A collagen-related peptide regulates phospholipase Cgamma2 via phosphatidylinositol 3-kinase in human platelets

The collagen receptor glycoprotein VI (GPVI) induces platelet activation through a similar pathway to that used by immune receptors. In the present study we have investigated the role of phosphatidylinositol 3-kinase (PI 3-kinase) in GPVI signalling. Our results show that collagen-related peptide {CRP: [GCP*(GPP*)(10)GCP*G](n); P*=hydroxyproline}, which is selective to GPVI, induces formation of phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P(3)] and phosphatidylinositol 3,4-bisphosphate [PI(3, 4)P(2)] in platelets. The increase in the two 3-phosphorylated lipids is inhibited completely by wortmannin and by LY294002, two structurally unrelated inhibitors of PI 3-kinase. The formation of inositol phosphates and phosphatidic acid (PA), two markers of phospholipase C (PLC) activation, by CRP are inhibited by between 50 and 85% in the presence of wortmannin and LY294002. This is associated with inhibition of elevation of intracellular Ca(2+) ([Ca(2+)](i)) and aggregation. Wortmannin and LY294002 also partially inhibit elevation of Ca(2+) by CRP in murine megakaryocytes. Microinjection of the pleckstrin-homology PH domain of Bruton's tyrosine kinase, which binds selectively to PI(3,4, 5)P(3), but not the R28C (Arg(28)-->Cys) mutant which binds to PI(3, 4,5)P(3) with low affinity, also inhibits elevation of [Ca(2+)](i) in megakaryocytes, suggesting that it is this lipid species which mediates the action of the PI 3-kinase pathway. Studies in platelets show that the action of wortmannin and LY294002 is not mediated through an alteration in tyrosine phosphorylation of PLCgamma2. These results demonstrate that PI 3-kinase is required for full activation of PLCgamma2 by GPVI in platelets and megakaryocytes.

Evidence that phospholipase C-gamma2 interacts with SLP-76, Syk, Lyn, LAT and the Fc receptor gamma-chain after stimulation of the collagen receptor glycoprotein VI in human platelets

Platelet activation by collagen is mediated by the sequential tyrosine phosphorylation of the Fc receptor gamma-chain (FcR gamma-chain), which is part of the collagen receptor glycoprotein VI, the tyrosine kinase Syk and phospholipase C-gamma2 (PLC-gamma2). In this study tyrosine-phosphorylated proteins that associate with PLC-gamma2 after stimulation by a collagen-related peptide (CRP) were characterized using glutathione S-transferase fusion proteins of PLC-gamma2 Src homology (SH) domains and by immunoprecipitation of endogenous PLC-gamma2. The majority of the tyrosine-phosphorylated proteins that associate with PLC-gamma2 bind to its C-terminal SH2 domain. These were found to include PLC-gamma2, Syk, SH2-domain-containing leucocyte protein of 76 kDa (SLP-76), Lyn, linker for activation of T cells (LAT) and the FcR gamma-chain. Direct association was detected between PLC-gamma2 and SLP-76, and between PLC-gamma2 and LAT upon CRP stimulation of platelets by far-Western blotting. FcR gamma-chain and Lyn were found to co-immunoprecipitate with PLC-gamma2 as well as with unidentified 110-kDa and 75-kDa phosphoproteins. The absence of an in vivo association between Syk and PLC-gamma2 in platelets is in contrast with that for PLC-gamma1 and Syk in B cells. The in vivo function of PLC-gamma2 SH2 domains was examined through measurement of Ca2+ increases in mouse megakaryocytes that had been microinjected with recombinant proteins. This revealed that the C-terminal SH2 domain is involved in the regulation of PLC-gamma2. These data indicate that the C-terminal SH2 domain of PLC-gamma2 is important for PLC-gamma2 regulation through possible interactions with SLP-76, Syk, Lyn, LAT and the FcR gamma-chain.

Characterization of the major superoxide dismutase of Staphylococcus aureus and its role in starvation survival, stress resistance, and pathogenicity

A Staphylococcus aureus mutant (SPW1) which is unable to survive long-term starvation was shown to have a transposon insertion within a gene homologous to the sodA family of manganese-dependent superoxide dismutases (SOD). Whole-cell lysates of the parental 8325-4 strain demonstrated three zones of SOD activity by nondenaturing gel electrophoresis. The activities of two of these zones were dependent on manganese for activity and were absent in SPW1. The levels of SOD activity and sodA expression were growth-phase dependent, occurring most during postexponential phase. This response was also dependent on the level of aeration of the culture, with highest activity and expression occurring only under high aeration. Expression of sodA and, consequently, SOD activity could be induced by methyl viologen but only during the transition from exponential- to postexponential-phase growth. SPW1 was less able to survive amino acid limitation and acid stress but showed no alteration in pathogenicity in a mouse abscess model of infection compared to the parental strain 8325-4.

Collagen mediates changes in intracellular calcium in primary mouse megakaryocytes through syk-dependent and -independent pathways

We have characterized changes in [Ca2+]i in primary mouse megakaryocytes in response to fibrillar collagen and in response to cross-linking of the collagen receptor, the integrin alpha2beta1. The response to collagen was markedly different from that seen to a triple helical collagen-related peptide (CRP), which signals via the tyrosine kinases p59(fyn) and p72(syk). This peptide binds to the collagen receptor glycoprotein VI (GPVI), but not to the integrin alpha2beta1. Collagen elicited a sustained increase in [Ca2+]i composed primarily of influx of extracellular Ca2+ with some Ca2+ release from internal stores. In contrast to CRP, this response was only partially (approximately 30%) inhibited by the src-family kinase inhibitor PP1 (10 micromol/L) or by microinjection of the tandem SH2 domains of p72(syk). Collagen also caused an increase in [Ca2+]i in megakaryocytes deficient in either p59(fyn) or p72(syk), although the response was reduced by approximately 40% in both cases: Cross-linking of the alpha2 integrin increased [Ca2+]i in these cells exclusively via Ca2+ influx. This response was reduced by approximately 50% after PP1 pretreatment, but was significantly increased in fyn-deficient megakaryocytes. Collagen therefore increases [Ca2+]i in mouse megakaryocytes via multiple receptors, including GPVI, which causes Ca2+ mobilization, and alpha2beta1, which stimulates a substantial influx of extracellular Ca2+.

Monomeric (glycine-proline-hydroxyproline)10 repeat sequence is a partial agonist of the platelet collagen receptor glycoprotein VI

We have previously reported that a triple-helical, collagen-related peptide (CRP; also known as CRP-XL) containing a glycine-proline-hydroxyproline (GPP*) repeat motif and cross-linked through cysteine residues at its N-terminus and C-terminus is a powerful stimulus of platelet aggregation and secretion through the surface receptor glycoprotein VI (GPVI). The activation of platelets is associated with tyrosine phosphorylation of the tyrosine kinase Syk and phospholipase C gamma2 (PLCgamma2). We now report that the non-cross-linked backbone of CRP, monomeric CRP (mCRP), stimulates the tyrosine phosphorylation of Syk and PLCgamma2 in platelets and induces the weak secretion of [3H]5-hydroxytryptamine ([3H]5-HT) and aggregation. The action of mCRP does not seem to be due to spontaneous cross-linking, because alkylation of the cysteine residues leads to an increase in activity. The tripeptide backbone of CRP, GPP*10 (in which P* represents hydroxyproline) also stimulates platelet shape change and the weak tyrosine phosphorylation of Syk and PLCgamma2, but is unable to induce aggregation or secretion. The monomeric peptides partly inhibit the release of [3H]5-HT by CRP, suggesting that they are partial agonists of the collagen receptor GPVI. These results demonstrate that GPP* present as a repeat motif is sufficient to activate the platelet collagen receptor GPVI but that the cross-linking of monomers brings about an increase in activity.

Tyrosine phosphorylation of SLP-76 is downstream of Syk following stimulation of the collagen receptor in platelets

Collagen-related peptide (CRP), a collagen homologue, induces platelet activation through a tyrosine kinase-dependent pathway, leading to sequential tyrosine phosphorylation of Fc receptor (FcR) gamma-chain, Syk, and phospholipase C-gamma2. Here we report that CRP and the platelet low affinity immune receptor FcgammaRIIA stimulate tyrosine phosphorylation of the T cell adapter SLP-76, whereas the G protein-coupled receptor agonist thrombin induces only minor tyrosine phosphorylation. This suggests that SLP-76 has a specific role downstream of receptors that signal via an immunoreceptor tyrosine-based activation motif. Immunoprecipitation studies demonstrate association of SLP-76 with SLAP-130, Vav, Fyn, Lyn, and the FcR gamma-chain in CRP-stimulated platelets. Several of these proteins, including SLP-76, undergo tyrosine phosphorylation in in vitro kinase assays performed on SLP-76 immunoprecipitates. Tyrosine phosphorylation of all of these proteins in the in vitro kinase assay was abrogated by the Src family kinase inhibitor PP1, suggesting that it is mediated by either Fyn or Lyn. The physiological significance of this is uncertain, however, since tyrosine phosphorylation of SLP-76 in vivo is not altered in either Fyn- or Lyn-deficient platelets. CRP stimulation of Syk-deficient platelets demonstrated that in vivo tyrosine phosphorylation of SLP-76 is downstream of Syk. The absence of Syk in the SLP-76 immunoprecipitates raises the possibility that another protein is responsible for bringing SLP-76 to Syk. Candidates for this include those proteins that co-immunoprecipitate with SLP-76, including the FcR gamma-chain. Tyrosine phosphorylation of PLC-gamma2 and Ca2+ mobilization is markedly attenuated in SLP-76-deficient platelets following CRP stimulation, suggesting that the adapter plays a critical role in the regulation of the phospholipase. The increase in tyrosine phosphorylation of SLAP-130 in response to CRP is also inhibited in SLP-76-deficient platelets, placing it downstream of SLP-76. This work identifies SLP-76 as an important adapter molecule that is regulated by Syk and lies upstream of SLAP-130 and PLC-gamma2 in CRP-stimulated platelets.

Evidence for the involvement of p59fyn and p53/56lyn in collagen receptor signalling in human platelets

The binding of collagen to platelet glycoprotein VI (GPVI) leads to the subsequent activation of phospholipase Cgamma2 through a pathway that is dependent on the Fc receptor gamma (FcR gamma) chain and the tyrosine kinase p72syk. We have investigated the role of platelet Src-family kinases in this signalling pathway. The selective Src-family kinase inhibitor PP1 prevented collagen-stimulated increases in whole-cell tyrosine phosphorylation and tyrosine phosphorylation of the FcR gamma chain and p72syk. A similar set of observations was made for a collagen-related peptide (CRP), which binds to GPVI but not to the integrin alpha2beta1 (GPIa/IIa). These effects were seen at a concentration of PP1 that inhibited platelet aggregation, dense granule release and Ca2+ mobilization induced by CRP, but not aggregation and Ca2+ mobilization mediated by the G-protein-coupled receptor agonist thrombin. After stimulation by CRP or collagen, the Src-family kinases p59fyn and p53/56lyn became associated with several tyrosine-phosphorylated proteins including the FcR gamma chain. This was not true of the other platelet Src-family kinases. The association between the FcR gamma chain and p59fyn was also seen under basal conditions, and was stable only in the weak detergent Brij96 but not in Nonidet P40, suggesting a non-SH2-dependent interaction. These results provide strong evidence for the involvement of p59fyn and p53/56lyn in signalling via GPVI, with p59fyn possibly acting upstream of FcR gamma chain phosphorylation.

Fetal hemorrhage and platelet dysfunction in SLP-76-deficient mice

The adapter protein SLP-76 is expressed in T lymphocytes and hematopoietic cells of the myeloid lineage, and is known to be a substrate of the protein tyrosine kinases that are activated after ligation of the T-cell antigen receptor. Transient overexpression of SLP-76 in a T-cell line potentiates transcriptional activation after T-cell receptor ligation, while loss of SLP-76 expression abrogates several T-cell receptor-dependent signaling pathways. Mutant mice that lack SLP-76 manifest a severe block at an early stage of thymocyte development, implicating SLP-76 in signaling events that promote thymocyte maturation. While it is clear that SLP-76 plays a key role in development and activation of T lymphocytes, relatively little is understood regarding its role in transducing signals initiated after receptor ligation in other hematopoietic cell types. In this report, we describe fetal hemorrhage and perinatal mortality in SLP-76-deficient mice. Although megakaryocyte and platelet development proceeds normally in the absence of SLP-76, collagen-induced platelet aggregation and granule release is markedly impaired. Furthermore, treatment of SLP-76-deficient platelets with collagen fails to elicit tyrosine phosphorylation of phospholipase C-gamma2 (PLC-gamma2), suggesting that SLP-76 functions upstream of PLC-gamma2 activation. These data provide one potential mechanism for the fetal hemorrhage observed in SLP-76-deficient mice and reveal that SLP-76 expression is required for optimal receptor-mediated signal transduction in platelets as well as T lymphocytes.

CtaA of Staphylococcus aureus is required for starvation survival, recovery, and cytochrome biosynthesis

A Staphylococcus aureus mutant (SPW3) apparently unable to survive long-term starvation was shown to have a transposon insertion within a gene homologous to ctaA of Bacillus subtilis which encodes a heme A synthase. Analysis of the cytochrome profiles of SPW3 revealed the absence of heme A-containing cytochromes compared to the parental 8325-4 strain. SPW3 demonstrated a 100-fold reduction in the ability to survive starvation induced by glucose limitation, under aerated conditions, compared to 8325-4. Analysis of starved cultures revealed that greater than 90% of the cells which demonstrated metabolism (as shown by rhodamine 123 accumulation) were unable to recover and form colonies on agar. Analysis of the lag phase and initial growth kinetics of those cells which could recover also showed a defect. This recovery defect could be partially alleviated by the inclusion of catalase in the recovery medium, indicating the probable involvement of oxidative stress. SPW3 also exhibited reduced colony size similar to that of a small-colony variant, increased resistance to aminoglycoside antibiotics, and reduced hemolysin and toxic shock syndrome toxin 1 production, but no alteration in the ability to form lesions in a subcutaneous mouse infection model.

The p85 subunit of phosphatidylinositol 3-kinase associates with the Fc receptor gamma-chain and linker for activitor of T cells (LAT) in platelets stimulated by collagen and convulxin

There is extensive evidence to show that phosphatidylinositol 3-kinase plays an important role in signaling by the immune family of receptors, which has recently been extended to include the platelet collagen receptor, glycoprotein VI. In this report we present two potential mechanisms for the regulation of this enzyme on stimulation of platelets by collagen. We show that on stimulation with collagen, the regulatory subunit of phosphatidylinositol 3-kinase associates with the tyrosine-phosphorylated form of the adapter protein linker for activator of T Cells (LAT) and the tyrosine-phosphorylated immunoreceptor tyrosine-based activation motif of the Fc receptor gamma-chain (a component of the collagen receptor complex that includes glycoprotein VI). The associations of the Fc receptor gamma-chain and LAT with p85 are rapid and supported by the Src-homology 2 domains of the regulatory subunit. We did not obtain evidence to support previous observations that the regulatory subunit of phosphatidylinositol 3-kinase is regulated through association with the tyrosine kinase Syk. The present results provide a molecular basis for the regulation of the p85/110 form of phosphatidylinositol 3-kinase by GPVI, the collagen receptor that underlies activation.

Direct inhibition of cyclooxygenase-1 and -2 by the kinase inhibitors SB 203580 and PD 98059. SB 203580 also inhibits thromboxane synthase

The kinase inhibitors SB 203580 and PD 98059 have been reported to be specific inhibitors of the 38- and 42/44-kDa mitogen-activated protein kinase (MAPK) pathways, respectively. In this study, the two inhibitors were found to decrease platelet aggregation induced by low concentrations of arachidonic acid, suggesting that they also interfere with the metabolism of arachidonic acid to thromboxane A2. In support of this, SB 203580 and PD 98059 inhibited the conversion of exogenous [3H]arachidonic acid to [3H]thromboxane in intact platelets. Measurement of platelet cyclooxygenase-1 activity following immunoprecipitation revealed that SB 203580 and PD 98059 are direct inhibitors of this enzyme. Both compounds were shown to inhibit purified cyclooxygenase-1 and -2 by a reversible mechanism. In addition, SB 203580 (but not PD 98059) inhibited platelet aggregation induced by prostaglandin H2 and the conversion of prostaglandin H2 to thromboxane A2 in intact platelets. SB 203580 also inhibited this pathway in platelet microsome preparations, suggesting a direct inhibitory effect on thromboxane synthase. These results demonstrate that direct effects of the two kinase inhibitors on active arachidonic acid metabolites have to be excluded before using these compounds for the investigation of MAPKs in signal transduction pathways. This is of particular relevance to studies on the regulation of cytosolic phospholipase A2 as these two MAPKs are capable of phosphorylating cytosolic phospholipase A2, thereby increasing its intrinsic activity.

A role for Bruton's tyrosine kinase (Btk) in platelet activation by collagen

Bruton's tyrosine kinase (Btk) is essential for normal B-cell receptor signalling. The lack of expression of functional Btk in humans leads to the B-cell deficiency X-linked agammaglobulinaemia (XLA). We report here that Btk is also important for signalling via the collagen receptor glycoprotein VI (GPVI) in platelets. GPVI is coupled to the Fc receptor gamma chain (FcRgamma). The FcRgamma-chain contains a consensus sequence known as the immune-receptor tyrosine-based activation motif (ITAM). Tyrosine phosphorylation of the ITAM upon GPVI stimulation is the initial step in the regulation of phospholipase C gamma2 (PLCgamma2) isoforms via the tyrosine kinase p72(Syk) (Syk) in platelets. Here we show that collagen and a collagen-related peptide (CRP), which binds to GPVI but does not bind to the integrin alpha2beta1, induced Btk tyrosine phosphorylation in platelets. Aggregation, dense granule secretion and calcium mobilisation were significantly diminished but not completely abolished in platelets from XLA patients in response to collagen and CRP. These effects were associated with a reduction in tyrosine phosphorylation of PLCgamma2. In contrast, aggregation and secretion stimulated by thrombin in Btk-deficient platelets were not significantly altered. Our results demonstrate that Btk is important for collagen signalling via GPVI, but is not essential for thrombin-mediated platelet activation.

RECAP--retrosynthetic combinatorial analysis procedure: a powerful new technique for identifying privileged molecular fragments with useful applications in combinatorial chemistry

The use of combinatorial chemistry for the generation of new lead molecules is now a well established strategy in the drug discovery process. Central to the use of combinatorial chemistry is the design and availability of high quality building blocks which are likely to afford hits from the libraries that they generate. Herein we describe "RECAP" (Retrosynthetic Combinatorial Analysis Procedure), a new computational technique designed to address this building block issue. RECAP electronically fragments molecules based on chemical knowledge. When applied to databases of biologically active molecules this allows the identification of building block fragments rich in biologically recognized elements and privileged motifs and structures. This allows the design of building blocks and the synthesis of libraries rich in biological motifs. Application of RECAP to the Derwent World Drug Index (WDI) and the molecular fragments/ building blocks that this generates are discussed. We also describe a WDI fragment knowledge base which we have built which stores the drug motifs and mention its potential application in structure based drug design programs.

Characterization of the starvation-survival response of Staphylococcus aureus

The starvation-survival response of Staphylococcus aureus as a result of glucose, amino acid, phosphate, or multiple-nutrient limitation was investigated. Glucose and multiple-nutrient limitation resulted in the loss of viability of about 99 to 99.9% of the population within 2 days. The remaining surviving cells developed increased survival potential, remaining viable for months. Amino acid or phosphate limitation did not lead to the development of a stable starvation-survival state, and cells became nonculturable within 7 days. For multiple-nutrient limitation, the development of the starvation-survival state was cell density dependent. Starvation survival was associated with a decrease in cell size and increase in resistance to acid shock and oxidative stress. There was no evidence for the formation of a viable but nonculturable state during starvation as demonstrated by flow cytometry. Long-term survival of cells was dependent on cell wall and protein biosynthesis. Analysis of [35S]methionine incorporation and labelled proteins demonstrated that differential protein synthesis occurred deep into starvation.

Fire in the operating room during tracheostomy

BACKGROUND

Fire in the modern operating room is still a constant danger today despite the usual absence of the historically explosive anesthetic gases, cyclopropane, and ether. During a tracheostomy, three conditions are present that will support an explosive or combustive event: heat, fuel and oxygen.

METHODS

We report three routine tracheostomy cases during which a fire was ignited. One patient had a minor burn during the cauterizing of a bleeding vessel. There was a visible flame in all cases and, in one case, the cloth drapes ignited. There were no serious airway injuries to any patients and all had a complete and uneventful recovery. We duplicated the same conditions in our laboratory.

RESULTS

We discuss each case and give effective techniques to prevent ignition during surgery in the future. For our study, we reproduced flames in a cadaver trachea using halothane and the electrocautery in an oxygen-rich environment. We describe a protocol that has effectively prevented tracheostomy fires in our institution and may decrease the risk during other procedures as well. Also, we reviewed the literature to provide insight into the magnitude of the problem.

CONCLUSIONS

High-oxygen concentration, the presence of fuels such as suture and tissue, and an igniting spark from the electrocautery device produce the proper conditions for a fire during tracheostomy. Taking the proper precautions can minimize if not eliminate this risk.

Identification of the phosphorylation sites of cytosolic phospholipase A2 in agonist-stimulated human platelets and HeLa cells

The present study identifies the phosphorylation sites of the 85-kDa cytosolic phospholipase A2 (cPLA2) in human platelets and HeLa cells. Tryptic digests of 32P-phosphorylated and -immunoprecipitated cPLA2 were analyzed by microbore high performance liquid chromatography and two-dimensional phosphopeptide mapping against synthetic phosphopeptide standards. Thrombin stimulated significant phosphorylation of platelet cPLA2 at two sites, Ser-505 and Ser-727. Exclusive phosphorylation on these two sites was also seen in collagen-stimulated platelets and HeLa cells stimulated with interferon-alpha or arsenite; no tyrosine phosphorylation was detected. The inhibitor of the 38-kDa stress-activated protein kinase (p38(mapk)), SB 203580, reduced phosphorylation of both Ser-505 and Ser-727 by 50 and 60%, respectively, in thrombin-stimulated platelets. An additional p38(mapk) inhibitor SB 202190 also partially (60%) inhibited the phosphorylation of cPLA2 in arsenite-stimulated HeLa cells. These studies extend the previous work on the identification of multiple phosphorylation sites on cPLA2 expressed in a baculovirus/insect cell system to cPLA2 in mammalian cells stimulated with physiological agonists. They also underscore the necessity of high resolution phosphopeptide mapping combined with microbore high performance liquid chromatography for quantification of phosphorylation levels, which has lead to the conclusion that Ser-505 and Ser-727 are common phosphorylation sites on cPLA2 in different mammalian cells stimulated with multiple agonists.

Syk and Fyn are required by mouse megakaryocytes for the rise in intracellular calcium induced by a collagen-related peptide

Stimulation of platelets by collagen leads to activation of a tyrosine kinase cascade resulting in secretion and aggregation. We have recently shown that this pathway involves rapid tyrosine phosphorylation of an Fc receptor gamma chain, which contains an immunoreceptor tyrosine-based activation motif (ITAM), enabling interaction with the tandem SH2 domains of the tyrosine kinase Syk. Activation of Syk lies upstream of tyrosine phosphorylation of phospholipase Cgamma2. In the present study we sought to test directly the role of the ITAM/Syk interaction and the role of the Src-related kinases in collagen receptor signaling using mouse megakaryocytes. We demonstrate that the calcium-mobilizing action of a collagen-related peptide (CRP) is kinase-dependent, inhibited by the microinjection of the tandem SH2 domains of Syk and abolished in Syk-deficient mice. Furthermore, the CRP response is abolished by the Src family kinase inhibitor PP1 and inhibited in Fyn-deficient mice. In contrast, the calcium response to the G-protein-linked receptor agonist thrombin is not significantly altered under these conditions. These results provide direct evidence of the functional importance of Fyn and Syk in collagen receptor signaling and support the megakaryocyte as a model for the study of proteins involved in this pathway.

Glycoprotein VI is the collagen receptor in platelets which underlies tyrosine phosphorylation of the Fc receptor gamma-chain

We have recently shown that collagen activates platelets through a pathway dependent on the Fc receptor gamma-chain and the tyrosine kinase Syk. We report here that the Fc receptor gamma-chain and the candidate collagen receptor glycoprotein VI (GPVI) co-associate. Furthermore, cross-linking GPVI stimulates a similar pattern of tyrosine phosphorylation to that stimulated by collagen, including tyrosine phosphorylation of Fc receptor gamma-chain. These results support a model where GPVI couples collagen-stimulation of platelets to phosphorylation of the Fc receptor gamma-chain leading to activation of Syk and phospholipase Cgamma2.

The Fc receptor gamma-chain and the tyrosine kinase Syk are essential for activation of mouse platelets by collagen

Activation of mouse platelets by collagen is associated with tyrosine phosphorylation of multiple proteins including the Fc receptor gamma-chain, the tyrosine kinase Syk and phospholipase Cgamma2, suggesting that collagen signals in a manner similar to that of immune receptors. This hypothesis has been tested using platelets from mice lacking the Fc receptor gamma-chain or Syk. Tyrosine phosphorylation of Syk and phospholipase Cgamma2 by collagen stimulation is absent in mice lacking the Fc receptor gamma-chain. Tyrosine phosphorylation of phospholipase Cgamma2 by collagen stimulation is also absent in mice platelets which lack Syk, although phosphorylation of the Fc receptor gamma-chain is maintained. In contrast, tyrosine phosphorylation of platelet proteins by the G protein-coupled receptor agonist thrombin is maintained in mouse platelets deficient in Fc receptor gamma-chain or Syk. The absence of Fc receptor gamma-chain or Syk is accompanied by a loss of secretion and aggregation responses in collagen- but not thrombin-stimulated platelets. These observations provide the first direct evidence of an essential role for the immunoreceptor tyrosine-based activation motif (ITAM) in signalling by a non-immune receptor stimulus.

Phosphorylation and activation of cytosolic phospholipase A2 by 38-kDa mitogen-activated protein kinase in collagen-stimulated human platelets

Phosphorylation and activation of cytosolic phospholipase A2 (PLA2) can occur independently of the activation of 42/44-kDa mitogen-activated protein (MAP) kinase in human platelets. We have investigated the hypothesis that the stress-activated p38 MAP kinase plays a role in the regulation of cytosolic PLA2. The specific inhibitor of p38 MAP kinase, SB 203580 [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl) imidazole], completely blocked the collagen-stimulated phosphorylation of cytosolic PLA2 in the presence of a cyclooxygenase blocker, and reduced the release of [3H]arachidonic acid by low concentrations of collagen. Stimulation of platelets with collagen (100 microg/ml) enhanced in vitro PLA2 activity of platelet lysates twofold over basal levels. In vitro PLA2 activity was reduced to basal levels when platelets were stimulated in the presence of SB 203580, but not in the presence of an inhibitor of the kinase that activates p42/p44 MAP kinase. SB 203580 only partially inhibited phosphorylation of cytosolic PLA2 in platelets that had not been treated with a cyclooxygenase blocker indicating that secondary stimulation by thromboxane A2 induces cytosolic PLA2 phosphorylation, by kinase(s) other than p38 MAP kinase. Under these conditions, inhibition of p42/p44 MAP kinase did not result in a reduction of cytosolic PLA2 phosphorylation, which is in agreement with the results obtained in the presence of cyclooxygenase blockers. In contrast to collagen, both p38 MAP kinase and p42/p44 MAP kinase participated in the phosphorylation of cytosolic PLA2 in platelets stimulated by cross-linking of the low-affinity receptor for immune complexes, Fc gammaRIIA. The present results demonstrate an important role for p38 MAP kinase in the regulation of cytosolic PLA2 activity in collagen-stimulated human platelets.

A novel inhibitory action of wheat germ agglutinin on phospholipase C in HEL and MEG-01 cell lines

Stimulation of HEL megakaryocytic cells by Fc gammaRIIA crosslinking is associated with tyrosine phosphorylation of syk and phospholipase C gamma2 (PLCgamma2) and is accompanied by formation of inositol phosphates and release of intracellular Ca2+. These responses are inhibited by the kinase inhibitors, staurosporine and ST271. In contrast, the G-protein receptor agonist, thrombin induces formation of inositol phosphates and release of intracellular calcium without an increase in tyrosine phosphorylation. The plant lectin wheat germ agglutinin (WGA) stimulates tyrosine phosphorylation of syk and PLCgamma2 but surprisingly does not stimulate formation of inositol phosphates and induce release of intracellular Ca2+. WGA also inhibited formation of inositol phosphates and release of intracellular Ca2+ by Fc gammaRIIA crosslinking and thrombin-stimulation. A similar inhibitory effect of WGA was observed against elevation of Ca2+ by the same two stimuli in MEG-01 megakaryotic cells. The results demonstrate a novel pathway of inhibition of PLC on crosslinking of cell surface proteins that is not present in platelets.

A collagen-like peptide stimulates tyrosine phosphorylation of syk and phospholipase C gamma2 in platelets independent of the integrin alpha2beta1

Activation of platelets by collagen is mediated through a tyrosine kinase-dependent pathway that is associated with phosphorylation of the Fc receptor gamma chain, the tyrosine kinase syk, and phospholipase C gamma2 (PLC gamma2). We recently described a collagen-related triple-helical peptide (CRP) with the sequence GCP*(GPP*)GCP*G (single letter amino acid code: P* = hydroxyproline; Morton et al, Biochem J306:337, 1995). The cross-linked peptide is a potent stimulus of platelet activation but, unlike collagen, does not support alpha2beta1-mediated, Mg2+-dependent adhesion, suggesting that its action is independent of the integrin alpha2beta1. This finding suggests the existence of a platelet receptor other than alpha2beta1 that underlies activation. In the present study, we show that CRP stimulates tyrosine phosphorylation of the same pattern of proteins in platelets as collagen, including syk and PLC gamma2. Protein tyrosine phosphorylation induced by CRP is not altered in the absence of Mg2+ or the presence of monoclonal antibodies (MoAbs) to the integrin alpha2beta1 (MoAb 6F1 and MoAb 13), conditions that prevent the interaction of collagen with the integrin. In contrast, phosphorylation of syk and PLC gamma2 by collagen is partially reduced by MoAb 6F1 and MoAb 13 or by removal of Mg2+. This may reflect a direct role of alpha2beta1 in collagen-induced signaling events or an indirect role in which the integrin facilitates the binding of collagen to its signaling receptor. The results show an alpha2beta1-independent pathway of platelet activation by CRP that involves phosphorylation of syk and PLC gamma2. This pathway appears to contribute to platelet activation by collagen.

p38 mitogen-activated protein kinase phosphorylates cytosolic phospholipase A2 (cPLA2) in thrombin-stimulated platelets. Evidence that proline-directed phosphorylation is not required for mobilization of arachidonic acid by cPLA2

The Ca2+-sensitive 85-kDa cytosolic phospholipase A2 (cPLA2) is responsible for thrombin-stimulated mobilization of arachidonic acid for the synthesis of thromboxane A2 in human platelets. We have previously shown that thrombin activates p38 kinase, a recently discovered new member of the mitogen-activated protein kinase family (Kramer, R. M., Roberts, E. F., Strifler, B. A., and Johnstone, E. M. (1995) J. Biol. Chem. 270, 27395-27398) and also induces phosphorylation of cPLA2, thereby increasing its intrinsic catalytic activity. In the present study we have examined the role of p38 kinase in the phosphorylation and activation of cPLA2 in stimulated platelets. We have observed that activation of p38 kinase accompanies receptor-mediated events in platelets and coincides with cPLA2 phosphorylation. Furthermore, in the presence of inhibitors of p38 kinase, the proline-directed phosphorylation of cPLA2 was completely blocked in platelets stimulated with the thrombin receptor agonist peptide SFLLRN and was suppressed during the early (up to 2 min) phase of platelet stimulation caused by thrombin. Unexpectedly, we found that prevention of proline-directed phosphorylation of cPLA2 in stimulated platelets did not attenuate its ability to release arachidonic acid from platelet phospholipids. We conclude that: 1) cPLA2 is a physiological target of p38 kinase; 2) p38 kinase is involved in the early phosphorylation of cPLA2 in stimulated platelets; and 3) proline-directed phosphorylation of cPLA2 is not required for its receptor-mediated activation.