Abstract-Slc26a4 (Pds, pendrin) is an anion transporter expressed in the apical region of type B and non-A, non-B intercalated cells of the distal nephron. It is upregulated by aldosterone analogues and is critical in the development of mineralocorticoid-induced hypertension. Thus, Slc26a4 expression and its role in blood pressure and fluid and electrolyte homeostasis was explored during NaCl restriction, a treatment model in which aldosterone is appropriately increased. Ultrastructural immunolocalization, balance studies, and cortical collecting ducts (CCDs) perfused in vitro were used. With moderate physiological NaCl restriction, Slc26a4 expression in the apical plasma membrane increased 2-to 3-fold in type B intercalated cells. Because Slc26a4 transports Cl Ϫ , we tested whether NaCl balance differs in Slc26a4 (ϩ/ϩ) (Ϫ/Ϫ) mice had evidence of relative vascular volume depletion because they had a higher arterial pH, hematocrit, and blood urea nitrogen than wild-type mice. With moderate NaCl restriction, blood pressure was similar in Slc26a4 (ϩ/ϩ) and Slc26a4 (Ϫ/Ϫ) mice. However, on a severely restricted intake of NaCl, Slc26a4 (Ϫ/Ϫ) mice were hypotensive relative to wild-type mice. We conclude that Slc26a4 is upregulated with NaCl restriction and is critical in the maintenance of acid-base balance and in the renal conservation of Cl Ϫ and water during NaCl restriction.

Prior RNA sequencing (RNA-seq) studies have identified complete transcriptomes for most renal epithelial cell types. The exceptions are the cell types that make up the renal collecting duct, namely intercalated cells (ICs) and principal cells (PCs), which account for only a small fraction of the kidney mass, but play critical physiological roles in the regulation of blood pressure, extracellular fluid volume, and extracellular fluid composition. To enrich these cell types, we used FACS that employed well-established lectin cell surface markers for PCs and type B ICs, as well as a newly identified cell surface marker for type A ICs, c-Kit. Single-cell RNA-seq using the IC- and PC-enriched populations as input enabled identification of complete transcriptomes of A-ICs, B-ICs, and PCs. The data were used to create a freely accessible online gene-expression database for collecting duct cells. This database allowed identification of genes that are selectively expressed in each cell type, including cell-surface receptors, transcription factors, transporters, and secreted proteins. The analysis also identified a small fraction of hybrid cells expressing aquaporin-2 and anion exchanger 1 or pendrin transcripts. In many cases, mRNAs for receptors and their ligands were identified in different cells (e.g., chiefly in PCs vs. chiefly in ICs), suggesting signaling cross-talk among the three cell types. The identified patterns of gene expression among the three types of collecting duct cells provide a foundation for understanding physiological regulation and pathophysiology in the renal collecting duct.

Pendrin is an anion exchanger expressed in type B intercalated cells of the cortical collecting duct (CCD). Whether pendrin localizes to other nephron segments with intercalated cells is unknown. Moreover, whether pendrin is expressed in proximal tubule is debated. Thus the distribution of pendrin mRNA and protein expression in mouse kidney was investigated by using light and electron microscopic immunohistochemistry and quantitative real-time PCR. We observed that pendrin mRNA is expressed mainly in cortex. Within cortex, pendrin mRNA is at least fivefold higher in CCD and the connecting tubule (CNT) than in the other segments. Pendrin protein was observed in a subset of cells within the distal convoluted tubule as well as in type B and in non-A-non-B intercalated cells of the CNT and CCD. In type B intercalated cells, pendrin immunoreactivity was highest in apical cytoplasmic vesicles with little immunolabel along the apical plasma membrane. In non-A-non-B intercalated cells, intense pendrin immunoreactivity was detected along the apical plasma membrane. These differences in the subcellular distribution of pendrin immunolabel were confirmed by morphometric analysis. In conclusion, pendrin is expressed in the mouse distal convoluted tubule, CCD, and CNT along the apical plasma membrane of non-A-non-B intercalated cells and in subapical cytoplasmic vesicles of type B intercalated cells.

Ammonia is both produced and transported by renal epithelial cells, and it regulates renal ion transport. Recent studies have identified a family of putative ammonium transporters; mRNA for two members of this family, Rh B-glycoprotein (RhBG) and Rh C-glycoprotein (RhCG), is expressed in the kidney. The purpose of this study was to determine the cellular location of RhBG and RhCG protein in the mouse kidney. We generated RhBG- and RhCG-specific anti-peptide antibodies. Immunoblot analysis confirmed that both proteins were expressed in the mouse kidney. RhBG localization with immunohistochemistry revealed discrete basolateral labeling in the connecting segment (CNT) and in the majority of initial collecting tubule (ICT) and cortical collecting duct (CCD) cells. In the outer medullary collecting duct (OMCD) and inner medullary collecting duct (IMCD) only a subpopulation of cells exhibited basolateral immunoreactivity. Colocalization of RhBG with carbonic anhydrase II, the thiazide-sensitive transporter, and the anion exchangers AE1 and pendrin demonstrated RhBG immunoreactivity in all CNT cells and all CCD and ICT principal cells. In the ICT and CCD, basolateral RhBG immunoreactivity is also present in A-type intercalated cells but not in pendrin-positive CCD intercalated cells. In the OMCD and IMCD, only intercalated cells exhibit RhBG immunoreactivity. Immunoreactivity for a second putative ammonium transporter, RhCG, was present in the apical region of cells with almost the same distribution as RhBG. However, RhCG immunoreactivity was present in all CCD cells, and it was present in outer stripe OMCD principal cells, in addition to OMCD and IMCD intercalated cells. Thus the majority of RhBG and RhCG protein expression is present in the same epithelial cell types in the CNT and collecting duct but with opposite polarity. These findings suggest that RhBG and RhCG may play important and cell-specific roles in ammonium transport and signaling in these regions of the kidney.

Abstract-Pendrin is an anion exchanger expressed along the apical plasma membrane and apical cytoplasmic vesicles of type B and of non-A, non-B intercalated cells of the distal convoluted tubule, connecting tubule, and cortical collecting duct. Thus, Pds (Slc26a4) is a candidate gene for the putative apical anion-exchange process of the type B intercalated cell. Because apical anion exchange-mediated transport is upregulated with deoxycorticosterone pivalate (DOCP), we tested whether Pds mRNA and protein expression in mouse kidney were upregulated after administration of this aldosterone analogue by using quantitative real-time polymerase chain reaction as well as light and electron microscopic immunolocalization. In kidneys from DOCP-treated mice, Pds mRNA increased 60%, whereas pendrin protein expression in the apical plasma membrane increased 2-fold in non-A, non-B intercalated cells and increased 6-fold in type B cells. Because pendrin transports HCO 3 -and Cl -, we tested whether DOCP treatment unmasks abnormalities in acid-base or NaCl balance in Pds (-/-) mice. In the absence of DOCP, arterial pH, systolic blood pressure, and body weight were similar in Pds (ϩ/ϩ) and Pds (-/-) mice. After DOCP treatment, weight gain and hypertension were observed in Pds (ϩ/ϩ) but not in Pds (-/-) mice. Moreover, after DOCP administration, metabolic alkalosis was more severe in Pds (-/-) than Pds (ϩ/ϩ) mice. We conclude that pendrin is upregulated with aldosterone analogues and is critical in the pathogenesis of mineralocorticoid-induced hypertension and metabolic alkalosis.

Ferritin plays a central role in iron metabolism and is made of 24 subunits of 2 types: heavy chain and light chain. The ferritin heavy chain (FtH) has ferroxidase activity that is required for iron incorporation and limiting toxicity. The purpose of this study was to investigate the role of FtH in acute kidney injury (AKI) and renal iron handling by using proximal tubule-specific FtH-knockout mice (FtH PT-/-mice). FtH PT-/-mice had significant mortality, worse structural and functional renal injury, and increased levels of apoptosis in rhabdomyolysis and cisplatin-induced AKI, despite significantly higher expression of heme oxygenase-1, an antioxidant and cytoprotective enzyme. While expression of divalent metal transporter-1 was unaffected, expression of ferroportin (FPN) was significantly lower under both basal and rhabdomyolysis-induced AKI in FtH PT-/-mice. Apical localization of FPN was disrupted after AKI to a diffuse cytosolic and basolateral pattern. FtH, regardless of iron content and ferroxidase activity, induced FPN. Interestingly, urinary levels of the iron acceptor proteins neutrophil gelatinase-associated lipocalin, hemopexin, and transferrin were increased in FtH PT-/-mice after AKI. These results underscore the protective role of FtH and reveal the critical role of proximal tubule FtH in iron trafficking in AKI.

Pendrin (encoded by Pds, Slc26a4) is a Cl−/HCO3− exchanger expressed in the apical regions of type B and non-A, non-B intercalated cells of kidney and mediates renal Cl− absorption, particularly when upregulated. Aldosterone increases blood pressure by increasing absorption of both Na+ and Cl− through increased protein abundance and function of Na+ transporters, such as the epithelial Na+ channel (ENaC) and the Na+-Cl− cotransporter (NCC), as well as Cl− transporters, such as pendrin. Because aldosterone analogs do not increase blood pressure in Slc26a4−/− mice, we asked whether Na+ excretion and Na+ transporter protein abundance are altered in kidneys from these mutant mice. Thus wild-type and Slc26a4-null mice were given a NaCl-replete, a NaCl-restricted, or NaCl-replete diet and aldosterone or aldosterone analogs. Abundance of the major renal Na+ transporters was examined with immunoblots and immunohistochemistry. Slc26a4-null mice showed an impaired ability to conserve Na+ during dietary NaCl restriction. Under treatment conditions in which circulating aldosterone is increased, α-, β-, and 85-kDa γ-ENaC subunit protein abundances were reduced 15–35%, whereas abundance of the 70-kDa fragment of γ-ENaC was reduced ∼70% in Slc26a4-null relative to wild-type mice. Moreover, ENaC-dependent changes in transepithelial voltage were much lower in cortical collecting ducts from Slc26a4-null than from wild-type mice. Thus, in kidney, ENaC protein abundance and function are modulated by pendrin or through a pendrin-dependent downstream event. The reduced ENaC protein abundance and function observed in Slc26a4-null mice contribute to their lower blood pressure and reduced ability to conserve Na+ during NaCl restriction.

Renal ammonia metabolism and transport mediates a central role in acid-base homeostasis. In contrast to most renal solutes, the majority of renal ammonia excretion derives from intrarenal production, not from glomerular filtration. Renal ammoniagenesis predominantly results from glutamine metabolism, which produces 2 NH4+ and 2 HCO3− for each glutamine metabolized. The proximal tubule is the primary site for ammoniagenesis, but there is evidence for ammoniagenesis by most renal epithelial cells. Ammonia produced in the kidney is either excreted into the urine or returned to the systemic circulation through the renal veins. Ammonia excreted in the urine promotes acid excretion; ammonia returned to the systemic circulation is metabolized in the liver in a HCO3−-consuming process, resulting in no net benefit to acid-base homeostasis. Highly regulated ammonia transport by renal epithelial cells determines the proportion of ammonia excreted in the urine versus returned to the systemic circulation. The traditional paradigm of ammonia transport involving passive NH3 diffusion, protonation in the lumen and NH4+ trapping due to an inability to cross plasma membranes is being replaced by the recognition of limited plasma membrane NH3 permeability in combination with the presence of specific NH3-transporting and NH4+-transporting proteins in specific renal epithelial cells. Ammonia production and transport are regulated by a variety of factors, including extracellular pH and K+, and by several hormones, such as mineralocorticoids, glucocorticoids and angiotensin II. This coordinated process of regulated ammonia production and transport is critical for the effective maintenance of acid-base homeostasis.