Isolation of human platelet glycoproteins

Megakaryocyte colony stimulating activity in allogenic bone marrow recipients prepared with busulfan and cyclophosphamide

Increased megakaryocyte colony stimulating activity (MK-CSA) has been reported after total body irradiation (TBI) for bone marrow transplant (BMT). We studied the effect of a busulfan (Bu) and cyclophosphamide (Cy) marrow transplant conditioning regimen, without radiation, on MK-CSA production. Initial screening of MK-CSA was done on previously collected and banked sera from 14 BMT patients. MK-CSA was expressed as the ability to stimulate growth of megakaryocyte progenitors (CFU-MK) in standard plasma clot cultures. In the initial samples, MK-CSA peaked at day 7. This preliminary data led to a prospective study of MK-CSA and clinical parameters in seven allogeneic recipients. MK-CSA activity increased from day -7 pre-transplant (2.9 +/- 1.7 CFU-MK/10(5) NATD, mean +/- SD) to day 0 (10.3 +/- 4.7 CFU-MK) and peaked by day 9 post-transplant (20.6 +/- 6.4 CFU-MK). MK-CSA activity decreased in all seven patients by day 21 at which time five of seven patients studied had recovery of platelet counts to greater than 100 x 10(9)/l. MK-CSA activity rose rapidly in both groups of sera after the initiation of this non-irradiation, BMT preparative regimen. High MK-CSA levels, early after transplant, may contribute to the rapid platelet recovery in some patients.

The cloning and biological characterization of recombinant human interleukin 11

Interleukin 11 (IL-11) is a pleiotropic hematopoietic growth factor with stimulatory effects on multiple hematopoietic progenitor cells. An immortalized primate bone marrow stromal cell line was established to facilitate the analysis of interactions between hematopoietic progenitors and the microenvironment. This line was found to produce a novel growth factor that directed the proliferation of a murine plasmacytoma cell line. A cDNA encoding this activity was isolated through functional expression cloning in mammalian cells. Preliminary characterization of the cytokine revealed that IL-11 stimulated T cell-dependent B cell immunoglobulin secretion and synergized with IL-3 in murine megakaryocyte formation. Subsequent analysis of the clone has demonstrated that it also synergizes with IL-3 and steel factor in the stimulation of early progenitors and affects early megakaryocyte formation and maturation. In addition, IL-11 induces secretion of acute phase proteins in the liver and may function in the hematopoietic microenvironment as an adipogenic antagonist in a paracrine manner. Further in vitro and in vivo analysis of IL-11 will be necessary to determine the biologic function and potential therapeutic use of the cytokine.

Megaloblastic hematopoiesis in vitro. Interaction of anti-folate receptor antibodies with hematopoietic progenitor cells leads to a proliferative response independent of megaloblastic changes

We tested the hypothesis that anti-placental folate receptor (PFR) antiserum-mediated effects on hematopoietic progenitor cells in vitro of increased cell proliferation and megaloblastic morphology were independent responses. We determined that (a) purified IgG from anti-PFR antiserum reacted with purified apo- and holo-PFR and specifically immunoprecipitated a single (44-kD) iodinated moiety on cell surfaces of low density mononuclear cells (LDMNC); (b) when retained in culture during in vitro hematopoiesis, anti-PFR IgG (in contrast to PFR-neutralized anti-PFR IgG and nonimmune IgG) consistently led to increased cloning efficiency of colony forming unit-erythroid (CFU-E), burst forming unit-E (BFU-E), CFU-granulocyte macrophage (CFU-GM), and CFU-GEM megakaryocyte (CFU-GEMM), and objectively defined megaloblastic changes in orthochromatic normoblasts from CFU-E- and BFU-E-derived colonies; (c) when anti-PFR antiserum was removed after initial (less than 1 h) incubation with LDMNC, a cell proliferation response was induced, but megaloblastic changes were not evident. (d) Conversely, delay at 4 degrees C for 24 h before long-term plating with antiserum resulted in megaloblastosis without increased cell proliferation; (e) however, 500-fold molar excess extracellular folate concentrations completely abrogated the expected anti-PFR antiserum-induced megaloblastic changes, without altering cell proliferative responses. Thus, although cell proliferative and megaloblastic changes are induced after short-term and prolonged interaction of antibody with folate receptors on hematopoietic progenitors, respectively, they are independent effects.

Interaction of monocytes and T cells in the regulation of normal human megakaryocytopoiesis in vitro: role of IL-1 and IL-2

Autologous or allogeneic peripheral blood T cells can stimulate the human megakaryocyte progenitor cell (CFU-Meg)-derived colony formation in a dose-dependent fashion in agar cultures of nonadherent (NA), T cell-depleted (NT) bone marrow (BM) cells. Low concentrations of monocytes and T cells can collaborate in the stimulation of CFU-Meg colony formation or in the production of megakaryocyte colony stimulating factor (Meg-CSF) by T cells in the presence of mitogens or IL-2. Monocytes alone can produce only negligible Meg-CSF under any conditions. When monocyte conditioned medium (CM) was added to T cell-stimulated NA, NT BM cell cultures, CFU-Meg colony growth was appreciably increased compared with that stimulated by T cells alone. Dose-dependent increase in CFU-Meg colony growth was noted when varying concentrations of IL-1 were added to T cell-stimulated NA, NT cell cultures, although IL-1 itself could support no CFU-Meg colony growth in the absence of T cells. These data suggest that a synergistic interaction between T cells and monocytes during the production of Meg-CSF by T cells could be partly mediated by IL-1. IL-2 was found to stimulate Meg-CSF production by T cells in the presence or absence of mitogens. IL-2-stimulated Meg-CSF production by T cells was augmented by the addition of monocytes. Although IL-2 itself had no stimulatory effect on CFU-Meg colony growth, dramatic augmentation in the CFU-Meg colony number was noted when IL-2 was added to T cell-stimulated NA, NT cell cultures. High concentrations of monocytes and prostaglandin E (PGE) inhibited the CFU-Meg colony formation. These results suggest that IL-1 and IL-2 may play a stimulatory role on the normal human in vitro megakaryocytopoiesis, and may be involved in the development of reactive thrombocytosis and bone marrow megakaryocytic hyperplasia in various inflammatory diseases.

The alpha and beta chains of human platelet glycoprotein Ib are both transmembrane proteins containing a leucine-rich amino acid sequence

The primary structure of the beta chain of human glycoprotein Ib (GPIb), the platelet receptor for von Willebrand factor, has been established by a combination of cDNA cloning and amino acid sequence analysis. A lambda phage cDNA expression library prepared from human erythroleukemia cells (HEL cells) was screened with a radiolabeled affinity-purified rabbit polyclonal antibody to the beta chain of GPIb. Eighteen positive clones were isolated and plaque-purified and the nucleotide sequences of three were determined. The composite sequence spanned 968 nucleotides and included a 5' untranslated region of 22 nucleotides, an open reading frame of 618 nucleotides encoding a signal peptide of 28 amino acids and a mature protein of 181 amino acids, a stop codon, and a 3' noncoding region of 307 nucleotides. The 3' noncoding sequence also contained a polyadenylylation signal (AATAAA) 14 nucleotides upstream from the poly(A) tail of 18 nucleotides. Edman degradation of the intact beta chain and of peptides produced by chemical cleavage yielded amino acid sequences spanning 76 residues that were identical to those predicted from the cDNA. The amino-terminal region of the beta chain contains a leucine-rich sequence of 24 amino acids that is similar to a sequence that occurs as seven tandem repeats in the alpha chain of GPIb and nine tandem repeats in leucine-rich alpha 2-glycoprotein. The leucine-rich sequence in the beta chain of GPIb is flanked on both sides by amino acid sequences that are similar to those flanking the leucine-rich tandem repeats of the alpha chain of GPIb and leucine-rich alpha 2-glycoprotein. The amino-terminal region of the beta chain of GPIb is followed by a transmembrane segment of 25 amino acids and an intracellular segment of 34 amino acids at the carboxyl terminus of the protein. The intracellular segment contains an unpaired cysteine and two potential sites for phosphorylation by cAMP-dependent protein kinase.

Glycoprotein Ib beta is the only phosphorylated major membrane glycoprotein in human platelets

Platelets were metabolically labelled with 32P and the phosphoproteins examined by two-dimensional non-reduced/reduced gel electrophoresis and isoelectric-focusing/gel electrophoresis. Comparison with similar separations of surface-labelled platelets showed that the only major glycoprotein which is phosphorylated is the beta-subunit of glycoprotein Ib, indicating that this subunit contains a cytoplasmic segment. The identification was confirmed using immunoblotting with an antibody to the beta-subunit. Phosphoserine was the principal phosphorylation site, with some phosphothreonine, but phosphotyrosine was absent. No quantitative or qualitative differences could be detected in the phosphorylation of glycoprotein Ib beta from resting or activated platelets. These results exclude changes in phosphorylation of the major platelet membrane glycoproteins as a method of signal transmission by these receptors.

Transitory hypomegakaryocytic thrombocytopenia: aetiological association with ethanol abuse and implications regarding regulation of human megakaryocytopoiesis

We studied a patient with a long history of ethanol abuse who presented to the hospital with profound weakness, anaemia and thrombocytopenia. Evaluation of these problems revealed the patient's bone marrow to be hypercellular but severely iron depleted and almost totally devoid of morphologically recognizable megakaryocytes. However, we were able to detect the presence of non-morphologically recognizable, immature megakaryocytes in the same sample using an immunochemical detection technique. This circumstance allowed us to study the relative importance of both megakaryocyte maturation and peripheral blood platelet count on the production of megakaryocyte colony stimulating activity (Meg-CSA), a putative regulator of the megakaryocyte colony forming unit (CFU-M). The results of our investigations disclosed a rapid decline in serum Meg-CSA levels which preceded recovery of the platelet count and appeared to coincide with the maturation of megakaryocytes into the morphologically recognizable pool. The effect of ETOH on the patient's CFU-M cloning efficiency was also studied. ETOH in amounts up to 454 mg/dl did not inhibit cloning of the patient's peripheral blood CFU-M in plasma clot cultures. Our results suggest that regulation of Meg-CSA production is a complex function which appears to be dependent on a number of factors including the level of megakaryocyte maturation in the marrow. We also speculate that ethanol associated thrombocytopenia may occasionally be brought about by a disruption in the process of megakaryocyte maturation at the level of a progenitor more mature than the CFU-M.

A method for purifying the platelet membrane glycoprotein IIb-IIIa complex

A method has been developed for the rapid isolation of platelet membrane glycoproteins (GP) IIb and IIIa. This method produces an excellent yield and does not require the prior isolation of platelet membranes. Outdated platelets were washed and solubilized in Triton X-100. Concanavalin A affinity chromatography was used to purify a platelet glycoprotein fraction. The concanavalin A-retained glycoproteins were eluted and adsorbed with a heparin-Sepharose column to remove a major contaminant, thrombospondin. Sephacryl S-300 gel filtration was used as the final purification step to remove most fibrinogen and low-molecular-weight contaminants. Wheat germ agglutinin affinity chromatography was used to completely remove trace amounts of fibrinogen. The purified GP IIb and GP IIIa were analyzed by sucrose gradient sedimentation and found to consist of heterodimer complexes.

Effects of diamide and dibucaine on platelet glycoprotein Ib, actin-binding protein and cytoskeleton

During extraction of platelets by 1% Triton X-100, the actin-binding protein (platelet filamin) and a 230 kDa protein are degraded by a calcium-activated thiol protease. Occurrence of degradation products of Mr 190 000 (HF-1) and 90 000 (HF-2) is a sensitive indicator of this proteolysis, and can be used to decide whether reduced amounts of the actin-binding protein in extracts are due to proteolysis or to incorporation in the Triton-insoluble (cytoskeletal) fraction. Diamide, which is a sulfhydryl-oxidizing protein cross-linker, inhibits the calcium-activated protease, polymerizes the actin-binding protein and the 230 kDa protein, increases the incorporation of glycoprotein Ib into the cytoskeletal fraction, and inhibits platelet agglutination induced by bovine von Willebrand factor. Inhibition of platelet agglutination by pretreatment with diamide is partly reversed by dibucaine which activates the calcium-activated protease. These observations are in accordance with a working hypothesis that interactions of glycoprotein Ib with cytoskeleton affect, and possibly regulate, its receptor function in the intact platelet.

Purification and partial characterization of a megakaryocyte colony-stimulating factor from human plasma

Human plasma obtained from patients with hypomegakaryocytic thrombocytopenia contains a factor that promotes megakaryocyte colony formation by normal human marrow cells. This megakaryocyte colony-stimulating factor was purified from such a plasma specimen. A four-step purification scheme which included ammonium sulfate precipitation, diethylaminoethyl-Sepharose chromatography, affinity chromatography on wheat germ lectin-Sepharose 6MB, and reverse-phase high performance liquid chromatography resulted in a recovery of 16.6% of the initial biological activity and an increase in specific activity by 3,489-fold. The purified protein produced a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Purified megakaryocyte colony-stimulating factor was capable of promoting megakaryocyte colony formation at a concentration of 7.6 X 10(-8) M. Megakaryocyte colony-stimulating factor was shown to be a glycoprotein and had an apparent 46,000 mol wt. Deglycosylation of megakaryocyte colony-stimulating factor by treatment with trifluoromethane-sulfonate resulted in the loss of its ability to promote megakaryocyte colony formation. Megakaryocyte colony-stimulating factor appears to be an important regulator of in vitro human megakaryocytopoiesis at the level of the colony-forming unit megakaryocyte and may be of importance physiologically.

Assay of an activity in the serum of patients with disorders of thrombopoiesis that stimulates formation of megakaryocytic colonies

We have recently described an in vitro clonal assay system for human megakaryocyte-progenitor cells or megakaryocytic colony-forming units (CFU-M). Serum specimens from patients with quantitative platelet disorders were screened for the capacity to alter in vitro megakaryocyte-colony formation. Serum from 11 patients with hypomegakaryocytic thrombocytopenia significantly enhanced the formation of CFU-M-derived colonies (200 to 1840 per cent). Neither serum from eight patients with thrombocytopenia and normal or increased numbers of marrow megakaryocytes nor serum from 11 patients with thrombocytosis altered colony formation. This stimulatory activity has been termed megakaryocytic-colony-stimulating activity (Meg-CSA). The number of megakaryocytic colonies formed was directly proportional to the quantity of stimulatory serum added. Meg-CSA levels appeared to be inversely related to marrow megakaryocyte numbers. The variations in Meg-CSA levels that were detected in different disease states suggest that alterations in the production of this stem-cell regulator have physiologic importance.

Regulation of human megakaryocytopoiesis. An in vito analysis

We have recently described an assay system for human peripheral blood megakaryocyte colony-forming unit cells (CFU-M) using an anti-platelet glycoprotein antiserum probe to define megakaryocyte colonies grown in vitro. This system was applied to study the nature and regulation of human bone marrow CFU-M. In the absence of a specific megakaryocyte growth-promoting factor, 12.4 +/- 3.0 (means +/- SEM) megakaryocyte colonies were cloned per 5 X 10(5) cells cultured. Colonies were present after 6 d of incubation reaching peak numbers between days 10 and 14 and slowly decreasing thereafter. Erythropoietin in concentrations of up to 4 U/ml failed to augment colony numbers. Also failing to enhance megakaryocyte colony plating efficiency were media containing burst-promoting activity and colony-stimulating activity. A medium conditioned by human embryonic kidney cells, which has been previously demonstrated to contain thrombopoietin, also had no effect on megakaryocyte colony numbers. In contrast, sera from three patients with severe aplastic anemia produced significant enhancement of CFU-M-derived colony formation in vitro. Both the number of megakaryocyte colonies present and the number of megakaryocytes per colony were increased in proportion to the final concentration of aplastic anemia serum. In the presence of 10% aplastic anemia serum, cultured megakaryocyte colony numbers were linear with respect to the number of bone marrow mononuclear cells plated suggesting a clonal origin of each of the colonies. This in vitro assay for bone marrow CFU-M is a reliable means by which to study the regulation of human megakaryocytopoiesis. Initial data suggest that megakaryocyte production is stimulated by a factor detectable in aplastic anemia serum that may be distinct from other known hematopoietic stem cell regulators.

Relationship between glycocalicin and glycoprotein Ib of human platelets

Asialoglycoprotein Ib and asialoglycocalicin have been isolated from the membranes and from the supernatant, respectively, after sonication of neuraminidase-treated platelets, by lectin affinity chromatography on peanut agglutinin. The isolated asialoglycoprotein Ib had an apparent molecular weight of 160,000 when not reduced and 150,000 when reduced, whereas the asialoglycocalicin had an apparent molecular weight of 150,000, both reduced and unreduced, on sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Both preparations contained only trace amounts of impurities. The asialoglycoprotein Ib and asialoglycocalicin in both the unreduced and reduced states were separated by gel electrophoresis, radioiodinated in gel slices, and digested with trypsin, and the digests were analyzed by two-dimensional high-voltage electrophoresis and thin-layer chromatography followed by autoradiography. The tryptic peptide maps showed great similarities between glycocalicin and glycoprotein Ib, with the latter (both unreduced and reduced) containing additional peptides, supporting the idea that glycocalicin is derived from glycoprotein Ib. The unreduced glycoprotein Ib contained additional peptides compared to the reduced due to the disulfide-bond-linked beta component. There were also slight differences between unreduced and reduced glycocalicin, indicating that at least one intramolecular disulfide bond is present.

Characterization of human blood platelet membrane proteins and glycorproteins by their isoelectric point (pI) and apparent molecular weight using two-dimensional electrophoresis and surface-labelling techniques

Intact human blood platelets were radioactively labelled at the surface by techniques specific for proteins or glycoproteins. Labelled platelet samples were analyzed by a high-resolution two-demensional separation system involving isoelectric focusing in the first dimension and discontinuous sodium dodecyl sulphate-polyacrylamide gel electrophoresis in the second. The major platelet membrane glycoprotein (GP) bands (Ib, IIb, IIIa and IIIb) were found to be highly heterogeneous even after removal of terminal sialic acid residues. Lactoperoxidase-catalyzed iodination of platelets showed that the major labelled proteins (Ib, IIb, IIIa and IIIb) had altered isoelectric points (pI) and molecular weights after neuraminidase treatment. A number of membrane glycoproteins previously undetected by one-dimensional gel electrophoresis were demonstrated and good evidence provided that the major platelet surface proteins are glycosylated.