LY2087101 and dFBr share transmembrane binding sites in the (α4)3(β2)2 Nicotinic Acetylcholine Receptor

Positive allosteric modulators (PAMs) of nicotinic acetylcholine receptors (nAChRs) have potential therapeutic application in neuropathologies associated with decrease in function or loss of nAChRs. In this study, we characterize the pharmacological interactions of the nAChRs PAM, LY2087101, with the α4β2 nAChR using mutational and computational analyses. LY2087101 potentiated ACh-induced currents of low-sensitivity (α4)3(β2)2 and high-sensitivity (α4)2(β2)3 nAChRs with similar potencies albeit to a different maximum potentiation (potentiation I _max  = ~840 and 450%, respectively). Amino acid substitutions within the α4 subunit transmembrane domain [e.g. α4Leu256 and α4Leu260 within the transmembrane helix 1 (TM1); α4Phe316 within the TM3; and α4Gly613 within TM4] significantly reduced LY2087101 potentiation of (α4)3(β2)2 nAChR. The locations of these amino acid residues and LY2087101 computational docking analyses identify two LY2087101 binding sites: an intrasubunit binding site within the transmembrane helix bundle of α4 subunit at the level of α4Leu260/α4Phe316 and intersubunit binding site at the α4:α4 subunit interface at the level of α4Leu256/α4Ile315 with both sites extending toward the extracellular end of the transmembrane domain. We also show that desformylflustrabromine (dFBr) binds to these two sites identified for LY2087101. These results provide structural information that are pertinent to structure-based design of nAChR allosteric modulators.