Researchers developed a new bioassay to study complement-mediated synaptic damage in neuromuscular junctions (NMJs) of patients with myasthenia gravis (MG).

“The strict spatial restriction of the complement activation at the native NMJ makes our new model highly relevant for MG,” said the authors of the new study published in the Journal of Neuroscience Methods.

They induced severe, complement-mediated damage at myofibres in NMJs by incubating ex vivo mouse muscle/nerve preparations with a monoclonal antibody against the acetylcholine receptor (AChR) and 33% of normal human serum (NHS). Hemidiaphragm contraction experiments revealed a decrease in muscle contraction of >95%.

The authors observed similar complement-mediated and NMJ-restricted damage when using purified immunoglobulin G from a patient with MG. In this case, muscle contraction decreased by nearly 70%. Moreover, they observed myofibre damage in NMJs.

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Methodologically, the use of the AChR monoclonal antibody did not change stimulation-evoked contraction per se, suggesting that its binding to AChR was not able to directly inhibit AChR function in acute settings, so the authors added 33% NHS to induce complement activation by the preformed antigenic complexes.

The effect in neuromuscular transmission was confirmed to be complement-mediated as the authors observed C6 molecule and membrane attack complex (MAC) deposition at the damaged NMJs.

“With this ex vivo MG model, NMJ-restricted antigenic complex-triggered complement activation and damage can be studied without interference of other potential pathological effects of AChR autoantibodies, i.e. functional block and/or cross-linking of AChRs,” the authors said.

MAC is composed of C5b, C6, C7, C8, and multiple C9 molecules that form a membrane pore. It damages the postsynaptic membrane, reducing the local amount of AChR and decreasing endplate potentials. The formation of MAC was shown to be prevented by eculizumab, a C5-targeted antibody that has recently been approved for clinical use in MG.

Reference

Plomp JJ, Huijbers MGM, Verschuuren JJGM, Borodovsky A. A bioassay for neuromuscular junction-restricted complement activation by myasthenia gravis acetylcholine receptor antibodies. J Neurosci Methods. Published online March 2, 2022. doi:10.1016/j.jneumeth.2022.109551