Glycoprotein IIb-IIIa Is Expressed on Avian Multilineage Hematopoietic Progenitor Cells

Temporal-Spatial Resolution Fate Mapping Reveals Distinct Origins for Embryonic and Adult Microglia in Zebrafish

Microglia are CNS resident macrophages, and they play important roles in neural development and function. Recent studies have suggested that murine microglia arise from a single source, the yolk sac (YS), yet these studies lack spatial resolution to define the bona fide source(s) for microglia. Here, using light-induced high temporal-spatial resolution fate mapping, we challenge this single-source view by showing that microglia in zebrafish arise from multiple sources. The embryonic/larval microglia originate from the rostral blood island (RBI) region, the equivalent of mouse YS for myelopoiesis, whereas the adult microglia arise from the ventral wall of dorsal aorta (VDA) region, a tissue also producing definitive hematopoiesis in mouse. We further show that the VDA-region-derived microglia are Runx1 dependent, but cMyb independent, and developmentally regulated differently from the RBI region-derived microglia. Our study establishes a new paradigm for investigating the development and function of distinct microglia populations.

Whole-transcriptome analysis of endothelial to hematopoietic stem cell transition reveals a requirement for Gpr56 in HSC generation

Using highly sensitive RNAseq to examine the whole transcriptome of enriched aortic hematopoietic stem cells and endothelial cells, the authors find G-protein–coupled receptor, Gpr56, is required to generate the first HSCs during endothelial to hematopoietic cell transition.

Hematopoietic stem cells (HSCs) are generated via a natural transdifferentiation process known as endothelial to hematopoietic cell transition (EHT). Because of small numbers of embryonal arterial cells undergoing EHT and the paucity of markers to enrich for hemogenic endothelial cells (ECs [HECs]), the genetic program driving HSC emergence is largely unknown. Here, we use a highly sensitive RNAseq method to examine the whole transcriptome of small numbers of enriched aortic HSCs, HECs, and ECs. Gpr56, a G-coupled protein receptor, is one of the most highly up-regulated of the 530 differentially expressed genes. Also, highly up-regulated are hematopoietic transcription factors, including the “heptad” complex of factors. We show that Gpr56 (mouse and human) is a target of the heptad complex and is required for hematopoietic cluster formation during EHT. Our results identify the processes and regulators involved in EHT and reveal the surprising requirement for Gpr56 in generating the first HSCs.

How the avian model has pioneered the field of hematopoietic development

The chicken embryo has a long history as a key model in developmental biology. Because of its distinctive developmental characteristics, it has contributed to major breakthroughs in the field of hematopoiesis. Among these, the discovery of B lymphocytes and the three rounds of thymus colonization; the embryonic origin of hematopoietic stem cells and the traffic between different hematopoietic organs; and the existence of two distinct endothelial cell lineages one angioblastic, restricted to endothelial cell production, and another, hemangioblastic, able to produce both endothelial and hematopoietic cells, should be cited. The avian model has also contributed to substantiate the endothelial-to-hematopoietic transition associated with aortic hematopoiesis and the existence of the allantois as a hematopoietic organ. Because the immune system develops relatively late in aves, the avian embryo is used to probe the tissue-forming potential of mouse tissues through mouse-into-chicken chimeras, providing insights into early mouse development by circumventing the lethality associated with some genetic strains. Finally, the avian embryo can be used to investigate the differentiation potential of human ES cells in the context of a whole organism. The combinations of classic approaches with the development of powerful genetic tools make the avian embryo a great and versatile model.

CD41 is developmentally regulated and differentially expressed on mouse hematopoietic stem cells

CD41 expression is associated with the earliest stages of mouse hematopoiesis. It is notably expressed on some cells of the intra-aortic hematopoietic clusters, an area where the first adult-repopulating hematopoietic stem cells (HSCs) are generated. Although it is generally accepted that CD41 expression marks the onset of primitive/definitive hematopoiesis, there are few published data concerning its expression on HSCs. It is as yet uncertain whether HSCs express CD41 throughout development, and if so, to what level. We performed a complete in vivo transplantation analysis with yolk sac, aorta, placenta, and fetal liver cells, sorted based on CD41 expression level. Our data show that the earliest emerging HSCs in the aorta express CD41 in a time-dependent manner. In contrast, placenta and liver HSCs are CD41⁻. Thus, differential and temporal expression of CD41 by HSCs in the distinct hematopoietic territories suggests a developmental/dynamic regulation of this marker throughout development.

Whole embryo imaging of hematopoietic cell emergence and migration

The use of transgenic mice in which tissue or lineage-specific, cell-restricted promoters drive fluorescent reporters has recently been reported as a means to follow the in vivo migration of various hematopoietic cells during murine development. At present there is limited ability of these approaches to image the emergence of the first hematopoietic cell subsets due to lack of unique markers that define those hematopoietic cells. We have utilized whole embryo analysis via immunostaining and confocal laser-scanning microscopic (CLSM) imaging to define the emergence of the first hematopoietic elements in the yolk sac of the developing conceptus. The methods employed to examine yolk sac hematopoiesis may be applied to hematopoietic cell emergence in the embryo proper or fetal liver in the generation of a complete map of hematopoietic ontogeny.

Novel monoclonal antibodies recognise guinea fowl thrombocytes

This paper introduces two novel monoclonal antibodies, designated GTr1 and GTr2, which recognise guinea fowl thrombocyte surface antigen(s). The antibodies were tested in embryos and adult birds. GTr1 and GTr2 staining emerged at embryonic days 12 and 7, respectively. After embryonic day 12 there was no difference in staining pattern between the two monoclonal antibodies. The isotype of the antibodies is IgG1. The antibodies did not react with any other haematopoietic cells of guinea fowl, and there was no species cross-reaction with chicken, turkey and quail. The antibodies can be used in interspecies chimeric and parabiotic experiments to identify cells of guinea fowl origin.

Platelet glycoprotein IIIa gene expression in normal and malignant megakaryopoiesis

The platelet glycoprotein GPIIb/IIIa functions as a receptor for fibrinogen in platelet aggregation process and is an example of an early megakaryocytic marker. One of a chronic myeloproliferative disorder, essential thrombocythemia, is caused by abnormal megakaryopoiesis. Due to the lack of reliable method for the diagnosis of that disease and the importance of GPIIIa as a marker for identifying early megakaryocytes, the expression level of GPIIIa in mononuclear and CD34(+) cells and during megakaryopoiesis was compared between normal individuals and patients with essential thrombocythemia. For this purpose, surface markers GPIIIa and CD34 were analyzed with flow cytometer, and GPIIIa expression level was measured with real-time polymerase chain reaction (PCR) method. Mononuclear and CD34(+) cells from normal individuals and patients were isolated, analyzed, and seeded into serum-free medium Stemspantrade mark Medium enriched with IL-6, IL-3, thrombopoietin, and stem cell factor. The difference between normal individuals and patients was noticed in the expression level of GPIIIa in the CD34(+) cells and in the time course of cell surface markers. CD34(+) cells from patients has 33% higher of GPIIIa antigens on the surface and 34% higher GPIIIa messenger RNA (mRNA) expression level. The negative effect of IL-3 on the maturation of megakaryocytes was not noticed; there were 56.46% of megakaryoblasts at the end of the cultivation, and after 14 days of culturing, 111.09 times increase of GPIIIa mRNA in patients was detected. This study is therefore offering the method that could serve as reliable tool for discriminating ET from other similar myeloproliferative disorders.

Ontogeny of the hematopoietic system

Blood cells are constantly produced in the bone marrow (BM) of adult mammals. This constant turnover ultimately depends on a rare population of progenitors that displays self-renewal and multilineage differentiation potential, the hematopoietic stem cells (HSCs). It is generally accepted that HSCs are generated during embryonic development and sequentially colonize the fetal liver, the spleen, and finally the BM. Here we discuss the experimental evidence that argues for the extrinsic origin of HSCs and the potential locations where HSC generation might occur. The identification of the cellular components playing a role in the generation process, in these precise locations, will be important in understanding the molecular mechanisms involved in HSC production from undifferentiated mesoderm.

Glycoprotein Ibalpha promoter drives megakaryocytic lineage-restricted expression after hematopoietic stem cell transduction using a self-inactivating lentiviral vector

Megakaryocytic (MK) lineage is an attractive target for cell/gene therapy approaches, aiming at correcting platelet protein deficiencies. However, MK cells are short-lived cells, and their permanent modification requires modification of hematopoietic stem cells with an integrative vector such as a lentiviral vector. Glycoprotein (Gp) IIb promoter, the most studied among the MK regulatory sequences, is also active in stem cells. To strictly limit transgene expression to the MK lineage after transduction of human CD34(+) hematopoietic cells with a lentiviral vector, we looked for a promoter activated later during MK differentiation. Human cord blood, bone marrow, and peripheral-blood mobilized CD34(+) cells were transduced with a human immunodeficiency virus-derived self-inactivating lentiviral vector encoding the green fluorescent protein (GFP) under the transcriptional control of GpIbalpha, GpIIb, or EF1alpha gene regulatory sequences. Both GpIbalpha and GpIIb promoters restricted GFP expression (analyzed by flow cytometry and immunoelectron microscopy) in MK cells among the maturing progeny of transduced cells. However, only the GpIbalpha promoter was strictly MK-specific, whereas GpIIb promoter was leaky in immature progenitor cells not yet engaged in MK cell lineage differentiation. We thus demonstrate the pertinence of using a 328-base-pair fragment of the human GpIbalpha gene regulatory sequence, in the context of a lentiviral vector, to tightly restrict transgene expression to the MK lineage after transduction of human CD34(+) hematopoietic cells. Disclosure of potential conflicts of interest is found at the end of this article.

Hematopoietic potential of the pre-fusion allantois

We previously showed that the fetal component of the placenta has a vigorous hematopoietic activity. Whether this organ is an environmental niche where hematopoietic stem cells (HSC) proliferate and become committed to various lineages, or whether it is also a site for HSC emergence, was left open. This issue can be addressed only if the components that will give rise to the placenta are tested prior to vascularization. The fetal part of the placenta forms through the fusion of the allantois and the chorionic plate around the stage of 7 somite pairs. The allantois, a mesodermal rudiment that provides fetal blood vessels to the placenta, was retrieved before fusion. We found in this rudiment expression of CD41, a known marker of early embryonic hematopoietic progenitors. c-Kit encoding a progenitor specific receptor was also expressed. Significantly, as early as the 1-2 somite stage, the allantois yielded erythroid, myeloid and multipotent clonogenic progenitors, when pre-cultured in toto prior to seeding in a semisolid medium. These results provide evidence that the allantois has hematopoietic potential per se. Whether this potential also involves the ability to produce HSC is still to be determined.

Integrin alphaIIbbeta3 induces the adhesion and activation of mast cells through interaction with fibrinogen

Integrin alphaIIb, a well-known marker of megakaryocyte-platelet lineage, has been recently recognized on hemopoietic progenitors. We now demonstrate that integrin alphaIIbbeta3 is highly expressed on mouse and human mast cells including mouse bone marrow-derived mast cells, peritoneal mast cells, and human cord blood-derived mast cells, and that its binding to extracellular matrix proteins leads to enhancement of biological functions of mast cells in concert with various stimuli. With exposure to various stimuli, including cross-linking of FcepsilonRI and stem cell factor, mast cells adhered to extracellular matrix proteins such as fibrinogen and von Willebrand factor in an integrin alphaIIbbeta3-dependent manner. In addition, the binding of mast cells to fibrinogen enhanced proliferation, cytokine production, and migration and induced uptake of soluble fibrinogen in response to stem cell factor stimulation, implicating integrin alphaIIbbeta3 in a variety of mast cell functions. In conclusion, mouse and human mast cells express functional integrin alphaIIbbeta3.

The embryonic origins of hematopoietic stem cells: a tale of hemangioblast and hemogenic endothelium

The developmental origin of hematopoietic stem cells has been for decades the subject of great interest. Once thought to emerge from the yolk sac, hematopoietic stem cells have now been shown to originate from the embryonic aorta. Increasing evidence suggests that hematopoietic stem cells are produced from an endothelial intermediate designated by the authors as hemangioblast or hemogenic endothelium. Recently, the allantois in the avian embryo and the placenta in the mouse embryo were shown to be a site of hematopoietic cell production/expansion and thus appear to play a critical role in the formation of the hematopoietic system. In this review we shall give an overview of the data obtained from human, mouse and avian models on the cellular origins of the hematopoietic system and discuss some aspects of the molecular mechanisms controlling hematopoietic cell production.

Genesis of hematopoietic stem cells in vitro and in vivo: new insights into developmental maturation

Hematopoietic stem cells first arise in the mammalian embryo in a primitive state, not capable of reconstituting hematopoiesis in irradiated adult recipients. As development proceeds, these cells eventually mature to acquire definitive, adult characteristics, including adult reconstitution ability. Mouse embryonic stem cells induced to undergo hematopoiesis in vitro readily generate primitive hematopoietic stem cells but rarely generate the definitive type. Recent work has stimulated a new appreciation of the events involved in the developmental maturation of hematopoietic stem cells. Application of this knowledge to in vitro differentiation systems will be critical to the successful development of hematopoietic therapies from embryonic stem cells.

GPIIb (CD41) integrin is expressed on mast cells and influences their adhesion properties

OBJECTIVES

GPIIb integrin expression has been found on platelets and megakaryocytes, and more recently on immature hematopoietic progenitors. We set out to investigate expression of GPIIb in other hematopoietic cell lineages and, having detected it on mast cells, aimed to determine what possible role it might perform.

METHODS

We have made use of cultured human and murine bone marrow mast cells (BMMC) in order to characterize the expression of GPIIb. Further, BMMC cultures from wild type and GPIIb deficient (gpIIb-/-) mice were used for comparison of the adhesive properties mediated by this receptor. Finally, peritoneal mast cells were analyzed from both wild type and (gpIIb-/-) mice.

RESULTS

We demonstrate expression of GPIIb on cultured BMMC. Using cells derived from mice homozygous for a null allele of gbIIb we show that the absence of GPIIb has no effect on mast cells with respect to a number of measures of cell growth and differentiation. However, loss of GPIIb on BMMC results in an increase in surface expression of aV integrin, the alternative partner of GPIIIa.

CONCLUSION

The results in this study demonstrate that GPIIb is expressed in human and murine mast cells. A function for GPIIb on mast cells is suggested by the altered adhesion of gbIIb-/- BMMC to fibronectin- and vitronectin-coated surfaces. Moreover, comparison of mast cells from the peritoneal cavity of wild type and gbIIb-/- mice indicates that GPIIb could influence the in vivo differentiation or homing of tissue mast cells.

From the hemangioblast to self-tolerance: a series of innovations gained from studies on the avian embryo

During the last decades of the 20th century, studies on the vertebrate hematopoietic and immune systems have largely been performed, on mammalian models. The mouse has been the preferred material for several cogent reasons: (i) numerous well defined genetic strains are available; (ii) this species has been and still is instrumental in the study of gene activity through transgenesis; and (iii) in vitro culture techniques and in vivo assays for blood cells together with a wide array of antibodies and nucleic acid probes have been developed to investigate the cellular interactions occurring during hematopoiesis and immune reactivity. However, important and fundamental notions have emerged from using another higher vertebrate model, the avian embryo. The distinction among small lymphocytes of two populations, the T and B lymphocytes, endowed with different roles in adaptive immunity and dependant on different environments for their specification, has relied on experiments carried out in birds. The avian model has been critical for the analysis of the origin and traffic of hematopoietic precursor cells. It allowed the demonstration that both hematopoietic and angioblastic lineages arise from a common precursor, a cell whose existence had been proposed but never undoubtedly proven, the hemangioblast. Finally a form of thymus-dependant 'dominant' tolerance was demonstrated on the basis of experiments in the avian embryo, which initiated a large current of studies on 'regulatory T-cells'. Work in this model during the last decades has relied strongly on the construction of chimeras between quail and chick embryos that allowed a refined analysis of cell behaviour during embryogenesis. The novel perception of developmental neuropoiesis and immunopoiesis that followed proved to be largely applicable to lower and higher vertebrates, notably mammals.

A Monoclonal Antibody against Chicken Thrombocytes Reacts with the Cells of Thrombocyte Lineage

A new mouse monoclonal antibody (mAb), HUKT was raised against chicken peripheral blood thrombocytes. The mAb HUKT appeared to detect a specific marker on the surface of chicken thrombocytes. Flow cytometry (FCM) analysis revealed that it did not react with cells from the normal thymus, bursa of Fabricius, six kinds of chicken cell lines, chicken erythrocytes or human platelets. In addition, HUKT(+) cells in peripheral blood leukocytes (PBL) were CD45(low), Bu-1a(-) and CD3(-) cells. Immunoblotting analysis showed that the molecule recognized by HUKT is a monomer with an apparent molecular weight of 150 kDa under non-reducing and reducing conditions. Tissue distribution studies revealed that only cells of thrombocyte lineage in bone marrow and embryonic blood cells were stained by HUKT. The HUKT mAb presented here may be useful for both ontogenetic studies of thrombocyte lineage and immunological studies in the chicken.

The glycoprotein IIb molecule is expressed on early murine hematopoietic progenitors and regulates their numbers in sites of hematopoiesis

The alpha integrin GPIIb is a marker of hematopoietic progenitors. Using a marking strategy based on Cre-loxP technology to trace the fate of GPIIb-expressing cells, we show that GPIIb is expressed during early definitive embryonic hematopoiesis. However, the marked fetal population is distinct from the hematopoietic cells that predominate in the adult, suggesting that at least two waves of progenitors arise concurrently or consecutively in the fetus. Furthermore, using an inactivated allele of gpIIb, we provide evidence for a functional role of GPIIb on progenitors. We observe an increase in hematopoietic progenitors in the yolk sac, fetal liver, and bone marrow, an effect which may, in part, be explained by loss of binding to fibronectin.

Vasculogenesis and the search for the hemangioblast

Embryonic endothelial cells (EC) are generated by two mechanisms, vasculogenesis and angiogenesis (1). The term vasculogenesis describes the de novo emergence of EC progenitors from the mesoderm, whereas angiogenesis corresponds to the generation of EC by sprouting from the pre-existing vascular network. Until recently, it was thought that vasculogenesis was restricted to the period of embryonic development, whereas in the adult, only angiogenesis contributed to EC proliferation. The discovery of circulating EC progenitors in adult bone marrow and peripheral blood has suggested that additional mechanisms besides angiogenesis can occur in the adult, and therefore have renewed interest in the embryonic origin and the development of these progenitor cells. Vasculogenesis in the chick embryo has been studied since the beginning of the 20th century. During early development, vasculogenesis is intimately linked to the emergence of hematopoietic cells (HC). The existence of a common precursor for both EC and HC, termed "hemangioblast," was postulated (2). The purpose of this review is to summarize the experimental evidence concerning the emergence of EC and HC during embryonic life.

Platelet glycoprotein complex Ia/IIa antibodies cause neonatal alloimmune thrombocytopenia but do not inhibit megakaryopoiesis and platelet recovery after allogeneic cord blood stem cell transplantation

A sibling cord blood (CB) transplantation was performed in a boy with Wiskott-Aldrich syndrome. The CB (31 x 10(6) CD34(+) cells) derived from a newborn sister with neonatal alloimmune thrombocytopenia (NAIT) with 40,000 platelets/microl, caused by a maternal anti-HPA-5b and HLA-A2 antibody. Maternal serum did not inhibit clonogenicity after in vitro testing of megakaryopoiesis. Accordingly, this CB was accepted for sibling transplantation. The transplantation showed a good course with fast and sustained hematopoietic reconstitution (granulocytes >500/microl on day +16, platelets >50,000/microl on day +30). This case demonstrates a successful CB transplantation from a donor suffering from NAIT.

CD9 and megakaryocyte differentiation

It is shown that the tetraspanin CD9 has a complex pattern of distribution in hematopoietic cells and is heterogeneously expressed on human bone marrow CD34(+) cells. CD34(high)CD38(low)Thy1(+) primitive progenitors are contained in the population with intermediate CD9 expression, thus suggesting that CD9 expression may precede CD38 appearance. Cell sorting shows that colony-forming unit (CFU)-GEMM and CFU-GM are present in high proportions in this fraction and in the fraction with the lowest CD9 expression. Cells with the highest level of CD9 are committed to the B-lymphoid or megakaryocytic (MK) lineages, as shown by the co-expression of either CD19 or CD41/GPIIb and by their strong potential to give rise to CFU-MK. In liquid cultures, CD9(high)CD41(neg) cells give rise to cells with high CD41 expression as early as 2 days, and this was delayed by at least 3 to 4 days for the CD9(mid) cells; few CD41(high) cells could be detected in the CD9(low) cell culture, even after 6 days. Antibody ligation of cell surface CD9 increased the number of human CFU-MK progenitors and reduced the production of CD41(+) megakaryocytic cells in liquid culture. This was associated with a decreased expression of MK differentiation antigens and with an alteration of the membrane structure of MK cells. Altogether these data show a precise regulation of CD9 during hematopoiesis and suggest a role for this molecule in megakaryocytic differentiation, possibly by participation in membrane remodeling. (Blood. 2001;97:1982-1989)

Different expression of CD41 on human lymphoid and myeloid progenitors from adults and neonates

The glycoprotein (Gp) IIb/IIIa integrin, also called CD41, is the platelet receptor for fibrinogen and several other extracellular matrix molecules. Recent evidence suggests that its expression is much wider in the hematopoietic system than was previously thought. To investigate the precise expression of the CD41 antigen during megakaryocyte (MK) differentiation, CD34(+) cells from cord blood and mobilized blood cells from adults were grown for 6 days in the presence of stem cell factor and thrombopoietin. Two different pathways of differentiation were observed: one in the adult and one in the neonate cells. In the neonate samples, early MK differentiation proceeded from CD34(+)CD41(-) through a CD34(-)CD41(+)CD42(-) stage of differentiation to more mature cells. In contrast, in the adult samples, CD41 and CD42 were co-expressed on a CD34(+) cell. The rare CD34(+)CD41(+)CD42(-) cell subset in neonates was not committed to MK differentiation but contained cells with all myeloid and lymphoid potentialities along with long-term culture initiating cells (LTC-ICs) and nonobese diabetic/severe combined immune-deficient repopulating cells. In the adult samples, the CD34(+)CD41(+)CD42(-) subset was enriched in MK progenitors, but also contained erythroid progenitors, rare myeloid progenitors, and some LTC-ICs. All together, these results demonstrate that the CD41 antigen is expressed at a low level on primitive hematopoietic cells with a myeloid and lymphoid potential and that its expression is ontogenically regulated, leading to marked differences in the surface antigenic properties of differentiating megakaryocytic cells from neonates and adults. (Blood. 2001;97:2023-2030)

Characterization of Mpl mutants using primary megakaryocyte-lineage cells from mpl−/−mice: a new system for Mpl structure–function studies

Mpl is the thrombopoietin (TPO) receptor. The current molecular understanding of how Mpl activation stimulates proliferation of megakaryocyte-lineage cells is based largely on the engineered expression of Mpl in nonmegakaryocyte-lineage cell lines. However, the relevance of these findings to Mpl signaling in primary megakaryocyte-lineage cells remains largely unknown. Therefore, a system was developed to study Mpl function in primarympl−/−megakaryocyte-lineage cells. Expressing avian retroviral receptors on the surfaces of mammalian cells overcomes their natural block to avian retroviral infection; 815 bp of human GPIIb regulatory sequence was used to generate transgenic mice with megakaryocyte-lineage expression of the subgroup A avian leukosis virus receptor, TVA. Avian retroviral infection of unfractionated bone marrow from these mice is restricted to megakaryocyte-lineage cells. The transgenic mice were crossed to anmpl−/−background generatingGPIIb-tva+mpl−/−mice. By using avian retroviruses to express wild-type or mutant Mpl on the surfaces of primary megakaryocyte-lineage cells, it was demonstrated that (1) the 10 membrane-proximal, cytoplasmic amino acids of Mpl are required for TPO-induced proliferation; (2) Y582F mutation confers a proliferative advantage over wild-type Mpl and imparts a constitutive anti-apoptotic signal; (3) truncating the 50 C-terminal Mpl amino acids reduces but does not eliminate TPO-induced mitogen-activated protein kinase activation, yet it does not alter the synergistic effect of stem cell factor on TPO-induced proliferation; and (4) TPO-induced proliferation of early, primary megakaryocyte-lineage cells does not require Stat-5 phosphorylation. The system reported provides an improved approach for Mpl structure–function studies, and the method can be applied to any hematopoietic lineage.

The αIIbβ3 integrin and GPIb-V-IX complex identify distinct stages in the maturation of CD34+cord blood cells to megakaryocytes

Megakaryocytopoiesis is a complex multistep process involving cell division, endoreplication, and maturation and resulting in the release of platelets into the blood circulation. Megakaryocytes (MK) progressively express lineage-restricted proteins, some of which play essential roles in platelet physiology. Glycoprotein (GP)Ib-V-IX (CD42) and GPIIb (CD41) are examples of MK-specific proteins having receptor properties essential for platelet adhesion and aggregation. This study defined the progressive expression of the GPIb-V-IX complex during in vitro MK maturation and compared it to that of GPIIb, an early MK marker. Human cord blood CD34+ progenitor cells were cultured in the presence of cytokines inducing megakaryocytic differentiation. GPIb-V-IX expression appeared at day 3 of culture and was strictly dependent on MK cytokine induction, whereas GPIIb was already present in immature CD34+ cells. Analysis by flow cytometry and of the messenger RNA level both showed that GPV appeared 1 day later than GPIb-IX. Microscopy studies confirmed the late appearance of GPV, which was principally localized in the cytoplasm when GPIb-IX was found on the cell surface, suggesting a delayed program of GPV synthesis and trafficking. Cell sorting studies revealed that the CD41+GPV+ population contained 4N and 8N cells at day 7, and was less effective than CD41+GPV− cells in generating burst-forming units of erythrocytes or MK colonies. This study shows that the subunits of the GPIb-V-IX complex represent unique surface markers of MK maturation. The genes coding for GPIb-IX and GPV are useful tools to study megakaryocytopoiesis and for tissue-specific or conditional expression in mature MK and platelets.

MHC class II and c-kit expression allows rapid enrichment of T-cell progenitors from total bone marrow cells

T-cell progenitors in the embryonic bone marrow express the tyrosine kinase receptor c-kit. RR5, an anti-MHC class II β chain monoclonal antibody, subdivides this c-kit positive population. Intrathymic transfer experiments showed that most of the T-cell progenitors belong to the MHC class II+/c-kit+ bone marrow population in the embryo and young adult. On transplantation, these bone marrow progenitors lose this expression and differentiate into CD4 CD8 T lymphocytes. In contrast, erythroid progenitors are restricted to the MHC class II−/c-kit+ population. The MHC class II+/c-kit+ pro-T cells are metabolically active, because they stain brightly with rhodamin 123. Their cyclin A and B expression level suggests that they are in the mitotic phase of the cell cycle. Thus, we define an easy sorting protocol, which allows enrichment of T-cell progenitors from total bone marrow hemopoietic cells.

Identification of a GATA-overlapping sequence within the enhancer of the murine GPIIb promoter that induces transcriptional deregulation in human K562 cells

The human and the murine glycoprotein platelet IIb (GPIIb) promoters are megakaryocyte specific in human and murine cell systems, respectively. Here we show that the murine promoter is, however, highly active when transfected in K562 human cells in which the human promoter is almost inactive. A murine promoter, in which the enhancer element was replaced by the human, retrieves its megakaryocytic specificity in human cell lines. The human and murine GATA-binding sites located in the enhancer region display slight sequence divergence next to the consensus GATA core sequence. Gel shift experiments show that, although the murine and the human GATA sequences both bind GATA-1, the murine sequence alone forms an additional complex (B) not detected with the human sequence. When the murine GATA-containing region is replaced by the human in the context of the murine GPIIb promoter, megakaryocyte specificity is restored in the human cell lines. A G nucleotide 3′ to GATA appears crucial because its substitution abrogates B but not GATA-1 binding and restores megakaryocyte specificity to the murine promoter. Conversely, substitution of the human GATA-1 binding sequence by its murine homologue that binds both GATA-1 and complex B induces an abnormal activity for the human promoter in K562 cells. Altogether, our data suggest that limited changes in the GATA-containing enhancer of the GPIIb promoter can induce the recruitment of accessory proteins that could be involved in alteration of a megakaryocyte-restricted gene activation program.

Thrombasthenic mice generated by replacement of the integrin αIIb gene: demonstration that transcriptional activation of this megakaryocytic locus precedes lineage commitment

To analyze the transcriptional activity of the gene encoding the α subunit of the platelet integrin αIIbβ3during the hematopoietic differentiation, mice were produced in which the herpes virus thymidine kinase (tk) was introduced in this megakaryocytic specific locus using homologous recombination technology. This provided a convenient manner in which to induce the eradication of particular hematopoietic cells expressing the targeted gene. Results of progenitor cell cultures and long-term bone marrow (BM) assays showed that the growth of a subset of stem cells was reduced in the presence of the antiherpetic drug ganciclovir, demonstrating that the activation of the toxic gene occurs before the commitment to the megakaryocytic lineage. Furthermore theknock-in of the tk gene into the αIIb locus resulted in the knock-out of the αIIb gene in homozygous mice. Cultures of BM cells of these animals, combined with ultrastructural analysis, established that the αIIbglycoprotein is dispensable for lineage commitment and megakaryocytic maturation. Platelets collected from αIIb-deficient mice failed to bind fibrinogen, to aggregate, and to retract a fibrin clot. Moreover, platelet α-granules did not contain fibrinogen. Consistent with these characteristics, the mice displayed bleeding disorders similar to those in humans with Glanzmann thrombasthenia.