Megakaryocytopoiesis is the cellular developmental process that leads to platelet production. At least two humoral growth factors may be necessary for megakaryocyte proliferation and maturation. One is a megakaryocyte-colony stimulating factor (MK-CSF) which induces the proliferation and differentiation of megakaryocyte progenitors, and the second, thrombopoietin, is a megakaryocyte maturation factor. Neither of these factors has been fully characterized. The proto-oncogene c-mpl, an orphan member of the haematopoietin receptor family, is specifically involved in megakaryocyte regulation. Here we present evidence that the c-mpl-encoded receptor binds a ligand (c-Mpl ligand) which is a humoral factor implicated in platelet homeostasis. Our results suggest that c-Mpl ligand, thrombopoietin and MK-CSF might be the same molecule.
We studied de novo protein biosynthesis in platelets of normal adult donors and in newly formed platelets isolated from splenectomized patients with idiopathic thrombocytopenic purpura (ITP). After metabolic labelling of platelet proteins, performed with different radiolabelled amino acids or carbohydrates, a tenfold increase in incorporation of radioactivity into trichloroacetic-acid-precipitable material was obtained with ITP platelets compared to control platelets. Electron microscopic studies of ITP platelets revealed the presence of rough endoplasmic reticulum and polyribosomes, providing morphological evidence for protein synthesis. SDS-PAGE of radiolabelled ITP platelet proteins followed by autoradiography showed that [35S]methionine and [3H]leucine were incorporated into almost all Coomassie-blue-stained proteins whereas [3H]cdrbohydrates only labelled a few bands. Using crossed-immunoelectrophoresis we identified some of the labelled platelet compounds and demonstrated that major membrane glycoproteins (GPIb, IIb, ZIIa) and alpha-granule proteins, such as fibrinogen, thrombospondin, albumin and von Willebrand factor, were synthesized in newly formed circulating platelets.Human platelets play an essential role in hemostasis and thrombosis. Following vascular injury, platelets adhere to exposed subendothelial tissue, become activated and secrete the content of their storage organelles. Released alpha-granule proteins, such as fibrinogen, von Willebrand factor, thrombospondin and factor V, interact with specific platelet membrane receptors to induce aggregation, thrombin conversion and fibrin formation for eventual clot retraction [I], whereas released platelet-derived growth factors promote tissue repair processes [2].Since human platelets are anucleated cytoplasmic fragments of megakaryocytes with a short lifespan in the peripheral blood, synthesis of platelet proteins is thought to occur almost solely in the precursor cell, the megdkaryocyte. Platelet-specific membrane glycoproteins have indeed been identified in megakaryocytes [3 -61, which are also known to contain platelet-related proteins such as von Willebrand factor [7] [12]. However, the site of biosynthesis of some platelet alpha-granule proteins, such as thrombospondin, albumin or immunoglobulins [ 131, remains uncertain since several of these proteins are not specific to platelets and are synthesized by other cell types, e.g. hepatocytes, endothelial cells, fibroblasts or lymphocytes. Biosynthesis therefore could occur either in megakaryocytes, where the proteins are packaged into alpha granules for subsequent release from circulating platelets, or in other cell types followed by an active uptake of the preformed proteins by the megakaryocyte, or possibly in both megakaryocytes and other cells. stable messenger RNA derived from nucleated megakaryocytes is present in circulating platelets [14-251. This biosynthetic activity has, however, been considered to be rather vestigial. Recently Belloc et al. reported increased protein synthesis and larg...
We have used ultrathin cryosectioning and immunogold cytochemistry to study the position of α-granules in the endocytic and biosynthetic pathways in megakaryocytes and platelets. Morphologically, we distinguished three types of granules; so-called multivesicular bodies type I (MVB I) with internal vesicles only, granules with internal vesicles and an electron dense matrix (MVB II), and the α-granules with mainly a dense content and often internal membrane vesicles at their periphery. The MVBs were prominent in cultured megakaryocytes and the megakaryoblastic cell line CHRF-288, but were less numerous in bone marrow megakaryocytes and platelets, whereas α-granules were most prominent in mature bone marrow megakaryocytes and in platelets. The internalization kinetics of bovine serum albumin-gold particles and of fibrinogen positioned the MVB subtypes and α-granules sequentially in the endocytic pathway. MVBs contained the secretory proteins von Willebrand factor (vWF) and β-thromboglobulin (β-TG), the platelet-specific membrane protein P-selectin, and the lysosomal membrane protein CD63. Within the MVBs, endocytosed fibrinogen and endogenous β-TG were restricted to the matrix, while vWF was predominantly associated with internal vesicles. CD63 was also observed in association with internal membrane vesicles in the α-granules. These observations, and the gradual morphologic transition from granules containing vesicles to granules containing predominantly dense material, suggest that MVBs represent a developmental stage in α-granule maturation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.