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

Targeting Janus Kinases and Signal Transducer and Activator of Transcription 3 to Treat Inflammation, Fibrosis, and Cancer: Rationale, Progress, and Caution

Before it was molecularly cloned in 1994, acute-phase response factor or signal transducer and activator of transcription (STAT)3 was the focus of intense research into understanding the mammalian response to injury, particularly the acute-phase response. Although known to be essential for liver production of acute-phase reactant proteins, many of which augment innate immune responses, molecular cloning of acute-phase response factor or STAT3 and the research this enabled helped establish the central function of Janus kinase (JAK) family members in cytokine signaling and identified a multitude of cytokines and peptide hormones, beyond interleukin-6 and its family members, that activate JAKs and STAT3, as well as numerous new programs that their activation drives. Many, like the acute-phase response, are adaptive, whereas several are maladaptive and lead to chronic inflammation and adverse consequences, such as cachexia, fibrosis, organ dysfunction, and cancer. Molecular cloning of STAT3 also enabled the identification of other noncanonical roles for STAT3 in normal physiology, including its contribution to the function of the electron transport chain and oxidative phosphorylation, its basal and stress-related adaptive functions in mitochondria, its function as a scaffold in inflammation-enhanced platelet activation, and its contributions to endothelial permeability and calcium efflux from endoplasmic reticulum. In this review, we will summarize the molecular and cellular biology of JAK/STAT3 signaling and its functions under basal and stress conditions, which are adaptive, and then review maladaptive JAK/STAT3 signaling in animals and humans that lead to disease, as well as recent attempts to modulate them to treat these diseases. In addition, we will discuss how consideration of the noncanonical and stress-related functions of STAT3 cannot be ignored in efforts to target the canonical functions of STAT3, if the goal is to develop drugs that are not only effective but safe. SIGNIFICANCE STATEMENT: Key biological functions of Janus kinase (JAK)/signal transducer and activator of transcription (STAT)3 signaling can be delineated into two broad categories: those essential for normal cell and organ development and those activated in response to stress that are adaptive. Persistent or dysregulated JAK/STAT3 signaling, however, is maladaptive and contributes to many diseases, including diseases characterized by chronic inflammation and fibrosis, and cancer. A comprehensive understanding of JAK/STAT3 signaling in normal development, and in adaptive and maladaptive responses to stress, is essential for the continued development of safe and effective therapies that target this signaling pathway.

Impact of Itga2-Gp6-double collagen receptor deficient mice for bone marrow megakaryocytes and platelets

The two main collagen receptors on platelets, GPVI and integrin α2β1, play an important role for the recognition of exposed collagen at sites of vessel injury, which leads to platelet activation and subsequently stable thrombus formation. Both receptors are already expressed on megakaryocytes, the platelet forming cells within the bone marrow. Megakaryocytes are in permanent contact with collagen filaments in the marrow cavity and at the basal lamina of sinusoids without obvious preactivation. The role of both collagen receptors for megakaryocyte maturation and thrombopoiesis is still poorly understood. To investigate the function of both collagen receptors, we generated mice that are double deficient for Gp6 and Itga2. Flow cytometric analyses revealed that the deficiency of both receptors had no impact on platelet number and led to the expected lack in GPVI responsiveness. Integrin activation and degranulation ability was comparable to wildtype mice. By immunofluorescence microscopy, we could demonstrate that both wildtype and double-deficient megakaryocytes were overall normally distributed within the bone marrow. We found megakaryocyte count and size to be normal, the localization within the bone marrow, the degree of maturation, as well as their association to sinusoids were also unaltered. However, the contact of megakaryocytes to collagen type I filaments was decreased at sinusoids compared to wildtype mice, while the interaction to type IV collagen was unaffected. Our results imply that GPVI and α2β1 have no influence on the localization of megakaryocytes within the bone marrow, their association to the sinusoids or their maturation. The decreased contact of megakaryocytes to collagen type I might at least partially explain the unaltered platelet phenotype in these mice, since proplatelet formation is mediated by these receptors and their interaction to collagen. It is rather likely that other compensatory signaling pathways and receptors play a role that needs to be elucidated.

Role of Platelet Glycoprotein VI and Tyrosine Kinase Syk in Thrombus Formation on Collagen-Like Surfaces

Platelet interaction with collagens, via von Willebrand factor, is a potent trigger of shear-dependent thrombus formation mediated by subsequent engagement of the signaling collagen receptor glycoprotein (GP)VI, enforced by integrin α₂β₁. Protein tyrosine kinase Syk is central in the GPVI-induced signaling pathway, leading to elevated cytosolic Ca²⁺. We aimed to determine the Syk-mediated thrombogenic activity of several collagen peptides and (fibrillar) type I and III collagens. High-shear perfusion of blood over microspots of these substances resulted in thrombus formation, which was assessed by eight parameters and was indicative of platelet adhesion, activation, aggregation, and contraction, which were affected by the Syk inhibitor PRT-060318. In platelet suspensions, only collagen peptides containing the consensus GPVI-activating sequence (GPO)_n and Horm-type collagen evoked Syk-dependent Ca²⁺ rises. In whole blood under flow, Syk inhibition suppressed platelet activation and aggregation parameters for the collagen peptides with or without a (GPO)_n sequence and for all of the collagens. Prediction models based on a regression analysis indicated a mixed role of GPVI in thrombus formation on fibrillar collagens, which was abolished by Syk inhibition. Together, these findings indicate that GPVI-dependent signaling through Syk supports platelet activation in thrombus formation on collagen-like structures regardless of the presence of a (GPO)_n sequence.

The molecular mechanisms involved in lectin-induced human platelet aggregation

We have compared the effect of three legume lectins, wheat germ agglutinin (WGA), Phaseolus vulgaris agglutinin (PHA) and Lens culinaris agglutinin (LCA), on the function of human platelets. We have found that WGA is more active than PHA in stimulating platelet activation/aggregation, while LCA has no effect. Studies on the mechanisms involved show that WGA and PHA induce phosphorylation/activation of PLCγ2 and increase [Ca2+]i. For the first time, it has been shown that Src/Syk pathway, the adapter protein SLP-76 and the exchange protein VAV, participate in the PLCγ2 activation by these lectins. Moreover WGA and PHA stimulate the PI3K/AKT pathway. PI3K, through its product phosphatidylinositol-3,4,5-trisphosphate activates Bruton's tyrosine kinase (BTK) and contributes to PLCγ2 activation. In conclusion, our findings suggest that PLCγ2 activation induced by WGA and PHA is regulated by Src/Syk and by PI3K/BTK pathways through their concerted action.

Serum ubiquitin via CXC chemokine receptor 4 triggered cyclooxygenase-1 ubiquitination possibly involved in the pathogenesis of aspirin resistance

Extracellular ubiquitin (Ub) with platelet aggregation property was found higher in acute myocardial infarction (AMI) patients. Here we detected the platelet functions and serum Ub levels in 250 AMI patients and 50 healthy volunteers before and after aspirin treatment. The influence of serum Ub on platelet functions was determined in vitro. We found that 47 out of 250 AMI patients showed aspirin resistance (AR) and 203 showed aspirin sensitivity (AS). During hospitalization, AR group had higher serum Ub levels than the AS group or the healthy group, and the serum Ub levels was related to the rates of thrombosis events. The patients with higher serum Ub levels showed that the platelets had more ubiquitinated platelets, higher contents of ubiquitinated proteins and ubiquitinated cyclooxygenase-1 (COX-1). The levels of ubiquitinated COX-1 in the platelets was inversely correlated with acetylated COX-1, the separated ubiquitinated COX-1 activity was approximately twofold or fourfold higher than the total COX-1(ubiquitinated COX-1 and COX-1) or COX-1. In vitro, we found that extracellular Ub, via the CXC chemokine receptor 4 (CXCR4) pathway, facilitated COX-1 to be ubiquitined and prevented aspirin to acetylate its target. Platelets had higher levels of ubiquitinated COX-1 showing poor response to aspirin. Such results were not detected in Ub-free serum or ovalbumin incubated platelets. Serum Ub, via the CXCR4 pathway, facilitated COX-1 to be ubiquitined and activated the platelets possibly involved in the pathogenesis of AR.

Calcium-signal facilitates herpes simplex virus type 1 nuclear transport through slingshot 1 and calpain-1 activation

Herpes simplex virus type 1 (HSV-1) can establish its latency in neurons and is associated with virus-induced pathological neurodegeneration in the nervous system. Here we show that viral penetration-induced calcium release facilitated HSV-1 intracellular trafficking through activating slingshot 1 (SSH), a phosphatase regulating actin filament dynamics. More detailed studies revealed that phospholipase C gamma 1, and the inositol 1,4,5-trisphosphate receptor isoform 1 were required for SSH activation. Besides, calpain-1, a calcium-dependent cysteine protease, was involved in viral intracellular migration. These results may lead to new targets for antiviral therapy.

The Drosophila TRPP cation channel, PKD2 and Dmel/Ced-12 act in genetically distinct pathways during apoptotic cell clearance

Apoptosis, a genetically programmed cell death, allows for homeostasis and tissue remodelling during development of all multi-cellular organisms. Phagocytes swiftly recognize, engulf and digest apoptotic cells. Yet, to date the molecular mechanisms underlying this phagocytic process are still poorly understood. To delineate the molecular mechanisms of apoptotic cell clearance in Drosophila, we have carried out a deficiency screen and have identified three overlapping phagocytosis-defective mutants, which all delete the fly homologue of the ced-12 gene, known as Dmel\ced12. As anticipated, we have found that Dmel\ced-12 is required for apoptotic cell clearance, as for its C. elegans and mammalian homologues, ced-12 and elmo, respectively. However, the loss of Dmel\ced-12 did not solely account for the phenotypes of all three deficiencies, as zygotic mutations and germ line clones of Dmel\ced-12 exhibited weaker phenotypes. Using a nearby genetically interacting deficiency, we have found that the polycystic kidney disease 2 gene, pkd2, which encodes a member of the TRPP channel family, is also required for phagocytosis of apoptotic cells, thereby demonstrating a novel role for PKD2 in this process. We have also observed genetic interactions between pkd2, simu, drpr, rya-r44F, and retinophilin (rtp), also known as undertaker (uta), a gene encoding a MORN-repeat containing molecule, which we have recently found to be implicated in calcium homeostasis during phagocytosis. However, we have not found any genetic interaction between Dmel\ced-12 and simu. Based on these genetic interactions and recent reports demonstrating a role for the mammalian pkd-2 gene product in ER calcium release during store-operated calcium entry, we propose that PKD2 functions in the DRPR/RTP pathway to regulate calcium homeostasis during this process. Similarly to its C. elegans homologue, Dmel\Ced-12 appears to function in a genetically distinct pathway.

Release of intracellular calcium stores facilitates coxsackievirus entry into polarized endothelial cells

Group B coxsackieviruses (CVB) are associated with viral-induced heart disease and are among the leading causes of aseptic meningitis worldwide. Here we show that CVB entry into polarized brain microvasculature and aortic endothelial cells triggers a depletion of intracellular calcium stores initiated through viral attachment to the apical attachment factor decay-accelerating factor. Calcium release was dependent upon a signaling cascade that required the activity of the Src family of tyrosine kinases, phospholipase C, and the inositol 1,4,5-trisphosphate receptor isoform 3. CVB-mediated calcium release was required for the activation of calpain-2, a calcium-dependent cysteine protease, which controlled the vesicular trafficking of internalized CVB particles. These data point to a specific role for calcium signaling in CVB entry into polarized endothelial monolayers and highlight the unique signaling mechanisms used by these viruses to cross endothelial barriers.

Enteroviruses are associated with a number of diverse syndromes such as myocarditis, febrile illness, and are the main causative agents of aseptic meningitis. No effective therapeutics exist to combat non-poliovirus enterovirus infections. A better understanding of the mechanisms by which these viruses infect host cells could lead to the design of effective therapeutic interventions. In this study, we found that intracellular calcium stores in polarized endothelial monolayers are depleted upon exposure to coxsackievirus B (CVB) and that this release is mediated by viral attachment to its receptor decay-accelerating factor. We also discovered that the calcium release requires the activation of signaling molecules involved in calcium signaling such as Src tyrosine kinases, phospholipase C, and the inositol 1,4,5-trisphosphate receptor isoform 3 on the ER membrane. Furthermore, we found that a calcium-activated cystein protease, calpain-2, was activated and necessary for proper viral trafficking inside the cell. Interestingly, we found that this signaling cascade was critical for CVB internalization into the endothelium, but was not involved in CVB entry into the epithelium. This is an important advance in our understanding of how enteroviruses hijack host endothelial cell signaling mechanisms in order to facilitate their entry and eventual spread.

Analysis of Fyn function in hemostasis and alphaIIbbeta3-integrin signaling

Recent studies have shown that Src-family kinases (SFKs) play an important role in mediating integrin signalling, and the beta3 subunit of alphaIIbbeta3 integrin has been shown to interact with multiple SFK members. Here, we analyzed the interactions and functional consequences of Fyn and Src binding to alphaIIbbeta3. Fyn associated with the beta3 subunit in resting and thrombin-aggregated platelets, whereas interaction between Src and alphaIIbbeta3 was seen predominantly in resting but not in thrombin-aggregated platelets. We have also observed that Fyn but not Src localized to focal adhesions in CHO cells adherent to fibrinogen through alphaIIbbeta3. On the basis of these differences, we wanted to determine the sequence requirements for the interaction of Fyn and Src within the beta3-cytoplasmic domain. Whereas Src association required the C-terminal region of beta3, Fyn continued to interact with mutants that could no longer associate with Src and that contained as few as 13 membrane-proximal amino acids of the beta3-cytoplasmic tail. Using deletion mutants of beta3-cytoplasmic tails expressed as GST-fusion proteins, we narrowed down the Fyn-binding site even further to the amino acid residues 721-725 (IHDRK) of the beta3-cytoplasmic domain. On the basis of these observations, we explored whether Fyn-/- mice exhibited any abnormalities in hemostasis and platelet function. We found that Fyn-/- mice significantly differed in their second bleeding times compared with wild-type mice, and platelets from Fyn-/- mice exhibited delayed spreading on fibrinogen-coated surfaces. Using mutant forms of Fyn, it appears that its kinase activity is required for its localization to focal adhesions and to mediate alphaIIbbeta3-dependent cell spreading. Our results suggest that Fyn and Src have distinct requirements for interaction with alphaIIbbeta3; and, consequently, the two SFK can mediate different functional responses.

Roles of the C-terminal tyrosine residues of LAT in GPVI-induced platelet activation: insights into the mechanism of PLC gamma 2 activation

Linker for activation of T cells (LAT) is an adaptor protein required for organization of the signaling machinery downstream of the platelet collagen receptor, the glycoprotein VI (GPVI). Here, we investigated the effect of LAT mutations on specific signaling pathways and on platelet functions in response to GPVI triggering by convulxin (Cvx). Using mice containing tyrosine to phenylalanine mutations of the adaptor, we show the crucial role played by the tyrosine residues at positions 175, 195, and 235 in the phosphorylation of LAT and in the whole pattern of protein tyrosine phosphorylation in response to Cvx. These 3 C-terminal tyrosine residues are important to recruit the tyrosine kinase Fyn, which may be involved in LAT phosphorylation. Efficient phosphoinositide 3-kinase (PI3K) activation requires the 3 C-terminal tyrosine residues of LAT but not its tyrosine 136. Interestingly, single mutation of the tyrosine 136 results in the loss of phospholipase C gamma2 (PLCgamma2) activation without affecting its PI3K-dependent membrane association, and is sufficient to impair platelet responses to Cvx. Thus, activation of PLCgamma2 via GPVI is dependent on 2 complementary events: its interaction with the tyrosine 136 of LAT and its membrane location, which itself requires events mediated by the 3 C-terminal tyrosines of LAT.

Trophoblast adhesion of the peri-implantation mouse blastocyst is regulated by integrin signaling that targets phospholipase C

Integrin signaling modulates trophoblast adhesion to extracellular matrices during blastocyst implantation. Fibronectin (FN)-binding activity on the apical surface of trophoblast cells is strengthened after elevation of intracellular Ca(2+) downstream of integrin ligation by FN. We report here that phosphoinositide-specific phospholipase C (PLC) mediates Ca(2+) signaling in response to FN. Pharmacological agents used to antagonize PLC (U73122) or the inositol phosphate receptor (Xestospongin C) inhibited FN-induced elevation of intracellular Ca(2+) and prevented the upregulation of FN-binding activity. In contrast, inhibitors of Ca(2+) influx through either voltage-gated or non-voltage-gated Ca(2+) channels were without effect. Inhibition of protein tyrosine kinase activity by genistein, but not G-protein inhibition by suramin, blocked FN-induced intracellular Ca(2+) signaling and upregulation of adhesion, consistent with involvement of PLC-gamma. Confocal immunofluorescence imaging of peri-implantation blastocysts demonstrated that PLC-gamma2, but not PLC-gamma1 nor PLC-beta1, accumulated near the outer surface of the embryo. Phosphotyrosine site-directed antibodies revealed phosphorylation of PLC-gamma2, but not PLC-gamma1, upon integrin ligation by FN. These data suggest that integrin-mediated activation of PLC-gamma to initiate phosphoinositide signaling and intracellular Ca(2+) mobilization is required for blastocyst adhesion to FN. Signaling cascades regulating PLC-gamma could, therefore, control a critical feature of trophoblast differentiation during peri-implantation development.

Kinase activation through dimerization by human SH2-B

The isoforms of SH2-B, APS, and Lnk form a family of signaling proteins that have been described as activators, mediators, or inhibitors of cytokine and growth factor signaling. We now show that the three alternatively spliced isoforms of human SH2-B readily homodimerize in yeast two-hybrid and cellular transfections assays, and this is mediated specifically by a unique domain in its amino terminus. Consistent with previous reports, we further show that the SH2 domains of SH2-B and APS bind JAK2 at Tyr813. These findings suggested a model in which two molecules of SH2-B or APS homodimerize with their SH2 domains bound to two JAK2 molecules, creating heterotetrameric JAK2-(SH2-B)2-JAK2 or JAK2-(APS)2-JAK2 complexes. We further show that APS and SH2-B isoforms heterodimerize. At lower levels of SH2-B or APS expression, dimerization approximates two JAK2 molecules to induce transactivation. At higher relative concentrations of SH2-B or APS, kinase activation is blocked. SH2-B or APS homodimerization and SH2-B/APS heterodimerization thus provide direct mechanisms for activating and inhibiting JAK2 and other kinases from the inside of the cell and for potentiating or attenuating cytokine and growth factor receptor signaling when ligands are present.

Rapid ubiquitination of Syk following GPVI activation in platelets

Spleen tyrosine kinase (Syk) activation is a key intermediate step in the activation of platelets by the physiologic agonist collagen. We have found that Syk is rapidly ubiquitinated upon activation of platelets by collagen, collagen-related peptide (CRP), and convulxin. The Src family kinase inhibitors prevented Syk phosphorylation and its ubiquitination, indicating that the process is downstream of Src kinases. The ubiquitination of Syk did not cause degradation of the protein as evidenced by the lack of effect of proteasomal and lysosomal inhibitors. We separated ubiquitinated Syk from its nonubiquitinated counterpart and used an in vitro kinase assay to compare their activities. We found that the ubiquitinated Syk appeared to be about 5-fold more active. Using a phosphospecific antibody to Syk (Tyr525/Tyr526) that measures activated Syk, we found that most (60%-75%) of the active Syk is in the ubiquitinated fraction. This result explains the apparent high specific activity of ubiquitinated Syk. In c-Cbl-deficient mice, Syk is not ubiquitinated, implicating c-Cbl as the E3 ligase involved in Syk ubiquitination. Furthermore, Syk is not dephosphorylated in these mice. We propose that c-Cbl plays a regulatory role in glycoprotein VI (GPVI)/Fc receptor gamma (FcRgamma)-chain-dependent platelet activation through its interaction with Syk.

Inhibitory effect of GBH on platelet aggregation through inhibition of intracellular Ca2+ mobilization in activated human platelets

Geiji-Bokryung-Hwan (GBH) was studied on antiplatelet activity in human platelet suspensions. GBH consists of the 5 herbs Cinnamomi Ramulus, Poria Cocos, Mountan Cortex Radicis, Paeoniae Radix, and Persicae Semen, which have been used in herbal medicine for thousands of years for atherosclerosis. The mechanism involved in the antiplatelet activity of GBH in human platelet suspensions was investigated. GBH inhibited platelet aggregation and Ca2+ mobilization in a concentration-dependent manner without increasing intracellular cyclic AMP and cyclic GMP. GBH had no inhibitory effect on thromboxane B2 (TXB2) production in cell-free systems. Collagen-related peptide (CRP)-induced Ca2+ mobilization is regulated by phospholipase C-2 (PLC-gamma2) activation. We evaluated the effect of GBH on tyrosine phosphorylation of PLC-gamma2 and the production of inositol-1,4,5-trisphosphate (IP3). GBH at concentrations that inhibited platelet aggregation and Ca2+ mobilization had no effects on tyrosine phosphorylation of PLC-gamma2 or on the formation of IP3 induced by CRP. Similar results were obtained with thrombin-induced platelet activation. GBH inhibited platelet aggregation and Ca2+ mobilization induced by thrombin without affecting the production of IP3. We then evaluated the effect of GBH on the binding of IP3 to its receptor. GBH at high concentrations partially blocked the binding of IP3 to its receptor. Therefore, the results suggested that GBH suppresses Ca2+ mobilization at a step distal to IP3 formation. GBH may provide a good tool for investigating Ca2+ mobilization.

The role of Tec family kinases in myeloid cells

Members of the Tec kinase family (Bmx, Btk, Itk, Rlk and Tec) are primarily expressed in the hematopoietic system and form, after the Src kinase family, the second largest class of non-receptor protein tyrosine kinases. During lymphocyte development and activation Tec kinases have important functions in signaling pathways downstream of the antigen receptors. Tec family kinases are also expressed in cells of the myeloid lineage. However, with the exception of mast cells and platelets, their biological role in the myeloid system is only poorly understood. This review summarizes the current knowledge about the function of Tec family kinases in hematopoietic cells of the myeloid lineage.

Integrin alpha IIb beta 3-dependent calcium signals regulate platelet-fibrinogen interactions under flow. Involvement of phospholipase C gamma 2

Platelet adhesion to fibrinogen is important for platelet aggregation and thrombus growth. In this study we have examined the mechanisms regulating platelet adhesion on immobilized fibrinogen under static and shear conditions. We demonstrate that integrin alpha IIb beta 3 engagement of immobilized fibrinogen is sufficient to induce an oscillatory calcium response, necessary for lamellipodial formation and platelet spreading. Released ADP increases the proportion of platelets exhibiting a cytosolic calcium response but is not essential for calcium signaling or lamellipodial extension. Pretreating platelets with the Src kinase inhibitor PP2, the inositol 1,4,5-trisphosphate (IP3) receptor antagonist 2-aminoethoxydiphenyl borate (APB-2), or the phospholipase C (PLC) inhibitor U73122 abolished calcium signaling and platelet spreading, suggesting a major role for Src kinase-regulated PLC isoforms in these processes. Analysis of PLC gamma 2-/- mouse platelets revealed a major role for this isoform in regulating cytosolic calcium flux and platelet spreading on fibrinogen. Under flow conditions, platelets derived from PLC gamma 2-/- mice formed less stable adhesive interactions with fibrinogen, particularly in the presence of ADP antagonists. Our studies define an important role for PLC gamma 2 in integrin alpha IIb beta 3-dependent calcium flux, necessary for stable platelet adhesion and spreading on fibrinogen. Furthermore, they establish an important cooperative signaling role for PLC gamma 2 and ADP in regulating platelet adhesion efficiency on fibrinogen.

Tumour necrosis factor-alpha potentiates CR3-induced respiratory burst by activating p38 MAP kinase in human neutrophils

CR3 and Fc gamma Rs are the main receptors involved in the phagocytic process leading to engulfment and killing of microbes by production of reactive oxygen intermediates (ROI) and degranulation. Various inflammatory mediators, such as tumour necrosis factor-alpha (TNF-alpha) and lipopolysaccharide (LPS), are known to prime neutrophils leading to increased bactericidal responses, but the underlying mechanism of priming has only been partially elucidated. The purpose of this study was to investigate how TNF-alpha primes neutrophils for subsequent stimuli via either CR3 or Fc gamma R. The receptors were specifically activated with pansorbins (protein-A-positive Staphylococcus aureus) coated with anti-CR3, anti-Fc gamma RIIa, or anti-Fc gamma RIIIb monoclonal antibody. Activation of neutrophils with these particles resulted in ROI production as measured by chemiluminescence. Anti-CR3 pansorbins induced the most prominent ROI production in neutrophils. TNF-alpha potentiated the CR3-mediated respiratory burst but had little effect on that mediated by Fc gamma Rs. The priming effect of TNF-alpha on CR3-mediated ROI production is associated with an increased activation of p38 MAPK as well as tyrosine phosphorylation of p72(syk). Pretreatment of neutrophils with the inhibitors for p38 MAPK and p72(syk) markedly suppressed the respiratory burst induced by CR3. Furthermore, TNF-alpha induced about a three-fold increase in the expression of CR3 in neutrophils, an effect which is blocked by the p38 MAPK inhibitor. Taken together, these results showed that TNF-alpha potentiates the CR3-mediated respiratory burst in neutrophils not only by triggering a p38 MAPK-dependent up-regulation of CD11b/CD18 but also by modulating the signalling pathways.

Role of a JAK3-dependent biochemical signaling pathway in platelet activation and aggregation

Here we provide experimental evidence that identifies JAK3 as one of the regulators of platelet function. Treatment of platelets with thrombin induced tyrosine phosphorylation of the JAK3 target substrates STAT1 and STAT3. Platelets from JAK3-deficient mice displayed a decrease in tyrosine phosphorylation of STAT1 and STAT3. In accordance with these data, pretreatment of human platelets with the JAK3 inhibitor WHI-P131 markedly decreased the base-line enzymatic activity of constitutively active JAK3 and abolished the thrombin-induced tyrosine phosphorylation of STAT1 and STAT3. Following thrombin stimulation, WHI-P131-treated platelets did not undergo shape changes indicative of activation such as pseudopod formation. WHI-P131 inhibited thrombin-induced degranulation/serotonin release as well as platelet aggregation. Highly effective platelet inhibitory plasma concentrations of WHI-P131 were achieved in mice without toxicity. WHI-P131 prolonged the bleeding time of mice in a dose-dependent manner and improved event-free survival in a mouse model of thromboplastin-induced generalized and invariably fatal thromboembolism. To our knowledge, WHI-P131 is the first anti-thrombotic agent that prevents platelet aggregation by inhibiting JAK3.

Expression and function of the collagen receptor GPVI during megakaryocyte maturation

In this report, the expression and function of the platelet collagen receptor glycoprotein VI (GPVI) were studied in human megakaryocytes during differentiation and maturation of mobilized blood and cord blood derived CD34(+) cells. By flow cytometry, using an anti-GPVI monoclonal antibody or convulxin, a GPVI-specific ligand, GPVI was detected only on CD41(+) cells including some CD41(+)/CD34(+) cells, suggesting expression at a stage of differentiation similar to CD41. These results were confirmed at the mRNA level using reverse transcription-polymerase chain reaction. GPVI expression was low during megakaryocytic differentiation but increased in the more mature megakaryocytes (CD41(high)). As in platelets, megakaryocyte GPVI associates with the Fc receptor gamma chain (FcRgamma). The FcR gamma chain was detected at the RNA and protein level at all stages of megakaryocyte maturation preceding the expression of GPVI. The other collagen receptor, alpha(2)beta(1) integrin (CD49b/CD29), had a pattern of expression similar to GPVI. Megakaryocytic GPVI was recognized as a 55-kDa protein by immunoblotting and ligand blotting, and thus it presented a slightly lower apparent molecular mass than platelet GPVI (58 kDa). Megakaryocytes began to adhere to immobilized convulxin via GPVI after only 8-10 days of culture, at a time when megakaryocytes were maturing. At this stage of maturation, they also adhered to immobilized collagen by alpha(2)beta(1) integrin-dependent and -independent mechanisms. Convulxin induced a very similar pattern of protein tyrosine phosphorylation in megakaryocytes and platelets including Syk, FcRgamma, and PLC(gamma)2. Our results showed that GPVI is expressed early during megakaryocytic differentiation but functionally allows megakaryocyte adherence to collagen only at late stages of differentiation when its expression increases.

Platelet endothelial cell adhesion molecule-1 serves as an inhibitory receptor that modulates platelet responses to collagen

Platelet responses to collagen are mediated by the combined actions of the integrin alpha2beta1, which serves as a major collagen-binding receptor, and the GPVI/FcRgamma-chain complex, which transmits collagen-specific activation signals into the cell interior through the action of an immunoreceptor tyrosine-based activation motif within the cytoplasmic domain of the FcRgamma-chain. Despite much progress in identifying components of the signaling pathway responsible for collagen-induced platelet activation, virtually nothing is known about the regulatory elements that modulate this important hemostatic event. PECAM-1, a recently recognized member of the inhibitory receptor family, contains a functional immunoreceptor tyrosine-based inhibitory motif within its cytoplasmic domain that, when tyrosine phosphorylated, recruits and activates the protein-tyrosine phosphatase, SHP-2. To test the hypothesis that PECAM-1 functions to regulate GPVI/FcRgamma-chain-mediated platelet activation, the responses of wild-type versus PECAM-1-deficient murine platelets to GPVI-specific agonists were compared. Four distinct GPVI/FcRgamma-chain-dependent responses were found to be significantly exaggerated in platelets derived from PECAM-1-deficient mice, including Mg++-independent adhesion to immobilized fibrillar collagen, collagen-induced platelet aggregation, platelet aggregation induced by the GPVI-specific agonist collagen-related peptide, and GPVI/FcRgamma-chain-induced dense granule secretion. Together, these data provide compelling evidence that PECAM-1 modulates platelet responses to collagen, and they implicate this novel member of the inhibitory receptor family in the regulation of primary hemostasis.

Phosphatidylinositol 3-kinase-dependent translocation of phospholipase Cgamma2 in mouse megakaryocytes is independent of Bruton tyrosine kinase translocation

Activation of the collagen receptor glycoprotein VI (GPVI) by a collagen-related peptide (CRP) induces stimulation of platelets and megakaryocytes through the phosphatidylinositol (PI) 3-kinase-dependent pathway leading to activation of Bruton tyrosine kinase (Btk) and phospholipase Cgamma2 (PLCgamma2). Here, we present evidence that both proteins undergo PI 3-kinase-dependent translocation to the plasma membrane on CRP stimulation that is markedly inhibited by wortmannin and LY294002. Translocation of PLCgamma2 but not Btk is also seen in megakaryocytes from X-linked immunodeficiency mice, which have a mutation that reduces the affinity of the pleckstrin homology (PH) domain of Btk for PI 3,4,5-trisphosphate (PI 3,4,5-P3). Activation of PC12 cells by epidermal growth factor (EGF) results in increased PI 3-kinase activity and high PI 3,4,5-P3 levels that trigger translocation of the green fluorescent protein (GFP)-labeled PH of Btk, but not the GFP-labeled PH and tandem Src homology 2 (SH2) domains of PLCgamma2. In contrast to the results with CRP, the G protein-coupled receptor agonist thrombin stimulates PI 3-kinase-independent translocation of Btk but not PLCgamma2. In conclusion, these results demonstrate that in mouse megakaryocytes, CRP leads to PI 3-kinase-dependent translocation of PLCgamma2 and Btk that are independent of one another, whereas thrombin only induces translocation of Btk through a pathway that is independent of PI 3-kinase activity.

Rhodocytin induces platelet aggregation by interacting with glycoprotein Ia/IIa (GPIa/IIa, Integrin alpha 2beta 1). Involvement of GPIa/IIa-associated src and protein tyrosine phosphorylation

Although glycoprotein Ia/IIa (GPIa/IIa, integrin alpha(2)beta(1)) has established its role as a collagen receptor, it remains unclear whether GPIa/IIa mediates activation signals. In this study, we show that rhodocytin, purified from the Calloselasma rhodostoma venom, induces platelet aggregation, which can be blocked by anti-GPIa monoclonal antibodies. Studies with rhodocytin-coupled beads and liposomes loaded with recombinant GPIa/IIa demonstrated that rhodocytin directly binds to GPIa/IIa independently of divalent cations. In vitro kinase assays and Western blotting of GPIa immunoprecipitates revealed that Src and Lyn constitutively associate with GPIa/IIa and that Src activity increases transiently after rhodocytin stimulation. Src specifically associates with p130 Crk-associated substrate (Cas) in a manner dependent upon Cas phosphorylation, suggesting that Src is responsible for Cas tyrosine phosphorylation. While all these phenomena occur early after rhodocytin stimulation in a cAMP-resistant manner, tyrosine phosphorylation of Syk and phospholipase Cgamma2, intracellular Ca(2+) mobilization, and platelet aggregation occur later in a cAMP-sensitive manner. Cytochalasin D, which interferes with actin polymerization and blocks receptor clustering, inhibits all the rhodocytin-mediated signals we examined in this study. We suggest that rhodocytin, by clustering GPIa/IIa, activates GPIa/IIa-associated Src, which then mediates downstream activation signals.

Outside-in signaling pathway linked to CD146 engagement in human endothelial cells

CD146 (S-Endo 1 Ag or MUC18) is a transmembrane glycoprotein expressed on endothelial cells on the whole vascular tree. CD146 is located at the intercellular junction where it plays a role in the cohesion of the endothelial monolayer. CD146 engagement initiates an outside-in signaling pathway involving the protein tyrosine kinases FYN and FAK as well as paxillin. Here we report that CD146 engagement by its specific monoclonal antibody in human umbilical vein endothelial cells induces a Ca(2+) influx that is sensitive to thapsigargin and EGTA treatment, indicating that CD146 engagement initiates a store-operated calcium mobilization. In addition, biochemical and pharmacological analysis revealed that CD146 engagement initiates the tyrosine phosphorylation of phospholipase C-gamma, Pyk2, and p130(Cas). Pharmacological inhibition of Ca(2+) flux with 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acetoxymethyl ester and EGTA indicated that an increase in Ca(2+) is required for Pyk2 and p130(Cas) tyrosine phosphorylation. Moreover, a complex association was observed between Pyk2, p130(Cas), and paxillin. These results indicate that CD146 is coupled to a FYN-dependent pathway that triggers Ca(2+) flux via phospholipase C-gamma activation leading subsequently to the tyrosine phosphorylation of downstream targets such as Pyk2, p130(Cas), FAK, and paxillin. In addition to its role in cell-cell adhesion, CD146 is a signaling molecule involved in the dynamics of actin cytoskeleton rearrangement.

Evidence for two distinct epitopes within collagen for activation of murine platelets

It has recently been shown that the monoclonal antibody JAQ1 to murine glycoprotein VI (GPVI) can cause aggregation of mouse platelets upon antibody cross-linking and that collagen-induced platelet aggregation can be inhibited by preincubation of platelets with JAQ1 in the absence of cross-linking (Nieswandt, B., Bergmeier, W., Schulte, V., Rackebrandt, K., Gessner, J. E., and Zirngibl, H. (2000) J. Biol. Chem. 275, 23998-24002). In the present study, we have shown that cross-linking of GPVI by JAQ1 results in tyrosine phosphorylation of the same profile of proteins as that induced by collagen, including the Fc receptor (FcR) gamma-chain, Syk, LAT, SLP-76, and phospholipase C gamma 2. In contrast, platelet aggregation and tyrosine phosphorylation of these proteins were inhibited when mouse platelets were preincubated with JAQ1 in the absence of cross-linking and were subsequently stimulated with a collagen-related peptide (CRP) that is specific for GPVI and low concentrations of collagen. However, at higher concentrations of collagen, but not CRP, aggregation of platelets and tyrosine phosphorylation of the above proteins (except for the adapter LAT) is re-established despite the presence of JAQ1. These observations suggest that a second activatory binding site, which is distinct from the CRP binding site on GPVI on mouse platelets, is occupied in the presence of high concentrations of collagen. Although this could be a second site on GPVI that is activated by a novel motif within the collagen molecule, the absence of LAT phosphorylation in response to collagen in the presence of JAQ1 suggests that this is more likely to be caused by activation of a second receptor that is also coupled to the FcR gamma-chain. The possibility that this response is mediated by a receptor that is not coupled to FcR gamma-chain is excluded on the grounds that aggregation is absent in platelets from FcR gamma-chain-deficient mice.

Invasion of human epithelial cells by Pseudomonas aeruginosa involves src-like tyrosine kinases p60Src and p59Fyn

Pseudomonas aeruginosa plays a major role in respiratory tract infections or sepsis in patients with cystic fibrosis or upon suppression of the immune system. Several P. aeruginosa strains have been shown to be internalized by human epithelial cells; however, the molecular mechanisms of the invasion process are poorly characterized. Here, we show that the internalization of P. aeruginosa into human epithelial cells results in and requires activation of the Src-like tyrosine kinases p59Fyn and p60Src and the consequent tyrosine phosphorylation of several eukaryotic proteins. The significance of Src-like tyrosine kinase activation is shown by an almost complete blockade of P. aeruginosa internalization, but not adhesion, upon inhibition of Src-like tyrosine kinases. Likewise, inhibition of P. aeruginosa binding to CFTR, which has been shown to block P. aeruginosa internalization, prevents Src and Fyn activation, supporting a pivotal role of Src-like tyrosine kinases for invasion by P. aeruginosa.

Inhibition by diallyl trisulfide, a garlic component, of intracellular Ca(2+) mobilization without affecting inositol-1,4, 5-trisphosphate (IP(3)) formation in activated platelets

Garlic has been used in herbal medicine for thousands of years. Some reports have shown that garlic has protective effects against atherosclerosis and inhibits platelet function. In this study, we investigated the mechanism by which diallyl trisulfide (DT), a component of garlic, inhibits platelet function. DT inhibited platelet aggregation and Ca(2+) mobilization in a concentration-dependent manner without increasing intracellular cyclic AMP and cyclic GMP. DT also had no inhibitory effects on thromboxane A(2) (TXA(2)) production in cell-free systems. Collagen-related peptide (CRP)-induced Ca(2+) mobilization is regulated by phospholipase C-gamma2 (PLC-gamma2) activation. We evaluated the effect of DT on tyrosine phosphorylation of PLC-gamma2 and the production of inositol-1,4,5-trisphosphate (IP(3)). DT at concentrations that inhibited platelet aggregation and Ca(2+) mobilization had no effects on tyrosine phosphorylation of PLC-gamma2 or on the formation of IP(3) induced by CRP. Similar results were obtained with thrombin-induced platelet activation. DT inhibited platelet aggregation and Ca(2+) mobilization induced by thrombin without affecting the production of IP(3.) We then evaluated the effect of DT on the binding of IP(3) to its receptor. DT at high concentrations partially blocked the binding of IP(3) to its receptor. Taken together, our findings suggest that the agent suppresses Ca(2+) mobilization at a step distal to IP(3) formation. DT may provide a good tool for investigating Ca(2+) mobilization.

Interaction of linker for activation of T cells with multiple adapter proteins in platelets activated by the glycoprotein VI-selective ligand, convulxin

The snake venom toxin convulxin activates platelets through the collagen receptor glycoprotein VI (GPVI)/Fc receptor gamma-chain (FcR gamma-chain) complex leading to tyrosine phosphorylation and activation of the tyrosine Syk and phospholipase Cgamma2 (PLCgamma2). In the present study, we demonstrate that convulxin is a considerably more powerful agonist than collagen or the GPVI-selective collagen-related peptide (CRP). Confirmation that the response to convulxin is mediated solely via Syk was provided by studies on Syk-deficient platelets. The increase in phosphorylation of the FcR gamma-chain is associated with marked increases in tyrosine phosphorylation of downstream proteins including Syk, linker for activation of T cells (LAT), SLP-76, and PLCgamma2. The transmembrane adapter LAT coprecipitates with SLP-76 and PLCgamma2, as well as with a number of other adapter proteins, some of which have not been previously described in platelets, including Cbl, Grb2, Gads, and SKAP-HOM. Gads is constitutively associated with SLP-76 and is probably the protein bridging its association with LAT. There was no detectable association between Grb2 and SLP-76 in control or stimulated cells, suggesting that the interaction of LAT with Grb2 is present in a separate complex to that of LAT-Gads-SLP-76. These results show that the trimeric convulxin stimulates a much greater phosphorylation of the FcR gamma-chain and subsequent downstream responses relative to CRP and collagen, presumably because of its ability to cause a greater degree of cross-linking of GPVI. The adapter LAT appears to play a critical role in recruiting a number of other adapter proteins to the surface membrane in response to activation of GPVI, presumably at sites of glycolipid-enriched microdomains, enabling an organized signaling cascade that leads to platelet activation.

Expression of the collagen receptor glycoprotein VI during megakaryocyte differentiation

This study examined the expression of the platelet collagen receptor glycoprotein VI (GPVI) in megakaryocyte cell lines and primary megakaryocytes by reverse transcriptase-polymerase chain reaction and by flow cytometry and ligand blotting using the snake venom toxin convulxin. Expression of GPVI is increased in the megakaryoblastic cell lines HEL and CMK on differentiation with the phorbol ester phorbol 12-myristate 13-acetate (PMA), along with the Fc receptor γ-chain (FcR γ-chain). The increase in GPVI expression is associated with marked potentiation of tyrosine phosphorylation and Ca++ elevation in response to convulxin. Syk, linker for activated T cells, and phospholipase Cγ2 (PLCγ2) are among the proteins tyrosine phosphorylated on convulxin stimulation in PMA-differentiated HEL cells. Studies on primary murine megakaryocytes grown in vitro confirmed that GPVI is up-regulated in parallel with functional activation, assessed by measurement of [Ca++]i, during differentiation. The results demonstrate that expression of GPVI is up-regulated along with the FcR γ-chain during differentiation of megakaryocytes.

Expression of the collagen receptor glycoprotein VI during megakaryocyte differentiation

This study examined the expression of the platelet collagen receptor glycoprotein VI (GPVI) in megakaryocyte cell lines and primary megakaryocytes by reverse transcriptase-polymerase chain reaction and by flow cytometry and ligand blotting using the snake venom toxin convulxin. Expression of GPVI is increased in the megakaryoblastic cell lines HEL and CMK on differentiation with the phorbol ester phorbol 12-myristate 13-acetate (PMA), along with the Fc receptor γ-chain (FcR γ-chain). The increase in GPVI expression is associated with marked potentiation of tyrosine phosphorylation and Ca++ elevation in response to convulxin. Syk, linker for activated T cells, and phospholipase Cγ2 (PLCγ2) are among the proteins tyrosine phosphorylated on convulxin stimulation in PMA-differentiated HEL cells. Studies on primary murine megakaryocytes grown in vitro confirmed that GPVI is up-regulated in parallel with functional activation, assessed by measurement of [Ca++]i, during differentiation. The results demonstrate that expression of GPVI is up-regulated along with the FcR γ-chain during differentiation of megakaryocytes.

Structure, function, and control of phosphoinositide-specific phospholipase C

Phosphoinositide-specific phospholipase C (PLC) subtypes beta, gamma, and delta comprise a related group of multidomain phosphodiesterases that cleave the polar head groups from inositol lipids. Activated by all classes of cell surface receptor, these enzymes generate the ubiquitous second messengers inositol 1,4, 5-trisphosphate and diacylglycerol. The last 5 years have seen remarkable advances in our understanding of the molecular and biological facets of PLCs. New insights into their multidomain arrangement and catalytic mechanism have been gained from crystallographic studies of PLC-delta(1), while new modes of controlling PLC activity have been uncovered in cellular studies. Most notable is the realization that PLC-beta, -gamma, and -delta isoforms act in concert, each contributing to a specific aspect of the cellular response. Clues to their true biological roles were also obtained. Long assumed to function broadly in calcium-regulated processes, genetic studies in yeast, slime molds, plants, flies, and mammals point to specific and conditional roles for each PLC isoform in cell signaling and development. In this review we consider each subtype of PLC in organisms ranging from yeast to mammals and discuss their molecular regulation and biological function.

Sperm extract injection into ascidian eggs signals Ca(2+) release by the same pathway as fertilization

Injection of eggs of various species with an extract of sperm cytoplasm stimulates intracellular Ca(2+) release that is spatially and temporally like that occurring at fertilization, suggesting that Ca(2+) release at fertilization may be initiated by a soluble factor from the sperm. Here we investigate whether the signalling pathway that leads to Ca(2+) release in response to sperm extract injection requires the same signal transduction molecules as are required at fertilization. Eggs of the ascidian Ciona intestinalis were injected with the Src-homology 2 domains of phospholipase C gamma or of the Src family kinase Fyn (which act as specific dominant negative inhibitors of the activation of these enzymes), and the effects on Ca(2+) release at fertilization or in response to injection of a sperm extract were compared. Our findings indicate that both fertilization and sperm extract injection initiate Ca(2+) release by a pathway requiring phospholipase C gamma and a Src family kinase. These results support the hypothesis that, in ascidians, a soluble factor from the sperm cytoplasm initiates Ca(2+) release at fertilization, and indicate that the activating factor from the sperm may be a regulator, directly or indirectly, of a Src family kinase in the egg.

Phosphatidylinositol 3,4,5-trisphosphate regulates Ca(2+) entry via btk in platelets and megakaryocytes without increasing phospholipase C activity

The role of phosphatidylinositol 3,4,5-trisphosphate (PI3,4,5P(3)) and Btk in signalling by the collagen receptor glycoprotein VI was investigated. PI3,4,5P(3) was increased in platelets from mice deficient in the SH2 domain-containing inositol 5-phosphatase (SHIP), in response to collagen related peptide (CRP). Tyrosine phosphorylation and activation of phospholipase Cgamma2 (PLCgamma2) were unaltered in SHIP(-/-) platelets, whereas Btk was heavily tyrosine phosphorylated under basal conditions and maximally phosphorylated by low concentrations of CRP. There was an increase in basal Ca(2+), maximal expression of P-selectin, and potentiation of Ca(2+) and aminophospholipid exposure to CRP in SHIP(-/-) platelets in the presence of Ca(2+) (1 mM). Microinjection of PI3,4, 5P(3) into megakaryocytes caused a 3-fold increase in Ca(2+) in response to CRP, which was absent in X-linked immunodeficiency (Xid) mice, which have a mutation in the PH domain of Btk. There was a corresponding partial reduction in the sustained level of intracellular Ca(2+) in response to CRP in Xid mice but no change in PLC activity. These results demonstrate a novel pathway of Ca(2+) entry that involves PI3,4,5P(3) and Btk, and which is independent of increased PLC activity.

Evidence that fertilization activates starfish eggs by sequential activation of a Src-like kinase and phospholipase cgamma

Recent evidence has indicated a requirement for a Src family kinase in initiating Ca(2+) release at fertilization in starfish eggs (Giusti, A. F., Carroll, D. J., Abassi, Y. A., Terasaki, M., Foltz, K. R., and Jaffe, L. A. (1999) J. Biol. Chem. 274, 29318-29322). We now show that injection of Src protein into starfish eggs initiates Ca(2+) release and DNA synthesis, as occur at fertilization. These responses depend on the phosphorylation state of the Src protein; only the kinase active form is effective. Like Ca(2+) release at fertilization, the Ca(2+) release in response to Src protein injection is inhibited by prior injection of the SH2 domains of phospholipase Cgamma. These findings support the conclusion that in starfish, sperm-egg interaction causes egg activation by sequential activation of a Src-like kinase and phospholipase Cgamma. Injection of the SH2 domain of Src, which inhibits Ca(2+) release at fertilization, does not inhibit Ca(2+) release caused by Src protein injection. This indicates that the requirement for a Src SH2 domain interaction is upstream of Src activation in the pathway leading to Ca(2+) release at fertilization.

Requirement of a Src family kinase for initiating calcium release at fertilization in starfish eggs

Signal transduction leading to calcium release in echinoderm eggs at fertilization requires phospholipase Cgamma-mediated production of inositol trisphosphate (IP(3)), indicating that a tyrosine kinase is a likely upstream regulator. Because previous work has shown a fertilization-dependent association between the Src homology 2 (SH2) domains of phospholipase Cgamma and a Src family kinase, we examined whether a Src family kinase was required for Ca(2+) release at fertilization. To inhibit the function of kinases in this family, we injected starfish eggs with the SH2 domains of Src and Fyn kinases. This inhibited Ca(2+) release in response to fertilization but not in response to injection of IP(3). We further established the specificity of the inhibition by showing that the SH2 domains of several other tyrosine kinases (Abl, Syk, and ZAP-70), and the SH3 domain of Src, were not inhibitory. Also, a point-mutated Src SH2 domain, which has reduced affinity for phosphotyrosine, was a correspondingly less effective inhibitor of fertilization-induced Ca(2+) release. These results indicate that a Src family kinase, by way of its SH2 domain, links sperm-egg interaction to IP(3)-mediated Ca(2+) release at fertilization in starfish eggs.

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.

Collagen Mediates Changes in Intracellular Calcium in Primary Mouse Megakaryocytes Through syk-Dependent and -Independent Pathways

We have characterized changes in [Ca2+]iin primary mouse megakaryocytes in response to fibrillar collagen and in response to cross-linking of the collagen receptor, the integrin 2β1. The response to collagen was markedly different from that seen to a triple helical collagen-related peptide (CRP), which signals via the tyrosine kinases p59fyn and p72syk. This peptide binds to the collagen receptor glycoprotein VI (GPVI), but not to the integrin 2β1. 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 (∼30%) inhibited by the src-family kinase inhibitor PP1 (10 μmol/L) or by microinjection of the tandem SH2 domains of p72syk. Collagen also caused an increase in [Ca2+]i in megakaryocytes deficient in either p59fyn or p72syk, although the response was reduced by approximately 40% in both cases: Cross-linking of the 2 integrin increased [Ca2+]iin 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 2β1, which stimulates a substantial influx of extracellular Ca2+.

Collagen Mediates Changes in Intracellular Calcium in Primary Mouse Megakaryocytes Through syk-Dependent and -Independent Pathways

We have characterized changes in [Ca2+]iin primary mouse megakaryocytes in response to fibrillar collagen and in response to cross-linking of the collagen receptor, the integrin 2β1. The response to collagen was markedly different from that seen to a triple helical collagen-related peptide (CRP), which signals via the tyrosine kinases p59fyn and p72syk. This peptide binds to the collagen receptor glycoprotein VI (GPVI), but not to the integrin 2β1. 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 (∼30%) inhibited by the src-family kinase inhibitor PP1 (10 μmol/L) or by microinjection of the tandem SH2 domains of p72syk. Collagen also caused an increase in [Ca2+]i in megakaryocytes deficient in either p59fyn or p72syk, although the response was reduced by approximately 40% in both cases: Cross-linking of the 2 integrin increased [Ca2+]iin 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 2β1, which stimulates a substantial influx of extracellular Ca2+.

Signal transduction pathways mediated by glycoprotein Ia/IIa in human platelets: comparison with those of glycoprotein VI

Human platelets were activated either by glycoprotein (GP) Ia/IIa agonist (rhodocytin) or by a GPVI agonist (collagen-related peptide, CRP), and the intracellular signal transduction pathways were compared in the presence of various inhibitors. Rhodocytin isolated from Calloselasma rhodostoma venom was verified as a GPIa/IIa agonist, based on the inhibitory effects of three mAbs directed against GPIa. Platelet activation mediated by GPIa/IIa led to overt platelet aggregation, elevation of intracellular Ca2+, and tyrosine phosphorylation of several proteins, similar to that of GPVI. p72(syk) and phospholipase Cgamma2 (PLCgamma2) tyrosine phosphorylation were also observed with GPIa/IIa-mediated platelet aggregation, although they peaked slightly later than that of GPVI. In contrast to GPVI-mediated platelet activation, most of these phenomena induced by GPIa/IIa activation were markedly suppressed by acetylsalicylic acid (ASA) or cytochalasin D. These findings suggest that the requirements for thromboxane A2 (TXA2) production and actin polymerization, which are the characteristics of collagen-induced platelet activation, are derived from the GPIa/IIa-mediated signal transduction, but not from that of GPVI.

Alkalinization-induced K+ current of the mouse megakaryocyte

We have recently found that mouse megakaryocytes responded to extracellular alkalinization to pH > 8.0, generating a K+ current under voltage-clamped conditions with the whole cell recording mode of the patch-clamp technique. The purpose of this study was to physiologically and pharmacologically characterize the alkaline-dependent K+ conductance of the megakaryocyte membrane. The alkalinization-induced K+ current (I(ALK)) did not seem to be Ca2+-dependent since I(ALK) was allowed to be generated under intracellularly Ca2+-buffered conditions with 10 mM EGTA, which completely prevented the generation of caffeine-induced Ca2+-activated currents of mouse megakaryocytes; and no [Ca2+]i elevation was evoked by the alkalinization protocol in contrast to a significant increase in [Ca2+]i in response to caffeine when [Ca2+]i was measured with a fura 2 ratiometry. I(ALK) was strongly suppressed with tetraethylammonium (TEA), 4-aminopyridine (4-AP) and streptomycin (SM), but was completely resistant to quinidine (QND). The values of IC50 for the suppression of I(ALK) with TEA, 4-AP and SM were 5.6, 0.47 and 1.5 mM, respectively. Voltage-gated K+ currents (I(K)) of the same megakaryocyte preparation were weakly suppressed with TEA and 4-AP, while they were significantly suppressed with either SM or QND. These results suggest that mouse megakaryocytes possess K+ conductance that was activated by extracellular alkalinization and that probably differs from conventional K+ conductance in its pharmacological properties.

Interaction Between α5β1 Integrin and Secreted Fibronectin Is Involved in Macrophage Differentiation of Human HL-60 Myeloid Leukemia Cells

We examined the role of fibronectin (FN) and FN-binding integrins in macrophage differentiation. Increased FN and α5β1 integrin gene expression was observed in phorbol 12-myristate 13-acetate PMA-treated HL-60 cells and PMA- or macrophage-CSF-treated blood monocytes before the manifestation of macrophage markers. After treatment of HL-60 cells and monocytes, newly synthesized FN was released and deposited on the dishes. An HL-60 cell variant, HL-525, which is deficient in the protein kinase Cβ (PKC-β) and resistant to PMA-induced differentiation, failed to express FN after PMA treatment. Transfecting HL-525 cells with a PKC-β expression plasmid restored PMA-induced FN gene expression and macrophage differentiation. Untreated HL-525 cells (which have a high level of the α5β1 integrin) incubated on FN differentiated into macrophages. The percentage of cells having a macrophage phenotype induced by PMA in HL-60 cells, by FN in HL-525 cells, or by either PMA or macrophage-CSF in monocytes was reduced in the presence of mAbs to FN and α5β1 integrin. The integrin-signaling nonreceptor tyrosine kinase, p72Syk, was activated in PMA-treated HL-60 and FN-treated HL-525 cells. We suggest that macrophage differentiation involves the activation of PKC-β and expression of extracellular matrix proteins such as FN and the corresponding integrins, α5β1 integrin in particular. The stimulated cells, through the integrins, attach to substrates by binding to the deposited FN. This attachment, in turn, may through integrin signaling activate nonreceptor tyrosine kinases, including p72Syk, and later lead to expression of other genes involved in evoking the macrophage phenotype.

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.

Role of tyrosine phosphorylation in ligand-induced regulation of transcytosis of the polymeric Ig receptor

The polymeric Ig receptor (pIgR) transcytoses its ligand, dimeric IgA (dIgA), from the basolateral to the apical surface of epithelial cells. Although the pIgR is constitutively transcytosed in the absence of ligand, binding of dIgA stimulates transcytosis of the pIgR. We recently reported that dIgA binding to the pIgR induces translocation of protein kinase C, production of inositol triphosphate, and elevation of intracellular free calcium. We now report that dIgA binding causes rapid, transient tyrosine phosphorylation of several proteins, including phosphatidyl inositol-specific phospholipase C-gammal. Protein tyrosine kinase inhibitors or deletion of the last 30 amino acids of pIgR cytoplasmic tail prevents IgA-stimulated protein tyrosine kinase activation, tyrosine phosphorylation of phospholipase C-gammal, production of inositol triphosphate, and the stimulation of transcytosis by dIgA. Analysis of pIgR deletion mutants reveals that the same discrete portion of the cytoplasmic domain, residues 727-736 (but not the Tyr734), controls both the ability of pIgR to cause dIgA-induced tyrosine phosphorylation of the phospholipase C-gammal and to undergo dIgA-stimulated transcytosis. In addition, dIgA transcytosis can be strongly stimulated by mimicking phospholipase C-gammal activation. In combination with our previous results, we conclude that the protein tyrosine kinase(s) and phospholipase C-gammal that are activated upon dIgA binding to the pIgR control dIgA-stimulated pIgR transcytosis.