The human adult subtype ACh receptor channel has high Ca~(2+) permeability and predisposes to endplate Ca~(2+) overloading

摘要

Slow-channel congenital myasthenic syndrome, caused by mutations in subunits of the endplate ACh receptor (AChR), results in prolonged synaptic currents and excitotoxic injury of the postsynaptic region by Ca~(2+) overloading. The Ca~(2+) overloading could be due entirely to the prolonged openings of the AChR channel or could be abetted by enhanced Ca~(2+) permeability of the mutant channels. We therefore measured the fractional Ca~(2+) current, defined as the percentage of the total ACh-evoked current carried by Ca~(2+) ions (Pf), for AChRs harbouring the aG153S or the aV249F slow-channel mutation, and for wild-type human AChRs in which P_f has not yet been determined. Experiments were performed in transiently transfected GH4C1 cells and human myotubes with simultaneous recording of ACh-evoked whole-cell currents and fura-2 fluorescence signals. We found that the P_f of the wild-type human endplate AChR was unexpectedly high (P_f ~7%), but neither the alpha V249F nor the aG153S mutation altered P_f. Fetal human AChRs containing either the wild-type or the mutated a subunit had a much lower Pf (2-3%). We conclude that the Ca~(2+) permeability of human endplate AChRs is higher than that reported for any other human nicotinic AChR, with the exception of a7-containing AChRs (P_f > 10%); and that neither the alpha G153S nor the a V249F mutations affect the Pf of fetal or adult endplate AChRs. However, the intrinsically high Ca~(2+) permeability of human AChRs probably predisposes to development of the endplate myopathy when opening events of the AChR channel are prolonged by altered AChR-channel kinetics.