Critical period of neuromuscular development: Importance for a new treatment of SMA.

Findings from mice that had their Smn gene deleted and some copies of the human SMN2 gene introduced to produce SMN protein are summarized. Symptoms due to this manipulation can be corrected only by restoring the SMN protein expression in neurones and not in muscle. The changes in muscle and neuromuscular junction (NMJ) in these mutant mice are probably due to the malfunction of the neuronal component of the NMJ i.e. the nerve terminal. The reduction of transmitter release by nerve terminals in animals with reduced SMN protein supports this notion. There is a critical period during which the presence of the SMN protein is mandatory for the survival of the motor unit and the individual. This period coincides with the most important events involved in the development of the motor unit. Results from normal genetically unaffected rats and mice show that during a critical period of development the function of the nerve terminal and the release of transmitter play a crucial role in the development of the motor neurone and muscle. The possibility that targeting the function of the nerve terminal to overcome its inability to release transmitter could benefit patients with the deletion of the SMN gene.