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

Abstract

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.

Perisynaptic Schwann Cells at the Neuromuscular Synapse: Adaptable, Multitasking Glial Cells

“…1H). There are other probes that can also label mammalian PSCs, for example, LNX-1 (an E3 ubiquitin ligase) (Young et al 2005), NaV1.6 (Musarella et al 2006), TrkC (Hess et al 2007) in intact muscles, and antibodies to p75 neurotrophin receptor (Hassan et al 1994), GAP-43 (Woolf et al 1992), nestin (Kang et al 2007), or transcription factor zinc-finger proliferation 1 (Ellerton et al 2008) in denervated muscles.…”

Motor endplate disease affects neuromuscular junction maturation

“…, 2006) and, in contrast to neurons (Van Wart & Matthews, 2006; Vega et al. , 2008), no compensatory expression of other sodium channels occurs in these TSCs (Musarella et al. , 2006).…”

“…There is actually excess sprouting of motor nerve terminals (Brown et al. , 1980; Musarella et al. , 2006) similar to that associated with failed neuromuscular transmission due to intracellular injections of botulinum toxin (Duchen & Strich, 1968).…”

“…, 2000; Boiko et al. , 2001) as well as in terminal Schwann cells (TSCs), also known as perisynaptic Schwann cells, at neuromuscular junctions (NMJs; Musarella et al. , 2006).…”

“…, 2010). Strikingly, in terminally ill med mice [postnatal day (P) 19], the number of TSCs per NMJ is dramatically decreased (Musarella et al. , 2006) and, in contrast to neurons (Van Wart & Matthews, 2006; Vega et al.…”