Postnatal expression of transport proteins involved in acid-base transport in mouse kidney

Snapshots of nascent RNA reveal cell- and stimulus-specific responses to acute kidney injury

The current strategy to detect acute injury of kidney tubular cells relies on changes in serum levels of creatinine. Yet serum creatinine (sCr) is a marker of both functional and pathological processes and does not adequately assay tubular injury. In addition, sCr may require days to reach diagnostic thresholds, yet tubular cells respond with programs of damage and repair within minutes or hours. To detect acute responses to clinically relevant stimuli, we created mice expressing Rosa26-floxed-stop uracil phosphoribosyltransferase (Uprt) and inoculated 4-thiouracil (4-TU) to tag nascent RNA at selected time points. Cre-driven 4-TU-tagged RNA was isolated from intact kidneys and demonstrated that volume depletion and ischemia induced different genetic programs in collecting ducts and intercalated cells. Even lineage-related cell types expressed different genes in response to the 2 stressors. TU tagging also demonstrated the transient nature of the responses. Because we placed Uprt in the ubiquitously active Rosa26 locus, nascent RNAs from many cell types can be tagged in vivo and their roles interrogated under various conditions. In short, 4-TU labeling identifies stimulus-specific, cell-specific, and time-dependent acute responses that are otherwise difficult to detect with other technologies and are entirely obscured when sCr is the sole metric of kidney damage.

Deletion of the serine protease CAP2/Tmprss4 leads to dysregulated renal water handling upon dietary potassium depletion

The kidney needs to adapt daily to variable dietary K⁺ contents via various mechanisms including diuretic, acid-base and hormonal changes that are still not fully understood. In this study, we demonstrate that following a K⁺-deficient diet in wildtype mice, the serine protease CAP2/Tmprss4 is upregulated in connecting tubule and cortical collecting duct and also localizes to the medulla and transitional epithelium of the papilla and minor calyx. Male CAP2/Tmprss4 knockout mice display altered water handling and urine osmolality, enhanced vasopressin response leading to upregulated adenylate cyclase 6 expression and cAMP overproduction, and subsequently greater aquaporin 2 (AQP2) and Na⁺-K⁺-2Cl⁻ cotransporter 2 (NKCC2) expression following K⁺-deficient diet. Urinary acidification coincides with significantly increased H⁺,K⁺-ATPase type 2 (HKA2) mRNA and protein expression, and decreased calcium and phosphate excretion. This is accompanied by increased glucocorticoid receptor (GR) protein levels and reduced 11β-hydroxysteroid dehydrogenase 2 activity in knockout mice. Strikingly, genetic nephron-specific deletion of GR leads to the mirrored phenotype of CAP2/Tmprss4 knockouts, including increased water intake and urine output, urinary alkalinisation, downregulation of HKA2, AQP2 and NKCC2. Collectively, our data unveil a novel role of the serine protease CAP2/Tmprss4 and GR on renal water handling upon dietary K⁺ depletion.

Tubular immaturity causes erythropoietin-deficiency anemia of prematurity in preterm neonates

Kidneys are physiologically hypoxic due to huge oxygen consumption for tubular reabsorption. The physiological hypoxia makes the kidney an appropriate organ for sensitively detecting oxygen levels and producing erythropoietin (EPO). In preterm neonates, immature kidneys cannot produce sufficient EPO, which results in anemia of prematurity (AOP). The cause of EPO insufficiency in AOP has been unclear, therefore current therapeutic options are transfusion and injection of recombinant human EPO. This report shows that the cause of insufficient EPO production in AOP is elevated renal oxygen levels due to poor oxygen consumption by immature tubules. Neonatal mice with AOP showed low tubular transporter expression and elevated renal oxygen levels compared with those without AOP. Enhancing transporter expression in AOP mice induced renal hypoxia and EPO production. In preterm neonates, red blood cell counts, hemoglobin levels, and hematocrit levels correlated with tubular function, but not with serum creatinine, gestational age, or birth weight. Furthermore, pharmacological upregulation of hypoxia signaling ameliorated AOP in mice. These data suggest that tubular maturation with increased oxygen consumption is required for renal EPO production.

Colocalization of the (Pro)renin Receptor/Atp6ap2 with H+-ATPases in Mouse Kidney but Prorenin Does Not Acutely Regulate Intercalated Cell H+-ATPase Activity

The (Pro)renin receptor (P)RR/Atp6ap2 is a cell surface protein capable of binding and non-proteolytically activate prorenin. Additionally, (P)RR is associated with H⁺-ATPases and alternative functions in H⁺-ATPase regulation as well as in Wnt signalling have been reported. Kidneys express very high levels of H⁺-ATPases which are involved in multiple functions such as endocytosis, membrane protein recycling as well as urinary acidification, bicarbonate reabsorption, and salt absorption. Here, we wanted to localize the (P)RR/Atp6ap2 along the murine nephron, exmaine whether the (P)RR/Atp6ap2 is coregulated with other H⁺-ATPase subunits, and whether acute stimulation of the (P)RR/Atp6ap2 with prorenin regulates H⁺-ATPase activity in intercalated cells in freshly isolated collecting ducts. We localized (P)PR/Atp6ap2 along the murine nephron by qPCR and immunohistochemistry. (P)RR/Atp6ap2 mRNA was detected in all nephron segments with highest levels in the collecting system coinciding with H⁺-ATPases. Further experiments demonstrated expression at the brush border membrane of proximal tubules and in all types of intercalated cells colocalizing with H⁺-ATPases. In mice treated with NH₄Cl, NaHCO₃, KHCO₃, NaCl, or the mineralocorticoid DOCA for 7 days, (P)RR/Atp6ap2 and H⁺-ATPase subunits were regulated but not co-regulated at protein and mRNA levels. Immunolocalization in kidneys from control, NH₄Cl or NaHCO₃ treated mice demonstrated always colocalization of PRR/Atp6ap2 with H⁺-ATPase subunits at the brush border membrane of proximal tubules, the apical pole of type A intercalated cells, and at basolateral and/or apical membranes of non-type A intercalated cells. Microperfusion of isolated cortical collecting ducts and luminal application of prorenin did not acutely stimulate H⁺-ATPase activity. However, incubation of isolated collecting ducts with prorenin non-significantly increased ERK1/2 phosphorylation. Our results suggest that the PRR/Atp6ap2 may form a complex with H⁺-ATPases in proximal tubule and intercalated cells but that prorenin has no acute effect on H⁺-ATPase activity in intercalated cells.

Developmental changes in renal tubular transport-an overview

The adult kidney maintains a constant volume and composition of extracellular fluid despite changes in water and salt intake. The neonate is born with a kidney that has a small fraction of the glomerular filtration rate of the adult and immature tubules that function at a lower capacity than that of the mature animal. Nonetheless, the neonate is also able to maintain a constant extracellular fluid volume and composition. Postnatal renal tubular development was once thought to be due to an increase in the transporter abundance to meet the developmental increase in glomerular filtration rate. However, postnatal renal development of each nephron segment is quite complex. There are isoform changes of several transporters as well as developmental changes in signal transduction that affect the capacity of renal tubules to reabsorb solutes and water. This review will discuss neonatal tubular function with an emphasis on the differences that have been found between the neonate and adult. We will also discuss some of the factors that are responsible for the maturational changes in tubular transport that occur during postnatal renal development.

Harvest and primary culture of the murine aldosterone-sensitive distal nephron

The aldosterone-sensitive distal nephron (ASDN) exhibits axial heterogeneity in structure and function from the distal convoluted tubule to the medullary collecting duct. Ion and water transport is primarily divided between the cortex and medulla of the ASDN, respectively. Transcellular transport in this segment is highly regulated in health and disease and is integrated across different cell types. We currently lack an inexpensive, high-yield, and tractable technique to harvest and culture cells for the study of gene expression and physiological properties of mouse cortical ASDN. To address this need, we harvested tubules bound to Dolichos biflorus agglutinin lectin-coated magnetic beads from the kidney cortex and characterized these cell preparations. We determined that these cells are enriched for markers of distal convoluted tubule, connecting tubule, and cortical collecting duct, including principal and intercalated cells. In primary culture, these cells develop polarized monolayers with high resistance (1,000-1,500 Ω * cm(2)) and maintain expression and activity of key channels. These cells demonstrate an amiloride-sensitive short-circuit current that can be enhanced with aldosterone and maintain measurable potassium and anion secretion. Our method can be easily adopted to study the biology of the ASDN and to investigate phenotypic differences between wild-type and transgenic mouse models.

Therapeutic effect of prenatal alkalization and PTC124 in Na(+)/HCO3(-) cotransporter 1 p.W516* knock-in mice

We created Na(+)/HCO3(-) cotransporter 1 (NBCe1) p.W516* knock-in mice as a model of isolated proximal renal tubular acidosis showing early lethality associated with severe metabolic acidosis to investigate the therapeutic effects of prenatal alkalization or posttranscriptional control 124 (PTC124). NBCe1(W516*/W516*) mice were treated with non-alkalization (control, n=12), prenatal alkalization postcoitus (prenatal group, n=7) and postnatal alkalization from postnatal day 6 (postnatal group, n=12). Mutation-specific therapy, PTC124 (60 mg kg(-1)) or gentamicin (30 mg kg(-1)), was administered intraperitoneally from postnatal day 6. Blood and urine biochemistry, acid-base analysis, survival rate and renal histology were examined. NBCe1 protein, mRNA abundance and activity ex vivo were assessed after PTC124 and gentamicin treatment. Prenatal group mice had similar initial body weight to wild-type mice and achieved significant weight gain thereafter compared with controls. They had higher serum bicarbonate level (15.5 ± 1.4 vs 5.5 ± 0.1 mmol l(-1), P<0.05) on postnatal day 14 and better renal function, histology and survival rates (60.8 ± 23.5 vs 41.1 ± 15.8 days, P<0.05) than the postnatal group. Compared with the control and gentamicin therapies, PTC124 therapy significantly increased NBCe1 protein abundance despite unchanged mRNA transcription. Only PTC124 therapy significantly increased survival rate and partially rescued NBCe1 activity ex vivo. In NBCe1(W516*/W516*) mice, prenatal alkali therapy achieved higher survival rates and ameliorated organ dysfunction. PTC124 therapy for this nonsense mutation was partially effective in increasing NBCe1 expression and activity.

Uriniferous tubule: structural and functional organization

The uriniferous tubule is divided into the proximal tubule, the intermediate (thin) tubule, the distal tubule and the collecting duct. The present chapter is based on the chapters by Maunsbach and Christensen on the proximal tubule, and by Kaissling and Kriz on the distal tubule and collecting duct in the 1992 edition of the Handbook of Physiology, Renal Physiology. It describes the fine structure (light and electron microscopy) of the entire mammalian uriniferous tubule, mainly in rats, mice, and rabbits. The structural data are complemented by recent data on the location of the major transport- and transport-regulating proteins, revealed by morphological means(immunohistochemistry, immunofluorescence, and/or mRNA in situ hybridization). The structural differences along the uriniferous tubule strictly coincide with the distribution of the major luminal and basolateral transport proteins and receptors and both together provide the basis for the subdivision of the uriniferous tubule into functional subunits. Data on structural adaptation to defined functional changes in vivo and to genetical alterations of specified proteins involved in transepithelial transport importantly deepen our comprehension of the correlation of structure and function in the kidney, of the role of each segment or cell type in the overall renal function,and our understanding of renal pathophysiology.

The divergence, actions, roles, and relatives of sodium-coupled bicarbonate transporters

The mammalian Slc4 (Solute carrier 4) family of transporters is a functionally diverse group of 10 multi-spanning membrane proteins that includes three Cl-HCO3 exchangers (AE1-3), five Na(+)-coupled HCO3(-) transporters (NCBTs), and two other unusual members (AE4, BTR1). In this review, we mainly focus on the five mammalian NCBTs-NBCe1, NBCe2, NBCn1, NDCBE, and NBCn2. Each plays a specialized role in maintaining intracellular pH and, by contributing to the movement of HCO3(-) across epithelia, in maintaining whole-body pH and otherwise contributing to epithelial transport. Disruptions involving NCBT genes are linked to blindness, deafness, proximal renal tubular acidosis, mental retardation, and epilepsy. We also review AE1-3, AE4, and BTR1, addressing their relevance to the study of NCBTs. This review draws together recent advances in our understanding of the phylogenetic origins and physiological relevance of NCBTs and their progenitors. Underlying these advances is progress in such diverse disciplines as physiology, molecular biology, genetics, immunocytochemistry, proteomics, and structural biology. This review highlights the key similarities and differences between individual NCBTs and the genes that encode them and also clarifies the sometimes confusing NCBT nomenclature.

Regulation of two renal chloride transporters, AE1 and pendrin, by electrolytes and aldosterone

The renal handling of salt and protons and bicarbonate are intricately linked through shared transport mechanisms for sodium, chloride, protons, and bicarbonate. In the collecting duct, the regulated fine-tuning of salt and acid-base homeostasis is achieved by a series of transport proteins located in different cell types, intercalated and principal cells. Intercalated cells are considered to be of less importance for salt handling but recent evidence has suggested that the anion exchanger pendrin may participate in salt reabsorption and blood pressure regulation. Here, we examined the regulated expression of two functionally related but differentially expressed anion exchangers, AE1 and pendrin, by dietary electrolyte intake and aldosterone. Cortical expression of pendrin was regulated on mRNA and protein level. The combination of NaHCO₃ and DOCA enhanced pendrin mRNA and protein levels, whereas DOCA or NaHCO₃ alone had no effect. NaCl or KHCO₃ increased pendrin mRNA, KCl decreased its mRNA abundance. On protein level, NH₄Cl, NaCl, and KCl reduced pendrin expression, the other treatments were without effect. In contrast, AE1 mRNA or protein expression in kidney cortex was regulated by none of these treatments. In kidney medulla, NaHCO₃/DOCA or NaHCO₃ alone enhanced AE1 mRNA levels. AE1 protein abundance was increased by NH₄Cl, NaHCO₃/DOCA, and NaCl. Immunolocalization showed that during NH₄Cl treatment the relative number of AE1 positive cells was increased and pendrin expressing cells reduced. Thus, pendrin and AE1 are differentially regulated with distinct mechanisms that separately affect mRNA and protein levels. Pendrin is regulated by acidosis and chloride intake, whereas AE1 is enhanced by acidosis, NaCl, and the combination of DOCA and NaHCO₃.

The anion exchanger pendrin (SLC26A4) and renal acid-base homeostasis

The anion exchanger pendrin (Pds, SLC26A4) transports various anions including bicarbonate, chloride and iodide. In the kidney, pendrin is exclusively expressed on the luminal pole of bicarbonate-secretory type B intercalated cells. Genetic ablation of pendrin in mice abolishes luminal chloride-bicarbonate exchanger activity from type B intercalated cells suggesting that pendrin is the apical bicarbonate extruding pathway. The renal expression of pendrin is developmentally adapted and pendrin positive cells originate from both the uretric bud and mesenchyme. In adult kidney, pendrin expression and activity is regulated by systemic acid-base status, dietary electrolyte intake (mostly chloride), and hormones such as angiotensin II and aldosterone which can affect subcellular localization, the relative number of pendrin expressing cells, and the overall abundance consistent with a role of pendrin in maintaining normal acid-base homeostasis. This review summarizes recent findings on the role and regulation of pendrin in the context of the kidneys role in acid-base homeostasis in health and disease.

Expression of ammonia transporter family members, Rh B glycoprotein and Rh C glycoprotein, in the developing rat kidney

Ammonia metabolism is a primary component of acid-base homeostasis but is incompletely developed at time of birth. Rh B glycoprotein (Rhbg) and Rh C glycoprotein (Rhcg) are recently recognized ammonia transporter family members expressed in the mammalian kidney. This study's purpose was to establish the expression and localization of Rhbg and Rhcg during kidney development. We examined kidneys from fetal days 16 (E16), 18 (E18), and 20 (E20), and from the first 21 days of postnatal development. Rhbg was expressed initially at E18, with expression only in the connecting tubule (CNT); at E20, Rhbg was expressed in both the CNT and the medullary collecting duct (MCD). In contrast, Rhcg was first expressed at E16 with basal expression in the ureteric bud; at E18, it was expressed in a subset of CNT cells with an apical pattern, followed by apical and basolateral expression in the MCD at E20. In the cortex, Rhbg and Rhcg expression increased in the CNT before expression in the cortical collecting duct during fetal development. In the MCD, both Rhbg and Rhcg expression was initially in cells in the papillary tip, with gradual removal from the tip during the late fetal period and transition during the early neonatal period to an adult pattern with predominant expression in the outer MCD and only rare expression in cells in the initial inner MCD. Double-labeling with intercalated cell-specific markers identified that Rhbg and Rhcg were expressed initially in CNT cells, CNT A-type intercalated cells and non-A, non-B intercalated cells, and in MCD A-type intercalated cells. We conclude that expression of Rhbg and Rhcg parallels intercalated cell development and that immature Rhbg and Rhcg expression at birth contributes to incomplete ammonia excretion capacity.

The calcineurin inhibitor FK506 (tacrolimus) is associated with transient metabolic acidosis and altered expression of renal acid-base transport proteins

Calcineurin inhibitors like FK506 (tacrolimus) are routinely used for immunosuppression following transplantation. Its use is limited by many side effects, including renal tubular acidosis (RTA), mainly of the distal type. In this study, rats were treated with FK506 and at baseline (after 9 days) systemic acid-base status was similar to that in control animals. However, FK506-treated rats given NH4Cl in the drinking water for 2 days developed a more severe metabolic acidosis than control animals. Urine pH was more alkaline, but net acid excretion was normal. After 7 days of acid load, all differences related to acid-base homeostasis were equalized in both groups. Protein abundance of type IIa Na-Pi cotransporter, type 3 Na+/H+ exchanger, and electrogenic Na+-bicarbonate cotransporter, and both a4 and B2 subunits of the vacuolar H+-ATPase were reduced under baseline conditions, while induction of metabolic acidosis enhanced protein abundance of these transporters in FK506-treated animals. In parallel, protein expression of AE1 was reduced at baseline and increased together with pendrin during NH4Cl loading in FK506 rats. Protein abundance of the Na+-bicarbonate cotransporter NBCn1 was reduced under baseline conditions but remained downregulated during metabolic acidosis. Morphological analysis revealed an increase in the relative number of non-type A intercalated cells in the connecting tubule and cortical collecting duct at the expense of principal cells. Additionally, subcellular distribution of the a4 subunit of the vacuolar H+-ATPase was affected by FK506 with less luminal localization in the connecting tubule and outer medullary collecting duct. These data suggest that FK506 impacts on several major acid-base transport proteins in the kidney, and its use is associated with transient metabolic acidosis and altered expression of key renal acid-base transport proteins.

Interaction of Membrane Skeletal Protein, Protein 4.1B and p55, and Sodium Bicarbonate Cotransporter1 in Mouse Renal S1-S2 Proximal Tubules

Our recent studies demonstrated the localization of protein 4.1B, a member of the 4.1 skeletal membrane proteins, to the basolateral membranes of the S1-S2 renal proximal tubules. In the present studies, we investigated the presence of binding partners that could form a molecular complex with the 4.1B protein. Immunohistochemistry revealed the localization of p55, a membrane-associated guanylate kinase, and the sodium bicarbonate cotransporter1 (NBC1), to the basolateral membrane domain of S1-S2 in mouse renal proximal tubules. Using immunoprecipitation of kidney lysates with anti-p55 antibody, a positive band was blotted with anti-4.1B antibody. GST fusion proteins including the NBC1 and 4.1B regions were confirmed to bind with each other by electrophoresis after mixing. Both NBC1- and 4.1B-specific bands were detected in renal protein mixtures immunoprecipated by either anti-4.1B- or NBC1-specific antibodies. It is likely that NBC1, 4.1B, and p55 form a molecular complex in the basolateral membrane of the kidney S1-S2 proximal tubules. We propose that the 4.1B-containing membrane skeleton may play a role in regulating the Na+ and HCO3- reabsorption in S1-S2 proximal tubules.

Origin and fate of pendrin-positive intercalated cells in developing mouse kidney

Pendrin is an apical anion exchanger found in type B and nonA-nonB intercalated cells that is involved in bicarbonate secretion. The purpose of this study was to establish the origin and fate of pendrin-positive intercalated cells in the mouse kidney. Using immunohistochemistry, we found that pendrin-positive cells first appeared in the connecting tubule at embryonic day 14 (E14) and subsequently in the medullary collecting duct at E18. Most of the pendrin-positive cells in the connecting tubule were nonA-nonB intercalated cells, wheras those in the medullary collecting duct were type B intercalated cells. In the cortical collecting duct, pendrin-positive cells appeared in the inner part at day 4 after birth and in the outer part at day 7. Pendrin-positive cells gradually disappeared by apoptosis from the inner part of the medullary collecting duct two weeks after birth. Using 5-bromo-2'deoxy-uridine (BrdU) to follow cell proliferation, we determined that selective proliferation of pendrin-positive intercalated cells does not occur; instead, these cells may arise from undifferentiated precursor cells from separate foci, one in the connecting tubule and one in the collecting duct.

Effect of chronic elevated carbon dioxide on the expression of acid-base transporters in the neonatal and adult mouse

Several pulmonary and neurological conditions, both in the newborn and adult, result in hypercapnia. This leads to disturbances in normal pH homeostasis. Most mammalian cells maintain tight control of intracellular pH (pHi) using a group of transmembrane proteins that specialize in acid-base transport. These acid-base transporters are important in adjusting pHiduring acidosis arising from hypoventilation. We hypothesized that exposure to chronic hypercapnia induces changes in the expression of acid-base transporters. Neonatal and adult CD-1 mice were exposed to either 8% or 12% CO2for 2 wk. We used Western blot analysis of membrane protein fractions from heart, kidney, and various brain regions to study the response of specific acid-base transporters to CO2. Chronic CO2increased the expression of the sodium hydrogen exchanger 1 (NHE1) and electroneutral sodium bicarbonate cotransporter (NBCn1) in the cerebral cortex, heart, and kidney of neonatal but not adult mice. CO2increased the expression of electrogenic NBC (NBCe1) in the neonatal but not the adult mouse heart and kidney. Hypercapnia decreased the expression of anion exchanger 3 (AE3) in both the neonatal and adult brain but increased AE3 expression in the neonatal heart. We conclude that: 1) chronic hypercapnia increases the expression of the acid extruders NHE1, NBCe1 and NBCn1 and decreases the expression of the acid loader AE3, possibly improving the capacity of the cell to maintain pHiin the face of acidosis; and 2) the heterogeneous response of tissues to hypercapnia depends on the level of CO2and development.

Thyroid hormone deficiency alters expression of acid-base transporters in rat kidney

Hypothyroidism in humans is associated with incomplete distal renal tubular acidosis, presenting as the inability to respond appropriately to an acid challenge by excreting less acid. Here, we induced hypothyroidism in rats with methimazole (HYPO) and in one group substituted with l-thyroxine (EU). After 4 wk, acid-base status was similar in both groups. However, after 24 h acid loading with NH(4)Cl HYPO rats displayed a more pronounced metabolic acidosis. The expression of the Na(+)/H(+) exchanger NHE3, the Na(+)-phosphate cotransporter NaPi-IIa, and the B2 subunit of the vacuolar H(+)-ATPase was reduced in the brush-border membrane of the proximal tubule of the HYPO group, paralleled by a lower abundance of the Na(+)/HCO(3)(-) cotransporter NBCe1 and a higher expression of the acid-secretory type A intercalated cell-specific Cl(-)/HCO(3)(-) exchanger AE1. In contrast to control conditions, the expression of NBCe1 was increased in the HYPO group during metabolic acidosis. In addition, net acid excretion was similar in both groups. The relative number of type A intercalated cells was increased in the connecting tubule and cortical collecting duct of the HYPO group during acidosis. Thus thyroid hormones modulate the renal response to an acid challenge and alter the expression of several key acid-base transporters. Mild hypothyroidism is associated only with a very mild defect in renal acid handling, which appears to be mainly located in the proximal tubule and is compensated by the distal nephron.

Molecular basis of pregnancy-induced breast cancer protection

We have postulated that the lifetime protective effect of an early pregnancy against breast cancer is due to the complete differentiation of the mammary gland characterized by a specific genomic signature imprinted by the physiological process of pregnancy. In the present work, we show evidence that the breast tissue of postmenopausal parous women has had a shifting of stem cell 1 to stem cell 2 with a genomic signature different from similar structures derived from postmenopausal nulliparous women that have stem cell 1. Those genes that are significantly different are grouped in major categories on the basis of their putative functional significance. Among them are those gene transcripts related to immune surveillance, DNA repair, transcription, chromatin structure/activators/co-activators, growth factor and signal transduction pathway, transport and cell trafficking, cell proliferation, differentiation, cell adhesion, protein synthesis and cell metabolism. From these data, it was concluded that during pregnancy there are significant genomic changes that reflect profound alterations in the basic physiology of the mammary gland that explain the protective effect against carcinogenesis. The implication of this knowledge is that when the genomic signature of protection or refractoriness to carcinogenesis is acquired by the shifting of stem cell 1 to stem cell 2, the hormonal milieu induced by pregnancy or pregnancy-like conditions is no longer required. This is a novel concept that challenges the current knowledge that a chemopreventive agent needs to be given for a long period to suppress a metabolic pathway or abrogate the function of an organ.

Colonic anion secretory defects and metabolic acidosis in mice lacking the NBC1 Na+/HCO3- cotransporter

The NBC1 Na+/HCO3- cotransporter is expressed in many tissues, including kidney and intestinal epithelia. NBC1 mutations cause proximal renal tubular acidosis in humans, consistent with its role in HCO3- absorption in the kidney. In intestinal and colonic epithelia, NBC1 localizes to basolateral membranes and is thought to function in anion secretion. To test the hypothesis that NBC1 plays a role in transepithelial HCO3- secretion in the intestinal tract, null mutant (NBC1-/-) mice were prepared by targeted disruption of its gene (Slc4a4). NBC1-/- mice exhibited severe metabolic acidosis, growth retardation, reduced plasma Na+, hyperal-dosteronism, splenomegaly, abnormal dentition, intestinal obstructions, and death before weaning. Intracellular pH (pH(i)) was not altered in cAMP-stimulated epithelial cells of NBC1-/- cecum, but pH(i) regulation during sodium removal and readdition was impaired. Bioelectric measurements of NBC1-/- colons revealed increased amiloride-sensitive Na+ absorption. In Ringer solution containing both Cl- and HCO3-, the magnitude of cAMP-stimulated anion secretion was normal in NBC1-/- distal colon but increased in proximal colon, with the increase largely supported by enhanced activity of the basolateral NKCC1 Na+-K+-2Cl- cotransporter. Anion substitution studies in which carbonic anhydrase was inhibited and transepithelial anion conductance was limited to HCO3- revealed a sharp decrease in both cAMP-stimulated HCO3- secretion and SITS-sensitive current in NBC1-/- proximal colon. These results are consistent with the known function of NBC1 in HCO3- absorption in the kidney and demonstrate that NBC1 activity is a component of the basolateral mechanisms for HCO3- uptake during cAMP-stimulated anion secretion in the proximal colon.

Functional characterization of wild-type and a mutated form of SLC26A4 identified in a patient with Pendred syndrome

BACKGROUND

Malfunction of the SLC26A4 protein leads to prelingual deafness often associated with mild thyroid dysfunction and goiter. It is assumed that SLC26A4 acts as a chloride/anion exchanger responsible for the iodide organification in the thyroid gland, and conditioning of the endolymphatic fluid in the inner ear.

METHODS

Chloride uptake studies were made using HEK293-Phoenix cells expressing human wild type SLC26A4 (pendrin) and a mutant (SLC26A4(S28R)) we recently described in a patient with hypothyroidism, goiter and sensorineural hearing loss.

RESULTS

Experiments are summarized showing the functional characterization of wild type SLC26A4 and a mutant (S28R), which we described recently. This mutant protein is transposed towards the cell membrane, however, its transport capability is markedly reduced if compared to wild-type SLC26A4. Furthermore, we show that the SLC26A4 induced chloride uptake in HEK293-Phoenix cells competes with iodide, and, in addition, that the chloride uptake can be blocked by NPPB and niflumic acid, whereas DIDS is ineffective.

CONCLUSIONS

The functional characteristics of SLC26A4(S28R) we describe here, are consistent with the clinical phenotype observed in the patient from which the mutant was derived.

The forkhead transcription factor Foxi1 directly activates the AE4 promoter

Intercalated cells are highly specialized cells within the renal collecting duct epithelium and play an important role in systemic acid-base homoeostasis. Whereas type A intercalated cells secrete protons via an apically localized H+-ATPase, type B intercalated cells secrete HCO3-. Type B intercalated cells specifically express the HCO3-/Cl- exchanger AE4 (anion exchanger 4), encoded by Slc4a9. Mice with a targeted disruption of the gene for the forkhead transcription factor Foxi1 display renal tubular acidosis due to an intercalated cell-differentiation defect. Collecting duct cells in these mice are characterized by the absence of inter-calated cell markers including AE4. To test whether Slc4a9 is a direct target gene of Foxi1, an AE4 promoter construct was generated for a cell-based reporter gene assay. Co-transfection with the Foxi1 cDNA resulted in an approx. 100-fold activation of the AE4 promoter construct. By truncating the AE4 promoter at the 5'-end, we demonstrate that a fragment of approx. 462 bp upstream of the transcription start point is sufficient to mediate activation by Foxi1. Sequence analysis of this region revealed at least eight potential binding sites for Foxi1 in both sense and antisense orientation. Only one element was bound by recombinant Foxi1 protein in bandshift assays. Mutation of this site abolished both binding in bandshift assays and transcriptional activation by co-transfection of Foxi1 in the reporter gene assay. We thus identify the AE4 promoter as a direct target of Foxi1.

Ubiquitous and kidney-specific subunits of vacuolar H+-ATPase are differentially expressed during nephrogenesis

The vacuolar H(+)-ATPase (V-ATPase) is a ubiquitous multisubunit pump that is responsible for acidification of intracellular organelles. In the kidney, a particular form of V-ATPase, made of specific subunits isoforms, has been located at the plasma membrane of intercalated cells (IC). Mutations in genes encoding IC-specific subunits cause infant distal renal tubular acidosis (dRTA), suggesting that the segmental distribution of these subunits is acquired at birth or during early infancy. However, the comparative ontogeny of the IC-specific versus the ubiquitous subunits of V-ATPase and the mechanisms involved in their segmental expression remain unknown. Real-time reverse transcription-PCR, in situ hybridization, immunoblotting, immunostaining, and subcellular fractionation analyses characterized the expression and distribution of V-ATPase subunits, transcription factors, and differentiation markers during mouse nephrogenesis. Ubiquitous A, E1, B2, G1, and C1 subunits showed an early (embryonic day 13.5 [E13.5]) and stable expression throughout nephrogenesis, followed by a slight increase around birth. The developmental pattern of a1 was bimodal, with early induction, gradual decrease during organogenesis, and neonatal increase. These patterns contrasted with the later (from E15.5) and progressive expression of IC-specific a4, B1, G3, and C2 subunits, after the induction of the forkhead transcription factor Foxi1. From E15.5, Foxi1 mRNA was detected in IC, where it co-distributed with B1 in late nephrogenesis. Immunostaining showed that the distribution of ubiquitous E1 and B2 was acquired from E15.5, whereas a4 was located in IC during late nephrogenesis. Subcellular fractionation showed that in both fetal and mature (cortex and medulla) kidneys, E1 and a4 were located in endosomes. These data demonstrate a differential expression and a coordinate regulation of IC-specific versus ubiquitous V-ATPase subunits during nephrogenesis. They provide new insights into the complex regulation of V-ATPase subunits, the maturation of IC along the nephron, and the pathophysiology of hereditary dRTA.

Vacuolar H+-ATPase d2 subunit: molecular characterization, developmental regulation, and localization to specialized proton pumps in kidney and bone

The ubiquitous multisubunit vacuolar-type proton pump (H+- or V-ATPase) is essential for acidification of diverse intracellular compartments. It is also present in specialized forms at the plasma membrane of intercalated cells in the distal nephron, where it is required for urine acidification, and in osteoclasts, playing an important role in bone resorption by acid secretion across the ruffled border membrane. It was reported previously that, in human, several of the renal pump's constituent subunits are encoded by genes that are different from those that are ubiquitously expressed. These paralogous proteins may be important in differential functions, targeting or regulation of H+-ATPases. They include the d subunit, where d1 is ubiquitous whereas d2 has a limited tissue expression. This article reports on an investigation of d2. It was first confirmed that in mouse, as in human, kidney and bone are two of the main sites of d2 mRNA expression. d2 mRNA and protein appear later during nephrogenesis than does the ubiquitously expressed E1 subunit. Mouse nephron-segment reverse transcription-PCR revealed detectable mRNA in all segments except thin limb of Henle's loop and distal convoluted tubule. However, with the use of a novel d2-specific antibody, high-intensity d2 staining was observed only in intercalated cells of the collecting duct in fresh-frozen human kidney, where it co-localized with the a4 subunit in the characteristic plasma membrane-enhanced pattern. In human bone, d2 co-localized with the a3 subunit in osteoclasts. This different subunit association in different tissues emphasizes the possibility of the H+-ATPase as a future therapeutic target.

Renal vacuolar H+-ATPase

Vacuolar H(+)-ATPases are ubiquitous multisubunit complexes mediating the ATP-dependent transport of protons. In addition to their role in acidifying the lumen of various intracellular organelles, vacuolar H(+)-ATPases fulfill special tasks in the kidney. Vacuolar H(+)-ATPases are expressed in the plasma membrane in the kidney almost along the entire length of the nephron with apical and/or basolateral localization patterns. In the proximal tubule, a high number of vacuolar H(+)-ATPases are also found in endosomes, which are acidified by the pump. In addition, vacuolar H(+)-ATPases contribute to proximal tubular bicarbonate reabsorption. The importance in final urinary acidification along the collecting system is highlighted by monogenic defects in two subunits (ATP6V0A4, ATP6V1B1) of the vacuolar H(+)-ATPase in patients with distal renal tubular acidosis. The activity of vacuolar H(+)-ATPases is tightly regulated by a variety of factors such as the acid-base or electrolyte status. This regulation is at least in part mediated by various hormones and protein-protein interactions between regulatory proteins and multiple subunits of the pump.