The Nicotinic Acetylcholine Receptor and Its Pentameric Homologs: Toward an Allosteric Mechanism of Signal Transduction at the Atomic Level

Cotinine influences the effect of high and low nicotine concentrations on planarian motility differently

Previous work from our laboratory showed that cotinine, a nicotine metabolite, reverses three nicotine-induced behavioral effects in freshwater planarians: motility decrease, seizure-like movements, and withdrawal-like behaviors. The present work explored whether cotinine, a nicotine metabolite, antagonized the nicotine-induced effects on planarian motility in a concentration-dependent manner. We found that nicotine decreased planarian motility at nicotine concentrations above 60 μM but increased planarian velocity at concentrations equal to or below 50 μM, in agreement with previous data. Cotinine did not affect planarian motility at a concentration range between 250 and 2750 μM. Furthermore, we found that cotinine alleviated the 100 μM nicotine-induced motility decrease in a concentration-dependent manner and reversed the low nicotine concentration motility increase, albeit in a concentration-independent manner. The apparent concentration-dependent alleviation of >60 μM nicotine-induced motility decrease by cotinine suggests an orthosteric relationship between nicotine and cotinine. On the other hand, the evident concentration-independent cotinine alleviation of the increase in motility induced by 50 μM nicotine suggests an allosteric relationship. Our data is consistent with the existing literature about the relationship between nicotine and cotinine in various models, reinforcing the case for the usefulness of the planarian model in pharmacological studies.

The effect of unliganded gating on agonist response in nicotinic receptors

Understanding the agonist concentration-response curve (CRC) is the cornerstone in pharmacology. While CRC parameters, agonist potency (EC50) and efficacy (maximum response, Imax) are well-studied, the role of unliganded gating (minimum response, Imin) on CRC is often overlooked. This study explores the effect of unliganded gating on agonist response in muscle-type acetylcholine (ACh) receptors, focusing on the underexplored role of Imin in modulating EC50 and Imax. Three Gain-of-Function (GOF) mutations that increase, and two Loss-of-Function (LOF) mutations that decrease the unliganded gating equilibrium constant (L0) were studied using automated patch-clamp electrophysiology. GOF mutations enhanced agonist potency, whereas LOF mutations reduced it. The calculated CRC aligned well with empirical results, indicating that agonist CRC can be estimated from knowledge of L0. Reduction in agonist efficacy due to LOF mutations was calculated and subsequently validated using single-channel patch-clamp electrophysiology, a factor often obscured in normalized CRC. The study also evaluated the combined impact of mutations (L0) on CRC, confirming the predictive model. Further, no significant energetic coupling between distant residues (>15 Å) was found, indicating that the mutations' effects are localized and do not alter overall agonist affinity. These findings substantiate the role of unliganded gating in modulating agonist responses and establishes a predictive model for estimating CRC parameters from known changes in L0. The study highlights the importance of intrinsic activity in receptor theory.

Impact of a worker bee thoracic ganglion RIC-3 variant on the actions of acetylcholine and neonicotinoids on nicotinic receptors in Apis mellifera

A transmembrane thioredoxin (TMX3) enables the functional expression of insect nicotinic acetylcholine receptors (nAChRs) in Xenopus laevis oocytes, while co-factors RIC-3 and UNC-50 regulate the receptor expression level. RIC-3 (resistant to inhibitors of cholinesterase 3) has been shown to diversify by its differential mRNA splicing patterns. How such diversity influences neonicotinoid sensitivity of nAChRs of beneficial insect species remains poorly understood. We have identified a RIC-3 variant expressed most abundantly in the thoracic ganglia of honeybee (Apis mellifera) workers and investigated its effects on the functional expression and pharmacology of Amα1/Amα8/Amβ1 and Amα1/Amα2/Amα8/Amβ1 nAChRs expressed in X. laevis oocytes. The AmRIC-3 enhanced the response amplitude to the acetylcholine (ACh) of these A. mellifera nAChRs when its cRNA was injected into oocytes at low concentrations but suppressed the ACh response amplitude at high concentrations. Co-expression of the AmRIC-3 had a minimal impact on the affinity of ACh, but changed the efficacy of imidacloprid and clothianidin, suggesting that the presence and the level of RIC-3 expression can affect the nAChR responses to ACh and neonicotinoids, depending on nAChR subunit composition in honeybees. © 2024 Society of Chemical Industry.

Protein Painting Mass Spectrometry in the Discovery of Interaction Sites within the Acetylcholine Binding Protein

Nicotinic acetylcholine receptors (nAChRs) are a family of ligand-gated ion channel receptors that contribute to cognition, memory, and motor control in many organisms. The pharmacological targeting of these receptors, using small molecules or peptides, presents an important strategy for the development of drugs that can treat important human diseases, including neurodegenerative disorders. The Aplysia californica acetylcholine binding protein (Ac-AChBP) is a structural surrogate of the nAChR with high homology to the extracellular ligand binding domain of homopentameric nAChRs. In this study, we optimized protein-painting-based mass spectrometry to identify regions of interaction between the Ac-AChBP and several nAChR ligands. Using molecular dyes that adhere to the surface of a solubilized Ac-AChBP complex, we identified amino acid residues that constitute a contact site within the Ac-AChBP for α-bungarotoxin, choline, nicotine, and amyloid-β 1-42. By integrating innovation in protein painting mass spectrometry with computational structural modeling, we present a new experimental tool for analyzing protein interactions of the nAChR.