Characterization and localization of P2 receptors in rat submandibular gland acinar and duct cells

Pacing intracellular Ca2+ signals in exocrine acinar cells

An increase in intracellular [Ca2+ ] in exocrine acinar cells resident in the salivary glands or pancreas is a fundamental event that drives fluid secretion and exocytosis of proteins. Stimulation with secretagogues initiates Ca2+ signals with precise spatiotemporal properties thought to be important for driving physiological output. Both in vitro, in acutely isolated acini, and in vivo, in animals expressing genetically encoded indicators, individual cells appear specialized to initiate Ca2+ signals upon stimulation. Furthermore, these signals appear to spread to neighbouring cells. These properties are present in the absence of a conventional pacemaker mechanism dependent on the cyclical activation of Ca2+ -dependent or Ca2+ -conducting plasma membrane ion channels. In this article, we propose a model for 'pacing' intracellular Ca2+ signals in acinar cells based on the enhanced sensitivity of a subpopulation of individual cells and the intercellular diffusion through gap junctions of inositol 1,4,5-trisphosphate and Ca2+ to neighbouring cells.

Slow hydrogel matrix degradation enhances salivary gland mimetic phenotype

We recently developed a salivary gland tissue mimetic (SGm), comprised of salivary gland cells encapsulated in matrix metalloproteinase (MMP)-degradable poly(ethylene glycol) hydrogels within arrays of ∼320 µm diameter spherical cavities molded in PDMS. The SGm provides a functional and physiologically relevant platform well-suited to high-throughput drug screening for radioprotective compounds. However, the utility of the SGm would benefit from improved retention of acinar cell phenotype and function. We hypothesized that tuning biochemical cues presented within the PEG hydrogel matrix would improve maintenance of acinar cell phenotype and function by mimicking the natural extracellular matrix microenvironment of the intact gland. Hydrogels formed using slower-degrading MMP-sensitive peptide crosslinkers showed >2-fold increase in sphere number formed at 48 h, increased expression of acinar cell markers, and more robust response to calcium stimulation by the secretory agonist, carbachol, with reduced SGm tissue cluster disruption and outgrowth during prolonged culture. The incorporation of adhesive peptides containing RGD or IKVAV improved calcium flux response to secretory agonists at 14 days of culture. Tuning the hydrogel matrix improved cell aggregation, and promoted acinar cell phenotype, and stability of the SGm over 14 days of culture. Furthermore, combining this matrix with optimized media conditions synergistically prolonged the retention of the acinar cell phenotype in SGm. STATEMENT OF SIGNIFICANCE: Salivary gland (SG) dysfunction occurs due to off-target radiation due to head and neck cancer treatments. Progress in understanding gland dysfunction and developing therapeutic strategies for the SG are hampered by the lack of in vitro models, as salivary gland cells rapidly lose critical secretory function within 24 hours in vitro. Herein, we identify properties of poly(ethylene glycol) hydrogel matrices that enhance the secretory phenotype of SG tissue mimetics within the previously-described SG-microbubble tissue chip environment. Combining slow-degrading hydrogels with media conditions optimized for secretory marker expression further enhanced functional secretory response and secretory marker expression.

Xerostomia and Its Cellular Targets

Xerostomia, the subjective feeling of a dry mouth associated with dysfunction of the salivary glands, is mainly caused by radiation and chemotherapy, various systemic and autoimmune diseases, and drugs. As saliva plays numerous essential roles in oral and systemic health, xerostomia significantly reduces quality of life, but its prevalence is increasing. Salivation mainly depends on parasympathetic and sympathetic nerves, and the salivary glands responsible for this secretion move fluid unidirectionally through structural features such as the polarity of acinar cells. Saliva secretion is initiated by the binding of released neurotransmitters from nerves to specific G-protein-coupled receptors (GPCRs) on acinar cells. This signal induces two intracellular calcium (Ca2+) pathways (Ca2+ release from the endoplasmic reticulum and Ca2+ influx across the plasma membrane), and this increased intracellular Ca2+ concentration ([Ca2+]i) causes the translocation of the water channel aquaporin 5 (AQP5) to the apical membrane. Consequently, the GPCR-mediated increased [Ca2+]i in acinar cells promotes saliva secretion, and this saliva moves into the oral cavity through the ducts. In this review, we seek to elucidate the potential of GPCRs, the inositol 1,4,5-trisphosphate receptor (IP3R), store-operated Ca2+ entry (SOCE), and AQP5, which are essential for salivation, as cellular targets in the etiology of xerostomia.

Making Sense of Intracellular Nucleic Acid Sensing in Type I Interferon Activation in Sjögren's Syndrome

Primary Sjögren's syndrome (pSS) is a systemic autoimmune rheumatic disease characterized by dryness of the eyes and mucous membranes, which can be accompanied by various extraglandular autoimmune manifestations. The majority of patients exhibit persistent systemic activation of the type I interferon (IFN) system, a feature that is shared with other systemic autoimmune diseases. Type I IFNs are integral to anti-viral immunity and are produced in response to stimulation of pattern recognition receptors, among which nucleic acid (NA) receptors. Dysregulated detection of endogenous NAs has been widely implicated in the pathogenesis of systemic autoimmune diseases. Stimulation of endosomal Toll-like receptors by NA-containing immune complexes are considered to contribute to the systemic type I IFN activation. Accumulating evidence suggest additional roles for cytosolic NA-sensing pathways in the pathogenesis of systemic autoimmune rheumatic diseases. In this review, we will provide an overview of the functions and signaling of intracellular RNA- and DNA-sensing receptors and summarize the evidence for a potential role of these receptors in the pathogenesis of pSS and the sustained systemic type I IFN activation.

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.

Salivary extracellular vesicles can modulate purinergic signalling in oral tissues by combined ectonucleoside triphosphate diphosphohydrolases and ecto-5'-nucleotidase activities

We reported previously that the rat submandibular gland is able to release nanovesicles capable to hydrolyse millimolar concentrations of ATP, ADP and AMP in vitro. Here, we show that rat saliva also contains nanovesicles with the ability to hydrolyse ATP. Our aim was to identify and characterize vesicular nucleotidases by using kinetic, immunological and in silico approaches. Nucleotidase activity in the absence or presence of specific inhibitors allowed us to assume the participation of NTPDase1, -2 and -3, together with ecto-5'-nucleotidase, confirmed using specific antibodies. At neutral pH, initial ATPase activity would be mostly due to NTPDase2, which was thereafter inactivated, leaving NTPDase1 and NTPDase3 to hydrolyse ATP and ADP with an efficacy ATPase/ADPase around 2. Ecto-5'nucleotidase would be mainly responsible for AMP hydrolysis and adenosine accumulation. We proposed a kinetic model for NTPDase2 as a tool to isolate and analyse the turnover of this enzyme in the presence of different ATP concentrations, including those expected in extracellular media. Our study characterizes the ectonucleotidases carried by extracellular vesicles which contribute to modulate ATP and adenosine concentrations in the oral cavity, essential players in purinergic signalling.

Histamine stimulates secretion of extracellular vesicles with nucleotidase activity in rat submandibular gland

BACKGROUND

Extracellular vesicles released by different cells have been isolated from diverse fluids including saliva. We previously reported that rat submandibular glands secrete nanovesicles that catalyze hydrolysis of ATP, ADP and AMP, which are actors of the purinergic signaling system along with adenosine. Extracellular nucleotides like ATP and adenosine are involved in the regulation of inflammatory processes and apoptosis. Histamine, a widely distributed biogenic amine, is involved in inflammatory response.

OBJECTIVE

To test if activation of histamine receptors in rat submandibular gland promotes changes in the release of vesicles with nucleotidase activity that could modulate purinergic signaling.

METHODS

Rat submandibular glands were incubated in the absence or presence of histamine and JNJ7777120, an antagonist for H₄ receptors. Extracellular vesicles were isolated from incubation media by differential centrifugation. Vesicular nucleotidase activity was measured following Pi release by 3mM MgATP, MgADP or MgAMP.

RESULTS

Histamine increased the release of vesicles with nucleotidase activity in a concentration dependent manner. JNJ7777120 significantly reduced this effect. Vesicular nucleotidases obtained in the absence or presence of histamine promoted Pi production from ATP, ADP and AMP.

CONCLUSION

The results show a relationship between histamine and the regulation of purinergic signaling, which could be important in the modulation of inflammatory processes.

Rat submandibular glands secrete nanovesicles with NTPDase and 5'-nucleotidase activities

Extracellular nucleotides modulate a wide number of biological processes such as neurotransmission, platelet aggregation, muscle contraction, and epithelial secretion acting by the purinergic pathway. Nucleotidases as NTPDases and ecto-5'-nucleotidase are membrane-anchored proteins that regulate extracellular nucleotide concentrations. In a previous work, we have partially characterized an NTPDase-like activity expressed by rat submandibular gland microsomes, giving rise to the hypothesis that membrane NTPDases could be released into salivary ducts to regulate luminal nucleotide concentrations as was previously proposed for ovarian, prostatic, and pancreatic secretions. Present results show that rat submandibular glands incubated in vitro release membrane-associated NTPDase and ecto-5'-nucleotidase activities. Electron microscopy images show that released membranes presenting nucleotidase activity correspond to exosome-like vesicles which are also present at microsomal fraction. Both exosome release and nucleotidase activities are raised by adrenergic stimulation. Nucleotidase activities present the same kinetic characteristics than microsomal nucleotidase activity, corresponding mainly to the action of NTPDase2 and NTPDase3 isoforms as well as 5'-nucleotidase. This is consistent with Western blot analysis revealing the presence of these enzymes in the microsomal fraction.

Anoctamin 1 mediates thermal pain as a heat sensor

Vertebrates can sense and avoid noxious heat that evokes pain. Many thermoTRP channels are associated with temperature sensation. TRPV1 is a representative ion channel that is activated by noxious heat. Anoctamin 1 (ANO1) is a Cl- channel activated by calcium that is highly expressed in small sensory neurons, colocalized with markers for nociceptors, and most surprisingly, activated by noxious heat over 44oC. Although ANO1 is a Cl- channel, opening of this channel leads to depolarization of sensory neurons, suggesting a role in nociception. Indeed, the functional deletion of ANO1 in sensory neurons triggers the reduction in thermal pain sensation. Thus, it seems clear that ANO1 is a heat sensor in a nociceptive pathway. Since ANO1 modulators are developed for the purpose of treating chronic diseases such as cystic fibrosis, this finding is likely to predict unwanted effects and provide a guide for better developmental strategy

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.

Ectonucleotidases in the digestive system: focus on NTPDase3 localization

Extracellular nucleotides and adenosine are biologically active molecules that bind members of the P2 and P1 receptor families, respectively. In the digestive system, these receptors modulate various functions, including salivary, gastric, and intestinal epithelial secretion and enteric neurotransmission. The availability of P1 and P2 ligands is modulated by ectonucleotidases, enzymes that hydrolyze extracellular nucleotides into nucleosides. Nucleoside triphosphate diphosphohydrolases (NTPDases) and ecto-5'-nucleotidase are the dominant ectonucleotidases at physiological pH. While there is some information about the localization of ecto-5'-nucleotidase and NTPDase1 and -2, the distribution of NTPDase3 in the digestive system is unknown. We examined the localization of these ectonucleotidases, with a focus on NTPDase3, in the gastrointestinal tract and salivary glands. NTPDase1, -2, and -3 are responsible for ecto-ATPase activity in these tissues. Semiquantitative RT-PCR, immunohistochemistry, and in situ enzyme activity revealed the presence of NTPDase3 in some epithelial cells in serous acini of salivary glands and mucous acini and duct cells of sublingual salivary glands, in cells from the stratified esophageal and forestomach epithelia, and in some enteroendocrine cells of the gastric antrum. Interestingly, NTPDase2 and ecto-5'-nucleotidase are coexpressed with NTPDase3 in salivary gland cells and stratified epithelia. In the colon, neurons express NTPDase3 and glial cells express NTPDase2. Ca(2+) imaging experiments demonstrate that NTPDases regulate P2 receptor ligand availability in the enteric nervous system. In summary, the specific localization of NTPDase3 in the digestive system suggests functional roles of the enzyme, in association with NTPDase2 and ecto-5'-nucleotidase, in epithelial functions such as secretion and in enteric neurotransmission.

Extracellular ATP and P2Y2 receptors mediate intercellular Ca(2+) waves induced by mechanical stimulation in submandibular gland cells: Role of mitochondrial regulation of store operated Ca(2+) entry

Coordination of Ca(2+) signaling among cells contributes to synchronization of salivary gland cell function. However, mechanisms that underlie this signaling remain elusive. Here, intercellular Ca(2+) waves (ICW) in submandibular gland cells were investigated using Fura-2 fluorescence imaging. Mechanical stimulation of single cells induced ICW propagation from the stimulated cells through approximately 7 layers of cells or approximately 120microm. Our findings indicate that an extracellular ATP-dependent pathway is involved because the purinergic receptor antagonist suramin and the ATP hydrolyzing enzyme apyrase blocked ICW propagation. However, the gap junction uncoupler oleamide had no effect. ATP is released from mechanically stimulated cells possibly through opening of mechanosensitive maxi-anion channels, and does not appear to be directly linked to cytosolic Ca(2+). The ICW is propagated by diffusing ATP, which activates purinergic receptors in neighboring cells. This purinergic signaling induces a Ca(2+) transient that is dependent on Ca(2+) release via IP(3) receptors in the ER and store operated Ca(2+) entry (SOCE). Finally, inhibition of mitochondrial Ca(2+) uptake modified ICW indicating an important role of these organelles in this phenomenon. These studies increase our understanding of purinergic receptor signaling in salivary gland cells, and its role as a coordination mechanism of Ca(2+) signals induced by mechanical stimulation.

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.

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.

TMEM16A confers receptor-activated calcium-dependent chloride conductance

Calcium (Ca(2+))-activated chloride channels are fundamental mediators in numerous physiological processes including transepithelial secretion, cardiac and neuronal excitation, sensory transduction, smooth muscle contraction and fertilization. Despite their physiological importance, their molecular identity has remained largely unknown. Here we show that transmembrane protein 16A (TMEM16A, which we also call anoctamin 1 (ANO1)) is a bona fide Ca(2+)-activated chloride channel that is activated by intracellular Ca(2+) and Ca(2+)-mobilizing stimuli. With eight putative transmembrane domains and no apparent similarity to previously characterized channels, ANO1 defines a new family of ionic channels. The biophysical properties as well as the pharmacological profile of ANO1 are in full agreement with native Ca(2+)-activated chloride currents. ANO1 is expressed in various secretory epithelia, the retina and sensory neurons. Furthermore, knockdown of mouse Ano1 markedly reduced native Ca(2+)-activated chloride currents as well as saliva production in mice. We conclude that ANO1 is a candidate Ca(2+)-activated chloride channel that mediates receptor-activated chloride currents in diverse physiological processes.

Ca2+-Operated Transcriptional Networks: Molecular Mechanisms and In Vivo Models

Calcium is the most universal signal used by living organisms to convey information to many different cellular processes. In this review we present well-known and recently identified proteins that sense and decode the calcium signal and are key elements in the nucleus to regulate the activity of various transcriptional networks. When possible, the review also presents in vivo models in which the genes encoding these calcium sensors-transducers have been modified, to emphasize the critical role of these Ca2+-operated mechanisms in many physiological functions.

Characterisation of neurotransmitter-induced electrolyte transport in cockroach salivary glands by intracellular Ca2+, Na+ and pH measurements in duct cells

Ion-transporting acinar peripheral cells in cockroach salivary glands are innervated by dopaminergic and serotonergic fibres, but saliva-modifying duct cells are innervated only by dopaminergic fibres. We used microfluorometry to record intracellular Na+, Ca2+ and H+concentrations ([Na+]i, [Ca2+]iand pHi) in duct cells of two types of preparation, viz`lobes' consisting of acini with their duct system and `isolated ducts'without acini, in order to obtain information about the transporters involved in saliva secretion and/or modification. Our results indicate that (1)stimulation of lobes by dopamine (DA) causes a strong drop of pHiand increases in [Na+]i and[Ca2+]i in duct cells; (2) in contrast, DA stimulation of isolated ducts produces only a small pHi drop and no changes in[Na+]i and [Ca2+]i; (3)pHi and [Ca2+]i changes are also induced in duct cells by serotonin (5-HT) stimulation of lobes, but not isolated ducts;(4) in the absence of CO2/HCO3–, the DA-induced pHi drop is strongly reduced by removal of extracellular Cl– or inhibition of the Na+–K+–2Cl– cotransporter(NKCC); (5) in the presence of CO2/HCO3–, the DA-induced pHi drop is not reduced by NKCC inhibition, but rather by inhibition of the Cl–/HCO3–exchanger (AE), Na+/H+ exchanger (NHE) or carbonic anhydrase. We suggest that DA and 5-HT act predominantly on acinar peripheral cells. Their activity (secretion of primary saliva) seems to cause changes in ion concentrations in duct cells. NKCC and/or AE/NHE activities are necessary for pHi changes in duct cells; we consider that these transporters are involved in the secretion of the NaCl-rich primary saliva.

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.

Cyclic AMP regulates bicarbonate secretion in cholangiocytes through release of ATP into bile

BACKGROUND & AIMS

Bicarbonate secretion is a primary function of cholangiocytes. Either adenosine 3',5'-cyclic monophosphate (cAMP) or cytosolic Ca(2+) can mediate bicarbonate secretion, but these are thought to act through separate pathways. We examined the role of the inositol 1,4,5-trisphosphate receptor (InsP3R) in mediating bicarbonate secretion because this is the only intracellular Ca(2+) release channel in cholangiocytes.

METHODS

Intrahepatic bile duct units (IBDUs) were microdissected from rat liver then luminal pH was examined by confocal microscopy during IBDU microperfusion. Cyclic AMP was increased using forskolin or secretin, and Ca(2+) was increased using acetylcholine (ACh) or adenosine triphosphate (ATP). Apyrase was used to hydrolyze extracellular ATP, and suramin was used to block apical P2Y ATP receptors. In selected experiments, IBDUs were pretreated with short interfering RNA (siRNA) to silence expression of specific InsP3R isoforms.

RESULTS

Both cAMP and Ca(2+) agonists increased luminal pH. The effect of ACh on luminal pH was reduced by siRNA for basolateral (types I and II) but not apical (type III) InsP3R isoforms. The effect of forskolin on luminal pH was reduced by a cystic fibrosis transmembrane conductance regulator (CFTR) inhibitor and by siRNA for the type III InsP3R. Luminal apyrase or suramin blocked the effects of forskolin but not ACh on luminal pH.

CONCLUSIONS

Cyclic AMP-induced ductular bicarbonate secretion depends on an autocrine signaling pathway that involves CFTR, apical release of ATP, stimulation of apical nucleotide receptors, and then activation of apical, type III InsP3Rs. The primary role of CFTR in bile duct secretion may be to regulate secretion of ATP rather than to secrete chloride and/or bicarbonate.

Expression and localization of P2 nucleotide receptor subtypes during development of the lateral ventricular choroid plexus of the rat

The choroid plexuses secrete cerebrospinal fluid (CSF) and regulate the brain's internal environment via the blood-CSF barrier. The permeability properties of the blood-CSF interface have been studied previously in adult and immature brains, however, little is known about the development of CSF secretion and its modulation. ATP influences secretion in other epithelia via ionotropic P2X or metabotropic P2Y receptors. P2 receptors have frequently been found to be down-regulated in the postnatal period, suggesting a developmental role for purinergic and pyrimidine signalling. The present study investigated the expression of P2 receptors in lateral ventricular choroid plexus in relation to recent studies of aquaporin-1 expression and rapid expansion of the lateral ventricles in rat embryos. In the present study mRNAs for all known mammalian nucleotide receptor subtypes, except P2X(7), were identified from as early as E15. P2X(7) mRNA was detected from E18. Indications of differential expression patterns were observed for the different subtypes during development: an apparent increase in expression for P2Y(2) and P2X(7), a decline in P2X(1-2,4), no detectable difference in expression levels for P2X(6) and P2Y(12-13) and transient expression peaks for P2X(3,5) and P2Y(1,4,6,14). P2X(4,5,7) and P2Y(1,4) receptor proteins were detected immunohistochemically in the choroidal epithelium from early in development (E15 or E18). Their differing developmental profiles suggest specific roles in the development of CSF secretion that may have particular relevance for the rapid expansion of the ventricles that occurs in the embryo. P2X(5) and P2Y(6) were also detected in the developing neuropendyma from P0 and P9, respectively.

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.

Calcium signaling complexes in microdomains of polarized secretory cells

The highly polarized nature of epithelial cells in exocrine glands necessitates targeting, assembly into complexes and confinement of the molecules comprising the Ca(2+) signaling apparatus, to cellular microdomains. Such high degree of polarized localization has been shown for all Ca(2+) signaling molecules tested, including G protein coupled receptors and their associated proteins, Ca(2+) pumps, Ca(2+) influx channels at the plasma membrane and Ca(2+) release channels in the endoplasmic reticulum. Although the physiological significance of polarized Ca(2+) signaling is clear, little is known about the mechanism of targeting, assembly and retention of Ca(2+) signaling complexes in cellular microdomains. The present review attempts to summarize the evidence in favor of polarized expression of Ca(2+) signaling proteins at the apical pole of secretory cells with emphasis on the role of scaffolding proteins in the assembly and function of the Ca(2+) signaling complexes. The consequence of polarized enrichment of Ca(2+) signaling complexes at the apical pole is generation of an apical to basal pole gradient of cell responsiveness that, at low physiological agonist concentrations, limits Ca(2+) spikes to the apical pole, and when a Ca(2+) wave occurs, it always propagates from the apical to the basal pole. Our understanding of Ca(2+) signaling in microdomains is likely to increase rapidly with the application of techniques to controllably and selectively disrupt components of the complexes and apply high resolution recording techniques, such as TIRF microscopy to this problem.

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.

Membrane-specific expression of functional purinergic receptors in normal human nasal epithelial cells

Extracellular purines and pyrimidines regulate various physiological responses via the cell surface receptors known as purinoreceptors and may exert autocrine or paracrine effects on ion transport, fluid transport, ciliary beat frequency, and mucin secretion. Therefore, this study aims to investigate the expression patterns of the purinoreceptors in normal human nasal epithelial (NHNE) cells. In RT-PCR, the mRNAs for several P2X (P2X3, P2X4, P2X7) and P2Y (P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, P2Y12) receptors were identified in NHNE cells. Functional localizations of P2 receptors were investigated by measuring intracellular calcium concentration ([Ca2+]i) increases in membrane-specific manner using a double-perfusion chamber. Absence of the responses of alphabeta-methylene ATP and 2-methylthio-ATP excluded functionally active P2X3, P2X4, and P2Y1 receptors as far as [Ca2+]i increase is concerned. Applications with ATP and UTP revealed that luminal membranes of NHNE cells express P2Y2 and P2Y6 receptors and basolateral membranes express P2Y2 receptor. Expressions of P2Y2 and P2Y6 receptors in NHNE cells were further verified by immunoblotting using specific antibodies. In addition, the results with 2,3-O-(4-benzoyl)-benzoyl-ATP indicate that the P2Y11 receptor may be present on the luminal side. In conclusion, the NHNE cells express functionally active P2Y2, P2Y6, and P2Y11 receptors in a membrane-specific pattern, which may play an important role in the control of mucin and fluid secretion in NHNE cells.

Purinergic stimulation induces Ca2+-dependent activation of Na+-K+-2Cl- cotransporter in human nasal epithelia

Increasing evidence suggests that P2 receptors (P2Rs) in airway epithelial cells perform critical functions in auto- or paracrine regulation of fluid and mucus secretion. In the present study, we characterized the effects of P2R stimulation on Na(+)-K(+)-2Cl(-) cotransporter (NKCC) activity in normal human nasal epithelial (NHNE) cells. [Ca(2+)](i) and pH(i) were measured in primary cultures of NHNE cells using a double perfusion chamber, which enabled us to analyze membrane-specific transporter activities. NKCC activities were estimated by the pH(i) reduction due to Na(+)-dependent and bumetanide-sensitive intracellular uptake of NH(4)(+). NKCC activities were observed in the basolateral membrane, but not in the luminal membrane, of NHNE cells. Interestingly, P2Rs were expressed in both membranes, and the stimulation of either luminal or basolateral P2R increased NKCC activity. Blockades of luminal Cl(-) channels, basolateral K(+) channels, or protein kinase C did not affect the activation of NKCC by basolateral P2R stimulation. The effects of luminal P2R stimulation were partially reduced by Cl(-) channel blockers. However, chelation of intracellular Ca(2+) by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) treatment completely blocked the stimulatory effects of luminal and basolateral P2Rs on NKCC. In addition, increasing [Ca(2+)](i) by treatment with ionomycin-stimulated NKCC activity. These results provide evidence that stimulation of P2Rs directly activates basolateral NKCC by Ca(2+)-dependent pathways in NHNE cells, which is an important aspect of the purinergic regulation of ion and fluid secretions in human airway epithelia under physiologic and pathologic conditions.

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.

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

P2X7 receptors (P2X7Rs) affect many epithelial cell functions including transcellular ion transport, secretion, and cell death. Here we used parotid acinar and duct cells to reveal the unique cell-specific assembly and gating of the P2X7R channels. Immunolocalization indicated expression of P2X7Rs in the luminal membrane of both cell types. Stimulation with 5 mm ATP raised [Ca2+]i levels in a cell-specific manner and activated multiple currents. The current mediated by P2X7R was isolated by infusing the cells with high [EGTA]. The initial activation of acinar cell P2X7Rs by ATP was slow requiring approximately 2.5 min. Subsequent removal and addition of ATP, however, resulted in rapid inhibition and activation (gating) of the P2X7Rs. By contrast, P2X7Rs in duct cells displayed only rapid gating by ATP. Activation of P2X7Rs in both cell types was verified by (a) low Km for ATP, (b) sensitivity to external divalent ions, (c) lack of desensitization/inactivation, (d) permeability to Na+, and (e) inhibition by Brilliant Blue G, Cu2+, and pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid tetrasodium. The slow P2X7R activation in acinar cells was not affected by manipulation of exo-/endocytosis. Rather, disassembly or solidification of the actin cytoskeleton prior to incubation with ATP prevented channel assembly. Remarkably, after completion of the slow activation, manipulation of the actin cytoskeleton no longer affected gating by ATP. Accordingly, manipulation of the actin cytoskeleton had no effect on P2X7R gating by ATP in duct cells. We concluded that P2X7Rs are not active in resting acinar cells. On exposure to ATP, P2X7Rs are assembled into functional channels with the aid of the actin cytoskeleton. Once assembled, P2X7Rs are subject to rapid gating by ATP. Duct cell P2X7Rs are preassembled and therefore continually subject to rapid gating by ATP. This cell-specific behavior may reflect the specific function of P2X7Rs in the two cell types.

P2X7 receptor activates extracellular signal-regulated kinases ERK1 and ERK2 independently of Ca2+ influx

P2X7 nucleotide receptors modulate a spectrum of cellular events in various cells including epithelia, such as exocrine pancreas. Although the pharmacology and channel properties of the P2X7 receptors have been studied intensively, signal transduction pathways are relatively unknown. In this study we applied a heterologous expression system of rat P2X7 receptors in HEK-293 cells. We followed the receptor expression and function using the enhanced green fluorescent protein (EGFP) tag, activation of intracellular proteins and increases in cellular Ca2+. EGFP-P2X7 receptors localized to the plasma membrane, clusters within the membrane and intracellularly. Stimulation of P2X7 receptors in HEK-293 cells led to an activation of extracellular signal-regulated kinases ERK1 and ERK2 and this activation was seen after just 1 min of stimulation with ATP. Using C- and N-terminal P2X7-receptor mutants we show that the N-terminus is important in activation of ERKs, whereas deletion of the last 230 amino acids in the C-terminus did not effect ERK activation. On the other hand, Ca2+ entry was impaired in C-terminal but not in N-terminal mutants. In cell suspensions prepared from rat pancreas we show that P2X7 receptors also activate ERK1 and ERK2, indicating that these signalling pathways are also turned on in native epithelium.

Control of epithelial transport via luminal P2 receptors

P2 membrane receptors are specifically activated by extracellular nucleotides like ATP, ADP, UTP, and UDP. P2 receptors are subdivided into metabotropic P2Y and ionotropic P2X receptors. They are expressed in all tissues and induce a variety of biological effects. In epithelia, they are found in both the basolateral and the luminal membranes. Their widespread luminal expression in nearly all transporting epithelia and their effect on transport are summarized. The P2Y(2) receptor is a prominent luminal receptor in many epithelia. Other luminal P2 receptors include the P2X(7), P2Y(4), and P2Y(6) receptors. Functionally, luminal P2Y(2) receptor activation elicits differential effects on ion transport. In nearly all secretory epithelia, intracellular Ca(2+) concentration-activated ion conductances are stimulated by luminal nucleotides to induce Cl(-), K(+), or HCO(3)(-) secretion. This encompasses respiratory and various gastrointestinal epithelia or tissues like the conjunctiva of the eye and the epithelium of sweat glands. In the distal nephron, all active transport processes appear to be inhibited by luminal nucleotides. P2Y(2) receptors inhibit Ca(2+) and Na(+) absorption and K(+) secretion. Commonly, in all steroid-sensitive epithelia (lung, distal nephron, and distal colon), luminal ATP/UTP inhibits epithelial Na(+) channel-meditated Na(+) absorption. ATP is readily released from epithelial cells onto their luminal aspect, where ecto-nucleotidases promote their metabolism. Adenosine generated by the action of 5'-nucleotidase may elicit further effects on ion transport, often opposite those of ATP. ATP release from epithelia continues to be poorly understood. Integrated functional concepts for luminal P2 receptors are suggested: 1) luminal P2 receptors are part of an epithelial "secretory" defense mechanism; 2) they may be involved in the regulation of cell volume when transcellular solute transport is out of balance; 3) ATP and adenosine may be important autocrine/paracrine regulators mediating cellular protection and regeneration after ischemic cell damage; and 4) ATP and adenosine have been suggested to mediate renal cyst growth and enlargement in polycystic kidney disease.

A novel chloride conductance activated by extracellular ATP in mouse parotid acinar cells

Salivary gland fluid secretion is driven by transepithelial Cl- movement involving an apical Cl- channel whose molecular identity remains unknown. Extracellular ATP (ATP(o)) has been shown to activate a Cl- conductance (I(ATPCl)) in secretory epithelia; to gain further insight into I(ATPCl) in mouse parotid acinar cells, we investigated the effects of ATP(o) using the whole-cell patch-clamp technique. ATP(o) and 2'- and 3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate triethylammonium salt (Bz-ATP) produced concentration-dependent, time-independent Cl- currents with an EC50 of 160 and 15 microM, respectively. I(ATPCl) displayed a selectivity sequence of SCN- > I- = NO3- > Cl- > glutamate, similar to the Cl- channels activated by Ca2+, cAMP and cell swelling in acinar cells. In contrast, I(ATPCl) was insensitive to pharmacological agents that are known to inhibit these latter Cl- channels, was independent of Ca2+ and was not regulated by cell volume. Moreover, the I(ATPCl) magnitude from wild-type animals was comparable to that from mice with null mutations in the Cftr, Clcn3 and Clcn2 Cl- channel genes. Taken together, our results demonstrate that I(ATPCl) is distinct from the channels described previously in acinar cells. The activation of I(ATPCl) by Bz-ATP suggests that P2 nucleotide receptors are involved. However, inhibition of G-protein activation with GDP-beta-S failed to block I(ATPCl), and Cibacron Blue 3GA and 4,4'-diisothyocyanostilbene-2,2'-disulphonic disodium salt selectively inhibited the Na+ currents (presumably through P2X receptors) without altering I(ATPCl), suggesting that neither P2Y nor P2X receptors are likely to be involved in I(ATPCl) activation. We conclude that I(ATPCl) is not associated with Cl- channels previously characterized in mouse parotid acinar cells, nor is it dependent on P2 nucleotide receptor stimulation. I(ATPCl) expressed in acinar cells reflects the activation of a novel ATP-gated Cl- channel that may play an important physiological role in salivary gland fluid secretion.

ATP as a signaling molecule: the exocrine focus

Why and how do cells release ATP? It is not spilled energy. ATP becomes an extracellular regulator. Various cellular responses are initiated by purinergic receptors and signaling processes and are terminated by breakdown of ATP by ectonucleotidases. In epithelia, ATP regulates salt and water transport; other effects may be longer lasting.

Ca2+ activates cystic fibrosis transmembrane conductance regulator- and Cl- -dependent HCO3 transport in pancreatic duct cells

Pancreatic duct cells secrete bicarbonate-rich fluids, which are important for maintaining the patency of pancreatic ductal trees as well as intestinal digestive function. The bulk of bicarbonate secretion in the luminal membrane of duct cells is mediated by a Cl(-)-dependent mechanism (Cl(-)/HCO(3)(-) exchange), and we previously reported that the mechanism is CFTR-dependent and cAMP-activated (Lee, M. G., Choi, J. Y., Luo, X., Strickland, E., Thomas, P. J., and Muallem, S. (1999) J. Biol. Chem. 274, 14670-14677). In the present study, we provide comprehensive evidence that calcium signaling also activates the same CFTR- and Cl(-)-dependent HCO(3)(-) transport. ATP and trypsin evoked intracellular calcium signaling in pancreatic duct-derived cells through the activation of purinergic and protease-activated receptors, respectively. Cl(-)/HCO(3)(-) exchange activity was measured by recording pH(i) in response to [Cl(-)](o) changes of the perfusate. In perfusate containing high concentrations of K(+), which blocks Cl(-) movement through electrogenic or K(+)-coupled pathways, ATP and trypsin highly stimulated luminal Cl(-)/HCO(3)(-) exchange activity in CAPAN-1 cells expressing wild-type CFTR, but not in CFPAC-1 cells that have defective (DeltaF508) CFTR. Notably, adenoviral transfection of wild-type CFTR in CFPAC-1 cells completely restored the stimulatory effect of ATP on luminal Cl(-)/HCO(3)(-) exchange. In addition, the chelation of intracellular calcium by 1,2-bis(2-aminophenoxy)ethane-N,N,N,N'-tetraacetic acid (BAPTA) treatment abolished the effect of calcium agonists on luminal Cl(-)/HCO(3)(-) exchange. These results provide a molecular basis for calcium-induced bicarbonate secretion in pancreatic duct cells and highlight the importance of CFTR in epithelial bicarbonate secretion induced by various stimuli.

P2X7 receptors activate protein kinase D and p42/p44 mitogen-activated protein kinase (MAPK) downstream of protein kinase C

Protein kinase D (PKD), also called protein kinase Cmu (PKCmu), is a serine/threonine kinase that has unique enzymic and structural properties distinct from members of the PKC family of proteins. In freshly isolated rat parotid acinar salivary cells, extracellular ATP rapidly increased the activity and phosphorylation of PKD. The stimulation by ATP required high concentrations, was mimicked by the P2X(7) receptor ligand BzATP [2'- and 3'-O-(4-benzoylbenzoyl)ATP], and was blocked by Mg(2+) and 4,4'-di-isothiocyano-2,2'-stilbene disulphonate (DIDS), suggesting that activation of PKD was mediated by P2X(7) receptors, which are ligand-gated non-selective cation channels. Phorbol ester (PMA) and the activation of muscarinic and substance P receptors also increased PKD activity. PKC inhibitors blocked ligand-dependent PKD activation and phosphorylation, determined by in vitro phosphorylation studies and by phospho-specific antibodies to two activation loop sites (Ser(744) and Ser(748)) and an autophosphorylation site (Ser(916)). ATP and BzATP also increased the tyrosine phosphorylation and activity of PKCdelta, and these stimuli also increased extracellular signal-regulated protein kinase (ERK) 1/2 activity in a PKC-dependent manner. PKD activation was not promoted by pervanadate (an inhibitor of tyrosine phosphatases) and was not blocked by PP1 (an inhibitor of Src family kinases) or genistein (a tyrosine kinase inhibitor), suggesting that tyrosine kinases and phosphatases did not play a major role in PKD activation. P2X(7) receptor-mediated signalling events were not dependent on Ca(2+) entry. These studies indicate that PKC is involved in cellular signalling initiated by P2X(7) receptors as well as by G-protein-coupled receptors, and demonstrate that PKD and ERK1/2 are activated in similar PKC-dependent signalling pathways initiated by these diverse receptor types.

Calcium signalling in tissue: diversity and domain-specific integration of individual cell response in salivary glands

Organ function requires coordinated multicellular activities, which may require proper control of cell signalling dynamics at the supracellular level. By using high-speed confocal microscopy, we studied how calcium signalling is organised in the dissociated rat parotid gland. Salivary gland function is accomplished primarily by the compartmentalized epithelial domains, acini and ducts, the former involved in the production of primary saliva and the latter involved in its modification. Upon muscarinic stimulation with carbachol, both domains showed an increase in intracellular free calcium concentration ([Ca(2+)]i) with distinctive spatiotemporal kinetics, as indicated by the fluo-3 fluorescence. Acini responded initially, and the ducts followed with a time lag of more than 0.3 second. Cells comprising an acinus responded synchronously, whereas those in the ducts responded heterogeneously with respect to the latency period, magnitude of response and the requirement of extracellular calcium to raise [Ca(2+)]i. ATP also elicited a non-synchronous [Ca(2+)]i response in the duct domain, under a pattern different from that of carbachol. The synchronous oscillations seen in the acinar domain were made asynchronous by octanol, an agent known to inhibit gap-junction function. Accordingly, a gap junction component, connexin 32, was immunolocalised predominantly between the acinar cells. Moreover, expression of the type 2 inositol (1,4,5)-trisphosphate receptor [Ins(1,4,5)P(3)R] was homogeneous in the acinar domain but heterogeneous in the duct domain. Together, these data suggest that the calcium signalling system in salivary glands is constructed specifically according to the tissue architecture.

Luminal P2Y2 receptor-mediated inhibition of Na+ absorption in isolated perfused mouse CCD

Extracellular nucleotides regulate renal transport. A luminal P2Y2 receptor in mouse cortical collecting duct (CCD) principal cells has been demonstrated elsewhere. Herein the effects of adenosine triphosphate (ATP) and uridine triphosphate (UTP) on electrogenic Na+ absorption in perfused CCD of mice kept on a low-NaCl diet were investigated. Simultaneously, transepithelial voltage (V(te)), transepithelial resistance (R(te)), and fura-2 [Ca2+]i fluorescence were measured. Baseline parameters were V(te), -16.5 +/- 1.2 mV; R(te), 80.8 +/- 7.1 Omega cm2; and equivalent short-circuit current (I(sc)), -261.0 +/- 25.1 microA/cm2 (n = 45). Amiloride (10 microM) almost completely inhibited I(sc) to -3.9 +/- 3.8 microA/cm2 (n = 10). Luminal ATP (100 microM) reduced V(te) from -16.5 +/- 2.1 to -12.5 +/- 1.93 and increased R(te) from 113.1 +/- 16.2 to 123.8 +/- 16.7 Omega cm2, which resulted in a 31.7% inhibition of amiloride-sensitive I(sc) (n = 12). Similarly, luminal UTP reversibly reduced V(te) from -22.0 +/- 2.1 to -13.6 +/- 2.1 mV and increased R(te) from 48.4 +/- 5.3 to 59.2 +/- 7.1 Omega cm2, which resulted in 49.1% inhibition of Na+ absorption (n = 6). In parallel, luminal ATP and UTP elevated [Ca2+]i in CCD, increasing the fura-2 ratio by 2.7 +/- 0.7 and 4.0 +/- 1.2, respectively. Basolateral ATP and UTP (100 microM) also inhibited amiloride-sensitive I(sc) by 21.8 (n = 14) and 20.1% (n = 8), respectively. Inhibition of luminal nucleotide-induced [Ca2+]i increase by Ca2+ store depletion with cyclopiazonic acid (3 microM) did not affect nucleotide-mediated inhibition of Na+ transport (n = 7). No evidence indicated the activation of a luminal Ca2+-activated Cl- conductance, a phenomenon previously shown in M-1 CCD cells (J Physiol 524: 77-99, 2000). In essence, these data indicate that luminal ATP and UTP, most likely via P2Y2 receptors, mediate inhibition of amiloride-sensitive I(sc) in perfused mouse CCD. This inhibition appears to occurs independently of an increase of cytosolic Ca2+.

Evidence for basolateral P2Y(6) receptors along the rat proximal tubule: functional and molecular characterization

In this study, the distribution of P2Y(6) receptor mRNA in rat nephron segments was investigated and a functional approach was used to analyze basolateral protein expression. Reverse transcription-PCR studies revealed more intense expression of P2Y(6) receptor mRNA in the proximal tubule and the thick ascending limb of Henle's loop, less intense expression in the thin descending limb and the cortical and outer medullary collecting ducts, and no detectable expression in either the thin ascending limb or the inner medullary collecting duct. Dose-dependent calcium responses to basolateral administration of UDP (a selective agonist for the P2Y(6) receptor) were observed in the proximal tubule but not in any of the other segments studied. In the proximal tubule, intracellular calcium concentration changes induced by UDP were associated with increased production of inositol phosphates, as were those induced by ATP and norepinephrine. However, UDP-induced intracellular calcium concentration changes were different, exhibiting no plateau after the initial peak; moreover, a single stimulation with a high concentration of UDP induced full desensitization of the UDP-sensitive calcium pathway but did not alter the responsiveness of the proximal tubule to ADP (a specific P2Y(1) receptor agonist), ATP or norepinephrine. In summary, this report demonstrates that P2Y(6) receptor mRNA is expressed in most segments of the rat nephron but that basolateral expression of the protein is restricted to the proximal tubule, where the receptor is coexpressed with the P2Y(1) receptor. The differences in the distributions of P2Y(6) receptor mRNA and UDP responses may indicate the presence of luminal receptors in other nephron segments.

P2X(7) receptors in rat parotid acinar cells: formation of large pores

1. Permeabilization of cells mediated by P2X(7) receptors occurs to varied degrees in native and heterologous expression systems. Previous studies on P2X(7) receptors in parotid acinar cells suggested that ATP does not permeabilize these cells. 2. Modification of the assay conditions showed that ATP permeabilizes freshly dissociated rat parotid acinar cells to the fluorescent dye YOPRO-1. 3. The pharmacological and physiological properties of this effect indicate that permeabilization is mediated by the P2X(7) receptor. Adenosine 5'-triphosphate (ATP) and 3'-O-(4-benzoyl)benzoyl adenosine 5'-triphosphate (BzBzATP) were effective agonists with EC(50) values of 49.3 and 0.6 microM, respectively. 4. Permeabilization was best observed in low divalent cation concentrations and at physiological temperatures. Previous studies failed to detect permeabilization because of the sensitivity of this effect to temperature and divalent cations. 5. An important consideration in understanding the effect of divalent cations is that the fluorescence of YOPRO-1/nucleic acid complexes is directly quenched by addition of divalent cations. This must be considered if quantitative study of the interaction of divalent cations with P2X(7) receptors is carried out using fluorescent DNA-binding dyes. 6. In summary, our data show that P2X(7) receptors in parotid acinar cells can form large pores in the plasma membrane. This property likely contributes to signalling and may be cytotoxic and have particular significance in damaged or inflamed salivary glands.

Induction of neuronal cell death by Rab5A-dependent endocytosis of alpha-synuclein

The presynaptic alpha-synuclein is a prime suspect for contributing to Lewy pathology and clinical aspects of diseases, including Parkinson's disease, dementia with Lewy bodies, and a Lewy body variant of Alzheimer's disease. Here we examined the pathogenic mechanism of neuronal cell death induced by alpha-synuclein. The exogenous addition of alpha-synuclein caused a marked decrease of cell viability in primary and immortalized neuronal cells. The neuronal cell death appeared to be correlated with the Rab5A-specific endocytosis of alpha-synuclein that subsequently caused the formation of Lewy body-like intracytoplasmic inclusions. This was further supported by the fact that the expression of GTPase-deficient Rab5A resulted in a significant decrease of its cytotoxicity as a result of incomplete endocytosis of alpha-synuclein.

Intracellular calcium mobilization induces immediate early gene pip92 via Src and mitogen-activated protein kinase in immortalized hippocampal cells

Regulation of intracellular calcium levels plays a central role in cell survival, proliferation, and differentiation. A cell-permeable, tumor-promoting thapsigargin elevates the intracellular calcium levels by inhibiting endoplasmic reticulum Ca(2+)-ATPase. The Src-tyrosine kinase family is involved in a broad range of cellular responses ranging from cell growth and cytoskeletal rearrangement to differentiation. The immediate early gene pip92 is induced in neuronal cell death as well as cell growth and differentiation. To resolve the molecular mechanism of cell growth by intracellular calcium mobilization, we have examined the effect of thapsigargin and subsequent intracellular calcium influx on pip92 expression in immortalized rat hippocampal H19-7 cells. An increase of intracellular calcium ion levels induced by thapsigargin stimulated the expression of pip92 in H19-7 cells. Transient transfection of the cells with kinase-inactive mitogen-activated protein kinase kinase (MEK) and Src kinase or pretreatment with the chemical MEK inhibitor PD98059 significantly inhibited pip92 expression induced by thapsigargin. When constitutively active v-Src or MEK was overexpressed, the transcriptional activity of the pip92 gene was markedly increased. Dominant inhibitory Raf-1 blocked the transcriptional activity of pip92 induced by thapsigargin. The transcription factor Elk1 is activated during thapsigargin-induced pip92 expression. Taken together, these results suggest that an increase of intracellular calcium ion levels by thapsigargin stimulates the pip92 expression via Raf-MEK-extracellular signal-regulated protein kinase- as well as Src kinase-dependent signaling pathways.

The luminal P2Y receptor in the isolated perfused mouse cortical collecting duct

Extracellular nucleotides regulate renal ion transport. With the use of in vitro perfusion and [Ca(2+)](i) imaging, this study investigated whether mouse and rabbit cortical collecting ducts (CCD) respond to luminal nucleotides. In mouse CCD, luminal ATP (EC(50): 10 microM) and UTP (EC(50): 9.7 microM) increased [Ca(2+)](i) with an initial peak and a plateau. To make certain that basolateral P2 receptors were not activated by luminal nucleotides via leak diffusion, luminal trypsin (1 microM), a known agonist for basolateral proteinase-activated receptors, was perfused. Mouse CCD that were responsive to luminal ATP were nonresponsive to luminal trypsin but always showed [Ca(2+)](i) elevations by basolateral trypsin (10 or 100 nM). Luminal alpha,beta- and beta,gamma-methylene ATP, 2-methyl-S-ATP, ADP, UDP, and 2',3'-O-4-benzoylbenzoyl ATP had no effect (100 microM, n = 9). Without external Ca(2+), luminal ATP still stimulated a [Ca(2+)](i) increase. Mouse CCD also responded to basolateral ATP (EC(50): 23 microM) and UTP (EC(50): 23 microM) with smaller [Ca(2+)](i) elevations. Confocal microscopy of perfused CCD showed that luminal ATP (100 microM) rapidly increased [Ca(2+)](i) in nearly all cells (n = 6) and the same cells that responded to luminal ATP responded to basolateral ATP (100 microM). In contrast, rabbit CCD did not respond to luminal ATP/UTP (n = 8) despite ATP's known effect from the basolateral side (EC(50): 34 microM). These data indicate the expression of luminal P2Y receptors (probably P2Y(2)) in principal cells of mouse CCD but not in rabbit CCD.

Salivary gland P2 nucleotide receptors

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

Bromoenol lactone enhances the permeabilization of rat submandibular acinar cells by P2X7 agonists

The permeabilizing effect of P2X(7) agonists was tested in rat submandibular acinar cells using the uptake of ethidium bromide as an index. The uptake of ethidium bromide by acini incubated at 37 degrees C in the presence of 1 mM ATP increased with time and reached after 5 min about 10% of maximal uptake measured in the presence of digitonin. The response to ATP was dose-dependent (half-maximal concentration around 40 microM) and it was decreased when the temperature was lowered to 25 degrees C. Benzoyl-ATP reproduced the response to ATP (half-maximal concentration around 10 microM). UTP or 2-methylthioATP had no effect. The permeabilization in response to ATP was blocked by oxidized ATP and by magnesium and inhibited by Coomassie blue. ATP increased the activity of a calcium-insensitive phospholipase A(2) (iPLA(2)). Bromoenol lactone (BEL) inhibited the iPLA(2) stimulated by ATP but potentiated the uptake of ethidium bromide in response to the purinergic agonist. From these results it is concluded that the activation of P2X(7) receptors permeabilizes rat submandibular acinar cells. The pore-forming activity of the receptor might be negatively regulated by the concomitant activation of the iPLA(2) by the receptor.

Basolateral store-operated Ca(2+)-entry in polarized human bronchial and colonic epithelial cells

Bronchial epithelial cells respond to extracellular nucleotides from the luminal and basolateral side activating Cl- secretion via [Ca2+]i increase. In this study we investigated the differences of apically (ap) and basolaterally (bl) stimulated [Ca2+]i signals in polarized human bronchial epithelial cells (16HBE14o-). Specifically we investigated the localization of 'capacitative Ca2+ entry' (CCE). 16HBE14o- cells grown on permeable filters were mounted into an Ussing chamber built for the simultaneous measurement of Fura-2 fluorescence and electrical properties. Application of ATP from both sides induced a rapid [Ca2+]i increase and subsequent sustained [Ca2+]i plateau due to transmembraneous Ca(2+)-influx. The use of different nucleotides revealed the following rank order or potency which was very similar for addition from the apical or basolateral side: UTP (EC50 ap: 4 microM, bl: 5 microM) > ATP (EC50 ap: 4 microM, bl: 10 microM) > ADP (n = 4-7 from both sides). 2-MeS-ATP, AMP, adenosine and beta gamma-methylene ATP were ineffective (n = 3 from both sides). The ATP- (ap and bl) induced Ca2+ influx was only abolished by removal of basolateral Ca2+. This was also true for receptor-independent activation of Ca(2+)-influx by intracellular Ca(2+)-store depletion with 2,5 Di-(tert-butyl)-1,4-benzohydroquinone (BHQ) (10 microM). Also in polarized T84 cells the basolateral carbachol and BHQ activated Ca2+ plateau was exclusively sensitive to removal of basolateral Ca2+. We propose that in all polarized epithelial cells the CCE entry pathway is located in the basolateral membrane. We furthermore suggest that Ca2+[i elevating agonists acting from the apical side of the epithelium lead to the opening of a basolateral CCE pathway.

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

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

Regulation of the Na+-K+(NH4+)-2Cl- cotransporter of rat submandibular glands

A cellular suspension from rat submandibular glands was exposed to different concentrations of NH4Cl, and the variations of the intracellular concentration of calcium ([Ca2+]i) and the intracellular pH (pHi) were measured using fura-2 and 2',7'-bis-(2-carboxy-ethyl)-5(6)-carboxyfluorescein. More than 5 mmol/l NH4Cl significantly increased the [Ca2+]i without affecting the response to 100 micromol/l carbachol. When exposed to 1 and 5 mmol/l NH4Cl, the cells acidified immediately. At 30 mmol/l, NH4Cl first alkalinized the cells and the pHi subsequently dropped. This drop reflects the uptake of NH4+ ions that dissociate to NH3 and H+ in the cytosol. These protons are exchanged for extracellular sodium by the Na+/H+ exchanger because the presence of an inhibitor of the exchanger in the medium increased the acidification induced by 1 mmol/l NH4Cl. Ouabain partly blocked the uptake of NH4+. In the combined presence of ouabain and bumetanide (an inhibitor of the Na+-K+-2Cl- cotransporter), 1 mmol/l NH4Cl alkalinized the cells. The contribution of the Na/K ATPase and the Na+-K+-2Cl- cotransporter in the uptake of NH4+ was independent of the presence of calcium in the medium. Isoproterenol increased the uptake of NH4+ by the cotransporter. Conversely, 1 mmol/l extracellular ATP blocked the basal uptake of NH4+ by the cotransporter. This inhibition was reversed by extracellular magnesium or Coomassie Blue. It was mimicked by benzoyl-ATP but not by CTP, GTP, UTP, ADP, or ADPbetaS. ATP only slightly inhibited the increase of cyclic AMP (-22%) by isoproterenol but fully blocked the stimulation of the cotransporter by the beta-adrenergic agonist. ATP increased the release of 3H-arachidonic acid from prelabeled cells but SK&F 96365, an imidazole-based cytochrome P450 inhibitor, did not affect the inhibition by ATP. It is concluded that the activation of a purinoceptor inhibits the basal and the cyclic AMP-stimulated activity of the Na+-K+-2Cl- cotransporter.

ATP-dependent activation of K(Ca) and ROMK-type K(ATP) channels in human submandibular gland ductal cells

[Ca(2+)](i) and membrane current were measured in human submandibular gland ductal (HSG) cells to determine the regulation of salivary cell function by ATP. 1-10 microM ATP activated internal Ca(2+) release, outward Ca(2+)-dependent K(+) channel (K(Ca)), and inward store-operated Ca(2+) current (I(SOC)). The subsequent addition of 100 microM ATP activated an inwardly rectifying K(+) current, without increasing [Ca(2+)](i). The K(+) current was also stimulated by ATP in cells treated with thapsigargin in a Ca(2+)-free medium and was blocked by glibenclamide and tolbutamide, but not by charybdotoxin. This suggests the involvement of a Ca(2+)-independent, sulfonylurea-sensitive K(+) channel (K(ATP)). UTP mimicked the low [ATP] effects, while benzoyl-ATP activated internal Ca(2+) release, a Ca(2+) influx pathway, and K(Ca). Thus, ATP acts via P(2U) (P2Y(2)) and P(2Z) (P2X(7)) receptors to increase [Ca(2+)](i) and activate K(Ca), but not K(ATP). Importantly, (i) ROMK1 and the cystic fibrosis transmembrane regulator protein (but not SUR1, SUR2A, or SUR2B) and (ii) cAMP-stimulated Cl(-) and K(+) currents were detected in HSG cells. These data demonstrate for the first time that a ROMK-type K(ATP) channel is present in salivary gland duct cells that is regulated by extracellular ATP and possibly by the cystic fibrosis transmembrane regulator. This reveals a potentially novel mechanism for K(+) secretion in these cells.

Regulation by P2 agonists of the intracellular calcium concentration in epithelial cells freshly isolated from rat trachea

Epithelial cells were isolated from rat trachea by incubation of the organ in a calcium-free medium. The intracellular concentration of calcium ([Ca(2+)](i)) was measured with the calcium-sensitive fluorescent dye fura2. In resting conditions, the cells maintained a low [Ca(2+)](i) in spite of the presence of millimolar concentration of calcium in the incubation medium. These cells had retained intracellular stores of calcium which were emptied after exposure of the cells to thapsigargin, an inhibitor of intracellular calcium ATPases. Substance P (125 nM) transiently increased 2.5-fold the [Ca(2+)](i). ATP (1 mM) doubled the [Ca(2+)](i) after a few seconds and further induced a sustained increase of the [Ca(2+)](i). Coomassie blue fully blocked the response to ATP and extracellular magnesium only inhibited the delayed response to ATP. Among purinergic analogs, only benzoyl-ATP (Bz-ATP), an agonist on P2X ionotropic purinergic receptors, reproduced the response to ATP. UTP and 2-methylthioATP (two agonists on P2Y metabotropic purinergic receptors) transiently increased the [Ca(2+)](i). Thapsigargin, ATP and Bz-ATP increased the uptake of extracellular calcium. RT-PCR analysis revealed that two metabotropic receptors (P2Y(1) and P2Y(2)) and two ionotropic receptors (P2X(4) and P2X(7)) were expressed by the cells present in the suspension. It is concluded that purinergic agonists can modulate the response of rat tracheal epithelial cells by several mechanisms. The activation of metabotropic receptors should mobilize intracellular IP(3)-sensitive calcium pools. The activation of the ionotropic receptors should not only open a non-specific cation channel leading to the entry of calcium but should also induce the formation of pores in cells expressing the P2X(7) receptors, which could be deleterious to these cells.

Multiple functional P2X and P2Y receptors in the luminal and basolateral membranes of pancreatic duct cells

Purinergic receptors in the basolateral and luminal membranes of the pancreatic duct can act by a feedback mechanism to coordinate transport activity in the two membranes during ductal secretion. The goal of the present work was to identify and localize the functional P2 receptors (P2R) in the rat pancreatic duct. The lack of selective agonists and/or antagonists for any of the cloned P2R dictated the use of molecular and functional approaches to the characterization of ductal P2R. For the molecular studies, RNA was prepared from microdissected pancreatic intralobular ducts and was shown to be free of mRNA for amylase and endothelial nitric oxide synthase (markers for acinar and endothelial cells, respectively). A new procedure is described to obtain an enriched preparation of single duct cells suitable for electrophysiological studies. Localization of P2R was achieved by testing the effect of various P2R agonists on intracellular Ca(2+) concentration ([Ca(2+)](i)) of microperfused intralobular ducts. RT-PCR analysis suggested the expression of six subtypes of P2R in the pancreatic duct: three P2YR and three P2XR. Activation of Cl(-) current by various nucleotides and coupling of the receptors activated by these nucleotides to G proteins confirmed the expression of multiple P2R in duct cells. Measurement of [Ca(2+)](i) in microperfused intralobular ducts suggested the expression of P2X(1)R, P2X(4)R, probably P2X(7)R, and as yet unidentified P2YR, possibly P2Y(1)R, in the basolateral membrane. Expression of P2Y(2)R, P2Y(4)R, and P2X(7)R was found in the luminal membrane. The unprecedented expression of such a variety of P2R in one cell type, many capable of activating Cl(-) channels, suggests that these receptors may have an important role in pancreatic duct cell function.