The chemokine KC regulates HGF-stimulated epithelial cell morphogenesis

Hepatocyte growth factor (HGF) induces migration, proliferation, and branching in renal epithelial cells from the inner medullary collecting duct (mIMCD-3 cells). Microarray analysis after HGF stimulation of these cells revealed upregulation of the chemokine KC. We found that both the message and protein levels of KC are increased after HGF treatment and that mIMCD-3 cells express the KC receptor CXCR2. Treatment with KC results in stimulation of mIMCD-3 cell proliferation but has no effect on basal rates of cell migration or branching morphogenesis. In contrast to its known stimulatory effect on neutrophil migration, KC markedly inhibits HGF-mediated cell migration and branching morphogenesis, resulting in shorter tubules with fewer branch points. Examination of the mechanism of this effect reveals that KC does not alter phosphorylation of the c-met receptor or the initial activation of the MAPK or phosphoinositide 3-kinase (PI 3-K) signaling pathways. However, sustained activation of the PI 3-K pathway by HGF was inhibited by treatment with KC, and mimicking this effect by treatment with LY-294002 2 h after HGF stimulation reproduced the inhibition of HGF-stimulated branching morphogenesis. These data demonstrate that HGF-mediated KC production can act in an autocrine fashion to downregulate excessive branching and migration of renal epithelial cells in response to HGF, while still supporting cell proliferation. These characteristics may play a role in modulating the response to HGF during developmental tubule formation and/or during the repair of the tubular architecture following injury.

Polycystins: what polycystic kidney disease tells us about sperm

Experimental evidence indicates that the membrane-associated proteins polycystin-1 and polycystin-2 operate as a receptor-calcium channel complex that regulates signaling pathways essential for modulation of renal tubulogenesis. Polycystic kidney disease is characterized by defective renal tubular structure and results from mutations in either PKD1 or PKD2 genes. Recent data suggest that polycystin-1 and polycystin-2 might localize to primary cilium in principal cells of renal collecting tubules and are thought to act as mechanosensors of fluid flow and contents. Ciliary bending by fluid flow or mechanical stimulation induce Ca(2+) release from intracellular stores, presumably to modulate ion influx in response to tubular fluid flow. Polycystins are also emerging as playing a significant role in sperm development and function. Drosophila polycystin-2 is associated with the head and tail of mature sperm. Targeted disruption of the PKD2 homolog results in nearly complete male sterility without disrupting spermatogenesis. Mutant sperm are motile but are unable to reach the female storage organs (seminal receptacles and spermathecae). The sea urchin polycystin-1-equivalent suPC2 colocalizes with the polycystin-1 homolog REJ3 to the plasma membrane over the acrosomal vesicle. This localization site suggests that the suPC2-REJ3 complex may function as a cation channel mediating acrosome reaction when sperm contact the jelly layer surrounding the egg at fertilization. Future studies leading to the identification of specific ligands for polycystins, including the signaling pathways, might define the puzzling relationship between renal tubular morphogenesis and sperm development and function.

Structure and regulation of the mDot1 gene, a mouse histone H3 methyltransferase

The nucleotide sequence data reported have been deposited in the DDBJ, EMBL, GenBank(R) and GSDB Nucleotide Sequence Databases under accession numbers AY196089, AY196090, AY376663, AY377920 and AY376664. Recently, a new class of histone methyltransferases that plays an indirect role in chromatin silencing by targeting a conserved lysine residue in the nucleosome core was described, namely the Dot1 (disruptor of telomeric silencing) family [Feng, Wang, Ng, Erdjument-Bromage, Tempst, Struhl and Zhang (2002) Curr. Biol. 12, 1052-1058; van Leeuwen, Gafken and Gottschling (2002) Cell (Cambridge, Mass.) 109, 745-756; Ng, Feng, Wang, Erdjument-Bromage, Tempst, Zhang and Struhl (2002) Genes Dev. 16, 1518-1527]. In the present study, we report the isolation, genomic organization and in vivo expression of a mouse Dot1 homologue (mDot1). Expressed sequence tag analysis identified five mDot1 mRNAs (mDot1a-mDot1e) derived from alternative splicing. mDot1a and mDot1b encode 1540 and 1114 amino acids respectively, whereas mDot1c-mDot1e are incomplete at the 5'-end. mDot1a is closest to its human counterpart (hDot1L), sharing 84% amino acid identity. mDot1b is truncated at its N- and C-termini and contains an internal deletion. The five mDot1 isoforms are encoded by 28 exons on chromosome 10qC1, with exons 24 and 28 further divided into two and four sections respectively. Alternative splicing occurs in exons 3, 4, 12, 24, 27 and 28. Northern-blot analysis with probes corresponding to the methyltransferase domain or the mDot1a-coding region detected 7.6 and 9.5 kb transcripts in multiple tissues, but only the 7.6 kb transcript was evident in mIMCD3-collecting duct cells. Transfection of mDot1a-EGFP constructs (where EGFP stands for enhanced green fluorescent protein) into human embryonic kidney (HEK)-293T or mIMCD3 cells increased the methylation of H3-K79 but not H3-K4, -K9 or -K36. Furthermore, DMSO induced mDot1 gene expression and methylation specifically at H3-K79 in mIMCD3 cells in a time- and dose-dependent manner. Collectively, these results add new members to the Dot1 family and show that mDot1 is involved in a DMSO-mediated signal-transduction pathway in collecting duct cells.

Application of difference gel electrophoresis to the identification of inner medullary collecting duct proteins

In this study, we present a standardized approach to purification of native inner medullary collecting duct (IMCD) cells from rat kidney for proteomic analysis and apply the approach to identification of abundant proteins utilizing two-dimensional difference gel electrophoresis (DIGE) coupled with matrix-assisted laser desorption-ionization-time of flight mass spectrometry. Fractionation of inner medullary cell suspensions by low-speed centrifugation gave a highly purified IMCD cell fraction in which aquaporin-2 was enriched 10-fold. When DIGE was initially applied to rat inner medullas fractionated into IMCD cells (labeled with Cy3) and non-IMCD cells (labeled with Cy5), we identified 50 highly abundant proteins expressed in the IMCD cells. These proteins, identifiable without subcellular fractionation, included chiefly enzymes, structural proteins, and signaling intermediates. An additional 35 proteins were found predominantly in the non-IMCD cell types. Proteins that were highly enriched in the IMCD fraction included cytokeratin 8, cytokeratin 18, transglutaminase II, aminopeptidase B, T-plastin, heat shock protein (HSP) 27, HSP70, and lactate dehydrogenase A. Semiquantitative immunoblotting and immunohistochemistry confirmed relative expression levels and distribution of selected proteins. An additional 40 IMCD proteins were identified in separate experiments aimed at further enrichment of proteins through optimization of sample loading. These studies document the applicability of a standardized approach to purification of IMCD cells for proteomic analysis of IMCD proteins and demonstrate the feasibility of large scale identification of proteins in the native IMCD cell.

Conditional gene expression in renal collecting duct epithelial cells: use of the inducible Cre-lox system

The renal collecting duct plays a key role in control of ion and fluid homeostasis. Genes encoding for ion transporters, hormone receptors, or regulatory proteins specifically expressed in the collecting duct are mutated in several genetic diseases with altered blood pressure. Suitable cellular models expressing genes in a conditional way should represent attractive systems for structure-function analyses and generation of appropriate physiopathological models of related diseases. However, generation of such systems remains laborious and quite inefficient. We adapted and improved a conditional Cre-lox-inducible system in the highly differentiated aldosterone-sensitive rat cortical collecting duct (RCCD2) cell line. The inducible MerCreMer recombinase allowed tight control and high levels of transgene expression, whereas flanking a selection marker with two loxP sites strongly improved the selection procedure. We have used this system to conditionally express an enhanced green fluorescent protein-tagged human mineralocorticoid receptor. In the future, this will allow structure-function analyses as well as mineralocorticoid receptor trafficking studies in these epithelial cells, which retain the features of the native collecting duct. Improvements in the conditional Cre-lox expression system have potentially wide applications in other epithelial or nonepithelial cell lines.

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.

Purinergic-induced signaling in C11-MDCK cells inhibits the secretory Na-K-Cl cotransporter

Purinergic inhibition of Na-K-Cl cotransport has been noted in various renal epithelial cells derived from the collecting tubule, including Madin-Darby canine kidney (MDCK) cells. In recent studies, we have observed purinergic inhibition of Na-K-Cl cotransport in C11-MDCK subclones (alpha-intercalated-like cells). Interestingly, Na-K-Cl cotransport activity was also detected in C7-MDCK subclones (principal-like cells) but was not affected by ATP. In this investigation, we have transfected the human Na-K-Cl cotransporter (huNKCC1) in both C11 and C7 cells to determine whether these differences in NKCC regulation by ATP were due to cell-specific purinoceptor signaling pathways or to cell-specific isoforms/splice variants of the transporter. In both cell lines, we found that endogenous as well as huNKCC1-derived cotransport activity was restricted to the basolateral side. In addition, we were able to show that extracellular application of 100 microM ATP or 100 microM UTP abolished NKCC activity in both mock- and huNKCC1-transfected C11 cells but not in mock- and huNKCC1-transfected C7 cells; in C11 cells, intriguingly, this inhibition was not affected by inhibitors of RNA and protein synthesis and occurred even though expression levels of UTP-sensitive P2Y2-, P2Y4-, and P2Y6-purinoceptors were not different from those observed in C7 cells. These results suggest that C11 cells express an undetermined type of UTP-sensitive P2-purinoceptors or a unique P2Y-purinoceptor-triggered signaling cascade that leads to inhibition of NKCC1.

Effect of dDAVP on basolateral cell surface water permeability in the outer medullary collecting duct

We report a novel approach for assessing the volume of living cells which allows quantitative, high-resolution characterization of dynamic changes in cell volume while retaining the cell functionality. The aim of this study was to evaluate the short-term effect of vasopressin on basolateral cell surface water permeability in the outer medullary collecting duct (OMCD). The permeability of the basolateral cell membrane was determined in the tubules where the apical membrane was blocked with oil injected into the lumen. The apparent coefficient of water permeability (Pf) was evaluated by measuring the cell swelling after the step from hypertonic to isotonic medium (600 mosm to 300 mosm). Desmopressin (dDAVP) induced an increase of the basolateral Pf from 113.7+/-8.5 microm/s in control cells to 186.6+/-11.4 mum/s in micro-dissected fragments of the OMCD incubated in vitro (10(-7) M dDAVP, 30 min at 37 degrees C) (P<0.05). Mercury caused pronounced inhibition of basolateral water permeability (26.0+/-6.9 microm/s; P<0.05). The effect of mercury (1.0 mM HgCl2) was reversible: after washing the fragments with PBS for 20 min, Pf values were restored to the control levels (125.0+/-9.5 microm/s). The results of the study indicate the existence of a mechanism controlling the osmotic water permeability of the basolateral cell membrane in the OMCD epithelium.

Prostaglandin E2 stimulates sodium reabsorption in MDCK C7 cells, a renal collecting duct principal cell model

We examined the direct epithelial effects of the major product of arachidonic acid metabolism in the kidney, prostaglandin E(2) (PGE(2)), on ion transport and signal transduction in the hormone-sensitive Madin-Darby canine kidney (MDCK) C7 subclone as a model of renal collecting duct principal cells. MDCK C7 cells were grown on microporous permeable filter supports and mounted in Ussing-type chambers. Reverse transcriptase (RT)-PCR and sequencing were used to determine E-prostanoid (EP) receptor expression. Basolateral and, about 14-fold less potent, apical addition of PGE(2) increased short-circuit current (I(sc)) in a concentration-dependent manner. This ion transport was biphasic with a rapid peak not detectable under chloride-free conditions. The remaining, stably elevated current was unaffected by furosemide, hydrochlorothiazide, ethylisopropanol amiloride, and 5-nitro-2-(3-phenyl-propyl-amino)benzoic acid (NPPB). In contrast, apical amiloride (10 microM) significantly decreased I(sc), indicating sodium reabsorption. The effect of PGE(2) was attenuated in the presence of vasopressin. Agonists acting by cAMP elevation like dibutyryl-cAMP and theophylline also induced an amiloride-sensitive ion transport with similar kinetics as PGE(2). Moreover, PGE(2) rapidly increased intracellular cAMP levels. RT-PCR demonstrated mRNA expression of the epithelial sodium channel (ENaC), and of the EP2 receptor in MDCK C7 cells. Accordingly, EP2 receptor agonist butaprost mimicked PGE(2) epithelial action. In conclusion, PGE(2) induces amiloride-sensitive sodium reabsorption in MDCK C7 monolayers. This ion transport is most likely mediated by EP2 receptor activation leading to increased intracellular cAMP levels. Therefore, PGE(2) might also contribute to Na(+) reabsorption in the mammalian collecting duct.

Effect of flow and stretch on the [Ca2+]i response of principal and intercalated cells in cortical collecting duct

An acute increase in tubular fluid flow rate in the microperfused cortical collecting duct (CCD), associated with a approximately 20% increase in tubular diameter, leads to an increase in intracellular Ca2+ concentration ([Ca2+]i)in both principal and intercalated cells (Woda CB, Leite M Jr, Rohatgi R, and Satlin LM. Am J Physiol Renal Physiol 283: F437-F446, 2002). The apical cilium present in principal but not intercalated cells has been proposed to be a flow sensor. To determine whether flow across the cilium and/or epithelial stretch mediates the [Ca2+]i response, CCDs from New Zealand White rabbits were microperfused in vitro, split-open (to isolate the effect of flow across cilia), or occluded (to examine the effect of stretch and duration/magnitude of the flow impulse), and [Ca2+]i was measured using fura 2. In perfused and occluded CCDs, a rapid (<1 s) but not slow (>3 min) increase in luminal flow rate and/or circumferential stretch led to an approximately threefold increase in [Ca2+]i in both principal and intercalated cells within approximately 10 s. This response was mediated by external Ca2+ entry and inositol 1,4,5-trisphosphate-mediated release of cell Ca2+ stores. In split-open CCDs, an increase in superfusate flow led to an approximately twofold increase in [Ca2+]i in both cell types within approximately 30 s. These experimental findings are interpreted using mathematical models to predict the fluid stress on the apical membranes of the CCD and the forces and torques on and deformation of the cilia. We conclude that rapid increases in luminal flow rate and circumferential stretch, leading to shear or hydrodynamic impulses at the cilium or apical membrane, lead to increases in [Ca2+]i in both principal and intercalated cells.

Adenosine biosynthesis in the collecting duct

Adenosine regulates tubular transport in collecting ducts (CDs); however, the sources of adenosine that modulate ion transport in CDs are unknown. The extracellular cAMP-adenosine pathway refers to the conversion of cAMP to AMP by ectophosphodiesterase, followed by metabolism of AMP to adenosine by ecto-5'-nucleotidase, with all steps occurring in the extracellular compartment. The goal of this study was to assess whether the extracellular cAMP-adenosine pathway exists in CDs. Studies were conducted in both freshly isolated CDs and in CD cells in culture (first passage) that were derived from isolated CDs. Purity of CDs was confirmed by microscopy, by Western blotting for aquaporin-1, aquaporin-2, bumetanide-sensitive cotransporter type 1, and thiazide-sensitive cotransporter; and by reverse transcription-polymerase chain reaction for adenosine receptors. Both freshly isolated CDs and CD cells in culture converted exogenous cAMP to AMP and adenosine. In both freshly isolated CDs and CD cells in culture, conversion of cAMP to AMP and adenosine was affected by a broad-spectrum phosphodiesterase inhibitor (3-isobutyl-1-methylxanthine), an ectophosphodiesterase inhibitor (1,3-dipropyl-8-p-sulfophenylxanthine), and a blocker of ecto-5'-nucleotidase (alpha,beta-methylene-adenosine-5'-diphosphate) in a manner consistent with exogenous cAMP being processed by the extracellular cAMP-adenosine pathway. In CD cells in culture, stimulation of adenylyl cyclase increased extracellular concentrations of cAMP, AMP, and adenosine, and these changes were also modulated by the aforementioned inhibitors in a manner consistent with the extracellular cAMP-adenosine pathway. In conclusion, the extracellular cAMP-adenosine pathway is an important source of adenosine in CDs.

Decreased amiloride-sensitive Na+ absorption in collecting duct principal cells isolated from BPK ARPKD mice

The main feature of polycystic kidney diseases (PKD) is formation and progressive enlargement of renal cysts. Alterations in epithelial cell proliferation, extracellular matrix, and ion transport are thought to contribute to cyst enlargement and loss of renal function. Abnormal Cl- secretion is implicated in cyst enlargement in autosomal dominant PKD (ADPKD), but little is known about transport abnormalities in autosomal recessive PKD (ARPKD). We developed a method to isolate collecting duct (CD) principal cells (site of the lesion in ARPKD) from normal and ARPKD mice. A transgenic mouse (Hoxb7/GFP) in which enhanced green fluorescent protein (GFP) is expressed in CDs was bred with an ARPKD mouse (BPK), and GFP-positive cells from normal and cystic mice were selected by fluorescence-activated cell sorting. GFP-positive CD cells (>95 +/- 3%) obtained from either normal or cystic mice formed high-resistance, polarized epithelial monolayers. Expression patterns for marker proteins and the presence of a central cilium confirmed that the monolayers are composed of principal cells. Under basal conditions, the Cl- secretory responses elicited by elevation of cAMP or calcium were not significantly different between normal and cystic monolayers. In contrast, the amiloride-sensitive short-circuit current was significantly reduced in monolayers of cells isolated from cystic mice (12.9 +/- 1.6 microA/cm2; n = 10) compared with monolayers of cells isolated from normal mice (27.3 +/- 3.4 microA/cm2; n = 12). The results of these studies suggest that epithelial sodium channel-mediated sodium absorption is decreased in principal cells of ARPKD CD cysts and that the reduction in sodium absorption may contribute to the accumulation of luminal fluid.

P2Y2 receptor-stimulated release of prostaglandin E2 by rat inner medullary collecting duct preparations

Extracellular nucleotides, acting through the P2Y2 receptor and the associated phosphoinositide-Ca2+ signaling pathway, inhibit AVP-stimulated osmotic water permeability in rat inner medullary collecting duct (IMCD). Because a rise in intracellular Ca2+ is frequently associated with enhanced arachidonic acid metabolism, we examined the effect of activation of the P2Y2 receptor on release of PGE2 in freshly prepared rat IMCD suspensions. Unstimulated IMCD released moderate, but significant, amounts of PGE2, which were more sensitive to cyclooxygenase (COX)-2 than COX-1 inhibition. Agonist activation of P2Y2 receptor by adenosine 5'-O-(3-thiotriphosphate) enhanced release of PGE2 from IMCD in a time- and concentration-dependent fashion. Purinergic-stimulated release of PGE2 was completely blocked by nonspecific COX inhibitors (flurbiprofen and 2-acetoxyphenylhept-2-ynyl sulfide). Differential COX inhibition studies revealed that purinergic-stimulated release of PGE2 was more sensitive to a COX-1-specific inhibitor (valeroyl salicylate) than a COX-2-specific inhibitor (NS-398). Thus purinergic stimulation resulted in significantly more release of PGE2 in the presence of COX-2 inhibitor than COX-1 inhibitor. If it is assumed that increased release of PGE2 is related to its increased production, our results suggest that purinergic stimulation of IMCD results in enhanced production and release of PGE2 in a COX-1-dependent fashion. Because PGE2 is known to affect transport of water, salt, and urea in IMCD, interaction of the purinergic system with the prostanoid system in IMCD can modulate handling of water, salt, and urea by IMCD and, thus, may constitute an AVP-independent regulatory mechanism.

Increased expression of H+-ATPase in inner medullary collecting duct of aquaporin-1-deficient mice

Phenotype analysis has demonstrated that aquaporin-1 (AQP1) null mice are polyuric and manifest a urinary concentrating defect because of an inability to create a hypertonic medullary interstitium. We report here that deletion of AQP1 is also associated with a decrease in urinary pH from 6.15 +/- (SE) 0.1 to 5.63 +/- 0.07. To explore the mechanism of the decrease in urinary pH, we examined the expression of H+-ATPase in kidneys of AQP1 null mice. There was strong labeling for H+-ATPase in intercalated cells and proximal tubule cells in both AQP1 null and wild-type mice. Strong H+-ATPase immunostaining was also present in the apical plasma membrane of inner medullary collecting duct (IMCD) cells in AQP1 null mice, whereas no H+-ATPase labeling was observed in IMCD cells in wild-type mice. In addition, there was an increase in the prevalence of type A intercalated cells in the IMCD of AQP1 null mice, suggesting that the deletion of intercalated cells from the IMCD, which normally occurs during postnatal kidney development, was impaired. Western blot analysis of H+-ATPase expression in the different regions of the kidney demonstrated a significant increase in H+-ATPase protein in the inner medulla of AQP1 null mice compared with wild-type mice. There were no changes in H+-ATPase expression in the cortex or outer medulla. These results represent the first demonstration of apical H+-ATPase immunoreactivity in IMCD cells in vivo and suggest that the decrease in urinary pH observed in AQP1 null mice is due to upregulation of H+-ATPase in the IMCD. The induction of H+-ATPase expression in IMCD cells of AQP1 null mice may be related to the chronically low interstitial osmolality in these animals. The challenge will be to identify the molecular signal(s) responsible for the de novo H+-ATPase expression.

High NaCl causes Mre11 to leave the nucleus, disrupting DNA damage signaling and repair

High NaCl causes DNA double-strand breaks and cell cycle arrest, but the mechanism of its genotoxicity has been unclear. In this study, we describe a novel mechanism that contributes to this genotoxicity. The Mre11 exonuclease complex is a central component of DNA damage response. This complex assembles at sites of DNA damage, where it processes DNA ends for subsequent activation of repair and initiates cell cycle checkpoints. However, this does not occur with DNA damage caused by high NaCl. Rather, following high NaCl, Mre11 exits from the nucleus, DNA double-strand breaks accumulate in the S and G2 phases of the cell cycle, and DNA repair is inhibited. Furthermore, the exclusion of Mre11 from the nucleus by high NaCl persists following UV or ionizing radiation, also preventing DNA repair in response to those stresses, as evidenced by absence of H2AX phosphorylation at places of DNA damage and by impaired repair of damaged reporter plasmids. Activation of chk1 by phosphorylation on Ser345 generally is required for DNA damage-induced cell cycle arrest. However, chk1 does not become phosphorylated during high NaCl-induced cell cycle arrest. Also, high NaCl prevents ionizing and UV radiation-induced phosphorylation of chk1, but cell cycle arrest still occurs, indicating the existence of alternative mechanisms for the S and G2/M delays. DNA breaks that occur normally during processes such as DNA replication and transcription, as well as damages to DNA induced by genotoxic stresses, ordinarily are rapidly repaired. We propose that inhibition of this repair by high NaCl results in accumulation of DNA damage, accounting for the genotoxicity of high NaCl, and that cell cycle delay induced by high NaCl slows accumulation of DNA damage until the DNA damage-response network can be reactivated.

Angiotensin I conversion to angiotensin II stimulates cortical collecting duct sodium transport

Angiotensin (Ang) II directly stimulates epithelial sodium channel activity in the rabbit cortical collecting duct. Because Ang I and converting enzyme analogues might be present in the distal nephron, this raises the possibility of intraluminal generation of Ang II. Conversion of Ang I to Ang II was monitored by Ang II-dependent changes in intracellular sodium concentration as a reflection of sodium transport across the apical membrane. This involved imaging-based fluorescence microscopy with sodium-binding benzofuran isophthalate in isolated, perfused, cortical collecting-duct segments from rabbit kidney. Principal and intercalated cells were differentiated by rhodamine-conjugated peanut lectin. Control principal cell intracellular sodium concentration, during perfusion with 25 mmol/L NaCl and zero sodium in the bath plus monensin (10(-5) mol/L) averaged 5.8+/-0.14 mmol/L (n=156). The increase in intracellular sodium concentration, when luminal NaCl was increased from 25 to 150 mmol/L, was elevated by 3.5-fold in the presence of intraluminal Ang I (10(-6) mol/L). Also, the effects of Ang I on sodium transport were not significantly different from the effects of Ang II (10(-9) mol/L). Ang I was used in micromolar concentrations to ensure that there was sufficient substrate available for conversion to Ang II. Inhibition of the angiotensin-converting enzyme with captopril reduced the stimulatory effect of Ang I. These results suggest that intraluminal conversion of Ang I to Ang II can occur in the cortical collecting duct, resulting in enhanced apical sodium entry.

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).

Beta2-adrenoceptor activation inhibits Shiga toxin2-induced apoptosis of renal tubular epithelial cells

Apoptosis is regulated by several pathways, such as caspases, mitogen activated protein kinase (MAPK) and cAMP/cAMP-dependent protein kinase A (PKA) cascade. This study investigated the effect of beta(2)-adrenoceptor activation on Shiga toxin (Stx)2-induced apoptosis in renal tubular cells and the contribution of these signalling pathways. Cultured human adenocarcinoma-derived tubular cells were exposed to Stx2 (64 pg/mL) for 2-24hr following the addition of the beta(2)-adrenoceptor agonist (terbutaline) to the incubation medium. Stx2-induced apoptosis and its amelioration by beta(2)-adrenoceptor activation was confirmed using DNA degradation assays and by flow cytometry for annexin V, mitochondrial membrane potential and caspase(-3 and -7) activity. Exposure of cells to Stx2 for 24hr increased the DNA fragmentation to 11.6+/-0.9%, compared to 3.3+/-0.2% in control cells (P<0.05) but was decreased to approximately 5-7% (P<0.05) in the presence of terbutaline. Furthermore, Stx2-stimulated apoptosis, detected by TUNEL, annexin V and mitochondrial potential, was inhibited by terbutaline (P<0.05) which was prevented by cAMP-PKA inhibitors and a beta(2)-adrenoceptor antagonist. However, inhibition of Stx2-mediated caspase activity by terbutaline was partially blocked by cAMP-PKA inhibitors. On the other hand, p38MAPK inhibition by terbutaline prevented Stx2-induced apoptosis and caspase activity through a cAMP-independent pathway via beta(2)-adrenoceptor. These data indicate that beta(2)-adrenoceptor activation can inhibit Stx2-induced apoptosis of the cells, which may be caused by a reduction in caspase activity through cAMP-PKA activation and the p38MAPK pathway.

Toxicity of acetaminophen, salicylic acid, and caffeine for first-passage rat renal inner medullary collecting duct cells

Chronic excess ingestion of nonsteroid anti-inflammatory drugs causes renal medullary necrosis. Previously, using an immortalized line of mouse inner medullary collecting ducts cells (mIMCD3), we found that acetaminophen, salicylic acid, and caffeine are toxic, and the effects of acetaminophen and caffeine are strongly additive. Furthermore, toxicity was greater in proliferating than in nonproliferating cells. Important limitations were that mIMCD3 cells do not readily tolerate the high concentrations of salt and urea normally present in renal inner medullas and proliferate much more rapidly than inner medullary cells in vivo. Thus, these cells may not serve as an appropriate model for the in vivo IMCD. The present studies address these limitations by using passage-1 rat inner medullary collecting duct (p1rIMCD) cells, which tolerate high salt and urea and become contact inhibited when confluent. At 640 mOsmol/kg (the lowest normal inner medullary osmolality), the drugs, singly and in combination, reduce the number of proliferating (i.e., subconfluent) p1rIMCD cells more than they do confluent cells. Effects of acetaminophen and caffeine are strongly additive. Addition of as little as 0.1 mM caffeine significantly enhances the toxicity of acetaminophen plus salicylic acid. With confluent cells at 640 mOsmol/kg and very slowly growing cells at 1370 mOsmol/kg, combinations of drugs that include acetaminophen increase proliferation, accompanied by DNA damage and apoptosis. We conclude that these drugs are toxic to renal inner medullary collecting duct cells under the conditions of high osmolality normally present in the inner medulla, that combinations of the drugs are more toxic than are the drugs individually, and that the toxicity includes induction of proliferation of these cells that are otherwise quiescent in the presence of high osmolality.

Terminal differentiation of intercalated cells: the role of hensin

During the response to metabolic acidosis, the intercalated cell of the collecting tubule converts from one that secretes HCO3(-) to one that absorbs HCO3(-) by H(+) secretion. The molecular basis of this complex change in phenotype was studied in an immortalized intercalated cell line. We found that it was induced by secretion, polymerization, and deposition of a protein, which we termed hensin, into the extracellular matrix. Surprisingly, this change in phenotype is identical to terminal differentiation of epithelial cells in that it recapitulated all the characteristics of terminal differentiation, including a change in cell shape, acquisition of specialized apical structures (microvilli and ruffles), and the ability to secrete and endocytose materials in a regulated manner from the apical membrane. Hensin is expressed in most epithelia, and others have discovered that it is deleted in a large number of epithelial tumors. These results suggest that conversion of polarity of the intercalated cells represents a process of terminal differentiation.

Dual influence of aldosterone on AQP2 expression in cultured renal collecting duct principal cells

In the renal collecting duct (CD) the major physiological role of aldosterone is to promote Na+ reabsorption. In addition, aldosterone may also influence CD water permeability elicited by vasopressin (AVP). We have previously shown that endogenous expression of the aquaporin-2 (AQP2) water channel in immortalized mouse cortical CD principal cells (mpkCCDC14) grown on filters is dramatically increased by administration of physiological concentrations of AVP. In the present study, we investigated the influence of aldosterone on AQP2 expression in mpkCCDC14 cells by RNase protection assay and Western blot analysis. Aldosterone reduced AQP2 mRNA and protein expression when administered together with AVP for short periods of time (< or =24 h). For longer periods of time, however, aldosterone increased AQP2 protein expression despite sustained low expression levels of AQP2 mRNA. Both events were dependent on mineralocorticoid receptor occupancy because they were both induced by a low concentration of aldosterone (10-9 m) and were abolished by the mineralocorticoid receptor antagonist canrenoate. Inhibition of lysosomal AQP2 protein degradation increased AQP2 protein expression in AVP-treated cells, an effect that was potentiated by aldosterone. Finally, both aldosterone and actinomycin D delayed AQP2 protein decay following AVP washout, but in a non-cumulative manner. Taken together, our data suggest that aldosterone tightly modulates AQP2 protein expression in cultured mpkCCDC14 cells by increasing AQP2 protein turnover while maintaining low levels of AQP2 mRNA expression.

Expression and characterization of human transient receptor potential melastatin 3 (hTRPM3)

Transient receptor potential (TRP) cation-selective channels are an emerging class of proteins that are involved in a variety of important biological functions including pain transduction, thermosensation, mechanoregulation, and vasorelaxation. Utilizing a bioinformatics approach, we have identified the full-length human TRPM3 (hTRPM3) as a member of the TRP family. The hTRPM3 gene is comprised of 24 exons and maps to human chromosome 9q-21.12. hTRPM3 is composed of 1555 amino acids and possesses the characteristic six-transmembrane domain of the TRP family. hTRPM3 is expressed primarily in kidney and, at lesser levels, in brain, testis, and spinal cord as demonstrated by quantitative RT-PCR and Northern blotting. In situ hybridization in human kidney demonstrated that hTRPM3 mRNA expression is predominantly found in the collecting tubular epithelium. Heterologous expression of hTRPM3 in human embryonic kidney cells (HEK 293) showed that hTRPM3 is localized to the cell membrane. hTRPM3-expressing cells exhibited Ca2+ concentration-dependent Ca2+ entry. Depletion of intracellular Ca2+ stores by lowering extracellular Ca2+ concentration and treatment with the Ca2+-ATPase inhibitor thapsigargin or the muscarinic receptor agonist carbachol further augmented hTRPM3-mediated Ca2+ entry. The nonselective Ca2+ channel blocker, lanthanide gadolinium (Gd3+), partially inhibited hTRPM3-mediated Ca2+ entry. These results are consistent with the hypothesis that hTRPM3 mediates a Ca2+ entry pathway that apparently is distinct from the endogenous Ca2+ entry pathways present in HEK 293 cells.

Terminal differentiation of epithelia from trophectoderm to the intercalated cell: the role of hensin

The intercalated cells of the collecting tubules of mammalian kidneys were discovered by Haggege and Richet to change their morphology in response to a variety of physiologic stimuli related to changes in acid base status. Recent studies showed that the conversion of beta to alpha intercalated cell under the influence of acidification of the medium is due to the deposition of hensin in the extracellular matrix of these cells and activation of a novel inductive signal transduction pathway. The conversion of beta to alpha cells is shown to be a process of terminal differentiation. Hensin is secreted as a monomer, and activation of the cell induces two activities that convert it to a dimer by folding and into a fiber by bundling of the folded dimers by galectin 3. Only the fiber is functional. Hensin is expressed in most epithelial cells, and its staining pattern suggests that it might be involved in the terminal differentiation of most epithelia. There is loss of heterozygosity of hensin in a large number of epithelial and neural tumors, making it likely that it is a tumor suppressor gene.

Immortalized renal proximal and collecting duct cell lines derived from transgenic mice harboring L-type pyruvate kinase promoters as tools for pharmacological and toxicological studies

Targeted oncogenesis in transgenic mice, where an oncogene is placed under the control of the regulatory sequences of a cell-specific gene, has been used to derive lines of differentiated kidney epithelial cells derived from proximal or distal tubules or from the collecting duct. These renal cell lines were obtained from kidneys of transgenic mice harboring the large-T and little-t antigens placed under the control of regulatory sequences of the L-type pyruvate kinase gene. This review summarizes the main properties of these differentiated cell lines, which are useful ex vivo cell systems for pharmacological and toxicological studies.

Crystal attachment to injured renal collecting duct cells: influence of urine proteins and pH

BACKGROUND

The attachment of crystals to injured kidney epithelium is thought to be a necessary event in the development of urolithiasis. In vivo, the crystals are coated with urinary macromolecules that define the surface properties of the crystals. The present study examines the influence of coating of calcium oxalate crystals with urinary macromolecules on their attachment to both healthy (polarized) and injured (nonpolarized) primary inner medullary collecting duct (IMCD) cells.

METHODS

Calcium oxalate monohydrate (COM) and calcium oxalate dihydrate (COD) crystals were coated with urine macromolecules by incubating the crystals in urine from normal healthy volunteers at pH 5, 6, and 7. The level of attachment of the coated crystals to IMCD cells was also determined at pH 5, 6, and 7. The adsorbed proteins were extracted from the crystal surfaces and separated by gel electrophoresis.

RESULTS

The coating of calcium oxalate crystals with urine proteins greatly reduced the attachment of crystals to both control and injured IMCD cells. At pH levels below 6, the crystals readily attached to injured cells. Extraction and separation of the adsorbed proteins showed that both COM and COD crystals adsorbed a similar array of proteins. At pH 5 and 6, several trace proteins were adsorbed to the crystals and were not apparent at pH 7.

CONCLUSION

The coating of crystals with urine macromolecules greatly reduces the attachment of the crystals to normal healthy epithelia. The coating and attachment of the crystals below pH 6 enhances the attachment to injured cells. The enhanced crystal attachment could possibly be associated with one or more proteins adsorbed to the crystal surface that are not adsorbed to the crystals at higher pH.

Adaptation of murine inner medullary collecting duct (IMCD3) cell cultures to hypertonicity

Recently, we have adapted IMCD3 cell cultures to survive under increasing hypertonic conditions (i.e., 600 and 900 mOsmol/kg H(2)O). In adapted cells, ATPase activity is increased by one order of magnitude, while the expression of the alpha and beta subunit is increased by a factor of 4 to 5 over controls (300 mOsmol/kg H(2)O). Corresponding increases in mRNAs were also detected. The gamma subunit has been described as being uniquely expressed in some areas of the kidney, but never in cell cultures (even those derived from kidney tissues). However, the gamma subunit was detected at the protein and mRNA levels in the adapted IMCD3 cells. In contrast to the alpha and beta subunits, the levels of gamma protein and mRNA expression continue to increase as a function of the media ion concentration. We have also demonstrated that signaling pathways that upregulate the alpha, beta, and gamma subunits are very different. Increasing concentrations of the PI3 kinase inhibitor, LY294002, resulted in a dose-dependent reduction in the expression of the gamma subunit, with total abolition at 10 micro M. However, LY294002 had no significant effect on the expression of the alpha subunit. Inhibition of the JNK2 (but not of the JNK1) pathways by dominant negative transfections abolished the upregulation of the gamma, but not the a subunit. Failure to upregulate the expression of the gamma subunit was associated with a marked decrease in cell viability upon stress.

Cycloheximide increases glucocorticoid-stimulated alpha -ENaC mRNA in collecting duct cells by p38 MAPK-dependent pathway

Aldosterone and glucocorticoids (GCs) stimulate Na(+) reabsorption in the collecting ducts by increasing the activity of the epithelial Na(+) channel (ENaC). Our laboratory has used Madin-Darby canine kidney-C7 cells to demonstrate that this effect is associated with an increase in alpha-ENaC gene transcription (Mick VE, Itani OA, Loftus RW, Husted RF, Schmidt TJ, and Thomas CP, Mol Endocrinol 15: 575-588, 2001). Cycloheximide (CHX) superinduced the GC-stimulated alpha-ENaC expression in a dose-dependent manner, but had no effect on basal or aldosterone-stimulated alpha-ENaC expression, whereas anisomycin inhibited basal and corticosteroid-stimulated alpha-ENaC expression. The superinduction of alpha-ENaC expression was also seen with hypotonicity, was blocked by RU-38486, and was independent of protein synthesis. CHX had no effect on alpha-ENaC mRNA half-life, confirming that its effect was via an increase in alpha-ENaC transcription. The effect of CHX and hypotonicity on alpha-ENaC expression was abolished by SB-202190, indicating an effect mediated via p38 MAPK. Consistent with this scheme, CHX increased pp38 and MKK6, an upstream activator of p38, stimulated alpha-ENaC promoter activity. These data confirm a model in which CHX activates p38 in Madin-Darby canine kidney-C7 cells to increase alpha-ENaC gene transcription in a GC-dependent manner.

Chronic hyperosmolarity mediates constitutive expression of molecular chaperones and resistance to injury

Renal medullary cells are exposed to elevated and variable osmolarities and low oxygen tension. Despite the harsh environment, these cells are resistant to the effects of many harmful events. To test the hypothesis that this resistance is a consequence of these cells developing a stress tolerance phenotype to survive in this milieu, we created osmotically tolerant cells [hypertonic (HT) cells] by gradually adapting murine inner medullary collecting duct 3 cells to hyperosmotic medium containing NaCl and urea. HT cells have a reduced DNA synthesis rate, with the majority of cells arrested in the G(0)/G(1) phase of the cell cycle, and show constitutive expression of heat shock protein 70 that is proportional to the degree of hyperosmolarity. Unlike acute hyperosmolarity, chronic hyperosmolarity failed to activate MAPKs. Moreover, HT cells acquired protein translational tolerance to further stress treatment, suggesting that HT cells have an osmotolerant phenotype that is analogous to thermotolerance but is a permanent condition. In addition to osmotic shock, HT cells were more resistant to heat, H(2)O(2), cyclosporin, and apoptotic inducers, compared with isotonic murine inner medullary duct 3 cells, but less resistant to amphotericin B and cadmium. HT cells demonstrate that in renal medullary cells, hyperosmotic stress activates biological processes that confer cross-tolerance to other stressful conditions.

Hormone-regulated transepithelial Na+ transport in mammalian CCD cells requires SGK1 expression

To study the role of serum and glucocorticoid-inducible kinase-1 (SGK1) in mammalian cells, we compared Na(+) transport rates in wild-type (WT) M1 cortical collecting duct cells with M1 populations stably expressing human full-length SGK1, NH(2)-terminal truncated (DeltaN-60) SGK1, "kinase-dead" (K127M) SGK1, and cells that have downregulated levels of SGK1 mRNA (antisense SGK1). Basal rates of transepithelial Na(+) transport were highest in full-length SGK1 populations, compared among the above populations. Dexamethasone treatment increased Na(+) transport in WT and full-length SGK1 cells 2.7- and 2-fold, respectively. Modest stimulation of Na(+) absorption was detected after dexamethasone treatment in DeltaN-60 SGK1 populations. However, DeltaN-60 SGK1 transport rates remained substantially lower than WT values. Importantly, a combination of high insulin, dexamethasone, and serum failed to significantly stimulate Na(+) transport in antisense or K127M SGK1 cells. Additionally, expression of antisense SGK1 significantly decreased transepithelial resistance values. Overall, we concluded that SGK1 is a critical component in corticosteroid-regulated Na(+) transport in mammalian cortical collecting duct cells. Furthermore, our data suggest that the NH(2) terminus of SGK1 may contain a Phox homology-like domain that may be necessary for effective Na(+) transport.

Inhibition of Na+,K+-ATPase by ouabain triggers epithelial cell death independently of inversion of the [Na+]i/[K+]i ratio

Treatment with ouabain led to massive death of principal cells from collecting ducts (C7-MDCK), indicated by cell swelling, loss of mitochondrial function, an irregular pattern of DNA degradation, and insensitivity to pan-caspase inhibitor. Equimolar substitution of extracellular Na(+) by K(+) or choline(+) sharply attenuated the effect of ouabain on intracellular Na(+) and K(+) content but did not protect the cells from death in the presence of ouabain. In contrast to ouabain, inhibition of the Na(+)/K(+) pump in K(+)-free medium increased Na(+)(i) content but did not affect cell survival. In control and K(+)-free medium, ouabain triggered half-maximal cell death at concentrations of approximately 0.5 and 0.05 microM, respectively, which was consistent with elevation of Na(+)/K(+) pump sensitivity to ouabain in K(+)-depleted medium. Our results show for the first time that the death of ouabain-treated renal epithelial cells is independent of the inhibition of Na(+)/K(+) pump-mediated ion fluxes and the [Na(+)](i)]/[K(+)](i) ratio.

Ontogeny of purinergic receptor-regulated Ca2+ signaling in mouse cortical collecting duct epithelium

Changes in ATP-induced increase in [Ca2+](i) during collecting duct ontogeny were studied in primary monolayer cultures of mouse ureteric bud (UB) and cortical collecting duct (CCD) cells by Fura-PE3 fluorescence ratio imaging. In UB (embryonic day E14 and postnatal day P1) the ATP-stimulated increase (EC(50) approximately 1 microM) in fluorescence ratio (DeltaR(ATP)) was independent of extracellular Ca2+ and insensitive to the P2 purinoceptor-antagonist suramin (1 mM). From day P7 onward when CCD morphogenesis had been completed DeltaR(ATP) increased and became dependent on extracellular Ca2+. This ATP-stimulated Ca2+ entry into CCD cells was non-capacitative and suramin (1 mM)-insensitive, but sensitive to nifedipine (30 microM) and enhanced by Bay K8644 (15 microM), a blocker and an agonist of L-type Ca2+ channels, respectively. Quantitative RT-PCR demonstrated similar mRNA expression of L-type Ca2+ channel alpha1-subunit, P2Y(1), P2Y(2), and P2X(4b) purinoceptors in UB and CCD monolayers while the abundance of P2X(4) mRNA increased with CCD morphogenesis. In conclusion, both embryonic and postnatal cells express probably P2Y(2)-stimulated Ca2+ release from intracellular stores. With development, the CCD epithelium acquires ATP-stimulated Ca2+ entry via L-type Ca2+ channels. This pathway might by mediated by the increasing expression of P2X(4)-receptors resulting in an increasing ATP-dependent membrane depolarization and activation of L-type Ca2+ channels.

Disordered calcium crystal handling in antisense CLC-5-treated collecting duct cells

Dent's disease, an X-linked tubulopathy secondary to defects in chloride channel CLC-5, is characterised by low molecular weight proteinuria, hypercalciuria, nephrocalcinosis, and renal stones. Mechanisms leading to nephrocalcinosis are unknown. Using a murine collecting duct cell line (mIMCD-3), we confirm endogenous expression of mCLC-5. During transfection of antisense CLC-5, we observe a reduction in CLC-5 protein expression that correlates with a reduction in the number of acidic endosomal compartments, as determined by quantitative analysis of confocal microscope images using LysoTracker Red. Using wheat germ agglutinin-lectin as an endocytic marker, an arrest of endocytosis is observed in antisense CLC-5 treated cells. Exposure of the cell surface to calcium oxalate crystals results in crystal agglomeration in a minority of sense CLC-5 transfectants (45%) and all antisense CLC-5 transfectants. We conclude that expression of CLC-5 in mIMCD-3 cells allows acidification of endosomes and endocytosis, and that disruption of CLC-5 expression causes abnormal crystal agglomeration.

Sp1 and Sp3 transcription factors synergistically regulate HGF receptor gene expression in kidney

We investigated the expression pattern and underlying mechanism that controls hepatocyte growth factor (HGF) receptor (c-met) expression in normal kidney and a variety of kidney cells. Immunohistochemical staining showed widespread expression of c-met in mouse kidney, a pattern closely correlated with renal expression of Sp1 and Sp3 transcription factors. In vitro, all types of kidney cells tested expressed different levels of c-met, which was tightly proportional to the cellular abundances of Sp1 and Sp3. Both Sp1 and Sp3 bound to the multiple GC boxes in the promoter region of the c-met gene. Coimmunoprecipitation suggested a physical interaction between Sp1 and Sp3. Functionally, Sp1 markedly stimulated c-met promoter activity. Although Sp3 only weakly activated the c-met promoter, its combination with Sp1 synergistically stimulated c-met transcription. Conversely, deprivation of Sp proteins by transfection of decoy Sp1 oligonucleotide or blockade of Sp1 binding with mithramycin A inhibited c-met expression. The c-met receptor in all types of kidney cells was functional and induced protein kinase B/Akt phosphorylation in a distinctly dynamic pattern after HGF stimulation. These results indicate that members of the Sp family of transcription factors play an important role in regulating constitutive expression of the c-met gene in all types of renal cells. Our findings suggest that HGF may have a broader spectrum of target cells and possess wider implications in kidney structure and function than originally thought.

Epidermal growth factor inhibits amiloride-sensitive sodium absorption in renal collecting duct cells

The effects of the ERK pathway on electrogenic transepithelial Na(+) absorption by renal collecting duct cells were determined. Approximately 90% of the unstimulated short-circuit current (15 +/- 1 microA/cm(2), n = 10) across conditionally immortalized murine collecting duct epithelial cells (mCT1) is amiloride sensitive and is likely mediated by apical epithelial Na(+) channels. Chronic exposure (24 h) of the epithelial monolayers to either EGF (50 ng/ml) or transforming growth factor-alpha (TGF-alpha; 20 ng/ml) reduced amiloride-sensitive short-circuit current by >60%. The inhibitory effect of EGF on Na(+) absorption was not due to inhibition of basolateral Na(+)-K(+)-ATPase, because the pump current elicited by permeabilization of apical membrane with nystatin was not reduced by EGF. Chronic exposure of the mCT1 cells to EGF (20 ng/ml, 24 h) elicited a 70-85% decrease in epithelial Na(+) channel subunit mRNA levels. Exposure of mCT1 cells to either EGF (20 ng/ml) or PMA (150 nM) induced rapid phosphorylation of p42/p44 (ERK1/2) and pretreatment of the monolayers with PD-98059 (an ERK kinase inhibitor; 30 microM) prevented phosphorylation of p42/p44. Similarly, pretreatment of mCT1 monolayers with PD-98059 prevented the EGF- and PMA-induced inhibition of amiloride-sensitive Na(+) absorption. The results of these studies demonstrate that amiloride-sensitive Na(+) absorption by renal collecting duct cells is regulated by the ERK pathway. This pathway may play a role in alterations in ion transport that occur in polycystic kidney disease.

Cyclic AMP is sufficient for triggering the exocytic recruitment of aquaporin-2 in renal epithelial cells

The initial response of renal epithelial cells to the antidiuretic hormone arginine vasopressin (AVP) is an increase in cyclic AMP. By applying immunofluorescence, cell membrane capacitance and transepithelial water flux measurements we show that cAMP alone is sufficient to elicit the antidiuretic cellular response in primary cultured epithelial cells from renal inner medulla, namely the transport of aquaporin-2 (AQP2)-bearing vesicles to, and their subsequent fusion with, the plasma membrane (AQP2 shuttle). The AQP2 shuttle is evoked neither by AVP-independent Ca(2+) increases nor by AVP-induced Ca(2+) increases. However, clamping cytosolic Ca(2+) concentrations below resting levels at 25 nM inhibited exocytosis. Exocytosis was confined to a slow monophasic response, and readily releasable vesicles were missing. Analysis of endocytic capacitance steps revealed that cAMP does not decelerate the retrieval of AQP2 from the plasma membrane. Our data suggest that cAMP initiates an early step, namely the transport of AQP2-bearing vesicles towards the plasma membrane, and do not support a regulatory function for Ca(2+) in the AQP2 shuttle.

Renal principal cell-specific expression of green fluorescent protein in transgenic mice

The purpose of this study is to develop transgenic mice with principal cell-specific expression of green fluorescent protein (GFP). After the cloning and sequencing of the mouse aquaporin-2 (AQP2) gene, 9.5 kb of the promoter were used to drive expression of GFP in transgenic mice. In transgenic mice, GFP was selectively expressed in principal cells of the renal collecting duct and not in intercalated cells. Expression was increased by dehydration of mice. AQP2 and GFP expression was maintained in primary cultures of renal medulla that were stimulated with cAMP or vasopressin analogs. GFP-expressing cells were then isolated by fluorescence-activated cell sorting. RT-PCR analysis showed expression of AQP2, AQP3, AQP4, vasopressin type 2 receptor, and cAMP response element binding protein but not H+-ATPase B1 subunit or anion exchanger 1. After expansion of these cells in culture, RT-PCR analysis showed continued expression of the same genes. This pattern of gene expression is that of principal cells rather than intercalated cells. This transgenic mouse model can be used in future studies of gene expression during the development, differentiation, and maturation of renal principal cells.

Electrolyte and fluid secretion by cultured human inner medullary collecting duct cells

Inner medullary collecting ducts (IMCD) are the final nephron segments through which urine flows. To investigate epithelial ion transport in human IMCD, we established primary cell cultures from initial (hIMCD(i)) and terminal (hIMCD(t)) inner medullary regions of human kidneys. AVP, PGE(2), and forskolin increased cAMP in both hIMCD(i) and hIMCD(t) cells. The effects of AVP and PGE2 were greatest in hIMCD(i); however, forskolin increased cAMP to the same extent in hIMCD(i) and hIMCD(t). Basal short-circuit current (I(SC)) of hIMCD(i) monolayers was 1.4 +/- 0.5 microA/cm2 and was inhibited by benzamil, a Na+ channel blocker. 8-Bromo-cAMP, AVP, PGE(2), and forskolin increased I(SC); the current was reduced by blocking PKA, apical Cl- channels, basolateral NKCC1 (a Na+ - K+ - 2Cl- cotransporter), and basolateral Cl-/HCO(3)(-) exchangers. In fluid transport studies, hIMCD(i) monolayers absorbed fluid in the basal state and forskolin reversed net fluid transport to secretion. In hIMCD(t) monolayers, basal current was not different from zero and cAMP had no effect on I(SC). We conclude that AVP and PGE2 stimulate cAMP-dependent Cl- secretion by hIMCD(i) cells, but not hIMCD(t) cells, in vitro. We suggest that salt secretion at specialized sites along human collecting ducts may be important in the formation of the final urine.

11beta-hydroxyprogesterone acts as a mineralocorticoid agonist in stimulating Na+ absorption in mammalian principal cortical collecting duct cells

The binding of mineralocorticoid hormones to the mineralocorticoid receptor is the first step in a cascade of events leading to the stimulation of Na(+) reabsorption by renal cortical collecting duct (CCD) principal cells. The agonist properties of mineralocorticoid hormones are linked to contacts between their 21-hydroxyl group and Asn770, a residue of the ligand-binding domain of the human mineralocorticoid receptor (hMR). Here, we investigate whether the presence of a hydroxyl group at position 11, 17, or 20 could also alter the activity of progesterone (P), a mineralocorticoid antagonist without the 21-hydroxyl group. Both 17 alpha-hydroxyprogesterone (17OHP) and 20 alpha-hydroxyprogesterone (20OHP) antagonized the aldosterone-induced trans-activation activity (IC(50): 17OHP, 10(-7) M; 20OHP, 10(-8) M) of the hMR transiently expressed in COS-7 cells lacking steroid receptors. In cultured mouse mpkCCD(cl4) principal cells, 17OHP and 20OHP also prevented the aldosterone-stimulated amiloride-sensitive component of the short-circuit current (Ams I(sc)), reflecting Na(+) absorption mediated by the epithelial Na(+) channel (ENaC). In contrast, 11 beta-hydroxyprogesterone (11OHP) activated the transiently expressed hMR in COS-7 cells in a dose-dependent manner (ED(50): 10(-8) M) and, like aldosterone, stimulated Ams I(sc) in mpkCCD(cl4) cells. Docking 11OHP within the hMR-ligand-binding domain homology model revealed that the agonist activity of 11OHP is caused by contacts between its 11 beta-hydroxyl group and Asn770. Furthermore, 11OHP was unable to activate the mutant hMR/N770A, in which Ala is substituted for Asn at position 770. These findings demonstrate that in the absence of the 21-hydroxyl group, the 11 beta-hydroxyl group can produce the contact with the hMR-Asn770 required for the hMR activation leading to stimulated Na(+) absorption.

Basolateral adrenoceptor activation mediates noradrenaline-induced Cl- secretion in M-1 mouse cortical collecting duct cells

Noradrenaline (NA) released from efferent renal sympathetic nerves may directly affect renal tubular transport. Here we examined the effect of NA on transepithelial ion transport of cultured M-1 mouse cortical collecting duct cells using equivalent short-circuit current ( I(SC)) measurements. Steady-state I(SC) averaged 87.5+/-2.9 microA cm(-2) (n=185) and was reduced by 97.1+/-0.1% (n=80) by apical amiloride (100 microM) confirming that the predominant electrogenic transport across M-1 monolayers is sodium absorption via the epithelial sodium channel (ENaC). Basolateral addition of NA (10 microM) induced a biphasic change in I(SC) characterized by an initial transient peak increase of 18.1+/-0.9 microA cm(-2) with a subsequent decline to a plateau 1.4+/-0.3 microA cm(-2) (n=20) above baseline. Apical application of NA had no effect. The response to basolateral NA was concentration dependent and was preserved in the presence of apical amiloride. In contrast, the response was largely reduced in the presence of apical diphenylamine-2-carboxylic acid (1 mM) and in the absence of extracellular Cl(-). The peak response to NA was reduced in the presence of the alpha-adrenoceptor antagonist phentolamine (100 microM), whereas the beta-antagonist propranolol (100 microM) reduced the secondary plateau phase while failing to influence the peak response. The alpha(1)-adrenoceptor-selective antagonists prazosin (10 nM) and corynanthine (1 microM) reduced the NA-induced peak response by about 75% and 70%, respectively, while the alpha(2)-adrenoceptor antagonist yohimbine (100 nM) was ineffective. We conclude that in M-1 cells NA stimulates Cl(-) secretion probably involving both alpha(1)- and beta-adrenoceptors located basolaterally.

Regulation of the expression of the Cl-/anion exchanger pendrin in mouse kidney by acid-base status

BACKGROUND

Pendrin belongs to a superfamily of Cl-/anion exchangers and is expressed in the inner ear, the thyroid gland, and the kidney. In humans, mutations in pendrin cause Pendred syndrome characterized by sensorineural deafness and goiter. Recently pendrin has been localized to the apical side of non-type A intercalated cells of the cortical collecting duct, and reduced bicarbonate secretion was demonstrated in a pendrin knockout mouse model. To investigate a possible role of pendrin in modulating acid-base transport in the cortical collecting duct, we examined the regulation of expression of pendrin by acid-base status in mouse kidney.

METHODS

Mice were treated orally either with an acid or bicarbonate load (0.28 mol/L NH4Cl or NaHCO3) or received a K+-deficient diet for one week. Immunohistochemistry and Western blotting was performed.

RESULTS

Acid-loading caused a reduction in pendrin protein expression levels within one day and decreased expression to 23% of control levels after one week. Concomitantly, pendrin protein was shifted from the apical membrane to the cytosol, and the relative abundance of pendrin positive cells declined. Similarly, in chronic K+-depletion, known to elicit a metabolic alkalosis, pendrin protein levels decreased and pendrin expression was shifted to an intracellular pool with the relative number of pendrin positive cells reduced. In contrast, following oral bicarbonate loading pendrin was found exclusively in the apical membrane and the relative number of pendrin positive cells increased.

CONCLUSIONS

These results are in agreement with a potential role of pendrin in bicarbonate secretion and regulation of acid-base transport in the cortical collecting duct.

NF-kappaB inhibits transcription of the H(+)-K(+)-ATPase alpha(2)-subunit gene: role of histone deacetylases

The H(+)-K(+)-ATPase alpha(2) (HKalpha(2)) gene plays a central role in potassium homeostasis, yet little is known about its transcriptional control. We recently demonstrated that the proximal promoter confers basal transcriptional activity in mouse inner medullary collecting duct 3 cells. We sought to determine whether the kappaB DNA binding element at -104 to -94 influences basal HKalpha(2) gene transcription in these cells. Recombinant NF-kappaB p50 footprinted the region -116/-94 in vitro. Gel shift and supershift analysis revealed NF-kappaB p50- and p65-containing DNA-protein complexes in nuclear extracts of mouse inner medullary collecting duct 3 cells. A promoter-luciferase construct with a mutated -104/-94 NF-kappaB element exhibited higher activity than the wild-type promoter in transfection assays. Overexpression of NF-kappaB p50, p65, or their combination trans-repressed the HKalpha(2) promoter. The histone deacetylase (HDAC) inhibitor trichostatin A partially reversed NF-kappaB-mediated trans-repression of the HKalpha(2) promoter. HDAC6 overexpression inhibited HKalpha(2) promoter activity, and HDAC6 coimmunoprecipitated with NF-kappaB p50 and p65. These results suggest that HDAC6, recruited to the DNA protein complex, acts with NF-kappaB to suppress HKalpha(2) transcription and identify NF-kappaB p50 and p65 as novel binding partners for HDAC6.

Three GADD45 isoforms contribute to hypertonic stress phenotype of murine renal inner medullary cells

Mammalian renal inner medullary (IM) cells routinely face and resist hypertonic stress. Such stress causes DNA damage to which IM cells respond with cell cycle arrest. We report that three growth arrest and DNA damage-inducible 45 (GADD45) isoforms (GADD45alpha, GADDD45beta, and GADD45gamma) are induced by acute hypertonicity in murine IM cells. Maximum induction occurs 16-18 h after the onset of hypertonicity. GADD45gamma is induced more strongly (7-fold) than GADD45beta (3-fold) and GADD45alpha (2-fold). GADD45alpha and GADD45beta protein induction is more pronounced and stable compared with the corresponding transcripts. Hypertonicity of various forms (NaCl, KCl, sorbitol, or mannitol) always induces GADD45 transcripts, whereas nonhypertonic hyperosmolality (urea) has no effect. Actinomycin D does not prevent hypertonic GADD45 induction, indicating that mRNA stabilization is the mechanism that mediates this induction. GADD45 induction patterns in IM cells exposed to 10 different stresses suggest isoform specificity, but similar functions, of individual isoforms during hypertonicity, heat shock, and heavy metal stress, when GADD45gamma induction is strongest (17-fold). These data associate all known GADD45 isoforms with the hypertonicity phenotype of renal IM cells.

Apical H(+)/base transporters mediating bicarbonate absorption and pH(i) regulation in the OMCD

The outer medullary collecting duct (OMCD) plays an important role in mediating transepithelial HCO transport [J(HCO(3)(-))] and urinary acidification. HCO absorption by type A intercalated cells in the OMCD inner stripe (OMCD(is)) segment is thought to by mediated by an apical vacuolar H(+)-ATPase and H(+)-K(+)-ATPase coupled to a basolateral Cl(-)-HCO exchanger (AE1). Besides these Na(+)-independent transporters, previous studies have shown that OMCD(is) type A intercalated cells have an apical electroneutral EIPA-sensitive, DIDS-insensitive Na(+)-HCO cotransporter (NBC3); a basolateral Na(+)/H(+) antiporter; and a basolateral Na(+)-K(+)-ATPase. In this study, we reexamined the Na(+) dependence of transepithelial Na(+) transport in the OMCD(is) and determined the role of apical NBC3 in intracellular (pH(i)) regulation in OMCD(is) type A intercalated cells. Control tubules absorbed HCO at a rate of approximately 13 pmol. min(-1). mm(-1). Lowering luminal Na(+) from 140 to 40 mM decreased [J(HCO(3)(-))] by approximately 15% without a change in transepithelial potential (V(te)). Furthermore, 50 microM EIPA (lumen) also decreased [J(HCO(3)(-))] by approximately 13% without a change in V(te). The effect of lowering luminal Na(+) and adding EIPA were not additive. These results demonstrate that [J(HCO(3)(-))] in the OMCD(is) is in part Na(+) dependent. In separate experiments, the pH(i) recovery rate after an NH prepulse was monitored in single type A intercalated cells with confocal fluorescence microscopy. The pH(i) recovery rate was approximately 0.21 pH/min in Na(+)-containing solutions and decreased to approximately 0.16 pH/min with EIPA (50 microM, lumen). In tubules perfused/bathed without Na(+), luminal Na(+) addition resulted in a pH(i) recovery rate of approximately 0.36 pH/min, whereas the Na(+)-independent recovery rate was approximately 0.16 pH/min. EIPA (50 microM, lumen) decreased the Na(+)-dependent pH(i) recovery rate to approximately 0.07 pH/min. The Na(+)-independent recovery rate was decreased to approximately 0.06 pH/min by bafilomycin (10 nM, lumen) and to approximately 0.10 pH/min using Schering 28080 (10 microM, lumen). These findings indicate that NBC3 contributes to pH(i) regulation in OMCD(is) type A intercalated cells and plays only a minor role in mediating [J(HCO(3)(-))] in the OMCD(is).

Immunocytochemical localization of pendrin in intercalated cell subtypes in rat and mouse kidney

Recent studies have demonstrated that a novel anion exchanger, pendrin, is expressed in the apical domain of type B intercalated cells in the mammalian collecting duct. The purpose of this study was 1) to determine the expression and distribution of pendrin along the collecting duct and connecting tubule of mouse and rat kidney and establish whether pendrin is expressed in the non-A-non-B intercalated cells and 2) to determine the intracellular localization of pendrin in the different populations of intercalated cells by immunoelectron microscopy. A peptide-derived affinity-purified antibody was generated that specifically recognized pendrin in immunoblots of rat and mouse kidney. Immunohistochemistry and confocal laser scanning microscopy demonstrated the presence of pendrin in apical domains of all type B intercalated cells in mouse and rat connecting tubule and collecting duct. In addition, strong pendrin immunostaining was observed in non-A-non-B intercalated cells. There was no labeling of type A intercalated cells. Immunoelectron microscopy demonstrated that pendrin was located in the apical plasma membrane and intracellular vesicles of both type B intercalated cells and non-A-non-B cells; the latter was identified by the presence of H(+)-ATPase in the apical plasma membrane. The results of this study demonstrate that both pendrin and H(+)-ATPase are expressed in the apical plasma membrane of non-A-non-B intercalated cells, suggesting that these cells are capable of both HCO and proton secretion. Furthermore, the presence of pendrin in both the apical plasma membrane and the apical intracellular vesicles of type B and non-A-non-B intercalated cells suggests that HCO secretion may be regulated by trafficking of pendrin between the two membrane compartments.

Epithelial Na channels and short-term renal response to salt deprivation

To test the role of epithelial Na channels in the day-to-day regulation of renal Na excretion, rats were infused via osmotic minipumps with the Na channel blocker amiloride at rates that achieved drug concentrations of 2-5 microM in the lumen of the distal nephron. Daily Na excretion rates were unchanged, although amiloride-treated animals tended to excrete more Na in the afternoon and less in the late evening than controls. When the rats were given a low-Na diet, Na excretion rates were elevated in the amiloride-treated group within 4 h and remained higher than controls for at least 48 h. Adrenalectomized animals responded similarly to the low-Na diet. In contrast, rats infused with polythiazide at rates designed to inhibit NaCl transport in the distal tubule were able to conserve Na as well as did the controls. Injection of aldosterone (2 microg/100 g body wt) decreased Na excretion in control animals after a 1-h delay. This effect was largely abolished in amiloride-treated rats. On the basis of quantitative analysis of the results, we conclude that activation of amiloride-sensitive channels by mineralocorticoids accounts for 50-80% of the immediate natriuretic response of the kidney to a reduction in Na intake. Furthermore, the channels are necessary to achieve minimal rates of Na excretion during more chronic Na deprivation.

The polycystic kidney disease proteins, polycystin-1, polycystin-2, polaris, and cystin, are co-localized in renal cilia

Recent evidence has suggested an association between structural and/or functional defects in the primary apical cilium of vertebrate epithelia and polycystic kidney disease (PKD). In Caenorhabditis elegans, the protein orthologues of the PKD-related proteins, polycystin-1 (LOV-1), polycystin-2 (PKD2), and polaris (OSM-5), co-localize in the cilia of male-specific sensory neurons, and defects in these proteins cause abnormalities of cilia structure and/or function. This study sought to determine whether the mammalian polycystins are expressed in primary cilia of renal epithelia and whether these proteins co-localize with polaris and cystin, the newly described, cilia-associated protein that is disrupted in the cpk mouse. To begin to address this issue, the expression of the protein products encoded by the PKD1, PKD2, Tg737, and cpk genes were examined in mouse cortical collecting duct (mCCD) cells using an immunofluorescence-based approach with a series of previously well-characterized antibodies. The mCCD cells were grown on cell culture inserts to optimize cell polarization and cilia formation. The data demonstrate co-localization in cilia of polycystin-1 and polycystin-2, which are the principal proteins involved in autosomal dominant polycystic kidney disease, with polaris and cystin, which are proteins that are disrupted in the Tg737(orpk)and cpk mouse models of autosomal recessive polycystic kidney disease, respectively. These data add to a growing body of evidence that suggests that primary cilium plays a key role in normal physiologic functions of renal epithelia and that defects in ciliary function contribute to the pathogenesis of PKD.

CD2-associated protein directly interacts with the actin cytoskeleton

CD2-associated protein (CD2AP) is an adapter protein associating with several membrane proteins, including nephrin, mutated in congenital nephrotic syndrome of the Finnish type, and polycystin-2, mutated in type 2 autosomal dominant polycystic kidney disease. Both proteins have critical roles in the maintenance of the integrity of the nephrons. Previous studies have suggested a role for CD2AP in the regulation of the organization of the actin cytoskeleton, but it has not been known whether the postulated association between CD2AP and actin is direct or mediated by other proteins. In this study, we address this question by using various cellular and biochemical approaches. We show that CD2AP and F-actin partially colocalize in cultured cells and that disruption of the actin cytoskeleton results in disorganization of endogenous CD2AP. Using cytoskeletal fractionation by differential centrifugation, we demonstrate that a proportion of CD2AP associates with the actin cytoskeleton. Furthermore, using pure actin and purified CD2AP fusion proteins in an F-actin coprecipitation assay, we show that CD2AP directly associates with filamentous actin and that this interaction is mediated by means of the COOH terminus of CD2AP. The present results suggest that CD2AP is involved in the regulation of the actin cytoskeleton and indicate that CD2AP may act as a direct adapter between the actin cytoskeleton and cell membrane proteins, such as nephrin and polycystin-2. Alterations in these interactions could explain some of the pathophysiological changes in congenital nephrotic syndrome and polycystic kidney disease.

Functional involvement of VAMP/synaptobrevin-2 in cAMP-stimulated aquaporin 2 translocation in renal collecting duct cells

The involvement of soluble N-ethylmaleimide sensitive factor-attachment protein receptor (SNARE) proteins in the cAMP-induced exocytosis of aquaporin 2 (AQP2)-containing vesicles was investigated in AQP2-transfected renal CD8 cells. RT-PCR and western blot analysis confirmed the presence of the SNARE homologs VAMP/synaptobrevin-2, syntaxin-1, syntaxin-4 and SNAP-23 in CD8 cells. Tetanus neurotoxin (TeNT) was efficient in cleaving synaptobrevin-like protein both in vitro and in intact CD8 cells incubated with the toxin. TeNT treatment in intact CD8 cells completely abolished cAMP-stimulated AQP2 targeting to the plasma membrane, as assessed by quantification of cell-surface immunoreactivity to an anti-AQP2 antibody raised against a peptide reproducing the extracellular AQP2 C-loop. These results represent the first evidence for the functional involvement of VAMP-2 in cAMP-induced AQP2 exocytosis in renal cells.