ALS/FTD Mutation-Induced Phase Transition of FUS Liquid Droplets and Reversible Hydrogels into Irreversible Hydrogels Impairs RNP Granule Function

TLDR

The mechanisms by which mutations in FUS and other RNA‑binding proteins cause ALS and FTD remain controversial. The authors propose that FUS low‑complexity domains drive reversible assembly into membrane‑free liquid droplets and hydrogel‑like structures, and that blocking the formation of fibrillar hydrogels can reduce neurotoxicity, offering a therapeutic strategy. Their model posits that FUS low‑complexity domains mediate reversible assembly into membrane‑free liquid droplets and hydrogel‑like structures. Mutations in FUS trigger a phase transition to insoluble fibrillar hydrogels that sequester ribonucleoprotein granule components, impair local protein synthesis in axon terminals, and are necessary and sufficient for neurotoxicity in a C.

Abstract

The mechanisms by which mutations in FUS and other RNA binding proteins cause ALS and FTD remain controversial. We propose a model in which low-complexity (LC) domains of FUS drive its physiologically reversible assembly into membrane-free, liquid droplet and hydrogel-like structures. ALS/FTD mutations in LC or non-LC domains induce further phase transition into poorly soluble fibrillar hydrogels distinct from conventional amyloids. These assemblies are necessary and sufficient for neurotoxicity in a C. elegans model of FUS-dependent neurodegeneration. They trap other ribonucleoprotein (RNP) granule components and disrupt RNP granule function. One consequence is impairment of new protein synthesis by cytoplasmic RNP granules in axon terminals, where RNP granules regulate local RNA metabolism and translation. Nuclear FUS granules may be similarly affected. Inhibiting formation of these fibrillar hydrogel assemblies mitigates neurotoxicity and suggests a potential therapeutic strategy that may also be applicable to ALS/FTD associated with mutations in other RNA binding proteins.