Characterization of P2X7 purinergic receptors and their function in rat lacrimal gland

Genomic DNA activates the AIM2 inflammasome and STING pathways to induce inflammation in lacrimal gland myoepithelial cells

PURPOSE

Primary Sjögren's syndrome (pSS) is an autoimmune disease that mainly attacks the lacrimal glands causing severe aqueous-deficient dry eye. Clinical evidence indicates the DNA sensing mechanism in the pathogenesis of pSS. The purpose of the present study is to determine the pro-inflammatory effect of self-genomic DNA (gDNA) on myoepithelial cells (MECs), which along with acinar and ductal cells is a major cell type of the lacrimal gland.

METHOD

MECs primary culture was acquired from female C57BL6J mice. Genomic DNA was extracted from the spleen of the same animal. The MECs were challenged with self-gDNA. The cytokine secretion was detected using supernatant by enzyme-linked immunosorbent assay (ELISA). The activation of inflammasomes was determined using FAM-FLICA. Cryosections of NOD.B10.H2b mouse model of pSS were obtained for immunofluorescence microscopy (IF), with Balb/C as control.

RESULT

Treatment with gDNA activated AIM2 inflammasome assembly and function, leading to secretion of interleukin (IL)-1β and IL-18 in MECs. The stimulation of IL-1β secretion by gDNA appeared to be solely at the post-translational level, whereas IL-18 secretion was a combination of increased protein synthesis and post-translational modification. Genomic DNA also induced the activation of STimulators of INterferon Genes (STING), which correlated to the activation of STING in the lacrimal gland from the NOD.B10.H2b mouse. STING activation led to the secretion of IFN-β via Nuclear Factor-κB (NF-κB). The IFN-β further enhances the secretion of IL-1β. The contractility of MECs was disabled by treatment with gDNA or poly AnT, independent of the level of intracellular [Ca2+].

CONCLUSION

Self-gDNA induces a proinflammatory response in lacrimal gland MECs by activating both the AIM2 inflammasome and STING and thus may contribute to the pathogenesis of pSS.

Novel Insights Into Muscarinic and Purinergic Responses in Primary Cultures of Rat Lacrimal Gland Myoepithelial Cells

Purpose

The functional characteristics of receptors that regulate lacrimal gland myoepithelial cells are still somewhat unclear. To date, mainly muscarinic receptors have been of interest; however, further knowledge is needed regarding their expression and functional roles. For this purpose, primary cultures of rat lacrimal gland myoepithelial cells were established and examined functionally.

Methods

Rat lacrimal glands were excised, minced, and further digested, yielding mixtures of cells that were seeded in culturing flasks. After 4-6 weeks, primary monocultures of myoepithelial cells were established, verified by immunocytochemistry. The cells were stained for all muscarinic receptor subtypes (M1–M5) and examined functionally regarding intracellular [Ca²⁺] responses upon activation of muscarinic receptors. For methodological verification, purinergic functional responses were also studied.

Results

Expression of muscarinic receptor subtypes M2-M5 was detected, whereas expression of muscarinic M1 receptors could not be shown. Activation of muscarinic receptors by the non-selective muscarinic agonist methacholine (3 × 10⁻¹¹–10⁻³ M) did not cause a significant increase in intracellular [Ca²⁺]. However, activation of purinergic receptors by the non-selective purinergic agonist ATP (10⁻⁸–10⁻³ M) caused a concentration-dependent increase in intracellular [Ca²⁺] that could be blocked by the P2 antagonists PPADS and suramin.

Conclusions

Primary cultures of rat lacrimal gland myoepithelial cells were established that displayed a heterogeneous expression of muscarinic receptors. Purinergic functional responses demonstrated a viable cell population. Upon treatment with methacholine, no significant increase in intracellular [Ca²⁺] could be detected, indicating that cholinergic activation of myoepithelial cells occurs via other intracellular messengers or is dependent on interaction with other cell types.

Evaluation of the Presence and Functional Importance of Nucleoside Transporters in Lacrimal Gland for Tear Disposition of Intravenously Injected Substrate in Rabbits

Purpose: Purpose of the current study was to assess the presence and functionality of the nucleoside transporters in the lacrimal gland for the tear disposition of its substrate given intravenously in rabbits.Materials and Methods: Rabbits were divided into two groups - control and blocker pretreated. The blocker pretreated group received 5 mg/kg of dipyridamole 30 min before ribavirin (substrate), which was given at a dose of 2.5 mg/kg. All the treatments were given intravenously. Blood and tear samples were collected at 5, 15, 30, 60, 90, 120, 180, 240, 300 and 360 min (n = 4; each time point) after substrate administration. Tear samples were collected on Schirmer's strips, and plasma was separated immediately after blood collection. All the samples were stored at -80°C until analysis by LC-MS/MS.Results: Plasma ribavirin concentration for blocker pretreated group showed significantly (p < .05) higher levels at 5, 15, 30, 60, 120, 180 and 300 min as compared to the control group. Similarly, tear ribavirin concentration for blocker pretreated group also showed a significant (p < .05) increase at 5, 15, 60, 90, 180, 240 and 300 min compared to the control group. Plasma and tear AUC_(0-6) for blocker pretreated group was 1.7 (p < .001) and 2.42 (p < .001) folds higher in a significant manner as compared to the control group, respectively. Percentage penetration of ribavirin from plasma to tears was also different between control and blocker pretreated group. Permeation ratio of ribavirin from plasma to tear for blocker pretreated group was found to be 1.4-folds higher in a significant (p < .05) manner.Conclusion: It is evident from the results that nucleoside transporters are present in lacrimal gland. The blocker treatment induced increase in tear transport of ribavirin indicates the possibility of the presence of nucleoside transporters on the apical side of lacrimal acinar cells in the uptake position.

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 P2X7 receptor and pannexin-1 are involved in glucose-induced autocrine regulation in β-cells

Extracellular ATP is an important short-range signaling molecule that promotes various physiological responses virtually in all cell types, including pancreatic β-cells. It is well documented that pancreatic β-cells release ATP through exocytosis of insulin granules upon glucose stimulation. We hypothesized that glucose might stimulate ATP release through other non-vesicular mechanisms. Several purinergic receptors are found in β-cells and there is increasing evidence that purinergic signaling regulates β-cell functions and survival. One of the receptors that may be relevant is the P2X7 receptor, but its detailed role in β-cell physiology is unclear. In this study we investigated roles of the P2X7 receptor and pannexin-1 in ATP release, intracellular ATP, Ca²⁺ signals, insulin release and cell proliferation/survival in β-cells. Results show that glucose induces rapid release of ATP and significant fraction of release involves the P2X7 receptor and pannexin-1, both expressed in INS-1E cells, rat and mouse β-cells. Furthermore, we provide pharmacological evidence that extracellular ATP, via P2X7 receptor, stimulates Ca²⁺ transients and cell proliferation in INS-1E cells and insulin secretion in INS-1E cells and rat islets. These data indicate that the P2X7 receptor and pannexin-1 have important functions in β-cell physiology, and should be considered in understanding and treatment of diabetes.

TFOS DEWS II pain and sensation report

Pain associated with mechanical, chemical, and thermal heat stimulation of the ocular surface is mediated by trigeminal ganglion neurons, while cold thermoreceptors detect wetness and reflexly maintain basal tear production and blinking rate. These neurons project into two regions of the trigeminal brain stem nuclear complex: ViVc, activated by changes in the moisture of the ocular surface and VcC1, mediating sensory-discriminative aspects of ocular pain and reflex blinking. ViVc ocular neurons project to brain regions that control lacrimation and spontaneous blinking and to the sensory thalamus. Secretion of the main lacrimal gland is regulated dominantly by autonomic parasympathetic nerves, reflexly activated by eye surface sensory nerves. These also evoke goblet cell secretion through unidentified efferent fibers. Neural pathways involved in the regulation of meibomian gland secretion or mucin release have not been identified. In dry eye disease, reduced tear secretion leads to inflammation and peripheral nerve damage. Inflammation causes sensitization of polymodal and mechano-nociceptor nerve endings and an abnormal increase in cold thermoreceptor activity, altogether evoking dryness sensations and pain. Long-term inflammation and nerve injury alter gene expression of ion channels and receptors at terminals and cell bodies of trigeminal ganglion and brainstem neurons, changing their excitability, connectivity and impulse firing. Perpetuation of molecular, structural and functional disturbances in ocular sensory pathways ultimately leads to dysestesias and neuropathic pain referred to the eye surface. Pain can be assessed with a variety of questionaires while the status of corneal nerves is evaluated with esthesiometry and with in vivo confocal microscopy.

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.

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.

Signaling Pathways of Purinergic Receptors and Their Interactions with Cholinergic and Adrenergic Pathways in the Lacrimal Gland

PURPOSE

Purinergic receptors play a key role in the function of the lacrimal gland (LG) as P1 purinergic receptors A₁, A_2A, and A_2B, P2X_1-7 receptors, and many of the P2Y receptors are expressed.

METHODS

This review examines the current knowledge of purinergic receptors in the LG as well as the signaling pathways activated by these receptors.

RESULTS

These receptors are expressed on the acinar, ductal, and myoepithelial cells. Considerable crosstalk exists between the pathways activated by P2X₇ receptors with those activated by M₃ muscarinic or α_1D adrenergic receptors. The mechanism of the crosstalk between P2X₇ and M₃ muscarinic receptors differs from that of the crosstalk between P2X₇ and α_1D adrenergic receptors.

CONCLUSIONS

Understanding purinergic receptors and how they modulate protein secretion could play a key role in normal and pathological responses of the LG.

The role of dinucleoside polyphosphates on the ocular surface and other eye structures

Dinucleoside polyphosphates comprises a group of dinucleotides formed by two nucleosides linked by a variable number of phosphates, abbreviated NpnN (where n represents the number of phosphates). These compounds are naturally occurring substances present in tears, aqueous humour and in the retina. As the consequence of their presence, these dinucleotides contribute to many ocular physiological processes. On the ocular surface, dinucleoside polyphosphates can stimulate tear secretion, mucin release from goblet cells and they help epithelial wound healing by accelerating cell migration rate. These dinucleotides can also stimulate the presence of proteins known to protect the ocular surface against microorganisms, such as lysozyme and lactoferrin. One of the latest discoveries is the ability of some dinucleotides to facilitate the paracellular way on the cornea, therefore allowing the delivery of compounds, such as antiglaucomatous ones, more easily within the eye. The compound Ap₄A has been described being abnormally elevated in patient's tears suffering of dry eye, Sjogren syndrome, congenital aniridia, or after refractive surgery, suggesting this molecule as biomarker for dry eye condition. At the intraocular level, some diadenosine polyphosphates are abnormally elevated in glaucoma patients, and this can be related to the stimulation of a P2Y₂ receptor that increases the chloride efflux and water movement in the ciliary epithelium. In the retina, the dinucleotide dCp₄U, has been proven to be useful to help in the recovery of retinal detachments. Altogether, dinucleoside polyphosphates are a group of compounds which present relevant physiological actions but which also can perform promising therapeutic benefits.

Homodimeric anoctamin-1, but not homodimeric anoctamin-6, is activated by calcium increases mediated by the P2Y1 and P2X7 receptors

The P2X7 receptor (P2X7R) is a ligand-gated ion channel that conducts Na(+), K(+), and Ca(2+) when activated by extracellular ATP. In various cell types, such as secretory epithelia, the P2X7R is co-expressed with Ca(2+)-dependent Cl(-) channels of the TMEM16/anoctamin family. Here, we studied whether the P2X7R and TMEM16A/anoctamin-1 (Ano1) or TMEM16F/anoctamin-6 (Ano6) interact functionally and physically, using oocytes of Xenopus laevis and Ambystoma mexicanum (Axolotl) for heterologous expression. As a control, we co-expressed anoctamin-1 with the P2Y1 receptor (P2Y1R), which induces the release of Ca(2+) from intracellular stores via activating phospholipase C through coupling to Gαq. We found that co-expression of anoctamin-1 with the P2Y1R resulted in a small transient increase in Cl(-) conductance in response to ATP. Co-expression of anoctamin-1 with the P2X7R resulted in a large sustained increase in Cl(-) conductance via Ca(2+) influx through the ATP-opened P2X7R in Xenopus and in Axolotl oocytes, which lack endogenous Ca(2+)-dependent Cl(-) channels. P2Y1R- or P2X7R-mediated stimulation of Ano1 was primarily functional, as demonstrated by the absence of a physically stable interaction between Ano1 and the P2X7R. In the pancreatic cell line AsPC-1, we found the same functional Ca(2+)-dependent interaction of P2X7R and Ano1. The P2X7R-mediated sustained activation of Ano1 may be physiologically relevant to the time course of stimulus-secretion coupling in secretory epithelia. No such increase in Cl(-) conductance could be elicited by activating the P2X7 receptor in either Xenopus oocytes or Axolotl oocytes co-expressing Ano6. The lack of function of Ano6 can, at least in part, be explained by its poor cell-surface expression, resulting from a relatively inefficient exit of the homodimeric Ano6 from the endoplasmic reticulum.

Purines in the eye: recent evidence for the physiological and pathological role of purines in the RPE, retinal neurons, astrocytes, Müller cells, lens, trabecular meshwork, cornea and lacrimal gland

This review highlights recent findings that describ how purines modulate the physiological and pathophysiological responses of ocular tissues. For example, in lacrimal glands the cross-talk between P2X7 receptors and both M3 muscarinic receptors and α1D-adrenergic receptors can influence tear secretion. In the cornea, purines lead to post-translational modification of EGFR and structural proteins that participate in wound repair in the epithelium and influence the expression of matrix proteins in the stroma. Purines act at receptors on both the trabecular meshwork and ciliary epithelium to modulate intraocular pressure (IOP); ATP-release pathways of inflow and outflow cells differ, possibly permitting differential modulation of adenosine delivery. Modulators of trabecular meshwork cell ATP release include cell volume, stretch, extracellular Ca(2+) concentration, oxidation state, actin remodeling and possibly endogenous cardiotonic steroids. In the lens, osmotic stress leads to ATP release following TRPV4 activation upstream of hemichannel opening. In the anterior eye, diadenosine polyphosphates such as Ap4A act at P2 receptors to modulate the rate and composition of tear secretion, impact corneal wound healing and lower IOP. The Gq11-coupled P2Y1-receptor contributes to volume control in Müller cells and thus the retina. P2X receptors are expressed in neurons in the inner and outer retina and contribute to visual processing as well as the demise of retinal ganglion cells. In RPE cells, the balance between extracellular ATP and adenosine may modulate lysosomal pH and the rate of lipofuscin formation. In optic nerve head astrocytes, mechanosensitive ATP release via pannexin hemichannels, coupled with stretch-dependent upregulation of pannexins, provides a mechanism for ATP signaling in chronic glaucoma. With so many receptors linked to divergent functions throughout the eye, ensuring the transmitters remain local and stimulation is restricted to the intended target may be a key issue in understanding how physiological signaling becomes pathological in ocular disease.

Loss of P2X7 nucleotide receptor function leads to abnormal fat distribution in mice

The P2X7 receptor is an ATP-gated cation channel expressed by a number of cell types. We have shown previously that disruption of P2X7 receptor function results in downregulation of osteogenic markers and upregulation of adipogenic markers in calvarial cell cultures. In the present study, we assessed whether loss of P2X7 receptor function results in changes to adipocyte distribution and lipid accumulation in vivo. Male P2X7 loss-of-function (KO) mice exhibited significantly greater body weight and epididymal fat pad mass than wild-type (WT) mice at 9 months of age. Fat pad adipocytes did not differ in size, consistent with adipocyte hyperplasia rather than hypertrophy. Histological examination revealed ectopic lipid accumulation in the form of adipocytes and/or lipid droplets in several non-adipose tissues of older male KO mice (9-12 months of age). Ectopic lipid was observed in kidney, extraorbital lacrimal gland and pancreas, but not in liver, heart or skeletal muscle. Specifically, lacrimal gland and pancreas from 12-month-old male KO mice had greater numbers of adipocytes in perivascular, periductal and acinar regions. As well, lipid droplets accumulated in the renal tubular epithelium and lacrimal acinar cells. Blood plasma analyses revealed diminished total cholesterol levels in 9- and 12-month-old male KO mice compared with WT controls. Interestingly, no differences were observed in female mice. Moreover, there were no significant differences in food consumption between male KO and WT mice. Taken together, these data establish novel in vivo roles for the P2X7 receptor in regulating adipogenesis and lipid metabolism in an age- and sex-dependent manner.

Nucleotides in the eye: focus on functional aspects and therapeutic perspectives

The presence and activity of nucleotides and dinucleotides in the physiology of most, if not all, organisms, from bacteria to humans, have been recognized by the scientific community, and the eye is no exception. Nucleotides in the dynamic fluids interact with many ocular structures, such as the tears and aqueous humor. Moreover, high concentrations of nucleotides in these secretions may reflect disease states such as dry eye and glaucoma. Apart from the nucleotide concentration in these fluids, P2 purinergic receptors have been described on the ocular surface (cornea and conjunctiva), anterior pole (ciliary body, trabecular meshwork), and posterior pole (retina). P2X and P2Y purinergic receptors are essential in maintaining the homeostasis of ocular processes, such as tear secretion, aqueous humor production, or retinal modulation. When they are functioning properly, they allow the eye to do its job (to see), but in some cases, a lack or an excess of nucleotides or a malfunction in the corresponding purinergic receptors leads to disease. This Perspective is focused on the nucleotides and dinucleotides and the P2 purinergic receptors in the eye and how they contribute to normal and disease states. We also emphasize the action of nucleotides and their receptors and antagonists as potential therapeutic agents.

Increase of intracellular Ca2+ by purinergic receptors in cultured rat lacrimal gland myoepithelial cells

PURPOSE

To isolate and characterize cultured myoepithelial cells (MECs) from rat lacrimal gland and determine which purinergic receptor subtypes are present and functional in MECs.

METHODS

Rat lacrimal glands were subjected to collagenase digestion, and MECs were grown. RT-PCR was performed for the purinergic receptors P2X(7), P2Y(1), P2Y(11), and P2Y(13) on RNA isolated from the MECs. Immunofluorescence experiments were performed with antibodies against MEC markers and P2X(7), P2Y(1), P2Y(11), and P2Y(13) purinergic receptors. Proteins from MECs were separated using Western blot analysis techniques. In addition, cells were incubated with Fura 2 tetra acetoxymethyl ester, and intracellular [Ca(2+)] ([Ca(2+)](i)) was determined in response to P2 purinergic agonists.

RESULTS

MECs expressed the MEC proteins α-smooth muscle actin, vimentin, α-actinin, and adenylyl cyclase II. RT-PCR, Western blot, and immunofluorescence techniques demonstrated the presence of the purinergic receptors P2X(7), P2Y(1), P2Y(11), and P2Y(13). The purinergic agonists ATP, benzoylbenzoyl ATP (BzATP), α,β methylene ATP, UTP, 2-methylthioATP (MeSATP), and ATPγS increased [Ca(2+)](i). As BzATP binds to the P2X(7) receptor, specific characteristics of this receptor were investigated. Neither inhibitors of P2X(7) receptors nor removal of extracellular Mg(2+) or Ca(2+) had an effect on the BzATP-stimulated increase in [Ca(2+)](i). Repeated applications of BzATP desensitized this response. Inhibitors for P2Y(1), P2Y(11), and P2Y(13) each decreased the BzATP-stimulated increase in [Ca(2+)](i) with the P2Y(1) inhibitor most effective.

CONCLUSIONS

MECs can be isolated from rat lacrimal glands, and they express P2X(7), P2Y(1), P2Y(11), and P2Y(13) purinergic receptors. Surprisingly, BzATP binds the P2Y(1) receptor, which is primarily responsible for the BzATP-stimulated increase in [Ca(2+)](i).

P2Y purinoceptors induce changes in intracellular calcium in acinar cells of rat lacrimal glands

Adenosine 5'-triphosphate (ATP) is an extracellular signal that regulates various cellular functions. Cellular secretory activities are enhanced by ATP as well as by cholinergic and adrenergic stimuli. The present study aimed to determine which purinoceptors play a role in ATP-induced changes in the intracellular concentration of calcium ions ([Ca²⁺](i)) and in the fine structure of acinar cells of rat lacrimal glands. ATP induced exocytotic structures, vacuolation and an increase in [Ca²⁺](i) in acinar cells. The removal of extracellular Ca²⁺ or the use of Ca²⁺ channel blockers partially inhibited the ATP-induced [Ca²⁺](i) increase. U73122 (an antagonist of PLC) and heparin (an antagonist of IP₃ receptors) did not completely inhibit the ATP-induced [Ca²⁺](i) increase. P1 purinoceptor agonists did not induce any changes in [Ca²⁺](i), whereas suramin (an antagonist of P2 receptors) completely inhibited ATP-induced changes in [Ca²⁺](i). A P2Y receptor agonist, 2-MeSATP, induced a strong increase in [Ca²⁺](i), although UTP (a P2Y₂,₄,₆ receptor agonist) had no effect, and reactive blue 2 (a P2Y receptor antagonist) resulted in partial inhibition. The potency order of ATP analogs (2-MeSATP > ATP >>> UTP) suggested that P2Y₁ played a significant role in the cellular response to ATP. BzATP (a P2X₇ receptor agonist) induced a small increase in [Ca²⁺](i), but α,β-meATP (a P2X₁,₃ receptor agonist) had no effect. RT-PCR indicated that P2X₂,₃,₄,₅,₆,₇ and P2Y₁,₂,₄,₁₂,₁₄ are expressed in acinar cells. In conclusion, the response of acinar cells to ATP is mediated by P2Y (especially P2Y₁) as well as by P2X purinoceptors.

Interaction of alpha1D-adrenergic and P2X(7) receptors in the rat lacrimal gland and the effect on intracellular [Ca2+] and protein secretion

PURPOSE

To determine whether α(1D)-adrenergic receptors (α(1D)-AR) and P2X(7) receptors interact by determining their effect on ATP release, intracellular [Ca(2+)] ([Ca(2+)](i)), and protein secretion in rat lacrimal gland acini.

METHODS

Exorbital lacrimal glands from male Sprague-Dawley rats were divided into pieces or digested with collagenase to form acini. With the use of an imaging system, [Ca(2+)](i) was measured in acini loaded with fura-2. 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 P2X(7) receptor agonist, (benzoylbenzoyl)adenosine 5' triphosphate (BzATP) or the α(1D)-AR agonist phenylephrine with or without antagonist preincubation.

RESULTS

Phenylephrine increased ATP release from pieces in a time-dependent manner. The α(1D)-AR antagonist BMY7378 blocked the BzATP-stimulated increase in [Ca(2+)](i) but not in peroxidase secretion. The P2X(7) antagonist A438079 blocked the phenylephrine-stimulated increase in [Ca(2+)](i) but not peroxidase secretion. The increase in [Ca(2+)](i) caused by phenylephrine and BzATP used simultaneously or sequentially was additive, as was the increase in peroxidase secretion. The inhibition of protein kinase C isoforms or calcium calmodulin kinase II did not alter the BzATP-induced increase in [Ca(2+)](i).

CONCLUSIONS

The authors conclude that activation of α(1D)-AR releases ATP, which induces P2X(7) receptors to increase [Ca(2+)](i) but not to stimulate protein secretion. P2X(7) receptors in turn activate α(1D)-AR to increase [Ca(2+)](i) but not to stimulate protein secretion. Furthermore, α(1D)-AR compared with P2X(7) receptors use different cellular mechanisms to increase [Ca(2+)](i) and cause protein secretion.

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.

Inhibition of the P2X7 receptor reduces cystogenesis in PKD

The P2X7 receptor participates in purinergic signaling, which may promote the progression of ADPKD. We examined the effects of a P2X7 receptor antagonist and a P2X7 receptor agonist on cyst development in a zebrafish model of polycystic kidney disease in which we knocked down pkd2 by morpholinos. We used live wt-1b pronephric-specific GFP-expressing zebrafish embryos to directly observe changes in the pronephros. Exposure of pkd2-morphant zebrafish to a P2X7 receptor antagonist (oxidized ATP [OxATP]) significantly reduced the frequency of the cystic phenotype compared with either exposure to a P2X7 receptor agonist (BzATP) or with no treatment (P < 0.01). Histology confirmed improvement of glomerular cysts in OxATP-treated pkd2 morphants. OxATP also reduced p-ERK activity and cell proliferation in pronephric kidneys in pkd2 morphants. Inhibition of P2X7 with an additional specific antagonist (A-438079), and through morpholino-mediated knockdown of p2rx7, confirmed these effects. In conclusion, blockade of the P2X7 receptor reduces cyst formation via ERK-dependent pathways in a zebrafish model of polycystic kidney disease, suggesting that P2X7 antagonists may have therapeutic potential in ADPKD.

Identification of P2X₃ and P2X₇ purinergic receptors activated by ATP in rat lacrimal gland

PURPOSE. To identify the type of purinergic receptors activated by adenosine triphosphate (ATP) in rat lacrimal gland and to determine their role in protein secretion. METHODS. Purinergic receptors were identified by RT-PCR, Western blot analysis, and immunofluorescence techniques. Acini from rat lacrimal gland were isolated by collagenase digestion. Acini were incubated with the fluorescence indicator fura-2 tetra-acetoxylmethyl ester, and intracellular [Ca(2+)] ([Ca(2+)](i)) was determined. Protein secretion was measured by fluorescence assay. RESULTS. The authors previously showed that P2X(7)receptors were functional in the lacrimal gland. In this study, they show that P2X(1-4) and P2X(6)receptors were identified in the lacrimal gland by RT-PCR, Western blot, and immunofluorescence analyses. P2X(5) receptors were not detected. ATP increased [Ca(2+)](i) and protein secretion in a concentration-dependent manner. Removal of extracellular Ca(2+) significantly reduced the ATP-stimulated increase in [Ca(2+)](i). Repeated applications of ATP caused desensitization of the [Ca(2+)](i) response. Incubation with the P2X(1) receptor inhibitor NF023 did not alter ATP-stimulated [Ca(2+)](i). Incubation with zinc, which potentiates P2X(2) and P2X(4) receptor responses, or lowering the pH to 6.8, which potentiates P2X(2) receptor responses, did not alter the ATP-stimulated [Ca(2+)](i). P2X(3) receptor inhibitors A-317491 and TNP-ATP significantly decreased ATP-stimulated [Ca(2+)](i) and protein secretion, whereas the P2X(3) receptor agonist α,β methylene ATP significantly increased them. The P2X(7) receptor inhibitor A438079 had no effect on ATP-stimulated [Ca(2+)](i) at 10(-6) M but did have an effect at 10(-4) M. CONCLUSIONS. Purinergic receptors P2X(1-4) and P2X(6) are present in the lacrimal gland. ATP uses P2X(3) and P2X(7) receptors to stimulate an increase in [Ca(2+)](i) and protein secretion.

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.

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.