Differential modulation of the phases of a Ca2+ spike by the store Ca2+-ATPase in human umbilical vein endothelial cells

Reduced SERCA activity underlies dysregulation of Ca2+ homeostasis under atmospheric O2 levels

Unregulated increases in cellular Ca²⁺ homeostasis are a hallmark of pathophysiological conditions and a key trigger of cell death. Endothelial cells cultured under physiologic O₂ conditions (5% O₂) exhibit a reduced cytosolic Ca²⁺ response to stimulation. The mechanism for reduced plateau [Ca²⁺]_i upon stimulation was due to increased sarco/endoplasmic reticulum Ca²⁺ ATPase (SERCA)-mediated reuptake rather than changes in Ca²⁺ influx capacity. Agonist-stimulated phosphorylation of the SERCA regulatory protein phospholamban was increased in cells cultured under 5% O₂. Elevation of cytosolic and mitochondrial [Ca²⁺] and cell death after prolonged ionomycin treatment, as a model of Ca²⁺ overload, were lower when cells were cultured long-term under 5% compared with 18% O₂. This protection was abolished by cotreatment with the SERCA inhibitor cyclopiazonic acid. Taken together, these results demonstrate that culturing cells under hyperoxic conditions reduces their ability to efficiently regulate [Ca²⁺]_i, resulting in greater sensitivity to cytotoxic stimuli.-Keeley, T. P., Siow, R. C. M., Jacob, R., Mann, G. E. Reduced SERCA activity underlies dysregulation of Ca²⁺ homeostasis under atmospheric O₂ levels.

Calcineurin is part of a negative feedback loop in the InsP3/Ca²⁺ signalling pathway in blowfly salivary glands

The ubiquitous InsP3/Ca(2+) signalling pathway is modulated by diverse mechanisms, i.e. feedback of Ca(2+) and interactions with other signalling pathways. In the salivary glands of the blowfly Calliphora vicina, the hormone serotonin (5-HT) causes a parallel rise in intracellular [Ca(2+)] and [cAMP] via two types of 5-HT receptors. We have shown recently that cAMP/protein kinase A (PKA) sensitizes InsP3-induced Ca(2+) release. We have now identified the protein phosphatase that counteracts the effect of PKA on 5-HT-induced InsP3/Ca(2+) signalling. We demonstrate that (1) tautomycin and okadaic acid, inhibitors of protein phosphatases PP1 and PP2A, have no effect on 5-HT-induced Ca(2+) signals; (2) cyclosporin A and FK506, inhibitors of Ca(2+)/calmodulin-activated protein phosphatase calcineurin, cause an increase in the frequency of 5-HT-induced Ca(2+) oscillations; (3) the sensitizing effect of cyclosporin A on 5-HT-induced Ca(2+) responses does not involve Ca(2+) entry into the cells; (4) cyclosporin A increases InsP3-dependent Ca(2+) release; (5) inhibition of PKA abolishes the effect of cyclosporin A on the 5-HT-induced Ca(2+) responses, indicating that PKA and calcineurin act antagonistically on the InsP3/Ca(2+) signalling pathway. These findings suggest that calcineurin provides a negative feedback on InsP3/Ca(2+) signalling in blowfly salivary glands, counteracting the effect of PKA and desensitizing the signalling cascade at higher 5-HT concentrations.

Update on vascular endothelial Ca2+ signalling: A tale of ion channels, pumps and transporters

A monolayer of endothelial cells (ECs) lines the lumen of blood vessels and forms a multifunctional transducing organ that mediates a plethora of cardiovascular processes. The activation of ECs from as state of quiescence is, therefore, regarded among the early events leading to the onset and progression of potentially lethal diseases, such as hypertension, myocardial infarction, brain stroke, and tumor. Intracellular Ca²⁺ signals have long been know to play a central role in the complex network of signaling pathways regulating the endothelial functions. Notably, recent work has outlined how any change in the pattern of expression of endothelial channels, transporters and pumps involved in the modulation of intracellular Ca²⁺ levels may dramatically affect whole body homeostasis. Vascular ECs may react to both mechanical and chemical stimuli by generating a variety of intracellular Ca²⁺ signals, ranging from brief, localized Ca²⁺ pulses to prolonged Ca²⁺ oscillations engulfing the whole cytoplasm. The well-defined spatiotemporal profile of the subcellular Ca²⁺ signals elicited in ECs by specific extracellular inputs depends on the interaction between Ca²⁺ releasing channels, which are located both on the plasma membrane and in a number of intracellular organelles, and Ca²⁺ removing systems. The present article aims to summarize both the past and recent literature in the field to provide a clear-cut picture of our current knowledge on the molecular nature and the role played by the components of the Ca²⁺ machinery in vascular ECs under both physiological and pathological conditions.

eNOS activation and NO function: pregnancy adaptive programming of capacitative entry responses alters nitric oxide (NO) output in vascular endothelium--new insights into eNOS regulation through adaptive cell signaling

In pregnancy, vascular nitric oxide (NO) production is increased in the systemic and more so in the uterine vasculature, thereby supporting maximal perfusion of the uterus. This high level of functionality is matched in the umbilical vein, and in corresponding disease states such as pre-eclampsia, reduced vascular responses are seen in both uterine artery and umbilical vein. In any endothelial cell, NO actually produced by endothelial NO synthase (eNOS) is determined by the maximum capacity of the cell (eNOS expression levels), eNOS phosphorylation state, and the intracellular [Ca(2+)](i) concentration in response to circulating hormones or physical forces. Herein, we discuss how pregnancy-specific reprogramming of NO output is determined as much by pregnancy adaptation of [Ca(2+)](i) signaling responses as it is by eNOS expression and phosphorylation. By examining the changes in [Ca(2+)](i) signaling responses from human hand vein endothelial cells, uterine artery endothelial cells, and human umbilical vein endothelial cells in (where appropriate) nonpregnant, normal pregnant, and pathological pregnant (pre-eclamptic) state, it is clear that pregnancy adaptation of NO output occurs at the level of sustained phase 'capacitative entry' [Ca(2+)](i) response, and the adapted response is lacking in pre-eclamptic pregnancies. Moreover, gap junction function is an essential permissive regulator of the capacitative response and impairment of NO output results from any inhibitor of gap junction function, or capacitative entry using TRPC channels. Identifying these [Ca(2+)](i) signaling mechanisms underlying normal pregnancy adaptation of NO output not only provides novel targets for future treatment of diseases of pregnancy but may also apply to other common forms of hypertension.

Cumulated Ca2+ spike duration underlies Ca2+ oscillation frequency-regulated NFκB transcriptional activity

[Ca2+]i oscillations drive downstream events, like transcription, in a frequency-dependent manner. Why [Ca2+]i oscillation frequency regulates transcription has not been clearly revealed. A variation in [Ca2+]i oscillation frequency apparently leads to a variation in the time duration of cumulated [Ca2+]i elevations or cumulated [Ca2+]i spike duration. By manipulating [Ca2+]i spike duration, we generated a series of [Ca2+]i oscillations with the same frequency but different cumulated [Ca2+]i spike durations, as well as [Ca2+]i oscillations with the different frequencies but the same cumulated [Ca2+]i spike duration. Molecular assays demonstrated that, when generated in ‘artificial’ models alone, under physiologically simulated conditions or repetitive pulses of agonist exposure, [Ca2+]i oscillation regulates NFκB transcriptional activity, phosphorylation of IκBα and Ca2+-dependent gene expression all in a way actually dependent on cumulated [Ca2+]i spike duration whether or not frequency varies. This study underlines that [Ca2+]i oscillation frequency regulates NFκB transcriptional activity through cumulated [Ca2+]i spike-duration-mediated IκBα phosphorylation.

Ca2+ release from the endoplasmic reticulum of NY-ESO-1-specific T cells is modulated by the affinity of TCR and by the use of the CD8 coreceptor

Although several cancer immunotherapy strategies are based on the use of analog peptides and on the modulation of the TCR affinity of adoptively transferred T cells, it remains unclear whether tumor-specific T cell activation by strong and weak TCR stimuli evoke different Ca(2+) signatures from the Ca(2+) intracellular stores and whether the amplitude of Ca(2+) release from the endoplasmic reticulum (ER) can be further modulated by coreceptor binding to peptide/MHC. In this study, we combined functional, structural, and kinetic measurements to correlate the intensity of Ca(2+) signals triggered by the stimulation of the 1G4 T cell clone specific to the tumor epitope NY-ESO-1(157-165). Two analogs of the NY-ESO-1(157-165) peptide, having similar affinity to HLA-A2 molecules, but a 6-fold difference in binding affinity for the 1G4 TCR, resulted in different Ca(2+) signals and T cell activation. 1G4 stimulation by the stronger stimulus emptied the ER of stored Ca(2+), even in the absence of CD8 binding, resulting in sustained Ca(2+) influx. In contrast, the weaker stimulus induced only partial emptying of stored Ca(2+), resulting in significantly diminished and oscillatory Ca(2+) signals, which were enhanced by CD8 binding. Our data define the range of TCR/peptide MHC affinities required to induce depletion of Ca(2+) from intracellular stores and provide insights into the ability of T cells to tailor the use of the CD8 coreceptor to enhance Ca(2+) release from the ER. This, in turn, modulates Ca(2+) influx from the extracellular environment, ultimately controlling T cell activation.

Investigating cADPR and NAADP in intact and broken cell preparations

The body of literature characterizing cyclic adenosine diphosphoribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) as Ca2+-mobilizing second messengers is growing apace. However, their unique properties may, for the uninitiated, make them difficult to work with. This article reviews many of the available techniques (and associated pitfalls) for investigating these nucleotide messengers, predominantly focusing upon optical techniques using fluorescent reporters to measure Ca2+ in the cytosol as well as Ca2+ or pH within the lumen of intracellular organelles.

Ca2+ extrusion in aged smooth muscle cells

We investigated the effects of aging in Ca(2+) extrusion mechanisms in smooth muscle bladder cells from 4 and 20-24-month-old guinea pigs using fluorescence microscopy and fura-2. Cells were challenged with a pulse of KCl immediately before perfusion with a Ca(2+) free solution containing no inhibitors (control, untreated cells) or inhibitors of plasma membrane Ca(2+) pump (PMCA, 1mM La(3+)), Na(+)/Ca(2+) exchanger (NCX, 1 microM SEA0400) or the sarcoendoplasmic Ca(2+) pump (SERCA, 1 microM thapsigargin). Treatment of young adult cells with the inhibitors allowed estimating a relative contribution of 55% for NCX, 27% for PMCA and 31% for SERCA. Combination of two inhibitors at the same time showed the presence of interaction between extrusion mechanisms. In aged cells the [Ca(2+)](i) extrusion was impaired due to decrease of PMCA activity, as revealed by the loss of effect of La(3+), and to inhibitory interactions between NCX and SERCA activities, indicated by acceleration of decay in response to their respective inhibitors. In conclusion, in smooth muscle cells aging decreases the overall Ca(2+) extrusion activity and modifies the interactions between the activities of the main Ca(2+) removing mechanisms.

Tachyphylaxis to the inhibitory effect of L-type channel blockers on ACh-induced [Ca2+]i oscillations in porcine tracheal myocytes

Discrepancies about the role of L-type voltage-gated calcium channels (VGCC) in acetylcholine (ACh)-induced [Ca(2+)](i) oscillations in tracheal smooth muscle cells (TSMCs) have been seen in recent reports. We demonstrate here that ACh-induced [Ca(2+)](i) oscillations in TMCS were reversibly inhibited by three VGCC blockers, nicardipine, nifedipine and verapamil. Prolonged (several minutes) application of VGCC blockers, led to tachyphylaxis; that is, [Ca(2+)](i) oscillations resumed, but at a lower frequency. Brief (15-30 s) removal of VGCC blockers re-sensitized [Ca(2+)](i) oscillations to inhibition by the agents. Calcium oscillations tolerant to VGCC blockers were abolished by KB-R7943, an inhibitor of the reverse mode of Na(+)/Ca(2+) exchanger (NCX). KB-R7943 alone also abolished ACh-induced [Ca(2+)](i) oscillations. Enhancement of the reverse mode of NCX via removing extracellular Na(+) reversed inhibition of ACh-induced [Ca(2+)](i) oscillations by VGCC blockers. Inhibition of non-selective cation channels using Gd(3+) slightly reduced the frequency of ACh-induced [Ca(2+)](i) oscillations, but did not prevent the occurrence of tachyphylaxis. Altogether, these results suggest that VGCC and the reverse mode of NCX are two primary Ca(2+) entry pathways for maintaining ACh-induced [Ca(2+)](i) oscillations in TSMCs. The two pathways complement each other, and may account for tachyphylaxis of ACh-induced [Ca(2+)](i) oscillations to VGCC blockers.

Depletion of intracellular Ca2+ stores enhances flow-induced vascular dilatation in rat small mesenteric artery

The effect of depleting intracellular Ca2+ stores on flow-induced vascular dilatation and the mechanism responsible for the vasodilatation were examined in rat isolated small mesenteric arteries. The arteries were pressurized to 50 mmHg and preconstricted with phenylephrine. Intraluminal flow reversed the effect of phenylephrine, resulting in vasodilatation. Flow dilatation consisted of an initial transient peak followed by a sustained plateau phase. The magnitude of dilatation was markedly reduced by removing Ca2+ from the intraluminal flow medium. Depletion of intracellular Ca2+ stores with either cyclopiazonic acid (CPA, 2 microM) or 1,4-dihydroxy-2,5-di-tert-butylbenzene (BHQ, 10 microM) significantly augmented the magnitude of flow dilatation. Flow-induced endothelial cell Ca2+ influx was also markedly enhanced in arteries pretreated with CPA or BHQ.Flow-induced dilatation was insensitive to Nw-nitro-L-arginine methyl ester (100 microM) plus indomethacin (3 microM) or to oxyhemoglobin (3 microM), but was markedly reduced by 30 mM extracellular K+ or 2 mM tetrabutylammonium (TBA), suggesting an involvement of EDHF. Catalase at 1200 U ml-1 abolished the flow-induced dilatation, while the application of exogenous H2O2 (90-220 microM) induced relaxation in phenylephrine-preconstricted arteries. Relaxation to exogenous H2O2 was blocked in the presence of 30 mM extracellular K+, and H2O2 (90 microM) hyperpolarized the smooth muscle cells, indicating that H2O2 can act as an EDHF. In conclusion, flow-induced dilatation in rat mesenteric arteries can be markedly enhanced by prior depletion of intracellular Ca2+ stores. Furthermore, these data are consistent with a role for H2O2 as the vasodilator involved.

Transcription of the sarcoplasmic/endoplasmic reticulum Ca2+-ATPase type 3 gene, ATP2A3, is regulated by the calcineurin/NFAT pathway in endothelial cells

Histamine, known to induce Ca2+ oscillations in endothelial cells, was used to alter Ca2+ cycling. Treatment of HUVEC (human umbilical-vein endothelial cell)-derived EA.hy926 cells with histamine for 1-3 days increased the levels of SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase) 3, but not of SERCA 2b, transcripts and proteins. Promoter-reporter gene assays demonstrated that this increase in expression was due to activation of SERCA 3 gene transcription. The effect of histamine was abolished by mepyramine, but not by cimetidine, indicating that the H1 receptor, but not the H2 receptor, was involved. The histamine-induced up-regulation of SERCA 3 was abolished by cyclosporin A and by VIVIT, a peptide that prevents calcineurin and NFAT (nuclear factor of activated T-cells) from interacting, indicating involvement of the calcineurin/NFAT pathway. Histamine also induced the nuclear translocation of NFAT. NFAT did not directly bind to the SERCA 3 promoter, but activated Ets-1 (E twenty-six-1), which drives the expression of the SERCA 3 gene. Finally, cells treated with histamine and loaded with fura 2 exhibited an improved capacity in eliminating high cytosolic Ca2+ concentrations, in accordance with an increase in activity of a low-affinity Ca2+-ATPase, like SERCA 3. Thus chronic treatment of endothelial cells with histamine up-regulates SERCA 3 transcription. The effect of histamine is mediated by the H1R (histamine 1 receptor) and involves activation of the calcineurin/NFAT pathway. By increasing the rate of Ca2+ sequestration, up-regulation of SERCA 3 counteracts the cytosolic increase in Ca2+ concentration.

Annihilation of limit-cycle oscillations by identification of critical perturbing stimuli via mixed-integer optimal control

We present a novel model-based mixed-integer optimal control method to automatically identify the strength and timing of critical external stimuli leading to the transient annihilation of limit-cycle oscillators. Biochemical oscillators of this type play a central role in regulating cellular rhythms. Their specific manipulation is a promising perspective to control biological functions by drugs and tailored treatment strategies. We demonstrate our new optimal control approach in an application to a biochemical model for oscillatory calcium signal transduction.

Ca2+-induced Ca2+ release by activation of inositol 1,4,5-trisphosphate receptors in primary pancreatic beta-cells

The effect of sarcoendoplasmic reticulum Ca(2+)-ATPase (SERCA) inhibition on the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) was studied in primary insulin-releasing pancreatic beta-cells isolated from mice, rats and human subjects as well as in clonal rat insulinoma INS-1 cells. In Ca(2+)-deficient medium the individual primary beta-cells reacted to the SERCA inhibitor cyclopiazonic acid (CPA) with a slow rise of [Ca(2+)](i) followed by an explosive transient elevation. The [Ca(2+)](i) transients were preferentially observed at low intracellular concentrations of the Ca(2+) indicator fura-2 and were unaffected by pre-treatment with 100 microM ryanodine. Whereas 20mM caffeine had no effect on basal [Ca(2+)](i) or the slow rise in response to CPA, it completely prevented the CPA-induced [Ca(2+)](i) transients as well as inositol 1,4,5-trisphosphate-mediated [Ca(2+)](i) transients in response to carbachol. In striking contrast to the primary beta-cells, caffeine readily mobilized intracellular Ca(2+) in INS-1 cells under identical conditions, and such mobilization was prevented by ryanodine pre-treatment. The results indicate that leakage of Ca(2+) from the endoplasmic reticulum after SERCA inhibition is feedback-accelerated by Ca(2+)-induced Ca(2+) release (CICR). In primary pancreatic beta-cells this CICR is due to activation of inositol 1,4,5-trisphosphate receptors. CICR by ryanodine receptor activation may be restricted to clonal beta-cells.

Evidence that Ca(2+) cycling by the plasma membrane Ca(2+)-ATPase increases the 'excitability' of the extracellular Ca(2+)-sensing receptor

The extracellular Ca(2+)-sensing receptor (CaR) is a widely expressed G-protein-coupled receptor that translates information about [Ca(2+)] in the extracellular milieu to the interior of the cell, usually via intracellular Ca(2+) signaling pathways. Using fura-2 imaging of cytoplasmic [Ca(2+)], we observed that HEK293 cells expressing CaR oscillated readily under conditions permissive for CaR activation. Spiking was also triggered in the absence of external Ca(2+) by the CaR agonist spermine (1 mM). Oscillating cells were typically located in clusters of closely apposed cells, but Ca(2+) spiking was insensitive to the gap junction inhibitor 18alpha-glycyrrhetinic acid. We hypothesized that Ca(2+) signals might be amplified, in part, through a positive feedback loop in which Ca(2+) extrusion via the plasma membrane Ca(2+)-ATPase (PMCA) activates CaRs on the same cell or adjacent cells through local increases in [Ca(2+)](out). In support of this idea, addition of exogenous Ca(2+) buffers (keeping free [Ca(2+)](out) constant) attenuated or eliminated Ca(2+) signals (manifested as oscillations), as did PMCA inhibitors (HgCl(2), orthovanadate and Caloxin 2A1). Measurement of extracellular [Ca(2+)] using the near membrane probe fura-C(18) revealed that external [Ca(2+)] rose following receptor activation, sometimes displaying an oscillatory pattern. Our data suggest that PMCA-mediated cycling of Ca(2+) across the plasma membrane leads to localized increases in [Ca(2+)](out) that increase the excitability of CaR.

Role of endothelial Ca2+ stores in the regulation of hydraulic conductivity of Rana microvessels in vivo

Vascular permeability is regulated by endothelial cytosolic Ca(2+) concentration ([Ca(2+)](i)). To determine whether vascular permeability is dependent on extracellular Ca(2+) influx or release of Ca(2+) from stores, hydraulic conductivity (L(p)) was measured in single perfused frog mesenteric microvessels in the presence and absence of Ca(2+) influx and store depletion. Prevention of Ca(2+) reuptake into stores by sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) inhibition increased L(p) in the absence of extracellular Ca(2+) influx. L(p) was further increased when Ca(2+) influx was restored. Depletion of the Ca(2+) stores with ionomycin and SERCA inhibition increased L(p) in the presence and the absence of extracellular Ca(2+) influx. However, store depletion in itself did not significantly increase L(p) in the absence of active Ca(2+) release from stores into the cytoplasm. There was a significant positive correlation between baseline permeability and the magnitude of the responses to both Ca(2+) store release and Ca(2+) influx, indicating that the Ca(2+) regulating properties of the endothelial cells may regulate the baseline L(p). To investigate the role of Ca(2+) stores in regulation of L(p), the relationship between SERCA inhibition and store release was studied. The magnitude of the L(p) increase during SERCA inhibition significantly and inversely correlated with that during store release by Ca(2+) ionophore, implying that the degree of store depletion regulates the size of the increase on L(p). These data show that microvascular permeability in vivo can be increased by agents that release Ca(2+) from stores in the absence of Ca(2+) influx. They also show that capacitative Ca(2+) entry results in increased L(p) and that the size of the permeability increase can be regulated by the degree of Ca(2+) release.

Three novel sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) 3 isoforms. Expression, regulation, and function of the membranes of the SERCA3 family

Sarco/endoplasmic reticulum Ca2+-ATPases (SERCAs) pump Ca2+ into the endoplasmic reticulum. Recently, three human SERCA3 (h3a-c) proteins and a previously unknown rat SERCA3 (r3b/c) mRNA have been described. Here, we (i) document two novel human SERCA3 splice variants h3d and h3e, (ii) provide data for the expression and mechanisms regulating the expression of all known SERCA3 variants (r3a, r3b/c, and h3a-e), and (iii) show functional characteristics of the SERCA3 isoforms. h3d and h3e are issued from the insertion of an additional penultimate exon 22 resulting in different carboxyl termini for these variants. Distinct distribution patterns of the SERCA3 gene products were observed in a series of cell lines of hematopoietic, epithelial, embryonic origin, and several cancerous types, as well as in panels of rat and human tissues. Hypertension and protein kinase C, calcineurin, or retinoic acid receptor signaling pathways were found to differently control rat and human splice variant expression, respectively. Stable overexpression of each variant was performed in human embryonic kidney 293 cells, and the SERCA3 isoforms were fully characterized. All SERCA3 isoforms were found to pump Ca2+ with similar affinities. However, they modulated the cytosolic Ca2+ concentration ([Ca2+]c) and the endoplasmic reticulum Ca2+ content ([Ca2+]er) in different manners. A newly generated polyclonal antibody and a pan-SERCA3 antibody proved the endogenous expression of the three novel SERCA3 proteins, h3d, h3e, and r3b/c. All these data suggest that the SERCA3 gene products have a more widespread role in cellular Ca2+ signaling than previously appreciated.

Structural and functional relationships between Ca2+ puffs and mitochondria in Xenopus oocytes

Ca2+ uptake and release from endoplasmic reticulum (ER) and mitochondrial Ca2+ stores play important physiological and pathological roles, and these processes are shaped by interactions that depend on the structural intimacy between these organelles. Here we investigate the morphological and functional relationships between mitochondria, ER, and the sites of intracellular Ca2+ release in Xenopus laevis oocytes by combining confocal imaging of local Ca2+ release events ("Ca2+ puffs") with mitochondrial localization visualized using vital dyes and subcellularly targeted fluorescent proteins. Mitochondria and ER are localized in cortical bands approximately 6-8 microm wide, with the mitochondria arranged as densely packed "islands" interconnected by discrete strands. The ER is concentrated more superficially than mitochondria, and the mean separation between Ca2+ puff sites and mitochondria is approximately 2.3 microm. However, a subpopulation of Ca2+ puff sites is intimately associated with mitochondria (approximately 28% within <600 nm), a greater number than expected if Ca2+ puff sites were randomly distributed. Ca2+ release sites close to mitochondria exhibit lower Ca2+ puff activity than Ca2+ puff sites in regions with lower mitochondrial density. Furthermore, Ca2+ puff sites in close association with mitochondria rarely serve as the sites for Ca2+ wave initiation. We conclude that mitochondria play important roles in regulating local ER excitability, Ca2+ wave initiation, and, thereby, spatial patterning of global Ca2+ signals.

Preeclampsia is associated with altered Ca2+ regulation and NO production in human fetal venous endothelial cells

Preeclampsia (PE) is a leading cause of maternal hypertension in pregnancy, fetal growth restriction, premature birth, and fetal and maternal mortality (1). Activation and dysfunction of the maternal and fetal endothelium in PE may be the consequence of increased oxidative stress associated with circulating lipid peroxides (2-4), and in cases of severe maternal hypertension, uterine and umbilical artery waveforms are abnormal (5). We have investigated PE-associated abnormalities in the regulation of intracellular Ca2+ ([Ca2+]i) and cyclic guanosine monophosphate (cGMP) production (index of nitric oxide [NO]) in human fetal umbilical vein endothelial cells. Basal [Ca2+]i was slightly elevated in PE cells, whereas agonist-stimulated Ca2+ entry was reduced in cells from PE compared with normal term or age-matched preterm pregnancies. Furthermore, PE cells exhibited a decreased permeability to Ba2+ but an increased permeability to Mn2+ and Gd3+, suggesting that PE is associated with phenotypic alterations in fetal endothelial cation channel(s). Basal and histamine-stimulated cGMP levels were elevated in PE compared with preterm or normal cells, implying an increased NO production in PE. However, immunoblots for endothelial NO synthase (eNOS) and soluble guanylyl cyclase (sGC) revealed reduced eNOS expression in PE and preterm cells, with negligible changes in sGC levels. This study provides important and novel insights into abnormalities of fetal endothelial cells isolated from women with PE, reveal ing an altered cation membrane permeability and activity of eNOS-sGC pathway. As these changes are sustained in culture in vitro, this may reflect long-term "programming" of the fetal cardiovascular system.

Nitric oxide inhibits capacitative Ca2+ entry and enhances endoplasmic reticulum Ca2+ uptake in bovine vascular endothelial cells

In vascular endothelial cells, elevation of cytosolic free calcium concentration ([Ca2+]i) causes activation of nitric oxide synthase (NOS) and release of nitric oxide (NO). The goal of the study was to characterize the interplay between [Ca2+]i and NO production in this cell type. Simultaneous measurements of [Ca2+]i and intracellular NO concentration ([NO]i) in cultured bovine vascular endothelial cells (CPAE cell line) with the fluorescent indicators fura-2 and DAF-2, respectively, revealed that Ca2+ influx following agonist-induced intracellular Ca2+ store depletion (capacitative Ca2+ entry, CCE) represents the preferential Ca2+ source for the activation of the Ca2+-calmodulin-dependent endothelial NOS (eNOS). Exposure to the NO donor sodium nitroprusside (SNP) showed that high NO levels suppressed CCE and had an inhibitory effect on Ca2+ extrusion by the plasmalemmal Ca2+-ATPase. This inhibitory effect on CCE was mimicked by the membrane-permeant cGMP analogue 8-bromo-cGMP, but was reversed by the NO scavenger haemoglobin and prevented by the inhibitor of the NO-sensitive guanylate cyclase ODQ. Brief exposure to SNP reduced the peak of ATP-induced Ca2+ release from the endoplasmic reticulum (ER) and accelerated Ca2+ reuptake into the ER. Prolonged incubation with SNP resulted in enhanced Ca2+ loading of the ER, as revealed by direct measurements of store content with the ER-entrapped low-affinity Ca2+ indicator mag-fura-2. The results suggest that in vascular endothelial cells, NO synthesis is under autoregulatory control that involves NO-dependent [Ca2+]i regulation. Via cGMP-dependent inhibition of CCE and acceleration of Ca2+ sequestration into the ER, NO can lower [Ca2+]i and therefore exert an autoregulatory negative feedback on its own Ca2+-dependent synthesis.

Effects of noise on the off rate of Ca(2+) binding proteins in a coupled biochemical cell system

We investigated numerically the kinetic properties of calcium binding proteins by using a three-calcium store model and discussed the response of a two-way coupled biochemical cell system, whose subsystems were coupled via diffusion-like cytosolic calcium transfer through gap junctions, to the external stimulation. When we used noise to modulate the off rate of the Ca(2+) binding proteins in simulation, an SR-like phenomenon of synchronous oscillations is observed. In addition, the interaction involving noise, coupling and the dynamics of Ca(2+) binding proteins is discussed.

Stimulus-dependent control of inositol 1,4,5-trisphosphate-induced Ca(2+) oscillation frequency by the endoplasmic reticulum Ca(2+)-ATPase

In many cell types, receptor stimulation evokes cytosolic calcium oscillations with a frequency that depends on agonist dose. Previous studies demonstrated controversial effects of changing the activity of the endoplasmic reticulum Ca(2+)-ATPase upon the frequency of oscillations. By numerical simulations, we found that the model of De Young and Keizer (J. Keizer and G.W. De Young, 1994, J. Theor. Biol. 166: 431-442), unlike other models, can explain the observed discrepancies, assuming that the different experiments were performed at different stimulus levels. According to model predictions, partial inhibition of internal calcium pumps is expected to increase frequency at low stimulus strength and should have an opposite effect at strong stimuli. Similar results were obtained using an analytical estimation of oscillation period, based on calcium-dependent channel activation and inactivation. In experiments on HeLa cells, 4 nM thapsigargin increased the frequency of calcium oscillations induced by 1 and 2.5 microM histamine but had no effect on supramaximally stimulated cells. In HEp-2 cells, 2 nM thapsigargin slowed down the rapid, ATP-induced oscillations. Our results suggest that in the investigated cell types, the De Young-Keizer model based on inositol 1,4,5-trisphosphate-dependent calcium-induced calcium release can properly describe intracellular calcium oscillations.

Cyclosporin A inhibits inositol 1,4,5-trisphosphate-dependent Ca2+ signals by enhancing Ca2+ uptake into the endoplasmic reticulum and mitochondria

Cytosolic Ca(2+) ([Ca(2+)](i)) oscillations may be generated by the inositol 1,4,5-trisphosphate receptor (IP(3)R) driven through cycles of activation/inactivation by local Ca(2+) feedback. Consequently, modulation of the local Ca(2+) gradients influences IP(3)R excitability as well as the duration and amplitude of the [Ca(2+)](i) oscillations. In the present work, we demonstrate that the immunosuppressant cyclosporin A (CSA) reduces the frequency of IP(3)-dependent [Ca(2+)](i) oscillations in intact hepatocytes, apparently by altering the local Ca(2+) gradients. Permeabilized cell experiments demonstrated that CSA lowers the apparent IP(3) sensitivity for Ca(2+) release from intracellular stores. These effects on IP(3)-dependent [Ca(2+)](i) signals could not be attributed to changes in calcineurin activity, altered ryanodine receptor function, or impaired Ca(2+) fluxes across the plasma membrane. However, CSA enhanced the removal of cytosolic Ca(2+) by sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA), lowering basal and inter-spike [Ca(2+)](i). In addition, CSA stimulated a stable rise in the mitochondrial membrane potential (DeltaPsi(m)), presumably by inhibiting the mitochondrial permeability transition pore, and this was associated with increased Ca(2+) uptake and retention by the mitochondria during a rise in [Ca(2+)](i). We suggest that CSA suppresses local Ca(2+) feedback by enhancing mitochondrial and endoplasmic reticulum Ca(2+) uptake, these actions of CSA underlie the lower IP(3) sensitivity found in permeabilized cells and the impaired IP(3)-dependent [Ca(2+)](i) signals in intact cells. Thus, CSA binding proteins (cyclophilins) appear to fine tune agonist-induced [Ca(2+)](i) signals, which, in turn, may adjust the output of downstream Ca(2+)-sensitive pathways.

Autocrine action and its underlying mechanism of nitric oxide on intracellular Ca2+ homeostasis in vascular endothelial cells

The rise in cytosolic Ca(2+) concentration (Ca(2+)(i)) in vascular endothelial cells (ECs) activates the production and release of nitric oxide (NO). NO modifies Ca(2+)(i) homeostasis in many types of nonendothelial cells. However, its effect on endothelial Ca(2+)(i) homeostasis at basal and excited states remains unclear. In the present study, to elucidate the effect of NO on basal Ca(2+)(i), inositol 1,4,5-trisphosphate-induced Ca(2+)(i) release (IICR) was blocked by expressing an antisense against type-1 inositol 1,4,5-trisphosphate receptors or by microinjecting heparin to individual ECs, and the effects of NO that was released by and diffused from adjacent IICR-intact ECs were recorded. After ATP or bradykinin stimulation, IICR-inhibited ECs showed a marked reduction of basal Ca(2+)(i), which was abolished by N(G)-monomethyl-l-arginine monoacetate pretreatment. The reduction disappeared in sparsely seeded ECs. Exogenous NO gas mimicked the effect of ATP or bradykinin to reduce basal Ca(2+)(i). Blocking plasma membrane Ca(2+)-ATPase (PMCA), but not Na(+)-Ca(2+) exchange or sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase, suppressed the reduction, indicating that the reduction resulted from a NO-dependent potentiation of PMCA. To elucidate the effect of NO on elevated Ca(2+)(i), ATP-, bradykinin-, or thapsigargin-evoked Ca(2+)(i) response in the presence and absence of NO production was compared in adjacent IICR-intact ECs. NO was found to potentiate PMCA, which, in turn, greatly attenuated agonist-evoked Ca(2+)(i) elevation. NO also potentiated Ca(2+) influx, which markedly increased the sustained phase of Ca(2+)(i) elevation and possibly NO production. NO did not affect other Ca(2+)(i)-elevating and Ca(2+)(i)-sequestrating components. Thus, NO-dependent potentiation of PMCA is crucial for Ca(2+)(i) homeostasis over a wide Ca(2+)(i) range.

Ca(2+)-dependent activation of c-jun NH(2)-terminal kinase in primary rabbit proximal tubule epithelial cells

Previous work from this laboratory demonstrated that arachidonic acid activates c-jun NH(2)-terminal kinase (JNK) through oxidative intermediates in a Ca(2+)-independent manner (Cui X and Douglas JG. Arachidonic acid activates c-jun N-terminal kinase through NADPH oxidase in rabbit proximal tubular epithelial cells. Proc Natl Acad Sci USA 94: 3771-3776, 1997.). We now report that JNK can also be activated via a Ca(2+)-dependent mechanism by agents that increase the cytosolic Ca(2+) concentration (Ca(2+) ionophore A(23187), Ca(2+)-ATPase inhibitor thapsigargin) or deplete intracellular Ca(2+) stores [intracellular Ca(2+) chelator 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-AM]. The activation of JNK by BAPTA-AM occurs despite a decrease in cytosolic Ca(2+) concentration as detected by the indicator dye fura 2, but appears to be related to Ca(2+) metabolism, because modification of BAPTA with two methyl groups increases not only the chelation affinity for Ca(2+), but also the potency for JNK activation. BAPTA-AM stimulates Ca(2+) influx across the plasma membrane, and the resulting local Ca(2+) increases are probably involved in activation of JNK because Ca(2+) influx inhibitors (SKF-96365, nifedipine) and lowering of the free extracellular Ca(2+) concentration with EGTA reduce the BAPTA-induced JNK activation.

Hepatitis B virus-related insertional mutagenesis implicates SERCA1 gene in the control of apoptosis

We have used the Hepatitis B Virus DNA genome as a probe to identify genes clonally mutated in vivo, in human liver cancers. In a tumor, HBV-DNA was found to be integrated into the gene encoding Sarco/Endoplasmic Reticulum Calcium ATPase (SERCA), which pumps calcium, an important intracellular messenger for cell viability and growth, from the cytosol to the endoplasmic reticulum. The HBV X gene promoter cis-activates chimeric HBV X/SERCA1 transcripts, with splicing of SERCA1 exon 11, encoding C-terminally truncated SERCA1 proteins. Two chimeric HBV X/SERCA1 proteins accumulate in the tumor and form dimers. In vitro analyses have demonstrated that these proteins localize to the ER, determine its calcium depletion and induce cell death. We have also shown that these biological effects are related to expression of the SERCA, rather than of the viral moiety. This report involves for the first time the expression of mutated SERCA proteins in vivo in a tumor cell proliferation and in vitro in the control of cell viability. Oncogene (2000).

Luteinizing hormone (LH) drives diverse intracellular calcium second messenger signals in isolated porcine ovarian thecal cells: preferential recruitment of intracellular Ca2+ oscillatory cells by higher concentrations of LH

The present study examines Ca2+ second messenger signaling driven by LH in isolated porcine thecal cells. To this end, we implemented semiquantitative fluorescent (fura-2) videomicroscopic imaging of single thecal cells in vitro. Stimulation of 388 cells with LH (5 microg/ml) elicited an intracellular Ca2+ ([Ca2+]i) signal in 85+/-5.3% of individual thecal cells (n = 11 experiments). Among 337 LH-responsive cells, we identified four predominant temporal modes of [Ca2+]i signaling: 1) [Ca2+]i oscillations with periodicities of 0.5 to 4.5 min(-1) (63+/-4.5%), 2) a [Ca2+]i spike followed by a sustained plateau (17+/-2.6%), 3) a [Ca2+]i spike only (5.8+/-2.6%); and 4) a [Ca2+]i plateau only (3.8+/-1.5%). The prevalence, but not the amplitude or frequency, of LH-induced [Ca2+]i oscillations in thecal cells was dependent on the agonist concentration. Reduced availability of extracellular Ca2+ induced by treatment with EGTA or cobaltous chloride did not block the initiation, but reversibly abolished ongoing [Ca2+]i oscillations (72% of cells) or increased the mean [Ca2+]i interspike periodicity from 1.09+/-0.16 to 0.59+/-0.07 min(-1) (P < 0.05). Putative phospholipase C inhibition with U-73122 (10 microM) also abolished or frequency-damped LH-driven [Ca2+]i oscillations in 95+/-4.7% of cells. [Ca2+]i oscillations in thecal cells were not abrogated by overnight pretreatment with pertussis toxin. We conclude that 1) thecal cells (unlike earlier findings in granulosa cells) manifest a diverse array of [Ca2+]i signaling responses to LH at the single cell level; 2) LH can dose dependently recruit an increasing number of individually [Ca2+]i oscillating thecal cells; 3) extracellular Ca2+ is required for LH to sustain (but not initiate) frequent and high amplitude [Ca2+] oscillations in thecal cells; and 4) these signaling actions of LH are mediated via phospholipase C, but not a pertussis-toxin sensitive mechanism. Accordingly, the present data extend the apparent complexity of LH-induced [Ca2+]i second messenger signaling and identify at the single cell level LH's dose-responsive drive of [Ca2+]i oscillations in gonadal cells.

Histamine-induced Ca2+ oscillations in a human endothelial cell line depend on transmembrane ion flux, ryanodine receptors and endoplasmic reticulum Ca2+-ATPase

Using single cell microfluorometry to monitor changes in bulk Ca2+ concentration ([Ca2+]bulk) and the whole-cell configuration of the patch clamp technique to measure K+ currents (voltage clamp) and membrane potential (current clamp), the mechanisms of histamine-induced Ca2+ oscillations in the umbilical vein endothelial cell-derived cell line EA.hy926 were studied. In single cells, histamine (10 microM) evoked sinusoidal Ca2+ oscillations in low extracellular Ca2+ concentrations ([Ca2+]o = 10-30 microM). In contrast, histamine did not initiate Ca2+ oscillations either in the absence of extracellular Ca2+ (10 microM EGTA) or in the presence of 2.5 mM extracellular Ca2+. Ca2+ oscillations were accompanied by rhythmic activation of Ca2+-activated K+ (KCa) channels and membrane hyperpolarization of 18.1 +/- 3.9 mV. Hence, cell depolarization with 70 mM extracellular K+ or the inhibition of non-selective cation channels (NSCCs) and KCa channels by 10 microM Loe 908 and 10 mM tetrabutylammonium prevented histamine-evoked Ca2+ oscillations. Preventing Na+-Ca2+ exchange (NCX) by 10 microM 2', 4'-dichlorobenzamil, or removal of extracellular Na+, abolished histamine-induced Ca2+ oscillations. Lowering the extracellular Na+ concentration and thus promoting the reversed mode of NCX (3Na+ out and 1Ca2+ in) increased the amplitude and frequency of histamine-induced Ca2+ oscillations by 25 and 13 %, respectively. Hence, in the absence of extracellular Ca2+, 10 microM histamine induced an elevation of intracellular Na+ concentration in certain subplasmalemmal domains. The inhibitor of sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) 2,5-di-tert-butyl-1, 4-benzo-hydroquinone (15 microM) prevented histamine-induced Ca2+ oscillations. In addition, blockage of ryanodine-sensitive Ca2+ release (RsCR) by 25 microM ryanodine blunted Ca2+ oscillations. In endothelial cells that were treated for 16 h with 10 microM nocodazole to collapse the superficial endoplasmic reticulum (sER), no histamine-induced Ca2+ oscillations were found. We conclude that in low [Ca2+]o conditions histamine-induced Ca2+ oscillations depend on transmembrane Na+ loading through NSCCs that leads to Ca2+ entry via NCX. Cation influx is controlled by KCa channel activity that triggers membrane hyperpolarization and, thus, provides the driving force for cation influx. Hence, the Ca2+ entering needs to be sequestrated via SERCA into sER to become released by RsCR to evoke Ca2+ spiking. These data further support our previous work on localized Ca2+ signalling as a key phenomenon in endothelial Ca2+ homeostasis.

Microscopic properties of elementary Ca2+ release sites in non-excitable cells

BACKGROUND

Elementary Ca2+ signals, such as 'Ca2+ puffs', that arise from the activation of clusters of inositol 1 ,4,5,-trisphosphate (InsP3) receptors are the building blocks for local and global Ca2+ signalling. We previously found that one, or a few, Ca2+ puff sites within agonist-stimulated cells act as 'pacemakers' to initiate global Ca2+ waves. The factors that distinguish these pacemaker Ca2+ puff sites from the other Ca2+ release sites that simply participate in Ca2+ wave propagation are unknown.

RESULTS

The spatiotemporal properties of Ca2+ puffs were investigated using confocal microscopy of fluo3-loaded HeLa cells. The same pacemaker Ca2+ puff sites were activated during stimulation of cells with different agonists. The majority of agonist-stimulated pacemaker Ca2+ puffs originated in a perinuclear location. The positions of such Ca2+ puff sites were stable for up to 2 hours, and were not affected by disruption of the actin cytoskeleton. A similar perinuclear distribution of Ca2+ puff sites was also observed when InsP3 receptors were directly stimulated with thimerosal or membrane-permeant InsP3 esters. Immunostaining indicated that the perinuclear position of pacemaker Ca2+ puffs was not due to the localised expression of InsP3 receptors.

CONCLUSIONS

The pacemaker Ca2+ puff sites that initiate Ca2+ responses are temporally and spatially stable within cells. These Ca2+ release sites are distinguished from their neighbours by an intrinsically higher InsP3 sensitivity.