Mutations linked to autosomal dominant nocturnal frontal lobe epilepsy affect allosteric Ca2+ activation of the alpha 4 beta 2 nicotinic acetylcholine receptor.

Extracellular Ca(2+) robustly potentiates the acetylcholine response of alpha4beta2 nicotinic receptors. Rat orthologs of five mutations linked to autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE)-alpha4(S252F), alpha4(S256L), alpha4(+L264), beta2(V262L), and beta2(V262M)-reduced 2 mM Ca(2+) potentiation of the alpha4beta2 1 mM acetylcholine response by 55 to 74%. To determine whether altered allosteric Ca(2+) activation or enhanced Ca(2+) block caused this reduction, we coexpressed the rat ADNFLE mutations with an alpha4 N-terminal mutation, alpha4(E180Q), that abolished alpha4beta2 allosteric Ca(2+) activation. In each case, Ca(2+) inhibition of the double mutants was less than that expected from a Ca(2+) blocking mechanism. In fact, the effects of Ca(2+) on the ADNFLE mutations near the intracellular end of the M2 region-alpha4(S252F) and alpha4(S256L)-were consistent with a straightforward allosteric mechanism. In contrast, the effects of Ca(2+) on the ADNFLE mutations near the extracellular end of the M2 region-alpha4(+L264)beta2, beta2(V262L), and beta2(V262M)-were consistent with a mixed mechanism involving both altered allosteric activation and enhanced block. However, the effects of 2 mM Ca(2+) on the alpha4beta2, alpha4(+L264)beta2, and alpha4beta2(V262L) single-channel conductances, the effects of membrane potential on the beta2(V262L)-mediated reduction in Ca(2+) potentiation, and the effects of eliminating the negative charges in the extracellular ring on this reduction failed to provide any direct evidence of mutant-enhanced Ca(2+) block. Moreover, analyses of the alpha4beta2, alpha4(S256L), and alpha4(+L264) Ca(2+) concentration-potentiation relations suggested that the ADNFLE mutations reduce Ca(2+) potentiation of the alpha4beta2 acetylcholine response by altering allosteric activation rather than by enhancing block.

Five ADNFLE mutations reduce the Ca2+ dependence of the mammalian alpha4beta2 acetylcholine response.

Five nicotinic acetylcholine receptor (nAChR) mutations are currently linked to autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE). The similarity of their clinical symptoms suggests that a common functional anomaly of the mutations underlies ADNFLE seizures. To identify this anomaly, we constructed rat orthologues (S252F, +L264, S256L, V262L, V262M) of the human ADNFLE mutations, expressed them in Xenopus oocytes with the appropriate wild-type (WT) subunit (alpha4 or beta2), and studied the Ca2+ dependence of their ACh responses. All the mutations significantly reduced 2 mM Ca2+-induced increases in the 30 microM ACh response (P < 0.05). Consistent with a dominant mode of inheritance, this reduction persisted in oocytes injected with a 1:1 mixture of mutant and WT cRNA. BAPTA injections showed that the reduction was not due to a decrease in the secondary activation of Ca2+-activated Cl- currents. The S256L mutation also abolished 2 mM Ba2+ potentiation of the ACh response. The S256L, V262L and V262M mutations had complex effects on the ACh concentration-response relationship but all three mutations shifted the concentration-response relationship to the left at [ACh] >= 30 microM. Co-expression of the V262M mutation with a mutation (E180Q) that abolished Ca2+ potentiation resulted in 2 mM Ca2+ block, rather than potentiation, of the 30 microM ACh response, suggesting that the ADNFLE mutations reduce Ca2+ potentiation by enhancing Ca2+ block of the alpha4beta2 nAChR. Ca2+ modulation may prevent presynaptic alpha4beta2 nAChRs from overstimulating glutamate release at central excitatory synapses during bouts of synchronous, repetitive activity. Reducing the Ca2+ dependence of the ACh response could trigger seizures by increasing alpha4beta2-mediated glutamate release during such bouts.