Human soluble prorenin receptor expressed in mouse renal collecting duct shows sex-specific effect on cardiorenal function

Soluble prorenin receptor (sPRR), a component of the renin-angiotensin system (RAS), has been identified as a plasma biomarker for hypertension and cardiovascular diseases in humans. Despite studies showing that sPRR in the kidney is produced by tubular cells in the renal collecting duct (CD), its biological actions modulating cardiorenal function in physiological conditions remain unknown. Therefore, the objective of our study was to investigate whether CD-derived human sPRR (HsPRR) expression influences cardiorenal function and examine sex and circadian differences. Thus, we investigated the status of the intrarenal RAS, water and electrolyte balance, renal filtration capacity, and blood pressure (BP) regulation in CD-HsPRR and control (CTL) mice. CD-HsPRR mice were generated by breeding human sPRR-Myc-tag mice with Hoxb7/Cre mice. Renal sPRR expression increased in CD-HsPRR mice, but circulating sPRR and RAS levels were unchanged compared with CTL mice. Only female littermates expressing CD-HsPRR showed 1) increased 24-h BP, 2) an impaired BP response to an acute dose of losartan and attenuated angiotensin II (ANG II)-induced hypertension, 3) reduced angiotensin-converting enzyme activity and ANG II content in the renal cortex, and 4) decreased glomerular filtration rate, with no changes in natriuresis and kaliuresis despite upregulation of the β-subunit of the epithelial Na+ channel in the renal cortex. These cardiorenal alterations were displayed only during the active phase of the day. Taken together, these data suggest that HsPRR could interact with ANG II type 1 receptors mediating sex-specific, ANG II-independent renal dysfunction and a prohypertensive phenotype in a sex-specific manner.NEW & NOTEWORTHY We successfully generated a humanized mouse model that expresses human sPRR in the collecting duct. Collecting duct-derived human sPRR did not change circulating sPRR and RAS levels but increased daytime BP in female mice while showing an attenuated angiotensin II-dependent pressor response. These findings may aid in elucidating the mechanisms by which women show uncontrolled BP in response to antihypertensive treatments targeting the RAS, improving approaches to reduce uncontrolled BP and chronic kidney disease incidences in women.

Vasopressin V2 receptor, tolvaptan, and ERK1/2 phosphorylation in the renal collecting duct

Tolvaptan, a vasopressin antagonist selective for the V2-subtype vasopressin receptor (V2R), is widely used in the treatment of hyponatremia and autosomal-dominant polycystic kidney disease (ADPKD). Its effects on signaling in collecting duct cells have not been fully characterized. Here, we perform RNA-seq in a collecting duct cell line (mpkCCD). The data show that tolvaptan inhibits the expression of mRNAs that were previously shown to be increased in response to vasopressin including aquaporin-2, but also reveals mRNA changes that were not readily predictable and suggest off-target actions of tolvaptan. One such action is activation of the MAPK kinase (ERK1/ERK2) pathway. Prior studies have shown that ERK1/ERK2 activation is essential in the regulation of a variety of cellular and physiological processes and can be associated with cell proliferation. In immunoblotting experiments, we demonstrated that ERK1/ERK2 phosphorylation in mpkCCD cells was significantly reduced by vasopressin, in contrast to the increases seen in non-collecting-duct cells overexpressing V2R in prior studies. We also found that tolvaptan has a strong effect to increase ERK1/ERK2 phosphorylation in the presence of vasopressin and that tolvaptan's effect to increase ERK1/ERK2 phosphorylation is absent in mpkCCD cells in which both protein kinase A (PKA)-catalytic subunits have been deleted. Thus, it appears that the tolvaptan effect to increase ERK activation is PKA-dependent and is not due to an off-target effect of tolvaptan. We conclude that in cells expressing V2R at endogenous levels: 1) vasopressin decreases ERK1/ERK2 activation; 2) in the presence of vasopressin, tolvaptan increases ERK1/ERK2 activation; and 3) these effects are PKA-dependent.NEW & NOTEWORTHY Vasopressin is a key hormone that regulates the function of the collecting duct of the kidney. ERK1 and ERK2 are enzymes that play key roles in physiological regulation in all cells. The authors used collecting duct cell cultures to investigate the effects of vasopressin and the vasopressin receptor antagonist tolvaptan on ERK1 and ERK2 phosphorylation and activation.

Human iPSC-derived renal collecting duct organoid model cystogenesis in ADPKD

In autosomal dominant polycystic kidney disease (ADPKD), renal cyst lesions predominantly arise from collecting ducts (CDs). However, relevant CD cyst models using human cells are lacking. Although previous reports have generated in vitro renal tubule cyst models from human induced pluripotent stem cells (hiPSCs), therapeutic drug candidates for ADPKD have not been identified. Here, by establishing expansion cultures of hiPSC-derived ureteric bud tip cells, an embryonic precursor that gives rise to CDs, we succeed in advancing the developmental stage of CD organoids and show that all CD organoids derived from PKD1-/- hiPSCs spontaneously develop multiple cysts, clarifying the initiation mechanisms of cystogenesis. Moreover, we identify retinoic acid receptor (RAR) agonists as candidate drugs that suppress in vitro cystogenesis and confirm the therapeutic effects on an ADPKD mouse model in vivo. Therefore, our in vitro CD cyst model contributes to understanding disease mechanisms and drug discovery for ADPKD.

Acid base homeostasis and serum bicarbonate concentration in syndrome of inappropriate anti-diuretic hormone secretion (SIADH) with hyponatremia

The Syndrome of Inappropriate ADH secretion (SIADH) presents with excess ADH release caused by a range of conditions; including pneumonia, brain tumors, certain lung cancers, and diseases of the hypothalamus. It presents with significant reduction in both sodium and chloride concentrations in the blood. However, reports examining the acid base status indicate a normal serum bicarbonate concentration and systemic acid base homeostasis. The mechanisms for the absence of abnormalities in acid base homeostasis remain speculative. This mini review is highlighting the recent advances in renal molecular physiology to provide answers for the maintenance of acid base status and serum bicarbonate in a physiological range.

Generation of Atp6v1g3-Cre mice for investigation of intercalated cells and the collecting duct

Kidney intercalated cells (ICs) maintain acid-base homeostasis and recent studies have demonstrated that they function in the kidney's innate defense. To study kidney innate immune function, ICs have been enriched using vacuolar ATPase (V-ATPase) B1 subunit (Atp6v1b1)-Cre (B1-Cre) mice. Although Atp6v1b1 is considered kidney specific, it is expressed in multiple organ systems, both in mice and humans, raising the possibility of off-target effects when using the Cre-lox system. We have recently shown using single-cell RNA sequencing that the gene that codes for the V-ATPase G3 subunit (mouse gene: Atp6v1g3; human gene: ATP6V1G3; protein abbreviation: G3) mRNA is selectively enriched in human kidney ICs. In this study, we generated Atp6v1g3-Cre (G3-Cre) reporter mice using CRISPR/CAS technology and crossed them with Tdtomatoflox/flox mice. The resultant G3-Cre+Tdt+ progeny was evaluated for kidney specificity in multiple tissues and found to be highly specific to kidney cells with minimal or no expression in other organs evaluated compared with B1-Cre mice. Tdt+ cells were flow sorted and were enriched for IC marker genes on RT-PCR analysis. Next, we crossed these mice to ihCD59 mice to generate an IC depletion mouse model (G3-Cre+ihCD59+/+). ICs were depleted in these mice using intermedilysin, which resulted in lower blood pH, suggestive of a distal renal tubular acidosis phenotype. The G3-Cre mice were healthy, bred normally, and produce regular-sized litter. Thus, this new "IC reporter" mice can be a useful tool to study ICs.NEW & NOTEWORTHY This study details the development, validation, and experimental use of a new mouse model to study the collecting duct and intercalated cells. Kidney intercalated cells are a cell type increasingly recognized to be important in several human diseases including kidney infections, acid-base disorders, and acute kidney injury.

Polycystin-1 Interacting Protein-1 (CU062) Interacts with the Ectodomain of Polycystin-1 (PC1)

The PKD1 gene, encoding protein polycystin-1 (PC1), is responsible for 85% of cases of autosomal dominant polycystic kidney disease (ADPKD). PC1 has been shown to be present in urinary exosome-like vesicles (PKD-ELVs) and lowered in individuals with germline PKD1 mutations. A label-free mass spectrometry comparison of urinary PKD-ELVs from normal individuals and those with PKD1 mutations showed that several proteins were reduced to a degree that matched the decrease observed in PC1 levels. Some of these proteins, such as polycystin-2 (PC2), may be present in a higher-order multi-protein assembly with PC1-the polycystin complex (PCC). CU062 (Q9NYP8) is decreased in ADPKD PKD-ELVs and, thus, is a candidate PCC component. CU062 is a small glycoprotein with a signal peptide but no transmembrane domain and can oligomerize with itself and interact with PC1. We investigated the localization of CU062 together with PC1 and PC2 using immunofluorescence (IF). In nonconfluent cells, all three proteins were localized in close proximity to focal adhesions (FAs), retraction fibers (RFs), and RF-associated extracellular vesicles (migrasomes). In confluent cells, primary cilia had PC1/PC2/CU062 + extracellular vesicles adherent to their plasma membrane. In cells exposed to mitochondrion-decoupling agents, we detected the development of novel PC1/CU062 + ring-like structures that entrained swollen mitochondria. In contact-inhibited cells under mitochondrial stress, PC1, PC2, and CU062 were observed on large, apically budding extracellular vesicles, where the proteins formed a reticular network on the membrane. CU062 interacts with PC1 and may have a role in the identification of senescent mitochondria and their extrusion in extracellular vesicles.

Adhesion GPCR Gpr126 (Adgrg6) Expression Profiling in Zebrafish, Mouse, and Human Kidney

Adhesion G protein-coupled receptors (aGPCRs) comprise the second-largest class of GPCRs, the most common target for approved pharmacological therapies. aGPCRs play an important role in development and disease and have recently been associated with the kidney. Several aGPCRs are expressed in the kidney and some aGPCRs are either required for kidney development or their expression level is altered in diseased kidneys. Yet, general aGPCR function and their physiological role in the kidney are poorly understood. Here, we characterize in detail Gpr126 (Adgrg6) expression based on RNAscope® technology in zebrafish, mice, and humans during kidney development in adults. Gpr126 expression is enriched in the epithelial linage during nephrogenesis and persists in the adult kidney in parietal epithelial cells, collecting ducts, and urothelium. Single-cell RNAseq analysis shows that gpr126 expression is detected in zebrafish in a distinct ionocyte sub-population. It is co-detected selectively with slc9a3.2, slc4a4a, and trpv6, known to be involved in apical acid secretion, buffering blood or intracellular pH, and to maintain high cytoplasmic Ca2+ concentration, respectively. Furthermore, gpr126-expressing cells were enriched in the expression of potassium transporter kcnj1a.1 and gcm2, which regulate the expression of a calcium sensor receptor. Notably, the expression patterns of Trpv6, Kcnj1a.1, and Gpr126 in mouse kidneys are highly similar. Collectively, our approach permits a detailed insight into the spatio-temporal expression of Gpr126 and provides a basis to elucidate a possible role of Gpr126 in kidney physiology.

Absence of E-Cadherin and β-Catenin in the Basal Plasma Membrane of Collecting Duct Cells During NDI Development and Recovery

Lithium (Li) induces severe polyuria and polydipsia in up to 40% of patients undergoing Li treatment. In rats, Li treatment induces a reversible cellular remodeling of the collecting duct (CD), decreasing the fraction of principal-to-intercalated cells. To investigate the potential role of adherens junction proteins, we performed immunohistochemistry on kidney cross-sections from rats treated with Li as well as rats undergoing recovery on a normal diet following 4 weeks of Li-treatment. We performed immunoelectron microscopy on cryosections to determine the ultrastructural localizations. Immunohistochemistry showed that E-cadherin and β-catenin were present in both the lateral and basal plasma membrane domains of CD cells. Immunoelectron microscopy confirmed that β-catenin was localized both to the lateral and the basal plasma membrane. The basal localization of both proteins was absent from a fraction of mainly principal cells after 10 and 15 days of Li-treatment. After 4 weeks of Li-treatment few to no cells were absent of E-cadherin and β-catenin at the basal plasma membrane. After 12 and 19 days of recovery some cells exhibited an absence of basal localization of both proteins. Thus, the observed localizational changes of E-cadherin and β-catenin appear before the cellular remodeling during both development and recovery from Li-NDI.

The transcription factor Foxi1 promotes expression of V-ATPase and Gpr116 in M-1 cells

The diverse functions of each nephron segment rely on the coordinated action of specialized cell populations that are uniquely defined by their transcriptional profile. In the collecting duct, there are two critical and distinct cell populations: principal cells and intercalated cells. Principal cells play key roles in the regulation of water, Na+, and K+, whereas intercalated cells are best known for their role in acid-base homeostasis. Currently, there are no in vitro systems that recapitulate the heterogeneity of the collecting ducts, which limits high-throughput and replicate investigations of genetic and physiological phenomena. Here, we demonstrated that the transcription factor Foxi1 is sufficient to alter the transcriptional identity of M-1 cells, a murine cortical collecting duct cell line. Specifically, overexpression of Foxi1 induces the expression of intercalated cell transcripts including Gpr116, Atp6v1b1, Atp6v1g3, Atp6v0d2, Slc4a9, and Slc26a4. These data indicate that overexpression of Foxi1 differentiates M-1 cells toward a non-A, non-B type intercalated cell phenotype and may provide a novel in vitro tool to study transcriptional regulation and physiological function of the renal collecting duct.NEW & NOTEWORTHY Transfection of M-1 cells with the transcription factor Foxi1 generates cells that express V-ATPase and Gpr116 as well as other genes associated with renal intercalated cells. This straightforward and novel in vitro system could be used to study processes including transcriptional regulation and cell specification and differentiation in renal intercalated cells.

Human ureteric bud organoids recapitulate branching morphogenesis and differentiate into functional collecting duct cell types

Directed differentiation of human pluripotent stem cells (hPSCs) into functional ureteric and collecting duct (CD) epithelia is essential to kidney regenerative medicine. Here we describe highly efficient, serum-free differentiation of hPSCs into ureteric bud (UB) organoids and functional CD cells. The hPSCs are first induced into pronephric progenitor cells at 90% efficiency and then aggregated into spheres with a molecular signature similar to the nephric duct. In a three-dimensional matrix, the spheres form UB organoids that exhibit branching morphogenesis similar to the fetal UB and correct distal tip localization of RET expression. Organoid-derived cells incorporate into the UB tips of the progenitor niche in chimeric fetal kidney explant culture. At later stages, the UB organoids differentiate into CD organoids, which contain >95% CD cell types as estimated by single-cell RNA sequencing. The CD epithelia demonstrate renal electrophysiologic functions, with ENaC-mediated vectorial sodium transport by principal cells and V-type ATPase proton pump activity by FOXI1-induced intercalated cells.

Differentiated mouse kidney tubuloids as a novel in vitro model to study collecting duct physiology

Kidney tubuloids are cell models that are derived from human or mouse renal epithelial cells and show high similarities with their in vivo counterparts. Tubuloids grow polarized in 3D, allow for long-term expansion, and represent multiple segments of the nephron, as shown by their gene expression pattern. In addition, human tubuloids form tight, functional barriers and have been succesfully used for drug testing. Our knowledge of mouse tubuloids, on the other hand, is only minimal. In this study, we further characterized mouse tubuloids and differentiated them towards the collecting duct, which led to a significant upregulation of collecting duct-specific mRNAs of genes and protein expression, including the water channel AQP2 and the sodium channel ENaC. Differentiation resulted in polarized expression of collecting duct water channels AQP2 and AQP3. Also, a physiological response to desmopressin and forskolin stimulation by translocation of AQP2 to the apical membrane was demonstrated. Furthermore, amiloride-sensitive ENaC-mediated sodium uptake was shown in differentiated tubuloids using radioactive tracer sodium. This study demonstrates that mouse tubuloids can be differentiated towards the collecting duct and exhibit collecting duct-specific function. This illustrates the potential use of mouse kidney tubuloids as novel in vitro models to study (patho)physiology of kidney diseases.

TRPML1-Induced Lysosomal Ca2+ Signals Activate AQP2 Translocation and Water Flux in Renal Collecting Duct Cells

Lysosomes are acidic Ca²⁺ storage organelles that actively generate local Ca²⁺ signaling events to regulate a plethora of cell functions. Here, we characterized lysosomal Ca²⁺ signals in mouse renal collecting duct (CD) cells and we assessed their putative role in aquaporin 2 (AQP2)-dependent water reabsorption. Bafilomycin A1 and ML-SA1 triggered similar Ca²⁺ oscillations, in the absence of extracellular Ca²⁺, by alkalizing the acidic lysosomal pH or activating the lysosomal cation channel mucolipin 1 (TRPML1), respectively. TRPML1-dependent Ca²⁺ signals were blocked either pharmacologically or by lysosomes' osmotic permeabilization, thus indicating these organelles as primary sources of Ca²⁺ release. Lysosome-induced Ca²⁺ oscillations were sustained by endoplasmic reticulum (ER) Ca²⁺ content, while bafilomycin A1 and ML-SA1 did not directly interfere with ER Ca²⁺ homeostasis per se. TRPML1 activation strongly increased AQP2 apical expression and depolymerized the actin cytoskeleton, thereby boosting water flux in response to an hypoosmotic stimulus. These effects were strictly dependent on the activation of the Ca²⁺/calcineurin pathway. Conversely, bafilomycin A1 led to perinuclear accumulation of AQP2 vesicles without affecting water permeability. Overall, lysosomal Ca²⁺ signaling events can be differently decoded to modulate Ca²⁺-dependent cellular functions related to the dock/fusion of AQP2-transporting vesicles in principal cells of the CD.

Navigating the Current Landscape of Non-Clear Cell Renal Cell Carcinoma: A Review of the Literature

Non-clear cell renal cell carcinoma (nccRCC) is an entity comprised of a heterogeneous constellation of RCC subtypes. Genomic profiling has broadened our understanding of molecular pathogenic mechanisms unique to individual nccRCC subtypes. To date, clinical trials evaluating the use of immunotherapies and targeted therapies have predominantly been conducted in patients with clear cell histology. A comprehensive review of the literature has been undertaken in order to describe molecular pathogenic mechanisms pertaining to each nccRCC subtype, and concisely summarise findings from therapeutic trials conducted in the nccRCC space.

The proximal tubule through an NBCe1-dependent mechanism regulates collecting duct phenotypic and remodeling responses to acidosis

The renal response to acid-base disturbances involves phenotypic and remodeling changes in the collecting duct. This study examines whether the proximal tubule controls these responses. We examined mice with genetic deletion of proteins present only in the proximal tubule, either the A variant or both A and B variants of isoform 1 of the electrogenic Na+-bicarbonate cotransporter (NBCe1). Both knockout (KO) mice have spontaneous metabolic acidosis. We then determined the collecting duct phenotypic responses to this acidosis and the remodeling responses to exogenous acid loading. Despite the spontaneous acidosis in NBCe1-A KO mice, type A intercalated cells in the inner stripe of the outer medullary collecting duct (OMCDis) exhibited decreased height and reduced expression of H+-ATPase, anion exchanger 1, Rhesus B glycoprotein, and Rhesus C glycoprotein. Combined kidney-specific NBCe1-A/B deletion induced similar changes. Ultrastructural imaging showed decreased apical plasma membrane and increased vesicular H+-ATPase in OMCDis type A intercalated cell in NBCe1-A KO mice. Next, we examined the collecting duct remodeling response to acidosis. In wild-type mice, acid loading increased the proportion of type A intercalated cells in the connecting tubule (CNT) and OMCDis, and it decreased the proportion of non-A, non-B intercalated cells in the connecting tubule, and type B intercalated cells in the cortical collecting duct (CCD). These changes were absent in NBCe1-A KO mice. We conclude that the collecting duct phenotypic and remodeling responses depend on proximal tubule-dependent signaling mechanisms blocked by constitutive deletion of proximal tubule NBCe1 proteins.NEW & NOTEWORTHY This study shows that the proximal tubule regulates collecting duct phenotypic and remodeling responses to acidosis.

Collecting duct renin regulates potassium homeostasis in mice

AIM

The kaliuretic action of the renin-angiotensin-aldosterone system (RAAS) is well established as highlighted by hyperkalemia side effect of RAAS inhibitors but such action is usually ascribed to systemic RAAS. The present study addresses the involvement of intrarenal RAAS in K+ homeostasis with emphasis on locally generated renin within the collecting duct (CD).

METHODS

Wild-type (Floxed) and CD-specific deletion of renin (CD renin KO) mice were treated for 7 days with a high K+ (HK) diet to investigate the role of CD renin in kaliuresis regulation and further define the underlying mechanism with emphasis on analysis of intrarenal aldosterone biosynthesis.

RESULTS

In floxed mice, renin levels were elevated in the renal medulla and urine following a 1-week HK diet, indicating activation of the intrarenal renin. CD renin KO mice had blunted HK-induced intrarenal renin response and developed impaired kaliuresis and elevated plasma K+ level (4.45 ± 0.14 vs. 3.89 ± 0.04 mM, p < 0.01). In parallel, HK-induced intrarenal aldosterone and CYP11B2 expression along with expression of renal outer medullary K+ channel (ROMK), calcium-activated potassium channel subunit alpha-1 (α-BK), α-Na+ -K+ -ATPase, and epithelial sodium channel (β-ENaC and cleaved-γ-ENaC) expression were all significantly blunted in CD renin KO mice in contrast to the unaltered responses of plasma aldosterone and adrenal CYP11B2.

CONCLUSION

Taken together, these results support a kaliuretic action of CD renin during HK intake.

Proteomics and AQP2 regulation

The advent of modern quantitative protein mass spectrometry techniques around the turn of the 21st century has contributed to a revolution in biology referred to as 'systems biology'. These methods allow identification and quantification of thousands of proteins in a biological specimen, as well as detection and quantification of post-translational protein modifications including phosphorylation. Here, we discuss these methodologies and show how they can be applied to understand the effects of the peptide hormone vasopressin to regulate the molecular water channel aquaporin-2. The emerging picture provides a detailed framework for understanding the molecular mechanisms involved in water balance disorders.

NOXA1-dependent NADPH oxidase 1 signaling mediates angiotensin II activation of the epithelial sodium channel

The activity of the epithelial Na+ channel (ENaC) in principal cells of the distal nephron fine-tunes renal Na+ excretion. The renin-angiotensin-aldosterone system modulates ENaC activity to control blood pressure, in part, by influencing Na+ excretion. NADPH oxidase activator 1-dependent NADPH oxidase 1 (NOXA1/NOX1) signaling may play a key role in angiotensin II (ANG II)-dependent activation of ENaC. The present study aimed to explore the role of NOXA1/NOX1 signaling in ANG II-dependent activation of ENaC in renal principal cells. Patch-clamp electrophysiology and principal cell-specific Noxa1 knockout (PC-Noxa1 KO) mice were used to determine the role of NOXA1/NOX1 signaling in ANG II-dependent activation of ENaC. The activity of ENaC in the luminal plasma membrane of principal cells was quantified in freshly isolated split-opened tubules using voltage-clamp electrophysiology. ANG II significantly increased ENaC activity. This effect was robust and observed in response to both acute (40 min) and more chronic (48-72 h) ANG II treatment of isolated tubules and mice, respectively. Inhibition of ANG II type 1 receptors with losartan abolished ANG II-dependent stimulation of ENaC. Similarly, treatment with ML171, a specific inhibitor of NOX1, abolished stimulation of ENaC by ANG II. Treatment with ANG II failed to increase ENaC activity in principal cells in tubules isolated from the PC-Noxa1 KO mouse. Tubules from wild-type littermate controls, though, retained their ability to respond to ANG II with an increase in ENaC activity. These results indicate that NOXA1/NOX1 signaling mediates ANG II stimulation of ENaC in renal principal cells. As such, NOXA1/NOX1 signaling in the distal nephron plays a central role in Na+ homeostasis and control of blood pressure, particularly as it relates to regulation by the renin-ANG II axis.NEW & NOTEWORTHY Activity of the epithelial Na+ channel (ENaC) in the distal nephron fine-tunes renal Na+ excretion. Angiotensin II (ANG II) has been reported to enhance ENaC activity. Emerging evidence suggests that NADPH oxidase (NOX) signaling plays an important role in the stimulation of ENaC by ANG II in principal cells. The present findings indicate that NOX activator 1/NOX1 signaling mediates ANG II stimulation of ENaC in renal principal cells.

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.

Chronic activation of vasopressin-2 receptors induces hypertension in Liddle mice by promoting Na+ and water retention

The renin-angiotensin-aldosterone and arginine vasopressin-V2 receptor-aquaporin-2 (AQP2) systems converge on the epithelial Na+ channel (ENaC) to regulate blood pressure and plasma tonicity. Although it is established that V2 receptors initiate renal water reabsorption through AQP2, whether V2 receptors can also induce renal Na+ retention through ENaC and raise blood pressure remains an open question. We hypothesized that a specific increase in V2 receptor-mediated ENaC activity can lead to high blood pressure. Our approach was to test effects of chronic activation of V2 receptors in Liddle mice, a genetic mouse model of high ENaC activity, and compare differences in ENaC activity, urine Na+ excretion, and blood pressure with control mice. We found that ENaC activity was elevated in Liddle mice and could not be stimulated further by administration of desmopressin (dDAVP), a V2 receptor-specific agonist. In contrast, Liddle mice showed higher levels of expression of AQP2 and aquaporin-3, but they could still respond to dDAVP infusion by increasing phospho-AQP2 expression. With dDAVP infusion, Liddle mice excreted smaller urine volume and less urine Na+ and developed higher blood pressure compared with control mice; this hypertension was attenuated with administration of the ENaC inhibitor benzamil. We conclude that V2 receptors contribute to hypertension in the Liddle mouse model by promoting primary Na+ and concomitant water retention.NEW & NOTEWORTHY Liddle syndrome is a classic model for hypertension from high epithelial Na+ channel (ENaC) activity. In the Liddle mouse model, vasopressin-2 receptors stimulate both ENaC and aquaporin-2, which increases Na+ and water retention to such an extent that hypertension ensues. Liddle mice will preserve plasma tonicity at the expense of a higher blood pressure; these data highlight the inherent limitation in which the kidney must use ENaC as a pathway to regulate both plasma tonicity and blood pressure.

Uromodulin Regulates Murine Aquaporin-2 Activity via Thick Ascending Limb-Collecting Duct Cross-Talk during Water Deprivation

Uromodulin, a urinary protein synthesized and secreted from the thick ascending limb (TAL) of the loop of Henle, is associated with hypertension through the activation of sodium reabsorption in the TAL. Uromodulin is a potential target for hypertension treatment via natriuresis. However, its biological function in epithelial cells of the distal nephron segment, particularly the collecting duct, remains unknown. Herein, we examined the regulation of uromodulin production during water deprivation in vivo as well as the effect of uromodulin on the activity of the water channel aquaporin−2 (AQP2) in vitro and in vivo using transgenic mice. Water deprivation upregulated uromodulin production; immunofluorescence experiments revealed uromodulin adhesion on the apical surface of the collecting duct. Furthermore, the activation of AQP2 was attenuated in mice lacking uromodulin. Uromodulin enhanced the phosphorylation and apical trafficking of AQP2 in mouse collecting duct cells treated with the vasopressin analog dDAVP. The uromodulin-induced apical trafficking of AQP2 was attenuated via endocytosis inhibitor treatment, suggesting that uromodulin activates AQP2 through the suppression of endocytosis. This study provides novel insights into the cross−talk between TAL and the collecting duct, and indicates that the modulation of uromodulin is a promising approach for diuresis and hypertension treatment.

Revisiting the relationship between (Pro)Renin receptor and the intrarenal RAS: focus on the soluble receptor

PURPOSE OF REVIEW

The (pro)renin receptor (PRR), also termed as ATPase H+ transporting accessory protein 2 (ATP6AP2), was originally cloned as a specific receptor for prorenin and renin [together called (pro)renin]. Given the wide tissue distribution of PRR, PRR was further postulated to act as a regulator of tissue renin. However, assigning a physiological role of PRR within the renin-angiotensin system (RAS) has been challenging largely due to its pleotropic functions in regulation of embryogenesis, autophagy, and H+ transport. The current review will summarize recent advances in understanding the roles of sPPR within the intrarenal RAS as well as those outside this local system.

RECENT FINDINGS

Site-1 protease (S1P) is a predominant source of sPPR at least in the kidney. So far most of the known physiological functions of PRR including renal handling of electrolytes and fluid and blood pressure are mediated by sPRR. In particular, sPRR serves as a positive regulator of collecting duct renin to activate the intrarenal RAS during water deprivation or angiotensin-II (AngII) infusion. However, PRR/sPRR can act in renin-independent manner under other circumstances.

SUMMARY

S1P-derived sPRR has emerged as a key regulator of kidney function and blood pressure and its relationship with the intrarenal RAS depends on the physiological context.

Analysis of the Hypoxic Response in a Mouse Cortical Collecting Duct-Derived Cell Line Suggests That Esrra Is Partially Involved in Hif1α-Mediated Hypoxia-Inducible Gene Expression in mCCDcl1 Cells

The kidney is strongly dependent on a continuous oxygen supply, and is conversely highly sensitive to hypoxia. Controlled oxygen gradients are essential for renal control of solutes and urine-concentrating mechanisms, which also depend on various hormones including aldosterone. The cortical collecting duct (CCD) is part of the aldosterone-sensitive distal nephron and possesses a key function in fine-tuned distal salt handling. It is well known that aldosterone is consistently decreased upon hypoxia. Furthermore, a recent study reported a hypoxia-dependent down-regulation of sodium currents within CCD cells. We thus investigated the possibility that cells from the cortical collecting duct are responsive to hypoxia, using the mouse cortical collecting duct cell line mCCDcl1 as a model. By analyzing the hypoxia-dependent transcriptome of mCCDcl1 cells, we found a large number of differentially-expressed genes (3086 in total logFC< −1 or >1) following 24 h of hypoxic conditions (0.2% O2). A gene ontology analysis of the differentially-regulated pathways revealed a strong decrease in oxygen-linked processes such as ATP metabolic functions, oxidative phosphorylation, and cellular and aerobic respiration, while pathways associated with hypoxic responses were robustly increased. The most pronounced regulated genes were confirmed by RT-qPCR. The low expression levels of Epas1 under both normoxic and hypoxic conditions suggest that Hif-1α, rather than Hif-2α, mediates the hypoxic response in mCCDcl1 cells. Accordingly, we generated shRNA-mediated Hif-1α knockdown cells and found Hif-1α to be responsible for the hypoxic induction of established hypoxically-induced genes. Interestingly, we could show that following shRNA-mediated knockdown of Esrra, Hif-1α protein levels were unaffected, but the gene expression levels of Egln3 and Serpine1 were significantly reduced, indicating that Esrra might contribute to the hypoxia-mediated expression of these and possibly other genes. Collectively, mCCDcl1 cells display a broad response to hypoxia and represent an adequate cellular model to study additional factors regulating the response to hypoxia.

Glucocorticoid Receptor Maintains Vasopressin Responses in Kidney Collecting Duct Cells

Water permeability of the kidney collecting ducts is regulated in part by the amount of the molecular water channel protein aquaporin-2 (AQP2), whose expression, in turn, is regulated by the pituitary peptide hormone vasopressin. We previously showed that stable glucocorticoid receptor knockdown diminished the vasopressin-induced Aqp2 gene expression in the collecting duct cell model mpkCCD. Here, we investigated the pathways regulated by the glucocorticoid receptor by comparing transcriptomes of the mpkCCD cells with or without stable glucocorticoid receptor knockdown. Glucocorticoid receptor knockdown downregulated 5,394 transcripts associated with 55 KEGG pathways including "vasopressin-regulated water reabsorption," indicative of positive regulatory roles of these pathways in the vasopressin-induced Aqp2 gene expression. Quantitative RT-PCR confirmed the downregulation of the vasopressin V2 receptor transcript upon glucocorticoid receptor knockdown. Glucocorticoid receptor knockdown upregulated 3,785 transcripts associated with 42 KEGG pathways including the "TNF signaling pathway" and "TGFβ signaling pathway," suggesting the negative regulatory roles of these pathways in the vasopressin-induced Aqp2 gene expression. Quantitative RT-PCR confirmed the upregulation of TNF and TGFβ receptor transcripts upon glucocorticoid receptor knockdown. TNF or TGFβ inhibitor alone, in the absence of vasopressin, did not induce Aqp2 gene transcription. However, TNF or TGFβ blunted the vasopressin-induced Aqp2 gene expression. In particular, TGFβ reduced vasopressin-induced increases in Akt phosphorylation without inducing epithelial-to-mesenchymal transition or interfering with vasopressin-induced apical AQP2 trafficking. In summary, our RNA-seq transcriptomic comparison revealed positive and negative regulatory pathways maintained by the glucocorticoid receptor for the vasopressin-induced Aqp2 gene expression.

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.

Functional role of histamine receptors in the renal cortical collecting duct cells

Histamine is an important immunomodulator, as well as a regulator of allergic inflammation, gastric acid secretion, and neurotransmission. Although substantial histamine level has been reported in the kidney, renal pathological and physiological effects of this compound have not been clearly defined. The goal of this study was to provide insight into the role of histamine-related pathways in the kidney, with emphasis on the collecting duct (CD), a distal part of the nephron important for the regulation of blood pressure. We report that all four histamine receptors (HRs) as well as enzymes responsible for histamine metabolism and synthesis are expressed in cultured mouse mpkCCD_cl4 cells, and histamine evokes a dose-dependent transient increase in intracellular Ca²⁺ in these cells. Furthermore, we observed a dose-dependent increase in cAMP in the CD cells in response to histamine. Short-circuit current studies aimed at measuring Na⁺ reabsorption via ENaC (epithelial Na⁺ channel) demonstrated inhibition of ENaC-mediated currents by histamine after a 4-hr incubation, and single-channel patch-clamp analysis revealed similar ENaC open probability before and after acute histamine application. The long-term (4 hr) effect on ENaC was corroborated in immunocytochemistry and qPCR, which showed a decrease in protein and gene expression for αENaC upon histamine treatment. In summary, our data highlight the functional importance of HRs in the CD cells and suggest potential implications of histamine in inflammation-related renal conditions. Further research is required to discern the molecular pathways downstream of HRs and assess the role of specific receptors in renal pathophysiology.

The Role of Vasopressin V2 Receptor in Drug-Induced Hyponatremia

Hyponatremia is frequently encountered in clinical practice and usually induced by renal water retention. Many medications are considered to be among the various causes of hyponatremia, because they either stimulate the release of arginine vasopressin (AVP) or potentiate its action in the kidney. Antidepressants, anticonvulsants, antipsychotics, diuretics, and cytotoxic agents are the major causes of drug-induced hyponatremia. However, studies addressing the potential of these drugs to increase AVP release from the posterior pituitary gland or enhance urine concentration through intrarenal mechanisms are lacking. We previously showed that in the absence of AVP, sertraline, carbamazepine, haloperidol, and cyclophosphamide each increased vasopressin V2 receptor (V2R) mRNA and aquaporin-2 (AQP2) protein and mRNA expression in primary cultured inner medullary collecting duct cells. The upregulation of AQP2 was blocked by the V2R antagonist tolvaptan or protein kinase A (PKA) inhibitors. These findings led us to conclude that the nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is the main mechanism of drug-induced hyponatremia. Previous studies have also shown that the V2R has a role in chlorpropamide-induced hyponatremia. Several other agents, including metformin and statins, have been found to induce antidiuresis and AQP2 upregulation through various V2R-independent pathways in animal experiments but are not associated with hyponatremia despite being frequently used clinically. In brief, drug-induced hyponatremia can be largely explained by AQP2 upregulation from V2R-cAMP-PKA signaling in the absence of AVP stimulation. This paper reviews the central and nephrogenic mechanisms of drug-induced hyponatremia and discusses the importance of the canonical pathway of AQP2 upregulation in drug-induced NSIAD.

α-Actinin 4 Links Vasopressin Short-Term and Long-Term Regulation of Aquaporin-2 in Kidney Collecting Duct Cells

Water permeability of the kidney collecting ducts is regulated by the peptide hormone vasopressin. Between minutes and hours (short-term), vasopressin induces trafficking of the water channel protein aquaporin-2 to the apical plasma membrane of the collecting duct principal cells to increase water permeability. Between hours and days (long-term), vasopressin induces aquaporin-2 gene expression. Here, we investigated the mechanisms that bridge the short-term and long-term vasopressin-mediated aquaporin-2 regulation by α-actinin 4, an F-actin crosslinking protein and a transcription co-activator of the glucocorticoid receptor. Vasopressin induced F-actin depolymerization and α-actinin 4 nuclear translocation in the mpkCCD collecting duct cell model. Co-immunoprecipitation followed by immunoblotting showed increased interaction between α-actinin 4 and glucocorticoid receptor in response to vasopressin. ChIP-PCR showed results consistent with α-actinin 4 and glucocorticoid receptor binding to the aquaporin-2 promoter. α-actinin 4 knockdown reduced vasopressin-induced increases in aquaporin-2 mRNA and protein expression. α-actinin 4 knockdown did not affect vasopressin-induced glucocorticoid receptor nuclear translocation, suggesting independent mechanisms of vasopressin-induced nuclear translocation of α-actinin 4 and glucocorticoid receptor. Glucocorticoid receptor knockdown profoundly reduced vasopressin-induced increases in aquaporin-2 mRNA and protein expression. In the absence of glucocorticoid analog dexamethasone, vasopressin-induced increases in glucocorticoid receptor nuclear translocation and aquaporin-2 mRNA were greatly reduced. α-actinin 4 knockdown further reduced vasopressin-induced increase in aquaporin-2 mRNA in the absence of dexamethasone. We conclude that glucocorticoid receptor plays a major role in vasopressin-induced aquaporin-2 gene expression that can be enhanced by α-actinin 4. In the absence of vasopressin, α-actinin 4 crosslinks F-actin underneath the apical plasma membrane, impeding aquaporin-2 membrane insertion. Vasopressin-induced F-actin depolymerization in one hand facilitates aquaporin-2 apical membrane insertion and in the other hand frees α-actinin 4 to enter the nucleus where it binds glucocorticoid receptor to enhance aquaporin-2 gene expression.

Generation of Distal Renal Segments Involves a Unique Population of Aqp2 + Progenitor Cells

BACKGROUND

Progenitor cells have clonogenicity, self-renewal, and multipotential capacity, and they can generate multiple types of cells during development. Evidence demonstrating the existence of such progenitor cells for renal distal segments is lacking.

METHODS

To identify Aqp2 + progenitor (AP) cells, we performed in vivo lineage tracing using both constitutive ( Aqp2Cre RFP/+ ) and Tamoxifen-inducible ( Aqp2 ECE/+ RFP/+ , Aqp2 ECE/+ Brainbow/+ , and Aqp2 ECE/+ Brainbow/Brainbow ) mouse models. Aqp2Cre RFP/+ mice were analyzed from E14.5 to adult stage. The inducible models were induced at P1 and examined at P3 and P42, respectively. Multiple segment- or cell-specific markers were used for high-resolution immunofluorescence confocal microscopy analyses to identify the cell types derived from Aqp2 + cells.

RESULTS

Both Aqp2Cre and Aqp2 ECE/+ faithfully indicate the activation of the endogenous Aqp2 promoter for lineage tracing. A subset of Aqp2 + cells behaves as potential AP. Aqp2Cre -based lineage tracing revealed that embryonic APs generate five types of cells, which form the late distal convoluted tubule (DCT2), connecting tubule segments 1 and 2 (CNT1 and CNT2, respectively), and collecting ducts (CDs). The α - and β -intercalated cells were apparently derived from embryonic AP in a stepwise manner. Aqp2 ECE/+ -based lineage tracing identified cells coexpressing Aqp2 and V-ATPase subunits B1 and B2 as the potential AP. Neonate APs generate daughter cells either inheriting their property (self-renewal) or evolving into various DCT2, CNT, or CD cells (multipotentiality), forming single cell-derived multiple-cell clones (clonogenicity) during development.

CONCLUSION

Our study demonstrates that unique Aqp2 + B1B2 + cells are the potential APs to generate DCT2, CNT, CNT2, and CD segments.

Expression of claudin-8 is induced by aldosterone in renal collecting duct principal cells

Fine tuning of Na⁺ reabsorption takes place along the aldosterone-sensitive distal nephron, which includes the collecting duct (CD), where it is mainly regulated by aldosterone. In the CD, Na⁺ reabsorption is mediated by the epithelial Na⁺ channel and Na⁺ pump (Na⁺-K⁺-ATPase). Paracellular ion permeability is mainly dependent on tight junction permeability. Claudin-8 is one of the main tight junction proteins expressed along the aldosterone-sensitive distal nephron. We have previously shown a coupling between transcellular Na⁺ reabsorption and paracellular Na⁺ barrier. We hypothesized that aldosterone controls the expression levels of both transcellular Na⁺ transporters and paracellular claudin-8 in a coordinated manner. Here, we show that aldosterone increased mRNA and protein levels as well as lateral membrane localization of claudin-8 in cultured CD principal cells. The increase in claudin-8 mRNA levels in response to aldosterone was prevented by preincubation with 17-hydroxyprogesterone, a mineralocorticoid receptor antagonist, and by inhibition of transcription with actinomycin D. We also showed that a low-salt diet, which stimulated aldosterone secretion, was associated with increased claudin-8 abundance in the mouse kidney. Reciprocally, mice subjected to a high-salt diet, which inhibits aldosterone secretion, or treated with spironolactone, a mineralocorticoid receptor antagonist, displayed decreased claudin-8 expression. Inhibition of glycogen synthase kinase-3, Lyn, and Abl signaling pathways prevented the effect of aldosterone on claudin-8 mRNA and protein abundance, suggesting that signaling of protein kinases plays a permissive role on the transcriptional activity of the mineralocorticoid receptor. This study shows that signaling via multiple protein kinases working in concert mediates aldosterone-induced claudin-8 expression in the CD.NEW & NOTEWORTHY In this study, we showed that aldosterone modulates claudin-8 expression in cultured collecting duct principal cells and in the mouse kidney. The upregulation of claudin-8 expression in response to aldosterone is dependent on at least glycogen synthase kinase-3, Lyn, and Abl signaling pathways, indicating the participation of multiple protein kinases to the effect of aldosterone.

Collecting Duct-Specific CR6-Interacting Factor-1-Deletion Aggravates Renal Inflammation and Fibrosis Induced by Unilateral Ureteral Obstruction

Although inflammation and fibrosis, which are key mechanisms of chronic kidney disease, are associated with mitochondrial damage, little is known about the effects of mitochondrial damage on the collecting duct in renal inflammation and fibrosis. To generate collecting duct-specific mitochondrial injury mouse models, CR6-interacting factor-1 (CRIF1) flox/flox mice were bred with Hoxb7-Cre mice. We evaluated the phenotype of these mice. To evaluate the effects on unilateral ureteral obstruction (UUO)-induced renal injury, we divided the mice into the following four groups: a CRIF1flox/flox (wild-type (WT)) group, a CRIF1flox/flox-Hob7 Cre (CRIF1-KO) group, a WT-UUO group, and a CRIF1-KO UUO group. We evaluated the blood and urine chemistries, inflammatory and fibrosis markers, light microscopy, and electron microscopy of the kidneys. The inhibition of Crif1 mRNA in mIMCD cells reduced oxygen consumption and membrane potential. No significant differences in blood and urine chemistries were observed between WT and CRIF1-KO mice. In UUO mice, monocyte chemoattractant protein-1 and osteopontin expression, number of F4/80 positive cells, transforming growth factor-β and α-smooth muscle actin staining, and Masson's trichrome staining were significantly higher in the kidneys of CRIF1-KO mice compared with the kidneys of WT mice. In sham mice, urinary 8-hydroxydeoxyguanosine (8-OHDG) was higher in CRIF1-KO mice than in WT mice. Moreover, CRIF1-KO sham mice had increased 8-OHDG-positive cell recruitment compared with WT-sham mice. CRIF1-KO-UUO kidneys had increased recruitment of 8-OHDG-positive cells compared with WT-UUO kidneys. In conclusion, collecting duct-specific mitochondrial injury increased oxidative stress. Oxidative stress associated with mitochondrial damage may aggravate UUO-induced renal injury.

Role of collecting duct principal cell NOS1β in sodium and potassium homeostasis

The nitric oxide (NO)-generating enzyme, NO synthase-1β (NOS1β), is essential for sodium (Na+ ) homeostasis and blood pressure control. We previously showed that collecting duct principal cell NOS1β is critical for inhibition of the epithelial sodium channel (ENaC) during high Na+ intake. Previous studies on freshly isolated cortical collecting ducts (CCD) demonstrated that exogenous NO promotes basolateral potassium (K+ ) conductance through basolateral channels, presumably Kir 4.1 (Kcnj10) and Kir 5.1 (Kcnj16). We, therefore, investigated the effects of NOS1β knockout on Kir 4.1/Kir 5.1 channel activity. Indeed, in CHO cells overexpressing NOS1β and Kir 4.1/Kir 5.1, the inhibition of NO signaling decreased channel activity. Male littermate control and principal cell NOS1β knockout mice (CDNOS1KO) on a 7-day, 4% NaCl diet (HSD) were used to detect changes in basolateral K+ conductance. We previously demonstrated that CDNOS1KO mice have high circulating aldosterone despite a high-salt diet and appropriately suppressed renin. We observed greater Kir 4.1 cortical abundance and significantly greater Kir 4.1/Kir 5.1 single-channel activity in the principal cells from CDNOS1KO mice. Moreover, blocking aldosterone action with in vivo spironolactone treatment resulted in lower Kir 4.1 abundance and greater plasma K+ in the CDNOS1KO mice compared to controls. Lowering K+ content in the HSD prevented the high aldosterone and greater plasma Na+ of CDNOS1KO mice and normalized Kir 4.1 abundance. We conclude that during chronic HSD, lack of NOS1β leads to increased plasma K+ , enhanced circulating aldosterone, and activation of ENaC and Kir 4.1/Kir 5.1 channels. Thus, principal cell NOS1β is required for the regulation of both Na+ and K+ by the kidney.

Bayesian identification of candidate transcription factors for the regulation of Aqp2 gene expression

Aquaporin-2 (Aqp2) gene transcription is strongly regulated by vasopressin in the renal collecting duct. However, the transcription factors (TFs) responsible for the regulation of expression of Aqp2 remain largely unknown. We used Bayes' theorem to integrate several -omics data sets to stratify the 1,344 TFs present in the mouse genome with regard to probabilities of regulating Aqp2 gene transcription. Also, we carried out new RNA sequencing experiments mapping the time course of vasopressin-induced changes in the transcriptome of mpkCCD cells to identify TFs that change in tandem with Aqp2. The analysis identified 17 of 1,344 TFs that are most likely to be involved in the regulation of Aqp2 gene transcription. These TFs included eight that have been proposed in prior studies to play a role in Aqp2 regulation, viz., Cebpb, Elf1, Elf3, Ets1, Jun, Junb, Nfkb1, and Sp1. The remaining nine represent new candidates for future studies (Atf1, Irf3, Klf5, Klf6, Mef2d, Nfyb, Nr2f6, Stat3, and Nr4a1). Conspicuously absent is CREB (Creb1), which has been widely proposed to mediate vasopressin-induced regulation of Aqp2 gene transcription (Nielsen S, Frokiaer J, Marples D, Kwon TH, Agre P, Knepper MA. Physiol Rev 82: 205-244, 2002; Kortenoeven ML, Fenton RA. Biochim Biophys Acta 1840: 1533-1549, 2014; Bockenhauer D, Bichet DG. Nat Rev Nephrol 11: 576-588, 2015; Pearce D, Soundararajan R, Trimpert C, Kashlan OB, Deen PM, Kohan DE. Clin J Am Soc Nephrol 10: 135-146, 2015). Instead, another CREB-like TF, Atf1, ranked fourth among all TFs. RNA sequencing time-course experiments showed a rapid increase in Aqp2 mRNA, within 3 h of vasopressin exposure. This response was matched by an equally rapid increase in the abundance of the mRNA coding for Cebpb, which we have shown by chromatin immunoprecipitation-sequencing studies to bind downstream from the Aqp2 gene. The identified TFs provide a roadmap for future studies to understand regulation of Aqp2 gene expression.NEW & NOTEWORTHY Abetted by the advent of systems biology-based ("-omics") techniques in the 21st century, there has been a massive expansion of published data relevant to virtually every physiological question. The authors have developed a large-scale data integration approach based on the application of Bayes'' theorem. In the current work, they integrated 12 different -omics data sets to identify the transcription factors most likely to mediate vasopressin-dependent regulation of transcription of the aquaporin-2 gene.

Acidosis-induced activation of distal nephron principal cells triggers Gdf15 secretion and adaptive proliferation of intercalated cells

AIM

Type A intercalated cells of the renal collecting duct participate in the maintenance of the acid/base balance through their capacity to adapt proton secretion to homeostatic requirements. We previously showed that increased proton secretion stems in part from the enlargement of the population of proton secreting cells in the outer medullary collecting duct through division of fully differentiated cells, and that this response is triggered by growth/differentiation factor 15. This study aimed at deciphering the mechanism of acid load-induced secretion of Gdf15 and its mechanism of action.

METHODS

We developed an original method to evaluate the proliferation of intercalated cells and applied it to genetically modified or pharmacologically treated mice under basal and acid-loaded conditions.

RESULTS

Gdf15 is secreted by principal cells of the collecting duct in response to the stimulation of vasopressin receptors. Vasopressin-induced production of cAMP triggers activation of AMP-stimulated kinases and of Na,K-ATPase, and induction of p53 and Gdf15. Gdf15 action on intercalated cells is mediated by ErbB2 receptors, the activation of which triggers the expression of cyclin d1, of p53 and anti-proliferative genes, and of Egr1.

CONCLUSION

Acidosis-induced proliferation of intercalated cells results from a cross talk with principal cells which secrete Gdf15 in response to their stimulation by vasopressin. Thus, vasopressin is a major determinant of the collecting duct cellular homeostasis as it promotes proliferation of intercalated cells under acidosis conditions and of principal cells under normal acid-base status.

α-Ketoglutarate Upregulates Collecting Duct (Pro)renin Receptor Expression, Tubular Angiotensin II Formation, and Na+ Reabsorption During High Glucose Conditions

Diabetes mellitus (DM) causes high glucose (HG) levels in the plasma and urine. The (pro)renin receptor (PRR) is a key regulator of renal Na⁺ handling. PRR is expressed in intercalated (IC) cells of the collecting duct (CD) and binds renin to promote angiotensin (Ang) II formation, thereby contributing to Na⁺ reabsorption. In DM, the Kreb's cycle is in a state of suppression in most tissues. However, in the CD, expression of glucose transporters is augmented, boosting the Kreb's cycle and consequently causing α-ketoglutarate (αKG) accumulation. The αKG receptor 1 (OXGR1) is a Gq-coupled receptor expressed on the apical membrane of IC cells of the CD. We hypothesize that HG causes αKG secretion and activation of OXGR1, which increases PRR expression in CD cells. This effect then promotes intratubular AngII formation and Na⁺ reabsorption. To test this hypothesis, streptozotocin (STZ)-induced diabetic mice were treated with or without montelukast (ML), an OXGR1 antagonist, for 6 days. STZ mice had higher urinary αKG and PRR expression along with augmented urinary AngII levels and Na⁺ retention. Treatment with ML prevented all these effects. Similarly, primary cultured inner medullary CD cells treated with HG showed increased PRR expression, while OXGR1 antagonist prevented this effect. αKG increases PRR expression, while treatments with ML, PKC inhibition, or intracellular Ca²⁺ depletion impair this effect. In silico analysis suggested that αKG binds to mouse OXGR1. These results indicate that HG conditions promote increased levels of intratubular αKG and OXGR1-dependent PRR upregulation, which impact AngII formation and Na⁺ reabsorption.

A Modular Differentiation System Maps Multiple Human Kidney Lineages from Pluripotent Stem Cells

Recent studies using human pluripotent stem cells (hPSCs) have developed protocols to induce kidney-lineage cells and reconstruct kidney organoids. However, the separate generation of metanephric nephron progenitors (NPs), mesonephric NPs, and ureteric bud (UB) cells, which constitute embryonic kidneys, in in vitro differentiation culture systems has not been fully investigated. Here, we create a culture system in which these mesoderm-like cell types and paraxial and lateral plate mesoderm-like cells are separately generated from hPSCs. We recapitulate nephrogenic niches from separately induced metanephric NP-like and UB-like cells, which are subsequently differentiated into glomeruli, renal tubules, and collecting ducts in vitro and further vascularized in vivo. Our selective differentiation protocols should contribute to understanding the mechanisms underlying human kidney development and disease and also supply cell sources for regenerative therapies.

Deletion of renal Nedd4-2 abolishes the effect of high sodium intake (HS) on Kir4.1, ENaC, and NCC and causes hypokalemia during high HS

Neural precursor cell expressed developmentally downregulated protein 4-2 (Nedd4-2) regulates the expression of Kir4.1, thiazide-sensitive NaCl cotransporter (NCC), and epithelial Na⁺ channel (ENaC) in the aldosterone-sensitive distal nephron (ASDN), and Nedd4-2 deletion causes salt-sensitive hypertension. We now examined whether Nedd4-2 deletion compromises the effect of high-salt (HS) diet on Kir4.1, NCC, ENaC, and renal K⁺ excretion. Immunoblot analysis showed that HS diet decreased the expression of Kir4.1, Ca²⁺-activated large-conductance K⁺ channel subunit-α (BKα), ENaCβ, ENaCγ, total NCC, and phospho-NCC (at Thr⁵³) in floxed neural precursor cell expressed developmentally downregulated gene 4-like (Nedd4l^fl/fl) mice, whereas these effects were absent in kidney-specific Nedd4-2 knockout (Ks-Nedd4-2 KO) mice. Renal clearance experiments also demonstrated that Nedd4-2 deletion abolished the inhibitory effect of HS diet on hydrochlorothiazide-induced natriuresis. Patch-clamp experiments showed that neither HS diet nor low-salt diet had an effect on Kir4.1/Kir5.1 currents of the distal convoluted tubule in Nedd4-2-deficient mice, whereas we confirmed that HS diet inhibited and low-salt diet increased Kir4.1/Kir5.1 activity in Nedd4l^flox/flox mice. Nedd4-2 deletion increased ENaC currents in the ASDN, and this increase was more robust in the cortical collecting duct than in the distal convoluted tubule. Also, HS-induced inhibition of ENaC currents in the ASDN was absent in Nedd4-2-deficient mice. Renal clearance experiments showed that HS intake for 2 wk increased the basal level of renal K⁺ excretion and caused hypokalemia in Ks-Nedd4-2-KO mice but not in Nedd4l^flox/flox mice. In contrast, plasma Na⁺ concentrations were similar in Nedd4l^flox/flox and Ks-Nedd4-2 KO mice on HS diet. We conclude that Nedd4-2 plays an important role in mediating the inhibitory effect of HS diet on Kir4.1, ENaC, and NCC and is essential for maintaining normal renal K⁺ excretion and plasma K⁺ ranges during long-term HS diet.NEW & NOTEWORTHY The present study suggests that Nedd4-2 is involved in mediating the inhibitory effect of high salt (HS) diet on Kir4.1/kir5.1 in the distal convoluted tubule, NaCl cotransporter function, and epithelial Na⁺ channel activity and that Nedd4-2 plays an essential role in maintaining K⁺ homeostasis in response to a long-term HS diet. This suggests the possibility that HS intake could lead to hypokalemia in subjects lacking proper Nedd4-2 E3 ubiquitin ligase activity in aldosterone-sensitive distal nephron.

High salt intake induces collecting duct HDAC1-dependent NO signaling

We reported that high salt (HS) intake stimulates renal collecting duct (CD) endothelin (ET) type B receptor (ETBR)/nitric oxide (NO) synthase 1β (NOS1β)-dependent NO production inhibiting the epithelial sodium channel (ENaC) promoting natriuresis. However, the mechanism underlying the HS-induced increase of NO production is unclear. Histone deacetylase 1 (HDAC1) responds to increased fluid flow, as can occur in the CD during HS intake. The renal inner medulla (IM), in particular the IMCD, has the highest NOS1 activity within the kidney. Hence, we hypothesized that HS intake provokes HDAC1 activation of NO production in the IM. HS intake for 1 wk significantly increased HDAC1 abundance in the IM. Ex vivo treatment of dissociated IM from HS-fed mice with a selective HDAC1 inhibitor (MS-275) decreased NO production with no change in ET-1 peptide or mRNA levels. We further investigated the role of the ET-1/ETBR/NOS1β signaling pathway with chronic ETBR blockade (A-192621). Although NO was decreased and ET-1 levels were elevated in the dissociated IM from HS-fed mice treated with A-192621, ex vivo MS-275 did not further change NO or ET-1 levels suggesting that HDAC1-mediated NO production is regulated at the level or downstream of ETBR activation. In split-open CDs from HS-fed mice, patch clamp analysis revealed significantly higher ENaC activity after MS-275 pretreatment, which was abrogated by an exogenous NO donor. Moreover, flow-induced increases in mIMCD-3 cell NO production were blunted by HDAC1 or calcium inhibition. Taken together, these findings indicate that HS intake induces HDAC1-dependent activation of the ETBR/NO pathway contributing to the natriuretic response.

Human Pluripotent Stem Cell-Derived Kidney Organoids with Improved Collecting Duct Maturation and Injury Modeling

Maximizing the potential of human kidney organoids for drug testing and regenerative medicine and to model development and disease requires addressing cell immaturity, the lack of a mature collecting system, and off-target cell types. By independently generating two kidney progenitor cell populations-metanephric mesenchyme and ureteric bud (UB)-like cells-we could generate kidney organoids with a collecting system. We also identify the hormones aldosterone and arginine vasopressin (AVP) as critical to promote differentiation of collecting duct cell types including both principal cells (PCs) and intercalated cells (ICs). The resulting PCs express aquaporin-2 (AQP2) protein, which undergoes translocation to the apical membrane after vasopressin or forskolin stimulation. By single-cell RNA sequencing (scRNA-seq), we demonstrate improved proximal tubule maturation and reduced off-target cell populations. We also show appropriate downregulation of progenitor cell types, improved modeling of tubular injury, the presence of urothelium (Uro), and the ability of Notch pathway modulation to regulate PC:IC ratios during organoid development.

Overexpression of TGF-β1 induces renal fibrosis and accelerates the decline in kidney function in polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by the presence of numerous fluid-filled cysts, extensive fibrosis, and the progressive decline in kidney function. Transforming growth factor-β1 (TGF-β1), an important mediator for renal fibrosis and chronic kidney disease, is overexpressed by cystic cells compared with normal kidney cells; however, its role in PKD pathogenesis remains undefined. To investigate the effect of TGF-β1 on cyst growth, fibrosis, and disease progression, we overexpressed active TGF-β1 specifically in collecting ducts (CDs) of phenotypic normal (Pkd1^RC/+) and Pkd1^RC/RC mice. In normal mice, CD-specific TGF-β1 overexpression caused tubule dilations by 5 wk of age that were accompanied by increased levels of phosphorylated SMAD3, α-smooth muscle actin, vimentin, and periostin; however, it did not induce overt cyst formation by 20 wk. In Pkd1^RC/RC mice, CD overexpression of TGF-β1 increased cyst epithelial cell proliferation. However, extensive fibrosis limited cyst enlargement and caused contraction of the kidneys, leading to a loss of renal function and a shortened lifespan of the mice. These data demonstrate that TGF-β1-induced fibrosis constrains cyst growth and kidney enlargement and accelerates the decline of renal function, supporting the hypothesis that a combined therapy that inhibits renal cyst growth and fibrosis will be required to effectively treat ADPKD.

Protein kinase A catalytic-α and catalytic-β proteins have nonredundant regulatory functions

Vasopressin regulates osmotic water transport in the renal collecting duct by protein kinase A (PKA)-mediated control of the water channel aquaporin-2 (AQP2). Collecting duct principal cells express two seemingly redundant PKA catalytic subunits, PKA catalytic α (PKA-Cα) and PKA catalytic β (PKA-Cβ). To identify the roles of these two protein kinases, we carried out deep phosphoproteomic analysis in cultured mpkCCD cells in which either PKA-Cα or PKA-Cβ was deleted using CRISPR-Cas9-based genome editing. Controls were cells carried through the genome editing procedure but without deletion of PKA. TMT mass tagging was used for protein mass spectrometric quantification. Of the 4,635 phosphopeptides that were quantified, 67 phosphopeptides were significantly altered in abundance with PKA-Cα deletion, whereas 21 phosphopeptides were significantly altered in abundance with PKA-Cβ deletion. However, only four sites were changed in both. The target proteins identified in PKA-Cα-null cells were largely associated with cell membranes and membrane vesicles, whereas target proteins in PKA-Cβ-null cells were largely associated with the actin cytoskeleton and cell junctions. In contrast, in vitro incubation of mpkCCD proteins with recombinant PKA-Cα and PKA-Cβ resulted in virtually identical phosphorylation changes. In addition, analysis of total protein abundances in in vivo samples showed that PKA-Cα deletion resulted in a near disappearance of AQP2 protein, whereas PKA-Cβ deletion did not decrease AQP2 abundance. We conclude that PKA-Cα and PKA-Cβ serve substantially different regulatory functions in renal collecting duct cells and that differences in phosphorylation targets may be due to differences in protein interactions, e.g., mediated by A-kinase anchor proteins, C-kinase anchoring proteins, or PDZ binding.

Gentamicin-Induced Acute Kidney Injury in an Animal Model Involves Programmed Necrosis of the Collecting Duct

BACKGROUND

Gentamicin is a potent aminoglycoside antibiotic that targets gram-negative bacteria, but nephrotoxicity limits its clinical application. The cause of gentamicin-induced AKI has been attributed mainly to apoptosis of the proximal tubule cells. However, blocking apoptosis only partially attenuates gentamicin-induced AKI in animals.

METHODS

Mice treated with gentamicin for 7 days developed AKI, and programmed cell death pathways were examined using pharmacologic inhibitors and in RIPK3-deficient mice. Effects in porcine and murine kidney cell lines were also examined.

RESULTS

Gentamicin caused a low level of apoptosis in the proximal tubules and significant ultrastructural alterations consistent with necroptosis, occurring predominantly in the collecting ducts (CDs), including cell and organelle swelling and rupture of the cell membrane. Upregulation of the key necroptotic signaling molecules, mixed lineage kinase domain-like pseudokinase (MLKL) and receptor-interacting serine/threonine-protein kinase 3 (RIPK3), was detected in gentamicin-treated mice and in cultured renal tubule cells. In addition, gentamicin induced apical accumulation of total and phosphorylated MLKL (pMLKL) in CDs in mouse kidney. Inhibiting a necroptotic protein, RIPK1, with necrostatin-1 (Nec-1), attenuated gentamicin-induced necrosis and upregulation of MLKL and RIPK3 in mice and cultured cells. Nec-1 also alleviated kidney inflammation and fibrosis, and significantly improved gentamicin-induced renal dysfunction in mice. Furthermore, deletion of RIPK3 in the Ripk3 ^-/- mice significantly attenuated gentamicin-induced AKI.

CONCLUSIONS

A previously unrecognized role of programmed necrosis in collecting ducts in gentamicin-induced kidney injury presents a potential new therapeutic strategy to alleviate gentamicin-induced AKI through inhibiting necroptosis.

Developmental loss, but not pharmacological suppression, of renal carbonic anhydrase 2 results in pyelonephritis susceptibility

Carbonic anhydrase II knockout (Car2^-/-) mice have depleted numbers of renal intercalated cells, which are increasingly recognized to be innate immune effectors. We compared pyelonephritis susceptibility following reciprocal renal transplantations between Car2^-/- and wild-type mice. We examined the effect of pharmacological CA suppression using acetazolamide in an experimental murine model of urinary tract infection. Car2^-/- versus wild-type mice were compared for differences in renal innate immunity. In our transplant scheme, mice lacking CA-II in the kidney had increased pyelonephritis risk. Mice treated with acetazolamide had lower kidney bacterial burdens at 6 h postinfection, which appeared to be due to tubular flow from diuresis because comparable results were obtained when furosemide was substituted for acetazolamide. Isolated Car2^-/- kidney cells enriched for intercalated cells demonstrated altered intercalated cell innate immune gene expression, notably increased calgizzarin and insulin receptor expression. Intercalated cell number and function along with renal tubular flow are determinants of pyelonephritis risk.

Epac1 null mice have nephrogenic diabetes insipidus with deficient corticopapillary osmotic gradient and weaker collecting duct tight junctions

AIM

The cAMP-mediator Epac1 (RapGef3) has high renal expression. Preliminary observations revealed increased diuresis in Epac1^-/- mice. We hypothesized that Epac1 could restrict diuresis by promoting transcellular collecting duct (CD) water and urea transport or by stabilizing CD paracellular junctions to reduce osmolyte loss from the renal papillary interstitium.

METHODS

In Epac1^-/- and Wt C57BL/6J mice, renal papillae, dissected from snap-frozen kidneys, were assayed for the content of key osmolytes. Cell junctions were analysed by transmission electron microscopy. Urea transport integrity was evaluated by urea loading with 40% protein diet, endogenous vasopressin production was manipulated by intragastric water loading and moderate dehydration and vasopressin type 2 receptors were stimulated selectively by i.p.-injected desmopressin (dDAVP). Glomerular filtration rate (GFR) was estimated as [¹⁴ C]inulin clearance. The glomerular filtration barrier was evaluated by urinary albumin excretion and microvascular leakage by the renal content of time-spaced intravenously injected ¹²⁵ I- and ¹³¹ I-labelled albumin.

RESULTS

Epac1^-/- mice had increased diuresis and increased free water clearance under antidiuretic conditions. They had shorter and less dense CD tight junction (TJs) and attenuated corticomedullary osmotic gradient. Epac1^-/- mice had no increased protein diet-induced urea-dependent osmotic diuresis, and expressed Wt levels of aquaporin-2 (AQP-2) and urea transporter A1/3 (UT-A1/3). Epac1^-/- mice had no urinary albumin leakage and unaltered renal microvascular albumin extravasation. Their GFR was moderately increased, unless when treated with furosemide.

CONCLUSION

Our results conform to the hypothesis that Epac1-dependent mechanisms protect against diabetes insipidus by maintaining renal papillary osmolarity and the integrity of CD TJs.

Cellular plasticity: A mechanism for homeostasis in the kidney

Cellular plasticity is a topical subject with interest spanning a wide range of fields from developmental biology to regenerative medicine. Even the nomenclature is a subject of debate, and the underlying mechanisms are still under investigation. On top of injury repair, cell plasticity is a constant physiological process in adult organisms and tissues, in response to homeostatic challenges. In this review we discuss two examples of plasticity for the maintenance of homeostasis in the renal system-namely the renin-producing juxtaglomerular cells (JG cells) and cortical collecting duct (CCD) cells. JG cells show plasticity through recruitment mechanisms, answering the demand for an increase in renin production. In the CCD, cells appear to have the ability to transdifferentiate between principal and intercalated cells to help maintain the highly regulated solute transport levels of that segment. These two cases highlight the complexity of plasticity processes and the role they can play in the kidney.

A Systematic Review of Systemic Treatment Options for Advanced Non-Clear Cell Renal Cell Carcinoma

INTRODUCTION

There have been a number of recent advances in the management of advanced clear cell renal cell carcinoma (ccRCC). However, the majority of these studies excluded patients with non-clear cell RCC (nccRCC), and optimal management of nccRCC remains unknown.

MATERIALS AND METHODS

A systematic review of the literature was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines to evaluate systemic treatment options in locally advanced or metastatic nccRCC between 2000-2019. Randomized controlled trials, single-arm phase II-IV trials, and prospective analyses of medication access programs were included. The primary outcome measures were progression free survival (PFS), overall survival (OS), and objective response rate (ORR).

RESULTS

A total of 31 studies were included in the final analysis. There was the highest level of evidence to support first-line treatment of nccRCC with sunitinib. Additional single-arm trials support the use of other vascular endothelial growth factor (VEGF) inhibitors with axitinib and pazopanib, as well as mammalian target of rapamycin (mTOR) inhibition with temsirolimus or everolimus +/- bevacizumab. Immune checkpoint inhibition has an emerging role in nccRCC, but optimal sequencing of available options is not clear. Prospective data to support the use of newer immunotherapy combinations are lacking. Treatment for collecting duct carcinoma remains platinum-based chemotherapy.

CONCLUSIONS

The availability of randomized trials in nccRCC is limited, and most studies include outcomes for nccRCC as a group, making conclusions about efficacy by subtype difficult. This systematic review supports consensus guidelines recommending sunitinib or clinical trial enrollment as preferred first-line treatment options for nccRCC, but also suggests a more nuanced approach to management and new options for therapy such as immune checkpoint inhibition.

Soluble (pro)renin receptor regulation of ENaC involved in aldosterone signaling in cultured collecting duct cells

We have previously shown that activation of (pro)renin receptor (PRR) induces epithelial Na+ channel (ENaC) activity in cultured collecting duct cells. Here, we examined the role of soluble PRR (sPRR), the cleavage product of PRR in ENaC regulation, and further tested its relevance to aldosterone signaling. In cultured mpkCCD cells, administration of recombinant histidine-tagged sPRR (sPRR-His) at 10 nM within minutes induced a significant and transient increase in the amiloride-sensitive short-circuit current as assessed using the Ussing chamber technique. The acute ENaC activation was blocked by the NADPH oxidase 1/4 inhibitor GKT137892 and siRNA against Nox4 but not the β-catenin inhibitor ICG-001. In primary rat inner medullary collecting duct cells, administration of sPRR-His at 10 nM for 24 h induced protein expression of the α-subunit but not β- or γ-subunits of ENaC, in parallel with upregulation of mRNA expression as well as promoter activity of the α-subunit. The transcriptional activation of α-ENaC was dependent on β-catenin signaling. Consistent results obtained by epithelial volt ohmmeter measurement of equivalent current and Ussing chamber determination of short-circuit current showed that aldosterone-induced transepithelial Na+ transport was inhibited by the PRR decoy inhibitor PRO20 and PF-429242, an inhibitor of sPRR-generating enzyme site-1 protease, and the response was restored by the addition of sPRR-His. Medium sPRR was elevated by aldosterone and inhibited by PF-429242. Taken together, these results demonstrate that sPRR induces two phases of ENaC activation via distinct mechanisms and functions as a mediator of the natriferic action of aldosterone.

Collecting system-specific deletion of Kcnj10 predisposes for thiazide- and low-potassium diet-induced hypokalemia

The basolateral potassium channel KCNJ10 (Kir4.1), is expressed in the renal distal convoluted tubule and controls the activity of the thiazide-sensitive sodium chloride cotransporter. Loss-of-function mutations of KCNJ10 cause EAST/SeSAME syndrome with salt wasting and severe hypokalemia. KCNJ10 is also expressed in the principal cells of the collecting system. However, its pathophysiological role in this segment has not been studied in detail. To address this, we generated the mouse model AQP2cre:Kcnj10flox/flox with a deletion of Kcnj10 specifically in the collecting system (collecting system-Kcnj10-knockout). Collecting system-Kcnj10-knockout mice responded normally to standard and high potassium diet. However, this knockout exhibited a higher kaliuresis and lower plasma potassium than control mice when treated with thiazide diuretics. Likewise, collecting systemKcnj10-knockout displayed an inadequately high kaliuresis and renal sodium retention upon dietary potassium restriction. In this condition, these knockout mice became hypokalemic due to insufficient downregulation of the epithelial sodium channel (ENaC) and the renal outer medullary potassium channel (ROMK) in the collecting system. Consistently, the phenotype of collecting system-Kcnj10-knockout was fully abrogated by ENaC inhibition with amiloride and ameliorated by genetic inactivation of ROMK in the collecting system. Thus, KCNJ10 in the collecting system contributes to the renal control of potassium homeostasis by regulating ENaC and ROMK. Hence, impaired KCNJ10 function in the collecting system predisposes for thiazide and low potassium diet-induced hypokalemia and likely contributes to the pathophysiology of renal potassium loss in EAST/SeSAME syndrome.

Claudins in the Renal Collecting Duct

The renal collecting duct fine-tunes urinary composition, and thereby, coordinates key physiological processes, such as volume/blood pressure regulation, electrolyte-free water reabsorption, and acid-base homeostasis. The collecting duct epithelium is comprised of a tight epithelial barrier resulting in a strict separation of intraluminal urine and the interstitium. Tight junctions are key players in enforcing this barrier and in regulating paracellular transport of solutes across the epithelium. The features of tight junctions across different epithelia are strongly determined by their molecular composition. Claudins are particularly important structural components of tight junctions because they confer barrier and transport properties. In the collecting duct, a specific set of claudins (Cldn-3, Cldn-4, Cldn-7, Cldn-8) is expressed, and each of these claudins has been implicated in mediating aspects of the specific properties of its tight junction. The functional disruption of individual claudins or of the overall barrier function results in defects of blood pressure and water homeostasis. In this concise review, we provide an overview of the current knowledge on the role of the collecting duct epithelial barrier and of claudins in collecting duct function and pathophysiology.

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.