The human adult subtype ACh receptor channel has high Ca2+ permeability and predisposes to endplate Ca2+ overloading

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 Ca2+ overloading. The Ca2+ overloading could be due entirely to the prolonged openings of the AChR channel or could be abetted by enhanced Ca2+ permeability of the mutant channels. We therefore measured the fractional Ca2+ current, defined as the percentage of the total ACh-evoked current carried by Ca2+ ions (Pf), for AChRs harbouring the alphaG153S or the alphaV249F slow-channel mutation, and for wild-type human AChRs in which Pf 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 Pf of the wild-type human endplate AChR was unexpectedly high (Pf approximately 7%), but neither the alphaV249F nor the alphaG153S mutation altered Pf. Fetal human AChRs containing either the wild-type or the mutated alpha subunit had a much lower Pf (2-3%). We conclude that the Ca2+ permeability of human endplate AChRs is higher than that reported for any other human nicotinic AChR, with the exception of alpha7-containing AChRs (Pf > 10%); and that neither the alphaG153S nor the alphaV249F mutations affect the Pf of fetal or adult endplate AChRs. However, the intrinsically high Ca2+ 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.