Mitochondrial Porin in Bioenergetic Defects in Huntingtons Disease

Indiana University Indianapolis, Indianapolis IN

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Abstract

DESCRIPTION (provided by applicant): Huntington's Disease (HD) is an inherited, neurodegenerative disorder associated with the abnormal expansion of CAG triplet that encodes a polyglutamine domain in huntingtin, a 350 kDa protein expressed in various tissues. A mechanistic link between Htt gene mutation and neuronal loss leading to neurological abnormalities in HD has not yet been determined, but mitochondrial dysfunction has emerged as a causal factor involved in HD pathogenesis. Despite extensive studies, the mechanisms of mitochondrial dysfunction in HD remain unclear. The overall objectives of the proposed study are to clarify the role of mitochondrial porin, also known as voltage-dependent anion channel (VDAC), in mutant huntingtin (mHtt)-induced mitochondrial dysfunction and abnormal mitochondrial fragmentation in mHtt-expressing neurons. In the proposed study, we will test a novel hypothesis that mHtt binds to VDAC and inhibits metabolite transport across the OMM, leading to mitochondrial dysfunction, Ca2+ handling defects, mitochondrial oxidative stress, and augmented mitochondrial fission. We will address the following questions: (1) Does mHtt diminish VDAC transport activity by binding to the channel? (2) Is VDAC inhibition accountable for respiratory suppression, depolarization, and accumulation of superoxide anion O2¿ - in mitochondria exposed to mHtt? (3) Does mHtt result in increased susceptibility to mitochondrial Ca2+-induced injury and decreased Ca2+ uptake capacity by inhibiting VDAC? (4) Does VDAC inhibition lead to mitochondrial oxidative stress and augmented mitochondrial fission in cultured neurons expressing mHtt? To answer these questions we will use VDAC-reconstituted giant proteoliposomes in conjunction with electrophysiological patch-clamp technique and glutathione-S-transferase (GST)-polyQ fusion proteins. We will use synaptic and non-synaptic purified brain mitochondria isolated from wild-type mice and transgenic and knock-in HD mouse models in combination with modern pharmacological, biochemical, and bioenergetic methodologies. To analyze mitochondrial dynamics, we will use live-cell, laser spinning-disk confocal microscopy followed by sophisticated image processing and quantitative 3D image rendering applied to cultured striatal and cortical neurons derived from wild-type and HD mice with mitochondria visualized by mitochondrially targeted fluorescent proteins. At the conclusion of this research program, we will establish the role of VDAC inhibition in mitochondrial dysfunction, Ca2+ handling defects, mitochondrial oxidative stress, and augmented fission in mitochondria exposed to mHtt. Thus, our study will provide novel, vital knowledge about molecular mechanisms of mitochondrial dysfunction in HD and build a platform for future HD research. This will lay a solid foundation for creating treatments aimed at improving mitochondrial functioning and neuronal survival in HD. Most importantly, this will immensely help in the development of new therapeutic strategies to alleviate neurological deficits in HD and significantly diminish suffering of HD patients, improve quality of their life, and lessen the emotional and financial burden on the family and the whole society.