Motor endplate disease affects neuromuscular junction maturation

Abstract

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.

Single amino acid deletion in transmembrane segment D4S6 of sodium channel Scn8a (Nav1.6) in a mouse mutant with a chronic movement disorder

“…It was recently reported that the neuronal and muscular components of the NMJ are defective in Scn8a −/− null mice (Caillol et al, 2012). The null NMJs lack the characteristic “pretzel” shape of the mature neuromuscular junction and the nerve terminals fail to develop complex arborization.…”

Functional decline at the aging neuromuscular junction is associated with altered laminin-α4 expression

“…(Bitplane, South Windsor, CT, USA) was used to analyze Z-stacked images for volume measure-ment of synaptic vesicles and postsynaptic endplate as previously conducted [33, 56]. Synaptic vesicles targeted by mouse anti-SV2 stained with Alexa Fluor 488-conjugated secondary antibody and postsynaptic AChR endplates stained with Alexa Fluor 555 α-BTX, were rendered in 3D based on fluorescent intensity using the surface-rendering module as previously described [33, 56, 57]. The mouse anti-SV2 antibody is highly specific for the binding of synaptic vesicles and it has been widely used for the staining of synaptic vesicles in other studies [33, 52, 56].…”

“…Synaptic vesicles targeted by mouse anti-SV2 stained with Alexa Fluor 488-conjugated secondary antibody and postsynaptic AChR endplates stained with Alexa Fluor 555 α-BTX, were rendered in 3D based on fluorescent intensity using the surface-rendering module as previously described [33, 56, 57]. The mouse anti-SV2 antibody is highly specific for the binding of synaptic vesicles and it has been widely used for the staining of synaptic vesicles in other studies [33, 52, 56]. Interactive histogram based volumetric pixels was used for thresholding of both SV2 and AChR labelling in order to accurately render 3D solid reconstructions without creating artifacts [33, 57].…”

ALS as a distal axonopathy: molecular mechanisms affecting neuromuscular junction stability in the presymptomatic stages of the disease

“…Recent evidence has demonstrated impaired neuromuscular transmission in presymptomatic (4–6 weeks old) ALS mice compared to age-matched controls (Rocha et al, 2013). Impaired synaptic transmission and other electrophysiological changes in electrical excitability or conduction velocity disrupt the necessary interactions between the nerve terminal, muscle fibers and TSCs required for appropriate postnatal formation of the NMJ, which can lead to a delay in maturation of the NMJ in presymptomatic animals (Blijham et al, 2007; Van Zundert et al, 2008; Caillol et al, 2012). During development, appropriate electrical activity is essential to get the correct distribution of the acetylcholine receptors (AChR) on the muscle cell membrane for NMJ maturation at that location (Goldman et al, 1988), thus one could speculate that the abnormal age-appropriate activity seen at the NMJ of presymptomatic ALS mice could alter the subcellular localization of the AChRs and cause a destabilization of the affected NMJ.…”