S3226, a novel inhibitor of Na+/H+ exchanger subtype 3 in various cell types

Proton-driven sodium secretion in a saline water animal

Aquatic animals residing in saline habitats either allow extracellular sodium concentration to conform to environmental values or regulate sodium to lower levels. The latter strategy requires an energy-driven process to move sodium against a large concentration gradient to eliminate excess sodium that diffuses into the animal. Previous studies of invertebrate and vertebrate species indicate a sodium pump, Na+/K+ ATPase, powers sodium secretion. We provide the first functional evidence of a saline-water animal, Aedes taeniorhynchus mosquito larva, utilizing a proton pump to power this process. Vacuolar-type H+ ATPase (VHA) protein is highly expressed on the apical membrane of the posterior rectal cells, and in situ sodium flux across this epithelium increases significantly in larvae held in higher salinity and is sensitive to Bafilomycin A1, an inhibitor of VHA. We also report the first evidence of splice variants of the sodium/proton exchanger, NHE3, with both high and low molecular weight variants highly expressed on the apical membrane of the posterior rectal cells. Evidence of NHE3 function was indicated with in situ sodium transport significantly inhibited by a NHE3 antagonist, S3226. We propose that the outward proton pumping by VHA establishes a favourable electromotive gradient to drive sodium secretion via NHE3 thus producing a hyperosmotic, sodium-rich urine. This H+- driven Na+ secretion process is the primary mechanism of ion regulation in salt-tolerant culicine mosquito species and was first investigated over 80 years ago.

Ion uptake in naturally acidic water

The first studies on ion regulation in fish exposed to low pH, which were inspired by the Acid Rain environmental crisis, seemed to indicate that ion transport at the gills was completely and irreversibly inhibited at pH 4.0-4.5 and below. However, work on characid fish native to the Rio Negro, a naturally acidic, blackwater tributary of the Amazon River, found that they possess ion transport mechanisms that are completely insensitive to pHs as low as 3.25. As more species were examined it appeared that pH-insensitive transport was a trait shared by many, if not most, species in the Order Characiformes. Subsequently, a few other species of fish have been shown to be able to transport ions at low pH, in particular zebrafish (Danio rerio), which show rapid recovery of Na+ uptake at pH 4.0 after initial inhibition. Measurements of rates of Na+ transport during exposure to pharmacological agents that inhibit various transport proteins suggested that characiform fish do not utilize the generally accepted mechanisms for Na+ transport that rely on some form of H+ extrusion. Examination of zebrafish transport at low pH suggest the rapid recovery may be due to a novel Na+/K+ exchanger, but after longer term exposure they may rely on a coupling of Na+/H+ exchangers and NH3 excretion. Further work is needed to clarify these mechanisms of transport and to find other acid-tolerant species to fully gain an appreciation of the diversity of physiological mechansisms involved.

Acidosis defense mechanisms in the preimplantation stages of embryos in BALB/c strain mice

Regulation of intracellular pH (pHi) is an important homeostatic function of cells. There are three major pHi regulatory mechanisms: the HCO3-/Cl- exchanger (AE), which alleviates alkalosis, and the Na+/H+ exchanger (NHE) and Na+,HCO3-/Cl- exchanger (NDBCE), both of which counteract acidosis. NHE activity, which is high at the germinal vesicle stage of oocyte, is inhibited during meiotic maturation, while this inhibition is abolished when the oocyte reaches the pronuclear (PN) stage of the zygote. On the other hand, we have previously found that NDBCE performs complementary regulation against acidosis during meiotic maturation. Additionally, we found that AE activity, which is a defense mechanism against alkalosis, gradually decreases during preimplantation period of embryonic development. Considering that NHE activity is inhibited during meiotic maturation and AE activity gradually decreases during embryonic development stages, we investigated whether NHE and NDBCE activities, both of which act against acidosis, functionally change from the PN zygote to the blastocyst stage of the embryo and identified these pH-regulating proteins at the molecular level in mice of the Balb/c strain. PN zygotes, two-cell (2-c), four-cell (4-c), morula and blastocyst stage embryos were obtained from 5-8-week-old, sexually mature female Balb/c mice by using the classical superovulation procedure. pHi was recorded by using the microspectrofluorometric technique on zygotes and embryos simultaneously loaded with the pH-sensitive fluorophore, 2',7'-Bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). The activities of NHE and NDBCE were determined from the recovery curve of induced-acidosis in bicarbonate-free and bicarbonate-containing media, respectively. Specific inhibitors such as cariporide (1 μM), S3226 (1 and 10 μM), EIPA (1, 5, and 25 μM), and amiloride (1 mM) were used to functionally identify NHE isoforms, and the nonspecific inhibitor 4,4'-diisocyanatostilbene-2,2' disulphonic acid, disodium salt (DIDS) was used to confirm NDBCE activity. The isoforms of the pHi-regulatory proteins were also identified by molecular biology using real-time PCR. We found that NHE activity was high at all embryonic stages, and differences between stages were not significant. Functional and molecular findings indicated that isoforms of NHE 1 and 5 are present in the blastocyst, whereas isoforms of NHE 1, 3, and 4 are functional at earlier embryonic stages. Although the contribution of NDBCE activity to recovery from induced-acidosis was detected at all embryonic stages, it was significant only in the PN zygote and the 2-c embryo. This finding was confirmed by molecular analysis, which detected the expression of SLC4A8 encoding NDBCE at all embryonic stages. In conclusion, NHE is an active and important defense mechanism against acidosis and is encoded by at least two protein isoforms in all stages of the Balb/c strain of mice. NDBCE has a supportive function in all embryonic stages, especially in the PN zygote and the 2-c embryo. Preimplantation stage embryos have effective mechanisms to defend against acidosis in response to their metabolic end products (increased acid load) and the acidic environment in utero.

The effect of claudin-15 deletion on cationic selectivity and transport in paracellular pathways of the cecum and large intestine

The large intestine plays a pivotal role in water and electrolyte balance. Paracellular transport may play a role in ion transport mechanisms in the cecum and large intestine; however, these molecular mechanisms and their physiological roles have not been fully studied. Claudin-15 forms a cation channel in tight junctions in the small intestine, but its role in the cecum and large intestine has not been investigated. This study aimed to explore the physiological role of claudin-15 in the cecum and large intestine using claudin-15 (Cldn15) KO mice. Electrical conductance, short-circuit current, Na⁺ flux, and dilution potential were assessed in isolated tissue preparations mounted in Ussing chambers. The induced short-circuit current of short-chain fatty acids, which are fermentative products in the intestinal tract, was also measured. Compared to wild type mice, the electrical conductance and paracellular Na⁺ flux was decreased in the cecum, but not the middle large intestine, while in both the cecum and the middle large intestine, paracellular Na⁺ permeability was decreased in Cldn15 KO mice. These results suggest that claudin-15 is responsible for Na⁺ permeability in the tight junctions of the cecum and large intestine and decreased Na⁺ permeability in the cecum may cause impaired absorption function.

Contact-dependent, polarized acidification response during neutrophil-epithelial interactions

Neutrophil (PMN) infiltration during active inflammation imprints changes in the local tissue environment. Such responses are often accompanied by significant extracellular acidosis that result in predictable transcriptional responses. In this study, we explore the mechanisms involved in inflammatory acidification as a result of PMN-intestinal epithelial cell (IEC) interactions. Using recently developed tools, we revealed that PMN transepithelial migration (TEM)-associated inflammatory acidosis is dependent on the total number of PMNs present during TEM and is polarized toward the apical surface. Extending these studies, we demonstrate that physical separation of the PMNs and IECs prevented acidification, whereas inhibition of PMN TEM using neutralizing antibodies enhanced extracellular acidification. Utilizing pharmaceutical inhibitors, we demonstrate that the acidification response is independent of myeloperoxidase and dependent on reactive oxygen species generated during PMN TEM. In conclusion, inflammatory acidosis represents a polarized PMN-IEC-dependent response by an as yet to be fully determined mechanism.

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.

Discovery of Tenapanor: A First-in-Class Minimally Systemic Inhibitor of Intestinal Na+/H+ Exchanger Isoform 3

We present herein the design, synthesis, and optimization of gut-restricted inhibitors of Na⁺/H⁺ exchanger isoform 3 (NHE3). NHE3 is predominantly expressed in the kidney and gastrointestinal tract where it acts as the major absorptive sodium transporter. We desired minimally systemic agents that would block sodium absorption in the gastrointestinal tract but avoid exposure in the kidney. Starting with a relatively low-potency highly bioavailable hit compound (1), potent and minimally absorbed NHE3 inhibitors were designed, culminating with the discovery of tenapanor (28). Tenapanor has been approved by the U.S. Food and Drug Administration (FDA) for the treatment of irritable bowel syndrome with constipation in adults.

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

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

New Insights into the Critical Importance of Intratubular Na+/H+ Exchanger 3 and Its Potential Therapeutic Implications in Hypertension

PURPOSE OF REVIEW

The sodium (Na⁺) and hydrogen (H⁺) exchanger 3 (NHE3), known as solute carrier family 9 member 3 (SLC9A3), mediates active transcellular Na⁺ and bicarbonate reabsorption in the small intestine of the gut and proximal tubules of the kidney. The purpose of this article is to review and discuss recent findings on the critical roles of intestinal and proximal tubule NHE3 in maintaining basal blood pressure (BP) homeostasis and their potential therapeutic implications in the development of angiotensin II (Ang II)-dependent hypertension.

RECENT FINDINGS

Recently, our and other laboratories have generated or used novel genetically modified mouse models with whole-body, kidney-specific, or proximal tubule-specific deletion of NHE3 to determine the critical roles and underlying mechanisms of NHE3 in maintaining basal BP homeostasis and the development of Ang II-induced hypertension at the whole-body, kidney, or proximal tubule levels. The new findings demonstrate that NHE3 contributes to about 10 to 15 mmHg to basal blood pressure levels, and that deletion of NHE3 at the whole-kidney or proximal tubule level, or pharmacological inhibition of NHE3 at the kidney level with an orally absorbable NHE3 inhibitor AVE-0657, attenuates ~ 50% of Ang II-induced hypertension in mice. The results support the proof-of-concept hypothesis that NHE3 plays critical roles in physiologically maintaining normal BP and in the development of Ang II-dependent hypertension. Our results also strongly suggest that NHE3 in the proximal tubules of the kidney may be therapeutically targeted to treat poorly controlled hypertension in humans.

Arsenic trioxide-increased MDCK cells proliferation requires activator protein 1-mediated increase of the sodium/proton exchanger 1 activity

The release of protons (H⁺) occurs via the Na⁺/H⁺ exchanger isoform 1 (NHE1) leading to a stable intracellular pH (pHi) in MDCK cells. Chronic intake of arsenic trioxide (ATO), in the drinking water, associated with higher morbidity and mortality in neoplastic tissues. ATO increased NHE1 expression and activity, resulting in intracellular alkalization and higher MDCK cells proliferation. Since the pro-proliferative transcription factor activator protein 1 (AP-1) gets activated by al alkaline intracellular pH, a phenomenon paralleled by higher NHEs activity, we asked whether ATO-increased MDCK cells proliferation involves AP-1-dependent NHE1 activation. Cells were exposed (48 h) to ATO (0.05 μmol/L), SR11302 (1 μmol/L, AP-1 inhibitor), HOE-694 (100 nmol/L, NHE1 inhibitor) and EIPA (50 μmol/L, NHE1/NHE3 inhibitor) in the presence of S3226 (10 μmol/L, NHE3 inhibitor), concanamycin A (0.1 μmol/L, V-ATPases inhibitor), and Schering (10 μmol/L, H⁺/K⁺-ATPase inhibitor). [³H]Thymidine incorporation, cell counting, wound healing assay, and AP-1 activity were determined. The pHi was measured in cells pre-loaded (10 min) with 2,7-bicarboxyethyl-5,6-carboxyfluorescein acetoxymethyl ester (12 mmol/L) and exposed to NH₄Cl (20 mmol/L). Basal pHi and recovery rate (dpHi/dt), intracellular buffer capacity (βi) and H⁺ flux (J_H⁺) were determined. NHE1 protein abundance was measured by Western blotting and immunofluorescence. ATO increased the cell growth (1.5 fold), basal pHi (0.4 pHi units), dpHi/dt (1.8 fold), J_H⁺ (1.4 fold), AP-1 activity and NHE1 protein abundance (1.3 fold). ATO also increased (1.5 fold) the nuclear/perinuclear NHE1 immunosignal. SR11302 and HOE-694 blocked ATO effects. Thus, ATO-increased proliferation resulted from AP-1-dependent NHE1 activation in MDCK cells.

Na+-Coupled Nutrient Cotransport Induced Luminal Negative Potential and Claudin-15 Play an Important Role in Paracellular Na+ Recycling in Mouse Small Intestine

Many nutrients are absorbed via Na⁺ cotransport systems, and therefore it is predicted that nutrient absorption mechanisms require a large amount of luminal Na⁺. It is thought that Na⁺ diffuses back into the lumen via paracellular pathways to support Na⁺ cotransport absorption. However, direct experimental evidence in support of this mechanism has not been shown. To elucidate this, we took advantage of claudin-15 deficient (cldn15^-/-) mice, which have been shown to have decreased paracellular Na⁺ permeability. We measured glucose-induced currents (ΔI_sc) under open- and short-circuit conditions and simultaneously measured changes in unidirectional ²²Na⁺ fluxes (ΔJ^Na) in Ussing chambers. Under short-circuit conditions, application of glucose resulted in an increase in ΔI_sc and unidirectional mucosal to serosal ²²Na⁺ (∆J^Na_MS) flux in both wild-type and cldn15^-/- mice. However, under open-circuit conditions, ΔI_sc was observed but ∆J^Na_MS was strongly inhibited in wild-type but not in cldn15^-/- mice. In addition, in the duodenum of mice treated with cholera toxin, paracellular Na⁺ conductance was decreased and glucose-induced ∆J^Na_MS increment was observed under open-circuit conditions. We concluded that the Na⁺ which is absorbed by Na⁺-dependent glucose cotransport is recycled back into the lumen via paracellular Na⁺ conductance through claudin-15, which is driven by Na⁺ cotransport induced luminal negativity.

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

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

Pharmacological reversal of renal cysts from secretion to absorption suggests a potential therapeutic strategy for managing autosomal dominant polycystic kidney disease

Autosomal-dominant polycystic kidney disease (ADPKD) induces a secretory phenotype, resulting in multiple fluid-filled cysts. We have previously demonstrated that VX-809, a corrector of the cystic fibrosis transmembrane conductance regulator (CFTR), reduces cyst growth. Here, we show that in normal mice CFTR is located within the cells and also at the apical and basolateral membranes. However, in polycystic kidney disease (pkd1)-knockout mice, CFTR was located at the plasma membrane, consistent with its role in cAMP-dependent fluid secretion. In cystic mice, VX-809 treatment increased CFTR levels at the apical membrane and reduced its association with the endoplasmic reticulum. Surprisingly, VX-809 treatment significantly increased CFTR's co-localization with the basolateral membrane in cystic mice. Na⁺/H⁺ exchanger 3 (NHE3) is present in pkd1-knockout and normal mice and in proximal tubule-derived, cultured pkd1-knockout cells. VX-809 increased the expression, activity, and apical plasma membrane localization of NHE3. Co-localization of epithelial sodium channel (ENaC) with the plasma membrane was reduced in cysts in pkd1-knockout mice, consistent with an inability of the cysts to absorb fluid. Interestingly, in the cystic mice, VX-809 treatment increased ENaC levels at the apical plasma membrane consistent with fluid absorption. Thus, VX-809 treatment of pkd1-null mouse kidneys significantly affected CFTR, NHE3, and ENaC, altering the cyst phenotype from one poised toward fluid secretion toward one more favorable for absorption. VX-809 also altered the location of CFTR but not of NHE3 or ENaC in normal mice. Given that VX-809 administration is safe, it may have potential utility for treating patients with ADPKD.

Solute carrier family 9, subfamily A, member 3 (SLC9A3)/sodium-hydrogen exchanger member 3 (NHE3) dysregulation and dilated intercellular spaces in patients with eosinophilic esophagitis

BACKGROUND

Eosinophilic esophagitis (EoE) is characterized by histopathologic modifications of esophageal tissue, including eosinophil-rich inflammation, basal zone hyperplasia, and dilated intercellular spaces (DIS). The underlying molecular processes that drive the histopathologic features of EoE remain largely unexplored.

OBJECTIVE

We sought to investigate the involvement of solute carrier family 9, subfamily A, member 3 (SLC9A3) in esophageal epithelial intracellular pH (pHi) and DIS formation and the histopathologic features of EoE.

METHODS

We examined expression of esophageal epithelial gene networks associated with regulation of pHi in the EoE transcriptome of primary esophageal epithelial cells and an in vitro esophageal epithelial 3-dimensional model system (EPC2-ALI). Molecular and cellular analyses and ion transport assays were used to evaluate the expression and function of SLC9A3.

RESULTS

We identified altered expression of gene networks associated with regulation of pHi and acid-protective mechanisms in esophageal biopsy specimens from pediatric patients with EoE (healthy subjects, n = 6; patients with EoE, n = 10). The most dysregulated gene central to regulating pHi was SLC9A3. SLC9A3 expression was increased within the basal layer of esophageal biopsy specimens from patients with EoE, and expression positively correlated with disease severity (eosinophils/high-power field) and DIS (healthy subjects, n = 10; patients with EoE, n = 10). Analyses of esophageal epithelial cells revealed IL-13-induced, signal transducer and activator of transcription 6-dependent SLC9A3 expression and Na+-dependent proton secretion and that SLC9A3 activity correlated positively with DIS formation. Finally, we showed that IL-13-mediated, Na+-dependent proton secretion was the primary intracellular acid-protective mechanism within the esophageal epithelium and that blockade of SLC9A3 transport abrogated IL-13-induced DIS formation.

CONCLUSIONS

SLC9A3 plays a functional role in DIS formation, and pharmacologic interventions targeting SLC9A3 function may suppress the histopathologic manifestations in patients with EoE.

Luminal Na+ homeostasis has an important role in intestinal peptide absorption in vivo

Intestinal cell line studies indicated luminal Na⁺ homeostasis is essential for proton-coupled peptide absorption, because the driving force of PepT1 activity is supported by the apical Na⁺/H⁺ exchanger NHE3. However, there is no direct evidence demonstrating the importance of in vivo luminal Na⁺ for peptide absorption in animal experiments. To investigate the relationship between luminal Na⁺ homeostasis and peptide absorption, we took advantage of claudin 15-deficient (cldn15^-/-) mice, whereby Na⁺ homeostasis is disrupted. We quantitatively assessed the intestinal segment responsible for peptide absorption using radiolabeled nonhydrolyzable dipeptide (glycylsarcosine, Gly-Sar) and nonabsorbable fluid phase marker polyethylene glycol (PEG) 4000 in vivo. In wild-type (WT) mice, the concentration ratio of Gly-Sar to PEG 4000 decreased in the upper jejunum, suggesting the upper jejunum is responsible for peptide absorption. Gly-Sar absorption was decreased in the jejunum of cldn15^-/- mice. To elucidate the mechanism underlining these impairments, a Gly-Sar-induced short-circuit ( I_sc) current was measured. In WT mice, increments of Gly-Sar-induced I_sc were inhibited by the luminal application of a NHE3-specific inhibitor S3226 in a dose-dependent fashion. In contrast to in vivo experiments, robust Gly-Sar-induced I_sc increments were observed in the jejunal mucosa of cldn15^-/- mice. Gly-Sar-induced I_sc was inhibited by S3226 or a reduction of luminal Na⁺ concentration, which mimics low luminal Na⁺ concentrations in vivo . Our study demonstrates that luminal Na⁺ homeostasis is important for peptide absorption in native epithelia and that there is a cooperative functional relationship between PepT1 and NHE3. NEW & NOTEWORTHY Our study is the first to demonstrate that luminal Na⁺ homeostasis is important for proton-coupled peptide absorption in in vivo animal experiments.

Linaclotide improves gastrointestinal transit in cystic fibrosis mice by inhibiting sodium/hydrogen exchanger 3

Gastrointestinal dysfunction in cystic fibrosis (CF) is a prominent source of pain among patients with CF. Linaclotide, a guanylate cyclase C (GCC) receptor agonist, is a US Food and Drug Administration-approved drug prescribed for chronic constipation but has not been widely used in CF, as the cystic fibrosis transmembrane conductance regulator (CFTR) is the main mechanism of action. However, anecdotal clinical evidence suggests that linaclotide may be effective for treating some gastrointestinal symptoms in CF. The goal of this study was to determine the effectiveness and mechanism of linaclotide in treating CF gastrointestinal disorders using CF mouse models. Intestinal transit, chloride secretion, and intestinal lumen fluidity were assessed in wild-type and CF mouse models in response to linaclotide. CFTR and sodium/hydrogen exchanger 3 (NHE3) response to linaclotide was also evaluated. Linaclotide treatment improved intestinal transit in mice carrying either F508del or null Cftr mutations but did not induce detectable Cl^- secretion. Linaclotide increased fluid retention and fluidity of CF intestinal contents, suggesting inhibition of fluid absorption. Targeted inhibition of sodium absorption by the NHE3 inhibitor tenapanor produced improvements in gastrointestinal transit similar to those produced by linaclotide treatment, suggesting that inhibition of fluid absorption by linaclotide contributes to improved gastrointestinal transit in CF. Our results demonstrate that linaclotide improves gastrointestinal transit in CF mouse models by increasing luminal fluidity through inhibiting NHE3-mediated sodium absorption. Further studies are necessary to assess whether linaclotide could improve CF intestinal pathologies in patients. GCC signaling and NHE3 inhibition may be therapeutic targets for CF intestinal manifestations. NEW & NOTEWORTHY Linaclotide's primary mechanism of action in alleviating chronic constipation is through cystic fibrosis transmembrane conductance regulator (CFTR), negating its use in patients with cystic fibrosis (CF). For the first time, our findings suggest that in the absence of CFTR, linaclotide can improve fluidity of the intestinal lumen through the inhibition of sodium/hydrogen exchanger 3. These findings suggest that linaclotide could improve CF intestinal pathologies in patients.

Cholera toxin inhibits SNX27-retromer-mediated delivery of cargo proteins to the plasma membrane

Cholera toxin (CT) causes severe diarrhea by increasing intracellular cAMP leading to a PKA-dependent increase in Cl^- secretion through CFTR and decreased Na⁺ absorption through inhibition of Na⁺/H⁺ exchanger 3 (NHE3; also known as SLC9A3). The mechanism(s) by which CT inhibits NHE3 is partially understood, although no drug therapy has been successful at reversing this inhibition. We now describe that CT phosphorylates an amino acid in the PDZ domain of SNX27, which inhibits SNX27-mediated trafficking of NHE3 from the early endosomes to the plasma membrane (PM), and contributes to reduced basal NHE3 activity through a mechanism that involves reduced PM expression and reduced endocytic recycling. Importantly, mutagenesis studies (Ser to Asp) showed that the effect of this phosphorylation of SNX27 phenocopies the effects seen upon loss of SNX27 function, affecting PM trafficking of cargo proteins that bind SNX27-retromer. Additionally, CT destabilizes retromer function by decreasing the amount of core retromer proteins. These effects of CT can be partially rescued by enhancing retromer stability by using 'pharmacological chaperones'. Moreover, pharmacological chaperones can be used to increase basal and cholera toxin-inhibited NHE3 activity and fluid absorption by intestinal epithelial cells.This article has an associated First Person interview with the first author of the paper.

The Myth of Water and Salt: From Aquaretics to Tenapanor

The impact of water intake has been studied in several renal diseases. For example, increasing water intake is useful to prevent primary and secondary nephrolithiasis. In autosomal dominant polycystic kidney disease, arginine vasopressin (AVP) is involved in the progression of the disease, and water intake could play a therapeutic role by inhibiting the synthesis of AVP, but its efficacy is still controversial. Conversely, the use of aquaretics, which are antagonists of AVP V2 receptors, results in the reduction of the increase rate of total kidney volume with a slower decline of glomerular filtration rate. In chronic kidney disease, AVP contributes to glomerular hyperfiltration, arterial hypertension, and synthesis of renin, resulting in renal sclerosis. Increased water intake could reduce AVP activation determining a potential protective effect on the kidney, but its efficacy has not yet been clearly demonstrated. On the other side, sodium and potassium play an important role in the control of arterial blood pressure and are involved in the development and progression of chronic kidney disease. Reduction of sodium intake and increase of potassium intake determine a decrease of arterial blood pressure with a beneficial effect on the kidney; however, adherence to sodium restriction is very poor. Regarding this, sodium-hydrogen exchanger isoform 3 inhibitors may reduce sodium absorption in the gut. The most recent sodium-hydrogen exchanger isoform 3 inhibitor, known as tenapanor, reduces extracellular fluid volume, left ventricular hypertrophy, albuminuria, and blood pressure in experimental studies and increases fecal loss of sodium in humans.

Molecular architecture underlying fluid absorption by the developing inner ear

Mutations of SLC26A4 are a common cause of hearing loss associated with enlargement of the endolymphatic sac (EES). Slc26a4 expression in the developing mouse endolymphatic sac is required for acquisition of normal inner ear structure and function. Here, we show that the mouse endolymphatic sac absorbs fluid in an SLC26A4-dependent fashion. Fluid absorption was sensitive to ouabain and gadolinium but insensitive to benzamil, bafilomycin and S3226. Single-cell RNA-seq analysis of pre- and postnatal endolymphatic sacs demonstrates two types of differentiated cells. Early ribosome-rich cells (RRCs) have a transcriptomic signature suggesting expression and secretion of extracellular proteins, while mature RRCs express genes implicated in innate immunity. The transcriptomic signature of mitochondria-rich cells (MRCs) indicates that they mediate vectorial ion transport. We propose a molecular mechanism for resorption of NaCl by MRCs during development, and conclude that disruption of this mechanism is the root cause of hearing loss associated with EES.

Splicing variation of Long-IRBIT determines the target selectivity of IRBIT family proteins

IRBIT [inositol 1,4,5-trisphosphate receptor (IP₃R) binding protein released with inositol 1,4,5-trisphosphate (IP₃)] is a multifunctional protein that regulates several target molecules such as ion channels, transporters, polyadenylation complex, and kinases. Through its interaction with multiple targets, IRBIT contributes to calcium signaling, electrolyte transport, mRNA processing, cell cycle, and neuronal function. However, the regulatory mechanism of IRBIT binding to particular targets is poorly understood. Long-IRBIT is an IRBIT homolog with high homology to IRBIT, except for a unique N-terminal appendage. Long-IRBIT splice variants have different N-terminal sequences and a common C-terminal region, which is involved in multimerization of IRBIT and Long-IRBIT. In this study, we characterized IRBIT and Long-IRBIT splice variants (IRBIT family). We determined that the IRBIT family exhibits different mRNA expression patterns in various tissues. The IRBIT family formed homo- and heteromultimers. In addition, N-terminal splicing of Long-IRBIT changed the protein stability and selectivity to target molecules. These results suggest that N-terminal diversity of the IRBIT family and various combinations of multimer formation contribute to the functional diversity of the IRBIT family.

Discovery and Optimization of 1-Phenoxy-2-aminoindanes as Potent, Selective, and Orally Bioavailable Inhibitors of the Na+/H+ Exchanger Type 3 (NHE3)

The design, synthesis, and structure-activity relationship of 1-phenoxy-2-aminoindanes as inhibitors of the Na⁺/H⁺ exchanger type 3 (NHE3) are described based on a hit from high-throughput screening (HTS). The chemical optimization resulted in the discovery of potent, selective, and orally bioavailable NHE3 inhibitors with 13d as best compound, showing high in vitro permeability and lacking CYP2D6 inhibition as main optimization parameters. Aligning 1-phenoxy-2-aminoindanes onto the X-ray structure of 13d then provided 3D-QSAR models for NHE3 inhibition capturing guidelines for optimization. These models showed good correlation coefficients and allowed for activity estimation. In silico ADMET models for Caco-2 permeability and CYP2D6 inhibition were also successfully applied for this series. Moreover, docking into the CYP2D6 X-ray structure provided a reliable alignment for 3D-QSAR models. Finally 13d, renamed as SAR197, was characterized in vitro and by in vivo pharmacokinetic (PK) and pharmacological studies to unveil its potential for reduction of obstructive sleep apneas.

An animal homolog of plant Mep/Amt transporters promotes ammonia excretion by the anal papillae of the disease vector mosquito Aedes aegypti

The transcripts of three putative ammonia (NH3/NH4 (+)) transporters, Rhesus-like glycoproteins AeRh50-1, AeRh50-2 and Amt/Mep-like AeAmt1 were detected in the anal papillae of larval Aedes aegypti Quantitative PCR studies revealed 12-fold higher transcript levels of AeAmt1 in anal papillae relative to AeRh50-1, and levels of AeRh50-2 were even lower. Immunoblotting revealed AeAmt1 in anal papillae as a pre-protein with putative monomeric and trimeric forms. AeAmt1 was immunolocalized to the basal side of the anal papillae epithelium where it co-localized with Na(+)/K(+)-ATPase. Ammonium concentration gradients were measured adjacent to anal papillae using the scanning ion-selective electrode technique (SIET) and used to calculate ammonia efflux by the anal papillae. dsRNA-mediated reductions in AeAmt1 decreased ammonia efflux at larval anal papillae and significantly increased ammonia levels in hemolymph, indicating a principal role for AeAmt1 in ammonia excretion. Pharmacological characterization of ammonia transport mechanisms in the anal papillae suggests that, in addition to AeAmt1, the ionomotive pumps V-type H(+)-ATPase and Na(+)/K(+)-ATPase as well as NHE3 are involved in ammonia excretion at the anal papillae.

Human Enteroids as a Model of Upper Small Intestinal Ion Transport Physiology and Pathophysiology

BACKGROUND & AIMS

Human intestinal crypt-derived enteroids are a model of intestinal ion transport that require validation by comparison with cell culture and animal models. We used human small intestinal enteroids to study neutral Na(+) absorption and stimulated fluid and anion secretion under basal and regulated conditions in undifferentiated and differentiated cultures to show their functional relevance to ion transport physiology and pathophysiology.

METHODS

Human intestinal tissue specimens were obtained from an endoscopic biopsy or surgical resections performed at Johns Hopkins Hospital. Crypts were isolated, enteroids were propagated in culture, induced to undergo differentiation, and transduced with lentiviral vectors. Crypt markers, surface cell enzymes, and membrane ion transporters were characterized using quantitative reverse-transcription polymerase chain reaction, immunoblot, or immunofluorescence analyses. We used multiphoton and time-lapse confocal microscopy to monitor intracellular pH and luminal dilatation in enteroids under basal and regulated conditions.

RESULTS

Enteroids differentiated upon withdrawal of WNT3A, yielding decreased crypt markers and increased villus-like characteristics. Na(+)/H(+) exchanger 3 activity was similar in undifferentiated and differentiated enteroids, and was affected by known inhibitors, second messengers, and bacterial enterotoxins. Forskolin-induced swelling was completely dependent on cystic fibrosis transmembrane conductance regulator and partially dependent on Na(+)/H(+) exchanger 3 and Na(+)/K(+)/2Cl(-) cotransporter 1 inhibition in undifferentiated and differentiated enteroids. Increases in cyclic adenosine monophosphate with forskolin caused enteroid intracellular acidification in HCO3(-)-free buffer. Cyclic adenosine monophosphate-induced enteroid intracellular pH acidification as part of duodenal HCO3(-) secretion appears to require cystic fibrosis transmembrane conductance regulator and electrogenic Na(+)/HCO3(-) cotransporter 1.

CONCLUSIONS

Undifferentiated or crypt-like, and differentiated or villus-like, human enteroids represent distinct points along the crypt-villus axis; they can be used to characterize electrolyte transport processes along the vertical axis of the small intestine. The duodenal enteroid model showed that electrogenic Na(+)/HCO3(-) cotransporter 1 might be a target in the intestinal mucosa for treatment of secretory diarrheas.

Acidic pH and short-chain fatty acids activate Na+ transport but differentially modulate expression of Na+/H+ exchanger isoforms 1, 2, and 3 in omasal epithelium

Low sodium content in feed and large amounts of salivary sodium secretion are essential requirements to efficient sodium reabsorption in the dairy cow. It is already known that Na(+)/H(+) exchange (NHE) of the ruminal epithelium plays a key role in Na(+) absorption, and its function is influenced by the presence of short-chain fatty acids (SCFA) and mucosal pH. By contrast, the functional role and regulation of NHE in omasal epithelium have not been completely understood. In the present study, we used model studies in small ruminants (sheep and goats) to investigate NHE-mediated Na(+) transport and the effects of pH and SCFA on NHE activity in omasal epithelium and on the expression of NHE isoform in omasal epithelial cells. Conventional Ussing chamber technique, primary cell culture, quantitative PCR, and Western blot were used. In native omasal epithelium of sheep, the Na(+) transport was electroneutral, and it was inhibited by the specific NHE3 inhibitor 3-[2-(3-guanidino-2-methyl-3-oxo-propenyl)-5-methyl-phenyl]-N-isopropylidene-2-methyl-acrylamide dihydrochloride, which decreased mucosal-to-serosal, serosal-to-mucosal, and net flux rates of Na(+) by 80% each. The application of low mucosal pH (6.4 or 5.8) in the presence of SCFA activated the Na(+) transport across omasal epithelium of sheep compared with that at pH 7.4. In cultured omasal epithelial cells of goats, mRNA and protein of NHE1, NHE2, and NHE3 were detected. The application of SCFA increased NHE1 mRNA and protein expression, which was most prominent when the culture medium pH decreased from 7.4 to 6.8. At variance, the mRNA and protein expression of NHE2 and NHE3 were decreased with low pH and SCFA, which was contrary to the published data from ruminal epithelial studies. In conclusion, this paper shows that (1) NHE1, NHE2, and NHE3 are expressed in omasal epithelium; (2) NHE3 mediates the major portion of transepithelial Na(+) transport in omasal epithelium; and (3) SCFA and acidic pH acutely activate Na(+) transport but suppress the expression of NHE2 and NHE3 in the longer term. By contrast, the expression of NHE1 is increased by SCFA and acidic pH, indicating a prominent role for NHE1 in the regulation of intracellular pH of omasal epithelium. Our results suggest a regulatable Na(+) absorption in ruminal and omasal epithelium. It is of benefit for intracellular pH homeostasis and highly relevant to dairy cows fed on high-concentrate diets.

Short- and long-term regulation of intestinal Na+/H+ exchange by Toll-like receptors TLR4 and TLR5

Inappropriate activation of pattern recognition receptors has been described as a potential trigger in the development of inflammatory bowel disease (IBD). In this study, we evaluated the activity and expression of Na(+)/H(+) exchanger (NHE) subtypes in T84 intestinal epithelial cells during Toll-like receptor 4 (TLR4) activation by monophosphoryl lipid A and TLR5 by flagellin. NHE activity and intracellular pH were evaluated by spectrofluorescence. Additionally, kinase activities were evaluated by ELISA, and siRNA was used to specifically inhibit adenylyl cyclase (AC). Monophosphoryl lipid A (MPLA) (0.01-50.00 μg/ml) and flagellin (10-500 ng/ml) inhibited NHE1 activity in a concentration-dependent manner (MPLA short term -25.2 ± 5.0%, long term -31.9 ± 4.0%; flagellin short term -14.9 ± 2.0%, long term -19.1 ± 2.0%). Both ligands triggered AC3, PKA, PLC, and PKC signal molecules. Long-term exposure to flagellin and MPLA induced opposite changes on NHE3 activity; flagellin increased NHE3 activity (∼10%) with overexpression of membrane protein, whereas MPLA decreased NHE3 activity (-17.3 ± 3.0%). MPLA and flagellin simultaneously had synergistic effects on NHE activity. MPLA and flagellin impaired pHi recovery after intracellular acidification. The simultaneous exposure to MPLA and flagellin induced a substantial pHi reduction (-0.55 ± 0.03 pH units). Activation of TLR4 and TLR5 exerts marked inhibition of NHE1 activity in intestinal epithelial cells. Transduction mechanisms set into motion during TLR4-mediated and long-term TLR5-mediated inhibition of NHE1 activity involve AC3, PKA, PLC, and PKC. However, short- and long-term TLR4 activation and TLR5 activation might use different signaling pathways. The physiological alterations on intestinal epithelial cells described here may be useful in the development of better IBD therapeutics.

Proximal tubule NHE3 activity is inhibited by beta-arrestin-biased angiotensin II type 1 receptor signaling

Physiological concentrations of angiotensin II (ANG II) upregulate the activity of Na+/H+ exchanger isoform 3 (NHE3) in the renal proximal tubule through activation of the ANG II type I (AT1) receptor/G protein-coupled signaling. This effect is key for maintenance of extracellular fluid volume homeostasis and blood pressure. Recent findings have shown that selective activation of the beta-arrestin-biased AT1 receptor signaling pathway induces diuresis and natriuresis independent of G protein-mediated signaling. This study tested the hypothesis that activation of this AT1 receptor/beta-arrestin signaling inhibits NHE3 activity in proximal tubule. To this end, we determined the effects of the compound TRV120023, which binds to the AT1R, blocks G-protein coupling, and stimulates beta-arrestin signaling on NHE3 function in vivo and in vitro. NHE3 activity was measured in both native proximal tubules, by stationary microperfusion, and in opossum proximal tubule (OKP) cells, by Na+-dependent intracellular pH recovery. We found that 10−7 M TRV120023 remarkably inhibited proximal tubule NHE3 activity both in vivo and in vitro. Additionally, stimulation of NHE3 by ANG II was completely suppressed by TRV120023 both in vivo as well as in vitro. Inhibition of NHE3 activity by TRV120023 was associated with a decrease in NHE3 surface expression in OKP cells and with a redistribution from the body to the base of the microvilli in the rat proximal tubule. These findings indicate that biased signaling of the beta-arrestin pathway through the AT1 receptor inhibits NHE3 activity in the proximal tubule at least in part due to changes in NHE3 subcellular localization.

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.

Increased Epithelial Sodium Channel Activity Contributes to Hypertension Caused by Na+-HCO3- Cotransporter Electrogenic 2 Deficiency

The gene SLC4A5 encodes the Na(+)-HCO3 (-) cotransporter electrogenic 2, which is located in the distal nephron. Genetically deleting Na(+)-HCO3 (-) cotransporter electrogenic 2 (knockout) causes Na(+)-retention and hypertension, a phenotype that is diminished with alkali loading. We performed experiments with acid-loaded mice and determined whether overactive epithelial Na(+) channels (ENaC) or the Na(+)-Cl(-) cotransporter causes the Na(+) retention and hypertension in knockout. In untreated mice, the mean arterial pressure was higher in knockout, compared with wild-type (WT); however, treatment with amiloride, a blocker of ENaC, abolished this difference. In contrast, hydrochlorothiazide, an inhibitor of Na(+)-Cl(-) cotransporter, decreased mean arterial pressure in WT, but not knockout. Western blots showed that quantity of plasmalemmal full-length ENaC-α was significantly higher in knockout than in WT. Amiloride treatment caused a 2-fold greater increase in Na(+) excretion in knockout, compared with WT. In knockout, but not WT, amiloride treatment decreased plasma [Na(+)] and urinary K(+) excretion, but increased hematocrit and plasma [K(+)] significantly. Micropuncture with microelectrodes showed that the [K(+)] was significantly higher and the transepithelial potential (Vte) was significantly lower in the late distal tubule of the knockout compared with WT. The reduced Vte in knockout was amiloride sensitive and therefore revealed an upregulation of electrogenic ENaC-mediated Na(+) reabsorption in this segment. These results show that, in the absence of Na(+)-HCO3 (-) cotransporter electrogenic 2 in the late distal tubule, acid-loaded mice exhibit disinhibition of ENaC-mediated Na(+) reabsorption, which results in Na(+) retention, K(+) wasting, and hypertension.

Renal nerve stimulation leads to the activation of the Na+/H+ exchanger isoform 3 via angiotensin II type I receptor

Renal nerve stimulation at a low frequency (below 2 Hz) causes water and sodium reabsorption via α1-adrenoreceptor tubular activation, a process independent of changes in systemic blood pressure, renal blood flow, or glomerular filtration rate. However, the underlying mechanism of the reabsorption of sodium is not fully understood. Since the sympathetic nervous system and intrarenal ANG II appear to act synergistically to mediate the process of sodium reabsorption, we hypothesized that low-frequency acute electrical stimulation of the renal nerve (ESRN) activates NHE3-mediated sodium reabsorption via ANG II AT1 receptor activation in Wistar rats. We found that ESRN significantly increased urinary angiotensinogen excretion and renal cortical ANG II content, but not the circulating angiotensinogen levels, and also decreased urinary flow and pH and sodium excretion via mechanisms independent of alterations in creatinine clearance. Urinary cAMP excretion was reduced, as was renal cortical PKA activity. ESRN significantly increased NHE3 activity and abundance in the apical microvillar domain of the proximal tubule, decreased the ratio of phosphorylated NHE3 at serine 552/total NHE3, but did not alter total cortical NHE3 abundance. All responses mediated by ESRN were completely abolished by a losartan-mediated AT1 receptor blockade. Taken together, our results demonstrate that higher NHE3-mediated proximal tubular sodium reabsorption induced by ESRN occurs via intrarenal renin angiotensin system activation and triggering of the AT1 receptor/inhibitory G-protein signaling pathway, which leads to inhibition of cAMP formation and reduction of PKA activity.

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.

Structural dynamics and regulation of the mammalian SLC9A family of Na⁺/H⁺ exchangers

Mammalian Na⁺/H⁺ exchangers of the SLC9A family are widely expressed and involved in numerous essential physiological processes. Their primary function is to mediate the 1:1 exchange of Na⁺ for H⁺ across the membrane in which they reside, and they play central roles in regulation of body, cellular, and organellar pH. Their function is tightly regulated through mechanisms involving interactions with multiple protein and lipid-binding partners, phosphorylations, and other posttranslational modifications. Biochemical and mutational analyses indicate that the SLC9As have a short intracellular N-terminus, 12 transmembrane (TM) helices necessary and sufficient for ion transport, and a C-terminal cytoplasmic tail region with essential regulatory roles. No high-resolution structures of the SLC9As exist; however, models based on crystal structures of the bacterial NhaAs support the 12 TM organization and suggest that TMIV and XI may form a central part of the ion-translocation pathway, whereas pH sensing may involve TMII, TMIX, and several intracellular loops. Similar to most ion transporters studied, SLC9As likely exist as coupled dimers in the membrane, and this appears to be important for the well-studied cooperativity of H⁺ binding. The aim of this work is to summarize and critically discuss the currently available evidence on the structural dynamics, regulation, and binding partner interactions of SLC9As, focusing in particular on the most widely studied isoform, SLC9A1/NHE1. Further, novel bioinformatic and structural analyses are provided that to some extent challenge the existing paradigm on how ions are transported by mammalian SLC9As.

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.

Elevated carbon dioxide upregulates NBCn1 Na+/HCO3(-) cotransporter in human embryonic kidney cells

The NBCn1 Na(+)/HCO3(-) cotransporter catalyzes the electroneutral movement of 1 Na(+):1 HCO3(-) into kidney cells. We characterized the intracellular pH (pHi) regulation in human embryonic kidney cells (HEK) subjected to NH4Cl prepulse acid loading, and we examined the NBCn1 expression and function in HEK cells subjected to 24-h elevated Pco2 (10-15%). After acid loading, in the presence of HCO3(-), ∼50% of the pHi recovery phase was blocked by the Na(+)/H(+) exchanger inhibitors EIPA (10-50 μM) and amiloride (1 mM) and was fully cancelled by 30 μM EIPA under nominally HCO3(-)-free conditions. In addition, in the presence of HCO3(-), pHi recovery after acid loading was completely blocked when Na(+) was omitted in the buffer. pHi recovery after acidification in HEK cells was repeated in the presence of the NBC inhibitor S0859, and the pHi recovery was inhibited by S0859 in a dose-dependent manner (Ki = 30 μM, full inhibition at 60 μM), which confirmed NBC Na(+)/HCO3(-) cotransporter activation. NBCn1 expression increased threefold after 24-h exposure of cultured HEK cells to 10% CO2 and sevenfold after exposure to 15% CO2, examined by immunoblots. Finally, exposure of HEK cells to high CO2 significantly increased the HCO3(-)-dependent recovery of pHi after acid loading. We conclude that HEK cells expressed the NBCn1 Na(+)/HCO3(-) cotransporter as the only HCO3(-)-dependent mechanism responsible for cellular alkaline loading. NBCn1, which expresses in different kidney cell types, was upregulated by 24-h high-Pco2 exposure of HEK cells, and this upregulation was accompanied by increased NBCn1-mediated HCO3(-) transport.

Acidosis-mediated regulation of the NHE1 isoform of the Na⁺/H⁺ exchanger in renal cells

The mammalian Na⁺/H⁺ exchanger isoform 1 (NHE1) is a ubiquitous plasma membrane protein that regulates intracellular pH by removing a proton in exchange for extracellular sodium. Renal tissues are subject to metabolic and respiratory acidosis, and acidosis has been shown to acutely activate NHE1 activity in other cell types. We examined if NHE1 is activated by acute acidosis in HEK293 and Madin-Darby canine kidney (MDCK) cells. Acute sustained intracellular acidosis (SIA) activated NHE1 in both cell types. We expressed wild-type and mutant NHE1 cDNAs in MDCK cells. All the cDNAs had a L163F/G174S mutation, which conferred a 100-fold resistance to EMD87580, an NHE1-specific inhibitor. We assayed exogenous NHE1 activity while inhibiting endogenous activity with EMD87580 and while inhibiting the NHE3 isoform of the Na⁺/H⁺ exchanger using the isoform-specific inhibitor S3226. We examined the activation and phosphorylation of the wild-type and mutant NHE1 proteins in response to SIA. In MDCK cells we demonstrated that the amino acids Ser⁷⁷¹, Ser⁷⁷⁶, Thr⁷⁷⁹, and Ser⁷⁸⁵ are important for NHE1 phosphorylation and activation after acute SIA. SIA activated ERK-dependent pathways in MDCK cells, and this was blocked by treatment with the MEK inhibitor U0126. Treatment with U0126 also blocked activation of NHE1 by SIA. These results suggest that acute acidosis activates NHE1 in mammalian kidney cells and that in MDCK cells this activation occurs through an ERK-dependent pathway affecting phosphorylation of a distinct set of amino acids in the cytosolic regulatory tail of NHE1.

Biological activities of guanidine compounds, 2008 - 2012 update

INTRODUCTION

Compounds incorporating guanidine moiety have found many practical applications in diverse areas of chemistry, such as nucleophilic organocatalysis, anion recognition and coordination chemistry. Moreover, guanidine functional group is found in natural products, pharmaceuticals and cosmetic ingredients produced by synthetic methods. Thus, knowledge of their biological activities and therapeutic uses is of utmost importance for researchers involved in drug discovery processes.

AREAS COVERED

In this review the authors highlight the continued development and therapeutic applications of newly synthesized guanidine-containing compounds including small peptides and peptidomimetics incorporating arginine. The review presents patents and patent applications filed in the years 2008 - 2012 with emphasis placed on new mechanisms of pharmacological action of guanidine derivatives.

EXPERT OPINION

While guanidines are often thought of as strong organic bases and compounds hydrophilic in nature, over the last 4 years there has been an enormous increase in discovery of new promising lead structures with guanidine core, suitable for development of potential drugs acting at central nervous system, anti-inflammatory agents, anti-diabetic and chemotherapeutic agents as well as cosmetics.

Lactobacillus acidophilus upregulates intestinal NHE3 expression and function

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

Fructose acutely stimulates NHE3 activity in kidney proximal tubule

BACKGROUND/AIMS

Fructose causes a sodium-sensitive hypertension and acutely reduces the urinary Na+ excretion, suggesting that it may regulate the activity of renal tubular sodium transporters. NHE3 is highly expressed in proximal tubule (PT), along with proteins that mediate fructose transport and metabolism. The present work was outlined to investigate whether fructose modulates proximal NHE3 activity and to elucidate the molecular mechanisms underlying this modulation.

METHODS/RESULTS

Using in vivo stationary microperfusion, we observed that fructose stimulates NHE3 mediated JHCO3- reabsorption. The MAPK pathway is not involved in this activation, as demonstrated by using of MEK/MAPK inhibitors, whereas experiments using a PKA inhibitor suggest that PKA inhibition plays a role in this response. These results were confirmed in vitro by measuring the cell pH recovery rate after NH4Cl pulse in LLC-PK1, a pig PT cell line, which showed reduced cAMP levels and NHE3 phosphorylation at serine-552 (PKA consensus site) after fructose treatment.

CONCLUSIONS

NHE3 activity is stimulated by fructose, which increases proximal tubule Na+ reabsorption. The molecular mechanisms involved in this process are mediated, at least in part, by downregulation of the PKA signaling pathway. Future studies are needed to address whether fructose-stimulated NHE3 activity may contribute to renal injury and hypertension.

OSR1-sensitive small intestinal Na+ transport

The oxidative stress responsive kinase 1 (OSR1) contributes to WNK (with no K)-dependent regulation of renal tubular salt transport, renal salt excretion, and blood pressure. Little is known, however, about a role of OSR1 in the regulation of intestinal salt transport. The present study thus explored whether OSR1 is expressed in intestinal tissue and whether small intestinal Na(+)/H(+) exchanger (NHE), small intestinal Na(+)-glucose cotransport (SGLT1), and/or colonic epithelium Na(+) channel (ENaC) differ between knockin mice carrying one allele of WNK-resistant OSR1 (osr1(+/KI)) and wild-type mice (osr1(+/+)). OSR1 protein abundance was determined by Western blotting, cytosolic pH from BCECF fluorescence, NHE activity from Na(+)-dependent realkalinization following an ammonium pulse, SGLT1 activity from glucose-induced current, and colonic ENaC activity from amiloride-sensitive transepithelial current in Ussing chamber experiments. As a result, OSR1 protein was expressed in small intestine of both osr1(+/KI) mice and osr1(+/+) mice. Daily fecal Na(+), K(+), and H(2)O excretion and jejunal SGLT1 activity were lower, whereas small intestinal NHE activity and colonic ENaC activity were higher in osr1(+/KI) mice than in osr1(+/+) mice. NHE3 inhibitor S-3226 significantly reduced NHE activity in both genotypes but did not abrogate the difference between the genotypes. Plasma osmolarity, serum antidiuretic hormone, plasma aldosterone, and plasma corticosterone concentrations were similar in both genotypes. Small intestinal NHE3 and colonic α-ENaC protein abundance were not significantly different between genotypes, but colonic phospho-β-ENaC (ser633) was significantly higher in osr1(+/KI) mice. In conclusion, OSR1 is expressed in intestine and partial WNK insensitivity of OSR1 increases intestinal NHE activity and colonic ENaC activity.

Mechanisms underlying the inhibitory effects of uroguanylin on NHE3 transport activity in renal proximal tubule

We previously demonstrated that uroguanylin (UGN) significantly inhibits Na(+)/H(+) exchanger (NHE)3-mediated bicarbonate reabsorption. In the present study, we aimed to elucidate the molecular mechanisms underlying the action of UGN on NHE3 in rat renal proximal tubules and in a proximal tubule cell line (LLC-PK(1)). The in vivo studies were performed by the stationary microperfusion technique, in which we measured H(+) secretion in rat renal proximal segments, through a H(+)-sensitive microelectrode. UGN (1 μM) significantly inhibited the net of proximal bicarbonate reabsorption. The inhibitory effect of UGN was completely abolished by either the protein kinase G (PKG) inhibitor KT5823 or by the protein kinase A (PKA) inhibitor H-89. The effects of UGN in vitro were found to be similar to those obtained by microperfusion. Indeed, we observed that incubation of LLC-PK(1) cells with UGN induced an increase in the intracellular levels of cAMP and cGMP, as well as activation of both PKA and PKG. Furthermore, we found that UGN can increase the levels of NHE3 phosphorylation at the PKA consensus sites 552 and 605 in LLC-PK(1) cells. Finally, treatment of LLC-PK(1) cells with UGN reduced the amount of NHE3 at the cell surface. Overall, our data suggest that the inhibitory effect of UGN on NHE3 transport activity in proximal tubule is mediated by activation of both cGMP/PKG and cAMP/PKA signaling pathways which in turn leads to NHE3 phosphorylation and reduced NHE3 surface expression. Moreover, this study sheds light on mechanisms by which guanylin peptides are intricately involved in the maintenance of salt and water homeostasis.

The SLC36 family of proton-coupled amino acid transporters and their potential role in drug transport

Members of the solute carrier (SLC) 36 family are involved in transmembrane movement of amino acids and derivatives. SLC36 consists of four members. SLC36A1 and SLC36A2 both function as H(+) -coupled amino acid symporters. SLC36A1 is expressed at the luminal surface of the small intestine but is also commonly found in lysosomes in many cell types (including neurones), suggesting that it is a multipurpose carrier with distinct roles in different cells including absorption in the small intestine and as an efflux pathway following intralysosomal protein breakdown. SLC36A1 has a relatively low affinity (K(m) 1-10 mM) for its substrates, which include zwitterionic amino and imino acids, heterocyclic amino acids and amino acid-based drugs and derivatives used experimentally and/or clinically to treat epilepsy, schizophrenia, bacterial infections, hyperglycaemia and cancer. SLC36A2 is expressed at the apical surface of the human renal proximal tubule where it functions in the reabsorption of glycine, proline and hydroxyproline. SLC36A2 also transports amino acid derivatives but has a narrower substrate selectivity and higher affinity (K(m) 0.1-0.7 mM) than SLC36A1. Mutations in SLC36A2 lead to hyperglycinuria and iminoglycinuria. SLC36A3 is expressed only in testes and is an orphan transporter with no known function. SLC36A4 is widely distributed at the mRNA level and is a high-affinity (K(m) 2-3 µM) transporter for proline and tryptophan. We have much to learn about this family of transporters, but from current knowledge, it seems likely that their function will influence the pharmacokinetic profiles of amino acid-based drugs by mediating transport in both the small intestine and kidney.

Increased NHE3 abundance and transport activity in renal proximal tubule of rats with heart failure

Heart failure (HF) is associated with a reduced effective circulating volume that drives sodium and water retention and extracellular volume expansion. We therefore hypothesized that Na+/H+ exchanger isoform 3 (NHE3), the major apical transcellular pathway for sodium reabsorption in the proximal tubule, is upregulated in an experimental model of HF. HF was induced in male rats by left ventricle radiofrequency ablation. Sham-operated rats (sham) were used as controls. At 6 wk after surgery, HF rats exhibited cardiac dysfunction with a dramatic increase in left ventricular end-diastolic pressure. By means of stationary in vivo microperfusion and pH-dependent sodium uptake, we demonstrated that NHE3 transport activity was significantly higher in the proximal tubule of HF compared with sham rats. Increased NHE3 activity was paralleled by increased renal cortical NHE3 expression at both protein and mRNA levels. In addition, the baseline PKA-dependent NHE3 phosphorylation at serine 552 was reduced in renal cortical membranes of rats with HF. Collectively, these results suggest that NHE3 is upregulated in the proximal tubule of HF rats by transcriptional, translational, and posttranslational mechanisms. Enhanced NHE3-mediated sodium reabsorption in the proximal tubule may contribute to extracellular volume expansion and edema, the hallmark feature of HF. Moreover, our study emphasizes the importance of undertaking a cardiorenal approach to contain progression of cardiac disease.

Enterohemorrhagic Escherichia coli infection stimulates Shiga toxin 1 macropinocytosis and transcytosis across intestinal epithelial cells

Gastrointestinal infection with Shiga toxins producing enterohemorrhagic Escherichia coli causes the spectrum of gastrointestinal and systemic complications, including hemorrhagic colitis and hemolytic uremic syndrome, which is fatal in ∼10% of patients. However, the molecular mechanisms of Stx endocytosis by enterocytes and the toxins cross the intestinal epithelium are largely uncharacterized. We have studied Shiga toxin 1 entry into enterohemorrhagic E. coli-infected intestinal epithelial cells and found that bacteria stimulate Shiga toxin 1 macropinocytosis through actin remodeling. This enterohemorrhagic E. coli-caused macropinocytosis occurs through a nonmuscle myosin II and cell division control 42 (Cdc42)-dependent mechanism. Macropinocytosis of Shiga toxin 1 is followed by its transcytosis to the basolateral environment, a step that is necessary for its systemic spread. Inhibition of Shiga toxin 1 macropinocytosis significantly decreases toxin uptake by intestinal epithelial cells and in this way provides an attractive, antibiotic-independent strategy for prevention of the harmful consequences of enterohemorrhagic E. coli infection.

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

Trafficking and regulation of the epithelial brush border membrane (BBM) Na+/H+ exchanger 3 (NHE3) in the intestine involves interaction with four different members of the NHERF family in a signal-dependent and possibly segment-specific fashion. The aim of this research was to study the role of NHERF2 (E3KARP) in intestinal NHE3 BBM localization and second messenger-mediated and receptor-mediated inhibition of NHE3. Immunolocalization of NHE3 in WT mice revealed predominant microvillar localization in jejunum and colon, a mixed distribution in the proximal ileum but localization near the terminal web in the distal ileum. The terminal web localization of NHE3 in the distal ileum correlated with reduced acid-activated NHE3 activity (fluorometrically assessed). NHERF2 ablation resulted in a shift of NHE3 to the microvilli and higher basal fluid absorption rates in the ileum, but no change in overall NHE3 protein or mRNA expression. Forskolin-induced NHE3 inhibition was preserved in the absence of NHERF2, whereas Ca2+ ionophore- or carbachol-mediated inhibition was abolished. Likewise, Escherichia coli heat stable enterotoxin peptide (STp) lost its inhibitory effect on intestinal NHE3. It is concluded that in native murine intestine, the NHE3 adaptor protein NHERF2 plays important roles in tethering NHE3 to a position near the terminal web and in second messenger inhibition of NHE3 in a signal- and segment-specific fashion, and is therefore an important regulator of intestinal fluid transport.

Evidence for NHE3-mediated Na transport in sheep and bovine forestomach

Na absorption across the cornified, multilayered, and squamous rumen epithelium is mediated by electrogenic amiloride-insensitive transport and by electroneutral Na transport. High concentrations of amiloride (>100 μM) inhibit Na transport, indicating Na+/H+ exchange (NHE) activity. The underlying NHE isoform for transepithelial Na absorption was characterized by mucosal application of the specific inhibitor HOE642 for NHE1 and S3226 for NHE3 in Ussing chamber studies with isolated epithelia from bovine and sheep forestomach. S3226 (1 μM; NHE3 inhibitor) abolished electroneutral Na transport under control conditions and also the short-chain fatty acid-induced increase of Na transport via NHE. However, HOE642 (30 μM; NHE1 inhibitor) did not change Na transport rates. NHE3 was immunohistochemically localized in membranes of the upper layers toward the lumen. Expression of NHE1 and NHE3 has been previously demonstrated by RT-PCR, and earlier experiments with isolated rumen epithelial cells have shown the activity of both NHE1 and NHE3. Obviously, both isoforms are involved in the regulation of intracellular pH, pHi. However, transepithelial Na transport is only mediated by apical uptake via NHE3 in connection with extrusion of Na by the basolaterally located Na-K-ATPase. The missing involvement of NHE1 in transepithelial Na transport suggests that the proposed “job sharing” in epithelia between these two isoforms probably also applies to forestomach epithelia: NHE3 for transepithelial transport and NHE1 for, among others, pHi and volume regulation.

Occludin S408 phosphorylation regulates tight junction protein interactions and barrier function

Occludin S408 phosphorylation regulates interactions between occludin, ZO-1, and select claudins to define tight junction molecular structure and barrier function.

Although the C-terminal cytoplasmic tail of the tight junction protein occludin is heavily phosphorylated, the functional impact of most individual sites is undefined. Here, we show that inhibition of CK2-mediated occludin S408 phosphorylation elevates transepithelial resistance by reducing paracellular cation flux. This regulation requires occludin, claudin-1, claudin-2, and ZO-1. S408 dephosphorylation reduces occludin exchange, but increases exchange of ZO-1, claudin-1, and claudin-2, thereby causing the mobile fractions of these proteins to converge. Claudin-4 exchange is not affected. ZO-1 domains that mediate interactions with occludin and claudins are required for increases in claudin-2 exchange, suggesting assembly of a phosphorylation-sensitive protein complex. Consistent with this, binding of claudin-1 and claudin-2, but not claudin-4, to S408A occludin tail is increased relative to S408D. Finally, CK2 inhibition reversed IL-13–induced, claudin-2–dependent barrier loss. Thus, occludin S408 dephosphorylation regulates paracellular permeability by remodeling tight junction protein dynamic behavior and intermolecular interactions between occludin, ZO-1, and select claudins, and may have therapeutic potential in inflammation-associated barrier dysfunction.

Dipeptidyl peptidase IV inhibition attenuates blood pressure rising in young spontaneously hypertensive rats

OBJECTIVES

The present study aimed to assess the effect of the specific dipeptidyl peptidase IV (DPPIV) inhibitor sitagliptin on blood pressure and renal function in young prehypertensive (5-week-old) and adult spontaneously hypertensive rats (SHRs; 14-week-old).

METHODS

Sitagliptin (40 mg/kg twice daily) was given by oral gavage to young (Y-SHR + IDPPIV) and adult (A-SHR + IDPPIV) SHRs for 8 days. Kidney function was assessed daily and compared with age-matched vehicle-treated SHR (Y-SHR and A-SHR) and with normotensive Wistar-Kyoto rats (Y-WKY and A-WKY). Arterial blood pressure was measured in these animals at the end of the experimental protocol. Additionally, Na/H exchanger isoform 3 (NHE3) function and expression in microvilli membrane vesicles were assessed in young animals.

RESULTS

Mean arterial blood pressure of Y-SHR + IDPPIV was significantly lower than that of Y-SHR (104 ± 3 vs. 123 ± 5 mmHg, P < 0.01) and was similar to Y-WKY (94 ± 4 mmHg, P > 0.05). Compared to Y-SHR, Y-SHR + IDPPIV exhibited enhanced cumulative urinary flow and sodium excretion and decreased NHE3 activity and expression in proximal tubule microvilli. In the A-SHR, sitagliptin treatment had no significant effect on either renal function or arterial blood pressure.

CONCLUSION

Our data suggest that DPPIV inhibition attenuates blood pressure rising in young prehypertensive SHRs, partially by inhibiting NHE3 activity in renal proximal tubule.

Mechanisms mediating the diuretic and natriuretic actions of the incretin hormone glucagon-like peptide-1

Glucagon-like peptide-1 (GLP-1) is a gut incretin hormone considered a promising therapeutic agent for type 2 diabetes because it stimulates beta cell proliferation and insulin secretion in a glucose-dependent manner. Cumulative evidence supports a role for GLP-1 in modulating renal function; however, the mechanisms by which GLP-1 induces diuresis and natriuresis have not been completely established. This study aimed to define the cellular and molecular mechanisms mediating the renal effects of GLP-1. GLP-1 (1 μg·kg(-1)·min(-1)) was intravenously administered in rats for the period of 60 min. GLP-1-infused rats displayed increased urine flow, fractional excretion of sodium, potassium, and bicarbonate compared with those rats that received vehicle (1% BSA/saline). GLP-1-induced diuresis and natriuresis were also accompanied by increases in renal plasma flow and glomerular filtration rate. Real-time RT-PCR in microdissected rat nephron segments revealed that GLP-1 receptor-mRNA expression was restricted to glomerulus and proximal convoluted tubule. In rat renal proximal tubule, GLP-1 significantly reduced Na(+)/H(+) exchanger isoform 3 (NHE3)-mediated bicarbonate reabsorption via a protein kinase A (PKA)-dependent mechanism. Reduced proximal tubular bicarbonate flux rate was associated with a significant increase of NHE3 phosphorylation at the PKA consensus sites in microvillus membrane vesicles. Taken together, these data suggest that GLP-1 has diuretic and natriuretic effects that are mediated by changes in renal hemodynamics and by downregulation of NHE3 activity in the renal proximal tubule. Moreover, our findings support the view that GLP-1-based agents may have a potential therapeutic use not only as antidiabetic drugs but also in hypertension and other disorders of sodium retention.