Abstract
Nine neurodegenerative diseases, including spinocer-ebellar 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 Drosoph-ila, 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 photorecep-tors. While co-expression of HSP70 with its co-factor HDJ2 efficiently suppressed mutant ataxin-7 aggrega-tion 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 trans-fected cells. We propose that differences in the aggrega-tion process might account for the different effects of chaperone overexpression in cellular and animal models of polyglutamine diseases. Spinocerebellar ataxia type 7 (SCA7) 1 is a dominantly inherited neurodegenerative disorder, characterized by late-onset neuronal loss in cerebellum, brainstem, and retina (1). SCA7 is caused by an abnormal expansion of a polyglutamine (poly(Q)) tract (38-460 repeats) in the SCA7 gene product ataxin-7 (2), a 892-amino acid protein recently identified as a new component of a multisubunit transcriptional complex (3). Eight other neurodegenerative diseases are caused by a CAG/poly(Q) repeat expansion, including Huntington disease (HD), denta-torubro-pallidoluysian atrophy, spinobulbar muscular atrophy (SBMA) and spinocerebellar ataxia types 1, 2, 3, 6, and 17. Genetic and molecular studies indicate that poly(Q) tracts confer a novel toxic function to the otherwise unrelated proteins, but the mechanisms by which poly(Q) expansions lead to neu-rodegeneration remain unclear (4, 5). All of these diseases are characterized by continuous accumulation of mutant proteins in insoluble aggregates, typically forming nuclear inclusions (NIs) (6). NIs have been shown to stain positively for ubiquitin, chaperones (mainly heat shock proteins), and proteasome subunits, in various cellular and animal models of poly(Q) diseases, as well as in post-mortem patient brains (7, 8). Several SCA7 mouse models, which reproduce many features of the human situation, display numerous NIs consistently stained for ubiquitin, proteasome sub-units, and chaperones (HSP70, HSC70, and HDJ2) (9-12). In SCA7 patient brains, HDJ2, ubiquitin, and 19 S proteasome subunits co-localize with NIs (13-15). Furthermore, the levels of several chaperones, including the HSP70 and the HSP40 chaperones HDJ1 and HDJ2, were shown to be reduced in SCA7 and/or HD mouse models (16, 17). Altogether, these findings suggested that protein misfolding and impaired clearance might be a common pathogenic event in poly(Q) diseases. Accordingly, several studies showed that overexpression of chaperones in cells reduces aggregate formation and, in some cases, suppresses poly(Q) toxicity (reviewed in Refs. 8 and 18). Genetic screens, in Drosophila models overexpressing mutant ataxin-1 or a pure 127Q stretch, identified molecular chaperones and components of the ubiquitin-proteasome pathway as strong modulators of poly(Q)-induced toxicity (19, 20). In particular , deletion of HSP70 genes exacerbated SCA1 or SBMA fly phenotypes, whereas overexpression of HSP70 or HDJ1 suppressed neurodegeneration in various Drosophila models (19-24). Furthermore, Chan et al. (25) showed that HSP70 and HDJ1 synergistically suppressed poly(Q) toxicity in a SCA3 fly model. These chaperones work together in an ATP-dependent manner, notably to help refold denaturated and aggregated proteins. HSP40 co-chaperones recognize abnormally folded polypeptide substrates, present them to HSP70, and stimulate HSP70 ATPase activity (26).