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 e{open} subunit, worsening the pathological cascade. To gain further support to the hypothesis that the e{open} subunit plays a crucial role in controlling Ca²⁺ permeability of endplate AChR-channel, in this work we measured the fractional Ca²⁺ current (P_f, 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 e{open} subunits detected in patients (α_N217K, α_S226Y, α_C418W, β_V266A, β_V266M, e{open}_I257F, e{open}_V265A and e{open}_L269F). We confirm that only mutations in the e{open} subunit altered Ca²⁺ permeability of AChR-channels, with e{open}_L269F increasing P_f (10% vs. 7% of wild type AChR) and e{open}_I257F decreasing it (to 4.6%). We also found that, for e{open}_L269F-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.