Expression of ALS-linked SOD1 Mutation in Motoneurons or Myotubes Induces Differential Effects on Neuromuscular Function In vitro

•Myotubes expressing the ALS-causing SOD1 mutant show reduced contraction frequency.•The presence of mutated SOD1 in myotube leads to reduced levels of the slow myosin heavy chain isoform.•Expression of SOD1G93A in motoneurons impairs the formation of functional neuromuscular junctions. Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease that selectively affects upper and lower motoneurons. Dismantlement of the neuromuscular junction (NMJ) is an early pathological hallmark of the disease whose cellular origin remains still debated. We developed an in vitro NMJ model to investigate the differential contribution of motoneurons and muscle cells expressing ALS-causing mutation in the superoxide dismutase 1 (SOD1) to neuromuscular dysfunction. The primary co-culture system allows the formation of functional NMJs and fosters the expression of the ALS-sensitive fast fatigable type II-b myosin heavy chain (MHC) isoform. Expression of SOD1G93A in myotubes does not prevent the formation of a functional NMJ but leads to decreased contraction frequency and lowers the slow type I MHC isoform transcript levels. Expression of SOD1G93A in both motoneurons and myotubes or in motoneurons alone however alters the formation of a functional NMJ. Our results strongly suggest that motoneurons are a major factor involved in the process of NMJ dismantlement in an experimental model of ALS.