Molecular mechanisms of allosteric modulation of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChR) are part of the Cys-loop family of ligand-gated ion channels, and are implicated in a wide variety of neurological disorders such as nicotine addiction, schizophrenia, and cognitive dysfunction. Therefore, they represent a critical molecular target for drug development and targeted therapeutic intervention. Positive allosteric modulators (PAMs) of ligand-gated ion channels have a unique therapeutic potential because they enhance synaptic transmission without disrupting the endogenous timing mechanisms. This research focused on the neuronal alpha7 nicotinic receptor because they are located both pre- and postsynaptically and can modulate glutamatergic and dopaminergic release in the brain regions involved in drug-seeking behaviors. Understanding the molecular mechanisms by which allosteric modulators enhance activation of neuronal nicotinic acetylcholine receptors is therefore critically important to the development of new drugs for research and therapeutics.Experiments with the Substituted Cysteine Accessibility Method indicate that two chemically different positive allosteric modulators, PNU-120596 and permeable divalent cations, cause structural transitions (or changes in local electrostatic potential) in the extracellular ligand binding domain of the alpha7 nicotinic receptor that are similar but not identical to those caused by the agonist, acetylcholine. These results suggest that positive allosteric modulators share a conserved mechanism to enhance receptor gating that is unrelated to the chemical structure of the molecule.As an additional approach to study gating of the nicotinic receptors, I developed homology models derived from the structures of bacterial Cys-loop receptors in the closed and open states. A comparison of electrophysiological MTSEA modification data against in silico calculations of solvent accessibility and electrostatic potential showed that electrostatic potential in the extracellular ligand-binding domain of the alpha7 nAChR is a better predictor of receptor gating from the closed to open states.Overall, this body of work has shown that positive allosteric modulators and agonists of the alpha7 nAChR induce similar conformational changes in the extracellular-ligand binding domain of the receptor by reducing the large electronegative potential energy along the ion-permeation pathway. A unifying model of receptor gating (electrostatic compensation) and future experiments designed to test this model are discussed.