Huntingtin and the Synapse

BDNF and TRiC-inspired reagent rescue cortical synaptic deficits in a mouse model of Huntington's disease

Synaptic changes are early manifestations of neuronal dysfunction in Huntington's disease (HD). However, the mechanisms by which mutant HTT protein impacts synaptogenesis and function are not well understood. Herein we explored HD pathogenesis in the BACHD mouse model by examining synaptogenesis and function in long term primary cortical cultures. At DIV14 (days in vitro), BACHD cortical neurons showed no difference from WT neurons in synaptogenesis as revealed by colocalization of a pre-synaptic (Synapsin I) and a post-synaptic (PSD95) marker. From DIV21 to DIV35, BACHD neurons showed progressively reduced colocalization of Synapsin I and PSD95 relative to WT neurons. The deficits were effectively rescued by treatment of BACHD neurons with BDNF. The recombinant apical domain of CCT1 (ApiCCT1) yielded a partial rescuing effect. BACHD neurons also showed culture age-related significant functional deficits as revealed by multielectrode arrays (MEAs). These deficits were prevented by BDNF, whereas ApiCCT1 showed a less potent effect. These findings are evidence that deficits in BACHD synapse and function can be replicated in vitro and that BDNF or a TRiC-inspired reagent can potentially be protective against these changes in BACHD neurons. Our findings support the use of cellular models to further explicate HD pathogenesis and potential treatments.

Therapeutic potential of thymoquinone and its nanoformulations in neuropsychological disorders: a comprehensive review on molecular mechanisms in preclinical studies

Thymoquinone (THQ) and its nanoformulation (NFs) have emerged as promising candidates for the treatment of neurological diseases due to their diverse pharmacological properties, which include anti-inflammatory, antioxidant, and neuroprotective effects. In this study, we conducted an extensive search across reputable scientific websites such as PubMed, ScienceDirect, Scopus, and Google Scholar to gather relevant information. The antioxidant and anti-inflammatory properties of THQ have been observed to enhance the survival of neurons in affected areas of the brain, leading to significant improvements in behavioral and motor dysfunctions. Moreover, THQ and its NFs have demonstrated the capacity to restore antioxidant enzymes and mitigate oxidative stress. The primary mechanism underlying THQ's antioxidant effects involves the regulation of the Nrf2/HO-1 signaling pathway. Furthermore, THQ has been found to modulate key components of inflammatory signaling pathways, including toll-like receptors (TLRs), nuclear factor-κB (NF-κB), interleukin 6 (IL-6), IL-1β, and tumor necrosis factor alpha (TNFα), thereby exerting anti-inflammatory effects. This comprehensive review explores the various beneficial effects of THQ and its NFs on neurological disorders and provides insights into the underlying mechanisms involved.

Mechanisms of synapse-to-nucleus calcium signalling in striatal neurons and impairments in Huntington's disease

Huntington's disease (HD) is a monogenic disorder with autosomal dominant inheritance. In HD patients, neurons in the striatum and cortex degenerate, leading to motor, psychiatric and cognitive disorders. Dysregulated synaptic function and calcium handling are common in many neurodegenerative diseases, including HD. N-methyl-D-aspartate (NMDA) receptor function is enhanced in HD at extrasynaptic sites, altering the balance of calcium-dependent neuronal survival versus death signalling pathways. Endoplasmic reticulum (ER) calcium handling is also abnormal in HD. The ER, which is continuous with the nuclear envelope, is purportedly involved in nuclear calcium signalling; based on this, we hypothesised that nuclear calcium signalling is altered in HD. We explored this hypothesis with calcium imaging techniques, including simultaneous epifluorescent imaging of cytosolic and nuclear calcium using jRCaMP1b and GCaMP3 sensors, respectively, in striatal spiny projection neurons in cortical-striatal co-cultures from the YAC128 mouse model of HD. Our data show contributions from a variety of calcium channels to nuclear calcium signalling. NMDA receptors (NMDARs) play an essential role in initiating action potential-dependent calcium signalling to the nucleus, and ryanodine receptors (RyR) contribute to both cytosolic and nuclear calcium signals. Unlike previous reports in glutamatergic hippocampal and cortical neurons, we found that in GABAergic striatal neurons, L-type voltage-gated calcium channels (CaV) contribute to cytosolic, but not nuclear calcium signalling. Calcium imaging also suggests impairments in nuclear calcium signalling in HD striatal neurons, where spontaneous action potential-dependent calcium transients in the nucleus were smaller in YAC128 striatal neurons compared to those of wild-type (WT). Our results elucidate mechanisms involved in action potential-dependent nuclear calcium signalling in GABAergic striatal neurons, and have revealed a clear deficit in this signalling in HD.

Neuroimaging to Facilitate Clinical Trials in Huntington's Disease: Current Opinion from the EHDN Imaging Working Group

 Neuroimaging is increasingly being included in clinical trials of Huntington's disease (HD) for a wide range of purposes from participant selection and safety monitoring, through to demonstration of disease modification. Selection of the appropriate modality and associated analysis tools requires careful consideration. On behalf of the EHDN Imaging Working Group, we present current opinion on the utility and future prospects for inclusion of neuroimaging in HD trials. Covering the key imaging modalities of structural-, functional- and diffusion- MRI, perfusion imaging, positron emission tomography, magnetic resonance spectroscopy, and magnetoencephalography, we address how neuroimaging can be used in HD trials to: 1) Aid patient selection, enrichment, stratification, and safety monitoring; 2) Demonstrate biodistribution, target engagement, and pharmacodynamics; 3) Provide evidence for disease modification; and 4) Understand brain re-organization following therapy. We also present the challenges of translating research methodology into clinical trial settings, including equipment requirements and cost, standardization of acquisition and analysis, patient burden and invasiveness, and interpretation of results. We conclude, that with appropriate consideration of modality, study design and analysis, imaging has huge potential to facilitate effective clinical trials in HD.

HAP40 modulates mutant Huntingtin aggregation and toxicity in Huntington's disease mice

Huntington's disease (HD) is a monogenic neurodegenerative disease, caused by the CAG trinucleotide repeat expansion in exon 1 of the Huntingtin (HTT) gene. The HTT gene encodes a large protein known to interact with many proteins. Huntingtin-associated protein 40 (HAP40) is one that shows high binding affinity with HTT and functions to maintain HTT conformation in vitro. However, the potential role of HAP40 in HD pathogenesis remains unknown. In this study, we found that the expression level of HAP40 is in parallel with HTT but inversely correlates with mutant HTT aggregates in mouse brains. Depletion of endogenous HAP40 in the striatum of HD140Q knock-in (KI) mice leads to enhanced mutant HTT aggregation and neuronal loss. Consistently, overexpression of HAP40 in the striatum of HD140Q KI mice reduced mutant HTT aggregation and ameliorated the behavioral deficits. Mechanistically, HAP40 preferentially binds to mutant HTT and promotes Lysine 48-linked ubiquitination of mutant HTT. Our results revealed that HAP40 is an important regulator of HTT protein homeostasis in vivo and hinted at HAP40 as a therapeutic target in HD treatment.

Huntington's Disease: Complex Pathogenesis and Therapeutic Strategies

Huntington's disease (HD) arises from the abnormal expansion of CAG repeats in the huntingtin gene (HTT), resulting in the production of the mutant huntingtin protein (mHTT) with a polyglutamine stretch in its N-terminus. The pathogenic mechanisms underlying HD are complex and not yet fully elucidated. However, mHTT forms aggregates and accumulates abnormally in neuronal nuclei and processes, leading to disruptions in multiple cellular functions. Although there is currently no effective curative treatment for HD, significant progress has been made in developing various therapeutic strategies to treat HD. In addition to drugs targeting the neuronal toxicity of mHTT, gene therapy approaches that aim to reduce the expression of the mutant HTT gene hold great promise for effective HD therapy. This review provides an overview of current HD treatments, discusses different therapeutic strategies, and aims to facilitate future therapeutic advancements in the field.

Environmental enrichment improves social isolation-induced memory impairment: The possible role of ITSN1-Reelin-AMPA receptor signaling pathway

Environmental enrichment (EE) through combination of social and non-biological stimuli enhances activity-dependent synaptic plasticity and improves behavioural performance. Our earlier studies have suggested that EE resilience the stress induced depression/ anxiety-like behaviour in Indian field mice Mus booduga. This study was designed to test whether EE reverses the social isolation (SI) induced effect and improve memory. Field-caught mice M. booduga were subjected to behaviour test (Direct wild, DW), remaining animals were housed under SI for ten days and then housed for short-term at standard condition (STSC)/ long-term at standard condition (LTSC) or as group in EE cage. Subsequently, we have examined reference, working memory and expression of genes associated with synaptic plasticity. Our analysis have shown that EE reversed SI induced impairment in reference, working memory and other accompanied changes i.e. increased level of Intersectin 1 (ITSN1), Huntingtin (Htt), Synaptotagmin -IV (SYT4), variants of brain-derived neurotrophic factor (Bdnf - III), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor (GluR1) expression, and decreased variants of Bdnf (IV), BDNF, Reelin, Apolipoprotein E receptor 2 (ApoER2), very low-density lipoprotein receptor (VLDLR), Src family tyrosine kinase (SFKs), Disabled protein (Dab)-1, Protein kinase B (PKB/Akt), GluR2, Mitogen-activated protein kinase (MAPK) and Extracellular signal-regulated kinase (ERK1/2) expression. In addition, SI induced reduction in BDNF expressing neurons in dentate gyrus of hippocampus reversed by EE. Further, we found that SI decreases small neuro-active molecules such as Benzenedicarboxylic acid, and increases 2-Pregnene in the hippocampus and feces reversed by EE. Overall, this study demonstrated that EE is effectively reversed the SI induced memory impairment by potentially regulating the molecules associated with the ITSN1-Reelin-AMPA receptor pathway to increase synaptic plasticity.

Cannabinerol (CBNR) Influences Synaptic Genes Associated with Cytoskeleton and Ion Channels in NSC-34 Cell Line: A Transcriptomic Study

Cannabinoids are receiving great attention as a novel approach in the treatment of cognitive and motor disabilities, which characterize neurological disorders. To date, over 100 phytocannabinoids have been extracted from Cannabis sativa, and some of them have shown neuroprotective properties and the capacity to influence synaptic transmission. In this study, we investigated the effects of a less-known phytocannabinoid, cannabinerol (CBNR), on neuronal physiology. Using the NSC-34 motor-neuron-like cell line and next-generation sequencing analysis, we discovered that CBNR influences synaptic genes associated with synapse organization and specialization, including genes related to the cytoskeleton and ion channels. Specifically, the calcium, sodium, and potassium channel subunits (Cacna1b, Cacna1c, Cacnb1, Grin1, Scn8a, Kcnc1, Kcnj9) were upregulated, along with genes related to NMDAR (Agap3, Syngap1) and calcium (Cabp1, Camkv) signaling. Moreover, cytoskeletal and cytoskeleton-associated genes (Actn2, Ina, Trio, Marcks, Bsn, Rtn4, Dgkz, Htt) were also regulated by CBNR. These findings highlight the important role played by CBNR in the regulation of synaptogenesis and synaptic transmission, suggesting the need for further studies to evaluate the neuroprotective role of CBNR in the treatment of synaptic dysfunctions that characterize motor disabilities in many neurological disorders.

Transmission-selective muscle pathology induced by the active propagation of mutant huntingtin across the human neuromuscular synapse

Neuron-to-neuron transmission of aggregation-prone, misfolded proteins may potentially explain the spatiotemporal accumulation of pathological lesions in the brains of patients with neurodegenerative protein-misfolding diseases (PMDs). However, little is known about protein transmission from the central nervous system to the periphery, or how this propagation contributes to PMD pathology. To deepen our understanding of these processes, we established two functional neuromuscular systems derived from human iPSCs. One was suitable for long-term high-throughput live-cell imaging and the other was adapted to a microfluidic system assuring that connectivity between motor neurons and muscle cells was restricted to the neuromuscular junction. We show that the Huntington's disease (HD)-associated mutant HTT exon 1 protein (mHTTEx1) is transmitted from neurons to muscle cells across the human neuromuscular junction. We found that transmission is an active and dynamic process that starts before aggregate formation and is regulated by synaptic activity. We further found that transmitted mHTTEx1 causes HD-relevant pathology at both molecular and functional levels in human muscle cells, even in the presence of the ubiquitous expression of mHTTEx1. In conclusion, we have uncovered a causal link between mHTTEx1 synaptic transmission and HD pathology, highlighting the therapeutic potential of blocking toxic protein transmission in PMDs.

Copper enhances aggregational toxicity of mutant huntingtin in a Drosophila model of Huntington's Disease

Huntington's disease (HD) is a progressive neurodegenerative disorder with clinical presentations of moderate to severe cognitive, motor, and psychiatric disturbances. HD is caused by the trinucleotide repeat expansion of CAG of the huntingtin (HTT) gene. The mutant HTT protein containing pathological polyglutamine (polyQ) extension is prone to misfolding and aggregation in the brain. It has previously been observed that copper and iron concentrations are increased in the striata of post-mortem human HD brains. Although it has been shown that the accumulation of mutant HTT protein can interact with copper, the underlying HD progressive phenotypes due to copper overload remains elusive. Here, in a Drosophila model of HD, we showed that copper induces dose-dependent aggregational toxicity and enhancement of Htt-induced neurodegeneration. Specifically, we found that copper increases mutant Htt aggregation, enhances the accumulation of Thioflavin S positive β-amyloid structures within Htt aggregates, and consequently alters autophagy in the brain. Administration of copper chelator D-penicillamine (DPA) through feeding significantly decreases β-amyloid aggregates in the HD pathological model. These findings reveal a direct role of copper in potentiating mutant Htt protein-induced aggregational toxicity, and further indicate the potential impact of environmental copper exposure in the disease onset and progression of HD.

Selection of appropriate non-clinical animal models to ensure translatability of novel AAV-gene therapies to the clinic

Gene Therapy Medicinal Products consist of a recombinant nucleic acid intended for the modulation or manipulation of a genetic sequence. A single administration of a novel gene therapy has the potential to be curative, with a durable long-term benefit to patients. Adeno-associated viral vectors have become the viral vector of choice for in vivo delivery of therapeutic transgenes as they are mildly immunogenic, can effectively transduce a variety of human tissues and cells, and have low levels of genomic integration. Central to the effective translation of data generated in discovery studies to the clinic is the selection of appropriate animal species for pivotal non-clinical studies. This review aims to support the selection of appropriate animal models for non-clinical studies to advance the development of novel adeno-associated virus gene therapies.

Saccharomyces cerevisiae as a Model for Studying Human Neurodegenerative Disorders: Viral Capsid Protein Expression

In this article, we briefly describe human neurodegenerative diseases (NDs) and the experimental models used to study them. The main focus is the yeast Saccharomyces cerevisiae as an experimental model used to study neurodegenerative processes. We review recent experimental data on the aggregation of human neurodegenerative disease-related proteins in yeast cells. In addition, we describe the results of studies that were designed to investigate the molecular mechanisms that underlie the aggregation of reporter proteins. The advantages and disadvantages of the experimental approaches that are currently used to study the formation of protein aggregates are described. Special attention is given to the similarity between aggregates that form as a result of protein misfolding and viral factories-special structural formations in which viral particles are formed inside virus-infected cells. A separate part of the review is devoted to our previously published study on the formation of aggregates upon expression of the insect densovirus capsid protein in yeast cells. Based on the reviewed results of studies on NDs and related protein aggregation, as well as viral protein aggregation, a new experimental model system for the study of human NDs is proposed. The core of the proposed system is a comparative transcriptomic analysis of changes in signaling pathways during the expression of viral capsid proteins in yeast cells.

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.

Selective perforant-pathway atrophy in Huntington disease: MRI analysis of hippocampal subfields

INTRODUCTION

While individuals with Huntington disease (HD) show memory impairment that indicates hippocampal dysfunction, the available literature does not consistently identify structural evidence for involvement of the whole hippocampus but rather suggests that hippocampal atrophy may be confined to certain hippocampal subregions.

METHODS

We processed T1-weighted MRI from IMAGE-HD study using FreeSurfer 7.0 and compared the volumes of the hippocampal subfields among 36 early motor symptomatic (symp-HD), 40 pre-symptomatic (pre-HD), and 36 healthy control individuals across three timepoints over 36 months.

RESULTS

Mixed-model analyses revealed significantly lower subfield volumes in symp-HD, compared with pre-HD and control groups, in the subicular regions of the perforant-pathway: presubiculum, subiculum, dentate gyrus, tail, and right molecular layer. These adjoining subfields aggregated into a single principal component, which demonstrated an accelerated rate of atrophy in the symp-HD. Volumes between pre-HD and controls did not show any significant difference. In the combined HD groups, CAG repeat length and disease burden score were associated with presubiculum, molecular layer, tail, and perforant-pathway subfield volumes. Hippocampal left tail and perforant-pathway subfields were associated with motor onset in the pre-HD group.

CONCLUSIONS

Hippocampal subfields atrophy in early symptomatic HD affects key regions of the perforant-pathway, which may implicate the distinctive memory impairment at this stage of illness. Their volumetric associations with genetic and clinical markers suggest the selective susceptibility of these subfields to mutant Huntingtin and disease progression.

Altered exocytosis of inhibitory synaptic vesicles at single presynaptic terminals of cultured striatal neurons in a knock-in mouse model of Huntington's disease

Huntington's disease (HD) is a progressive dominantly inherited neurodegenerative disease caused by the expansion of a cytosine-adenine-guanine (CAG) trinucleotide repeat in the huntingtin gene, which encodes the mutant huntingtin protein containing an expanded polyglutamine tract. One of neuropathologic hallmarks of HD is selective degeneration in the striatum. Mechanisms underlying selective neurodegeneration in the striatum of HD remain elusive. Neurodegeneration is suggested to be preceded by abnormal synaptic transmission at the early stage of HD. However, how mutant huntingtin protein affects synaptic vesicle exocytosis at single presynaptic terminals of HD striatal neurons is poorly understood. Here, we measured synaptic vesicle exocytosis at single presynaptic terminals of cultured striatal neurons (mainly inhibitory neurons) in a knock-in mouse model of HD (zQ175) during electrical field stimulation using real-time imaging of FM 1-43 (a lipophilic dye). We found a significant decrease in bouton density and exocytosis of synaptic vesicles at single presynaptic terminals in cultured striatal neurons. Real-time imaging of VGAT-CypHer5E (a pH sensitive dye conjugated to an antibody against vesicular GABA transporter (VGAT)) for inhibitory synaptic vesicles revealed a reduction in bouton density and exocytosis of inhibitory synaptic vesicles at single presynaptic terminals of HD striatal neurons. Thus, our results suggest that the mutant huntingtin protein decreases bouton density and exocytosis of inhibitory synaptic vesicles at single presynaptic terminals of striatal neurons, causing impaired inhibitory synaptic transmission, eventually leading to the neurodegeneration in the striatum of HD.

Functional unknomics: Systematic screening of conserved genes of unknown function

The human genome encodes approximately 20,000 proteins, many still uncharacterised. It has become clear that scientific research tends to focus on well-studied proteins, leading to a concern that poorly understood genes are unjustifiably neglected. To address this, we have developed a publicly available and customisable "Unknome database" that ranks proteins based on how little is known about them. We applied RNA interference (RNAi) in Drosophila to 260 unknown genes that are conserved between flies and humans. Knockdown of some genes resulted in loss of viability, and functional screening of the rest revealed hits for fertility, development, locomotion, protein quality control, and resilience to stress. CRISPR/Cas9 gene disruption validated a component of Notch signalling and 2 genes contributing to male fertility. Our work illustrates the importance of poorly understood genes, provides a resource to accelerate future research, and highlights a need to support database curation to ensure that misannotation does not erode our awareness of our own ignorance.

Detection of proteomic alterations at different stages in a Huntington's disease mouse model via matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) imaging

Huntington's disease (HD) is a progressive and irreversible neurodegenerative disease leading to the inability to carry out daily activities and for which no cure exists. The underlying mechanisms of the disease have not been fully elucidated yet. Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) allows the spatial information of proteins to be obtained upon the tissue sections without homogenisation. In this study, we aimed to examine proteomic alterations in the brain tissue of an HD mouse model with MALDI-MSI coupled to LC-MS/MS system. We used 3-, 6- and 12-month-old YAC128 mice representing pre-stage, mild stage and pathological stage of the HD and their non-transgenic littermates, respectively. The intensity levels of 89 proteins were found to be significantly different in YAC128 in comparison to their control mice in the pre-stage, 83 proteins in the mild stage, and 82 proteins in the pathological stage. Among them, Tau, EF2, HSP70, and NogoA proteins were validated with western blot analysis. In conclusion, the results of this study have provided remarkable new information about the spatial proteomic alterations in the HD mouse model, and we suggest that MALDI-MSI is an excellent technique for identifying such regional proteomic changes and could offer new perspectives in examining complex diseases.

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.

Age-dependent increase in tau phosphorylation at serine 396 in Huntington's disease pre-frontal cortex

Background

To date, it is still controversial whether tau phosphorylation plays a role in Huntington's disease (HD), as previous studies demonstrated either no alterations or increases in phosphorylated tau (pTau) in HD post-mortem brain and mouse models.

Objectives

The goal of this study was to determine whether total tau and pTau levels are altered in HD.

Methods

Immunohistochemistry, cellular fractionations, and western blots were used to measure tau and pTau levels in a large cohort of HD and control post-mortem prefrontal cortex (PFC). Furthermore, western blots were performed to assess tau, and pTau levels in HD and control isogenic embryonic stem cell (ESC)-derived cortical neurons and neuronal stem cells (NSCs). Similarly, western blots were used to assess tau and pTau in Htt Q111 and transgenic R6/2 mice. Lastly, total tau levels were assessed in HD and healthy control plasma using Quanterix Simoa assay.

Results

Our results revealed that, while there was no difference in tau or pTau levels in HD PFC compared to controls, tau phosphorylated at S396 levels were increased in PFC samples from HD patients 60 years or older at time of death. Additionally, tau and pTau levels were not changed in HD ESC-derived cortical neurons and NSCs. Similarly, tau or pTau levels were not altered in Htt Q111 and transgenic R6/2 mice compared to wild-type littermates. Lastly, tau levels were not changed in plasma from a small cohort of HD patients compared to controls.

Conclusion

Together these findings demonstrate that pTau-S396 levels increase significantly with age in HD PFC.

Paternal Western diet causes transgenerational increase in food consumption in Drosophila with parallel alterations in the offspring brain proteome and microRNAs

Several lines of evidence indicate that ancestral diet might play an important role in determining offspring's metabolic traits. However, it is not yet clear whether ancestral diet can affect offspring's food choices and feeding behavior. In the current study, taking advantage of Drosophila model system, we demonstrate that paternal Western diet (WD) increases offspring food consumption up to the fourth generation. Paternal WD also induced alterations in F1 offspring brain proteome. Using enrichment analyses of pathways for upregulated and downregulated proteins, we found that upregulated proteins had significant enrichments in terms related to translation and translation factors, whereas downregulated proteins displayed enrichments in small molecule metabolic processes, TCA cycles, and electron transport chain (ETC). Using MIENTURNET miRNA prediction tool, dme-miR-10-3p was identified as the top conserved miRNA predicted to target proteins regulated by ancestral diet. RNAi-based knockdown of miR-10 in the brain significantly increased food consumption, implicating miR-10 as a potential factor in programming feeding behavior. Together, these findings suggest that ancestral nutrition may influence offspring feeding behavior through alterations in miRNAs.

The SphK1/S1P Axis Regulates Synaptic Vesicle Endocytosis via TRPC5 Channels

Sphingosine-1-phosphate (S1P), a bioactive sphingolipid concentrated in the brain, is essential for normal brain functions, such as learning and memory and feeding behaviors. Sphingosine kinase 1 (SphK1), the primary kinase responsible for S1P production in the brain, is abundant within presynaptic terminals, indicating a potential role of the SphK1/S1P axis in presynaptic physiology. Altered S1P levels have been highlighted in many neurologic diseases with endocytic malfunctions. However, it remains unknown whether the SphK1/S1P axis may regulate synaptic vesicle endocytosis in neurons. The present study evaluates potential functions of the SphK1/S1P axis in synaptic vesicle endocytosis by determining effects of a dominant negative catalytically inactive SphK1. Our data for the first time identify a critical role of the SphK1/S1P axis in endocytosis in both neuroendocrine chromaffin cells and neurons from mice of both sexes. Furthermore, our Ca2+ imaging data indicate that the SphK1/S1P axis may be important for presynaptic Ca2+ increases during prolonged stimulations by regulating the Ca2+ permeable TRPC5 channels, which per se regulate synaptic vesicle endocytosis. Collectively, our data point out a critical role of the regulation of TRPC5 by the SphK1/S1P axis in synaptic vesicle endocytosis.SIGNIFICANCE STATEMENT Sphingosine kinase 1 (SphK1), the primary kinase responsible for brain sphingosine-1-phosphate (S1P) production, is abundant within presynaptic terminals. Altered SphK1/S1P metabolisms has been highlighted in many neurologic disorders with defective synaptic vesicle endocytosis. However, whether the SphK1/S1P axis may regulate synaptic vesicle endocytosis is unknown. Here, we identify that the SphK1/S1P axis regulates the kinetics of synaptic vesicle endocytosis in neurons, in addition to controlling fission-pore duration during single vesicle endocytosis in neuroendocrine chromaffin cells. The regulation of the SphK1/S1P axis in synaptic vesicle endocytosis is specific since it has a distinguished signaling pathway, which involves regulation of Ca2+ influx via TRPC5 channels. This discovery may provide novel mechanistic implications for the SphK1/S1P axis in brain functions under physiological and pathologic conditions.

Defective Mitochondrial Dynamics and Protein Degradation Pathways Underlie Cadmium-Induced Neurotoxicity and Cell Death in Huntington's Disease Striatal Cells

Exposure to heavy metals, including cadmium (Cd), can induce neurotoxicity and cell death. Cd is abundant in the environment and accumulates in the striatum, the primary brain region selectively affected by Huntington's disease (HD). We have previously reported that mutant huntingtin protein (mHTT) combined with chronic Cd exposure induces oxidative stress and promotes metal dyshomeostasis, resulting in cell death in a striatal cell model of HD. To understand the effect of acute Cd exposure on mitochondrial health and protein degradation pathways, we hypothesized that expression of mHTT coupled with acute Cd exposure would cooperatively alter mitochondrial bioenergetics and protein degradation mechanisms in striatal STHdh cells to reveal novel pathways that augment Cd cytotoxicity and HD pathogenicity. We report that mHTT cells are significantly more susceptible to acute Cd-induced cell death as early as 6 h after 40 µM CdCl₂ exposure compared with wild-type (WT). Confocal microscopy, biochemical assays, and immunoblotting analysis revealed that mHTT and acute Cd exposure synergistically impair mitochondrial bioenergetics by reducing mitochondrial potential and cellular ATP levels and down-regulating the essential pro-fusion proteins MFN1 and MFN2. These pathogenic effects triggered cell death. Furthermore, Cd exposure increases the expression of autophagic markers, such as p62, LC3, and ATG5, and reduces the activity of the ubiquitin-proteasome system to promote neurodegeneration in HD striatal cells. Overall, these results reveal a novel mechanism to further establish Cd as a pathogenic neuromodulator in striatal HD cells via Cd-triggered neurotoxicity and cell death mediated by an impairment in mitochondrial bioenergetics and autophagy with subsequent alteration in protein degradation pathways.

Neuroprotectant Effects of Hibiscetin in 3-Nitropropionic Acid-Induced Huntington's Disease via Subsiding Oxidative Stress and Modulating Monoamine Neurotransmitters in Rats Brain

BACKGROUND

Previously reported data suggest that hibiscetin, isolated from roselle, contains delphinidin-3-sambubioside and cyanidin-3-sambubioside including anthocyanidins and has a broad range of physiological effects. In this study, we aim to analyze the effect of hibiscetin neuroprotective ability in rats against 3-nitropropionic acid (3-NPA)-induced Huntington's disease (HD).

METHODS

To investigate possible toxicities in animals, oral acute toxicity studies of hibiscetin were undertaken, and results revealed the safety of hibiscetin in animals with a maximum tolerated dose. Wistar rats were divided into four groups (n = 6); (group-1) treated with normal saline, (group-2) hibiscetin (10 mg/kg) only, (group-3) 3-NPA only, and (group-4) 3-NPA +10 mg/kg hibiscetin. The efficacy of hibiscetin 10 mg/kg was studied with the administration of 3-NPA doses for the induction of experimentally induced HD symptoms in rats. The mean body weight (MBW) was recorded at end of the study on day 22 to evaluate any change in mean body weight. Several biochemical parameters were assessed to support oxidative stress (GSH, SOD, CAT, LPO, GR, and GPx), alteration in neurotransmitters (DOPAC, HVA, 5-HIAA, norepinephrine, serotonin, GABA, and dopamine), alterations in BDNF and cleaved caspase (caspase 3) activity. Additionally, inflammatory markers, i.e., tumor necrosis factor alpha (TNF-α), interleukins beta (IL-1β), and myeloperoxidase (MPO) were evaluated.

RESULTS

The hibiscetin-treated group exhibits a substantial restoration of MBW than the 3-NPA control group. Furthermore, 3-NPA caused a substantial alteration in biochemical, neurotransmitter monoamines, and neuroinflammatory parameters which were restored successfully by hibiscetin.

CONCLUSION

The current study linked the possible role of hibiscetin by offering neuroprotection in experimental animal models.

TREM2 and Microglia Contribute to the Synaptic Plasticity: from Physiology to Pathology

Synapses are bridges for information transmission in the central nervous system (CNS), and synaptic plasticity is fundamental for the normal function of synapses, contributing substantially to learning and memory. Numerous studies have proven that microglia can participate in the occurrence and progression of neurodegenerative diseases (NDDs), such as Alzheimer's disease (AD), by regulating synaptic plasticity. In this review, we summarize the main characteristics of synapses and synaptic plasticity under physiological and pathological conditions. We elaborate the origin and development of microglia and the two well-known microglial signaling pathways that regulate synaptic plasticity. We also highlight the unique role of triggering receptor expressed on myeloid cells 2 (TREM2) in microglia-mediated regulation of synaptic plasticity and its relationship with AD. Finally, we propose four possible ways in which TREM2 is involved in regulating synaptic plasticity. This review will help researchers understand how NDDs develop from the perspective of synaptic plasticity.

Huntingtin turnover: modulation of huntingtin degradation by cAMP-dependent protein kinase A (PKA) phosphorylation of C-HEAT domain Ser2550

Huntington's disease (HD) is a neurodegenerative disorder caused by an inherited unstable HTT CAG repeat that expands further, thereby eliciting a disease process that may be initiated by polyglutamine-expanded huntingtin or a short polyglutamine-product. Phosphorylation of selected candidate residues is reported to mediate polyglutamine-fragment degradation and toxicity. Here to support the discovery of phosphosites involved in the life-cycle of (full-length) huntingtin, we employed mass spectrometry-based phosphoproteomics to systematically identify sites in purified huntingtin and in the endogenous protein by proteomic and phosphoproteomic analyses of members of an HD neuronal progenitor cell panel. Our results bring total huntingtin phosphosites to 95, with more located in the N-HEAT domain relative to numbers in the Bridge and C-HEAT domains. Moreover, phosphorylation of C-HEAT Ser2550 by cAMP-dependent protein kinase (PKA), the top hit in kinase activity screens, was found to hasten huntingtin degradation, such that levels of the catalytic subunit (PRKACA) were inversely related to huntingtin levels. Taken together, these findings highlight categories of phosphosites that merit further study and provide a phosphosite kinase pair (pSer2550-PKA) with which to investigate the biological processes that regulate huntingtin degradation and thereby influence the steady state levels of huntingtin in HD cells.

Pharmacological Co-Activation of TrkB and TrkC Receptor Signaling Ameliorates Striatal Neuropathology and Motor Deficits in Mouse Models of Huntington's Disease

BACKGROUND

Loss of neurotrophic support in the striatum, particularly reduced brain-derived neurotrophic factor (BDNF) levels, contributes importantly to Huntington's disease (HD) pathogenesis. Another neurotrophin (NT), NT-3, is reduced in the cortex of HD patients; however, its role in HD is unknown. BDNF and NT-3 bind with high affinity to the tropomyosin receptor-kinases (Trk) B and TrkC, respectively. Targeting TrkB/TrkC may be an effective HD therapeutic strategy, as multiple links exist between their signaling pathways and HD degenerative mechanisms. We developed a small molecule ligand, LM22B-10, that activates TrkB and TrkC to promote cell survival.

OBJECTIVE

This study aimed to determine if upregulating TrkB/TrkC signaling with LM22B-10 would alleviate the HD phenotype in R6/2 and Q140 mice.

METHODS

LM22B-10 was delivered by concomitant intranasal-intraperitoneal routes to R6/2 and Q140 mice and then motor performance and striatal pathology were evaluated.

RESULTS

NT-3 levels, TrkB/TrkC phosphorylation, and AKT signaling were reduced in the R6/2 striatum; LM22B-10 counteracted these deficits. LM22B-10 also reduced intranuclear huntingtin aggregates, dendritic spine loss, microglial activation, and degeneration of dopamine- and cyclic AMP-regulated phosphoprotein with a molecular weight of 32 kDa (DARPP-32) and parvalbumin-containing neurons in the R6/2 and/or Q140 striatum. Moreover, both HD mouse models showed improved motor performance after LM22B-10 treatment.

CONCLUSIONS

These results reveal an NT-3/TrkC signaling deficiency in the striatum of R6/2 mice, support the idea that targeting TrkB/TrkC alleviates HD-related neurodegeneration and motor dysfunction, and suggest a novel, disease-modifying, multi-target strategy for treating HD.

Age-Dependent Increase in Tau Phosphorylation at Serine 396 in Huntington's Disease Prefrontal Cortex

BACKGROUND

To date, it is still controversial whether tau phosphorylation plays a role in Huntington's disease (HD), as previous studies demonstrated either no alterations or increases in phosphorylated tau (pTau) in HD postmortem brain and mouse models.

OBJECTIVE

The goal of this study was to determine whether total tau and pTau levels are altered in HD.

METHODS

Immunohistochemistry, cellular fractionations, and western blots were used to measure total tau and pTau levels in a large cohort of HD and control postmortem prefrontal cortex (PFC). Furthermore, western blots were performed to assess tau, and pTau levels in HD and control isogenic embryonic stem cell (ESC)-derived cortical neurons and neuronal stem cells (NSCs). Similarly, western blots were used to assess tau and pTau levels in HttQ111 and transgenic R6/2 mice. Lastly, total tau levels were assessed in HD and healthy control plasma using Quanterix Simoa assay.

RESULTS

Our results revealed that, while there was no difference in total tau or pTau levels in HD PFC compared to controls, the levels of tau phosphorylated at S396 were increased in PFC samples from HD patients 60 years or older at time of death. Additionally, tau and pTau levels were not changed in HD ESC-derived cortical neurons and NSCs. Similarly, total tau or pTau levels were not altered in HttQ111 and transgenic R6/2 mice compared to wild-type littermates. Lastly, tau levels were not changed in plasma from a small cohort of HD patients compared to controls.

CONCLUSIONS

Together these findings demonstrate that pTau-S396 levels increase significantly with age in HD PFC.

Crosstalk between cilia and autophagy: implication for human diseases

Macroautophagy/autophagy is a self-degradative process necessary for cells to maintain their energy balance during development and in response to nutrient deprivation. Autophagic processes are tightly regulated and have been found to be dysfunctional in several pathologies. Increasing experimental evidence points to the existence of an interplay between autophagy and cilia. Cilia are microtubule-based organelles protruding from the cell surface of mammalian cells that perform a variety of motile and sensory functions and, when dysfunctional, result in disorders known as ciliopathies. Indeed, selective autophagic degradation of ciliary proteins has been shown to control ciliogenesis and, conversely, cilia have been reported to control autophagy. Moreover, a growing number of players such as lysosomal and mitochondrial proteins are emerging as actors of the cilia-autophagy interplay. However, some of the published data on the cilia-autophagy axis are contradictory and indicate that we are just starting to understand the underlying molecular mechanisms. In this review, the current knowledge about this axis and challenges are discussed, as well as the implication for ciliopathies and autophagy-associated disorders.

Transcriptome Alterations Caused by Social Defeat Stress of Various Durations in Mice and Its Relevance to Depression and Posttraumatic Stress Disorder in Humans: A Meta-Analysis

The research on molecular causes of stress-associated psychopathologies is becoming highly important because the number of people with depression, generalized anxiety disorder and posttraumatic stress disorders (PTSDs) is steadily increasing every year. Investigation of molecular mechanisms in animal models opens up broad prospects for researchers, but relevant molecular signatures can differ significantly between patients and animal models. In our work, we for the first time carried out a meta-analysis of transcriptome changes in the prefrontal cortex of C57BL/6 mice after 10 and 30 days of social defeat stress (SDS). We then examined possible correlations of these alterations with transcriptome changes found in post-mortem samples from patients with depression or PTSD. Although transcriptional signatures of human psychiatric disorders and SDS did not overlap substantially, our results allowed us to identify the most reproducible changes seen after SDS of various durations. In addition, we were able to identify the genes involved in susceptibility to SDS after 10 days of stress. Taken together, these data help us to elucidate the molecular changes induced by SDS depending on its duration as well as their relevance to the alterations found in depression or PTSD in humans.

Microarray profiling of hypothalamic gene expression changes in Huntington’s disease mouse models

Structural changes and neuropathology in the hypothalamus have been suggested to contribute to the non-motor manifestations of Huntington’s disease (HD), a neurodegenerative disorder caused by an expanded cytosine-adenine-guanine (CAG) repeat in the huntingtin (HTT) gene. In this study, we investigated whether hypothalamic HTT expression causes transcriptional changes. Hypothalamic RNA was isolated from two different HD mouse models and their littermate controls; BACHD mice with ubiquitous expression of full-length mutant HTT (mHTT) and wild-type mice with targeted hypothalamic overexpression of either wild-type HTT (wtHTT) or mHTT fragments. The mHTT and wtHTT groups showed the highest number of differentially expressed genes compared to the BACHD mouse model. Gene Set Enrichment Analysis (GSEA) with leading-edge analysis showed that suppressed sterol- and cholesterol metabolism were shared between hypothalamic wtHTT and mHTT overexpression. Most distinctive for mHTT overexpression was the suppression of neuroendocrine networks, in which qRT-PCR validation confirmed significant downregulation of neuropeptides with roles in feeding behavior; hypocretin neuropeptide precursor (Hcrt), tachykinin receptor 3 (Tacr3), cocaine and amphetamine-regulated transcript (Cart) and catecholamine-related biological processes; dopa decarboxylase (Ddc), histidine decarboxylase (Hdc), tyrosine hydroxylase (Th), and vasoactive intestinal peptide (Vip). In BACHD mice, few hypothalamic genes were differentially expressed compared to age-matched WT controls. However, GSEA indicated an enrichment of inflammatory- and gonadotropin-related processes at 10 months. In conclusion, we show that both wtHTT and mHTT overexpression change hypothalamic transcriptome profile, specifically mHTT, altering neuroendocrine circuits. In contrast, the ubiquitous expression of full-length mHTT in the BACHD hypothalamus moderately affects the transcriptomic profile.

Balancing neuronal circuits

Correcting synaptic defects in development delays Huntington's disease symptoms in older mice.

Huntingtin coordinates dendritic spine morphology and function through cofilin-mediated control of the actin cytoskeleton

Compelling evidence indicates that in Huntington's disease (HD), mutation of huntingtin (HTT) alters several aspects of early brain development such as synaptogenesis. It is not clear to what extent the partial loss of wild-type HTT function contributes to these abnormalities. Here we investigate the function of HTT in the formation of spines. Although larger spines normally correlate with more synaptic activity, cell-autonomous depletion of HTT leads to enlarged spines but reduced excitatory synaptic function. We find that HTT is required for the proper turnover of endogenous actin and to recruit AMPA receptors at active synapses; loss of HTT leads to LIM kinase (LIMK) hyperactivation, which maintains cofilin in its inactive state. HTT therefore influences actin dynamics through the LIMK-cofilin pathway. Loss of HTT uncouples spine structure from synaptic function, which may contribute to the ultimate development of HD symptoms.

Synaptic and functional alterations in the development of mutant huntingtin expressing hiPSC‐derived neurons

Huntington’s disease (HD) is a monogenic disease that results in a combination of motor, psychiatric, and cognitive symptoms. It is caused by a CAG trinucleotide repeat expansion in the exon 1 of the huntingtin (HTT) gene, which results in the production of a mutant HTT protein (mHTT) with an extended polyglutamine tract (PolyQ). Severe motor symptoms are a hallmark of HD and typically appear during middle age; however, mild cognitive and personality changes often occur already during early adolescence. Wild-type HTT is a regulator of synaptic functions and plays a role in axon guidance, neurotransmitter release, and synaptic vesicle trafficking. These functions are important for proper synapse assembly during neuronal network formation. In the present study, we assessed the effect of mHTT exon1 isoform on the synaptic and functional maturation of human induced pluripotent stem cell (hiPSC)-derived neurons. We used a relatively fast-maturing hiPSC line carrying a doxycycline-inducible pro-neuronal transcription factor, (iNGN2), and generated a double transgenic line by introducing only the exon 1 of HTT, which carries the mutant CAG (mHTTEx1). The characterization of our cell lines revealed that the presence of mHTTEx1 in hiPSC-derived neurons alters the synaptic protein appearance, decreases synaptic contacts, and causes a delay in the development of a mature neuronal activity pattern, recapitulating some of the developmental alterations observed in HD models, nonetheless in a shorted time window. Our data support the notion that HD has a neurodevelopmental component and is not solely a degenerative disease.

Altered m6A RNA methylation contributes to hippocampal memory deficits in Huntington's disease mice

N6-methyladenosine (m6A) regulates many aspects of RNA metabolism and is involved in learning and memory processes. Yet, the impact of a dysregulation of post-transcriptional m6A editing on synaptic impairments in neurodegenerative disorders remains unknown. Here we investigated the m6A methylation pattern in the hippocampus of Huntington’s disease (HD) mice and the potential role of the m6A RNA modification in HD cognitive symptomatology. m6A modifications were evaluated in HD mice subjected to a hippocampal cognitive training task through m6A immunoprecipitation sequencing (MeRIP-seq) and the relative levels of m6A-modifying proteins (FTO and METTL14) by subcellular fractionation and Western blot analysis. Stereotaxic CA1 hippocampal delivery of AAV-shFTO was performed to investigate the effect of RNA m6A dysregulation in HD memory deficits. Our results reveal a m6A hypermethylation in relevant HD and synaptic related genes in the hippocampal transcriptome of Hdh^+/Q111 mice. Conversely, m6A is aberrantly regulated in an experience-dependent manner in the HD hippocampus leading to demethylation of important components of synapse organization. Notably, the levels of RNA demethylase (FTO) and methyltransferase (METTL14) were modulated after training in the hippocampus of WT mice but not in Hdh^+/Q111 mice. Finally, inhibition of FTO expression in the hippocampal CA1 region restored memory disturbances in symptomatic Hdh^+/Q111 mice. Altogether, our results suggest that a differential RNA methylation landscape contributes to HD cognitive symptoms and uncover a role of m6A as a novel hallmark of HD.

Molecular Pathophysiological Mechanisms in Huntington's Disease

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

SUMO-modifying Huntington's disease

Small ubiquitin-like modifiers, SUMOs, are proteins that are conjugated to target substrates and regulate their functions in a post-translational modification called SUMOylation. In addition to its physiological roles, SUMOylation has been implicated in several neurodegenerative diseases, such as Alzheimer's, Parkinson's, and Huntington's diseases (HD). HD is a neurodegenerative monogenetic autosomal dominant disorder caused by a mutation in the CAG repeat of the huntingtin (htt) gene, which expresses a mutant Htt protein more susceptible to aggregation and toxicity. Besides Htt, other SUMO ligases, enzymes, mitochondrial and autophagic components are also important for the progression of the disease. Here we review the main aspects of Htt SUMOylation and its role in cellular processes involved in the pathogenesis of HD.

Polyglutamine Expansion in Huntingtin and Mechanism of DNA Damage Repair Defects in Huntington’s Disease

Emerging evidence suggests that DNA repair deficiency and genome instability may be the impending signs of many neurological diseases. Genome-wide association (GWAS) studies have established a strong correlation between genes that play a role in DNA damage repair and many neurodegenerative diseases, including Huntington’s disease (HD), and several other trinucleotides repeat expansion-related hereditary ataxias. Recently, many reports have documented a significant role played by the DNA repair processes in aging and in modifying many neurodegenerative diseases, early during their progression. Studies from our lab and others have now begun to understand the mechanisms that cause defective DNA repair in HD and surprisingly, many proteins that have a strong link to known neurodegenerative diseases seem to be important players in these cellular pathways. Mutations in huntingtin (HTT) gene that lead to polyglutamine repeat expansion at the N-terminal of HTT protein has been shown to disrupt transcription-coupled DNA repair process, a specialized DNA repair process associated with transcription. Due to the recent progress made in understanding the mechanisms of DNA repair in relation to HD, in this review, we will mainly focus on the mechanisms by which the wild-type huntingtin (HTT) protein helps in DNA repair during transcription, and the how polyglutamine expansions in HTT impedes this process in HD. Further studies that identify new players in DNA repair will help in our understanding of this process in neurons. Furthermore, it should help us understand how various DNA repair mechanism(s) coordinate to maintain the normal physiology of neurons, and provide insights for the development of novel drugs at prodromal stages of these neurodegenerative diseases.

Dynamics of huntingtin protein interactions in the striatum identifies candidate modifiers of Huntington disease

Huntington disease (HD) is a monogenic neurodegenerative disorder with one causative gene, huntingtin (HTT). Yet, HD pathobiology is multifactorial, suggesting that cellular factors influence disease progression. Here, we define HTT protein-protein interactions (PPIs) perturbed by the mutant protein with expanded polyglutamine in the mouse striatum, a brain region with selective HD vulnerability. Using metabolically labeled tissues and immunoaffinity purification-mass spectrometry, we establish that polyglutamine-dependent modulation of HTT PPI abundances and relative stability starts at an early stage of pathogenesis in a Q140 HD mouse model. We identify direct and indirect PPIs that are also genetic disease modifiers using in-cell two-hybrid and behavioral assays in HD human cell and Drosophila models, respectively. Validated, disease-relevant mHTT-dependent interactions encompass mediators of synaptic neurotransmission (SNAREs and glutamate receptors) and lysosomal acidification (V-ATPase). Our study provides a resource for understanding mHTT-dependent dysfunction in cortico-striatal cellular networks, partly through impaired synaptic communication and endosomal-lysosomal system. A record of this paper's Transparent Peer Review process is included in the supplemental information.

The evolutionary history of the polyQ tract in huntingtin sheds light on its functional pro-neural activities

Huntington's disease is caused by a pathologically long (>35) CAG repeat located in the first exon of the Huntingtin gene (HTT). While pathologically expanded CAG repeats are the focus of extensive investigations, non-pathogenic CAG tracts in protein-coding genes are less well characterized. Here, we investigated the function and evolution of the physiological CAG tract in the HTT gene. We show that the poly-glutamine (polyQ) tract encoded by CAGs in the huntingtin protein (HTT) is under purifying selection and subjected to stronger selective pressures than CAG-encoded polyQ tracts in other proteins. For natural selection to operate, the polyQ must perform a function. By combining genome-edited mouse embryonic stem cells and cell assays, we show that small variations in HTT polyQ lengths significantly correlate with cells' neurogenic potential and with changes in the gene transcription network governing neuronal function. We conclude that during evolution natural selection promotes the conservation and purity of the CAG-encoded polyQ tract and that small increases in its physiological length influence neural functions of HTT. We propose that these changes in HTT polyQ length contribute to evolutionary fitness including potentially to the development of a more complex nervous system.

Non-Cell Autonomous and Epigenetic Mechanisms of Huntington's Disease

Huntington's disease (HD) is a rare neurodegenerative disorder caused by an expansion of CAG trinucleotide repeat located in the exon 1 of Huntingtin (HTT) gene in human chromosome 4. The HTT protein is ubiquitously expressed in the brain. Specifically, mutant HTT (mHTT) protein-mediated toxicity leads to a dramatic degeneration of the striatum among many regions of the brain. HD symptoms exhibit a major involuntary movement followed by cognitive and psychiatric dysfunctions. In this review, we address the conventional role of wild type HTT (wtHTT) and how mHTT protein disrupts the function of medium spiny neurons (MSNs). We also discuss how mHTT modulates epigenetic modifications and transcriptional pathways in MSNs. In addition, we define how non-cell autonomous pathways lead to damage and death of MSNs under HD pathological conditions. Lastly, we overview therapeutic approaches for HD. Together, understanding of precise neuropathological mechanisms of HD may improve therapeutic approaches to treat the onset and progression of HD.

CX3CL1/CX3CR1 signaling targets for the treatment of neurodegenerative diseases

Neuroinflammation was initially thought of as a consequence of neurodegenerative disease pathology, but more recently it is becoming clear that it plays a significant role in the development and progression of disease. Thus, neuroinflammation is seen as a realistic and valuable therapeutic target for neurodegeneration. Neuroinflammation can be modulated by neuron-glial signaling through various soluble factors, and one such critical modulator is Fractalkine or C-X3-C Motif Chemokine Ligand 1 (CX3CL1). CX3CL1 is produced in neurons and is a unique chemokine that is initially translated as a transmembrane protein but can be proteolytically processed to generate a soluble chemokine. CX3CL1 has been shown to signal through its sole receptor CX3CR1, which is located on microglial cells within the central nervous system (CNS). Although both the membrane bound and soluble forms of CX3CL1 appear to interact with CX3CR1, they do seem to have different signaling capabilities. It is believed that the predominant function of CX3CL1 within the CNS is to reduce the proinflammatory response and many studies have shown neuroprotective effects. However, in some cases CX3CL1 appears to be promoting neurodegeneration. This review focusses on presenting a comprehensive overview of the complex nature of CX3CL1/CX3CR1 signaling in neurodegeneration and how it may present as a therapeutic in some neurodegenerative diseases but not others. The role of CX3CL1/CXCR1 is reviewed in the context of Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), ischemia, retinopathies, spinal cord and neuropathic pain, traumatic brain injury, amyotrophic lateral sclerosis, multiple sclerosis, and epilepsy.