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.     

miR-25 alleviates polyQ-mediated cytotoxicity by silencing ATXN3

MicroRNAs (miRNAs) have been reported to play significant roles in the pathogenesis of various polyQ diseases. This study aims to investigate the regulation of ATXN3 gene expression by miRNA. We found that miR-25 reduced both wild-type and polyQ-expanded mutant ataxin-3 protein levels by interacting with the 3′UTR of ATXN3 mRNA. miR-25 also increased cell viability, decreased early apoptosis, and downregulated the accumulation of mutant ataxin-3 protein aggregates in SCA3/MJD cells. These novel results shed light on the potential role of miR-25 in the pathogenesis of SCA3/MJD, and provide a possible therapeutic intervention for this disorder.     

Autophagy: PolyQ toxic fragment turnover

Recent studies have highlighted the importance of the lysosome in degrading proteins that misfold in neurodegenerative diseases. In this study we explore the role for autophagy in the clearance of an N-terminal caspase-7-generated fragment of ataxin-7, a protein with a pathogenic polyglutamine (polyQ) expansion in the neurodegenerative disease spinocerebellar ataxia 7 (SCA7). Using both cellular and transgenic mouse models of SCA7 we show that the stability of wild-type ataxin-7 is modified by macroautophagy, but not by proteasomal, inhibition, whereas both autophagy and proteasomal degradation have little effect on polyQ-expanded ataxin-7. We also create a post-translational modification-deficient ataxin-7 mutant that has increased protein turnover of both wild-type and polyQ-expanded ataxin-7, mediated through the autophagy pathway. Histological analysis reveals that wild-type ataxin-7 colocalizes with markers of chaperone-mediated autophagy (CMA) and macroautophagy, indicating that both of these mechanisms may play a role in the clearance of ataxin-7. Furthermore, there is an increase in LC3, a marker of autophagy initiation, in the cerebellum of SCA7 transgenic mice. Our findings indicate that the ataxin-7 fragment may be cleared via autophagy and that this process is altered in SCA7. Identification of the different types of autophagy involved in ataxin-7 turnover and the influence of post-translational modifications on these processes will be pursued in future studies.     

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, beta-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.     

The structural plasticity of SCA7 domains defines their differential nucleosome‐binding properties

SAGA (Spt–Ada–Gcn5 acetyltransferase), a coactivator complex involved in chromatin remodelling, harbours both histone acetylation and deubiquitination activities. ATXN7/Sgf73 and ATXN7L3, two subunits of the SAGA deubiquitination module, contain an SCA7 domain characterized by an atypical zinc‐finger. We show that the yeast Sgf73–SCA7 domain is not required to recruit Sgf73 into SAGA. Instead, it binds to nucleosomes, a property that is conserved in the human ATXN7–SCA7 domain but is lost in the ATXN7L3 domain. The solution structures of the SCA7 domain of both ATXN7 and ATXN7L3 reveal a new, common zinc‐finger motif at the heart of two distinct folds, providing a molecular basis for the observed functional differences.