Polyalanine Expansion in PABPN1 Alters the Structure and Dynamics of Its Nuclear Aggregates in Differentiated Muscle Cells

Intracellular protein aggregation is a hallmark of aging and contributes to pathology in some age-associated diseases. In hereditary adult-onset neuromuscular diseases (NMDs), protein aggregates play a key role in disease onset and progression. The wild-type Poly(A) binding protein nuclear 1 (PABPN1) forms benign nuclear aggregates, whereas a short trinucleotide expansion leads to the formation of pathogenic aggregates, a hallmark of Oculopharyngeal Muscular Dystrophy (OPMD). In OPMD, the mutant PABPN1 causes skeletal muscle weakness. So far, the structural differences between benign and pathogenic protein aggregates and their effects on muscle cell biology remain poorly understood. We employed an array of advanced imaging modalities to explore the morphological differences between nuclear aggregates formed by non-pathogenic and pathogenic PABPN1 variants. Through analyses spanning micro- to nanoscale, we identified distinct structural features of aggregates formed by wild-type and expanded PABPN1. We demonstrate that these differences were more pronounced in differentiated muscle cells compared to proliferating cells. We further linked the structural features of PABPN1 aggregates to muscle cell biology, namely alterations in mitochondrial function and proteasomal activity. Our findings provide new insights into the structural distinctions between pathogenic and non-pathogenic aggregates and their implications for cellular dysfunction in NMDs.