Na+/H+ exchanger 1 is regulated via its lipid-interacting domain, which functions as a molecular switch: a pharmacological approach using indolocarbazole compounds

Polyunsaturated ω3 fatty acids prevent the cardiac hypertrophy in hypertensive rats

It has been demonstrated that supplementation with the two main omega 3 polyunsaturated fatty acids (ω3 FAs), docosahexaenoic acid (DHA), and eicosapentaenoic acid (EPA), leads to modifications in the cardiac physiology. ω3 FAs can affect the membrane's lipid composition, as well as proteins' location and/or function. The Na⁺/H⁺ exchanger (NHE1) is an integral membrane protein involved in the maintenance of intracellular pH and its hyperactivity has been associated with the development of various cardiovascular diseases such as cardiac hypertrophy. Our aim was to determine the effect of ω3 FAs on systolic blood pressure (SBP), lipid profiles, NHE1 activity, and cardiac function in spontaneously hypertensive rats (SHR) using Wistar rats (W) as normotensive control. After weaning, the rats received orally ω3 FAs (200 mg/kg body mass/day/ 4 months). We measured SBP, lipid profiles, and different echocardiography parameters, which were used to calculate cardiac hypertrophy index, systolic function, and ventricular geometry. The rats were sacrificed, and ventricular cardiomyocytes were obtained to measure NHE1 activity. While the treatment with ω3 FAs did not affect the SBP, lipid analysis of plasma revealed a significant decrease in omega-6/omega-3 ratio, correlated with a significant reduction in left ventricular mass index in SHR. The NHE1 activity was significantly higher in SHR compared with W. While in W the NHE1 activity was similar in both groups, a significant decrease in NHE1 activity was detected in SHRs supplemented with ω3 FAs, reaching values comparable with W. Altogether, these findings revealed that diet supplementation with ω3 FAs since early age prevents the development of cardiac hypertrophy in SHR, perhaps by decreasing NHE1 activity, without altering hemodynamic overload.

The HVCN1 voltage-gated proton channel contributes to pH regulation in canine ventricular myocytes

KEY POINTS

Intracellular pH (pH_i ) regulation is crucial for cardiac function, as acidification depresses contractility and causes arrhythmias. H⁺ ions are generated in cardiomyocytes from metabolic processes and particularly from CO₂ hydration, which has been shown to facilitate CO₂ -venting from mitochondria. Currently, the NHE1 Na⁺ /H⁺ exchanger is viewed as the dominant H⁺ -extrusion mechanism in cardiac muscle. We show that the HVCN1 voltage-gated proton channel is present and functional in canine ventricular myocytes, and that HVCN1 and NHE1 both contribute to pH_i regulation. HVCN1 provides an energetically-efficient mechanism of H⁺ -extrusion that would not cause Na⁺ -loading, which can cause pathology, and that could contribute to transport-mediated CO₂ disposal. These results provide a major advance in our understanding of pH_i regulation in cardiac muscle.

ABSTRACT

Regulation of intracellular pH (pH_i ) in cardiomyocytes is crucial for cardiac function; however, currently known mechanisms for direct or indirect extrusion of acid from cardiomyocytes seem insufficient for energetically-efficient extrusion of the massive H⁺ loads generated under in vivo conditions. In cardiomyocytes, voltage-sensitive H⁺ channel activity mediated by the HVCN1 proton channel would be a highly efficient means of disposing of H⁺ , while avoiding Na⁺ -loading, as occurs during direct acid extrusion via Na⁺ /H⁺ exchange or indirect acid extrusion via Na⁺ -HCO₃ ^- cotransport. PCR and immunoblotting demonstrated expression of HVCN1 mRNA and protein in canine heart. Patch clamp analysis of canine ventricular myocytes revealed a voltage-gated H⁺ current that was highly H⁺ -selective. The current was blocked by external Zn²⁺ and the HVCN1 blocker 5-chloro-2-guanidinobenzimidazole (ClGBI). Both the gating and Zn²⁺ blockade of the current were strongly influenced by the pH gradient across the membrane. All characteristics of the observed current were consistent with the known hallmarks of HVCN1-mediated H⁺ current. Inhibition of HVCN1 and the NHE1 Na⁺ /H⁺ exchanger, singly and in combination, showed that either mechanism is largely sufficient to maintain pH_i in beating cardiomyocytes, but that inhibition of both activities causes rapid acidification. These results show that HVCN1 is expressed in canine ventricular myocytes and provides a major H⁺ -extrusion activity, with a capacity similar to that of NHE1. In the beating heart in vivo, this activity would allow Na⁺ -independent extrusion of H⁺ during each action potential and, when functionally coupled with anion transport mechanisms, could facilitate transport-mediated CO₂ disposal. Abstract figure legend The HVCN1 proton channel is expressed in canine ventricular myocytes and contributes to H⁺ extrusion. This article is protected by copyright. All rights reserved.

Dynamic Na+/H+ exchanger 1 (NHE1) - calmodulin complexes of varying stoichiometry and structure regulate Ca2+-dependent NHE1 activation

Calmodulin (CaM) engages in Ca²⁺-dependent interactions with numerous proteins, including a still incompletely understood physical and functional interaction with the human Na⁺/H⁺-exchanger NHE1. Using nuclear magnetic resonance (NMR) spectroscopy, isothermal titration calorimetry, and fibroblasts stably expressing wildtype and mutant NHE1, we discovered multiple accessible states of this functionally important complex existing in different NHE1:CaM stoichiometries and structures. We determined the NMR solution structure of a ternary complex in which CaM links two NHE1 cytosolic tails. In vitro, stoichiometries and affinities could be tuned by variations in NHE1:CaM ratio and calcium ([Ca²⁺]) and by phosphorylation of S648 in the first CaM-binding α-helix. In cells, Ca²⁺-CaM-induced NHE1 activity was reduced by mimicking S648 phosphorylation and by mutation of the first CaM-binding α-helix, whereas it was unaffected by inhibition of Akt, one of several kinases phosphorylating S648. Our results demonstrate a diversity of NHE1:CaM interaction modes and suggest that CaM may contribute to NHE1 dimerization and thereby augment NHE1 regulation. We propose that a similar structural diversity is of relevance to many other CaM complexes.

Structure and elevator mechanism of the mammalian sodium/proton exchanger NHE9

Na+ /H+ exchangers (NHEs) are ancient membrane-bound nanomachines that work to regulate intracellular pH, sodium levels and cell volume. NHE activities contribute to the control of the cell cycle, cell proliferation, cell migration and vesicle trafficking. NHE dysfunction has been linked to many diseases, and they are targets of pharmaceutical drugs. Despite their fundamental importance to cell homeostasis and human physiology, structural information for the mammalian NHE was lacking. Here, we report the cryogenic electron microscopy structure of NHE isoform 9 (SLC9A9) from Equus caballus at 3.2 Å resolution, an endosomal isoform highly expressed in the brain and associated with autism spectrum (ASD) and attention deficit hyperactivity (ADHD) disorders. Despite low sequence identity, the NHE9 architecture and ion-binding site are remarkably similar to distantly related bacterial Na+ /H+  antiporters with 13 transmembrane segments. Collectively, we reveal the conserved architecture of the NHE ion-binding site, their elevator-like structural transitions, the functional implications of autism disease mutations and the role of phosphoinositide lipids to promote homodimerization that, together, have important physiological ramifications.

The intracellular lipid-binding domain of human Na+/H+ exchanger 1 forms a lipid-protein co-structure essential for activity

Dynamic interactions of proteins with lipid membranes are essential regulatory events in biology, but remain rudimentarily understood and particularly overlooked in membrane proteins. The ubiquitously expressed membrane protein Na+/H+-exchanger 1 (NHE1) regulates intracellular pH (pHi) with dysregulation linked to e.g. cancer and cardiovascular diseases. NHE1 has a long, regulatory cytosolic domain carrying a membrane-proximal region described as a lipid-interacting domain (LID), yet, the LID structure and underlying molecular mechanisms are unknown. Here we decompose these, combining structural and biophysical methods, molecular dynamics simulations, cellular biotinylation- and immunofluorescence analysis and exchanger activity assays. We find that the NHE1-LID is intrinsically disordered and, in presence of membrane mimetics, forms a helical αα-hairpin co-structure with the membrane, anchoring the regulatory domain vis-a-vis the transport domain. This co-structure is fundamental for NHE1 activity, as its disintegration reduced steady-state pHi and the rate of pHi recovery after acid loading. We propose that regulatory lipid-protein co-structures may play equally important roles in other membrane proteins.

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.

Yeast recombinant production of intact human membrane proteins with long intrinsically disordered intracellular regions for structural studies

Membrane proteins exist in lipid bilayers and mediate solute transport, signal transduction, cell-cell communication and energy conversion. Their activities are fundamental for life, which make them prominent subjects of study, but access to only a limited number of high-resolution structures complicates their mechanistic understanding. The absence of such structures relates mainly to difficulties in expressing and purifying high quality membrane protein samples in large quantities. An additional layer of complexity stems from the presence of intra- and/or extra-cellular domains constituted by unstructured intrinsically disordered regions (IDR), which can be hundreds of residues long. Although IDRs form key interaction hubs that facilitate biological processes, these are regularly removed to enable structural studies. To advance mechanistic insight into intact intrinsically disordered membrane proteins, we have developed a protocol for their purification. Using engineered yeast cells for optimized expression and purification, we have purified to homogeneity two very different human membrane proteins each with >300 residues long IDRs; the sodium proton exchanger 1 and the growth hormone receptor. Subsequent to their purification we have further explored their incorporation into membrane scaffolding protein nanodiscs, which will enable future structural studies.

Overexpression of Na+/H+ exchanger 1 specifically induces cell death in human iPS cells via sustained activation of the Rho kinase ROCK

Understanding the specific properties of human induced pluripotent stem cells (iPSCs) is important for quality control of iPSCs. Having incidentally discovered that overexpression of plasma membrane Na⁺/H⁺ exchanger 1 (NHE1) induces cell death in iPSCs, we investigated the mechanism of NHE1-induced cell death. Doxycycline-induced NHE1 overexpression arrested cell growth, and nearly all cells were killed by a necrotic process within 72 h. NHE1 overexpression led to sustained activation of Rho-associated coiled-coil kinase (ROCK), accompanied by dramatic changes in cell shape, cell elongation, and swelling of peripheral cells in iPSC colonies, as well as marked stress fiber formation. The ROCK inhibitor Y27632 reduced NHE1-induced cell death. ROCK-dependent phenotypes were suppressed by a loss-of-function mutation of NHE1 and inhibited by an inhibitor of NHE1 activity, indicating that NHE1-mediated transport activity is required. Moreover, ROCK was activated by trimethylamine treatment-mediated cytosolic alkalinization and accumulated in the plasma membrane near NHE1 in peripheral iPSCs of cell colonies. By contrast, cell death did not occur in mesendoderm-like cells that had differentiated from iPSCs, indicating that the NHE1-mediated effects were specific for iPSCs. These results suggest that NHE1 overexpression specifically induces death of iPSCs via sustained ROCK activation, probably caused by an increase in local pH near NHE1. Finally, monensin, a Na⁺/H⁺ exchange ionophore, selectively killed iPSCs, suggesting that monensin could help eliminate iPSCs that remain after differentiation, a strategy that might be useful for improving regenerative medicine.

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.

Structural and Functional Changes in the Na+/H+ Exchanger Isoform 1, Induced by Erk1/2 Phosphorylation

The human Na⁺/H⁺ exchanger isoform 1 (NHE1) is a plasma membrane transport protein that plays an important role in pH regulation in mammalian cells. Because of the generation of protons by intermediary metabolism as well as the negative membrane potential, protons accumulate within the cytosol. Extracellular signal-regulated kinase (ERK)-mediated regulation of NHE1 is important in several human pathologies including in the myocardium in heart disease, as well as in breast cancer as a trigger for growth and metastasis. NHE1 has a N-terminal, a 500 amino acid membrane domain, and a C-terminal 315 amino acid cytosolic domain. The C-terminal domain regulates the membrane domain and its effects on transport are modified by protein binding and phosphorylation. Here, we discuss the physiological regulation of NHE1 by ERK, with an emphasis on the critical effects on structure and function. ERK binds directly to the cytosolic domain at specific binding domains. ERK also phosphorylates NHE1 directly at multiple sites, which enhance NHE1 activity with subsequent downstream physiological effects. The NHE1 cytosolic regulatory tail possesses both ordered and disordered regions, and the disordered regions are stabilized by ERK-mediated phosphorylation at a phosphorylation motif. Overall, ERK pathway mediated phosphorylation modulates the NHE1 tail, and affects the activity, structure, and function of this membrane protein.

Roles of pH in control of cell proliferation

Precise spatiotemporal regulation of intracellular pH (pH_i ) is a prerequisite for normal cell function, and changes in pH_i or pericellular pH (pH_e ) exert important signalling functions. It is well established that proliferation of mammalian cells is dependent on a permissive pH_i in the slightly alkaline range (7.0-7.2). It is also clear that mitogen signalling in nominal absence of HCO3- is associated with an intracellular alkalinization (~0.3 pH unit above steady-state pH_i ), which is secondary to activation of Na⁺ /H⁺ exchange. However, it remains controversial whether this increase in pH_i is part of the mitogenic signal cascade leading to cell cycle entry and progression, and whether it is relevant under physiological conditions. Furthermore, essentially all studies of pH_i in mammalian cell proliferation have focused on the mitogen-induced G0-G1 transition, and the regulation and roles of pH_i during the cell cycle remain poorly understood. The aim of this review is to summarize and critically discuss the possible roles of pH_i and pH_e in cell cycle progression. While the focus is on the mammalian cell cycle, important insights from studies in lower eukaryotes are also discussed. We summarize current evidence of links between cell cycle progression and pH_i and discuss possible pH_i - and pH_e sensors and signalling pathways relevant to mammalian proliferation control. The possibility that changes in pH_i during cell cycle progression may be an integral part of the checkpoint control machinery is explored. Finally, we discuss the relevance of links between pH and proliferation in the context of the perturbed pH homoeostasis and acidic microenvironment of solid tumours.

The effects of angiotensin-(1-7) on the exchanger NHE3 and on [Ca2+]i in the proximal tubules of spontaneously hypertensive rats

The acute effects of angiotensin-1-7 [ANG-(1-7)] on the reabsorptive bicarbonate flow (J[Formula: see text]) were evaluated using stationary microperfusion in vivo in the proximal tubules of spontaneously hypertensive rats (SHR) and their normotensive controls, Wistar-Kyoto (WKY) rats, using a microelectrode sensitive to H⁺ In WKY rats, the control J[Formula: see text] was 2.40 ± 0.10 nmol·cm^-2·s^-1 (n = 120); losartan (10^-7 M) or A779 (10^-6 M, a specific Mas antagonist), alone or in combination with losartan, decreased the J[Formula: see text] ANG-(1-7) had biphasic effects on J[Formula: see text]: at 10^-9 M, it inhibited, and at 10^-6, it stimulated the flow. S3226 [10^-6 M, a specific Na⁺-H⁺ exchanger 3 (NHE3) antagonist] decreased J[Formula: see text] and changed the stimulatory effect of ANG-(1-7) to an inhibitory one but did not alter the inhibitory action of ANG-(1-7). In SHR, the control J[Formula: see text] was 2.04 ± 0.13 nmol·cm^-2·s^-1 (n = 56), and A779 and/or losartan reduced the flow. ANG-(1-7) at 10^-9 M increased J[Formula: see text], and ANG-(1-7) at 10^-6 M reduced it. The effects of A779, losartan, and S3226 on the J[Formula: see text] were similar to those found in WKY rats, which indicated that in SHR, the ANG-(1-7) action on the NHE3 was via Mas and ANG II type 1. The cytosolic calcium in the WKY or SHR rats was ~100 nM and was increased by ANG-(1-7) at 10^-9 or 10^-6 M. In hypertensive animals, a high plasma level of ANG-(1-7) inhibited NHE3 in the proximal tubule, which mitigated the hypertension caused by the high plasma level of ANG II.

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.

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

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

Na(+)/H(+) antiporter (NHE1) and lactate/H(+) symporters (MCTs) in pH homeostasis and cancer metabolism

The Na(+)/H(+)-exchanger NHE1 and the monocarboxylate transporters MCT1 and MCT4 are crucial for intracellular pH regulation, particularly under active metabolism. NHE1, a reversible antiporter, uses the energy provided by the Na(+) gradient to expel H(+) ions generated in the cytosol. The reversible H(+)/lactate(-) symporters MCT1 and 4 cotransport lactate and proton, leading to the net extrusion of lactic acid in glycolytic tumors. In the first two sections of this article we review important features and remaining questions on the structure, biochemical function and cellular roles of these transporters. We then use a fully-coupled mathematical model to simulate their relative contribution to pH regulation in response to lactate production, as it occurs in highly hypoxic and glycolytic tumor cells. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

Isoform-specific phosphorylation-dependent regulation of connexin hemichannels

Connexins form gap junction channels made up of two connexons (hemichannels) from adjacent cells. Unopposed hemichannels may open toward the extracellular space upon stimulation by, e.g., removal of divalent cations from the extracellular solution and allow isoform-specific transmembrane flux of fluorescent dyes and physiologically relevant molecules, such as ATP and ions. Connexin (Cx)43 and Cx30 are the major astrocytic connexins. Protein kinase C (PKC) regulates Cx43 in its cell-cell gap junction configuration and may also act to keep Cx43 hemichannels closed. In contrast, the regulation of Cx30 hemichannels by PKC is unexplored. To determine phosphorylation-dependent regulation of Cx30 and Cx43 hemichannels, these were heterologously expressed in Xenopus laevis oocytes and opened with divalent cation-free solution. Inhibition of PKC activity did not affect hemichannel opening of either connexin. PKC activation had no effect on Cx43-mediated hemichannel activity, whereas both dye uptake and current through Cx30 hemichannels were reduced. We detected no PKC-induced connexin internalization from the plasma membrane, indicating that PKC reduced Cx30 hemichannel activity by channel closure. In an attempt to resolve the PKC phosphorylation site(s) on Cx30, alanine mutations of putative cytoplasmic PKC consensus sites were created to prevent phosphorylation (T5A, T8A, T102A, S222A, S225A, S239A, and S258A). These Cx30 mutants responded to PKC activation, suggesting that Cx30 hemichannels are not regulated by phosphorylation of a single site. In conclusion, Cx30, but not Cx43, hemichannels close upon PKC activation, illustrating that connexin hemichannels display not only isoform-specific permeability profiles but also isoform-specific regulation by PKC.

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.

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.