MUTATION CAUSING AUTOSOMAL-DOMINANT NOCTURNAL FRONTAL-LOBE EPILEPSY ALTERS CA2-4-BETA-2 NICOTINIC ACETYLCHOLINE-RECEPTORS( PERMEABILITY, CONDUCTANCE, AND GATING OF HUMAN ALPHA)

A mutation (S247F) in the channel-lining domain (M2) of the alpha 4 ni cotinic acetylcholine receptor (AChR) subunit has previously been link ed genetically to autosomal dominant nocturnal frontal lobe epilepsy ( ADNFLE). To better understand the functional significance of this muta tion, we characterized the properties of mutant and wild-type human al pha 4 beta 2 AChRs expressed in Xenopus oocytes. Both had similar expr ession levels and EC50 values for ACh and nicotine. Substantial use-de pendent functional upregulation was found for mutant alpha 4 beta 2 AC hRs, but not for wild type. Mutant AChR responses showed faster desens itization, slower recovery from desensitization, less inward rectifica tion, and virtually no Ca2+ permeability as compared with wild-type al pha 4 beta 2 AChRs. Addition of the alpha 5 subunit restored Ca2+ perm eability to the mutant alpha 4 beta 2 alpha 5 AChRs. At -80 mV, wild-t ype alpha 4 beta 2 AChR single channel currents exhibited two conducta nces, each with two mean open times (gamma(1) = 17 pS, tau(1) = 3.7 ms ec, and tau(2) = 23.4 msec; gamma(2) = 28 pS, tau(1) = 1.9 msec, and t au(2) = 8.1 msec), In contrast, mutant AChRs exhibited only one conduc tance of 11 pS, with tau(1) = 1.9 msec and tau(2) = 4.1 msec. The net effect of the mutation is to reduce AChR function. This could result i n the hyperexcitability characteristic of epilepsy if the mutant AChRs were part of an inhibitory circuit, e.g., presynaptically regulating the release of GABA. In the minority of AChRs containing the alpha 5 s ubunit, the overall functionality of these AChRs could be maintained d espite the mutation in the alpha 4 subunit.