Vol 6. Issue 20 / June 19, 2006
By Eric Sauter
Zaverio Ruggeri has been looking at blood platelets for over 35 years—he began his postdoctoral work in hematology in 1970—so by now he knows a thing or two. The knowledge he has accumulated and continues to accumulate is coming along well. As a matter of fact, it’s gone global.
“I would say our view of what platelets do in several different aspects of biology has become much more global,” he says. “The concepts have become accepted—this is another part of globalization, except in biology, not finance.”
Platelets in all mammals are tiny bits of cytoplasm crucial to the formation of blood clots that stop you from bleeding to death when you’re cut or injured. Produced in the bone marrow, platelets are the smallest cell-like structures in the blood and circulate by the millions; 150 to 400 million per milliliter is normal. Their ability to stick and stay is also implicated in several diseases, particularly atherothrombosis, in which they block arteries and cause heart attacks and strokes.
Ruggeri, who heads the Division of Experimental Hemostasis and Thrombosis in The Scripps Research Institute’s Department of Molecular and Experimental Medicine as well as directing the Roon Research Center for Arteriosclerosis and Thrombosis, works with colleagues and collaborators to further define the mechanisms that make platelets function as they do, for good and ill.
More Than Just Clots
Recently, though, his global view of what platelets can do has taken something of a surprising turn. Through his close collaboration with Scripps Research Associate Professor Luca Guidotti, the two Italian-born scientists have discovered that platelets do more than just form clots. They play a key role in regulating the body’s immune response.
“From everything we have learned in the last few years,” Ruggeri says, “platelets seem to play a fundamental biological role that has to do with reporting on the condition of the vessel wall. Because they form a direct interface between blood and the body, platelets have the ability to detect whether endothelial cells [which line blood vessels] are normal or not. If something pathological occurs, like a viral infection, this affects the endothelial cells. Circulating white blood cells—leucocytes—then exit the system and attack the site of the infection. But now we find that the white cells, or at least some types of white cells, don’t really know where to go unless there are platelets around to direct them to the site of the infection.”
This new picture of platelets as a combination early-warning system and route planners first emerged through his work with Guidotti, whose studies had uncovered the fact that platelets were an unusual and highly unexpected factor in the pathogenesis of liver infections (such as those caused by hepatitis B) mediated by cytotoxic T lymphocytes (CTLs). It was those findings that gave a confirming boost to the emerging hypothesis that platelets were necessary for virus-specific CTLs to control the infection in peripheral organs.
“Our work together—initiated by Luca Guidotti’s observation in the mouse hepatitis model—has opened up a whole new field of research,” Ruggeri says. “For someone who has been looking at platelets for many, many years, this has been a surprise for me because it has to do with the immune response. With Luca Guidotti we have been able to show that when activated platelets contribute to liver disease—but also to viral clearance—by promoting the recruitment of virus-specific CTLs into the liver. This is likely dependent on specific interactions between platelets and CTLs—which may favor CTLs moving from blood vessels into the infected cells. Once there, the CTLS may perform pathogenetic or antiviral effector functions or both. This new knowledge could lead to new ways to treat the disease.”
There was also the recent discovery—by Scripps Research scientists B. Felding-Habermann and Kim Janda—that platelets have a critical role to play in metastatic breast cancer. An integral membrane protein (an integrin called the vitronectin receptor), which exists in an activated state on aggressive tumor cells, interacts with platelets in the blood. This integrin-platelet combination helps tumor metastasis by clumping together in vulnerable organs and by helping the process of angiogenesis in new tumors.
A New Definition
As a result of these new discoveries, Ruggeri’s definition of platelets has been both broadened and simplified: “It seems to me now that platelets are created to do many things—to stop bleeding, to help the immune system do the right job. To me, they are all part of the same scenario. My opinion now, which is very simple to conceptualize once you finally understand the mechanism, is that platelets are designed to recognize and discriminate between healthy and unhealthy endothelial cells. That’s it.”
Given that simple but elegant definition, platelets are perfectly designed for the job. In circulation they remain close to the vessel wall, basically covering the endothelial cells as a constantly moving layer—there may be one quarter to one trillion endothelial cells in the body, and one to two trillion platelets in the blood.
The obvious question—why do we need so many?—receives the newly formed yet obvious answer: Because that’s what it takes to look after the endothelium.
As Ruggeri points out, the normal volume of platelets goes above and beyond our need for blood clotting. Serious bleeding is a very rare thing, even when there is major trauma—provided that circulating platelets are at least one quarter of their normal number.
“You have to have so many platelets because they’re needed to survey the endothelium,” he says. “Certainly there must have been evolutionary pressure from infectious agents like viruses to make them evolve into what they are now. Perhaps in a few years we may more fully understand the kinds of pressures that were needed to create platelets.”
The Power of Blood
This unexpected turn in understanding—this really serendipitous break—emerges from what he calls “classic studies of platelets, all this accumulated knowledge that will allow us to understand these other mechanisms” and echoes Ruggeri’s own reasons for his interest in blood in the first place.
“I got into the study of blood through medicine and patients because—well, actually I wanted to become a physicist originally but that’s another story. Like many things in life, it was serendipity. I was talking to a young doctor in Milan who had just come back from training at Oxford where he studied patients with hemophilia. I became fascinated by the disease. That led to studying clotting in Italy and England and treating patients with hemophilia but also those with von Willebrand disease. When I saw that, it intrigued me so much that I said, ‘I will not look at anymore patients until I know more about this disease.’”
Von Willebrand disease, the most common bleeding disorder in existence, is caused by a deficiency of von Willebrand Factor (VWF), a blood protein that aids platelets in clotting. Ruggeri has spent the majority of his years in science delineating the structure and function of von Willebrand Factor and various related mechanisms of cell adhesion, piling up the knowledge that will help define this entirely new field of platelet mediation in immune-mediated and viral pathogenesis.
Ruggeri is also deeply immersed in another new field, nanotechnology. As part of the Program of Excellence in Nanomedicine (PEN), a collaborative partnership between the Burnham Institute, the California Nanosystems Institute at UC Santa Barbara, and Scripps Research, Ruggeri is helping to design and develop a range of nanodevices that could be injected into the circulatory system to target injured or diseased areas of the vasculature. These nanodevices will deliver sensors that monitor and report on the damage or, alternatively, deliver targeted therapeutic drugs that would be released at the site.
“We’re working on a kind of lipid vesicle, developed by the bioengineers at Santa Barbara, which is between 50 and a few hundred nanometers in size—much smaller than platelets,” Ruggeri says. “We will coat these vesicles with molecules that allow them to stick to the surface of damaged endothelial cells and to other platelets that have become activated—powerful glues with very high specificity. Right now, we’re testing different molecules. We use our flow models to see if we have the right molecules that will stick effectively in the right places.”
Right now, the project is fully funded and is expected to run for the next four years. Ruggeri doesn’t know if they will have the nanodevices ready by then but he’s anxious to get started regardless. “If we don’t have them ready in four years, we will still be a long way toward developing nanodevices that can be targeted,” he says.
And while he doesn’t follow popular culture—this is a man who while he was in high school in Italy read Greek and Roman literature in their original languages and spent much of his time translating Greek and Latin texts into modern Italian—Ruggeri does seem to understand the fantastical pull that blood has on us all.
“I think that the interest in blood is easy to explain,” he says. “Even though the discovery of our circulation system was relatively recent—the 17th century—blood itself has been at the center of everything magical and mythical from the very origin of human beings. Spilling blood means losing your life. Really, blood is life itself.”
Spoken like a man who might know a thing or two about it.
Send comments to: mikaono[at]scripps.edu