Interference of Crx-dependent transcription by ataxin-7 involves interaction between the glutamine regions and requires the ataxin-7 carboxy-terminal region for nuclear localization

Abstract

Spinocerebellar ataxia type 7 (SCA7) is an inherited neurodegenerative disorder caused by expansion of a polyglutamine tract in the ataxin-7 protein. A unique feature of SCA7 is degeneration of photoreceptor cells in the retina, resulting in cone-rod dystrophy. In an SCA7 transgenic mouse model that we developed, it was found that the cone-rod dystrophy involves altered photoreceptor gene expression due to interference with Crx, a homeodomain transcription factor containing a glutamine-rich region. To determine the basis of the Crx-ataxin-7 interaction, Crx and ataxin-7 truncation and point mutants were generated, and the ability of mutant versions of either protein to co-immunoprecipitate the normal version of the other protein was tested. Thus Crx's ataxin-7 interaction domain was localized to its glutamine-rich region and ataxin-7's Crx binding domain was mapped to its glutamine tract. The importance of each protein's respective glutamine region for a productive interaction was confirmed by performing Crx transactivation assays in HEK293 cells and correlating the extent of Crx transcription interference with the intactness of each protein's glutamine region. It was also established that ataxin-7 must localize to the nucleus to repress Crx transactivation, and the likely nuclear localization signals were mapped to ataxin-7's carboxy-terminal region. Finally, using chromatin immunoprecipitation, it was demonstrated that Crx and ataxin-7 engage in a functionally significant interaction by co-occupying the promoter and enhancer regions of Crx-regulated retinal genes in vivo. The results suggest that one mechanism of SCA7 disease pathogenesis is transcription dysregulation, and that Crx transcription interference is a predominant factor in SCA7 cone-rod dystrophy retinal degeneration.

Preventing polyglutamine-induced activation of c-Jun delays neuronal dysfunction in a mouse model of SCA7 retinopathy

“…Crx, which is reduced in R7E mice, acts as a critical regulator of both cone and rod photoreceptor genes. Its transcriptional activity was decreased in another SCA7 transgenic mouse model (Chen et al, 2004b), although it was not impaired in the SCA7 knock-in mice (Yoo et al, 2003). Thus, altered Crx expression may contribute to functional changes in both rod and cone photoreceptors in SCA7 disease.…”

“…Accumulation of polyQ-ataxin-7 in the retina of these mice results in decreased expression of phototransduction genes [e.g., rhodopsin (Rho), rod transducin (Gnat1) and phosphodiesterase β (Pdeb)] (Yoo et al, 2003;Helmlinger et al, 2004b). The expression of rod-specific transcriptional regulators, including cone-rod homeobox (Crx) and neural retina leucine zipper (Nrl), is also reduced (La Spada et al, 2001;Chen et al, 2004b;Abou-Sleymane et al, 2006). As mutations in CRX and NRL lead to human retinal dystrophies (Freund et al, 1997;Sohocki et al, 1998;Bessant et al, 1999;Nishiguchi et al, 2004), SCA7 retinopathy may probably be a consequence of the reduced expression of phototransduction genes.…”

Polyglutamine tracts as modulators of transcriptional activation from yeast to mammals

“…It has been reported that polyQ domains of proteins can interact with other polyQ-containing or glutamine-rich transcription factors (Chen et al , 2004 ). To test for polyQ-polyQ interactions in vivo , we generated a mammalian two-hybrid system…”

Regulation of photoreceptor gene expression by Crx-associated transcription factor network

“…Expansion of the poly-glutamine tract of ataxin-7 is associated with a dominant neurological disorder, spinocerebellar ataxia type 7 (SCA7), which features cone-rod dystrophy-like retinal degeneration similar to the pathology linked to CRX mutations. Animal model studies and in vitro functional analysis suggest that Crx is a STAGA-dependent transcription activator (La Spada et al, 2001;Chen et al, 2004;Palhan et al, 2005). A polyglutamine-expanded ataxin-7 disrupts Gcn5 HAT activity, resulting in hypoacetylation of histones on Crx target genes and transcription impairment in a SCA7 transgenic mouse model.…”