Dysregulation of alternative splicing underlies synaptic defects in familial amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is an incurable neurodegenerative disease characterized by the degeneration of upper and lower motor neurons, progressive wasting and paralysis of voluntary muscles. A hallmark of ALS is the frequent nuclear loss and cytoplasmic accumulation of RNA binding proteins (RBPs) in motor neurons (MN), which leads to aberrant alternative splicing regulation. However, whether altered splicing patterns are also present in familial models of ALS without mutations in RBP-encoding genes has not been investigated yet. Herein, we found that altered splicing of synaptic genes is a common trait of familial ALS MNs. Similar deregulation was also observed in hSOD1G93A MN-like cells. In silico analysis identified the potential regulators of these pre-mRNAs, including the RBP Sam68. Immunofluorescence analysis and biochemical fractionation experiments revealed that Sam68 accumulates in the cytoplasmic insoluble ribonucleoprotein fraction of MN. Remarkably, the synaptic splicing events deregulated in ALS MNs were also affected in Sam68-/- spinal cords. Recombinant expression of Sam68 protein was sufficient to rescue these splicing changes in ALS hSOD1G93A MN-like cells. Hence, our study highlights an aberrant function of Sam68, which leads to splicing changes in synaptic genes and may contribute to the MN phenotype that characterizes ALS.