Modulation of biomolecular aggregate morphology and condensate infectivity.

Neurodegenerative diseases are characterized by pathological aggregates exhibiting distinct morphologies, such as neurofibrillary tangles and dense circular Lewy body-like structures in Alzheimer's disease, and round hyaline gel-like inclusions and skein-like filaments in amyotrophic lateral sclerosis. However, the mechanisms driving the formation of these diverse morphological structures remain poorly understood. Employing advanced microscopy, including fluorescence lifetime imaging, we investigated condensate aging and aggregation mechanisms of the prion-like domain of hnRNPA1 (A1PrD), a ribonucleoprotein implicated in both disorders. Using a simplified system across various salt and RNA conditions, we demonstrate that homotypic and heterotypic interactions between A1PrD and RNA significantly influence aggregate morphology and amyloid fibril formation, yielding diverse structures including thin fibrils, solid gels, and filamentous starburst aggregates. By tracking aggregate morphogenesis, we observed shifts in fluorescence lifetimes that reflect differences in condensate microenvironments, highlighting distinct homotypic and heterotypic interaction dynamics. Our findings indicate that amyloid fibril formation can initiate within fluid condensates or at the interfaces of solid gels. Moreover, early amyloid-rich fluid starbursts demonstrated the capability to fuse with or recruit younger amyloid-poor droplets, exemplifying prion-like infectivity and accelerating fibril formation. Collectively, our study provides evidence that the interplay between solution composition and the kinetic balance of liquid-liquid phase separation, gelation, and fibrillation contributes to the diverse pathological aggregate morphologies observed in neurodegenerative diseases.