Striatal Chloride Dysregulation and Impaired GABAergic Signaling Due to Cation-Chloride Cotransporter Dysfunction in Huntington’s Disease

Intracellular chloride (Cl–) levels in mature neurons must be tightly regulated for the maintenance of fast synaptic inhibition. In the mature central nervous system (CNS), synaptic inhibition is primarily mediated by gamma-amino butyric acid (GABA), which binds to Cl– permeable GABAA receptors (GABAARs). The intracellular Cl– concentration is primarily maintained by the antagonistic actions of two cation-chloride cotransporters (CCCs): Cl–-importing Na+-K+-Cl– co-transporter-1 (NKCC1) and Cl– -exporting K+-Cl– co-transporter-2 (KCC2). In mature neurons in the healthy brain, KCC2 expression is higher than NKCC1, leading to lower levels of intracellular Cl–, and Cl– influx upon GABAAR activation. However, in neurons of the immature brain or in neurological disorders such as epilepsy and traumatic brain injury, impaired KCC2 function and/or enhanced NKCC1 expression lead to intracellular Cl– accumulation and GABA-mediated excitation. In Huntington’s disease (HD), KCC2- and NKCC1-mediated Cl–-regulation are also altered, which leads to GABA-mediated excitation and contributes to the development of cognitive and motor impairments. This review summarizes the role of Cl– (dys)regulation in the healthy and HD brain, with a focus on the basal ganglia (BG) circuitry and CCCs as potential therapeutic targets in the treatment of HD.

Investigating Antisense Transcription at the HTT Locus

Antisense transcription is an important mechanism of gene expression regulation. Antisense RNAs play a role in mRNA processing, translation and epigenetic modifications of DNA and histones in the locus of their origin, leading to gene silencing. HTT is a widely expressed gene, the mutation of which causes Huntington’s disease. The product of the gene plays an important role in many cell processes, such as intracellular trafficking, cell division, autophagy, and others. An antisense transcription has been found at the HTT 5’-region. The HTT-AS gene has been reported to affect HTT expression in a Dicer-dependent manner. In this study, we analyzed extensive data from RNA-seq experiments for antisense transcription at the HTT locus. Antisense transcripts corresponding to the HTT-AS gene were not found. However, we revealed a number of antisense transcripts in different parts of the locus that may take part in the regulation and functioning of the HTT gene. Keywords: antisense transcription, HTT-AS, HTTregulation, Huntington’s disease

Oxidative Stress Research on Huntington's Disease Neurons Using Genetically Encoded Biosensors

Many neurodegenerative diseases, including Huntington’s disease (HD), are associated with oxidative stress in the neurons of the brain. Genetically encoded biosensorsare useful for studying these processesin vitro. Human cell cultures expressing the biosensors can serve as a cell model for developing and testing effective agents that reduce oxidative stress. In this work, transgenes encoding biosensors of glutathione oxidative potential(Grx1-roGFP2) with cytoplasmic and mitochondrial localization were introduced into human induced pluripotent stem cells of a healthy donor and an HD patient using CRISPR/Cas9-mediated genome editing. The cells were subsequently differentiated into medium spiny neurons of the striatum. The expression of the biosensors was detected in the iPSCs, neuronal precursors and mature neurons.The obtained cells could be used to study the redox potential of glutathione in HD neurons and to screen for new drug compounds aimed at reducing oxidative stress. Keywords: genetically encoded biosensors, Huntington’s disease, induced pluripotent stem cells, medium spiny neurons, oxidative stress, glutathione, Grx1-roGFP2

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.

Skeletal muscle regeneration is altered in the R6/2 mouse model of Huntington's disease

Huntington's disease (HD) is caused by CAG repeat expansion in the huntingtin (HTT) gene. Skeletal muscle wasting alongside central pathology is a well-recognized phenomenon seen in patients with HD and HD mouse models. HD muscle atrophy progresses with disease and affects prognosis and quality of life. Satellite cells, progenitors of mature skeletal muscle fibers, are essential for proliferation, differentiation, and repair of muscle tissue in response to muscle injury or exercise. In this study, we aim to investigate the effect of mutant HTT on the differentiation and regeneration capacity of HD muscle by employing in vitro mononuclear skeletal muscle cell isolation and in vivo acute muscle damage model in R6/2 mice. We found that, similar to R6/2 adult mice, neonatal R6/2 mice also exhibit a significant reduction in myofiber width and morphological changes in gastrocnemius and soleus muscles compared to WT mice. Cardiotoxin (CTX)-induced acute muscle damage in R6/2 and WT mice showed that the Pax7+ satellite cell pool was dampened in R6/2 mice at 4 weeks post-injection, and R6/2 mice exhibited an altered inflammatory profile in response to acute damage. Our results suggest that, in addition to the mutant HTT degenerative effects in mature muscle fibers, expression of mutant HTT in satellite cells might alter developmental and regenerative processes to contribute to the progressive muscle mass loss in HD. Taken together, the results presented here encourage further studies evaluating the underlying mechanisms of satellite cell dysfunction in HD mouse models.

Impact of ER Stress and ER-Mitochondrial Crosstalk in Huntington’s Disease

Accumulation of misfolded proteins is a common phenomenon of several neurodegenerative diseases. The misfolding of proteins due to abnormal polyglutamine (PolyQ) expansions are linked to the development of PolyQ diseases including Huntington’s disease (HD). Though the genetic basis of PolyQ repeats in HD remains prominent, the primary molecular basis mediated by PolyQ toxicity remains elusive. Accumulation of misfolded proteins in the ER or disruption of ER homeostasis causes ER stress and activates an evolutionarily conserved pathway called Unfolded protein response (UPR). Protein homeostasis disruption at organelle level involving UPR or ER stress response pathways are found to be linked to HD. Due to dynamic intricate connections between ER and mitochondria, proteins at ER-mitochondria contact sites (mitochondria associated ER membranes or MAMs) play a significant role in HD development. The current review aims at highlighting the most updated information about different UPR pathways and their involvement in HD disease progression. Moreover, the role of MAMs in HD progression has also been discussed. In the end, the review has focused on the therapeutic interventions responsible for ameliorating diseased states via modulating either ER stress response proteins or modulating the expression of ER-mitochondrial contact proteins.

More than Just a Brain Disorder: A Five-Point Manifesto for Psychological Care for People with Huntington’s Disease

Huntington’s disease (HD) is a rare and complex condition where affected individuals, family members, caregivers, and clinicians face a number of both long-term and fluctuating challenges. The predominant biomedical framework adopted in HD to date has traditionally viewed it as a brain disorder first and foremost. As a consequence, one of the most challenging aspects of the condition—psychological difficulties and their care—is often not given the emphasis it deserves in everyday clinical practice. Here, we propose a manifesto outlining five points to address the quality, effectiveness, availability, and accessibility of psychological care in HD. These include (1) Listening to People with HD, (2) Reformulating Difficulties Psychologically, (3) Exploring New Interventions, (4) Increasing Psychological Provision, and (5) Learning from Other Conditions. As the search for a cure continues, we hope that this manifesto will create a new impetus towards refining the current approach to psychological difficulties in HD and ultimately improve the quality of life of the tens of thousands of families affected by HD worldwide.