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For certain mammals sleeping is the only way to cope with winter's cold. And although this hibernation strategy is common, it isn't trivial. Before descending into a long winter's nap, a mammal's body temperature falls to near freezing. During hibernation, the creature periodically wakes up, warms up and sinks back into its chilly slumber. What scientists haven't been able to understand is exactly how the animal's body sustains such rapid temperature shifts without injury: cell membranes are generally susceptible to cold damage, which causes some of their components to separate. A paper published today in the journal Nature sheds light on the matter.
Researchers at the National Institutes of Health took tissue samples from hibernating ground squirrels during both cold slumbers and warm awakenings to investigate structural changes inside cells from their central nervous systems. As it turned out, light and electron microscopy revealed a curious difference. When the squirrels were cold, their cells showed little slits; when warm, they didn't. The slits, it appears, correspond to a reorganization of the internal cell membrane's lipid and protein components. In the cold, proteins are relocated away from the membrane's saturated fatty acids, which congeal, moving instead to areas with unsaturated fatty acids, which remain liquid. In this way, the proteins can continue to perform their life-sustaining activities. Understanding this reversible, temperature-induced rearrangement, the authors say, "should help in the study of the effects of severe cold on non-hibernating species (including humans) and in the cryopreservation of cells."