Quinacrine binds to the lipid-protein interface of the Torpedo acetylcholine receptor: a fluorescence study

Multimodal Investigation into the Interaction of Quinacrine with Microcavity-Supported Lipid Bilayers

Quinacrine is a versatile drug that is widely recognized for its antimalarial action through its inhibition of the phospholipase enzyme. It also has antianthelmintic and antiprotozoan activities and is a strong DNA binder that may be used to combat multidrug resistance in cancer. Despite extensive cell-based studies, a detailed understanding of quinacrine's influence on the cell membrane, including permeability, binding, and rearrangement at the molecular level, is lacking. Herein, we apply microcavity-suspended lipid bilayers (MSLBs) as in vitro models of the cell membrane comprising DOPC, DOPC:Chol(3:1), and DOPC:SM:Chol(2:2:1) to investigate the influence of cholesterol and intrinsic phase heterogeneity induced by mixed-lipid composition on the membrane interactions of quinacrine. Using electrochemical impedance spectroscopy (EIS) and surface-enhanced Raman spectroscopy (SERS) as label-free surface-sensitive techniques, we have studied quinacrine interaction and permeability across the different MSLBs. Our EIS data reveal that the drug is permeable through ternary DOPC:SM:Chol and DOPC-only bilayer compositions. In contrast, the binary cholesterol/DOPC membrane arrested permeation, yet the drug binds or intercalates at this membrane as reflected by an increase in membrane impedance. SERS supported the EIS data, which was utilized to gain structural insights into the drug-membrane interaction. Our SERS data also provides a simple but powerful label-free assessment of drug permeation because a significant SERS enhancement of the drug's Raman signature was observed only if the drug accessed the plasmonic interior of the pore cavity passing through the membrane. Fluorescent lifetime correlation spectroscopy (FLCS) provides further biophysical insight, revealing that quinacrine binding increases the lipid diffusivity of DOPC and the ternary membrane while remarkably decreasing the lipid diffusivity of the DOPC:Chol membrane. Overall, because of its adaptability to multimodal approaches, the MSLB platform provides rich and detailed insights into drug-membrane interactions, making it a powerful tool for in vitro drug screening.

Synthetic and natural inhibitors of phospholipases A2: their importance for understanding and treatment of neurological disorders

Phospholipases A2 (PLA2) are a diverse group of enzymes that hydrolyze membrane phospholipids into arachidonic acid and lysophospholipids. Arachidonic acid is metabolized to eicosanoids (prostaglandins, leukotrienes, thromboxanes), and lysophospholipids are converted to platelet-activating factors. These lipid mediators play critical roles in the initiation, maintenance, and modulation of neuroinflammation and oxidative stress. Neurological disorders including excitotoxicity; traumatic nerve and brain injury; cerebral ischemia; Alzheimer's disease; Parkinson's disease; multiple sclerosis; experimental allergic encephalitis; pain; depression; bipolar disorder; schizophrenia; and autism are characterized by oxidative stress, inflammatory reactions, alterations in phospholipid metabolism, accumulation of lipid peroxides, and increased activities of brain phospholipase A2 isoforms. Several old and new synthetic inhibitors of PLA2, including fatty acid trifluoromethyl ketones; methyl arachidonyl fluorophosphonate; bromoenol lactone; indole-based inhibitors; pyrrolidine-based inhibitors; amide inhibitors, 2-oxoamides; 1,3-disubstituted propan-2-ones and polyfluoroalkyl ketones as well as phytochemical based PLA2 inhibitors including curcumin, Ginkgo biloba and Centella asiatica extracts have been discovered and used for the treatment of neurological disorders in cell culture and animal model systems. The purpose of this review is to summarize information on selective and potent synthetic inhibitors of PLA2 as well as several PLA2 inhibitors from plants, for treatment of oxidative stress and neuroinflammation associated with the pathogenesis of neurological disorders.

The purinergic neurotransmitter revisited: a single substance or multiple players?

The past half century has witnessed tremendous advances in our understanding of extracellular purinergic signaling pathways. Purinergic neurotransmission, in particular, has emerged as a key contributor in the efficient control mechanisms in the nervous system. The identity of the purine neurotransmitter, however, remains controversial. Identifying it is difficult because purines are present in all cell types, have a large variety of cell sources, and are released via numerous pathways. Moreover, studies on purinergic neurotransmission have relied heavily on indirect measurements of integrated postjunctional responses that do not provide direct information for neurotransmitter identity. This paper discusses experimental support for adenosine 5'-triphosphate (ATP) as a neurotransmitter and recent evidence for possible contribution of other purines, in addition to or instead of ATP, in chemical neurotransmission in the peripheral, enteric and central nervous systems. Sites of release and action of purines in model systems such as vas deferens, blood vessels, urinary bladder and chromaffin cells are discussed. This is preceded by a brief discussion of studies demonstrating storage of purines in synaptic vesicles. We examine recent evidence for cell type targets (e.g., smooth muscle cells, interstitial cells, neurons and glia) for purine neurotransmitters in different systems. This is followed by brief discussion of mechanisms of terminating the action of purine neurotransmitters, including extracellular nucleotide hydrolysis and possible salvage and reuptake in the cell. The significance of direct neurotransmitter release measurements is highlighted. Possibilities for involvement of multiple purines (e.g., ATP, ADP, NAD(+), ADP-ribose, adenosine, and diadenosine polyphosphates) in neurotransmission are considered throughout.

Evidence for modulatory sites at the lipid-protein interface of the human multidrug transporter P-glycoprotein

The human multidrug transporter P-glycoprotein (Pgp or ABCB1) sets up pharmacological barriers to many clinically important drugs, a therapeutic remedy for which has yet to be formulated. For the rational design of mechanism-based inhibitors (or modulators), it is necessary to map the potential sites for modulator interaction and understand their modes of communication with the other functional domains of Pgp. In this study, combining directed mutagenesis with homology modeling, we provide evidence of two modulator-specific sites at the lipid protein interface of Pgp. Targeting 21 variant positions in the COOH-terminal transmembrane (TM) regions, we find residues M948 (in TM11) and F983, M986, V988, and Q990 (all four in TM12) critically involved in substrate-site modulation by a thioxanthene-based allosteric modulator cis-(Z)-flupentixol. Interestingly, for ATP-site modulation by the same modulator, only two (M948 and Q990) of those four residues appear indispensable, together with two additional residues, T837 and I864 in TM9 and TM10, respectively, suggesting independent modes of communication linking the allosteric site with the substrate binding and ATPase domains. None of the seven residues identified prove to be critical for modulation of the substrate or ATP sites by Pgp modulators that are transported by the pump, such as cyclosporin A or verapamil, indicating their specificity for cis-(Z)-flupentixol. On the other hand, ATP-site modulation by verapamil proves to be highly sensitive to replacement at positions F716 (in TM7) and I765 (in TM8), and to a more moderate extent at I764 and L772 (both in TM8). Homology modeling based on the known crystal structures of the bacterial multidrug transporter SAV1866 and the mouse Pgp homologue maps the identified residues primarily at the lipid-protein interface of Pgp, in two spatially distinct modulator-specific clusters. The two modulatory sites demonstrate negative synergism in influencing ATP hydrolysis, consolidating their spatial distinctness. Because Pgp is known to recruit drug molecules directly from the lipid bilayer, identification of modulatory sites at the lipid-protein interface and at the same time outside the conventional central drug binding cavity is mechanistically revealing.

Dynamic Structural Investigations on the Torpedo Nicotinic Acetylcholine Receptor by Time-Resolved Photoaffinity Labeling

An increasing number of high-resolution structures of membrane-embedded ion channels (or soluble homologues) have emerged during the last couple of years. The most pressing need now is to understand the complex mechanism underlying ion-channel function. Time-resolved photoaffinity labeling is a suitable tool for investigating the molecular function of membrane proteins, especially when high-resolution structures of related proteins are available. However until now this methodology has only been used on the Torpedo nicotinic acetylcholine receptor (nAChR). nAChRs are allosteric cation-selective receptor channels that are activated by the neurotransmitter acetylcholine (ACh) and implicated in numerous physiological and pathological processes. Time-resolved photoaffinity labeling has already enabled local motions of nAChR subdomains (i.e. agonist binding sites, ion channel, subunit interface) to be understood at the molecular level, and has helped to explain how small molecules can exert their physiological effect, an important step toward the development of drug design. Recent analytical and technical improvements should allow the application of this powerful methodology to other membrane proteins in the near future.

C-terminus of a long α-neurotoxin is highly mobile when bound to the nicotinic acetylcholine receptor: A time-resolved fluorescence anisotropy approach

To better understand how alpha-neurotoxins interact with the acetylcholine receptor, four fluorescein isothiocyanate derivatives of the siamemsis alpha-cobratoxin were prepared (conjugated to the epsilon-amino group in Lys(23), Lys(35), Lys(49), or Lys(69)) and the time-resolved fluorescence anisotropy of each conjugate was measured free in solution and bound to the Torpedo acetylcholine receptor. All the conjugated reporter groups displayed a high and comparable level of mobility free in solution. When receptor bound, on the other hand, significant differences in the conformational dynamics of the reporter groups were observed with the C-terminal Lys(69) derivative displaying by far the greatest mobility strongly suggesting that the C-terminal domain of the bound neurotoxin is highly mobile and does not participate in the toxin-nAChR binding surface. Additionally, this study demonstrates the utility of time-resolved fluorescence anisotropy to characterize the interaction of heteroproteins.

Lysophosphatidylcholine Decreases Locomotor Activities and Dopamine Turnover Rate in Rats

Lysophosphatidylcholine (lyso-PTC), a secondary product of S-adenosylmethionine (SAM)-dependent phosphatidylethanolamine (PTE) methylation, is a potent cytotoxin and might be involved in the pathogenesis of Parkinson's disease (PD). Our previous studies showed that the injection of SAM into the brain caused PD-like changes in rodents. Moreover, 1-methyl-4-phenylpyridinium (MPP+), a Parkinsonism-inducing agent, increased lyso-PTC formation via the stimulation of PTE methylation pathway. These results indicate a possible role of lyso-PTC in the PD-like changes seen following the injection of SAM or MPP+. In the present study, lyso-PTC was injected into the lateral ventricle of rats and locomotor activities and the biogenic amine levels were measured to evaluate the effects of lyso-PTC on the dopaminergic system. Quinacrine, a phospholipase A2 (PLA2) inhibitor, was employed to determine its protective effect on SAM-induced PD-like changes by the inhibition of lyso-PTC formation. The results showed that 1 h after the injection, 0.4 and 0.8 micromol of lyso-PTC increased striatal dopamine (DA) by 20 and 24%, decreased 3,4-dihydroxyphenylacetic acid (DOPAC) by 37 and 45% and decreased homovanilic acid (HVA) by 24 and 13%, respectively. Consequently, dopamine turnover rate, (DOPAC + HVA)/DA, was significantly reduced by 44 and 48% in the rat striatum. Meanwhile, the administration of 0.4 or 0.8 micromol of lyso-PTC decreased movement time by 52 and 63%, total distance by 44 and 48% and the number of movements by 43 and 64%, respectively. Quinacrine attenuated SAM-induced hypokinesia without affecting SAM metabolism prior to its action on rat brain. The results obtained indicate that the hypokinesia observed following the administration of lyso-PTC might be related to the decline in DA turnover in the striatum in response to lyso-PTC exposure. The present study suggests that inhibitory effects of lyso-PTC on dopaminergic neurotransmission is one of the contributing factors in SAM and MPP+-induced PD-like changes.

Quinacrine blocks PrP (106-126)-formed channels

We investigated the action of the acridine derivative, quinacrine (QC), which has been shown to act as a noncompetitive channel inhibitor. The main effects of QC are voltage- and concentration-dependent changes in the kinetics of the prion protein fragment (PrP[106-126])-formed cation channels. The current-voltage relationships show that the maximal current (I) was not affected whereas the physiologically important mean current (I') was reduced as a result of changes in channel kinetics. These findings suggest that QC acts on the open state of the channels. The half-inhibitory concentration (IC50) for the dose-dependent effects of [QC]cis on the kinetic parameters of the PrP(106-126)-formed cation channel shows a reduction in the ratios Po(QC)/Po, Fo(QC)/Fo, and To(QC)/To, whereas Tc(QC)/Tc increases. Of these ratios, Po(QC)/Po was more sensitive than the others. The corresponding IC50 for these ratios were 51, 94, 86, and 250 microM QC, respectively. The QC-induced changes in the kinetic parameters were more apparent at positive voltages. IC50 values for Po were 95, 75, and 51 microM at +20, +80, and +140 mV, respectively. The fact that QC induced changes in the kinetics of this channel, although the conductance of the channel remained unchanged, indicates that QC may bind at the mouth of the channel via a mechanism known as fast channel block. The QC-induced changes in the kinetic parameters of this channel suggest that they are pathophysiologically significant because these channels could be the mechanisms by which amyloids induce membrane damage in vivo.

The noncompetitive inhibitor quinacrine modifies the desensitization kinetics of muscle acetylcholine receptors

Quinacrine has been shown to act as a noncompetitive inhibitor of the nicotinic acetylcholine receptor (nAChR). However, its mechanism of action is still a matter of controversy. We analyzed in detail the action of quinacrine at both the single-channel and macroscopic current levels. The main effect of quinacrine is a profound concentration-dependent decrease in both the frequency of opening events and the duration of clusters elicited by high acetylcholine concentrations. Quinacrine also significantly increases (40-fold at 30 microM) the decay rate of macroscopic currents elicited by rapid perfusion of acetylcholine to outside-out patches. This decay is still well-described by a single exponential. Quinacrine has very little effect on the peak amplitude of the response, suggesting that it acts mainly on open channels. The recovery from desensitization after removal of acetylcholine is delayed in the presence of quinacrine. Results from both single-channel and macroscopic current recordings indicate that quinacrine increases the rate of nAChR desensitization and stabilizes the desensitized state. Interestingly, in equilibrium agonist-binding assays, quinacrine does not promote the typical high-affinity desensitized state. Thus, quinacrine seems to induce an intermediate state exhibiting the permeability but not the agonist binding properties of desensitization.

Characterization of the dizocilpine binding site on the nicotinic acetylcholine receptor

Although the dissociative anesthetic dizocilpine [(+)-MK-801] inhibits nicotinic acetylcholine receptor (AChR) function in a noncompetitive manner, the location of the dizocilpine binding site(s) has yet to be clearly established. Thus, to characterize the binding site for dizocilpine on the AChR we examined 1) the dissociation constant (K(d)) and stoichiometry of [(3)H]dizocilpine binding; 2) the displacement of dizocilpine radioligand binding by noncompetitive inhibitors (NCIs) and conversely dizocilpine displacement of fluorescent and radiolabeled NCIs from their respective high-affinity binding sites on the AChR; and 3) photoaffinity labeling of the AChR using (125)I-dizocilpine. The results establish that one high-affinity (K(d) = 4.8 microM) and several (3-6) low-affinity (K(d) = approximately 140 microM) binding sites exist for dizocilpine on the desensitized and resting AChR, respectively. The binding of the fluorescent NCIs ethidium, quinacrine, and crystal violet as well as [(3)H]thienylcyclohexylpiperidine was inhibited by dizocilpine on desensitized AChRs. However, Schild-type analyses indicate that only the inhibition of quinacrine in the desensitized state seems to be mediated by a mutually exclusive action. Photoaffinity labeling of the AChR by (125)I-dizocilpine was primarily restricted to the alpha1 subunit and subsequent mapping revealed that the principal sites of labeling are localized to the M4 (approximately 70%) and M1 (30%) transmembrane domains. Collectively, the data indicate that the high-affinity dizocilpine binding site is not located in the lumen of the ion channel but probably near the quinacrine binding locus at a nonluminal domain in the AChR desensitized state.

Paramagnetic fluorescence quenching in a model membrane: a consideration of lifetime and temperature

To expand our understanding of paramagnetic quenching in membranes, the relationship between fluorophore excited-state lifetime (tau), temperature, and the collisional quenching was studied. Specifically, the ability of tempo to quench the steady-state and time-resolved emission from five lipophilic fluorophores (diphenylhexatriene, perylene, phenanthrene, pyrene, and triphenylene) partitioned into egg phosphatidylcholine (EggPC) liposomes was examined. Also, the temperature dependence of spin-labeled androstane to quench the emission (steady-state and time-resolved) from perylene in EggPC liposomes was determined. Unexpectedly, in EggPC liposomes, the apparent quenching efficiency decreased with increasing tau until the effect leveled off above approximately 20 ns. Moreover, in EggPC liposomes, dynamic quenching decreased with increasing temperature. The results suggest that in membranes, paramagnetic quenching is more complex than generally recognized.

Binding of quinacrine to acidic phospholipids and pancreatic phospholipase A2. Effects on the catalytic activity of the enzyme

Binding of quinacrine to phospholipids and porcine pancreatic phospholipase A2 (PLA2) was investigated using fluorescence resonance energy transfer, Langmuir films, assay for the enzymatic activity, and molecular modeling. No significant binding of this drug to the zwitterionic phosphatidylcholine was observed whereas a high affinity for acidic phospholipids was revealed by quenching of pyrene-labeled phospholipid analogues. Partial reversal of this binding was observed due to the addition of 4 mM CaCl2. Quinacrine efficiently and independently of the lipid surface pressure penetrated into monolayers of phosphatidylglycerol while only a weak penetration into phosphatidylcholine films was evident. Quinacrine also bound to eosin-labeled PLA2, and the addition of 4 mM CaCl2 reversed this interaction almost completely. In the presence of acidic phospholipids both the drug and the enzyme were attached to the lipid surface. Studies on the influence of quinacrine on the activity of PLA2 toward pyrene-labeled phospholipid analogues revealed that the hydrolysis of phosphatidylcholine was progressively reduced as a function of increasing [quinacrine]. At low [CaCl2] and low quinacrine:lipid molar ratios (<1:5) quinacrine enhanced slightly the rate of hydrolysis of acidic phospholipids whereas at higher drug:lipid molar ratios (>1:2) an inhibition was observed. In the presence of 1 mM CaCl2 quinacrine inhibited PLA2-catalyzed hydrolysis of phosphatidylglycerol only when the drug:lipid molar ratio exceeded 1:1. The presence of 4 mM CaCl2 abolished nearly completely the inhibition with all the substrate analogues used. Our data suggest that the inhibition of PLA2 by quinacrine is due to its binding to the enzyme. This is supported also by molecular modeling which suggested a binding site for quinacrine close to the active site and Ca2+ binding site of the enzyme. Importantly, our data indicate that quinacrine binds avidly to acidic phospholipids and their presence may influence the drug-enzyme interaction and the inhibition of the enzyme action. Accordingly, presence of quinacrine may interfere also with other processes that require the presence of acidic lipids and/or Ca2+, such as the function of the nicotinic acetylcholine receptor.

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

Topology of ligand binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of both alpha subunits there exist the binding sites for agonists such as the neurotransmitter acetylcholine (ACh) and for competitive antagonists such as d-tubocurarine. Agonists trigger the channel opening upon binding while competitive antagonists compete for the former ones and inhibit its pharmacological action. Identification of all residues involved in recognition and binding of agonist and competitive antagonists is a primary objective in order to understand which structural components are related to the physiological function of the AChR. The picture for the localisation of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are mainly located on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are sequentially identical, the observed high and low affinity for agonists on the receptor is conditioned by the interaction of the alpha subunit with the delta or the gamma chain, respectively. This relationship is opposite for curare-related drugs. This molecular interaction takes place probably at the interface formed by the different subunits. The principal component for the agonist/competitive antagonist binding sites involves several aromatic residues, in addition to the cysteine pair at 192-193, in three loops-forming binding domains (loops A-C). Other residues such as the negatively changed aspartates and glutamates (loop D), Thr or Tyr (loop E), and Trp (loop F) from non-alpha subunits were also found to form the complementary component of the agonist/competitive antagonist binding sites. Neurotoxins such as alpha-, kappa-bungarotoxin and several alpha-conotoxins seem to partially overlap with the agonist/competitive antagonist binding sites at multiple point of contacts. The alpha subunits also carry the binding site for certain acetylcholinesterase inhibitors such as eserine and for the neurotransmitter 5-hydroxytryptamine which activate the receptor without interacting with the classical agonist binding sites. The link between specific subunits by means of the binding of ACh molecules might play a pivotal role in the relative shift among receptor subunits. This conformational change would allow for the opening of the intrinsic receptor cation channel transducting the external chemical signal elicited by the agonist into membrane depolarisation. The ion flux activity can be inhibited by non-competitive inhibitors (NCIs). For this kind of drugs, a population of low-affinity binding sites has been found at the lipid-protein interface of the AChR. In addition, several high-affinity binding sites have been found to be located at different rings on the M2 transmembrane domain, namely luminal binding sites. In this regard, the serine ring is the locus for exogenous NCIs such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222, phencyclidine, and trifluoromethyliodophenyldiazirine. Trifluoromethyliodophenyldiazirine also binds to the valine ring, which is the postulated site for cembranoids. Additionally, the local anaesthetic meproadifen binding site seems to be located at the outer or extracellular ring. Interestingly, the M2 domain is also the locus for endogenous NCIs such as the neuropeptide substance P and the neurotransmitter 5-hydroxytryptamine. In contrast with this fact, experimental evidence supports the hypothesis for the existence of other NCI high-affinity binding sites located not at the channel lumen but at non-luminal binding domains. (ABSTRACT TRUNCATED)

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.

Quinacrine and ethidium bromide bind the same locus on the nicotinic acetylcholine receptor from Torpedo californica

Quinacrine is a noncompetitive antagonist of the nicotinic acetylcholine receptor (AChR) which displays severalfold fluorescent enhancement upon binding to AChR-rich membranes from Torpedo californica electric organ. It is demonstrated that the fluorescence enhancement comprises two components: specific interaction at a high-affinity binding site on the AChR, and interaction with the lipid bilayer. The interaction with the lipid bilayer can be attenuated by other noncompetitive antagonists, but at concentrations substantially higher than those required for binding to the AChR. It is further shown that quinacrine can inhibit the binding of [3H]phencyclidine and [3H]ethidium in a manner fully consistent with simple competitive inhibition. The data support a model for high-affinity quinacrine binding to the same, single locus of the acetylcholine receptor as phencyclidine and ethidium. This site is likely within the lumen of the ion channel.

Quinacrine noncompetitive inhibitor binding site localized on the Torpedo acetylcholine receptor in the open state

Open-channel blockers of the nicotinic acetylcholine receptor (nAcChR) are widely thought to act sterically by entering and "plugging" the open channel of the nAcChR. However, quinacrine, a fluorescent open-channel blocker, has been recently shown to bind to the nAcChR at a site near the lipid bilayer while the receptor is in a closed, desensitized state, suggesting that at least one open-channel blocker might act allosterically outside the channel [Valenzuela et al. (1992) J. Biol. Chem. 267, 8238]. To determine whether or not quinacrine also binds near the lipid bilayer when the receptor is in an open state, a short-range lipophilic quencher (5-doxylstearate, 5-SA) was used to assess the proximity of the nAcChR-bound quinacrine to the lipid bilayer while the receptor was transiently open by an agonist. Initial experiments using a stopped-flow instrument established the conditions required to monitor a portion of the changes in quinacrine fluorescence associated with its binding to the receptor in the open state. 5-SA (80 microM) reduced the amplitude of the rapid agonist-induced change in quinacrine emission to 44% +/- 12% of the control value, indicating that the quinacrine was binding to a site proximal to the membrane-partitioned 5-SA. Control experiments established that 5-SA had no effect on the ability of the receptor to undergo agonist-induced conformational changes, suggesting that little, if any, 5-SA distributed into the channel lumen and perturbed the functional activity of the receptor. Together, the results indicate that quinacrine binds to a site on the open receptor that is in contact with the lipid bilayer and not in the channel lumen.

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.

Agonist-induced displacement of quinacrine from its binding site on the nicotinic acetylcholine receptor: plausible agonist membrane partitioning mechanism

It was previously demonstrated that high concentrations of cholinergic agonists such as acetylcholine (ACh), carbamylcholine (CCh), suberyldicholine (SubCh) and spin-labelled acetylcholine (SL-ACh) displaced quinacrine from its high-affinity binding site located at the lipid-protein interface of the nicotinic acetylcholine receptor (AChR) (Anas, H. R. and Johnson, D. A. (1995) Biochemistry, 34, 1589-1595). In order to account for the agonist self-inhibitory binding site which overlaps, at least partially, with the quinacrine binding site, we determined the partition coefficient (Kp) of these agonists relative to the local anaesthetic tetracaine in AChR native membranes from Torpedo californica electric organ by examining (1) the ability of tetracaine and SL-ACh to quench membrane-partitioned 1-pyrenedecanoic acid (C10-Py) monomer fluorescence, and (2) the ability of ACh, CCh and SubCh to induce an increase in the excimer/monomer ratio of C10-Py-labelled AChR membrane fluorescence. To further assess the differences in agonist accessibility to the quinacrine binding site, we calculated the agonist concentration in the lipid membrane (CM) at an external agonist concentration high enough to inhibit 50% of quinacrine binding (IC50), which in turn was obtained by agonist back titration of AChR-bound quinacrine. Initial experiments established that high agonist concentrations do not affect either transmembrane proton concentration equilibria (pH) of AChR membrane suspension or AChR-bound quinacrine fluorescence spectra. The agonist membrane partitioning experiments indicated relatively small (< or = 20) Kp values relative to tetracaine. These values follow the order: SL-ACh>SubCh>>CCh-ACh. A direct correlation was observed between Kp and the apparent inhibition constant (Ki) for agonists to displace AChR-bound quinacrine. Particularly, agonist with high KpS such as SL-ACh and SubCh showed low Ki values, and this relationship was opposite for CCh and ACh. The calculated CM values indicated significant (between 7 and 54 mM) agonist accessibility to lipid membrane. By themselves, these results support the conjecture that agonist self-inhibition seems to be mediated by the quinacrine binding site via a membrane approach mechanism. The existence of an agonist self-inhibitory binding site, not located in the channel lumen would indicate an allosteric mechanism of ion channel inhibition; however, we can not discard that the process of agonist self-inhibition can also be mediated by a steric blockage of the ion channel.

Mutations in the M1 region of the nicotinic acetylcholine receptor alter the sensitivity to inhibition by quinacrine

1. Site directed mutagenesis was used to alter the structure of Torpedo californica nicotinic acetylcholine receptor (nAChR) and to identify amino acid residues which contribute to noncompetitive inhibition by quinacrine. Mutant receptors were expressed in Xenopus laevis oocytes injected with in vitro synthesized mRNA and the whole cell currents induced by acetylcholine (ACh) were recorded by two electrode voltage clamp. 2. A series of mutations of a highly conserved Arg at position 209 of the alpha subunit of Torpedo californica nAChR revealed that positively charged amino acids are required for functional receptor expression. Mutation of Arg to Lys (alpha R209K) or His (alpha R209H) at position 209 shifted the EC50 for ACh slightly from 5 microM to 12 microM and increased the normalized maximal channel activity 8.5- and 3.2-fold, respectively. 3. These mutations altered the sensitivity of nAChR to noncompetitive inhibition by quinacrine. The extent of inhibition of ion channel function by quinacrine was decreased as pH increased in both wild type and mutant nAChR suggesting that the doubly charged form of quinacrine was responsible for the inhibition. 4. Further mutations at different positions of the alpha subunit suggest the contribution of Pro and Tyr residues at positions 211 and 213 to quinacrine inhibition whereas mutations alpha I210A and alpha L212A did not have any effects. None of these mutations changed the sensitivity of nAChR to inhibition by a different noncompetitive inhibitor, chlorpromazine. 5. These findings support a hypothesis that the quinacrine binding site is located in the lumen of the ion channel. In addition, the quantitative effect of point mutations at alternate positions on the sensitivity of quinacrine inhibition suggests that the secondary structure at the beginning of M1 region might be beta sheet structure.

The histrionicotoxin-sensitive ethidium binding site is located outside of the transmembrane domain of the nicotinic acetylcholine receptor: a fluorescence study

A novel, relatively photostable, long-wavelength fluorescent membrane probe, N-(Texas Red sulfonyl)-5(and 6)-dodecanoylamine (C12-Texas Red), was synthesized and used as an electronic energy acceptor for Förster fluorescence resonance energy transfer (FRET) between ethidium bound to a histrionicotoxin-sensitive binding site on the Torpedo nicotinic acetylcholine receptor (AChR) and the lipid membrane surface. FRET from membrane-partitioned 5-(N-dodecanoylamino)fluorescein (C12-fluorescein) to the membrane-partitioned C12-Texas Red was also determined with a parallel set of cuvettes to (1) compare FRET results with a donor in a known position in the membrane and (2) assess the surface density of the membrane-partitioned C12-Texas Red. Stern-Volmer analysis of the FRET results showed that C12-Texas Red quenched membrane-partitioned C12-fluorescein fluorescence 2.9 times more effectively than it quenched the receptor-bound ethidium fluorescence even though the Förster critical distances for the two donor-acceptor pairs were very similar (49.9 and 54.3 A, respectively). Analysis of the ethidium to C12-Texas Red FRET as a function of acceptor surface density with the assumptions that the donor is attached along the major axis of symmetry of a cylindrical protein embedded perpendicularly into the membrane (On-Axis FRET model) suggested that the distance of closest approach between the receptor-bound ethidium and the membrane surface was approximately 52 A. Because the minimum distance between the surface of the lipid-membrane domain and the major symmetry axis of the AChR is approximately 28 A, the FRET results strongly suggest that the ethidium binding site is not located near the entrance of the luminal transmembrane domain is generally assumed.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ethanol on structural parameters of rat brain membranes: relationship to genetic differences in ethanol sensitivity

Fluorescent probes located in different membrane regions were used to evaluate effects of ethanol (50 and 100 mM) on structural parameters (protein distribution, fluidity of total and annular lipid, and thickness of the bilayer) of synaptic plasma membranes (SPMs) from brain cortex of High-Alcohol Sensitivity (HAS) and Low-Alcohol Sensitivity (LAS) rats. An experimental procedure based on radiationless energy transfer from tryptophan of membrane proteins to pyrene, 1,3-bis-(1-pyrene)propane(pyr-C3-pyr), or 1,6-diphenyl-1,3,5-hexatriene (DPH), as well as pyr-C3-pyr monomer-eximer formation and DPH polarization, and energy transfer from pyrene monomers to 1-anilinonaphthalene-8-sulfonic acid (ANSA) was utilized. The efficiency of energy transfer from tryptophan to pyrene was sensitive to protein clustering induced in SPMs by concanavalin A. Efficiency of energy transfer from pyrene monomers to ANSA was different for vesicles of dimyristoyl phosphatidyl choline, dipalmitoyl phosphatidyl choline, and distearoyl phosphatidyl choline, consistent with differences in the thickness of these lipid bilayers. Without ethanol, there were no significant differences between the structural parameters of SPMs from HAS and from LAS rats. Addition of ethanol (50 mM) changed protein distribution (increased clustering) only in membranes from HAS rats and had no effect on the structure of membranes from LAS rats. A larger concentration of ethanol (100 mM) changed the fluidity of annular and total lipid in both lines of rats, but changed protein distribution and decreased thickness of the membranes from HAS rats with no effect on these parameters in SPMs from LAS animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Voltage dependencies of the effects of chlorpromazine on the nicotinic receptor channel from mouse muscle cell line So18

The effects of chlorpromazine (CPZ) on nicotinic acetylcholine receptor (nAChR) were re-investigated by patch-clamp recordings on a mouse muscle cell line: (1) CPZ decreased the channel-opening frequency and, thus, acted as a closed-channel blocker. This effect was independent of the membrane potential and was consistent with an enhanced desensitization of the nAChR. (2) In addition, CPZ decreased the mean channel open time of the nAChR in a concentration- and voltage-dependent manner and, thus, behaved as an open-channel blocker. The latter effect supports the notion that CPZ binds to a site within the nAChR ionic channel.

Structure of nicotinic acetylcholine receptors

Nicotinic acetylcholine (ACh) receptors convert the binding of ACh into the opening of a cation-conducting channel. New information about the regions of the receptor most immediately involved in its function, namely the ACh-binding sites, the gate and the channel, has come from two approaches. One is the identification by labelling and by mutagenesis of residues contributing to these regions. Another is the determination of the three-dimensional structure of the receptor by electron microscopy. Although the identification of functionally relevant residues is incomplete and residues cannot yet be resolved in the three-dimensional structure, the two approaches are converging. There is still room in the gap for speculation.