Nuclear Protein Aggregates Disrupt RNA Processing and Alter Biomechanics in a Muscle Cell Model of OPMD.

Aggregation of RNA-binding proteins (RBPs) is a hallmark of several age-related neuromuscular diseases. However, our understanding of how these aggregates drive dysfunction is often limited by the use of non-disease-relevant models. Oculopharyngeal muscular dystrophy (OPMD) is caused by a short alanine expansion mutation in the PABPN1 gene, which leads to nuclear aggregation of the protein. To investigate how these aggregates impair muscle cell function, we developed a muscle cell model with inducible expression of the pathogenic PABPN1 (A16) variant and confirmed its relevance to OPMD. Using subcellular fractionation combined with mass spectrometry and RNA sequencing, we examined the molecular consequences of nuclear PABPN1 aggregation. In the cytoplasmic fraction, we observed significant impairments in cellular metabolism and biomechanics. In the nuclear fraction, RNA metabolism was broadly disrupted, and additional RBPs were significantly enriched in insoluble aggregates. Importantly, mRNAs trapped within the aggregates were associated with impaired nuclear export and decreased translation efficiency, and the pathogenic PABPN1 variant led to reduced endogenous PABPN1 levels. Our findings support a model in which OPMD pathology arises from reduced levels of soluble PABPN1 due to nuclear aggregation and establish a mechanistic link between RBP aggregation and muscle cell dysfunction, highlighting shared pathological pathways across neuromuscular and neurodegenerative diseases.