Cortical Collecting Duct Cells Research Articles

Lovastatin attenuates hypertension induced by renal tubule-specific knockout of ATP-binding cassette transporter A1, by inhibiting epithelial sodium channels.

We have shown that cholesterol is synthesized in the principal cells of renal cortical collecting ducts (CCD) and stimulates the epithelial sodium channels (ENaC). Here we have determined whether lovastatin, a cholesterol synthesis inhibitor, can antagonize the hypertension induced by activated ENaC, following deletion of the cholesterol transporter (ATP-binding cassette transporter A1; ABCA1). We selectively deleted ABCA1 in the principal cells of mouse CCD and used the cell-attached patch-clamp technique to record ENaC activity. Western blot and immunofluorescence staining were used to evaluate protein expression levels. Systolic BP was measured with the tail-cuff method. Specific deletion of ABCA1 elevated BP and ENaC single-channel activity in the principal cells of CCD in mice. These effects were antagonized by lovastatin. ABCA1 deletion elevated intracellular cholesterol levels, which was accompanied by elevated ROS, increased expression of serum/glucocorticoid regulated kinase 1 (Sgk1), phosphorylated neural precursor cell-expressed developmentally down-regulated protein 4-2 (Nedd4-2) and furin, along with shorten the primary cilium, and reduced ATP levels in urine. These data suggest that specific deletion of ABCA1 in principal cells increases BP by stimulating ENaC channels via a cholesterol-dependent pathway which induces several secondary responses associated with oxidative stress, activated Sgk1/Nedd4-2, increased furin expression, and reduced cilium-mediated release of ATP. As ABCA1 can be blocked by cyclosporine A, these results suggest further investigation of the possible use of statins to treat CsA-induced hypertension.

Murine epithelial sodium (Na+) channel regulation by biliary factors

The epithelial sodium channel (ENaC) mediates Na+ transport in several epithelia, including the aldosterone-sensitive distal nephron, distal colon, and biliary epithelium. Numerous factors regulate ENaC activity, including extracellular ligands, post-translational modifications, and membrane-resident lipids. However, ENaC regulation by bile acids and conjugated bilirubin, metabolites that are abundant in the biliary tree and intestinal tract and are sometimes elevated in the urine of individuals with advanced liver disease, remains poorly understood. Here, using a Xenopus oocyte-based system to express and functionally study ENaC, we found that, depending on the bile acid used, bile acids both activate and inhibit mouse ENaC. Whether bile acids were activating or inhibiting was contingent on the position and orientation of specific bile acid moieties. For example, a hydroxyl group at the 12-position and facing the hydrophilic side (12α-OH) was activating. Taurine-conjugated bile acids, which have reduced membrane permeability, affected ENaC activity more strongly than did their more membrane-permeant unconjugated counterparts, suggesting that bile acids regulate ENaC extracellularly. Bile acid-dependent activation was enhanced by amino acid substitutions in ENaC that depress open probability and was precluded by proteolytic cleavage that increases open probability, consistent with an effect of bile acids on ENaC open probability. Bile acids also regulated ENaC in a cortical collecting duct cell line, mirroring the results in Xenopus oocytes. We also show that bilirubin conjugates activate ENaC. These results indicate that ENaC responds to compounds abundant in bile and that their ability to regulate this channel depends on the presence of specific functional groups.

OR10-1 Regulation of Glucocorticoid Metabolism: A Novel Function for Thyroid Hormones?

Glucocorticoid hormone metabolism and action are regulated by the 11-beta hydroxysteroid dehydrogenase (11βHSD) isozymes: the 11βHSD2, mostly expressed in the distal nephron, converts cortisol [F] into cortisone [E] in humans or corticosterone [B] into 11-dehydrocorticosterone [A] in rodents (11-dehydro derivatives being inactive compounds), and the 11βHSD1, ubiquitously expressed but abundant in the liver, catalyzes the opposite reaction. Under pathophysiological conditions of severe hypothyroidism, altered glucocorticoid metabolism has been observed, with decreased [F] to [E] conversion1,2. However, direct functional relationship between these two hormonal pathways has never been demonstrated to date. Using bioinformatics analyses, we identified five thyroid hormone response elements in the promoter region of the murine hsd11b2 gene. Therefore, we aimed at investigating whether thyroid hormones (T3) directly regulate expression and/or activity of the 11βHSD2 enzyme. We used three complementary models: human and mouse translational studies and molecular analyses in HEK293T cells and in fully differentiated KC3AC1 cortical collecting duct cells. Children and adults in hypo- or hyperthyroidism status were first compared either to age- and sex-matched euthyroid controls, or to themselves after reaching a euthyroid status. The urinary [E]/[F] ratio measured by LC-MS/MS method, was used as an index of renal 11βHSD2 activity. Interestingly, a 60% decrease in the [E]/[F] ratio was observed in hypothyroid patients, corroborating our hypothesis (p<0.05). Next, a mouse model of hyperthyroidism, generated by administrating T3 in drinking water, led to a significant 10% increase in renal 11βHSD2 mRNA and protein levels compared to wild type mice (n=10, p<0.05). Furthermore, we demonstrated in HEK293T cells that T3 transactivates the mouse hsd11b2 promoter cloned upstream the luciferase reporter gene via the Thyroid Receptor α1(TRα1). Finally, we showed that T3 exposure induces a 50% increase in 11βHSD2 mRNA levels in KC3AC1 cells, in a dose-dependent and time-dependent manner (as early as 6 h) (p<0.01). This induction was abrogated by Actinomycin D or DRB, two inhibitors of transcription, underlining a transcriptional regulatory mechanism. In addition, 11βHSD2 enzymatic activity, quantified by the [F] to [E] conversion ratio measured by LC-MS/MS in cell supernatants, increased significantly by 20% (p<0.05) after 24 h exposure to 100 nM T3. ChIP experiments further demonstrated a T3-dependent specific recruitment of the TRα1 onto the hsd11b2 promoter region. Altogether, our findings constitute the first demonstration that thyroid hormones directly regulate expression and activity of the renal 11βHSD2 enzyme, thereby controlling glucocorticoid metabolism and action.

ENaC activation by bile acids depends on specific moieties, but not on membrane permeability

The epithelial sodium channel (ENaC) mediates Na+ transport in several epithelia, including the aldosterone‐sensitive distal nephron, distal colon, and biliary epithelium. Numerous factors regulate the activity of the channel, including extracellular ligands, post‐translational modifications, and membrane‐resident lipids. Bile acids are abundant in the biliary tree and intestinal tract, and can be elevated in the urine of patients with advanced liver disease. Using Xenopus oocytes, we found that bile acids both activated and inhibited mouse ENaC, dependent on the bile acid. Whether bile acids were activating or inhibiting depended on the position and stereochemistry of specific moieties. Taurine conjugated bile acids had stronger effects than their more membrane permeant unconjugated counterparts, suggesting that bile acids regulate ENaC extracellularly. Bile acids that increased ENaC currents had a hydroxyl group at position 12, facing the hydrophilic side. Bile acids that decreased ENaC currents had a hydroxyl group at position 6. Bile acid dependent activation of ENaC currents was mildly voltage‐dependent, suggesting that regulation occurs in the outer leaflet of the membrane. Bile acids also regulated ENaC in a cortical collecting duct cell line, mirroring results in Xenopus oocytes. These results suggest that bile acids interact directly with ENaC near the interface between the outer leaflet and the extracellular solution.Support or Funding InformationThis work was supported by NIDDK, National Institutes of Health, Grant R01 DK098204 (to O.B.K), and a grant from the Pittsburgh Liver Research Center. The Pittsburgh Center for Kidney Research was supported by Grant DK P30 DK079307 from NIDDK, National Institutes of Health.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Atorvastatin does not ameliorate lithium‐induced nephrogenic diabetes insipidus in mice

Lithium (Li) is an important mood‐stabilizing drug used in the treatment of affective disorders. However, as a significant adverse effect patients can develop Nephrogenic Diabetes Insipidus (NDI). NDI is characterized by the inability of the kidney to concentrate urine in response to vasopressin, resulting in severe polyuria and an increased long‐term risk of chronic kidney disease. Li‐NDI is associated with downregulation of the water channel AQP2 and a cellular remodeling of the kidney collecting duct. Effective and safe treatment of Li‐NDI is still lacking. Statins are cholesterol‐lowering drugs, which appear to increase AQP2 membrane‐translocation and improve urine concentration ability in other NDI models.This project will investigate if atorvastatin is able to prevent the Li‐induced changes in mice using a mouse cortical collecting duct cell line (mCCD) and C57BL/6 mice.Biotinylation assays showed that acute (1hr) treatment with simvastatin (p&lt;0.05) or fluvastatin (p&lt;0.0001) increased AQP2 membrane accumulation compared to controls in the mCCD cells. This confirms that the mCCD cell line respond to acute statin treatment as shown before for these two statins in other cellular and animal models. In order to see whether chronic statin treatment abolish the effects of Li, cells were treated with Li and Li/atorvastatin for 48 hrs. Li reduced total AQP2 levels (p&lt;0.01) compared to controls as expected, but combined Li/atorvastatin treatment did not prevent the Li‐induced downregulation of AQP2. Chronic (21 days) Li treatment of mice increased urine output (p&lt;0.0001) and reduced urine osmolality (p&lt;0.0001) compared to controls (receiving normal food). Atorvastatin (given in combination with Li for 21 days) decreased levels of triglycerides in the blood (p&lt;0.05) compared to controls. Combined Li/atorvastatin treatment did not prevent the effects of Li on urine output and urine osmolality. In kidney inner medulla, western blotting showed that Li reduced total AQP2 levels (p&lt;0.001) and increased pS261‐AQP2 levels (p&lt;0.05), and further showed a tendency to pERK1/2 upregulation (p=0.06) compared to controls as shown previously. However, combined Li/atorvastatin treatment did not abolish any of these changes. Immunohistochemistry further showed that atorvastatin did not prevent the Li‐induced increase in intercalated cells and increased proliferation in the inner medulla.In conclusion, both chronic in vitro and in vivo experiments showed that atorvastatin does not appear to have positive effects on prevention of Li‐NDI in mice.Support or Funding InformationDanish Medical Research Council, Beckett Fonden, Aase and Ejnar Danielsens Fonden, Knud and Edith Eriksens MindefondThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Видовые особенности негеномной реакции главных клеток собирательных трубок почек крыс и мышей на альдостерон

We have previously shown the fast nongenomic effect of aldosterone on intracellular sodium ([Na+]i) in the principal cells of rat kidney cortical collecting duct (CCD) in low sodium outer medium (14 mM NaCl). The method of fluorescence microscopy with the intracellular dye Sodium Green was used to study the rate of [Na+]i changes in response to external sodium shift (from 137 to 14 mM ) in Wistar rat and C57bl/6 mouse principal cells. In the presence of aldosterone (10 nM), the level of intracellular sodium in the rat principal cells was higher than in control at the low sodium medium (14 mM NaCl) (р˂0.05). In the principal cells of the mice kidney CCD, the response to a sharp change in the sodium level in the outer medium was significantly faster than we have found in rat, and the nongenomic response to aldosterone (10 nM) was weaker than in rats (р˂0.05). The data obtained for the first time demonstrated the differences in the fast nongenomic aldosterone effects in the principal cells of rat and mice CCD on intracellular sodium balance.

β1Pix exchange factor stabilizes the ubiquitin ligase Nedd4-2 and plays a critical role in ENaC regulation by AMPK in kidney epithelial cells

Our previous work has established that the metabolic sensor AMP-activated protein kinase (AMPK) inhibits the epithelial Na+ channel (ENaC) by promoting its binding to neural precursor cell-expressed, developmentally down-regulated 4-2, E3 ubiquitin protein ligase (Nedd4-2). Here, using MS analysis and in vitro phosphorylation, we show that AMPK phosphorylates Nedd4-2 at the Ser-444 (Xenopus Nedd4-2) site critical for Nedd4-2 stability. We further demonstrate that the Pak-interacting exchange factor β1Pix is required for AMPK-mediated inhibition of ENaC-dependent currents in both CHO and murine kidney cortical collecting duct (CCD) cells. Short hairpin RNA-mediated knockdown of β1Pix expression in CCD cells attenuated the inhibitory effect of AMPK activators on ENaC currents. Moreover, overexpression of a β1Pix dimerization-deficient mutant unable to bind 14-3-3 proteins (Δ602-611) increased ENaC currents in CCD cells, whereas overexpression of WT β1Pix had the opposite effect. Using additional immunoblotting and co-immunoprecipitation experiments, we found that treatment with AMPK activators promoted the binding of β1Pix to 14-3-3 proteins in CCD cells. However, the association between Nedd4-2 and 14-3-3 proteins was not consistently affected by AMPK activation, β1Pix knockdown, or overexpression of WT β1Pix or the β1Pix-Δ602-611 mutant. Moreover, we found that β1Pix is important for phosphorylation of the aforementioned Nedd4-2 site critical for its stability. Overall, these findings elucidate novel molecular mechanisms by which AMPK regulates ENaC. Specifically, they indicate that AMPK promotes the assembly of β1Pix, 14-3-3 proteins, and Nedd4-2 into a complex that inhibits ENaC by enhancing Nedd4-2 binding to ENaC and its degradation.

Endogenous hepcidin synthesis protects the distal nephron against hemin and hemoglobin mediated necroptosis

Hemoglobinuria is associated with kidney injury in various hemolytic pathologies. Currently, there is no treatment available and its pathophysiology is not completely understood. Here we studied the potential detrimental effects of hemoglobin (Hb) exposure to the distal nephron (DN). Involvement of the DN in Hb kidney injury was suggested by the induction of renal hepcidin synthesis (p < 0.001) in mice repeatedly injected with intravenous Hb. Moreover, the hepcidin induction was associated with a decline in urinary kidney injury markers 24p3/NGAL and KIM1, suggesting a role for hepcidin in protection against Hb kidney injury. We demonstrated that uptake of Hb in the mouse cortical collecting duct cells (mCCDcl1) is mediated by multi-protein ligand receptor 24p3R, as indicated by a significant 90% reduction in Hb uptake (p < 0.001) after 24p3R silencing. Moreover, incubation of mCCDcl1 cells with Hb or hemin for 4 or 24 h resulted in hepcidin synthesis and increased mRNA expression of markers for oxidative, inflammatory and ER stress, but no cell death as indicated by apoptosis staining. A protective role for cellular hepcidin against Hb-induced injury was demonstrated by aggravation of oxidative, inflammatory and ER stress after 4 h Hb or hemin incubation in hepcidin silenced mCCDcl1 cells. Hepcidin silencing potentiated hemin-mediated cell death that could be diminished by co-incubation of Nec-1, suggesting that endogenous hepcidin prevents necroptosis. Combined, these results demonstrate that renal hepcidin synthesis protects the DN against hemin and hemoglobin-mediated injury.

AKAPs-PKA disruptors increase AQP2 activity independently of vasopressin in a model of nephrogenic diabetes insipidus

Congenital nephrogenic diabetes insipidus (NDI) is characterized by the inability of the kidney to concentrate urine. Congenital NDI is mainly caused by loss-of-function mutations in the vasopressin type 2 receptor (V2R), leading to impaired aquaporin-2 (AQP2) water channel activity. So far, treatment options of congenital NDI either by rescuing mutant V2R with chemical chaperones or by elevating cyclic adenosine monophosphate (cAMP) levels have failed to yield effective therapies. Here we show that inhibition of A-kinase anchoring proteins (AKAPs) binding to PKA increases PKA activity and activates AQP2 channels in cortical collecting duct cells. In vivo, the low molecular weight compound 3,3′-diamino-4,4′-dihydroxydiphenylmethane (FMP-API-1) and its derivatives increase AQP2 activity to the same extent as vasopressin, and increase urine osmolality in the context of V2R inhibition. We therefore suggest that FMP-API-1 may constitute a promising lead compound for the treatment of congenital NDI caused by V2R mutations.

β1Pix Stabilizes Nedd4‐2 and Plays a Critical Role in ENaC Regulation by AMPK in Kidney Epithelial Cells

The epithelial Na+ channel (ENaC) is expressed in many salt‐reabsorbing epithelia at the apical membrane, including in the aldosterone‐sensitive distal nephron of the kidney where it is the final regulator of Na+ balance, blood volume, and blood pressure. Our previous work has established that the metabolic sensor AMP‐activated protein kinase (AMPK) inhibits ENaC by promoting the binding of the ubiquitin ligase Nedd4‐2 to ENaC. Here, we have further demonstrated that functional β1Pix, a Rho‐GEF signaling protein, is required for AMPK‐dependent ENaC inhibition in CHO cells. Epithelial volt‐ohmmeter measurements were performed to examine the role of β1Pix in regulating ENaC‐dependent currents in polarized kidney cortical collecting duct (CCD) cells. Lentiviral shRNA‐mediated knockdown of β1Pix expression in mpkCCDcl1 cells attenuated the inhibitory effect of AMPK activators on ENaC currents. Moreover, overexpression of a β1Pix dimerization‐deficient mutant unable to bind 14‐3‐3 proteins (Δ602‐611) increased ENaC currents in mpkCCDc14 cells, whereas overexpression of wild‐type β1Pix had an opposite effect. Through additional immunoblotting and co‐immunoprecipitation studies, we explored the role of β1Pix in the regulation of ENaC by AMPK and the interplay of β1Pix, 14‐3‐3 proteins and Nedd4‐2. Treatment with AMPK activators promoted the binding of β1Pix to 14‐3‐3 proteins in CCD cells. However, the association between Nedd4‐2 and 14‐3‐3 proteins was not significantly altered by AMPK activation, β1Pix knockdown, or overexpression of wild‐type or β1Pix‐Δ602–611 mutant in cells. Moreover, β1Pix knockdown or overexpression of β1Pix‐Δ602–611 inhibited Nedd4‐2 protein expression in CCD cells. This inhibition was associated with a parallel reduction in Nedd4‐2 phosphorylation at Ser‐328, a site that is phosphorylated by several kinases including AMPK which is critical for cellular Nedd4‐2 protein stability. Decreased Nedd4‐2 protein stability in the setting of β1Pix knockdown was confirmed by cycloheximide chase studies. Together, these results suggest that functional β1Pix is important for Nedd4‐2 Ser‐328 phosphorylation and thus Nedd4‐2 stability. Overall, our findings suggest that AMPK activation promotes, potentially via AMPK‐dependent β1Pix phosphorylation, the association of β1Pix, 14‐3‐3 proteins, and Nedd4‐2 into a complex that inhibits ENaC by enhancing Nedd4‐2 binding to ENaC followed by ENaC ubiquitination and targeting for degradation. This regulation of ENaC and other epithelial transport proteins by AMPK may have a pro‐survival role by inhibiting energy‐consuming ion transport processes during conditions of ischemia and other cellular stresses that occur with acute kidney injury.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Cyclosporin A Induces Hypertension via a Cholesterol‐ and ENaC‐Dependent Mechanism

Hypertension related to the use of immunosuppressive drugs, particularly cyclosporin A (CsA), presents one of the major problems in the management of transplant recipients. Our recent studies suggest a hypothesis that CsA may cause hypertension by stimulating the renal epithelial sodium channel (ENaC) via a pathway associated with elevated intracellular cholesterol (Cho). To further determine the mechanism, we performed confocal microscopy and patch‐clamp experiments using cultured cortical collecting duct (CCD) principal cells and measured the blood pressure in mice. Confocal data show that CsA and Cho increased both the length of microvilli of CCD cells where ENaC is located and the levels of phosphatidylinositol‐4,5‐bisphosphate (PIP2) in microvilli. Conversely, inhibition of Cho synthesis with lovastatin decreased the levels of PIP2, a well‐known ENaC activator. Patch‐clamp data shows that Cho increased ENaC activity in a PIP2‐dependent manner. Interestingly, CsA elevated the blood pressure in mice and the elevation was abolished by either inhibition of Cho synthesis with lovastatin or blockade of ENaC with amiloride. These results suggest that CsA increases the blood pressure of mice by stimulating ENaC via a pathway associated with the elevation of intracellular Cho and PIP2 in the principal cells of CCD. However, it remains to be determined whether statins or amiloride can be used for the treatment of CsA‐induced hypertension.Support or Funding Information(Supported by NIH R01DK100582 to H.M.)This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Diversity of ENaC expression in different tissues and different gender

The epithelial sodium channel (ENaC) plays an important role in different organs such as kidney, lung, and colon. We performed Western blot experiments using the tissues from these organs of mice to determine whether ENaC expression is dependent on either tissue types or gender. Our data show that the kidney expressed more α‐ENaC than the lung and the colon whereas the lung expressed more β‐ and γ‐ENaC than the kidney and more γ‐ENaC than the colon. The diversity of ENaC expression may account for the different roles of ENaC in these organs. Our data also show that a 60 kD band was consistently detected with antibody to γ‐ENaC in the male mouse kidney, but barely seen in the female mouse kidney. The expression of this band was not altered by low sodium diet. It was also observed in cultured cortical collecting duct cells which were from the male mouse kidney. Interestingly, this band was not observed in the lung of any gender, but was the only band found in the colon of both male and female mouse. To determine whether this band represents a novel cleaved γ‐ENaC, we examined the enzymes that can cleave γ‐ENaC. The data show that the expression levels of prostasin, furin, kallikrein‐2, and plasmin were not significantly different between male and female, but the levels of prostasin was significantly different between the tissues of kidney and lung. These results suggest that there is a diversity of ENaC expression in different tissues and different gender. However, it remains to be further determined whether the 60 kD band represents a novel cleaved γ‐ENaC.Support or Funding InformationNIH R01‐DK100582 to H.M.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Stabilization of Hypoxia Inducible Factor by Cobalt Chloride Can Alter Renal Epithelial Transport

Around 600,000 people in the United States alone have polycystic kidney disease (PKD) which is characterized by the formation of fluid‐filled pouch‐like structures, called cysts. These cysts tend to expand over the years, finally leading to kidney failure. Previously, kidney cysts were reported to be hypoxic [1]. Cyst expansion, stagnant fluid accumulation, and insufficient vascular supply can result in localized chronic ischemia‐hypoxia in cysts and as well as in normal epithelia adjacent to a cyst. We hypothesize that in normal epithelia near a cyst, the stabilization of Hypoxia Inducible Factor 1α (HIF1α), a major regulator of cellular response to hypoxia, can cause altered paracellular and transcellular transport in normal renal epithelia, transforming the normal absorptive epithelia to a secretory and leaky phenotype, leading to cyst expansion.Using cobalt chloride (CoCl2), HIF1α was stabilized in cellular nucleus of a mouse cortical collecting duct cell line (mCCD 1296 (d)), which resulted in an increased level of erythropoietin, an effector molecule of HIF1α. The mCCD monolayers have high transepithelial resistance (TER) value around 3000 Ω‐cm2 and around 95% amiloride sensitive voltage value. Equivalent current (Ieq) was calculated to compare active ion transport and FITC‐conjugated dextran molecules (3 and 70 kDa) were used to quantify transepithelial transport in terms of permeability values. MTT assay was used in order to make sure CoCl2 treated and untreated groups maintained same levels of metabolically active cells. Our results showed that TER values decreased significantly after 48 and 72 hours of HIF stabilization by CoCl2. The decrease of TER value was consistent with the increase in the permeability of large‐size 70 kDa FITC‐dextran molecules, supporting HIF stabilization actually altered paracellular transport. Western blot analysis showed HIF stabilization caused a significant decrease in the protein level of one of the major tight junction proteins, zonula occludin 1 (ZO1). Decrease in the ZO1 protein level was consistent with the decreased TER value and the increased paracellular permeability. HIF stabilization by CoCl2 for 48 hours caused almost complete loss of active sodium ion transport, and very interestingly, 72 hours of HIF stabilization caused a switch in the direction of active ion transport. Western blot analysis showed that HIF stabilization caused a decrease in the protein level of sodium‐potassium‐ATPase (Na+‐K+‐ATPase) α1 subunit, the catalytic subunit of the enzyme responsible for active sodium ion transport. Decreased level of Na+‐K+‐ATPase α1 subunit due to HIF stabilization was consistent with the loss of active sodium ion transport. Thus, HIF stabilization by CoCl2 can affect Na+‐K+‐ATPase α1 subunit and ZO1 protein levels, leading to alterations of transcellular and paracellular transport through normal kidney epithelial monolayers. Thus, our results indicate that localized ischemia‐hypoxia can transform a normal absorptive epithelia to a secretory and leaky epithelia and may contribute to cyst expansion.Support or Funding InformationThis research was supported by the Dr. John Vitullos Pilot and Bridge Funding Program at the Center for Gene Regulation in Health and Disease (GRHD), NIH DK092716 award (both to AR) and Cleveland State University Dissertation Research Award to SN.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Potassium Can Regulate Its Own Excretion in Collecting Duct Cells: Role of mTORC2, SGK1 and ENaC

BackgroundThe control of potassium (K+) excretion is central to the homeostatic response to variation in environmental K+ in all multicellular organisms. In mammals, the serine‐threonine kinase SGK1 plays a central role in regulating K+ excretion by stimulating electrogenic Na+ transport in the kidney tubules through the epithelial Na+ channel, ENaC. The resulting increase in lumen negative electrical potential enhances the driving force for K+ secretion through apical membrane K+ channels. It is well established that SGK1 phosphorylation state and activity are controlled by the kinase mTORC2, however, the central physiological inputs that control this phosphorylation are poorly characterized. We postulated that K+ itself might act directly in kidney tubule cells to regulate SGK1 through effects on mTORC2 activity.MethodsmpkCCD cortical collecting duct (CCD) cells were grown on Transwell filters, and treated for 4 h with aldosterone to increase SGK1 expression and adapted to either 1 mM or 5 mM [K+]. At t = 0, basolateral [K+] was changed from 1 to 5 mM or from 5 mM to 1 mM, followed by a return to 5 mM in the presence or absence of either an SGK1 (GSK650394) or mTOR inhibitor. Additional experiments were performed using patch clamp to detect ENaC currents in CCD isolated from mice and subjected to acute change in bath [K+] from 5 mM to 1 mM. SGK1 phosphorylation state in response to changes in [K+] was also assessed in wild type and WNK1 deficient HEK‐293 cells.ResultsIn mpkCCD cells, raising basolateral [K+] from 1 to 5 mM increased (approximately 3‐fold), while lowering [K+] from 5 mM to 1 mM decreased (approximately 4.5‐fold) SGK1 phosphorylation (p &lt; 0.01 for both manipulations). Changing apical [K+] had no significant effect. Further, shifting basolateral [K+] from 5 mM to 1 mM markedly reduced ENaC‐dependent Na+ current, and returning [K+] to 5 mM induced a rapid increase in Na+ current. The stimulatory effects on both SGK1 phosphorylation and Na+ current were dependent on mTOR and SGK1 in that they were inhibited by the mTOR inhibitor, AZD 8055 and the SGK1 inhibitor, GSK650394. SGK1 phosphorylation was also strongly stimulated in a graded fashion by raising medium [K+] in HEK‐293 cells. Interestingly, the K+ stimulatory effect on SGK1 was lost in WNK1‐deficient HEK‐293 cells. Finally, in patch clamp performed on cortical collecting duct isolated from mice, ENaC currents were significantly greater in the presence of 5 mM (330 +/−25 pA, N=5) than in the presence of 1 mM (250 +/− 20 pA, N=5) [K+] (p &lt; 0.05).ConclusionThese data strongly support the idea that changes in extracellular [K+] act directly in CCD cells to modulate mTORC2‐dependent SGK1 phosphorylation resulting in altered ENaC‐mediated Na+ transport, and thereby stimulating K+ secretion. This effect appears to be highly dependent on WNK1. In light of recent evidence that Na‐Cl cotransporter (NCC) is directly regulated in distal convoluted tubule cells by extracellular fluid [K+], these data support a new model of coordinated regulation of Na+ transport between distal convoluted tubule and CCD, which is directly modulated by local renal [K+].Support or Funding InformationAm Heart Assoc. 17GRNT33410667;NIH/NIDDK, R01‐DK56695This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

CHIP regulates Aquaporin‐2 Quality Control and Body Water Homeostasis

The importance of the kidney distal convoluted tubule (DCT) and cortical collecting duct (CCD) are highlighted by various water and electrolyte disorders that arise when the unique transport properties of these segments are disturbed. Despite this critical role, little is known about which proteins play a regulatory role in these cells and how these cells can be regulated by individual physiological stimuli. By combining proteomics, bioinformatics, and cell biology approaches alongside gene targeted animal models, we found an essential role of the E3 ubiquitin ligase CHIP for modulating the abundance of the water channel aquaporin‐2 (AQP2). CHIP is highly expressed throughout the collecting duct, modulated in abundance by vasopressin, interacts with AQP2, Hsp70 and Hsc70 and can directly ubiquitylate AQP2 in vitro. shRNA knockdown of CHIP in CCD cells increased AQP2 protein half‐life and reduced AQP2 ubiquitylation, resulting in greater AQP2 and phosphorylated AQP2 levels. Plasma membrane abundance of AQP2 was increased in CHIP knockdown cells. CHIP knockout mice or novel CRISPR/Cas9 mice without CHIP E3 ligase activity had greater AQP2 abundance and altered renal water handling, with decreased water intake and urine volume, alongside higher urine osmolality. No significant changes in other water or sodium transporting proteins were observed in the gene‐modified mice. In summary, CHIP has a regulatory role for AQP2 and subsequently renal water handling.Support or Funding InformationQ.W. is supported by European Union Horizon 2020 Marie Skłodowska‐Curie Individual Fellowship project 705682, Danish Medical Research Council grant 6110‐00118B, and Lundbeck Foundation grant R192‐2015‐804. M.L.A.K. is supported by Danish Medical Research Council grant 1333‐00279. Funding is provided by National Institutes of Health grant R37 HL065619 (to C.P. and J.C.S.). Funding is also provided by the Novo Nordisk Foundation (R.A.F.), the Lundbeck Foundation (R.A.F.), and the Danish Medical Research Council (R.A.F.).This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Knockout of mitochondrial voltage-dependent anion channel type 3 increases reactive oxygen species (ROS) levels and alters renal sodium transport

It has been suggested that voltage-dependent anion channels (VDACs) control the release of superoxide from mitochondria. We have previously shown that reactive oxygen species (ROS) such as superoxide (O2̇̄) and hydrogen peroxide (H2O2) stimulate epithelial sodium channels (ENaCs) in sodium-transporting epithelial tissue, including cortical collecting duct (CCD) principal cells. Therefore, we hypothesized that VDACs could regulate ENaC by modulating cytosolic ROS levels. Herein, we find that VDAC3-knockout(KO) mice can maintain normal salt and water balance on low-salt and high-salt diets. However, on a high-salt diet for 2 weeks, VDAC3-KO mice had significantly higher systolic blood pressure than wildtype mice. Consistent with this observation, after a high-salt diet for 2 weeks, ENaC activity in VDAC3-KO mice was significantly higher than wildtype mice. EM analysis disclosed a significant morphological change of mitochondria in the CCD cells of VDAC3-KO mice compared with wildtype mice, which may have been caused by mitochondrial superoxide overload. Of note, compared with wildtype animals, ROS levels in VDAC3-KO animals fed a normal or high-salt diet were consistently and significantly increased in renal tubules. Both the ROS scavenger 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine (TEMPOL) and the mitochondrial ROS scavenger (2-(2,2,6,6-tetramethylpiperidin-1-oxyl-4-ylamino)-2-oxoethyl)triphenylphosphonium chloride (mito-TEMPO) could reverse the effect of high-salt on ENaC activity and systolic blood pressure in the VDAC3-KO mice. Mito-TEMPO partially correct the morphological changes in VDAC3-KO mice. Our results suggest that knocking out mitochondrial VDAC3 increases ROS, alters renal sodium transport, and leads to hypertension.

AQP2 can modulate the pattern of Ca2+ transients induced by store-operated Ca2+ entry under TRPV4 activation.

There is increasing evidence indicating that aquaporins (AQPs) exert an influence in cell signaling by the interplay with the TRPV4 Ca2+ channel. Ca2+ release from intracellular stores and plasma membrane hyperpolarization due to opening of Ca2+ -activated potassium channels (KCa) are events that have been proposed to take place downstream of TRPV4 activation. A major mechanism for Ca2+ entry, activated after depletion of intracellular Ca2+ stores and driven by electrochemical forces, is the store-operated Ca2+ entry (SOCE). The consequences of the interplay between TRPV4 and AQPs on SOCE have not been yet investigated. The aim of our study was to test the hypothesis that AQP2 can modulate SOCE by facilitating the interaction of TRPV4 with KCa channels in renal cells. Using fluorescent probe techniques, we studied intracellular Ca2+ concentration and membrane potential in response to activation of TRPV4 in two rat cortical collecting duct cell lines (RCCD1 ), one not expressing AQPs (WT-RCCD1 ) and the other transfected with AQP2 (AQP2-RCCD1 ). We found that AQP2 co-immunoprecipitates with TRPV4 and with the small-conductance potassium channel (SK3). We also showed that AQP2 is crucial for the activation of SK3 by TRPV4, leading to hyperpolarization of the plasma membrane. This seems to be relevant to modulate the magnitude of SOCE and is accompanied by TRPV4 translocation to the plasma membrane only in AQP2 expressing cells. These findings open the perspective to further investigate whether the interplay between different AQPs with TRPV4 and KCa channels can be an important mechanism to modulate SOCE with physiological relevance.

Downregulation of the Cl-/HCO3-Exchanger Pendrin in Kidneys of Mice with Cystic Fibrosis: Role in the Pathogenesis of Metabolic Alkalosis

Background/Aims: Patients with cystic fibrosis (CF) are prone to the development of metabolic alkalosis; however, the pathogenesis of this life threatening derangement remains unknown. We hypothesized that altered acid base transport machinery in the kidney collecting duct underlies the mechanism of impaired bicarbonate elimination in the CF kidney. Methods: Balance studies in metabolic cages were performed in WT and CFTR knockout (CF) mice with the intestinal rescue in response to bicarbonate loading or salt restriction, and the expression levels and cellular distribution of acid base and electrolyte transporters in the proximal tubule, collecting duct and small intestine were examined by western blots, northern blots and/or immunofluorescence labeling. Results: Baseline parameters, including acid-base and systemic vascular volume status were comparable in WT and CF mice, as determined by blood gas, kidney renin expression and urine chloride excretion. Compared with WT animals, CF mice demonstrated a significantly higher serum HCO₃^- concentration (22.63 in WT vs. 26.83 mEq/l in CF mice; n=4, p=0.013) and serum pH (7.33 in WT vs. 7.42 in CF mice; n=4, p=0.00792) and exhibited impaired kidney HCO₃^- excretion (urine pH 8.10 in WT vs. 7.35 in CF mice; n=7, p=0.00990) following a 3-day oral bicarbonate load. When subjected to salt restriction, CF mice developed a significantly higher serum HCO₃^- concentration vs. WT animals (29.26 mEq/L in CF mice vs. 26.72 in WT; n=5, p=0.0291). Immunofluorescence labeling demonstrated a profound reduction in the apical expression of the Cl^-/HCO₃^- exchanger pendrin in cortical collecting duct cells and western and northern blots indicated diminished plasma membrane abundance and mRNA expression of pendrin in CF kidneys. Conclusions: We propose that patients with cystic fibrosis are prone to the development of metabolic alkalosis secondary to the inactivation of the bicarbonate secreting transporter pendrin, specifically during volume depletion, which is a common occurrence in CF patients.

CHIP Regulates Aquaporin-2 Quality Control and Body Water Homeostasis.

The importance of the kidney distal convoluted tubule (DCT) and cortical collecting duct (CCD) is highlighted by various water and electrolyte disorders that arise when the unique transport properties of these segments are disturbed. Despite this critical role, little is known about which proteins have a regulatory role in these cells and how these cells can be regulated by individual physiologic stimuli. By combining proteomics, bioinformatics, and cell biology approaches, we found that the E3 ubiquitin ligase CHIP is highly expressed throughout the collecting duct; is modulated in abundance by vasopressin; interacts with aquaporin-2 (AQP2), Hsp70, and Hsc70; and can directly ubiquitylate the water channel AQP2 in vitro shRNA knockdown of CHIP in CCD cells increased AQP2 protein t1/2 and reduced AQP2 ubiquitylation, resulting in greater levels of AQP2 and phosphorylated AQP2. CHIP knockdown increased the plasma membrane abundance of AQP2 in these cells. Compared with wild-type controls, CHIP knockout mice or novel CRISPR/Cas9 mice without CHIP E3 ligase activity had greater AQP2 abundance and altered renal water handling, with decreased water intake and urine volume, alongside higher urine osmolality. We did not observe significant changes in other water- or sodium-transporting proteins in the gene-modified mice. In summary, these results suggest that CHIP regulates AQP2 and subsequently, renal water handling.

Dexamethasone enhances vasopressin-induced aquaporin-2 gene expression in the mpkCCD cells

The mouse cortical collecting duct cell (mpkCCD) has been an instrumental cell model for studying vasopressin-mediated aquaporin-2 regulation. This cell line was first developed by Vandewalle's group from a transgenic mouse carrying the transforming SV40 antigens driven by the pyruvate kinase promoter. To immortalize the cells, four hormone supplements (dexamethasone, epidermal growth factor, insulin, and triiodothyronine) were used to enhance SV40 antigen expression; however, these hormones appear to have various effects on aquaporin-2 gene expression in the cells. Here, we evaluated the effects of each hormone supplement and found that dexamethasone enhanced vasopressin-induced aquaporin-2 gene expression at both mRNA and protein levels in a dose- and time-dependent manner, without affecting mRNA or protein stability. The effects of dexamethasone were attributed largely to enhanced aquaporin-2 mRNA transcription in association with an enhanced aquaporin-2 promoter activity. Dexamethasone did not affect vasopressin-regulated aquaporin-2 phosphorylation and trafficking. In summary, we optimized the conditions to enhance vasopressin-induced endogenous aquaporin-2 gene expression in the mpkCCD cells. By increasing the amount of aquaporin-2 protein in the cells, our method will facilitate the study of aquaporin-2 cell physiology regulated by vasopressin.