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

The molecular basis of the dichotomous functionality of MAP4K4 in proliferation and cell motility control in cancer

The finely tuned integration of intra- and extracellular cues by components of the mitogen-activated protein kinase (MAPK) signaling pathways controls the mutually exclusive phenotypic manifestations of uncontrolled growth and tumor cell dissemination. The Ser/Thr kinase MAP4K4 is an upstream integrator of extracellular cues involved in both proliferation and cell motility control. Initially identified as an activator of the c-Jun N-terminal kinase (JNK), the discovery of diverse functions and additional effectors of MAP4K4 beyond JNK signaling has considerably broadened our understanding of this complex kinase. The implication of MAP4K4 in the regulation of cytoskeleton dynamics and cell motility provided essential insights into its role as a pro-metastatic kinase in cancer. However, the more recently revealed role of MAP4K4 as an activator of the Hippo tumor suppressor pathway has complicated the understanding of MAP4K4 as an oncogenic driver kinase. To develop a better understanding of the diverse functions of MAP4K4 and their potential significance in oncogenesis and tumor progression, we have collected and assessed the current evidence of MAP4K4 implication in molecular mechanisms that control proliferation and promote cell motility. A better understanding of these mechanisms is particularly relevant in the brain, where MAP4K4 is highly expressed and under pathological conditions either drives neuronal cell death in neurodegenerative diseases or cell dissemination in malignant tumors. We review established effectors and present novel interactors of MAP4K4, which offer mechanistic insights into MAP4K4 function and may inspire novel intervention strategies. We discuss possible implications of novel interactors in tumor growth and dissemination and evaluate potential therapeutic strategies to selectively repress pro-oncogenic functions of MAP4K4.

Role of Genetic Mutations of the Na+/H+ Exchanger Isoform 1, in Human Disease and Protein Targeting and Activity

The mammalian Na⁺/H⁺ exchanger isoform one (NHE1) is a plasma membrane protein that is ubiquitously present in human cells. It functions to regulate intracellular pH removing an intracellular proton in exchange for one extracellular sodium and is involved in heart disease and in promoting metastasis in cancer. It is made of a 500 amino acid membrane domain plus a 315 amino acid, regulatory cytosolic tail. The membrane domain is thought to have 12 transmembrane segments and a large membrane-associated extracellular loop. Early studies demonstrated that in mice, disruption of the NHE1 gene results in locomotor ataxia and a phenotype of slow-wave epilepsy. Defects included a progressive neuronal degeneration. Growth and reproductive ability were also reduced. Recent studies have identified human autosomal homozygous recessive mutations in the NHE1 gene (SLC9A1) that result in impaired development, ataxia and other severe defects, and explain the cause of the human disease Lichtenstein-Knorr syndrome. Other human mutations have been identified that are stop codon polymorphisms. These cause short non-functional NHE1 proteins, while other genetic polymorphisms in the NHE1 gene cause impaired expression of the NHE1 protein, reduced activity, enhanced protein degradation or altered kinetic activation of the protein. Since NHE1 plays a key role in many human physiological functions and in human disease, genetic polymorphisms of the protein that significantly alter its function and are likely play significant roles in varying human phenotypes and be involved in disease.

Polarized Expression of Ion Channels and Solute Carrier Family Transporters on Heterogeneous Cultured Human Corneal Endothelial Cells

Purpose

To clarify the expression profiles of ion channels and transporters of metabolic substrates among heterogeneous cultured human corneal endothelial cells (cHCECs) distinct in their effectiveness in reconstituting the corneal endothelium.

Methods

Integrated proteomics for cell lysates by liquid chromatography–tandem mass spectrometry was carried out from three aliquots of cHCECs enriched in either cluster of definition (CD)44^−/+ (mature) cHCECs or CD44^++/+++ cell-state transition (CST) cHCECs. The expression profiles of cations/anions, monocarboxylic acid transporters (MCTs), and solute carrier (SLC) family proteins, as well as carbonic anhydrases (CAs), were investigated.

Results

The polarized expression of cations/anions, MCTs, and SLC family proteins, as well as CAs, was clarified for mature and CST cHCECs. Most SLC4 family members, including SLC4A11 and SLC4A4 (NBCe1), were upregulated in the CST cHCECs, whereas SLC9A1 (Na⁺/H⁺ exchanger isoform one [NHE1]) and CA5B were detected only in the mature cHCECs. In addition, SLC25A42, catalyzing the entry of coenzyme A into the mitochondria, and SLC25A18, functioning as a mitochondrial glutamate carrier 2 (both relevant for providing the substrates for mitochondrial bioenergetics), were selectively expressed in the mature cHCECs.

Conclusions

Our findings may suggest the relevance of qualifying the polarized expression of these ion channels and transporter-like proteins to ensure not only the suitability but also the in vivo biological functionality of cHCECs selected for use in a cell-injection therapy.

Intracellular pH Regulates Cancer and Stem Cell Behaviors: A Protein Dynamics Perspective

The International Society of Cancer Metabolism (ISCaM) meeting on Cancer Metabolic Rewiring, held in Braga Portugal in October 2019, provided an outstanding forum for investigators to present current findings and views, and discuss ideas and future directions on fundamental biology as well as clinical translations. The first session on Cancer pH Dynamics was preceded by the opening keynote presentation from our group entitled Intracellular pH Regulation of Protein Dynamics: From Cancer to Stem Cell Behaviors. In this review we introduce a brief background on intracellular pH (pHi) dynamics, including how it is regulated as well as functional consequences, summarize key findings included in our presentation, and conclude with perspectives on how understanding the role of pHi dynamics in stem cells can be relevant for understanding how pHi dynamics enables cancer progression.

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

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

Na+/H+ exchanger and cardiac hypertrophy

Reactive cardiac hypertrophy (CH) is an increase in heart mass in response to hemodynamic overload. Exercise-induced CH emerges as an adaptive response with improved cardiac function, in contrast to pathological CH that represents a risk factor for cardiovascular health. The Na⁺/H⁺ exchanger (NHE-1) is a membrane transporter that not only regulates intracellular pH but also intracellular Na⁺ concentration. In the scenario of cardiovascular diseases, myocardial NHE-1 is activated by a variety of stimuli, such as neurohumoral factors and mechanical stress, leading to intracellular Na⁺ overload and activation of prohypertrophic cascades. NHE-1 hyperactivity is intimately linked to heart diseases, including ischemia-reperfusion injury, maladaptive CH and heart failure. In this review, we will present evidence to support that the NHE-1 hyperactivity constitutes a "switch on/off" for the pathological phenotype during CH development. We will also discuss some classical and novel strategies to avoid NHE-1 hyperactivity, and that are therefore worthwhile to improve cardiovascular health.

Targeted genomic CRISPR-Cas9 screen identifies MAP4K4 as essential for glioblastoma invasion

Among high-grade brain tumors, glioblastoma is particularly difficult to treat, in part due to its highly infiltrative nature which contributes to the malignant phenotype and high mortality in patients. In order to better understand the signaling pathways underlying glioblastoma invasion, we performed the first large-scale CRISPR-Cas9 loss of function screen specifically designed to identify genes that facilitate cell invasion. We tested 4,574 genes predicted to be involved in trafficking and motility. Using a transwell invasion assay, we discovered 33 genes essential for invasion. Of the 11 genes we selected for secondary testing using a wound healing assay, 6 demonstrated a significant decrease in migration. The strongest regulator of invasion was mitogen-activated protein kinase 4 (MAP4K4). Targeting of MAP4K4 with single guide RNAs or a MAP4K4 inhibitor reduced migration and invasion in vitro. This effect was consistent across three additional patient derived glioblastoma cell lines. Analysis of epithelial-mesenchymal transition markers in U138 cells with lack or inhibition of MAP4K4 demonstrated protein expression consistent with a non-invasive state. Importantly, MAP4K4 inhibition limited migration in a subset of human glioma organotypic slice cultures. Our results identify MAP4K4 as a novel potential therapeutic target to limit glioblastoma invasion.

2-Aminopyridine-Based Mitogen-Activated Protein Kinase Kinase Kinase Kinase 4 (MAP4K4) Inhibitors: Assessment of Mechanism-Based Safety

Studies have linked the serine-threonine kinase MAP4K4 to the regulation of a number of biological processes and/or diseases, including diabetes, cancer, inflammation, and angiogenesis. With a majority of the members of our lead series (e.g., 1) suffering from time-dependent inhibition (TDI) of CYP3A4, we sought design avenues that would eliminate this risk. One such approach arose from the observation that carboxylic acid-based intermediates employed in our discovery efforts retained high MAP4K4 inhibitory potency and were devoid of the TDI risk. The medicinal chemistry effort that led to the discovery of this central nervous system-impaired inhibitor together with its preclinical safety profile is described.

A Histidine Cluster in the Cytoplasmic Domain of the Na-H Exchanger NHE1 Confers pH-sensitive Phospholipid Binding and Regulates Transporter Activity

The Na-H exchanger NHE1 contributes to intracellular pH (pH_i) homeostasis in normal cells and the constitutively increased pH_i in cancer. NHE1 activity is allosterically regulated by intracellular protons, with greater activity at lower pH_i However, the molecular mechanism for pH-dependent NHE1 activity remains incompletely resolved. We report that an evolutionarily conserved cluster of histidine residues located in the C-terminal cytoplasmic domain between two phosphatidylinositol 4,5-bisphosphate binding sites (PI(4,5)P₂) of NHE1 confers pH-dependent PI(4,5)P₂ binding and regulates NHE1 activity. A GST fusion of the wild type C-terminal cytoplasmic domain of NHE1 showed increased maximum PI(4,5)P₂ binding at pH 7.0 compared with pH 7.5. However, pH-sensitive binding is abolished by substitutions of the His-rich cluster to arginine (RXXR3) or alanine (AXXA3), mimicking protonated and neutral histidine residues, respectively, and the RXXR3 mutant had significantly greater PI(4,5)P₂ binding than AXXA3. When expressed in cells, NHE1 activity and pH_i were significantly increased with NHE1-RXXR3 and decreased with NHE1-AXXA3 compared with wild type NHE1. Additionally, fibroblasts expressing NHE1-RXXR3 had significantly more contractile actin filaments and focal adhesions compared with fibroblasts expressing wild type NHE1, consistent with increased pH_i enabling cytoskeletal remodeling. These data identify a molecular mechanism for pH-sensitive PI(4,5)P₂ binding regulating NHE1 activity and suggest that the evolutionarily conserved cluster of four histidines in the proximal cytoplasmic domain of NHE1 may constitute a proton modifier site. Moreover, a constitutively activated NHE1-RXXR3 mutant is a new tool that will be useful for studying how increased pH_i contributes to cell behaviors, most notably the biology of cancer cells.

Map4k4 Signaling Nodes in Metabolic and Cardiovascular Diseases

Mitogen-activated kinase kinase kinase kinase 4 (Map4k4), originally identified in small interfering (si)RNA screens and characterized by tissue-specific gene deletions, is emerging as a regulator of glucose homeostasis and cardiovascular health. Recent studies have shown that Map4k4 gene ablation or inhibition of its kinase activity attenuates hyperglycemia and plaque formation in mouse models of insulin resistance and atherosclerosis, and suggest roles for Map4k4 in multiple signaling systems, including NFκB activation, small GTPase regulation, the Hippo cascade, and regulation of cell dynamics by FERM domain proteins. This new and promising area of inquiry raises key questions that need to be addressed, such as defining which of the above or other effectors mediate Map4k4 control of metabolic and vascular functions, and identifying upstream activators of Map4k4.

MAP4K4 Is a Threonine Kinase That Phosphorylates FARP1

Mitogen-activated protein kinase 4 (MAP4K4) regulates the MEK kinase cascade and is implicated in cytoskeletal rearrangement and migration; however, identifying MAP4K4 substrates has remained a challenge. To ascertain MAP4K4-dependent phosphorylation events, we combined phosphoproteomic studies of MAP4K4 inhibition with in vitro assessment of its kinase specificity. We identified 235 phosphosites affected by MAP4K4 inhibition in cells and found that pTP and pSP motifs were predominant among them. In contrast, in vitro assessment of kinase specificity showed that MAP4K4 favors a pTL motif. We showed that MAP4K4 directly phosphorylates and coimmunoprecipitates with FERM, RhoGEF, and pleckstrin domain-containing protein 1 (FARP1). MAP4K4 inhibition in SH-SY5Y cells increases neurite outgrowth, a process known to involve FARP1. As FARP1 and MAP4K4 both contribute to cytoskeletal rearrangement, the results suggest that MAP4K4 exerts some of its effects on the cytoskeleton via phosphorylation of FARP1.

Na+-H+ exchanger-1 (NHE1) regulation in kidney proximal tubule

The ubiquitously expressed plasma membrane Na(+)-H(+) exchanger NHE1 is a 12 transmembrane-spanning protein that directs important cell functions such as homeostatic intracellular volume and pH control. The 315 amino acid cytosolic tail of NHE1 binds plasma membrane phospholipids and multiple proteins that regulate additional, ion-translocation independent functions. This review focuses on NHE1 structure/function relationships, as well as the role of NHE1 in kidney proximal tubule functions, including pH regulation, vectorial Na(+) transport, cell volume control and cell survival. The implications of these functions are particularly critical in the setting of progressive, albuminuric kidney diseases, where the accumulation of reabsorbed fatty acids leads to disruption of NHE1-membrane phospholipid interactions and tubular atrophy, which is a poor prognostic factor for progression to end stage renal disease. This review amplifies the vital role of the proximal tubule NHE1 Na(+)-H(+) exchanger as a kidney cell survival factor.

The Nck-interacting kinase NIK increases Arp2/3 complex activity by phosphorylating the Arp2 subunit

The nucleating activity of the Arp2/3 complex promotes the assembly of branched actin filaments that drive plasma membrane protrusion in migrating cells. Arp2/3 complex binding to nucleation-promoting factors of the WASP and WAVE families was previously thought to be sufficient to increase nucleating activity. However, phosphorylation of the Arp2 subunit was recently shown to be necessary for Arp2/3 complex activity. We show in mammary carcinoma cells that mutant Arp2 lacking phosphorylation assembled with endogenous subunits and dominantly suppressed actin filament assembly and membrane protrusion. We also report that Nck-interacting kinase (NIK), a MAP4K4, binds and directly phosphorylates the Arp2 subunit, which increases the nucleating activity of the Arp2/3 complex. In cells, NIK kinase activity was necessary for increased Arp2 phosphorylation and plasma membrane protrusion in response to epidermal growth factor. NIK is the first kinase shown to phosphorylate and increase the activity of the Arp2/3 complex, and our findings suggest that it integrates growth factor regulation of actin filament dynamics.

The Ser/Thr kinase MAP4K4 drives c-Met-induced motility and invasiveness in a cell-based model of SHH medulloblastoma

Medulloblastoma (MB) comprises four molecularly and genetically distinct subgroups of embryonal brain tumors that develop in the cerebellum. MB mostly affects infants and children and is difficult to treat because of frequent dissemination of tumor cells within the leptomeningeal space. A potential promoter of cell dissemination is the c-Met proto-oncogene receptor tyrosine kinase, which is aberrantly expressed in many human tumors including MB. Database analysis showed that c-Met is highly expressed in the sonic hedgehog (SHH) subgroup and in a small subset of Group 3 and Group 4 MB tumors. Using a cell-based three-dimensional cell motility assay combined with live-cell imaging, we investigated whether the c-Met ligand HGF could drive dissemination of MB cells expressing high levels of c-Met, and determined downstream effector mechanisms of this process. We detected variable c-Met expression in different established human MB cell lines, and we found that in lines expressing high c-Met levels, HGF promoted cell dissemination and invasiveness. Specifically, HGF-induced c-Met activation enhanced the capability of the individual cells to migrate in a JNK-dependent manner. Additionally, we identified the Ser/Thr kinase MAP4K4 as a novel driver of c-Met-induced invasive cell dissemination. This increased invasive motility was due to MAP4K4 control of F-actin dynamics in structures required for migration and invasion. Thus, MAP4K4 couples growth factor signaling to actin cytoskeleton regulation in tumor cells, suggesting that MAP4K4 could present a promising novel target to be evaluated for treating growth factor-induced dissemination of MB tumors of different subgroups and of other human cancers.

Talin regulates moesin-NHE-1 recruitment to invadopodia and promotes mammary tumor metastasis

Invadopodia are actin-rich protrusions that degrade the extracellular matrix and are required for stromal invasion, intravasation, and metastasis. The role of the focal adhesion protein talin in regulating these structures is not known. Here, we demonstrate that talin is required for invadopodial matrix degradation and three-dimensional extracellular matrix invasion in metastatic breast cancer cells. The sodium/hydrogen exchanger 1 (NHE-1) is linked to the cytoskeleton by ezrin/radixin/moesin family proteins and is known to regulate invadopodium-mediated matrix degradation. We show that the talin C terminus binds directly to the moesin band 4.1 ERM (FERM) domain to recruit a moesin-NHE-1 complex to invadopodia. Silencing talin resulted in a decrease in cytosolic pH at invadopodia and blocked cofilin-dependent actin polymerization, leading to impaired invadopodium stability and matrix degradation. Furthermore, talin is required for mammary tumor cell motility, intravasation, and spontaneous lung metastasis in vivo. Thus, our findings provide a novel understanding of how intracellular pH is regulated and a molecular mechanism by which talin enhances tumor cell invasion and metastasis.

Na⁺/H⁺ exchangers and intracellular pH in perinatal brain injury

Encephalopathy consequent on perinatal hypoxia–ischemia occurs in 1–3 per 1,000 term births in the UK and frequently leads to serious and tragic consequences that devastate lives and families, with huge financial burdens for society. Although the recent introduction of cooling represents a significant advance, only 40 % survive with normal neurodevelopmental function. There is thus a significant unmet need for novel, safe, and effective therapies to optimize brain protection following brain injury around birth. The Na⁺/H⁺ exchanger (NHE) is a membrane protein present in many mammalian cell types. It is involved in regulating intracellular pH and cell volume. NHE1 is the most abundant isoform in the central nervous system and plays a role in cerebral damage after hypoxia–ischemia. Excessive NHE activation during hypoxia–ischemia leads to intracellular Na⁺ overload, which subsequently promotes Ca²⁺ entry via reversal of the Na⁺/Ca²⁺ exchanger. Increased cytosolic Ca²⁺ then triggers the neurotoxic cascade. Activation of NHE also leads to rapid normalization of pH_i and an alkaline shift in pH_i. This rapid recovery of brain intracellular pH has been termed pH paradox as, rather than causing cells to recover, this rapid return to normal and overshoot to alkaline values is deleterious to cell survival. Brain pH_i changes are closely involved in the control of cell death after injury: an alkalosis enhances excitability while a mild acidosis has the opposite effect. We have observed a brain alkalosis in 78 babies with neonatal encephalopathy serially studied using phosphorus-31 magnetic resonance spectroscopy during the first year after birth (151 studies throughout the year including 56 studies of 50 infants during the first 2 weeks after birth). An alkaline brain pH_i was associated with severely impaired outcome; the degree of brain alkalosis was related to the severity of brain injury on MRI and brain lactate concentration; and a persistence of an alkaline brain pH_i was associated with cerebral atrophy on MRI. Experimental animal models of hypoxia–ischemia show that NHE inhibitors are neuroprotective. Here, we review the published data on brain pH_i in neonatal encephalopathy and the experimental studies of NHE inhibition and neuroprotection following hypoxia–ischemia.

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

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

Roles of ion transport in control of cell motility

Cell motility is an essential feature of life. It is essential for reproduction, propagation, embryonic development, and healing processes such as wound closure and a successful immune defense. If out of control, cell motility can become life-threatening as, for example, in metastasis or autoimmune diseases. Regardless of whether ciliary/flagellar or amoeboid movement, controlled motility always requires a concerted action of ion channels and transporters, cytoskeletal elements, and signaling cascades. Ion transport across the plasma membrane contributes to cell motility by affecting the membrane potential and voltage-sensitive ion channels, by inducing local volume changes with the help of aquaporins and by modulating cytosolic Ca(2+) and H(+) concentrations. Voltage-sensitive ion channels serve as voltage detectors in electric fields thus enabling galvanotaxis; local swelling facilitates the outgrowth of protrusions at the leading edge while local shrinkage accompanies the retraction of the cell rear; the cytosolic Ca(2+) concentration exerts its main effect on cytoskeletal dynamics via motor proteins such as myosin or dynein; and both, the intracellular and the extracellular H(+) concentration modulate cell migration and adhesion by tuning the activity of enzymes and signaling molecules in the cytosol as well as the activation state of adhesion molecules at the cell surface. In addition to the actual process of ion transport, both, channels and transporters contribute to cell migration by being part of focal adhesion complexes and/or physically interacting with components of the cytoskeleton. The present article provides an overview of how the numerous ion-transport mechanisms contribute to the various modes of cell motility.

Aberrant dynamin 2-dependent Na(+) /H(+) exchanger-1 trafficking contributes to cardiomyocyte apoptosis

Sarcolemmal Na⁺/H⁺ exchanger 1 (NHE1) activity is essential for the intracellular pH (pH_i) homeostasis in cardiac myocytes. Emerging evidence indicates that sarcolemmal NHE1 dysfunction was closely related to cardiomyocyte death, but it remains unclear whether defective trafficking of NHE1 plays a role in the vital cellular signalling processes. Dynamin (DNM), a large guanosine triphosphatase (GTPase), is best known for its roles in membrane trafficking events. Herein, using co-immunoprecipitation, cell surface biotinylation and confocal microscopy techniques, we investigated the potential regulation on cardiac NHE1 activity by DNM. We identified that DNM2, a cardiac isoform of DNM, directly binds to NHE1. Overexpression of a wild-type DNM2 or a dominant-negative DNM2 mutant with defective GTPase activity in adult rat ventricular myocytes (ARVMs) facilitated or retarded the internalization of sarcolemmal NHE1, whereby reducing or increasing its activity respectively. Importantly, the increased NHE1 activity associated with DNM2 deficiency led to ARVMs apoptosis, as demonstrated by cell viability, terminal deoxynucleotidyl transferase–mediated dUTP nick-end labelling assay, Bcl-1/Bax expression and caspase-3 activity, which were effectively rescued by pharmacological inhibition of NHE1 with zoniporide. Thus, our results demonstrate that disruption of the DNM2-dependent retrograde trafficking of NHE1 contributes to cardiomyocyte apoptosis.

Mitochondrial NHE1: a newly identified target to prevent heart disease

Mitochondrial damage has been associated with early steps of cardiac dysfunction in heart subjected to ischemic stress, oxidative stress and hypertrophy. A common feature for the mitochondrial deterioration is the loss of the mitochondrial membrane potential (ΔΨ m) with the concomitant irreversible opening of the mitochondrial permeability transition pore (MPTP) which follows the mitochondrial Ca²⁺ overload, and the subsequent mitochondrial swelling. We have recently characterized the expression of the Na⁺/H⁺ exchanger 1 (mNHE1) in mitochondrial membranes. This surprising observation provided a unique target for the prevention of the Ca²⁺-induced MPTP opening, based on the inhibition of the NHE1 m. In this line, inhibition of NHE1 m activity and/or reduction of NHE1 m expression decreased the Ca²⁺-induced mitochondrial swelling and the release of reactive oxygen species (ROS) in isolated cardiac mitochondria and preserved the ΔΨ m in isolated cardiomyocytes. Mitochondrial NHE1 thus represents a novel target to prevent cardiac disease, opening new avenues for future research.

Inside out: targeting NHE1 as an intracellular and extracellular regulator of cancer progression

The sodium hydrogen exchanger isoform one is a critical regulator of intracellular pH, serves as an anchor for the formation of cytoplasmic signaling complexes, and modulates cytoskeletal organization. There is a growing interest in the potential for sodium hydrogen exchanger isoform one as a therapeutic target against cancer. Sodium hydrogen exchanger isoform one transport drives formation of membrane protrusions essential for cell migration and contributes to the establishment of a tumor microenvironment that leads to the rearrangement of the extracellular matrix further supporting tumor progression. Here, we focus on the potential impact that an inexpensive, $100 genome would have in identifying prospective therapeutic targets to treat tumors based upon changes in gene expression and variation of sodium hydrogen exchanger isoform one regulators. In particular, we will focus on the ezrin, radixin, moesin family proteins, calcineurin B homologous proteins, Ras/Raf/MEK/ERK signaling, and phosphoinositide signaling as they relate to the regulation of sodium hydrogen exchanger isoform one in cancer progression.

Regulation of the cardiac Na⁺/H⁺ exchanger in health and disease

The Na(+) gradient produced across the cardiac sarcolemma by the ATP-dependent Na(+)-pump is a constant source of energy for Na(+)-dependent transporters. The plasma membrane Na(+)/H(+) exchanger (NHE) is one such secondary active transporter, regulating intracellular pH, Na(+) concentration, and cell volume. NHE1, the major isoform found in the heart, is activated in response to a variety of stimuli such as hormones and mechanical stress. This important characteristic of NHE1 is intimately linked to heart diseases, including maladaptive cardiac hypertrophy and subsequent heart failure, as well as acute ischemic-reperfusion injury. NHE1 activation results in elevation of pH and intracellular Na(+) concentration, which potentially enhance downstream signaling cascades in the myocardium. Therefore, in addition to determining the mechanism underlying regulation of NHE1 activity, it is important to understand how the ionic signal produced by NHE1 is transmitted to the downstream targets. Extensive studies have identified many accessory factors that interact with NHE1. Here, we have summarized the recent progress on understanding the molecular mechanism underlying NHE1 regulation and have shown a possible signaling pathway leading to cardiac remodeling, which is initiated from NHE1. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

Regulation of the Na+/H+ Exchanger (NHE1) in Breast Cancer Metastasis

The pH gradient in normal cells is tightly controlled by the activity of various pH-regulatory membrane proteins including the isoform protein of the Na(+)/H(+) exchanger (NHE1). NHE1 is constitutively active in a neoplastic microenvironment, dysregulating pH homeostasis and altering the survival, differentiation, and proliferation of cancer cells, thereby causing them to become tumorigenic. Cytoplasmic alkalinization in breast cancer cells occurs as a result of increased NHE1 activity and, while much is known about the pathophysiologic role of NHE1 in tumor progression with regard to ion flux, the regulation of its activity on a molecular level is only recently becoming evident. The membrane domain of NHE1 is sufficient for ion exchange. However, its activity is regulated through the phosphorylation of key amino acids in the cytosolic domain as well as by its interaction with other intracellular proteins and lipids. Here, we review the importance of these regulatory sites and what role they may play in the disrupted functionality of NHE1 in breast cancer metastasis.

Role of NHE1 in Nociception

Intracellular pH is a fundamental parameter to cell function that requires tight homeostasis. In the absence of any regulation, excessive acidification of the cytosol would have the tendency to produce cellular damage. Mammalian Na(+)/H(+) exchangers (NHEs) are electroneutral Na(+)-dependent proteins that exchange extracellular Na(+) for intracellular H(+). To date, there are 9 identified NHE isoforms where NHE1 is the most ubiquitous member, known as the housekeeping exchanger. NHE1 seems to have a protective role in the ischemia-reperfusion injury and other inflammatory diseases. In nociception, NHE1 is found in neurons along nociceptive pathways, and its pharmacological inhibition increases nociceptive behavior in acute pain models at peripheral and central levels. Electrophysiological studies also show that NHE modulates electrical activity of primary nociceptive terminals. However, its role in neuropathic pain still remains controversial. In humans, NHE1 may be responsible for inflammatory bowel diseases since its expression is reduced in Crohn's disease and ulcerative colitis. The purpose of this work is to provide a review of the evidence about participation of NHE1 in the nociceptive processing.

Role of ion channels and transporters in cell migration

Cell motility is central to tissue homeostasis in health and disease, and there is hardly any cell in the body that is not motile at a given point in its life cycle. Important physiological processes intimately related to the ability of the respective cells to migrate include embryogenesis, immune defense, angiogenesis, and wound healing. On the other side, migration is associated with life-threatening pathologies such as tumor metastases and atherosclerosis. Research from the last ≈ 15 years revealed that ion channels and transporters are indispensable components of the cellular migration apparatus. After presenting general principles by which transport proteins affect cell migration, we will discuss systematically the role of channels and transporters involved in cell migration.

Phosphorylation and activation of the plasma membrane Na+/H+ exchanger (NHE1) during osmotic cell shrinkage

The Na(+)/H(+)Exchanger isoform 1 (NHE1) is a highly versatile, broadly distributed and precisely controlled transport protein that mediates volume and pH regulation in most cell types. NHE1 phosphorylation contributes to Na(+)/H(+) exchange activity in response to phorbol esters, growth factors or protein phosphatase inhibitors, but has not been observed during activation by osmotic cell shrinkage (OCS). We examined the role of NHE1 phosphorylation during activation by OCS, using an ideal model system, the Amphiuma tridactylum red blood cell (atRBC). Na(+)/H(+) exchange in atRBCs is mediated by an NHE1 homolog (atNHE1) that is 79% identical to human NHE1 at the amino acid level. NHE1 activity in atRBCs is exceptionally robust in that transport activity can increase more than 2 orders of magnitude from rest to full activation. Michaelis-Menten transport kinetics indicates that either OCS or treatment with the phosphatase inhibitor calyculin-A (CLA) increase Na(+) transport capacity without affecting transport affinity (K(m)=44 mM) in atRBCs. CLA and OCS act non-additively to activate atNHE1, indicating convergent, phosphorylation-dependent signaling in atNHE1 activation. In situ(32)P labeling and immunoprecipitation demonstrates that the net phosphorylation of atNHE1 is increased 4-fold during OCS coinciding with a more than 2-order increase in Na(+) transport activity. This is the first reported evidence of increased NHE1 phosphorylation during OCS in any vertebrate cell type. Finally, liquid chromatography and mass spectrometry (LC-MS/MS) analysis of atNHE1 immunoprecipitated from atRBC membranes reveals 9 phosphorylated serine/threonine residues, suggesting that activation of atNHE1 involves multiple phosphorylation and/or dephosphorylation events.

Cortactin phosphorylation regulates cell invasion through a pH-dependent pathway

Invadopodia are invasive protrusions with proteolytic activity uniquely found in tumor cells. Cortactin phosphorylation is a key step during invadopodia maturation, regulating Nck1 binding and cofilin activity. The precise mechanism of cortactin-dependent cofilin regulation and the roles of this pathway in invadopodia maturation and cell invasion are not fully understood. We provide evidence that cortactin-cofilin binding is regulated by local pH changes at invadopodia that are mediated by the sodium-hydrogen exchanger NHE1. Furthermore, cortactin tyrosine phosphorylation mediates the recruitment of NHE1 to the invadopodium compartment, where it locally increases the pH to cause the release of cofilin from cortactin. We show that this mechanism involving cortactin phosphorylation, local pH increase, and cofilin activation regulates the dynamic cycles of invadopodium protrusion and retraction and is essential for cell invasion in 3D. Together, these findings identify a novel pH-dependent regulation of cell invasion.

Saccharomyces cerevisiae glucose signalling regulator Mth1p regulates the organellar Na+/H+ exchanger Nhx1p

Organelle-localized NHEs (Na+/H+ exchangers) are found in cells from yeast to humans and contribute to organellar pH regulation by exporting H+ from the lumen to the cytosol coupled to an H+ gradient established by vacuolar H+-ATPase. The mechanisms underlying the regulation of organellar NHEs are largely unknown. In the present study, a yeast two-hybrid assay identified Mth1p as a new binding protein for Nhx1p, an organellar NHE in Saccharomyces cerevisiae. It was shown by an in vitro pull-down assay that Mth1p bound to the hydrophilic C-terminal half of Nhx1p, especially to the central portion of this region. Mth1p is known to bind to the cytoplasmic domain of the glucose sensor Snf3p/Rgt2p and also functions as a negative transcriptional regulator. Mth1p was expressed in cells grown in a medium containing galactose, but was lost (possibly degraded) when cells were grown in medium containing glucose as the sole carbon source. Deletion of the MTH1 gene increased cell growth compared with the wild-type when cells were grown in a medium containing galactose and with hygromycin or at an acidic pH. This resistance to hygromycin or acidic conditions was not observed for cells grown with glucose as the sole carbon source. Gene knockout of NHX1 increased the sensitivity to hygromycin and acidic pH. The increased resistance to hygromycin was reproduced by truncation of the Mth1p-binding region in Nhx1p. These results implicate Mth1p as a novel regulator of Nhx1p that responds to specific extracellular carbon sources.

A human MAP kinase interactome

Mitogen Activated Protein Kinase (MAPK) pathways form the backbone of signal transduction within the mammalian cell. Here, we apply a systematic experimental and computational approach to map 2,269 interactions between human MAPK-related proteins and other cellular machinery and to assemble these data into functional modules. A core network of 641 interactions is supported by multiple lines of evidence including conservation with yeast. Using siRNA knockdowns, we reveal that a significant number of novel interactors can modulate MAPK mediated signaling. We uncover the Na-H exchanger NHE1 as a scaffold for a novel set of MAPKs, link HSP90 chaperones to MAPK pathways, and identify MUC12 as the human analogue to the yeast signaling mucin Msb2. This study makes available a large resource of MAPK interactions along with the accompanying clone libraries. It illustrates a methodology for probing signaling networks based on functional refinement of experimentally-derived protein interaction maps.

Mechanisms of the regulation of the intestinal Na+/H+ exchanger NHE3

A major of Na⁺ absorptive process in the proximal part of intestine and kidney is electroneutral exchange of Na⁺ and H⁺ by Na⁺/H⁺ exchanger type 3 (NHE3). During the past decade, significant advance has been achieved in the mechanisms of NHE3 regulation. A bulk of the current knowledge on Na⁺/H⁺ exchanger regulation is based on heterologous expression of mammalian Na⁺/H⁺ exchangers in Na⁺/H⁺ exchanger deficient fibroblasts, renal epithelial, and intestinal epithelial cells. Based on the reductionist's approach, an understanding of NHE3 regulation has been greatly advanced. More recently, confirmations of in vitro studies have been made using animals deficient in one or more proteins but in some cases unexpected findings have emerged. The purpose of this paper is to provide a brief overview of recent progress in the regulation and functions of NHE3 present in the luminal membrane of the intestinal tract.

Directional cell migration and chemotaxis in wound healing response to PDGF-AA are coordinated by the primary cilium in fibroblasts

Cell motility and migration play pivotal roles in numerous physiological and pathophysiological processes including development and tissue repair. Cell migration is regulated through external stimuli such as platelet-derived growth factor-AA (PDGF-AA), a key regulator in directional cell migration during embryonic development and a chemoattractant during postnatal migratory responses including wound healing. We previously showed that PDGFRalpha signaling is coordinated by the primary cilium in quiescent cells. However, little is known about the function of the primary cilium in cell migration. Here we used micropipette analysis to show that a normal chemosensory response to PDGF-AA in fibroblasts requires the primary cilium. In vitro and in vivo wound healing assays revealed that in ORPK mouse (IFT88(Tg737Rpw)) fibroblasts, where ciliary assembly is defective, chemotaxis towards PDGF-AA is absent, leading to unregulated high speed and uncontrolled directional cell displacement during wound closure, with subsequent defects in wound healing. These data suggest that in coordination with cytoskeletal reorganization, the fibroblast primary cilium functions via ciliary PDGFRalpha signaling to monitor directional movement during wound healing.

Measurement of Intracellular pH

The activity of most cellular processes is sensitive to pH. Cells therefore tightly control cytosol pH within narrow bounds. Measurement of cytosolic pH is of interest in studying many processes, including pH regulatory transport proteins. Key approaches that have been used to determine intracellular pH include pH-sensitive microelectrodes, nuclear magnetic resonance, and pH-sensitive fluorescent proteins. Here we review these approaches while providing details on the use of pH-sensitive fluorescent dyes to measure cytosolic pH.

Sensors and regulators of intracellular pH

Protons dictate the charge and structure of macromolecules and are used as energy currency by eukaryotic cells. The unique function of individual organelles therefore depends on the establishment and stringent maintenance of a distinct pH. This, in turn, requires a means to sense the prevailing pH and to respond to deviations from the norm with effective mechanisms to transport, produce or consume proton equivalents. A dynamic, finely tuned balance between proton-extruding and proton-importing processes underlies pH homeostasis not only in the cytosol, but in other cellular compartments as well.

The sodium-hydrogen exchanger NHE1 is an Akt substrate necessary for actin filament reorganization by growth factors

The kinase Akt mediates signals from growth factor receptors for increased cell proliferation, survival, and migration, which contribute to the positive effects of Akt in cancer progression. Substrates are generally inhibited when phosphorylated by Akt; however, we show phosphorylation of the plasma membrane sodium-hydrogen exchanger NHE1 by Akt increases exchanger activity (H(+) efflux). Our data fulfill criteria for NHE1 being a bona fide Akt substrate, including direct phosphorylation in vitro, using mass spectrometry and Akt phospho-substrate antibodies to identify Ser(648) as the Akt phosphorylation site and loss of increased exchanger phosphorylation and activity by insulin and platelet-derived growth factor in fibroblasts expressing a mutant NHE1-S648A. How Akt induces actin cytoskeleton remodeling to promote cell migration and tumor cell metastasis is unclear, but disassembly of actin stress fibers by platelet-derived growth factor and insulin and increased proliferation in growth medium are inhibited in fibroblasts expressing NHE1-S648A. We predict that other functions shared by Akt and NHE1, including cell growth and survival, might be regulated by increased H(+) efflux.

The Na+/H+ exchanger NHE1 is required for directional migration stimulated via PDGFR-alpha in the primary cilium

We previously demonstrated that the primary cilium coordinates platelet-derived growth factor (PDGF) receptor (PDGFR) α–mediated migration in growth-arrested fibroblasts. In this study, we investigate the functional relationship between ciliary PDGFR-α and the Na⁺/H⁺ exchanger NHE1 in directional cell migration. NHE1 messenger RNA and protein levels are up-regulated in NIH3T3 cells and mouse embryonic fibroblasts (MEFs) during growth arrest, which is concomitant with cilium formation. NHE1 up-regulation is unaffected in Tg737^orpk MEFs, which have no or very short primary cilia. In growth-arrested NIH3T3 cells, NHE1 is activated by the specific PDGFR-α ligand PDGF-AA. In wound-healing assays on growth-arrested NIH3T3 cells and wild-type MEFs, NHE1 inhibition by 5′-(N-ethyl-N-isopropyl) amiloride potently reduces PDGF-AA–mediated directional migration. These effects are strongly attenuated in interphase NIH3T3 cells, which are devoid of primary cilia, and in Tg737^orpk MEFs. PDGF-AA failed to stimulate migration in NHE1-null fibroblasts. In conclusion, stimulation of directional migration in response to ciliary PDGFR-α signals is specifically dependent on NHE1 activity, indicating that NHE1 activation is a critical event in the physiological response to PDGFR-α stimulation.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Cofilin is a pH sensor for actin free barbed end formation: role of phosphoinositide binding

Newly generated actin free barbed ends at the front of motile cells provide sites for actin filament assembly driving membrane protrusion. Growth factors induce a rapid biphasic increase in actin free barbed ends, and we found both phases absent in fibroblasts lacking H(+) efflux by the Na-H exchanger NHE1. The first phase is restored by expression of mutant cofilin-H133A but not unphosphorylated cofilin-S3A. Constant pH molecular dynamics simulations and nuclear magnetic resonance (NMR) reveal pH-sensitive structural changes in the cofilin C-terminal filamentous actin binding site dependent on His133. However, cofilin-H133A retains pH-sensitive changes in NMR spectra and severing activity in vitro, which suggests that it has a more complex behavior in cells. Cofilin activity is inhibited by phosphoinositide binding, and we found that phosphoinositide binding is pH-dependent for wild-type cofilin, with decreased binding at a higher pH. In contrast, phosphoinositide binding by cofilin-H133A is attenuated and pH insensitive. These data suggest a molecular mechanism whereby cofilin acts as a pH sensor to mediate a pH-dependent actin filament dynamics.

Structural model and functional significance of pH-dependent talin-actin binding for focal adhesion remodeling

Actin filament binding by the focal adhesion (FA)-associated protein talin stabilizes cell-substrate adhesions and is thought to be rate-limiting in cell migration. Although F-actin binding by talin is known to be pH-sensitive in vitro, with lower affinity at higher pH, the functional significance of this pH dependence is unknown. Because increased intracellular pH (pH(i)) promotes cell migration and is a hallmark of metastatic carcinomas, we asked whether it increases FA remodeling through lower-affinity talin-actin binding. Talin contains several actin binding sites, but we found that only the COOH-terminal USH-I/LWEQ module showed pH-dependent actin binding, with lower affinity and decreased maximal binding at higher pH. Molecular dynamics simulations and NMR of this module revealed a structural mechanism for pH-dependent actin binding. A cluster of titratable amino acids with upshifted pK(a) values, including His-2418, was identified at one end of the five-helix bundle distal from the actin binding site. Protonation of His-2418 induces changes in the conformation and dynamics of the remote actin binding site. Structural analyses of a mutant talin-H2418F at pH 6.0 and 8.0 suggested changes different from the WT protein, and we confirmed that actin binding by talin-H2418F was relatively pH-insensitive. In motile fibroblasts, increasing pH(i) decreased FA lifetime and increased the migratory rate. However, expression of talin-H2418F increased lifetime 2-fold and decreased the migratory rate. These data identify a molecular mechanism for pH-sensitive actin binding by talin and suggest that FA turnover is pH-dependent and in part mediated by pH-dependent affinity of talin for binding actin.

Apoptosis-induced alkalinization by the Na+/H+ exchanger isoform 1 is mediated through phosphorylation of amino acids Ser726 and Ser729

Apoptosis is a complex process essential for normal tissue development and cellular homeostasis. While biochemical events that occur late in the apoptotic process are better characterized, early physiological changes that initiate the progression of cell death remain poorly understood. Previously, we observed that lymphocytes, undergoing apoptosis in response to growth factor withdrawal, experienced a rapid and transient rise in cytosolic pH. We found that the protein responsible was the pH-regulating, plasma membrane protein Na(+)/H(+) exchanger isoform 1 (NHE1), and that its activity was impeded by inhibition of the stress-activated kinase, p38 MAP kinase. In the current study, we examined how NHE1 is activated during apoptosis. We identified the phosphorylation sites on NHE1 that regulate its alkalinizing activity in response to a cell death stimulus. Performing targeted mutagenesis, we observed that substitution of Ser726 and Ser729 for alanines produced a mutant form of NHE1 that did not alkalinize in response to an apoptotic stimulus, and expression of which protected cells from serum withdrawal- induced death. In contrast, substitution of Ser726 and Ser729 for glutamic acids raised the basal pH and induced susceptibility to death. Analysis of serine phosphorylation showed that phosphorylation of NHE1 during apoptosis decreased upon mutation of Ser726 and Ser729. Our findings thus confirm a necessary function for NHE1 during apoptosis and reveal the critical regulatory sites that when phosphorylated mediate the alkalinizing activity of NHE1 in the early stages of a cell death response.

Nerve growth factor inhibits Na+/H+ exchange and formula absorption through parallel phosphatidylinositol 3-kinase-mTOR and ERK pathways in thick ascending limb

In the medullary thick ascending limb, inhibiting the basolateral NHE1 Na(+)/H(+) exchanger with nerve growth factor (NGF) induces actin cytoskeleton remodeling that secondarily inhibits apical NHE3 and transepithelial HCO(3)(-) absorption. The inhibition by NGF is mediated 50% through activation of extracellular signal-regulated kinase (ERK). Here we examined the signaling pathway responsible for the remainder of the NGF-induced inhibition. Inhibition of HCO(3)(-) absorption was reduced 45% by the phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin or LY294002 and 50% by rapamycin, a specific inhibitor of mammalian target of rapamycin (mTOR), a downstream effector of PI3K. The combination of a PI3K inhibitor plus rapamycin did not cause a further reduction in the inhibition by NGF. In contrast, the combination of a PI3K inhibitor plus the MEK/ERK inhibitor U0126 completely eliminated inhibition by NGF. Rapamycin decreased NGF-induced inhibition of basolateral NHE1 by 45%. NGF induced a 2-fold increase in phosphorylation of Akt, a PI3K target linked to mTOR activation, and a 2.2-fold increase in the activity of p70 S6 kinase, a downstream effector of mTOR. p70 S6 kinase activation was blocked by wortmannin and rapamycin, consistent with PI3K, mTOR, and p70 S6 kinase in a linear pathway. Rapamycin-sensitive inhibition of NHE1 by NGF was associated with an increased level of phosphorylated mTOR in the basolateral membrane domain. These findings indicate that NGF inhibits HCO(3)(-) absorption in the medullary thick ascending limb through the parallel activation of PI3K-mTOR and ERK signaling pathways, which converge to inhibit NHE1. The results identify a role for mTOR in the regulation of Na(+)/H(+) exchange activity and implicate NHE1 as a possible downstream effector contributing to mTOR's effects on cell growth, proliferation, survival, and tumorigenesis.

Calcineurin B homologous protein 3 promotes the biosynthetic maturation, cell surface stability, and optimal transport of the Na+/H+ exchanger NHE1 isoform

Calcineurin B homologous protein (CHP) 1 and 2 are Ca(2+)-binding proteins that modulate several cellular processes, including cytoplasmic pH by positively regulating plasma membrane-type Na(+)/H(+) exchangers (NHEs). Recently another CHP-related protein, termed tescalcin or CHP3, was also shown to interact with the ubiquitous NHE1 isoform, but seemingly suppressed its activity. However, the precise physical and functional nature of this association was not examined in detail. In this study, biochemical and cellular studies were undertaken to further delineate this relationship. Glutathione S-transferase-NHE1 fusion protein pulldown assays revealed that full-length CHP3 binds directly to the proximal juxtamembrane C-terminal region (amino acids 505-571) of rat NHE1 in the same region that binds CHP1 and CHP2. The interaction was further validated by coimmunoprecipitation and coimmunolocalization experiments using full-length CHP3 and wild-type NHE1 in transfected Chinese hamster ovary AP-1 cells. Simultaneous mutation of four hydrophobic residues within this region ((530)FLDHLL(535)) to either Ala, Gln, or Arg (FL-A, FL-Q, or FL-R) abrogated this interaction both in vitro and in intact cells. The NHE1 mutants were sorted properly to the cell surface but showed markedly reduced (FL-A) or minimal (FL-R and FL-Q) activity. Interestingly, and contrary to an earlier finding, ectopic coexpression of CHP3 up-regulated the cell surface activity of wild-type NHE1. This stimulation was not observed with the CHP3 binding-defective mutants. Mechanistically, overexpression of CHP3 did not alter the H(+) sensitivity of wild-type NHE1 but rather promoted its biosynthetic maturation and half-life at the cell surface, thereby increasing the steady-state abundance of functional NHE1 protein.

NHE-1: a molecular target for signalling and cell matrix interactions

The activation of sodium/hydrogen exchanger (NHE) is associated with a variety of cell functions like cell adhesion, migration, proliferation, and apoptosis. Since its discovery, 9 NHE isoforms have been identified, but the most widely spread and the most important for the cellular functions is NHE-1. This ubiquitously expressed sodium/hydrogen exchanger (NHE-1) plays a central housekeeping role in all cells regulating cell volume and internal pH (pHi). At physiological pHi, NHE-1 is essentially inactive but it is extremely sensitive to pHi changes, being rapidly activated by small intracellular hydrogen concentration increases. NHE-1 activity can be stimulated via a series of cell surface receptors, including tyrosine kinase, G-protein-coupled, and integrin receptors. These signals converge, regulating the affinity of the internal hydrogen-binding site. NHE-1 also is a plasma membrane-anchoring protein for the cytoskeleton. Cytoskeleton anchoring of NHE-1 is important for cell adhesion to extracellular matrix proteins and cell migration. Moreover, NHE-1 plays the role of a "scaffold" for the building of various intracellular signaling molecule clusters.

The primary cilium coordinates signaling pathways in cell cycle control and migration during development and tissue repair

Cell cycle control and migration are critical processes during development and maintenance of tissue functions. Recently, primary cilia were shown to take part in coordination of the signaling pathways that control these cellular processes in human health and disease. In this review, we present an overview of the function of primary cilia and the centrosome in the signaling pathways that regulate cell cycle control and migration with focus on ciliary signaling via platelet-derived growth factor receptor alpha (PDGFRalpha). We also consider how the primary cilium and the centrosome interact with the extracellular matrix, coordinate Wnt signaling, and modulate cytoskeletal changes that impinge on both cell cycle control and cell migration.

Positive feedback between Cdc42 activity and H+ efflux by the Na-H exchanger NHE1 for polarity of migrating cells

A fundamental feature of cell polarity in response to spatial cues is asymmetric amplification of molecules generated by positive feedback signaling. We report a positive feedback loop between the guanosine triphosphatase Cdc42, a central determinant in eukaryotic cell polarity, and H⁺ efflux by Na-H⁺ exchanger 1 (NHE1), which is necessary at the front of migrating cells for polarity and directional motility. In response to migratory cues, Cdc42 is not activated in fibroblasts expressing a mutant NHE1 that lacks H⁺ efflux, and wild-type NHE1 is not activated in fibroblasts expressing mutationally inactive Cdc42-N17. H⁺ efflux by NHE1 is not necessary for release of Cdc42–guanosine diphosphate (GDP) from Rho GDP dissociation inhibitor or for the membrane recruitment of Cdc42 but is required for GTP binding by Cdc42 catalyzed by a guanine nucleotide exchange factor (GEF). Data indicate that GEF binding to phosphotidylinositol 4,5–bisphosphate is pH dependent, suggesting a mechanism for how H⁺ efflux by NHE1 promotes Cdc42 activity to generate a positive feedback signal necessary for polarity in migrating cells.

Cell surface levels of organellar Na+/H+ exchanger isoform 6 are regulated by interaction with RACK1

In mammalian cells, four Na(+)/H(+) exchangers (NHE6 - NHE9) are localized to intracellular compartments. NHE6 and NHE9 are predominantly localized to sorting and recycling endosomes, NHE7 to the trans-Golgi network, and NHE8 to the mid-trans-Golgi stacks. The unique localization of NHEs may contribute to establishing organelle-specific pH values and ion homeostasis in cells. Mechanisms underlying the regulation and targeting of organellar NHEs are largely unknown. We identified an interaction between NHE9 and RACK1 (receptor for activated C kinase 1), a cytoplasmic scaffold protein, by yeast two-hybrid screening using the NHE9 C terminus as bait. The NHE9 C terminus is exposed to the cytoplasm, verifying that the interaction is topologically possible. The binding region was further delineated to the central region of the NHE9 C terminus. RACK1 also bound NHE6 and NHE7, but not NHE8, in vitro. Endogenous association between NHE6 and RACK1 was confirmed by co-immunoprecipitation and co-localization in HeLa cells. The luminal pH of the recycling endosome was elevated in RACK1 knockdown cells, accompanied by a decrease in the amount of NHE6 on the cell surface, although the total level of NHE6 was not significantly altered. These results indicate that RACK1 plays a role in regulating the distribution of NHE6 between endosomes and the plasma membrane and contributes to maintaining luminal pH of the endocytic recycling compartments.