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In Vivo Expression of Murine Platelet Glycoprotein Ib

H. Fujita, S. Russell, H. Tran, J. Ware

Glycoprotein (GP) receptors within the platelet membrane are essential for initiating platelet adhesion and aggregation on thrombogenic surfaces. In response to vascular injury, these receptors provide platelets with 2 essential properties: the ability to bind adhesive substrates exposed at the site of injury (adhesion) and the ability to recruit additional platelets to form a thrombus (aggregation). It is becoming increasingly evident that defined rheological conditions govern the physiologic relevance of specific receptor-ligand interactions along with fundamentally distinct molecular mechanisms for individual receptors and their ligands.

Among platelet receptors, the GP Ib-IX-V complex is important because it initiates thrombus formation over a wide range of flow conditions through an initial interaction with the adhesive ligand von Willebrand factor. The importance of this receptor-ligand interaction is best exemplified by congenital bleeding disorders caused by the lack of either the receptor or the ligand, the Bernard-Soulier syndrome and von Willebrand disease, respectively. Additionally, the GP Ib component of the GP Ib-IX-V complex contains a binding site for -thrombin, and recent studies have strengthened the concept that the interaction between -thrombin and GP Ib is of biological relevance.

We recently cloned and sequenced a 5371-bp fragment of mouse DNA containing the homolog to the gene for human platelet GP Ib. The complete nucleotide sequence of the mouse gene revealed an exon-intron arrangement strikingly similar to that of the gene for human GP Ib. Characterization of the gene for mouse GP Ib enabled us to systematically evaluate expression of GP Ib in nonhematopoietic organs. This study was warranted by a number of in vitro observations suggesting that GP Ib mRNA and protein expression may be induced by cytokines in endothelial cells and smooth muscle cells. However, an in vivo role for a GP Ib-IX-V receptor has not been established beyond that described for normal megakaryocyte-platelet physiology and hemostasis.

Northern blot analysis of mouse organs revealed high levels of GP Ib mRNA in bone marrow, and a similar pattern of expression occurred in mice containing a luciferase transgene under the control of the murine GP Ib promoter. Consistently high levels of luciferase activity were detected in the 2 hematopoietic organs of mice: bone marrow and spleen. Reproducible but low levels of luciferase activity were present in heart, aorta, and lung. Among circulating blood cells, the luciferase activity was exclusively localized in platelets. No increase in GP Ib mRNA or luciferase activity was observed after treatment of mice with lipopolysaccharides or TNF-.

We conclude that the murine GP Ib promoter supports a high level of gene expression in megakaryocytes and can express heterologous proteins, allowing an in vivo manipulation of platelet-specific proteins in the unique environment of a blood platelet. Unquestionably, studies of the GP Ib-IX-V complex are defining essential aspects of normal hemostasis and thrombosis and are providing key information on the molecular mechanisms that govern the formation of pathologic platelet thrombi.

PUBLICATIONS

Fujita, H., Hashimoto, Y., Russell, S., Zieger, B., Ware, J. In vivo expression of murine platelet glycoprotein Ib. Blood 92:488, 1998.

Ware, J. Molecular analyses of the platelet glycoprotein Ib-IX-V receptor. Thromb. Haemost. 79:466, 1998.

Ware, J. Platelet receptors for von Willebrand factor: I. The glycoprotein Ib-IX-V. In: von Willebrand Factor and the Mechanism of Platelet Function. Ruggeri, Z.M. (Ed.). Springer-Verlag, New York, 1998, p. 111.

Yagi, M., Zieger, B., Roth, G.J., Ware, J. Structure and expression of the human septin gene hCDCrel-1. Gene 212:229, 1998.