Ultracytochemistry of human blood platelets

Integrin-dependent control of translation: engagement of integrin alphaIIbbeta3 regulates synthesis of proteins in activated human platelets

Integrins are widely expressed plasma membrane adhesion molecules that tether cells to matrix proteins and to one another in cell-cell interactions. Integrins also transmit outside-in signals that regulate functional responses of cells, and are known to influence gene expression by regulating transcription. In previous studies we found that platelets, which are naturally occurring anucleate cytoplasts, translate preformed mRNA transcripts when they are activated by outside-in signals. Using strategies that interrupt engagement of integrin alphaIIbbeta3 by fibrinogen and platelets deficient in this integrin, we found that alphaIIbbeta3 regulates the synthesis of B cell lymphoma 3 (Bcl-3) when platelet aggregation is induced by thrombin. We also found that synthesis of Bcl-3, which occurs via a specialized translation control pathway regulated by mammalian target of rapamycin (mTOR), is induced when platelets adhere to immobilized fibrinogen in the absence of thrombin and when integrin alphaIIbbeta3 is engaged by a conformation-altering antibody against integrin alphaIIbbeta3. Thus, outside-in signals delivered by integrin alphaIIbbeta3 are required for translation of Bcl-3 in thrombin-stimulated aggregated platelets and are sufficient to induce translation of this marker protein in the absence of thrombin. Engagement of integrin alpha2beta1 by collagen also triggered synthesis of Bcl-3. Thus, control of translation may be a general mechanism by which surface adhesion molecules regulate gene expression.

Storage lesion of human platelets as revealed by ultrathin sections and freeze-fracture replicas

We report the ultrastructural changes occurring in human platelets during eight days of storage. Extension of pseudopodia is frequently observed, but a concentration of organelles in the centre of the platelets is found only in a minor fraction (approximately 5%). Striking changes can be observed in both the granules and the open canalicular system. In fresh platelets, the latter often has the form of stacked membranes that have no lumen, but these membranes separate and spread with increasing storage time. However, the openings of this system on the outer surface of the platelet remain unchanged. Some of these features differ from the morphological description of platelets activated by thrombin or ADP, and suggest that the storage lesion is the result of a prolonged weak activation that leads to an incomplete release reaction within the first five days.

Morphological changes in thrombocytes during blood bank storage. An ultrastructural morphometric study

Monitoring the storage lesion of platelet concentrates (PC) is commonly done by determining metabolic parameters, release of cytoplasmic markers and granules contents and watching the light microscopical appearance of platelets. Descriptions of ultrastructural changes are rare, and so we ascertained the changes in two important organelles, the alpha- and lysosomal granules and the open canalicular system (OCS), by morphometric analysis. Single donor PC's were prepared with a CS-3000 plus (Baxter) processor and stored under routine conditions. The volume fraction of the granules displayed a decrease to 43% after 8 days of storage, and the OCS fraction increased to 143%. Moreover, changes in the morphology of granules and the development of OCS channels are described, and ballooning and even complete lysis of some platelets could be observed after only three days. Furthermore, the membrane flux in the formation of pseudopodia is discussed. Our results stress the need to establish better preparation and storage conditions for platelet concentrates.

Ultrastructural investigations on the question of mechanical activation of blood platelets

The present study addresses the question whether platelets are activated by mechanical stresses as they occur in pathologically accelerated blood flow. Their potential mechanoreceptive properties were tested by subjecting human platelets to defined fluid mechanical forces for periods of milliseconds. Platelet activation was assessed by quantitative morphology, revealing besides activated platelets, irreversibly ballooned, lytic platelets. However, this morphometrically documented "shear activation" of platelets can be suppressed almost completely by the addition of enzyme-substrate systems, capable of removing adenosine diphosphate from the suspending medium. This is in keeping with a recent study from our laboratory showing that the behaviour of lactic dehydrogenase as marker for platelet lysis and beta-thromboglobulin as release marker refuted earlier data, suggesting a direct activation of platelets by shear. It is concluded that former evidence of "shear induced platelet activation" must be interpreted as the consequence of lytic damage to a small proportion of platelets.

Kinetic characterization and substrate requirement for the Ca2+ uptake system in platelet membrane

An ATP-dependent mechanism for Ca2+ uptake in human platelet membrane fractions has been identified and characterized. Ca2+ uptake into a membrane fraction is shown to be stimulated at low concentrations of ATP and Ca2+ and to require magnesium ions. Initial rate kinetics, using Eadie-Scatchard analysis, indicated a single class of calcium uptake sites in the presence of ATP, with a Kd for free [Ca2+] of 0.145 microM. Ca2+ uptake in the presence of several ATP concentrations demonstrates that ATP binds to at least two sites, representing high and low affinities of 3.21 and 80.1 microM, respectively. The neuroleptic drug fluphenazine inhibited ATP-stimulated calcium uptake (IC50 = 55 microM), suggesting this ATP-dependent Ca2+ uptake system may provide a useful ion-transport model with which to study neuroleptic therapy in humans.