Simin Saffaripour

Induction of specific storage organelles by von Willebrand factor propolypeptide.

Endothelial cells store the multimeric adhesive glycoprotein von Willebrand factor (vWf), which promotes the formation of a platelet plug at the site of vessel injury. To investigate the packaging of vWf into the granules called Weibel-Palade bodies, we expressed pro-vWf cDNA and cDNA lacking the prosequence in a variety of cell lines. Storage granules formed only in cells that contain a regulated pathway of secretion. Furthermore, packaging required the prosequence. Pro-vWf, lacking the C-terminal region involved in interchain disulfide bonding, formed granules. We conclude that the signal for storage is universal in that an adhesive glycoprotein can be stored by a hormone-secreting cell; the storage of vWf is independent of its covalent multimeric structure; the unusual rod shape of Weibel-Palade bodies is due to vWf; and the vWf propolypeptide is necessary for the formation of vWf storage granules.

Defect in regulated secretion of P-selectin affects leukocyte recruitment in von Willebrand factor-deficient mice

Stimulation of endothelial cells by various inflammatory mediators leads to release of Weibel–Palade bodies and therefore to exocytosis of both P-selectin (adhesion receptor for leukocytes) and von Willebrand factor (vWf) (platelet ligand). The potential role of vWf in leukocyte recruitment was investigated with the use of vWf-deficient mice. We report a strong reduction of leukocyte rolling in venules of vWf-deficient mice. Similarly, vWf deficiency led to a decrease in neutrophil recruitment in a cytokine-induced meningitis model as well as in early skin wounds. In all instances with an antibody that preferentially recognizes plasma membrane P-selectin, we observed a dramatic reduction in P-selectin expression at the cell surface of vWf-deficient endothelium. With confocal microscopy, we found that the typical rodlike shape of the Weibel–Palade body is missing in vWf −/− endothelial cells and that part of the P-selectin content in the vWf −/− cells colocalized with LAMP-1, a lysosomal marker. However, intracellular P-selectin levels were similar in tumor necrosis factor α- and lipopolysaccharide-activated cells of both genotypes. We conclude that the absence of vWf, as found in severe von Willebrand disease, leads to a defect in Weibel–Palade body formation. This defect results in decreased P-selectin translocation to the cell surface and reduced leukocyte recruitment in early phases of inflammation.

Antithrombotic activity of TNF-α

Basic and clinical observations suggest that thrombosis and inflammation are closely related. Here we addressed the role played by TNF-α in thrombus formation and growth in an in vivo mouse model. Using intravital microscopy, we show that systemic administration of TNF-α at doses found in sepsis transiently inhibits thrombus formation and delays arterial occlusion upon vascular injury. These results were reflected in a prolonged bleeding time. Platelets isolated from the TNF-α–treated mice showed a marked decrease in fibrinogen binding and P-selectin expression as well as reduced platelet aggregation in response to various agonists. In contrast, in vitro treatment of platelets with TNF-α did not affect their function. TNF receptor 1– and 2–deficient mice exhibited normal thrombogenesis in the presence of TNF-α. Additionally, the inhibitory effect of TNF-α was lost either after treatment with NG-monomethyl-L-arginine, an inhibitor of NO production, or in mice deficient for iNOS. These results indicate that under inflammatory conditions, when leukocytes need free passage to transmigrate into tissues, TNF-α decreases platelet activation and inhibits thrombi formation. This effect is not exerted directly on platelets but mediated through the rapid generation of NO in the vessel wall.