Functional Analysis of Acidic Amino Acids in the Cytosolic Tail of the Na+/H+Exchanger†

Roles of the Na+/H+ Exchanger Isoform 1 and Urokinase in Prostate Cancer Cell Migration and Invasion

Prostate cancer is a leading cause of cancer-associated deaths in men over 60 years of age. Most patients are killed by tumor metastasis. Recent evidence has implicated a role of the tumor microenvironment and urokinase plasminogen activator (uPA) in cancer cell migration, invasion, and metastasis. Here, we examine the role of the Na⁺/H⁺ exchanger isoform 1 (NHE1) and uPA in DU 145 prostate cancer cell migration and colony formation. Knockout of NHE1 reduced cell migration. The effects of a series of novel NHE1/uPA hexamethylene-amiloride-based inhibitors with varying efficacy towards NHE1 and uPA were examined on prostate cancer cells. Inhibition of NHE1-alone, or with inhibitors combining NHE1 or uPA inhibition-generally did not prevent prostate cancer cell migration. However, uPA inhibition-but not NHE1 inhibition-prevented anchorage-dependent colony formation. Application of inhibitors at concentrations that only saturate uPA inhibition decreased tumor invasion in vivo. The results suggest that while knockout of NHE1 affects cell migration, these effects are not due to NHE1-dependent proton translocation. Additionally, while neither NHE1 nor uPA activity was critical in cell migration, only uPA activity appeared to be critical in anchorage-dependent colony formation of DU 145 prostate cancer cells and invasion in vivo.

Amino Acids 785, 787 of the Na+/H+ Exchanger Cytoplasmic Tail Modulate Protein Activity and Tail Conformation

The mammalian Na⁺/H⁺ exchanger isoform 1 (NHE1) is a plasma membrane protein ubiquitously present in humans. It regulates intracellular pH by removing an intracellular proton in exchange for an extracellular sodium. It consists of a 500 amino acid membrane domain plus a 315 amino acid, regulatory cytosolic tail. Here, we investigated the effect of mutation of two amino acids of the regulatory tail, Ser⁷⁸⁵ and Ser⁷⁸⁷, that were similar in location and context to two amino acids of the Arabidopsis Na⁺/H⁺ exchanger SOS1. Mutation of these two amino acids to either Ala or phosphomimetic Glu did not affect surface targeting but led to a slight reduction in the level of protein expressed. The activity of the NHE1 protein was reduced in the phosphomimetic mutations and the effect was due to a decrease in Vmax activity. The Ser to Glu mutations also caused a change in the apparent molecular weight of both the full-length protein and of the cytosolic tail of NHE1. A conformational change in this region was indicated by differential trypsin sensitivity. We also found that a peptide containing amino acids 783–790 bound to several more proximal regions of the NHE1 tail in in vitro protein interaction experiments. The results are the first characterization of these two amino acids and show that they have significant effects on enzyme kinetics and the structure of the NHE1 protein.

The C-terminal tail of the plant endosomal-type NHXs plays a key role in its function and stability

Typically, Na⁺/H⁺ antiporters (NHXs) possess a conserved N-terminus for cation binding and exchange and a hydrophilic C-terminus for regulating the antiporter activity. Plant endosomal-type NHXs play important roles in protein trafficking, as well as K⁺ and vesicle pH homeostasis, however the role of the C-terminal tail remains unclear. Here, the function of MnNHX6, an endosomal-type NHX in mulberry, was investigated using heterologous expression in yeast. Functional and localization analyses of C-terminal truncation and mutations in MnNHX6 revealed that the C-terminal conserved region was responsible for the function and stability of the protein and its hydrophobicity, which is a key domain requirement. Nuclear magnetic resonance spectroscopy provided direct structural evidence and yeast two-hybrid screening indicated that this functional domain was also necessary for interaction with sorting nexin 1. Our findings demonstrate that although the C-terminal tail of MnNHX6 is intrinsically disordered, the C-terminal conserved region may be an important part of the external mouth of this transporter, which controls protein function and stability by serving as an inter-molecular cork with a chain mechanism. These findings improve our understanding of the roles of the C-terminal tail of endosomal-type NHXs in plants and the ion transport mechanism of NHX-like antiporters.

Amino Acids 563-566 of the Na+/H+ Exchanger Isoform 1 C-Terminal Cytosolic Tail Prevent Protein Degradation and Stabilize Protein Expression and Activity

Isoform one of the mammalian Na⁺/H⁺ exchanger is a plasma membrane protein that is ubiquitously present in humans. It regulates intracellular pH through the removal of one intracellular proton in exchange for a single extracellular sodium. It consists of a 500 amino acid membrane domain plus a 315 amino acid, C-terminal tail. We examined amino acids of the C-terminal tail that are important in the targeting and activity of the protein. A previous study demonstrated that stop codon polymorphisms can result in decreased activity, expression, targeting and enhanced protein degradation. Here, we determine elements that are critical in these anomalies. A series of progressive deletions of the C-terminal tail demonstrated a progressive decrease in activity and targeting, though these remained until a final drop off with the deletion of amino acids 563–566. The deletion of the ⁵⁶²LIAGERS⁵⁶⁸ sequence or the alteration to the ⁵⁶²LAAAARS⁵⁶⁸ sequence caused the decreased protein expression, aberrant targeting, reduced activity and enhanced degradation of the Na⁺/H⁺ exchanger (NHE1) protein. The ⁵⁶²LIAGERS⁵⁶⁸ sequence bound to other regions of the C-terminal cytosolic domain. We suggest this region is necessary for the activity, targeting, stability, and expression of the NHE1 protein. The results define a new sequence that is important in maintenance of NHE1 protein levels and activity.

Proteins with calmodulin-like domains: structures and functional roles

The appearance of modular proteins is a widespread phenomenon during the evolution of proteins. The combinatorial arrangement of different functional and/or structural domains within a single polypeptide chain yields a wide variety of activities and regulatory properties to the modular proteins. In this review, we will discuss proteins, that in addition to their catalytic, transport, structure, localization or adaptor functions, also have segments resembling the helix-loop-helix EF-hand motifs found in Ca2+-binding proteins, such as calmodulin (CaM). These segments are denoted CaM-like domains (CaM-LDs) and play a regulatory role, making these CaM-like proteins sensitive to Ca2+ transients within the cell, and hence are able to transduce the Ca2+ signal leading to specific cellular responses. Importantly, this arrangement allows to this group of proteins direct regulation independent of other Ca2+-sensitive sensor/transducer proteins, such as CaM. In addition, this review also covers CaM-binding proteins, in which their CaM-binding site (CBS), in the absence of CaM, is proposed to interact with other segments of the same protein denoted CaM-like binding site (CLBS). CLBS are important regulatory motifs, acting either by keeping these CaM-binding proteins inactive in the absence of CaM, enhancing the stability of protein complexes and/or facilitating their dimerization via CBS/CLBS interaction. The existence of proteins containing CaM-LDs or CLBSs substantially adds to the enormous versatility and complexity of Ca2+/CaM signaling.

Temperature-dependent increase in the calcium sensitivity and acceleration of activation of ANO6 chloride channel variants

Anoctamin-6 (ANO6) belongs to a family of calcium (Ca2+)-activated chloride channels (CaCCs), with three splicing variants (V1, V2, and V5) showing plasma membrane expression. Unlike other CaCCs, ANO6 requires a non-physiological intracellular free calcium concentration ([Ca2+]i > 1 μM) and several minutes for full activation under a whole-cell patch clamp. Therefore, its physiological role as an ion channel is uncertain and it is more commonly considered a Ca2+-dependent phospholipid scramblase. Here, we demonstrate that physiological temperature (37 °C) increases ANO6 Ca2+ sensitivity under a whole-cell patch clamp; V1 was activated by 1 μM [Ca2+]i, whereas V2 and V5 were activated by 300 nM [Ca2+]i. Increasing the temperature to 42 °C led to activation of all ANO6 variants by 100 nM [Ca2+]i. The delay time for activation of the three variants was significantly shortened at 37 °C. Notably, the temperature-dependent Ca2+-sensitisation of ANO6 became insignificant under inside-out patch clamp, suggesting critical roles of unknown cytosolic factors. Unlike channel activity, 27 °C but not 37 °C (physiological temperature) induced the scramblase activity of ANO6 at submicromolar [Ca2+]i (300 nM), irrespective of variant type. Our results reveal a physiological ion conducting property of ANO6 at 37 °C and suggest that ANO6 channel function acts separately from its scramblase activity.

Sodium/calcium exchanger is involved in apoptosis induced by H2S in tumor cells through decreased levels of intracellular pH

We explored possibility that sodium/calcium exchanger 1 (NCX1) is involved in pH modulation and apoptosis induction in GYY4137 treated cells. We have shown that although 10 days treatment with GYY4137 did not significantly decreased volume of tumors induced by colorectal cancer DLD1 cells in nude mice, it already induced apoptosis in these tumors. Treatment of DLD1 and ovarian cancer A2780 cells with GYY4137 resulted in intracellular acidification in a concentration-dependent manner. We observed increased mRNA and protein expression of both, NCX1 and sodium/hydrogen exchanger 1 (NHE1) in DLD1-induced tumors from GYY4137-treated mice. NCX1 was coupled with NHE1 in A2780 and DLD1 cells and this complex partially disintegrated after GYY4137 treatment. We proposed that intracellular acidification is due to uncoupling of NCX1/NHE1 complex rather than blocking of the reverse mode of NCX1, probably due to internalization of NHE1. Results might contribute to understanding molecular mechanism of H₂S-induced apoptosis in tumor cells.

Stop Codon Polymorphisms in the Human SLC9A1 Gene Disrupt or Compromise Na+/H+ Exchanger Function

The NHE1 isoform of the mammalian Na+/H+ exchanger is a ubiquitous plasma membrane protein that regulates intracellular pH in mammalian cells by removing one intracellular proton in exchange for one extracellular sodium. Deletion of the NHE1 gene (SLC9A1) affects the growth and motor ability of mice and humans but mutations and polymorphisms of the gene are only beginning to be characterized. NHE1 has a cytosolic C-terminal regulatory tail of approximately 315 amino acids and a 500 amino acid membrane domain. We examined the functional effects of three human stop codon mutations at amino acids 321, 449 and 735 in comparison with a mutant that had a shortened tail region (543 stop codon). The short mutants, 321, 449 and 543 stop codon mutant proteins, lost NHE1 activity and expression, and did not target to the plasma membrane. Protein for these short mutants was more rapidly degraded than the wild type and 735 ending proteins. The 735 terminating mutant, with the membrane domain and much of the cytosolic tail, had reduced protein expression and activity. The results demonstrate that early stop codon polymorphisms have significant and deleterious effects on the activity of the SLC9A1 protein product. The 735-NHE1 mutant, without the last 80 amino acids, had more minor defects. Surprisingly, retention of a proximal 43 amino acids adjacent to the membrane domain did little to maintain NHE1 expression, targeting and activity.

Emerging roles of the single EF-hand Ca2+ sensor tescalcin in the regulation of gene expression, cell growth and differentiation

Tescalcin (TESC, also known as calcineurin-homologous protein 3, CHP3) is a 24-kDa EF-hand Ca²⁺-binding protein that has recently emerged as a regulator of cell differentiation and growth. The TESC gene has also been linked to human brain abnormalities, and high expression of tescalcin has been found in several cancers. The expression level of tescalcin changes dramatically during development and upon signal-induced cell differentiation. Recent studies have shown that tescalcin is not only subjected to up- or down-regulation, but also has an active role in pathways that drive cell growth and differentiation programs. At the molecular level, there is compelling experimental evidence showing that tescalcin can directly interact with and regulate the activities of the Na⁺/H⁺ exchanger NHE1, subunit 4 of the COP9 signalosome (CSN4) and protein kinase glycogen-synthase kinase 3 (GSK3). In hematopoetic precursor cells, tescalcin has been shown to couple activation of the extracellular signal-regulated kinase (ERK) cascade to the expression of transcription factors that control cell differentiation. The purpose of this Commentary is to summarize recent efforts that have served to characterize the biochemical, genetic and physiological attributes of tescalcin, and its unique role in the regulation of various cellular functions.

Activation of the Na+/H+ exchanger in isolated cardiomyocytes through β-Raf dependent pathways. Role of Thr653 of the cytosolic tail

The mammalian Na⁺/H⁺ exchanger isoform 1 (NHE1) is a ubiquitous plasma membrane protein that is a key regulator of intracellular pH in isolated cardiomyocytes. A 500 amino acid membrane domain removes protons and is regulated by a 315 amino acid cytosolic domain. In the myocardium, aberrant regulation of NHE1 contributes to ischemia reperfusion damage and to heart hypertrophy. We examined mechanisms of regulation of NHE1 in the myocardium by endothelin and β-Raf. Endothelin stimulated NHE1 activity and activated Erk-dependent pathways. Inhibition of β-Raf reduced NHE1 activity and Erk-pathway activation. We demonstrated that myocardial β-Raf binds to the C-terminal 182 amino acids of the NHE1 protein and that β-Raf is associated with NHE1 in intact cardiomyocytes. NHE1 was phosphorylated in vivo and the protein kinase inhibitor sorafenib reduced NHE1 phosphorylation levels. Immunoprecipitates of β-Raf from cardiomyocytes phosphorylated the C-terminal 182 amino acids of NHE1 and mass spectrometry analysis showed that amino acid Thr⁶⁵³ was phosphorylated. Mutation of this amino acid to Ala resulted in defective activity while mutation to Asp restored the activity. The results demonstrate that Thr⁶⁵³ is an important regulatory amino acid of NHE1 that is activated through β-Raf dependent pathways by phosphorylation either directly or indirectly by β-Raf, and this affects NHE1 activity.

A novel human mutation in the SLC9A1 gene results in abolition of Na+/H+ exchanger activity

The SLC9A1 gene, the Na+/H+ exchanger isoform 1 is the principal plasma membrane Na+/H+ exchanger of mammalian cells and functions by exchanging one intracellular proton for one extracellular sodium. The human protein is 815 amino acids in length. Five hundred N-terminal amino acids make up the transport domain of the protein and are believed to form 12 transmembrane segments. Recently, a genetic mutation of the Na+/H+ exchanger isoform 1, N266H, was discovered in a human patient through exome sequencing. We examined the effect of this mutation on expression, targeting and activity of the Na+/H+ exchanger. Mutant N266H protein was expressed in AP-1 cells, which lack their endogenous Na+/H+ exchanger protein. Targeting of the mutant protein to the cell surface was normal and expression levels were only slightly reduced relative to the wild type protein. However, the N266H mutant protein had no detectable Na+/H+ exchanger activity. A histidine residue at this location may disrupt the cation binding site or the pore of the Na+/H+ exchanger protein.

The EF-hand calcium-binding protein tescalcin is a potential oncotarget in colorectal cancer

Tescalcin (TESC) is an EF-hand calcium binding protein that is differentially expressed in several tissues, however it is not reported that the expression and functional roles of TESC in colorectal cancer. Levels of messenger RNA (mRNA) and protein expression of TESC in colorectal cancer tissues were assessed using RT-PCR, real time PCR, immunohistochemistry, and clinicopathologic analyses. Quantitative analysis of TESC levels in serum specimens was performed using sandwich ELISA. Colorectal cancer cells transfected with TESC small interfering RNA and short hairpin RNA were examined in cell proliferation assays, phospho-MAPK array, and mouse xenograft models. Here we demonstrated that TESC is overexpressed in colorectal cancer (CRC), but was not expressed in normal mucosa and premalignant dysplastic lesions. Furthermore, serum TESC levels were elevated in patients with CRC. Knockdown of TESC inhibited the Akt-dependent NF-κB pathway and decreased cell survival in vitro. Depletion of TESC reduced tumor growth in a CRC xenograft model. Thus, TESC is a potential diagnostic marker and oncotarget in colorectal cancer.

Functional role of arginine 425 in the mammalian Na+/H+ exchanger

Na+/H+ exchanger isoform 1 (NHE1) is the principal plasma membrane Na+/H+ exchanger of mammalian cells and functions by exchanging one intracellular proton for one extracellular sodium ion. Critical transmembrane segments of Na+/H+ exchangers have discontinuous transmembrane helices, which result in a dipole within the membrane. Amino acid R425 has been suggested to play an important role in neutralizing one such helix dipole. To investigate this hypothesis, R425 was mutated to alanine, glutamine, histidine, or lysine and the mutant NHE1 proteins were expressed and characterized in NHE1-deficient cells. The R425A and R425E mutants exhibited complete loss of expression of mature, fully glycosylated NHE1, reduced expression overall, and greatly reduced cell surface targeting. The cell surface targeting, expression, and activity of the R425H and R425K mutant proteins were also impaired, though residual NHE1 activity remained. When reduced targeting and expression were accounted for, the R425H and R425K mutant proteins had activity similar to that of the wild-type protein. The results suggest that R425 is critical for NHE1 expression, targeting, and activity and that replacement with another basic residue can rescue activity. The findings are consistent with a role for R425 in both neutralizing a helix dipole and maintaining NHE1 structure and function.

Characterization of human mutations in phosphorylatable amino acids of the cytosolic regulatory tail of SLC9A1

The NHE1 isoform of the mammalian Na(+)/H(+) exchanger is a ubiquitous plasma membrane protein that regulates intracellular pH in cells by removing one intracellular proton in exchange for one extracellular sodium. Genetic defects in NHE1 have been shown to affect the growth and motor ability of mice, but mutations in humans have not been studied. NHE1 has a cytosolic C-terminal regulatory domain of approximately 300 amino acids. We investigated the functional effects of two human mutations found in the regulatory phosphorylatable amino acids Ser(703) and Ser(771). A Ser703Pro mutant protein had essentially the same activity, expression, and targeting as the wild type NHE1 protein. In contrast, the Ser771Pro protein had reduced activity and expression of NHE1 protein, though cell surface targeting was normal. In dual pulse assays the Ser771Pro mutant was not further activated by sustained intracellular acidosis but displayed an unusual activation by brief pulses of acidosis. The results demonstrate that the Ser771Pro human genetic mutation has significant and detrimental physiological effects on the activity of the NHE1 protein, SLC9A1.

Calmodulin-dependent binding to the NHE1 cytosolic tail mediates activation of the Na+/H+ exchanger by Ca2+ and endothelin

The mammalian Na(+)/H(+) exchanger isoform 1 (NHE1) is a ubiquitous plasma membrane protein that regulates intracellular pH by removing a single proton (H(+)) in exchange for one extracellular Na(+). The human protein contains a ∼500-amino acid membrane domain and a regulatory, ∼315-amino acid cytosolic domain. NHE1 is activated by a number of hormones including endothelin (ET) and by Ca(2+). The regulatory tail possesses an inhibitory calmodulin (CaM)-binding domain, and inhibition of NHE1 is relieved by binding of a Ca(2+)-CaM complex. We examined the dynamics of ET-1 and Ca(2+) regulation of binding to NHE1 in vivo. CFP was linked to the NHE1 protein cytoplasmic COOH terminus. This was stably transfected into AP-1 cells that are devoid of their own NHE1 protein. The protein was expressed and targeted properly and retained NHE1 activity comparable to the wild-type protein. We examined the in vivo coupling of NHE1 to CaM by Förster resonance energy transfer using CaM linked to the fluorescent protein Venus. CaM interaction with NHE1 was dynamic. Removal of serum reduced CaM interaction with NHE1. Addition of the Ca(2+) ionophore ionomycin increased the interaction between CaM and NHE1. We expressed an ET receptor in AP-1 cells and also found a time-dependent association of NHE1 with CaM in vivo that was dependent on ET treatment. The results are the first demonstration of the in vivo association of NHE1 and CaM through ET-dependent signaling pathways.

Structure of human Na+/H+ exchanger NHE1 regulatory region in complex with calmodulin and Ca2+

The ubiquitous mammalian Na(+)/H(+) exchanger NHE1 has critical functions in regulating intracellular pH, salt concentration, and cellular volume. The regulatory C-terminal domain of NHE1 is linked to the ion-translocating N-terminal membrane domain and acts as a scaffold for signaling complexes. A major interaction partner is calmodulin (CaM), which binds to two neighboring regions of NHE1 in a strongly Ca(2+)-dependent manner. Upon CaM binding, NHE1 is activated by a shift in sensitivity toward alkaline intracellular pH. Here we report the 2.23 Å crystal structure of the NHE1 CaM binding region (NHE1(CaMBR)) in complex with CaM and Ca(2+). The C- and N-lobes of CaM bind the first and second helix of NHE1(CaMBR), respectively. Both the NHE1 helices and the Ca(2+)-bound CaM are elongated, as confirmed by small angle x-ray scattering analysis. Our x-ray structure sheds new light on the molecular mechanisms of the phosphorylation-dependent regulation of NHE1 and enables us to propose a model of how Ca(2+) regulates NHE1 activity.

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

The Na(+)/H(+) exchanger isoform 1 is a ubiquitously expressed integral membrane protein. It resides on the plasma membrane of cells and regulates intracellular pH in mammals by extruding an intracellular H(+) in exchange for one extracellular Na(+). We characterized structural and functional aspects of the transmembrane segment (TM) VI (residues 227-249) by using cysteine scanning mutagenesis and high resolution NMR. Each residue of TM VI was mutated to cysteine in the background of the cysteineless NHE1 protein, and the sensitivity to water-soluble sulfhydryl-reactive compounds (2-(trimethylammonium)ethyl)methanethiosulfonate (MTSET) and (2-sulfonatoethyl)methanethiosulfonate (MTSES) was determined for those residues with significant activity remaining. Three residues were essentially inactive when mutated to Cys: Asp(238), Pro(239), and Glu(247). Of the remaining residues, proteins with the mutations N227C, I233C, and L243C were strongly inhibited by MTSET, whereas amino acids Phe(230), Gly(231), Ala(236), Val(237), Ala(244), Val(245), and Glu(248) were partially inhibited by MTSET. MTSES did not affect the activity of the mutant NHE1 proteins. The structure of a peptide representing TM VI was determined using high resolution NMR spectroscopy in dodecylphosphocholine micelles. TM VI contains two helical regions oriented at an approximate right angle to each other (residues 229-236 and 239-250) surrounding a central unwound region. This structure bears a resemblance to TM IV of the Escherichia coli protein NhaA. The results demonstrate that TM VI of NHE1 is a discontinuous pore-lining helix with residues Asn(227), Ile(233), and Leu(243) lining the translocation pore.

Up- or downregulation of tescalcin in HL-60 cells is associated with their differentiation to either granulocytic or macrophage-like lineage

Tescalcin is a 25-kDa EF-hand Ca(2+)-binding protein that is differentially expressed in several mammalian tissues. Previous studies demonstrated that expression of this protein is essential for differentiation of hematopoietic precursor cell lines and primary stem cells into megakaryocytes. Here we show that tescalcin is expressed in primary human granulocytes and is upregulated in human promyelocytic leukemia HL-60 cells that have been induced to differentiate along the granulocytic lineage. However, during induced macrophage-like differentiation of HL-60 cells the expression of tescalcin is downregulated. The decrease in expression is associated with a rapid drop in tescalcin mRNA level, whereas upregulation occurs via a post-transcriptional mechanism. Tescalcin is necessary for HL-60 differentiation into granulocytes as its knockdown by shRNA impairs the ability of HL-60 cells to acquire the characteristic phenotypes such as phagocytic activity and generation of reactive oxygen species measured by respiratory burst assay. Both up- and downregulation of tescalcin require activation of the MEK/ERK cascade. It appears that commitment of HL-60 cells toward granulocytic versus macrophage-like lineage correlates with expression of tescalcin and kinetics of ERK activation. In retinoic acid-induced granulocytic differentiation, the activation of ERK and upregulation of tescalcin occurs slowly (16-48 h). In contrast, in PMA-induced macrophage-like differentiation the activation of ERK is rapid (15-30 min) and tescalcin is downregulated. These studies indicate that tescalcin is one of the key gene products that is involved in switching differentiation program in some cell types.

Expression and evolutionary conservation of the tescalcin gene during development

The tescalcin gene (Tesc) encodes an EF-hand calcium-binding protein that interacts with the sodium/hydrogen exchanger, NHE1. Previous studies indicated that Tesc was expressed in mouse embryonic testis, but not in ovary, during the critical period of testis and ovary determination. In this paper we compared the expression of Tesc in embryonic tissues of chicken and mouse. Tesc expression was sexually dimorphic in the embryonic gonads of both mouse and chicken. Tescalcin (TESC) was detected in both Sertoli cells and germ cells. In the embryonic brain of both mouse and chicken, Tesc was highly expressed in the nasal placode and in fibers extending from the olfactory epithelium to the primordial olfactory bulb. Tesc was expressed in the embryonic heart of both chicken and mouse. In mouse Tesc expression was also detected in embryonic adrenal. These studies indicate very specific expression of Tesc in various tissues in chicken and mouse during embryologic development, and conservation of Tesc expression in both species.

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.

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).

Regulatory Binding Partners and Complexes of NHE3

NHE3 is the brush-border (BB) Na+/H+exchanger of small intestine, colon, and renal proximal tubule which is involved in large amounts of neutral Na+absorption. NHE3 is a highly regulated transporter, being both stimulated and inhibited by signaling that mimics the postprandial state. It also undergoes downregulation in diarrheal diseases as well as changes in renal disorders. For this regulation, NHE3 exists in large, multiprotein complexes in which it associates with at least nine other proteins. This review deals with short-term regulation of NHE3 and the identity and function of its recognized interacting partners and the multiprotein complexes in which NHE3 functions.

Physiological role and regulation of the Na+/H+ exchanger

In mammalian eukaryotic cells, the Na+/H+ exchanger is a family of membrane proteins that regulates ions fluxes across membranes. Plasma membrane isoforms of this protein extrude 1 intracellular proton in exchange for 1 extracellular sodium. The family of Na+/H+ exchangers (NHEs) consists of 9 known isoforms, NHE1-NHE9. The NHE1 isoform was the first discovered, is the best characterized, and exists on the plasma membrane of all mammalian cells. It contains an N-terminal 500 amino acid membrane domain that transports ions, plus a 315 amino acid C-terminal, the intracellular regulatory domain. The Na+/H+ exchanger is regulated by both post-translational modifications including protein kinase-mediated phosphorylation, plus by a number of regulatory-binding proteins including phosphatidylinositol-4,5-bisphosphate, calcineurin homologous protein, ezrin, radixin and moesin, calmodulin, carbonic anhydrase II, and tescalcin. The Na+/H+ exchanger is involved in a variety of complex physiological and pathological events that include regulation of intracellular pH, cell movement, heart disease, and cancer. This review summarizes recent advances in the understanding of the physiological role and regulation of this protein.

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 characterization of transmembrane segment VII of the Na+/H+ exchanger isoform 1

The Na(+)/H(+) exchanger isoform 1 is an integral membrane protein that regulates intracellular pH by exchanging one intracellular H(+) for one extracellular Na(+). It is composed of an N-terminal membrane domain of 12 transmembrane segments and an intracellular C-terminal regulatory domain. We characterized the structural and functional aspects of the critical transmembrane segment VII (TM VII, residues 251-273) by using alanine scanning mutagenesis and high resolution NMR. Each residue of TM VII was mutated to alanine, the full-length protein expressed, and its activity characterized. TM VII was sensitive to mutation. Mutations at 13 of 22 residues resulted in severely reduced activity, whereas other mutants exhibited varying degrees of decreases in activity. The impaired activities sometimes resulted from low expression and/or low surface targeting. Three of the alanine scanning mutant proteins displayed increased, and two displayed decreased resistance to the Na(+)/H(+) exchanger isoform 1 inhibitor EMD87580. The structure of a peptide of TM VII was determined by using high resolution NMR in dodecylphosphocholine micelles. TM VII is predominantly alpha-helical, with a break in the helix at the functionally critical residues Gly(261)-Glu(262). The relative positions and orientations of the N- and C-terminal helical segments are seen to vary about this extended segment in the ensemble of NMR structures. Our results show that TM VII is a critical transmembrane segment structured as an interrupted helix, with several residues that are essential to both protein function and sensitivity to inhibition.

Insights into sperm cell motility signaling through sNHE and the CatSpers

Successful natural reproduction normally requires vigorously motile spermatozoa. Using a signal peptide trapping strategy, we identified two new genes, a putative sperm Na+/H+ exchanger (sNHE) and the putative cation channel CatSper2, with unique and essential roles in sperm motility. Disruption of the sNHE or CatSper2 genes in mice caused male infertility due to immotile spermatozoa or failed motility hyperactivation, respectively, without other apparent abnormalities. The immotility phenotype of the sNHE null spermatozoa appears to result from an intimate association of sNHE and the atypical adenylyl cyclase (sAC), while a failure of calcium entry requiring an apparent CatSper1 and -2 heteromeric ion channel correlates with a hyperactivation defect in these null animals. The specific expression of sNHE and the CatSpers in spermatozoa and their required function in cell motility make them excellent potential targets for the development of novel male contraceptives.