Platelets can bind and phagocytose infectious microorganisms and so enable their transport for a prolonged time. To investigate the subcellular events of these interactions, platelets were incubated either with Staphylococcus aureus or with HIV and analyzed by electron microscopy (EM) and immuno-EM. HIV and bacteria internalization occurred exclusively within platelets showing morphological evidence of activation. Platelet activation enhanced the degree of bacterial internalization. Immunolabeling revealed that the engulfing vacuoles and the open canalicular system (OCS) were composed of distinct antigens. The engulfing vacuoles eventually became the site of prominent alpha-granule release. In platelets incubated with HIV, characteristic endocytic vacuoles were identified close to the plasma membrane, tightly surrounding 1 or 2 HIV particles. Virus particles were also located within the OCS. Immunogold labeling for the viral core protein p24 confirmed the presence of HIV within platelets. Finally, examination of platelets from a patient with acquired immunodeficiency syndrome and high viremia suggested that HIV endocytosis may also occur in vivo.

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...

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