Aberrant histone acetylation, altered transcription, and retinal degeneration in a Drosophila model of polyglutamine disease are rescued by CREB-binding protein

Gene Alterations Induced by Glutamine (Q) Encoding CAG Repeats Associated with Neurodegeneration

Several studies have been reported linking the role of polyglutamine (polyQ) disease-associated proteins with altered gene regulation induced by an unstable trinucleotide (CAG) repeat. Owing to their dynamic nature of expansion, these DNA repeats form secondary structures interfering with the normal cellular mechanisms like replication and transcription and, thereby, have become the underlying cause of numerous neurodegenerative disorders involving mental retardation and/or muscular or neuronal degeneration. Despite the widespread expression of the disease-causing protein, specific subsets of neurons are susceptible to specific patterns of inheritance and clinical symptoms. Although this cell-type selectivity is still elusive and less understood, it has been found that aberrant transcriptional regulation is one of the primary causes of polyQ diseases where the functions of histone-modifying complexes are disrupted. Besides, epigenetic modifications play a critical role in the pathogenesis of these diseases. In this chapter, we will be delving into how these polyQ repeats induce the self-assembly and aggregation of altered carrier proteins based on gene alterations, causing neuronal toxicity and cellular deaths. Besides, genomic instability in CAG repeats due to altered chromatin-related enzymes will be highlighted, along with epigenetic changes present in many polyQ disorders. Understanding the underlying molecular mechanisms in the root cause of these disorders will culminate in identifying therapeutic approaches for the treatment of these neurodegenerative disorders.

Acetylation State of Lysine 14 of Histone H3.3 Affects Mutant Huntingtin Induced Pathogenesis

Huntington's Disease (HD) is a fatal neurodegenerative disorder caused by the expansion of a polyglutamine-coding CAG repeat in the Huntingtin gene. One of the main causes of neurodegeneration in HD is transcriptional dysregulation that, in part, is caused by the inhibition of histone acetyltransferase (HAT) enzymes. HD pathology can be alleviated by increasing the activity of specific HATs or by inhibiting histone deacetylase (HDAC) enzymes. To determine which histone's post-translational modifications (PTMs) might play crucial roles in HD pathology, we investigated the phenotype-modifying effects of PTM mimetic mutations of variant histone H3.3 in a Drosophila model of HD. Specifically, we studied the mutations (K→Q: acetylated; K→R: non-modified; and K→M: methylated) of lysine residues K9, K14, and K27 of transgenic H3.3. In the case of H3.3K14Q modification, we observed the amelioration of all tested phenotypes (viability, longevity, neurodegeneration, motor activity, and circadian rhythm defects), while H3.3K14R had the opposite effect. H3.3K14Q expression prevented the negative effects of reduced Gcn5 (a HAT acting on H3K14) on HD pathology, while it only partially hindered the positive effects of heterozygous Sirt1 (an HDAC acting on H3K14). Thus, we conclude that the Gcn5-dependent acetylation of H3.3K14 might be an important epigenetic contributor to HD pathology.

Modifier pathways in polyglutamine (PolyQ) diseases: from genetic screens to drug targets

Polyglutamine (PolyQ) diseases include a group of inherited neurodegenerative disorders caused by unstable expansions of CAG trinucleotide repeats in the coding region of specific genes. Such genetic alterations produce abnormal proteins containing an unusually long PolyQ tract that renders them more prone to aggregate and cause toxicity. Although research in the field in the last years has contributed significantly to the knowledge of the biological mechanisms implicated in these diseases, effective treatments are still lacking. In this review, we revisit work performed in models of PolyQ diseases, namely the yeast Saccharomyces cerevisiae, the nematode worm Caenorhabditis elegans and the fruit fly Drosophila melanogaster, and provide a critical overview of the high-throughput unbiased genetic screens that have been performed using these systems to identify novel genetic modifiers of PolyQ diseases. These approaches have revealed a wide variety of cellular processes that modulate the toxicity and aggregation of mutant PolyQ proteins, reflecting the complexity of these disorders and demonstrating how challenging the development of therapeutic strategies can be. In addition to the unbiased large-scale genetic screenings in non-vertebrate models, complementary studies in mammalian systems, closer to humans, have contributed with novel genetic modifiers of PolyQ diseases, revealing neuronal function and inflammation as key disease modulators. A pathway enrichment analysis, using the human orthologues of genetic modifiers of PolyQ diseases clustered modifier genes into major themes translatable to the human disease context, such as protein folding and transport as well as transcription regulation. Innovative genetic strategies of genetic manipulation, together with significant advances in genomics and bioinformatics, are taking modifier genetic studies to more realistic disease contexts. The characterization of PolyQ disease modifier pathways is of extreme relevance to reveal novel therapeutic possibilities to delay disease onset and progression in patients.

Pan-neuronal expression of human mutant huntingtin protein in Drosophila impairs immune response of hemocytes

Huntington's disease (HD) is a late-onset; progressive, dominantly inherited neurological disorder marked by an abnormal expansion of polyglutamine (poly Q) repeats in Huntingtin (HTT) protein. The pathological effects of mutant Huntingtin (mHTT) are not restricted to the nervous system but systemic abnormalities including immune dysregulation have been evidenced in clinical and experimental settings of HD. Indeed, mHTT is ubiquitously expressed and could induce cellular toxicity by directly acting on immune cells. However, it is still unclear if selective expression of mHTT exon1 in neurons could induce immune responses and hemocytes' function. In the present study, we intended to monitor perturbations in the hemocytes' population and their physiological functions in Drosophila, caused by pan-neuronal expression of mHTT protein. A measure of hemocyte count and their physiological activities caused by pan-neuronal expression of mHTT protein highlighted the extent of immune dysregulation occurring with disease progression. We found that pan-neuronal expression of mHTT significantly alters crystal cells and plasmatocyte count in larvae and adults with disease progression. Interestingly, plasmatocytes isolated from diseased conditions exhibit a gradual decline in phagocytic activity ex vivo at progressive stages of the disease as compared to age-matched control groups. In addition, diseased flies displayed elevated reactive oxygen species (ROS) in circulating plasmatocytes at the larval stage and in sessile plasmatocytes of hematopoietic pockets at terminal stages of disease. These findings strongly implicate that neuronal expression of mHTT alone is sufficient to induce non-cell-autonomous immune dysregulation in vivo.

Mutant Ataxin-3-Containing Aggregates (MATAGGs) in Spinocerebellar Ataxia Type 3: Dynamics of the Disorder

Spinocerebellar ataxia type 3 (SCA3) is the most common type of SCA worldwide caused by abnormal polyglutamine expansion in the coding region of the ataxin-3 gene. Ataxin-3 is a multi-faceted protein involved in various cellular processes such as deubiquitination, cytoskeletal organisation, and transcriptional regulation. The presence of an expanded poly(Q) stretch leads to altered processing and misfolding of the protein culminating in the production of insoluble protein aggregates in the cell. Various post-translational modifications affect ataxin-3 fibrillation and aggregation. This review provides an exhaustive assessment of the various pathogenic mechanisms undertaken by the mutant ataxin-3-containing aggregates (MATAGGs) for disease induction and neurodegeneration. This includes in-depth discussion on MATAGG dynamics including their formation, role in neuronal pathogenesis, and the debate over the toxic v/s protective nature of the MATAGGs in disease progression. Additionally, the currently available therapeutic strategies against SCA3 have been reviewed. The shift in the focus of such strategies, from targeting the steps that lead to or reduce aggregate formation to targeting the expression of mutant ataxin-3 itself via RNA-based therapeutics, has also been presented. We also discuss the intriguing promise that various growth and neurotrophic factors, especially the insulin pathway, hold in the modulation of SCA3 progression. These emerging areas show the newer directions through which SCA3 can be targeted including various preclinical and clinical trials. All these advances made in the last three decades since the discovery of the ataxin-3 gene have been critically reviewed here.

Genetic Screen in Adult Drosophila Reveals That dCBP Depletion in Glial Cells Mitigates Huntington Disease Pathology through a Foxo-Dependent Pathway

Huntington’s disease (HD) is a progressive and fatal autosomal dominant neurodegenerative disease caused by a CAG repeat expansion in the first exon of the huntingtin gene (HTT). In spite of considerable efforts, there is currently no treatment to stop or delay the disease. Although HTT is expressed ubiquitously, most of our knowledge has been obtained on neurons. More recently, the impact of mutant huntingtin (mHTT) on other cell types, including glial cells, has received growing interest. It is currently unclear whether new pathological pathways could be identified in these cells compared to neurons. To address this question, we performed an in vivo screen for modifiers of mutant huntingtin (HTT-548-128Q) induced pathology in Drosophila adult glial cells and identified several putative therapeutic targets. Among them, we discovered that partial nej/dCBP depletion in these cells was protective, as revealed by strongly increased lifespan and restored locomotor activity. Thus, dCBP promotes the HD pathology in glial cells, in contrast to previous opposite findings in neurons. Further investigations implicated the transcriptional activator Foxo as a critical downstream player in this glial protective pathway. Our data suggest that combinatorial approaches combined to specific tissue targeting may be required to uncover efficient therapies in HD.

Acetyl-CoA Metabolism and Histone Acetylation in the Regulation of Aging and Lifespan

Acetyl-CoA is a metabolite at the crossroads of central metabolism and the substrate of histone acetyltransferases regulating gene expression. In many tissues fasting or lifespan extending calorie restriction (CR) decreases glucose-derived metabolic flux through ATP-citrate lyase (ACLY) to reduce cytoplasmic acetyl-CoA levels to decrease activity of the p300 histone acetyltransferase (HAT) stimulating pro-longevity autophagy. Because of this, compounds that decrease cytoplasmic acetyl-CoA have been described as CR mimetics. But few authors have highlighted the potential longevity promoting roles of nuclear acetyl-CoA. For example, increasing nuclear acetyl-CoA levels increases histone acetylation and administration of class I histone deacetylase (HDAC) inhibitors increases longevity through increased histone acetylation. Therefore, increased nuclear acetyl-CoA likely plays an important role in promoting longevity. Although cytoplasmic acetyl-CoA synthetase 2 (ACSS2) promotes aging by decreasing autophagy in some peripheral tissues, increased glial AMPK activity or neuronal differentiation can stimulate ACSS2 nuclear translocation and chromatin association. ACSS2 nuclear translocation can result in increased activity of CREB binding protein (CBP), p300/CBP-associated factor (PCAF), and other HATs to increase histone acetylation on the promoter of neuroprotective genes including transcription factor EB (TFEB) target genes resulting in increased lysosomal biogenesis and autophagy. Much of what is known regarding acetyl-CoA metabolism and aging has come from pioneering studies with yeast, fruit flies, and nematodes. These studies have identified evolutionary conserved roles for histone acetylation in promoting longevity. Future studies should focus on the role of nuclear acetyl-CoA and histone acetylation in the control of hypothalamic inflammation, an important driver of organismal aging.

The S6k/4E-BP mediated growth promoting sub-pathway of insulin signalling cascade is essential to restrict pathogenesis of poly(Q) disorders in Drosophila

Human neurodegenerative polyglutamine [poly(Q)] disorders, such as Huntington's disease (HD) and spinocerebellar ataxias (SCA), are characterised by an abnormal expansion of CAG repeats in the affected gene. The mutated proteins misfold and aggregate to form inclusion bodies that sequester important factors involved in cellular transcription, growth, stress and autophagic response and other essential functions. The insulin signalling pathway has been demonstrated as a major modifier and a potential drug target to ameliorate the poly(Q) mediated neurotoxicity in various model systems. Insulin signalling cascade harbours several downstream sub-pathways, which are synergistically involved in discharging indispensable biological functions such as growth and proliferation, metabolism, autophagy, regulation of cell death pathways etc. Hence, it is difficult to conclude whether the mitigation of poly(Q) neurotoxicity is an accumulative outcome of the insulin cascade, or the result of a specific sub-pathway. For the first time, we report that the ligand binding domain of insulin receptor mediated downstream growth promoting sub-pathway plays the pivotal role in operating the rescue event. We show that the growth promoting activity of insulin cascade is essential to minimize the abundance of inclusion bodies, to restrict neurodegeneration, and to restore the cellular transcriptional balance. Subsequently, we noted the involvement of the mTOR/S6k/4E-BP candidates in mitigating poly(Q) mediated neurotoxicity. Due to the conserved cellular functioning of the insulin cascade across species, and availability of several growth promoting molecules, our results in Drosophila poly(Q) models indicate towards a possibility of designing novel therapeutic strategies to restrict the pathogenesis of devastating human poly(Q) disorders.

Hat1 acetylates histone H4 and modulates the transcriptional program in Drosophila embryogenesis

Post-translational modifications of histone proteins play a pivotal role in DNA packaging and regulation of genome functions. Histone acetyltransferase 1 (Hat1) proteins are conserved enzymes that modify histones by acetylating lysine residues. Hat1 is implicated in chromatin assembly and DNA repair but its role in cell functions is not clearly elucidated. We report the generation and characterization of a Hat1 loss-of-function mutant in Drosophila. Hat1 mutants are viable and fertile with a mild sub-lethal phenotype showing that Hat1 is not essential in fruit flies. Lack of Hat1 results in the near complete loss of histone H4 lysine (K) 5 and K12 acetylation in embryos, indicating that Hat1 is the main acetyltransferase specific for these marks in this developmental stage. We found that Hat1 function and the presence of these acetyl marks are not required for the nuclear transport of histone H4 as histone variant His4r retained its nuclear localization both in Hat1 mutants and in His4r-K5R-K12R double point mutants. RNA-seq analysis of embryos indicate that in Hat1 mutants over 2000 genes are dysregulated and the observed transcriptional changes imply a delay in the developmental program of gene expression.

Expression of Human Mutant Huntingtin Protein in Drosophila Hemocytes Impairs Immune Responses

The pathogenic effect of mutant HTT (mHTT) which causes Huntington disease (HD) are not restricted to nervous system. Such phenotypes include aberrant immune responses observed in the HD models. However, it is still unclear how this immune dysregulation influences the innate immune response against pathogenic infection. In the present study, we used transgenic Drosophila melanogaster expressing mutant HTT protein (mHTT) with hemocyte-specific drivers and examined the immune responses and hemocyte function. We found that mHTT expression in the hemocytes did not affect fly viability, but the numbers of circulating hemocytes were significantly decreased. Consequently, we observed that the expression of mHTT in the hemocytes compromised the immune responses including clot formation and encapsulation which lead to the increased susceptibility to entomopathogenic nematode and parasitoid wasp infections. In addition, mHTT expression in Drosophila macrophage-like S2 cells in vitro reduced ATP levels, phagocytic activity and the induction of antimicrobial peptides. Further effects observed in mHTT-expressing cells included the altered production of cytokines and activation of JAK/STAT signaling. The present study shows that the expression of mHTT in Drosophila hemocytes causes deficient cellular and humoral immune responses against invading pathogens. Our findings provide the insight into the pathogenic effects of mHTT in the immune cells.

A genetically encoded 19F NMR probe for lysine acetylation

Advances in acetylated protein-protein/DNA interactions depend on the development of a novel NMR (nuclear magnetic resonance) probe to study the conformational changes of acetylated proteins. However, the method for detecting the acetylated protein conformation is underdeveloped. Herein, an acetyllysine mimic has been exploited for detecting the conformational changes of acetylated p53-protein/DNA interactions by genetic code expansion and 19F NMR. This 19F NMR probe shows high structural similarity to acetyllysine and could not be deacetylated by sirtuin deacetylase in vitro/vivo. Moreover, acetylation of p53 K164 is reported to be deacetylated by SIRT2 for the first time.

Transcriptional Dysregulation and Post-translational Modifications in Polyglutamine Diseases: From Pathogenesis to Potential Therapeutic Strategies

Polyglutamine (polyQ) diseases are hereditary neurodegenerative disorders caused by an abnormal expansion of a trinucleotide CAG repeat in the coding region of their respective associated genes. PolyQ diseases mainly display progressive degeneration of the brain and spinal cord. Nine polyQ diseases are known, including Huntington's disease (HD), spinal and bulbar muscular atrophy (SBMA), dentatorubral-pallidoluysian atrophy (DRPLA), and six forms of spinocerebellar ataxia (SCA). HD is the best characterized polyQ disease. Many studies have reported that transcriptional dysregulation and post-translational disruptions, which may interact with each other, are central features of polyQ diseases. Post-translational modifications, such as the acetylation of histones, are closely associated with the regulation of the transcriptional activity. A number of groups have studied the interactions between the polyQ proteins and transcription factors. Pharmacological drugs or genetic manipulations aimed at correcting the dysregulation have been confirmed to be effective in the treatment of polyQ diseases in many animal and cellular models. For example, histone deaceylase inhibitors have been demonstrated to have beneficial effects in cases of HD, SBMA, DRPLA, and SCA3. In this review, we describe the transcriptional and post-translational dysregulation in polyQ diseases with special focus on HD, and we summarize and comment on potential treatment approaches targeting disruption of transcription and post-translation processes in these diseases.

Cognitive dysfunction in Huntington's disease: mechanisms and therapeutic strategies beyond BDNF

One of the main focuses in Huntington's disease (HD) research, as well as in most neurodegenerative diseases, is the development of new therapeutic strategies, as currently there is no treatment to delay or prevent the progression of the disease. Neuronal dysfunction and neuronal death in HD are caused by a combination of interrelated pathogenic processes that lead to motor, cognitive and psychiatric symptoms. Understanding how mutant huntingtin impacts on a plethora of cellular functions could help to identify new molecular targets. Although HD has been classically classified as a neurodegenerative disease affecting voluntary movement, lately cognitive dysfunction is receiving increased attention as it is very invalidating for patients. Thus, an ambitious goal in HD research is to find altered molecular mechanisms that contribute to cognitive decline. In this review, we have focused on those findings related to corticostriatal and hippocampal cognitive dysfunction in HD, as well as on the underlying molecular mechanisms, which constitute potential therapeutic targets. These include alterations in synaptic plasticity, transcriptional machinery and neurotrophic and neurotransmitter signaling.

Epigenetic profiles in polyglutamine disorders

The dominant polyglutamine (polyQ) disorders are a group of progressive and incurable neurodegenerative disorders, which are caused by unstable expanded CAG trinucleotide repeats in the coding regions of their respective causative genes. The most prevalent polyQ disorders worldwide are Huntington’s disease and spinocerebellar ataxia type 3. Epigenetic mechanisms, such as DNA methylation, histone modifications and chromatin remodeling and noncoding RNA regulation, regulate gene expression or genome function. Epigenetic dysregulation has been suggested to play a pivotal role in the pathogenesis of polyQ disorders. Here, we summarize the current knowledge of epigenetic changes present in several representative polyQ disorders and discuss the potentiality of miRNAs as therapeutic targets for the clinic therapy of these disorders.

Spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is an adult-onset degenerative disorder of the neuromuscular system resulting in slowly progressive weakness and atrophy of the proximal limb and bulbar muscles. The disease is caused by the expansion of a CAG/glutamine tract in the amino-terminus of the androgen receptor. That SBMA exclusively affects males reflects the fact that critical pathogenic events are hormone-dependent. These include translocation of the polyglutamine androgen receptor from the cytoplasm to the nucleus and unfolding of the mutant protein. Studies of the pathology of SBMA subjects have revealed nuclear aggregates of the mutant androgen receptor, loss of lower motor neurons in the brainstem and spinal cord, and both neurogenic and myopathic changes in skeletal muscle. Mechanisms underlying disease pathogenesis include toxicity in both lower motor neurons and skeletal muscle, where effects on transcription, intracellular transport, and mitochondrial function have been documented. Therapies to treat SBMA patients remain largely supportive, although experimental approaches targeting androgen action or promoting degradation of the mutant androgen receptor protein or the encoding RNA are under active study.

The CAG-polyglutamine repeat diseases: a clinical, molecular, genetic, and pathophysiologic nosology

Throughout the genome, unstable tandem nucleotide repeats can expand to cause a variety of neurologic disorders. Expansion of a CAG triplet repeat within a coding exon gives rise to an elongated polyglutamine (polyQ) tract in the resultant protein product, and accounts for a unique category of neurodegenerative disorders, known as the CAG-polyglutamine repeat diseases. The nine members of the CAG-polyglutamine disease family include spinal and bulbar muscular atrophy (SBMA), Huntington disease, dentatorubral pallidoluysian atrophy, and six spinocerebellar ataxias (SCA 1, 2, 3, 6, 7, and 17). All CAG-polyglutamine diseases are dominantly inherited, with the exception of SBMA, which is X-linked, and many CAG-polyglutamine diseases display anticipation, which is defined as increasing disease severity in successive generations of an affected kindred. Despite widespread expression of the different polyQ-expanded disease proteins throughout the body, each CAG-polyglutamine disease strikes a particular subset of neurons, although the mechanism for this cell-type selectivity remains poorly understood. While the different genes implicated in these disorders display amino acid homology only in the repeat tract domain, certain pathologic molecular processes have been implicated in almost all of the CAG-polyglutamine repeat diseases, including protein aggregation, proteolytic cleavage, transcription dysregulation, autophagy impairment, and mitochondrial dysfunction. Here we highlight the clinical and molecular genetic features of each distinct disorder, and then discuss common themes in CAG-polyglutamine disease pathogenesis, closing with emerging advances in therapy development.

Pharmacological Therapies for Machado-Joseph Disease

Machado-Joseph disease (MJD), also known as Spinocerebellar Ataxia type 3 (SCA3), is the most common autosomal dominant ataxia worldwide. MJD integrates a large group of disorders known as polyglutamine diseases (polyQ). To date, no effective treatment exists for MJD and other polyQ diseases. Nevertheless, researchers are making efforts to find treatment possibilities that modify the disease course or alleviate disease symptoms. Since neuroimaging studies in mutation carrying individuals suggest that in nervous system dysfunction begins many years before the onset of any detectable symptoms, the development of therapeutic interventions becomes of great importance, not only to slow progression of manifest disease but also to delay, or ideally prevent, its onset. Potential therapeutic targets for MJD and polyQ diseases can be divided into (i) those that are aimed at the polyQ proteins themselves, namely gene silencing, attempts to enhance mutant protein degradation or inhibition/prevention of aggregation; and (ii) those that intercept the toxic downstream effects of the polyQ proteins, such as mitochondrial dysfunction and oxidative stress, transcriptional abnormalities, UPS impairment, excitotoxicity, or activation of cell death. The existence of relevant animal models and the recent contributions towards the identification of putative molecular mechanisms underlying MJD are impacting on the development of new drugs. To date only a few preclinical trials were conducted, nevertheless some had very promising results and some candidate drugs are close to being tested in humans. Clinical trials for MJD are also very few to date and their results not very promising, mostly due to trial design constraints. Here, we provide an overview of the pharmacological therapeutic strategies for MJD studied in animal models and patients, and of their possible translation into the clinical practice.

Remodeling of heterochromatin structure slows neuropathological progression and prolongs survival in an animal model of Huntington's disease

Huntington's disease (HD) is an autosomal-dominant inherited neurological disorder caused by expanded CAG repeats in exon 1 of the Huntingtin (HTT) gene. Altered histone modifications and epigenetic mechanisms are closely associated with HD suggesting that transcriptional repression may play a pathogenic role. Epigenetic compounds have significant therapeutic effects in cellular and animal models of HD, but they have not been successful in clinical trials. Herein, we report that dSETDB1/ESET, a histone methyltransferase (HMT), is a mediator of mutant HTT-induced degeneration in a fly HD model. We found that nogalamycin, an anthracycline antibiotic and a chromatin remodeling drug, reduces trimethylated histone H3K9 (H3K9me3) levels and pericentromeric heterochromatin condensation by reducing the expression of Setdb1/Eset. H3K9me3-specific ChIP-on-ChIP analysis identified that the H3K9me3-enriched epigenome signatures of multiple neuronal pathways including Egr1, Fos, Ezh1, and Arc are deregulated in HD transgenic (R6/2) mice. Nogalamycin modulated the expression of the H3K9me3-landscaped epigenome in medium spiny neurons and reduced mutant HTT nuclear inclusion formation. Moreover, nogalamycin slowed neuropathological progression, preserved motor function, and extended the life span of R6/2 mice. Together, our results indicate that modulation of SETDB1/ESET and H3K9me3-dependent heterochromatin plasticity is responsible for the neuroprotective effects of nogalamycin in HD and that small compounds targeting dysfunctional histone modification and epigenetic modification by SETDB1/ESET may be a rational therapeutic strategy in HD.

Post-translational Modifications and Protein Quality Control in Motor Neuron and Polyglutamine Diseases

Neurodegenerative diseases, including motor neuron and polyglutamine (polyQ) diseases, are a broad class of neurological disorders. These diseases are characterized by neuronal dysfunction and death, and by the accumulation of toxic aggregation-prone proteins in the forms of inclusions and micro-aggregates. Protein quality control is a cellular mechanism to reduce the burden of accumulation of misfolded proteins, a function that results from the coordinated actions of chaperones and degradation systems, such as the ubiquitin-proteasome system (UPS) and autophagy-lysosomal degradation system. The rate of turnover, aggregation and degradation of the disease-causing proteins is modulated by post-translational modifications (PTMs), such as phosphorylation, arginine methylation, palmitoylation, acetylation, SUMOylation, ubiquitination, and proteolytic cleavage. Here, we describe how PTMs of proteins linked to motor neuron and polyQ diseases can either enhance or suppress protein quality control check and protein aggregation and degradation. The identification of molecular strategies targeting these modifications may offer novel avenues for the treatment of these yet incurable diseases.

Safety and efficacy of valproic acid treatment in SCA3/MJD patients

BACKGROUND

Spinocerebellar ataxia type 3/Machado-Joseph disease (SCA3/MJD) is one of 10 known polyglutamine (polyQ) diseases. In Drosophila and rat models of polyQ diseases, histone deacetylation (HDAC) inhibitors improved locomotor function and survival time by increasing histone acetylation levels and modulating gene expression. Valproic acid (VPA) is a pan-HDAC inhibitor used clinically to treat bipolar and seizure disorders. We evaluated the clinical safety and efficacy of VPA treatment for SCA3/MJD patients.

METHODS

First, a randomized, open-label, dose-escalation method was used to evaluate tolerance to single-dose VPA administration in 12 SCA3/MJD patients. Patients were randomly assigned to three groups of four subjects, each with an oral dosage of 400 mg, 600 mg, or 800 mg (twice daily (bid) for one day). VPA was well-tolerated for one-dose by all patient groups. Second, a randomized, double-blind, placebo-controlled, dose-controlled study evaluated the safety and efficacy of multi-dose VPA (oral administration, twice daily (bid) for 12 weeks) in 36 SCA3/MJD patients. Patients received either low-dose VPA (800 mg/day), high-dose VPA (1200 mg/day), or placebo (n = 12 subjects per group). Symptoms were evaluated using the Scale for Assessment and Rating of Ataxia (SARA).

RESULTS

Multi-dose VPA treatment improved SARA measures of locomotor function. Major adverse effects included dizziness and loss of appetite.

CONCLUSIONS

VPA is a potentially beneficial agent for the treatment of SCA3/MJD. These results also provide insight into possible future therapeutics for polyQ diseases.

Quantitative Measurement of Histone Tail Acetylation Reveals Stage-Specific Regulation and Response to Environmental Changes during Drosophila Development

Histone modification plays a major role in regulating gene transcription and ensuring the healthy development of an organism. Numerous studies have suggested that histones are dynamically modified during developmental events to control gene expression levels in a temporal and spatial manner. However, the study of histone acetylation dynamics using currently available techniques is hindered by the difficulty of simultaneously measuring acetylation of the numerous potential sites of modification present in histones. Here, we present a methodology that allows us to combine mass spectrometry-based histone analysis with Drosophila developmental genetics. Using this system, we characterized histone acetylation patterns during multiple developmental stages of the fly. Additionally, we utilized this analysis to characterize how treatments with pharmacological agents or environmental changes such as γ-irradiation altered histone acetylation patterns. Strikingly, γ-irradiation dramatically increased the level of acetylation at H3K18, a site linked to DNA repair via nonhomologous end joining. In mutant fly strains deficient in DNA repair proteins, however, this increase in the level of H3K18 acetylation was lost. These results demonstrate the efficacy of our combined mass spectrometry system with a Drosophila model system and provide interesting insight into the changes in histone acetylation during development, as well as the effects of both pharmacological and environmental agents on global histone acetylation.

Drosophila Eye Model to Study Neuroprotective Role of CREB Binding Protein (CBP) in Alzheimer's Disease

BACKGROUND

The progressive neurodegenerative disorder Alzheimer's disease (AD) manifests as loss of cognitive functions, and finally leads to death of the affected individual. AD may result from accumulation of amyloid plaques. These amyloid plaques comprising of amyloid-beta 42 (Aβ42) polypeptides results from the improper cleavage of amyloid precursor protein (APP) in the brain. The Aβ42 plaques have been shown to disrupt the normal cellular processes and thereby trigger abnormal signaling which results in the death of neurons. However, the molecular-genetic mechanism(s) responsible for Aβ42 mediated neurodegeneration is yet to be fully understood.

METHODOLOGY/PRINCIPAL FINDINGS

We have utilized Gal4/UAS system to develop a transgenic fruit fly model for Aβ42 mediated neurodegeneration. Targeted misexpression of human Aβ42 in the differentiating photoreceptor neurons of the developing eye of transgenic fly triggers neurodegeneration. This progressive neurodegenerative phenotype resembles Alzheimer's like neuropathology. We identified a histone acetylase, CREB Binding Protein (CBP), as a genetic modifier of Aβ42 mediated neurodegeneration. Targeted misexpression of CBP along with Aβ42 in the differentiating retina can significantly rescue neurodegeneration. We found that gain-of-function of CBP rescues Aβ42 mediated neurodegeneration by blocking cell death. Misexpression of Aβ42 affects the targeting of axons from retina to the brain but misexpression of full length CBP along with Aβ42 can restore this defect. The CBP protein has multiple domains and is known to interact with many different proteins. Our structure function analysis using truncated constructs lacking one or more domains of CBP protein, in transgenic flies revealed that Bromo, HAT and polyglutamine (BHQ) domains together are required for the neuroprotective function of CBP. This BHQ domain of CBP has not been attributed to promote survival in any other neurodegenerative disorders.

CONCLUSIONS/SIGNIFICANCE

We have identified CBP as a genetic modifier of Aβ42 mediated neurodegeneration. Furthermore, we have identified BHQ domain of CBP is responsible for its neuroprotective function. These studies may have significant bearing on our understanding of genetic basis of AD.

Nemo-like kinase is a novel regulator of spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is a progressive neuromuscular disease caused by polyglutamine expansion in the androgen receptor (AR) protein. Despite extensive research, the exact pathogenic mechanisms underlying SBMA remain elusive. In this study, we present evidence that Nemo-like kinase (NLK) promotes disease pathogenesis across multiple SBMA model systems. Most remarkably, loss of one copy of Nlk rescues SBMA phenotypes in mice, including extending lifespan. We also investigated the molecular mechanisms by which NLK exerts its effects in SBMA. Specifically, we have found that NLK can phosphorylate the mutant polyglutamine-expanded AR, enhance its aggregation, and promote AR-dependent gene transcription by regulating AR-cofactor interactions. Furthermore, NLK modulates the toxicity of a mutant AR fragment via a mechanism that is independent of AR-mediated gene transcription. Our findings uncover a crucial role for NLK in controlling SBMA toxicity and reveal a novel avenue for therapy development in SBMA.

DOI:http://dx.doi.org/10.7554/eLife.08493.001

Spinal and bulbar muscular atrophy (SBMA) is an inherited disease that eventually leads to degeneration in motor neurons and weakness in muscles. It is caused by a specific genetic mutation in the gene that encodes the androgen receptor protein, which leads to the production of a mutant protein that is larger than normal. Similar mutations in other genes can lead to the development of other so-called ‘polyglutamine’ diseases such as Huntington's disease and spinocerebellar ataxia. However, the precise details of how these mutations lead to disease symptoms are not known, and there are currently no effective ways of treating these conditions.

Previous research has shown that an enzyme called Nemo-like kinase (or NLK for short) regulates the normal androgen receptor in cancer cells. NLK has kinase activity, that is, it adds phosphate molecules to other proteins to regulate their activity. Todd et al. used human cells, fruit flies, and mice as model systems to investigate whether NLK is involved in the development of SBMA.

The experiments show that NLK promotes the development of features associated with SBMA in all three models. The kinase activity of NLK is required for these features to develop. Todd et al. also found that NLK can bind to and add phosphate molecules to the mutant version of the androgen receptor protein. This causes the mutant androgen receptor proteins to accumulate and increases the ability of the mutant proteins to activate particular genes.

Todd et al.'s findings suggest that NLK promotes the development of SBMA by interacting with the mutant androgen receptor. Previous studies have shown that NLK is able to modulate the development of spinocerebellar ataxia type 1, which suggests that NLK may also play an important role in other polyglutamine diseases. The next challenge will be to fully understand the role of NLK in these diseases, which may aid future efforts to develop new treatments.

DOI:http://dx.doi.org/10.7554/eLife.08493.002

Eusocial insects as emerging models for behavioural epigenetics

Understanding the molecular basis of how behavioural states are established, maintained and altered by environmental cues is an area of considerable and growing interest. Epigenetic processes, including methylation of DNA and post-translational modification of histones, dynamically modulate activity-dependent gene expression in neurons and can therefore have important regulatory roles in shaping behavioural responses to environmental cues. Several eusocial insect species - with their unique displays of behavioural plasticity due to age, morphology and social context - have emerged as models to investigate the genetic and epigenetic underpinnings of animal social behaviour. This Review summarizes recent studies in the epigenetics of social behaviour and offers perspectives on emerging trends and prospects for establishing genetic tools in eusocial insects.

Using membrane-targeted green fluorescent protein to monitor neurotoxic protein-dependent degeneration of Drosophila eyes

Age-related neurodegeneration has been studied extensively through the use of model organisms, including the genetically versatile Drosophila melanogaster. Various neurotoxic proteins have been expressed in fly eyes to approximate degeneration occurring in humans, and much has been learned from this heterologous system. Although Drosophila expedites scientific research through rapid generational times and relative inexpensiveness, one factor that can hinder analyses is the examination of milder forms of degeneration caused by some toxic proteins in fly eyes. Whereas several disease proteins cause massive degeneration that is easily observed by examining the external structure of the fly eye, others cause mild degeneration that is difficult to observe externally and requires laborious histological preparation to assess and monitor. Here, we describe a sensitive fluorescence-based method to observe, monitor, and quantify mild Drosophila eye degeneration caused by various proteins, including the polyglutamine disease proteins ataxin-3 (spinocerebellar ataxia type 3) and huntingtin (Huntington's disease), mutant α-synuclein (Parkinson's disease), and Aβ42 (Alzheimer's disease). We show that membrane-targeted green fluorescent protein reports degeneration robustly and quantitatively. This simple yet powerful technique, which is amenable to large-scale screens, can help accelerate studies to understand age-related degeneration and to find factors that suppress it for therapeutic purposes.

Histone acetylation and CREB binding protein are required for neuronal resistance against ischemic injury

Epigenetic transcriptional regulation by histone acetylation depends on the balance between histone acetyltransferase (HAT) and deacetylase activities (HDAC). Inhibition of HDAC activity provides neuroprotection, indicating that the outcome of cerebral ischemia depends crucially on the acetylation status of histones. In the present study, we characterized the changes in histone acetylation levels in ischemia models of focal cerebral ischemia and identified cAMP-response element binding protein (CREB)–binding protein (CBP) as a crucial factor in the susceptibility of neurons to ischemic stress. Both neuron-specific RNA interference and neurons derived from CBP heterozygous knockout mice showed increased damage after oxygen-glucose deprivation (OGD) in vitro. Furthermore, we demonstrated that ischemic preconditioning by a short (5 min) subthreshold occlusion of the middle cerebral artery (MCA), followed 24 h afterwards by a 30 min occlusion of the MCA, increased histone acetylation levels in vivo. Ischemic preconditioning enhanced CBP recruitment and histone acetylation at the promoter of the neuroprotective gene gelsolin leading to increased gelsolin expression in neurons. Inhibition of CBP's HAT activity attenuated neuronal ischemic preconditioning. Taken together, our findings suggest that the levels of CBP and histone acetylation determine stroke outcome and are crucially associated with the induction of an ischemia-resistant state in neurons.

Lysine acetyltransferases CBP and p300 as therapeutic targets in cognitive and neurodegenerative disorders

Neuropsychiatric pathologies, including neurodegenerative diseases and neurodevelopmental syndromes, are frequently associated with dysregulation of various essential cellular mechanisms, such as transcription, mitochondrial respiration and protein degradation. In these complex scenarios, it is difficult to pinpoint the specific molecular dysfunction that initiated the pathology or that led to the fatal cascade of events that ends with the death of the neuron. Among the possible original factors, epigenetic dysregulation has attracted special attention. This review focuses on two highly related epigenetic factors that are directly involved in a number of neurological disorders, the lysine acetyltransferases CREB-binding protein (CBP) and E1A-associated protein p300 (p300). We first comment on the role of chromatin acetylation and the enzymes that control it, particularly CBP and p300, in neuronal plasticity and cognition. Next, we describe the involvement of these proteins in intellectual disability and in different neurodegenerative diseases. Finally, we discuss the potential of ameliorative strategies targeting CBP/p300 for the treatment of these disorders.

Expansion of the polyQ repeats in THAP11 forms intranuclear aggregation and causes cell G0/G1 arrest

Polyglutamine diseases are a group of neurodegenerative disorders caused by expansion of a CAG repeat that encodes polyglutamine in each respective disease gene. The transcription factor THAP11, a member of THAP family, is involved in cell growth, ES cell pluripotency and embryogenesis. Previous studies suggest that THAP11 protein contains a 29-residue repeat polyglutamine motif and the number of polyglutamine ranges from 20 to 41 in Indian population. We have investigated the CAG numbers at the THAP11 locus in normal individuals and neurodegenerative disease patients of Chinese Han population and a 38Q expansion (THAP11(38Q)) was found in patients. Using fluorescence confocal-based cell imaging, THAP11(38Q) protein formed intranuclear inclusions easier than THAP11(29Q) in PC12 cells. Enhanced toxicity was investigated in THAP11(38Q)-expressing cells by growth inhibition and G0/G1 arrest. CREB-mediated transcription activity was inhibited by THAP11(38Q). The transcription factor, TBP, coactivator CBP, and chaperon protein, HSP70, could be recruited to THAP11(38Q). These results indicate that expansion of the polyglutamine in THAP11 forms intracellular aggregation and is toxic in PC12 cells, suggesting a putative role of THAP11 in polyglutamine disease.

Alleviating neurodegeneration in Drosophila models of PolyQ diseases

Polyglutamine (polyQ) diseases are a group of neurodegenerative conditions, induced from CAG trinucleotide repeat expansion within causative gene respectively. Generation of toxic proteins, containing polyQ-expanded tract, is the key process to cause neurodegeneration. Till now, although polyQ diseases remain uncurable, numerous therapeutic strategies with great potential have been examined and have been proven to be effective against polyQ diseases, including diverse small biological molecules and many pharmacological compounds mainly through prevention on formation of aggregates and inclusions, acceleration on degradation of toxic proteins and regulation of cellular function. We review promising therapeutic strategies by using Drosophila models of polyQ diseases including HD, SCA1, SCA3 and SBMA.

Pathogenic mechanisms and therapeutic strategies in spinobulbar muscular atrophy

We review the genetic and clinical features of spinobulbar muscular atrophy (SBMA), a progressive neuromuscular disorder caused by a CAG/glutamine tract expansion in the androgen receptor. SBMA was the first polyglutamine disease to be discovered, and we compare and contrast it with related degenerative disorders of the nervous system caused by expanded glutamine tracts. We review the cellular and animals models that have been most widely used to study this disorder, and highlight insights into disease pathogenesis derived from this work. These model systems have revealed critical aspects of the disease, including its hormone dependence, a feature that underlies disease occurrence only in men with the mutant allele. We discuss how this and other findings have been translated to clinical trials for SBMA patients, and examine emerging therapeutic targets that have been identified by recent work.

Drosophila Myc, a novel modifier suppresses the poly(Q) toxicity by modulating the level of CREB binding protein and histone acetylation

Polyglutamine or poly(Q) disorders are dominantly inherited neurodegenerative diseases characterised by progressive loss of neurons in cerebellum, basal ganglia and cortex in adult human brain. Overexpression of human form of mutant SCA3 protein with 78 poly(Q) repeats leads to the formation of inclusion bodies and increases the cellular toxicity in Drosophila eye. The present study was directed to identify a genetic modifier of poly(Q) diseases that could be utilised as a potential drug target. The initial screening process was influenced by the fact of lower prevalence of cancer among patients suffering with poly(Q) disorders which appears to be related to the intrinsic biological factors. We investigated if Drosophila Myc (a homologue of human cMyc proto-oncogene) harbours intrinsic property of suppressing cellular toxicity induced by an abnormally long stretch of poly(Q). We show for the first time that targeted overexpression of Drosophila Myc (dMyc) mitigates the poly(Q) toxicity in eye and nervous systems. Upregulation of dMyc results in a significant reduction in accumulation of inclusion bodies with residual poly(Q) aggregates localising into cytoplasm. We demonstrate that dMyc mediated suppression of poly(Q) toxicity is achieved by alleviating the cellular level of CBP and improved histone acetylation, resulting restoration of transcriptional machinery which are otherwise abbreviated due to poly(Q) disease conditions. Moreover, our study also provides a rational justification of the enigma of poly(Q) patients showing resistance to the predisposition of cancer.

What's wrong with epigenetics in Huntington's disease?

Huntington's disease (HD) can be considered the paradigm of epigenetic dysregulation in neurodegenerative disorders. In this review, we attempted to compile the evidence that indicates, on the one hand, that several epigenetic marks (histone acetylation, methylation, ubiquitylation, phosphorylation and DNA modifications) are altered in multiple models and in postmortem patient samples, and on the other hand, that pharmacological treatments aimed to reverse such alterations have beneficial effects on HD phenotypic and biochemical traits. However, the working hypotheses regarding the biological significance of epigenetic dysregulation in this disease and the mechanisms of action of the tested ameliorative strategies need to be refined. Understanding the complexity of the epigenetics in HD will provide useful insights to examine the role of epigenetic dysregulation in other neuropathologies, such as Alzheimer's or Parkinson's diseases.

Acetyltransferases (HATs) as targets for neurological therapeutics

The acetylation of histone and non-histone proteins controls a great deal of cellular functions, thereby affecting the entire organism, including the brain. Acetylation modifications are mediated through histone acetyltransferases (HAT) and deacetylases (HDAC), and the balance of these enzymes regulates neuronal homeostasis, maintaining the pre-existing acetyl marks responsible for the global chromatin structure, as well as regulating specific dynamic acetyl marks that respond to changes and facilitate neurons to encode and strengthen long-term events in the brain circuitry (e.g., memory formation). Unfortunately, the dysfunction of these finely-tuned regulations might lead to pathological conditions, and the deregulation of the HAT/HDAC balance has been implicated in neurological disorders. During the last decade, research has focused on HDAC inhibitors that induce a histone hyperacetylated state to compensate acetylation deficits. The use of these inhibitors as a therapeutic option was efficient in several animal models of neurological disorders. The elaboration of new cell-permeant HAT activators opens a new era of research on acetylation regulation. Although pathological animal models have not been tested yet, HAT activator molecules have already proven to be beneficial in ameliorating brain functions associated with learning and memory, and adult neurogenesis in wild-type animals. Thus, HAT activator molecules contribute to an exciting area of research.

Purification of transcripts and metabolites from Drosophila heads

For the last decade, we have tried to understand the molecular and cellular mechanisms of neuronal degeneration using Drosophila as a model organism. Although fruit flies provide obvious experimental advantages, research on neurodegenerative diseases has mostly relied on traditional techniques, including genetic interaction, histology, immunofluorescence, and protein biochemistry. These techniques are effective for mechanistic, hypothesis-driven studies, which lead to a detailed understanding of the role of single genes in well-defined biological problems. However, neurodegenerative diseases are highly complex and affect multiple cellular organelles and processes over time. The advent of new technologies and the omics age provides a unique opportunity to understand the global cellular perturbations underlying complex diseases. Flexible model organisms such as Drosophila are ideal for adapting these new technologies because of their strong annotation and high tractability. One challenge with these small animals, though, is the purification of enough informational molecules (DNA, mRNA, protein, metabolites) from highly relevant tissues such as fly brains. Other challenges consist of collecting large numbers of flies for experimental replicates (critical for statistical robustness) and developing consistent procedures for the purification of high-quality biological material. Here, we describe the procedures for collecting thousands of fly heads and the extraction of transcripts and metabolites to understand how global changes in gene expression and metabolism contribute to neurodegenerative diseases. These procedures are easily scalable and can be applied to the study of proteomic and epigenomic contributions to disease.

New routes to therapy for spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA), also known as Kennedy's disease, is a genetically inherited neuromuscular disorder characterized by loss of lower motor neurons in the brainstem and spinal cord and skeletal muscle fasciculation, weakness, and atrophy. SBMA is caused by expansion of a polyglutamine (polyQ) tract in the gene coding for the androgen receptor (AR). PolyQ expansions cause at least eight other neurological disorders, which are collectively known as polyQ diseases. SBMA is unique in the family of polyQ diseases in that the disease manifests fully in male individuals only. The sex specificity of SBMA is the result of the interaction between mutant AR and its natural ligand, testosterone. Here, we will discuss emerging therapeutic perspectives for SBMA in light of recent findings regarding disease pathogenesis.

Sodium valproate alleviates neurodegeneration in SCA3/MJD via suppressing apoptosis and rescuing the hypoacetylation levels of histone H3 and H4

Spinocerebellar ataxia type 3 (SCA3) also known as Machado-Joseph Disease (MJD), is one of nine polyglutamine (polyQ) diseases caused by a CAG-trinucelotide repeat expansion within the coding sequence of the ATXN3 gene. There are no disease-modifying treatments for polyQ diseases. Recent studies suggest that an imbalance in histone acetylation may be a key process leading to transcriptional dysregulation in polyQ diseases. Because of this possible imbalance, the application of histone deacetylase (HDAC) inhibitors may be feasible for the treatment of polyQ diseases. To further explore the therapeutic potential of HDAC inhibitors, we constructed two independent preclinical trials with valproic acid (VPA), a promising therapeutic HDAC inhibitor, in both Drosophila and cell SCA3 models. We demonstrated that prolonged use of VPA at specific dose partly prevented eye depigmentation, alleviated climbing disability, and extended the average lifespan of SCA3/MJD transgenic Drosophila. We found that VPA could both increase the acetylation levels of histone H3 and histone H4 and reduce the early apoptotic rate of cells without inhibiting the aggregation of mutant ataxin-3 proteins in MJDtr-Q68- expressing cells. These results collectively support the premise that VPA is a promising therapeutic agent for the treatment of SCA3 and other polyQ diseases.

Oral administration of the pimelic diphenylamide HDAC inhibitor HDACi 4b is unsuitable for chronic inhibition of HDAC activity in the CNS in vivo

Histone deacetylase (HDAC) inhibitors have received considerable attention as potential therapeutics for a variety of cancers and neurological disorders. Recent publications on a class of pimelic diphenylamide HDAC inhibitors have highlighted their promise in the treatment of the neurodegenerative diseases Friedreich’s ataxia and Huntington’s disease, based on efficacy in cell and mouse models. These studies’ authors have proposed that the unique action of these compounds compared to hydroxamic acid-based HDAC inhibitors results from their unusual slow-on/slow-off kinetics of binding, preferentially to HDAC3, resulting in a distinctive pharmacological profile and reduced toxicity. Here, we evaluate the HDAC subtype selectivity, cellular activity, absorption, distribution, metabolism and excretion (ADME) properties, as well as the central pharmacodynamic profile of one such compound, HDACi 4b, previously described to show efficacy in vivo in the R6/2 mouse model of Huntington’s disease. Based on our data reported here, we conclude that while the in vitro selectivity and binding mode are largely in agreement with previous reports, the physicochemical properties, metabolic and p-glycoprotein (Pgp) substrate liability of HDACi 4b render this compound suboptimal to investigate central Class I HDAC inhibition in vivo in mouse per oral administration. A drug administration regimen using HDACi 4b dissolved in drinking water was used in the previous proof of concept study, casting doubt on the validation of CNS HDAC3 inhibition as a target for the treatment of Huntington’s disease. We highlight physicochemical stability and metabolic issues with 4b that are likely intrinsic liabilities of the benzamide chemotype in general.

Polyglutamine (polyQ) disorders: the chromatin connection

Polyglutamine (PolyQ)-related diseases are dominant late-onset genetic disorders that are manifested by progressive neurodegeneration, leading to behavioral and physical impairments. An increased body of evidence suggests that chromatin structure and epigenetic regulation are involved in disease pathology. PolyQ diseases often display an aberrant transcriptional regulation due to the disrupted function of histone-modifying complexes and altered interactions of the polyQ-extended proteins with chromatin-related factors. In this review we describe recent findings relating to the role of chromatin in polyQ diseases. We discuss the involvement of epigenetic-related factors and chromatin structure in genomic instability of CAG repeats; we describe changes in the expression and regulation of chromatin-related enzymes and in the levels and patterns of histone modifications in disease state; we illustrate the potential beneficial effects of different histone deacetylase (HDAC) inhibitors for the treatment of polyQ diseases, and we end by describing the potential use of human pluripotent stem cells and their differentiated derivatives for modeling polyQ diseases in vitro. Taken together, these accumulating studies strongly suggest that disrupted chromatin regulation may be directly involved with the pathophysiology of polyQ-related diseases.

LANP mediates neuritic pathology in Spinocerebellar ataxia type 1

Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant neurodegenerative disease that results from a pathogenic glutamine-repeat expansion in the protein ataxin-1 (ATXN1). Although the functions of ATXN1 are still largely unknown, there is evidence to suggest that ATXN1 plays a role in regulating gene expression, the earliest process known to go awry in SCA1 mouse models. In this study, we show that ATXN1 reduces histone acetylation, a post-translational modification of histones associated with enhanced transcription, and represses histone acetyl transferase-mediated transcription. In addition, we find that depleting the Leucine-rich Acidic Nuclear Protein (LANP)-an ATXN1 binding inhibitor of histone acetylation-reverses aspects of SCA1 neuritic pathology.

Current status of treatment of spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is the first member identified among polyglutamine diseases characterized by slowly progressive muscle weakness and atrophy of the bulbar, facial, and limb muscles pathologically associated with motor neuron loss in the spinal cord and brainstem. Androgen receptor (AR), a disease-causing protein of SBMA, is a well-characterized ligand-activated transcription factor, and androgen binding induces nuclear translocation, conformational change and recruitment of coregulators for transactivation of AR target genes. Some therapeutic strategies for SBMA are based on these native functions of AR. Since ligand-induced nuclear translocation of mutant AR has been shown to be a critical step in motor neuron degeneration in SBMA, androgen deprivation therapies using leuprorelin and dutasteride have been developed and translated into clinical trials. Although the results of these trials are inconclusive, renewed clinical trials with more sophisticated design might prove the effectiveness of hormonal intervention in the near future. Furthermore, based on the normal function of AR, therapies targeted for conformational changes of AR including amino-terminal (N) and carboxy-terminal (C) (N/C) interaction and transcriptional coregulators might be promising. Other treatments targeted for mitochondrial function, ubiquitin-proteasome system (UPS), and autophagy could be applicable for all types of polyglutamine diseases.

SIRT1 modulates aggregation and toxicity through deacetylation of the androgen receptor in cell models of SBMA

Posttranslational protein modifications can play a major role in disease pathogenesis; phosphorylation, sumoylation, and acetylation modulate the toxicity of a variety of proteotoxic proteins. The androgen receptor (AR) is substantially modified, in response to hormone binding, by phosphorylation, sumoylation, and acetylation; these modifications might thus contribute to DHT-dependent polyglutamine (polyQ)-expanded AR proteotoxicity in spinal and bulbar muscular atrophy (SBMA). SIRT1, a nuclear protein and deacetylase of the AR, is neuroprotective in many neurodegenerative disease models. Our studies reveal that SIRT1 also offers protection against polyQ-expanded AR by deacetylating the AR at lysines 630/632/633. This finding suggested that nuclear AR acetylation plays a role in the aberrant metabolism and toxicity of polyQ-expanded AR. Subsequent studies revealed that the polyQ-expanded AR is hyperacetylated and that pharmacologic reduction of acetylation reduces mutant AR aggregation. Moreover, genetic mutation to inhibit polyQ-expanded AR acetylation of lysines 630/632/633 substantially decreased its aggregation and completely abrogated its toxicity in cell lines and motor neurons. Our studies also reveal one means by which the AR acetylation state likely modifies polyQ-expanded AR metabolism and toxicity, through its effect on DHT-dependent AR stabilization. Overall, our findings reveal a neuroprotective function of SIRT1 that operates through its deacetylation of polyQ-expanded AR and highlight the potential of both SIRT1 and AR acetylation as powerful therapeutic targets in SBMA.

Reduced neuronal expression of ribose-5-phosphate isomerase enhances tolerance to oxidative stress, extends lifespan, and attenuates polyglutamine toxicity in Drosophila

Aging and age-related diseases can be viewed as the result of the lifelong accumulation of stress insults. The identification of mutant strains and genes that are responsive to stress and can alter longevity profiles provides new therapeutic targets for age-related diseases. Here we reported that a Drosophila strain with reduced expression of ribose-5-phosphate isomerase (rpi), EP2456, exhibits increased resistance to oxidative stress and enhanced lifespan. In addition, the strain also displays higher levels of NADPH. The knockdown of rpi in neurons by double-stranded RNA interference recapitulated the lifespan extension and oxidative stress resistance in Drosophila. This manipulation was also found to ameliorate the effects of genetic manipulations aimed at creating a model for studying Huntington's disease by overexpression of polyglutamine in the eye, suggesting that modulating rpi levels could serve as a treatment for normal aging as well as for polyglutamine neurotoxicity.

Epigenetics in nucleotide repeat expansion disorders

Over the past 20 years, nucleotide repeat expansion disorders have informed our broader understanding of neurodevelopmental and neurodegenerative disease. This is especially true with regard to the contributions of epigenetic mechanisms to neurologic disease pathogenesis. In this review, the authors describe a few of the myriad ways in which epigenetic processes underlie aspects of repeat expansion disorder pathophysiology and discuss how therapies targeted at epigenetic modulation hold promise for many of these disorders.

ATRX induction by mutant huntingtin via Cdx2 modulates heterochromatin condensation and pathology in Huntington's disease

Aberrant chromatin remodeling is involved in the pathogenesis of Huntington's disease (HD) but the mechanism is not known. Herein, we report that mutant huntingtin (mtHtt) induces the transcription of alpha thalassemia/mental retardation X linked (ATRX), an ATPase/helicase and SWI/SNF-like chromatin remodeling protein via Cdx-2 activation. ATRX expression was elevated in both a cell line model and transgenic model of HD, and Cdx-2 occupancy of the ATRX promoter was increased in HD. Induction of ATRX expanded the size of promyelocytic leukemia nuclear body (PML-NB) and increased trimethylation of H3K9 (H3K9me3) and condensation of pericentromeric heterochromatin, while knockdown of ATRX decreased PML-NB and H3K9me3 levels. Knockdown of ATRX/dXNP improved the hatch rate of fly embryos expressing mtHtt (Q127). ATRX/dXNP overexpression exacerbated eye degeneration of eye-specific mtHtt (Q127) expressing flies. Our findings suggest that transcriptional alteration of ATRX by mtHtt is involved in pericentromeric heterochromatin condensation and contributes to the pathogenesis of HD.

Long-term memory deficits in Huntington's disease are associated with reduced CBP histone acetylase activity

Huntington's disease (HD) is an autosomal dominant progressive neurodegenerative disorder caused by an expanded CAG/polyglutamine repeat in the coding region of the huntingtin (htt) gene. Although HD is classically considered a motor disorder, there is now considerable evidence that early cognitive deficits appear in patients before the onset of motor disturbances. Here we demonstrate early impairment of long-term spatial and recognition memory in heterozygous HD knock-in mutant mice (Hdh(Q7/Q111)), a genetically accurate HD mouse model. Cognitive deficits are associated with reduced hippocampal expression of CREB-binding protein (CBP) and diminished levels of histone H3 acetylation. In agreement with reduced CBP, the expression of CREB/CBP target genes related to memory, such c-fos, Arc and Nr4a2, was significantly reduced in the hippocampus of Hdh(Q7/Q111) mice compared with wild-type mice. Finally, and consistent with a role of CBP in cognitive impairment in Hdh(Q7/Q111) mice, administration of the histone deacetylase inhibitor trichostatin A rescues recognition memory deficits and transcription of selective CREB/CBP target genes in Hdh(Q7/Q111) mice. These findings demonstrate an important role for CBP in cognitive dysfunction in HD and suggest the use of histone deacetylase inhibitors as a novel therapeutic strategy for the treatment of memory deficits in this disease.

Assessing the contribution of heterogeneous distributions of oligomers to aggregation mechanisms of polyglutamine peptides

Polyglutamine aggregation is associated with neurodegeneration in nine different disorders. The effects of polyglutamine length and peptide concentration on the kinetics of aggregation were previously analyzed using a homogeneous nucleation model that assumes the presence of a single bottleneck along the free energy profile G(n), where n denotes the number of polyglutamine molecules. The observation of stable, soluble oligomers as intermediates along aggregation pathways is refractory to the assumptions of homogeneous nucleation. Furthermore, the analysis of in vitro kinetic data using a specific variant of homogeneous nucleation leads to confounding observations such as fractional and/or negative values for estimates of the critical nucleus size. Here, we show that the homogeneous nucleation model is inherently robust and is unlikely to yield fractional values if the underlying process is strictly homogeneous with a free energy profile G(n) that displays a sharp maximum at n=n*, where n* corresponds to the critical nucleus. Conversely, a model that includes oligomers of different size and different potentials for supporting turnover into fibrils yields estimates of fractional and/or negative nucleus sizes when the kinetic data are analyzed using the assumption of a homogeneous process. This model provides a route to reconcile independent observations of heterogeneous distributions of oligomers and other non-fibrillar aggregates with results obtained from analysis of aggregation kinetics using the assumption of a homogeneous nucleation model. In the new model, the mechanisms of fibril assembly are governed by the relative stabilities of two types of oligomers viz., fibril-competent and fibril-incompetent oligomers, the size of the smallest fibril competent oligomer, and rates for conformational conversion within different oligomers.

Neurotoxic effects of androgens in spinal and bulbar muscular atrophy

Expansion of polyglutamine tracts in nine different genes causes selective neuronal degeneration through unknown mechanisms. Expansion of polyglutamine in the androgen receptor is responsible for spinal and bulbar muscular atrophy (SBMA), a neuromuscular disorder characterized by the loss of lower motor neurons in the brainstem and spinal cord. A unique feature of SBMA in the family of polyglutamine diseases is sex specificity. SBMA fully manifests only in males. SBMA is a disease triggered by the binding of polyglutamine androgen receptor to its natural ligand testosterone. Recent evidence has emerged showing that the expanded polyglutamine tract itself is not the only determinant of disease pathogenesis. There is evidence that both the native structure and function of the disease protein strongly influence the pathogenicity of mutant protein. Here, we review recent progress in the understanding of disease pathogenesis and advancements towards development of potential therapeutic strategies for SBMA.