SCA7 mouse models show selective stabilization of mutant ataxin-7 and similar cellular responses in different neuronal cell types

The molecular mechanisms of spinocerebellar ataxias for DNA repeat expansion in disease

Spinocerebellar ataxias (SCAs) are a heterogenous group of neurodegenerative disorders which commonly inherited in an autosomal dominant manner. They cause muscle incoordination due to degeneration of the cerebellum and other parts of nervous system. Out of all the characterized (>50) SCAs, 14 SCAs are caused due to microsatellite repeat expansion mutations. Repeat expansions can result in toxic protein gain-of-function, protein loss-of-function, and/or RNA gain-of-function effects. The location and the nature of mutation modulate the underlying disease pathophysiology resulting in varying disease manifestations. Potential toxic effects of these mutations likely affect key major cellular processes such as transcriptional regulation, mitochondrial functioning, ion channel dysfunction and synaptic transmission. Involvement of several common pathways suggests interlinked function of genes implicated in the disease pathogenesis. A better understanding of the shared and distinct molecular pathogenic mechanisms in these diseases is required to develop targeted therapeutic tools and interventions for disease management. The prime focus of this review is to elaborate on how expanded 'CAG' repeats contribute to the common modes of neurotoxicity and their possible therapeutic targets in management of such devastating disorders.

UPS writes a new saga of SAGA

SAGA (Spt-Ada-Gcn5-Acetyltransferase), an evolutionarily conserved transcriptional co-activator among eukaryotes, is a large multi-subunit protein complex with two distinct enzymatic activities, namely HAT (Histone acetyltransferase) and DUB (De-ubiquitinase), and is targeted to the promoter by the gene-specific activator proteins for histone covalent modifications and PIC (Pre-initiation complex) formation in enhancing transcription (or gene activation). Targeting of SAGA to the gene promoter is further facilitated by the 19S RP (Regulatory particle) of the 26S proteasome (that is involved in targeted degradation of protein via ubiquitylation) in a proteolysis-independent manner. Moreover, SAGA is also recently found to be regulated by the 26S proteasome in a proteolysis-dependent manner via the ubiquitylation of its Sgf73/ataxin-7 component that is required for SAGA's integrity and DUB activity (and hence transcription), and is linked to various diseases including neurodegenerative disorders and cancer. Thus, SAGA itself and its targeting to the active gene are regulated by the UPS (Ubiquitin-proteasome system) with implications in diseases.

A novel ubiquitin-proteasome system regulation of Sgf73/ataxin-7 that maintains the integrity of the coactivator SAGA in orchestrating transcription

Ataxin-7 maintains the integrity of Spt-Ada-Gcn5-Acetyltransferase (SAGA), an evolutionarily conserved coactivator in stimulating preinitiation complex (PIC) formation for transcription initiation, and thus, its upregulation or downregulation is associated with various diseases. However, it remains unknown how ataxin-7 is regulated that could provide new insights into disease pathogenesis and therapeutic interventions. Here, we show that ataxin-7's yeast homologue, Sgf73, undergoes ubiquitylation and proteasomal degradation. Impairment of such regulation increases Sgf73's abundance, which enhances recruitment of TATA box-binding protein (TBP) (that nucleates PIC formation) to the promoter but impairs transcription elongation. Further, decreased Sgf73 level reduces PIC formation and transcription. Thus, Sgf73 is fine-tuned by ubiquitin-proteasome system (UPS) in orchestrating transcription. Likewise, ataxin-7 undergoes ubiquitylation and proteasomal degradation, alteration of which changes ataxin-7's abundance that is associated with altered transcription and cellular pathologies/diseases. Collectively, our results unveil a novel UPS regulation of Sgf73/ataxin-7 for normal cellular health and implicate alteration of such regulation in diseases.

RNA Foci Formation in a Retinal Glial Model for Spinocerebellar Ataxia Type 7

Spinocerebellar ataxia type 7 (SCA7) is a neurodegenerative disorder characterized by cerebellar ataxia and retinopathy. SCA7 is caused by a CAG expansion in the ATXN7 gene, which results in an extended polyglutamine (polyQ) tract in the encoded protein, the ataxin-7. PolyQ expanded ataxin-7 elicits neurodegeneration in cerebellar Purkinje cells, however, its impact on the SCA7-associated retinopathy remains to be addressed. Since Müller glial cells play an essential role in retinal homeostasis, we generate an inducible model for SCA7, based on the glial Müller MIO-M1 cell line. The SCA7 pathogenesis has been explained by a protein gain-of-function mechanism, however, the contribution of the mutant RNA to the disease cannot be excluded. In this direction, we found nuclear and cytoplasmic foci containing mutant RNA accompanied by subtle alternative splicing defects in MIO-M1 cells. RNA foci were also observed in cells from different lineages, including peripheral mononuclear leukocytes derived from SCA7 patient, suggesting that this molecular mark could be used as a blood biomarker for SCA7. Collectively, our data showed that our glial cell model exhibits the molecular features of SCA7, which makes it a suitable model to study the RNA toxicity mechanisms, as well as to explore therapeutic strategies aiming to alleviate glial dysfunction.

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

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

Key Modulators of the Stress Granule Response TIA1, TDP-43, and G3BP1 Are Altered by Polyglutamine-Expanded ATXN7

Spinocerebellar ataxia type 7 (SCA7) and other polyglutamine (polyQ) diseases are caused by expansions of polyQ repeats in disease-specific proteins. Aggregation of the polyQ proteins resulting in various forms of cellular stress, that could induce the stress granule (SG) response, is believed to be a common pathological mechanism in these disorders. SGs can contribute to cell survival but have also been suggested to exacerbate disease pathology by seeding protein aggregation. In this study, we show that two SG-related proteins, TDP-43 and TIA1, are sequestered into the aggregates formed by polyQ-expanded ATXN7 in SCA7 cells. Interestingly, mutant ATXN7 also localises to induced SGs, and this association altered the shape of the SGs. In spite of this, neither the ability to induce nor to disassemble SGs, in response to arsenite stress induction or relief, was affected in SCA7 cells. Moreover, we could not observe any change in the number of ATXN7 aggregates per cell following SG induction, although a small, non-significant, increase in total aggregated ATXN7 material could be detected using filter trap. However, mutant ATXN7 expression in itself increased the speckling of the SG-nucleating protein G3BP1 and the SG response. Taken together, our results indicate that the SG response is induced, and although some key modulators of SGs show altered behaviour, the dynamics of SGs appear normal in the presence of mutant ATXN7.

The Molecular Basis of Spinocerebellar Ataxia Type 7

Spinocerebellar ataxia (SCA) type 7 (SCA7) is caused by a CAG trinucleotide repeat expansion in the ataxin 7 (ATXN7) gene, which results in polyglutamine expansion at the amino terminus of the ATXN7 protein. Although ATXN7 is expressed widely, the best characterized symptoms of SCA7 are remarkably tissue specific, including blindness and degeneration of the brain and spinal cord. While it is well established that ATXN7 functions as a subunit of the Spt Ada Gcn5 acetyltransferase (SAGA) chromatin modifying complex, the mechanisms underlying SCA7 remain elusive. Here, we review the symptoms of SCA7 and examine functions of ATXN7 that may provide further insights into its pathogenesis. We also examine phenotypes associated with polyglutamine expanded ATXN7 that are not considered symptoms of SCA7.

Consensus Paper: Strengths and Weaknesses of Animal Models of Spinocerebellar Ataxias and Their Clinical Implications

Spinocerebellar ataxias (SCAs) represent a large group of hereditary degenerative diseases of the nervous system, in particular the cerebellum, and other systems that manifest with a variety of progressive motor, cognitive, and behavioral deficits with the leading symptom of cerebellar ataxia. SCAs often lead to severe impairments of the patient's functioning, quality of life, and life expectancy. For SCAs, there are no proven effective pharmacotherapies that improve the symptoms or substantially delay disease progress, i.e., disease-modifying therapies. To study SCA pathogenesis and potential therapies, animal models have been widely used and are an essential part of pre-clinical research. They mainly include mice, but also other vertebrates and invertebrates. Each animal model has its strengths and weaknesses arising from model animal species, type of genetic manipulation, and similarity to human diseases. The types of murine and non-murine models of SCAs, their contribution to the investigation of SCA pathogenesis, pathological phenotype, and therapeutic approaches including their advantages and disadvantages are reviewed in this paper. There is a consensus among the panel of experts that (1) animal models represent valuable tools to improve our understanding of SCAs and discover and assess novel therapies for this group of neurological disorders characterized by diverse mechanisms and differential degenerative progressions, (2) thorough phenotypic assessment of individual animal models is required for studies addressing therapeutic approaches, (3) comparative studies are needed to bring pre-clinical research closer to clinical trials, and (4) mouse models complement cellular and invertebrate models which remain limited in terms of clinical translation for complex neurological disorders such as SCAs.

SCA7 Mouse Cerebellar Pathology Reveals Preferential Downregulation of Key Purkinje Cell-Identity Genes and Shared Disease Signature with SCA1 and SCA2

Spinocerebellar ataxia type 7 (SCA7) is an inherited neurodegenerative disease mainly characterized by motor incoordination because of progressive cerebellar degeneration. SCA7 is caused by polyglutamine expansion in ATXN7, a subunit of the transcriptional coactivator SAGA, which harbors histone modification activities. Polyglutamine expansions in specific proteins are also responsible for SCA1-SCA3, SCA6, and SCA17; however, the converging and diverging pathomechanisms remain poorly understood. Using a new SCA7 knock-in mouse, SCA7140Q/5Q, we analyzed gene expression in the cerebellum and assigned gene deregulation to specific cell types using published datasets. Gene deregulation affects all cerebellar cell types, although at variable degree, and correlates with alterations of SAGA-dependent epigenetic marks. Purkinje cells (PCs) are by far the most affected neurons and show reduced expression of 83 cell-type identity genes, including these critical for their spontaneous firing activity and synaptic functions. PC gene downregulation precedes morphologic alterations, pacemaker dysfunction, and motor incoordination. Strikingly, most PC genes downregulated in SCA7 have also decreased expression in SCA1 and SCA2 mice, revealing converging pathomechanisms and a common disease signature involving cGMP-PKG and phosphatidylinositol signaling pathways and LTD. Our study thus points out molecular targets for therapeutic development, which may prove beneficial for several SCAs. Furthermore, we show that SCA7140Q/5Q males and females exhibit the major disease features observed in patients, including cerebellar damage, cerebral atrophy, peripheral nerves pathology, and photoreceptor dystrophy, which account for progressive impairment of behavior, motor, and visual functions. SCA7140Q/5Q mice represent an accurate model for the investigation of different aspects of SCA7 pathogenesis.SIGNIFICANCE STATEMENT Spinocerebellar ataxia 7 (SCA7) is one of the several forms of inherited SCAs characterized by cerebellar degeneration because of polyglutamine expansion in specific proteins. The ATXN7 involved in SCA7 is a subunit of SAGA transcriptional coactivator complex. To understand the pathomechanisms of SCA7, we determined the cell type-specific gene deregulation in SCA7 mouse cerebellum. We found that the Purkinje cells are the most affected cerebellar cell type and show downregulation of a large subset of neuronal identity genes, critical for their spontaneous firing and synaptic functions. Strikingly, the same Purkinje cell genes are downregulated in mouse models of two other SCAs. Thus, our work reveals a disease signature shared among several SCAs and uncovers potential molecular targets for their treatment.

What is the Pathogenic CAG Expansion Length in Huntington's Disease?

Huntington's disease (HD) (OMIM 143100) is caused by an expanded CAG repeat tract in the HTT gene. The inherited CAG length is known to expand further in somatic and germline cells in HD subjects. Age at onset of the disease is inversely correlated with the inherited CAG length, but is further modulated by a series of genetic modifiers which are most likely to act on the CAG repeat in HTT that permit it to further expand. Longer repeats are more prone to expansions, and this expansion is age dependent and tissue-specific. Given that the inherited tract expands through life and most subjects develop disease in mid-life, this implies that in cells that degenerate, the CAG length is likely to be longer than the inherited length. These findings suggest two thresholds- the inherited CAG length which permits further expansion, and the intracellular pathogenic threshold, above which cells become dysfunctional and die. This two-step mechanism has been previously proposed and modelled mathematically to give an intracellular pathogenic threshold at a tract length of 115 CAG (95% confidence intervals 70- 165 CAG). Empirically, the intracellular pathogenic threshold is difficult to determine. Clues from studies of people and models of HD, and from other diseases caused by expanded repeat tracts, place this threshold between 60- 100 CAG, most likely towards the upper part of that range. We assess this evidence and discuss how the intracellular pathogenic threshold in manifest disease might be better determined. Knowing the cellular pathogenic threshold would be informative for both understanding the mechanism in HD and deploying treatments.

Gene Deregulation and Underlying Mechanisms in Spinocerebellar Ataxias With Polyglutamine Expansion

Polyglutamine spinocerebellar ataxias (polyQ SCAs) include SCA1, SCA2, SCA3, SCA6, SCA7, and SCA17 and constitute a group of adult onset neurodegenerative disorders caused by the expansion of a CAG repeat sequence located within the coding region of specific genes, which translates into polyglutamine tract in the corresponding proteins. PolyQ SCAs are characterized by degeneration of the cerebellum and its associated structures and lead to progressive ataxia and other diverse symptoms. In recent years, gene and epigenetic deregulations have been shown to play a critical role in the pathogenesis of polyQ SCAs. Here, we provide an overview of the functions of wild type and pathogenic polyQ SCA proteins in gene regulation, describe the extent and nature of gene expression changes and their pathological consequences in diseases, and discuss potential avenues to further investigate converging and distinct disease pathways and to develop therapeutic strategies.

Impairment of Lysosome Function and Autophagy in Rare Neurodegenerative Diseases

Rare genetic diseases affect a limited number of patients, but their etiology is often known, facilitating the development of reliable animal models and giving the opportunity to investigate physiopathology. Lysosomal storage disorders are a group of rare diseases due to primary alteration of lysosome function. These diseases are often associated with neurological symptoms, which highlighted the importance of lysosome in neurodegeneration. Likewise, other groups of rare neurodegenerative diseases also present lysosomal alteration. Lysosomes fuse with autophagosomes and endosomes to allow the degradation of their content thanks to hydrolytic enzymes. It has emerged that alteration of the autophagy–lysosome pathway could play a critical role in neuronal death in many neurodegenerative diseases. Using a repertoire of selected rare neurodegenerative diseases, we highlight that a variety of alterations of the autophagy–lysosome pathway are associated with neuronal death. Yet, in most cases, it is still unclear why alteration of this pathway can lead to neurodegeneration.

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Lysosome function is impaired in many rare neurodegenerative diseases, making it a convergent point for these diseases.

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Impaired lysosome function is associated with alteration of the autophagy pathway.

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Autophagy–lysosome pathway can be impaired at various steps in different rare neurodegenerative diseases.

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The mechanisms linking impaired autophagy–lysosome pathway to neurodegeneration are still not fully elucidated.

Motor Performances of Spontaneous and Genetically Modified Mutants with Cerebellar Atrophy

Chance discovery of spontaneous mutants with atrophy of the cerebellar cortex has unearthed genes involved in optimizing motor coordination. Rotorod, stationary beam, and suspended wire tests are useful in delineating behavioral phenotypes of spontaneous mutants with cerebellar atrophy such as Grid2^Lc, Grid2^ho, Rora^sg, Agtpbp1^pcd, Reln^rl, and Dab1^scm. Likewise, transgenic or null mutants serving as experimental models of spinocerebellar ataxia (SCA) are phenotyped with the same tests. Among experimental models of autosomal dominant SCA, rotorod deficits were reported in SCA1 to 3, SCA5 to 8, SCA14, SCA17, and SCA27 and stationary beam deficits in SCA1 to 3, SCA5, SCA6, SCA13, SCA17, and SCA27. Beam tests are sensitive to experimental therapies of various kinds including molecules affecting glutamate signaling, mesenchymal stem cells, anti-oligomer antibodies, lentiviral vectors carrying genes, interfering RNAs, or neurotrophic factors, and interbreeding with other mutants.

Molecular Targets and Therapeutic Strategies in Spinocerebellar Ataxia Type 7

Spinocerebellar ataxia type 7 (SCA7) is a rare autosomal dominant neurodegenerative disorder characterized by progressive neuronal loss in the cerebellum, brainstem, and retina, leading to cerebellar ataxia and blindness as major symptoms. SCA7 is due to the expansion of a CAG triplet repeat that is translated into a polyglutamine tract in ATXN7. Larger SCA7 expansions are associated with earlier onset of symptoms and more severe and rapid disease progression. Here, we summarize the pathological and genetic aspects of SCA7, compile the current knowledge about ATXN7 functions, and then focus on recent advances in understanding the pathogenesis and in developing biomarkers and therapeutic strategies. ATXN7 is a bona fide subunit of the multiprotein SAGA complex, a transcriptional coactivator harboring chromatin remodeling activities, and plays a role in the differentiation of photoreceptors and Purkinje neurons, two highly vulnerable neuronal cell types in SCA7. Polyglutamine expansion in ATXN7 causes its misfolding and intranuclear accumulation, leading to changes in interactions with native partners and/or partners sequestration in insoluble nuclear inclusions. Studies of cellular and animal models of SCA7 have been crucial to unveil pathomechanistic aspects of the disease, including gene deregulation, mitochondrial and metabolic dysfunctions, cell and non-cell autonomous protein toxicity, loss of neuronal identity, and cell death mechanisms. However, a better understanding of the principal molecular mechanisms by which mutant ATXN7 elicits neurotoxicity, and how interconnected pathogenic cascades lead to neurodegeneration is needed for the development of effective therapies. At present, therapeutic strategies using nucleic acid-based molecules to silence mutant ATXN7 gene expression are under development for SCA7.

Lentiviral vector-mediated overexpression of mutant ataxin-7 recapitulates SCA7 pathology and promotes accumulation of the FUS/TLS and MBNL1 RNA-binding proteins

Background

We used lentiviral vectors (LVs) to generate a new SCA7 animal model overexpressing a truncated mutant ataxin-7 (MUT ATXN7) fragment in the mouse cerebellum, in order to characterize the specific neuropathological and behavioral consequences of the genetic defect in this brain structure.

Results

LV-mediated overexpression of MUT ATXN7 into the cerebellum of C57/BL6 adult mice induced neuropathological features similar to that observed in patients, such as intranuclear aggregates in Purkinje cells (PC), loss of synaptic markers, neuroinflammation, and neuronal death. No neuropathological changes were observed when truncated wild-type ataxin-7 (WT ATXN7) was injected. Interestingly, the local delivery of LV-expressing mutant ataxin-7 (LV-MUT-ATXN7) into the cerebellum of wild-type mice also mediated the development of an ataxic phenotype at 8 to 12 weeks post-injection. Importantly, our data revealed abnormal levels of the FUS/TLS, MBNL1, and TDP-43 RNA-binding proteins in the cerebellum of the LV-MUT-ATXN7 injected mice. MUT ATXN7 overexpression induced an increase in the levels of the pathological phosphorylated TDP-43, and a decrease in the levels of soluble FUS/TLS, with both proteins accumulating within ATXN7-positive intranuclear inclusions. MBNL1 also co-aggregated with MUT ATXN7 in most PC nuclear inclusions. Interestingly, no MBNL2 aggregation was observed in cerebellar MUT ATXN7 aggregates. Immunohistochemical studies in postmortem tissue from SCA7 patients and SCA7 knock-in mice confirmed SCA7-induced nuclear accumulation of FUS/TLS and MBNL1, strongly suggesting that these proteins play a physiopathological role in SCA7.

Conclusions

This study validates a novel SCA7 mouse model based on lentiviral vectors, in which strong and sustained expression of MUT ATXN7 in the cerebellum was found sufficient to generate motor defects.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-016-0123-2) contains supplementary material, which is available to authorized users.

Quantification of Ataxin-3 and Ataxin-7 aggregates formed in vivo in Drosophila reveals a threshold of aggregated polyglutamine proteins associated with cellular toxicity

Polyglutamine diseases are nine dominantly inherited neurodegenerative pathologies caused by the expansion of a polyglutamine domain in a protein responsible for the disease. This expansion leads to protein aggregation, inclusion formation and toxicity. Despite numerous studies focusing on the subject, whether soluble polyglutamine proteins are responsible for toxicity or not remains debated. To focus on this matter, we evaluated the level of soluble and insoluble truncated pathological Ataxin-3 in vivo in Drosophila, in presence or absence of two suppressors (i.e. Hsp70 and non-pathological Ataxin-3) and along aging. Suppressing truncated Ataxin-3-induced toxicity resulted in a lowered level of aggregated polyglutamine protein. Interestingly, aggregates accumulated as flies aged and reached a maximum level when cell death was detected. Our results were similar with two other pathological polyglutamine proteins, namely truncated Ataxin-7 and full-length Ataxin-3. Our data suggest that accumulation of insoluble aggregates beyond a critical threshold could be responsible for toxicity.

Poly(Q) Expansions in ATXN7 Affect Solubility but Not Activity of the SAGA Deubiquitinating Module

Spinocerebellar ataxia type 7 (SCA7) is a debilitating neurodegenerative disease caused by expansion of a polyglutamine [poly(Q)] tract in ATXN7, a subunit of the deubiquitinase (DUB) module (DUBm) in the SAGA complex. The effects of ATXN7-poly(Q) on DUB activity are not known. To address this important question, we reconstituted the DUBm in vitro with either wild-type ATXN7 or a pathogenic form, ATXN7-92Q NT, with 92 Q residues at the N terminus (NT). We found that both forms of ATXN7 greatly enhance DUB activity but that ATXN7-92Q NT is largely insoluble unless it is incorporated into the DUBm. Cooverexpression of DUBm components in human astrocytes also promoted the solubility of ATXN7-92Q, inhibiting its aggregation into nuclear inclusions that sequester DUBm components, leading to global increases in ubiquitinated H2B (H2Bub) levels. Global H2Bub levels were also increased in the cerebellums of mice in a SCA7 mouse model. Our findings indicate that although ATXN7 poly(Q) expansions do not change the enzymatic activity of the DUBm, they likely contribute to SCA7 by initiating aggregates that sequester the DUBm away from its substrates.

Allele-specific silencing of mutant Ataxin-7 in SCA7 patient-derived fibroblasts

Polyglutamine (polyQ) disorders are inherited neurodegenerative conditions defined by a common pathogenic CAG repeat expansion leading to a toxic gain-of-function of the mutant protein. Consequences of this toxicity include activation of heat-shock proteins (HSPs), impairment of the ubiquitin-proteasome pathway and transcriptional dysregulation. Several studies in animal models have shown that reducing levels of toxic protein using small RNAs would be an ideal therapeutic approach for such disorders, including spinocerebellar ataxia-7 (SCA7). However, testing such RNA interference (RNAi) effectors in genetically appropriate patient cell lines with a disease-relevant phenotype has yet to be explored. Here, we have used primary adult dermal fibroblasts from SCA7 patients and controls to assess the endogenous allele-specific silencing of ataxin-7 by two distinct siRNAs. We further identified altered expression of two disease-relevant transcripts in SCA7 patient cells: a twofold increase in levels of the HSP DNAJA1 and a twofold decrease in levels of the de-ubiquitinating enzyme, UCHL1. After siRNA treatment, the expression of both genes was restored towards normal levels. To our knowledge, this is the first time that allele-specific silencing of mutant ataxin-7, targeting a common SNP, has been demonstrated in patient cells. These findings highlight the advantage of an allele-specific RNAi-based therapeutic approach, and indicate the value of primary patient-derived cells as useful models for mechanistic studies and for measuring efficacy of RNAi effectors on a patient-to-patient basis in the polyQ diseases.

Loss of Drosophila Ataxin-7, a SAGA subunit, reduces H2B ubiquitination and leads to neural and retinal degeneration

The SAGA chromatin-modifying complex plays a critical role in gene regulation and has been implicated in processes such as carcinogenesis and neurodegeneration. SAGA bears both acetyltransferase and deubiquitinase activities, and Ataxin-7 anchors the deubiquitinase activity to the larger complex. Workman and colleagues now show that in contrast to yeast, loss of Drosophila Ataxin-7 results in a global reduction in H2B ubiquitination, an effect conserved in human cells. Furthermore, reduced Ataxin-7 results in neural and retinal degeneration, impaired movement, and decreased life span.

The Spt–Ada–Gcn5–acetyltransferase (SAGA) chromatin-modifying complex possesses acetyltransferase and deubiquitinase activities. Within this modular complex, Ataxin-7 anchors the deubiquitinase activity to the larger complex. Here we identified and characterized Drosophila Ataxin-7 and found that reduction of Ataxin-7 protein results in loss of components from the SAGA complex. In contrast to yeast, where loss of Ataxin-7 inactivates the deubiquitinase and results in increased H2B ubiquitination, loss of Ataxin-7 results in decreased H2B ubiquitination and H3K9 acetylation without affecting other histone marks. Interestingly, the effect on ubiquitination was conserved in human cells, suggesting a novel mechanism regulating histone deubiquitination in higher organisms. Consistent with this mechanism in vivo, we found that a recombinant deubiquitinase module is active in the absence of Ataxin-7 in vitro. When we examined the consequences of reduced Ataxin-7 in vivo, we found that flies exhibited pronounced neural and retinal degeneration, impaired movement, and early lethality.

Histone deacetylase-3 interacts with ataxin-7 and is altered in a spinocerebellar ataxia type 7 mouse model

Spinocerebellar ataxia type 7 (SCA7) is caused by a toxic polyglutamine (polyQ) expansion in the N-terminus of the protein ataxin-7. Ataxin-7 has a known function in the histone acetylase complex, Spt/Ada/Gcn5 acetylase (STAGA) chromatin-remodeling complex. We hypothesized that some histone deacetylase (HDAC) family members would impact the posttranslational modification of normal and expanded ataxin-7 and possibly modulate ataxin-7 function or neurotoxicity associated with the polyQ expansion. Interestingly, when we coexpressed each HDAC family member in the presence of ataxin-7 we found that HDAC3 increased the posttranslational modification of normal and expanded ataxin-7. Specifically, HDAC3 stabilized ataxin-7 and increased modification of the protein. Further, HDAC3 physically interacts with ataxin-7. The physical interaction of HDAC3 with normal and polyQ-expanded ataxin-7 affects the toxicity in a polyQ-dependent manner. We detect robust HDAC3 expression in neurons and glia in the cerebellum and an increase in the levels of HDAC3 in SCA7 mice. Consistent with this we found altered lysine acetylation levels and deacetylase activity in the brains of SCA7 transgenic mice. This study implicates HDAC3 and ataxin-7 interaction as a target for therapeutic intervention in SCA7, adding to a growing list of neurodegenerative diseases that may be treated by HDAC inhibitors.

Inhibition of autophagy via p53-mediated disruption of ULK1 in a SCA7 polyglutamine disease model

Spinocerebellar ataxia type 7 (SCA7) is one of nine neurodegenerative disorders caused by expanded polyglutamine domains. These so-called polyglutamine (polyQ) diseases are all characterized by aggregation. Reducing the level of aggregating polyQ proteins via pharmacological activation of autophagy has been suggested as a therapeutic approach. However, recently, evidence implicating autophagic dysfunction in these disorders has also been reported. In this study, we show that the SCA7 polyglutamine protein ataxin-7 (ATXN7) reduces the autophagic activity via a previously unreported mechanism involving p53-mediated disruption of two key proteins involved in autophagy initiation. We show that in mutant ATXN7 cells, an increased p53-FIP200 interaction and co-aggregation of p53-FIP200 into ATXN7 aggregates result in decreased soluble FIP200 levels and subsequent destabilization of ULK1. Together, this leads to a decreased capacity for autophagy induction via the ULK1-FIP200-Atg13-Atg101 complex. We also show that treatment with a p53 inhibitor, or a blocker of ATXN7 aggregation, can restore the soluble levels of FIP200 and ULK1, as well as increase the autophagic activity and reduce ATXN7 toxicity. Understanding the mechanism behind polyQ-mediated inhibition of autophagy is of importance if therapeutic approaches based on autophagy stimulation should be developed for these disorders.

Reelin is a target of polyglutamine expanded ataxin-7 in human spinocerebellar ataxia type 7 (SCA7) astrocytes

Spinocerebellar ataxia type 7 (SCA7) is an autosomal-dominant neurodegenerative disorder that results from polyglutamine expansion of the ataxin-7 (ATXN7) protein. Remarkably, although mutant ATXN7 is expressed throughout the body, pathology is restricted primarily to the cerebellum and retina. One major goal has been to identify factors that contribute to the tissue specificity of SCA7. Here we describe the development and use of a human astrocyte cell culture model to identify reelin, a factor intimately involved in the development and maintenance of Purkinje cells and the cerebellum as a whole, as an ATXN7 target gene. We found that polyglutamine expansion decreased ATXN7 occupancy, which correlated with increased levels of histone H2B monoubiquitination, at the reelin promoter. Treatment with trichostatin A, but not other histone deacetylase inhibitors, partially restored reelin transcription and promoted the accumulation of mutant ATXN7 into nuclear inclusions. Our findings suggest that reelin could be a previously unknown factor involved in the tissue specificity of SCA7 and that trichostatin A may ameliorate deleterious effects of the mutant ATXN7 protein by promoting its sequestration away from promoters into nuclear inclusions.

Mouse models of polyglutamine diseases: review and data table. Part I

Polyglutamine (polyQ) disorders share many similarities, such as a common mutation type in unrelated human causative genes, neurological character, and certain aspects of pathogenesis, including morphological and physiological neuronal alterations. The similarities in pathogenesis have been confirmed by findings that some experimental in vivo therapy approaches are effective in multiple models of polyQ disorders. Additionally, mouse models of polyQ diseases are often highly similar between diseases with respect to behavior and the features of the disease. The common features shared by polyQ mouse models may facilitate the investigation of polyQ disorders and may help researchers explore the mechanisms of these diseases in a broader context. To provide this context and to promote the understanding of polyQ disorders, we have collected and analyzed research data about the characterization and treatment of mouse models of polyQ diseases and organized them into two complementary Excel data tables. The data table that is presented in this review (Part I) covers the behavioral, molecular, cellular, and anatomic characteristics of polyQ mice and contains the most current knowledge about polyQ mouse models. The structure of this data table is designed in such a way that it can be filtered to allow for the immediate retrieval of the data corresponding to a single mouse model or to compare the shared and unique aspects of many polyQ models. The second data table, which is presented in another publication (Part II), covers therapeutic research in mouse models by summarizing all of the therapeutic strategies employed in the treatment of polyQ disorders, phenotypes that are used to examine the effects of the therapy, and therapeutic outcomes.

Genetically engineered mouse models of the trinucleotide-repeat spinocerebellar ataxias

The spinocerebellar ataxias (SCAs) are dominantly inherited disorders that primarily affect coordination of motor function but also frequently involve other brain functions. The models described in this review address mechanisms of trinucleotide-repeat expansions, particularly those relating to polyglutamine expression in the mutant proteins. Modeling chronic late-onset human ataxias in mice is difficult because of their short life-span. While this potential hindrance has been partially overcome by using over-expression of the mutant gene, and/or worsening of the mutation by increasing the length of the trinucleotide repeat expansion, interpretation of results from such models and extrapolation to the human condition should be cautious. Nevertheless, genetically engineered murine models of these diseases have enhanced our understanding of the pathogenesis of many of these conditions. A common theme in many of the polyglutamine-repeat diseases is nuclear localization of mutant protein, with resultant effects on gene regulation. Conditional mutant models and transgenic knock-down therapy have demonstrated the potential for reversibility of disease when production of mutant protein is halted. Several other genetically engineered murine models of SCA also have begun to show utility in the identification and assessment of more classical drug-based therapeutic modalities.

Differential degradation of full-length and cleaved ataxin-7 fragments in a novel stable inducible SCA7 model

Spinocerebellar ataxia type 7 (SCA7) is one of nine neurodegenerative disorders caused by expanded polyglutamine repeats, and a common toxic gain-of-function mechanism has been proposed. Proteolytic cleavage of several polyglutamine proteins has been identified and suggested to modulate the polyglutamine toxicity. In this study, we show that full-length and cleaved fragments of the SCA7 disease protein ataxin-7 (ATXN7) are differentially degraded. We found that the ubiquitin–proteosome system (UPS) was essential for the degradation of full-length endogenous ATXN7 or transgenic full-length ATXN7 with a normal or expanded glutamine repeat in both HEK 293T and stable PC12 cells. However, a similar contribution by UPS and autophagy was found for the degradation of proteolytically cleaved ATXN7 fragments. Furthermore, in our novel stable inducible PC12 model, induction of mutant ATXN7 expression resulted in toxicity and this toxicity was worsened by inhibition of either UPS or autophagy. In contrast, pharmacological activation of autophagy could ameliorate the ATXN7-induced toxicity. Based on our findings, we propose that both UPS and autophagy are important for the reduction of mutant ataxin-7-induced toxicity, and enhancing ATXN7 clearance through autophagy could be used as a potential therapeutic strategy in SCA7.

RNA therapy for polyglutamine neurodegenerative diseases

Polyglutamine neurodegenerative diseases result from the expansion of a trinucleotide CAG repeat, encoding a polyglutamine tract in the disease-causing protein. The process by which each polyglutamine protein exerts its toxicity is complex, involving a variety of mechanisms including transcriptional dysregulation, proteasome impairment and mitochondrial dysfunction. Thus, the most effective and widely applicable therapies are likely to be those designed to eliminate production of the mutant protein upstream of these deleterious effects. RNA-based approaches represent promising therapeutic strategies for polyglutamine diseases, offering the potential to suppress gene expression in a sequence-specific manner at the transcriptional and post-transcriptional levels. In particular, gene silencing therapies capable of discrimination between mutant and wildtype alleles, based on disease-linked polymorphisms or CAG repeat length, might prove crucial in cases where a loss of wild type function is deleterious. Novel methods, such as gene knockdown and replacement, seek to eliminate the technical difficulties associated with allele-specific silencing by avoiding the need to target specific mutations. With a variety of RNA technologies currently being developed to target multiple facets of polyglutamine pathogenesis, the emergence of an effective therapy seems imminent. However, numerous technical obstacles associated with design, discrimination and delivery must be overcome before RNA therapy can be effectively applied in the clinical setting.

Spinocerebellar ataxia type 7

Spinocerebellar ataxia type 7 (SCA7) is associated with progressive blindness, dominant transmission, and marked anticipation. SCA7 represents one of the polyglutamine expansion diseases with increase of CAG repeats. The gene maps to chromosome 3p12-p21.1. Normal values of CAG repeats range from 4 to 18. The SCA7 gene encodes a protein of largely unknown function, called ataxin-7. SCA7 is reported in many countries and ethnic groups. Its phenotypic expression depends on the number of expanded repeats. The infantile phenotype is very severe, with more than 100 repeats. The classic type has 50 to 55 repeats and is characterized by a combination of visual and ataxic disturbances lasting for 20-40 years.When the number of CAG repeats is between 36 and 43, the evolution is much slower, with few or no retinal abnormalities. A CAG repeat number from 18 to 35 is asymptomatic but predisposes to the development of the disorder when expanding to the pathological range through transmission. The diagnosis is made by molecular genetics. The neuropathology of the disorder includes atrophy of the spinocerebellar pathways, pyramidal tracts, and motor nuclei in the brainstem and spinal cord, a cone-rod sytrophy of the retina, and ataxin-7 immunoreactive neuronal intranuclear inclusions. The neuropathological features vary as a function of the number of CAG repeats. Present research deals mainly with the study of ataxin-7 in transfected neural cells and transgenic mouse models.

Comparison of an expanded ataxia interactome with patient medical records reveals a relationship between macular degeneration and ataxia

Spinocerebellar ataxias 6 and 7 (SCA6 and SCA7) are neurodegenerative disorders caused by expansion of CAG repeats encoding polyglutamine (polyQ) tracts in CACNA1A, the alpha1A subunit of the P/Q-type calcium channel, and ataxin-7 (ATXN7), a component of a chromatin-remodeling complex, respectively. We hypothesized that finding new protein partners for ATXN7 and CACNA1A would provide insight into the biology of their respective diseases and their relationship to other ataxia-causing proteins. We identified 118 protein interactions for CACNA1A and ATXN7 linking them to other ataxia-causing proteins and the ataxia network. To begin to understand the biological relevance of these protein interactions within the ataxia network, we used OMIM to identify diseases associated with the expanded ataxia network. We then used Medicare patient records to determine if any of these diseases co-occur with hereditary ataxia. We found that patients with ataxia are at 3.03-fold greater risk of these diseases than Medicare patients overall. One of the diseases comorbid with ataxia is macular degeneration (MD). The ataxia network is significantly (P= 7.37 × 10⁻⁵) enriched for proteins that interact with known MD-causing proteins, forming a MD subnetwork. We found that at least two of the proteins in the MD subnetwork have altered expression in the retina of Ataxin-7^266Q/+ mice suggesting an in vivo functional relationship with ATXN7. Together these data reveal novel protein interactions and suggest potential pathways that can contribute to the pathophysiology of ataxia, MD, and diseases comorbid with ataxia.

Histone acetylation, acetyltransferases, and ataxia--alteration of histone acetylation and chromatin dynamics is implicated in the pathogenesis of polyglutamine-expansion disorders

Eukaryotic chromosomal DNA is packaged into nucleosomes to form a dynamic structure known as chromatin. The compaction of DNA within chromatin poses a unique hindrance with regards to the accessibility of the DNA to enzymes involved in replication, transcriptional regulation, and repair. The physical structure and physiological activity of chromatin are regulated through a diverse set of posttranslational modifications, histone exchange, and structural remodeling. Of the covalent chromatin modifications, the acetylation of lysine residues within histone proteins by acetyltransferase enzymes, such as GCN5, is one of the most prevalent and important steps in the regulation of chromatin function. Alteration of histone acetyltransferase activity can easily result in the dysregulation of gene transcription and ultimately the onset of a disease state. Many transcription factors contain polyglutamine regions within their primary sequence. Mutations resulting in the elongation of these polyglutamine tracts are associated with a disease family known as the polyglutamine expansion disorders. Spinocerebellar ataxia type 7 (SCA7) is one of the nine diseases that are grouped in this family and is caused by polyglutamine expansion of the ataxin-7 protein, which is a component of the GCN5-containing human SAGA histone acetyltransferase complex. Mutation of ataxin-7 in this manner has been shown to disrupt the structural integrity of the SAGA complex and result in aberrant chromatin acetylation patterns at the promoters of genes involved in the normal function of tissues that are affected by the disease. The specific aspects of molecular pathology are not currently understood; however, studies carried out in laboratory systems ranging from the budding yeast Saccharomyces cerevisiae to transgenic mouse models and cultured human cells are poised to allow for the elucidation of disease mechanisms and subsequent therapeutic approaches.

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.

Molecular pathogenesis and cellular pathology of spinocerebellar ataxia type 7 neurodegeneration

Spinocerebellar ataxia type 7 (SCA7) is unique among CAG/polyglutamine (polyQ) repeat diseases due to dramatic intergenerational instability in repeat length and an associated cone-rod dystrophy retinal degeneration phenotype. SCA7 is caused by a polyQ expansion in the protein ataxin-7. Like other neurodegenerative diseases caused by polyQ expansion mutations, the spectrum of clinical severity and disease progression worsens with increasing polyQ length. Several potential mechanisms for the molecular pathogenesis of polyQ-expanded ataxin-7 have been suggested. These include, but are not limited to, alteration of endogenous ataxin-7 function, abnormal processing and stability of polyQ ataxin-7, and alteration of transcriptional regulation via interaction of polyQ-expanded ataxin-7 with other transcriptional regulators. Ataxin-7's normal function as a transcription factor may contribute to the selective vulnerability of specific cellular populations in SCA7, and the resolution of the mechanistic basis of this pathogenic cascade is a major focus of SCA7 disease research. PolyQ-expanded ataxin-7 can cause non-cell autonomous neurodegeneration in cerebellar Purkinje cells. Advances in understanding SCA7's molecular basis have led to important insights into cell-type specific neurodegeneration. We expect that further study of ataxin-7 normal function, insights into the molecular basis of SCA7 neurodegeneration, and the development of therapeutic interventions for SCA7 will greatly influence related endeavors directed at other CAG/polyQ repeat diseases.

Glial cells as intrinsic components of non-cell-autonomous neurodegenerative disease

A lesson from dominantly inherited forms of diverse neurodegenerative diseases, including amyotrophic lateral sclerosis, spinocerebellar ataxia and Huntington's disease, is that the selective dysfunction or death of the neuronal population most at risk in each disease is not mediated solely by damage from the mutant protein within the target neurons. The disease-causing toxic process, which in each case is caused by mutation in a gene that is widely or ubiquitously expressed, involves damage done by mutant proteins within the non-neuronal glial cells of the central nervous system, especially astrocytes and microglia. The disease mechanism is non-cell-autonomous, with toxicity derived from glia as a prominent contributor driving disease progression and in some instances even disease initiation.

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.

Proteolytic cleavage of ataxin-7 by caspase-7 modulates cellular toxicity and transcriptional dysregulation

Spinocerebellar ataxia type 7 (SCA7) is a polyglutamine (polyQ) disorder characterized by specific degeneration of cerebellar, brainstem, and retinal neurons. Although they share little sequence homology, proteins implicated in polyQ disorders have common properties beyond their characteristic polyQ tract. These include the production of proteolytic fragments, nuclear accumulation, and processing by caspases. Here we report that ataxin-7 is cleaved by caspase-7, and we map two putative caspase-7 cleavage sites to Asp residues at positions 266 and 344 of the ataxin-7 protein. Site-directed mutagenesis of these two caspase-7 cleavage sites in the polyQ-expanded form of ataxin-7 produces an ataxin-7 D266N/D344N protein that is resistant to caspase cleavage. Although ataxin-7 displays toxicity, forms nuclear aggregates, and represses transcription in human embryonic kidney 293T cells in a polyQ length-dependent manner, expression of the non-cleavable D266N/D344N form of polyQ-expanded ataxin-7 attenuated cell death, aggregate formation, and transcriptional interference. Expression of the caspase-7 truncation product of ataxin-7-69Q or -92Q, which removes the putative nuclear export signal and nuclear localization signals of ataxin-7, showed increased cellular toxicity. We also detected N-terminal polyQ-expanded ataxin-7 cleavage products in SCA7 transgenic mice similar in size to those generated by caspase-7 cleavage. In a SCA7 transgenic mouse model, recruitment of caspase-7 into the nucleus by polyQ-expanded ataxin-7 correlated with its activation. Our results, thus, suggest that proteolytic processing of ataxin-7 by caspase-7 may contribute to SCA7 disease pathogenesis.

A conditional pan-neuronal Drosophila model of spinocerebellar ataxia 7 with a reversible adult phenotype suitable for identifying modifier genes

Spinocerebellar ataxia 7 (SCA7) is a neurodegenerative disease caused by a polyglutamine (polyQ) expansion in the ataxin 7 (ATXN7) protein, a member of a multiprotein complex involved in histone acetylation. We have created a conditional Drosophila model of SCA7 in which expression of truncated ATXN7 (ATXN7T) with a pathogenic polyQ expansion is induced in neurons in adult flies. In this model, mutant ATXN7T accumulated in neuronal intranuclear inclusions containing ubiquitin, the 19S proteasome subunit, and HSP70 (heat shock protein 70), as in patients. Aggregation was accompanied by a decrease in locomotion and lifespan but limited neuronal death. Disaggregation of the inclusions, when expression of expanded ATXN7T was stopped, correlated with improved locomotor function and increased lifespan, suggesting that the pathology may respond to treatment. Lifespan was then used as a quantitative marker in a candidate gene approach to validate the interest of the model and to identify generic modulators of polyQ toxicity and specific modifiers of SCA7. Several molecular pathways identified in this focused screen (proteasome function, unfolded protein stress, caspase-dependent apoptosis, and histone acetylation) were further studied in primary neuronal cultures. Sodium butyrate, a histone deacetylase inhibitor, improved the survival time of the neurons. This model is therefore a powerful tool for studying SCA7 and for the development of potential therapies for polyQ diseases.

Polyglutamine-expanded ataxin-7 decreases nuclear translocation of NF-κB p65 and impairs NF-κB activity by inhibiting proteasome activity of cerebellar neurons

Our recent study indicated that polyglutamine-expanded ataxin-7-Q75 induced apoptotic death of cultured cerebellar neurons by downregulating Bcl-x(L) expression and activating mitochondrial apoptotic cascade. Mutant polyglutamine-expanded proteins are believed to impair the proteolytic function of ubiquitin-proteasome system by sequestering components of proteasomes. Proteasome degradation of IkappaBalpha permits nuclear translocation of NF-kappaB and is required for continuous NF-kappaB activity, which supports the survival of cultured cerebellar neurons by inducing Bcl-x(L) expression. Thus, we tested the hypothesis that mutant ataxin-7-Q75 causes proteasome dysfunction and impairs NF-kappaB activity, leading to reduced Bcl-x(L) expression, caspase activation and cerebellar neuronal death. EMSA assays indicate that DNA-binding activity of NF-kappaB was significantly decreased in cerebellar neurons expressing ataxin-7-Q75. Similar to mutant ataxin-7-Q75, NF-kappaB inhibitor APEQ induced cerebellar neuronal death by decreasing Bcl-x(L) expression and activating caspase-9. Mutant ataxin-7-Q75 inhibited the proteolytic activity of proteasomes in cerebellar neurons. Proteasome inhibitor MG132 also caused cerebellar neuronal death by decreasing Bcl-x(L) expression and activating caspase-9. Both ataxin-7-Q75 and MG132 caused the cytosolic accumulation of IkappaBalpha in cerebellar neurons. Mutant ataxin-7-Q75 or MG132 increased the cytosolic level of NF-kappaB p65 and decreased the nuclear NF-kappaB p65 level. Our study provides the evidence that polyglutamine-expanded ataxin-7-Q75 decreases nuclear translocation of NF-kappaB p65 and impairs NF-kappaB activity by inhibiting proteasome activity of cerebellar neurons.

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.

SAGA-associated Sgf73p facilitates formation of the preinitiation complex assembly at the promoters either in a HAT-dependent or independent manner in vivo

Although Sgf73p, a yeast homologue of human Sca7p, has recently been implicated as a new component of Spt-Ada-Gcn5-acetyltransferase (SAGA), its association with SAGA and functional role in regulation of transcription remain unknown in vivo. Here, using a chromatin immunoprecipitation (ChIP) assay, we show in vivo that, like SAGA, Sgf73p is recruited to the upstream activating sequence (UAS) of a SAGA-dependent gene, GAL1, in an activator-dependent manner. Further, Sgf73p is required for recruitment of SAGA to the GAL1 UAS, and facilitates formation of the preinitiation complex (PIC) assembly at the GAL1 promoter. When PIC is not formed in Deltasgf73, histone H3 is not evicted from the GAL1 promoter. Interestingly, PIC formation at GAL1 is not regulated by histone H3 acetylation or histone acetyltransferase (HAT) activity of SAGA. Similarly, Sgf73p facilitates PIC formation at another SAGA-dependent gene, ADH1, independent of histone H3 acetylation or HAT. In contrast, Sgf73p stimulates PIC formation at PHO84 (a SAGA-dependent gene), in a HAT-dependent-manner. Collectively, our data reveal that Sgf73p is required for SAGA recruitment, and stimulates PIC formation either in a HAT-dependent or -independent manner, providing significant information on how Sgf73p and possibly human Sca7p function physiologically.

PML clastosomes prevent nuclear accumulation of mutant ataxin-7 and other polyglutamine proteins

The pathogenesis of spinocerebellar ataxia type 7 and other neurodegenerative polyglutamine (polyQ) disorders correlates with the aberrant accumulation of toxic polyQ-expanded proteins in the nucleus. Promyelocytic leukemia protein (PML) nuclear bodies are often present in polyQ aggregates, but their relation to pathogenesis is unclear. We show that expression of PML isoform IV leads to the formation of distinct nuclear bodies enriched in components of the ubiquitin-proteasome system. These bodies recruit soluble mutant ataxin-7 and promote its degradation by proteasome-dependent proteolysis, thus preventing the aggregate formation. Inversely, disruption of the endogenous nuclear bodies with cadmium increases the nuclear accumulation and aggregation of mutant ataxin-7, demonstrating their role in ataxin-7 turnover. Interestingly, β-interferon treatment, which induces the expression of endogenous PML IV, prevents the accumulation of transiently expressed mutant ataxin-7 without affecting the level of the endogenous wild-type protein. Therefore, clastosomes represent a potential therapeutic target for preventing polyQ disorders.

Ataxin-7 can export from the nucleus via a conserved exportin-dependent signal

Spinocerebellar ataxia type 7 is a progressive neurodegenerative disorder caused by a CAG DNA triplet repeat expansion leading to an expanded polyglutamine tract in the ataxin-7 protein. Ataxin-7 appears to be a transcription factor and a component of the STAGA transcription coactivator complex. Here, using live cell imaging and inverted fluorescence recovery after photobleaching, we demonstrate that ataxin-7 has the ability to export from the nucleus via the CRM-1/exportin pathway and that ataxin-7 contains a classic leucine-type nuclear export signal (NES). We have precisely defined the location of this NES in ataxin-7 and found it to be fully conserved in all vertebrate species. Polyglutamine expansion was seen to reduce the nuclear export rate of mutant ataxin-7 relative to wild-type ataxin-7. Subtle point mutation of the NES in polyglutamine expanded ataxin-7 increased toxicity in primary cerebellar neurons in a polyglutamine length-dependent manner in the context of full-length ataxin-7. Our results add ataxin-7 to a growing list of polyglutamine disease proteins that are capable of nuclear shuttling, and we define an activity of ataxin-7 in the STAGA complex of trafficking between the nucleus and cytoplasm.

Mouse models of Machado-Joseph disease and other polyglutamine spinocerebellar ataxias

Machado-Joseph disease (MJD), also called spinocerebellar ataxia type 3, is caused by mutant ataxin-3 with a polyglutamine expansion. Although there is no treatment available at present to cure or delay the onset of MJD, mouse models have been generated to facilitate the development of a therapy. In this review, the published reports on mouse models of MJD and other polyglutamine spinocerebellar ataxias are compared. Based on these studies, the following approaches will be discussed as candidate treatments for MJD: 1) interfering with the formation of the mutant ataxin-3 cleavage fragment and possibly aggregate or inclusions, 2) reducing the disease protein nuclear localization, and 3) decreasing mutant ataxin-3 expression in neurons.

Polyglutamine-expanded spinocerebellar ataxia-7 protein disrupts normal SAGA and SLIK histone acetyltransferase activity

Histone acetyltransferases have been shown to participate in many essential cellular processes, particularly those associated with activation of transcription. SAGA (Spt-Ada-Gcn5 acetyltransferase) and SLIK (SAGA-like) are two highly homologous multisubunit histone acetyltransferase complexes that were originally identified in the yeast Saccharomyces cerevisiae. Here, we identify the protein Sgf73/Sca7 as a component of SAGA and SLIK, and a homologue of the human SCA7-encoded protein ataxin-7, which, in its polyglutamine expanded pathological form, is responsible for the neurodegenerative disease spinocerebellar ataxia 7 (SCA7). Our findings indicate that yeast Sca7 is necessary for the integrity and function of both SAGA and SLIK, and that the human ataxin-7 is able to compliment the loss of Sca7 in yeast. A polyglutamine-expanded version of ataxin-7 assembles a SAGA complex that is depleted of critical proteins that regulate the ability of SAGA to acetylate nucleosomes. These observations have significant implications for the function of the human Sca7 protein in disease pathogenesis.

Phosphorylated tau and the neurodegenerative foldopathies

Many studies have implicated phosphorylated tau in the Alzheimer disease process. However, the cellular fate of phosphorylated tau has only recently been described. Recent work has shown that tau phosphorylation at substrate sites for the kinases Cdk5 and GSK3-beta can trigger the binding of tau to the chaperones Hsc70 and Hsp27. The binding of phosphorylated tau to Hsc70 implied that the complex may be a substrate for the E3 ligase CHIP and this possibility was experimentally verified. The presence of this system in cells suggests that phosphorylated tau may hold toxic dangers for cell viability, and the response of the cell is to harness a variety of protective mechanisms. These include binding to chaperones, which may prevent more toxic conformations of the protein, ubiquitination which will direct the protein to the proteasome, segregation of tau aggregates from the cellular machinery, and recruitment of Hsp27 which will confer anti-apoptotic properties to the cell.

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.

Fine-structural analysis and connexin expression in the retina of a transgenic model of Huntington's disease

Recent studies indicate that the visual system appears more frequently affected in polyglutamine diseases than expected previously. Here, we investigated retinal degenerations in adult transgenic R6/2 mice, a model for Huntington's disease (HD). Light microscopical analysis revealed retinal dystrophy all over the retina, with central areas showing major effects. Electron microscopical analysis showed strong degenerations of outer and inner photoreceptor segments, shrinkage of photoreceptor cell somata, and signs of degeneration in photoreceptor terminals in the outer plexiform layer. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling showed hints of apoptosis. Mutant huntingtin and ubiquitin were expressed in all classes of retinal neurons, the pigment epithelium, and to a minor extent in neuropil structures. For investigating possible links to functional impairments in the rod-cone pathway, expression levels of three connexins (Cx) were compared in R6/2 and wildtype mice retinae. In R6/2 mice, expression of Cx36, the major neuronal connexin in the retina, was slightly reduced in the outer plexiform layer, indicating affected photoreceptor terminals as detected at the electron microscopical level. In contrast, Cx45, a putative neuronal connexin in the retina, was remarkably reduced in the inner plexiform layer of R6/2 mice. This result corresponded to fainter signals of Cx45 mRNA as documented by in situ hybridization and to a lower level of mCx45 cDNA as obtained by polymerase chain reaction after reverse transcription, suggesting functional deficits in spatial processing of Cx45-mediated gap junction coupling due to transgene-induced retinal degenerations. Thus, it is important to clarify the meaning of visual involvement in HD.

Hsp70 and Hsp40 chaperones do not modulate retinal phenotype in SCA7 mice

Nine neurodegenerative diseases, including spinocerebellar ataxia type 7 (SCA7), are caused by the expansion of polyglutamine stretches in the respective disease-causing proteins. A hallmark of these diseases is the aggregation of expanded polyglutamine-containing proteins in nuclear inclusions that also accumulate molecular chaperones and components of the ubiquitin-proteasome system. Manipulation of HSP70 and HSP40 chaperone levels has been shown to suppress aggregates in cellular models, prevent neuronal death in Drosophila, and improve to some extent neurological symptoms in mouse models. An important issue in mammals is the relative expression levels of toxic and putative rescuing proteins. Furthermore, overexpression of both HSP70 and its co-factor HSP40/HDJ2 has never been investigated in mice. We decided to address this question in a SCA7 transgenic mouse model that progressively develops retinopathy, similar to SCA7 patients. To co-express HSP70 and HDJ2 with the polyglutamine protein, in the same cell type, at comparable levels and with the same time course, we generated transgenic mice that express the heat shock proteins specifically in rod photoreceptors. While co-expression of HSP70 with its co-factor HDJ2 efficiently suppressed mutant ataxin-7 aggregation in transfected cells, they did not prevent either neuronal toxicity or aggregate formation in SCA7 mice. Furthermore, nuclear inclusions in SCA7 mice were composed of a cleaved mutant ataxin-7 fragment, whereas they contained the full-length protein in transfected cells. We propose that differences in the aggregation process might account for the different effects of chaperone overexpression in cellular and animal models of polyglutamine diseases.