Cloning and pharmacological characterization of a fourth P2X receptor subtype widely expressed in brain and peripheral tissues including various endocrine tissues

Role of microglia and P2X4 receptors in chronic pain

This study summarizes current understanding of the role of microglia and P2X4 receptor in chronic pain including neuropathic pain and of their therapeutic potential.

Pain plays an indispensable role as an alarm system to protect us from dangers or injuries. However, neuropathic pain, a debilitating pain condition caused by damage to the nervous system, persists for a long period even in the absence of dangerous stimuli or after injuries have healed. In this condition, pain becomes a disease itself rather than the alarm system and is often resistant to currently available medications. A growing body of evidence indicates that microglia, a type of macrophages residing in the central nervous system, play a crucial role in the pathogenesis of neuropathic pain. Whenever microglia in the spinal cord detect a damaging signal within the nervous system, they become activated and cause diverse alterations that change neural excitability, leading to the development of neuropathic pain. For over a decade, several lines of molecular and cellular mechanisms that define microglial activation and subsequently altered pain transmission have been proposed. In particular, P2X4 receptors (a subtype of purinergic receptors) expressed by microglia have been investigated as an essential molecule for neuropathic pain. In this review article, we describe our understanding of the mechanisms by which activated microglia cause neuropathic pain through P2X4 receptors, their involvement in several pathological contexts, and recent efforts to develop new drugs targeting microglia and P2X4 receptors.

P2X Electrophysiology and Surface Trafficking in Xenopus Oocytes

Xenopus oocytes serve as a standard heterologous expression system for the study of various ligand-gated ion channels including ATP P2X receptors. Here we describe the whole-cell two-electrode voltage clamp and biotinylation/Western blotting techniques to investigate the functional properties and surface trafficking from P2X-expressing oocytes.

Role of Extracellular Adenosine Triphosphate in Human Skin

Background:

The nucleotide adenosine triphosphate (ATP) has long been known to drive and participate in countless intracellular processes. Extracellular ATP and its metabolite adenosine have also been shown to exert a variety of effects on nearly every cell type in human skin. Knowledge of the sources and effects of extracellular ATP in human skin may help shape new therapies for skin injury, inflammation, and numerous other cutaneous disorders.

Objective:

The objective of this review is to introduce the reader to current knowledge regarding the sources and effects of extracellular ATP in human skin and to outline areas in which further research is necessary to clarify the nature and mechanism of these effects.

Conclusion:

Extracellular ATP seems to play a direct role in triggering skin inflammatory, regenerative, and fibrotic responses to mechanical injury, an indirect role in melanocyte proliferation and apoptosis, and a complex role in Langerhans cell-directed adaptive immunity.

P2X4 Receptor Reporter Mice: Sparse Brain Expression and Feeding-Related Presynaptic Facilitation in the Arcuate Nucleus

P2X4 receptors are ATP-gated cation channels that are widely expressed in the nervous system. To identify P2X4 receptor-expressing cells, we generated BAC transgenic mice expressing tdTomato under the control of the P2X4 receptor gene (P2rx4). We found sparse populations of tdTomato-positive neurons in most brain areas with patterns that matched P2X4 mRNA distribution. tdTomato expression within microglia was low but was increased by an experimental manipulation that triggered microglial activation. We found surprisingly high tdTomato expression in the hypothalamic arcuate nucleus (Arc) (i.e., within parts of the neural circuitry controlling feeding). Immunohistochemistry and genetic crosses of P2rx4 tdTomato mice with cell-specific GFP reporter lines showed that the tdTomato-expressing cells were mainly AgRP-NPY neurons and tanycytes. There was no electrophysiological evidence for functional expression of P2X4 receptors on AgRP-NPY neuron somata, but instead, we found clear evidence for functional presynaptic P2X4 receptor-mediated responses in terminals of AgRP-NPY neurons onto two of their postsynaptic targets (Arc POMC and paraventricular nucleus neurons), where ATP dramatically facilitated GABA release. The presynaptic responses onto POMC neurons, and the expression of tdTomato in AgRP-NPY neurons and tanycytes, were significantly decreased by food deprivation in male mice in a manner that was partially reversed by the satiety-related peptide leptin. Overall, we provide well-characterized tdTomato reporter mice to study P2X4-expressing cells in the brain, new insights on feeding-related regulation of presynaptic P2X4 receptor responses, and the rationale to explore extracellular ATP signaling in the control of feeding behaviors.

SIGNIFICANCE STATEMENT

Cells expressing ATP-gated P2X4 receptors have proven problematic to identify and study in brain slice preparations because P2X4 expression is sparse. To address this limitation, we generated and characterized BAC transgenic P2rx4 tdTomato reporter mice. We report the distribution of tdTomato-expressing cells throughout the brain and particularly strong expression in the hypothalamic arcuate nucleus. Together, our studies provide a new, well-characterized tool with which to study P2X4 receptor-expressing cells. The electrophysiological studies enabled by this mouse suggest previously unanticipated roles for ATP and P2X4 receptors in the neural circuitry controlling feeding.

Key sites for P2X receptor function and multimerization: overview of mutagenesis studies on a structural basis

P2X receptors constitute a seven-member family (P2X1-7) of extracellular ATP-gated cation channels of widespread expression. Because P2X receptors have been implicated in neurological, inflammatory and cardiovascular diseases, they constitute promising drug targets. Since the first P2X cDNA sequences became available in 1994, numerous site-directed mutagenesis studies have been conducted to disclose key sites of P2X receptor function and oligomerization. The publication of the 3-Å crystal structures of the zebrafish P2X4 (zfP2X4) receptor in the homotrimeric apo-closed and ATP-bound open states in 2009 and 2012, respectively, has ushered a new era by allowing for the interpretation of the wealth of molecular data in terms of specific three-dimensional models and by paving the way for designing more-decisive experiments. Thanks to these structures, the last five years have provided invaluable insight into our understanding of the structure and function of the P2X receptor class of ligandgated ion channels. In this review, we provide an overview of mutagenesis studies of the pre- and post-crystal structure eras that identified amino acid residues of key importance for ligand binding, channel gating, ion flow, formation of the pore and the channel gate, and desensitization. In addition, the sites that are involved in the trimerization of P2X receptors are reviewed based on mutagenesis studies and interface contacts that were predicted by the zfP2X4 crystal structures.

Purinergic signalling in endocrine organs

There is widespread involvement of purinergic signalling in endocrine biology. Pituitary cells express P1, P2X and P2Y receptor subtypes to mediate hormone release. Adenosine 5'-triphosphate (ATP) regulates insulin release in the pancreas and is involved in the secretion of thyroid hormones. ATP plays a major role in the synthesis, storage and release of catecholamines from the adrenal gland. In the ovary purinoceptors mediate gonadotrophin-induced progesterone secretion, while in the testes, both Sertoli and Leydig cells express purinoceptors that mediate secretion of oestradiol and testosterone, respectively. ATP released as a cotransmitter with noradrenaline is involved in activities of the pineal gland and in the neuroendocrine control of the thymus. In the hypothalamus, ATP and adenosine stimulate or modulate the release of luteinising hormone-releasing hormone, as well as arginine-vasopressin and oxytocin. Functionally active P2X and P2Y receptors have been identified on human placental syncytiotrophoblast cells and on neuroendocrine cells in the lung, skin, prostate and intestine. Adipocytes have been recognised recently to have endocrine function involving purinoceptors.

A new role for P2X4 receptors as modulators of lung surfactant secretion

In recent years, P2X receptors have attracted increasing attention as regulators of exocytosis and cellular secretion. In various cell types, P2X receptors have been found to stimulate vesicle exocytosis directly via Ca(2+) influx and elevation of the intracellular Ca(2+) concentration. Recently, a new role for P2X4 receptors as regulators of secretion emerged. Exocytosis of lamellar bodies (LBs), large storage organelles for lung surfactant, results in a local, fusion-activated Ca(2+) entry (FACE) in alveolar type II epithelial cells. FACE is mediated via P2X4 receptors that are located on the limiting membrane of LBs and inserted into the plasma membrane upon exocytosis of LBs. The localized Ca(2+) influx at the site of vesicle fusion promotes fusion pore expansion and facilitates surfactant release. In addition, this inward-rectifying cation current across P2X4 receptors mediates fluid resorption from lung alveoli. It is hypothesized that the concomitant reduction in the alveolar lining fluid facilitates insertion of surfactant into the air-liquid interphase thereby "activating" it. These findings constitute a novel role for P2X4 receptors in regulating vesicle content secretion as modulators of the secretory output during the exocytic post-fusion phase.

Unraveling the effects of 1,25OH2D3 on global gene expression in pancreatic islets

INTRODUCTION

Vitamin D deficiency has been linked to type 1 and 2 diabetes, whereas supplementation may prevent both diseases. However, the extent of the effects of vitamin D or its metabolites directly on pancreatic islets is still largely unknown. The aim of the present study was to investigate how active vitamin D, 1,25(OH)2D3, affects beta cells directly by establishing its effects on global gene expression in healthy murine islets.

MATERIALS AND METHODS

Pancreatic islets were isolated from 2 to 3 week old C57BL/6 mice and cultured in vitro with 1,25(OH)2D3 or vehicle for 6 and 24h. Total RNA was extracted from the islets and the effects on global gene expression were analyzed using Affymetrix microarrays.

RESULTS AND DISCUSSION

Exposure to 1,25(OH)2D3 compared to vehicle resulted in 306 and 151 differentially expressed genes after 6 and 24h, respectively (n=4, >1.3-fold, p<0.02). Of these 220 were up-regulated, whereas 86 displayed a decreased expression after 6h. Furthermore, expression levels were increased for 124 and decreased for 27 genes following 24h of exposure. Formation of intercellular junctions, cytoskeletal organization, and intracellular trafficking as well as lipid metabolism and ion transport were among the most affected gene classes. Effects on several genes already identified as being part of vitamin D signaling in other cell types were observed along with genes known to affect insulin release, although with our assay we were not able to detect any effects of 1,25(OH)2D3 on glucose-stimulated insulin release from healthy pancreatic islets.

CONCLUSION

The effects of 1,25(OH)2D3 on the expression of cytoskeletal and intracellular trafficking genes along with genes involved in ion transport may influence insulin exocytosis. However, an effect of 1,25(OH)2D3 on insulin release could not be detected for healthy islets in contrast to islets subjected to pathological conditions such as cytokine exposure and vitamin D deficiency as suggested by other studies. Thus, in addition to previously identified tolerogenic effects on the immune system, 1,25(OH)2D3 may affect basic functions of pancreatic beta cells, with the potential to render them more resistant to the detrimental conditions encountered during type 1 and 2 diabetes. This article is part of a Special Issue entitled 'Vitamin D Workshop'.

Purinergic signalling in the pancreas in health and disease

Pancreatic cells contain specialised stores for ATP. Purinergic receptors (P2 and P1) and ecto-nucleotidases are expressed in both endocrine and exocrine calls, as well as in stromal cells. The pancreas, especially the endocrine cells, were an early target for the actions of ATP. After the historical perspective of purinergic signalling in the pancreas, the focus of this review will be the physiological functions of purinergic signalling in the regulation of both endocrine and exocrine pancreas. Next, we will consider possible interaction between purinergic signalling and other regulatory systems and their relation to nutrient homeostasis and cell survival. The pancreas is an organ exhibiting several serious diseases - cystic fibrosis, pancreatitis, pancreatic cancer and diabetes - and some are associated with changes in life-style and are increasing in incidence. There is upcoming evidence for the role of purinergic signalling in the pathophysiology of the pancreas, and the new challenge is to understand how it is integrated with other pathological processes.

Molecular and functional properties of P2X receptors--recent progress and persisting challenges

ATP-gated P2X receptors are trimeric ion channels that assemble as homo- or heteromers from seven cloned subunits. Transcripts and/or proteins of P2X subunits have been found in most, if not all, mammalian tissues and are being discovered in an increasing number of non-vertebrates. Both the first crystal structure of a P2X receptor and the generation of knockout (KO) mice for five of the seven cloned subtypes greatly advanced our understanding of their molecular and physiological function and their validation as drug targets. This review summarizes the current understanding of the structure and function of P2X receptors and gives an update on recent developments in the search for P2X subtype-selective ligands. It also provides an overview about the current knowledge of the regulation and modulation of P2X receptors on the cellular level and finally on their physiological roles as inferred from studies on KO mice.

Involvement of P2X receptors in the regulation of insulin secretion, proliferation and survival in mouse pancreatic β-cells

In order to clarify the functional role of ionotropic purinergic (P2X) receptors in pancreatic β-cells, we examined the effect of several P2 receptor agonists and antagonists on insulin secretion by mouse pancreatic islets, mouse Beta-TC6 cell proliferation and survival of dispersed islet cells in culture. Reverse transcription-polymerase chain reaction (RT-PCR) analysis showed the expression of mRNAs of P2X(4) receptor in mouse islets and P2X(1), P2X(2), P2X(3), P2X(4), P2X(5) and P2X(7) receptors in Beta-TC6 cells. The presence of P2X(4) receptor proteins in islets and Beta-TC6 cells was confirmed by immunofluorescent staining and Western blot analysis. We have previously found that the functional P2Y(1) receptor but not P2Y(2) and P2Y(4) receptors was present in islets. In this study we found that a nonspecific P2 receptor agonist, ATP (1 μM) stimulated insulin secretion by islets in the presence of high glucose (20 mM) in culture. The effect of ATP was partially inhibited by a P2 receptor antagonist PPADS as well as a P2Y(1) receptor antagonist MRS2179. In addition, a P2X(4) receptor potentiator ivermectin per se augmented glucose-induced insulin secretion and slightly potentiated the effect of ATP. These results suggested the involvement of P2Y(1)and P2X receptors. We also found that ATP inhibited proliferation of Beta-TC6 cells in a concentration-dependent manner during 72 h culture. The inhibitory effect of ATP was completely reversed by PPADS and partially by treating cells with small interfering RNA targeted for P2X(4) receptor mRNA. Furthermore, we found that the phosphorylation of the extracellular signal-regulated kinase 1 and 2 (ERK1/2) was suppressed by treatment with ATP in Beta-TC6 cells. In addition, we found that ATP reduced the cell viability and DNA synthesis of islet cells in culture. The effect of ATP on the cell viability was blocked by PPADS or MRS2179. These results suggested that P2X receptors as well as the P2Y(1) receptor played a role in the modulation of insulin secretion, proliferation and cell viability in mouse pancreatic β-cells.

Pharmacological properties and physiological function of a P2X-like current in single proximal tubule cells isolated from frog kidney

Although previous studies have provided evidence for the expression of P2X receptors in renal proximal tubule, only one cell line study has provided functional evidence. The current study investigated the pharmacological properties and physiological role of native P2X-like currents in single frog proximal tubule cells using the whole-cell patch-clamp technique. Extracellular ATP activated a cation conductance (P2X_f) that was also Ca²⁺-permeable. The agonist sequence for activation was ATP = αβ-MeATP > BzATP = 2-MeSATP, and P2X_f was inhibited by suramin, PPADS and TNP-ATP. Activation of P2X_f attenuated the rundown of a quinidine-sensitive K⁺ conductance, suggesting that P2X_f plays a role in K⁺ channel regulation. In addition, ATP/ADP apyrase and inhibitors of P2X_f inhibited regulatory volume decrease (RVD). These data are consistent with the presence of a P2X receptor that plays a role in the regulation of cell volume and K⁺ channels in frog renal proximal tubule cells.

P2X4 receptors in activated C8-B4 cells of cerebellar microglial origin

We investigated the properties and regulation of P2X receptors in immortalized C8-B4 cells of cerebellar microglial origin. Resting C8-B4 cells expressed virtually no functional P2X receptors, but largely increased functional expression of P2X4 receptors within 2–6 h of entering the activated state. Using real-time polymerase chain reaction, we found that P2X4 transcripts were increased during the activated state by 2.4-fold, but this increase was not reflected by a parallel increase in total P2X4 proteins. In resting C8-B4 cells, P2X4 subunits were mainly localized within intracellular compartments, including lysosomes. We found that cell surface P2X4 receptor levels increased by ∼3.5-fold during the activated state. This change was accompanied by a decrease in the lysosomal pool of P2X4 proteins. We next exploited our findings with C8-B4 cells to investigate the mechanism by which antidepressants reduce P2X4 responses. We found little evidence to suggest that several antidepressants were antagonists of P2X4 receptors in C8-B4 cells. However, we found that moderate concentrations of the same antidepressants reduced P2X4 responses in activated microglia by affecting lysosomal function, which indirectly reduced cell surface P2X4 levels. In summary, our data suggest that activated C8-B4 cells express P2X4 receptors when the membrane insertion of these proteins by lysosomal secretion exceeds their removal, and that antidepressants indirectly reduce P2X4 responses by interfering with lysosomal trafficking.

Antidepressants inhibit P2X4 receptor function: a possible involvement in neuropathic pain relief

BACKGROUND

Neuropathic pain is characterized by pain hypersensitivity to innocuous stimuli (tactile allodynia) that is nearly always resistant to known treatments such as non-steroidal anti-inflammatory drugs or even opioids. It has been reported that some antidepressants are effective for treating neuropathic pain. However, the underlying molecular mechanisms are not well understood. We have recently demonstrated that blocking P2X4 receptors in the spinal cord reverses tactile allodynia after peripheral nerve injury in rats, implying that P2X4 receptors are a key molecule in neuropathic pain. We investigated a possible role of antidepressants as inhibitors of P2X4 receptors and analysed their analgesic mechanism using an animal model of neuropathic pain.

RESULTS

Antidepressants strongly inhibited ATP-mediated Ca2+ responses in P2X4 receptor-expressing 1321N1 cells, which are known to have no endogenous ATP receptors. Paroxetine exhibited the most powerful inhibition of calcium influx via rat and human P2X4 receptors, with IC50 values of 2.45 microM and 1.87 microM, respectively. Intrathecal administration of paroxetine produced a striking antiallodynic effect in an animal model of neuropathic pain. Co-administration of WAY100635, ketanserin or ondansetron with paroxetine induced no significant change in the antiallodynic effect of paroxetine. Furthermore, the antiallodynic effect of paroxetine was observed even in rats that had received intrathecal pretreatment with 5,7-dihydroxytryptamine, which dramatically depletes spinal 5-hydroxytryptamine.

CONCLUSION

These results suggest that paroxetine acts as a potent analgesic in the spinal cord via a mechanism independent of its inhibitory effect on serotonin transporters. Powerful inhibition on P2X4 receptors may underlie the analgesic effect of paroxetine, and it is possible that some antidepressants clinically used in patients with neuropathic pain show antiallodynic effects, at least in part via their inhibitory effects on P2X4 receptors.

P2 purinergic signalling in the pancreatic beta-cell: control of insulin secretion and pharmacology

Extracellular adenosine triphosphate is able to modulate pancreatic beta-cell function, acting on P2 purinergic ionotropic (P2X) and metabotropic (P2Y) receptors. Physiologically, ATP entrains beta-cells into a common rhythm by coordinating Ca(2+) oscillations; it plays a central role in insulin secretion pulsatility. ATP also triggers a positive feedback signal amplifying glucose-induced insulin release, which argues for a potential pharmacological application. ATP has consistently been shown to increase cytoplasmic free calcium concentration, notably in human tissue. Acting on P2X receptors, of which different molecular subtypes are expressed in beta-cells, it leads to a transient insulin release that may involve a closure of K(ATP) channels or a rapidly decaying inward current. Activation of G-protein-coupled P2Y receptors triggers different signalling pathways and amplifies insulin release in a glucose-dependent way. It has recently been shown that pancreatic beta-cells express different molecular subtypes of receptors, which may explain the complex interaction of P2Y ligands on high- and low-affinity binding sites. Despite the complexity of this purinergic pharmacology, consistent pre-clinical data suggest the potential of P2Y receptor agonists as drug candidates for type 2 diabetes.

Dual effect of ATP on glucose-induced insulin secretion in HIT-T15 cells

OBJECTIVES

: The study investigated the dual effect of purinergic nucleotides on the secretion of insulin from pancreatic beta cells.

METHODS

: The level of insulin secretion in HIT-T15 cells of static incubation was measured using a radioimmunoassay.

RESULTS

: The adenine nucleotides reduced the level of glucose-induced insulin secretion in a concentration-dependent manner, and the relative potency order (IC50; muM) was BzATP (6.9) > ATP (20.4) >/= alpha, beta-methylene ATP (23.3) >/= 2-methylthio-ATP (24.9). Suramin and PPADS (200 muM), which are blockers of the purinergic receptors, had a little influence on the activity of ATP. However, the inhibitory effect of ATP was reversed by preincubation with oxidized ATP (200 muM), which is a P2X7 antagonist. The level of insulin secretion in these preincubated cells exposed to the purinergic nucleotides increased in the following order: ATP > alpha, beta-methylene ATP >/= 2-methylthio-ATP. A pretreatment with foskolin and PDBu (100 nM) potentiated the increasing effect of ATP on insulin secretion. The Western blotting showed the expression of P2X7 and P2Y11 receptors.

CONCLUSIONS

: Purinergic stimulation has inhibitory activity on glucose-dependent insulin secretion through the activation of the P2X7 receptor, whereas it has enhancing effect through the activation of the P2Y11 receptor in HIT-T15 cells.

Volume-activated chloride channels in mice Leydig cells

Production and secretion of testosterone in Leydig cells are mainly controlled by the luteinizing hormone (LH). Biochemical evidences suggest that the activity of Cl(-) ions can modulate the steroidogenic process, but the specific ion channels involved are not known. Here, we extend the characterization of Cl(-) channels in mice Leydig cells (50-60 days old) by describing volume-activated Cl(-) currents (I(Cl,swell)). The amplitude of I(Cl,swell) is dependent on the osmotic gradient across the cell membrane, with an apparent EC(50) of approximately 75 mOsm. These currents display the typical biophysical signature of volume-activated anion channels (VRAC): dependence on intracellular ATP, outward rectification, inactivation at positive potentials, and selectivity sequence (I(- )> Cl(- )> F(-)). Staurosporine (200 nM) did not block the activation of I(Cl,swell). The block induced by 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB; 128 microM), SITS (200 microM), ATP (500 microM), pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate (PPADS; 100 miccroM), and Suramin (10 microM) were described by the permeant blocker model with apparent dissociation constant at 0 mV K(do) and fractional distance of the binding site (delta) of 334 microM and 47 %, 880 microM and 35 %, 2,100 microM and 49%, 188 microM and 27%, and 66.5 microM and 49%, respectively. These numbers were derived from the peak value of the currents. We conclude that I(Cl,swell) in Leydig cells are activated independently of purinergic stimulation, that Suramin and PPADS block these currents by a direct interaction with VRAC and that ATP is able to permeate this channel.

Electrophysiological and immunocytochemical evidence for P2X purinergic receptors in pancreatic beta cells

OBJECTIVES

Glucose-induced insulin secretion from pancreatic beta cells is modulated by several hormones and transmitters, namely adenosine triphosphate (ATP) via purinergic receptors. Although P2Y receptors are well documented in beta cells, the presence of P2X receptors remains elusive. We present the first electrophysiological evidence for the presence of P2X receptors in single beta cells of different species.

METHODS

Ionic currents were recorded from voltage-clamped beta cells near their resting potential using the perforated (nystatin) whole-cell patch-clamp configuration. Receptors were detected by immunocytochemistry.

RESULTS

When bathed in substimulatory (2 mM) glucose, mouse beta cells, isolated from islets displaying immunochemical colocalization of P2X1 or P2X3 receptors and insulin, developed large (approximately 250 pA/pF), rapidly activating, and then biexponentially decaying (tau1, approximately 20 milliseconds/tau2, approximately 1 second) inward currents on exposure to micromolar concentrations of ATP and alpha,beta-methylene ATP. The ATP also evoked inward currents (100-300 pA/pF) from porcine and human beta cells, albeit with a slower and more complex inactivation pattern.

CONCLUSIONS

The ATP-gated ion channels are present in pancreatic beta cells from different species. Specifically, mouse beta cells express rapidly desensitizing P2X1 and P2X3 receptors. Paracrine or neural activation of these receptors may contribute to the initial outburst of glucose- or acetylcholine-evoked insulin release, thus enhancing the islet secretory response.

Purinergic receptors in the endocrine and exocrine pancreas

The pancreas is a complex gland performing both endocrine and exocrine functions. In recent years there has been increasing evidence that both endocrine and exocrine cells possess purinergic receptors, which influence processes such as insulin secretion and epithelial ion transport. Most commonly, these processes have been viewed separately. In beta cells, stimulation of P2Y(1) receptors amplifies secretion of insulin in the presence of glucose. Nucleotides released from secretory granules could also contribute to autocrine/paracrine regulation in pancreatic islets. In addition to P2Y(1) receptors, there is also evidence for other P2 and adenosine receptors in beta cells (P2Y(2), P2Y(4), P2Y(6), P2X subtypes and A(1) receptors) and in glucagon-secreting alpha cells (P2X(7), A(2) receptors). In the exocrine pancreas, acini release ATP and ATP-hydrolysing and ATP-generating enzymes. P2 receptors are prominent in pancreatic ducts, and several studies indicate that P2Y(2), P2Y(4), P2Y(11), P2X(4) and P2X(7) receptors could regulate secretion, primarily by affecting Cl(-) and K(+) channels and intracellular Ca(2+) signalling. In order to understand the physiology of the whole organ, it is necessary to consider the full complement of purinergic receptors on different cells as well as the structural and functional relation between various cells within the whole organ. In addition to the possible physiological function of purinergic receptors, this review analyses whether the receptors could be potential therapeutic targets for drug design aimed at treatment of pancreatic diseases.

Characteristics of P2X7-like receptor activated by adenosine triphosphate in HIT-T15 cells

OBJECTIVES

The study examined the presence of a P2X7 receptor subtype and its functional roles in pancreatic beta cells.

METHODS

In a hamster beta-cell line, HIT-T15 cells, purinergic stimulation was investigated using fluorometry, electrophysiology, flow cytometry, and electrophoresis.

RESULTS

Adenosine triphosphate (ATP) and 2'-3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate (BzATP) increased in the intracellular free Ca2+ concentration, with an EC50 of 398.0 and 136.6 microM, respectively. Preincubation with oxidized ATP, a P2X7 receptor antagonist, inhibited the ATP- and BzATP-induced increase in the intracellular Ca2+ level. The BzATP-induced increase in the intracellular Ca2+ level was dependent on the extracellular Ca2+ concentration. The extracellular Mg2+ had a significant effect on the ATP-induced increase in the intracellular Ca2+ level. The ATP also induced depolarization like high potassium chloride. In the voltage-clamp experiments, ATP evoked inward currents, which were reversed at almost 0 mV. The ATP stimulated the slow influx of ethidium bromide, indicating permeability to larger molecules. Flow cytometry showed that the number of hypodiploid cells (A0), which are indicative of apoptosis, increased when the cells were exposed to ATP for 24 hours. The ATP also induced DNA fragmentation.

CONCLUSIONS

These results suggest that the HIT-T15 cells have endogenous P2X7-like receptors and that purinergic stimulation increased the level of intracellular Ca2+, depolarization, inward current, permeability, and apoptosis.

Pharmacology of P2X channels

Significant progress in understanding the pharmacological characteristics and physiological importance of homomeric and heteromeric P2X channels has been achieved in recent years. P2X channels, gated by ATP and most likely trimerically assembled from seven known P2X subunits, are present in a broad distribution of tissues and are thought to play an important role in a variety of physiological functions, including peripheral and central neuronal transmission, smooth muscle contraction, and inflammation. The known homomeric and heteromeric P2X channels can be distinguished from each other on the basis of pharmacological differences when expressed recombinantly in cell lines, but whether this pharmacological classification holds true in native cells and in vivo is less well-established. Nevertheless, several potent and selective P2X antagonists have been discovered in recent years and shown to be efficacious in various animal models including those for visceral organ function, chronic inflammatory and neuropathic pain, and inflammation. The recent advancement of drug candidates targeting P2X channels into human trials, confirms the medicinal exploitability of this novel target family and provides hope that safe and effective medicines for the treatment of disorders involving P2X channels may be identified in the near future.

P2X receptors in mouse Leydig cells

ATP-activated currents were studied in Leydig cells of mice with the patch-clamp technique. Whole cell currents were rapidly activating and slowly desensitizing (55% decrement from the peak value on exposure to 100 microM ATP for 60 s), requiring 3 min of washout to recover 100% of the response. The concentration-response relationships for ATP, adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS), and 2-methylthio-ATP (2-MeS-ATP) were described by the Hill equation with a concentration evoking 50% of maximal ATP response (K(d)) of 44, 110, and 637 microM, respectively, and a Hill coefficient of 2. The order of efficacy of agonists was ATP >or= ATPgammaS > 2-MeS-ATP > 2',3'-O-(4-benzoylbenzoyl)-ATP (BzATP). alphabeta-Methylene-ATP (alphabeta-MeATP), GTP, UTP, cAMP, and adenosine were ineffective. Suramin and pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) blocked the responses in a concentration-dependent manner. The ATP-activated currents were dependent on extracellular pH, being maximal at pH 6.5 and decreasing with both acidification and alkalinization (apparent dissociation constant (pK(a)) of 5.9 and 7.4, respectively). The whole cell current-voltage relationship showed inward rectification and reversed near 0 mV. Experiments performed in bi-ionic conditions for measurement of reversal potentials showed that this channel is highly permeable to calcium [permeability (P)(Ca)/P(Na) = 5.32], but not to chloride (P(Cl)/P(Na) = 0.03) or N-methyl-D-glucamine (NMDG) (P(NMDG)/P(Na) = 0.09). Unitary currents recorded in outside-out patches had a chord conductance of 27 pS (between -90 and -50 mV) and were inward rectifying. The average current passing through the excised patch decreased with time [time constant (tau) = 13 s], resembling desensitization of the macroscopic current. These findings indicate that the ATP receptor present in Leydig cells shows properties most similar to those of cloned homomeric P2X(2).

Developmental downregulation of P2X3 receptors in motoneurons of the compact formation of the nucleus ambiguus

Motoneurons of the compact division of the nucleus ambiguus (cNA) are the final output neurons of the swallowing pattern generator. Thus, their normal function is critical to neonatal survival. To explore the role of purinergic signaling in modulating the excitability of these motoneurons during development, immunohistochemical and whole-cell recording techniques were used to characterize expression patterns of ionotropic P2X receptors and the effects of ATP on cNA motoneurons. Medullary slices containing the cNA were prepared from neonatal (P0-4) and juvenile (P15-21) rats. In neonatal cNA motoneurons, local application of 1 mM ATP produced a large (-133 +/- 17 pA; n = 78), desensitizing, inward current that was mimicked by 1 mM alpha,beta meATP and 2meSATP, and inhibited by the P2 antagonist, PPADS (5 microM), and the P2X3 antagonist, A-317481 (0.1-1 mM). In juvenile cNA motoneurons, 1 mM ATP produced negligible currents, while 10 mM ATP produced small (-59 +/- 14 pA; n = 42), primarily non-desensitizing currents. Immunohistochemistry demonstrated that in the neonate, the expression of P2X3 was robust, P2X2 and P2X5 moderate, P2X4 and P2X6 weak, and P2X1 absent. In the juvenile cNA, only low levels of P2X5 and P2X6 labeling were detected. These data indicate that P2X receptors in cNA motoneurons are profoundly downregulated during the first two postnatal weeks, and suggest a role for the purinoceptor system, particularly P2X3 receptors, in the control of esophageal motor networks during early postnatal periods.

Distinct intracellular Ca2+ response to extracellular adenosine triphosphate in pancreatic beta-cells in rats and mice

Extracellular adenosine triphosphate (ATP) has distinct effects on insulin secretion from pancreatic beta-cells between rats and mice. Using a confocal microscope, we compared changes between rats and mice in cytosolic free calcium concentration ([Ca2+]c) in pancreatic beta-cells stimulated by extracellular ATP. Extracellular ATP (50 microM) induced calcium release from intracellular calcium stores by activating P2Y receptors in both rat and mouse beta-cells. The intracellular calcium release stimulated by extracellular ATP is significantly smaller in amplitude and longer in duration in rat beta-cells than in mouse. In response to extracellular ATP, rat beta-cells activate store-operated calcium entry following intracellular calcium release. This response is lacking in mouse beta-cells. Rat and mouse beta-cells both responded to 9 mM glucose by increasing [Ca2+]c. This increase, however, was pronounced only in the rat beta-cells. In 9 mM glucose, extracellular ATP induced a pronounced calcium release above the increased level of [Ca2+]c in rat beta-cells. In mouse beta-cells, however, extracellular ATP did not exhibit calcium release on top of the increased level of [Ca2+]c in 9 mM glucose. These results demonstrate distinct responses between rat and mouse beta-cells to extracellular ATP under the condition of low and high glucose. Considering that extracellular ATP inhibits insulin secretion from mouse beta-cells but stimulates insulin secretion from rat beta-cells, we suggest that store-operated Ca2+ entry may be related to exocytosis in pancreatic rat beta-cells.

Extracellular ATP as a signaling molecule for epithelial cells

The charge of this invited review is to present a convincing case for the fact that cells release their ATP for physiological reasons. Many of our "purinergic" colleagues as well as ourselves have experienced resistance to this concept, because it is teleologically counter-intuitive. This review serves to integrate the three main tenets of extracellular ATP signaling: ATP release from cells, ATP receptors on cells, and ATP receptor-driven signaling within cells to affect cell or tissue physiology. First principles will be discussed in the Introduction concerning extracellular ATP signaling. All possible cellular mechanisms of ATP release will then be presented. Use of nucleotide and nucleoside scavengers as well as broad-specificity purinergic receptor antagonists will be presented as a method of detecting endogenous ATP release affecting a biological endpoint. Innovative methods of detecting released ATP by adapting luciferase detection reagents or by using "biosensors" will be presented. Because our laboratory has been primarily interested in epithelial cell physiology and pathophysiology for several years, the role of extracellular ATP in regulation of epithelial cell function will be the focus of this review. For ATP release to be physiologically relevant, receptors for ATP are required at the cell surface. The families of P2Y G protein-coupled receptors and ATP-gated P2X receptor channels will be introduced. Particular attention will be paid to P2X receptor channels that mediate the fast actions of extracellular ATP signaling, much like neurotransmitter-gated channels versus metabotropic heptahelical neurotransmitter receptors that couple to G proteins. Finally, fascinating biological paradigms in which extracellular ATP signaling has been implicated will be highlighted. It is the goal of this review to convert and attract new scientists into the exploding field of extracellular nucleotide signaling and to convince the reader that extracellular ATP is indeed a signaling molecule.

P2X4 receptors induced in spinal microglia gate tactile allodynia after nerve injury

Pain after nerve damage is an expression of pathological operation of the nervous system, one hallmark of which is tactile allodynia-pain hypersensitivity evoked by innocuous stimuli. Effective therapy for this pain is lacking, and the underlying mechanisms are poorly understood. Here we report that pharmacological blockade of spinal P2X4 receptors (P2X4Rs), a subtype of ionotropic ATP receptor, reversed tactile allodynia caused by peripheral nerve injury without affecting acute pain behaviours in naive animals. After nerve injury, P2X4R expression increased strikingly in the ipsilateral spinal cord, and P2X4Rs were induced in hyperactive microglia but not in neurons or astrocytes. Intraspinal administration of P2X4R antisense oligodeoxynucleotide decreased the induction of P2X4Rs and suppressed tactile allodynia after nerve injury. Conversely, intraspinal administration of microglia in which P2X4Rs had been induced and stimulated, produced tactile allodynia in naive rats. Taken together, our results demonstrate that activation of P2X4Rs in hyperactive microglia is necessary for tactile allodynia after nerve injury and is sufficient to produce tactile allodynia in normal animals. Thus, blocking P2X4Rs in microglia might be a new therapeutic strategy for pain induced by nerve injury.

Expression and function of P2X purinoceptors in rat histaminergic neurons

(1) The pharmacology of ATP responses and the expression pattern of seven known subunits of the P2X receptor were investigated in individual histaminergic neurons of the tuberomamillary nucleus (TM). (2) ATP (3-1000 micro M) evoked fast non-desensitizing inward currents in TM neurons. 2-methylthioATP (2MeSATP) displayed the same efficacy but a lower potency, EC(50)s 84 micro M versus 48 micro M, when compared with ATP. Adenosine-diphosphate (ADP), uridine-triphosphate (UTP) and alpha beta methylene-ATP (alphabeta-meATP) were inactive. (3) ATP-mediated whole cell currents were potentiated by acidification of the recording solution (pH 7.5 and 6.6 were compared). (4) Single-cell RT-PCR (scRT-PCR) analysis revealed that the P2X(2) receptor is expressed in all PCR-positive neurons. Each of the P2X(1), P2X(3), P2X(4), P2X(5) and P2X(6) mRNAs were detected in less than 35% of the cells. (5) Suramin antagonized ATP responses with an IC(50) of 4.2 micro M and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 1 micro M) reduced ATP responses to 43% of control, when antagonists were pre-applied 90s before the agonist. Cibacron blue (3 micro M) given together with ATP potentiated control responses by 67%, but inhibited it to 10% after pre-application. (6) 2',3'-O-(2,4,6-Trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP) antagonized ATP responses with an IC(50) of 7 micro M. (7) Pharmacological properties of ATP responses together with scRT-PCR data suggest that P2X(2) is the major purinoceptor on the soma of TM neurons, however the presence of heteromeric P2X(2/5) receptors in some neurons cannot be excluded.

Molecular physiology of P2X receptors

P2X receptors are membrane ion channels that open in response to the binding of extracellular ATP. Seven genes in vertebrates encode P2X receptor subunits, which are 40-50% identical in amino acid sequence. Each subunit has two transmembrane domains, separated by an extracellular domain (approximately 280 amino acids). Channels form as multimers of several subunits. Homomeric P2X1, P2X2, P2X3, P2X4, P2X5, and P2X7 channels and heteromeric P2X2/3 and P2X1/5 channels have been most fully characterized following heterologous expression. Some agonists (e.g., alphabeta-methylene ATP) and antagonists [e.g., 2',3'-O-(2,4,6-trinitrophenyl)-ATP] are strongly selective for receptors containing P2X1 and P2X3 subunits. All P2X receptors are permeable to small monovalent cations; some have significant calcium or anion permeability. In many cells, activation of homomeric P2X7 receptors induces a permeability increase to larger organic cations including some fluorescent dyes and also signals to the cytoskeleton; these changes probably involve additional interacting proteins. P2X receptors are abundantly distributed, and functional responses are seen in neurons, glia, epithelia, endothelia, bone, muscle, and hemopoietic tissues. The molecular composition of native receptors is becoming understood, and some cells express more than one type of P2X receptor. On smooth muscles, P2X receptors respond to ATP released from sympathetic motor nerves (e.g., in ejaculation). On sensory nerves, they are involved in the initiation of afferent signals in several viscera (e.g., bladder, intestine) and play a key role in sensing tissue-damaging and inflammatory stimuli. Paracrine roles for ATP signaling through P2X receptors are likely in neurohypophysis, ducted glands, airway epithelia, kidney, bone, and hemopoietic tissues. In the last case, P2X7 receptor activation stimulates cytokine release by engaging intracellular signaling pathways.

Role of adenine nucleotides in insulin secretion from MIN6 pseudoislets

Insulin secretion from MIN6 cells configured as cell aggregates by culture on a gelatin substrate (pseudoislets) is enhanced compared to that of MIN6 cells grown as monolayers on tissue culture plastic, indicating the importance of beta-cell-to-beta-cell proximity for insulin release. In this study we have shown that glucose induced a biphasic release of insulin from pseudoislets, whereas the amplitude and duration of the responses of equivalent monolayer cells were much reduced. Purinergic aqonists have been implicated in intercellular communication between beta-cells, so we investigated whether adenine nucleotides co-released with insulin are responsible for the enhanced secretory responses of pseudoislets. We have demonstrated that MIN6 cells express purinergic A(1) and P2Y receptors, and that adenine nucleotides increased [Ca(2+)](i) with an efficacy of agonists being ATP > ADP > AMP. However, neither suramin nor the more selective A(1) antagonist 1,3-dipropyl-8-cyclopentylxanthine reduced glucose-induced insulin secretion from pseudoislets, and stimulation of monolayer cells with a range of adenine nucleotides did not enhance glucose-induced secretion. These results suggest that enhanced secretion from MIN6 pseudoislets is not due to increased paracrine/autocrine action of adenine nucleotides.

P2X purinergic receptor channel expression and function in bovine aortic endothelium

We examined bovine aortic endothelial cells (BAECs) for the functional expression of P2X receptors, the ATP-gated cation channels. We identified the P2X subtypes present in BAECs using RT-PCR. mRNA was present for only three of seven family members: P2X4, P2X5, and P2X7. We then characterized agonist-activated currents in whole cell and outside-out patch recordings using 2-methyl-thio-ATP (MeSATP) as a P2X4 and P2X5 receptor agonist and 2',3'-O-(4-benzoylbenzoyl)ATP (BzATP) as a P2X7 receptor agonist. MeSATP (10-20 microM) produced current with characteristics of P2X4 receptors. The current was an inwardly rectifying current, reversed near 0 mV, slowly desensitized, was not blocked by suramin (300 microM) or reactive blue (60 microM), and had a single channel conductance of 36 pS. BzATP (10-100 microM), on the other hand, activated a 9-pS channel with sustained activity in the continued presence of the agonist. BzATP-activated current was blocked by reactive blue (60 microM) and by suramin (approximately 50% block at 300 microM). We confirmed, by immunocytochemistry, the presence of P2X4 and P2X7 protein. The agonists failed, however, to induce significant uptake of the large molecule YO-PRO, indicating the lack of pore development that has been demonstrated for P2X7 and P2X4 in response to agonist in some cell types.

Heterogeneity of the functional expression of P2X3 and P2X2/3 receptors in the primary nociceptive neurons of rat

The properties and functional expression of the purinergic receptors in small (nociceptive) neurons acutely isolated from the DRG of rat were studied using whole-cell patch-clamp recording. The responses of small DRG neurons to ATP exhibited diverse kinetics and could be subdivided into three types: rapid, slow and mixed kinetics responses. Their affinities to agonists allowed to identify the responsible receptors as P2X3 ("fast") and heteromeric P2X2/3 ("slow") subtypes. The expression of different responses dramatically varied both on the neuron-to-neuron and animal-to-animal basis. Out of 744 neurons tested 24% of cells demonstrated predominance of functional P2X2/3 receptors, 44% had mixed representation and in 32% of cells P2X3 receptors dominated. All the animals tested (110) could be subdivided into 3 groups: in 19% of animals the response of each cell to ATP was mediated by P2X2/3 receptors, both types of ATP-evoked currents were found in 58% of animals and only in 23% of the animals P2X3 receptors dominated. Our results argue with exclusive role of P2X3 receptors in purinergic signaling in primary nociceptive neurons.

Pharmacology of cloned P2X receptors

There are seven P2X receptor cDNAs currently known. Six homomeric (P2X1, P2X2, P2X3, P2X4, P2X5, P2X7) and three heteromeric (P2X2/P2X3, P2X4/P2X6, P2X1/P2X5) P2X receptor channels have been characterized in heterologous expression systems. Homomeric P2X1 and P2X3 receptors are readily distinguishable by their rapid desensitization, the agonist action of alpha beta methyleneATP, and the block by 2',3'-O-(2,4,6-trinitrophenyl)-ATP. P2X2 receptors are unique among homomeric forms in their potentiation by low pH. Homomeric P2X4 receptors are much less sensitive to antagonism by suramin and pyridoxal 5-phosphate-6-azo-2',4'-disulfonic acid. Homomeric P2X7 receptors are the only form in which 2',3'-O-(4-benzoylbenzoyl)-ATP is more potent than ATP. The heteromeric P2X2/P2X3 receptor resembles P2X2 in slow desensitization kinetics and potentiation by low pH and is similar to P2X3 with respect to agonism by alpha beta methyleneATP and block by 2',3'-O-(2,4,6-trinitrophenyl)-ATP. Other agonists, antagonists, and ions that can be used to differentiate among the receptors are discussed.

Inhibition by ethanol of rat P2X(4) receptors expressed in Xenopus oocytes

1. The effect of ethanol on the function of P2X(4) receptors expressed in Xenopus oocytes was studied using two-electrode voltage-clamp recording. 2. The amplitude of current activated by 1 microM ATP was decreased by ethanol in a concentration-dependent manner over the concentration range 1 - 500 mM. The concentration of ethanol that produced 50% inhibition (IC(50)) of current activated by 1 microM ATP was 58 mM. 3. Ethanol inhibition of ATP-activated current was not dependent on membrane potential from -60 to +20 mV, and ethanol did not change the reversal potential of ATP-activated current. 4. Ethanol, 50 mM, shifted the ATP concentration-response curve to the right, increasing the EC(50) for ATP from 9.1 to 16.0 microM, but did not reduce the maximal response to ATP. 5. The results suggest that ethanol may inhibit P2X(4) receptors by decreasing the apparent affinity of the binding site for ATP. 6. Since the P2X(4) receptor is the most abundant P2X subunit in the brain, these receptors could be important effectors of ethanol action in the central nervous system.

Procedures to characterize and study P2Z/P2X7 purinoceptor: flow cytometry as a promising practical, reliable tool

The expression of P2Z/P2X7 purinoceptor in different cell types is well established. This receptor is a member of the ionotropic P2X receptor family, which is composed by seven cloned receptor subtypes (P2X1 - P2X7). Interestingly, the P2Z/P2X7 has a unique feature of being linked to a non-selective pore which allows the passage of molecules up to 900 Da depending on the cell type. Early studies of P2Z/P2X7 purinoceptor were exclusively based on classical pharmacological studies but the recent tools of molecular biology have enriched the analysis of the receptor expression. The majority of assays and techniques chosen so far to study the expression of P2Z/P2X7 receptor explore directly or indirectly the effects of the opening of P2Z/P2X7 linked pore. In this review we describe the main techniques used to study the expression and functionality of P2Z/P2X7 receptor. Additionally, the increasing need and importance of a multifunctional analysis of P2Z/P2X7 expression based on flow cytometry technology is discussed, as well as the adoption of a more complete analysis of P2Z/P2X7 expression involving different techniques.

Salivary gland P2 nucleotide receptors

The effects of ATP on salivary glands have been recognized since 1982. Functional and pharmacological studies of the P2 nucleotide receptors that mediate the effects of ATP and other extracellular nucleotides have been supported by the cloning of receptor cDNAs, by the expression of the receptor proteins, and by the identification in salivary gland cells of multiple P2 receptor subtypes. Currently, there is evidence obtained from pharmacological and molecular biology approaches for the expression in salivary gland of two P2X ligand-gated ion channels, P2Z/P2X7 and P2X4, and two P2Y G protein-coupled receptors, P2Y1 and P2Y2. Activation of each of these receptor subtypes increases intracellular Ca2+, a second messenger with a key role in the regulation of salivary gland secretion. Through Ca2+ regulation and other mechanisms, P2 receptors appear to regulate salivary cell volume, ion and protein secretion, and increased permeability to small molecules that may be involved in cytotoxicity. Some localization of the various salivary P2 receptor subtypes to specific cells and membrane subdomains has been reported, along with evidence for the co-expression of multiple P2 receptor subtypes within specific salivary acinar or duct cells. However, additional studies in vivo and with intact organ preparations are required to define clearly the roles the various P2 receptor subtypes play in salivary gland physiology and pathology. Opportunities for eventual utilization of these receptors as pharmacotherapeutic targets in diseases involving salivary gland dysfunction appear promising.

Purinoceptors evoke different electrophysiological responses in pancreatic ducts. P2Y inhibits K(+) conductance, and P2X stimulates cation conductance

In epithelia, extracellular nucleotides are often associated with regulation of ion transporters, especially Cl(-) channels. In this study, we investigated which purinoceptors are present in native pancreatic ducts and how they regulate ion transport. We applied whole-cell patch-clamp recordings, intracellular Ca(2+) and pH measurements, and reverse transcription-polymerase chain reaction (RT-PCR) analysis. The data show two types of purinoceptors and cellular responses. UTP and ATP produced large Ca(2+) transients, a decrease in intracellular pH, 8-10-mV depolarization of the membrane voltage, and a decrease in the whole-cell conductance. The membrane effects were due to closure of K(+) channels, as confirmed by dependence on extracellular K(+). UTP/ATP effects could be associated with P2Y(2) purinoceptors, and RT-PCR revealed mRNAs for P2Y(2) and P2Y(4) receptors. On the other hand, 2', 3'-O-4-benzoylbenzoyl-ATP induced Ca(2+) influx and approximately 20-mV depolarization of the membrane voltage with a concomitant increase in the whole-cell conductance. These effects were dependent on extracellular Na(+), not Cl(-), indicating opening of cation channels associated with P2X(7) purinoceptors. RT-PCR showed mRNAs for P2X(7) and P2X(4) receptors. In microperfused ducts, luminal (but not basolateral) ATP caused large depolarizations of membrane voltages recorded with microelectrodes, consistent with luminal localization of P2X(7) receptors. Thus, P2Y(2) (and possibly P2Y(4)) purinoceptors inhibit K(+) channels and may not support secretion in native ducts. P2X(7) (and possibly P2X(4)) receptors are associated with cation channels and may contribute to regulation of secretion.

Identification of amino acids within the P2X2 receptor C-terminus that regulate desensitization

1. The ATP-activated P2X2(a) and P2X2(b) receptor splice variants, which differ only in their C-terminal sequences, desensitize at different rates. We used mutational analysis to investigate the involvement of the C-terminal region in receptor desensitization. Rat wild-type and mutant P2X2 receptors were expressed in Xenopus oocytes and currents were measured using the two-electrode voltage-clamp technique. 2. Truncating P2X2 at the Lys369 splice site increased the rate of desensitization by >100-fold. Recovery from desensitization was slowed by approximately 5-fold. 3. Addition of Val370 onto the C-terminus of the truncated receptor slowed desensitization by approximately 70-fold. Point mutations that substituted either smaller or larger hydrophobic amino acids for Val370, within the P2X2(a) splice variant, had profound effects on the rate of desensitization. The rate decreased with increasing hydrophobicity but was not dependent upon the precise structure of the side group. 4. A mutant receptor, with only nine amino acids, Val-Asp-Pro-Lys-Gly-Leu-Ala-Gln-Leu, beyond the Lys369 splice site, desensitized at a similar rate to P2X2(a). Injection of the peptide of this sequence into oocytes expressing P2X2(a) increased the rate of desensitization, whereas the eight-residue peptide lacking the valine had no effect. 5. Neutralizing lysines in the vicinity of the splice site increased the rate of receptor desensitization. Substituting glutamine for Lys365 produced the greatest effect ( approximately 30-fold increase), whereas mutating lysines that were further upstream or downstream of this position had progressively less of an effect. 6. We conclude that the C-terminal splice site of the P2X2 receptor is located within a region that is critically involved in regulating the rate of receptor desensitization. The valine at position 370 interacts with an intracellular hydrophobic site to slow the rate of desensitization. Nearby lysines may facilitate this interaction.

Single channel properties of P2X2 purinoceptors

The single channel properties of cloned P2X2 purinoceptors expressed in human embryonic kidney (HEK) 293 cells and Xenopus oocytes were studied in outside-out patches. The mean single channel current-voltage relationship exhibited inward rectification in symmetric solutions with a chord conductance of approximately 30 pS at -100 mV in 145 mM NaCl. The channel open state exhibited fast flickering with significant power beyond 10 kHz. Conformational changes, not ionic blockade, appeared responsible for the flickering. The equilibrium constant of Na+ binding in the pore was approximately 150 mM at 0 mV and voltage dependent. The binding site appeared to be approximately 0.2 of the electrical distance from the extracellular surface. The mean channel current and the excess noise had the selectivity: K+ > Rb+ > Cs+ > Na+ > Li+. ATP increased the probability of being open (Po) to a maximum of 0.6 with an EC50 of 11.2 microM and a Hill coefficient of 2.3. Lowering extracellular pH enhanced the apparent affinity of the channel for ATP with a pKa of approximately 7.9, but did not cause a proton block of the open channel. High pH slowed the rise time to steps of ATP without affecting the fall time. The mean single channel amplitude was independent of pH, but the excess noise increased with decreasing pH. Kinetic analysis showed that ATP shortened the mean closed time but did not affect the mean open time. Maximum likelihood kinetic fitting of idealized single channel currents at different ATP concentrations produced a model with four sequential closed states (three binding steps) branching to two open states that converged on a final closed state. The ATP association rates increased with the sequential binding of ATP showing that the binding sites are not independent, but positively cooperative. Partially liganded channels do not appear to open. The predicted Po vs. ATP concentration closely matches the single channel current dose-response curve.

Functional and molecular diversity of purinergic ion channel receptors

P2X receptors are membrane ion channels gated by extracellular adenosine 5'-triphosphate (ATP); nucleotides also activate a family of seven transmembrane G protein-coupled receptors (P2Y). P2X receptors are widely expressed on mammalian cells, where they can be broadly differentiated into three groups. The first group is almost equally well activated by ATP and its analog alpha beta methyleneATP (alpha beta meATP), whereas a second group is not activated by alpha beta meATP. A third-group type of receptor (termed P2Z) is distinguished by the fact that the channel opening is followed by cell permeabilization and lysis if the agonist application is continued for more than a few seconds. Seven cDNAs have been cloned that encode P2X receptor subunits. When expressed individually in heterologous systems, P2X1 and P2X3 subunits form channels activated by ATP or alpha beta meATP; whereas P2X2, P2X4, and P2X5 form channels activated by ATP but not alpha beta meATP. P2X6 receptors do not express readily, and P2X7 receptors correspond closely in their properties to P2Z. Further phenotypes can be produced when two subunits are coexpressed, indicating hetero-multimerization. This chapter compares the properties of the native P2X receptors with those of the cloned and expressed subunits.

Molecular cloning, functional characterization and possible cooperativity between the murine P2X4 and P2X4a receptors

We have cloned and functionally characterised the mouse orthologue of the P2X4 receptor, mP2X4, and a splice variant of this receptor, mP2X4a. mP2X4 is 388 amino acids in length and shares 94% and 87% identity with the rat and human P2X4 receptors, respectively, while mP2X4a is 361 amino acids in length and lacks a 27-amino acid region in the extracellular domain corresponding to exon 6 of the known P2X receptor gene structures. When expressed in Xenopus laevis oocytes, mP2X4 produces a rapid inward current in response to ATP with an EC50 of 1.68+/-0.2 microM, consistent with the affinity of the rat and human P2X4 receptors for ATP. This agonist response is potentiated by the P2X receptor antagonists suramin, Reactive blue 2 and, over a limited concentration range, by PPADS. Although mP2X4a forms a poorly functional homomeric receptor, it appears able to interact with the full-length mP2X4 subunit to result in a functional channel with a reduced affinity for ATP. These results suggest a possible role for splice variants of P2X receptors in the formation of functional heteromeric ion channels.

Modulation of ATP-responses at recombinant rP2X4 receptors by extracellular pH and zinc

The modulatory effects of extracellular H+ and Zn2+ were tested against ATP-responses at rat P2X4 (rP2X4) receptors expressed in Xenopus oocytes under voltage-clamp conditions. ATP (0.1-100 microM, at pH 7.5), evoked inward currents via rP2X4 receptors (EC50 value, 4.1+/-0.98 microM; nH, 1.2+/-0.1). ATP potency was reduced 2 fold, at pH 6.5, without altering maximal activity. ATP potency was reduced by a further 4 fold, at pH 5.5, and the maximal activity of ATP was also reduced. Alkaline conditions (pH 8.0) had no effect on ATP-responses. Zn2+ (100 nM - 10 microM) potentiated ATP-responses at the rP2X4 receptor by 2 fold, whereas higher concentrations (30 microM - 1 mM) inhibited ATP-responses. Zn2+ potentiation was due to an increase in ATP potency, whereas its inhibitory action was due to a reduction in ATP efficacy. Zn2+ modulation of ATP-responses was pH-dependent. At pH 6.5, the bell-shaped curve for Zn2+ was shifted to the right by 1 log unit. At pH 5.5, Zn2+ potentiation was abolished and its inhibitory effect reduced considerably. Suramin (50 microM) also potentiated ATP-responses at rP2X4 receptors. Neither H+ (pH 6.5 and 5.5), Zn2+ (10-100 microM) or a combination of both failed to reveal an inhibitory action of suramin at rP2X4 receptors. In conclusion, H+ and Zn2+ exerted opposite effects on the rP2X4 receptor by lowering and raising agonist potency, respectively. H+ (> or = 3 microM) and Zn2+ (> or = 30 microM) also reduces agonist efficacy by lowering the number of rP2X4 receptors available for activation. The striking differences between the modulatory actions of H+ and Zn2+ at rP2X4 and rP2X2 receptors are discussed.

Cibacron blue allosterically modulates the rat P2X4 receptor

We have used whole-cell patch clamp electrophysiology to characterise the actions of the P2 antagonist, cibacron blue, on the rat recombinant P2X4 receptor, stably expressed in human embryonic kidney 293 (HEK293) cells. In single cells, adenosine triphosphate (ATP) evoked inward currents, but the response was subject to considerable run down which precluded obtaining quantitative data. However, when recordings were made from cells that were part of a group of 20-40 electrically coupled cells (cell rafts), run-down of current was not observed and reproducible responses could be obtained. When studied using cell rafts, cibacron blue was a weak antagonist of the rat P2X4 receptor (IC50 > 300 microM) when co-applied with ATP. However, when cell rafts were preincubated with low concentrations of cibacron blue (3-30 microM) for 5 min prior to ATP addition, cibacron blue increased responses to ATP by increasing its potency (up to 4-fold) without affecting the maximum current. Potentiation of ATP-evoked currents was also observed following washout of high, inhibitory concentrations of cibacron blue (300 microM). In contrast to these effects on P2X4 receptors, cibacron blue inhibited the ATP-induced response in both single cells and rafts of HEK293 cells expressing the P2X2 receptor (IC50 approximately 600-800 nM). The effects of cibacron blue on the P2X4 receptor were quantitatively similar to those of Zn2+ which also increased ATP-evoked currents by decreasing the EC50 of ATP (up to 3.5-fold). These data are consistent with the concept that cibacron blue, like zinc, allosterically regulates the function of the P2X4 receptor.

Evidence for two different types of P2 receptors stimulating insulin secretion from pancreatic B cell

Adenine nucleotides have been shown to stimulate insulin secretion by acting on P2 receptors of the P2Y type. Since there have been some discrepancies in the insulin response of different analogues of ATP and ADP, we investigated whether two different types of P2 receptors exist on pancreatic B cells. The effects of alpha,beta-methylene ATP, which is more specific for the P2X subtype, were studied in vitro in pancreatic islets and isolated perfused pancreas from rats, in comparison with the potent P2Y receptor agonist ADPbetaS. In isolated islets, incubated with a slightly stimulating glucose concentration (8.3 mM), alpha,beta-me ATP (200 microM) and ADPbetaS (50 microM) similarly stimulated insulin secretion; by contrast, under a non stimulating glucose concentration (3 mM), alpha,beta-me ATP was still effective whereas ADPbetaS was not. In the same way, in islets perifused with 3 mM glucose, alpha,beta-me ATP but not ADPbetaS induced a partial but significant reduction in the peak 86Rb efflux induced by the ATP-dependent potassium channel opener diazoxide. In the isolated pancreas, perfused with a non stimulating glucose concentration (4.2 mM), ADPbetaS and alpha,beta-me ATP (5-50 microM), administered for 10 min, induced an immediate, transient and concentration-dependent increase in the insulin secretion; their relative potency was not significantly different. In contrast, with a slightly stimulating glucose concentration (8.3 mM), ADPbetaS was previously shown to be 100 fold more potent than alpha,beta-me ATP. Furthermore, at 4.2 mM glucose a second administration of alpha,beta-me ATP was ineffective. In the same way, ADPbetaS was also able to desensitize its own insulin response. At 3 mM glucose, alpha,beta-me ATP as well as ADPbetaS (50 microM) induced a transient stimulation of insulin secretion and down regulated the action of each other. These results give evidence that pancreatic B cells, in addition to P2Y receptors, which potentiate glucose-induced insulin secretion, are provided with P2X receptors, which transiently stimulate insulin release at low non-stimulating glucose concentration and slightly affect the potassium conductance of the membrane.