Innervation and cargo-specific axonal transport impairments in FUS-ALS mice with gain and loss of function [RNA-seq]

Mutations in the RNA-binding protein FUS cause aggressive, early-onset forms of amyotrophic lateral sclerosis (ALS), leading to the progressive degeneration of the neuromuscular system. These mutations drive nuclear depletion and cytoplasmic mislocalisation of FUS, resulting in both loss and gain of function. We investigated these mechanisms and their impact on key pathophysiological processes in vivo, using a novel FUS-ALS mouse model. Adult mice presented widespread transcriptomic alterations consistent with nuclear loss of function, while translatome analysis revealed specific deficits associated with aberrant FUS-dependent cytoplasmic partitioning and gain of function. Mutant FUS expression selectively impaired neuromuscular junction (NMJ) innervation in FUS-ALS females. However, NMJ reinnervation following sciatic nerve crush was equally perturbed in homozygous mutants of both sexes, indicating impaired regeneration and damage responses. Additionally, we observed cargo-specific alterations in axonal transport, a process critical for neuronal function and maintenance; in vivo mitochondrial transport was impaired in homozygous FUS-ALS mice, whereas the retrograde transport of signalling endosomes was again only affected in mutant females, strengthening the link between endosomal transport impairment and NMJ denervation in ALS. Altogether, our findings establish FUS-ALS homozygous mice as a valuable model for investigating early cellular impairments driving ALS pathology, and identify broader and sex-dependent motor neuron dysfunction in FUS-ALS pathogenesis. Overall design: spinal cord RNA-seq from Fus+/+ and Fus?14/?14 (1 and 3 month old) and Fus-/- (3 month old). All animals derive from C57BL/6J × DBA/2J intercrossing and are F1.