UC San Francisco researchers have identified a molecular mechanism that may play an important role in activating platelets, the blood cells that coagulate to stop bleeding but also cause the clots leading to heart attack and stroke. The finding, published in the August 13 issue of Nature, requires further investigation, said the senior author of the study, Shaun R. Coughlin, MD, PhD, director of the Cardiovascular Research Institute (CRVI) and professor of medicine and cellular and molecular pharmacology at UCSF, but it could provide a new avenue for developing drugs for preventing heart attack and stroke.
The study, conducted in mice and on human platelets, explored the way in which the enzyme known as thrombin, one of several factors known to activate platelets, stimulates the blood cells into action. The initial observations in mice led the researchers to a potentially profound finding for humans.
In the mouse study, the researchers determined that thrombin is able to activate platelets by latching onto, and cleaving, a newly identified receptor in the membrane of the cells. When the enzyme (a protease) binds to the receptor, it instigates the transmembrane signaling that prompts platelet aggregation. The researchers named the receptor protease activated receptor (PAR) 4.
The investigators had previously identified a PAR in human platelets, which they named PAR1. They also had previously identified a PAR in mouse platelets, which they named PAR3.
In the current study, they created a mouse model with platelets lacking PAR3 and found that while the mice had a markedly delayed and diminished response to thrombin activation the response was not totally absent. The discovery of PAR4 explained the continued response. "We determined that PAR3 is necessary for normal thrombin responses in mouse platelets, but that PAR4 does contribute to thrombin signaling," said Coughlin.
The discovery that PAR3 and PAR4 appear to act as a dual-receptor system in mice led the researchers to examine whether human platelets also contain PAR4. The discovery that they do suggests that, in human platelets, PAR1 and PAR4 also act as a dual-receptor system. And this suggestion opens up the possibility of designing drugs tailored to this system, said Coughlin.
"It may be necessary to block both PAR1 and PAR4 in human platelets to achieve an antithrombotic effect," he said. "Alternatively," he said, "the existence of a second receptor may provide a useful margin of safety for such potentially powerful therapeutic agents."
Numerous questions about the thrombin system remain to be answered, said Coughlin. The two-receptor system may simply provide redundancy of an important molecular mechanism. Alternatively, it may provide a mechanism for responding to proteases other than thrombin or to thrombin itself over a wider range of concentrations.
"We're initially interested in seeing if there are more receptors involved in the response of platelets to thrombin or whether these two receptors fully account for thrombin signaling," he said.
A still larger question, said Coughlin, is how important the thrombin system as a whole is for activating platelets. Laboratory studies indicate that thrombin is a potent stimulator of platelets, but other factors contribute to platelet aggregation, as well. "Thrombin is a potent activator of platelets under experimental conditions, but other activators are known, and the question of what happens when platelets in living animals or people cannot respond to thrombin has not been answered. We suspect thrombin plays a critical role, but that role has yet to be proven conclusively."
Co-investigators of the UCSF study were Mark L. Kahn, MD, an assistant research physician, CVRI, Yao-Wu Zheng, PhD, an associate specialist, CVRI, Wei Huang, MD, a staff research associate, CVRI, Violeta Bigornia, BS, a staff research associate, CVRI, Dewan Zeng, PhD, formerly a postdoctoral fellow, CVRI, Stephen Moff, formerly a Sarnoff Fellow, CVRI, Robert V. Farese Jr., MD, an assistant investigator, Gladstone Institute for Cardiovascular Disease, and Carmen Tam, BS, a staff research associate, CVRI.