Distinctive effects of nicotinic receptor intracellular-loop mutations associated with nocturnal frontal lobe epilepsy. (March 2016)

• Record Type: Journal Article
• Authors: Weltzin, Maegan M.; Lindstrom, Jon M.; Lukas, Ronald J.; Whiteaker, Paul
• Is Part Of: Neuropharmacology. Volume 102(2016)
• Page Start: 158
• Page End: 173
• Publication Date: 2016-03
• Subjects: Nicotinic acetylcholine receptor -- Nocturnal frontal lobe epilepsy -- α4β2 isoform -- Cytoplasmic loop; nAChR nicotinic acetylcholine receptor -- NFLE nocturnal frontal lobe epilepsy -- ADNFLE autosomal dominant nocturnal frontal lobe epilepsy -- HS high sensitivity -- LS low sensitivity -- TEVC two-electrode voltage clamp -- (ACh) acetylcholine -- TM transmembrane -- C2 intracellular cytoplasmic loop -- DHβE dihydro-β-erythroidine hydrobromide; Acetylcholine chloride (PubChem CID: 6060) -- (−)-nicotine hydrogen tartrate salt (PubChem CID: 89594) -- cytisine (PubChem CID: 10235) -- sazetidine-A (PubChem CID: 11983356) -- Dihydro-β-erythroidine (PubChem CID: 31762) -- mecamylamine hydrochloride (PubChem CID: 13221); Neuropsychopharmacology -- Periodicals; Autonomic Agents -- Periodicals; Neuropsychopharmacologie -- Périodiques; Neuropsychopharmacology; Periodicals; Electronic journals; 615.78
• DOI: 10.1016/j.neuropharm.2015.11.004
• Languages: English
• ISSNs: 0028-3908

Abstract

Abstract: Previously characterized nicotinic acetylcholine receptor (nAChR) autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE)-associated mutations are found in α2, α4 and β2 subunit transmembrane (TM) domains. They predominantly increase ACh potency and, for β2-subunit mutants, increase macroscopic currents. Two recently-identified mutations, α4(R336H) and β2(V337G), located in the intracellular cytoplasmic loop (C2) have been associated with non-familial NFLE. Effects of these mutations on α4β2-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; (α4)2 (β2)3 ] or low-sensitivity [LS; (α4)3 (β2)2 ] via 1:10 or 30:1 [α4:β2] cRNA injection ratios, respectively. An unbiased (1:1 [α4:β2] cRNA) injection ratio was also used to study potential shifts in isoform expression. α4(R336H)-containing receptors showed significant increases in maximal ACh-induced currents (Imax ) in all preparations (140% increase compared to wild type control). β2(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. α4β2-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 β2-subunit-specific [ 125 I]mAb 295 immunolabeling, nAChR cell-surface expression was determined. Antibody binding studies revealed that the β2(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. Graphical abstract: Highlights: Unlike α4β2 nAChR ADNFLE TM-domain mutations, C2 NFLE-associated mutations do not alter ACh potency. C2 NFLE-associated mutations significantly enhance ACh-induced peak currents in both the α4β2 nAChR HS- and LS-isoforms. C2 NFLE-associated mutations favor the expression of the α4β2 HS-isoform. The β2(V337G) C2 NFLE-associated mutation significantly reduced receptor cell surface expression. The C2 mutations significantly enhanced the function per unit of receptor when expressed in the α4β2 nAChR HS-isoform.