Numerous growth factors, cytokines, and chemokines are secreted by human CD34(+) cells, myeloblasts, erythroblasts, and megakaryoblasts and regulate normal hematopoiesis in an autocrine/paracrine manner

The aim of this study was to explore further the hypothesis that early stages of normal human hematopoiesis might be coregulated by autocrine/paracrine regulatory loops and by cross-talk among early hematopoietic cells. Highly purified normal human CD34(+) cells and ex vivo expanded early colony-forming unit-granulocyte-macrophage (CFU-GM)-derived, burst forming unit-erythroid (BFU-E)-derived, and CFU-megakaryocyte (CFU-Meg)-derived cells were phenotyped for messenger RNA expression and protein secretion of various growth factors, cytokines, and chemokines to determine the biological significance of this secretion. Transcripts were found for numerous growth factors (kit ligand [KL], FLT3 ligand, fibroblast growth factor-2 [FGF-2], vascular endothelial growth factor [VEGF], hepatocyte growth factor [HGF], insulinlike growth factor-1 [IGF-1], and thrombopoietin [TPO]); cytokines (tumor necrosis factor-alpha, Fas ligand, interferon alpha, interleukin 1 [IL-1], and IL-16); and chemokines (macrophage inflammatory protein-1alpha [MIP-1alpha], MIP-1beta, regulated upon activation, normal T cell expressed and secreted [RANTES], monocyte chemotactic protein-3 [MCP-3], MCP-4, IL-8, interferon-inducible protein-10, macrophage-derived chemokine [MDC], and platelet factor-4 [PF-4]) to be expressed by CD34(+) cells. More importantly, the regulatory proteins VEGF, HGF, FGF-2, KL, FLT3 ligand, TPO, IL-16, IGF-1, transforming growth factor-beta1 (TGF-beta1), TGF-beta2, RANTES, MIP-1alpha, MIP-1beta, IL-8, and PF-4 were identified in media conditioned by these cells. Moreover, media conditioned by CD34(+) cells were found to inhibit apoptosis and slightly stimulate the proliferation of other freshly isolated CD34(+) cells; chemo-attract CFU-GM- and CFU-Meg-derived cells as well as other CD34(+) cells; and, finally, stimulate the proliferation of human endothelial cells. It was also demonstrated that these various hematopoietic growth factors, cytokines, and chemokines are expressed and secreted by CFU-GM-, CFU-Meg-, and BFU-E-derived cells. It is concluded that normal human CD34(+) cells and hematopoietic precursors secrete numerous regulatory molecules that form the basis of intercellular cross-talk networks and regulate in an autocrine and/or a paracrine manner the various stages of normal human hematopoiesis.

Platelet-derived microparticles bind to hematopoietic stem/progenitor cells and enhance their engraftment

Because human CD34+ and murine Sca-1+ hematopoietic stem-progenitor cells (HSPCs) express platelet-binding sialomucin P-selectin (CD162) and integrin Mac-1 (CD11b-CD18) antigen, it was inferred that these cells might interact with platelets. As a result of this interaction, microparticles derived from platelets (PMPs) may transfer many platelet antigens (CD41, CD61, CD62, CXCR4, PAR-1) to the surfaces of HSPCs. To determine the biologic significance of the presence of PMPs on human CD34+ and murine Sca-1+ cells, their expressions on mobilized peripheral blood (mPB) and on nonmobilized PB- and bone marrow (BM)-derived CD34+ cells were compared. In addition, the effects of PMPs on the proliferation of CD34+ and Sca-1+ cells and on adhesion of HSPCs to endothelium and immobilized SDF-1 were studied. Finally, the hematopoietic reconstitution of lethally irradiated mice receiving transplanted BM mononuclear cells covered or not covered with PMPs was examined. It was found that PMPs are more numerous on mPB than on BM CD34+ cells, do not affect the clonogenicity of human and murine HSPCs, and increase adhesion of these cells to endothelium and immobilized SDF-1. Moreover, murine BM cells covered with PMPs engrafted lethally irradiated mice significantly faster than those not covered, indicating that PMPs play an important role in the homing of HSPCs. This could explain why in a clinical setting human mPB HSPCs (densely covered with PMPs) engraft more rapidly than BM HSPCs (covered with fewer PMPs). These findings indicate a new role for PMPs in stem cell transplantation and may have clinical implications for the optimization of transplantations.

Coagulation facilitates tumor cell spreading in the pulmonary vasculature during early metastatic colony formation

Coagulation has long been known to facilitate metastasis. To pinpoint the steps where coagulation might play a role in the metastasis, we used three-dimensional visualization of direct infusion of fluorescence labeled antibody to observe the interaction of tumor cells with platelets and fibrinogen in isolated lung preparations. Tumor cells arrested in the pulmonary vasculature were associated with a clot composed of both platelets and fibrin(ogen). Initially, the cells attached to the pulmonary vessels were rounded. Over the next 2 to 6 hours, they spread on the vessel surface. The associated clot was lysed coincident with tumor cell spreading. To assess the importance of clot formation, we inhibited coagulation with hirudin, a potent inhibitor of thrombin. The number of tumor cells initially arrested in the lung of hirudin-treated mice was essentially the same as in control mice. However, tumor cell spreading and subsequent retention of the tumor cells in the lung was markedly inhibited in the anticoagulated mice. These associations of the tumor cells with platelets were independent of tumor cell expression of P-selectin ligands. This work identifies tumor cell spreading onto the vascular surface as an important component of the metastatic cascade and implicates coagulation in this process.

Role of platelet surface PF4 antigenic complexes in heparin-induced thrombocytopenia pathogenesis: diagnostic and therapeutic implications

Heparin-induced thrombocytopenia (HIT) antibodies recognize complexes between heparin and platelet factor 4 (PF4). Heparin and PF4 bind HIT antibodies only over a narrow molar ratio. We explored the involvement of platelet surface-bound PF4 as an antigen in the pathogenesis of experimental HIT. We show that cell-surface PF4 complexes are also antigenic only over a restricted concentration range of PF4. Heparin is not required for HIT antibody binding but shifts the concentration of PF4 needed for optimal surface antigenicity to higher levels. These data are supported by in vitro studies involving both human and murine platelets with exogenous recombinant human (h) PF4 and either an anti-PF4-heparin monoclonal antibody (KKO) or HIT immunoglobulin. Injection of KKO into transgenic mice expressing different levels of hPF4 demonstrates a correlation between the severity of the thrombocytopenia and platelet hPF4 expression. Therapeutic interventions in this model using high-dose heparin or protamine sulfate support the pathogenic role of surface PF4 antigenic complexes in the etiology of HIT. We believe that this focus on surface PF4 advances our understanding of the pathogenesis of HIT, suggests ways to identify patients at high risk to develop HIT upon heparin exposure, and offers new therapeutic strategies.

Loss of signaling through the G protein, Gz, results in abnormal platelet activation and altered responses to psychoactive drugs

Heterotrimeric G proteins mediate the earliest step in cell responses to external events by linking cell surface receptors to intracellular signaling pathways. G(z) is a member of the G(i) family of G proteins that is prominently expressed in platelets and brain. Here, we show that deletion of the alpha subunit of G(z) in mice: (i) impairs platelet aggregation by preventing the inhibition of cAMP formation normally seen at physiologic concentrations of epinephrine, and (ii) causes the mice to be more resistant to fatal thromboembolism. Loss of G(zalpha) also results in greatly exaggerated responses to cocaine, reduces the analgesic effects of morphine, and abolishes the effects of widely used antidepressant drugs that act as catecholamine reuptake inhibitors. These changes occur despite the presence of other G(ialpha) family members in the same cells and are not accompanied by detectable compensatory changes in the level of expression of other G protein subunits. Therefore, these results provide insights into receptor selectivity among G proteins and a model for understanding platelet function and the effects of psychoactive drugs.

CXC chemokine ligand 4 (Cxcl4) is a platelet-derived mediator of experimental liver fibrosis

Liver fibrosis is a major cause of morbidity and mortality worldwide. Platelets are involved in liver damage, but the underlying molecular mechanisms remain elusive. Here, we investigate the platelet-derived chemokine (C-X-C motif) ligand 4 (CXCL4) as a molecular mediator of fibrotic liver damage. Serum concentrations and intrahepatic messenger RNA of CXCL4 were measured in patients with chronic liver diseases and mice after toxic liver injury. Platelet aggregation in early fibrosis was determined by electron microscopy in patients and by immunohistochemistry in mice. Cxcl4(-/-) and wild-type mice were subjected to two models of chronic liver injury (CCl(4) and thioacetamide). The fibrotic phenotype was analyzed by histological, biochemical, and molecular analyses. Intrahepatic infiltration of immune cells was investigated by fluorescence-activated cell sorting, and stellate cells were stimulated with recombinant Cxcl4 in vitro. The results showed that patients with advanced hepatitis C virus-induced fibrosis or nonalcoholic steatohepatitis had increased serum levels and intrahepatic CXCL4 messenger RNA concentrations. Platelets were found directly adjacent to collagen fibrils. The CCl(4) and thioacetamide treatment led to an increase of hepatic Cxcl4 levels, platelet activation, and aggregation in early fibrosis in mice. Accordingly, genetic deletion of Cxcl4 in mice significantly reduced histological and biochemical liver damage in vivo, which was accompanied by changes in the expression of fibrosis-related genes (Timp-1 [tissue inhibitor of matrix metalloproteinase 1], Mmp9 [matrix metalloproteinase 9], Tgf-beta [transforming growth factor beta], IL10 [interleukin 10]). Functionally, Cxcl4(-/-) mice showed a strongly decreased infiltration of neutrophils (Ly6G) and CD8(+) T cells into the liver. In vitro, recombinant murine Cxcl4 stimulated the proliferation, chemotaxis, and chemokine expression of hepatic stellate cells.

CONCLUSION

The results underscore an important role of platelets in chronic liver damage and imply a new target for antifibrotic therapies.

Transgenic mice studies demonstrate a role for platelet factor 4 in thrombosis: dissociation between anticoagulant and antithrombotic effect of heparin

The platelet-specific chemokine platelet factor 4 (PF4) is released in large amounts at sites of vascular injury. PF4 binds to heparin with high affinity, but its in vivo biologic role has not been defined. We studied the role of PF4 in thrombosis using heterozygote and homozygote PF4 knock-out mice (mPF4(+/-) and mPF4(-/-), respectively) and transgenic mice overexpressing human PF4 (hPF4(+)). None of these lines had an overt bleeding diathesis, but in a FeCl(3) carotid artery thrombosis model, all showed impaired thrombus formation. This defect in thrombus formation in the mPF4(-/-) animals was corrected by infusing hPF4 over a narrow concentration range. The thrombotic defect in the mPF4(+/-) and mPF4(-/-) animals was particularly sensitive to infusions of the negatively charged anticoagulant heparin. However, the same amount of heparin paradoxically normalized thrombus formation in the hPF4(+) animals, although these animals were anticoagulated systemically. Upon infusion of the positively charged protein, protamine sulfate, the reverse was observed with mPF4(+/-) and mPF4(-/-) animals having improved thrombosis, with the hPF4(+) animals having worsened thrombus formation. These studies support an important role for PF4 in thrombosis, and show that neutralization of PF4 is an important component of heparin's anticoagulant effect. The mechanisms underlying these observations of PF4 biology and their clinical implications remain to be determined.

Factor VIII ectopically expressed in platelets: efficacy in hemophilia A treatment

Activated platelets release their granule content in a concentrated fashion at sites of injury. We examined whether ectopically expressed factor VIII in developing megakaryocytes would be stored in alpha-granules and whether its release from circulating platelets would effectively ameliorate bleeding in a factor VIIInull mice model. Using the proximal glycoprotein 1b alpha promoter to drive expression of a human factor VIII cDNA construct, transgenic lines were established. One line had detectable human factor VIII that colocalizes with von Willebrand factor in platelets. These animals had platelet factor VIII levels equivalent to 3% to 9% plasma levels, although there was no concurrent plasma human factor VIII detectable. When crossed onto a factor VIIInull background, whole blood clotting time was partially corrected, equivalent to a 3% correction level. In a cuticular bleeding time study, these animals also had only a partial correction, but in an FeCl3 carotid artery, thrombosis assay correction was equivalent to a 50% to 100% level. These studies show that factor VIII can be expressed and stored in platelet alpha-granules. Our studies also suggest that platelet-released factor VIII is at least as potent as an equivalent plasma level and perhaps even more potent in an arterial thrombosis model.

Megakaryocyte precursors, megakaryocytes and platelets express the HIV co-receptor CXCR4 on their surface: determination of response to stromal-derived factor-1 by megakaryocytes and platelets

Thrombocytopenia is a late complication of human immunodeficiency virus (HIV) infection. The chemokine receptor CXCR4 has been shown to be a co-receptor for lymphocyte-tropic HIV-1 strains. CXCR4 is also a natural receptor for the chemokine SDF-1. We have previously shown that CXCR1 and CXCR2 are present on megakaryocytes and platelets. Although interleukin-8 (IL-8) and other chemokines that bind to these two receptors do not activate platelets, they are able to inhibit megakaryocytopoiesis, presumably through these receptors. We therefore examined whether CXCR4 is present on developing and mature megakaryocytes and on platelets. Reverse transcription-polymerase chain reaction (RT-PCR) demonstrated the presence of CXCR4 message. Immature and mature alphaIIbbeta3+ megakaryocytes, and platelets were also positive for CXCR4 by flow cytometric studies using a CXCR4-specific antibody. We then tested whether SDF-1 can affect the biology of these cells. CD34+ cells and immature alphaIIbbeta3+ cells responded to SDF-1 as indicated by Ca2+ mobilization and chemotaxis. However, mature megakaryocytes failed to demonstrate either of these responses, in spite of their continued ability to bind 125I-SDF-1. Further, SDF-1 failed to inhibit megakaryocyte colony growth. Platelets bound 125I-SDF-1 with a K(D) similar to the affinity seen for CXCR4 on other cells, yet SDF-1 did not aggregate washed platelets nor augment aggregation by low-dose ADP or thrombin. SDF-1 also failed to stimulate Ca2+ mobilization, granular release or expression of P-selectin in platelets. Accordingly, although our studies demonstrate that CD34+ precursors, megakaryocytes and platelets all express CXCR4 and bind SDF-1, biological effects were only demonstrable of SDF-1 on CD34+ precursors. The potential biological implications of CXCR4 expression on maturing megakaryocytes and platelets in normal individuals and following HIV infection are discussed.

Neutrophil accumulation and NET release contribute to thrombosis in HIT

Heparin-induced thrombocytopenia (HIT) is an immune-mediated thrombocytopenic disorder associated with a severe prothrombotic state. We investigated whether neutrophils and neutrophil extracellular traps (NETs) contribute to the development of thrombosis in HIT. Using an endothelialized microfluidic system and a murine passive immunization model, we show that HIT induction leads to increased neutrophil adherence to venous endothelium. In HIT mice, endothelial adherence is enhanced immediately downstream of nascent venous thrombi, after which neutrophils undergo retrograde migration via a CXCR2-dependent mechanism to accumulate into the thrombi. Using a microfluidic system, we found that PF4 binds to NETs, leading them to become compact and DNase resistant. PF4-NET complexes selectively bind HIT antibodies, which further protect them from nuclease digestion. In HIT mice, inhibition of NET formation through Padi4 gene disruption or DNase treatment limited venous thrombus size. PAD4 inactivation did affect arterial thrombi or severity of thrombocytopenia in HIT. Thus, neutrophil activation contributes to the development of venous thrombosis in HIT by enhancing neutrophil-endothelial adhesion and neutrophil clot infiltration, where incorporated PF4-NET-HIT antibody complexes lead to thrombosis propagation. Inhibition of neutrophil endothelial adhesion, prevention of neutrophil chemokine-dependent recruitment of neutrophils to thrombi, or suppression of NET release should be explored as strategies to prevent venous thrombosis in HIT.

Platelet factor 4 is a negative autocrine in vivo regulator of megakaryopoiesis: clinical and therapeutic implications

Platelet factor 4 (PF4) is a negative regulator of megakaryopoiesis in vitro. We have now examined whether PF4 regulates megakaryopoiesis in vivo by studying PF4 knockout mice and transgenic mice that overexpress human (h) PF4. Steady-state platelet count and thrombocrit in these animals was inversely related to platelet PF4 content. Growth of megakaryocyte colonies was also inversely related to platelet PF4 content. Function-blocking anti-PF4 antibody reversed this inhibition of megakaryocyte colony growth, indicating the importance of local PF4 released from developing megakaryocytes. The effect of megakaryocyte damage and release of PF4 on 5-fluorouracil-induced marrow failure was then examined. Severity of thrombocytopenia and time to recovery of platelet counts were inversely related to initial PF4 content. Recovery was faster and more extensive, especially in PF4-overexpressing mice, after treatment with anti-PF4 blocking antibodies, suggesting a means to limit the duration of such a chemotherapy-induced thrombocytopenia, especially in individuals with high endogenous levels of PF4. We found that approximately 8% of 250 healthy adults have elevated (> 2 times average) platelet PF4 content. These individuals with high levels of platelet PF4 may be especially sensitive to developing thrombocytopenia after bone marrow injury and may benefit from approaches that block the effects of released PF4.

Infusion of mature megakaryocytes into mice yields functional platelets

Thrombopoiesis, the process by which circulating platelets arise from megakaryocytes, remains incompletely understood. Prior studies suggest that megakaryocytes shed platelets in the pulmonary vasculature. To better understand thrombopoiesis and to develop a potential platelet transfusion strategy that is not dependent upon donors, of which there remains a shortage, we examined whether megakaryocytes infused into mice shed platelets. Infused megakaryocytes led to clinically relevant increases in platelet numbers. The released platelets were normal in size, displayed appropriate surface markers, and had a near-normal circulating half-life. The functionality of the donor-derived platelets was also demonstrated in vivo. The infused megakaryocytes mostly localized to the pulmonary vasculature, where they appeared to shed platelets. These data suggest that it may be unnecessary to generate platelets from ex vivo grown megakaryocytes to achieve clinically relevant increases in platelet numbers.

Platelet factor 4 limits Th17 differentiation and cardiac allograft rejection

Th cells are the major effector cells in transplant rejection and can be divided into Th1, Th2, Th17, and Treg subsets. Th differentiation is controlled by transcription factor expression, which is driven by positive and negative cytokine and chemokine stimuli at the time of T cell activation. Here we discovered that chemokine platelet factor 4 (PF4) is a negative regulator of Th17 differentiation. PF4-deficient and platelet-deficient mice had exaggerated immune responses to cardiac transplantation, including increased numbers of infiltrating Th17 cells and increased plasma IL-17. Although PF4 has been described as a platelet-specific molecule, we found that activated T cells also express PF4. Furthermore, bone marrow transplantation experiments revealed that T cell-derived PF4 contributes to a restriction in Th17 differentiation. Taken together, the results of this study demonstrate that PF4 is a key regulator of Th cell development that is necessary to limit Th17 differentiation. These data likely will impact our understanding of platelet-dependent regulation of T cell development, which is important in many diseases, in addition to transplantation.

Platelet signal transduction defect with Galpha subunit dysfunction and diminished Galphaq in a patient with abnormal platelet responses

G proteins play a major role in signal transduction upon platelet activation. We have previously reported a patient with impaired agonist-induced aggregation, secretion, arachidonate release, and Ca2+ mobilization. Present studies demonstrated that platelet phospholipase A2 (cytosolic and membrane) activity in the patient was normal. Receptor-mediated activation of glycoprotein (GP) IIb-IIIa complex measured by flow cytometry using antibody PAC-1 was diminished despite normal amounts of GPIIb-IIIa on platelets. Ca2+ release induced by guanosine 5'-[gamma-thio]triphosphate (GTP[gammaS]) was diminished in the patient's platelets, suggesting a defect distal to agonist receptors. GTPase activity (a function of alpha-subunit) in platelet membranes was normal in resting state but was diminished compared with normal subjects on stimulation with thrombin, platelet-activating factor, or the thromboxane A2 analog U46619. Binding of 35S-labeled GTP[gammaS] to platelet membranes was decreased under both basal and thrombin-stimulated states. Iloprost (a stable prostaglandin I2 analog) -induced rise in cAMP (mediated by Galphas) and its inhibition (mediated by Galphai) by thrombin in the patient's platelet membranes were normal. Immunoblot analysis of Galpha subunits in the patient's platelet membranes showed a decrease in Galphaq (<50%) but not Galphai, Galphaz, Galpha12, and Galpha13. These studies provide evidence for a hitherto undescribed defect in human platelet G-protein alpha-subunit function leading to impaired platelet responses, and they provide further evidence for a major role of Galphaq in thrombin-induced responses.

Binding of stromal derived factor-1alpha (SDF-1alpha) to CXCR4 chemokine receptor in normal human megakaryoblasts but not in platelets induces phosphorylation of mitogen-activated protein kinase p42/44 (MAPK), ELK-1 transcription factor and serine/threonine kinase AKT

The aim of this study was to identify pathways which are involved in signal transduction from the CXCR4 receptor stimulated by stromal derived factor-1alpha (SDF-1alpha) in human malignant hematopoietic cells and normal megakaryoblasts. First, we found that activation of CXCR4 in human T cell lines (Jurkat and ATL-2) rapidly induced phosphorylation of mitogen-activated protein kinases (MAPK) (p44 ERK-1 and p42 ERK-2). Next, we became interested in CXCR4-mediated signaling in normal hematopoietic cells, and employed human megakaryoblasts, which highly express CXCR4 as a model. We found that stimulation of these cells with SDF-1alpha led to the phosphorylation of MAPK and serine/threonine kinase AKT as well. Activation of MAPK further led to the phosphorylation of the nuclear transcription factor ELK-1. Phosphorylation of ELK-1 in megakaryoblasts implies that phosphorylated MAPK translocate from cytoplasm into the nucleus where they may phosphorylate some nuclear proteins. Note that neither MAPK nor AKT was phosphorylated in normal human platelets after stimulation by SDF-1. We conclude that both MAPK and AKT are involved in signal transduction pathways from the CXCR4 receptor in malignant and normal human hematopoietic cells. The biological consequences of MAPK, ELK-1 and AKT phosphorylation in megakaryoblasts after stimulation with SDF-1alpha require further studies.

Interaction of disintegrins with the alpha IIb beta 3 receptor on resting and activated human platelets

Viper venom disintegrins contain the RGD/KGD motif. They inhibit platelet aggregation and cell adhesion, but show structural and functional heterogeneity. We investigated the interaction of four prototypic disintegrins with alpha IIb beta 3 expressed on the surface of resting and activated platelets. The binding affinity (Kd) of 125I-albolabrin, 125I-echistatin, 125I-bitistatin and 125I-eristostatin toward resting platelets was 294, 153, 48 and 18 nM respectively. The Kd value for albolabrin decreased 3-fold and 6-fold after ADP- or thrombin-induced activation. The Kd values for bitistatin and echistatin also decreased with ADP, but there was no further decrease with thrombin. In contrast, eristostatin bound with the same high affinity to resting and activated platelets. The pattern of fluorescein isothiocyanate (FITC)-eristostatin and FITC-albolabrin binding to resting and activated platelets was consistent with observations using radiolabelled material. Eristostatin showed faster and more irreversible binding to platelets, and greater potency compared with albolabrin in inducing conformational neo-epitopes in beta 3. The anti-alpha IIb beta 3 monoclonal antibody OP-G2 that is RGD-dependent inhibited disintegrin binding to activated platelets more strongly than binding to resting platelets and it inhibited the binding to platelets of albolabrin more strongly than eristostatin. The specificity of disintegrin interaction with alpha IIb beta 3 was confirmed by demonstrating cross-linking of these peptides to alpha IIb beta 3 on normal platelets, but not to thrombasthenic platelets deficient in alpha IIb beta 3.

Antithrombotic thrombocytes: ectopic expression of urokinase-type plasminogen activator in platelets

Arterial occlusive disorders are a leading cause of human morbidity. We hypothesized that ectopic expression of fibrinolytic proteins in platelets could be used to favorably alter the hemostatic balance at sites of thrombosis. To test our hypothesis, we directed murine urokinase-type plasminogen activator transgene expression to platelets using a platelet factor 4 promoter. Urokinase was selectively expressed and stored in the platelets of these mice. These transgenic mice had altered platelet biology and a bleeding diathesis similar to that seen in patients with Quebec platelet disorder, affirming the role of ectopic urokinase expression as the etiology of this inherited disease. These mice were resistant to the development of occlusive carotid artery thrombosis in the absence of systemic fibrinolysis and displayed rapid resolution of pulmonary emboli. Moreover, transfusion of urokinase-expressing platelets into wild-type mice prevented formation of occlusive arterial thrombi. These studies show the feasibility of delivering fibrinolytic agents to sites of incipient thrombus formation through selective storage in platelets and offer a new strategy to prevent thrombosis and hemorrhage.

RGD-containing peptides inhibit fibrinogen binding to platelet alpha(IIb)beta3 by inducing an allosteric change in the amino-terminal portion of alpha(IIb)

To determine the molecular basis for the insensitivity of rat alpha(IIb)beta(3) to inhibition by RGD-containing peptides, hybrids of human and rat alpha(IIb)beta(3) and chimeras of alpha(IIb)beta(3) in which alpha(IIb) was composed of portions of human and rat alpha(IIb) were expressed in Chinese hamster ovary cells and B lymphocytes, and the ability of the tetrapeptide RGDS to inhibit fibrinogen binding to the various forms of alpha(IIb)beta(3) was measured. These measurements indicated that sequences regulating the sensitivity of alpha(IIb)beta(3) to RGDS are located in the seven amino-terminal repeats of alpha(IIb). Moreover, replacing the first three or four (but not the first two) repeats of rat alpha(IIb) with the corresponding human sequences enhanced sensitivity to RGDS, whereas replacing the first two or three repeats of human alpha(IIb) with the corresponding rat sequences had little or no effect. Nevertheless, RGDS bound to Chinese hamster ovary cells expressing alpha(IIb)beta(3) regardless whether the alpha(IIb) in the heterodimers was human, rat, or a rat-human chimera. These results indicate that the sequences determining the sensitivity of alpha(IIb)beta(3) to RGD-containing peptides are located in the third and fourth amino-terminal repeats of alpha(IIb). Because RGDS binds to both human and rat alpha(IIb)beta(3), the results suggest that differences in RGDS sensitivity result from differences in the allosteric changes induced in these repeats following RGDS binding.

Thrombospondin-induced adhesion of human platelets

Washed human unactivated platelets attached and spread on thrombospondin (TSP)-coated microtiter plates. Platelet adhesion was promoted by divalent cations Mn2+, Mg2+, and Ca2+ as compared to buffer having all divalent cations complexed with EDTA. TSP-dependent adhesion was inhibited by anti-TSP fab fragments, an anti-TSP monoclonal antibody, an RGD-containing peptide, complex-specific anti-glycoprotein (GP)IIb-IIIa monoclonal antibodies (A2A9 or AP-2) and anti-VLA-2 monoclonal antibodies (6F1 and Gi9), but not by rabbit preimmune fab fragments, mouse IgG, an anti-GPIIIa monoclonal antibody, or monoclonal antibodies against either the human vitronectin receptor, glycocalicin, or GPIV. At saturating concentrations, anti-GPIIb-IIIa inhibited adhesion by 40-60%. Glanzman's thrombasthenic platelets, which lack GPIIb-IIIa, adhered to TSP to the same extent as anti-GPIIb-IIIa-treated normal platelets or 40-60% as well as untreated normal platelets. Antibody 6F1 (5-10 micrograms/ml) inhibited platelet adhesion of both normal and thrombasthenic platelets by 84-100%. Both VLA-2 antibodies also inhibited collagen-induced platelet adhesion, but had no effect on fibronectin-induced adhesion of normal platelets. These data indicate that platelets specifically adhere to TSP and that this adhesion is mediated through GPIIb-IIIa and/or VLA-2.

Targeting of a mutant plasminogen activator to circulating red blood cells for prophylactic fibrinolysis

Chemical coupling to carrier red blood cells (RBCs) converts tissue type plasminogen activator (tPA) from a problematic therapeutic into a safe agent for thromboprophylaxis. The goal of this study was to develop a more clinically relevant recombinant biotherapeutic by fusing a mutant tPA with a single-chain antibody fragment (scFv) with specificity for glycophorin A (GPA) on mouse RBCs. The fusion construct (anti-GPA scFv/PA) bound specifically to mouse but not human RBCs and activated plasminogen; this led to rapid and stable attachment of up to 30,000 copies of anti-GPA scFv/PA per mouse RBC that were thereby endowed with high fibrinolytic activity. Binding of anti-GPA scFv/PA neither caused RBC aggregation, hemolysis, uptake in capillary-rich lungs or in the reticuloendothelial system nor otherwise altered the circulation of RBCs. Over 40% of labeled anti-GPA scFv/PA injected in mice bound to RBC, which markedly prolonged its intravascular circulation and fibrinolytic activity compared with its nontargeted PA counterpart, anti-GPA scFv/PA, but not its nontargeted PA analog, prevented thrombotic occlusion in FeCl(3) models of vascular injury. These results provide proof-of-principle for the development of a recombinant PA variant that binds to circulating RBC and provides thromboprophylaxis by use of a clinically relevant approach.

Lumican Inhibits SNAIL-Induced Melanoma Cell Migration Specifically by Blocking MMP-14 Activity

Lumican, a small leucine rich proteoglycan, inhibits MMP-14 activity and melanoma cell migration in vitro and in vivo. Snail triggers epithelial-mesenchymal transitions endowing epithelial cells with migratory and invasive properties during tumor progression. The aim of this work was to investigate lumican effects on MMP-14 activity and migration of Snail overexpressing B16F1 (Snail-B16F1) melanoma cells and HT-29 colon adenocarcinoma cells. Lumican inhibits the Snail induced MMP-14 activity in B16F1 but not in HT-29 cells. In Snail-B16F1 cells, lumican inhibits migration, growth, and melanoma primary tumor development. A lumican-based strategy targeting Snail-induced MMP-14 activity might be useful for melanoma treatment.

Localization of distal regulatory domains in the megakaryocyte-specific platelet basic protein/platelet factor 4 gene locus

The genes for the related human (h) chemokines, PBP (platelet basic protein) and PF4 (platelet factor 4), are within 5.3 kilobases (kb) of each other and form a megakaryocyte-specific gene locus. The hypothesis was considered that the PBP and PF4 genes share a common distal regulatory region(s) that leads to their high-level megakaryocyte-specific expression in vivo. This study examined PBP and PF4 expression in transgenic mice using 4 distinct human PBP/PF4 gene locus constructs. These studies showed that within the region studied there was sufficient information to regulate tissue-specific expression of both hPBP and hPF4. Indeed this region contained sufficient DNA information to lead to expression levels of PBP and PF4 comparable to the homologous mouse genes in a position-independent, copy number-dependent fashion. These studies also indicated that the DNA domains that led to this expression were distinct for the 2 genes; hPBP expression is regulated by a region that is 1.5 to 4.4 kb upstream of that gene. Expression of hPF4 is regulated by a region that is either intergenic between the 2 genes or immediately downstream of the hPF4 gene. Comparison of the available human and mouse sequences shows conserved flanking region domains containing potential megakaryocyte-related transcriptional factor DNA-binding sites. Further analysis of these regulatory regions may identify enhancer domains involved in megakaryopoiesis that may be useful in the selective expression of other genes in megakaryocytes and platelets as a strategy for regulating hemostasis, thrombosis, and inflammation. (Blood. 2001;98:610-617)

beta-Amyloid protein induces platelet aggregation and supports platelet adhesion

The amyloid precursor protein (APP) is found in many cells including neurons, endothelial cells and blood platelets. Beta-amyloid protein (beta AP) is derived from APP and is deposited in brain and in cerebral microvasculature of individuals with Alzheimer's disease. In this study we demonstrate that beta AP interacts with human blood platelets. We found that human beta AP peptide (1-40) fibrils aggregate platelets and support their adhesion, and these interactions are mediated through platelet membrane integrin receptors.

Fc-modified HIT-like monoclonal antibody as a novel treatment for sepsis

Sepsis is characterized by multiorgan system dysfunction that occurs because of infection. It is associated with high morbidity and mortality and is in need of improved therapeutic interventions. Neutrophils play a crucial role in sepsis, releasing neutrophil extracellular traps (NETs) composed of DNA complexed with histones and toxic antimicrobial proteins that ensnare pathogens, but also damage host tissues. At presentation, patients often have a significant NET burden contributing to the multiorgan damage. Therefore, interventions that inhibit NET release would likely be ineffective at preventing NET-based injury. Treatments that enhance NET degradation may liberate captured bacteria and toxic NET degradation products (NDPs) and likely be of limited therapeutic benefit as well. We propose that interventions that stabilize NETs and sequester NDPs may be protective in sepsis. We showed that platelet factor 4 (PF4), a platelet-associated chemokine, binds and compacts NETs, increasing their resistance to DNase I. We now show that PF4 increases NET-mediated bacterial capture, reduces the release of NDPs, and improves outcome in murine models of sepsis. A monoclonal antibody KKO which binds to PF4-NET complexes, further enhances DNase resistance. However, the Fc portion of this antibody activates the immune response and increases thrombotic risk, negating any protective effects in sepsis. Therefore, we developed an Fc-modified KKO that does not induce these negative outcomes. Treatment with this antibody augmented the effects of PF4, decreasing NDP release and bacterial dissemination and increasing survival in murine sepsis models, supporting a novel NET-targeting approach to improve outcomes in sepsis.

Platelet-Specific Chemokines Contribute to the Pathogenesis of Acute Lung Injury

Platelets and neutrophils contribute to the development of acute lung injury (ALI). However, the mechanism by which platelets make this contribution is incompletely understood. We investigated whether the two most abundant platelet chemokines, CXCL7, which induces neutrophil chemotaxis and activation, and CXCL4, which does neither, mediate ALI through complementary pathogenic pathways. To examine the role of platelet-derived chemokines in the pathogenesis of ALI using Cxcl7^-/- and Cxcl4^-/- knockout mice and mice that express human CXCL7 or CXCL4, we measured levels of chemokines in these mice. ALI was then induced by acid aspiration, and the severity of injury was evaluated by histology and by the presence of neutrophils and protein in the bronchoalveolar lavage fluid. Pulmonary vascular permeability was studied in vivo by measuring extravasation of fluorescently labeled dextran. Murine CXCL7, both recombinant and native protein released from platelets, can be N-terminally processed by cathepsin G to yield a biologically active CXCL7 fragment. Although Cxcl7^-/- mice are protected from lung injury through the preservation of endothelial/epithelial barrier function combined with impaired neutrophils transmigration, Cxcl4^-/- mice are protected through improved barrier function without affecting neutrophils transmigration to the airways. Sensitivity to ALI is restored by transgenic expression of CXCL7 or CXCL4. Platelet-derived CXCL7 and CXCL4 contribute to the pathogenesis of ALI through complementary effects on neutrophil chemotaxis and through activation and vascular permeability.