Mutant huntingtin N-terminal fragments of specific size mediate aggregation and toxicity in neuronal cells

Protective Proteolysis in Huntington's Disease: Unraveling the Role of Post-Translational Myristoylation of Huntingtin in Autophagy

 Huntington's disease (HD) is a devastating neurodegenerative disorder characterized by impaired motor function and cognitive decline, ultimately leading to death. HD is caused by a polyglutamine expansion in the N-terminal region of the huntingtin (HTT) protein, which is linked to decreased HTT turnover, increased HTT proteolysis, increased HTT aggregation, and subsequent neuronal death. In this review, we explore the mechanism of the protective effect of blocking HTT proteolysis at D586, which has been shown to rescue the HD phenotype in HD mouse models. Until recently, the mechanism remained unclear. Herein, we discuss how blocking HTT proteolysis at D586 promotes HTT turnover by correcting autophagy, and making HTT a better autophagy substrate, through post-translational myristoylation of HTT at G553.

Kinetic models reveal the interplay of protein production and aggregation

Protein aggregation is a key process in the development of many neurodegenerative disorders, including dementias such as Alzheimer's disease. Significant progress has been made in understanding the molecular mechanisms of aggregate formation in pure buffer systems, much of which was enabled by the development of integrated rate laws that allowed for mechanistic analysis of aggregation kinetics. However, in order to translate these findings into disease-relevant conclusions and to make predictions about the effect of potential alterations to the aggregation reactions by the addition of putative inhibitors, the current models need to be extended to account for the altered situation encountered in living systems. In particular, in vivo, the total protein concentrations typically do not remain constant and aggregation-prone monomers are constantly being produced but also degraded by cells. Here, we build a theoretical model that explicitly takes into account monomer production, derive integrated rate laws and discuss the resulting scaling laws and limiting behaviours. We demonstrate that our models are suited for the aggregation-prone Huntington's disease-associated peptide HttQ45 utilizing a system for continuous in situ monomer production and the aggregation of the tumour suppressor protein P53. The aggregation-prone HttQ45 monomer was produced through enzymatic cleavage of a larger construct in which a fused protein domain served as an internal inhibitor. For P53, only the unfolded monomers form aggregates, making the unfolding a rate-limiting step which constitutes a source of aggregation-prone monomers. The new model opens up possibilities for a quantitative description of aggregation in living systems, allowing for example the modelling of inhibitors of aggregation in a dynamic environment of continuous protein synthesis.

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.

Beneficial behavioral effects of chronic cerebral dopamine neurotrophic factor (CDNF) infusion in the N171-82Q transgenic model of Huntington's disease

Huntington's disease (HD) is a progressive inherited neurological disease characterized by the degeneration of basal ganglia and the accumulation of mutant huntingtin (mHtt) aggregates in specific brain areas. Currently, there is no treatment for halting the progression of HD. Cerebral dopamine neurotrophic factor (CDNF) is a novel endoplasmic reticulum located protein with neurotrophic factor properties that protects and restores dopamine neurons in rodent and non-human primate models of Parkinson's disease. Our recent study showed that CDNF improves motor coordination and protects NeuN positive cells in a Quinolinic acid toxin rat model of HD. Here we have investigated the effect of chronic intrastriatal CDNF administration on behavior and mHtt aggregates in the N171-82Q mouse model of HD. Data showed that CDNF did not significantly decrease the number of mHtt aggregates in most brain regions studied. Notably, CDNF significantly delayed the onset of symptoms and improved motor coordination in N171-82Q mice. Furthermore, CDNF increased BDNF mRNA level in hippocampus in vivo in the N171-82Q model and BDNF protein level in cultured striatal neurons. Collectively our results indicate that CDNF might be a potential drug candidate for the treatment of HD.

Widespread alterations in microRNA biogenesis in human Huntington’s disease putamen

Altered microRNA (miRNA) expression is a common feature of Huntington’s disease (HD) and could participate in disease onset and progression. However, little is known about the underlying causes of miRNA disruption in HD. We and others have previously shown that mutant Huntingtin binds to Ago2, a central component of miRNA biogenesis, and disrupts mature miRNA levels. In this study, we sought to determine if miRNA maturation per se was compromised in HD. Towards this end, we characterized major miRNA biogenesis pathway components and miRNA maturation products (pri-miRNA, pre-miRNA, and mature) in human HD (N = 41, Vonsattel grades HD2-4) and healthy control (N = 25) subjects. Notably, the striatum (putamen) and cortex (BA39) from the same individuals were analyzed in parallel. We show that Ago2, Drosha, and Dicer were strongly downregulated in human HD at the early stages of the disease. Using a panel of HD-related miRNAs (miR-10b, miR-196b, miR-132, miR-212, miR-127, miR-128), we uncovered various types of maturation defects in the HD brain, the most prominent occurring at the pre-miRNA to mature miRNA maturation step. Consistent with earlier findings, we provide evidence that alterations in autophagy could participate in miRNA maturation defects. Notably, most changes occurred in the striatum, which is more prone to HTT aggregation and neurodegeneration. Likewise, we observed no significant alterations in miRNA biogenesis in human HD cortex and blood, strengthening tissue-specific effects. Overall, these data provide important clues into the underlying mechanisms behind miRNA alterations in HD-susceptible tissues. Further investigations are now required to understand the biological, diagnostic, and therapeutic implications of miRNA/RNAi biogenesis defects in HD and related neurodegenerative disorders.

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.

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.

Interaction of huntingtin with PRMTs and its subsequent arginine methylation affects HTT solubility, phase transition behavior and neuronal toxicity

Huntington's disease (HD) is an incurable neurodegenerative disorder caused by a CAG expansion in the huntingtin gene (HTT). Post-translational modifications of huntingtin protein (HTT), such as phosphorylation, acetylation and ubiquitination, have been implicated in HD pathogenesis. Arginine methylation/dimethylation is an important modification with an emerging role in neurodegeneration; however, arginine methylation of HTT remains largely unexplored. Here we report nearly two dozen novel arginine methylation/dimethylation sites on the endogenous HTT from human and mouse brain and human cells suggested by mass spectrometry with data-dependent acquisition. Targeted quantitative mass spectrometry identified differential arginine methylation at specific sites in HD patient-derived striatal precursor cell lines compared to normal controls. We found that HTT can interact with several type I protein arginine methyltransferases (PRMTs) via its N-terminal domain. Using a combination of in vitro methylation and cell-based experiments, we identified PRMT4 (CARM1) and PRMT6 as major enzymes methylating HTT at specific arginines. Alterations of these methylation sites had a profound effect on biochemical properties of HTT rendering it less soluble in cells and affected its liquid-liquid phase separation and phase transition patterns in vitro. We found that expanded HTT 1-586 fragment can form liquid-like assemblies, which converted into solid-like assemblies when the R200/205 methylation sites were altered. Methyl-null alterations increased HTT toxicity to neuronal cells, while overexpression of PRMT 4 and 6 was beneficial for neuronal survival. Thus, arginine methylation pathways that involve specific HTT-modifying PRMT enzymes and modulate HTT biochemical and toxic properties could provide targets for HD-modifying therapies.

A novel and accurate full-length HTT mouse model for Huntington's disease

Here, we report the generation and characterization of a novel Huntington's disease (HD) mouse model BAC226Q by using a bacterial artificial chromosome (BAC) system, expressing full-length human HTT with ~226 CAG-CAA repeats and containing endogenous human HTT promoter and regulatory elements. BAC226Q recapitulated a full-spectrum of age-dependent and progressive HD-like phenotypes without unwanted and erroneous phenotypes. BAC226Q mice developed normally, and gradually exhibited HD-like psychiatric and cognitive phenotypes at 2 months. From 3 to 4 months, BAC226Q mice showed robust progressive motor deficits. At 11 months, BAC226Q mice showed significant reduced life span, gradual weight loss and exhibited neuropathology including significant brain atrophy specific to striatum and cortex, striatal neuronal death, widespread huntingtin inclusions, and reactive pathology. Therefore, the novel BAC226Q mouse accurately recapitulating robust, age-dependent, progressive HD-like phenotypes will be a valuable tool for studying disease mechanisms, identifying biomarkers, and testing gene-targeting therapeutic approaches for HD.

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 New Chemoenzymatic Semisynthetic Approach Provides Insight into the Role of Phosphorylation beyond Exon1 of Huntingtin and Reveals N-Terminal Fragment Length-Dependent Distinct Mechanisms of Aggregation

Huntington's disease is a neurodegenerative disorder caused by the expansion of a polyglutamine repeat (>36Q) in the N-terminal domain of the huntingtin protein (Htt), which renders the protein or fragments thereof more prone to aggregate and form inclusions. Although several Htt N-terminal fragments of different lengths have been identified within Htt inclusions, most studies on the mechanisms, sequence, and structural determinants of Htt aggregation have focused on the Httexon1 (Httex1). Herein, we investigated the aggregation properties of mutant N-terminal Htt fragments of various lengths (Htt171, Htt140, and Htt104) in comparison to mutant Httex1 (mHttex1). We also present a new chemoenzymatic semisynthetic strategy that enables site-specific phosphorylation of Htt beyond Httex1. These advances yielded insights into how post-translational modifications (PTMs) and structured domains beyond Httex1 influence aggregation mechanisms, kinetics, and fibril morphology of longer N-terminal Htt fragments. We demonstrate that phosphorylation at T107 significantly slows the aggregation of mHtt171, whereas phosphorylation at T107 and S116 accelerates the aggregation, underscoring the importance of crosstalk between different PTMs. The mHtt171 proteins aggregate via a different mechanism and form oligomers and fibrillar aggregates with morphological properties that are distinct from that of mHttex1. These observations suggest that different N-terminal fragments could have distinct aggregation mechanisms and that a single polyQ-targeting antiaggregation strategy may not effectively inhibit the aggregation of all N-terminal Htt fragments. Finally, our results underscore the need for further studies to investigate the aggregation mechanisms of Htt fragments and how the various fragments interact with each other and influence Htt toxicity and disease progression.

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.

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.

PolyQ-expanded proteins impair cellular proteostasis of ataxin-3 through sequestering the co-chaperone HSJ1 into aggregates

Polyglutamine (polyQ) expansion of proteins can trigger protein misfolding and amyloid-like aggregation, which thus lead to severe cytotoxicities and even the respective neurodegenerative diseases. However, why polyQ aggregation is toxic to cells is not fully elucidated. Here, we took the fragments of polyQ-expanded (PQE) ataxin-7 (Atx7) and huntingtin (Htt) as models to investigate the effect of polyQ aggregates on the cellular proteostasis of endogenous ataxin-3 (Atx3), a protein that frequently appears in diverse inclusion bodies. We found that PQE Atx7 and Htt impair the cellular proteostasis of Atx3 by reducing its soluble as well as total Atx3 level but enhancing formation of the aggregates. Expression of these polyQ proteins promotes proteasomal degradation of endogenous Atx3 and accumulation of its aggregated form. Then we verified that the co-chaperone HSJ1 is an essential factor that orchestrates the balance of cellular proteostasis of Atx3; and further discovered that the polyQ proteins can sequester HSJ1 into aggregates or inclusions in a UIM domain-dependent manner. Thereby, the impairment of Atx3 proteostasis may be attributed to the sequestration and functional loss of cellular HSJ1. This study deciphers a potential mechanism underlying how PQE protein triggers proteinopathies, and also provides additional evidence in supporting the hijacking hypothesis that sequestration of cellular interacting partners by protein aggregates leads to cytotoxicity or neurodegeneration.

Site-Specific Phosphorylation of Huntingtin Exon 1 Recombinant Proteins Enabled by the Discovery of Novel Kinases

Post-translational modifications (PTMs) within the first 17 amino acids (Nt17) of exon 1 of the Huntingtin protein (Httex1) play important roles in modulating its cellular properties and functions in health and disease. In particular, phosphorylation of threonine and serine residues (T3, S13, and/or S16) has been shown to inhibit Htt aggregation in vitro and inclusion formation in cellular and animal models of Huntington's disease (HD). In this paper, we describe a new and simple methodology for producing milligram quantities of highly pure wild-type or mutant Httex1 proteins that are site-specifically phosphorylated at T3 or at both S13 and S16. This advance was enabled by 1) the discovery and validation of novel kinases that efficiently phosphorylate Httex1 at S13 and S16 (TBK1), at T3 (GCK) or T3 and S13 (TNIK and HGK), and 2) the development of an efficient methodology for producing recombinant native Httex1 proteins by using a SUMO-fusion expression and purification strategy.[26] As a proof of concept, we demonstrate how this method can be applied to produce Httex1 proteins that are both site-specifically phosphorylated and fluorescently or isotopically labeled. Together, these advances should increase access to these valuable tools and expand the range of methods and experimental approaches that can be used to elucidate the mechanisms by which phosphorylation influences Httex1 or HTT structure, aggregation, interactome, and function(s) in health and disease.

Antisense oligonucleotide therapeutics in neurodegenerative diseases: the case of polyglutamine disorders

Polyglutamine (polyQ) disorders are a group of nine neurodegenerative diseases that share a common genetic cause, which is an expansion of CAG repeats in the coding region of the causative genes that are otherwise unrelated. The trinucleotide expansion encodes for an expanded polyQ tract in the respective proteins, resulting in toxic gain-of-function and eventually in neurodegeneration. Currently, no disease-modifying therapies are available for this group of disorders. Nevertheless, given their monogenic nature, polyQ disorders are ideal candidates for therapies that target specifically the gene transcripts. Antisense oligonucleotides (ASOs) have been under intense investigation over recent years as gene silencing tools. ASOs are small synthetic single-stranded chains of nucleic acids that target specific RNA transcripts through several mechanisms. ASOs can reduce the levels of mutant proteins by breaking down the targeted transcript, inhibit mRNA translation or alter the maturation of the pre-mRNA via splicing correction. Over the years, chemical optimization of ASO molecules has allowed significant improvement of their pharmacological properties, which has in turn made this class of therapeutics a very promising strategy to treat a variety of neurodegenerative diseases. Indeed, preclinical and clinical strategies have been developed in recent years for some polyQ disorders using ASO therapeutics. The success of ASOs in several animal models, as well as encouraging results in the clinic for Huntington's disease, points towards a promising future regarding the application of ASO-based therapies for polyQ disorders in humans, offering new opportunities to address unmet medical needs for this class of disorders. This review aims to present a brief overview of key chemical modifications, mechanisms of action and routes of administration that have been described for ASO-based therapies. Moreover, it presents a review of the most recent and relevant preclinical and clinical trials that have tested ASO therapeutics in polyQ disorders.

Reactive Species in Huntington Disease: Are They Really the Radicals You Want to Catch?

Huntington disease (HD) is a neurodegenerative condition and one of the so-called rare or minority diseases, due to its low prevalence (affecting 1–10 of every 100,000 people in western countries). The causative gene, HTT, encodes huntingtin, a protein with a yet unknown function. Mutant huntingtin causes a range of phenotypes, including oxidative stress and the activation of microglia and astrocytes, which leads to chronic inflammation of the brain. Although substantial efforts have been made to find a cure for HD, there is currently no medical intervention able to stop or even delay progression of the disease. Among the many targets of therapeutic intervention, oxidative stress and inflammation have been extensively studied and some clinical trials have been promoted to target them. In the present work, we review the basic research on oxidative stress in HD and the strategies used to fight it. Many of the strategies to reduce the phenotypes associated with oxidative stress have produced positive results, yet no substantial functional recovery has been observed in animal models or patients with the disease. We discuss possible explanations for this and suggest potential ways to overcome it.

Truncation of mutant huntingtin in knock-in mice demonstrates exon1 huntingtin is a key pathogenic form

Polyglutamine expansion in proteins can cause selective neurodegeneration, although the mechanisms are not fully understood. In Huntington’s disease (HD), proteolytic processing generates toxic N-terminal huntingtin (HTT) fragments that preferentially kill striatal neurons. Here, using CRISPR/Cas9 to truncate full-length mutant HTT in HD140Q knock-in (KI) mice, we show that exon 1 HTT is stably present in the brain, regardless of truncation sites in full-length HTT. This N-terminal HTT leads to similar HD-like phenotypes and age-dependent HTT accumulation in the striatum in different KI mice. We find that exon 1 HTT is constantly generated but its selective accumulation in the striatum is associated with the age-dependent expression of striatum-enriched HspBP1, a chaperone inhibitory protein. Our findings suggest that tissue-specific chaperone function contributes to the selective neuropathology in HD, and highlight the therapeutic potential in blocking generation of exon 1 HTT.

The mechanisms by which mutant Huntington protein Htt leads to selective neurodegeneration are not fully understood. Here, using gene editing in HD140Q knock-in mice, the authors show that exon1 Htt is a critical pathological form of the protein.

Application of yeast to studying amyloid and prion diseases

Amyloids are fibrous cross-β protein aggregates that are capable of proliferation via nucleated polymerization. Amyloid conformation likely represents an ancient protein fold and is linked to various biological or pathological manifestations. Self-perpetuating amyloid-based protein conformers provide a molecular basis for transmissible (infectious or heritable) protein isoforms, termed prions. Amyloids and prions, as well as other types of misfolded aggregated proteins are associated with a variety of devastating mammalian and human diseases, such as Alzheimer's, Parkinson's and Huntington's diseases, transmissible spongiform encephalopathies (TSEs), amyotrophic lateral sclerosis (ALS) and transthyretinopathies. In yeast and fungi, amyloid-based prions control phenotypically detectable heritable traits. Simplicity of cultivation requirements and availability of powerful genetic approaches makes yeast Saccharomyces cerevisiae an excellent model system for studying molecular and cellular mechanisms governing amyloid formation and propagation. Genetic techniques allowing for the expression of mammalian or human amyloidogenic and prionogenic proteins in yeast enable researchers to capitalize on yeast advantages for characterization of the properties of disease-related proteins. Chimeric constructs employing mammalian and human aggregation-prone proteins or domains, fused to fluorophores or to endogenous yeast proteins allow for cytological or phenotypic detection of disease-related protein aggregation in yeast cells. Yeast systems are amenable to high-throughput screening for antagonists of amyloid formation, propagation and/or toxicity. This review summarizes up to date achievements of yeast assays in application to studying mammalian and human disease-related aggregating proteins, and discusses both limitations and further perspectives of yeast-based strategies.

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.

Abrogation of prenucleation, transient oligomerization of the Huntingtin exon 1 protein by human profilin I

Human profilin I reduces aggregation and concomitant toxicity of the polyglutamine-containing N-terminal region of the huntingtin protein encoded by exon 1 (htt^ex1) and responsible for Huntington's disease. Here, we investigate the interaction of profilin with htt^ex1 using NMR techniques designed to quantitatively analyze the kinetics and equilibria of chemical exchange at atomic resolution, including relaxation dispersion, exchange-induced shifts, and lifetime line broadening. We first show that the presence of two polyproline tracts in htt^ex1, absent from a shorter huntingtin variant studied previously, modulates the kinetics of the transient branched oligomerization pathway that precedes nucleation, resulting in an increase in the populations of the on-pathway helical coiled-coil dimeric and tetrameric species (τ_ex ≤ 50 to 70 μs), while leaving the population of the off-pathway (nonproductive) dimeric species largely unaffected (τ_ex ∼750 μs). Next, we show that the affinity of a single molecule of profilin to the polyproline tracts is in the micromolar range (K _diss ∼ 17 and ∼ 31 μM), but binding of a second molecule of profilin is negatively cooperative, with the affinity reduced ∼11-fold. The lifetime of a 1:1 complex of htt^ex1 with profilin, determined using a shorter huntingtin variant containing only a single polyproline tract, is shown to be on the submillisecond timescale (τ _ex ∼ 600 μs and K _diss ∼ 50 μM). Finally, we demonstrate that, in stable profilin-htt^ex1 complexes, the productive oligomerization pathway, leading to the formation of helical coiled-coil htt^ex1 tetramers, is completely abolished, and only the pathway resulting in "nonproductive" dimers remains active, thereby providing a mechanistic basis for how profilin reduces aggregation and toxicity of htt^ex1.

Brain Microbiota in Huntington's Disease Patients

One of the most important challenges facing medical science is to better understand the cause of neuronal pathology in neurodegenerative diseases. Such is the case for Huntington's disease (HD), a genetic disorder primarily caused by a triplet expansion in the Huntingtin gene (HTT). Although aberrant HTT is expressed from embryogenesis, it remains puzzling as to why the onset of disease symptoms manifest only after several decades of life. In the present study, we investigated the possibility of microbial infection in brain tissue from patients with HD, reasoning that perhaps mutated HTT could be deleterious for immune cells and neural tissue, and could facilitate microbial colonization. Using immunohistochemistry approaches, we observed a variety of fungal structures in the striatum and frontal cortex of seven HD patients. Some of these fungi were found in close proximity to the nucleus, or even as intranuclear inclusions. Identification of the fungal species was accomplished by next-generation sequencing (NGS). Interestingly, some genera, such as Ramularia, appeared unique to HD patients, and have not been previously described in other neurodegenerative diseases. Several bacterial species were also identified both by PCR and NGS. Notably, a curved and filamentous structure that immunoreacts with anti-bacterial antibodies was characteristic of HD brains and has not been previously observed in brain tissue from neurodegenerative patients. Prevalent bacterial genera included Pseudomonas, Acinetobacter, and Burkholderia. Collectively, our results represent the first attempt to identify the brain microbiota in HD. Our observations suggest that microbial colonization may be a risk factor for HD and might explain why the onset of the disease appears after several decades of life. Importantly, they may open a new field of investigation and could help in the design of new therapeutic strategies for this devastating disorder.

Global Proteome and Ubiquitinome Changes in the Soluble and Insoluble Fractions of Q175 Huntington Mice Brains

Huntington's disease is caused by a polyglutamine repeat expansion in the huntingtin protein which affects the function and folding of the protein, and results in intracellular protein aggregates. Here, we examined whether this mutation leads to altered ubiquitination of huntingtin and other proteins in both soluble and insoluble fractions of brain lysates of the Q175 knock-in Huntington's disease mouse model and the Q20 wild-type mouse model. Ubiquitination sites are detected by identification of Gly-Gly (diGly) remnant motifs that remain on modified lysine residues after digestion. We identified K6, K9, K132, K804, and K837 as endogenous ubiquitination sites of soluble huntingtin, with wild-type huntingtin being mainly ubiquitinated at K132, K804, and K837. Mutant huntingtin protein levels were strongly reduced in the soluble fraction whereas K6 and K9 were mainly ubiquitinated. In the insoluble fraction increased levels of huntingtin K6 and K9 diGly sites were observed for mutant huntingtin as compared with wild type. Besides huntingtin, proteins with various roles, including membrane organization, transport, mRNA processing, gene transcription, translation, catabolic processes and oxidative phosphorylation, were differently expressed or ubiquitinated in wild-type and mutant huntingtin brain tissues. Correlating protein and diGly site fold changes in the soluble fraction revealed that diGly site abundances of most of the proteins were not related to protein fold changes, indicating that these proteins were differentially ubiquitinated in the Q175 mice. In contrast, both the fold change of the protein level and diGly site level were increased for several proteins in the insoluble fraction, including ubiquitin, ubiquilin-2, sequestosome-1/p62 and myo5a. Our data sheds light on putative novel proteins involved in different cellular processes as well as their ubiquitination status in Huntington's disease, which forms the basis for further mechanistic studies to understand the role of differential ubiquitination of huntingtin and ubiquitin-regulated processes in Huntington's disease.

Huntingtin Lowering Strategies for Disease Modification in Huntington's Disease

Huntington's disease is caused by an abnormally expanded CAG repeat expansion in the HTT gene, which confers a predominant toxic gain of function in the mutant huntingtin (mHTT) protein. There are currently no disease-modifying therapies available, but approaches that target proximally in disease pathogenesis hold great promise. These include 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. Improvements in the delivery and distribution of such agents as well as the development of objective biomarkers of disease and of HTT lowering pharmacodynamic outcomes have brought these potential therapies to the forefront of Huntington's disease research, with clinical trials in patients already underway.

Huntingtin's N-Terminus Rearrangements in the Presence of Membranes: A Joint Spectroscopic and Computational Perspective

Huntington's disease is a neurodegenerative disorder resulting from an expanded polyglutamine (polyQ) repeat of the Huntingtin (Htt) protein. Affected tissues often contain aggregates of the N-terminal Htt exon 1 (Htt-Ex1) fragment. The N-terminal N17 domain proximal to the polyQ tract is key to enhance aggregation and modulate Htt toxicity. Htt-Ex1 is intrinsically disordered, yet it has been postulated that under physiological conditions membranes induce the N17 to adopt an α-helical structure, which then plays a key role in regulating Htt protein aggregation. The present study leverages the recently available assignment of NMR peaks in an N17Q17 construct, in order to provide a look into the changes occurring in vitro upon exposing this fragment to various brain extract fragments as well as to synthetic bilayers. Residue-specific changes were observed by 3D HNCO NMR, whose nature was further clarified with ancillary CD and aggregation studies, as well as with molecular dynamic calculations. From this combination of measurements and computations, a unified picture emerges, whereby transient structures consisting of α-helices spanning a fraction of the N17 residues form during N17Q17-membrane interactions. These interactions are fairly dynamic, but they qualitatively mimic more rigid variants that have been discussed in the literature. The nature of these interactions and their potential influence on the aggregation process of these kinds of constructs under physiological conditions are briefly assessed.

RNA Related Pathology in Huntington's Disease

This chapter summarises research investigating the expression of huntingtin sense and anti-sense transcripts, the effect of the mutation on huntingtin processing as well as the more global effect of the mutation on the coding and non-coding transcriptomes. The huntingtin gene is ubiquitously expressed, although expression levels vary between tissues and cell types. A SNP that affects NF-ĸB binding in the huntingtin promoter modulates the expression level of huntingtin transcripts and is associated with the age of disease onset. Incomplete splicing between exon 1 and exon 2 has been shown to result in the expression of a small polyadenylated mRNA that encodes the highly pathogenic exon 1 huntingtin protein. This occurs in a CAG-repeat length dependent manner in all full-length mouse models of HD as well as HD patient post-mortem brains and fibroblasts. An antisense transcript to huntingtin is generated that contains a CUG repeat that is expanded in HD patients. In myotonic dystrophy, expanded CUG repeats form RNA foci in cell nuclei that bind specific proteins (e.g. MBL1). Short, pure CAG RNAs of approximately 21 nucleotides that have been processed by DICER can inhibit the translation of other CAG repeat containing mRNAs. The HD mutation affects the transcriptome at the level of mRNA expression, splicing and the expression of non-coding RNAs. Finally, expanded repetitive stretched of nucleotides can lead to RAN translation, in which the ribosome translates from the expanded repeat in all possible reading frames, producing proteins with various poly-amino acid tracts. The extent to which these events contribute to HD pathogenesis is largely unknown.

Huntingtin protein: A new option for fixing the Huntington's disease countdown clock

Huntington's disease is a dreadful, incurable disorder. It springs from the autosomal dominant mutation in the first exon of the HTT gene, which encodes for the huntingtin protein (HTT) and results in progressive neurodegeneration. Thus far, all the attempted approaches to tackle the mutant HTT-induced toxicity causing this disease have failed. The mutant protein comes with the aberrantly expanded poly-glutamine tract. It is primarily to blame for the build-up of β-amyloid-like HTT aggregates, deleterious once broadened beyond the critical ∼35-37 repeats threshold. Recent experimental findings have provided valuable information on the molecular basis underlying this HTT-driven neurodegeneration. These findings indicate that the poly-glutamine siding regions and many post-translation modifications either abet or counter the poly-glutamine tract. This review provides an overall, up-to-date insight into HTT biophysics and structural biology, particularly discussing novel pharmacological options to specifically target the mutated protein and thus inhibit its functions and toxicity.

The cryo-electron microscopy structure of huntingtin

Huntingtin (Htt) is a large (348 kDa) protein, essential for embryonic development and involved in diverse cellular activities such as vesicular transport, endocytosis, autophagy and transcription regulation1,2. While an integrative understanding of Htt's biological functions is lacking, the large number of identified interactors suggests that Htt serves as a protein-protein interaction hub1,3,4. Furthermore, Huntington’s disease is caused by a mutation in the Htt gene, resulting in a pathogenic expansion of a polyglutamine (polyQ) repeat at the N-terminus of Htt5,6. However, only limited structural information on Htt is currently available. Here we employed cryo-electron microscopy (cryo-EM) to determine the structure of full-length human Htt in a complex with HAP40/F8A7 to 4 Å resolution. Htt is largely α-helical and consists of three major domains. The N- and C-terminal domains contain multiple HEAT repeats arranged in a solenoid fashion. These domains are connected by a smaller bridge domain containing different types of tandem repeats. HAP40 is also largely α-helical and has a tetratricopeptide repeat (TPR)-like organization. HAP40 binds in a cleft contacting the three Htt domains by hydrophobic and electrostatic interactions, thereby stabilizing Htt conformation. These data rationalize previous biochemical results and pave the way for an improved understanding of Htt’s diverse cellular functions.

Sphingomyelin and GM1 Influence Huntingtin Binding to, Disruption of, and Aggregation on Lipid Membranes

Huntington disease (HD) is an inherited neurodegenerative disease caused by the expansion beyond a critical threshold of a polyglutamine (polyQ) tract near the N-terminus of the huntingtin (htt) protein. Expanded polyQ promotes the formation of a variety of oligomeric and fibrillar aggregates of htt that accumulate into the hallmark proteinaceous inclusion bodies associated with HD. htt is also highly associated with numerous cellular and subcellular membranes that contain a variety of lipids. As lipid homeostasis and metabolism abnormalities are observed in HD patients, we investigated how varying both the sphingomyelin (SM) and ganglioside (GM1) contents modifies the interactions between htt and lipid membranes. SM composition is altered in HD, and GM1 has been shown to have protective effects in animal models of HD. A combination of Langmuir trough monolayer techniques, vesicle permeability and binding assays, and in situ atomic force microscopy (AFM) were used to directly monitor the interaction of a model, synthetic htt peptide and a full-length htt-exon1 recombinant protein with model membranes comprised of total brain lipid extract (TBLE) and varying amounts of exogenously added SM or GM1. The addition of either SM or GM1 decreased htt insertion into the lipid monolayers. However, TBLE vesicles with an increased SM content were more susceptible to htt-induced permeabilization, whereas GM1 had no effect on permeablization. Pure TBLE bilayers and TBLE bilayers enriched with GM1 developed regions of roughened, granular morphologies upon exposure to htt-exon1, but plateau-like domains with a smoother appearance formed in bilayers enriched with SM. Oligomeric aggregates were observed on all bilayer systems regardless of induced morphology. Collectively, these observations suggest that the lipid composition and its subsequent effects on membrane material properties strongly influence htt binding and aggregation on lipid membranes.

HSP90 recognizes the N-terminus of huntingtin involved in regulation of huntingtin aggregation by USP19

Huntington’s disease (HD) is caused by aberrant expansion of polyglutamine (polyQ) in the N-terminus of huntingtin (Htt). Our previous study has demonstrated that HSP90 is involved in the triage decision of Htt, but how HSP90 recognizes and regulates Htt remains elusive. We investigated the interaction between HSP90 and the N-terminal fragments of Htt (Htt-N), such as the N-terminal 90-residue fragment (Htt-N90). Our results showed that HSP90 binds to the N-terminal extreme of Htt-N in a sequence just ahead of the polyQ tract. Structural integration of the middle and C-terminal domains of HSP90 is essential for interacting with Htt-N90, and the dimerization mediated by the C-terminal domain facilitates this interaction. Moreover, ubiquitin-specific protease 19 (USP19), a deubiquitinating enzyme interacting with HSP90, up-regulates the protein level of Htt-N90 and consequently promotes its aggregation, whereas disruption of the interaction between Htt-N90 and HSP90 attenuates the effect of USP19 on Htt-N90. Thus, HSP90 interacts with Htt-N90 on the N-terminal amphipathic α-helix, and then recruits USP19 to modulate the protein level and aggregation of Htt-N90. This study provides mechanistic insights into the recognition between HSP90 and the N-terminus of Htt, and the triage decision for the Htt protein by the HSP90 chaperone system.

Optimization of trans-Splicing for Huntington's Disease RNA Therapy

Huntington's disease (HD) is a devastating neurodegenerative disorder caused by a polyglutamine (polyQ) expansion in exon 1 of the Huntingtin (HTT) gene. We have previously demonstrated that spliceosome-mediated trans-splicing is a viable molecular strategy to specifically reduce and repair mutant HTT (mtHTT). Here, the targeted tethering efficacy of the pre-mRNA trans-splicing modules (PTM) in HTT was optimized. Various PTMs that targeted the 3′ end of HTT intron 1 or the intron 1 branch point were shown trans-splice into an HTT mini-gene, as well as the endogenous HTT pre-mRNA. PTMs that specifically target the endogenous intron 1 branch point increased the trans-splicing efficacy from 1–5 to 10–15%. Furthermore, lentiviral expression of PTMs in a human HD patient iPSC-derived neural culture significantly reversed two previously established polyQ-length dependent phenotypes. These results suggest that pre-mRNA repair of mtHTT could hold therapeutic benefit and it demonstrates an alternative platform to correct the mRNA product produced by the mtHTT allele in the context of HD.

Post-translational modifications clustering within proteolytic domains decrease mutant huntingtin toxicity

Huntington's disease (HD) is caused in large part by a polyglutamine expansion within the huntingtin (Htt) protein. Post-translational modifications (PTMs) control and regulate many protein functions and cellular pathways, and PTMs of mutant Htt are likely important modulators of HD pathogenesis. Alterations of selected numbers of PTMs of Htt fragments have been shown to modulate Htt cellular localization and toxicity. In this study, we systematically introduced site-directed alterations in individual phosphorylation and acetylation sites in full-length Htt constructs. The effects of each of these PTM alteration constructs were tested on cell toxicity using our nuclear condensation assay and on mitochondrial viability by measuring mitochondrial potential and size. Using these functional assays in primary neurons, we identified several PTMs whose alteration can block neuronal toxicity and prevent potential loss and swelling of the mitochondria caused by mutant Htt. These PTMs included previously described sites such as serine 116 and newly found sites such as serine 2652 throughout the protein. We found that these functionally relevant sites are clustered in protease-sensitive domains throughout full-length Htt. These findings advance our understanding of the Htt PTM code and its role in HD pathogenesis. Because PTMs are catalyzed by enzymes, the toxicity-modulating Htt PTMs identified here may be promising therapeutic targets for managing HD.

Assembly of Huntingtin headpiece into α-helical bundles

Protein aggregation is a hallmark of neurodegenerative disorders. In this group of brain-related disorders, a disease-specific "host" protein or fragment misfolds and adopts a metastatic, aggregate-prone conformation. Often, this misfolded conformation is structurally and thermodynamically different from its native state. Intermolecular contacts, which arise in this non-native state, promote aggregation. In this regard, understanding the molecular principles and mechanisms that lead to the formation of such a non-native state and further promote the formation of the critical nucleus for fiber growth is essential. In this study, the authors analyze the aggregation propensity of Huntingtin headpiece (htt^NT), which is known to facilitate the polyQ aggregation, in relation to the helix mediated aggregation mechanism proposed by the Wetzel group. The authors demonstrate that even though htt^NT displays a degenerate conformational spectrum on its own, interfaces of macroscopic or molecular origin can promote the α-helix conformation, eliminating all other alternatives in the conformational phase space. Our findings indicate that htt^NT molecules do not have a strong orientational preference for parallel or antiparallel orientation of the helices within the aggregate. However, a parallel packed bundle of helices would support the idea of increased polyglutamine concentration, to pave the way for cross-β structures.

Aggregation landscapes of Huntingtin exon 1 protein fragments and the critical repeat length for the onset of Huntington's disease

Huntington's disease (HD) is a neurodegenerative disease caused by an abnormal expansion in the polyglutamine (polyQ) track of the Huntingtin (HTT) protein. The severity of the disease depends on the polyQ repeat length, arising only in patients with proteins having 36 repeats or more. Previous studies have shown that the aggregation of N-terminal fragments (encoded by HTT exon 1) underlies the disease pathology in mouse models and that the HTT exon 1 gene product can self-assemble into amyloid structures. Here, we provide detailed structural mechanisms for aggregation of several protein fragments encoded by HTT exon 1 by using the associative memory, water-mediated, structure and energy model (AWSEM) to construct their free energy landscapes. We find that the addition of the N-terminal 17-residue sequence ([Formula: see text]) facilitates polyQ aggregation by encouraging the formation of prefibrillar oligomers, whereas adding the C-terminal polyproline sequence ([Formula: see text]) inhibits aggregation. The combination of both terminal additions in HTT exon 1 fragment leads to a complex aggregation mechanism with a basic core that resembles that found for the aggregation of pure polyQ repeats using AWSEM. At the extrapolated physiological concentration, although the grand canonical free energy profiles are uphill for HTT exon 1 fragments having 20 or 30 glutamines, the aggregation landscape for fragments with 40 repeats has become downhill. This computational prediction agrees with the critical length found for the onset of HD and suggests potential therapies based on blocking early binding events involving the terminal additions to the polyQ repeats.

Mechanisms underlying neurodegeneration in Huntington disease: applications to novel disease-modifying therapies

The CAG repeat expansion mutation that causes Huntington Disease (HD) was discovered more than 20 years ago, yet no treatment has yet been developed to stop the relentless course of the disease. Nonetheless, substantial progress has been made in understanding HD pathogenesis. We review insights that have been gleaned from HD genetics, metabolism, and pathology; HD mouse and cell models; the structure, function and post-translational modification of normal and mutant huntingtin (htt) protein; gene expression profiles in HD cells and tissue; the neurotoxicy of mutant htt RNA; and the expression of an antisense transcript from the HD locus. We conclude that rationale therapeutics for HD is within sight, though many questions remain to be answered.

N-Terminal Fragments of Huntingtin Longer than Residue 170 form Visible Aggregates Independently to Polyglutamine Expansion

BACKGROUND

A hallmark of Huntington's disease is the progressive aggregation of full length and N-terminal fragments of polyglutamine (polyQ)-expanded Huntingtin (Htt) into intracellular inclusions. The production of N-terminal fragments appears important for enabling pathology and aggregation; and hence the direct expression of a variety of N-terminal fragments are commonly used to model HD in animal and cellular models.

OBJECTIVE

It remains unclear how the length of the N-terminal fragments relates to polyQ - mediated aggregation. We investigated the fundamental intracellular aggregation process of eight different-length N-terminal fragments of Htt in both short (25Q) and long polyQ (97Q).

METHODS

N-terminal fragments were fused to fluorescent proteins and transiently expressed in mammalian cell culture models. These included the classic exon 1 fragment (90 amino acids) and longer forms of 105, 117, 171, 513, 536, 552, and 586 amino acids based on wild-type Htt (of 23Q) sequence length nomenclature.

RESULTS

N-terminal fragments of less than 171 amino acids only formed inclusions in polyQ-expanded form. By contrast the longer fragments formed inclusions irrespective of Q-length, with Q-length playing a negligible role in extent of aggregation. The inclusions could be classified into 3 distinct morphological categories. One type (Type A) was universally associated with polyQ expansions whereas the other two types (Types B and C) formed independently of polyQ length expansion.

CONCLUSIONS

PolyQ-expansion was only required for fragments of less than 171 amino acids to aggregate. Longer fragments aggregated predominately through a non-polyQ mechanism, involving at least one, and probably more distinct clustering mechanisms.

Dopamine exacerbates mutant Huntingtin toxicity via oxidative-mediated inhibition of autophagy in SH-SY5Y neuroblastoma cells: Beneficial effects of anti-oxidant therapeutics

Neuronal cell death in Huntington's Disease (HD) is associated with the abnormal expansions of a polyglutamine (polyQ) tract in the huntingtin protein (Htt) at the N-terminus that causes the misfolding and aggregation of the mutated protein (mHtt). Autophagy-lysosomal degradation of Htt aggregates may protect the neurons in HD. HD patients eventually manifest parkinsonian-like symptoms, which underlie defects in the dopaminergic system. We hypothesized that dopamine (DA) exacerbates the toxicity in affected neurons by over-inducing an oxidative stress that negatively impinges on the autophagy clearance of mHtt and thus precipitating neuronal cell death. Here we show that the hyper-expression of mutant (>113/150) polyQ Htt is per se toxic to dopaminergic human neuroblastoma SH-SY5Y cells, and that DA exacerbates this toxicity leading to apoptosis and secondary necrosis. DA toxicity is mediated by ROS production (mainly anion superoxide) that elicits a block in the formation of autophagosomes. We found that the pre-incubation with N-Acetyl-l-Cysteine (a quinone reductase inducer) or Deferoxamine (an iron chelator) prevents the generation of ROS, restores the autophagy degradation of mHtt and preserves the cell viability in SH-SY5Y cells expressing the polyQ Htt and exposed to DA. The present findings suggest that DA-induced impairment of autophagy underlies the parkinsonism in HD patients. Our data provide a mechanistic explanation of the DA toxicity in dopaminergic neurons expressing the mHtt and support the use of anti-oxidative stress therapeutics to restore protective autophagy in order to slow down the neurodegeneration in HD patients.

An Intrabody Drug (rAAV6-INT41) Reduces the Binding of N-Terminal Huntingtin Fragment(s) to DNA to Basal Levels in PC12 Cells and Delays Cognitive Loss in the R6/2 Animal Model

Huntington's disease (HD) is a fatal progressive disease linked to expansion of glutamine repeats in the huntingtin protein and characterized by the progressive loss of cognitive and motor function. We show that expression of a mutant human huntingtin exon-1-GFP fusion construct results in nonspecific gene dysregulation that is significantly reduced by 50% due to coexpression of INT41, an intrabody specific for the proline-rich region of the huntingtin protein. Using stable PC12 cell lines expressing either inducible human mutant huntingtin (mHtt, Q73) or normal huntingtin (nHtt, Q23), we investigated the effect of rAAV6-INT41, an adeno-associated virus vector with the INT41 coding sequence, on the subcellular distribution of Htt. Compartmental fractionation 8 days after induction of Htt showed a 6-fold increased association of a dominate N-terminal mHtt fragment with DNA compared to N-terminal nHtt. Transduction with rAAV6-INT41 reduced DNA binding of N-terminal mHtt 6.5-fold in the nucleus and reduced nuclear translocation of the detected fragments. Subsequently, when rAAV6-INT41 is delivered to the striatum in the R6/2 mouse model, treated female mice exhibited executive function statistically indistinguishable from wild type, accompanied by reductions in Htt aggregates in the striatum, suggesting that rAAV6-INT41 is promising as a gene therapy for Huntington's disease.

Astrocytes are key but indirect contributors to the development of the symptomatology and pathophysiology of Huntington's disease

Huntington's disease (HD) is a fatal neurodegenerative disease in which an early and selective vulnerability of striatal Spiny Projection Neurons is observed. However, several studies have highlighted the implication of glial cells, and in particular astrocytes, in the pathophysiological mechanisms of this disease. A better understanding of the respective contributions of neurons and astrocytes in HD is needed and would be important for the development of new therapeutic approaches. Today, no comparable in vivo models expressing the mutant HTT selectively in astrocytes or in neurons are available. In this study, we developed comparable cell-type specific mouse models expressing a fragment of Huntingtin specifically in neurons, astrocytes, or in both cell populations of the adult mouse basal ganglia circuit. This approach allowed us to characterize behavioral alterations occurring as soon as 4 weeks postinjection. Interestingly, less severe but significant behavioral alterations were also observed in the two cell-type specific models. We further showed that astrocytes are less affected by mHTT compared to neurons, in particular concerning mHTT aggregation. Additionally, a more indirect contribution of astrocytes compared to neurons was observed in several pathophysiological mechanisms such as astrogliosis and neuronal dysfunction. Finally, we showed that direct and indirect transcriptional alterations within the glial glutamatergic clearing system are caused by astrocytic and neuronal expression of mHTT, respectively. We anticipate that our study will help to better understand the contributions of astrocytes to HD and guide future therapeutic efforts. GLIA 2016;64:1841-1856.

PRMT5- mediated symmetric arginine dimethylation is attenuated by mutant huntingtin and is impaired in Huntington's disease (HD)

Abnormal protein interactions of mutant huntingtin (Htt) triggered by polyglutamine expansion are thought to mediate Huntington's disease (HD) pathogenesis. Here, we explored a functional interaction of Htt with protein arginine methyltransferase 5 (PRMT5), an enzyme mediating symmetrical dimethylation of arginine (sDMA) of key cellular proteins, including histones, and spliceosomal Sm proteins. Gene transcription and RNA splicing are impaired in HD. We demonstrated PRMT5 and Htt interaction and their co-localization in transfected neurons and in HD brain. As a result of this interaction, normal (but to a lesser extend mutant) Htt stimulated PRMT5 activity in vitro. SDMA of histones H2A and H4 was reduced in the presence of mutant Htt in primary cultured neurons and in HD brain, consistent with a demonstrated reduction in R3Me2s occupancy at the transcriptionally repressed promoters in HD brain. SDMA of another PRMT5 substrate, Cajal body marker coilin, was also reduced in the HD mouse model and in human HD brain. Finally, compensation of PRMT5 deficiency by ectopic expression of PRMT5/MEP50 complexes, or by the knock-down of H4R3Me2 demethylase JMJD6, reversed the toxic effects of mutant Htt in primary cortical neurons, suggesting that PRMT5 deficiency may mediate, at least in part, HD pathogenesis. These studies revealed a potential new mechanism for disruption of gene expression and RNA processing in HD, involving a loss of normal function of Htt in facilitation of PRMT5, supporting the idea that epigenetic regulation of gene transcription may be involved in HD and highlighting symmetric dimethylation of arginine as potential new therapeutic target.

Cholesterol Modifies Huntingtin Binding to, Disruption of, and Aggregation on Lipid Membranes

Huntington's disease (HD) is an inherited neurodegenerative disease caused by abnormally long CAG-repeats in the huntingtin gene that encode an expanded polyglutamine (polyQ) domain near the N-terminus of the huntingtin (htt) protein. Expanded polyQ domains are directly correlated to disease-related htt aggregation. Htt is found highly associated with a variety of cellular and subcellular membranes that are predominantly comprised of lipids. Since cholesterol homeostasis is altered in HD, we investigated how varying cholesterol content modifies the interactions between htt and lipid membranes. A combination of Langmuir trough monolayer techniques, vesicle permeability and binding assays, and in situ atomic force microscopy were used to directly monitor the interaction of a model, synthetic htt peptide and a full-length htt-exon1 recombinant protein with model membranes comprised of total brain lipid extract (TBLE) and varying amounts of exogenously added cholesterol. As the cholesterol content of the membrane increased, the extent of htt insertion decreased. Vesicles containing extra cholesterol were resistant to htt-induced permeabilization. Morphological and mechanical changes in the bilayer associated with exposure to htt were also drastically altered by the presence of cholesterol. Disrupted regions of pure TBLE bilayers were grainy in appearance and associated with a large number of globular aggregates. In contrast, morphological changes induced by htt in bilayers enriched in cholesterol were plateau-like with a smooth appearance. Collectively, these observations suggest that the presence and amount of cholesterol in lipid membranes play a critical role in htt binding and aggregation on lipid membranes.

Nuclear retention of full-length HTT RNA is mediated by splicing factors MBNL1 and U2AF65

Huntington's disease (HD) is caused by a CAG repeat expansion in the huntingtin (HTT) gene. Recent evidence suggests that HD is a consequence of multimodal, non-mutually exclusive mechanisms of pathogenesis that involve both HTT protein- and HTT RNA-triggered mechanisms. Here we provide further evidence for the role of expanded HTT (expHTT) RNA in HD by demonstrating that a fragment of expHTT is cytotoxic in the absence of any translation and that the extent of cytotoxicity is similar to the cytotoxicity of an expHTT protein fragment encoded by a transcript of similar length and with a similar repeat size. In addition, full-length (FL) expHTT is retained in the nucleus. Overexpression of the splicing factor muscleblind-like 1 (MBNL1) increases nuclear retention of expHTT and decreases the expression of expHTT protein in the cytosol. The splicing and nuclear export factor U2AF65 has the opposite effect, decreasing expHTT nuclear retention and increasing expression of expHTT protein. This suggests that MBNL1 and U2AF65 play a role in nuclear export of expHTT RNA.

Ultrasensitive measurement of huntingtin protein in cerebrospinal fluid demonstrates increase with Huntington disease stage and decrease following brain huntingtin suppression

Quantitation of huntingtin protein in the brain is needed, both as a marker of Huntington disease (HD) progression and for use in clinical gene silencing trials. Measurement of huntingtin in cerebrospinal fluid could be a biomarker of brain huntingtin, but traditional protein quantitation methods have failed to detect huntingtin in cerebrospinal fluid. Using micro-bead based immunoprecipitation and flow cytometry (IP-FCM), we have developed a highly sensitive mutant huntingtin detection assay. The sensitivity of huntingtin IP-FCM enables accurate detection of mutant huntingtin protein in the cerebrospinal fluid of HD patients and model mice, demonstrating that mutant huntingtin levels in cerebrospinal fluid reflect brain levels, increasing with disease stage and decreasing following brain huntingtin suppression. This technique has potential applications as a research tool and as a clinical biomarker.

Huntingtin proteolysis releases non-polyQ fragments that cause toxicity through dynamin 1 dysregulation

Cleavage of mutant huntingtin (HTT) is an essential process in Huntington's disease (HD), an inherited neurodegenerative disorder. Cleavage generates N-ter fragments that contain the polyQ stretch and whose nuclear toxicity is well established. However, the functional defects induced by cleavage of full-length HTT remain elusive. Moreover, the contribution of non-polyQ C-terminal fragments is unknown. Using time- and site-specific control of full-length HTT proteolysis, we show that specific cleavages are required to disrupt intramolecular interactions within HTT and to cause toxicity in cells and flies. Surprisingly, in addition to the canonical pathogenic N-ter fragments, the C-ter fragments generated, that do not contain the polyQ stretch, induced toxicity via dilation of the endoplasmic reticulum (ER) and increased ER stress. C-ter HTT bound to dynamin 1 and subsequently impaired its activity at ER membranes. Our findings support a role for HTT on dynamin 1 function and ER homoeostasis. Proteolysis-induced alteration of this function may be relevant to disease.

Ectopic expression of the striatal-enriched GTPase Rhes elicits cerebellar degeneration and an ataxia phenotype in Huntington's disease

Huntington's disease (HD) is caused by an expansion of glutamine repeats in the huntingtin protein (mHtt) that invokes early and prominent damage of the striatum, a region that controls motor behaviors. Despite its ubiquitous expression, why certain brain regions, such as the cerebellum, are relatively spared from neuronal loss by mHtt remains unclear. Previously, we implicated the striatal-enriched GTPase, Rhes (Ras homolog enriched in the striatum), which binds and SUMOylates mHtt and increases its solubility and cellular cytotoxicity, as the cause for striatal toxicity in HD. Here, we report that Rhes deletion in HD mice (N171-82Q), which express the N-terminal fragment of human Htt with 82 glutamines (Rhes(-/-)/N171-82Q), display markedly reduced HD-related behavioral deficits, and absence of lateral ventricle dilatation (secondary to striatal atrophy), compared to control HD mice (N171-82Q). To further validate the role of GTPase Rhes in HD, we tested whether ectopic Rhes expression would elicit a pathology in a brain region normally less affected in HD. Remarkably, ectopic expression of Rhes in the cerebellum of N171-82Q mice, during the asymptomatic period led to an exacerbation of motor deficits, including loss of balance and motor incoordination with ataxia-like features, not apparent in control-injected N171-82Q mice or Rhes injected wild-type mice. Pathological and biochemical analysis of Rhes-injected N171-82Q mice revealed a cerebellar lesion with marked loss of Purkinje neuron layer parvalbumin-immunoreactivity, induction of caspase 3 activation, and enhanced soluble forms of mHtt. Similarly reintroducing Rhes into the striatum of Rhes deleted Rhes(-/-)Hdh(150Q/150Q) knock-in mice, elicited a progressive HD-associated rotarod deficit. Overall, these studies establish that Rhes plays a pivotal role in vivo for the selective toxicity of mHtt in HD.

Integration-independent Transgenic Huntington Disease Fragment Mouse Models Reveal Distinct Phenotypes and Life Span in Vivo

The cascade of events that lead to cognitive decline, motor deficits, and psychiatric symptoms in patients with Huntington disease (HD) is triggered by a polyglutamine expansion in the N-terminal region of the huntingtin (HTT) protein. A significant mechanism in HD is the generation of mutant HTT fragments, which are generally more toxic than the full-length HTT. The protein fragments observed in human HD tissue and mouse models of HD are formed by proteolysis or aberrant splicing of HTT. To systematically investigate the relative contribution of the various HTT protein proteolysis events observed in vivo, we generated transgenic mouse models of HD representing five distinct proteolysis fragments ending at amino acids 171, 463, 536, 552, and 586 with a polyglutamine length of 148. All lines contain a single integration at the ROSA26 locus, with expression of the fragments driven by the chicken β-actin promoter at nearly identical levels. The transgenic mice N171-Q148 and N552-Q148 display significantly accelerated phenotypes and a shortened life span when compared with N463-Q148, N536-Q148, and N586-Q148 transgenic mice. We hypothesized that the accelerated phenotype was due to altered HTT protein interactions/complexes that accumulate with age. We found evidence for altered HTT complexes in caspase-2 fragment transgenic mice (N552-Q148) and a stronger interaction with the endogenous HTT protein. These findings correlate with an altered HTT molecular complex and distinct proteins in the HTT interactome set identified by mass spectrometry. In particular, we identified HSP90AA1 (HSP86) as a potential modulator of the distinct neurotoxicity of the caspase-2 fragment mice (N552-Q148) when compared with the caspase-6 transgenic mice (N586-Q148).