Peripheral circadian gene activity is altered during hibernation in the thirteen-lined ground squirrel

Many small mammals living in seasonally cold environments rely on hibernation, utilizing strong metabolic rate suppression and a slow consumption of adipose reserves to survive the winter months. The circannual rhythm of hibernation is well known but less is known about the role of the circadian clock while animals are in torpor for weeks at a time. We hypothesized that due to strong global suppression of transcription and translation in the torpid state, that circadian clock activity would likewise be suppressed in peripheral tissues during hibernation. However, the present study indicates that peripheral circadian clock activity persists during torpor. Using 13-lined ground squirrels (Ictidomys tridecemlineatus) as the model, this study analyzed transcript and protein responses by clock components, comparing euthermic control animals with squirrels in deep torpor for >3 days (subcutaneous body temperature 5-8 °C). The data show tissue specific responses by mRNA transcript levels: (a) no significant changes in transcript abundance in liver of control versus torpid squirrels, (b) a strong increase in Nr1d1 levels in white adipose during torpor, and (c) five significant transcript changes in skeletal muscle during torpor (increased Bmal1, Clock, Cry1 and Nr1d1 but decreased Per1). Levels of core clock proteins (BMAL1, CRY2, PER2, and casein kinases CK1δ and CK1ε) were also assessed across five time points of the torpor/arousal cycle showing both tissue- and time-dependent changes in clock proteins that were most prominent in liver and white adipose and indicating that peripheral clocks are still active in tissues over the torpor/arousal cycle.