Mouse model of type II Bartter's syndrome. II. Altered expression of renal sodium- and water-transporting proteins

Macrophages preserve endothelial cell specialization in the adrenal gland to modulate aldosterone secretion and blood pressure

Macrophages play crucial roles in organ-specific functions and homeostasis. In the adrenal gland, macrophages closely associate with sinusoidal capillaries in the aldosterone-producing zona glomerulosa. We demonstrate that macrophages preserve capillary specialization and modulate aldosterone secretion. Using macrophage-specific deletion of VEGF-A, single-cell transcriptomics, and functional phenotyping, we found that the loss of VEGF-A depletes PLVAP+ fenestrated endothelial cells in the zona glomerulosa, leading to increased basement membrane collagen IV deposition and subendothelial fibrosis. This results in increased aldosterone secretion, called "haptosecretagogue" signaling. Human aldosterone-producing adenomas also show capillary rarefaction and basement membrane thickening. Mice with myeloid cell-specific VEGF-A deletion exhibit elevated serum aldosterone, hypokalemia, and hypertension, mimicking primary aldosteronism. These findings underscore macrophage-to-endothelial cell signaling as essential for endothelial cell specialization, adrenal gland function, and blood pressure regulation, with broader implications for other endocrine organs.

A novel mouse model for familial hypocalciuric hypercalcemia (FHH1) reveals PTH-dependent and independent CaSR defects

The Calcium-sensing receptor (CaSR) senses extracellular calcium, regulates parathyroid hormone (PTH) secretion, and has additional functions in various organs related to systemic and local calcium and mineral homeostasis. Familial hypocalciuric hypercalcemia type I (FHH1) is caused by heterozygous loss-of-function mutations in the CaSR gene, and is characterized by the combination of hypercalcemia, hypocalciuria, normal to elevated PTH, and facultatively hypermagnesemia and mild bone mineralization defects. To date, only heterozygous Casr null mice have been available as model for FHH1. Here we present a novel mouse FHH1 model identified in a large ENU-screen that carries an c.2579 T > A (p.Ile859Asn) variant in the Casr gene (CasrBCH002 mice). In order to dissect direct effects of the genetic variant from PTH-dependent effects, we crossed CasrBCH002 mice with PTH deficient mice. Heterozygous CasrBCH002 mice were fertile, had normal growth and body weight, were hypercalcemic and hypermagnesemic with inappropriately normal PTH levels and urinary calcium excretion replicating some features of FHH1. Hypercalcemia and hypermagnesemia were independent from PTH and correlated with higher expression of claudin 16 and 19 in kidneys. Likewise, reduced expression of the renal TRPM6 channel in CasrBCH002 mice was not dependent on PTH. In bone, mutations in Casr rescued the bone phenotype observed in Pth null mice by increasing osteoclast numbers and improving the columnar pattern of chondrocytes in the growth zone. In summary, CasrBCH002 mice represent a new model to study FHH1 and our results indicate that only a part of the phenotype is driven by PTH.

Lectin-mediated, time-efficient, and high-yield sorting of different morphologically intact nephron segments

The kidney is a highly complex organ equipped with a multitude of miniscule filter-tubule units called nephrons. Each nephron can be subdivided into multiple segments, each with its own morphology and physiological function. To date, conventional manual approaches to isolate specific nephron segments are very laborious, time-consuming, often limited to only a specific segment, and typically have low yield. Here, we describe a novel, unconventional method that is superior in many aspects to previous protocols by combining low-cost fluorophore-conjugated lectins or agglutinins (Flaggs) with flow sorting. This allows the simultaneous separation of different nephron segments with preserved 3D morphology from mouse or human samples in under 3 h. Using a 200-µm nozzle and 5 psi, glomeruli, proximal, or distal convoluted tubules are sorted with Cy3-labeled Sambucus Nigra agglutinin (SNA-Cy3), Fluorescein-labeled Lotus Tetragonolobus lectin (LTL-FITC), or Pacific Blue-labeled soybean agglutinin (SBA-PB), respectively. Connecting tubules and collecting ducts are sorted by double-positive SBA-PB and SNA-Cy3 signals, while thick ascending limb segments are characterized by the absence of any Flaggs labeling. From two mouse kidneys, this yields 37-521 ng protein/s or 0.71-16.71 ng RNA/s, depending on the specific nephron segment. The purity of sorted segments, as assessed by mRNA expression level profiling of 15 genes, is very high with a 96.1-fold median enrichment across all genes and sorted segments. In summary, our method represents a simple, straightforward, cost-effective, and widely applicable tool yielding high amounts of pure and morphologically largely intact renal tubule materials with the potential to propel nephron segment-specific research.

Fluxomics reveals cellular and molecular basis of increased renal ammoniagenesis

The kidney plays a critical role in excreting ammonia during metabolic acidosis and liver failure. The mechanisms behind this process have been poorly explored. The present study combines results of in vivo experiments of increased total ammoniagenesis with systems biology modeling, in which eight rats were fed an amino acid-rich diet (HD group) and eight a normal chow diet (AL group). We developed a method based on elementary mode analysis to study changes in amino acid flux occurring across the kidney in increased ammoniagenesis. Elementary modes represent minimal feasible metabolic paths in steady state. The model was used to predict amino acid fluxes in healthy and pre-hyperammonemic conditions, which were compared to experimental fluxes in rats. First, we found that total renal ammoniagenesis increased from 264 ± 68 to 612 ± 87 nmol (100 g body weight)^-1 min^-1 in the HD group (P = 0.021) and a concomitated upregulation of NKCC2 ammonia and other transporters in the kidney. In the kidney metabolic model, the best predictions were obtained with ammonia transport as an objective. Other objectives resulting in a fair correlation with the measured fluxes (correlation coefficient >0.5) were growth, protein uptake, urea excretion, and lysine and phenylalanine transport. These predictions were improved when specific gene expression data were considered in HD conditions, suggesting a role for the mitochondrial glycine pathway. Further studies are needed to determine if regulation through the mitochondrial glycine pathway and ammonia transporters can be modulated and how to use the kidney as a therapeutic target in hyperammonemia.

Tubular IL-1β Induces Salt Sensitivity in Diabetes by Activating Renal Macrophages

BACKGROUND

Chronic renal inflammation has been widely recognized as a major promoter of several forms of high blood pressure including salt-sensitive hypertension. In diabetes, IL (interleukin)-6 induces salt sensitivity through a dysregulation of the epithelial sodium channel. However, the origin of this inflammatory process and the molecular events that culminates with an abnormal regulation of epithelial sodium channel and salt sensitivity in diabetes are largely unknown.

METHODS

Both in vitro and in vivo approaches were used to investigate the molecular and cellular contributors to the renal inflammation associated with diabetic kidney disease and how these inflammatory components interact to develop salt sensitivity in db/db mice.

RESULTS

Thirty-four-week-old db/db mice display significantly higher levels of IL-1β in renal tubules compared with nondiabetic db/+ mice. Specific suppression of IL-1β in renal tubules prevented salt sensitivity in db/db mice. A primary culture of renal tubular epithelial cells from wild-type mice releases significant levels of IL-1β when exposed to a high glucose environment. Coculture of tubular epithelial cells and bone marrow-derived macrophages revealed that tubular epithelial cell-derived IL-1β promotes the polarization of macrophages towards a proinflammatory phenotype resulting in IL-6 secretion. To evaluate whether macrophages are the cellular target of IL-1β in vivo, diabetic db/db mice were transplanted with the bone marrow of IL-1R1 (IL-1 receptor type 1) knockout mice. db/db mice harboring an IL-1 receptor type 1 knockout bone marrow remained salt resistant, display lower renal inflammation and lower expression and activity of epithelial sodium channel compared with db/db transplanted with a wild-type bone marrow.

CONCLUSIONS

Renal tubular epithelial cell-derived IL-1β polarizes renal macrophages towards a proinflammatory phenotype that promotes salt sensitivity through the accumulation of renal IL-6. When tubular IL-1β synthesis is suppressed or in db/db mice in which immune cells lack the IL-1R1, macrophage polarization is blunted resulting in no salt-sensitive hypertension.

Activation of the Hypoxia-Inducible Factor Pathway Inhibits Epithelial Sodium Channel-Mediated Sodium Transport in Collecting Duct Principal Cells

BACKGROUND

Active sodium reabsorption is the major factor influencing renal oxygen consumption and production of reactive oxygen species (ROS). Increased sodium reabsorption uses more oxygen, which may worsen medullary hypoxia and produce more ROS via enhanced mitochondrial ATP synthesis. Both mechanisms may activate the hypoxia-inducible factor (HIF) pathway. Because the collecting duct is exposed to low oxygen pressure and variations of active sodium transport, we assessed whether the HIF pathway controls epithelial sodium channel (ENaC)-dependent sodium transport.

METHODS

We investigated HIF's effect on ENaC expression in mpkCCD cl4 cells (a model of collecting duct principal cells) using real-time PCR and western blot and ENaC activity by measuring amiloride-sensitive current. We also assessed the effect of hypoxia and sodium intake on abundance of kidney sodium transporters in wild-type and inducible kidney tubule-specific Hif1α knockout mice.

RESULTS

In cultured cells, activation of the HIF pathway by dimethyloxalylglycine or hypoxia inhibited sodium transport and decreased expression of β ENaC and γ ENaC, as well as of Na,K-ATPase. HIF1 α silencing increased β ENaC and γ ENaC expression and stimulated sodium transport. A constitutively active mutant of HIF1 α produced the opposite effect. Aldosterone and inhibition of the mitochondrial respiratory chain slowly activated the HIF pathway, suggesting that ROS may also activate HIF. Decreased γ ENaC abundance induced by hypoxia in normal mice was abolished in Hif1α knockout mice. Similarly, Hif1α knockout led to increased γ ENaC abundance under high sodium intake.

CONCLUSIONS

This study reveals that γ ENaC expression and activity are physiologically controlled by the HIF pathway, which may represent a negative feedback mechanism to preserve oxygenation and/or prevent excessive ROS generation under increased sodium transport.

Sex-specific adaptations to high-salt diet preserve electrolyte homeostasis with distinct sodium transporter profiles

Kidneys continuously filter an enormous amount of sodium and adapt kidney Na+ reabsorption to match Na+ intake to maintain circulatory volume and electrolyte homeostasis. Males (M) respond to high-salt (HS) diet by translocating proximal tubule Na+/H+ exchanger isoform 3 (NHE3) to the base of the microvilli, reducing activated forms of the distal NaCl cotransporter (NCC) and epithelial Na+ channel (ENaC). Males (M) and females (F) on normal-salt (NS) diet present sex-specific profiles of "transporters" (cotransporters, channels, pumps, and claudins) along the nephron, e.g., F exhibit 40% lower NHE3 and 200% higher NCC abundance than M. We tested the hypothesis that adaptations to HS diet along the nephron will, likewise, exhibit sexual dimorphisms. C57BL/6J mice were fed for 15 days with 4% NaCl diet (HS) versus 0.26% NaCl diet (NS). On HS, M and F exhibited normal plasma [Na+] and [K+], similar urine volume, Na+, K+, and osmolal excretion rates normalized to body weight. In F, like M, HS lowered abundance of distal NCC, phosphorylated NCC, and cleaved (activated) forms of ENaC. The adaptations associated with achieving electrolyte homeostasis exhibit sex-dependent and independent mechanisms. Sex differences in baseline "transporters" abundance persist during HS diet, yet the fold changes during HS diet (normalized to NS) are similar along the distal nephron and collecting duct. Sex-dependent differences observed along the proximal tubule during HS show that female kidneys adapt differently from patterns reported in males, yet achieve and maintain fluid and electrolyte homeostasis.

A five amino acids deletion in NKCC2 of C57BL/6 mice affects analysis of NKCC2 phosphorylation but does not impact kidney function

Aim

The phosphorylation level of the furosemide‐sensitive Na⁺‐K⁺‐2Cl⁻ cotransporter (NKCC2) in the thick ascending limb (TAL) is used as a surrogate marker for NKCC2 activation and TAL function. However, in mice, analyses of NKCC2 phosphorylation with antibodies against phosphorylated threonines 96 and 101 (anti‐pT96/pT101) give inconsistent results. We aimed (a) to elucidate these inconsistencies and (b) to develop a phosphoform‐specific antibody that ensures reliable detection of NKCC2 phosphorylation in mice.

Methods

Genetic information, molecular biology, biochemical techniques and mouse phenotyping was used to study NKCC2 and kidney function in two commonly used mouse strains (ie 129Sv and in C57BL/6 mice). Moreover, a new phosphoform‐specific mouse NKCC2 antibody was developed and characterized.

Results

Amino acids sequence alignment revealed that C57BL/6 mice have a strain‐specific five amino acids deletion (ΔF97‐T101) in NKCC2 that diminishes the detection of NKCC2 phosphorylation with previously developed pT96/pT101 NKCC2 antibodies. Instead, the antibodies cross‐react with the phosphorylated thiazide‐sensitive NaCl cotransporter (NCC), which can obscure interpretation of results. Interestingly, the deletion in NKCC2 does not impact on kidney function and/or expression of renal ion transport proteins as indicated by the analysis of the F2 generation of crossbred 129Sv and C57BL/6 mice. A newly developed pT96 NKCC2 antibody detects pNKCC2 in both mouse strains and shows no cross‐reactivity with phosphorylated NCC.

Conclusion

Our work reveals a hitherto unappreciated, but essential, strain difference in the amino acids sequence of mouse NKCC2 that needs to be considered when analysing NKCC2 phosphorylation in mice. The new pNKCC2 antibody circumvents this technical caveat.

Differential localization patterns of claudin 10, 16, and 19 in human, mouse, and rat renal tubular epithelia

Functional properties of the paracellular pathway depend critically on the set of claudins (CLDN) expressed at the tight junction. Two syndromes are causally linked to loss-of-function mutations of claudins: hypohidrosis, electrolyte imbalance, lacrimal gland dysfunction, ichthyosis, and xerostomia (HELIX) syndrome caused by genetic variations in the CLDN10 gene and familial hypomagnesemia with hypercalciuria and nephrocalcinosis caused by genetic variations in the CLDN16 or CLDN19 genes. All three genes are expressed in the kidney, particularly in the thick ascending limb (TAL). However, localization of these claudins in humans and rodents remains to be delineated in detail. We studied the segmental and subcellular expression of CLDN10, CLDN16, and CLDN19 in both paraffin-embedded and frozen kidney sections from the adult human, mouse, and rat using immunohistochemistry and immunofluorescence, respectively. Here, CLDN10 was present in a subset of medullary and cortical TAL cells, localizing to basolateral domains and tight junctions in human and rodent kidneys. Weak expression was detected at the tight junction of proximal tubular cells. CLDN16 was primarily expressed in a subset of TAL cells in the cortex and outer stripe of outer medulla, restricted to basolateral domains and tight junctional structures in both human and rodent kidneys. CLDN19 predominantly colocalized with CLDN16 in tight junctions and basolateral domains of the TAL but was also found in basolateral and junctional domains in more distal sites. CLDN10 expression at tight junctions almost never overlapped with that of CLND16 and CLDN19, consistent with distinct junctional pathways with different permeation profiles in both human and rodent kidneys.NEW & NOTEWORTHY This study used immunohistochemistry and immunofluorescence to investigate the distribution of claudin 10, 16, and 19 in the human, mouse, and rat kidney. The findings showed distinct junctional pathways in both human and rodent kidneys, supporting the existence of different permeation profiles in all species investigated.

Benzamil-mediated urine alkalization is caused by the inhibition of H+-K+-ATPases

Epithelial Na⁺ channel (ENaC) blockers elicit acute and substantial increases of urinary pH. The underlying mechanism remains to be understood. Here, we evaluated if benzamil-induced urine alkalization is mediated by an acute reduction in H⁺ secretion via renal H⁺-K⁺-ATPases (HKAs). Experiments were performed in vivo on HKA double-knockout and wild-type mice. Alterations in dietary K⁺ intake were used to change renal HKA and ENaC activity. The acute effects of benzamil (0.2 µg/g body wt, sufficient to block ENaC) on urine flow rate and urinary electrolyte and acid excretion were monitored in anesthetized, bladder-catheterized animals. We observed that benzamil acutely increased urinary pH (ΔpH: 0.33 ± 0.07) and reduced NH₄⁺ and titratable acid excretion and that these effects were distinctly enhanced in animals fed a low-K⁺ diet (ΔpH: 0.74 ± 0.12), a condition when ENaC activity is low. In contrast, benzamil did not affect urine acid excretion in animals kept on a high-K⁺ diet (i.e., during high ENaC activity). Thus, urine alkalization appeared completely uncoupled from ENaC function. The absence of benzamil-induced urinary alkalization in HKA double-knockout mice confirmed the direct involvement of these enzymes. The inhibitory effect of benzamil was also shown in vitro for the pig α₁-isoform of HKA. These results suggest a revised explanation of the benzamil effect on renal acid-base excretion. Considering the conditions used here, we suggest that it is caused by a direct inhibition of HKAs in the collecting duct and not by inhibition of the ENaC function.NEW & NOTEWORTHY Bolus application of epithelial Na⁺ channel (EnaC) blockers causes marked and acute increases of urine pH. Here, we provide evidence that the underlying mechanism involves direct inhibition of the H⁺-K⁺ pump in the collecting duct. This could provide a fundamental revision of the previously assumed mechanism that suggested a key role of ENaC inhibition in this response.

ENaC expression correlates with the acute furosemide-induced K+ excretion

BACKGROUND

In the aldosterone-sensitive distal nephron (ASDN), epithelial sodium channel (ENaC)-mediated Na+ absorption drives K+ excretion. K+ excretion depends on the delivery of Na+ to the ASDN and molecularly activated ENaC. Furosemide is known as a K+ wasting diuretic as it greatly enhances Na+ delivery to the ASDN. Here, we studied the magnitude of acute furosemide-induced kaliuresis under various states of basal molecular ENaC activity.

METHODS

C57/Bl6J mice were subjected to different dietary regimens that regulate molecular ENaC expression and activity levels. The animals were anesthetized and bladder-catheterized. Diuresis was continuously measured before and after administration of furosemide (2 µg/g BW) or benzamil (0.2 µg/g BW). Flame photometry was used to measure urinary [Na+ ] and [K+ ]. The kidneys were harvested and, subsequently, ENaC expression and cleavage activation were determined by semiquantitative western blotting.

RESULTS

A low K+ and a high Na+ diet markedly suppressed ENaC protein expression, cleavage activation, and furosemide-induced kaliuresis. In contrast, furosemide-induced kaliuresis was greatly enhanced in animals fed a high K+ or low Na+ diet, conditions with increased ENaC expression. The furosemide-induced diuresis was similar in all dietary groups.

CONCLUSION

Acute furosemide-induced kaliuresis differs greatly and depends on the a priori molecular expression level of ENaC. Remarkably, it can be even absent in animals fed a high Na+ diet, despite a marked increase of tubular flow and urinary Na+ excretion. This study provides auxiliary evidence that acute ENaC-dependent K+ excretion requires both Na+ as substrate and molecular activation of ENaC.

Coordinate adaptations of skeletal muscle and kidney to maintain extracellular [K+] during K+-deficient diet

Extracellular fluid (ECF) potassium concentration ([K+]) is maintained by adaptations of kidney and skeletal muscle, responses heretofore studied separately. We aimed to determine how these organ systems work in concert to preserve ECF [K+] in male C57BL/6J mice fed a K+-deficient diet (0K) versus 1% K+ diet (1K) for 10 days (n = 5-6/group). During 0K feeding, plasma [K+] fell from 4.5 to 2 mM; hindlimb muscle (gastrocnemius and soleus) lost 28 mM K+ (from 115 ± 2 to 87 ± 2 mM) and gained 27 mM Na+ (from 27 ± 0.4 to 54 ± 2 mM). Doubling of muscle tissue [Na+] was not associated with inflammation, cytokine production or hypertension as reported by others. Muscle transporter adaptations in 0K- versus 1K-fed mice, assessed by immunoblot, included decreased sodium pump α2-β2 subunits, decreased K+-Cl- cotransporter isoform 3, and increased phosphorylated (p) Na+,K+,2Cl- cotransporter isoform 1 (NKCC1p), Ste20/SPS-1-related proline-alanine rich kinase (SPAKp), and oxidative stress-responsive kinase 1 (OSR1p) consistent with intracellular fluid (ICF) K+ loss and Na+ gain. Renal transporters' adaptations, effecting a 98% reduction in K+ excretion, included two- to threefold increased phosphorylated Na+-Cl- cotransporter (NCCp), SPAKp, and OSR1p abundance, limiting Na+ delivery to epithelial Na+ channels where Na+ reabsorption drives K+ secretion; and renal K sensor Kir 4.1 abundance fell 25%. Mass balance estimations indicate that over 10 days of 0K feeding, mice lose ~48 μmol K+ into the urine and muscle shifts ~47 μmol K+ from ICF to ECF, illustrating the importance of the concerted responses during K+ deficiency.

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.

Time-course of sodium transport along the nephron in nephrotic syndrome: The role of potassium

The mechanism of sodium retention and its location in kidney tubules may vary with time in nephrotic syndrome (NS). We studied the mechanisms of sodium retention in transgenic POD-ATTAC mice, which display an inducible podocyte-specific apoptosis. At day 2 after the induction of NS, the increased abundance of NHE3 and phosphorylated NCC in nephrotic mice compared with controls suggest that early sodium retention occurs mainly in the proximal and distal tubules. At day 3, the abundance of NHE3 normalized, phosphorylated NCC levels decreased, and cleavage and apical localization of γ-ENaC increased in nephrotic mice. These findings indicate that sodium retention shifted from the proximal and distal tubules to the collecting system. Increased cleavage and apical localization of γ-ENaC persisted at day 5 in nephrotic mice when hypovolemia resolved and steady-state was reached. Sodium retention and γ-ENaC cleavage were independent of the increased plasma levels of aldosterone. Nephrotic mice displayed decreased glomerular filtration rate and urinary potassium excretion associated with hyperkaliemia at day 3. Feeding nephrotic mice with a low potassium diet prevented hyperkaliemia, γ-ENaC cleavage, and led to persistent increased phosphorylation of NCC. These results suggest that potassium homeostasis is a major determinant of the tubular site of sodium retention in nephrotic mice.

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.

Nephrotic syndrome is associated with increased plasma K+ concentration, intestinal K+ losses, and attenuated urinary K+ excretion: a study in rats and humans

The present study tested the hypotheses that nephrotic syndrome (NS) leads to renal K⁺ loss because of augmented epithelial Na⁺ channel (ENaC) activity followed by downregulation of renal K⁺ secretory pathways by suppressed aldosterone. The hypotheses were addressed by determining K⁺ balance and kidney abundance of K⁺ and Na⁺ transporter proteins in puromycin aminonucleoside (PAN)-induced rat nephrosis. The effects of amiloride and angiotensin II type 1 receptor and mineralocorticoid receptor (MR) antagonists were tested. Glucocorticoid-dependent MR activation was tested by suppression of endogenous glucocorticoid with dexamethasone. Urine and plasma samples were obtained from pediatric patients with NS in acute and remission phases. PAN-induced nephrotic rats had ENaC-dependent Na⁺ retention and displayed lower renal K⁺ excretion but elevated intestinal K⁺ secretion that resulted in less cumulated K⁺ in NS. Aldosterone was suppressed at day 8. The NS-associated changes in intestinal, but not renal, K⁺ handling responded to suppression of corticosterone, whereas angiotensin II type 1 receptor and MR blockers and amiloride had no effect on urine K⁺ excretion during NS. In PAN-induced nephrosis, kidney protein abundance of the renal outer medullary K⁺ channel and γ-ENaC were unchanged, whereas the Na⁺-Cl^- cotransporter was suppressed and Na⁺-K⁺-ATPase increased. Pediatric patients with acute NS displayed suppressed urine Na⁺-to-K⁺ ratios compared with remission and elevated plasma K⁺ concentration, whereas fractional K⁺ excretion did not differ. Acute NS is associated with less cumulated K⁺ in a rat model, whereas patients with acute NS have elevated plasma K⁺ and normal renal fractional K⁺ excretion. In NS rats, K⁺ balance is not coupled to ENaC activity but results from opposite changes in renal and fecal K⁺ excretion with a contribution from corticosteroid MR-driven colonic secretion.

Learning Physiology From Inherited Kidney Disorders

The identification of genes causing inherited kidney diseases yielded crucial insights in the molecular basis of disease and improved our understanding of physiological processes that operate in the kidney. Monogenic kidney disorders are caused by mutations in genes coding for a large variety of proteins including receptors, channels and transporters, enzymes, transcription factors, and structural components, operating in specialized cell types that perform highly regulated homeostatic functions. Common variants in some of these genes are also associated with complex traits, as evidenced by genome-wide association studies in the general population. In this review, we discuss how the molecular genetics of inherited disorders affecting different tubular segments of the nephron improved our understanding of various transport processes and of their involvement in homeostasis, while providing novel therapeutic targets. These include inherited disorders causing a dysfunction of the proximal tubule (renal Fanconi syndrome), with emphasis on epithelial differentiation and receptor-mediated endocytosis, or affecting the reabsorption of glucose, the handling of uric acid, and the reabsorption of sodium, calcium, and magnesium along the kidney tubule.

The serine-threonine kinase PIM3 is an aldosterone-regulated protein in the distal nephron

The mineralocorticoid hormone aldosterone plays a crucial role in the control of Na+ and K+ balance, blood volume, and arterial blood pressure, by acting in the aldosterone-sensitive distal nephron (ASDN) and stimulating a complex transcriptional, translational, and cellular program. Because the complexity of the aldosterone response is still not fully appreciated, we aimed at identifying new elements in this pathway. Here, we demonstrate that the expression of the proto-oncogene PIM3 (Proviral Integration Site of Moloney Murine Leukemia Virus 3), a serine/threonine kinase belonging to the calcium/calmodulin-regulated group of kinases, is stimulated by aldosterone in vitro (mCCDcl1 cells), ex vivo (mouse kidney slices), and in vivo in mice. Characterizing a germline Pim3-/- mouse model, we found that these mice have an upregulated Renin-Angiotensin-Aldosterone System (RAAS), with high circulating aldosterone and plasma renin activity levels on both standard or Na+ -deficient diet. Surprisingly, we did not observe any obvious salt-losing phenotype in Pim3 KO mice as shown by normal blood pressure, plasma and urinary electrolytes, as well as unchanged expression levels of the major Na+ transport proteins. These observations suggest that the potential effects of the loss of the Pim3 gene are physiologically compensated. Indeed, the 2 other family members of the PIM kinase family, PIM1 and PIM2 are upregulated in the kidney of Pim3-/- mice, and may therefore be involved in such compensation. In conclusion, our data demonstrate that the PIM3 kinase is a novel aldosterone-induced protein, but its precise role in aldosterone-dependent renal homeostasis remains to be determined.

Podocyte Injury Augments Intrarenal Angiotensin II Generation and Sodium Retention in a Megalin-Dependent Manner

We have previously shown that podocyte injury increases the glomerular filtration of liver-derived Agt (angiotensinogen) and the generation of intrarenal Ang II (angiotensin II) and that the filtered Agt is reabsorbed by proximal tubules in a manner dependent on megalin. In the present study, we aimed to study the role of megalin in the generation of renal Ang II and sodium handling during nephrotic syndrome. We generated proximal tubule-specific megalin KO (knockout) mice and crossed these animals with NEP25 mice, in which podocyte-specific injury can be induced by injection of the immunotoxin LMB2. Without podocyte injury, renal Agt staining was markedly diminished and urinary Agt increased in KO mice. However, renal Ang II was similar between KO and control mice on average: 117 (95% CI, 101-134) versus 101 (95% CI, 68-133) fmol/g tissue. We next tested the effect of megalin KO on intrarenal Ang II generation with podocyte injury. Control NEP25 mice showed markedly increased renal Agt staining and renal Ang II levels: 450 (336-565) fmol/g tissue. Megalin KO/NEP25 mice showed markedly diminished Agt reabsorption and attenuated renal Ang II: 199 (156-242) fmol/g tissue (P<0.001). Compared with control NEP25 mice, megalin KO/NEP25 mice excreted 5-fold more sodium in the urine. Western blot analysis showed that megalin KO decreased NHE3 and the cleaved α and γ forms of Epithelial Na Channel. These data indicate that Agt reabsorbed by proximal tubules via megalin in nephrotic syndrome is converted to Ang II, which may contribute to sodium retention and edema formation by activating NHE3 and Epithelial Na Channel.

Protein Phosphatase 1 Inhibitor-1 Mediates the cAMP-Dependent Stimulation of the Renal NaCl Cotransporter

BACKGROUND

A number of cAMP-elevating hormones stimulate phosphorylation (and hence activity) of the NaCl cotransporter (NCC) in the distal convoluted tubule (DCT). Evidence suggests that protein phosphatase 1 (PP1) and other protein phosphatases modulate NCC phosphorylation, but little is known about PP1's role and the mechanism regulating its function in the DCT.

METHODS

We used ex vivo mouse kidney preparations to test whether a DCT-enriched inhibitor of PP1, protein phosphatase 1 inhibitor-1 (I1), mediates cAMP's effects on NCC, and conducted yeast two-hybrid and coimmunoprecipitation experiments in NCC-expressing MDCK cells to explore protein interactions.

RESULTS

Treating isolated DCTs with forskolin and IBMX increased NCC phosphorylation via a protein kinase A (PKA)-dependent pathway. Ex vivo incubation of mouse kidney slices with isoproterenol, norepinephrine, and parathyroid hormone similarly increased NCC phosphorylation. The cAMP-induced stimulation of NCC phosphorylation strongly correlated with the phosphorylation of I1 at its PKA consensus phosphorylation site (a threonine residue in position 35). We also found an interaction between NCC and the I1-target PP1. Moreover, PP1 dephosphorylated NCC in vitro, and the PP1 inhibitor calyculin A increased NCC phosphorylation. Studies in kidney slices and isolated perfused kidneys of control and I1-KO mice demonstrated that I1 participates in the cAMP-induced stimulation of NCC.

CONCLUSIONS

Our data suggest a complete signal transduction pathway by which cAMP increases NCC phosphorylation via a PKA-dependent phosphorylation of I1 and subsequent inhibition of PP1. This pathway might be relevant for the physiologic regulation of renal sodium handling by cAMP-elevating hormones, and may contribute to salt-sensitive hypertension in patients with endocrine disorders or sympathetic hyperactivity.

Molecular mechanisms for the regulation of blood pressure by potassium

It has been well documented that the amount of potassium in the diet is associated with blood pressure levels in the population: the higher the potassium consumption, the lower the blood pressure and the cardiovascular mortality. In the last few years certain mechanisms for potassium regulation of salt reabsorption in the kidney have been elucidated at the molecular level. In this work we discuss the evidence demonstrating the relationship between potassium intake and blood pressure levels in human populations and in animal models, as well as the experimental data that reveal the effects of potassium on transepithelial Na⁺ reabsorption in different nephron segments. We also discuss the physiological relevance of K⁺-induced natriuresis, and finally, we focus on the molecular mechanisms by which extracellular potassium modulates the activity of the renal NaCl cotransporter, which is the mechanism that has been best dissected so far.

The renal and blood pressure response to low sodium diet in P2X4 receptor knockout mice

In the kidney, purinergic (P2) receptor-mediated ATP signaling has been shown to be an important local regulator of epithelial sodium transport. Appropriate sodium regulation is crucial for blood pressure (BP) control and disturbances in sodium balance can lead to hypo- or hypertension. Links have already been established between P2 receptor signaling and the development of hypertension, attributed mainly to vascular and/or inflammatory effects. A transgenic mouse model with deletion of the P2X4 receptor (P2X4-/- ) is known to have hypertension, which is thought to reflect endothelial dysfunction and impaired nitric oxide (NO) release. However, renal function in this model has not been characterized; moreover, studies in vitro have shown that the P2X4 receptor can regulate renal epithelial Na+ channel (ENaC) activity. Therefore, in the present study we investigated renal function and sodium handling in P2X4-/- mice, focusing on ENaC-mediated Na+ reabsorption. We confirmed an elevated BP in P2X4-/- mice compared with wild-type mice, but found that ENaC-mediated Na+ reabsorption is no different from wild-type and does not contribute to the raised BP observed in the knockout. However, when P2X4-/- mice were placed on a low sodium diet, BP normalized. Plasma aldosterone concentration tended to increase according to sodium restriction status in both genotypes; in contrast to wild-types, P2X4-/- mice did not show an increase in functional ENaC activity. Thus, although the increased BP in P2X4-/- mice has been attributed to endothelial dysfunction and impaired NO release, there is also a sodium-sensitive component.

Hydronephrosis is associated with elevated plasmin in urine in pediatric patients and rats and changes in NCC and γ-ENaC abundance in rat kidney

Obstruction of urine flow at the level of the pelvo-ureteric junction (UPJO) and subsequent development of hydronephrosis is one of the most common congenital renal malformations. UPJO is associated with development of salt-sensitive hypertension, which is set by the obstructed kidney, and with a stimulated renin-angiotensin-aldosterone system (RAAS) in rodent models. This study aimed at investigating the hypothesis that 1) in pediatric patients with UPJO the RAAS is activated before surgical relief of the obstruction; 2) in rats with UPJO the RAAS activation is reflected by increased abundance of renal aldosterone-stimulated Na transporters; and 3) the injured UPJO kidney allows aberrant filtration of plasminogen, leading to proteolytic activation of the epithelial Na channel γ-subunit (γ-ENaC). Hydronephrosis resulting from UPJO in pediatric patients and rats was associated with increased urinary plasminogen-to-creatinine ratio. In pediatric patients, plasma renin, angiotensin II, urine and plasma aldosterone, and urine soluble prorenin receptor did not differ significantly before or after surgery, or compared with controls. Increased plasmin-to-plasminogen ratio was seen in UPJO rats. Intact γ-ENaC abundance was not changed in UPJO kidney, whereas low-molecular cleavage product abundance increased. The Na-Cl cotransporter displayed significantly lower abundance in the UPJO kidney compared with the nonobstructed contralateral kidney. The Na-K-ATPase α-subunit was unaltered. Treatment with an angiotensin-converting enzyme inhibitor (8 days, captopril) significantly lowered blood pressure in UPJO rats. It is concluded that the RAAS contributes to hypertension following partial obstruction of urine flow at the pelvo-ureteric junction with potential contribution from proteolytic activation of ENaC.

Arginase-II negatively regulates renal aquaporin-2 and water reabsorption

Type-II l-arginine:ureahydrolase, arginase-II (Arg-II), is abundantly expressed in the kidney. The physiologic role played by Arg-II in the kidney remains unknown. Herein, we report that in mice that are deficient in Arg-II (Arg-II^−/−), total and membrane-associated aquaporin-2 (AQP2) protein levels were significantly higher compared with wild-type (WT) controls. Water deprivation enhanced Arg-II expression, AQP2 levels, and membrane association in collecting ducts. Effects of water deprivation on AQP2 were stronger in Arg-II^−/− mice than in WT mice. Accordingly, a decrease in urine volume and an increase in urine osmolality under water deprivation were more pronounced in Arg-II^−/− mice than in WT mice, which correlated with a weaker increase in plasma osmolality in Arg-II^−/− mice. There was no difference in vasopressin release under water deprivation conditions between either genotype of mice. Although total AQP2 and phosphorylated AQP2-S256 levels (mediated by PKA) in kidneys under water deprivation conditions were significantly higher in Arg-II^−/− mice compared with WT animals, there is no difference in the ratio of AQP2-S256:AQP2. In cultured mouse collecting duct principal mCCD_cl1 cells, expression of both Arg-II and AQP2 were enhanced by the vasopressin type 2 receptor agonist, desamino-d-arginine vasopressin (dDAVP). Silencing Arg-II enhanced the expression and membrane association of AQP2 by dDAVP without influencing cAMP levels. In conclusion, in vivo and in vitro experiments demonstrate that Arg-II negatively regulates AQP2 and the urine-concentrating capability in kidneys via a mechanism that is not associated with the modulation of the cAMP pathway.—Huang, J., Montani, J.-P., Verrey, F., Feraille, E., Ming, X.-F., Yang, Z. Arginase-II negatively regulates renal aquaporin-2 and water reabsorption.

Acute genetic ablation of pendrin lowers blood pressure in mice

Background

Pendrin, the chloride/bicarbonate exchanger of β-intercalated cells of the renal connecting tubule and the collecting duct, plays a key role in NaCl reabsorption by the distal nephron. Therefore, pendrin may be important for the control of extracellular fluid volume and blood pressure.

Methods

Here, we have used a genetic mouse model in which the expression of pendrin can be switched-on in vivo by the administration of doxycycline. Pendrin can also be rapidly removed when doxycycline administration is discontinued. Therefore, our genetic strategy allows us to test selectively the acute effects of loss of pendrin function.

Results

We show that acute loss of pendrin leads to a significant decrease of blood pressure. In addition, acute ablation of pendrin did not alter significantly the acid-base status or blood K +  concentration.

Conclusion

By using a transgenic mouse model, avoiding off-target effects related to pharmacological compounds, this study suggests that pendrin could be a novel target to treat hypertension.

Plasma Potassium Determines NCC Abundance in Adult Kidney-Specific γENaC Knockout

The amiloride-sensitive epithelial sodium channel (ENaC) and the thiazide-sensitive sodium chloride cotransporter (NCC) are key regulators of sodium and potassium and colocalize in the late distal convoluted tubule of the kidney. Loss of the αENaC subunit leads to a perinatal lethal phenotype characterized by sodium loss and hyperkalemia resembling the human syndrome pseudohypoaldosteronism type 1 (PHA-I). In adulthood, inducible nephron-specific deletion of αENaC in mice mimics the lethal phenotype observed in neonates, and as in humans, this phenotype is prevented by a high sodium (HNa⁺)/low potassium (LK⁺) rescue diet. Rescue reflects activation of NCC, which is suppressed at baseline by elevated plasma potassium concentration. In this study, we investigated the role of the γENaC subunit in the PHA-I phenotype. Nephron-specific γENaC knockout mice also presented with salt-wasting syndrome and severe hyperkalemia. Unlike mice lacking αENaC or βΕΝaC, an HNa⁺/LK⁺ diet did not normalize plasma potassium (K⁺) concentration or increase NCC activation. However, when K⁺ was eliminated from the diet at the time that γENaC was deleted, plasma K⁺ concentration and NCC activity remained normal, and progressive weight loss was prevented. Loss of the late distal convoluted tubule, as well as overall reduced βENaC subunit expression, may be responsible for the more severe hyperkalemia. We conclude that plasma K⁺ concentration becomes the determining and limiting factor in regulating NCC activity, regardless of Na⁺ balance in γENaC-deficient mice.

Dietary sodium induces a redistribution of the tubular metabolic workload

Key points

Body Na⁺ content is tightly controlled by regulated urinary Na⁺ excretion.

The intrarenal mechanisms mediating adaptation to variations in dietary Na⁺ intake are incompletely characterized.

We confirmed and expanded observations in mice that variations in dietary Na⁺ intake do not alter the glomerular filtration rate but alter the total and cell‐surface expression of major Na⁺ transporters all along the kidney tubule.

Low dietary Na⁺ intake increased Na⁺ reabsorption in the proximal tubule and decreased it in more distal kidney tubule segments.

High dietary Na⁺ intake decreased Na⁺ reabsorption in the proximal tubule and increased it in distal segments with lower energetic efficiency.

The abundance of apical transporters and Na⁺ delivery are the main determinants of Na⁺ reabsorption along the kidney tubule.

Tubular O₂ consumption and the efficiency of sodium reabsorption are dependent on sodium diet.

Abstract

Na⁺ excretion by the kidney varies according to dietary Na⁺ intake. We undertook a systematic study of the effects of dietary salt intake on glomerular filtration rate (GFR) and tubular Na⁺ reabsorption. We examined the renal adaptive response in mice subjected to 7 days of a low sodium diet (LSD) containing 0.01% Na⁺, a normal sodium diet (NSD) containing 0.18% Na⁺ and a moderately high sodium diet (HSD) containing 1.25% Na⁺. As expected, LSD did not alter measured GFR and increased the abundance of total and cell‐surface NHE3, NKCC2, NCC, α‐ENaC and cleaved γ‐ENaC compared to NSD. Mathematical modelling predicted that tubular Na⁺ reabsorption increased in the proximal tubule but decreased in the distal nephron because of diminished Na⁺ delivery. This prediction was confirmed by the natriuretic response to diuretics targeting the thick ascending limb, the distal convoluted tubule or the collecting system. On the other hand, HSD did not alter measured GFR but decreased the abundance of the aforementioned transporters compared to NSD. Mathematical modelling predicted that tubular Na⁺ reabsorption decreased in the proximal tubule but increased in distal segments with lower transport efficiency with respect to O₂ consumption. This prediction was confirmed by the natriuretic response to diuretics. The activity of the metabolic sensor adenosine monophosphate‐activated protein kinase (AMPK) was related to the changes in tubular Na⁺ reabsorption. Our data show that fractional Na⁺ reabsorption is distributed differently according to dietary Na⁺ intake and induces changes in tubular O₂ consumption and sodium transport efficiency.

Body Na⁺ content is tightly controlled by regulated urinary Na⁺ excretion.

The intrarenal mechanisms mediating adaptation to variations in dietary Na⁺ intake are incompletely characterized.

We confirmed and expanded observations in mice that variations in dietary Na⁺ intake do not alter the glomerular filtration rate but alter the total and cell‐surface expression of major Na⁺ transporters all along the kidney tubule.

Low dietary Na⁺ intake increased Na⁺ reabsorption in the proximal tubule and decreased it in more distal kidney tubule segments.

High dietary Na⁺ intake decreased Na⁺ reabsorption in the proximal tubule and increased it in distal segments with lower energetic efficiency.

The abundance of apical transporters and Na⁺ delivery are the main determinants of Na⁺ reabsorption along the kidney tubule.

Tubular O₂ consumption and the efficiency of sodium reabsorption are dependent on sodium diet.

Uninephrectomy and apical fluid shear stress decrease ENaC abundance in collecting duct principal cells

Acute nephron reduction such as after living kidney donation may increase the risk of hypertension. Uninephrectomy induces major hemodynamic changes in the remaining kidney, resulting in rapid increase of single-nephron glomerular filtration rate (GFR) and fluid delivery in the distal nephron. Decreased sodium (Na) fractional reabsorption after the distal tubule has been reported after uninephrectomy in animals preserving volume homeostasis. In the present study, we thought to specifically explore the effect of unilateral nephrectomy on epithelial Na channel (ENaC) subunit expression in mice. We show that γ-ENaC subunit surface expression was specifically downregulated after uninephrectomy, whereas the expression of the aldosterone-sensitive α-ENaC and α₁-Na-K-ATPase subunits as well as of kidney-specific Na-K-Cl cotransporter isoform and Na-Cl cotransporter were not significantly altered. Because acute nephron reduction induces a rapid increase of single-nephron GFR, resulting in a higher tubular fluid flow, we speculated that local mechanical factors such as fluid shear stress (FSS) were involved in Na reabsorption regulation after uninephrectomy. We further explore such hypothesis in an in vitro model of FSS applied on highly differentiated collecting duct principal cells. We found that FSS specifically downregulates β-ENaC and γ-ENaC subunits at the transcriptional level through an unidentified heat-insensitive paracrine basolateral factor. The primary cilium as a potential mechanosensor was not required. In contrast, protein kinase A and calcium-sensitive cytosolic phospholipase A₂ were involved, but we could not demonstrate a role for cyclooxygenase or epoxygenase metabolites.

Renal localization and regulation by dietary phosphate of the MCT14 orphan transporter

MCT14 is an orphan transporter belonging to the SLC16 transporter family mediating the transport of monocarboxylates, aromatic amino acids, creatine, and thyroid hormones. The expression, tissue localization, regulation, and function of MCT14 are unknown. In mouse MCT14 mRNA abundance is highest in kidney. Using a newly developed and validated antibody, MCT14 was localized to the luminal membrane of the thick ascending limb of the loop of Henle colocalizing in the same cells with uromodulin and NKCC2. MCT14 mRNA and protein was found to be highly regulated by dietary phosphate intake in mice being increased by high dietary phosphate intake at both mRNA and protein level. In order to identify the transport substrate(s), we expressed MCT14 in Xenopus laevis oocytes where MCT14 was integrated into the plasma membrane. However, no transport was discovered for the classic substrates of the SLC16 family nor for phosphate. In summary, MCT14 is an orphan transporter regulated by phosphate and highly enriched in kidney localizing to the luminal membrane of one specific nephron segment.

Aldosterone Modulates the Association between NCC and ENaC

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

Combined mutation in Vhl, Trp53 and Rb1 causes clear cell renal cell carcinoma in mice

Clear cell renal cell carcinomas (ccRCC) frequently exhibit inactivation of the VHL tumour suppressor gene and often harbour multiple copy number alterations in genes that regulate cell cycle progression. We show here that modelling these genetic alterations by combined renal epithelium-specific deletion of Vhl, Trp53 and Rb1 in mice caused ccRCC. These tumours arose from proximal tubule epithelial cells and shared molecular markers and mRNA expression profiles with human ccRCC. Exome sequencing revealed that mouse and human ccRCCs exhibit recurrent mutations in genes associated with the primary cilium, uncovering a mutational convergence on this organelle and implicating a subset of ccRCCs as genetic ciliopathies. Different mouse tumours responded differently to standard therapies for advanced human ccRCC, mimicking the range of clinical behaviours in the human disease. Inhibition of HIF-α transcription factors with Acriflavine as third line therapy had therapeutic effects in some tumours, providing pre-clinical evidence for further investigation of HIF-α inhibition as a ccRCC treatment. This autochthonous mouse ccRCC model represents a tool to investigate the biology of ccRCC and to identify new treatment strategies.

Endothelin-1 mediates natriuresis but not polyuria during vitamin D-induced acute hypercalcaemia

KEY POINTS

Hypercalcaemia can occur under various pathological conditions, such as primary hyperparathyroidism, malignancy or granulomatosis, and it induces natriuresis and polyuria in various species via an unknown mechanism. A previous study demonstrated that hypercalcaemia induced by vitamin D in rats increased endothelin (ET)-1 expression in the distal nephron, which suggests the involvement of the ET system in hypercalcaemia-induced effects. In the present study, we demonstrate that, during vitamin D-induced hypercalcaemia, the activation of ET system by increased ET-1 is responsible for natriuresis but not for polyuria. Vitamin D-treated hypercalcaemic mice showed a blunted response to amiloride, suggesting that epithelial sodium channel function is inhibited. We have identified an original pathway that specifically mediates the effects of vitamin D-induced hypercalcaemia on sodium handling in the distal nephron without affecting water handling.

ABSTRACT

Acute hypercalcaemia increases urinary sodium and water excretion; however, the underlying molecular mechanism remains unclear. Because vitamin D-induced hypercalcaemia increases the renal expression of endothelin (ET)-1, we hypothesized that ET-1 mediates the effects of hypercalcaemia on renal sodium and water handling. Hypercalcaemia was induced in 8-week-old, parathyroid hormone-supplemented, male mice by oral administration of dihydrotachysterol (DHT) for 3 days. DHT-treated mice became hypercalcaemic and displayed increased urinary water and sodium excretion compared to controls. mRNA levels of ET-1 and the transcription factors CCAAT-enhancer binding protein β and δ were specifically increased in the distal convoluted tubule and downstream segments in DHT-treated mice. To examine the role of the ET system in hypercalcaemia-induced natriuresis and polyuria, mice were treated with the ET-1 receptor antagonist macitentan, with or without DHT. Mice treated with both macitentan and DHT displayed hypercalcaemia and polyuria similar to that in mice treated with DHT alone; however, no increase in urinary sodium excretion was observed. To identify the affected sodium transport mechanism, we assessed the response to various diuretics in control and DHT-treated hypercalcaemic mice. Amiloride, an inhibitor of the epithelial sodium channel (ENaC), increased sodium excretion to a lesser extent in DHT-treated mice compared to control mice. Mice treated with either macitentan+DHT or macitentan alone had a similar response to amiloride. In summary, vitamin D-induced hypercalcaemia increases the renal production of ET-1 and decreases ENaC activity, which is probably responsible for the rise in urinary sodium excretion but not for polyuria.

Extracellular K+ rapidly controls NaCl cotransporter phosphorylation in the native distal convoluted tubule by Cl- -dependent and independent mechanisms

KEY POINTS

High dietary potassium (K⁺ ) intake dephosphorylates and inactivates the NaCl cotransporter (NCC) in the renal distal convoluted tubule (DCT). Using several ex vivo models, we show that physiological changes in extracellular K⁺ , similar to those occurring after a K⁺ rich diet, are sufficient to promote a very rapid dephosphorylation of NCC in native DCT cells. Although the increase of NCC phosphorylation upon decreased extracellular K⁺ appears to depend on cellular Cl^- fluxes, the rapid NCC dephosphorylation in response to increased extracellular K⁺ is not Cl^- -dependent. The Cl^- -dependent pathway involves the SPAK/OSR1 kinases, whereas the Cl^- independent pathway may include additional signalling cascades.

ABSTRACT

A high dietary potassium (K⁺ ) intake causes a rapid dephosphorylation, and hence inactivation, of the thiazide-sensitive NaCl cotransporter (NCC) in the renal distal convoluted tubule (DCT). Based on experiments in heterologous expression systems, it was proposed that changes in extracellular K⁺ concentration ([K⁺ ]_ex ) modulate NCC phosphorylation via a Cl^- -dependent modulation of the with no lysine (K) kinases (WNK)-STE20/SPS-1-44 related proline-alanine-rich protein kinase (SPAK)/oxidative stress-related kinase (OSR1) kinase pathway. We used the isolated perfused mouse kidney technique and ex vivo preparations of mouse kidney slices to test the physiological relevance of this model on native DCT. We demonstrate that NCC phosphorylation inversely correlates with [K⁺ ]_ex , with the most prominent effects occurring around physiological plasma [K⁺ ]. Cellular Cl^- conductances and the kinases SPAK/OSR1 are involved in the phosphorylation of NCC under low [K⁺ ]_ex . However, NCC dephosphorylation triggered by high [K⁺ ]_ex is neither blocked by removing extracellular Cl^- , nor by the Cl^- channel blocker 4,4'-diisothiocyano-2,2'-stilbenedisulphonic acid. The response to [K⁺ ]_ex on a low extracellular chloride concentration is also independent of significant changes in SPAK/OSR1 phosphorylation. Thus, in the native DCT, [K⁺ ]_ex directly and rapidly controls NCC phosphorylation by Cl^- -dependent and independent pathways that involve the kinases SPAK/OSR1 and a yet unidentified additional signalling mechanism.

Prostaglandin-E2 Mediated Increase in Calcium and Phosphate Excretion in a Mouse Model of Distal Nephron Salt Wasting

Contribution of salt wasting and volume depletion to the pathogenesis of hypercalciuria and hyperphosphaturia is poorly understood. Pendrin/NCC double KO (pendrin/NCC-dKO) mice display severe salt wasting under basal conditions and develop profound volume depletion, prerenal renal failure, and metabolic alkalosis and are growth retarded. Microscopic examination of the kidneys of pendrin/NCC-dKO mice revealed the presence of calcium phosphate deposits in the medullary collecting ducts, along with increased urinary calcium and phosphate excretion. Confirmatory studies revealed decreases in the expression levels of sodium phosphate transporter-2 isoforms a and c, increases in the expression of cytochrome p450 family 4a isotypes 12 a and b, as well as prostaglandin E synthase 1, and cyclooxygenases 1 and 2. Pendrin/NCC-dKO animals also had a significant increase in urinary prostaglandin E2 (PGE-2) and renal content of 20-hydroxyeicosatetraenoic acid (20-HETE) levels. Pendrin/NCC-dKO animals exhibit reduced expression levels of the sodium/potassium/2chloride co-transporter 2 (NKCC2) in their medullary thick ascending limb. Further assessment of the renal expression of NKCC2 isoforms by quantitative real time PCR (qRT-PCR) reveled that compared to WT mice, the expression of NKCC2 isotype F was significantly reduced in pendrin/NCC-dKO mice. Provision of a high salt diet to rectify volume depletion or inhibition of PGE-2 synthesis by indomethacin, but not inhibition of 20-HETE generation by HET0016, significantly improved hypercalciuria and salt wasting in pendrin/NCC dKO mice. Both high salt diet and indomethacin treatment also corrected the alterations in NKCC2 isotype expression in pendrin/NCC-dKO mice. We propose that severe salt wasting and volume depletion, irrespective of the primary originating nephron segment, can secondarily impair the reabsorption of salt and calcium in the thick ascending limb of Henle and/or proximal tubule, and reabsorption of sodium and phosphate in the proximal tubule via processes that are mediated by PGE-2.

The sodium chloride cotransporter (NCC) and epithelial sodium channel (ENaC) associate

The thiazide-sensitive sodium chloride cotransporter (NCC) and the epithelial sodium channel (ENaC) are two of the most important determinants of salt balance and thus systemic blood pressure. Abnormalities in either result in profound changes in blood pressure. There is one segment of the nephron where these two sodium transporters are coexpressed, the second part of the distal convoluted tubule. This is a key part of the aldosterone-sensitive distal nephron, the final regulator of salt handling in the kidney. Aldosterone is the key hormonal regulator for both of these proteins. Despite these shared regulators and coexpression in a key nephron segment, associations between these proteins have not been investigated. After confirming apical localization of these proteins, we demonstrated the presence of functional transport proteins and native association by blue native PAGE. Extensive coimmunoprecipitation experiments demonstrated a consistent interaction of NCC with α- and γ-ENaC. Mammalian two-hybrid studies demonstrated direct binding of NCC to ENaC subunits. Fluorescence resonance energy transfer and immunogold EM studies confirmed that these transport proteins are within appropriate proximity for direct binding. Additionally, we demonstrate that there are functional consequences of this interaction, with inhibition of NCC affecting the function of ENaC. This novel finding of an association between ENaC and NCC could alter our understanding of salt transport in the distal tubule.

Renal tubular SGK1 deficiency causes impaired K+ excretion via loss of regulation of NEDD4-2/WNK1 and ENaC

The stimulation of postprandial K(+) clearance involves aldosterone-independent and -dependent mechanisms. In this context, serum- and glucocorticoid-induced kinase (SGK)1, a ubiquitously expressed kinase, is one of the primary aldosterone-induced proteins in the aldosterone-sensitive distal nephron. Germline inactivation of SGK1 suggests that this kinase is fundamental for K(+) excretion under conditions of K(+) load, but the specific role of renal SGK1 remains elusive. To avoid compensatory mechanisms that may occur during nephrogenesis, we used inducible, nephron-specific Sgk1(Pax8/LC1) mice to assess the role of renal tubular SGK1 in K(+) regulation. Under a standard diet, these animals exhibited normal K(+) handling. When challenged by a high-K(+) diet, they developed severe hyperkalemia accompanied by a defect in K(+) excretion. Molecular analysis revealed reduced neural precursor cell expressed developmentally downregulated protein (NEDD)4-2 phosphorylation and total expression. γ-Epithelial Na(+) channel (ENaC) expression and α/γENaC proteolytic processing were also decreased in mutant mice. Moreover, with no lysine kinase (WNK)1, which displayed in control mice punctuate staining in the distal convoluted tubule and diffuse distribution in the connecting tubule/cortical colleting duct, was diffused in the distal convoluted tubule and less expressed in the connecting tubule/collecting duct of Sgk(Pax8/LC1) mice. Moreover, Ste20-related proline/alanine-rich kinase phosphorylation, and Na(+)-Cl(-) cotransporter phosphorylation/apical localization were reduced in mutant mice. Consistent with the altered WNK1 expression, increased renal outer medullary K(+) channel apical localization was observed. In conclusion, our data suggest that renal tubular SGK1 is important in the regulation of K(+) excretion via the control of NEDD4-2, WNK1, and ENaC.

The mineralocorticoid receptor (MR) regulates ENaC but not NCC in mice with random MR deletion

Aldosterone binds to the mineralocorticoid receptor (MR) and increases renal Na(+) reabsorption via up-regulation of the epithelial Na(+) channel (ENaC) and the Na(+)-K(+)-ATPase in the collecting system (CS) and possibly also via the NaCl cotransporter (NCC) in the distal convoluted tubule (DCT). However, whether aldosterone directly regulates NCC via MR or indirectly through systemic alterations remains controversial. We used mice with deletion of MR in ∼20 % of renal tubule cells (MR/X mice), in which MR-positive (MR(wt)) and -negative (MR(ko)) cells can be studied side-by-side in the same physiological context. Adult MR/X mice showed similar mRNA and protein levels of renal ion transport proteins to control mice. In MR/X mice, no differences in NCC abundance and phosphorylation was seen between MR(wt) and MR(ko) cells and dietary Na(+) restriction up-regulated NCC to similar extent in both groups of cells. In contrast, MR(ko) cells in the CS did not show any detectable alpha-ENaC abundance or apical targeting of ENaC neither on control diet nor in response to dietary Na(+) restriction. Furthermore, Na(+)-K(+)-ATPase expression was unaffected in MR(ko) cells of the DCT, while it was lost in MR(ko) cells of the CS. In conclusion, MR is crucial for ENaC and Na(+)-K(+)-ATPase regulation in the CS, but is dispensable for NCC and Na(+)-K(+)-ATPase regulation in the DCT.

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.

Romk1 Knockout Mice Do Not Produce Bartter Phenotype but Exhibit Impaired K Excretion

Romk knock-out mice show a similar phenotype to Bartter syndrome of salt wasting and dehydration due to reduced Na-K-2Cl-cotransporter activity. At least three ROMK isoforms have been identified in the kidney; however, unique functions of any of the isoforms in nephron segments are still poorly understood. We have generated a mouse deficient only in Romk1 by selective deletion of the Romk1-specific first exon using an ES cell Cre-LoxP strategy and examined the renal phenotypes, ion transporter expression, ROMK channel activity, and localization under normal and high K intake. Unlike Romk(-/-) mice, there was no Bartter phenotype with reduced NKCC2 activity and increased NCC expression in Romk1(-/-) mice. The small conductance K channel (SK) activity showed no difference of channel properties or gating in the collecting tubule between Romk1(+/+) and Romk1(-/-) mice. High K intake increased SK channel number per patch and increased the ROMK channel intensity in the apical membrane of the collecting tubule in Romk1(+/+), but such regulation by high K intake was diminished with significant hyperkalemia in Romk1(-/-) mice. We conclude that 1) animal knockouts of ROMK1 do not produce Bartter phenotype. 2) There is no functional linking of ROMK1 and NKCC2 in the TAL. 3) ROMK1 is critical in response to high K intake-stimulated K(+) secretion in the collecting tubule.

Severe Salt-Losing Syndrome and Hyperkalemia Induced by Adult Nephron-Specific Knockout of the Epithelial Sodium Channel α-Subunit

Systemic pseudohypoaldosteronism type 1 (PHA-1) is a severe salt-losing syndrome caused by loss-of-function mutations of the amiloride-sensitive epithelial sodium channel (ENaC) and characterized by neonatal life-threatening hypovolemia and hyperkalemia. The very high plasma aldosterone levels detected under hypovolemic or hyperkalemic challenge can lead to increased or decreased sodium reabsorption, respectively, through the Na(+)/Cl(-) cotransporter (NCC). However, the role of ENaC deficiency remains incompletely defined, because constitutive inactivation of individual ENaC subunits is neonatally lethal in mice. We generated adult inducible nephron-specific αENaC-knockout mice (Scnn1a(Pax8/LC1)) that exhibit hyperkalemia and body weight loss when kept on a regular-salt diet, thus mimicking PHA-1. Compared with control mice fed a regular-salt diet, knockout mice fed a regular-salt diet exhibited downregulated expression and phosphorylation of NCC protein, despite high plasma aldosterone levels. In knockout mice fed a high-sodium and reduced-potassium diet (rescue diet), although plasma aldosterone levels remained significantly increased, NCC expression returned to control levels, and body weight, plasma and urinary electrolyte concentrations, and excretion normalized. Finally, shift to a regular diet after the rescue diet reinstated the symptoms of severe PHA-1 syndrome and significantly reduced NCC phosphorylation. In conclusion, lack of ENaC-mediated sodium transport along the nephron cannot be compensated for by other sodium channels and/or transporters, only by a high-sodium and reduced-potassium diet. We further conclude that hyperkalemia becomes the determining factor in regulating NCC activity, regardless of sodium loss, in the ENaC-mediated salt-losing PHA-1 phenotype.

Acetazolamide Attenuates Lithium-Induced Nephrogenic Diabetes Insipidus

To reduce lithium-induced nephrogenic diabetes insipidus (lithium-NDI), patients with bipolar disorder are treated with thiazide and amiloride, which are thought to induce antidiuresis by a compensatory increase in prourine uptake in proximal tubules. However, thiazides induced antidiuresis and alkalinized the urine in lithium-NDI mice lacking the sodium-chloride cotransporter, suggesting that inhibition of carbonic anhydrases (CAs) confers the beneficial thiazide effect. Therefore, we tested the effect of the CA-specific blocker acetazolamide in lithium-NDI. In collecting duct (mpkCCD) cells, acetazolamide reduced the cellular lithium content and attenuated lithium-induced downregulation of aquaporin-2 through a mechanism different from that of amiloride. Treatment of lithium-NDI mice with acetazolamide or thiazide/amiloride induced similar antidiuresis and increased urine osmolality and aquaporin-2 abundance. Thiazide/amiloride-treated mice showed hyponatremia, hyperkalemia, hypercalcemia, metabolic acidosis, and increased serum lithium concentrations, adverse effects previously observed in patients but not in acetazolamide-treated mice in this study. Furthermore, acetazolamide treatment reduced inulin clearance and cortical expression of sodium/hydrogen exchanger 3 and attenuated the increased expression of urinary PGE2 observed in lithium-NDI mice. These results show that the antidiuresis with acetazolamide was partially caused by a tubular-glomerular feedback response and reduced GFR. The tubular-glomerular feedback response and/or direct effect on collecting duct principal or intercalated cells may underlie the reduced urinary PGE2 levels with acetazolamide, thereby contributing to the attenuation of lithium-NDI. In conclusion, CA activity contributes to lithium-NDI development, and acetazolamide attenuates lithium-NDI development in mice similar to thiazide/amiloride but with fewer adverse effects.

Potassium and Its Discontents: New Insight, New Treatments

Hyperkalemia is common in patients with impaired kidney function or who take drugs that inhibit the renin-angiotensin-aldosterone axis. During the past decade, substantial advances in understanding how the body controls potassium excretion have been made, which may lead to improved standard of care for these patients. Renal potassium disposition is primarily handled by a short segment of the nephron, comprising part of the distal convoluted tubule and the connecting tubule, and regulation results from the interplay between aldosterone and plasma potassium. When dietary potassium intake and plasma potassium are low, the electroneutral sodium chloride cotransporter is activated, leading to salt retention. This effect limits sodium delivery to potassium secretory segments, limiting potassium losses. In contrast, when dietary potassium intake is high, aldosterone is stimulated. Simultaneously, potassium inhibits the sodium chloride cotransporter. Because more sodium is then delivered to potassium secretory segments, primed by aldosterone, kaliuresis results. When these processes are disrupted, hyperkalemia results. Recently, new agents capable of removing potassium from the body and treating hyperkalemia have been tested in clinical trials. This development suggests that more effective and safer approaches to the prevention and treatment of hyperkalemia may be on the horizon.

Rab-GAP TBC1D4 (AS160) is dispensable for the renal control of sodium and water homeostasis but regulates GLUT4 in mouse kidney

The Rab GTPase-activating protein TBC1D4 (AS160) controls trafficking of the glucose transporter GLUT4 in adipocytes and skeletal muscle cells. TBC1D4 is also highly abundant in the renal distal tubule, although its role in this tubule is so far unknown. In vitro studies suggest that it is involved in the regulation of renal transporters and channels such as the epithelial sodium channel (ENaC), aquaporin-2 (AQP2), and the Na+-K+-ATPase. To assess the physiological role of TBC1D4 in the kidney, wild-type (TBC1D4+/+) and TBC1D4-deficient (TBC1D4-/-) mice were studied. Unexpectedly, neither under standard nor under challenging conditions (low Na+/high K+, water restriction) did TBC1D4-/- mice show any difference in urinary Na+ and K+ excretion, urine osmolarity, plasma ion and aldosterone levels, and blood pressure compared with TBC1D4+/+ mice. Also, immunoblotting did not reveal any change in the abundance of major renal sodium- and water-transporting proteins [Na-K-2Cl cotransporter (NKCC2) NKCC2, NaCl cotransporter (NCC), ENaC, AQP2, and the Na+-K+-ATPase]. However, the abundance of GLUT4, which colocalizes with TBC1D4 along the distal nephron of TBC1D4+/+ mice, was lower in whole kidney lysates of TBC1D4-/- mice than in TBC1D4+/+ mice. Likewise, primary thick ascending limb (TAL) cells isolated from TBC1D4-/- mice showed an increased basal glucose uptake and an abrogated insulin response compared with TAL cells from TBC1D4+/+ mice. Thus, TBC1D4 is dispensable for the regulation of renal Na+ and water transport, but may play a role for GLUT4-mediated basolateral glucose uptake in distal tubules. The latter may contribute to the known anaerobic glycolytic capacity of distal tubules during renal ischemia.

ENaC activity in collecting ducts modulates NCC in cirrhotic mice

Cirrhosis is a frequent and severe disease, complicated by renal sodium retention leading to ascites and oedema. A better understanding of the complex mechanisms responsible for renal sodium handling could improve clinical management of sodium retention. Our aim was to determine the importance of the amiloride-sensitive epithelial sodium channel (ENaC) in collecting ducts in compensate and decompensate cirrhosis. Bile duct ligation was performed in control mice (CTL) and collecting duct-specific αENaC knockout (KO) mice, and ascites development, aldosterone plasma concentration, urinary sodium/potassium ratio and sodium transporter expression were compared. Disruption of ENaC in collecting ducts (CDs) did not alter ascites development, urinary sodium/potassium ratio, plasma aldosterone concentrations or Na,K-ATPase abundance in CCDs. Total αENaC abundance in whole kidney increased in cirrhotic mice of both genotypes and cleaved forms of α and γ ENaC increased only in ascitic mice of both genotypes. The sodium chloride cotransporter (NCC) abundance was lower in non-ascitic KO, compared to non-ascitic CTL, and increased when ascites appeared. In ascitic mice, the lack of αENaC in CDs induced an upregulation of total ENaC and NCC and correlated with the cleavage of ENaC subunits. This revealed compensatory mechanisms which could also take place when treating the patients with diuretics. These compensatory mechanisms should be considered for future development of therapeutic strategies.

Essential amino acid transporter Lat4 (Slc43a2) is required for mouse development

Amino acid (AA) uniporter Lat4 (Slc43a2) mediates facilitated diffusion of branched-chain AAs, methionine and phenylalanine, although its physiological role and subcellular localization are not known. We report that Slc43a2 knockout mice were born at expected Mendelian frequency but displayed an ∼10% intrauterine growth retardation and low amniotic fluid AAs, suggesting defective transplacental transport. Postnatal growth was strongly reduced, with premature death occurring within 9 days such that further investigations were made within 3 days of birth. Lat4 immunofluorescence showed a strong basolateral signal in the small intestine, kidney proximal tubule and thick ascending limb epithelial cells of wild-type but not Slc43a2 null littermates and no signal in liver and skeletal muscle. Experiments using Xenopus laevis oocytes demonstrated that Lat4 functioned as a symmetrical low affinity uniporter with a K₀.₅ of ∼5 mm for both in- and efflux. Plasma AA concentration was decreased in Slc43a2 null pups, in particular that of non-essential AAs alanine, serine, histidine and proline. Together with an increased level of plasma long chain acylcarnitines and a strong alteration of liver gene expression, this indicates malnutrition. Attempts to rescue pups by decreasing the litter size or by nutrients injected i.p. did not succeed. Radioactively labelled leucine but not lysine given per os accumulated in the small intestine of Slc43a2null pups, suggesting the defective transcellular transport of Lat4 substrates. In summary, Lat4 is a symmetrical uniporter for neutral essential AAs localizing at the basolateral side of (re)absorbing epithelia and is necessary for early nutrition and development.

Dietary potassium and the renal control of salt balance and blood pressure

Dietary potassium (K(+)) intake has antihypertensive effects, prevents strokes, and improves cardiovascular outcomes. The underlying mechanism for these beneficial effects of high K(+) diets may include vasodilation, enhanced urine flow, reduced renal renin release, and negative sodium (Na(+)) balance. Indeed, several studies demonstrate that dietary K(+) intake induces renal Na(+) loss despite elevated plasma aldosterone. This review briefly highlights the epidemiological and experimental evidences for the effects of dietary K(+) on arterial blood pressure. It discusses the pivotal role of the renal distal tubule for the regulation of urinary K(+) and Na(+) excretion and blood pressure and highlights that it depends on the coordinated interaction of different nephron portions, epithelial cell types, and various ion channels, transporters, and ATPases. Moreover, we discuss the relevance of aldosterone and aldosterone-independent factors in mediating the effects of an altered K(+) intake on renal K(+) and Na(+) handling. Particular focus is given to findings suggesting that an aldosterone-independent downregulation of the thiazide-sensitive NaCl cotransporter significantly contributes to the natriuretic and antihypertensive effect of a K(+)-rich diet. Last but not least, we refer to the complex signaling pathways enabling the kidney to adapt its function to the homeostatic needs in response to an altered K(+) intake. Future work will have to further address the underlying cellular and molecular mechanism and to elucidate, among others, how an altered dietary K(+) intake is sensed and how this signal is transmitted to the different epithelial cells lining the distal tubule.