Effects of Cyclic Tensile Forces on the Expression of Vascular Endothelial Growth Factor (VEGF) and Macrophage-colony-stimulating Factor (M-CSF) in Murine Osteoblastic MC3T3-E1 Cells

It has been reported that vascular endothelial growth factor (VEGF), expressed by osteoblasts, can induce osteoclast recruitment and thus affects bone remodeling. The purpose of this study was to investigate the effects of cyclic tensile forces on the expression of VEGF and macrophage-colony-stimulating factor (M-CSF) in osteoblastic MC3T3-E1 cells. VEGF and M-CSF gene expression and protein concentration were determined by real-time PCR and enzyme-linked immunoassay. The expression of VEGF and M-CSF mRNA in the experimental group was higher than in the control group. The increase in the concentration of VEGF and M-CSF protein in the experimental group was time-dependent. Moreover, gadolinium (an S-A channel inhibitor), but not nifedipine (L-Type Ca2+ channel blocker), treatment reduced the concentration of VEGF and M-CSF mRNA and protein in the experimental groups. These findings suggest that cyclic tensile forces increase the expression of VEGF and M-CSF in osteoblastic MC3T3-E1 cells via a stretch-activated channel (S-A channel).

Amyloid beta protein deposition in osteopetrotic (op/op) mice is reduced by injections of macrophage colony stimulating factor

The deposition of amyloid beta (Abeta) protein is a neuropathological change that characterizes Alzheimer's disease. Animals with the osteopetrosis (op/op) mutation suffer from a general skeletal sclerosis, a significantly reduced number of macrophages and osteoclasts in various tissues, and have no systemic macrophage colony stimulating factor (M-CSF). This study examined the effect that M-CSF injections had on Abeta deposition and microglial cell distribution in the brains of normal and op/op mice. Abeta-positive plaques were detected in the cerebral cortex of op/op mice, but not in normal mice. M-CSF reduced the numbers of Abeta-positive plaques in op/op mice. The microglial cell population was reduced in op/op mice compared with normal mice, and M-CSF increased the numbers to 65.8% of that observed in normal mice. Our results suggest that a clearer understanding of the role that microglial cells play in Abeta deposition may help determine the mechanisms involved in the pathogenesis of Alzheimer's disease.

Influence of sex hormone disturbances on the internal structure of the mandible in newborn mice

It has not yet been clarified how sex hormones affect craniofacial bone development immediately after birth. The purpose of this study was to examine the effects of sex hormone deficiency on craniofacial bone development immediately after birth, in terms of the internal structure of the mandible in newborn mice with orchiectomy (ORX) and ovariectomy (OVX). ORX, OVX and a sham-operation were performed on 40 five-day-old C57BL/6J mice. Eight weeks after surgery, each mandible was subjected to histomorphometric analysis of trabecular (Tr) and cortical (Ct) bone mineral density (BMD) by peripheral quantitative computed tomography (pQCT). In the experimental groups, a significant reduction in BMD was found in comparison with the control groups. In histomorphometric analysis, the number of tartrate-resistant acid phosphatase (TRAP)-positive cells in the condyle and the thickness of the condylar cartilage layer was significantly greater in the experimental mice than in the controls. Trabecular bone volume of the condyle measured on azocarmine-aniline blue (AZAN) sections was significantly less in the experimental mice than in the controls. These results indicate that mandibular growth is inhibited by sex hormone disturbances and the relevant internal structures changed. The findings show that sex hormones are one of the key determinants of mandibular growth and development immediately after birth.

Effects of sex hormone disturbances on craniofacial growth in newborn mice

It is well-known that sex hormones influence bone metabolism. However, it remains unclear as to how sex hormones affect bone growth in newborn mice. In this study, we performed orchiectomy (ORX) and ovariectomy (OVX) on newborn mice, and examined the effects on craniofacial growth morphometrically. ORX and OVX were performed on five-day-old C57BL/6J mice. Four weeks after surgery, lateral cephalograms were taken of all of the mice, with the use of a rat and mouse cephalometer. Cephalometric analysis of the craniofacial skeleton was performed by means of a personal computer. Inhibition of craniofacial growth was found in the experimental groups but not in the sham-operated groups. In the nasomaxillary bone and mandible, the amount of growth was significantly reduced. These results suggest that craniofacial growth is inhibited by sex hormone disturbances not only in puberty but also immediately after birth.

NHE6 protein possesses a signal peptide destined for endoplasmic reticulum membrane and localizes in secretory organelles of the cell

The NHE6 protein is a unique Na(+)/H(+) exchanger isoform believed to localize in mitochondria. It possesses a hydrophilic N-terminal portion that is rich in positively charged residues and many hydrophobic segments. In the present study, signal sequences in the NHE6 molecule were examined for organelle localization and membrane topogenesis. When the full-length protein was expressed in COS7 cells, it localized in the endoplasmic reticulum and on the cell surface. Furthermore, the protein was fully N-glycosylated. When green fluorescent protein was fused after the second (H2) or third (H3) hydrophobic segment, the fusion proteins were targeted to the endoplasmic reticulum (ER) membrane. The localization pattern was the same as that of fusion proteins in which green fluorescent protein was fused after H2 of NHE1. In an in vitro system, H1 behaved as a signal peptide that directs the translocation of the following polypeptide chain and is then processed off. The next hydrophobic segment (H2) halted translocation and eventually became a transmembrane segment. The N-terminal hydrophobic segment (H1) of NHE1 also behaved as a signal peptide. Cell fractionation studies using antibodies against the 15 C-terminal residues indicated that NHE6 protein localized in the microsomal membranes of rat liver cells. All of the NHE6 molecules in liver tissue possess an endoglycosidase H-resistant sugar chain. These findings indicate that NHE6 protein is targeted to the ER membrane via the N-terminal signal peptide and is sorted to organelle membranes derived from the ER membrane.

A mechanism of Ca2+ release from Ca2+ stores coupling to the Na+/Ca2+ exchanger in cultured smooth muscle cells

We previously observed Ca2+ release from intracellular Ca2+ stores caused by reduction in extracellular Na+ concentration ([Na+]o). The purpose of this study was to determine whether lowering [Na+]o can elicit Ca2+ release from Ca2+ stores via the Na+/Ca2+ exchanger and to elucidate the mechanisms related to the Ca2+ release pathway in cultured longitudinal smooth muscle cells obtained from guinea pig ileum. Low [Na+]o-induced Ca2+ release was inhibited by antisense oligodeoxynucleotides for Na+/Ca2+ exchanger type 1 (anti-NCX). Application of anti-NCX to cells attenuated both the number of Ca2+ responding cells and the expression of the exchanger. Moreover, microinjection of heparin, a blocker of inositol 1,4,5-trisphosphate (IP3) receptors, into the cells inhibited low [Na+]o-induced Ca2+ release. These findings suggest that low [Na+]o-induced Ca2+ release occurs through an IP3-induced Ca2+ release mechanism due to changes in the Ca2+ flux regulated by the Na+/Ca2+ exchanger.

Stretch-activated cation channels in skeletal muscle myotubes from sarcoglycan-deficient hamsters

Deficiency of delta-sarcoglycan (delta-SG), a component of the dystrophin-glycoprotein complex, causes cardiomyopathy and skeletal muscle dystrophy in Bio14.6 hamsters. Using cultured myotubes prepared from skeletal muscle of normal and Bio14.6 hamsters (J2N-k strain), we investigated the possibility that the delta-SG deficiency may lead to alterations in ionic conductances, which may ultimately lead to myocyte damage. In cell-attached patches (with Ba(2+) as the charge carrier), an approximately 20-pS channel was observed in both control and Bio14.6 myotubes. This channel is also permeable to K(+) and Na(+) but not to Cl(-). Channel activity was increased by pressure-induced stretch and was reduced by GdCl(3) (>5 microM). The basal open probability of this channel was fourfold higher in Bio14.6 myotubes, with longer open and shorter closed times. This was mimicked by depolymerization of the actin cytoskeleton. In intact Bio14.6 myotubes, the unidirectional basal Ca(2+) influx was enhanced compared with control. This Ca(2+) influx was sensitive to GdCl(3), signifying that stretch-activated cation channels may have been responsible for Ca(2+) influx in Bio14.6 hamster myotubes. These results suggest a possible mechanism by which cell damage might occur in this animal model of muscular dystrophy.

[Pharmaceutical agents for calcium-induced cell damage (Na+/Ca2+ exchange inhibitor)]

Recently, a novel selective inhibitor of the Ca(2+) influx mode of the Na(+)/Ca(2+) exchanger, KB-R7943, has been developed. This compound is expected to be a pharmaceutical agent that offers effective protection against ischemia/reperfusion-associated injury in several organs such as heart and kidney. Here, we summarize pharmacological profiles of KB-R7943, the molecular mechanism of its action, and its future prospect as a novel pharmaceutical agent.

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.

Stretch-induced cell damage in sarcoglycan-deficient myotubes

Sarcoglycans (SGs) are components of the dystrophin-glycoprotein complex, genetic defects in which cause skeletal muscle dystrophy and cardiomyopathy in humans and animals. To obtain insight into the roles of SGs, we characterized properties of myotubes prepared from cells of the rat L6 line or primary myoblast cultures of rat gastrocnemius muscle that were made SG-deficient by treatment with antisense oligodeoxynucleotides (AS-ODNs). Immunoblot and immunoprecipitation analyses revealed that dystrophin and its remaining associated proteins were tightly associated in these cells despite SG deficiency. 45Ca2+ influx into SG AS-ODN-treated L6 myotubes under resting conditions was significantly higher (1.7-fold at 6 min) than in controls, suggesting that Ca2+ influx is activated in these SG-deficient myotubes. When these cells were subjected to cyclic elongation of up to 20% for 1 h, a marked increase in creatine phosphokinase (CK) release into the medium was observed. Nifedipine, tranilast, FK506 and E64 or intracellular loading with 1,2-bis(2-aminophenoxy)ethane- N,N,N',N'-tetraacetic acid, tetrakis(acetoxymethyl)ester (BAPTA/AM) reduced the stretch-induced CK release; a raised extracellular [Ca2+] increased CK release. The stretch-induced damage to SG-deficient myotubes thus appears to be caused by alterations in cell Ca2+ homeostasis. A similar abnormality in Ca2+ handling has been reported for myoctes from mdx mice or dystrophin-deficient patients, in whom SGs are also greatly reduced or absent. Thus it is possible that SG deficiency may play a critical role in the pathology of dystrophin-deficient muscle.

Structural domains influencing sensitivity to isothiourea derivative inhibitor KB-R7943 in cardiac Nna(+)/Ca(2+) exchanger

KB-R7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate) is a potent and selective Na(+)/Ca(2+) exchange (NCX) inhibitor that is 3-fold more inhibitory to NCX3 than to NCX1 or NCX2. Here we searched for amino acid residues that may form the KB-R7943 receptor in the exchanger by analyzing the function of chimeras between NCX1 and NCX3 as well as of their site-directed mutants. We found that the highly conserved alpha-2 repeat of the exchanger is almost exclusively responsible for the difference in drug response of the isoforms. Such difference was mostly reproduced by single substitutions of residues in the alpha-2 repeat (V820G or Q826V in NCX1 and A809V or A809I in NCX3), suggesting their importance in drug sensitivity. Cysteine scanning mutagenesis of the alpha-2 repeat of NCX1 identified one residue (Gly833) that caused a large (> or = 30-fold) reduction in drug sensitivity. We found that the Gly-to-Thr substitution caused even larger reduction in drug sensitivity. Interestingly, extracellularly applied KB-R7943 at 0.8 microM markedly inhibited the whole-cell outward exchange current, whereas the drug applied intracellularly at 30 microM did not. These results suggest that KB-R7943 inhibits the exchanger from the external side in intact cells and that a region of the alpha-2 repeat of NCX1 containing Gly833 may participate in the formation of the drug receptor. Because we suggested previously that Gly833 is accessible from the inside of a cell, the results raised an interesting possibility that this residue may alter its position during Na(+)/Ca(2+) exchange in such a way that it becomes accessible to external drug.

Calcineurin homologous protein as an essential cofactor for Na+/H+ exchangers

The Na+/H+ exchangers (NHEs) comprise a family of transporters that catalyze cell functions such as regulation of the pH and volume of a cell and epithelial absorption of Na+ and bicarbonate. Ubiquitous calcineurin B homologous protein (CHP or p22) is co-localized and co-immunoprecipitated with expressed NHE1, NHE2, or NHE3 independently of its myristoylation and Ca2+ binding, and its binding site was identified as the juxtamembrane region within the carboxyl-terminal cytoplasmic domain of exchangers. CHP binding-defective mutations of NHE1-3 or CHP depletion by injection of the competitive CHP-binding region of NHE1 into Xenopus oocytes resulted in a dramatic reduction (>90%) in the Na+/H+ exchange activity. The data suggest that CHP serves as an essential cofactor, which supports the physiological activity of NHE family members.

Second mutations rescue point mutant of the Na(+)/H(+) exchanger NHE1 showing defective surface expression

We studied the effect of point mutation within the putative 11th transmembrane domain (TM11) of the Na(+)/H(+) exchanger NHE1 on the plasma membrane expression. Of the 19 mutants tested, two mutants (Tyr454 or Arg458 replaced by Cys) were retained in the endoplasmic reticulum. Interestingly, Y454C was expressed on the cell surface when one of the endogenous cysteine residues at position 8, 133, 421, or 477 was substituted with alanine. Random mutagenesis at Cys8 and its surrounding residues in the cytosolic N-tail revealed that replacement of Cys8 with Ala was the only identified single residue mutation that rescued Y454C. These results suggest that the abnormal conformation of the region of TM11 containing the Y454C mutation is compensated by the second mutation within other domains such as the N-tail. This approach may provide evidence for the interdomain interaction in NHE1.

The Na+/Ca2+ exchanger NCX1 has oppositely oriented reentrant loop domains that contain conserved aspartic acids whose mutation alters its apparent Ca2+ affinity

We examined the membrane topology and functional importance of residues in regions of the Na(+)/Ca(2+) exchanger NCX1 encompassing the conserved internal alpha repeats by substituted cysteine scanning analysis and kinetic analysis of site-directed mutants. The results suggest that both the alpha-1 repeat and a region encompassing the alpha-2 repeat and its immediately C-terminal segment contain reentrant loop domains, each oriented in an opposite direction with respect to the membrane. We found that single or multiple mutations of six residues including Asn-125 and conserved aspartates Asp-130, Asp-825, and Asp-829 in the alpha repeat reentrant domains reduce the apparent affinity of the exchanger for extracellular Ca(2+) by up to 6-fold. In contrast, the triple cysteine mutation D130C/D825C/D829C did not influence the current-voltage (I-V) relationship of the exchange current. Cysteine accessibility scanning with different thiol modifiers suggested that N125C, D130C, and D825C may be located in a restricted aqueous space in the membrane accessible only to ions when examined with external probes, although N125C and D825C were previously shown to be internally accessible during exchange reaction. The results suggest that these reentrant domains in the alpha repeats may participate in the formation of the ion transport pathway in the exchanger with some of the aspartates possibly lining it or located close to it.

Targeted disruption of Na+/Ca2+ exchanger gene leads to cardiomyocyte apoptosis and defects in heartbeat

Ca(2+), which enters cardiac myocytes through voltage-dependent Ca(2+) channels during excitation, is extruded from myocytes primarily by the Na(+)/Ca(2+) exchanger (NCX1) during relaxation. The increase in intracellular Ca(2+) concentration in myocytes by digitalis treatment and after ischemia/reperfusion is also thought to result from the reverse mode of the Na(+)/Ca(2+) exchange mechanism. However, the precise roles of the NCX1 are still unclear because of the lack of its specific inhibitors. We generated Ncx1-deficient mice by gene targeting to determine the in vivo function of the exchanger. Homozygous Ncx1-deficient mice died between embryonic days 9 and 10. Their hearts did not beat, and cardiac myocytes showed apoptosis. No forward mode or reverse mode of the Na(+)/Ca(2+) exchange activity was detected in null mutant hearts. The Na(+)-dependent Ca(2+) exchange activity as well as protein content of NCX1 were decreased by approximately 50% in the heart, kidney, aorta, and smooth muscle cells of the heterozygous mice, and tension development of the aortic ring in Na(+)-free solution was markedly impaired in heterozygous mice. These findings suggest that NCX1 is required for heartbeats and survival of cardiac myocytes in embryos and plays critical roles in Na(+)-dependent Ca(2+) handling in the heart and aorta.

Transport of magnesium by two isoforms of the Na+-Ca2+ exchanger expressed in CCL39 fibroblasts

Cytoplasmic concentrations of Ca2+ ([Ca2+]i) and Mg2+ ([Mg2+]i) were measured with fluorescent indicators in CCL39 cells, a cell line established from Chinese hamster lung fibroblasts, transfected with complementary deoxyribonucleic acid (cDNA) of the Na+-Ca2+ exchanger isolated either from canine heart (NCX1) or from rat brain (NCX3). Raising extracellular [Mg2+] to 10 mM increased Mg2+ influx and the resultant change in [Mg2+]i (delta[Mg2+]i) was monitored with furaptra under Ca2+-free conditions. In control (vector-transfected) cells, delta[Mg2+]i at 45 min was similar with or without extracellular Na+ (130 mM or 0 mM) and when [Na+]i was raised by 1 mM ouabain treatment. delta[Mg2+]i in NCX1-transfected cells was attenuated significantly in the presence of 130 mM Na+, but became comparable to (or slightly larger than) that in control cells on either removal of extracellular Na+ or treatment with 1 mM ouabain. Cells expressing NCX3 showed an intermediate dependence of delta[Mg2+]i on Na+, probably reflecting a lower degree of expression of the exchanger protein. Extracellular Na+-dependent changes in [Ca2+]i (measured with fura-2 in the presence of extracellular Ca2+ and 10 microM ionomycin, a Ca2+ ionophore) were minimal in control cells, marked in the NCX1-transfected cells and intermediate in the NCX3-transfected cells. These results suggest that the Na+-Ca2+ exchanger (either NCX1 or NCX3) can transport Mg2+ and may play a role in the extrusion of magnesium from cells.

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

Physiological functions of the intracellular regulatory domains of the Na(+)/Ca(2+) exchanger NCX1 were studied by examining Ca(2+) handling in CCL39 cells expressing a low-affinity Ca(2+) regulatory site mutant (D447V/D498I), an exchanger inhibitory peptide (XIP) region mutant displaying no Na(+) inactivation (XIP-4YW), or a mutant lacking most of the central cytoplasmic loop (Delta246-672). We found that D447V/D498I was unable to efficiently extrude Ca(2+) from the cytoplasm, particularly during a small rise in intracellular Ca(2+) concentration induced by the physiological agonist alpha-thrombin or thapsigargin. The same mutant took up Ca(2+) much less efficiently than the wild-type NCX1 in Na(+)-free medium when transfectants were not loaded with Na(+), although it appeared to take up Ca(2+) normally in transfectants preloaded with Na(+). XIP-4YW and, to a lesser extent, Delta246-672, but not NCX1 and D447V/D498I, markedly accelerated the loss of viability of Na(+)-loaded transfectants. Furthermore, XIP-4YW was not activated by phorbol ester, whereas XIP-4YW and D447V/D498I were resistant to inhibition by ATP depletion. The results suggest that these regulatory domains play important roles in the physiological and pathological Ca(2+) handling by NCX1, as well as in the regulation of NCX1 by protein kinase C or ATP depletion.

A novel topology model of the human Na(+)/H(+) exchanger isoform 1

The membrane topology of the human Na(+)/H(+) exchanger isoform 1 (NHE1) was assessed by substituted cysteine accessibility analysis. Eighty-three cysteine residues were individually introduced into a functional cysteineless NHE1, and these mutants were expressed in the exchanger-deficient PS120 cells. The topological disposition of introduced cysteines was determined by labeling with a biotinylated maleimide in the presence or absence of preincubation with the membrane-impermeable sulfhydryl reagent, 2-trimethylammoniumethyl-methanethiosulfonate in streptolysin O-permeabilized or nonpermeabilized cells. We proposed a new model for the topology of NHE1 that is significantly different from the model derived from hydropathy analysis. In this model, NHE1 is composed of 12 transmembrane segments (TMs) with the N and C termini located in the cytosol. The large, last extracellular loop in the membrane domain of the original model was suggested to comprise an intracellular loop, a new transmembrane segment (TM11), and an extracellular loop in the new model. Interestingly, cysteines at 183 and 184 and at 324 and 325 mapped to intracellular loops connecting TMs 4 and 5 (IL2) and TMs 8 and 9 (IL4), respectively, were accessible to sulfhydryl reagents from the outside. Furthermore, exchange activities of two mutants, R180C and Q181C, within IL2 were markedly inhibited by external MTSET. These data suggest that part of IL2 or IL4 may be located in a pore-lining region that is accessible from either side of the membrane and involved in ion transport.

Lithium activates mammalian Na+/H+ exchangers: isoform specificity and inhibition by genistein

Replacement of external NaCl with LiCl induced cytoplasmic alkalinization in CCL-39 cells and rat L6 myoblasts expressing the endogenous Na+/H+ exchanger isoform NHE1. This Li+-induced alkalinization is due to activation of the Na+/H+ exchanger because it was completely inhibited by 100 microM ethylisopropylamiloride (apparent Kd=1 microM) and because it did not occur in exchanger-deficient PS120 cells. The effect of Li+ was not mimicked by Na+, K+, Cs+ and choline+. Li+ caused cytoplasmic alkalinization in PS120 cells expressing NHE1 or NHE2, but not NHE3, when Li+ was added to cells at a concentration high enough to saturate their external transport sites as predicted from Li+ affinities. Li+ did not induce phosphatidylinositol (PI) turnover or intracellular Ca2+ mobilization. Li+-induced alkalinization was not affected by protein kinase C down-regulation, loss of glycogen synthase kinase 3beta caused by antisense oligonucleotide treatment, or pretreatment with calphostin C, pertussis toxin, MEK inhibitor PD98059 and PI3-kinase inhibitor LY294002. However, it was markedly suppressed by the tyrosine kinase inhibitor genistein (10 microM). Thus, externally added Li+ activates NHE1 and NHE2 via a mechanism possibly involving a tyrosine kinase, causing an increase in cytoplasmic pH that could potentially affect various cell functions.

Chimeric analysis of Na(+)/Ca(2+) exchangers NCX1 and NCX3 reveals structural domains important for differential sensitivity to external Ni(2+) or Li(+)

Externally applied Ni(2+), which apparently competes with Ca(2+) in all three isoforms of Na(+)/Ca(2+) exchanger, inhibits exchange activity of NCX1 or NCX2 with a 10-fold higher affinity than that of NCX3, whereas stimulation of exchange by external Li(+) is significantly greater in NCX2 and NCX3 than in NCX1 (Iwamoto, T., and Shigekawa, M. (1998) Am. J. Physiol. 275, C423-C430). Here we identified structural domains in the exchanger that confer differential sensitivity to Ni(2+) or Li(+) by measuring intracellular Na(+)-dependent (45)Ca(2+) uptake in CCL39 cells stably expressing NCX1/NCX3 chimeras or mutants. We found that two segments in the exchanger corresponding mostly to the internal alpha-1 and alpha-2 repeats are individually responsible for the alteration of Ni(2+) sensitivity, both together accounting for approximately 80% of the difference between NCX1 and NCX3. In contrast, the segment corresponding to the alpha-2 repeat fully accounts for the differential Li(+) sensitivity between the isoforms. The Ni(2+) sensitivity was mimicked, respectively, by simultaneous substitution of two amino acids in the alpha-1 repeat (N125G/T127I in NCX1 and G159N/I161T in NCX3) and substitution of one amino acid in the alpha-2 repeat (V820A in NCX1 and A809V in NCX3). On the other hand, the Li(+) sensitivity was mimicked by double substitution mutation in the alpha-2 repeat (V820A/Q826V in NCX1 and A809V/V815Q in NCX3). Single substitution mutations at Asn(125) and Val(820) of NCX1 caused significant alterations in the interactions of the exchanger with Ca(2+) and Ni(2+), and Ni(2+) and Li(+), respectively, although the extent of alteration varied depending on the nature of side chains of substituted residues. Since the above four important residues are mostly in the putative loops of the alpha repeats, these regions might form an ion interaction domain in the exchanger.

Unique topology of the internal repeats in the cardiac Na+/Ca2+ exchanger

Hydropathy analysis predicts 11 transmembrane helices in the cardiac Na+/Ca2+ exchanger. Using cysteine susceptibility analysis and epitope tagging, we here studied the membrane topology of the exchanger, in particular of the highly conserved internal alpha-1 and alpha-2 repeats. Unexpectedly, we found that the connecting loop in the alpha-1 repeat forms a re-entrant membrane loop with both ends facing the extracellular side and one residue (Asn-125) being accessible from the inside and that the region containing the alpha-2 repeat is mostly accessible from the cytoplasm. Together with other data, we propose that the exchanger may consist of nine transmembrane helices.

Protein kinase C-dependent regulation of Na+/Ca2+ exchanger isoforms NCX1 and NCX3 does not require their direct phosphorylation

We compared the phosphorylation-dependent regulation of three mammalian Na+/Ca2+ exchanger isoforms (NCX1-NCX3) expressed in CCL39 fibroblasts that have little endogenous activity. Na+i-dependent 45Ca2+ uptake into NCX1- or NCX3-expressing cells, but not that into NCX2-expressing cells, was significantly enhanced by phorbol 12-myristate 13-acetate (PMA) or platelet-derived growth factor-BB, which was abolished by pretreatment of cells with calphostin C or a prior long exposure to PMA. This suggests that NCX1 or NCX3, but not NCX2, is stimulated by a pathway involving protein kinase C (PKC). Immunoprecipitation experiments using [32P]orthophosphate-labeled cells revealed that both NCX2 and NCX3 proteins were phosphorylated to a much lesser extent than the NCX1 protein in unstimulated cells and that the extent of phosphorylation was not increased by treatment with PKC activators, although NCX1 phosphorylation was enhanced significantly. Using site-directed mutagenesis, we identified three phosphorylation sites in the NCX1 protein in the PMA-stimulated cells to be Ser-249, Ser-250, and Ser-357 with Ser-250 being predominantly phosphorylated. We found that the NCX1 mutant with these serine residues substituted with alanine still maintained a normal response to PMA. In contrast, the NCX1 or NCX3 mutant, with the large central cytoplasmic loop deleted, lost the responsiveness to PMA. These results suggest that the PKC-dependent regulation of NCX1 or NCX3 requires the central cytoplasmic loop but does not require the direct phosphorylation of the exchanger.

Differential inhibition of Na+/Ca2+ exchanger isoforms by divalent cations and isothiourea derivative

We compared the properties of three mammalian Na+/Ca2+ exchanger isoforms, NCX1, NCX2, and NCX3, by analyzing the effects of Ni2+ and other cations as well as the recently identified inhibitor isothiourea derivatives on intracellular Na+-dependent 45Ca2+ uptake into CCL-39 (Dede) fibroblasts stably expressing each isoform. All these NCX isoforms had similar affinities for the extracellular transport substrates Ca2+ and Na+. Ni2+ inhibited 45Ca2+ uptake by competing with Ca2+ for the external transport site, with 10-fold less affinity in NCX3 than in NCX1 or NCX2. Ni2+ and Co2+ were most efficient in such discrimination of NCX isoforms, although their inhibitory potencies were less than those of La3+ and Cd2+. The monovalent cation Li+ stimulated 45Ca2+ uptake rate by all NCX isoforms similarly with low affinity, although the extent of stimulation was somewhat smaller in NCX1. On the other hand, the isothiourea derivative KB-R7943 was threefold more inhibitory to NCX3 than to NCX1 or NCX2. Thus distinct differences in the kinetic and pharmacological properties were detected between NCX3 and the other two isoforms.

Na+/Ca2+ exchanger overexpression impairs calcium signaling in fibroblasts: inhibition of the [Ca2+] increase at the cell periphery and retardation of cell adhesion

We examined the Ca2+ handling property and cell function of CCL39 fibroblasts highly overexpressing the cardiac isoform (NCX1) of Na+/ Ca2+ exchanger. In NCX1 transfectants in 146 mM Na+, ionomycin, alpha-thrombin or thapsigargin only produced a small transient increase in [Ca2+]i compared to the large increase seen in control cells, although resting [Ca2+]i was not significantly different between these cells. In Na+-free medium, in contrast, the [Ca2+]i responses in NCX1 transfectants and control cells stimulated with these agents were not different, indicating that the Ca2+ content of the intracellular store(s) does not decrease on NCX1 transfection. The expression levels of the endoplasmic reticulum and plasma membrane Ca2+-ATPases, and thrombin- or serum-stimulated cell growth were not altered in NCX1 transfectants. The latter finding suggests that Ca2+ signaling in the nucleus is not impaired appreciably. On fluorescence imaging and confocal microscopy, we found that [Ca2+] did not increase in the peripheral cytoplasm of these cells treated with alpha-thrombin in Na+-containing medium. In these NCX1 transfectants, activation of the plasma membrane Ca2+-activated K+ channels by thrombin or ionomycin was markedly suppressed, and the integrin-mediated adhesion to substrate was significantly delayed compared with control cells. NCX1-overexpressing CCL39 cells thus seem to be a good model with which we can study the Ca2+-regulated membrane processes under physiologically relevant conditions.

Regulation of the Na+/H+ exchanger in fibroblasts overexpressing the Na+/Ca2+ exchanger

To assess the role of Ca2+ in regulation of the Na+/H+ exchanger (NHE1),we used CCL-39 fibroblasts overexpressing the Na+/Ca2+ exchanger (NCX1). Expression of NCX1 markedly inhibited the transient cytoplasmic Ca2+ rise and long-lasting cytoplasmic alkalinization (60-80% inhibition) induced by alpha-thrombin. In contrast, coexpression of NCX1 did not inhibit this alkalinization in cells expressing the NHE1 mutant with the calmodulin (CaM)-binding domain deleted (amino acids 637-656), suggesting that the effect of NCX1 transfection involves Ca2+-CaM binding. Expression of NCX1 only slightly inhibited platelet-derived growth factor BB-induced alkalinization and did not affect hyperosmolarity- or phorbol 12-myristate 13-acetate-induced alkalinization. Downregulation of protein kinase C (PKC) inhibited thrombin-induced alkalinization partially in control cells and abolished it completely in NCX1-transfected cells, suggesting that the thrombin effect is mediated exclusively via Ca2+ and PKC. On the other hand, deletion mutant study revealed that PKC-dependent regulation occurs through a small cytoplasmic segment (amino aids 566-595). These data suggest that a mechanism involving direct Ca2+-CaM binding lasts for a relatively long period after agonist stimulation, despite apparent short-lived Ca2+ mobilization, and further support our previous conclusion that Ca2+- and PKC-dependent mechanisms are mediated through distinct segments of the NHE1 cytoplasmic domain.

Alpha1-syntrophin has distinct binding sites for actin and calmodulin

Overlay and co-sedimentation assays using recombinant alpha1-syntrophin proteins revealed that two regions of alpha1-syntrophin, i.e. aa 274-315 and 449-505, contain high-affinity binding sites for F-actin (Kd 0.16-0.45 microM), although only a single high-affinity site (Kd 0.35 microM) was detected in the recombinant full-length syntrophin. We also found that actomyosin fractions prepared from both cardiac and skeletal muscle contain proteins recognized by anti-syntrophin antibody. These data suggest a novel role for syntrophin as an actin binding protein, which may be important for the function of the dystrophin-glycoprotein complex or for other cell functions. We also found that alpha1-syntrophin binds calmodulin at two distinct sites with high (Kd 15 nM) and low (Kd 0.3 microM) affinity.

Bidirectional signaling between sarcoglycans and the integrin adhesion system in cultured L6 myocytes

The rat L6 skeletal muscle cell line was used to study expression of the dystrophin-containing glycoprotein complex and its interaction with the integrin system involved in the cell-matrix adhesion reaction. A complex of dystrophin and its associated proteins was fully expressed in L6 myotubes, from which anti-dystrophin or anti-alpha-sarcoglycan co-precipitated integrin alpha 5 beta 1 and other focal adhesion-associated proteins vinculin, talin, paxillin, and focal adhesion kinase. Immunostaining and confocal microscopy revealed that dystrophin, alpha-sarcoglycan, integrin alpha 5 beta 1, and vinculin exhibited overlapping distribution in the sarcolemma, especially at focal adhesion-like, spotty structures. Adhesion of cells to fibronectin- or collagen type I-coated dishes resulted in induction of tyrosine phosphorylation of alpha- and gamma-sarcoglycans but not beta-sarcoglycan. The same proteins were also tyrosine-phosphorylated when L6 cells in suspension were exposed to Arg-Gly-Asp-Ser peptide. All of these tyrosine phosphorylations were inhibited by herbimycin A. On the other hand, treatment of L6 myotubes with alpha- and gamma-sarcoglycan antisense oligodeoxynucleotides resulted in complete disappearance of alpha- and gamma-sarcoglycans and in significant reduction of levels of the associated focal adhesion proteins, which caused about 50% reduction of cell adhesion. These results indicate the existence of bidirectional communication between the dystrophin-containing complex and the integrin adhesion system in cultured L6 myocytes.

Calmodulin-binding autoinhibitory domain controls "pH-sensing" in the Na+/H+ exchanger NHE1 through sequence-specific interaction

The calmodulin (CaM)-binding domain reduces the affinity of the Na+/H+ exchanger NHE1 for intracellular H+ by exerting an autoinhibitory function in quiescent cells. We replaced this domain (aa 637-656) with homologous segments from other NHE isoforms (NHE2 and 4) or functionally similar regions from other sources (Na+/Ca2+ exchanger, CaM-dependent protein kinase II, plasma membrane Ca2+-pump, or CaM-binding peptide Trp3). The NHE-1-, NHE2-, and NHE4-segments bound CaM with Kds of 16, 130, and 27 nM, respectively. These chimeric molecules were expressed in the exchanger-deficient cell PS120. NHE1 with incorporated NHE2-segment was activated in response to Ca2+-mobilizing agents ionomycin and thrombin resulting in an alkaline shift of the intracellular pH (pHi)-dependence of 22Na+ uptake, as was the case with the intact rat NHE2. In contrast, incorporation of the NHE4-segment or other CaM-binding segments induced a constitutive alkaline shift of pHi-dependence with concomitant abolishment of Ca2+-dependent activation, indicating that these segments could not function as an autoinhibitory domain in NHE1. Detailed analyses revealed that Leu639, Lys651 and Tyr652, conserved in the NHE1- and NHE2-segments, but not in the NHE4-segment, are important for the autoinhibition. Furthermore, 125I-labeled CaM-binding peptide from NHE1 was efficiently crosslinked to the NHE1 protein, suggesting that the inhibitory domain physically interacts with part(s) of the molecule. Together, these findings support the notion that the reduction of H+ affinity in Na+/H+ exchange occurs through a mechanism involving a highly sequence-specific interaction of the inhibitory domain with its putative acceptor in NHE1.

Whole-cell currents from the cloned canine cardiac Na+/Ca2+ exchanger NCX1 overexpressed in a fibroblast cell CCL39

A conventional patch-clamp technique was used to record the whole-cell current from the cloned canine cardiac Na+/Ca2+ exchanger NCX1 overexpressed in a fibroblast cell. Ca2+ was extracellularly applied to the Na+-loaded cell to activate the outward current by operating the reverse mode of NCX1. No measurable outward current was ever elicited from the nontransfected cell. Na+/Ca2+ exchange blocker 5 mM Ni2+ or 3 microM KB-R7943 that was applied extracellularly abolished the outward current. With 140 mM external Li+ (replacing Na+), the outward current was transient during the Ca2+ application. In contrast, with 140 mM external Na+, the outward current was maintained without any inactivation during the Ca2+ application. I-V relations predicted from the whole-cell clamp protocols used were obtained both before and during the Ca2+ application. The exchanger whole-cell currents are thus successfully detectable from NCX1 which is overexpressed in this stable transfectant system.

mRNA expression and cDNA sequences of beta- and gamma-sarcoglycans are normal in cardiomyopathic hamster heart

In BIO14.6 cardiomyopathic hamster heart, the dystrophin-glycoprotein complex is disrupted and sarcoglycans are greatly reduced in abundance. We examined whether the gene expression of beta- and gamma-sarcoglycans is indeed defective in this hamster. We found that mRNA expression for these proteins and the cDNA sequences of their coding regions are identical in both normal and myopathic hamster cardiomyocytes. The results strongly suggest that defect in a currently unknown sarcoglycan-associated protein(s) is responsible for the deficiency of the sarcoglycan complex that leads to muscle cell necrosis in the myopathic hamster.

Identification of cytoplasmic subdomains that control pH-sensing of the Na+/H+ exchanger (NHE1): pH-maintenance, ATP-sensitive, and flexible loop domains

To precisely identify the cytoplasmic subdomains that are responsible for the intracellular pH (pHi)-sensitivity, ATP depletion-induced inhibition and Ca2+ activation of the Na+/H+ exchanger (NHE1), we generated a set of deletion mutants of carboxyl-terminated cytoplasmic domain and expressed them in the exchanger-deficient cell line PS120. We evaluated pHi-sensitivity of these mutants by measuring the resting pHi in cells placed in an acidic medium (pH 6.0) and pHi-dependence of 5-(N-ethyl-N-isopropyl)amiloride-sensitive 22Na+ uptake. Detailed analysis revealed that the cytoplasmic domain of NHE1 is consists of at least four subdomains in terms of pHi-sensitivity of the unstimulated NHE1: I, aa 516-590/595; II, aa 596-635; III aa 636-659; and IV, aa 660-815. Subdomains II and IV were silent for pHi-sensitivity. Subdomain I had a pHi-maintenance function, preserving pHi-sensitivity in a physiological range, whereas subdomain III, overlapping with the high affinity calmodulin (CaM)-binding site, exhibited an autoinhibitory function. Deletion of subdomain I abolished the decrease of pHi-sensitivity induced by cell ATP depletion, indicating that domain I plays a crucial role in this phenomenon. Deletion of subdomain III rendered the inhibition by ATP depletion less efficient, suggesting the possible interaction between subdomains I and III. On the other hand, tandem elongation of subdomain II by insertion did not affect either the inhibitory function of domain III or the removal of this inhibition by ionomycin or thrombin. However, deletion of subdomain II partially abolished the inhibitory effect of subdomain III. Subdomain II thus seems to function as a mobile "flexible loop," permitting the CaM-binding subdomain III to exert its normal function. These findings, together with our previous data, support a concept that cell ATP, Ca2+, and growth factors regulate NHE1 via a mechanism involving direct or indirect interactions of specific cytoplasmic subdomains with the "H(+)-modifier site.".

Molecular physiology of vertebrate Na+/H+ exchangers

This review describes recent progress concerning the molecular aspects of the Na+/H+ exchanger. The Na+/H+ exchanger is an important regulator for intracellular pH, cell volume, and transepithelial Na+ transport. It exists in virtually all cells with cell type-dependent pattern of isoform expression, and it is regulated in response to a variety of extracellular stimuli, among them not only agonists such as growth factors and hormones but also mechanical stimuli such as osmotic stress and cell spreading. Thus this transporter is also an excellent model to study the signal transduction. Since the first molecular cloning of the Na+/H+ exchanger, detailed studies revealed many interesting features of this transporter. At present, at least five different isoforms of the Na+/H+ exchanger are known. These isoforms differ in tissue localization, sensitivity of inhibitors, and mode of transcriptional and posttranscriptional regulation, allowing them to participate in different physiological processes. We have only started to understand an intriguing mechanism underlying these functional differences among the exchanger isoforms. Because the Na+/H+ exchanger is relatively simple in terms of its kinetic features, e.g., a simple 1:1 stoichiometry of Na+ and H+ and no input of metabolic energy such as ATP hydrolysis, the study of its structural and mechanistic aspects would also serve as a good model to understand the general mechanism of various ion transporters.

Dystrophin-glycoprotein complex purified from hamster cardiac muscle. Comparison of the complexes from cardiac and skeletal muscles of hamster and rabbit

The dystrophin-glycoprotein complex was isolated from hamster ventricular muscle by a method involving homogenization of muscle directly in the presence of 1% digitonin, followed by chromatography on succinylated wheat germ agglutinin agarose, Diethyl aminoethyl (DEAE) cellulose, and/or immunoaffinity agarose. Protein yield of the DEAE cellulose-purified dystrophin-glycoprotein complex was 120 +/- 30 (n = 3) micrograms per 5 g hamster ventricular muscle. The cardiac dystrophin-glycoprotein complex, unlike the skeletal muscle counterpart, could not be solubilized from a microsomal fraction with digitonin or some other detergents. By sodium dodecyl sulfate gel electrophoresis, protein composition of the dystrophin-glycoprotein complexes from hamster cardiac muscle was found to be significantly different from that of rabbit skeletal muscle which has been extensively studied. This difference mainly arises from the species difference, because in hamster the cardiac and skeletal muscle complexes exhibited essentially the same protein composition. In rabbit, on the other hand, there are differences between the cardiac and skeletal complexes in the relative abundance of 60 and 64 kDa proteins and in the apparent M(r) of alpha-dystroglycan. We found that the content of the dystrophin-glycoprotein complex, estimated by quantitative immunoblot assay, is at least 5 times more abundant in cardiac than in skeletal muscle in hamster and rabbit.

Generation of cell transfectants expressing cardiac calcium ion channel and calcium indicator protein aequorin

Chinese hamster ovary (CHO) cells stably coexpressing cardiac calcium ion channel [L-type calcium channel or ryanodine receptor (RyR)] and the calcium-sensitive bioluminescent protein aequorin were generated by transfecting aequorin cDNA. In a selected clone, C1-17, carrying the L-type calcium channel, depolarization induced by high concentration of K+ produces aequorin luminescence. In another clone, R3-7, carrying RyR, caffeine produces aequorin luminescence. In the presence of selective calcium ion channel blockers, the aequorin luminescence was inhibited in a dose-dependent manner. These results indicate that functionally expressed calcium ion channels in these transformants can be monitored through the activation of endogenous aequorin luminescence following a physiological signal similar to that of native calcium channel. Moreover, the aequorin system compared very well with Fura-2 measurements. Thus, the recombinant cell models, which expressed cloned calcium channel and aequorin, will contribute to the elucidation of Ca2+ movement through the cell surface and intracellular calcium ion channels.

A novel isothiourea derivative selectively inhibits the reverse mode of Na+/Ca2+ exchange in cells expressing NCX1

No.7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate), a selective inhibitor of the Na+/Ca2+ exchanger (NCX1), has been newly synthesized. It dose-dependently inhibited Na+i-dependent 45Ca2+ uptake and Na+i-dependent [Ca2+]i increase in cardiomyocytes, smooth muscle cells, and NCX1-transfected fibroblasts (IC50 = 1.2-2.4 microM). Inhibition was observed without prior incubation with the agent and was completely reversed by washing cells with buffer for 1 min. Interestingly, No.7943 was much less potent in inhibiting Na+o-dependent 45Ca2+ efflux and Na+o-induced [Ca2+]i decline (IC50 = >30 microM), indicating that it selectively blocks the reverse mode of Na+/Ca2+ exchange in intact cells. In cardiac sarcolemmal preparations consisting mostly of inside-out vesicles, the agent inhibited Na+i-dependent 45Ca2+ uptake and Na+o-dependent 45Ca2+ efflux with similar, but slightly lower, potencies (IC50 = 5.4-13 microM). Inhibition was noncompetitive with respect to Ca2+ and Na+ in both cells and sarcolemmal vesicles. These results suggest that No.7943 primarily acts on external exchanger site(s) other than the transport sites in intact cells, although it is able to inhibit the exchanger from both sides of the plasma membrane. No.7943 at up to 10 microM does not affect many other ion transporters nor several cardiac action potential parameters. This agent at these concentrations also did not influence either diastolic [Ca2+]i or spontaneous beating in cardiomyocytes. Furthermore, No.7943 markedly inhibited Ca2+ overloading into cardiomyocytes under the Ca2+ paradox conditions. Thus, No.7943 is not only useful as a tool with which to study the transport mechanism and physiological role of the Na+/Ca2+ exchanger but also has therapeutic potential as a selective blocker of excessive Ca2+ influx mediated via the Na+/Ca2+ exchanger under pathological conditions.

Phosphorylation-dependent regulation of cardiac Na+/Ca2+ exchanger via protein kinase C

The cardiac Na+/Ca2+ exchanger (NCX1) plays a major role in the extrusion of Ca2+ from cardiomyocytes. We studied the role of protein phosphorylation in the regulation of cardiac NCX1 using CCL39 stably overexpressing the canine cardiac NCX1 and rat neonatal cardiomyocytes. In both cell types, the NCX1 protein immunoprecipitated with a chicken anti-NCX1 antibody exhibited a significant basal phosphorylation that was further enhanced by treatment with endothelin-1, acidic fibroblast growth factor, phorbol 12-myristate 13-acetate, or okadaic acid. In contrast, calphostin C, K252a, or EGTA inhibited the phosphorylation. The phosphorylation occurred on two major tryptic phosphopeptides (P1 and P2) exclusively on serine residues. Evidence is presented suggesting that P2 was derived from an N-terminal half (amino acids 240-475) of the central cytoplasmic domain of NCX1 and was phosphorylated directly by protein kinase C (PKC). The agents that increased NCX1 phosphorylation significantly enhanced both the forward and reverse modes of Na+/Ca2+ exchange. This exchange activation exhibited a very good correlation with the NCX1 phosphorylation. In NCX1-transfected cells, PKC down-regulation following prolonged exposure to phorbol 12-myristate 13-acetate abolished the acidic fibroblast growth factor-induced activation of exchange activity. On the other hand, cell ATP depletion reduced the exchange activity and abolished the effects of the above agents on exchange activity. These results indicate that the cardiac NCX1 is up-regulated by PKC-catalyzed phosphorylation. The cardiac NCX1 thus could play an important role in the previously reported negative inotropic actions of phorbol esters and other PKC-activating agents.

Mechanisms of reoxygenation-induced calcium overload in cardiac myocytes: dependence on pHi

This study investigated the selective effects of intracellular (pHi) or extracellular change in pH on reoxygenation-induced Ca2+ overload in simulated myocardial hypoxia. Experiments were performed in cultured cardiomyocytes isolated from the ventricle of neonatal ICR mouse. A model of chemical hypoxia with 2 mM sodium cyanide was developed to mimic the ATP depletion of hypoxia. This chemical hypoxia was "reoxygenated" and the dynamics in intracellular Ca2+ concentration ([Ca2+]i) and pHi were monitored using the fluorescent dyes fura-2 and 2', 7'-bis (2-carboxyethyl)-5(6)-carboxyfluorescein, respectively. During a 40-min chemical hypoxia, pHi progressively fell from 7.2 to 6.6. Reoxygenation with control solution caused rapid recovery of pHi and a marked increase in [Ca2+]i (1884 +/- 136 nM). Intracellular acidotic reoxygenation produced by lactate apparently prolonged the time course of pHi recovery and significantly suppressed reoxygenation-induced Ca2+ overload (1170 +/- 118 nM, P = 0.008). Extracellular acidotic reoxygenation with 2-(N-morpholino) ethanesulfonic acid (pK = 5.96) buffer somewhat suppressed the Ca2+ overload; however, the maximal value of [Ca2+]i was not reduced significantly compared with the control (1790 +/- 122 nM, P = 0.130). In addition, inhibition of Na(+)-H+ exchange by amiloride potentiated prolongation of intracellular acidosis during reoxygenation and resulted in a minimal increase in [Ca2+]i (985 +/- 102 nM, P = 0.004). These results suggest that reoxygenation-induced Ca2+ overload is closely correlated with intracellular pH in the initial phase of reoxygenation, and the protective effects of extracellular acidosis is principally mediated by intracellular acidification of reoxygenated cardiomyocytes.

Cytoplasmic domain of the ubiquitous Na+/H+ exchanger NHE1 can confer Ca2+ responsiveness to the apical isoform NHE3

The Na+/H+ exchanger isoforms NHE1 and NHE3 are regulated differently by various stimuli. Calcium has been recognized as one of the major second messengers in such exchanger regulation. We previously proposed that Ca(2+)-induced activation of NHE1 occurs via displacement of its autoinhibitory domain from the H+ modifier site due to direct binding of Ca2+/calmodulin. To further validate this hypothesis, the functional role of the cytoplasmic domain was studied in both wild-type and chimeric exchangers, i.e. NHE1, NHE3, NHE1 with the cytoplasmic domain of NHE3 (N1N3), and NHE3 with the cytoplasmic domain of NHE1 (N3N1). After expression in exchanger-deficient fibroblasts (PS120), early response (< 80 s) to external stimuli was assessed as 5-(N-ethyl-N-isopropyl)amiloride-sensitive 22Na+ uptake. Among stimuli tested (ionomycin, alpha-thrombin, phorbol ester, hyperosmotic stress, and platelet-derived growth factor) that are all known to activate NHE1, only ionomycin and thrombin induced a significant intracellular Ca2+ mobilization and early activation of 22Na+ uptake, implying that Ca2+ is a main regulator of NHE1 in the early phase of the agonist response. However, all the stimuli did not activate NHE3 or N1N3. In contrast, a significant stimulation of 22Na+ uptake in response to ionomycin and thrombin was observed in N3N1, accompanied by an alkaline shift of pHi sensitivity (approximately 0.2 pH units). Deletion of the cytoplasmic calmodulin-binding domain within N3N1 resulted in a constitutive alkaline shift of pHi sensitivity and abolished the activation by ionomycin and thrombin. Together, these data reinforce our concept of Ca(2+)-induced activation of NHE1. Furthermore, they provide evidence for a functional interaction of the autoinhibitory domain of NHE1 with the H(+)-modifier site of a different isoform, NHE3.

Detachment of cultured cells from the substratum induced by the neutrophil-derived oxidant NH2Cl: synergistic role of phosphotyrosine and intracellular Ca2+ concentration

The neutrophil-derived, membrane-permeating oxidant, NH2Cl, (but not the non-membrane-permeating chloramine, taurine-NHCl) induced detachment of fetal mouse cardiac myocytes and other cell types (fibroblasts, epithelial cells, and endothelial cells) from the culture dish, concomitant with cell shrinkage ("peeling off"). Stimulated human neutrophils also induced peeling off of cultured mouse cardiac myocytes when the latter were pretreated with inhibitors of .OH and elastase. Immunofluorescence microscopy revealed that the NH2Cl-induced peeling off of WI-38 fibroblasts is accompanied by disorganization of integrin alpha 5 beta 1, vinculin, stress fibers, and phosphotyrosine (p-Tyr)-containing proteins. Decrease in the content of the p-Tyr-containing proteins of the NH2Cl-treated cells was analyzed by immunoblotting techniques. Coating of fibronectin on the culture dish prevented both NH2Cl-induced peeling off and a decrease in p-Tyr content. Preincubation with a protein-tyrosine phosphatase inhibitor, sodium orthovanadate (Na3VO4), also prevented NH2Cl-induced peeling off, suggesting that dephosphorylation of p-Tyr is necessary for peeling off. NH2Cl-induced peeling off was accompanied by an increase in intracellular Ca2+ concentration ([Ca2+]i) in mouse cardiac myocytes and WI-38 fibroblasts. The absence of extracellular Ca2+ prevented both NH2Cl-induced peeling off and increased [Ca2+]i, both of which did occur on subsequent incubation of the cells in Ca2+-containing medium. These observations suggest that an increase in [Ca2+]i is also necessary for peeling off. Depletion of microsomal and cytosolic Ca2+ by incubation with the microsomal Ca2+-ATPase inhibitor 2',5'-di(tert-butyl)-1,4-benzohydroquinone (BHQ) plus EGTA prevented both NH2Cl-induced increases in [Ca2+]i and peeling off. Direct inhibition of microsomal Ca2+ pump activity by NH2Cl may participate in the NH2Cl-induced [Ca2+]i increment. A combination of p-Tyr dephosphorylation by genistein (an inhibitor of tyrosine kinase) and an increase in [Ca2+]i by BHQ could also induce peeling off. All these observations suggest a synergism between p-Tyr dephosphorylation and increased [Ca2+]i in NH2Cl-induced peeling off.

Growth factor-induced phosphorylation and activation of aortic smooth muscle Na+/Ca2+ exchanger

Although the Na+/Ca2+ exchanger is one of the major Ca2+ extrusion systems in excitable tissues, little is known about its regulation via protein phosphorylation. We now present evidence that the Na+/Ca2+ exchanger is phosphorylated in quiescent and growth factor-stimulated cultured aortic smooth muscle cells. The Na+/Ca2+ exchanger was isolated from 32P-labeled cells by immunoprecipitation with a specific polyclonal antibody. Phosphorylation of the exchanger was increased by up to 1.7-fold in response to platelet-derived growth factor-BB (PDGF-BB), alpha-thrombin, or phorbol 12-myristate 13-acetate (PMA). However, angiotensin II did not enhance the phosphorylation significantly. The extent of phosphorylation appeared to correlate with the growth factor-induced increase in cell 1,2-diacylglycerol. At least four phosphopeptides (P1 to P4) were detected by tryptic phosphopeptide map analysis of the phosphorylated exchanger, suggesting that phosphorylation occurred at multiple sites. PDGF-BB and PMA increased phosphorylation of the same phosphopeptides (in particular P1). Phosphorylated amino acids were exclusively serine residues in both quiescent and stimulated cells. We found that growth factors enhanced Na+/Ca2+ exchange activity and that there was a good correlation between the growth factor-induced stimulations of phosphorylation and exchange activity. PDGF-BB-induced activation of the exchanger was abolished by prior long treatment of cells with PMA. These results suggest that the Na+/Ca2+ exchanger is activated by protein kinase C-dependent phosphorylation in response to growth factors in vascular smooth muscle cells.

Ca(2+)-ATPase distributes differently in cardiac sarcolemma than dihydropyridine receptor alpha 1 subunit and Na+/Ca2+ exchanger

We have investigated the distribution of the sarcolemmal Ca2+ transporters in hamster and dog ventricular myocytes by immunocytochemical and membrane fractionation techniques. The data suggest that the DHP receptor alpha 1 subunit and the Na+/Ca2+ exchanger are present in surface sarcolemma as well as T-tubule membranes located at the cardiac dyads. Compared with these Ca2+ transporters, the sarcolemmal Ca(2+)-ATPase is much less abundant in the latter fraction. Thus the sarcolemmal Ca(2+)-ATPase seems to be located predominantly in surface sarcolemma.

Multiple mechanisms of arachidonic acid release in Chinese hamster ovary cells transfected with cDNA of substance P receptor

We investigated the release of [3H]arachidonic acid ([3H]AA) and its relationship to the formation of [3H]inositol trisphosphate ([3H]IP3) elicited by substance P (SP) in prelabeled Chinese hamster ovary cells stably expressing the SP receptor. Activation of the SP receptor resulted in a concentration- and time-dependent stimulation of [3H]AA release. Half-maximal release was obtained at 10(-9) M, comparable to that for [3H]IP3 formation reported previously, and the maximal release effected by 0.1 microM SP was 8 to 10-fold above the basal value. Both the [3H]AA release and the [3H]-IP3 accumulation stimulated in the cells by 0.1 microM SP were concentration-dependently blocked with the specific SP receptor antagonist CP-96,345, with IC50 values of 2.5 and 0.4 microM, respectively. The time course of [3H]AA release showed a biphasic pattern: an initial rapid release essentially independent of Ca2+, followed by a sustained release markedly suppressed by removal of extracellular Ca2+ or chelation of intracellular Ca2+ with 1,2-bis(2-aminophenoxyethane)-N,N,N',N'-tetraacetic acid (BAPTA). While pretreatment with pertussis toxin (200 ng/mL, 6 hr) did not block [3H]IP3 formation, it did reduce [3H]AA release by 50% at 1 and 10 min after SP stimulation. Treatment of the cells with a phorbol ester, a protein kinase C activator, augmented the SP-stimulated [H]AA release, and sphingosine, a protein kinase C inhibitor, reversed the phorbol ester-potentiated [3H]AA release, but not the release stimulated by SP alone, suggesting a synergistic effect of protein kinase C on SP-stimulated AA release. These results demonstrate that SP, acting at the SP receptor, stimulates [3H]AA release via mechanisms that are (1) mediated by a pertussis toxin-sensitive G-protein, (2) dependent on extracellular Ca2+, and (3) enhanced by activation of protein kinase C.

Regulation of Ca2+ entry in rat aortic smooth muscle cells in primary culture

We characterized Ca2+ entry in rat aortic smooth muscle cells (SMCs) maintained in primary culture by measuring uptake of 45Ca2+ or Mn2+ from a normal balanced salt solution and the extracellular Ca(2+)-induced increase in the intracellular Ca2+ concentration ([Ca2+]i) in a medium [high pH (pH 8.8)/high Mg2+ (20 mM) medium containing a sarcoplasmic reticulum Ca(2+)-ATPase inhibitor, thapsigargin] that inhibits removal of Ca2+ from the cytoplasm. Such measurements in the presence or absence of a dihydropyridine (DHP) calcium channel antagonist (PN200-110) or hyperpolarizing agent (valinomycin) revealed that DHP-sensitive voltage-gated Ca2+ channels (VGCCs) are activated in these SMCs under resting conditions and that DHP-sensitive Ca2+ entry occurs mostly via these VGCCs. We found that receptor stimulation by endothelin-1 in these SMCs resulted in activation of neither DHP-sensitive nor -insensitive Ca2+ entry, but rather resulted in marked suppression of the former. Utilizing the DHP-sensitive extracellular Ca(2+)-induced increase in [Ca2+]i as a monitor of activity of the DHP-sensitive VGCCs, we investigated the effects of protein kinase activators and phosphatase inhibitors on the regulation of these VGCCs. We found that the DHP-sensitive VGCCs were inhibited by endothelin-1 through the activation of protein kinase C. We also found that they were inhibited by 8Br-cGMP, okadaic acid, and calyculin A.

The Na+/H+ exchanger isoform 1 (NHE1) is a novel member of the calmodulin-binding proteins. Identification and characterization of calmodulin-binding sites

The Na+/H+ exchange activity (NHE1 human isoform) is rapidly activated in response to growth factors and hyperosmotic stress. To get insight into the mechanism of NHE1 activation, we studied the direct interaction of a ubiquitous Ca(2+)-dependent regulatory factor, calmodulin (CaM) with NHE1. Binding experiments with CaM-Sepharose, as well as fluorescence measurements with dansylated CaM, revealed that the NHE1 cytoplasmic domain strongly binds CaM in a Ca(2+)-dependent manner. Fusion protein analysis with deletion mutants provided evidence for high (Kd approximately 20 nM) and intermediate (Kd approximately 350 nm) affinity CaM-binding sites located in neighboring regions of NHE1 (amino acids 636-656 and 657-700). To assess a regulatory role of CaM-binding sites, several cDNAs having deletion and point mutations in the high affinity site were generated and expressed in the exchanger-deficient fibroblast cell line PS120. Deletion and point mutations of positively charged residues of the high affinity CaM-binding site resulted in up to 50 and 80% reductions of cytoplasmic alkalinization caused by growth factors (alpha-thrombin, etc.) and 100 mM sucrose, respectively. In these mutants, the reduction in alkalinization was apparently in proportion to that of the CaM-binding ability. These results suggest that binding of Ca2+/CaM to the high affinity site is involved at least partly in the activation of NHE1 in response to different extracellular signals.

Mutation of calmodulin-binding site renders the Na+/H+ exchanger (NHE1) highly H(+)-sensitive and Ca2+ regulation-defective

The ubiquitous plasma membrane Na+/H+ exchanger (NHE1) is rapidly activated in response to various extracellular signals. To understand how the intracellular Ca2+ is involved in this activation process, we investigated the effect of Ca2+ ionophore ionomycin on activity of the wild-type or mutant NHE1 expressed in the exchanger-deficient fibroblasts (PS120). In wild-type transfectants, a short (up to 1 min) incubation with ionomycin induced a significant alkaline shift (approximately 0.2 pH unit) in the intracellular pH (pHi) dependence of the rate of 5-(N-ethyl-N-isopropyl) amiloride-sensitive 22Na+ uptake, without changes in the cell volume and phosphorylation state of NHE1. Mutations that prevented calmodulin (CaM) binding to a high affinity binding region (region A, amino acids 636-656) rendered NHE1 constitutively active by inducing a similar alkaline shift in pHi dependence of Na+/H+ exchange. These same mutations abolished the ionomycin-induced NHE1 activation. These data suggest that CaM-binding region A functions as an "autoinhibitory domain" and that Ca2+/CaM activates NHE1 by binding to region A and thus abolishing its inhibitory effect. Furthermore, we found that a short stimulation with thrombin and ionomycin had apparently no additive effects on the alkaline shift in the pHi dependence of Na+/H+ exchange and that deletion of region A also abolished such an alkaline shift induced by a short thrombin stimulation. The results strongly suggest that the early thrombin response and the ionomycin response share the same activation mechanism. Based on these data and the results shown in the accompanying paper (Bertrand, B., Wakabayashi, S., Ikeda, T., Pouysségur, J., and Shigekawa, M. (1994) J. Biol. Chem. 269, 13703-13709), we propose that CaM is one of the major "signal transducers" that mediate distinct extracellular signals to the "pHi sensor" of NHE1.

Growth factor activation and "H(+)-sensing" of the Na+/H+ exchanger isoform 1 (NHE1). Evidence for an additional mechanism not requiring direct phosphorylation

Growth factors stimulate the Na+/H+ exchange activity (NHE1 human isoform) and at the same time increase the phosphorylation state of the exchanger at serine residues. To determine the role of NHE1 phosphorylation, a set of deletion and point mutants has been generated. Functional characterization of deletion mutants expressed in fibroblastic cells revealed that the cytoplasmic region between amino acids 567 and 635 is required for both growth factor-induced cytoplasmic alkalinization and maintenance of high intracellular pH (pHi) sensitivity. In contrast to the loss of growth factor activation and pHi sensitivity caused by the deletion of amino acids 567-635, the same deletion had no apparent effect on the pattern of growth factor-induced phosphorylation. In addition, individual replacement of any of the serine residues between amino acids 567 and 635 with alanine also had no effect on growth factor activation of the exchange activity. Comparison of phosphopeptide maps for the wild type with those for the internal deletion mutant exchangers and the expressed cytoplasmic domain revealed that all major in vivo phosphorylation sites including growth factor-sensitive ones map to the cytoplasmic tail (amino acids 636-815). Deletion of these sites preserves high pHi sensitivity and reduces by only 50% growth factor-induced cytoplasmic alkalinization. Taken together, these data support the existence of a mechanism not requiring direct phosphorylation of NHE1, by which growth factor signals transmit to the "H(+)-sensor" and control the set point value of the exchanger. We propose that a regulatory factor(s) controls NHE1, presumably through its interaction with the critical cytoplasmic region prior to amino acid 635.