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

Mechanistic insights into auxin-enhancing polycyclic aromatic hydrocarbon uptake by wheat roots: Evidence from in situ intracellular pH and root-surface H+ flux

Polycyclic aromatic hydrocarbons (PAHs) are a group of extremely carcinogenic organic pollutants. Our previous findings have demonstrated that plant roots actively take up PAHs through co-transport with H+ ions. Auxin serves as a pivotal regulator of plant growth and development. However, it remains unclear whether the hormone can enhance the uptake of PAHs by plant roots. Hence, the wheat root exposed to PAHs with/without auxins was set to investigate how the auxin promotes the PAHs uptake by roots. In our study, auxin could significantly enhance the uptake of PAHs after 4 h of exposure. After the addition of auxin, the root tissue cytoplasmic pH value was decreased and the H+ influx was observed, indicating that the extracellular space was alkalinized in a short time. The increased H+ influx rate enhanced the uptake of PAHs. In addition, the H+-ATPase activity was also increased, suggesting that auxin activated two distinct and antagonistic H+ flux pathways, and the H+ influx pathway was dominant. Our findings offer important information for exploring the mechanism underlying auxin regulation of PAHs uptake and the phytoremediation of PAH-contaminated soil and water.

NHERF1/EBP50 immunoexpression in renal cell carcinomas and oncocytomas with ultrastructural analysis of clear cell renal cell carcinoma

Background

Na+/H+ exchanger (NHE) maintains the alkaline pH of epithelial cells working at the cellular membrane and exchanging H+/Na+ ions. In renal tubular epithelial cells, the reabsorption of NaCl is implemented by NHE3 isoform, which is regulated by NHE regulatory factor-1 (NHERF1). Normally situated at the apical zones of proximal tubular cells, NHERF1 participates in cytoskeletal reorganization and signal transduction facilitating structural stability and ion exchange. Based on an extensive search in English literature, NHERF1/EBP50 immunoexpression has been studied in breast, colon, and other tumors with only one study on 21 cases of renal cell carcinomas (RCC).

Methods

Using NHERF1/EBP50 immunohistochemistry (IHC) on 64 (82%) RCCs (34 clear cells, 21 papillary and 9 chromophobe types) and 14 (18%) oncocytomas, we evaluated and scored NHERF1/EBP50 immunoexpression depending on the World Health Organization (WHO)/International Society of Urological Pathology (ISUP) grading system followed by ultrastructural identification of microlumen-like structures (MLS) in clear cell renal cell carcinomas (ccRCC).

Results

Staining patterns varied throughout the tumors and within individual tumors. Only ccRCC showed unique MLS within the cytoplasm of tumor cells. All neoplasia-transformed tubular cells, regardless of the tumor grade and stage, had altered immunoexpression of NHERF1/EBP50 ranging from complete absence to aberrant expression in the luminal cell membrane, nuclear or cytoplasmic localizations.

Conclusions

Only ccRCC showed unique dot-like condensations of immunostaining/MLS at membranous, submembranous, and paranuclear localizations. The latter two localizations were mainly observed in the combined WHO/ISUP grade 1 and 2 group compared to the combined group of grade 3 and 4 tumor samples (P=0.0146 and P<0.0001, respectively). Ultrastructurally, the MLS were identified as thick microvilli trapped by a single-layer membrane, displaced into the cytoplasm and ranging from 400 nm to 3.5 µm. These significant ultrastructural reorganizations may contribute to tumor progression, metastasis, and drug resistance.

SLC26A3 (DRA) is stimulated in a synergistic, intracellular Ca2+-dependent manner by cAMP and ATP in intestinal epithelial cells

In polarized intestinal epithelial cells, downregulated in adenoma (DRA) is an apical Cl-/[Formula: see text] exchanger that is part of neutral NaCl absorption under baseline conditions, but in cyclic adenosine monophosphate (cAMP)-driven diarrheas, it is stimulated and contributes to increased anion secretion. To further understand the regulation of DRA in conditions mimicking some diarrheal diseases, Caco-2/BBE cells were exposed to forskolin (FSK) and adenosine 5'-triphosphate (ATP). FSK and ATP stimulated DRA in a concentration-dependent manner, with ATP acting via P2Y1 receptors. FSK at 1 µM and ATP at 0.25 µM had minimal to no effect on DRA given individually; however, together, they stimulated DRA to levels seen with maximum concentrations of FSK and ATP alone. In Caco-2/BBE cells expressing the Ca2+ indicator GCaMP6s, ATP increased intracellular Ca2+ (Ca2+i) in a concentration-dependent manner, whereas FSK (1 µM), which by itself did not significantly alter Ca2+i, followed by 0.25 µM ATP produced a large increase in Ca2+ that was approximately equal to the elevation caused by 1 µM ATP. 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester) (BAPTA-AM) pretreatment prevented the ATP and FSK/ATP synergistically increased the DRA activity and the increase in Ca2+i caused by FSK/ATP. FSK/ATP synergistic stimulation of DRA was similarly observed in human colonoids. In Caco-2/BBE cells, subthreshold concentrations of FSK (cAMP) and ATP (Ca2+) synergistically increased Ca2+i and stimulated DRA activity with both being blocked by BAPTA-AM pretreatment. Diarrheal diseases, such as bile acid diarrhea, in which both cAMP and Ca2+ are elevated, are likely to be associated with stimulated DRA activity contributing to increased anion secretion, whereas separation of DRA from Na+/H+ exchanger isoform-3 (NHE3) contributes to reduced NaCl absorption.NEW & NOTEWORTHY The BB Cl-/[Formula: see text] exchanger DRA takes part in both neutral NaCl absorption and stimulated anion secretion. Using intestinal cell line, Caco-2/BBE high concentrations of cAMP and Ca2+ individually stimulated DRA activity, whereas low concentrations, which had no/minimal effect, synergistically stimulated DRA activity that required a synergistic increase in intracellular Ca2+. This study increases understanding of diarrheal diseases, such as bile salt diarrhea, in which both cAMP and elevated Ca2+ are involved.

Identification of Intestinal NaCl Absorptive-Anion Secretory Cells: Potential Functional Significance

Use of human enteroids studied in the undifferentiated and differentiated state that mimic the intestinal crypt and villus, respectively, has allowed studies of multiple enterocyte populations, including a large population of enterocytes that are transitioning from the crypt to the villus. This population expresses NHE3, DRA, and CFTR, representing a combination of Na absorptive and anion secretory functions. In this cell population, these three transporters physically interact, which affects their baseline and regulated activities. A study of this cell population and differentiated Caco-2 cells transduced with NHE3 and endogenously expressing DRA and CFTR has allowed an understanding of previous studies in which cAMP seemed to stimulate and inhibit DRA at the same time. Understanding the contributions of these cells to overall intestinal transport function as part of the fasting and post-prandial state and their contribution to the pathophysiology of diarrheal diseases and some conditions with constipation will allow new approaches to drug development.

An epidermal patch for the simultaneous monitoring of haemodynamic and metabolic biomarkers

Monitoring the effects of daily activities on the physiological responses of the body calls for wearable devices that can simultaneously track metabolic and haemodynamic parameters. Here we describe a non-invasive skin-worn device for the simultaneous monitoring of blood pressure and heart rate via ultrasonic transducers and of multiple biomarkers via electrochemical sensors. We optimized the integrated device so that it provides mechanical resiliency and flexibility while conforming to curved skin surfaces, and to ensure reliable sensing of glucose in interstitial fluid and of lactate, caffeine and alcohol in sweat, without crosstalk between the individual sensors. In human volunteers, the device captured physiological effects of food intake and exercise, in particular the production of glucose after food digestion, the consumption of glucose via glycolysis, and increases in blood pressure and heart rate compensating for oxygen depletion and lactate generation. Continuous and simultaneous acoustic and electrochemical sensing via integrated wearable devices should enrich the understanding of the body's response to daily activities, and could facilitate the early prediction of abnormal physiological changes.

Nephrin-Ephrin-B1-Na+/H+ Exchanger Regulatory Factor 2-Ezrin-Actin Axis Is Critical in Podocyte Injury

Ephrin-B1 is one of the critical components of the slit diaphragm of kidney glomerular podocyte. However, the precise function of ephrin-B1 is unclear. To clarify the function of ephrin-B1, ephrin-B1-associated molecules were studied. RNA-sequencing analysis suggested that Na⁺/H⁺ exchanger regulatory factor 2 (NHERF2), a scaffolding protein, is associated with ephrin-B1. NHERF2 was expressed at the apical area and the slit diaphragm, and interacted with the nephrin-ephrin-B1 complex at the slit diaphragm. The nephrin-ephrin-B1-NHERF2 complex interacted with ezrin bound to F-actin. NHERF2 bound ephrin-B1 via its first postsynaptic density protein-95/disks large/zonula occludens-1 domain, and podocalyxin via its second postsynaptic density protein-95/disks large/zonula occludens-1 domain. Both in vitro analyses with human embryonic kidney 293 cells and in vivo study with rat nephrotic model showed that stimulaiton of the slit diaphragm, phosphorylation of nephrin and ephrin-B1, and dephosphorylation of NHERF2 and ezrin, disrupted the linkages of ephrin-B1-NHERF2 and NHERF2-ezrin. It is conceivable that the linkage of nephrin-ephrin-B1-NHERF2-ezrin-actin is a novel critical axis in the podocytes. Ephrin-B1 phosphorylation also disrupted the linkage of an apical transmembrane protein, podocalyxin, with NHERF2-ezrin-actin. The phosphorylation of ephrin-B1 and the consequent dephosphorylation of NHERF2 are critical initiation events leading to podocyte injury.

Altered absorptive function in the gall bladder during cholesterol gallstone formation is associated with abnormal NHE3 complex formation

Dysfunction of the Na⁺/H⁺ exchanger 3 (NHE3) contributes to the formation of cholesterol gallstones. We aimed to investigate whether NHE3 dysfunction is associated with abnormalities in NHE3 complex formation. We fed C57BL/6 mice with control or lithogenic diet and study the expression of NHE3, ezrin, and Na⁺/H⁺ exchanger regulatory factor 1 (NHERF1) in the gallbladder (GB) using RT-PCR and western blot. Immunofluorescence and immunoprecipitation were performed to investigate the interactions of NHE3 with ezrin or NHERF1. To explore the initiating factor that leads to NHE3 dysfunction, we stimulated cholangiocarcinoma cells with taurochenodeoxycholate (TCDC) and/or forskolin. The effects of TCDC on the expression of NHE3 regulatory proteins, as well as their bindings to NHE3, were detected by western blot and immunoprecipitation. Enzyme-linked immunosorbent assay was used to study the regulation of cAMP production by TCDC. The expression of NHERF1 and ezrin phosphorylation level were increased in the gallbladder epithelial cells (GBECs) of C57BL/6 mice with cholesterol gallstones. Immunofluorescence studies demonstrated that the subcellular localization of ezrin and NHERF1 were similar to that of NHE3 in GBECs. Immunoprecipitation revealed that ezrin formed macrocomplex with NHE3, which were enhanced after gallstone formation. TCDC increased forskolin-induced cAMP accumulation, and NHERF1 and PKCα expression in cholangiocarcinoma cells. Under the synergistic effect of forskolin, TCDC stimulated ezrin phosphorylation, with enhanced interaction between ezrin and NHE3. The formation of cholesterol gallstones is associated with abnormal formation of NHE3 complexes. Decreased biliary TCDC may be an initiating factor that leads to abnormal GB absorption.

NHERF1 Is Required for Localization of PMCA2 and Suppression of Early Involution in the Female Lactating Mammary Gland

Prior studies have demonstrated that the calcium pump, plasma membrane calcium ATPase 2 (PMCA2), mediates calcium transport into milk and prevents mammary epithelial cell death during lactation. PMCA2 also regulates cell proliferation and cell death in breast cancer cells, in part by maintaining the receptor tyrosine kinase ErbB2/HER2 within specialized plasma membrane domains. Furthermore, the regulation of PMCA2 membrane localization and activity in breast cancer cells requires its interaction with the PDZ domain-containing scaffolding molecule sodium-hydrogen exchanger regulatory factor (NHERF) 1. In this study, we asked whether NHERF1 also interacts with PMCA2 in normal mammary epithelial cells during lactation. Our results demonstrate that NHERF1 expression is upregulated during lactation and that it interacts with PMCA2 at the apical membrane of secretory luminal epithelial cells. Similar to PMCA2, NHERF1 expression is rapidly reduced by milk stasis after weaning. Examining lactating NHERF1 knockout (KO) mice showed that NHERF1 contributes to the proper apical location of PMCA2, for proper apical-basal polarity in luminal epithelial cells, and that it participates in the suppression of Stat3 activation and the prevention of premature mammary gland involution. Additionally, we found that PMCA2 also interacts with the closely related scaffolding molecule, NHERF2, at the apical membrane, which likely maintains PMCA2 at the plasma membrane of mammary epithelial cells in lactating NHERF1KO mice. Based on these data, we conclude that, during lactation, NHERF1 is required for the proper expression and apical localization of PMCA2, which, in turn, contributes to preventing the premature activation of Stat3 and the lysosome-mediated cell death pathway that usually occur only early in mammary involution.

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

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

cAMP Stimulates SLC26A3 Activity in Human Colon by a CFTR-Dependent Mechanism That Does Not Require CFTR Activity

BACKGROUND & AIMS

SLC26A3 (DRA) is an electroneutral Cl^-/HCO₃^- exchanger that is present in the apical domain of multiple intestinal segments. An area that has continued to be poorly understood is related to DRA regulation in acute adenosine 3',5'-cyclic monophosphate (cAMP)-related diarrheas, in which DRA appears to be both inhibited as part of NaCl absorption and stimulated to contribute to increased HCO₃^- secretion. Different cell models expressing DRA have shown that cAMP inhibits, stimulates, or does not affect its activity.

METHODS

This study re-evaluated cAMP regulation of DRA using new tools, including a successful knockout cell model, a specific DRA inhibitor (DRA_inh-A250), specific antibodies, and a transport assay that did not rely on nonspecific inhibitors. The studies compared DRA regulation in colonoids made from normal human colon with regulation in the colon cancer cell line, Caco-2.

RESULTS

DRA is an apical protein in human proximal colon, differentiated colonoid monolayers, and Caco-2 cells. It is glycosylated and appears as 2 bands. cAMP (forskolin) acutely stimulated DRA activity in human colonoids and Caco-2 cells. In these cells, DRA is the predominant apical Cl^-/HCO₃^- exchanger and is inhibited by DRA_inh-A250 with a median inhibitory concentration of 0.5 and 0.2 μmol/L, respectively. However, there was no effect of cAMP in HEK293/DRA cells that lacked a cystic fibrosis transmembrane conductance regulator (CFTR). When CFTR was expressed in HEK293/DRA cells, cAMP also stimulated DRA activity. In all cases, cAMP stimulation of DRA was not inhibited by CFTR_inh-172.

CONCLUSIONS

DRA is acutely stimulated by cAMP by a process that is CFTR-dependent, but appears to be one of multiple regulatory effects of CFTR that does not require CFTR activity.

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

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

Regulation of Transporters and Channels by Membrane-Trafficking Complexes in Epithelial Cells

The vectorial secretion and absorption of fluid and solutes by epithelial cells is dependent on the polarized expression of membrane solute transporters and channels at the apical and basolateral membranes. The establishment and maintenance of this polarized expression of transporters and channels are affected by divers protein-trafficking complexes. Moreover, regulation of the magnitude of transport is often under control of physiological stimuli, again through the interaction of transporters and channels with protein-trafficking complexes. This review highlights the value in utilizing transporters and channels as cargo to characterize core trafficking machinery by which epithelial cells establish and maintain their polarized expression, and how this machinery regulates fluid and solute transport in response to physiological stimuli.

Roles of NHERF Family of PDZ-Binding Proteins in Regulating GPCR Functions

Multicellular organisms are equipped with an array of G-protein-coupled receptors (GPCRs) that mediate cell-cell signaling allowing them to adapt to environmental cues and ultimately survive. This is mechanistically possible through complex intracellular GPCR machinery that encompasses a vast network of proteins. Within this network, there is a group called scaffolding proteins that facilitate proper localization of signaling proteins for a quick and robust GPCR response. One protein family within this scaffolding group is the PSD-95/Dlg/ZO-1 (PDZ) family which is important for GPCR localization, internalization, recycling, and downstream signaling. Although the PDZ family of proteins regulate the functions of several receptors, this chapter focuses on a subfamily within the PDZ protein family called the Na⁺/H⁺ exchanger regulatory factors (NHERFs). Here we extensively review the predominantly characterized roles of NHERFs in renal phosphate absorption, intestinal ion regulation, cancer progression, and immune cell functions. Finally, we discuss the future perspectives and possible clinical application of targeting NHERFs in several disorders.

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

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

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

PDZ domain-dependent regulation of NHE3 protein by both internal Class II and C-terminal Class I PDZ-binding motifs

NHE3 directly binds Na⁺/H⁺ exchanger regulatory factor (NHERF) family scaffolding proteins that are required for many aspects of NHE3 regulation. The NHERFs bind both to an internal region (amino acids 586-660) of the NHE3 C terminus and to the NHE3 C-terminal four amino acids. The internal NHERF-binding region contains both putative Class I (-⁵⁹²SAV-) and Class II (-⁵⁹⁵CLDM-) PDZ-binding motifs (PBMs). Point mutagenesis showed that only the Class II motif contributes to NHERF binding. In this study, the roles in regulation of NHE3 activity of these two PBMs were investigated, revealing the following findings. 1) Interaction occurred between these binding sites because mutation of either removed nearly all NHERF binding. 2) Mutations in either significantly reduced basal NHE3 activity. Total and percent plasma membrane (PM) NHE3 protein expression was reduced in the C-terminal but not in the internal PBD mutation. 3) cGMP- and Ca²⁺-mediated inhibition of NHE3 was impaired in both the internal and the C-terminal PBM mutations. 4) There was a significant reduction in half-life of the PM pool of NHE3 in only the internal PBM mutation but no change in total NHE3 half-life in either. 5) There were some differences in NHE3-associating proteins in the two PBM mutations. In conclusion, NHE3 binds to NHERF proteins via both an internal Class II PBM and C-terminal Class I PBM, which interact. The former determines NHE3 stability in the PM, and the latter determines total expression and percent PM expression.

Reduced sodium/proton exchanger NHE3 activity causes congenital sodium diarrhea

Congenital sodium diarrhea (CSD) refers to an intractable diarrhea of intrauterine onset with high fecal sodium loss. CSD is clinically and genetically heterogeneous. Syndromic CSD is caused by SPINT2 mutations. While we recently described four cases of the non-syndromic form of CSD that were caused by dominant activating mutations in intestinal receptor guanylate cyclase C (GC-C), the genetic cause for the majority of CSD is still unknown. Therefore, we aimed to determine the genetic cause for non-GC-C non-syndromic CSD in 18 patients from 16 unrelated families applying whole-exome sequencing and/or chromosomal microarray analyses and/or direct Sanger sequencing. SLC9A3 missense, splicing and truncation mutations, including an instance of uniparental disomy, and whole-gene deletion were identified in nine patients from eight families with CSD. Two of these nine patients developed inflammatory bowel disease (IBD) at 4 and 16 years of age. SLC9A3 encodes Na(+)/H(+) antiporter 3 (NHE3), which is the major intestinal brush-border Na(+)/H(+) exchanger. All mutations were in the NHE3 N-terminal transport domain, and all missense mutations were in the putative membrane-spanning domains. Identified SLC9A3 missense mutations were functionally characterized in plasma membrane NHE null fibroblasts. SLC9A3 missense mutations compromised NHE3 activity by reducing basal surface expression and/or loss of basal transport function of NHE3 molecules, whereas acute regulation was normal. This study identifies recessive mutations in NHE3, a downstream target of GC-C, as a cause of CSD and implies primary basal NHE3 malfunction as a predisposition for IBD in a subset of patients.

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.

The NHERF2 sequence adjacent and upstream of the ERM-binding domain affects NHERF2-ezrin binding and dexamethasone stimulated NHE3 activity

The microvillar localization of Na⁺-H⁺ exchanger regulatory factor (NHERF)1/2 requires not only ezrin, radixin and moesin (ERM)-binding domain (EBD) but also a newly defined ERM-binding regulatory sequence (EBRS) that modulates NHERF1/2–ezrin binding. NHERF2 EBRS is also regulated by phosphorylation, which affects NHE3 (Na⁺-H⁺ exchanger 3) stimulation by dexamethasone.

In the brush border of intestinal and kidney epithelial cells, scaffolding proteins ezrin, Na⁺-H⁺ exchanger regulatory factor (NHERF)1 and NHERF2 play important roles in linking transmembrane proteins to the cytoskeleton and assembling signalling regulatory complexes. The last 30 carboxyl residues of NHERF1 and NHERF2 form the EBDs [ezrin, radixin and moesin (ERM)-binding domain]. The current study found that NHERF1/2 contain an ERM-binding regulatory sequence (EBRS), which facilitates the interaction between the EBD and ezrin. The EBRSs are located within 24 and 19 residues immediately upstream of EBDs for NHERF1 and NHERF2 respectively. In OK (opossum kidney) epithelial cells, EBRSs are necessary along with the EBD to distribute NHERF1 and NHERF2 exclusively to the apical domain. Furthermore, phosphorylation of Ser³⁰³ located in the EBRS of NHERF2, decreases the binding affinity for ezrin, dislocates apical NHERF2 into the cytosol and increases the NHERF2 microvillar mobility rate. Moreover, increased phosphorylation of Ser³⁰³ was functionally significant preventing acute stimulation of NHE3 (Na⁺-H⁺ exchanger 3) activity by dexamethasone.

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.

Cyclic GMP kinase II (cGKII) inhibits NHE3 by altering its trafficking and phosphorylating NHE3 at three required sites: identification of a multifunctional phosphorylation site

The epithelial brush-border Na(+)/H(+) exchanger NHE3 is acutely inhibited by cGKII/cGMP, but how cGKII inhibits NHE3 is unknown. This study tested the hypothesis that cGMP inhibits NHE3 by phosphorylating it and altering its membrane trafficking. Studies were carried out in PS120/NHERF2 and in Caco-2/Bbe cells overexpressing HA-NHE3 and cGKII, and in mouse ileum. NHE3 activity was measured with 2',7'-bis(carboxyethyl)-S-(and 6)carboxyfluorescein acetoxy methylester/fluorometry. Surface NHE3 was determined by cell surface biotinylation. Identification of NHE3 phosphorylation sites was by iTRAQ/LC-MS/MS with TiO2 enrichment and immunoblotting with specific anti-phospho-NHE3 antibodies. cGMP/cGKII rapidly inhibited NHE3, which was associated with reduced surface NHE3. cGMP/cGKII increased NHE3 phosphorylation at three sites (rabbit Ser(554), Ser(607), and Ser(663), equivalent to mouse Ser(552), Ser(605), and Ser(659)), all of which had to be present at the same time for cGMP to inhibit NHE3. NHE3-Ser(663) phosphorylation was not necessary for cAMP inhibition of NHE3. Dexamethasone (4 h) stimulated wild type NHE3 activity and increased surface expression but failed to stimulate NHE3 activity or increase surface expression when NHE3 was mutated to either S663A or S663D. We conclude that 1) cGMP inhibition of NHE3 is associated with phosphorylation of NHE3 at Ser(554), Ser(607), and Ser(663), all of which are necessary for cGMP/cGKII to inhibit NHE3. 2) Dexamethasone stimulates NHE3 by phosphorylation of a single site, Ser(663). The requirement for three phosphorylation sites in NHE3 for cGKII inhibition, and for phosphorylation of one of these sites for dexamethasone stimulation of NHE3, is a unique example of regulation by phosphorylation.

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.

Myosin VI mediates the movement of NHE3 down the microvillus in intestinal epithelial cells

The intestinal brush border Na(+)/H(+) exchanger NHE3 is tightly regulated through changes in its endocytosis and exocytosis. Myosin VI, a minus-end-directed actin motor, has been implicated in endocytosis at the inter-microvillar cleft and during vesicle remodeling in the terminal web. Here, we asked whether myosin VI also regulates NHE3 movement down the microvillus. The basal NHE3 activity and its surface amount, determined by fluorometry of the ratiometric pH indicator BCECF and biotinylation assays, respectively, were increased in myosin-VI-knockdown (KD) Caco-2/Bbe cells. Carbachol (CCH) and forskolin (FSK) stimulated NHE3 endocytosis in control but not in myosin VI KD cells. Importantly, immunoelectron microscopy results showed that NHE3 was preferentially localized in the basal half of control microvilli but in the distal half in myosin VI KD cells. Treatment with dynasore duplicated some aspects of myosin VI KD: it increased basal surface NHE3 activity and prevented FSK-induced NHE3 endocytosis. However, NHE3 had an intermediate distribution along the microvillus (between that in myosin VI KD and untreated cells) in dynasore-treated cells. We conclude that myosin VI is required for basal and stimulated endocytosis of NHE3 in intestinal cells, and suggest that myosin VI also moves NHE3 down the microvillus.

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

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

Carbachol-mediated endocytosis of NHE3 involves a clathrin-independent mechanism requiring lipid rafts and Cdc42

BACKGROUND

In intestinal epithelial cells, acute regulation of the brush border Na(+)/H(+) exchanger, NHE3, usually occurs by changes in endocytosis and/or exocytosis. Constitutive NHE3 endocytosis involves clathrin. Carbachol (CCH), which elevates intracellular Ca(2+) ([Ca(2+)]i), decreases NHE3 activity and stimulates endocytosis; however, the mechanism involved in calcium-mediated endocytosis of NHE3 is unclear. A pool of NHE3 resides in lipid rafts, which contributes to basal, but not cAMP-mediated, NHE3 trafficking, suggesting that an alternative mechanism exists for NHE3 endocytosis. Cdc42 was demonstrated to play an integral role in some cases of cholesterol-sensitive, clathrin-independent endocytosis. Therefore, the current study was designed to test the hypotheses that (1) clathrin-mediated endocytosis (CME) is involved in constitutive, but not CCH-mediated, endocytosis of NHE3, and (2) CCH-mediated endocytosis of NHE3 occurs through a lipid raft, activated Cdc42-dependent pathway that does not involve clathrin.

METHODS

The role of Cdc42 and lipid rafts on NHE3 activity and endocytosis were investigated in polarized Caco-2/BBe cells using pharmacological and shRNA knockdown approaches.

RESULTS

Basal NHE3 activity was increased in the presence of CME blockers (chlorpromazine; K(+) depletion) supporting previous reports that constitutive NHE3 endocytosis is clathrin dependent. In contrast, CCH-inhibition of NHE3 activity was abolished in Caco-2/BBe cells treated with MβCD (to disrupt lipid rafts) as well as in Cdc42 knockdown cells but was unaffected by CME blockers.

CONCLUSION

CCH-mediated inhibition of NHE3 activity is not dependent on clathrin and involves lipid rafts and requires Cdc42.

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

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

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

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

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

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

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

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

Regulation of expression and function of scavenger receptor class B, type I (SR-BI) by Na+/H+ exchanger regulatory factors (NHERFs)

Scavenger receptor class B, type I (SR-BI) binds HDL and mediates selective delivery of cholesteryl esters (CEs) to the liver, adrenals, and gonads for product formation (bile acids and steroids). Because relatively little is known about SR-BI posttranslational regulation in steroidogenic cells, we examined the roles of Na(+)/H(+) exchanger regulatory factors (NHERFs) in regulating SR-BI expression, SR-BI-mediated selective CE uptake, and steroidogenesis. NHERF1 and NHERF2 mRNA and protein are expressed at varying levels in model steroidogenic cell lines and the adrenal, with only low expression of PDZK1 (NHERF3) and NHERF4. Dibutyryl cyclic AMP decreased NHERF1 and NHERF2 and increased SR-BI mRNA expression in primary rat granulosa cells and MLTC-1 cells, whereas ACTH had no effect on NHERF1 and NHERF2 mRNA levels but decreased their protein levels in rat adrenals. Co-immunoprecipitation, colocalization, bimolecular fluorescence complementation, and mutational analysis indicated that SR-BI associates with NHERF1 and NHERF2. NHERF1 and NHERF2 down-regulated SR-BI protein expression through inhibition of its de novo synthesis. NHERF1 and NHERF2 also inhibited SR-BI-mediated selective CE transport and steroidogenesis, which were markedly attenuated by partial deletions of the PDZ1 or PDZ2 domain of NHERF1, the PDZ2 domain of NHERF2, or the MERM domains of NHERF1/2 or by gene silencing of NHERF1/2. Moreover, an intact COOH-terminal PDZ recognition motif (EAKL) in SR-BI is needed. Transient transfection of hepatic cell lines with NHERF1 or NHERF2 caused a significant reduction in endogenous protein levels of SR-BI. Collectively, these data establish NHERF1 and NHERF2 as SR-BI protein binding partners that play a negative role in the regulation of SR-BI expression, selective CE transport, and steroidogenesis.

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.

Roles for NHERF1 and NHERF2 on the regulation of C3a receptor signaling in human mast cells

Background

The anaphylatoxin C3a binds to the G protein coupled receptor (GPCR, C3aR) and activates divergent signaling pathways to induce degranulation and cytokine production in human mast cells. Adapter proteins such as the Na⁺/H⁺ exchange regulatory factor (NHERF1 and NHERF2) have been implicated in regulating functions of certain GPCRs by binding to the class I PDZ (PSD-95/Dlg/Zo1) motifs present on their cytoplasmic tails. Although C3aR possesses a class I PDZ motif, the possibility that it interacts with NHERF proteins to modulate signaling in human mast cells has not been determined.

Methodology/Principal Findings

Using reverse transcription PCR and Western blotting, we found that NHERF1 and NHERF2 are expressed in human mast cell lines (HMC-1, LAD2) and CD34⁺-derived primary human mast cells. Surprisingly, however, C3aR did not associate with these adapter proteins. To assess the roles of NHERFs on signaling downstream of C3aR, we used lentiviral shRNA to stably knockdown the expression of these proteins in human mast cells. Silencing the expression of NHERF1 and NHERF2 had no effect on C3aR desensitization, agonist-induced receptor internalization, ERK/Akt phosphorylation or chemotaxis. However, loss of NHERF1 and NHERF2 resulted in significant inhibition of C3a-induced mast cell degranulation, NF-κB activation and chemokine production.

Conclusion/Significance

This study demonstrates that although C3aR possesses a class I PDZ motif, it does not associate with NHERF1 and NHERF2. Surprisingly, these proteins provide stimulatory signals for C3a-induced degranulation, NF-κB activation and chemokine generation in human mast cells. These findings reveal a new level of complexity for the functional regulation of C3aR by NHERFs in human mast cells.

Small intestinal ion transport

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

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.

An old herbal medicine with a potentially new therapeutic application in inflammatory bowel disease

Inflammatory Bowel Disease (IBD) is a chronic and frequently disabling inflammatory disorder of the intestine. New developments in IBD therapy are primarily focused on biologic treatments; however, they are both expensive and associated with significant side effects. Here, we provide the first preclinical evidence that YunNan BaiYao (YNBY), a well-known traditional Chinese herbal remedy frequently used for treating hemorrhages and wounds, can effectively alleviate experimental colitis. Oral administration of YNBY in drinking water significantly reduced the disease activities of both DSS- and TNBS-induced experimental colitis. Mechanistic studies revealed that the effectiveness of YNBY was not due to an anti-bacterial function since YNBY had no effect on E. coli growth. Rather, it exhibited an anti-inflammatory or immunosuppressive function: In the DSS colitis model, YNBY treatment decreased the levels of several pro-inflammatory cytokines in colonic mucosa, including TNFα, IL-12p40, and IL-17. Similar cytokine changes were also observed in mouse serum, suggesting that systemic changes in general reflect the changes in the affected colon. Significant down-regulation of IL-12p40 and IL-17, in addition to IFNγ, was also seen in TNBS-colitis model. Another potential mechanism for the anti-inflammatory effects of YNBY involves the selective suppression of pro-inflammatory immune cells: YNBY effectively suppressed the growth of multiple T- and B-lymphocytes, including Molt-4, Jurkat, and EBV-transformed human B-lymphocytes, more potently than 6-mecaptopurine (6-MP) and 5-aminosalicylic acid (5-ASA), two of the most commonly used first-line drugs in IBD therapy. In sharp contrast, YNBY exhibited no cytotoxicity to colonic epithelial cells (Caco-2 cells), even at the concentration 10-fold higher than that used in the lymphocyte model; and instead promoted cell spreading and wound healing. These results strongly suggest that YNBY not only has effective anti-inflammatory properties through suppressing lymphocyte growth and pro-inflammatory cytokine expression, but also can promote intestinal epithelial wound-healing and repair. Therefore, YNBY demonstrates strong potential as an alternative herbal therapy for IBD.

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.

Chloride channel (Clc)-5 is necessary for exocytic trafficking of Na+/H+ exchanger 3 (NHE3)

ClC-5, a chloride/proton exchanger, is predominantly expressed and localized in subapical endosomes of the renal proximal tubule. Mutations of the CLCN5 gene cause Dent disease. The symptoms of Dent disease are replicated in Clcn5 knock-out mice. Absence of ClC-5 in mice is associated with reduced surface expression of NHE3 in proximal tubules. The molecular basis for this change is not fully understood. In this study, we investigated the mechanisms by which ClC-5 regulates trafficking of NHE3. Whether ClC-5-dependent endocytosis, exocytosis, or both contributed to the altered distribution of NHE3 was examined. First, NHE3 activity in proximal tubules of wild type (WT) and Clcn5 KO mice was determined by two-photon microscopy. Basal and dexamethasone-stimulated NHE3 activity of Clcn5 KO mice was decreased compared with that seen in WT mice, whereas the degree of inhibition of NHE3 activity by increasing cellular concentration of cAMP (forskolin) or Ca(2+) (A23187) was not different in WT and Clcn5 KO mice. Second, NHE3-dependent absorption of HCO(3)(-), measured by single tubule perfusion, was reduced in proximal tubules of Clcn5 KO mice. Third, by cell surface biotinylation, trafficking of NHE3 was examined in short hairpin RNA (shRNA) plasmid-transfected opossum kidney cells. Surface NHE3 was reduced in opossum kidney cells with reduced expression of ClC-5, whereas the total protein level of NHE3 did not change. Parathyroid hormone decreased NHE3 surface expression, but the extent of decrease and the rate of endocytosis observed in both scrambled and ClC-5 knockdown cells were not significantly different. However, the rates of basal and dexamethasone-stimulated exocytosis of NHE3 were attenuated in ClC-5 knockdown cells. These results show that ClC-5 plays an essential role in exocytosis of NHE3.

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.