Muscle autoantibodies in myasthenia gravis: beyond diagnosis?

Myasthenia gravis is an autoimmune disorder of the neuromuscular junction. A number of molecules, including ion channels and other proteins at the neuromuscular junction, may be targeted by autoantibodies leading to abnormal neuromuscular transmission. In approximately 85% of patients, autoantibodies, directed against the postsynaptic nicotinic acetylcholine receptor can be detected in the serum and confirm the diagnosis, but in general, do not precisely predict the degree of weakness or response to therapy. Antibodies to the muscle-specific tyrosine kinase are detected in approximately 50% of generalized myasthenia gravis patients who are seronegative for anti-acetylcholine receptor antibodies, and levels of anti-muscle-specific tyrosine kinase antibodies do appear to correlate with disease severity and treatment response. Antibodies to other muscle antigens may be found in the subsets of myasthenia gravis patients, potentially providing clinically useful diagnostic information, but their utility as relevant biomarkers (measures of disease state or response to treatment) is currently unclear.

Antibodies against Wnt receptor of muscle-specific tyrosine kinase in myasthenia gravis

Muscle-specific tyrosine kinase (MuSK) antibodies are detected in a proportion of myasthenia gravis (MG) patients who are negative for acetylcholine receptor (AChR) antibodies and have prominent bulbar weakness and crises. In the MuSK ectodomains, the immunoglobulin-like 1 and 2 domains (Ig1/2) mediate the agrin-Lrp4-MuSK signaling and the cysteine-rich domain (CRD) mediates the Wnt-MuSK-Dishevelled signaling; both contribute to AChR clustering. Immunoblotting against recombinant proteins showed MuSK Ig1/2 antibodies in 33 anti-AChR-negative MG patients; 10 patients of them (30%) were additionally positive for MuSK CRD antibodies. The result suggests that MuSK antibodies have heterogeneity in their binding to functional domains of MuSK.

Muscle specific kinase autoantibodies cause synaptic failure through progressive wastage of postsynaptic acetylcholine receptors

In myasthenia gravis muscle weakness is caused by autoantibodies against components of the neuromuscular junction. Patient autoantibodies against muscle specific kinase (MuSK) deplete MuSK from the postsynaptic membrane and reproduce signs of myasthenia gravis when injected into mice. Here we have examined the time-course of structural and functional changes that lead up to synaptic failure. C57Bl6J mice received daily injections of anti-MuSK patient IgG for 15 days. Mice began to lose weight from day 12 and demonstrated whole-body weakness by day 14. Electromyography indicated synaptic impairment from day 6 in the gastrocnemius muscle and from day 10 in the diaphragm muscle. Confocal microscopy revealed linear declines in the area and density of postsynaptic acetylcholine receptors (3-5% per day) from day 1 through day 15 of the injection series in all five muscles examined. Intracellular recordings from the diaphragm muscle revealed comparable progressive declines in the amplitude of the endplate potential and miniature endplate potential of 3-4% per day. Neither quantal content nor the postsynaptic action potential threshold changed significantly over the injection series. The inverse relationship between the quantal amplitude of a synapse and its quantal content disappeared only late in the injection series (day 10). Our results suggest that the primary myasthenogenic action of anti-MuSK IgG is to cause wastage of postsynaptic acetylcholine receptor density. Consequent reductions in endplate potential amplitudes culminated in failure of neuromuscular transmission.