Mitochondrial sensitivity and altered calcium handling underlie enhanced NMDA-induced apoptosis in YAC128 model of Huntington's disease.
Expansion of a CAG repeat in the Huntington's disease (HD) gene results in progressive neuronal loss, particularly of striatal medium-sized spiny neurons (MSNs). Studies in human HD autopsy brain tissue, as well as cellular and animal models of HD, suggest that increased activity of NMDA-type glutamate receptors and altered mitochondrial function contribute to selective neuronal degeneration. In this regard, the YAC128 mouse model, expressing full-length human huntingtin with 128 glutamine repeats, has been the focus of much interest. Although NMDA-induced apoptosis is enhanced in YAC128 MSNs, here we report that the initial steps in the death signaling pathway, including NMDA receptor (NMDAR) current and cytosolic Ca2+ loading, are similar to those observed in wild-type MSNs. In contrast, we found that the NMDAR-mediated Ca2+ load triggered a strikingly enhanced loss of mitochondrial membrane potential in YAC128 MSNs, suggesting that NMDAR signaling via the mitochondrial apoptotic pathway is altered. This effect was accompanied by impaired cytosolic Ca2+ clearance after removal of NMDA, a difference that was not apparent after high potassium-evoked depolarization-mediated Ca2+ entry. Inhibition of the mitochondrial permeability transition (mPT) reduced peak cytosolic Ca2+ and mitochondrial depolarization evoked by NMDA in YAC128 MSNs but not wild-type MSNs. Hence, in contrast to YAC models with moderate CAG expansions, the enhanced NMDA-induced apoptosis in YAC128 MSNs is predominantly determined by augmented mitochondrial sensitivity to Ca2+-induced activation of the mPT. These results suggest that the CAG repeat length influences the mechanism by which mHtt enhances NMDAR-mediated excitotoxicity.
Fernandes
Herman B
HB
Department of Psychiatry and Brain Research Centre, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z3.
Baimbridge
Kenneth G
KG
Church
John
J
Hayden
Michael R
MR
Raymond
Lynn A
LA
D000818:Animals; D017209:Apoptosis / Q000187:drug effects* / Q000235:genetics; D001865:Bongkrekic Acid / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000187:drug effects* / Q000235:genetics; D002478:Cells, Cultured; D016572:Cyclosporine / Q000494:pharmacology; D004195:Disease Models, Animal; D018690:Excitatory Amino Acid Agonists / Q000494:pharmacology; D005456:Fluorescent Dyes; D006816:Huntington Disease / Q000235:genetics / Q000378:metabolism / Q000503:physiopathology*; D007476:Ionophores / Q000494:pharmacology; D051379:Mice; D008818:Mice, Neurologic Mutants; D008928:Mitochondria / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D016202:N-Methylaspartate / Q000633:toxicity*; D009498:Neurotoxins / Q000633:toxicity; D018408:Patch-Clamp Techniques; D016194:Receptors, N-Methyl-D-Aspartate / Q000187:drug effects / Q000378:metabolism; D019680:Trinucleotide Repeat Expansion / Q000235:genetics
D018690:Excitatory Amino Acid Agonists; D005456:Fluorescent Dyes; D007476:Ionophores; D009498:Neurotoxins; D016194:Receptors, N-Methyl-D-Aspartate; D001865:Bongkrekic Acid; D016202:N-Methylaspartate; D016572:Cyclosporine; D002118:Calcium