American Society of Nephrology Kidney TREKS Program

In response to decreasing numbers of individuals entering into nephrology fellowships, the American Society of Nephrology launched Kidney TREKS (Tutored Research and Education for Kidney Scholars) to stimulate interest in nephrology among medical students, graduate students, and postdoctoral fellows. The program combines a one-week intensive exposure to kidney physiology with a longitudinal mentorship program at the participants' home institutions. Ten years in, an analysis was conducted to assess its effectiveness. We surveyed participants to assess their opinions regarding nephrology before and after the course and followed them longitudinally to determine their career choices. TREKS applicants who were not selected to participate were used as a comparison group. 381 people participated in the program and 242 completed the survey. After TREKS, both medical students and graduate students showed increased interest in nephrology, with rank scores of 5.6±0.2 pre- to 7.5±0.1 post-course for medical students (mean ± standard deviation, n=189, p=0.001) and 7.3±0.3 to 8.7±0.3 (n=53, p=0.001) for graduate students. In long term follow-up, TREKS medical students chose a nephrology pipeline residency at a higher rate than medical students overall (57% vs. 31%, p=0.01) and TREKS applicants who did not participate (47% vs. 31%, p=0.04). Nephrology fellowship rates for these groups exceeded the general population but did not significantly differ between TREKS participants and applicants. PhD students and postdoctoral TREKS participants had a higher rate of participating in nephrology research compared to TREKS applicants (66% vs. 30%, p=0.01). In summary, the ASN Kidney TREKS program has demonstrated that it can improve interest in nephrology in the short term and increase the number of individuals going into nephrology careers. This long-term effect is most evident in PhD students and postdoctoral participants. Further study is needed to assess the impact of TREKS on enrollment in nephrology fellowship programs.

P2X7-Mediated Antigen-Independent Activation of CD8+ T Cells Promotes Salt-Sensitive Hypertension

BACKGROUND

CD8+ T cells (CD8Ts) have been implicated in hypertension. However, the specific mechanisms are not fully understood. In this study, we explore the contribution of the P2X7 (purinergic receptor P2X7) receptor to CD8T activation and subsequent promotion of sodium retention in the kidney.

METHODS

We used mouse models of hypertension. Wild type were used as genetic controls, OT1 and Rag2/OT1 mice were utilized to determine antigen dependency, and P2X7-knockout mice were studied to define the role of P2X7 in activating CD8Ts and promoting hypertension. Blood pressure was monitored continuously and kidneys were obtained at different experimental end points. Freshly isolated CD8Ts from mice for activation assays and ATP stimulation. CD8T activation-induced promotion of sodium retention was explored in cocultures of CD8Ts and mouse DCTs.

RESULTS

We found that OT1 and Rag2/OT1 mice, which are nonresponsive to common antigens, still developed hypertension and CD8T-activation in response to deoxycorticosterone acetate/salt treatment, similar to wild-type mice. Further studies identified the P2X7 receptor on CD8Ts as a possible mediator of this antigen-independent activation of CD8Ts in hypertension. Knockout of the P2X7 receptor prevented calcium influx and cytokine production in CD8Ts. This finding was associated with reduced CD8T-DCT stimulation, reversal of excessive salt retention in DCTs, and attenuated development of salt-sensitive hypertension.

CONCLUSIONS

Our findings suggest a novel mechanism by which CD8Ts are activated in hypertension to exacerbate salt retention and infer that the P2X7 receptor on CD8Ts may represent a new therapeutic target to attenuate T-cell-mediated immunopathology in hypertension.

Reimagining Nephrology Fellowship Education to Meet the Future Needs of Nephrology: A Report of the American Society of Nephrology Task Force on the Future of Nephrology

The American Society of Nephrology (ASN) Task Force on the Future of Nephrology was established in April 2022 in response to requests from the American Board of Internal Medicine and the Accreditation Council for Graduate Medical Education regarding training requirements in nephrology. Given recent changes in kidney care, ASN also charged the task force with reconsidering all aspects of the specialty's future to ensure that nephrologists are prepared to provide high-quality care for people with kidney diseases. The task force engaged multiple stakeholders to develop 10 recommendations focused on strategies needed to promote: ( 1 ) just, equitable, and high-quality care for people living with kidney diseases; ( 2 ) the value of nephrology as a specialty to nephrologists, the future nephrology workforce, the health care system, the public, and government; and ( 3 ) innovation and personalization of nephrology education across the scope of medical training. This report reviews the process, rationale, and details (the "why" and the "what") of these recommendations. In the future, ASN will summarize the "how" of implementing the final report and its 10 recommendations.

Interleukin 6 mediated activation of the mineralocorticoid receptor in the aldosterone-sensitive distal nephron

Hypertension is characterized by increased sodium (Na+) reabsorption along the aldosterone-sensitive distal nephron (ASDN) as well as chronic systemic inflammation. Interleukin-6 (IL-6) is thought to be a mediator of this inflammatory process. Interestingly, increased Na+ reabsorption within the ASDN does not always correlate with increases in aldosterone (Aldo), the primary hormone that modulates Na+ reabsorption via the mineralocorticoid receptor (MR). Thus, understanding how increased ASDN Na+ reabsorption may occur independent of Aldo stimulation is critical. Here, we show that IL-6 can activate the MR by activating Rac1 and stimulating the generation of reactive oxygen species (ROS) with a consequent increase in thiazide-sensitive Na+ uptake. Using an in vitro model of the distal convoluted tubule (DCT2), mDCT15 cells, we observed nuclear translocation of eGFP-tagged MR after IL-6 treatment. To confirm the activation of downstream transcription factors, mDCT15 cells were transfected with mineralocorticoid response element (MRE)-luciferase reporter constructs; then treated with vehicle, Aldo, or IL-6. Aldosterone or IL-6 treatment increased luciferase activity that was reversed with MR antagonist cotreatment, but IL-6 treatment was reversed by Rac1 inhibition or ROS reduction. In both mDCT15 and mpkCCD cells, IL-6 increased amiloride-sensitive transepithelial Na+ current. ROS and IL-6 increased 22Na+ uptake via the thiazide-sensitive sodium chloride cotransporter (NCC). These results are the first to demonstrate that IL-6 can activate the MR resulting in MRE activation and that IL-6 increases NCC-mediated Na+ reabsorption, providing evidence for an alternative mechanism for stimulating ASDN Na+ uptake during conditions where Aldo-mediated MR stimulation may not occur.

The IFNγ-PDL1 Pathway Enhances CD8T-DCT Interaction to Promote Hypertension

BACKGROUND

Renal T cells contribute importantly to hypertension, but the underlying mechanism is incompletely understood. We reported that CD8Ts directly stimulate distal convoluted tubule cells (DCTs) to increase sodium chloride co-transporter expression and salt reabsorption. However, the mechanistic basis of this pathogenic pathway that promotes hypertension remains to be elucidated.

METHODS

We used mouse models of DOCA+salt (DOCA) treatment and adoptive transfer of CD8⁺ T cells (CD8T) from hypertensive animals to normotensive animals in in-vivo studies. Co-culture of mouse DCTs and CD8Ts was used as in-vitro model to test the effect of CD8T activation in promoting sodium chloride co-transporter-mediated sodium retention and to identify critical molecular players contributing to the CD8T-DCT interaction. IFNγ (interferon γ)-KO mice and mice receiving renal tubule-specific knockdown of PDL1 were used to verify in-vitro findings. Blood pressure was continuously monitored via radio-biotelemetry, and kidney samples were saved at experimental end points for analysis.

RESULTS

We identified critical molecular players and demonstrated their roles in augmenting the CD8T-DCT interaction leading to salt-sensitive hypertension. We found that activated CD8Ts exhibit enhanced interaction with DCTs via IFN-γ-induced upregulation of MHC-I and PDL1 in DCTs, thereby stimulating higher expression of sodium chloride co-transporter in DCTs to cause excessive salt retention and progressive elevation of blood pressure. Eliminating IFN-γ or renal tubule-specific knockdown of PDL1 prevented T cell homing into the kidney, thereby attenuating hypertension in 2 different mouse models.

CONCLUSIONS

Our results identified the role of activated CD8Ts in contributing to increased sodium retention in DCTS through the IFN-γ-PDL1 pathway. These findings provide a new mechanism for T cell involvement in the pathogenesis of hypertension and reveal novel therapeutic targets.

Calcineurin A-α suppression drives nuclear factor-κB-mediated NADPH oxidase-2 upregulation

Calcineurin inhibitors (CNIs) are vital immunosuppressive therapies in the management of inflammatory conditions. A long-term consequence is nephrotoxicity. In the kidneys, the primary, catalytic calcineurin (CnA) isoforms are CnAα and CnAβ. Although the renal phenotype of CnAα-/- mice substantially mirrors CNI-induced nephrotoxicity, the mechanisms downstream of CnAα are poorly understood. Since NADPH oxidase-2 (Nox2)-derived oxidative damage has been implicated in CNI-induced nephrotoxicity, we hypothesized that CnAα inhibition drives Nox2 upregulation and promotes oxidative stress. To test the hypothesis, Nox2 regulation was investigated in kidneys from CnAα-/-, CnAβ-/-, and wild-type (WT) littermate mice. To identify the downstream mediator of CnAα, nuclear factor of activated T cells (NFAT) and NF-κB regulation was examined. To test if Nox2 is transcriptionally regulated via a NF-κB pathway, CnAα-/- and WT renal fibroblasts were treated with the NF-κB inhibitor caffeic acid phenethyl ester. Our findings showed that cyclosporine A treatment induced Nox2 upregulation and oxidative stress. Furthermore, Nox2 upregulation and elevated ROS generation occurred only in CnAα-/- mice. In these mice, NF-κB but not NFAT activity was increased. In CnAα-/- renal fibroblasts, NF-κB inhibition prevented Nox2 upregulation and reactive oxygen species (ROS) generation. In conclusion, these findings indicate that 1) CnAα loss stimulates Nox2 upregulation, 2) NF-κB is a novel CnAα-regulated transcription factor, and 3) NF-κB mediates CnAα-induced Nox2 and ROS regulation. Our results demonstrate that CnAα plays a key role in Nox2 and ROS generation. Furthermore, these novel findings provide evidence of divergent CnA isoform signaling pathways. Finally, this study advocates for CnAα-sparing CNIs, ultimately circumventing the CNI nephrotoxicity.NEW & NOTEWORTHY A long-term consequence of calcineurin inhibitors (CNIs) is oxidative damage and nephrotoxicity. This study indicates that NF-κB is a novel calcineurin-regulated transcription factor that is activated with calcineurin inhibition, thereby driving oxidative damage in CNI nephropathy. These findings provide additional evidence of divergent calcineurin signaling pathways and suggest that selective CNIs could improve the long-term outcomes of patients by mitigating renal side effects.

Stimulatory Role of SPAK Signaling in the Regulation of Large Conductance Ca2+-Activated Potassium (BK) Channel Protein Expression in Kidney

SPS1-related proline/alanine-rich kinase (SPAK) plays important roles in regulating the function of numerous ion channels and transporters. With-no-lysine (WNK) kinase phosphorylates SPAK kinase to active the SPAK signaling pathway. Our previous studies indicated that WNK kinases regulate the activity of the large-conductance Ca²⁺-activated K⁺ (BK) channel and its protein expression via the ERK1/2 signaling pathway. It remains largely unknown whether SPAK kinase directly modulates the BK protein expression in kidney. In this study, we investigated the effect of SPAK on renal BK protein expression in both HEK293 cells and mouse kidney. In HEK293 cells, siRNA-mediated knockdown of SPAK expression significantly reduced BK protein expression and increased ERK1/2 phosphorylation, whereas overexpression of SPAK significantly enhanced BK expression and decreased ERK1/2 phosphorylation in a dose-dependent manner. Knockdown of ERK1/2 prevented SPAK siRNA-mediated inhibition of BK expression. Similarly, pretreatment of HEK293 cells with either the lysosomal inhibitor bafilomycin A1 or the proteasomal inhibitor MG132 reversed the inhibitory effects of SPAK knockdown on BK expression. We also found that there is no BK channel activity in PCs of CCD in SPAK KO mice using the isolated split-open tubule single-cell patching. In addition, we found that BK protein abundance in the kidney of SPAK knockout mice was significantly decreased and ERK1/2 phosphorylation was significantly enhanced. A high-potassium diet significantly increased BK protein abundance and SPAK phosphorylation levels, while reducing ERK1/2 phosphorylation levels. These findings suggest that SPAK enhances BK protein expression by reducing ERK1/2 signaling-mediated lysosomal and proteasomal degradations of the BK channel.

14-3-3γ, a novel regulator of the large-conductance Ca2+-activated K+ channel

14-3-3γ is a small protein regulating its target proteins through binding to phosphorylated serine/threonine residues. Sequence analysis of large-conductance Ca²⁺-activated K⁺ (BK) channels revealed a putative 14-3-3 binding site in the COOH-terminal region. Our previous data showed that 14-3-3γ is widely expressed in the mouse kidney. Therefore, we hypothesized that 14-3-3γ has a novel role in the regulation of BK channel activity and protein expression. We used electrophysiology, Western blot analysis, and coimmunoprecipitation to examine the effects of 14-3-3γ on BK channels both in vitro and in vivo. We demonstrated the interaction of 14-3-3γ with BK α-subunits (BKα) by coimmunoprecipitation. In human embryonic kidney-293 cells stably expressing BKα, overexpression of 14-3-3γ significantly decreased BK channel activity and channel open probability. 14-3-3γ inhibited both total and cell surface BKα protein expression while enhancing ERK1/2 phosphorylation in Cos-7 cells cotransfected with flag-14-3-3γ and myc-BK. Knockdown of 14-3-3γ by siRNA transfection markedly increased BKα expression. Blockade of the ERK1/2 pathway by incubation with the MEK-specific inhibitor U0126 partially abolished 14-3-3γ-mediated inhibition of BK protein expression. Similarly, pretreatment of the lysosomal inhibitor bafilomycin A1 reversed the inhibitory effects of 14-3-3γ on BK protein expression. Furthermore, overexpression of 14-3-3γ significantly increased BK protein ubiquitination in embryonic kidney-293 cells stably expressing BKα. Additionally, 3 days of dietary K⁺ challenge reduced 14-3-3γ expression and ERK1/2 phosphorylation while enhancing renal BK protein expression and K⁺ excretion. These data suggest that 14-3-3γ modulates BK channel activity and protein expression through an ERK1/2-mediated ubiquitin-lysosomal pathway.

Sympathetic regulation of NCC in norepinephrine-evoked salt-sensitive hypertension in Sprague-Dawley rats

Salt sensitivity of blood pressure is characterized by inappropriate sympathoexcitation and renal Na⁺ reabsorption during high salt intake. In salt-resistant animal models, exogenous norepinephrine (NE) infusion promotes salt-sensitive hypertension and prevents dietary Na⁺-evoked suppression of the Na⁺-Cl^- cotransporter (NCC). Studies of the adrenergic signaling pathways that modulate NCC activity during NE infusion have yielded conflicting results implicating α₁- and/or β-adrenoceptors and a downstream kinase network that phosphorylates and activates NCC, including with no lysine kinases (WNKs), STE20/SPS1-related proline-alanine-rich kinase (SPAK), and oxidative stress response 1 (OxSR1). In the present study, we used selective adrenoceptor antagonism in NE-infused male Sprague-Dawley rats to investigate the differential roles of α₁- and β-adrenoceptors in sympathetically mediated NCC regulation. NE infusion evoked salt-sensitive hypertension and prevented dietary Na⁺-evoked suppression of NCC mRNA, protein expression, phosphorylation, and in vivo activity. Impaired NCC suppression during high salt intake in NE-infused rats was paralleled by impaired suppression of WNK1 and OxSR1 expression and SPAK/OxSR1 phosphorylation and a failure to increase WNK4 expression. Antagonism of α₁-adrenoceptors before high salt intake or after the establishment of salt-sensitive hypertension restored dietary Na⁺-evoked suppression of NCC, resulted in downregulation of WNK4, SPAK, and OxSR1, and abolished the salt-sensitive component of hypertension. In contrast, β-adrenoceptor antagonism attenuated NE-evoked hypertension independently of dietary Na⁺ intake and did not restore high salt-evoked suppression of NCC. These findings suggest that a selective, reversible, α₁-adenoceptor-gated WNK/SPAK/OxSR1 NE-activated signaling pathway prevents dietary Na⁺-evoked NCC suppression, promoting the development and maintenance of salt-sensitive hypertension.

Zinc deficiency induces hypertension by promoting renal Na+ reabsorption

Zn²⁺ deficiency (ZnD) is a common comorbidity of many chronic diseases. In these settings, ZnD exacerbates hypertension. Whether ZnD alone is sufficient to alter blood pressure (BP) is unknown. To explore the role of Zn²⁺ in BP regulation, adult mice were fed a Zn²⁺-adequate (ZnA) or a Zn²⁺-deficient (ZnD) diet. A subset of ZnD mice were either returned to the ZnA diet or treated with hydrochlorothiazide (HCTZ), a Na⁺-Cl^- cotransporter (NCC) inhibitor. To reduce intracellular Zn²⁺ in vitro, mouse distal convoluted tubule cells were cultured in N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN, a Zn²⁺ chelator)- or vehicle (DMSO)-containing medium. To replete intracellular Zn²⁺, TPEN-exposed cells were then cultured in Zn²⁺-supplemented medium. ZnD promoted a biphasic BP response, characterized by episodes of high BP. BP increases were accompanied by reduced renal Na⁺ excretion and NCC upregulation. These effects were reversed in Zn²⁺-replete mice. Likewise, HCTZ stimulated natriuresis and reversed BP increases. In vitro, Zn²⁺ depletion increased NCC expression. Furthermore, TPEN promoted NCC surface localization and Na⁺ uptake activity. Zn²⁺ repletion reversed TPEN effects on NCC. These data indicate that 1) Zn²⁺ contributes to BP regulation via modulation of renal Na⁺ transport, 2) renal NCC mediates ZnD-induced hypertension, and 3) NCC is a Zn²⁺-regulated transporter that is upregulated with ZnD. This study links dysregulated renal Na⁺ handling to ZnD-induced hypertension. Furthermore, NCC is identified as a novel mechanism by which Zn²⁺ regulates BP. Understanding the mechanisms of ZnD-induced BP dysregulation may have an important therapeutic impact on hypertension.

Abstract P284: Increased Blood Pressure in Interleukin-6 Infused Mice is Secondary to Reduced Urinary Sodium Excretion and Not Vascular Dysfunction

Hypertension (HTN) is associated with increased renal sodium (Na + ) reabsorption, vascular and endothelial dysfunction and an increase in cytokines, such as interleukin 6 (IL-6). We have previously shown that IL-6 increases blood pressure (BP), and increases the expression/activity of distal nephron Na + transport proteins via the mineralocorticoid receptor. We hypothesized that IL-6 increases Na + reabsorption, and induces endothelial dysfunction, leading to HTN. All mice were fed high salt food, and osmotic minipumps were implanted containing IL-6 (16 ng/hr, 0.01% BSA/saline), or vehicle/sham surgery (Ctl). To allow for early urine collection, IL-6 infusion was delayed by adding catheters to the minipumps during metabolic cage studies. Tail cuff plethysmography was used to measure systolic BP. Concentration response curves (CRC) to phenylephrine (Phe) and acetylcholine (ACh) were performed on aorta (Ao) and mesenteric arteries (Mes) from IL-6 or Ctl mice. IL-6 infused mice had increased BP by day 3 (D3), as compared to Ctl. Total urine volume (U Vol ) on D1 of infusion was reduced in IL-6 mice (U Vol 17.3±4 vs. 29.5±3 mL/day/per 100g body weight (BW); p<0.05), as compared to Ctl. This corresponded with a reduction in total urinary Na + excretion (U Na 81±19 IL-6 vs . 139±13 mg/day/BW Ctl; p<0.05). By D2, no differences in U Na were observed, and by D3 U Na levels were increasing in IL-6 infused mice. Similar trends for U Vol were observed. These results corresponded with our observed increases in BP. CRCs (D3) to Phe were not changed in Ao. However, Mes from IL-6 mice exhibited a surprising reduction in Phe-mediated contraction (136±7% vs. 155±5%; p<0.05), compared to Ctl. Similarly, Mes from IL-6 mice exhibited an increase in ACh-mediated relaxation responses (EC 50 -7.5±0.12 vs . -6.9±0.15; p<0.05). These data suggest that IL-6 increases BP via an early (D1) reduction in U Na /U Vol , followed by steady state recovery and pressure natriuresis (D3). Additionally, the increased relaxation and reduced contractility observed in the resistance arteries may suggest a compensatory response to the increased BP. Together, our studies suggest that in the early stages of HTN, cytokines may cause changes in renal Na + reabsorption via activation of distal Na + transporters.

Abstract P499: Protein Kinase Cα Deletion Causes Hypotension Due to Decreased Vascular Contractility

Protein kinase Cα (PKCα) regulates multiple cell signaling pathways, including those that impact blood pressure. PKCα activation increases vascular smooth muscle contractility, yet reduces cardiac contractility. PKCα has also been shown to modulate nephron ion transport. We have shown that PKCα deletion leads to hypotension, with compensatory increases in sodium retention. Here, we hypothesized that PKCα deficiency reduces vascular contractility, leading to decreased mean arterial pressure (MAP). MAP, measured by telemetry, was decreased in PKC KO (≈12 mmHg) compared to PKC control (PKC CTL) mice. Aorta and mesenteric arteries were isolated, and concentration response curves (CRCs) to phenylephrine (Phe), acetylcholine (ACh) or sodium nitroprusside (SNP) were performed in the presence of vehicle or the following inhibitors: L-NAME or indomethacin (NOS, COX inhibitor, resp. ). CRCs to KCL were performed to assess receptor-independent vascular responses. In aorta, we observed a striking reduction in KCl-mediated contraction (5.8±0.3mN vs. 10.4±1.1mN control, **p<0.01). PKC KO aorta and mesenteric arteries had decreased contractile responses to Phe, as compared to control (aorta, 12.7±0.5mN R max vs. 16.3±0.5mN R max , and mesenteric 9.9±0.3mN R max vs. 11.8±0.6mN R max ; n=4, **p<0.01), revealing a role for reduced vascular contractility. Endothelium-mediated relaxation responses to ACh were also increased in PKC KO mice, as compared to control (59.3±6.8% R max vs. 45.4±3.2% R max , n=4, *p<0.05). Interestingly, NOS inhibition increased contractility in mesenteric arteries from PKC KO mice (8.55±2.65mN R max vs. 6.95±0.39mN R max control, n=4, ***p<0.001). However, PKC KO aorta had an enhanced response to COX inhibition (12.2±0.7mN R max vs. 10.1±0.6mN R max control, n=4, *p<0.05) suggesting that PKCα may be negatively regulating NOS in mesenteric arteries, and COX-mediated prostaglandin production in the aorta. No differences were observed in the relaxation responses to SNP. These data suggest that global deletion of PKCα results in hypotension due to decreased vascular contractility, and loss of PKCα-mediated inhibition of endothelial relaxing factors. Thus, systemic targeting of PKCα may be beneficial for the reduction of MAP.

Regulation of Lung Epithelial Sodium Channels by Cytokines and Chemokines

Acute lung injury leading to acute respiratory distress (ARDS) is a global health concern. ARDS patients have significant pulmonary inflammation leading to flooding of the pulmonary alveoli. This prevents normal gas exchange with consequent hypoxemia and causes mortality. A thin fluid layer in the alveoli is normal. The maintenance of this thin layer results from fluid movement out of the pulmonary capillaries into the alveolar interstitium driven by vascular hydrostatic pressure and then through alveolar tight junctions. This is then balanced by fluid reabsorption from the alveolar space mediated by transepithelial salt and water transport through alveolar cells. Reabsorption is a two-step process: first, sodium enters via sodium-permeable channels in the apical membranes of alveolar type 1 and 2 cells followed by active extrusion of sodium into the interstitium by the basolateral Na⁺, K⁺-ATPase. Anions follow the cationic charge gradient and water follows the salt-induced osmotic gradient. The proximate cause of alveolar flooding is the result of a failure to reabsorb sufficient salt and water or a failure of the tight junctions to prevent excessive movement of fluid from the interstitium to alveolar lumen. Cytokine- and chemokine-induced inflammation can have a particularly profound effect on lung sodium transport since they can alter both ion channel and barrier function. Cytokines and chemokines affect alveolar amiloride-sensitive epithelial sodium channels (ENaCs), which play a crucial role in sodium transport and fluid reabsorption in the lung. This review discusses the regulation of ENaC via local and systemic cytokines during inflammatory disease and the effect on lung fluid balance.

Aldosterone Modulates the Association between NCC and ENaC

Distal sodium transport is a final step in the regulation of blood pressure. As such, understanding how the two main sodium transport proteins, the thiazide-sensitive sodium chloride cotransporter (NCC) and the epithelial sodium channel (ENaC), are regulated is paramount. Both are expressed in the late distal nephron; however, no evidence has suggested that these two sodium transport proteins interact. Recently, we established that these two sodium transport proteins functionally interact in the second part of the distal nephron (DCT2). Given their co-localization within the DCT2, we hypothesized that NCC and ENaC interactions might be modulated by aldosterone (Aldo). Aldo treatment increased NCC and αENaC colocalization (electron microscopy) and interaction (coimmunoprecipitation). Finally, with co-expression of the Aldo-induced protein serum- and glucocorticoid-inducible kinase 1 (SGK1), NCC and αENaC interactions were increased. These data demonstrate that Aldo promotes increased interaction of NCC and ENaC, within the DCT2 revealing a novel method of regulation for distal sodium reabsorption.

CD8+ T cells stimulate Na-Cl co-transporter NCC in distal convoluted tubules leading to salt-sensitive hypertension

Recent studies suggest a role for T lymphocytes in hypertension. However, whether T cells contribute to renal sodium retention and salt-sensitive hypertension is unknown. Here we demonstrate that T cells infiltrate into the kidney of salt-sensitive hypertensive animals. In particular, CD8⁺ T cells directly contact the distal convoluted tubule (DCT) in the kidneys of DOCA-salt mice and CD8⁺ T cell-injected mice, leading to up-regulation of the Na-Cl co-transporter NCC, p-NCC and the development of salt-sensitive hypertension. Co-culture with CD8⁺ T cells upregulates NCC in mouse DCT cells via ROS-induced activation of Src kinase, up-regulation of the K⁺ channel Kir4.1, and stimulation of the Cl^- channel ClC-K. The last event increases chloride efflux, leading to compensatory chloride influx via NCC activation at the cost of increasing sodium retention. Collectively, these findings provide a mechanism for adaptive immunity involvement in the kidney defect in sodium handling and the pathogenesis of salt-sensitive hypertension.

NADPH oxidase-2 mediates zinc deficiency-induced oxidative stress and kidney damage

Zn²⁺ deficiency (ZnD) is comorbid with chronic kidney disease and worsens kidney complications. Oxidative stress is implicated in the detrimental effects of ZnD. However, the sources of oxidative stress continue to be identified. Since NADPH oxidases (Nox) are the primary enzymes that contribute to renal reactive oxygen species generation, this study's objective was to determine the role of these enzymes in ZnD-induced oxidative stress. We hypothesized that ZnD promotes NADPH oxidase upregulation, resulting in oxidative stress and kidney damage. To test this hypothesis, wild-type mice were pair-fed a ZnD or Zn²⁺-adequate diet. To further investigate the effects of Zn²⁺ bioavailability on NADPH oxidase regulation, mouse tubular epithelial cells were exposed to the Zn²⁺ chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) or vehicle followed by Zn²⁺ supplementation. We found that ZnD diet-fed mice develop microalbuminuria, electrolyte imbalance, and whole kidney hypertrophy. These markers of kidney damage are accompanied by elevated Nox2 expression and H₂O₂ levels. In mouse tubular epithelial cells, TPEN-induced ZnD stimulates H₂O₂ generation. In this in vitro model of ZnD, enhanced H₂O₂ generation is prevented by NADPH oxidase inhibition with diphenyleneiodonium. Specifically, TPEN promotes Nox2 expression and activation, which are reversed when intracellular Zn²⁺ levels are restored following Zn²⁺ supplementation. Finally, Nox2 knockdown by siRNA prevents TPEN-induced H₂O₂ generation and cellular hypertrophy in vitro. Together, these findings reveal that Nox2 is a Zn²⁺-regulated enzyme that mediates ZnD-induced oxidative stress and kidney hypertrophy. Understanding the specific mechanisms by which ZnD contributes to kidney damage may have an important impact on the treatment of chronic kidney disease.

Abstract 125: Interleukin 6 Activates the Mineralocorticoid Receptor via Rac1, Increasing Sodium Chloride Cotransporter Expression and Activity

Hypertension is characterized by increased sodium (Na + ) reabsorption along the aldosterone-sensitive distal nephron (ASDN), as well as a chronic systemic inflammation. Interleukin-6 (IL-6) is thought to be a mediator of this inflammatory process. Interestingly, increased Na + reabsorption within the ASDN does not always correlate with increases in serum aldosterone (Aldo), the hormone that modulates Na + reabsorption via the mineralocorticoid receptor (MR). Thus, understanding how increased MR-mediated Na + reabsorption may occur independent of Aldo stimulation is critical. We hypothesize that IL-6 transactivates the MR via Rac1, increasing sodium chloride cotransporter (NCC) expression and activity. To demonstrate IL-6-mediated MR activation via Rac1, mDCT15 cells were transfected with mineralocorticoid response element (MRE)-luciferase reporter constructs, treated with vehicle, Aldo (100nM), IL-6 (100ng/mL) and/or Rac1 inhibitor (EHT1864, 20uMol), and luciferase assays performed (48hrs). To determine if IL-6 increases NCC expression, in vivo , we perfused kidneys with IL-6 (intra-renal, 16ng/hr, 1-3d) and the cortex was isolated for WB analysis of total NCC expression. To determine direct IL-6 mediated effects on NCC activity, we performed thiazide-sensitive 22 Na + -uptake studies in cell monolayers in presence of: vehicle, IL-6 (100ng/mL for 45min or 6hrs) or IL-6+MR antagonist (spironolactone, 20uMol). IL-6 treatment significantly increased luciferase activity (2.6±0.9 fold/MRE-only), which was reduced with Rac1 inhibition (0.2±0.08, p<0.01, n=8-10) demonstrating a Rac1-dependent activation of MR via IL-6. In the kidney cortex, IL-6 infusion increased total NCC expression (>2.5 fold) after day 3, as compared to vehicle. In addition, 22 Na + -uptake studies (n=6) revealed an IL-6-mediated increase in Na + transport (1927±40 nmol/mg/20min vs. 1503±7 nmol/mg/20min, p<0.0001), which was reduced with the MR antagonist (1721±25nmol/mg, p<0.001). These data suggest that IL-6 activates the MR via Rac1, leading to increased NCC expression and activity. These data provide evidence for alternate mechanisms of increased ASDN Na + uptake during inflammatory and hypertensive conditions, independently of Aldo-mediated signaling.

WNK3 Kinase Enhances the Sodium Chloride Cotransporter Expression via an ERK 1/2 Signaling Pathway

BACKGROUND

WNK kinase is a serine/threonine kinase that plays an important role in normal blood pressure homeostasis. WNK3 was previously found to enhance the activity of sodium chloride cotransporter (NCC) in Xenopus oocyte. However, the mechanism through which it works remains unclear.

METHODS

Using overexpression and siRNA knock-down techniques, the effects of WNK3 on NCC in both Cos-7 and mouse distal convoluted cells were analyzed by Western blot.

RESULTS

We found that WNK3 significantly increased NCC protein expression in a dose-dependent manner. NCC protein expression in Cos-7 cells was markedly decreased after 2 h treatment with protease inhibitor, cycloheximide (CHX) in the NCC alone group, but was significantly decreased after 8 h treatment of CHX in the WNK3 + NCC group. WNK3 significantly increased NCC protein expression in both NCC alone and WNK3 + NCC groups regardless the overnight treatments of bafilomycin A1, a proton pump inhibitor, suggesting that WNK3-mediated increased NCC expression is not dependent on the lysosomal pathway. We further found that WNK3 group had a quicker NCC recovery than the control group using CHX pulse assay, suggesting that WNK3 increases NCC protein synthesis. WNK3 enhanced NCC protein level while reducing ERK 1/2 phosphorylation. In addition, knock-down of ERK 1/2 expression reversed WNK3-mediated increase of NCC expression.

CONCLUSION

These results suggest that WNK3 enhances NCC protein expression by increasing NCC synthesis via an ERK 1/2-dependent signaling pathway.

Interleukin-17A Regulates Renal Sodium Transporters and Renal Injury in Angiotensin II-Induced Hypertension

Angiotensin II-induced hypertension is associated with an increase in T-cell production of interleukin-17A (IL-17A). Recently, we reported that IL-17A(-/-) mice exhibit blunted hypertension, preserved natriuresis in response to a saline challenge, and decreased renal sodium hydrogen exchanger 3 expression after 2 weeks of angiotensin II infusion compared with wild-type mice. In the current study, we performed renal transporter profiling in mice deficient in IL-17A or the related isoform, IL-17F, after 4 weeks of Ang II infusion, the time when the blood pressure reduction in IL-17A(-/-) mice is most prominent. Deficiency of IL-17A abolished the activation of distal tubule transporters, specifically the sodium-chloride cotransporter and the epithelial sodium channel and protected mice from glomerular and tubular injury. In human proximal tubule (HK-2) cells, IL-17A increased sodium hydrogen exchanger 3 expression through a serum and glucocorticoid-regulated kinase 1-dependent pathway. In mouse distal convoluted tubule cells, IL-17A increased sodium-chloride cotransporter activity in a serum and glucocorticoid-regulated kinase 1/Nedd4-2-dependent pathway. In both cell types, acute treatment with IL-17A induced phosphorylation of serum and glucocorticoid-regulated kinase 1 at serine 78, and treatment with a serum and glucocorticoid-regulated kinase 1 inhibitor blocked the effects of IL-17A on sodium hydrogen exchanger 3 and sodium-chloride cotransporter. Interestingly, both HK-2 and mouse distal convoluted tubule 15 cells produce endogenous IL-17A. IL17F had little or no effect on blood pressure or renal sodium transporter abundance. These studies provide a mechanistic link by which IL-17A modulates renal sodium transport and suggest that IL-17A inhibition may improve renal function in hypertension and other autoimmune disorders.

PTH modulation of NCC activity regulates TRPV5 Ca2+ reabsorption

Since parathyroid hormone (PTH) is known to increase transient receptor potential vanilloid (TRPV)5 activity and decrease Na(+)-Cl(-) cotransporter (NCC) activity, we hypothesized that decreased NCC-mediated Na(+) reabsorption contributes to the enhanced TRPV5 Ca(2+) reabsorption seen with PTH. To test this, we used mDCT15 cells expressing functional TRPV5 and ruthenium red-sensitive (45)Ca(2+) uptake. PTH increased (45)Ca(2+) uptake to 8.8 ± 0.7 nmol·mg(-1)·min(-1) (n = 4, P < 0.01) and decreased NCC activity from 75.4 ± 2.7 to 20.3 ± 1.3 nmol·mg(-1)·min(-1) (n = 4, P < 0.01). Knockdown of Ras guanyl-releasing protein (RasGRP)1 had no baseline effect on (45)Ca(2+) uptake but significantly attenuated the response to PTH from a 45% increase (6.0 ± 0.2 to 8.7 ± 0.4 nmol·mg(-1)·min(-1)) in control cells to only 20% in knockdown cells (6.1 ± 0.1 to 7.3 ± 0.2 nmol·mg(-1)·min(-1), n = 4, P < 0.01). Inhibition of PKC and PKA resulted in further attenuation of the PTH effect. RasGRP1 knockdown decreased the magnitude of the TRPV5 response to PTH (7.9 ± 0.1 nmol·mg(-1)·min(-1) for knockdown compared with 9.1 ± 0.1 nmol·mg(-1)·min(-1) in control), and the addition of thiazide eliminated this effect (a nearly identical 9.0 ± 0.1 nmol·mg(-1)·min(-1)). This indicates that functionally active NCC is required for RasGRP1 knockdown to impact the PTH effect on TRPV5 activity. Knockdown of with no lysine kinase (WNK)4 resulted in an attenuation of the increase in PTH-mediated TRPV5 activity. TRPV5 activity increased by 36% compared with 45% in control (n = 4, P < 0.01 between PTH-treated groups). PKC blockade further attenuated the PTH effect, whereas combined PKC and PKA blockade in WNK4KD cells abolished the effect. We conclude that modulation of NCC activity contributes to the response to PTH, implying a role for hormonal modulation of NCC activity in distal Ca(2+) handling.

Pendrin localizes to the adrenal medulla and modulates catecholamine release

Pendrin (Slc26a4) is a Cl(-)/HCO3 (-) exchanger expressed in renal intercalated cells and mediates renal Cl(-) absorption. With pendrin gene ablation, blood pressure and vascular volume fall, which increases plasma renin concentration. However, serum aldosterone does not significantly increase in pendrin-null mice, suggesting that pendrin regulates adrenal zona glomerulosa aldosterone production. Therefore, we examined pendrin expression in the adrenal gland using PCR, immunoblots, and immunohistochemistry. Pendrin protein was detected in adrenal lysates from wild-type but not pendrin-null mice. However, immunohistochemistry and qPCR of microdissected adrenal zones showed that pendrin was expressed in the adrenal medulla, rather than in cortex. Within the adrenal medulla, pendrin localizes to both epinephrine- and norepinephrine-producing chromaffin cells. Therefore, we examined plasma catecholamine concentration and blood pressure in wild-type and pendrin-null mice under basal conditions and then after 5 and 20 min of immobilization stress. Under basal conditions, blood pressure was lower in the mutant than in the wild-type mice, although epinephrine and norepinephrine concentrations were similar. Catecholamine concentration and blood pressure increased markedly in both groups with stress. With 20 min of immobilization stress, epinephrine and norepinephrine concentrations increased more in pendrin-null than in wild-type mice, although stress produced a similar increase in blood pressure in both groups. We conclude that pendrin is expressed in the adrenal medulla, where it blunts stress-induced catecholamine release.

Abstract 101: Interleukin 17A Upregulates Both Renal Proximal and Distal Tubule Sodium Transporters in Angiotensin II-dependent Hypertension

We have previously shown that angiotensin II (Ang II)-induced hypertension is associated with an increase in T cell production of interleukin 17A (IL17A), and that IL17A promotes hypertension and end-organ damage. However, the precise mechanism is unknown. Recently, we reported that Ang II infusion into C56Bl/6J wild type (WT) mice blunted the rate of natriuresis following an acute saline challenge, while the rate of salt and water excretion in IL17A-/- mice was unaffected by Ang II. Following 2 weeks of Ang II infusion (490 ng/kg/min), proximal tubule sodium hydrogen exchanger 3 (NHE3) abundance was depressed in IL17A-/- but not WT mice, suggesting enhanced pressure natriuresis in IL17A-/- mice. We then performed renal transporter profiling on mice deficient in IL17A, or the related isoform IL17F, after prolonged (4 weeks) of Ang II infusion (490 ng/kg/min), a time when the blood pressure reduction in IL17A-/- mice is most prominent. Interestingly, at this time, deficiency of IL17A, but not IL17F, blunted the activation of distal tubule transporters, specifically sodium-chloride cotransporter (NCC) and the epithelial sodium channel (ENaC). We hypothesized that IL17A directly modulates renal sodium transporters as a mechanism to regulate salt and water excretion and hypertension. To test this hypothesis, we treated cultured human renal proximal tubule cells and mouse distal convoluted tubule (mDCT15) cells with recombinant IL17A or IL17F. We found that IL17A, but not IL17F, increased NHE3 protein levels (1.4-fold, p=0.003) and SGK1 mRNA expression (3.9-fold, p=0.01). In mDCT15 cells, IL17A but not IL17F, increased NCC activity as measured by thiazide-inhibited sodium uptake (1.78 vs 1.62 μmol/mg/20min, p<0.001), and this increase was significantly blunted with an SGK1 inhibitor (GSK 650394) and in cells lacking Nedd4-2 (an E3 ubiquitin ligase downstream of SGK1). Moreover, in mDCT15 cells, acute IL17A treatment caused phosphorylation of SGK1 on Ser78. These studies are the first to describe a mechanistic link by which IL17A modulates renal sodium transporters and suggests that targeting IL17A may improve renal function and slow the progression to renal failure in hypertension and other autoimmune disorders.

Mechanisms of angiotensin II stimulation of NCC are time-dependent in mDCT15 cells

Angiotensin II (ANG II) increases thiazide-sensitive sodium-chloride cotransporter (NCC) activity both acutely and chronically. ANG II has been implicated as a switch that turns WNK4 from an inhibitor of NCC into an activator of NCC, and ANG II's effect on NCC appears to require WNK4. Chronically, ANG II stimulation of NCC results in an increase in total and phosphorylated NCC, but the role of NCC phosphorylation in acute ANG II actions is unclear. Here, using a mammalian cell model with robust native NCC activity, we corroborate the role that ANG II plays in WNK4 regulation and clarify the role of Ste20-related proline alanine-rich kinase (SPAK)-induced NCC phosphorylation in ANG II action. ANG II was noted to have a biphasic effect on NCC, with a peak increase in NCC activity in the physiologic range of 10(-11) M ANG II. This effect was apparent as early as 15 min and remained sustained through 120 min. These changes correlated with significant increases in NCC surface protein expression. Knockdown of WNK4 expression sharply attenuated the effect of ANG II. SPAK knockdown did not affect ANG II action at early time points (15 and 30 min), but it did attenuate the response at 60 min. Correspondingly, NCC phosphorylation did not increase at 15 or 30 min, but increased significantly at 60 min. We therefore conclude that within minutes of an increase in ANG II, NCC is rapidly trafficked to the cell surface in a phosphorylation-independent but WNK4-dependent manner. Then, after 60 min, ANG II induces SPAK-dependent phosphorylation of NCC.

Compensatory renal hypertrophy following uninephrectomy is calcineurin-independent

Calcineurin is a calcium-dependent phosphatase that is involved in many cellular processes including hypertrophy. Inhibition or genetic loss of calcineurin blocks pathological cardiac hypertrophy and diabetic renal hypertrophy. However, calcineurin does not appear to be involved in physiological cardiac hypertrophy induced by exercise. The role of calcineurin in a compensatory, non-pathological model of renal hypertrophy has not been tested. Therefore, in this study, we examined activation of calcineurin and the effect of calcineurin inhibition or knockout on compensatory hypertrophy following uninephrectomy (UNX). UNX induces ∼15% increase in the size of the remaining kidney; the data show no change in the generation of reactive oxygen species (ROS), Nox4 or transforming growth factor-β expression confirming the model as one of compensatory hypertrophy. Next, analyses of the remaining kidney reveal that total calcineurin activity is increased, and, to a lesser extent, transcriptional activity of the calcineurin substrate nuclear factor of activated T cell is up-regulated following UNX. However, inhibition of calcineurin with cyclosporine failed to prevent compensatory renal hypertrophy. Likewise, hypertrophy was comparable to WT in mice lacking either isoform of the catalytic subunit of calcineurin (CnAα−/− or CnAβ−/−). In conclusion, similar to its role in the heart, calcineurin is required for pathological but not compensatory renal hypertrophy. This separation of signalling pathways could therefore help further define key factors necessary for pathological hypertrophy including diabetic nephropathy.

Abstract 318: The Role of PKC-alpha in NCC-Mediated Hypertension

The sodium-chloride cotransporter (NCC) is an essential regulator of sodium transport in the distal convoluted tubule (DCT); functional aberrations will lead to dysregulation of blood pressure. Although this role is well-accepted, the mechanisms accounting for increased distal nephron sodium reabsorption is not completely understood. Previously published data from the Pharmacogenomic Evaluation of Antihypertensive Responses (PEAR) Study has demonstrated that a polymorphism in PKC-alpha may lead to an increased sensitivity to thiazide diuretics. To investigate whether PKC-alpha may be altering blood pressure via NCC, PKC-alpha KO (PKC KO) and PKC Control (Ctl) mice were used to measure blood pressure via radiotelemetry after normal and high salt (HS, 4%) dietary manipulations and Western blot analysis performed using kidney lysates. Using radiotelemetry (DSI) measurements obtained over an 18 hour ‘active’ time period, we report a significant increase in mean arterial pressure (MAP) in the PKC KO mice as compared to control (123.2±1.1 mmHg vs. 102.0±0.77 mmHg, p<0.001). With HS feeding, MAP increased in both groups; in addition MAP remained significantly higher in the PKC KO mice as compared to the Ctl after 21 days (139.5±0.3 mmHg vs. 124.7±1.1 mmHg, p<0.01). To determine a possible mechanism for this phenomenon, whole kidney lysate was used to assay for total NCC protein. Total NCC protein was almost 3 fold higher in the PKC KO mice as compared to Ctl (in arbitrary units, 208±16 vs. 61±30, p<0.01). Herein we report that lack of PKC-alpha leads to an increase in MAP, which may be accounted for by increased total NCC expression and a subsequent increase in distal nephron reabsorption. We believe that PKC-alpha may be negative regulator of NCC protein expression, and that aberrations in this pathway will lead to hypertension.

A role for the circadian clock protein Per1 in the regulation of the NaCl co-transporter (NCC) and the with-no-lysine kinase (WNK) cascade in mouse distal convoluted tubule cells

It has been well established that blood pressure and renal function undergo circadian fluctuations. We have demonstrated that the circadian protein Per1 regulates multiple genes involved in sodium transport in the collecting duct of the kidney. However, the role of Per1 in other parts of the nephron has not been investigated. The distal convoluted tubule (DCT) plays a critical role in renal sodium reabsorption. Sodium is reabsorbed in this segment through the actions of the NaCl co-transporter (NCC), which is regulated by the with-no-lysine kinases (WNKs). The goal of this study was to test if Per1 regulates sodium transport in the DCT through modulation of NCC and the WNK kinases, WNK1 and WNK4. Pharmacological blockade of nuclear Per1 entry resulted in decreased mRNA expression of NCC and WNK1 but increased expression of WNK4 in the renal cortex of mice. These findings were confirmed by using Per1 siRNA and pharmacological blockade of Per1 nuclear entry in mDCT15 cells, a model of the mouse distal convoluted tubule. Transcriptional regulation was demonstrated by changes in short lived heterogeneous nuclear RNA. Chromatin immunoprecipitation experiments demonstrated interaction of Per1 and CLOCK with the promoters of NCC, WNK1, and WNK4. This interaction was modulated by blockade of Per1 nuclear entry. Importantly, NCC protein expression and NCC activity, as measured by thiazide-sensitive, chloride-dependent (22)Na uptake, were decreased upon pharmacological inhibition of Per1 nuclear entry. Taken together, these data demonstrate a role for Per1 in the transcriptional regulation of NCC, WNK1, and WNK4.

Insulin increases the functional activity of the renal NaCl cotransporter

OBJECTIVES

Insulin is recognized to increase renal salt reabsorption in the distal nephron and hyperinsulinemic states have been shown to be associated with increased expression of the renal NaCl cotransporter (NCC). However, the effect of insulin on NCC functional activity has not been reported.

METHODS

Using a heterologous expression system of Xenopus laevis oocytes, a mouse distal convoluted cell line, mDCT15 cells, endogenously expressing NCC, and an ex-vivo kidney perfusion technique, we assessed the effect of insulin on the activity and phosphorylation of NCC. The signaling pathway involved was analyzed.

RESULTS

In Xenopus oocytes insulin increases the activity of NCC together with its phosphorylation at threonine residue 58. Activation of NCC by insulin was also observed in mDCT15 cells. Additionally, insulin increased the NCC phosphorylation in kidney under the ex-vivo perfusion technique. In oocytes and mDCT15 cells, insulin effect on NCC was prevented with inhibitors of phosphatidylinositol 3-kinase (PI3K), mTORC2, and AKT1 kinases, but not by inhibitors of MAP or mTORC1 kinases, suggesting that PI3K-mTORC2-AKT1 is the intracellular pathway required. Additionally, activation of NCC by insulin was not affected by wild-type or mutant versions of with no lysine kinase 1, with no lysine kinase 4, or serum glucocorticoid kinase 1, but it was no longer observed in the presence of wild-type or the dominant negative, catalytically inactive with no lysine kinase 3, implicating this kinase in the process.

CONCLUSION

Insulin induces activation and phosphorylation of NCC. This effect could play an important role in arterial hypertension associated with hyperinsulinemic states, such as obesity, metabolic syndrome, or type 2 diabetes mellitus.

Aldosterone acutely stimulates NCC activity via a SPAK-mediated pathway

Hypertension is a leading cause of morbidity and mortality worldwide, and disordered sodium balance has long been implicated in its pathogenesis. Aldosterone is perhaps the key regulator of sodium balance and thus blood pressure. The sodium chloride cotransporter (NCC) in the distal convoluted tubule of the kidney is a major site of sodium reabsorption and plays a key role in blood pressure regulation. Chronic exposure to aldosterone increases NCC protein expression and function. However, more acute effects of aldosterone on NCC are unknown. In our salt-abundant modern society where chronic salt deprivation is rare, understanding the acute effects of aldosterone is critical. Here, we examined the acute effects (12-36 h) of aldosterone on NCC in the rodent kidney and in a mouse distal convoluted tubule cell line. Studies demonstrated that aldosterone acutely stimulated NCC activity and phosphorylation without affecting total NCC abundance or surface expression. This effect was dependent upon the presence of the mineralocorticoid receptor and serum- and glucocorticoid-regulated kinase 1 (SGK1). Furthermore, STE20/SPS-1-related proline/alanine-rich kinase (SPAK) phosphorylation also increased, and gene silencing of SPAK eliminated the effect of aldosterone on NCC activity. Aldosterone administration via a minipump in adrenalectomized rodents confirmed an increase in NCC phosphorylation without a change in NCC total protein. These data indicate that acute aldosterone-induced SPAK-dependent phosphorylation of NCC increases individual transporter activity.

A new model of the distal convoluted tubule

The Na(+)-Cl(-) cotransporter (NCC) in the distal convoluted tubule (DCT) of the kidney is a key determinant of Na(+) balance. Disturbances in NCC function are characterized by disordered volume and blood pressure regulation. However, many details concerning the mechanisms of NCC regulation remain controversial or undefined. This is partially due to the lack of a mammalian cell model of the DCT that is amenable to functional assessment of NCC activity. Previously reported investigations of NCC regulation in mammalian cells have either not attempted measurements of NCC function or have required perturbation of the critical without a lysine kinase (WNK)/STE20/SPS-1-related proline/alanine-rich kinase regulatory pathway before functional assessment. Here, we present a new mammalian model of the DCT, the mouse DCT15 (mDCT15) cell line. These cells display native NCC function as measured by thiazide-sensitive, Cl(-)-dependent (22)Na(+) uptake and allow for the separate assessment of NCC surface expression and activity. Knockdown by short interfering RNA confirmed that this function was dependent on NCC protein. Similar to the mammalian DCT, these cells express many of the known regulators of NCC and display significant baseline activity and dimerization of NCC. As described in previous models, NCC activity is inhibited by appropriate concentrations of thiazides, and phorbol esters strongly suppress function. Importantly, they display release of WNK4 inhibition of NCC by small hairpin RNA knockdown. We feel that this new model represents a critical tool for the study of NCC physiology. The work that can be accomplished in such a system represents a significant step forward toward unraveling the complex regulation of NCC.

Parathyroid hormone (PTH) regulates the sodium chloride cotransporter via Ras guanyl releasing protein 1 (Ras-GRP1) and extracellular signal-regulated kinase (ERK)1/2 mitogen-activated protein kinase (MAPK) pathway

The sodium chloride cotransporter (NCC) is the principal salt absorptive pathway in the mammalian distal convoluted tubule (DCT) and is the site of action of thiazide diuretics. Using a mammalian cell model system to assess NCC function, we demonstrated previously that Ras guanyl releasing protein 1 (Ras-GRP1) mediates phorbol ester-induced suppression of the function and surface expression of NCC in a protein kinase C (PKC)-independent and extracellular signal-regulated kinase (ERK)1/2-dependent manner. Given that phorbol esters are functional analogs of diacylglycerol (DAG), this finding suggested a potential physiologic regulation of NCC by DAG. The parathyroid hormone (PTH) receptor is a G-protein-coupled receptor that is expressed in the DCT and activates PLC resulting in the generation of DAG. In this article, we demonstrate that PTH suppresses NCC function via a PLC/Ras-GRP1/ERK pathway. A functional assessment of NCC measuring thiazide-sensitive (22)Na(+) flux revealed that PTH suppresses NCC function. The inhibition of PLC prevented the suppression of NCC, indicating that PLC was necessary for this effect. Inhibitors of PKC and protein kinase A (PKA) had no effect on this suppression, but mitogen-activated protein kinase (MAPK) inhibitors prevented the PTH effect completely. Ras-GRP1 activates the MAPK pathway though activation of the small G-protein Ras. Gene silencing of Ras-GRP1 prevented the PTH-mediated suppression of NCC activity, the activation of the H-Ras isoform of Ras, and the activation of ERK1/2 MAPK. This finding confirmed the critical role of Ras-GRP1 in mediating the PTH-induced suppression of NCC activity through stimulation of the MAPK pathway.

Nedd4-2 modulates renal Na+-Cl- cotransporter via the aldosterone-SGK1-Nedd4-2 pathway

Regulation of renal Na(+) transport is essential for controlling blood pressure, as well as Na(+) and K(+) homeostasis. Aldosterone stimulates Na(+) reabsorption by the Na(+)-Cl(-) cotransporter (NCC) in the distal convoluted tubule (DCT) and by the epithelial Na(+) channel (ENaC) in the late DCT, connecting tubule, and collecting duct. Aldosterone increases ENaC expression by inhibiting the channel's ubiquitylation and degradation; aldosterone promotes serum-glucocorticoid-regulated kinase SGK1-mediated phosphorylation of the ubiquitin-protein ligase Nedd4-2 on serine 328, which prevents the Nedd4-2/ENaC interaction. It is important to note that aldosterone increases NCC protein expression by an unknown post-translational mechanism. Here, we present evidence that Nedd4-2 coimmunoprecipitated with NCC and stimulated NCC ubiquitylation at the surface of transfected HEK293 cells. In Xenopus laevis oocytes, coexpression of NCC with wild-type Nedd4-2, but not its catalytically inactive mutant, strongly decreased NCC activity and surface expression. SGK1 prevented this inhibition in a kinase-dependent manner. Furthermore, deficiency of Nedd4-2 in the renal tubules of mice and in cultured mDCT(15) cells upregulated NCC. In contrast to ENaC, Nedd4-2-mediated inhibition of NCC did not require the PY-like motif of NCC. Moreover, the mutation of Nedd4-2 at either serine 328 or 222 did not affect SGK1 action, and mutation at both sites enhanced Nedd4-2 activity and abolished SGK1-dependent inhibition. Taken together, these results suggest that aldosterone modulates NCC protein expression via a pathway involving SGK1 and Nedd4-2 and provides an explanation for the well-known aldosterone-induced increase in NCC protein expression.

RasGRP1 stimulation enhances ubiquitination and endocytosis of the sodium-chloride cotransporter

The sodium-chloride cotransporter (NCC) is the principal salt-absorptive pathway in the distal convoluted tubule. Recently, we described a novel pathway of NCC regulation in which phorbol esters (PE) stimulate Ras guanyl-releasing protein 1 (RasGRP1), triggering a cascade ultimately activating ERK1/2 MAPK and decreasing NCC cell surface expression (Ko B, Joshi LM, Cooke LL, Vazquez N, Musch MW, Hebert SC, Gamba G, Hoover RS. Proc Natl Acad Sci USA 104: 20120-20125, 2007). Little is known about the mechanisms which underlie these effects on NCC activity. Regulation of NCC via changes in NCC surface expression has been reported, but endocytosis of NCC has not been demonstrated. In this study, utilizing biotinylation, internalization assays, and a dynamin dominant-negative construct, we demonstrate that the regulation of NCC by PE occurs via an enhancement in internalization of NCC and is dynamin dependent. In addition, immunoprecipitation of NCC and subsequent immunoblotting for ubiquitin showed increased ubiquitination of NCC with phorbol ester treatment. MEK1/2 inhibitors and gene silencing of RasGRP1 indicated that this effect was dependent on RasGRP1 and ERK1/2 activation. Inhibition of ubiquitination prevents any PE-mediated decrease in NCC surface expression as measured by biotinylation or NCC activity as measured by radiotracer uptake. These findings confirmed that the PE effect on NCC is mediated by endocytosis of NCC. Furthermore, ubiquitination of NCC is essential for this process and this ubiquitination is dependent upon RasGRP1-mediated ERK1/2 activation.

cAMP-dependent activation of the renal-specific Na+-K+-2Cl- cotransporter is mediated by regulation of cotransporter trafficking

The murine apical bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransporter gene (mBSC1) exhibits two spliced isoform products that differ at the COOH-terminal domain. A long COOH-terminal isoform (L-mBSC1) encodes the Na(+)-K(+)-2Cl(-) cotransporter, and a short isoform (S-mBSC1) exerts a dominant-negative effect on L-mBSC1 cotransporter activity that is abrogated by cAMP. However, the mechanism of this dominant-negative effect was not clear. In this study, we used confocal microscopic analysis of an enhanced green fluorescent protein (EGFP) fusion construct (L-mBSC1-EGFP) expressed to characterize the surface expression of the L-BSC1 isoform in Xenopus laevis oocytes. Functional expression was also assessed in L-mBSC1-injected oocytes by measuring the bumetanide-sensitive (86)Rb(+) uptake. Oocytes injected with L-mBSC1-EGFP cRNA developed a distinct plasma membrane-associated fluorescence that colocalized with the fluorescent membrane dye FM 4-64. The fluorescence intensity in L-mBSC1-EGFP oocytes did not change after cAMP was added to the extracellular medium. In contrast, L-mBSC1-EGFP fluorescence intensity was reduced in a dose-dependent manner, with coexpression of S-mBSC1. The inhibitory effect of S-mBSC1 was abrogated by cAMP. Finally, the exocytosis inhibitor colchicine blocked the effect of cAMP on the L-mBSC1-EGFP/S-mBSC1-coinjected oocytes. All changes in L-mBSC1 surface expression correlated with modification of bumetanide-sensitive (86)Rb(+) uptake. Our data suggest that the dominant-negative effect of S-mBSC1 on L-mBSC1 transport function is due to the effects of the cotransporter on trafficking.

N-Glycosylation at two sites critically alters thiazide binding and activity of the rat thiazide-sensitive Na(+):Cl(-) cotransporter

The rat thiazide-sensitive Na-Cl cotransporter (rNCC) is expressed in the renal distal convoluted tubule and is the site of action of an important class of antihypertensive agents, the thiazide diuretics. The amino acid sequence contains two potential N-linked glycosylation consensus sites, N404 and N424. Either enzymatic deglycosylation or tunicamycin reduced the cotransporter to its core molecular weight (113 kD). Glycosylation site single mutants expressed in oocytes ran as thick bands at 115 kD, consistent with the high-mannose glycoprotein. The double mutant produced the single thin 113-kD band seen in the deglycosylated cotransporter. Functional expression of cotransporters in Xenopus laevis oocytes revealed that the mutants displayed drastically decreased thiazide-sensitive (22)Na(+) uptake compared with wild-type NCC. Analysis of enhanced green fluorescence protein (EGFP)-tagged cotransporters demonstrated that this decrease in function is predominantly secondary to decreased surface expression. The elimination of glycosylation in the double mutant increased thiazide sensitivity by more than two orders of magnitude and also increased Cl(-) affinity. Thus, we have demonstrated that rNCC is N-glycosylated in vivo at two sites, that glycosylation is essential for efficient function and surface expression of the cotransporter, and that the elimination of glycosylation allows much greater access of thiazide diuretics to their binding site.

Molecular pathogenesis of inherited hypertension with hyperkalemia: the Na-Cl cotransporter is inhibited by wild-type but not mutant WNK4

Mutations in the serine-threonine kinases WNK1 and WNK4 [with no lysine (K) at a key catalytic residue] cause pseudohypoaldosteronism type II (PHAII), a Mendelian disease featuring hypertension, hyperkalemia, hyperchloremia, and metabolic acidosis. Both kinases are expressed in the distal nephron, although the regulators and targets of WNK signaling cascades are unknown. The Cl(-) dependence of PHAII phenotypes, their sensitivity to thiazide diuretics, and the observation that they constitute a "mirror image" of the phenotypes resulting from loss of function mutations in the thiazide-sensitive Na-Cl cotransporter (NCCT) suggest that PHAII may result from increased NCCT activity due to altered WNK signaling. To address this possibility, we measured NCCT-mediated Na(+) influx and membrane expression in the presence of wild-type and mutant WNK4 by heterologous expression in Xenopus oocytes. Wild-type WNK4 inhibits NCCT-mediated Na-influx by reducing membrane expression of the cotransporter ((22)Na-influx reduced 50%, P < 1 x 10(-9), surface expression reduced 75%, P < 1 x 10(-14) in the presence of WNK4). This inhibition depends on WNK4 kinase activity, because missense mutations that abrogate kinase function prevent this effect. PHAII-causing missense mutations, which are remote from the kinase domain, also prevent inhibition of NCCT activity, providing insight into the pathophysiology of the disorder. The specificity of this effect is indicated by the finding that WNK4 and the carboxyl terminus of NCCT coimmunoprecipitate when expressed in HEK 293T cells. Together, these findings demonstrate that WNK4 negatively regulates surface expression of NCCT and implicate loss of this regulation in the molecular pathogenesis of an inherited form of hypertension.

The electroneutral cation-chloride cotransporters

Electroneutral cation-chloride cotransporters are widely expressed and perform a variety of physiological roles. A novel gene family of five members, encompassing a Na+-Cl- transporter, two Na+-K+-2Cl- transporters and two K+-Cl- cotransporters, encodes these membrane proteins; homologous genes have also been identified in a prokaryote and a number of lower eukaryotes. The cotransporter proteins share a common predicted membrane topology, with twelve putative transmembrane segments flanked by long hydrophilic N- and C-terminal cytoplasmic domains. The molecular identification of these transporters has had a significant impact on the study of their function, regulation and pathophysiology.