The epithelial sodium channel in hypertension

Hydrogen Sulfide Prevents Advanced Glycation End-Products Induced Activation of the Epithelial Sodium Channel

Advanced glycation end-products (AGEs) are complex and heterogeneous compounds implicated in diabetes. Sodium reabsorption through the epithelial sodium channel (ENaC) at the distal nephron plays an important role in diabetic hypertension. Here, we report that H₂S antagonizes AGEs-induced ENaC activation in A6 cells. ENaC open probability (P_O) in A6 cells was significantly increased by exogenous AGEs and that this AGEs-induced ENaC activity was abolished by NaHS (a donor of H₂S) and TEMPOL. Incubating A6 cells with the catalase inhibitor 3-aminotriazole (3-AT) mimicked the effects of AGEs on ENaC activity, but did not induce any additive effect. We found that the expression levels of catalase were significantly reduced by AGEs and both AGEs and 3-AT facilitated ROS uptake in A6 cells, which were significantly inhibited by NaHS. The specific PTEN and PI3K inhibitors, BPV_(pic) and LY294002, influence ENaC activity in AGEs-pretreated A6 cells. Moreover, after removal of AGEs from AGEs-pretreated A6 cells for 72 hours, ENaC P_O remained at a high level, suggesting that an AGEs-related “metabolic memory” may be involved in sodium homeostasis. Our data, for the first time, show that H₂S prevents AGEs-induced ENaC activation by targeting the ROS/PI3K/PTEN pathway.

Chronic regulation of the renal Na(+)/H(+) exchanger NHE3 by dopamine: translational and posttranslational mechanisms

The intrarenal autocrine/paracrine dopamine (DA) system contributes to natriuresis in response to both acute and chronic Na(+) loads. While the acute DA effect is well described, how DA induces natriuresis chronically is not known. We used an animal and a cell culture model to study the chronic effect of DA on a principal renal Na(+) transporter, Na(+)/H(+) exchanger-3 (NHE3). Intraperitoneal injection of Gludopa in rats for 2 days elevated DA excretion and decreased total renal cortical and apical brush-border NHE3 antigen. Chronic treatment of an opossum renal proximal cell line with DA decreased NHE3 activity, cell surface and total cellular NHE3 antigen, but not NHE3 transcript. The decrease in NHE3 antigen was dose and time dependent with maximal inhibition at 16-24 h and half maximal effect at 3 × 10(-7) M. This is in contradistinction to the acute effect of DA on NHE3 (half maximal at 2 × 10(-6) M), which was not associated with changes in total cellular NHE3 protein. The DA-induced decrease in total NHE3 protein was associated with decrease in NHE3 translation and mediated by cis-sequences in the NHE3 5'-untranslated region. DA also decreased cell surface and total cellular NHE3 protein half-life. The DA-induced decrease in total cellular NHE3 was partially blocked by proteasome inhibition but not by lysosome inhibition, and DA increased ubiquitylation of total and surface NHE3. In summary, chronic DA inhibits NHE3 with mechanisms distinct from its acute action and involves decreased NHE3 translation and increased NHE3 degradation, which are novel mechanisms for NHE3 regulation.

Hydrogen peroxide stimulates the epithelial sodium channel through a phosphatidylinositide 3-kinase-dependent pathway

Recent studies indicate that oxidative stress mediates salt-sensitive hypertension. To test the hypothesis that the renal epithelial sodium channel (ENaC) is a target of oxidative stress, patch clamp techniques were used to determine whether ENaC in A6 distal nephron cells is regulated by hydrogen peroxide (H(2)O(2)). In the cell-attached configuration, H(2)O(2) significantly increased ENaC open probability (P(o)) and single-channel current amplitude but not the unit conductance. High concentrations of exogenous H(2)O(2) are required to elevate intracellular H(2)O(2), probably because catalase, the enzyme that promotes the decomposition of H(2)O(2) to H(2)O and O(2), is highly expressed in A6 cells. The effect of H(2)O(2) on ENaC P(o) was enhanced by 3-aminotriazole, a catalase inhibitor, and abolished by overexpression of catalase, indicating that intracellular H(2)O(2) levels are critical to produce the effect. However, H(2)O(2) did not directly activate ENaC in inside-out patches. The effects of H(2)O(2) on ENaC P(o) and amiloride-sensitive Na(+) current were abolished by inhibition of phosphatidylinositide 3-kinase (PI3K). Confocal microscopy data showed that H(2)O(2) elevated phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)) in the apical membrane by stimulating PI3K. Because ENaC is stimulated by PI(3,4,5)P(3), these data suggest that H(2)O(2) stimulates ENaC via PI3K-mediated increases in apical PI(3,4,5)P(3).

Functional significance of channels and transporters expressed in the inner ear and kidney

A number of ion channels and transporters are expressed in both the inner ear and kidney. In the inner ear, K+cycling and endolymphatic K+, Na+, Ca2+, and pH homeostasis are critical for normal organ function. Ion channels and transporters involved in K+cycling include K+channels, Na+-2Cl−-K+cotransporter, Na+/K+-ATPase, Cl−channels, connexins, and K+/Cl−cotransporters. Furthermore, endolymphatic Na+and Ca2+homeostasis depends on Ca2+-ATPase, Ca2+channels, Na+channels, and a purinergic receptor channel. Endolymphatic pH homeostasis involves H+-ATPase and Cl−/HCO3−exchangers including pendrin. Defective connexins (GJB2 and GJB6), pendrin (SLC26A4), K+channels (KCNJ10, KCNQ1, KCNE1, and KCNMA1), Na+-2Cl−-K+cotransporter (SLC12A2), K+/Cl−cotransporters (KCC3 and KCC4), Cl−channels (BSND and CLCNKA + CLCNKB), and H+-ATPase (ATP6V1B1 and ATPV0A4) cause hearing loss. All these channels and transporters are also expressed in the kidney and support renal tubular transport or signaling. The hearing loss may thus be paralleled by various renal phenotypes including a subtle decrease of proximal Na+-coupled transport (KCNE1/KCNQ1), impaired K+secretion (KCNMA1), limited HCO3−elimination (SLC26A4), NaCl wasting (BSND and CLCNKB), renal tubular acidosis (ATP6V1B1, ATPV0A4, and KCC4), or impaired urinary concentration (CLCNKA). Thus, defects of channels and transporters expressed in the kidney and inner ear result in simultaneous dysfunctions of these seemingly unrelated organs.

Rab proteins regulate epithelial sodium channel activity in colonic epithelial HT-29 cells

ENaC, the sodium-selective amiloride-sensitive epithelial channel, mediates electrogenic sodium re-absorption in tight epithelia and is deeply associated with human hypertension. The ENaC expression at plasma membrane requires the regulated transport, processing, and macromolecular assembly in a defined and highly compartmentalized manner. Ras-related Rab GTPases regulate intracellular trafficking during endocytosis, regulated exocytosis, and secretion. To evaluate the role of these proteins in regulating amiloride-sensitive sodium channel activity, multiple Rab isoforms 3, 5, 6, and Rab27a were expressed in HT-29 cells. Rab3 and Rab27a inhibited ENaC currents, while the expression of other Rab isoforms failed to elicit any statistically significant effect on amiloride-sensitive currents. The immunoprecipitation experiments suggest protein-protein interaction of Rab3 and Rab27a with epithelial sodium channel. Biotinylation studies revealed that modulation of ENaC function is due to the reduced apical expression of channel proteins. Study also indicates that Rabs do not appear to affect the steady-state level of total cellular ENaC. Alternatively, introduction of isoform-specific small inhibitory RNA (SiRNA) reversed the Rab-dependent inhibition of amiloride-sensitive currents. These observations point to the involvement of multiple Rab proteins in ENaC transport through intracellular routes like exocytosis, recycling from ER to plasma membrane or degradation and thus serve as potential target for human hypertension.

Impact of αENaC Polymorphisms on the Risk of Ischemic Cerebrovascular Events: A Multicenter Case–Control Study

Background: Amiloride-sensitive epithelial sodium channels (ENaCs) are important candidates in the development of hypertension, which is a major risk factor for stroke. Two single-nucleotide polymorphisms (SNPs) in the gene that encodes the ENac α-subunit (αENaC) have been identified. We evaluated those SNPs for a possible association with ischemic cerebrovascular events (ICEs).

Methods and Results: We genotyped 1399 patients with ICEs [median age, 70 years; interquartile range, 58–78 years; 745 (53%) men] and 1076 control individuals without vascular disease [47 (39–58) years; 557 (52%) men] for the SNPs Trp493Arg and Ala663Thr. The SNP frequencies at nucleotide 3977 (Trp493Arg) in the αENaC gene were significantly different in patients and controls. Carriers of 493Arg had a 1.78-fold increased risk (95% confidence interval, 1.02–3.12) for ICEs compared with Trp/Trp carriers. Interaction analysis revealed that the relative risk was even higher in women in the lowest age tertile [adjusted odds ratio, 3.26 (1.10–9.72)].

Conclusions: Carriers of the 493Arg allele are at increased risk for ICEs compared with Trp/Trp carriers. The effect is independent of traditional vascular risk factors and is particularly evident in younger women. The Trp493Arg variant in αENaC may represent an important candidate genetic susceptibility factor in the development of ICEs.

Hereditary disorders of potassium homeostasis

There has been a dramatic recent increase in the understanding of the renal epithelial transport systems with the identification, cloning and characterization of a large number of membrane transport proteins. The aim of this chapter is to integrate this body of knowledge with the understanding of the clinical disorders that accompany gain, loss or dysregulation of function of these transport systems. The specific focus is on the best-defined human clinical syndromes in which there are derangements in potassium (K(+)) homeostasis. The focus is on inherited syndromes, rather than on acquired syndromes due to tubular transport defects, and the therapeutic approaches address chronic derangements of K(+) homeostasis rather than acute interventions directed at life-threatening hyperkalaemia.

Renal genetic disorders related to K+ and Mg2+

The recent knowledge of the renal epithelial transport systems has exploded with the identification, cloning, and characterization of a large number of membrane transport proteins. The fundamental aspects of these transporters are beginning to emerge at the molecular level and are summarized in the accompanying contributions in this volume of the Annual Review of Physiology. The aim of my review is to integrate this body of knowledge with the understanding of the clinical disorders of human mineral homeostasis that accompany gain, loss, or dysregulation of function of these transport systems. The specific focus is on the best defined human clinical syndromes in which there are derangements in K(+) and Mg(2+) homeostasis.

Liddle syndrome: genetics and mechanisms of Na+ channel defects

Our current understanding of Na+ transport defects has been greatly expanded over the last several years and has provided new insights into unusual clinical syndromes resulting from mutations of specific ion transporters. These genetic disorders affect Na+ balance, with both Na+ retaining and Na+ wasting conditions being the consequence. A major focus of these studies has been the epithelial sodium channel (ENaC), which can be directly affected by mutations (eg, Liddle syndrome, autosomal recessive pseudohypoaldosteronism, type I) or by changes in the response to (autosomal recessive pseudohypoaldosteronism, type I), or production of mineralocorticoids (apparent mineralocorticoid excess syndrome, glucocorticoid-remediable aldosteronism). As a result, we now have clearly defined syndromes in which ENaC activity is "dysregulated" with subsequent development of disorders of systemic blood pressure that can be attributed to a primary renal mechanisms. The focus of the current review is on Liddle syndrome ("pseudoaldosteronism").

Genetic forms of human hypertension

Our basic understanding of sodium mechanisms provides unique insights into epithelial transport processes, and unusual clinical syndromes can arise from mutations of these ion transporters. These genetic disorders affect sodium balance, with both sodium-retaining and sodium-wasting conditions being the consequence. A major focus of such studies has been the epithelial sodium channel, which can be activated by mutations in the channel subunits or mineralocorticoid receptor, and changes in the response to or production of mineralocorticoids. As a result, there are now clearly defined Mendelian syndromes in which epithelial sodium channel activity is 'dysregulated', with the subsequent development of systemic hypertension with suppressed plasma renin activity that can be attributed to a primary renal mechanism. Applying these insights to the far more common disorder of low renin hypertension may shed new light on the underlying pathophysiology of this common form of human hypertension, and more clearly define the interactions of dietary constituents such as sodium and potassium in the regulation of blood pressure.

Aldosterone-related genetic effects in hypertension

Our basic understanding of Na(+) transport mechanisms provides unique insights into epithelial transport processes. Unusual clinical syndromes can arise from mutations of these ion transporters. These genetic disorders affect Na(+) balance, resulting in both N(a+) retaining and Na(+) wasting conditions. A major focus has been the epithelial sodium channel (ENaC), which can be activated by mutations (eg, Liddle's syndrome), changes in the response to mineralocorticoids (apparent mineralocorticoid excess syndrome), or production of mineralocorticoids (glucocorticoid-remediable aldosteronism). As a result, we now have clearly defined Mendelian syndromes in which ENaC activity is "dysregulated." This dysregulation leads to systemic hypertension associated with suppressed plasma renin activity, which can be attributed to a primary renal mechanism. Applying these insights to the far more common disorder of low-renin hypertension may shed new light on the underlying pathophysiology of this common form of human hypertension.