A novel microtubule protein in the marginal band of human blood platelets

Collagen binding induces changes in its platelet integrin receptor alpha2beta1

Integrins can signal upon binding of their ligand, presumably because of conformational changes induced by ligand-binding. It has been postulated that ligand binding causes changes in the affinity of the integrin to its ligand. In order to test for ligand-induced change in the affinity of platelet alpha2beta1 to collagen, labelled viable platelets were passaged through a column of fibrillar collagen and stringent lysis conditions were used to remove all low-affinity receptors. A high-affinity fraction left on the collagen could be eluted with dithiothreitol (DTT) and 2% Sodium dodecyl sulfate (SDS). Antibodies raised against this fraction, identified alpha2beta1 by Western-blotting. Functional tests performed with the antibodies confirmed the involvement of the high-affinity proteins in platelet-collagen interactions attributed to alpha2beta1: inhibition of collagen-specific platelet adhesion and aggregation. EDTA, chaotropic agents or low pH did not elute the high affinity fraction of alpha2beta1. However, DTT followed by acetic acid did, which indicates that the steps necessary to disrupt the high-affinity collagen alpha2beta1 bond are reduction of disulfide bond(s) followed by disruption of electrostatic interactions. Our data 2 1 suggest that (i) ligand binding induces the formation of a new disulfide bond in a fraction of alpha2beta1, (ii) that this bond is an intrareceptor, and (iii) that this change increases the affinity of the receptor to its ligand.

Inherited thrombocytopenias: toward a molecular understanding of disorders of platelet production

PURPOSE OF REVIEW

To review the defined syndromes of inherited thrombocytopenia and discuss new genetic data for several disorders that shed light on the process of megakaryopoiesis.

RECENT FINDINGS

The genes responsible for several inherited thrombocytopenias have been recently discovered, including congenital amegakaryocytic leukemia, amegakaryocytic thrombocytopenia with radio-ulnar synostosis, familial platelet syndrome with predisposition to acute myelogenous leukemia, Paris-Trousseau, Wiskott-Aldrich syndrome, and the May-Hegglin, Sebastian, Epstein, and Fechner syndromes. These clinical syndromes, combined with studies in mouse and in vitro models, reveal the importance of these genes for normal hematopoiesis.

SUMMARY

Although inherited syndromes of thrombocytopenia are rare, characterization of mutations in these disorders has contributed greatly to our understanding of megakaryocyte and platelet development. A systematic registry of congenitally thrombocytopenic individuals would almost certainly lead to new genetic discoveries.

Affinity modulation in platelet alpha 2 beta 1 following ligand binding

In order to test for ligand-induced change in the affinity of platelet alpha 2 beta 1 to collagen we passaged labeled viable platelets through a column of fibrillar collagen and used stringent lysis conditions to remove all low-affinity receptors. A high affinity fraction left on the collagen could be eluted with DTT and 2% SDS. Antibodies raised against it Western-blotted alpha 2 beta 1. Functional tests performed with the antibodies confirmed the involvement of the high affinity proteins in platelet-collagen interactions attributed to alpha 2 beta 1: inhibition of collagen-specific platelet adhesion and aggregation. EDTA, chaotropic agents or low pH did not elute the high affinity fraction of alpha 2 beta 1. However, DTT followed by acetic acid did. Our data suggest that 1) ligand binding induces the formation of a new disulfide bond in a fraction of alpha 2 beta 1, 2) that this bond is intrareceptor and 3) that this change increases the affinity of the receptor to its ligand.

Protein kinase C inhibitors suppress disc-sphere changes of human platelets, as assessed with the shape-change parameter

Changes in the shape of human platelets and the biochemical mechanism responsible were evaluated, using the shape-change parameter. Neither the Na+/H+ exchanger, nor intracellular or extracellular calcium ions (Ca2+) affected disc-sphere changes induced by low doses of thrombin. Treatment with inhibitors of Ca2+ mobilization, calmodulin or mysoin light-chain kinase had no significant effect on the shape change of platelets. Staurosporine and H-7, both of which are inhibitors of protein kinase C, inhibited disc-sphere changes at low concentrations. Moreover, calphostin C, a specific inhibitor of protein kinase C, effectively inhibited thrombin-induced shape change, as assessed by the shape-change parameter, in a dose-dependent manner. 1-Oleoyl-2-acetyl-glycerol and 1,2-dioctanoyl-glycerol, which are synthetic protein kinase C activators, induced shape changes similar to those induced by thrombin. A decrease in the surface area of platelet images on scanning electron micrographs was used to quantify the disc-sphere transformation. The mean platelet areas was significantly decreased after stimulation with thrombin or 1-oleoyl-2-acetyl-glycerol. Pretreatment with H-7 inhibited the thrombin-induced disc-sphere change, as assessed by changes in the platelet surface area. Our results obtained with various inhibitors suggest that thrombin-induced platelet change, as assessed by the shape-change parameter, are associated with activation of protein kinase C.

Microtubule organization by cross-linking and bundling proteins

To understand microtubule function the factors regulating their spatial organization and their interaction with cellular organelles, including other microtubules, must be elucidated. Many proteins are implicated in these organizational events and the known consequences of their actions within the cell are increasing. For example, the function of microtubule bundles at the surfaces of polarized cells has recently received attention, as has the action in cortical rotation of a transient arrangement of microtubules found beneath the vegetal surface of fertilized frog eggs. The in vivo association of microtubules during early Xenopus oogenesis has added interest as microtubules bundled in cell-free extracts are protected against the action of a severing protein found in this animal. A 52 kDa F-actin bundling protein purified from Physarum polycephalum organizes microtubules and causes the cobundling of microtubules and microfilaments. These observations, in concert with others that are presented, emphasize the diversity within the family of microtubule cross-linking proteins. The challenge is to determine which proteins are relevant from a physiological perspective, to ascertain their molecular mechanisms of action and to describe how they affect cytoplasmic organization and cell function. To realize this objective, the proteins which cross-link and bundle microtubules must be investigated by techniques which reveal different but related aspects of their properties. Cloning and sequencing of genes for cross-linking proteins, their subcellular localization especially as microtubule-related changes in cell morphology are occurring and the application of genetic studies are necessary. Study of the neural MAP provides the best example of just how powerful current experimental approaches are and at the same time shows their limits. The neural MAP have long been noted for their enhancement of tubulin assembly and microtubule stability. Their spatial distribution has been studied during the morphogenesis of neural cells. Sequencing of cloned genes has revealed the functional domains of neural MAP including carboxy-terminal microtubule-binding sites. Similarities to microtubule binding proteins from other cell types stimulate interest in the neural MAP and further suggest their importance in microtubule organization. For example, MAP4 enjoys a wide cellular distribution and has microtubule-binding sequences very similar to those in the neural MAP. Moreover, the nontubulin proteins of marginal bands are immunologically related to neural MAP, indicating shared structural/functional domains. Even with these findings the mechanism by which neural MAP cross-link microtubules remains uncertain. Indeed, some researchers express doubt that microtubule cross-linking is actually a function of neural MAP in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)