Loss of PDZ-adaptor protein NHERF2 affects membrane localization and cGMP- and [Ca2+]- but not cAMP-dependent regulation of Na+/H+ exchanger 3 in murine intestine

cGMP-dependent kinase 2, Na+/H+ exchanger NHE3, and PDZ-adaptor NHERF2 co-assemble in apical membrane microdomains

AIM

Trafficking, membrane retention, and signal-specific regulation of the Na+/H+ exchanger 3 (NHE3) are modulated by the Na+/H+ Exchanger Regulatory Factor (NHERF) family of PDZ-adapter proteins. This study explored the assembly of NHE3 and NHERF2 with the cGMP-dependent kinase II (cGKII) within detergent-resistant membrane microdomains (DRMs, "lipid rafts") during in vivo guanylate cycle C receptor (Gucy2c) activation in murine small intestine.

METHODS

Small intestinal brush border membranes (siBBMs) were isolated from wild type, NHE3-deficient, cGMP-kinase II-deficient, and NHERF2-deficient mice, after oral application of the heat-stable Escherichia coli toxin (STa) analog linaclotide. Lipid raft and non-raft fractions were separated by Optiprep density gradient centrifugation of Triton X-solubilized siBBMs. Confocal microscopy was performed to study NHE3 redistribution after linaclotide application in vivo.

RESULTS

In the WT siBBM, NHE3, NHERF2, and cGKII were strongly raft associated. The raft association of NHE3, but not of cGKII, was NHERF2 dependent. After linaclotide application to WT mice, lipid raft association of NHE3 decreased, that of cGKII increased, while that of NHERF2 did not change. NHE3 expression in the BBM shifted from a microvillar to a terminal web region. The linaclotide-induced decrease in NHE3 raft association and in microvillar abundance was abolished in cGKII-deficient mice, and strongly reduced in NHERF2-deficient mice.

CONCLUSION

NHE3, cGKII, and NHERF2 form a lipid raft-associated signal complex in the siBBM, which mediates the inhibition of salt and water absorption by Gucy2c activation. NHERF2 enhances the raft association of NHE3, which is essential for its close interaction with the exclusively raft-associated activated cGKII.

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.

The role of NHE3 (Slc9a3) in oxalate and sodium transport by mouse intestine and regulation by cAMP

Intestinal oxalate transport involves Cl⁻/HCO₃⁻ exchangers but how this transport is regulated is not currently known. NHE3 (Slc9a3), an apical Na⁺/H⁺ exchanger, is an established target for regulation of electroneutral NaCl absorption working in concert with Cl⁻/HCO₃⁻ exchangers. To test whether NHE3 could be involved in regulation of intestinal oxalate transport and renal oxalate handling we compared urinary oxalate excretion rates and intestinal transepithelial fluxes of ¹⁴C‐oxalate and ²²Na⁺ between NHE3 KO and wild‐type (WT) mice. NHE3 KO kidneys had lower creatinine clearance suggesting reduced GFR, but urinary oxalate excretion rates (µmol/24 h) were similar compared to the WT but doubled when expressed as a ratio of creatinine. Intestinal transepithelial fluxes of ¹⁴C‐oxalate and ²²Na⁺ were measured in the distal ileum, cecum, and distal colon. The absence of NHE3 did not affect basal net transport rates of oxalate or sodium across any intestinal section examined. Stimulation of intracellular cAMP with forskolin (FSK) and 3‐isobutyl‐1‐methylxanthine (IBMX) led to an increase in net oxalate secretion in the WT distal ileum and cecum and inhibition of sodium absorption in the cecum and distal colon. In NHE3 KO cecum, cAMP stimulation of oxalate secretion was impaired suggesting the possibility of a role for NHE3 in this process. Although, there is little evidence for a role of NHE3 in basal intestinal oxalate fluxes, NHE3 may be important for cAMP stimulation of oxalate in the cecum and for renal handling of oxalate.

Impairment of electroneutral Na+ transport and associated downregulation of NHE3 contributes to the development of diarrhea following in vivo challenge with Brachyspira spp

The effect of Brachyspira hyodysenteriae and Brachyspira hampsonii spirochetosis on Na⁺ transport was assessed in the colon to determine its contribution to diarrheal disease in pigs following experimental infection. Electrogenic and electroneutral Na⁺ absorption was assessed in Ussing chambers by radiolabeled ²²Na flux and pharmacological inhibitory studies. Basal radiolabeled ²²Na flux experiments revealed that mucosal-to-serosal flux (J_ms) was significantly impaired in B. hyodysenteriae and B. hampsonii-diseased pigs. Inhibition of epithelial sodium channel via amiloride did not significantly reduce electrogenic short-circuit current (I_sc) in the proximal, apex, and distal colonic segments of diseased pigs over control pigs, suggesting that a loss of electroneutral Na⁺ absorption is responsible for diarrheal development. These findings were further supported by significant downregulation of Na⁺/H⁺ exchanger (NHE1, NHE2, and NHE3) mRNA expression in the proximal, apex, and distal colonic segments paired with decreased protein expression of the critical NHE3 isoform. The decrease in NHE3 mRNA expression appears not to be attributed to the host's cytokine response as human IL-1α did not modify NHE3 mRNA expression in Caco-2 cells. However, a whole cell B. hampsonii lysate significantly downregulated NHE3 mRNA expression and significantly increased p38 phosphorylation in Caco-2 cells. Together these findings provide a likely mechanism for the spirochete-induced malabsorptive diarrhea, indicated by a decrease in electroneutral Na⁺ absorption in the porcine colon due to Brachyspira's ability to inhibit NHE3 transcription, resulting in diarrheal disease.NEW & NOTEWORTHY This research demonstrates that diarrheal disease caused by two infectious spirochete spp. is a result of impaired electroneutral Na⁺ absorption via Na⁺/H⁺ exchanger 3 (NHE3) in the porcine colon. Our findings suggest that the decrease in NHE3 mRNA and protein is not likely a result of the host's cytokine response. Rather, it appears that these two Brachyspira spp. directly inhibit the transcription and translation of NHE3, resulting in the development of diarrhea.

NHERF3 is necessary for Escherichia coli heat-stable enterotoxin-induced inhibition of NHE3: differences in signaling in mouse small intestine and Caco-2 cells

Enterotoxigenic Escherichia coli (ETEC) is a leading cause of childhood death from diarrhea and the leading cause of Traveler's diarrhea. E. coli heat-stable enterotoxin (ST) is a major virulence factor of ETEC and inhibits the brush border Na/H exchanger NHE3 in producing diarrhea. NHE3 regulation involves multiprotein signaling complexes that form on its COOH terminus. In this study, the hypothesis was tested that ST signals via members of the Na/H exchanger regulatory factor (NHERF) family of scaffolding proteins, NHERF2, which had been previously shown to have a role, and now with concentration on a role for NHERF3. Two models were used: mouse small intestine and Caco-2/BBe cells. In both models, ST rapidly increased intracellular cGMP, inhibited NHE3 activity, and caused a quantitatively similar decrease in apical expression of NHE3. The transport effects were NHERF3 and NHERF2 dependent. Also, mutation of the COOH-terminal amino acids of NHERF3 supported that NHERF3-NHERF2 heterodimerization was likely to account for this dual dependence. The ST increase in cGMP in both models was partially dependent on NHERF3. The intracellular signaling pathways by which ST-cGMP inhibits NHE3 were different in mouse jejunum (activation of cGMP kinase II, cGKII) and Caco-2 cells, which do not express cGKII (elevation of intracellular Ca²⁺ concentration [Ca²⁺]_i). The ST elevation of [Ca²⁺]_i was from intracellular stores and was dependent on NHERF3-NHERF2. This study shows that intracellular signaling in the same diarrheal model in multiple cell types may be different; this has implications for therapeutic strategies, which often assume that models have similar signaling mechanisms.

Both NHERF3 and NHERF2 are necessary for multiple aspects of acute regulation of NHE3 by elevated Ca2+, cGMP, and lysophosphatidic acid

The intestinal epithelial brush border Na⁺/H⁺ exchanger NHE3 accounts for a large component of intestinal Na absorption. NHE3 is regulated during digestion by signaling complexes on its COOH terminus that include the four multi-PDZ domain-containing NHERF family proteins. All bind to NHE3 and take part in different aspects of NHE3 regulation. Because the roles of each NHERF appear to vary on the basis of the cell model or intestinal segment studied and because of our recent finding that a NHERF3-NHERF2 heterodimer appears important for NHE3 regulation in Caco-2 cells, we examined the role of NHERF3 and NHERF2 in C57BL/6 mouse jejunum using homozygous NHERF2 and NHERF3 knockout mice. NHE3 activity was determined with two-photon microscopy and the dual-emission pH-sensitive dye SNARF-4F. The jejunal apical membrane of NHERF3-null mice appeared similar to wild-type (WT) mice in surface area, microvillus number, and height, which is similar to results previously reported for jejunum of NHERF2-null mice. NHE3 basal activity was not different from WT in either NHERF2- or NHERF3-null jejunum, while d-glucose-stimulated NHE3 activity was reduced in NHERF2, but similar to WT in NHERF3 KO. LPA stimulation and UTP (elevated Ca²⁺) and cGMP inhibition of NHE3 were markedly reduced in both NHERF2- and NHERF3-null jejunum. Forskolin inhibited NHE3 in NHERF3-null jejunum, but the extent of inhibition was reduced compared with WT. The forskolin inhibition of NHE3 in NHERF2-null mice was too inconsistent to determine whether there was an effect and whether it was altered compared with the WT response. These results demonstrate similar requirement for NHERF2 and NHERF3 in mouse jejunal NHE3 regulation by LPA, Ca²⁺, and cGMP. The explanation for the similarity is not known but is consistent with involvement of a brush-border NHERF3-NHERF2 heterodimer or sequential NHERF-dependent effects in these aspects of NHE3 regulation. NEW & NOTEWORTHY NHERF2 and NHERF3 are apical membrane multi-PDZ domain-containing proteins that are involved in regulation of intestinal NHE3. This study demonstrates that NHERF2 and NHERF3 have overlapping roles in NHE3 stimulation by LPA and inhibition by elevated Ca²⁺ and cGMP. These results are consistent with their role being as a NHERF3-NHERF2 heterodimer or via sequential NHERF-dependent signaling steps, and they begin to clarify a role for multiple NHERF proteins in NHE3 regulation.

Phosphorylation of NHE3-S719 regulates NHE3 activity through the formation of multiple signaling complexes

CK2 regulates NHE3 by phosphorylating a single C-terminal amino acid, which, when mutated, reduces basal NHE3 activity and its acute stimulation and inhibition. It also is necessary for binding of proteins throughout the C-terminus, which means that it determines the C-terminal structure.

Casein kinase 2 (CK2) binds to the NHE3 C-terminus and constitutively phosphorylates a downstream site (S719) that accounts for 40% of basal NHE3 activity. The role of CK2 in regulation of NHE3 activity in polarized Caco-2/bbe cells was further examined by mutation of NHE3-S⁷¹⁹ to A (not phosphorylated) or D (phosphomimetic). NHE3-S719A but not -S719D had multiple changes in NHE3 activity: 1) reduced basal NHE3 activity—specifically, inhibition of the PI3K/AKT-dependent component; 2) reduced acute stimulation of NHE3 activity by LPA/LPA₅R stimulation; and 3) reduced acute inhibition of NHE3 activity—specifically, elevated Ca²⁺ related (carbachol/Ca²⁺ ionophore), but there was normal inhibition by forskolin and hyperosmolarity. The S719A mutant had reduced NHE3 complex size, reduced expression in lipid rafts, increased BB mobile fraction, and reduced binding to multiple proteins that bind throughout the NHE3 intracellular C-terminus, including calcineurin homologous protein, the NHERF family and SNX27 (related PDZ domains). These studies show that phosphorylation of the NHE3 at a single amino acid in the distal part of the C-terminus affects multiple aspects of NHE3 complex formation and changes the NHE3 lipid raft distribution, which cause changes in specific aspects of basal as well as acutely stimulated and inhibited Na⁺/H⁺ exchange activity.

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.

Mucosal immunity to pathogenic intestinal bacteria

The intestinal mucosa is a particularly dynamic environment in which the host constantly interacts with trillions of commensal microorganisms, known as the microbiota, and periodically interacts with pathogens of diverse nature. In this Review, we discuss how mucosal immunity is controlled in response to enteric bacterial pathogens, with a focus on the species that cause morbidity and mortality in humans. We explain how the microbiota can shape the immune response to pathogenic bacteria, and we detail innate and adaptive immune mechanisms that drive protective immunity against these pathogens. The vast diversity of the microbiota, pathogens and immune responses encountered in the intestines precludes discussion of all of the relevant players in this Review. Instead, we aim to provide a representative overview of how the intestinal immune system responds to pathogenic bacteria.

Secretory diarrhoea: mechanisms and emerging therapies

Diarrhoeal disease remains a major health burden worldwide. Secretory diarrhoeas are caused by certain bacterial and viral infections, inflammatory processes, drugs and genetic disorders. Fluid secretion across the intestinal epithelium in secretory diarrhoeas involves multiple ion and solute transporters, as well as activation of cyclic nucleotide and Ca(2+) signalling pathways. In many secretory diarrhoeas, activation of Cl(-) channels in the apical membrane of enterocytes, including the cystic fibrosis transmembrane conductance regulator and Ca(2+)-activated Cl(-) channels, increases fluid secretion, while inhibition of Na(+) transport reduces fluid absorption. Current treatment of diarrhoea includes replacement of fluid and electrolyte losses using oral rehydration solutions, and drugs targeting intestinal motility or fluid secretion. Therapeutics in the development pipeline target intestinal ion channels and transporters, regulatory proteins and cell surface receptors. This Review describes pathogenic mechanisms of secretory diarrhoea, current and emerging therapeutics, and the challenges in developing antidiarrhoeal therapeutics.

The distinct roles of anion transporters Slc26a3 (DRA) and Slc26a6 (PAT-1) in fluid and electrolyte absorption in the murine small intestine

The mixing of gastric and pancreatic juice subjects the jejunum to unique ionic conditions with high luminal CO₂ tension and HCO₃⁻ concentration. We investigated the role of the small intestinal apical anion exchangers PAT-1 (Slc26a6) and DRA (Slc26a3) in basal and CO₂/HCO₃⁻-stimulated jejunal fluid absorption. Single pass perfusion of jejunal segments was performed in anaesthetised wild type (WT) as well as in mice deficient in DRA, PAT-1, Na⁺/H⁺ exchanger 3 (NHE3) or NHE2, and in carbonic anhydrase II (CAII). Unbuffered saline (pH 7.4) perfusion of WT jejunum resulted in fluid absorption and acidification of the effluent. DRA-deficient jejunum absorbed less fluid than WT, and acidified the effluent more strongly, consistent with its action as a Cl⁻/HCO₃⁻ exchanger. PAT-1-deficient jejunum also absorbed less fluid but resulted in less effluent acidification. Switching the luminal solution to a 5 % CO₂/HCO₃⁻ buffered solution (pH 7.4), resulted in a decrease in jejunal enterocyte pH_i in all genotypes, an increase in luminal surface pH and a strong increase in fluid absorption in a PAT-1- and NHE3- but not DRA-, CAII, or NHE2-dependent fashion. Even in the absence of luminal Cl⁻, luminal CO₂/HCO₃⁻ augmented fluid absorption in WT, CAII, NHE2- or DRA-deficient, but not in PAT-1- or NHE3-deficient mice, indicating the likelihood that PAT-1 serves to import HCO₃⁻ and NHE3 serves to import Na⁺ under these circumstances. The results suggest that PAT-1 plays an important role in jejunal Na⁺HCO₃^– reabsorption, while DRA absorbs Cl⁻ and exports HCO₃⁻ in a partly CAII-dependent fashion. Both PAT-1 and DRA significantly contribute to intestinal fluid absorption and enterocyte acid/base balance but are activated by different ion gradients.

Evidence for a causal link between adaptor protein PDZK1 downregulation and Na⁺/H⁺ exchanger NHE3 dysfunction in human and murine colitis

A dysfunction of the Na(+)/H(+) exchanger isoform 3 (NHE3) significantly contributes to the reduced salt absorptive capacity of the inflamed intestine. We previously reported a strong decrease in the NHERF family member PDZK1 (NHERF3), which binds to NHE3 and regulates its function in a mouse model of colitis. The present study investigates whether a causal relationship exists between the decreased PDZK1 expression and the NHE3 dysfunction in human and murine intestinal inflammation. Biopsies from the colon of patients with ulcerative colitis, murine inflamed ileal and colonic mucosa, NHE3-transfected Caco-2BBe colonic cells with short hairpin RNA (shRNA) knockdown of PDZK1, and Pdzk1-gene-deleted mice were studied. PDZK1 mRNA and protein expression was strongly decreased in inflamed human and murine intestinal tissue as compared to inactive disease or control tissue, whereas that of NHE3 or NHERF1 was not. Inflamed human and murine intestinal tissues displayed correct brush border localization of NHE3 but reduced acid-activated NHE3 transport activity. A similar NHE3 transport defect was observed when PDZK1 protein content was decreased by shRNA knockdown in Caco-2BBe cells or when enterocyte PDZK1 protein content was decreased to similar levels as found in inflamed mucosa by heterozygote breeding of Pdzk1-gene-deleted and WT mice. We conclude that a decrease in PDZK1 expression, whether induced by inflammation, shRNA-mediated knockdown, or heterozygous breeding, is associated with a decreased NHE3 transport rate in human and murine enterocytes. We therefore hypothesize that inflammation-induced loss of PDZK1 expression may contribute to the NHE3 dysfunction observed in the inflamed intestine.

Differential association of the Na+/H+ Exchanger Regulatory Factor (NHERF) family of adaptor proteins with the raft- and the non-raft brush border membrane fractions of NHE3

BACKGROUND/AIMS

Trafficking, brush border membrane (BBM) retention, and signal-specific regulation of the Na+/H+ exchanger NHE3 is regulated by the Na+/H+ Exchanger Regulatory Factor (NHERF) family of PDZ-adaptor proteins, which enable the formation of multiprotein complexes. It is unclear, however, what determines signal specificity of these NHERFs. Thus, we studied the association of NHE3, NHERF1 (EBP50), NHERF2 (E3KARP), and NHERF3 (PDZK1) with lipid rafts in murine small intestinal BBM.

METHODS

Detergent resistant membranes ("lipid rafts") were isolated by floatation of Triton X-incubated small intestinal BBM from a variety of knockout mouse strains in an Optiprep step gradient. Acid-activated NHE3 activity was measured fluorometrically in BCECF-loaded microdissected villi, or by assessment of CO2/HCO3(-) mediated increase in fluid absorption in perfused jejunal loops of anethetized mice.

RESULTS

NHE3 was found to partially associate with lipid rafts in the native BBM, and NHE3 raft association had an impact on NHE3 transport activity and regulation in vivo. NHERF1, 2 and 3 were differentially distributed to rafts and non-rafts, with NHERF2 being most raft-associated and NHERF3 entirely non-raft associated. NHERF2 expression enhanced the localization of NHE3 to membrane rafts. The use of acid sphingomyelinase-deficient mice, which have altered membrane lipid as well as lipid raft composition, allowed us to test the validity of the lipid raft concept in vivo.

CONCLUSIONS

The differential association of the NHERFs with the raft-associated and the non-raft fraction of NHE3 in the brush border membrane is one component of the differential and signal-specific NHE3 regulation by the different NHERFs.

Loss of Slc26a9 anion transporter alters intestinal electrolyte and HCO3(-) transport and reduces survival in CFTR-deficient mice

Slc26a9 is an anion transporter that is strongly expressed in the stomach and lung. Slc26a9 variants were recently found associated with a higher incidence of meconium ileus in cystic fibrosis (CF) infants, raising the question whether Slc26a9 is expressed in the intestine and what its functional role is. Slc26a9 messenger RNA (mRNA) was found highly expressed in the mucosae of the murine and human upper gastrointestinal tract, with an abrupt decrease in expression levels beyond the duodenum. Absence of SLC26a9 expression strongly increased the intestinally related mortality in cystic fibrosis transmembrane conductance regulator (CFTR)-deficient mice. Proximal duodenal JHCO3(-) and fluid secretion were reduced in the absence of Slc26a9 expression. In the proximal duodenum of young Slc26a9 KO mice, the glands and villi/crypts were elongated and proliferation was enhanced. This difference was lost with ageing, as were the alterations in fluid movement, whereas the reduction in JHCO3(-) remained. Laser dissection followed by qPCR suggested Slc26a9 expression to be crypt-predominant in the duodenum. In summary, deletion of Slc26a9 caused bicarbonate secretory and fluid absorptive changes in the proximal duodenal mucosa and increased the postweaning death rates in CFTR-deficient mice. Functional alterations in the duodenum were most prominent at young ages. We assume that the association of meconium ileus and Slc26a9 variants may be related to maldigestion and impaired downstream signaling caused by loss of upper GI tract digestive functions, aggravating the situation of lack of secretion and sticky mucus at the site of obstruction in CF intestine.

NHERF2/NHERF3 protein heterodimerization and macrocomplex formation are required for the inhibition of NHE3 activity by carbachol

NHERF1, NHERF2, and NHERF3 belong to the NHERF (Na(+)/H(+) exchanger regulatory factor) family of PSD-95/Discs-large/ZO-1 (PDZ) scaffolding proteins. Individually, each NHERF protein has been shown to be involved in the regulation of multiple receptors or transporters including Na(+)/H(+) exchanger 3 (NHE3). Although NHERF dimerizations have been reported, results have been inconsistent, and the physiological function of NHERF dimerizations is still unknown. The current study semiquantitatively compared the interaction strength among all possible homodimerizations and heterodimerizations of these three NHERF proteins by pulldown and co-immunoprecipitation assays. Both methods showed that NHERF2 and NHERF3 heterodimerize as the strongest interaction among all NHERF dimerizations. In vivo NHERF2/NHERF3 heterodimerization was confirmed by FRET and FRAP (fluorescence recovery after photobleach). NHERF2/NHERF3 heterodimerization is mediated by PDZ domains of NHERF2 and the C-terminal PDZ domain recognition motif of NHERF3. The NHERF3-4A mutant is defective in heterodimerization with NHERF2 and does not support the inhibition of NHE3 by carbachol. This suggests a role for NHERF2/NHERF3 heterodimerization in the regulation of NHE3 activity. In addition, both PDZ domains of NHERF2 could be simultaneously occupied by NHERF3 and another ligand such as NHE3, α-actinin-4, and PKCα, promoting formation of NHE3 macrocomplexes. This study suggests that NHERF2/NHERF3 heterodimerization mediates the formation of NHE3 macrocomplexes, which are required for the inhibition of NHE3 activity by carbachol.

Slc26a3 deficiency is associated with loss of colonic HCO3 (-) secretion, absence of a firm mucus layer and barrier impairment in mice

AIM

Downregulated in adenoma (DRA, Slc26a3) is a member of the solute carrier family 26 (SLC26), family of anion transporters, which is mutated in familial chloride-losing diarrhoea (CLD). Besides Cl(-) -rich diarrhoea, CLD patients also have a higher-than-average incidence of intestinal inflammation. In a search for potential explanations for this clinical finding, we investigated colonic electrolyte transport, the mucus layer and susceptibility against dextran sodium sulphate (DSS)-induced colitis in Slc26a3(-/-) mice.

METHODS

HCO3 (-) secretory (JHCO3 (-) ) and fluid absorptive rates were measured by single-pass perfusion in vivo and in isolated mid-distal colonic mucosa in Ussing chambers in vitro. Colonocyte intracellular pH (pHi ) was assessed fluorometrically, the mucus layer by immunohistochemistry and colitis susceptibility by the addition of DSS to the drinking water.

RESULTS

HCO3 (-) secretory (JHCO3- ) and fluid absorptive rates were strongly reduced in Slc26a3(-/-) mice compared to wild-type (WT) littermates. Despite an increase in sodium/hydrogen exchanger 3 (NHE3) mRNA and protein expression, and intact acid-activation of NHE3, the high colonocyte pH in Slc26a3(-/-) mice prevented Na(+) /H(+) exchange-mediated fluid absorption in vivo. Mucin 2 (MUC2) immunohistochemistry revealed the absence of a firm mucus layer, implying that alkaline secretion and/or an absorptive flux may be necessary for optimal mucus gel formation. Slc26a3(-/-) mice were highly susceptible to DSS damage.

CONCLUSIONS

Deletion of DRA results in severely reduced colonic HCO3 (-) secretory rate, a loss of colonic fluid absorption, a lack of a firmly adherent mucus layer and a severely reduced colonic mucosal resistance to DSS damage. These data provide potential pathophysiological explanations for the increased susceptibility of CLD patients to intestinal inflammation.

NHERF2 is crucial in ERM phosphorylation in pulmonary endothelial cells

Background

EBP50 and NHERF2 adaptor proteins are incriminated in various signaling pathways of the cell. They can bind ERM proteins and mediate ERM-membrane protein interactions.

Results

Binding of ERM to EBP50 and NHERF2 was compared in pulmonary artery endothelial cells by immunoprecipitation. NHERF2 associates with all three ERM, but EBP50 appeared to be a weak binding partner if at all. Furthermore, we detected co-localization of NHERF2 and phospho-ERM at the cell membrane and in the filopodia of dividing cells. Silencing of NHERF2 prevented agonist or angiogenesis induced phosphorylation of ERM, while overexpression of the adaptor elevated the phosphorylation level of ERM, likely catalyzed by Rho kinase 2, which co-immunoprecipitated with NHERF2/ERM in control EC, but did not bind to ERM in NHERF2 depleted cells. Dependence of ERM phosphorylation on NHERF2 was also shown in Matrigel tube formation assay, and NHERF2 was proved to be important in angiogenesis as well. Furthermore, when NHERF2 was depleted or cells were overexpressing a mutant form of NHERF2 unable to bind ERM, we found attenuated cell attachment with ECIS measurements, while it was supported by overexpression of wild type NHERF2.

Conclusions

Pivotal role of NHERF2 in the phosphorylation process of ERM in pulmonary artery endothelial cells is shown. We propose that NHERF2 provides a common anchoring surface for ERM and Rho kinase 2. Our results demonstrate the essential role of NHERF2 in endothelial cell adhesion/migration and angiogenesis.

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.

Molecular transport machinery involved in orchestrating luminal acid-induced duodenal bicarbonate secretion in vivo

The duodenal villus brush border membrane expresses several ion transporters and/or channels, including the solute carrier 26 anion transporters Slc26a3 (DRA) and Slc26a6 (PAT-1), the Na(+)/H(+) exchanger isoform 3 (NHE3), as well as the anion channels cystic fibrosis transmembrane conductance regulator (CFTR) and Slc26a9. Using genetically engineered mouse models lacking Scl26a3, Slc26a6, Slc26a9 or Slc9a3 (NHE3), the study was carried out to assess the role of these transporters in mediating the protective duodenal bicarbonate secretory response (DBS-R) to luminal acid; and to compare it to their role in DBS-R elicited by the adenylyl cyclase agonist forskolin. While basal DBS was reduced in the absence of any of the three Slc26 isoforms, the DBS-R to forskolin was not altered. In contrast, the DBS-R to a 5 min exposure to luminal acid (pH 2.5) was strongly reduced in the absence of Slc26a3 or Slc26a9, but not Slc26a6. CFTR inhibitor [CFTR(Inh)-172] reduced the first phase of the acid-induced DBS-R, while NHE3 inhibition (or knockout) abolished the sustained phase of the DBS-R. Luminal acid exposure resulted in the activation of multiple intracellular signalling pathways, including SPAK, AKT and p38 phosphorylation. It induced a biphasic trafficking of NHE3, first rapidly into the brush border membrane, followed by endocytosis in the later stage. We conclude that the long-lasting DBS-R to luminal acid exposure activates multiple duodenocyte signalling pathways and involves changes in trafficking and/or activity of CFTR, Slc26 isoforms Slc26a3 and Slc26a9, and NHE3.

Carbachol-induced MUC17 endocytosis is concomitant with NHE3 internalization and CFTR membrane recruitment in enterocytes

We have reported that transmembrane mucin MUC17 binds PDZ protein PDZK1, which retains MUC17 apically in enterocytes. MUC17 and transmembrane mucins MUC3 and MUC12 are suggested to build the enterocyte apical glycocalyx. Carbachol (CCh) stimulation of the small intestine results in gel-forming mucin secretion from goblet cells, something that requires adjacent enterocytes to secrete chloride and bicarbonate for proper mucin formation. Surface labeling and confocal imaging demonstrated that apically expressed MUC17 in Caco-2 cells and Muc3(17) in murine enterocytes were endocytosed upon stimulation with CCh. Relocation of MUC17 in response to CCh was specific as MUC3 and MUC12 did not relocate following CCh stimulation. MUC17 colocalized with PDZK1 under basal conditions, while MUC17 relocated to the terminal web and into early endosomes after CCh stimulation. CCh stimulation concomitantly internalized the Na(+/)H(+) exchanger 3 (NHE3) and recruited cystic fibrosis transmembrane conductance regulator (CFTR) to the apical membranes, a process that was important for CFTR-mediated bicarbonate secretion necessary for proper gel-forming mucin unfolding. The reason for the specific internalization of MUC17 is not understood, but it could limit the diffusion barrier for ion secretion caused by the apical enterocyte glycocalyx or alternatively act to sample luminal bacteria. Our results reveal well-orchestrated mucus secretion and trafficking of ion channels and the MUC17 mucin.

PLC-γ directly binds activated c-Src, which is necessary for carbachol-mediated inhibition of NHE3 activity in Caco-2/BBe cells

Elevated levels of intracellular Ca(2+) ([Ca(2+)]i) inhibit Na(+)/H(+) exchanger 3 (NHE3) activity in the intact intestine. We previously demonstrated that PLC-γ directly binds NHE3, an interaction that is necessary for [Ca(2+)]i inhibition of NHE3 activity, and that PLC-γ Src homology 2 (SH2) domains may scaffold Ca(2+) signaling proteins necessary for regulation of NHE3 activity. [Ca(2+)]i regulation of NHE3 activity is also c-Src dependent; however, the mechanism by which c-Src is involved is undetermined. We hypothesized that the SH2 domains of PLC-γ might link c-Src to NHE3-containing complexes to mediate [Ca(2+)]i inhibition of NHE3 activity. In Caco-2/BBe cells, carbachol (CCh) decreased NHE3 activity by ∼40%, an effect abolished with the c-Src inhibitor PP2. CCh treatment increased the amount of active c-Src as early as 1 min through increased Y(416) phosphorylation. Coimmunoprecipitation demonstrated that c-Src associated with PLC-γ, but not NHE3, under basal conditions, an interaction that increased rapidly after CCh treatment and occurred before the dissociation of PLC-γ and NHE3 that occurred 10 min after CCh treatment. Finally, direct binding to c-Src only occurred through the PLC-γ SH2 domains, an interaction that was prevented by blocking the PLC-γ SH2 domain. This study demonstrated that c-Src 1) activity is necessary for [Ca(2+)]i inhibition of NHE3 activity, 2) activation occurs rapidly (∼1 min) after CCh treatment, 3) directly binds PLC-γ SH2 domains and associates dynamically with PLC-γ under elevated [Ca(2+)]i conditions, and 4) does not directly bind NHE3. Under elevated [Ca(2+)]i conditions, PLC-γ scaffolds c-Src into NHE3-containing multiprotein complexes before dissociation of PLC-γ from NHE3 and subsequent endocytosis of NHE3.

NHERF2 protein mobility rate is determined by a unique C-terminal domain that is also necessary for its regulation of NHE3 protein in OK cells

Na(+)/H(+) exchanger regulatory factor (NHERF) proteins are a family of PSD-95/Discs-large/ZO-1 (PDZ)-scaffolding proteins, three of which (NHERFs 1-3) are localized to the brush border in kidney and intestinal epithelial cells. All NHERF proteins are involved in anchoring membrane proteins that contain PDZ recognition motifs to form multiprotein signaling complexes. In contrast to their predicted immobility, NHERF1, NHERF2, and NHERF3 were all shown by fluorescence recovery after photobleaching/confocal microscopy to be surprisingly mobile in the microvilli of the renal proximal tubule OK cell line. Their diffusion coefficients, although different among the three, were all of the same magnitude as that of the transmembrane proteins, suggesting they are all anchored in the microvilli but to different extents. NHERF3 moves faster than NHERF1, and NHERF2 moves the slowest. Several chimeras and mutants of NHERF1 and NHERF2 were made to determine which part of NHERF2 confers the slower mobility rate. Surprisingly, the slower mobility rate of NHERF2 was determined by a unique C-terminal domain, which includes a nonconserved region along with the ezrin, radixin, moesin (ERM) binding domain. Also, this C-terminal domain of NHERF2 determined its greater detergent insolubility and was necessary for the formation of larger multiprotein NHERF2 complexes. In addition, this NHERF2 domain was functionally significant in NHE3 regulation, being necessary for stimulation by lysophosphatidic acid of activity and increased mobility of NHE3, as well as necessary for inhibition of NHE3 activity by calcium ionophore 4-Br-A23187. Thus, multiple functions of NHERF2 require involvement of an additional domain in this protein.

A role of intestine in hypertension: mechanism of suppression of intestinal Na-H exchanger isoform-3 in spontaneously hypertensive rats

The main objective of this study was to investigate the role and the underlying mechanism of Na-H exchanger-3 (NHE-3) expression in spontaneously hypertensive rat (SHR) intestine. Expression of colonic and ileal NHE-3 isoform, its regulatory factor-1 (NHERF-1) and cyclic GMP kinase II (cGKII) were examined using western blot analysis. Since NHE-3 activity is regulated by its abundance on the plasma membrane, its levels were also examined in lipid rafts-enriched membrane fractions. The lipid rafts fractions were characterized by examining the concentration of flotillin-1 and caveolin-1, total protein, and cholesterol. Twelve-weeks-old SHR used in this study developed significant hypertension, proteinuria, and renal and cardiac hypertrophy. These changes were significantly reversed by captopril treatment. There was a significant decrease in the levels of NHE-3 and NHERF-1 proteins, and sodium pump activity, but an increase in the cGKII levels in both tissues from SHR. Reduction in NHERF-1 levels was reversed by captopril but not of the other proteins. Cholesterol profile was significantly different in SHR colon as compared to normo-tensive Wistar Kyoto rats. These findings suggest that suppression of NHE-3 in intestine is a counteracting mechanism of hypertension and is regulated by NHERF-1 through cGKII activation in SHR. NHE-3 suppression together with decrease in the sodium pump activity would accumulate intracellular Na(+) and may contribute to the reported hypertension-induced tissue damage in the GI-tract.

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.

Deciphering the mechanisms of the Na+/H+ exchanger-3 regulation in organ dysfunction

The Na+/H+ exchanger-3 (NHE3) belongs to the mammalian NHE protein family and catalyzes the electro-neutral exchange of extracellular sodium for intracellular proton across cellular membranes. Its transport function is of essential importance for the maintenance of the body's salt and water homeostasis as well as acid-base balance. Indeed, NHE3 activity is finely regulated by a variety of stimuli, both acutely and chronically, and its transport function is fundamental for a multiplicity of severe and world-wide infection-pathological conditions. This review aims to provide a concise overview of NHE3 physiology and discusses the role of NHE3 in clinical conditions of prominent importance, specifically in hypertension, diabetic nephropathy, heart failure, acute kidney injury, and diarrhea. Study of NHE3 function in models of these diseases has contributed to the deciphering of mechanisms that control the delicate ion balance disrupted in these disorders. The majority of the findings indicate that NHE3 transport function is activated before the onset of hypertension and inhibited thereafter; NHE3 transport function is also upregulated in diabetic nephropathy and heart failure, while it is reported to be downregulated in acute kidney injury and in diarrhea. The molecular mechanisms activated during these pathological conditions to regulate NHE3 transport function are examined with the aim of linking NHE3 dysfunction to the analyzed clinical disorders.

The electroneutral Na⁺:HCO₃⁻ cotransporter NBCn1 is a major pHi regulator in murine duodenum

Duodenocyte pHi control and HCO3 − secretion protects the proximal duodenum against damage by gastric acid. The molecular details of duodenocyte pH control are not well understood. A selective duodenal expression (within the upper GI tract) has been reported for the electroneutral Na+:HCO3 − cotransporter NBCn1 (Slc4a7). We aimed to determine the role of NBCn1 and NBCe2 in duodenocyte intracellular pH regulation as well as basal and agonist-stimulated duodenal bicarbonate secretion (JHCO3 −), exploiting mouse models of genetic slc4a7 and slc4a5 disruption. Basal and forskolin (FSK)-stimulated JHCO3 − was measured by single-pass perfusion in the duodenum of slc4a7−/− and slc4a7+/+ as well as slc4a5−/− and slc4a5+/+ mice in vivo, and by pH-stat titration in isolated duodenal mucosa in vitro. Duodenocyte HCO3 − uptake rates were fluorometrically assessed after acidification of intact villi and of isolated duodenocytes. Slc4a7−/− mice displayed significantly lower basal and FSK-stimulated duodenal HCO3 − secretion than slc4a7+/+ littermates in vivo. FSK-stimulated HCO3 − secretion was significantly reduced in slc4a7−/− isolated duodenal mucosa. Na+- and HCO3 −-dependent base uptake rates were significantly decreased in slc4a7−/− compared with slc4a7+/+ villus duodenocytes when measured in intact villi. Carbonic anhydrase (CA)-mediated CO2 hydration played no apparent role as a HCO3 − supply mechanism for basal or FSK-stimulated secretion in the slc4a7+/+ duodenum, but was an important alternative HCO3 − supply mechanism in the slc4a7−/− duodenum. NBCe2 (Slc4a5) displayed markedly lower duodenal mRNA expression levels, and its disruption did not interfere with duodenal HCO3 − secretion. The electroneutral Na+:HCO3 − cotransporter NBCn1 (slc4a7) is a major duodenal HCO3 − importer that supplies HCO3 − during basal and FSK-stimulated HCO3 − secretion.

Small intestinal ion transport

PURPOSE OF REVIEW

In this review, we focus on the recent (March 2010 to September 2011) advances in small intestinal ion transport, with particular emphasis on sodium, chloride, bicarbonate, and calcium transport mechanisms under physiological and pathophysiological conditions.

RECENT FINDINGS

Knockout of NHERF1 and NHERF2 allowed translation of the data largely derived from the in-vitro models into a living organism. These studies also expand our knowledge about the complexity of intestinal transporter interactomes, define the role for scaffolding proteins in basal and regulated apical transport, and help identify potential targets for pharmacological approaches. We continue to accumulate novel information about the function and regulation of NHE3 (including its role in regulating paracellular Ca2+ flux), NHE8, as well as about the complexity of the intestinal Cl- and HCO3- transport in health and disease.

SUMMARY

Thanks to the new genetically engineered mouse models, a significant progress has been made in our understanding of the role of NHERF proteins in regulation of intestinal Na+ absorption. Significant novel data on the coordinated function of bicarbonate, chloride, and sodium transporters contributes to our current views of the integrative physiology of the small intestinal electrolyte transport.

NHERF-2 maintains endothelial homeostasis

The Na(+)/H(+) exchanger regulatory factor-2 (NHERF-2) is an integral component of almost all endothelial cells (ECs), yet its endothelial function is not known. Here, we found that NHERF-2, is a key regulator of endothelial homeostasis because NHERF-2-silenced ECs proliferate at a much higher rate even in the absence of mitogens such as VEGF compared with control ECs. We further show that the hyperproliferation phenotype of NHERF-2-silenced EC is because of an accelerated cell cycle that is probably caused by a combination of the following factors: increased cytoplasmic calcium, increased expression of c-Myc, increased expression of cyclin D1, and reduced expression of p27. Using an experimental mouse model of human hemangioma, we found that the endothelial neoplasms derived from NHERF-2-silenced cells were much larger in volume than those derived from control cells. Thus, NHERF-2 is a negative regulator of endothelial proliferation and may have important roles in endothelial homeostasis and vascular modeling.

NHERF2 is necessary for basal activity, second messenger inhibition, and LPA stimulation of NHE3 in mouse distal ileum

To test the hypothesis that Na(+)/H(+) exchanger (NHE) regulatory factor 2 (NHERF2) is necessary for multiple aspects of acute regulation of NHE3 in intact mouse small intestine, distal ileal NHE3 activity was determined using two-photon microscopy/SNARF-4F in a NHERF2-null mouse model. The NHERF2-null mouse ileum had shorter villi, deeper crypts, and decreased epithelial cell number. Basal rates of NHE3 activity were reduced in NHERF2-null mice, which was associated with a reduced percentage of NHE3 in the apical domain and an increase in intracellular NHE3 amount but no change in total level of NHE3 protein. cAMP, cGMP, and elevated Ca(2+) due to apical exposure to UTP all inhibited NHE3 activity in wild-type mouse ileum but not in NHERF2-null mice, while inhibition by hyperosmolarity occurred normally. The cAMP-increased phosphorylation of NHE3 at aa 552; levels of PKAIIα and cGMP-dependent protein kinase II (cGKII); and elevation of Ca(2+) were similar in wild-type and NHERF2-null mouse ileum. Luminal lysophosphatidic acid (LPA) stimulated NHE3 in wild-type but not in NHERF2-null ileum. In conclusion, 1) there are subtle structural abnormalities in the small intestine of NHERF2-null mouse which include fewer villus epithelial cells; 2) the decreased basal NHE3 activity and reduced brush border NHE3 amount in NHERF2-null mice show that NHERF2 is necessary for normal basal trafficking or retention of NHE3 in the apical domain; 3) hyperosmolar inhibition of NHE3 occurs similarly in wild-type and NHERF2-null ileum, demonstrating that some inhibitory mechanisms of NHE3 are not NHERF2 dependent; 4) cAMP inhibition of NHE3 is NHERF2 dependent at a step downstream of cAMP/PKAII phosphorylation of NHE3 at aa 552; 5) cGMP- and UTP-induced inhibition of NHE3 are NHERF2 dependent at steps beyond cGKII and the UTP-induced increase of intracellular Ca(2+); and 6) LPA stimulation of NHE3 is also NHERF2 dependent.

NHERF1 and NHERF2 are necessary for multiple but usually separate aspects of basal and acute regulation of NHE3 activity

Na(+)/H(+) exchanger 3 (NHE3) is expressed in the brush border (BB) of intestinal epithelial cells and accounts for the majority of neutral NaCl absorption. It has been shown that the Na(+)/H(+) exchanger regulatory factor (NHERF) family members of multi-PDZ domain-containing scaffold proteins bind to the NHE3 COOH terminus and play necessary roles in NHE3 regulation in intestinal epithelial cells. Most studies of NHE3 regulation have been in cell models in which NHERF1 and/or NHERF2 were overexpressed. We have now developed an intestinal Na(+) absorptive cell model in Caco-2/bbe cells by expressing hemagglutinin (HA)-tagged NHE3 with an adenoviral infection system. Roles of NHERF1 and NHERF2 in NHE3 regulation were determined, including inhibition by cAMP, cGMP, and Ca(2+) and stimulation by EGF, with knockdown (KD) approaches with lentivirus (Lenti)-short hairpin RNA (shRNA) and/or adenovirus (Adeno)-small interfering RNA (siRNA). Stable infection of Caco-2/bbe cells by NHERF1 or NHERF2 Lenti-shRNA significantly and specifically reduced NHERF protein expression by >80%. NHERF1 KD reduced basal NHE3 activity, while NHERF2 KD stimulated NHE3 activity. siRNA-mediated (transient) and Lenti-shRNA-mediated (stable) gene silencing of NHERF2 (but not of NHERF1) abolished cGMP- and Ca(2+)-dependent inhibition of NHE3. KD of NHERF1 or NHERF2 alone had no effect on cAMP inhibition of NHE3, but KD of both simultaneously abolished the effect of cAMP. The stimulatory effect of EGF on NHE3 was eliminated in NHERF1-KD but occurred normally in NHERF2-KD cells. These findings show that both NHERF2 and NHERF1 are involved in setting NHE3 activity. NHERF2 is necessary for cGMP-dependent protein kinase (cGK) II- and Ca(2+)-dependent inhibition of NHE3. cAMP-dependent inhibition of NHE3 activity requires either NHERF1 or NHERF2. Stimulation of NHE3 activity by EGF is NHERF1 dependent.