Modulation by propranolol of the uptake of ethidium bromide by rat submandibular acinar cells exposed to a P2X(7) agonist or to maitotoxin

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.

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.

Regulation by CRAMP of the responses of murine peritoneal macrophages to extracellular ATP

Peritoneal macrophages were isolated from wild type (WT) mice and from mice invalidated for the P2X(7) receptor (KO) which had been pretreated with thioglycolate. In cells from WT mice, 1 mM ATP increased the intracellular concentration of calcium ([Ca(2+)](i)), the uptake of ethidium bromide, the production of reactive oxygen species (ROS), the secretion of IL-1beta, the release of oleic acid and of lactate dehydrogenase; it decreased the intracellular concentration of potassium ([K(+)](i)). In KO mice, ATP transiently increased the [Ca(2+)](i) confirming that the P2X(7) receptor is a major receptor of peritoneal macrophages. WKYMVm, an agonist of receptors for formylated peptides (FPR) also increased the [Ca(2+)](i) in murine macrophages. The slight increase of the [Ca(2+)](i) was strongly potentiated by ivermectin confirming the expression of functional P2X(4) receptors by murine peritoneal macrophages. CRAMP, the unique antimicrobial peptide derived from cathelin in mouse inhibited all the responses coupled to P2X(7) receptors in macrophages from WT mice. Agonists for FPR had no effect on the increase of the [Ca(2+)](i) in response to ATP. CRAMP had no effect on the increase of the [Ca(2+)](i) evoked by a combination of ATP and ivermectin in macrophages from P2X(7)-KO mice. In summary CRAMP inhibits the responses secondary to the activation of the murine P2X(7) receptors expressed by peritoneal macrophages. This inhibition is not mediated by FPR receptors and is specific since CRAMP has no effect on the response coupled to P2X(4) receptors. It can thus be concluded that the interaction between P2X(7) receptors and cathelin-derived antimicrobial peptides is species-specific, in some cases (man) positive in others (mouse) negative.

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.

Pharmacological differentiation of the P2X7 receptor and the maitotoxin-activated cationic channel

The ATP-P2X(7) receptor subtype and a maitotoxin-activated ion channel were studied to determine factors which identify them as separate entities in the control of a cytotolytic pore. Activation of ATP-P2X(7) receptors with 2'-3'-O-(benzylbenzyl) ATP (BzATP) or maitotoxin ion channels resulted in influx of ethidium bromide and cell death. Maitotoxin (25-250 pM)-induced ethidium bromide uptake and cell death was sensitive to extracellular Ca(2+), the ionic composition of the buffer, reduced by the calmodulin inhibitor W7, (N-(s-aminohexyl)-5-chloro-1-naphthalenesulfonamide), (10-100 microM) but unaffected by the ATP-P2X(7) receptor antagonist oxidized ATP, (adenosine 5'-triphosphate periodate oxidized sodium salt) (oATP). BzATP (10-200 microM)-induced ethidium bromide uptake and cell death were inhibited by oATP, unaffected by W7, inhibited by high ionic concentrations but only slightly dependant on external Ca(2+). These results are consistent with the existence of a pharmacological mechanism for controlling cell death consisting of an ATP-P2X(7) receptor, a maitotoxin-activated ion channel and a cytolytic pore.

Are second messengers crucial for opening the pore associated with P2X7 receptor?

Stimulation of the P2X7 receptor by ATP induces cell membrane depolarization, increase in intracellular Ca2+ concentration, and, in most cases, permeabilization of the cell membrane to molecules up to 900 Da. After the activation of P2X7, at least two phenomena occur: the opening of low-conductance (8 pS) cationic channels and pore formation. At least two conflicting hypotheses have been postulated to reconcile these findings: 1) the P2X7 pore is formed as a result of gradual permeability increase (dilation) of cationic channels, and 2) the P2X7 pore represents a distinct channel, possibly activated by a second messenger and not directly by extracellular nucleotides. In this study, we investigated whether second messengers are necessary to open the pore associated with the P2X7 receptor in cells that expressed the pore activity by using the patch-clamp technique in whole cell and cell-attached configurations in conjunction with fluorescent imaging. In peritoneal macrophages and 2BH4 cells, we detected permeabilization and single-channel currents in the cell-attached configuration when ATP was applied outside the membrane patch in a condition in which oxidized ATP and Lucifer yellow were maintained within the pipette. Our data support Ca2+ as a second messenger associated with pore formation because the permeabilization depended on the presence of intracellular Ca2+ and was blocked by BAPTA-AM. In addition, MAPK inhibitors (SB-203580 and PD-98059) blocked the permeabilization and single-channel currents in these cells. Together our data indicate that the P2X7 pore depends on second messengers such as Ca2+ and MAP kinases.

Regulation of phospholipase D by P2X7 receptors in submandibular ductal cells

ATP (1 mM) increased the phospholipase D (PLD) activity of rat submandibular gland (RSMG) ductal cells in a concentration-dependent and calcium-sensitive manner. The response to ATP was reproduced by benzoylbenzoyl-ATP (Bz-ATP, 100 microM) and also partly by adenosine 5'-(gamma-thio)triphosphate (ATPgammaS, 1 mM). A similar stimulation was observed in control mice (P2X7R+/+ mice) but not in mice lacking the P2X7 receptors (P2X7R-/- mice). Oxidized ATP and Coomassie blue or the addition of magnesium or nickel to the incubation medium inhibited the response to ATP. The stimulation of PLD by purinergic agonist was inhibited by about 50% by calphostin C and chelerythrine, two protein kinase C (PKC) inhibitors. The stimulation of PLD by Bz-ATP and by o-tetradecanoylphorbol 13-acetate (TPA), a phorbol ester which activates PKC, were not additive. From these results we can conclude that the activation of P2X7 receptors in RSMG ductal cells is coupled to the activation of a PLD. This activation is partly mediated by protein kinase C.

Glu496Ala polymorphism of human P2X7 receptor does not affect its electrophysiological phenotype

A glutamate to alanine exchange at amino acid position 496 of the human P2X(7) receptor was recently shown to be associated with a loss of function in human B lymphocytes in terms of ATP-induced ethidium(+) uptake, Ba(2+) influx, and induction of apoptosis (Gu BJ, Zhang WY, Worthington RA, Sluyter R, Dao-Ung P, Petrou S, Barden JA, and Wiley JS. J Biol Chem 276: 11135-11142, 2001). Here we analyzed the effect of the Glu(496) to Ala exchange on the channel properties of the human P2X(7) receptor expressed in Xenopus oocytes with the two-microelectrode voltage-clamp technique. The amplitudes of ATP-induced whole cell currents characteristic of functional expression, kinetic properties including ATP concentration dependence, and permeation behavior were not altered by this amino acid exchange. Also in HEK293 cells, the Ala(496) mutant mediated typical P2X(7) receptor-dependent currents like the parent Glu(496) hP2X(7) receptor. Because the function of the P2X(7) receptor as an ATP-gated channel for small cations including Ba(2+) remained unaffected by this mutation, we conclude that Glu(496) plays a critical role in pore formation but does not determine the ion channel properties of the human P2X(7) receptor.