The neurodegenerative diseases Huntington disease, dentatorubropallidoluysian atrophy, spinocerebellar atrophy type 3, and spinal bulbar muscular atrophy are caused by expansion of a polyglutamine tract within their respective gene products. There is increasing evidence that generation of truncated proteins containing an expanded polyglutamine tract may be a key step in the pathogenesis of these disorders. We now report that, similar to huntingtin, atrophin-1, ataxin-3, and the androgen receptor are cleaved in apoptotic extracts. Furthermore, each of these proteins is cleaved by one or more purified caspases, cysteine proteases involved in apoptotic death. The CAG length does not modulate susceptibility to cleavage of any of the full-length proteins. Our results suggest that by generation of truncated polyglutamine-containing proteins, caspase cleavage may represent a common step in the pathogenesis of each of these neurodegenerative diseases.

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Huntington's disease is a neurodegenerative disorder caused by CAG expansion that results in expansion of a polyglutamine tract at the extreme N terminus of huntingtin (htt). htt with polyglutamine expansion is proapoptotic in different cell types. Here, we show that caspase inhibitors diminish the toxicity of htt. Additionally, we define htt itself as an important caspase substrate by generating a site-directed htt mutant that is resistant to caspase-3 cleavage at positions 513 and 530 and to caspase-6 cleavage at position 586. In contrast to cleavable htt, caspase-resistant htt with an expanded polyglutamine tract has reduced toxicity in apoptotically stressed neuronal and nonneuronal cells and forms aggregates at a much reduced frequency. These results suggest that inhibiting caspase cleavage of htt may therefore be of potential therapeutic benefit in Huntington's disease.Huntington's disease (HD) 1 is a progressive neurodegenerative disorder caused by polyglutamine expansion in the N terminus of htt (1). The cardinal neuropathological feature of HD is the selective neuronal loss of ␥-aminobutyric acid-ergic medium spiny neostriatal neurons and large projection neurons in cortical layers V and VI (2-4). The detection of DNA strand breaks in affected regions of HD patient brains (5-7) suggests that neurodegeneration occurs by an apoptotic mechanism and suggests that caspases could play an important role in HD.Caspases are cysteine aspartic acid proteases that cleave specific target proteins during apoptotic death (8). We have previously shown that huntingtin is cleaved in apoptotic cells and by recombinant caspase-3 (9), and expression of truncated htt fragments with expanded polyglutamine repeats is known to be toxic to cells (10 -14). These observations led to the development of the toxic fragment hypothesis (15), which postulates that proteolytic cleavage of htt liberates toxic fragments containing the expanded polyglutamine tract that are neurotoxic and that stimulate additional proteolytic activity.Evidence of htt cleavage in HD includes the presence of N-terminal htt fragments in patient brains (16) as well as in yeast artificial chromosome transgenic mice that express fulllength, expanded human htt (17). htt cleavage in the yeast artificial chromosome transgenic mice occurs in the cytoplasm, after which the N-terminal fragments are imported into the nucleus (17).In vitro, htt is cleaved by caspase-3 at two sites yielding N-terminal fragments of 70 and 80 kDa for htt with 15 glutamines and 90 and 100 kDa for htt with 138 glutamines (9, 18). These fragments are also generated when htt is incubated with apoptotic extracts (9, 18) and accumulate from endogenous htt in apoptotic cells (19). Taken together, these results suggest that caspase-3 is likely to contribute to the generation of N-terminal htt fragments.Further support for the toxic fragment hypothesis can be obtained by testing whether preventing the formation of Nterminal htt fragments lessens the toxicity of htt. Here we show abrogation of htt c...

In Huntington disease, polyglutamine expansion of the protein huntingtin (Htt) leads to selective neurodegenerative loss of medium spiny neurons throughout the striatum by an unknown apoptotic mechanism. Binding of Hip-1, a protein normally associated with Htt, is reduced by polyglutamine expansion. Free Hip-1 binds to a hitherto unknown polypeptide, Hippi (Hip-1 protein interactor), which has partial sequence homology to Hip-1 and similar tissue and subcellular distribution. The availability of free Hip-1 is modulated by polyglutamine length within Htt, with disease-associated polyglutamine expansion favouring the formation of pro-apoptotic Hippi-Hip-1 heterodimers. This heterodimer can recruit procaspase-8 into a complex of Hippi, Hip-1 and procaspase-8, and launch apoptosis through components of the 'extrinsic' cell-death pathway. We propose that Htt polyglutamine expansion liberates Hip-1 so that it can form a caspase-8 recruitment complex with Hippi. This novel non-receptor-mediated pathway for activating caspase-8 might contribute to neuronal death in Huntington disease.

Expansion of a polyglutamine sequence in the N terminus of huntingtin is the gain-of-function event that causes Huntington's disease. This mutation affects primarily the medium-size spiny neurons of the striatum. Huntingtin is expressed in many neuronal and non-neuronal cell types, implying a more general function for the wild-type protein. Here we report that wild-type huntingtin acts by protecting CNS cells from a variety of apoptotic stimuli, including serum withdrawal, death receptors, and pro-apoptotic Bcl-2 homologs. This protection may take place at the level of caspase-9 activation. The full-length protein also modulates the toxicity of the poly-Q expansion. Cells expressing full-length mutant protein are susceptible to fewer death stimuli than cells expressing truncated mutant huntingtin.