Protein ubiquitination and deubiquitination in neurological disorders

Abstract Protein ubiquitination is a posttranslational modification that is essential for nervous system function. UB requires the utilization of a series of enzymes called E1, E2, and E3 that enable the covalent addition of ubiquitin (UB) to to its cellular targets. Protein ubiquitination is also a highly reversible process which is handled by enzymes called deubiquitinases (DUBs) that cut and/or edit UB chains on substrates. While numerous neurological disorders are associated with mutations in the UB enzyme machinery, the role of protein ubiquitination under non-diseased states and the intricate mechanisms by which proteins become ubiquitinated and deubiquitinated in the nervous system remain vastly uncharacterized. Using an unbiased proteomics screen for substrates of the E3 UB ligase RNF216, we identified the DUB OTUD4 and proteins within the OTUD4 interaction network as substrates. Remarkably, mutations in OTUD4 and RNF216 have been identified in Gordon Holmes syndrome (GHS), where patients have neuroendocrine dysfunction, neurodegeneration, and dementia. Increases in OTUD4 mRNA and protein were also identified in individuals with frontotemporal lobar degeneration. Our preliminary data show that RNF216 regulates OTUD4 ubiquitination, which is reduced in the OTUD4 GHS- associated mutation. Moreover, OTUD4 was found to de-ubiquitinate RNF216, which was increased in the GHS- associated mutation. In the proposed work, we will test the hypothesis that excessive OTUD4 DUB activity is a driving factor in neurodegenerative disease. In Aim 1, we will determine if RNF216 ubiquitination of OTUD4 is destabilizing and if the OTUD4 GHS mutation has enhanced stability. In Aim 2, we will determine if the interaction between OTUD4 and RNF216 would enable OTUD4 to function as a direct DUB of RNF216 and affect its stability and catalytic activity for conjugating UB to substrate proteins. In Aim 3, we will determine if OTUD4 gain-of- function increases neuroinflammation and leads to reproductive impairments, and if reductions in its activity can restore these deficiencies in GHS models. Completion of these aims will increase our understanding on how E3s and DUBs are coordinated in the nervous system. Our work will also outline a mechanism to explain how mutations and increased activity of OTUD4 drive GHS pathologies, and will identify new therapeutic targets for the treatment of GHS and other related disorders.