Proteolytic Cleavage of Ataxin-7 by Caspase-7 Modulates Cellular Toxicity and Transcriptional Dysregulation

Abstract

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. Spinocerebellar ataxia type 7 (SCA7)4 is an autosomal dominant polyglutamine disorder clinically characterized by progressive ataxia and blindness. In the disease state, highly specific cell death ultimately occurs, most notably in the photoreceptor cells of the retina, Purkinje cells, dentate nuclei, and granule cells of the cerebellum, inferior olivary nuclei, and pontine neurons (1-3). SCA7 is a member of a family of "polyglutamine disorders." The retinal degeneration distinguishes SCA7 from the other polyglutamine diseases, although like many of the other polyQ disease proteins, ataxin-7 is widely expressed throughout the central nervous system. These autosomal dominant neurodegenerative diseases include Huntington disease (4), spinocerebellar ataxias type 1, 2, 3, 6, and 17 (SCA1, SCA2, SCA3 or Machado-Joseph disease, SCA6, SCA17) (5-8), dentatorubropallidoluysian atrophy (9), and spinal bulbar muscular atrophy (Kennedy disease) (10). The resultant increase in polyQ tract length derived from the CAG expansion within the gene product appears to exert direct toxicity on neuronal populations. Common mechanisms of disease pathogenesis include transcriptional dysregulation (11, 12) and proteolysis to produce toxic fragments (13-18). Neurodegeneration of specific neuronal populations found for each disease has been proposed to involve cell-specific...

Inhibition of Autophagy via p53-Mediated Disruption of ULK1 in a SCA7 Polyglutamine Disease Model

“…Proteolytic processing of ATXN7, believed to occur in the nucleus, has been observed in SCA7 and is thought to generate smaller more toxic polyQ fragments (Garden et al, 2002;Young et al, 2007;Yu et al, 2012;Yvert et al, 2001). We have previously shown that in the stable PC12 cell model used in this study, full-length ATXN7 is mainly found in the nucleus, whereas ATXN7 fragments localize and aggregate both in the nucleus and the cytoplasm .…”

Mimicking proteasomal release of polyglutamine peptides initiates aggregation and toxicity

“…Proteolytic fragments containing expanded polyQ tracts are more aggregation-prone than original fulllength proteins, as has been shown for huntingtin (Cooper et al, 1998), androgen receptor , ataxin 3 (Haacke et al, 2006) and ataxin 7 (Young et al, 2007). Recently, it was also postulated that an expanded polyQ fragment was expressed in SCA8, because antisense transcription resulted in polyQ inclusions (Ikeda et al, 2007).…”

“…The presence of proteolytic protein fragments harbouring a polyQ tract in aggregates (DiFiglia et al, 1997;Goti et al, 2004;Li et al, 1998;Schmidt et al, 1998) has led to the 'toxic fragment hypothesis', which states that proteolytic fragments of polyQexpanded huntingtin (Cooper et al, 1998), androgen receptor or certain ataxins (Haacke et al, 2006;Ikeda et al, 1996;Young et al, 2007) initiate protein aggregation and induce neuronal toxicity. Full-length polyQ proteins aggregate, but at a much slower rate than their proteolytic fragments .…”

Caspase-7 Cleavage of Kaposi Sarcoma-associated Herpesvirus ORF57 Confers a Cellular Function against Viral Lytic Gene Expression

“…Recent studies have suggested that substrates can have a substantial preference for caspase-7 over caspase-3 or vice versa. The reported substrates with specificity for caspase-7, but not for the closely related caspase-3, are Nogo-B (SSTD2S) (38), claspin (DEYD2G) (39), and Ataxin-7 (PKMD2G and FDPD2I) (40). A substrate preference for caspase-3 over caspase-7 is found in protein phosphatase-1 inhibitor-3 (DTVD2X) (41) and in excitatory amino acid transporter EAAT2 (DTID2S) (42).…”