Initial events in FcϵRI signal transduction
1994; Elsevier BV; Volume: 94; Issue: 6 Linguagem: Inglês
10.1016/0091-6749(94)90323-9
ISSN1097-6825
AutoresAndrew M. Scharenberg, Jean‐Pierre Kinet,
Tópico(s)Asthma and respiratory diseases
ResumoIgE-mediated mast cell or basophil activation occurs through the interaction of multivalent antigen with antigen-specific IgE bound to high-affinity IgE receptors (FcϵRI). The result of cellular activation is the release of vasoactive mediators, such as histamine and serotonin, from cytoplasmic secretory granules, as well as the production and secretion of prostaglandins and various cytokines.1Gordon JR Burd PR Galli SJ Mast cells as a source of multifunctional cytokines.Immunol Today. 1990; 11: 458-464Abstract Full Text PDF PubMed Scopus (171) Google Scholar There is great interest in the intermediate events that propagate the signal from the cell surface to the secretory granules and the nucleus because of the potential therapeutic benefit of being able to interrupt them. FcϵRI is a tetrameric member of the antigen receptor family consisting of one α chain, one β chain, and two γ chain subunits.2Ravetch JV Kinet JP Fc receptors.Annu Rev Immunol. 1991; 9: 457-492Crossref PubMed Scopus (1275) Google Scholar The α chain contains one membrane-spanning region and the single IgE binding site, but apparently does not contribute to the signal transduction properties of the receptor, because its cytoplasmic tail is dispensable for normal receptor function.3Alber G Miller L Jelsema CL Varin-Blank N Metzger H Structure-function relationships in the mast cell high affinity receptor for IgE. Role of the cytoplasmic domains and of the beta subunit.J Biol Chem. 1991; 266: 22613-22620Abstract Full Text PDF PubMed Google Scholar The β chain has four membrane-spanning domains, with both its amino- and carboxy-terminals protruding into the cell cytoplasm. The γ chain has one membrane-spanning region and a long cytoplasmic tail but essentially no extracellular amino-terminal portion. The γ chain is closely related to the ζ and η chains of the T-cell receptor, and all three exist as disulfide bond–linked dimers within their holoreceptors.4Kinet JP The gamma-zeta dimers of Fc receptors as connectors to signal transduction.Curr Opin Immunol. 1992; 4: 43-48Crossref PubMed Scopus (50) Google Scholar Neither the β chain nor the γ chain dimer has intrinsic enzymatic activity, but alterations in the carboxy-terminal cytoplasmic tails of either can profoundly affect the ability of the holoreceptor to properly transduce signals.5Miller L Alber G Varin-Blank N Ludowyke R Metzger H Transmembrane signalling in P815 mastocytoma cells by transfected IgE receptors.J Biol Chem. 1988; 205: 7819-7826Google Scholar, 6Jouvin MH Adamczewski M Numerof R Letourneur O Valle A Kinet JP Differential control of the tyrosine kinases lyn and syk by the two signaling chains of the high affinity immunoglobulin E receptor.J Biol Chem. 1994; 269: 5918-5925Abstract Full Text PDF PubMed Google Scholar This article comprises a description of what is known about the molecular events that occur in the first few seconds after cross-linking of FcϵRI. It is presented in loosely chronologic order, from the earliest measurable events that occur after contact with multivalent antigen up to and including the elevation of intracellular calcium. For the purposes of this discussion, these events are broadly divided into cross-linking of FcϵRI, tyrosine phosphorylation, inositol phospholipid turnover, and calcium pulse generation. FcϵRI-mediated transmembrane signaling is initiated by the process of receptor cross-linking, but the molecular interactions by which this occurs are not yet well understood. What is known is that binding of two or more liganded FcϵRI (FcϵRI with bound IgE) to the same multivalent antigen molecule requires that the receptors move physically closer. Indeed, scanning electron micrographs demonstrate that in the first 60 seconds or so after exposure to dinitrophenol 24-bovine serum albumin, liganded receptors are redistributed from a random order into primarily chain-like aggregates.7Seagrave JC Pfeiffer JR Wofsy C Oliver JO Relationship of IgE receptor topography to secretion in RBL-2H3 mast cells.J Cell Phys. 1991; 148: 139-151Crossref PubMed Scopus (54) Google Scholar Within the first 1 to 2 minutes, addition of hapten effectively "deaggregates" receptors as assayed by the release of multivalent antigen from the cell surface. At later time points, addition of hapten is not able to deaggregate receptors or cause the release of multivalent antigen from the cell surface.8Seagrave JC Deanin GG Martin JC Davis BH Oliver JO DNP-phycobiliproteins, fluorescent antigens to study properties of antigen-IgE-receptor complexes on RBL-2H3 rat mast cells.Cytometry. 1987; 8: 287-295Crossref PubMed Scopus (40) Google Scholar Tyrosine phosphorylation of multiple cellular substrates is one of the first detectable events that occurs after receptor aggregation, occurring within 5 to 15 seconds after contact of sensitized cells with multivalent antigen.9Benhamou M Gutking JS Robbins KC Siraganian RP Tyrosine phosphorylation coupled to IgE receptor-mediated signal transduction and histamine release.Proc Natl Acad Sci U S A. 1990; 87: 5327-5332Crossref PubMed Scopus (199) Google Scholar, 10Paolini R Jouvin MH Kinet JP Phosphorylation and dephosphorylation of the high affinity receptor for immunoglobulin E immediately after receptor engagement and disengagement.Nature. 1991; 353: 855-858Crossref PubMed Scopus (220) Google Scholar Tyrosine kinases with plausible roles in these phosphorylation events include the src-related kinase lyn11Eiseman E Bolen JB Engagement of the high affinity IgE receptor activates src protein-related tyrosine kinases.Nature. 1992; 355: 78-80Crossref PubMed Scopus (415) Google Scholar and syk.12Taniguchi T Kobayashi T Kondo J et al.Molecular cloning of a porcine gene syk that encodes a 72-kda protein tyrosine kinase showing high susceptibility to proteolysis.J Biol Chem. 1991; 266: 15790-15796Abstract Full Text PDF PubMed Google Scholar, 13Benhamou M Ryba NJ Kihara N Nishikata H Siraganian RP Protein-tyrosine kinase p72syk in high affinity IgE receptor signaling. Identification as a component of pp72 and association with the receptor gamma chain after receptor aggregation.J Biol Chem. 1993; 268: 23318-23324Abstract Full Text PDF PubMed Google Scholar The most prominent cellular tyrosine phosphoproteins detectable after receptor cross-linking include the β and γ subunits of FcϵRI and the syk tyrosine kinase itself.10Paolini R Jouvin MH Kinet JP Phosphorylation and dephosphorylation of the high affinity receptor for immunoglobulin E immediately after receptor engagement and disengagement.Nature. 1991; 353: 855-858Crossref PubMed Scopus (220) Google Scholar, 14Hutchcroft JE Geahlen RL Deanin GG Oliver JO FcεRI-mediated tyrosine phosphorylation and activation of the 72 kd protein tyrosine kinase PTK72 in RBL-2H3 rat tumor mast cells.Proc Natl Acad Sci U S A. 1992; 89: 9107-9111Crossref PubMed Scopus (205) Google Scholar In addition, tyrosine phosphorylation of phospholipase C (PLC) γ1, PLCγ2, and Vav have been demonstrated,15Li W Deanin GG Margolis B Schlessinger J, Oliver JM. FcεRI-mediated tyrosine phosphorylation of multiple proteins including phospholipase C gamma 1 and the receptor beta/gamma2 complex in RBL-2H3 cells.Mol Cell Biol. 1992; 12: 3176-3182Crossref PubMed Google Scholar, 16Margolis B Hu P Katzav S et al.Tyrosine phosphorylation of the Vav proto-oncogene product: a potential transcriptional regulator with SH2 and SH3 domains.Nature. 1992; 356: 71-74Crossref PubMed Scopus (303) Google Scholar each with a time course similar to that of the receptor subunits. One functional role of tyrosine phosphorylation is to promote specific interactions between proteins with phosphorylated tyrosine residues and other proteins with regions called src homology 2 (SH2) domains. SH2 domains were first described on the basis of a strong homology between members of the src tyrosine kinase family within their amino-terminal region, but have now been found in many proteins, essentially all of which have some role in signal transduction. They have been shown to bind with high affinities to sequences that contain an upstream negative charge, followed within a few residues by a phosphorylated tyrosine.17Moran MF Koch CA Anderson D et al.Src homology region 2 domains direct protein-protein interactions in signal transduction.Proc Natl Acad Sci U S A. 1990; 87: 8622-8626Crossref PubMed Scopus (330) Google Scholar, 18Songyang Zhou Shoelson SE et al.SH2 domains recognize specific phosphopeptide sequences.Cell. 1993; 72: 767-778Abstract Full Text PDF PubMed Scopus (2378) Google Scholar The affinity of a particular SH2 domain for a particular phosphoryl-tyrosine–containing peptide sequence depends especially on the 3 to 4 residues directly adjacent to and downstream from the phosphorylated tyrosine: particular domains have preferences for particular downstream residues. The src-related tyrosine kinase lyn is similar to other src family kinases in having an N-terminal myristylation site and a negative regulatory tyrosine residue at its carboxy-terminus. The original description of lyn sparked interest because of its hematopoietic cell (particularly B-cell lineage) specificity. Homology to another src family member, lck, which has a demonstrated role in T-cell activation through its association with CD4 and CD8 molecules, was revealed later. The involvement of lyn in FcϵRI signaling was demonstrated initially by using immune complex kinase assays to show an increase in FcϵRI-associated lyn activity after cell activation.11Eiseman E Bolen JB Engagement of the high affinity IgE receptor activates src protein-related tyrosine kinases.Nature. 1992; 355: 78-80Crossref PubMed Scopus (415) Google Scholar More recently, it has been demonstrated that lyn is associated with the β chain of FcϵRI before cell activation and that at least one of its signaling roles is to phosphorylate FcϵRI β and γ subunits.6Jouvin MH Adamczewski M Numerof R Letourneur O Valle A Kinet JP Differential control of the tyrosine kinases lyn and syk by the two signaling chains of the high affinity immunoglobulin E receptor.J Biol Chem. 1994; 269: 5918-5925Abstract Full Text PDF PubMed Google Scholar, 19Oliver JM Burg DL Deanin GG McLaughlin JL Geahlen RL Role of the protein tyrosine kinase, syk, in signal transduction in mast cells: inhibition of FcεRI-mediated signalling by the syk selective inhibitor piceatannol.J Biol Chem. 1994; (in press)Google Scholar By analogy with the role of lck in T-cell receptor (TCR) signaling, it is likely that lyn phosphorylates the syk tyrosine kinase as well.20Iwashima M Irving BA Van Oers NS Chan AC Weiss A Sequential interactions of the TCR with two distinct cytoplasmic tyrosine kinase.Science. 1994; 263: 1136-1139Crossref PubMed Scopus (2) Google Scholar The syk tyrosine kinase was first described by Taniguchi et al.12Taniguchi T Kobayashi T Kondo J et al.Molecular cloning of a porcine gene syk that encodes a 72-kda protein tyrosine kinase showing high susceptibility to proteolysis.J Biol Chem. 1991; 266: 15790-15796Abstract Full Text PDF PubMed Google Scholar after its isolation and cloning from porcine splenic B cells. The presence of two SH2 domains in its N-terminal half placed it in a new family of tyrosine kinases. It was soon demonstrated that a 72 kd tyrosine kinase became associated with FcϵRI on receptor cross-linking14Hutchcroft JE Geahlen RL Deanin GG Oliver JO FcεRI-mediated tyrosine phosphorylation and activation of the 72 kd protein tyrosine kinase PTK72 in RBL-2H3 rat tumor mast cells.Proc Natl Acad Sci U S A. 1992; 89: 9107-9111Crossref PubMed Scopus (205) Google Scholar in rat basophilic leukemia (RBL)-2H3 cells and that this was, in fact, the rat homologue of syk.13Benhamou M Ryba NJ Kihara N Nishikata H Siraganian RP Protein-tyrosine kinase p72syk in high affinity IgE receptor signaling. Identification as a component of pp72 and association with the receptor gamma chain after receptor aggregation.J Biol Chem. 1993; 268: 23318-23324Abstract Full Text PDF PubMed Google Scholar Syk is also the major nonreceptor tyrosine phosphoprotein observed after FcϵRI cross-linking. No clear regulatory role has been assigned to the tyrosine phosphorylation, which syk incurs on FcϵRI cross-linking, although this does correlate with a tremendous increase in its in vitro kinase activity.14Hutchcroft JE Geahlen RL Deanin GG Oliver JO FcεRI-mediated tyrosine phosphorylation and activation of the 72 kd protein tyrosine kinase PTK72 in RBL-2H3 rat tumor mast cells.Proc Natl Acad Sci U S A. 1992; 89: 9107-9111Crossref PubMed Scopus (205) Google Scholar In addition, although no definitive substrates have yet been assigned to syk, it is likely that its activation has a critical role in the sequence of events leading to the calcium pulse, possibly through tyrosine phosphorylation and activation of PLCγ isozymes (see below).21Kolanus W Romero C Seed B T cell activation by clustered tyrosine kinases.Cell. 1993; 74: 171-183Abstract Full Text PDF PubMed Scopus (302) Google Scholar The identities of other possible syk substrates and an understanding of the role of syk in downstream signaling events are under investigation. Tyrosine phosphorylation of cross-linked FcϵRI subunits occurs with "antigen receptor activation motifs" (ARAMs), which are present in the carboxy-terminal cytoplasmic tail of the β chain and in the cytoplasmic tail of the γ chain. ARAMs were first noted by Reth22Reth M Antigen receptor tail clue.Nature. 1989; 338: 383-384Crossref PubMed Scopus (1160) Google Scholar as a striking correspondence between the spacings of five residues, including two precisely spaced tyrosine residues, within the cytoplasmic tails of a variety of antigen receptor structures. Their relevance was first experimentally demonstrated by Romeo and Seed,23Romeo C Seed B Sequence requirements for induction of cytolysis by the T cell antigen/Fc receptor zeta chain.Cell. 1992; 68: 1037-1046Abstract Full Text PDF Scopus (262) Google Scholar and independently by Irving and Weiss,24Irving BA Weiss A The cytoplasmic domain of the T cell receptor zeta chain is sufficient to couple to receptor-associated signal transduction pathways.Cell. 1991; 64: 891-901Abstract Full Text PDF PubMed Scopus (621) Google Scholar by showing that TCR ζ-chain–derived ARAMs were both necessary and sufficient to direct cross-linking–mediated calcium mobilization and cytolysis when expressed as part of chimeric transmembrane molecules in cultured T cells. For FcϵRI, all three ARAMs (one in the β carboxy-terminal tail and one in each γ tail) become tyrosine-phosphorylated within 5 seconds of cell activation and return to their baseline phosphorylation state after hapten is added, suggesting that this phosphorylation is related to cell activation. As noted above, several lines of evidence suggest that it is the lyn tyrosine kinase that is responsible for FcϵRI ARAM phosphorylation in RBL.6Jouvin MH Adamczewski M Numerof R Letourneur O Valle A Kinet JP Differential control of the tyrosine kinases lyn and syk by the two signaling chains of the high affinity immunoglobulin E receptor.J Biol Chem. 1994; 269: 5918-5925Abstract Full Text PDF PubMed Google Scholar, 19Oliver JM Burg DL Deanin GG McLaughlin JL Geahlen RL Role of the protein tyrosine kinase, syk, in signal transduction in mast cells: inhibition of FcεRI-mediated signalling by the syk selective inhibitor piceatannol.J Biol Chem. 1994; (in press)Google Scholar The relevance of ARAM tyrosine phosphorylation may be to direct binding of SH2-containing proteins to activated receptors, thereby resulting in the assembly of a "signaling complex" of receptor-associated proteins.4Kinet JP The gamma-zeta dimers of Fc receptors as connectors to signal transduction.Curr Opin Immunol. 1992; 4: 43-48Crossref PubMed Scopus (50) Google Scholar, 25Paolini R Numerof R Kinet JP Phosphorylation/dephosphorylation of high affinity IgE receptors: a mechanism for coupling/uncoupling a large signaling complex.Proc Natl Acad Sci U S A. 1992; 89: 10733-10737Crossref PubMed Scopus (46) Google Scholar, 26Weiss A Signal transduction by lymphocyte antigen receptors.Cell. 1994; 76: 263-274Abstract Full Text PDF PubMed Scopus (1948) Google Scholar Both PLCγ isozymes (PLCγ1 and PLCγ2) are tyrosine-phosphorylated after FcϵRI cross-linking,15Li W Deanin GG Margolis B Schlessinger J, Oliver JM. FcεRI-mediated tyrosine phosphorylation of multiple proteins including phospholipase C gamma 1 and the receptor beta/gamma2 complex in RBL-2H3 cells.Mol Cell Biol. 1992; 12: 3176-3182Crossref PubMed Google Scholar, 27Atkinson TP Kaliner MA Hohman RJ Phospholipase C-gamma 1 is translocated to the membrane of rat basophilic leukemia cells in response to aggregation of IgE receptors.J Immunol. 1992; 148: 2194-2200PubMed Google Scholar a finding of considerable interest when first described because of the role of PLC isozymes in generating phosphatidyl inositol turnover. Both of these isozymes have two SH2 domains and an SH3 domain, in addition to the requisite phospholipase domain. At this time, it is not known what role their SH2 domains play in terms of directing binding to other tyrosine-phosphorylated proteins, although some investigators have been able to show an association of PLCγ1 with src family kinases.28Nakanishi O Shibasaki F Hidaka M Homma Y Takenawa T Phospholipase C gamma 1 associates with viral and cellular src kinases.J Biol Chem. 1993; 268: 10754-10759Abstract Full Text PDF PubMed Google Scholar By analogy with growth factor receptor–mediated PLCγ1 tyrosine phosphorylation, FcϵRI-mediated tyrosine phosphorylation would be expected to result in activation of the enzymatic activity of PLCγ1 (see below for further discussion).29Kim HK Kim JW Zilberstein A et al.PDGF stimulation of inositol phospholipid hydrolysis requires PLC-gamma1 phosphorylation on tyrosine residues 783 and 1254.Cell. 1991; 65: 435-441Abstract Full Text PDF PubMed Scopus (444) Google Scholar Vav, a proto-oncogene involved in the ras-activating pathway, has recently been identified as an FcϵRI-induced phosphoprotein.16Margolis B Hu P Katzav S et al.Tyrosine phosphorylation of the Vav proto-oncogene product: a potential transcriptional regulator with SH2 and SH3 domains.Nature. 1992; 356: 71-74Crossref PubMed Scopus (303) Google Scholar, 30Gulbins E Coggeshall KM Baier G Katsav S Burn P Altman A Tyrosine kinase stimulated guanine nucleotide exchange activity of Vav in T cell activation.Science. 1993; 260: 822-825Crossref PubMed Scopus (229) Google Scholar Its role in FcϵRI-mediated signal transduction has not yet been completely defined, but by analogy with its role in TCR-mediated signal transduction, Vav likely regulates signaling via the ras/MAP kinase pathway. The prominent role that tyrosine kinases play in FcϵRI-mediated signaling, the return of β chain, γ chain, and syk tyrosine phosphorylation to baseline levels after the addition of hapten to activated cells, and a plethora of data derived from studies with pharmacologic tyrosine phosphatase inhibitors, all imply critical roles in FcϵRI signaling for tyrosine phosphatases.10Paolini R Jouvin MH Kinet JP Phosphorylation and dephosphorylation of the high affinity receptor for immunoglobulin E immediately after receptor engagement and disengagement.Nature. 1991; 353: 855-858Crossref PubMed Scopus (220) Google Scholar, 31Adamczewski M Paolini R Kinet JP Evidence for two distinct phosphorylation pathways activated by high affinity immunoglobulin E receptors.J Biol Chem. 1992; 267: 18126-18132Abstract Full Text PDF PubMed Google Scholar, 32Valle A, Kinet JP. Hydrogen peroxide desensitizes antigen-induced exocytosis in a mast cell line despite activation of FcεRI-associated tyrosine kinases (submitted for publication).Google Scholar It is likely that the CD45 transmembrane tyrosine phosphatase plays some role in maintaining mast cells in an activatable state. This is based on data that suggest that monoclonal antibodies, which cross-link CD45 on human basophils, inhibit histamine release, and on data implicating CD45 in signaling through the TCR or transfected FcϵRI in cultured T cells.33Adamczewski M, Numerof R, Kinet J-P. Regulation by CD-45 of the activation-induced phosphorylation of high affinity IgE receptor β and γ chains (submitted for publication).Google Scholar, 35Hook WA Berenstein EH Zinsser FU Fischler C Siraganian RP Monoclonal antibodies to the leukocyte common antigen inhibit IgE-mediated histamine release from human basophils.J Immunol. 1991; 147: 2670-2676PubMed Google Scholar The involvement of other tyrosine phosphatases, including many that contain SH2 domains, is an area of intense interest. The production of inositol phospholipids begins almost immediately after the onset of tyrosine phosphorylation in FcϵRI activated RBL cells. The PLCγ1 and PLCγ2 isozymes appear to be responsible for FcϵRI-mediated inositol phospholipid turnover, because they become tyrosine-phosphorylated, and hence probably activated, soon after cell activation. Concurrent with their tyrosine phosphorylation, PLCγ1 and PLCγ2 are translocated from the soluble to the particulate (membrane) fraction, allowing these enzymes to colocalize with their substrates.27Atkinson TP Kaliner MA Hohman RJ Phospholipase C-gamma 1 is translocated to the membrane of rat basophilic leukemia cells in response to aggregation of IgE receptors.J Immunol. 1992; 148: 2194-2200PubMed Google Scholar Whether or how the tyrosine phosphorylation is related to the translocation is unknown. The time course of FcϵRI-stimulated inositol phospholipid turnover in RBL closely parallels that of the calcium pulse and has been described in detail.36Cunha-Melo RJ Dean NM Moyer JD Maeyama K Beaven MA The kinetics of phosphoinositide hydrolysis in rat basophilic leukemia cells varies with the type of IgE receptor cross linking agent used.J Biol Chem. 1987; 262: 11455-11463Abstract Full Text PDF PubMed Google Scholar In addition to the primary rise in 1,4,5 inositol trisphosphate produced by membrane phospholipid hydrolysis by PLCγ1 and PLCγ2, subsequent increases in the 3′ phosphate derivatives occur as a result of the activity of phosphatidyl inositol 3′ kinase. This is then followed by gradual accumulation of bis- and monophosphate derivatives by sequential dephosphorylation. Although the role of 1,4,5 inositol trisphosphate in causing release of calcium from intracellular stores has been well established, the metabolic or signaling roles of other inositol phosphates, particularly 3' derivatives, are unknown. The initial calcium rise occurs through inositol 1,4,5-trisphophate–mediated release of calcium from intracellular stores,37Prentki M Biden TJ Janjic D Irvine RF Berridge MJ Wollheim CB Rapid mobilization of Ca2+ from rat insulinoma microsomes by inositol-1,4,5-trisphosphate.Nature. 1984; 309: 562-564Crossref PubMed Scopus (241) Google Scholar whereas the majority of the later sustained calcium rise is attributable to influx of extracellular calcium.38Beaven MA Roger J Moore JP Hesketh TR Smith GA Metcalfe J The mechanism of the calcium signal and correlation with histamine release in 2H3 cells.J Biol Chem. 1984; 259: 7129-7136Abstract Full Text PDF PubMed Google Scholar A low molecular weight calcium secretion factor, which is produced on depletion of intracellular calcium stores, has recently been described, but its role in the generation of the extracellular calcium influx generated by FcϵRI cross-linking remains to be established.39Randriamampita C Tsien RY Emptying of intracellular Ca2+ stores releases a novel small messenger that stimulates Ca2+ influx.Nature. 1993; 264: 809-814Crossref Scopus (786) Google Scholar The consensal view of the time course of the rise in intracellular calcium has evolved over the past few years as technologic advances have allowed more sophisticated measurements. Initial descriptions focused on the population calcium signal measured in a bulk spectrofluorimeter, which appears as a monotonic rise from a baseline concentration around 80 nm to a peak around 400 nm by 30 seconds after FcϵRI cross-linking, followed by a slow decrease to a plateau over the next 10 minutes, and finally a gradual decay back to baseline.38Beaven MA Roger J Moore JP Hesketh TR Smith GA Metcalfe J The mechanism of the calcium signal and correlation with histamine release in 2H3 cells.J Biol Chem. 1984; 259: 7129-7136Abstract Full Text PDF PubMed Google Scholar More recent descriptions have used single cell measurements and single cell imaging to document a remarkable cell-to-cell heterogeneity in the morphology of the calcium flux.40Millard PJ Gross D Webb W Fewtrell C Imaging asynchronous changes in intracellular calcium in individual stimulated tumor mast cells.Proc Natl Acad Sci U S A. 1988; 85: 1854-1858Crossref PubMed Scopus (70) Google Scholar, 41Millard PJ Ryan TA Webb WA Fewtrell C Immunoglobulin E receptor cross linking induces oscillations in intracellular free calcium in individual tumor mast cells.J Biol Chem. 1989; 264: 19730-19739Abstract Full Text PDF PubMed Google Scholar There is typically a delay from contact of appropriately sensitized RBL cells with multivalent antigen to onset of the rise in calcium. The length of this delay varies remarkably in an otherwise apparently homogenous preparation of cells, from nearly zero to more than a minute, depending to some extent on the strength of the cross-linking stimulus. Once the intracellular calcium level starts to rise, it does so extremely rapidly and subsequently decays to a plateau, during which repetitive spikes or oscillations occur, up to the magnitude of the original rise. In some systems similar oscillations appear to correlate with changes in overall intracellular inositol levels,42Harootunian AT Kao JPY Paranjape S Tsien RY Generation of calcium oscillations in fibroblasts by positive feedback between calcium and IP3.Science. 1991; 251: 75-78Crossref PubMed Scopus (251) Google Scholar but their genesis in mast cells and relevance to FcϵRI-mediated signaling remain a mystery. Our current model of the events that occur in the first few seconds after FcϵRI cross-linking focuses primarily on the role of tyrosine phosphorylation and its ability to direct specific protein-protein interactions through SH2 domains. Contact of a mast cell bearing appropriately liganded FcϵRI with multivalent antigen results in the approximation of receptors initially into chains. The proximity of receptors in these chains allows the phosphorylation of their ARAMs by the lyn tyrosine kinase. ARAM phosphorylation results in binding of syk specifically to cross-linked receptors and its probable subsequent phosphorylation and activation by lyn. Activated syk then phosphorylates and activates PLCγ1 and PLCγ2, resulting in their activation and translocation to the membrane. The presence of active PLCγ1 and PLCγ2 on the cell membrane results in hydrolysis of membrane phosphatidyl inositol and the production of 1,4,5 inositol triphosphate. Inositol 1,4,5 triphosphate diffuses to the sarcoplasmic reticulum and causes the release of sequestered calcium. This model represents a snapshot of the current body of knowledge about FcϵRI-mediated signal transduction. Given the rapid pace of research in this field, it will likely be incorrect or incomplete in at least some respects by the time of publication. Ideally, the information presented here should provide a framework on which to build for those interested in learning more about FcϵRI in particular and multisubunit antigen receptors in general. We thank Dr. Robert Numerof for critical reading of the manuscript.
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