Ca2+ and cAMP-activated Cl- conductances mediate Cl- secretion in a mouse renal inner medullary collecting duct cell line

Activation of farnesoid X receptor retards expansion of renal collecting duct cell-derived cysts via inhibition of CFTR-mediated Cl- secretion

The transcription factor farnesoid X receptor (FXR) regulates energy metabolism. Specifically, FXR functions to regulate cystic fibrosis transmembrane conductance regulator (CFTR)-mediated Cl- secretion in intestinal epithelial cells. Therefore, this study aimed to investigate the role of FXR in CFTR-mediated Cl- secretion in renal tubular cells and to further elucidate its effects on renal cyst formation and growth. CFTR-mediated Cl- transport was evaluated via short-circuit current (ISC) measurements in Madin-Darby canine kidney (MDCK) cell monolayers and primary rat inner medullary collecting duct cells. The role of FXR in renal cyst formation and growth was determined by the MDCK cell-derived cyst model. Incubation with synthesized (GW4064) and endogenous (CDCA) FXR ligands reduced CFTR-mediated Cl- secretion in a concentration- and time-dependent manner. The inhibitory effect of FXR ligands was not due to the result of reduced cell viability and was attenuated by cotreatment with an FXR antagonist. FXR activation significantly decreased CFTR protein but not its mRNA. In addition, FXR activation inhibited CFTR-mediated Cl- secretion in primary renal collecting duct cells. FXR activation decreased ouabain-sensitive ISC without altering Na+-K+-ATPase mRNA and protein levels. Furthermore, FXR activation significantly reduced the number of cysts and renal cyst expansion. These inhibitory effects were correlated with a decrease in the expression of protein synthesis regulators mammalian target of rapamycin/S6 kinase. This study shows that FXR activation inhibits Cl- secretion in renal cells via inhibition of CFTR expression and retards renal cyst formation and growth. The discoveries point to a physiological role of FXR in the regulation of CFTR and a potential therapeutic application in polycystic kidney disease treatment.NEW & NOTEWORTHY The present study reveals that farnesoid X receptor (FXR) activation reduces microcyst formation and enlargement. This inhibitory effect of FXR activation is involved with decreased cell proliferation and cystic fibrosis transmembrane conductance regulator-mediated Cl- secretion in renal collecting duct cells. FXR might represent a novel target for the treatment of autosomal dominant polycystic kidney disease.

Increases in cytosolic Ca2+ induce dynamin- and calcineurin-dependent internalisation of CFTR

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated, apical anion channel that regulates ion and fluid transport in many epithelia including the airways. We have previously shown that cigarette smoke (CS) exposure to airway epithelia causes a reduction in plasma membrane CFTR expression which correlated with a decrease in airway surface hydration. The effect of CS on CFTR was dependent on an increase in cytosolic Ca2+. However, the underlying mechanism for this Ca2+-dependent, internalisation of CFTR is unknown. To gain a better understanding of the effect of Ca2+ on CFTR, we performed whole cell current recordings to study the temporal effect of raising cytosolic Ca2+ on CFTR function. We show that an increase in cytosolic Ca2+ induced a time-dependent reduction in whole cell CFTR conductance, which was paralleled by a loss of cell surface CFTR expression, as measured by confocal and widefield fluorescence microscopy. The decrease in CFTR conductance and cell surface expression were both dynamin-dependent. Single channel reconstitution studies showed that raising cytosolic Ca2+ per se had no direct effect on CFTR. In fact, the loss of CFTR plasma membrane activity correlated with activation of calcineurin, a Ca2+-dependent phosphatase, suggesting that dephosphorylation of CFTR was linked to the loss of surface expression. In support of this, the calcineurin inhibitor, cyclosporin A, prevented the Ca2+-induced decrease in cell surface CFTR. These results provide a hitherto unrecognised role for cytosolic Ca2+ in modulating the residency of CFTR at the plasma membrane through a dynamin- and calcineurin-dependent mechanism.

TMEM16A is a Ca(2+) -activated Cl(-) channel expressed in the renal collecting duct

AIM

In the renal collecting ducts, ATP stimulates a Ca(2+) -activated chloride current. The identity of the channel responsible for the current under physiological conditions is not known and it was hypothesized that TMEM16a is a relevant candidate in the renal collecting duct.

METHODS

The cortical collecting duct cell line M-1 was used as a model of the collecting duct. The ATP induced Ca(2+) signalling was imaged in cells loaded with Ca(2+) -sensitive fluorescent probes using confocal laser-scanning fluorescence microscopy. Chloride current was determined by mounting M-1 cell layers in Ussing chamber. The expression of TMEM16a in human kidney was tested by immunohistochemistry.

RESULTS

M-1 cells displayed a transient increase in intracellular Ca(2+) concentration in response to 100 nm ATP. This response was completely blocked by addition of 100 μm suramin, indicating that ATP signals through purinergic P2 receptors. Apical addition of 100 nm ATP induced a Cl(-) current, which was blocked by suramin, DPC and the cysteine-modifying compound MTSET. M-1 cells were found to express TMEM16a at the mRNA and protein level. Functionally, it was found that knock-down of TMEM16a expression in M-1 cells inhibited the ATP induced Cl(-) -current. In human and mouse kidney sections, TMEM16a protein expression was localized to the collecting duct, and TMEM16a was found to be excreted in human urinary exosomes.

CONCLUSION

TMEM16a is a Ca(2+) -activated Cl(-) channel expressed in the collecting ducts.

Prostaglandin E2 induces chloride secretion through crosstalk between cAMP and calcium signaling in mouse inner medullary collecting duct cells

Under conditions of high dietary salt intake, prostaglandin E2 (PGE2) production is increased in the collecting duct and promotes urinary sodium chloride (NaCl) excretion; however, the molecular mechanisms by which PGE2 increases NaCl excretion in this context have not been clearly defined. We used the mouse inner medullary collecting duct (mIMCD)-K2 cell line to characterize mechanisms underlying PGE2-regulated NaCl transport. When epithelial Na(+) channels were inhibited, PGE2 exclusively stimulated basolateral EP4 receptors to increase short-circuit current (Isc(PGE2)). We found that Isc(PGE2) was sensitive to inhibition by H-89 and CFTR-172, indicating that EP4 receptors signal through protein kinase A to induce Cl(-) secretion via cystic fibrosis transmembrane conductance regulator (CFTR). Unexpectedly, we also found that Isc(PGE2) was sensitive to inhibition by BAPTA-AM (Ca(2+) chelator), 2-aminoethoxydiphenyl borate (2-APB) (inositol triphosphate receptor blocker), and flufenamic acid (FFA) [Ca(2+)-activated Cl(-) channel (CACC) inhibitor], suggesting that EP4 receptors also signal through Ca(2+) to induce Cl(-) secretion via CACC. Additionally, we observed that PGE2 stimulated an increase in Isc through crosstalk between cAMP and Ca(2+) signaling; BAPTA-AM or 2-APB inhibited a component of Isc(PGE2) that was sensitive to CFTR-172 inhibition; H-89 inhibited a component of Isc(PGE2) that was sensitive to FFA inhibition. Together, our findings indicate that PGE2 activates basolateral EP4 receptors and signals through both cAMP and Ca(2+) to stimulate Cl(-) secretion in IMCD-K2 cells. We propose that these signaling pathways, and the crosstalk between them, may provide a concerted mechanism for enhancing urinary NaCl excretion under conditions of high dietary NaCl intake.

The chloride channel/transporter Slc26a9 regulates the systemic arterial pressure and renal chloride excretion

Apical chloride secretory pathways in the kidney medullary collecting duct are thought to play an important role in the modulation of final urine composition and regulation of systemic vascular volume and/or blood pressure. However, the molecular identity of these molecules has largely remained unknown. Here, we demonstrate that Slc26a9, an electrogenic chloride channel/transporter, is localized on the apical membrane of principal cells in the kidney medullary collecting duct and mediates chloride secretion. Mice with the genetic deletion of Slc26a9 show significant reduction in renal chloride excretion when fed a diet high in salt or subjected to water deprivation. Arterial pressure measurements indicated that Slc26a9 knockout (Slc26a9(-/-)) mice are hypertensive under baseline conditions and increase their blood pressure further within 48 h of switching to a high-salt diet. These results suggest that Slc26a9 plays an important role in renal chloride/fluid excretion and arterial pressure regulation. We propose that impaired SLC26A9 activity in humans may interfere with the excretion of excess salt and result in hypertension.

Collecting duct-specific knockout of adenylyl cyclase type VI causes a urinary concentration defect in mice

Collecting duct (CD) adenylyl cyclase VI (AC6) has been implicated in arginine vasopressin (AVP)-stimulated renal water reabsorption. To evaluate the role of CD-derived AC6 in regulating water homeostasis, mice were generated with CD-specific knockout (KO) of AC6 using the Cre/loxP system. CD AC6 KO and controls were studied under normal water intake, chronically water loaded, or water deprived; all of these conditions were repeated in the presence of continuous administration of 1-desamino-8-d-arginine vasopressin (DDAVP). During normal water intake or after water deprivation, urine osmolality (U(osm)) was reduced in CD AC6 KO animals vs. controls. Similarly, U(osm) was decreased in CD AC6 KO mice vs. controls after water deprivation+DDAVP administration. Pair-fed (with controls) CD AC6 KO mice also had lower urine osmolality vs. controls. There were no detectable differences between KO and control animals in fluid intake or urine volume under any conditions. CD AC6 KO mice did not have altered plasma AVP levels vs. controls. AVP-stimulated cAMP accumulation was reduced in acutely isolated inner medullary CD (IMCD) from CD A6 KO vs. controls. Medullary aquaporin-2 (AQP2) protein expression was lower in CD AC6 KO mice vs. controls. There were no differences in urinary urea excretion or IMCD UT-A1 expression; however, IMCD UT-A3 expression was reduced in CD AC6 KO mice vs. controls. In summary, AC6 in the CD regulates renal water excretion, most likely through control of AVP-stimulated cAMP accumulation and AQP2.

Activation of P2Y1 and P2Y2 receptors induces chloride secretion via calcium-activated chloride channels in kidney inner medullary collecting duct cells

Dysregulation of urinary sodium chloride (NaCl) excretion can result in extracellular fluid (ECF) volume expansion and hypertension. Recent studies demonstrated that urinary nucleotide excretion increases in mice ingesting a high-salt diet and that these increases in extracellular nucleotides can signal through P2Y(2) receptors in the kidney collecting duct to inhibit epithelial Na(+) channels (ENaC). However, under conditions of ECF volume expansion brought about by high-dietary salt intake, ENaC activity should already be suppressed. We hypothesized that alternative pathways exist by which extracellular nucleotides control renal NaCl excretion. We used an inner medullary collecting duct (mIMCD-K2) cell line in an Ussing chamber system as a model to study additional ion transport pathways that are regulated by extracellular nucleotides. When ENaC was inhibited, the addition of adenosine triphosphate (ATP) to the basal side of cell sheets activated both P2Y(1) and P2Y(2) receptors, inducing a transient increase in short-circuit current (I(sc)); addition of ATP to the apical side activated only P2Y(2) receptors, inducing first a transient and then a sustained increase in I(sc). The ATP-induced increases in I(sc) were blocked by pretreatment with a phospholipase C (PLC) inhibitor, a calcium (Ca(2+)) chelator, or Ca(2+)-activated Cl(-) channel (CACC) inhibitors, suggesting that ATP signals through both PLC and intracellular Ca(2+) to activate CACC. We propose that P2Y(1) and P2Y(2) receptors operate in tandem in IMCD cells to provide an adaptive mechanism for enhancing urinary NaCl excretion in the setting of high-dietary NaCl intake.

Fluid transport and cystogenesis in autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most frequent inherited nephropathy. The development and enlargement of cysts in ADPKD requires tubular cell proliferation, abnormalities in the extracellular matrix and transepithelial fluid secretion. Multiple studies have suggested that fluid secretion across ADPKD cyst-lining cells is driven by the transepithelial secretion of chloride, mediated by the apical CFTR channel and specific basolateral transporters. The whole secretory process is stimulated by increased levels of cAMP in the cells, probably reflecting modifications in the intracellular calcium homeostasis and abnormal stimulation of the vasopressin V2 receptor. This review will focus on the pathophysiology of fluid secretion in ADPKD cysts, starting with classic, morphological and physiological studies that were followed by investigations of the molecular mechanisms involved and therapeutic trials targeting these pathways in cellular and animal models and ADPKD patients. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

P2 receptors in renal pathophysiology

Our knowledge and understanding of the P2 receptor signalling system in the kidney have increased significantly in the last ten years. The broad range of physiological roles proposed for this receptor system and the variety of P2 receptor subtypes found in the kidney suggest that any disturbance of function may contribute to several pathological processes. So far, most reports of a possible pathophysiological role for this system in the kidney have focussed on polycystic kidney disease, where abnormal P2 receptor signalling might be involved in cyst expansion and disease progression, and on the P2X(7) receptor, a unique P2X subtype, which when activated enhances inflammatory cytokine release and production, and also cell death. Expression of this particular receptor is upregulated in some forms of chronic renal injury and inflammatory diseases. Further studies of adenosine triphosphate signalling and P2 receptor expression in renal disorders could provide us with novel insights into the role of these receptors in both normal and abnormal kidney function.

A voltage-dependent Ca2+ influx pathway regulates the Ca2+-dependent Cl(-) conductance of renal IMCD-3 cells

We have previously shown that the membrane conductance of mIMCD-3 cells at a holding potential of 0 mV is dominated by a Ca2+-dependent Cl(-) current (I(CLCA)). Here we report that I(CLCA) activity is also voltage dependent and that this dependence on voltage is linked to the opening of a novel Al3+-sensitive, voltage-dependent, Ca2+ influx pathway. Using whole-cell patch-clamp recordings at a physiological holding potential (-60 mV), ICLCA was found to be inactive and resting currents were predominantly K+ selective. However, membrane depolarization to 0 mV resulted in a slow, sigmoidal, activation of ICLCA (T(0.5) approximately 500 s), while repolarization in turn resulted in a monoexponential decay in I(CLCA) (T (0.5) approximately 100 s). The activation of I(CLCA) by depolarization was reduced by lowering extracellular Ca2+ and completely inhibited by buffering cytosolic Ca2+ with EGTA, suggesting a role for Ca2+ influx in the activation of I(CLCA). However, raising bulk cytosolic Ca2+ at -60 mV did not produce sustained I(CLCA) activity. Therefore I(CLCA) is dependent on both an increase in intracellular Ca2+ and depolarization to be active. We further show that membrane depolarization is coupled to opening of a Ca2+ influx pathway that displays equal permeability to Ca2+ and Ba2+ ions and that is blocked by extracellular Al3+ and La3+. Furthermore, Al3+ completely and reversibly inhibited depolarization-induced activation of ICLCA, thereby directly linking Ca2+ influx to activation of I(CLCA). We speculate that during sustained membrane depolarization, calcium influx activates ICLCA which functions to modulate NaCl transport across the apical membrane of IMCD cells.

Purinergic signaling in the lumen of a normal nephron and in remodeled PKD encapsulated cysts

The nephron is the functional unit of the kidney. Blood and plasma are continually filtered within the glomeruli that begin each nephron. Adenosine 5' triphosphate (ATP) and its metabolites are freely filtered by each glomerulus and enter the lumen of each nephron beginning at the proximal convoluted tubule (PCT). Flow rate, osmolality, and other mechanical or chemical stimuli for ATP secretion are present in each nephron segment. These ATP-release stimuli are also different in each nephron segment due to water or salt permeability or impermeability along different luminal membranes of the cells that line each nephron segment. Each of the above stimuli can trigger additional ATP release into the lumen of a nephron segment. Each nephron-lining epithelial cell is a potential source of secreted ATP. Together with filtered ATP and its metabolites derived from the glomerulus, secreted ATP and adenosine derived from cells along the nephron are likely the principal two of several nucleotide and nucleoside candidates for renal autocrine and paracrine ligands within the tubular fluid of the nephron. This minireview discusses the first principles of purinergic signaling as they relate to the nephron and the urinary bladder. The review discusses how the lumen of a renal tubule presents an ideal purinergic signaling microenvironment. The review also illustrates how remodeled and encapsulated cysts in autosomal dominant polycystic kidney disease (ADPKD) and remodeled pseudocysts in autosomal recessive PKD (ARPKD) of the renal collecting duct likely create an even more ideal microenvironment for purinergic signaling. Once trapped in these closed microenvironments, purinergic signaling becomes chronic and likely plays a significant epigenetic and detrimental role in the secondary progression of PKD, once the remodeling of the renal tissue has begun. In PKD cystic microenvironments, we argue that normal purinergic signaling within the lumen of the nephron provides detrimental acceleration of ADPKD once remodeling is complete.

Cl- secretion in ATP-treated renal epithelial C7-MDCK cells is mediated by activation of P 2Y1 receptors, phospholipase A2 and protein kinase A

This study examines the mechanism of P 2Y-induced Cl- secretion in monolayers of C7-Madin-Darby canine kidney (MDCK) cells triggered by basolateral application of ATP and measured as transcellular short current (I(SC)). Both ATP-induced arachidonic acid (AA) synthesis and I(SC) in ATP-treated cells were abolished by the phosholipase A2 (PLA2) inhibitor, AACOCF3. The cyclo-oxygenase inhibitor indomethacin decreased I(SC) and cAMP production in ATP-treated cells with an IC50 of approximately 0.3 microm. ATP led to rapid activation of cAMP-dependent protein kinase A (PKA), as estimated by phosphorylation of a vasodilator-stimulated phosphoprotein. PKA activity and I(SC) evoked by ATP, as well as by prostaglandin E1 (PGE1), were diminished in the presence of the PKA inhibitor H-89 or an adenovirus-mediated expression of PKA-inhibitor protein, PKI. In contrast, indomethacin completely blocked the increment of PKA and I(SC) triggered by ATP and AA, but did not affect PKA activation and I(SC) detected with PGE1. The kinetics of [Ca2+]i elevation in ATP- and thapsigargin-treated cells were similar and suppressed by the Ca(2+)i chelator BAPTA. Neither baseline nor maximal increment of ATP-induced I(SC) was affected by thapsigargin and BAPTA. Real-time PCR showed that C7 cells express more mRNA for P 2Y1 and P 2Y2 than for other P 2Y receptor subtypes. The rank order of potency (2MeSATP > ATP > ADP >> UTP) indicates that P 2Y1 rather than P 2Y2 receptors contribute to PKA and I(SC) activation. Viewed collectively, these data show that Cl- secretion in C7-MDCK monolayers treated with basolateral ATP is triggered by P 2Y1 receptors and is mediated by subsequent [Ca2+]i-independent activation of PLA2 and PKA.

Structure and function of CLCA proteins

CLCA proteins were discovered in bovine trachea and named for a calcium-dependent chloride conductance found in trachea and in other secretory epithelial tissues. At least four closely located gene loci in the mouse and the human code for independent isoforms of CLCA proteins. Full-length CLCA proteins have an unprocessed mass ratio of approximately 100 kDa. Three of the four human loci code for the synthesis of membrane-associated proteins. CLCA proteins affect chloride conductance, epithelial secretion, cell-cell adhesion, apoptosis, cell cycle control, mucus production in asthma, and blood pressure. There is a structural and probable functional divergence between CLCA isoforms containing or not containing beta4-integrin binding domains. Cell cycle control and tumor metastasis are affected by isoforms with the binding domains. These isoforms are expressed prominently in smooth muscle, in some endothelial cells, in the central nervous system, and also in secretory epithelial cells. The isoform with disrupted beta4-integrin binding (hCLCA1, pCLCA1, mCLCA3) alters epithelial mucus secretion and ion transport processes. It is preferentially expressed in secretory epithelial tissues including trachea and small intestine. Chloride conductance is affected by the expression of several CLCA proteins. However, the dependence of the resulting electrical signature on the expression system rather than the CLCA protein suggests that these proteins are not independent Ca2+-dependent chloride channels, but may contribute to the activity of chloride channels formed by, or in conjunction with, other proteins.

Calcium-activated chloride channels

Calcium-activated chloride channels (CaCCs) play important roles in cellular physiology, including epithelial secretion of electrolytes and water, sensory transduction, regulation of neuronal and cardiac excitability, and regulation of vascular tone. This review discusses the physiological roles of these channels, their mechanisms of regulation and activation, and the mechanisms of anion selectivity and conduction. Despite the fact that CaCCs are so broadly expressed in cells and play such important functions, understanding these channels has been limited by the absence of specific blockers and the fact that the molecular identities of CaCCs remains in question. Recent status of the pharmacology and molecular identification of CaCCs is evaluated.

Exploration of the basolateral chloride channels in the renal tubule using

Chloride channels located on the basolateral membrane are known to be involved in chloride absorption in several parts of the renal tubule, and particularly in the thick ascending limb and distal convoluted tubule. The data available suggest that the ClC-K channels play the major role in this process. We provide here a description of the electrophysiological properties of these channels, still very incomplete at this stage, and we attempt to compare ClC-Ks to three chloride channels that we have identified in the basolateral membrane of microdissected fragments of the mouse renal tubule using the patch-clamp technique. Based on anion selectivity and dependence on external pH and calcium shown by the ClC-Ks, we propose candidate ClC-K1 and ClC-K2 in native tissue. We also discuss the possibility that chloride channels that do not belong to the ClC family may also be involved in the absorption of chloride across the cortical thick ascending limb.

PGE2 stimulates Cl- secretion in murine M-1 cortical collecting duct cells in an autocrine manner

Prostaglandin E2 (PGE2) is thought to be an important modulator of renal ion and water transport, but its effects remain complex and incompletely understood. Here we examined the effects of PGE2 on transepithelial ion transport of M-1 mouse cortical collecting duct cells using short-circuit current (ISC) measurements. Basolateral addition of PGE2 (1 microM) produced a transient peak increase in ISC of 6.3+/-0.8 microA cm(-2) (n=11), followed by a sustained plateau. The PGE2-evoked response was preserved in the presence of 100 micro M apical amiloride with an average peak increase of 10.6+/-1.0 microA cm(-2) (n=23). However, it was greatly diminished in both the presence of apical diphenylamine-2-carboxylic acid (DPC, 1 mM) and the absence of extracellular Cl-, indicating that Cl- secretion had been stimulated. Basolateral PGE2 induced a concentration dependent response, with an EC50 of about 8 nM. Apical addition of PGE2 elicited an ISC response similar to that observed with basolateral PGE2. Furthermore, apical exposure to arachidonic acid (AA) produced a similar increase in ISC, which could be prevented by the cyclooxygenase inhibitor indomethacin, while AA failed to exert an additional effect in the presence of PGE2. Using RT-PCR, we confirmed the expression of the PGE2 (EP) receptor subtypes EP1, EP3 and EP4 but not of EP2 in cultured M-1 CCD cells. We conclude that M-1 cells express functional cyclooxygenase activity and can generate PGE2 which acts in an autocrine manner, causing Cl- secretion.

Extracellular ATP-induced calcium signaling in mIMCD-3 cells requires both P2X and P2Y purinoceptors

Kidney tubules are targets for the activation of locally released nucleotides through multiple P2 receptor types. Activation of these P2 receptors modulates cellular Ca(2+) signaling and downstream cellular function. The purpose of this study was to determine whether P2 receptors were present in mIMCD-3 cells, a mouse inner medullary collecting duct cell line, and if so, to examine their link with intracellular Ca(2+) homeostasis. To monitor intracellular Ca(2+) concentration ([Ca(2+)](i)), experiments were conducted using the fluorescent dye fura 2. ATP (0.1-100 microM) produced a dose-dependent increase in [Ca(2+)](i) in a physiological Ca(2+)-containing solution, with an EC(50) of 2.5 microM. The P2-receptor antagonist PPADS reduced the effect of ATP on [Ca(2+)](i), and the P1-receptor agonist adenosine caused only a small increase in [Ca(2+)](i). Preincubation of cells with the phospholipase C antagonist U-73122 blocked the ATP-induced increase in [Ca(2+)](i), indicating P2Y receptors were involved in this process. In a Ca(2+)-free bath solution, thapsigargin and ATP induced intracellular Ca(2+) release from an identical pool. Nucleotides caused an increase in [Ca(2+)](i) in the potency order of UTP = ATP > ATP gamma S > ADP > UDP that is best fitted with the P2Y(2) subtype profile. Although the P2Y agonist UTP induced a similar large transient increase in [Ca(2+)](i) as did ATP, a small but sustained increase in [Ca(2+)](i) occurred only in ATP-stimulated cells, suggesting the role of P2X receptors in Ca(2+) influx. The sustained increase in [Ca(2+)](i) could be blocked by either nonselective cation channel blockers Gd(3+) or P2X antagonists PPADS and PPNDS. Furthermore, when either Gd(3+) or PPNDS was applied to the bath solution before ATP application, the ATP-induced increase in [Ca(2+)](i) was significantly reduced. Both RT-PCR and Western blotting corroborated the presence of P2X(1) and P2Y(2) receptors. These studies demonstrate that mIMCD-3 cells have both P2X and P2Y subtype receptors and that the activation of both P2X and P2Y receptors by extracellular ATP appears to be required to regulate intracellular Ca(2+) signaling.

Kinetics and regulation of a Ca2+-activated Cl- conductance in mouse renal inner medullary collecting duct cells

Using the whole cell patch-clamp technique, a Ca2+-activated Cl- conductance (CaCC) was transiently activated by extracellular ATP (100 microM) in primary cultures of mouse inner medullary collecting duct (IMCD) cells and in the mouse IMCD-K2 cell line. ATP also transiently increased intracellular Ca2+ concentration ([Ca2+]i) from 100 nM to peak values of approximately 750 nM in mIMCD-K2 cells, with a time course similar to the ATP-induced activation and decay of the CaCC. Removal of extracellular Ca2+ had no major effect on the peak Cl- conductance or the increase in [Ca2+]i induced by ATP, suggesting that Ca2+ released from intracellular stores directly activates the CaCC. In mIMCD-K2 cells, a rectifying time- and voltage-dependent current was observed when [Ca2+]i was fixed via the patch pipette to between 100 and 500 nM. Maximal activation occurred at approximately 1 microM [Ca2+]i, with currents losing any kinetics and displaying a linear current-voltage relationship. From Ca2+-dose-response curves, an EC50 value of approximately 650 nM at -80 mV was obtained, suggesting that under physiological conditions the CaCC would be near fully activated by mucosal nucleotides. Noise analysis of whole cell currents in mIMCD-K2 cells suggests a single-channel conductance of 6-8 pS and a density of approximately 5,000 channels/cell. In conclusion, the CaCC in mouse IMCD cells is a low-conductance, nucleotide-sensitive Cl- channel, whose activity is tightly coupled to changes in [Ca2+]i over the normal physiological range.

The isolated polycystin-1 cytoplasmic COOH terminus prolongs ATP-stimulated Cl- conductance through increased Ca2+ entry

The precise steps leading from mutation of the polycystic kidney disease (PKD1) gene to the autosomal dominant polycystic kidney disease (ADPKD) phenotype remain to be established. Fluid accumulation is a requirement for cyst expansion in ADPKD, suggesting that abnormal fluid secretion into the cyst lumen might play a role in disease. In this study, we sought to establish a link between polycystin-1 (the PKD1 gene product) and ATP-stimulated Cl- secretion in renal tubule cells. To do this, we performed a whole cell patch-clamp analysis of the effects of expression of the isolated cytoplasmic COOH-terminus of polycystin-1 in stably transfected mouse cortical collecting duct cells. The truncated polycystin-1 fusion protein prolonged the duration of ATP-stimulated Cl- conductance and intracellular Ca2+ responses. Both effects were dependent on extracellular Ca2+. It was determined that expression of the truncated polycystin-1 fusion protein introduced, or activated, an ATP-induced Ca2+ entry pathway that was undetectable in transfection control cell lines. Our findings are concordant with increasing evidence for a role of polycystin-1 in cell Ca2+ homeostasis and indicate that dysregulated Ca2+ entry might promote Cl- secretion and cyst expansion in ADPKD.

Characterization of Ca2+-activated Cl- currents in mouse kidney inner medullary collecting duct cells

Ca2+-activated Cl- (ClCa) channels were characterized biophysically and pharmacologically in a mouse kidney inner medullary collecting duct cell line, IMCD-K2. Whole cell recording was performed with symmetrical N-methyl-d-glucamine chloride (NMDG)-Cl in the intracellular and extracellular solutions, and the intracellular Ca2+ concentration ([Ca2+]i) was adjusted with Ca2+-EGTA buffers. The amplitude of the current was dependent on [Ca2+]i. [Ca2+]i <800 nM strongly activated outwardly rectifying Cl- currents, whereas high Ca2+ (21 microM) elicited time-independent currents that did not rectify. The currents activated at low [Ca2+] exhibited time-dependent activation and deactivation. The affinity of the channel for Ca2+ was voltage dependent. The EC50 for Ca2+ was approximately 0.4 microM at +100 mV and approximately 1.0 microM at -100 mV. The Cl- channel blocker niflumic acid in the bath equally inhibited both inward and outward currents reversibly, with a Ki = 7.6 microM. DIDS, diphenylamine-2-carboxylic acid, and anthracene-9-carboxylic acid reversibly inhibited outward currents in a voltage-dependent manner. DTT slowly inhibited the currents, but tamoxifen did not. Comparing the biophysical and pharmacological properties, we conclude that IMCD-K2 cells express the same type of ClCa channels as those we have described in detail in Xenopus laevis oocytes (Qu Z and Hartzell HC. J Biol Chem 276: 18423-18429, 2001).

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.

Autosomal dominant polycystic kidney disease: molecular genetics and pathophysiology

In autosomal dominant polycystic kidney disease (ADPKD), the precise steps leading to cyst formation and loss of renal function remain uncertain. Pathophysiologic studies have suggested that renal tubule epithelial cells form cysts as a consequence of increased proliferation, dedifferentiation, and transition to a secretory pattern of transepithelial-fluid transport. Since the cloning of two genes implicated in ADPKD, there has been an explosion of information about the functions of the gene products polycystin 1 and 2. In this review, we discuss what is known of the functions of the polycystins and how this information is providing important insights into the molecular pathogenesis of ADPKD.

Autocrine extracellular purinergic signaling in epithelial cells derived from polycystic kidneys

ATP and its metabolites are potent autocrine agonists that act extracellularly within tissues to affect epithelial function. In polycystic kidneys, renal tubules become dilated and/or encapsulated as cysts, creating abnormal microenvironments for autocrine signaling. Previously, our laboratory has shown that high-nanomolar to micromolar quantities of ATP are released from cell monolayers in vitro and detectable in cyst fluids from microdissected human autosomal dominant polycystic kidney (ADPKD) cysts. Here, we show enhanced ATP release from autosomal recessive polycystic kidney (ARPKD) and ADPKD epithelial cell models. RT-PCR and immunoblotting for P2Y G protein-coupled receptors and P2X purinergic receptor channels show expression of mRNA and/or protein for multiple subtypes from both families. Assays of cytosolic Ca(2+) concentration and secretory Cl(-) transport show P2Y and P2X purinergic receptor-mediated stimulation of Cl(-) secretion via cytosolic Ca(2+)-dependent signaling. Therefore, we hypothesize that autocrine purinergic signaling may augment detrimentally cyst volume expansion in ADPKD or tubule dilation in ARPKD, accelerating disease progression.

Protease-activated receptor-1 stimulates Ca(2+)-dependent Cl(-) secretion in human intestinal epithelial cells

The thrombin receptor, protease-activated receptor-1 (PAR-1), has wide tissue distribution and is involved in many physiological functions. Because thrombin is in the intestinal lumen and mucosa during inflammation, we sought to determine PAR-1 expression and function in human intestinal epithelial cells. RT-PCR showed PAR-1 mRNA expression in SCBN cells, a nontransformed duodenal epithelial cell line. Confluent SCBN monolayers mounted in Ussing chambers responded to PAR-1 activation with a Cl(-)-dependent increase in short-circuit current. The secretory effect was blocked by BaCl2 and the Ca(2+)-ATPase inhibitor thapsigargin, but not by the L-type Ca(2+) channel blocker verapamil or DIDS, the nonselective inhibitor of Ca(2+)-dependent Cl(-) transport. Responses to thrombin and PAR-1-activating peptides exhibited auto- and crossdesensitization. Fura 2-loaded SCBN cells had increased fluorescence after PAR-1 activation, indicating increased intracellular Ca(2+). RT-PCR showed that SCBN cells expressed mRNA for the cystic fibrosis transmembrane conductance regulator (CFTR) and hypotonicity-activated Cl(-) channel-2 but not for the Ca(2+)-dependent Cl(-) channel-1. PAR-1 activation failed to increase intracellular cAMP, suggesting that the CFTR channel is not involved in the Cl(-) secretory response. Our data demonstrate that PAR-1 is expressed on human intestinal epithelial cells and regulates a novel Ca(2+)-dependent Cl(-) secretory pathway. This may be of clinical significance in inflammatory intestinal diseases with elevated thrombin levels.

Extracellular nucleotide signaling along the renal epithelium

During the past two decades, several cell membrane receptors, which preferentially bind extracellular nucleotides, and their analogs have been identified. These receptors, collectively known as nucleotide receptors or "purinergic" receptors, have been characterized and classified on the basis of their biological actions, their pharmacology, their molecular biology, and their tissue and cell distribution. For these receptors to have biological and physiological relevance, nucleotides must be released from cells. The field of extracellular ATP release and signaling is exploding, as assays to detect this biological process increase in number and ingenuity. Studies of ATP release have revealed a myriad of roles in local regulatory (autocrine or paracrine) processes in almost every tissue in the body. The regulatory mechanisms that these receptors control or modulate have physiological and pathophysiological roles and potential therapeutic applications. Only recently, however, have ATP release and nucleotide receptors been identified along the renal epithelium of the nephron. This work has set the stage for the study of their physiological and pathophysiological roles in the kidney. This review provides a comprehensive presentation of these issues, with a focus on the renal epithelium.

Expression of cystic fibrosis transmembrane conductance regulator in rat efferent duct epithelium

The expression of cystic fibrosis transmembrane conductance regulator (CFTR) was studied in rat efferent ducts. Under whole-cell patch-clamp condition, efferent duct cells responded to intracellular cAMP with a rise in inward current. The cAMP-activated current exhibited a linear I-V relationship and time- and voltage-independent characteristics. The current was inhibited by the Cl(-) channel blocker diphenylamine 2,2'-dicarboxylic acid (DPC) in a voltage-dependent manner and reversed at 24 +/- 0.5 mV, close to the equilibrium potential for Cl(-) (30 mV), suggesting that the current was Cl(-) selective. The cAMP-activated current displayed a permeability sequence of Br(-) > Cl(-) > I(-). Short-circuit current measurement in cultured rat efferent duct epithelia also revealed a cAMP-activated inward current inhibitable by DPC. These electrophysiological properties of the cAMP-activated Cl(-) conductance in the efferent duct were consistent with those reported for CFTR. In support of the functional studies, reverse transcription polymerase chain reaction revealed the presence of CFTR message in cultured efferent duct epithelium. Immunohistochemical studies in intact rats also demonstrated CFTR protein at the apical membrane of the principal cells of efferent duct. CFTR may play a role in modulating fluid transport in the efferent duct.

Role of CFTR in autosomal recessive polycystic kidney disease

An extensive body of in vitro data implicates epithelial chloride secretion, mediated through cystic fibrosis transmembrane conductance regulator (CFTR) protein, in generating or maintaining fluid filled cysts in MDCK cells and in human autosomal dominant polycystic kidney disease (ADPKD). In contrast, few studies have addressed the pathophysiology of fluid secretion in cyst formation and enlargement in autosomal recessive polycystic kidney disease (ARPKD). Murine models of targeted disruptions or deletions of specific genes have created opportunities to examine the role of individual gene products in normal development and/or disease pathophysiology. The creation of a murine model of CF, which lacks functional CFTR protein, provides the opportunity to determine whether CFTR activity is required for renal cyst formation in vivo. Therefore, this study sought to determine whether renal cyst formation could be prevented by genetic complementation of the BPK murine model of ARPKD with the CFTR knockout mouse. The results of this study reveal that in animals that are homozygous for the cystic gene (bpk), the lack of functional CFTR protein on the apical surface of cystic epithelium does not provide protection against cyst growth and subsequent decline in renal function. Double mutant mice (bpk -/-; cftr -/-) developed massively enlarged kidneys and died, on average, 7 d earlier than cystic, non-CF mice (bpk -/-; cftr +/+/-). This suggests fundamental differences in the mechanisms of transtubular fluid secretion in animal models of ARPKD compared with ADPKD.

The alpha(1G)-subunit of a voltage-dependent Ca(2+) channel is localized in rat distal nephron and collecting duct

The molecular type and localization of calcium channels along the nephron are not well understood. In the present study, we assessed the distribution of the recently identified alpha(1G)-subunit encoding a voltage-dependent calcium channel with T-type characteristics. Using a RNase protection assay, alpha(1G)-mRNA levels in kidney regions were determined as inner medulla >> outer medulla congruent with cortex. RT-PCR analysis of microdissected rat nephron segments revealed alpha(1G) expression in the distal convoluted tubule (DCT), in the connecting tubule and cortical collecting duct (CT+CCD), and inner medullary collecting duct (IMCD). alpha(1G) mRNA was expressed in the IMCD cell line mIMCD-3. Single- and double-labeling immunohistochemistry and confocal laser microscopy on semithin paraffin sections of rat kidneys by using an anti-alpha(1G) antibody demonstrated a distinct labeling at the apical plasma membrane domains of DCT cells, CT principal cells, and IMCD principal cells.

Bradykinin regulation of salt transport across mouse inner medullary collecting duct epithelium involves activation of a Ca(2+)-dependent Cl(-) conductance

The mechanism by which bradykinin regulates renal epithelial salt transport has been investigated using a mouse inner medullary renal collecting duct cell-line mIMCD-K2. Using fura-2 loaded mIMCD-K2 cells bradykinin (100 nM) has been shown to induce a transient increase in intracellular Ca(2+) via activation of bradykinin B2 receptors localized to both the apical and basolateral epithelial cell surfaces. In mIMCD-K2 epithelial cell-layers clamped in Ussing chambers, 100 nM bradykinin via apical and basolateral bradykinin B2 receptors stimulated a transient increase in inward short-circuit current (I:(sc)) of similar duration to the increase in intracellular Ca(2+). Replacements of the bathing solution Na(+) by the impermeant cation, N-methyl-D-glucamine and of Cl(-) and HCO(3)(-) by the impermeant anion gluconate at either the apical (no reduction) or basal bathing solutions (abolition of the response) are consistent with the bradykinin-stimulated increase in inward I:(sc) resulting from basal to apical Cl(-) (anion) secretion. Using the slow whole cell configuration of the patch-clamp technique, bradykinin was shown to activate a transient Cl(-) selective whole cell current which showed time-dependent activation at positive membrane potentials and time-dependent inactivation at negative membrane potentials. These currents were distinct from those activated by forskolin (CFTR), but identical to those activated by exogenous ATP and are therefore consistent with bradykinin activation of a Ca(2+)-dependent Cl(-) conductance. The molecular identity of the Ca(2+)-dependent Cl(-) conductance has been investigated by an RT - PCR approach. Expression of an mRNA transcript with 96% identity to mCLCA1/2 was confirmed, however an additional but distinct mRNA transcript with only 81% of the identity to mCLCA1/2 was identified.