Early aldosterone effect in distal colon by transcriptional regulation of ENaC subunits

Revising the Roles of Aldosterone in Vascular Physiology and Pathophysiology: From Electocortin to Baxdrostat

Aldosterone was initially identified as a hormone primarily related to regulation of fluid and electrolyte homeostasis. However, over the past 20 years there has been an increasing appreciation of its role in regulation of vascular function and pathophysiology in the setting of hypertension, atherosclerosis, and heart failure. This review highlights recent advances in our understanding the biology of aldosterone as it relates to the pathophysiology and the management of vascular disease-especially related to hypertension. The review focuses on 3 key areas: 1) advances in our understanding of the cellular mechanisms by which aldosterone mediates its cellular effects, 2) identification of the hidden epidemic of aldosteronism as a mediator of hypertension, and 3) appreciating new therapeutic advances in the clinical pharmacology of aldosterone inhibition in cardiovascular and renal disease.

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.

cCPE Fusion Proteins as Molecular Probes to Detect Claudins and Tight Junction Dysregulation in Gastrointestinal Cell Lines, Tissue Explants and Patient-Derived Organoids

Claudins regulate paracellular permeability, contribute to epithelial polarization and are dysregulated during inflammation and carcinogenesis. Variants of the claudin-binding domain of Clostridium perfringens enterotoxin (cCPE) are highly sensitive protein ligands for generic detection of a broad spectrum of claudins. Here, we investigated the preferential binding of YFP- or GST-cCPE fusion proteins to non-junctional claudin molecules. Plate reader assays, flow cytometry and microscopy were used to assess the binding of YFP- or GST-cCPE to non-junctional claudins in multiple in vitro and ex vivo models of human and rat gastrointestinal epithelia and to monitor formation of a tight junction barrier. Furthermore, YFP-cCPE was used to probe expression, polar localization and dysregulation of claudins in patient-derived organoids generated from gastric dysplasia and gastric cancer. Live-cell imaging and immunocytochemistry revealed cell polarity and presence of tight junctions in glandular organoids (originating from intestinal-type gastric cancer and gastric dysplasia) and, in contrast, a disrupted diffusion barrier for granular organoids (originating from discohesive tumor areas). In sum, we report the use of cCPE fusion proteins as molecular probes to specifically and efficiently detect claudin expression, localization and tight junction dysregulation in cell lines, tissue explants and patient-derived organoids of the gastrointestinal tract.

Dietary Na+ depletion up-regulates NKCC1 expression and enhances electrogenic Cl- secretion in rat proximal colon

The corticosteroid hormone, aldosterone, markedly enhances K+ secretion throughout the colon, a mechanism critical to its role in maintaining overall K+ balance. Previous studies demonstrated that basolateral NKCC1 was up-regulated by aldosterone in the distal colon specifically to support K+ secretion-which is distinct from the more well-established role of NKCC1 in supporting luminal Cl- secretion. However, considerable segmental variability exists between proximal and distal colonic ion transport processes, especially concerning their regulation by aldosterone. Furthermore, delineating such region-specific effects has important implications for the management of various gastrointestinal pathologies. Experiments were therefore designed to determine whether aldosterone similarly up-regulates NKCC1 in the proximal colon to support K+ secretion. Using dietary Na+ depletion as a model of secondary hyperaldosteronism in rats, we found that proximal colon NKCC1 expression was indeed enhanced in Na+-depleted (i.e., hyperaldosteronemic) rats. Surprisingly, electrogenic K+ secretion was not detectable by short-circuit current (ISC) measurements in response to either basolateral bumetanide (NKCC1 inhibitor) or luminal Ba2+ (non-selective K+ channel blocker), despite enhanced K+ secretion in Na+-depleted rats, as measured by 86Rb+ fluxes. Expression of BK and IK channels was also found to be unaltered by dietary Na+ depletion. However, bumetanide-sensitive basal and agonist-stimulated Cl- secretion (ISC) were significantly enhanced by Na+ depletion, as was CFTR Cl- channel expression. These data suggest that NKCC1-dependent secretory pathways are differentially regulated by aldosterone in proximal and distal colon. Development of therapeutic strategies in treating pathologies related to aberrant colonic K+/Cl- transport-such as pseudo-obstruction or ulcerative colitis-may benefit from these findings.

Epithelial Sodium Channel δ Subunit Is Expressed in Human Arteries and Has Potential Association With Hypertension

BACKGROUND

Elevated expression and increased activity of vascular epithelial sodium channel (ENaC) can result in vascular dysfunction in small animal models. However, there is limited or no knowledge on expression and function of ENaC channels in human vasculature. Hence, this study explored the expression and function of ENaC in human arteries and their association with hypertension.

METHODS

Human internal mammary artery (IMA) and aorta were obtained from cardiovascular patients undergoing coronary artery bypass graft surgery. Expression of the ENaC subunit was analyzed by polymerase chain reaction, Western blot, and immunohistochemistry. ENaC function was observed by patch-clamp electrophysiology in endothelial cells isolated from IMA. Levels of ENaC subunit expression levels were compared between arteries from normotensive, uncontrolled hypertensive, and controlled hypertensive patients.

RESULTS

For the first time, expression of α, β, γ, and δ was detected at mRNA and protein levels in human IMA and aorta. Single-channel patch-clamp recordings identified both αβγ- and δβγ-like channel conductance in primary endothelial cells isolated and cultured from IMA. Reduced expression of the δ subunit was observed in controlled hypertensive IMA, whereas reduced expression of γ-ENaC was observed in controlled hypertensive aorta.

CONCLUSIONS

These data suggest that functional ENaC channels are expressed in human arteries and their expression levels are associated with hypertension.

Function and Regulation of the Epithelial Na+ Channel ENaC

The Epithelial Na⁺ Channel, ENaC, comprised of 3 subunits (αβγ, or sometimes δβγENaC), plays a critical role in regulating salt and fluid homeostasis in the body. It regulates fluid reabsorption into the blood stream from the kidney to control blood volume and pressure, fluid absorption in the lung to control alveolar fluid clearance at birth and maintenance of normal airway surface liquid throughout life, and fluid absorption in the distal colon and other epithelial tissues. Moreover, recent studies have also revealed a role for sodium movement via ENaC in nonepithelial cells/tissues, such as endothelial cells in blood vessels and neurons. Over the past 25 years, major advances have been made in our understanding of ENaC structure, function, regulation, and role in human disease. These include the recently solved three-dimensional structure of ENaC, ENaC function in various tissues, and mutations in ENaC that cause a hereditary form of hypertension (Liddle syndrome), salt-wasting hypotension (PHA1), or polymorphism in ENaC that contributes to other diseases (such as cystic fibrosis). Moreover, great strides have been made in deciphering the regulation of ENaC by hormones (e.g., the mineralocorticoid aldosterone, glucocorticoids, vasopressin), ions (e.g., Na⁺ ), proteins (e.g., the ubiquitin-protein ligase NEDD4-2, the kinases SGK1, AKT, AMPK, WNKs & mTORC2, and proteases), and posttranslational modifications [e.g., (de)ubiquitylation, glycosylation, phosphorylation, acetylation, palmitoylation]. Characterization of ENaC structure, function, regulation, and role in human disease, including using animal models, are described in this article, with a special emphasis on recent advances in the field. © 2021 American Physiological Society. Compr Physiol 11:1-29, 2021.

Increased colonic K+ excretion through inhibition of the H,K-ATPase type 2 helps reduce plasma K+ level in a murine model of nephronic reduction

Hyperkalemia is frequently observed in patients at the end-stage of chronic kidney disease (CKD), and has possible harmful consequences on cardiac function. Many strategies are currently used to manage hyperkalemia, one consisting of increasing fecal K⁺ excretion through the administration of cation-exchange resins. In this study, we explored another more specific method of increasing intestinal K⁺ secretion by inhibiting the H,K-ATPase type 2 (HKA2), which is the main colonic K⁺ reabsorptive pathway. We hypothetised that the absence of this pump could impede the increase of plasma K⁺ levels following nephronic reduction (N5/6) by favoring fecal K⁺ secretion. In N5/6 WT and HKA2KO mice under normal K⁺ intake, the plasma K⁺ level remained within the normal range, however, a load of K⁺ induced strong hyperkalemia in N5/6 WT mice (9.1 ± 0.5 mM), which was significantly less pronounced in N5/6 HKA2KO mice (7.9 ± 0.4 mM, p < 0.01). This was correlated to a higher capacity of HKA2KO mice to excrete K⁺ in their feces. The absence of HKA2 also increased fecal Na⁺ excretion by inhibiting its colonic ENaC-dependent absorption. We also showed that angiotensin-converting-enzyme inhibitor like enalapril, used to treat hypertension during CKD, induced a less severe hyperkalemia in N5/6 HKA2KO than in N5/6 WT mice. This study therefore provides the proof of concept that the targeted inhibition of HKA2 could be a specific therapeutic maneuver to reduce plasma K⁺ levels in CKD patients.

Campylobacter concisus Impairs Sodium Absorption in Colonic Epithelium via ENaC Dysfunction and Claudin-8 Disruption

The epithelial sodium channel (ENaC) can increase the colonic absorptive capacity for salt and water. Campylobacter concisus is a common pathogenic epsilonproteobacterium, causing enteritis and diarrhea. It can induce barrier dysfunction in the intestine, but its influence on intestinal transport function is still unknown. Therefore, our study aimed to characterize C. concisus effects on ENaC using the HT-29/B6-GR/MR (epithelial cell line HT-29/B6 transfected with glucocorticoid and mineralocorticoid receptors) cell model and mouse colon. In Ussing chambers, C. concisus infection inhibited ENaC-dependent Na⁺ transport as indicated by a reduction in amiloride-sensitive short circuit current (−55%, n = 15, p < 0.001). This occurred via down-regulation of β- and γ-ENaC mRNA expression and ENaC ubiquitination due to extracellular signal-regulated kinase (ERK)1/2 activation, predicted by Ingenuity Pathway Analysis (IPA). In parallel, C. concisus reduced the expression of the sealing tight junction (TJ) protein claudin-8 and induced claudin-8 redistribution off the TJ domain of the enterocytes, which facilitates the back leakage of Na⁺ ions into the intestinal lumen. In conclusion, C. concisus caused ENaC dysfunction via interleukin-32-regulated ERK1/2, as well as claudin-8-dependent barrier dysfunction—both of which contribute to Na⁺ malabsorption and diarrhea.

Intestinal Mineralocorticoid Receptor Contributes to Epithelial Sodium Channel-Mediated Intestinal Sodium Absorption and Blood Pressure Regulation

Background

Mineralocorticoid receptor (MR) has pathological roles in various cell types, including renal tubule cells, myocytes, and smooth muscle cells; however, the role of MR in intestinal epithelial cells (IECs) has not been sufficiently evaluated. The intestine is the sensing organ of ingested sodium; accordingly, intestinal MR is expected to have essential roles in blood pressure (BP) regulation.

Methods and Results

We generated IEC‐specific MR knockout (IEC‐MR‐KO) mice. With a standard diet, fecal sodium excretion was 1.5‐fold higher in IEC‐MR‐KO mice, with markedly decreased colonic expression of β‐ and γ‐epithelial sodium channel, than in control mice. Urinary sodium excretion in IEC‐MR‐KO mice decreased by 30%, maintaining sodium balance; however, a low‐salt diet caused significant reductions in body weight and BP in IEC‐MR‐KO mice, and plasma aldosterone exhibited a compensatory increase. With a high‐salt diet, intestinal sodium absorption markedly increased to similar levels in both genotypes, without an elevation in BP. Deoxycorticosterone/salt treatment elevated BP and increased intestinal sodium absorption in both genotypes. Notably, the increase in BP was significantly smaller in IEC‐MR‐KO mice than in control mice. The addition of the MR antagonist spironolactone to deoxycorticosterone/salt treatment eliminated the differences in BP and intestinal sodium absorption between genotypes.

Conclusions

Intestinal MR regulates intestinal sodium absorption in the colon and contributes to BP regulation. These regulatory effects are associated with variation in epithelial sodium channel expression. These findings suggest that intestinal MR is a new target for studying the molecular mechanism of hypertension and cardiovascular diseases.

Genomic and rapid effects of aldosterone: what we know and do not know thus far

Aldosterone is the most known mineralocorticoid hormone synthesized by the adrenal cortex. The genomic pathway displayed by aldosterone is attributed to the mineralocorticoid receptor (MR) signaling. Even though the rapid effects displayed by aldosterone are long known, our knowledge regarding the receptor responsible for such event is still poor. It is intense that the debate whether the MR or another receptor-the "unknown receptor"-is the receptor responsible for the rapid effects of aldosterone. Recently, G protein-coupled estrogen receptor-1 (GPER-1) was elegantly shown to mediate some aldosterone-induced rapid effects in several tissues, a fact that strongly places GPER-1 as the unknown receptor. It has also been suggested that angiotensin receptor type 1 (AT1) also participates in the aldosterone-induced rapid effects. Despite this open question, the relevance of the beneficial effects of aldosterone is clear in the kidneys, colon, and CNS as aldosterone controls the important water reabsorption process; on the other hand, detrimental effects displayed by aldosterone have been reported in the cardiovascular system and in the kidneys. In this line, the MR antagonists are well-known drugs that display beneficial effects in patients with heart failure and hypertension; it has been proposed that MR antagonists could also play an important role in vascular disease, obesity, obesity-related hypertension, and metabolic syndrome. Taken altogether, our goal here was to (1) bring a historical perspective of both genomic and rapid effects of aldosterone in several tissues, and the receptors and signaling pathways involved in such processes; and (2) critically address the controversial points within the literature as regarding which receptor participates in the rapid pathway display by aldosterone.

ENaC Dysregulation Through Activation of MEK1/2 Contributes to Impaired Na+ Absorption in Lymphocytic Colitis

BACKGROUND

Lymphocytic colitis (LC) causes watery diarrhea. We aimed to identify mechanisms of altered Na absorption and regulatory inputs in patients with LC by examining the epithelial Na channel (ENaC) function as the predominant Na transport system in human distal colon.

METHODS

Epithelial Na channel function and regulation was analyzed in biopsies from sigmoid colon of patients with LC and in rat distal colon in Ussing chambers. ENaC-subunit expression was measured by real-time PCR and RNA sequencing. Correction factors for subepithelial resistance contributions were determined by impedance spectroscopy. Upstream regulators in LC were determined by RNA sequencing.

RESULTS

Epithelial Na channel-mediated electrogenic Na transport was inhibited despite aldosterone stimulation in human sigmoid colon of patients with LC. The increase in γ-ENaC mRNA expression in response to aldosterone was MEK1/2-dependently reduced in LC, since it could be restored toward normal by MEK1/2 inhibition through U0126. Parallel experiments for identification of signaling in rat distal colon established MEK1/2 to be activated by a cytokine cocktail of TNFα, IFNγ, and IL-15, which were identified as the most important regulators in the upstream regulator analysis in LC.

CONCLUSIONS

In the sigmoid colon of patients with LC, the key effector cytokines TNFα, IFNγ, and IL-15 inhibited γ-ENaC upregulation in response to aldosterone through a MEK1/2-mediated pathway. This prevents ENaC to reach its maximum transport capacity and results in Na malabsorption which contributes to diarrhea.

Long-term use of lithium and risk of colorectal adenocarcinoma: a nationwide case-control study

Background:

Lithium accumulates in the colon and inhibits the enzyme GSK-3β that possesses anti-carcinogenic effects. We therefore examined the association between lithium use and colorectal cancer risk in a nationwide study.

Methods:

We used the Danish Cancer Registry to identify all patients diagnosed with incident colorectal adenocarcinoma during 2000–2012 (n=36 248). Using a matched case–control approach, we estimated the association between long-term use (⩾5 years) of lithium and risk of colorectal adenocarcinoma using conditional logistic regression.

Results:

Long-term use of lithium was similar among cases (0.22%) and controls (0.20%), yielding an odds ratio of 1.13 (95% confidence interval (CI), 0.89–1.43) for colorectal adenocarcinoma. Dose–response, subgroup and other subanalyses returned neutral associations. However, ORs differed for colorectal subsites (proximal colon: 1.01 (95% CI, 0.66–1.55; distal colon: 1.52 (95% CI, 1.05–2.20); and rectum: 0.80 (95% CI, 0.50–1.30).

Conclusions:

Lithium use was not associated with an overall increased risk of colorectal adenocarcinoma. The variation by subsite warrants further investigation.

Interleukin-13 affects the epithelial sodium channel in the intestine by coordinated modulation of STAT6 and p38 MAPK activity

KEY POINTS

Interleukin-13 (IL-13) causes intestinal epithelial barrier dysfunction, and is implicated in the pathogenesis of Th2-driven intestinal inflammation (e.g. ulcerative colitis). However, it is unclear whether the epithelial sodium channel (ENaC) - the main limiting factor for sodium absorption in the distal colon - is also influenced by IL-13 and if so, by what mechanism(s). We demonstrate in an intestinal cell model as well as in mouse distal colon that IL-13 causes reduced ENaC activity. We show that IL-13 impairs ENaC-dependent sodium transport by activating the JAK1/2-STAT6 signalling pathway. These results improve our understanding of the mechanisms through which IL-13 functions as a key effector cytokine in ulcerative colitis, thereby contributing to the distinct pathology of this disease.

ABSTRACT

Interleukin-13 (IL-13) has been strongly implicated in the pathogenesis of ulcerative colitis, possibly by disrupting epithelial integrity. In the distal colon, the epithelial sodium channel (ENaC) is an important factor in the regulation of sodium absorption, and therefore plays a critical role in minimizing intestinal sodium and water losses. In the present study, we investigated whether IL-13 also acts as a potent modulator of epithelial sodium transport via ENaC, and the signalling components involved. The effect of IL-13 on ENaC was examined in HT-29/B6-GR/MR human colon cells, as well as in mouse distal colon, by measuring amiloride-sensitive short-circuit current (ISC ) in Ussing chambers. The expression levels of ENaC subunits and the cellular components that contribute to ENaC activity were analysed by qRT-PCR and promoter gene assay. We show that IL-13, in both the cell model and in native intestinal tissue, impaired epithelial sodium absorption via ENaC (JNa ) as a result of decreased transcription levels of β- and γ-ENaC subunits and SGK1, a post-translational regulator of ENaC activity, due to impaired promoter activity. The reduction in JNa was prevented by inhibition of JAK1/2-STAT6 signalling. This inhibition also affected the IL-13-induced decrease in p38 MAPK phosphorylation. The contribution of STAT6 to IL-13-mediated ENaC inactivation was confirmed in a STAT6(-/-) mouse model. In conclusion, these results indicate that IL-13, the levels of which are elevated in ulcerative colitis, contributes to impaired ENaC activity via modulation of the STAT6/p38 MAPK pathways.

Epithelial sodium transport and its control by aldosterone: the story of our internal environment revisited

Transcription and translation require a high concentration of potassium across the entire tree of life. The conservation of a high intracellular potassium was an absolute requirement for the evolution of life on Earth. This was achieved by the interplay of P- and V-ATPases that can set up electrochemical gradients across the cell membrane, an energetically costly process requiring the synthesis of ATP by F-ATPases. In animals, the control of an extracellular compartment was achieved by the emergence of multicellular organisms able to produce tight epithelial barriers creating a stable extracellular milieu. Finally, the adaptation to a terrestrian environment was achieved by the evolution of distinct regulatory pathways allowing salt and water conservation. In this review we emphasize the critical and dual role of Na(+)-K(+)-ATPase in the control of the ionic composition of the extracellular fluid and the renin-angiotensin-aldosterone system (RAAS) in salt and water conservation in vertebrates. The action of aldosterone on transepithelial sodium transport by activation of the epithelial sodium channel (ENaC) at the apical membrane and that of Na(+)-K(+)-ATPase at the basolateral membrane may have evolved in lungfish before the emergence of tetrapods. Finally, we discuss the implication of RAAS in the origin of the present pandemia of hypertension and its associated cardiovascular diseases.

Rapid aldosterone actions on epithelial sodium channel trafficking and cell proliferation

Aldosterone regulates blood pressure through its effects on the kidney and the cardiovascular system. Dysregulation of aldosterone signalling can result in hypertension which in turn can lead to chronic pathologies of the kidney such as renal fibrosis and nephropathy. Aldosterone acts by binding to the mineralocorticoid receptor (MR), which acts as a ligand-dependent transcription factor in target tissues such as segments of the distal nephron including the connecting tubule and cortical collecting duct (CCD). Aldosterone also promotes the activation of protein kinase signalling cascades that are coupled to growth factor receptors and act directly on specific substrates in the cell membrane or cytoplasm. The rapid actions of aldosterone can also modulate gene expression through the phosphorylation of transcription factors. Aldosterone is a key regulator of Na(+) conservation in the distal nephron, largely through multiple mechanisms that modulate the activity of the epithelial Na(+) channel (ENaC). Aldosterone transcriptionally up-regulates the ENaCα subunit and also up regulates serum and glucocorticoid-regulated kinase-1 (SGK1) that indirectly regulates the ubiquitination of ENaC subunits. Aldosterone promotes the activation of protein kinase D1 (PKD1) which can modify the activity of ENaC and other transporters through effects on sub-cellular trafficking. In M1-CCD cells, early sub-cellular trafficking causes the redistribution of ENaC subunits within minutes of treatment with aldosterone. ENaC subunits can also interact directly with phosphatidylinositide signalling intermediates in the membrane and the mechanism by which PKD isoforms regulate protein trafficking is through the control of vesicle fission from the trans Golgi network by activation of phosphatidylinositol 4-kinaseIIIβ (PI4KIIIβ).

Bile acids increase the activity of the epithelial Na+ channel

The epithelial Na(+) channel (ENaC) is a key regulator of Na(+) absorption in various epithelia including the distal nephron and the distal colon. ENaC is a constitutively active channel, but its activity is modulated by a number of mechanisms. These include proteolytic activation, ubiquitination and cell surface expression, phosphorylation, intracellular Na(+) concentration, and shear stress. ENaC is related to the bile acid-sensitive ion channel (BASIC), a channel that is expressed in the epithelial cells of bile ducts. BASIC is activated by millimolar concentrations of extracellular bile acids. Bile acids are synthesized by the liver and secreted into the duodenum to aid lipolysis. A large fraction of the secreted bile acids is absorbed by the ileum and recirculated into the liver, but a small fraction passes the colon and is excreted. Bile acids can influence the ion transport processes in the intestinal tract including the colon. In this study, we show that various bile acids present in rat bile potently and reversibly increase the activity of rat ENaC expressed in Xenopus oocytes, suggesting that bile acids are natural modulators of ENaC activity.

Direct control of Na(+)-K(+)-2Cl(-)-cotransport protein (NKCC1) expression with aldosterone

Sodium/potassium/chloride cotransporter (NKCC1) proteins play important roles in Na(+) and K(+) concentrations in key physiological systems, including cardiac, vascular, renal, nervous, and sensory systems. NKCC1 levels and functionality are altered in certain disease states, and tend to decline with age. A sensitive, effective way of regulating NKCC1 protein expression has significant biotherapeutic possibilities. The purpose of the present investigation was to determine if the naturally occurring hormone aldosterone (ALD) could regulate NKCC1 protein expression. Application of ALD to a human cell line (HT-29) revealed that ALD can regulate NKCC1 protein expression, quite sensitively and rapidly, independent of mRNA expression changes. Utilization of a specific inhibitor of mineralocorticoid receptors, eplerenone, implicated these receptors as part of the ALD mechanism of action. Further experiments with cycloheximide (protein synthesis inhibitor) and MG132 (proteasome inhibitor) revealed that ALD can upregulate NKCC1 by increasing protein stability, i.e., reducing ubiquitination of NKCC1. Having a procedure for controlling NKCC1 protein expression opens the doors for therapeutic interventions for diseases involving the mis-regulation or depletion of NKCC1 proteins, for example during aging.

Ion transport and barrier function are disturbed in microscopic colitis

In this paper, we identify mechanisms of watery diarrhea in microscopic colitis (MC). Biopsies from the sigmoid colon of patients with collagenous colitis and treated lymphocytic colitis were analyzed in miniaturized Ussing chambers for electrogenic sodium transport and barrier function with one-path impedance spectroscopy. Cytometric bead arrays (CBA) served to analyze cytokine profiles. In active MC, electrogenic sodium transport was diminished and epithelial resistance decreased. CBA revealed a Th1 cytokine profile featuring increased IFN-γ, TNF-α, and IL-1β levels. After four weeks of steroid treatment with budesonide, electrogenic sodium transport recovered while epithelial barrier defects remained. Diarrhea in MC results at least in part from a combination of impaired electrogenic sodium transport and barrier defects. From a therapeutic perspective it can be postulated that the functional importance of loss of ions may be higher than that caused by barrier impairment.

High salt intake down-regulates colonic mineralocorticoid receptors, epithelial sodium channels and 11β-hydroxysteroid dehydrogenase type 2

Besides the kidneys, the gastrointestinal tract is the principal organ responsible for sodium homeostasis. For sodium transport across the cell membranes the epithelial sodium channel (ENaC) is of pivotal relevance. The ENaC is mainly regulated by mineralocorticoid receptor mediated actions. The MR activation by endogenous 11β-hydroxy-glucocorticoids is modulated by the 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2). Here we present evidence for intestinal segment specific 11β-HSD2 expression and hypothesize that a high salt intake and/or uninephrectomy (UNX) affects colonic 11β-HSD2, MR and ENaC expression. The 11β-HSD2 activity was measured by means of 3H-corticosterone conversion into 3H-11-dehydrocorticosterone in Sprague Dawley rats on a normal and high salt diet. The activity increased steadily from the ileum to the distal colon by a factor of about 3, an observation in line with the relevance of the distal colon for sodium handling. High salt intake diminished mRNA and protein of 11β-HSD2 by about 50% (p<0.001) and reduced the expression of the MR (p<0.01). The functionally relevant ENaC-β and ENaC-γ expression, a measure of mineralocorticoid action, diminished by more than 50% by high salt intake (p<0.001). The observed changes were present in rats with and without UNX. Thus, colonic epithelial cells appear to contribute to the protective armamentarium of the mammalian body against salt overload, a mechanism not modulated by UNX.

Aldosterone induces active K⁺ secretion by enhancing mucosal expression of Kcnn4c and Kcnma1 channels in rat distal colon

Although both Kcnn4c and Kcnma1 channels are present on colonic mucosal membranes, only Kcnma1 has been suggested to mediate K(+) secretion in the colon. Therefore, studies were initiated to investigate the relative roles of Kcnn4c and Kcnma1 in mediating aldosterone (Na-free diet)-induced K(+) secretion. Mucosal to serosal (m-s), serosal to mucosal (s-m), and net (86)Rb(+) (K(+) surrogate) fluxes as well as short circuit currents (I(sc); measure of net ion movement) were measured under voltage clamp condition in rat distal colon. Active K(+) absorption, but not K(+) secretion, is present in normal, while aldosterone induces active K(+) secretion (1.04 ± 0.26 vs. -1.21 ± 0.15 μeq·h(-1)·cm(-2); P < 0.001) in rat distal colon. Mucosal VO(4) (a P-type ATPase inhibitor) inhibited the net K(+) absorption in normal, while it significantly enhanced net K(+) secretion in aldosterone animals. The aldosterone-induced K(+) secretion was inhibited by the mucosal addition of 1) either Ba(2+) (a nonspecific K(+) channel blocker) or charybdotoxin (CTX; a common Kcnn4 and Kcnma1 channel blocker) by 89%; 2) tetraethyl ammonium (TEA) or iberiotoxin (IbTX; a Kcnma1 channel blocker) by 64%; and 3) TRAM-34 (a Kcnn4 channel blocker) by 29%. TRAM-34, but not TEA, in the presence of IbTX further significantly inhibited the aldosterone-induced K(+) secretion. Thus the aldosterone-induced Ba(2+)/CTX-sensitive K(+) secretion consists of IbTX/TEA-sensitive (Kcnma1) and IbTX/TEA-insensitive fractions. TRAM-34 inhibition of the IbTX-insensitive fraction is consistent with the aldosterone-induced K(+) secretion being mediated partially via Kcnn4c. Western and quantitative PCR analyses indicated that aldosterone enhanced both Kcnn4c and Kcnma1α protein expression and mRNA abundance. In vitro exposure of isolated normal colonic mucosa to aldosterone also enhanced Kcnn4c and Kcnma1α mRNA levels, and this was prevented by exposure to actinomycin D (an RNA synthesis inhibitor). These observations indicate that aldosterone induces active K(+) secretion by enhancing mucosal Kcnn4c and Kcnma1 expression at the transcriptional level.

Severe defects in absorptive ion transport in distal colons of mice that lack ClC-2 channels

BACKGROUND & AIMS

The fluid secretion model predicts that intestinal obstruction disorders can be alleviated by promoting epithelial Cl(-) secretion. The adenosine 3',5'-cyclic monophosphate (cAMP)-activated anion channel CFTR mediates Cl(-)-dependent fluid secretion in the intestine. Although the role of the ClC-2 channel has not been determined in the intestine, this voltage-gated Cl(-) channel might compensate for the secretory defects observed in patients with cystic fibrosis and other chronic constipation disorders. We investigated whether mice that lack ClC-2 channels (Clcn2(-/-)) have defects in intestinal ion transport.

METHODS

Immunolocalization and immunoblot analyses were used to determine the cellular localization and the amount of ClC-2 expressed in mouse early distal colon (EDC) and late distal colon (LDC). Colon sheets from wild-type and Clcn2(-/-) littermates were mounted in Ussing chambers to determine transepithelial bioelectrical parameters and Na(+), K(+), and Cl(-) fluxes.

RESULTS

Expression of ClC-2 was higher in the basolateral membrane of surface cells in the EDC compared with the LDC, with little expression in crypts. Neither cAMP nor Ca(2+)-induced secretion of Cl(-) was affected in the EDC or LDC of Clcn2(-/-) mice, whereas the amiloride-sensitive short-circuit current was increased approximately 3-fold in Clcn2(-/-) EDC compared with control littermates. Conversely, electroneutral Na(+), K(+), and Cl(-) absorption was dramatically reduced in colons of Clcn2(-/-) mice.

CONCLUSIONS

Basolateral ClC-2 channels are required for colonic electroneutral absorption of NaCl and KCl. The increase in the amiloride-sensitive short-circuit current in Clcn2(-/-) mice revealed a compensatory mechanism that is activated in the colons of mice that lack the ClC-2 channel.

ENaCs and ASICs as therapeutic targets

The epithelial Na(+) channel (ENaC) and acid-sensitive ion channel (ASIC) branches of the ENaC/degenerin superfamily of cation channels have drawn increasing attention as potential therapeutic targets in a variety of diseases and conditions. Originally thought to be solely expressed in fluid absorptive epithelia and in neurons, it has become apparent that members of this family exhibit nearly ubiquitous expression. Therapeutic opportunities range from hypertension, due to the role of ENaC in maintaining whole body salt and water homeostasis, to anxiety disorders and pain associated with ASIC activity. As a physiologist intrigued by the fundamental mechanics of salt and water transport, it was natural that Dale Benos, to whom this series of reviews is dedicated, should have been at the forefront of research into the amiloride-sensitive sodium channel. The cloning of ENaC and subsequently the ASIC channels has revealed a far wider role for this channel family than was previously imagined. In this review, we will discuss the known and potential roles of ENaC and ASIC subunits in the wide variety of pathologies in which these channels have been implicated. Some of these, such as the role of ENaC in Liddle's syndrome are well established, others less so; however, all are related in that the fundamental defect is due to inappropriate channel activity.

The use of plasma aldosterone and urinary sodium to potassium ratio as translatable quantitative biomarkers of mineralocorticoid receptor antagonism

BACKGROUND

Accumulating evidence supports the role of the mineralocorticoid receptor (MR) in the pathogenesis of diabetic nephropathy. These findings have generated renewed interest in novel MR antagonists with improved selectivity against other nuclear hormone receptors and a potentially reduced risk of hyperkalemia. Characterization of novel MR antagonists warrants establishing translatable biomarkers of activity at the MR receptor. We assessed the translatability of urinary sodium to potassium ratio (Na+/K+) and plasma aldosterone as biomarkers of MR antagonism using eplerenone (Inspra®), a commercially available MR antagonist. Further we utilized these biomarkers to demonstrate antagonism of MR by PF-03882845, a novel compound.

METHODS

The effect of eplerenone and PF-03882845 on urinary Na+/K+ and plasma aldosterone were characterized in Sprague-Dawley rats and spontaneously hypertensive rats (SHR). Additionally, the effect of eplerenone on these biomarkers was determined in healthy volunteers. Drug exposure-response data were modeled to evaluate the translatability of these biomarkers from rats to humans.

RESULTS

In Sprague-Dawley rats, eplerenone elicited a rapid effect on urinary Na+/K+ yielding an EC50 that was within 5-fold of the functional in vitro IC50. More importantly, the effect of eplerenone on urinary Na+/K+ in healthy volunteers yielded an EC50 that was within 2-fold of the EC50 generated in Sprague-Dawley rats. Similarly, the potency of PF-03882845 in elevating urinary Na+/K+ in Sprague-Dawley rats was within 3-fold of its in vitro functional potency. The effect of MR antagonism on urinary Na+/K+ was not sustained chronically; thus we studied the effect of the compounds on plasma aldosterone following chronic dosing in SHR. Modeling of drug exposure-response data for both eplerenone and PF-03882845 yielded EC50 values that were within 2-fold of that estimated from modeling of drug exposure with changes in urinary sodium and potassium excretion. Importantly, similar unbound concentrations of eplerenone in humans and SHR rats yielded the same magnitude of elevations in aldosterone, indicating a good translatability from rat to human.

CONCLUSIONS

Urinary Na+/K+ and plasma aldosterone appear to be translatable biomarkers of MR antagonism following administration of single or multiple doses of compound, respectively.

TRIAL REGISTRATION

For clinical study reference EE3-96-02-004, this study was completed in 1996 and falls out scope for disclosure requirements. Clinical study reference A6141115: http://clinicaltrials.gov, http://NIHclinicaltrails.gov; NCTID: NCT00990223.

Glucocorticoid receptor is indispensable for physiological responses to aldosterone in epithelial Na+ channel induction via the mineralocorticoid receptor in a human colonic cell line

The epithelial Na+ channel (ENaC) plays a crucial role in electrogenic Na(+) absorption in the distal colon. ENaC induction via the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR) is differentially regulated by modulatory components. As most existing epithelial cell lines including colonic epithelial cell lines miss the co-expression of functional GR and MR, signaling on ENaC is only poorly characterized regarding the interplay of glucocorticoids and mineralocorticoids. In the present study, we show that GR expression and activity are indispensable for MR-dependent induction of ENaC-mediated Na(+) transport. Cooperativity of the two receptors has been studied in the highly differentiated, epithelial colonic cell line HT-29/B6-GR/MR which is equipped with the complete receptor repertoire of both GR and MR due to stable transfection. In contrast to HT-29/B6 cells solely expressing the MR, this cell line displays a physiological response to aldosterone regarding ENaC induction. To achieve this, a pre-incubation step with the GR agonist dexamethasone was required to allow for the subsequent stimulation of ENaC by aldosterone. As a result of cooperative effects between the activated GR and the MR, MR protein levels were elevated and MR-dependent transcription of ENaC subunits β and γ was increased. As an additional mechanism involved, transcription of SGK-1 (serum- and glucocorticoid-induced kinase 1) and GILZ (glucocorticoid-induced leucin zipper)--both essential for the insertion of ENaC into the apical enterocyte membrane--were also augmented by the activated MR.

Functional and molecular expression of epithelial sodium channels in cultured human endolymphatic sac epithelial cells

HYPOTHESIS

Epithelial sodium channels are expressed in cultured human endolymphatic sac (ES) epithelial (HESE) cells and epithelial sodium channel (ENaC) expression is suppressed by interleukin 1beta.

BACKGROUND

The ES is part of the membranous labyrinth in the inner ear that plays an important role in maintaining homeostasis of the endolymphatic fluid system. However, the exact mechanism of fluid volume regulation is not yet known.

METHODS

The ES specimens were harvested during acoustic neuroma surgery (n = 13) using the translabyrinthine approach and were subcultured with high-epidermal growth factor (25 ng/mL) media.

RESULTS

The serially passaged HESE cells differentiated into a monolayer of confluent cells and some of the cultured cells had features of mitochondria-rich cells. Reverse transcription-polymerase chain reaction revealed that ENaC subunits are expressed in the cultured HESE cells. We also confirmed the presence of an ENaC-dependent short-circuit current in the cultured HESE cells. Interestingly, ENaC mRNA expression and ENaC-dependent current decreased after treatment with interleukin 1beta (10 nmol/L for 24 h).

CONCLUSION

These findings suggest that ENaC plays an important role in fluid absorption in the human ES and that its function may be altered during inflammatory conditions.

Glucocorticoids and tumor necrosis factor-alpha synergize to induce absorption by the epithelial sodium channel in the colon

BACKGROUND & AIMS

The epithelial sodium channel (ENaC) mediates electrogenic sodium absorption in distal colon. In patients with inflammatory bowel disease (IBD), ENaC induction is impaired, mainly through transcriptional suppression by proinflammatory cytokines such as tumor necrosis factor (TNF)-alpha. Glucocorticoid therapy rapidly increases sodium absorption; we investigated the molecular mechanisms underlying the interaction among TNF-alpha, glucocorticoids, and ENaC induction.

METHODS

ENaC-mediated sodium transport in glucocorticoid receptor (GR)-expressing HT-29/B6 cells and rat distal colon, under the influence of the synthetic glucocorticoid dexamethasone and TNF-alpha, was quantified in Ussing chambers. ENaC messenger RNA (mRNA) levels were monitored by real-time polymerase chain reaction. GR transactivation and expression were investigated by gene reporter, immunoblot, and confocal immunofluorescence microscopy analyses. The GR mRNA half-life was determined. Signaling pathways were characterized using mitogen-activated protein kinase inhibitors.

RESULTS

Dexamethasone not only prevented TNF-alpha-mediated ENaC suppression but caused synergistic induction of ENaC-dependent sodium absorption in HT-29/B6-GR cells and rat distal colon. This synergy resulted from TNF-alpha-mediated increases in GR protein levels because of GR mRNA stabilization and subsequent GR transactivation by dexamethasone. As a consequence, transcription of the ENaC beta- and gamma-subunits was up-regulated, increasing ENaC-dependent sodium absorption. p38 Mitogen-activated protein kinase is required for this synergistic effect: p38 inhibition blocked the increase in GR protein expression and ENaC-dependent sodium absorption.

CONCLUSIONS

TNF-alpha and dexamethasone induce ENaC, explaining the rapid and intense proabsorptive effect of glucocorticoid therapies.

Acid-base disturbances in gastrointestinal disease

Disruption of normal gastrointestinal function as a result of infection, hereditary or acquired diseases, or complications of surgical procedures uncovers its important role in acid-base homeostasis. Metabolic acidosis or alkalosis may occur, depending on the nature and volume of the unregulated losses that occur. Investigation into the specific pathophysiology of gastrointestinal disorders has provided important new insights into the normal physiology of ion transport along the gut and has also provided new avenues for treatment. This review provides a brief overview of normal ion transport along the gut and then discusses the pathophysiology and treatment of the metabolic acid-base disorders that occur when normal gut function is disrupted.

Aldosterone increases KCa1.1 (BK) channel-mediated colonic K+ secretion

Mammalian K(+) homeostasis results from highly regulated renal and intestinal absorption and secretion, which balances the unregulated K(+) intake. Aldosterone is known to enhance both renal and colonic K(+) secretion. In mouse distal colon K(+) secretion occurs exclusively via luminal K(Ca)1.1 (BK) channels. Here we investigate if aldosterone stimulates colonic K(+) secretion via BK channels. Luminal Ba(2+) and iberiotoxin (IBTX)-sensitive electrogenic K(+) secretion was measured in Ussing chambers. In vivo aldosterone was augmented via a high K(+) diet. High K(+) diet led to a 2-fold increase of luminal Ba(2+) and IBTX-sensitive short-circuit current in distal mouse colonic mucosa. This effect was absent in BK alpha-subunit-deficient (BK(-/-)) mice. The resting and diet-induced K(+) secretion was stimulated by luminal ionomycin. In BK(-/-) mice luminal ionomycin did not stimulate K(+) secretion. In vitro addition of aldosterone likewise triggered a 2-fold increase in K(+) secretion, which was inhibited by the mineralocorticoid receptor antagonist spironolactone and the BK channel blocker IBTX. Semi-quantification of mRNA from colonic crypts showed up-regulation of BK alpha- and beta(2)-subunits in high K(+) diet mice. The BK channel could be detected luminally in colonic crypt cells by immunohistochemistry. The expression level of the channel in the luminal membrane was strongly up-regulated in K(+)-loaded animals. Taken together, these data strongly suggest that aldosterone-induced K(+) secretion occurs via increased expression of luminal BK channels.

Altered ENaC expression leads to impaired sodium absorption in the noninflamed intestine in Crohn's disease

BACKGROUND & AIMS

Crohn's disease (CD) is a chronic inflammatory bowel disease. In this study, we have investigated sodium absorption via epithelial sodium channels (ENaC) in the macroscopically noninflamed colon in active CD.

METHODS

Sodium transport via ENaC was investigated in Ussing chambers using biopsy specimens of sigmoid colon from controls and active CD limited to the small intestine. ENaC messenger RNA expression and subcellular localization were studied by real-time polymerase chain reaction and confocal microscopy. Effects of proinflammatory cytokines on ENaC and signaling via mitogen-activated protein kinases were investigated in rat distal colon. Therapeutic inhibition of mitogen-activated protein kinases was studied in CD biopsy specimens.

RESULTS

Electrogenic sodium absorption via ENaC was strongly impaired in the macroscopically noninflamed CD colon because of reduced gamma-ENaC transcription, whereas subcellular localization of ENaC was not changed. In contrast to impaired epithelial sodium transport, epithelial barrier function was not altered in noninflamed CD colon, indicating that paracellular leak flux of ions did not contribute to decreased sodium absorption. Exposure of rat distal colon to tumor necrosis factor alpha led to reduced electrogenic sodium absorption because of impaired transcriptional gamma-ENaC induction, which resembled the changes found in CD. Tumor necrosis factor alpha effects were dependent on extracellular signal-regulated kinase 1/2 but not p38 or c-Jun-N-terminal kinase because inhibition of mitogen-activated protein kinase/extracellular regulated kinase (MEK)1/2 but not inhibition of p38 or c-Jun-N-terminal kinase prevented suppression of ENaC. Finally, therapeutic inhibition of MEK1/2 restored electrogenic sodium absorption in CD.

CONCLUSIONS

In CD, macroscopically noninflamed colon contributes to diarrhea via impaired ENaC-mediated sodium absorption. Inhibition of extracellular signal-regulated kinase might serve as a potential therapeutic strategy for CD diarrhea.

The mineralocorticoid receptor: insights into its molecular and (patho)physiological biology

The last decade has witnessed tremendous progress in the understanding of the mineralocorticoid receptor (MR), its molecular mechanism of action, and its implications for physiology and pathophysiology. After the initial cloning of MR, and identification of its gene structure and promoters, it now appears as a major actor in protein-protein interaction networks. The role of transcriptional coregulators and the determinants of mineralocorticoid selectivity have been elucidated. Targeted oncogenesis and transgenic mouse models have identified unexpected sites of MR expression and novel roles for MR in non-epithelial tissues. These experimental approaches have contributed to the generation of new cell lines for the characterization of aldosterone signaling pathways, and have also facilitated a better understanding of MR physiology in the heart, vasculature, brain and adipose tissues. This review describes the structure, molecular mechanism of action and transcriptional regulation mediated by MR, emphasizing the most recent developments at the cellular and molecular level. Finally, through insights obtained from mouse models and human disease, its role in physiology and pathophysiology will be reviewed. Future investigations of MR biology should lead to new therapeutic strategies, modulating cell-specific actions in the management of cardiovascular disease, neuroprotection, mineralocorticoid resistance, and metabolic disorders.

Aldosterone responsiveness of the epithelial sodium channel (ENaC) in colon is increased in a mouse model for Liddle's syndrome

Liddle's syndrome is an autosomal dominant form of human hypertension, caused by gain-of-function mutations of the epithelial sodium channel (ENaC) which is expressed in aldosterone target tissues including the distal colon. We used a mouse model for Liddle's syndrome to investigate ENaC-mediated Na+ transport in late distal colon by measuring the amiloride-sensitive transepithelial short circuit current (Delta I SC-Ami) ex vivo. In Liddle mice maintained on a standard salt diet, Delta I SC-Ami was only slightly increased but plasma aldosterone (P Aldo) was severely suppressed. Liddle mice responded to a low or a high salt diet by increasing or decreasing, respectively, their P Aldo and Delta I SC-Ami. However, less aldosterone was required in Liddle animals to achieve similar or even higher Na+ transport rates than wild-type animals. Indeed, the ability of aldosterone to stimulate Delta I SC-Ami was about threefold higher in Liddle animals than in the wild-type controls. Application of aldosterone to colon tissue in vitro confirmed that ENaC stimulation by aldosterone was not only preserved but enhanced in Liddle mice. Aldosterone-induced transcriptional up-regulation of the channel's beta- and gamma-subunit (beta ENaC and gamma ENaC) and of the serum- and glucocorticoid-inducible kinase 1 (SGK1) was similar in colon tissue from Liddle and wild-type animals, while aldosterone had no transcriptional effect on the alpha-subunit (alpha ENaC). Moreover, Na+ feedback regulation was largely preserved in colon tissue of Liddle animals. In conclusion, we have demonstrated that in the colon of Liddle mice, ENaC-mediated Na+ transport is enhanced with an increased responsiveness to aldosterone. This may be pathophysiologically relevant in patients with Liddle's syndrome, in particular on a high salt diet, when suppression of P Aldo is likely to be insufficient to reduce Na+ absorption to an appropriate level.

Discordant effects of corticosteroids and expression of subunits on ENaC activity

In renal distal nephron and airway epithelial cells, adrenocortical steroids increase epithelial Na+ channel (ENaC) activity and also markedly increase the expression of the α-subunit. The present experiments were designed to reconstitute this steroid effect in ENaC-expressing cells by overexpressing the subunits whose expression is enhanced by corticosteroids. In renal collecting duct monolayers, corticosteroids increased ENaC activity 5- to 8-fold, endogenous α-ENaC mRNA and protein ∼10-fold, and β-ENaC protein and mRNA 1.2- to 2-fold. γ-ENaC expression was unchanged. To determine whether this increase in expression was sufficient to increase ENaC activity, we used a regulated adenovirus system to increase expression of each subunit alone and in combination. Unexpectedly, increased expression of the α- and/or β-subunit had no effect on ENaC activity in collecting duct cells or lung epithelial cells. In contrast, a small increase in γ-ENaC expression increased ENaC activity about threefold. This increase in activity was additive to the effect of steroids. Thus, even though corticosteroids strongly increase α-ENaC expression and moderately increase β-ENaC expression, these effects are not, by themselves, sufficient to increase ENaC activity. Knockdown experiments are consistent with the idea that the increased expression of α-ENaC is necessary for the full steroid effect on ENaC. Increased expression of γ-ENaC and corticosteroid treatment enhances ENaC activity by parallel, noninteracting pathways. These results underscore the importance of other actions of steroid hormones for long-term enhancement of ENaC activity and raise new possibilities for regulation of ENaC activity by γ-ENaC expression.

Butyrate induces intestinal sodium absorption via Sp3-mediated transcriptional up-regulation of epithelial sodium channels

BACKGROUND & AIMS

The epithelial sodium channel (ENaC) is the rate-limiting factor for colonic electrogenic sodium absorption. This study aimed to investigate ENaC regulation by butyrate, a short-chain fatty acid (SCFA) produced by intestinal bacteria.

METHODS

ENaC was examined in HT-29/B6 cells and glucocorticoid receptor(GR)-transfected HT-29/B6 cells (HT-29/B6-GR) by reverse-transcription polymerase chain reaction, real-time polymerase chain reaction, and confocal microscopy. ENaC promoters were investigated by deletion/mutation analysis, electrophoretic mobility shift assays, and quantitative chromatin immunoprecipitation. Sodium transport of HT-29/B6-GR cells and rat distal colon was quantified in Ussing chambers.

RESULTS

Butyrate up-regulated beta- and gamma-ENaC mRNA expression in HT-29/B6 cells and induced transcription from beta- and gamma-ENaC promoter constructs. The gamma-ENaC promoter could also be induced by the SCFA propionate but not by acetate. Deletion/mutation assays revealed that activation of the gamma-ENaC promoter depended on 2 GC boxes, which were shown to bind Sp1 and Sp3 in vitro. Although both transcription factors increased butyrate-mediated gamma-ENaC transcription upon overexpression, chromatin immunoprecipitation revealed that only Sp3 binds to the gamma-ENaC promoter in vivo and that Sp3 binding is enhanced by butyrate. Transcriptional ENaC induction by butyrate led to synthesis of gamma-ENaC subunits, but correct targeting of ENaC channels to the apical cell membrane was dependent on corticosteroid hormones. Finally, butyrate substantially increased electrogenic sodium absorption via ENaC in the presence of corticosteroid hormones in HT-29/B6-GR cells and in rat distal colon.

CONCLUSIONS

Concerted action of SCFA and corticosteroid hormones is required for induction of ENaC and maintenance of intestinal electrogenic sodium absorption.

Restoration of ENaC expression by glucocorticoid receptor transfection in human HT-29/B6 colon cells

The epithelial sodium channel (ENaC) controls colonic sodium absorption. So far, investigation of ENaC was limited by an unexplained lack of steroid-dependent ENaC expression in cultured intestinal cells, which we aimed to resolve. HT-29/B6 cells constitutively expressed the alpha-ENaC subunit, while beta- and gamma-ENaC subunits could not be detected due to deficient basal as well as corticosteroid-induced transcription. This was due to a lack of expression of both activating and inhibiting isoforms of glucocorticoid receptor (GR-alpha, -beta) and mineralocorticoid receptor. Stable transfection of GR-alpha restored intestine-specific glucocorticoid upregulation of beta- and gamma-ENaC in HT-29/B6 cells, which was followed by intact targeting of ENaC channels to the apical cell membrane and dose-dependent induction of electrogenic sodium absorption. In conclusion, ENaC deficiency is due to a lack of steroid receptors and not the consequence of a crypt-like phenotype of cultured intestinal cells. By stable GR transfection we obtained a model, in which ENaC regulation can be studied.

Rab4 GTP/GDP modulates amiloride-sensitive sodium channel (ENaC) function in colonic epithelia

The sodium-selective amiloride-sensitive epithelial sodium channel (ENaC) mediates electrogenic sodium re-absorption in tight epithelia. ENaC expression at the plasma membrane requires regulated transport, processing, and macromolecular assembly of subunit proteins in a defined and highly compartmentalized manner. Ras-related Rab GTPases monitor these processes in a highly regulated sequence of events. In order to evaluate the role of Rab proteins in ENaC function, Rab4 wild-type (WT), the GTPase-deficient mutant Rab4Q67L, and the dominant negative GDP-locked mutant Rab4S22N were over-expressed in the colon cancer cell line, HT-29 and amiloride-sensitive currents were recorded. Rab4 over-expression inhibited amiloride-sensitive currents. The effect was reversed by introducing Rab4-neutralizing antibody and Rab4 specific SiRNA. The GDP-locked Rab4 mutant inhibited, while GTPase-deficient mutant moderately stimulated amiloride-sensitive currents. Active status of Rab4 was confirmed by GTP overlay assay, while its expression was verified by Western blotting. Immunoprecipitation and pull-down assay suggest protein-protein interaction between Rab4 and ENaC. In addition, the functional modulation coincides with concomitant changes in ENaC expression at the cell surface and in intracellular pool. We propose that Rab4 is a critical element that regulates ENaC function by mechanisms that include GTP-GDP status, recycling, and expression level. Our observations imply that channel expression in apical membranes of epithelial cell system incorporates RabGTPase as an essential determinant of channel function and adds an exciting paradigm to ENaC therapeutics.

Immunohistochemical localization of the epithelial sodium channel in the rat inner ear

Endolymph in membranous labyrinth is a K+-rich and Na+-poor fluid, and perilymph is conversely Na+-rich and K+-poor. Electrolyte transport between endolymph and perilymph is important for regulation of volume and osmotic pressure of the labyrinth. The epithelial sodium channel (ENaC) is a good candidate protein for Na+ transport in the tight epithelia, which has been well demonstrated in other tissues such as kidney, colon and lung. The purpose of the present study was to investigate the cellular localization of ENaC subunits in the rat inner ear immunohistochemically with the specific polyclonal rabbit antibodies against the rat alpha-, beta- and gamma-ENaC. All three subunits of ENaC were extensively labeled in the cochlea including the stria vascularis, spiral ligament, organ of Corti, spiral limbus, Reissner's membrane and spiral ganglion, and in the vestibule including the sensory epithelia and stroma cells of the macula utriculi, macula sacculi and ampullary crest. In conclusion, our results suggest that functional ENaC in the labyrinth may work in concert with other Na+ and K+ transport molecules to regulate endolymph and to maintain homeostasis in the inner ear.

IL-1beta and TNFalpha regulate sodium absorption in rat distal colon

The epithelial Na+ channel (ENaC) provides the main absorptive pathway of the distal large intestine. This study aimed to characterize regulatory influences of cytokines in rat late distal colon. After 6 h incubation with either IL1beta, TNFalpha, IFNgamma, or combinations of TNFalpha and IFNgamma, ENaC was measured as electrogenic Na+ transport after 8 h induction by 3 nM aldosterone (JNa) in totally stripped specimens in the Ussing chamber. Subsequently, alpha-, beta-, and gamma-ENaC subunit mRNAs were analyzed by Northern blotting. The gamma-ENaC promoter was cloned and characterized by reporter gene assays. IL-1beta and TNFalpha, but not interferon-gamma, decreased JNa. In parallel, beta- and gamma-ENaC transcription was inhibited, whereas alpha-ENaC was unaffected. gamma-ENaC promoter activity was inhibited by IL-1beta and TNFalpha but not by IFNgamma. We conclude that the pro-inflammatory cytokines IL-1beta and TNFalpha inhibit electrogenic sodium absorption in rat distal colon by mRNA expression regulation of the beta- and gamma-ENaC subunits.

The epithelial sodium channel: from molecule to disease

Genetic analysis has demonstrated that Na absorption in the aldosterone-sensitive distal nephron (ASDN) critically determines extracellular blood volume and blood pressure variations. The epithelial sodium channel (ENaC) represents the main transport pathway for Na+ absorption in the ASDN, in particular in the connecting tubule (CNT), which shows the highest capacity for ENaC-mediated Na+ absorption. Gain-of-function mutations of ENaC causing hypertension target an intracellular proline-rich sequence involved in the control of ENaC activity at the cell surface. In animal models, these ENaC mutations exacerbate Na+ transport in response to aldosterone, an effect that likely plays an important role in the development of volume expansion and hypertension. Recent studies of the functional consequences of mutations in genes controlling Na+ absorption in the ASDN provide a new understanding of the molecular and cellular mechanisms underlying the pathogenesis of salt-sensitive hypertension.

Expression of ENaC and serum- and glucocorticoid-induced kinase 1 in the rat intestinal epithelium

Increase in epithelium sodium channel (ENaC) activity induced by aldosterone in the distal tubule of the kidney has been attributed to serum- and glucocorticoid-induced kinase 1 (sgk1). The distal colon constitutes another classical aldosterone-responsive epithelium that expresses both ENaC and sgk1 in an aldosterone-dependent manner. However, the site of expression and the temporal relationship of the aldosterone induction of these two proteins have not been investigated. Here, we examined the distribution and abundance of sgk1 in the rat intestine under basal conditions and after changes in the concentration of aldosterone and glucocorticoids. Results indicate that sgk1 is expressed in the distal colon and also in the ileum and jejunum. Abundance of sgk1 was high in control animals, and it did not change significantly after sodium depletion or after a single dose of aldosterone; however, it decreased after adrenalectomy. In contrast, the three subunits of ENaC were markedly induced in the distal colon by acute and chronic increases in aldosterone levels. Results indicate differential regulation of sgk and ENaC subunits by aldosterone in the distal colon. Distribution of sgk1 in the intestine beyond the aldosterone-responsive segments suggests that sgk1 may additionally regulate other sodium transporters in the intestinal epithelium.

Regulation of epithelial ion transport by aldosterone through changes in gene expression

The year 2003 marks the 50th year since the unfolding of the chemical structures of both aldosterone and DNA. Since the recognition in the early 1960's that aldosterone and its cousin cortisol act through DNA binding proteins that alter gene transcription, research on these corticosteroid hormones and their receptors has attracted fervent attention, both for their importance in endocrine physiology, and as model systems for understanding gene regulation. Recently, aldosterone has emerged as arguably the single most important physiological regulator of extracellular fluid volume and blood pressure in mammals, and has been implicated in a variety of disease states in humans. Moreover, its principal receptor, the mineralocorticoid receptor is increasingly recognized as an important therapeutic target for the treatment of hypertension and congestive heart failure, as well as an important model system for understanding aspects of gene regulation. This increased insight into the functional and pathophysiologic importance of aldosterone has been accompanied by increased insight into its cellular and molecular mechanisms of action. Aldosterone acts in a variety of epithelial and non-epithelial tissues to influence extracellular fluid volume, blood pressure, salt appetite, and can under the appropriate conditions cause cardiac fibrosis. This review will address the current view of aldosterone's molecular mechanism of action in epithelia focusing primarily on the classical MR and on a particular MR target gene, SGK1.

Amiloride-sensitive epithelial Na+channel currents in surface cells of rat rectal colon

Surface cells of the mammalian distal colon are shown to molecularly express the amiloride-sensitive epithelial Na+channel composed of three homologous subunits (α-, β-, and γ-ENaC). However, because basic electrophysiological properties of amiloride-sensitive Na+channels expressed in these cells are largely unknown at the cellular level, functional evidence for the involvement of the subunits in the native channels is incomplete. Using electrophysiological techniques, we have now characterized functional properties of native ENaC in surface cells of rectal colon (RC) of rats fed a normal Na+diet. Ussing chamber experiments showed that apical amiloride inhibited a basal short-circuit current in mucosal preparation of RC with an apparent half-inhibition constant ( Ki) value of 0.20 μM. RT-PCR analysis confirmed the presence of transcripts of α-, β-, and γ-rENaC in rectal mucosa. Whole cell patch-clamp experiments in surface cells of intact crypts acutely isolated from rectal mucosa identified an inward cationic current, which was inhibited by amiloride with a Kivalue of 0.12 μM at a membrane potential of –64 mV, the inhibition being weakly voltage dependent. Conductance ratios of the currents were Li+(1.8) > Na+(1) >> K+(≈0), respectively. Amiloride-sensitive current amplitude was almost the same at 15 or 150 mM extracellular Na+, suggesting a high Na+affinity for current activation. These results are consistent with the hypothesis that a heterooligomer composed of α-, β-, and γ-ENaC may be the molecular basis of the native channels, which are responsible for amiloride-sensitive electrogenic Na+absorption in rat rectal colon.

Mechanisms of diarrhea in the interleukin-2-deficient mouse model of colonic inflammation

Colitis in interleukin-2-deficient (IL-2(-/-)) mice resembles ulcerative colitis in humans. We studied epithelial transport and barrier function in IL-2(-/-) mice and used this model to characterize mechanisms of diarrhea during intestinal inflammation. (22)Na(+) and (36)Cl(-) fluxes were measured in proximal colon. Net Na(+) flux was reduced from 4.0 +/- 0.5 to 0.8 +/- 0.5 micromol.h(-1).cm(-2), which was paralleled by diminished mRNA and protein expression of the Na(+)/H(+) exchanger NHE3. Net Cl(-) flux was also decreased from 2.2 +/- 1.6 to -2.7 +/- 0.6 micromol.h(-1).cm(-2), indicating impaired Na(+)-Cl(-) absorption. In distal colon, aldosterone-induced electrogenic Na(+) absorption was 6.1 +/- 0.9 micromol.h(-1).cm(-2) in controls and was abolished in IL-2(-/-) mice. Concomitantly, mRNA expression of beta- and gamma-subunits of the epithelial sodium channel (ENaC) was reduced. Epithelial barrier was studied in proximal colon by impedance technique and mannitol fluxes. In contrast to ulcerative colitis, epithelial resistance was increased and mannitol fluxes were decreased in IL-2(-/-) mice. This was in accord with the findings of reduced ion transport as well as increased expression of tight junction proteins occludin and claudin-1, -2, -3, and -5. In conclusion, the IL-2(-/-) mucosa exhibits impaired electroneutral Na(+)-Cl(-) absorption and electrogenic Na(+) transport due to reduced mRNA and protein expression of NHE3 and ENaC beta- and gamma-subunit mRNA. This represents a model of early intestinal inflammation with absorptive dysfunction due to impaired transport protein expression/function while epithelial barrier is still intact. Therefore, this model is ideal to study regulation of transporter expression independent of barrier defects.

Prostaglandin E2 induces upregulation of Na+ transport across Xenopus lung epithelium

The apical mucus on pulmonary epithelia is not only critical for physiological functions such as gas exchange or inflammatory processes, but also contains surfactants and multiple molecules that mediate cellular responses. A tight control of transepithelial ion transport maintains viscosity of this layer and, e.g., the amiloride-sensitive sodium channels (ENaCs) in lung epithelia of vertebrates are the most important regulatory sites for transcellular sodium uptake. Dysfunction of this sodium transport results in reduced liquid absorption and causes massive problems with gas exchange. We used dissected lungs of Xenopus laevis in Ussing chambers to investigate the influence of prostaglandin E2 (PGE2) on the regulation of short-circuit current (ISC) and amiloride-sensitive sodium absorption (Iami). Apical application of PGE2 (1 microM) increased ISC by 38% and Iami by approximately 60%. In contrast, a different prostaglandin, PGI2, neither affected ISC nor Iami. Forskolin increased current to a similar magnitude and preincubation of the lung with an RP-isomer of cyclic AMP, an inhibitor of protein kinase A (PKA), abolished the effects of both PGE2 and forskolin. Transepithelial Na+ uptake was also upregulated by the prostaglandin receptor agonists misoprostol and sulprostone. The Iami in Xenopus oocytes that heterologously expressed ENaCs was not affected by PGE2.