Bear-ly a clot in sight: Potential thrombosis drug target identified thanks to hibernating bears

Blood clots present something of a paradox. Flights as short as five hours can leave even relatively healthy people with deep vein thrombosis, or blood clots in the extremities. Meanwhile, bears can hibernate for months on end without developing them—and people who are chronically immobilized with spinal cord injuries have the same risk of clotting as those who aren’t. What gives?

A new study provides answers—and, potentially, a path to treating or preventing clots. In the results of an article published April 13 in Science, a research team led by scientists from Denmark’s Aarhus University and Aarhus University Hospital described how studying the blood of bears, humans with chronic immobility and healthy volunteers led them to discover how waxing and waning levels of a protein protect against blood clots in veins (also known as venous thromboembolism, or VTE).

“This protein is the key to a natural mechanism to protect the body against blood clots when it cannot move,” Ole Frøbert, M.D., Ph.D., a cardiologist at Aarhus who co-led the study, said in a press release. “This finding is very exciting as it has the potential to be of great importance for people at risk of developing blood clots due to inactivity.”

The protein at the heart of the study is heat shock protein 47, or HSP47. HSP47 is important to collagen folding and inflammatory processes, during which it presents on the surface of platelets and facilitates connections between clotting proteins and immune cells called neutrophils. But while there was already evidence that HSP47 is involved in VTE, the new study is the first to show that levels of it are directly linked to the reduced risk of blood clots in immobile animals, including humans.

The findings came about over the course of Frøbert and colleagues’ multifaceted research project. It began in the woods of middle Sweden, where the scientists took electrocardiograms of 13 sedated bears during winter hibernation and the following active period in the summer. Their work showed that while the bears were still prone to blood clots when awake, they didn’t develop them while hibernating. This suggested that something was changing in their blood during the winter.

Blood samples taken from the bears during the two periods offered some clues. While there were only slight changes in levels of coagulation factors and blood plasma proteins, levels of platelets and neutrophils were reduced in the winter compared with the summer. The platelets stuck together less, too, even when stimulated by peptides that would cause them to aggregate under normal conditions.

To figure out the mechanism behind the changes, the scientists studied the bears’ blood using a technique called mass spectrometry proteomics, which characterizes proteins based on their weight. While they found several that fluctuated between seasons, HSP47 had the most dramatic shift. Winter levels of HSP47 were 55-fold lower than in the summer, a change 10 times greater than the second most down-regulated protein.

Next, the researchers sought to define exactly how lowered HSP47 levels prevented VTE. After developing a mouse model that was deficient in the protein, they ran experiments designed to stimulate blood clot formation. Thromboembolism frequency and size were lower in experimental mice than in the control group, as was the number of neutrophil extracellular traps, or NETs. NETs are sheets of DNA-histone strands and proteins that form during inflammation and collect microbes, red blood cells and platelets, which become blood clots.

Further experiments using human HSP47 illuminated its role in NET formation. The researchers showed that when HSP47 is presented on platelets, it helps them bind to the blood enzyme thrombin. Thrombin triggers platelet aggregation and the formation of NETs, ultimately leading to blood clots. On top of that, they found that HSP47 also has an independent role in NET formation. When it’s separated from platelets, it binds to neutrophil toll-like receptor 2, which triggers NETs to form.

The team then conducted studies on blood taken from eight people who were immobilized due to chronic spinal cord injuries and compared them with samples from eight non-disabled individuals. They saw a pattern similar to the one they had seen in bears—immobilized participants had lower levels of HSP47 compared to controls.

Two final experiments on people and pigs gave insight into how immobilization affects HSP47 levels. First, healthy, non-disabled volunteers were placed on bed rest for 27 days, with blood samples taken before and after. HSP47 expression dropped dramatically over the period, just as it did in the bears during and post-hibernation. They saw similar results in the pigs: Lactating mother pigs who were prevented from moving for between 21 to 28 days after giving birth had lower levels of HSP47 than controls who could roam freely. One potential caveat: The control pigs hadn’t recently given birth, which could affect HSP47 levels.

The study comes with other limitations, too. None of the human participants, including individuals who were chronically immobilized, had risk factors that predisposed them to VTE. Typically, VTE associated with immobilization happens in patients who have cardiovascular conditions, illness or trauma, as hematology researcher Mirta Schattner, Ph.D., pointed out in a perspective article that accompanied the study in Science. Plus, there are other causes of VTE besides immobility, the researchers noted in their paper.

“We did not investigate the role of HSP47 in other thromboembolic environments such as cancer or coagulation disorders,” the scientists wrote. “Nevertheless, patients at risk of VTE beyond immobilization might benefit from treatments mimicking the here described thromboprotective mechanism.”

Limitations aside, the researchers believe their work could foster new anti-thrombotic drug development as well as new guidelines for treating VTE. Future studies will look at whether some patients spontaneously reduce HSP47 protein levels under certain conditions, such as strokes, which could negate the need for blood thinning medications and their inherent bleeding risk.