Spinocerebellar ataxia (SCA) type 7 is an inherited neurodegenerative disorder caused by expansion of a polyglutamine tract within the ataxin-7 protein. To determine the molecular basis of polyglutamine neurotoxicity in this and other related disorders, we produced SCA7 transgenic mice that express ataxin-7 with 24 or 92 glutamines in all neurons of the CNS, except for Purkinje cells. Transgenic mice expressing ataxin-7 with 92 glutamines (92Q) developed a dramatic neurological phenotype presenting as a gait ataxia and culminating in premature death. Despite the absence of expression of polyglutamine-expanded ataxin-7 in Purkinje cells, we documented severe Purkinje cell degeneration in 92Q SCA7 transgenic mice. We also detected an N-terminal truncation fragment of ataxin-7 in transgenic mice and in SCA7 patient material with both anti-ataxin-7 and anti-polyglutamine specific antibodies. The appearance of truncated ataxin-7 in nuclear aggregates correlates with the onset of a disease phenotype in the SCA7 mice, suggesting that nuclear localization and proteolytic cleavage may be important features of SCA7 pathogenesis. The non-cell-autonomous nature of the Purkinje cell degeneration in our SCA7 mouse model indicates that polyglutamine-induced dysfunction in adjacent or connecting cell types contributes to the neurodegeneration.

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.

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 wildtype 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 fulllength 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. Spinocerebellar ataxia type 7 (SCA7)4 is a dominantly inherited neurodegenerative disorder characterized by loss of neurons in the cerebellum, brain stem, and retina (1). SCA7 is a member of a family of neurodegenerative diseases in which a CAG DNA triplet repeat expansion results in polyglutamine expansion in the gene product (2). Other members of this polyglutamine expansion disease family include Huntington disease, spinobulbar muscle atrophy, dentatorubral pallidoluysian atrophy, and spinocerebellar ataxia types 1, 2, 3, 6, and 17 (3, 4). One unique feature of SCA7 is the loss of photoreceptor neurons in the retina leading to cone-rod dystrophy (5). The mutant ataxin-7 protein can have polyglutamine repeats from 38 to 300 residues in length (6). Ataxin-7 subcellular localization has been seen to be primarily nuclear with nuclear import signals (NLSs) defined in both the central (7), and carboxyl-terminal regions of the protein (8). Within the nucleus, ataxin-7 is known to be a subunit of the mammalian GCN5 histone acetyltransferase STAGA transcription coactivator complex (9). Ataxin-7 directly binds GCN5, and mutant ataxin-7 can inhibit the histone acetyltransferase activity of STAGA (10). Although the precise biological function of ataxin-7 is unknown, mutant ataxin-7 is known to interfere with Crx-dependent transcription of retinal photoreceptor-specific genes (11, 12). Ataxin-7 interacts with TFTC/STAGA protein subunits through a central evolutionarily conserved block of residues that have defined an ataxin-7 homology family in species ranging from human to yeast (9). In Saccharomyces cerevisiae, the yeast ataxin-7 homolog, Sgf73, is a member of the SAGA and SLIK histone acetyltransferase complexes (13). Ataxin-7 and the Crx homeodomain transcription factor interact via glutamine regions in each p...

Spinocerebellar ataxia type 7 (SCA7) is one member of a growing list of neurodegenerative disorders that are all caused by CAG repeat expansions that produce disease by encoding elongated polyglutamine tracts in a variety of apparently unrelated proteins. In this review, we provide an overview of our efforts to determine the molecular basis of polyglutamine neurotoxicity in SCA7 by modeling this polyglutamine repeat disorder in mice. We discuss how our SCA7 mouse model develops a phenotype that is reminiscent of the retinal and cerebellar disease pathology seen in human patients. All of these findings are considered in the context of numerous other models of polyglutamine disease pathology in mice and other organisms, together with various other in vitro and biochemical studies. We present the competing hypotheses of polyglutamine disease pathogenesis, and explain how our studies of SCA7 brainstem and retinal degeneration using this mouse model have yielded insights into possible mechanisms and pathways of polyglutamine disease pathology. In addition to illustrating how our SCA7 mouse model has allowed us to develop and advance notions of disease pathogenesis, we propose a model of polyglutamine molecular pathology that attempts to integrate the key observations in the field. We close by describing why our SCA7 mouse model should be useful for the next phase of polyglutamine disease research – the development of therapies, and predict that this stage of experimentation will continue to rely heavily on the mouse.

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