UBC Theses and Dissertations
Novel, peptide-mediated neuroprotective strategies for Huntington's Disease Girling, Kimberly Dayle
Huntington’s disease (HD) is an inherited neurodegenerative disease with progressive striatal loss. No treatments exist, though availability of predictive testing offers possibility for early intervention. Peptides represent an exciting way to interact with molecular signaling. My thesis investigates three potential early targets for preventative HD therapies: NMDAR-mediated PTEN nuclear translocation, caspase-6 activation, and peptide-mediated mutant huntingtin (mHTT) knockdown. Excitotoxicity via N-methyl-D-aspartate receptor (NMDAR) over-activation has a role in HD pathogenesis. We recently demonstrated that nuclear translocation of phosphatase and tensin homolog deleted on chromosome ten (PTEN) is a critical step in NMDAR-mediated excitotoxicity, and blocking PTEN nuclear translocation with peptide Tat-K13 prevents excitotoxic neuronal death. Given the role of NMDAR-mediated excitoxicity in HD, PTEN nuclear translocation may have a role in excitotoxic neuron death in HD. Here, PTEN nuclear translocation was associated with NMDAR-mediated death in cultured HD neurons. I also detected small increases in nuclear PTEN in HD transgenic mouse brains vs control. Interestingly, Tat-K13 effectively blocked PTEN translocation and prevented excitotoxicity in cortex/hippocampus, but not striatum, in vitro and in vivo, suggesting differential mechanisms of PTEN nuclear translocation. Caspase activation downstream of NMDARs may be critically involved in excitotoxicity. Caspase-6 (casp6) particularly, has roles in pathogenesis of HD and other conditions. Using NMDA-induced excitotoxicity in cultured neurons, I demonstrated early increase in caspase profiles via mRNA, protein and activity. Casp6 is elevated and activated first, followed by caspase-8 and caspase-3. Casp6 substrate huntingtin, and novel casp6 substrates STK3 and DAXX, are cleaved in similar temporal patterns post-NMDA, pointing to casp6 is an initiator caspase in NMDA-mediated apoptotic cascades and a potential therapeutic target. Many studies suggest that reducing mHTT protein may be an effective HD therapy. Our lab recently developed a technique for targeted protein degradation using peptides that signal proteins for lysosome- or proteasome-mediated degradation. Utilizing mHTT-binding domains paired with degradation sequences, I designed and tested mHTT-degradation peptides. In cultured neurons from HD transgenic mice, mHTT-degradation peptides led to robust mHTT knockdown, at least in part due to degradation. mHTT degradation peptides were ineffective in vivo, but pointed to potential for development of peptide-mediated mHTT-lowering therapies.
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