The neuromuscular junction (NMJ) is the critical synapse for functional muscle contraction. NMJ dysfunction is implicated in multiple neuromuscular diseases (NMDs), including peripheral neuropathies. We investigated multiple mouse models of rare NMDs to assay the NMJ, laying groundwork for future mechanistic and preclinical studies.
We used repetitive nerve stimulation (RNS) to measure electromyography decrement and immunofluorescence to assess neuromuscular junction anatomy in proximal and distal muscles. Seven mouse models were evaluated at pre-, post-, or late-onset disease stages. The following models exhibited RNS decrement or highly unique NMJ morphology: Gars (CMT2D), Nefl (CMT2E), and Nadk2 (rare NMD). Other models did not display substantially affected NMJs.
Gars mice have previously been shown to display RNS decrement and NMJ denervation, as well as reduced motoneuronal mitochondria numbers at the NMJ; these data have been replicated here. RNS data in Nefl mice do not show significant decrement during post- or late-onset – despite this, the motoneuron axon termini show immense swelling with morphologically intact endplate bands. Nadk2 mice display decrement shortly after disease onset (5 weeks), however synaptic dysfunction and NMJ denervation appears to improve 1 month later, indicating an acute phase of denervation. Surprisingly, this pattern only holds in one distal muscle – nearby muscles exhibit severe denervation in late disease onset (11 weeks). The time course and spectrum of these changes is being evaluated. Pre-synaptic release defects at the NMJ were established in Gars mice via two-electrode voltage clamp (TEVC), providing a basis for comparison with the present studies.
Nefl mice will be evaluated in more detail using TEVC at individual neuromuscular junctions and transmission electron microscopy. This data will provide insight into molecular mechanisms, as well as possible targets for therapeutics. Further investigation into the role of mitochondria mediating the phenotypes across all three models is also warranted.