Cortical distal nephron Cl(-) transport in volume homeostasis and blood pressure regulation

The study of intercalated cells using ex vivo techniques: primary cell culture, cell lines, kidney slices, and organoids

This review summarizes methods to study kidney intercalated cell (IC) function ex vivo. While important for acid-base homeostasis, IC dysfunction is often not recognized clinically until it becomes severe. The advantage of using ex vivo techniques is that they allow for the differential evaluation of IC function in controlled environments. Although in vitro kidney tubular perfusion is a classical ex vivo technique to study IC, here we concentrate on primary cell cultures, immortalized cell lines, and ex vivo kidney slices. Ex vivo techniques are useful in evaluating IC signaling pathways that allow rapid responses to extracellular changes in pH, CO2, and bicarbonate (HCO3-). However, these methods for IC work can also be challenging, as cell lines that recapitulate IC do not proliferate easily in culture. Moreover, a "pure" IC population in culture does not necessarily replicate its collecting duct (CD) environment, where ICs are surrounded by the more abundant principal cells (PCs). It is reassuring that many findings obtained in ex vivo IC systems signaling have been largely confirmed in vivo. Some of these newly identified signaling pathways reveal that ICs are important for regulating NaCl reabsorption, thus suggesting new frontiers to target antihypertensive treatments. Moreover, recent single-cell characterization studies of kidney epithelial cells revealed a dual developmental origin of IC, as well as the presence of novel CD cell types with certain IC characteristics. These exciting findings present new opportunities for the study of IC ex vivo and will likely rediscover the importance of available tools in this field.NEW & NOTEWORTHY The study of kidney intercalated cells has been limited by current cell culture and kidney tissue isolation techniques. This review is to be used as a reference to select ex vivo techniques to study intercalated cells. We focused on the use of cell lines and kidney slices as potential useful models to study membrane transport proteins. We also review how novel collecting duct organoids may help better elucidate the role of these intriguing cells.

A comprehensive mouse kidney atlas enables rare cell population characterization and robust marker discovery

The kidney's cellular diversity is on par with its physiological intricacy; yet identifying cell populations and their markers remains challenging. Here, we created a comprehensive atlas of the healthy adult mouse kidney (MKA: Mouse Kidney Atlas) by integrating 140.000 cells and nuclei from 59 publicly available single-cell and single-nuclei RNA-sequencing datasets from eight independent studies. To harmonize annotations across datasets, we built a hierarchical model of the cell populations. Our model allows the incorporation of novel cell populations and the refinement of known profiles as more datasets become available. Using MKA and the learned model of cellular hierarchies, we predicted previously missing cell annotations from several studies. The MKA allowed us to identify reproducible markers across studies for poorly understood cell types and transitional states, which we verified using existing data from micro-dissected samples and spatial transcriptomics.

Modus operandi of ClC-K2 Cl- Channel in the Collecting Duct Intercalated Cells

The renal collecting duct is known to play a critical role in many physiological processes, including systemic water-electrolyte homeostasis, acid-base balance, and the salt sensitivity of blood pressure. ClC-K2 (ClC-Kb in humans) is a Cl^--permeable channel expressed on the basolateral membrane of several segments of the renal tubule, including the collecting duct intercalated cells. ClC-Kb mutations are causative for Bartters' syndrome type 3 manifested as hypotension, urinary salt wasting, and metabolic alkalosis. However, little is known about the significance of the channel in the collecting duct with respect to the normal physiology and pathology of Bartters' syndrome. In this review, we summarize the available experimental evidence about the signaling determinants of ClC-K2 function and the regulation by systemic and local factors as well as critically discuss the recent advances in understanding the collecting-duct-specific roles of ClC-K2 in adaptations to changes in dietary Cl^- intake and maintaining systemic acid-base homeostasis.

Potential of soluble (pro)renin receptor in kidney disease: can it go beyond a biomarker?

(Pro)renin receptor (PRR), also termed ATPase H+-transporting accessory protein 2 (ATP6AP2), is a type I transmembrane receptor and is capable of binding and activating prorenin and renin. Apart from its association with the renin-angiotensin system, PRR has been implicated in diverse developmental, physiological, and pathophysiological processes. Within the kidney, PRR is predominantly expressed in the distal nephron, particularly the intercalated cells, and activation of renal PRR contributes to renal injury in various rodent models of chronic kidney disease. Moreover, recent evidence demonstrates that PRR is primarily cleaved by site-1 protease to produce 28-kDa soluble PRR (sPRR). sPRR seems to mediate most of the known pathophysiological functions of renal PRR through modulating the activity of the intrarenal renin-angiotensin system and provoking proinflammatory and profibrotic responses. Not only does sPRR activate renin, but it also directly binds and activates the angiotensin II type 1 receptor. This review summarizes recent advances in understanding the roles and mechanisms of sPRR in the context of renal pathophysiology.

Identification of IQGAP1 as a SLC26A4 (Pendrin)-Binding Protein in the Kidney

Background: Several members of the SLC26A family of transporters, including SLC26A3 (DRA), SLC26A5 (prestin), SLC26A6 (PAT-1; CFEX) and SLC26A9, form multi-protein complexes with a number of molecules (e.g., cytoskeletal proteins, anchoring or adaptor proteins, cystic fibrosis transmembrane conductance regulator, and protein kinases). These interactions provide regulatory signals for these molecules. However, the identity of proteins that interact with the Cl^-/HCO₃ ^- exchanger, SLC26A4 (pendrin), have yet to be determined. The purpose of this study is to identify the protein(s) that interact with pendrin. Methods: A yeast two hybrid (Y2H) system was employed to screen a mouse kidney cDNA library using the C-terminal fragment of SLC26A4 as bait. Immunofluorescence microscopic examination of kidney sections, as well as co-immunoprecipitation assays, were performed using affinity purified antibodies and kidney protein extracts to confirm the co-localization and interaction of pendrin and the identified binding partners. Co-expression studies were carried out in cultured cells to examine the effect of binding partners on pendrin trafficking and activity. Results: The Y2H studies identified IQ motif-containing GTPase-activating protein 1 (IQGAP1) as a protein that binds to SLC26A4's C-terminus. Co-immunoprecipitation experiments using affinity purified anti-IQGAP1 antibodies followed by western blot analysis of kidney protein eluates using pendrin-specific antibodies confirmed the interaction of pendrin and IQGAP1. Immunofluorescence microscopy studies demonstrated that IQGAP1 co-localizes with pendrin on the apical membrane of B-intercalated cells, whereas it shows basolateral expression in A-intercalated cells in the cortical collecting duct (CCD). Functional and confocal studies in HEK-293 cells, as well as confocal studies in MDCK cells, demonstrated that the co-transfection of pendrin and IQGAP1 shows strong co-localization of the two molecules on the plasma membrane along with enhanced Cl^-/HCO₃ ^- exchanger activity. Conclusion: IQGAP1 was identified as a protein that binds to the C-terminus of pendrin in B-intercalated cells. IQGAP1 co-localized with pendrin on the apical membrane of B-intercalated cells. Co-expression of IQGAP1 with pendrin resulted in strong co-localization of the two molecules and increased the activity of pendrin in the plasma membrane in cultured cells. We propose that pendrin's interaction with IQGAP1 may play a critical role in the regulation of CCD function and physiology, and that disruption of this interaction could contribute to altered pendrin trafficking and/or activity in pathophysiologic states.

Angiotensin II increases activity of the ClC-K2 Cl- channel in collecting duct intercalated cells by stimulating production of reactive oxygen species

The renal collecting duct plays a critical role in setting urinary volume and composition, with principal cells transporting Na+ and K+ and intercalated cells mediating Cl- reabsorption. Published evidence implies Angiotensin II (Ang II) is a potent regulator of the collecting duct apical transport systems in response to systemic volume depletion. However, virtually nothing is known about Ang II actions on the basolateral conductance of principal and intercalated cells. Here, we combined macroscopic and single channel patch clamp recordings from freshly isolated mouse collecting ducts with biochemical and fluorescence methods to demonstrate an acute stimulation of the basolateral Cl- conductance and specifically the ClC-K2 Cl- channel by nanomolar Ang II concentrations in intercalated cells. In contrast, Ang II did not exhibit measurable effects on the basolateral conductance and on Kir4.1/5.1 potassium channel activity in principal cells. Although both Ang II receptors AT1 and AT2 are expressed in collecting duct cells, we show that AT1 receptors were essential for stimulatory actions of Ang II on ClC-K2. Moreover, AT1R-/- mice had decreased renal ClC-K2 expression. We further demonstrated that activation of NADPH oxidases is the major signaling pathway downstream of Ang II-AT1R that leads to stimulation of ClC-K2. Treatment of freshly isolated collecting ducts with Ang II led to production of reactive oxygen species on the same timescale as single channel ClC-K2 activation. Overall, we propose that Ang II-dependent regulation of ClC-K2 in intercalated cells is instrumental for stimulation of Cl- reabsorption by the collecting duct, particularly during hypovolemic states.

Simultaneous influence of sex and age on blood pressure difference between supine and sitting body positions

OBJECTIVES

Doctors' offices and outpatient departments typically measure blood pressure (BP) with the patient in a sitting position, whereas inpatient departments also use the supine position. As sex and age influence the autonomic function associated with BP regulation our study compared BP measurements in supine and sitting positions for men and women of different ages.

METHODS

We included 91 men and 118 women (≥18 years) without severe diseases. Hypertension was not an exclusion criterion because it is common in older persons. Mean left brachial BP and heart rate were determined by a digital sphygmomanometer in supine position as well as in sitting position before and after hand force measurement.

RESULTS

In a supine position women had slightly lower diastolic BP values than men. After sitting up, the diastolic BP increased in nearly all subjects. This increase was greater in women older than 50 years than for aged-matched men. In contrast to diastolic BP the systolic BP increased after sitting up in only two thirds of the subjects. Especially in women younger than 50 years the systolic BP often did not increase but decreased in response to postural change. The pulse pressure was mostly reduced after sitting up. This reduction was more pronounced in women than men independent of age and physical effort (i.e. hand force measurement). The sitting position also caused an increased heart rate, which was independent of sex and age.

CONCLUSION

Postural changes in the systolic and diastolic BP simultaneously depend on sex and age that needs to be considered for BP measurements in supine and sitting body positions.

The Renal Physiology of Pendrin-Positive Intercalated Cells

Intercalated cells (ICs) are found in the connecting tubule and the collecting duct. Of the three IC subtypes identified, type B intercalated cells are one of the best characterized and known to mediate Cl- absorption and HCO3- secretion, largely through the anion exchanger pendrin. This exchanger is thought to act in tandem with the Na+-dependent Cl-/HCO3- exchanger, NDCBE, to mediate net NaCl absorption. Pendrin is stimulated by angiotensin II and aldosterone administration via the angiotensin type 1a and the mineralocorticoid receptors, respectively. It is also stimulated in models of metabolic alkalosis, such as with NaHCO3 administration. In some rodent models, pendrin-mediated HCO3- secretion modulates acid-base balance. However, of probably more physiological or clinical significance is the role of these pendrin-positive ICs in blood pressure regulation, which occurs, at least in part, through pendrin-mediated renal Cl- absorption, as well as their effect on the epithelial Na+ channel, ENaC. Aldosterone stimulates ENaC directly through principal cell mineralocorticoid hormone receptor (ligand) binding and also indirectly through its effect on pendrin expression and function. In so doing, pendrin contributes to the aldosterone pressor response. Pendrin may also modulate blood pressure in part through its action in the adrenal medulla, where it modulates the release of catecholamines, or through an indirect effect on vascular contractile force. In addition to its role in Na+ and Cl- balance, pendrin affects the balance of other ions, such as K+ and I-. This review describes how aldosterone and angiotensin II-induced signaling regulate pendrin and the contribution of pendrin-positive ICs in the kidney to distal nephron function and blood pressure.

Metabolic Alkalosis: A Brief Pathophysiologic Review

Metabolic alkalosis is a very commonly encountered acid-base disorder that may be generated by a variety of exogenous and/or endogenous, pathophysiologic mechanisms. Multiple mechanisms are also responsible for the persistence, or maintenance, of metabolic alkalosis. Understanding these generation and maintenance mechanisms helps direct appropriate intervention and correction of this disorder. The framework utilized in this review is based on the ECF volume-centered approach popularized by Donald Seldin and Floyd Rector in the 1970s.  Although many subsequent scientific discoveries have advanced our understanding of the pathophysiology of metabolic alkalosis, that framework continues to be a valuable and relatively straightforward diagnostic and therapeutic model.

Intercalated Cells of the Kidney Collecting Duct in Kidney Physiology

The epithelium of the kidney collecting duct (CD) is composed mainly of two different types of cells with distinct and complementary functions. CD principal cells traditionally have been considered to have a major role in Na⁺ and water regulation, while intercalated cells (ICs) were thought to largely modulate acid-base homeostasis. In recent years, our understanding of IC function has improved significantly owing to new research findings. Thus, we now have a new model for CD transport that integrates mechanisms of salt and water reabsorption, K⁺ homeostasis, and acid-base status between principal cells and ICs. There are three main types of ICs (type A, type B, and non-A, non-B), which first appear in the late distal convoluted tubule or in the connecting segment in a species-dependent manner. ICs can be detected in CD from cortex to the initial part of the inner medulla, although some transport proteins that are key components of ICs also are present in medullary CD, cells considered inner medullary. Of the three types of ICs, each has a distinct morphology and expresses different complements of membrane transport proteins that translate into very different functions in homeostasis and contributions to CD luminal pro-urine composition. This review includes recent discoveries in IC intracellular and paracrine signaling that contributes to acid-base regulation as well as Na⁺, Cl^-, K⁺, and Ca²⁺ homeostasis. Thus, these new findings highlight the potential role of ICs as targets for potential hypertension treatments.

Aldosterone controls primary cilium length and cell size in renal collecting duct principal cells

Primary cilia are nonmotile sensory organelles found on the surface of almost all kidney tubule epithelial cells. Being exposed to the tubular lumen, primary cilia are thought to be chemo- and mechanosensors of luminal composition and flux, respectively. We hypothesized that, Na⁺ transport and primary cilia exist in a sensory functional connection in mature renal tubule epithelial cells. Our results demonstrate that primary cilium length is reduced in mineralocorticoid receptor (MR) knockout (KO) mice in a cell autonomous manner along the aldosterone-sensitive distal nephron (ADSN) compared with wild type (as µm ± SEM; 3.1 ± 0.2 vs 4.0 ± 0.1). In mouse cortical collecting duct (mCCD)_cl1 cells, which are a model of collecting duct (CD) principal cells, changes in Na⁺ transport intensity were found to mediate primary cilium length in response to aldosterone (as µm ± SEM: control: 2.7 ± 0.9 vs aldosterone treated: 3.8 ± 0.8). Cilium length was positively correlated with the availability of IFT88, a major intraflagellar anterograde transport complex B component, which is stabilized in response to exposure to aldosterone treatment. This suggests that the abundance of IFT88 is a regulated, rate limiting factor in the elongation of primary cilia. As previously observed in vivo, aldosterone treatment increased cell volume of cultured CD principal cells. Knockdown of IFT88 prevents ciliogenesis and inhibits the adaptive increase in cell size that was observed in response to aldosterone treatment. In conclusion, our results reveal a functional connection between Na⁺ transport, primary cilia, and cell size, which may play a key role in the morphological and functional adaptation of the CD to sustained changes in active Na⁺ reabsorption due to variations in aldosterone secretion.

Electrolyte transport in the renal collecting duct and its regulation by the renin-angiotensin-aldosterone system

Distal nephron of the kidney plays key roles in fluid volume and electrolyte homeostasis by tightly regulating reabsorption and excretion of Na⁺, K⁺, and Cl^- Studies to date demonstrate the detailed electrolyte transport mechanisms in principal cells of the cortical collecting duct, and their regulation by renin-angiotensin-aldosterone system (RAAS). In recent years, however, accumulating data indicate that intercalated cells, another cell type that is present in the cortical collecting duct, also play active roles in the regulation of blood pressure. Notably, pendrin in β-intercalated cells not only controls acid/base homeostasis, but is also one of the key components controlling salt and K⁺ transport in distal nephron. We have recently shown that pendrin is regulated by the co-ordinated action of angiotensin II (AngII) and aldosterone, and at the downstream of AngII, mammalian target of rapamycin (mTOR) signaling regulates pendrin through inhibiting the kinase unc51-like-kinase 1 and promoting dephosphorylation of mineralocorticoid receptor (MR). In this review, we summarize recent advances in the current knowledge on the salt transport mechanisms in the cortical collecting duct, and their regulation by the RAAS.

"I don't get no respect": the role of chloride in acute kidney injury

Acute kidney injury (AKI) is a major public health problem that complicates 10-40% of hospital admissions. Importantly, AKI is independently associated with increased risk of progression to chronic kidney disease, end-stage renal disease, cardiovascular events, and increased risk of in-hospital and long-term mortality. The chloride content of intravenous fluid has garnered much attention over the last decade, as well as its association with excess use and adverse outcomes, including AKI. Numerous studies show that changes in serum chloride concentration, independent of serum sodium and bicarbonate, are associated with increased risk of AKI, morbidity, and mortality. This comprehensive review details the complex renal physiology regarding the role of chloride in regulating renal blood flow, glomerular filtration rate, tubuloglomerular feedback, and tubular injury, as well as the findings of clinical research related to the chloride content of intravenous fluids, changes in serum chloride concentration, and AKI. Chloride is underappreciated in both physiology and pathophysiology. Although the exact mechanism is debated, avoidance of excessive chloride administration is a reasonable treatment option for all patients and especially in those at risk for AKI. Therefore, high-risk patients and those with "incipient" AKI should receive balanced solutions rather than normal saline to minimize the risk of AKI.

Nephrolithiasis secondary to inherited defects in the thick ascending loop of henle and connecting tubules

Twin and genealogy studies suggest a strong genetic component of nephrolithiasis. Likewise, urinary traits associated with renal stone formation were found to be highly heritable, even after adjustment for demographic, anthropometric and dietary covariates. Recent high-throughput sequencing projects of phenotypically well-defined cohorts of stone formers and large genome-wide association studies led to the discovery of many new genes associated with kidney stones. The spectrum ranges from infrequent but highly penetrant variants (mutations) causing mendelian forms of nephrolithiasis (monogenic traits) to common but phenotypically mild variants associated with nephrolithiasis (polygenic traits). About two-thirds of the genes currently known to be associated with nephrolithiasis code for membrane proteins or enzymes involved in renal tubular transport. The thick ascending limb of Henle and connecting tubules are of paramount importance for renal water and electrolyte handling, urinary concentration and maintenance of acid-base homeostasis. In most instances, pathogenic variants in genes involved in thick ascending limb of Henle and connecting tubule function result in phenotypically severe disease, frequently accompanied by nephrocalcinosis with progressive CKD and to a variable degree by nephrolithiasis. The aim of this article is to review the current knowledge on kidney stone disease associated with inherited defects in the thick ascending loop of Henle and the connecting tubules. We also highlight recent advances in the field of kidney stone genetics that have implications beyond rare disease, offering new insights into the most common type of kidney stone disease, i.e., idiopathic calcium stone disease.

Hyperkalemic Forms of Renal Tubular Acidosis: Clinical and Pathophysiological Aspects

In contrast to distal type I or classic renal tubular acidosis (RTA) that is associated with hypokalemia, hyperkalemic forms of RTA also occur usually in the setting of mild-to-moderate CKD. Two pathogenic types of hyperkalemic metabolic acidosis are frequently encountered in adults with underlying CKD. One type, which corresponds to some extent to the animal model of selective aldosterone deficiency (SAD) created experimentally by adrenalectomy and glucocorticoid replacement, is manifested in humans by low plasma and urinary aldosterone levels, reduced ammonium excretion, and preserved ability to lower urine pH below 5.5. This type of hyperkalemic RTA is also referred to as type IV RTA. It should be noted that the mere deficiency of aldosterone when glomerular filtration rate is completely normal only causes a modest decline in plasma bicarbonate which emphasizes the importance of reduced glomerular filtration rate in the development of the hyperchloremic metabolic acidosis associated with SAD. Another type of hyperkalemic RTA distinctive from SAD in which plasma aldosterone is not reduced is referred to as hyperkalemic distal renal tubular acidosis because urine pH cannot be reduced despite acidemia or after provocative tests aimed at increasing sodium-dependent distal acidification such as the administration of sodium sulfate or loop diuretics with or without concurrent mineralocorticoid administration. This type of hyperkalemic RTA (also referred to as voltage-dependent distal renal tubular acidosis) has been best described in patients with obstructive uropathy and resembles the impairment in both hydrogen ion and potassium secretion that are induced experimentally by urinary tract obstruction and when sodium transport in the cortical collecting tubule is blocked by amiloride.

The Role of Intercalated Cell Nedd4-2 in BP Regulation, Ion Transport, and Transporter Expression

BackgroundNedd4-2 is an E3 ubiquitin-protein ligase that associates with transport proteins, causing their ubiquitylation, and then internalization and degradation. Previous research has suggested a correlation between Nedd4-2 and BP. In this study, we explored the effect of intercalated cell (IC) Nedd4-2 gene ablation on IC transporter abundance and function and on BP.Methods We generated IC Nedd4-2 knockout mice using Cre-lox technology and produced global pendrin/Nedd4-2 null mice by breeding global Nedd4-2 null (Nedd4-2^-/- ) mice with global pendrin null (Slc26a4^-/- ) mice. Mice ate a diet with 1%-4% NaCl; BP was measured by tail cuff and radiotelemetry. We measured transepithelial transport of Cl^- and total CO₂ and transepithelial voltage in cortical collecting ducts perfused in vitro Transporter abundance was detected with immunoblots, immunohistochemistry, and immunogold cytochemistry.Results IC Nedd4-2 gene ablation markedly increased electroneutral Cl^-/HCO₃^- exchange in the cortical collecting duct, although benzamil-, thiazide-, and bafilomycin-sensitive ion flux changed very little. IC Nedd4-2 gene ablation did not increase the abundance of type B IC transporters, such as AE4 (Slc4a9), H⁺-ATPase, barttin, or the Na⁺-dependent Cl^-/HCO₃^- exchanger (Slc4a8). However, IC Nedd4-2 gene ablation increased CIC-5 total protein abundance, apical plasma membrane pendrin abundance, and the ratio of pendrin expression on the apical membrane to the cytoplasm. IC Nedd4-2 gene ablation increased BP by approximately 10 mm Hg. Moreover, pendrin gene ablation eliminated the increase in BP observed in global Nedd4-2 knockout mice.Conclusions IC Nedd4-2 regulates Cl^-/HCO₃^- exchange in ICs., Nedd4-2 gene ablation increases BP in part through its action in these cells.

α-Ketoglutarate drives electroneutral NaCl reabsorption in intercalated cells by activating a G-protein coupled receptor, Oxgr1

PURPOSE OF REVIEW

This review describes the recent discoveries about a powerful electroneutral NaCl reabsorption mechanism in intercalated cells, and its regulation by an intrarenal metabolite paracrine, α-ketoglutartate, and the G-protein coupled receptor, Oxgr1.

RECENT FINDINGS

The distal nephron fine-tunes sodium, chloride, potassium, hydrogen, bicarbonate and water transport to maintain electrolyte homeostasis and blood pressure. Intercalated cells have been traditionally viewed as the professional regulators of acid-base balance, but recent studies reveal that a specific population of intercalated cells, identified by the pendrin-transporter, have a surprising role in the regulation of salt balance. The pendrin-positive intercalated cells (PP-ICs) facilitate electroneutral NaCl reabsorption through the cooperative activation of multitransport protein network. α-Ketoglutartate is synthesized and secreted into the proximal tubule lumen in the combined state of metabolic alkalosis and intravascular volume contraction to activate Oxgr1 in PP-IC, which in turn activates the multitransport protein network to drive salt reabsorption and bicarbonate secretion by these cells.

SUMMARY

Recent studies identify a novel salt transport pathway in intercalated cells that is activated by an intrarenal paracrine system, α-ketoglutartate/Oxgr1. Activation of the paracrine system and transport pathway, particularly during alkalosis and volume contraction, mitigates deleterious salt wasting while restoring acid-base balance.

α-Ketoglutarate stimulates pendrin-dependent Cl- absorption in the mouse CCD through protein kinase C

α-Ketoglutarate (α-KG) is a citric acid cycle intermediate and a glutamine catabolism product. It is also the natural ligand of 2-oxoglutarate receptor 1 (OXGR1), a G_q protein-coupled receptor expressed on the apical membrane of intercalated cells. In the cortical collecting duct (CCD), Cl^-/[Formula: see text] exchange increases upon α-KG binding to the OXGR1. To determine the signaling pathway(s) by which α-KG stimulates Cl^- absorption, we examined α-KG-stimulated Cl^- absorption in isolated perfused mouse CCDs. α-KG increased electroneutral Cl^- absorption in CCDs from wild-type mice but had no effect on Cl^- absorption in pendrin knockout mice. Because G_q protein-coupled receptors activate PKC, we hypothesized that α-KG stimulates Cl^- absorption through PKC. If so, PKC agonists should mimic, whereas PKC inhibitors should abolish, α-KG-stimulated Cl^- absorption. Like α-KG, PKC agonist (phorbol-12,13-dibutyrate, 500 nM) application increased Cl^- absorption in wild-type but not in pendrin null CCDs. Moreover, PKC inhibitors (2.5 mM GF109203X and 20 nM calphostin C), Ca²⁺ chelators (BAPTA, 10-20 μM), or PKC-α or -δ gene ablation eliminated α-KG-stimulated Cl^- absorption. We have shown that STE20/SPS-1-related proline-alanine-rich protein kinase (SPAK) gene ablation increases urinary α-KG excretion, renal pendrin abundance, and CCD Cl^- absorption. However, in SPAK null CCDs, Cl^- absorption was not activated further by luminal α-KG application nor was Cl^- absorption reduced with the PKC inhibitor GF109203 . Thus SPAK gene ablation likely acts through a PKC-independent pathway to produce a chronic adaptive increase in pendrin function. In conclusion, α-KG stimulates pendrin-dependent Cl^-/[Formula: see text] exchange through a mechanism dependent on PKC and Ca²⁺ that involves PKC-α and PKC-δ.

CaSR signaling down-regulates AQP2 expression via a novel microRNA pathway in pendrin and NaCl cotransporter knockout mice

High concentrations of urinary calcium counteract vasopressin action via the activation of the calcium-sensing receptor (CaSR) that is expressed in the luminal membrane of collecting duct cells, which impairs the trafficking of aquaporin-2 (AQP2). Pendrin/NaCl cotransporter double-knockout (dKO) mice display significant calcium wasting and develop severe volume depletion, despite increased circulating vasopressin levels. We hypothesized that the CaSR-mediated impairment of AQP2 expression/trafficking underlies vasopressin resistance in dKO mice. Compared with wild-type mice, in renal inner medulla, dKO mice had reduced total AQP2 sensitive to proteasome inhibitors, higher levels of AQP2-pS261, ubiquitinated AQP2, and p38-MAPK, an enzyme that is activated by CaSR signaling and known to phosphorylate AQP2 at Ser261. CaSR inhibition with the calcilytic NPS2143 reversed these effects, which indicates that CaSR mediates the up-regulation of AQP2-pS261, ubiquitination, and degradation. Of note, dKO mice demonstrated significantly higher AQP2-targeting miRNA-137 that was reduced upon CaSR inhibition, supporting a critical role for CaSR in the down-regulation of AQP2 expression. Our data indicate that CaSR signaling reduces AQP2 abundance both via AQP2-targeting miRNA-137 and the p38-MAPK/AQP2-pS261/ubiquitination/proteasomal axis. These effects may contribute to the reduced renal concentrating ability that has been observed in dKO mice and underscore a physiologic mechanism of the CaSR-dependent regulation of AQP2 abundance via a novel microRNA pathway.-Ranieri, M., Zahedi, K., Tamma, G., Centrone, M., Di Mise, A., Soleimani, M., Valenti, G. CaSR signaling down-regulates AQP2 expression via a novel microRNA pathway in pendrin and NaCl cotransporter knockout mice.

Renal intercalated cells and blood pressure regulation

Type B and non-A, non-B intercalated cells are found within the connecting tubule and the cortical collecting duct. Of these cell types, type B intercalated cells are known to mediate Cl- absorption and HCO3- secretion largely through pendrin-dependent Cl-/HCO3- exchange. This exchange is stimulated by angiotensin II administration and is also stimulated in models of metabolic alkalosis, for instance after aldosterone or NaHCO3 administration. In some rodent models, pendrin-mediated HCO3- secretion modulates acid-base balance. However, the role of pendrin in blood pressure regulation is likely of more physiological or clinical significance. Pendrin regulates blood pressure not only by mediating aldosterone-sensitive Cl- absorption, but also by modulating the aldosterone response for epithelial Na+ channel (ENaC)-mediated Na+ absorption. Pendrin regulates ENaC through changes in open channel of probability, channel surface density, and channels subunit total protein abundance. Thus, aldosterone stimulates ENaC activity through both direct and indirect effects, the latter occurring through its stimulation of pendrin expression and function. Therefore, pendrin contributes to the aldosterone pressor response. Pendrin may also modulate blood pressure in part through its action in the adrenal medulla, where it modulates the release of catecholamines, or through an indirect effect on vascular contractile force. This review describes how aldosterone and angiotensin II-induced signaling regulate pendrin and the contributory role of pendrin in distal nephron function and blood pressure.

Dietary K+ and Cl- independently regulate basolateral conductance in principal and intercalated cells of the collecting duct

The renal collecting duct contains two distinct cell types, principal and intercalated cells, expressing potassium K_ir4.1/5.1 (KCNJ10/16) and chloride ClC-K2 (ClC-Kb in humans) channels on their basolateral membrane, respectively. Both channels are thought to play important roles in controlling systemic water-electrolyte balance and blood pressure. However, little is known about mechanisms regulating activity of K_ir4.1/5.1 and ClC-K2/b. Here, we employed patch clamp analysis at the single channel and whole cell levels in freshly isolated mouse collecting ducts to investigate regulation of K_ir4.1/5.1 and ClC-K2/b by dietary K⁺ and Cl^- intake. Treatment of mice with high K⁺ and high Cl^- diet (6% K⁺, 5% Cl^-) for 1 week significantly increased basolateral K⁺-selective current, single channel K_ir4.1/5.1 activity and induced hyperpolarization of basolateral membrane potential in principal cells when compared to values in mice on a regular diet (0.9% K⁺, 0.5% Cl^-). In contrast, basolateral Cl^--selective current and single channel ClC-K2/b activity was markedly decreased in intercalated cells under this condition. Substitution of dietary K⁺ to Na⁺ in the presence of high Cl^- exerted a similar inhibiting action of ClC-K2/b suggesting that the channel is sensitive to variations in dietary Cl^- per se. Cl^--sensitive with-no-lysine kinase (WNK) cascade has been recently proposed to orchestrate electrolyte transport in the distal tubule during variations of dietary K⁺. However, co-expression of WNK1 or its major downstream effector Ste20-related proline-alanine-rich kinase (SPAK) had no effect on ClC-Kb over-expressed in Chinese hamster ovary (CHO) cells. Treatment of mice with high K⁺ diet without concomitant elevations in dietary Cl^- (6% K⁺, 0.5% Cl^-) elicited a comparable increase in basolateral K⁺-selective current, single channel K_ir4.1/5.1 activity in principal cells, but had no significant effect on ClC-K2/b activity in intercalated cells. Furthermore, stimulation of aldosterone signaling by Deoxycorticosterone acetate (DOCA) recapitulated the stimulatory actions of high K⁺ intake on K_ir4.1/5.1 channels in principal cells but was ineffective to alter ClC-K2/b activity and basolateral Cl^- conductance in intercalated cells. In summary, we report that variations of dietary K⁺ and Cl^- independently regulate basolateral potassium and chloride conductance in principal and intercalated cells. We propose that such discrete mechanism might contribute to fine-tuning of urinary excretion of electrolytes depending on dietary intake.

Emerging Targets of Diuretic Therapy

Diuretics are commonly prescribed for treatment in patients with hypertension, edema, or heart failure. Studies on hypertensive and salt-losing disorders and on urea transporters have contributed to better understanding of mechanisms of renal salt and water reabsorption and their regulation. Proteins involved in the regulatory pathways are emerging targets for diuretic and aquaretic therapy. Integrative high-throughput screening, protein structure analysis, and chemical modification have identified promising agents for preclinical testing in animals. These include WNK-SPAK inhibitors, ClC-K channel antagonists, ROMK channel antagonists, and pendrin and urea transporter inhibitors. We discuss the potential advantages and side effects of these potential diuretics.

30 YEARS OF THE MINERALOCORTICOID RECEPTOR: Mineralocorticoid receptor and NaCl transport mechanisms in the renal distal nephron

A key role of aldosterone and mineralocorticoid receptor is to regulate fluid volume and K⁺ homeostasis in the body by acting on the renal distal nephron. Global responses of the kidney to elevated aldosterone levels are determined by the coordinate action of different constituent tubule cells, including principal cells, intercalated cells and distal convoluted tubule cells. Recent studies on genetic mutations causing aldosterone overproduction have identified the molecules involved in aldosterone biosynthesis in the adrenal gland, and there is also increasing evidence for mechanisms and signaling pathways regulating the balance between renal NaCl reabsorption and K⁺ secretion, the two major effects of aldosterone. In particular, recent studies have demonstrated that mineralocorticoid receptor in intercalated cells is selectively regulated by phosphorylation, which prevents ligand binding and activation. Moreover, the ubiquitin ligase complex composed of Kelch-like 3 and Cullin 3 acts downstream of angiotensin II and plasma K⁺ alterations, regulating Na-Cl cotransporter independently of aldosterone in distal convoluted tubule cells. These and other effects are integrated to produce appropriate kidney responses in a high-aldosterone state, and are implicated in fluid and electrolyte disorders in humans. This review summarizes the current knowledge on mechanisms modulating mineralocorticoid receptor and its downstream effectors in the distal nephron.

Hypokalemia and Pendrin Induction by Aldosterone

Aldosterone plays an important role in regulating Na-Cl reabsorption and blood pressure. Epithelial Na⁺ channel, Na⁺-Cl^- cotransporter, and Cl^-/HCO₃^- exchanger pendrin are the major mediators of Na-Cl transport in the aldosterone-sensitive distal nephron. Existing evidence also suggests that plasma K⁺ concentration affects renal Na-Cl handling. In this study, we posited that hypokalemia modulates the effects of aldosterone on pendrin in hyperaldosteronism. Chronic aldosterone infusion in mice increased pendrin levels at the plasma membrane, and correcting hypokalemia in this model almost completely blocked pendrin upregulation. However, hypokalemia induced by a low-K⁺ diet resulted in pendrin downregulation along with reduced plasma aldosterone levels, indicating that both hypokalemia and aldosterone excess are necessary for pendrin induction. In contrast, decreased plasma K⁺ levels were sufficient to increase Na⁺-Cl^- cotransporter levels. We found that phosphorylation of mineralocorticoid receptor that prevents aldosterone binding in intercalated cells was suppressed by hypokalemia, which resulted in enhanced pendrin response to aldosterone, explaining the coordinated action of aldosterone and hypokalemia in pendrin regulation. Finally, to address the physiological significance of our observations, we administered aldosterone to mice lacking pendrin. Notably, plasma K⁺ levels were significantly lower in pendrin knockout mice (2.7±0.1 mmol/L) than in wild-type mice (3.0±0.1 mmol/L) after aldosterone infusion, demonstrating that pendrin alleviates hypokalemia in a state of aldosterone excess. These data indicate that the decreased plasma K⁺ levels promote pendrin induction by aldosterone, which, in concert with Na⁺-Cl^- cotransporter, counteracts the progression of hypokalemia but promotes hypertension in primary aldosterone excess.

Glucocorticoid-induced leucine zipper protein regulates sodium and potassium balance in the distal nephron

Glucocorticoid induced leucine zipper protein (GILZ) is an aldosterone-regulated protein that controls sodium transport in cultured kidney epithelial cells. Mice lacking GILZ have been reported previously to have electrolyte abnormalities. However, the mechanistic basis has not been explored. Here we provide evidence supporting a role for GILZ in modulating the balance of renal sodium and potassium excretion by regulating the sodium-chloride cotransporter (NCC) activity in the distal nephron. Gilz^-/- mice have a higher plasma potassium concentration and lower fractional excretion of potassium than wild type mice. Furthermore, knockout mice are more sensitive to NCC inhibition by thiazides than are the wild type mice, and their phosphorylated NCC expression is higher. Despite increased NCC activity, knockout mice do not have higher blood pressure than wild type mice. However, during sodium deprivation, knockout mice come into sodium balance more quickly, than do the wild type, without a significant increase in plasma renin activity. Upon prolonged sodium restriction, knockout mice develop frank hyperkalemia. Finally, in HEK293T cells, exogenous GILZ inhibits NCC activity at least in part by inhibiting SPAK phosphorylation. Thus, GILZ promotes potassium secretion by inhibiting NCC and enhancing distal sodium delivery to the epithelial sodium channel. Additionally, Gilz^-/- mice have features resembling familial hyperkalemic hypertension, a human disorder that manifests with hyperkalemia associated variably with hypertension.

New perspective of ClC-Kb/2 Cl- channel physiology in the distal renal tubule

Since its identification as the underlying molecular cause of Bartter's syndrome type 3, ClC-Kb (ClC-K2 in rodents, henceforth it will be referred as ClC-Kb/2) is proposed to play an important role in systemic electrolyte balance and blood pressure regulation by controlling basolateral Cl(-) exit in the distal renal tubular segments from the cortical thick ascending limb to the outer medullary collecting duct. Considerable experimental and clinical effort has been devoted to the identification and characterization of disease-causing mutations as well as control of the channel by its cofactor, barttin. However, we have only begun to unravel the role of ClC-Kb/2 in different tubular segments and to reveal the regulators of its expression and function, e.g., insulin and IGF-1. In this review we discuss recent experimental evidence in this regard and highlight unexplored questions critical to understanding ClC-Kb/2 physiology in the kidney.

Small-Molecule Inhibitors of Pendrin Potentiate the Diuretic Action of Furosemide

Pendrin is a Cl^-/HCO₃^- exchanger expressed in type B and non-A, non-B intercalated cells in the distal nephron, where it facilitates Cl^- absorption and is involved in Na⁺ absorption and acid-base balance. Pendrin-knockout mice show no fluid-electrolyte abnormalities under baseline conditions, although mice with double knockout of pendrin and the Na⁺/Cl^- cotransporter (NCC) manifest profound salt wasting. Thus, pendrin may attenuate diuretic-induced salt loss, but this function remains unconfirmed. To clarify the physiologic role of pendrin under conditions not confounded by gene knockout, and to test the potential utility of pendrin inhibitors for diuretic therapy, we tested in mice a small-molecule pendrin inhibitor identified from a high-throughput screen. In vitro, a pyrazole-thiophenesulfonamide, PDS_inh-C01, inhibited Cl^-/anion exchange mediated by mouse pendrin with a 50% inhibitory concentration of 1-3 µM, without affecting other major kidney tubule transporters. Administration of PDS_inh-C01 to mice at predicted therapeutic doses, determined from serum and urine pharmacokinetics, did not affect urine output, osmolality, salt excretion, or acid-base balance. However, in mice treated acutely with furosemide, administration of PDS_inh-C01 produced a 30% increase in urine output, with increased Na⁺ and Cl^- excretion. In mice treated long term with furosemide, in which renal pendrin is upregulated, PDS_inh-C01 produced a 60% increase in urine output. Our findings clarify the role of pendrin in kidney function and suggest pendrin inhibition as a novel approach to potentiate the action of loop diuretics. Such combination therapy might enhance diuresis and salt excretion for treatment of hypertension and edema, perhaps including diuretic-resistant edema.

Insulin and IGF-1 activate Kir4.1/5.1 channels in cortical collecting duct principal cells to control basolateral membrane voltage

Potassium Kir4.1/5.1 channels are abundantly expressed at the basolateral membrane of principal cells in the cortical collecting duct (CCD), where they are thought to modulate transport rates by controlling transepithelial voltage. Insulin and insulin-like growth factor-1 (IGF-1) stimulate apically localized epithelial sodium channels (ENaC) to augment sodium reabsorption in the CCD. However, little is known about their actions on potassium channels localized at the basolateral membrane. In this study, we implemented patch-clamp analysis in freshly isolated murine CCD to assess the effect of these hormones on Kir4.1/5.1 at both single channel and cellular levels. We demonstrated that K(+)-selective conductance via Kir4.1/5.1 is the major contributor to the macroscopic current recorded from the basolateral side in principal cells. Acute treatment with 10 μM amiloride (ENaC blocker), 100 nM tertiapin-Q (TPNQ; ROMK inhibitor), and 100 μM ouabain (Na(+)-K(+)-ATPase blocker) failed to produce a measurable effect on the macroscopic current. In contrast, Kir4.1 inhibitor nortriptyline (100 μM), but not fluoxetine (100 μM), virtually abolished whole cell K(+)-selective conductance. Insulin (100 nM) markedly increased the open probability of Kir4.1/5.1 and nortriptyline-sensitive whole cell current, leading to significant hyperpolarization of the basolateral membrane. Inhibition of the phosphatidylinositol 3-kinase cascade with LY294002 (20 μM) abolished action of insulin on Kir4.1/5.1. IGF-1 had similar stimulatory actions on Kir4.1/5.1-mediated conductance only when applied at a higher (500 nM) concentration and was ineffective at 100 nM. We concluded that both insulin and, to a lesser extent, IGF-1 activate Kir4.1/5.1 channel activity and open probability to hyperpolarize the basolateral membrane, thereby facilitating Na(+) reabsorption in the CCD.

Context-dependent mechanisms modulating aldosterone signaling in the kidney

The aldosterone-mineralocorticoid receptor (MR) system serves as the major regulator of fluid homeostasis, and is an important drug target for the treatment of hypertension, heart failure, and chronic kidney disease. While the ligand aldosterone plays a central role in facilitating MR activity, recent studies have revealed that MR signaling is modulated through distinct mechanisms at the levels of the receptor and the downstream targets. Notably, phosphorylation of the ligand-binding domain in MR regulates the ability of the receptor to bind to ligand in renal intercalated cells, providing an additional layer of regulation that allows the cell-selective control of MR signaling. These mechanisms are involved in the context-dependent effects of aldosterone in the distal nephron. In this article, the recent progress in the understanding of mechanisms regulating the action of aldosterone is discussed, focusing on the connecting tubules and collecting duct in the kidney.

The role of pendrin in blood pressure regulation

Pendrin is a Na(+)-independent Cl(-)/HCO3(-) exchanger found in the apical regions of type B and non-A, non-B intercalated cells within the aldosterone-sensitive region of the nephron, i.e., the distal convoluted tubule (DCT), the connecting tubule (CNT), and the cortical collecting duct (CCD). Type B intercalated cells mediate Cl(-) absorption and HCO3(-) secretion primarily through pendrin-mediated Cl(-)/HCO3(-) exchange. This exchanger is upregulated with angiotensin II administration and in models of metabolic alkalosis, such as following administration of aldosterone or NaHCO3. In the absence of pendrin-mediated HCO3(-) secretion, an enhanced alkalosis is observed following aldosterone or NaHCO3 administration. However, probably of more significance is the role of pendrin in the pressor response to aldosterone. Pendrin mediates Cl(-) absorption and modulates aldosterone-induced Na(+) absorption mediated by the epithelial Na channel (ENaC). Pendrin changes ENaC activity by changing both channel open probability (Po) and surface density (N), at least partly by altering luminal HCO3(-) and ATP concentration. Thus aldosterone and angiotensin II stimulate pendrin expression and function, which stimulates ENaC activity, thereby contributing to the pressor response of these hormones. However, pendrin may modulate blood pressure partly through its extrarenal effects. For example, pendrin is expressed in the adrenal medulla, where it modulates catecholamine release. The increase in catecholamine release observed with pendrin gene ablation likely contributes to the increment in vascular contractile force observed in the pendrin null mouse. This review summarizes the signaling mechanisms that regulate pendrin abundance and function as well as the contribution of pendrin to distal nephron function.

The role of pendrin in renal physiology

Pendrin is a Na(+)-independent Cl(-)/HCO3(-) exchanger that localizes to type B and non-A, non-B intercalated cells, which are expressed within the aldosterone-sensitive region of the nephron, i.e., the distal convoluted tubule, the connecting tubule, and the cortical collecting duct. Type B cells mediate Cl(-) absorption and HCO3(-) secretion primarily through pendrin-mediated Cl(-)/HCO3(-) exchange. At least in some treatment models, pendrin acts in tandem with the Na(+)-dependent Cl(-)/HCO3(-) exchanger (NDCBE) encoded by Slc4a8 to mediate NaCl absorption. The pendrin-mediated Cl(-)/HCO3(-) exchange process is greatly upregulated in models of metabolic alkalosis, such as following aldosterone administration or dietary NaHCO3 loading. It is also upregulated by angiotensin II. In the absence of pendrin [Slc26a4 (-/-) or pendrin null mice], aldosterone-stimulated NaCl absorption is reduced, which lowers the blood pressure response to aldosterone and enhances the alkalosis that follows the administration of this steroid hormone. Pendrin modulates aldosterone-induced Na(+) absorption by changing ENaC abundance and function through a kidney-specific mechanism that does not involve changes in the concentration of a circulating hormone. Instead, pendrin changes ENaC abundance and function at least in part by altering luminal HCO3(-) and ATP concentrations. Thus, aldosterone and angiotensin II also stimulate pendrin expression and function, which likely contributes to the pressor response of these hormones. This review summarizes the contribution of the Cl(-)/HCO3(-) exchanger pendrin in distal nephron function.

Pappa2 is linked to salt-sensitive hypertension in Dahl S rats

A 1.37 Mbp region of chromosome 13 previously identified by exclusion mapping was consistently associated with a reduction of salt-induced hypertension in the Dahl salt-sensitive (SS) rat. This region contained five genes that were introgressed from the salt-insensitive Brown Norway (BN) rat. The goal of the present study was to further narrow that region to identify the gene(s) most likely to protect from salt-induced hypertension. The studies yielded a subcongenic SS rat strain containing a 0.71 Mbp insert from BN (26-P strain) in which salt-induced hypertension was reduced by 24 mmHg. The region contained two protein-coding genes (Astn1 and Pappa2) and a microRNA (miR-488). Pappa2 mRNA in the renal cortex of the protected 26-P was 6- to 10-fold greater than in SS fed a 0.4% NaCl diet but was reduced to levels observed in SS when fed 8.0% NaCl diet for 7 days. Compared with brain nuclei (NTS, RVLM, CVLM) and the adrenal gland, Pappa2 in the renal cortex was the only gene found to be differentially expressed between SS and 26-P and that responded to changes of salt diet. Immunohistochemistry studies found Pappa2 localized in the cytosol of the epithelial cells of the cortical thick ascending limbs. In more distal segments of the renal tubules, it was observed within tubular lumens and most notably bound to the apical membranes of the intercalated cells of collecting ducts. We conclude that we have identified a variant form of Pappa2 that can protect against salt-induced hypertension in the Dahl S rat.

Molecular mechanisms and regulation of urinary acidification

The H(+) concentration in human blood is kept within very narrow limits, ~40 nmol/L, despite the fact that dietary metabolism generates acid and base loads that are added to the systemic circulation throughout the life of mammals. One of the primary functions of the kidney is to maintain the constancy of systemic acid-base chemistry. The kidney has evolved the capacity to regulate blood acidity by performing three key functions: (i) reabsorb HCO3(-) that is filtered through the glomeruli to prevent its excretion in the urine; (ii) generate a sufficient quantity of new HCO3(-) to compensate for the loss of HCO3(-) resulting from dietary metabolic H(+) loads and loss of HCO3(-) in the urea cycle; and (iii) excrete HCO3(-) (or metabolizable organic anions) following a systemic base load. The ability of the kidney to perform these functions requires that various cell types throughout the nephron respond to changes in acid-base chemistry by modulating specific ion transport and/or metabolic processes in a coordinated fashion such that the urine and renal vein chemistry is altered appropriately. The purpose of the article is to provide the interested reader with a broad review of a field that began historically ~60 years ago with whole animal studies, and has evolved to where we are currently addressing questions related to kidney acid-base regulation at the single protein structure/function level.

Physiological role of NBCe2 in the regulation of electrolyte transport in the distal nephron

The electrogenic Na(+)-HCO3 (-) cotransporter 2 (NBCe2) is a newly discovered protein in the distal nephron. Our understanding is minimal regarding its physiological role in renal electrolyte transport. In this mini-review, we summarize the potential function of NBCe2 in the regulation of blood pressure, acid-base, and K(+) and Ca(2+) transport in the distal nephron.

Integrated compensatory network is activated in the absence of NCC phosphorylation

Thiazide diuretics are used to treat hypertension; however, compensatory processes in the kidney can limit antihypertensive responses to this class of drugs. Here, we evaluated compensatory pathways in SPAK kinase-deficient mice, which are unable to activate the thiazide-sensitive sodium chloride cotransporter NCC (encoded by Slc12a3). Global transcriptional profiling, combined with biochemical, cell biological, and physiological phenotyping, identified the gene expression signature of the response and revealed how it establishes an adaptive physiology. Salt reabsorption pathways were created by the coordinate induction of a multigene transport system, involving solute carriers (encoded by Slc26a4, Slc4a8, and Slc4a9), carbonic anhydrase isoforms, and V-type H⁺-ATPase subunits in pendrin-positive intercalated cells (PP-ICs) and ENaC subunits in principal cells (PCs). A distal nephron remodeling process and induction of jagged 1/NOTCH signaling, which expands the cortical connecting tubule with PCs and replaces acid-secreting α-ICs with PP-ICs, were partly responsible for the compensation. Salt reabsorption was also activated by induction of an α-ketoglutarate (α-KG) paracrine signaling system. Coordinate regulation of a multigene α-KG synthesis and transport pathway resulted in α-KG secretion into pro-urine, as the α-KG-activated GPCR (Oxgr1) increased on the PP-IC apical surface, allowing paracrine delivery of α-KG to stimulate salt transport. Identification of the integrated compensatory NaCl reabsorption mechanisms provides insight into thiazide diuretic efficacy.

Crosstalk between (Pro)renin receptor and COX-2 in the renal medulla during angiotensin II-induced hypertension

Angiotensin II (AngII) is an octapeptide hormone that plays a central role in regulation of sodium balance, plasma volume, and blood pressure. Its role in the pathogenesis of hypertension is highlighted by the wide use of inhibitors of the renin-angiotensin system (RAS) as the first-line antihypertensive therapy. However, despite intensive investigation, the mechanism of AngII-induced hypertension is still incompletely understood. Although diverse pathways are likely involved, increasing evidence suggests that the activation of intrarenal RAS may represent a dominant mechanism of AngII-induced hypertension. (Pro)renin receptor (PRR), a potential regulator of intrarenal RAS, is expressed in the intercalated cells of the collecting duct (CD) and induced by AngII, in parallel with increased renin in the principal cells of the CD. Activation of PRR elevated PGE2 release and COX-2 expression in renal inner medullary cells whereas COX-2-derived PGE2via the EP4 receptor mediates the upregulation of PRR during AngII infusion, thus forming a vicious cycle. The mutually stimulatory relationship between PRR and COX-2 in the distal nephron may play an important role in mediating AngII-induced hypertension.

Collecting duct intercalated cell function and regulation

Intercalated cells are kidney tubule epithelial cells with important roles in the regulation of acid-base homeostasis. However, in recent years the understanding of the function of the intercalated cell has become greatly enhanced and has shaped a new model for how the distal segments of the kidney tubule integrate salt and water reabsorption, potassium homeostasis, and acid-base status. These cells appear in the late distal convoluted tubule or in the connecting segment, depending on the species. They are most abundant in the collecting duct, where they can be detected all the way from the cortex to the initial part of the inner medulla. Intercalated cells are interspersed among the more numerous segment-specific principal cells. There are three types of intercalated cells, each having distinct structures and expressing different ensembles of transport proteins that translate into very different functions in the processing of the urine. This review includes recent findings on how intercalated cells regulate their intracellular milieu and contribute to acid-base regulation and sodium, chloride, and potassium homeostasis, thus highlighting their potential role as targets for the treatment of hypertension. Their novel regulation by paracrine signals in the collecting duct is also discussed. Finally, this article addresses their role as part of the innate immune system of the kidney tubule.

Generation of WNK1 knockout cell lines by CRISPR/Cas-mediated genome editing

Sodium-coupled SLC12 cation chloride cotransporters play important roles in cell volume and chloride homeostasis, epithelial fluid secretion, and renal tubular salt reabsorption. These cotransporters are phosphorylated and activated indirectly by With-No-Lysine (WNK) kinases through their downstream effector kinases, Ste20- and SPS1-related proline alanine-rich kinase (SPAK) and oxidative stress-responsive kinase 1 (OSR1). Multiple WNK kinases can coexist within a single cell type, although their relative contributions to SPAK/OSR1 activation and salt transport remain incompletely understood. Deletion of specific WNKs from cells that natively express a functional WNK-SPAK/OSR1 network will help resolve these knowledge gaps. Here, we outline a simple method to selectively knock out full-length WNK1 expression from mammalian cells using RNA-guided clustered regularly interspaced short palindromic repeats/Cas9 endonucleases. Two clonal cell lines were generated by using a single-guide RNA (sgRNA) targeting exon 1 of the WNK1 gene, which produced indels that abolished WNK1 protein expression. Both cell lines exhibited reduced endogenous WNK4 protein abundance, indicating that WNK1 is required for WNK4 stability. Consistent with an on-target effect, the reduced WNK4 abundance was associated with increased expression of the KLHL3/cullin-3 E3 ubiquitin ligase complex and was rescued by exogenous WNK1 overexpression. Although the morphology of the knockout cells was indistinguishable from control, they exhibited low baseline SPAK/OSR1 activity and failed to trigger regulatory volume increase after hypertonic stress, confirming an essential role for WNK1 in cell volume regulation. Collectively, our data show how this new, powerful, and accessible gene-editing technology can be used to dissect and analyze WNK signaling networks.

Mineralocorticoid-induced sodium appetite and renal salt retention: evidence for common signaling and effector mechanisms

An increase in renal sodium chloride (salt) retention and an increase in sodium appetite are the body's responses to salt restriction or depletion in order to restore salt balance. Renal salt retention and increased sodium appetite can also be maladaptive and sustain the pathophysiology in conditions like salt-sensitive hypertension and chronic heart failure. Here we review the central role of the mineralocorticoid aldosterone in both the increase in renal salt reabsorption and sodium appetite. We discuss the working hypothesis that aldosterone activates similar signaling and effector mechanisms in the kidney and brain, including the mineralocorticoid receptor, the serum- and glucocorticoid-induced kinase SGK1, the ubiquitin ligase NEDD4-2, and the epithelial sodium channel ENaC. The latter also mediates the gustatory salt sensing in the tongue, which is required for the manifestation of increased salt intake. Effects of aldosterone on both the brain and kidney synergize with the effects of angiotensin II. Thus, mineralocorticoids appear to induce similar molecular pathways in the kidney, brain, and possibly tongue, which could provide opportunities for more effective therapeutic interventions. Inhibition of renal salt reabsorption is compensated by stimulation of salt appetite and vice versa; targeting both mechanisms should be more effective. Inhibiting the arousal to consume salty food may improve a patient's compliance to reducing salt intake. While a better understanding of the molecular mechanisms is needed and will provide new therapeutic options, current pharmacological interventions that target both salt retention and sodium appetite include mineralocorticoid receptor antagonists and potentially inhibitors of angiotensin II and ENaC.

IGF-1 and insulin exert opposite actions on ClC-K2 activity in the cortical collecting ducts

Despite similar stimulatory actions on the epithelial sodium channel (ENaC)-mediated sodium reabsorption in the distal tubule, insulin promotes kaliuresis, whereas insulin-like growth factor-1 (IGF-1) causes a reduction in urinary potassium levels. The factors contributing to this phenomenon remain elusive. Electrogenic distal nephron ENaC-mediated Na(+) transport establishes driving force for Cl(-) reabsorption and K(+) secretion. Using patch-clamp electrophysiology, we document that a Cl(-) channel is highly abundant on the basolateral plasma membrane of intercalated cells in freshly isolated mouse cortical collecting duct (CCD) cells. The channel has characteristics attributable to the ClC-K2: slow gating kinetics, conductance ∼10 pS, voltage independence, Cl(-)>NO3 (-) anion selectivity, and inhibition/activation by low/high pH, respectively. IGF-1 (100 and 500 nM) acutely stimulates ClC-K2 activity in a reversible manner. Inhibition of PI3-kinase (PI3-K) with LY294002 (20 μM) abrogates activation of ClC-K2 by IGF-1. Interestingly, insulin (100 nM) reversibly decreases ClC-K2 activity in CCD cells. This inhibitory action is independent of PI3-K and is mediated by stimulation of a mitogen-activated protein kinase-dependent cascade. We propose that IGF-1, by stimulating ClC-K2 channels, promotes net Na(+) and Cl(-) reabsorption, thus reducing driving force for potassium secretion by the CCD. In contrast, inhibition of ClC-K2 by insulin favors coupling of Na(+) reabsorption with K(+) secretion at the apical membrane contributing to kaliuresis.

The multiple roles of pendrin in the kidney

The [Formula: see text] exchanger pendrin (SLC26A4, PDS) is located on the apical membrane of B-intercalated cells in the kidney cortical collecting duct and the connecting tubules and mediates the secretion of bicarbonate and the reabsorption of chloride. Given its dual function of bicarbonate secretion and chloride reabsorption in the distal tubules, it was thought that pendrin plays important roles in systemic acid-base balance and electrolyte and vascular volume homeostasis under basal conditions. Mice with the genetic deletion of pendrin or humans with inactivating mutations in PDS gene, however, do not display excessive salt and fluid wasting or altered blood pressure under baseline conditions. Very recent reports have unmasked the basis of incongruity between the mild phenotype in mutant mice and the role of pendrin as an important player in salt reabsorption in the distal tubule. These studies demonstrate that pendrin and the Na-Cl cotransporter (NCC; SLC12A3) cross compensate for the loss of each other, therefore masking the role that each transporter plays in salt reabsorption under baseline conditions. In addition, pendrin regulates calcium reabsorption in the distal tubules. Furthermore, combined deletion of pendrin and NCC not only causes severe volume depletion but also results in profound calcium wasting and luminal calcification in medullary collecting ducts. Based on studies in pathophysiological states and the examination of genetically engineered mouse models, the evolving picture points to important roles for pendrin (SLC26A4) in kidney physiology and in disease states. This review summarizes recent advances in the characterization of pendrin and the multiple roles it plays in the kidney, with emphasis on its essential roles in several diverse physiological processes, including chloride homeostasis, vascular volume and blood pressure regulation, calcium excretion and kidney stone formation.

Chemical and Physical Sensors in the Regulation of Renal Function

In order to assess the status of the volume and composition of the body fluid compartment, the kidney monitors a wide variety of chemical and physical parameters. It has recently become clear that the kidney's sensory capacity extends well beyond its ability to sense ion concentrations in the forming urine. The kidney also keeps track of organic metabolites derived from a surprising variety of sources and uses a complex interplay of physical and chemical sensing mechanisms to measure the rate of fluid flow in the nephron. Recent research has provided new insights into the nature of these sensory mechanisms and their relevance to renal function.

Mineralocorticoid receptor phosphorylation regulates ligand binding and renal response to volume depletion and hyperkalemia

Nuclear receptors are transcription factors that regulate diverse cellular processes. In canonical activation, ligand availability is sufficient to produce receptor binding, entraining downstream signaling. The mineralocorticoid receptor (MR) is normally activated by aldosterone, which is produced in both volume depletion and hyperkalemia, states that require different homeostatic responses. We report phosphorylation at S843 in the MR ligand-binding domain that prevents ligand binding and activation. In kidney, MR(S843-P) is found exclusively in intercalated cells of the distal nephron. In volume depletion, angiotensin II and WNK4 signaling decrease MR(S843-P) levels, whereas hyperkalemia increases MR(S843-P). Dephosphorylation of MR(S843-P) results in aldosterone-dependent increases of the intercalated cell apical proton pump and Cl(-)/HCO3(-) exchangers, increasing Cl(-) reabsorption and promoting increased plasma volume while inhibiting K(+) secretion. These findings reveal a mechanism regulating nuclear hormone receptor activity and implicate selective MR activation in intercalated cells in the distinct adaptive responses to volume depletion and hyperkalemia.