“Many people assumed that as bears go through hyperphagia, fat just sort of accumulates and sits there as a fuel reservoir,” explained Jansen. “In fact, our studies have shown that the fatty tissue is far from inert. Fat is actually very metabolically active being driven by the expression of over 1000 unique genes in fat during hibernation as compared to the level of expression seen during the normal seasonal activities.
While other studies in other species have looked at gene expression in tissues before and during hibernation, the work until now has never been done with grizzlies. The WSU results while somewhat expected, far exceeded the level of differing genetic expression seen before.
For centuries, people have been fascinated with various species known to hibernate. Science fiction writers describe fantastic space journeys and hibernation states employed with humans. Medically-induced comas in humans get them past extraordinary traumatic or disease states, organs are cooled for storage and transport, and scientists continue to wonder if hibernation could be induced as a therapeutic tool. A wide variety of species have been studied, including those that ‘hibernate’ in the warmer months and ones that hibernate in winter and whose body temperature can sometimes drop to near freezing. But not bears. Bears appear to be more like humans.
With grizzlies, the observable details of hibernation are astounding. During hibernation for nearly five months, grizzlies maintain only a slightly lowered body temperature and essentially do not eat, urinate, or defecate. They do however, give birth and produce milk and all the while they do not lose significant bone or muscle mass. In metabolic terms, during hibernation they are the ultimate recyclers of the waste products that mammals usually have to eliminate or suffer with toxicity from their buildup.
“What fat’s entire role is in the overall hyperphagia and hibernation process remains an exciting area to continue to explore.”
Differential gene expression also means genes may be upregulated or downregulated depending on the gene, sort of like a panel of light switches being on or off. Of the genes expressed in fatty tissue, more than 2000 were upregulated and about 1800 were downregulated in hibernation compared to the active season.
Fat is the tissue that fuels hibernation and probably orchestrates the sparing of other tissues. But, despite the calorie intake and fat accumulation, bears do not suffer the same negative effects people do. Furthermore, they reverse the process by switching genes on and off based upon the season.
Jansen went on to explain that during the active period and subsequent hyperphagia, the genetic expression varied among the tissues studied. While many genes in fat were being differentially expressed, there were no genes being expressed like that in muscle tissue and only three were expressed differentially in liver tissue.
Among the discoveries was that all three tissues studied had dynamic gene expression changes occurring during hibernation. Perhaps more importantly, they discovered there was a subset of the same genes in all three tissues making the same changes at the same time.
New RNA sequencing-based genetic research shows grizzlies express a larger number of genes in preparation for, and during hibernation to cope with such stressors, than do any other species studied.