Nicotinic acetylcholine receptors (nAChRs) are important drug discovery targets for development of therapeutics against cognitive disorders and e.g. pain. Through the years, this discovery process has been hampered by poor translation from in-vitro to in-vivo situations and this may be founded in a poor mechanistic understanding of how drugs work at the receptors. In a recent study we have discovered an overlooked orthosteric binding site for acetylcholine (ACh) in the interface between two a4 subunits in the (a4)3(b2)2 receptor (1) which is the most abundant of two existing a4b2 receptors in the brain. The acknowledgement of this additional site is important for understanding of structure activity relationships and it is the key to development of super agonists and modulators. We have used a combination of structural and functional tools to study the molecular interactions of a stoichiometry selective modulator NS9283 which acts selectively at the a4a4 interface of a4b2 nAChRs (2). From an X-ray structure of NS9283 with the acetylcholine binding proteins (AChBP) in combination with mutational data it is evident that the modulator in fact not an allosteric modulator but rather a co-agonist. Further, using engineered AChBPs in which an a4a4-like binding is introduced by mutation, we are able to show significant differences in the binding site that relate to efficacy of agonists. Collectively, the insight and tools presented can form the basis for future a4b2 nAChR drug discovery ventures but it also has general implications for other ligand gated ion channels.
(1) Timmermann DB et al., Br J Pharmacol, 167, 164-82, 20, 2012
(2) Harpsøe K, Ahring PK et al., J Neuroscience, 31, 10759-66, 2011