The Nck-interacting kinase (NIK) phosphorylates the Na+-H+ exchanger NHE1 and regulates NHE1 activation by platelet-derived growth factor

Physical interactions and functional coupling between Daxx and sodium hydrogen exchanger 1 in ischemic cell death

Daxx, a death domain-associated protein, is implicated in ischemic cell death. To clarify the mechanism of cell death mediated by Daxx, a yeast two-hybrid assay was performed. Sodium hydrogen exchanger isoform 1 (NHE1) was identified as a Daxx-interacting protein. During ischemic stress, Daxx translocates from the nucleus to the cytoplasm, where it colocalizes with NHE1. Daxx binds to the ezrin/radixin/moesin-interacting domain of NHE1, in competition with ezrin. Consistent with this finding, transfection of the constitutively cytoplasmic mutant, Daxx(W621A), inhibited ezrin-mediated Akt-1 activation. Moreover, transfection of Daxx(W621A), but not the Daxx(S667A) mutant that is confined to the nucleus, accelerated pH(i) recovery from an acid load, indicating that the cytoplasmic protein activates NHE1. Based on the results, we propose that ischemic insult triggers the nucleocytoplasmic translocation of Daxx, following which cytoplasmic Daxx stimulates the NHE1 transporter activity and suppresses activation of the NHE1-ezrin-Akt-1 pathway. Our data support a novel molecular function of Daxx as an upstream regulator of NHE1 in ischemic cell death.

Beyond ion translocation: structural functions of the sodium-hydrogen exchanger isoform-1

PURPOSE OF REVIEW

The sodium-hydrogen exchanger isoform-1 (NHE1) functions in intracellular pH and cell volume homeostasis by catalyzing an electroneutral exchange of extracellular sodium and intracellular hydrogen. Recent studies have revealed the structural functions of NHE1 as an anchor for actin filaments and a scaffold for an ensemble of signaling proteins. This review highlights how these functions contribute to NHE1 regulation of biochemical events and cell behaviors.

RECENT FINDINGS

New data confirming nontransport structural functions of NHE1 suggest reexamining how NHE1 regulates cell functions. Cell survival, cell substrate adhesion, and organization of the actin cytoskeleton are confirmed to be regulated through actin anchoring by NHE1 and likely by NHE1-dependent scaffolding of signaling proteins. A role for NHE1 in mechanotransduction is emerging and a challenge of future studies is to determine whether structural functions of NHE1 are important for mechanoresponsiveness.

SUMMARY

This review highlights evidence for the nontransport functions of NHE1 and describes how the structural functions are integrated with ion translocation to regulate a range of cellular processes. Nontransporting features of NHE1 are analogous to recently observed nonconducting actions of ion channels in regulating cell behaviors and represent an emerging paradigm of ion transporters as multifunctional proteins.

Regulation of mitogen-activated protein kinase pathways by the plasma membrane Na+/H+ exchanger, NHE1

The mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 MAPK, play a major role in the regulation of pivotal cellular processes such as cell death/survival balance, cell cycle progression, and cell migration. MAPK activity is regulated by a three-tiered phosphorelay system, which is in turn regulated by a complex network of signaling events and scaffolding proteins. The ubiquitous plasma membrane Na(+)/H(+) exchanger NHE1 is activated by, and implicated in, the physiological/pathophysiological responses to many of the same stimuli that modulate MAPK activity. While under some conditions, NHE1 is regulated by MAPKs, a number of studies have, conversely, implicated NHE1 in the regulation of MAPK activity. Here, we discuss the current evidence indicating the involvement of NHE1 in MAPK regulation, the mechanisms by which this may occur, and the possible physiological and pathophysiological relevance of this phenomenon.

Novel functional interaction between Na+/H+exchanger 1 and tyrosine phosphatase SHP-2

Besides being a intracellular pH (pHi) regulator, Na+/H+exchanger (NHE)1 has recently been postulated as a membrane scaffold that assembles protein complexes and coordinates various signaling pathways. The aim of the present study was to uncover NHE1 interactive partners and study their functional implications. NHE1 interactive partners were screened in the mouse brain with a signal transduction AntibodyArray. Ten of 400 tested proteins appeared to be potentially associated with NHE1. These partners have been shown to be involved in either cell proliferative or apoptotic pathways. The interactions between NHE1 and Src homology 2 domain-containing protein tyrosine phosphatase (SHP-2), Bin1, and heat shock protein (HSP)70 were reciprocally confirmed by coimmunoprecipitation. Moreover, in vitro binding data have shown that NHE1 COOH terminus interacts directly with SHP-2. The functional significance of the association between NHE1 and SHP-2 was further investigated by measuring pHi, cell proliferation, and cell death with the fluorescent dye BCECF, [3H]thymidine incorporation, and medium lactate dehydrogenase activity, respectively. Our results revealed that cells with SHP-2 overexpression exhibited a higher steady-state pHiand a faster, NHE1-dependent pHirecovery rate from acid load in HEPES buffer. In addition, SHP-2 overexpression diminished the HOE-642-induced inhibition of cell proliferation and protected cells from hypoxic injury, especially in the presence of HOE-642. Together, our findings demonstrate that SHP-2 not only is physically associated with NHE1 but also modulates NHE1 functions such as pHiregulation, cell proliferation, and cell death under hypoxia.

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.

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.

Solution Structure of the Cytoplasmic Region of Na+/H+ Exchanger 1 Complexed with Essential Cofactor Calcineurin B Homologous Protein 1

Na+/H+ exchanger 1 (NHE1) regulates intracellular pH, Na+ content, and cell volume. Calcineurin B homologous protein 1 (CHP1) serves as an essential cofactor that facilitates NHE1 exchange activity under physiological conditions by direct binding to the cytoplasmic juxtamembrane region of NHE1. Here we describe the solution structure of the cytoplasmic juxtamembrane region of NHE1 complexed with CHP1. The region of NHE1 forms an amphipathic helix, which is induced by CHP1 binding, and CHP1 possesses a large hydrophobic cleft formed by EF-hand helices. The apolar side of the NHE1 helix participates in extensive hydrophobic interactions with the cleft of CHP1. We suggest that helix formation of the cytoplasmic region of NHE1 by CHP1 is a prerequisite for generating the active form of NHE1. The molecular recognition detailed in this study also provides novel insight into the target binding mechanism of EF-hand proteins.

The role of carbonic anhydrases in renal physiology

Carbonic anhydrase (CA) catalyzes the reversible hydration of CO(2). CA is expressed in most segments of the kidney. CAII and CAIV predominate in human and rabbit kidneys; in rodent kidneys, CAXII, and CAXIV are also present. CAIX is expressed by renal cell carcinoma (RCC). Most of these isoforms, except for rodent CAIV, have high turnover rates. CAII is a cytoplasmic enzyme, whereas the others are membrane-associated; CAIV is anchored by glycosylphosphatidylinositol linkage. Membrane polarity is apical for CAXIV, basolateral for CAXII, and apical and basolateral for CAIV. Luminal membrane CAs facilitate the dehydration of carbonic acid (H(2)CO(3)) that is formed when secreted protons combine with filtered bicarbonate. Basolateral CA enhances the efflux of bicarbonate via dehydration of H(2)CO(3). CAII and CAIV can associate with bicarbonate transporters (e.g., AE1, kNBC1, NBC3, and SCL26A6), and proton antiporter, NHE1 in a membrane protein complex called a transport metabolon. CAXII and CAXIV may also be associated with transporters in normal kidney and CAIX in RCCs. The multiplicity of CAs implicates their importance in acid-base and other solute transport along the nephron. For example, CAII on the cytoplasmic face and CAIV on the extracellular surface provide the 'push' and 'pull' for bicarbonate transport by supplying and dissipating substrate respectively.

Ca2+-Calmodulin and Janus Kinase 2 Are Required for Activation of Sodium-Proton Exchange by the Gi-Coupled 5-Hydroxytryptamine1aReceptor

The type 1 sodium-proton exchanger (NHE-1) is expressed ubiquitously and regulates key cellular functions, including mitogenesis, cell volume, and intracellular pH. Despite its importance, the signaling pathways that regulate NHE-1 remain incompletely defined. In this work, we present evidence that stimulation of the 5-hydroxytryptamine 1A (5-HT1A) receptor results in the formation of a signaling complex that includes activated Janus kinase 2 (Jak2), Ca2+/calmodulin (CaM), and NHE-1, and which involves tyrosine phosphorylation of CaM. The signaling pathway also involves rapid agonist-induced association of CaM and NHE-1 as assessed by coimmunoprecipitation studies and by bioluminescence resonance energy transfer studies in living cells. We propose that NHE-1 is activated through this pathway: 5-HT1A receptor --> G(i2)alpha and/or G(i3)alpha --> Jak2 activation --> tyrosine phosphorylation of CaM --> increased binding of CaM to NHE-1 --> induction of a conformational change in NHE-1 that unmasks an obscured proton-sensing and/or proton-transporting region of NHE-1 --> activation of NHE-1. The G(i/o)-coupled 5-HT1A receptor now joins a handful of Gq-coupled receptors and hypertonic shock as upstream activators of this emerging pathway. In the course of this work, we have presented clear evidence that CaM can be activated through tyrosine phosphorylation in the absence of a significant role for elevated intracellular Ca2+. We have also shown for the first time that the association of CaM with NHE-1 in living cells is a dynamic process.

The Nck-interacting kinase phosphorylates ERM proteins for formation of lamellipodium by growth factors

The mammalian Ste20-like Nck-interacting kinase (NIK) and its orthologs Misshapen in Drosophila and Mig-15 in Caenorhabditis elegans have a conserved function in regulating cell morphology, although through poorly understood mechanisms. We report two previously unrecognized actions of NIK: regulation of lamellipodium formation by growth factors and phosphorylation of the ERM proteins ezrin, radixin, and moesin. ERM proteins regulate cell morphology and plasma membrane dynamics by reversibly anchoring actin filaments to integral plasma membrane proteins. In vitro assays show that NIK interacts directly with ERM proteins, binding their N termini and phosphorylating a conserved C-terminal threonine. In cells, NIK and phosphorylated ERM proteins localize at the distal margins of lamellipodia, and NIK activity is necessary for phosphorylation of ERM proteins induced by EGF and PDGF, but not by thrombin. Lamellipodium extension in response to growth factors is inhibited in cells expressing a kinase-inactive NIK, suppressed for NIK expression with siRNA oligonucleotides, or expressing ezrin T567A that cannot be phosphorylated. These data suggest that direct phosphorylation of ERM proteins by NIK constitutes a signaling mechanism controlling growth factor-induced membrane protrusion and cell morphology.

Transcriptional stimulation of the human NHE3 promoter activity by PMA: PKC independence and involvement of the transcription factor EGR-1

NHE3 (Na+/H+ exchanger 3) is essential for Na+ absorption in the ileum and is expressed in a cell-specific manner in the apical membrane of the intestinal epithelial cells. In the present study, we report the stimulatory effect of PMA on the hNHE3 (human NHE3) transcription. Pretreatment with actinomycin D or cycloheximide blocked the up-regulation of the NHE3 mRNA by PMA, indicating that the increased level of NHE3 mRNA expression is regulated by transcriptional activation and is dependent on de novo protein synthesis. 5'-Deletion of the promoter region and transfection analysis in C2BBe1 cells revealed that the PMA effect is mediated through a GC-rich DNA region between nt -88 and -69. Gel mobility-shift assays demonstrated that in nuclear extracts from C2BBe1 cells grown under the basal growth conditions, Sp1 (stimulating protein-1) and Sp3 interact with this GC-rich DNA region, while, in PMA-treated nuclear extracts, PMA-induced EGR-1 (early growth response gene product 1) transcription factor binds to the same site. Binding of EGR-1 diminished the Sp1 and Sp3 interactions with this promoter region significantly. Co-transfection of Sp1 or Sp3 into SL2 cells activated the NHE3-reporter constructs, suggesting that Sp1 and Sp3 act as positive regulators of the NHE3 expression. In addition, overexpression of EGR-1 was sufficient to transactivate the NHE3-reporter gene activity, and knockdown of EGR-1 with gene-specific small interfering RNA resulted in inhibition of the PMA-induced up-regulation of the endogenous NHE3 mRNA expression. Furthermore, the PKC (protein kinase C) inhibitor chelerythrine chloride did not affect PMA-induced NHE3 promoter activity, suggesting that PMA stimulation of the hNHE3 gene expression may be PKC-independent.

Inhibiting p90 ribosomal S6 kinase prevents (Na+)-H+ exchanger-mediated cardiac ischemia-reperfusion injury

BACKGROUND

Pharmacological and genetic studies indicate that the (Na+)-H+ exchanger isoform 1 (NHE1) plays a critical role in myocardial ischemia and reperfusion (I/R) injury. We found that p90 ribosomal S6 kinase (RSK) phosphorylated serine 703 of NHE1, stimulating 14-3-3 binding and NHE1 activity. Therefore, we hypothesized that inhibiting RSK in cardiomyocytes would prevent NHE1 activation and decrease I/R-mediated injury.

METHODS AND RESULTS

To examine the role of RSK in vivo, we generated transgenic mice with cardiac-specific overexpression of dominant negative RSK (DN-RSK-TG). DN-RSK-TG hearts demonstrated normal basal cardiac function and morphology. However, myocardial infarction (left coronary artery occlusion for 45 minutes) in DN-RSK-TG hearts was significantly reduced at 24 hours of reperfusion from 46.9+/-5.6% area at risk in nontransgenic littermate controls to 26.0+/-4.2% in DN-RSK-TG (P<0.01). Cardiomyocyte apoptosis was significantly reduced after I/R in DN-RSK (0.9+/-0.2%) compared with nontransgenic littermate controls (6.2+/-2.6%). Importantly, activation of RSK and interaction of 14-3-3 with NHE1, necessary for agonist-stimulated NHE1 activity, were increased by I/R and inhibited by 70% in DN-RSK-TG (P<0.01). Next, we transduced rat neonatal cardiomyocytes with adenovirus-expressing DN-RSK (Ad.DN-RSK) and measured NHE1 activity. The baseline rate of pH recovery in acid-loaded cells was equal in cells expressing LacZ or DN-RSK. However, NHE1 activation by 100 micromol/L H2O2 was significantly inhibited in cells expressing DN-RSK (0.16+/-0.02 pH units/min) compared with Ad.LacZ (0.49+/-0.13 pH units/min). Apoptosis induced by 12 hours of anoxia followed by 24 hours' reoxygenation was significantly reduced in cells expressing Ad.DN-RSK (18.6+/-2.0%) compared with Ad.LacZ (29.3+/-5.4%).

CONCLUSIONS

In summary, RSK is a novel regulator of cardiac NHE1 activity by phosphorylating NHE1 serine 703 and a new pathological mediator of I/R injury in the heart.

Role of the Na/H exchanger NHE1 in cell migration

Cell migration plays a basic role in many physiological and pathophysiological processes such as embryogenesis, immune defence, wound healing or metastasis. The activity of the ubiquitously expressed NHE1 isoform of the plasma membrane Na+/H+ exchanger is one of the requirements for directed locomotion of migrating cells and also contributes to cell adhesion. The mechanisms by which NHE1 is involved in cell migration are multiple. NHE1 contributes to cell migration by affecting the cell volume, by regulating the intracellular pH and thereby the assembly and activity of cytoskeletal elements, by anchoring the cytoskeleton to the plasma membrane, by signalling, by regulating gene expression and by controlling cell adhesion. The present article gives a review of the different ways in which NHE1 is involved in and contributes to cell migration. These different mechanisms complement one another forming an intricate, integrative process.

The Na+/H+ exchanger NHE1 in stress-induced signal transduction: implications for cell proliferation and cell death

The ubiquitous plasma membrane Na+/H+ exchanger NHE1 is highly conserved across vertebrate species and is extensively characterized as a major membrane transport mechanism in the regulation of cellular pH and volume. In recent years, the understanding of the role of NHE1 in regulating cell function has expanded from one of a household protein involved in ion homeostasis to that of a multifaceted regulator and/or modulator of a wide variety of cell functions. NHE1 plays pivotal roles in response to a number of important physiological stress conditions which, in addition to cell shrinkage and acidification, include hypoxia and mechanical stimuli, such as cell stretch. It has recently become apparent that NHE1-mediated modulation of, e.g., cell migration, morphology, proliferation, and death results not only from NHE1-mediated changes in pHi, cell volume, and/or [Na+]i, but also from direct protein-protein interactions with, e.g., ezrin/radixin/moesin (ERM) proteins and regulation of cellular signaling events, including the activity of mitogen-activated protein kinases (MAPKs) and Akt/protein kinase B (PKB). The aim of this review is to present and discuss new findings implicating NHE1 activation as a central signaling event activated by stress conditions and modulating cell proliferation and death. The pathophysiological importance of NHE1 in modulating the balance between cell proliferation and cell death in cancer and in ischemia/severe hypoxia will also be briefly addressed.

Ste20-type kinases: evolutionarily conserved regulators of ion transport and cell volume

Ste20 serine/threonine kinases regulate fundamental cellular processes including the cell cycle, apoptosis, and stress responses. Recent studies in Caenorhabditis elegans and mammals demonstrate that Ste20 kinases also function in cell volume sensing and Cl- transport regulation. Yeast Ste20 initiates a shrinkage activated MAPK cascade that regulates organic osmolyte accumulation. Ste20 kinases thus play evolutionarily conserved roles in cellular volume sensing and regulation. They may also function in systemic osmotic homeostasis and to link cell-cycle events with cell volume.

Functional identification of kinases essential for T-cell activation through a genetic suppression screen

Activation of T-cells by antigens initiates a complex series of signal-transduction events that are critical for immune responses. While kinases are key mediators of signal transduction networks, several of which have been well characterized in T-cell activation, the functional roles of other kinases remain poorly defined. To address this deficiency, we developed a genetic screen to survey the functional roles of kinases in antigen mediated T-cell activation. A retroviral library was constructed that expressed genetic suppressor elements (GSEs) comprised of peptides and antisense nucleotides derived from kinase cDNAs including members of the STE, CAMK, AGC, CMGC, RGC, TK, TKL, Atypical, and Lipid kinase groups. The retroviral library was expressed in Jurkat T-cells and analyzed for their effect on T-cell activation as monitored by CD69 expression. Jurkat cells were activated by antigen presenting cells treated with superantigen, and sorted for a CD69 negative phenotype by flow cytometry. We identified 19 protein kinases that were previously implicated in T-cell signaling processes and 12 kinases that were not previously linked to T-cell activation. To further validate our approach, we characterized the role of the protein kinase MAP4K4 that was identified in the screen. siRNA studies showed a role for MAP4K4 in antigen mediated T-cell responses in Jurkat and primary T-cells. In addition, by analyzing multiple promoter elements using reporter assays, we have shown that MAP4K4 is implicated in the activation of the TNF-alpha promoter. Our results suggest that this methodology could be used to survey the function of the entire kinome in T-cell activation.

MOLECULAR PHYSIOLOGY OF INTESTINAL N+/H+EXCHANGE

The sodium/hydrogen exchange (NHE) gene family plays an integral role in neutral sodium absorption in the mammalian intestine. The NHE gene family is comprised of nine members that are categorized by cellular localization (i.e., plasma membrane or intracellular). In the gastrointestinal (GI) tract of multiple species, there are resident plasma membrane isoforms including NHE1 (basolateral) and NHE2 (apical), recycling isoforms (NHE3), as well as intracellular isoforms (NHE6, 7, 9). NHE3 recycles between the endosomal compartment and the apical plasma membrane and functions in both locations. NHE3 regulation occurs during normal digestive processes and is often inhibited in diarrheal diseases. The C terminus of NHE3 binds multiple regulatory proteins to form large protein complexes that are involved in regulation of NHE3 trafficking to and from the plasma membrane, turnover number, and protein phosphorylation. NHE1 and NHE2 are not regulated by trafficking. NHE1 interacts with multiple regulatory proteins that affect phosphorylation; however, whether NHE1 exists in large multi-protein complexes is unknown. Although intestinal and colonic sodium absorption appear to involve at least NHE2 and NHE3, future studies are necessary to more accurately define their relative contributions to sodium absorption during human digestion and in pathophysiological conditions.

The Ste20-like kinase misshapen functions together with Bicaudal-D and dynein in driving nuclear migration in the developing drosophila eye

Nuclear translocation, driven by the motility apparatus consisting of the cytoplasmic dynein motor and microtubules, is essential for cell migration during embryonic development. Bicaudal-D (Bic-D), an evolutionarily conserved dynein-interacting protein, is required for developmental control of nuclear migration in Drosophila. Nothing is known about the signaling events that coordinate the function of Bic-D and dynein during development. Here, we show that Misshapen (Msn), the fly homolog of the vertebrate Nck-interacting kinase is a component of a novel signaling pathway that regulates photoreceptor (R-cell) nuclear migration in the developing Drosophila compound eye. Msn, like Bic-D, is required for the apical migration of differentiating R-cell precursor nuclei. msn displays strong genetic interaction with Bic-D. Biochemical studies demonstrate that Msn increases the phosphorylation of Bic-D, which appears to be necessary for the apical accumulation of both Bic-D and dynein in developing R-cell precursor cells. We propose that Msn functions together with Bic-D to regulate the apical localization of dynein in generating directed nuclear migration within differentiating R-cell precursor cells.

Na+/H+exchanger NHE1 as plasma membrane scaffold in the assembly of signaling complexes

The plasma membrane Na+/H+exchanger NHE1 has an established function in intracellular pH and cell volume homeostasis by catalyzing electroneutral influx of extracellular Na+and efflux of intracellular H+. A second function of NHE1 as a structural anchor for actin filaments through its direct binding of the ezrin, radixin, and moesin (ERM) family of actin-binding proteins was recently identified. ERM protein binding and actin anchoring by NHE1 are necessary to retain the localization of NHE1 in specialized plasma membrane domains and to promote cytoskeleton-dependent processes, including actin filament bundling and cell-substrate adhesions. This review explores a third function of NHE1, as a plasma membrane scaffold in the assembly of signaling complexes. Through its coordinate functions in H+efflux, actin anchoring, and scaffolding, we propose that NHE1 promotes protein interactions and activities, assembles signaling complexes in specialized plasma membrane domains, and coordinates divergent signaling pathways.

Comparative biology of the ubiquitous Na+/H+ exchanger, NHE1: lessons from erythrocytes

By virtue of their electroneutral exchange of intracellular H+ for extracellular Na+, the Na+/H+ exchangers (NHE1-NHE8) play a pivotal role in many physiological processes. This review focuses on the ubiquitous plasma membrane isoform, NHE1. Particular attention is given to the roles and regulation of NHE1 in erythrocytes, in their own right and as model systems, but pertinent findings from non-erythroid cells are also discussed. NHE1 plays a key role in the regulation of cell volume and pH, and consequently in the control of such diverse processes as blood O2/CO2 transport, and cell proliferation, motility, and survival. Disturbances in NHE1 function are involved in important pathological states such as hypoxic cell damage and cancer development. NHE1 has a predicted topology of 12 transmembrane domains, and a hydrophilic C-terminus thought to be the major site for NHE1 regulation. NHE1 is highly conserved throughout the vertebrate phylum, particularly in the transmembrane region and the proximal part of the C-terminus. In non-erythroid, and probably also in erythroid cells, this part of the hydrophilic C-terminus interacts with multiple binding partners important for NHE1 function. Erythrocyte NHE1s from mammalian, amphibian, and teleost species are activated by cell shrinkage, decreased pH(i), inhibition of Ser/Thr protein phosphatases, and activation of Ser/Thr protein kinases, i.e., many of the stimuli activating NHE1 in non-erythroid cells. In erythrocytes of many lower vertebrates, NHE1 is activated during hypoxia and is an important modulator of hemoglobin oxygen affinity. Sensitivity of NHE1 to oxygenation status has recently been described also in non-erythroid mammalian cells.

Expression profile of genes regulated by activity of the Na-H exchanger NHE1

BACKGROUND

In mammalian cells changes in intracellular pH (pHi), which are predominantly controlled by activity of plasma membrane ion exchangers, regulate a diverse range of normal and pathological cellular processes. How changes in pHi affect distinct cellular processes has primarily been determined by evaluating protein activities and we know little about how pHi regulates gene expression.

RESULTS

A global profile of genes regulated in mammalian fibroblasts by decreased pHi induced by impaired activity of the plasma membrane Na-H exchanger NHE1 was characterized by using cDNA microarrays. Analysis of selected genes by quantitative RT-PCR, TaqMan, and immunoblot analyses confirmed results obtained from cDNA arrays. Consistent with established roles of pHi and NHE1 activity in cell proliferation and oncogenic transformation, grouping regulated genes into functional categories and biological pathways indicated a predominant number of genes with altered expression were associated with growth factor signaling, oncogenesis, and cell cycle progression.

CONCLUSION

A comprehensive analysis of genes selectively regulated by pHi provides insight on candidate targets that might mediate established effects of pHi on a number of normal and pathological cell functions.

Hypertonicity activates Na+/H+ exchange through Janus kinase 2 and calmodulin

The type 1 sodium-hydrogen exchanger (NHE-1) is a ubiquitous electroneutral membrane transporter that is activated by hypertonicity in many cells. NHE-1 may be an important pathway for Na(+) entry during volume restoration, yet the molecular mechanisms underlying the osmotic regulation of NHE-1 are poorly understood. In the present study we conducted a screen for important signaling molecules that could be involved in hypertonicity-induced activation of NHE-1 in CHO-K1 cells. Hypertonicity rapidly activated NHE-1 in a concentration-dependent manner as assessed by proton microphysiometry and by measurements of intracellular pH on a FLIPR (fluorometric imaging plate reader). Inhibitors of Ca(2+)/calmodulin (CaM) and Janus kinase 2 (Jak2) attenuated this activation, whereas neither calcium chelation nor inhibitors of protein kinase C, the Ras-ERK1/2 pathway, Src kinase, and Ca(2+)/calmodulin-dependent enzymes had significant effects. Hypertonicity also resulted in the rapid tyrosine phosphorylation of Jak2 and STAT3 (the major substrate of Jak2) and CaM. Phosphorylation of Jak2 and CaM were blocked by AG490, an inhibitor of Jak2. Immunoprecipitation studies showed that hypertonicity stimulates the assembly of a signaling complex that includes CaM, Jak2, and NHE-1. Formation of the complex could be blocked by AG490. Thus, we propose that hypertonicity induces activation of NHE-1 in CHO-K1 cells in large part through the following pathway: hypertonicity --> Jak2 phosphorylation and activation --> tyrosine phosphorylation of CaM --> association of CaM with NHE-1 --> NHE-1 activation.

Mutations of Arg440 and Gly455/Gly456 oppositely change pH sensing of Na+/H+ exchanger 1

To identify important amino acid residues involved in intracellular pH (pH(i)) sensing of Na(+)/H(+) exchanger 1, we produced single-residue substitution mutants in the region of the exchanger encompassing the putative 11th transmembrane segment (TM11) and its adjacent intracellular (intracellular loop (IL) 5) and extracellular loops (extracellular loop 6). Substitution of Arg(440) in IL5 with other residues except positively charged Lys caused a large shift in pH(i) dependence of (22)Na(+) uptake to an acidic side, whereas substitution of Gly(455) or Gly(456) within the highly conserved glycine-rich sequence of TM11 shifted pH(i) dependence to an alkaline side. The observed alkaline shift was larger with substitution of Gly(455) with residues with increasing sizes, suggesting the involvement of the steric effect. Interestingly, mutation of Arg(440) (R440D) abolished the ATP depletion-induced acidic shift in pH(i) dependence of (22)Na(+) uptake as well as the cytoplasmic alkalinization induced by various extracellular stimuli, whereas with that of Gly(455) (G455Q) these functions were preserved. These mutant exchangers did not alter apparent affinities for extracellular transport substrates Na(+) and H(+) and the inhibitor 5-(N-ethyl-N-isopropyl)amiloride. These results suggest that positive charge at Arg(440) is required for normal pH(i) sensing, whereas mutation-induced perturbation of the TM11 structure may be involved in the effects of Gly mutations. Thus, both Arg(440) in IL5 and Gly residues in the conserved segment of TM11 appear to constitute important elements for proper functioning of the putative "pH(i) sensor" of Na(+)/H(+) exchanger 1.

Cell migration requires both ion translocation and cytoskeletal anchoring by the Na-H exchanger NHE1

Directed cell movement is a multi-step process requiring an initial spatial polarization that is established by asymmetric stimulation of Rho GTPases, phosphoinositides (PIs), and actin polymerization. We report that the Na-H exchanger isoform 1 (NHE1), a ubiquitously expressed plasma membrane ion exchanger, is necessary for establishing polarity in migrating fibroblasts. In fibroblasts, NHE1 is predominantly localized in lamellipodia, where it functions as a plasma membrane anchor for actin filaments by its direct binding of ezrin/radixin/moesin (ERM) proteins. Migration in a wounding assay was impaired in fibroblasts expressing NHE1 with mutations that independently disrupt ERM binding and cytoskeletal anchoring or ion transport. Disrupting either function of NHE1 impaired polarity, as indicated by loss of directionality, mislocalization of the Golgi apparatus away from the orientation of the wound edge, and inhibition of PI signaling. Both functions of NHE1 were also required for remodeling of focal adhesions. Most notably, lack of ion transport inhibited de-adhesion, resulting in trailing edges that failed to retract. These findings indicate that by regulating asymmetric signals that establish polarity and by coordinating focal adhesion remodeling at the cell front and rear, cytoskeletal anchoring by NHE1 and its localized activity in lamellipodia act cooperatively to integrate cues for directed migration.

Expression of calcineurin B homologous protein 2 protects serum deprivation-induced cell death by serum-independent activation of Na+/H+ exchanger

The calcineurin B homologous protein (designated CHP1) has been shown to be a common essential cofactor for the plasma membrane Na(+)/H(+) exchangers (NHEs) (Pang, T., Su, X., Wakabayashi, S., and Shigekawa, M. (2001) J. Biol. Chem. 276, 17367-17372). In this study, we characterized the function of another isoform of CHP (designated CHP2) that has a 61% amino acid identity with CHP1. CHP2, like CHP1, conferred the ability to NHEs 1-3 to express a high exchange activity by binding to the juxtamembrane region of the cytoplasmic domain of the exchanger, but it interacts more strongly (approximately 5-fold) with NHE1 than does CHP1. Although CHP1 is expressed ubiquitously at relatively high levels, CHP2 expression was extremely low in most human tissues but was higher in tumor cells. We produced stable cell clones overexpressing either CHP1 or CHP2 in which one of them is predominantly bound to NHE1. Serum (10%) induced a significant cytoplasmic alkalinization (0.1-0.2 pH unit) in cells co-expressing CHP1 and NHE1 but not in cells co-expressing CHP2 and NHE1. In the latter, pH(i) was high (7.4-7.5) even in the absence of serum, suggesting that NHE1 was already activated. Surprisingly, most (>80%) of CHP2/NHE1 cells unlike CHP1/NHE1 cells were viable even after long serum starvation (>7 days). Thus, the expression of CHP2 appears to protect cells from serum deprivation-induced death by increasing pH(i). These properties of CHP2/NHE1 cells are similar to those of malignantly transformed cells. We propose that serum-independent activation of NHE1 by bound CHP2 is one of the key mechanisms for the maintenance of high pH(i) and the resistance to serum deprivation-induced cell death in malignantly transformed cells.

The changing face of the Na+/H+ exchanger, NHE1: structure, regulation, and cellular actions

The NHE family of ion exchangers includes six isoforms (NHE1-NHE6) that function in an electroneutral exchange of intracellular H(+) for extracellular Na(+). This review focuses on the only ubiquitously expressed isoform, NHE1, which is localized at the plasma membrane where it plays a critical role in intracellular pH (pHi) and cell volume homeostasis. All NHE isoforms share a similar topology: an N-terminus of 12 transmembrane (TM) alpha-helices that collectively function in ion exchange, and a C-terminal cytoplasmic regulatory domain that modulates transport activity by the TM domain. Extracellular signals, mediated by diverse classes of cell-surface receptors, regulate NHE1 activity through distinct signaling networks that converge to directly modify the C-terminal regulatory domain. Modifications in the C-terminus, including phosphorylation and the binding of regulatory proteins, control transport activity by altering the affinity of the TM domain for intracellular H(+). Recently, it was determined that NHE1 also functions as a membrane anchor for the actin-based cytoskeleton, independently of its role in ion translocation. Through its effects on pHi homeostasis, cell volume, and the actin cortical network, NHE1 regulates a number of cell behaviors, including adhesion, shape determination, migration, and proliferation.