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Huntington disease (HD) is a progressive autosomal dominant neurodegenerative disorder in which patients typically present with uncontrolled involuntary movements and subsequent cognitive decline. In 1993, a CAG trinucleotide repeat expansion in the coding region of the huntingtin (HTT) gene was identified as the cause of this disorder. This extended CAG repeat results in production of HTT protein with an expanded polyglutamine tract, leading to pathogenic HTT protein conformers that are resistant to protein turnover, culminating in cellular toxicity and neurodegeneration. Research into the mechanistic basis of HD has highlighted a role for bioenergetics abnormalities stemming from mitochondrial dysfunction, and for synaptic defects, including impaired neurotransmission and excitotoxicity. Interference with transcription regulation may underlie the mitochondrial dysfunction. Current therapies for HD are directed at treating symptoms, as there are no disease-modifying therapies. Commonly prescribed drugs for involuntary movement control include tetrabenazine, a potent and selective inhibitor of vesicular monoamine transporter 2 that depletes synaptic monoamines, and olanzapine, an atypical neuroleptic that blocks the dopamine D2 receptor. Various drugs are used to treat non-motor features. The HD therapeutic pipeline is robust, as numerous efforts are underway to identify disease-modifying treatments, with some small compounds and biological agents moving into clinical trials. Especially encouraging are dosage reduction strategies, including antisense oligonucleotides, and molecules directed at transcription dysregulation. Given the depth and breadth of current HD drug development efforts, there is reason to believe that disease-modifying therapies for HD will emerge, and this achievement will have profound implications for the entire neurotherapeutics field., (© 2017 Wiley Periodicals, Inc.)

Huntington's disease (HD) is a progressive neurodegenerative disorder caused by a CAG trinucleotide repeat expansion in the huntingtin (HTT) gene, which encodes a polyglutamine tract in the HTT protein. We found that peroxisome proliferator-activated receptor delta (PPAR-δ) interacts with HTT and that mutant HTT represses PPAR-δ-mediated transactivation. Increased PPAR-δ transactivation ameliorated mitochondrial dysfunction and improved cell survival of neurons from mouse models of HD. Expression of dominant-negative PPAR-δ in the central nervous system of mice was sufficient to induce motor dysfunction, neurodegeneration, mitochondrial abnormalities and transcriptional alterations that recapitulated HD-like phenotypes. Expression of dominant-negative PPAR-δ specifically in the striatum of medium spiny neurons in mice yielded HD-like motor phenotypes, accompanied by striatal neuron loss. In mouse models of HD, pharmacologic activation of PPAR-δ using the agonist KD3010 improved motor function, reduced neurodegeneration and increased survival. PPAR-δ activation also reduced HTT-induced neurotoxicity in vitro and in medium spiny-like neurons generated from stem cells derived from individuals with HD, indicating that PPAR-δ activation may be beneficial in HD and related disorders.

Autophagy is the cellular process by which proteins, macromolecules, and organelles are targeted to and degraded by the lysosome. Given that neurodegenerative diseases involve the production of misfolded proteins that cannot be degraded by the protein quality-control systems of the cell, the autophagy pathway is now the focus of intense scrutiny, because autophagy is primarily responsible for maintaining normal cellular proteostasis in the central nervous system (CNS). Huntington's disease (HD) is an inherited CAG-polyglutamine repeat disorder, resulting from the production and accumulation of misfolded huntingtin (Htt) protein. HD shares key features with common neurodegenerative disorders, such as Alzheimer's disease (AD) and Parkinson's disease (PD) and, thus, belongs to a large class of disorders known as neurodegenerative proteinopathies. Multiple independent lines of research have documented alterations in autophagy function in HD, and numerous studies have demonstrated a potential role for autophagy modulation as a therapeutic intervention. In this review, we consider the evidence for autophagy dysfunction in HD, and delineate different targets and mechanistic pathways that might account for the autophagy abnormalities detected in HD. We assess the utility of autophagy modulation as a treatment modality in HD, and suggest guidelines and caveats for future therapy development directed at the autophagy pathway in HD and related disorders., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Expansion of CAG/CTG trinucleotide repeats causes numerous inherited neurological disorders, including Huntington's disease (HD), several spinocerebellar ataxias and myotonic dystrophy type 1. Expanded repeats are genetically unstable with a propensity to further expand when transmitted from parents to offspring. For many alleles with expanded repeats, extensive somatic mosaicism has been documented. For CAG repeat diseases, dramatic instability has been documented in the striatum, with larger expansions noted with advancing age. In contrast, only modest instability occurs in the cerebellum. Using microarray expression analysis, we sought to identify the genetic basis of these regional instability differences by comparing gene expression in the striatum and cerebellum of aged wild-type C57BL/6J mice. We identified eight candidate genes enriched in cerebellum, and validated four--Pcna, Rpa1, Msh6 and Fen1--along with a highly associated interactor, Lig1. We also explored whether expression levels of mismatch repair (MMR) proteins are altered in a line of HD transgenic mice, R6/2, that is known to show pronounced regional repeat instability. Compared with wild-type littermates, MMR expression levels were not significantly altered in R6/2 mice regardless of age. Interestingly, expression levels of these candidates were significantly increased in the cerebellum of control and HD human samples in comparison to striatum. Together, our data suggest that elevated expression levels of DNA replication and repair proteins in cerebellum may act as a safeguard against repeat instability, and may account for the dramatically reduced somatic instability present in this brain region, compared with the marked instability observed in the striatum.

Aims: Dynamin-related protein1 (Drp1) is a large GTPase that mediates mitochondrial fission. We recently reported in Alzheimer's disease (AD) that S-nitrosylation of Drp1 (forming S-nitroso [SNO]-Drp1) results in GTPase hyperactivity and mitochondrial fragmentation, thus impairing bioenergetics and inducing synaptic damage and neuronal loss. Here, since aberrant mitochondrial dynamics are also key features of Huntington's disease (HD), we investigated whether formation of SNO-Drp1 contributes to the pathogenesis of HD in cell-based and animal models., Results: We found that expression of mutant huntingtin (mutHTT) protein in primary cultured neurons triggers significant production of nitric oxide (NO). Consistent with this result, increased levels of SNO-Drp1 were found in the striatum of a transgenic mouse model of HD as well as in human postmortem brains from HD patients. Using specific fluorescence markers, we found that formation of SNO-Drp1 induced excessive mitochondrial fragmentation followed by loss of dendritic spines, signifying synaptic damage. These neurotoxic events were significantly abrogated after transfection with non-nitrosylatable mutant Drp1(C644A), or by the blocking of NO production using an nitric oxide synthase inhibitor. These findings suggest that SNO-Drp1 is a key mediator of mutHTT toxicity, and, thus, may represent a novel drug target for HD., Innovation and Conclusion: Our findings indicate that aberrant S-nitrosylation of Drp1 is a prominent pathological feature of neurodegenerative diseases such as AD and HD. Moreover, the SNO-Drp1 signaling pathway links mutHTT neurotoxicity to a malfunction in mitochondrial dynamics, resulting in neuronal synaptic damage in HD.

Huntington disease (HD) results from CAG repeats that encode expanded polyglutamine tracts in the HTT/huntingtin protein. HD belongs to a large category of inherited and sporadic neurodegenerative disorders in which production of a misfolded protein initiates the pathogenic cascade. Previous studies have shown that misfolded proteins become resistant to cellular protein turnover pathways by eluding and disabling the ubiquitin-proteasome system (UPS) and autophagy-lysosome pathway. Based upon earlier work implicating impaired PPARGC1A function in HD, we derived inducible PPARGC1A mice and crossed them with HD mice. We found that PPARGC1A overexpression can rescue HD neurological phenotypes and neurodegeneration. An unexpected outcome of the rescue was the virtual elimination of huntingtin aggregates, and we found that PPARGC1A-mediated aggregate elimination required the autophagy pathway. Moreover, we observed decreased expression of transcription factor EB (TFEB), a master regulator of the autophagy-lysosome pathway, in HD cells and mice, and documented PPARGC1A co-activation of TFEB in these model systems, noting that PPARGC1A is upstream of TFEB in promoting proteostasis. These findings underscore the importance of bioenergetics and autophagy in neurodegeneration, and indicate that PPARGC1A promotes mitochondrial quality control to support high-energy production states in cells, such as neurons. As impaired energy production and altered protein-organelle quality control appear inextricably linked in disorders such as HD, Parkinson disease, and Alzheimer disease, efforts directed at enhancing PPARGC1A and TFEB action may represent viable strategies for therapy development in neurodegeneration.

Huntington's disease (HD) is caused by CAG repeat expansions in the huntingtin (htt) gene, yielding proteins containing polyglutamine repeats that become misfolded and resist degradation. Previous studies demonstrated that mutant htt interferes with transcriptional programs coordinated by the peroxisome proliferator-activated receptor γ (PPARγ) coactivator 1α (PGC-1α), a regulator of mitochondrial biogenesis and oxidative stress. We tested whether restoration of PGC-1α could ameliorate the symptoms of HD in a mouse model. We found that PGC-1α induction virtually eliminated htt protein aggregation and ameliorated HD neurodegeneration in part by attenuating oxidative stress. PGC-1α promoted htt turnover and the elimination of protein aggregates by activating transcription factor EB (TFEB), a master regulator of the autophagy-lysosome pathway. TFEB alone was capable of reducing htt aggregation and neurotoxicity, placing PGC-1α upstream of TFEB and identifying these two molecules as important therapeutic targets in HD and potentially other neurodegenerative disorders caused by protein misfolding.

Neurons are specialized cells with unique features, including a constant high demand for energy. Mitochondria satisfy this constant demand, and are emerging as a central target for dysfunction in neurodegenerative disorders, such as Huntington's disease (HD) and Parkinson's disease. PPARγ co-activator-1α (PGC-1α) is a transcription co-activator for nuclear receptors such as the PPARs, and thereby coordinates a number of gene expression programs to promote mitochondrial biogenesis and oxidative phosphorylation. Studies of PGC-1α knock-out mice have yielded important insights into the role of PGC-1α in normal nervous system function and potentially neurological disease. HD is caused by a polyglutamine repeat expansion in the huntingtin protein, and decades of work have established mitochondrial dysfunction as a key feature of HD pathogenesis. However, after the discovery of the HD gene, numerous reports produced strong evidence for altered transcription in HD. In 2006, a series of studies revealed that PGC-1α transcription interference contributes to HD neurodegeneration, linking the nuclear transcriptionopathy with the mitochondrial dysfunction. Subsequent work has strengthened this view, and further extended the role of PGC-1α within the CNS. Within the last year, studies of Parkinson's disease, another involuntary movement disorder long associated with mitochondrial dysfunction, have shown that PGC-1α dysregulation is contributing to its pathogenesis. As PGC-1α is likely also important for aging, a process with considerable relevance to neuron function, translational studies aimed at developing therapies based upon the PGC-1α pathway as a high priority target are underway., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Huntington's disease-like-2 (HDL2) is a phenocopy of Huntington's disease caused by CTG/CAG repeat expansion at the Junctophilin-3 (JPH3) locus. The mechanisms underlying HDL2 pathogenesis remain unclear. Here we developed a BAC transgenic mouse model of HDL2 (BAC-HDL2) that exhibits progressive motor deficits, selective neurodegenerative pathology, and ubiquitin-positive nuclear inclusions (NIs). Molecular analyses reveal a promoter at the transgene locus driving the expression of a CAG repeat transcript (HDL2-CAG) from the strand antisense to JPH3, which encodes an expanded polyglutamine (polyQ) protein. Importantly, BAC-HDL2 mice, but not control BAC mice, accumulate polyQ-containing NIs in a pattern strikingly similar to those in the patients. Furthermore, BAC mice with genetic silencing of the expanded CUG transcript still express HDL2-CAG transcript and manifest polyQ pathogenesis. Finally, studies of HDL2 mice and patients revealed CBP sequestration into NIs and evidence for interference of CBP-mediated transcriptional activation. These results suggest overlapping polyQ-mediated pathogenic mechanisms in HD and HDL2., (Copyright © 2011 Elsevier Inc. All rights reserved.)

The article by McConoughey et al in the current issue of EMBO Molecular Medicine examines the contribution of transglutaminase 2 (TG2) to Huntington's disease (HD) pathogenesis. The authors find that TG2 inhibition can ameliorate HD neurodegeneration, and thereby elevate the status of transglutaminases (TGs) to a major therapeutic target-not because of their well-known activity in mutant protein aggregation, but instead based upon their ability to epigenetically modulate transcription and energy production. While the reintroduction of TG inhibition as a therapy for HD may evoke feelings of déjà vu, the outcome this time around could go in a dramatically different direction.

Huntington's disease (HD) is a fatal, dominantly inherited disorder caused by polyglutamine repeat expansion in the huntingtin (htt) gene. Here, we observe that HD mice develop hypothermia associated with impaired activation of brown adipose tissue (BAT). Although sympathetic stimulation of PPARgamma coactivator 1alpha (PGC-1alpha) was intact in BAT of HD mice, uncoupling protein 1 (UCP-1) induction was blunted. In cultured cells, expression of mutant htt suppressed UCP-1 promoter activity; this was reversed by PGC-1alpha expression. HD mice showed reduced food intake and increased energy expenditure, with dysfunctional BAT mitochondria. PGC-1alpha is a known regulator of mitochondrial function; here, we document reduced expression of PGC-1alpha target genes in HD patient and mouse striatum. Mitochondria of HD mouse brain show reduced oxygen consumption rates. Finally, HD striatal neurons expressing exogenous PGC-1alpha were resistant to 3-nitropropionic acid treatment. Altered PGC-1alpha function may thus link transcription dysregulation and mitochondrial dysfunction in HD.

A role for apoptotic mediators in neurodegenerative disease has long been considered, but the identity of the putative effectors and the basis of their actions in neurodegeneration remain ill-defined. Huntington's disease (HD) is a polyglutamine disorder whose pathogenesis likely involves mitochondrial dysfunction. In this issue of Neuron, Bae et al. present data implicating p53 in HD pathogenesis. This intriguing study may thus provide a link between nuclear transcription dysregulation and mitochondrial abnormalities in HD.

Recent evidence indicates that transcriptional abnormalities may play an important role in the pathophysiology of polyglutamine diseases. In the present study, we have explored the extent to which polyglutamine-related changes in gene expression may be independent of protein context by comparing mouse models of dentatorubral-pallidoluysian atrophy (DRPLA) and Huntington's disease (HD). Microarray gene expression profiling was conducted in mice of the same background strain in which the same promoter was employed to direct the expression of full-length atrophin-1 or partial huntingtin transproteins (At-65Q or N171-82Q mice). A large number of overlapping gene expression changes were observed in the cerebella of At-65Q and N171-82Q mice. Six of the gene expression changes common to both huntingtin and atrophin-1 transgenic mice were also observed in the cerebella of mouse models expressing full-length mutant ataxin-7 or the androgen receptor. These results demonstrate that some of the gene expression effects of expanded polyglutamine proteins occur independently of protein context.

Abstract Two decades ago, the recognition of protein misfolding and aggregate accumulation as defining features of neurodegenerative disease set the stage for a thorough examination of how protein quality control is maintained in neurons and in other non-neuronal cells in the central nervous system (CNS). Autophagy, a pathway of cellular self-digestion, has emerged as especially important for CNS proteostasis, and autophagy dysregulation has been documented as a defining feature of neurodegeneration in Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington's disease (HD). Transcription factor EB (TFEB) is one of the main transcriptional regulators of autophagy, as it promotes the expression of genes required for autophagosome formation, lysosome biogenesis, and lysosome function, and it is highly expressed in CNS. Over the last 7 years, TFEB has received considerable attention and TFEB dysfunction has been implicated in the pathogenesis of numerous neurodegenerative disorders. In this review, we delineate the current understanding of how TFEB dysregulation is involved in neurodegeneration, highlighting work done on AD, PD, HD, X-linked spinal & bulbar muscular atrophy, and amyotrophic lateral sclerosis. Because TFEB is a central node in defining autophagy activation status, efforts at understanding the basis for TFEB dysfunction are yielding insights into how TFEB might be targeted for therapeutic application, which may represent an exciting opportunity for the development of a treatment modality with broad application to neurodegeneration. [ABSTRACT FROM AUTHOR]

The article reports on the activation of peroxisome proliferator–activated receptors (PPARs) by the U.S. Food and Drug Administration-approved drug bexarotene promoting neuroprotective in cellular models of Huntington's disease (HD), a progressive neurodegenerative disorder. Mitochondrial and protein quality control in cellular models of HD was examined.

The mechanisms involved in microsatellite repeat instability and the consequent pathways that lead to disease have fascinated investigators from a wide range of disciplines for over a decade. This fascination was sparked by the discovery of dynamic mutations as the cause of a number of important and enigmatic human genetic diseases, including fragile X syndrome, myotonic dystrophy and Huntington disease. A primary focus of current research that was discussed at the conference is the molecular basis of microsatellite repeat instability. Both the composition of the repeat sequence and the length of the repeat are primary determinants of the likelihood that a repeat will expand.