The P2Z receptor is responsible for adenosine triphosphate (ATP)-dependent lysis of macrophages through the formation of membrane pores permeable to large molecules. Other ATP-gated channels, the P2X receptors, are permeable only to small cations. Here, an ATP receptor, the P2X7 receptor, was cloned from rat brain and exhibited both these properties. This protein is homologous to other P2X receptors but has a unique carboxyl-terminal domain that was required for the lytic actions of ATP. Thus, the P2X7 (or P2Z) receptor is a bifunctional molecule that could function in both fast synaptic transmission and the ATP-mediated lysis of antigen-presenting cells.

ATP is a known mediator of inflammatory and neuropathic pain. However, the mechanisms by which specific purinergic receptors contribute to chronic pain states are still poorly characterized. Here, we demonstrate that in response to peripheral nerve injury, P2X 4 receptors (P2X 4 R) are expressed de novo by activated microglia in the spinal cord. Using in vitro and in vivo models, we provide direct evidence that P2X 4 R stimulation leads to the release of BDNF from activated microglia and, most likely phosphorylation of the NR1 subunit of NMDA receptors in dorsal horn neurons of the spinal cord. Consistent with these findings, P2X4-deficient mice lack mechanical hyperalgesia induced by peripheral nerve injury and display impaired BDNF signaling in the spinal cord. Furthermore, ATP stimulation is unable to stimulate BDNF release from P2X 4 -deficient mice microglia in primary cultures. These results indicate that P2X 4 R contribute to chronic pain through a central inflammatory pathway. P2X 4 R might thus represent a potential therapeutic target to limit microglia-mediated inflammatory responses associated with brain injury and neurodegenerative disorders.

P2X receptors are ligand-gated ion channels activated by the binding of extracellular adenosine 5'-triphosphate (ATP). Brief (< 1 s) applications of ATP to nodose ganglion neurons or to cells transfected with P2X2 or P2X4 receptor cDNAs induce the opening of a channel selectively permeable to small cations within milliseconds. We now show that, during longer ATP application (10-60 s), the channel also becomes permeable to much larger cations such as N-methyl-D-glucamine and the propidium analog YO-PRO-1. This effect is enhanced in P2X2 receptors carrying point mutations in the second transmembrane segment. Progressive dilation of the ion-conducting pathway during prolonged activation reveals a mechanism by which ionotropic receptors may alter neuronal function.

Microglia cells are the resident macrophages of the CNS, and their activation plays a critical role in inflammatory reactions associated with many brain disorders, including ischemia, Alzheimer's and Parkinson's diseases, and epilepsy. However, the changes of microglia functional properties in epilepsy have rarely been studied. Here, we used a model of status epilepticus (SE) induced by intraperitoneal kainate injections to characterize the properties of microglial cells in hippocampal slices from CX3CR1 eGFP/ϩ mice. SE induced within 3 h an increased expression of inflammatory mediators in the hippocampus, followed by a modification of microglia morphology, a microglia proliferation, and a significant neurodegeneration in CA1. Changes in electrophysiological intrinsic membrane properties of hippocampal microglia were detected at 24 -48 h after SE with, in particular, the appearance of new voltage-activated potassium currents. Consistent with the observation of an upregulation of purinergic receptor mRNAs in the hippocampus, we also provide pharmacological evidence that microglia membrane currents mediated by the activation of P2 receptors, including P2X 7 , P2Y 6 , and P2Y 12 , were increased 48 h after SE. As a functional consequence of this modification of purinergic signaling, motility of microglia processes toward a source of P2Y 12 receptor agonist was twice as fast in the epileptic hippocampus. This study is the first functional description of microglia activation in an in vivo model of inflammation and provides evidence for the existence of a particular microglial activation state after a status epilepticus.

P2X receptor subunits have intracellular N and C termini, two membrane-spanning domains, and an extracellular loop of about 280 amino acids. We expressed the rat P2X 2 receptor in human embryonic kidney cells, and used alanine-scanning mutagenesis on 30 residues with polar side chains conserved among the seven rat P2X receptor subunits. This identified a region proximal to the first transmembrane domain which contained 2 lysine residues that were critical for the action of ATP (Lys 69 and Lys 71 ). We substituted cysteines in this region (Asp 57 to Asp 71 ) and found that for S65C and I67C ATPevoked currents were inhibited by methanethiosulfonates. At I67C, the inhibition by negatively charged ethylsulfonate and pentylsulfonate derivatives could be overcome by increasing the ATP concentration, consistent with a reduced affinity of ATP binding. The inhibitory action of the methanethiosulfonates was prevented by pre-exposure to ATP, suggesting occlusion of the binding site. Finally, introduction of negative charges into the receptor by mutagenesis at this position (I67E and I67D) also gave receptors in which the ATP concentration-response curve was right-shifted. The results suggest that residues close to Ile 67 contribute to the ATP-binding site.P2X receptors are ligand-gated ion channels in the plasma membrane (1). They are homomeric or heteromeric proteins, formed by assembly of subunits named P2X 1 -P2X 7 . Current evidence suggests that three subunits form a channel (or a multiple of three) (2, 3). The ligand for the P2X receptors is ATP, acting from the extracellular milieu. In this way ATP released from cells functions as a synaptic transmitter, an autocrine or paracrine signal, in a wide range of mammalian tissues (4). Other membrane proteins which bind ATP from the extracellular aspect include G-protein-coupled P2Y receptors, and degradative enzymes such as ectoATPase (4).The individual subunits of the P2X receptor seem unrelated in amino acid sequence to other ion channels, and indeed to other proteins. The proteins range in length from 379 to 595 amino acids. The P2X 2 receptor is normally glycosylated (Asn 183 , Asn 239 , and Asn 298 ), and experiments with the introduction of additional glycosylation sites by mutagenesis indicate that most of the protein is extracellular (the ectodomain: residues 50 -330) (5, 6). The N terminus and C terminus lie within the cell, and the residues 30 -50 and 330 -352 form membrane-spanning domains. Three kinds of approaches have been made to determine which parts of the molecule contribute to which of the functional properties of the channel, but none of these have addressed the ATP-binding site. First, the substituted cysteine accessibility method has been used to show that residues in and around the second membrane-spanning domain contribute to the formation of permeation pathway for cations (7,8); in particular, Thr 336 appears to lie in the outer vestibule of the pore and Asn 349 is internal to the gate of the pore (7). Second, desensitization is profound in the ca...

Buell et al., 1996b) or prevent glycosylation (N184S Development, Plan-les-Ouates, 1228 Geneva, Switzerland in P2X 1 ; our unpublished observations). All subunits 1 Corresponding author except P2X 6 express readily in heterologous systems and are presumed to form channels as homomultimers (Collo P2X receptors are ion channels opened by extracellular et al., 1996). In the case of P2X 2 and P2X 3 receptors, ATP. The seven subunits currently known are encoded heteromeric channel assembly has also been shown (Lewis by different genes. It is thought that each subunit has et al., 1995). two transmembrane domains, a large extracellular This overall topology of the P2X channel appears to be loop, and intracellular N-and C-termini, a topology very different from that established for the other two major which is fundamentally different from that of other families of ligand-gated channels (nicotinic acetylcholine ligand-gated channels such as nicotinic acetylcholine or family and glutamate family); those are pentamers, in which glutamate receptors. We used the substituted cysteine each subunit has four or three membrane-spanning segments accessibility method to identify parts of the molecule (Numa, 1989; Karlin, 1993; Hollman et al., 1994). The P2X that form the ionic pore of the P2X 2 receptor. Amino receptors more closely resemble in structure that proposed acids preceding and throughout the second hydrofor the epithelial sodium channels (ENaC), which also have phobic domain (316-354) were mutated individually a large extracellular loop flanked by two hydrophobic to cysteine, and the DNAs were expressed in HEK293regions (Canessa et al., 1994a,b;Rossier et al., 1994; North, cells. For three of the 38 residues (I328C, N333C, 1996b). Epithelial sodium channels belong to a larger family T336C), currents evoked by ATP were inhibited by related by sequence homology which includes presumed extracellular application of methanethiosulfonates of channel proteins (degenerins) from Caenorhabditis elegans either charge (ethyltrimethylammonium, ethylsulfon- (Hong and Driscoll, 1994;Huang and Chalfie, 1994) and a ate) suggesting that they lie in the outer vestibule of recently described ligand-gated channel from Helix aspersa the pore. For two further substitutions (L338C, D349C) (Lingueglia et al., 1995). There is no detectable similarity only the smaller ethylamine derivative inhibited the of sequence between P2X receptors and the ENaC-based current. L338C was accessible to cysteine modification family (North, 1996b). The stoichiometry is not yet known whether or not the channel was opened by ATP, but for any of these channels with two hydrophobic domains D349C was inhibited only when ATP was concurrently per subunit. applied. The results indicate that part of the pore of Our current understanding of the structure and function the P2X receptor is formed by the second hydrophobic of P2X receptors remains primitive, and specifically we domain, and that L338 and D349 are on either side of do not know which regions of the molecule co...

A cDNA was isolated from a human monocyte library that encodes the P2X 7 receptor; the predicted protein is 80% identical to the rat receptor. Whole cell recordings were made from human embryonic kidney cells transfected with the human cDNA and from human macrophages. Brief applications (1-3 s) of ATP and 2,3-(4-benzoyl)-benzoyl-ATP elicited cation-selective currents. When compared with the rat P2X 7 receptor, these effects required higher concentrations of agonists, were more potentiated by removal of extracellular magnesium ions, and reversed more rapidly on agonist removal. Longer applications of agonists permeabilized the cells, as evidenced by uptake of the propidium dye YO-PRO1, but this was less marked than for cells expressing the rat P2X 7 receptor. Expression of chimeric molecules indicated that some of the differences between the rat and human receptor could be reversed by exchanging the intracellular C-terminal domain of the proteins.Cell surface receptors for ATP can be divided into metabotropic (P2Y/P2U) and ionotropic (P2X) classes. The metabotropic class belong to the superfamily of G protein-coupled receptors with seven transmembrane segments; the ionotropic class are ligand-gated channels, currently thought to be multisubunit proteins with two transmembrane domains per subunit (for review see Ref. 1). P2Z receptors have been distinguished from other P2 receptors in three main ways (2-4). First, activation of this receptor leads not only to an inward ionic current but also to cell permeabilization. Second, 2Ј,3Ј-(4-benzoyl)benzoyl ATP is the most effective agonist, and ATP itself is of rather low potency. Third, responses are strongly inhibited by extracellular magnesium ions, which has been interpreted to indicate that ATP Ϫ4 is the active agonist (for review see Ref. 5).A seventh member of the P2X receptor family was isolated recently from a rat cDNA library that, when expressed in human embryonic kidney (HEK293) cells, exhibits these three properties (6). This receptor (rP2X 7 ) 1 is thus considered to represent a P2Z receptor. The protein is structurally related to other members of the P2X family; there is 35-40% amino acid identity in the region of homology, but the C terminus is 239 amino acids long in the rP2X 7 receptor compared with 27-120 amino acids in the others. The rP2X 7 receptor functions both as a channel permeable to small cations and as a cytolytic pore (6). Brief applications of ATP (1-2 s) transiently open the channel, and this is generally similar in properties to other P2X receptors. Repeated or prolonged applications of agonist cause cell permeabilization; reducing the extracellular magnesium concentration much potentiates this effect. The permeabilization involves the cytoplasmic C terminus of the protein because it does not occur with a P2X 7 receptor lacking the last 177 residues, although this truncation does not affect the function as a small cation channel. The P2Z receptor has been implicated in lysis of antigenpresenting cells by cytotoxic T lymphocytes, in the mi...

P2X 4 purinergic receptors are calcium-permeable, ATP-activated ion channels. In the CA1 area of the hippocampus, they are located at the subsynaptic membrane somewhat peripherally to AMPA receptors. The possible role of P2X 4 receptors has been difficult to elucidate because of the lack of selective antagonists. Here we report the generation of a P2X 4 receptor knock-out mouse and show that long-term potentiation (LTP) at Schaffer collateral synapses is reduced relative to that in wild-type mice. Ivermectin, which selectively potentiates currents at P2X 4 , was found to increase LTP in wild-type mice but had no effect in P2X 4 knock-out mice. We suggest that calcium entry through subsynaptic P2X 4 receptors during high-frequency stimulation contributes to synaptic strengthening.