Functional interaction of FUS with SMN: a common pathogenic pathway in two motor neuron diseases

Motor neuron diseases (MNDs) form a heterogeneous group of pathologies characterised by the progressive degeneration of motor neurons. More and more genetic factors associated with MND encode proteins that have a function in RNA metabolism, suggesting that disturbed RNA metabolism could be a common underlying problem in several, perhaps all, forms of MND, even if the particular step in RNA metabolism that is vulnerable in motor neurons remains unknown. FUS, a nuclear protein supposed to have several functions in DNA and RNA metabolism, forms cytoplasmic aggregates in cells affected by amyotrophic lateral sclerosis (ALS), and mutations disturbing the nuclear import of FUS cause the disease. We engineered mouse motorneuronal NSC34 cells to express wild-type FUS, as well as variants mutated in the C-terminal region and associated to familial ALS (R514G, R521G), a combination of the two single mutants (R514G/R521G), and a truncation mutant associated to a juvenile and aggressive form of familial ALS (R495X), and we showed that our cellular model well recapitulates the FUS-ALS phenotype of mislocalisaton and aggregation. It is extremely likely that the FUS cytoplasmic aggregates are cytotoxic because they trap important factors; the nature of these factors, however, remains to be elucidated. In this study we showed that mutated FUS colocalise with Stress Granules upon oxidative stress induction. Most importantly, mis-localised, aggregated FUS colocalises and associates with SMN, the causative factor in spinal muscular atrophy (SMA). SMN is known to have a crucial role in the biogenesis and localisation of the spliceosomal snRNPs, which are essential assembly modules of the splicing machinery. Our results indicate that FUS and SMN work on the same pathway, as FUS binds to SMN and 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 and that such alterations could represent a common pathogenic patway for ALS, SMA and – perhaps – other MNDs.