Structural answers and persistent questions about how nicotinic receptors work

Abstract

The electron diffraction structure of nicotinic acetylcholine receptor (nAChR) from Torpedo marmorata and the X-ray crystallographic structure of acetylcholine binding protein (AChBP) are providing new answers to persistent questions about how nAChRs function as biophysical machines and as participants in cellular and systems physiology. New high-resolution information about nAChR structures might come from advances in crystallography and NMR, from extracellular domain nAChRs as high fidelity models, and from prokaryotic nicotinoid proteins. At the level of biophysics, structures of different nAChRs with different pharmacological profiles and kinetics will help describe how agonists and antagonists bind to orthosteric binding sites, how allosteric modulators affect function by binding outside these sites, how nAChRs control ion flow, and how large cytoplasmic domains affect function. At the level of cellular and systems physiology, structures of nAChRs will help characterize interactions with other cellular components, including lipids and trafficking and signaling proteins, and contribute to understanding the roles of nAChRs in addiction, neurodegeneration, and mental illness. Understanding nAChRs at an atomic level will be important for designing interventions for these pathologies. KeywordsAcetylcholine; Addiction; Neurodegeneration; Nicotine; Protein Design; Protein Folding; Protein Structure; Cys-Loop Receptors; Review INTRODUCTIONNicotinic acetylcholine receptors (nAChRs) are integral-membrane, neurotransmitteractivated ion channels in the central and peripheral nervous systems that participate in signal transmission associated with the physiological release of acetylcholine (1). As cationselective ion channels, they depolarize transmembrane voltage when they transition from the closed resting state to the open cation-conducting state by binding agonist. Continued exposure to agonist causes transition from the open state to the closed desensitized state. More than four decades of research have answered many structural questions about nAChRs. Reviews published across these decades reveal the progression of questions and answers (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15). Current understanding of the structural basis of how nAChRs work is relatively advanced compared to other areas of ion channel biophysics (16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27). Despite this level of understanding, however, questions persist, not only about biophysical properties but also about the structural basis of how nAChRs affect cellular and systems physiology (28). NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptBy reviewing research literature published primarily between 2003 and mid-2007, the goals of this review are to describe the current state of understanding of the structures of nAChRs and to describe questions for which atomic-level structural answers still await. WHICH STRUCTURAL FEATURES DEFINE NACHRS AND OTHER CYS-LOOP RECEPTORS?Mammalian genomes contain 16 nAChR subunits ...

Loop 3 of Short Neurotoxin II is an Additional Interaction Site with Membrane-bound Nicotinic Acetylcholine Receptor as Detected by Solid-state NMR Spectroscopy

“…These differences may account for the low affinity of the short forms to neuronal nAChR and AChBP. [1,[3][4][5][6][7][8][9][10][11][12][13] C]glycerol as sole nitrogen and carbon sources, respectively. Gene expression was induced by increasing temperature to 37°C.…”

“…A prominent long-studied representative is the nicotinic acetylcholine receptor (nAChR), which functions as ligand-gated ion channel and is found in postsynaptic membranes of the central nervous system and in the neuromuscular endplate. [1][2][3][4][5] The muscle-type receptor is formed by five subunits with the stoichiometry α 2 βγδ, whereas neuronal types of the nAChR contain only subunits of the α-type or a combination of α-and β-subunits. In many biochemical/ biophysical studies, receptor preparations from the electric organ of the ray torpedo, which resembles the muscle type, were used.…”

[3H]Epibatidine Photolabels Non-equivalent Amino Acids in the Agonist Binding Site of Torpedo and α4β2 Nicotinic Acetylcholine Receptors

“…nAChRs from vertebrate skeletal muscle and the electric organs of Torpedo rays are heteropentamers of homologous subunits with a stoichiometry of 2␣:␤: ␥(⑀):␦ that are arranged pseudosymmetrically around central cation-selective ion channels (1,2). There are 12 mammalian neuronal nAChR subunit genes: nine neuronal ␣ subunits (␣2-␣10) and three neuronal ␤ subunits (␤2-␤4).…”

Molecular actions of smoking cessation drugs at α4β2 nicotinic receptors defined in crystal structures of a homologous binding protein

“…However, significant progress has been made since the discovery of water-soluble pentameric acetylcholine binding proteins (AChBPs) from snails and the subsequent elucidation of their high-resolution ligandbound crystal structures. AChBPs are homologs of the extracellular domain of nAChRs and are the best-studied structural models of ligand recognition by the extracellular domain of nAChRs (17)(18)(19).…”