Primary megakaryocytes reveal a role for transcription factor NF-E2 in integrin alpha IIb beta 3 signaling

Integrins and their role in megakaryocyte development and function

Mature megakaryocytes, the platelet precursors, originate from hematopoietic stem cell progenitors, which, once committed to this lineage, undergo endomitosis leading to polyploidization. The process entails repeated rounds of DNA replication without cell division, yielding polyploid cells. Supporting the cell's developmental process and various cellular functions are integrin receptors, a conduit of communication between the extracellular environment and the cell actin cytoskeleton. Integrins are heterodimers of α and β subunits, where different combinations of the known 18 α and 8 β subunits confer specificity to the receptor. Integrin ligands range from extracellular matrices through soluble ligands, infectious agents, and counterreceptors, to cells. In this review, we describe the different integrins expressed on bone marrow megakaryocytes and their attributed roles in lineage development and cellular functions, including adhesion, spreading, proplatelet formation, and functional interaction with other cells. Pathologies associated with dysregulated megakaryocyte integrin expression are also reviewed.

Conditional CRISPR-mediated deletion of Lyn kinase enhances differentiation and function of iPSC-derived megakaryocytes

BACKGROUND

Thrombocytopenia leading to life-threatening excessive bleeding can be treated by platelet transfusion. Currently, such treatments are totally dependent on donor-derived platelets. To support future applications in the use of in vitro-derived platelets, we sought to identify genes whose manipulation might improve the efficiency of megakaryocyte production and resulting hemostatic effectiveness. Disruption of Lyn kinase has previously been shown to improve cell survival, megakaryocyte ploidy and TPO-mediated activation in mice, but its role in human megakaryocytes and platelets has not been examined.

METHODS

To analyze the role of Lyn at defined differentiation stages during human megakaryocyte differentiation, conditional Lyn-deficient cells were generated using CRISPR/Cas9 technology in iPS cells. The efficiency of Lyn-deficient megakaryocytes to differentiate and become activated in response to a range of platelet agonists was analyzed in iPSC-derived megakaryocytes.

RESULTS

Temporally controlled deletion of Lyn improved the in vitro differentiation of hematopoietic progenitor cells into mature megakaryocytes, as measured by the rate and extent of appearance of CD41+ CD42+ cells. Lyn-deficient megakaryocytes also demonstrated improved hemostatic effectiveness, as reported by their ability to mediate clot formation in rotational thromboelastometry. Finally, Lyn-deficient megakaryocytes produced increased numbers of platelet-like particles (PLP) in vitro.

CONCLUSIONS

Conditional deletion of Lyn kinase increases the hemostatic effectiveness of megakaryocytes and their progeny as well as improving their yield. Adoption of this system during generation of in vitro-derived platelets may contribute to both their efficiency of production and their ability to support hemostasis.

Megakaryopoiesis and Platelet Biology: Roles of Transcription Factors and Emerging Clinical Implications

Platelets play a critical role in hemostasis and thrombus formation. Platelets are small, anucleate, and short-lived blood cells that are produced by the large, polyploid, and hematopoietic stem cell (HSC)-derived megakaryocytes in bone marrow. Approximately 3000 platelets are released from one megakaryocyte, and thus, it is important to understand the physiologically relevant mechanism of development of mature megakaryocytes. Many genes, including several key transcription factors, have been shown to be crucial for platelet biogenesis. Mutations in these genes can perturb megakaryopoiesis or thrombopoiesis, resulting in thrombocytopenia. Metabolic changes owing to inflammation, ageing, or diseases such as cancer, in which platelets play crucial roles in disease development, can also affect platelet biogenesis. In this review, I describe the characteristics of platelets and megakaryocytes in terms of their differentiation processes. The role of several critical transcription factors have been discussed to better understand the changes in platelet biogenesis that occur during disease or ageing.

RasGRP2 Structure, Function and Genetic Variants in Platelet Pathophysiology

RasGRP2 is calcium and diacylglycerol-regulated guanine nucleotide exchange factor I that activates Rap1, which is an essential signaling-knot in "inside-out" αIIbβ3 integrin activation in platelets. Inherited platelet function disorder caused by variants of RASGRP2 represents a new congenital bleeding disorder referred to as platelet-type bleeding disorder-18 (BDPLT18). We review here the structure of RasGRP2 and its functions in the pathophysiology of platelets and of the other cellular types that express it. We will also examine the different pathogenic variants reported so far as well as strategies for the diagnosis and management of patients with BDPLT18.

Site-specific phosphorylation regulates the functions of kindlin-3 in a variety of cells

Studies of isolated cells, mice, and humans have demonstrated the vital role of the FERM domain protein kindlin-3 in integrin activation in certain hematopoietic and non-hematopoietic cells, consequent to binding to integrin β-subunits. To explore regulatory mechanisms, we developed a monoclonal antibody that selectively recognizes the phosphorylated form of Ser⁴⁸⁴ (pS⁴⁸⁴) in kindlin-3. Activation of platelets, HEL megakaryocytic-like cells and BT549 breast cancer cells led to enhanced expression of pS⁴⁸⁴ as assessed by immunofluorescence or Western blotting. In platelets, pS⁴⁸⁴ rose rapidly and transiently upon stimulation. When a mutant form of kindlin-3, T⁴⁸²S⁴⁸⁴/AA kindlin-3, was transduced into mouse megakaryocytes, it failed to support activation of integrin α_IIbβ₃, whereas wild-type kindlin-3 did. In MDA-MB231 breast cancer cells, expression of T⁴⁸²S⁴⁸⁴/AA kindlin-3 suppressed cell spreading, migration, invasion, and VEGF production. Wild-type kindlin-3 expressing cells markedly increased tumor growth in vivo, whereas T⁴⁸²S⁴⁸⁴/AA kindlin-3 significantly blunted tumor progression. Thus, our data establish that a unique phosphorylation event in kindlin-3 regulates its cellular functions.

Knock-in mice bearing constitutively active αIIb(R990W) mutation develop macrothrombocytopenia with severe platelet dysfunction

BACKGROUND

To date, several mutations that induce constitutive activation of integrin αIIbβ3 have been identified in congenital macrothrombocytopenia. Of these, αIIb(R995W) is the most prevalent mutation observed in Japanese patients with αIIbβ3-related congenital macrothrombocytopenia.

OBJECTIVE AND METHODS

The present study aimed to explore the effects of constitutive activation of the αIIb(R995W) mutation on platelet production, morphology, and function. We generated αIIb(R990W) knock-in (KI) mice corresponding to human αIIb(R995W).

RESULTS

Platelet counts of heterozygous (hetero) and homozygous (homo) KI mice were decreased by ~10% and ~25% relative to those of wild-type (WT) mice, respectively, with increase in platelet size. Decrease in absolute reticulated platelet numbers in steady state, delayed recovery from thrombocytopenia induced by anti-platelet antibody and impaired response to exogenous thrombopoietin administration suggested impaired platelet production in KI mice. WT and KI mice showed no significant differences in the number of megakaryocytes and ploidy of megakaryocytes, whereas proplatelet formation was significantly impaired in homo mice. We observed a slight but significant reduction in platelet lifespan in homo mice. The homo mice showed dramatic reduction in αIIbβ3 expression in platelets, which was accompanied by severe in vivo and in vitro platelet dysfunction.

CONCLUSION

The αIIb(R990W) KI mice developed macrothrombocytopenia, which was primarily attributed to impaired proplatelet formation. In addition, homo KI mice showed marked downregulation in αIIbβ3 expression in platelets with severe impaired platelet function, similar to Glanzmann thrombasthenia.

SHARPIN at the nexus of integrin, immune, and inflammatory signaling in human platelets

Platelets mediate primary hemostasis, and recent work has emphasized platelet participation in immunity and inflammation. The function of the platelet-specific integrin αIIbβ3 as a fibrinogen receptor in hemostasis is well defined, but the roles of αIIbβ3 or integrin-associated proteins in nonhemostatic platelet functions are poorly understood. Here we show that human platelets express the integrin-associated protein SHARPIN with functional consequences. In leukocytes, SHARPIN interacts with integrin α cytoplasmic tails, and it is also an obligate member of the linear ubiquitin chain assembly complex (LUBAC), which mediates Met1 linear ubiquitination of proteins leading to canonical NF-κB activation. SHARPIN interacted with αIIb in pull-down and coimmunoprecipitation assays. SHARPIN was partially localized, as was αIIbβ3, at platelet edges, and thrombin stimulation induced more central SHARPIN localization. SHARPIN also coimmunoprecipitated from platelets with the two other proteins comprising LUBAC, the E3 ligase HOIP and HOIL-1. Platelet stimulation with thrombin or inflammatory agonists, including lipopolysaccharide or soluble CD40 ligand (sCD40L), induced Met1 linear ubiquitination of the NF-κB pathway protein NEMO and serine-536 phosphorylation of the p65 RelA subunit of NF-κB. In human megakaryocytes and/or platelets derived from induced pluripotent stem (iPS) cells, SHARPIN knockdown caused increased basal and agonist-induced fibrinogen binding to αIIbβ3 as well as reduced Met1 ubiquitination and RelA phosphorylation. Moreover, these SHARPIN knockdown cells exhibited increased surface expression of MHC class I molecules and increased release of sCD40L. These results establish that SHARPIN functions in the human megakaryocyte/platelet lineage through protein interactions at the nexus of integrin and immune/inflammatory signaling.

Megakaryocytes compensate for Kit insufficiency in murine arthritis

The growth factor receptor Kit is involved in hematopoietic and nonhematopoietic development. Mice bearing Kit defects lack mast cells; however, strains bearing different Kit alleles exhibit diverse phenotypes. Herein, we investigated factors underlying differential sensitivity to IgG-mediated arthritis in 2 mast cell-deficient murine lines: KitWsh/Wsh, which develops robust arthritis, and KitW/Wv, which does not. Reciprocal bone marrow transplantation between KitW/Wv and KitWsh/Wsh mice revealed that arthritis resistance reflects a hematopoietic defect in addition to mast cell deficiency. In KitW/Wv mice, restoration of susceptibility to IgG-mediated arthritis was neutrophil independent but required IL-1 and the platelet/megakaryocyte markers NF-E2 and glycoprotein VI. In KitW/Wv mice, platelets were present in numbers similar to those in WT animals and functionally intact, and transfer of WT platelets did not restore arthritis susceptibility. These data implicated a platelet-independent role for the megakaryocyte, a Kit-dependent lineage that is selectively deficient in KitW/Wv mice. Megakaryocytes secreted IL-1 directly and as a component of circulating microparticles, which activated synovial fibroblasts in an IL-1-dependent manner. Transfer of WT but not IL-1-deficient megakaryocytes restored arthritis susceptibility to KitW/Wv mice. These findings identify functional redundancy among Kit-dependent hematopoietic lineages and establish an unanticipated capacity of megakaryocytes to mediate IL-1-driven systemic inflammatory disease.

NF-E2, FLI1 and RUNX1 collaborate at areas of dynamic chromatin to activate transcription in mature mouse megakaryocytes

Mutations in mouse and human Nfe2, Fli1 and Runx1 cause thrombocytopenia. We applied genome-wide chromatin dynamics and ChIP-seq to determine these transcription factors’ (TFs) activities in terminal megakaryocyte (MK) maturation. Enhancers with H3K4me2-marked nucleosome pairs were most enriched for NF-E2, FLI and RUNX sequence motifs, suggesting that this TF triad controls much of the late MK program. ChIP-seq revealed NF-E2 occupancy near previously implicated target genes, whose expression is compromised in Nfe2-null cells, and many other genes that become active late in MK differentiation. FLI and RUNX were also the motifs most enriched near NF-E2 binding sites and ChIP-seq implicated FLI1 and RUNX1 in activation of late MK, including NF-E2-dependent, genes. Histones showed limited activation in regions of single TF binding, while enhancers that bind NF-E2 and either RUNX1, FLI1 or both TFs gave the highest signals for TF occupancy and H3K4me2; these enhancers associated best with genes activated late in MK maturation. Thus, three essential TFs co-occupy late-acting cis-elements and show evidence for additive activity at genes responsible for platelet assembly and release. These findings provide a rich dataset of TF and chromatin dynamics in primary MK and explain why individual TF losses cause thrombopocytopenia.

Megakaryocyte- and megakaryocyte precursor-related gene therapies

Hematopoietic stem cells (HSCs) can be safely collected from the body, genetically modified, and re-infused into a patient with the goal to express the transgene product for an individual's lifetime. Hematologic defects that can be corrected with an allogeneic bone marrow transplant can theoretically also be treated with gene replacement therapy. Because some genetic disorders affect distinct cell lineages, researchers are utilizing HSC gene transfer techniques using lineage-specific endogenous gene promoters to confine transgene expression to individual cell types (eg, ITGA2B for inherited platelet defects). HSCs appear to be an ideal target for platelet gene therapy because they can differentiate into megakaryocytes which are capable of forming several thousand anucleate platelets that circulate within blood vessels to establish hemostasis by repairing vascular injury. Platelets play an essential role in other biological processes (immune response, angiogenesis) as well as diseased states (atherosclerosis, cancer, thrombosis). Thus, recent advances in genetic manipulation of megakaryocytes could lead to new and improved therapies for treating a variety of disorders. In summary, genetic manipulation of megakaryocytes has progressed to the point where clinically relevant strategies are being developed for human trials for genetic disorders affecting platelets. Nevertheless, challenges still need to be overcome to perfect this field; therefore, strategies to increase the safety and benefit of megakaryocyte gene therapy will be discussed.

Regulation and function of the NFE2 transcription factor in hematopoietic and non-hematopoietic cells

The NFE2 transcription factor was identified over 25 years ago. The NFE2 protein forms heterodimers with small MAF proteins, and the resulting complex binds to regulatory elements in a large number of target genes. In contrast to other CNC transcription family members including NFE2L1 (NRF1), NFE2L2 (NRF2) and NFE2L3 (NRF3), which are widely expressed, earlier studies had suggested that the major sites of NFE2 expression are hematopoietic cells. Based on cell culture studies it was proposed that this protein acts as a critical regulator of globin gene expression. However, the knockout mouse model displayed only mild erythroid abnormalities, while the major phenotype was a defect in megakaryocyte biogenesis. Indeed, absence of NFE2 led to severely impaired platelet production. A series of recent data, also summarized here, shed new light on the various functional roles of NFE2 and the regulation of its activity. NFE2 is part of a complex regulatory network, including transcription factors such as GATA1 and RUNX1, controlling megakaryocytic and/or erythroid cell function. Surprisingly, it was recently found that NFE2 also has a role in non-hematopoietic tissues, such as the trophoblast, in which it is also expressed, as well as the bone, opening the door to new research areas for this transcription factor. Additional data showed that NFE2 function is controlled by a series of posttranslational modifications. Important strides have been made with respect to the clinical significance of NFE2, linking this transcription factor to hematological disorders such as polycythemias.

Rasa3 controls megakaryocyte Rap1 activation, integrin signaling and differentiation into proplatelet

Rasa3 is a GTPase activating protein of the GAP1 family which targets Ras and Rap1. Ubiquitous Rasa3 catalytic inactivation in mouse results in early embryonic lethality. Here, we show that Rasa3 catalytic inactivation in mouse hematopoietic cells results in a lethal syndrome characterized by severe defects during megakaryopoiesis, thrombocytopenia and a predisposition to develop preleukemia. The main objective of this study was to define the cellular and the molecular mechanisms of terminal megakaryopoiesis alterations. We found that Rasa3 catalytic inactivation altered megakaryocyte development, adherence, migration, actin cytoskeleton organization and differentiation into proplatelet forming megakaryocytes. These megakaryocyte alterations were associated with an increased active Rap1 level and a constitutive integrin activation. Thus, these mice presented a severe thrombocytopenia, bleeding and anemia associated with an increased percentage of megakaryocytes in the bone marrow, bone marrow fibrosis, extramedular hematopoiesis, splenomegaly and premature death. Altogether, our results indicate that Rasa3 catalytic activity controls Rap1 activation and integrin signaling during megakaryocyte differentiation in mouse.

Megakaryocytes are the bone marrow cellular precursors of circulating blood platelets and give rise to nascent platelets by forming branching filaments called proplatelets. Terminal differentiation of round megakaryocytes into branched proplatelet forming megakaryocytes is a complex cytoskeletal-driven process which is affected in rare human familial thrombocytopenias. Interactions of megakaryocytes with extracellular matrix proteins are essential in this process since constitutive megakaryocyte integrin activity caused by specific mutations in ITGA2B or ITGB3 genes encoding for extracellular matrix protein receptors may result in abnormal adherent megakaryocytes, defect in proplatelet formation and thrombocytopenia. Here, we show that Rasa3, a GTPase activating protein of the GAP1 family, controls Rap1 activation and integrin signaling during megakaryocyte differentiation. We found that Rasa3 catalytic inactivation in mice altered megakaryocyte development, adherence, migration, actin cytoskeleton organization and differentiation into proplatelet. Thus, these mice presented a severe thrombocytopenia, bleeding and anemia.

Mechanisms underlying platelet function defect in a pedigree with familial platelet disorder with a predisposition to acute myelogenous leukemia: potential role for candidate RUNX1 targets

BACKGROUND

Familial platelet disorder with a predisposition to acute myelogenous leukemia (FPD/AML) is an inherited platelet disorder caused by a germline RUNX1 mutation and characterized by thrombocytopenia, a platelet function defect, and leukemia predisposition. The mechanisms underlying FPD/AML platelet dysfunction remain incompletely clarified. We aimed to determine the contribution of platelet structural abnormalities and defective activation pathways to the platelet phenotype. In addition, by using a candidate gene approach, we sought to identify potential RUNX1-regulated genes involved in these defects.

METHODS

Lumiaggregometry, α-granule and dense granule content and release, platelet ultrastructure, αIIb β3 integrin activation and outside-in signaling were assessed in members of one FPD/AML pedigree. Expression levels of candidate genes were measured and luciferase reporter assays and chromatin immunoprecipitation were performed to study NF-E2 regulation by RUNX1.

RESULTS

A severe decrease in platelet aggregation, defective αIIb β3 integrin activation and combined αδ storage pool deficiency were found. However, whereas the number of dense granules was markedly reduced, α-granule content was heterogeneous. A trend towards decreased platelet spreading was found, and β3 integrin phosphorylation was impaired, reflecting altered outside-in signaling. A decrease in the level of transcription factor p45 NF-E2 was shown in platelet RNA and lysates, and other deregulated genes included RAB27B and MYL9. RUNX1 was shown to bind to the NF-E2 promoter in primary megakaryocytes, and wild-type RUNX1, but not FPD/AML mutants, was able to activate NF-E2 expression.

CONCLUSIONS

The FPD/AML platelet function defect represents a complex trait, and RUNX1 orchestrates platelet function by regulating diverse aspects of this process. This study highlights the RUNX1 target NF-E2 as part of the molecular network by which RUNX1 regulates platelet biogenesis and function.

NF-E2 p45 is important for establishing normal function of platelets

NF-E2 is a heterodimeric transcription factor consisting of p45 and small Maf subunits. Since p45(-/-) mice display severe thrombocytopenia, p45 is recognized as a critical regulator of platelet production from megakaryocytes. To identify direct p45 target genes in megakaryocytes, we used chromatin immunoprecipitation (ChIP) sequencing to analyze the genome-wide chromatin occupancy of p45 in primary megakaryocytes. p45 target gene candidates obtained from the analysis are implicated in the production and function of platelets. Two of these genes, Selp and Myl9, were verified as direct p45 targets through multiple approaches. Since P-selectin, encoded by Selp, plays a critical role in platelet function during thrombogenesis, we tested whether p45 determines the intrinsic reactivity and potency of platelets generated from megakaryocytes. Mice expressing a hypomorphic p45 mutant instead of wild-type p45 in megakaryocytes (p45(-/-):ΔNTD-Tg mice) displayed platelet hypofunction accompanied by mild thrombocytopenia. Furthermore, lung metastasis of melanoma cells, which requires platelet activation, was repressed in p45(-/-):ΔNTD-Tg mice compared to control mice, validating the impaired function of platelets produced from p45(-/-):ΔNTD-Tg megakaryocytes. By activating genes in megakaryocytes that mediate platelet production and function, p45 determines the quantity and quality of platelets.

Transcription factors in late megakaryopoiesis and related platelet disorders

Cell type-specific transcription factors regulate the repertoire of genes expressed in a cell and thereby determine its phenotype. The differentiation of megakaryocytes, the platelet progenitors, from hematopoietic stem cells is a well-known process that can be mimicked in culture. However, the efficient formation of platelets in culture remains a challenge. Platelet formation is a complicated process including megakaryocyte maturation, platelet assembly and platelet shedding. We hypothesize that a better understanding of the transcriptional regulation of this process will allow us to influence it such that sufficient numbers of platelets can be produced for clinical applications. After an introduction to gene regulation and platelet formation, this review summarizes the current knowledge of the regulation of platelet formation by the transcription factors EVI1, GATA1, FLI1, NFE2, RUNX1, SRF and its co-factor MKL1, and TAL1. Also covered is how some platelet disorders including myeloproliferative neoplasms, result from disturbances of the transcriptional regulation. These disorders give us invaluable insights into the crucial role these transcription factors play in platelet formation. Finally, there is discussion of how a better understanding of these processes will be needed to allow for efficient production of platelets in vitro.

ACTN1 mutations cause congenital macrothrombocytopenia

Congenital macrothrombocytopenia (CMTP) is a heterogeneous group of rare platelet disorders characterized by a congenital reduction of platelet counts and abnormally large platelets, for which CMTP-causing mutations are only found in approximately half the cases. We herein performed whole-exome sequencing and targeted Sanger sequencing to identify mutations that cause CMTP, in which a dominant mode of transmission had been suspected but for which no known responsible mutations have been documented. In 13 Japanese CMTP-affected pedigrees, we identified six (46%) affected by ACTN1 variants cosegregating with CMTP. In the entire cohort, ACNT1 variants accounted for 5.5% of the dominant forms of CMTP cases and represented the fourth most common cause in Japanese individuals. Individuals with ACTN1 variants presented with moderate macrothrombocytopenia with anisocytosis but were either asymptomatic or had only a modest bleeding tendency. ACTN1 encodes α-actinin-1, a member of the actin-crosslinking protein superfamily that participates in the organization of the cytoskeleton. In vitro transfection experiments in Chinese hamster ovary cells demonstrated that altered α-actinin-1 disrupted the normal actin-based cytoskeletal structure. Moreover, transduction of mouse fetal liver-derived megakaryocytes with disease-associated ACTN1 variants caused a disorganized actin-based cytoskeleton in megakaryocytes, resulting in the production of abnormally large proplatelet tips, which were reduced in number. Our findings provide an insight into the pathogenesis of CMTP.

Agonist stimulation, talin-1, and kindlin-3 are crucial for α(IIb)β(3) activation in a human megakaryoblastic cell line, CMK

Platelet integrin α(IIb)β(3) activation is regulated by inside-out signaling via agonist stimulation. However, when α(IIb)β(3) was exogenously expressed in cell lines such as Chinese hamster ovarian cells, physiological agonists hardly induced α(IIb)β(3) activation. To overcome this disadvantage, we characterized the functional regulation of endogenously expressed α(IIb)β(3) in a megakaryoblastic cell line, CMK, employing an initial velocity assay for PAC-1 binding. We firstly demonstrated that protease-activated receptor 1-activating peptide induced robust, but transient α(IIb)β(3) activation in CMK cells with high glycoprotein-Ib expression. Stable talin-1 or kindlin-3 knockdown cells confirmed that the protease-activated receptor 1-activating peptide-induced α(IIb)β(3) activation was dependent on talin-1 and kindlin-3 expression. In sharp contrast to exogenously expressed α(IIb)β(3) in Chinese hamster ovarian cells, transient overexpression of full-length talin (FL-talin) or talin-head domain (THD) alone did not activate α(IIb)β(3) in CMK cells, but required agonist stimulation. Similarly, kindlin-3 overexpression alone did not induce α(IIb)β(3) activation, but it significantly augmented agonist-induced α(IIb)β(3) activation. Several mutants of FL-talin and THD suggested that the head-rod interaction was critical for autoinhibition of talin-1, and the interaction between the THD and the membrane-proximal region of the β(3) cytoplasmic tail was essential for talin-mediated α(IIb)β(3) activation. In addition, THD and kindlin-3 cooperatively augmented protease-activated receptor 1-induced α(IIb)β(3) activation. We proposed that the CMK cell line is an attractive platform for investigating agonist-, talin-1-, and kindlin-3- dependent α(IIb)β(3) activation.

The endocytic adaptor protein Disabled-2 is required for cellular uptake of fibrinogen

Endocytosis is pivotal for uptake of fibrinogen from plasma into megakaryocytes and platelet α-granules. Due to the complex adaptor and cargo contents in endocytic vehicles, the underlying mechanism of fibrinogen uptake is not yet completely elucidated. In this study, we investigated whether the endocytic adaptor protein Disabled-2 (DAB2) mediates fibrinogen uptake in an adaptor-specific manner. By employing primary megakaryocytes and megakaryocytic differentiating human leukemic K562 cells as the study models, we found that fibrinogen uptake is associated with the expression of integrin αIIbβ3 and DAB2 and is mediated through clathrin-dependent manner. Accordingly, constitutive and inducible knockdown of DAB2 by small interfering RNA reduced fibrinogen uptake for 53.2 ± 9.8% and 59.0 ± 10.7%, respectively. Culturing the cells in hypertonic solution or in the presence of clathrin inhibitor chlorpromazine abrogated clathrin-dependent endocytosis and diminished the uptake of fibrinogen. Consistent with these findings, 72.2 ± 0.2% of cellular DAB2 was colocalized with clathrin, whereas 56.4±4.1% and 54.6 ± 2.0% of the internalized fibrinogen were colocalized with clathrin and DAB2, respectively. To delineate whether DAB2 mediates fibrinogen uptake in an adaptor-specific manner, K562 stable cell lines with knockdown of the adaptor protein-2 (AP-2) or double knockdown of AP-2/DAB2 were established. The AP-2 knockdown cells elicited normal fibrinogen uptake activity but the uptake of collagen was diminished. In addition, collagen uptake was further reduced in DAB2/AP-2 knockdown cells. These findings thereby define an adaptor-specific mechanism in the control of fibrinogen uptake and implicate that DAB2 is the key adaptor in the clathrin-associated endocytic complexes to mediate fibrinogen internalization.

Platelet formation

Thrombocytopenia is the underlying cause of a number of major clinical conditions and genetic disorders worldwide. While therapeutic agents that bind and stimulate the thrombopoietin receptor are currently available, the development of drugs that directly stimulate megakaryocytes to generate platelets has lagged behind. To improve the management of thrombocytopenia, we will need to define the cell biological pathways that drive the production of platelets from megakaryocytes. This review integrates the latest research of platelet biogenesis and focuses on the molecular pathways that power and regulate proplatelet production.

Talin and signaling through integrins

Integrin adhesion receptors are essential for the development and functioning of multicellular animals. Integrins mediate cell adhesion to the extracellular matrix and to counter-receptors on adjacent cells, and the ability of integrins to bind extracellular ligands is regulated in response to intracellular signals that act on the short cytoplasmic tails of integrin subunits. Integrin activation, the rapid conversion of integrin receptors from low to high affinity, requires binding of talin to integrin β tails and, once bound, talin provides a connection from activated integrins to the actin cytoskeleton. A wide range of experimental approaches have contributed to the current understanding of the importance of talin in integrin signaling. Here, we describe two methods that have been central to our investigations of talin; a biochemical assay that has allowed characterization of interactions between integrin cytoplasmic tails and talin, and a fluorescent-activated cell-sorting procedure to assess integrin activation in cultured cells expressing talin domains, mutants, dominant negative constructs, or shRNA.

Inhibitory effects of megakaryocytic cells in prostate cancer skeletal metastasis

Prostate cancer cells commonly spread through the circulation, but few successfully generate metastatic foci in bone. Osteoclastic cellular activity has been proposed as an initiating event for skeletal metastasis. Megakaryocytes (MKs) inhibit osteoclastogenesis, which could have an impact on tumor establishment in bone. Given the location of mature MKs at vascular sinusoids, they may be the first cells to physically encounter cancer cells as they enter the bone marrow. Identification of the interaction between MKs and prostate cancer cells was the focus of this study. K562 (human MK precursors) and primary MKs derived from mouse bone marrow hematopoietic precursor cells potently suppressed prostate carcinoma PC-3 cells in coculture. The inhibitory effects were specific to prostate carcinoma cells and were enhanced by direct cell-cell contact. Flow cytometry for propidium iodide (PI) and annexin V supported a proapoptotic role for K562 cells in limiting PC-3 cells. Gene expression analysis revealed reduced mRNA levels for cyclin D1, whereas mRNA levels of apoptosis-associated specklike protein containing a CARD (ASC) and death-associated protein kinase 1 (DAPK1) were increased in PC-3 cells after coculture with K562 cells. Recombinant thrombopoietin (TPO) was used to expand MKs in the marrow and resulted in decreased skeletal lesion development after intracardiac tumor inoculation. These novel findings suggest a potent inhibitory role of MKs in prostate carcinoma cell growth in vitro and in vivo. This new finding, of an interaction of metastatic tumors and hematopoietic cells during tumor colonization in bone, ultimately will lead to improved therapeutic interventions for prostate cancer patients.

PAF-acetylhydrolase expressed during megakaryocyte differentiation inactivates PAF-like lipids

Platelet activating factor (PAF) and PAF-like lipids induce inflammatory responses in target cells. These lipid mediators are inactivated by PAF-acetylhydrolase (PAF-AH). The PAF signaling system affects the growth of hematopoietic CD34(+) cells, but roles for PAF-AH in this process are unknown. Here, we investigated PAF-AH function during megakaryopoiesis and found that human CD34(+) cells accumulate this enzymatic activity as they differentiate toward megakaryocytes, consistent with the expression of mRNA and protein for the plasma PAF-AH isoform. Inhibition of endogenous PAF-AH activity in differentiated megakaryocytes increased formation of lipid mediators that signaled the PAF receptor (PAFR) in fully differentiated human cells such as neutrophils, as well as megakaryocytes themselves. PAF-AH also controlled megakaryocyte alpha(IIb)beta(3)-dependent adhesion, cell spreading, and mobility that relied on signaling through the PAFR. Together these data suggest that megakaryocytes generate PAF-AH to modulate the accumulation of intracellular phospholipid mediators that may detrimentally affect megakaryocyte development and function.

Dynamin 3 participates in the growth and development of megakaryocytes

High-density oligonucleotide microarrays were used to compare gene expression profiles from uncultured CD34+/CD38lo cells and culture-derived megakaryocytes (MKs). As previously published, three replicate microarray data sets from three different sources of organ donor marrow were analyzed using the software program Rosetta Resolver. After setting a stringent p value of <or=0.001 with a fold change cutoff of three or more in expression level, dynamin 3 (DNM3) was identified to be differentially expressed during the course of MK development with a mean fold-change of 8.2+/-2.1 (mean+/-standard deviation). DNM3 is a member of a family of mechanochemical enzymes (DNM1, DNM2, and DNM3) known for their participation in membrane dynamics by hydrolyzing nucleotides to link cellular membranes to the actin cytoskeleton. Real-time quantitative polymerase chain reaction confirmed that DNM3 increased by 20.7-+/-3.4-fold (n=4, p=0.09) during megakaryocytopoiesis and Western blot analysis showed that DNM3 protein was expressed in human MKs. Confocal microscopy revealed that DNM3 was distributed diffusely throughout the cytoplasm of MKs with a punctate appearance in proplatelet processes. Immunogold electron microscopy also showed that DNM3 is widely distributed in the cytoplasm of MKs, with no apparent localization to specific organelles. The open reading frame of DNM3 was cloned from culture-derived human MKs and determined to be 100% identical to the protein encoded by the DNM3 transcript variant ENST00000367731 published in the Ensemble database. Overexpression of DNM3 in umbilical cord blood CD34+ cells resulted in an increase in total nucleated cells, an amplification of total colony-forming cells and colony-forming unit-megakaryocytes, and a concomitant increase in the expression of nuclear factor erythroid 2 (NF-E2) and beta-tubulin. Together these findings provide the first evidence that a member of the dynamin family of mechanochemical enzymes is present in human MKs and indicate that DNM3 is an excellent candidate for playing an important role in mediating cytoskeleton and membrane changes that occur during MK/platelet development.

Novel approach for formation of platelet-like particles from mouse embryonic stem cells without using feeder cells

Megakaryocytes (MKs) and platelet-like particles (PLPs) have generally been obtained by culturing embryonic stem (ES) cells over feeder cells. However, using feeder cells need many labor-consuming processes and the MK and PLP fractions obtained are often contaminated by such cells and their fragments. Here we describe our new culture system for differentiating mouse ES cells to MKs and PLPs without using feeder cells. ES cells are differentiated to cells with MK-like morphology and properties, including proplatelet formation, high ploidy (>8N), and CD41 expression. The culture medium contained PLPs expressing platelet glycoproteins, CD41 and GPIb. Integrin alpha(IIb)beta(3) of PLPs can be activated by thrombin. Addition of the metalloproteinase inhibitor TAPI-2 to the culture increased the surface expression of GPIbalpha and augmented the adhesion of PLPs to immobilized von Willebrand factor through decreasing the shedding of GPIbalpha. Thus our mouse ES cells culture system is a suitable and efficient method for obtaining MKs and functional PLPs that obviates the need for feeder cells.

Characterization of calcium- and integrin-binding protein 1 (CIB1) knockout platelets: potential compensation by CIB family members

Platelet aggregation requires activation of the alphaIIbbeta3 integrin, an event regulated by the integrin cytoplasmic tails. CIB1 binds to the cytoplasmic tail of the integrin alphaIIb subunit. Previous over-expression and knockdown studies in murine megakaryocytes demonstrated that CIB1 inhibits integrin alphaIIbbeta3 activation. Here we analyzed Cib1(-/-) mice to determine the function of CIB1 in platelets in vitro and in vivo. We found that although these mice had no overt platelet phenotype, mRNA level of CIB1 homolog CIB3 was increased in Cib1(-/-) megakaryocytes. In vitro binding experiments showed that recombinant CIB1, -2 and -3 bound specifically to an alphaIIb cytoplasmic tail peptide. Subsequent protein modeling experiments indicated that CIBs 1-3 each have a highly conserved hydrophobic binding pocket. Therefore, the potential exists for compensation for the loss of CIB1 by these CIB family members, thereby preventing pathologic thrombus formation in Cib1(-/-) mice.

The CXC-chemokine CXCL4 interacts with integrins implicated in angiogenesis

The human CXC-chemokine CXCL4 is a potent inhibitor of tumor-induced angiogenesis. Considering that CXCL4 is sequestered in platelet alpha-granules and released following platelet activation in the vicinity of vessel wall injury, we tested the hypothesis that CXCL4 might function as a ligand for integrins. Integrins are a family of adhesion receptors that play a crucial role in angiogenesis by regulating early angiogenic processes, such as endothelial cell adhesion and migration. Here, we show that CXCL4 interacts with alphavbeta3 on the surface of alphavbeta3-CHO. More importantly, human umbilical vein endothelial cells adhere to immobilized CXCL4 through alphavbeta3 integrin, and also through other integrins, such as alphavbeta5 and alpha5beta1. We further demonstrate that CXCL4-integrin interaction is of functional significance in vitro, since immobilized CXCL4 supported endothelial cell spreading and migration in an integrin-dependent manner. Soluble CXCL4, in turn, inhibits integrin-dependent endothelial cell adhesion and migration. As a whole, our study identifies integrins as novel receptors for CXCL4 that may contribute to its antiangiogenic effect.

Mechanisms and consequences of agonist-induced talin recruitment to platelet integrin alphaIIbbeta3

Platelet aggregation requires agonist-induced αIIbβ3 activation, a process mediated by Rap1 and talin. To study mechanisms, we engineered αIIbβ3 Chinese hamster ovary (CHO) cells to conditionally express talin and protease-activated receptor (PAR) thrombin receptors. Human PAR1 or murine PAR4 stimulation activates αIIbβ3, which was measured with antibody PAC-1, indicating complete pathway reconstitution. Knockdown of Rap1–guanosine triphosphate–interacting adaptor molecule (RIAM), a Rap1 effector, blocks this response. In living cells, RIAM overexpression stimulates and RIAM knockdown blocks talin recruitment to αIIbβ3, which is monitored by bimolecular fluorescence complementation. Mutations in talin or β3 that disrupt their mutual interaction block both talin recruitment and αIIbβ3 activation. However, one talin mutant (L325R) is recruited to αIIbβ3 but cannot activate it. In platelets, RIAM localizes to filopodia and lamellipodia, and, in megakaryocytes, RIAM knockdown blocks PAR4-mediated αIIbβ3 activation. The RIAM-related protein lamellipodin promotes talin recruitment and αIIbβ3 activity in CHO cells but is not expressed in megakaryocytes or platelets. Thus, talin recruitment to αIIbβ3 by RIAM mediates agonist-induced αIIbβ3 activation, with implications for hemostasis and thrombosis.

Protein phosphatase 2A negatively regulates integrin alpha(IIb)beta(3) signaling

Integrin alpha(IIb)beta(3) activation is critical for platelet physiology and is controlled by signal transduction through kinases and phosphatases. Compared with kinases, a role for phosphatases in platelet integrin alpha(IIb)beta(3) signaling is less understood. We report that the catalytic subunit of protein phosphatase 2A (PP2Ac) associates constitutively with the integrin alpha(IIb)beta(3) in resting platelets and in human embryonal kidney 293 cells expressing alpha(IIb)beta(3). The membrane proximal KVGFFKR sequence within the cytoplasmic domain of integrin alpha(IIb) is sufficient to support a direct interaction with PP2Ac. Fibrinogen binding to alpha(IIb)beta(3) during platelet adhesion decreased integrin-associated PP2A activity and increased the phosphorylation of a PP2A substrate, vasodilator associated phosphoprotein. Overexpression of PP2Ac(alpha) in 293 cells decreased alpha(IIb)beta(3)-mediated adhesion to immobilized fibrinogen. Conversely, small interference RNA mediated knockdown of endogenous PP2Ac(alpha) expression in 293 cells, enhanced extracellular signal-regulated kinase (ERK1/2) and p38 activation, and accelerated alpha(IIb)beta(3) adhesion to fibrinogen and von Willebrand factor. Inhibition of ERK1/2, but not p38 activation, abolished the increased adhesiveness of PP2Ac (alpha)-depleted 293 cells to fibrinogen. Furthermore, knockdown of PP2A(calpha) expression in bone marrow-derived murine megakaryocytes increased soluble fibrinogen binding induced by protease-activated receptor 4-activating peptide. These studies demonstrate that PP2Ac (alpha) can negatively regulate integrin alpha(IIb)beta(3) signaling by suppressing the ERK1/2 signaling pathway.

Silencing of a targeted protein in in vivo platelets using a lentiviral vector delivering short hairpin RNA sequence

OBJECTIVE

Because platelets are anucleate cells having a limited life span, direct gene manipulation cannot in principle be used to investigate the involvement of a specific signal transduction pathway in platelet activation. In this study, we examined whether the expression of a short hairpin RNA (shRNA) sequence in hematopoietic stem cells is maintained during megakaryocyte differentiation, thus resulting in inhibition of targeted protein in platelets.

METHODS AND RESULTS

To identify platelets derived from transduced stem cells, we generated a lentiviral vector that simultaneously expresses the shRNA sequence driven by the U6 promoter and GFP under the control of the glycoprotein (GP) Ib alpha promoter. Transplantation of mouse bone marrow cells transduced with the vector facilitated specifically mark platelets derived from the transduced cells. Transplantation of cells transduced with shRNA sequence targeting integrin alphaIIb caused a significant reduction of integrin alphaIIb beta3 (alphaIIb beta3) expression in GFP-positive platelets. It also inhibited alphaIIb beta3 activation assessed by the binding of JON/A, an antibody that recognizes activated alphaIIb beta3. Talin-1 silencing by the same method resulted in normal alphaIIb beta3 expression but deficient inside-out alphaIIb beta3 signaling.

CONCLUSIONS

shRNA expression driven by the U6 promoter is preserved during megakaryopoiesis. This method facilitates functional analysis of targeted protein in platelet activation.

Calcium-diacylglycerol guanine nucleotide exchange factor I gene mutations associated with loss of function in canine platelets

Calcium-Diacylglycerol Guanine Nucleotide Exchange Factor I (CalDAG-GEFI) has been implicated in platelet aggregation signaling in CalDAG-GEFI knockouts. Functional mutations were identified in the gene encoding for CalDAG-GEFI in 3 dog breeds. Affected dogs experienced epistaxis, gingival bleeding, and petechiation. Platelet number, von Willebrand factor, clot retraction, and coagulation screening assays were normal, whereas bleeding time tests were prolonged. Platelet aggregation and release responses to all agonists, except thrombin, were markedly impaired. Platelet membranes had normal concentrations of integrin alphaIIb-beta3; however, ADP-induced fibrinogen binding by activated platelets was markedly impaired. Forskolin-stimulated platelets exhibited a marked increase in intraplatelet cAMP associated with impaired phosphodiesterase (PDE) activity, whereas levels of extractable phosphoinositides were 1.5-fold to 2-fold higher in thrombin-stimulated affected platelets. DNA analysis of the CalDAG-GEFI gene in affected dogs documented the existence of 3 distinct mutations within portions of the CalDAG-GEFI gene encoding for structurally conserved regions within the catalytic domain of the protein. The mutations are predicted to result in either lack of synthesis, enhanced degradation, or marked impairment of protein function. The dysfunctional profile of canine platelets observed in mutant dogs putatively links CalDAG-GEFI and its target Rap1 or other Ras family member, for the first time, to a role in pathways that regulate cAMP PDE activity and thrombin-stimulated phosphoinositide anchoring or metabolism. The finding of distinct functional mutations in 3 dog breeds suggests that mutations in the CalDAG-GEFI gene may be implicated in similar defects in human patients with congenital platelet disorders having primary secretion defects of unknown etiology.

Tickling the tails: cytoplasmic domain proteins that regulate integrin αIIbβ3 activation

PURPOSE OF REVIEW

Integrin alphaIIbbeta3 activation is essential for platelet aggregation and related hemostatic events. In recent years, intense effort has been put forward to understand the molecular mechanisms regulating platelet integrin alphaIIbbeta3 activation. Here we review the current models of alphaIIbbeta3 activation and highlight the potential regulatory roles of proteins that interact directly with the alphaIIbbeta3 cytoplasmic domains, with emphasis on the alphaIIb cytoplasmic domain binding protein, CIB1.

RECENT FINDINGS

Mutational and crystallographic studies reveal the importance of integrin transmembrane and cytoplasmic domains in propagating bidirectional signaling events. Proteins that interact directly with the integrin cytoplasmic domains may play important roles in mediating these signaling events. Of particular interest is the interaction between CIB1 and the alphaIIb tail which may function to negatively regulate alphaIIbbeta3 activation. In addition, a number of CIB1 interacting proteins have been identified, including p21-activated kinase and serum-inducible kinase, which may act in concert with CIB1 to regulate platelet function.

SUMMARY

Understanding the molecular mechanisms underlying integrin activation will be important in developing novel therapies to regulate platelet function in cardiovascular disease. Discussion of recent developments in elucidating the mechanism of integrin activation, with particular focus on the platelet integrin alphaIIbbeta3, is provided in this review.

The effect of prostacyclin agonists on the differentiation of phorbol ester treated human erythroleukemia cells

Phorbol-12-myristate-13-acetate (PMA) induces megakaryocytopoeisis in human erythroleukemia (HEL) cells which is characterized by the increase in cell size, increase in nuclear polyploidization and expression of megakaryocyte marker, CD41. However, upon treatment with 100 nM of selective prostacyclin (IP) agonist beraprost inhibits the induced differentiation. Moreover, selective non-prostanoid IP agonist, BMY 45778 prevents PMA induced megakaryocytopoeisis in HEL cells similarly, while prostaglandin E(2) and specific EP(3) agonist sulprostone have no effect. Thus, IP receptor is involved. Furthermore, adenylate cyclase activator forskolin and cAMP analog dibutyryl-cAMP also prevented PMA induced megakaryocytopoeisis in HEL cells. Thus, IP agonists inhibition of PMA induced megakaryocytopoeisis in HEL cells may involve a cAMP dependent pathway.

Expression analysis of primary mouse megakaryocyte differentiation and its application in identifying stage-specific molecular markers and a novel transcriptional target of NF-E2

Megakaryocyte (MK) differentiation is well described in morphologic terms but its molecular counterparts and the basis for platelet release are incompletely understood. We profiled mRNA expression in populations of primary mouse MKs representing successive differentiation stages. Genes associated with DNA replication are highly expressed in young MKs, in parallel with endomitosis. Intermediate stages are characterized by disproportionate expression of genes associated with the cytoskeleton, cell migration, and G-protein signaling, whereas terminally mature MKs accumulate hemostatic factors, including many membrane proteins. We used these expression profiles to extract a reliable panel of molecular markers for MKs of early, intermediate, or advanced differentiation and establish the value of this marker panel using mouse models of defective thrombopoiesis resulting from absence of GATA1, NF-E2, or tubulin beta1. Computational analysis of the promoters of late-expressed MK genes identified new candidate targets for NF-E2, a critical transcriptional regulator of platelet release. One such gene encodes the kinase adaptor protein LIMS1/PINCH1, which is highly expressed in MKs and platelets and significantly reduced in NF-E2-deficient cells. Transactivation studies and chromatin immunoprecipitation implicate Lims1 as a direct target of NF-E2 regulation. Attribution of stage-specific genes, in combination with various applications, thus constitutes a powerful way to study MK differentiation and platelet biogenesis.

A product of their environment: do megakaryocytes rely on extracellular cues for proplatelet formation?

Megakaryocytes have long been observed to form abundant filamentous extensions called proplatelets. A strong body of evidence strongly suggests these proplatelets are the mechanism by which platelets are released into the vasculature. Despite the recent advances in understanding proplatelet architecture, surprisingly little attention has been paid to identifying the ways in which the bone marrow environment regulates proplatelet formation. This review summarises this field and how these findings suggest a spatial and temporal regulation to ensure that platelets are produced in the correct location.

Platelet integrin adhesive functions and signaling

Integrin-mediated cellular events affect all cell types and functions, in physiological as well as pathological settings. Blood platelets, because of their unique nature, have proven to be a powerful cell model with which to study the adhesive and signaling properties of integrins. The characterization of the structural and molecular mechanisms regulating the main platelet integrin, alphaIIbbeta3, has provided some essential clues as to how integrins are regulated in general. The present chapter details the various protocols and reagents currently in use in our laboratory to study alphaIIbbeta3 adhesive responses and signaling in both human and murine cell models.

Megakaryocyte development and platelet production

Megakaryocytopoiesis involves the commitment of haematopoietic stem cells, and the proliferation, maturation and terminal differentiation of the megakaryocytic progenitors. Circulating levels of thrombopoietin (TPO), the primary growth-factor for the megakaryocyte (MK) lineage, induce concentration-dependent proliferation and maturation of MK progenitors by binding to the c-Mpl receptor and signalling induction. Decreased platelet turnover rates results in increased concentration of free TPO, enabling the compensatory response of marrow MKs to increased platelet production. C-Mpl activity is orchestrated by a complex cascade of signalling molecules that induces the action of specific transcription factors to drive MK proliferation and maturation. Mature MKs form proplatelet projections that are fragmented into circulating particles. Newly developed thrombopoietic agents operating via c-Mpl receptor may prove useful in supporting platelet production in thrombocytopenic state. Herein, we review the regulation of megakaryocytopoiesis and platelet production in normal and disease state, and the new approaches to thrombopoietic therapy.

Megakaryocytes derived from human embryonic stem cells: a genetically tractable system to study megakaryocytopoiesis and integrin function

BACKGROUND

The platelet fibrinogen receptor, a heterodimer consisting of integrin subunits alpha(IIb) and beta(3), is required for platelet aggregation, spreading, and hemostasis. Platelet agonists such as thrombin and adenosine diphosphate (ADP) lead to the activation of alpha(IIb)beta(3), thereby enhancing its affinity and avidity for binding fibrinogen (inside-out signaling). Furthermore, fibrinogen binding to alpha(IIb)beta(3) triggers cytoskeletal changes and granule release (outside-in signaling).

AIM

Genetic approaches to characterize the molecular pathways involved in alpha(IIb)beta(3) signaling are not possible with anucleate blood platelets. Therefore, we have established an OP9 stromal cell co-culture system to generate megakaryocytes from human embryonic stem cells (hESCs).

RESULTS

alpha(IIb)beta(3) activation, measured by soluble fibrinogen binding to hESC-derived megakaryocytes, /GPIbalpha(+) cells, is readily detectable following stimulation with known platelet agonists. Dose-response curves for peptide agonists specific for the two platelet thrombin receptors, protease-activated receptor 1 (PAR1) and PAR4, show a relative responsiveness that mirrors that of human platelets, and sub-maximal ADP responses are augmented by epinephrine. Moreover, hESC-derived megakaryocytes undergo lamellipodia formation, actin filament assembly, and vinculin localization at focal adhesions when plated on a fibrinogen-coated surface, characteristic of alpha(IIb)beta(3) outside-in signaling. Undifferentiated hESCs genetically modified by lentiviral infection can be cloned and maintained in an undifferentiated state and then differentiated into megakaryocytes capable of alpha(IIb)beta(3) activation.

CONCLUSION

Using hESCs, we have developed a renewable source of human megakaryocytes, and a genetically tractable system for studying megakaryocytopoiesis and alpha(IIb)beta(3) signaling in the native cellular environment.

CIB1 is an endogenous inhibitor of agonist-induced integrin alphaIIbbeta3 activation

In response to agonist stimulation, the alphaIIbbeta3 integrin on platelets is converted to an active conformation that binds fibrinogen and mediates platelet aggregation. This process contributes to both normal hemostasis and thrombosis. Activation of alphaIIbbeta3 is believed to occur in part via engagement of the beta3 cytoplasmic tail with talin; however, the role of the alphaIIb tail and its potential binding partners in regulating alphaIIbbeta3 activation is less clear. We report that calcium and integrin binding protein 1 (CIB1), which interacts directly with the alphaIIb tail, is an endogenous inhibitor of alphaIIbbeta3 activation; overexpression of CIB1 in megakaryocytes blocks agonist-induced alphaIIbbeta3 activation, whereas reduction of endogenous CIB1 via RNA interference enhances activation. CIB1 appears to inhibit integrin activation by competing with talin for binding to alphaIIbbeta3, thus providing a model for tightly controlled regulation of alphaIIbbeta3 activation.

Multiple roles of Rap1 in hematopoietic cells: complementary versus antagonistic functions

Small G proteins serve as critical control points in signal transduction, integrating a wide range of stimuli to dictate discrete cellular outcomes. The outcomes of small G-protein signaling can both potentiate and antagonize one another. Studies in hematopoietic cells have uncovered multiple functions for the small G protein, Rap1 (Ras-proximate-1). Because Rap1 can regulate cell proliferation, differentiation, and adhesion through distinct mechanisms, it serves as a paradigm for the need for tight cellular control of small G-protein function. Rap1 has received recent attention for its role in enhancing integrin-dependent signals. This action of Rap1 augments a variety of processes that characterize hematopoietic-cell function, including aggregation, migration, extravasation, and homing to target tissues. Rap1 may also regulate cellular differentiation and proliferation via pathways that are distinct from those mediating adhesion, and involve regulation of the mitogen-activated protein (MAP) kinase or ERK (extracellular signal-regulated kinase) cascade. These actions of Rap1 occur in selected cell types to enhance or diminish ERK signaling, depending on the expression pattern of the MAP kinase kinase kinases of the Raf family: Raf-1 and B-Raf. This review will examine the functions of Rap1 in hematopoietic cells, and focus on 3 cellular scenarios where the multiple actions of Rap1 function have been proposed. Recent studies implicating Rap1 in the maturation of megakaryocytes, the pathogenesis of chronic myelogenous leukemia (CML), and activation of peripheral T cells will receive particular attention.

Differential effects of a novel IFN-ζ/limitin and IFN-α on signals for Daxx induction and Crk phosphorylation that couple with growth control of megakaryocytes

OBJECTIVE

Although a novel IFN-zeta/limitin uses IFN-alpha/beta receptor, it lacks some common activities of type I IFNs. We compared effects on megakaryocyte proliferation and differentiation as well as signals for their biological activities.

MATERIALS AND METHODS

Recombinant IFN-zeta/limitin and IFN-alpha titrated with a cytopathic effect dye binding assay, were used in this study. Colony assays and serum-free suspension cultures for megakaryocytes were performed to compare their growth inhibitory effects. To analyze signals, megakaryocytes cultured in serum-free suspension cultures were stimulated and Western blotted with the indicated antibody.

RESULTS

Both IFN-zeta/limitin and IFN-alpha suppressed the proliferation of megakaryocyte progenitors without influencing their differentiation. However, much higher concentrations of IFN-zeta/limitin were required for the growth inhibition than IFN-alpha. The growth inhibition by IFN-zeta/limitin and IFN-alpha was significantly reduced when either Tyk2 or STAT1 was disrupted. In addition, the antisense oligonucleotides against Crk and Daxx, downstream molecules of Tyk2, greatly rescued the IFN-zeta/limitin- and IFN-alpha-induced reduction of megakaryocyte colony numbers. In cultured megakaryocytes, IFN-zeta/limitin induced the expression of SOCS-1 as strongly as IFN-alpha. However, IFN-zeta/limitin induced weaker phosphorylation of Crk and lower induction of Daxx than IFN-alpha.

CONCLUSIONS

Weaker signals for Crk and Daxx may participate in less megakaryocyte suppressive activity of IFN-zeta/limitin and may distinguish IFN-zeta/limitin from IFN-alpha in megakaryocytes. Our results extend the understanding about thrombocytopenia in patients with IFN-alpha treatment as well as the possibility for the clinical application of human homologue of IFN-zeta/limitin or an engineered cytokine with useful features of the IFN-zeta/limitin structure.

A critical function for B-Raf at multiple stages of myelopoiesis

Raf kinases play an integral role in the classic mitogen-activated protein (MAP) kinase (Raf/MEK/extracellular signal-related kinase [ERK]) intracellular signaling cascade, but their role in specific developmental processes is largely unknown. Using a genetic approach, we have identified a role for B-Raf during hematopoietic progenitor cell development and during megakaryocytopoiesis. Fetal liver and in vitro embryonic stem (ES) cell-derived myeloid progenitor development is quantitatively impaired in the absence of B-Raf. Biochemical data suggest that this phenotype is due to the loss of a normally occurring rise in B-Raf expression and associated ERK1/2 activation during hematopoietic progenitor cell formation. However, the presence of B-raf-/- ES cell-derived myeloid progenitors in the bone marrow of adult chimeric mice indicates the lack of an obligate cell-autonomous requirement for B-Raf in myeloid progenitor development. The lack of B-Raf also impairs megakaryocytopoiesis. Thrombopoietin (Tpo)-induced in vitro expansion of ES cell-derived megakaryocyte-lineage cells fails to occur in the absence of B-Raf. Moreover, this quantitative in vitro defect in megakaryocyte-lineage expansion is mirrored by chimeric mice data that show reduced B-raf-/- genotype contribution in megakaryocytes relative to its contribution in myeloid progenitors. Together, these data suggest that B-Raf plays a cell-autonomous role in megakaryocytopoiesis and a permissive role in myeloid progenitor development.

Expression and subcellular localization of WAVE isoforms in the megakaryocyte/platelet lineage

WAVE isoforms, which consist of WAVE-1, WAVE-2 and WAVE-3, are members of the Wiskott-Aldrich syndrome protein (WASP) family. They are implicated in the regulation of actin-cytoskeletal reorganization downsteam of the small GTPase, Rac. Since platelet attachment to extracellular matrices leads to filopodial and lamellipodial extension, we examined the expression and subcellular localization of WAVEs in platelets. Employing primary megakaryocytic cells derived from cord blood as well as megakaryocytic cell lines, we also examined their expression during megakaryocytic differentiation. Immunoblotting and immunohistochemical analysis revealed that platelets expressed WAVE-1 and WAVE-2, whereas WAVE-3 expression was hardly to be detected. WAVE-1 expression was associated with megakaryocytic differentiation, whereas WAVE-2 and WAVE-3 expression was not changed by the differentiation. In adhered platelets both WAVE-1 and WAVE-2 were localized at the edge of the lamellipodia and at the tips of filopodia. In WASP-deficient platelets we found normal lamellipodial formation and localization of WAVE-1 and WAVE-2 at the edges of lamellipodia. Furthermore, we demonstrated that WAVE-1 and WAVE-2 moved from a detergent-soluble cytosolic fraction to insoluble cytoskeleton fraction after platelet aggregation. These results suggest that WAVE-1 and WAVE-2 regulate actin reorganization during platelet spreading and aggregate formation.

Competition for talin results in trans-dominant inhibition of integrin activation

The ability of integrin adhesion receptors to undergo rapid changes in affinity for their extracellular ligands (integrin activation) is essential for the development and function of multicellular animals and is dependent on interactions between the integrin beta subunit-cytoplasmic tail and the cytoskeletal protein talin. Cross-talk among different integrins and between integrins and other receptors impacts many cellular processes including adhesion, spreading, migration, clot retraction, proliferation, and differentiation. One form of integrin cross-talk, transdominant inhibition of integrin activation, occurs when ligand binding to one integrin inhibits the activation of a second integrin. This may be relevant clinically in a number of settings such as during platelet adhesion, leukocyte trans-migration, and angiogenesis. Here we report that competition for talin underlies the trans-dominant inhibition of integrin activation. This conclusion is based on our observations that (i). beta tails selectively defective in talin binding are unable to mediate trans-dominant inhibition, (ii). trans-dominant inhibition can be reversed by overexpression of integrin binding and activating fragments of talin, and (iii). expression of another non-integrin talin-binding protein, phosphatidylinositol phosphate kinase type Igamma-90, also inhibits integrin activation. Thus, the sequestration of talin by the suppressive species is both necessary and sufficient for trans-dominant inhibition of integrin activation.

Caspase-12: a developmental link between G-protein-coupled receptors and integrin alphaIIbbeta3 activation

Fibrinogen binding by integrin alphaIIbbeta3 is promoted by platelet agonists that increase the affinity and avidity of alphaIIbbeta3 for fibrinogen through a process called "inside-out" signaling. Having previously demonstrated that inside-out activation of alphaIIbbeta3 is defective in murine megakaryocytes that lack the transcription factor NF-E2, we screened for NF-E2-regulated genes that affect alphaIIbbeta3 activation. Caspase-12 is the most down-regulated gene we identified in NF-E2(-/-) megakaryocytes. Therefore, the role of this protein in alphaIIbbeta3 activation was determined using platelets from caspase-12(-/-) mice. Despite wild-type levels of alphaIIbbeta3, caspase-12(-/-) platelets exhibit reduced fibrinogen binding to alphaIIbbeta3 following stimulation by adenosine diphosphate (ADP) or protease-activated receptor 4 (PAR4) receptor-activating peptide. The defect in alphaIIbbeta3 activation is associated with decreased cytosolic free calcium and inositol triphosphate levels, and with reduced aggregation, despite wild-type phospholipase Cbeta expression levels. In contrast, agonist-induced surface expression of P-selectin, suppression of cAMP levels following ADP stimulation, and spreading on immobilized fibrinogen are unimpaired. Moreover, although caspase-12 is highly expressed in mature megakaryocytes, it is undetectable in platelets. Taken together, these studies establish that caspase-12 expression in murine megakaryocytes is regulated, directly or indirectly, by NF-E2, and suggest that caspase-12 participates in the development of fully functional signaling pathways linking some G-protein-coupled receptors to alphaIIbbeta3 activation.

Raf-1 is not required for megakaryocytopoiesis or TPO-induced ERK phosphorylation

Thrombopoietin stimulates extracellular signal-related kinase 1/2 (ERK1/2) phosphorylation in megakaryocytes, and the classic mitogen-activated protein (MAP) kinase (Raf/mitogen-induced extracellular kinase [MEK]/ERK) pathway has been implicated directly and indirectly to play a critical role in megakaryocytopoiesis. However, the involvement of specific Raf family members in megakaryocytopoiesis is unknown. raf-1-/- mice were therefore used to directly determine the role of Raf-1 in megakaryocytopoiesis. Surprisingly, raf-1-/- mice have a modestly higher platelet count than their raf-1+/+ littermates. Nonetheless, the absence of Raf-1 does not alter thrombopoietin-induced expansion of primary megakaryocyte-lineage cells, the development of apoptotic megakaryocytes in the presence or absence of thrombopoietin, or the development of megakaryocyte DNA ploidy distribution. Moreover, raf-1-/- megakaryocytes do not have a compensatory increase in A-Raf or B-Raf expression, and thrombopoietin-induced ERK1/2 phosphorylation is similar in raf-1-/- and raf-1+/+ megakaryocytes. These unexpected findings demonstrate that Raf-1 is dispensable for megakaryocytopoiesis, and for thrombopoietin-induced ERK1/2 activation in primary megakaryocyte-lineage cells.