Researchers at Washington University School of Medicine in St. Louis have a new explanation for why and how this pair embraces abnormally to cause the disease. Their work is published in the July issue of Biophysical Journal.
The study suggests that the disease occurs because a defective form of vWF causes chemical bonds to persist longer than they should, thereby holding vWF and blood platelets together in flowing blood when they shouldn't. That is, the defect in the vWF protein changes the kinetics of the chemical bonds that form between the protein and the platelets.
"This is the first time that a naturally occurring disease has been linked to an alteration in the kinetic properties of a chemical bond," says study leader Thomas G. Diacovo, M.D., assistant professor of pediatrics and of pathology and immunology. "The finding should give us a better understanding of how normal platelets function and of the delicate balance that exists between these blood-clotting elements, disturb that balance, and the whole system falls apart."
For the past 25 years, scientists have tried to explain why platelets normally adhere to vWF at sites of vascular injury but not in flowing blood. Most of them believe that docking sites on vWF undergo a change in shape after the protein adheres to a site of vessel injury. This change presumably allows passing platelets to attach to vWF. Biology has many examples of such conformational changes that transform molecules from an inactive to an active state. People with von Willebrand's disease have an altered form of vWF, in which one amino acid in the protein has been replaced by another. Proponents of the conformational-change theory believe that the abnormal amino acid changes the shape of vWF, thereby causing it to bind with platelets when it shouldn't.
"It seems like a reasonable mechanism," says Diacovo, "but the evidence isn't there."
Diacovo suspected the answer lay in bond kinetics, which refers to how fast a bond can form and then dissociate. Earlier work on selectins, a family of proteins critical for recruiting circulating white blood to inflamed blood vessel walls, revealed that bond kinetics are responsible for controlling the interaction between these cells and the vessel wall. Diacovo theorized that the same was true for vWF and platelets.
To test the idea, Diacovo and a team of researchers studied the interaction of vWF and platelets using a variety of experiments, including some that involved mice and others that used flow chambers to duplicate the forces acting on platelets and vWF in circulating blood. The investigators discovered that a few bonds probably do form between vWF and platelets as they flow along in the bloodstream in healthy individuals. However, the number of bonds formed at any one time are too few to stabilize the attachment of platelets to vWF because as soon as a bond forms, it rapidly releases. In a particular form of von Willebrand's disease, however, the bonds last significantly longer than normal. That extra time allows many additional bonds to form, which stabilizes the interaction and locks the proteins and platelets together.
"It's not just a brief touch-and-go," says Diacovo. "Rather, one bond forms and before it breaks, two, three and four more have formed."
As more bonds form, small aggregates of platelets and vWF develop. These aggregates are cleaned from the blood, probably in the spleen, says Diacovo, which reduces the amount of vWF and the number of platelets in the blood. Consequently, people with von Willebrand's disease have a mild to moderate bleeding disorder. They bruise easily and simple nosebleeds can continue for several hours or days before finally healing.
In addition to providing insight into platelet function in normal individuals and in people with von Willebrand's disease, the findings may also help researchers develop new kinds of anti-thrombotic drugs. The results also may pose a new way to classify molecules, called adhesion receptors, found on the surface of cells.
Diacovo was recognized for this research with the 2002 Young Investigator's Prize in Thrombosis from the American Heart Association.
Doggett TA, Girdhar G, Lawshe A, Schmidtke DW, Laurenzi IJ, Diamond SL, Diacovo TG. Selectin-like kinetics and biomechanics promote rapid platelet adhesion in flow: the GPI balpha-vWF tether bond. Biophysical Journal, 83(1):194-205, July 2002.
Funding from the National Heart, Lung and Blood Institute and from the American Heart Association supported this research.
The full-time and volunteer faculty of Washington University School of Medicine are the physicians and surgeons of Barnes-Jewish and St. Louis Children's hospitals. The School of Medicine is one of the leading medical research, teaching and patient-care institutions in the nation. Through its affiliations with Barnes-Jewish and St. Louis Children's hospitals, the School of Medicine is linked to BJC HealthCare.