The findings could lead to more personalized approaches to controlling platelet activity during heart attacks and other vascular emergencies and diseases.
Researchers at the UNC School of Medicine have found that the blood platelet protein Rasa3 is critical to the success of the common anti-platelet drug Plavix, which breaks up blood clots during heart attacks and other arterial diseases.
Platelets / non-active state
Platelet / activated, sticky to form clot
The discovery, published in the Journal of Clinical Investigation, details how Rasa3 is part of a cellular pathway crucial for platelet activity during clot formation. Understanding the protein’s role could also prove vital in the development of new compounds aimed at altering platelet function.
“We believe these findings could lead to improved strategies for treatment following a heart attack and a better understanding of why people respond differently to anti-platelet drugs, such as aspirin and Plavix,” said Wolfgang Bergmeier, PhD, professor of biochemistry and biophysics, member of the McAllister Heart Institute at UNC, and senior author of the paper.
The research, which was conducted in mice, may also open the door to developing antidotes to Plavix, which was the second-best selling drug in the world prior to its patent expiring in 2012. It is still prescribed under its generic name clopidogrel to millions of people with heart disease, peripheral vascular disease, and cerebrovascular disease.
However, the drug’s anti-platelet effect increases the risk of bleeding in patients and makes emergency surgery too risky because Plavix affects the ability of platelets to prevent blood loss after vascular injury. An antidote would bypass the need to wait until the kidneys eliminate the drug from circulation.
Since the 1970s, scientists knew that clopidogrel had an anti-clotting effect on platelets. In 2001, they found the compound’s target – a cell receptor called P2Y12. As Plavix was developed into a multi-billion-dollar drug, scientists still didn’t know how this receptor communicated with other proteins in the cell pathways important for platelet activation. This also meant they didn’t know why people responded differently to the drug.
Researchers have since learned that the receptor P2Y12 communicates with a small protein called Rap1, which is like a cellular switch. In platelets, this switch is typically off, which keeps platelets in a non-sticky state. In this quiet state, the 2.5 trillion platelets can patrol blood vessels and arteries without sticking to the endothelium – or inside wall – of, say, a coronary artery. If there’s a problem in the endothelium, the Rap1 switch is flipped and platelets morph into super sticky cells that clot fast to keep blood from gushing into tissue.
This is crucial when we have a severe injury or even a cut. But this clotting also happens during a heart attack, when a massive clot is the last thing a person with heart disease needs.
In the arteries that feed blood to the heart, plaque builds over time. But this buildup isn’t typically the cause of heart attacks; they occur when the plaque ruptures and platelets rush in to plug the rupture. This clotting blocks the artery, which blocks oxygen from entering the heart. And that causes the heart attack.
To counteract the effects of the clot, Plavix hits its P2Y12 target to flip the Rap1 switch back to the off position so the platelets return to their quiet, non-sticky state. Aspirin also helps keep platelets from sticking.
Until now, no one knew how hitting the P2Y12 receptor triggered the Rap1 protein to switch off. The experiments conducted by the Bergmeier lab show that the Rasa3 protein is a crucial player in this process.
“Platelets live in unique environment and they need to be very sensitive to changes in that environment,” Bergmeier said. “They are ready to jump into action almost without anything happening. You could say they’re in a preloaded state. But for that to be possible, they need a breaking system that keeps the platelets in the off state so that they don’t do anything until they absolutely have to.”
Rasa3 is a key part of that breaking system, and Plavix makes sure that the break stays on.
Think of a platelet like a circuit with Rap1-GDP representing the off state and Rap1-GTP representing the on state. In between, there are proteins called exchange factors (GEFs), which flip on the platelet’s Rap1 machinery. The proteins needed to switch off Rap1 are called GAPs. (see illustration)
Using deep sequencing techniques, Bergmeier’s team found that Rasa3 was the only highly expressed GAP gene for Rap1 in platelets. He thought that a malfunctioning Rasa3 protein would lead to platelet activation and clearance from circulation.
His team knocked out Rasa3 in mice to show that the offspring had no platelets and could not survive. The researchers then used mice from The Jackson Laboratory to study platelets in mice with a Rasa3 mutation. These mice had 3 to 5 percent of the typical platelet count. Bergmeier’s team found that the rest of the platelets were being activated and cleared from circulation.
When the researchers disabled the major GEF proteins, the platelet counts rose to normal amounts in the mice. This showed that a tightly controlled balance between GEF and GAP proteins, especially Rasa3, is vital for platelet activity.
At the sites of vascular injury there’s a shift in this balance inside a platelet that makes the cells very sticky. Plavix ensures that Rasa3 cannot be turned off in platelets; the drug irreversibly limits the cell’s ability to stick. It keeps the cell’s breaking system perpetually on.
“These experiments show that this Rap1 GEF-GAP pathway is crucial for platelets to jump into action to plug a hole in the endothelium,” Bergmeier said. “And now we know that Rasa3 is a critical negative regulator, a break, on the process.”
Bergmeier added, “We have good reason to believe that the Rap1 switch, controlled by the same GEF and GAP proteins, also regulates the active state of human platelets. We expect this research will provide critical information for improving anti-platelet therapies, possibly including approaches that eliminate some of the patient-to-patient variability and the increased bleeding risk associated with current anti-platelet drugs.”
Co-first authors of the study are David Paul, PhD, and Lucia Stefanini, PhD, both postdoctoral fellows in the Bergmeier lab when this research was conducted. Stefanini is now member of the Institute for Cardiovascular and Metabolic Research at the University of Reading in the United Kingdom. The co-corresponding author, along with Bergmeier, is Luanne Peters, PhD, professor at The Jackson Laboratory.
Other UNC authors include Kathleen Caron, PhD, professor and chair of the department of cell biology and physiology; Nigel Mackman, PhD, the John C. Parker Professor of Medicine and director of the McAllister Heart Institute; Matthew Parrott, PhD, assistant professor of radiology and member of the UNC Biomedical Research Imaging Center; Todd Getz, PhD, former UNC graduate student and current ORISE research fellow the U.S. Army Institute of Surgical Research; Yacine Boulaftali, PhD, and Caterina Casari, PhD, both postdoctoral fellows in the Bergmeier lab; and graduate student Dan Kechele.
This research was supported through grants from the National Institutes of Health, The American Heart Association, the European Hematology Association, and the International Society of Thrombosis and Hemostasis.
Science Communications Manager
Mark Derewicz | newswise
Newly designed molecule binds nitrogen
23.02.2018 | Julius-Maximilians-Universität Würzburg
Atomic Design by Water
23.02.2018 | Max-Planck-Institut für Eisenforschung GmbH
A newly developed laser technology has enabled physicists in the Laboratory for Attosecond Physics (jointly run by LMU Munich and the Max Planck Institute of Quantum Optics) to generate attosecond bursts of high-energy photons of unprecedented intensity. This has made it possible to observe the interaction of multiple photons in a single such pulse with electrons in the inner orbital shell of an atom.
In order to observe the ultrafast electron motion in the inner shells of atoms with short light pulses, the pulses must not only be ultrashort, but very...
A group of researchers led by Andrea Cavalleri at the Max Planck Institute for Structure and Dynamics of Matter (MPSD) in Hamburg has demonstrated a new method enabling precise measurements of the interatomic forces that hold crystalline solids together. The paper Probing the Interatomic Potential of Solids by Strong-Field Nonlinear Phononics, published online in Nature, explains how a terahertz-frequency laser pulse can drive very large deformations of the crystal.
By measuring the highly unusual atomic trajectories under extreme electromagnetic transients, the MPSD group could reconstruct how rigid the atomic bonds are...
Quantum computers may one day solve algorithmic problems which even the biggest supercomputers today can’t manage. But how do you test a quantum computer to...
For the first time, a team of researchers at the Max-Planck Institute (MPI) for Polymer Research in Mainz, Germany, has succeeded in making an integrated circuit (IC) from just a monolayer of a semiconducting polymer via a bottom-up, self-assembly approach.
In the self-assembly process, the semiconducting polymer arranges itself into an ordered monolayer in a transistor. The transistors are binary switches used...
Breakthrough provides a new concept of the design of molecular motors, sensors and electricity generators at nanoscale
Researchers from the Institute of Organic Chemistry and Biochemistry of the CAS (IOCB Prague), Institute of Physics of the CAS (IP CAS) and Palacký University...
15.02.2018 | Event News
13.02.2018 | Event News
12.02.2018 | Event News
23.02.2018 | Physics and Astronomy
23.02.2018 | Health and Medicine
23.02.2018 | Physics and Astronomy