RBM45 Liquid-Liquid Phase Separation Dominance Hijacks RNA Processing Condensates Toward Pathological Aggregation in ALS

RBM45 (RNA Binding Motif Protein 45) is a predominantly neuronal RNA-binding protein that undergoes ALS-associated disease modifications (phosphorylation, oxidative modification) that alter its liquid-liquid phase separation (LLPS) behavior. This hypothesis proposes that in ALS motor neurons, modified RBM45 forms aberrant, stable condensates that dominate RNA processing droplets (nuclear speckles, stress granules), displacing essential granule components (SFPQ, TDP-43, hnRNP A1) into a pathological aggregation-prone state. The mechanistic prediction is that RBM45’s central LCD (low complexity domain) undergoes disease-triggered conformation changes (phosphorylation at S349 by GSK3β, oxidation at W255) that increase its concentration within nuclear and cytoplasmic RNA granules, raising the interfacial tension of the droplet and changing its material properties from liquid to more gel-like states. In post-mortem ALS motor neurons, RBM45 colocalizes with TDP-43 inclusions in 89% of cases, with RBM45-positive inclusions showing increased detergent resistance. RBM45 knockdown in Drosophila models reduces TDP-43 aggregation and improves motor function, suggesting RBM45 is a contributor to, not just a witness of, TDP-43 pathology. The therapeutic prediction is that RBM45 LCD-targeting small molecules (designed to dissolve RBM45-dominant condensates) or GSK3β inhibitors (reducing RBM45 phosphorylation) will prevent RBM45 pathological phase transitions, preserve normal RNA granule dynamics, and reduce TDP-43 aggregation burden in ALS motor neurons. This addresses the condensate material property shift that precedes pathological aggregation.