Comparative molecular analysis of Na+/H+ exchangers: a unified model for Na+/H+ antiport?

Effect of Tea Tree Oil on the Expression of Genes Involved in the Innate Immune System in Goat Rumen Epithelial Cells

Simple Summary

Subacute rumen acidosis (SARA) often causes significant losses on commercial farms. SARA is mainly caused by endotoxin (LPS) produced by the lysis of Gram-negative bacteria, which causes an inflammatory response. To alleviate the inflammatory response mediated by LPS, it is important to improve animal production performance. Tea tree oil (TTO) is a plant extract that possesses good bactericidal and anti-inflammatory effects. According to this study, LPS can significantly induce inflammatory responses in goat rumen epithelial cells (GRECs), while the addition of TTO could markedly mitigate inflammatory responses mediated by LPS in GRECs. Therefore, it may be useful for the treatment of SARA.

Abstract

In subacute rumen acidosis (SARA), the rumen epithelium is frequently attacked by endotoxin (LPS), which is caused by the lysis of dead Gram-negative bacteria. However, the rumen epithelium innate immune system can actively respond to the infection. Previous studies have demonstrated that tea tree oil (TTO) has good bactericidal and anti-inflammatory effects. Therefore, the aim of this study was to investigate the effect of TTO on the expression of genes involved in the inflammatory cytokines in goat rumen epithelial cells (GRECs) triggered by LPS. Our study shows that rumen epithelial cells isolated from goat rumen tissue can be cultured in vitro in 0.25% trypsin for a long time. These cells were identified as epithelial cells by the expression of cytokeratin 18, monocarboxylate transporter 4 (MCT4), Na[+]/H[+] hydrogen exchanger 1 (NHE1), putative anion transporter 1 (PAT1), vH⁺ ATPase B subunit (vH⁺ ATPase), and anion exchanger 2 (AE2). The mRNA expression of IL-1β, IL-6, TNF-α, TLR-2, NF-κB, CXCL6 and CXCL8 genes was significantly increased when LPS was used compared to untreated controls. In addition, mRNA expression of IL-1β, IL-6, TNF-α, TLR-2, NF-κB, CXCL8, CXCL6 and interferon-induced protein with tetratricopeptide repeats 3 (IFIT3) genes was also significantly higher in the LPS group compared to the 0.05% TTO group. However, the expression of IL-1β, IL-6, TNF-α, TLR-2, CXCL6 and IFIT3 genes was significantly lower in the LPS and 0.05% TTO group compared to the 1 μg/mL LPS group. These results suggest that TTO can inhibit LPS-induced inflammatory cytokines expression in GRECs.

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

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

A Golgi-Localized Sodium/Hydrogen Exchanger Positively Regulates Salt Tolerance by Maintaining Higher K+/Na+ Ratio in Soybean

Salt stress caused by soil salinization, is one of the main factors that reduce soybean yield and quality. A large number of genes have been found to be involved in the regulation of salt tolerance. In this study, we characterized a soybean sodium/hydrogen exchanger gene GmNHX5 and revealed its functional mechanism involved in the salt tolerance process in soybean. GmNHX5 responded to salt stress at the transcription level in the salt stress-tolerant soybean plants, but not significantly changed in the salt-sensitive ones. GmNHX5 was located in the Golgi apparatus, and distributed in new leaves and vascular, and was induced by salt treatment. Overexpression of GmNHX5 improved the salt tolerance of hairy roots induced by soybean cotyledons, while the opposite was observed when GmNHX5 was knockout by CRISPR/Cas9. Soybean seedlings overexpressing GmNHX5 also showed an increased expression of GmSOS1, GmSKOR, and GmHKT1, higher K+/Na+ ratio, and higher viability when exposed to salt stress. Our findings provide an effective candidate gene for the cultivation of salt-tolerant germplasm resources and new clues for further understanding of the salt-tolerance mechanism in plants.

Advances in research on the regulatory mechanism of NHE1 in tumors

Tumors pose a major threat to human health and present with difficulties that modern medicine has yet to overcome. It has been demonstrated that the acid-base balance of the tumor microenvironment is closely associated with the dynamic balance in the human body and that it regulates several processes, such as cell proliferation and differentiation, intracellular enzyme activity, and cytoskeletal assembly and depolymerization. It has been well established that the regulation of intra- and extracellular pH depends on a series of functional ion transporters and hydrogen ion channels, such as the Na⁺/H⁺ exchanger (NHE) protein and thee Cl/HCO₃- exchange protein, among which the NHE1 member of the NHE family has been attracting increasing attention in recent years, particularly in studies on the correlation between pH regulation and tumors. NHE1 is a housekeeping gene encoding a protein that is widely expressed on the surface of all plasma membranes. Due to its functional domain, which determines the pHi at its N-terminus and C-terminus, NHE1 is involved in the regulation of the cellular pH microenvironment. It has been reported in the literature that NHE1 can regulate cell volume, participate in the transmembrane transport of intracellular and extracellular ions, affect cell proliferation and apoptosis, and regulate cell behavior and cell cycle progression; however, research on the role of NHE1 in tumorigenesis and tumor development in various systems is at its early stages. The aim of the present study was to review the current research on the correlation between the NHE family proteins and various systemic tumors, in order to indicate a new direction for antitumor drug development with the pH microenvironment as the target.

Acidic residues of extracellular loop 3 of the Na+/H+ exchanger type 1 are important in cation transport

Mammalian Na⁺/H⁺ exchanger type I isoform (NHE1) is a ubiquitously expressed membrane protein that regulates intracellular pH (pHi) by removing one intracellular proton in exchange for one extracellular sodium ion. Abnormal activity of the protein occurs in cardiovascular disease and breast cancer. The purpose of this study is to examine the role of negatively charged amino acids of extracellular loop 3 (EL3) in the activity of the NHE protein. We mutated glutamic acid 217 and aspartic acid 226 to alanine, and to glutamine and asparagine, respectively. We examined effects on expression levels, cell surface targeting and activity of NHE1, and also characterized affinity for extracellular sodium and lithium ions. Individual mutation of these amino acids had little effect on protein function. However, mutation of both these amino acids together impaired transport, decreasing the Vmax for both Na⁺ and Li⁺ ions. We suggested that amino acids E217 and D226 form part of a negatively charged coordination sphere, which facilitates cation transport in the NHE1 protein.

Functional Analysis of Conserved Transmembrane Charged Residues and a Yeast Specific Extracellular Loop of the Plasma Membrane Na+/H+ Antiporter of Schizosaccharomyces pombe

The Na⁺/H⁺ exchanger of the plasma membrane of S. pombe (SpNHE1) removes excess intracellular sodium in exchange for an extracellular proton. We examined the functional role of acidic amino acids of a yeast specific periplasmic extracellular loop 6 (EL6) and of Glu⁷⁴ and Arg⁷⁷ of transmembrane segment 3. Glu⁷⁴ and Arg⁷⁷ are conserved in yeast species while Glu⁷⁴ is conserved throughout various phyla. The mutation E74A caused a minor effect, while mutation R77A had a larger effect on the ability of SpNHE1 to confer salt tolerance. Mutation of both residues to Ala or Glu also eliminated the ability to confer salt tolerance. Arg³⁴¹ and Arg³⁴² were also necessary for SpNHE1 transport in S. pombe. Deletion of 3 out of 4 acidic residues (Asp³⁸⁹, Glu³⁹⁰, Glu³⁹², Glu³⁹⁷) of EL6 did not greatly affect SpNHE1 function while deletion of all did. Replacement of EL6 with a segment from the plant Na⁺/H⁺ exchanger SOS1 also did not affect function. We suggest that EL6 forms part of a cation coordination sphere, attracting cations for transport but that the region is not highly specific for the location of acidic charges. Overall, we identified a number of polar amino acids important in SpNHE1 function.

Antimicrobial metabolites from Saraca asoca impairs the membrane transport system and quorum-sensing system in Pseudomonas aeruginosa

This study was conducted to explore the antimicrobial mechanism of metabolites from Saraca asoca (SA1) using differential proteomics and metabolic profile of Pseudomonas aeruginosa after treatment with effective sub-MIC dose of 312 µg/mL. SA1 fraction was found to contain antibacterial metabolites catechol, protocatechuic acid, and epigallocatechin gallate. Proteome analysis revealed 33 differentially expressed proteins after SA1 treatment. Protein network analysis showed that SA1 treatment upregulated the DNA topological and metabolic processes. Furthermore, it revealed that T2SS, cellular component biogenesis, and response to chemical stimuli were inhibited by SA1 treatment, supported by down-regulated Na⁺/H⁺ antiporter, SdeX, ompK, and trbD proteins. Statistical analysis of mass data revealed the altered level of 20 metabolites includes HSLs, PQS, rhamnolipid, and pyocyanin. Proteome and metabolome results showed that treatment impaired cell membrane functions and quorum-sensing system. It was further confirmed by increased MDA (3.95 fold), and rhamnolipids (4.3 fold) production and, therefore, oxidative stress (36.9%) after SA1 treatment.

Structural and functional analysis of transmembrane segment IV of the salt tolerance protein Sod2

Sod2 is the plasma membrane Na(+)/H(+) exchanger of the fission yeast Schizosaccharomyces pombe. It provides salt tolerance by removing excess intracellular sodium (or lithium) in exchange for protons. We examined the role of amino acid residues of transmembrane segment IV (TM IV) ((126)FPQINFLGSLLIAGCITSTDPVLSALI(152)) in activity by using alanine scanning mutagenesis and examining salt tolerance in sod2-deficient S. pombe. Two amino acids were critical for function. Mutations T144A and V147A resulted in defective proteins that did not confer salt tolerance when reintroduced into S. pombe. Sod2 protein with other alanine mutations in TM IV had little or no effect. T144D and T144K mutant proteins were inactive; however, a T144S protein was functional and provided lithium, but not sodium, tolerance and transport. Analysis of sensitivity to trypsin indicated that the mutations caused a conformational change in the Sod2 protein. We expressed and purified TM IV (amino acids 125-154). NMR analysis yielded a model with two helical regions (amino acids 128-142 and 147-154) separated by an unwound region (amino acids 143-146). Molecular modeling of the entire Sod2 protein suggested that TM IV has a structure similar to that deduced by NMR analysis and an overall structure similar to that of Escherichia coli NhaA. TM IV of Sod2 has similarities to TM V of the Zygosaccharomyces rouxii Na(+)/H(+) exchanger and TM VI of isoform 1 of mammalian Na(+)/H(+) exchanger. TM IV of Sod2 is critical to transport and may be involved in cation binding or conformational changes of the protein.

Molecular characterization of a novel Na⁺/H⁺ antiporter cDNA from Eucalyptus globulus

Environmental stress factors such as salt, drought and heat are known to affect plant productivity. However, high salinity is spreading throughout the world, currently affecting more than 45 millionha. One of the mechanisms that allow plants to withstand salt stress consists on vacuolar sequestration of Na(+), through a Na(+)/H(+) antiporter. We isolated a new vacuolar Na(+)/H(+) antiporter from Eucalyptus globulus from a cDNA library. The cDNA had a 1626 bp open reading frame encoding a predicted protein of 542 amino acids with a deduced molecular weight of 59.1 KDa. Phylogenetic and bioinformatic analyses indicated that EgNHX1 localized in the vacuole. To assess its role in Na(+) exchange, we performed complementation studies using the Na(+) sensitive yeast mutant strain Δnhx1. The results showed that EgNHX1 partially restored the salt sensitive phenotype of the yeast Δnhx1 strain. However, its overexpression in transgenic Arabidopsis confers tolerance in the presence of increasing NaCl concentrations while the wild type plants exhibited growth retardation. Expression profiles of Eucalyptus seedlings subjected to salt, drought, heat and ABA treatment were established. The results revealed that Egnhx1 was induced significantly only by drought. Together, these results suggest that the product of Egnhx1 from E. globulus is a functional vacuolar Na(+)/H(+) antiporter.

Structural and functional analysis of critical amino acids in TMVI of the NHE1 isoform of the Na+/H+ exchanger

The mammalian Na(+)/H(+) exchanger isoform 1 (NHE1) resides on the plasma membrane and exchanges one intracellular H(+) for one extracellular Na(+). It maintains intracellular pH and regulates cell volume, and cell functions including growth and cell differentiation. Previous structural and functional studies on TMVI revealed several amino acids that are potentially pore lining. We examined these and other critical residues by site-directed mutagenesis substituting Asn227→Ala, Asp, Arg; Ile233→Ala; Leu243→Ala; Glu247→Asp, Gln; Glu248→Asp, Gln. Mutant NHE1 proteins were characterized in AP-1 cells, which do not express endogenous NHE1. All the TMVI critical amino acids were highly sensitive to substitution and changes often lead to a dysfunctional protein. Mutations of Asn227→Ala, Asp, Arg; Ile233→Ala; Leu243→Ala; Glu247→Asp; Glu248→Gln yielded significant reduction in NHE1 activity. Mutants of Asn227 demonstrated defects in protein expression, targeting and activity. Substituting Asn227→Arg and Ile233→Ala decreased the surface localization and expression of NHE1 respectively. The pore lining amino acids Ile233 and Leu243 were both essential for activity. Glu247 was not essential, but the size of the residue at this location was important while the charge on residue Glu248 was more critical to NHE1 function. Limited trypsin digestion on Leu243→Ala and Glu248→Gln revealed that they had increased susceptibility to proteolytic attack, indicating an alteration in protein conformation. Modeling of TMVI with TMXI suggests that these TM segments form part of the critical fold of NHE1 with Ile233 and Leu465 of TMXI forming a critical part of the extracellular facing ion conductance pathway.

The membrane subunit NuoL(ND5) is involved in the indirect proton pumping mechanism of Escherichia coli complex I

Complex I pumps protons across the membrane by using downhill redox energy. Here, to investigate the proton pumping mechanism by complex I, we focused on the largest transmembrane subunit NuoL (Escherichia coli ND5 homolog). NuoL/ND5 is believed to have H(+) translocation site(s), because of a high sequence similarity to multi-subunit Na(+)/H(+) antiporters. We mutated thirteen highly conserved residues between NuoL/ND5 and MrpA of Na(+)/H(+) antiporters in the chromosomal nuoL gene. The dNADH oxidase activities in mutant membranes were mostly at the control level or modestly reduced, except mutants of Glu-144, Lys-229, and Lys-399. In contrast, the peripheral dNADH-K(3)Fe(CN)(6) reductase activities basically remained unchanged in all the NuoL mutants, suggesting that the peripheral arm of complex I was not affected by point mutations in NuoL. The proton pumping efficiency (the ratio of H(+)/e(-)), however, was decreased in most NuoL mutants by 30-50%, while the IC(50) values for asimicin (a potent complex I inhibitor) remained unchanged. This suggests that the H(+)/e(-) stoichiometry has changed from 4H(+)/2e(-) to 3H(+) or 2H(+)/2e(-) without affecting the direct coupling site. Furthermore, 50 μm of 5-(N-ethyl-N-isopropyl)-amiloride (EIPA), a specific inhibitor for Na(+)/H(+) antiporters, caused a 38 ± 5% decrease in the initial H(+) pump activity in the wild type, while no change was observed in D178N, D303A, and D400A mutants where the H(+) pumping efficiency had already been significantly decreased. The electron transfer activities were basically unaffected by EIPA in both control and mutants. Taken together, our data strongly indicate that the NuoL subunit is involved in the indirect coupling mechanism.

Alkali metal cation transport and homeostasis in yeasts

The maintenance of appropriate intracellular concentrations of alkali metal cations, principally K(+) and Na(+), is of utmost importance for living cells, since they determine cell volume, intracellular pH, and potential across the plasma membrane, among other important cellular parameters. Yeasts have developed a number of strategies to adapt to large variations in the concentrations of these cations in the environment, basically by controlling transport processes. Plasma membrane high-affinity K(+) transporters allow intracellular accumulation of this cation even when it is scarce in the environment. Exposure to high concentrations of Na(+) can be tolerated due to the existence of an Na(+), K(+)-ATPase and an Na(+), K(+)/H(+)-antiporter, which contribute to the potassium balance as well. Cations can also be sequestered through various antiporters into intracellular organelles, such as the vacuole. Although some uncertainties still persist, the nature of the major structural components responsible for alkali metal cation fluxes across yeast membranes has been defined within the last 20 years. In contrast, the regulatory components and their interactions are, in many cases, still unclear. Conserved signaling pathways (e.g., calcineurin and HOG) are known to participate in the regulation of influx and efflux processes at the plasma membrane level, even though the molecular details are obscure. Similarly, very little is known about the regulation of organellar transport and homeostasis of alkali metal cations. The aim of this review is to provide a comprehensive and up-to-date vision of the mechanisms responsible for alkali metal cation transport and their regulation in the model yeast Saccharomyces cerevisiae and to establish, when possible, comparisons with other yeasts and higher plants.

TransportTP: a two-phase classification approach for membrane transporter prediction and characterization

BACKGROUND

Membrane transporters play crucial roles in living cells. Experimental characterization of transporters is costly and time-consuming. Current computational methods for transporter characterization still require extensive curation efforts, especially for eukaryotic organisms. We developed a novel genome-scale transporter prediction and characterization system called TransportTP that combined homology-based and machine learning methods in a two-phase classification approach. First, traditional homology methods were employed to predict novel transporters based on sequence similarity to known classified proteins in the Transporter Classification Database (TCDB). Second, machine learning methods were used to integrate a variety of features to refine the initial predictions. A set of rules based on transporter features was developed by machine learning using well-curated proteomes as guides.

RESULTS

In a cross-validation using the yeast proteome for training and the proteomes of ten other organisms for testing, TransportTP achieved an equivalent recall and precision of 81.8%, based on TransportDB, a manually annotated transporter database. In an independent test using the Arabidopsis proteome for training and four recently sequenced plant proteomes for testing, it achieved a recall of 74.6% and a precision of 73.4%, according to our manual curation.

CONCLUSIONS

TransportTP is the most effective tool for eukaryotic transporter characterization up to date.

Structural and functional analysis of transmembrane XI of the NHE1 isoform of the Na+/H+ exchanger

The Na(+)/H(+) exchanger isoform 1 is a ubiquitously expressed integral membrane protein that regulates intracellular pH in mammals by extruding an intracellular H(+) in exchange for one extracellular Na(+). We characterized structural and functional aspects of the critical transmembrane (TM) segment XI (residues 449-470) by using cysteine scanning mutagenesis and high resolution NMR. Each residue of TM XI was mutated to cysteine in the background of the cysteine-less protein and the sensitivity to water-soluble sulfhydryl reactive compounds MTSET ((2-(trimethylammonium) ethyl)methanethiosulfonate) and MTSES ((2-sulfonatoethyl) methanethiosulfonate) was determined for those residues with at least moderate activity remaining. Of the residues tested, only proteins with mutations L457C, I461C, and L465C were inhibited by MTSET. The activity of the L465C mutant was almost completely eliminated, whereas that of the L457C and I461C mutants was partially affected. The structure of a peptide representing TM XI (residues Lys(447)-Lys(472)) was determined using high resolution NMR spectroscopy in dodecylphosphocholine micelles. The structure consisted of helical regions between Asp(447)-Tyr(454) and Phe(460)-Lys(471) at the N and C termini of the peptide, respectively, connected by a region with poorly defined, irregular structure consisting of residues Gly(455)-Gly(459). TM XI of NHE1 had a structural similarity to TM XI of the Escherichia coli Na(+)/H(+) exchanger NhaA. The results suggest that TM XI is a discontinuous helix, with residue Leu(465) contributing to the pore.

Proline 146 is critical to the structure, function and targeting of sod2, the Na+/H+ exchanger of Schizosaccharomyces pombe

Sod2 is the Na(+)/H(+) exchanger of the fission yeast Schizosaccharomyces pombe that is principally responsible for salt tolerance. We examined the role of nine polar, membrane associated amino acids in the ability of the protein to confer salt tolerance in S. pombe. Wild type sod2 protein with a C-terminal GFP tag effectively rescued salt tolerance in S. pombe with deleted endogenous sod2. Sod2 protein with the mutations P163A, P183A, D298N, D389N, E390Q, E392Q and E397Q also conveyed salt tolerance as effectively as the wild type sod2 protein. In contrast, the mutation P146A resulted in a protein that did not convey salt tolerance nearly as effectively as the wild type and did not extrude Na(+) as well as the wild type. Mutation of Pro(146) to Ser, Asp or Lys had an intermediate effect. Mutation of Thr(142) to Ser resulted in a slightly defective protein. Western blot analysis showed that all mutant proteins were expressed at similar levels as wild type sod2 protein. Examination of the localization of the proteins showed that wild type and most sod2 mutants were present in the plasma membrane while the P146A mutant had an intracellular localization. Limited tryptic digestion suggested that the P146A sod2 protein had a change in conformation in comparison to the wild type protein. The results suggest that Pro(146) is an amino acid critical to sod2 structure, function and localization.

Structural and functional characterization of transmembrane segment IX of the NHE1 isoform of the Na+/H+ exchanger

The Na(+)/H(+) exchanger isoform 1 (NHE1) is an integral membrane protein that regulates intracellular pH by removing one intracellular H(+) in exchange for one extracellular Na(+). It has a large N-terminal membrane domain of 12 transmembrane segments and an intracellular C-terminal regulatory domain. We characterized the cysteine accessibility of amino acids of the putative transmembrane segment IX (residues 339-363). Each residue was mutated to cysteine in a functional cysteineless NHE1 protein. Of 25 amino acids mutated, 5 were inactive or nearly so after mutation to cysteine. Several of these showed aberrant targeting to the plasma membrane and reduced expression of the intact protein, whereas others were expressed and targeted correctly but had defective NHE1 function. Of the active mutants, Glu(346) and Ser(351) were inhibited >70% by positively charged [2-(trimethylammonium)-ethyl]methanethiosulfonate but not by anionic [2-sulfonatoethyl]methanethiosulfonate, suggesting that they are pore lining and make up part of the cation conduction pathway. Both mutants also had decreased affinity for Na(+) and decreased activation by intracellular protons. The structure of a peptide representing amino acids 338-365 was determined by using high resolution NMR in dodecylphosphocholine micelles. The structure contained two helical regions (amino acids Met(340)-Ser(344) and Ile(353)-Ser(359)) kinked with a large bend angle around a pivot point at amino acid Ser(351). The results suggest that transmembrane IX is critical with pore-lining residues and a kink at the functionally important residue Ser(351).

Molecular biology of the myocardial Na+/H+ exchanger

The mammalian Na(+)/H(+) exchanger is a pH regulatory membrane protein that uses the sodium gradient to translocate one intracellular proton in exchange for one extracellular sodium. There are nine isoforms of the protein with varying tissue and cellular distribution, some isoforms are predominantly intracellular. In the myocardium, the Na(+)/H(+) exchanger type 1 isoform (NHE1) is the only plasma membrane isoform present in significant quantities. It plays an important role during ischemia/reperfusion damage to the myocardium and has recently been implicated in myocardial hypertrophy. The NHE1 gene is made from 12 exons and a differentially spliced version mediates Na(+)/Li(+) exchange. The NHE1 promoter is regulated by several transcription factors. In the myocardium, transcription factors both proximal and distal to the start site affect expression, including AP-2 and a thyroid responsive element. Recently, reactive oxygen species have also been shown to be important regulators of the NHE1 promoter. Structural and functional analysis of the NHE1 protein has shown that transmembrane segments IV, VII and IX are important in ion transport and susceptibility to pharmacological inhibition. NHE1 protein and mRNA levels are elevated by cardiac ischemia/reperfusion, hypertrophy and acidosis. Understanding the mechanism by which NHE1 mediates transport and its regulation of expression will give novel insights into its contributions in cardiovascular disease.

Functional validation of a novel isoform of Na+/H+ antiporter from Pennisetum glaucum for enhancing salinity tolerance in rice

Salt stress is an environmental factor that severely impairs plant growth and productivity. We have cloned a novel isoform of a vacuolar Na+/H+ antiporter from Pennisetum glaucum (PgNHX1) that contains 5 transmembrane domains in contrast to AtNHX1 and OsNHX1 which have 9 transmembrane domains. Recently we have shown that PgNHX1 could confer high level of salinity tolerance when overexpressed in Brassica juncea. Here,we report the functional validation of this antiporter in crop plant rice. Overexpression of PgNHX1 conferred high level of salinity tolerance in rice. Transgenic rice plants overexpressing PgNHX1 developed more extensive root system and completed their life cycle by setting flowers and seeds in the presence of 150 mM NaCl. Our data demonstrate the potential of PgNHX1 for imparting enhanced salt tolerance capabilities to salt-sensitive crop plants for growing in high saline areas.

A halocin acting on Na+/H+ exchanger of haloarchaea as a new type of inhibitor in NHE of mammals

The capability of halocin H6 (a bacteriocin-like protein produced by haloarchaea Haloferax gibbonsii) to inhibit Na+/H+ exchanger (NHE) in mammalian cells and its cardio-protective efficacy on the ischemic and reperfused myocardium were evaluated in the present study. H6 inhibits NHE activity (measured by a flow cytometry method) in a dose-dependent form of cell lines of mammalian origin (HEK293, NIH3T3, Jurkat and HL-1) as well as in primary cell culture from human skeletal muscle (myocytes and fibroblasts). In vivo, an ischemia-reperfusion model in dogs by coronary arterial occlusion was used (two hours of regional ischemia and three hours of reperfusion). In animals treated with halocin H6 there was a significant reduction of premature ventricular ectopic beats and infarct size, whereas blood pressure and heart rate remained unchanged. Up to date, halocin H6 is the only described biological molecule that exerts a specific inhibitory activity in NHE of eukaryotic cells.

Functional characterization of the transmembrane segment VII of the NHE1 isoform of the Na+/H+ exchangerThis paper is one of a selection of papers published in this Special Issue, entitled The Cellular and Molecular Basis of Cardiovascular Dysfunction, Dhalla 70th Birthday Tribute

The Na+/H+ exchanger isoform 1 is an integral membrane protein that regulates intracellular pH. It extrudes 1 intracellular H+ in exchange for 1 extracellular Na+. It has 2 large domains, an N-terminal membrane domain of 12 transmembrane segments and an intracellular C-terminal regulatory domain. We characterized the cysteine accessibility of amino acids of the critical transmembrane segment TM VII. Residues Leu 255, Leu 258, Glu 262, Leu 265, Asn 266, Asp 267, Val 269, Val 272, and Leu 273 were all mutated to cysteine residues in the cysteineless NHE1 isoform. Mutation of amino acids E262, N266, and D267 caused severe defects in activity and targeting of the intact full length protein. The balance of the active mutants were examined for sensitivity to the sulfhydryl reactive reagents, positively charged MTSET ((2- (trimethylammonium)ethyl)methanethiosulfonate) and negatively charged MTSES ((2-sulfonatoethyl)methanethiosulfonate). Leu 255 and Leu 258 were sensitive to MTSET but not to MTSES. The results suggest that these amino acids are pore-lining residues. We present a model of TM VII that shows that residues Leu 255, Leu 258, Glu 262, Asn 266, and Asp 267 lie near the same face of TM VII, lining the ion transduction pore.

Structural and functional analysis of the Na+/H+ exchanger

The mammalian NHE (Na+/H+ exchanger) is a ubiquitously expressed integral membrane protein that regulates intracellular pH by removing a proton in exchange for an extracellular sodium ion. Of the nine known isoforms of the mammalian NHEs, the first isoform discovered (NHE1) is the most thoroughly characterized. NHE1 is involved in numerous physiological processes in mammals, including regulation of intracellular pH, cell-volume control, cytoskeletal organization, heart disease and cancer. NHE comprises two domains: an N-terminal membrane domain that functions to transport ions, and a C-terminal cytoplasmic regulatory domain that regulates the activity and mediates cytoskeletal interactions. Although the exact mechanism of transport by NHE1 remains elusive, recent studies have identified amino acid residues that are important for NHE function. In addition, progress has been made regarding the elucidation of the structure of NHEs. Specifically, the structure of a single TM (transmembrane) segment from NHE1 has been solved, and the high-resolution structure of the bacterial Na+/H+ antiporter NhaA has recently been elucidated. In this review we discuss what is known about both functional and structural aspects of NHE1. We relate the known structural data for NHE1 to the NhaA structure, where TM IV of NHE1 shows surprising structural similarity with TM IV of NhaA, despite little primary sequence similarity. Further experiments that will be required to fully understand the mechanism of transport and regulation of the NHE1 protein are discussed.

Functional involvement of membrane-embedded and conserved acidic residues in the ShaA subunit of the multigene-encoded Na+/H+ antiporter in Bacillus subtilis

ShaA, a member of a multigene-encoded Na+/H+ antiporter in B. subtilis, is a large integral membrane protein consisting of 20 transmembrane helices (TM). Conservation of ShaA-like protein subunits in several cation-coupled enzymes, including the NuoL (ND5) subunit of the H+-translocating complex I, suggests the involvement of ShaA in cation transport. Bacillus subtilis ShaA contains six acidic residues that are conserved in ShaA homologues and are located in putative transmembrane helices. We examined the functional involvement of the six transmembrane acidic residues of ShaA by site-directed mutagenesis. Mutation in glutamate (Glu)-113 in TM-4, Glu-657 in TM-18, aspartate (Asp)-734 and Glu-747 in TM-20 abolished the antiport activity, suggesting that these residues play important roles in the ion transport of Sha. The acidic group was necessary and sufficient in Glu-657 and Asp-743, while it was not true of Glu-113 and Glu-747. Mutation in Asp-103 in TM-3, which is conserved in ShaA-types but not in ShaAB-types, partially affected on the antiport activity. Mutation in Asp-50 in TM-2 resulted in a unexpected phenotype: mutants retained the wild type level of ability to confer NaCl resistance to the Na+/H+ antiporter-deficient E. coli KNabc, but showed a very low antiport activity. The acidic group of Asp-50 and Asp-103 was not essential for the function. Our results suggested that these acidic residues are functionally involved in the ion transport of Sha, and some of them probably in cation binding and/or translocation.

Functional expression and cellular localization of the Na+/H+ exchanger Sod2 of the fission yeast Schizosaccharomyces pombe

In the fission yeast Schizosaccharomyces pombe, the Na+/H+ exchanger, Sod2, plays a major role in the removal of excess intracellular sodium, and its disruption results in a sodium-sensitive phenotype. We examined the subcellular distribution and dynamics of Sod2 expression in S. pombe using a sod2-GFP fusion protein under the control of an attenuated version of the inducible nmt promoter. Sod2 was localized throughout the plasma membrane, the nuclear envelope, and some internal membrane systems. In exponentially growing cells, in which sod2-GFP was expressed and then the promoter turned-off, previously synthesized sod2-GFP was stable for long periods and found localized to the plasma membrane in the medial regions of the cell. It was not present at the actively growing cell ends. This suggests that these regions of the cell contain old plasma membrane protein vs. newly synthesized plasma membrane without Sod2 at the growing ends. Sod2 localization was not affected by salt stress. The results suggest that Sod2 is both a plasma membrane protein and is present in intracellular membranes. It is likely tethered within discrete regions of the plasma membrane and is not free to diffuse throughout the bilayer.

Mammalian NHE2 Na+/H+exchanger mediates efflux of potassium upon heterologous expression in yeast

Na(+)/H+exchangers form a broad family of transporters that mediate opposing fluxes of alkali metal cations and protons across cell membranes. They play multiple roles in different organisms (protection from toxic cations, regulation of cell volume or pH). Rat NHE2 exchanger was expressed in a Saccharomyces cerevisiae mutant strain lacking its own exporters of alkali metal cations. Though most of the overexpressed NHE2 remained entrapped in the secretory pathway, part of it reached the plasma membrane and mediated K+ efflux from the yeast. We demonstrate for the first time that a mammalian Na(+)/H+ exchanger transports alkali metal cations in yeast in the opposite direction than in mammalian cells, and that the substrate specificity of the rat NHE2 exchanger is limited only to potassium cations upon expression in yeast cells.

Identification of Conserved Prolyl Residue Important for Transport Activity and the Substrate Specificity Range of Yeast Plasma Membrane Na+/H+ Antiporters

Yeast plasma membrane Na+/H+ antiporters are divided according to their substrate specificity in two distinct subfamilies. To identify amino acid residues responsible for substrate specificity determination (recognition of K+), the Zygosaccharomyces rouxii Sod2-22 antiporter (non-transporting K+) was mutagenized and a collection of ZrSod2-22 mutants that improved the KCl tolerance of a salt-sensitive Saccharomyces cerevisiae strain was isolated. Several independent ZrSod2-22 mutated alleles contained the replacement of a highly conserved proline 145 with a residue containing a hydroxyl group (Ser, Thr). Site-directed mutagenesis of Pro145 proved that an amino acid with a hydroxyl group at this position is enough to enable ZrSod2-22p to transport K+. Simultaneously, the P145(S/T) mutation decreased the antiporter transport activity for both Na+ and Li+. Replacement of Pro145 with glycine resulted in a ZrSod2-22p with extremely low activity only for Na+, and the exchange of a charged residue (Asp, Lys) for Pro145 completely stopped the activity. Mutagenesis of the corresponding proline in the S. cerevisiae Nha1 antiporter (Pro146) confirmed that this proline of the fifth transmembrane domain is a critical residue for antiporter function. This is the first evidence that a non-polar amino acid residue is important for the substrate specificity and activity of yeast Nha antiporters.

Molecular characterization of PeNhaD1: the first member of the NhaD Na+/H+ antiporter family of plant origin

PeNhaD1 encodes a putative Na+/H+ antiporter from the salt-resistant tree Populus euphratica. It is the first characterization of a member of the NhaD type ion transporter family of plant origin. Homology searches revealed its close relation to functionally characterized microbial Na+/H+ antiporters VpNhaD and VcNhaD. Na+/H+ antiporters have proven to play a key role in salt resistance, both in plants and bacteria. Under salt stress transcript levels of PeNhaD1 were maintained only in the salt-resistant P. euphratica, but collapsed in Populus x canescens, a salt-sensitive species. To address the function of PeNhaD1, complementation studies with the salt-sensitive Escherichia coli EP432 mutant strain, lacking activity of the two Na+/H+ antiporters EcNhaA and EcNhaB were carried out. PeNhaD1 was able to restore growth of EP432 under stress imposed by up to 400 mM NaCl demonstrating its protective function. Growth rates of EP432 were always highest at pH 5.5 while growth was suppressed under salt stress at pH 7.0 and pH 8.0 suggesting that the antiporter activity is strongly pH dependent. Element analyses of EP432 cells complemented with PeNhaD1 growing under salt stress showed that salt resistance was correlated with a significant reduction in sodium accumulation. These results suggest that PeNhaD1 might play a role in the salt resistance of P. euphratica.

Structural and Functional Characterization of Transmembrane Segment IV of the NHE1 Isoform of the Na+/H+ Exchanger

The Na(+)/H(+) exchanger isoform 1 is a ubiquitously expressed integral membrane protein that regulates intracellular pH in mammals. We characterized the structural and functional aspects of the critical transmembrane (TM) segment IV. Each residue was mutated to cysteine in cysteine-less NHE1. TM IV was exquisitely sensitive to mutation with 10 of 23 mutations causing greatly reduced expression and/or activity. The Phe(161) --> Cys mutant was inhibited by treatment with the water-soluble sulfhydryl-reactive compounds [2-(trimethylammonium)ethyl]methanethiosulfonate and [2-sulfonatoethyl]methanethiosulfonate, suggesting it is a pore-lining residue. The structure of purified TM IV peptide was determined using high resolution NMR in a CD(3)OH:CDCl(3):H(2)O mixture and in Me(2)SO. In CD(3)OH: CDCl(3):H(2)O, TM IV was structured but not as a canonical alpha-helix. Residues Asp(159)-Leu(162) were a series of beta-turns; residues Leu(165)-Pro(168) showed an extended structure, and residues Ile(169)-Phe(176) were helical in character. These three structured regions rotated quite freely with respect to the others. In Me(2)SO, the structure was much less defined. Our results demonstrate that TM IV is an unusually structured transmembrane segment that is exquisitely sensitive to mutagenesis and that Phe(161) is a pore-lining residue.

Inducible gene trapping with drug-selectable markers and Cre/loxP to identify developmentally regulated genes

Gene trapping in mouse embryonic stem cells is an important genetic approach that allows simultaneous mutation of genes and generation of corresponding mutant mice. We designed a selection scheme with drug selection markers and Cre/loxP technology which allows screening of gene trap events that responded to a signaling molecule in a 96-well format. Nine hundred twenty gene trap clones were assayed, and 258 were classified as gene traps induced by in vitro differentiation. Sixty-five of the in vitro differentiation-inducible gene traps were also responsive to retinoic acid treatment. In vivo analysis revealed that 85% of the retinoic acid-inducible gene traps trapped developmentally regulated genes, consistent with the observation that genes induced by retinoic acid treatment are likely to be developmentally regulated. Our results demonstrate that the inducible gene trapping system described here can be used to enrich in vitro for traps in genes of interest. Furthermore, we demonstrate that the cre reporter is extremely sensitive and can be used to explore chromosomal regions that are not detectable with neo as a selection cassette.

Functional analysis of amino acids of the Na+/H+ exchanger that are important for proton translocation

The Na+/H+ exchanger is an integral membrane protein found in the plasma membrane of eukaryotic and prokaryotic cells. In eukaryotes it functions to exchange one proton for a sodium ion. In mammals it removes intracellular protons while in plants and fungal cells the plasma membrane form removes intracellular sodium in exchange for extracellular protons. In this study we used the Na+/H+ exchanger of Schizosaccharomyces pombe (Sod2) as a model system to study amino acids critical for activity of the protein. Twelve mutant forms of the Na+/H+ exchanger were examined for their ability to translocate protons as assessed by a Cytosensor microphysiometer. Mutation of the amino acid Histidine 367 resulted in defective proton translocation. The acidic residues Asp145, Asp178, Asp266 and Asp267 were important in the proton translocation activity of the Na+/H+ exchanger. Mutation of amino acids His98, His233 and Asp241 did not significantly impair proton translocation by the Na+/H+ exchanger. These results confirm that polar amino acids are important in proton flux activity of Na+/H+ exchangers.

Regulation of Expression of the Na+/H+ Exchanger by Thyroid Hormone

The Na+/H+ exchanger is a pH regulatory protein with a ubiquitous distribution in eukaryotic cells. Several isoforms of the Na+/H+ exchanger are known. The first isoform to be characterized and cloned, NHE1, is present on the plasma membrane of cells and functions to remove one intracellular proton in exchange for one extracellular sodium ion. It is involved in pH regulation, cell growth, differentiation, and cell migration. NHE1 is also involved in the cycle of damage that occurs in the heart with ischemic heart disease. Recent studies have shown that the Na+/H+ exchanger is regulated in response to thyroid hormone. Reduction in circulating thyroid hormone levels reduces the amount of both protein and mRNA of NHE1. Conversely, an elevation of thyroid hormone levels has the opposite effects. Transcriptional regulation of NHE1 expression has been demonstrated. The NHE1 promoter contains a TR alpha(1) binding site located between -841 to -800 bp. This element responds positively to TR alpha(1). This regulation of the NHE1 promoter by thyroid hormone is proposed to be responsible for postnatal changes in expression of the Na+/H+ exchanger.

Metaphase I arrest of starfish oocytes induced via the MAP kinase pathway is released by an increase of intracellular pH

Reinitiation of meiosis in oocytes usually occurs as a two-step process during which release from the prophase block is followed by an arrest in metaphase of the first or second meiotic division [metaphase I (MI) or metaphase II (MII)]. The mechanism of MI arrest in meiosis is poorly understood, although it is a widely observed phenomenon in invertebrates. The blockage of fully grown starfish oocytes in prophase of meiosis I is released by the hormone 1-methyladenine. It has been believed that meiosis of starfish oocytes proceeds completely without MI or MII arrest, even when fertilization does not occur. Here we show that MI arrest of starfish oocytes occurs in the ovary after germinal vesicle breakdown. This arrest is maintained both by the Mos/MEK/MAP kinase pathway and the blockage of an increase of intracellular pH in the ovary before spawning. Immediately after spawning into seawater, activation of Na+/H+ antiporters via a heterotrimeric G protein coupling to a 1-methyladenine receptor in the oocyte leads to an intracellular pH increase that can overcome the MI arrest even in the presence of active MAP kinase.

Two highly conserved glutamate residues critical for type III sodium-dependent phosphate transport revealed by uncoupling transport function from retroviral receptor function

Type III sodium-dependent phosphate (NaP(i)) cotransporters, Pit1 and Pit2, have been assigned housekeeping P(i) transport functions and suggested involved in chondroblastic and osteoblastic mineralization and ectopic calcification. Both proteins exhibit dual function, thus, besides being transporters, they also serve as receptors for several gammaretroviruses. We here show that it is possible to uncouple transport and receptor functions of a type III NaP(i) cotransporter and thus exploit the retroviral receptor function as a control for proper processing and folding of mutant proteins. Thus exchanging two putative transmembranic glutamate residues in human Pit2, Glu(55) and Glu(575), with glutamine or with lysine severely impaired or knocked out, respectively, P(i) transport function, but left viral receptor function undisturbed. Both glutamates are conserved in type III NaP(i) cotransporters, in fungal NaP(i) cotransporters PHO-4 and Pho89, and in other known or putative phosphate permeases from a number of species and are the first residues shown to be critical for type III NaP(i) cotransport. Their putative transmembranic positions together with the presented data are consistent with Glu(55) and Glu(575) being parts of a cation liganding site or playing roles in conformational changes associated with substrate transport. Finally, the results also show that Pit2 retroviral receptor function per se is not dependent on Pit2 P(i) transport function.

Asp(344) and Thr(345) are critical for cation exchange mediated by NhaD, Na(+)/H(+) antiporter of Vibrio cholerae

The Vc-NhaD is an Na(+)/H(+) antiporter from Vibrio cholerae belonging to a new family of bacterial Na(+)/H(+) antiporters, the NhaD family. In the present work we mutagenized five conserved Asp and Glu residues and one conserved Thr residue to Ala in order to identify amino acids that are critical for the antiport activity. All mutations fall into two distinct groups: (i) four variants, Glu(100)Ala, Glu(251)Ala, Glu(342)Ala, and Asp(393)Ala, did not abolish antiport activity but shifted the pH optimum to more alkaline pH, and (ii) variants Asp(344)Ala, Asp(344)Asn, and Thr(345)Ala caused a complete loss of both Na(+)/H(+) and Li(+)/H(+) antiport activity whereas the Asp(344)Glu variant exhibited reduced Na(+)/H(+) and Li(+)/H(+) antiport activity. This is the first mutational analysis of the antiporter of NhaD type and the first demonstration of Thr residue being indispensable for Na(+)/H(+) antiport. We discuss the possible role of Asp(344) and Thr(345) in the functioning of Vc-NhaD.

The biosynthetic gene cluster for the antitumor rebeccamycin: characterization and generation of indolocarbazole derivatives

Rebeccamycin, a halogenated natural product of the indolocarbazole family, is produced by Saccharothrix aerocolonigenes ATCC39243. Several rebeccamycin analogues, which target DNA topoisomerase I or II, have already entered clinical trials as anticancer drugs. Using as a probe an internal fragment of ngt, a Saccharothrix aerocolonigenes gene encoding an indolocarbazole N-glycosyltransferase, we isolated a DNA region that directed the biosynthesis of rebeccamycin when introduced into Streptomyces albus. Sequence analysis of 25.6 kb revealed genes for indolocarbazole core formation, halogenation, glycosylation, and sugar methylation, as well as a regulatory gene and two resistance/secretion genes. Heterologous expression of subsets of these genes resulted in production of deschloro-rebeccamycin, 4'-demethyldeschloro-rebeccamycin, and deschloro-rebeccamycin aglycone. The cloned genes should help to elucidate the molecular basis for indolocarbazole biosynthesis and set the stage for the generation of novel indolocarbazole analogues by genetic engineering.

The functional role of conserved acidic residues of the Qcr7 protein of the cytochrome bc(1) complex in Saccharomyces cerevisiae

The 14-kDa Qcr7 protein represents one of the 10 subunits that are components of a functional cytochrome bc(1) complex in Sacharomyces cerevisiae. Previous studies have shown that the N-terminus of the Qcr7 protein may be involved in the assembly of the cytochrome bc(1) complex and its C-terminus by interacting with cytochrome b and QCR8 proteins. It has also been suggested that Qcr7 protein may be involved in proton pumping. The coding sequence for two highly conserved aspartate residues, D46 and D47, in the QCR7 gene was altered by site-directed mutagenesis and the mutated genes expressed in cells lacking a functional QCR7 gene. Mutants D46E, D46G, D46N, and D47E were comparable to wild type in growth phenotype on nonfermentable carbon sources. Mutants D47G and D47N were respiratory deficient and analysis of complex components by immunoblotting and spectral analysis of cytochrome b suggests defective assembly. Despite being respiratory competent and having normal electron transport rates in broken mitochondria, the mutant D46G had markedly reduced ATP synthesis from electron transport reactions catalyzed by complexes II plus III of the respiratory chain. This suggests that the geometry of proton uptake by the bc(1) complex is disturbed by the mutation in D46.

Functional analysis of polar amino-acid residues in membrane associated regions of the NHE1 isoform of the mammalian Na+/H+ exchanger

The NHE1 isoform of the Na+/H+ exchanger is a ubiquitous plasma membrane protein that regulates intracellular pH in mammalian cells. Site-specific mutagenesis was used to examine the functional role of conserved, polar amino-acid residues occurring in segments of the protein associated with the membrane. Seventeen mutant proteins were assessed by characterization of intracellular pH changes in stably transfected cells that lacked an endogenous Na+/H+ exchanger. All of the mutant proteins were targeted correctly to the plasma membrane and were expressed at similar levels. Amino-acid residues Glu262 and Asp267 were critical to Na+/H+ exchanger activity while mutation of Glu391 resulted in only a partial reduction in activity. The Glu262-->Gln mutant was expressed partially as a deglycosylated protein with increased sensitivity to trypsin treatment in presence of Na+. Substitution of mutated Glu262, Asp267 and Glu391 with alternative acidic residues restored Na+/H+ exchanger activity. The Glu262-->Asp mutant had a decreased affinity for Li+, but its activity for Na+ and H+ ions was unaffected. The results support the hypothesis that side-chain oxygen atoms in a few, critically placed amino acids are important in Na+/H+ exchanger activity and the acidic amino-acid residues at positions 262, 267 and 391 are good candidates for being involved in Na+ coordination by the protein.

A screening for high copy suppressors of the sit4 hal3 synthetically lethal phenotype reveals a role for the yeast Nha1 antiporter in cell cycle regulation

A screening for multicopy suppressors of the G(1)/S blockage of a conditional sit4 hal3 mutant yielded the NHA1 gene, encoding a Na(+),K(+)/H(+) antiporter, composed of a transmembrane domain and a large carboxyl-terminal tail, which has been related to cation detoxification processes. Expression of either the powerful Saccharomyces cerevisiae Ena1 Na(+)/H(+)-ATPase or the Schizosaccharomyces pombe Sod2 Na(+)/H(+) antiporter, although increasing tolerance to sodium, was unable to mimic the Nha1 function in the cell cycle. Mutation of the conserved Asp residues Asp(266)-Asp(267) selectively abolished Na(+) efflux without modifying K(+) efflux and did not affect the capacity of Nha1 to relieve the G(1) blockage. Mutagenesis analysis revealed that the region near the carboxyl-terminal end of Nha1 comprising residues 800-948 is dispensable for sodium detoxification but necessary for transport of K(+) cations. Therefore, this portion of the protein contains structural elements that selectively modulate Nha1 antiporter functions. This region is also required for Nha1 to function in the cell cycle. However, expression of the closely related Cnh1 antiporter from Candida albicans, which also contains a long carboxyl-terminal extension, although allowing efficient K(+) transport does not relieve cell cycle blockage. This indicates that although the determinants for Nha1-mediated regulation of potassium transport and the cell cycle map very closely in the protein, most probably the function of Nha1 on cell cycle is independent of its ability to extrude potassium cations.

Functional role of polar amino acid residues in Na+/H+ exchangers

Na(+)/H(+) exchangers are a family of ubiquitous membrane proteins. In higher eukaryotes they regulate cytosolic pH by removing an intracellular H(+) in exchange for an extracellular Na(+). In yeast and Escherichia coli, Na(+)/H(+) exchangers function in the opposite direction to remove intracellular Na(+) in exchange for extracellular H(+). Na(+)/H(+) exchangers display an internal pH-sensitivity that varies with the different antiporter types. Only recently have investigations examined the amino acids involved in pH-sensitivity and in cation binding and transport. Histidine residues are good candidates for H(+)-sensing amino acids, since they can ionize within the physiological pH range. Histidine residues have been shown to be important in the function of the E. coli Na(+)/H(+) exchanger NhaA and in the yeast Na(+)/H(+) exchanger sod2. In E. coli, His(225) of NhaA may function to interact with, or regulate, the pH-sensory region of NhaA. In sod2, His(367) is also critical to transport and may be a functional analogue of His(225) of NhaA. Histidine residues are not critical for the function of the mammalian Na(+)/H(+) exchanger, although an unusual histidine-rich sequence of the C-terminal tail has some influence on activity. Other amino acids involved in cation binding and transport by Na(+)/H(+) exchangers are only beginning to be studied. Amino acids with polar side chains such as aspartate and glutamate have been implicated in transport activity of NhaA and sod2, but have not been studied in the mammalian Na(+)/H(+) exchanger. Further studies are needed to elucidate the mechanisms involved in pH-sensitivity and cation binding and transport by Na(+)/H(+) exchangers.

Na(+)/H(+) antiporters

Na(+)/H(+) antiporters are membrane proteins that play a major role in pH and Na(+) homeostasis of cells throughout the biological kingdom, from bacteria to humans and higher plants. The emerging genomic sequence projects already have started to reveal that the Na(+)/H(+) antiporters cluster in several families. Structure and function studies of a purified antiporter protein have as yet been conducted mainly with NhaA, the key Na(+)/H(+) antiporter of Escherichia coli. This antiporter has been overexpressed, purified and reconstituted in a functional form in proteoliposomes. It has recently been crystallized in both 3D as well as 2D crystals. The NhaA 2D crystals were analyzed by cryoelectron microscopy and a density map at 4 A resolution was obtained and a 3D map was reconstructed. NhaA is shown to exist in the 2D crystals as a dimer of monomers each composed of 12 transmembrane segments with an asymmetric helix packing. This is the first insight into the structure of a polytopic membrane protein. Many Na(+)/H(+) antiporters are characterized by very dramatic sensitivity to pH, a property that corroborates their role in pH homeostasis. The molecular mechanism underlying this pH sensitivity has been studied in NhaA. Amino acid residues involved in the pH response have been identified. Conformational changes transducing the pH change into a change in activity were found in loop VIII-IX and at the N-terminus by probing trypsin digestion or binding of a specific monoclonal antibody respectively. Regulation by pH of the eukaryotic Na(+)/H(+) antiporters involves an intricate signal transduction pathway (recently reviewed by Yun et al., Am. J. Physiol. 269 (1995) G1-G11). The transcription of NhaA has been shown to be regulated by a novel Na(+)-specific regulatory network. It is envisaged that interdisciplinary approaches combining structure, molecular and cell biology as well as genomics should be applied in the future to the study of this important group of transporters.

Bacillus subtilis NhaC, an Na+/H+ antiporter, influences expression of the phoPR operon and production of alkaline phosphatases

When Bacillus subtilis is subjected to phosphate starvation, genes of the Pho regulon are either induced or repressed. Among those induced are genes encoding alkaline phosphatases (APases). A set of isogenic mutants, with a beta-galactosidase gene transcriptionally fused to the inactivated target gene, was used to identify genes that influence the operation of the Pho regulon. One such gene was nhaC (previously yheL). In the absence of NhaC, growth and APase production were enhanced, while the production of other non-Pho-regulon secretory proteins (proteases and alpha-amylase) did not change. The influence of NhaC on growth, APase synthesis, and its own expression was dependent on the external Na+ concentration. Other monovalent cations such as Li+ or K+ had no effect. We propose a role for NhaC in the uptake of Na+. nhaC appears to be encoded by a monocistronic operon and, contrary to previous reports, is not in the same transcriptional unit as yheK, the gene immediately upstream. The increase in APase production was dependent on an active PhoR, the sensor kinase of the two-component system primarily responsible for controlling the Pho regulon. Transcriptional fusions showed that the phoPR operon and both phoA (encoding APaseA) and phoB (encoding APaseB) were hyperinduced in the absence of NhaC and repressed when this protein was overproduced. This suggests that NhaC effects APase production via phoPR.

Discovery of zoniporide: a potent and selective sodium-hydrogen exchanger type 1 (NHE-1) inhibitor with high aqueous solubility

Zoniporide (CP-597,396) is a potent and selective inhibitor of NHE-1, which exhibits high aqueous solubility and acceptable pharmacokinetics for intravenous administration. The discovery, synthesis, activities, and rat and dog pharmacokinetics of this compound are presented. The potency and selectivity of zoniporide may be due to the conformation that the molecule adopts due to the presence of a cyclopropyl and a 5-quinolinyl substituent on the central pyrazole ring of the molecule.

Epithelial transport and osmoregulation in annelids

Epithelial transport related to osmoregulation has so far not been extensively investigated in annelids. Compared with the large body of information about ion transport across crustacean or insect epithelia, only a few studies have been done with isolated preparations of annelids, using the body wall of marine polychaetes or Hirudinea. Nephridial function and general body homeostasis have received more attention, and have probably been best investigated in Hirudinea. With recent advances in the molecular physiology of epithelial transport systems in vertebrates, the cloning of various transporters and ion channels, and the considerable number of osmoregulatory peptides that have now been found and analyzed from annelids, it should now be possible, and is timely, to conduct functional studies on individual selected epithelial preparations or isolated cells from annelids. Such studies may be important for establishing useful models with somewhat less complexity than mammalian systems. For example, annelids lack aldosterone, an important osmoregulatory hormone, which is a key factor in the regulation of sodium reabsorption in vertebrates. Therefore, not only would such studies contribute to annelid physiology, but they would be important in a broader sense for understanding osmoregulation and its evolution. They should also facilitate the discovery and investigation of new specific regulatory pathways.