Abstract
Hibernation is a natural adaptation that allows certain mammals to survive physiological extremes that are lethal to humans. Near freezing body temperatures, heart rates of 3–10 beats per minute, absence of food consumption, and depressed metabolism are characteristic of hibernation torpor bouts that are periodically interrupted by brief interbout arousals (IBAs). The molecular basis of torpor induction is unknown, however starved mice overexpressing the metabolic hormone fibroblast growth factor 21 (FGF21) promote fat utilization, reduce body temperature, and readily enter torpor–all hallmarks of mammalian hibernation. In this study we cloned FGF21 from the naturally hibernating thirteen-lined ground squirrel (Ictidomys tridecemlineatus) and found that levels of FGF21 mRNA in liver and FGF21 protein in serum are elevated during hibernation torpor bouts and significantly elevated during IBAs compared to summer active animals. The effects of artificially elevating circulating FGF21 concentrations 50 to 100-fold via adenoviral-mediated overexpression were examined at three different times of the year. This is the first time that a transgenic approach has been used in a natural hibernator to examine mechanistic aspects of hibernation. Surgically implanted transmitters measured various metrics of the hibernation phenotype over a 7-day period including changes in motor activity, heart rate and core body temperature. In April fed-state animals, FGF21 overexpression decreased blood insulin and free fatty acid concentrations, effects similar to those seen in obese mice. However, elevated FGF21 concentrations did not cause torpor in these fed-state animals nor did they cause torpor or affect metabolic parameters in fasted-state animals in March/April, August or October. We conclude that FGF21 is strongly regulated during torpor and IBA but that its overexpression is not sufficient to cause torpor in naturally hibernating ground squirrels.
Highlights
Hibernation is characterized by physiological changes that cause certain mammals to enter a state of suspended animation called torpor
Thirteen-lined Ground Squirrel fibroblast growth factor 21 (FGF21) Sequence The thirteen-lined ground squirrel FGF21 sequence was isolated by low stringency PCR followed by 39 and 59 RACE using primers derived from conserved FGF21 sequences seen in multiple species (Figure 1A)
We studied FGF21 function in a model hibernating mammal, the thirteen-lined ground squirrel (Ictidomys tridecemlineatus)
Summary
Hibernation is characterized by physiological changes that cause certain mammals to enter a state of suspended animation called torpor. In small hibernators such as ground squirrels torpor is characterized by heart rate rates of 3–10 beats per minute, body temperatures of 4–7uC, and oxygen consumption that is approximately 2% of the active state (reviewed in [1]). In this paper we examine a member of the fibroblast growth factor (FGF) family of proteins, fibroblast growth factor 21 (FGF21), as a potential regulator of hibernation in the thirteenlined ground squirrel (Ictidomys tridecemlinatus). FGF21 binds fibroblast growth factor receptors (FGFR) -1c, -2c, and -3c, with the strongest interactions with FGFR-1c in the presence of another protein, bKlotho [14,15,16,17]. FGF21 binds fibroblast growth factor receptors (FGFR) -1c, -2c, and -3c, with the strongest interactions with FGFR-1c in the presence of another protein, bKlotho [14,15,16,17]. bKlotho is a necessary component for FGF21 binding to the FGF receptors and is required for downstream signaling [15,16,17,18]
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