Physiological functions of the regulatory domains of the cardiac Na(+)/Ca(2+) exchanger NCX1

Cardiac Na+/Ca2+ exchange stimulators among cardioprotective drugs

We previously reviewed our study of the pharmacological properties of cardiac Na+/Ca2+ exchange (NCX1) inhibitors among cardioprotective drugs, such as amiodarone, bepridil, dronedarone, cibenzoline, azimilide, aprindine, and benzyl-oxyphenyl derivatives (Watanabe et al. in J Pharmacol Sci 102:7-16, 2006). Since then we have continued our studies further and found that some cardioprotective drugs are NCX1 stimulators. Cardiac Na+/Ca2+ exchange current (INCX1) was stimulated by nicorandil (a hybrid ATP-sensitive K+ channel opener), pinacidil (a non-selective ATP-sensitive K+ channel opener), flecainide (an antiarrhythmic drug), and sodium nitroprusside (SNP) (an NO donor). Sildenafil (a phosphodiesterase-5 inhibitor) further increased the pinacidil-induced augmentation of INCX1. In paper, here I review the NCX stimulants that enhance NCX function among the cardioprotective agents we examined such as nicorandil, pinacidil, SNP, sildenafil and flecainide, in addition to atrial natriuretic (ANP) and dofetilide, which were reported by other investigators.

Nicorandil stimulates a Na⁺/Ca²⁺ exchanger by activating guanylate cyclase in guinea pig cardiac myocytes

Nicorandil, a hybrid of an ATP-sensitive K(+) (KATP) channel opener and a nitrate generator, is used clinically for the treatment of angina pectoris. This agent has been reported to exert antiarrhythmic actions by abolishing both triggered activity and spontaneous automaticity in an in vitro study. It is well known that delayed afterdepolarizations (DADs) are caused by the Na(+)/Ca(2+) exchange current (I NCX). In this study, we investigated the effect of nicorandil on the cardiac Na(+)/Ca(2+) exchanger (NCX1). We used the whole-cell patch clamp technique and the Fura-2/AM (Ca(2+) indicator) method to investigate the effect of nicorandil on I NCX in isolated guinea pig ventricular myocytes and CCL39 fibroblast cells transfected with dog heart NCX1. Nicorandil enhanced I NCX in a concentration-dependent manner. The EC50 (half-maximum concentration for enhancement of the drug) values were 15.0 and 8.7 μM for the outward and inward components of I NCX, respectively. 8-Bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP), a membrane-permeable analog of guanosine 3',5'-cyclic monophosphate (cGMP), enhanced I NCX. 1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a guanylate cyclase inhibitor (10 μM), completely abolished the nicorandil-induced I NCX increase. Nicorandil increased I NCX in CCL39 cells expressing wild-type NCX1 but did not affect mutant NCX1 without a long intracellular loop between transmembrane segments (TMSs) 5 and 6. Nicorandil at 100 μM abolished DADs induced by electrical stimulation with ouabain. Nicorandil enhanced the function of NCX1 via guanylate cyclase and thus may accelerate Ca(2+) exit via NCX1. This may partially contribute to the cardioprotection by nicorandil in addition to shortening action potential duration (APD) by activating KATP channels.

Insulin effects on cardiac Na+/Ca2+ exchanger activity: role of the cytoplasmic regulatory loop

Insulin can alter myocardial contractility, in part through an effect on the cardiac sarcolemmal Na(+)/Ca(2+) exchanger (NCX), but little is known about its mechanism of action. The large cytoplasmic domain (f-loop) of NCX is required for regulation by various intracellular factors, and we have shown previously that residues 562-679 are determinants of NCX inhibition by exchanger inhibitory peptide (XIP). Here we show that the same f-loop deletion eliminates the enhancement of NCX current by insulin, and we examine the signal pathways involved in the insulin response. NCX current (I(NCX)) was measured in freshly isolated or cultured (up to 48 h) adult guinea pig myocytes and in myocytes expressing canine NCX1.1 with the 562-679 f-loop deletion (NCX-(Delta562-679)) via adenoviral gene transfer. I(NCX) was recorded by whole-cell patch clamp as the Ni(2+)-sensitive current at 37 degrees C with intracellular Ca(2+) buffered. Insulin (1 microm) increased I(NCX) (at +80 mV) by 110 and 83% in fresh and cultured myocytes, respectively, whereas in myocytes expressing NCX-(Delta562-679) the response was eliminated (with 100 microm XIP included to suppress any native guinea pig I(NCX)). The insulin effect on I(NCX) was not inhibited by wortmannin, a nitric-oxide synthase inhibitor, or disruption of caveolae but was blocked by chelerythrine, implicating protein kinase C, but not phosphatidylinositol-3-kinase, in the mechanism. The insulin effect was also not additive with phosphatidylinositol-4,5-bisphosphate-induced activation of I(NCX). The finding that the 562-670 f-loop domain is implicated in both XIP and receptor-mediated modulation of NCX highlights its important role in acute physiological or pathophysiological regulation of Ca(2+) balance in the heart.

A comparison of adenovirally delivered molecular methods to inhibit Na+/Ca2+ exchange

The ability to use molecular biology tools to down-regulate Na+/Ca2+ exchanger (NCX) expression will allow us to better understand the regulation of Ca(i)2+ and contractility in heart. Three different techniques to deplete NCX expression were compared: short hairpin RNA (shRNA), antisense RNA and exchanger inhibitory peptide expression via adenoviral transfection. Our results demonstrate that the most efficient method to deplete NCX expression and activity from cardiomyocytes is shRNA. It is also possible to replace the endogenous NCX with alternative isoforms or mutant forms of the NCX. Adenovirally delivered shRNA is an efficient tool for the study of the NCX and could be adapted for many other cardiac proteins.

Down regulation of small intestinal ion transport in PDZK1- (CAP70/NHERF3) deficient mice

The PDZ-binding protein PDZK1 (CAP70/PDZ-dc-1/NHERF3) in vitro binds to cystic fibrosis transmembrane conductance regulator (CFTR), the anion exchangers SLC26A3 and SLC26A6 and the Na(+)/H(+) exchanger NHE3, all of which are major transport proteins for intestinal anion secretion and salt absorption. This study was undertaken to search for a role of PDZK1 in regulating electrolyte transport in native murine small intestine. Short circuit current (I (SC)) and HCO-(3) secretory rate (J(HCO-)(3)) were measured to assess electrogenic anion secretion; (22)Na(+) fluxes to assess sodium absorption in isolated small intestine. NHE3, CFTR, as well as NHERF1, NHERF2, and PDZK1 messenger RNA (mRNA) expression levels, and NHE3 total enterocyte and brush border membrane (BBM) protein abundance were determined by quantitative polymerase chain reaction (PCR) and Western analysis. NHE3 localization was performed by immunohistochemistry. In pdzk1 -/- jejunal mucosa, basal net Na(+) absorption as well as the inhibition of Na(+) absorption by forskolin was significantly reduced. In pdzk1 -/- duodenal mucosa, identical basal I (SC) and (J(HCO-)(3)) but a significant, yet mild, reduction of forskolin-stimulated Delta(J(HCO-)(3)) and DeltaI (SC) was observed compared to +/+ tissue. Tissue conductance, morphological features, and the DeltaI (SC) and increase in (22)Na(+) absorption in response to luminal glucose was identical in pdzk1 +/+ and -/- small intestine, ruling out a general absorptive defect. While CFTR mRNA expression levels were unchanged, NHE3 mRNA expression levels were significantly increased in small intestinal mucosa of pdzk1 -/- mice. Total enterocyte and BBM abundance was not significantly different, suggesting an increased NHE3 turnover, possibly due to reduced NHE3 membrane retention time. Lack of the PDZ-adapter protein PDZK1 in murine small intestine causes a mild reduction in maximal CFTR activation, but a severe defect in electroneutral Na(+) absorption.

Vascular Na+/Ca2+exchanger: implications for the pathogenesis and therapy of salt-dependent hypertension

The Na+/Ca2+exchanger is an ion transporter that exchanges Na+and Ca2+in either Ca2+efflux or Ca2+influx mode, depending on membrane potential and transmembrane ion gradients. In arterial smooth muscle cells, the Na+/Ca2+exchanger is thought to participate in the maintenance of vascular tone by regulating cytosolic Ca2+concentration. Recent pharmacological and genetic engineering studies have revealed that the Ca2+influx mode of vascular Na+/Ca2+exchanger type-1 (NCX1) is involved in the pathogenesis of salt-dependent hypertension. SEA0400, a specific Na+/Ca2+exchange inhibitor that preferentially blocks the Ca2+influx mode, lowers arterial blood pressure in salt-dependent hypertensive models, but not in normotensive rats or other types of hypertensive rats. Furthermore, heterozygous mice with reduced expression of NCX1 are resistant to development of salt-dependent hypertension, whereas transgenic mice with vascular smooth muscle-specific overexpression of NCX1 readily develop hypertension after high-salt loading. SEA0400 reverses the cytosolic Ca2+elevation and vasoconstriction induced by nanomolar ouabain, as well as humoral factors in salt-loaded animals. One possibility is that circulating endogenous cardiotonic steroids may be necessary for NCX1-mediated hypertension. These findings help to explain how arterial smooth muscle cells in blood vessels contribute to salt-elicited blood pressure elevation and suggest that NCX1 inhibitors might be therapeutically useful for salt-dependent hypertension.

Na+/H+ Exchanger Regulatory Factor Isoform 1 Overexpression Modulates Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Expression and Activity in Human Airway 16HBE14o- Cells and Rescues ΔF508 CFTR Functional Expression in Cystic Fibrosis Cells*

There is evidence that cystic fibrosis transmembrane conductance regulator (CFTR) interacting proteins play critical roles in the proper expression and function of CFTR. The Na(+)/H(+) exchanger regulatory factor isoform 1 (NHERF1) was the first identified CFTR-binding protein. Here we further clarify the role of NHERF1 in the regulation of CFTR activity in two human bronchial epithelial cell lines: the normal, 16HBE14o-, and the homozygous DeltaF508 CFTR, CFBE41o-. Confocal analysis in polarized cell monolayers demonstrated that NHERF1 distribution was associated with the apical membrane in 16HBE14o- cells while being primarily cytoplasmic in CFBE41o- cells. Transfection of 16HBE14o- monolayers with vectors encoding for wild-type (wt) NHERF1 increased both apical CFTR expression and apical protein kinase A (PKA)-dependent CFTR-mediated chloride efflux, whereas transfection with NHERF1 mutated in the binding groove of the PDZ domains or truncated for the ERM domain inhibited both the apical CFTR expression and the CFTR-dependent chloride efflux. These data led us to hypothesize an important role for NHERF1 in regulating CFTR localization and stability on the apical membrane of 16HBE14o- cell monolayers. Importantly, wt NHERF1 overexpression in confluent DeltaF508 CFBE41o- and DeltaF508 CFT1-C2 cell monolayers induced both a significant redistribution of CFTR from the cytoplasm to the apical membrane and a PKA-dependent activation of CFTR-dependent chloride secretion.

Indirect regulation of the intestinal H+-coupled amino acid transporter hPAT1 (SLC36A1)

A H(+)-coupled amino acid transporter has been characterised functionally at the brush border membrane of the human intestinal cell line Caco-2. This carrier, hPAT1 (human Proton-coupled Amino acid Transporter 1) or SLC36A1, has been identified recently at the molecular level and hPAT1 protein is localised to the brush border membrane of human small intestine. hPAT1 transports both amino acids (e.g., beta-alanine) and therapeutic agents (e.g., D-cycloserine). In human Caco-2 cells, hPAT1 function (H(+)/amino acid symport) is associated with a decrease in intracellular pH (pH(i)), which selectively activates the Na(+)/H(+) exchanger NHE3, and thus maintains pH(i) and the driving force for hPAT1 function (the H(+) electrochemical gradient). This study provides the first evidence for regulation of hPAT1 function. Activation of the cAMP/protein kinase A pathway in Caco-2 cell monolayers either using pharmacological tools (forskolin, 8-br-cAMP, [(11,22,28)Ala]VIP) or physiological activators (the neuropeptides VIP and PACAP) inhibited hPAT1 function (beta-alanine uptake) at the apical membrane. Under conditions where NHE3 is inactive (the absence of Na(+), apical pH 5.5, the presence of the NHE3 inhibitor S1611) no regulation of beta-alanine uptake is observed. Forskolin and VIP inhibit pH(i) recovery (NHE3 function) from beta-alanine-induced intracellular acidification. Immunocytochemistry localises NHERF1 (NHE3 regulatory factor 1) to the apical portion of Caco-2 cells where it will interact with NHE3 and allow PKA-mediated phosphorylation of NHE3. In conclusion, we have shown that amino acid uptake via hPAT1 is inhibited by activators of the cAMP pathway indirectly through inhibition of NHE3 activity.

Sodium–calcium exchange inhibitors: therapeutic potential in cardiovascular diseases

Intracellular calcium ions (Ca2+) are the key regulators in cardiac and arterial functions during the contraction–relaxation cycle. Myocyte Ca2+ imbalance thus produces mechanical dysfunction, electrical instability (arrhythmia) and muscle remodeling. The sodium–calcium exchanger (NCX) is one of the major Ca2+-handling proteins in myocytes. Evidence is currently accumulating to suggest that NCX1 is upregulated in various cardiovascular diseases. Recently developed benzyloxyphenyl NCX inhibitors effectively prevent myocardial ischemia/reperfusion injury and salt-sensitive hypertension in animal models. Furthermore, several experiments with genetically engineered mice provide compelling evidence that these diseases are triggered by pathologic Ca2+ entry through NCX1 in cardiac and arterial myocytes, respectively. Thus, NCX inhibitors may have therapeutic potential as novel cardiovascular drugs for myocardial reperfusion injury and salt-sensitive hypertension. However, the efficacy of NCX inhibitors, as well as the role of NCX1, in heart failure or arrhythmias requires more detailed study.

Identification of an endogenous inhibitor of the cardiac Na+/Ca2+ exchanger, phospholemman

Rapid and precise control of Na(+)/Ca(2+) exchanger (NCX1) activity is essential in the maintenance of beat-to-beat Ca(2+) homeostasis in cardiac myocytes. Here, we show that phospholemman (PLM), a 15-kDa integral sarcolemmal phosphoprotein, is a novel endogenous protein inhibitor of cardiac NCX1. Using a heterologous expression system that is devoid of both endogenous PLM and NCX1, we first demonstrated by confocal immunofluorescence studies that both exogenous PLM and NCX1 co-localized at the plasma membrane. Reciprocal co-immunoprecipitation studies revealed specific protein-protein interaction between PLM and NCX1. The functional consequences of direct association of PLM with NCX1 was the inhibition of NCX1 activity, as demonstrated by whole-cell patch clamp studies to measure NCX1 current density and radiotracer flux assays to assess Na(+)-dependent (45)Ca(2+) uptake. Inhibition of NCX1 by PLM was specific, because a single mutation of serine 68 to alanine in PLM resulted in a complete loss of inhibition of NCX1 current, although association of the PLM mutant with NCX1 was unaltered. In native adult cardiac myocytes, PLM co-immunoprecipitated with NCX1. We conclude that PLM, a member of the FXYD family of small ion transport regulators known to modulate Na(+)-K(+)-ATPase, also regulates Na(+)/Ca(2+) exchange in the heart.

NHERF and regulation of the renal sodium-hydrogen exchanger NHE3

The sodium-hydrogen exchanger 3 (NHE3) isoform is the major regulated sodium transporter in the proximal convoluted tubule of the kidney. Study of the regulation of NHE3 by hormonal stimuli has identified a number of PDZ adaptor proteins that form an apical/subapical membrane scaffold that binds NHE3 and facilitates down-regulation of its activity in response to cAMP and activation of protein kinase A. The precise relation of proximal tubule adaptor proteins such as sodium-hydrogen exchanger regulatory factor-1 (NHERF-1), NHERF-2, and PDZ domain-containing-protein-1 (PDZK1) with each other and with protein targets such as NHE3 has been evolving with the development of specific reagents and genetically altered animals. In this review, we trace the discovery of NHERF-1 and NHERF-2, and update our current understanding of the relation between these proteins and the regulation and trafficking of NHE3.

Calcineurin inhibits Na+/Ca2+ exchange in phenylephrine-treated hypertrophic cardiomyocytes

The cardiac Na(+)/Ca(2+) exchanger (NCX1) is the predominant mechanism for the extrusion of Ca(2+) from beating cardiomyocytes. The role of protein phosphorylation in the regulation of NCX1 function in normal and diseased hearts remains unclear. In our search for proteins that interact with NCX1 using a yeast two-hybrid screen, we found that the C terminus of calcineurin Abeta, containing the autoinhibitory domain, binds to the beta1 repeat of the central cytoplasmic loop of NCX1 that presumably constitutes part of the allosteric Ca(2+) regulatory site. The association of NCX1 with calcineurin was significantly increased in the BIO14.6 cardiomyopathic hamster heart compared with that in the normal control. In hypertrophic neonatal rat cardiomyocytes subjected to chronic phenylephrine treatment, we observed a marked depression of NCX activity measured as the rate of Na(+)(i)-dependent (45)Ca(2+) uptake or the rate of Na(+)(o)-dependent (45)Ca(2+) efflux. Depressed NCX activity was partially and independently reversed by the acute inhibition of calcineurin and protein kinase C activities with little effect on myocyte hypertrophic phenotypes. Studies of NCX1 deletion mutants expressed in CCL39 cells were consistent with the view that the beta1 repeat is required for the action of endogenous calcineurin and that the large cytoplasmic loop may be required to maintain the interaction of the enzyme with its substrate. Our data suggest that NCX1 is a novel regulatory target for calcineurin and that depressed NCX activity might contribute to the etiology of in vivo cardiac hypertrophy and dysfunction occurring under conditions in which both calcineurin and protein kinase C are chronically activated.

Defective Parathyroid Hormone Regulation of NHE3 Activity and Phosphate Adaptation in Cultured NHERF-1-/- Renal Proximal Tubule Cells

The present experiments using primary cultures of renal proximal tubule cells derived from wild-type and NHERF-1 knockout animals examines the regulation of NHE3 by phenylthiohydantoin (PTH) and the regulation of phosphate transport in response to alterations in the media content of phosphate. Forskolin (34.8 +/- 6.2%) and PTH (29.7 +/- 1.8%) inhibited NHE3 activity in wild-type proximal tubule cells but neither forskolin (-3.2 +/- 3.3%) nor PTH (-16.6 +/- 8.1%) inhibited NHE3 activity in NHERF-1(-/-) cells. Using adenovirus-mediated gene transfer, expression of NHERF-1 in NHERF-1(-/-) proximal tubule cells restored the inhibitory response to forskolin (28.2 +/- 3.0%) and PTH (33.2 +/- 3.9%). Compared with high phosphate media, incubation of wild-type cells in low phosphate media resulted in a 36.0 +/- 6.3% higher rate of sodium-dependent phosphate transport and a significant increase in the abundance of Npt2a and PDZK1. NHERF-1(-/-) cells, on the other hand, had lower rates of sodium-dependent phosphate uptake and low phosphate media did not stimulate phosphate transport. Npt2a expression was not affected by the phosphate content of the media in NHERF-1 null cells although low phosphate media up-regulated PDZK1 abundance. Primary cultures of mice proximal tubule cells retain selected regulatory pathways observed in intact kidneys. NHERF-1(-/-) proximal tubule cells demonstrate defective regulation of NHE3 by PTH and indicate that reintroduction of NHERF-1 repairs this defect. NHERF-1(-/-) cells also do not adapt to alterations in the phosphate content of the media indicating that the defect resides within the cells of the proximal tubule and is not dependent on systemic factors.

κ Opioid Receptor Interacts with Na+/H+-exchanger Regulatory Factor-1/Ezrin-Radixin-Moesin-binding Phosphoprotein-50 (NHERF-1/EBP50) to Stimulate Na+/H+ Exchange Independent of Gi/Go Proteins

We previously showed that Na(+)/H(+)-exchanger regulatory factor-1/Ezrin-radixin-moesin-binding phosphoprotein-50 (NHERF-1/EBP50) co-immunoprecipitated with the human kappa opioid receptor (hKOR) and that its overexpression blocked the kappa agonist U50,488H-induced hKOR down-regulation by enhancing recycling. Here, we show that glutathione S-transferase (GST)-hKOR C-tail interacted with purified NHERF-1/EBP50, whereas GST or GST-C-tails of micro or delta opioid receptors did not. GST-hKOR C-tail, but not GST, bound HA-NHERF-1/EBP50 transfected into Chinese hamster ovary cells and endogenous NHERF-1/EBP50 in opossum kidney proximal tubule epithelial cells (OK cells). The PDZ domain I, but not II, of NHERF-1/EBP50 was involved in the interaction. Association of NHERF-1/EBP50 with hKOR C-tail enhanced oligomerization of NHERF-1/EBP50. NHERF-1/EBP50 was previously shown to regulate Na(+)/H(+)-exchanger 3 (NHE3) activities in OK cells. We found stimulation of OK cells with U50,488H significantly enhanced Na(+)/H(+) exchange, which was blocked by naloxone but not by pertussis toxin pretreatment, indicating it is mediated by KORs but independent of G(i)/G(o) proteins. In OKH cells, a subclone of OK cells expressing a much lower level of NHERF-1/EBP50, U50,488H had no effect on Na(+)/H(+) exchange, although it enhanced p44/42 mitogen-activated protein kinase phosphorylation via G(i)/G(o) proteins similar to that in OK cells. Stable transfection of NHERF-1/EBP50 into OKH cells restored the stimulatory effect of U50,488H upon Na(+)/H(+) exchange. Thus, NHERF-1/EBP50 binds directly to KOR, and this association plays an important role in accelerating Na(+)/H(+) exchange. We hypothesize that binding of the KOR to NHERF-1/EBP50 facilitates oligomerization of NHERF-1/EBP50, leading to stimulation of NHE3. This study provides the first direct evidence that a G protein-coupled receptor through association with NHERF-1/EBP-50 stimulates NHE3.

The NHE1 Na+/H+ exchanger recruits ezrin/radixin/moesin proteins to regulate Akt-dependent cell survival

Apoptosis results in cell shrinkage and intracellular acidification, processes opposed by the ubiquitously expressed NHE1 Na(+)/H(+) exchanger. In addition to mediating Na(+)/H(+) transport, NHE1 interacts with ezrin/radixin/moesin (ERM), which tethers NHE1 to cortical actin cytoskeleton to regulate cell shape, adhesion, motility, and resistance to apoptosis. We hypothesize that apoptotic stress activates NHE1-dependent Na(+)/H(+) exchange, and NHE1-ERM interaction is required for cell survival signaling. Apoptotic stimuli induced NHE1-regulated Na(+)/H(+) transport, as demonstrated by ethyl-N-isopropyl-amiloride-inhibitable, intracellular alkalinization. Ectopic NHE1, but not NHE3, expression rescued NHE1-null cells from apoptosis induced by staurosporine or N-ethylmaleimide-stimulated KCl efflux. When cells were subjected to apoptotic stress, NHE1 and phosphorylated ERM physically associated within the cytoskeleton-enriched fraction, resulting in activation of the pro-survival kinase, Akt. NHE1-associated Akt activity and cell survival were inhibited in cells expressing ERM binding-deficient NHE1, dominant negative ezrin constructs, or ezrin mutants with defective binding to phosphoinositide 3-kinase, an upstream regulator of Akt. We conclude that NHE1 promotes cell survival by dual mechanisms: by defending cell volume and pH(i) through Na(+)/H(+) exchange and by functioning as a scaffold for recruitment of a signalplex that includes ERM, phosphoinositide 3-kinase, and Akt.

Molecular determinants of Na+/Ca2+ exchange (NCX1) inhibition by SEA0400

SEA0400 is a potent and selective Na(+)/Ca(2+) exchanger (NCX) inhibitor. We evaluated the inhibitory effects of SEA0400 on Na(+)(i)-dependent (45)Ca(2+) uptake and whole-cell Na(+)/Ca(2+) exchange currents in NCX-transfected fibroblasts. SEA0400 preferentially inhibited (45)Ca(2+) uptake by NCX1 compared with inhibitions by NCX2, NCX3, and NCKX2. SEA0400 also selectively blocked outward exchange currents from NCX1 transfectants. We searched for regions that may form the SEA0400 receptor in the NCX1 molecule by NCX1/NCX3 chimeric analysis. The results suggest that the first intracellular loop and the fifth transmembrane segment are mostly responsible for the differential drug responses between NCX1 and NCX3. Further site-directed mutagenesis revealed that multiple mutations at Phe-213 markedly reduced sensitivity to SEA0400 without affecting that to KB-R7943. We also found that Gly-833-to-Cys mutation (within the alpha-2 repeat) greatly reduced the inhibition by SEA0400, but unexpectedly the NCX1 chimera with an alpha-2 repeat from NCKX2 possessed normal drug sensitivity. In addition, exchangers with mutated exchanger inhibitory peptide regions, which display either undetectable or accelerated Na(+)-dependent inactivation, had a markedly reduced sensitivity or hypersensitivity to SEA0400, respectively. To verify the efficacy of the NCX inhibitor, we examined the renoprotective effect of SEA0400 in a hypoxic injury model using porcine renal tubular cells. SEA0400 protected against hypoxia/reoxygenation-induced cell damage in tubular cells expressing wild-type NCX1 but not in cells expressing SEA0400-insensitive mutants. These results suggest that Phe-213, Gly-833, and residues that eliminate Na(+)-dependent inactivation are critical determinants for the inhibition by SEA0400, and their mutants are very useful for checking the pharmacological importance of NCX inhibition by SEA0400.

NHERF-1 uniquely transduces the cAMP signals that inhibit sodium-hydrogen exchange in mouse renal apical membranes

Sodium-hydrogen exchanger regulatory factor isoform-1 (NHERF-1) and NHERF-2 are two structurally related PDZ-domain-containing protein adapters that effectively transduce cyclic AMP (cAMP) signals that inhibit NHE3, the sodium-hydrogen exchanger isoform present at the apical surface of kidney and gut epithelia. The mouse renal proximal tubule expresses both NHERF isoforms, suggesting their redundant functions as regulators of renal electrolyte metabolism. To define the role of NHERF-1 in the physiological control of NHE3, we analyzed NHE3 activity in isolated brush border membrane (BBM) preparations from renal proximal tubules of wild-type (WT) and NHERF-1 (-/-) mice. Basal Na(+)-H(+) exchange was indistinguishable in BBMs from WT and NHERF-1 (-/-) mice (0.96+/-0.08 and 0.95+/-0.10 nmol/mg protein/10 s, respectively). Activation of membrane bound cAMP-dependent protein kinase (PKA) by cAMP inhibited NHE3 activity in WT BBMs (0.55+/-0.07 nmol/mg protein/10 s or 40+/-9%, P<0.01) but had no discernible effect on Na(+)-H(+) exchange in the NHERF-1 (-/-) BBM (0.97+/-0.07 nmol/mg protein/10 s; P=not significant). This was associated with a significant decrease in cAMP-stimulated phosphorylation of NHE3 immunoprecipitated from solubilized NHERF-1 (-/-) BBMs. As the protein levels for NHE3, NHERF-2, PKA and ezrin were not changed in the NHERF-1 (-/-) BBMs, the data suggest a unique role for NHERF-1 in cAMP-mediated inhibition of NHE3 activity in the renal proximal tubule of the mouse.

Regulation of GTP-binding protein alpha q (Galpha q) signaling by the ezrin-radixin-moesin-binding phosphoprotein-50 (EBP50)

Although ezrin-radixin-moesin-binding phosphoprotein 50 (EBP50) is a PDZ domain-containing protein known to bind to various channels, receptors, cytoskeletal elements, and cytoplasmic proteins, there is still very little evidence for a role of EBP50 in the regulation of receptor signal transduction. In this report, we show that EBP50 inhibits the phospholipase C (PLC)-beta-mediated inositol phosphate production of a Galpha(q)-coupled receptor as well as PLC-beta activation by the constitutively active Galpha(q)-R183C mutant. Coimmunoprecipitation experiments revealed that EBP50 interacts with Galpha(q) and to a greater extent with Galpha(q)-R183C. Agonist stimulation of the thromboxane A(2) receptor (TP receptor) resulted in an increased interaction between EBP50 and Galpha(q), suggesting that EBP50 preferentially interacts with activated Galpha(q). We also demonstrate that EBP50 inhibits Galpha(q) signaling by preventing the interaction between Galpha(q) and the TP receptor and between activated Galpha(q) and PLC-beta1. Investigation of the EBP50 regions involved in Galpha(q) binding indicated that its two PDZ domains are responsible for this interaction. This study constitutes the first demonstration of an interaction between a G protein alpha subunit and another protein through a PDZ domain, with broad implications in the regulation of diverse physiological systems.

Regulation of G protein-coupled receptor signaling by scaffold proteins

The actions of many hormones and neurotransmitters are mediated through stimulation of G protein-coupled receptors. A primary mechanism by which these receptors exert effects inside the cell is by association with heterotrimeric G proteins, which can activate a wide variety of cellular enzymes and ion channels. G protein-coupled receptors can also interact with a number of cytoplasmic scaffold proteins, which can link the receptors to various signaling intermediates and intracellular effectors. The multicomponent nature of G protein-coupled receptor signaling pathways makes them ideally suited for regulation by scaffold proteins. This review focuses on several specific examples of G protein-coupled receptor-associated scaffolds and the roles they may play in organizing receptor-initiated signaling pathways in the cardiovascular system and other tissues.

The molecular determinants of ionic regulatory differences between brain and kidney Na+/Ca2+ exchanger (NCX1) isoforms

The Na(+)/Ca(2+) exchanger gene NCX1 undergoes alternative splicing leading to several isoforms that differ in a small portion of the large cytoplasmic loop. This loop is involved in many regulatory processes of NCX1, including ionic regulation by the transported substrates Na(+) and Ca(2+). High intracellular Ca(2+) can alleviate intracellular Na(+)-dependent inactivation in exon A (NCX1.4)-containing isoforms but not in those containing the mutually exclusive exon B (NCX1.3). Giant excised patches from Xenopus oocytes expressing various NCX1 constructs were used to examine the specific amino acids responsible for these observed regulatory differences. Using a chimeric approach, the region responsible was narrowed down to the small central part of exon A (IDDEEYEKNKTF). Replacing the second aspartic acid of this sequence with arginine (the corresponding amino acid in exon B) in an exon A background completely prevented the effect of Ca(2+) on intracellular Na(+)-dependent inactivation. Mutating the second lysine to cysteine (exon B) had a similar, but only partial, effect. The converse double mutant, but neither single mutation alone, introduced into an exon B background (arginine to aspartic acid and cysteine to lysine) was able to restore the NCX1.4 regulatory phenotype. These data demonstrate that aspartic acid 610 and lysine 617 (using the rat NCX1.4 numbering scheme) are critical molecular determinants of the unique Ca(2+) regulatory properties of NCX1.4.

Epithelial inducible nitric-oxide synthase is an apical EBP50-binding protein that directs vectorial nitric oxide output

Nitric oxide (NO), produced via inducible NO synthase (iNOS), can modulate polarized epithelial processes such as solute transport. Given the high reactivity of NO, we hypothesized that optimal NO regulation of polarized epithelial functions is achieved through compartmentalization of iNOS, allowing local NO delivery to its molecular targets. Here, we show that iNOS localizes to the apical domain of epithelial cells within a submembranous protein complex tightly bound to cortical actin. We further show that iNOS can bind to the apical PDZ protein, EBP50 (ezrin-radixin-moesin-binding phosphoprotein 50), an interaction that is dependent on the last three COOH-terminal amino acids of iNOS, SAL, but requires the presence of additional unknown cellular proteins. Mutation of these three COOH-terminal residues abolishes the iNOS-EBP50 interaction and disrupts the apical association of iNOS in transfected cells, showing that this COOH-terminal motif is essential for the correct localization of iNOS in epithelial cells. Apically localized iNOS directs vectorial NO production at the apical proximal tubule epithelial cell surface. These studies define human epithelial iNOS as an apical EBP50-binding protein and suggest that the physical association of iNOS with EBP50 might allow precise NO modulation of EBP50-associated protein functions.

Cystic fibrosis transmembrane conductance regulator-dependent up-regulation of Kir1.1 (ROMK) renal K+ channels by the epithelial sodium channel

The epithelial sodium channel (ENaC) and the secretory potassium channel (Kir1.1/ROMK) are expressed in the apical membrane of renal collecting duct principal cells where they provide the rate-limiting steps for Na(+) absorption and K(+) secretion. The cystic fibrosis transmembrane conductance regulator (CFTR) is thought to regulate the function of both ENaC and Kir1.1. We hypothesized that CFTR may provide a regulatory link between ENaC and Kir1.1. In Xenopus laevis oocytes co-expressing both ENaC and CFTR, the CFTR currents were 3-fold larger than those in oocytes expressing CFTR alone due to an increased expression of CFTR in the plasma membrane. ENaC was also able to increase Kir1.1 currents through an increase in surface expression, but only in the presence of CFTR. In the absence of CFTR, co-expression of ENaC was without effect on Kir1.1. ENaC-mediated CFTR-dependent up-regulation of Kir1.1 was reduced with a Liddle's syndrome mutant of ENaC. Furthermore, ENaC co-expressed with CFTR was without effect on the closely related K(+) channel, Kir4.1. We conclude that ENaC up-regulates Kir1.1 in a CFTR-dependent manner. CFTR may therefore provide the mechanistic link that mediates the coordinated up-regulation of Kir1.1 during the stimulation of ENaC by hormones such as aldosterone or antidiuretic hormone.

Inhibitory effect of aprindine on Na+/Ca2+ exchange current in guinea-pig cardiac ventricular myocytes

1. Using the whole-cell voltage clamp technique, the effect of aprindine on Na+/Ca2+ exchange current (I(NCX)) was examined in guinea-pig single cardiac ventricular myocytes and CCL39 fibroblasts expressing a dog cardiac Na+/Ca2+ exchanger (NCX1). 2. I(NCX) was recorded by ramp pulses from the holding potential of -60 mV with the external solution containing 140 mM Na+ and 1 mM Ca2+, and the pipette solution containing 20 mM Na+, 20 mM BAPTA and 13 mM Ca2+ (433 nM free Ca2+). 3. External application of aprindine suppressed I(NCX) in a concentration-dependent manner. The IC50 values of outward (measured at 50 mV) and inward (measured at -100 mV) I(NCX) components were 48.8 and 51.8 microM with Hill coefficients of 1.3 and 1, respectively. 4. Intracellular application of trypsin via the pipette solution did not change the blocking effect of aprindine, suggesting that aprindine does not affect the exchanger from the cytoplasmic side. 5. Aprindine inhibited I(NCX) of a mutant NCX1 with a deletion of amino acids 247 - 671 in the large intracellular domain between the transmembrane segments 5 and 6 in a similar manner to that of the wild-type, suggesting that the site of aprindine inhibition is not in the large intracellular domain of NCX1. 6. A kinetic study indicated that aprindine was cooperatively competitive with KB-R7943, another inhibitor of NCX and that aprindine was a competitive inhibitor with respect to external Ca2+. 7. We conclude that aprindine may modestly inhibit I(NCX) in a therapeutic range of concentrations (around 2.5 approximately 6.9 microM) possibly at an external or intra-membranous site of the exchanger.

Role of the cytoskeleton in mediating cAMP-dependent protein kinase inhibition of the epithelial Na+/H+ exchanger NHE3

The Na(+)/H(+) exchanger NHE3 isoform mediates the entry of Na(+) into epithelial cells of the kidney and gastrointestinal tract. Hormones and pharmacological agents that activate cAMP-dependent protein kinase A (PKA) are potent inhibitors of native and ectopically expressed NHE3 in epithelial and Chinese hamster ovary AP-1 cells, respectively. Previous studies have shown that acute inhibition is coupled to direct phosphorylation of the exchanger, but this only partly accounts for the observed effects. In this report, we show that inhibition of NHE3 activity by forskolin, an activator of adenylate cyclase, occurs without changes in surface expression of the exchanger but is associated with altered cytoskeletal structure. This effect resembles that obtained with cytochalasin D or latrunculin B, actin disrupting agents that also inhibit NHE3. Such similarities prompted us to further investigate the relationship between PKA-induced inhibition of the exchanger and changes in the actin cytoskeleton. Inhibition of NHE3 by cytochalasin D does not require PKA, because the inhibitory effect is preserved in a mutant NHE3 that is not phosphorylated by PKA and in cells pretreated with the PKA inhibitor H89. In contrast, involvement of actin in the effect of cAMP on the exchanger is supported by the following observations: (i) jasplakinolide, an F-actin stabilizer, prevents the inhibition caused by forskolin, and (ii) constitutively active forms of RhoA and Rho kinase interfere with actin disruption by forskolin and also decrease inhibition of the transporter. These results suggest that reorganization of the cytoskeleton by PKA is involved in mediating inhibition of NHE3.

Regulation of the sodium transporters NHE3, NKCC2 and NCC in the kidney

The regulation of sodium transport in the kidney is important for maintenance of extracellular fluid volume and arterial blood-pressure regulation. The major sodium transporters and channels in individual renal tubule segments have been identified via physiological techniques, and complementary DNAs for all of the key sodium transporters and channels expressed along the renal tubule have been cloned. Complementary DNA probes and antibodies are now being used to investigate the molecular basis of renal tubule sodium-transport regulation. This review summarizes some of the major observations made in the past year that are relevant to the regulation of the major sodium transporters in the proximal tubule (the type 3 sodium-hydrogen exchanger, NHE3), the thick ascending limb of Henle (the bumetanide-sensitive sodium-potassium-chloride cotransporter, NKCC2), and the distal convoluted tubule (the thiazide-sensitive sodium-chloride cotransporter, NCC).

Molecular aspects in the regulation of renal inorganic phosphate reabsorption: the type IIa sodium/inorganic phosphate co-transporter as the key player

The type IIa sodium/inorganic phosphate co-transporter is the rate-limiting inorganic phosphate transport pathway in renal brush-border membranes, and is thus a key player in overall inorganic phosphate homeostasis. Its regulation is mostly associated with membrane retrieval/reinsertion (traffic) of the transport protein. This membrane traffic is controlled by specific 'motifs' at the level of the transporter protein and probably involves interacting proteins (e.g. for scaffolding, regulation or sorting). The intracellular signaling mechanisms (e.g. the involvement of kinases) and the involvement of the cytoskeleton are not yet understood. Hereditary alterations in renal inorganic phosphate handling can be associated with factors controlling the expression of the brush-border type IIa sodium/inorganic phosphate co-transporter.

Cardiac Na + -Ca 2+ Exchange

Abstract —The Na + -Ca 2+ exchanger (NCX) is one of the essential regulators of Ca 2+ homeostasis in cardiomyocytes and thus an important modulator of the cardiac contractile function. The purpose of this review is to survey recent advances in cardiac NCX research, with particular emphasis on molecular and pharmacological aspects. The NCX function is thought to be regulated by a variety of cellular factors. However, data obtained by use of different experimental systems often appear to be in conflict. Where possible, we endeavor to provide a rational interpretation of such data. We also provide a summary of current work relating to the structure and function of the cardiac NCX. Recent molecular studies of the NCX protein are beginning to shed light on structural features of the ion translocation pathway in the NCX membrane domain, which seems likely to be formed, at least partly, by the phylogenetically conserved α-1 and α-2 repeat structures and their neighboring membrane-spanning segments. Finally, we discuss new classes of NCX inhibitors with improved selectivity. One of these, 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate (KB-R7943), appears to exhibit unique selectivity for Ca 2+ -influx–mode NCX activity. Data obtained with these inhibitors should provide a basis for designing more selective and clinically useful drugs targeting NCX.

Interaction of the type IIa Na/Pi cotransporter with PDZ proteins

The type IIa Na(+)-dependent inorganic phosphate (Na/P(i)) cotransporter is localized in the apical membrane of proximal tubular cells and is regulated by an endocytotic pathway. Because molecular processes such as apical sorting, internalization, or subsequent degradation might be assisted by associated proteins, a yeast two-hybrid screen against the C-terminal, cytosolic tail of type IIa cotransporter was designed. Most of the potential proteins found belonged to proteins with multiple PDZ modules and were either identical/related to PDZK1 or identical to NHERF-1. Yeast trap truncation assays confined the peptide-protein association to the C-terminal amino acid residues TRL of type IIa cotransporter and to single PDZ domains of each identified protein, respectively. The specificity of these interactions were confirmed in yeast by testing other apical localized transmembraneous proteins. Moreover, the type IIa protein was recovered in vitro by glutathione S-transferase-fused PDZ proteins from isolated renal brush border membranes or from type IIa-expressing oocytes. Further, these PDZ proteins are immunohistochemically detected either in the microvilli or in the subapical compartment of proximal tubular cells. Our results suggest that the type IIa Na/P(i) cotransporter interacts with various PDZ proteins that might be responsible for the apical sorting, parathyroid hormone controlled endocytosis or the lysosomal sorting of internalized type IIa cotransporter.

Inhibition of sodium-calcium exchange by ceramide and sphingosine

Na(+)/Ca(2+) exchange activity in Chinese hamster ovary cells expressing the bovine cardiac Na(+)/Ca(2+) exchanger was inhibited by the short chain ceramide analogs N-acetylsphingosine and N-hexanoylsphingosine (5-15 micrometer). The sphingolipids reduced exchange-mediated Ba(2+) influx by 50-70% and also inhibited the Ca(2+) efflux mode of exchange activity. The biologically inactive ceramide analog N-acetylsphinganine had only modest effects on exchange activity. Cells expressing the Delta(241-680) and Delta(680-685) deletion mutants of the Na(+)/Ca(2+) exchanger were not inhibited by ceramide; these mutants show defects in both Na(+)-dependent and Ca(2+)-dependent regulatory behavior. Another mutant, which was defective only in Na(+)-dependent regulation, was as sensitive to ceramide inhibition as the wild-type exchanger. Inhibition of exchange activity by ceramide was time-dependent and was accelerated by depletion of internal Ca(2+) stores. Sphingosine (2.5 micrometer) also inhibited the Ca(2+) influx and efflux modes of exchange activity in cells expressing the wild-type exchanger; sphingosine did not affect Ba(2+) influx in the Delta(241-680) mutant. The effects of the exogenous sphingolipids were reproduced by blocking cellular ceramide utilization pathways, suggesting that exchange activity is inhibited by increased levels of endogenous ceramide and/or sphingosine. We propose that sphingolipids impair Ca(2+)-dependent activation of the exchanger and that in cardiac myocytes, this process serves as a feedback mechanism that links exchange activity to the diastolic concentration of cytosolic Ca(2+).