A reassessment of the effects of luminal [Ca2+] on inositol 1,4,5-trisphosphate-induced Ca2+ release from internal stores

The interplay between associated proteins, redox state and Ca2+ in the intraluminal ER compartment regulates the IP3 receptor

There have been in the last three decades repeated publications indicating that the inositol 1,4,5-trisphosphate receptor (IP3R) is regulated not only by cytosolic Ca2+ but also by intraluminal Ca2+. Although most studies indicated that a decreasing intraluminal Ca2+ level led to an inhibition of the IP3R, a number of publications reported exactly the opposite effect, i.e. an inhibition of the IP3R by high intraluminal Ca2+ levels. Although intraluminal Ca2+-binding sites on the IP3Rs were reported, a regulatory role for them was not demonstrated. It is also well known that the IP3R is regulated by a vast array of associated proteins, but only relatively recently proteins were identified that can be linked to the regulation of the IP3R by intraluminal Ca2+. The first to be reported was annexin A1 that is proposed to associate with the second intraluminal loop of the IP3R at high intraluminal Ca2+ levels and to inhibit the IP3R. More recently, ERdj5/PDIA19 reductase was described to reduce an intraluminal disulfide bridge of IP3R1 only at low intraluminal Ca2+ levels and thereby to inhibit the IP3R. Annexin A1 and ERdj5/PDIA19 can therefore explain most of the experimental results on the regulation of the IP3R by intraluminal Ca2+. Further studies are needed to provide a fuller understanding of the regulation of the IP3R from the intraluminal side. These findings underscore the importance of the state of the endoplasmic reticulum in the control of IP3R activity.

Ligand sensitivity of type-1 inositol 1,4,5-trisphosphate receptor is enhanced by the D2594K mutation

Inositol 1,4,5-trisphosphate receptor (IP₃R) and ryanodine receptor (RyR) are homologous cation channels that mediate release of Ca²⁺ from the endoplasmic/sarcoplasmic reticulum (ER/SR) and thereby are involved in many physiological processes. In previous studies, we determined that when the D2594 residue, located at or near the gate of the IP₃R type 1, was replaced by lysine (D2594K), a gain of function was obtained. This mutant phenotype was characterized by increased IP₃ sensitivity. We hypothesized the IP₃R1-D2594 determines the ligand sensitivity of the channel by electrostatically affecting the stability of the closed and open states. To test this possibility, the relationship between the D2594 site and IP₃R1 regulation by IP₃, cytosolic, and luminal Ca²⁺ was determined at the cellular, subcellular, and single-channel levels using fluorescence Ca²⁺ imaging and single-channel reconstitution. We found that in cells, D2594K mutation enhances the IP₃ ligand sensitivity. Single-channel IP₃R1 studies revealed that the conductance of IP₃R1-WT and -D2594K channels is similar. However, IP₃R1-D2594K channels exhibit higher IP₃ sensitivity, with substantially greater efficacy. In addition, like its wild type (WT) counterpart, IP₃R1-D2594K showed a bell-shape cytosolic Ca²⁺-dependency, but D2594K had greater activity at each tested cytosolic free Ca²⁺ concentration. The IP₃R1-D2594K also had altered luminal Ca²⁺ sensitivity. Unlike IP₃R1-WT, D2594K channel activity did not decrease at low luminal Ca²⁺ levels. Taken together, our functional studies indicate that the substitution of a negatively charged residue by a positive one at the channels' pore cytosolic exit affects the channel's gating behavior thereby explaining the enhanced ligand-channel's sensitivity.

Termination of Ca2+ puffs during IP3-evoked global Ca2+ signals

We previously described that cell-wide cytosolic Ca²⁺ transients evoked by inositol trisphosphate (IP₃) are generated by two modes of Ca²⁺ liberation from the ER; 'punctate' release via an initial flurry of transient Ca²⁺ puffs from local clusters of IP₃ receptors, succeeded by a spatially and temporally 'diffuse' Ca²⁺ liberation. Those findings were derived using statistical fluctuation analysis to monitor puff activity which is otherwise masked as global Ca²⁺ levels rise. Here, we devised imaging approaches to resolve individual puffs during global Ca²⁺ elevations to better investigate the mechanisms terminating the puff flurry. We find that puffs contribute about 40% (∼90 attomoles) of the total Ca²⁺ liberation, largely while the global Ca²⁺ signal rises halfway to its peak. The major factor terminating punctate Ca²⁺ release is an abrupt decline in puff frequency. Although the amplitudes of large puffs fall during the flurry, the amplitudes of more numerous small puffs remain steady, so overall puff amplitudes decline only modestly (∼30%). The Ca²⁺ flux through individual IP₃ receptor/channels does not measurably decline during the flurry, or when puff activity is depressed by pharmacological lowering of Ca²⁺ levels in the ER lumen, indicating that the termination of punctate release is not a simple consequence of reduced driving force for Ca²⁺ liberation. We propose instead that the gating of IP₃ receptors at puff sites is modulated such that their openings become suppressed as the bulk [Ca²⁺] in the ER lumen falls during global Ca²⁺ signals.

Calcium Release Channels: Structure and Function of IP3 Receptors and Ryanodine Receptors

Ca²⁺-release channels are giant membrane proteins that control the release of Ca²⁺ from the endoplasmic and sarcoplasmic reticulum. The two members, ryanodine receptors (RyRs) and inositol-1,4,5-trisphosphate Receptors (IP₃Rs), are evolutionarily related and are both activated by cytosolic Ca²⁺. They share a common architecture, but RyRs have evolved additional modules in the cytosolic region. Their massive size allows for the regulation by tens of proteins and small molecules, which can affect the opening and closing of the channels. In addition to Ca²⁺, other major triggers include IP₃ for the IP₃Rs, and depolarization of the plasma membrane for a particular RyR subtype. Their size has made them popular targets for study via electron microscopic methods, with current structures culminating near 3Å. The available structures have provided many new mechanistic insights int the binding of auxiliary proteins and small molecules, how these can regulate channel opening, and the mechanisms of disease-associated mutations. They also help scrutinize previously proposed binding sites, as some of these are now incompatible with the structures. Many questions remain around the structural effects of post-translational modifications, additional binding partners, and the higher-order complexes these channels can make in situ. This review summarizes our current knowledge about the structures of Ca²⁺-release channels and how this informs on their function.

Role of Serosal TRPV4-Constituted SOCE Mechanism in Secretagogues-Stimulated Intestinal Epithelial Anion Secretion

As little is known about the role of calcium (Ca²⁺) signaling mediating the small intestinal epithelial anion secretion, we aimed to study its regulatory role in secretagogue-stimulated duodenal anion secretion and the underlying molecular mechanisms. Therefore, intestinal anion secretion from native mouse duodenal epithelia was examined with Ussing chambers to monitor PGE₂-, 5-HT-, and CCh-induced short-circuit currents (I_sc). PGE₂ (10 μM) and 5-HT (10 μM) induced mouse duodenal I_sc, markedly attenuated by serosal Ca²⁺-free solution and selective blockers of store-operated Ca²⁺ channels on the serosal side of the duodenum. Furthermore, PGE₂- and 5-HT-induced duodenal I_sc was also inhibited by ER Ca²⁺ chelator TPEN. However, dantrolene, a selective blocker of ryanodine receptors, inhibited PGE₂-induced duodenal I_sc, while LiCl, an inhibitor of IP₃ production, inhibited 5-HT-induced I_sc. Moreover, duodenal I_sc response to the serosal applications of both PGE₂ and 5-HT was significantly attenuated in transient receptor potential vanilloid 4 (TRPV4) knockout mice. Finally, mucosal application of carbachol (100 μM) also induced duodenal I_sc via selective activation of muscarinic receptors, which was significantly inhibited in serosal Ca²⁺-free solution but neither in mucosal Ca²⁺-free solution nor by nifedipine. Therefore, the serosal TRPV4-constituted SOCE mechanism is likely universal for the most common and important secretagogues-induced and Ca²⁺-dependent intestinal anion secretion. These findings will enhance our knowledge about gastrointestinal (G.I.) epithelial physiology and the associated G.I. diseases, such as diarrhea and constipation.

Small intestinal glucose and sodium absorption through calcium-induced calcium release and store-operated Ca2+ entry mechanisms

BACKGROUND AND PURPOSE

Luminal glucose enhances intestinal Ca²⁺ absorption through apical Ca_v 1.3 channels necessary for GLUT2-mediated glucose absorption. As these reciprocal mechanisms are not well understood, we investigated the regulatory mechanisms of intestinal [Ca²⁺ ]_cyt and SGLT1-mediated Na⁺ -glucose co-transports.

EXPERIMENTAL APPROACH

Glucose absorption and channel expression were examined in mouse upper jejunal epithelium using an Ussing chamber, immunocytochemistry and Ca²⁺ and Na⁺ imaging in single intestinal epithelial cells.

KEY RESULTS

Glucose induced jejunal I_sc via Na⁺ -glucose cotransporter 1 (SGLT1) operated more efficiently in the presence of extracellular Ca²⁺ . A crosstalk between luminal Ca²⁺ entry via plasma Ca_v 1.3 channels and the ER Ca²⁺ release through ryanodine receptor (RYR) activation in small intestinal epithelial cell (IEC) or Ca²⁺ -induced Ca²⁺ release (CICR) mechanism was involve in Ca²⁺ -mediated jejunal glucose absorption. The ER Ca²⁺ release through RyR triggered basolateral Ca²⁺ entry or store-operated Ca²⁺ entry (SOCE) mechanism and the subsequent Ca²⁺ entry via Na⁺ /Ca²⁺ exchanger 1 (NCX1) were found to be critical in Na⁺ -glucose cotransporter-mediated glucose absorption. Blocking RyR, SOCE and NCX1 inhibited glucose induced [Na⁺ ]_cyt and [Ca²⁺ ]_cyt in single IEC and protein expression and co-localization of STIM1/Orai1, RyR1 and NCX1 were detected in IEC and jejunal mucosa.

CONCLUSION AND IMPLICATIONS

Luminal Ca²⁺ influx through Ca_v 1.3 triggers the CICR through RyR1 to deplete the ER Ca²⁺ , which induces the basolateral STIM1/Orai1-mediated SOCE mechanism and the subsequent Ca²⁺ entry via NCX1 to regulate intestinal glucose uptake via Ca²⁺ signalling. Targeting these mechanisms in IEC may help to modulate blood glucose and sodium in the metabolic disease.

ER-luminal [Ca2+] regulation of InsP3 receptor gating mediated by an ER-luminal peripheral Ca2+-binding protein

Modulating cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i) by endoplasmic reticulum (ER)-localized inositol 1,4,5-trisphosphate receptor (InsP₃R) Ca²⁺-release channels is a universal signaling pathway that regulates numerous cell-physiological processes. Whereas much is known regarding regulation of InsP₃R activity by cytoplasmic ligands and processes, its regulation by ER-luminal Ca²⁺ concentration ([Ca²⁺]_ER) is poorly understood and controversial. We discovered that the InsP₃R is regulated by a peripheral membrane-associated ER-luminal protein that strongly inhibits the channel in the presence of high, physiological [Ca²⁺]_ER. The widely-expressed Ca²⁺-binding protein annexin A1 (ANXA1) is present in the nuclear envelope lumen and, through interaction with a luminal region of the channel, can modify high-[Ca²⁺]_ER inhibition of InsP₃R activity. Genetic knockdown of ANXA1 expression enhanced global and local elementary InsP₃-mediated Ca²⁺ signaling events. Thus, [Ca²⁺]_ER is a major regulator of InsP₃R channel activity and InsP₃R-mediated [Ca²⁺]_i signaling in cells by controlling an interaction of the channel with a peripheral membrane-associated Ca²⁺-binding protein, likely ANXA1.

Molecular mechanisms of caffeine-mediated intestinal epithelial ion transports

BACKGROUND AND PURPOSE

As little is known about the effect of caffeine, one of the most widely consumed substances worldwide, on intestinal function, we aimed to study its action on intestinal anion secretion and the underlying molecular mechanisms.

EXPERIMENTAL APPROACH

Anion secretion and channel expression were examined in mouse duodenal epithelium by Ussing chambers and immunocytochemistry. Ca²⁺ imaging was also performed in intestinal epithelial cells (IECs).

KEY RESULTS

Caffeine (10 mM) markedly increased mouse duodenal short-circuit current (I_sc ), which was attenuated by a removal of either Cl^- or HCO₃ ^- , Ca²⁺ -free serosal solutions and selective blockers of store-operated Ca²⁺ channels (SOC/Ca²⁺ release-activated Ca²⁺ channels), and knockdown of Orai1 channels on the serosal side of duodenal tissues. Caffeine induced SOC entry in IEC, which was inhibited by ruthenium red and selective blockers of SOC. Caffeine-stimulated duodenal I_sc was inhibited by the endoplasmic reticulum Ca²⁺ chelator (N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine), selective blockers (ruthenium red and dantrolene) of ryanodine receptors (RyR), and of Ca²⁺ -activated Cl^- channels (niflumic acid and T16A). There was synergism between cAMP and Ca²⁺ signalling, in which cAMP/PKA promoted caffeine/Ca²⁺ -mediated anion secretion. Expression of STIM1 and Orai1 was detected in mouse duodenal mucosa and human IECs. The Orai1 proteins were primarily co-located with the basolateral marker Na⁺ , K⁺ -ATPase.

CONCLUSIONS AND IMPLICATIONS

Caffeine stimulated intestinal anion secretion mainly through the RyR/Orai1/Ca²⁺ signalling pathway. There is synergism between cAMP/PKA and caffeine/Ca²⁺ -mediated anion secretion. Our findings suggest that a caffeine-mediated RyR/Orai1/Ca²⁺ pathway could provide novel potential drug targets to control intestinal anion secretion.

TPEN prevents rapid pacing-induced calcium overload and nitration stress in HL-1 myocytes

INTRODUCTION

Atrial fibrillation (AF) is the most common cardiac arrhythmia. However, the current drug interference of antiarrhythmia has limited efficacy and off-target effects. Accumulating evidence has implicated a potential role of nitration stress in the pathogenesis of AF. The aim of the study was to determine whether TPEN provided antinitration effects on atrial myocytes during AF, especially under circumstances of nitration stress.

METHODS

We utilized a rapid paced HL-1 cells model for AF. The changes of electrophysiological characteristics and structure of paced HL-1 cells were determined by a patch clamp and a TEM method. The effects of TPEN on pacing and ONOO(-) pretreated HL-1 cells were examined using MTT assay, TUNEL technique, confocal microscope experiment, and Western blot analysis.

RESULTS

The results revealed that ONOO(-) reduced the viability of HL-1 cells in a dose-dependent manner, and 1 μmol/L TPEN significantly ameliorated the damage caused by 50 μmol/L ONOO(-) (P < 0.05). Pacing and/or ONOO(-) -induced marked shortening of APD, myolysis, and nuclear condensation. TPEN inhibited the Ca(2+) overload induced by rapid pacing (P < 0.05) and ONOO(-) stimulation (P < 0.05). The application of TPEN significantly prevented the protein nitration caused by pacing or pacing plus ONOO(-) (P < 0.05). Additionally, pacing in combination with ONOO(-) treatment led to increase in apoptosis in HL-1 cells (P < 0.01), which could be reduced by pretreatment with TPEN (P < 0.05).

CONCLUSIONS

TPEN prevents Ca(2+) overload and nitration stress in HL-1 atrial myocytes during rapid pacing and circumstances of nitration stress.

"Store-operated" cAMP signaling contributes to Ca2+-activated Cl- secretion in T84 colonic cells

Apical cAMP-dependent CFTR Cl(-) channels are essential for efficient vectorial movement of ions and fluid into the lumen of the colon. It is well known that Ca(2+)-mobilizing agonists also stimulate colonic anion secretion. However, CFTR is apparently not activated directly by Ca(2+), and the existence of apical Ca(2+)-dependent Cl(-) channels in the native colonic epithelium is controversial, leaving the identity of the Ca(2+)-activated component unresolved. We recently showed that decreasing free Ca(2+) concentration ([Ca(2+)]) within the endoplasmic reticulum (ER) lumen elicits a rise in intracellular cAMP. This process, which we termed "store-operated cAMP signaling" (SOcAMPS), requires the luminal ER Ca(2+) sensor STIM1 and does not depend on changes in cytosolic Ca(2+). Here we assessed the degree to which SOcAMPS participates in Ca(2+)-activated Cl(-) transport as measured by transepithelial short-circuit current (Isc) in polarized T84 monolayers in parallel with imaging of cAMP and PKA activity using fluorescence resonance energy transfer (FRET)-based reporters in single cells. In Ca(2+)-free conditions, the Ca(2+)-releasing agonist carbachol and Ca(2+) ionophore increased Isc, cAMP, and PKA activity. These responses persisted in cells loaded with the Ca(2+) chelator BAPTA-AM. The effect on Isc was enhanced in the presence of the phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX), inhibited by the CFTR inhibitor CFTRinh-172 and the PKA inhibitor H-89, and unaffected by Ba(2+) or flufenamic acid. We propose that a discrete component of the "Ca(2+)-dependent" secretory activity in the colon derives from cAMP generated through SOcAMPS. This alternative mode of cAMP production could contribute to the actions of diverse xenobiotic agents that disrupt ER Ca(2+) homeostasis, leading to diarrhea.

Regulatory Mechanisms of Endoplasmic Reticulum Resident IP3 Receptors

Dysregulated calcium signaling and accumulation of aberrant proteins causing endoplasmic reticulum stress are the early sign of intra-axonal pathological events in many neurodegenerative diseases, and apoptotic signaling is initiated when the stress goes beyond the maximum threshold level of endoplasmic reticulum. The fate of the cell to undergo apoptosis is controlled by Ca2(+) signaling and dynamics at the level of the endoplasmic reticulum. Endoplasmic reticulum resident inositol 1,4,5-trisphosphate receptors (IP3R) play a pivotal role in cell death signaling by mediating Ca2(+) flux from the endoplasmic reticulum into the cytosol and mitochondria. Hence, many prosurvival and prodeath signaling pathways and proteins affect Ca2(+) signaling by directly targeting IP3R channels, which can happen in an IP3R-isoform-dependent manner. Here, in this review, we summarize the regulatory mechanisms of inositol triphosphate receptors in calcium regulation and initiation of apoptosis during unfolded protein response.

Inositol 1,4,5-trisphosphate receptor-isoform diversity in cell death and survival

Cell-death and -survival decisions are critically controlled by intracellular Ca(2+) homeostasis and dynamics at the level of the endoplasmic reticulum (ER). Inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) play a pivotal role in these processes by mediating Ca(2+) flux from the ER into the cytosol and mitochondria. Hence, it is clear that many pro-survival and pro-death signaling pathways and proteins affect Ca(2+) signaling by directly targeting IP3R channels, which can happen in an IP3R-isoform-dependent manner. In this review, we will focus on how the different IP3R isoforms (IP3R1, IP3R2 and IP3R3) control cell death and survival. First, we will present an overview of the isoform-specific regulation of IP3Rs by cellular factors like IP3, Ca(2+), Ca(2+)-binding proteins, adenosine triphosphate (ATP), thiol modification, phosphorylation and interacting proteins, and of IP3R-isoform specific expression patterns. Second, we will discuss the role of the ER as a Ca(2+) store in cell death and survival and how IP3Rs and pro-survival/pro-death proteins can modulate the basal ER Ca(2+) leak. Third, we will review the regulation of the Ca(2+)-flux properties of the IP3R isoforms by the ER-resident and by the cytoplasmic proteins involved in cell death and survival as well as by redox regulation. Hence, we aim to highlight the specific roles of the various IP3R isoforms in cell-death and -survival signaling. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

Oncogenic K-Ras suppresses IP₃-dependent Ca²⁺ release through remodelling of the isoform composition of IP₃Rs and ER luminal Ca²⁺ levels in colorectal cancer cell lines

The GTPase Ras is a molecular switch engaged downstream of G-protein-coupled receptors and receptor tyrosine kinases that controls multiple cell-fate-determining signalling pathways. Ras signalling is frequently deregulated in cancer, underlying associated changes in cell phenotype. Although Ca(2+) signalling pathways control some overlapping functions with Ras, and altered Ca(2+) signalling pathways are emerging as important players in oncogenic transformation, how Ca(2+) signalling is remodelled during transformation and whether it has a causal role remains unclear. We have investigated Ca(2+) signalling in two human colorectal cancer cell lines and their isogenic derivatives in which the allele encoding oncogenic K-Ras (G13D) was deleted by homologous recombination. We show that agonist-induced Ca(2+) release from the endoplasmic reticulum (ER) intracellular Ca(2+) stores is enhanced by loss of K-Ras(G13D) through an increase in the Ca(2+) content of the ER store and a modification of the abundance of inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) subtypes. Consistently, uptake of Ca(2+) into mitochondria and sensitivity to apoptosis was enhanced as a result of K-Ras(G13D) loss. These results suggest that suppression of Ca(2+) signalling is a common response to naturally occurring levels of K-Ras(G13D), and that this contributes to a survival advantage during oncogenic transformation.

Regulation of inositol 1,4,5-trisphosphate receptors during endoplasmic reticulum stress

The endoplasmic reticulum (ER) performs multiple functions in the cell: it is the major site of protein and lipid synthesis as well as the most important intracellular Ca(2+) reservoir. Adverse conditions, including a decrease in the ER Ca(2+) level or an increase in oxidative stress, impair the formation of new proteins, resulting in ER stress. The subsequent unfolded protein response (UPR) is a cellular attempt to lower the burden on the ER and to restore ER homeostasis by imposing a general arrest in protein synthesis, upregulating chaperone proteins and degrading misfolded proteins. This response can also lead to autophagy and, if the stress can not be alleviated, to apoptosis. The inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) and IP3-induced Ca(2+) signaling are important players in these processes. Not only is the IP3R activity modulated in a dual way during ER stress, but also other key proteins involved in Ca(2+) signaling are modulated. Changes also occur at the structural level with a strengthening of the contacts between the ER and the mitochondria, which are important determinants of mitochondrial Ca(2+) uptake. The resulting cytoplasmic and mitochondrial Ca(2+) signals will control cellular decisions that either promote cell survival or cause their elimination via apoptosis. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

Permeant calcium ion feed-through regulation of single inositol 1,4,5-trisphosphate receptor channel gating

The ubiquitous inositol 1,4,5-trisphosphate (InsP(3)) receptor (InsP(3)R) Ca(2+) release channel plays a central role in the generation and modulation of intracellular Ca(2+) signals, and is intricately regulated by multiple mechanisms including cytoplasmic ligand (InsP(3), free Ca(2+), free ATP(4-)) binding, posttranslational modifications, and interactions with cytoplasmic and endoplasmic reticulum (ER) luminal proteins. However, regulation of InsP(3)R channel activity by free Ca(2+) in the ER lumen ([Ca(2+)](ER)) remains poorly understood because of limitations of Ca(2+) flux measurements and imaging techniques. Here, we used nuclear patch-clamp experiments in excised luminal-side-out configuration with perfusion solution exchange to study the effects of [Ca(2+)](ER) on homotetrameric rat type 3 InsP(3)R channel activity. In optimal [Ca(2+)](i) and subsaturating [InsP(3)], jumps of [Ca(2+)](ER) from 70 nM to 300 µM reduced channel activity significantly. This inhibition was abrogated by saturating InsP(3) but restored when [Ca(2+)](ER) was raised to 1.1 mM. In suboptimal [Ca(2+)](i), jumps of [Ca(2+)](ER) (70 nM to 300 µM) enhanced channel activity. Thus, [Ca(2+)](ER) effects on channel activity exhibited a biphasic dependence on [Ca(2+)](i). In addition, the effect of high [Ca(2+)](ER) was attenuated when a voltage was applied to oppose Ca(2+) flux through the channel. These observations can be accounted for by Ca(2+) flux driven through the open InsP(3)R channel by [Ca(2+)](ER), raising local [Ca(2+)](i) around the channel to regulate its activity through its cytoplasmic regulatory Ca(2+)-binding sites. Importantly, [Ca(2+)](ER) regulation of InsP(3)R channel activity depended on cytoplasmic Ca(2+)-buffering conditions: it was more pronounced when [Ca(2+)](i) was weakly buffered but completely abolished in strong Ca(2+)-buffering conditions. With strong cytoplasmic buffering and Ca(2+) flux sufficiently reduced by applied voltage, both activation and inhibition of InsP(3)R channel gating by physiological levels of [Ca(2+)](ER) were completely abolished. Collectively, these results rule out Ca(2+) regulation of channel activity by direct binding to the luminal aspect of the channel.

The luminal Ca(2+) chelator, TPEN, inhibits NAADP-induced Ca(2+) release

The regulation of Ca(2+) release by luminal Ca(2+) has been well studied for the ryanodine and IP(3) receptors but has been less clear for the NAADP-regulated channel. In view of conflicting reports, we have re-examined the issue by manipulating luminal Ca(2+) with the membrane-permeant, low affinity Ca(2+) buffer, TPEN, and monitoring NAADP-induced Ca(2+) release in sea urchin egg homogenate. NAADP-induced Ca(2+) release was almost entirely blocked by TPEN (IC(50) 17-25μM) which suppressed the maximal extent of Ca(2+) release without altering NAADP sensitivity. In contrast, Ca(2+) release via IP(3) receptors was 3- to 30-fold less sensitive to TPEN whereas that evoked by ionomycin was essentially unaffected. The effect of TPEN on NAADP-induced Ca(2+) release was not due to an increase in the luminal pH or chelation of trace metals since it could not be mimicked by NH(4)Cl or phenanthroline. The fact that TPEN had no effect upon ionophore-induced Ca(2+) release also argued against a substantial reduction in the driving force for Ca(2+) efflux. We propose that, in the sea urchin egg, luminal Ca(2+) is important for gating native NAADP-regulated two-pore channels.

Ca(2+) signalling by IP(3) receptors

The Ca(2) (+) signals evoked by inositol 1,4,5-trisphosphate (IP(3)) are built from elementary Ca(2) (+) release events involving progressive recruitment of IP(3) receptors (IP(3)R), intracellular Ca(2) (+) channels that are expressed in almost all animal cells. The smallest events ('blips') result from opening of single IP(3)R. Larger events ('puffs') reflect the near-synchronous opening of a small cluster of IP(3)R. These puffs become more frequent as the stimulus intensity increases and they eventually trigger regenerative Ca(2) (+) waves that propagate across the cell. This hierarchical recruitment of IP(3)R is important in allowing Ca(2) (+) signals to be delivered locally to specific target proteins or more globally to the entire cell. Co-regulation of IP(3)R by Ca(2) (+) and IP(3), the ability of a single IP(3)R rapidly to mediate a large efflux of Ca(2) (+) from the endoplasmic reticulum, and the assembly of IP(3)R into clusters are key features that allow IP(3)R to propagate Ca(2) (+) signals regeneratively. We review these properties of IP(3)R and the structural basis of IP(3)R behavior.

Inositol 1,4,5-trisphosphate and its receptors

Activation of cells by many extracellular agonists leads to the production of inositol 1,4,5-trisphosphate (IP₃). IP₃ is a global messenger that easily diffuses in the cytosol. Its receptor (IP₃R) is a Ca(2+)-release channel located on intracellular membranes, especially the endoplasmic reticulum (ER). The IP₃R has an affinity for IP(3) in the low nanomolar range. A prime regulator of the IP₃R is the Ca(2+) ion itself. Cytosolic Ca(2+) is considered as a co-agonist of the IP₃R, as it strongly increases IP(3)R activity at concentrations up to about 300 nM. In contrast, at higher concentrations, cytosolic Ca(2+) inhibits the IP₃R. Also the luminal Ca(2+) sensitizes the IP₃R. In higher organisms three genes encode for an IP₃R and additional diversity exists as a result of alternative splicing mechanisms and the formation of homo- and heterotetramers. The various IP₃R isoforms have a similar structure and a similar function, but due to differences in their affinity for IP₃, their variable sensitivity to regulatory parameters, their differential interaction with associated proteins, and the variation in their subcellular localization, they participate differently in the formation of intracellular Ca(2+) signals and this affects therefore the physiological consequences of these signals.

STIM1, but not STIM2, is required for proper agonist-induced Ca2+ signaling

The stromal interaction molecules STIM1 and STIM2 sense a decreasing Ca(2+) concentration in the lumen of the endoplasmic reticulum and activate Ca(2+) channels in the plasma membrane. In addition, at least 2 reports suggested that STIM1 may also interact with the inositol 1,4,5-trisphosphate (IP(3)) receptor. Using embryonic fibroblasts from Stim1(-/-), Stim2(-/-) and wild-type mice, we now tested the hypothesis that STIM1 and STIM2 would also regulate the IP(3) receptor. We investigated whether STIM1 or STIM2 would be the luminal Ca(2+) sensor that controls the loading dependence of the IP(3)-induced Ca(2+) release. Partial emptying of the stores in plasma-membrane permeabilized cells resulted in an increased EC(50) and a decreased Hill coefficient for IP(3)-induced Ca(2+) release. This effect occurred both in the presence and absence of STIM proteins, indicating that these proteins were not the luminal Ca(2+) sensor for the IP(3) receptor. Although Stim1(-/-) cells displayed a normal IP(3)-receptor function, agonist-induced Ca(2+) release was reduced. This finding suggests that the presence of STIM1 is required for proper agonist-induced Ca(2+) signaling. Our data do not provide experimental evidence for the suggestion that STIM proteins would directly control the function of the IP(3) receptor.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

Dynamic interaction of hTRPC6 with the Orai1-STIM1 complex or hTRPC3 mediates its role in capacitative or non-capacitative Ca(2+) entry pathways

TRPC (canonical transient receptor potential) channel subunits have been shown to assemble into homo- or hetero-meric channel complexes, including different Ca2+-handling proteins, required for the activation of CCE (capacitative Ca2+ entry) or NCCE (non-CCE) pathways. In the present study we found evidence for the dynamic interaction between endogenously expressed hTRPC6 (human TRPC6) with either both Orai1 and STIM1 (stromal interaction molecule 1) or hTRPC3 to participate in CCE or NCCE. Electrotransjection of cells with an anti-hTRPC6 antibody, directed towards the C-terminal region, reduces CCE induced by TPEN [N,N,N',N'-tetrakis-(2-pyridylmethyl)-ethylenediamine], which reduces the intraluminal free Ca2+ concentration. Cell stimulation with thrombin or extensive Ca2+-store depletion by TG (thapsigargin)+ionomycin enhanced the interaction between hTRPC6 and the CCE proteins Orai1 and STIM1. In contrast, stimulation with the diacylglycerol analogue OAG (1-oleoyl-2-acetyl-sn-glycerol) displaces hTRPC6 from Orai1 and STIM1 and enhances the association between hTRPC6 and hTRPC3. The interaction between hTRPC6 and hTRPC3 was abolished by dimethyl-BAPTA [1,2-bis-(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid] loading, which indicates that this phenomenon is Ca2+-dependent. These findings support the hypothesis that hTRPC6 participates both in CCE and NCCE through its interaction with the Orai1-STIM1 complex or hTRPC3 respectively.

Probing cellular dynamics with a chemical signal generator

Observations of material and cellular systems in response to time-varying chemical stimuli can aid the analysis of dynamic processes. We describe a microfluidic “chemical signal generator,” a technique to apply continuously varying chemical concentration waveforms to arbitrary locations in a microfluidic channel through feedback control of the interface between parallel laminar (co-flowing) streams. As the flow rates of the streams are adjusted, the channel walls are exposed to a chemical environment that shifts between the individual streams. This approach can be used to probe the dynamic behavior of objects or substances adherent to the interior of the channel. To demonstrate the technique, we exposed live fibroblast cells to ionomycin, a membrane-permeable calcium ionophore, while assaying cytosolic calcium concentration. Through the manipulation of the laminar flow interface, we exposed the cells' endogenous calcium handling machinery to spatially-contained discrete and oscillatory intracellular disturbances, which were observed to elicit a regulatory response. The spatiotemporal precision of the generated signals opens avenues to previously unapproachable areas for potential investigation of cell signaling and material behavior.

Store-operated cyclic AMP signalling mediated by STIM1

Depletion of Ca(2+) from the endoplasmic reticulum (ER) results in activation of plasma membrane Ca(2+) entry channels. This 'store-operated' process requires translocation of a transmembrane ER Ca(2+) sensor protein, stromal interaction molecule 1 (STIM1), to sites closely apposed to Ca(2+) channels at the cell surface. However, it is not known whether a reduction in Ca(2+) stores is coupled to other signalling pathways by this mechanism. We found that lowering the concentration of free Ca(2+) in the ER, independently of the cytosolic Ca(2+) concentration, also led to recruitment of adenylyl cyclases. This resulted in enhanced cAMP accumulation and PKA activation, measured using FRET-based cAMP indicators. Translocation of STIM1 was required for efficient coupling of ER Ca(2+) depletion to adenylyl cyclase activity. We propose the existence of a pathway (store-operated cAMP signalling or SOcAMPS) in which the content of internal Ca(2+) stores is directly connected to cAMP signalling through a process that involves STIM1.

Intraluminal calcium as a primary regulator of endoplasmic reticulum function

The concentration of Ca2+ inside the lumen of endoplasmic reticulum (ER) regulates a vast array of spatiotemporally distinct cellular processes, from intracellular Ca2+ signals to intra-ER protein processing and cell death. This review summarises recent data on the mechanisms of luminal Ca2+-dependent regulation of Ca2+ release and uptake as well as ER regulation of cellular adaptive processes. In addition we discuss general biophysical properties of the ER membrane, as trans-endomembrane Ca2+ fluxes are subject to basic electrical forces, determined by factors such as the membrane potential of the ER and the ease with which Ca2+ fluxes are able to change this potential (i.e. the resistance of the ER membrane). Although these electrical forces undoubtedly play a fundamental role in shaping [Ca2+](ER) dynamics, at present there is very little direct experimental information about the biophysical properties of the ER membrane. Further studies of how intraluminal [Ca2+] is regulated, best carried out with direct measurements, are vital for understanding how Ca2+ orchestrates cell function. Direct monitoring of [Ca2+](ER) under conditions where the cytosolic [Ca2+] is known may also help to capture elusive biophysical information about the ER, such as the potential difference across the ER membrane.

Inositol 1,4,5-trisphosphate receptor movement is restricted by addition of elevated levels of O-linked sugar

The inositol 1,4,5-trisphosphate receptor (InsP3R) is a versatile, ubiquitous intracellular calcium channel. Traditionally, visualizing the InsP3R in live cells involves attaching a fluorescent marker to either terminal of the protein, but the termini themselves contain binding sites for accessory molecules and proteins. Using random transposition, constructs have been developed that express the type I InsP3R with green fluorescent protein (GFP) inserted at various points within its sequence. We have used two of these constructs, one in the ligand-binding domain, and another in the regulatory domain, to investigate InsP3R dynamics within the endoplasmic reticulum. We present evidence that endogenous calcium signaling is maintained when these constructs are expressed. In addition, by measuring the fluorescent recovery after photobleaching of a subcellular region, we demonstrate that treatment with 8mM N-acetylglucosamine (GlcNAc), known to lead to increased O-linked GlcNAcylation of proteins, leads to a reduction in the ability of the InsP3R to travel laterally within the endoplasmic reticulum. Each construct serves as the control for the other one, suggesting that this decrease in mobility is not specific to the insertion site of GFP within the InsP3R. These constructs represent a new tool that will allow us to follow receptor turnover and translocation events.

Dynamics of a three-variable nonlinear model of vasomotion: comparison of theory and experiment

The effects of pharmacological interventions that modulate Ca(2+) homeodynamics and membrane potential in rat isolated cerebral vessels during vasomotion (i.e., rhythmic fluctuations in arterial diameter) were simulated by a third-order system of nonlinear differential equations. Independent control variables employed in the model were [Ca(2+)] in the cytosol, [Ca(2+)] in intracellular stores, and smooth muscle membrane potential. Interactions between ryanodine- and inositol 1,4,5-trisphosphate-sensitive intracellular Ca(2+) stores and transmembrane ion fluxes via K(+) channels, Cl(-) channels, and voltage-operated Ca(2+) channels were studied by comparing simulations of oscillatory behavior with experimental measurements of membrane potential, intracellular free [Ca(2+)] and vessel diameter during a range of pharmacological interventions. The main conclusion of the study is that a general model of vasomotion that predicts experimental data can be constructed by a low-order system that incorporates nonlinear interactions between dynamical control variables.

Cell membrane permeable esters of D-myo-inositol 1,4,5-trisphosphate

d-myo-inositol 1,4,5-trisphosphate (Ins(1,4,5)P3, or IP3) is a ubiquitous second messenger that regulates cytosolic Ca2+ activities ([Ca2+]i). To study this signaling branch in intact cells, we have synthesized a caged and cell permeable derivative of IP3, ci-IP3/PM, from myo-inositol in 9 steps. Ci-IP3/PM is a homologue of cm-IP3/PM, a caged and cell permeable IP3 ester developed earlier. In ci-IP3/PM, 2- and 3-hydroxyl groups of myo-inositiol are protected by an isopropylidene group; whereas in cm-IP3/PM, a methoxymethylene is used. Ci-IP3/PM can be loaded into cells non-invasively to high concentrations without activating IP3 receptors (IP3Rs). UV uncaging of loaded ci-IP3 released i-IP3, a potent agonist of IP3Rs, and evoked Ca2+ release from internal stores. Interestingly, elevations of [Ca2+]i by i-IP3 lasted longer than [Ca2+]i transients by m-IP3, the uncaging product of cm-IP3. To understand this difference, we measured the metabolic stability of i-IP3 and m-IP3. Like natural IP3 which is known to be rapidly metabolized in cells, m-IP3 could only be detected within several seconds after uncaging cm-IP3. In contrast, i-IP3 was metabolized at a much slower rate. By exploiting different metabolic rates of m-IP3 and i-IP3, we developed two procedures for activating IP3Rs in cells without UV uncaging. The first method involves photolyzing ci-IP3/PM in vitro to generate i-IP3/PM. Successive additions of low micromolar i-IP3/PM to NIH 3T3 cells caused graded Ca2+ releases, confirming that "quantal Ca2+ release" occurs in fully intact cells with normal ATP supplies and undisrupted endoplasmic reticulum. The second technique utilizes two photon uncaging. After locally illuminating cells loaded with cm-IP3 with femtosecond-pulsed near-infrared light (730 nm), we observed a burst of Ca2+ activity in the uncaging area. This local Ca2+ rise rapidly propagated across cells and could be repeated many times in different sub-cellular locations to produce artificial Ca2+ oscillations of defined amplitudes and frequencies. The complementary advantages of these IP3 prodrugs should provide new approaches for studying IP3-Ca2+ signaling in intact cell populations with high spatiotemporal resolutions.

Thiol-oxidant monochloramine mobilizes intracellular Ca2+ in parietal cells of rabbit gastric glands

In Helicobacter pylori-induced gastritis, oxidants are generated through the interactions of bacteria in the lumen, activated granulocytes, and cells of the gastric mucosa. In this study we explored the ability of one such class of oxidants, represented by monochloramine (NH(2)Cl), to serve as agonists of Ca(2+) accumulation within the parietal cell of the gastric gland. Individual gastric glands isolated from rabbit mucosa were loaded with fluorescent reporters for Ca(2+) in the cytoplasm (fura-2 AM) or intracellular stores (mag-fura-2 AM). Conditions were adjusted to screen out contributions from metal cations such as Zn(2+), for which these reporters have affinity. Exposure to NH(2)Cl (up to 200 microM) led to dose-dependent increases in intracellular Ca(2+) concentration ([Ca(2+)](i)), in the range of 200-400 nM above baseline levels. These alterations were prevented by pretreatment with the oxidant scavenger vitamin C or a thiol-reducing agent, dithiothreitol (DTT), which shields intracellular thiol groups from oxidation by chlorinated oxidants. Introduction of vitamin C during ongoing exposure to NH(2)Cl arrested but did not reverse accumulation of Ca(2+) in the cytoplasm. In contrast, introduction of DTT or N-acetylcysteine permitted arrest and partial reversal of the effects of NH(2)Cl. Accumulation of Ca(2+) in the cytoplasm induced by NH(2)Cl is due to release from intracellular stores, entry from the extracellular fluid, and impaired extrusion. Ca(2+)-handling proteins are susceptible to oxidation by chloramines, leading to sustained increases in [Ca(2+)](i). Under certain conditions, NH(2)Cl may act not as an irritant but as an agent that activates intracellular signaling pathways. Anti-NH(2)Cl strategies should take into account different effects of oxidant scavengers and thiol-reducing agents.

Ca2+ release from the sarcoplasmic reticulum activated by the low affinity Ca2+ chelator TPEN in ventricular myocytes

The Ca2+ content of the sarcoplasmic reticulum (SR) of cardiac myocytes is thought to play a role in the regulation and termination of SR Ca2+ release through the ryanodine receptors (RyRs). Experimentally altering the amount of Ca2+ within the SR with the membrane-permeant low affinity Ca2+ chelator TPEN could improve our understanding of the mechanism(s) by which SR Ca2+ content and SR Ca2+ depletion can influence Ca2+ release sensitivity and termination. We applied laser-scanning confocal microscopy to examine SR Ca2+ release in freshly isolated ventricular myocytes loaded with fluo-3, while simultaneously recording membrane currents using the whole-cell patch-clamp technique. Following application of TPEN, local spontaneous Ca2+ releases increased in frequency and developed into cell-wide Ca2+ waves. SR Ca2+ load after TPEN application was found to be reduced to about 60% of control. Isolated cardiac RyRs reconstituted into lipid bilayers exhibited a two-fold increase of their open probability. At the low concentration used (20-40microTPEN did not significantly inhibit the SR-Ca2+-ATPase in SR vesicles. These results indicate that TPEN, traditionally used as a low affinity Ca2+ chelator in intracellular Ca2+ stores, may also act directly on the RyRs inducing an increase in their open probability. This in turn results in an increased Ca2+ leak from the SR leading to its Ca2+ depletion. Lowering of SR Ca2+ content may be a mechanism underlying the recently reported cardioprotective and antiarrhythmic features of TPEN.

Modulation of Ca(2+) release and Ca(2+) oscillations in HeLa cells and fibroblasts by mitochondrial Ca(2+) uniporter stimulation

The recent availability of activators of the mitochondrial Ca(2+) uniporter allows direct testing of the influence of mitochondrial Ca(2+) uptake on the overall Ca(2+) homeostasis of the cell. We show here that activation of mitochondrial Ca(2+) uptake by 4,4',4''-(4-propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol (PPT) or kaempferol stimulates histamine-induced Ca(2+) release from the endoplasmic reticulum (ER) and that this effect is enhanced if the mitochondrial Na(+)-Ca(2+) exchanger is simultaneously inhibited with CGP37157. This suggests that both Ca(2+) uptake and release from mitochondria control the ability of local Ca(2+) microdomains to produce feedback inhibition of inositol 1,4,5-trisphosphate receptors (InsP(3)Rs). In addition, the ability of mitochondria to control Ca(2+) release from the ER allows them to modulate cytosolic Ca(2+) oscillations. In histamine stimulated HeLa cells and human fibroblasts, both PPT and kaempferol initially stimulated and later inhibited oscillations, although kaempferol usually induced a more prolonged period of stimulation. Both compounds were also able to induce the generation of Ca(2+) oscillations in previously silent fibroblasts. Our data suggest that cytosolic Ca(2+) oscillations are exquisitely sensitive to the rates of mitochondrial Ca(2+) uptake and release, which precisely control the size of the local Ca(2+) microdomains around InsP(3)Rs and thus the ability to produce feedback activation or inhibition of Ca(2+) release.

The endoplasmic reticulum: folding, calcium homeostasis, signaling, and redox control

The endoplasmic reticulum (ER) plays a major role in regulating synthesis, folding, and orderly transport of proteins. It is also essentially involved in various cellular signaling processes, primarily by its function as a dynamic Ca(2+) store. Compared to the cytosol, oxidizing conditions are found in the ER that allow oxidation of cysteine residues in nascent polypeptide chains to form intramolecular disulfide bonds. However, compounds and enzymes such as PDI that catalyze disulfide bonds become reduced and have to be reoxidized for further catalytic cycles. A number of enzymes, among them products of the ERO1 gene, appear to provide oxidizing equivalents, and oxygen appears to be the final oxidant in aerobic living organisms. Thus, protein oxidation in the ER is connected with generation of reactive oxygen species (ROS). Changes in the redox state and the presence of ROS also affect the Ca(2+) homeostasis by modulating the functionality of ER-based channels and buffering chaperones. In addition, a close relationship exists between oxidative stress and ER stress, which both may activate signaling events leading to a rebalance of folding capacity and folding demand or to cell death. Thus, redox homeostasis appears to be a prerequisite for proper functioning of the ER.

‘Quantal’ Ca2+release reassessed - a clue to oscillation and synchronization

Ca(2+) release from intracellular Ca(2+) stores, a pivotal event in Ca(2+) signaling, is a 'quantal' process; it terminates after a rapid release of a fraction of stored Ca(2+). To explain the 'quantal' nature, 'all-or-none' model and 'steady-state' model were proposed. This article shortly reviews these hypotheses and considers a recently proposed mechanism, 'luminal potential' model, in which the membrane potential of Ca(2+) store regulates Ca(2+) efflux. By reassessing the 'quantal' nature, other important features of Ca(2+) signaling, oscillation and synchronization, are highlighted. The mechanism for 'quantal' Ca(2+) release may underlie the temporal and spatial control of Ca(2+) signaling.

Voltage- and Ca2+-activated potassium channels in Ca2+ store control Ca2+ release

Ca2+ release from Ca2+ stores is a 'quantal' process; it terminates after a rapid release of stored Ca2+. To explain the quantal nature, it has been supposed that a decrease in luminal Ca2+ acts as a 'brake' on store release. However, the mechanism for the attenuation of Ca2+ efflux remains unknown. We show that Ca2+ release is controlled by voltage- and Ca2+-activated potassium channels in the Ca2+ store. The potassium channel was identified as the big or maxi-K (BK)-type, and was activated by positive shifts in luminal potential and luminal Ca2+ increases, as revealed by patch-clamp recordings from an exposed nuclear envelope. The blockage or closure of the store BK channel due to Ca2+ efflux developed lumen-negative potentials, as revealed with an organelle-specific voltage-sensitive dye [DiOC5(3); 3,3'-dipentyloxacarbocyanine iodide], and suppressed Ca2+ release. The store BK channels are reactivated by Ca2+ uptake by Ca2+ pumps regeneratively with K+ entry to allow repetitive Ca2+ release. Indeed, the luminal potential oscillated bistably by approximately 45 mV in amplitude. Our study suggests that Ca2+ efflux-induced store BK channel closures attenuate Ca2+ release with decreases in counter-influx of K+.

Modulation of agonist-induced Ca2+ release by SR Ca2+ load: direct SR and cytosolic Ca2+ measurements in rat uterine myocytes

Release of Ca2+ from sarcoplasmic reticulum (SR) is one of the most important mechanisms of smooth muscle stimulation by a variety of physiologically active substances. Agonist-induced Ca2+ release is considered to be dependent on the Ca2+ content of the SR, although the mechanism underlying this dependence is unclear. In the present study, the effect of SR Ca2+ load on the amplitude of [Ca2+]i transients elicited by application of the purinergic agonist ATP was examined in uterine smooth muscle cells isolated from pregnant rats. Measurement of intraluminal Ca2+ level ([Ca2+]L) using a low affinity Ca indicator, mag-fluo-4, revealed that incubation of cells in a high-Ca2+ (10 mM) extracellular solution leads to a substantial increase in [Ca2+]L (SR overload). However, despite increased SR Ca2+ content this did not potentiate ATP-induced [Ca2+]i transients. Repetitive applications of ATP in the absence of extracellular Ca2+, as well as prolonged incubation in Ca2+-free solution without agonist, depleted the [Ca2+]L (SR overload). In contrast to overload, partial depletion of the SR substantially reduced the amplitude of Ca2+ release. ATP-induced [Ca2+]i transients were completely abolished when SR Ca2+ content was decreased below 80% of its normal value indicating a steep dependence of the IP3-mediated Ca2+ release on the Ca2+ load of the store. Our results suggest that in uterine smooth muscle cells decrease in the SR Ca2+ load below its normal resting level substantially reduces the IP3-mediated Ca2+ release, while Ca2+ overload of the SR has no impact on such release.

Egg activation events are regulated by the duration of a sustained [Ca2+]cyt signal in the mouse

Although the dynamics of oscillations of cytosolic Ca2+ concentration ([Ca2+]cyt) play important roles in early mammalian development, the impact of the duration when [Ca2+]cyt is elevated is not known. To determine the sensitivity of fertilization-associated responses [i.e., cortical granule exocytosis, resumption of the cell cycle, Ca2+/calmodulin-dependent protein kinase II (CaMKII) activity, recruitment of maternal mRNAs] and developmental competence of the parthenotes to the duration of a [Ca2+]cyt transient, unfertilized mouse eggs were subjected to a prolonged [Ca2+]cyt change for 15, 25, or 50 min by means of repetitive Ca2+ electropermeabilization at 2-min intervals. The initiation and completion of fertilization-associated responses are correlated with the duration of time in which the [Ca2+]cyt is elevated, with the exception that autonomous CaMKII activity is down-regulated with prolonged elevated [Ca2+]cyt. Activated eggs from 25- or 50-min treatments readily develop to the blastocyst stage with no sign of apoptosis or necrosis and some implant. Ca2+ influx into unfertilized eggs causes neither Ca2+ release from intracellular stores nor rapid removal of cytosolic Ca2+. Thus, the total Ca2+ signal input appears to be an important regulatory parameter that ensures completion of fertilization-associated events and oocytes have a surprising degree of tolerance for a prolonged change in [Ca2+]cyt.

Dual sensitivity of sarcoplasmic/endoplasmic Ca2+-ATPase to cytosolic and endoplasmic reticulum Ca2+ as a mechanism of modulating cytosolic Ca2+ oscillations

The effects of ER (endoplasmic reticulum) Ca2+ on cytosolic Ca2+ oscillations in pancreatic acinar cells were investigated using mathematical models of the Ca2+ oscillations. We first examined the mathematical model of SERCA (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase) to reproduce the highly co-operative inhibitory effect of Ca2+ in the ER lumen on ER Ca2+ uptake in the acinar cells. The model predicts that luminal Ca2+ would most probably inhibit the conversion of the conformation state with luminal Ca2+-binding sites (E2) into the conformation state with cytoplasmic Ca2+-binding sites (E1). The SERCA model derived from this prediction showed dose-response relationships to cytosolic and luminal Ca2+ concentrations that were consistent with the experimental data from the acinar cells. According to a mathematical model of cytosolic Ca2+ oscillations based on the modified SERCA model, a small decrease in the concentration of endoplasmic reticulum Ca2+ (approx. 20% of the total) was sufficient to abolish the oscillations. When a single type of IP3R (IP3 receptor) was included in the model, store depletion decreased the spike frequency. However, the frequency became less sensitive to store depletion when we added another type of IP3R with higher sensitivity to the concentration of free Ca2+ in the cytosol. Bifurcation analysis of the mathematical model showed that the loss of Ca2+ from the ER lumen decreased the sensitivity of cytosolic Ca2+ oscillations to IP3 [Ins(1,4,5)P3]. The addition of a high-affinity IP3R did not alter this property, but significantly decreased the sensitivity of the spike frequency to IP3. Our mathematical model demonstrates how luminal Ca2+, through its effect on Ca2+ uptake, can control cytosolic Ca2+ oscillations.

Initiation of embryonic cardiac pacemaker activity by inositol 1,4,5-trisphosphate-dependent calcium signaling

In the adult, the heart rate is driven by spontaneous and repetitive depolarizations of pacemaker cells to generate a firing of action potentials propagating along the conduction system and spreading into the ventricles. In the early embryo before E9.5, the pacemaker ionic channel responsible for the spontaneous depolarization of cells is not yet functional. Thus the mechanisms that initiate early heart rhythm during cardiogenesis are puzzling. In the absence of a functional pacemaker ionic channel, the oscillatory nature of inositol 1,4,5-trisphosphate (InsP3)-induced intracellular Ca2+ signaling could provide an alternative pacemaking mechanism. To test this hypothesis, we have engineered pacemaker cells from embryonic stem (ES) cells, a model that faithfully recapitulates early stages of heart development. We show that InsP3-dependent shuttle of free Ca2+ in and out of the endoplasmic reticulum is essential for a proper generation of pacemaker activity during early cardiogenesis and fetal life.

Deoxycholic acid activates protein kinase C and phospholipase C via increased Ca2+ entry at plasma membrane

BACKGROUND & AIMS

Secondary bile acids like deoxycholic acid (DCA) are well-established tumor promoters that may exert their pathologic actions by interfering with intracellular signaling cascades.

METHODS

We evaluated the effects of DCA on Ca2+ signaling in BHK-21 fibroblasts using fura-2 and mag-fura-2 to measure cytoplasmic and intraluminal internal stores [Ca2+], respectively. Furthermore, green fluorescent protein (GFP)-based probes were used to monitor time courses of phospholipase C (PLC) activation (pleckstrin-homology [PH]-PLCdelta-GFP), and translocation of protein kinase C (PKC) and a major PKC substrate, myristolated alanine-rich C-kinase substrate (MARCKS).

RESULTS

DCA (50-250 micromol/L) caused profound Ca2+ release from intracellular stores of intact or permeabilized cells. Correspondingly, DCA increased cytoplasmic Ca2+ to levels that were approximately 120% of those stimulated by Ca2+-mobilizing agonists in the presence of external Ca2+, and approximately 60% of control in Ca2+-free solutions. DCA also caused dramatic translocation of PH-PLCdelta-GFP, and conventional, Ca2+/diacylglycerol (DAG)-dependent isoforms of PKC (PKC-betaI and PKC-alpha), and MARCKS-GFP, but only in Ca2+-containing solutions. DCA had no effect on localization of a novel (PKCdelta) or an atypical (PKCzeta) PKC isoform.

CONCLUSIONS

Data are consistent with a model in which DCA directly induces both Ca2+ release from internal stores and persistent Ca2+ entry at the plasma membrane. The resulting microdomains of high Ca2+ levels beneath the plasma membrane appear to directly activate PLC, resulting in modest InsP 3 and DAG production. Furthermore, the increased Ca2+ entry stimulates vigorous recruitment of conventional PKC isoforms to the plasma membrane.

N,N,N′,N′-Tetrakis(2-pyridylmethyl)-ethylenediamine Improves Myocardial Protection against Ischemia by Modulation of Intracellular Ca2+ Homeostasis

N,N,N',N'-Tetrakis(2-pyridylmethyl)-ethylenediamine (TPEN), a transition-metal chelator, was recently found to protect against myocardial ischemia-reperfusion injury. The goals of this study were to investigate the in vivo antiarrhythmic and antifibrillatory potential of TPEN in rats and guinea pigs and to study the in vitro effects of TPEN on calcium homeostasis in cultured newborn rat cardiac cells in normoxia and hypoxia. We demonstrated on an in vivo rat model of ischemia-reperfusion that TPEN abolishes ventricular fibrillation incidence and mortality and decreases the incidence and duration of ventricular tachycardia. To elucidate the mechanism of cardioprotection by TPEN, contraction, synchronization, and intracellular calcium level were examined in vitro. We have shown for the first time that TPEN prevented the increase in intracellular Ca(2+) levels ([Ca(2+)](i)) caused by hypoxia and abolished [Ca(2+)](i) elevation caused by high extracellular Ca(2+) levels ([Ca(2+)](o)) or by caffeine. Addition of TPEN returned synchronized beating of cardiomyocytes desynchronized by [Ca(2+)](o) elevation. To discover the mechanism by which TPEN reduces [Ca(2+)](i) in cardiomyocytes, the cells were treated with thapsigargin, which inhibits Ca(2+) uptake into the sarcoplasmic reticulum (SR). TPEN successfully reduced [Ca(2+)](i) elevated by thapsigargin, indicating that TPEN did not sequester Ca(2+) in the SR. However, TPEN did not reduce [Ca(2+)](i) in the Na(+)-free medium in which the Na(+)/Ca(2+) exchanger was inhibited. Taken together, the results show that activation of sarcolemmal Na(+)/Ca(2+) exchanger by TPEN increases Ca(2+) extrusion from the cytoplasm of cardiomyocytes, preventing cytosolic Ca(2+) overload, which explains the beneficial effects of TPEN on postischemic cardiac status.

Subtype-Specific and ER Lumenal Environment-Dependent Regulation of Inositol 1,4,5-Trisphosphate Receptor Type 1 by ERp44

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are intracellular channel proteins that mediate Ca(2+) release from the endoplasmic reticulum (ER) and are involved in many biological processes and diseases. IP(3)Rs are differentially regulated by a variety of cytosolic proteins, but their regulation by ER lumenal protein(s) remains largely unexplored. In this study, we found that ERp44, an ER lumenal protein of the thioredoxin family, directly interacts with the third lumenal loop of IP(3)R type 1 (IP(3)R1) and that the interaction is dependent on pH, Ca(2+) concentration, and redox state: the presence of free cysteine residues in the loop is required. Ca(2+)-imaging experiments and single-channel recording of IP(3)R1 activity with a planar lipid bilayer system demonstrated that IP(3)R1 is directly inhibited by ERp44. Thus, ERp44 senses the environment in the ER lumen and modulates IP(3)R1 activity accordingly, which should in turn contribute to regulating both intralumenal conditions and the complex patterns of cytosolic Ca(2+) concentrations.