Hibernating mammals are amazing! How in the world do they prevent blood clots when they not only reduce their body temperature but also their blood flow during hibernation? That question is precisely what a new study published in Physiological Genomics examined.
Typically, cold temperatures are known to activate the process of blood clotting by stimulating platelets. Platelets are sticky cells without a nucleus that circulate in the blood. When a blood vessel wall is injured, they stick to the wall and group together to form a blood clot to prevent blood loss. Similarly, in cold exposure, platelets stick together to form clots. Remarkably, the body temperature of 13-lined ground squirrels, Ictidomys tridecemlineatus, drops to as low as 4-8°C during hibernation and their heart rate slows to only 3-5 beats per minute. These torpor bouts are regularly interrupted by periods of arousal during which their body temperatures increase to 35-38°C and their heart rate climbs to 200-300 beats per minute. Yet, they somehow avoid forming blood clots during hibernation. To understand how they do it, researchers compared platelet proteins from ground squirrels during the summer (when the animals are active), fall (when they begin to enter hibernation), and in the winter (during hibernation).
During torpor, the amount of circulating platelets decreases by about 90% in ground squirrels and the platelets are resistant to cold-induced damage. In contrast, human platelets need to be stored at a relatively warm 22°C to avoid clotting. That means they are only stored for 3-4 days to avoid risk of bacterial contamination. In comparing platelets from ground squirrels to humans, the researchers noted that while the platelets were similar, several differences in protein composition may account for resistance to blood clot formation in ground squirrels. Among other differences, they noted that platelets from hibernating ground squirrels had decreases in 6 proteins that are involved in activating platelets. These findings may help researchers identify ways to better preserve human platelets for transfusions.
S Cooper, PA Wilmarth, JM Cunliffe, J Klimek, J Pang, ST Yunga, J Minnier, A Reddy, L David, JE Aslan. Platelet proteome dynamics in hibernating 13-lined ground squirrels. Physiological Genomics. 53(11): 473-485, 2021.