Mislocalised FUS mutants stall spliceosomal snRNPs in the cytoplasm

Genes encoding RNA-binding proteins have frequently been implicated in various motor neuron diseases, but the particular step in RNA metabolism that is vulnerable in motor neurons remains unknown. FUS, a nuclear protein, forms cytoplasmic aggregates in cells affected by amyotrophic lateral sclerosis (ALS), and mutations disturbing the nuclear import of FUS cause the disease. It is extremely likely that the cytoplasmic aggregates are cytotoxic because they trap important factors; the nature of these factors, however, remains to be elucidated. Here we show that FUS associates in a neuronal cell line with SMN, the causative factor in spinal muscular atrophy (SMA). The two genes work on the same pathway, as FUS binds to spliceosomal snRNPs downstream of the SMN function. Pathogenic FUS mutations do not disturb snRNP binding. Instead, cytoplasmic mislocalisation of FUS causes partial mis-localisation of snRNAs to the cytoplasm, which in turn causes a change in the behaviour of the alternative splicing machinery. FUS, and especially its mutations, thus have a similar effect as SMN1 deletion in SMA, suggesting that motor neurons could indeed be particularly sensitive to changes in alternative splicing.