A person may develop blood clots after lying down for just a few days, but how come a bear is fine after lying down for half a year?

After feeding themselves well, brown bears will begin hibernation, which can last for several months. After hibernation, the bears will have no major health problems except hunger.

The same bear before hibernation in October (left) and after hibernation in April of the following year (right) | Image source: WSU Bear Center

However, no matter how tired you are, if you try to sleep for such a long time, your body will suffer from blood clots, muscle atrophy, bone loss, bedsores, and other problems. Not to mention lying down for seven months, even sitting in a car or plane for more than ten hours will make your body uncomfortable, and the risk of blood clots will increase accordingly.

So why don't bears worry about blood clots when they lie down for months? How do they do it?

The answer lies in a special protein: Hibernating bears can remain motionless for months without suffering any harm because they produce less of a protein that helps their blood clot, thus preventing the formation of deadly blood clots, a study finds.

Scientists, doctors and veterinarians around the world are exploring hibernation in animals, hoping to use the findings to develop drugs to treat cardiovascular disease and other human diseases. Some space agencies and the military are also investing in research into hibernation to help astronauts better cope with space travel, perform long missions, or treat soldiers who are unable to move after injuries.

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Drawing blood from a brown bear

Although they are studying the secrets of brown bear hibernation, in the summer, the research team begins flying helicopters over Sweden and Scandinavia to look for brown bears.

When a small, agile helicopter landed above the target bear, the brown bear immediately raised its head, looked with its small eyes almost hidden in its hair, and then started running. Under the operation of professionals, the tranquilizer gun was shot into the bear, and it immediately fell asleep . Afterwards, the veterinarian would rush over to check whether the bear was anesthetized, while a few hundred meters away, another group of researchers waited anxiously.

The researchers have little time, as anesthesia lasts about an hour. Once they get the definitive "OK," they kneel around the furry beasts in a predetermined order and begin taking samples. Each sample is collected, processed, and frozen within minutes.

Within an hour, the mission was accomplished and everyone retreated immediately, with one of the veterinarians giving the bear an antidote so it could continue to eat blueberries.

**The most important sample was bear blood. **Because blood cells degenerate quickly outside the body, the researchers shipped centrifuges and other lab equipment from Germany to a country house in Sweden. " If you work with blood and platelets, you have to be very fast ," said Tobias Petzold, a cardiologist on the project.

The research team tracked a dozen brown bears, all of which were equipped with GPS collars, which helped scientists find the bears' caves during the winter. In the forest, the researchers checked the bears' locations using radio direction-finding antennas and maps on electronic devices. Then, a group of researchers from countries including Germany, France, Denmark, Sweden and Norway began to head to the caves where the bears hibernate.

Trekking through the snowy forest to find hibernating bears and draw their blood is no easy task. " Sometimes when we arrive, the bears wake up and run away. Bears can be quite dangerous animals when you crawl into their dens, " said Jon Arnemo, a professor of applied ecology at the Inland Norway University of Applied Sciences who worked on the project.

The bears were successfully anesthetized and pulled out of their dens to collect samples, and then carefully returned to their dens to continue hibernating. The following summer, they tracked the same bears and continued to collect blood samples.

Protein "HSP47" is more abundant in summer and less in winter

The research team had previously found that bear platelets, which are essential for blood clots to form, are less likely to stick together in the winter . Now, by comparing summer and winter samples, the researchers have noticed seasonal differences in the bears' blood that could explain why the bears' platelets are less sticky in the winter.

The answer lies in a protein called HSP47—which is abundant in the bears' blood in the summer but almost disappears in the winter.

This protein appears on the surface of platelets and helps blood cells stick together. When blood clots form in wounds, they stop the body from bleeding and help the wound heal. However, when blood clots in veins and cannot dissolve naturally, it can form a deadly blood clot. In this process, HSP47 on platelets activates neutrophils (a type of white blood cell), causing them to form a "net" that captures proteins, pathogens and cells, leading to blood clots.

Because hibernating bears produce less HSP47, their blood is less likely to form these “webs” and therefore does not clot as easily. | Image source: Reference [3]

“By carefully examining the proteins in (brown bear) platelets, we found that the protein HSP47 was almost absent in the winter. When we deleted the gene that controls the production of the protein HSP47 in mice, we found that the mice were almost unable to form blood clots ,” said Ole Frøbert, a cardiologist at Örebro University Hospital in Sweden, who has studied the brown bears for more than a decade.

"This finding is very exciting because it could be important for people who are at risk for blood clots due to inactivity. "

Image source: Reference [3]

This research result has given researchers great prospects for developing new anti-thrombotic drugs. Venous thromboembolism mainly includes deep vein thrombosis and pulmonary embolism, which is the main cause of human morbidity and mortality. Several drugs have been widely used to prevent thrombosis, but the common feature of these drugs is that they are all associated with the risk of bleeding , which may even be life-threatening in severe cases.

Studying the brown bear's ability to hibernate may not only help us develop drugs to prevent blood clots with fewer side effects, but may also help humans solve the public health problems of obesity and overweight.

Perhaps one day, we will be able to use the secret of bear hibernation to realize space travel as described in science fiction works.

References

[1] Researchers decode the secret of bears in pursuit of new treatment against blood clots. (2023, November 09). Aarhus University. Retrieved from https://biomed.au.dk/display/artikel/researchers-decode-the-secret-of-bears-in-pursuit-of-new-treatment-against-blood-clots

[2] Now we know why hibernating bears don't get blood clots. Retrieved from https://www.science.org/content/article/now-we-know-why-hibernating-bears-don-t-get-blood-clots

[3] Fröbert, O., Frøbert, AM, Kindberg, J., Arnemo, JM, & Overgaard, MT (2020). The brown bear as a translational model for sedentary lifestyle-related diseases. J. Intern. Med., 287(3), 263–270. doi: 10.1111/joim.12983

[4] Grandoni, D. (2023). Learning to sleep like a bear could save your life. Washington Post. Retrieved from https://www.washingtonpost.com/climate-environment/2023/12/23/bear-hibernation-research-health-care

[5] Following the trail of the bear's secrets - English. Retrieved from https://www.inn.no/english/research/research-news/following-the-trail-of-the-bears-secrets

[6] Hibernation | Bear Project. Retrieved from https://www.brownbearproject.com/hibernation

[7] Hibernation of brown bears: clues to protect against thrombosis. (2023, April 14). Ludwig-Maximilians-Universität München, Geschwister-Scholl-Platz 1, 80539 München. Retrieved from https://www.lmu.de/en/newsroom/news-overview/news/hibernation-of-brown-bears-clues-to-protect-against-thrombosis.html

[8] ScienceNordic. (2018, April 10). Retrieved from https://www.sciencenordic.com/animals--plants-denmark-forskerzonen/gut-bacteria-keeps-bears-healthily-obese/1454922

[9] WildSweden - wildlife adventures in Sweden. Retrieved from https://www.wildsweden.com/facts-about-bears-in-sweden

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Editor: Zhong Yanping

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