Broken Vein? Call a Cell.

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by Angela Spivey

he first stage, initiation, happens on a cell that has tissue factor on its surface. Tissue factor is a protein that stays on the membrane of a cell at all times. Your blood normally never touches tissue factor—unless a blood vessel breaks. Then plasma in the blood is exposed to tissue-factor-bearing cells, and the tissue factor binds to one of the clotting factors in the plasma, factor VII, which becomes activated or turned on. (The activated form of a factor is noted with a lowercase "a," as in VIIa.)

Tissue factor and factor VIIa are what are known as an enzyme and a cofactor. An enzyme speeds up a chemical reaction, and its cofactor helps the enzyme do its job faster. So, bound together, the tissue factor and factor VIIa activate factor IX and X (creating IXa and Xa). These combine with other factors, continuing until factor Xa combines with factor Va, which changes an enzyme called prothrombin into its active form, thrombin. Thrombin is one of the compounds that starts the clotting process. But at this stage just a small amount is produced—not enough to cause a clot.

It is enough thrombin, though, to begin activating platelets that have been circulating in the blood in an inactive state. This is the second stage, called amplification, in which the action moves to the platelet surface. Once activated, platelets release their internal stores—sort of turn themselves inside out. One of the things that’s released is a small molecule called adenosine diphosphate (ADP), which will bind to a receptor on another platelet and activate it. "So the activated platelet is sending an inactivated platelet a signal—‘stick down here,’" Monroe says. The activated platelets go directly to the site of the bleeding, where they form a sticky mass—the first soft clot that you see after you cut yourself.

In the final stage, propagation, activated factors combine with their cofactors and bind to sites on the surfaces of platelets. One crucial part of this stage is when IXa and VIIIa combine to form Xa, which helps produce a large burst of thrombin. The large amount of thrombin sends signals that turn a factor called fibrinogen into its active form, fibrin, which weaves through the platelet clump to form a scab.

The model provides clues as to why people can’t clot without factor IX but blood in a test tube can. In the last stage, activated factor IX helps make the crucial Xa on the platelet surface. But factor Xa is also produced earlier, by factor VIIa and tissue factor. So why can’t this Xa compensate for the Xa normally made by IXa? In a test tube, it can. But in a person, the first Xa is produced on a tissue-factor-bearing cell. For the Xa to promote clotting, it must move from the tissue-factor-bearing cell to the platelet surface. But inhibitors in the plasma probably stop it before it can get there, Roberts says.

t a more minute level, Leslie Parise, professor of pharmacology, studies how thrombin signals the platelet to bind to fibrinogen. When thrombin or ADP activate their receptors, some of the signals created inside the platelet turn on a glycoprotein called GP IIb-IIIa. It is GP IIb-IIIa that actually binds to fibrinogen, forming that first soft clot at a site of blood vessel injury, Parise says. Her lab has identified a new protein that they think may be involved in regulating the activation of the receptor for GP IIb-IIIa. "The platelet tightly regulates the activation of this glycoprotein," Parise says, "because if it was activated, our blood would clot and we’d be having heart attacks all the time."

Roberts’ group thinks that clotting is also kept in check by endothelial cells (those that line the inside of the blood vessels). Normally, platelets don’t stick to endothelial cells because these cells produce an enzyme that degrades ADP. So if a platelet accidentally gets activated, this enzyme "chews up" the ADP so that it can’t signal more platelets to activate, which could cause a clot that could block blood flow to the heart or lungs.

As many details as are known about this process, there as just as many that are a mystery. For instance, scientists know that the binding sites for clotting factors exist on the platelet surface because they can measure the sites’ activity, Monroe says. But no one has identified the protein structures of most of the sites, much less sequenced them or cloned them. And researchers don’t know yet whether these sites are true receptors that signal changes inside the cell or simply binding sites (click here for a diagram).

As these researchers learn more about how cells control clotting, they hope they will learn more about how we can control it. "Right now, your control mechanisms are keeping you alive," Roberts says. "If we can understand the normal mechanism, then we can figure out what goes wrong, what is it that causes disease."

Next: To clot or not

© 2001 Endeavors, The University of North Carolina at Chapel Hill. All rights reserved.