Activation of volume-regulated Cl(-) channels by ACh and ATP in Xenopus follicles

Purinergic signalling - a possible mechanism for KCNQ1 channel response to cell volume challenges

AIM

A number of K(+) channels are regulated by small, fast changes in cell volume. The mechanisms underlying cell volume sensitivity are not known, but one possible mechanism could be purinergic signalling. Volume activated ATP release could trigger signalling pathways that subsequently lead to ion channel stimulation and cell volume back-regulation. Our aim was to investigate whether volume sensitivity of the voltage-gated K(+) channel, KCNQ1, is dependent on ATP release and regulation by purinergic signalling.

METHODS

We used Xenopus oocytes heterologously expressing human KCNQ1, KCNE1, water channels (AQP1) and P2Y2 receptors. ATP release was monitored by a luciferin-luciferase assay and ion channel conductance was recorded by two-electrode voltage clamp.

RESULTS

The luminescence assay showed that oocytes released ATP in response to mechanical, hypoosmotic stimuli and hyperosmotic stimuli. Basal ATP release was approx. three times higher in the KCNQ1 + AQP1 and KCNQ1 injected oocytes compared to the non-injected ones. Exogenously added ATP (0.1 mm) did not have any substantial effect on volume-induced KCNQ1 currents. Nevertheless, apyrase decreased all currents by about 50%. Suramin inhibited about 23% of the KCNQ1 volume sensitivity. Expression of P2Y2 receptors stimulated endogenous Cl(-) channels, but it also led to 68% inhibition of the KCNQ1 currents. Adenosine (0.1 mm) also inhibited the KCNQ1 currents by about 56%.

CONCLUSION

Xenopus oocytes release ATP in response to mechanical stimuli and cell volume changes. Purinergic P2 and P1 receptors confer some of the KCNQ1 channel volume sensitivity, although endogenous adenosine receptors and expressed P2Y2 receptors do so in the negative direction.

Endogenous transport systems in the Xenopus laevis oocyte plasma membrane

Oocytes of the South African clawed frog Xenopus laevis are widely used as a heterologous expression system for the characterization of transport systems such as passive and active membrane transporters, receptors and a whole plethora of other membrane proteins originally derived from animal or plant tissues. The large size of the oocytes and the high degree of expression of exogenous mRNA or cDNA makes them an optimal tool, when compared with other expression systems such as yeast, Escherichia coli or eukaryotic cell lines, for the expression and functional characterization of membrane proteins. This easy to handle expression system is becoming increasingly attractive for pharmacological research. Commercially available automated systems that microinject mRNA into the oocytes and perform electrophysiological measurements fully automatically allow for a mass screening of new computer designed drugs to target membrane transport proteins. Yet, the oocytes possess a large variety of endogenous membrane transporters and it is absolutely mandatory to distinguish the endogenous transporters from the heterologous, expressed transport systems. Here, we review briefly the endogenous membrane transport systems of the oocytes.

Evolutionary origins of the purinergic signalling system

Purines appear to be the most primitive and widespread chemical messengers in the animal and plant kingdoms. The evidence for purinergic signalling in plants, invertebrates and lower vertebrates is reviewed. Much is based on pharmacological studies, but important recent studies have utilized the techniques of molecular biology and receptors have been cloned and characterized in primitive invertebrates, including the social amoeba Dictyostelium and the platyhelminth Schistosoma, as well as the green algae Ostreococcus, which resemble P2X receptors identified in mammals. This suggests that contrary to earlier speculations, P2X ion channel receptors appeared early in evolution, while G protein-coupled P1 and P2Y receptors were introduced either at the same time or perhaps even later. The absence of gene coding for P2X receptors in some animal groups [e.g. in some insects, roundworms (Caenorhabditis elegans) and the plant Arabidopsis] in contrast to the potent pharmacological actions of nucleotides in the same species, suggests that novel receptors are still to be discovered.

Autocrine signaling involved in cell volume regulation: the role of released transmitters and plasma membrane receptors

Cell volume regulation is a basic homeostatic mechanism transcendental for the normal physiology and function of cells. It is mediated principally by the activation of osmolyte transport pathways that result in net changes in solute concentration that counteract cell volume challenges in its constancy. This process has been described to be regulated by a complex assortment of intracellular signal transduction cascades. Recently, several studies have demonstrated that alterations in cell volume induce the release of a wide variety of transmitters including hormones, ATP and neurotransmitters, which have been proposed to act as extracellular signals that regulate the activation of cell volume regulatory mechanisms. In addition, changes in cell volume have also been reported to activate plasma membrane receptors (including tyrosine kinase receptors, G-protein coupled receptors and integrins) that have been demonstrated to participate in the regulatory process of cell volume. In this review, we summarize recent studies about the role of changes in cell volume in the regulation of transmitter release as well as in the activation of plasma membrane receptors and their further implications in the regulation of the signaling machinery that regulates the activation of osmolyte flux pathways. We propose that the autocrine regulation of Ca2+-dependent and tyrosine phosphorylation-dependent signaling pathways by the activation of plasma membrane receptors and swelling-induced transmitter release is necessary for the activation/regulation of osmolyte efflux pathways and cell volume recovery. Furthermore, we emphasize the importance of studying these extrinsic signals because of their significance in the understanding of the physiology of cell volume regulation and its role in cell biology in vivo, where the constraint of the extracellular space might enhance the autocrine or even paracrine signaling induced by these released transmitters.

Epithelium and/or theca are required for ATP-elicited K+ current in follicle-enclosed Xenopus oocytes

The Xenopus follicular cell membrane is endowed with ATP-sensitive K+ channels, which are operated by various transmitters. These generate the ionic response named IK,cAMP via a mechanism that involves intracellular cAMP synthesis. It is known that opening these K+ channels favors oocyte maturation. Follicle stimulation by adenosine (Ado) or ATP consistently generates a strong IK,cAMP via activation of P1 and P3 purinergic receptors; however, ATP can also inhibit IK,cAMP, apparently acting on a third receptor type. Here, we show that IK,cAMP might be elicited by ATP released within the follicle, and that current activation by ATP was entirely dependent on the presence of epithelial and/or theca layers. Morphological studies confirmed that removal of epithelium/theca in these follicles (e.t.r.) was complete, and activation of fast Cl- (Fin) currents by ATP in e.t.r. follicles confirmed that communication between oocyte and follicular cells remained unchanged. Thus, dependence on epithelium/theca was specific for ATP-elicited K+ current. Using UTP and betagamma-MeATP as specific purinergic agents for IK,cAMP inhibition and activation, respectively, it was found that inhibition of IK,cAMP elicited by ATP or UTP was robustly present in e.t.r. follicles but was absent or strongly decreased in whole follicles (w.f.). Accordingly, this indicated that in w.f., epithelium and/or theca downregulated the IK,cAMP inhibition evoked by ATP, and that this control mechanism was absent in e.t.r. follicles. We suggest that this notable action on follicular cells involves one or both of two mechanisms, a paracrine transmitter released from epithelial and/or theca layers and action of ecto-ATPases.

Mechanisms of the ATP potentiation of hyposmotic taurine release in Swiss 3T3 fibroblasts

Reducing osmolarity by 35% increased (3)H-taurine efflux from Swiss 3T3 fibroblasts from 0.5% to a peak of 5.7%. The presence of ATP (10-100 microM; EC(50) 1.5 microM) increased taurine efflux up to 10%, and decreased the set point for hyposmotically stimulated taurine release (HTR). ATP potentiation was mimicked by UTP, reduced by addition of suramin and pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) and unaffected by ADP, beta,gamma-methylene-ATP (beta,gamma-ATP) or 2-methylthio-ATP (Me-ATP), suggesting its mediation by purinergic P2Y(2) and P2Y(4) metabotropic receptors. Under isosmotic conditions ATP increased the cytosolic [Ca(2+)] ([Ca(2+)](i)) markedly, but did not increase taurine release. HTR was independent of external Ca(2+) but was reduced (by 56-59%) by BAPTA-AM, thapsigargin-induced depletion of intracellular Ca(2+) stores, or phospholipase C (PLC) inhibition. Blockade of calmodulin (CaM) or calmodulin kinase II (CaMKII) reduced HTR by 54% and 76%, respectively. The ATP-mediated potentiation was prevented fully by all these treatments. HTR was reduced by 30-50% by blockers of protein tyrosine kinases (AG18), phosphoinositide 3-kinase (PI3K) (wortmannin), p21rho (toxin B), p21rho-kinase (Y27632) and the stress-activated kinase p38 (PD169316). ATP-mediated potentiation was reduced similarly by these blockers. Simultaneous inhibition of PI3K and CaMKII abolished HTR. Altogether, these results suggest a modulatory effect of ATP, probably exerted by a potentiation of the Ca(2+)-dependent fraction of HTR. This fraction has as signalling elements a PLC-dependent [Ca(2+)](i) increase, resulting from Ca(2+) released from thapsigargin-sensitive internal stores, followed by activation of CaM/CaMKII reactions. The Ca(2+)/ATP effect operates only when the Ca(2+)-independent, tyrosine kinase-mediated pathway is already activated. Suggested elements of cross-talk between the two pathways are PLC, PI3K and CaMKII.

P2Y purinergic receptor regulation of CFTR chloride channels in mouse cardiac myocytes

The intracellular signalling pathways and molecular mechanisms responsible for P2-purinoceptor-mediated chloride (Cl(-)) currents (I(Cl,ATP)) were studied in mouse ventricular myocytes. In standard NaCl-containing extracellular solutions, extracellular ATP (100 microm) activated two different currents, I(Cl,ATP) with a linear I-V relationship in symmetrical Cl(-) solutions, and an inwardly rectifying cation conductance (cationic I(ATP)). Cationic I(ATP) was selectively inhibited by Gd(3+) and Zn(2+), or by replacement of extracellular NaCl by NMDG; I(Cl,ATP) was Cl(-) selective, and inhibited by replacement of extracellular Cl(-) by Asp(-); both currents were prevented by suramin or DIDS pretreatment. In GTPgammaS-loaded cells, I(Cl,ATP) was irreversibly activated by ATP, but cationic I(ATP) was still regulated reversibly. GDPbetaS prevented activation of the I(Cl,ATP,) even though pertussis toxin pretreatment did not modulate I(Cl,ATP). These results suggest that activation of I(Cl,ATP) occurs via a G-protein coupled P2Y purinergic receptor. The I(Cl,ATP) persistently activated by GTPgammaS, was inhibited by glibenclamide but not by DIDS, thus exhibiting known pharmacological properties of cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channels. In ventricular cells of cftr(-/-) mice, extracellular ATP activated cationic I(ATP), but failed to activate any detectable I(Cl,ATP). These results provide compelling evidence that activation of CFTR Cl(-) channels in mouse heart are coupled to G-protein coupled P2Y purinergic receptors.

Release of ATP induced by hypertonic solutions in Xenopus oocytes

ATP mediates intercellular communication. Mechanical stress and changes in cell volume induce ATP release from various cell types, both secretory and non-secretory. In the present study, we stressed Xenopus oocytes with a hypertonic solution enriched in mannitol (300 mM). We measured simultaneously ATP release and ionic currents from a single oocyte. A decrease in cell volume, the activation of an inward current and ATP release were coincident. We found two components of ATP release: the first was associated with granule or vesicle exocytosis, because it was inhibited by tetanus neurotoxin, and the second was related to the inward current. A single exponential described the correlation between ATP release and the hypertonic-activated current. Gadolinium ions, which block mechanically activated ionic channels, inhibited the ATP release and the inward current but did not affect the decrease in volume. Oocytes expressing CFTR (cystic fibrosis transmembrane regulator) released ATP under hypertonic shock, but ATP release was significantly inhibited in the first component: that related to granule exocytosis. Since the ATP measured is the balance between ATP release and ATP degradation by ecto-enzymes, we measured the nucleoside triphosphate diphosphohydrolase (NTPDase) activity of the oocyte surface during osmotic stress, as the calcium-dependent hydrolysis of ATP, which was inhibited by more than 50 % in hypertonic conditions. The best-characterized membrane protein showing NTPDase activity is CD39. Oocytes injected with an antisense oligonucleotide complementary to CD39 mRNA released less ATP and showed a lower amplitude in the inward current than those oocytes injected with water.

ATP stimulation of P2X(7) receptors activates three different ionic conductances on cultured mouse Schwann cells

Extracellular ATP, by acting on P2 purinergic receptors, is a potent mediator of cell-to-cell communication both within and between the nervous and the immune systems. We show here by patch-clamp recording, fluorescent dye uptake and immunocytochemistry that, in cultured mouse Schwann cells, ATP activates a P2X(7) receptor associated with three different ionic conductances. In control conditions, ATP activated an inward current (I(ATP)) with a low potency (EC(50), 7.2 mM). Replacing ATP either by the ATP analogue 2',3'-O-(4-benzoyl-4-benzoyl)-ATP (BzATP) or by the tetraacidic form ATP(4-) potentiated the inward current (ATP(4-) EC(50), 375 microM). ATP and BzATP currents were strongly reduced by periodate oxidized ATP (oATP), an antagonist of P2X(7) receptors. IATP was a mixed current composed of a nonselective cationic conductance, a cationic conductance selective for K(+) and an anionic conductance selective for Cl(-). The activation of the K(+) conductance was dependent on an influx of Ca(2+), and was blocked by charybdotoxin (ChTX) and tetraethylammonium (TEA), two potent antagonists of large conductance Ca(2+)-activated K(+) channels (BK channels). The activation of the Cl(-) conductance was insensitive to Ca(2+) but required the presence of K(+). Total removal of K(+) blocked both the Ca(2+)-activated K(+) conductance and the Cl(-) conductance, unveiling the P2X(7) nonselective cationic conductance. The P2X(7) receptor was localized by immunocytochemistry using a polyclonal antibody, anti-P2X(7), whilst its expression and functionality were both detected by the uptake of Lucifer Yellow. This receptor could regulate the synthesis and the release of cytokines by Schwann cells during pathophysiological events.

Basic experimental studies and clinical aspects of gadolinium salts and chelates

Gadolinium is a lanthanide that has in recent years become more commonly present in our society. Organic chelates of gadolinium are increasingly used as contrast agents for the imaging of body fluids. Although adverse reactions to these agents are uncommon, it is known that gadolinium salts can bring about a wide variety of changes in physiology. Gadolinium chloride is widely used experimentally as an inhibitor of stretch-activated ion channels and physiological responses of tissues to mechanical stimulation. It is also employed as a selective inhibitor of macrophages in vivo. In this review, the known biochemical actions of gadolinium are brought together with its in vivo pharmacology and toxicology.

Brefeldin A block of integrin-dependent mechanosensitive ATP release from Xenopus oocytes reveals a novel mechanism of mechanotransduction

Many animal cells release ATP into the extracellular medium, and often this release is mechanosensitive. However, the mechanisms underlying this release are not well understood. Using the luciferin-luciferase bioluminescent assay we demonstrate that a Xenopus oocyte releases ATP at a basal rate approximately 0.01 fmol/s, and gentle mechanical stimulation can increase this to 50 fmol/s. Brefeldin A, nocodazole, and progesterone-induced- maturation block basal and mechanosensitive ATP release. These treatments share the common feature of disrupting the Golgi complex and vesicle trafficking to the cell surface and thereby block protein secretion and membrane protein insertion. We propose that ATP release occurs when protein transport vesicles enriched in ATP fuse with the plasma membrane. Collagenase, integrin-binding peptides, and cytochalasin D also block ATP release, indicating that extracellular, membrane and cytoskeletal elements are involved in the release process. Elevation of intracellular Ca(2+) does not evoke ATP release but potentiates mechanosensitive ATP release. Our study indicates a novel mechanism of mechanotransduction that would allow cells to regulate membrane trafficking and protein transport/secretion in response to mechanical loading.

Molecular basis of mechanotransduction in living cells

The simplest cell-like structure, the lipid bilayer vesicle, can respond to mechanical deformation by elastic membrane dilation/thinning and curvature changes. When a protein is inserted in the lipid bilayer, an energetic cost may arise because of hydrophobic mismatch between the protein and bilayer. Localized changes in bilayer thickness and curvature may compensate for this mismatch. The peptides alamethicin and gramicidin and the bacterial membrane protein MscL form mechanically gated (MG) channels when inserted in lipid bilayers. Their mechanosensitivity may arise because channel opening is associated with a change in the protein's membrane-occupied area, its hydrophobic mismatch with the bilayer, excluded water volume, or a combination of these effects. As a consequence, bilayer dilation/thinning or changes in local membrane curvature may shift the equilibrium between channel conformations. Recent evidence indicates that MG channels in specific animal cell types (e.g., Xenopus oocytes) are also gated directly by bilayer tension. However, animal cells lack the rigid cell wall that protects bacteria and plants cells from excessive expansion of their bilayer. Instead, a cortical cytoskeleton (CSK) provides a structural framework that allows the animal cell to maintain a stable excess membrane area (i.e., for its volume occupied by a sphere) in the form of membrane folds, ruffles, and microvilli. This excess membrane provides an immediate membrane reserve that may protect the bilayer from sudden changes in bilayer tension. Contractile elements within the CSK may locally slacken or tighten bilayer tension to regulate mechanosensitivity, whereas membrane blebbing and tight seal patch formation, by using up membrane reserves, may increase membrane mechanosensitivity. In specific cases, extracellular and/or CSK proteins (i.e., tethers) may transmit mechanical forces to the process (e.g., hair cell MG channels, MS intracellular Ca(2+) release, and transmitter release) without increasing tension in the lipid bilayer.