Motor endplate disease affects neuromuscular junction maturation.

Postnatal formation of the neuromuscular synapse requires complex interactions among nerve terminal, muscle fibres and terminal Schwann cells. In motor endplate disease (med) mice, neuromuscular transmission is severely impaired without alteration of axonal conduction and a lethal paralytic phenotype occurs during the postnatal period. The med phenotype appears at a crucial stage of the neuromuscular junction development, corresponding to the increase in terminal Schwann cell number, the elimination of the multiple innervations and the pre- and postsynaptic maturation. Here we investigated the early cellular and molecular consequences of the med mutation on neuromuscular junction development. We observed that cellular defects preceded overt clinical phenotype. The first detectable cellular effect of the mutation at the onset of the clinical phenotype was a drastic reduction in the number of terminal Schwann cells, in part due to an increase in glial apoptosis, and a delayed maturation of motor endplates. We also showed that, in terminally ill animals, mono-innervation was not achieved, synaptic vesicles had accumulated in the presynaptic compartment and, finally, the size of motor endplates was reduced. All together, our findings suggested that the clinical weakness in these mutant mice was likely to be related to postnatal structural abnormalities of the neuromuscular junction maturation.

Expression of Nav1.6 sodium channels by Schwann cells at neuromuscular junctions: role in the motor endplate disease phenotype.

In addition to their role in action potential generation and fast synaptic transmission in neurons, voltage-dependent sodium channels can also be active in glia. Terminal Schwann cells (TSCs) wrap around the nerve terminal arborization at the neuromuscular junction, which they contribute to shape during development and in the postdenervation processes. Using fluorescent in situ hybridization (FISH), immunofluorescence, and confocal microscopy, we detected the neuronal Nav1.6 sodium channel transcripts and proteins in TSCs in normal adult rats and mice. Nav1.6 protein co-localized with the Schwann cell marker S-100 but was not detected in the SV2-positive nerve terminals. The med phenotype in mice is due to a mutation in the SCN8A gene resulting in loss of Nav1.6 expression. It leads to early onset in postnatal life of defects in neuromuscular transmission with minimal alteration of axonal conduction. Strikingly, in mutant mice, the nonmyelinated pre-terminal region of axons showed abundant sprouting at neuromuscular junctions, and most of the alpha-bungarotoxin-labeled endplates were devoid of S-100- or GFAP-positive TSCs. Using specific antibodies against the Nav1.2 and Nav1.6 sodium channels, ankyrin G and Caspr 1, and a pan sodium channel antibody, we found that a similar proportion of ankyrin G-positive nodes of Ranvier express sodium channels in mutant and wild-type animals and that nodal expression of Nav1.2 persists in med mice. Our data supports the hypothesis that the lack of expression of Nav1.6 in Schwann cells at neuromuscular junctions might play a role in the med phenotype.