Surface expression of sodium channels and transporters in rat kidney: effects of dietary sodium

Epithelial Na + Channel Activation after Bile Duct Ligation with Mineralocorticoid Receptor Blockade

Key Points

Bile acids activate the epithelial Na+ channel (ENaC), which may lead to subsequent fluid retention in liver disease. Bile duct ligation with spironolactone increased ENaC-dependent Na+ and fluid retention without hormone-linked increased ENaC abundance. Counteracting bile acid ENaC activation may be effective for treating fluid retention in liver disease.

Background

Sodium and fluid retention in liver disease is classically thought to result from reduced effective circulating volume and stimulation of the renin-angiotensin-aldosterone system. However, evidence of fluid retention in patients without renin-angiotensin-aldosterone system activation suggests the involvement of additional mechanisms. In vitro , bile acids activate the epithelial Na+ channel (ENaC) found in the aldosterone-sensitive distal nephron. If this occurs in vivo , ENaC may become activated in liver disease even with antagonism of aldosterone signaling.

Methods

To test this, we performed bile duct ligation to induce liver disease and increase circulating bile acids in mice given spironolactone to antagonize aldosterone signaling. We analyzed effects on blood, urine, and body composition. We also determined the effects of taurocholic acid, a primary conjugated bile acid elevated in liver disease, on ion fluxes in microperfused rabbit collecting ducts.

Results

Bile duct ligation increased benzamil-sensitive natriuresis compared with sham, indicating ENaC activation. These effects were not explained by effects on ENaC expression, cleavage, or localization. Bile duct–ligated mice also gained significantly more fluid than sham-operated animals. Blocking ENaC reversed fluid gains in bile duct–ligated mice but had no effect in shams. In dissected collecting ducts from rabbits, which express ENaC, taurocholic acid stimulated net Na+ absorption.

Conclusions

Our results provide experimental evidence for a novel aldosterone-independent mechanism for sodium and fluid retention in liver disease.

Loss of the alpha subunit distal furin cleavage site blunts ENaC activation following Na+ restriction

Epithelial Na+ channels (ENaCs) are activated by proteolysis of the α and γ subunits at specific sites flanking embedded inhibitory tracts. To examine the role of α subunit proteolysis in channel activation in vivo, we generated mice lacking the distal furin cleavage site in the α subunit (αF2M mice). On a normal Na+ control diet, no differences in ENaC protein abundance in kidney or distal colon were noted between wild-type (WT) and αF2M mice. Patch-clamp analyses revealed similar levels of ENaC activity in kidney tubules, while no physiologically relevant differences in blood chemistry or aldosterone levels were detected. Male αF2M mice did exhibit diminished ENaC activity in the distal colon, as measured by amiloride-sensitive short-circuit current (ISC). Following dietary Na+ restriction, WT and αF2M mice had similar natriuretic and colonic ISC responses to amiloride. However, single-channel activity was significantly lower in kidney tubules from Na+-restricted αF2M mice compared with WT littermates. ENaC α and γ subunit expression in kidney and distal colon were also enhanced in Na+-restricted αF2M vs. WT mice, in association with higher aldosterone levels. These data provide evidence that disrupting α subunit proteolysis impairs ENaC activity in vivo, requiring compensation in response to Na+ restriction. KEY POINTS: The epithelial Na+ channel (ENaC) is activated by proteolytic cleavage in vitro, but key questions regarding the role of ENaC proteolysis in terms of whole-animal physiology remain to be addressed. We studied the in vivo importance of this mechanism by generating a mouse model with a genetic disruption to a key cleavage site in the ENaC's α subunit (αF2M mice). We found that αF2M mice did not exhibit a physiologically relevant phenotype under normal dietary conditions, but have impaired ENaC activation (channel open probability) in the kidney during salt restriction. ENaC function at the organ level was preserved in salt-restricted αF2M mice, but this was associated with higher aldosterone levels and increased expression of ENaC subunits, suggesting compensation was required to maintain homeostasis. These results provide the first evidence that ENaC α subunit proteolysis is a key regulator of channel activity in vivo.

Control of ENaC ubiquitination

Ubiquitination influences the expression of the epithelial Na+ channel (ENaC). We assessed the mechanisms of selective ubiquitination of the mature, cleaved form of γENaC in both native rodent kidneys and Fisher rat thyroid (FRT) cells expressing the channel heterologously. In both models, singly cleaved and fully cleaved γENaCs were strongly ubiquitinated, implying that the second cleavage releasing an inhibitory peptide was not essential for the process. To see whether location of the protein in or near the apical membrane rather than cleavage per se influences ubiquitination, we studied mutants of γENaC in which cleavage sites are abolished. These subunits were ubiquitinated only when coexpressed with α- and βENaC, facilitating trafficking through the Golgi apparatus. To test whether reaching the apical surface is necessary we performed in situ surface biotinylation and measured ENaC ubiquitination in the apical membrane of rat kidney. Ubiquitination of cleaved γENaC was similar in whole kidney and surface fractions, implying that both apical and subapical channels could be modified. In FRT cells, inhibiting clathrin-mediated endocytosis with Dyngo-4a increased both total and ubiquitinated γENaC at the cell surface. Finally, we tested the idea that increased intracellular Na+ could stimulate ubiquitination. Administration of amiloride to block Na+ entry through the channels did not affect ubiquitination of γENaC in either FRT cells or the rat kidney. However, presumed large increases in cellular Na+ produced by monensin in FRT cells or acute Na+ repletion in rats increased ubiquitination and decreased overall ENaC expression.NEW & NOTEWORTHY We have explored the mechanisms underlying the ubiquitination of the γ subunit of epithelial Na+ channel (ENaC), a process believed to control channel internalization and degradation. We previously reported that the mature, cleaved form of the subunit is selectively ubiquitinated. Here we show that this specificity arises not from the cleavage state of the protein but from its location in the cell. We also show that under some conditions, increased intracellular Na+ can stimulate ENaC ubiquitination.

Influence of proteolytic cleavage of ENaC's γ subunit upon Na+ and K+ handling

The epithelial Na+ channel (ENaC) γ subunit is essential for homeostasis of Na+, K+, and body fluid. Dual γ subunit cleavage before and after a short inhibitory tract allows dissociation of this tract, increasing channel open probability (PO), in vitro. Cleavage proximal to the tract occurs at a furin recognition sequence (143RKRR146, in the mouse γ subunit). Loss of furin-mediated cleavage prevents in vitro activation of the channel by proteolysis at distal sites. We hypothesized that 143RKRR146 mutation to 143QQQQ146 (γQ4) in 129/Sv mice would reduce ENaC PO, impair flow-stimulated flux of Na+ (JNa) and K+ (JK) in perfused collecting ducts, reduce colonic amiloride-sensitive short-circuit current (ISC), and impair Na+, K+, and body fluid homeostasis. Immunoblot of γQ4/Q4 mouse kidney lysates confirmed loss of a band consistent in size with the furin-cleaved proteolytic fragment. However, γQ4/Q4 male mice on a low Na+ diet did not exhibit altered ENaC PO or flow-induced JNa, though flow-induced JK modestly decreased. Colonic amiloride-sensitive ISC in γQ4/Q4 mice was not altered. γQ4/Q4 males, but not females, exhibited mildly impaired fluid volume conservation when challenged with a low Na+ diet. Blood Na+ and K+ were unchanged on a regular, low Na+, or high K+ diet. These findings suggest that biochemical evidence of γ subunit cleavage should not be used in isolation to evaluate ENaC activity. Furthermore, factors independent of γ subunit cleavage modulate channel PO and the influence of ENaC on Na+, K+, and fluid volume homeostasis in 129/Sv mice, in vivo.NEW & NOTEWORTHY The epithelial Na+ channel (ENaC) is activated in vitro by post-translational proteolysis. In vivo, low Na+ or high K+ diets enhance ENaC proteolysis, and proteolysis is hypothesized to contribute to channel activation in these settings. Using a mouse expressing ENaC with disruption of a key proteolytic cleavage site, this study demonstrates that impaired proteolytic activation of ENaC's γ subunit has little impact upon channel open probability or the ability of mice to adapt to low Na+ or high K+ diets.

Epithelial Na + Channels Function as Extracellular Sensors

The epithelial Na + channel (ENaC) resides on the apical surfaces of specific epithelia in vertebrates and plays a critical role in extracellular fluid homeostasis. Evidence that ENaC senses the external environment emerged well before the molecular identity of the channel was reported three decades ago. This article discusses progress toward elucidating the mechanisms through which specific external factors regulate ENaC function, highlighting insights gained from structural studies of ENaC and related family members. It also reviews our understanding of the role of ENaC regulation by the extracellular environment in physiology and disease. After familiarizing the reader with the channel's physiological roles and structure, we describe the central role protein allostery plays in ENaC's sensitivity to the external environment. We then discuss each of the extracellular factors that directly regulate the channel: proteases, cations and anions, shear stress, and other regulators specific to particular extracellular compartments. For each regulator, we discuss the initial observations that led to discovery, studies investigating molecular mechanism, and the physiological and pathophysiological implications of regulation. © 2024 American Physiological Society. Compr Physiol 14:5407-5447, 2024.

Proteolytic Cleavage of the ENaC γ Subunit - Impact Upon Na + and K + Handling

The ENaC gamma subunit is essential for homeostasis of Na + , K + , and body fluid. Dual subunit cleavage before and after a short inhibitory tract allows dissociation of this tract, increasing channel open probability (P O ), in vitro . Cleavage proximal to the tract occurs at a furin recognition sequence ( 143 RKRR 146 in mouse). Loss of furin-mediated cleavage prevents in vitro activation of the channel by proteolysis at distal sites. We hypothesized that 143 RKRR 146 mutation to 143 QQQQ 146 ( Q4 ) in 129/Sv mice would reduce ENaC P O , impair flow-stimulated flux of Na + (J Na ) and K + (J K ) in perfused collecting ducts, reduce colonic amiloride-sensitive short circuit current (I SC ), and impair Na + , K + , and body fluid homeostasis. Immunoblot of Q4/Q4 mouse kidney lysates confirmed loss of a band consistent in size with the furin-cleaved proteolytic fragment. However, Q4/Q4 male mice on a low Na + diet did not exhibit altered ENaC P O or flow-induced J Na , though flow-induced J K modestly decreased. Colonic amiloride-sensitive I SC in Q4/Q4 mice was not altered. Q4/Q4 males, but not females, exhibited mildly impaired fluid volume conservation when challenged with a low Na + diet. Blood Na + and K + were unchanged on a regular, low Na + , or high K + diet. These findings suggest that biochemical evidence of gamma subunit cleavage should not be used in isolation to evaluate ENaC activity. Further, factors independent of gamma subunit cleavage modulate channel P O and the influence of ENaC on Na + , K + , and fluid volume homeostasis in 129/Sv mice, in vivo .

Sex differences in renal transporters: assessment and functional consequences

Mammalian kidneys are specialized to maintain fluid and electrolyte homeostasis. The epithelial transport processes along the renal tubule that match output to input have long been the subject of experimental and theoretical study. However, emerging data have identified a new dimension of investigation: sex. Like most tissues, the structure and function of the kidney is regulated by sex hormones and chromosomes. Available data demonstrate sex differences in the abundance of kidney solute and electrolyte transporters, establishing that renal tubular organization and operation are distinctly different in females and males. Newer studies have provided insights into the physiological consequences of these sex differences. Computational simulations predict that sex differences in transporter abundance are likely driven to optimize reproduction, enabling adaptive responses to the nutritional requirements of serial pregnancies and lactation - normal life-cycle changes that challenge the ability of renal transporters to maintain fluid and electrolyte homeostasis. Later in life, females may also undergo menopause, which is associated with changes in disease risk. Although numerous knowledge gaps remain, ongoing studies will provide further insights into the sex-specific mechanisms of sodium, potassium, acid-base and volume physiology throughout the life cycle, which may lead to therapeutic opportunities.

Mice lacking γENaC palmitoylation sites maintain benzamil-sensitive Na+ transport despite reduced channel activity

Epithelial Na+ channels (ENaCs) control extracellular fluid volume by facilitating Na+ absorption across transporting epithelia. In vitro studies showed that Cys-palmitoylation of the γENaC subunit is a major regulator of channel activity. We tested whether γ subunit palmitoylation sites are necessary for channel function in vivo by generating mice lacking the palmitoylated cysteines (γC33A,C41A) using CRISPR/Cas9 technology. ENaCs in dissected kidney tubules from γC33A,C41A mice had reduced open probability compared with wild-type (WT) littermates maintained on either standard or Na+-deficient diets. Male mutant mice also had higher aldosterone levels than WT littermates following Na+ restriction. However, γC33A,C41A mice did not have reduced amiloride-sensitive Na+ currents in the distal colon or benzamil-induced natriuresis compared to WT mice. We identified a second, larger conductance cation channel in the distal nephron with biophysical properties distinct from ENaC. The activity of this channel was higher in Na+-restricted γC33A,C41A versus WT mice and was blocked by benzamil, providing a possible compensatory mechanism for reduced prototypic ENaC function. We conclude that γ subunit palmitoylation sites are required for prototypic ENaC activity in vivo but are not necessary for amiloride/benzamil-sensitive Na+ transport in the distal nephron or colon.

High dietary K+ intake inhibits proximal tubule transport

The impact of chronic dietary K+ loading on proximal tubule (PT) function was measured using free-flow micropuncture along with measurements of overall kidney function, including urine volume, glomerular filtration rate, and absolute and fractional Na+ and K+ excretion in the rat. Feeding animals a diet with 5% KCl [high K+ (HK)] for 7 days reduced glomerular filtration rate by 29%, increased urine volume by 77%, and increased absolute K+ excretion by 202% compared with rats on a 1% KCl [control K+ (CK)] diet. HK did not change absolute Na+ excretion but significantly increased fraction excretion of Na+ (1.40% vs. 0.64%), indicating that fractional Na+ absorption is reduced by HK. PT reabsorption was assessed using free-flow micropuncture in anesthetized animals. At 80% of the accessible length of the PT, measurements of inulin concentration indicated volume reabsorption of 73% and 54% in CK and HK, respectively. At the same site, fractional PT Na+ reabsorption was 66% in CK animals and 37% in HK animals. Fractional PT K+ reabsorption was 66% in CK and 37% in HK. To assess the role of Na+/H+ exchanger isoform 3 (NHE3) in mediating these changes, we measured NHE3 protein expression in total kidney microsomes as well as surface membranes using Western blots. We found no significant changes in protein in either cell fraction. Expression of the Ser552 phosphorylated form of NHE3 was also similar in CK and HK animals. Reduction in PT transport may facilitate K+ excretion and help balance Na+ excretion by shifting Na+ reabsorption from K+-reabsorbing to K+-secreting nephron segments.NEW & NOTEWORTHY In rats fed a diet rich in K+, proximal tubules reabsorbed less fluid, Na+, and K+ compared with those in animals on a control diet. Glomerular filtration rates also decreased, probably due to glomerulotubular feedback. These reductions may help to maintain balance of the two ions simultaneously by shifting Na+ reabsorption to K+-secreting nephron segments.

Kir4.1 deletion prevents salt-sensitive hypertension in early streptozotocin-induced diabetic mice via Na + -Cl - cotransporter in the distal convoluted tubule

OBJECTIVES

Functional impairment of renal sodium handling and blood pressure (BP) homeostasis is an early characteristic manifestation of type 1 diabetes. However, the underlying mechanisms remain unclear.

METHODS

Metabolic cages, radio-telemetry, immunoblotting, and electrophysiology were utilized to examine effects of high salt (8% NaCl, HS) intake on Na + /K + balance, BP, Na + -Cl - cotransporter (NCC) function, and basolateral K + channel activity in the distal convoluted tubule (DCT) under diabetic conditions.

RESULTS

Improper Na + balance, hypernatremia, and a mild but significant increase in BP were found in streptozotocin (STZ)-induced diabetic mice in response to HS intake for 7 days. Compared to the vehicle, STZ mice showed increased Kir4.1 expression and activity in the DCT, a more negative membrane potential, higher NCC abundance, and enhanced hydrochlorothiazide-induced natriuretic effect. However, HS had no significant effect on basolateral Kir4.1 expression/activity and DCT membrane potential, or NCC activity under diabetic conditions, despite a downregulation in phosphorylated NCC abundance. In contrast, HS significantly downregulated the expression of Na + -H + exchanger 3 (NHE3) and cleaved epithelial sodium channel-γ in STZ mice, despite an increase in NHE3 abundance after STZ treatment. Kir4.1 deletion largely abolished STZ-induced upregulation of NCC expression and prevented BP elevation during HS intake. Interestingly, HS causes severe hypokalemia in STZ-treated kidney-specific Kir4.1 knockout (Ks-Kir4.1 KO) mice and lead to death within a few days, which could be attributed to a higher circulating aldosterone level.

CONCLUSIONS

We concluded that Kir4.1 is required for upregulating NCC activity and may be essential for developing salt-sensitive hypertension in early STZ-induced diabetes.

miRNA-23a modulates sodium-hydrogen exchanger 1 expression: studies in medullary thick ascending limb of salt-induced hypertensive rats

BACKGROUND

The kidney is the main organ in the pathophysiology of essential hypertension. Although most bicarbonate reabsorption occurs in the proximal tubule, the medullary thick ascending limb (mTAL) of the nephron also maintains acid-base balance by contributing to 25% of bicarbonate reabsorption. A crucial element in this regulation is the sodium-hydrogen exchanger 1 (NHE1), a ubiquitous membrane protein controlling intracellular pH, where proton extrusion is driven by the inward sodium flux. MicroRNA (miRNA) expression of hypertensive patients significantly differs from that of normotensive subjects. The aim of this study was to determine the functional role of miRNA alterations at the mTAL level.

METHODS

By miRNA microarray analysis, we identified miRNA expression profiles in isolated mTALs from high sodium intake-induced hypertensive rats (HSD) versus their normotensive counterparts (NSD). In vitro validation was carried out in rat mTAL cells.

RESULTS

Five miRNAs involved in the onset of salt-sensitive hypertension were identified, including miR-23a, which was bioinformatically predicted to target NHE1 mRNA. Data demonstrated that miRNA-23a is downregulated in the mTAL of HSD rats while NHE1 is upregulated. Consistently, transfection of an miRNA-23a mimic in an mTAL cell line, using a viral vector, resulted in NHE1 downregulation.

CONCLUSION

NHE1, a protein involved in sodium reabsorption at the mTAL level and blood pressure regulation, is upregulated in our model. This was due to a downregulation of miRNA-23a. Expression levels of this miRNA are influenced by high sodium intake in the mTALs of rats. The downregulation of miRNA-23a in humans affected by essential hypertension corroborate our data and point to the potential role of miRNA-23a in the regulation of mTAL function following high salt intake.

Inverse Salt Sensitivity of Blood Pressure: Mechanisms and Potential Relevance for Prevention of Cardiovascular Disease

PURPOSE OF REVIEW

To review the etiology of inverse salt sensitivity of blood pressure (BP).

RECENT FINDINGS

Both high and low sodium (Na+) intake can be associated with increased BP and cardiovascular morbidity and mortality. However, little is known regarding the mechanisms involved in the increase in BP in response to low Na+ intake, a condition termed inverse salt sensitivity of BP, which affects approximately 15% of the adult population. The renal proximal tubule is important in regulating up to 70% of renal Na+ transport. The renin-angiotensin and renal dopaminergic systems play both synergistic and opposing roles in the regulation of Na+ transport in this nephron segment. Clinical studies have demonstrated that individuals express a "personal salt index" (PSI) that marks whether they are salt-resistant, salt-sensitive, or inverse salt-sensitive. Inverse salt sensitivity results in part from genetic polymorphisms in various Na+ regulatory genes leading to a decrease in natriuretic activity and an increase in renal tubular Na+ reabsorption leading to an increase in BP. This article reviews the potential mechanisms of a new pathophysiologic entity, inverse salt sensitivity of BP, which affects approximately 15% of the general adult population.

Expression of ENaC subunits in epithelia

The epithelial Na+ channel (ENaC) is a heterotrimeric protein whose assembly, trafficking, and function are highly regulated. To better understand the biogenesis and activation of the channel, we quantified the expression of individual subunits of ENaC in rat kidneys and colon using calibrated Western blots. The estimated abundance for the three subunits differed by an order of magnitude with the order γENaC ∼ βENaC ≫ αENaC in both organs. Transcript abundance in the kidney, measured with digital-drop PCR and RNAseq, was similar for the three subunits. In both organs, the calculated protein expression of all subunits was much larger than that required to account for maximal Na+ currents measured in these cells, implying a large excess of subunit protein. Whole-kidney biotinylation indicated that at least 5% of β and γ subunits in the kidney and 3% in the colon were expressed on the surface under conditions of salt restriction, which maximizes ENaC-dependent Na+ transport. This indicates a 10- to 100-fold excess of βENaC and γENaC subunits at the surface relative to the requirement for channel activity. We conclude that these epithelia make much more ENaC protein than is required for the physiological function of the channel. This could facilitate rapid regulation of the channels at the cell surface by insuring a large population of inactive, recruitable subunits.

Activation by cleavage of the epithelial Na+ channel α and γ subunits independently coevolved with the vertebrate terrestrial migration

Vertebrates evolved mechanisms for sodium conservation and gas exchange in conjunction with migration from aquatic to terrestrial habitats. Epithelial Na⁺ channel (ENaC) function is critical to systems responsible for extracellular fluid homeostasis and gas exchange. ENaC is activated by cleavage at multiple specific extracellular polybasic sites, releasing inhibitory tracts from the channel’s α and γ subunits. We found that proximal and distal polybasic tracts in ENaC subunits coevolved, consistent with the dual cleavage requirement for activation observed in mammals. Polybasic tract pairs evolved with the terrestrial migration and the appearance of lungs, coincident with the ENaC activator aldosterone, and appeared independently in the α and γ subunits. In summary, sites within ENaC for protease activation developed in vertebrates when renal Na⁺ conservation and alveolar gas exchange were required for terrestrial survival.

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.

Cleavage state of γENaC in mouse and rat kidneys

Extracellular proteases can activate the epithelial Na channel (ENaC) by cleavage of the γ subunit. Here, we investigated the cleavage state of the channel in the kidneys of mice and rats on a low-salt diet. We identified the cleaved species of channels expressed in Fisher rat thyroid cells by coexpressing the apical membrane-bound protease channel-activating protease 1 (CAP1; prostasin). To compare the peptides produced in the heterologous system with those in the mouse kidney, we treated both lysates with PNGaseF to remove N-linked glycosylation. The apparent molecular mass of the smallest COOH-terminal fragment of γENaC (52 kDa) was indistinguishable from that of the CAP1-induced species in Fisher rat thyroid cells. Similar cleaved peptides were observed in total and cell surface fractions of the rat kidney. This outcome suggests that most of the subunits at the surface have been processed by extracellular proteases. This was confirmed using nonreducing gels, in which the NH₂- and COOH-terminal fragments of γENaC are linked by a disulfide bond. Under these conditions, the major cleaved form in the rat kidney had an apparent molecular mass of 56 kDa, ∼4 kDa lower than that of the full-length form, consistent with excision of a short peptide by two proteolytic events. We conclude that the most abundant γENaC species in the apical membrane of rat and mouse kidneys on a low-Na diet is the twice-cleaved, presumably activated form.NEW & NOTEWORTHY We have identified the major aldosterone-dependent cleaved form of the epithelial Na channel (ENaC) γ subunit in the kidney as a twice-cleaved peptide. This form appears to be identical in size with a subunit cleaved in vitro by the extracellular protease channel-activating protease 1 (prostasin). In the absence of reducing agents, it has an overall molecular mass less than that of the intact subunit, consistent with the excision of an inhibitory domain.

WNK4 kinase: from structure to physiology

With no lysine kinase-4 (WNK4) belongs to a serine-threonine kinase family characterized by the atypical positioning of its catalytic lysine. Despite the fact that WNK4 has been found in many tissues, the majority of its study has revolved around its function in the kidney, specifically as a positive regulator of the thiazide-sensitive NaCl cotransporter (NCC) in the distal convoluted tubule of the nephron. This is explained by the description of gain-of-function mutations in the gene encoding WNK4 that causes familial hyperkalemic hypertension. This disease is mainly driven by increased downstream activation of the Ste20/SPS1-related proline-alanine-rich kinase/oxidative stress responsive kinase-1-NCC pathway, which increases salt reabsorption in the distal convoluted tubule and indirectly impairs renal K⁺ secretion. Here, we review the large volume of information that has accumulated about different aspects of WNK4 function. We first review the knowledge on WNK4 structure and enumerate the functional domains and motifs that have been characterized. Then, we discuss WNK4 physiological functions based on the information obtained from in vitro studies and from a diverse set of genetically modified mouse models with altered WNK4 function. We then review in vitro and in vivo evidence on the different levels of regulation of WNK4. Finally, we go through the evidence that has suggested how different physiological conditions act through WNK4 to modulate NCC activity.

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.

Sodium-hydrogen exchanger regulatory factor-1 (NHERF1) confers salt sensitivity in both male and female models of hypertension in aging

Hypertension is a risk factor for premature death and roughly 50% of hypertensive patients are salt-sensitive. The incidence of salt-sensitive hypertension increases with age. However, the mechanisms of salt-sensitive hypertension are not well understood. We had demonstrated decreased renal sodium‑hydrogen exchanger regulatory factor 1 (NHERF1) expression in old salt-resistant F344 rats. Based on those studies we hypothesized that NHERF1 expression is required for the development of some forms of salt-sensitive hypertension. To address this hypothesis, we measured blood pressure in NHERF1 expressing salt-sensitive 4-mo and 24-mo-old male and female Fischer Brown Norway (FBN) rats male and female 18-mo-old NHERF1 knock-out (NHERF1-/-) mice and wild-type (WT) littermates on C57BL/6J background after feeding high salt (8% NaCl) diet for 7 days. Our data demonstrate that 8% salt diet increased blood pressure in both male and female 24-mo-old FBN rats but not in 4-mo-old FBN rats and in 18-mo-old male and female WT mice but not in NHERF1-/- mice. Renal dopamine 1 receptor (D1R) expression was decreased in 24-mo-old rats, compared with 4-mo-old FBN rats. However, sodium chloride cotransporter (NCC) expression increased in 24-mo-old FBN rats. In FBN rats, age had no effect on NaK ATPase α1 and NKCC2 expression. By contrast, high salt diet increased the renal expressions of NKCC2, and NCC in 24-mo-old FBN rats. High salt diet also increased NKCC2 and NCC expression in WT mice but not NHERF1-/- mice. Our data suggest that renal NHERF1 expression confers salt sensitivity with aging, associated with increased expression of sodium transporters.

Regulating ENaC's gate

Epithelial Na+ channels (ENaCs) are members of a family of cation channels that function as sensors of the extracellular environment. ENaCs are activated by specific proteases in the biosynthetic pathway and at the cell surface and remove embedded inhibitory tracts, which allows channels to transition to higher open-probability states. Resolved structures of ENaC and an acid-sensing ion channel revealed highly organized extracellular regions. Within the periphery of ENaC subunits are unique domains formed by antiparallel β-strands containing the inhibitory tracts and protease cleavage sites. ENaCs are inhibited by Na+ binding to specific extracellular site(s), which promotes channel transition to a lower open-probability state. Specific inositol phospholipids and channel modification by Cys-palmitoylation enhance channel open probability. How these regulatory factors interact in a concerted manner to influence channel open probability is an important question that has not been resolved. These various factors are reviewed, and the impact of specific factors on human disorders is discussed.

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.

Lovastatin attenuates hypertension induced by renal tubule-specific knockout of ATP-binding cassette transporter A1, by inhibiting epithelial sodium channels

BACKGROUND AND PURPOSE

We have shown that cholesterol is synthesized in the principal cells of renal cortical collecting ducts (CCD) and stimulates the epithelial sodium channels (ENaC). Here we have determined whether lovastatin, a cholesterol synthesis inhibitor, can antagonize the hypertension induced by activated ENaC, following deletion of the cholesterol transporter (ATP-binding cassette transporter A1; ABCA1).

EXPERIMENTAL APPROACH

We selectively deleted ABCA1 in the principal cells of mouse CCD and used the cell-attached patch-clamp technique to record ENaC activity. Western blot and immunofluorescence staining were used to evaluate protein expression levels. Systolic BP was measured with the tail-cuff method.

KEY RESULTS

Specific deletion of ABCA1 elevated BP and ENaC single-channel activity in the principal cells of CCD in mice. These effects were antagonized by lovastatin. ABCA1 deletion elevated intracellular cholesterol levels, which was accompanied by elevated ROS, increased expression of serum/glucocorticoid regulated kinase 1 (Sgk1), phosphorylated neural precursor cell-expressed developmentally down-regulated protein 4-2 (Nedd4-2) and furin, along with shorten the primary cilium, and reduced ATP levels in urine.

CONCLUSIONS AND IMPLICATIONS

These data suggest that specific deletion of ABCA1 in principal cells increases BP by stimulating ENaC channels via a cholesterol-dependent pathway which induces several secondary responses associated with oxidative stress, activated Sgk1/Nedd4-2, increased furin expression, and reduced cilium-mediated release of ATP. As ABCA1 can be blocked by cyclosporine A, these results suggest further investigation of the possible use of statins to treat CsA-induced hypertension.

Kir4.1/Kir5.1 in the DCT plays a role in the regulation of renal K+ excretion

The aim of this mini review is to provide an overview regarding the role of inwardly rectifying potassium channel 4.1 (Kir4.1)/Kir5.1 in regulating renal K+ excretion. Deletion of Kir4.1 in the kidney inhibited thiazide-sensitive NaCl cotransporter (NCC) activity in the distal convoluted tubule (DCT) and slightly suppressed Na-K-2Cl cotransporter (NKCC2) function in the thick ascending limb (TAL). Moreover, increased dietary K+ intake inhibited, whereas decreased dietary K+ intake stimulated, the basolateral potassium channel (a Kir4.1/Kir5.1 heterotetramer) in the DCT. The alteration of basolateral potassium conductance is essential for the effect of dietary K+ intake on NCC because deletion of Kir4.1 in the DCT abolished the effect of dietary K+ intake on NCC. Since potassium intake-mediated regulation of NCC plays a key role in regulating renal K+ excretion and potassium homeostasis, the deletion of Kir4.1 caused severe hypokalemia and metabolic alkalosis under control conditions and even during increased dietary K+ intake. Finally, recent studies have suggested that the angiotensin II type 2 receptor (AT2R) and bradykinin-B2 receptor (BK2R) are involved in mediating the effect of high dietary K+ intake on Kir4.1/Kir5.1 in the DCT.

Collecting duct principal, but not intercalated, cell prorenin receptor regulates renal sodium and water excretion

The collecting duct is the predominant nephron site of prorenin and prorenin receptor (PRR) expression. We previously demonstrated that the collecting duct PRR regulates epithelial Na+ channel (ENaC) activity and water transport; however, which cell type is involved remains unclear. Herein, we examined the effects of principal cell (PC) or intercalated cell (IC) PRR deletion on renal Na+ and water handling. PC or IC PRR knockout (KO) mice were obtained by crossing floxed PRR mice with mice harboring Cre recombinase under the control of the AQP2 or B1 subunit of the H+ ATPase promoters, respectively. PC KO mice had reduced renal medullary ENaC-α abundance and increased urinary Na+ losses on a low-Na+ diet compared with controls. Conversely, IC KO mice had no apparent differences in Na+ balance or ENaC abundance compared with controls. Acute treatment with prorenin increased ENaC channel number and open probability in acutely isolated cortical collecting ducts from control and IC PRR KO, but not PC PRR KO, mice. Furthermore, compared with controls, PC KO, but not IC KO mice, had increased urine volume, reduced urine osmolality, and reduced abundance of renal medullary AQP2. Taken together, these findings indicate that PC, but not IC, PRR modulates ENaC activity, urinary Na+ excretion, and water transport.

Na restriction activates epithelial Na channels in rat kidney through two mechanisms and decreases distal Na+ delivery

KEY POINTS

Dietary Na restriction, through the mineralocorticoid aldosterone, acts on epithelial Na channels via both fast (24 h) and slow (5-7 days) mechanisms in the kidney. The fast effect entails increased proteolytic processing and trafficking of channel protein to the apical membrane. It is rapidly reversible by the mineralocorticoid receptor antagonist eplerenone and is largely lost when tubules are studied ex vivo. The slow effect does not require increased processing or surface expression, is refractory to acute eplerenone treatment, and is preserved ex vivo. Both slow and fast effects contribute to Na retention in vivo. Increased Na⁺ reabsorption in the proximal tubule also promotes Na conservation under conditions of chronic dietary Na restriction, reducing Na⁺ delivery to the distal nephron.

ABSTRACT

Changes in the activity of the epithelial Na channel (ENaC) help to conserve extracellular fluid volume. In rats fed a low-salt diet, proteolytic processing of ENaC increased within 1 day, and was almost maximal after 3 days. The rapid increase in the abundance of cleaved αENaC and γENaC correlated with decreased urinary Na⁺ excretion and with increased ENaC surface expression. By contrast, ENaC activity, measured ex vivo in isolated cortical collecting ducts, increased modestly after 3 days and required 5 days to reach maximal levels. The mineralocorticoid receptor antagonist eplerenone reversed the increase in cleaved γENaC and induced natriuresis after 1 or 3 days but failed to alter either ENaC currents or Na⁺ excretion after 7 days of Na restriction. We conclude that Na depletion, through aldosterone, stimulates ENaC via independent fast and slow mechanisms. In vivo, amiloride-induced natriuresis increased after 1 day of Na depletion. By contrast, hydrochlorothiazide (HCTZ)-induced natriuresis decreased gradually over 7 days, consistent with increased ability of ENaC activity to compensate for decreased Na⁺ reabsorption in the distal convoluted tubule. Administration of amiloride and HCTZ together increased Na⁺ excretion less in Na-depleted compared to control animals, indicating decreased delivery of Na⁺ to the distal nephron when dietary Na is restricted. Measurements of creatinine and Li⁺ clearances indicated that increased Na reabsorption by the proximal tubules is responsible for the decreased delivery. Thus, Na conservation during chronic dietary salt restriction entails enhanced transport by both proximal and distal nephron segments.

Hydrochlorothiazide and acute urinary acidification: The "voltage hypothesis" of ENaC-dependent H+ secretion refuted

AIM

The "voltage hypothesis" of H⁺ secretion states that urinary acidification following increased Na⁺ delivery to the collecting duct (CD) is ENaC dependent leading to transepithelial voltage-dependent increase in H⁺ secretion. We recently showed that furosemide acidifies the urine independently of ENaC activity. If the voltage hypothesis holds, hydrochlorothiazide (HCT) must acidify the urine. We here tested the acute effect of HCT on urine pH under normal and high ENaC expression.

METHODS

Mice subjected to a control or a low-Na⁺ diet were anesthetized and infused (0.5 mL h^-1 ) with saline. Catheterization of the urinary bladder allowed real-time measurement of diuresis and urine pH. Mice received either HCT (1 mg mL^-1 ) or vehicle. Urinary Na⁺ and K⁺ excretions were determined by flame photometry. ENaC expression levels were measured by semi-quantitative Western blotting.

RESULTS

(1) HCT increased diuresis and natriuresis in both diet groups. (2) K⁺ excretion rates increased after HCT administration from 18.6 ± 1.3 to 31.7 ± 2.5 μmol h^-1 in the control diet group and from 23.0 ± 1.3 to 48.7 ± 3.0 μmol h^-1 in the low-Na⁺ diet group. (3) Mice fed a low-Na⁺ diet showed a marked upregulation of ENaC. (4) Importantly, no acute changes in urine pH were observed after the administration of HCT in either group.

CONCLUSION

Acute administration of HCT has no effect on urine pH. Similarly, substantial functional and molecular upregulation of ENaC did not cause HCT to acutely change urine pH. Thus, an increased Na⁺ load to the CD does not alter urine pH. This supports our previous finding and likely falsifies the voltage hypothesis of H⁺ secretion.

Dominant functional role of the novel phosphorylation site S811 in the human renal NaCl cotransporter

The NaCl cotransporter (NCC) is essential for electrolyte homeostasis and control of blood pressure. The human SLC12A3 gene, which encodes NCC, gives rise to 3 isoforms, of which only the shortest isoform [NaCl cotransporter isoform 3 (NCC₃)] has been studied extensively. All NCC isoforms share key phosphorylation sites at T55 and T60 that are essential mediators of NCC function. Recently, a novel phosphorylation site at S811 was identified in isoforms 1 and 2 [NaCl cotransporter splice variant (NCC_SV)], which are only present in humans and higher primates. The aim of the current study, therefore, is to investigate the role of S811 phosphorylation in the regulation of NCC by a combination of biochemical and fluorescent microscopy analyses. We demonstrate that hypotonic low-chloride buffer increases S811 phosphorylation, whereas phosphorylation-deficient S811A mutant hinders phosphorylation at T55 and T60 in NCC_SV and NCC₃. NCC_SV S811A impairs NCC₃ activity in a dominant-negative fashion, although it does not affect plasma membrane abundance. This effect may be explained by the heterodimerization of NCC_SV with NCC₃. Taken together, our study highlights the dominant-negative effect of NCC_SV on T55 and T60 phosphorylation and NCC activity. Here, we reveal a new function of NCC_SV in humans that broadens the understanding on NCC regulation in blood pressure control.-Tutakhel, O. A. Z., Bianchi, F., Smits, D. A., Bindels, R. J. M., Hoenderop, J. G. J., van der Wijst, J. Dominant functional role of the novel phosphorylation site S811 in the human renal NaCl cotransporter.

Epithelial Na+ Channel Regulation by Extracellular and Intracellular Factors

Epithelial Na⁺ channels (ENaCs) are members of the ENaC/degenerin family of ion channels that evolved to respond to extracellular factors. In addition to being expressed in the distal aspects of the nephron, where ENaCs couple the absorption of filtered Na⁺ to K⁺ secretion, these channels are found in other epithelia as well as nonepithelial tissues. This review addresses mechanisms by which ENaC activity is regulated by extracellular factors, including proteases, Na⁺, and shear stress. It also addresses other factors, including acidic phospholipids and modification of ENaC cytoplasmic cysteine residues by palmitoylation, which enhance channel activity by altering interactions of the channel with the plasma membrane.

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.

Responses of distal nephron Na+ transporters to acute volume depletion and hyperkalemia

We assessed effects of acute volume reductions induced by administration of diuretics in rats. Direct block of Na⁺ transport produced changes in urinary electrolyte excretion. Adaptations to these effects appeared as alterations in the expression of protein for the distal nephron Na⁺ transporters NCC and ENaC. Two hours after a single injection of furosemide (6 mg/kg) or hydrochlorothiazide (HCTZ; 30 mg/kg) Na⁺ and K⁺ excretion increased but no changes in the content of activated forms of NCC (phosphorylated on residue T53) or ENaC (cleaved γ-subunit) were detected. In contrast, amiloride (0.6 mg/kg) evoked a similar natriuresis that coincided with decreased pT53NCC and increased cleaved γENaC. Alterations in posttranslational membrane protein processing correlated with an increase in plasma K⁺ of 0.6-0.8 mM. Decreased pT53NCC occurred within 1 h after amiloride injection, whereas changes in γENaC were slower and were blocked by the mineralocorticoid receptor antagonist spironolactone. Increased γENaC cleavage correlated with elevation of the surface expression of the subunit as assessed by in situ biotinylation. Na depletion induced by 2 h of furosemide or HCTZ treatment increases total NCC expression without affecting ENaC protein. However, restriction of Na intake for 10 h (during the day) or 18 h (overnight) increased the abundance of both total NCC and of cleaved α- and γENaC. We conclude that the kidneys respond acutely to hyperkalemic challenges by decreasing the activity of NCC while increasing that of ENaC. They respond to hypovolemia more slowly, increasing Na⁺ reabsorptive capacities of both of these transporters.

Albuminuria is associated with an increased prostasin in urine while aldosterone has no direct effect on urine and kidney tissue abundance of prostasin

The proteinase prostasin is a candidate mediator for aldosterone-driven proteolytic activation of the epithelial sodium channel (ENaC). It was hypothesized that the aldosterone-mineralocorticoid receptor (MR) pathway stimulates prostasin abundance in kidney and urine. Prostasin was measured in plasma and urine from type 2 diabetic patients with resistant hypertension (n = 112) randomized to spironolactone/placebo in a clinical trial. Prostasin protein level was assessed by immunoblotting in (1) human and rat urines with/without nephrotic syndrome, (2) human nephrectomy tissue, (3) urine and kidney from aldosterone synthase-deficient (AS^-/-) mice and ANGII- and aldosterone-infused mice, and in (4) kidney from adrenalectomized rats. Serum aldosterone concentration related to prostasin concentration in urine but not in plasma. Plasma prostasin concentration increased significantly after spironolactone compared to control. Urinary prostasin and albumin related directly and were reduced by spironolactone. In patients with nephrotic syndrome, urinary prostasin protein was elevated compared to controls. In rat nephrosis, proteinuria coincided with increased urinary prostasin, unchanged kidney tissue prostasin, and decreased plasma prostasin while plasma aldosterone was suppressed. Prostasin protein abundance in human nephrectomy tissue was similar across gender and ANGII inhibition regimens. Prostasin urine abundance was not different in AS^-/- and aldosterone-infused mice. Prostasin kidney level was not different from control in adrenalectomized rats and AS^-/- mice. We found no evidence for a direct relationship between mineralocorticoid receptor signaling and kidney and urine prostasin abundance. The reduction of urinary prostasin in spironolactone-treated patients is most likely the result of an improved glomerular filtration barrier function and generally reduced proteinuria.

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

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

SGK1-dependent ENaC processing and trafficking in mice with high dietary K intake and elevated aldosterone

We examined renal Na and K transporters in mice with deletions in the gene encoding the aldosterone-induced protein SGK1. The knockout mice were hyperkalemic, and had altered expression of the subunits of the epithelial Na channel (ENaC). The kidneys showed decreased expression of the cleaved forms of the γENaC subunit, and the fully glycosylated form of the βENaC subunits when animals were fed a high-K diet. Knockout animals treated with exogenous aldosterone also had reduced subunit processing and diminished surface expression of βENaC and γENaC. Expression of the three upstream Na transporters NHE3, NKCC2, and NCC was reduced in both wild-type and knockout mice in response to K loading. The activity of ENaC measured as whole cell amiloride-sensitive current (I_Na) in principal cells of the cortical collecting duct (CCD) was minimal under control conditions but was increased by a high-K diet to a similar extent in knockout and wild-type animals. I_Na in the connecting tubule also increased similarly in the two genotypes in response to exogenous aldosterone administration. The activities of both ROMK channels in principal cells and BK channels in intercalated cells of the CCD were unaffected by the deletion of SGK1. Acute treatment of animals with amiloride produced similar increases in Na excretion and decreases in K excretion in the two genotypes. The absence of changes in ENaC activity suggests compensation for decreased surface expression. Altered K balance in animals lacking SGK1 may reflect defects in ENaC-independent K excretion.

Renal tubular epithelial cell prorenin receptor regulates blood pressure and sodium transport

The physiological significance of the renal tubular prorenin receptor (PRR) has been difficult to elucidate due to developmental abnormalities associated with global or renal-specific PRR knockout (KO). We recently developed an inducible renal tubule-wide PRR KO using the Pax8/LC1 transgenes and demonstrated that disruption of renal tubular PRR at 1 mo of age caused no renal histological abnormalities. Here, we examined the role of renal tubular PRR in blood pressure (BP) regulation and Na(+) excretion and investigated the signaling mechanisms by which PRR regulates Na(+) balance. No detectable differences in BP were observed between control and PRR KO mice fed normal- or low-Na(+) diets. However, compared with controls, PRR KO mice had elevated plasma renin concentration and lower cumulative Na(+) balance with normal- and low-Na(+) intake. PRR KO mice had an attenuated hypertensive response and reduced Na(+) retention following angiotensin II (ANG II) infusion. Furthermore, PRR KO mice had significantly lower epithelial Na(+) channel (ENaC-α) expression. Treatment with mouse prorenin increased, while PRR antagonism decreased, ENaC activity in isolated split-open collecting ducts (CD). The prorenin effect was prevented by protein kinase A and Akt inhibition, but unaffected by blockade of AT1, ERK1/2, or p38 MAPK pathways. Taken together, these data indicate that renal tubular PRR, likely via direct prorenin/renin stimulation of PKA/Akt-dependent pathways, stimulates CD ENaC activity. Absence of renal tubular PRR promotes Na(+) wasting and reduces the hypertensive response to ANG II.

Regulation of ENaC trafficking in rat kidney

The epithelial Na channel (ENaC) forms a pathway for Na(+) reabsorption in the distal nephron, and regulation of these channels is essential for salt homeostasis. In the rat kidney, ENaC subunits reached the plasma membrane in both immature and fully processed forms, the latter defined by either endoglycosidase H-insensitive glycosylation or proteolytic cleavage. Animals adapted to a low-salt diet have increased ENaC surface expression that is specific for the mature forms of the subunit proteins and is similar (three- to fourfold) for α, β, and γENaC. Kidney membranes were fractionated using differential centrifugation, sucrose-gradient separation, and immunoabsorption. Endoplasmic reticulum membranes, isolated using an antibody against calnexin, expressed immature γENaC, and the content decreased with Na depletion. Golgi membranes, isolated with an antibody against the cis-Golgi protein GM130, expressed both immature and processed γENaC; Na depletion increased the content of processed γENaC in this fraction by 3.8-fold. An endosomal compartment isolated using an antibody against Rab11 contained both immature and processed γENaC; the content of processed subunit increased 2.4-fold with Na depletion. Finally, we assessed the content of γENaC in the late endocytic compartments indirectly using urinary exosomes. All of the γENaC in these exosomes was in the fully cleaved form, and its content increased by 4.5-fold with Na depletion. These results imply that stimulation of ENaC surface expression results at least in part from increased rates of formation of fully processed subunits in the Golgi and subsequent trafficking to the apical membrane.

Mini-review: regulation of the renal NaCl cotransporter by hormones

The renal thiazide-sensitive NaCl cotransporter, NCC, is the major pathway for salt reabsorption in the distal convoluted tubule. The activity of this cotransporter is critical for regulation of several physiological variables such as blood pressure, serum potassium, acid base metabolism, and urinary calcium excretion. Therefore, it is not surprising that numerous hormone-signaling pathways regulate NCC activity to maintain homeostasis. In this review, we will provide an overview of the most recent evidence on NCC modulation by aldosterone, angiotensin II, vasopressin, glucocorticoids, insulin, norepinephrine, estradiol, progesterone, prolactin, and parathyroid hormone.

Roles and Regulation of Renal K Channels

More than two dozen types of potassium channels, with different biophysical and regulatory properties, are expressed in the kidney, influencing renal function in many important ways. Recently, a confluence of discoveries in areas from human genetics to physiology, cell biology, and biophysics has cast light on the special function of five different potassium channels in the distal nephron, encoded by the genes KCNJ1, KCNJ10, KCNJ16, KCNMA1, and KCNN3. Research aimed at understanding how these channels work in health and go awry in disease has transformed our understanding of potassium balance and provided new insights into mechanisms of renal sodium handling and the maintenance of blood pressure. This review focuses on recent advances in this rapidly evolving field.

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.

Sodium retention and volume expansion in nephrotic syndrome: implications for hypertension

Sodium retention is a major clinical feature of nephrotic syndrome. The mechanisms responsible for sodium retention in this setting have been a subject of debate for years. Excessive sodium retention occurs in some individuals with nephrotic syndrome in the absence of activation of the renin-angiotensin-aldosterone system, suggesting an intrinsic defect in sodium excretion by the kidney. Recent studies have provided new insights regarding mechanisms by which sodium transporters are activated by factors present in nephrotic urine. These mechanisms likely have a role in the development of hypertension in nephrotic syndrome, where hypertension may be difficult to control, and provide new therapeutic options for the management of blood pressure and edema in the setting of nephrotic syndrome.

Integrated control of Na transport along the nephron

The kidney filters vast quantities of Na at the glomerulus but excretes a very small fraction of this Na in the final urine. Although almost every nephron segment participates in the reabsorption of Na in the normal kidney, the proximal segments (from the glomerulus to the macula densa) and the distal segments (past the macula densa) play different roles. The proximal tubule and the thick ascending limb of the loop of Henle interact with the filtration apparatus to deliver Na to the distal nephron at a rather constant rate. This involves regulation of both filtration and reabsorption through the processes of glomerulotubular balance and tubuloglomerular feedback. The more distal segments, including the distal convoluted tubule (DCT), connecting tubule, and collecting duct, regulate Na reabsorption to match the excretion with dietary intake. The relative amounts of Na reabsorbed in the DCT, which mainly reabsorbs NaCl, and by more downstream segments that exchange Na for K are variable, allowing the simultaneous regulation of both Na and K excretion.

Control of ENaC-mediated sodium reabsorption in the distal nephron by Bradykinin

Kinins, such as Bradykinin (BK), are peptide hormones of the kallikrein-kinin system. Apart from being a vasodilator, BK also increases urinary sodium excretion to reduce systemic blood pressure. It is becoming appreciated that BK modulates function of the epithelial Na(+) channel in the distal part of the renal nephron to affect tubular sodium reabsorption. In this chapter, we outline the molecular details, as well as discuss the physiological relevance of this regulation for the whole organism sodium homeostasis and setting chronic blood pressure.

Distal convoluted tubule

The distal convoluted tubule (DCT) is a short nephron segment, interposed between the macula densa and collecting duct. Even though it is short, it plays a key role in regulating extracellular fluid volume and electrolyte homeostasis. DCT cells are rich in mitochondria, and possess the highest density of Na+/K+-ATPase along the nephron, where it is expressed on the highly amplified basolateral membranes. DCT cells are largely water impermeable, and reabsorb sodium and chloride across the apical membrane via electroneurtral pathways. Prominent among this is the thiazide-sensitive sodium chloride cotransporter, target of widely used diuretic drugs. These cells also play a key role in magnesium reabsorption, which occurs predominantly, via a transient receptor potential channel (TRPM6). Human genetic diseases in which DCT function is perturbed have provided critical insights into the physiological role of the DCT, and how transport is regulated. These include Familial Hyperkalemic Hypertension, the salt-wasting diseases Gitelman syndrome and EAST syndrome, and hereditary hypomagnesemias. The DCT is also established as an important target for the hormones angiotensin II and aldosterone; it also appears to respond to sympathetic-nerve stimulation and changes in plasma potassium. Here, we discuss what is currently known about DCT physiology. Early studies that determined transport rates of ions by the DCT are described, as are the channels and transporters expressed along the DCT with the advent of molecular cloning. Regulation of expression and activity of these channels and transporters is also described; particular emphasis is placed on the contribution of genetic forms of DCT dysregulation to our understanding.

International Union of Basic and Clinical Pharmacology. XCI. structure, function, and pharmacology of acid-sensing ion channels and the epithelial Na+ channel

The epithelial Na(+) channel (ENaC) and the acid-sensing ion channels (ASICs) form subfamilies within the ENaC/degenerin family of Na(+) channels. ENaC mediates transepithelial Na(+) transport, thereby contributing to Na(+) homeostasis and the maintenance of blood pressure and the airway surface liquid level. ASICs are H(+)-activated channels found in central and peripheral neurons, where their activation induces neuronal depolarization. ASICs are involved in pain sensation, the expression of fear, and neurodegeneration after ischemia, making them potentially interesting drug targets. This review summarizes the biophysical properties, cellular functions, and physiologic and pathologic roles of the ASIC and ENaC subfamilies. The analysis of the homologies between ENaC and ASICs and the relation between functional and structural information shows many parallels between these channels, suggesting that some mechanisms that control channel activity are shared between ASICs and ENaC. The available crystal structures and the discovery of animal toxins acting on ASICs provide a unique opportunity to address the molecular mechanisms of ENaC and ASIC function to identify novel strategies for the modulation of these channels by pharmacologic ligands.

Acute effects of aldosterone on the epithelial Na channel in rat kidney

The acute effects of aldosterone administration on epithelial Na channels (ENaC) in rat kidney were examined using electrophysiology and immunodetection. Animals received a single injection of aldosterone (20 μg/kg body wt), which reduced Na excretion over the next 3 h. Channel activity was assessed in principal cells of cortical collecting ducts as amiloride-sensitive whole cell clamp current (INa). INa averaged 100 pA/cell, 20-30% of that reported for the same preparation under conditions of chronic stimulation. INa was negligible in control animals that did not receive hormone. The acute physiological response correlated with changes in ENaC processing and trafficking. These effects included increases in the cleaved forms of α-ENaC and γ-ENaC, assessed by Western blot, and increases in the surface expression of β-ENaC and γ-ENaC measured after surface protein biotinylation. These changes were qualitatively and quantitatively similar to those of chronic stimulation. This suggests that altered trafficking to or from the apical membrane is an early response to the hormone and that later increases in channel activity require stimulation of channels residing at the surface.

Prostasin interacts with the epithelial Na+ channel and facilitates cleavage of the γ-subunit by a second protease

During maturation, the α- and γ-subunits of the epithelial Na+ channel (ENaC) undergo proteolytic processing by furin. Cleavage of the γ-subunit by furin at the consensus site γRKRR143 and subsequent cleavage by a second protease at a distal site strongly activate the channel. For example, coexpression of prostasin with ENaC increases both channel function and cleavage at the γRKRK186 site. We generated a polyclonal antibody that recognizes the region 144-186 in the γ-subunit (anti-γ43) to determine whether prostasin promotes the release of the intervening tract between the putative furin and γRKRK186 cleavage sites. Anti-γ43 precipitated both full-length (93 kDa) and furin-processed (83 kDa) γ-subunits from extracts obtained from oocytes expressing αβHA-γ-V5 channels, but only the full-length (93 kDa) γ-subunit from oocytes expressing αβHA-γ-V5 channels and either wild-type or a catalytically inactive prostasin. Although both wild-type and catalytically inactive prostasin activated ENaCs in an aprotinin-sensitive manner, only wild-type prostasin bound to aprotinin beads, suggesting that catalytically inactive prostasin facilitates the cleavage of the γ-subunit by an endogenous protease in Xenopus oocytes. As dietary salt restriction increases cleavage of the renal γ-subunit, we assessed release of the 43-mer inhibitory tract on rats fed a low-Na+ diet. We found that a low-Na+ diet increased γ-subunit cleavage detected with the anti-γ antibody and dramatically reduced the fraction precipitated with the anti-γ43 antibody. Our results suggest that the inhibitory tract dissociates from the γ-subunit in kidneys from rats on a low-Na+ diet.

ENaC activity and expression is decreased in the lungs of protein kinase C-α knockout mice

We used a PKC-α knockout model to investigate the regulation of alveolar epithelial Na(+) channels (ENaC) by PKC. Primary alveolar type II (ATII) cells were subjected to cell-attached patch clamp. In the absence of PKC-α, the open probability (Po) of ENaC was decreased by half compared with wild-type mice. The channel density (N) was also reduced in the knockout mice. Using in vivo biotinylation, membrane localization of all three ENaC subunits (α, β, and γ) was decreased in the PKC-α knockout lung, compared with the wild-type. Confocal microscopy of lung slices showed elevated levels of reactive oxygen species (ROS) in the lungs of the PKC-α knockout mice vs. the wild-type. High levels of ROS in the knockout lung can be explained by a decrease in both cytosolic and mitochondrial superoxide dismutase activity. Elevated levels of ROS in the knockout lung activates PKC-δ and leads to reduced dephosphorylation of ERK1/2 by MAP kinase phosphatase, which in turn causes increased internalization of ENaC via ubiquitination by the ubiquitin-ligase Nedd4-2. In addition, in the knockout lung, PKC-δ activates ERK, causing a decrease in ENaC density at the apical alveolar membrane. PKC-δ also phosphorylates MARCKS, leading to a decrease in ENaC Po. The effects of ROS and PKC-δ were confirmed with patch-clamp experiments on isolated ATII cells in which the ROS scavenger, Tempol, or a PKC-δ-specific inhibitor added to patches reversed the observed decrease in ENaC apical channel density and Po. These results explain the decrease in ENaC activity in PKC-α knockout lung.

Regulation of renal potassium secretion: molecular mechanisms

A new understanding of renal potassium balance has emerged as the molecular underpinnings of potassium secretion have become illuminated, highlighting the key roles of apical potassium channels, renal outer medullary potassium channel (ROMK) and Big Potassium (BK), in the aldosterone-sensitive distal nephron and collecting duct. These channels act as the final-regulated components of the renal potassium secretory machinery. Their activity, number, and driving forces are precisely modulated to ensure potassium excretion matches dietary potassium intake. Recent identification of the underlying regulatory mechanisms at the molecular level provides a new appreciation of the physiology and reveals a molecular insight to explain the paradoxic actions of aldosterone on potassium secretion. Here, we review the current state of knowledge in the field.