Change of voltage‐gated sodium channel repertoire in skeletal muscle of a MuSK myasthenia gravis mouse model

Muscle‐specific kinase myasthenia gravis (MuSK MG) is caused by autoantibodies against MuSK in the neuromuscular junction (NMJ). MuSK MG patients have fluctuating, fatigable skeletal muscle weakness, in particular of bulbar muscles. Severity differs greatly between patients, in spite of comparable autoantibody levels. One explanation for inter‐patient and inter‐muscle variability in sensitivity might be variations in compensatory muscle responses. Previously, we developed a passive transfer mouse model for MuSK MG. In preliminary ex vivo experiments, we observed that muscle contraction of some mice, in particular those with milder myasthenia, had become partially insensitive to inhibition by μ‐Conotoxin‐GIIIB, a blocker of skeletal muscle NaV1.4 voltage‐gated sodium channels. We hypothesised that changes in NaV channel expression profile, possibly co‐expression of (μ‐Conotoxin‐GIIIB insensitive) NaV1.5 type channels, might lower the muscle fibre's firing threshold and facilitate neuromuscular synaptic transmission. To test this hypothesis, we here performed passive transfer in immuno‐compromised mice, using ‘high’, ‘intermediate’ and ‘low’ dosing regimens of purified MuSK MG patient IgG4. We compared myasthenia levels, μ‐Conotoxin‐GIIIB resistance and muscle fibre action potential characteristics and firing thresholds. High‐ and intermediate‐dosed mice showed clear, progressive myasthenia, not seen in low‐dosed animals. However, diaphragm NMJ electrophysiology demonstrated almost equal myasthenic severities amongst all regimens. Nonetheless, low‐dosed mouse diaphragms showed a much higher degree of μ‐Conotoxin‐GIIIB resistance. This was not explained by upregulation of Scn5a (the NaV1.5 gene), lowered muscle fibre firing thresholds or histologically detectable upregulated NaV1.5 channels. It remains to be established which factors are responsible for the observed μ‐Conotoxin‐GIIIB insensitivity and whether the NaV repertoire change is compensatory beneficial or a bystander effect. Nerve stimulation‐evoked contraction of muscles from mild MuSK myasthenia gravis mice (injected with low doses patient IgG4) appeared partly insensitive to μ‑Conotoxin‐GIIIB. This blocker of NaV1.4 voltage‐gated sodium channels normally completely eliminates muscle contraction. We studied neuromuscular transmission and possible compensatory expression of NaV1.5 (cardiac‐type) channels but showed this was not the case. Which other Na channel types or related factors are involved r