Adrenal gene expression dynamics support hibernation in 13-lined ground squirrels

Hibernation is a natural model of extreme physiology in a mammal. Throughout winter, small hibernators repeatedly undergo rapid, dramatic swings in body temperature, perfusion, and oxygen delivery. To gain insight into the molecular mechanisms that support homeostasis despite the numerous challenges posed by this dynamic physiology, we collected 13-lined ground squirrel adrenal glands from at least five individuals representing six key timepoints across the year using body temperature telemetry. Differentially expressed genes were identified using RNA-seq, revealing both strong seasonal and torpor-arousal cycle effects on gene expression. Two novel findings emerge from this study. First, transcripts encoding multiple genes involved in steroidogenesis decreased seasonally. Taken together with morphometric analyses, the data are consistent with preservation of mineralocorticoids but suppression of glucocorticoid and androgen output throughout winter hibernation. Second, a temporally orchestrated, serial gene expression program unfolds across the brief arousal periods. This program initiates during early rewarming with the transient activation of a set of immediate early response (IER) genes, comprised of both transcription factors and the RNA degradation proteins that assure their rapid turnover. This pulse in turn activates a cellular stress response program to restore proteostasis comprised of protein turnover, synthesis, and folding machinery. These and other data support a general model for gene expression across the torpor-arousal cycle that is facilitated in synchrony with whole body temperature shifts; induction of the immediate early response upon rewarming activates a proteostasis program followed by a restored tissue-specific gene expression profile enabling renewal, repair, and survival of the torpid state.NEW & NOTEWORTHY This pioneer study of adrenal gland gene expression dynamics in hibernating ground squirrels leverages the power of RNA-seq on multiple precisely timed samples to demonstrate: 1) steroidogenesis is seasonally reorganized to preserve aldosterone at the expense of glucocorticoids and androgens throughout winter hibernation; 2) a serial gene expression program unfolds during each short arousal whereby immediate early response genes induce the gene expression machinery that restores proteostasis and the cell-specific expression profile before torpor reentry.