Ataxin-3 is transported into the nucleus and associates with the nuclear matrix

ATXN3: a multifunctional protein involved in the polyglutamine disease spinocerebellar ataxia type 3

ATXN3 is a ubiquitin hydrolase (or deubiquitinase, DUB), product of the ATXN3 gene, ubiquitously expressed in various cell types including peripheral and neuronal tissues and involved in several cellular pathways. Importantly, the expansion of the CAG trinucleotides within the ATXN3 gene leads to an expanded polyglutamine domain in the encoded protein, which has been associated with the onset of the spinocerebellar ataxia type 3, also known as Machado-Joseph disease, the most common dominantly inherited ataxia worldwide. ATXN3 has therefore been under intensive investigation for decades. In this review, we summarize the main functions of ATXN3 in proteostasis, DNA repair and transcriptional regulation, as well as the emerging role in regulating chromatin structure. The mentioned molecular functions of ATXN3 are also reviewed in the context of the pathological expanded form of ATXN3.

The deubiquitinase function of ataxin-3 and its role in the pathogenesis of Machado-Joseph disease and other diseases

Machado-Joseph disease (MJD) is a devastating and incurable neurodegenerative disease characterised by progressive ataxia, difficulty speaking and swallowing. Consequently, affected individuals ultimately become wheelchair dependent, require constant care, and face a shortened life expectancy. The monogenic cause of MJD is expansion of a trinucleotide (CAG) repeat region within the ATXN3 gene, which results in polyglutamine (polyQ) expansion within the resultant ataxin-3 protein. While it is well established that the ataxin-3 protein functions as a deubiquitinating (DUB) enzyme and is therefore critically involved in proteostasis, several unanswered questions remain regarding the impact of polyQ expansion in ataxin-3 on its DUB function. Here we review the current literature surrounding ataxin-3's DUB function, its DUB targets, and what is known regarding the impact of polyQ expansion on ataxin-3's DUB function. We also consider the potential neuroprotective effects of ataxin-3's DUB function, and the intersection of ataxin-3's role as a DUB enzyme and regulator of gene transcription. Ataxin-3 is the principal pathogenic protein in MJD and also appears to be involved in cancer. As aberrant deubiquitination has been linked to both neurodegeneration and cancer, a comprehensive understanding of ataxin-3's DUB function is important for elucidating potential therapeutic targets in these complex conditions. In this review, we aim to consolidate knowledge of ataxin-3 as a DUB and unveil areas for future research to aid therapeutic targeting of ataxin-3's DUB function for the treatment of MJD and other diseases.

CBP-Mediated Acetylation of Importin α Mediates Calcium-Dependent Nucleocytoplasmic Transport of Selective Proteins in Drosophila Neurons

For proper function of proteins, their subcellular localization needs to be monitored and regulated in response to the changes in cellular demands. In this regard, dysregulation in the nucleocytoplasmic transport (NCT) of proteins is closely associated with the pathogenesis of various neurodegenerative diseases. However, it remains unclear whether there exists an intrinsic regulatory pathway(s) that controls NCT of proteins either in a commonly shared manner or in a target-selectively different manner. To dissect between these possibilities, in the current study, we investigated the molecular mechanism regulating NCT of truncated ataxin-3 (ATXN3) proteins of which genetic mutation leads to a type of polyglutamine (polyQ) diseases, in comparison with that of TDP-43. In Drosophila dendritic arborization (da) neurons, we observed dynamic changes in the subcellular localization of truncated ATXN3 proteins between the nucleus and the cytosol during development. Moreover, ectopic neuronal toxicity was induced by truncated ATXN3 proteins upon their nuclear accumulation. Consistent with a previous study showing intracellular calcium-dependent NCT of TDP-43, NCT of ATXN3 was also regulated by intracellular calcium level and involves Importin α3 (Imp α3). Interestingly, NCT of ATXN3, but not TDP-43, was primarily mediated by CBP. We further showed that acetyltransferase activity of CBP is important for NCT of ATXN3, which may acetylate Imp α3 to regulate NCT of ATXN3. These findings demonstrate that CBP-dependent acetylation of Imp α3 is crucial for intracellular calcium-dependent NCT of ATXN3 proteins, different from that of TDP-43, in Drosophila neurons.

Identification of the calpain-generated toxic fragment of ataxin-3 protein provides new avenues for therapy of Machado-Joseph disease| Spinocerebellar ataxia type 3

AIMS

Machado-Joseph disease (MJD) is the most frequent dominantly inherited cerebellar ataxia worldwide. Expansion of a CAG trinucleotide in the MJD1 gene translates into a polyglutamine tract within ataxin-3, which upon proteolysis may lead to MJD. The aim of this work was to understand the in vivo contribution of calpain proteases to the pathogenesis of MJD. Therefore, we investigated (a) the calpain cleavage sites in ataxin-3 protein, (b) the most toxic ataxin-3 fragment generated by calpain cleavage and (c) whether targeting calpain cleavage sites of mutant ataxin-3 could be a therapeutic strategy for MJD.

METHODS

We generated truncated and calpain-resistant constructs at the predicted cleavage sites of ataxin-3 using inverse PCR mutagenesis. Lentiviral vectors encoding these constructs were transduced in the adult mouse brain prior to western blot and immunohistochemical analysis 5 and 8 weeks later.

RESULTS

We identified the putative calpain cleavage sites for both wild-type and mutant ataxin-3 proteins. The mutation of these sites eliminated the formation of the toxic fragments, namely, the 26-kDa fragment, the major contributor for striatal degeneration. Nonetheless, reducing the formation of both the 26- and 34-kDa fragments was required to preclude the intranuclear localisation of ataxin-3. A neuroprotective effect was observed upon mutagenesis of calpain cleavage sites within mutant ataxin-3 protein.

CONCLUSIONS

These findings suggest that the calpain system should be considered a target for MJD therapy. The identified calpain cleavage sites will contribute to the design of targeted drugs and genome editing systems for those specific locations.

Polyglutamine spinocerebellar ataxias: emerging therapeutic targets

INTRODUCTION

Six of the most frequent dominantly inherited spinocerebellar ataxias (SCAs) worldwide - SCA1, SCA2, SCA3, SCA6, SCA7, and SCA17 - are caused by an expansion of a polyglutamine (polyQ) tract in the corresponding proteins. While the identification of the causative mutation has advanced knowledge on the pathogenesis of polyQ SCAs, effective therapeutics able to mitigate the severe clinical manifestation of these highly incapacitating disorders are not yet available.

AREAS COVERED

This review provides a comprehensive and critical perspective on well-established and emerging therapeutic targets for polyQ SCAs; it aims to inspire prospective drug discovery efforts.

EXPERT OPINION

The landscape of polyQ SCAs therapeutic targets and strategies includes (1) the mutant genes and proteins themselves, (2) enhancement of endogenous protein quality control responses, (3) abnormal protein-protein interactions of the mutant proteins, (4) disturbed neuronal function, (5) mitochondrial function, energy availability and oxidative stress, and (6) glial dysfunction, growth factor or hormone imbalances. Challenges include gaining a clearer definition of therapeutic targets for the drugs in clinical development, the discovery of novel drug-like molecules for challenging key targets, and the attainment of a stronger translation of preclinical findings to the clinic.

The Mechanisms of Nuclear Proteotoxicity in Polyglutamine Spinocerebellar Ataxias

Polyglutamine (polyQ) spinocerebellar ataxias (SCAs) are the most prevalent subset of SCAs and share the aberrant expansion of Q-encoding CAG repeats within the coding sequences of disease-responsible genes as their common genetic cause. These polyQ SCAs (SCA1, SCA2, SCA3, SCA6, SCA7, and SCA17) are inherited neurodegenerative diseases characterized by the progressive atrophy of the cerebellum and connected regions of the nervous system, which leads to loss of fine muscle movement coordination. Upon the expansion of polyQ repeats, the mutated proteins typically accumulate disproportionately in the neuronal nucleus, where they sequester various target molecules, including transcription factors and other nuclear proteins. However, it is not yet clearly understood how CAG repeat expansion takes place or how expanded polyQ proteins accumulate in the nucleus. In this article, we review the current knowledge on the molecular and cellular bases of nuclear proteotoxicity of polyQ proteins in SCAs and present our perspectives on the remaining issues surrounding these diseases.

Machado-Joseph Disease: A Stress Combating Deubiquitylating Enzyme Changing Sides

Machado-Joseph disease (MJD), also known as Spinocerebellar ataxia type 3 (SCA3), is an autosomal dominant inheritable neurodegenerative disorder. After a long pre-symptomatic period, this late-onset disease progressively disables patients and typically leads to premature death. Neuronal loss in specific regions of the cerebellum, brainstem and basal ganglia as well as the spinal cord explains the spectra of debilitating neurological symptoms, most strikingly progressive limb, and gait ataxia. The genetic cause of MJD is a polyglutamine (polyQ) repeat expansion in the gene that encodes ataxin-3. This polyQ-containing protein displays a well-defined catalytic activity as ataxin-3 is a deubiquitylating enzyme that removes and disassembles ubiquitin chains from specific substrates. While mutant ataxin-3 with an expanded polyQ repeat induces cellular stress due to its propensity to aggregate, the native functions of wild-type ataxin-3 are linked to the cellular countermeasures against the very same stress conditions inflicted by polyQ-containing and other aggregation-prone proteins. Hence, a mixture of gain-of-function and loss-of-function mechanisms are likely to contribute to the neuronal demise observed in MJD. In this review, we discuss the intimate link between ataxin-3 and cellular stress and its relevance for therapeutic intervention in MJD.

Machado-Joseph disease/spinocerebellar ataxia type 3: lessons from disease pathogenesis and clues into therapy

Machado-Joseph disease (MJD), also known as spinocerebellar ataxia type 3 (SCA3), is an incurable disorder, widely regarded as the most common form of spinocerebellar ataxia in the world. MJD/SCA3 arises from mutation of the ATXN3 gene, but this simple monogenic cause contrasts with the complexity of the pathogenic mechanisms that are currently admitted to underlie neuronal dysfunction and death. The aberrantly expanded protein product - ataxin-3 - is known to aggregate and generate toxic species that disrupt several cell systems, including autophagy, proteostasis, transcription, mitochondrial function and signalling. Over the years, research into putative therapeutic approaches has often been devoted to the development of strategies that counteract disease at different stages of cellular pathogenesis. Silencing the pathogenic protein, blocking aggregation, inhibiting toxic proteolytic processing and counteracting dysfunctions of the cellular systems affected have yielded promising ameliorating results in studies with cellular and animal models. The current review analyses the available studies dedicated to the investigation of MJD/SCA3 pathogenesis and the exploration of possible therapeutic strategies, focusing primarily on gene therapy and pharmacological approaches rooted on the molecular and cellular mechanisms of disease.

Loss of the Spinocerebellar Ataxia type 3 disease protein ATXN3 alters transcription of multiple signal transduction pathways

Spinocerebellar ataxia type 3 (SCA3) is a dominantly inherited neurodegenerative disorder caused by a polyglutamine-encoding CAG repeat expansion in the ATXN3 gene which encodes the deubiquitinating enzyme, ATXN3. Several mechanisms have been proposed to explain the pathogenic role of mutant, polyQ-expanded ATXN3 in SCA3 including disease protein aggregation, impairment of ubiquitin-proteasomal degradation and transcriptional dysregulation. A better understanding of the normal functions of this protein may shed light on SCA3 disease pathogenesis. To assess the potential normal role of ATXN3 in regulating gene expression, we compared transcriptional profiles in WT versus Atxn3 null mouse embryonic fibroblasts. Differentially expressed genes in the absence of ATXN3 contribute to multiple signal transduction pathways, suggesting a status switch of signaling pathways including depressed Wnt and BMP4 pathways and elevated growth factor pathways such as Prolactin, TGF-β, and Ephrin pathways. The Eph receptor A3 (Efna3), a receptor protein-tyrosine kinase in the Ephrin pathway that is highly expressed in the nervous system, was the most differentially upregulated gene in Atxn3 null MEFs. This increased expression of Efna3 was recapitulated in Atxn3 knockout mouse brainstem, a selectively vulnerable brain region in SCA3. Overexpression of normal or expanded ATXN3 was sufficient to repress Efna3 expression, supporting a role for ATXN3 in regulating Ephrin signaling. We further show that, in the absence of ATXN3, Efna3 upregulation is associated with hyperacetylation of histones H3 and H4 at the Efna3 promoter, which in turn is induced by decreased levels of HDAC3 and NCoR in ATXN3 null cells. Together, these results reveal a normal role for ATXN3 in transcriptional regulation of multiple signaling pathways of potential relevance to disease processes in SCA3.

Divalproex sodium modulates nuclear localization of ataxin-3 and prevents cellular toxicity caused by expanded ataxin-3

BACKGROUND & AIMS

Spinocerebellar ataxia type 3 (SCA3), also known as Machado-Joseph disease (MJD), is an autosomal dominantly inherited neurodegenerative disorder and the most common form of SCA worldwide. It is caused by the expansion of a polyglutamine (polyQ) tract in the ataxin-3 protein. Nuclear localization of the affected protein is a key event in the pathology of SCA3 via affecting nuclear organization, transcriptional dysfunction, and seeding aggregations, finally causing neurodegeneration and cell death. So far, there is no effective therapy to prevent or slow the progression of SCA3.

METHODS

In this study, we explored the effect of divalproex sodium as an HDACi in SCA3 cell models and explored how divalproex sodium interferes with pathogenetic processes causing SCA3.

RESULTS

We found that divalproex sodium rescues the hypoacetylation levels of histone H3 and attenuates cellular cytotoxicity induced by expanded ataxin-3 partly via preventing nuclear transport of ataxin-3 (particularly heat shock-dependent).

CONCLUSION

Our study provides novel insights into the mechanisms of action of divalproex sodium as a possible treatment for SCA3, beyond the known regulation of transcription.

Experimental and Clinical Strategies for Treating Spinocerebellar Ataxia Type 3

Spinocerebellar ataxia type 3 (SCA3), or Machado-Joseph disease (MJD), is an autosomal dominant neurodegenerative disorder caused by the expansion of a polyglutamine (polyQ) tract in the ataxin-3 protein. To date, there is no effective therapy available to prevent progression of this disease. However, clinical strategies for alleviating various symptoms are imperative to promote a better quality of life for SCA3/MJD patients. Furthermore, experimental therapeutic strategies, including gene silencing or mutant protein clearance, mutant polyQ protein modification, stabilizing the native protein conformation, rescue of cellular dysfunction and neuromodulation to slow the progression of SCA3/MJD, have been developed. In this study, based on the current knowledge, I detail the clinical and experimental therapeutic strategies for treating SCA3/MJD, paying particular attention to drug discovery.

The business of deubiquitination - location, location, location

A majority of proteins in the cell can be modified by ubiquitination, thereby altering their function or stability. This ubiquitination is controlled by both ubiquitinating and deubiquitinating enzymes (DUBs). The number of ubiquitin ligases exceeds that of DUBs by about eightfold, indicating that DUBs may have much broader substrate specificity. Despite this, DUBs have been shown to have quite specific physiological functions. This functional specificity is likely due to very precise regulation of activity arising from the sophisticated use of all mechanisms of enzyme regulation. In this commentary, we briefly review key features of DUBs with more emphasis on regulation. In particular, we focus on localization of the enzymes as a critical regulatory mechanism which when integrated with control of expression, substrate activation, allosteric regulation, and post-translational modifications results in precise spatial and temporal deubiquitination of proteins and therefore specific physiological functions. Identification of compounds that target the structural elements in DUBs that dictate localization may be a more promising approach to development of drugs with specificity of action than targeting the enzymatic activity, which for most DUBs is dependent on a thiol group that can react non-specifically with many compounds in large-scale screening.

Ataxin-3 protein and RNA toxicity in spinocerebellar ataxia type 3: current insights and emerging therapeutic strategies

Ataxin-3 is a ubiquitously expressed deubiqutinating enzyme with important functions in the proteasomal protein degradation pathway and regulation of transcription. The C-terminus of the ataxin-3 protein contains a polyglutamine (PolyQ) region that, when mutationally expanded to over 52 glutamines, causes the neurodegenerative disease spinocerebellar ataxia 3 (SCA3). In spite of extensive research, the molecular mechanisms underlying the cellular toxicity resulting from mutant ataxin-3 remain elusive and no preventive treatment is currently available. It has become clear over the last decade that the hallmark intracellular ataxin-3 aggregates are likely not the main toxic entity in SCA3. Instead, the soluble PolyQ containing fragments arising from proteolytic cleavage of ataxin-3 by caspases and calpains are now regarded to be of greater influence in pathogenesis. In addition, recent evidence suggests potential involvement of a RNA toxicity component in SCA3 and other PolyQ expansion disorders, increasing the pathogenic complexity. Herein, we review the functioning of ataxin-3 and the involvement of known protein and RNA toxicity mechanisms of mutant ataxin-3 that have been discovered, as well as future opportunities for therapeutic intervention.

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.

Valosin-containing protein (VCP/p97) is an activator of wild-type ataxin-3

Alterations in the ubiquitin-proteasome system (UPS) have been reported in several neurodegenerative disorders characterized by protein misfolding and aggregation, including the polylgutamine diseases. Machado-Joseph disease (MJD) or Spinocerebellar Ataxia type 3 is caused by a polyglutamine-encoding CAG expansion in the ATXN3 gene, which encodes a 42 kDa deubiquitinating enzyme (DUB), ataxin-3. We investigated ataxin-3 deubiquitinating activity and the functional relevance of ataxin-3 interactions with two proteins previously described to interact with ataxin-3, hHR23A and valosin-containing protein (VCP/p97). We confirmed ataxin-3 affinity for both hHR23A and VCP/p97. hHR23A and ataxin-3 were shown to co-localize in discrete nuclear foci, while VCP/p97 was primarily cytoplasmic. hHR23A and VCP/p97 recombinant proteins were added, separately or together, to normal and expanded ataxin-3 in in vitro deubiquitination assays to evaluate their influence on ataxin-3 activity. VCP/p97 was shown to be an activator specifically of wild-type ataxin-3, exhibiting no effect on expanded ataxin-3, In contrast, we observed no significant alterations in ataxin-3 enzyme kinetics or substrate preference in the presence of hHR23A alone or in combination with VCP. Based on our results we propose a model where ataxin-3 normally functions with its interactors to specify the cellular fate of ubiquitinated proteins.

Identification of human proteins that modify misfolding and proteotoxicity of pathogenic ataxin-1

Proteins with long, pathogenic polyglutamine (polyQ) sequences have an enhanced propensity to spontaneously misfold and self-assemble into insoluble protein aggregates. Here, we have identified 21 human proteins that influence polyQ-induced ataxin-1 misfolding and proteotoxicity in cell model systems. By analyzing the protein sequences of these modifiers, we discovered a recurrent presence of coiled-coil (CC) domains in ataxin-1 toxicity enhancers, while such domains were not present in suppressors. This suggests that CC domains contribute to the aggregation- and toxicity-promoting effects of modifiers in mammalian cells. We found that the ataxin-1-interacting protein MED15, computationally predicted to possess an N-terminal CC domain, enhances spontaneous ataxin-1 aggregation in cell-based assays, while no such effect was observed with the truncated protein MED15ΔCC, lacking such a domain. Studies with recombinant proteins confirmed these results and demonstrated that the N-terminal CC domain of MED15 (MED15CC) per se is sufficient to promote spontaneous ataxin-1 aggregation in vitro. Moreover, we observed that a hybrid Pum1 protein harboring the MED15CC domain promotes ataxin-1 aggregation in cell model systems. In strong contrast, wild-type Pum1 lacking a CC domain did not stimulate ataxin-1 polymerization. These results suggest that proteins with CC domains are potent enhancers of polyQ-mediated protein misfolding and aggregation in vitro and in vivo.

Cell biology of spinocerebellar ataxia

Ataxia is a neurological disorder characterized by loss of control of body movements. Spinocerebellar ataxia (SCA), previously known as autosomal dominant cerebellar ataxia, is a biologically robust group of close to 30 progressive neurodegenerative diseases. Six SCAs, including the more prevalent SCA1, SCA2, SCA3, and SCA6 along with SCA7 and SCA17 are caused by expansion of a CAG repeat that encodes a polyglutamine tract in the affected protein. How the mutated proteins in these polyglutamine SCAs cause disease is highly debated. Recent work suggests that the mutated protein contributes to pathogenesis within the context of its “normal” cellular function. Thus, understanding the cellular function of these proteins could aid in the development of therapeutics.

Machado-Joseph disease/spinocerebellar ataxia type 3

Machado-Joseph disease (MJD), also known as spinocerebellar ataxia type 3 (SCA3), may be the most common dominantly inherited ataxia in the world. Here I will review historical, clinical, neuropathological, genetic, and pathogenic features of MJD, and finish with a brief discussion of present, and possible future, treatment for this currently incurable disorder. Like many other dominantly inherited ataxias, MJD/SCA3 shows remarkable clinical heterogeneity, reflecting the underlying genetic defect: an unstable CAG trinucleotide repeat that varies in size among affected persons. This pathogenic repeat in MJD/SCA3 encodes an expanded tract of the amino acid glutamine in the disease protein, which is known as ataxin-3. MJD/SCA3 is one of nine identified polyglutamine neurodegenerative diseases which share features of pathogenesis centered on protein misfolding and accumulation. The specific properties of MJD/SCA3 and its disease protein are discussed in light of what is known about the entire class of polyglutamine diseases.

Toward understanding Machado-Joseph disease

Machado-Joseph disease (MJD), also known as spinocerebellar ataxia type 3 (SCA3), is the most common inherited spinocerebellar ataxia and one of many polyglutamine neurodegenerative diseases. In MJD, a CAG repeat expansion encodes an abnormally long polyglutamine (polyQ) tract in the disease protein, ATXN3. Here we review MJD, focusing primarily on the function and dysfunction of ATXN3 and on advances toward potential therapies. ATXN3 is a deubiquitinating enzyme (DUB) whose highly specialized properties suggest that it participates in ubiquitin-dependent proteostasis. By virtue of its interactions with VCP, various ubiquitin ligases and other ubiquitin-linked proteins, ATXN3 may help regulate the stability or activity of many proteins in diverse cellular pathways implicated in proteotoxic stress response, aging, and cell differentiation. Expansion of the polyQ tract in ATXN3 is thought to promote an altered conformation in the protein, leading to changes in interactions with native partners and to the formation of insoluble aggregates. The development of a wide range of cellular and animal models of MJD has been crucial to the emerging understanding of ATXN3 dysfunction upon polyQ expansion. Despite many advances, however, the principal molecular mechanisms by which mutant ATXN3 elicits neurotoxicity remain elusive. In a chronic degenerative disease like MJD, it is conceivable that mutant ATXN3 triggers multiple, interconnected pathogenic cascades that precipitate cellular dysfunction and eventual cell death. A better understanding of these complex molecular mechanisms will be important as scientists and clinicians begin to focus on developing effective therapies for this incurable, fatal disorder.

HDAC inhibitor sodium butyrate reverses transcriptional downregulation and ameliorates ataxic symptoms in a transgenic mouse model of SCA3

Spinocerebellar ataxia type 3 (SCA3) is an autosomal dominant neurodegenerative disease caused by polyglutamine-expanded ataxin-3. Previously, we prepared a SCA3 animal model by generating transgenic mice expressing disease-causing ataxin-3-Q79. Mutant ataxin-3-Q79 caused cerebellar malfunction of SCA3 transgenic mice by downregulating cerebellar mRNA expressions of proteins involved in synaptic transmission, signal transduction or regulating neuronal survival/differentiation. Histone acetylation, which is controlled by histone acetyltransferase and histone deacetylase (HDAC), plays an important role in regulating transcriptional activity. In the present study, we tested the hypothesis that ataxin-3-Q79 causes cerebellar transcriptional downregulation by inducing histone hypoacetylation and that HDAC inhibitor sodium butyrate (SB) alleviates ataxic symptoms of SCA3 transgenic mice by reversing ataxin-3-Q79-induced histone hypoacetylation and transcriptional repression. Compared to wild-type mice, H3 and H4 histones were hypoacetylated in the cerebellum of 6- to 8-month-old ataxin-3-Q79 transgenic mice, which displayed transcriptional downregulation and ataxic symptoms. Daily intraperitoneal administration of SB significantly reversed ataxin-3-Q79-induced histone hypoacetylation and transcriptional downregulation in the cerebellum of SCA3 transgenic mice. SB treatment also delayed the onset of ataxic symptoms, ameliorated neurological phenotypes and improved the survival rate of ataxin-3-Q79 transgenic mice. The present study provides the evidence that mutant ataxin-3-Q79 causes cerebellar transcriptional repression and ataxic symptoms of SCA3 transgenic mice by inducing hypoacetylation of histones H3 and H4. Our results suggest that sodium butyrate might be a promising therapeutic agent for SCA3.

CK2 and GSK3 phosphorylation on S29 controls wild-type ATXN3 nuclear uptake

In the present work we show that murine ATXN3 (ATXN3Q6) nuclear uptake is promoted by phosphorylation on serine 29, a highly conserved residue inside the Josephin domain. Both casein kinase 2 (CK2) and glycogen synthase kinase 3 (GSK3) are able to carry out phosphorylation on this residue. S29 phosphorylation was initially assessed in vitro on purified ATXN3Q6, and subsequently confirmed in transfected COS-7 cells, by MS analysis. Site-directed mutagenesis of S29 to an alanine was shown to strongly reduce nuclear uptake, in COS-7 transiently transfected cells overexpressing ATXN3Q6, while substitution with phospho-mimic aspartic acid restored the wild-type phenotype. Finally, treatment with CK2 and GSK3 inhibitors prevented S29 phosphorylation and strongly inhibited nuclear uptake, showing that both kinases are involved in ATXN3Q6 subcellular sorting. Although other authors have previously addressed this issue, we show for the first time that ATXN3 is phosphorylated inside the Josephin domain and that S29 phosphorylation is involved in nuclear uptake of ATXN3.

Silencing ataxin-3 mitigates degeneration in a rat model of Machado-Joseph disease: no role for wild-type ataxin-3?

Machado-Joseph disease or spinocerebellar ataxia type 3 (MJD/SCA3) is a fatal, autosomal dominant disorder caused by a cytosine-adenine-guanine expansion in the coding region of the MJD1 gene. RNA interference has potential as a therapeutic approach but raises the issue of the role of wild-type ataxin-3 (WT ATX3) in MJD and of whether the expression of the wild-type protein must be maintained. To address this issue, we both overexpressed and silenced WT ATX3 in a rat model of MJD. We showed that (i) overexpression of WT ATX3 did not protect against MJD pathology, (ii) knockdown of WT ATX3 did not aggravate MJD pathology and that (iii) non-allele-specific silencing of ataxin-3 strongly reduced neuropathology in a rat model of MJD. Our findings indicate that therapeutic strategies involving non-allele-specific silencing to treat MJD patients may be safe and effective.

Proteotoxic stress increases nuclear localization of ataxin-3

Spinocerebellar ataxia type 3 (SCA3)/Machado Joseph disease results from expansion of the polyglutamine domain in ataxin-3 (Atx3). Atx3 is a transcriptional co-repressor, as well as a deubiquitinating enzyme that appears to function in cellular pathways involved in protein homeostasis. In this study, we show that interactions of Atx3 with valosin-containing protein and hHR23B are dynamic and modulated by proteotoxic stresses. Heat shock, a general proteotoxic stress, also induced wild-type and pathogenic Atx3 to accumulate in the nucleus. Mapping studies showed that two regions of Atx3, the Josephin domain and the C-terminus, regulated heat shock-induced nuclear localization. Heat shock-induced nuclear localization of Atx3 was not affected by a casein kinase-2 inhibitor or by mutating a predicted nuclear localization signal. However, serine-111 of Atx3 was required for nuclear localization of the Josephin domain and regulated nuclear localization of full-length Atx3. Atx3 null cells were more sensitive to toxic effects of heat shock suggesting that Atx3 had a protective function in the cellular response to heat shock. Importantly, we found that oxidative stress also induced nuclear localization of Atx3; both wild-type and pathogenic Atx3 accumulated in the nucleus of SCA3 patient fibroblasts following oxidative stress. Heat shock and oxidative stress are the first processes identified that increase nuclear localization of Atx3. Observations in this study provide new and important insights for understanding SCA3 pathology as the nucleus is likely a key site for early pathogenesis.

A transgenic mouse model of spinocerebellar ataxia type 3 resembling late disease onset and gender-specific instability of CAG repeats

Spinocerebellar ataxia type 3 (SCA3), or Machado-Joseph disease (MJD), is caused by the expansion of a polyglutamine repeat in the ataxin-3 protein. We generated a mouse model of SCA3 expressing ataxin-3 with 148 CAG repeats under the control of the huntingtin promoter, resulting in ubiquitous expression throughout the whole brain. The model resembles many features of the disease in humans, including a late onset of symptoms and CAG repeat instability in transmission to offspring. We observed a biphasic progression of the disease, with hyperactivity during the first months and decline of motor coordination after about 1 year of age; however, intranuclear aggregates were not visible at this age. Few and small intranuclear aggregates appeared first at the age of 18 months, further supporting the claim that neuronal dysfunction precedes the formation of intranuclear aggregates.

The pathogenic mechanisms of polyglutamine diseases and current therapeutic strategies

Expansion of CAG trinucleotide repeat within the coding region of several genes results in the production of proteins with expanded polyglutamine (PolyQ) stretch. The expression of these pathogenic proteins leads to PolyQ diseases, such as Huntington's disease or several types of spinocerebellar ataxias. This family of neurodegenerative disorders is characterized by constant progression of the symptoms and molecularly, by the accumulation of mutant proteins inside neurons causing their dysfunction and eventually death. So far, no effective therapy actually preventing the physical and/or mental decline has been developed. Experimental therapeutic strategies either target the levels or processing of mutant proteins in an attempt to prevent cellular deterioration, or they are aimed at the downstream pathologic effects to reverse or ameliorate the caused damages. Certain pathomechanistic aspects of PolyQ disorders are discussed here. Relevance of disease models and recent knowledge of therapeutic possibilities is reviewed and updated.

Nucleocytoplasmic shuttling activity of ataxin-3

Spinocerebellar ataxia type-3, also known as Machado-Joseph Disease (MJD), is one of many inherited neurodegenerative disorders caused by polyglutamine-encoding CAG repeat expansions in otherwise unrelated genes. Disease protein misfolding and aggregation, often within the nucleus of affected neurons, characterize polyglutamine disorders. Several evidences have implicated the nucleus as the primary site of pathogenesis for MJD. However, the molecular determinants for the nucleocytoplasmic transport of human ataxin-3 (Atx3), the protein which is mutated in patients with MJD, are not characterized.

In order to characterize the nuclear shuttling activity of Atx3, we performed yeast nuclear import assays and found that Atx3 is actively imported into the nucleus, by means of a classical nuclear localizing sequence formed by a cluster of lysine and arginine residues. On the other hand, when active nuclear export was inhibited using leptomycin B, a specific inhibitor of the nuclear export receptor CRM1, both endogenous Atx3 and transfected GFP-Atx3 accumulated inside the nucleus of a subpopulation of COS-7 cells, whereas both proteins are normally predominant in the cytoplasm.

Additionally, using a Rev(1.4)-GFP nuclear export assay, we performed an extensive analysis of six putative aliphatic nuclear export motifs identified in Atx3 amino acid sequence. Although none of the tested peptide sequences were found to drive nuclear export when isolated, we have successfully mapped the region of Atx3 responsible for its CRM1-independent nuclear export activity. Curiously, the N-terminal Josephin domain alone is exported into the cytoplasm, but the nuclear export activity of Atx3 is significantly enhanced in a longer construct that is truncated after the two ubiquitin interaction motifs, upstream from the polyQ tract.

Our data show that Atx3 is actively imported to and exported from the cell nucleus, and that its nuclear export activity is dependent on a motif located at its N-terminal region. Since pathological Atx3 aggregates in the nucleus of affected neurons in MJD, and there is in vivo evidence that nuclear localization of Atx3 is required for the manifestation of symptoms in MJD, defects in the nucleocytoplasmic shuttling activity of the protein may be involved in the nuclear accumulation and aggregation of expanded Atx3.

Casein kinase 2 interacts with and phosphorylates ataxin-3

OBJECTIVE

Machado-Joseph disease (MJD)/Spinocerebellar ataxia type 3 (SCA3) is an autosomal dominant neurodegenerative disorder caused by an expansion of polyglutamine tract near the C-terminus of the MJD1 gene product, ataxin-3. The precise mechanism of the MJD/SCA3 pathogenesis remains unclear. A growing body of evidence demonstrates that phosphorylation plays an important role in the pathogenesis of many neurodegenerative diseases. However, few kinases are known to phosphorylate ataxin-3. The present study is to explore whether ataxin-3 is a substrate of casein kinase 2 (CK2).

METHODS

The interaction between ataxin-3 and CK2 was identified by glutathione S-transferase (GST) pull-down assay and co-immunoprecipition assay. The phosphorylation of ataxin-3 by CK2 was measured by in vitro phosphorylation assays. Results (1) Both wild type and expanded ataxin-3 interacted with CK2alpha and CK2beta in vitro. (2) In 293 cells, both wild type and expanded ataxin-3 interacted with CK2beta, but not CK2alpha. (3) CK2 phosphorylated wild type and expanded ataxin-3.

CONCLUSION

Ataxin-3 is a substrate of protein kinase CK2.

Polyglutamine-expanded ataxin-3 causes cerebellar dysfunction of SCA3 transgenic mice by inducing transcriptional dysregulation

In the present study, we prepared a SCA3 animal model by generating transgenic mice expressing polyglutamine-expanded ataxin-3-Q79. Ataxin-3-Q79 was expressed in brain areas implicated in SCA3 neurodegeneration, including cerebellum, pontine nucleus and substantia nigra. Ataxin-3-Q79 transgenic mice displayed motor dysfunction with an onset age of 5-6 months, and neurological symptoms deteriorated in the following months. A prominent neuronal loss was not found in the cerebellum of 10 to 11-month-old ataxin-3-Q79 mice displaying pronounced ataxic symptoms, suggesting that instead of neuronal demise, ataxin-3-Q79 causes neuronal dysfunction of the cerebellum and resulting ataxia. To test the involvement of transcriptional dysregulation in ataxin-3-Q79-induced cerebellar malfunction, microarray analysis and real-time RT-PCR assays were performed to identify altered cerebellar mRNA expressions of ataxin-3-Q79 mice. Compared to non-transgenic mice or mice expressing wild-type ataxin-3-Q22, 10 to 11-month-old ataxin-3-Q79 mice exhibited downregulated mRNA expressions of proteins involved in glutamatergic neurotransmission, intracellular calcium signaling/mobilization or MAP kinase pathways, GABA(A/B) receptor subunits, heat shock proteins and transcription factor regulating neuronal survival and differentiation. Upregulated expressions of Bax, cyclin D1 and CDK5-p39, which may mediate neuronal death, were also observed in ataxin-3-Q79 transgenic mice. The involvement of transcriptional abnormality in initiating the pathological process of SCA3 was indicated by the finding that 4 to 5-month-old ataxin-3-Q79 mice, which did not display neurological phenotype, exhibited downregulated mRNA levels of genes involved in glutamatergic signaling and signal transduction. Our study suggests that polyglutamine-expanded ataxin-3 causes cerebellar dysfunction and ataxia by disrupting the normal pattern of gene transcriptions.

Nuclear bodies in neurodegenerative disease

Neurodegenerative diseases are characterized by a relentlessly progressive loss of the functional and structural integrity of the central nervous system. In many cases, these diseases arise sporadically and the causes are unknown. The abnormal aggregation of protein within the cytoplasm or the nucleus of brain cells represents a unifying pathological feature of these diseases. There is increasing evidence for nuclear dysfunction in neurodegenerative diseases. How this relates to protein aggregation in the context of "cause and effect" remains to be determined in most cases. Co-ordinated nuclear function is predicated on the activity of distinct nuclear subdomains, or nuclear bodies, each responsible for a specific function. If nuclear dysfunction represents an important etiopathological feature in neurodegenerative disease, then this should be reflected by functional and/or morphological alterations in this nuclear compartmentalization. For most neurodegenerative diseases, evidence for nuclear dysfunction, with attendant consequences for nuclear architecture, is only beginning to emerge. In this review, I will discuss neurodegenerative diseases in the context of nuclear dysfunction and, more specifically, alterations in nuclear bodies. Although research in this field is in its infancy, identifying alterations in the nucleus in neurodegenerative disease has potentially profound implications for elucidating the pathogenesis of these disorders.

Striatal and nigral pathology in a lentiviral rat model of Machado-Joseph disease

Machado-Joseph disease (MJD) is a fatal, dominant neurodegenerative disorder. MJD results from polyglutamine repeat expansion in the MJD-1 gene, conferring a toxic gain of function to the ataxin-3 protein. In this study, we aimed at overexpressing ataxin-3 in the rat brain using lentiviral vectors (LV), to generate an in vivo MJD genetic model and, to study the disorder in defined brain regions: substantia nigra, an area affected in MJD, cortex and striatum, regions not previously reported to be affected in MJD. LV encoding mutant or wild-type human ataxin-3 was injected in the brain of adult rats and the animals were tested for behavioral deficits and neuropathological abnormalities. Striatal pathology was confirmed in transgenic mice and human tissue. In substantia nigra, unilateral overexpression of mutant ataxin-3 led to: apomorphine-induced turning behavior; formation of ubiquitinated ataxin-3 aggregates; alpha-synuclein immunoreactivity; and loss of dopaminergic markers (TH and VMAT2). No neuropathological changes were observed upon wild-type ataxin-3 overexpression. Mutant ataxin-3 expression in striatum and cortex, resulted in accumulation of misfolded ataxin-3, and within striatum, loss of neuronal markers. Striatal pathology was confirmed by observation in MJD transgenic mice of ataxin-3 aggregates and substantial reduction of DARPP-32 immunoreactivity and, in human striata, by ataxin-3 inclusions, immunoreactive for ubiquitin and alpha-synuclein. This study demonstrates the use of LV encoding mutant ataxin-3 to produce a model of MJD and brings evidence of striatal pathology, suggesting that this region may contribute to dystonia and chorea observed in some MJD patients and may represent a target for therapies.

Study of subcellular localization and proteolysis of ataxin-3

In this work we investigate subcellular localization and proteolytic cleavage of different forms of ataxin-3 (AT-3), the protein responsible for spinocerebellar ataxia type 3. Normal (AT-3Q6 and AT-3Q26) and pathological (AT-3Q72) ataxins-3, as well as two truncated forms lacking poly-Q, were studied. Full-length proteins were also expressed as C14A mutants, in order to assess whether AT-3 autoproteolytic activity was involved in its fragmentation. We found that both normal and pathological proteins localized in the cytoplasm and in the nucleus, as expected, but also in the mitochondria. Microsequencing showed that all ataxins-3 underwent the same proteolytic cleavage, removing the first 27 amino acids. Interestingly, while normal ataxins were further cleaved at a number of caspase sites, pathological AT-3 was proteolyzed to a much lesser extent. This may play a role in the pathogenesis, hampering degradation of aggregation-prone expanded AT-3. In addition, autolytic cleavage was apparently not involved in AT-3 proteolysis.

Inclusion formation by ataxins -1, -2, -3, and -7

The authors studied inclusion formation in vitro using transiently transfected PC12 cells, with epitope-tagged and untagged full-length and truncated wild-type and expanded ataxins -1, -2, -3, and -7. At 72 hours, no inclusions were seen with wild-type full-length or truncated ataxins -2, -3, or -7, and only one with ataxin-1. Truncation abolished nuclear localization of ataxins -1 and -7, and allowed nuclear entry of ataxin-2. Of the expanded ataxins, only -1 and -2 formed inclusions, and those of ataxin-2 were rare and exclusively cytoplasmic. Truncation resulted in inclusion formation by ataxins -3 and -7, increased ataxin-1 inclusions, and enabled formation of nuclear ataxin-2 inclusions. There was no recruitment of wild-type ataxin-1 to expanded ataxin-1 inclusions.

Trinucleotide repeat disorders

The discovery that expansion of unstable repeats can cause a variety of neurological disorders has changed the landscape of disease-oriented research for several forms of mental retardation, Huntington disease, inherited ataxias, and muscular dystrophy. The dynamic nature of these mutations provided an explanation for the variable phenotype expressivity within a family. Beyond diagnosis and genetic counseling, the benefits from studying these disorders have been noted in both neurobiology and cell biology. Examples include insight about the role of translational control in synaptic plasticity, the role of RNA processing in the integrity of muscle and neuronal function, the importance of Fe-S-containing enzymes for cellular energy, and the dramatic effects of altering protein conformations on neuronal function and survival. It is exciting that within a span of 15 years, pathogenesis studies of this class of disorders are beginning to reveal pathways that are potential therapeutic targets.

Inactivation of the mouse Atxn3 (ataxin-3) gene increases protein ubiquitination

Spinocerebellar ataxia type 3 is a neurodegenerative disease caused by expansion of a polyglutamine domain in the protein ataxin-3 (ATXN3). Physiological functions of ATXN3 presumably include ubiquitin protease and transcriptional corepressor activity. To gain insight into the function of ATXN3 and to test the hypothesis that loss of ATXN3 contributes to the pathology in SCA3 we generated Atxn3 knockout (ko) mice by targeted mutagenesis. Loss of Atxn3 did not affect viability or fertility and Atxn3 ko mice displayed no overt abnormalities. On the accelerating Rotarod Atxn3 ko mice performed as well as wildtype (wt) animals, but reduced exploratory behavior in the open field suggested a sense of heightened anxiety. While no gross deficits were apparent upon morphological examination, we found increased levels of ubiquitinated proteins in Atxn3 ko tissues. Thus Atxn3 ko mice provide the first in vivo reference to the deubiquitinating activity of ATXN3.

Nuclear localization of ataxin-3 is required for the manifestation of symptoms in SCA3: in vivo evidence

Spinocerebellar ataxia type 3 (SCA3) is an autosomal dominantly inherited neurodegenerative disorder caused by the expansion of a CAG repeat in the MJD1 gene resulting in an expanded polyglutamine repeat in the ataxin-3 protein. To study the course of the disease, we generated transgenic mice for SCA3 using full-length ataxin-3 constructs containing 15, 70, or 148 CAG repeats, respectively. Control mice (15 CAGs) were phenotypically normal and had no neuropathological findings. However, mice transgenic for ataxin-3 with expanded polyglutamine repeats were severely affected by a strong neurological phenotype with tremor, behavioral deficits, strongly reduced motor and exploratory activity, a hunchback, and premature death at 3 to 6 months of age. Neuropathological examination by immunohistochemical staining revealed ubiquitin- and ataxin-3-positive intranuclear inclusion bodies in a multitude of neurons. Directing ataxin-3 with 148 CAGs to the nucleus revealed an even more pronounced phenotype with more inclusions and earlier death, whereas mice transgenic with the same construct but attached to a nuclear export signal developed a milder phenotype with less inclusions. These studies indicate that nuclear localization of ataxin-3 is required for the manifestation of symptoms in SCA3 in vivo.

Abnormalities of the nucleus and nuclear inclusions in neurodegenerative disease: a work in progress

Neurodegenerative diseases are characterized pathologically by the abnormal accumulation of pathogenic protein species within the cell. Several neurodegenerative diseases feature intranuclear protein aggregation in the form of intranuclear inclusion bodies. Studies of these intranuclear inclusions are providing important clues regarding the cellular pathophysiology of these diseases, as exemplified by recent progress in defining the genetic basis of a subset of frontotemporal dementia cases. The precise role of intranuclear inclusion bodies in disease pathogenesis is currently a focus of debate. The present review provides an overview of the diverse family of neurodegenerative diseases in which nuclear inclusions form part of the neuropathological spectrum. In addition, current pathogenetic concepts relevant to these diseases will be reviewed and arguments for and against a protective role for intranuclear inclusions will be presented. The relationship of pathological intranuclear inclusions to functional intranuclear bodies will also be discussed. Finally, by analogy with pathological intranuclear inclusions, I will speculate on the possibility that intranuclear protein aggregation may represent a constitutive cellular protective mechanism occurring in neurons under physiological conditions.

Functional genomics and biochemical characterization of the C. elegans orthologue of the Machado-Joseph disease protein ataxin-3

Machado-Joseph disease (MJD) is the most common dominant spinocerebellar ataxia. MJD is caused by a CAG trinucleotide expansion in the ATXN3 gene, which encodes a protein named ataxin-3. Ataxin-3 has been proposed to act as a deubiquitinating enzyme in the ubiquitin-proteasome pathway and to be involved in transcriptional repression; nevertheless, its precise biological function(s) remains unknown. To gain further insight into the function of ataxin-3, we have identified the Caenorhabditis elegans orthologue of the ATXN3 gene and characterized its pattern of expression, developmental regulation, and subcellular localization. We demonstrate that, analogous to its human orthologue, C. elegans ataxin-3 has deubiquitinating activity in vitro against polyubiquitin chains with four or more ubiquitins, the minimum ubiquitin length for proteasomal targeting. To further evaluate C. elegans ataxin-3, we characterized the first known knockout animal models both phenotypically and biochemically, and found that the two C. elegans strains were viable and displayed no gross phenotype. To identify a molecular phenotype, we performed a large-scale microarray analysis of gene expression in both knockout strains. The data revealed a significant deregulation of core sets of genes involved in the ubiquitin-proteasome pathway, structure/motility, and signal transduction. This gene identification provides important clues that can help elucidate the specific biological role of ataxin-3 and unveil some of the physiological effects caused by its absence or diminished function.

Polyglutamine neurodegenerative diseases and regulation of transcription: assembling the puzzle

The polyglutamine disorders are a class of nine neuro-degenerative disorders that are inherited gain-of-function diseases caused by expansion of a translated CAG repeat. Even though the disease-causing proteins are widely expressed, specific collections of neurons are more susceptible in each disease, resulting in characteristic patterns of pathology and clinical symptoms. One hypothesis poses that altered protein function is fundamental to pathogenesis, with protein context of the expanded polyglutamine having key roles in disease-specific processes. This review will focus on the role of the disease-causing polyglutamine proteins in gene transcription and the extent to which the mutant proteins induce disruption of transcription.

Polyglutamine-expanded ataxin-3 activates mitochondrial apoptotic pathway by upregulating Bax and downregulating Bcl-xL

Spinocerebellar ataxia type 3 (SCA3) is an autosomal dominant neurodegenerative disease caused by polyglutamine-expanded ataxin-3. In the present study, we expressed disease-causing mutant ataxin-3-Q79 in neuronal cultures of cerebellum, striatum and substantia nigra by using recombinant adenoviruses. Subsequently, SCA3 cellular model was used to investigate the molecular mechanism by which ataxin-3-Q79 causes neuronal death. TUNEL staining studies showed that ataxin-3-Q79 induced apoptotic death of cerebellar, striatal or substantia nigra neurons. Ataxin-3-Q79 activated caspase-3 and caspase-9 without inducing the formation of active caspase-8. Ataxin-3-Q79 promoted mitochondrial release of cytochrome c and Smac, which was preceded by the upregulation of Bax protein and downregulation of Bcl-x(L) protein expression. Real-time TaqMan RT-PCR assays demonstrated that ataxin-3-Q79 upregulated Bax mRNA level and downregulated Bcl-xL mRNA expression in striatal, cerebellar and substantia nigra neurons. Our results suggest that polyglutamine-expanded ataxin-3-Q79 activates mitochondrial apoptotic pathway and induces neuronal death by upregulating Bax expression and downregulating Bcl-xL expression.

Absence of spinocerebellar ataxia type 3/Machado-Joseph disease within ataxic patients in the Czech population

Although spinocerebellar ataxia type 3 (SCA3)/Machado-Joseph disease is the most common type of SCA worldwide, we did not identify any cases of the disease amongst SCA patients in the Czech population. It has been proposed that the prevalence of large normal alleles correlates with the frequency of various types of SCA. We have therefore attempted to resolve the absence of SCA3 in our population by investigating, within 204 normal chromosomes, the frequency and nature of CAG repeats as well as two intragenic polymorphisms. We found that large normal alleles with more than 33 CAG repeats were observed at a frequency of only 0.49%. Whereas most of the expanded alleles worldwide have the CA haplotype, this was the least common (5.4%) variant observed in our study, although it was associated with a larger mean CAG repeat length (26.9). We postulate that the absence of SCA3 in the Czech population might be explained by the lack of large normal alleles and consequently a relatively small reservoir for aberrant CAG expansions at the SCA3 locus.

Ectoine alters subcellular localization of inclusions and reduces apoptotic cell death induced by the truncated Machado–Joseph disease gene product with an expanded polyglutamine stretch

Protein misfolding is considered a key event in the pathogenesis of polyglutamine disease such as Machado-Joseph disease (MJD). Overexpression of chaperone proteins and the application of chemical chaperones are reported to suppress polyglutamine induced cytotoxicity in vitro and in vivo. The effects of compatible solutes, which are osmoprotectants in bacteria and possess the action in stabilizing proteins under stress, have not, to our knowledge, been studied. We explored the protective effects of the compatible solutes ectoine, hydroxyectoine, and betaine on apoptotic cell death produced by the truncated MJD gene product with an expanded polyglutamine tract in cultured neuro2a cells. Ectoine, but not hydroxyectoine or betaine, decreased large cytoplasmic inclusions and increased the frequency of nuclear inclusions. Immunoblot analysis showed that ectoine reduced the total amount of aggregates. Despite the presence of nuclear inclusions, apoptotic features were less frequently observed after ectoine application. Our findings suggest that ectoine, a natural osmoprotectant in bacteria, may function as a novel molecule protecting cells from polyglutamine-induced toxicity.

A mutant ataxin-3 putative-cleavage fragment in brains of Machado-Joseph disease patients and transgenic mice is cytotoxic above a critical concentration

Machado-Joseph disease (MJD) is an inherited neurodegenerative disorder caused by ataxin-3 with a polyglutamine expansion. It is proposed that a toxic cleavage fragment of mutant ataxin-3 alternatively spliced isoform mjd1a triggers neurodegeneration, although this fragment has not yet been detected in the brains of MJD patients or in animal models. We have now generated transgenic mice expressing human mutant (Q71) or normal (Q20) ataxin-3 mjd1a under the control of the mouse prion promoter. Q71 transgenic mice expressing mutant ataxin-3 mjd1a above a critical level developed a phenotype similar to MJD including progressive postural instability, gait and limb ataxia, weight loss, premature death, neuronal intranuclear inclusions, and decreased tyrosine hydroxylase-positive neurons in the substantia nigra (determined by unbiased stereology). Q20 transgenic mice had normal behavior and pathology. Brains from sick Q71 transgenic mice contained an abundant mutant ataxin-3 mjd1a putative-cleavage fragment (Fragment), which was scarce in normal Q71 transgenic mice. Reactivity of the Fragment with a panel of antibodies and comigration with truncations of mutant ataxin-3 revealed that it contained residues C terminal to amino acid 221 to include the polyglutamine expansion. A similar portion of mutant ataxin-3 mjd1a expressed in transfected neuroblastoma cells was toxic above a critical concentration. The Fragment was more abundant in two affected brain regions of MJD patients. Thus, we have developed a murine model for mutant ataxin-3 mjd1a toxicity and identified a putative-cleavage fragment of the disease protein in the brains of these transgenic mice and MJD patients that is cytotoxic above a critical concentration.

The roles of proteolysis and nuclear localisation in the toxicity of the polyglutamine diseases. A review

The polyglutamine disorders consist of a group of nine neurodegenerative diseases with overlapping phenotypes, but which affect distinct neuronal subsets, causing neuronal dysfunction and death. In the majority of these, the causative proteins share no homology to other known proteins, or to each other apart from the polyglutamine tract. The polyglutamine tracts themselves are toxic over a disease-specific threshold, and this common feature has suggested a common pathogenesis. The pathogenic mechanism(s) of this group of diseases is hotly debated, with proteolytic cleavage and nuclear accumulation both popular hypotheses. Such cleavage is thought to release toxic fragments containing an expanded polyglutamine tract, and may itself facilitate entry of cytoplasmic polyglutamine proteins to the nucleus. Numerous downstream effects including accumulation and apoptotic activation, misfolding, aggregation, and sequestration of other proteins including transcription factors and chaperones may then be initiated. It is uncertain whether all of the polyglutamine proteins undergo cleavage in vivo. Even in those in which proteolysis has been demonstrated, it remains unclear to what extent this also occurs in the wild-type proteins, or whether it is dependent on, or increased by, the expanded polyglutamine tract. Similarly, in at least one of these disorders (spinocerebellar ataxia type 6), nuclear localisation has not been demonstrated. The contradictory evidence for the production and role of proteolytic fragments and for nuclear localisation in toxicity, reviewed in this article, suggests that neither may be uniformly necessary steps in the pathogenesis of this group of diseases, and that, for all their apparent similarities, the exact pathogenic mechanisms may not be identical in each.

Structural and functional analysis of ataxin-2 and ataxin-3

Spinocerebellar ataxia types 2 (SCA2) and 3 (SCA3) are autosomal-dominantly inherited, neurodegenerative diseases caused by CAG repeat expansions in the coding regions of the genes encoding ataxin-2 and ataxin-3, respectively. To provide a rationale for further functional experiments, we explored the protein architectures of ataxin-2 and ataxin-3. Using structure-based multiple sequence alignments of homologous proteins, we investigated domains, sequence motifs, and interaction partners. Our analyses focused on presumably functional amino acids and the construction of tertiary structure models of the RNA-binding Lsm domain of ataxin-2 and the deubiquitinating Josephin domain of ataxin-3. We also speculate about distant evolutionary relationships of ubiquitin-binding UIM, GAT, UBA and CUE domains and helical ANTH and UBX domain extensions.

Neuronal intranuclear hyaline inclusion disease

Neuronal intranuclear hyaline inclusion disease (NIHID) is a rare neurodegenerative disorder characterized pathologically by the presence of eosinophilic intranuclear inclusions in neuronal and glial cells. It has been considered to be a heterogeneous disease entity because the clinical pictures of previously described cases were highly variable. In the present review, reported NIHID cases have been categorized into three clinical subgroups according to onset and disease duration, and the clinical phenotype of each subgroup is discussed. Neuronal intranuclear inclusions (NII) in NIHID are ubiquitinated and their prevalence is inversely correlated with neuronal loss, suggesting that NII formation is a protective mechanism involving the ubiquitin-proteasome-dependent proteolytic pathway. In several polyglutamine diseases, disease-related proteins containing abnormally expanded polyglutamine tracts aggregate in neuronal nuclei, resulting in NII formation. The similarity between NII in NIHID and polyglutamine diseases suggests that they are formed during a common proteolysis-related process that takes place in the nucleus. Although the pathogenetic mechanism underlying NIHID remains unknown, the data reviewed here suggest that it might be related to accumulation of as yet unidentified abnormal proteins or dysfunction of the intranuclear ubiquitin-proteasome pathway.

Misfolding promotes the ubiquitination of polyglutamine-expanded ataxin-3, the defective gene product in SCA3/MJD

A major hallmark of the polyglutamine diseases is the formation of neuronal intranuclear inclusions (NIIs) of the disease proteins that are ubiquitinated and often associated with various chaperones and proteasome components. Recently, misfolding has come to be considered one of the primary factors for polyglutamine protein aggregation, although, the nature of misfolding and the relationship between misfolding and ubiquitination of the expanded polyglutamine protein is not yet known. By using ataxin-3, the defective gene product of SCA3/MJD, we demonstrate here that the misfolding propensity and the cellular toxicity of a polyglutamine protein is directly proportional to the length of the glutamine repeats and inversely dependent on the size of the corresponding protein. The size of the polyglutamine bearing protein also inversely influences the binding of 1C2 antibody (an antibody that selectively recognizes polyglutamine expansion) to the polyglutamine protein and determines the minimum length of glutamine expansion to be recognized by 1C2 antibody, which suggests that the critical pathological range of glutamine repeats could also be dependent on the size of the corresponding protein. Ataxin-3 (both full length and truncated) with normal glutamine repeats are not ubiquitinated, however, ataxin-3 with expanded polyglutamine is ubiquitinated and the ubiquitination depends on the misfolding propensity of the polyglutamine expanded ataxin-3.