Functional and structural analysis of acetylcholine receptor-rich membranes after negative staining

Allosteric modulation of Torpedo nicotinic acetylcholine receptor ion channel activity by noncompetitive agonists

Similar to other neuroreceptors of the vertebrate central nervous system, the nicotinic acetylcholine receptor (nAChR) is subject to modulatory control by allosterically acting ligands. Of particular interest in this regard are allosteric ligands that enhance the sensitivity of the receptor to its natural agonist acetylcholine (ACh), as such ligands could be useful as drugs in diseases associated with impaired nicotinic neurotransmission. Here we discuss the action of a novel class of nAChR ligands which act as allosterically potentiating ligands (APL) on the nicotinic responses induced by ACh and competitive agonists. In addition, APLs also act as noncompetitive agonists of very low efficacy, and as direct blockers of ACh-activated channels. These actions are observed with nAChRs from brain, muscle and electric tissue, and they depend on the structure of the APL and the concentration range applied. We focus here on Torpedo nAChR because (i) the unusual pharmacology of these ligands was first discovered with this system, and (ii) large quantities of this receptor are readily available for biochemical studies.

Photoaffinity labeling of Torpedo acetylcholine receptor by physostigmine

The plant alkaloid physostigmine, an established anti-cholinesterase agent of the carbamate type, has recently been shown to bind to the nicotinic acetylcholine receptor from Torpedo marmorata electrocytes [Okonjo, K. O., Kuhlmann, J. & Maelicke, A. (1991) Eur. J. Biochem. 200, 671-677]. Pharmacological studies of physostigmine-induced ion flux into nicotinic-acetylcholine-receptor-rich membrane vesicles, indicated distinct binding sites for physostigmine and acetylcholine. As shown in this study by photoaffinity labeling with [phenyl-(n)-3H](-)physostigmine, the physostigmine-binding site is located within the same subunit (alpha polypeptide) of the receptor as the acetylcholine-binding site. Using a variety of proteolytic cleavage conditions for the purified alpha polypeptide, several [3H]physostigmine-labeled peptides were isolated and sequenced. From the radioactivity released in the course of the Edman degradations of the labeled peptides, it was found that the label was associated in all cases with Lys125. These results identify a novel ligand-binding site for the Torpedo nicotinic acetylcholine receptor that is different in location from binding sites identified previously for acetylcholine, its established agonists and antagonists, and direct channel blockers.

Physostigmine and neuromuscular transmission

Single channel studies carried out in cultured rat myoballs and cultured hippocampal neurons, and ion flux studies performed on Torpedo electrocyte membrane vesicles, showed that physostigmine (Phy), a well-established acetylcholinesterase inhibitor, interacts directly with nicotinic acetylcholine receptors (nAChR). Low concentrations (0.1 microM) of Phy activate the receptor integral channel, whereas higher concentrations blocked the channel in its opened state. In contrast to channel activation by acetylcholine (ACh) and classical cholinergic agonists, however, Phy was capable of activating the nAChR channel even when the ACh binding sites were blocked by competitive antagonists, such as alpha-neurotoxins and d-tubocurarine, or when the nAChR was desensitized by preincubation with high concentrations of ACh. The binding site at which Phy binds and activates the nAChR was mapped. It was located within the N-terminal extracellular region of the alpha-polypeptide, in close proximity to the binding site of the natural transmitter. These data identify a novel binding site at nAChRs from many species and tissues that may be involved in receptor 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.

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).

The electron microscopy of the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor is a glycoprotein occurring in the electric tissue of the electric ray Torpedo sp. and the electric eel Electrophorus electricus in postsynaptic membranes in high densities. Since these membranes can be easily prepared they have been, since their discovery, a favourable object for electron microscopists. The receptor protein appears in negatively stained membranes as a ring with a diameter of about 75 A. With improved techniques of preparing membranes which contain the receptor molecules in two-dimensional crystalline arrays and especially with computer-aided image processing, the ring appeared as an arrangement of five maxima (representing probably the five receptor subunits) with a five-fold axis of pseudosymmetry perpendicular to the membrane plane. At present the resolution obtained is better than 20 A, enough to depict the receptor's overall shape and dimensions but not enough to resolve functional moieties, as for example the selectivity filter and the gating device of the ion channel, which is an integral part of the receptor complex. The receptor-rich membranes turned out to be models for developing and comparing image processing methods. In this article some of these methods, especially the Circular Harmonic Averaging method, are critically reviewed.

Comparative electron microscopic studies of single biomolecules negatively stained and freeze-dried metal-shadowed

This paper deals with the development of physical fixation and contrasting procedures for the electron microscopic structure analysis of isolated biomolecules. It has been shown on isolated antibodies (IgG) that these preparation methods give better preservation of biomolecules than the commonly practiced chemical fixation and staining techniques. With regard to morphology and size, the data from the electron microscopic structure analysis of the tested antibodies (IgG) are in good agreement with those from X-ray structure analysis. With isolated ribosomal 40S subunits, the influence of chemical fixation and staining techniques on both the structure preservation and contrast medium distribution has been demonstrated.