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Mechanisms of modulation of N-methyl-D-aspartate (NMDA) receptors by mutant huntingtin Fan, Mannie Man Yee

Abstract

Evidence supports a role for neuronal damage arising from excessive activation of glutamate receptors (especially the NMDA subtype) in the pathogenesis of Huntington's disease (HD), although clinical trials involving NMDA receptor inhibition have mostly failed. Further understanding of the underlying molecular mechanisms is therefore needed to refine therapeutic approaches. My work has focused on the elucidation of molecular pathways linking huntingtin (htt) to NMDA receptors (NMDARs). I found that NMDAR subunits NR1/NR2B are shifted from internal pools to the plasma membrane, with faster NMDAR insertion to the surface in striatal neurons from YAC72 HD mice. YAC72 striatum shows a relative enrichment of NR1 C2' isoforms in the vesicle/microsome-enriched fraction and preferential association of these isoforms with NR2B subunits, suggesting alternative splicing of NR1 may favour faster forward trafficking of receptors to potentiate NMDAR current and toxicity in this HD mouse model. I demonstrated co-localization and interaction of the htt-interacting protein HIP-1 and actin-crosslinking α-actinin proteins, together in a complex with NMDARs in mouse striatal neurons and forebrain tissue. In fact, HIP-1 can directly interact with α-actinin, thus providing a physical link between htt and NMDARs. To broaden the search for candidate proteins mediating the effects of mutant htt (mhtt) on NMDAR function, I collaborated with Kinexus Bioinformatics Corp. to examine expression patterns of a variety of protein kinases, phosphatases, and heat shock/stress proteins, in HEK cells over-expressing NR1/NR2B-type NMDARs and wild-type or mutant htt. Multiple proteins involved in the heat shock response pathway, including Hsp-70 and CK2, show altered subcellular distribution with co-expression of mhtt and NR1/NR2B, an effect potentiated by NMDAR stimulation. CK2 expression is also elevated in striatal tissue from YAC HD mice, and its activity plays a protective role against NMDAR toxicity. Altered expression of stress response proteins in the presence of mhtt and NR1/NR2B may reflect anattempt to mitigate mhtt-induced sensitivity to excitotoxicity. Findings from this thesis provide additional insight into the molecular mechanisms underlying increased NMDAR-mediated excitotoxicity in the YAC mouse model of HD. Target molecules and pathways identified in this work may contribute to the optimization of strategies for the treatment of HD.

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