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.
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.
Pubmed ID: 22649225 RIS Download
Age Factors | Amino Acids | Animals | Aspartic Acid | Body Weight | Brain | Caspase 6 | Cells, Cultured | Corpus Striatum | Disease Models, Animal | Embryo, Mammalian | Exploratory Behavior | Female | Gene Expression Regulation | Huntingtin Protein | Huntington Disease | Magnetic Resonance Imaging | Mice | Mice, Inbred C57BL | Mice, Knockout | Mice, Mutant Strains | Motor Activity | Nerve Tissue Proteins | Neurons | Proteolysis | RNA, Small Interfering | Rotarod Performance Test | Trinucleotide Repeat Expansion | Ubiquitination