Phospholipase C activity affinity purifies with the Torpedo nicotinic acetylcholine receptor

Pursuing High-Resolution Structures of Nicotinic Acetylcholine Receptors: Lessons Learned from Five Decades

Since their discovery, nicotinic acetylcholine receptors (nAChRs) have been extensively studied to understand their function, as well as the consequence of alterations leading to disease states. Importantly, these receptors represent pharmacological targets to treat a number of neurological and neurodegenerative disorders. Nevertheless, their therapeutic value has been limited by the absence of high-resolution structures that allow for the design of more specific and effective drugs. This article offers a comprehensive review of five decades of research pursuing high-resolution structures of nAChRs. We provide a historical perspective, from initial structural studies to the most recent X-ray and cryogenic electron microscopy (Cryo-EM) nAChR structures. We also discuss the most relevant structural features that emerged from these studies, as well as perspectives in the field.

Tools for Understanding Nanoscale Lipid Regulation of Ion Channels

Anionic phospholipids are minor but prominent components of the plasma membrane that are necessary for ion channel function. Their persistence in bulk membranes, in particular phosphatidylinositol 4,5-bisphosphate (PIP2), initially suggested they act as channel cofactors. However, recent technologies have established an emerging system of nanoscale signaling to ion channels based on lipid compartmentalization (clustering), direct lipid binding, and local lipid dynamics that allow cells to harness lipid heterogeneity to gate ion channels. The new tools to study lipid binding are set to transform our view of the membrane and answer important questions surrounding ion channel-delimited processes such as mechanosensation.

A Review on the Receptor-ligand Molecular Interactions in the Nicotinic Receptor Signaling Systems

Nicotine is regarded as the main active addictive ingredient in tobacco products driving continued tobacco abuse behavior (smoking) to the addiction behavior, whereas nicotinic acetylcholine receptors (nAChR) is the crucial effective apparatus or molecular effector of nicotine and acetylcholine and other similar ligands. Many nAChR subunits have been revealed to bind to either neurotransmitters or exogenous ligands, such as nicotine and acetylcholine, being involved in the nicotinic receptor signal transduction. Therefore, the nicotinic receptor signalling molecules and the receptor-ligand molecular interactions between nAChRs and their ligands are universally regarded as crucial mediators of cellular functions and drug targets in medical treatment and clinical diagnosis. Given numerous endeavours have been made in defining the roles of nAChRs in response to nicotine and other addictive drugs, this review focuses on studies and reports in recent years on the receptor-ligand interactions between nAChR receptors and ligands, including lipid-nAChR and protein-nAChR molecular interactions, relevant signal transduction pathways and their molecular mechanisms in the nicotinic receptor signalling systems. All the references were carefully retrieved from the PubMed database by searching key words "nicotine", "acetylcholine", "nicotinic acetylcholine receptor(s)", "nAChR*", "protein and nAChR", "lipid and nAChR", "smok*" and "tobacco". All the relevant referred papers and reports retrieved were fully reviewed for manual inspection. This effort intend to get a quick insight and understanding of the nicotinic receptor signalling and their molecular interactions mechanisms. Understanding the cellular receptor-ligand interactions and molecular mechanisms between nAChRs and ligands will lead to a better translational and therapeutic operations and outcomes for the prevention and treatment of nicotine addiction and other chronic drug addictions in the brain's reward circuitry.

A Closely Associated Phospholipase C Regulates Cation Channel Function through Phosphoinositide Hydrolysis

In the hemaphroditic sea snail, Aplysia californica, reproduction is initiated when the bag cell neurons secrete egg-laying hormone during a protracted afterdischarge. A source of depolarization for the afterdischarge is a voltage-gated, nonselective cation channel, similar to transient receptor potential (TRP) channels. Once the afterdischarge is triggered, phospholipase C (PLC) is activated to hydrolyze phosphatidylinositol-4,5-bisphosphate (PIP₂) into diacylglycerol (DAG) and inositol trisphosphate (IP₃). We previously reported that a DAG analog, 1-oleoyl-2-acetyl-sn-glycerol (OAG), activates a prominent, inward whole-cell cationic current that is enhanced by IP₃ To examine the underlying mechanism, we investigated the effect of exogenous OAG and IP₃, as well as PLC activation, on cation channel activity and voltage dependence in excised, inside-out patches from cultured bag cell neurons. OAG transiently elevated channel open probability (P_O) when applied to excised patches; however, coapplication of IP₃ prolonged the OAG-induced response. In patches exposed to OAG and IP₃, channel voltage dependence was left-shifted; this was also observed with OAG, but not to the same extent. Introducing the PLC activator, m-3M3FBS, to patches increased channel P_O, suggesting PLC may be physically linked to the channels. Accordingly, blocking PLC with U-73122 ablated the m-3M3FBS-induced elevation in P_O Treatment with m-3M3FBS left-shifted cation channel voltage dependence to a greater extent than exogenous OAG and IP₃ Finally, OAG and IP₃ potentiated the stimulatory effect of PKC, which is also associated with the channel. Thus, the PLC-PKC signaling system is physically localized such that PIP₂ breakdown products liberated during the afterdischarge modulate the cation channel and temporally influence neuronal activity.SIGNIFICANCE STATEMENT Using excised patches from Aplysia bag cell neurons, we present the first evidence of a nonselective cation channel physically associating with phospholipase C (PLC) at the single-channel level. PLC-mediated breakdown of phospholipids generates diacylglycerol and inositol trisphosphate, which activate the cation channel. This is mimicked by exogenous lipids; furthermore, these second messengers left-shift channel voltage dependence and enhance the response of the channel to protein kinase C. PLC-mediated lipid signaling controls single-channel currents to ensure depolarization is maintained for an extended period of firing, termed the afterdischarge, when the bag cell neurons secrete egg-laying hormone to trigger reproduction.

Nicotinic acetylcholine receptor-lipid interactions: Mechanistic insight and biological function

Membrane lipids are potent modulators of the nicotinic acetylcholine receptor (nAChR) from Torpedo. Lipids influence nAChR function by both conformational selection and kinetic mechanisms, stabilizing varying proportions of activatable versus non-activatable conformations, as well as influencing the transitions between these conformational states. Of note, some membranes stabilize an electrically silent uncoupled conformation that binds agonist but does not undergo agonist-induced conformational transitions. The uncoupled nAChR, however, does transition to activatable conformations in relatively thick lipid bilayers, such as those found in lipid rafts. In this review, we discuss current understanding of lipid-nAChR interactions in the context of increasingly available high resolution structural and functional data. These data highlight different sites of lipid action, including the lipid-exposed M4 transmembrane α-helix. Current evidence suggests that lipids alter nAChR function by modulating interactions between M4 and the adjacent transmembrane α-helices, M1 and M3. These interactions have also been implicated in both the folding and trafficking of nAChRs to the cell surface. We review current mechanistic understanding of lipid-nAChR interactions, and highlight potential biological roles for lipid-nAChR interactions in modulating the synaptic response. This article is part of a Special Issue entitled: Lipid-protein interactions.

The effect of centrally injected CDP-choline on respiratory system; involvement of phospholipase to thromboxane signaling pathway

CDP-choline is an endogenous metabolite in phosphatidylcholine biosynthesis. Exogenous administration of CDP-choline has been shown to affect brain metabolism and to exhibit cardiovascular, neuroendocrine neuroprotective actions. On the other hand, little is known regarding its respiratory actions and/or central mechanism of its respiratory effect. Therefore the current study was designed to investigate the possible effects of centrally injected CDP-choline on respiratory system and the mediation of the central cholinergic receptors and phospholipase to thromboxane signaling pathway on CDP-choline-induced respiratory effects in anaesthetized rats. Intracerebroventricularly (i.c.v.) administration of CDP-choline induced dose- and time-dependent increased respiratory rates, tidal volume and minute ventilation of male anaesthetized Spraque Dawley rats. İ.c.v. pretreatment with atropine failed to alter the hyperventilation responses to CDP-choline whereas mecamylamine, cholinergic nicotinic receptor antagonist, mepacrine, phospholipase A2 inhibitor, and neomycin phospholipase C inhibitor, blocked completely the hyperventilation induced by CDP-choline. In addition, central pretreatment with furegrelate, thromboxane A2 synthesis inhibitor, also partially blocked CDP-choline-evoked hyperventilation effects. These data show that centrally administered CDP-choline induces hyperventilation which is mediated by activation of central nicotinic receptors and phospholipase to thromboxane signaling pathway.

Brain phospholipase C, diacylglycerol lipase and monoacylglycerol lipase are involved in (±)-epibatidine-induced activation of central adrenomedullary outflow in rats

We previously reported that intracerebroventricularly (i.c.v.) administered (±)-epibatidine (a potent agonist of nicotinic acetylcholine receptors) (1, 5 and 10 nmol/animal) dose-dependently elevated plasma levels of noradrenaline and adrenaline and that this response was reduced by i.c.v. administered indomethacin (cyclooxygenase inhibitor) and abolished by bilateral adrenalectomy, indicating the involvement of brain arachidonic acid, as a substrate of cyclooxygenase, in this alkaloid-induced secretion of both catecholamines from the adrenal medulla in rats. Arachidonic acid is mainly released by the action of phospholipase A(2), but is also released by a phospholipase C-, diacylglycerol lipase- and monoacylglycerol lipase-mediated pathway. In the present study, (±)-epibatidine (5 nmol/animal, i.c.v.)-induced elevation of plasma catecholamines was not influenced by pretreatment with mepacrine (phospholipase A(2) inhibitor) (1.1 and 2.2 μmol/animal, i.c.v.), but was effectively reduced by pretreatment with U-73122 (1-[6-[[(17 β)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione) (phospholipase C inhibitor) (10 and 30 nmol/animal, i.c.v.), RHC-80267 [1,6-bis(cyclohexyloximinocarbonylamino)hexane] (diacylglycerol lipase inhibitor) (1.3 and 2.6 μmol/animal, i.c.v.), MAFP (methyl arachidonoyl fluorophosphonate) (monoacylglycerol lipase inhibitor) (0.7 and 1.4 μmol/animal, i.c.v.) or JZL184 [4-nitrophenyl 4-(dibenzo[d][1,3]dioxol-5-yl(hydroxy)methyl)piperidine-1-carboxylate] (selective monoacylglycerol lipase inhibitor) (0.7 and 1.4 μmol/animal, i.c.v.). Immunohistochemical studies demonstrated that (±)-epibatidine (10 nmol/animal, i.c.v.) activates spinally projecting neurons expressing monoacylglycerol lipase in the rat hypothalamic paraventricular nucleus, a control center of central sympatho-adrenomedullary outflow. Taken together, the brain phospholipase C-, diacylglycerol lipase- and monoacylglycerol lipase-mediated pathway seems to be involved in the centrally administered (±)-epibatidine-induced activation of central adrenomedullary outflow in rats.

Molecular mechanisms of acetylcholine receptor-lipid interactions: from model membranes to human biology

Lipids are potent modulators of the Torpedo nicotinic acetylcholine receptor. Lipids influence nicotinic receptor function by allosteric mechanisms, stabilizing varying proportions of pre-existing resting, open, desensitized, and uncoupled conformations. Recent structures reveal that lipids could alter function by modulating transmembrane α-helix/α-helix packing, which in turn could alter the conformation of the allosteric interface that links the agonist-binding and transmembrane pore domains-this interface is essential in the coupling of agonist binding to channel gating. We discuss potential mechanisms by which lipids stabilize different conformational states in the context of the hypothesis that lipid-nicotinic receptor interactions modulate receptor function at biological synapses.

Cations mediate interactions between the nicotinic acetylcholine receptor and anionic lipids

Interactions between the nicotinic acetylcholine receptor (nAChR) and phosphatidic acid (PA) are bidirectional in that membranes containing PA are effective at stabilizing an agonist-responsive nAChR, whereas incorporation of the nAChR into the same membranes leads to a substantial increase in lipid lateral packing density. A previous study suggested that the ability of PA to adopt a dianionic ionization state is key. We monitored the ionization state of PA in both reconstituted and protein-free membranes. In model membranes composed of PA and 3:2 (mol/mol) phosphatidylcholine (PC)/PA, the monoanionic-to-dianionic transition of PA was detected with a pKa of 8.7 and 6.5, respectively. In the reconstituted 3:2 PC/PA membranes, however, PA was stabilized in a monoanionic state at pH values up to 10. Although dianionic PA does not play a role in nAChR function, we found that both the stabilization of monoanionic PA and the concentration of other cations at the bilayer surface can account for changes in bilayer physical properties that are observed upon incorporation of the nAChR into 3:2 PC/PA membranes. A nAChR-induced concentration of cations at the bilayer surface likely mediates interactions between the nAChR and the anionic lipids in its membrane environment.