Apical Na+/H+ exchange near the base of mouse colonic crypts

The Role of Plasma Membrane Sodium/Hydrogen Exchangers in Gastrointestinal Functions: Proliferation and Differentiation, Fluid/Electrolyte Transport and Barrier Integrity

The five plasma membrane Na⁺/H⁺ exchanger (NHE) isoforms in the gastrointestinal tract are characterized by distinct cellular localization, tissue distribution, inhibitor sensitivities, and physiological regulation. NHE1 (Slc9a1) is ubiquitously expressed along the gastrointestinal tract in the basolateral membrane of enterocytes, but so far, an exclusive role for NHE1 in enterocyte physiology has remained elusive. NHE2 (Slc9a2) and NHE8 (Slc9a8) are apically expressed isoforms with ubiquitous distribution along the colonic crypt axis. They are involved in pH_i regulation of intestinal epithelial cells. Combined use of a knockout mouse model, intestinal organoid technology, and specific inhibitors revealed previously unrecognized actions of NHE2 and NHE8 in enterocyte proliferation and differentiation. NHE3 (Slc9a3), expressed in the apical membrane of differentiated intestinal epithelial cells, functions as the predominant nutrient-independent Na⁺ absorptive mechanism in the gut. The new selective NHE3 inhibitor (Tenapanor) allowed discovery of novel pathophysiological and drug-targetable NHE3 functions in cystic-fibrosis associated intestinal obstructions. NHE4, expressed in the basolateral membrane of parietal cells, is essential for parietal cell integrity and acid secretory function, through its role in cell volume regulation. This review focuses on the expression, regulation and activity of the five plasma membrane Na⁺/H⁺ exchangers in the gastrointestinal tract, emphasizing their role in maintaining intestinal homeostasis, or their impact on disease pathogenesis. We point to major open questions in identifying NHE interacting partners in central cellular pathways and processes and the necessity of determining their physiological role in a system where their endogenous expression/activity is maintained, such as organoids derived from different parts of the gastrointestinal tract.

Functional characterization of the sodium/hydrogen exchanger 8 and its role in proliferation of colonic epithelial cells

Intestinal NaCl, HCO₃^-, and fluid absorption are strongly dependent on apical Na⁺/H⁺ exchange. The intestine expresses three presumably apical sodium-hydrogen exchanger (NHE) isoforms: NHE2, NHE3, and NHE8. We addressed the role of NHE8 [solute carrier 9A8 (SLC9A8)] and its interplay with NHE2 (SLC9A2) in luminal proton extrusion during acute and chronic enterocyte acidosis and studied the differential effects of NHE8 and NHE2 on enterocyte proliferation. In contrast to NHE3, which was upregulated in differentiated versus undifferentiated colonoids, the expression of NHE2 and NHE8 remained constant during differentiation of colonoids and Caco2Bbe cells. Heterogeneously expressed Flag-tagged rat (r)Nhe8 and human (h)NHE8 translocated to the apical membrane of Caco2Bbe cells. rNhe8 and hNHE8, when expressed in NHE-deficient PS120 fibroblasts showed higher sensitivity to HOE642 compared to NHE2. Lentiviral shRNA knockdown of endogenous NHE2 in Caco2Bbe cells (C2Bbe/shNHE2) resulted in a decreased steady-state intracellular pH (pH_i), an increased NHE8 mRNA expression, and augmented NHE8-mediated apical NHE activity. Lentiviral shRNA knockdown of endogenous NHE8 in Caco2Bbe cells (C2Bbe/shNHE8) resulted in a decreased steady-state pH_i as well, accompanied by decreased NHE2 mRNA expression and activity, which together contributed to reduced apical NHE activity in the NHE8-knockdown cells. Chronic acidosis increased NHE8 but not NHE2 mRNA expression. Alterations in NHE2 and NHE8 expression/activity affected proliferation, with C2Bbe/shNHE2 cells having lower and C2Bbe/shNHE8 having higher proliferative capacity, accompanied by amplified ERK1/2 signaling pathway and increased EGFR expression in the latter cell line. Thus, both Na⁺/H⁺ exchangers have distinct functions during cellular homeostasis by triggering different signaling pathways to regulate cellular proliferation and pH_i control.

The Role of pHi in Intestinal Epithelial Proliferation-Transport Mechanisms, Regulatory Pathways, and Consequences

During the maturation of intestinal epithelial cells along the crypt/surface axis, a multitude of acid/base transporters are differentially expressed in their apical and basolateral membranes, enabling processes of electrolyte, macromolecule, nutrient, acid/base and fluid secretion, and absorption. An intracellular pH (pHi)-gradient is generated along the epithelial crypt/surface axis, either as a consequence of the sum of the ion transport activities or as a distinctly regulated entity. While the role of pHi on proliferation, migration, and tumorigenesis has been explored in cancer cells for some time, emerging evidence suggests an important role of the pHi in the intestinal stem cells (ISCs) proliferative rate under physiological conditions. The present review highlights the current state of knowledge about the potential regulatory role of pHi on intestinal proliferation and differentiation.

Exploring the impact of intestinal ion transport on the gut microbiota

The gut microbiota and the host are intimately connected. The host physiology dictates the intestinal environment through regulation of pH, ion concentration, mucus production, etc., all of which exerts a selective pressure on the gut microbiota. Since different regions of the gastrointestinal tract are characterized by their own physicochemical conditions, distinct microbial communities are present in these locations. While it is widely accepted that the intestinal microbiome influences the host (tight junctions, cytokine/immune responses, diarrhea, etc.), the reciprocal interaction of the host on the microbiome is under-explored. This review aims to address these gaps in knowledge by focusing on how the host intestinal ion transport influences the luminal environment and thereby modulates the gut microbiota composition.

Expression, Localization and Functional Activity of the Major Na⁺/H⁺ Exchange Isoforms Expressed in the Intestinal Cell Line Caco-2BBe

Background/Aims

Enterocytes express a number of NHE isoforms with presumed localization in the apical (NHE2, 3 and 8) or basolateral (NHE1) membrane. Functional activity and localization of enterocyte NHE isoforms were assessed using fully differentiated Caco-2BBe cells, whose genetic expression profile closely resembles mature enterocytes.

Methods

The activity of the different NHEs was analyzed by fluorometric pH_i-metry in a perfusion chamber with separate apical and basolateral perfusion, using specific inhibitors and shRNA knockdown of NHE2. The expression of the NHEs and of other relevant acid extrusion transporters was quantified by qPCR.

Results

Quantitative comparison of the mRNA expression levels of the different NHE isoforms in 14 day-differentiated Caco-2BBe cells showed the following order: NHE2>NHE8>NHE3>NHE1. Acid-activated NHE exchange rates in the basolateral membrane were >6-fold higher than in the apical membrane. 79 ± 3 % of the acid-activated basolateral Na⁺/H⁺ exchange rate displayed a NHE1-typical inhibitor profile, and no NHE2/3/8 typical activity could be observed. Analysis of the apical Na⁺/H⁺ exchange rates revealed that approximately 51 ± 3 % of the total apical activity displayed a NHE2/8-typical inhibitor profile and 31 ± 6 % a NHE3-typical inhibitor profile. Because no selective NHE2 inhibitor is available, a stable NHE2 knockdown cell line (C2NHE2KD) was generated. C2NHE2KD displayed a reduced NHE2-typical apical Na⁺/H⁺ exchange rate and maintained a lower steady-state pH_i, despite high expression levels of other acid extruders, in particular NBCn1 (Slc4a7).

Conclusion

Differentiated Caco-2BBe cells display particularly high mRNA expression levels of NHE2, which can be functionally identified in the apical membrane. Although at low intracellular pH, NHE2 transport rate was far lower than that of NHE1. NHE2 activity was nevertheless essential for the maintenance of the steady-state pH_i of these cells.

SLC9 Gene Family: Function, Expression, and Regulation

The Slc9 family of Na⁺ /H⁺ exchangers (NHEs) plays a critical role in electroneutral exchange of Na⁺ and H⁺ in the mammalian intestine as well as other absorptive and secretory epithelia of digestive organs. These transport proteins contribute to the transepithelial Na⁺ and water absorption, intracellular pH and cellular volume regulation as well as the electrolyte, acid-base, and fluid volume homeostasis at the systemic level. They also influence the function of other membrane transport mechanisms, affect cellular proliferation and apoptosis as well as cell migration, adherence to the extracellular matrix, and tissue repair. Additionally, they modulate the extracellular milieu to facilitate other nutrient absorption and to regulate the intestinal microbial microenvironment. Na⁺ /H⁺ exchange is inhibited in selected gastrointestinal diseases, either by intrinsic factors (e.g., bile acids, inflammatory mediators) or infectious agents and associated bacterial toxins. Disrupted NHE activity may contribute not only to local and systemic electrolyte imbalance but also to the disease severity via multiple mechanisms. In this review, we describe the cation proton antiporter superfamily of Na⁺ /H⁺ exchangers with a particular emphasis on the eight SLC9A isoforms found in the digestive tract, followed by a more integrative description in their roles in each of the digestive organs. We discuss regulatory mechanisms that determine the function of Na⁺ /H⁺ exchangers as pertinent to the digestive tract, their regulation in pathological states of the digestive organs, and reciprocally, the contribution of dysregulated Na⁺ /H⁺ exchange to the disease pathogenesis and progression. © 2018 American Physiological Society. Compr Physiol 8:555-583, 2018.

Extracellular acidosis stimulates NHE2 expression through activation of transcription factor Egr-1 in the intestinal epithelial cells

Na(+)/H(+) exchangers (NHEs) play important roles in regulating internal pH (pHi), cell volume and neutral Na(+) absorption in the human intestine. Earlier studies have shown that low extracellular pH (pHe) and metabolic acidosis increases the expression and function of NHE1-3 genes. However, transcriptional mechanisms involved remained unknown. Therefore, we investigated the molecular mechanisms underlying acid-induced NHE2 expression in C2BBe1 and SK-CO15 intestinal epithelial cells. Assessing total RNA and protein by RT-PCR and Western blot analysis, respectively, displayed significant increases in the NHE2 mRNA and protein levels in cells exposed to acidic media (pH 6.5 and 6.7) compared to normal medium. Acid treatment was also associated with a significant enhancement in NHE2 transport activity. Quantification of the heterogeneous nuclear RNA indicated that the rate of NHE2 transcription was increased in response to acid. Furthermore, acid caused a significant increase in NHE2 promoter activity confirming transcriptional upregulation. Through functional and mutational studies the acid-response element was mapped to a 15-nucleotide GC-rich sequence at bp -337 to -323 upstream from the transcription start site. We previously identified this element as an overlapping Egr-1/Sp1/Egr-1 motif that was essential for the NHE2 upregulation by mitogen-induced transcription factor Egr-1. Cells exposed to acid exhibited a temporal increase in Egr-1 mRNA and protein expression. These events were followed by Egr-1 nuclear accumulation, as detected by immunofluorescence microscopy, and potentiated its in vitro and in vivo interaction with the NHE2 promoter. Disruption of ESE motif and knockdown of Egr-1 expression by targeted small interfering RNA abrogated the acid-induced NHE2 transcriptional activity. These data indicate that the acid-dependent NHE2 stimulation is implemented by transcriptional upregulation of NHE2 via acid-induced Egr-1 in the intestinal epithelial cells.

In vivo epithelial wound repair requires mobilization of endogenous intracellular and extracellular calcium

We report that a localized intracellular and extracellular Ca(2+) mobilization occurs at the site of microscopic epithelial damage in vivo and is required to mediate tissue repair. Intravital confocal/two-photon microscopy continuously imaged the surgically exposed stomach mucosa of anesthetized mice while photodamage of gastric epithelial surface cells created a microscopic lesion that healed within 15 min. Transgenic mice with an intracellular Ca(2+)-sensitive protein (yellow cameleon 3.0) report that intracellular Ca(2+) selectively increases in restituting gastric epithelial cells adjacent to the damaged cells. Pretreatment with U-73122, indomethacin, 2-aminoethoxydiphenylborane, or verapamil inhibits repair of the damage and also inhibits the intracellular Ca(2+) increase. Confocal imaging of Fura-Red dye in luminal superfusate shows a localized extracellular Ca(2+) increase at the gastric surface adjacent to the damage that temporally follows intracellular Ca(2+) mobilization. Indomethacin and verapamil also inhibit the luminal Ca(2+) increase. Intracellular Ca(2+) chelation (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid/acetoxymethyl ester, BAPTA/AM) fully inhibits intracellular and luminal Ca(2+) increases, whereas luminal calcium chelation (N-(2-hydroxyetheyl)-ethylendiamin-N,N,N'-triacetic acid trisodium, HEDTA) blocks the increase of luminal Ca(2+) and unevenly inhibits late-phase intracellular Ca(2+) mobilization. Both modes of Ca(2+) chelation slow gastric repair. In plasma membrane Ca-ATPase 1(+/-) mice, but not plasma membrane Ca-ATPase 4(-/-) mice, there is slowed epithelial repair and a diminished gastric surface Ca(2+) increase. We conclude that endogenous Ca(2+), mobilized by signaling pathways and transmembrane Ca(2+) transport, causes increased Ca(2+) levels at the epithelial damage site that are essential to gastric epithelial cell restitution in vivo.

Extracellular pH regulates zinc signaling via an Asp residue of the zinc-sensing receptor (ZnR/GPR39)

Zinc activates a specific Zn(2+)-sensing receptor, ZnR/GPR39, and thereby triggers cellular signaling leading to epithelial cell proliferation and survival. Epithelial cells that express ZnR, particularly colonocytes, face frequent changes in extracellular pH that are of physiological and pathological implication. Here we show that the ZnR/GPR39-dependent Ca(2+) responses in HT29 colonocytes were maximal at pH 7.4 but were reduced by about 50% at pH 7.7 and by about 62% at pH 7.1 and were completely abolished at pH 6.5. Intracellular acidification did not attenuate ZnR/GPR39 activity, indicating that the pH sensor of this protein is located on an extracellular domain. ZnR/GPR39-dependent activation of extracellular-regulated kinase (ERK)1/2 or AKT pathways was abolished at acidic extracellular pH of 6.5. A similar inhibitory effect was monitored for the ZnR/GPR39-dependent up-regulation of Na(+)/H(+) exchange activity at pH 6.5. Focusing on residues putatively facing the extracellular domain, we sought to identify the pH sensor of ZnR/GPR39. Replacing the histidine residues forming the Zn(2+) binding site, His(17) or His(19), or other extracellular-facing histidines to alanine residues did not abolish the pH dependence of ZnR/GPR39. In contrast, replacing Asp(313) with alanine resulted in similar Ca(2+) responses triggered by ZnR/GPR39 at pH 7.4 or 6.5. This mutant also showed similar activation of ERK1/2 and AKT pathways, and ZnR-dependent up-regulation of Na(+)/H(+) exchange at pH 7.4 and pH 6.5. Substitution of Asp(313) to His or Glu residues restored pH sensitivity of the receptor. This indicates that Asp(313), which was shown to modulate Zn(2+) binding, is an essential residue of the pH sensor of GPR39. In conclusion, ZnR/GPR39 is tuned to sense physiologically relevant changes in extracellular pH that thus regulate ZnR-dependent signaling and ion transport activity.

Zinc sensing receptor signaling, mediated by GPR39, reduces butyrate-induced cell death in HT29 colonocytes via upregulation of clusterin

Zinc enhances epithelial proliferation, protects the digestive epithelial layer and has profound antiulcerative and antidiarrheal roles in the colon. Despite the clinical significance of this ion, the mechanisms linking zinc to these cellular processes are poorly understood. We have previously identified an extracellular Zn²⁺ sensing G-protein coupled receptor (ZnR) that activates Ca²⁺ signaling in colonocytes, but its molecular identity as well as its effects on colonocytes' survival remained elusive. Here, we show that Zn²⁺, by activation of the ZnR, protects HT29 colonocytes from butyrate induced cell death. Silencing of the G-protein coupled receptor GPR39 expression abolished ZnR-dependent Ca²⁺ release and Zn²⁺-dependent survival of butyrate-treated colonocytes. Importantly, GPR39 also mediated ZnR-dependent upregulation of Na⁺/H⁺ exchange activity as this activity was found in native colon tissue but not in tissue obtained from GPR39 knock-out mice. Although ZnR-dependent upregulation of Na⁺/H⁺ exchange reduced the cellular acid load induced by butyrate, it did not rescue HT29 cells from butyrate induced cell death. ZnR/GPR39 activation however, increased the expression of the anti-apoptotic protein clusterin in butyrate-treated cells. Furthermore, silencing of clusterin abolished the Zn²⁺-dependent survival of HT29 cells. Altogether, our results demonstrate that extracellular Zn²⁺, acting through ZnR, regulates intracellular pH and clusterin expression thereby enhancing survival of HT29 colonocytes. Moreover, we identify GPR39 as the molecular moiety of ZnR in HT29 and native colonocytes.

Trefoil factor 2 requires Na/H exchanger 2 activity to enhance mouse gastric epithelial repair

Trefoil factor (TFF) peptides are pivotal for gastric restitution after surface epithelial damage, but TFF cellular targets that promote cell migration are poorly understood. Conversely, Na/H exchangers (NHE) are often implicated in cellular migration but have a controversial role in gastric restitution. Using intravital microscopy to create microscopic lesions in the mouse gastric surface epithelium and directly measure epithelial restitution, we evaluated whether TFFs and NHE isoforms share a common pathway to promote epithelial repair. Blocking Na/H exchange (luminal 10 μm 5-(N-ethyl-N-isopropyl) amiloride or 25 μm HOE694) slows restitution 72-83% in wild-type or NHE1(-/-) mice. In contrast, HOE694 has no effect on the intrinsically defective gastric restitution in NHE2(-/-) mice or TFF2(-/-) mice. In TFF2(-/-) mice, NHE2 protein is reduced 23%, NHE2 remains localized to apical membranes of surface epithelium, and NHE1 protein amount or localization is unchanged. The action of topical rat TFF3 to accelerate restitution in TFF2(-/-) mice was inhibited by AMD3100 (CXCR4 receptor antagonist). Furthermore, rat TFF3 did not rescue restitution when NHE2 was inhibited [TFF2(-/-) mice +HOE694, or NHE2(-/-) mice]. HOE694 had no effect on pH at the juxtamucosal surface before or after damage. We conclude that functional NHE2, but not NHE1, is essential for mouse gastric epithelial restitution and that TFFs activate epithelial repair via NHE2.

Basolateral ion transporters involved in colonic epithelial electrolyte absorption, anion secretion and cellular homeostasis

Electrolyte transporters located in the basolateral membrane of the colonic epithelium are increasingly appreciated as elaborately regulated components of specific transport functions and cellular homeostasis: During electrolyte absorption, Na(+) /K(+) ATPase, Cl⁻ conductance, Cl⁻/HCO₃⁻ exchange, K(+) /Cl⁻ cotransport and K(+) channels are candidates for basolateral Na(+) , Cl⁻ and K(+) extrusion. The process of colonic anion secretion involves basolateral Na(+) /K(+) /2Cl⁻ , and probably also Na(+) /HCO₃⁻ cotransport, as well as Na(+) /K(+) ATPase and K(+) channels to supply substrate, stabilize the membrane potential and generate driving force respectively. Together with a multitude of additional transport systems, Na(+) /H(+) exchange and Na(+) /HCO₃⁻ cotransport have been implicated in colonocyte pH(i) and volume homeostasis. The purpose of this article is to summarize recently gathered information on the molecular identity, function and regulation of the involved basolateral transport systems in native tissue. Furthermore, we discuss how these findings can help to integrate these systems into the transport function and the cellular homoeostasis of colonic epithelial cells. Finally, disturbances of basolateral electrolyte transport during disease states such as mucosal inflammation will be reviewed.

Segregation of Na/H exchanger-3 and Cl/HCO3 exchanger SLC26A3 (DRA) in rodent cecum and colon

The colon is believed to absorb NaCl via the coupled operation of apical Na/H exchanger-3 (NHE3) and Cl/HCO(3) exchanger SLC26A3 (DRA). Efficient coupling requires that NHE3 and DRA operate in close proximity within common luminal and cytosolic microenvironments. Thus we examined whether these proteins coexist along the apical margin of surface enterocytes by quantitative immunofluorescence microscopy in consecutive colon segments from nonfasted mice and rats. The cecocolonic profiles of NHE3 and DRA expression were roughly inverse; NHE3 was highest in proximal colon (PC) and negligible in distal colon, whereas DRA was absent in early PC and highest in the late midcolon, and DRA was prominent in the cecum whereas NHE3 was not. NHE3 and DRA coexisted only in the middle third of the colon. The consequences of unpaired NHE3/DRA expression on mucosal surface (subscript MS) pH and Na(+) concentration ([Na(+)]) were assessed in nonfasted rats in situ using miniature electrodes. In the cecum, where only DRA is expressed, pH(MS) was approximately 7.5, markedly higher than underlaying stool (6.3), consistent with net HCO(3)(-) secretion. In the early PC, where NHE3 is not expressed with DRA, pH(MS) was acidic (6.2), consistent with unopposed H(+) secretion. [Na(+)](MS) was approximately 60 mM in the cecum, decreased along the PC to approximately 20 mM, and declined further to approximately 10 mM distally. Cl(-) was secreted into the PC, then reabsorbed distally. Our results suggest a model in which 1) unpaired DRA activity in the cecum maintains an alkaline mucosal surface that could neutralize fermentative H(+); 2) unpaired NHE3 activity in the early PC preserves an acidic mucosal surface that could energize short-chain fatty acid absorption; and 3) coupled NHE3/DRA activities in the midcolon allow for vigorous NaCl absorption at a neutral pH(MS).

Mechanisms of the regulation of the intestinal Na+/H+ exchanger NHE3

A major of Na⁺ absorptive process in the proximal part of intestine and kidney is electroneutral exchange of Na⁺ and H⁺ by Na⁺/H⁺ exchanger type 3 (NHE3). During the past decade, significant advance has been achieved in the mechanisms of NHE3 regulation. A bulk of the current knowledge on Na⁺/H⁺ exchanger regulation is based on heterologous expression of mammalian Na⁺/H⁺ exchangers in Na⁺/H⁺ exchanger deficient fibroblasts, renal epithelial, and intestinal epithelial cells. Based on the reductionist's approach, an understanding of NHE3 regulation has been greatly advanced. More recently, confirmations of in vitro studies have been made using animals deficient in one or more proteins but in some cases unexpected findings have emerged. The purpose of this paper is to provide a brief overview of recent progress in the regulation and functions of NHE3 present in the luminal membrane of the intestinal tract.

Role of wild-type estrogen receptor-beta in mitochondrial cytoprotection of cultured normal male and female human lens epithelial cells

The influence of sexual category as a modifier of cellular function is underinvestigated. Whether sex differences affect estrogen-mediated mitochondrial cytoprotection was determined using cell cultures of normal human lens epithelia (nHLE) from postmortem male and female donors. Experimental indicators assessed included differences in estrogen receptor-beta (ERbeta) isoform expression, receptor localization in mitochondria, and estrogen-mediated prevention of loss of mitochondrial membrane potential using the potentiometric fluorescent compound JC-1 after nHLE were exposed to peroxide. The impact of wild-type ERbeta (wtERbeta1) was also assessed using wtERbeta1 siRNA to suppress expression. A triple-primer PCR assay was employed to determine the proportional distribution of the receptor isoforms (wtERbeta1, -beta2, and -beta5) from the total ERbeta message pool in male and female cell cultures. Irrespective of sex, nHLE express wtERbeta1 and the ERbeta2 and ERbeta5 splice variants in similar ratios. Confocal microscopy and immunofluorescence revealed localization of the wild-type receptor in peripheral mitochondrial arrays and perinuclear mitochondria as well as nuclear staining in both cell populations. The ERbeta2 and ERbeta5 isoforms were distributed primarily in the nucleus and cytosol, respectively; no association with the mitochondria was detected. Both male and female nHLE treated with E(2) (1 muM) displayed similar levels of protection against peroxide-induced oxidative stress. In conjunction with acute oxidative insult, RNA suppression of wtERbeta1 elicited the collapse of mitochondrial membrane potential and markedly diminished the otherwise protective effects of E(2). Thus, whereas the estrogen-mediated prevention of mitochondrial membrane permeability transition is sex independent, the mechanism of estrogen-induced mitochondrial cytoprotection is wtERbeta1 dependent.

NHE1, NHE2, and NHE4 contribute to regulation of cell pH in T84 colon cancer cells

The isoforms of the Na+/H+ exchanger present in T84 human colon cells were identified by functional and molecular methods. Cell pH was measured by fluorescence microscopy using the probe BCECF. Based on the pH recovery after an ammonium pulse and determination of buffering capacity of these cells, the rate of H+ extrusion (JH) was 3.68 mM/min. After the use of the amiloride derivative HOE-694 at 25 microM, which inhibits the isoforms NHE1 and NHE2, there remained 43% of the above transport rate, the nature of which was investigated. Evidence of the presence of NHE1, NHE2, and NHE4 was obtained by reverse transcriptase polymerase chain reaction (RT-PCR) (mRNA) and Western blot. There was no decrease of JH by the NHE3 inhibitor S3226 (1 microM) and no evidence of this isoform by RT-PCR was found. The following functional evidence for the presence of NHE4 was obtained: 25 microM EIPA abolished JH entirely, but NHE4 was not inhibited at 10 microM; substitution of Na by K increased the remainder, a property of NHE4; hypertonicity also increased this fraction of JH. Cl--dependent NHE was not detected: in 0 Cl- solutions JH was increased and not reduced. In 0 Cl- cell volume decreased significantly, which was abolished by the Cl- channel blocker NPPB, indicating that the 0 Cl- effect was because of reduction of cell volume. In conclusion, T84 human colon cells contain three isoforms of the Na+/H+ exchanger, NHE1, NHE2, and NHE4, but not the Cl-dependent NHE.

Regulatory Binding Partners and Complexes of NHE3

NHE3 is the brush-border (BB) Na+/H+exchanger of small intestine, colon, and renal proximal tubule which is involved in large amounts of neutral Na+absorption. NHE3 is a highly regulated transporter, being both stimulated and inhibited by signaling that mimics the postprandial state. It also undergoes downregulation in diarrheal diseases as well as changes in renal disorders. For this regulation, NHE3 exists in large, multiprotein complexes in which it associates with at least nine other proteins. This review deals with short-term regulation of NHE3 and the identity and function of its recognized interacting partners and the multiprotein complexes in which NHE3 functions.

Short-term effect on intestinal epithelial Na+/H+ exchanger by Giα1,2-coupled 5-HT1A and Gq/11-coupled 5-HT2 receptors

The present study evaluated the effect of 5-hydroxytryptamine (5-HT) on intestinal Na(+)/H(+) exchanger (NHE) activity and the cellular signaling pathways involved in T84 cells. T84 cells express endogenous NHE1 and NHE2 proteins, detected by immunoblotting, but not NHE3. The rank order for inhibition of NHE activity in acid-loaded T84 cells was 5-(N-ethyl-N-isopropyl)-amiloride (EIPA; IC(50)=519 [465, 579] nM)>cariporide (IC(50)=630 [484, 819] nM)>amiloride (IC(50)=19 [16, 24] microM); the NHE3 inhibitor S3226 was found to be devoid of effect. This different inhibitory sensitivity indicates that both NHE1 and NHE2 isoforms may play an active role in Na(+)-dependent intracellular pH (pH(i)) recovery in T84 cells. Short-term exposure (0.5 h) of T84 cells to 5-HT increased NHE activity in a concentration-dependent manner. The stimulation induced by 5-HT (30 microM) was partially inhibited by both WAY 100135 (300 nM) and ketanserin (300 nM), antagonists of 5-HT(1A) and 5-HT(2) receptors, respectively. NHE activity was significantly increased by 8-OH-DPAT and alpha-methyl-5-HT, agonists of, respectively, 5-HT(1A) and 5-HT(2) receptors. An incubation of T84 cells with anti-G(s) and anti-G(beta) antibodies complexed with lipofectin did not prevent the 5-HT-induced stimulation of NHE activity. Overnight treatment with anti-G(ialpha1,2) and anti-G(q/11) antibodies complexed with lipofectin blocked the stimulatory effect induced by 8-OH-DPAT and alpha-methyl-5-HT, respectively. It is concluded that in T84 cells 5-HT enhances intestinal NHE activity through stimulation of G(ialpha1,2)-coupled 5-HT(1A) and G(q/11)-coupled 5-HT(2) receptors.

PGE2 reduces net water and chloride absorption from the rat colon by targeting the Na+/H+ exchanger and the Na+ K+ 2Cl- cotransporter

An effect of PGE2 on water and chloride absorption was already established in a previous work. This study is an attempt to find the mechanism of action of the prostaglandin by investigating the involvement of three major transporters namely the Na+ -K+ ATPase, the Na+/H+ exchanger and the Na+ K+ 2Cl- cotransporter. Rats were injected with PGE2 and 15 min later, the colon was perfused in situ with Krebs Ringer buffer, and net water and chloride absorption were determined. When the involvement of the cotransporter and/or the exchanger was investigated, animals were injected with, respectively, furosemide and amiloride 10 min before PGE2. Superficial and crypt colonocytes were then isolated and the protein expression of the Na+ -K+ ATPase and the Na+ K+ 2Cl- was determined by western blot analysis. The effect of PGE2 on the pump activity in presence or absence of the transporters' inhibitors was also studied. PGE2 decreased net water and chloride absorption from the colon, increased the Na+ -K+ ATPase activity in superficial cells and reduced it in crypt cells. The prostaglandin was found to stimulate secretion in superficial cells by targeting the Na+ K+ 2Cl- symporter, and reduce absorption in crypt cells by targeting the Na+/H+ antiporter. Changes in the activity of the pump are secondary to changes in the activity of the other transporters.

NHE3 inhibition by cAMP and Ca2+ is abolished in PDZ-domain protein PDZK1-deficient murine enterocytes

The PDZ-binding protein PDZK1 (NHERF3/CAP70/PDZ-dc-1) in vitro binds to NHE3, but its role in the regulation of NHE3 activity in native enterocytes is unknown. This study was undertaken to understand the physiological role of PDZK1 in regulating NHE3 activity in native murine colonic enterocytes. NHE3 transport rates were assessed fluorometrically in BCECF-loaded colonic crypts in the NHE3-expressing cryptal openings by measuring acid-activated, Na+-dependent, Hoe 642-insensitive proton efflux rates. NHE3 mRNA expression levels and NHE3 total enterocyte and brush border membrane (BBM) protein abundance were determined by quantitative PCR and Western analysis and immunohistochemistry. In pdzk1-/- colonic surface cells, acid-activated NHE3 transport rates were strongly reduced, and the inhibitory effect of forskolin and ionomcyin was virtually abolished. Hyperosmolarity, on the other hand, still had an inhibitory effect. In addition, the NHE3-selective inhibitor S1611 inhibited acid-activated NHE3 activity in pdzk1-/- and +/+mice, suggesting that functional NHE3 is present in pdzk1-deficient colonocytes. NHE1 and NHE2 activity was not altered in pdzk1-/- colonic crypts. Immunohistochemistry revealed apical NHE3 staining in pdzk1-/- and +/+proximal colon, and Western blot analysis revealed no difference in NHE3 abundance in colonic enterocyte homogenate as well as brush border membrane. Lack of the PDZ-adaptor protein PDZK1 in murine proximal colonic enterocytes does not influence NHE3 abundance or targeting to the apical membrane, but abolishes NHE3 regulation by cAMPergic and Ca2+ -dependent pathways. It leaves NHE3 inhibition by hyperosmolarity intact, suggesting an important and selective role for PDZK1 in the agonist-mediated regulation of intestinal NHE3 activity.

Different ionic conditions prompt NHE2 and NHE3 translocation to the plasma membrane

We tested whether NHE3 and NHE2 Na(+)/H(+) exchanger isoforms were recruited to the plasma membrane (PM) in response to changes in ion homeostasis. NHE2-CFP or NHE3-CFP fusion proteins were functional Na(+)/H(+) exchangers when transiently expressed in NHE-deficient PS120 fibroblasts. Confocal morphometry of cells whose PM was labeled with FM4-64 measured the fractional amount of fusion protein at the cell surface. In resting cells, 10-20% of CFP fluorescence was at PM and stable over time. A protocol commonly used to activate the Na(+)/H(+) exchange function (NH(4)-prepulse acid load sustained in Na(+)-free medium), increased PM percentages of PM NHE3-CFP and NHE2-CFP. Separation of cellular acidification from Na(+) removal revealed that only NHE3-CFP translocated when medium Na(+) was removed, and only NHE2-CFP translocated when the cell was acidified. NHE2/NHE3 chimeric proteins demonstrate that the Na(+)-removal response element resides predominantly in the NHE3 cytoplasmic tail and is distinct from the acidification response sequence of NHE2.

Fluid flow in the juxtaglomerular interstitium visualized in vivo

Earlier electron microscopy studies demonstrated morphological signs of fluid flow in the juxtaglomerular apparatus (JGA), including fenestrations of the afferent arteriole (AA) endothelium facing renin granular cells. We aimed to directly visualize fluid flow in the JGA, the putative function of the fenestrated endothelium, using intravital multiphoton microscopy of Munich-Wistar rats and C57BL6 mice. Renin content of the AA correlated strongly with the length of the fenestrated, filtering AA segment. Fluorescence of the extracellular fluid marker lucifer yellow (LY) injected into the cannulated femoral vein in bolus was followed in the renal cortex by real-time imaging. LY was detected in the interstitium around the JG AA before the plasma LY filtered into Bowman's capsule and early proximal tubule. The fluorescence intensity of LY in the JGA interstitium was 17.9 ± 3.5% of that in the AA plasma ( n = 6). The JGA fluid flow was oscillatory, consisting of two components: a fast (one every 5–10 s) and a slow (one every 45–50 s) oscillation, most likely due to the rapid transmission of both the myogenic and tubuloglomerular feedback (TGF)-mediated hemodynamic changes. LY was also detected in the distal tubular lumen about 2–5 s later than in the AA, indicating the flow of JGA interstitial fluid through the macula densa. In the isolated microperfused JGA, blocking the early proximal tubule with a micropipette caused significant increases in MD cell volume by 62 ± 4% ( n = 4) and induced dilation of the intercellular lateral spaces. In summary, significant and dynamic fluid flow exists in the JGA which may help filter the released renin into the renal interstitium (endocrine function). It may also modulate TGF and renin signals in the JGA (hemodynamic function).

Prostaglandin-mediated inhibition of Na+/H+ exchanger isoform 2 stimulates recovery of barrier function in ischemia-injured intestine

Prostaglandins stimulate repair of the ischemia-injured intestinal barrier in the porcine ileum through a mechanism involving cAMP-dependent Cl- secretion and inhibition of electroneutral Na+/H+ exchanger (NHE) activity. In the present study, we focused on the role of individual NHE isoforms in the recovery of barrier function. Ischemia-injured porcine ileal mucosa was mounted on Ussing chambers. Short-circuit current (I(sc)), transepithelial electrical resistance (TER), and isotopic fluxes of 22Na were measured in response to PGE2 and selective inhibitors of epithelial NHE isoforms. Immunoassays were used to assess the expression of NHE isoforms. Forty-five minutes of intestinal ischemia resulted in a 45% reduction in TER (P < 0.01). Near-complete restitution occurred within 60 min. Inhibition of NHE2 with HOE-694 (25 microM) added to the mucosal surface of the injured ileum stimulated significant elevations in TER, independent of changes in I(sc) and histological evidence of restitution. Pharmacological inhibition of NHE3 or NHE1 with mucosal S-3226 (20 microM) or serosal cariporide (25 microM), respectively, had no effect. Ischemia-injured tissues treated with mucosal S-3226 or HOE-694 exhibited equivalent reductions in mucosal-to-serosal fluxes of 22Na+ (by approximately 35%) compared with nontreated ischemia-injured control tissues (P < 0.05). Intestinal ischemia resulted in increased expression of the cytoplasmic NHE regulatory factor EBP50 in NHE2 but not in NHE3 immunoprecipitates. Selective inhibition of NHE2, and not NHE3, induces recovery of barrier function in the ischemia-injured intestine.

NHE2 is the main apical NHE in mouse colonic crypts but an alternative Na+-dependent acid extrusion mechanism is upregulated in NHE2-null mice

The mechanism of apical Na(+)-dependent H(+) extrusion in colonic crypts is controversial. With the use of confocal microscopy of the living mouse distal colon loaded with BCECF or SNARF-5F (fluorescent pH sensors), measurements of intracellular pH (pH(i)) in epithelial cells at either the crypt base or colonic surface were reported. After cellular acidification, the addition of luminal Na(+) stimulated similar rates of pH(i) recovery in cells at the base of distal colonic crypts of wild-type or Na(+)/H(+) exchanger isoform 2 (NHE2)-null mice. In wild-type crypts, 20 microM HOE694 (NHE2 inhibitor) blocked 68-75% of the pH(i) recovery rate, whereas NHE2-null crypts were insensitive to HOE694, the NHE3-specific inhibitor S-1611 (20 microM), or the bicarbonate transport inhibitor 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS; 1 mM). A general NHE inhibitor, 5-(N-ethyl-N-isopropyl)amiloride (EIPA; 20 microM), inhibited pH(i) recovery in NHE2-null mice (46%) but less strongly than in wild-type mice (74%), suggesting both EIPA-sensitive and -insensitive compensatory mechanisms. Transepithelial Na(+) leakage followed by activation of basolateral NHE1 could confound the outcomes; however, the rates of Na(+)-dependent pH(i) recovery were independent of transepithelial leakiness to lucifer yellow and were unchanged in NHE1-null mice. NHE2 was immunolocalized on apical membranes of wild-type crypts but not NHE2-null tissue. NHE3 immunoreactivity was near the colonic surface but not at the crypt base in NHE2-null mice. Colonic surface cells from wild-type mice demonstrated S1611- and HOE694-sensitive pH(i) recovery in response to luminal sodium, confirming a functional role for both NHE3 and NHE2 at this site. We conclude that constitutive absence of NHE2 results in a compensatory increase in a Na(+)-dependent, EIPA-sensitive acid extruder distinct from NHE1, NHE3, or SITS-sensitive transporters.

Task-based imaging of colon cancer in the Apc(Min/+) mouse model

Optical coherence tomography (OCT), laser-induced fluorescence (LIF), and laser-scanning confocal microscopy (LSCM) were used for the task of multimodal study of healthy and adenomatous mouse colon. The results from each modality were compared with histology, which served as the gold standard. The Apc(Min/+) genetic mouse model of colon cancer was compared with wild-type mice. In addition, a special diet was used for the task of studying the origins of a 680 nm autofluorescent signal that was previously observed in colon. The study found close agreement among each of the modalities and with histology. All four modalities were capable of identifying diseased tissue accurately. The OCT and LSCM images provided complementary structural information about the tissue, while the autofluorescence signal measured by LIF and LSCM provided biochemical information. OCT and LIF were performed in vivo and nondestructively, while the LSCM and histology required extraction of the tissue. The magnitude of the 680 nm signal correlates with chlorophyll content in the mouse diet, suggesting that the autofluorescent compound is a dietary metabolite.

Quantitative imaging of basic functions in renal (patho)physiology

Multiphoton fluorescence microscopy offers the advantages of deep optical sectioning of living tissue with minimal phototoxicity and high optical resolution. More importantly, dynamic processes and multiple functions of an intact organ can be visualized in real time using noninvasive methods, and quantified. These studies aimed to extend existing methods of multiphoton fluorescence imaging to directly observe and quantify basic physiological parameters of the kidney including glomerular filtration rate (GFR) and permeability, blood flow, urinary concentration/dilution, renin content and release, as well as more integrated and complex functions like the tubuloglomerular feedback (TGF)-mediated oscillations in glomerular filtration and tubular flow. Streptozotocin-induced diabetes significantly increased single-nephron GFR (SNGFR) from 32.4 +/- 0.4 to 59.5 +/- 2.5 nl/min and glomerular permeability to a 70-kDa fluorophore approximately eightfold. The loop diuretic furosemide 2-fold diluted and increased approximately 10-fold the volume of distal tubular fluid, while also causing the release of 20% of juxtaglomerular renin content. Significantly higher speeds of individual red blood cells were measured in intraglomerular capillaries (16.7 +/- 0.4 mm/s) compared with peritubular vessels (4.7 +/- 0.2 mm/s). Regular periods of glomerular contraction-relaxation were observed, resulting in oscillations of filtration and tubular flow rate. Oscillations in proximal and distal tubular flow showed similar cycle times ( approximately 45 s) to glomerular filtration, with a delay of approximately 5-10 and 25-30 s, respectively. These innovative technologies provide the most complex, immediate, and dynamic portrayal of renal function, clearly depicting the components and mechanisms involved in normal physiology and pathophysiology.

Absorption and paracellular visualization of fluorescein, a hydrosoluble probe, in intact house sparrows (Passer domesticus)

We describe a method to visualize the cellular location of compounds during absorption by the small intestine in intact animals. First, we employed pharmacokinetic methodology to measure the fractional absorption of sodium fluorescein, a small (MW = 376) water-soluble molecule that is widely used as hydrophilic marker molecule for paracellular permeability studies. Based on the hypothesis that the paracellular pathway acts as a sieve, we predicted that fluorescein absorption would be considerable, but less than that of passively absorbed L-glucose which is a smaller molecule (MW = 180). When the two compounds were gavaged into house sparrows simultaneously, the birds absorbed significantly less fluorescein (42 +/- 8%) than L-glucose (82 +/- 7%), as predicted, and absorptions of the two were correlated as one would predict if they shared the same pathway. We removed intestinal tissue 10 min after gavage with sodium fluorescein and determined the cellular location of the compound's fluorescence using confocal laser microscopy. The fluorescent signal was found primarily in the paracellular space. In contrast, in the same type of experiment using instead the similar-sized fluorescent lipophilic compound rhodamine 123 (MW = 381), most fluorescence appeared inside enterocytes, as expected for a compound that diffuses across the apical membrane. Thus, results from all the experiments are consistent with the hypothesis that hydrophilic fluorescein is absorbed primarily via a paracellular pathway. These methods could be applied to visualize absorption pathways of other compounds in other intact animals.

Differential expression and comparative subcellular localization of estrogen receptor beta isoforms in virally transformed and normal cultured human lens epithelial cells

A number of variants of the wild-type (wt) estrogen receptor beta (ERbeta-1) coexist in a wide range of tissues. In the human these include, together with others, the expression of several isoforms (ERbeta-2-ERbeta-5) due to alternative splicing of exons encoding the carboxy terminus. In this study, we determined whether virally transformed cell cultures of human lens epithelial cells (HLE-B3) express both full length (or wt) and variant isoforms of ERbeta in comparison to normal secondary cultures of human lens epithelial cells (nHLE) and furthermore, identify the subcellular localization of the wtERbeta-1 and ERbeta isoform variants in HLE-B3 and nHLE cells, as well as from human breast adenocarcinoma cells (MCF-7) which provided a positive control. ERbeta isoform mRNA expression was evaluated by coupled RT-PCR. Subcellular localization of ERbeta isoforms was determined on formaldehyde-fixed, Saponin-permeabilized cells using conventional immunofluorescence techniques and affinity purified polyclonal antibodies specific for ERbeta-1 as well as to two of the truncated carboxy terminus isoforms (beta-2 and beta-5). Total RNA was extracted from HLE-B3 and nHLE cells and lens tissue, as well as from human breast adenocarcinoma cells (MCF-7) and subjected to RT-PCR using specific estrogen receptor primers intended to distinguish ERbeta-1-ERbeta-5 mRNA. The PCR products corresponded to wtERbeta-1 as well as to the isoform variants beta-2 and beta-5. The proportional distribution of wtERbeta-1, beta-2 and beta-5 PCR products differed between the normal lens epithelial cells and the SV-40 transformed lens epithelial cell line; the nHLE being similar to lens tissue with respect to relative expression of ERbeta isoform cDNAs. Confocal microscopy and immunofluorescence revealed ERbeta-2 was distributed throughout the cytosol and was associated with the nucleus of all cells examined, although sporadic immunostaining was observed with the nuclei of MCF-7. Prominent immunostaining of ERbeta-1 appeared in the mitochondria (along with weaker staining in the nucleus) of all cell types as authenticated by co-localization with Mitotrack-633. ERbeta-5 immunostaining was diffuse in the cytosol and also associated with the nuclei of all cell types. The differential subcellular partitioning of ERbeta-1 to the mitochondria and ERbeta-2 to the nucleus suggests a new aspect of regulation and function of the estrogen signalling system.

Action of ANG II and ANP on colon epithelial cells

The interaction of angiotensin II (ANG II) and atrial natriuretic peptide (ANP) on intracellular pH (pH(i)) and calcium ([Ca2+](i)) was investigated in T84 cells (a permanent cell line derived from human colon epithelium) using the fluorescent stains BCECF/AM and Fluo 4/AM, respectively. pH(i) recovery rate mediated by the Na(+)/H+ exchanger (NHE) was examined following an NH4Cl pulse. Under control conditions pH(i) recovered at 0.114+/-0.005 pH units/min (n=35). ANG II (10(-12) or 10(-9) M) increased this value, whilst ANG II (10(-7) M) decreased it. These effects of ANG II were impaired by simultaneous addition of 1 microM or 25 microM HOE-694, indicating that the stimulatory and inhibitory effects of ANG II on pH(i) recovery are mediated in part via the NHE1 and NHE2 isoforms. ANG II increased [Ca2+]i concentration-dependently. ANP (10(-6) M) or dimethyl-BAPTA/AM (50 microM) blocked the effects of ANG II on [Ca2+]i and on the rate of pH(i) recovery. Thapsigargin (10(-5) M) enhanced the effect of ANG II on [Ca2+]i and reversed its stimulatory effect on the rate of pH(i) recovery to an inhibitory one. External Ca(2+)-free solution did not affect the effects of ANG II on these parameters. These data suggest that the [Ca2+]i increase induced by ANG II is dependent on intracellular calcium stores. They are compatible with the demonstration of two sites on the C-terminal of the Na(+)/H+ exchanger, one stimulating Na(+)/H+ activity by increases of [Ca2+]i in the lower range (at 10(-12) or 10(-9) M ANG II) and the other inhibiting this activity at high [Ca2+]i levels (at 10(-7) M ANG II). ANP or dimethyl-BAPTA/AM, by impairing the pathway mediating the increase in [Ca2+]i, block both the stimulatory and inhibitory effects of ANG II.

Serotonin inhibits Na+/H+ exchange activity via 5-HT4 receptors and activation of PKC alpha in human intestinal epithelial cells

BACKGROUND & AIMS

Increased serotonin levels have been implicated in the pathophysiology of diarrhea associated with celiac and inflammatory diseases. However, the effects of serotonin on Na+ /H+ exchange (NHE) activity in the human intestine have not been investigated fully. The present studies examined the acute effects of 5-hydroxytryptamine (5-HT) on NHE activity using Caco-2 cells as an in vitro model.

METHODS

Caco-2 cells were treated with 5-HT (.1 micromol/L, 1 h) and NHE activity was measured as ethyl-isopropyl-amiloride (EIPA)-sensitive 22Na uptake. The effect of 5-HT receptor-specific agonists and antagonists was examined. The role of signaling intermediates in 5-HT-mediated effects on NHE activity was elucidated using pharmacologic inhibitors and immunoblotting.

RESULTS

NHE activity was inhibited significantly (approximately 50%-75%, P < .05) by .1 micromol/L 5-HT via inhibition of maximal velocity (Vmax) without any changes in apparent affinity (Km) for the substrate Na+ . NHE inhibition involved a decrease of both NHE2 and NHE3 activities. Studies using specific inhibitors and agonists showed that the effects of 5-HT were mediated by 5-HT4 receptors. 5-HT-mediated inhibition of NHE activity was dependent on phosphorylation of phospholipase C gamma 1 (PLC gamma 1) via activation of src-kinases. Signaling pathways downstream of PLC gamma 1 involved increase of intracellular Ca 2+ levels and subsequent activation of protein kinase C alpha (PKC alpha). The effects of 5-HT on NHE activity were not cell-line specific because T84 cells also showed NHE inhibition.

CONCLUSIONS

A better understanding of the regulation of Na+ absorption by 5-HT offers the potential for providing insights into molecular and cellular mechanisms involved in various diarrheal and inflammatory disorders.

Flow cytometric kinetic assay of the activity of Na+/H+ antiporter in mammalian cells

BACKGROUND

The Na(+)/H(+) exchanger (NHE) of mammalian cells is an integral membrane protein that extrudes H(+) ion in exchange for extracellular Na(+) and plays a crucial role in the regulation of intracellular pH (pHi). Thus, when pHi is lowered, NHE extrudes protons at a rate depending of pHi that can be expressed as pH units/s.

METHODS

To abolish the activity of other cellular pH-restoring systems, cells were incubated in bicarbonate-free Dulbecco's modified Eagle's medium buffered with HEPES. Flow cytometry was used to determine pHi with 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein acetoxymethyl ester or 5-(and-6)-carboxy SNARF-1 acetoxymethyl ester acetate, and the appropriate fluorescence ratios were measured. The calibration of fluorescence ratios versus pHi was established by using ionophore nigericin. The activity of NHE was calculated by a kinetic flow cytometric assay as the slope at time 0 of the best-fit curve of pHi recovery versus time after intracellular acidification with a pulse of exogenous sodium propionate.

RESULTS

The kinetic method allowed determination of the pHi-dependent activity of NHE in cell lines and primary cell cultures. NHE activity values were demonstrated to be up to 0.016 pH units/s within the pHi range of 7.3 to 6.3. The inhibition of NHE activity by the specific inhibitor ethyl isopropyl amiloride was easily detected by this method.

CONCLUSIONS

The assay conditions can be used to relate variations in pHi with the activity of NHE and provide a standardized method to compare between different cells, inhibitors, models of ischemia by acidification, and other relevant experimental or clinical situations.

MOLECULAR PHYSIOLOGY OF INTESTINAL N+/H+EXCHANGE

The sodium/hydrogen exchange (NHE) gene family plays an integral role in neutral sodium absorption in the mammalian intestine. The NHE gene family is comprised of nine members that are categorized by cellular localization (i.e., plasma membrane or intracellular). In the gastrointestinal (GI) tract of multiple species, there are resident plasma membrane isoforms including NHE1 (basolateral) and NHE2 (apical), recycling isoforms (NHE3), as well as intracellular isoforms (NHE6, 7, 9). NHE3 recycles between the endosomal compartment and the apical plasma membrane and functions in both locations. NHE3 regulation occurs during normal digestive processes and is often inhibited in diarrheal diseases. The C terminus of NHE3 binds multiple regulatory proteins to form large protein complexes that are involved in regulation of NHE3 trafficking to and from the plasma membrane, turnover number, and protein phosphorylation. NHE1 and NHE2 are not regulated by trafficking. NHE1 interacts with multiple regulatory proteins that affect phosphorylation; however, whether NHE1 exists in large multi-protein complexes is unknown. Although intestinal and colonic sodium absorption appear to involve at least NHE2 and NHE3, future studies are necessary to more accurately define their relative contributions to sodium absorption during human digestion and in pathophysiological conditions.

Extracellular zinc triggers ERK-dependent activation of Na+/H+ exchange in colonocytes mediated by the zinc-sensing receptor

Extracellular zinc promotes cell proliferation and its deficiency leads to impairment of this process, which is particularly important in epithelial cells. We have recently characterized a zinc-sensing receptor (ZnR) linking extracellular zinc to intracellular release of calcium. In the present study, we addressed the role of extracellular zinc, acting via the ZnR, in regulating the MAP kinase pathway and Na+/H+ exchange in colonocytes. We demonstrate that Ca2+ release, mediated by the ZnR, induces phosphorylation of ERK1/2, which is highly metal-specific, mediated by physiological concentrations of extracellular Zn2+ but not by Cd2+, Fe2+, Ni2+, or Mn2+. Desensitization of the ZnR by Zn2+, is followed by approximately 90% inhibition of the Zn2+ -dependent ERK1/2 phosphorylation, indicating that the ZnR is a principal link between extracellular Zn2+ and ERK1/2 activation. Application of both the IP3 pathway and PI 3-kinase antagonists largely inhibited Zn2+ -dependent ERK1/2 phosphorylation. The physiological significance of the Zn2+ -dependent activation of ERK1/2 was addressed by monitoring Na+/H+ exchanger activity in HT29 cells and in native colon epithelium. Preincubation of the cells with zinc was followed by robust activation of Na+/H+ exchange, which was eliminated by cariporide (0.5 microm); indicating that zinc enhances the activity of NHE1. Activation of NHE1 by zinc was totally blocked by the ERK1/2 inhibitor, U0126. Prolonged acidification, in contrast, stimulates NHE1 by a distinct pathway that is not affected by extracellular Zn2+ or inhibitors of the MAP kinase pathway. Desensitization of ZnR activity eliminates the Zn2+ -dependent, but not the prolonged acidification-dependent activation of NHE1, indicating that Zn2+ -dependent activation of H+ extrusion is specifically mediated by the ZnR. Our results support a role for extracellular zinc, acting through the ZnR, in regulating multiple signaling pathways that affect pH homeostasis in colonocytes. Furthermore activation of both, ERK and NHE1, by extracellular zinc may provide the mechanism linking zinc to enhanced cell proliferation.

Differential regulation of Na+/H+ exchange isoform activities by enteropathogenic E. coli in human intestinal epithelial cells

Enteropathogenic Escherichia coli (EPEC) is an important human intestinal foodborne pathogen associated with diarrhea, especially in infants and young children. Although EPEC produces characteristic attaching and effacing lesions and loss of microvilli, the pathophysiology of EPEC-associated diarrhea, particularly during early infection, remains elusive. The present studies were designed to examine the direct effects of EPEC infection on intestinal absorption via Na(+)/H(+) exchanger (NHE) isoforms. Caco-2 cells were infected with EPEC strain E2348/69 or nonpathogenic E. coli HB101 for a period of 60 to 120 min. Total NHE activity was significantly increased at 60 min, reaching approximately threefold increase after 90 min of EPEC infection. Similar findings were seen in HT-29 cells and T84 cells indicating that the response was not cell-line specific. Most surprising was the differential regulation of NHE2 and NHE3 by EPEC. Marked activation of NHE2 (300%) occurred, whereas significant inhibition ( approximately 50%) of NHE3 activity was induced. The activity of basolateral isoform NHE1 was also significantly increased in response to EPEC infection. Mutations that disrupted the type III secretion system (TTSS) ablated the effect of EPEC on the activity of both NHE2 and NHE3. These results suggest that EPEC, through a TTSS-dependent mechanism, exerts differential effects on NHE isoform activity in intestinal epithelial cells. Additionally, NHEs do not appear to play any role in EPEC-mediated inflammation, because the NHE inhibitors amiloride and 5-(N-ethyl-N-isopropyl)amiloride did not prevent EPEC-mediated IkappaBalpha degradation.

The Na+/H+ exchanger isoform 2 is the predominant NHE isoform in murine colonic crypts and its lack causes NHE3 upregulation

The Na(+)/H(+) exchanger isoform NHE2 is highly expressed in the intestinal tract, but its physiological role has remained obscure. The aim of this study was to define its expression, location, and regulatory properties in murine colon and to look for the compensatory changes in NHE2 (-/-) colon that allow normal histology and absorptive function. To this end, we measured murine proximal colonic surface and crypt cell NHE1, NHE2, and NHE3 expression levels, transport rates in response to acid, hyperosmolarity and cAMP in murine proximal colonic crypts, as well as changes in transcript levels and acid-activated NHE activity in NHE2 (-/-) crypts. We found that NHE2 was expressed most abundantly in crypts, NHE1 equally in crypts and surface cells, and NHE3 much stronger in surface cells. NHE2, like NHE1, was activated by low intracellular pH (pH(i)), hyperosmolarity, and cAMP, whereas NHE3 was activated only by low pH(i). Crypts isolated from NHE2 (-/-) mice displayed increased acid-activated NHE1- and NHE3-attributable Na(+)/H(+) exchange activity, no change in NHE1 expression, and NHE3 expression levels twice as high as in normal littermates. No change in cellular ultrastructure was found in NHE2 (-/-) colon. Our results demonstrate high NHE2 expression in the crypts and suggest a role for NHE2 in cryptal pH(i) and volume homeostasis.

Cdx1 or Cdx2 expression activates E-cadherin-mediated cell-cell adhesion and compaction in human COLO 205 cells

A mature columnar intestinal epithelium develops late in embryogenesis and is maintained throughout the life of the organism. Although the mechanisms driving intestine-specific gene expression have been well studied, those promoting the acquisition of cell-cell junctions, columnar morphogenesis, and polarization have been less studied. The Cdx homeodomain transcription factors (Cdx1 and Cdx2) regulate intestine-specific gene expression and intestinal epithelial differentiation. We report here that Cdx expression induces E-cadherin activity and cell-cell adhesion in human COLO 205 cancer cells. Within days of Cdx1 or Cdx2 expression, a new homotypic cell-cell adhesion phenotype is induced. This is a specific response to Cdx, inasmuch as a Cdx1 mutant failed to elicit the effect. Additionally, Cdx-expressing COLO 205 cells demonstrate a reduced proliferative capacity and an increase in the mRNA expression of differentiation-associated genes. Electron micrographs of these cells demonstrate induction of tight, adherens, and desmosomal junctions, as well as a columnar shape and apical microvilli. Investigations of the adhesion phenotype determined that it was Ca(2+) dependent and could be blocked by an E-cadherin-blocking antibody. However, E-cadherin protein levels and intracellular distribution were unchanged. Cdx expression restored the ability of the cell membranes to adhere and undergo compaction. We conclude that Cdx1 or Cdx2 expression is sufficient to induce an E-cadherin-dependent adhesion of COLO 205 cells. This adhesion is associated with polarization and cell-cell membrane compaction, as well as induction of a differentiated gene-expression pattern. Ascertaining the mechanism for this novel Cdx effect may yield insight into the development of mature colonic epithelium.

High concentration of glucose inhibits glomerular endothelial eNOS through a PKC mechanism

Kidney glomeruli are important targets of diabetic nephropathy. We hypothesized a high concentration of glucose could suppress glomerular endothelial nitric oxide synthase (eNOS) by a protein kinase C (PKC) mechanism, as has been found in other tissues. Mouse kidney slices (150-200 microm) were bathed in Hanks' solution with 100 microM L-arginine and exposed to either 5 or 20-30 mM D-glucose. Immunofluorescence identified only eNOS in normal mouse glomeruli. Measurements of glomerular NO concentration with NO-sensitive fluorescent dye (4,5-diaminofluorescein diacetate) using confocal microscopy and NO-sensitive microelectrodes verified that resting glomeruli had active production of NO that was inhibited by N(G)-nitro-L-arginine methyl ester. High-concentration (20-30 mM) D-glucose inhibited 60-70% of the NO production within 15-30 min; L-glucose at the same concentration did not have any effect. Inhibition of PKC-beta with 100 nM ruboxistaurin prevented eNOS suppression in high-glucose media. Activation of PKC with 100 nM phorbol ester also suppressed the glomerular NO concentration. We concluded that eNOS in the renal glomerular capillary endothelial cells is suppressed by activity of PKC at high-glucose concentrations comparable to those in diabetic animals and humans. The consequence is a rapid decline in the generation of NO in the glomerular endothelial cells in the presence of a high concentration of glucose.

Oral rehydration therapy: new explanations for an old remedy

Diarrheal diseases are among the most devastating illnesses globally, but the introduction of oral rehydration therapy has reduced mortality due to diarrhea from >5 million children, under the age of 5, in 1978 to 1.3 million in 2002. Variations of this simple therapy of salts and sugars are prevalent in traditional remedies in cultures world-wide, but only in the past four decades have the scientific bases for these remedies begun to be elucidated. This review aims to provide a broad understanding of the cellular basis of oral rehydration therapy. The features integral to the success of oral rehydration therapy are active glucose transport in the small intestine, commensal bacteria, and short-chain fatty acid transport in the colon. The review examines these processes and their regulation and considers new approaches that might supplement oral rehydration therapy in controlling diarrheal diseases.

Subcellular distribution of native estrogen receptor α and β subtypes in cultured human lens epithelial cells

The expression and intracellular localization of the estrogen receptor (ER) subtypes, ERalpha and ERbeta were examined in cultured human lens epithelial cells (HLE-B3). ERalpha and ERbeta mRNA expression was evaluated by coupled reverse transcription-polymerase chain reaction (RT-PCR) and Southern blot analysis. Subcellular localization of ERalpha and ERbeta was determined on formaldehyde-fixed, Saponin-permeabilized cells using conventional immunofluorescence techniques, as well as immunodetection of differential cellular components after subjecting the cultured cells to fractionation by sucrose gradient centrifugation. Using RT-PCR, ER species specific primers distinguished mRNA from total RNA extracted from HLE-B3 cells, as well as from human breast adenocarcinoma cells (MCF-7), which provided a positive control. The 286-bp (ERalpha) and 167-bp (ERbeta) PCR products were verified by sequence analysis. Southern blot analysis using internal oligonucleotides directed to specific primer pairs for ERalpha and ERbeta, respectively, further confirmed the authenticity of the PCR products. HLE-B3 cells expressed ERalpha and ERbeta in association with the nucleus and ERbeta in the mitochondria. That the mitochondrial-enriched subfraction correlated with the presence of the ERbeta subtype was confirmed by Western blot analysis. The differential subcellular partitioning of ERalpha and ERbeta subtypes lends a new aspect to the estrogen signalling system wherein mitochondrial stabilization, possibly through ER binding, may play a causal role in the maintenance of cellular integrity. The occurrence of ER subtypes in human lens epithelial cells suggests that estrogen plays a role in the physiology of the lens. The comparative nuclear and mitochondrial distribution of ERalpha and ERbeta raises the interesting probability that the two receptors, dependent upon subcellular localization, may have differential cytoprotective potential, and by inference, disparity in their mechanisms of action.

Urea movement across mouse colonic plasma membranes is mediated by UT-A urea transporters

BACKGROUND & AIMS

Urea is a major nitrogen source for commensal bacteria that inhabit the large intestine. UT-A urea transporters mediate urea movement across plasma membranes. The aim of this study was to determine whether UT-A proteins are expressed in the mouse colon and, if so, whether they have a functional role in transcellular urea transport.

METHODS

Mouse colonic UT-A transporters were investigated with Northern blot analysis, immunoblotting, immunolocalization, and refractive light flux experiments.

RESULTS

Northern blot analysis showed that 4 UT-A transcripts were present in mouse colon. Two peptide-targeted polyclonal antibodies showed the presence of UT-A immunoreactive proteins in mouse colon. Antiserum ML446 targeted to the N-terminus of mouse UT-A1 detected proteins of 34 and 48 kilodaltons. Antiserum ML194 targeted to the C-terminus of mouse UT-A1 detected proteins of 48, 75, and 100 kilodaltons. Immunolocalization studies using ML446 showed the presence of UT-A proteins in cells throughout the colonic crypts. ML194 specifically stained cells located in the proliferative and stem regions of the lower portion of colonic crypts. Differential centrifugation and immunoblotting of colonic epithelia showed that UT-A proteins were present in plasma membrane-enriched fractions. Refractive light flux experiments using colonic plasma membrane vesicles showed a significant urea flux, which was completely inhibited by the UT-A inhibitor phloretin.

CONCLUSIONS

Functional UT-A transporters are expressed in the plasma membranes of mouse colon, indicating that these proteins may play a key role in host/bacterial interaction.

NHE3 inhibition activates duodenal bicarbonate secretion in the rat

We examined the effect of inhibition of Na+/H+ exchange (NHE) on duodenal bicarbonate secretion (DBS) in rats to further understand DBS regulation. DBS was measured by using the pH-stat method and by using CO2-sensitive electrodes. 5-(N,N-dimethyl)-amiloride (50 microM; DMA), a concentration that selectively inhibits the NHE isoforms NHE1 and NHE2, but not NHE3, did not affect DBS. Nevertheless, 3 mM DMA, a higher concentration that inhibits NHE1, NHE2, and NHE3, significantly increased DBS. Moreover, S1611 and S3226, both specific inhibitors of NHE3 only, or perfusion with Na+-free solutions, dose dependently increased DBS, as measured by pH-stat and CO2-sensitive electrode, without affecting intracellular pH. Coperfusion with 0.1 microM indomethacin, 0.5 mM DIDS, or 1 mM methazolamide did not affect S3226-induced DBS. Nevertheless, coperfusion with 0.1 and 0.3 mM 5-nitro-2-(3-phenylpropylamino) benzoic acid, which inhibits the cystic fibrosis transmembrane conductor regulator (CFTR), dose dependently inhibited S3226-induced DBS. In conclusion, only specific apical NHE3 inhibition increased DBS, whereas prostaglandin synthesis, Na+-HCO3- cotransporter activation, or intracellular HCO3- formation by carbonic anhydrase was not involved. Because NHE3 inhibition-increased DBS was inhibited by an anion channel inhibitor and because reciprocal CFTR regulation has been previously shown between NHE3 and apical membrane anion transporters, we speculate that NHE3 inhibition increased DBS by altering anion transporter function.

The future of GI and liver research: editorial perspectives: I. Visions of epithelial research

Epithelial cells are gatekeepers that sit at the interface between two compartments. By controlling the flow of molecules and information between two compartments, epithelial cells provide unique benefit to the body. This article provides a brief appraisal of our current knowledge about the functions of gastrointestinal epithelial cells as a functionally diverse set of cells mediating transepithelial transport and as a continually renewing layer of cells. The convergence of new methodologies in laser capture microdissection, microarray analyses, microscopic analyses, and generation of mutant animals provides an exciting template for future research.

Transcriptional regulation of the rat NHE3 gene. Functional interactions between GATA-5 and Sp family transcription factors

Expression of sodium-hydrogen exchanger isoform 3 (NHE3) in the intestinal and renal epithelium plays a critical role in sodium absorption and acid/base homeostasis. To decipher rat NHE3 gene regulation, its cis-acting regulatory elements and associated transcription factors were characterized by transient transfection of Caco-2, IEC-6, Qt6, and Drosophila SL2 cells. Deletion and mutational analyses demonstrated that the atypical TATA box located at bp -26/-31 was not necessary for promoter activity, and that a -20/+8-bp fragment represents a functional initiator. Within the 81-bp upstream region, three Sp transcription factor binding sites were critical because their mutation drastically reduced promoter activity. The roles of Sp1 and Sp3 were further demonstrated by electromobility shift assay and by transactivation of the NHE3 promoter in SL2 cells by forced expression of Sp1 and Sp3. Both of these transcription factors were found to act synergistically with GATA-5 bound to a GATA box in exon 1 (+20/+23 bp). These studies demonstrate that rat NHE3 promoter is initiator-driven and controlled mainly by Sp1 and Sp3, which functionally interact with GATA-5. This interaction represents a novel regulatory mechanism, which is likely to participate in a gradient of intestinal gene expression along the crypt-villus axis.