Cell-specific behavior of P2X7 receptors in mouse parotid acinar and duct cells

P2 Receptors as Therapeutic Targets in the Salivary Gland: From Physiology to Dysfunction

Although often overlooked in our daily lives, saliva performs a host of necessary physiological functions, including lubricating and protecting the oral cavity, facilitating taste sensation and digestion and maintaining tooth enamel. Therefore, salivary gland dysfunction and hyposalivation, often resulting from pathogenesis of the autoimmune disease Sjögren's syndrome or from radiotherapy of the head and neck region during cancer treatment, severely reduce the quality of life of afflicted patients and can lead to dental caries, periodontitis, digestive disorders, loss of taste and difficulty speaking. Since their initial discovery in the 1970s, P2 purinergic receptors for extracellular nucleotides, including ATP-gated ion channel P2X and G protein-coupled P2Y receptors, have been shown to mediate physiological processes in numerous tissues, including the salivary glands where P2 receptors represent a link between canonical and non-canonical saliva secretion. Additionally, extracellular nucleotides released during periods of cellular stress and inflammation act as a tissue alarmin to coordinate immunological and tissue repair responses through P2 receptor activation. Accordingly, P2 receptors have gained widespread clinical interest with agonists and antagonists either currently undergoing clinical trials or already approved for human use. Here, we review the contributions of P2 receptors to salivary gland function and describe their role in salivary gland dysfunction. We further consider their potential as therapeutic targets to promote physiological saliva flow, prevent salivary gland inflammation and enhance tissue regeneration.

The Role of the P2X7 Receptor in Ocular Stresses: A Potential Therapeutic Target

The P2X7 receptor is expressed in both anterior and posterior segments of the eyeball. In the ocular surface, the P2X7 receptor is activated in case of external aggressions: preservatives and surfactants induce the activation of P2X7 receptors, leading to either apoptosis, inflammation, or cell proliferation. In the retina, the key endogenous actors of age-related macular degeneration, diabetic retinopathy, and glaucoma act through P2X7 receptors’ activation and/or upregulation of P2X7 receptors’ expression. Different therapeutic strategies aimed at the P2X7 receptor exist. P2X7 receptor antagonists, such as divalent cations and Brilliant Blue G (BBG) could be used to target either the ocular surface or the retina, as long as polyunsaturated fatty acids may exert their effects through the disruption of plasma membrane lipid rafts or saffron that reduces the response evoked by P2X7 receptor stimulation. Treatments against P2X7 receptor activation are proposed by using either eye drops or food supplements.

Airway epithelial anion secretion and barrier function following exposure to fungal aeroallergens: role of oxidative stress

Aeroallergens produced by Alternaria alternata can elicit life-threatening exacerbations of asthma in patients sensitized to this fungus. In this study, the effect of Alternaria on ion transport mechanisms underlying mucociliary clearance and airway epithelial barrier function was investigated in human airway epithelial cells. Apical exposure to Alternaria induced an increase in anion secretion that was inhibited by blockers of CFTR and Ca²⁺-activated Cl^- channels. Stimulation of anion secretion was dependent on Ca²⁺ uptake from the apical solution. Alternaria exposure also produced an increase in reactive oxygen species (ROS) that was blocked by pretreatment with the oxidant scavenger glutathione (GSH). GSH and the NADPH oxidase inhibitor/complex 1 electron transport inhibitor diphenylene iodonium chloride (DPI) blocked ATP release and the increase in intracellular [Ca²⁺] evoked by AlternariaAlternaria also decreased transepithelial resistance, and a portion of this effect was dependent on the increase in ROS. However, the Alternaria-induced increase in unidirectional dextran (molecular mass = 4,000 Da) flux across the epithelium could not be accounted for by increased oxidative stress. These results support the conclusion that oxidative stress induced by Alternaria was responsible for regulating Ca²⁺-dependent anion secretion and tight junction electrical resistance that would be expected to affect mucociliary clearance.

P2Y purinoceptors mediate ATP-induced changes in intracellular calcium and amylase release in acinar cells of mouse parotid glands

Adenosine 5'-triphosphate (ATP) can act as an extracellular signal that regulates various cellular functions. The present study aimed to determine which purinoceptors play a role in ATP-induced changes in intracellular Ca(2+) ([Ca(2+)]i) and amylase secretion in mouse parotid glands. ATP induced a steep increase in [Ca(2+)]i in acinar cells. The removal of extracellular Ca(2+) or the use of Ca(2+) channel blockers slightly inhibited this increase. Inhibition of PLCγ by U73122 and of IP3 by xestospongin C did not completely block this increase. The purinoceptor antagonists suramin and reactive blue-2 strongly inhibited the ATP-induced changes in [Ca(2+)]i. 2-MeSATP induced a strong increase in [Ca(2+)]i, while Bz-ATP induced a small [Ca(2+)]i increase, and UTP and α,β-MeATP had no effect. The potency order of ATP analogs (2-MeSATP > ATP >> UTP) suggested that P2Y1 and P2Y12 play a significant role in the cellular response to ATP. RT-PCR revealed that P2X2,4,7 and P2Y1,2,10,12,14 were expressed in acinar cells. Ca(2+)-dependent exocytotic secretion of amylase was detected in parotid glands. These findings indicated that ATP activates P2Y receptors more than P2X receptors at low concentrations. Thus, P2Y receptors were found to be the main receptors involved in Ca(2+)-related cell homeostasis and amylase secretion in mouse parotid glands.

Aquaporins in Salivary Glands: From Basic Research to Clinical Applications

Salivary glands are involved in saliva secretion that ensures proper oral health. Aquaporins are expressed in salivary glands and play a major role in saliva secretion. This review will provide an overview of the salivary gland morphology and physiology of saliva secretion, and focus on the expression, subcellular localization and role of aquaporins under physiological and pathophysiological conditions, as well as clinical applications involving aquaporins. This review is highlighting expression and localization of aquaporins in human, rat and mouse, the most studied species and is pointing out possible difference between major salivary glands, i.e., parotid, submandibular and sublingual glands.

Purinergic signalling in the gastrointestinal tract and related organs in health and disease

Purinergic signalling plays major roles in the physiology and pathophysiology of digestive organs. Adenosine 5'-triphosphate (ATP), together with nitric oxide and vasoactive intestinal peptide, is a cotransmitter in non-adrenergic, non-cholinergic inhibitory neuromuscular transmission. P2X and P2Y receptors are widely expressed in myenteric and submucous enteric plexuses and participate in sympathetic transmission and neuromodulation involved in enteric reflex activities, as well as influencing gastric and intestinal epithelial secretion and vascular activities. Involvement of purinergic signalling has been identified in a variety of diseases, including inflammatory bowel disease, ischaemia, diabetes and cancer. Purinergic mechanosensory transduction forms the basis of enteric nociception, where ATP released from mucosal epithelial cells by distension activates nociceptive subepithelial primary afferent sensory fibres expressing P2X3 receptors to send messages to the pain centres in the central nervous system via interneurons in the spinal cord. Purinergic signalling is also involved in salivary gland and bile duct secretion.

Purinergic receptor P2X₇: a novel target for anti-inflammatory therapy

Purinergic receptors, also known as purinoceptors, are ligand gated membrane ion channels involved in many cellular functions. Among all identified purinergic receptors, P2X₇ subform is unique since it induces the caspase activity, cytokine secretion, and apoptosis. The distribution of P2X₇ receptors, and the need of high concentration of ATP required to activate this receptor exhibited its ability to function as 'danger' sensor associated with tissue inflammation and damage. Further, the modulation of other signalling pathways associated with P2X₇ has also been proposed to play an important role in the control of macrophage functions and inflammatory responses, especially towards lipopolysaccharides. Experimentally, researchers have also observed the decreased severity of inflammatory responses in P2X₇ receptor expressing gene (P2RX₇) knockout (KO) phenotypes. Therefore, newly developed potent antagonists of P2X₇ receptor would serve as novel therapeutic agents to combat various inflammatory conditions. In this review article, we tried to explore various aspects of P2X₇ receptors including therapeutic potential, and recent discoveries and developments of P2X₇ receptor antagonists.

The P2X7 receptor-inflammasome complex has a role in modulating the inflammatory response in primary Sjögren's syndrome

OBJECTIVE

Innate and adaptive immunity may contribute to gland dysfunction in patients with primary Sjögren's syndrome (pSS). The P2X7 receptor (P2X7 R)-NLRP3 inflammasome complex modulates the release of the inflammatory cytokines IL-1β and IL-18. The presence of P2X7 R in salivary glands suggests an interesting scenario for the initiation and amplification of the innate immune response in pSS. Therefore, the aim of this study was to assess the role of the P2X7 R-NLRP3 inflammasome in pSS.

SUBJECTS AND METHODS

Twenty-one consecutive patients with pSS according to the American-European Consensus Group criteria and 15 patients with sicca syndrome (i.e. without Sjögren's syndrome, non-SS) were enrolled in this study, together with six control (CTL) subjects. Expression of the P2X7R-NLRP3 platform and IL-18 was determined by real-time PCR and western blotting in gland specimens and peripheral lymphomonocytes; data were related to patients\x92 clinical, serological and histopathological characteristics. The presence of IL-18 was determined in gland and saliva samples.

RESULTS

P2X7 R expression was significantly higher in salivary glands from individuals with pSS than in those from non-SS and CTL subjects. Accordingly, the gene expression levels of the inflammasome components NLRP3, ASC and caspase-1 were significantly higher in pSS gland specimens, and this was paralleled by an increased expression of mature IL-18 in pSS saliva samples. The expression of both the P2X7 R and the inflammasome components was a marker of disease-related glandular involvement, being increased in patients with anti-Ro/SSA positivity and correlated with focus score.

CONCLUSION

The results of this study suggest an involvement of the P2X7 R-inflammasome-caspase-1-IL-18 axis in the development of pSS exocrinopathy. This finding provides the basis for studying the complex mechanisms underlying pSS, as well as for developing novel potential therapeutic strategies.

Is pannexin the pore associated with the P2X7 receptor?

The P2X7 receptor (P2X7R), an ATP-gated cation channel, is expressed predominantly in leukocytes. Activation of P2X7R has been implicated in the formation of a cytolytic pore (i.e., a large conductance channel) that allows the passage of molecules up to 900 Da in macrophages. At least two hypotheses have been presented to explain the conversion of a nonselective cation channel to a cytolytic pore. One hypothesis suggests that the pore is a separate molecular structure activated by P2X7R, and the second asserts that this is an intrinsic property of P2X7R (pore dilation). Based on connexin knockout and hemichannel antagonist studies, some groups have concluded that connexins and pannexins, the hemichannel-forming proteins in vertebrates, are fundamental components of the large conductance channel associated with P2X7R. Dye uptake and electrophysiology experiments were used to evaluate the efficacy and specificity of some hemichannel antagonists under conditions known to open the large conductance channel associated with P2X7R. Hemichannel antagonists and interference RNA (RNAi) targeting pannexin-1 did not affect P2X7R macroscopic currents [ATP, 1,570±189 pA; ATP+100 μM carbenoxolone (CBX), 1,498±100 pA; ATP+1 mM probenecid (Prob), 1,522±9 pA] or dye uptake in a FACS assay (ATP, 63±5%; ATP+100 μM CBX, 51.51±8.4%; ATP+1 mM Prob, 57.7±4.3%) in mouse macrophages. These findings strongly suggest that the high-permeability pore evident after prolonged P2X7R activation does not occur through connexin or pannexin hemichannels in murine macrophages. Another membrane protein may be involved in P2X7R pore formation.

The second transmembrane domain of P2X7 contributes to dilated pore formation

Activation of the purinergic receptor P2X7 leads to the cellular permeability of low molecular weight cations. To determine which domains of P2X7 are necessary for this permeability, we exchanged either the C-terminus or portions of the second transmembrane domain (TM2) with those in P2X1 or P2X4. Replacement of the C-terminus of P2X7 with either P2X1 or P2X4 prevented surface expression of the chimeric receptor. Similarly, chimeric P2X7 containing TM2 from P2X1 or P2X4 had reduced surface expression and no permeability to cationic dyes. Exchanging the N-terminal 10 residues or C-terminal 14 residues of the P2X7 TM2 with the corresponding region of P2X1 TM2 partially restored surface expression and limited pore permeability. To further probe TM2 structure, we replaced single residues in P2X7 TM2 with those in P2X1 or P2X4. We identified multiple substitutions that drastically changed pore permeability without altering surface expression. Three substitutions (Q332P, Y336T, and Y343L) individually reduced pore formation as indicated by decreased dye uptake and also reduced membrane blebbing in response to ATP exposure. Three others substitutions, V335T, S342G, and S342A each enhanced dye uptake, membrane blebbing and cell death. Our results demonstrate a critical role for the TM2 domain of P2X7 in receptor function, and provide a structural basis for differences between purinergic receptors.

Molecular mechanism of pancreatic and salivary gland fluid and HCO3 secretion

Fluid and HCO(3)(-) secretion is a vital function of all epithelia and is required for the survival of the tissue. Aberrant fluid and HCO(3)(-) secretion is associated with many epithelial diseases, such as cystic fibrosis, pancreatitis, Sjögren's syndrome, and other epithelial inflammatory and autoimmune diseases. Significant progress has been made over the last 20 years in our understanding of epithelial fluid and HCO(3)(-) secretion, in particular by secretory glands. Fluid and HCO(3)(-) secretion by secretory glands is a two-step process. Acinar cells secrete isotonic fluid in which the major salt is NaCl. Subsequently, the duct modifies the volume and electrolyte composition of the fluid to absorb the Cl(-) and secrete HCO(3)(-). The relative volume secreted by acinar and duct cells and modification of electrolyte composition of the secreted fluids varies among secretory glands to meet their physiological functions. In the pancreas, acinar cells secrete a small amount of NaCl-rich fluid, while the duct absorbs the Cl(-) and secretes HCO(3)(-) and the bulk of the fluid in the pancreatic juice. Fluid secretion appears to be driven by active HCO(3)(-) secretion. In the salivary glands, acinar cells secrete the bulk of the fluid in the saliva that is driven by active Cl(-) secretion and contains high concentrations of Na(+) and Cl(-). The salivary glands duct absorbs both the Na(+) and Cl(-) and secretes K(+) and HCO(3)(-). In this review, we focus on the molecular mechanism of fluid and HCO(3)(-) secretion by the pancreas and salivary glands, to highlight the similarities of the fundamental mechanisms of acinar and duct cell functions, and to point out the differences to meet gland-specific secretions.

Distinct contributions by ionotropic purinoceptor subtypes to ATP-evoked calcium signals in mouse parotid acinar cells

There is emerging consensus that P2X₄ and P2X₇ ionotropic purinoceptors (P2X₄R and P2X₇R) are critical players in regulating [Ca²⁺]i dynamics and fluid secretion in the salivary gland. In contrast, details regarding their compartmentalization and selective activation, contributions to the spatiotemporal properties of intracellular signals and roles in regulating protein exocytosis and ion channel activity have remained largely undefined. To address these concerns, we profiled mouse parotid acinar cells using live-cell imaging to follow the spatial and temporal features of ATP-evoked Ca²⁺ dynamics and exocytotic activity. Selective activation of P2X7Rs revealed an apical-to-basal [Ca²⁺]i signal that initiated at the sub-luminal border and propagated with a wave speed estimated at 17.3 ± 4.3 μm s⁻¹ (n =6). The evoked Ca²⁺ spike consisted of Ca²⁺ influx and Ca²⁺-induced Ca²⁺ release from intracellular Ca²⁺ channels. In contrast, selective activation of P2X₄Rs induced a Ca²⁺ signal that initiated basally and propagated toward the lumen with a wave speed of 4.3 ± 0.2 μm s⁻¹ (n =8) that was largely independent of intracellular Ca²⁺ channel blockade. Consistent with these observations, P2X₇R expression was enriched in the sub-luminal regions of acinar cells while P2X₄R appeared localized to basal areas. In addition, we showed that P2X₄R and P2X₇R activation evokes exocytosis in parotid acinar cells. Our studies also demonstrate that the P2X₄R-mediated [Ca²⁺]i rise and subsequent protein exocytosis was enhanced by ivermectin (IVR). Thus, in addition to furthering our understanding of salivary gland physiology, this study identifies P2X₄R as a potential target for treatment of salivary hypofunction diseases.

Identification of human P2X1 receptor-interacting proteins reveals a role of the cytoskeleton in receptor regulation

P2X1 receptors are ATP-gated ion channels expressed by smooth muscle and blood cells. Carboxyl-terminally His-FLAG-tagged human P2X1 receptors were stably expressed in HEK293 cells and co-purified with cytoskeletal proteins including actin. Disruption of the actin cytoskeleton with cytochalasin D inhibited P2X1 receptor currents with no effect on the time course of the response or surface expression of the receptor. Stabilization of the cytoskeleton with jasplakinolide had no effect on P2X1 receptor currents but decreased receptor mobility. P2X2 receptor currents were unaffected by cytochalasin, and P2X1/2 receptor chimeras were used to identify the molecular basis of actin sensitivity. These studies showed that the intracellular amino terminus accounts for the inhibitory effects of cytoskeletal disruption similar to that shown for lipid raft/cholesterol sensitivity. Stabilization of the cytoskeleton with jasplakinolide abolished the inhibitory effects of cholesterol depletion on P2X1 receptor currents, suggesting that lipid rafts may regulate the receptor through stabilization of the cytoskeleton. These studies show that the cytoskeleton plays an important role in P2X1 receptor regulation.

Purinergic signalling in epithelial ion transport: regulation of secretion and absorption

Intracellular ATP, the energy source for many reactions, is crucial for the activity of plasma membrane pumps and, thus, for the maintenance of transmembrane ion gradients. Nevertheless, ATP and other nucleotides/nucleosides are also extracellular molecules that regulate diverse cellular functions, including ion transport. In this review, I will first introduce the main components of the extracellular ATP signalling, which have become known as the purinergic signalling system. With more than 50 components or processes, just at cell membranes, it ranks as one of the most versatile signalling systems. This multitude of system components may enable differentiated regulation of diverse epithelial functions. As epithelia probably face the widest variety of potential ATP-releasing stimuli, a special attention will be given to stimuli and mechanisms of ATP release with a focus on exocytosis. Subsequently, I will consider membrane transport of major ions (Cl(-) , HCO(3)(-) , K(+) and Na(+) ) and integrate possible regulatory functions of P2Y2, P2Y4, P2Y6, P2Y11, P2X4, P2X7 and adenosine receptors in some selected epithelia at the cellular level. Some purinergic receptors have noteworthy roles. For example, many studies to date indicate that the P2Y2 receptor is one common denominator in regulating ion channels on both the luminal and basolateral membranes of both secretory and absorptive epithelia. In exocrine glands though, P2X4 and P2X7 receptors act as cation channels and, possibly, as co-regulators of secretion. On an organ level, both receptor types can exert physiological functions and together with other partners in the purinergic signalling, integrated models for epithelial secretion and absorption are emerging.

Preservation of differentiation and clonogenic potential of human hematopoietic stem and progenitor cells during lyophilization and ambient storage

Progenitor cell therapies show great promise, but their potential for clinical applications requires improved storage and transportation. Desiccated cells stored at ambient temperature would provide economic and practical advantages over approaches employing cell freezing and subzero temperature storage. The objectives of this study were to assess a method for loading the stabilizing sugar, trehalose, into hematopoietic stem and progenitor cells (HPC) and to evaluate the effects of subsequent freeze-drying and storage at ambient temperature on differentiation and clonogenic potential. HPC were isolated from human umbilical cord blood and loaded with trehalose using an endogenous cell surface receptor, termed P2Z. Solution containing trehalose-loaded HPC was placed into vials, which were transferred to a tray freeze-dryer and removed during each step of the freeze-drying process to assess differentiation and clonogenic potential. Control groups for these experiments were freshly isolated HPC. Control cells formed 1450+/-230 CFU-GM, 430+/-140 BFU-E, and 50+/-40 CFU-GEMM per 50 microL. Compared to the values for the control cells, there was no statistical difference observed for cells removed at the end of the freezing step or at the end of primary drying. There was a gradual decrease in the number of CFU-GM and BFU-E for cells removed at different temperatures during secondary drying; however, there were no significant differences in the number of CFU-GEMM. To determine storage stability of lyophilized HPC, cells were stored for 4 weeks at 25 degrees C in the dark. Cells reconstituted immediately after lyophilization produced 580+/-90 CFU-GM ( approximately 40%, relative to unprocessed controls p<0.0001), 170+/-70 BFU-E (approximately 40%, p<0.0001), and 41+/-22 CFU-GEMM (approximately 82%, p = 0.4171), and cells reconstituted after 28 days at room temperature produced 513+/-170 CFU-GM (approximately 35%, relative to unprocessed controls, p<0.0001), 112+/-68 BFU-E (approximately 26%, p<0.0001), and 36+/-17 CFU-GEMM ( approximately 82%, p = 0.2164) These studies are the first to document high level retention of CFU-GEMM following lyophilization and storage for 4 weeks at 25 degrees C. This type of flexible storage stability would potentially permit the ability to ship and store HPC without the need for refrigeration.

Rodent cortical astroglia express in situ functional P2X7 receptors sensing pathologically high ATP concentrations

ATP is an important neuronal and astroglial signaling molecule in the brain. In the present study, brain slices were prepared from the prefrontal cortex (PFC) of Wistar rats and 2 lines of mice. P2X₇ receptor immunoreactivity was colocalized with astro- and microglial but not neuronal markers. Whole-cell patch-clamp recordings showed that, in astroglial cells, dibenzoyl-ATP (BzATP) and ATP caused inward currents, near the resting membrane potential. The inactivity of α,β-methylene ATP, as well as the potency increases of BzATP and ATP in a low divalent cation (X²(+))-containing superfusion medium suggested the involvement of P2X₇ receptors. This idea was corroborated by the inhibition of these current responses by PPADS, Brilliant Blue G, A 438079, and calmidazolium. Ivermectin, trinitrophenyl-adenosine-5'-triphosphate, and cyclopentyl-dipropylxanthine did not alter the agonist effects. The reversal potential of BzATP was near 0 mV, indicating the opening of cationic receptor channels. In a low X²(+) superfusion medium, ATP-induced current responses in PFC astroglial cells of wild-type mice but not of the P2X₇ knockouts. Hence, cortical astroglia of rats and mice possess functional P2X₇ receptors. These receptors may participate in necrotic/apoptotic or proliferative reactions after stimulation by large quantities of ATP released by central nervous system injury.

Potentiation of the glutamatergic synaptic input to rat locus coeruleus neurons by P2X7 receptors

Locus coeruleus (LC) neurons in a rat brain slice preparation were superfused with a Mg(2+)-free and bicuculline-containing external medium. Under these conditions, glutamatergic spontaneous excitatory postsynaptic currents (sEPSCs) were recorded by means of the whole-cell patch-clamp method. ATP, as well as its structural analogue 2-methylthio ATP (2-MeSATP), both caused transient inward currents, which were outlasted by an increase in the frequency but not the amplitude of the sEPSCs. PPADS, but not suramin or reactive blue 2 counteracted both effects of 2-MeSATP. By contrast, α,β-methylene ATP (α,β-meATP), UTP and BzATP did not cause an inward current response. Of these latter agonists, only BzATP slightly facilitated the sEPSC amplitude and strongly potentiated its frequency. PPADS and Brilliant Blue G, as well as fluorocitric acid and aminoadipic acid prevented the activity of BzATP. Furthermore, BzATP caused a similar facilitation of the miniature (m)EPSC (recorded in the presence of tetrodotoxin) and sEPSC frequencies (recorded in its absence). Eventually, capsaicin augmented the frequency of the sEPSCs in a capsazepine-, but not PPADS-antagonizable, manner. In conclusion, the stimulation of astrocytic P2X7 receptors appears to lead to the outflow of a signalling molecule, which presynaptically increases the spontaneous release of glutamate onto LC neurons from their afferent fibre tracts. It is suggested, that the two algogenic compounds ATP and capsaicin utilise separate receptor systems to potentiate the release of glutamate and in consequence to increase the excitability of LC neurons.

Effect of P2X(7) receptor knockout on exocrine secretion of pancreas, salivary glands and lacrimal glands

The purinergic P2X(7) receptors are expressed in different cell types where they have varied functions, including regulation of cell survival. The P2X(7) receptors are also expressed in exocrine glands, but their integrated role in secretion is unclear. The aim of our study was to determine whether the P2X(7) receptors affect fluid secretion in pancreas, salivary glands and tear glands. We monitored gland secretions in in vivo preparations of wild-type and P2X(7)(-/-) (Pfizer) mice stimulated with pilocarpine. In cell preparations from pancreas, parotid and lacrimal glands we measured ATP release and intracellular Ca(2+) activity using Fura-2. The data showed that pancreatic secretion and salivary secretions were reduced in P2X(7)(-/-) mice, and in contrast, tear secretion was increased in P2X(7)(-/-) mice. The secretory phenotype was also dependent on the sex of the animal, such that males were more dependent on the P2X(7) receptor expression. ATP release in all cell preparations could be elicited by carbachol and other agonists, and this was independent of the P2X(7) receptor expression. ATP and carbachol increased intracellular Ca(2+) activity, but responses depended on the gland type, presence of the P2X(7) receptor and the sex of the animal. Together, these results demonstrate that cholinergic stimulation leads to release of ATP that can via P2X(7) receptors up-regulate pancreatic and salivary secretion but down-regulate tear secretion. Our data also indicate that there is an interaction between purinergic and cholinergic receptor signalling and that function of the P2X(7) receptor is suppressed in females. We conclude that the P2X(7) receptors are important in short-term physiological regulation of exocrine gland secretion.

Spontaneous Ca2+ oscillations via purinergic receptors elicit transient cell swelling in rat parotid ducts

Rat parotid ductal cells were found to exhibit spontaneous Ca(2+) oscillations. These oscillatory Ca(2+) responses were observed during continuous perfusion with physiological salt solution at 37 degrees C in the absence of calcium mobilizing agonist stimulation. These Ca(2+) oscillations were completely blocked by the purinergic receptor inhibitors, pyridoxal phosphate-6-azo (benzene-2,4-disulfonic acid) (PPADS) and suramin, but were not blocked by the muscarinic antagonist, atropine, nor the alpha-adrenergic antagonist, phentolamine. Simultaneous observation with fura-2 fluorescence and differential interference contrast (DIC) images showed that the spontaneous elevations of [Ca(2+)]i were well correlated with the shape changes of the ductal cells. Using a plasma membrane fluorescence probe, we found that the changes in DIC images reflected spontaneous cell swelling of ductal cells. Electron microscopic analysis after Ca(2+) imaging indicated that the spontaneously oscillating duct cells contained numerous granules at the luminal side, which is characteristic of the granular duct cells. These results indicate that the spontaneous [Ca(2+)]i increase occurs through purinergic receptors, and activates Ca(2+)-dependent ion transporters and/or channels. Our findings present the possibility that spontaneous Ca(2+) oscillations via purinergic receptors are involved in the regulation of the electrolyte composition of saliva in resting states.

Differences in the Ca2+ response resulting from neurotransmitter stimulations of rat parotid acini and ducts

There are few data available regarding the differences in intracellular Ca2+ responses of parotid acinar and ductal cells. This study investigated the Ca2+ mobilization that was induced by the chemical stimulation of acinar and ductal cells from rat parotid glands. In fura-2 loaded parotid cells, carbachol increased the intracellular Ca2+ concentration ([Ca2+](i)) to a greater extent in the acinar cells than in the ductal cells, but noradrenaline increased the [Ca2+](i) in the ductal cells more than in the acinar cells. Although there was no difference in the alpha1-adrenergic receptor agonist phenylephrine-induced Ca2+ mobilization between acini and ducts, the beta-adrenergic receptor agonist isoproterenol increased the [Ca2+](i) in only the ductal cells. Additionally, the effects of non-adrenergic, non-cholinergic neurotransmitters were investigated. Substance P and ATP increased the [Ca2+](i) in parotid acini and/or ducts. A substance P-induced Ca2+ response was observed in only acini, while the ATP-induced Ca2+ response was significantly higher in ducts than in acini. These results suggest that parotid acini have a greater sensitivity to cholinergic and substance P stimulation and a lesser sensitivity to beta-adrenergic and ATP stimulation than the ductal cells. In light of these results, substance P and isoproterenol will be useful for identifying parotid acini and ducts, respectively.

Spontaneous oscillations in intracellular Ca(2+) concentration via purinergic receptors elicit transient cell swelling in rat parotid ducts

Using multiphoton microscopy, we established that rat parotid ductal cells exhibit spontaneous oscillations in intracellular Ca(2+) concentration ([Ca(2+)](i)). These oscillatory Ca(2+) responses were observed during continuous perfusion with a physiological salt solution at 37 degrees C in the absence of calcium mobilizing agonist stimulation. The timing and patterns of these spontaneous Ca(2+) oscillations varied among individual ductal cells, and the average number of Ca(2+) responses in a single responding ductal cell was 2.1 in a 10-min recording period. High-speed scanning (0.6 s/image) revealed that most spontaneous elevations in [Ca(2+)](i) were initiated at the luminal side of ductal cells and spread toward the basal side within 2 s. Electron microscopic analysis after Ca(2+) imaging indicated that spontaneously oscillating ducts contained numerous granules at the luminal side, which is characteristic of granular ducts. These Ca(2+) oscillations were completely blocked by the purinergic receptor inhibitors 4-[[4-formyl-5-hydroxy-6-methyl-3-[(phosphonooxy)methyl]-2-pyridinyl]azo]-1,3-benzenedisulfonic acid (PPADS) and suramin but were not blocked by the muscarinic antagonist atropine or the alpha-adrenergic antagonist phentolamine. Simultaneous observation of fura-2 fluorescence and differential interference contrast (DIC) images showed that spontaneous elevations of [Ca(2+)](i) were well correlated with changes in shape of ductal cells. Using a plasma membrane fluorescence probe, SynaptoGreen C4, we found that the changes in DIC images reflected spontaneous cell swelling of ductal cells. Our findings present the possibility that purinergic receptors mediate spontaneous Ca(2+) oscillations in parotid ductal cells and regulate electrolyte reabsorption from the primary saliva in the resting state.

Identification and characterization of a novel variant of the human P2X(7) receptor resulting in gain of function

The P2X(7) receptor exhibits significant allelic polymorphism in humans, with both loss and gain of function variants potentially impacting on a variety of infectious and inflammatory disorders. At least five loss-of-function polymorphisms (G150R, R307Q, T357S, E496A, and I568N) and two gain-of-function polymorphisms (H155Y and Q460R) have been identified and characterized to date. In this study, we used RT-PCR cloning to isolate and characterize P2X(7) cDNA clones from human PBMCs and THP-1 cells. A previously unreported variant with substitutions of V80M and A166G was identified. When expressed in HEK293 cells, this variant exhibited heightened sensitivity to the P2X(7) agonist (BzATP) relative to the most frequent allele, as shown by pore formation measured by fluorescent dye uptake into cells. Mutational analyses showed that A166G alteration was critical for the gain-of-function change, while V80M was not. Full-length variants with multiple previously identified nonsynonymous SNPs (H155Y, H270R, A348T, and E496A) were also identified. Distinct functional phenotypes of the P2X(7) variants or mutants constructed with multiple polymorphisms were observed. Gain-of-function variations (A166G or H155Y) could not rescue the loss-of-function E496A polymorphism. Synergistic effects of the gain-of-function variations were also observed. We also identified the A348T alteration as a weak gain-of-function variant. Thus, these results identify the new gain-of-function variant A166G and demonstrate that multiple-gene polymorphisms contribute to functional phenotypes of the human P2X(7) receptor. Furthermore, the results demonstrate that the C-terminal of the cysteine-rich domain 1 of P2X(7) is critical for regulation of P2X(7)-mediated pore formation.

Expression of purinergic receptors and modulation of P2X7 function by the inflammatory cytokine IFNgamma in human epithelial cells

The cervical epithelial cell line, HeLa, is one of the oldest and most commonly used cell lines in cell biology laboratories. Although a truncated P2X(7) receptor has recently been identified in HeLa cells, the expression of other purinergic receptors or the function of the P2X(7) protein has not been characterized. We here show that HeLa cells express transcripts for most P2X and P2Y purinergic receptors. Treatment of cells with ATP or other P2X(7) agonists does not stimulate cell death, but can induce atypical calcium fluxes and ion currents. Cervical epithelial cells represent an important target for sexually-transmitted pathogens and are commonly exposed to pro-inflammatory cytokines such as IFNgamma. Stimulation of HeLa cells with IFNgamma upregulates expression of P2X(7) mRNA and full-length protein, modifies ATP-dependent calcium fluxes, and renders the cells sensitive to ATP-induced apoptosis, which can be blocked by a P2X(7) antagonist. IFNgamma treatment also increased dramatically the sensitivity of the intestinal epithelial cell line, HCT8, to ATP-induced apoptosis. Significantly, IFNgamma also stimulated P2X(7) expression on human intestinal tissues. Responses to other purinergic receptor ligands suggest that HeLa cells may also express functional P2Y(1), P2Y(2) and P2Y(6) receptors, which could be relevant for modulating ion homeostasis in the cells.

Sphingosine-1-phosphate receptors stimulate macrophage plasma-membrane actin assembly via ADP release, ATP synthesis and P2X7R activation

Eukaryotic plasma membranes assemble actin filaments within seconds of activation of many receptors, especially during chemotaxis. Here, serum or sphingosine-1-phosphate stimulation of J774 and RAW macrophages released ADP within seconds into the extracellular medium, along with an adenylate kinase activity that converted ADP to ATP. ATP then activated the P2X7 receptor (P2X7R) that was necessary for a peak of plasma-membrane actin assembly within 5 to 10 seconds in P2X7R-expressing J774, RAW and primary macrophages. Neither actin assembly nor characteristic P2X7R channel activity was seen in response to ATP in P2X7R-knockout macrophages, as detected by patch-clamp analysis. Since P2X7R has been shown previously to form a macromolecular complex with actin we propose that it is involved in the membrane assembly of actin. Our data reveal a surprisingly rapid and complex relay of signaling and externalization events that precede and control actin assembly induced by sphingosine-1-phosphate. The overall model we present is strongly supported by the data presented in the accompanying paper that focuses on latex bead phagosomes.

Lipids regulate P2X7-receptor-dependent actin assembly by phagosomes via ADP translocation and ATP synthesis in the phagosome lumen

Latex bead phagosomes isolated from J774 macrophages polymerize actin. We show here that five lipids--phosphatidylinositol-4-phosphate, phosphatidylinositol-(4,5)-bisphosphate, sphingosine-1-phosphate (S1P), ceramide-1-phosphate and phosphatidic acid--stimulate both actin assembly and transport of ADP across the phagosomal membrane into the lumen. Once there, this ADP is converted to ATP by adenylate kinase activity. High luminal ATP concentrations correlated well with phagosome actin assembly under different conditions. The ATP-binding P2X7 receptor (P2X7R) was detected in phagosomes. Although S1P stimulated actin assembly by phagosomes from P2X7R-containing bone marrow macrophages, S1P-stimulated actin assembly was inhibited in phagosomes from cells lacking P2X7R. We propose that luminal ATP accumulates in response to selected lipids and activates the P2X7R that signals across the phagosomal membrane to trigger actin assembly on the cytoplasmic membrane surface. In the accompanying paper by Kuehnel et al. (doi:10.1242/jcs.034207), more evidence is provided in support of this model from the analysis of actin assembly at the plasma membrane of intact macrophages.

Purinergic P2X7 receptors mediate ATP-induced saliva secretion by the mouse submandibular gland

Salivary glands express multiple isoforms of P2X and P2Y nucleotide receptors, but their in vivo physiological roles are unclear. P2 receptor agonists induced salivation in an ex vivo submandibular gland preparation. The nucleotide selectivity sequence of the secretion response was BzATP >> ATP > ADP >> UTP, and removal of external Ca(2+) dramatically suppressed the initial ATP-induced fluid secretion ( approximately 85%). Together, these results suggested that P2X receptors are the major purinergic receptor subfamily involved in the fluid secretion process. Mice with targeted disruption of the P2X(7) gene were used to evaluate the role of the P2X(7) receptor in nucleotide-evoked fluid secretion. P2X(7) receptor protein and BzATP-activated inward cation currents were absent, and importantly, purinergic receptor agonist-stimulated salivation was suppressed by more than 70% in submandibular glands from P2X(7)-null mice. Consistent with these observations, the ATP-induced increases in [Ca(2+)](i) were nearly abolished in P2X(7)(-/-) submandibular acinar and duct cells. ATP appeared to also act through the P2X(7) receptor to inhibit muscarinic-induced fluid secretion. These results demonstrate that the ATP-sensitive P2X(7) receptor regulates fluid secretion in the mouse submandibular gland.

The P2X7 receptor channel pore dilates under physiological ion conditions

Activation of the purinergic P2X₇ receptor leads to the rapid opening of an integral ion channel that is permeable to small cations. This is followed by a gradual increase in permeability to fluorescent dyes by integrating the actions of the pannexin-1 channel. Here, we show that during the prolonged agonist application a rapid current that peaked within 200 ms was accompanied with a slower current that required tens of seconds to reach its peak. The secondary rise in current was observed under different ionic conditions and temporally coincided with the development of conductivity to larger organic cations. The biphasic response was also observed in cells with blocked pannexin channels and in cells not expressing these channels endogenously. The biphasic current was preserved in N-terminal T15A, T15S, and T15V mutants that have low or no permeability to organic cations, reflecting enhanced permeability to inorganic cations. In contrast, the T15E, T15K, and T15W mutants, and the Δ18 mutant with deleted P2X₇ receptor–specific 18–amino acid C-terminal segment, were instantaneously permeable to organic cations and generated high amplitude monophasic currents. These results indicate that the P2X₇ receptor channel dilates under physiological ion conditions, leading to generation of biphasic current, and that this process is controlled by residues near the intracellular side of the channel pore.

Species and agonist dependent zinc modulation of endogenous and recombinant ATP-gated P2X7 receptors

Zinc (Zn2+) and copper (Cu2+) are key signalling molecules in the immune system and regulate the activity of many ion channels. Both Zn2+ and Cu2+ potently inhibit rat P2X7 receptors via a binding site identified by mutagenesis. Here we show that extracellular Cu2+ also potently inhibits mouse P2X7 receptors. By contrast, the receptor expression system and agonist strongly influence the action of extracellular Zn2+ at mouse P2X7 receptors. Consistent with previous reports, Zn2+ inhibits recombinant rat P2X7 receptors. However, recombinant mouse P2X7 receptors are potentiated by Zn2+ when activated by ATP4- but inhibited when stimulated with the ATP analogue BzATP4-. Endogenous murine macrophage P2X7 receptors are not modulated by Zn2+ when stimulated by ATP4- however Zn2+ inhibits BzATP4- mediated responses. In summary, these findings provide a fundamental insight into the differential actions of Zn2+ and Cu2+ between different P2X7 receptor species.

Na+ modulates anion permeation and block of P2X7 receptors from mouse parotid glands

We previously reported that mouse parotid acinar cells display anion conductance (I(ATPCl)) when stimulated by external ATP in Na+-free extracellular solutions. It has been suggested that the P2X7 receptor channel (P2X7R) might underlie I(ATPCl). In this work we show that I (ATPCl) can be activated by ATP, ADP, AMP-PNP, ATPgammaS and CTP. This is consistent with the nucleotide sensitivity of P2X7R. Accordingly, acinar cells isolated from P2X7R( -/- ) mice lacked I(ATPCl). Experiments with P2X7R heterologously expressed resulted in ATP-activated currents (I(ATP-P2X7)) partially carried by anions. In Na(+)-free solutions, I (ATP-P2X7) had an apparent anion permeability sequence of SCN(-) > I(-) congruent with NO3(-) > Br(-) > Cl(-) > acetate, comparable to that reported for I(ATPCl) under the same conditions. However, in the presence of physiologically relevant concentrations of external Na+, the Cl(-) permeability of I(ATP-P2X7) was negligible, although permeation of Br(-) or SCN(-) was clearly resolved. Relative anion permeabilities were not modified by addition of 1 mM: carbenoxolone, a blocker of Pannexin-1. Moreover, cibacron blue 3GA, which blocks the Na(+) current activated by ATP in acinar cells but not I(ATPCl), blocked I(ATP-P2X7) in a dose-dependent manner when Na+ was present but failed to do so in tetraethylammonium containing solutions. Thus, our data indicate that P2X7R is fundamental for I(ATPCl) generation in acinar cells and that external Na+ modulates ion permeability and conductivity, as well as drug affinity, in P2X7R.

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.

Multi-photon microscopic imaging of rat parotid ducts demonstrates cellular heterogeneity in Ca2+ responsiveness

The heterogeneity of salivary ductal cells, with regard to their sensitivity to Ca(2+)-mobilizing agonists, was visualized by multi-photon microscopy. Stimulation of isolated parotid ducts with 0.1 and 1 microM epinephrine (Epi) elevated the intracellular Ca(2+) levels ([Ca(2+)](i)) in approximately 30% and >90% of the ductal cells, respectively. Of the 0.1 microM Epi-responsive cells, 80% responded rapidly to subsequent stimulation with 1 microM Epi. Similarly, threshold concentrations (0.5 or 1 microM) of phenylephrine (PhL), carbachol (CCh) or ATP, induced responses in approximately 20% of the ductal cells, and subsequent stimulations with 10 microM of the same agonist activated approximately 80% of ductal cells. These observations indicate that parotid ducts contain a certain subpopulation of cells, which exhibits particularly high sensitivity to these Ca(2+)-mobilizing agonists, compared to the remaining ductal cells. Sequential stimulation with threshold concentrations of PhL, CCh, and ATP induced Ca(2+) responses in approximately 33% of ductal cells. Of these responsive cells, the majority (69%) could only respond to one of the three agonists; while a small minority (9%) were capable of responding to all three agonists. These results indicate that low concentrations of PhL, CCh, and ATP activate different subpopulations of parotid ductal cells.

Lysophosphatidylcholine potentiates Ca2+ influx, pore formation and p44/42 MAP kinase phosphorylation mediated by P2X7 receptor activation in mouse microglial cells

The P2X7 receptor (P2X7R) is an ATP-gated ion channel highly expressed in microglia. P2X7R plays important roles in inflammatory responses in the brain. However, little is known about the mechanisms regulating its functions in microglia. Lysophosphatidylcholine (LPC), an inflammatory phospholipid that promotes microglial activation, may have some relevance to P2X7R signaling in terms of microglial function. In this study, we examined its effects on P2X7R signaling in a mouse microglial cell line (MG6) and primary microglia. LPC facilitated the sustained increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)) through P2X7R channels activated by ATP or BzATP. The potentiated increase in [Ca(2+)](i) was actually inhibited by P2X7R antagonists, brilliant blue G and oxidized ATP. The potentiating effect of LPC was not observed with P2Y receptor systems, which are also expressed in MG6 cells. G2A, a receptor for LPC, was expressed in MG6 cells, but not involved in the facilitating effect of LPC on the P2X7R-mediated change in [Ca(2+)](i). Furthermore, LPC enhanced the P2X7R-associated formation of membrane pores and the activation of p44/42 mitogen-activated protein kinase. These results suggest that LPC may regulate microglial functions in the brain by enhancing the sensitivity of P2X7R.

Modulation of P2X7 receptor functions by polymyxin B: crucial role of the hydrophobic tail of the antibiotic molecule

BACKGROUND AND PURPOSE

P2X7 is a membrane receptor for extracellular ATP which is highly expressed in dendritic cells, macrophages and microglia where it mediates pro-inflammatory responses. The antibiotic polymyxin B, which binds to and neutralizes the toxic residue of bacterial lipopolysaccharide, greatly amplifies cellular responses mediated by the P2X7 receptor. However, the molecular mechanism involved is so far unknown.

EXPERIMENTAL APPROACH

We investigated the effects of polymyxin B and polymyxin B nonapeptide (PMBN) which is the deacylated amino derivative of polymyxin B lacking the N-terminal fatty amino acid 6-methylheptanoic/octanoic-Dab residue, in human macrophages and HEK293 cells stably expressing the human P2X7 receptor (HEK293-hP2X7). Differences between the two antibiotics were assessed by monitoring the following: nucleotide-induced cytoplasmic free Ca2+ concentration changes, plasma membrane permeability changes, lactate dehydrogenase activity, cell morphology changes. Western blot and microscopic analyses of P2X7GFP-expressing cells were also performed.

KEY RESULTS

In contrast to polymyxin B, the polymyxin B nonapeptide was unable to potentiate: a) the ATP-induced Ca2+ increase, b) pore formation and consequently ATP-mediated plasma membrane permeabilization; c) ATP-dependent cytotoxicity. Moreover, in contrast to polymyxin B, polymyxin B nonapeptide did not affect aggregation of the P2X7 receptor subunits and it did not potentiate P2X7-dependent cell fusion.

CONCLUSIONS AND IMPLICATIONS

The effects of polymyxin B depended on the presence of its N-terminal fatty amino acid 6-methylheptanoic/octanoic-Dab residue as deletion of this residue abolished polymyxin B-dependent modulation of ATP-triggered responses. These findings are important in the search for allosteric modulators of the P2X7 receptor.

P2X7 and phospholipid signalling: The search of the “missing link” in epithelial cells

The purinergic receptor P2X(7) is widely expressed in epithelial cells. This receptor shares in common with the other P2X receptors the ability to form a non-selective cation channel. On the other hand, the COOH terminus of P2X(7) seems to allow this receptor to couple to a spectrum of downstream effectors responsible for the regulation of cell death and pore formation among other functions. However, the coupling of P2X(7) to these downstream effectors, as well as the identity of possible adapters directly interacting with the receptor, remains poorly understood. Here we review the ability of P2X(7) to activate phospholipid signalling pathways in epithelial cells and propose this step as a possible link between the receptor and other downstream effectors. The P2X(7) ability to control the cellular levels of several lipid messengers (PA, AA, DAG, ceramide, etc.) through the modulation of phospholipases (C, A(2), D) and neutral sphingomyelinase is described. These pathways are sometimes regulated independently of the channel function of the receptor. Recent data concerning P2X(7) localization in lipid rafts is also discussed in relation to the coupling to these pathways and dissociation from channel function.

Oxidized ATP decreases beta-actin expression and intracellular superoxide concentrations in RBA-2 type-2 astrocytes independently of P2X7 receptor

Periodate-oxidized 2',3'-dialdehyde ATP (oxidized ATP) has been used extensively as a selective antagonist at P2X(7) receptors, although P2X(7)-independent actions on pro-inflammatory cytokine release have also been reported. Because P2X(7) receptors in astrocytes have been suggested as potential targets of anti-inflammatory drug therapy, we examined the effect of oxidized ATP on beta-actin expression and superoxide production of RBA-2 type-2 astrocytes known to possess P2X(7) receptors. Oxidized ATP per se decreased beta-actin expression time and dose dependently. Treatment with oxidized ATP for 8 h caused an approximately 50% decrease in beta-actin expression whereas other P2 receptor antagonists, brilliant blue G (BBG), suramin and pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), were not effective. In addition, oxidized ATP per se decreased the intracellular superoxide concentration, whereas ATP and the P2X(7) receptor-selective agonist 3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate (BzATP) stimulated intracellular superoxide production, an effect inhibited by oxidized ATP. In addition, oxidized ATP neither affected cellular viability nor affected interleukin-1beta, converting enzyme (ICE)-like protease activity in these astrocytes. To further elucidate the mechanism, the effects of oxidized ATP on intracellular superoxide concentration and beta-actin expression were examined in a P2X(7) receptor-negative astrocyte cell line, IA-1g1. Oxidized ATP-induced a time-dependent decrease in intracellular superoxide concentration whereas oxidized ATP had no effect on beta-actin expression. Nevertheless, oxidized ATP altered f-actin cytoskeleton arrangement in IA-1g1 astrocytes. Taken together, these results indicate that oxidized ATP per se caused a cell specific decrease in beta-actin expression in RBA-2 type-2 astrocytes. In addition, oxidized ATP decreased intracellular superoxide concentrations and altered f-actin cytoskeleton arrangement in both P2X(7) receptor-positive and -negative astrocytes. Thus, we conclude from these results that the effects of oxidized ATP on actin and superoxide are mediated through mechanisms that are at least in part, independent of P2X(7) receptors.

P2X7 receptor stimulation of membrane internalization in a thyrocyte cell line

Using fluorescent membrane markers, we have previously shown that extracellular ATP stimulates both exocytosis and membrane internalization in the Fisher rat thyroid cell line FRTL. In this study, we examine the actions of ATP using whole-cell recording conditions that favor stimulation of membrane internalization. ATP stimulation of the P2X(7) receptor activated a reversible, Ca(2+)-permeable, cation conductance that slowly increased in size without changes in ion selectivity. ATP also induced a delayed irreversible decrease in cell capacitance (C(m)) that was equivalent to an 8% decrease in membrane surface area. Addition of guanosine 5'-0-2-thiodiphosphate to the pipette solution inhibited the ATP-induced decrease in C(m) without affecting channel activation. The effects of ATP on membrane conductance were mimicked by 2',3'-O-(4-benzoylbenzoyl)-ATP, but not by UTP, adenosine, or 2-methylthio-ATP, and were inhibited by pyridoxal phosphate-6-azophenyl-2'4'-disulfonic acid, adenosine 5'-triphosphate-2'3'-dialdehyde, and Cu(2+). The capacitance decrease persisted in Na(+)-, Ca(2+)- and Cl(-)-free external saline or with Ca(2+)-free pipette solution. It is concluded that ATP activation of the inotropic P2X(7) receptor stimulates membrane internalization by a mechanism that involves intracellular GTP, but does not require internal Ca(2+) or influx of Na(+) or Ca(2+) through the receptor-gated channel.

P2X(7) receptor: Death or life?

The P2X₇ plasma membrane receptor is an intriguing molecule that is endowed with the ability to kill cells, as well as to activate many responses and even stimulate proliferation. Here, the authors give an overview on the multiplicity and complexity of P2X₇-mediated responses, discussing recent information on this receptor. Particular attention has been paid to early and late signs of apoptosis and necrosis linked to activation of the receptor and to the emerging field of P2X₇ function in carcinogenesis.

Regulation of the P2X7 receptor permeability to large molecules by extracellular Cl- and Na+

Upon continuous stimulation, the pore of the monovalent cation-selective P2X7 receptor (P2X7R) expands to accommodate large molecules such as N-methyl-D-glucamine (NMDG+). How the change in P2X7R permeability is regulated is not known. Here we report that extracellular Cl- (Cl-(o)) regulates the outward current, whereas extracellular Na+ (Na+(o)) regulates the inward current of large molecules by P2X7Rs. The P2X7R-mediated current was measured in parotid acinar and duct cells of wild type and P2X7R-/- mice and in HEK293 cells expressing the human or mouse P2X7R isoforms. In symmetrical NaCl, triethylammonium chloride, and NMDG+ chloride solutions, the P2X7R current followed a linear current/voltage relationship. In symmetrical NaCl, removal of Cl-(o) reduced the inward Na+ current by approximately 35% and the outward Na+ current by only 10%. By contrast, in the absence of Na+(i) and the presence of Na+(o) or NMDG+(o), the removal of Cl-(o) reduced the inward Na+ or NMDG+ currents by 35% but the outward NMDG+ current by >95%. The effect of Cl-(o) was half-maximal at approximately 60 mm. Reducing Cl-(i) from 150 to 10 mm did not reproduce the effects of Cl-(o). All currents were eliminated in P2X7R-/- cells and reproduced by expressing the P2X7Rs in HEK cells. These findings suggest that Cl-(o) primarily regulates the outward P2X7R current of large molecules. When cells dialyzed with NMDG+ were stimulated in the presence of Na+(o), subsequent removal of Na+(o) resulted in a strongly outward rectifying NMDG+ current, indicating maintained high selectivity for Na+ over NMDG+. During continuous incubation in Na+-free medium, the permeability of the P2X7Rs to NMDG+ gradually increased. On the other hand, when the cells were incubated in symmetrical NMDG+ and only then stimulated with ATP, the NMDG+ current by P2X7Rs followed a linear current/voltage relationship and did not change with time. These findings suggest that the P2X7R has a "Na+(o) memory" and that Na+(o) regulates the inward permeability of P2X7Rs to large molecules. The novel regulation of P2X7R outward and inward permeability to large molecules by Cl-(o) and Na+(o), respectively, may have an important protective function, particularly in secretory epithelial cells.

P2X7-like receptor subunits enhance excitatory synaptic transmission at central synapses by presynaptic mechanisms

Recent studies demonstrate that P2X7 receptor subunits (P2X7RS) are present at central and peripheral synapses and suggest that P2X7RS can regulate transmitter release. In brainstem slices from 15 to 26 day old pentobarbitone-anesthetized mice, we examined the effect of P2X7RS activation on excitatory postsynaptic currents (EPSCs) recorded from hypoglossal motoneurons using whole-cell patch clamp techniques. After blockade of most P2X receptors with suramin (which is inactive at P2X7RS) and of adenosine receptors with 8-phenyltheophylline (8PT), bath application of the P2X receptor agonist 3'-0-(4-benzoyl)ATP (BzATP) elicited a 40.5+/-16.0% (mean+/-S.E.M., n = 8, P = 0.039) increase in evoked EPSC amplitude and significantly reduced paired pulse facilitation of evoked EPSCs. This response to BzATP (with suramin and 8PT present) was completely blocked by prior application of Brilliant Blue G (200 nM or 2 microM), a P2X7RS antagonist. In contrast, BzATP application with suramin and 8PT present did not alter miniature EPSC frequency or amplitude when action potentials were blocked with tetrodotoxin. These electrophysiological results suggest that P2X7RS activation increases central excitatory transmitter release via presynaptic mechanisms, confirming previous indirect measures of enhanced transmitter release. We suggest that possible presynaptic mechanisms underlying enhancement of evoked transmitter release by P2X7RS activation are modulation of action potential width or an increase in presynaptic terminal excitability, due to subthreshold membrane depolarization which increases the number of terminals releasing transmitter in response to stimulation.

cAMP potentiates ATP-evoked calcium signaling in human parotid acinar cells

In salivary acinar cells, intracellular calcium ([Ca(2+)](i)) signaling plays an important role in eliciting fluid secretion through the activation of Ca(2+)-activated ionic conductances. Ca(2+) and cAMP have synergistic effects on fluid secretion such that peak secretion is elicited following activation of both parasympathetic and sympathetic pathways. We have recently demonstrated that cAMP exerts effects on Ca(2+) release, through protein kinase A (PKA)-mediated phosphorylation of inositol 1,4,5-trisphosphate receptors (InsP(3)R) in mouse parotid acinar cells. To extend these findings, in the present study cross-talk between Ca(2+) signaling and cAMP pathways in human parotid acinar cells was investigated. In human parotid acinar cells, carbachol stimulation evoked increases in the [Ca(2+)](i) and the initial peak amplitude was enhanced following PKA activation, consistent with reports from mouse parotid. Stimulation with ATP also evoked an increase in [Ca(2+)](i). The ATP-evoked Ca(2+) elevation was largely dependent on extracellular Ca(2+), suggesting the involvement of the P2X family of purinergic receptors. Pharmacological elevation of cAMP resulted in a approximately 5-fold increase in the peak [Ca(2+)](i) change evoked by ATP stimulation. This enhanced [Ca(2+)](i) increase was not dependent on intracellular release from InsP(3)R or ryanodine receptors, suggesting a direct effect on P2XR. Reverse transcription-polymerase chain reaction and Western blot analysis confirmed the presence of P2X(4)R and P2X(7)R mRNA and protein in human parotid acinar cells. ATP-activated cation currents were studied using whole cell patch clamp techniques in HEK-293 cells, a null background for P2XR. Raising cAMP resulted in a approximately 4.5-fold enhancement of ATP-activated current in HEK-293 cells transfected with P2X(4)R DNA but had no effects on currents in cells expressing P2X(7)R. These data indicate that in human parotid acinar cells, in addition to modulation of Ca(2+) release, Ca(2+) influx through P2X(4)R may constitute a further locus for the synergistic effects of Ca(2+) and PKA activation.

A P2X7 receptor stimulates plasma membrane trafficking in the FRTL rat thyrocyte cell line

Thyroid cells express a variety of P2Y and P2X purinergic receptor subtypes. G protein-coupled P2Y receptors influence a wide variety of thyrocyte-specific functions; however, functional P2X receptor-gated channels have not been observed. In this study, we used whole cell patch-clamp recording and fluorescence imaging of the plasma membrane marker FM1-43 to examine the effects of extracellular ATP on membrane permeability and trafficking in the Fisher rat thyroid cell line FRTL. We found a cation-selective current that was gated by ATP and 2',3'-O-(4-benzoylbenzoyl)-ATP but not by UTP. The ATP-evoked currents were inhibited by pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid, adenosine 5'-triphosphate-2',3'-dialdehyde, 100 microM Zn(2+), and 50 microM Cu(2+). Fluorescence imaging revealed pronounced, temperature-sensitive stimulation of exocytosis and membrane internalization by ATP with the same pharmacological profile as observed for activation of current. The EC(50) for ATP stimulation of internalization was 440 microM in saline containing 2 mM Ca(2+) and 2 mM Mg(2+), and 33 microM in low-Mg(2+), nominally Ca(2+)-free saline. Overall, the results are most consistent with activation of a P2X(7) receptor by ATP(4-). However, low permeability to N-methyl-d-glucamine(+) and the propidium cation YO-PRO-1 indicates absence of the cytolytic pore that often accompanies P2X(7) receptor activation. ATP stimulation of internalization occurs in Na(+)-free, Ca(2+)-free, or low-Mg(2+) saline and therefore does not depend on cation influx through the ATP-gated channel. We conclude that ATP activation of a P2X(7) receptor stimulates membrane internalization in FRTL cells via a transduction pathway that does not depend on cation influx.

Subpopulation of store-operated Ca2+ channels regulate Ca2+-induced Ca2+ release in non-excitable cells

Ca2+-induced Ca2+ release (CICR) is a well characterized activity in skeletal and cardiac muscles mediated by the ryanodine receptors. The present study demonstrates CICR in the non-excitable parotid acinar cells, which resembles the mechanism described in cardiac myocytes. Partial depletion of internal Ca2+ stores leads to a minimal activation of Ca2+ influx. Ca2+ influx through this pathway results in an explosive mobilization of Ca2+ from the majority of the stores by CICR. Thus, stimulation of parotid acinar cells in Ca2+ -free medium with 0.5 microm carbachol releases approximately 5% of the Ca2+ mobilizable by 1 mm carbachol. Addition of external Ca2+ induced the same Ca2+ release observed in maximally stimulated cells. Similar results were obtained by a short treatment with 2.5-10 microm cyclopiazonic acid, an inhibitor of the sarco/endoplasmic reticulum Ca2+ ATPase pump. The Ca2+ release induced by the addition of external Ca2+ was largely independent of IP(3)Rs because it was reduced by only approximately 30% by the inhibition of the inositol 1,4,5-trisphosphate receptors with caffeine or heparin. Measurements of Ca2+ -activated outward current and [Ca2+](i) suggested that most CICR triggered by Ca2+ influx occurred away from the plasma membrane. Measurement of the response to several concentrations of cyclopiazonic acid revealed that Ca2+ influx that regulates CICR is associated with a selective portion of the internal Ca2+ pool. The minimal activation of Ca2+ influx by partial store depletion was confirmed by the measurement of Mn2+ influx. Inhibition of Ca2+ influx with SKF96365 or 2-aminoethoxydiphenyl borate prevented activation of CICR observed on addition of external Ca2+. These findings provide evidence for activation of CICR by Ca2+ influx in non-excitable cells, demonstrate a previously unrecognized role for Ca2+ influx in triggering CICR, and indicate that CICR in non-excitable cells resembles CICR in cardiac myocytes with the exception that in cardiac cells Ca2+ influx is mediated by voltage-regulated Ca2+ channels whereas in non-excitable cells Ca2+ influx is mediated by store-operated channels.