Cleavage at the Caspase-6 Site Is Required for Neuronal Dysfunction and Degeneration Due to Mutant Huntingtin

Abstract

Cleavage of huntingtin (htt) has been characterized in vitro, and accumulation of caspase cleavage fragments represents an early pathological change in brains of Huntington's disease (HD) patients. However, the relationship between htt proteolysis and the pathogenesis of HD is unknown. To determine whether caspase cleavage of htt is a key event in the neuronal dysfunction and selective neurodegeneration in HD, we generated YAC mice expressing caspase-3- and caspase-6-resistant mutant htt. Mice expressing mutant htt, resistant to cleavage by caspase-6 but not caspase-3, maintain normal neuronal function and do not develop striatal neurodegeneration. Furthermore, caspase-6-resistant mutant htt mice are protected against neurotoxicity induced by multiple stressors including NMDA, quinolinic acid (QA), and staurosporine. These results are consistent with proteolysis of htt at the caspase-6 cleavage site being an important event in mediating neuronal dysfunction and neurodegeneration and highlight the significant role of htt proteolysis and excitotoxicity in HD.

Generation of an indestructible cyclin B1 by caspase-6-dependent cleavage during mitotic catastrophe

“…In support of this, we found that cyclin B1 was a poor substrate for purified caspase-3 in vitro (our unpublished data). Cyclin B1 was likely to be cleaved by caspase-6 (Figure 5e), with a putative cleavage site (ILVD 123 k) resembling that of Huntingtin (IVLDk) (Graham et al, 2006). Caspase-6 is essential for the cleavage of nuclear lamins (Orth et al, 1996;Takahashi et al, 1996) and for chromatin condensation (Ruchaud et al, 2002), especially in cells that express lamin A/C (which is cleaved exclusively by caspase-6) (Slee et al, 2001).…”

“…As Asp123 resembles a caspase-6 cleavage site (such as IVLDk in Huntingtin; Graham et al, 2006), we next examined whether specific inhibition of caspase-6 could affect cyclin B1 cleavage. Figure 5e shows that cyclin B1 cleavage was abolished with a caspase-6-specific inhibitor (Ac-VEID-CHO).…”

Caspase substrates

“…Importantly, caspase-3 does this not by activating caspase-9 directly, but by removing the neoepitope exosite required for caspase-9 inhibition, and therefore this event can be thought of as 'derepression' rather than 'activation'. 64 Although the function of HTT is unknown, overexpression of wild-type HTT has shown a protective phenotype under models of neuronal stress. 65 Significantly, caspase cleavage of HTT is reported to not only remove HTT's protective function, but also generates a toxic byproduct that sensitizes neurons to further stressors such as excitotoxic stimulation.…”

Neuroprotection for Huntington’s Disease: Ready, Set, Slow

“…60 Most evidence points to a proximal toxicity residing in mutant huntingtin or its proteolytic fragments and their soluble interactions with other proteins, including huntingtin itself or the hundreds of other proteins that have been demonstrated to associate with huntingtin. 61 Proximal events mediated by mutant huntingtin in turn trigger cascades of both damaging and compensatory molecular processes and genetic programs.…”