Families of ion channels with two hydrophobic segments

Nucleotide receptors in the nervous system. An abundant component using diverse transduction mechanisms

Extracellular nucleotides achieve their role as cell-to-cell communicators by acting at cell surface transmembrane receptors-the P2 receptors. Before molecular cloning led to the isolation of any P2-receptor sequence, a small number of receptor types had been proposed on the basis of pharmacological evidence. The application of molecular biology to this field of receptor research has indicated that a great underestimation of the number of receptor subtypes and of their abundance had occurred. There are now known to be seven characterized P2Y (G protein linked) receptors and the same number again of P2X receptors of the transmitter-gated ion channel type. In this review, we discuss the properties of these cloned receptors, their distribution within the nervous system, and their methods of signal transduction.

Baculovirus expression provides direct evidence for heteromeric assembly of P2X2 and P2X3 receptors

P2X2 and P2X3 are subunits of P2X receptors, cation channels opened by binding extracellular ATP. cDNAs encoding P2X2 and P2X3 receptor subunits, each with one of two C-terminal epitope tags, were cloned into baculovirus. Virally infected insect cells (Spodoptera frugiperda) expressed moderate to high levels of the corresponding proteins, as detected by Western blotting, by the specific binding of [35S]ATP and by whole-cell recordings of membrane current evoked by ATP or alphabetamethylene-ATP. In cells infected at the same time with two viruses encoding P2X2 and P2X3 receptors, the two proteins could be cross-immunoprecipitated with antibodies specific for either of the epitope tags. Whole-cell recordings from these cells showed that ATP and alphabetamethylene-ATP evoked currents with agonist sensitivity and desensitization quite distinct from those observed when P2X2 or P2X3 receptors were expressed alone. The results offer a method to express large amounts of P2X receptor protein, and they provide direct evidence that P2X2 and P2X3 subunits assemble to form heteromeric channels having distinct properties from those formed as homomers.

Effects of divalent cations, protons and calmidazolium at the rat P2X7 receptor

The P2X7 receptor is a uniquely bifunctional molecule through which ATP can open a small cationic channel typical of ionotropic receptors and also induce a large pore permeable to high molecular weight molecules (> 600 Da). Activation of this large pore can lead to cell lysis within 1-2 min. We asked whether pharmacological differences existed between the cationic channel and the cell permeabilizing pore by measuring whole-cell currents and uptake of a propidium dye (YO-PRO; Mw 629) in HEK293 cells stably expressing the rat P2X7 receptor, and comparing the actions of divalent cations and protons in these two assays. Currents in response to 2'-3'-(O)-(4-benzoyl benzoyl) ATP (BzATP, 30 microM) were inhibited by extracellular calcium, magnesium, zinc, copper and protons with half-maximal inhibitory concentrations (IC50) of 2.9 mM, 0.5 mM, 11 microM, 0.5 microM and 0.4 microM, respectively. The inhibition was voltage independent in each case. YO-PRO uptake induced by BzATP was also inhibited with similar IC50 values. The rank order of potency of a range of divalents was Cu2+ > Cd2+ = Zn2+ > Ni2+ >> Mg2+ = Co2+ > Mn2+ > Ca2+ = Ba2+ >> Sr2+. These results suggest that these divalent cations and protons all act primarily as allosteric modulators to alter the affinity of ATP binding to the P2X7 receptor. In contrast, extracellular (but not intracellular) calmidazolium inhibited the BzATP-evoked current by up to 90% (IC50 = 15 nM) but had no effect on YO-PRO uptake. Thus, calmidazolium can block activation of the ionic channel but this does not prevent the formation of the large permeabilizing pore.

ATP-mediated cytotoxicity in microglial cells

Microglial cells are known to express purinergic receptors for extracellular ATP of both the P2Y and P2X subtypes. Functional studies have shown that both primary mouse microglial cells and the N9 and N13 microglial cell lines express the pore-forming P2Z/P2X7 receptor. Here we identify the presence of this receptor in N9 and N13 cells with a specific polyclonal Ab and show that microglial cells expressing the P2Z/P2X7 receptor are exquisitively sensitive to ATP-mediated cytotoxicity while clones selected for the lack of this receptor are resistant. Transfection of HEK293 cells with P2X7 (but not P2X2) receptor cDNA confers susceptibility to ATP-mediated cytotoxicity. Morphological and biochemical analysis suggests that ATP-dependent cell death in microglial cells occurs by apoptosis. Finally, microglial cells release ATP via a non-lytic mechanism when activated by bacterial endotoxin, thus suggesting the operation of a purinergic autocrine/paracrine loop.

Immunohistochemical study of the P2X2 and P2X3 receptor subunits in rat and monkey sensory neurons and their central terminals

Of the cloned P2X receptor subunits, six are expressed in sensory neurons, suggesting that the native channels may be heteromultimers with diverse composition. It has been proposed that P2X2 and P2X3 form heteromultimers in sensory neurons. We further tested this hypothesis by examining the relationship of P2X2 and P2X3 immunocytochemically. In rat dorsal root and nodose ganglia, P2X2- and P2X3-immunoreactivity (-ir) were highly colocalized, although single-labeled cells were also present. In dorsal root ganglia (DRG), in some cases P2X2-ir appeared to be present in satellite cells. In dorsal horn of spinal cord, at low magnification the laminar localization of P2X2- and P2X3-ir overlapped, but at high magnification colocalization was rarely observed. In contrast, in the solitary tract and its nucleus (NTS), colocalization of P2X2- and P2X3-ir was seen at low and high magnification. These results suggest that the relationship of P2X2- and P2X3-ir is different in nodose and dorsal root ganglia and might reflect differences in the targeting of P2X receptors in different sensory neurons. In monkey, P2X2-ir was observed in DRG neurons and satellite cells and in dorsal horn of spinal cord. P2X3-ir was also seen in DRG neurons. However, the presence of P2X2-ir in NTS as well as the presence of P2X3-ir in spinal cord and NTS could not be established definitively. These results suggest species differences, although a more extensive study of primate sensory systems is necessary.

Structure and chromosomal mapping of the mouse P2X3 gene

P2X3 is one of seven cloned ATP-gated non-selective cation channels. We have isolated a full-length mouse P2X3 gene from a phage lambda-129/Sv genomic library. The gene consists of 12 exons spanning a locus of approximately 40 kb. No significant similarities have been found between the genomic organisation of the mouse P2X3 gene and genes encoding other ion channels. The encoded mouse P2X3 protein consists of 397 amino acids and shows 99% identity with rat P2X3. Using RNase protection and primer extension assays, multiple transcription initiation sites have been mapped in the mouse P2X3 promoter to a region 162-168 bp upstream of the translation initiation codon. The P2X3 gene has been mapped to mouse chromosome 2p by fluorescence in situ hybridisation. The RAG locus-associated gene T160 is located 1.8 kb upstream of the transcription start site of mouse P2X3 gene. The promoter region of the mouse P2X3 gene lacks a conventional TATA and CCAAT consensus sites, and initiator elements. P2X3 is the first member of the P2X gene family to be completely characterised.

Spontaneous cell fusion in macrophage cultures expressing high levels of the P2Z/P2X7 receptor

Mouse and human macrophages express a plasma membrane receptor for extracellular ATP named P2Z/P2X7. This molecule, recently cloned, is endowed with the intriguing property of forming an aqueous pore that allows transmembrane fluxes of hydrophylic molecules of molecular weight below 900. The physiological function of this receptor is unknown. In a previous study we reported experiments suggesting that the P2Z/P2X7 receptor is involved in the formation of macrophage-derived multinucleated giant cells (MGCs; Falzoni, S., M. Munerati, D. Ferrari, S. Spisani, S. Moretti, and F. Di Virgilio. 1995. J. Clin. Invest. 95:1207- 1216). We have selected several clones of mouse J774 macrophages that are characterized by either high or low expression of the P2Z/P2X7 receptor and named these clones P2Zhyper or P2Zhypo, respectively. P2Zhyper, but not P2Zhypo, cells grown to confluence in culture spontaneously fuse to form MGCs. As previously shown for human macrophages, fusion is inhibited by the P2Z/P2X7 blocker oxidized ATP. MGCs die shortly after fusion through a dramatic process of cytoplasmic sepimentation followed by fragmentation. These observations support our previous hypothesis that the P2Z/P2X7 receptor is involved in macrophage fusion.

Molecular characterization and pharmacological properties of the human P2X3 purinoceptor

Using PCR and library screening techniques, a cDNA encoding an ATP ligand-gated channel has been isolated from human heart. The full-length cDNA encodes a protein 397 amino acids long which shows a high amino-acid sequence identity with the rat P2X3 purinoceptor (93%). By fluorescence in situ hybridization, the human P2X3 gene has been mapped to region q12 of chromosome 11. Tissue distribution analysis of human P2X3 receptor mRNA shows a restricted expression pattern, i.e. transcripts are limited to the spinal cord and heart. This result contrasts with the distribution of the rat P2X3 receptor which was detected exclusively in sensory neurons of trigeminal, dorsal root and nodose ganglia. Heterologous expression of human P2X3 cRNA in Xenopus oocytes generates a fast desensitizing ATP-activated channel with pharmacological properties resembling the profile of the rat homologue receptor. Thus, the order of agonist potency is 2MeSATP > ATP > alphabeta-meATP > CTP > betagamma-meATP approximately ADP. Moreover, ATP-evoked currents on human P2X3 receptor are efficiently blocked in a reversible manner by the purinoceptor antagonists, suramin and PPADS.

Identification of amino acid residues contributing to the pore of a P2X receptor

P2X receptors are ion channels opened by extracellular ATP. The seven subunits currently known are encoded by different genes. It is thought that each subunit has two transmembrane domains, a large extracellular loop, and intracellular N- and C-termini, a topology which is fundamentally different from that of other ligand-gated channels such as nicotinic acetylcholine or glutamate receptors. We used the substituted cysteine accessibility method to identify parts of the molecule that form the ionic pore of the P2X2 receptor. Amino acids preceding and throughout the second hydrophobic domain (316-354) were mutated individually to cysteine, and the DNAs were expressed in HEK293 cells. For three of the 38 residues (I328C, N333C, T336C), currents evoked by ATP were inhibited by extracellular application of methanethiosulfonates of either charge (ethyltrimethylammonium, ethylsulfonate) suggesting that they lie in the outer vestibule of the pore. For two further substitutions (L338C, D349C) only the smaller ethylamine derivative inhibited the current. L338C was accessible to cysteine modification whether or not the channel was opened by ATP, but D349C was inhibited only when ATP was concurrently applied. The results indicate that part of the pore of the P2X receptor is formed by the second hydrophobic domain, and that L338 and D349 are on either side of the channel 'gate'.

Nucleotide receptors

Adenosine 5'-triphosphate (ATP) and/or related nucleotides act at both ionotropic (P2X) and metabotropic (P2Y) receptors. P2X receptor subunits (P2X1-P2X7) form ligand-gated cation channels, as homomultimers or heteromultimers. Recent work indicates that P2X3 subunits participate in channels expressed by nociceptive sensory neurons, and that the second of the two transmembrane domains of each subunit contributes to the ion permeation pathway. P2X7 subunits form large cytolytic pores in addition to cation channels; they have been found in macrophages and brain microglia. P2Y receptors form a distinct subset of G-protein-coupled receptors; most couple through G proteins to phospholipase C, but inhibition of adenylate cyclase and N-type Ca2+ channels, and activation of K+ channels also occurs. Expressed P2Y receptors have generally been distinguished pharmacologically by the rank order of effectiveness of agonists; some prefer pyrimidines to purines. Recent studies suggest that it is important to use purified nucleotides in such classifications. Several P2Y receptors have a very widespread tissue distribution.

Expression of the P2X2 receptor subunit of the ATP-gated ion channel in the retina

The site of extracellular ATP signalling in the retina was investigated by examining expression of the P2X2 receptor (P2X2R) subunit which assembles to form ATP-gated ion channels. Indirect in situ RT-PCR in situ hybridization localized the presence of mRNA for the P2X2R subunit within the soma of photoreceptors, inner nuclear layer neurones and the retina ganglion cells. Use of an antiserum specific for the P2X2R subunit confirmed the expression of the protein by these cells and demonstrated a particularly dense immunolabelling within the inner plexiform layer containing the dendritic processes of the retina ganglion cells. The outer segment of the photoreceptors also exhibited P2X2R-like immunoreactivity. The extensive expression of ATP-gated ion channel protein within the retina suggests that extracellular ATP plays diverse neurohumoral roles in regulation of visual processing and cellular homeostasis.

Characterization of recombinant human P2X4 receptor reveals pharmacological differences to the rat homologue

We isolated a cDNA from human brain encoding a purinergic receptor that shows a high degree of homology to the rat P2X4 receptor (87% identity). By fluorescence in situ hybridization, the human P2X4 gene has been mapped to region q24.32 of chromosome 12. Tissue distribution analysis of human P2X4 transcripts demonstrates a broad expression pattern in that the mRNA was detected not only in brain but also in all tissues tested. Heterologous expression of the human P2X4 receptor in Xenopus laevis oocytes and human embryonic kidney 293 cells evoked an ATP-activated channel. Simultaneous whole-cell current and Fura-2 fluorescence measurements in human embronic kidney 293 cells transfected with human P2X4 cDNA allowed us to determine the fraction of the current carried by Ca2: this was approximately 8%, demonstrating a high Ca2+ permeability. Low extracellular Zn2+ concentrations (5-10 microM) increase the apparent gating efficiency of human P2X4 by ATP without affecting the maximal response. However, raising the concentration of the divalent cation (> 100 microM) inhibits the ATP-evoked current in a non-voltage-dependent manner. The human P2X4 receptor displays a very similar agonist potency profile to that of rat P2X4 (ATP > > 2-methylthio-ATP > or = CTP > alpha, beta-methylene-ATP > dATP) but has a notably higher sensitivity for the antagonists suramin, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid, and bromphenol blue. Chimeric constructs between human and rat isoforms as well as single-point mutations were engineered to map the regions responsible for the different sensitivity to suramin and pyridoxal-phosphate-6-azophenyl-2'4'-disulfonic acid.

Identification of amino acid residues in the alpha, beta, and gamma subunits of the epithelial sodium channel (ENaC) involved in amiloride block and ion permeation

The amiloride-sensitive epithelial Na channel (ENaC) is a heteromultimeric channel made of three alpha beta gamma subunits. The structures involved in the ion permeation pathway have only been partially identified, and the respective contributions of each subunit in the formation of the conduction pore has not yet been established. Using a site-directed mutagenesis approach, we have identified in a short segment preceding the second membrane-spanning domain (the pre-M2 segment) amino acid residues involved in ion permeation and critical for channel block by amiloride. Cys substitutions of Gly residues in beta and gamma subunits at position beta G525 and gamma G537 increased the apparent inhibitory constant (Ki) for amiloride by > 1,000-fold and decreased channel unitary current without affecting ion selectivity. The corresponding mutation S583 to C in the alpha subunit increased amiloride Ki by 20-fold, without changing channel conducting properties. Coexpression of these mutated alpha beta gamma subunits resulted in a non-conducting channel expressed at the cell surface. Finally, these Cys substitutions increased channel affinity for block by external Zn2+ ions, in particular the alpha S583C mutant showing a Ki for Zn2+ of 29 microM. Mutations of residues alpha W582L, or beta G522D also increased amiloride Ki, the later mutation generating a Ca2+ blocking site located 15% within the membrane electric field. These experiments provide strong evidence that alpha beta gamma ENaCs are pore-forming subunits involved in ion permeation through the channel. The pre-M2 segment of alpha beta gamma subunits may form a pore loop structure at the extracellular face of the channel, where amiloride binds within the channel lumen. We propose that amiloride interacts with Na+ ions at an external Na+ binding site preventing ion permeation through the channel pore.

Domains of P2X receptors involved in desensitization

ATP-gated ion channels (P2X receptors) are abundantly expressed in both neuronal and nonneuronal tissues, where they can serve as postsynaptic receptors. The response to ATP shows marked desensitization in some tissues but not others. Currents induced by ATP in Xenopus oocytes expressing cloned P2X1 (or P2X3) receptor had strong desensitization, whereas currents in cells expressing P2X2 receptors desensitized relatively little (90% vs. 14% decline of current in a 10-s application). In chimeric receptors, substitution into the P2X1 receptor of either one of two 34-residue segments from the P2X2 receptor removed the desensitization; these segments included the first or the second hydrophobic domain. In contrast, desensitization was introduced into the P2X2 receptor only by providing both these segments of the P2X1 (or P2X3) receptor. This suggests that desensitization requires interaction between the two hydrophobic domains of the receptor, and supports the view that these are membrane-spanning segments.

Ionic permeability of, and divalent cation effects on, two ATP-gated cation channels (P2X receptors) expressed in mammalian cells

1. Complementary DNAs for the ATP-gated ion channel subunits P2X1 (from human bladder) and P2X2 (from rat phaeochromocytoma (PC12) cells) were used to express the receptors in human embryonic kidney cells by stable transfection, and in Chinese hamster ovary cells by viral infection. 2. Membrane currents evoked by ATP were recorded by the whole-cell patch clamp method. The reversal potential of the current was measured with various intracellular and extracellular solutions and used to compute the relative permeability of the P2X receptor channels. 3. There was no difference between the two receptors with respect to their permeability to monovalent organic cations. The relative permeabilities (PX/PNa) were 2.3, 1.0, 1.0, 0.95, 0.72, 0.5, 0.29, 0.16, 0.04 and 0.03 for guanidinium, potassium, sodium, methylamine, caesium, dimethylamine, 2-methylethanolamine, tris(hydroxymethyl)-aminomethane, tetraethylammonium and N-methyl-D-glucamine, respectively (values for P2X2 receptor). 4. The calcium permeability of P2X1 receptors was greater than that of P2X2 receptors. Under biionic conditions (112 mM calcium outside, 154 mM sodium inside), PCa/PNa values were 3.9 and 2.2, respectively (corrected for ionic activities). 5. ATP-evoked currents in cells expressing the P2X2 receptor were strongly inhibited when the extracellular calcium concentration was increased (0.3-30 mM); the action of ATP could be restored by increasing the ATP concentration. ATP-evoked currents in cells expressing the P2X1 receptor were not inhibited by such increases in the extracellular calcium concentration.