Role of EPS8-mediated RAC activation in disease-related protein aggregation

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Aging is a major risk factor for neurodegenerative diseases that involve protein aggregation, including amyotrophic lateral sclerosis, Huntington’s, Parkinson’s and Alzheimer’s. The discovery of pathways that slow down the aging process has revolutionized the field of molecular gerontology, suggesting the realistic possibility of creating therapeutics to prevent age-related neurodegenerative diseases. However, the mechanisms underlying aging and their impact on age-related diseases are only beginning to be unraveled at the molecular level. In the previous DFG grant, we found a remodeling of the ubiquitinated proteome during aging, which was ameliorated by longevity paradigms such as dietary restriction and reduced insulin signaling. Notably, aging caused a global loss of ubiquitination that was triggered by increased deubiquitinase (DUB) activity. Because ubiquitination can tag proteins for recognition by the proteasome, a fundamental question was whether deficits in targeted degradation influence longevity. By integrating data from worms with a defective proteasome, we identified proteasomal targets that accumulate with age owing to decreased ubiquitination and subsequent degradation. Lowering the levels of age-dysregulated proteasome targets prolonged longevity, whereas preventing their degradation shortened lifespan. Among the proteasomal targets, we found the IFB-2 intermediate filament and the EPS-8 modulator of RAC signaling. While increased levels of IFB-2 promoted the loss of intestinal integrity, upregulation of EPS-8 hyperactivated RAC in muscle and neurons, and led to alterations in the actin cytoskeleton and protein kinase JNK. An exciting hypothesis raised by these results is whether regulatory proteins that escape proteasomal degradation with aging have a role in the development of age-related disorders that involve protein aggregation. Indeed, our preliminary results indicate that lowering EPS-8/RAC signaling, but not IFB-2, prevents aggregation of proteins linked with neurodegenerative diseases such as Huntington’s and amyotrophic lateral sclerosis in C. elegans disease models. In this renewal application, we propose to define the impact of EPS-8/RAC in disease-related protein aggregation and subsequent physiological consequences using different C. elegans and human cell models. Then, we will define whether hyperactivated EPS8/RAC signaling modulates disease-related protein aggregation through its role in actin and/or JNK regulation. Finally, we will define the DUBs that deubiquitinate and suppress proteasomal degradation of EPS-8. Importantly, our preliminary data indicate that knockdown of USP4, a DUB that becomes upregulated with age, phenocopies the beneficial effects of lowering EPS8/RAC signaling in C. elegans and human cells. Together, our findings could lead to converging therapeutic approaches for distinct age-related neurodegenerative disorders.