Purification of an inositol 1,4,5-trisphosphate-binding calreticulin-containing intracellular compartment of HL-60 cells

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.

Molecular characteristics and expression analysis of calreticulin in Chinese shrimp Fenneropenaeus chinensis

Calreticulin (CRT), as an endoplasmic reticulum luminal resident protein, plays important roles in Ca(2+) homeostasis and molecular chaperoning. CRT on the surface of the cell can modulate cell adhesion, phagocytosis and integrin-dependent Ca(2+) signaling. The full length cDNA of calreticulin (FcCRT) was cloned from Chinese shrimp Fenneropenaeus chinensis. It consists of 1672 bp with an open reading frame of 1221 bp, encoding 406 amino acids. This is the first reported cDNA sequence of calreticulin in Crustacea. The deduced amino acid sequence of FcCRT showed high identity with those of Bombyx mori (88%), Drosophila melanogaster (83%), Mus musculus (82%) and Homo sapiens (82%). Highest expression of FcCRT was detected in ovary by Northern blot and in situ hybridization. Different mRNA levels of FcCRT were detected at various molting stages. Expression of FcCRT was induced significantly after 3 h of heat shock treatment, reached the maximum at 4 h and dropped after that. Differential expression profiles of FcCRT were observed in hepatopancreas and haemocytes when shrimp were challenged by white spot syndrome virus (WSSV). From the above results, we inferred that FcCRT might play important roles in Ca(2+) homeostasis, chaperoning and immune function in shrimp.

Calnexin deficiency and endoplasmic reticulum stress-induced apoptosis

In this study, we used calnexin-deficient cells to investigate the role of this protein in ER stress-induced apoptosis. We found that calnexin-deficient cells are relatively resistant to ER stress-induced apoptosis. However, caspase 3 and 8 cleavage and cytochrome c release were unchanged in these cells, indicating that ER to mitochondria "communication" during apoptotic stimulation is not affected in the absence of calnexin. The Bcl-2:Bax ratio was also not significantly changed in calnexin-deficient cells regardless of whether the ER stress was induced with thapsigargin or not. Ca(2+) homeostasis and ER morphology were unaffected by the lack of calnexin, but ER stress-induced Bap31 cleavage was significantly inhibited. Immunoprecipitation experiments revealed that Bap31 forms complexes with calnexin, which may play a role in apoptosis. The results suggest that calnexin may not play a role in the initiation of the ER stress but that the protein has an effect on later apoptotic events via its influence on Bap31 function.

Calreticulin: one protein, one gene, many functions

The endoplasmic reticulum (ER) plays a critical role in the synthesis and chaperoning of membrane-associated and secreted proteins. The membrane is also an important site of Ca(2+) storage and release. Calreticulin is a unique ER luminal resident protein. The protein affects many cellular functions, both in the ER lumen and outside of the ER environment. In the ER lumen, calreticulin performs two major functions: chaperoning and regulation of Ca(2+) homoeostasis. Calreticulin is a highly versatile lectin-like chaperone, and it participates during the synthesis of a variety of molecules, including ion channels, surface receptors, integrins and transporters. The protein also affects intracellular Ca(2+) homoeostasis by modulation of ER Ca(2+) storage and transport. Studies on the cell biology of calreticulin revealed that the ER membrane is a very dynamic intracellular compartment affecting many aspects of cell physiology.

Localization of intracellular Ca2+ stores in HeLa cells during infection with Chlamydia trachomatis

Chlamydia trachomatis elementary bodies (EBs) enter epithelial cells within membrane-bound endosomes that aggregate with each other in a calcium-regulated process, but avoid fusion with lysosomes. Annexin III but not I translocates to chlamydial aggregates and inclusions. In this study, we localize the intracellular Ca2+ stores during the course of infection by analyzing the distribution of three intracellular Ca2+ store proteins: calreticulin, type-1 inositol-1,4, 5-trisphosphate receptor (IP3-R), and Sarcoplasmic/Endoplasmic Reticulum Ca2+ ATPase type 2 (SERCA2) in HeLa cells infected with C. trachomatis serovar L2. In uninfected cells, immunofluorescence staining of the proteins showed a fine granular distributed pattern for all three proteins. After infection with C. trachomatis, calreticulin was found at the periphery of chlamydial aggregates and inclusions from 3 to 48 hours post-infection. In infected cells, SERCA2 was intimately associated with chlamydial inclusions after 3 and 24 hours, but not after 48 hours. Moreover, IP3-R was translocated to and colocalized with EB aggregates and chlamydial inclusions and had a distribution very similar to that of SERCA 2. After 24 hours incubation with chlamydiae, there was a local accumulation of [Ca2+]i (105+/-17 nM) in the proximity of chlamydial inclusions, compared to 50+/-13 nM in other parts of the cell cytoplasm. In the absence of extracellular Ca2+, this local accumulation of Ca2+ increased to 295+/-50 nM after adding 50 microM ATP, and to a similar extent after adding 100 nM thapsigargin (Tg). These data indicate that during infection of HeLa cells with chlamydiae, intracellular Ca2+ stores are redistributed, causing local accumulation of Ca2+ in the vicinity of chlamydial inclusions. These changes may trigger the association of certain proteins such as annexins with chlamydia-containing vesicles, and thereby regulation of membrane-membrane interaction during endosome aggregation and inclusion formation.

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

RBL-2H3 rat basophilic leukemia cells were homogenized and fractionated. A fraction F3 obtained by differential centrifugation was 6-fold enriched in [3H]-inositol 1,4,5-trisphosphate (InsP3) binding activity, while the NADH-cytochrome c oxidoreductase and sulphatase-C activities were only 3.8- and 2.9-fold enriched, respectively. Furthermore, the three InsP3 receptor (InsP3R) isoforms, two sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) isoforms (2b and 3) as well as four Ca2+ binding proteins (calreticulin, calnexin, protein disulfide isomerase (PDI) and BiP), were present in this fraction. Fraction F3 was, therefore, further purified on a discontinuous sucrose density gradient, and the 3 resulting fractions were analyzed. The InsP3 binding sites were distributed over the gradient and did not co-migrate with the RNA. We examined the relative content of the three InsP3R isoforms, of both SERCA2b and 3, as well as that of the four Ca2+ binding proteins in fraction F3 and the sucrose density gradient fractions. InsP3R-1 and InsP3R-2 showed a similar distribution, with the highest level in the light and intermediate density fractions. InsP3R-3 distributed differently, with the highest level in the intermediate density fraction. Both SERCA isoforms distributed similarly to InsP3R-1 and InsP3R-2. SERCA3 was present at a very low level in the high density fraction. Calreticulin and BiP showed a pattern similar to that of InsP3R-1 and InsP3R-2 and the SERCAs. PDI was clearly enriched in the light density fraction while calnexin was broadly distributed. These results indicate a heterogeneous distribution of the three InsP3R isoforms, the two SERCA isoforms and the four Ca2+ binding proteins investigated. This heterogeneity may underlie specialization of the Ca2+ stores and the subsequent initiation of intracellular Ca2+ signals.

Calreticulin Biosynthesis and Processing in Human Myeloid Cells: Demonstration of Signal Peptide Cleavage and N-Glycosylation

Calreticulin is a soluble endoplasmic reticulum protein comprising the major storage reservoir for inositol trisphosphate-releasable calcium. Although its highly conserved primary structure and a wide range of functions have been well described, less attention has been paid to its biosynthesis, particularly in human tissues. We report analyses of synthesis, proteolytic processing and glycosylation of human calreticulin. In both HL-60 and PLB-985 myeloid cell lines calreticulin was immunoprecipitated as a single 60-kD species without evidence of precursor forms. However, in vitro cell-free synthesis produced a 62-kD primary translation product, which in the presence of microsomal membranes, was processed by cotranslational signal peptide cleavage to a 60-kD species that comigrated with mature calreticulin produced in myeloid cells. Neither tunicamycin treatment of the cells nor endoglycosidase digestion of calreticulin resulted in any forms other than the 60-kD protein on sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis, suggesting that the potential site for N-glycosylation at asparagine-327 was unmodified. However, oxidative derivatization of carbohydrate components with digoxigenin showed that human calreticulin produced in either HL-60 cells or Sf9 insect cells is glycosylated, indicating that glycosylated and nonglycosylated human calreticulin have indistinguishable electrophoretic mobilities. Direct measurement by phenol-H2SO4 confirmed the presence of carbohydrate on recombinant human calreticulin. These data show that human myeloid calreticulin undergoes cotranslational signal peptide cleavage and posttranslational N-linked glycosylation. Although glycosylation of calreticulin has been shown in rat liver and bovine liver and brain, it has been reported to be lacking in other tissues including human lymphocytes.

Calreticulin Biosynthesis and Processing in Human Myeloid Cells: Demonstration of Signal Peptide Cleavage and N-Glycosylation

Calreticulin is a soluble endoplasmic reticulum protein comprising the major storage reservoir for inositol trisphosphate-releasable calcium. Although its highly conserved primary structure and a wide range of functions have been well described, less attention has been paid to its biosynthesis, particularly in human tissues. We report analyses of synthesis, proteolytic processing and glycosylation of human calreticulin. In both HL-60 and PLB-985 myeloid cell lines calreticulin was immunoprecipitated as a single 60-kD species without evidence of precursor forms. However, in vitro cell-free synthesis produced a 62-kD primary translation product, which in the presence of microsomal membranes, was processed by cotranslational signal peptide cleavage to a 60-kD species that comigrated with mature calreticulin produced in myeloid cells. Neither tunicamycin treatment of the cells nor endoglycosidase digestion of calreticulin resulted in any forms other than the 60-kD protein on sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis, suggesting that the potential site for N-glycosylation at asparagine-327 was unmodified. However, oxidative derivatization of carbohydrate components with digoxigenin showed that human calreticulin produced in either HL-60 cells or Sf9 insect cells is glycosylated, indicating that glycosylated and nonglycosylated human calreticulin have indistinguishable electrophoretic mobilities. Direct measurement by phenol-H2SO4 confirmed the presence of carbohydrate on recombinant human calreticulin. These data show that human myeloid calreticulin undergoes cotranslational signal peptide cleavage and posttranslational N-linked glycosylation. Although glycosylation of calreticulin has been shown in rat liver and bovine liver and brain, it has been reported to be lacking in other tissues including human lymphocytes.

Calcium-containing organelles display unique reactivity to chemical stimulation in cultured hippocampal neurons

Cultured rat hippocampal neurons grown on glass coverslips for 1-3 weeks were loaded with the calcium-sensitive fluorescent dye Fluo-3 and viewed with a confocal laser scanning microscope. Large pyramidal-shaped neurons were found to contain dye-accumulating organelles in their somata, primarily around nuclei and near the base of their primary dendrites. These organelles varied in size and increased in density over weeks in culture, and were not colocalized with the endoplasmic reticulum or with mitochondria. The Fluo-3 fluorescence in these calcium-containing organelles (CCOs) was transiently quenched by exposure to Mn2+, indicating that the dye is a genuine [Ca2+] reporter and is not just a site of accumulating Fluo-3 dye. Recovery of fluorescence in the CCOs after washout of Mn2+ involved activation of a thapsigargin-sensitive process. CCOs responded to stimuli that evoke a rise of cytosolic [Ca2+] ([Ca]i) in a unique manner; perfusion of caffeine caused a prolonged rise of [Ca] in the CCOs ([Ca]C), whereas it caused only a transient rise of [Ca]i. Pulse application of caffeine also caused a faster effect on [Ca]C than on [Ca]i. Glutamate caused a transient rise of both [Ca]i and [Ca]C, followed by a prolonged fall of only [Ca]C to below rest level. This fall was blocked by preincubation with thapsigargin. Ryanodine blocked the cytosolic effects of caffeine but not its effect on [C]C. A clear distinction between CCOs and the known calcium stores was seen in digitonin-permeabilized cells; in these, remaining Fluo-3 reported changes in store calcium, i.e., caffeine caused a reduction in Fluo-3 fluorescence in permeabilized cells, whereas it still caused an increase in [Ca]C. A possible role of CCOs in regulation of release of calcium from ryanodine-sensitive stores was indicated by the observation that CCO-containing cells exhibited a larger and faster response to caffeine than cells that did not have them. We propose that CCOs constitute a unique functional compartment involved in release of calcium from calcium-sensitive stores.

Calcium-containing organelles display unique reactivity to chemical stimulation in cultured hippocampal neurons

Cultured rat hippocampal neurons grown on glass coverslips for 1-3 weeks were loaded with the calcium-sensitive fluorescent dye Fluo-3 and viewed with a confocal laser scanning microscope. Large pyramidal-shaped neurons were found to contain dye-accumulating organelles in their somata, primarily around nuclei and near the base of their primary dendrites. These organelles varied in size and increased in density over weeks in culture, and were not colocalized with the endoplasmic reticulum or with mitochondria. The Fluo-3 fluorescence in these calcium-containing organelles (CCOs) was transiently quenched by exposure to Mn2+, indicating that the dye is a genuine [Ca2+] reporter and is not just a site of accumulating Fluo-3 dye. Recovery of fluorescence in the CCOs after washout of Mn2+ involved activation of a thapsigargin-sensitive process. CCOs responded to stimuli that evoke a rise of cytosolic [Ca2+] ([Ca]i) in a unique manner; perfusion of caffeine caused a prolonged rise of [Ca] in the CCOs ([Ca]C), whereas it caused only a transient rise of [Ca]i. Pulse application of caffeine also caused a faster effect on [Ca]C than on [Ca]i. Glutamate caused a transient rise of both [Ca]i and [Ca]C, followed by a prolonged fall of only [Ca]C to below rest level. This fall was blocked by preincubation with thapsigargin. Ryanodine blocked the cytosolic effects of caffeine but not its effect on [C]C. A clear distinction between CCOs and the known calcium stores was seen in digitonin-permeabilized cells; in these, remaining Fluo-3 reported changes in store calcium, i.e., caffeine caused a reduction in Fluo-3 fluorescence in permeabilized cells, whereas it still caused an increase in [Ca]C. A possible role of CCOs in regulation of release of calcium from ryanodine-sensitive stores was indicated by the observation that CCO-containing cells exhibited a larger and faster response to caffeine than cells that did not have them. We propose that CCOs constitute a unique functional compartment involved in release of calcium from calcium-sensitive stores.

Mechanisms responsible for quantal Ca2+ release from inositol trisphosphate-sensitive calcium stores

Activation of cells by hormones, growth factors or neurotransmitters leads to an increased production of inositol trisphosphate (InsP3) and, after activation of the InsP3 receptor (InsP3R), to Ca2+ release from intracellular Ca2+ stores. The release of intracellular Ca2+ is characterised by a graded response when submaximal doses of agonists are used. The basic phenomenon, called "quantal Ca2+ release", is that even the maintained presence of a submaximal dose of agonist or of InsP3 for long time periods (up to 20 min) provokes only a partial release of Ca2+. This partial, or quantal, release phenomenon is due to the fact that the initially very rapid InsP3-induced Ca2+ release eventually develops into a much slower release phase. Physiologically, quantal release allows the Ca2+ stores to function as increment detectors and to induce local Ca2+ responses. The basic mechanism for quantal release of Ca2+ is presently not known. Possible mechanisms to explain the quantal behaviour of InsP3- induced Ca2+ release include the presence of InsP3Rs with varying sensitivities for InsP3, heterogeneous InsP3R distribution, intrinsic inactivation of the InsP3Rs, and regulation of the InsP3Rs by Ca2+ store content. This article reviews critically the evidence for the various mechanisms and evaluates their functional importance. A Ca2+-mediated conformational change of the InsP3R is most likely the key feature of the mechanism for quantal Ca2+ release, but the exact mode of operation remains unclear. It should also be pointed out that in intact cells more than one mechanism can be involved.

Organization of Ca2+ stores in myeloid cells: association of SERCA2b and the type-1 inositol-1,4,5-trisphosphate receptor

In this study, we have analysed the relationship between Ca2+ pumps and Ins(1,4,5)P3-sensitive Ca2+ channels in myeloid cells. To study whether sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase (SERCA)-type Ca(2+)-ATPases are responsible for Ca2+ uptake into Ins(1,4,5)P3-sensitive Ca2+ stores, we used the three structurally unrelated inhibitors thapsigargin, 2,5-di-t-butylhydroquinone and cyclopiazonic acid. In HL-60 cells, all three compounds precluded formation of the phosphorylated intermediate of SERCA-type Ca(2+)-ATPases. They also decreased, in parallel, ATP-dependent Ca2+ accumulation and the amount of Ins(1,4,5)P3-releasable Ca2+. Immunoblotting with subtype-directed antibodies demonstrated that HL-60 cells contain the Ca2+ pump SERCA2 (subtype b), and the Ca(2+)-release-channel type-1 Ins(1,4,5)P3 receptor. In subcellular fractionation studies, SERCA2 and type-1 Ins(1,4,5)P3 receptor co-purified. Immunofluorescence studies demonstrated that both type-1 Ins(1,4,5)P3 receptor and SERCA2 were evenly distributed throughout the cell in moving neutrophils. During phagocytosis both proteins translocated to the periphagosomal space. Taken together, our results suggest that in myeloid cells (i) SERCA-type Ca(2+)-ATPases function as Ca2+ pumps of Ins(1,4,5)P3-sensitive Ca2+ stores, and (ii) SERCA2 and type-1 Ins(1,4,5)P3 receptor reside either in the same or two tightly associated subcellular compartments.

Overexpression of calreticulin increases intracellular Ca2+ storage and decreases store-operated Ca2+ influx

The widely distributed and highly conserved Ca(2+)-binding protein calreticulin has been suggested to play a role as a Ca2+ storage protein of intracellular Ca+ stores. To test this hypothesis, we have generated a mouse L fibroblast cell line stably transfected with a calreticulin expression vector. The calreticulin content of the overexpressers was increased by 1.6 +/- 0.2-fold compared with mock-transfected cells. The total cellular Ca2+ content of calreticulin-overexpressing and control cells, as assessed by equilibrium 45Ca+2 uptake, was 141 +/- 8 and 67 +/- 6 pmol of Ca2+/10(6) cells, respectively (i.e. a 2.1 +/- 0.2-fold increase in the Ca2+ content of calreticulin-overexpressing cells). Over 80% of the increased Ca2+ content was found within thapsigargin-sensitive Ca2+ stores. The pattern of calreticulin distribution, revealed by immunofluorescence microscopy, showed an endoplasmic reticulum-like pattern and was identical in overexpressers and control cells. In overexpressers, cytosolic free [Ca2+] elevations due to Ca2+ release were enhanced when either ATP or a combination of ionomycin and thapsigargin was used as a stimulus. In contrast, thapsigargin-induced Ca2+ and Mn2+ influxes from the extracellular space were markedly diminished in calreticulin-overexpressing cells, suggesting an active involvement of calreticulin in the regulation of store-operated Ca2+ influx.

Intracellular calcium stores and inositol 1,4,5-trisphosphate receptor in rat liver cells

The D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] receptor was localized by immunofluorescence experiments in situ in liver cryosections. Two anti-Ins(1,4,5)P3 receptor antibodies (against the 14 C-terminal residues of the type 1 receptor or against the entire cerebellar receptor) weakly decorated the whole cytoplasm, and a more intense labelling was observed at the periphery of the hepatocytes, particularly beneath the canalicular and the sinusoidal domains of the plasma membrane (PM). Antibodies against calreticulin, the Ca2+ pump (SERCA2b) or endoplasmic reticulum (ER) membranes homogeneously labelled the cytoplasm and the subplasmalemmal area. These data indicate that the ER can be divided into at least two specialized subregions: one is located throughout most of the cytoplasm and contains markers of the rough ER (RER), calreticulin, SERCA2b and a low density of Ins(1,4,5)P3 receptor, and the other is confined to the periphery of the cells and contains calreticulin, Ca2+ pump, RER markers and a high density of Ins(1,4,5)P3 receptor. A membrane fraction enriched in Ins(1,4,5)P3 receptor and in markers of the PM was immuno-adsorbed with the antibody against the C-terminal end of the Ins(1,4,5)P3 receptor and pelleted with Sepharose protein A. The immuno-isolated material was enriched in Ins(1,4,5)P3 receptor, but none of the markers of the ER or of the PM could be detected. This suggests that the Ins(1,4,5)P3 receptor is localized on discrete domains of the ER membrane beneath the canalicular and the sinusoidal membranes, where it was found at higher densities than the other markers.

Highly cooperative Ca2+ elevations in response to Ins(1,4,5)P3 microperfusion through a patch-clamp pipette

To study the initial kinetics of Ins(1,4,5)P3-induced [Ca2+]i elevations with a high time resolution and to avoid the problem of cell-to-cell heterogeneity, we have used the combined patch-clamp/microfluorimetry technique. The mathematical description of the microperfusion of Ins(1,4,5)P3 and the subsequent Ca2+ release consists of a monoexponential decay (cytosolic Ins(1,4,5)P3 concentration) and a Hill equation (Ins(1,4,5)P3 dose-response curve). Two additional Hill equations and an integration were necessary to include a putative dependence of Ins(1,4,5)P3-induced Ca2+ release on [Ca2+]i. Best-fitting analysis assuming [Ca2+]i-independent Ca2+ release yielded Hill coefficients between 4 and 12. The high cooperativity was also observed with the poorly metabolizable analog Ins(2,4,5)P3 and was independent of extracellular [Ca2+]. Best-fitting analysis including a positive [Ca2+]i feedback suggested a cooperativity on the level of Ins(1,4,5)P3-induced channel opening (n = 2) and an enhancement of Ins(1,4,5)P3-induced Ca2+ release by [Ca2+]i. In summary, the onset kinetics of Ins(1,4,5)P3-induced [Ca2+]i elevations in single HL-60 granulocytes showed a very high cooperativity, presumably because of a cooperativity on the level of channel opening and a positive Ca2+ feedback, but not because of Ca2+ influx or Ins(1,4,5)P3 metabolism. This high cooperativity, acting in concert with negative feedback mechanisms, might play an important role in the fine-tuning of the cellular Ca2+ signal.

Heteroligomers of type-I and type-III inositol trisphosphate receptors in WB rat liver epithelial cells

We have previously shown that a 222-kDa polypeptide co-immunoprecipitates together with the type-I myoinositol 1,4,5-trisphosphate receptor (IP3R) in WB rat liver epithelial cell extracts, when the immunoprecipitation is carried out with a type-I isoform specific antibody (Joseph, S. K. (1994) J. Biol. Chem. 269, 5673-5679). Utilizing isoform-specific antibodies raised to unique sequences within the COOH-terminal region of IP3 receptors, we now report that the co-immunoprecipitating 222-kDa polypeptide is the type-III IP3R isoform and that type-III IP3R antibodies (Abs) can co-immunoprecipitate the type-I IP3R isoform. Co-immunoprecipitation of IP3R isoforms was not due to cross-reactivity of the antibodies for the following reasons: (a) on immunoblots the type-III antibodies did not cross-react with type-I IP3R and vice versa; (b) inclusion of the COOH-terminal type-III peptide had no effect on the ability of type-I IP3R Ab to co-immunoprecipitate the type-III IP3R but blocked the ability of type-III IP3R Ab to coimmunoprecipitate the type-I isoform; and (c) crude hepatocyte lysates contain undetectable amounts of type-III IP3R, and immunoprecipitation with type-III IP3R Ab does not co-immunoprecipitate any other isoforms. However, type-I and type-II IP3R isoforms were co-immunoprecipitated by their respective antibodies in hepatocyte lysates. Sucrose density gradient analysis of WB cell lysates indicated that the co-immunoprecipitating fraction is exclusively located at the density expected for tetrameric receptors, suggesting that co-immunoprecipitation was not a reflection of the nonspecific aggregation of IP3R isoforms. Phosphorylation of either type-I or type-III immunoprecipitates by protein kinase A indicated that only the type-I IP3R could be phosphorylated in vitro. Fractionation of WB cell membranes and immunofluorescence studies showed that the type-I and type-III isoforms have very similar sub-cellular localizations. We conclude that the WB cell contains both type-I and type-III IP3R isoforms and that a proportion of these receptors exist as heterotetramers.

Luminal calcium regulation of calcium release from sarcoplasmic reticulum

This article discusses how changes in luminal calcium concentration affect calcium release rates from triad-enriched sarcoplasmic reticulum vesicles, as well as single channel opening probability of the ryanodine receptor/calcium release channels incorporated in bilayers. The possible participation of calsequestrin, or of other luminal proteins of sarcoplasmic reticulum in this regulation is addressed. A comparison with the regulation by luminal calcium of calcium release mediated by the inositol 1,4,5-trisphosphate receptor/calcium channel is presented as well.

The whoosh and trickle of calcium signalling

The importance of phospholipase C catalysed hydrolysis of phosphatidylinositol-(4,5)bisphosphate (PtdIns(4,5)P2) to inositol-(1,4,5)trisphosphate (Ins(1,4,5)P3) and sn-1,2-diacylglycerol in the signal transduction pathways of eukaryote cells, in response to extracellular stimuli, is now widely recognised. Although nearly 60 naturally occurring inositol phosphates have been identified in mammalian cells, mobilisation of Ca2+ from the intracellular stores has been most commonly attributed to the generation of Ins(1,4,5)P3 [1]. However, there is increasing evidence for the presence of ryanodine receptors (RyR) in non-excitable cells and for cADP-ribose (cADPr) as the signalling molecule responsible for Ca2+ release via the RyR. But what is the purpose for the co-existence of these two intracellular Ca2+ channels in non-excitable cells and why are they so heterogeneous in their distribution? These questions were explored at the recent International Symposium Calcium Signalling in Inflammatory Cells. Depletion of the intracellular Ca2+ pools is followed by entry of Ca2+ into the cell across the plasma membrane, but the mechanism(s) underlying this 'capacitative Ca2+ entry' is not well understood. Many potential signalling pathways which may account for capacitative Ca2+ entry have been proposed although none have been unanimously accepted. New developments in the elucidation of the mechanism responsible for capacitative Ca2+ entry and how Ca2+ entry is regulated, together with progress in the characterisation of plasma membrane Ca2+ entry channels were also discussed at this symposium.

Rapid heparin-sensitive Ca2+ release following Ca(2+)-ATPase inhibition in intact HL-60 granulocytes. Evidence for Ins(1,4,5)P3-dependent Ca2+ cycling across the membrane of Ca2+ stores

In many cell types, emptying of intracellular Ca2+ stores after application of inhibitors of the intracellular Ca(2+)-ATPase (e.g. thapsigargin) is astonishingly rapid. It was the aim of this study to elucidate the underlying mechanism. We first compared thapsigargin-induced emptying of intracellular Ca2+ stores in intact and homogenized HL-60 granulocytes. Thapsigargin-induced Ca2+ release was rapid in intact cells (33.9 +/- 4.9% of store content/min), but it was slow in permeabilized or homogenized cells (7.7 +/- 3.9 and 12 +/- 3.8% of store content/min respectively). To study whether the Ins(1,4,5)P3 receptor might be involved in thapsigargin-induced Ca2+ release, we tested the effect of heparin, a competitive Ins(1,4,5)P3 antagonist. In homogenized and permeabilized preparations, heparin did not interfere with thapsigargin-induced Ca2+ release. In contrast, when introduced into intact cells by an endocytosis/osmotic-shock procedure, heparin, but not the inactive de-N-sulphated heparin, decreased the rate of Ca2+ release by approx. 70%. Heparin inhibited Ca2+ release in response to the Ins(1,4,5)P3-generating receptor agonist N-formylmethionyl-leucyl-phenylalanine (f-MLP) (50 nM) and to thapsigargin (50 nM) at comparable concentrations. Heparin inhibition was competitive for f-MLP-induced, but not for thapsigargin-induced, Ca2+ release. In permeabilized cells, the addition of low Ins(1,4,5)P3 concentrations before thapsigargin increased the rate of thapsigargin-induced Ca2+ release 4-fold. Taken together, our results suggest that the rapid Ca(2+)-ATPase-inhibitor-induced Ca2+ release is due to a partial activation of the Ins(1,4,5)P3 receptor in resting cells. This implies Ca2+ cycling across the membrane of Ins(1,4,5)P3-sensitive Ca2+ stores in resting cells.

The inositol 1,4,5-trisphosphate receptor is localized on specialized sub-regions of the endoplasmic reticulum in rat liver

Inositol 1,4,5-trisphosphate (InsP3) is involved in the mobilization of Ca2+ from intracellular non-mitochondrial stores. In rat liver, it has been shown that the InsP3-binding site co-purifies with the plasma membrane. This suggests that in the liver the InsP3 receptor (InsP3R) associates with plasma membrane. We studied the subcellular distribution of the liver InsP3R by measuring the maximal binding capacity of [3H]InsP3 and using antibodies against the 14 C-terminal residues of the type 1 InsP3R. The antibodies recognized a large amount of an InsP3R protein of 260 kDa in a membrane fraction which is also enriched with [3H]InsP3-binding sites and with markers of the basal, the lateral and the bile-canalicular membrane and the plasma-membrane Ca2+ pump (PMCA). The fractions enriched in markers of the endoplasmic reticulum (ER) and the Ca2+ pump of the ER (SERCA2b) contained low levels of InsP3 receptors. The immunofluorescent labelling of cultured hepatocytes with anti-InsP3R antibodies indicated that the receptor is concentrated in the perinuclear area and in some regions near the plasma membrane. The fraction enriched with InsP3R is also contaminated with markers of the ER and with SERCA2b. It was exposed to alkaline medium (pH 10.5) to extract endogenous actin and membrane-associated proteins before being subfractionated by Percoll-gradient centrifugation. The alkaline treatment allowed partial separation of the markers of the ER from the markers of the plasma membrane. The InsP3R was recovered in the heavy subfraction, which was also enriched with markers for the ER and with the SERCA2b and contained low levels of markers of the plasma membrane. These data indicate that the InsP3R is neither localized on the plasma membrane itself nor homogeneously distributed on the ER membrane. This supports the view that part of the receptor is localized on a specialized sub-region of the ER which interacts with the plasma membrane.

Plasmalemmal dihydropyridine receptors modify the function of subplasmalemmal inositol 1,4,5-trisphosphate receptors: a hypothesis

Experimental observations on rat glomerulosa cells inspired a model which postulates that plasmalemmal dihydropyridine receptors are in juxtaposition and interaction with inositol 1,4,5-trisphosphate receptors in subplasmalemmal calciosomes. Activation of dihydropyridine receptors promotes the Ca2+ releasing effect of inositol 1,4,5-trisphosphate. The most important observations compatible with the model are the following: (1) angiotensin II does not influence Ca2+ influx during the peak phase of Ca2+ signal; (2) dihydropyridine drugs modify the initial peak of the Ca2+ signal induced by angiotensin II; (3) inhibitors of the dihydropyridine receptor reduce the initial Ca2+ signal also in the presence of 5 mM Ni2+, an inhibitor of voltage dependent Ca2+ influx; and (4) changes in extracellular K+ concentration within the physiological range also modify the cytoplasmic Ca2+ response to angiotensin II.

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.

Inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate binding sites in smooth muscle

1. We have previously demonstrated that activation of M3 muscarinic receptors increases inositol 1,4,5-trisphosphate (InsP3) and inositol 1,3,4,5-tetrakisphosphate (InsP4) accumulation in colonic smooth muscle. 2. In the present study, we demonstrate the existence of InsP3 and InsP4 binding sites in colonic circular smooth muscle by use of radioligand binding methods. Both [3H]-InsP3 and [3H]-InsP4 bound rapidly and reversibly to a single class of saturable sites in detergent-solubilized colonic membranes with affinities of 5.04 +/- 1.03 nM and 3.41 +/- 0.78 nM, respectively. The density of [3H]-InsP3 binding sites was 335.3 +/- 19.3 fmol mg-1 protein which was approximately 2.5 fold greater than that of [3H]-InsP4 sites (127.3 +/- 9.1 fmol mg-1 protein). 3. The two high affinity inositol phosphate binding sites exhibited markedly different pH optima for binding of each radioligand. At pH 9.0, specific [3H]-InsP3 binding was maximal, whereas [3H]-InsP4 binding was only 10% that of [3H]-InsP3. Conversely, at pH 5.0, [3H]-InsP4 binding was maximal, while [3H]-InsP3 binding was reduced to 15% of its binding at pH 9.0. 4. InsP3 was about 20 fold less potent (KI = 50.7 +/- 8.3 nM) than InsP4 in competing for [3H]-InsP4 binding sites and could compete for only 60% of [3H]-InsP4 specific binding. InsP4 was also capable of high affinity competition with [3H]-InsP3 binding (KI = 103.5 +/- 1.5 nM), and could compete for 100% of [3H]-InsP3 specific binding. 5. [3H]-InsP3 binding in subcellular fractions separated by discontinuous sucrose density gradients followed NADPH-cytochrome c reductase activity, suggesting an intracellular localization for the majority of InsP3 receptors in this tissue, whereas [3H]-InsP4 binding appeared to be equally distributed between plasma membrane and intracellular membrane populations.6. These results suggest the existence of distinct and specific InsP3 and InsP4 binding sites which may represent the physiological receptors for these second messengers; differences in the subcellular distribution of these receptors may contribute to differences in their putative physiological roles.

Inositol 1,4,5-trisphosphate binding sites copurify with the putative Ca-storage protein calreticulin in rat liver

Rat liver was homogenized and subjected to differential centrifugation. When the low speed nuclear pellet was processed on a Percoll gradient, plasma membrane markers and Ins(1,4,5)P3 binding activity purified together. The high speed (microsomal) fraction was subfractionated by sucrose density gradient centrifugation, resulting in 10-fold enrichment of [32P]-Ins(1,4,5)P3 binding. In the sucrose density gradient fractions there was an inverse relationship between the enrichment of plasma membrane markers and Ins(1,4,5)P3 binding sites. Endoplasmic reticulum markers showed a moderate enrichment in the fractions displaying high Ins(1,4,5)P3 binding activity. Calcium binding proteins in the homogenate and in the microsomal subfractions were separated by SDS/PAGE. A 60 kD protein, stained metachromatically with Stains-All was identified as calreticulin with immunoblotting. Its enrichment pattern was similar to that of Ins(1,4,5)P3 binding sites, indicating the co-existence of these two elements of Ca(2+)-metabolism in the same intracellular compartment in the liver.

Intracellular Ca2+ pools in Jurkat T-lymphocytes

Jurkat T-lymphocytes comprise at least four intracellular Ca2+ pools. Pool I was agonist-sensitive and contained 23 +/- 8% (n = 18) of the total Ca(2+)-storage capacity, as shown in intact cells in the presence of EGTA. The time courses of the agonist-induced formation of Ins(1,4,5)P3 and of the Ca2+ release from pool I were nearly superimposable, indicating that the agonist-sensitive pool I is emptied by Ins(1,4,5)P3. Likewise, in permeabilized cells, the size of the Ins(1,4,5)P3-sensitive Ca2+ pool I was 27 +/- 11% (n = 14). Pool II contained 26 +/- 5% (n = 9) of intracellularly stored Ca2+ and was liberated by thapsigargin, an inhibitor of the endoplasmic-reticulum (ER) Ca(2+)-ATPase. Addition of thapsigargin before addition of agonist abolished the agonist-induced Ca2+ release in both intact and permeabilized cells, indicating that pool I is a subcompartment of the ER Ca2+ pool. The content of this ER Ca2+ pool (pools I and II) amounted to 51 +/- 15% (n = 9) in intact cells and 49 +/- 16% (n = 16) in permeabilized cells. Caffeine released Ca2+ even when the ER pool (pools I and II) was emptied by previous addition of thapsigargin, indicating the presence of a third pool independent of pools I and II. Pool III contained 23 +/- 6% (n = 8) in intact cells, but 41 +/- 8% (n = 5) in permeabilized cells. The remaining intracellularly stored Ca2+ was released by addition of the Ca2+ ionophore ionomycin. This fourth pool contained 27 +/- 8% (n = 9) in intact cells, but less than 10% in permeabilized cells. The size of pool III was increased when pools I and II were emptied before addition of caffeine, whereas the size of pool IV was decreased under such conditions. In conclusion, this first comprehensive description of intracellular Ca2+ pools in Jurkat T-lymphocytes demonstrates the presence of four different Ca2+ pools, provides estimates of their sizes and describes relationships between each other. Release of Ca2+ from pool I [Ins(1,4,5)P3-sensitive] has previously been shown to play a major role in T-cell activation, whereas the physiological role of pools II-IV remains to be established.

Intracellular Ca2+ stores of rat cerebellum: heterogeneity within and distinction from endoplasmic reticulum

Rat cerebellum microsomes were subfractionated on isopycnic linear sucrose (20-42%)-density gradients. The distribution of endoplasmic reticulum (ER) markers (RNA, signal-sequence receptor alpha, calnexin, calreticulin, the immunoglobulin-binding protein Bip) and markers of intracellular rapidly exchanging Ca2+ stores [Ca2+ channels sensitive to either Ins(1,4,5)P3 or ryanodine) was investigated biochemically and immunologically. The comparison indicates that: (a) vesicles bearing the InsP3 receptor were separated from those bearing the ryanodine receptor; (b) ER markers, i.e. Bip, calnexin, signal-sequence receptor alpha, RNA, did not sediment as either InsP3 or ryanodine receptors did; (c) calreticulin, an intralumenal low-affinity high-capacity Ca(2+)-binding protein, had a widespread distribution, similar to that of Bip and calnexin, and was present in Purkinje, granule, Golgi and stellate neurons, as indicated by immunofluorescent labelling of cerebellum cortex cryosections. The present results show that the ER is not a homogeneous entity, and that Ca2+ stores are heterogeneous insofar as InsP3 receptors and ryanodine receptors are segregated, either to discrete intracellular organelles or to specialized ER subcompartments.

Long-term sensitization training in Aplysia leads to an increase in calreticulin, a major presynaptic calcium-binding protein

Long-term memory for sensitization in Aplysia requires new protein and RNA synthesis. Here, we identify a late protein as calreticulin, the major Ca(2+)-binding protein of the lumen of the endoplasmic reticulum. An antiserum against Aplysia calreticulin reveals an enrichment of calreticulin immunoreactivity in presynaptic varicosities. Quantitative S1 nuclease analysis indicates that the steady-state level of calreticulin mRNA in Aplysia sensory neurons increases during the maintenance phase of long-term sensitization. The finding that this mRNA increases in expression late, some time after training, is consistent with the idea that long-term neuromodulatory changes underlying sensitization may depend on a cascade of gene expression in which the induction of early regulatory genes leads to the expression of late effector genes.