Mitochondrial stress responses in neurodegeneration

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Neurodegeneration is an increasing and unmitigated disease burden in our aging population. Damaged mitochondria accumulate in post-mitotic cells such as neurons and are associated with the common neurodegenerative disorders, most notably sporadic Parkinsons disease (PD). Additionally, damaged mitochondria are drivers of neurodegeneration in monogenic forms of PD (due to mutations in PINK1, PRKN, and CHCHD26), as well as monogenic forms of amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and myopathy (due to mutations in CHCHD10). Preventing (or in some cases exacerbating) neurodegeneration, the cell has evolved stress responses to handle damaged mitochondria. These include selective degradation by autophagy (mitophagy), isolation by inhibition of their fusion, and signaling to the cytosol and nucleus through the integrated stress response (ISR). In mammals, these responses are initiated by stress sensing proteins within mitochondria, including PINK1 and DELE1, which are stabilized in the setting of mitochondrial import arrest, and OMA1, an inner membrane peptidase that is activated by mitochondrial depolarization. A more complete understanding of these mitochondrial stress responses may uncover novel targets for the treatment of neurodegenerative disorders. The overall goal of our program is to characterize mitochondrial stress responses in neurodegeneration and develop precision therapy through the study of monogenic disorders, with an emphasis on the disease-causing genes PINK1, PRKN, CHCHD2, and CHCHD10. To this end, we propose three complementary aims that draw on strengths of the intramural program and my perspective as a cell biologist and neurologist: SPECIFIC AIM 1: To determine mitochondrial stress responses in and develop therapies for CHCHD2 and CHCHD10 related neurodegeneration (60% Effort). Mutations in the CHCHD2 and CHCHD10 paralogs share a pathogenetic mechanism of protein misfolding within mitochondrial cristae. In knock-in (KI) mouse models and engineered and patient cell lines (including iPSC), we are determining the normal function of CHCHD2 and CHCHD10 and the pathogenesis of these disorders. Additionally, in a collaboration with NCATS we are developing allele specific antisense oligonucleotide (ASO) treatments for ALS and other forms of neurodegeneration due to mutations in CHCHD10. We have been evaluating a family with ALS due to a mutation in CHCHD10 within the clinical center. Finally, we have identified a protective mitochondrial stress response against mutant CHCHD10 protein misfolding, which is mediated by the stress-induced peptidase OMA1. This response includes activation of the ISR through cleavage of DELE1 by OMA1, but also inhibits mitochondrial fusion through cleavage of OPA1. We are determining the specific role of the mito-ISR in protecting against mitochondrial damage. We have additionally generated KO mouse models to dissect which OMA1 substrates mediate this protection in vivo. SPECIFIC AIM 2: To identify modulators of and pathogenic mutations in the PINK1-Parkin pathway (25% Effort). Through comprehensive analysis of PRKN in NIH PD clinic patients and public datasets, we have found that PRKN missense variants are common, but most are not functionally characterized. We have developed a novel FACS based reporter of Parkin activity, which we are using to determine which PRKN variants are benign, and which are loss of function in a pooled format. We are additionally using this reporter to identify modulators of the PINK1-Parkin pathway in genome-wide screens. Finally, we are establishing the molecular diagnosis of patients with early onset and/or familial PD at the NIH and U Penn. SPECIFIC AIM 3: To develop novel methods to measure protein turnover in models of neurodegeneration (15% Effort). Using dynamic proteomics and nanoscale metabolic imaging, we are developing novel methods to assess mitophagy and other mitochondrial stress responses in models of neurodegenerative disease and myopathy. These methods will complement efforts in other aims, in addition to their broader applications.