Analogs of α-conotoxin MII are selective for α6-containing nicotinic acetylcholine receptors

Neuronal nicotinic acetylcholine receptors (nAChRs) both mediate direct cholinergic synaptic transmission and modulate synaptic transmission by other neurotransmitters. Novel ligands are needed as probes to discriminate among structurally related nAChR subtypes. alpha-Conotoxin MII, a selective ligand that discriminates among a variety of nAChR subtypes, fails to discriminate well between some subtypes containing the closely related alpha3 and alpha6 subunits. Structure-function analysis of alpha-conotoxin MII was performed in an attempt to generate analogs with preference for alpha6-containing [alpha6* (asterisks indicate the possible presence of additional subunits)] nAChRs. Alanine substitution resulted in several analogs with decreased activity at alpha3* versus alpha6* nAChRs heterologously expressed in Xenopus laevis oocytes. From the initial analogs, a series of mutations with two alanine substitutions was synthesized. Substitution at His9 and Leu15 (MII[H9A;L15A]) resulted in a 29-fold lower IC50 at alpha6beta4 versus alpha3beta4 nAChRs. The peptide had a 590-fold lower IC50 for alpha6/alpha3beta2 versus alpha3beta2 and a 2020-fold lower IC50 for alpha6/alpha3beta2beta3 versus alpha3beta2 nAChRs. MII[H9A;L15A] had little or no activity at alpha2beta2, alpha2beta4, alpha3beta4, alpha4beta2, alpha4beta4, and alpha7 nAChRs. Functional block by MII[H9A;L15A] of rat alpha6/alpha3beta2beta3 nAChRs (IC50 = 2.4 nM) correlated well with the inhibition constant of MII[H9A;L15A] for [I-125]alpha-conotoxin MII binding to putative alpha6beta2* nAChRs in mouse brain homogenates (K-i = 3.3 nM). Thus, structure- function analysis of alpha-conotoxin MII enabled the creation of novel selective antagonists for discriminating among nAChRs containing alpha3 and alpha6 subunits.