Human iPSC-derived spinal neurons carrying the ALS FUS (P525L) mutation exhibit lower response to inhibitory neurotransmitters

Amyotrophic lateral sclerosis (ALS) is a progressive neuromuscular disorder characterized by motoneurons degeneration. Functional studies have linked ALS to hyperexcitability and excitotoxicity, but the cause of the disease is unknown, though familial ALS cases are linked to pathogenic variants in several genes, including SOD1 , TARDBP and FUS . Here we focused on the effect of the severe FUS (P525L) mutation on the functional properties of human spinal neurons derived from induced pluripotent stem cells (hiPSCs). This mutation delayed functional maturation, as revealed by the observation that mutated neurons showed alterations of membrane potential, reduced spontaneous synaptic activity, and altered action potentials at early differentiation stages. FUS (P525L) mutation was associated with a significant alteration of inhibitory signalling transmission: mutated neurons showed a significantly lower current response to GABA and glycine compared to control isogenic WT neurons of the same age. Also, glutamatergic currents exhibited a different temporal evolution in control and mutated neurons, but at a lower extent in comparison to inhibitory neurotransmitters. The decrease in the glycine-evoked currents was confirmed by the reduction of the expression of the a1 subunit of glycine receptor, measured by immunofluorescence assay. Similar functional alterations were measured in spinal neurons differentiated form a second hiPSC line, confirming the causative role of the FUS (P525L) mutation. Our data indicate that the FUS (P525L) mutation reduces the maturation rates and the function of hiPSC-derived spinal neurons, with a strong decrease of inhibitory transmission, which may affect the excitatory/inhibitory balance, possibly predisposing to excitotoxicity and neurodegeneration.