Muscle‐specific kinase (MuSK) autoantibodies suppress the MuSK pathway and ACh receptor retention at the mouse neuromuscular junction

Key points Myasthenic anti‐muscle‐specific‐kinase (MuSK) IgG was injected into mice to study its effect upon the MuSK signalling pathway and the homeostasis of postsynaptic acetylcholine receptor packing at the neuromuscular junction. Densities of MuSK, activated Src kinase, phosphorylated ACh receptors and rapsyn were all reduced at motor endplates while β‐dystroglycan was unaffected. Pulse‐labelling showed that the slow decline in junctional ACh receptor density could be explained largely by diminished retention of ACh receptors within the postsynaptic membrane scaffold. The results suggest that anti‐MuSK IgG reduces the density of MuSK, associated tyrosine phosphorylation and retention of junctional ACh receptors within the postsynaptic membrane. Muscle‐specific kinase (MuSK) autoantibodies from myasthenia gravis patients can block the activation of MuSK in vitro and/or reduce the postsynaptic localization of MuSK. Here we use a mouse model to examine the effects of MuSK autoantibodies upon some key components of the postsynaptic MuSK pathway and upon the regulation of junctional ACh receptor (AChR) numbers. Mice became weak after 14 daily injections of anti‐MuSK‐positive patient IgG. The intensity and area of AChR staining at the motor endplate was markedly reduced. Pulse‐labelling of AChRs revealed an accelerated loss of pre‐existing AChRs from postsynaptic AChR clusters without a compensatory increase in incorporation of (newly synthesized) replacement AChRs. Large, postsynaptic AChR clusters were replaced by a constellation of tiny AChR microaggregates. Puncta of AChR staining also appeared in the cytoplasm beneath the endplate. Endplate staining for MuSK, activated Src, rapsyn and AChR were all reduced in intensity. In the tibialis anterior muscle there was also evidence that phosphorylation of the AChR β‐subunit‐Y390 was reduced at endplates. In contrast, endplate staining for β‐dystroglycan (through which rapsyn couples AChR to the synaptic basement membrane) remained intense. The results suggest that anti‐MuSK IgG suppresses the endplate density of MuSK, thereby down‐regulating MuSK signalling activity and the retention of junctional AChRs locally within the postsynaptic membrane scaffold.