Heterologous desensitization of muscarinic receptors by P2Z purinoceptors in rat parotid acinar 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.

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.

The pannexins: past and present

The pannexins (Panxs) are a family of chordate proteins homologous to the invertebrate gap junction forming proteins named innexins. Three distinct Panx paralogs (Panx1, Panx2, and Panx3) are shared among the major vertebrate phyla, but they appear to have suppressed (or even lost) their ability to directly couple adjacent cells. Connecting the intracellular and extracellular compartments is now widely accepted as Panx's primary function, facilitating the passive movement of ions and small molecules along electrochemical gradients. The tissue distribution of the Panxs ranges from pervasive to very restricted, depending on the paralog, and are often cell type-specific and/or developmentally regulated within any given tissue. In recent years, Panxs have been implicated in an assortment of physiological and pathophysiological processes, particularly with respect to ATP signaling and inflammation, and they are now considered to be a major player in extracellular purinergic communication. The following is a comprehensive review of the Panx literature, exploring the historical events leading up to their discovery, outlining our current understanding of their biochemistry, and describing the importance of these proteins in health and disease.

ATP inhibits Ins(1,4,5)P3-evoked Ca2+ release in smooth muscle via P2Y1 receptors

Adenosine 5′-triphosphate (ATP) mediates a variety of biological functions following nerve-evoked release, via activation of either G-protein-coupled P2Y- or ligand-gated P2X receptors. In smooth muscle, ATP, acting via P2Y receptors (P2YR), may act as an inhibitory neurotransmitter. The underlying mechanism(s) remain unclear, but have been proposed to involve the production of inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃] by phospholipase C (PLC), to evoke Ca²⁺ release from the internal store and stimulation of Ca²⁺-activated potassium (K_Ca) channels to cause membrane hyperpolarization. This mechanism requires Ca²⁺ release from the store. However, in the present study, ATP evoked transient Ca²⁺ increases in only ~10% of voltage-clamped single smooth muscle cells. These results do not support activation of K_Ca as the major mechanism underlying inhibition of smooth muscle activity. Interestingly, ATP inhibited Ins(1,4,5)P₃-evoked Ca²⁺ release in cells that did not show a Ca²⁺ rise in response to purinergic activation. The reduction in Ins(1,4,5)P₃-evoked Ca²⁺ release was not mimicked by adenosine and therefore, cannot be explained by hydrolysis of ATP to adenosine. The reduction in Ins(1,4,5)P₃-evoked Ca²⁺ release was, however, also observed with its primary metabolite, ADP, and blocked by the P2Y₁R antagonist, MRS2179, and the G protein inhibitor, GDPβS, but not by PLC inhibition. The present study demonstrates a novel inhibitory effect of P2Y₁R activation on Ins(1,4,5)P₃-evoked Ca²⁺ release, such that purinergic stimulation acts to prevent Ins(1,4,5)P₃-mediated increases in excitability in smooth muscle and promote relaxation.

Cholinergic agonists activate P2X7 receptors to stimulate protein secretion by the rat lacrimal gland

PURPOSE

To determine the interaction of M3 muscarinic receptors (M3AChR) and P2X(7) receptors to increase intracellular [Ca2+] ([Ca2+]i) and stimulate protein secretion in rat lacrimal gland cells.

METHODS

Exorbital lacrimal glands from male Sprague-Dawley rats were divided into pieces or digested with collagenase to form acinar clumps. [Ca2+]i was measured using an imaging system in acini incubated with fura-2/AM. Adenosine triphosphate (ATP) release was determined using the luciferin-luciferase reaction. Peroxidase secretion, our index for protein secretion, was measured spectrophotometrically. Acini were stimulated with the P2X7 receptor agonist, (benzoylbenzoyl)adenosine 5' triphosphate (BzATP), cholinergic agonist carbachol, or the activator of conventional and novel PKC isoforms, phorbol 12-myristate 13-acetate (PMA).

RESULTS

The increase in [Ca2+]i caused by carbachol and BzATP used simultaneously was less than additive, but the increase in protein secretion was additive. The M3AChR antagonist atropine blocked the BzATP-stimulated increase in [Ca2+]i and peroxidase secretion. The P2X7 antagonist did not alter the carbachol-stimulated increase in [Ca2+]i or peroxidase. PMA- and BzATP-stimulated increases in [Ca2+]i were additive. Neither constitutively active PKCα, dominant-negative PKCα, nor PKCε altered BzATP-stimulated increases in [Ca2+]i. Carbachol increased ATP release from lacrimal gland pieces but not from acini.

CONCLUSIONS

In lacrimal gland cells, the activation of M3AChRs stimulates P2X7 receptors to increase [Ca2+]i and protein secretion. The underlying mechanisms are unknown but could include the release of ATP or intracellular interactions not mediated by PKC isoforms. In addition, M3AChRs use signaling pathways that overlap with those used by P2X7 receptors to increase [Ca2+]i, but they also use signaling pathways not used by P2X7 receptors to stimulate protein secretion.

The P2X7 receptor-pannexin-1 complex decreases muscarinic acetylcholine receptor-mediated seizure susceptibility in mice

Pannexin-1 (Panx1) plays a role in the release of ATP and glutamate in neurons and astrocytes. Panx1 can be opened at the resting membrane potential by extracellular ATP via the P2X7 receptor (P2X7R). Panx1 opening has been shown to induce neuronal death and aberrant firing, but its role in neuronal activity has not been established. Here, we report the role of the P2X7R-Panx1 complex in regulating muscarinic acetylcholine 1 (M1) receptor function. P2X7R knockout (P2X7-/-) mice showed greater susceptibility to seizures induced by pilocarpine (PILO), an M1 receptor agonist, than their WT littermates, despite having similar levels of hippocampal M1 receptor expression. This hypersensitivity to PILO in the P2X7-/- mice did not involve the GABA or glutamate system. Both administration of P2X7R antagonists and gene silencing of P2X7R or Panx1 in WT mice increased PILO-induced seizure susceptibility in a process mediated by PKC via intracellular Ca2+ release. Therefore, we suggest that the P2X7R-Panx1 complex may play an important role as a negative modulator of M1 receptor-mediated seizure activity in vivo.

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.

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.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

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.

Problems caused by high concentration of ATP on activation of the P2X7 receptor in bone marrow cells loaded with the Ca2+ fluorophore fura-2

Fura-2 is one of the most used fluorophore for measuring intracellular calcium concentration ([Ca2+]i). In mouse bone marrow cell suspensions ATP produces a biphasic effect: till 1 mM, ATP produces increases in [Ca2+]i; from 1 mM on an increase is observed, that is followed by the decrease in the 340/380 nm ratio (R340/380). At high ATP (4 mM) concentration fura-2 leaked from loaded bone marrow cell suspensions. We observed that ATP decreases fluorescence in the absorption and excitation spectra of fura-2, consequently the emitted one is decreased including the isobestic point (360 nm). ATP analogs: BzATP, ATPyS and UTP, but not alphabetaATP, ADP or AMP, promote decrease of fluorescence in the isobestic point of fura-2. The physical/chemical process that reduces the absorption and excitation of fura-2 by ATP is unknown. The P2X7 inhibitors, Mg2+ (5 mM), OxATP (300 microM) and Brilliant Blue (100 nM), blocked the efflux of fura-2 and ATP-induced R340/380 decrease. The J774 cell line and mononuclear cells with a higher expression of P2X7 receptors show the same decrease in R340/380 as that induced by ATP. In the HL-60 cell line, myeloid cells and erythroblasts extracted from bone marrow, such effect does not occur. It is concluded that the use of the fluorescent Ca2+ indicator fura-2 does not allow the correct measurement of [Ca2+]i in these cells in the presence of a higher concentration of ATP which activated the P2X7 receptor.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Identification of cyclic ADP-ribose-dependent mechanisms in pancreatic muscarinic Ca(2+) signaling using CD38 knockout mice

We showed that muscarinic acetylcholine (ACh)-stimulation increased the cellular content of cADPR in the pancreatic acinar cells from normal mice but not in those from CD38 knockout mice. By monitoring ACh-evoked increases in the cytosolic Ca(2+) concentration ([Ca(2+)](i)) using fura-2 microfluorimetry, we distinguished and characterized the Ca(2+) release mechanisms responsive to cADPR. The Ca(2+) response from the cells of the knockout mice (KO cells) lacked two components of the muscarinic Ca(2+) release present in wild mice. The first component inducible by the low concentration of ACh contributed to regenerative Ca(2+) spikes. This component was abolished by ryanodine treatment in the normal cells and was severely impaired in KO cells, indicating that the low ACh-induced regenerative spike responses were caused by cADPR-dependent Ca(2+) release from a pool regulated by a class of ryanodine receptors. The second component inducible by the high concentration of ACh was involved in the phasic Ca(2+) response, and it was not abolished by ryanodine treatment. Overall, we conclude that muscarinic Ca(2+) signaling in pancreatic acinar cells involves a CD38-dependent pathway responsible for two cADPR-dependent Ca(2+) release mechanisms in which the one sensitive to ryanodine plays a crucial role for the generation of repetitive Ca(2+) spikes.

Adenosine triphosphate-induced heterologous desensitization of endothelin-1- and glutamate-evoked calcium increases in cultured rat cortical astrocytes

In rat cortical astrocytes, we investigated the occurrence of cross-talks between purinoceptor and endothelin (ET) receptor, or glutamate receptor. The treatments of adenosine triphosphate (ATP), ET-1, and glutamate induced the increase of intracellular calcium level in the astrocytes. In repetitive additions of ATP to astrocytes, the second application of ATP exhibited comparable amplitude of calcium response, but the stimulation with ATP completely blocked subsequent ET-1- or glutamate-evoked calcium responses showing complete heterologous desensitization. In contrast, ET-1 and glutamate failed to desensitize the response elicited by ATP. Preincubation with sphingosine, a protein kinase C (PKC) inhibitor, reversed the ATP-induced desensitization of ET-1- and glutamate-evoked calcium responses. Taken together, these results demonstrate the resistance of purinoceptor to homologous desensitization, and unidirectional desensitization between ATP and other receptors such as ET and glutamate receptors, suggesting a dominant role of purinoceptor in modulating calcium signal of astrocytes.

Regulation of acetylcholine binding by ATP at the muscarinic M(1) receptor in intact CHO cells

ATP may have a modulatory effect on cholinergic transmission, as it is known that ATP is released as a co-transmitter with acetylcholine from nerve terminals. The ability of ATP to influence the binding of acetylcholine to the M(1) muscarinic acetylcholine receptor expressed in intact CHO cells was investigated. In competition binding experiments, acetylcholine completely inhibited the binding of [3H]N-methylscopolamine, but yielded a shallow competition isotherm that was best described in terms of two affinity states. When these experiments were repeated in the presence of 1 mM ATP, the acetylcholine competition curve was better described in terms of a single, low-affinity state with a Hill slope not significantly different from unity. This modulatory effect of ATP was completely reversed by the addition of the P(2) purinoceptor antagonist, suramin, to the assay medium. When the competition between the muscarinic receptor antagonist, atropine, and [3H]N-methylscopolamine was investigated, however, ATP was unable to modulate the binding of atropine, which was consistent with a one-site binding model in each instance. In contrast to the intact cell studies, ATP did not affect either affinity state of acetylcholine binding when studied in homogenate preparations. The results of the present study indicate that ATP, acting via endogenously expressed purinoceptors, is able to influence agonist binding to the M(1) muscarinic acetylcholine receptor via a cross-talk that requires the functional integrity of intact CHO cells.