An Unusual Pattern of Ligand-Receptor Interactions for theα7 Nicotinic Acetylcholine Receptor, with Implications for the Binding of Varenicline

Abstract

The a7 nicotinic acetylcholine receptor shows broad pharmacology, complicating the development of subtype-specific nicotinic receptor agonists. Here we use unnatural amino acid mutagenesis to characterize binding to a7 by the smoking cessation drug varenicline (Chantix; Pfizer, Groton, CT), an a4b2-targeted agonist that shows full efficacy and modest potency at the a7 receptor. We find that unlike binding to its target receptor, varenicline does not form a cation-p interaction with TrpB, further supporting a unique binding mode for the cationic amine of nicotinic agonists at the a7 receptor. We also evaluate binding to the complementary face of the receptor's binding site by varenicline, the endogenous agonist acetylcholine, and the potent nicotine analog epibatidine. Interestingly, we find no evidence for functionally important interactions involving backbone NH and CO groups thought to bind the canonical agonist hydrogen bond acceptor of the nicotinic pharmacophore, perhaps reflecting a lesser importance of this pharmacophore element for a7 binding. We also show that the Trp55 and Leu119 side chains of the binding site's complementary face are important for the binding of the larger agonists epibatidine and varenicline, but dispensable for binding of the smaller, endogenous agonist acetylcholine.

Therapeutic Potential ofα7 Nicotinic Acetylcholine Receptors

“…Other studies have shown that acetylcholinesterase inhibitors effectively modulate the acute colonic inflammation in colitis possibly by an increase in the cholinergic tone of the colon (Miceli and Jacobson, 2003). Vagal nerve stimulation via ACh also ameliorates disease activity (van der Zanden et al, 2009). In this same study, it has been shown that vagotomized a7 nAChR knockout mice display more severe colitis than wild-type mice.…”

“…Computer modeling further pointed to the role of fractional hydrophobicity/ hydrophilicity of the LBD and the contribution of the cation-p interaction of the ligand's basic nitrogen atom with tryptophan 149 of the LBD (Kombo and Bencherif, 2013). The LBD interaction with a ligand is, however, probably more complex, as revealed by studies conducted using nonnatural amino acids that point to an unusual interaction of varenicline with a7 nAChRs and the absence of a cation-p interaction with a tryptophan residue (Van Arnam et al, 2013). This study further revealed that the leucine at position 119 and tryptophan at position 55 are important for the binding of large molecules such as epibatidine or varenicline but are dispensable for binding of smaller molecules such as ACh.…”

Biomolecular recognition of antagonists by α7 nicotinic acetylcholine receptor: Antagonistic mechanism and structure–activity relationships studies

“…Alzheimer's disease, epilepsy, neurodegeneration and cognitive deficits in Parkinson's disease, and schizophrenia (Changeux and Edelstein, 2005;Zhang et al, 2012;van Arnam and Dougherty, 2014). Normally, it has yielded the viewpoint that neuronal nAChRs exercise a principally regulative effect in the CNS, and this notion has been ratified by several biological proofs (van Arnam et al, 2013;Schaaf, 2014). As consequence, neuronal nAChRs have attracted increasingly attention of multidisciplinary scientists and further become the vital target receptors for the developments of various pharmaceuticals (Fraenkel et al, 1996;Petersen et al, 2015).…”

An Unaltered Orthosteric Site and a Network of Long-Range Allosteric Interactions for PNU-120596 in α7 Nicotinic Acetylcholine Receptors

“…We also applied a previously described strategy for evaluating the two key H-bonding interactions at the agonist binding site (Tavares, et al, 2012; Van Arnam, et al, 2013). Briefly, α-hydroxy analogues of α-amino acids are incorporated in ways that are known to strongly modulate the hydrogen bonding ability of the protein backbone (Figure S1).…”

“…Non-canonical amino acid mutagenesis provides high-resolution data that complement those from X-ray crystallography. The key binding interactions at the agonist binding site of nAChRs – a cation-π interaction and two hydrogen bonds – can be probed in ways that would be sensitive to ligand displacements of <1 Å (Dougherty and Van Arnam, 2014; Tavares, et al, 2012; Van Arnam, et al, 2013). We have therefore applied non-canonical amino acid mutagenesis to ask whether the presence of a PAM in any way modulates these binding interactions at the orthosteric site.…”