Modulation of the Ca2+ permeability of human endplate acetylcholine receptor-channel

In slow-channel congenital myasthenic syndrome, point mutations of the endplate acetylcholine receptor (AChR) prolong channel openings, leading to excessive Ca2+ entry with ensuing endplate degeneration and myasthenic symptoms. The Ca2+ permeability of the human endplate AChR-channel is quite high, and is further increased by two slow-channel mutations in its ɛ subunit, worsening the pathological cascade. To gain further support to the hypothesis that the ɛ subunit plays a crucial role in controlling Ca2+ permeability of endplate AChR-channel, in this work we measured the fractional Ca2+ current (Pf, i.e., the percentage of the total current carried by Ca2+ ions) of a panel of AChR carrying slow-channel mutations in the α, β and ɛ subunits detected in patients (αN217K, αS226Y, αC418W, βV266A, βV266M, ɛI257F, ɛV265A and ɛL269F). We confirm that only mutations in the ɛ subunit altered Ca2+ permeability of AChR-channels, with ɛL269F increasing Pf (10% vs. 7% of wild type AChR) and ɛI257F decreasing it (to 4.6%). We also found that, for ɛL269F-AChR, the Ca2+ permeability and ACh-induced cell death can be normalized by clinically relevant concentrations of salbutamol or verapamil, providing the first evidence that the Ca2+ permeability of AChR-channels can be modulated and this treatment may provide protection against excitotoxic insults.