As our population ages, neurodegenerative disorders such as Parkinson disease (PD) comprise a major societal burden. While mechanisms for PD etiology are still emerging, evidence of mitochondrial dysfunction in the pathogenesis of this disease is abundant. Another component of PD pathology is the protein a-synuclein (a-syn); it is found within Lewy Body inclusions, yet causes of cellular toxicity remain unclear. A strategy that mitochondria employ for managing stress is to engage the mitochondrial unfolded protein response (UPRmt), which coordinates nuclear expression of chaperones and proteases that translocate to the mitochondria to handle damaged and/or unfolded proteins. When activated in response to acute stressors, the UPRmt re- establishes homeostasis and promotes cell survival. However, it can become dysregulated when challenged with a long-term genetic stressor such as misfolded a-syn and becomes cytotoxic. Notably, molecular variants of a-syn can interact with TOMM20, an outer mitochondrial membrane protein, and initiate a physical block of mitochondrial protein import. We speculate that the increased UPRmt response observed in a-syn-expressing neurons is a consequence of blocked mitochondrial import. Although attention to a role for mitochondrial quality control in neurodegenerative disease has proven increasingly insightful, there is a pivotal gap that remains to be addressed in demonstrating a direct functional correlation between dysregulated UPRmt activity and neurodegeneration. Importantly, our research illustrates an insidious aspect of mitochondrial signaling in which the UPRmt pathway exacerbates disruption of dopaminergic neurons in vivo, resulting in the neuron loss characteristic of PD. Our approach exploits the expedience of genetic manipulation in Caenorhabditis elegans research, and the rigor with which large, isogenic populations can be scored for neurodegeneration with unprecedented accuracy, at the single-neuron level. We will systematically investigate combinations of transgenic worms co-expressing structural variants of a-syn and transcription factors that activate the UPRmt to discern functional requirements for UPRmt activation with neurodegeneration as the primary endpoint. The studies in Aim 1 will investigate the hypothesis that the a-syn-TOMM20 mitochondrial import block triggers the UPRmt pathway and will explore a role for dopamine in potentially exacerbating the deleterious consequences of this process. As a distinct strategy, Aim 2 will involve the identification of molecular components associated with UPRmt signaling through a forward genetic screening strategy that takes advantage of a strain we have generated that reveals an uncharacterized compensatory mechanism for UPRmt induction. Phenotypic bioassays and genetic screening using C. elegans are routinely conducted by undergraduates in our lab and will serve as an excellent training opportunity for students through this R15 proposal. These studies represent a timely and mechanistic strategy towards defining nuclear-mitochondrial dynamics, specifically with respect to dopaminergic neurodegeneration, with potential to inform a translational path for therapeutic development.
Parkinson Disease (PD) is the most common movement disorder affecting over 1 million Americans, yet underlying causes of this neurodegenerative disease have largely eluded medical science. This proposal is designed to address an unmet challenge of discerning how neurons respond to mitochondrial stress and how this can lead to enhanced susceptibility to neurodegeneration. Understanding the mechanisms underlying this cytotoxicity will be explored using genetics and a whole animal model system whereby outcomes include limiting mitochondrial damage and neurodegeneration in PD.
