Compositional control of ageing kinetics in TDP-43 condensates

Biomolecular compartments orchestrate the spatiotemporal organisation of cells. The spontaneous assembly of proteins and nucleic acids through liquid-liquid phase separation into biomolecular condensates has been shown to ubiquitously contribute to the functional compartmentalisation of the cytoplasm and nucleoplasm. However, some condensates can undergo an additional phase transition from functional liquid states to pathological solid-like assemblies (i.e., ageing). This liquid-to-solid transition, driven by the accumulation of protein cross- β -sheet structures, represents a hallmark of multiple neurodegenerative disorders. In this study, we employ Molecular Dynamics simulations to explore the role of various biomolecules in regulating the ageing kinetics of condensates scaffolded by TDP-43, a key RNA-binding protein linked to amyotrophic lateral sclerosis and frontotemporal dementia. We find that the recruitment of arginine-rich peptides, such as those produced by the C9orf72 gene, accelerates the nucleation of cross- β -sheet structures. In contrast, the inclusion of poly-Uridine RNA and the HSP70 chaperone significantly slows the emergence of these structures. Remarkably, we observe a correlation between the compactness of the low-complexity domain of TDP-43— which drives the transition to cross- β -sheet structures—and the condensate ageing kinetics as we vary the composition of the condensates. Moreover, we find that near-interfacial regions of TDP-43 condensates exhibit faster β -sheet transitions than the bulk core of the condensate. Together, our findings underscore the critical role of client biomolecules in modulating protein conformational ensembles and intermolecular interactions, thereby controlling the propensity of condensates to transition into harmful solid-like states.