Mapping of a binding site for ATP within the extracellular region of the Torpedo nicotinic acetylcholine receptor beta-subunit

Evolutionary origins of the purinergic signalling system

Purines appear to be the most primitive and widespread chemical messengers in the animal and plant kingdoms. The evidence for purinergic signalling in plants, invertebrates and lower vertebrates is reviewed. Much is based on pharmacological studies, but important recent studies have utilized the techniques of molecular biology and receptors have been cloned and characterized in primitive invertebrates, including the social amoeba Dictyostelium and the platyhelminth Schistosoma, as well as the green algae Ostreococcus, which resemble P2X receptors identified in mammals. This suggests that contrary to earlier speculations, P2X ion channel receptors appeared early in evolution, while G protein-coupled P1 and P2Y receptors were introduced either at the same time or perhaps even later. The absence of gene coding for P2X receptors in some animal groups [e.g. in some insects, roundworms (Caenorhabditis elegans) and the plant Arabidopsis] in contrast to the potent pharmacological actions of nucleotides in the same species, suggests that novel receptors are still to be discovered.

Acetylcholine nicotinic receptors: finding the putative binding site of allosteric modulators using the "blind docking" approach

Allosteric potentiation of acetylcholine nicotinic receptors is considered to be one of the most promising approaches for the treatment of Alzheimer's disease. However, the exact localization of the allosteric binding site and the potentiation mechanism at the molecular level are presently unknown. We have performed the "blind docking" of three known allosteric modulators (galanthamine, codeine and eserine) with the Acetylcholine Binding Protein and models of human alpha7, alpha3beta4 and alpha4beta2 nicotinic receptors, created by homology modeling. Three putative binding sites were identified in the channel pore, each one showing different affinities for the ligands. One of these sites is localized opposite to the agonist binding site and is probably implicated in the potentiation process. On the basis of these results, a possible mechanism for nicotinic acetylcholine receptor (nAChRs) activation is proposed. The present findings may represent an important advance for understanding the allosteric modulation mechanism of nAChRs. [Figure: see text].

Allosterically potentiating ligands of nicotinic receptors as a treatment strategy for Alzheimer's disease

One of the most prominent cholinergic deficit in Alzheimer's disease (AD) is the reduced number of nicotinic acetylcholine receptors (nAChR) in the hippocampus and cortex of AD patients, as compared to age-matched controls. This deficit results in reduced nicotinic cholinergic excitation which may not only impair postsynaptic depolarization but also presynaptic neurotransmitter release and Ca2+-dependent intracellular signaling, including transcriptional activity. Presently, the most common approach to correct the nicotinic cholinergic deficit in AD is the application of cholinesterase inhibitors. Due to the resulting increase in synaptic acetylcholine levels, both in concentration and time, additional nAChR molecules, e.g. those more distant from the ACh release sites, could be activated. As an obvious disadvantage, this approach affects cholinergic neurotransmission as a whole, including muscarinic neurotransmission. As a novel and alternative approach, a treatment strategy which exclusively targets nicotinic receptors is suggested. The strategy is based on a group of modulating ligands of nicotinic receptors, named allosterically potentiating ligands (APL), which increase the probability of channel opening induced by ACh and nicotinic agonists, and in addition decrease receptor desensitization. The action of APL on nicotinic receptors is reminiscent of that of benzodiazepines on GABA(A) receptors and of that of glycine on the NMDA-subtype of glutamate receptor. Representative nicotinic APL are the plant alkaloids physostigmine, galanthamine and codeine, and the neurotransmitter serotonin (5HT). The potentiating effect of APL on nicotinic neurotransmission has been shown by whole-cell patch-clamp studies in natural murine and human neurons, and in murine and human cell lines expressing various subtypes of neuronal nAChR.

Non-additive interaction between nicotinic cholinergic and P2X purine receptors in guinea-pig enteric neurons in culture

1. Acetylcholine (ACh)-activated currents and their interaction with ATP-activated currents were studied in primary cultures of myenteric neurons from guinea-pig small intestine using patch clamp techniques. Peak currents caused by co-application of ACh (1 mM) and ATP (300 microM) were 78 +/- 2 % of the sum of currents activated by each agonist alone (P < 0.05, n = 29). Reversal potentials measured during co-application of ACh and ATP did not differ from those measured during application of ACh or ATP alone. Addition of BAPTA (10 mM) to the pipette solution or replacement of extracellular Ca2+ with Na+ did not prevent occlusion. 2. Responses caused by co-application of 5-HT (300 microM), acting at 5-HT3 receptors, and ACh (3 mM) or ATP (1 mM) were additive (94 +/- 3 or 96 +/- 4 %, respectively, of the sum of currents activated by 5-HT and ACh or ATP alone; P > 0.05). Currents caused by GABA (1 mM), acting at GABAA receptors, and ACh (3 mM) or ATP (1 mM) were also additive (105 +/- 4 or 100 +/- 3 %, respectively, of the sum of currents activated by GABA and ACh or GABA and ATP applied separately; P > 0. 05). 3. Single channel currents caused by ACh and ATP in the same outside-out patches were less than additive (85 +/- 10 % of the predicted sum, P < 0.05). 4. P2X receptors and nicotinic cholinergic receptors (nAChRs) are linked in a mutually inhibitory manner in guinea-pig myenteric neurons. The functional interaction does not involve ligand binding sites, Ca2+-dependent mechanisms, a change in the driving force for Na+ or cytoplasmic signalling mechanisms.

Expression and renaturation of the N-terminal extracellular domain of torpedo nicotinic acetylcholine receptor alpha-subunit

The N-terminal extracellular region (amino acids 1-209) of the alpha-subunit of the nicotinic acetylcholine receptor (nAChR) from Torpedo marmorata electric tissue was expressed as inclusion bodies in Escherichia coli using the pET 3a vector. Employing a novel protocol of unfolding and refolding, in the absence of detergent, a water-soluble globular protein of 25 kDa was obtained displaying approximately 15% alpha-helical and 45% beta-structure. The fragment bound alpha-[3H]bungarotoxin in 1:1 stoichiometry with a KD value of 0.5 nM as determined from kinetic measurements (4 nM from equilibrium binding). The kinetics of association of toxin and fragment were of second order, with a similar rate constant (8.2 x 10(5) M-1 s-1) as observed previously for the membrane-bound heteropentameric nAChR. Binding of small ligands was demonstrated by competition with alpha-[3H]bungarotoxin yielding the following KI values: acetylcholine, 69 microM; nicotine, 0.42 microM; anatoxin-a, 3 miroM; tubocurarine, 400 microM; and methyllycaconitine, 0.12 microM. The results demonstrate that the N-terminal extracellular region of the nAChR alpha-subunit forms a self-assembling domain that functionally expresses major elements of the ligand binding sites of the receptor.

Metabotropic receptors for ATP and UTP: exploring the correspondence between native and recombinant nucleotide receptors

In the past five years, an extended series (P2Y1-n) of metabotropic nucleotide (P2) receptors has been cloned from vertebrate tissues; these receptors are activated by either ATP or UTP, or both nucleotides. While certain cloned P2Y receptors appear to correspond functionally to particular native P2 receptor phenotypes, such pharmacological phenotypes could be explained by either a combination of several members of the P2Y1-n series being coexpressed in the same tissue or the existence of novel, uncloned P2Y subtypes. Here, Brian King, Andrea Townsend-Nicholson and Geoffrey Burnstock review recent findings on the matter of pharmacological relationships between native P2 and cloned P2Y receptors.