Integrins: dynamic scaffolds for adhesion and signaling in platelets

DL-3-n-butylphthalide inhibits platelet activation via inhibition of cPLA2-mediated TXA2 synthesis and phosphodiesterase

Aberrant platelet activation plays a critical role in the pathogenesis of heart attack and stroke. DL-3-n-butylphthalide (NBP) has been approved in China to treat stroke with multiple mechanisms. The anti-stroke effects of NBP may be related to its antiplatelet effects reported in rats in addition to its antioxidative, antiapoptotic, and angiogenic effects. However, the effects and the underlying mechanisms of NBP on human platelets are not yet clear. In this study, we found that NBP concentration-dependently inhibited human platelet aggregation and ATP release induced by ADP, thrombin, U46619, arachidonic acid, or collagen. NBP also inhibited PAC-1 binding induced by ADP or thrombin and platelet spreading on immobilized fibrinogen. NBP reduced TXA2 synthesis induced by thrombin or collagen via inhibiting cPLA2 phosphorylation, concomitantly with a marked decrease in intracellular calcium mobilization. Moreover, NBP also inhibited human platelet phosphodiesterase (PDE) and elevated 3,5-cyclic adenosine monophosphate level in platelets. In conclusion, NBP significantly inhibits human platelet activation via inhibition of cPLA2-mediated TXA2 synthesis and PDE, and may be effective as an antiplatelet drug to treat other arterial thrombotic diseases.

EGFL7 is expressed in bone microenvironment and promotes angiogenesis via ERK, STAT3, and integrin signaling cascades

Angiogenesis plays a pivotal role in bone formation, remodeling, and fracture healing. The regulation of angiogenesis in the bone microenvironment is highly complex and orchestrated by intercellular communication between bone cells and endothelial cells. Here, we report that EGF-like domain 7 (EGFL7), a member of the epidermal growth factor (EGF) repeat protein superfamily is expressed in both the osteoclast and osteoblast lineages, and promotes endothelial cell activities. Addition of exogenous recombinant EGFL7 potentiates SVEC (simian virus 40-transformed mouse microvascular endothelial cell line) cell migration and tube-like structure formation in vitro. Moreover, recombinant EGFL7 promotes angiogenesis featuring web-like structures in ex vivo fetal mouse metatarsal angiogenesis assay. We show that recombinant EGFL7 induces phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2), signal transducer and activator of transcription 3 (STAT3), and focal adhesion kinase (FAK) in SVEC cells. Inhibition of ERK1/2 and STAT3 signaling impairs EGFL7-induced endothelial cell migration, and angiogenesis in fetal mouse metatarsal explants. Bioinformatic analyses indicate that EGFL7 contains a conserved RGD/QGD motif and EGFL7-induced endothelial cell migration is significantly reduced in the presence of RGD peptides. Moreover, EGFL7 gene expression is significantly upregulated during growth plate injury repair. Together, these results demonstrate that EGFL7 expressed by bone cells regulates endothelial cell activities through integrin-mediated signaling. This study highlights the important role that EGFL7, like EGFL6, expressed in bone microenvironment plays in the regulation of angiogenesis in bone.

Antiplatelet activity of loureirin A by attenuating Akt phosphorylation: In vitro studies

Loureirin A is a flavonoid extracted from Dragon׳s Blood that has been used to promote blood circulation and remove stasis in Chinese traditional medicine. However, the mechanisms of these effects are not fully understood. We explored the anti-platelet activity and underlying mechanism of loureirin A in vitro. Our results indicated that loureirin A negatively affected agonist-induced platelet aggregation such as collagen, collagen-related peptide (CRP), ADP and thrombin. Loureirin A inhibited collagen-induced platelet ATP secretion and thrombin-stimulated P-selectin expression in a dose-dependent manner. Platelet spreading on immobilized fibrinogen was significantly impaired in the presence of loureirin A. Immunoblotting analysis indicated that 100μM of loureirin A almost completely eliminated collagen-induced Akt phosphorylation at Ser473. Interestingly, a submaximal dose (50μM) of loureirin A had an additive inhibitory effect with the phosphoinositide 3-kinase (PI3K) inhibitor Ly294002 on collage-induced Akt phosphorylation in platelets. Taken together, loureirin A had an inhibitory effect on platelet activation, perhaps through an impairment of PI3K/Akt signaling.

Targeting integrin and integrin signaling in treating thrombosis

The critical roles of integrins in thrombosis have enabled the successful development and clinical use of the first generation of integrin antagonists as represented by abciximab (Reopro), eptifibatide (Integrilin), and tirofiban (Aggrastat). These integrin αIIbβ3 antagonists are not only potent antithrombotics but also have significant side effects. In particular, their induction of ligand-induced integrin conformational changes is associated with thrombocytopenia. Increased bleeding risk prevents integrin antagonists from being used at higher doses and in patients at risk for bleeding. To address the ligand-induced conformational changes caused by current integrin antagonists, compounds that minimally induce conformational changes in integrin αIIbβ3 have been developed. Recent studies on the mechanisms of integrin signaling suggest that selectively targeting integrin outside-in signaling mechanisms allows for potent inhibition of thrombosis, while maintaining hemostasis in animal models.

Effect of Cordycepin-Enriched WIB801C from Cordyceps militaris Suppressing Fibrinogen Binding to Glycoprotein IIb/IIIa

In this study, we investigated the effects of cordycepin-enriched (CE)-WIB801C, a n-butanol extract of Cordyceps militaris-hypha on collagen-stimulated platelet aggregation. CE-WIB801C dose dependently inhibited collagen-induced platelet aggregation, and had a synergistic effect together with cordycepin (W-cordycepin) from CE-WIB801C on the inhibition of collagen-induced platelet aggregation. CE-WIB801C and cordycepin stimulated the phosphorylation of VASP (Ser(157)) and the dephosphorylation of PI3K and Akt, and inhibited the binding of fibrinogen to glycoprotein IIb/IIIa (αIIb/β3) and the release of ATP and serotonin in collagen-induced platelet aggregation. A-kinase inhibitor Rp-8-Br-cAMPS reduced CE-WIB801C-, and cordycepin-increased VASP (Ser(157)) phosphorylation, and increased CE-WIB801C-, and cordycepin-inhibited the fibrinogen binding to αIIb/β3. Therefore, we demonstrate that CE-WIB801C-, and cordycepin-inhibited fibrinogen binding to αIIb/β3 are due to stimulation of cAMP-dependent phosphorylation of VASP (Ser(157)), and inhibition of PI3K/Akt phosphorylation. These results strongly indicate that CE-WIB801C and cordycepin may have preventive or therapeutic potential for platelet aggregation-mediated diseases, such as thrombosis, myocardial infarction, atherosclerosis, and ischemic cerebrovascular disease.

Rap1-GTP-interacting adaptor molecule (RIAM) is dispensable for platelet integrin activation and function in mice

Platelet aggregation at sites of vascular injury is essential for hemostasis but also thrombosis. Platelet adhesiveness is critically dependent on agonist-induced inside-out activation of heterodimeric integrin receptors by a mechanism involving the recruitment of talin-1 to the cytoplasmic integrin tail. Experiments in heterologous cells have suggested a critical role of Rap1-guanosine triphosphate-interacting adaptor molecule (RIAM) for talin-1 recruitment and thus integrin activation, but direct in vivo evidence to support this has been missing. We generated RIAM-null mice and found that they are viable, fertile, and apparently healthy. Unexpectedly, platelets from these mice show unaltered β3- and β1-integrin activation and consequently normal adhesion and aggregation responses under static and flow conditions. Similarly, hemostasis and arterial thrombus formation were indistinguishable between wild-type and RIAM-null mice. These results reveal that RIAM is dispensable for integrin activation and function in mouse platelets, strongly suggesting the existence of alternative mechanisms of talin-1 recruitment.

The adaptor protein Swiprosin-1/EFhd2 is dispensable for platelet function in mice

Background

Platelets are anuclear cell fragments derived from bone marrow megakaryocytes that safeguard vascular integrity, but may also cause pathological vessel occlusion. Reorganizations of the platelet cytoskeleton and agonist-induced intracellular Ca²⁺-mobilization are crucial for platelet hemostatic function. EF-hand domain containing 2 (EFhd2, Swiprosin-1) is a Ca²⁺-binding cytoskeletal adaptor protein involved in actin remodeling in different cell types, but its function in platelets is unknown.

Objective

Based on the described functions of EFhd2 in immune cells, we tested the hypothesis that EFhd2 is a crucial adaptor protein for platelet function acting as a regulator of Ca²⁺-mobilization and cytoskeletal rearrangements.

Methods and Results

We generated EFhd2-deficient mice and analyzed their platelets in vitro and in vivo. Efhd2^-/- mice displayed normal platelet count and size, exhibited an unaltered in vivo life span and showed normal Ca²⁺-mobilization and activation/aggregation responses to classic agonists. Interestingly, upon stimulation of the immunoreceptor tyrosine-based activation motif-coupled receptor glycoprotein (GP) VI, Efhd2^-/- platelets showed a slightly increased coagulant activity. Furthermore, absence of EFhd2 had no significant impact on integrin-mediated clot retraction, actomyosin rearrangements and spreading of activated platelets on fibrinogen. In vivo EFhd2-deficiency resulted in unaltered hemostatic function and unaffected arterial thrombus formation.

Conclusion

These results show that EFhd2 is not essential for platelet function in mice indicating that other cytoskeletal adaptors may functionally compensate its loss.

Depletion of STAT5 blocks TEL-SYK-induced APMF-type leukemia with myelofibrosis and myelodysplasia in mice

The spleen tyrosine kinase (SYK) was identified as an oncogenic driver in a broad spectrum of hematologic malignancies. The in vivo comparison of three SYK containing oncogenes, SYK(wt), TEL-SYK and IL-2-inducible T-cell kinase (ITK)-SYK revealed a general myeloexpansion and the establishment of three different hematologic (pre)diseases. SYK(wt) enhanced the myeloid and T-cell compartment, without leukemia/lymphoma development. ITK-SYK caused lethal T-cell lymphomas and the cytoplasmic TEL-SYK fusion induced an acute panmyelosis with myelofibrosis-type acute myeloid leukemia (AML) with up to 50% immature megakaryoblasts infiltrating bone marrow, spleen and liver, additional MPN features (myelofibrosis and granulocyte expansion) and MDS stigmata with megakaryocytic and erythroid dysplasia. LKS cells were reduced and all subsets (LT/ST/MPP) showed reduced proliferation rates. SYK inhibitor treatment (R788) of diseased TEL-SYK mice reduced leukocytosis, spleen and liver infiltration, enhanced the hematocrit and prolonged survival time, but could not significantly reduce myelofibrosis. Stat5 was identified as a major downstream mediator of TEL-SYK in vitro as well as in vivo. Consequently, targeted deletion of Stat5 in vivo completely abrogated TEL-SYK-induced AML and myelofibrosis development, proving Stat5 as a major driver of SYK-induced transformation. Our experiments highlight the important role of SYK in AML and myelofibrosis and prove SYK and STAT5 inhibitors as potent treatment options for those diseases.

Paired immunoglobulin-like receptor B regulates platelet activation

Murine paired immunoglobulin-like receptors B (PIRB), as the ortholog of human leukocyte immunoglobulin-like receptor B2 (LILRB2), is involved in a variety of biological functions. Here, we found that PIRB and LILRB2 were expressed in mouse and human platelets, respectively. PIRB intracellular domain deletion (PIRB-TM) mice had thrombocythemia and significantly higher proportions of megakaryocytes in bone marrow. Agonist-induced aggregation and spreading on immobilized fibrinogen were facilitated in PIRB-TM platelets. The rate of clot retraction in platelet-rich plasma containing PIRB-TM platelets was also increased. Characterization of signaling confirmed that PIRB associated with phosphatases Shp1/2 in platelets. The phosphorylation of Shp1/2 was significantly downregulated in PIRB-TM platelets stimulated with collagen-related peptide (CRP) or on spreading. The results further revealed that the phosphorylation levels of the linker for activation of T cells, SH2 domain-containing leukocyte protein of 76kDa, and phospholipase C were enhanced in PIRB-TM platelets stimulated with CRP. The phosphorylation levels of FAK Y397 and integrin β3 Y759 were also enhanced in PIRB-TM platelet spread on fibrinogen. The PIRB/LILRB2 ligand angiopoietin-like-protein 2 (ANGPTL2) was expressed and stored in platelet α-granules. ANGPTL2 inhibited agonist-induced platelet aggregation and spreading on fibrinogen. The data presented here reveal that PIRB and its ligand ANGPTL2 possess an antithrombotic function by suppressing collagen receptor glycoprotein VI and integrin αIIbβ3-mediated signaling.

Integrin αII b tail distal of GFFKR participates in inside-out αII b β3 activation

BACKGROUND

Increases in ligand binding to integrins (activation) play critical roles in platelet and leukocyte function. Integrin activation requires talin and kindlin binding to integrin β cytoplasmic tails. Research has focused on the conserved GFFKR motif in integrin αII b tails, integrin β cytoplasmic tails and the binding partners of β tails. However, the roles of αII b tail distal of GFFKR motif are unexplored.

OBJECTIVE

To investigate the role of αII b tail distal of GFFKR in talin-mediated inside-out integrin signaling.

METHODS

We used model cell systems to examine the role of αII b tail distal of GFFKR in bidirectional αII b β3 signaling and αII b β3 -talin interactions.

RESULTS

Deletion of amino acid residues after the GFFKR motif in αII b tail moderately decreased β3 (D723R)-induced activation, abolished talin-induced αII b β3 activation in model cells, and inhibited agonist-induced αII b β3 activation in megakaryocytic cells. Furthermore, residues in αII b tail distal of GFFKR did not affect outside-in αII b β3 signaling or αII b β3 -talin interaction. Addition of non-homologous or non-specific amino acids to the GFFKR motif restored αII b β3 activation in model cells and in megakaryocytic cells. Molecular modeling indicates that β3 -bound talin sterically clashes with the αII b tail in the αII b β3 complexes, potentially disfavoring the α-β interactions that keep αII b β3 inactive.

CONCLUSION

The αII b tail sequences distal of GFFKR participate in talin-mediated inside-out αII b β3 activation through its steric clashes with β3 -bound talin.

Structural and mechanical functions of integrins

Integrins are ubiquitously expressed cell surface receptors that play a critical role in regulating the interaction between a cell and its microenvironment to control cell fate. These molecules are regulated either via their expression on the cell surface or through a unique bidirectional signalling mechanism. However, integrins are just the tip of the adhesome iceberg, initiating the assembly of a large range of adaptor and signalling proteins that mediate the structural and signalling functions of integrin. In this review, we summarise the structure of integrins and mechanisms by which integrin activation is controlled. The different adhesion structures formed by integrins are discussed, as well as the mechanical and structural roles integrins play during cell migration. As the function of integrin signalling can be quite varied based on cell type and context, an in depth understanding of these processes will aid our understanding of aberrant adhesion and migration, which is often associated with human pathologies such as cancer.

The functional role of platelets in the regulation of angiogenesis

Functionally, platelets are primarily recognized as key regulators of thrombosis and hemostasis. Upon vessel injury, the typically quiescent platelet interacts with subendothelial matrix to regulate platelet adhesion, activation and aggregation, with subsequent induction of the coagulation cascade forming a thrombus. Recently, however, newly described roles for platelets in the regulation of angiogenesis have emerged. Platelets possess an armory of pro- and anti-angiogenic proteins, which are actively sequestered and highly organized in α-granule populations. Platelet activation facilitates their release, eliciting potent angiogenic responses through mechanisms that appear to be tightly regulated. In conjunction, the release of platelet-derived phospholipids and microparticles has also earned merit as synergistic regulators of angiogenesis. Consequently, platelets have been functionally implicated in a range of angiogenesis-dependent processes, including physiological roles in wound healing, vascular development and blood/lymphatic vessel separation, whilst facilitating aberrant angiogenesis in a range of diseases including cancer, atherosclerosis and diabetic retinopathy. Whilst the underlying mechanisms are only starting to be elucidated, significant insights have been established, suggesting that platelets represent a promising therapeutic strategy in diseases requiring angiogenic modulation. Moreover, anti-platelet therapies targeting thrombotic complications also exert protective effects in disorders characterized by persistent angiogenesis.

On the activation of integrin αIIbβ3: outside-in and inside-out pathways

Integrin αIIbβ3 is a member of the integrin family of transmembrane proteins present on the plasma membrane of platelets. Integrin αIIbβ3 is widely known to regulate the process of thrombosis via activation at its cytoplasmic side by talin and interaction with the soluble fibrinogen. It is also reported that three groups of interactions restrain integrin family members in the inactive state, including a set of salt bridges on the cytoplasmic side of the transmembrane domain of the integrin α- and β-subunits known as the inner membrane clasp, hydrophobic packing of a few transmembrane residues on the extracellular side between the α- and β-subunits that is known as the outer membrane clasp, and the key interaction group of the βA domain (located on the β-subunit head domain) with the βTD (proximal to the plasma membrane on the β-subunit). However, molecular details of this key interaction group as well as events that lead to detachment of the βTD and βA domains have remained ambiguous. In this study, we use molecular dynamics models to take a comprehensive outside-in and inside-out approach at exploring how integrin αIIbβ3 is activated. First, we show that talin's interaction with the membrane-proximal and membrane-distal regions of integrin cytoplasmic-transmembrane domains significantly loosens the inner membrane clasp. Talin also interacts with an additional salt bridge (R734-E1006), which facilitates integrin activation through the separation of the integrin's α- and β-subunits. The second part of our study classifies three types of interactions between RGD peptides and the extracellular domains of integrin αIIbβ3. Finally, we show that the interaction of the Arg of the RGD sequence may activate integrin via disrupting the key interaction group between K350 on the βA domain and S673/S674 on the βTD.

Akt signaling in platelets and thrombosis

Akt is a Ser-Thr kinase with pleiotropic effects on cell survival, growth and metabolism. Recent evidence from gene-deletion studies in mice, and analysis of human platelets treated with Akt inhibitors, suggest that Akt regulates platelet activation, with potential consequences for thrombosis. Akt activation is regulated by the level of phosphoinositide 3-phosphates, and proteins that regulate concentrations of this lipid also regulate Akt activation and platelet function. Although the effectors through which Akt contributes to platelet activation are not definitively known, several candidates are discussed, including endothelial nitric oxide synthase, glycogen synthase kinase 3β, phosphodiesterase 3A and the integrin β(3) tail. Selective inhibitors of Akt isoforms or of proteins that contribute to its activation, such as individual PI3K isoforms, may make attractive targets for antithrombotic therapy. This review summarizes the current literature describing Akt activity and its regulation in platelets, including speculation regarding the future of Akt or its regulatory pathways as targets for the development of antithrombotic therapies.

The potential for the use of nanofeaturing in medical devices

This review looks at potential developments in medical devices which may be based upon nanofeaturing implant and tissue engineering scaffolds, and describes the basic science upon which such expectations are based.

Dok-2 adaptor protein regulates the shear-dependent adhesive function of platelet integrin αIIbβ3 in mice

The Dok proteins are a family of adaptor molecules that have a well defined role in regulating cellular migration, immune responses, and tumor progression. Previous studies have demonstrated that Doks-1 to 3 are expressed in platelets and that Dok-2 is tyrosine-phosphorylated downstream of integrin αIIbβ3, raising the possibility that it participates in integrin αIIbβ3 outside-in signaling. We demonstrate that Dok-2 in platelets is primarily phosphorylated by Lyn kinase. Moreover, deficiency of Dok-2 leads to dysregulated integrin αIIbβ3-dependent cytosolic calcium flux and phosphatidylinositol(3,4)P2 accumulation. Although agonist-induced integrin αIIbβ3 affinity regulation was unaltered in Dok-2(-/-) platelets, Dok-2 deficiency was associated with a shear-dependent increase in integrin αIIbβ3 adhesive function, resulting in enhanced platelet-fibrinogen and platelet-platelet adhesive interactions under flow. This increase in adhesion was restricted to discoid platelets and involved the shear-dependent regulation of membrane tethers. Dok-2 deficiency was associated with an increased rate of platelet aggregate formation on thrombogenic surfaces, leading to accelerated thrombus growth in vivo. Overall, this study defines an important role for Dok-2 in regulating biomechanical adhesive function of discoid platelets. Moreover, they define a previously unrecognized prothrombotic mechanism that is not detected by conventional platelet function assays.

The integrin-linked kinase-PINCH-parvin complex supports integrin αIIbβ3 activation

Integrin-linked kinase (ILK) is an important signaling regulator that assembles into the heteroternary complex with adaptor proteins PINCH and parvin (termed the IPP complex). We recently reported that ILK is important for integrin activation in a Chinese hamster ovary (CHO) cell system. We previously established parental CHO cells expressing a constitutively active chimeric integrin (αIIbα6Bβ3) and mutant CHO cells expressing inactive αIIbα6Bβ3 due to ILK deficiency. In this study, we further investigated the underlying mechanisms for ILK-dependent integrin activation. ILK-deficient mutant cells had trace levels of PINCH and α-parvin, and transfection of ILK cDNA into the mutant cells increased not only ILK but also PINCH and α-parvin, resulting in the restoration of αIIbα6Bβ3 activation. In the parental cells expressing active αIIbα6Bβ3, ILK, PINCH, and α-parvin were co-immunoprecipitated, indicating the formation of the IPP complex. Moreover, short interfering RNA (siRNA) experiments targeting PINCH-1 or both α- and β-parvin mRNA in the parent cells impaired the αIIbα6Bβ3 activation as well as the expression of the other components of the IPP complex. In addition, ILK mutants possessing defects in either PINCH or parvin binding failed to restore αIIbα6Bβ3 activation in the mutant cells. Kindlin-2 siRNA in the parental cells impaired αIIbα6Bβ3 activation without disturbing the expression of ILK. For CHO cells stably expressing wild-type αIIbβ3 that is an inactive form, overexpression of a talin head domain (THD) induced αIIbβ3 activation and the THD-induced αIIbβ3 activation was impaired by ILK siRNA through a significant reduction in the expression of the IPP complex. In contrast, overexpression of all IPP components in the αIIbβ3-expressing CHO cells further augmented THD-induced αIIbβ3 activation, whereas they did not induce αIIbβ3 activation without THD. These data suggest that the IPP complex rather than ILK plays an important role and supports integrin activation probably through stabilization of the active conformation.

Swing-out of the β3 hybrid domain is required for αIIbβ3 priming and normal cytoskeletal reorganization, but not adhesion to immobilized fibrinogen

Structural and functional analyses of integrin αIIbβ3 has implicated swing-out motion of the β3 hybrid domain in αIIbβ3 activation and ligand binding. Using data from targeted molecular dynamics (TMD) simulations, we engineered two disulfide-bonded mutant receptors designed to limit swing-out (XS-O). XS-O mutants cannot bind the high Mr ligand fibrinogen in the presence of an activating mAb or after introducing mutations into the αIIb subunit designed to simulate inside-out signaling. They also have reduced capacity to be “primed” to bind fibrinogen by pretreatment with eptifibatide. They can, however, bind the small RGD venom protein kistrin. Despite their inability to bind soluble fibrinogen, the XS-O mutants can support adhesion to immobilized fibrinogen, although such adhesion does not initiate outside-in signaling leading to normal cytoskeletal reorganization. Collectively, our data further define the biologic role of β3 hybrid domain swing-out in both soluble and immobilized high Mr ligand binding, as well as in priming and outside-in signaling. We also infer that swing-out is likely to be a downstream effect of receptor extension.

A critical role for the regulation of Syk from agglutination to aggregation in human platelets

Agglucetin, a tetrameric glycoprotein (GP) Ibα agonist from Formosan Agkistrodon acutus venom, has been characterized as an agglutination inducer in human washed platelets (WPs). In platelet-rich plasma (PRP), agglucetin dramatically elicits a biphasic response of agglutination and subsequent aggregation. For clarifying the intracellular signaling events from agglutination to aggregation in human platelets, we examined the essential signaling molecules involved through the detection of protein tyrosine phosphorylation (PTP). In WPs, an anti-GPIbα monoclonal antibody (mAb) AP1, but not a Src kinase inhibitor PP1, completely inhibited agglucetin-induced agglutination. However, PP1 but not AP1 had a potent suppression on platelet aggregation by a GPVI activator convulxin. The PTP analyses showed agglucetin alone can cause a weak pattern involving sequential phosphorylation of Lyn/Fyn, Syk, SLP-76 and phospholipase Cγ2 (PLCγ2). Furthermore, a Syk-selective kinase inhibitor, piceatannol, significantly suppressed the aggregating response in agglucetin-activated PRP. Analyzed by flow cytometry, the binding capacity of fluorophore-conjugated PAC-1, a mAb recognizing activated integrin αIIbβ3, was shown to increase in agglucetin-stimulated platelets. Again, piceatannol but not PP1 had a concentration-dependent suppression on agglucetin-induced αIIbβ3 exposure. Moreover, the formation of signalosome, including Syk, SLP-76, VAV, adhesion and degranulation promoting adapter protein (ADAP) and PLCγ2, are required for platelet aggregation in agglucetin/fibrinogen-activated platelets. In addition, GPIbα-ligation via agglucetin can substantially promote the interactions between αIIbβ3 and fibrinogen. Therefore, the signal pathway of Lyn/Fyn/Syk/SLP-76/ADAP/VAV/PLCγ2/PKC is sufficient to trigger platelet aggregation in agglucetin/fibrinogen-pretreated platelets. Importantly, Syk may function as a major regulator for the response from GPIbα-initiated agglutination to integrin αIIbβ3-dependent aggregation in human platelets.

Adjunctive input to the nuclear thyroid hormone receptor from the cell surface receptor for the hormone

At thyroid hormone response elements on specific genes, complexes of nuclear thyroid hormone receptors (TRs) and 3,5,3'-triiodo-L-thyronine (T(3)), coactivator or corepressor nucleoproteins, and histone acetylases or deacetylases mediate genomic effects of the hormone. Nongenomic effects of the hormone are those whose initiation does not primarily depend upon formation of the TR-T(3) complex. Among the nongenomic effects of thyroid hormone are a set of actions initiated at a cell surface receptor on integrin αvβ3 that are relevant to a) intracellular trafficking of proteins, including TRβ1, b) serine phosphorylation and acetylation of this nuclear receptor, c) assembly within the nucleus of complexes of coactivators and corepressor, and d) transcription of specific genes, including that for TRβ1. These actions initiated at αvβ3 are reviewed here and appear to be adjunctive to the genomic actions of the TR-T(3) complex.

Growth factor receptor-bound protein 2 contributes to (hem)immunoreceptor tyrosine-based activation motif-mediated signaling in platelets

RATIONALE

Platelets are anuclear cell fragments derived from bone marrow megakaryocytes (MKs) that safeguard vascular integrity but may also cause pathological vessel occlusion. One major pathway of platelet activation is triggered by 2 receptors that signal through an (hem)immunoreceptor tyrosine-based activation motif (ITAM), the activating collagen receptor glycoprotein (GP) VI and the C-type lectin-like receptor 2 (CLEC-2). Growth factor receptor-bound protein 2 (Grb2) is a ubiquitously expressed adapter molecule involved in signaling processes of numerous receptors in different cell types, but its function in platelets and MKs is unknown.

OBJECTIVE

We tested the hypothesis that Grb2 is a crucial adapter protein in (hem)immunoreceptor tyrosine-based activation motif signaling in platelets.

METHODS AND RESULTS

Here, we show that genetic ablation of Grb2 in MKs and platelets did not interfere with MK differentiation or platelet production. However, Grb2-deficiency severely impaired glycoprotein VI-mediated platelet activation because of defective stabilization of the linker of activated T-cell (LAT) signalosome and activation of downstream signaling proteins that resulted in reduced adhesion, aggregation, and coagulant activity on collagen in vitro. Similarly, CLEC-2-mediated signaling was impaired in Grb2-deficient platelets, whereas the cells responded normally to stimulation of G protein-coupled receptors. In vivo, this selective (hem)immunoreceptor tyrosine-based activation motif signaling defect resulted in prolonged bleeding times but affected arterial thrombus formation only after concomitant treatment with acetylsalicylic acid, indicating that defective glycoprotein VI signaling in the absence of Grb2 can be compensated through thromboxane A2-induced G protein-coupled receptor signaling pathways.

CONCLUSIONS

These results reveal an important contribution of Grb2 in (hem)immunoreceptor tyrosine-based activation motif signaling in platelets in hemostasis and thrombosis by stabilizing the LAT signalosome.

2,3,5,4'-tetrahydroxystilbene-2-O-β-D-glucoside (THSG) attenuates human platelet aggregation, secretion and spreading in vitro

INTRODUCTION

2,3,5,4'-tetrahydroxystilbene-2-O-β-D-glucoside(THSG) is a water-soluble component of the rhizome extract from the traditional Chinese herb Polygonum multiflorum. Recent studies have demonstrated that THSG has potent anti-oxidative and anti-inflammatory effects. In this study, we investigated the anti-platelet aggregation, secretion and spreading of THSG with different methods. The purpose was to explore the anti-platelet effect of THSG and the underlying mechanism.

MATERIALS AND METHODS

We investigated the anti-platelet activity of THSG on platelet aggregation induced by collagen (2 μg/mL), thrombin(0.04U/mL), U46619 (3 μM) and ADP (2 μM). ATP secretion induced by collagen (2 μg/mL) was also investigated. P-selectin expression and PAC-1 binding were measured by flow cytometry. In addition, human platelet spreading on immobilized fibrinogen and immunoblotting were also tested.

RESULTS

THSG dose-dependently inhibited platelet aggregation and ATP secretion induced by collagen. It inhibited platelet P-selectin expression and PAC-1 binding induced by thrombin(0.1U/mL). THSG also inhibited human platelet spreading on immobilized fibrinogen, a process mediated by platelet outside-in signaling. Western blot analysis showed that THSG could inhibit platelet Fc γ RIIa, Akt(Ser473)and GSK3β(Ser9) phosphorylation.

CONCLUSIONS

Our study indicates that THSG has potent anti-platelet activity to collagen induced aggregation. THSG is likely to exert protective effects in platelet-associated thromboembolic disorders by modulating human platelet.

The mechanism of kindlin-mediated activation of integrin αIIbβ3

Increased ligand binding to cellular integrins ("activation") plays important roles in processes such as development, cell migration, extracellular matrix assembly, tumor metastasis, hemostasis, and thrombosis. Integrin activation encompasses both increased integrin monomer affinity and increased receptor clustering and depends on integrin-talin interactions. Loss of kindlins results in reduced activation of integrins. Kindlins might promote talin binding to integrins through a cooperative mechanism; however, kindlins do not increase talin association with integrins. Here, we report that, unlike talin head domain (THD), kindlin-3 has little effect on the affinity of purified monomeric αIIbβ3, and it does not enhance activation by THD. Furthermore, studies with ligands of varying valency show that kindlins primarily increase cellular αIIbβ3 avidity rather than monomer affinity. In platelets or nucleated cells, loss of kindlins markedly reduces αIIbβ3 binding to multivalent but not monovalent ligands. Finally, silencing of kindlins reduces the clustering of ligand-occupied αIIbβ3 as revealed by total internal reflection fluorescence and electron microscopy. Thus, in contrast to talins, kindlins have little primary effect on integrin αIIbβ3 affinity for monovalent ligands and increase multivalent ligand binding by promoting the clustering of talin-activated integrins.

A directional switch of integrin signalling and a new anti-thrombotic strategy

Integrins are critical in thrombosis and hemostasis¹. Antagonists of the platelet integrin α_IIbβ₃ are potent anti-thrombotic drugs, but also have the life-threatening adverse effect of bleeding^2,3. It is thus desirable to develop new antagonists that do not cause bleeding. Integrins transmit signals bidirectionally^4,5. Inside-out signaling activates integrins via a talin-dependent mechanism^6,7. Integrin ligation mediates thrombus formation and outside-in signaling^8,9, which requires Gα₁₃ and greatly expands thrombi. Here we show that Gα₁₃ and talin bind to mutually exclusive, but distinct sites within the integrin β₃ cytoplasmic domain in opposing waves. The first talin binding wave mediates inside-out signaling and also “ligand-induced integrin activation”, but is not required for outside-in signaling. Integrin ligation induces transient talin dissociation and Gα₁₃ binding to an ExE motif, which selectively mediates outside-in signaling and platelet spreading. The second talin binding wave is associated with clot retraction. An ExE motif-based inhibitor of Gα₁₃-integrin interaction selectively abolishes outside-in signaling without affecting integrin ligation, and suppresses occlusive arterial thrombosis without affecting bleeding time. Thus, we have discovered a novel mechanism for the directional switch of integrin signaling and, based on this mechanism, we designed a potent new anti-thrombotic that does not cause bleeding.

Rac regulates collagen-induced HSP27 phosphorylation via p44/p42 MAP kinase in human platelets

We previously reported that the collagen-induced phosphorylation of heat shock protein (HSP) 27 via p44/p42 mitogen-activated protein (MAP) kinase is sufficient to induce the secretion of platelet-derived growth factor (PDGF)-AB and the release of soluble CD40 ligand (sCD40L) from human platelets. It has been shown that Rac, which belongs to the Rho family of small GTPases, is involved in the collagen-induced platelet aggregation. In this study, we investigated the role of Rac in the collagen-stimulated release of PDGF-AB and sCD40L in human platelets. Human blood was donated from healthy volunteers and platelet-rich plasma was obtained from the blood samples. The samples were then treated with 1.0 µg/ml collagen for 0, 1, 3, or 5 min and Rac1 activity was determined using the Rac1 Activation Assay kit. We found that collagen stimulated the activation of Rac in human platelets in a time-dependent manner. However, pre-treatment with NSC23766, a selective inhibitor of Rac-guanine nucleotide exchange factor interaction, reduced the collagen-induced platelet aggregation. NSC23766 markedly attenuated not only the collagen-induced p44/p42 MAP kinase phosphorylation, but also the phosphorylation of HSP27 at three serine residues (Ser-15, Ser-78 and Ser-82). In addition, the collagen‑induced release of PDGF-AB and sCD40L was significantly suppressed by NSC23766 in a dose-dependent manner. These results strongly suggest that Rac regulates the collagen-induced HSP27 phosphorylation via p44/p42 MAP kinase in human platelets, resulting in the stimulation of PDGF-AB secretion and the release of sCD40L.

Differences in α-β transmembrane domain interactions among integrins enable diverging integrin signaling

Integrins are transmembrane adhesion molecules composed of α and β subunits. In humans, 24 integrins are expressed in a tissue-specific manner. Each integrin plays a specific role within a tissue type to control cell adhesion. We previously found that the degree of transmembrane domain (TMD) interaction between the integrin αIIb and β3 subunits is reversely correlated with the affinity of integrin αIIbβ3 to its ligand. Here, we examined the TMD interactions of various integrins, including α4β1, αLβ2, α5β1, αVβ1, αIIbβ3, and αVβ3. Our findings revealed that the degree of the TMD interactions in integrins α4β1 and αLβ2 expressed in immune cells was low and in integrins αIIbβ3 and αVβ3 expressed in platelets was high, while integrins α5β1 and αVβ1 that are expressed in most adherent cells displayed intermediate TMD interactions. We identified sequence variation within the N-terminal TMD region as a factor responsible for the observed differential degree of TMD interaction among integrins. When the N-terminal interaction that was missing in integrin α5β1 was restored with mutagenesis, the increase in TMD interaction inhibited the outside-in but not inside-out signaling of integrin α5β1 and also accelerated the speed of cell migration. We suggest, therefore, that the degree of TMD interaction is designed to accommodate the specific, desired function of each integrin.

Computational study on thrombus formation regulated by platelet glycoprotein and blood flow shear

Thrombogenesis results from the interaction between glycoprotein receptors and their ligands, although a thrombus is affected by multiple factors such as blood flow, platelet interactions, and changes in ligand characteristics. In this study, we propose a platelet adhesion and aggregation model, focusing on the interaction between the glycoprotein receptor and its ligand. First, we conducted thrombogenesis simulations to compare physiological and pathological conditions. The results suggested that simulations of thrombogenesis differed in distribution, volume, and stability of the thrombus based on disorders of platelet adhesion, aggregation, and the activation. For example, distribution and volume were affected by the activation of GPIIb/IIIa with a GPIb/IX/V deficiency. The thrombus was also unstable, but formed from the upstream side of the injured site, with a GPIIb/IIIa deficiency. Second, we investigated thrombogenesis enhanced by the shear-induced platelet aggregation (SIPA) mechanism. The results demonstrated that the degree of SIPA decreased gradually with thrombus growth in a straight vessel. This result suggests that SIPA is a key hemostasis mechanism in an injured healthy arteriole, although it can lead to the formation of an occlusive thrombus in stenosed vessels.

Dual mechanism of integrin αIIbβ3 closure in procoagulant platelets

BACKGROUND

Inactivation of integrin αIIbβ3 reverses platelet aggregate formation upon coagulation.

RESULTS AND CONCLUSION

Platelets from patient (Scott) and mouse (Capn1(-/-) and Ppif(-/-)) blood reveal a dual mechanism of αIIbβ3 inactivation: by calpain-2 cleavage of integrin-associated proteins and by cyclophilin D/TMEM16F-dependent phospholipid scrambling.

SIGNIFICANCE

These data provide novel insight into the switch mechanisms from aggregating to procoagulant platelets. Aggregation of platelets via activated integrin αIIbβ3 is a prerequisite for thrombus formation. Phosphatidylserine-exposing platelets with a key role in the coagulation process disconnect from a thrombus by integrin inactivation via an unknown mechanism. Here we show that αIIbβ3 inactivation in procoagulant platelets relies on a sustained high intracellular Ca(2+), stimulating intracellular cleavage of the β3 chain, talin, and Src kinase. Inhibition of calpain activity abolished protein cleavage, but only partly suppressed αIIbβ3 inactivation. Integrin αIIbβ3 inactivation was unchanged in platelets from Capn1(-/-) mice, suggesting a role of the calpain-2 isoform. Scott syndrome platelets, lacking the transmembrane protein TMEM16F and having low phosphatidylserine exposure, displayed reduced αIIbβ3 inactivation with the remaining activity fully dependent on calpain. In platelets from Ppif(-/-) mice, lacking mitochondrial permeability transition pore (mPTP) formation, agonist-induced phosphatidylserine exposure and αIIbβ3 inactivation were reduced. Treatment of human platelets with cyclosporin A gave a similar phenotype. Together, these data point to a dual mechanism of αIIbβ3 inactivation via calpain(-2) cleavage of integrin-associated proteins and via TMEM16F-dependent phospholipid scrambling with an assistant role of mPTP formation.

Integrin αIIbβ3: from discovery to efficacious therapeutic target

From the initial description of platelets in 1882, their propensity to aggregate and to contribute to thrombosis was apparent. Indeed, excessive platelet aggregation is associated with myocardial infarction and other thrombotic diseases whereas Glanzmann thrombasthenia, in which platelet aggregation is reduced, is a bleeding syndrome. Over the last half of the 20th century, many investigators have provided insights into the cellular and molecular basis for platelet aggregation. The major membrane protein on platelets, integrin αIIbβ3, mediates this response by rapidly transiting from its resting to an activated state in which it serves as a receptor for ligands that can bridge platelets together. Monoclonal antibodies, natural products, and small peptides were all shown to inhibit αIIbβ3 dependent platelet aggregation, and these inhibitors became the forerunners of antagonists that proceeded through preclinical testing and into large patient trials to treat acute coronary syndromes, particularly in the context of percutaneous coronary interventions. Three such αIIbβ3 antagonists, abciximab, eptifibatide, and tirofiban, received Food and Drug Administration approval. Over the past 15 years, millions of patients have been treated with these αIIbβ3 antagonists and many lives have been saved by their administration. With the side effect of increased bleeding and the development of new antithrombotic drugs, the use of αIIbβ3 antagonists is waning. Nevertheless, they are still widely used for the prevention of periprocedural thrombosis during percutaneous coronary interventions. This review focuses on the biology of αIIbβ3, the development of its antagonists, and some of the triumphs and shortcomings of αIIbβ3 antagonism.

Harnessing the platelet signaling network to produce an optimal hemostatic response

Once released into the circulation by megakaryocytes, circulating platelets can undergo rapid activation at sites of vascular injury and resist unwarranted activation, which can lead to heart attacks and strokes. Historically, the signaling mechanisms underlying the regulation of platelet activation have been approached as a collection of individual pathways unique to agonist. This review takes a different approach, casting platelet activation as the product of a signaling network, in which activating and restraining mechanisms interact in a flexible network that regulates platelet adhesiveness, cohesion between platelets, granule secretion, and the formation of a stable hemostatic thrombus.

CD41 expression marks myeloid-biased adult hematopoietic stem cells and increases with age

The hematopoietic stem cell (HSC) compartment is heterogeneous, yet our understanding of the identities of different HSC subtypes is limited. Here we show that platelet integrin CD41 (αIIb), currently thought to only transiently mark fetal HSCs, is expressed on an adult HSC subtype that accumulates with age. CD41+ HSCs were largely quiescent and exhibited myeloerythroid and megakaryocyte gene priming, governed by Gata1, whereas CD41- HSCs were more proliferative and exhibited lymphoid gene priming. When isolated without the use of blocking antibodies, CD41+ HSCs possessed long-term repopulation capacity on serial transplantations and showed a marked myeloid bias compared with CD41- HSCs, which yielded a more lymphoid-biased progeny. CD41-knockout (KO) mice displayed multilineage hematopoietic defects coupled with decreased quiescence and survival of HSCs, suggesting that CD41 is functionally relevant for HSC maintenance and hematopoietic homeostasis. Transplantation experiments indicated that CD41-KO-associated defects are long-term transplantable, HSC-derived and, in part, mediated through the loss of platelet mass leading to decreases in HSC exposure to important platelet released cytokines, such as transforming growth factor β1. In summary, our data provide a novel marker to identify a myeloid-biased HSC subtype that becomes prevalent with age and highlights the dogma of HSC regulation by their progeny.

Megakaryocyte-specific deletion of the protein-tyrosine phosphatases Shp1 and Shp2 causes abnormal megakaryocyte development, platelet production, and function

The SH2 domain-containing protein-tyrosine phosphatases Shp1 and Shp2 have been implicated in regulating signaling from a variety of platelet and megakaryocyte receptors. In this study, we investigate the functions of Shp1 and Shp2 in megakaryocytes and platelets. Megakaryocyte/platelet (MP)-specific deletion of Shp1 in mice resulted in platelets being less responsive to collagen-related peptide due to reduced GPVI expression and signaling via the Src family kinase (SFK)-Syk-PLCγ2 pathway, and fibrinogen due to reduced SFK activity. By contrast, deletion of Shp2 in the MP lineage resulted in macrothrombocytopenia and platelets being hyper-responsive to anti-CLEC-2 antibody and fibrinogen. Shp1- and Shp2-deficient megakaryocytes had partial blocks at 2N/4N ploidy; however, only the latter exhibited reduced proplatelet formation, thrombopoietin, and integrin signaling. Mice deficient in both Shp1 and Shp2 were severely macrothrombocytopenic and had reduced platelet surface glycoprotein expression, including GPVI, αIIbβ3, and GPIbα. Megakaryocytes from these mice were blocked at 2N/4N ploidy and did not survive ex vivo. Deletion of the immunoreceptor tyrosine-based inhibition motif-containing receptor G6b-B in the MP lineage phenocopied multiple features of Shp1/2-deficient mice, suggesting G6b-B is a critical regulator of Shp1 and Shp2. This study establishes Shp1 and Shp2 as major regulators of megakaryocyte development, platelet production, and function.

GDF-15 prevents platelet integrin activation and thrombus formation

BACKGROUND

Integrin-mediated platelet function plays an important role in primary hemostasis. Growth-differentiation factor 15 (GDF-15) has been shown to inhibit β(2) -integrin activation in leukocytes.

METHODS

We investigated the effect of GDF-15 on platelet integrin activation in vitro and in different in vivo models of thrombus formation.

RESULTS

GDF-15(-/-) mice showed an accelerated thrombus formation and a reduced survival rate after collagen-induced pulmonary thromboembolism. In reconstitution experiments, recombinant GDF-15 decelerated thrombus formation and prolonged the bleeding time. In vitro experiments demonstrated that GDF-15 pretreated, agonist-stimulated platelets showed decreased binding to fibrinogen in flow chamber assays and reduced activation of β(1) - and β(3) -integrins in flow cytometry experiments. Pretreating human and mouse platelets with GDF-15 reduced platelet aggregation. Mechanistically, GDF-15 prevents agonist-induced Rap1- dependent α(II) (b) β(3) activation by activating PKA. Platelet P-selectin expression and dense granule secretion after stimulation were unaffected by GDF-15, indicating a specific effect of GDF-15 on integrin activation.

CONCLUSION

GDF-15 specifically inhibits platelet integrin activation. These findings may have profound clinical implications for the treatment of hemostatic conditions involving platelets.

The proline-rich tyrosine kinase Pyk2 regulates platelet integrin αIIbβ3 outside-in signaling

BACKGROUND

The proline-rich tyrosine kinase Pyk2 is a focal adhesion kinase expressed in blood platelets, and is activated downstream of G-protein coupled receptors as well as integrin α2β1.

OBJECTIVE

In this study we have investigated the involvement of Pyk2 in integrin αIIbβ3 outside-in signaling in human and murine platelets.

METHODS

We analyzed the stimulation of intracellular signaling pathways in platelets from Pyk2 knockout mice adherent to immobilized fibrinogen.

RESULTS

Pyk2 was rapidly phosphorylated and activated in human and murine platelets adherent to fibrinogen through integrin αIIbβ3. Activation of Pyk2 was Src-dependent, but did not require phospholipase Cγ2 activity. Platelets from Pyk2 knockout mice showed a defective ability to adhere and spread on fibrinogen, in association with a dramatic reduction of phosphatidylinositol 3-kinase (PI3K) activation and Akt phosphorylation. Pharmacological and genetic analysis demonstrated that integrin αIIbβ3 engagement selectively stimulated the β-isoform of PI3K (PI3Kβ), and that, as for Pyk2, PI3Kβ activation required Src family kinases activity, but not phospholipase Cγ2. In fibrinogen-adherent platelets, both Pyk2 and PI3Kβ were necessary for stimulation of the small GTPase Rap1b, a regulator of cell adhesion and spreading. Integrin αIIbβ3 engagement triggered the association of the PI3Kβ regulatory subunit p85 with the adaptor protein c-Cbl, which was mediated by the p85 SH3 domain, and was independent of c-Cbl tyrosine phosphorylation. However, p85-associated c-Cbl was tyrosine phosphorylated by activated Pyk2 in fibrinogen adherent platelets.

CONCLUSIONS

These results identify a novel pathway of integrin αIIbβ3 outside-in signaling and recognize the tyrosine kinase Pyk2 as a major regulator of platelet adhesion and spreading on fibrinogen.

Cooperative integrin/ITAM signaling in platelets enhances thrombus formation in vitro and in vivo

The integrin family is composed of a series of 24 αβ heterodimer transmembrane adhesion receptors that mediate cell-cell and cell-extracellular matrix interactions. Adaptor molecules bearing immunoreceptor tyrosine-based activation motifs (ITAMs) have recently been shown to cooperate with specific integrins to increase the efficiency of transmitting ligand-binding-induced signals into cells. In human platelets, Fc receptor γ-chain IIa (FcγRIIa) has been identified as an ITAM-bearing transmembrane receptor responsible for mediating "outside-in" signaling through αIIbβ3, the major adhesion receptor on the platelet surface. To explore the importance of FcγRIIa in thrombosis and hemostasis, we subjected FcγRIIa-negative and FcγRIIa-positive murine platelets to a number of well-accepted models of platelet function. Compared with their FcγRIIa-negative counterparts, FcγRIIa-positive platelets exhibited increased tyrosine phosphorylation of Syk and phospholipase Cγ2 and increased spreading upon interaction with immobilized fibrinogen, retracted a fibrin clot faster, and showed markedly enhanced thrombus formation when perfused over a collagen-coated flow chamber under conditions of arterial and venous shear. They also displayed increased thrombus formation and fibrin deposition in in vivo models of vascular injury. Taken together, these data establish FcγRIIa as a physiologically important functional conduit for αIIbβ3-mediated outside-in signaling, and suggest that modulating the activity of this novel integrin/ITAM pair might be effective in controlling thrombosis.

Platelet-leukocyte crosstalk: Linking proinflammatory responses to procoagulant state

Platelet activation is known to be associated with the release of a vast array of chemokines and proinflammatory lipids which induce pleiotropic effects on a wide variety of tissues and cells, including leukocytes. During thrombosis, the recruitment of leukocytes to activated platelets is considered an important step which not only links thrombosis to inflammatory responses but may also enhance procoagulant state. This phenomenon is highly regulated and influenced by precise mutual interactions between the cells at site of vascular injury and thrombi formation. Platelet-leukocyte interaction involves a variety of mediators including adhesion molecules, chemokines and chemoattractant molecules, shed proteins, various proinflammatory lipids and other materials. The current review addresses the detailed mechanisms underlying platelet-leukocyte crosstalk. This includes their adhesive interactions, transcellular metabolisms, induced tissue factor activity and neutrophil extracellular traps formation as well as the impacts of these phenomena in modulation of the proinflammatory and procoagulant functions in a reciprocal manner that enhances the physiological responses.

Headpiece domain of dematin regulates calcium mobilization and signaling in platelets

Dematin is a broadly expressed membrane cytoskeletal protein that has been well characterized in erythrocytes and to a lesser extent in non-erythroid cells. However, dematin's function in platelets is not known. Here, we show that dematin is abundantly expressed in both human and mouse platelets. Platelets harvested from the dematin headpiece knock-out (HPKO) mouse model exhibit a striking defect in the mobilization of calcium in response to multiple agonists of platelet activation. The reduced calcium mobilization in HPKO platelets is associated with concomitant inhibition of platelet aggregation and granule secretion. Integrin α(IIb)β(3) activation in response to agonists is attenuated in the HPKO platelets. The mutant platelets show nearly normal spreading on fibrinogen and an unaltered basal cAMP level; however, the clot retraction was compromised in the mutant mice. Immunofluorescence analysis indicated that dematin is present both at the dense tubular system and plasma membrane fractions of platelets. Proteomic analysis of dematin-associated proteins in human platelets identified inositol 1,4,5-trisphosphate 3-kinase isoform B (IP3KB) as a binding partner, which was confirmed by immunoprecipitation analysis. IP3KB, a dense tubular system protein, is a major regulator of calcium homeostasis. Loss of the dematin headpiece resulted in a decrease of IP3KB at the membrane and increased levels of IP3KB in the cytosol. Collectively, these findings unveil dematin as a novel regulator of internal calcium mobilization in platelets affecting multiple signaling and cytoskeletal functions. Implications of a conserved role of dematin in the regulation of calcium homeostasis in other cell types will be discussed.

Distinct roles for the α , β and γ1 isoforms of protein phosphatase 1 in the outside-in αIIbβ3 integrin signalling-dependent functions

Although protein kinases and phosphatases participate in integrin αIIbβ3 signalling, whether integrin functions are regulated by the catalytic subunit of protein phosphatase 1(PP1c)isoforms are unclear. We show that siRNA mediated knockdown of all PP1c isoforms(α, β and γ1)in 293 αIIbβ3 cells decreased adhesion to immobilised fibrinogen and fibrin clot retraction. Selective knockdown of only PP1cγ1 did not alter adhesion or clot retraction, while depletion of PP1cβ decreased both functions. Unexpectedly, knockdown of PP1cα enhanced αIIbβ3 adhesion to fibrinogen and clot retraction. Protein interaction studies revealed that all PP1c isoforms can interact with the integrin αIIb subunit. Phospho-profiling studies revealed an enhanced activation of mitogen-activated protein kinase (MAPK) p38 in the PP1cα depleted cells. Enhanced adhesive phenotype displayed by the PP1cα-depleted 293 αIIbβ3 cells was blocked by pharmacological inhibition of p38. Conversely, the decreased adhesion of PP1cα overexpressing cells was rescued by the expression of constitutively active p38α or p38γ. Thus, PP1c isoforms have distinct contribution to the outside-in αIIbβ3 signalling-dependent functions in 293 αIIbβ3 cells. Moreover, PP1cα negatively regulates integrin function by suppressing the p38 pathway.

The a1/a2 polymorphism of the glycoprotein IIIa gene and myocardial infarction in Caucasians with type 2 diabetes

A PlA1/A2 polymorphism of glycoprotein IIIa is known to be involved in the pathogenesis of coronary artery disease (CAD) and myocardial infarction (MI). The aim of this study was to investigate an association between the PlA1/A2 polymorphism of the glycoprotein IIIa gene and the risk of MI in Caucasians with type 2 diabetes. 549 Caucasians with type 2 diabetes were enrolled in the cross sectional retrospective case-control study: 224 patients with MI and 325 diabetic subjects without CAD. Blood biochemical analysis was performed. The polymerase chain reaction with restriction fragment length polymorphism was used for genetic analysis. Patients with MI were older (62 ± 11.8 vs. 58.5 ± 11.6 years; P < 0.001), and had a longer duration of type 2 diabetes (17.6 ± 8.9 vs. 15.1 ± 9.2; P = 0.01) compared to the diabetics without CAD. A significant difference in distribution of the A2A2 genotype of glycoprotein IIIa was not found between 224 diabetic patients with MI in comparison to 325 diabetics without CAD (11.6 vs. 14,1 %; n.s.). The diabetes duration and male sex were independent factors for the development of MI, whereas the PlA1/A2 polymorphism of glycoprotein IIIa was not. To conclude, The A2A2 genotype of the glycoprotein IIIa polymorphism was not associated with MI risk in Caucasians with type 2 diabetes.

Platelet function in health and disease: from molecular mechanisms, redox considerations to novel therapeutic opportunities

Increased oxidative stress appears to be of fundamental importance in the pathogenesis and development of several disease processes. Indeed, it is well known that reactive oxygen species (ROS) exert critical regulatory functions within the vascular wall, and it is, therefore, plausible that platelets represent a relevant target for their action. Platelet activation cascade (including receptor-mediated tethering to the endothelium, rolling, firm adhesion, aggregation, and thrombus formation) is tightly regulated. In addition to already well-defined platelet regulatory factors, ROS may participate in the regulation of platelet activation. It is already established that enhanced ROS release from the vascular wall can indirectly affect platelet activity by scavenging nitric oxide (NO), thereby decreasing the antiplatelet properties of endothelium. On the other hand, recent data suggest that platelets themselves generate ROS, which may evoke pro-thrombotic responses, triggering many biological processes participating in atherosclerosis initiation, progression, and complication. That oxidative stress may alter platelet function is conceivable when considering that antioxidants play a role in the prevention of cardiovascular disease, although the precise mechanism accounting for changes attributable to antioxidants in atherosclerosis remains unknown. It is possible that the effects of antioxidants may be a consequence of their enhancing or promoting the antiplatelet effects of NO derived from both endothelial cells and platelets. This review focuses on current knowledge regarding ROS-dependent regulation of platelet function in health and disease, and summarizes in vitro and in vivo evidence for their physiological and potential therapeutic relevance.

Platelets induce apoptosis during sepsis in a contact-dependent manner that is inhibited by GPIIb/IIIa blockade

Purpose

End-organ apoptosis is well-described in progressive sepsis and Multiple Organ Dysfunction Syndrome (MODS), especially where platelets accumulate (e.g. spleen and lung). We previously reported an acute sepsis-induced cytotoxic platelet phenotype expressing serine protease granzyme B. We now aim to define the site(s) of and mechanism(s) by which platelet granzyme B induces end-organ apoptosis in sepsis.

Methods

End-organ apoptosis in murine sepsis (i.e. polymicrobial peritonitis) was analyzed by immunohistochemistry. Platelet cytotoxicity was measured by flow cytometry following 90 minute ex vivo co-incubation with healthy murine splenocytes. Sepsis progression was measured via validated preclinical murine sepsis score.

Measurements and Main Results

There was evident apoptosis in spleen, lung, and kidney sections from septic wild type mice. In contrast, there was a lack of TUNEL staining in spleens and lungs from septic granzyme B null mice and these mice survived longer following induction of sepsis than wild type mice. In co-incubation experiments, physical separation of septic platelets from splenocytes by a semi-permeable membrane reduced splenocyte apoptosis to a rate indistinguishable from negative controls. Chemical separation by the platelet GPIIb/IIIa receptor inhibitor eptifibatide decreased apoptosis by 66.6±10.6% (p = 0.008). Mice treated with eptifibatide in vivo survived longer following induction of sepsis than vehicle control mice.

Conclusions

In sepsis, platelet granzyme B-mediated apoptosis occurs in spleen and lung, and absence of granzyme B slows sepsis progression. This process proceeds in a contact-dependent manner that is inhibited ex vivo and in vivo by the platelet GPIIb/IIIa receptor inhibitor eptifibatide. The GPIIb/IIIa inhibitors and other classes of anti-platelet drugs may be protective in sepsis.

ADP-stimulated activation of Akt during integrin outside-in signaling promotes platelet spreading by inhibiting glycogen synthase kinase-3β

OBJECTIVE

Integrins mediate platelet adhesion and transmit outside-in signals leading to platelet spreading. Phosphoinositide 3-kinases (PI3Ks) play a critical role in outside-in signaling and platelet spreading; however, the mechanisms of PI3K activation and function in outside-in signaling are unclear. We sought to determine the role of the Akt family of serine/threonine kinases and activation mechanisms of the PI3K/Akt pathway in outside-in signaling.

METHODS AND RESULTS

Akt inhibitors and Akt3 knockout inhibited platelet spreading on fibrinogen, indicating that Akt is important in integrin outside-in signaling. Akt inhibitors and Akt3 knockout also diminished integrin-dependent phosphorylation of glycogen synthase kinase-3β. Inhibition of glycogen synthase kinase-3β reversed the inhibitory effects of Akt3 knockout and inhibitors of Akt or PI3K on platelet spreading, indicating that glycogen synthase kinase-3β is a downstream target of Akt in outside-in signaling. Integrin-dependent activation of the PI3K-Akt pathway requires Src family kinase. Akt phosphorylation is also significantly inhibited in ADP receptor P2Y12 knockout platelets and further inhibited in P2Y12 knockout platelets treated with a P2Y1 antagonist. Consistently, P2Y12 knockout and P2Y1 inhibition together reduced platelet spreading.

CONCLUSIONS

These results demonstrate that integrin outside-in signaling and platelet spreading requires Src family kinase-dependent and ADP receptor-amplified activation of the PI3K-Akt-GSK-3β pathway.