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

Biochemical Pathways Delivering Distinct Glycosphingolipid Patterns in MDA-MB-231 and MCF-7 Breast Cancer Cells

The complex structure of glycosphingolipids (GSLs) supports their important role in cell function as modulators of growth factor receptors and glutamine transporters in plasma membranes. The aberrant composition of clustered GSLs within signaling platforms, so-called lipid rafts, inevitably leads to tumorigenesis due to disturbed growth factor signal transduction and excessive uptake of glutamine and other molecules needed for increased energy and structural molecule cell supply. GSLs are also involved in plasma membrane processes such as cell adhesion, and their transition converts cells from epithelial to mesenchymal with features required for cell migration and metastasis. Glutamine activates the mechanistic target of rapamycin complex 1 (mTORC1), resulting in nucleotide synthesis and proliferation. In addition, glutamine contributes to the cancer stem cell GD2 ganglioside-positive phenotype in the triple-negative breast cancer cell line MDA-MB-231. Thieno[2,3-b]pyridine derivative possesses higher cytotoxicity against MDA-MB-231 than against MCF-7 cells and induces a shift to aerobic metabolism and a decrease in S(6)nLc4Cer GSL-positive cancer stem cells in the MDA-MB-231 cell line. In this review, we discuss findings in MDA-MB-231, MCF-7, and other breast cancer cell lines concerning their differences in growth factor receptors and recent knowledge of the main biochemical pathways delivering distinct glycosphingolipid patterns during tumorigenesis and therapy.

Phosphoinositides take a central stage in regulating blood platelet production and function

Blood platelets are produced by megakaryocytes through a complex program of differentiation and play a critical role in hemostasis and thrombosis. These anucleate cells are the target of antithrombotic drugs that prevent them from clumping in cardiovascular disease conditions. Platelets also significantly contribute to various aspects of physiopathology, including interorgan communications, healing, inflammation, and thromboinflammation. Their production and activation are strictly regulated by highly elaborated mechanisms. Among them, those involving inositol lipids have drawn the attention of researchers. Phosphoinositides represent the seven combinatorially phosphorylated forms of the inositol head group of inositol lipids. They play a crucial role in regulating intracellular mechanisms, such as signal transduction, actin cytoskeleton rearrangements, and membrane trafficking, either by generating second messengers or by directly binding to specific domains of effector proteins. In this review, we will explore how phosphoinositides are implicated in controlling platelet production by megakaryocytes and in platelet activation processes. We will also discuss the diversity of phosphoinositides in platelets, their role in granule biogenesis and maintenance, as well as in integrin signaling. Finally, we will address the discovery of a novel pool of phosphatidylinositol 3-monophosphate in the outerleaflet of the plasma membrane of human and mouse platelets.

PI3K Isoform Signalling in Platelets

Platelets are unique anucleated blood cells that constantly patrol the vasculature to seal and prevent injuries in a process termed haemostasis. Thereby they rapidly adhere to the subendothelial matrix and recruit further platelets, resulting in platelet aggregates. Apart from their central role in haemostasis, they also kept some of their features inherited by their evolutionary ancestor-the haemocyte, which was also involved in immune defences. Together with leukocytes, platelets fight pathogenic invaders and guide many immune processes. In addition, they rely on several signalling pathways which are also relevant to immune cells. Among these, one of the central signalling hubs is the PI3K pathway. Signalling processes in platelets are unique as they lack a nucleus and therefore transcriptional regulation is absent. As a result, PI3K subclasses fulfil distinct roles in platelets compared to other cells. In contrast to leukocytes, the central PI3K subclass in platelet signalling is PI3K class Iβ, which underlines the uniqueness of this cell type and opens new ways for potential platelet-specific pharmacologic inhibition. An overview of platelet function and signalling with emphasis on PI3K subclasses and their respective inhibitors is given in this chapter.

Abivertinib inhibits megakaryocyte differentiation and platelet biogenesis

Abivertinib, a third-generation tyrosine kinase inhibitor, is originally designed to target epidermal growth factor receptor (EGFR)-activating mutations. Previous studies have shown that abivertinib has promising antitumor activity and a well-tolerated safety profile in patients with non-small-cell lung cancer. However, abivertinib also exhibited high inhibitory activity against Bruton's tyrosine kinase and Janus kinase 3. Given that these kinases play some roles in the progression of megakaryopoiesis, we speculate that abivertinib can affect megakaryocyte (MK) differentiation and platelet biogenesis. We treated cord blood CD34⁺ hematopoietic stem cells, Meg-01 cells, and C57BL/6 mice with abivertinib and observed megakaryopoiesis to determine the biological effect of abivertinib on MK differentiation and platelet biogenesis. Our in vitro results showed that abivertinib impaired the CFU-MK formation, proliferation of CD34⁺ HSC-derived MK progenitor cells, and differentiation and functions of MKs and inhibited Meg-01-derived MK differentiation. These results suggested that megakaryopoiesis was inhibited by abivertinib. We also demonstrated in vivo that abivertinib decreased the number of MKs in bone marrow and platelet counts in mice, which suggested that thrombopoiesis was also inhibited. Thus, these preclinical data collectively suggested that abivertinib could inhibit MK differentiation and platelet biogenesis and might be an agent for thrombocythemia.

Comparison of inhibitory effects of irreversible and reversible Btk inhibitors on platelet function

All irreversible Bruton tyrosine kinase (Btk) inhibitors including ibrutinib and acalabrutinib induce platelet dysfunction and increased bleeding risk. New reversible Btk inhibitors were developed, like MK-1026. The mechanism underlying increased bleeding tendency with Btk inhibitors remains unclear. We investigated the effects of ibrutinib, acalabrutinib and MK-1026 on platelet function in healthy volunteers, patients and Btk-deficient mice, together with off-target effects on tyrosine kinase phosphorylation. All inhibitors suppressed GPVI- and CLEC-2-mediated platelet aggregation, activation and secretion in a dose-dependent manner. Only ibrutinib inhibited thrombus formation on vWF-co-coated surfaces, while on collagen this was not affected. In blood from Btk-deficient mice, collagen-induced thrombus formation under flow was reduced, but preincubation with either inhibitor was without additional effects. MK-1026 showed less off-target effects upon GPVI-induced TK phosphorylation as compared to ibrutinib and acalabrutinib. In ibrutinib-treated patients, GPVI-stimulated platelet activation, and adhesion on vWF-co-coated surfaces were inhibited, while CLEC-2 stimulation induced variable responses. The dual inhibition of GPVI and CLEC-2 signalling by Btk inhibitors might account for the increased bleeding tendency, with ibrutinib causing more high-grade bleedings due to additional inhibition of platelet-vWF interaction. As MK-1026 showed less off-target effects and only affected activation of isolated platelets, it might be promising for future treatment.

Bleeding by Bruton Tyrosine Kinase-Inhibitors: Dependency on Drug Type and Disease

Bruton tyrosine kinase (Btk) is expressed in B-lymphocytes, myeloid cells and platelets, and Btk-inhibitors (BTKi) are used to treat patients with B-cell malignancies, developed against autoimmune diseases, have been proposed as novel antithrombotic drugs, and been tested in patients with severe COVID-19. However, mild bleeding is frequent in patients with B-cell malignancies treated with the irreversible BTKi ibrutinib and the recently approved 2nd generation BTKi acalabrutinib, zanubrutinib and tirabrutinib, and also in volunteers receiving in a phase-1 study the novel irreversible BTKi BI-705564. In contrast, no bleeding has been reported in clinical trials of other BTKi. These include the brain-penetrant irreversible tolebrutinib and evobrutinib (against multiple sclerosis), the irreversible branebrutinib, the reversible BMS-986142 and fenebrutinib (targeting rheumatoid arthritis and lupus erythematodes), and the reversible covalent rilzabrutinib (against pemphigus and immune thrombocytopenia). Remibrutinib, a novel highly selective covalent BTKi, is currently in clinical studies of autoimmune dermatological disorders. This review describes twelve BTKi approved or in clinical trials. By focusing on their pharmacological properties, targeted disease, bleeding side effects and actions on platelets it attempts to clarify the mechanisms underlying bleeding. Specific platelet function tests in blood might help to estimate the probability of bleeding of newly developed BTKi.

Inhibition of Bruton tyrosine kinase in patients with severe COVID-19

Patients with severe COVID-19 have a hyperinflammatory immune response suggestive of macrophage activation. Bruton tyrosine kinase (BTK) regulates macrophage signaling and activation. Acalabrutinib, a selective BTK inhibitor, was administered off-label to 19 patients hospitalized with severe COVID-19 (11 on supplemental oxygen; 8 on mechanical ventilation), 18 of whom had increasing oxygen requirements at baseline. Over a 10-14 day treatment course, acalabrutinib improved oxygenation in a majority of patients, often within 1-3 days, and had no discernable toxicity. Measures of inflammation - C-reactive protein and IL-6 - normalized quickly in most patients, as did lymphopenia, in correlation with improved oxygenation. At the end of acalabrutinib treatment, 8/11 (72.7%) patients in the supplemental oxygen cohort had been discharged on room air, and 4/8 (50%) patients in the mechanical ventilation cohort had been successfully extubated, with 2/8 (25%) discharged on room air. Ex vivo analysis revealed significantly elevated BTK activity, as evidenced by autophosphorylation, and increased IL-6 production in blood monocytes from patients with severe COVID-19 compared with blood monocytes from healthy volunteers. These results suggest that targeting excessive host inflammation with a BTK inhibitor is a therapeutic strategy in severe COVID-19 and has led to a confirmatory international prospective randomized controlled clinical trial.

Ca2+-activated cleavage of ezrin visualised dynamically in living myeloid cells during cell surface area expansion

The intracellular events underlying phagocytosis, a crucial event for innate immunity, are still unresolved. In order to test whether the reservoir of membrane required for the formation of the phagocytic pseudopodia is maintained by cortical ezrin, and that its cleavage is a key step in releasing this membrane, the cleavage of cortical ezrin was monitored within living phagocytes (the phagocytically competent cell line RAW264.7) through expressing two ezrin constructs with fluorescent protein tags located either inside the FERM or at the actin-binding domains. When ezrin is cleaved in the linker region by the Ca2+-activated protease calpain, separation of the two fluorophores would result. Experimentally induced Ca2+ influx triggered cleavage of peripherally located ezrin, which was temporally associated with cell expansion. Ezrin cleavage was also observed in the phagocytic pseudopodia during phagocytosis. Thus, our data demonstrates that peripheral ezrin is cleaved during Ca2+-influx-induced membrane expansion and locally within the extending pseudopodia during phagocytosis. This is consistent with a role for intact ezrin in maintaining folded membrane on the cell surface, which then becomes available for cell spreading and phagocytosis.

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.

Distinct pathways regulate Syk protein activation downstream of immune tyrosine activation motif (ITAM) and hemITAM receptors in platelets

Tyrosine kinase pathways are known to play an important role in the activation of platelets. In particular, the GPVI and CLEC-2 receptors are known to activate Syk upon tyrosine phosphorylation of an immune tyrosine activation motif (ITAM) and hemITAM, respectively. However, unlike GPVI, the CLEC-2 receptor contains only one tyrosine motif in the intracellular domain. The mechanisms by which this receptor activates Syk are not completely understood. In this study, we identified a novel signaling mechanism in CLEC-2-mediated Syk activation. CLEC-2-mediated, but not GPVI-mediated, platelet activation and Syk phosphorylation were abolished by inhibition of PI3K, which demonstrates that PI3K regulates Syk downstream of CLEC-2. Ibrutinib, a Tec family kinase inhibitor, also completely abolished CLEC-2-mediated aggregation and Syk phosphorylation in human and murine platelets. Furthermore, embryos lacking both Btk and Tec exhibited cutaneous edema associated with blood-filled vessels in a typical lymphatic pattern similar to CLEC-2 or Syk-deficient embryos. Thus, our data show, for the first time, that PI3K and Tec family kinases play a crucial role in the regulation of platelet activation and Syk phosphorylation downstream of the CLEC-2 receptor.

Class I PI 3-kinases signaling in platelet activation and thrombosis: PDK1/Akt/GSK3 axis and impact of PTEN and SHIP1

Class I phosphoinositide 3-kinases (PI3K) have been extensively studied in different models these last years and several isoforms are now promising drug targets to treat cancer and immune diseases. Blood platelets are non-nucleated cells critical for hemostasis and strongly involved in arterial thrombosis, a leading cause of death worldwide. Besides their role in hemostasis and thrombosis, platelets provide an interesting model to characterize the implication of the different isoforms of PI3K in signaling. They are specialized for regulated adhesion, particularly under high shear stress conditions found in arteries and use highly regulated signaling mechanisms to form and stabilize a thrombus. In this review we will highlight the role of class I PI3K in these processes and the pertinence of targeting them in the context of antithrombotic strategies but also the potential consequences on the bleeding risk of inhibiting the PI3K signaling in cancer therapy. The implication of upstream regulators of the most important isoforms of PI3K in platelets and their downstream effectors such as protein kinase B (PKB or Akt) and its target glycogen synthase kinase 3 (GSK3) will be discussed as well as the impact of PTEN and SHIP phosphatases as modulators of this pathway.

New insights into the regulation of neutrophil NADPH oxidase activity in the phagosome: a focus on the role of lipid and Ca(2+) signaling

SIGNIFICANCE

Reactive oxygen species, produced by the phagosomal NADPH oxidase of neutrophils, play a significant physiological role during normal defense. Their role is not only to kill invading pathogens, but also to act as modulators of global physiological functions of phagosomes. Given the importance of NADPH oxidase in the immune system, its activity has to be decisively controlled by distinctive mechanisms to ensure appropriate regulation at the phagosome.

RECENT ADVANCES

Here, we describe the signal transduction pathways that regulate phagosomal NADPH oxidase in neutrophils, with an emphasis on the role of lipid metabolism and intracellular Ca(2+) mobilization.

CRITICAL ISSUES

The potential involvement of Ca(2+)-binding S100A8 and S100A9 proteins, known to interact with the plasma membrane NADPH oxidase, is also considered.

FUTURE DIRECTIONS

Recent technical progress in advanced live imaging microscopy will permit to focus more accurately on phagosomal rather than plasma membrane NADPH oxidase regulation during neutrophil phagocytosis.

Distinct and overlapping functional roles of Src family kinases in mouse platelets

BACKGROUND AND OBJECTIVES

Src family kinases (SFKs) play a critical role in initiating and propagating signals in platelets. The aims of this study were to quantitate SFK members present in platelets and to analyze their contribution to platelet regulation using glycoprotein VI (GPVI) and intregrin αIIbβ3, and in vivo.

METHODS AND RESULTS

Mouse platelets express four SFKs, Fgr, Fyn, Lyn and Src, with Lyn expressed at a considerably higher level than the others. Using mutant mouse models, we demonstrate that platelet activation by collagen-related peptide (CRP) is delayed and then potentiated in the absence of Lyn, but only marginally reduced in the absence of Fyn or Fgr, and unaltered in the absence of Src. Compound deletions of Lyn/Src or Fyn/Lyn, but not of Fyn/Src or Fgr/Lyn, exhibit a greater delay in activation relative to Lyn-deficient platelets. Fibrinogen-adherent platelets show reduced spreading in the absence of Src, potentiation in the absence of Lyn, but no change in the absence of Fyn or Fgr. In mice double-deficient in Lyn/Src or Fgr/Lyn, the inhibitory role of Lyn on spreading on fibrinogen is lost. Lyn is the major SFK-mediating platelet aggregation on collagen at arterial shear and its absence leads to a reduction in thrombus size in a laser injury model.

CONCLUSION

These results demonstrate that SFKs share individual and overlapping roles in regulating platelet activation, with Lyn having a dual role in regulating GPVI signaling and an inhibitory role downstream of αIIbβ3, which requires prior signaling through Src.

PI 3-Kinase p110β regulation of platelet integrin α(IIb)β3

Hemopoietic cells express relatively high levels of the type I phosphoinositide (PI) 3-kinase isoforms, with p110δ and γ exhibiting specialized signaling functions in neutrophils, monocytes, mast cells, and lymphocytes. In platelets, p110β appears to be the dominant PI 3-kinase isoform regulating platelet activation, irrespective of the nature of the primary platelet activating stimulus. Based on findings with isoform-selective p110β pharmacological inhibitors and more recently with p110β-deficient platelets, p110β appears to primarily signal downstream of G(i)- and tyrosine kinase-coupled receptors. Functionally, inhibition of p110β kinase function leads to a marked defect in integrin α(IIb)β₃ adhesion and reduced platelet thrombus formation in vivo. This defect in platelet adhesive function is not associated with increased bleeding, suggesting that therapeutic targeting of p110β may represent a safe approach to reduce thrombotic complications in patients with cardiovascular disease.

Non-redundant roles of phosphoinositide 3-kinase isoforms alpha and beta in glycoprotein VI-induced platelet signaling and thrombus formation

Platelets are activated by adhesion to vascular collagen via the immunoglobulin receptor, glycoprotein VI (GPVI). This causes potent signaling toward activation of phospholipase Cgamma2, which bears similarity to the signaling pathway evoked by T- and B-cell receptors. Phosphoinositide 3-kinase (PI3K) plays an important role in collagen-induced platelet activation, because this activity modulates the autocrine effects of secreted ADP. Here, we identified the PI3K isoforms directly downstream of GPVI in human and mouse platelets and determined their role in GPVI-dependent thrombus formation. The targeting of platelet PI3Kalpha or -beta strongly and selectively suppressed GPVI-induced Ca(2+) mobilization and inositol 1,4,5-triphosphate production, thus demonstrating enhancement of phospholipase Cgamma2 by PI3Kalpha/beta. That PI3Kalpha and -beta have a non-redundant function in GPVI-induced platelet activation and thrombus formation was concluded from measurements of: (i) serine phosphorylation of Akt, (ii) dense granule secretion, (iii) intracellular Ca(2+) increases and surface expression of phosphatidylserine under flow, and (iv) thrombus formation, under conditions where PI3Kalpha/beta was blocked or p85alpha was deficient. In contrast, GPVI-induced platelet activation was insensitive to inhibition or deficiency of PI3Kdelta or -gamma. Furthermore, PI3Kalpha/beta, but not PI3Kgamma, contributed to GPVI-induced Rap1b activation and, surprisingly, also to Rap1b-independent platelet activation via GPVI. Together, these findings demonstrate that both PI3Kalpha and -beta isoforms are required for full GPVI-dependent platelet Ca(2+) signaling and thrombus formation, partly independently of Rap1b. This provides a new mechanistic explanation for the anti-thrombotic effect of PI3K inhibition and makes PI3Kalpha an interesting new target for anti-platelet therapy.

Lyn, PKC-delta, SHIP-1 interactions regulate GPVI-mediated platelet-dense granule secretion

Protein kinase C-delta (PKC-delta) is expressed in platelets and activated downstream of protease-activated receptors (PARs) and glycoprotein VI (GPVI) receptors. We have previously shown that PKC-delta positively regulates PAR-mediated dense granule secretion, whereas it negatively regulates GPVI-mediated dense granule secretion. We further investigated the mechanism of such differential regulation of dense granule release by PKC-delta in platelets. SH2 domain-containing inositol phosphatase-1 (SHIP-1) is phosphorylated on Y1020, a marker for its activation, upon stimulation of human platelets with PAR agonists SFLLRN and AYPGKF or GPVI agonist convulxin. GPVI-mediated SHIP-1 phosphorylation occurred rapidly at 15 seconds, whereas PAR-mediated phosphorylation was delayed, occurring at 1 minute. Lyn and SHIP-1, but not SHIP-2 or Shc, preferentially associated with PKC-delta on stimulation of platelets with a GPVI agonist, but not with a PAR agonist. In PKC-delta-null murine platelets, convulxin-induced SHIP-1 phosphorylation was inhibited. Furthermore, in Lyn null murine platelets, GPVI-mediated phosphorylations on Y-1020 of SHIP-1 and Y311 of PKC-delta were inhibited. In murine platelets lacking Lyn or SHIP-1, GPVI-mediated dense granule secretions are potentiated, whereas PAR-mediated dense granule secretions are inhibited. Therefore, we conclude that Lyn-mediated phosphorylations of PKC-delta and SHIP-1 and their associations negatively regulate GPVI-mediated dense granule secretion in platelets.

Protein kinase C[delta] differentially regulates platelet functional responses

OBJECTIVE

Protein Kinase C delta (PKCdelta) is expressed in platelets and activated downstream of protease-activated receptors (PAR)s and glycoprotein VI (GPVI) receptors. The purpose of this study was to investigate the role of PKCdelta in platelets.

METHODS AND RESULTS

We evaluated the role of PKCdelta in platelets using two approaches--pharmacological and molecular genetic approach. In human platelets pretreated with isoform selective antagonistic RACK peptide (delta V1-1)TAT, and in the murine platelets lacking PKCdelta, PAR4-mediated dense granule secretion was inhibited, whereas GPVI-mediated dense granule secretion was potentiated. These effects were statistically significant in the absence and presence of thromboxane A2 (TXA2). Furthermore, TXA2 generation was differentially regulated by PKCdelta. However, PKCdelta had a small effect on platelet P-selectin expression. Calcium- and PKC-dependent pathways independently activate fibrinogen receptor in platelets. When calcium pathways are blocked by dimethyl-BAPTA, AYPGKF-induced aggregation in PKCdelta null mouse platelets and in human platelets pretreated with (delta V1-1)TAT, was inhibited. In a FeCl3-induced injury in vivo thrombosis model, PKCdelta-/- mice occluded similar to their wild-type littermates.

CONCLUSIONS

Hence, we conclude that PKCdelta differentially regulates platelet functional responses such as dense granule secretion and TXA2 generation downstream of PARs and GPVI receptors, but PKCdelta deficiency does not affect the thrombus formation in vivo.

Regulation of superoxide production in neutrophils: role of calcium influx

Upon stimulation, activation of NADPH oxidase complexes in neutrophils produces a burst of superoxide anions contributing to oxidative stress and the development of inflammatory process. Store-operated calcium entry (SOCE), whereby the depletion of intracellular stores induces extracellular calcium influx, is known to be a crucial element of NADPH oxidase regulation. However, the mechanistic basis mediating SOCE is still only partially understood, as is the signal-coupling pathway leading to modulation of store-operated channels. This review emphasizes the role of calcium influx in the control of the NADPH oxidase and summarizes the current knowledge of pathways mediating this extracellular calcium entry in neutrophils. Such investigations into the cross-talk between NADPH oxidase and calcium might allow the identification of novel pharmacological targets with clinical use, particularly in inflammatory diseases.

The docking properties of SHIP2 influence both JIP1 tyrosine phosphorylation and JNK activity

SHIP2 (SH2-containing inositol polyphosphate 5-phosphatase 2) is an ubiquitously expressed phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P(3)) 5-phosphatase which contains various motifs susceptible to mediate protein-protein interaction. In cell models, evidence has been provided that SHIP2 plays a role in insulin and growth factor signaling, cytoskeletal organization, cell adhesion and migration. Herein we describe the c-Jun NH2-terminal kinase (JNK)-interacting protein 1 (JIP1) as a new protein partner of SHIP2. The interaction between SHIP2 and JIP1 was confirmed in both overexpression systems and native cells. Without modifying the association of JIP1 with the MAPKs in the scaffold complex and with no apparent change of Akt phosphorylation, SHIP2 positively modulated the MLK3/JIP1-mediated JNK1 activation. Moreover, SHIP2 positively regulated the tyrosine phosphorylation of JIP1. This up-regulation was prevented by inhibitors of the Src family and Abl kinases, PP2 and Glivec. The effects of SHIP2 on JNK activity and JIP1 tyrosine phosphorylation were independent of the SHIP2 phosphoinositide 5-phosphatase activity, as similar results were obtained when using a SHIP2 catalytic inactive mutant instead of wild-type SHIP2. Together, these data suggest that by its docking properties, SHIP2 can modulate JIP1-mediated JNK pathway signaling.

Deficiency of Src homology 2 domain-containing inositol 5-phosphatase 1 affects platelet responses and thrombus growth

Platelets are critical for normal hemostasis. Their deregulation can lead to bleeding or to arterial thrombosis, a primary cause of heart attack and ischemic stroke. Src homology 2 domain-containing inositol 5-phosphatase 1 (SHIP1) is a 5-phosphatase capable of dephosphorylating the phosphatidylinositol 3,4,5-trisphosphate second messenger into phosphatidylinositol 3,4-bisphosphate. SHIP1 plays a critical role in regulating the level of these 2 lipids in platelets. Using SHIP1-deficient mice, we found that its loss affects platelet aggregation in response to several agonists with minor effects on fibrinogen binding and beta(3) integrin tyrosine phosphorylation. Accordingly, SHIP1-null mice showed defects in arterial thrombus formation in response to a localized laser-induced injury. Moreover, these mice had a prolonged tail bleeding time. Upon stimulation, SHIP1-deficient platelets showed large membrane extensions, abnormalities in the open canalicular system, and a dramatic decrease in close cell-cell contacts. Interestingly, SHIP1 appeared to be required for platelet contractility, thrombus organization, and fibrin clot retraction. These data indicate that SHIP1 is an important element of the platelet signaling machinery to support normal hemostasis. To our knowledge, this is the first report unraveling an important function of SHIP1 in the activation of hematopoietic cells, in contrast to its well-documented role in the negative regulation of lymphocytes.

Vav family proteins are required for optimal regulation of PLCgamma2 by integrin alphaIIbbeta3

Vav proteins belong to the family of guanine-nucleotide-exchange factors for the Rho/Rac family of small G-proteins. In addition, they serve as important adapter proteins for the activation of PLCgamma (phospholipase Cgamma) isoforms by ITAM (immunoreceptor tyrosine-based activation motif) receptors, including the platelet collagen receptor GPVI (glycoprotein VI). Vav proteins are also regulated downstream of integrins, including the major platelet integrin alphaIIbbeta3, which has recently been shown to regulate PLCgamma2. In the present study, we have investigated the role of Vav family proteins in filopodia and lamellipodia formation on fibrinogen using platelets deficient in Vav1 and Vav3. Wild-type mouse platelets undergo a limited degree of spreading on fibrinogen, characterized by the formation of numerous filopodia and limited lamellipodia structures. Platelets deficient in Vav1 and Vav3 exhibit reduced filopodia and lamellipodia formation during spreading on fibrinogen. This is accompanied by reduced alphaIIbbeta3-mediated PLCgamma2 tyrosine phosphorylation and reduced Ca(2+) mobilization. In contrast, the G-protein agonist thrombin stimulates full spreading of control and Vav1/3-deficient platelets. Consistent with this, stimulation of F-actin (filamentous actin) formation and Rac activation by thrombin is not altered in Vav-deficient cells. These results demonstrate that Vav1 and Vav3 are required for optimal spreading and regulation of PLCgamma2 by integrin alphaIIbbeta3, but that their requirement is by-passed upon G-protein receptor activation.

Calmodulin potentiates G beta gamma activation of phospholipase C-beta3

Phospholipase C-beta (PLC-beta) isozymes (EC 3.1.4.11) hydrolyze the membrane phospholipid phosphatidylinositol-4,5-bisphosphate to generate intracellular second messenger signaling molecules inositol-1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) in response to receptor activation and other cellular stimuli. PLCbeta1 and PLCbeta3 isozymes were previously demonstrated to bind the calcium-sensitive molecule calmodulin [McCullar JS, Larsen SA, Millimaki RA, Filtz TM. Calmodulin is a phospholipase C-{beta} interacting protein. J Biol Chem 2003;278(36):33708-13]. We have now shown through fluorescence anisotropy that calmodulin/PLCbeta3 affinities increase with increasing calcium in a physiologically relevant concentration range. The bimolecular affinity constants for calmodulin interaction with PLCbeta1 or PLCbeta3 were estimated as 260 and 200 nM, respectively, from fluorescence anisotropy data. There was no effect of calmodulin on basal or G alpha q-stimulated catalytic activity for either isozyme. However, the interaction between calmodulin and PLCbeta3 leads to potentiation of activation by the G-protein beta gamma dimer in an in vitro assay. 1321N1 cells treated with calmodulin inhibitors concurrent with and post-stimulation of muscarinic receptors significantly reduced [3H]PIP hydrolysis. Together these data are suggestive of cooperative role for calmodulin in the G-protein beta gamma dimer-stimulated activity of PLCbeta3.

Molecular and electrophysiological characterization of transient receptor potential ion channels in the primary murine megakaryocyte

The molecular identity of platelet Ca(2+) entry pathways is controversial. Furthermore, the extent to which Ca(2+)-permeable ion channels are functional in these tiny, anucleate cells is difficult to assess by direct electrophysiological measurements. Recent work has highlighted how the primary megakaryocyte represents a bona fide surrogate for studies of platelet signalling, including patch clamp recordings of ionic conductances. We have now screened for all known members of the transient receptor potential (TRP) family of non-selective cation channels in murine megakaryocytes following individual selection of these rare marrow cells using glass micropipettes. RT-PCR detected messages for TRPC6 and TRPC1, which have been reported in platelets and megakaryocytic cell lines, and TRPM1, TRPM2 and TRPM7, which to date have not been demonstrated in cells of megakaryocytic/platelet lineage. Electrophysiological recordings demonstrated the presence of functional TRPM7, a constitutively active cation channel sensitive to intracellular Mg(2+), and TRPM2, an ADP-ribose-dependent cation channel activated by oxidative stress. In addition, the electrophysiological and pharmacological properties of the non-selective cation channels stimulated by the physiological agonist ADP are consistent with a major role for TRPC6 in this G-protein-coupled receptor-dependent Ca(2+) influx pathway. This study defines for the first time the principal TRP channels within the primary megakaryocyte, which represent candidates for Ca(2+) influx pathways activated by a diverse range of stimuli in the platelet and megakaryocyte.

CCL5 evokes calcium signals in microglia through a kinase-, phosphoinositide-, and nucleotide-dependent mechanism

Microglia, the resident macrophages of the CNS, are responsible for the innate immune response in the brain and participate in the pathogenesis of certain neurodegenerative disorders. Chemokines initiate activation and migration of microglia. The beta-chemokine CCL5 induces an elevation in intracellular calcium concentration ([Ca(2+)](i)) in human microglia. Here, we examined the signal transduction pathway linking activation of chemokine receptor CCR5 to an elevation in [Ca(2+)](i) in cultured microglia by using pharmacological approaches in combination with Fura-2-based digital imaging. The CCL5-induced response required Janus kinase (Jak) activity and the stimulation of an inhibitory G protein. Multiple downstream signaling pathways were involved, including phosphatidylinositol 3-kinase (PI3K), Bruton's tyrosine kinase (Btk), and phospholipase C (PLC)-mediated release of Ca(2+) from inositol 1,4,5-trisphosphate (IP(3))-sensitive stores. Activation of both the kinase and the lipase pathways was required for eliciting the Ca(2+) response. However, the majority of the [Ca(2+)](i) increase was derived from sources activated by NAD metabolites. Cyclic ADP-ribose (cADPR) evoked Ca(2+) release from intracellular stores, and ADPR evoked Ca(2+) influx via a nimodipine-sensitive channel. Thus, a multistep cascade couples CCR5 activation to Ca(2+) increases in human microglia. Because changes in [Ca(2+)](i) affect chemotaxis, secretion, and gene expression, pharmacologic modulation of this pathway may alter inflammatory and degenerative processes in the CNS.

Localised PtdIns(3,4,5)P3 or PtdIns(3,4)P2 at the phagocytic cup is required for both phagosome closure and Ca2+ signalling in HL60 neutrophils

Several events accompany integrin-mediated phagocytosis by myeloid cells. These include local pseudopod and phagocytic cup formation followed by Ca2+ signalling. However, there is also a role for localised phosphatidylinositol (3,4,5) trisphosphate [PtdIns(3,4,5)P3] production. Here we report that in neutrophilic HL-60 cells expressing PH-Akt-GFP, binding of iC3b-coated zymosan particles (2 μm in diameter) via β2 integrin induces an incomplete phagocytic cup to form before either PtdIns(3,4,5)P3 or phosphatidylinositol (3,4) bisphosphate [PtdIns(3,4)P2] production or Ca2+ signalling. These phosphoinositides then accumulated locally at the site of the phagocytic cup and Ca2+ signalling and phagosome closure follows immediately. Although photobleaching showed that PH-Akt-GFP was freely diffusible in the cytosol and able to dissociate from the phagocytic cup, it was restricted to the plasma membrane of the formed but open phagosome and failed to diffuse into the surrounding plasma membrane or neighbouring phagocytic cups even if connected. Inhibition of phosphoinositide (PI) 3-kinase or depletion of membrane cholesterol inhibited both Ca2+ signalling and phagosome closure, but had no effect on particle binding or phagocytic cup formation. We therefore conclude that PtdIns(3,4,5)P3 or PtdIns(3,4)P2 generation was not required for the events that initiate the formation of the phagocytic cup, but that anchoring of PtdIns(3,4,5)P3 at the phagocytic cup is an essential step for phagosome closure and Ca2+ signalling.

The Rab27a-binding protein, JFC1, regulates androgen-dependent secretion of prostate-specific antigen and prostatic-specific acid phosphatase

Two of the major proteins secreted by the prostate epithelium secretory cells are PSA (prostate-specific antigen) and PSAP (prostatic-specific acid phosphatase). The molecules involved in the secretory machinery of PSA and PSAP, and the regulation of this machinery, remain unknown. In the present paper, we provide evidence that JFC1 [synaptotagmin-like protein (slp1)], a Rab27a- and PtdIns(3,4,5)P3-binding protein, regulates the androgen-dependent secretion of PSAP and PSA in human LNCaP prostate carcinoma cells. Androgen-dependent PSAP secretion was significantly inhibited in cells that expressed the C2A domain of JFC1 [PtdIns(3,4,5)P3-binding-domain], but was unaffected by JFC1 overexpression. Conversely, PSA secretion was not inhibited by the C2A domain of JFC1. We show, using immunofluorescence analysis, that JFC1 co-localizes with PSAP, but rarely with PSA, in prostate granules, suggesting that JFC1 is part of the PSAP secretory machinery. However, PSA secretion was significantly increased in LNCaP cells that overexpressed JFC1, indicating that the secretion of PSA is susceptible to variations in the intracellular concentration of JFC1. Both PSAP and PSA secretion was increased by overexpression of wild-type Rab27a or the constitutively active Rab27aQ78L. The secretion of PSA was partially inhibited in the presence of LY294002, while the secretion of PSAP was completely abolished by the PI3K (phosphoinositide 3-kinase) inhibitor. This supports the view that PI3K plays a differential role in the secretion of prostate secretory markers. In conclusion, we present evidence that JFC1 differentially regulates the secretion of PSAP and PSA, and that Rab27a and PI3K play a central role in the exocytosis of prostate-specific markers.

Role of the p110delta PI 3-kinase in integrin and ITAM receptor signalling in platelets

We have investigated the function of the p110delta catalytic subunit of phosphoinositide 3-kinase (PI 3-kinase) in platelets using p110delta knock-out (p110delta(-/-)) mice and p110delta knock-in (p110delta(D910A/D910A)) mice, which express a catalytically inactive form of the enzyme. Aggregation to threshold concentrations of the GPVI-specific agonist, CRP, was partially reduced in p110delta(-/-) and p110delta(D910A/D910A) platelets. This inhibition was overcome by higher concentrations of CRP. The degree of inhibition was considerably weaker than that induced by LY294002 and wortmannin, which inhibit all isoforms of PI 3-kinase. p110delta(-/-) platelets showed decreased spreading on fibrinogen- or von Willebrand factor (VWF)-coated surfaces under static conditions, whereas they spread normally on collagen. LY294002 had a more pronounced inhibitory effect on spreading on all three surfaces. Adhesion and aggregate formation of p110delta(-/-) platelets to collagen or fibrinogen/VWF at intermediate/high rates of shear were normal. This study demonstrates a minor role for the p110delta catalytic subunit in mediating platelet activation by the collagen receptor GPVI and integrin alphaIIbeta3. The more pronounced inhibitory effect of LY294002 and wortmannin indicates that other isoforms of PI 3-kinase play a more significant role in signalling by the two platelet glycoprotein receptors.

Calcium oscillation and phosphatidylinositol 3-kinase positively regulate integrin alpha(IIb)beta3-mediated outside-in signaling

The frequency of calcium oscillation reveals the platelet activation status, however, the biological significance of the periodic calcium responses and methods of communication with other integrin-mediated signals are not clear. RGD-containing disintegrin rhodostomin coated substrates were employed to enhance platelet spreading and calcium oscillation through direct binding and clustering of the receptor integrin alpha(IIb)beta3. The results showed that the activation of phosphatidylinositol 3-kinase (PI3-K) and internal calcium pathways were crucial for alpha(IIb)beta3 outside-in signaling. PI3-K antagonists wortmannin and LY294002 inhibited disintegrin substrates and induced platelet spreading and calcium oscillation. At the same time, pretreatment of platelets with the microsomal calcium-ATPase inhibitor thapsigargin to deplete internal calcium stores severely impaired the calcium oscillation as well as PI3-K activation and spreading on disintegrin substrates. Because inhibition of one pathway could inhibit the other, our data indicates that PI3-K and calcium oscillation are synergistically operated and form a positive-feedback regulation in integrin alpha(IIb)beta3-mediated outside-in signaling.

Interplay between P2Y(1), P2Y(12), and P2X(1) receptors in the activation of megakaryocyte cation influx currents by ADP: evidence that the primary megakaryocyte represents a fully functional model of platelet P2 receptor signaling

The difficulty of conducting electrophysiologic recordings from the platelet has restricted investigations into the role of ion channels in thrombosis and hemostasis. We now demonstrate that the well-established synergy between P2Y(1) and P2Y(12) receptors during adenosine diphosphate (ADP)-dependent activation of the platelet alpha(IIb)beta(3) integrin also exists in murine marrow megakaryocytes, further supporting the progenitor cell as a bona fide model of platelet P2 receptor signaling. In patch clamp recordings, ADP (30 microM) stimulated a transient inward current at -70 mV, which was carried by Na(+) and Ca(2+) and was amplified by phenylarsine oxide, a potentiator of certain transient receptor potential (TRP) ion channels by phosphatidylinositol 4,5-bisphosphate depletion. This initial current decayed to a sustained phase, upon which repetitive transient inward cation currents with pre-dominantly P2X(1)-like kinetics were super-imposed. Abolishing P2X(1)-receptor activity prevented most of the repetitive currents, consistent with their activation by secreted adenosine triphosphate (ATP). Recordings in P2Y(1)-receptor-deficient megakaryocytes demonstrated an essential requirement of this receptor for activation of all ADP-evoked inward currents. However, P2Y(12) receptors, through the activation of PI3-kinase, played a synergistic role in both P2Y(1) and P2X(1)-receptor-dependent currents. Thus, direct stimulation of P2Y(1) and P2Y(12) receptors, together with autocrine P2X(1) activation, is responsible for the activation of nonselective cation currents by the platelet agonist ADP.

Platelet adhesion signalling and the regulation of thrombus formation

Platelets perform a central role in haemostasis and thrombosis. They adhere to subendothelial collagens exposed at sites of blood vessel injury via the glycoprotein (GP) Ib-V-IX receptor complex, GPVI and integrin alpha(2)beta(1). These receptors perform distinct functions in the regulation of cell signalling involving non-receptor tyrosine kinases (e.g. Src, Fyn, Lyn, Syk and Btk), adaptor proteins, phospholipase C and lipid kinases such as phosphoinositide 3-kinase. They are also coupled to an increase in cytosolic calcium levels and protein kinase C activation, leading to the secretion of paracrine/autocrine platelet factors and an increase in integrin receptor affinities. Through the binding of plasma fibrinogen and von Willebrand Factor to integrin alpha(IIb)beta(3), a platelet thrombus is formed. Although increasing evidence indicates that each of the adhesion receptors GPIb-V-IX and GPVI and integrins alpha(2)beta(1) and alpha(IIb)beta(3) contribute to the signalling that regulates this process, the individual roles of each are only beginning to be dissected. By contrast, adhesion receptor signalling through platelet endothelial cell adhesion molecule 1 (PECAM-1) is implicated in the inhibition of platelet function and thrombus formation in the healthy circulation. Recent studies indicate that understanding of platelet adhesion signalling mechanisms might enable the development of new strategies to treat and prevent thrombosis.

The relative role of PLCbeta and PI3Kgamma in platelet activation

Stimulation of platelet G protein-coupled receptors results in the cleavage of phosphatidylinositol 4,5-trisphosphate (PIP(2)) into inositol 1,4,5-trisphosphate and 1,2-diacylglycerol by phospholipase C (PLCbeta). It also results in the phosphorylation of PIP2 by the gamma isoform of phosphatidylinositol 3-kinase (PI3Kgamma) to synthesize phosphatidylinositol 3,4,5-trisphosphate. To understand the role of PIP2 in platelet signaling, we evaluated knock-out mice lacking 2 isoforms of PLCbeta (PLCbeta2 and PLCbeta3) or lacking the G(betagamma)-activated isoform of PI3K (PI3Kgamma). Both knock-out mice were unable to form stable thrombi in a carotid injury model. To provide a functional explanation, knock-out platelets were studied ex vivo. PLCbeta2/beta3-/- platelets failed to assemble filamentous actin, had defects in both secretion and mobilization of intracellular calcium, and were unable to form stable aggregates following low doses of agonists. Platelets lacking PI3Kgamma disaggregated following low-dose adenosine diphosphate (ADP) and had a mildly impaired ability to mobilize intracellular calcium. Yet, they exhibited essentially normal actin assembly and secretion. Remarkably, both PLCbeta2/beta3-/- and PI3Kgamma-/- platelets spread more slowly upon fibrinogen. These results suggest substantial redundancy in platelet signaling pathways. Nonetheless, the diminished ability of knock-out platelets to normally spread after adhesion and to form stable thrombi in vivo suggests that both PLCbeta2/beta3 and PI3Kgamma play vital roles in platelet cytoskeletal dynamics.

Emerging roles for P2X1 receptors in platelet activation

The platelet surface membrane possesses three P2 receptors activated by extracellular adenosine nucleotides; one member of the ionotropic receptor family (P2X(1)) and two members of the G-protein-coupled receptor family (P2Y(1) and P2Y(12)). P2Y(1) and P2Y(12) receptors have firmly established roles in platelet activation during thrombosis and haemostasis, whereas the importance of the P2X(1) receptor has been more controversial. However, recent studies have demonstrated that P2X(1) receptors can generate significant functional platelet responses alone and in synergy with other receptor pathways. In addition, studies in transgenic animals indicate an important role for P2X(1) receptors in platelet activation, particularly under conditions of shear stress and thus during arterial thrombosis. This review discusses the background behind discovery of P2X(1) receptors in platelets and their precursor cell, the megakaryocyte, and how signalling via these ion channels may participate in platelet activation.

The role of SHIP1 in macrophage programming and activation

The SHIP1 (SH2-containing inositol-5′-phosphatase 1) acts as a negative regulator of proliferation, survival and end cell activation in haemopoietic cells. It does so, at least in part, by translocating to membranes after extracellular stimulation and hydrolysing the phosphoinositide 3-kinase-generated second messenger, PtdIns(3,4,5)P3 to PtdIns(3,4)P2. SHIP1−/− mice have, as a result, an increased number of neutrophils and monocyte/macrophages because their progenitors display enhanced survival and proliferation. These mice also suffer from osteoporosis because of an increased number of hyperactive osteoclasts and a significant neutrophil infiltration of the lungs. Interestingly, SHIP1−/− mice do not display endotoxin tolerance and we have found that lipopolysaccharide-induced endotoxin tolerance is contingent on up-regulating SHIP1, through the production of autocrine-acting transforming growth factor-β, in bone-marrow-derived macrophages and mast cells. Intriguingly, unlike bone-marrow-derived macrophages, SHIP1−/− peritoneal and alveolar macrophages produce 10-fold less NO than wild-type macrophages because these in vivo-generated macrophages have very high arginase I levels and this enzyme competes with inducible nitric oxide synthase for the substrate L-arginine. It is probable that, in the face of chronically increased PtdIns(3,4,5)P3 levels in their myeloid progenitors, SHIP1−/− mice display a skewed development away from M1 (killer) macrophages (which have high inducible nitric oxide synthase levels and produce NO to kill microorganisms and tumour cells), towards M2 (healing) macrophages (which have high arginase levels and produce ornithine to promote host-cell growth and collagen formation). This skewing probably occurs to avoid septic shock and suggests that the phosphoinositide 3-kinase pathway plays a critical role in programming macrophages.

Vav1 and vav3 have critical but redundant roles in mediating platelet activation by collagen

Vav family proteins are guanine nucleotide exchange factors for the Rho/Rac family of small GTP-binding proteins. In addition, they have domains that mediate protein-protein interactions, including one Src homology 2 (SH2) and two Src homology 3 (SH3) domains. Vav1, Vav2, and Vav3 play a crucial role in the regulation of phospholipase C gamma (PLC gamma) isoforms by immuno-tyrosine-based activation motif (ITAM)-coupled receptors, including the T- and B-cell antigen receptors. We have reported in platelets, however, that Vav1 and Vav2 are not required for activation of PLC gamma 2 in response to stimulation of the ITAM-coupled collagen receptor glycoprotein VI (GPVI). Here we report that Vav3 is tyrosinephosphorylated upon activation of GPVI but that Vav3-deficient platelets also exhibit a normal response upon activation of the ITAM receptor. In sharp contrast, platelets deficient in both Vav1 and Vav3 show a marked inhibition of aggregation and spreading upon activation of GPVI, which is associated with a reduction in tyrosine phosphorylation of PLC gamma 2. The phenotype of Vav1/2/3 triple-deficient platelets is similar to that of Vav1/3 double-deficient cells. These results demonstrate that Vav3 and Vav1 play crucial but redundant roles in the activation of PLC gamma 2 by GPVI. This is the first time that absolute redundancy between two protein isoforms has been observed with respect to the regulation of PLC gamma 2 in platelets.

Tec Kinases Mediate Sustained Calcium Influx via Site-specific Tyrosine Phosphorylation of the Phospholipase Cγ Src Homology 2-Src Homology 3 Linker

Tyrosine phosphorylation of phospholipase Cgamma2 (PLCgamma2) is a crucial activation switch that initiates and maintains intracellular calcium mobilization in response to B cell antigen receptor (BCR) engagement. Although members from three distinct families of non-receptor tyrosine kinases can phosphorylate PLCgamma in vitro, the specific kinase(s) controlling BCR-dependent PLCgamma activation in vivo remains unknown. Bruton's tyrosine kinase (Btk)-deficient human B cells exhibit diminished inositol 1,4,5-trisphosphate production and calcium signaling despite a normal inducible level of total PLCgamma2 tyrosine phosphorylation. This suggested that Btk might modify a critical subset of residues essential for PLCgamma2 activity. To evaluate this hypothesis, we generated site-specific phosphotyrosine antibodies recognizing four putative regulatory residues within PLCgamma2. Whereas all four sites were rapidly modified in response to BCR engagement in normal B cells, Btk-deficient B cells exhibited a marked reduction in phosphorylation of the Src homology 2 (SH2)-SH3 linker region sites, Tyr(753) and Tyr(759). Phosphorylation of both sites was restored by expression of Tec, but not Syk, family kinases. In contrast, phosphorylation of the PLCgamma2 carboxyl-terminal sites, Tyr(1197) and Tyr(1217), was unaffected by the absence of functional Btk. Together, these data support a model whereby Btk/Tec kinases control sustained calcium signaling via site-specific phosphorylation of key residues within the PLCgamma2 SH2-SH3 linker.

SHIP1 and Lyn Kinase Negatively Regulate Integrin αIIbβ3 Signaling in Platelets

Integrin alpha(IIb)beta(3) plays a critical role in platelet function, promoting a broad range of functional responses including platelet adhesion, spreading, aggregation, clot retraction, and platelet procoagulant function. Signaling events operating downstream of this receptor (outside-in signaling) are important for these responses; however the mechanisms negatively regulating integrin alpha(IIb)beta(3) signaling remain ill-defined. We demonstrate here a major role for the Src homology 2 domain-containing inositol 5-phosphatase (SHIP1) and Src family kinase, Lyn, in this process. Our studies on murine SHIP1 knockout platelets have defined a major role for this enzyme in regulating integrin alpha(IIb)beta(3)-dependent phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) accumulation, necessary for a cytosolic calcium response and platelet spreading. SHIP1 phosphorylation and PtdIns(3,4,5)P(3) metabolism is partially regulated through Lyn kinase, resulting in an enhanced calcium flux and spreading response in Lyn-deficient mouse platelets. Analysis of platelet adhesion dynamics under physiological blood flow conditions revealed an important role for SHIP1 in regulating platelet adhesion on fibrinogen. Specifically, SHIP1-dependent PtdIns(3,4,5)P(3) metabolism down-regulates the stability of integrin alpha(IIb)beta(3)-fibrinogen adhesive bonds, leading to a decrease in the proportion of platelets forming shear-resistant adhesion contacts. These studies define a major role for SHIP1 and Lyn as negative regulators of integrin alpha(IIb)beta(3) adhesive and signaling function.

Reciprocal cross-talk between P2Y1 and P2Y12 receptors at the level of calcium signaling in human platelets

Adenosine diphosphate (ADP), an important platelet agonist, acts through 2 G-protein-coupled receptors (GPCRs), P2Y(1) and P2Y(12), which signal through Gq and Gi, respectively. There is increasing evidence for cross-talk between signaling pathways downstream of GPCRs and here we demonstrate cross-talk between these 2 ADP receptors in human platelets. We show that P2Y(12) contributes to platelet signaling by potentiating the P2Y(1)-induced calcium response. This potentiation is mediated by 2 mechanisms: inhibition of adenylate cyclase and activation of phosphatidylinositol 3 (PI 3)-kinase. Furthermore, the Src family kinase inhibitor PP1 selectively potentiates the contribution to the calcium response by P2Y(12), although inhibition of adenylate cyclase by P2Y(12) is unaffected. Using PP1 in combination with the inhibitor of PI 3-kinase LY294002, we show that Src negatively regulates the PI 3-kinase-mediated component of the P2Y(12) calcium response. Finally, we were able to show that Src kinase is activated through P2Y(1) but not P2Y(12). Taken together, we present evidence for a complex signaling interplay between P2Y(1) and P2Y(12), where P2Y(12) is able to positively regulate P2Y(1) action and P2Y(1) negatively regulates this action of P2Y(12). It is likely that this interplay between receptors plays an important role in maintaining the delicate balance between platelet activation and inhibition during normal hemostasis.

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.

Impaired platelet responses to thrombin and collagen in AKT-1-deficient mice

We investigated the role of Akt-1, one of the major downstream effectors of phosphoinositide 3-kinase (PI3K), in platelet function using mice in which the gene for Akt-1 had been inactivated. Using ex vivo techniques, we showed that Akt-1-deficient mice exhibited impaired platelet aggregation and spreading in response to various agonists. These differences were most apparent in platelets activated with low concentrations of thrombin. Although Akt-1 is not the predominant Akt isoform in mouse platelets, its absence diminished the amount of total phospho-Akt and inhibited increases in intracellular Ca(2+) concentration in response to thrombin. Moreover, thrombin-induced platelet alpha-granule release as well as release of adenosine triphosphate from dense granules was also defective in Akt-1-null platelets. Although the absence of Akt-1 did not influence expression of the major platelet receptors for thrombin and collagen, fibrinogen binding in response to these agonists was significantly reduced. As a consequence of impaired alpha(IIb)beta(3) activation and platelet aggregation, Akt-1 null mice showed significantly longer bleeding times than wild-type mice.

SHIP, SHIP2, and PTEN activities are regulated in vivo by modulation of their protein levels: SHIP is up-regulated in macrophages and mast cells by lipopolysaccharide

The phosphatidylinositol-3 kinase (PI3K) pathway plays a central role in regulating numerous biologic processes, including survival, adhesion, migration, metabolic activity, proliferation, differentiation, and end cell activation through the generation of the potent second messenger PI-3,4,5-trisphosphate (PI-3,4,5-P(3)). To ensure that activation of this pathway is appropriately suppressed/terminated, the ubiquitously expressed 54-kDa tumor suppressor PTEN hydrolyzes PI-3,4,5-P(3) to PI-4,5-P(2), whereas the 145-kDa hematopoietic-restricted SH2-containing inositol 5'-phosphatase SHIP (also known as SHIP1), the 104-kDa stem cell-restricted SHIP sSHIP, and the more widely expressed 150-kDa SHIP2 break it down to PI-3,4-P(2). In this review, we focus on the properties of these phospholipid phosphatases and summarize recent data showing that the activities of these negative regulators often are modulated by simply altering their protein levels. We also highlight the critical role that SHIP plays in lipopolysaccharide-induced macrophage activation and in endotoxin tolerance.

Tec regulates platelet activation by GPVI in the absence of Btk

The Tec family kinase Btk plays an important role in the regulation of phospholipase C gamma 2 (PLC gamma 2) downstream of the collagen receptor glycoprotein VI (GPVI) in human platelets. Platelets also express a second member of this family, Tec; however, its function has not been analyzed. To address the role of Tec, we analyzed Btk-/-, Tec-/-, and Btk/Tec double-deficient (Btk-/-/Tec-/-) platelets. Tec-/- platelets exhibit a minor reduction in aggregation to threshold concentrations of collagen or the GPVI-specific agonist collagen-related peptide (CRP), whereas responses to higher concentrations are normal. Tyrosine phosphorylation of PLC gamma 2 by collagen and CRP is not altered in Tec-/- platelets. However, Btk-/-/Tec-/- platelets exhibit a greater reduction in PLC gamma 2 phosphorylation than is seen in the absence of Btk, thus revealing an important role for Tec in this situation. Furthermore, Btk-/-/Tec-/- platelets fail to undergo an increase in Ca2+, aggregation, secretion, and spreading in response to collagen or CRP, whereas they aggregate normally to adenosine diphosphate (ADP) and spread on fibrinogen. A residual GPVI signal exists in the Btk-/-/Tec-/- platelets as CRP synergizes with ADP to mediate aggregation. These results demonstrate an essential requirement for Tec and Btk in platelet activation by GPVI and reveal a functional role for Tec in the regulation of PLC gamma 2 in the absence of Btk.

SH2-containing inositol 5-phosphatases 1 and 2 in blood platelets: their interactions and roles in the control of phosphatidylinositol 3,4,5-trisphosphate levels

Src homology domain 2-containing inositol 5-phosphatases 1 and 2 (SHIP1 and SHIP2) are capable of dephosphorylating the second messenger PtdIns(3,4,5) P3 (phosphatidylinositol 3,4,5-trisphosphate) and interacting with several signalling proteins. SHIP1 is essentially expressed in haematopoietic cells, whereas SHIP2, a closely related enzyme, is ubiquitous. In the present study, we show that SHIP1 and SHIP2 are expressed as functional PtdIns(3,4,5) P3 5-phosphatases in human blood platelets and are capable of interacting when these two lipid phosphatases are co-expressed, either naturally (platelets and A20 B lymphoma cells) or artificially (COS-7 cells). Using COS-7 cells transfected with deletion mutants of SHIP2, we demonstrate that the Src homology domain 2 of SHIP2 is the minimal and sufficient protein motif responsible for the interaction between the two phosphatases. These results prompted us to investigate the relative importance of SHIP1 and SHIP2 in the control of PtdIns(3,4,5) P3 levels in platelets using homozygous or heterozygous SHIP1- or SHIP2-deficient mice. Our results strongly suggest that SHIP1, rather than SHIP2, plays a major role in controlling PtdIns(3,4,5) P3 levels in response to thrombin or collagen activation of mouse blood platelets.

Oligomerization controls in tissue-specific manner ligand binding of native, affinity-purified p42IP4/centaurin α1 and cytohesins—proteins with high affinity for the messengers d-inositol 1,3,4,5-tetrakisphosphate/phosphatidylinositol 3,4,5-trisphosphate

Several distinct receptor proteins for the second messengers Ins(1,3,4,5)P(4) and PtdIns(3,4,5)P(3) are already known, such as the brain-specific p42(IP4), which we have previously cloned from different species, and cytohesins. However, it is still unclear whether proteins interacting with phosphoinositide and inositolpolyphosphate second messengers are regulated differently in different tissues. Here, we investigated these native proteins for comparison also from rat lung cytosol and purified them by PtdIns(3,4,5)P(3) affinity chromatography. Proteins selectively binding Ins(1,3,4,5)P(4) with high affinity also showed high affinity and specificity towards PtdIns(3,4,5)P(3). In lung cytosol, two prominent protein bands were found in the eluate from a PtdIns(3,4,5)P(3) affinity column. We identified these proteins by mass spectrometry as the cytohesin family of Arf guanosine nucleotide exchange factors (cytohesin 1, ARNO, GRP-1) and as Bruton's tyrosine kinase. Western blot analysis indicated that p42(IP4) was present in lung only at very low concentrations. Applying the affinity purification scheme established for rat lung cytosol to cytosol from rat brain, however, yielded only p42(IP4). We identified cytohesins in rat brain by Western blotting and PCR, but cytohesins surprisingly did not bind to the PtdIns(3,4,5)P(3)-affinity column. Gel filtration experiments of brain cytosol revealed that brain cytohesins are bound to large molecular weight complexes (150 to more than 500 kDa). Thus, we hypothesize that this finding explains why brain cytohesins apparently do not bind the inositolphosphate ligand. In lung cytosol, on the other hand, cytohesins occur as dimers. Gel filtration also showed that p42(IP4) in brain cytosol occurs as a monomer. Thus, oligomerization (homomeric or heteromeric) of InsP(4)/PtdInsP(3) binding proteins can modulate their function in a tissue-dependent manner because it can modify their ability to interact with the ligands.

Glycoprotein IIb/IIIa inhibition attenuates platelet-activating factor-induced platelet activation by reducing protein kinase C activity

Glycoprotein (GP)IIb/IIIa inhibition may abolish activated leukocyte-induced platelet activation, in which leukocyte-released platelet-activating factor (PAF) is a major mediator. The present study thus investigated if and how GPIIb/IIIa inhibitors interfere with PAF-induced platelet activation. Platelet and leukocyte activation were monitored by flow cytometry and immunoblotting. GPIIb/IIIa inhibitors (c7E3, non-peptide SR121566, and MAb RFGP56) attenuated PAF-induced, but not adenosine diphosphate (ADP)- or thrombin receptor activating peptide (TRAP)-induced platelet P-selectin expression in whole blood. GPIIb/IIIa blockade enhanced ADP- or TRAP-induced leukocyte CD11b expression, but not the response to PAF. GPIIb/IIIa blockade attenuated PAF-induced, but enhanced ADP- or TRAP-induced platelet-leukocyte aggregation. Under the present experimental conditions, thromboxane A2 receptor antagonism did not significantly influence PAF-induced platelet activation, and GPIIb/IIIa inhibition did not interfere with calcium mobilization/influx in platelets. Protein kinase C (PKC) blockade inhibited PAF-induced platelet P-selectin expression, and PAF-induced PKC activity was reduced by GPIIb/IIIa inhibition. PAF (=1 micro m) did not induce MEK 1/2 or ERK 1/2 phosphorylation, whilst thrombin induced marked responses, which were enhanced by GPIIb/IIIa blockade. Thus, GPIIb/IIIa inhibition attenuates PAF-induced platelet activation via inhibiting PKC activity. GPIIb/IIIa blockade enhances thrombin-induced platelet MEK 1/2 and ERK 1/2 activation, and augments ADP- and TRAP-induced leukocyte activation by enhancing platelet-leukocyte aggregation.