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Caspase-6 Activity in a BACHD Mouse Modulates Steady-State Levels of Mutant Huntingtin Protein But Is Not Necessary for Production of a 586 Amino Acid Proteolytic Fragment

The Buck Institute for Research on Aging, Novato, CA 94945, USA.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience (Impact Factor: 6.75). 05/2012; 32(22):7454-65. DOI: 10.1523/JNEUROSCI.6379-11.2012
Source: PubMed

ABSTRACT Huntington's disease (HD) is caused by a mutation in the huntingtin (htt) gene encoding an expansion of glutamine repeats at the N terminus of the Htt protein. Proteolysis of Htt has been identified as a critical pathological event in HD models. In particular, it has been postulated that proteolysis of Htt at the putative caspase-6 cleavage site (at amino acid Asp-586) plays a critical role in disease progression and pathogenesis. However, whether caspase-6 is indeed the essential enzyme that cleaves Htt at this site in vivo has not been determined. To evaluate, we crossed the BACHD mouse model with a caspase-6 knock-out mouse (Casp6(-/-)). Western blot and immunocytochemistry confirmed the lack of caspase-6 protein in Casp6(-/-) mice, regardless of HD genotype. We predicted the Casp6(-/-) mouse would have reduced levels of caspase-6 Htt fragments and increased levels of full-length Htt protein. In contrast, we found a significant reduction of full-length mutant Htt (mHtt) and fragments in the striatum of BACHD Casp6(-/-) mice. Importantly, we detected the presence of Htt fragments consistent with cleavage at amino acid Asp-586 of Htt in the BACHD Casp6(-/-) mouse, indicating that caspase-6 activity cannot fully account for the generation of the Htt 586 fragment in vivo. Our data are not consistent with the hypothesis that caspase-6 activity is critical in generating a potentially toxic 586 aa Htt fragment in vivo. However, our studies do suggest a role for caspase-6 activity in clearance pathways for mHtt protein.

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    • "Additionally , improved performance on the rotarod was shown for BACHD/ Casp6−/− mice (Gafni et al., 2012). Importantly, the mHTT-586 fragment is still present in BACHD/Casp6−/− mice, albeit at reduced levels by immunohistochemical analysis (Gafni et al., 2012). This suggests that other proteases may be capable of cleaving mHTT in the context of a constitutive knockout of Casp6 in the mouse. "
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    • "Moreover, medium spiny neurons (MSNs) expressing caspase-6 resistant mHtt showed a decreased susceptibility for NMDAR-induced excitotoxicity and no caspase-6 activation compared to MSNs expressing unmodified mHtt [56] [57] [58]. By contrast, two caspase-6 knockout HD mouse models showed that production of a 586 amino acid derived proteolytic fragment was not prevented in the brain, disagreeing with a direct involvement of caspase-6 in mHtt cleavage [59] [60]. "
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