P2 purinoceptors regulate calcium-activated chloride and fluid transport in 31EG4 mammary epithelia

The epithelial polarity axis controls the resting membrane potential and Cl- co-transport in breast glandular structures

The membrane potential (MP) controls cell homeostasis by directing molecule transport and gene expression. How the MP is set upon epithelial differentiation is unknown. Given that tissue architecture also controls homeostasis, we investigated the relationship between basoapical polarity and resting MP in three-dimensional culture of the HMT-3522 breast cancer progression. A microelectrode technique to measure MP and input resistance reveals that the MP is raised by gap junction intercellular communication (GJIC), which directs tight-junction mediated apical polarity, and is decreased by the Na+/K+/2Cl- (NKCC, encoded by SLC12A1 and SLC12A2) co-transporter, active in multicellular structures displaying basal polarity. In the tumor counterpart, the MP is reduced. Cancer cells display diminished GJIC and do not respond to furosemide, implying loss of NKCC activity. Induced differentiation of cancer cells into basally polarized multicellular structures restores widespread GJIC and NKCC responses, but these structures display the lowest MP. The absence of apical polarity, necessary for cancer onset, in the non-neoplastic epithelium is also associated with the lowest MP under active Cl- transport. We propose that the loss of apical polarity in the breast epithelium destabilizes cellular homeostasis in part by lowering the MP.

A biologically validated mathematical model for decoding epithelial apical, basolateral, and paracellular electrical properties

Epithelial tissues form selective barriers to ions, nutrients, waste products, and infectious agents throughout the body. Damage to these barriers is associated with conditions such as celiac disease, cystic fibrosis, diabetes, and age-related macular degeneration. Conventional electrophysiology measurements like transepithelial resistance can quantify epithelial tissue maturity and barrier integrity but are limited in differentiating between apical, basolateral, and paracellular transport pathways. To overcome this limitation, a combination of mathematical modeling, stem cell biology, and cell physiology led to the development of 3 P-EIS, a novel mathematical model and measurement technique. 3 P-EIS employs an intracellular pipette and extracellular electrochemical impedance spectroscopy to accurately measure membrane-specific properties of epithelia, without the constraints of prior models. 3 P-EIS was validated using electronic circuit models of epithelia with known resistances and capacitances, confirming a median error of 19% (interquartile range: 14%-26%) for paracellular and transcellular resistances and capacitances (n = 5). Patient stem cell-derived retinal pigment epithelium tissues were measured using 3 P-EIS, successfully isolating the cellular responses to adenosine triphosphate. 3 P-EIS enhances quality control in epithelial cell therapies and has extensive applicability in drug testing and disease modeling, marking a significant advance in epithelial physiology.NEW & NOTEWORTHY This interdisciplinary paper integrates mathematics, biology, and physiology to measure epithelial tissue's apical, basolateral, and paracellular transport pathways. A key advancement is the inclusion of intracellular voltage recordings using a sharp pipette, enabling precise quantification of relative impedance changes between apical and basolateral membranes. This enhanced electrochemical impedance spectroscopy technique offers insights into epithelial transport dynamics, advancing disease understanding, drug interactions, and cell therapies. Its broad applicability contributes significantly to epithelial physiology research.

P2 Purinergic Signaling in the Distal Lung in Health and Disease

The distal lung provides an intricate structure for gas exchange in mammalian lungs. Efficient gas exchange depends on the functional integrity of lung alveoli. The cells in the alveolar tissue serve various functions to maintain alveolar structure, integrity and homeostasis. Alveolar epithelial cells secrete pulmonary surfactant, regulate the alveolar surface liquid (ASL) volume and, together with resident and infiltrating immune cells, provide a powerful host-defense system against a multitude of particles, microbes and toxicants. It is well established that all of these cells express purinergic P2 receptors and that purinergic signaling plays important roles in maintaining alveolar homeostasis. Therefore, it is not surprising that purinergic signaling also contributes to development and progression of severe pathological conditions like pulmonary inflammation, acute lung injury/acute respiratory distress syndrome (ALI/ARDS) and pulmonary fibrosis. Within this review we focus on the role of P2 purinergic signaling in the distal lung in health and disease. We recapitulate the expression of P2 receptors within the cells in the alveoli, the possible sources of ATP (adenosine triphosphate) within alveoli and the contribution of purinergic signaling to regulation of surfactant secretion, ASL volume and composition, as well as immune homeostasis. Finally, we summarize current knowledge of the role for P2 signaling in infectious pneumonia, ALI/ARDS and idiopathic pulmonary fibrosis (IPF).

High-yield, automated intracellular electrophysiology in retinal pigment epithelia

BACKGROUND

Recent advancements with induced pluripotent stem cell-derived (iPSC) retinal pigment epithelium (RPE) have made disease modeling and cell therapy for macular degeneration feasible. However, current techniques for intracellular electrophysiology - used to validate epithelial function - are painstaking and require manual skill; limiting experimental throughput.

NEW METHOD

A five-stage algorithm, leveraging advances in automated patch clamping, systematically derived and optimized, improves yield and reduces skill when compared to conventional, manual techniques.

RESULTS

The automated algorithm improves yield per attempt from 17% (manually, n = 23) to 22% (automated, n = 120) (chi-squared, p = 0.004). Specifically for RPE, depressing the local cell membrane by 6 μm and electroporating (buzzing) just prior to this depth (5 μm) maximized yield.

COMPARISON WITH EXISTING METHOD

Conventionally, intracellular epithelial electrophysiology is performed by manually lowering a pipette with a micromanipulator, blindly, towards a monolayer of cells and spontaneously stopping when the magnitude of the instantaneous measured membrane potential decreased below a predetermined threshold. The new method automatically measures the pipette tip resistance during the descent, detects the cell surface, indents the cell membrane, and briefly buzzes to electroporate the membrane while descending, overall achieving a higher yield than conventional methods.

CONCLUSIONS

This paper presents an algorithm for high-yield, automated intracellular electrophysiology in epithelia; optimized for human RPE. Automation reduces required user skill and training while, simultaneously, improving yield. This algorithm could enable large-scale exploration of drug toxicity and physiological function verification for numerous kinds of epithelia.

P2Y receptor regulation of K2P channels that facilitate K+ secretion by human mammary epithelial cells

The objective of this study was to determine the molecular identity of ion channels involved in K+ secretion by the mammary epithelium and to examine their regulation by purinoceptor agonists. Apical membrane voltage-clamp experiments were performed on human mammary epithelial cells where the basolateral membrane was exposed to the pore-forming antibiotic amphotericin B dissolved in a solution with intracellular-like ionic composition. Addition of the Na+ channel inhibitor benzamil reduced the basal current, consistent with inhibition of Na+ uptake across the apical membrane, whereas the KCa3.1 channel blocker TRAM-34 produced an increase in current resulting from inhibition of basal K+ efflux. Treatment with two-pore potassium (K2P) channel blockers quinidine, bupivacaine and a selective TASK1/TASK3 inhibitor (PK-THPP) all produced concentration-dependent inhibition of apical K+ efflux. qRT-PCR experiments detected mRNA expression for nine K2P channel subtypes. Western blot analysis of biotinylated apical membranes and confocal immunocytochemistry revealed that at least five K2P subtypes (TWIK1, TREK1, TREK2, TASK1, and TASK3) are expressed in the apical membrane. Apical UTP also increased the current, but pretreatment with the PKC inhibitor GF109203X blocked the response. Similarly, direct activation of PKC with phorbol 12-myristate 13-acetate produced a similar increase in current as observed with UTP. These results support the conclusion that the basal level of K+ secretion involves constitutive activity of apical KCa3.1 channels and multiple K2P channel subtypes. Apical UTP evoked a transient increase in KCa3.1 channel activity, but over time caused persistent inhibition of K2P channel function leading to an overall decrease in K+ secretion.

Inflammatory Responses Regulating Alveolar Ion Transport during Pulmonary Infections

The respiratory epithelium is lined by a tightly balanced fluid layer that allows normal O₂ and CO₂ exchange and maintains surface tension and host defense. To maintain alveolar fluid homeostasis, both the integrity of the alveolar–capillary barrier and the expression of epithelial ion channels and pumps are necessary to establish a vectorial ion gradient. However, during pulmonary infection, auto- and/or paracrine-acting mediators induce pathophysiological changes of the alveolar–capillary barrier, altered expression of epithelial Na,K-ATPase and of epithelial ion channels including epithelial sodium channel and cystic fibrosis membrane conductance regulator, leading to the accumulation of edema and impaired alveolar fluid clearance. These mediators include classical pro-inflammatory cytokines such as TGF-β, TNF-α, interferons, or IL-1β that are released upon bacterial challenge with Streptococcus pneumoniae, Klebsiella pneumoniae, or Mycoplasma pneumoniae as well as in viral infection with influenza A virus, pathogenic coronaviruses, or respiratory syncytial virus. Moreover, the pro-apoptotic mediator TNF-related apoptosis-inducing ligand, extracellular nucleotides, or reactive oxygen species impair epithelial ion channel expression and function. Interestingly, during bacterial infection, alterations of ion transport function may serve as an additional feedback loop on the respiratory inflammatory profile, further aggravating disease progression. These changes lead to edema formation and impair edema clearance which results in suboptimal gas exchange causing hypoxemia and hypercapnia. Recent preclinical studies suggest that modulation of the alveolar–capillary fluid homeostasis could represent novel therapeutic approaches to improve outcomes in infection-induced lung injury.

Ca2+-activated Cl− channel currents in mammary secretory cells from lactating mouse

The Cl− secretion via Ca2+-activated Cl− channel (CaCC) is critical for fluid secretion in exocrine glands like the salivary gland. Also in the mammary gland, it has been hypothesized that CaCC plays an important role in the secretion of Cl− and aqueous phase of milk. However, there has been no evidence for the functional expression of CaCC in native mammary secretory (MS) cells of lactating animals. We therefore assessed membrane current in MS cells that were freshly isolated from lactating mice using whole cell patch-clamp techniques. In MS cells, we detected CaCC current that exhibited the following characteristics: 1) Ca2+-dependent activation at the concentrations of submicromolar range; 2) voltage-dependent activation; 3) slow kinetics for activation and deactivation; 4) outward rectification of the steady-state current; 5) anion permeability in the sequence of I− > NO3− > Br− > Cl− >> glutamate; 6) inhibition by Cl− channel blockers (niflumic acid, DIDS, and CaCCinh-A01). These characteristics of native CaCC current were similar to reported characteristics of heterologously expressed TMEM16A. RT-PCR analyses showed the expression of multiple CaCC channels including TMEM16A, Best1, and Best3 in the mammary glands of lactating mice. Immunohistochemical staining revealed the localization of TMEM16A protein at the apical membrane of the MS cells. Collectively, our data strongly suggest that MS cells functionally express CaCC, which is at least partly constituted by TMEM16A. The CaCC such as TMEM16A at the apical membrane of the MS cells may influence the quantity and/or quality of milk.

Loss of Panx1 Impairs Mammary Gland Development at Lactation: Implications for Breast Tumorigenesis

Pannexin1 (Panx1) subunits oligomerize to form large-pore channels between the intracellular and extracellular milieu that have been shown to regulate proliferation, differentiation and cell death mechanisms. These key cellular responses are ultimately necessary for normal tissue development and function but the role of Panx1 in development, differentiation and function in many tissues remains unexplored, including that of the breast. Panx1 was identified to be expressed in the mammary gland through western blot and immunofluorescent analysis and is dynamically upregulated during pregnancy and lactation. In order to evaluate the role of Panx1 in the context of mammary gland development and function, Panx1^-/- mice were evaluated in comparison to wild-type mice in the mammary glands of virgin, lactating and involuting mice. Our results revealed that Panx1 ablation did not affect virgin or involuting mammary glands following histological and whole mount analysis. Panx1 was necessary for timely alveolar development during early lactation based on a decreased number of alveolar lumen following histological analysis and reduced proliferation following Ki67 immunofluorescent labelling. Importantly, the loss of Panx1 in lactating mammary glands did not overtly affect epithelial or secretory differentiation of the mammary gland suggesting that Panx1 is not critical in normal mammary gland function. In addition, PANX1 mRNA expression was correlated with negative clinical outcomes in patients with breast cancer using in silico arrays. Together, our results suggest that Panx1 is necessary for timely alveolar development following the transition from pregnancy to lactation, which may have implications extending to patients with breast cancer.

Changes in Purines Concentration in the Cerebrospinal Fluid of Pregnant Women Experiencing Pain During Active Labor

Labor pain has been reported as a severe pain and can be considered as a model of acute visceral pain. It is well known that extracellular purines have an important role in pain signaling in the central nervous system. This study analyzes the relationship between extracellular purines and pain perception during active labor. A prospective observational study was performed. Cerebrospinal fluid (CSF) levels of the purines and their metabolites were compared between women at term pregnancy with labor pain (n = 49) and without labor pain (Caesarian section; n = 47). Control groups (healthy men and women without chronic or acute pain-n = 40 and 32, respectively) were also investigated. The CSF levels of adenosine were significantly lower in the labor pain group (P = 0.026) and negatively correlated with pain intensity measured by a visual analogue scale (r = -0.48, P = 0.0005). Interestingly, CSF levels of uric acid were significantly higher in healthy men as compared to women. Additionally, pregnant women showed increased CSF levels of ADP, GDP, adenosine and guanosine and reduced CSF levels of AMP, GTP, and uric acid as compared to non-pregnant women (P < 0.05). These findings suggest that purines, in special the nucleoside adenosine, are associated with pregnancy and labor pain.

Purinergic signalling in the reproductive system in health and disease

There are multiple roles for purinergic signalling in both male and female reproductive organs. ATP, released as a cotransmitter with noradrenaline from sympathetic nerves, contracts smooth muscle via P2X1 receptors in vas deferens, seminal vesicles, prostate and uterus, as well as in blood vessels. Male infertility occurs in P2X1 receptor knockout mice. Both short- and long-term trophic purinergic signalling occurs in reproductive organs. Purinergic signalling is involved in hormone secretion, penile erection, sperm motility and capacitation, and mucous production. Changes in purinoceptor expression occur in pathophysiological conditions, including pre-eclampsia, cancer and pain.

Functional expression of a Kir2.1-like inwardly rectifying potassium channel in mouse mammary secretory cells

K(+) channels in mammary secretory (MS) cells are believed to play a role in transcellular electrolyte transport and thus determining ionic composition of the aqueous phase of milk. However, direct evidence for specific K(+) channel activity in native MS cells is lacking at the single-cell level. Here, we show for the first time that an inwardly rectifying K(+) (Kir) channel is functionally expressed in fully differentiated MS cells that were freshly isolated from the mammary gland of lactating mice. Using the standard whole cell patch-clamp technique, we found that mouse MS cells consistently displayed a K(+) current, whose electrophysiological properties are similar to those previously reported for Kir2.x channels, particularly Kir2.1: 1) current-voltage relationship with strong inward rectification, 2) slope conductance approximately proportional to the square root of external K(+) concentration, 3) voltage- and time-dependent and high-affinity block by external Ba(2+), and 4) voltage-dependent inhibition by external Cs(+). Accordingly, RT-PCR analysis revealed the gene expression of Kir2.1, but not Kir2.2, Kir2.3, and Kir2.4, in lactating mouse mammary gland, and immunohistochemical staining showed Kir2.1 protein expression in the secretory cells. Cell-attached patch recordings from MS cells revealed that a 31-pS K(+) channel with strong inward rectification was likely active at the resting membrane potential. Collectively, the present work demonstrates that a functional Kir2.1-like channel is expressed in lactating mouse MS cells. We propose that the channel might be involved, at least in part, in secretion and/or preservation of ionic components of milk stored into the lumen of these cells.

BRMS1 Sensitizes Breast Cancer Cells to ATP-Induced Growth Suppression

Purinergic signaling may represent an effective target in cancer therapy because the expression of purinergic receptors is altered in many forms of cancer and extracellular nucleotides modulate cancer cell growth. We examined the effect of extracellular ATP on the growth of the metastatic breast carcinoma cell line MDA-MB-435 relative to an immortalized breast epithelial cell line, hTERT-HME1. We also investigated whether the metastasis suppressor gene BRMS1 alters the sensitivity of breast cancer cells to ATP. Exposure to ATP for 24 h decreased proliferation and induced apoptosis in hTERT-HME1. However, exposure to ATP did not decrease proliferation or induce apoptosis in MDA-MD-435 cells until 48 h of exposure and only at higher doses than were effective with hTERT-HME1, suggesting MDA-MB-435 cells were resistant to the antiproliferative and apoptosis-inducing effects of ATP. Exposure to ATP for 24 h induced a decrease in proliferation of MDA-MB-435 cells expressing BRMS1, similar to hTERT-HME1, but did not induce an increase in apoptosis. MDA-MB-435 cells expressed low levels of the purinergic receptor P2Y₂, as well as decreased ATP-induced cytosolic calcium mobilization, relative to hTERT-HME1. However, expressing BRMS1 in MDA-MB-435 cells restored P2Y₂ levels and ATP-induced cytosolic calcium mobilization such that they were similar to hTERT-HME1. These data suggest that BRMS1 increases the sensitivity of breast cancer cells to the antiproliferative, but not apoptosis-inducing effects of ATP and that this is at least partly mediated by increased expression of the P2Y₂ receptor.

Prolactin and dexamethasone regulate second messenger-stimulated cl(-) secretion in mammary epithelia

Mammary gland ion transport is essential for lactation and is regulated by prolactin and glucocorticoids. This study delineates the roles of prolactin receptors (PRLR) and long-term prolactin and dexamethasone (P-D)-mediation of [Ca²⁺]_i and Cl⁻ transport in HC-11 cells. P-D (24 h) suppressed ATP-induced [Ca²⁺]_i. This may be due to decreased Ca²⁺ entry since P-D decreased transient receptor potential channel 3 (TRPC3) but not secretory pathway Ca²⁺-ATPase 2 (SPCA2) mRNA. ATP increased Cl⁻ transport, measured by iodide (I⁻) efflux, in control and P-D-treated cells. P-D enhanced I⁻ efflux response to cAMP secretagogues without altering Cl⁻ channels or NKCC cotransporter expression. HC-11 cells contain only the long form of PRLR (PRLR-L). Since the short isoform, PRLR-S, is mammopoietic, we determined if transfecting PRLR-S (rs) altered PRLR-L-mediated Ca²⁺ and Cl⁻ transport. Untreated rs cells showed an attenuated [Ca²⁺]_i response to ATP with no further response to P-D, in contrast to vector-transfected (vtc) controls. P-D inhibited TRPC3 in rs and vtc cells but increased SPCA2 only in rs cells. As in wild-type, cAMP-stimulated Cl⁻ transport, in P-D-treated vtc and rs cells. In summary, 24 h P-D acts via PRLR-L to attenuate ATP-induced [Ca²⁺]_i and increase cAMP-activated Cl⁻ transport. PRLR-S fine-tunes these responses underscoring its mammopoietic action.

K(Ca)3.1 channels facilitate K+ secretion or Na+ absorption depending on apical or basolateral P2Y receptor stimulation

Human mammary epithelial (HME) cells express several P2Y receptor subtypes located in both apical and basolateral membranes. Apical UTP or ATP-γ-S stimulation of monolayers mounted in Ussing chambers evoked a rapid, but transient decrease in short circuit current (I(sc)), consistent with activation of an apical K+ conductance. In contrast, basolateral P2Y receptor stimulation activated basolateral K+ channels and increased transepithelial Na+ absorption. Chelating intracellular Ca2+ using the membrane-permeable compound BAPTA-AM, abolished the effects of purinoceptor activation on I(sc). Apical pretreatment with charybdotoxin also blocked the I(sc) decrease by >90% and similar magnitudes of inhibition were observed with clotrimazole and TRAM-34. In contrast, iberiotoxin and apamin did not block the effects of apical P2Y receptor stimulation. Silencing the expression of K(Ca)3.1 produced ∼70% inhibition of mRNA expression and a similar reduction in the effects of apical purinoceptor agonists on I(sc). In addition, silencing P2Y2 receptors reduced the level of P2Y2 mRNA by 75% and blocked the effects of ATP-γ-S by 65%. These results suggest that P2Y2 receptors mediate the effects of purinoceptor agonists on K+ secretion by regulating the activity of K(Ca)3.1 channels expressed in the apical membrane of HME cells. The results also indicate that release of ATP or UTP across the apical or basolateral membrane elicits qualitatively different effects on ion transport that may ultimately determine the [Na+]/[K+] composition of fluid within the mammary ductal network.

Tissue polarity-dependent control of mammary epithelial homeostasis and cancer development: an epigenetic perspective

The basoapical organization of monolayered epithelia is defined by the presence of hemidesmosomes at the basal cellular pole, where the cell makes contacts with the basement membrane, and tight junctions at the opposite apical pole. In the mammary gland, tight junctions seal cell-cell contacts against the lumen and separate the apical and basolateral cell membranes. This separation is critical to organize intracellular signaling pathways and the cytoskeleton. The study of the impact of the highly organized apical pole, and notably apical polarity regulators (Crb complex, Par complex, and Scrib, Dlg, Lgl proteins) and tight junction proteins on cell phenotype and gene expression has revealed an intricate relationship between apical polarity and the cell nucleus. The goal of this review is to highlight the role of the apical pole of the tissue polarity axis in the epigenetic control of tissue phenotype. The organization of the apical pole and its importance in mammary homeostasis and tumorigenesis will be emphasized before presenting how apical polarity proteins impact gene expression indirectly, by influencing signal transduction and the location of transcription regulators, and directly, by participating in chromatin-associated complexes. The relationship between apical polarity and cell nucleus organizations might explain how apical polarity proteins could switch from nuclear repressors to nuclear promoters of cancerous behavior following alterations in the apical pole. The impact of apical polarity proteins on epigenetic mechanisms of gene expression will be discussed in light of increased evidence supporting a role for apical polarity in the fate of breast neoplasms.

Expression cloning of TMEM16A as a calcium-activated chloride channel subunit

Calcium-activated chloride channels (CaCCs) are major regulators of sensory transduction, epithelial secretion, and smooth muscle contraction. Other crucial roles of CaCCs include action potential generation in Characean algae and prevention of polyspermia in frog egg membrane. None of the known molecular candidates share properties characteristic of most CaCCs in native cells. Using Axolotl oocytes as an expression system, we have identified TMEM16A as the Xenopus oocyte CaCC. The TMEM16 family of "transmembrane proteins with unknown function" is conserved among eukaryotes, with family members linked to tracheomalacia (mouse TMEM16A), gnathodiaphyseal dysplasia (human TMEM16E), aberrant X segregation (a Drosophila TMEM16 family member), and increased sodium tolerance (yeast TMEM16). Moreover, mouse TMEM16A and TMEM16B yield CaCCs in Axolotl oocytes and mammalian HEK293 cells and recapitulate the broad CaCC expression. The identification of this new family of ion channels may help the development of CaCC modulators for treating diseases including hypertension and cystic fibrosis.

Effect of basolateral adenosine triphosphate on chloride secretion by bovine oviductal epithelium

The composition of the fluid within the oviduct is largely determined by the secretory and absorptive activities of the oviduct epithelium. The present study explored the effects of basolateral nucleotide stimulation on ion transport in the bovine oviduct using the chamber short-circuit current technique. Basolateral application of ATP induced a rapid transient increase in ion secretion by oviduct epithelial monolayers in a concentration-dependent manner. The ATP-induced short-circuit current (I(SC)) response was preserved in the presence of amiloride, whereas it was reduced in the absence of extracellular chloride or in the presence of bumetanide. The channels underlying the chloride secretory response were identified as Ca(2+)-activated Cl(-) channels and CFTR. The ATP-induced Cl(-) secretory response was largely preserved in the absence of extracellular Ca(2+) but was significantly reduced in the presence of BAPTA-am (1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid-acetomethoxy ester), thapsigargin, or 2-APB (2-aminoethoxydiphenylborate), demonstrating an important role for intracellular Ca(2+) signaling in mediating these effects. A nucleotide potency profile of ATP = UTP (uridine triphosphate) > ADP, sensitivity to suramin, and cross-desensitization by basolateral UTP suggests that ATP exerted its effects on chloride secretion through the purinergic receptor P2Y, G protein-coupled 2, and the presence of the P2RY2 gene was confirmed by RT-PCR. These results provide strong evidence that purinergic signaling constitutes a key mechanism of regulating chloride secretion and thus fluid formation in the bovine oviduct.

Contribution of KCNQ1 to the regulatory volume decrease in the human mammary epithelial cell line MCF-7

Using the human mammary epithelial cell line MCF-7, we have investigated volume-activated changes in response to hyposmotic stress. Switching MCF-7 cells from an isosmotic to a hyposmotic solution resulted in an initial cell swelling response, followed by a regulatory volume decrease (RVD). This RVD response was inhibited by the nonselective K+ channel inhibitors Ba2+, quinine, and tetraethylammonium chloride, implicating K+ channel activity in this volume-regulatory mechanism. Additional studies using chromonol 293B and XE991 as inhibitors of the KCNQ1 K+ channel, and also a dominant-negative NH2-terminal truncated KCNQ1 isoform, showed complete abolition of the RVD response, suggesting that KCNQ1 plays an important role in regulation of cell volume in MCF-7 cells. We additionally confirmed that KCNQ1 mRNA and protein is expressed in MCF-7 cells, and that, when these cells are cultured as a polarized monolayer, KCNQ1 is located exclusively at the apical membrane. Whole cell patch-clamp recordings from MCF-7 cells revealed a small 293B-sensitive current under hyposmotic, but not isosmotic conditions, while recordings from mammalian cells heterologously expressing KCNQ1 alone or KCNQ1 with the accessory subunit KCNE3 reveal a volume-sensitive K+ current, inhibited by 293B. These data suggest that KCNQ1 may play important physiological roles in the mammary epithelium, regulating cell volume and potentially mediating transepithelial K+ secretion.

P2Y receptor regulation of sodium transport in human mammary epithelial cells

Primary human mammary epithelial (HME) cells were immortalized by stable, constitutive expression of the catalytic subunit of human telomerase. Purinergic receptors were identified by RT-PCR and quantitative RT-PCR from mRNA isolated from primary and immortalized cells grown to confluence on membrane filters. Several subtypes of P2Y receptor mRNA were identified including P2Y(1), P2Y(2), P2Y(4), and P2Y(6) receptors. RT-PCR experiments also revealed expression of A(2b) adenosine receptor mRNA in primary and immortalized cells. Confluent monolayers of HME cells exhibited a basal short-circuit current (I(sc)) that was abolished by amiloride and benzamil. When monolayers were cultured in the presence of hydrocortisone, mRNA expression of Na(+) channel (ENaC) alpha-, beta-, and gamma-subunits increased approximately threefold compared with that in cells grown without hydrocortisone. In addition, basal benzamil-sensitive Na(+) transport was nearly twofold greater in hydrocortisone-treated monolayers. Stimulation with UTP, UDP, or adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS) produced increases in intracellular calcium concentration that were significantly reduced following pretreatment with the calcium-chelating agent BAPTA-AM. Concentration-response relationships indicated that the rank order of potency for these agonists was UTP > UDP > ATPgammaS. Basolateral stimulation with UTP produced a rapid but transient increase in I(sc) that was significantly reduced if cells were pretreated with BAPTA-AM or benzamil. Moreover, basolateral treatment with either charybdotoxin or clotrimazole significantly inhibited the initial UTP-dependent increase in I(sc) and eliminated the sustained current response. These results indicate that human mammary epithelial cells express multiple P2 receptor subtypes and that Ca(2+) mobilization evoked by P2Y receptor agonists stimulates Na(+) absorption by increasing the activity of Ca(2+)-activated K(+) channels located in the basolateral membrane.

Physiology and pathophysiology of purinergic neurotransmission

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

Purinergic receptor expression and activation in first trimester and term human placenta

Intracellular calcium concentration ([Ca(2+)](i)) is an important signalling molecule in the human placenta and regulation of [Ca(2+)](i) must be tightly controlled to ensure normal cell function and in order to meet the changing demand for calcium with increased fetal growth over gestation. Little is known about the receptors and mechanisms involved in intracellular calcium signalling in the human placenta but in isolated cytotrophoblast cells members of the P2 purinergic receptor family have been shown to mediate an ATP-stimulated rise in [Ca(2+)](i). In this study we examined activation and expression of several of the purinergic receptor subtypes in human placental villous fragments at two stages of gestation, first trimester and term. We demonstrate mRNA and protein expression of the P2X(4), P2X(7) and P2Y(2) subtypes but found no evidence of P2Y(4) protein in the placenta. Using fluorescent calcium imaging we demonstrate that 300 microM ATP, 450 microM UTP and 300 microM BzATP significantly elevate [Ca(2+)](i) in villous fragments with a significant increase in agonist-induced response seen in the term compared to the first trimester fragments (ATP, P<0.0001; UTP, P=0.018; BzATP, P=0.015). The roles of the purinergic receptors within the human placenta are not known but it seems likely for this study that calcium handling through these receptors is altered with advancing gestation. This may be due to the need to meet increased fetal Ca(2+) requirements due to growth or as a secondary function to alterations in placental [Ca(2+)](i) signalling.

Pathophysiology and therapeutic potential of purinergic signaling

The concept of a purinergic signaling system, using purine nucleotides and nucleosides as extracellular messengers, was first proposed over 30 years ago. After a brief introduction and update of purinoceptor subtypes, this article focuses on the diverse pathophysiological roles of purines and pyrimidines as signaling molecules. These molecules mediate short-term (acute) signaling functions in neurotransmission, mechanosensory transduction, secretion and vasodilatation, and long-term (chronic) signaling functions in cell proliferation, differentiation, and death involved in development and regeneration. Plasticity of purinoceptor expression in pathological conditions is frequently observed, including an increase in the purinergic component of autonomic cotransmission. Recent advances in therapies using purinergic-related drugs in a wide range of pathological conditions will be addressed with speculation on future developments in the field.

Acidic ATP activates lymphocyte outwardly rectifying chloride channels via a novel pathway

Using whole-cell patch-clamp techniques we found that ATP activated an outwardly rectifying current in Daudi human B lymphoma cells under acidic conditions. The substitution of Cl- for gluconate(-) shifted the reversal potential, while Cl- channel blockers, 4,4'-diisothiocyanostibene-2,2'-disulfonic acid (DIDS) and 9-anthracene carboxylic acid (9-AC), blocked the current, indicating that ATP induces this current by activating the outwardly rectifying chloride channel (ORCC). The effect of ATP on ORCC was mimicked by ADP, but not by other P2 receptor agonists such as ATPgammaS (a poorly hydrolyzable analog of ATP), 2',3'-O-benzoyl-4-benzoyl-ATP (BzATP), and UTP. The ATP-induced ORCC current was completely blocked by 100 microM suramin (a P2 receptor antagonist), and was partially blocked by 100 microM pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid tetrasodium (PPADS), which is another P2 receptor antagonist. Neither inactivation of G proteins nor elimination of extracellular Ca2+ affected the ATP-induced current, indicating that G protein-coupled P2Y receptors and Ca(2+)-permeable P2X receptors are not involved. Based on the pharmacological profile and the fact that acidic conditions are required for ATP to activate the ORCC, we suggest that acidic ATP activates the lymphocyte ORCC via a novel pathway, which is not associated with any previously described purinergic receptors.

The effect of a hyposmotic shock and purinergic agonists on K+(Rb+) efflux from cultured human breast cancer cells

The effect of a hyposmotic shock and extracellular ATP on the efflux of K(+)(Rb(+)) from human breast cancer cell lines (MDA-MB-231 and MCF-7) has been examined. A hyposmotic shock increased the fractional efflux of K(+)(Rb(+)) from MDA-MB-231 cells via a pathway which was unaffected by Cl(-) replacement. Apamin, charybdotoxin or removing extracellular Ca(2+) had no effect on volume-activated K(+)(Rb(+)) efflux MDA-MB-231 cells. An osmotic shock also stimulated K(+)(Rb(+)) efflux from MCF-7 cells but to a much lesser extent than found with MDA-MB-231 cells. ATP-stimulated K(+)(Rb(+)) efflux from MDA-MB-231 cells in a dose-dependent fashion but had little effect on K(+)(Rb(+)) release from MCF-7 cells. ATP-stimulated K(+)(Rb(+)) efflux was only inhibited slightly by replacing Cl(-) with NO(3)(-). Removal of external Ca(2+) during treatment with ATP reduced the fractional efflux of K(+)(Rb(+)) in a manner suggesting a role for cellular Ca(2+) stores. Charybdotoxin, but neither apamin nor iberiotoxin, inhibited ATP-stimulated K(+)(Rb(+)) release from MDA-MB-231 cells. Suramin inhibited the ATP-activated efflux of K(+)(Rb(+)). UTP also stimulated K(+)(Rb(+)) efflux from MDA-MB-231 cells whereas ADP, AMP and adenosine were without effect. A combination of an osmotic shock and ATP increased the fractional efflux of K(+)(Rb(+)) to a level greater than the sum of the individual treatments. It appears that the hyposmotically-activated and ATP-stimulated K(+) efflux pathways are separate entities. However, there may be a degree of 'crosstalk' between the two pathways.

P2Y2 nucleotide receptors enhance alpha-secretase-dependent amyloid precursor protein processing

The amyloid precursor protein (APP) is proteolytically processed by beta- and gamma-secretases to release amyloid beta, the main component in senile plaques found in the brains of patients with Alzheimer disease. Alternatively, APP can be cleaved within the amyloid beta domain by alpha-secretase releasing the non-amyloidogenic product sAPP alpha, which has been shown to have neuroprotective properties. Several G protein-coupled receptors are known to activate alpha-secretase-dependent processing of APP; however, the role of G protein-coupled nucleotide receptors in APP processing has not been investigated. Here it is demonstrated that activation of the G protein-coupled P2Y2 receptor (P2Y2R) subtype expressed in human 1321N1 astrocytoma cells enhanced the release of sAPP alpha in a time- and dose-dependent manner. P2Y2 R-mediated sAPP alpha release was dependent on extracellular calcium but was not affected by 1,2-bis(2-aminophenoxy)ethane-N,N,N,-trimethylammonium salt, an intracellular calcium chelator, indicating that P2Y2R-stimulated intracellular calcium mobilization was not involved. Inhibition of protein kinase C (PKC) with GF109203 or by PKC down-regulation with phorbol ester pre-treatment had no effect on UTP-stimulated sAPP alpha release, indicating a PKC-independent mechanism. U0126, an inhibitor of the mitogen-activated protein kinase pathway, partially inhibited sAPPalpha release by UTP, whereas inhibitors of Src-dependent epidermal growth factor receptor transactivation by P2Y2Rs had no effect. The metalloprotease inhibitors phenanthroline and TAPI-2 and the furin inhibitor decanoyl-Arg-Val-Lys-Arg-chloromethylketone also diminished UTP-induced sAPP alpha release. Furthermore, small interfering RNA silencing of an endogenous adamalysin, ADAM10 or ADAM17/TACE, partially suppressed P2Y2R-activated sAPP alpha release, whereas treatment of cells with both ADAM10 and ADAM17/TACE small interfering RNAs completely abolished UTP-activated sAPP alpha release. These results may contribute to an understanding of the non-amyloidogenic processing of APP.

Cell swelling-induced ATP release is tightly dependent on intracellular calcium elevations

Mechanical stresses release ATP from a variety of cells by a poorly defined mechanism(s). Using custom-designed flow-through chambers, we investigated the kinetics of cell swelling-induced ATP secretion, cell volume and intracellular calcium changes in epithelial A549 and 16HBE14o- cells, and NIH/3T3 fibroblasts. Fifty per cent hypotonic shock triggered transient ATP release from cell confluent monolayers, which consistently peaked at around 1 min 45 s for A549 and NIH/3T3, and at 3 min for 16HBE14o- cells, then declined to baseline within the next 15 min. Whereas the release time course had a similar pattern for the three cell types, the peak rates differed significantly (294 +/- 67, 70 +/- 22 and 17 +/- 2.8 pmol min(-1) (10(6) cells)(-1), for A549, 16HBE14o- and NIH/3T3, respectively). The concomitant volume changes of substrate-attached cells were analysed by a 3-dimensional cell shape reconstruction method based on images acquired from two perpendicular directions. The three cell types swelled at a similar rate, reaching maximal expansion in 1 min 45 s, but differed in the duration of the volume plateau and regulatory volume decrease (RVD). These experiments revealed that ATP release does not correlate with either cell volume expansion and the expected activation of stretch-sensitive channels, or with the activation of volume-sensitive, 5-nitro-2-(3-phenylpropylamino) benzoic acid-inhibitable anion channels during RVD. By contrast, ATP release was tightly synchronized, in all three cell types, with cytosolic calcium elevations. Furthermore, loading A549 cells with the calcium chelator BAPTA significantly diminished ATP release (71% inhibition of the peak rate), while the calcium ionophore ionomycin triggered ATP release in the absence of cell swelling. Lowering the temperature to 10 degrees C almost completely abolished A549 cell swelling-induced ATP release (95% inhibition of the peak rate). These results strongly suggest that calcium-dependent exocytosis plays a major role in mechanosensitive ATP release.