The Nicotinic Acetylcholine Receptor and its Lipid Microenvironment

Cholesterol-Dependent Serotonin Insertion Controlled by Gangliosides in Model Lipid Membranes

Serotonin is distinct among synaptic neurotransmitters because it is amphipathic and released from synaptic vesicles at concentrations superior to its water solubility limit (270 mM in synaptic vesicles for a solubility limit of 110 mM). Hence, serotonin is mostly aggregated in the synaptic cleft, due to extensive aromatic stacking. This important characteristic has received scant attention, as most representations of the serotonergic synapse take as warranted that serotonin molecules are present as monomers after synaptic vesicle exocytosis. Using a combination of in silico and physicochemical approaches and a new experimental device mimicking synaptic conditions, we show that serotonin aggregates are efficiently dissolved by gangliosides (especially GM1) present in postsynaptic membranes. This initial interaction, driven by electrostatic forces, attracts serotonin from insoluble aggregates and resolves micelles into monomers. Serotonin also interacts with cholesterol via a set of CH-π and van der Waals interactions. Thus, gangliosides and cholesterol act together as a functional serotonin-collecting funnel on brain cell membranes. Based on this unique mode of interaction with postsynaptic membranes, we propose a new model of serotonergic transmission that takes into account the post-exocytosis solubilizing effect of gangliosides and cholesterol on serotonin aggregates.

Modulation of a rapid neurotransmitter receptor-ion channel by membrane lipids

Membrane lipids modulate the proteins embedded in the bilayer matrix by two non-exclusive mechanisms: direct or indirect. The latter comprise those effects mediated by the physicochemical state of the membrane bilayer, whereas direct modulation entails the more specific regulatory effects transduced via recognition sites on the target membrane protein. The nicotinic acetylcholine receptor (nAChR), the paradigm member of the pentameric ligand-gated ion channel (pLGIC) superfamily of rapid neurotransmitter receptors, is modulated by both mechanisms. Reciprocally, the nAChR protein exerts influence on its surrounding interstitial lipids. Folding, conformational equilibria, ligand binding, ion permeation, topography, and diffusion of the nAChR are modulated by membrane lipids. The knowledge gained from biophysical studies of this prototypic membrane protein can be applied to other neurotransmitter receptors and most other integral membrane proteins.

Fluorescence microscopy imaging of a neurotransmitter receptor and its cell membrane lipid milieu

Hampered by the diffraction phenomenon, as expressed in 1873 by Abbe, applications of optical microscopy to image biological structures were for a long time limited to resolutions above the ∼200 nm barrier and restricted to the observation of stained specimens. The introduction of fluorescence was a game changer, and since its inception it became the gold standard technique in biological microscopy. The plasma membrane is a tenuous envelope of 4 nm-10 nm in thickness surrounding the cell. Because of its highly versatile spectroscopic properties and availability of suitable instrumentation, fluorescence techniques epitomize the current approach to study this delicate structure and its molecular constituents. The wide spectral range covered by fluorescence, intimately linked to the availability of appropriate intrinsic and extrinsic probes, provides the ability to dissect membrane constituents at the molecular scale in the spatial domain. In addition, the time resolution capabilities of fluorescence methods provide complementary high precision for studying the behavior of membrane molecules in the time domain. This review illustrates the value of various fluorescence techniques to extract information on the topography and motion of plasma membrane receptors. To this end I resort to a paradigmatic membrane-bound neurotransmitter receptor, the nicotinic acetylcholine receptor (nAChR). The structural and dynamic picture emerging from studies of this prototypic pentameric ligand-gated ion channel can be extrapolated not only to other members of this superfamily of ion channels but to other membrane-bound proteins. I also briefly discuss the various emerging techniques in the field of biomembrane labeling with new organic chemistry strategies oriented to applications in fluorescence nanoscopy, the form of fluorescence microscopy that is expanding the depth and scope of interrogation of membrane-associated phenomena.

Binding sites for exogenous and endogenous non-competitive inhibitors of the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) is the paradigm of the neurotransmitter-gated ion channel superfamily. The pharmacological behavior of the AChR can be described as three basic processes that progress sequentially. First, the neurotransmitter acetylcholine (ACh) binds the receptor. Next, the intrinsically coupled ion channel opens upon ACh binding with subsequent ion flux activity. Finally, the AChR becomes desensitized, a process where the ion channel becomes closed in the prolonged presence of ACh. The existing equilibrium among these physiologically relevant processes can be perturbed by the pharmacological action of different drugs. In particular, non-competitive inhibitors (NCIs) inhibit the ion flux and enhance the desensitization rate of the AChR. The action of NCIs was studied using several drugs of exogenous origin. These include compounds such as chlorpromazine (CPZ), triphenylmethylphosphonium (TPMP+), the local anesthetics QX-222 and meproadifen, trifluoromethyl-iodophenyldiazirine (TID), phencyclidine (PCP), histrionicotoxin (HTX), quinacrine, and ethidium. In order to understand the mechanism by which NCIs exert their pharmacological properties several laboratories have studied the structural characteristics of their binding sites, including their respective locations on the receptor. One of the main objectives of this review is to discuss all available experimental evidence regarding the specific localization of the binding sites for exogenous NCIs. For example, it is known that the so-called luminal NCIs bind to a series of ring-forming amino acids in the ion channel. Particularly CPZ, TPMP+, QX-222, cembranoids, and PCP bind to the serine, the threonine, and the leucine ring, whereas TID and meproadifen bind to the valine and extracellular rings, respectively. On the other hand, quinacrine and ethidium, termed non-luminal NCIs, bind to sites outside the channel lumen. Specifically, quinacrine binds to a non-annular lipid domain located approximately 7 A from the lipid-water interface and ethidium binds to the vestibule of the AChR in a site located approximately 46 A away from the membrane surface and equidistant from both ACh binding sites. The non-annular lipid domain has been suggested to be located at the intermolecular interfaces of the five AChR subunits and/or at the interstices of the four (M1-M4) transmembrane domains. One of the most important concepts in neurochemistry is that receptor proteins can be modulated by endogenous substances other than their specific agonists. Among membrane-embedded receptors, the AChR is one of the best examples of this behavior. In this regard, the AChR is non-competitively modulated by diverse molecules such as lipids (fatty acids and steroids), the neuropeptide substance P, and the neurotransmitter 5-hydroxytryptamine (5-HT). It is important to take into account that the above mentioned modulation is produced through a direct binding of these endogenous molecules to the AChR. Since this is a physiologically relevant issue, it is useful to elucidate the structural components of the binding site for each endogenous NCI. In this regard, another important aim of this work is to review all available information related to the specific localization of the binding sites for endogenous NCIs. For example, it is known that both neurotransmitters substance P and 5-HT bind to the lumen of the ion channel. Particularly, the locus for substance P is found in the deltaM2 domain, whereas the binding site for 5-HT and related compounds is putatively located on both the serine and the threonine ring. Instead, fatty acid and steroid molecules bind to non-luminal sites. More specifically, fatty acids may bind to the belt surrounding the intramembranous perimeter of the AChR, namely the annular lipid domain, and/or to the high-affinity quinacrine site which is located at a non-annular lipid domain. Additionally, steroids may bind to a site located on the extracellular hydrophi

Where does cholesterol act during activation of the nicotinic acetylcholine receptor?

Why agonist-induced activation of the nicotinic acetylcholine receptor (nAcChoR) fails completely in the absence of cholesterol is unknown. Affinity-purified nAcChoRs from Torpedo reconstituted into 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine/1, 2-dioleoyl-sn-glycero-3-phosphate/steroid bilayers at mole ratios of 58:12:30 were used to distinguish between three regions of the membrane where cholesterol might act: the lipid bilayer, the lipid-protein interface, or sites within the protein itself. In the bilayer, the role of fluidity has been ruled out and certain neutral lipids can substitute for cholesterol [C. Sunshine, M.G. McNamee, Biochim. Biophys. Acta 1191 (1994) 59-64]; therefore, we first tested the hypothesis that flip-flop of cholesterol across the membrane is important; a plausible mechanism might be the relief of mechanical bending strain induced by a conformation change that expands the two leaflets of the bilayer asymmetrically. Cholesterol analogs prevented from flipping by charged groups attached to the 3-position's hydroxyl supported channel opening, contrary to this hypothesis. The second hypothesis is that interstitial cholesterol binding sites exist deep within the nAcChoR that must be occupied for channel opening to occur. When cholesterol hemisuccinate was covalently 'tethered' to the glycerol backbone of phosphatidylcholine, channel opening was still supported. Thus, if there are functionally important cholesterol sites, they must be very close to the lipid-protein interface and might be termed periannular.

The high-affinity quinacrine binding site is located at a non-annular lipid domain of the nicotinic acetylcholine receptor

This work deals with the localization of the high-affinity non-competitive quinacrine binding site on the muscle-type nicotinic acetylcholine receptor (AChR). Specifically, quantitative steady-state fluorescence spectroscopy is used to determine whether quinacrine binds to a site located at either the annular or the non-annular lipid domain. For this purpose, we measure the ability of spin-labelled phosphatidylcholine (SL-PC) to quench AChR-bound quinacrine, AChR-bound ethidium and membrane-partitioned 7-(9-anthroyloxy)stearate (7-AS) fluorescence. Additionally, we compare the accessibility of SL-PC which is considered to bind only to the annular lipid domain of the AChR with the accessibility of two non-annular domain-sensing lipids such as 5-doxylstearate (5-SAL) and spin-labelled androstane (ASL). Initial experiments using 7-AS established the experimental conditions for maximum SL-PC membrane partitioning. The non-specific quenching elicited by increasing turbidity of the sample after addition of SL-PC is corrected by means of parallel experiments with unlabelled egg yolk phosphatidylcholine. After correction, the SL-PC quenching experiments show the following order in quenching efficiency: 7-AS > quinacrine >> ethidium. The relative intrinsic sensitivity of quinacrine to TEMPO paramagnetic quenching in acetonitrile is considered to be approximately two times higher than that for 7-AS. Thus, SL-PC was found to be more accessible (about 5-fold) to the membrane-partitioned 7-AS than to the quinacrine locus. In addition, SL-PC was virtually not accessible to the high-affinity non-luminal binding site for ethidium. The relative capacity of SL-PC, 5-SAL, and ASL to quench AChR-bound quinacrine fluorescence indicated that the spin-labelled lipid accessibility to the quinacrine binding site follows the order: 5-SAL > ASL >> SL-PC. Examination of the effect of high concentrations of 5-SAL, of its unlabelled parent stearate, and of SL-PC on ethidium and quinacrine binding showed that: (a) both fatty acids displace quinacrine, but not ethidium, from its high-affinity binding site, however (b) 5-SAL was found to be more effective than stearate to displace quinacrine from its locus, whereas (c) SL-PC competes neither for the ethidium locus nor for the quinacrine binding site. The results suggest that the high-affinity binding site for quinacrine is located at a non-annular lipid domain of the AChR. This particular area has been considered to be located at the intramolecular interfaces of the five AChR subunits and/or at the interstices of the transmembrane domains.

Luminal and non-luminal non-competitive inhibitor binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of the alpha subunit exist the binding sites for agonists such as the neurotransmitter acetylcholine, which upon binding trigger the channel opening, and for competitive antagonists such as d-tubocurarine, which compete for the former inhibiting its pharmacological action. For non-competitive inhibitors, a population of low-affinity binding sites have been found at the lipid-protein interface of the nicotinic acetylcholine receptor. In addition, at the M2 transmembrane domain, several high-affinity binding sites have been found for non-competitive inhibitors such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222 and the hydrophobic probe trifluoromethyl-iodophenyldiazirine. They are known as luminal binding sites. Although the local anaesthetic meproadifen seems to be located between the hydrophobic domains M2-M3, this locus is considered to form part of the channel mouth, thus this site can also be called a luminal binding site. In contraposition, experimental evidences support the hypothesis of the existence of other high-affinity binding sites for non-competitive inhibitors located not at the channel lumen, but at non-luminal binding domains. Among them, we can quote the binding site for quinacrine, which is located at the lipid-protein interface of the alpha M1 domain, and the binding site for ethidium, which is believed to interact with the wall of the vestibule very far away from both the lumen channel and the lipid membrane surface. The aim of this review is to discuss these recent findings relative to both structurally and functionally relevant aspects of non-competitive inhibitors of the nicotinic acetylcholine receptor. We will put special emphasis on the description of the localization of molecules with non-competitive antagonist properties that bind with high-affinity to luminal and non-luminal domains. The information described herein was principally obtained by means of methods such as photolabelling and site-directed mutagenesis in combination with patch-clamp. Our laboratory has contributed with data obtained by using biophysical approaches such as paramagnetic electron spin resonance and quantitative fluorescence spectroscopy.

Thermal stability of Torpedo californica acetylcholine receptor in a cholesterol lipid environment

Controlled heating of acetylcholine receptor (AChR) vesicles inactivates the alpha-bungarotoxin (alpha-Bgtx) binding sites with a T50 (temperature at which 50% of the initial capacity to bind alpha-Bgtx remains) of 60 +/- 0.2 degrees C. The same value was obtained for receptor reconstituted in lipid vesicles from Torpedo electroplax where the % mol composition of cholesterol to phospholipid was 30. However, when the reconstitution was carried out in dioleoylphosphatidylcholine (DOPC), dioleoylphosphatidic acid (DOPA) vesicles (3:1 molar ratio), T50 of the curves decreased to 56 +/- 0.2 degrees C and no carbamylcholine stimulated 22Na+ flux was detected. Inclusion of cholesterol in the DOPC-DOPA vesicles increased the toxin binding site stability. The maximal T50 of the toxin binding curves was 63 +/- 0.1 degrees C when the % mol cholesterol/mol DOPC:DOPA in the vesicles was 33. Under these conditions AChR was able to translocate ions, a property that was lost upon heating at 46 degrees C. Preincubation of AChR in the presence of d-tubocurarine, tetracaine or procaine did not affect T50 values of toxin binding. However, a slight increment in thermal stability was found when the receptor was preincubated in the presence of carbamylcholine. The results show that cholesterol requirements for protecting against thermal inactivation of toxin binding and ion gating properties are different and the carbamylcholine-bound receptor may have a different conformation.