The liquid-to-solid transition of FUS is promoted by the condensate surface

TLDR

Macromolecules can phase‑separate into dynamic, membraneless condensates, yet their liquid‑to‑solid transition can cause irreversible aggregation linked to neurodegenerative disease, and the initiating mechanism remains unknown. The study investigates whether FUS condensates exhibit spatially heterogeneous liquid‑to‑solid transitions, with liquid and gel phases coexisting within the same condensate. The authors employed micropipette aspiration combined with spatial dynamic mapping and reflective confocal dynamic speckle microscopy to probe the inhomogeneous rheology of aging FUS condensates. The results show that the liquid‑to‑solid transition starts at the condensate surface and propagates inward, underscoring the interface as a key driver of pathological protein aggregation.

Abstract

A wide range of macromolecules can undergo phase separation, forming biomolecular condensates in living cells. These membraneless organelles are typically highly dynamic, formed reversibly, and carry out essential functions in biological systems. Crucially, however, a further liquid-to-solid transition of the condensates can lead to irreversible pathological aggregation and cellular dysfunction associated with the onset and development of neurodegenerative diseases. Despite the importance of this liquid-to-solid transition of proteins, the mechanism by which it is initiated in normally functional condensates is unknown. Here we show, by measuring the changes in structure, dynamics, and mechanics in time and space, that single-component FUS condensates do not uniformly convert to a solid gel, but rather that liquid and gel phases coexist simultaneously within the same condensate, resulting in highly inhomogeneous structures. Furthermore, our results show that this transition originates at the interface between the condensate and the dilute continuous phase, and once initiated, the gelation process propagates toward the center of the condensate. To probe such spatially inhomogeneous rheology during condensate aging, we use a combination of established micropipette aspiration experiments together with two optical techniques, spatial dynamic mapping and reflective confocal dynamic speckle microscopy. These results reveal the importance of the spatiotemporal dimension of the liquid-to-solid transition and highlight the interface of biomolecular condensates as a critical element in driving pathological protein aggregation.