Stable expression of the cardiac sodium-calcium exchanger in CHO cells

Sodium-calcium exchangers in the nucleus: an unexpected locus and an unusual regulatory mechanism

Whereas sodium-calcium exchangers (NCXs) have long been recognized as plasma membrane constituents that serve to maintain homeostatic concentrations of Ca2+ in the cytoplasm, they were recently shown to also occur in the nuclear envelope (NE) of neural and other cells where they function to regulate nuclear Ca2+. A unique feature of NCXs in the NE is their high-affinity binding to GM1 ganglioside, this association being required for optimal exchanger activity. The NCX-GM1 complex occurs in the inner membrane of the NE and transfers Ca2+ from the nucleoplasm to the NE lumen. In neuronal cells, nuclear GM1 levels are low prior to differentiation but increase rapidly as axonal outgrowth progresses. Cells from genetically altered mice lacking GM1 have limited ability to regulate nuclear Ca2+, and the mice themselves showed similar deficit as seen in their high susceptibility to kainite-induced seizures. These are attenuated by LIGA-20, a derivative of GM1 that enters the nuclear membrane and restores nuclear NCX activity to normal level.

Potentiation of a sodium-calcium exchanger in the nuclear envelope by nuclear GM1 ganglioside

Calcium is recognized as an important intracellular messenger with a pivotal role in the regulation of many cytosolic and nuclear processes. Gangliosides of various types, especially GM1, are known to have a role in some aspects of Ca2+ regulation, operating through a variety of mechanisms that are gradually coming to light. The present study provides evidence for a sodium-calcium exchanger in the nuclear envelope of NG108-15 neuroblastoma cells that is potently and specifically activated by GM1. Immunoblot analysis revealed an unusually tight association of GM1 with the exchanger in the nuclear envelope but not with that in the plasma membrane. Exchanger and associated GM1 were located in the inner membrane of the nuclear envelope, suggesting this system could function to transfer Ca2+ between nucleoplasm and the envelope lumen. The GM1-enhanced exchange was blocked by cholera toxin B subunit while C2-ceramide, a recently discovered inhibitor of the exchanger, blocked all transfer. Exchanger activity was significantly elevated in nuclei isolated from cells that were induced to differentiate by KCl + dibutyryl-cAMP, a treatment previously shown to promote up-regulation of nuclear GM1 in conjunction with axonogenesis. Similar enhancement was achieved by addition of exogenous GM1 to nuclei from undifferentiated cells. These results suggest a prominent role for nuclear GM1 in regulation of nuclear Ca2+ homeostasis.

The calcium-sensing receptor regulates calcium absorption in MDCK cells by inhibition of PMCA

Calcium transport across a monolayer of Madin-Darby canine kidney (MDCK) cells was measured in response to stimulation of the basal surface with calcium-sensing receptor (CaR) agonists. Stimulation of the CaR resulted in a time- and concentration-dependent inhibition of calcium transport but did not change transepithelial voltage or resistance. Inhibition of transport was not altered by pretreatment of cells with pertussis toxin but was blocked by the phospholipase C (PLC) inhibitor U-73122. To determine a potential mechanism by which the CaR could inhibit calcium transport, we measured activity of the plasma membrane calcium ATPase (PMCA). Stimulation of the CaR on the basal surface resulted in an inhibition of the PMCA in a concentration- and PLC-dependent manner. Thus stimulation of the CaR inhibits both calcium transport and PMCA activity through a PLC-dependent pathway. These studies provide the first direct evidence that calcium can inhibit its own transcellular absorption in a model of the distal tubule. In addition, they provide a potential mechanism for the CaR to inhibit calcium transport, inhibition of PMCA.

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+).

Feedback inhibition of sodium/calcium exchange by mitochondrial calcium accumulation

Chinese hamster ovary cells expressing the bovine cardiac Na(+)/Ca(2+) exchanger were subjected to two periods of 5 and 3 min, respectively, during which the extracellular Na(+) concentration ([Na(+)](o)) was reduced to 20 mm; these intervals were separated by a 5-min recovery period at 140 mm Na(+)(o). The cytosolic Ca(2+) concentration ([Ca(2+)](i)) increased during both intervals due to Na(+)-dependent Ca(2+) influx by the exchanger. However, the peak rise in [Ca(2+)](i) during the second interval was only 26% of the first. The reduced rise in [Ca(2+)](i) was due to an inhibition of Na(+)/Ca(2+) exchange activity rather than increased Ca(2+) sequestration since the influx of Ba(2+), which is not sequestered by internal organelles, was also inhibited by a prior interval of Ca(2+) influx. Mitochondria accumulated Ca(2+) during the first interval of reduced [Na(+)](o), as determined by an increase in fluorescence of the Ca(2+)-indicating dye rhod-2, which preferentially labels mitochondria. Agents that blocked mitochondrial Ca(2+) accumulation (uncouplers, nocodazole) eliminated the observed inhibition of exchange activity during the second period of low [Na(+)](o). Conversely, diltiazem, an inhibitor of the mitochondrial Na(+)/Ca(2+) exchanger, increased mitochondrial Ca(2+) accumulation and also increased the inhibition of exchange activity. We conclude that Na(+)/Ca(2+) exchange activity is regulated by a feedback inhibition process linked to mitochondrial Ca(2+) accumulation.

Mode-specific inhibition of sodium-calcium exchange during protein phosphatase blockade

The effects of the protein phosphatase inhibitors calyculin A and okadaic acid on Na(+)/Ca(2+) exchange activity were examined in transfected Chinese hamster ovary cells expressing the bovine cardiac Na(+)/Ca(2+) exchanger. Incubating the cells for 5-10 min with 100 nM calyculin A reduced exchange-mediated (45)Ca(2+) uptake or Ba(2+) influx by 50-75%. Half-maximal inhibition of (45)Ca(2+) uptake was observed at 15 nM calyculin A. The nonselective protein kinase inhibitors K252a and staurosporine provided partial protection against the effects of calyculin A. Okadaic acid, another protein phosphatase inhibitor, nearly completely blocked exchange-mediated Ba(2+) influx. Chinese hamster ovary cells expressing a mutant exchanger in which 420 out of 520 amino acid residues were deleted from the central hydrophilic domain of the exchanger remained sensitive to the inhibitory effects of calyculin A and okadaic acid. Surprisingly, Na(o)(+)-dependent Ca(2+) efflux appeared to be only modestly inhibited, if at all, by calyculin A or okadaic acid. We conclude that protein hyperphosphorylation during protein phosphatase blockade selectively inhibits the Ca(2+) influx mode of Na(+)/Ca(2+) exchange, probably by an indirect mechanism that does not involve phosphorylation of the exchanger itself.

Na+-Ca2+ exchange and Ca2+ efflux in transfected Chinese hamster ovary cells

The objective of this study was to assess the contribution of Na+-Ca2+ exchange activity to Ca2+ efflux at various cytosolic Ca2+ concentrations ([Ca2+]i) in transfected Chinese hamster cells expressing the bovine cardiac Na+-Ca2+ exchanger. Ionomycin was added to fura-2 loaded cells and the resulting [Ca2+]i transient was monitored in Ca2+-free media with or without extracellular Na+. The presence of Na+ reduced both the amplitude and duration of the [Ca2+]i transient. Na+ had similar effects when the peak of the [Ca2+]i transient was buffered to 100 nM by cytosolic EGTA, or when Ca2+ was slowly released from internal stores with thapsigargin. Ca2+ efflux following ionomycin addition was directly measured with extracellular fura-2 and followed a biphasic time course (t(1/2) approximately = 10 s and 90s). The proportion of total efflux owing to the rapid phase was increased by Na+ and reduced by EGTA-loading. Na+ accelerated the initial rate of Ca2+ efflux by 65% in unloaded cells but only by 16% in EGTA-loaded cells. In both cases, the stimulation by Na+ was less than expected, given the pronounced effects of Na+ on the [Ca2+]i transient. We conclude that the exchanger contributes importantly to Ca2+ efflux activity at all [Ca2+]i values above 40 nM. We also suggest that Ca2+ efflux pathways may involve non-cytosolic or local routes of Ca2+ traffic.

Excitation-contraction coupling in gastrointestinal and other smooth muscles

The main contributors to increases in [Ca2+]i and tension are the entry of Ca2+ through voltage-dependent channels opened by depolarization or during action potential (AP) or slow-wave discharge, and Ca2+ release from store sites in the cell by the action of IP3 or by Ca(2+)-induced Ca(2+)-release (CICR). The entry of Ca2+ during an AP triggers CICR from up to 20 or more subplasmalemmal store sites (seen as hot spots, using fluorescent indicators); Ca2+ waves then spread from these hot spots, which results in a rise in [Ca2+]i throughout the cell. Spontaneous transient releases of store Ca2+, previously detected as spontaneous transient outward currents (STOCs), are seen as sparks when fluorescent indicators are used. Sparks occur at certain preferred locations--frequent discharge sites (FDSs)--and these and hot spots may represent aggregations of sarcoplasmic reticulum scattered throughout the cytoplasm. Activation of receptors for excitatory signal molecules generally depolarizes the cell while it increases the production of IP3 (causing calcium store release) and diacylglycerols (which activate protein kinases). Activation of receptors for inhibitory signal molecules increases the activity of protein kinases through increases in cAMP or cGMP and often hyperpolarizes the cell. Other receptors link to tyrosine kinases, which trigger signal cascades interacting with trimeric G-protein systems.

Pharmacomechanical coupling: the role of calcium, G-proteins, kinases and phosphatases

The concept of pharmacomechanical coupling, introduced 30 years ago to account for physiological mechanisms that can regulate contraction of smooth muscle independently of the membrane potential, has since been transformed from a definition into what we now recognize as a complex of well-defined, molecular mechanisms. The release of Ca2+ from the SR by a chemical messenger, InsP3, is well known to be initiated not by depolarization, but by agonist-receptor interaction. Furthermore, this G-protein-coupled phosphatidylinositol cascade, one of many processes covered by the umbrella of pharmacomechanical coupling, is part of complex and general signal transduction mechanisms also operating in many non-muscle cells of diverse organisms. It is also clear that, although the major contractile regulatory mechanism of smooth muscle, phosphorylation/dephosphorylation of MLC20, is [Ca2+]-dependent, the activity of both the kinase and the phosphatase can also be modulated independently of [Ca2+]i. Sensitization to Ca2+ is attributed to inhibition of SMPP-1M, a process most likely dominated by activation of the monomeric GTP-binding protein RhoA that, in turn, activates Rho-kinase that phosphorylates the regulatory subunit of SMPP-1M and inhibits its myosin phosphatase activity. It is likely that the tonic phase of contraction activated by a variety of excitatory agonists is, at least in part, mediated by this Ca(2+)-sensitizing mechanism. Desensitization to Ca2+ can occur either through inhibitory phosphorylation of MLCK by other kinases or autophosphorylation and by activation of SMPP-1M by cyclic nucleotide-activated kinases, probably involving phosphorylation of a phosphatase activator. Based on our current understanding of the complexity of the many cross-talking signal transduction mechanisms that operate in cells, it is likely that, in the future, our current concepts will be refined, additional mechanisms of pharmacomechanical coupling will be recognized, and those contributing to the pathologenesis diseases, such as hypertension and asthma, will be identified.

Large-scale expression of recombinant cardiac sodium-calcium exchange in insect larvae

Recombinant bovine cardiac sodium-calcium exchange (NCX1) in a baculovirus construct was used to infect cabbage looper larvae (Trichoplusia ni). Infected larvae were homogenized and larvae membrane vesicles were purified. Western blot analysis indicated the presence of recombinant NCX1 protein in vesicles from infected larvae but not in controls. Vesicles from infected larvae expressed high levels of NCX1 activity (1.7 nmol Ca2+/mg protein/s) while vesicles from control larvae had no activity. NCX1 in larvae vesicles was bidirectional. Kinetic analysis yielded a Vmax of 3.6 nmol Ca2+/mg protein/s and a Km for Ca of 4.2 microM. NCX1 activity was inhibited by the exchange inhibitory peptide with an IC50 of 4 microM. These data demonstrate a novel and efficient method for the expression of large amounts of active recombinant NCX1 protein that has general application for expression and analysis of recombinant membrane proteins.

Interaction of the Na(+)-Ca2+ exchanger with small molecules on cell Ca2+ signaling

Interactions of the Na(+)-Ca2+ exchanger with small molecules on cell Ca2+ signaling were elucidated in Chinese hamster ovary (CHO) C1 cells, which transfected a control vector without any expression of the Na(+)-Ca2+ exchanger's gene while CHO CK1.4 cells transfected an expression vector encoding the bovine cardiac Na(+)-Ca2+ exchanger's cDNA, treated with lithium- or sodium-buffer medium respectively, by using L16(2)15 multifactorial orthogonal statistics and fura-2 fluorescence real-time imaging. In contrast to controls of Li(+)-treated C1 cells, the store-dependent Ca(2+)-influx (SDCI) was enhanced by either the Na(+)-Ca2+ exchanger, Na(+), 1-¿(beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl¿-1H-imidazole HCl (SK&F96365) or ouabain, and by interactions of the Na(+)-Ca2+ exchanger with either Na+, SK&F96365 or both SK&F96365 and ouabain; and ATP-induced Ca2+ release (AICR) was activated by SK&F96365 or Na+ alone, interactions of the Na(+)-Ca2+ exchanger with SK&F96365 or Na+, and an interaction between SK&F96365 and ouabain. The dramatic interaction of the Na(+)-Ca2+ exchanger with small molecules indicates that cell Ca2+ signaling is generated by inositol triphosphate (InsP3)-dependent pathways, allosteric effects of the G-protein coupled P2y&2u purinoceptor and multi-site recognition. Our findings provide meaningful clues for designing new strategies of cardiocerebral vascular oxidative diseases.

Mode-actions of the Na(+)-Ca2+ exchanger: from genes to mechanisms to a new strategy in brain disorders

Mode-actions of the Na(+)-Ca2+ exchanger from genes to mechanisms to a new strategy for brain disorders were comparatively studied in oxidative stress. In transfected Chinese hamster ovary (CHO) cells steadily expressing the Na(+)-Ca2+ exchanger's gene, Ca(2+)-efflux via an active mode of the Na(+)-Ca2+ exchanger was elicited by hydrogen peroxide (H2O2) after preincubation of the cell with a Ca(2+)-free medium, whereas Ca(2+)-influx via a reverse mode of the Na(+)-Ca2+ exchanger was dramatically evoked by H2O2 after preincubation of the cell with a Ca2+ medium, as a prelude to neuronal death. According to [45Ca2+] uptake of transfected CHO cells at given time intervals or extracellular Na+[Na+]o gradients, hyperbola, logarithmic and sigmoid curve equations of the Na(+)-Ca2+ exchanger's mode-actions were respectively defined in the absence and the presence of H2O2. The Na(+)-Ca2+ exchanger's conformational transition in oxidative stress was dominated by adenosine triphosphate (ATP)-dependent cytoskeletal redox modification, cation-pi interactions and secondary Ca2+ activation. These mechanisms were used to generate an intracellulary distributed tetra-cluster (named VISA931) for rescuing G-protein agonist-sensitive signal transduction and cortico-cerebral somatosensory evoke potential (SEP) from oxidation via activating forward operation of the Na(+)-Ca2+ exchanger, the beta-adrenergic and the P2-purinergic receptors, blocking Ca2+ influx and catalyzing the dismutation of superoxide anions (O2-.) to H2O2. In conclusion, knowledge-based drug design is a new strategy for developing promising candidates of neuroprotective agents.

Sensitivity of the synaptic membrane Na+/Ca2+ exchanger and the expressed NCX1 isoform to reactive oxygen species

Two plasma membrane proteins, the Na+/Ca2+ exchanger (NCX) and the Ca2+-ATPase, are major regulators of free intraneuronal Ca2+ levels as they are responsible for extrusion of Ca2+ from the intracellular to the extracellular medium. Because disruption of cellular Ca2+ regulation plays a role in damage occurring under conditions of oxidative stress, studies were conducted to assess the sensitivity of the NCX to reactive oxygen species (ROS). Exchanger activity in brain synaptic plasma membranes and in transfected CHO-K1 cells was inhibited following brief exposure to the peroxyl radical generating azo initiator 2,2'-azobis(2-amidinopropane)dihydrochloride (AAPH) and to peroxynitrite. Incubation with hydrogen peroxide did not alter NCX activity, even at 800 microM concentration. In CHO-K1 cells transiently transfected with the NCX1 isoform of the exchanger, AAPH treatment decreased the maximal transport capacity (Vmax), whereas the K(act) remained unchanged. Peroxynitrite led to an increase in K(act) with no change in Vmax. Loss of activity following exposure to either AAPH or peroxynitrite was associated with the formation of high molecular weight aggregates of NCX, and AAPH also caused fragmentation of the exchanger protein. These findings suggest that the NCX is sensitive to biologically relevant ROS and could be involved in the loss of Ca2+ homeostasis observed under oxidative stress.

Regulation of Na+/Ca2+ exchange activity by cytosolic Ca2+ in transfected Chinese hamster ovary cells

Transfected Chinese hamster ovary cells stably expressing the bovine cardiac Na+/Ca2+ exchanger (CK1.4 cells) were used to determine the range of cytosolic Ca2+ concentrations ([Ca2+]i) that activate Na+/Ca2+ exchange activity. Ba2+ influx was measured in fura 2-loaded, ionomycin-treated cells under conditions in which the intracellular Na+ concentration was clamped with gramicidin at approximately 20 mM. [Ca2+]i was varied by preincubating ionomycin-treated cells with either the acetoxymethyl ester of EGTA or medium containing 0-1 mM added CaCl2. The rate of Ba2+ influx increased in a saturable manner with [Ca2+]i, with the half-maximal activation value of 44 nM and a Hill coefficient of 1.6. When identical experiments were carried out with cells expressing a Ca2+-insensitive mutant of the exchanger, Ba2+ influx did not vary with [Ca2+]i. The concentration for activation of exchange activity was similar to that reported for whole cardiac myocytes but approximately an order of magnitude lower than that reported for excised, giant patches. The reason for the difference in Ca2+ regulation between whole cells and membrane patches is unknown.

Oligomycin inhibits store-operated channels by a mechanism independent of its effects on mitochondrial ATP

Inhibitors of mitochondrial oxidative metabolism have been proposed to interfere with Ca2+ influx mediated by store-operated channels (SOC), secondary to their effects on ATP production. We assessed SOC activity by 45Ca2+ influx and fluorimetric measurements of free Ca2+ or Mn2+ quench in thapsigargin-treated Chinese hamster ovary cells and Jurkat T-cells, and additionally by electrophysiological measurements of the Ca2+-release-activated Ca2+ current (Icrac) in Jurkat T-cells. Various mitochondrial antagonists were confirmed to inhibit SOC. However, the following evidence supported the proposal that oligomycin, in particular, exerts an inhibitory effect on SOC in addition to its known actions on mitochondria and Na+-pump activity: (i) the concentrations of oligomycin required to inhibit SOC-mediated Ca2+ influx or Icrac (half-inhibitory concentration approximately 2 microM) were nearly 50-fold higher than the concentrations that blocked mitochondrial ATP production; (ii) the rank order of potency of oligomycins A, B and C for decreasing SOC-mediated Ca2+ influx or Icrac differed from that known for inhibition of mitochondrial function; (iii) oligomycin blocked Icrac under voltage clamp and with intracellular Na+ and K+ concentrations fixed by dialysis from the patch pipette, arguing that the effect was not secondary to membrane polarization or pump activity; and (iv) fixing the cytosolic ATP concentration by dialysis from the patch pipette attenuated rotenone- but not oligomycin-mediated inhibition of Icrac. Oligomycin also blocked volume-activated Cl- currents, a profile common to some other known blockers of SOC that are not known mitochondrial inhibitors. These findings raise the possibility that oligomycin interacts directly with SOC, and thus may extend the known pharmacological profile for this type of Ca2+-influx pathway.

Identification, expression pattern and potential activity of Na/Ca exchanger isoforms in rat pancreatic B-cells

In the pancreatic B-cell, Na/Ca exchange displays a quite high capacity and participates in the control of cytosolic free Ca2+ concentration. The Na/Ca exchanger was recently cloned in various tissues. Two genes coding for two different exchangers (NCX1 and NCX2) have been identified and evidence for several isoforms for NCX1 shown. To characterize the isoform(s) expressed in pancreatic B-cells, a RT-PCR analysis was performed on mRNA from rat pancreatic islets, purified B-cells and insulinoma B-cells (RINm5F cells). PCR amplification did not yield the expected NCX2 DNA fragment but yielded 2 NCX1 bands, corresponding to NaCa3 and NaCa7, in the three preparations. NaCa3 and NaCa7 were equally expressed in pancreatic islets and purified B-cells. In RINm5F cells, NaCa3 expression did not differ from that in islet and purified B-cells but NaCa7 was 3 times less expressed. This lower expression was accompanied by a 3 times lower Na/Ca exchange activity in RINm5F cells compared to islet cells. Our data indicate the existence of 2 NCX1 isoforms but not of NCX2 in pancreatic B-cells. The difference in both the expression patterns of NCX1 isoforms and the activity of Na/Ca exchange in islet cells and RINm5F cells is compatible with a difference in activity between NaCa3 and NaCa7.

Interactions of Na+, H2O2 and the Na+-Ca2+ exchanger stimulate Ca2+ release in CK1.4 cells

1. The present study aimed to demonstrate that interaction of cations, hydrogen peroxide (H2O2) and the Na(+)-Ca2+ exchanger stimulate Ca2+ release and oscillations of cytosolic Ca2+[Ca2+]i in non-transfected Chinese Hamster Ovary (CHO) C1 cells and in transfected CHO (CK1.4) cells that contained an expression vector coding the Na(+)-Ca2+ exchanger sequence. 2. The [45Ca2+] uptake assay, fura-2 fluorescence imaging and 2(2) and 2(3) factorial orthogonal statistics provide comparative, direct, efficient, quantitative and transient methods to delineate the effects of such interactions on Ca2+ influx, Ca2+ release and [Ca2+]i in C1 and CK1.4 cells. 3. In contrast to the control of either Na(+)-, Ca2(+)- or H2O2-free or C1 cells, an elevated [45Ca2+] uptake was induced by Ca2+, Na+ and H2O2 individually and in combination, intra-cellular Ca2+ release was activated by H2O2, and by combinations of either H2O2 and Na+, H2O2 and the Na(+)-Ca2+ exchanger, Na+ and the Na(+)-Ca2+ exchanger or by H2O2, Na+ and the Na(+)-Ca2+ exchanger and a rise in [Ca2+]i was triggered by H2O2, Na+ and a combination of Na+ and the Na(+)-Ca2+ exchanger. 4. These results indicate that interactions between H2O2, Na+ and the Na(+)-Ca2+ exchanger stimulate intracellular Ca2+ mobilization via Ca2(+)-induced Ca2+ release mechanisms, ATP-activated G-protein coupled P2y-purinoceptor-sensitive pathways, Na(+)-Ca2+ exchanger-mediated Ca2+ influx and cation-pi interaction (a strong non-covalent force between the cation and the pi face of an aromatic structure in the transmembrane protein). 5. The present findings provide important clues for understanding Ca2+ signal transduction mechanisms from the plasma membrane to the endoplasmic reticulum.

Barium influx mediated by the cardiac sodium-calcium exchanger in transfected Chinese hamster ovary cells

We examined Ba2+ influx using isotopic and fura-2 techniques in transfected Chinese hamster ovary cells expressing the bovine cardiac Na+/Ca2+ exchanger (CK1.4 cells). Ba2+ competitively inhibited exchange-mediated 45Ca2+ uptake with a Ki approximately 3 mM. Ba2+ uptake was stimulated by pretreating the cells with ouabain and by removing extracellular Na+, as expected for Na+/Ba2+ exchange activity. The maximal velocity of Ba2+ accumulation was estimated to be 50% of that for Ca2+. When the monovalent cation ionophore gramicidin was used to equilibrate internal and external concentrations of Na+, Ba2+ influx was negligible in the absence of Na+ and increased to a maximum at 20-40 mM Na+. At higher Na+ concentrations, Ba2+ influx declined, presumably due to the competition between Na+ and Ba2+ for transport sites on the exchanger. Unlike Ca2+, Ba2+ did not appear to be taken up by intracellular organelles. Thus, 133Ba2+ uptake in ouabain-treated cells was not reduced by mitochondrial inhibitors such as-Cl-CCP or oligomycin-rotenone. Moreover, intracellular Ca2+ stores that had been depleted of Ca2+ by pretreatment of the cells with ionomycin (a Ca2+ ionophore) remained empty during a subsequent period of Ba2+ influx. Ca2+ uptake or release by intracellular organelles secondarily regulated exchange activity through alterations in [Ca2+]i. Exchange-mediated Ba2+ influx was inhibited when cytosolic [Ca2+] was reduced to 20 nM or less and was accelerated at cytosolic Ca2+ concentrations of 25-50 nM We conclude that (a) Ba2+ substitutes for Ca2+ as a transport substrate for the exchanger, (b) cytosolic Ba2+ does not appear to be sequestered by intracellular organelles, and (c) exchange-mediated Ba2+ influx is accelerated by low concentrations of cytosolic Ca2+.

Acceleration of sodium-calcium exchange activity during ATP-induced calcium release in transfected Chinese hamster ovary cells

The P2U purinergic agonist ATP (0.3 mM) elicited an increase in [Ca2+]i due to Ca2+ release from intracellular stores in transfected Chinese hamster ovary cells that express the bovine cardiac Na+/Ca2+ exchanger (CK1.4 cells). The following observations indicate that ATP-evoked Ca2+ release was accompanied by a Ca(2+)-dependent regulatory activation of Na+/Ca2+ exchange activity: Addition of extracellular Ca2+ (0.7 mM) 0-1 min after ATP evoked a dramatic rise in [Ca2+]i in Na(+)-free media (Li+ substitution) compared to Na(+)-containing media; no differences between Na(+)- and Li(+)-based media were observed with vector-transfected cells. In the presence of physiological concentrations of extracellular Na+ and Ca2+, the ATP-evoked rise in [Ca2+]i declined more rapidly in CK1.4 cells compared to control cells, but then attained a long-lived plateau of elevated [Ca2+]i which eventually came to exceed the declining [Ca2+]i values in control cells. ATP elicited a transient acceleration of exchange-mediated Ba2+ influx, consistent with regulatory activation of the Na+/Ca2+ exchanger. The acceleration of Ba2+ influx was not observed in vector-transfected control cells, or in CK1.4 cells in the absence of intracellular Na+ or when the Ca2+ content of the intracellular stores had been reduced by prior treatment with ionomycin. The protein kinase C activator phorbol 12-myristate 13-acetate attenuated the exchange-mediated rise in [Ca2+]i under Na(+)-free conditions, but did not inhibit the ATP-evoked stimulation of Ba2+ influx. The effects of PMA are therefore not due to inhibition of exchange activity, but probably reflect the influence of protein kinase C on other Ca2+ homeostatic mechanisms. We conclude that exchange activity is accelerated during ATP-evoked Ca2+ release from intracellular stores through regulatory activation by increased [Ca2+]i. In the absence of extracellular Ca2+, the stimulation of exchange activity is short-lived and follows the time course of the [Ca2+]i transient; in the presence of extracellular Ca2+, we suggest that the exchanger remains activated for a longer period of time, thereby stabilizing and prolonging the plateau phase of store-dependent Ca2+ entry.

Functional expression of Na-Ca exchanger clones measured with the fluorescent Ca(2+)-indicating dye fluo-3

The process of Ca2+ homeostasis is of prime importance to all cells because of the ubiquitous role of cytoplasmic Ca2+ as an intracellular messenger and the cytotoxicity of sustained elevated cytosolic Ca2+ concentrations. Two classes of plasma membrane proteins are responsible for maintaining cytosolic free Ca2+ in the submicromolar range against a very large electrochemical Ca2+ gradient across the plasma membrane, the ATP-driven Ca2+ pump and Na-Ca exchangers. Two types of Na-Ca exchangers are known, the 3Na:1Ca exchangers found in heart, brain, kidney, and most other tissues and the 4Na:1Ca+ 1K exchanger found in retinal rod and cone photoreceptors. Functional expression of Na-Ca(/K) exchangers is most often measured as 45Ca uptake in Na(+)-loaded cells or as Na-Ca exchange currents with the giant excised patch technique. In this study, two functional assays used to detect expression of the bovine heart Na-Ca exchanger in CHO cells are described. Both assays are based on measurements of cytosolic free Ca2+ with the fluorescent Ca(2+)-indicating dye fluo-3 and should be equally applicable in the study of functional expression of both Na-Ca and Na-Ca/K exchanger clones.

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.

Expression of Na(+)-Ca2+ exchanger with modified C-terminal hydrophobic domains and enhanced activity

A 6-Kb canine cDNA fragment complementary to the 5' region of the 7-Kb mRNA encoding the cardiac Na+-Ca2+ exchanger was expressed in human kidney 293 cells. The mRNA products were reverse transcribed and amplified by PCR. The determined DNA sequence of the amplified DNA fragments revealed the presence of an intron that was alternatively spliced. The partial exon sequence, located at the 3' end of the 6-Kb cDNA, was alternatively connected to bases 3198, 2821, 2620 and 1844 in four types of splicing products identified. In the largest product the adjoining exon was located after the putative stop codon of the regular sequence. In a second and third type of shortened transcripts, a hydrophobic sequence encoded by the spliced-in exon was linked with the 4th or the 5th extracellular loops, and could possibly replace transmembrane segments 9 or 11. In the fourth type of spliced transcript the in-frame exon sequence introduced one Leu followed by a stop codon in the large hydrophilic loop. Measurements of Ca2+ uptake in 293 cells expressing the modified exchanger indicated a higher activity in comparison with 293 cells expressing the 3.7-Kb cDNA, in which this alternative splicing does not occur. Deletion mutagenesis of the C-terminal region encoded by the spliced-in exon was performed to investigate its role in the enhancement of the transport activity.

Na-Ca exchange in circulating blood cells

The experiments with peripheral lymphocytes raise two provocative questions: is SDCI composed of Ca influx via both a Ca channel and Na-Ca exchanger?, and what is the role of Na-Ca exchange in lymphocytes? In regard to the first issue, the potential for this dual Ca influx pathway exists, inasmuch as both Ca store depletion (by exposure of cells to EGTA) and TG-treatment initiated Ca influx that was enhanced following reversal of the Na gradient. These data could be interpreted to suggest a role for Ca influx via the exchanger during lymphocyte activation. However, our ability to demonstrate Na-Ca exchange activity was facilitated by the removal of Ca sequestering or extrusion mechanisms, including SERCA Ca pumps and forward mode Na-Ca exchange. Thus, it seems likely that under physiological conditions the primary function of the exchanger is to mediate Ca efflux. In this regard, it might play a role in lymphocyte activation by limiting net Ca entry during the sustained phase of Ca mobilization. Since sustained Ca entry is critical for Ca-dependent processes including interleukin-2 production, exchange activity would be an important modulator of this process. Changes in membrane potential, intracellular [Na] and cytosolic pH could therefore regulate Cai through its effects on Na-Ca exchange activity. Future challenges include defining the role of the Na-Ca exchange in Cai homeostasis and characterizing its function in lymphocyte populations.

Sodium-calcium exchange and store-dependent calcium influx in transfected chinese hamster ovary cells expressing the bovine cardiac sodium-calcium exchanger. Acceleration of exchange activity in thapsigargin-treated cells

The effects of extracellular Na+ on store-dependent Ca2+ influx were compared for transfected Chinese hamster ovary cells expressing the bovine cardiac Na+-Ca2+ exchanger (CK1.4 cells) and vector-transfected control cells. Store-dependent Ca2+ influx was elicited by depletion of intracellular Ca2+ stores with ionomycin, thapsigargin, or extracellular ATP, a purinergic agonist. In each case, the rise in [Ca2+]i upon the addition of extracellular Ca2+ was reduced in CK1.4 cells compared with control cells at physiological [Na+]o. When Li+ or NMDG was substituted for Na+, the CK1.4 cells showed a greater rise in [Ca2+]i than control cells over the subsequent 3 min after the addition of Ca2+o. Under Na+-free conditions, SK&F 96365 (50 microM), a blocker of store-operated Ca2+ channels, nearly abolished the thapsigargin-induced rise in [Ca2+]i in the control cells but only partially inhibited this response in the CK1.4 cells. We conclude that in the CK1.4 cells, Ca2+ entry through store-operated channels was counteracted by Na+o-dependent Ca2+ efflux at physiological [Na+]o, whereas Ca2+ entry was enhanced through Na+i-dependent Ca2+ influx in the Na+-free medium. We examined the effects of thapsigargin on Ba2+ entry in the CK1.4 cells because Ba2+ is transported by the Na+-Ca2+ exchanger, but it enters these cells only poorly through store-operated channels, and it is not sequestered by intracellular organelles. Thapsigargin treatment stimulated Ba2+ influx in a Na+-free medium, consistent with an acceleration of Ba2+ entry through the Na+-Ca2+ exchanger. We conclude that organellar Ca2+ release induces a regulatory activation of Na+-Ca2+ exchange activity.

Isolation and characterization of a stable Chinese hamster ovary cell line overexpressing the plasma membrane Ca(2+)-ATPase

Stable Chinese hamster ovary (CHO) cell lines overexpressing the human plasma membrane Ca(2+)-ATPase (PMCA) were generated, and three independent cell clones were characterized in details. They overexpressed high amounts of active PMCA pump (15-20 times over the amount of endogenous PMCA) as indicated by experiments in which the formation of the phosphoenzyme intermediate and the uptake of Ca2+ by microsomes were measured. Immunocytochemistry experiments coupled to the biotinylation of the pump in the intact cells indicated the correct deliver of the expressed pump to the plasma membrane. The expressed pump was purified by affinity chromatography on calmodulin sepharose. The PMCA of transfected CHO cells promoted an increase of Ca2+ into the medium, after induction of Ca2+ release from the internal stores by activation of a purinergic receptor. An evident decrease of the activity of the endogenous sarcoplasmic reticulum Ca(2+)-ATPase pump was observed, probably related to the down-regulation of its expression. The cells overexpressing the PMCA pump had delayed recovery after trypsinization and plating. Their doubling time was, however, the same as CHO cells.

ATP-dependent regulation of sodium-calcium exchange in Chinese hamster ovary cells transfected with the bovine cardiac sodium-calcium exchanger

Chinese hamster ovary cells expressing the bovine cardiac Na/Ca exchanger were treated with ouabain to increase [Na+]i and stimulate Ca2+ influx by Na/Ca exchange. Depletion of cellular ATP inhibited 45Ca uptake by 40% or more and reduced the half-maximal Na+ concentration for inhibition of 45Ca uptake from 90 to 55 mM. ATP depletion also reduced the rate of rise in [Ca2+]i when [Na+]o was reduced and inhibited the decline in [Ca2+]i when high [Na+]o was restored. The effects of ATP depletion were either absent or reduced in cells expressing a mutant exchanger missing most of the cytosolic hydrophilic domain. We were unable to detect a phosphorylated form of the exchanger in immunoprecipitates from 32P-labeled cells. ATP depletion caused a breakdown in the actin cytoskeleton of the cells. Treatment of the cells with cytochalasin D mimicked the effects of ATP depletion on the [Na+] inhibition profile for 45Ca uptake. Thus, ATP depletion inhibits both the Ca2+ influx and Ca2+ efflux modes of Na/Ca exchange, and may alter the competitive interactions of extracellular Na+ and Ca2+ with the transporter. The latter effect appears to be related to changes in the actin cytoskeleton.

An alternative splicing site modifies the carboxyl-terminal trans-membrane domains of the Na+/Ca2+ exchanger

The 6-kilobase (kb) cDNA of pTB11 clone and its 5' fragment of 3.7 kb encoding the canine heart Na+/Ca2+ exchanger (Nicoll, D.A., Longoni, S., and Philipson, K.D. (1990) Science 250, 562-565) were transiently expressed in 293 cells to investigate the role of the 3'-"untranslated" region. Both fragments yielded high levels of expressed protein that were well incorporated in the membranes. Cells expressing the 6-kb cDNA produced rearranged transcripts of smaller than expected size. A 120-kDa polypeptide was produced in cells expressing the modified exchanger, and Ca2+ uptake was higher in this type of transfected cells. A constant stretch of nucleotides located at the 3' end of the 6 kb cDNA was found to be connected, by alternative RNA splicing, to four different upstream sequence positions. The deduced hydrophobic sequence of the spliced-in exon could replace the IX or the XI trans-membrane domain of the exchanger protein in two spliced isoforms. The new exon sequence was not completely included in the pTB11 insert, i.e. these two products were artificially truncated. The RNA processing of these two alternative 5'-splicing sites also occurred in tissues, as shown by RNase protection analysis. In a third type of isoform the splicing took place downstream of the originally proposed stop codon, whereas in a fourth type a stop codon was introduced after the V hydrophobic segment in the large intracellular loop.

Superficial buffer barrier function of smooth muscle sarcoplasmic reticulum

In smooth muscle the superficial sarcoplasmic reticulum accumulates a portion of the Ca2+ that enters cells through the plasmalemma and thus functions as a buffer barrier to Ca2+ entry into the myoplasm (superficial buffer barrier or SBB). In this review Cornelis van Breemen, Qian Chen and Ismail Laher summarize experimental support for the SBB, and discuss data indicating that: (1) contraction is related more to the rate than extent of Ca2+ entry; (2) refilling of sarcoplasmic reticulum from the extracellular space is mediated by Ca2+ influx and Ca2+ pumping by the sarcoplasmic reticulum Ca2+ pump; (3) the superficial sarcoplasmic reticulum unloads Ca2+ to the extracellular space by a multi step process that involves sequentially the opening of Ca2+ and inositol 1,4,5-trisphosphate [Ins(1,4,5,)P3] sensitive channels and Ca2+ extrusion by Na(+)-Ca2+ exchange; (4) the SBB generates a peripheral Ca2+ gradient; (5) Ca(2+)-mobilizing receptor agonists generate Ins(1,4,5)P3 which short circuits the SBB to increase the effectiveness of Ca2+ influx in raising [Ca2+]i and consequently increase smooth muscle contraction. A physiologically regulated SBB is thought to enhance the informational content of Ca2+ signalling and support variable reduction of smooth muscle tone. Pharmacological modulation of Ca2+ transport in the superficial sarcoplasmic reticulum therefore presents an alternative means of controlling smooth muscle tone dependent on Ca2+ entry.

Na+/Ca2+ exchange-mediated calcium entry in human lymphocytes

Regulation of cytosolic Ca2+ and cytosolic Na+ is critical for lymphocyte cation homeostasis and function. To examine the influence of cytosolic Na+ on Ca2+ regulation in human peripheral blood lymphocytes, Ca2+ entry and cytosolic Ca2+ (measured with fura-2) were monitored in cells in which cytosolic Na+ was increased and/or the Na+ gradient was decreased by reduction of external Na+ concentration. Ouabain-treated cells (0.1 mM for 30 min at 37 degrees C), suspended in Na(+)-free medium, showed a 30-65% increase in Ca2+ uptake compared to cells in 140 mM Na+ medium. Enhanced Ca2+ influx was entirely dependent on ouabain pretreatment and reversal of the Na+ gradient. Na pump inhibition or Na ionophore addition and subsequent exposure to Na(+)-free medium resulted in a sustained elevation of cytosolic Ca2+. As preincubation of cells in Ca(2+)-free medium further enhanced the ouabain-dependent increase in cytosolic Ca2+, the effects of the microsomal Ca(2+)-ATPase inhibitor thapsigargin on Ca2+ influx and cytosolic Ca2+ were studied. Thapsigargin stimulated Ca2+ entry following ouabain pretreatment and reversal of the Na+ gradient; the effects of thapsigargin were retained in the presence of LaCl3, a potent inhibitor of store-dependent calcium influx pathways. These results show lymphocytes demonstrate Na+/Ca2+ exchange activity and suggest the Na+/Ca2+ exchanger modulates cytosolic Ca2+ following intracellular Ca2+ store depletion.