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26.05.2010 Beyond Clotting: The Powers of Platelets

Platelets are known for thwarting blood loss, but new research shows these simplified cells defend against microbes and perform other duties—and they're also drug targets in sepsis and other conditions.

Photo: Coming unstuck. Platelets like these enmeshed in a blood clot perform numerous jobs in the body. CREDIT: © DENNIS KUNKEL MICROSCOPY INC./VISUALS UNLIMITED/CORBIS

Thirty years ago, researchers were convinced that they had platelets pegged. Every milliliter of our blood, the thinking went, harbors hundreds of millions of these cell fragments for just one reason: to save us from bleeding to death. If we suffer a cut or other injury, platelets swarm into action, forming a plug that seals the wound.

But in recent years, platelets have displayed powers no one imagined they had. They are healers that pour out growth factors and other soothing molecules that help damaged tissue rebuild. They are soldiers that spark the protective response known as inflammation, alert immune cells, and even attack microbial interlopers. They are long-haul truckers that pick up and deliver chemicals such as serotonin, which helps the liver regenerate after injury (Science, 7 April 2006, p. 104). They are even engineers, shaping the vascular system in newborns. "Platelets are certainly not just the Band-Aids of the bloodstream," says hematologist Joseph Italiano of Harvard Medical School in Boston.

Additional platelet functions continue to come to light, and biologists have just described a novel way that the body might make these multitalented cells—a finding that could one day ease the demand for donated blood. The new view of platelets as more complex and capable "has really got momentum now," says hematologist Andrew Weyrich of the University of Utah in Salt Lake City.

Yet for all their benefits, platelets will be the death of most of us, through blood clots that cause strokes and heart attacks. Platelets also foster cancer, rheumatoid arthritis, and other diseases.

That means platelet-inhibiting medicines originally developed to stall blood clotting might have broader benefits, notes cardiologist Susan Smyth of the University of Kentucky in Lexington. Her lab, for instance, is investigating whether the anticlotting agent clopidogrel also helps against sepsis, an often-fatal condition in which bacterial infections run rampant in the bloodstream.

Platelet factory

It's easy to see why researchers underestimated platelets. Like the already simplified red blood cell, a platelet lacks a nucleus and all the myriad genes it normally contains. Yet platelets are even smaller—less than one-third the diameter of red blood cells—and some scientists maintain they don't even qualify as cells.

But researchers have learned that platelets share many features and abilities with conventional cells—even, at least according to a controversial new study, the capacity to reproduce. Biologists have long thought that platelets, rather than forming through mitosis as most cells do, arise by breaking off from hulking bone marrow cells called megakaryocytes. Three years ago, scientists for the first time observed how this separation occurs in the bone marrow of living mice (Science, 21 September 2007, p. 1767). Tendrils from a megakaryocyte bore into a vessel, and the force of flowing blood pulls off pieces that further fission and eventually become platelets.

But is that the only way platelets are born? The question is pressing because no lab has succeeded in growing clinical quantities of platelets—and they are desperately needed for medical traumas that involve lots of bleeding and for cancer patients whose platelet counts have plummeted because of chemotherapy. The supply for transfusions comes entirely from donated blood and can be stored for only about 5 days.

In a paper published in January in the journal Blood, Weyrich and colleagues reported that they had caught platelets isolated from fresh blood in the act of reproducing. Instead of going through a conventional mitotic cell division, a platelet sends out a strand with one or more beadlike bulges. These progeny—which sometimes break off but often remain tethered to their parent—harbor standard platelet proteins and latch onto clotting proteins as they would at the site of a cut. Previous studies had noted such platelet necklaces in blood samples, but most researchers assumed they derived from megakaryocytes, not from platelets themselves.

Weyrich says the findings show that platelets have the capacity for independent replication, though whether this process happens often enough naturally to lift a person's platelet count remains unclear. Italiano, whose lab is trying to cultivate platelets, concurs. "I think there's something here," he says. "But the question is to what extent does it occur in the bloodstream."

Unexpected chemistry

Platelets are chock-full of biologically influential molecules—not just ingredients for blood clotting but a wealth of growth factors, immune system messengers, enzymes, and other compounds. Researchers have identified more than 1100 kinds of proteins inside platelets or on their surface. The assumption had been that a platelet, given its lack of a nucleus, inherited these proteins from the megakaryocyte from which it had broken off.

Yet platelets "are just as metabolically active as nucleated cells," says immunologist John Semple of St. Michael's Hospital in Toronto, Canada. Indeed, platelets contain mitochondria, which provide cells with energy for chemical reactions and protein synthesis, and ribosomes, the structures cells use to make proteins from amino acids.

Hints that platelets could build their own proteins began accumulating in the 1960s, when biologists discovered that the cells absorb free amino acids. And in 1988, researchers confirmed that platelets store small amounts of messenger RNA molecules (mRNAs). Yet some scientists objected that any protein production using these mRNAs was unimportant.

In 1998, however, Weyrich and colleagues demonstrated that platelets fashion a particular protein, Bcl-3, in response to a specific stimulus, indicating that platelets carefully manage what they manufacture. Since then, Weyrich's group and other researchers have revealed that platelets make additional key proteins, such as the cytokine interleukin-1β, which induces inflammation, in response to chemical signals. Still, researchers need to pin down how much of its protein repertoire a platelet makes and how much it receives from its megakaryocyte mother.

Cardiologist Jane Freedman of Boston University Medical Center cautions against giving platelets too much credit. "I think that platelets do a lot more than people think they do, but they don't have a nucleus" and thus can't perform all the functions of a conventional cell. Nonetheless, a clear picture is emerging, Weyrich says: "A platelet is dynamic and independent."

Attack of the killer platelets

Their most visible profession is curbing blood loss, but platelets also "moonlight," as Italiano puts it, at an assortment of jobs. In the January issue of Nature Medicine, for example, Steffen Massberg of the German Heart Centre Munich and colleagues reported a new developmental role for the cells. In mice, the researchers discovered, platelets ensure that the ductus arteriosus, a shunt that allows blood to bypass the lungs during fetal development, closes once the animals are born. Platelets form a clot in the ductus arteriosus and spur cells in the lining to form a permanent seal. The same mechanism seems to work in humans. The scientists retrospectively studied premature infants and found that those with low platelet counts were more likely to have a ductus arteriosus that remained open. If this connection is confirmed, it might lead to a new way to treat infants and premature babies, who are particularly susceptible to the condition.

Parent and offspring? Platelet chains like these, in which the beadlike bulges contain standard platelet proteins, have convinced some researchers that the cells can reproduce. CREDIT: H. SCHWERTZ ET AL., BLOOD (JANUARY 2010)

Platelets also appear to guard against microbial invasion. Six years ago, researchers discovered that, like pathogen-fighting macrophages and dendritic cells but unlike red blood cells, platelets sport Toll-like receptors (TLRs) that recognize molecular features of microbes. Semple and colleagues have found that when pathogens trip a platelet's TLRs, there's a surge in TNF, a compound that fuels inflammation, one of the body's most potent protections against infection.

Sometimes platelets play a more active role in defense, latching onto invaders. They usually turn their captives over to macrophages that can destroy the microbes. But recent work suggests that they can do the job themselves: By hooking onto the surface of a red blood cell, they can kill malaria parasites lurking inside—though how isn't clear (Science, 6 February 2009, p. 797).

Semple speculates that platelets are some of the most important pathogen detectors in the body. They are the second most numerous cells in the bloodstream, after red blood cells, and they circulate throughout the body and "pick up everything in the plasma," he says.

The immune talents of platelets are probably holdovers from an earlier era in animal evolution, when one cell type sufficed for microbial defense and hemostasis, or prevention of blood loss. Insects sport such multipurpose cells today, called hemocytes. "Platelets evolved to be hemostatic agents, but they kept their inflammatory functions," says Semple.

Gone septic

Platelets may help newborns and ward off microbes, but they also cause plenty of misery. More than half the people who died in the United States last year were killed by platelets. Most fatalities resulted when a blood clot orchestrated by the cells lodged in an artery and caused a heart attack or stroke. That's why aspirin, clopidogrel, and other antiplatelet agents have found widespread use in people with atherosclerosis or several other conditions.

Researchers have more recently found that another common killer owes a lot to platelets: sepsis, an inflammatory over-reaction to microbes in the blood that can devastate multiple organs. According to some estimates, sepsis kills more than 200,000 people in the United States every year.

Developing drugs to combat sepsis has proved difficult, but two recent studies suggest that targeting the effects of platelets could offer a novel strategy. In 2007, immunologist Paul Kubes of the University of Calgary in Canada and colleagues reported that during a bloodstream infection, platelets that have detected bacteria via their TLRs glom onto neutrophils, a type of bacteria-slaying white blood cell. In response, the immune cells release neutrophil extracellular traps (NETs), webs of DNA that ensnare some kinds of microbes. In a small blood vessel in the lungs or liver, several neutrophils could be clinging to the lining, spinning a curtain of sticky strands that detain bacteria trying to pass through. This defensive mechanism "may be a last gasp if you have to get rid of a systemic infection," says Kubes.

Yet NETs can "cause collateral damage," notes Kubes, by injuring the lining of blood vessels, which may harm the liver. When the team induced blood infections in rodents that lacked either platelets or neutrophils, the animals incurred less liver damage than did normal controls. Given this finding, Kubes speculates that drugs curbing platelet-triggered NET release could limit organ damage from sepsis.

Platelets gone wild are also at the heart of a sepsis complication called disseminated intravascular coagulation. This condition, which is responsible for up to half of sepsis deaths, involves bleeding and widespread clotting within blood vessels. In a 2008 study in Nature Medicine, molecular biologist Jamey Marth of the University of California, Santa Barbara, and colleagues solved a molecular mystery and revealed how the liver tries to combat this dangerous coagulation by removing platelets from circulation.

The researchers showed that some sepsis-causing bacteria release an enzyme called neuraminidase that prunes proteins on the surface of platelets. This modification might allow the bacteria to hook on and ride a platelet to a blood vessel injury, where they can infiltrate the surrounding tissue, Marth says. But the change also signals the body that a massive bacterial invasion is under way. And liver cells, using a protein called the Ashwell-Morell receptor, start grabbing and eliminating platelets sporting the altered proteins. The receptor is part of a last-ditch effort by the body to stop disseminated intravascular coagulation; no one had found a physiological function for this receptor despite 4 decades of study.

Marth and colleagues are hoping that administering drugs that emulate bacterial neuraminidase in the early stages of sepsis could jump-start platelet removal before disseminated intravascular coagulation sets in. They are now investigating that possibility in animals.

Platelet vs. joint

Platelets' Jekyll-and-Hyde nature may also be evident in rheumatoid arthritis (RA), an autoimmune attack on the lining of joints. Researchers have usually attributed the painful and sometimes crippling symptoms of RA to the actions of immune cells such as T cells and B cells. But platelets' facility for triggering inflammation recently inspired rheumatologist David Lee of Harvard Medical School and colleagues to check for an RA connection. The researchers induced a form of RA in mice and then injected some of the animals with an antibody that destroyed 95% of their platelets. The platelet-deficient animals suffered milder joint inflammation than did the other rodents (Science, 29 January, p. 580).

Done in by DNA. Bacteria (orange) caught in a mesh of DNA released by a neutrophil. Platelets that detect microbes can spur neutrophils to release these webs. CREDIT: MAX PLANCK INSTITUTE

However, when the researchers examined fluid from the joints of people with RA, platelets were scarce. Instead, the team detected myriad microparticles, sometimes termed platelet dust. These tiny capsules, discharged by stimulated platelets, brimmed with the inflammatory molecule IL-1.

Lee and colleagues postulate a series of unfortunate events that begins when platelets passing through blood vessels of an already inflamed joint react to cartilage, releasing microparticles. In turn, the IL-1 in the particles prompts cells within the joint to emit molecules that promote further inflammation. Platelets don't instigate RA, they just worsen it, concludes Jerry Ware, a hematologist at the University of Arkansas for Medical Sciences in Little Rock who collaborated with Lee.

The trigger for microparticle release is the receptor glycoprotein VI on the platelet surface, and the researchers speculate that blocking this receptor could deter platelets from unloading their inflammatory cargo. "This is a pathway that has not been previously targeted" in connection with RA, says Lee.

Cancer's little helpers

Cancer, too, seems to get an unfortunate boost from platelets. For example, they might aid tumors by promoting angiogenesis, the formation of new blood vessels. The late Judah Folkman of Harvard Medical School, who championed the importance of angiogenesis for tumor growth, pursued that possibility for a decade. Last year, in a paper that carried Folkman's name as a co-author, some of his former colleagues revealed that cancer cells change the chemical lineup of platelets.

As part of its first-aid kit for injured tissue, a platelet carries molecules that spur or block angiogenesis, packaged into containers called granules. In mice with multiple tumors, platelet granules accumulated more angiogenesis-promoting molecules. "The tumor somehow subverts the platelet into making pro-angiogenic granules," says Italiano, who was one of the study co-authors. The team now hopes to identify the cancer signals that influence platelets.

Besides furnishing new blood vessels for a tumor, platelets might help maintain them, notes vascular biologist Denisa Wagner of Harvard Medical School. A tumor's blood supply is prone to leaks. That's because to immune cells "a tumor is kind of like a wound," she says. They swarm into the newly forming blood vessels and trigger inflammation that can cause the walls to rupture. Four years ago, Wagner, Folkman, and colleagues reported that platelets prevented this vessel deterioration. And in 2008, Wagner and colleagues offered evidence that platelets must release some kind of protective molecule: Platelets that had discharged their chemical contents lost their vessel-saving powers. The identity of the molecular sealant remains a mystery, however.

Cancers also recruit platelets to shield them from the immune system, according to research by tumor biologist Joseph Palumbo of the Cincinnati Children's Hospital Medical Center in Ohio and his colleague Jay Degen. Five years ago, they and their colleagues found that platelets protect tumors from attack by natural killer cells, one of the body's stalwart defenses against cancer. Although this protection wasn't necessary for the original growth to survive, it was essential for the tumor to spread, or metastasize. Palumbo suspects that platelets release a chemical or chemicals that disarm natural killer cells, but they remain unidentified.

Researchers are just starting to try to turn their new knowledge about platelets into medical treatments. And Weyrich says we should expect yet more platelet functions to turn up: "I continue to be surprised by these guys."

Link:  http://www.sciencemag.org/cgi/content/full/328/5978/562
Source:  Science
Author:  Mitch Leslie
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