Direct spectrophotometric detection of cation flux in membrane vesicles: stopped-flow measurements of acetylcholine-receptor-mediated ion flux

Regulation of ion channel function by the host lipid bilayer examined by a stopped-flow spectrofluorometric assay

To examine the function of ligand-gated ion channels in a defined membrane environment, we developed a robust sequential-mixing fluorescence-based stopped-flow assay. Channel activity is determined using a channel-permeable quencher (e.g., thallium, Tl(+)) of a water-soluble fluorophore (8-aminonaphthalene-1,3,6-trisulfonic acid) encapsulated in large unilamellar vesicles in which the channel of interest has been reconstituted, which allows for rapid solution changes. To validate the method, we explored the activation of wild-type KcsA channel, as well as it's noninactivating (E71A) KcsA mutant, by extravesicular protons (H(+)). For both channel types, the day-to-day variability in the reconstitution yield (as judged from the time course of fluorescence quenching) is <10%. The activation curve for E71A KcsA is similar to that obtained previously using single-channel electrophysiology, and the activation curves for wild-type and E71A KcsA are indistinguishable, indicating that channel activation and inactivation are separate processes. We then investigated the regulation of KcsA activation by changes in lipid bilayer composition. Increasing the acyl chain length (from C18:1 to C22:1 in diacylphosphatidylcholine), but not the mole fraction of POPG (>0.25) in the bilayer-forming phospholipid mixture, alters KcsA H(+) gating. The bilayer-thickness-dependent shift in the activation curve is suggestive of a decrease in an apparent H(+) affinity and cooperativity. The control over bilayer environment and time resolution makes this method a powerful assay for exploring ligand activation and inactivation of ion channels, and how channel gating varies with changes in the channels' lipid bilayer environment or other regulatory processes.

Biochemical characterization of a novel channel-activating site on nicotinic acetylcholine receptors

We have studied the interaction of the reversible acetylcholine esterase inhibitor (-)physostigmine and several structurally related compounds with the nicotinic acetylcholine receptor (nAChR) from Torpedo marmorata electric tissue by means of ligand-induced ion flux into nAChR-rich membrane vesicles, direct binding studies and photoaffinity labeling. (-)Physostigmine acts as a channel-activating ligand at low concentrations and as a direct channel blocker at elevated concentrations. Channel activation is not inhibited by desensitizing concentrations of ACh or ACh-competitive ligands (including alpha-bungarotoxin and D-tubocurarine) but is inhibited by antibody FK1 and several other compounds. From photoaffinity labeling using tritiated physostigmine and mapping of the epitope for the Phy-competitive antibody FK1, the binding site for physostigmine is located within the alpha-subunit of the Torpedo nAChR and is distinct from the acetylcholine binding site. Our data suggest a second pathway of nAChR channel activation that may function physiologically as an allosteric control of receptor activity.

A second pathway of activation of the Torpedo acetylcholine receptor channel

We have studied the interaction of the reversible acetylcholine esterase inhibitor (-)physostigmine (D-eserine) with the nicotinic acetylcholine receptor (nAChR) from Torpedo marmorata electric tissue by means of ligand-induced ion flux into nAChR-rich membrane vesicles and of equilibrium binding. We find that (-) physostigmine induces cation flux (and also binds to the receptor) even in the presence of saturating concentrations of antagonists of acetylcholine, such as D-tubocurarine, alpha-bungarotoxin or antibody WF6. The direct action on the acetylcholine receptor is not affected by removal of the methylcarbamate function from the drug and thus is not due to carbamylation of the receptor. Antibodies FK1 and benzoquinonium antagonize channel activation (and binding) of eserine, suggesting that the eserine binding site(s) is separate from, but adjacent to, the acetylcholine binding site at the receptor. In addition to the channel activating site(s) with an affinity of binding in the 50 microM range, there exists a further class of low-affinity (Kd approximately mM) sites from which eserine acts as a direct blocker of the acetylcholine-activated channel. Our results suggest the existence of a second pathway of activation of the nAChR channel.

Imaging acetylcholine-receptor-induced influx of inorganic ions at single-cell resolution with ion microscopy

Ion microscopy was used to image neurotransmitter-induced tracer ion flux at single-cell resolution. A mammalian muscle cell line (BC3H1) expressing the nicotinic acetylcholine receptor was exposed to 2 mM CsCl, with and without the acetylcholine analog carbamylcholine. 133Cs+, 12C+, 40Ca+, 39K+, and 23Na+ secondary ion images revealing intracellular distribution of these elements were recorded with a CAMECA IMS-3f ion microscope from freeze-fractured freeze-dried BC3H1 cells. The ion images were digitized directly from the microchannel plate/fluorescent screen detector assembly of the ion microscope using a charge-coupled device imager. Submillimolar concentrations of cesium were easily imaged. Cesium images were normalized to carbon images for a direct comparison of carbamylcholine-exposed and control cells. Carbamylcholine-exposed cells showed significantly higher cesium influx than controls. Within the carbamylcholine-exposed cells, cell-to-cell heterogeneity for cesium influx was observed. Injured cells were identified by their potassium, sodium, and calcium signals and omitted from the quantitative analysis of the ion image data. This method should be useful for identifying cells from various regions of the nervous system containing receptors that control the translocation of monovalent cations, including Cs+. Among these neuronal receptors in the central nervous system are those activated by acetylcholine, glutamate, aspartate, or N-methyl-D-aspartate.

Desensitization is a property of the cholinergic binding region of the nicotinic acetylcholine receptor, not of the receptor-integral ion channel

The reversible acetylcholine esterase inhibitor (-)-physostigmine (eserine) is the prototype of a new class of nicotinic acetylcholine receptor (nAChR) activating ligands: it induces cation fluxes into nAChR-rich membrane vesicles from Torpedo marmorata electric tissue even under conditions of antagonist blocked acetylcholine binding sites (Okonjo, Kuhlmann, Maelicke, Neuron, in press). This suggests that eserine exerts its channel-activating property via binding sites at the nAChR separate from those of the natural transmitter. We now report that eserine can activate the channel even when the receptor has been preincubated (desensitized) with elevated concentrations of acetylcholine. Thus the conformational state of the receptor corresponding to desensitization is confined to the transmitter binding region, leaving the channel fully activatable-albeit only from other than the transmitter binding site(s).

High acetylcholine concentrations cause rapid inactivation before fast desensitization in nicotinic acetylcholine receptors from Torpedo

By using both a 3 to 4 ms quenched-86Rb+ flux assay and native acetylcholine receptor (AChR) rich electroplaque vesicles on which 50-60% of acetylcholine activation sites were blocked with alpha-BTX, we determined apparent rates of agonist-induced inactivation in AChR from Torpedo under conditions where measured flux response was directly proportional to initial 86Rb+ influx rate. Inactivation kinetics with acetylcholine in both the activating range (10 microM-10 mM) and the self-inhibiting range (15-100 mM) were measured at 4 degrees C. In the presence of 10 microM-1 mM acetylcholine, inactivation is characterized by a single exponential rate constant, kd (fast desensitization). Plots of kd vs. acetylcholine concentration display maximum kds [kd(max)] of 6.6-8.0 s-1, half-maximal kd at 102 +/- 16 microM, and a Hill coefficient of 1.6 +/- 0.3, closely paralleling the initial ion flux response of AChR. Thus, fast desensitization probably occurs from a doubly-liganded preopen state or the open channel state. In the self-inhibiting acetylcholine concentration range, inactivation is biphasic. A "rapid inactivation" phase is complete within 30 ms, followed by fast desensitization at a rate close to kd(max). Both the rate and extent of rapid inactivation increase with acetylcholine concentration, indicating that acetylcholine binds to its self-inhibition site with apparent kon approximately equal to 10(3) M-1s-1 and koff approximately equal to 40 s-1. This slow kon suggests either hindered access to the inhibitory allosteric site or that a fast binding step is followed by a slower conformational change leading to channel inhibition. Overall, our data suggest that acetylcholine binds preferentially to its inhibitory site when the receptor is in the open-channel conformation and that fast desensitization can occur from all multiple-liganded states.

Spectrophotometric detection of monovalent cation flux in cells: fluorescence microscope measurement of acetylcholine receptor-mediated ion flux in PC-12 cells

A new and convenient spectroscopic method for measuring monovalent cation flux in cells is described. The technique is based on fluorescence quenching of an entrapped fluorophore (anthracene-1,5-dicarboxylic acid) by Cs+. A conventional fluorescence microscope can be used to measure the Cs+ flux. The usefulness of the technique is illustrated by measurement of acetylcholine receptor-mediated Cs+ flux in PC-12 cells, a sympathetic neuronal cell line. The results are the same as those obtained when radioactive tracer ions were used. The technique is applicable to any transmembrane process in which Cs+ can substitute for either Na+ or K+. The method has been developed to identify the different neurotransmitter receptors that control the translocation of monovalent cations and to locate the cells in central nervous system cell preparations that contain these receptors. The advantage of an optical method over tracer ion methods for biochemical and pharmacological studies of transmembrane processes in cells is described.

Phosphorylation of the nicotinic acetylcholine receptor regulates its rate of desensitization

Recent studies have provided evidence for a role of protein phosphorylation in the regulation of the function of various potassium and calcium channels (for reviews, see refs 1, 2). As these ion channels have not yet been isolated and characterized, it has not been possible to determine whether phosphorylation of the ion channels themselves alters their properties or whether some indirect mechanism is involved. In contrast, the nicotinic acetylcholine receptor, a neurotransmitter-dependent ion channel, has been extensively characterized biochemically and has been shown to be directly phosphorylated. The phosphorylation of this receptor is catalysed by at least three different protein kinases (cyclic AMP-dependent protein kinase, protein kinase C and a tyrosine-specific protein kinase) on seven different phosphorylation sites. However, the functional significance of phosphorylation of the receptor has been unclear. We have now examined the functional effects of phosphorylation of the nicotinic acetylcholine receptor by cAMP-dependent protein kinase. We investigated the ion transport properties of the purified and reconstituted acetylcholine receptor before and after phosphorylation. We report here that phosphorylation of the nicotinic acetylcholine receptor on the gamma- and delta-subunits by cAMP-dependent protein kinase increases the rate of the rapid desensitization of the receptor, a process by which the receptor is inactivated in the presence of acetylcholine (ACh). These results provide the first direct evidence that phosphorylation of an ion channel protein modulates its function and suggest that phosphorylation of postsynaptic receptors in general may play an important role in synaptic plasticity.

Acetylcholine receptor inhibition by d-tubocurarine involves both a competitive and a noncompetitive binding site as determined by stopped-flow measurements of receptor-controlled ion flux in membrane vesicles

The issue of whether d-tubocurarine, the classical acetylcholine receptor inhibitor, inhibits the receptor by a competitive or noncompetitive mechanism has long been controversial. d-Tubocurarine, in this study, has been found to be both a competitive (KC = 120 nM) and a noncompetitive (KNC = 4 microM) inhibitor of receptor-mediated ion flux at zero transmembrane voltage in membrane vesicles prepared from Electrophorus electricus electroplax. A spectrophotometric stopped-flow method, based on fluorescence quenching of entrapped anthracene-1,5-disulfonic acid by Cs+, was used to measure both the rate coefficient of ion flux prior to receptor inactivation (desensitization) and the rate coefficient of the rapid inactivation process. Inhibition by d-tubocurarine of the initial rate of ion flux decreased with increasing acetylcholine concentration, consistent with competitive inhibition, but the inhibition by micromolar concentrations of d-tubocurarine could not be overcome with saturating concentrations of acetylcholine, consistent with noncompetitive inhibition. A minimum mechanism is proposed in which d-tubocurarine competes for one of the two acetylcholine activating sites and also binds to a noncompetitive site. The present data do not distinguish between one or two competitive sites, although one successfully accounts for all of the data. By variation of the acetylcholine concentration, the two types of sites could be studied in isolation. Binding of d-tubocurarine to the noncompetitive site does not change the rate of rapid receptor inactivation, whereas binding of d-tubocurarine to the competitive site decreases the rate of rapid inactivation by displacing acetylcholine, in agreement with the observation that d-tubocurarine does not inactivate (desensitize) the E. electricus receptor by itself.(ABSTRACT TRUNCATED AT 250 WORDS)

Cocaine, phencyclidine, and procaine inhibition of the acetylcholine receptor: characterization of the binding site by stopped-flow measurements of receptor-controlled ion flux in membrane vesicles

Noncompetitive inhibition of acetylcholine receptor-controlled ion translocation was studied in membrane vesicles prepared from both Torpedo californica and Electrophorus electricus electroplax. Ion flux was measured in the millisecond time region by using a spectrophotometric stopped-flow method, based on fluorescence quenching of entrapped anthracene-1,5-disulfonic acid by Cs+, and a quench-flow technique using 86Rb+. The rate coefficient of ion flux prior to receptor inactivation (desensitization), JA, was measured at different acetylcholine and inhibitor concentrations, in order to assess which active (nondesensitized) receptor forms bind noncompetitive inhibitors. The degree of inhibition of JA by the inhibitors studied (cocaine, procaine, and phencyclidine) was found to be independent of acetylcholine concentration. The results are consistent with a mechanism in which each compound inhibits by binding to a single site that exists with equal affinity on all active receptor forms. Mechanisms in which the inhibitors bind exclusively to the open-channel form of the receptor are excluded by the data. The same conclusions were reached in cocaine experiments at 0-mV and procaine experiments at -25-mV transmembrane voltage in T. californica vesicles. It had been previously shown that phencyclidine, in addition to decreasing JA (by binding to active receptors), also increases the rate of rapid receptor inactivation (desensitization) and changes the equilibrium between active and inactive receptors (by binding better to inactivated receptor than to active receptor in the closed or open conformations). These effects were not observed with cocaine or procaine. Here it is shown that despite these differential effects on inactivation, cocaine and phencyclidine bind to the same inhibitory site on active receptors (in E. electricus vesicles).(ABSTRACT TRUNCATED AT 250 WORDS)

Reconstitution of MIP26 from single human lenses into artificial membranes. I. Differences in pH sensitivity of cataractous vs. normal human lens fiber cell proteins

Reconstitution of the lens fiber cell protein known as MIP26 into liposomes composed of heterologous phospholipids was achieved; this protein renders the liposomes permeable to low molecular weight compounds. MIP26 from either bovine or human lenses was capable of forming channels in artificial membranes. The assay technique was sufficiently sensitive to allow reconstitution of MIP26 from single human lenses, enabling us to examine the function of channels from either cataractous or age-matched normal lenses. Decreases in pH can cause these channels to close, analogous to the hypothesized channel closing in the in vivo situation. The pH optimum of reconstituted channels in liposomes containing MIP26 from bovine lenses or normal human lenses is very sharp; but is substantially broadened if the liposomes contain MIP26 from cataractous human lenses. This latter result suggests a functional alteration in human lens membranes which is correlated with the development of human senile cataract.