD and other neurodegenerative diseases. This review summarizes the current state of knowledge of the mRNA targeting approaches such as antisense oligonucleotides and RNAi system; and the novel methods targeting mHTT and aggregates for degradation via the ubiquitin proteasome or the autophagy-lysosomal systems. These methods include the proteolysis-targeting chimera, Trim-Away, autophagosome-tethering compound, autophagy-targeting chimera, lysosome-targeting chimera and approach targeting mHTT for chaperone-mediated autophagy. These molecular strategies provide a knowledge-based approach to target HD and other neurodegenerative diseases at the origin.

Altered microRNA expression in animal models of Huntington's disease and potential therapeutic strategies

A review of recent animal models of Huntington's disease showed many microRNAs had altered expression levels in the striatum and cerebral cortex, and which were mostly downregulated. Among the altered microRNAs were miR-9/9*, miR-29b, miR-124a, miR-132, miR-128, miR-139, miR-122, miR-138, miR-23b, miR-135b, miR-181 (all downregulated) and miR-448 (upregulated), and similar changes had been previously found in Huntington's disease patients. In the animal cell studies, the altered microRNAs included miR-9, miR-9*, miR-135b, miR-222 (all downregulated) and miR-214 (upregulated). In the animal models, overexpression of miR-155 and miR-196a caused a decrease in mutant huntingtin mRNA and protein level, lowered the mutant huntingtin aggregates in striatum and cortex, and improved performance in behavioral tests. Improved performance in behavioral tests also occurred with overexpression of miR-132 and miR-124. In the animal cell models, overexpression of miR-22 increased the viability of rat primary cortical and striatal neurons infected with mutant huntingtin and decreased huntingtin -enriched foci of ≥ 2 µm. Also, overexpression of miR-22 enhanced the survival of rat primary striatal neurons treated with 3-nitropropionic acid. Exogenous expression of miR-214, miR-146a, miR-150, and miR-125b decreased endogenous expression of huntingtin mRNA and protein in Hdh^Q111/Hdh^Q111 cells. Further studies with animal models of Huntington's disease are warranted to validate these findings and identify specific microRNAs whose overexpression inhibits the production of mutant huntingtin protein and other harmful processes and may provide a more effective means of treating Huntington's disease in patients and slowing its progression.

The Cytotoxicity and Clearance of Mutant Huntingtin and Other Misfolded Proteins

Protein misfolding and aggregation are implicated in many neurodegenerative diseases. One of these diseases is Huntington's, which is caused by increased glutamine-encoding trinucleotide repeats within the Huntingtin gene. Like other misfolded proteins, mutated Huntingtin proteins with polyglutamine expansions are prone to aggregation. Misfolded proteins exist as soluble monomers, small aggregates, or as large insoluble inclusion bodies. Misfolded protein aggregates are believed to be cytotoxic by stressing the protein degradation machinery, disrupting membrane structure, or sequestering other proteins. We recently showed that expression of misfolded proteins lowers cellular free ubiquitin levels, which compromises the protein degradation machinery. Therefore, the efficient degradation of misfolded proteins is critical to preserve cell health. Cells employ two major mechanisms to degrade misfolded proteins. The first is the ubiquitin-proteasome system (UPS), which ubiquitinates and degrades misfolded proteins with the assistance of segregase Cdc48/p97. The UPS pathway is mainly responsible for the clearance of misfolded proteins present as monomers or smaller aggregates. The second pathway is macroautophagy/autophagy, in which protein aggregates or inclusion bodies are recruited into an autophagosome before transport to the vacuole/lysosome for degradation. This review is focused on the current understanding of the cytotoxicity of misfolded proteins as well as their clearance pathways, with a particular emphasis on mutant Huntingtin.

The Nt17 Domain and its Helical Conformation Regulate the Aggregation, Cellular Properties and Neurotoxicity of Mutant Huntingtin Exon 1

Converging evidence points to the N-terminal domain comprising the first 17 amino acids of the Huntingtin protein (Nt17) as a key regulator of its aggregation, cellular properties and toxicity. In this study, we further investigated the interplay between Nt17 and the polyQ domain repeat length in regulating the aggregation and inclusion formation of exon 1 of the Huntingtin protein (Httex1). In addition, we investigated the effect of removing Nt17 or modulating its local structure on the membrane interactions, neuronal uptake, and toxicity of monomeric or fibrillar Httex1. Our results show that the polyQ and Nt17 domains synergistically modulate the aggregation propensity of Httex1 and that the Nt17 domain plays important roles in shaping the surface properties of mutant Httex1 fibrils and regulating their poly-Q-dependent growth, lateral association and neuronal uptake. Removal of Nt17 or disruption of its transient helical conformations slowed the aggregation of monomeric Httex1 in vitro, reduced inclusion formation in cells, enhanced the neuronal uptake and nuclear accumulation of monomeric Httex1 proteins, and was sufficient to prevent cell death induced by Httex1 72Q overexpression. Finally, we demonstrate that the uptake of Httex1 fibrils into primary neurons and the resulting toxicity are strongly influenced by mutations and phosphorylation events that influence the local helical propensity of Nt17. Altogether, our results demonstrate that the Nt17 domain serves as one of the key master regulators of Htt aggregation, internalization, and toxicity and represents an attractive target for inhibiting Htt aggregate formation, inclusion formation, and neuronal toxicity.

Ubiquilin-2 differentially regulates polyglutamine disease proteins

Divergent protein context helps explain why polyglutamine expansion diseases differ clinically and pathologically. This heterogeneity may also extend to how polyglutamine disease proteins are handled by cellular pathways of proteostasis. Studies suggest, for example, that the ubiquitin-proteasome shuttle protein Ubiquilin-2 (UBQLN2) selectively interacts with specific polyglutamine disease proteins. Here we employ cellular models, primary neurons and mouse models to investigate the potential differential regulation by UBQLN2 of two polyglutamine disease proteins, huntingtin (HTT) and ataxin-3 (ATXN3). In cells, overexpressed UBQLN2 selectively lowered levels of full-length pathogenic HTT but not of HTT exon 1 fragment or full-length ATXN3. Consistent with these results, UBQLN2 specifically reduced accumulation of aggregated mutant HTT but not mutant ATXN3 in mouse models of Huntington's disease (HD) and spinocerebellar ataxia type 3 (SCA3), respectively. Normally a cytoplasmic protein, UBQLN2 translocated to the nuclei of neurons in HD mice but not in SCA3 mice. Remarkably, instead of reducing the accumulation of nuclear mutant ATXN3, UBQLN2 induced an accumulation of cytoplasmic ATXN3 aggregates in neurons of SCA3 mice. Together these results reveal a selective action of UBQLN2 toward polyglutamine disease proteins, indicating that polyglutamine expansion alone is insufficient to promote UBQLN2-mediated clearance of this class of disease proteins. Additional factors, including nuclear translocation of UBQLN2, may facilitate its action to clear intranuclear, aggregated disease proteins like HTT.

Transglutaminase 6 Is Colocalized and Interacts with Mutant Huntingtin in Huntington Disease Rodent Animal Models

Mammalian transglutaminases (TGs) catalyze calcium-dependent irreversible posttranslational modifications of proteins and their enzymatic activities contribute to the pathogenesis of several human neurodegenerative diseases. Although different transglutaminases are found in many different tissues, the TG6 isoform is mostly expressed in the CNS. The present study was embarked on/undertaken to investigate expression, distribution and activity of transglutaminases in Huntington disease transgenic rodent models, with a focus on analyzing the involvement of TG6 in the age- and genotype-specific pathological features relating to disease progression in HD transgenic mice and a tgHD transgenic rat model using biochemical, histological and functional assays. Our results demonstrate the physical interaction between TG6 and (mutant) huntingtin by co-immunoprecipitation analysis and the contribution of its enzymatic activity for the total aggregate load in SH-SY5Y cells. In addition, we identify that TG6 expression and activity are especially abundant in the olfactory tubercle and piriform cortex, the regions displaying the highest amount of mHTT aggregates in transgenic rodent models of HD. Furthermore, mHTT aggregates were colocalized within TG6-positive cells. These findings point towards a role of TG6 in disease pathogenesis via mHTT aggregate formation.

Cellular functions regulated by deubiquitinating enzymes in neurodegenerative diseases

Neurodegenerative diseases are one of the most common diseases in mankind. Although there are reports of several candidates that cause neurodegenerative diseases, the exact mechanism of pathogenesis is poorly understood. The ubiquitin-proteasome system (UPS) is an important posttranslational modification for protein degradation and control of homeostasis. Enzymes such as E1, E2, E3 ligases, and deubiquitinating enzymes (DUBs) participating in UPS, regulate disease-inducing proteins by controlling the degree of ubiquitination. Therefore, the development of treatments targeting enzymes for degenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and amyotrophic lateral sclerosis (ALS), is emerging as an attractive perspective. In particular, as DUBs are able to regulate one or more degenerative disease-related proteins, the potential as a therapeutic target is even more evident. DUBs influence the regulation of toxic proteins that cause neurodegenerative diseases by not only their removal, but also by regulating signals associated with mitophagy, autophagy, and endoplasmic reticulum-associated degradation (ERAD). In this review, we analyze not only the cellular processes of DUBs, which control neurodegenerative disease-inducing proteins, but also their potentials as a therapeutic agent for neurodegenerative diseases.

Investigating Crosstalk Among PTMs Provides Novel Insight Into the Structural Basis Underlying the Differential Effects of Nt17 PTMs on Mutant Httex1 Aggregation

Post-translational modifications (PTMs) within the first 17 amino acids (Nt17) of the Huntingtin protein (Htt) have been shown to inhibit the aggregation and attenuate the toxicity of mutant Htt proteins in vitro and in various models of Huntington’s disease. Here, we expand on these studies by investigating the effect of methionine eight oxidation (oxM8) and its crosstalk with lysine 6 acetylation (AcK6) or threonine 3 phosphorylation (pT3) on the aggregation of mutant Httex1 (mHttex1). We show that M8 oxidation delays but does not inhibit the aggregation and has no effect on the final morphologies of mHttex1aggregates. The presence of both oxM8 and AcK6 resulted in dramatic inhibition of Httex1 fibrillization. Circular dichroism spectroscopy and molecular dynamics simulation studies show that PTMs that lower the mHttex1 aggregation rate (oxM8, AcK6/oxM8, pT3, pT3/oxM8, and pS13) result in increased population of a short N-terminal helix (first eight residues) in Nt17 or decreased abundance of other helical forms, including long helix and short C-terminal helix. PTMs that did not alter the aggregation rate (AcK6) of mHttex1 exhibit a similar distribution of helical conformation as the unmodified peptides. These results show that the relative abundance of N- vs. C-terminal helical conformations and long helices, rather than the overall helicity of Nt17, better explains the effect of different Nt17 PTMs on mHttex1; thus, explaining the lack of correlation between the effect of PTMs on the overall helicity of Nt17 and mHttex1 aggregation in vitro. Taken together, our results provide novel structural insight into the differential effects of single PTMs and crosstalk between different PTMs in regulating mHttex1 aggregation.

Huntington's Chorea-a Rare Neurodegenerative Autosomal Dominant Disease: Insight into Molecular Genetics, Prognosis and Diagnosis

Huntington's disease is a neurodegenerative autosomal disease results due to expansion of polymorphic CAG repeats in the huntingtin gene. Phosphorylation of the translation initiation factor 4E-BP results in the alteration of the translation control leading to unwanted protein synthesis and neuronal function. Consequences of mutant huntington (mhtt) gene transcription are not well known. Variability of age of onset is an important factor of Huntington's disease separating adult and juvenile types. The factors which are taken into account are-genetic modifiers, maternal protection i.e excessive paternal transmission, superior ageing genes and environmental threshold. A major focus has been given to the molecular pathogenesis which includes-motor disturbance, cognitive disturbance and neuropsychiatric disturbance. The diagnosis part has also been taken care of. This includes genetic testing and both primary and secondary symptoms. The present review also focuses on the genetics and pathology of Huntington's disease.

DNAzyme Cleavage of CAG Repeat RNA in Polyglutamine Diseases

CAG repeat expansion is the genetic cause of nine incurable polyglutamine (polyQ) diseases with neurodegenerative features. Silencing repeat RNA holds great therapeutic value. Here, we developed a repeat-based RNA-cleaving DNAzyme that catalyzes the destruction of expanded CAG repeat RNA of six polyQ diseases with high potency. DNAzyme preferentially cleaved the expanded allele in spinocerebellar ataxia type 1 (SCA1) cells. While cleavage was non-allele-specific for spinocerebellar ataxia type 3 (SCA3) cells, treatment of DNAzyme leads to improved cell viability without affecting mitochondrial metabolism or p62-dependent aggresome formation. DNAzyme appears to be stable in mouse brain for at least 1 month, and an intermediate dosage of DNAzyme in a SCA3 mouse model leads to a significant reduction of high molecular weight ATXN3 proteins. Our data suggest that DNAzyme is an effective RNA silencing molecule for potential treatment of multiple polyQ diseases.

Small, Seeding-Competent Huntingtin Fibrils Are Prominent Aggregate Species in Brains of zQ175 Huntington's Disease Knock-in Mice

The deposition of mutant huntingtin (mHTT) protein aggregates in neurons of patients is a pathological hallmark of Huntington’s disease (HD). Previous investigations in cell-free and cell-based disease models showed mHTT exon-1 (mHTTex1) fragments with pathogenic polyglutamine (polyQ) tracts (>40 glutamines) to self-assemble into highly stable, β-sheet-rich protein aggregates with a fibrillar morphology. HD knock-in mouse models have not been extensively studied with regard to mHTT aggregation. They endogenously produce full-length mHTT with a pathogenic polyQ tract as well as mHTTex1 fragments. Here, we demonstrate that seeding-competent, fibrillar mHTT aggregates can be readily detected in brains of zQ175 knock-in HD mice. To do this, we applied a highly sensitive FRET-based protein amplification assay that is capable of detecting seeding-competent mHTT aggregate species down to the femtomolar range. Furthermore, we show that fibrillar structures with an average length of ∼200 nm can be enriched with aggregate-specific mouse and human antibodies from zQ175 mouse brain extracts through immunoprecipitations, confirming that such structures are formed in vivo. Together these studies indicate that small, fibrillar, seeding-competent mHTT structures are prominent aggregate species in brains of zQ175 mice.

Quantitative Exchange NMR-Based Analysis of Huntingtin-SH3 Interactions Suggests an Allosteric Mechanism of Inhibition of Huntingtin Aggregation

Huntingtin polypeptides (htt^ex1), encoded by exon 1 of the htt gene and containing an expanded polyglutamine tract, form fibrils that accumulate within neuronal inclusion bodies, resulting in the fatal neurodegenerative condition known as Huntington's disease. Htt^ex1 comprises three regions: a 16-residue N-terminal amphiphilic domain (NT), a polyglutamine tract of variable length (Q_n), and a polyproline-rich domain containing two polyproline tracts. The NT region of htt^ex1 undergoes prenucleation transient oligomerization on the sub-millisecond time scale, resulting in a productive tetramer that promotes self-association and nucleation of the polyglutamine tracts. Here we show that binding of Fyn SH3, a small intracellular proline-binding domain, to the first polyproline tract of htt^ex1Q₃₅ inhibits fibril formation by both NMR and a thioflavin T fluorescence assay. The interaction of Fyn SH3 with htt^ex1Q₇ was investigated using NMR experiments designed to probe kinetics and equilibria at atomic resolution, including relaxation dispersion, and concentration-dependent exchange-induced chemical shifts and transverse relaxation in the rotating frame. Sub-millisecond exchange between four species is demonstrated: two major states comprising free (P) and SH3-bound (PL) monomeric htt^ex1Q₇, and two sparsely populated dimers in which either both subunits (P₂L₂) or only a single subunit (P₂L) is bound to SH3. Binding of SH3 increases the helical propensity of the NT domain, resulting in a 25-fold stabilization of the P₂L₂ dimer relative to the unliganded P₂ dimer. The P₂L₂ dimer, in contrast to P₂, does not undergo any detectable oligomerization to a tetramer, thereby explaining the allosteric inhibition of htt^ex1 fibril formation by Fyn SH3.

Effects of flanking sequences and cellular context on subcellular behavior and pathology of mutant HTT

Huntington's disease (HD) is caused by an expansion of a poly glutamine (polyQ) stretch in the huntingtin protein (HTT) that is necessary to cause pathology and formation of HTT aggregates. Here we ask whether expanded polyQ is sufficient to cause pathology and aggregate formation. By addressing the sufficiency question, one can identify cellular processes and structural parameters that influence HD pathology and HTT subcellular behavior (i.e. aggregation state and subcellular location). Using Drosophila, we compare the effects of expressing mutant full-length human HTT (fl-mHTT) to the effects of mutant human HTTexon1 and to two commonly used synthetic fragments, HTT171 and shortstop (HTT118). Expanded polyQ alone is not sufficient to cause inclusion formation since full-length HTT and HTTex1 with expanded polyQ are both toxic although full-length HTT remains diffuse while HTTex1 forms inclusions. Further, inclusions are not sufficient to cause pathology since HTT171-120Q forms inclusions but is benign and co-expression of HTT171-120Q with non-aggregating pathogenic fl-mHTT recruits fl-mHTT to aggregates and rescues its pathogenicity. Additionally, the influence of sequences outside the expanded polyQ domain is revealed by finding that small modifications to the HTT118 or HTT171 fragments can dramatically alter their subcellular behavior and pathogenicity. Finally, mutant HTT subcellular behavior is strongly modified by different cell and tissue environments (e.g. fl-mHTT appears as diffuse nuclear in one tissue and diffuse cytoplasmic in another but toxic in both). These observations underscore the importance of cellular and structural context for the interpretation and comparison of experiments using different fragments and tissues to report the effects of expanded polyQ.