Scripps Research team sheds new light on formation of blood clots

Findings could lead to novel ways to prevent heart attack and stroke

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LA JOLLA, Calif.—In an effort that could lead to better drugs for preventing heart attacks, strokes and other clot-related conditions, Scripps Research Institute scientists have discovered new elements of the blood clot-formation process that help form a new model of clotting.

According to the old model, an injury to the wall of blood vessels causes smooth muscle cells to expose a clot-organizing protein called tissue factor. In the emerging new model, tissue factor exists on the surfaces of these smooth muscle cells, as well as on circulating immune cells, but in an inactive state, according to Wolfram Ruf, a professor at Scripps Research and one of the authors of the study, published by the Journal of Clinical Investigation in an advance, online edition June 13, 2011.

"In this study, we've shown that cell-surface receptor P2X7, which was known to promote inflammation when stimulated, also plays a major role in the clot-forming process by activating tissue factor," Ruf adds.

To better understand clot formation, Ruf and his colleagues performed a set of experiments on cultured mouse cells and transgenic mouse models. The team's investigation began with the P2X7 receptor, because of its known role in the inflammatory response that can lead to excessive clotting in sepsis, a severe illness in which the bloodstream is overwhelmed by bacteria.
Normally, when cells are damaged, they release large quantities of energy-storage molecules known as adenosine triphosphate (ATP). Previous research had hinted that when this freed ATP encounters passing immune cells, it serves as a damage signal, stimulating the immune cells' P2X7 receptors and causing the release of "microparticles" exposing the clot-promoting tissue factor. The new study showed that ATP can affect P2X7 receptors on both immune cells and smooth muscle cells.
To confirm the significance of the P2X7 receptor in the clot-forming process, the team bred transgenic mice that lacked functional P2X7 receptors, and found that these P2X7-knockout mice failed to form stable arterial blood clots when the vessel wall was exposed to a clot-inducing substance. Importantly, these mice did not suffer from uncontrollable bleeding.

"This suggests that clot-preventing drugs targeting the P2X7 pathway might not have unacceptable side effects," says Ruf.
In the cell experiments, the team found that the cascade of molecular events following P2X7 stimulation alters the activity of a thiol-targeting enzyme known as protein disulfide isomerase (PDI), which Ruf's previous studies had implicated as a possible activator of tissue factor. In the new study, the scientists demonstrated the importance of PDI in this process by showing that they could block clot formation in normal mice with anti-PDI antibodies.
Targeting the top of the clot-formation pathway by blocking the P2X7 receptor might have even broader beneficial effects, since the activation of this receptor occurs in a number of inflammatory disorders.
"Cardiovascular disease and heart attacks are caused by chronic inflammation as well as clot formation," Ruf notes, "so possibly P2X7 is a major explanation for the link between inflammation and thrombosis, as well as a good target for preventing these conditions."
In addition to Ruf, authors of the study, "P2X7 receptor signaling contributes to tissue factor–dependent thrombosis in mice," are first author Christian Furlan-Freguia, a research associate in the Scripps Research Department of Immunology and Microbial Science; Patrizia Marchese and Zaverio M. Ruggeri of the Scripps Research Department of Molecular and Experimental Medicine; and András Gruber of the Department of Biomedical Engineering and Department of Medicine at the Oregon Health and Sciences University in Portland.
This work was supported by the National Institutes of Health and the American Heart Association.

Original source: Scripps Research Institute

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