Gastrointestinal distribution and kinetic characterization of the sodium-hydrogen exchanger isoform 8 (NHE8)

The Expression of NHE8 in Liver and Its Role in Carbon Tetrachloride-Induced Liver Injury

BACKGROUND AND AIMS

Sodium-hydrogen exchanger 8 (NHE8) is expressed in array of tissues and has pleiotropic functions beyond simply exchanging sodium and hydrogen across cell membrane. This study investigates the expression pattern of liver NHE8 and its roles in carbon tetrachloride (CCl4)-induced liver injury.

METHODS

NHE8 expression pattern was investigated in mouse livers of different ages and in HepG2 cells. CCl4 was given to mice to determine NHE8 expression in CCl4-induced liver injury. Tumor necrosis factor (TNF)-α and interleukin (IL)-1β were used to treat HepG2 cells to evaluate their effect on NHE8 expression. The CCl4-induced acute and chronic liver injuries were also used in NHE8KO mice to determine the role of NHE8 deficiency in liver injury.

RESULTS

NHE8 was mainly detected in the peripheral area of hepatocytes in mouse liver and in HepG2 cells. The liver NHE8 expression was 47% of NHE1, and liver NHE8 expression was the lowest at suckling age and reached plateau at 4 weeks of age. Similar to dextran sulfate sodium colitis reduced intestinal NHE8, CCl4-induced acute liver injury also inhibited NHE8 expression. The absence of NHE8 in the liver displayed abnormal hepatocyte morphology and has elevated expression of IL-1β and Lgr5. However, unlike NHE8 deficiency enhanced dextran sulfate sodium-induced colon tissue damage, the absence of NHE8 in the liver did not exacerbate CCl4-induced liver injury. Although both TNF-α and IL-1β were elevated in CCl4-induced liver injury, they could not inhibit NHE8 expression in hepatocytes, which is in contrast with TNF-α-mediated NHE8 inhibition in the intestine.

CONCLUSION

Liver NHE8 has unique roles that are different from the intestine.

The Physiological Function and Potential Role of the Ubiquitous Na+/H+ Exchanger Isoform 8 (NHE8): An Overview Data

The Na⁺/H⁺ exchanger transporters (NHE) play an important role in various biologic processes including Na⁺ absorption, intracellular pH homeostasis, cell volume regulation, proliferation, and apoptosis. The wide expression pattern and cellular localization of NHEs make these proteins pivotal players in virtually all human tissues and organs. In addition, recent studies suggest that NHEs may be one of the primeval transport protein forms in the history of life. Among the different isoforms, the most well-characterized NHEs are the Na⁺/H⁺ exchanger isoform 1 (NHE1) and Na⁺/H⁺ exchanger isoform 3 (NHE3). However, Na⁺/H⁺ exchanger isoform 8 (NHE8) has been receiving attention based on its recent discoveries in the gastrointestinal tract. In this review, we will discuss what is known about the physiological function and potential role of NHE8 in the main organ systems, including useful overviews that could inspire new studies on this multifaceted protein.

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.

Roles of Endomembrane Alkali Cation/Proton Exchangers in Synaptic Function and Neurodevelopmental Disorders

Endomembrane alkali cation (Na⁺, K⁺)/proton (H⁺) exchangers (eNHEs) are increasingly associated with neurological disorders. These eNHEs play integral roles in regulating the luminal pH, processing, and trafficking of cargo along the secretory (Golgi and post-Golgi vesicles) and endocytic (early, recycling, and late endosomes) pathways, essential regulatory processes vital for neuronal development and plasticity. Given the complex morphology and compartmentalization of multipolar neurons, the contribution of eNHEs in maintaining optimal pH homeostasis and cargo trafficking is especially significant during periods of structural and functional development and remodeling. While the importance of eNHEs has been demonstrated in a variety of non-neuronal cell types, their involvement in neuronal function is less well understood. In this review, we will discuss their emerging roles in excitatory synaptic function, particularly as it pertains to cellular learning and remodeling. We will also explore their connections to neurodevelopmental conditions, including intellectual disability, autism, and attention deficit hyperactivity disorders.

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.

Na+/H+ exchangers differentially contribute to midgut fluid sodium and proton concentration in the sea urchin larva

Regulation of ionic composition and pH is a requisite of all digestive systems in the animal kingdom. Larval stages of the marine superphylum Ambulacraria, including echinoderms and hemichordates, were demonstrated to have highly alkaline conditions in their midgut with the underlying epithelial transport mechanisms being largely unknown. Using ion-selective microelectrodes, the present study demonstrated that pluteus larvae of the purple sea urchin have highly alkaline pH (pH ∼9) and low [Na+] (∼120 mmol l-1) in their midgut fluids, compared with the ionic composition of the surrounding seawater. We pharmacologically investigated the role of Na+/H+ exchangers (NHE) in intracellular pH regulation and midgut proton and sodium maintenance using the NHE inhibitor 5-(n-ethyl-n-isopropyl)amiloride (EIPA). Basolateral EIPA application decreased midgut pH while luminal application via micro-injections increased midgut [Na+], without affecting pH. Immunohistochemical analysis demonstrated a luminal localization of NHE-2 (SpSlc9a2) in the midgut epithelium. Specific knockdown of spslc9a2 using Vivo-Morpholinos led to an increase in midgut [Na+] without affecting pH. Acute acidification experiments in combination with quantitative PCR analysis and measurements of midgut pH and [Na+] identified two other NHE isoforms, Spslc9a7 and SpSlc9a8, which potentially contribute to the regulation of [Na+] and pH in midgut fluids. This work provides new insights into ion regulatory mechanisms in the midgut epithelium of sea urchin larvae. The involvement of NHEs in regulating pH and Na+ balance in midgut fluids shows conserved features of insect and vertebrate digestive systems and may contribute to the ability of sea urchin larvae to cope with changes in seawater pH.

An indisputable role of NHE8 in mucosal protection

The loss of the intestinal Na⁺/H⁺ exchanger isoform 8 (NHE8) results in an ulcerative colitis-like condition with reduction of mucin production and dysbiosis, indicating that NHE8 plays an important role in intestinal mucosal protection. The aim of this study was to investigate the potential rebalance of the altered microbiota community of NHE8-deficient mice via fecal microbiota transplantation (FMT) and feeding probiotic VSL#3. We also aimed to stimulate mucin production by sodium butyrate administration via enema. Data from 16S rRNA sequencing showed that loss of NHE8 contributes to colonic microbial dysbiosis with reduction of butyrate-producing bacteria. FMT increased bacterial adhesion in the colon in NHE8 knockout (NHE8KO) mice. Periodic-acid Schiff reagent (PAS) stain and quantitative PCR showed no changes in mucin production during FMT. In mice treated with the probiotic VSL#3, a reduction of Lactobacillus and segmented filamentous bacteria (SFB) in NHE8KO mouse colon was detected and an increase in goblet cell theca was observed. In NHE8KO mice receiving sodium butyrate (NaB), 1 mM NaB stimulated Muc2 expression without changing goblet cell theca, but 10 mM NaB induced a significant reduction of goblet cell theca without altering Muc2 expression. Furthermore, 5 mM and 10 mM NaB-treated HT29-MTX cells displayed increased apoptosis, while 0.5 mM NaB stimulated Muc2 gene expression. These data showed that loss of NHE8 leads to dysbiosis with reduction of butyrate-producing bacteria and FMT and VSL#3 failed to rebalance the microbiota in NHE8KO mice. Therefore, FMT, VSL#3, and NaB are not able to restore mucin production in the absence of NHE8 in the intestine.NEW & NOTEWORTHY Loss of Na⁺/H⁺ exchanger isoform 8 (NHE8), a Slc9 family of exchanger that contributes to sodium uptake, cell volume regulation, and intracellular pH homeostasis, resulted in dysbiosis with reduction of butyrate-producing bacteria and decrease of Muc2 production in the intestine in mice. Introducing fecal microbiota transplantation (FMT) and VSL#3 in NHE8 knockout (NHE8KO) mice failed to rebalance the microbiota in these mice. Furthermore, administration of FMT, VSL#3, and sodium butyrate was unable to restore mucin production in the absence of NHE8 in the intestine.

Mucosal Abnormalities in Children With Congenital Chloride Diarrhea-An Underestimated Phenotypic Feature?

Objectives and Study: Congenital chloride diarrhea (CCD) is a rare, autosomal recessive disorder caused by mutations in the SLC26A3 gene encoding a transmembrane chloride/bicarbonate ion exchanger mainly expressed in the apical brush border of the ileal and colonic epithelium. Lifelong, secretory, chloride-rich diarrhea and hypochloremic, hypokalemic metabolic alkalosis are characteristic. Histological evidence of bowel inflammation is not typically described in CCD and has only been reported in a few patients. Methods: We report four cases of CCD who received adequate resuscitation with appropriate replacement of their fecal salt and water losses. Three had associated inflammatory bowel changes at endoscopy. The index case of CCD who developed frankly bloodstained diarrhea aged 7 months was found to have histologically confirmed colitis at endoscopy. An electronic search of the hospital database to identify all patients with confirmed CCD was performed. A further three children underwent de novo diagnostic evaluation and treatment. A retrospective case note review was undertaken to determine the incidence and subtype of inflammatory bowel disease (IBD) by clinical, endoscopic, and histological means. Results: Four children with genetically confirmed CCD were identified, two being female. The first girl had a granulomatous colitis with ulceration. She went into remission with a combination of steroids and azathioprine. Immunosuppression was subsequently discontinued without a further flare of colitis. A second girl was found to have patchy inflammatory changes in the small bowel and focal active colitis. A third patient, a boy, demonstrated mild inflammatory changes in the small bowel with apoptotic debris and mild inflammation in the colon. A fourth patient did not develop intestinal inflammation. Conclusion: Our case series highlights the potential association of CCD with panenteric inflammation. While our cohort was small, CCD is rare and three out of four children referred to our tertiary referral center were affected. While early diagnosis and adequate salt replacement therapy are crucial in CCD management, the clinician should also be aware of bowel inflammation as a potential cause of failure of CCD therapy to control bowel symptomatology. Further insight is needed to understand the underlying patho-mechanism giving rise to bowel inflammation in this group.

The SLC9A-C Mammalian Na+/H+ Exchanger Family: Molecules, Mechanisms, and Physiology

Na⁺/H⁺ exchangers play pivotal roles in the control of cell and tissue pH by mediating the electroneutral exchange of Na⁺ and H⁺ across cellular membranes. They belong to an ancient family of highly evolutionarily conserved proteins, and they play essential physiological roles in all phyla. In this review, we focus on the mammalian Na⁺/H⁺ exchangers (NHEs), the solute carrier (SLC) 9 family. This family of electroneutral transporters constitutes three branches: SLC9A, -B, and -C. Within these, each isoform exhibits distinct tissue expression profiles, regulation, and physiological roles. Some of these transporters are highly studied, with hundreds of original articles, and some are still only rudimentarily understood. In this review, we present and discuss the pioneering original work as well as the current state-of-the-art research on mammalian NHEs. We aim to provide the reader with a comprehensive view of core knowledge and recent insights into each family member, from gene organization over protein structure and regulation to physiological and pathophysiological roles. Particular attention is given to the integrated physiology of NHEs in the main organ systems. We provide several novel analyses and useful overviews, and we pinpoint main remaining enigmas, which we hope will inspire novel research on these highly versatile proteins.

Genome-wide association analysis uncovers variants for reproductive variation across dog breeds and links to domestication

BACKGROUND AND OBJECTIVES

The diversity of eutherian reproductive strategies has led to variation in many traits, such as number of offspring, age of reproductive maturity and gestation length. While reproductive trait variation has been extensively investigated and is well established in mammals, the genetic loci contributing to this variation remain largely unknown. The domestic dog, Canis lupus familiaris is a powerful model for studies of the genetics of inherited disease due to its unique history of domestication. To gain insight into the genetic basis of reproductive traits across domestic dog breeds, we collected phenotypic data for four traits, cesarean section rate, litter size, stillbirth rate and gestation length, from primary literature and breeders' handbooks.

METHODOLOGY

By matching our phenotypic data to genomic data from the Cornell Veterinary Biobank, we performed genome-wide association analyses for these four reproductive traits, using body mass and kinship among breeds as covariates.

RESULTS

We identified 12 genome-wide significant associations between these traits and genetic loci, including variants near CACNA2D3 with gestation length, MSRB3 and MSANTD1 with litter size, SMOC2 with cesarean section rate and UFM1 with stillbirth rate. A few of these loci, such as CACNA2D3 and MSRB3, have been previously implicated in human reproductive pathologies, whereas others have been associated with domestication-related traits, including brachycephaly (SMOC2) and coat curl (KRT71).

CONCLUSIONS AND IMPLICATIONS

We hypothesize that the artificial selection that gave rise to dog breeds also influenced the observed variation in their reproductive traits. Overall, our work establishes the domestic dog as a system for studying the genetics of reproductive biology and disease.

LAY SUMMARY

The genetic contributors to variation in mammalian reproductive traits remain largely unknown. We took advantage of the domestic dog, a powerful model system, to test for associations between genome-wide variants and four reproductive traits (cesarean section rate, litter size, stillbirth rate and gestation length) that vary extensively across breeds. We identified associations at a dozen loci, including ones previously associated with domestication-related traits, suggesting that selection on dog breeds also influenced their reproductive traits.

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.

Na+/H+ exchanger (NHE) in Pacific white shrimp (Litopenaeus vannamei): Molecular cloning, transcriptional response to acidity stress, and physiological roles in pH homeostasis

Na⁺/H⁺ exchangers are the most common membrane proteins involved in the regulation of intracellular pH that concurrently transport Na⁺ into the cells and H⁺ out of the cells. In this study, the full-length cDNA of the Na⁺/H⁺ exchanger (NHE) from the Pacific white shrimp (Litopenaeus vannamei) was cloned. The LvNHE cDNA is 3167 bp long, contains a 5’-untranslated region (UTR) of 74 bp and a 3’-UTR of 456 bp and an open reading frame (ORF) of 2637 bp, coding for a protein of 878 amino acids with 11 putative transmembrane domains and a long cytoplasmic tail. LvNHE shows high sequence homology with mud crab NHE at the amino acid level. LvNHE mRNA was detected in the hepatopancreas, gill, eyestalk, skin, heart, intestine, muscle, brain and stomach, with the highest abundance in the intestine. In the shrimp intestinal fragment cultures exposed to gradually declining pH medium (from pH 8.0 to pH 6.4), the LvNHE mRNA expression was significantly stimulated, with the highest response when incubated in pH 7.0 medium for 6 h. To investigate the functional roles of LvNHE in pH regulation at the physiological and cellular levels, the LvNHE mRNA expression was silenced by siRNA knockdown. Upon low-pH challenge, the hemolymph pH was significantly reduced in the LvNHE mRNA knockdown shrimp. In addition, knockdown of LvNHE mRNA reduced the recovery capacity of intracellular pH in intestinal fragment cultures after acidification. Altogether, this study demonstrates the role of NHE in shrimp response to low pH stress and provides new insights into the acid/base homeostasis mechanisms of crustaceans.

NHE8 Deficiency Promotes Colitis-Associated Cancer in Mice via Expansion of Lgr5-Expressing Cells

BACKGROUND & AIMS

Lgr5 overexpression has been detected in colorectal cancers (CRCs), including some cases of colitis-associated CRCs. In colitis-associated CRCs, chronic inflammation is a contributing factor in carcinogenesis. We recently reported that intestinal Na⁺/H⁺ exchanger isoform 8 (NHE8) plays an important role in intestinal mucosal protection and that loss of NHE8 expression results in an ulcerative colitis-like condition. Therefore, we hypothesized that NHE8 may be involved in the development of intestinal tumors.

METHODS

We assessed NHE8 expression in human CRCs by immunohistochemistry and studied tumor burden in NHE8 knockout (KO) mice using an azoxymethane/dextran sodium sulfate colon cancer model. We also evaluated cell proliferation in HT29^NHE8KO cells and assessed tumor growth in NOD scid gamma (NSG) mice xenografted with HT29^NHE8KO cells. To verify if a relationship exists between Lgr5 and NHE8 expression, we analyzed Lgr5 expression in NHE8KO mice by polymerase chain reaction and in situ hybridization. Lgr5 expression and cell proliferation in the absence of NHE8 were confirmed in colonic organoid cultures. The expression of β-catenin and c-Myc also were analyzed to evaluate Wnt/β-catenin activation.

RESULTS

NHE8 was undetectable in human CRC tissues. Although only 9% of NHE8 wild-type mice showed tumorigenesis in the azoxymethane/dextran sodium sulfate colon cancer model, almost 10 times more NHE8KO mice (89%) developed tumors. In the absence of NHE8, a higher colony formation unit was discovered in HT29^NHE8KO cells. In NSG mice, larger tumors developed at the site where HT29^NHE8KO cells were injected compared with HT29^{NHE8 wild type} cells. Furthermore, NHE8 deficiency resulted in increased Lgr5 expression in the colon, in HT29-derived tumors, and in colonoids. The absence of NHE8 also increased Wnt/β-catenin activation.

CONCLUSIONS

NHE8 might be an intrinsic factor that regulates Wnt/β-catenin in the intestine.

Essential function of NHE8 in mouse retina demonstrated by AAV-mediated CRISPR/Cas9 knockdown

We studied the role of sodium/proton exchanger 8 (NHE8) in retinal pigment epithelium (RPE) and photoreceptor cells of adult mouse retina by using the clustered regularly interspaced short palindromic repeats (CRISPR)-associated endonuclease (Cas)9 from Neisseria meningitidis (Nm). Specific single guide RNAs (sgRNAs) were designed to knockdown the Slc9a8 gene, which encodes the NHE8. Nuclease null NmCas9 and sgRNAs were packaged respectively using adeno-associated viral vector (AAV), and delivered into mouse eyes in vivo by subretinal injection on wild-type mice of about four-week-old when mouse retina is fully developed. Eye samples were collected four weeks after injection for phenotype examination. Real-time PCR analysis demonstrated ∼38% reduction of NHE8 transcripts in retinas injected with AAV-knockdown sgRNA and AAV-Cas9. Loss of photoreceptor cells was found in eyes injected with AAV-knockdown sgRNA and AAV-Cas9 under either the human rhodopsin promoter or the minimal chicken β-actin promoter, while normal morphology was observed in control eyes injected with AAV-Cas9 and AAV-control sgRNA; immunostaining data showed degenerating photoreceptor cells and RPE cells in eyes injected with knockdown sgRNA and Cas9 AAVs. We further determined that mutant M120K-NHE8 displayed altered intracellular pH regulation in human RPE and primary mouse RPE cells using genetically encoded pH sensor pHluorin and that primary cultured NHE8 mutant RPE cells showed different pH titration curves. These results indicate that NHE8 plays essential function in both RPE and photoreceptor cells. NHE8 dysfunction either in photoreceptor or RPE is sufficient to cause retinal degeneration in adult mice at any age.

Pathophysiology of IBD associated diarrhea

Inflammatory bowel diseases broadly categorized into Crohn's disease (CD) and ulcerative colitis (UC), are chronic inflammatory disorders of the gastrointestinal tract with increasing prevalence worldwide. The etiology of the disease is complex and involves a combination of genetic, environmental, immunological and gut microbial factors. Recurring and bloody diarrhea is the most prevalent and debilitating symptom in IBD. The pathogenesis of IBD-associated diarrhea is multifactorial and is essentially an outcome of mucosal damage caused by persistent inflammation resulting in dysregulated intestinal ion transport, impaired epithelial barrier function and increased accessibility of the pathogens to the intestinal mucosa. Altered expression and/or function of epithelial ion transporters and channels is the principle cause of electrolyte retention and water accumulation in the intestinal lumen leading to diarrhea in IBD. Aberrant barrier function further contributes to diarrhea via leak-flux mechanism. Mucosal penetration of enteric pathogens promotes dysbiosis and exacerbates the underlying immune system further perpetuating IBD associated-tissue damage and diarrhea. Here, we review the mechanisms of impaired ion transport and loss of epithelial barrier function contributing to diarrhea associated with IBD.

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.

Polymorphisms associated with adalimumab and infliximab response in moderate-to-severe plaque psoriasis

Aim: This study evaluated the influence of pharmacogenetics in psoriatic patients treated with adalimumab and/or infliximab. Materials & methods: Prospective observational study evaluating the association of 124 polymorphisms with the response to adalimumab or infliximab (PASI75) in patients with moderate-to-severe plaque psoriasis at 3 months (n = 95) and 6 months of treatment (n = 90). Significant SNPs for univariate analysis were subjected to multivariate analysis. Results: Five SNPs were associated with PASI75 at 3 months: rs6661932 (IVL), rs2546890 (IL-12B), rs2145623 (NFKBIA), rs9304742 (ZNF816A) and rs645544 (SLC9A8). Furthermore, rs1061624 (TNFR1B) was associated with PASI75 at 6 months. Conclusion: Nevertheless, these biomarkers should be validated in large-scale studies before implementation in clinical practice.

Comprehensive Evaluation of Protein Coding Mononucleotide Microsatellites in Microsatellite-Unstable Colorectal Cancer

Approximately 15% of colorectal cancers exhibit microsatellite instability (MSI), which leads to accumulation of large numbers of small insertions and deletions (indels). Genes that provide growth advantage to cells via loss-of-function mutations in microsatellites are called MSI target genes. Several criteria to define these genes have been suggested, one of them being simple mutation frequency. Microsatellite mutation rate, however, depends on the length and nucleotide context of the microsatellite. Therefore, assessing the general impact of mismatch repair deficiency on the likelihood of mutation events is paramount when following this approach. To identify MSI target genes, we developed a statistical model for the somatic background indel mutation rate of microsatellites to assess mutation significance. Exome sequencing data of 24 MSI colorectal cancers revealed indels at 54 million mononucleotide microsatellites of three or more nucleotides in length. The top 105 microsatellites from 71 genes were further analyzed in 93 additional MSI colorectal cancers. Mutation significance and estimated clonality of mutations determined the most likely MSI target genes to be the aminoadipate-semialdehyde dehydrogenase AASDH and the solute transporter SLC9A8 Our findings offer a systematic profiling of the somatic background mutation rate in protein-coding mononucleotide microsatellites, allowing a full cataloging of the true targets of MSI in colorectal cancer. Cancer Res; 77(15); 4078-88. ©2017 AACR.

Pathophysiology of Intestinal Na+/H+ exchange

Several members of the SLC9A family of Na⁺/H⁺ exchangers are expressed in the gut, with varying expression patterns and cellular localization. Not only do they participate in the regulation of basic epithelial cell functions, including control of transepithelial Na⁺ absorption, intracellular pH (pH _i ), cell volume, and nutrient absorption, but also in cellular proliferation, migration, and apoptosis. Additionally, they modulate the extracellular milieu in order to facilitate other nutrient absorption and to regulate the intestinal microbial microenvironment. Na⁺/H⁺ exchangers are frequent targets of inhibition in gastrointestinal pathologies, either by intrinsic factors (e.g. bile acids, inflammatory mediators) or infectious agents and associated microbial toxins. Based on emerging evidence, disruption of NHE activity via impaired expression or function of respective isoforms may contribute not only to local and systemic electrolyte imbalance, but also to the disease severity via multiple mechanisms. Here, we review the current state of knowledge about the roles Na⁺/H⁺ exchangers play in the pathogenesis of disorders of diverse origin and affecting a range of GI tissues.

Role of epithelial ion transports in inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic inflammatory disorder with a complex pathogenesis. Diarrhea is a highly prevalent and often debilitating symptom of IBD patients that results, at least in part, from an intestinal hydroelectrolytic imbalance. Evidence suggests that reduced electrolyte absorption is more relevant than increased secretion to this disequilibrium. This systematic review analyses and integrates the current evidence on the roles of epithelial Na(+)-K(+)-ATPase (NKA), Na(+)/H(+) exchangers (NHEs), epithelial Na(+) channels (ENaC), and K(+) channels (KC) in IBD-associated diarrhea. NKA is the key driving force of the transepithelial ionic transport and its activity is decreased in IBD. In addition, the downregulation of apical NHE and ENaC and the upregulation of apical large-conductance KC all contribute to the IBD-associated diarrhea by lowering sodium absorption and/or increasing potassium secretion.

Intestinal NHE8 is highly expressed in goblet cells and its expression is subject to TNF-α regulation

While the intestine plays an important role in digestion and absorption, the mucus lining the epithelium represents a pivotal function in mucosal protection. Goblet cells are scattered in both the crypts and among enterocytes, and they secrete an important component of mucus, mucin. We have reported that sodium/hydrogen exchanger (NHE) 8 is a novel player in mucosal protection, since loss of NHE8 function resulted in reduced mucin production and increased bacterial adhesion. While NHE8 has been shown to be expressed in enterocytes and its expression is reduced during intestinal inflammation, nothing is known about the role of NHE8 in goblet cells. This current study is designed to define the expression of NHE8 and the role of TNF-α in the regulation of NHE8 in goblet cells. Using HT29-MTX cells as an in vitro model, we detected abundant NHE8 mRNA in goblet cells. Immunohistochemical staining localized NHE8 protein on the plasma membrane and in the intracellular compartments in goblet cells. Furthermore, NHE8 expression in goblet cells is regulated by the proinflammatory cytokine TNF-α. The expression of NHE8 in HT29-MTX cells was significantly reduced at both mRNA and protein levels in the presence of TNF-α. This inhibition of NHE8 mRNA expression could be blocked by the transcriptional inhibitor actinomycin D. Promoter reporter assay showed that NHE8 promoter activity was indeed reduced by TNF-α. Mechanistically, TNF-α reduced Sp3 protein binding to the human NHE8 basal promoter region. Therefore, NHE8 is expressed in goblet cells, and the inflammatory cytokine TNF-α downregulates NHE8 expression by a transcriptional mechanism.

Loss of NHE8 expression impairs intestinal mucosal integrity

The newest member of the Na(+)/H(+) exchanger (NHE) family, NHE8, is abundantly expressed at the apical membrane of the intestinal epithelia. We previously reported that mucin 2 expression was significantly decreased in the colon in NHE8(-/-) mice, suggesting that NHE8 is involved in intestinal mucosal protection. In this study, we further evaluated the role of NHE8 in intestinal epithelial protection after dextran sodium sulfate (DSS) challenge. Compared with wild-type mice, NHE8(-/-) mice have increased bacterial adhesion and inflammation, especially in the distal colon. NHE8(-/-) mice are also susceptible to DSS treatment. Real-time PCR detected a remarkable increase in the expression of IL-1β, IL-6, TNF-α, and IL-4 in DSS-treated NHE8(-/-) mice compared with DSS-treated wild-type littermates. Immunohistochemistry showed a disorganized epithelial layer in the colon of NHE8(-/-) mice. Periodic acid-Schiff staining showed a reduction in the number of mature goblet cells and the area of the goblet cell theca in NHE8(-/-) mice. Phyloxine/tartrazine staining revealed a decrease in functional Paneth cell population in the NHE8(-/-) small intestinal crypt. The expression of enteric defensins was also decreased in NHE8(-/-) mice. The reduced mucin production in goblet cells and antimicrobial peptides production in Paneth cells lead to disruption of the intestinal mucosa protection. Therefore, NHE8 may be involved in the establishment of intestinal mucosal integrity by regulating the functions of goblet and Paneth cells.

Sodium butyrate stimulates NHE8 expression via its role on activating NHE8 basal promoter activity

Butyrate is a major metabolite in colonic lumen. It is produced from bacterial fermentation of dietary fiber. Butyrate has been shown to stimulate electroneutral sodium absorption through its regulation on sodium/hydrogen exchanger 3 (NHE3). Although NHE8, the newest addition of intestinal NHE family, is involved in sodium absorption in the intestinal tract, whether butyrate modulates NHE8 expression in the intestinal epithelial cells is not known. In the current study, we showed that butyrate treatment strongly induced NHE8 protein and NHE8 mRNA expression in human intestinal epithelial cells. Transfection with the human NHE8 promoter reporter constructs showed that butyrate treatment stimulated reporter gene expression at an amount comparable with its stimulation of NHE8 mRNA expression. Interestingly, a similar result was also observed in human NHE8 promoter transfected cells after trichostatin (TSA) treatment. Gel mobility shift assay identified an enhanced Sp3 protein binding on the human NHE8 basal promoter region upon butyrate stimulation. Furthermore, Sp3 acetylation modification is involved in butyrate-mediated NHE8 activation in Caco-2 cells. Our findings suggest that the mechanism of butyrate action on NHE8 expression involves enhanced Sp3 interaction at the basal promoter region of the human NHE8 gene promoter to activate NHE8 gene transcription. Thus butyrate is involved in intestinal regulation of NHE8 resulting enhanced sodium absorption.

Na+/H+ exchangers

Tightly coupled exchange of Na(+) for H(+) occurs across the surface membrane of virtually all living cells. For years, the underlying molecular entity was unknown and the full physiological significance of the exchange process was not appreciated, but much knowledge has been gained in the last two decades. We now realize that, unlike most of the other transporters that specialize in supporting one specific function, Na(+)/H(+) exchangers (NHE) participate in a remarkable assortment of physiological processes, ranging from pH homeostasis and epithelial salt transport, to systemic and cellular volume regulation. In parallel, we have learned a great deal about the biochemistry and molecular biology of Na(+)/H(+) exchange. Indeed, it has now become apparent that exchange is mediated not by one, but by a diverse family of related yet distinct carriers (antiporters) sometimes present in different cell types and located in various intracellular compartments. Each one of these has unique structural features that dictate its functional role and mode of regulation. The biological relevance of Na(+)/H(+) exchange is emphasized by its evolutionary conservation; analogous exchangers are present from bacteria to man. Because of its wide distribution and versatile function, Na(+)/H(+) exchange has attracted an enormous amount of interest and therefore generated a vast literature. The vastness and complexity of the field has been compounded by the multiplicity of NHE isoforms. For reasons of space and in the spirit of this series, this overview is restricted to the family of mammalian NHEs.

Traditional and emerging roles for the SLC9 Na+/H+ exchangers

The SLC9 gene family encodes Na(+)/H(+) exchangers (NHEs). These transmembrane proteins transport ions across lipid bilayers in a diverse array of species from prokaryotes to eukaryotes, including plants, fungi, and animals. They utilize the electrochemical gradient of one ion to transport another ion against its electrochemical gradient. Currently, 13 evolutionarily conserved NHE isoforms are known in mammals [22, 46, 128]. The SLC9 gene family (solute carrier classification of transporters: www.bioparadigms.org) is divided into three subgroups [46]. The SLC9A subgroup encompasses plasmalemmal isoforms NHE1-5 (SLC9A1-5) and the predominantly intracellular isoforms NHE6-9 (SLC9A6-9). The SLC9B subgroup consists of two recently cloned isoforms, NHA1 and NHA2 (SLC9B1 and SLC9B2, respectively). The SLC9C subgroup consist of a sperm specific plasmalemmal NHE (SLC9C1) and a putative NHE, SLC9C2, for which there is currently no functional data [46]. NHEs participate in the regulation of cytosolic and organellar pH as well as cell volume. In the intestine and kidney, NHEs are critical for transepithelial movement of Na(+) and HCO3(-) and thus for whole body volume and acid-base homeostasis [46]. Mutations in the NHE6 or NHE9 genes cause neurological disease in humans and are currently the only NHEs directly linked to human disease. However, it is becoming increasingly apparent that members of this gene family contribute to the pathophysiology of multiple human diseases.

SLC9/NHE gene family, a plasma membrane and organellar family of Na⁺/H⁺ exchangers

This brief review of the human Na/H exchanger gene family introduces a new classification with three subgroups to the SLC9 gene family. Progress in the structure and function of this gene family is reviewed with structure based on homology to the bacterial Na/H exchanger NhaA. Human diseases which result from genetic abnormalities of the SLC9 family are discussed although the exact role of these transporters in causing any disease is not established, other than poorly functioning NHE3 in congenital Na diarrhea.

NHE8 plays an important role in mucosal protection via its effect on bacterial adhesion

The Na⁺/H⁺ exchanger NHE8 is expressed on the apical membrane of intestinal epithelial cells and is particularly abundant in the colon. Our previous study showed that Muc2 expression was significantly reduced in NHE8-knockout (NHE8-/-) mice, suggesting that NHE8 plays a role in mucosal protection in the colon. The current study confirms and extends our studies on the role of NHE8 in mucosal protection. The number of bacteria attached on the distal colon was significantly increased in NHE8-/- mice compared with their wild-type littermates. As expected, IL-4 expression was markedly increased in NHE8-/- mice compared with wild-type mice. Immunohistochemistry showed disorganization in the mucin layer of NHE8-/- mice, suggesting a possible direct bacteria-epithelia interaction. Furthermore, NHE8-/- mice were susceptible to dextran sodium sulfate-induced mucosal injury. In wild-type mice, dextran sodium sulfate treatment inhibited colonic NHE8 expression. In Caco-2 cells, the absence of NHE8 expression resulted in higher adhesion rates of Salmonella typhimurium but not Lactobacillus plantarum. Similarly, in vivo, S. typhimurium adhesion rate was increased in NHE8-/- mice compared with wild-type mice. Our study suggests that NHE8 plays important roles in protecting intestinal epithelia from infectious bacterial adherence.

NHE8 plays important roles in gastric mucosal protection

Sodium/hydrogen exchanger (NHE) 8 is an apically expressed membrane protein in the intestinal epithelial cells. It plays important roles in sodium absorption and bicarbonate secretion in the intestine. Although NHE8 mRNA has been detected in the stomach, the precise location and physiological role of NHE8 in the gastric glands remain unclear. In the current study, we successfully detected the expression of NHE8 in the glandular region of the stomach by Western blotting and located NHE8 protein at the apical membrane in the surface mucous cells by a confocal microscopic method. We also identified the expression of downregulated-in-adenoma (DRA) in the surface mucous cells in the stomach. Using NHE8(-/-) mice, we found that NHE8 plays little or no role in basal gastric acid production, yet NHE8(-/-) mice have reduced gastric mucosal surface pH and higher incidence of developing gastric ulcer. DRA expression was reduced significantly in the stomach in NHE8(-/-) mice. The propensity for gastric ulcer, reduced mucosal surface pH, and low DRA expression suggest that NHE8 is indirectly involved in gastric bicarbonate secretion and gastric mucosal protection.

Lactobacillus acidophilus upregulates intestinal NHE3 expression and function

A major mechanism of electroneutral NaCl absorption in the human ileum and colon involves coupling of Na(+)/H(+) and Cl(-)/HCO(3)(-) exchangers. Disturbances in these mechanisms have been implicated in diarrheal conditions. Probiotics such as Lactobacillus have been indicated to be beneficial in the management of gastrointestinal disorders, including diarrhea. However, the molecular mechanisms underlying antidiarrheal effects of probiotics have not been fully understood. We have previously demonstrated Lactobacillus acidophilus (LA) to stimulate Cl(-)/HCO3- exchange activity via an increase in the surface levels and expression of the Cl(-)/HCO3- exchanger DRA in vitro and in vivo. However, the effects of LA on NHE3, the Na(+)/H(+) exchanger involved in the coupled electroneutral NaCl absorption, are not known. Current studies were, therefore, undertaken to investigate the effects of LA on the function and expression of NHE3 and to determine the mechanisms involved. Treatment of Caco2 cells with LA or its conditioned culture supernatant (CS) for 8-24 h resulted in a significant increase in Na(+)/H(+) exchange activity, mRNA, and protein levels of NHE3. LA-CS upregulation of NHE3 function and expression was also observed in SK-CO15 cells, a human colonic adenocarcinoma cell line. Additionally, LA treatment increased NHE3 promoter activity, suggesting involvement of transcriptional mechanisms. In vivo, mice gavaged with live LA showed significant increase in NHE3 mRNA and protein expression in the ileum and colonic regions. In conclusion, LA-induced increase in NHE3 expression may contribute to the upregulation of intestinal electrolyte absorption and might underlie the potential antidiarrheal effects of probiotics.

Proximal tubule Na+/H+ exchanger activity in adult NHE8-/-, NHE3-/-, and NHE3-/-/NHE8-/- mice

NHE3 is the predominant Na(+)/H(+) exchanger on the brush-border membrane (BBM) of the proximal tubule in adults. However, NHE3 null mice still have significant renal BBM Na(+)/H(+) activity. NHE8 has been localized to the BBM of proximal tubules and is more highly expressed in neonates than adult animals. The relative role of NHE8 in adult renal H(+) transport is unclear. This study examined whether there was compensation by NHE8 in NHE3(-/-) mice and by NHE3 in NHE8(-/-) mice. NHE3(-/-) mice had significant metabolic acidosis, and renal BBM NHE8 protein abundance was greater in NHE3(-/-) mice than control mice, indicating that there may be compensation by NHE8 in NHE3(-/-) mice. NHE8(-/-) mice had serum bicarbonate levels and pH that were not different from controls. NHE3 protein expression on the BBM was greater in NHE8(-/-) mice than in wild-type mice, indicating that there may be compensation by NHE3 in NHE8(-/-) mice. Both BBM NHE3 and NHE8 protein abundance increased in response to acidosis. Blood pressure and Na(+)/H(+) exchanger activity were comparable in NHE8(-/-) mice to that of controls, but both were significantly lower in NHE3(-/-) mice compared with control mice. Compared with NHE3(-/-) mice, NHE3(-/-)/NHE8(-/-) mice had lower blood pressures. While serum bicarbonate was comparable in NHE3(-/-) mice and NHE3(-/-)/NHE8(-/-) mice, proximal tubule Na(+)/H(+) exchange activity was less in NHE3(-/-)/NHE8(-/-) mice compared with NHE3(-/-) mice. In conclusion, NHE3 is the predominant Na(+)/H(+) exchanger in adult mice. NHE8 may play a compensatory role in renal acidification and blood pressure regulation in NHE3(-/-) mice.

Impaired mucin synthesis and bicarbonate secretion in the colon of NHE8 knockout mice

Sodium/hydrogen exchanger 8 (NHE8), the newest member of the SLC9 family, is expressed at the apical membrane of the epithelial cells in the intestine and the kidney. Although NHE8 has been shown to be an important player for intestinal sodium absorption early in development, its physiological role in the intestine remains unclear. Here, we successfully created a NHE8 knockout (NHE8(-/-)) mouse model to study the function of this transporter in the intestinal tract. Embryonic stem cells containing interrupted NHE8 gene were injected into mouse blastocyst to produce NHE8(+/-) chimeras. NHE8(-/-) mice showed no lethality during embryonic and fetal development. These mice had normal serum sodium levels and no signs of diarrhea. Apically expressed NHE2 and NHE3 were increased in the small intestine of the NHE8(-/-) mice in compensation. The number of goblet cells and mucin (MUC)-positive cells in the colon was reduced in NHE8(-/-) mice along with mucosal pH, MUC2 expression as well as downregulated in adenoma (DRA) expression. Therefore, the role of NHE8 in the intestine involves both sodium absorption and bicarbonate secretion.

Human intestinal epithelial cell line SK-CO15 is a new model system to study Na(+)/H(+) exchanger 3

The Caco-2 cell line represents absorptive polarized intestinal epithelial cells that express multiple forms of Na(+)/H(+) exchanger (NHE) in their plasma membranes. Caco-2 cells express the major apical NHE isoform NHE3, but low NHE3 expression together with inefficient transfection often hamper intended studies. In this study, we examined whether SK-CO15 cells could be used to study NHE3 regulation. SK-CO15 cells grown on Transwell inserts developed polarized epithelial cells with microvilli. The transfection efficiency of SK-CO15 cells was markedly higher compared with Caco-2 cells, an advantage in gene transfer and knockout. SK-CO15 cells expressed NHE1, NHE2, and NHE3. NHE3 expression was significantly greater in these cells than Caco-2, and NHE3 comprised more than half of total NHE activity. Apical expression of NHE3 in SK-CO15 cells was confirmed by confocal immunofluorescence and surface biotinylation. NHE regulatory factors NHERF1 and NHERF2, which are important for regulation of NHE3 activity, were expressed in these cells. Stimulatory response of NHE3 in SK-CO15 cells was assessed by dexamethasone and lysophosphatidic acid (LPA). Treatment with dexamethasone for 24-48 h increased NHE3 expression and activity. Similarly to Caco-2 cells, SK-CO15 cells lacked the expression of the LPA receptor LPA(5,) but exogenous expression of LPA(5) resulted in acute stimulation of NHE3. Forskolin acutely inhibited NHE3 activity in SK-CO15 cells, further attesting the validity of these cells. We conclude that SK-CO15 cells with the amenity for transfection and high endogenous NHE3 expression are a new and better cell model for NHE3 regulatory investigation than widely used Caco-2 cells.

Functional role of NHE4 as a pH regulator in rat and human colonic crypts

To regulate ionic and fluid homeostasis, the colon relies upon a series of Na(+)-dependent transport proteins. Recent studies have identified a sodium/hydrogen exchanger (NHE) 4 (NHE4) protein in the gastrointestinal tract but to date there has been little description of its function. Additionally, we have previously shown that aldosterone can rapidly modulate Na(+)-dependent proton excretion via NHE proteins. In this study we examined the role of NHE4 in rat and human colonic crypts, determined the effect of aldosterone on NHE4 specifically, and explored the intracellular pathways leading to activation. Colonic samples were dissected from Sprague-Dawley rats. Human specimens were obtained from patients undergoing elective colon resections. Crypts were isolated using ethylenediaminetetraacetic acid and intracellular pH (pH(i)) changes were monitored using 2'-7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Crypts were exposed to 7 μM ethylisopropylamiloride or 400 μM amiloride, doses previously shown to inhibit NHE1 and NHE3 but allow NHE4 to remain active. Functional NHE4 activity was demonstrated in both rat and human colonic crypts. NHE4 activity was increased in the presence of 1 μM aldosterone. In the rat model, crypts were exposed to 100 μM 3-isobutyl-1-methylxanthine/1 μM forskolin and demonstrated a decrease in NHE4 activity with increased cAMP levels. No significant change in NHE4 activity was seen by increasing osmolarity. These results demonstrate functional NHE4 activity in the rat and human colon and an increase in activity by aldosterone. This novel exchanger is capable of modulating intracellular pH over a wide pH spectrum and may play an important role in maintaining cellular pH homeostasis.

Nucleotide sequence of the Na+/H+ exchanger-8 in patients with congenital sodium diarrhea

Sodium absorption by the intestine is mediated by brush border Na/H exchangers, which include the NHE3 and NHE8 isoforms. We demonstrated a maturational decrease in NHE8 and increase in NHE3 in mouse intestine mRNA abundance and brush border membrane protein abundance, indicating a developmental switch of isoforms. Congenital sodium diarrhea is a rare autosomal recessive disorder characterized by polyhydramnios, hyponatremia, metabolic acidosis, and diarrhea with a high sodium content. Previous studies using intestinal brush border membrane vesicles from patients with this disorder have demonstrated a decrease in Na/H exchanger activity. Because some patients with congenital sodium diarrhea improve with age and knowing the developmental switch from NHE8 to NHE3, NHE8 may be a candidate gene for this disorder. We sequenced NHE8 from 5 patients with this disorder and found no disease-causing homozygous mutations. Although brush border membrane Na/H exchange activity may be decreased, exonic mutations in NHE8 cannot account for this disorder in these subjects.

Transcriptional regulation of the intestinal luminal Na⁺ and Cl⁻ transporters

The epithelial apical membrane Na+/H+ exchangers [NHE (sodium hydrogen exchanger)2 and NHE3] and Cl-/HCO3- exchangers [DRA (down-regulated in adenoma) and PAT-1 (putative anion transporter 1)] are key luminal membrane transporters involved in electroneutral NaCl absorption in the mammalian intestine. During the last decade, there has been a surge of studies focusing on the short-term regulation of these electrolyte transporters, particularly for NHE3 regulation. However, the long-term regulation of the electrolyte transporters, involving transcriptional mechanisms and transcription factors that govern their basal regulation or dysregulation in diseased states, has only now started to unfold with the cloning and characterization of their gene promoters. The present review provides a detailed analysis of the core promoters of NHE2, NHE3, DRA and PAT-1 and outlines the transcription factors involved in their basal regulation as well as in response to both physiological (butyrate, protein kinases and probiotics) and pathophysiological (cytokines and high levels of serotonin) stimuli. The information available on the transcriptional regulation of the recently identified NHE8 isoform is also highlighted. Therefore the present review bridges a gap in our knowledge of the transcriptional mechanisms underlying the alterations in the gene expression of intestinal epithelial luminal membrane Na+ and Cl- transporters involved in electroneutral NaCl absorption. An understanding of the mechanisms of the modulation of gene expression of these transporters is important for a better assessment of the pathophysiology of diarrhoea associated with inflammatory and infectious diseases and may aid in designing better management protocols.

NHE2X3 DKO mice exhibit gender-specific NHE8 compensation

NHE8, the newest member of the sodium/hydrogen exchanger family, is expressed in the epithelial cells of the intestine and the kidney. Intestinal expression of NHE8 is significantly higher than that of NHE2 and NHE3 at a young age, suggesting that NHE8 is an important player for intestinal sodium absorption during early development. The current study was designed to explore if NHE8 plays a compensatory role for the loss of NHE2 and NHE3 function in NHE2X3 double-knockout (NHE2X3 DKO) mice. We further explored the regulatory mechanism(s) responsible for the change in NHE8 expression in NHE2X3 DKO mice. We found that >95% of NHE2X3 DKO mice survived through weanling. However, only 60% of male NHE2X3 DKO mice and 88% of female NHE2X3 DKO mice survived to 6 wk of life. We also found that the expression of NHE8 in wild-type female mice was higher compared with wild-type male mice after puberty. In NHE2X3 KDO mice, NHE8 expression was increased in females but not in males. Using Caco-2 cells as a model of the small intestine, we showed that testosterone inhibited endogenous NHE8 expression by reducing NHE8 mRNA synthesis, whereas estrogen had no effect on NHE8 expression. Thus our data show for the first time that intestinal NHE8 has a compensatory role in NHE2X3 DKO mice and this regulation is gender-dependent.

Recent advances in the molecular and functional characterization of acid/base and electrolyte transporters in the basolateral membranes of gastric and duodenal epithelial cells

All segments of the gastrointestinal tract are comprised of an elaborately folded epithelium that expresses a variety of cell types and performs multiple secretory and absorptive functions. While the apical membrane expresses the electrolyte transporters that secrete or absorb electrolytes and water, basolateral transporters regulate the secretory or absorptive rates. During gastric acid formation, Cl⁻/HCO₃⁻ and Na(+) /H(+) exchange and other transporters secure Cl⁻ re-supply as well as pH and volume regulation. Gastric surface cells utilize ion transporters to secrete HCO₃⁻, maintain pH(i) during a luminal acid load and repair damaged surface areas during the process of epithelial restitution. Na(+)/H(+) exchange and Na(+)/HCO₃⁻ cotransport serve basolateral acid/base import for gastroduodenal HCO₃⁻ secretion. The gastric and duodenal epithelium also absorbs salt and water. Recent molecular information on novel ion transporters expressed in the gastric and duodenal epithelium has exploded; however, a function has not been found yet for all transporters. The purpose of this review is to summarize current knowledge on the molecular identity and cellular function of basolateral ion transporters in the gastric and duodenal epithelium.

Somatostatin stimulates intestinal NHE8 expression via p38 MAPK pathway

Diarrhea is a common manifestation of gastrointestinal disorders. Diarrhea-induced losses of fluid and electrolyte could lead to dehydration and electrolyte imbalances, resulting in significant morbidity and mortality, especially in children living in developing countries. Somatostatin, a peptide hormone secreted by D-cells, plays an important role in regulating motility and intestinal Na(+) absorption. Although octreotide, a somatostatin analog, is used to treat diarrhea, its mechanisms of action are unclear. Here we showed that octreotide increased brush-border membrane Na(+)/H(+) exchanger 8 (NHE8) expression in the small intestine to the exclusion of other NHEs that participate in Na(+) absorption. The same effect also occurred in human intestinal cells (Caco-2). We found that the increase of NHE8 expression by somatostatin required p38 mitogen-activated protein kinase (MAPK) activation. Furthermore, the somatostatin receptor SSTR2 antagonist CYN154806 could abolish somatostatin-induced NHE8 expression and p38 MAPK phosphorylation. Thus our data provided the first concrete evidence indicating that somatostatin stimulates intestinal Na(+) absorption by increasing intestinal NHE8 expression through the SSTR2-p38 MAPK pathway.

Transcriptional inhibition of intestinal NHE8 expression by glucocorticoids involves Pax5

Sodium/hydrogen exchangers (NHEs) are a family of proteins that transport sodium ions into the cells by moving protons out of the cells. They play a major role in sodium absorption, cell volume regulation, and intracellular pH regulation. Three out of nine identified NHEs (NHE2, NHE3, and NHE8) are expressed on the apical membrane of intestinal epithelial cells. Glucocorticoids have been found to regulate NHE3 function in the intestine, but it is unknown if they have a similar function on NHE8 expression. Interestingly, high glucocorticoid levels in the intestine coincide chronologically with the change from high expression of NHE8 to high expression of NHE3. Studies were performed to explore the role of glucocorticoids on NHE8 expression during intestinal maturation. Brush-border membrane vesicles were isolated from intestinal epithelia, and Western blotting was performed to determine NHE8 protein expression of suckling male rats treated with methylpredisolone. Real-time PCR was used to quantitate NHE8 mRNA expression in rats and Caco-2 cells. Human NHE8 promoter activity was characterized through transfection of Caco-2 cells. Gel mobility shift assays (GMSAs) were used to identify the promoter sequences and the transcription factors involved in glucocorticoid-mediated regulation. Our results showed that the expression of NHE8 mRNA and protein was decreased in glucocorticoid-treated rats and human intestinal epithelial cells (Caco-2). The activity of the human NHE8 gene promoter transfected in Caco-2 cells was also reduced by glucocorticoid treatment. GMSAs suggested that the reduction in promoter activity in the presence of glucocorticoids was due to enhanced transcription factor Pax5 binding on the NHE8 proximal promoter region. In conclusion, this study showed that glucocorticoids inhibit NHE8 gene expression by increasing Pax5 binding on NHE8 gene promoter, suggesting an important role for Pax5 during intestinal maturation.

Effect of metabolic acidosis on neonatal proximal tubule acidification

The serum bicarbonate in neonates is lower than adults due in large part to a lower rate of proximal tubule acidification. It is unclear if the neonatal proximal tubule is functioning at maximal capacity or if the proximal tubule can respond to metabolic acidosis as has been described in adult proximal tubules. We find that neonatal mouse brush-border membranes have a lower Na(+)/H(+) exchanger (NHE) 3 protein abundance (neonate 0.11 ± 0.05 vs. adult 0.64 ± 0.07; P < 0.05) and a higher NHE8 protein abundance (neonate 1.0 ± 0.01 vs. adult 0.13 ± 0.09; P < 0.001) compared with adults. To examine if neonates can adapt to acidosis, neonatal mice were gavaged with either acid or vehicle for 4 days, resulting in a drop in serum bicarbonate from 19.5 ± 1.0 to 8.9 ± 0.6 meq/l (P < 0.001). Proximal convoluted tubule Na(+)/H(+) exchanger activity (dpH(i)/dt) was 1.68 ± 0.19 pH units/min in control tubules and 2.49 ± 0.60 pH units/min in acidemic neonatal mice (P < 0.05), indicating that the neonatal proximal tubule can respond to metabolic acidosis with an increase in Na(+)/H(+) exchanger activity. Similarly, brush-border membrane vesicles from neonatal rats had an increase in Na(+)/H(+) exchanger activity with acidemia that was almost totally inhibited by 10(-6) M 5-(N-ethyl-n-isopropyl)-amiloride, a dose that has little effect on NHE3 but inhibits NHE8. There was a significant increase in both NHE3 (vehicle 0.35 ± 0.07 vs. acid 0.73 ± 0.07; P < 0.003) and NHE8 brush-border membrane protein abundance (vehicle 0.41 ± 0.05 vs. acid 0.73 ± 0.06; P < 0.001) in acidemic mouse neonates compared with controls. A comparable increase in NHE3 and NHE8 was found in neonatal rats with acidosis. In conclusion, the neonatal proximal tubule can adapt to metabolic acidosis with an increase in Na(+)/H(+) exchanger activity.

The sodium/proton exchanger NHE8 regulates late endosomal morphology and function

The pH and lumenal environment of intracellular organelles is considered essential for protein sorting and trafficking through the cell. We provide the first evidence that a mammalian sodium (potassium)/proton exchanger, NHE8, plays a key role in the control of protein trafficking and endosome morphology.

The pH and lumenal environment of intracellular organelles is considered essential for protein sorting and trafficking through the cell. We provide the first evidence that a mammalian NHE sodium (potassium)/proton exchanger, NHE8, plays a key role in the control of protein trafficking and endosome morphology. At steady state, the majority of epitope-tagged NHE8 was found in the trans-Golgi network of HeLa M-cells, but a proportion was also localized to multivesicular bodies (MVBs). Depletion of NHE8 in HeLa M-cells with siRNA resulted in the perturbation of MVB protein sorting, as shown by an increase in epidermal growth factor degradation. Additionally, NHE8-depleted cells displayed striking perinuclear clustering of endosomes and lysosomes, and there was a ninefold increase in the cellular volume taken up by LAMP1/LBPA-positive, dense MVBs. Our data points to a role for the ion exchange activity of NHE8 being required to maintain endosome morphology, as overexpression of a nonfunctional point mutant protein (NHE8 E225Q) resulted in phenotypes similar to those seen after siRNA depletion of endogenous NHE8. Interestingly, we found that depletion of NHE8, despite its function as a sodium (potassium)/proton antiporter, did not affect the overall pH inside dense MVBs.

Toxin mediated diarrhea in the 21 century: the pathophysiology of intestinal ion transport in the course of ETEC, V. cholerae and rotavirus infection

An estimated 4 billion episodes of diarrhea occur each year. As a result, 2–3 million children and 0.5–1 million adults succumb to the consequences of this major healthcare concern. The majority of these deaths can be attributed to toxin mediated diarrhea by infectious agents, such as E. coli, V. cholerae or Rotavirus. Our understanding of the pathophysiological processes underlying these infectious diseases has notably improved over the last years. This review will focus on the cellular mechanism of action of the most common enterotoxins and the latest specific therapeutic approaches that have been developed to contain their lethal effects.

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.

NHE3 regulatory complexes

The epithelial brush border Na/H exchanger NHE3 is active under basal conditions and functions as part of neutral NaCl absorption in the intestine and renal proximal tubule, where it accounts for the majority of total Na absorbed. NHE3 is highly regulated. Both stimulation and inhibition occur post-prandially. This digestion related regulation of NHE3 is mimicked by multiple extracellular agonists and intracellular second messengers. The regulation of NHE3 depends on its C-terminal cytoplasmic domain, which acts as a scaffold to bind multiple regulatory proteins and links NHE3 to the cytoskeleton. The cytoskeletal association occurs by both direct binding to ezrin and by indirect binding via ezrin binding to the C-terminus of the multi-PDZ domain containing proteins NHERF1 and NHERF2. This is a review of the domain structure of NHE3 and of the scaffolding function and role in the regulation of NHE3 of the NHE3 C-terminal domain.

High rates of HCO3- secretion and Cl- absorption against adverse gradients in the marine teleost intestine: the involvement of an electrogenic anion exchanger and H+-pump metabolon?

Anion exchange contributes significantly to intestinal Cl(-) absorption in marine teleost fish and is thus vital for successful osmoregulation. This anion exchange process leads to high luminal HCO(3)(-) concentrations (up to approximately 100 mmol l(-1)) and high pH and results in the formation of CaCO(3) precipitates in the intestinal lumen. Recent advances in our understanding of the transport processes involved in intestinal anion exchange in marine teleost fish include the demonstration of a role for the H(+)-pump (V-ATPase) in apical H(+) extrusion and the presence of an electrogenic (nHCO(3)(-)/Cl(-)) exchange protein (SLC26a6). The H(+)-V-ATPase defends against cellular acidification, which might otherwise occur as a consequence of the high rates of base secretion. In addition, apical H(+) extrusion probably maintains lower HCO(3)(-) concentrations in the unstirred layer at the apical surface than in the bulk luminal fluids and thus facilitates continued anion exchange. Furthermore, H(+)-V-ATPase activity hyperpolarizes the apical membrane potential that provides the driving force for apical electrogenic nHCO(3)(-)/Cl(-) exchange, which appears to occur against both Cl(-) and HCO(3)(-) electrochemical gradients. We propose that a similar coupling between apical H(+) extrusion and nHCO(3)(-)/Cl(-) exchange accounts for Cl(-) uptake in freshwater fish and amphibians against very steep Cl(-) gradients.

Tumor necrosis factor-{alpha} downregulates intestinal NHE8 expression by reducing basal promoter activity

NHE8 transporter is a member of the sodium/hydrogen exchanger (NHE) family. This transporter protein is expressed at the apical membrane of epithelial cells of kidney and intestine and contributes to vectorial Na(+) transport in both tissues. Although NaCl absorption has been shown to be reduced in diarrhea associated with colitis and enteritis, little is known about the role of Na(+)/H(+) exchange and the involvement of NHE isoforms in the pathogenesis of inflammatory disorders and the mechanism of inflammation-associated diarrhea. This study investigated the role of NHE8 in the setting of inflammatory states. Jejunal mucosa was harvested from trinitrobenzene sulfonic acid (TNBS) colitis rats or lipopolysaccharide (LPS) rats for RNA extraction and brush-border membrane protein purification. The human NHE8 gene promoter was cloned from human genomic DNA and characterized in Caco-2 cells. The promoter was further used to study the mechanisms of TNF-alpha-mediated NHE8 expression downregulation in Caco-2 cells. Results from Western blot and real-time PCR indicated that NHE8 protein and mRNA were significantly reduced in TNBS rats and LPS rats. In Caco-2 cells, TNF-alpha produces similar reduction levels in the endogenous NHE8 mRNA expression observed in our in vivo studies. The downregulation of NHE8 expression mediated by TNF-alpha could be blocked by transcription inhibitor actinomycin D, suggesting the involvement of transcriptional regulation. Further studies indicated that the human NHE8 gene transcription could be activated by Sp3 transcriptional factor, and TNF-alpha inhibits human NHE8 expression by reducing Sp3 interaction at the minimal promoter region of the human NHE8 gene. In conclusion, our studies suggest that TNF-alpha decreases NHE8 expression in inflammation induced by TNBS and LPS, which may contribute to the diarrhea associated with inflammation.

Ion transport in the intestine

PURPOSE OF REVIEW

In recent years, the field of intestinal physiology has witnessed significant progress in our understanding of the expression and function of ion transport proteins and their genes under physiological and pathophysiological conditions. This review will present some of these most recent advances in the small intestinal ion transport mechanisms.

RECENT FINDINGS

One of the new and exciting aspects of this field has been the integration of function and structure of several intestinal transport processes. This is well exemplified by the discussed intricacies of intestinal bicarbonate secretion as well as the role of scaffolding PDZ proteins interacting with several transporters. We also discuss some of the most recent data pointing to the role of ion transporters in the pathogenesis of inflammation-associated diarrhea and their potential role in the maintenance of epithelial integrity.

SUMMARY

Mouse models deficient in some of the key genes encoding ion transporters and their adapter proteins continue to provide important clues into intestinal transport processes. Several of the new in-vivo findings revise or complement past paradigms, many of which were derived from in-vitro approaches. New data on the interdependent functions of multiple transporters, as exemplified here by intestinal bicarbonate secretion, increase the complexity of the intestinal ion transport mechanisms and continue to contribute to a more integrated view of the transport phenomena in the gut. Data from patients and mouse models of intestinal inflammation also increase our understanding of the pathophysiology of inflammation-associated diarrhea.

Down-regulated in adenoma Cl/HCO3 exchanger couples with Na/H exchanger 3 for NaCl absorption in murine small intestine

BACKGROUND & AIMS

Electroneutral NaCl absorption across small intestine contributes importantly to systemic fluid balance. Disturbances in this process occur in both obstructive and diarrheal diseases, eg, cystic fibrosis, secretory diarrhea. NaCl absorption involves coupling of Cl(-)/HCO(3)(-) exchanger(s) primarily with Na(+)/H(+) exchanger 3 (Nhe3) at the apical membrane of intestinal epithelia. Identity of the coupling Cl(-)/HCO(3)(-) exchanger(s) was investigated using mice with gene-targeted knockout (KO) of Cl(-)/HCO(3)(-) exchangers: Slc26a3, down-regulated in adenoma (Dra) or Slc26a6, putative anion transporter-1 (Pat-1).

METHODS

Intracellular pH (pH(i)) of intact jejunal villous epithelium was measured by ratiometric microfluoroscopy. Ussing chambers were used to measure transepithelial (22)Na(36)Cl flux across murine jejunum, a site of electroneutral NaCl absorption. Expression was estimated using immunofluorescence and quantitative polymerase chain reaction.

RESULTS

Basal pH(i) of DraKO epithelium, but not Pat-1KO epithelium, was alkaline, whereas pH(i) in the Nhe3KO was acidic relative to wild-type. Altered pH(i) was associated with robust Na(+)/H(+) and Cl(-)/HCO(3)(-) exchange activity in the DraKO and Nhe3KO villous epithelium, respectively. Contrary to genetic ablation, pharmacologic inhibition of Nhe3 in wild-type did not alter pH(i) but coordinately inhibited Dra. Flux studies revealed that Cl(-) absorption was essentially abolished (>80%) in the DraKO and little changed (<20%) in the Pat-1KO jejunum. Net Na(+) absorption was unaffected. Immunofluorescence demonstrated modest Dra expression in the jejunum relative to large intestine. Functional and expression studies did not indicate compensatory changes in relevant transporters.

CONCLUSIONS

These studies provide functional evidence that Dra is the major Cl(-)/HCO(3)(-) exchanger coupled with Nhe3 for electroneutral NaCl absorption across mammalian small intestine.