Structural and Functional Studies of the Modulator NS9283 Reveal Agonist-like Mechanism of Action at α4β2 Nicotinic Acetylcholine Receptors

Abstract

Background: Cys loop receptors can be modulated by exogenous compounds. Results: By combining x-ray crystallography, homology modeling, quantum mechanical calculations, and functional studies on ␣4␤2 nAChRs, the binding mode and modulatory mechanism of the ␣4␤2 nAChR modulator NS9283 were revealed. Conclusion: Modulatory actions occur by mimicking agonists in the ␣4-␣4 ACh-binding pocket. Significance: Increased understanding of modulator actions open new possibilities for rational drug design.

An Accessory Agonist Binding Site Promotes Activation of α4β2* Nicotinic Acetylcholine Receptors

“…NS9283 does not exceed the maximum activation efficiency of (␣4␤2) 2 ␣4 nAChRs by ACh (18). As expected, NS9283 is without effect on (␣4␤2) 2 ␤2 nAChRs for lack of the ␣4/␣4 binding site (17,19).…”

“…NS9283 cannot activate through its action on the ␣4/␣4 site alone. In combination with agonists at ␣4/␤2 sites, it produces the high probability of channel opening resulting from increased binding site occupancy (12,17). Because NS9283 achieves its effect by occupying a third ACh site, just as any other full agonist would at that site, it is not a modulator.…”

“…However, it was recently established that NS9283 is neither allosteric nor a modulator. It is not allosteric because NS9283 acts as a selective agonist at the ACh binding site formed at the ␣4/␣4 interface (16,17). NS9283 cannot activate through its action on the ␣4/␣4 site alone.…”

Targeting α4β2 Nicotinic Acetylcholine Receptors in Central Nervous System Disorders: Perspectives on Positive Allosteric Modulation as a Therapeutic Approach

“…In general, only few detailed kinetic studies with nAChR PAMs using experimental setups with ultra-fast application systems have been conducted, which leaves ample room for furthering our understanding of the molecular interactions between PAMs and the a4b2 receptor. Additionally, due to the small number of 1 As NS9283 binds in the orthosteric ACh binding site in the a4/a4 interface, a recent study has highlighted the need for revisiting the nomenclature with regard to positive modulators not binding in an allosteric site [21]. This point is acknowledged by the authors of this review; however, for readability in this review, NS9283 will be termed a PAM.…”

“…The lower ACh sensitivity of the LS receptor is believed to arise from a recently discovered low-sensitivity ACh binding site located in the a4/a4 interface only present in this stoichiometry [17,18]. Intriguingly, the high-and low-sensitive ACh binding sites in the LS receptor display differential responses to various nAChR ligands [19][20][21]. Additionally, the HS stoichiometry is more sensitive to nicotine-induced up-regulation and more sensitive to desensitization induced by low concentration of agonists [13,22], whereas the LS stoichiometry has a higher permeability to Ca 2+ [16] and more rapid desensitization kinetics [13] than the HS receptor.…”