Investigation of molecular mechanisms underlying Oculopharyngeal Muscular Dystrophy (OPMD)

Oculopharyngeal Muscular Dystrophy (OPMD) is a late-onset dominant/recessive myopathy caused by the expansion of a polyalanine repeat in exon 1 of the PABPN1 gene. The expression of expanded PABPN1 (expPABPN1) triggers the formation of insoluble nuclear aggregates within muscle fiber nuclei of OPMD patients. These aggregates are enriched in poly(A)RNA and sequester molecular chaperones, ubiquitin and proteasome subunits. In addition to these cellular components, we first identified two novel PABPN1 interacting partners, hnRNPA1 and hnRNPA/B that also localized to the insoluble expPABPN1 aggregates. However, only hnRNPA1 was observed in inclusions of OPMD patients' muscle fiber nuclei. Following this finding, we next established the involvement of the ubiquitin-proteasome pathway in the clearance of misfolded expPABPN1 and provided more insights into the beneficial role of molecular chaperones in OPMD. The inhibition of proteasome correlated with an increase in the aggregation of expPABPN1, suggesting a possible proteasome impairment in OPMD. Conversely, the overexpression of Hsp70 and Hsp40 coincided with a decrease in nuclear aggregates concomitant with a reduced cellular toxicity, suggesting the therapeutic potential of manipulating molecular chaperones levels. Finally, we demonstrated that soluble forms of expPABPN1 are the primary toxic species in OPMD. In the presence of endogenous HSPs, a decrease in expPABPN1 aggregation correlated with an increased cellular toxicity. A defect in polyadenylation or ubiquitination significantly increased expPABPN1 solubility and cell death. Using live-cell imaging, we observed that nuclear aggregates prolonged the survival of expPABPN1-expressing cells, which led us to speculate that protein aggregates are subnuclear structures that preserve cellular homeostasis by depleting the expPABPN1 from the nuclear soluble pool. We propose that the polyalanine expansion in expPABPN1 could enable aberrant protein-protein interactions that would compromise the cellular function of nuclear factors and the expression of genes essential for muscle integrity and differentiation. For instance, expPABPN1 might compromise the function of hnRNP proteins and lead to altered mRNA processing and nucleocytoplasmic export, which can be detrimental to the cell