Distinctive Effects Of Nicotinic Receptor Intracellular-Loop Mutations Associated With Nocturnal Frontal Lobe Epilepsy
Previously characterized nicotinic acetylcholine receptor (nAChR) autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE)-associated mutations are found in Î±2, Î±4 and Î²2 subunit transmembrane (TM) domains. They predominantly increase ACh potency and, for Î²2-subunit mutants, increase macroscopic currents. Two recently-identified mutations, Î±4(R336H) and Î²2(V337G), located in the intracellular cytoplasmic loop (C2) have been associated with non-familial NFLE. Effects of these mutations on Î±4Î²2-nAChR function and expression were studied for the first time, using two-electrode voltage clamp recordings in Xenopus laevis oocytes. Biased-ratio preparations elucidated the mutations' effects at alternate isoforms: high-sensitivity [HS; (Î±4)2(Î²2)3] or low-sensitivity [LS; (Î±4)3(Î²2)2] via 1:10 or 30:1 [Î±4:Î²2] cRNA injection ratios, respectively. An unbiased (1:1 [Î±4:Î²2] cRNA) injection ratio was also used to study potential shifts in isoform expression. Î±4(R336H)-containing receptors showed significant increases in maximal ACh-induced currents (Imax) in all preparations (140% increase compared to wild type control). Î²2(V337G)-containing receptors significantly increased Imax in the LS-favoring preparation (20% increase compared to control). Expression of either mutation consistently produced enrichment of HS-isoform expression in all preparations. Î±4Î²2-nAChR harboring either NFLE mutant subunit showed unchanged ACh, sazetidine-A, nicotine, cytisine and mecamylamine potency. However, both mutant subunits enhanced partial agonist efficacies in the LS-biased preparation. Using Î²2-subunit-specific [125I]mAb 295 immunolabeling, nAChR cell-surface expression was determined. Antibody binding studies revealed that the Î²2(V337G) mutation tended to reduce cell-surface expression, and function per receptor was significantly increased by either NFLE mutant subunit in HS-favoring preparations. These findings identify both common and differing features between TM- and C2-domain AD/NFLE-associated mutations. As we discuss, the shared features may be particularly salient to AD/NFLE etiology.
Digital Object Identifier (DOI)
Weltzin, Maegan M.; Lindstrom, Jon M.; Lukas, Ronald J.; and Whiteaker, Paul, "Distinctive Effects Of Nicotinic Receptor Intracellular-Loop Mutations Associated With Nocturnal Frontal Lobe Epilepsy" (2016). Neurobiology. 384.