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

Abstract

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.

• Authors: Nivalda O Rodrigues-Pinguet, Thierry J Pinguet, Antonio Figl, Henry A Lester, Bruce N Cohen
• Journal: Molecular pharmacology (Mol Pharmacol) Vol. 68 Issue 2 Pg. 487-501 (Aug 2005) ISSN: 0026-895X [Print] United States
• PMID: 15901849 (Publication Type: Comparative Study, Journal Article, Research Support, N.I.H., Extramural, Research Support, U.S. Gov't, P.H.S.)

Chemical References

• Receptors, Nicotinic
• nicotinic receptor alpha4beta2
• Calcium

Topics

• Allosteric Regulation (drug effects, physiology)
• Amino Acid Sequence
• Animals
• Calcium (pharmacology)
• Dose-Response Relationship, Drug
• Epilepsy, Frontal Lobe (genetics, metabolism)
• Female
• Molecular Sequence Data
• Mutation
• Rats
• Receptors, Nicotinic (genetics, metabolism)
• Xenopus laevis