A YAC mouse model for Huntington's disease with full-length mutant huntingtin, cytoplasmic toxicity, and selective striatal neurodegeneration

Huntingtin plays an essential role in the adult hippocampus

The consequences of non-pathogenic huntingtin (HTT) reduction in the mature brain are of substantial importance as clinical trials for numerous HTT-lowering therapies are underway; many of which are non-selective in that they reduce both mutant and wild type protein variants. In this study, we injected CaMKII-promoted AAV-Cre directly into the hippocampus of adult HTT floxed mice to explore the role of wild-type huntingtin (wtHTT) in adult hippocampal pyramidal neurons and the broader implications of its loss. Our findings reveal that wtHTT depletion results in profound macroscopic morphological abnormalities in hippocampal structure, accompanied by significant reactive gliosis. At the synaptic level, we identified a marked reduction in presynaptic terminals 1-2 months following wtHTT loss; this was contrasted by an increased density of postsynaptic mushroom spines and larger amplitudes of spontaneous excitatory postsynaptic currents, indicative of disrupted synaptic homeostasis. Furthermore, intrinsic neuronal excitability was significantly diminished in CA1 pyramidal neurons lacking wtHTT, and we observed a complete loss of NMDA receptor-dependent long-term potentiation. Unexpectedly, synapse density returned to control levels 6-8 months following wtHTT loss, despite the ongoing presence of macroscopic morphological abnormalities, altered anxiety-related behaviors and clear impairments in spatial learning and memory. Overall, these findings uncover a previously unrecognized role of wtHTT as a critical regulator of hippocampal function in the mature brain, and highlight the hippocampus as a potentially vulnerable region to the adverse effects of non-selective HTT reduction.

AMPA receptor diffusional trapping machinery as an early therapeutic target in neurodegenerative and neuropsychiatric disorders

Over the past two decades, there has been a growing recognition of the physiological importance and pathological implications surrounding the surface diffusion of AMPA receptors (AMPARs) and their diffusional trapping at synapses. AMPAR surface diffusion entails the thermally powered random Brownian lateral movement of these receptors within the plasma membrane, facilitating dynamic exchanges between synaptic and extrasynaptic compartments. This process also enables the activity-dependent diffusional trapping and accumulation of AMPARs at synapses through transient binding to synaptic anchoring slots. Recent research highlights the critical role of synaptic recruitment of AMPARs via diffusional trapping in fundamental neural processes such as the development of the early phases of long-term potentiation (LTP), contextual fear memory, memory consolidation, and sensory input-induced cortical remapping. Furthermore, studies underscore that regulation of AMPAR diffusional trapping is altered across various neurological disease models, including Huntington's disease (HD), Alzheimer's disease (AD), and stress-related disorders like depression. Notably, pharmacological interventions aimed at correcting deficits in AMPAR diffusional trapping have demonstrated efficacy in restoring synapse numbers, LTP, and memory functions in these diverse disease models, despite their distinct pathogenic mechanisms. This review provides current insights into the molecular mechanisms underlying the dysregulation of AMPAR diffusional trapping, emphasizing its role as a converging point for multiple pathological signaling pathways. We propose that targeting AMPAR diffusional trapping represents a promising early therapeutic strategy to mitigate synaptic plasticity and memory deficits in a spectrum of brain disorders, encompassing but not limited to HD, AD, and stress-related conditions. This approach underscores an integrated therapeutic target amidst the complexity of these neurodegenerative and neuropsychiatric diseases.

Lipidomics of Huntington's Disease: A Comprehensive Review of Current Status and Future Directions

BACKGROUND

Huntington's disease (HD) is a multifaceted neurological disorder characterized by the progressive deterioration of motor, cognitive, and psychiatric functions. Despite a limited understanding of its pathogenesis, research has implicated abnormal trinucleotide cytosine-adenine-guanine CAG repeat expansion in the huntingtin gene (HTT) as a critical factor. The development of innovative strategies is imperative for the early detection of predictive biomarkers, enabling timely intervention and mitigating irreversible cellular damage. Lipidomics, a comprehensive analytical approach, has emerged as an indispensable tool for systematically characterizing lipid profiles and elucidating their role in disease pathology.

METHOD

A MedLine search was performed to identify studies that use lipidomics for the characterization of HD. Search terms included "Huntington disease"; "lipidomics"; "biomarker discovery"; "NMR"; and "Mass spectrometry".

RESULTS

This review highlights the significance of lipidomics in HD diagnosis and treatment, exploring changes in brain lipids and their functions. Recent breakthroughs in analytical techniques, particularly mass spectrometry and NMR spectroscopy, have revolutionized brain lipidomics research, enabling researchers to gain deeper insights into the complex lipidome of the brain.

CONCLUSIONS

A comprehensive understanding of the broad spectrum of lipidomics alterations in HD is vital for precise diagnostic evaluation and effective disease management. The integration of lipidomics with artificial intelligence and interdisciplinary collaboration holds promise for addressing the clinical variability of HD.

Neuroglia in neurodegeneration: Alzheimer, Parkinson, and Huntington disease

The conspicuous rise of chronic neurodegenerative diseases, including Alzheimer (AD), Parkinson (PD), and Huntington (HD) diseases, is currently without disease-modifying therapies and accompanied by an excessive rate of unsuccessful clinical trials. This reflects a profound lack of understanding of the pathogenesis of these diseases, indicating that the current paradigms guiding disease modeling and drug development are in need of reconsideration. The role of neuroglia, namely astrocytes, microglial cells, and oligodendrocytes, in the pathogenesis of neurodegenerative diseases emerged during the last decades. This chapter provides the state-of-the-art update on the changes of astrocytes, microglial cells, and oligodendrocytes in AD, PD, and HD. A growing body of evidence suggests that homeostatic and defensive functions of glial cells are compromised at different disease stages, leading to increased susceptibility of neurons to noxious stimuli, eventually resulting in their malfunction and degeneration. Investments are needed in the generation of novel preclinical models suitable for studying glial pathology, in "humanizing" research, and in-depth investigation of glial cell alterations to slow down and, possibly, halt and prevent the rise of neurodegenerative disease. Targeting glial cells opens new therapeutic avenues to treat AD, PD, and HD.

Developmental and physiological impacts of pathogenic human huntingtin protein in the nervous system

Huntington's Disease (HD) is a neurodegenerative disorder, part of the nine identified inherited polyglutamine (polyQ) diseases. Most commonly, HD pathophysiology manifests in middle-aged adults with symptoms including progressive loss of motor control, cognitive decline, and psychiatric disturbances. Associated with the pathophysiology of HD is the formation of insoluble fragments of the huntingtin protein (htt) that tend to aggregate in the nucleus and cytoplasm of neurons. To track both the intracellular progression of the aggregation phenotype as well as the physiological deficits associated with mutant htt, two constructs of human HTT were expressed in the Drosophila melanogaster nervous system with varying polyQ lengths, non-pathogenic-htt (NP-htt) and pathogenic-htt (P-htt), with an N-terminal RFP tag for in vivo visualization. P-htt aggregates accumulate in the ventral nerve cord cell bodies as early as 24 h post hatching and significant aggregates form in the segmental nerve branches at 48 h post hatching. Organelle trafficking up- and downstream of aggregates formed in motor neurons showed severe deficits in trafficking dynamics. To explore putative downstream deficits of htt aggregation, ultrastructural changes of presynaptic motor neurons and muscles were assessed, but no significant effects were observed. However, the force and kinetics of muscle contractions were severely affected in P-htt animals, reminiscent of human chorea. Reduced muscle force production translated to altered locomotory behavior. A novel HD aggregation model was established to track htt aggregation throughout adulthood in the wing, showing similar aggregation patterns with larvae. Expressing P-htt in the adult nervous system resulted in significantly reduced lifespan, which could be partially rescued by feeding flies the mTOR inhibitor rapamycin. These findings advance our understanding of htt aggregate progression as well the downstream physiological impacts on the nervous system and peripheral tissues.

Oleuropein enhances proteasomal activity and reduces mutant huntingtin-induced cytotoxicity

Introduction

Huntington's disease (HD) is a hereditary neurodegenerative disorder that primarily affects the striatum, a brain region responsible for movement control. The disease is characterized by the mutant huntingtin (mHtt) proteins with an extended polyQ stretch, which are prone to aggregation. These mHtt aggregates accumulate in neurons and are the primary cause of the neuropathology associated with HD. To date, no effective cure for HD has been developed.

Methods

The immortalized STHdh Q111/Q111 striatal cell line, the mHtt-transfected wild-type STHdh Q7/Q7 striatal cell line, and N2a cells were used as Huntington's disease cell models. Flow cytometry was used to assess cellular reactive oxygen species and transfection efficiency. The CCK-8 assay was used to measure cell viability, while fluorescence microscopy was used to quantify aggregates. Immunoblotting analyses were used to evaluate the effects on protein expression.

Results

Polyphenols are natural antioxidants that offer neuroprotection in neurological disorders. In this study, we provide evidence that oleuropein, the primary polyphenol in olive leaves and olive oil, enhances cell viability in HD cell models, including. STHdh Q7/Q7 STHdh Q7/Q7 striatal cells, N2a cells ectopically expressing the truncated mHtt, and STHdh Q111/Q111 striatal cells expressing the full-length mHtt. Oleuropein effectively reduced both soluble and aggregated forms of mHtt protein in these HD model cells. Notably, the reduction of mHtt aggregates associated with oleuropein was linked to increased proteasome activity rather than changes in autophagic flux. Oleuropein seems to modulate proteasome activity through an unidentified pathway, as it did not affect the 20S proteasome catalytic β subunits, the proteasome regulator PA28γ, or multiple MAPK pathways.

Discussion

We demonstrated that oleuropein enhances the degradation of mHtt by increasing proteasomal protease activities and alleviates mHtt-induced cytotoxicity. Hence, we propose that oleuropein and potentially other polyphenols hold promise as a candidate for alleviating Huntington's disease.

Preclinical evaluation of stereopure antisense oligonucleotides for allele-selective lowering of mutant HTT

Huntington's disease (HD) is an autosomal dominant disease caused by the expansion of cytosine-adenine-guanine (CAG) repeats in one copy of the HTT gene (mutant HTT, mHTT). The unaffected HTT gene encodes wild-type HTT (wtHTT) protein, which supports processes important for the health and function of the central nervous system. Selective lowering of mHTT for the treatment of HD may provide a benefit over nonselective HTT-lowering approaches, as it aims to preserve the beneficial activities of wtHTT. Targeting a heterozygous single-nucleotide polymorphism (SNP) where the targeted variant is on the mHTT gene is one strategy for achieving allele-selective activity. Herein, we investigated whether stereopure phosphorothioate (PS)- and phosphoryl guanidine (PN)-containing oligonucleotides can direct allele-selective mHTT lowering by targeting rs362273 (SNP3). We demonstrate that our SNP3-targeting molecules are potent, durable, and selective for mHTT in vitro and in vivo in mouse models. Through comparisons with a surrogate for the nonselective investigational compound tominersen, we also demonstrate that allele-selective molecules display equivalent potency toward mHTT with improved durability while sparing wtHTT. Our preclinical findings support the advancement of WVE-003, an investigational allele-selective compound currently in clinical testing (NCT05032196) for the treatment of patients with HD.

Developmental and physiological impacts of pathogenic human huntingtin protein in the nervous system

Huntington's Disease (HD) is a neurodegenerative disorder, part of the nine identified inherited polyglutamine (polyQ) diseases. Most commonly, HD pathophysiology manifests in middle-aged adults with symptoms including progressive loss of motor control, cognitive decline, and psychiatric disturbances. Associated with the pathophysiology of HD is the formation of insoluble fragments of the huntingtin protein (htt) that tend to aggregate in the nucleus and cytoplasm of neurons. To track both the intracellular progression of the aggregation phenotype as well as the physiological deficits associated with mutant htt, two constructs of human HTT were expressed with varying polyQ lengths, non-pathogenic-htt (Q15, NP-htt) and pathogenic-htt (Q138, P-htt), with an N-terminal RFP tag for in vivo visualization. P-htt aggregates accumulate in the ventral nerve cord cell bodies as early as 24 hours post hatching and significant aggregates form in the segmental nerve branches at 48 hours post hatching. Organelle trafficking up-and downstream of aggregates formed in motor neurons showed severe deficits in trafficking dynamics. To explore putative downstream deficits of htt aggregation, ultrastructural changes of presynaptic motor neurons and muscles were assessed, but no significant effects were observed. However, the force and kinetics of muscle contractions were severely affected in P-htt animals, reminiscent of human chorea. Reduced muscle force production translated to altered locomotory behavior. A novel HD aggregation model was established to track htt aggregation throughout adulthood in the wing, showing similar aggregation patterns with larvae. Expressing P-htt in the adult nervous system resulted in significantly reduced lifespan, which could be partially rescued by feeding flies the mTOR inhibitor rapamycin. These findings advance our understanding of htt aggregate progression as well the downstream physiological impacts on the nervous system and peripheral tissues.

In vivo CRISPR base editing for treatment of Huntington's disease

Huntington's disease (HD) is an inherited and ultimately fatal neurodegenerative disorder caused by an expanded polyglutamine-encoding CAG repeat within exon 1 of the huntingtin (HTT) gene, which produces a mutant protein that destroys striatal and cortical neurons. Importantly, a critical event in the pathogenesis of HD is the proteolytic cleavage of the mutant HTT protein by caspase-6, which generates fragments of the N-terminal domain of the protein that form highly toxic aggregates. Given the role that proteolysis of the mutant HTT protein plays in HD, strategies for preventing this process hold potential for treating the disorder. By screening 141 CRISPR base editor variants targeting splice elements in the HTT gene, we identified platforms capable of producing HTT protein isoforms resistant to caspase-6-mediated proteolysis via editing of the splice acceptor sequence for exon 13. When delivered to the striatum of a rodent HD model, these base editors induced efficient exon skipping and decreased the formation of the N-terminal fragments, which in turn reduced HTT protein aggregation and attenuated striatal and cortical atrophy. Collectively, these results illustrate the potential for CRISPR base editing to decrease the toxicity of the mutant HTT protein for HD.

DNA methylation patterns of FKBP5 regulatory regions in brain and blood of humanized mice and humans

Humanized mouse models can be used to explore human gene regulatory elements (REs), which frequently lie in non-coding and less conserved genomic regions. Epigenetic modifications of gene REs, also in the context of gene x environment interactions, have not yet been explored in humanized mouse models. We applied high-accuracy measurement of DNA methylation (DNAm) via targeted bisulfite sequencing (HAM-TBS) to investigate DNAm in three tissues/brain regions (blood, prefrontal cortex and hippocampus) of mice carrying the human FK506-binding protein 5 (FKBP5) gene, an important candidate gene associated with stress-related psychiatric disorders. We explored DNAm in three functional intronic glucocorticoid-responsive elements (at introns 2, 5, and 7) of FKBP5 at baseline, in cases of differing genotype (rs1360780 single nucleotide polymorphism), and following application of the synthetic glucocorticoid dexamethasone. We compared DNAm patterns in the humanized mouse (N = 58) to those in human peripheral blood (N = 447 and N = 89) and human postmortem brain prefrontal cortex (N = 86). Overall, DNAm patterns in the humanized mouse model seem to recapitulate DNAm patterns observed in human tissue. At baseline, this was to a higher extent in brain tissue. The animal model also recapitulated effects of dexamethasone on DNAm, especially in peripheral blood and to a lesser extent effects of genotype on DNAm. The humanized mouse model could thus assist in reverse translation of human findings in psychiatry that involve genetic and epigenetic regulation in non-coding elements.

Mitochondrial Dysfunction: A Key Player in Brain Aging and Diseases

Mitochondria are thought to have become incorporated within the eukaryotic cell approximately 2 billion years ago and play a role in a variety of cellular processes, such as energy production, calcium buffering and homeostasis, steroid synthesis, cell growth, and apoptosis, as well as inflammation and ROS production. Considering that mitochondria are involved in a multitude of cellular processes, mitochondrial dysfunction has been shown to play a role within several age-related diseases, including cancers, diabetes (type 2), and neurodegenerative diseases, although the underlying mechanisms are not entirely understood. The significant increase in lifespan and increased incidence of age-related diseases over recent decades has confirmed the necessity to understand the mechanisms by which mitochondrial dysfunction impacts the process of aging and age-related diseases. In this review, we will offer a brief overview of mitochondria, along with structure and function of this important organelle. We will then discuss the cause and consequence of mitochondrial dysfunction in the aging process, with a particular focus on its role in inflammation, cognitive decline, and neurodegenerative diseases, such as Huntington's disease, Parkinson's disease, and Alzheimer's disease. We will offer insight into therapies and interventions currently used to preserve or restore mitochondrial functioning during aging and neurodegeneration.

Modeling Huntington's disease: An insight on in-vitro and in-vivo models

Huntington's disease is a neurodegenerative illness that causes neuronal death most extensively within the basal ganglia. There is a broad class of neurologic disorders associated with the expansion of polyglutamine (polyQ) repeats in numerous proteins. Several other molecular mechanisms have also been implicated in HD pathology, including brain-derived neurotrophic factor (BDNF), mitochondrial dysfunction, and altered synaptic plasticity in central spiny neurons. HD pathogenesis and the effectiveness of therapy approaches have been better understood through the use of animal models. The pathological manifestations of the disease were reproduced by early models of glutamate analog toxicity and mitochondrial respiration inhibition. Because the treatments available for HD are quite limited, it is important to have a definite preclinical model that mimics all the aspects of the disease. It can be used to study mechanisms and validate candidate therapies. Although there hasn't been much success in translating animal research into clinical practice, each model has something special to offer in the quest for a deeper comprehension of HD's neurobehavioral foundations. This review provides insight into various in-vitro-and in-vivo models of HD which may be useful in the screening of newer therapeutics for this incapacitating disorder.

Metabolomics: An Emerging "Omics" Platform for Systems Biology and Its Implications for Huntington Disease Research

Huntington's disease (HD) is a progressive, fatal neurodegenerative disease characterized by motor, cognitive, and psychiatric symptoms. The precise mechanisms of HD progression are poorly understood; however, it is known that there is an expansion of the trinucleotide cytosine-adenine-guanine (CAG) repeat in the Huntingtin gene. Important new strategies are of paramount importance to identify early biomarkers with predictive value for intervening in disease progression at a stage when cellular dysfunction has not progressed irreversibly. Metabolomics is the study of global metabolite profiles in a system (cell, tissue, or organism) under certain conditions and is becoming an essential tool for the systemic characterization of metabolites to provide a snapshot of the functional and pathophysiological states of an organism and support disease diagnosis and biomarker discovery. This review briefly highlights the historical progress of metabolomic methodologies, followed by a more detailed review of the use of metabolomics in HD research to enable a greater understanding of the pathogenesis, its early prediction, and finally the main technical platforms in the field of metabolomics.

Huntingtin Decreases Susceptibility to a Spontaneous Seizure Disorder in FVN/B Mice

Huntington disease (HD) is an adult-onset neurodegenerative disorder that is caused by a trinucleotide CAG repeat expansion in the HTT gene that codes for the protein huntingtin (HTT in humans or Htt in mice). HTT is a multi-functional, ubiquitously expressed protein that is essential for embryonic survival, normal neurodevelopment, and adult brain function. The ability of wild-type HTT to protect neurons against various forms of death raises the possibility that loss of normal HTT function may worsen disease progression in HD. Huntingtin-lowering therapeutics are being evaluated in clinical trials for HD, but concerns have been raised that decreasing wild-type HTT levels may have adverse effects. Here we show that Htt levels modulate the occurrence of an idiopathic seizure disorder that spontaneously occurs in approximately 28% of FVB/N mice, which we have called FVB/N Seizure Disorder with SUDEP (FSDS). These abnormal FVB/N mice demonstrate the cardinal features of mouse models of epilepsy including spontaneous seizures, astrocytosis, neuronal hypertrophy, upregulation of brain-derived neurotrophic factor (BDNF), and sudden seizure-related death. Interestingly, mice heterozygous for the targeted inactivation of Htt (Htt+/- mice) exhibit an increased frequency of this disorder (71% FSDS phenotype), while over-expression of either full length wild-type HTT in YAC18 mice or full length mutant HTT in YAC128 mice completely prevents it (0% FSDS phenotype). Examination of the mechanism underlying huntingtin's ability to modulate the frequency of this seizure disorder indicated that over-expression of full length HTT can promote neuronal survival following seizures. Overall, our results demonstrate a protective role for huntingtin in this form of epilepsy and provide a plausible explanation for the observation of seizures in the juvenile form of HD, Lopes-Maciel-Rodan syndrome, and Wolf-Hirschhorn syndrome. Adverse effects caused by decreasing huntingtin levels have ramifications for huntingtin-lowering therapies that are being developed to treat HD.

Limitations of Dual-Single Guide RNA CRISPR Strategies for the Treatment of Central Nervous System Genetic Disorders

Huntington's disease (HD) is a fatal neurodegenerative disorder caused by a toxic gain-of-function CAG expansion in the first exon of the huntingtin (HTT) gene. The monogenic nature of HD makes mutant HTT (mHTT) inactivation a promising therapeutic strategy. Single nucleotide polymorphisms frequently associated with CAG expansion have been explored to selectively inactivate mHTT allele using the CRISPR/Cas9 system. One of such allele-selective approaches consists of excising a region flanking the first exon of mHTT by inducing simultaneous double-strand breaks at upstream and downstream positions of the mHTT exon 1. The removal of the first exon of mHTT deletes the CAG expansion and important transcription regulatory sites, leading to mHTT inactivation. However, the frequency of deletion events is yet to be quantified either in vitro or in vivo. Here, we developed accurate quantitative digital polymerase chain reaction-based assays to assess HTT exon 1 deletion in vitro and in fully humanized HU97/18 mice. Our results demonstrate that dual-single guide RNA (sgRNA) strategies are efficient and that 67% of HTT editing events are leading to exon 1 deletion in HEK293T cells. In contrast, these sgRNA actively cleaved HTT in HU97/18 mice, but most editing events do not lead to exon 1 deletion (10% exon 1 deletion). We also showed that the in vivo editing pattern is not affected by CAG expansion but may potentially be due to the presence of multiple copies of wildtype (wt)/mHTT genes HU97/18 mice as well as the slow kinetics of AAV-mediated CRISPR/Cas9 delivery.

From Pathogenesis to Therapeutics: A Review of 150 Years of Huntington's Disease Research

Huntington's disease (HD) is a debilitating neurodegenerative genetic disorder caused by an expanded polyglutamine-coding (CAG) trinucleotide repeat in the huntingtin (HTT) gene. HD behaves as a highly penetrant dominant disorder likely acting through a toxic gain of function by the mutant huntingtin protein. Widespread cellular degeneration of the medium spiny neurons of the caudate nucleus and putamen are responsible for the onset of symptomology that encompasses motor, cognitive, and behavioural abnormalities. Over the past 150 years of HD research since George Huntington published his description, a plethora of pathogenic mechanisms have been proposed with key themes including excitotoxicity, dopaminergic imbalance, mitochondrial dysfunction, metabolic defects, disruption of proteostasis, transcriptional dysregulation, and neuroinflammation. Despite the identification and characterisation of the causative gene and mutation and significant advances in our understanding of the cellular pathology in recent years, a disease-modifying intervention has not yet been clinically approved. This review includes an overview of Huntington's disease, from its genetic aetiology to clinical presentation and its pathogenic manifestation. An updated view of molecular mechanisms and the latest therapeutic developments will also be discussed.

Early TNF-Dependent Regulation of Excitatory and Inhibitory Synapses on Striatal Direct Pathway Medium Spiny Neurons in the YAC128 Mouse Model of Huntington's Disease

Huntington's disease (HD) is a neurodegenerative disease caused by a polyglutamine expansion in the huntingtin gene. Neurodegeneration first occurs in the striatum, accompanied by an elevation in inflammatory cytokines. Using the presymptomatic male YAC128 HD model mouse, we examined the synaptic input onto the striatal medium spiny neurons to look for early changes that precede degeneration. We observed an increase in excitatory synaptic strength, as measured by AMPA/NMDA ratios, specifically on direct pathway D1 receptor expressing medium spiny neurons, with no changes on indirect pathway neurons. The changes in excitation were accompanied by a decrease in inhibitory synaptic strength, as measured by the amplitude of miniature inhibitory synaptic currents. The pro-inflammatory cytokine tumor necrosis factor alpha (TNF) was elevated in the striatum of YAC128 at the ages examined. Critically, the changes in excitatory and inhibitory inputs are both dependent on TNF signaling, as blocking TNF signaling genetically or pharmacological normalized synaptic strength. The observed changes in synaptic function are similar to the changes seen in D1 medium spiny neurons treated with high levels of TNF, suggesting that saturating levels of TNF exist in the striatum even at early stages of HD. The increase in glutamatergic synaptic strength and decrease in inhibitory synaptic strength would increase direct pathway neuronal excitability, which may potentiate excitotoxicity during the progress of HD.SIGNIFICANCE STATEMENT The striatum is the first structure to degenerate in Huntington's disease, but the early changes that presage the degeneration are not well defined. Here we identify early synaptic changes in the YAC128 mouse model of Huntington's disease specifically on a subpopulation of striatal neurons. These neurons have stronger excitatory synapses and weaker inhibitory inputs, and thus would increase the susceptibility to excitotoxicity. These changes are dependent on signaling by the pro-inflammatory cytokine TNFα. TNF is elevated even at early presymptomatic stages, and blocking TNF signaling even acutely will reverse the synaptic changes. This suggests early intervention could be important therapeutically.

Contribution of Glial Cells to Polyglutamine Diseases: Observations from Patients and Mouse Models

Neurodegenerative diseases are broadly characterized neuropathologically by the degeneration of vulnerable neuronal cell types in a specific brain region. The degeneration of specific cell types has informed on the various phenotypes/clinical presentations in someone suffering from these diseases. Prominent neurodegeneration of specific neurons is seen in polyglutamine expansion diseases including Huntington's disease (HD) and spinocerebellar ataxias (SCA). The clinical manifestations observed in these diseases could be as varied as the abnormalities in motor function observed in those who have Huntington's disease (HD) as demonstrated by a chorea with substantial degeneration of striatal medium spiny neurons (MSNs) or those with various forms of spinocerebellar ataxia (SCA) with an ataxic motor presentation primarily due to degeneration of cerebellar Purkinje cells. Due to the very significant nature of the degeneration of MSNs in HD and Purkinje cells in SCAs, much of the research has centered around understanding the cell autonomous mechanisms dysregulated in these neuronal cell types. However, an increasing number of studies have revealed that dysfunction in non-neuronal glial cell types contributes to the pathogenesis of these diseases. Here we explore these non-neuronal glial cell types with a focus on how each may contribute to the pathogenesis of HD and SCA and the tools used to evaluate glial cells in the context of these diseases. Understanding the regulation of supportive and harmful phenotypes of glia in disease could lead to development of novel glia-focused neurotherapeutics.

Huntingtin Overexpression Does Not Alter Overall Survival in Murine Cancer Models

A reduced incidence of various forms of cancer has been reported in Huntington's disease patients and may be due to pro-apoptotic effects of mutant huntingtin. We tested this hypothesis by assessing the effects of huntingtin protein overexpression on survival in two murine cancer models. We generated YAC HD mice containing human huntingtin transgenes with various CAG tract lengths (YAC18, YAC72, YAC128) on either an Msh2 or p53 null background which have increased cancer incidence. In both mouse models of cancer, the overexpression of either mutant or wild-type huntingtin had no significant effect on overall survival. These results do not support the hypothesis that mutant huntingtin expression is protective against cancer.

From kitchen to clinic: Pharmacotherapeutic potential of common spices in Indian cooking in age-related neurological disorders

Aging is described as an advanced time-related collection of changes that may negatively affect with the risk of several diseases or death. Aging is a main factor of several age-related neurological disorders, including neurodegenerative diseases (Alzheimer’s disease, Parkinson’s disease, and dementia), stroke, neuroinflammation, neurotoxicity, brain tumors, oxidative stress, and reactive oxygen species (ROS). Currently available medications for age-related neurological disorders may lead to several side effects, such as headache, diarrhea, nausea, gastrointestinal (GI) diseases, dyskinesia, and hallucinosis. These days, studies on plant efficacy in traditional medicine are being conducted because herbal medicine is affordable, safe, and culturally acceptable and easily accessible. The Indian traditional medicine system called Ayurveda uses several herbs and medicinal plants to treat various disorders including neurological disorders. This review aims to summarize the data on the neuroprotective potential of the following common Indian spices widely used in Ayurveda: cumin (Cuminum cyminum (L.), Apiaceae), black cumin (Nigella sativa (L.), Ranunculaceae), black pepper (Piper nigrum (L.), Piperaceae), curry leaf tree (Murraya koenigii (L.), Spreng Rutaceae), fenugreek (Trigonella foenum-graecum (L.), Fabaceae), fennel (Foeniculum vulgare Mill, Apiaceae), cardamom (Elettaria cardamomum (L.) Maton, Zingiberaceae), cloves (Syzygium aromaticum (L.) Merr. & L.M.Perry, Myrtaceae), and coriander (Coriandrum sativum (L.), Apiaceae) in age-related neurological disorders.

Insights into White Matter Defect in Huntington's Disease

Huntington's disease (HD) is an autosomal-dominant inherited progressive neurodegenerative disorder. It is caused by a CAG repeat expansion in the Huntingtin gene that is translated to an expanded polyglutamine (PolyQ) repeat in huntingtin protein. HD is characterized by mood swings, involuntary movement, and cognitive decline in the late disease stage. HD patients often die 15-20 years after disease onset. Currently, there is no cure for HD. Due to the striking neuronal loss in HD, most studies focused on the investigation of the predominantly neuronal degeneration in specific brain regions. However, the pathology of the white matter area in the brains of HD patients was also reported by clinical imaging studies, which showed white matter abnormalities even before the clinical onset of HD. Since oligodendrocytes form myelin sheaths around the axons in the brain, white matter lesions are likely attributed to alterations in myelin and oligodendrocyte-associated changes in HD. In this review, we summarized the evidence for white matter, myelin, and oligodendrocytes alterations that were previously observed in HD patients and animal models. We also discussed potential mechanisms for white matter changes and possible treatment to prevent glial dysfunction in HD.

A survey of protein interactions and posttranslational modifications that influence the polyglutamine diseases

The presence and aggregation of misfolded proteins has deleterious effects in the nervous system. Among the various diseases caused by misfolded proteins is the family of the polyglutamine (polyQ) disorders. This family comprises nine members, all stemming from the same mutation—the abnormal elongation of a polyQ repeat in nine different proteins—which causes protein misfolding and aggregation, cellular dysfunction and disease. While it is the same type of mutation that causes them, each disease is distinct: it is influenced by regions and domains that surround the polyQ repeat; by proteins with which they interact; and by posttranslational modifications they receive. Here, we overview the role of non-polyQ regions that control the pathogenicity of the expanded polyQ repeat. We begin by introducing each polyQ disease, the genes affected, and the symptoms experienced by patients. Subsequently, we provide a survey of protein-protein interactions and posttranslational modifications that regulate polyQ toxicity. We conclude by discussing shared processes and pathways that bring some of the polyQ diseases together and may serve as common therapeutic entry points for this family of incurable disorders.

mTOR Signalling Pathway: A Potential Therapeutic Target for Ocular Neurodegenerative Diseases

Recent advances in the research of the mammalian target of the rapamycin (mTOR) signalling pathway demonstrated that mTOR is a robust therapeutic target for ocular degenerative diseases, including age-related macular degeneration (AMD), diabetic retinopathy (DR), and glaucoma. Although the exact mechanisms of individual ocular degenerative diseases are unclear, they share several common pathological processes, increased and prolonged oxidative stress in particular, which leads to functional and morphological impairment in photoreceptors, retinal ganglion cells (RGCs), or retinal pigment epithelium (RPE). mTOR not only modulates oxidative stress but is also affected by reactive oxygen species (ROS) activation. It is essential to understand the complicated relationship between the mTOR pathway and oxidative stress before its application in the treatment of retinal degeneration. Indeed, the substantial role of mTOR-mediated autophagy in the pathogenies of ocular degenerative diseases should be noted. In reviewing the latest studies, this article summarised the application of rapamycin, an mTOR signalling pathway inhibitor, in different retinal disease models, providing insight into the mechanism of rapamycin in the treatment of retinal neurodegeneration under oxidative stress. Besides basic research, this review also summarised and updated the results of the latest clinical trials of rapamycin in ocular neurodegenerative diseases. In combining the current basic and clinical research results, we provided a more complete picture of mTOR as a potential therapeutic target for ocular neurodegenerative diseases.

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.

A CAG repeat-targeting artificial miRNA lowers the mutant huntingtin level in the YAC128 model of Huntington's disease

Among the many proposed therapeutic strategies for Huntington's disease (HD), allele-selective therapies are the most desirable but also the most challenging. RNA interference (RNAi) tools that target CAG repeats selectively reduce the mutant huntingtin level in cellular models of HD. The purpose of this study was to test the efficacy, selectivity, and safety of two vector-based RNAi triggers in an animal model of HD. CAG repeat-targeting short hairpin RNA (shRNA) and artificial miRNA (amiRNA) were delivered to the brains of YAC128 mice via intrastriatal injection of AAV5 vectors. Molecular tests demonstrated that both the shRNA and amiRNA reduced the mutant huntingtin level by 50% without influencing endogenous mouse huntingtin. In addition, a concentration-dependent reduction in HTT aggregates in the striatum was observed. In contrast to the shRNA, the amiRNA was well tolerated and did not show signs of toxicity during the course of the experiment up to 20 weeks post injection. Interestingly, amiRNA treatment reduced the spleen weight to values characteristic of healthy (WT) mice and improved motor performance on the static rod test. These preclinical data demonstrate that the CAG-targeting strategy and amiRNA could make an original and valuable contribution to currently used therapeutic approaches for HD.

A New Perspective on Huntington's Disease: How a Neurological Disorder Influences the Peripheral Tissues

Huntington's disease (HD) is a neurodegenerative disorder caused by a toxic, aggregation-prone expansion of CAG repeats in the HTT gene with an age-dependent progression that leads to behavioral, cognitive and motor symptoms. Principally affecting the frontal cortex and the striatum, mHTT disrupts many cellular functions. In fact, increasing evidence shows that peripheral tissues are affected by neurodegenerative diseases. It establishes an active crosstalk between peripheral tissues and the brain in different neurodegenerative diseases. This review focuses on the current knowledge of peripheral tissue effects in HD animal and cell experimental models and identifies biomarkers and mechanisms involved or affected in the progression of the disease as new therapeutic or early diagnostic options. The particular changes in serum/plasma, blood cells such as lymphocytes, immune blood cells, the pancreas, the heart, the retina, the liver, the kidney and pericytes as a part of the blood-brain barrier are described. It is important to note that several changes in different mouse models of HD present differences between them and between the different ages analyzed. The understanding of the impact of peripheral organ inflammation in HD may open new avenues for the development of novel therapeutic targets.

Neuroinflammation in Huntington's Disease: A Starring Role for Astrocyte and Microglia

Huntington's disease (HD) is a neurodegenerative genetic disorder caused by a CAG repeat expansion in the huntingtin gene. HD causes motor, cognitive, and behavioral dysfunction. Since no existing treatment affects the course of this disease, new treatments are needed. Inflammation is frequently observed in HD patients before symptom onset. Neuroinflammation, characterized by the presence of reactive microglia, astrocytes and inflammatory factors within the brain, is also detected early. However, in comparison to other neurodegenerative diseases, the role of neuroinflammation in HD is much less known. Work has been dedicated to altered microglial and astrocytic functions in the context of HD, but less attention has been given to glial participation in neuroinflammation. This review describes evidence of inflammation in HD patients and animal models. It also discusses recent knowledge on neuroinflammation in HD, highlighting astrocyte and microglia involvement in the disease and considering anti-inflammatory therapeutic approaches.

Temporal Phenotypic Changes in Huntington's Disease Models for Preclinical Studies

Background:

Mouse models bearing genetic disease mutations are instrumental in the development of therapies for genetic disorders. Huntington’s disease (HD) is a late-onset lethal dominant genetic disorder due to a CAG repeat within exon 1 of the Huntingtin (Htt) gene. Several mice were developed to model HD through the expression of a transgenic fragment (exon 1 of the human HTT), the knock-in mutation of the CAG repeat in the context of the mouse Htt gene, or the full-length HTT human gene. The different mouse models present distinct onset, symptoms, and progression of the disease.

Objective:

The objective of this study is to advise on the best behavioral tests to assess disease progression in three HD mouse models.

Methods:

We tested N171-82Q transgenic mice, zQ175 knock-in mice, and BACHD full-length mice in a comprehensive behavior test battery in early, mid-, and late disease stages.

Results:

We contrast and compare the models and the emerging phenotypes with the available literature. These results suggest the most effective behavioral tests and appropriate sample sizes to detect treatment efficacy in each model at the different ages. We provide options for early detection of motor deficits while minimizing testing time and training.

Conclusion:

This information will inform researchers in the HD field as to which mouse model, tests and sample sizes can accurately and sensitively detect treatment efficacy in preclinical HD research.

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

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

Ion channels and neuronal excitability in polyglutamine neurodegenerative diseases

Polyglutamine (polyQ) diseases are a family composed of nine neurodegenerative inherited disorders (NDDs) caused by pathological expansions of cytosine-adenine-guanine (CAG) trinucleotide repeats which encode a polyQ tract in the corresponding proteins. CAG polyQ repeat expansions produce neurodegeneration via multiple downstream mechanisms; among those the neuronal activity underlying the ion channels is affected directly by specific channelopathies or indirectly by secondary dysregulation. In both cases, the altered excitability underlies to gain- or loss-of-function pathological effects. Here we summarize the repertoire of ion channels in polyQ NDDs emphasizing the biophysical features of neuronal excitability and their pathogenic role. The aim of this review is to point out the value of a deeper understanding of those functional mechanisms and processes as crucial elements for the designing and targeting of novel therapeutic avenues.

Structural Plasticity of the Hippocampus in Neurodegenerative Diseases

Neuroplasticity is the capacity of neural networks in the brain to alter through development and rearrangement. It can be classified as structural and functional plasticity. The hippocampus is more susceptible to neuroplasticity as compared to other brain regions. Structural modifications in the hippocampus underpin several neurodegenerative diseases that exhibit cognitive and emotional dysregulation. This article reviews the findings of several preclinical and clinical studies about the role of structural plasticity in the hippocampus in neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, Huntington's disease, and multiple sclerosis. In this study, literature was surveyed using Google Scholar, PubMed, Web of Science, and Scopus, to review the mechanisms that underlie the alterations in the structural plasticity of the hippocampus in neurodegenerative diseases. This review summarizes the role of structural plasticity in the hippocampus for the etiopathogenesis of neurodegenerative diseases and identifies the current focus and gaps in knowledge about hippocampal dysfunctions. Ultimately, this information will be useful to propel future mechanistic and therapeutic research in 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.

Advances in Modeling Polyglutamine Diseases Using Genome Editing Tools

Polyglutamine (polyQ) diseases, including Huntington’s disease, are a group of late-onset progressive neurological disorders caused by CAG repeat expansions. Although recently, many studies have investigated the pathological features and development of polyQ diseases, many questions remain unanswered. The advancement of new gene-editing technologies, especially the CRISPR-Cas9 technique, has undeniable value for the generation of relevant polyQ models, which substantially support the research process. Here, we review how these tools have been used to correct disease-causing mutations or create isogenic cell lines with different numbers of CAG repeats. We characterize various cellular models such as HEK 293 cells, patient-derived fibroblasts, human embryonic stem cells (hESCs), induced pluripotent stem cells (iPSCs) and animal models generated with the use of genome-editing technology.

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.

Allele-Specific Knockdown of Mutant Huntingtin Protein via Editing at Coding Region Single Nucleotide Polymorphism Heterozygosities

Huntington's disease (HD) is a devastating, autosomal dominant neurodegenerative disease caused by a trinucleotide repeat expansion in the huntingtin (HTT) gene. Inactivation of the mutant allele by clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 based gene editing offers a possible therapeutic approach for this disease, but permanent disruption of normal HTT function might compromise adult neuronal function. Here, we use a novel HD mouse model to examine allele-specific editing of mutant HTT (mHTT), with a BAC97 transgene expressing mHTT and a YAC18 transgene expressing normal HTT. We achieve allele-specific inactivation of HTT by targeting a protein coding sequence containing a common, heterozygous single nucleotide polymorphism (SNP). The outcome is a marked and allele-selective reduction of mHTT protein in a mouse model of HD. Expression of a single CRISPR-Cas9 nuclease in neurons generated a high frequency of mutations in the targeted HD allele that included both small insertion/deletion (InDel) mutations and viral vector insertions. Thus, allele-specific targeting of InDel and insertion mutations to heterozygous coding region SNPs provides a feasible approach to inactivate autosomal dominant mutations that cause genetic disease.

Synaptic pathology in Huntington's disease: Beyond the corticostriatal pathway

Huntington's disease (HD) is a heritable, fatal neurodegenerative disorder caused by a mutation in the Huntingtin gene. It is characterized by chorea, as well as cognitive and psychiatric symptoms. Histopathologically, there is a massive loss of striatal projection neurons and less but significant loss in other areas throughout the cortico-basal ganglia-thalamocortical (CBGTC) loop. The mutant huntingtin protein has been implicated in numerous functions, including an important role in synaptic transmission. Most studies on anatomical and physiological alterations in HD have focused on striatum and cerebral cortex. However, based on recent CBGTC projectome evidence, the need to study other pathways has become increasingly clear. In this review, we examine the current status of our knowledge of morphological and electrophysiological alterations of those pathways in animal models of HD. Based on recent studies, there is accumulating evidence that synaptic disconnection, particularly along excitatory pathways, is pervasive and almost universal in HD, thus supporting a critical role of the huntingtin protein in synaptic transmission.

Therapeutic reversal of Huntington's disease by in vivo self-assembled siRNAs

Huntington’s disease is an autosomal-dominant neurodegenerative disease caused by CAG expansion in exon 1 of the huntingtin (HTT) gene. Since mutant huntingtin (mHTT) protein is the root cause of Huntington’s disease, oligonucleotide-based therapeutic approaches using small interfering RNAs (siRNAs) and antisense oligonucleotides designed to specifically silence mHTT may be novel therapeutic strategies for Huntington’s disease. Unfortunately, the lack of an effective in vivo delivery system remains a major obstacle to realizing the full potential of oligonucleotide therapeutics, especially regarding the delivery of oligonucleotides to the cortex and striatum, the most severely affected brain regions in Huntington’s disease.

In this study, we present a synthetic biology strategy that integrates the naturally existing exosome-circulating system with artificial genetic circuits for self-assembly and delivery of mHTT-silencing siRNA to the cortex and striatum. We designed a cytomegalovirus promoter-directed genetic circuit encoding both a neuron-targeting rabies virus glycoprotein tag and an mHTT siRNA.

After being taken up by mouse livers after intravenous injection, this circuit was able to reprogramme hepatocytes to transcribe and self-assemble mHTT siRNA into rabies virus glycoprotein-tagged exosomes. The mHTT siRNA was further delivered through the exosome-circulating system and guided by a rabies virus glycoprotein tag to the cortex and striatum. Consequently, in three mouse models of Huntington’s disease treated with this circuit, the levels of mHTT protein and toxic aggregates were successfully reduced in the cortex and striatum, therefore ameliorating behavioural deficits and striatal and cortical neuropathologies.

Overall, our findings establish a convenient, effective and safe strategy for self-assembly of siRNAs in vivo that may provide a significant therapeutic benefit for Huntington’s disease.

Involvement of CRMP2 in Regulation of Mitochondrial Morphology and Motility in Huntington's Disease

Mitochondrial morphology and motility (mitochondrial dynamics) play a major role in the proper functioning of distant synapses. In Huntington’s disease (HD), mitochondria become fragmented and less motile, but the mechanisms leading to these changes are not clear. Here, we found that collapsin response mediator protein 2 (CRMP2) interacted with Drp1 and Miro 2, proteins involved in regulating mitochondrial dynamics. CRMP2 interaction with these proteins inversely correlated with CRMP2 phosphorylation. CRMP2 was hyperphosphorylated in postmortem brain tissues of HD patients, in human neurons derived from induced pluripotent stem cells from HD patients, and in cultured striatal neurons from HD mouse model YAC128. At the same time, CRMP2 interaction with Drp1 and Miro 2 was diminished in HD neurons. The CRMP2 hyperphosphorylation and dissociation from Drp1 and Miro 2 correlated with increased fission and suppressed motility. (S)-lacosamide ((S)-LCM), a small molecule that binds to CRMP2, decreased its phosphorylation at Thr 509/514 and Ser 522 and rescued CRMP2’s interaction with Drp1 and Miro 2. This was accompanied by reduced mitochondrial fission and enhanced mitochondrial motility. Additionally, (S)-LCM exerted a neuroprotective effect in YAC128 cultured neurons. Thus, our data suggest that CRMP2 may regulate mitochondrial dynamics in a phosphorylation-dependent manner and modulate neuronal survival in HD.

Huntington's disease mouse models: unraveling the pathology caused by CAG repeat expansion

Huntington's disease (HD) is a neurodegenerative disease that results in motor and cognitive dysfunction, leading to early death. HD is caused by an expansion of CAG repeats in the huntingtin gene (HTT). Here, we review the mouse models of HD. They have been used extensively to better understand the molecular and cellular basis of disease pathogenesis as well as to provide non-human subjects to test the efficacy of potential therapeutics. The first and best-studied in vivo rodent model of HD is the R6/2 mouse, in which a transgene containing the promoter and exon 1 fragment of human HTT with 150 CAG repeats was inserted into the mouse genome. R6/2 mice express rapid, robust behavioral pathologies and display a number of degenerative abnormalities in neuronal populations most vulnerable in HD. The first conditional full-length mutant huntingtin (mHTT) mouse model of HD was the bacterial artificial chromosome (BAC) transgenic mouse model of HD (BACHD), which expresses human full-length mHTT with a mixture of 97 CAG-CAA repeats under the control of endogenous HTT regulatory machinery. It has been useful in identifying the role of mHTT in specific neuronal populations in degenerative processes. In the knock-in (KI) model of HD, the expanded human CAG repeats and human exon 1 are inserted into the mouse Htt locus, so a chimera of the full-length mouse protein with the N-terminal human portion is expressed. Many of aspects of the pathology and behavioral deficits in the KI model better mimic disease characteristics found in HD patients than other models. Accordingly, some have proposed that these mice may be preferable models of the disease over others. Indeed, as our understanding of HD advances, so will the design of animal models to test and develop HD therapies.

Nicotinamide Adenine Dinucleotide Phosphate Oxidases Are Everywhere in Brain Disease, but Not in Huntington's Disease?

Huntington's disease (HD) is an inherited neurodegenerative disorder characterized by neuronal loss and tissue atrophy mainly in the striatum and cortex. In the early stages of the disease, impairment of neuronal function, synaptic dysfunction and white matter loss precedes neuronal death itself. Relative to other neurodegenerative diseases such as Alzheimer's and Parkinson's disease and Amyotrophic Lateral Sclerosis, where the effects of either microglia or NADPH oxidases (NOXs) are recognized as important contributors to disease pathogenesis and progression, there is a pronounced lack of information in HD. This information void contrasts with evidence from human HD patients where blood monocytes and microglia are activated well before HD clinical symptoms (PET scans), and the clear signs of oxidative stress and inflammation in post mortem HD brain. Habitually, NOX activity and oxidative stress in the central nervous system (CNS) are equated with microglia, but research of the last two decades has carved out important roles for NOX enzyme function in neurons. Here, we will convey recent information about the function of NOX enzymes in neurons, and contemplate on putative roles of neuronal NOX in HD. We will focus on NOX-produced reactive oxygen species (ROS) as redox signaling molecules in/among neurons, and the specific roles of NOXs in important processes such as neurogenesis and lineage specification, neurite outgrowth and growth cone dynamics, and synaptic plasticity where NMDAR-dependent signaling, and long-term depression/potentiation are redox-regulated phenomena. HD animal models and induced pluripotent stem cell (iPSC) studies have made it clear that the very same physiological processes are also affected in HD, and we will speculate on possible roles for NOX in the pathogenesis and development of disease. Finally, we also take into account the limited information on microglia in HD and relate this to any contribution of NOX enzymes.

Decreased Interactions between Calmodulin and a Mutant Huntingtin Model Might Reduce the Cytotoxic Level of Intracellular Ca2+: A Molecular Dynamics Study

Mutant huntingtin (m-HTT) proteins and calmodulin (CaM) co-localize in the cerebral cortex with significant effects on the intracellular calcium levels by altering the specific calcium-mediated signals. Furthermore, the mutant huntingtin proteins show great affinity for CaM that can lead to a further stabilization of the mutant huntingtin aggregates. In this context, the present study focuses on describing the interactions between CaM and two huntingtin mutants from a biophysical point of view, by using classical Molecular Dynamics techniques. The huntingtin models consist of a wild-type structure, one mutant with 45 glutamine residues and the second mutant with nine additional key-point mutations from glutamine residues into proline residues (9P(EM) model). Our docking scores and binding free energy calculations show higher binding affinities of all HTT models for the C-lobe end of the CaM protein. In terms of dynamic evolution, the 9P(EM) model triggered great structural changes into the CaM protein’s structure and shows the highest fluctuation rates due to its structural transitions at the helical level from α-helices to turns and random coils. Moreover, our proposed 9P(EM) model suggests much lower interaction energies when compared to the 45Qs-HTT mutant model, this finding being in good agreement with the 9P(EM)’s antagonistic effect hypothesis on highly toxic protein–protein interactions.

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

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

Mitostasis, Calcium and Free Radicals in Health, Aging and Neurodegeneration

Mitochondria play key roles in ATP supply, calcium homeostasis, redox balance control and apoptosis, which in neurons are fundamental for neurotransmission and to allow synaptic plasticity. Their functional integrity is maintained by mitostasis, a process that involves mitochondrial transport, anchoring, fusion and fission processes regulated by different signaling pathways but mainly by the peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α). PGC-1α also favors Ca²⁺ homeostasis, reduces oxidative stress, modulates inflammatory processes and mobilizes mitochondria to where they are needed. To achieve their functions, mitochondria are tightly connected to the endoplasmic reticulum (ER) through specialized structures of the ER termed mitochondria-associated membranes (MAMs), which facilitate the communication between these two organelles mainly to aim Ca²⁺ buffering. Alterations in mitochondrial activity enhance reactive oxygen species (ROS) production, disturbing the physiological metabolism and causing cell damage. Furthermore, cytosolic Ca²⁺ overload results in an increase in mitochondrial Ca²⁺, resulting in mitochondrial dysfunction and the induction of mitochondrial permeability transition pore (mPTP) opening, leading to mitochondrial swelling and cell death through apoptosis as demonstrated in several neuropathologies. In summary, mitochondrial homeostasis is critical to maintain neuronal function; in fact, their regulation aims to improve neuronal viability and to protect against aging and neurodegenerative diseases.

Glutaminyl Cyclase, Diseases, and Development of Glutaminyl Cyclase Inhibitors

Pyroglutamate (pE) modification, catalyzed mainly by glutaminyl cyclase (QC), is prevalent throughout nature and is particularly important in mammals including humans for the maturation of hormones, peptides, and proteins. In humans, the upregulation of QC is involved in multiple diseases and conditions including Alzheimer's disease, Huntington's disease, melanomas, thyroid carcinomas, accelerated atherosclerosis, septic arthritics, etc. This upregulation catalyzes the generation of modified mediators such as pE-amyloid beta (Aß) and pE-chemokine ligand 2 (CCL2) peptides. Not surprisingly, QC has emerged as a reasonable target for the development of therapeutics to combat these diseases and conditions. In this manuscript the deleterious effects of upregulated QC resulting in disease manifestation are reviewed, along with progress on the development of QC inhibitors.

The Potential Role of Protein Kinase R as a Regulator of Age-Related Neurodegeneration

There is a growing evidence describing a decline in adaptive homeostasis in aging-related diseases affecting the central nervous system (CNS), many of which are characterized by the appearance of non-native protein aggregates. One signaling pathway that allows cell adaptation is the integrated stress response (ISR), which senses stress stimuli through four kinases. ISR activation promotes translational arrest through the phosphorylation of the eukaryotic translation initiation factor 2 alpha (eIF2α) and the induction of a gene expression program to restore cellular homeostasis. However, depending on the stimulus, ISR can also induce cell death. One of the ISR sensors is the double-stranded RNA-dependent protein kinase [protein kinase R (PKR)], initially described as a viral infection sensor, and now a growing evidence supports a role for PKR on CNS physiology. PKR has been largely involved in the Alzheimer’s disease (AD) pathological process. Here, we reviewed the antecedents supporting the role of PKR on the efficiency of synaptic transmission and cognition. Then, we review PKR’s contribution to AD and discuss the possible participation of PKR as a player in the neurodegenerative process involved in aging-related pathologies affecting the CNS.

Synthetic genomics for curing genetic diseases

From the beginning of the genome sequencing era, it has become increasingly evident that genetics plays a role in all diseases, of which only a minority are single-gene disorders, the most common target of current gene therapies. However, the majority of people have some kind of health problems resulting from congenital genetic mutations (over 6000 diseases have been associated to genes, https://www.omim.org/statistics/geneMap) and most genetic disorders are rare and only incompletely understood. The vision and techniques applied to the synthesis of genomes may help to address unmet medical needs from a chromosome and genome-scale perspective. In this chapter, we address the potential therapy of genetic diseases from a different outlook, in which we no longer focus on small gene corrections but on higher-order tools for genome manipulation. These will play a crucial role in the next years, as they prelude to a much deeper understanding of the architecture of the human genome and a more accurate modeling of human diseases, offering new therapeutic opportunities.

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

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