The expression of the chemorepellent Semaphorin 3A is selectively induced in terminal Schwann cells of a subset of neuromuscular synapses that display limited anatomical plasticity and enhanced vulnerability in motor neuron disease

Neuromuscular synapses differ markedly in their plasticity. Motor nerve terminals innervating slow muscle fibers sprout vigorously following synaptic blockage, while those innervating fast-fatigable muscle fibers fail to exhibit any sprouting. Here, we show that the axon repellent Semaphorin 3A is differentially expressed in terminal Schwann cells (TSCs) on different populations of muscle fibers: postnatal, regenerative and paralysis induced remodeling of neuromuscular connections is accompanied by increased expression of Sema3A selectively in TSCs on fast-fatigable muscle fibers. To our knowledge, this is the first demonstration of a molecular difference between TSCs on neuromuscular junctions of different subtypes of muscle fibers. Interestingly, also in a mouse model for amyotrophic lateral sclerosis (ALS), Sema3A is expressed at NMJs of fast-fatigable muscle fibers. We propose that expression of Sema3A by TSCs not only suppresses nerve terminal plasticity at specific neuromuscular synapses, but may also contribute to their early and selective loss in the motor neuron disease ALS.