Growth factor-induced process formation of megakaryocytes derived from CFU-MK

Cellular characterization of thrombocytes in Xenopus laevis with specific monoclonal antibodies

Platelets are produced from megakaryocytes (MKs) in the bone marrow. In contrast, most nonmammalian vertebrates have nucleated and spindle-shaped thrombocytes instead of platelets in their circulatory systems, and the presence of MKs as thrombocyte progenitors has not been verified. In developing a new animal model in adult African clawed frog (Xenopus laevis), we needed to distinguish nucleated thrombocytes and their progenitors from other blood cells, because the cellular morphology of activated thrombocytes resembles lymphocytes and other cells. We initially generated two monoclonal antibodies, T5 and T12, to X. laevis thrombocytes. Whereas T5 recognized both thrombocytes and leukocytes, T12 specifically reacted to spindle-shaped thrombocytes. The T12(+) thrombocytes displayed much higher DNA ploidy than nucleated erythrocytes, and they expressed CD41 and Fli-1. In the presence of CaCl2, adenosine diphosphate, thrombin, or various collagens, T12(+) thrombocytes exhibited aggregation. These thrombocytes were located predominantly in the hepatic sinusoids and the splenic red pulp, suggesting that both organs are the sites of thrombopoiesis. Notably, circulating thrombocytes exhibited lower DNA ploidy than hepatic thrombocytes. Intraperitoneal administration of T12 produced immune thrombocytopenia in frogs, which reached a nadir 4 days postinjection, followed by recovery, suggesting that humoral regulation maintained the number of circulating thrombocytes. Although differences between MKs and thrombocytes in X. laevis remain to be defined, our results provide further insight into MK development and thrombopoiesis in vertebrates.

Effects of Cytokines on Platelet Production From Blood and Marrow CD34+ Cells

The late stages of megakaryocytopoiesis, consisting of the terminal processes of cytoplasmic maturation and platelet shedding, remain poorly understood. A simple liquid culture system using CD34+ cells in serum-free medium has been developed to study the regulation of platelet production in vitro. Platelets produced in vitro were enumerated by flow cytometry. A truncated form of human Mpl-Ligand conjugated to polyethylene glycol (PEG-rHuMGDF) played a crucial role in both proplatelet formation and platelet production. A combination of stem cell factor (SCF), interleukin-3 (IL-3), and IL-6 was as potent as PEG-rHuMGDF for the growth of megakaryocytes (MKs). However, the number of proplatelet-displaying MKs and platelets was increased 10-fold when PEG-rHuMGDF was used. Peripheral blood mobilized CD34+ cells gave rise to a threefold augmentation of platelets compared with marrow CD34+ cells. This finding was related to the higher proliferative capacity of the former population because the proportion of proplatelet-displaying MKs was similar for both types of CD34+ cells. The production of platelets per MK from CD34+ cells was low, perhaps because of the low ploidy of the cultured MKs. This defect in polyploidization correlated with the degree of proliferation of MK progenitors induced by cytokines. In contrast, ploidy development closer to that observed in marrow MKs was observed in MKs derived from the low proliferative CD34+CD41+ progenitors and was associated with a twofold to threefold increment in platelet production per MK. As shown using this CD34+ CD41+ cell population, PEG-rHuMGDF was required throughout the culture period to potently promote platelet production, but was not involved directly in the process of platelet shedding. IL-3, SCF, and IL-6 alone had a very weak effect on proplatelet formation and platelet shedding. Surprisingly, when used in combination, these cytokines elicited a degree of platelet production which was decreased only 2.4-fold in comparison with PEG-rHuMGDF. This suggests that proplatelet formation may be inhibited by non-MK cells which contaminate the cultures when the entire CD34+ cell population is used. Cultured platelets derived from PEG-rHuMGDF– or cytokine combination-stimulated cultures had similar ultrastructural features and a nearly similar response to activation by thrombin. The data show that this culture system may be useful to study the effects of cytokines and the role of polyploidization on platelet production and function.

Effects of Cytokines on Platelet Production From Blood and Marrow CD34+ Cells

The late stages of megakaryocytopoiesis, consisting of the terminal processes of cytoplasmic maturation and platelet shedding, remain poorly understood. A simple liquid culture system using CD34+ cells in serum-free medium has been developed to study the regulation of platelet production in vitro. Platelets produced in vitro were enumerated by flow cytometry. A truncated form of human Mpl-Ligand conjugated to polyethylene glycol (PEG-rHuMGDF) played a crucial role in both proplatelet formation and platelet production. A combination of stem cell factor (SCF), interleukin-3 (IL-3), and IL-6 was as potent as PEG-rHuMGDF for the growth of megakaryocytes (MKs). However, the number of proplatelet-displaying MKs and platelets was increased 10-fold when PEG-rHuMGDF was used. Peripheral blood mobilized CD34+ cells gave rise to a threefold augmentation of platelets compared with marrow CD34+ cells. This finding was related to the higher proliferative capacity of the former population because the proportion of proplatelet-displaying MKs was similar for both types of CD34+ cells. The production of platelets per MK from CD34+ cells was low, perhaps because of the low ploidy of the cultured MKs. This defect in polyploidization correlated with the degree of proliferation of MK progenitors induced by cytokines. In contrast, ploidy development closer to that observed in marrow MKs was observed in MKs derived from the low proliferative CD34+CD41+ progenitors and was associated with a twofold to threefold increment in platelet production per MK. As shown using this CD34+ CD41+ cell population, PEG-rHuMGDF was required throughout the culture period to potently promote platelet production, but was not involved directly in the process of platelet shedding. IL-3, SCF, and IL-6 alone had a very weak effect on proplatelet formation and platelet shedding. Surprisingly, when used in combination, these cytokines elicited a degree of platelet production which was decreased only 2.4-fold in comparison with PEG-rHuMGDF. This suggests that proplatelet formation may be inhibited by non-MK cells which contaminate the cultures when the entire CD34+ cell population is used. Cultured platelets derived from PEG-rHuMGDF– or cytokine combination-stimulated cultures had similar ultrastructural features and a nearly similar response to activation by thrombin. The data show that this culture system may be useful to study the effects of cytokines and the role of polyploidization on platelet production and function.

Platelet-like particle formation in the human megakaryoblastic leukaemia cell lines, MEG-01 and MEG-01s

The platelet-sized particle formation in the human megakaryoblastic leukaemia cell line MEG-01 and its subline MEG-01s was examined. MEG-01 and MEG-01s cells spontaneously released platelet-sized particles into the culture medium, in which the cells occasionally extended cytoplasmic processes similar to those of megakaryocyte proplatelets. Scanning electron microscopic images showed cytoplasmic processes elongated from blebs on the MEG-01 and MEG-01s cell surface and were constricted between segments of platelet size. Immunofluorescence staining with anti-tubulin antibody showed that the cytoplasmic processes contained microtubules that were organized into a ring, which is a characteristic of circulating platelets. Some platelet-sized particles, probably released by ruptures at the sites of the process constriction, were metabolically active in an MTT assay (about 50%). Some particles also expressed the platelet-specific glycoproteins GPIIb, IIIa and GMP-140. Rarely, in response to thrombin, particles underwent a shape change from spherical to a shape with irregular membrane protrusions and fine filopodia, and aggregating with one another. The particles also had increased GMP-140 (P-selectin) expression with the addition of thrombin. These results show the usefulness of the MEG-01 and MEG-01s cell lines for the study of thrombopoiesis.

Effect of recombinant interleukin-6 and thrombopoietin on isolated guinea pig bone marrow megakaryocyte protein phosphorylation and proplatelet formation

Guinea pig bone marrow megakaryocytes were isolated and cultured on collagen gels to promote proplatelet formation. In control cultures 15.6% of the cells formed proplatelets. Both IL6 and TPO stimulated dose dependent increases in the percent of proplatelet forming cells up to 26.7% at 100ng/mal IL6 and 26.8% at 100 ng/ml TPO. IL1 and IL3 had no effect on proplatelet formation. IL3 in combination with IL6 and TPO blocked the increase in proplatelet formation observed with IL6 or TPO alone. IL3 was also found to stimulate thymidine incorporation in megakaryocytes. The role of phosphorylation in proplatelet formation was studied using certain inhibitors. The tyrosine kinase inhibitor genestien had no effect on proplatelet formation at concentrations up to 100 microg/ml. The phosphatase inhibitors calyculin A and okadaic acid both inhibited proplatelet formation. Studies on protein phosphorylation revealed that IL6, but not TPO, stimulated phosphorylation of JAK1, JAK2 and MAP kinase. TPO did stimulate tyrosine phosphorylation of Tyk-2. Although IBMX stimulated proplatelet formation, it inhibited phosphorylation of JAK1 and MAP kinase. Adhesion of megakaryocytes to collagen gel also inhibited phosphorylation of JAK1 and JAK2, while MAP kinase phosphorylation was unaffected. These data show that IL6 and TPO stimulate megakaryocyte proplatelet formation. In addition, although these cytokines increase phosphorylation of signal transduction proteins in the JAK/STAT pathway, it appears that a different signal transduction pathway regulated by a combination of phosphatase activity and cAMP levels, leads to proplatelet formation.

Ultrastructure of Platelet Formation by Human Megakaryocytes Cultured With the Mpl Ligand

The site and mechanism of platelet production by bone marrow megakaryocytes (MKs) has been the subject of extensive studies, but is still a matter of controversy. However, the recent discovery of the Mpl ligand (Mpl-l), also called megakaryocyte growth and development factor (MGDF ) or thrombopoietin, has resulted in considerable progress in the understanding of the maturation of the MK lineage. To better understand the mechanism of platelet production, we examined the late stage of MK maturation by electron microscopy in cells cultured in the presence of Mpl-l. Human bone marrow CD34+CD38+ cells, which contain late MK progenitors, were purified by flow cytometry and cultured in a serum-free liquid medium containing recombinant human Mpl-l (MGDF 10 ng/mL) for 7 days. In this system, the majority of cultured cells were large MKs with lobulated polyploid nuclei. The MKs displayed a smooth surface with harmonious cytoplasmic maturation and abundant, regularly distributed demarcation membranes and α-granules, and even some dense granules. Interestingly, approximately 30% of the MKs observed displayed morphologic evidence of platelet production: at optical microscopy, MKs formed long filamentous cytoplasmic extensions (proplatelets) that fragmented into platelet-sized particles. Moreover, flow cytometric analysis of this cultured cell population showed GPIIb-positive particles of the size of platelets. Electron microscopic observation showed that MKs producing platelets displayed thin pseudopods on the surface, and that the channels of the demarcation membrane system were dilated, allowing long strands of cytoplasm to extend from the cell periphery. These cytoplasmic strands displayed beading with constrictions separating platelet-sized segments; the more distal to the cell core, the smaller the fragments were. They eventually detached from the cell core into the culture medium either occasionally still elongated or, more often, separated into individual platelets. Parallel longitudinal and perpendicular microtubules were visualized in the constricted regions of these cytoplasmic strips; immunogold study of tubulin localization confirmed this subcellular distribution. On both sides of the constricted areas, vacuoles were noted, the fusion of which might have led to the detachment of individual platelets. Finally, in close proximity to the platelet-forming MKs, numerous microparticles were shed. Although some of these particles might correspond to transverse sections of pseudopods, this did not seem to be the case, since they were rarely seen around thrombin-stimulated MKs with surfaces bristled by numerous pseudopods. Flow cytometry showed that apart from shed cytoplasmic fragments of platelet size, numerous smaller particles strongly labeled for CD41 were also released by mature MKs. In conclusion, this study describes the ultrastructure of human platelet production in cultured MKs, involving the formation of proplatelets and the shedding of microparticles.

Synergistic effects of erythropoietin and interleukin-6 on the in vitro proplatelet process formation of rat megakaryocytes

The recombinant hemopoietic factors of megakaryocyte potentiator (MEG-POT) were studied to compare their activity in stimulating proplatelet process formation (PPF) with thrombopoietin (TPO, c-MpI ligand). For the assay, a highly enriched (> 95%) population of more than 90% viable megakaryocytes was isolated from rat bone marrow using the immunomagnetic beads method and cultured with fetal calf serum (FCS) or in a serum-free condition. Megakaryocytes developing slender beaded cytoplasmic processes (proplatelet processes) were observed on both inverted phase contract microscopy and scanning electron microscopy. A large number of proplatelet process clusters were dose-dependently formed with the addition of varying doses of recombinant erythropoietin (rEpo) and interleukin-6 (rIL-6) as well as TPO. Epo and IL-6 were demonstrated to act synergistically solely at low doses in the development of PPF (P < 0.05). Other recombinant factors such as IL-11, leukemia inhibitory factor (LIF) and erythroid differentiation factor (EDF) appeared weak or ineffective. From these in vitro observations, it was suggested that a synergism of Epo and IL-6 might play a significant role in the terminal stage of megakaryocyte maturation leading to platelet release.