NHE3 regulatory complexes

Na-H Exchanger Isoform-2 (NHE2) Mediates Butyrate-dependent Na+ Absorption in Dextran Sulfate Sodium (DSS)-induced Colitis

Diarrhea associated with ulcerative colitis (UC) occurs primarily as a result of reduced Na(+) absorption. Although colonic Na(+) absorption is mediated by both epithelial Na(+) channels (ENaC) and Na-H exchangers (NHE), inhibition of NHE-mediated Na(+) absorption is the primary cause of diarrhea in UC. As there are conflicting observations reported on NHE expression in human UC, the present study was initiated to identify whether NHE isoforms (NHE2 and NHE3) expression is altered and how Na(+) absorption is regulated in DSS-induced inflammation in rat colon, a model that has been used to study UC. Western blot analyses indicate that neither NHE2 nor NHE3 expression is altered in apical membranes of inflamed colon. Na(+) fluxes measured in vitro under voltage clamp conditions in controls demonstrate that both HCO3 (-)-dependent and butyrate-dependent Na(+) absorption are inhibited by S3226 (NHE3-inhibitor), but not by HOE694 (NHE2-inhibitor) in normal animals. In contrast, in DSS-induced inflammation, butyrate-, but not HCO3 (-)-dependent Na(+) absorption is present and is inhibited by HOE694, but not by S3226. These observations indicate that in normal colon NHE3 mediates both HCO3 (-)-dependent and butyrate-dependent Na(+) absorption, whereas DSS-induced inflammation activates NHE2, which mediates butyrate-dependent (but not HCO3 (-)-dependent) Na(+) absorption. In in vivo loop studies HCO3 (-)-Ringer and butyrate-Ringer exhibit similar rates of water absorption in normal rats, whereas in DSS-induced inflammation luminal butyrate-Ringer reversed water secretion observed with HCO3 (-)-Ringer to fluid absorption. Lumen butyrate-Ringer incubation activated NHE3-mediated Na(+) absorption in DSS-induced colitis. These observations suggest that the butyrate activation of NHE2 would be a potential target to control UC-associated diarrhea.

Rapamycin Inhibition of mTOR Reduces Levels of the Na+/H+ Exchanger 3 in Intestines of Mice and Humans, Leading to Diarrhea

BACKGROUND & AIMS

The immunosuppressant rapamycin frequently causes noninfectious diarrhea in organ transplant recipients. We investigated the mechanisms of this process.

METHODS

We performed a retrospective analysis of renal transplant recipients treated with rapamycin from 2003 through 2010 at Albany Medical College, collecting data on serum levels of rapamycin. Levels of the Na+/H+ exchanger 3 (NHE3) were measured in human ileal biopsy specimens from patients who did and did not receive rapamycin (controls), in ileum tissues from rats or mice given rapamycin, and in mice with intestine-specific disruption of mammalian target of rapamycin (Mtor) (mTOR(f/f):Villin-cre mice) or Atg7 (Atg7(flox/flox); Villin-Cre). Exchange activity and intestinal water absorption were measured using a pH-sensitive dye and small intestine perfusion, respectively.

RESULTS

Episodes of noninfectious diarrhea occurred in organ recipients after increases in serum levels of rapamycin. The expression of NHE3 was reduced in the ileal brush border of patients with diarrhea. In rats and mice, continuous administration of low doses of rapamycin reduced levels of NHE3 in intestinal tissues; this effect was not observed in mice with intestinal deletion of ATG7, indicating that autophagy is required for the reduction. Administration of single high doses of rapamycin to mice, to model the spikes in rapamycin levels that occur in patients with severe diarrheal episodes, resulted in reduced phosphorylation of S6 and AKT in ileal tissues, indicating inhibition of the mTOR complex (mTORC1 and mTORC2). The intestines of mice with intestine-specific deletion of mTOR were dilated and contained large amounts of liquid stools; they also had reduced levels of total NHE3 and NHERF1 compared with control mice. We observed a significant reduction in Na(+)/H(+) exchange activity in ileum tissues from these mice.

CONCLUSIONS

Rapamycin inhibition of mTOR reduces levels of NHE3 and Na(+)/H(+) exchange activity in intestinal tissues of patients and rodents. This process appears to require the autophagic activity mediated by ATG7. Loss of mTOR regulation of NHE3 could mediate the development of diarrhea in patients undergoing rapamycin therapy.

Cloning and characterization of Na(+)/H(+) Exchanger isoforms NHE2 and NHE3 from the gill of Pacific dogfish Squalus suckleyi

Na(+)/H(+) Exchanger (NHE) proteins mediate cellular and systemic homeostasis of sodium and acid and may be the major sodium uptake method for fishes. We cloned and sequenced NHE2 and NHE3 from the gill of the North Pacific Spiny Dogfish shark Squalus suckleyi and expressed them in functional form in NHE-deficient (AP-1) cell lines. Estimated IC50 for inhibition of NHE activity by amiloride and EIPA were 55 μmol l(-1) and 4.8 μmol l(-1), respectively, for NHE2 and 9 μmol l(-1) and 24 μmol l(-1), respectively, for NHE3. Phenamil at 100 μmol l(-1) caused less than 16% inhibition of activity for each isoform. Although the IC50 are similar for the two isoforms, dfNHE2 is less sensitive than human NHE2 to inhibition by amiloride and EIPA, while dfNHE3 is more sensitive than human NHE3. These IC50 estimates should be considered when selecting inhibitor doses for fishes and for reinterpretation of previous studies that use these pharmacological agents.

Sorting nexin 27 regulates basal and stimulated brush border trafficking of NHE3

In polarized epithelial cells, SNX27 regulates PDZ domain–directed trafficking of NHE3 from endosomes to the plasma membrane and increases the stability of brush border NHE3. This establishes SNX27 as an important regulator of polarized sorting in epithelial cells.

Sorting nexin 27 (SNX27) contains a PDZ domain that is phylogenetically related to the PDZ domains of the NHERF proteins. Studies on nonepithelial cells have shown that this protein is located in endosomes, where it regulates trafficking of cargo proteins in a PDZ domain–dependent manner. However, the role of SNX27 in trafficking of cargo proteins in epithelial cells has not been adequately explored. Here we show that SNX27 directly interacts with NHE3 (C-terminus) primarily through the SNX27 PDZ domain. A combination of knockdown and reconstitution experiments with wild type and a PDZ domain mutant (GYGF → GAGA) of SNX27 demonstrate that the PDZ domain of SNX27 is required to maintain basal NHE3 activity and surface expression of NHE3 in polarized epithelial cells. Biotinylation-based recycling and degradation studies in intestinal epithelial cells show that SNX27 is required for the exocytosis (not endocytosis) of NHE3 from early endosome to plasma membrane. SNX27 is also required to regulate the retention of NHE3 on the plasma membrane. The findings of the present study extend our understanding of PDZ-mediated recycling of cargo proteins from endosome to plasma membrane in epithelial cells.

Nonsynonymous single nucleotide polymorphisms of NHE3 differentially decrease NHE3 transporter activity

Genetic determinants appear to play a role in susceptibility to chronic diarrhea, but the genetic abnormalities involved have only been identified in a few conditions. The Na⁺/H⁺ exchanger 3 (NHE3) accounts for a large fraction of physiologic intestinal Na⁺ absorption. It is highly regulated through effects on its intracellular COOH-terminal regulatory domain. The impact of genetic variation in the NHE3 gene, such as single nucleotide polymorphisms (SNPs), on transporter activity remains unexplored. From a total of 458 SNPs identified in the entire NHE3 gene, we identified three nonsynonymous mutations (R474Q, V567M, and R799C), which were all in the protein's intracellular COOH-terminal domain. Here we evaluated whether these SNPs affect NHE3 activity by expressing them in a mammalian cell line that is null for all plasma membrane NHEs. These variants significantly reduced basal NHE3 transporter activity through a reduction in intrinsic NHE3 function in variant R474Q, abnormal trafficking in variant V567M, or defects in both intrinsic NHE3 function and trafficking in variant R799C. In addition, variants NHE3 R474Q and R799C failed to respond to acute dexamethasone stimulation, suggesting cells with these mutant proteins might be defective in NHE3 function during postprandial stimulation and perhaps under stressful conditions. Finally, variant R474Q was shown to exhibit an aberrant interaction with calcineurin B homologous protein (CHP), an NHE3 regulatory protein required for basal NHE3 activity. Taken together, these results demonstrate decreased transport activity in three SNPs of NHE3 and provide mechanistic insight into how these SNPs impact NHE3 function.

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.

Lysophosphatidic acid stimulation of NHE3 exocytosis in polarized epithelial cells occurs with release from NHERF2 via ERK-PLC-PKCδ signaling

The Na(+)/H(+) exchanger 3 (NHE3) is a brush border (BB) Na(+)/H(+) antiporter that accounts for the majority of physiologic small intestinal and renal Na(+) absorption. It is regulated physiologically and in disease via changes in endocytosis/exocytosis. Paradoxically, NHE3 is fixed to the microvillar (MV) actin cytoskeleton and has little basal mobility. This fixation requires NHE3 binding to the multi-PDZ domain scaffold proteins Na(+)/H(+) exchanger regulatory factor (NHERF)1 and NHERF2 and to ezrin. Coordinated release of NHE3 from the MV cytoskeleton has been demonstrated during both stimulation and inhibition of NHE3. However, the signaling molecules involved in coordinating NHE3 trafficking and cytoskeletal association have not been identified. This question was addressed by studying lysophosphatidic acid (LPA) stimulation of NHE3 in polarized renal proximal tubule opossum kidney (OK) cells that occurs via apical LPA5 receptors and is NHERF2 dependent and mediated by epidermal growth factor receptor (EGFR), Rho/Rho-associated kinase (ROCK), and ERK. NHE3 activity was determined by BCECF/fluorometry and NHE3 microvillar mobility by FRAP/confocal microscopy using NHE3-EGFP. Apical LPA (3 μM)/LPA5R stimulated NHE3 activity, increased NHE3 mobility, and decreased the NHE3/NHERF2 association. The LPA stimulation of NHE3 was also PKCδ dependent. PKCδ was necessary for LPA stimulation of NHE3 mobility and NHE3/NHERF2 association. Moreover, the LPA-induced translocation to the membrane of PKCδ was both ERK and phospholipase C dependent with ERK acting upstream of PLC. We conclude that LPA stimulation of NHE3 exocytosis includes a signaling pathway that regulates fixation of NHE3 to the MV cytoskeleton. This involves a signaling module consisting of ERK-PLC-PKCδ, which dynamically and reversibly releases NHE3 from NHERF2 to contribute to the changes in NHE3 MV mobility.

Functional role of glucose metabolism, osmotic stress, and sodium-glucose cotransporter isoform-mediated transport on Na+/H+ exchanger isoform 3 activity in the renal proximal tubule

Na(+)-glucose cotransporter 1 (SGLT1)-mediated glucose uptake leads to activation of Na(+)-H(+) exchanger 3 (NHE3) in the intestine by a process that is not dependent on glucose metabolism. This coactivation may be important for postprandial nutrient uptake. However, it remains to be determined whether SGLT-mediated glucose uptake regulates NHE3-mediated NaHCO3 reabsorption in the renal proximal tubule. Considering that this nephron segment also expresses SGLT2 and that the kidneys and intestine show significant variations in daily glucose availability, the goal of this study was to determine the effect of SGLT-mediated glucose uptake on NHE3 activity in the renal proximal tubule. Stationary in vivo microperfusion experiments showed that luminal perfusion with 5 mM glucose stimulates NHE3-mediated bicarbonate reabsorption. This stimulatory effect was mediated by glycolytic metabolism but not through ATP production. Conversely, luminal perfusion with 40 mM glucose inhibited NHE3 because of cell swelling. Notably, pharmacologic inhibition of SGLT activity by Phlorizin produced a marked inhibition of NHE3, even in the absence of glucose. Furthermore, immunofluorescence experiments showed that NHE3 colocalizes with SGLT2 but not SGLT1 in the rat renal proximal tubule. Collectively, these findings show that glucose exerts a bimodal effect on NHE3. The physiologic metabolism of glucose stimulates NHE3 transport activity, whereas, supraphysiologic glucose concentrations inhibit this exchanger. Additionally, Phlorizin-sensitive SGLT transporters and NHE3 interact functionally in the proximal tubule.

IRBIT: a regulator of ion channels and ion transporters

IRBIT (also called AHCYL1) was originally identified as a binding protein of the intracellular Ca(2+) channel inositol 1,4,5-trisphosphate (IP3) receptor and functions as an inhibitory regulator of this receptor. Unexpectedly, many functions have subsequently been identified for IRBIT including the activation of multiple ion channels and ion transporters, such as the Na(+)/HCO3(-) co-transporter NBCe1-B, the Na(+)/H(+) exchanger NHE3, the Cl(-) channel cystic fibrosis transmembrane conductance regulator (CFTR), and the Cl(-)/HCO3(-) exchanger Slc26a6. The characteristic serine-rich region in IRBIT plays a critical role in the functions of this protein. In this review, we describe the evolution, domain structure, expression pattern, and physiological roles of IRBIT and discuss the potential molecular mechanisms underlying the coordinated regulation of these diverse ion channels/transporters through IRBIT. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

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.

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.

Functional vacuolar ATPase (V-ATPase) proton pumps traffic to the enterocyte brush border membrane and require CFTR

Vacuolar ATPases (V-ATPases) are highly conserved proton pumps that regulate organelle pH. Epithelial luminal pH is also regulated by cAMP-dependent traffic of specific subunits of the V-ATPase complex from endosomes into the apical membrane. In the intestine, cAMP-dependent traffic of cystic fibrosis transmembrane conductance regulator (CFTR) channels and the sodium hydrogen exchanger (NHE3) in the brush border regulate luminal pH. V-ATPase was found to colocalize with CFTR in intestinal CFTR high expresser (CHE) cells recently. Moreover, apical traffic of V-ATPase and CFTR in rat Brunner's glands was shown to be dependent on cAMP/PKA. These observations support a functional relationship between V-ATPase and CFTR in the intestine. The current study examined V-ATPase and CFTR distribution in intestines from wild-type, CFTR(-/-) mice and polarized intestinal CaCo-2BBe cells following cAMP stimulation and inhibition of CFTR/V-ATPase function. Coimmunoprecipitation studies examined V-ATPase interaction with CFTR. The pH-sensitive dye BCECF determined proton efflux and its dependence on V-ATPase/CFTR in intestinal cells. cAMP increased V-ATPase/CFTR colocalization in the apical domain of intestinal cells and redistributed the V-ATPase Voa1 and Voa2 trafficking subunits from the basolateral membrane to the brush border membrane. Voa1 and Voa2 subunits were localized to endosomes beneath the terminal web in untreated CFTR(-/-) intestine but redistributed to the subapical cytoplasm following cAMP treatment. Inhibition of CFTR or V-ATPase significantly decreased pHi in cells, confirming their functional interdependence. These data establish that V-ATPase traffics into the brush border membrane to regulate proton efflux and this activity is dependent on CFTR in the intestine.

The biophysical and molecular basis of intracellular pH sensing by Na+/H+ exchanger-3

Epithelial Na(+)/H(+) exchanger-3 (NHE3) transport is fundamental for renal and intestinal sodium reabsorption. Cytoplasmic protons are thought to serve as allosteric modifiers of the exchanger and to trigger its transport through protein conformational change. This effect presupposes an intracellular pH (pHi) dependence of NHE3 activity, although the biophysical and molecular basis of NHE3 pHi sensitivity have not been defined. NHE3, when complexed with the calcineurin homologous protein-1 (CHP1), had a shift in pHi sensitivity (0.4 units) toward the acidic side in comparison with NHE3 alone, as measured by oscillating pH electrodes combined with whole-cell patch clamping. Indeed, CHP1 interaction with NHE3 inhibited NHE3 transport in a pHi -dependent manner. CHP1 binding to NHE3 also affected its acute regulation. Intracellular perfusion of peptide from the CHP1 binding region (or pHi modification to reduce the CHP1 amount bound to NHE3) was permissive and cooperative for dopamine inhibition of NHE3 but reversed that of adenosine. Thus, CHP1 interaction with NHE3 apparently establishes the exchanger set point for pHi, and modification in this set point is effective in the hormonal stimuli-mediated regulation of NHE3. CHP1 may serve as a regulatory cofactor for NHE3 conformational change, dependent on intracellular protonation.

Proximal tubular NHEs: sodium, protons and calcium?

Na⁺/H⁺ exchange activity in the apical membrane of the proximal tubule is fundamental to the reabsorption of Na⁺ and water from the filtrate. The role of this exchange process in bicarbonate reclamation and, consequently, the maintenance of acid-base homeostasis has been appreciated for at least half a century and remains a pillar of renal tubular physiology. More recently, apical Na⁺/H⁺ exchange, mediated by Na⁺/H⁺ exchanger isoform 3 (NHE3), has been implicated in proximal tubular reabsorption of Ca²⁺ and Ca²⁺ homeostasis in general. Overexpression of NHE3 increased paracellular Ca²⁺ flux in a proximal tubular cell model. Consistent with this observation, mice with genetic deletion of Nhe3 have a noticable renal Ca²⁺ leak. These mice also display decreased intestinal Ca²⁺ uptake and osteopenia. This review highlights the traditional roles of proximal tubular Na⁺/H⁺ exchange and summarizes recent novel findings implicating the predominant isoform, NHE3, in Ca²⁺ homeostasis.

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.

Amino acid secondary transporters: toward a common transport mechanism

Solute carriers (SLC) that transport amino acids are key players in health and diseases in humans. Their prokaryotic relatives are often involved in essential physiological processes in microorganisms, e.g. in homeostasis and acidic/osmotic stress response. High-resolution X-ray structures of the sequence-unrelated amino acid transporters unraveled a striking structural similarity between carriers, which were formerly assigned to different families. The highly conserved fold is characterized by two inverted structural repeats of five transmembrane helices each and indicates common mechanistic transport concepts if not an evolutionary link among a large number of amino acid transporters. Therefore, these transporters are classified now into the structural amino acid-polyamine-organocation superfamily (APCS). The APCS includes among others the mammalian SLC6 transporters and the heterodimeric SLC7/SLC3 transporters. However, it has to be noted that the APCS is not limited entirely to amino acid transporters but contains also transporters for, e.g. amino acid derivatives and sugars. For instance, the betaine-choline-carnitine transporter family of bacterial activity-regulated Na(+)- and H(+)-coupled symporters for glycine betaine and choline is also part of this second largest structural superfamily. The APCS fold provides different possibilities to transport the same amino acid. Arginine can be transported by an H(+)-coupled symport or by antiport mechanism in exchange against agmatine for example. The convergence of the mechanistic concept of transport under comparable physiological conditions allows speculating if structurally unexplored amino acid transporters, e.g. the members of the SLC36 and SLC38 family, belong to the APCS, too. In the kidney, which is an organ that depends critically on the regulated amino acid transport, these different SLC transporters have to work together to account for proper function. Here, we will summarize the basic concepts of Na(+)- and H(+)-coupled amino acid symport and amino acid-product antiport in the light of the respective physiological requirements.

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

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

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.

Gastrin induces sodium-hydrogen exchanger 3 phosphorylation and mTOR activation via a phosphoinositide 3-kinase-/protein kinase C-dependent but AKT-independent pathway in renal proximal tubule cells derived from a normotensive male human

Gastrin is natriuretic, but its renal molecular targets and signal transduction pathways are not fully known. In this study, we confirmed the existence of CCKBR (a gastrin receptor) in male human renal proximal tubule cells and discovered that gastrin induced S6 phosphorylation, a downstream component of the phosphatidylinositol 3 kinase (PI3 kinase)-mammalian target of rapamycin pathway. Gastrin also increased the phosphorylation of sodium-hydrogen exchanger 3 (NHE3) at serine 552, caused its internalization, and decreased its expression at the cell surface and NHE activity. The phosphorylation of NHE3 and S6 was dependent on PI3 kinases because it was blocked by 2 different PI3-kinase inhibitors, wortmannin and LY294,002. The phosphorylation of NHE3 and S6 was not affected by the protein kinase A inhibitor H-89 but was blocked by a pan-PKC (chelerythrine) and a conventional PKC (cPKC) inhibitor (Gö6976) (10 μM) and an intracellular calcium chelator, 1,2-bis-(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, tetra(acetoxymethyl)-ester, suggesting the importance of cPKC and intracellular calcium in the gastrin signaling pathway. The cPKC involved was probably PKCα because it was phosphorylated by gastrin. The gastrin-mediated phosphorylation of NHE3, S6, and PKCα was via phospholipase C because it was blocked by a phospholipase C inhibitor, U73122 (10 μM). The phosphorylation (activation) of AKT, which is usually upstream of mammalian target of rapamycin in the classic PI3 kinase-AKT-p70S6K signaling pathway, was not affected, suggesting that the gastrin-induced phosphorylation of NHE3 and S6 is dependent on both PI3 kinase and PKCα but not AKT.

Cortisol regulates Na+ uptake in zebrafish, Danio rerio, larvae via the glucocorticoid receptor

Unlike other freshwater fish previously examined, zebrafish are capable of increasing their rate of Na(+) uptake during chronic exposure to acidic water (pH 4). In the present study, the potential role of cortisol in the induction of Na(+) uptake during acid-exposure was investigated. When zebrafish larvae (4 days post-fertilization) were treated with waterborne cortisol, the rate of Na(+) uptake was significantly increased; this effect was blocked by co-incubating larvae with RU-486, an antagonist selective for the glucocorticoid receptor (GR). A similar induction in Na(+) uptake, which was also blocked by RU-486, was observed when larvae were treated with dexamethasone, a selective GR agonist. Conversely, treating larvae with aldosterone, a selective agonist for the mineralocorticoid receptor (MR) had no effect on Na(+) uptake. Acid-exposure increased whole body cortisol levels and translational knockdown of GR using antisense morpholinos prevented the full induction of Na(+) uptake during exposure to acidic water, further confirming the role of cortisol and GR in Na(+) uptake stimulation. Using immunohistochemistry, GR was localized to ionocytes known to be responsible for Na(+) uptake (HR-cells). Knockdown of Rhcg1, an apical membrane ammonia channel or Na(+)/H(+) exchanger 3b (NHE3b), proteins known to play an important role in facilitating Na(+) uptake in acidic water, prevented the stimulatory effects of cortisol treatment on Na(+) uptake, suggesting that cortisol regulates Na(+) uptake by stimulating an Rhcg1-NHE3b "functional metabolon".

Scaffold protein connector enhancer of kinase suppressor of Ras isoform 3 (CNK3) coordinates assembly of a multiprotein epithelial sodium channel (ENaC)-regulatory complex

Hormone regulation of ion transport in the kidney tubules is essential for fluid and electrolyte homeostasis in vertebrates. A large body of evidence has suggested that transporters and channels exist in multiprotein regulatory complexes; however, relatively little is known about the composition of these complexes or their assembly. The epithelial sodium channel (ENaC) in particular is tightly regulated by the salt-regulatory hormone aldosterone, which acts at least in part by increasing expression of the serine-threonine kinase SGK1. Here we show that aldosterone induces the formation of a 1.0-1.2-MDa plasma membrane complex, which includes ENaC, SGK1, and the ENaC inhibitor Nedd4-2, a key target of SGK1. We further show that this complex contains the PDZ domain-containing protein connector enhancer of kinase suppressor of Ras isoform 3 (CNK3). CNK3 physically interacts with ENaC, Nedd4-2, and SGK1; enhances the interactions among them; and stimulates ENaC function in a PDZ domain-dependent, aldosterone-induced manner. These results strongly suggest that CNK3 is a molecular scaffold, which coordinates the assembly of a multiprotein ENaC-regulatory complex and hence plays a central role in Na(+) homeostasis.

Calmodulin kinase II constitutively binds, phosphorylates, and inhibits brush border Na+/H+ exchanger 3 (NHE3) by a NHERF2 protein-dependent process

The epithelial brush border (BB) Na(+)/H(+) exchanger 3 (NHE3) accounts for most renal and intestinal Na(+) absorption. Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) inhibits NHE3 activity under basal conditions in intact intestine, acting in the BB, but the mechanism is unclear. We now demonstrate that in both PS120 fibroblasts and polarized Caco-2BBe cells expressing NHE3, CaMKII inhibits basal NHE3 activity, because the CaMKII-specific inhibitors KN-93 and KN-62 stimulate NHE3 activity. This inhibition requires NHERF2. CaMKIIγ associates with NHE3 between aa 586 and 605 in the NHE3 C terminus in a Ca(2+)-dependent manner, with less association when Ca(2+) is increased. CaMKII inhibits NHE3 by an effect on its turnover number, not changing surface expression. Back phosphorylation demonstrated that NHE3 is phosphorylated by CaMKII under basal conditions. This overall phosphorylation of NHE3 is not affected by the presence of NHERF2. Amino acids downstream of NHE3 aa 690 are required for CaMKII to inhibit basal NHE3 activity, and mutations of the three putative CaMKII phosphorylation sites downstream of aa 690 each prevented KN-93 stimulation of NHE3 activity. These studies demonstrate that CaMKIIγ is a novel NHE3-binding protein, and this association is reduced by elevated Ca(2+). CaMKII inhibits basal NHE3 activity associated with phosphorylation of NHE3 by effects requiring aa downstream of NHE3 aa 690 and of the CaMKII-binding site on NHE3. CaMKII binding to and phosphorylation of the NHE3 C terminus are parts of the physiologic regulation of NHE3 that occurs in fibroblasts as well as in the BB of an intestinal Na(+)-absorptive cell.

The interaction between megalin and ClC-5 is scaffolded by the Na⁺-H⁺ exchanger regulatory factor 2 (NHERF2) in proximal tubule cells

Albumin endocytosis in the proximal tubule is mediated by a number of proteins, including the scavenger receptor megalin/cubilin and the PSD-95/Dlg/ZO-1 (PDZ) scaffolds NHERF1 and NHERF2. In addition, in a number of in vitro and in vivo models, the loss of ClC-5 results in a decreased cell surface expression and whole cell level of megalin, suggesting an interaction between these two proteins in vivo. We investigated if ClC-5 and megalin interact directly, and as ClC-5 binds to NHERF2, we investigated if this PDZ scaffold was required for a megalin/ClC-5 complex. GST-pulldown and immunoprecipitation experiments using rat kidney lysate demonstrated an interaction between ClC-5 and megalin, which was mediated by their C-termini. As this interaction may be controlled by a scaffold protein, we characterised any interaction between megalin and NHERF2. Immunoprecipitation experiments indicated that megalin interacts with NHERF2 in vivo, and that this interaction was via an internal NHERF binding domain in the C-terminus of megalin and PDZ2 and the C-terminus of NHERF2. Silencing NHERF2 had no effect on megalin protein levels in the whole cell or plasma membrane. Using siRNA against NHERF2, we demonstrated that NHERF2 was required to facilitate the interaction between megalin and ClC-5. Using fusion proteins, we characterised a protein complex containing ClC-5 and megalin, which is scaffolded by NHERF2, in the absence of any other proteins. Importantly, these observations are the first to describe an interaction between megalin and ClC-5, which is scaffolded by NHERF2 in proximal tubule cells.

Developing novel antisecretory drugs to treat infectious diarrhea

Diarrhea, a disease of poverty and poor sanitation, kills an estimated two million children each year. Oral rehydration therapy is a very simple and inexpensive treatment that has significantly reduced mortality from secretory diarrhea caused by rotavirus, cholera and enterotoxigenic Escherichia coli. The efficacy and adoption of oral rehydration therapy would be enhanced by a drug that reduces fluid loss associated with these diseases and alleviates disease symptoms. Secretion and absorption by the intestine offer a number of potential drug targets to reduce fluid loss. Among these, the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel is the most attractive because it is the primary driver of secretion in cases of diarrhea caused by enterotoxigenic bacteria. CFTR can be inhibited by both natural products and synthetic small molecules. iOWH032 is a synthetic CFTR inhibitor that has recently entered clinical trials for this indication.

Dynamic Na+-H+ exchanger regulatory factor-1 association and dissociation regulate parathyroid hormone receptor trafficking at membrane microdomains

Na/H exchanger regulatory factor-1 (NHERF1) is a cytoplasmic PDZ (postsynaptic density 95/disc large/zona occludens) protein that assembles macromolecular complexes and determines the localization, trafficking, and signaling of select G protein-coupled receptors and other membrane-delimited proteins. The parathyroid hormone receptor (PTHR), which regulates mineral ion homeostasis and bone turnover, is a G protein-coupled receptor harboring a PDZ-binding motif that enables association with NHERF1 and tethering to the actin cytoskeleton. NHERF1 interactions with the PTHR modify its trafficking and signaling. Here, we characterized by live cell imaging the mechanism whereby NHERF1 coordinates the interactions of multiple proteins, as well as the fate of NHERF1 itself upon receptor activation. Upon PTHR stimulation, NHERF1 rapidly dissociates from the receptor and induces receptor aggregation in long lasting clusters that are enriched with the actin-binding protein ezrin and with clathrin. After NHERF1 dissociates from the PTHR, ezrin then directly interacts with the PTHR to stabilize the PTHR at the cell membrane. Recruitment of β-arrestins to the PTHR is delayed until NHERF1 dissociates from the receptor, which is then trafficked to clathrin for internalization. The ability of NHERF to interact dynamically with the PTHR and cognate adapter proteins regulates receptor trafficking and signaling in a spatially and temporally coordinated manner.

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.

Elevated calcium acutely regulates dynamic interactions of NHERF2 and NHE3 proteins in opossum kidney (OK) cell microvilli

The brush border (BB) Na(+)/H(+) exchanger NHE3 is rapidly activated or inhibited by changes in trafficking, which mimics renal and intestinal physiology. However, there is a paradox in that NHE3 has limited mobility in the BB due to its binding to the multi-PDZ domain containing the NHERF family. To allow increased endocytosis, as occurs with elevated intracellular Ca(2+), we hypothesized that NHE3 had to be, at least transiently, released from the BB cytoskeleton. Because NHERF1 and -2 are localized at the BB, where they bind NHE3 as well as the cytoskeleton, we tested whether either or both might dynamically interact with NHE3 as part of Ca(2+) signaling. We employed FRET to study close association of NHE3 and these NHERFs and fluorescence recovery after photobleaching to monitor NHE3 mobility in the apical domain in polarized opossum kidney cells. Under basal conditions, NHERF2 and NHE3 exhibited robust FRET signaling. Within 1 min of A23187 (0.5 μm) exposure, the NHERF2-NHE3 FRET signal was abolished, and BB NHE3 mobility was transiently increased. The dynamics in FRET signal and NHE3 mobility correlated well with a change in co-precipitation of NHE3 and NHERF2 but not NHERF1. We conclude the following. 1) Under basal conditions, NHE3 closely associates with NHERF2 in opossum kidney cell microvilli. 2) Within 1 min of elevated Ca(2+), the close association of NHE3-NHERF2 is abolished but is re-established in ∼60 min. 3) The change in NHE3-NHERF2 association is accompanied by an increased BB mobile fraction of NHE3, which contributes to inhibition of NHE3 transport activity via increased endocytosis.

Intestinal anion exchange in marine teleosts is involved in osmoregulation and contributes to the oceanic inorganic carbon cycle

Marine teleost fish osmoregulation involves seawater ingestion and intestinal fluid absorption. Solute coupled fluid absorption by the marine teleost fish intestine has long been believed to be the product of Na(+) and Cl(-) absorption via the Na(+) :K(+) :2Cl(-) co-transporter (NKCC2). However, the past decade has revealed that intestinal anion exchange contributes significantly to Cl(-) absorption, in exchange for HCO(3) (-) secretion, and that this process is important for intestinal water absorption. In addition to contributing to solute coupled water absorption intestinal anion exchange results in luminal precipitation of CaCO(3) which acts to reduce luminal osmotic pressure and thus assist water absorption. Most recently, activity of apical H(+) -pumps, especially in distal segments of the intestine have been suggested to not only promote anion exchange, but also to reduce luminal osmotic pressure by preventing excess HCO(3)(-) concentrations from accumulating in intestinal fluids, thereby aiding water absorption. The present review summarizes and synthesizes the most recent advances in our view of marine teleosts osmoregulation, including our emerging understanding of epithelial transport of acid-base equivalents in the intestine, the consequences for whole organism acid-base balance and finally the impact of piscine CaCO(3) formation on the global oceanic carbon cycle.

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.

Organellar Na+/H+ exchangers: novel players in organelle pH regulation and their emerging functions

Mammalian Na+/H+ exchangers (NHEs) play a fundamental role in cellular ion homeostasis. NHEs exhibit an appreciable variation in expression, regulation, and physiological function, dictated by their dynamics in subcellular localization and/or interaction with regulatory proteins. In recent years, a subgroup of NHEs consisting of four isoforms has been identified, and its members predominantly localize to the membranes of the Golgi apparatus and endosomes. These organellar NHEs constitute a family of transporters with an emerging function in the regulation of luminal pH and in intracellular membrane trafficking as expressed, for example, in cell polarity development. Moreover, specific roles of a variety of cofactors, regulating the intracellular dynamics of these transporters, are also becoming apparent, thereby providing further insight into their mechanism of action and overall functioning. Interestingly, organellar NHEs have been related to mental disorders, implying a potential role in the brain, thus expanding the physiological significance of these transporters.

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.

Dopamine regulation of Na+-K+-ATPase requires the PDZ-2 domain of sodium hydrogen regulatory factor-1 (NHERF-1) in opossum kidney cells

Na(+)-K(+)-ATPase activity in renal proximal tubule is regulated by several hormones including parathyroid hormone (PTH) and dopamine. The current experiments explore the role of Na(+)/H(+) exchanger regulatory factor 1 (NHERF-1) in dopamine-mediated regulation of Na(+)-K(+)-ATPase. We measured dopamine regulation of ouabain-sensitive (86)Rb uptake and Na(+)-K(+)-ATPase α1 subunit phosphorylation in wild-type opossum kidney (OK) (OK-WT) cells, OKH cells (NHERF-1-deficient), and OKH cells stably transfected with full-length human NHERF-1 (NF) or NHERF-1 constructs with mutated PDZ-1 (Z1) or PDZ-2 (Z2) domains. Treatment with 1 μM dopamine decreased ouabain-sensitive (86)Rb uptake, increased phosphorylation of Na(+)-K(+)-ATPase α1-subunit, and enhanced association of NHERF-1 with D1 receptor in OK-WT cells but not in OKH cells. Transfection with wild-type, full-length, or PDZ-1 domain-mutated NHERF-1 into OKH cells restored dopamine-mediated regulation of Na(+)-K(+)-ATPase and D1-like receptor association with NHERF-1. Dopamine did not regulate Na(+)-K(+)-ATPase or increase D1-like receptor association with NHERF-1 in OKH cells transfected with mutated PDZ-2 domain. Dopamine stimulated association of PKC-ζ with NHERF-1 in OK-WT and OKH cells transfected with full-length or PDZ-1 domain-mutated NHERF-1 but not in PDZ-2 domain-mutated NHERF-1-transfected OKH cells. These results suggest that NHERF-1 mediates Na(+)-K(+)-ATPase regulation by dopamine through its PDZ-2 domain.

NHE3 activity is dependent on direct phosphoinositide binding at the N terminus of its intracellular cytosolic region

The small intestinal BB Na(+)/H(+) antiporter NHE3 accounts for the majority of intestinal sodium and water absorption. It is highly regulated with both postprandial inhibition and stimulation sequentially occurring. Phosphatidylinositide 4,5-bisphosphate (PI(4,5)P(2)) and phosphatidylinositide 3,4,5-trisphosphate (PI(3,4,5)P(3)) binding is involved with regulation of multiple transporters. We tested the hypothesis that phosphoinositides bind NHE3 under basal conditions and are necessary for its acute regulation. His(6) proteins were made from the NHE3 C-terminal region divided into four parts as follows: F1 (amino acids 475-589), F2 (amino acids 590-667), F3 (amino acids 668-747), and F4 (amino acids 748-832) and purified by a nickel column. Mutations were made in the F1 region of NHE3 and cloned in pet30a and pcDNA3.1 vectors. PI(4,5)P(2) and PI(3,4,5)P(3) bound only to the NHE3 F1 fusion protein (amino acids 475-589) on liposomal pulldown assays. Mutations were made in the putative lipid binding region of the F1 domain and studied for alterations in lipid binding and Na(+)/H(+) exchange as follows: Y501A/R503A/K505A; F509A/R511A/R512A; R511L/R512L; R520/FR527F; and R551L/R552L. Our results indicate the following. 1) The F1 domain of the NHE3 C terminus has phosphoinositide binding regions. 2) Mutations of these regions alter PI(4,5)P(2) and PI(3,4,5)P(3) binding and basal NHE3 activity. 3) The magnitude of serum stimulation of NHE3 correlates with PI(4,5)P(2) and PI(3,4,5)P(3) binding of NHE3. 4) Wortmannin inhibition of PI3K did not correlate with PI(4,5)P(2) or PI(3,4,5)P(3) binding of NHE3. Two functionally distinct phosphoinositide binding regions (Tyr(501)-Arg(512) and Arg(520)-Arg(552)) are present in the NHE3 F1 domain; both regions are important for serum stimulation, but they display differences in phosphoinositide binding, and the latter but not the former alters NHE3 surface expression.

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.

Ion transport in the small intestine

PURPOSE OF REVIEW

The 2009 review on small intestinal ion transport, in this series, focused on recent advances in duodenal bicarbonate secretion, the importance of scaffolding proteins and the pathophysiology of inflammation-associated diarrhea. The current review focuses on advances in ion-coupled solute transport, the dynamic role of the paracellular pathway in transepithelial-fluid transport and of elucidating the cellular basis of diarrheas associated with enteric infections.

RECENT FINDINGS

In understanding the cellular pathophysiology underlying diarrheal diseases, there is increased focus on the role of altering Na absorptive mechanisms as well as the role of the paracellular pathway. This is not to minimize the role of Cl-secretory pathways, especially cystic fibrosis transmembrane conductance regulator (CFTR), which continues to have pleiotropic roles in modulating other transporters. The Na-glucose cotransporter (SGLT) was the first transporter ever to be cloned. Twenty-one years later, with another first, the crystal structure of the related Na-galactose transporter has been described and opens new avenues to understand structure-function relationships and intelligent drug design for transporters.

SUMMARY

Progress continues to be made on integrating information obtained from reductionist models into more complex in-vivo animal models and where possible in human studies. Recognition of the coordinated regulation of cellular Na absorptive and Cl-secretory pathways together with the paracellular route in health and disease will help develop a more holistic picture of the multifaceted nature of small intestinal ion transport.

Shiga toxin 1 interaction with enterocytes causes apical protein mistargeting through the depletion of intracellular galectin-3

Shiga toxins (Stx) 1 and 2 are responsible for intestinal and systemic sequelae of infection by enterohemorrhagic Escherichia coli (EHEC). However, the mechanisms through which enterocytes are damaged remain unclear. While secondary damage from ischemia and inflammation are postulated mechanisms for all intestinal effects, little evidence excludes roles for more primary toxin effects on intestinal epithelial cells. We now document direct pathologic effects of Stx on intestinal epithelial cells. We study a well-characterized rabbit model of EHEC infection, intestinal tissue and stool samples from EHEC-infected patients, and T84 intestinal epithelial cells treated with Stx1. Toxin uptake by intestinal epithelial cells in vitro and in vivo causes galectin-3 depletion from enterocytes by increasing the apical galectin-3 secretion. This Shiga toxin-mediated galectin-3 depletion impairs trafficking of several brush border structural proteins and transporters, including villin, dipeptidyl peptidase IV, and the sodium-proton exchanger 2, a major colonic sodium absorptive protein. The mistargeting of proteins responsible for the absorptive function might be a key event in Stx1-induced diarrhea. These observations provide new evidence that human enterocytes are directly damaged by Stx1. Conceivably, depletion of galectin-3 from enterocytes and subsequent apical protein mistargeting might even provide a means whereby other pathogens might alter intestinal epithelial absorption and produce diarrhea.