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.
The Buck Institute for Research on Aging, Novato, CA 94945, USA.
Juliette
J
Gafni
Theodora
T
Papanikolaou
Francesco
F
Degiacomo
Jennifer
J
Holcomb
Sylvia
S
Chen
Liliana
L
Menalled
Andrea
A
Kudwa
Jon
J
Fitzpatrick
Sam
S
Miller
Sylvie
S
Ramboz
Pasi I
PI
Tuunanen
Kimmo K
KK
Lehtimäki
X William
XW
Yang
Larry
L
Park
Seung
S
Kwak
David
D
Howland
Hyunsun
H
Park
Lisa M
LM
Ellerby
D000367:Age Factors; D000596:Amino Acids / Q000235:genetics / Q000378:metabolism; D000818:Animals; D001224:Aspartic Acid / Q000235:genetics / Q000378:metabolism*; D001835:Body Weight / Q000235:genetics; D001921:Brain / Q000378:metabolism / Q000473:pathology; D053178:Caspase 6 / Q000172:deficiency / Q000378:metabolism*; D002478:Cells, Cultured; D003342:Corpus Striatum / Q000166:cytology; D004195:Disease Models, Animal; D004622:Embryo, Mammalian; D005106:Exploratory Behavior / Q000502:physiology; D005260:Female; D005786:Gene Expression Regulation / Q000235:genetics*; D000071058:Huntingtin Protein; D006816:Huntington Disease / Q000235:genetics / Q000378:metabolism* / Q000473:pathology / Q000503:physiopathology*; D008279:Magnetic Resonance Imaging; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D008817:Mice, Mutant Strains; D009043:Motor Activity / Q000235:genetics; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism*; D009474:Neurons; D059748:Proteolysis; D034741:RNA, Small Interfering / Q000378:metabolism; D045442:Rotarod Performance Test; D019680:Trinucleotide Repeat Expansion / Q000235:genetics; D054875:Ubiquitination / Q000235:genetics
D000596:Amino Acids; C086055:HTT protein, human; D000071058:Huntingtin Protein; D009419:Nerve Tissue Proteins; D034741:RNA, Small Interfering; D001224:Aspartic Acid; D053178:Caspase 6