HDL3 activates phospholipase D in normal but not in glycoprotein IIb/IIIa-deficient platelets

Glycoprotein IIb/IIIa has been proposed as the platelet receptor for high density lipoproteins (HDL3). We characterized the HDL3-induced second messenger response in normal and glycoprotein IIb/IIIa-deficient platelets. In normal platelets physiological concentrations of HDL3 induced the time-dependent generation of phosphatidic acid in the absence of phosphoinositide turnover. The rise in phosphatidic acid preceded that of diacyglycerol which was inconsistent with phospholipase C/diacylglycerol kinase pathway being the source of phosphatidic acid and suggested the involvement of phospholipase D. In the presence of butanol, HDL3 stimulated the accumulation of phosphatidylbutanol, an unequivocal indicator of phospholipase D activity. No increase in phosphatidic acid, diacylglycerol, and phosphatidylbutanol was observed upon addition of HDL3 to glycoprotein IIb/IIIa-deficient platelets. We conclude that phosphatidic acid is generated in HDL3-stimulated platelets by phospholipase D and that glycoprotein IIb/IIIa is the receptor involved in this process.

Evidence that activation of platelet calpain is induced as a consequence of binding of adhesive ligand to the integrin, glycoprotein IIb-IIIa

Calpain (a Ca(2+)-dependent protease) is present in many cell types. Because it is present in the cytosol, the potential exists that it may regulate critical intracellular events by inducing crucial proteolytic cleavages. However, the concentrations of Ca2+ required to activate calpain are higher than those attained in the cytoplasm of most cells. Thus, the physiological importance of calpain and the mechanisms involved in its activation have remained elusive. In this study, we show that calpain rapidly moved to a peripheral location upon the addition of an agonist to suspensions of platelets, but it remained unactivated. We provide three lines of evidence that calpain was subsequently activated by a mechanism that required the binding of an adhesive ligand to the major platelet integrin, glycoprotein (GP) IIb-IIIa: calpain activation was prevented by RGDS, a tetrapeptide that inhibits the binding of adhesive ligand to GP IIb-IIIa; it was also prevented by monoclonal antibodies that inhibit adhesive ligand binding to GP IIb-IIIa; and its activation was markedly reduced in platelets from patients whose platelets have greatly reduced levels of functional GP IIb-IIIa. Thus, in platelets, binding of the extracellular domain of GP IIb-IIIa to its adhesive ligand can initiate a transmembrane signal that activates intracellular calpain. Because calpain is present in focal contacts of adherent cells, the interaction of integrins with adhesive ligands in the extracellular matrix may regulate activation of calpain in other cell types as well.

Inherited abnormalities of the platelet membrane and secretory granules

Platelet stimulus-activation-contraction-secretion coupling is linked to fundamental modifications in the biochemistry and ultrastructure of the platelet surface and the membranes enclosing storage organelles. It is not surprising, therefore, that membrane defects are common in platelets from patients with inherited hemorrhagic disorders caused by platelet dysfunction. In fact, it might be stated that all inherited disorders of platelet function are related directly or indirectly to abnormalities of membranes. The current review discusses the state of knowledge on inherited platelet membrane defects of the cell surface and storage organelles.

Development of procoagulant binding sites on the platelet surface

Activation of coagulation factor X by a complex of factors IXa-VIIIa and prothrombin by a complex of factor Xa.Va is markedly enhanced in the presence of a negatively-charged phospholipid surface. A suitable phospholipid surface is provided by a platelet lysate but not by a suspension of intact platelets, due to the internal localization of phosphatidylserine in the platelet membrane. Upon stimulation of platelets with a combination of collagen and thrombin, or calcium ionophore A23187 or treatment with diamide, alterations in the distribution of membrane phospholipids take place resulting in the exposure of significant amounts of phosphatidylserine at the platelet surface. As a consequence, an increased number of intrinsic factor X and prothrombinase complexes can be assembled at the platelet surface thus leading to an acceleration of factor Xa and thrombin formation. Studies with pathological platelets have shown that neither release nor aggregation are essential to provoke prothrombinase activity. The relatively high prothrombinase activity of non-stimulated Bernard-Soulier platelets is in agreement with the slightly altered phospholipid distribution in these platelets, in which more phosphatidylserine is exposed at the outer surface. Disturbances in the membrane bilayer structure as well as changes in the plasma membrane-cytoskeleton interaction are considered as possible explanations for the increased transbilayer movement of phosphatidylserine.