Distribution of inositol 1,4,5-trisphosphate receptor isoforms, SERCA isoforms and Ca2+ binding proteins in RBL-2H3 rat basophilic leukemia cells

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.

Ion channels regulating mast cell biology

Mast cells play a central role in the pathophysiology of asthma and related allergic conditions. Mast cell activation leads to the degranulation of preformed mediators such as histamine and the secretion of newly synthesised proinflammatory mediators such as leukotrienes and cytokines. Excess release of these mediators contributes to allergic disease states. An influx of extracellular Ca2+ is essential for mast cell mediator release. From the Ca2+ channels that mediate this influx, to the K+ , Cl- and transient receptor potential channels that set the cell membrane potential and regulate Ca2+ influx, ion channels play a critical role in mast cell biology. In this review we provide an overview of our current knowledge of ion channel expression and function in mast cells with an emphasis on how channels interact to regulate Ca2+ signalling.

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.

Regulators of Ca(2+) signaling in mast cells: potential targets for treatment of mast cell-related diseases?

A calcium signal is essential for degranulation, generation of eicosanoids and optimal production of cytokines in mast cells in response to antigen and other stimulants. The signal is initiated by phospholipase C-mediated production of inositol1,4,5-trisphosphate resulting in release of stored Ca(2+) from the endoplasmic reticulum (ER) and Golgi. Depletion of these stores activates influx of extracellular Ca(2+), usually referred to as store-operated calcium entry (SOCE), through the interaction of the Ca(2+)-sensor, stromal interacting molecule-1 (STIM1 ), in ER with Orai1(CRACM1) and transient receptor potential canonical (TRPC) channel proteins in the plasma membrane (PM). This interaction is enabled by microtubular-directed reorganization of ER to form ER/PM contact points or "punctae" in which STIM1 and channel proteins colocalize. The ensuing influx of Ca(2+) replenishes Ca(2+) stores and sustains elevated levels of cytosolic Ca(2+) ions-the obligatory signal for mast-cell activation. In addition, the signal can acquire spatial and dynamic characteristics (e.g., calcium puffs, waves, oscillations) that encode signals for specific functional outputs. This is achieved by coordinated regulation of Ca(2+) fluxes through ATP-dependent Ca(2+)-pumps and ion exchangers in mitochondria, ER and PM. As discussed in this chapter, studies in mast cells revealed much about the mechanisms described above but little about allergic and autoimmune diseases although studies in other types of cells have exposed genetic defects that lead to aberrant calcium signaling in immune diseases. Pharmacologic agents that inhibit or activate the regulatory components of calcium signaling in mast cells are also discussed along with the prospects for development of novel SOCE inhibitors that may prove beneficial in the treatment inflammatory mast-cell related diseases.

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.

Inositol 1,4,5-trisphosphate-induced Ca2+ signalling is involved in estradiol-induced breast cancer epithelial cell growth

Background

Ca²⁺ is a ubiquitous messenger that has been shown to be responsible for controlling numerous cellular processes including cell growth and cell death. Whereas the involvement of IP₃-induced Ca²⁺ signalling (IICS) in the physiological activity of numerous cell types is well documented, the role of IICS in cancer cells is still largely unknown. Our purpose was to characterize the role of IICS in the control of growth of the estrogen-dependent human breast cancer epithelial cell line MCF-7 and its potential regulation by 17β-estradiol (E₂).

Results

Our results show that the IP₃ receptor (IP₃R) inhibitors caffeine, 2-APB and xestospongin C (XeC) inhibited the growth of MCF-7 stimulated by 5% foetal calf serum or 10 nM E₂. Furthermore, Ca²⁺ imaging experiments showed that serum and E₂ were able to trigger, in a Ca²⁺-free medium, an elevation of internal Ca²⁺ in a 2-APB and XeC-sensitive manner. Moreover, the phospholipase C (PLC) inhibitor U-73122 was able to prevent intracellular Ca²⁺ elevation in response to serum, whereas the inactive analogue U-73343 was ineffective. Western-blotting experiments revealed that the 3 types of IP₃Rs are expressed in MCF-7 cells and that a 48 hours treatment with 10 nM E₂ elevated IP₃R3 protein expression level in an ICI-182,780 (a specific estrogen receptor antagonist)-dependent manner. Furthermore, IP₃R3 silencing by the use of specific small interfering RNA was responsible for a drastic modification of the temporal feature of IICS, independently of a modification of the sensitivity of the Ca²⁺ release process and acted to counteract the proliferative effect of 10 nM E₂.

Conclusions

Altogether, our results are in favour of a role of IICS in MCF-7 cell growth, and we hypothesize that the regulation of IP₃R3 expression by E₂ is involved in this effect.

Regulation of Ca2+ signaling with particular focus on mast cells

Calcium signals mediate diverse cellular functions in immunological cells. Early studies with mast cells, then a preeminent model for studying Ca2+-dependent exocytosis, revealed several basic features of calcium signaling in non-electrically excitable cells. Subsequent studies in these and other cells further defined the basic processes such as inositol 1,4,5-trisphosphate-mediated release of Ca2+ from Ca2+ stores in the endoplasmic reticulum (ER); coupling of ER store depletion to influx of external Ca2+ through a calcium-release activated calcium (CRAC) channel now attributed to the interaction of the ER Ca2+ sensor, stromal interacting molecule-1 (STIM1), with a unique Ca2+-channel protein, Orai1/CRACM1, and subsequent uptake of excess Ca2+ into ER and mitochondria through ATP-dependent Ca2+ pumps. In addition, transient receptor potential channels and ion exchangers also contribute to the generation of calcium signals that may be global or have dynamic (e.g., waves and oscillations) and spatial resolution for specific functional readouts. This review discusses past and recent developments in this field of research, the pharmacologic agents that have assisted in these endeavors, and the mast cell as an exemplar for sorting out how calcium signals may regulate multiple outputs in a single cell.

How many Ca2+ATPase isoforms are expressed in a cell type? A growing family of membrane proteins illustrated by studies in platelets

Ca(2+) signaling plays a key role in normal and abnormal platelet functions. Understanding platelet Ca(2+) signaling requires the knowledge of proteins involved in this process. Among these proteins are Ca(2+)ATPases or Ca(2+) pumps that deplete the cytosol of Ca(2+) ions. Here, we will particularly focus on two Ca(2+) pump families: the plasma membrane Ca(2+)ATPases (PMCAs) that extrude cytosolic Ca(2+) towards the extracellular medium and the sarco/endoplasmic reticulum Ca(2+)ATPases (SERCAs) that pump Ca(2+) into the endoplasmic reticulum (ER). In the present review, we will summarize data on platelet Ca(2+)ATPases including their identification and biogenesis. First of all, we will present the Ca(2+)ATPase genes and their isoforms expressed in platelets. We will especially focus on a member of the SERCA family, SERCA3, recently found to give rise to a number of species-specific isoforms. Next, we will describe the differences in Ca(2+)ATPase patterns observed in human and rat platelets. Last, we will analyze how the expression of Ca(2+)ATPase isoforms changes during megakaryocytic maturation and show that megakaryocytopoiesis is associated with a profound reorganization of the expression and/or activity of Ca(2+)ATPases. Taken together, these data provide new aspects of investigations to better understand normal and abnormal platelet Ca(2+) signaling.

Up-regulation of inositol 1,4,5-trisphosphate receptor type 1 is responsible for a decreased endoplasmic-reticulum Ca2+ content in presenilin double knock-out cells

Presenilins (PS) are proteins involved in the pathogenesis of autosomal-dominant familial cases of Alzheimer's disease. Mutations in PS are known to induce specific alterations in cellular Ca2+ signaling which might be involved in the pathogenesis of neurodegenerative diseases. Mouse embryonic fibroblasts (MEF) deficient in PS1 and PS2 (PS DKO) as well as the latter rescued with PS1 (Rescue), were used to investigate the underlying mechanism of these alterations in Ca2+ signaling. PS DKO cells were characterized by a decrease in the [Ca2+]ER as measured by ER-targeted aequorin luminescence and an increased level of type 1 inositol 1,4,5-trisphosphate receptor (IP3R1). The lower [Ca2+]ER was associated with an increase in a Ca2+ leak from the ER. The increased IP3R1 expression and the concomitant changes in ER Ca2+ handling were reversed in the Rescue cells. Moreover using RNA-interference mediated reduction of IP3R1 we could demonstrate that the up-regulation of this isoform was responsible for the increased Ca2+ leak and the lowered [Ca2+]ER PS DKO cells. Finally, we show that the decreased [Ca2+]ER in PS DKO cells was protective against apoptosis.

Agonist-evoked inositol trisphosphate receptor (IP3R) clustering is not dependent on changes in the structure of the endoplasmic reticulum

The size and number of IP3R (inositol 1,4,5-trisphosphate receptor) clusters located on the surface of the ER (endoplasmic reticulum) is hypothesized to regulate the propagation of Ca2+ waves in cells, but the mechanisms by which the receptors cluster are not understood. Using immunocytochemistry, live-cell imaging and heterologous expression of ER membrane proteins we have investigated IP3R clustering in the basophilic cell line RBL-2H3 following the activation of native cell-surface antigen receptors. IP3R clusters are present in resting cells, and upon receptor stimulation, form larger aggregates. Cluster formation and maintenance required the presence of extracellular Ca2+ in both resting and stimulated cells. Using transfection with a marker of the ER, we found that the ER itself also showed structural changes, leading to an increased number of 'hotspots', following antigen stimulation. Surprisingly, however, when we compared the ER hotspots and IP3R clusters, we found them to be distinct. Imaging of YFP (yellow fluorescent protein)-IP3R transfected in to living cells confirmed that IP3R clustering increased upon stimulation. Photobleaching experiments showed that the IP3R occupied a single contiguous ER compartment both before and after stimulation, suggesting a dynamic exchange of IP3R molecules between the clusters and the surrounding ER membrane. It also showed a decrease in the mobile fraction after cell activation, consistent with receptor anchoring within clusters. We conclude that IP3R clustering in RBL-2H3 cells is not simply a reflection of bulk-changes in ER structure, but rather is due to the receptor undergoing homotypic or heterotypic protein-protein interactions in response to agonist stimulation.

Receptor-operated Ca2+ entry mediated by TRPC3/TRPC6 proteins in rat prostate smooth muscle (PS1) cell line

Prostate smooth muscle cells predominantly express alpha1-adrenoceptors (alpha1-AR). alpha1-AR antagonists induce prostate smooth muscle relaxation and therefore they are useful therapeutic compounds for the treatment of benign prostatic hyperplasia symptoms. However, the Ca(2+) entry pathways associated with the activation of alpha1-AR in the prostate have yet to be elucidated. In many cell types, mammalian homologues of transient receptor potential (TRP) genes, first identified in Drosophila, encode TRPC (canonical TRP) proteins. They function as receptor-operated channels (ROCs) which are involved in various physiological processes such as contraction, proliferation, apoptosis, and differentiation. To date, the expression and function of TRPC channels have not been studied in prostate smooth muscle. In fura-2 loaded PS1 (a prostate smooth muscle cell line) which express endogenous alpha1A-ARs, alpha-agonists epinephrine (EPI), and phenylephrine (PHE) induced Ca(2+) influx which depended on the extracellular Ca(2+) and PLC activation but was independent of PKC activation. Thus, we have tested two membrane-permeable analogues of diacylglycerol (DAG), oleoyl-acyl-sn-glycerol (OAG) and 1,2-dioctanoyl-sn-glycerol (DOG). They initiated Ca(2+) influx whose properties were similar to those induced by the alpha-agonists. Sensitivity to 2-aminoethyl diphenylborate (2-APB), SKF-96365 and flufenamate implies that Ca(2+)-permeable channels mediated both alpha-agonist- and OAG-evoked Ca(2+) influx. Following the sarcoplasmic reticulum (SR) Ca(2+) store depletion by thapsigargin (Tg), a SERCA inhibitor, OAG and PHE were both still able to activate Ca(2+) influx. However, OAG failed to enhance Ca(2+) influx when added in the presence of an alpha-agonist. RT-PCR and Western blotting performed on PS1 cells revealed the presence of mRNAs and the corresponding TRPC3 and TRPC6 proteins. Experiments using an antisense strategy showed that both alpha-agonist- and OAG-induced Ca(2+) influx required TRPC3 and TRPC6, whereas the Tg-activated ("capacitative") Ca(2+) entry involved only TRPC3 encoded protein. It may be thus concluded that PS1 cells express TRPC3 and TRPC6 proteins which function as receptor- and store-operated Ca(2+) entry pathways.

Calcium release from the Golgi apparatus and the endoplasmic reticulum in HeLa cells stably expressing targeted aequorin to these compartments

Extracellular agonists mobilize Ca2+ from SERCA-comprising intracellular Ca2+ stores located in both the Golgi apparatus and the endoplasmic reticulum. Ca2+ release from both these compartments was studied in HeLa cells stably expressing the luminescent Ca2+ indicator aequorin specifically targeted to these compartments. Changes in lumenal [Ca2+] as detected by the aequorin measurements were correlated with parallel changes in total Ca2+ content of the stores. The latencies and initial rates of Ca2+ release from the Golgi apparatus and the endoplasmic reticulum were quite similar. However, maximal Ca2+ release measured with Golgi-targeted aequorin terminated faster than that from the endoplasmic reticulum. The rate and extent of Ca2+ depletion from both compartments correlated well with the peak amplitude of the cytosolic [Ca2+] rise. Time-course experiments further revealed that the peak of the cytosolic Ca2+ response occurred before the lumenal [Ca2+] reached its lowest level. We conclude that both the Golgi apparatus and the endoplasmic reticulum contribute to the rise in cytosolic [Ca2+] upon agonist stimulation, but the kinetics of the Ca2+ release are different.

Regulation of the phosphorylation of the inositol 1,4,5-trisphosphate receptor by protein kinase C

The various inositol 1,4,5-trisphosphate receptor (IP(3)R) isoforms are potential substrates for several protein kinases. We compared the in vitro phosphorylation of purified IP(3)R1 and IP(3)R3 by the catalytic subunit of protein kinase C (PKC). Phosphorylation of IP(3)R1 by PKC was about eight times stronger than that of IP(3)R3 under identical conditions. Protein kinase A strongly stimulated the PKC-induced phosphorylation of IP(3)R1. In contrast, Ca(2+) inhibited its phosphorylation (IC(50)<or=2microM) and this inhibition was further potentiated by calmodulin (CaM), while the Ca(2+)-independent CaM mutant CaM(1234) was ineffective. Ca(2+) and CaM, however, did not inhibit IP(3)R3 phosphorylation by PKC. Taken together, these findings show that Ca(2+) and CaM differentially regulate the PKC-mediated phosphorylation of IP(3)R1 and IP(3)R3 and are indicative for a role for the inhibition of IP(3)R1 phosphorylation by Ca(2+) and CaM in the negative slope of the bell-shaped effect of Ca(2+) on IP(3)R function.

Two types of store-operated Ca2+ channels with different activation modes and molecular origin in LNCaP human prostate cancer epithelial cells

The one or more coupling mechanisms of store-operated channels (SOCs) to endoplasmic reticulum (ER) Ca2+ store depletion as well as the molecular identity of SOCs per se still remain a mystery. Here, we demonstrate the co-existence of two populations of molecular distinct endogenous SOCs in LNCaP prostate cancer epithelial cells, which are preferentially activated by either active inositol 1,4,5-trisphosphate (IP3)-mediated or passive thapsigargin-facilitated store depletion and have different ER store content sensitivity. The first population, called SOC(CC) (for "conformational coupling"), is characterized by preferential IP3 receptor-dependent mode of activation, as judged from sensitivity to cytoskeleton modifications, and dominant contribution of transient receptor potential (TRP) TRPC1 within it. The second one, called SOC(CIF) (for "calcium influx factor"), depends on Ca(2+)-independent phospholipase A2 for activation with probable CIF involvement and is mostly represented by TRPC4. The previously identified SOC constituent in LNCaP cells, TRPV6, seems to play equal role in both SOC populations. These results provide new insight into the nature of SOCs and their representation in the single cell type as well as permit reconciliation of current SOC activation hypotheses.

Regulation of InsP3 receptor activity by neuronal Ca2+-binding proteins

Inositol 1,4,5-trisphosphate receptors (InsP(3)Rs) were recently demonstrated to be activated independently of InsP(3) by a family of calmodulin (CaM)-like neuronal Ca(2+)-binding proteins (CaBPs). We investigated the interaction of both naturally occurring long and short CaBP1 isoforms with InsP(3)Rs, and their functional effects on InsP(3)R-evoked Ca(2+) signals. Using several experimental paradigms, including transient expression in COS cells, acute injection of recombinant protein into Xenopus oocytes and (45)Ca(2+) flux from permeabilised COS cells, we demonstrated that CaBPs decrease the sensitivity of InsP(3)-induced Ca(2+) release (IICR). In addition, we found a Ca(2+)-independent interaction between CaBP1 and the NH(2)-terminal 159 amino acids of the type 1 InsP(3)R. This interaction resulted in decreased InsP(3) binding to the receptor reminiscent of that observed for CaM. Unlike CaM, however, CaBPs do not inhibit ryanodine receptors, have a higher affinity for InsP(3)Rs and more potently inhibited IICR. We also show that phosphorylation of CaBP1 at a casein kinase 2 consensus site regulates its inhibition of IICR. Our data suggest that CaBPs are endogenous regulators of InsP(3)Rs tuning the sensitivity of cells to InsP(3).

Colocalization of Ca2+-ATPase and GRP94 with p58 and the effects of thapsigargin on protein recycling suggest the participation of the pre-Golgi intermediate compartment in intracellular Ca2+ storage

We have studied the localization of functional components of cellular Ca2+ transport and storage and the effects of thapsigargin (TG), a specific inhibitor of the sarco-endoplasmic reticulum Ca2+-ATPase (SERCA), with respect to the p58-containing pre-Golgi intermediate compartment (IC). The depletion of Ca2+ stores in normal rat kidney (NRK) cells by TG abolished the retention of the KDEL-containing, Ca2+-binding, luminal ER chaperones GRP94/endoplasmin and GRP78/BiP, and resulted in the appearance of the proteins in the culture medium before inducing their synthesis. Immunolocalization of GRP94 in TG-treated cells showed that the protein was transported to the Golgi complex and, in parallel, the KDEL receptor was redistributed from the Golgi to p58-positive IC structures, but was not transported further to the ER. Similarly, p58 that normally cycles between the ER, IC, and cis-Golgi, was largely depleted from the cell periphery and arrested in large-sized IC elements and numerous vesicles or buds in the Golgi region, showing that TG selectively blocks its recycling from the IC back to the ER. Importantly, cell fractionation analyses and confocal fluorescence microscopy provided evidence that the IC elements in unperturbed cells contain SERCA and a considerable pool of GRP94. Thus, the observed effects of TG on protein retention and recycling can be explained by a change in the luminal Ca2+ concentration of the IC. Moreover, the compositional properties of the IC elements suggest that they participate in intracellular Ca2+ storage.

Localization and function of a calmodulin-apocalmodulin-binding domain in the N-terminal part of the type 1 inositol 1,4,5-trisphosphate receptor

Calmodulin (CaM) is a ubiquitous protein that plays a critical role in regulating cellular functions by altering the activity of a large number of proteins, including the d-myo-inositol 1,4,5-trisphosphate (IP3) receptor (IP3R). CaM inhibits IP3 binding in both the presence and absence of Ca2+ and IP3-induced Ca2+ release in the presence of Ca2+. We have now mapped and characterized a Ca2+-independent CaM-binding site in the N-terminal part of the type 1 IP3R (IP3R1). This site could be responsible for the inhibitory effects of CaM on IP3 binding. We therefore expressed the N-terminal 581 amino acids of IP3R1 as a His-tagged recombinant protein, containing the functional IP3-binding pocket. We showed that CaM, both in the presence and absence of Ca2+, inhibited IP3 binding to this recombinant protein with an IC50 of approx. 2 microM. Deletion of the N-terminal 225 amino acids completely abolished the effects of both Ca2+ and CaM on IP3 binding. We mapped the Ca2+-independent CaM-binding site to a recombinant glutathione S-transferase fusion protein containing the first 159 amino acids of IP3R1 and then made different synthetic peptides overlapping this region. We demonstrated that two synthetic peptides matching amino acids 49-81 and 106-128 bound CaM independently of Ca2+ and could reverse the inhibition of IP3 binding caused by CaM. This suggests that these sequences are components of a discontinuous Ca2+-independent CaM-binding domain, which is probably involved in the inhibition of IP3 binding by CaM.

Modification of store-operated channel coupling and inositol trisphosphate receptor function by 2-aminoethoxydiphenyl borate in DT40 lymphocytes

Store-operated channels (SOCs) provide an important means for mediating longer-term Ca(2+) signals and replenishment of Ca(2+) stores in a multitude of cell types. However, the coupling mechanism between endoplasmic reticulum stores to activate plasma membrane SOCs remains unknown. In DT40 chicken B lymphocytes, the permeant inositol trisphosphate receptor (InsP(3)R) modifier, 2-aminoethoxydiphenyl borate (2-APB), was a powerful activator of store-operated Ca(2+) entry between 1-10 microm. 2-APB activated authentic SOCs because the entry was totally selective for Ca(2+) (no detectable entry of Ba(2+) or Sr(2+) ions), and highly sensitive to La(3+) ions (IC(50) 30-100 nm). To assess the role of InsP(3)Rs in this response, we used the DT40 triple InsP(3)R-knockout (ko) cell line, DT40InsP(3)R-ko, in which the absence of full-length InsP(3)Rs or InsP(3)R fragments was verified by Western analysis using antibodies cross-reacting with N-terminal epitopes of all three chicken InsP(3)R subtypes. The 2-APB-induced activation of SOCs was identical in the DT40InsP(3)R-ko, cells indicating InsP(3)Rs were not involved. With both wild type (wt) and ko DT40 cells, 2-APB had no effect on Ca(2+) entry in store-replete cells, indicating that its action was restricted to SOCs in a store-coupled state. 2-APB induced a robust activation of Ca(2+) release from stores in intact DT40wt cells but not in DT40InsP(3)R-ko cells, indicating an InsP(3)R-mediated effect. In contrast, 2-APB blocked InsP(3)Rs in permeabilized DT40wt cells, suggesting that the stimulatory action of 2-APB was restricted to functionally coupled InsP(3)Rs in intact cells. Uncoupling of ER/PM interactions in intact cells by calyculin A-induced cytoskeletal rearrangement prevented SOC activation by store-emptying and 2-APB; this treatment completely prevented 2-APB-induced InsP(3)R activation but did not alter InsP(3)R activation mediated by phospholipase C-coupled receptor stimulation. The results indicate that the robust bifunctional actions of 2-APB on both SOCs and InsP(3)Rs are dependent on the coupled state of these channels and suggest that 2-APB may target the coupling machinery involved in mediating store-operated Ca(2+) entry.

The conserved sites for the FK506-binding proteins in ryanodine receptors and inositol 1,4,5-trisphosphate receptors are structurally and functionally different

We compared the interaction of the FK506-binding protein (FKBP) with the type 3 ryanodine receptor (RyR3) and with the type 1 and type 3 inositol 1,4,5-trisphosphate receptor (IP(3)R1 and IP(3)R3), using a quantitative GST-FKBP12 and GST-FKBP12.6 affinity assay. We first characterized and mapped the interaction of the FKBPs with the RyR3. GST-FKBP12 as well as GST-FKBP12.6 were able to bind approximately 30% of the solubilized RyR3. The interaction was completely abolished by FK506, strengthened by the addition of Mg(2+), and weakened in the absence of Ca(2+) but was not affected by the addition of cyclic ADP-ribose. By using proteolytic mapping and site-directed mutagenesis, we pinpointed Val(2322), located in the central modulatory domain of the RyR3, as a critical residue for the interaction of RyR3 with FKBPs. Substitution of Val(2322) for leucine (as in IP(3)R1) or isoleucine (as in RyR2) decreased the binding efficiency and shifted the selectivity to FKBP12.6; substitution of Val(2322) for aspartate completely abolished the FKBP interaction. Importantly, the occurrence of the valylprolyl residue as alpha-helix breaker was an important determinant of FKBP binding. This secondary structure is conserved among the different RyR isoforms but not in the IP(3)R isoforms. A chimeric RyR3/IP(3)R1, containing the core of the FKBP12-binding site of IP(3)R1 in the RyR3 context, retained this secondary structure and was able to interact with FKBPs. In contrast, IP(3)Rs did not interact with the FKBP isoforms. This indicates that the primary sequence in combination with the local structural environment plays an important role in targeting the FKBPs to the intracellular Ca(2+)-release channels. Structural differences in the FKBP-binding site of RyRs and IP(3)Rs may contribute to the occurrence of a stable interaction between RyR isoforms and FKBPs and to the absence of such interaction with IP(3)Rs.

Calcium puffs are generic InsP3-activated elementary calcium signals and are downregulated by prolonged hormonal stimulation to inhibit cellular calcium responses

Elementary Ca2+ signals, such as ‘Ca2+ puffs’, which arise from the activation of inositol 1,4,5-trisphosphate receptors, are building blocks for local and global Ca2+ signalling. We characterized Ca2+ puffs in six cell types that expressed differing ratios of the three inositol 1,4,5-trisphosphate receptor isoforms. The amplitudes, spatial spreads and kinetics of the events were similar in each of the cell types. The resemblance of Ca2+ puffs in these cell types suggests that they are a generic elementary Ca2+ signal and, furthermore, that the different inositol 1,4,5-trisphosphate isoforms are functionally redundant at the level of subcellular Ca2+ signalling. Hormonal stimulation of SH-SY5Y neuroblastoma cells and HeLa cells for several hours downregulated inositol 1,4,5-trisphosphate expression and concomitantly altered the properties of the Ca2+ puffs. The amplitude and duration of Ca2+ puffs were substantially reduced. In addition, the number of Ca2+ puff sites active during the onset of a Ca2+ wave declined. The consequence of the changes in Ca2+ puff properties was that cells displayed a lower propensity to trigger regenerative Ca2+ waves. Therefore, Ca2+ puffs underlie inositol 1,4,5-trisphosphate signalling in diverse cell types and are focal points for regulation of cellular responses.

Baseline cytosolic Ca2+ oscillations derived from a non-endoplasmic reticulum Ca2+ store

Cytosolic Ca(2+) oscillations can be due to cycles of release and re-uptake of internally stored Ca(2+). To investigate the nature of these Ca(2+) stores, we expressed the Pmr1 Ca(2+) pump of Caenorhabditis elegans in COS-1 cells and pretreated the cells with thapsigargin to prevent Ca(2+) uptake by the sarco(endo)plasmic reticulum Ca(2+)-ATPase. Pmr1 co-localized with the Golgi-specific 58K protein and was targeted to a Ca(2+) store that was less leaky for Ca(2+) than the endoplasmic reticulum and whose inositol trisphosphate receptors were less sensitive to inositol trisphosphate and ATP than those in the endoplasmic reticulum. ATP-stimulated Pmr1-overexpressing cells responded after a latency to extracellular Ca(2+) with a regenerative Ca(2+) signal, which could be prevented by caffeine. They also produced very stable ilimaquinone-sensitive baseline Ca(2+) spikes, even in the presence of thapsigargin. Such responses never occurred in non-transfected cells or in cells that overexpressed the type-1 sarco(endo)plasmic reticulum Ca(2+)-ATPase. Abortive Ca(2+) spikes also occurred in histamine-stimulated untransfected HeLa cells pretreated with thapsigargin, and they too were inhibited by ilimaquinone. We conclude that the Pmr1-induced Ca(2+) store, which probably corresponds to the Golgi compartment, can play a crucial role in setting up baseline Ca(2+) spiking.

Functional specialization of calreticulin domains

Calreticulin is a Ca2+-binding chaperone in the endoplasmic reticulum (ER), and calreticulin gene knockout is embryonic lethal. Here, we used calreticulin-deficient mouse embryonic fibroblasts to examine the function of calreticulin as a regulator of Ca2+ homeostasis. In cells without calreticulin, the ER has a lower capacity for Ca2+ storage, although the free ER luminal Ca2+ concentration is unchanged. Calreticulin-deficient cells show inhibited Ca2+ release in response to bradykinin, yet they release Ca2+ upon direct activation with the inositol 1,4,5-trisphosphate (InsP3). These cells fail to produce a measurable level of InsP3 upon stimulation with bradykinin, likely because the binding of bradykinin to its cell surface receptor is impaired. Bradykinin binding and bradykinin-induced Ca2+ release are both restored by expression of full-length calreticulin and the N + P domain of the protein. Expression of the P + C domain of calreticulin does not affect bradykinin-induced Ca2+ release but restores the ER Ca2+ storage capacity. Our results indicate that calreticulin may play a role in folding of the bradykinin receptor, which affects its ability to initiate InsP3-dependent Ca2+ release in calreticulin-deficient cells. We concluded that the C domain of calreticulin plays a role in Ca2+ storage and that the N domain may participate in its chaperone functions.

Recent developments in non-excitable cell calcium entry

Influx of calcium into cells following stimulation of cell surface receptors is a key process controlling cellular activity. However, despite intensive research, there is still no consensus on precisely how calcium entry is controlled in electrically no n-excitable cells. In particular, the regulation of depletion-activated or 'capacitative' calcium entry continues to be a focus of debate. Work published in the last 2 years has lent new impetus to the so-called 'conformational coupling' theory, although evidence for the existence of soluble messengers between the ER and the plasma membrane also continues to appear. In addition, there remains disagreement on whether intra-store [Ca(2+)] has to fall below a threshold before Ca(2+)entry is activated. A further major question is the identity of the putative depletion-operated Ca(2+)channel or channels. Here discussion has largely focussed on whether homologue(s) of the Drosophila TRP ('Transient Receptor Potential') protein is/are the elusive channel, or at least a part of it. Finally, it remains possible that Ca(2+)entry mechanisms other than depletion-activated channels may be important in agonist-evoked Ca(2+)influx. This commentary summarizes recent developments in the field, and highlights both current debates and critical unsolved questions.

Characterization and mapping of the 12 kDa FK506-binding protein (FKBP12)-binding site on different isoforms of the ryanodine receptor and of the inositol 1,4,5-trisphosphate receptor

We investigated the interaction of the 12 kDa FK506-binding protein (FKBP12) with two ryanodine-receptor isoforms (RyR1 and RyR3) and with two myo-inositol 1,4,5-trisphosphate (IP3) receptor isoforms (IP3R1 and IP3R3). Using glutathione S-transferase (GST)-FKBP12 affinity chromatography, we could efficiently extract RyR1 (42+/-7% of the solubilized RyR1) from terminal cisternae of skeletal muscle as well as RyR3 (32+/-4% of the solubilized RyR3) from RyR3-overexpressing HEK-293 cells. These interactions were completely abolished by FK506 (20 microM) but were largely unaffected by RyR-channel modulators. In contrast, neither IP3R1 nor IP3R3 from various sources, including rabbit cerebellum, A7r5 smooth-muscle cells and IP3R-overexpressing Sf9 insect cells from Spodoptera frugiperda, were retained on the GST-FKBP12 matrix. Moreover, immunoprecipitation experiments indicated a high-affinity interaction of FKBP12 with RyR1 but not with IP3R1. In order to determine the FKBP12-binding site, we fragmented both RyR1 and IP33R1 by limited proteolysis. We obtained a 45 kDa fragment of RyR1 that bound to the GST-FKBP12 matrix, indicating that it retained all requirements for FKBP12 binding. This fragment was identified by its interaction with antibody m34C and must therefore contain its epitope (amino acids 2756-2803). However, no fragment of IP3R1 was retained on the column. These molecular data are in agreement with the lack of correlation between FKBP12 and IP3R1 expression in various cell types. The observation that FKBP12 did not affect IP3-induced Ca2+ release but reduced caffeine-induced Ca2+ release also indicated that mature IP3R1 and IP3R3, in contrast to RyR1 and RyR3, did not display a specific, high-affinity interaction with FKBP12.

Mapping of the ATP-binding sites on inositol 1,4,5-trisphosphate receptor type 1 and type 3 homotetramers by controlled proteolysis and photoaffinity labeling

Submillimolar ATP concentrations strongly enhance the inositol 1,4,5-trisphosphate (IP(3))-induced Ca(2+) release, by binding specifically to ATP-binding sites on the IP(3) receptor (IP(3)R). To locate those ATP-binding sites on IP(3)R1 and IP(3)R3, both proteins were expressed in Sf9 insect cells and covalently labeled with 8-azido-[alpha-(32)P]ATP. IP(3)R1 and IP(3)R3 were then purified and subjected to a controlled proteolysis, and the labeled proteolytic fragments were identified by site-specific antibodies. Two fragments of IP(3)R1 were labeled, each containing one of the previously proposed ATP-binding sites with amino acid sequence GXGXXG (amino acids 1773-1780 and 2016-2021, respectively). In IP(3)R3, only one fragment was labeled. This fragment contained the GXGXXG sequence (amino acids 1920-1925), which is conserved in the three IP(3)R isoforms. The presence of multiple interaction sites for ATP was also evident from the IP(3)-induced Ca(2+) release in permeabilized A7r5 cells, which depended on ATP over a very broad concentration range from micromolar to millimolar.

Calreticulin is an interleukin-3-sensitive calcium-binding protein in human basophil leukocytes

BACKGROUND

IL-3 enhances basophil histamine release upon stimulation with any known secretagogue. The molecular mechanism behind this regulation is not known, although some observations suggest that IL-3 modulates the calcium part of the signal transduction mechanism. The inhibitory action of glucocorticoids on basophils can be reversed by stimulation with IL-3.

METHODS

Calcium-binding proteins in the basophil cell line KU812 were identified by two-dimensional gel electrophoresis, Calcium-overlay assay, N-terminal sequence analysis, and mass spectometry. The presence of the same proteins in purified human basophil leukocytes was established by comigration of KU812 and human basophil proteins on the two-dimensional gels. The expression of the calcium-binding proteins in the absence and presence of IL-3 and/or anti-IgE was determined by densitometric measurement of the spots on the two-dimensional gels.

RESULTS

Calreticulin was identified on the two-dimensional gel of KU812 proteins. A protein with exactly the same migration pattern was found on the gels of proteins from purified human basophils. Immunoblotting with a specific antihuman calreticulin antibody confirmed that this protein was calreticulin. Subsequent analysis showed that the expression of calreticulin in the basophils is upregulated twofold upon stimulation with rhIL-3, even in doses below those needed for enhancement of histamine release.

CONCLUSIONS

The expression of calreticulin in human basophil leukocytes is regulated by IL-3. Calreticulin is known to modulate IP3-dependent Ca2+ influx in different cell systems, and calreticulin overexpression inhibits steroid-induced transcriptional activation. Therefore, modulation of calreticulin expression may be one mechanism by which IL-3 exerts its effects on human basophils.

Expression of Ca(2+) Transport Genes in Platelets and Endothelial Cells in Hypertension

-Altered Ca(2+) handling is observed in different cells in essential hypertension. We investigated the expression of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) and inositol 1,4,5-trisphosphate receptor (IP(3)R) isoforms in platelets and aortic endothelial cells (EC) isolated from spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats by ratio reverse-transcriptase-polymerase chain reaction (RT-PCR) analysis and Western blotting. SERCA2b and SERCA3 were assessed at mRNA (EC and platelets) and at protein level (platelets). IP(3)R1, IP(3)R2, and IP(3)R3 mRNAs were demonstrated in both cell types, but only IP(3)R1 and IP(3)R2 proteins were detected in platelets. Compared with WKY, SHR EC and platelets showed higher SERCA3 and IP(3)R2 expression and lower IP(3)R1 expression. We then investigated the effect of lisinopril (20 mg. kg(-)(1). d(-)(1); 10-week treatment of 4-week-old rats or 2-week treatment of adult rats) and captopril (100 mg. kg(-)(1). d(-)(1); 2-week treatment of adult rats). Consequently, expression patterns of SERCAs and IP(3)Rs were significantly modified. Except for SERCAs mRNA in platelets, all differences between SHR and WKY disappeared. However, SERCA3 remained the predominant isoform. Both EC and platelets demonstrated a high equal expression of IP(3)R2 mRNA. IP(3)R1 was the predominant platelet protein isoform, as it was in untreated WKY. mRNA was also isolated from pancreatic islets of WKY and SHR, but no effect of either rat strain or of lisinopril treatment was observed on the expression of the studied genes. We hypothesize that the identical expression pattern of SERCAs and IP(3)Rs after treatment with ACE inhibitors represents a different nonhypertensive configuration, which, through changes in intracellular Ca(2+) handling, improves endothelial and platelet dysfunction in SHR but has no effect in WKY.

Green fluorescent protein-tagged sarco(endo)plasmic reticulum Ca2+-ATPase overexpression in Paramecium cells: isoforms, subcellular localization, biogenesis of cortical calcium stores and functional aspects

We have followed the time-dependent transfection of Paramecium cells with a vector containing the gene of green fluorescent protein (GFP) attached to the C-terminus of the PtSERCA1 gene. The outlines of alveolar sacs (ASs) are labelled, as is the endoplasmic reticulum (ER) throughout the cell. When GFP fluorescence is compared with previous anti-PtSERCA1 antibody labelling, the much wider distribution of GFP (ER+ASs) indicates that only a small amount of SERCA molecules is normally retained in the ER. A second isoform, PtSERCA2, also occurs and its C-terminal GFP-tagging results in the same distribution pattern. However, when GFP is inserted in the major cytoplasmic loop, PtSERCA1 and two fusion proteins are mostly retained in the ER, probably because of the presence of the overt C-terminal KKXX ER-retention signal and/or masking of a signal for transfer into ASs. On the overall cell surface, new SERCA molecules seem to be permanently delivered from the ER to ASs by vesicle transport, whereas in the fission zone of dividing cells ASs may form anew. In cells overexpressing PtSERCA1 (with C-terminal GFP) in ASs, [Ca2+]i regulation during exocytosis is not significantly different from controls, probably because their Ca2+ pump has to mediate only slow reuptake.

Basic properties of an inositol 1,4,5-trisphosphate-gated channel in carp olfactory cilia

In addition to the activation of cAMP-dependent pathways, odorant binding to its receptor can lead to inositol 1,4,5-trisphosphate (InsP3) production that may induce the opening of plasma membrane channels. We therefore investigated the presence and nature of such channels in carp olfactory cilia. Functional analysis was performed by reconstitution of the olfactory cilia in planar lipid bilayers (tip-dip method). In the presence of InsP3 (10 microM) and Ca2+ (100 nM), a current of 1.6 +/- 0.1 pA (mean +/- SEM, n = 4) was measured, using Ba2+ as charge carrier. The I/V curve displayed a slope conductance of 45 +/- 5 pS and a reversal potential of -29 mV indicating a higher selectivity for divalent cations. This current was characterized by two mean open times (3.0 +/- 0.4 ms and 42.0 +/- 2.6 ms, n = 4) and was strongly inhibited by ruthenium red (30 microM) or heparin (10 microg/mL). Importantly, the channel activity was closely dependent on the Ca2+ concentration, with the highest open probability (Po) at 100 nM Ca2+ (Po = 0.50 +/- 0.02, n = 4). Po is lower at both higher and lower Ca2+ concentrations. A structural identification of the channel was attempted by using a large panel of antibodies, raised against several InsP3 receptor (InsP3R)/Ca2+ release channel isoforms. The type 1 InsP3R was detected in carp cerebellum and whole brain, while a lower molecular mass InsP3R, which may correspond to type 2 or 3, was detected in heart, whole brain and the soma of the olfactory neurons. None of the antibodies, however, cross-reacted with olfactory cilia. Taken together, these results indicate that in carp olfactory cilia an InsP3-dependent channel is present, distinct from the classical InsP3Rs localized on intracellular membranes.

Effects of the immunosuppressant FK506 on intracellular Ca2+ release and Ca2+ accumulation mechanisms

The immunophilin FKBP12 associates with intracellular Ca2+ channels and this interaction can be disrupted by the immunosuppressant FK506. We have investigated the effect of FK506 on Ca2+ release and Ca2+ uptake in permeabilized cell types. Changes in medium free [Ca2+] were detected by the fluorescent Ca2+ indicator fluo-3 in digitonin-permeabilized SH-SY5Y human neuroblastoma cells, DT40 and R23-11 (i.e. triple inositol 1,4,5-trisphosphate (IP3) receptor knockout cells) chicken B lymphocytes and differentiated and undifferentiated BC3H1 skeletal muscle cells. 45Ca2+ fluxes were studied in saponin-permeabilized A7r5 rat smooth muscle cells. Addition of FK506 to permeabilized SH-SY5Y cells led to a sustained elevation of the medium [Ca2+] corresponding to approximately 30 % of the Ca2+ ionophore A23187-induced [Ca2+] rise. This rise in [Ca2+] was not dependent on mitochondrial activity. This FK506-induced [Ca2+] rise was related to the inhibition of the sarcoplasmic/endoplasmic reticulum Ca2+-Mg2+-ATPase (SERCA) Ca2+ pump. Oxalate-facilitated 45Ca2+ uptake in SH-SY5Y microsomes was inhibited by FK506 with an IC50 of 19 microM. The inhibition of the SERCA Ca2+ pump was not specific since several macrocyclic lactone compounds (ivermectin > FK506, ascomycin and rapamycin) were able to inhibit Ca2+ uptake activity. FK506 (10 microM) did not affect IP3-induced Ca2+ release in permeabilized SH-SY5Y and A7r5 cells, but enhanced caffeine-induced Ca2+ release via the ryanodine receptor (RyR) in differentiated BC3H1 cells. In conclusion, FK506 inhibited active Ca2+ uptake by the SERCA Ca2+ pump; in addition, FK506 enhanced intracellular Ca2+ release through the RyR, but it had no direct effect on IP3-induced Ca2+ release.

Inhibition of type I and III IP(3)Rs by TGF-beta is associated with impaired calcium release in mesangial cells

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) mediate cytosolic free calcium concentration ([Ca(2+)](c)) signals in response to a variety of agonists that stimulate mesangial cell contraction and proliferation. In the present study, we demonstrate that mesangial cells express both type I and III IP(3)Rs and that these receptors occupy different cellular locations. Chronic treatment with transforming growth factor-beta1 (TGF-beta1; 10 ng/ml, 24 h) leads to downregulation of both type I and III IP(3)Rs as measured by immunoblot and confocal analysis. TGF-beta1 treatment does not affect IP(3) levels, and downregulation of type I IP(3)R is not due to enhanced degradation of the protein, as the half-life of type I IP(3)R is unchanged in the presence or absence of TGF-beta1. Functional effects of TGF-beta1-induced downregulation of the IP(3)Rs were evaluated by measuring [Ca(2+)](c) changes in response to epidermal growth factor (EGF) in intact cells and sensitivity of [Ca(2+)](c) release to IP(3) in permeabilized cells. TGF-beta1 pretreatment led to a significant decrease of [Ca(2+)](c) release induced by EGF in intact cells and by submaximal IP(3) (400 nm) in permeabilized cells. Total IP(3)-sensitive [Ca(2+)](c) stores were not changed, as assessed by stimulation with maximal doses of IP(3) (10.5 microm) and thapsigargin-mediated calcium release in permeabilized cells. We conclude that prolonged exposure to TGF-beta1 leads to downregulation of both type I and III IP(3)Rs in mesangial cells and this is associated with impaired sensitivity to IP(3).

Differential modulation of inositol 1,4,5-trisphosphate receptor type 1 and type 3 by ATP

Binding of ATP to the inositol 1,4,5-trisphosphate receptor (IP(3)R) results in a more pronounced Ca(2+)release in the presence of inositol 1,4,5-trisphosphate (IP(3)). Two recently published studies demonstrated a different ATP sensitivity of IP(3)-induced Ca(2+)release in cell types expressing different IP(3)R isoforms. Cell types expressing mainly IP(3)R3 were less sensitive to ATP than cell types expressing mainly IP(3)R1 (Missiaen L, Parys JB, Sienaert I et al. Functional properties of the type 3 InsP(3)receptor in 16HBE14o- bronchial mucosal cells. J Biol Chem 1998;273: 8983-8986; Miyakawa T, Maeda A, Yamazawa T et al. Encoding of Ca(2+)signals by differential expression of IP(3)receptor subtypes. EMBO J 1999;18: 1303-1308). In order to investigate the difference in ATP sensitivity between IP(3)R isoforms at the molecular level, microsomes of Sf9 insect cells expressing full-size IP(3)R1 or IP(3)R3 were covalently labeled with ATP by using the photoaffinity label 8-azido[alpha-(32)P]ATP. ATP labeling of the IP(3)R was measured after immunoprecipitation of IP(3)Rs with isoform-specific antibodies, SDS-PAGE and Phosphorimaging. Unlabeled ATP inhibited covalent linking of 8-azido[alpha-(32)P]ATP to the recombinant IP(3)R1 and IP(3)R3 with an IC(50)of 1.6 microM and 177 microM, respectively. MgATP was as effective as ATP in displacing 8-azido[alpha-(32)P]ATP from the ATP-binding sites on IP(3)R1 and IP(3)R3, and in stimulating IP(3)-induced Ca(2+)release from permeabilized A7r5 and 16HBE14o- cells. The interaction of ATP with the ATP-binding sites on IP(3)R1 and IP(3)R3 was different from its interaction with the IP(3)-binding domains, since ATP inhibited IP(3)binding to the N-terminal 581 amino acids of IP(3)R1 and IP(3)R3 with an IC(50)of 353 microM and 4.0 mM, respectively. The ATP-binding sites of IP(3)R1 bound much better ATP than ADP, AMP and particularly GTP, while IP(3)R3 displayed a much broader nucleotide specificity. These results therefore provide molecular evidence for a differential regulation of IP(3)R1 and IP(3)R3 by ATP.

Substantial depletion of the intracellular Ca2+ stores is required for macroscopic activation of the Ca2+ release-activated Ca2+ current in rat basophilic leukaemia cells

1. Tight-seal whole-cell patch clamp experiments were performed to examine the ability of different intracellular Ca2+ mobilising agents to activate the Ca2+ release-activated Ca2+ current (ICRAC) in rat basophilic leukaemia (RBL-1) cells under conditions of weak cytoplasmic Ca2+ buffering. 2. Dialysis with a maximal concentration of inositol 1,4,5-trisphosphate (IP3) routinely failed to activate macroscopic ICRAC in low buffer (0.mM EGTA, BAPTA or dimethyl BAPTA), whereas it activated the current to its maximal extent in high buffer (10 mM EGTA). Dialysis with a poorly metabolisable analogue of IP3, with ionomycin, or with IP3 and ionomycin all failed to generate macroscopic ICRAC in low Ca2+ buffering conditions. 3. Dialysis with the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) pump blocker thapsigargin was able to activate ICRAC even in the presence of low cytoplasmic Ca2+ buffering, albeit at a slow rate. Exposure to IP3 together with the SERCA blockers thapsigargin, thapsigargicin or cyclopiazonic acid rapidly activated ICRAC in low buffer. 4. Following activation of ICRAC by intracellular dialysis with IP3 and thapsigargin in low buffer, the current was very selective for Ca2+ (apparent KD of 1 mM) Sr2+ and Ba2+ were less effective charge carriers and Na+ was not conducted to any appreciable extent. The ionic selectivity of ICRAC was very similar in low or high intracellular Ca2+ buffer. 5. Fast Ca2+-dependent inactivation of ICRAC occurred at a similar rate and to a similar extent in low or high Ca2+ buffer. Ca2+-dependent inactivation is not the reason why macroscopic ICRAC cannot be seen under conditions of low cytoplasmic Ca2+ buffering. 6. ICRAC could be activated by combining IP3 with thapsigargin, even in the presence of 100 microM Ca2+ and the absence of any exogenous Ca2+ chelator, where ATP and glutamate represented the only Ca2+ buffers in the pipette solution. 7. Our results suggest that a threshold exists within the IP3-sensitive Ca2+ store, below which intraluminal Ca2+ needs to fall before ICRAC activates. Possible models to explain the results are discussed.

Isoforms of the inositol 1,4,5-trisphosphate receptor are expressed in bovine oocytes and ovaries: the type-1 isoform is down-regulated by fertilization and by injection of adenophostin A

Mammalian fertilization is characterized by the presence of long-lasting intracellular calcium ([Ca2+]i) oscillations that are required to induce oocyte activation. One of the Ca2+ channels that may mediate this Ca2+ release is the inositol 1,4, 5-trisphosphate receptor (IP(3)R). Three isoforms of the receptor have been described, but their expression in oocytes and possible roles in mammalian fertilization are not well known. Using isoform-specific antibodies against IP(3)R types 1, 2, and 3 and Western analysis, we determined the isoforms that are expressed in bovine metaphase II oocytes and ovaries. In oocytes, all isoforms are expressed, but type 1 is present in overwhelmingly larger amounts and is likely responsible for the majority of Ca2+ release at fertilization. In ovarian microsomes, all three isoforms appear well expressed, suggesting the participation of all IP(3)R isoforms in ovarian Ca2+ signaling. We then investigated whether the reported cessation/reduction in amplitude of fertilization-associated [Ca2+]i oscillations, which is observed as pronuclear formation approaches, corresponded with down-regulation of the IP(3)R-1 isoform. Fertilization resulted in approximately 40% reduction in the amount of receptor by 16 h postinsemination. In addition, injection of adenophostin A, a potent IP(3)R agonist that elicits high-frequency [Ca2+]i oscillations in mammalian oocytes, induced similar reduction in receptor numbers. Together, these data show that 1) the three IP(3)R isoforms are expressed in bovine oocytes; 2) IP(3)R-1 is likely to mediate most of the Ca2+ release during fertilization; 3) its down-regulation may explain the decline in amplitude of sperm-induced [Ca2+]i rises as fertilization progresses toward pronuclear formation; and 4) agonists of the IP(3)R induce down-regulation of the type-1 receptor in oocytes similar to that evoked by fertilization.

Modulation of inositol 1,4,5-trisphosphate binding to the recombinant ligand-binding site of the type-1 inositol 1,4, 5-trisphosphate receptor by Ca2+ and calmodulin

A recombinant protein (Lbs-1) containing the N-terminal 581 amino acids of the mouse type 1 inositol 1,4,5-trisphosphate receptor (IP3R-1), including the complete IP3-binding site, was expressed in the soluble fraction of E. coli. The characteristics of IP3 binding to this protein were similar as observed previously for the intact IP3R-1. Ca2+ dose-dependently inhibited IP3 binding to Lbs-1 with an IC50 of about 200 nM. This effect represented a decrease in the affinity of Lbs-1 for IP3, because the Kd increased from 115 +/- 15 nM in the absence to 196 +/- 18 nM in the presence of 5 microM Ca2+. The maximal effect of Ca2+ on Lbs-1 (5 microM Ca2+, 42.0 +/- 6.4% inhibition) was similar to the maximal inhibition observed for microsomes of insect Sf9 cells expressing full-length IP3R-1 (33.8 +/- 10.2%). Conceivably, the two contiguous Ca2+-binding sites (residues 304-450 of mouse IP3R-1) previously found by us (Sienaert, I., Missiaen, L., De Smedt, H., Parys, J.B., Sipma, H., and Casteels, R. (1997) J. Biol. Chem. 272, 25899-25906) mediate the effect of Ca2+ on IP3 binding to IP3R-1. Calmodulin also dose-dependently inhibited IP3 binding to Lbs-1 with an IC50 of about 3 microM. Maximal inhibition (10 microM calmodulin, 43.1 +/- 5.9%) was similar as observed for Sf9-IP3R-1 microsomes (35.8 +/- 8.7%). Inhibition by calmodulin occurred independently of Ca2+ and was additive to the inhibitory effect of 5 microM Ca2+ (together 74.5 +/- 5.1%). These results suggest that the N-terminal ligand-binding region of IP3R-1 contains a calmodulin-binding domain that binds calmodulin independently of Ca2+ and that mediates the inhibition of IP3 binding to IP3R-1.

Tissue-specific expression and endogenous subcellular distribution of the inositol 1,3,4,5-tetrakisphosphate-binding proteins GAP1(IP4BP) and GAP1(m)

GAP1(IP4BP) and GAP1(m) belong to the GAP1 family of Ras GTPase-activating proteins that are candidate InsP4 receptors. Here we show they are ubiquitously expressed in human tissues and are likely to have tissue-specific splice variants. Analysis by subcellular fractionation of RBL-2H3 rat basophilic leukemia cells confirms that endogenous GAP1(IP4BP) is primarily localised to the plasma membrane, whereas GAP1(m) appears localised to the cytoplasm (cytosol and internal membranes) but not the plasma membrane. Subcellular fractionation did not indicate a specific co-localisation between membrane-bound GAP1(m) and several Ca2+ store markers, consistent with the lack of co-localisation between GAP1(m) and SERCA1 upon co-expression in COS-7 cells. This difference suggests that GAP1(m) does not reside at a site where it could regulate the ability of InsP4 to release intracellular Ca2+. As GAP1(m) is primarily localised to the cytosol of unstimulated cells it may be spatially regulated in order to interact with Ras at the plasma membrane.

Differential distribution of inositol trisphosphate receptor isoforms in mouse oocytes

In mammalian fertilization, inositol 1,4,5-trisphosphate receptor (IP3R)-dependent Ca2+ release is a crucial signaling event that originates from the vicinity of sperm-egg interaction and spreads as a wave throughout the egg cytoplasm. While it is known that Ca2+ is released by the type 1 IP3R in the egg cortex, the potential involvement of other isoform types responsible for the Ca2+ rise in the mouse egg (interior) and their spatial distribution are not known. In addition, the biochemical basis has not been definitively established for the development of increased sensitivity to inositol 1,4,5-trisphosphate (IP3) during meiotic maturation. Using specific antibodies to the type 1, 2, and 3 IP3R, we tested the hypotheses that different IP3R isoforms are responsible for the internal Ca2+ elevation and that they contribute to the maturation-associated acquisition of IP3 sensitivity. In both preovulatory oocytes and ovulated eggs of CF-1 mice, immunofluorescence revealed that types 1 and 2 isoforms were present in the cell cortex and interior. Type 1 was observed throughout the cytoplasm, and Western analysis indicated a 1.9-fold maturation-associated increase. In contrast, the signals detected for the type 2 (high-affinity) isoform and type 3 were present to a lesser extent, with type 2 restricted to isolated islands (similar to aggregates of vesicles detected by electron microscopy), which, in the cortex, may amplify early sperm-egg signaling events. The cortical-to-perinuclear localization of the receptor and cortical vesicle aggregates imply an efficient mechanism for propagating Ca2+ release from the cortex into the interior of the egg to activate development, and the isoform localization analysis indicates a clear spatial and biochemical heterogeneity. Types 1 and 2 isoforms were also present in granulosa cells.

Differential modulation of SERCA2 isoforms by calreticulin

In Xenopus laevis oocytes, overexpression of calreticulin suppresses inositol 1,4,5-trisphosphate-induced Ca2+ oscillations in a manner consistent with inhibition of Ca2+ uptake into the endoplasmic reticulum. Here we report that the alternatively spliced isoforms of the sarcoendoplasmic reticulum Ca2+-ATPase (SERCA)2 gene display differential Ca2+ wave properties and sensitivity to modulation by calreticulin. We demonstrate by glucosidase inhibition and site-directed mutagenesis that a putative glycosylated residue (N1036) in SERCA2b is critical in determining both the selective targeting of calreticulin to SERCA2b and isoform functional differences. Calreticulin belongs to a novel class of lectin ER chaperones that modulate immature protein folding. In addition to this role, we suggest that these chaperones dynamically modulate the conformation of mature glycoproteins, thereby affecting their function.

Agonist-induced down-regulation of type 1 and type 3 inositol 1,4,5-trisphosphate receptors in A7r5 and DDT1 MF-2 smooth muscle cells

Prolonged stimulation of rat A7r5 aortic smooth muscle cells with 3 microM vasopressin, or of hamster DDT1 MF-2 smooth muscle cells with 10 microM bradykinin or 100 microM histamine led within 4 h to a 40-50% down-regulation of the type 1 InsP3 receptor (InsP3R-1) and of the type 3 InsP3 receptor (InsP3R-3). InsP3R down-regulation was a cell- and agonist-specific process, since several other agonists acting on PLC-coupled receptors did not change the expression level of the InsP3R isoforms in these cell types and since no agonist-induced down-regulation of InsP3Rs was observed in HeLa cells. Down-regulation of InsP3Rs was prevented by an inhibitor of proteasomal protease activity, N-acetyl-Leu-Leu-norleucinal (ALLN). The Ca2+ channel blocker verapamil (2 microM) also induced InsP3R-1 down-regulation (43%) in A7r5 cells, which was inhibited by ALLN. In A7r5 cells transiently transfected with a cDNA construct, bearing a luciferase coding sequence under control of the rat InsP3R-1 promoter, reduced luciferase activity could be demonstrated upon stimulation of cells with vasopressin or verapamil. Thus, besides enhanced protein degradation, a reduction of InsP3R promoter activity might contribute to the down-regulation of InsP3Rs in A7r5 cells. We next investigated the effect of InsP3R down-regulation on Ca2+ responses in A7r5 cells. A rightward shift in the dose-response curve for InsP3-induced Ca2+ release was observed in permeabilized monolayers of vasopressin-pretreated A7r5 cells (EC50 630 nM and 400 nM for pretreated and non-pretreated cells, respectively). The Ca2+ responses to threshold doses of vasopressin were markedly reduced in intact vasopressin-pretreated cells. We conclude that prolonged agonist-exposure leads to down-regulation of InsP3Rs in A7r5 and DDT, MF-2 smooth muscle cells. The mechanism of down-regulation likely involves proteasomal degradation and reduction of InsP3R promoter activity. Moreover, down-regulation of InsP3Rs resulted in desensitization of Ca2+ release from InsP3 sensitive stores.