Abstract
Seasonal life-history events, such as migration, hibernation, and reproduction, depend on coordinated physiological changes. In vertebrates, a conserved thyroid hormone-signaling pathway in the hypothalamus is known to trigger many of these seasonal transitions. However, the broader processes and regulators modulating seasonal physiology are poorly defined. Recent research in Arctic ground squirrels (AGS, Urocitellus parryii) revealed that hypothalamic thyroid hormone signaling is activated, and markers of tanycytic remodeling are expressed in late hibernation in anticipation of springtime reproduction. We conducted RNA-sequencing on hypothalamic micropunches encompassing the arcuate nucleus, median eminence, pars tuberalis, and third ventricle in male and female AGS at early and late hibernation. We found substantial sex differences in the hypothalamic transcriptome across hibernation. Functional enrichment analysis of gene expression data revealed an upregulation of processes and pathways related to hormone transport and neurogenesis in females, whereas this was less apparent in males. Transcription factor binding site analysis of differentially expressed genes identified upstream regulators involved in glial cell differentiation, neuronal development, survival, and plasticity. Notably, many of the intersecting genes from these analyses were localized to specialized glial cells (tanycytes) lining the floor and walls of the third ventricle. Our findings support a model in which annual changes in gene expression rely on a progressive remodeling of tanycytes across hibernation. This remodeling may contribute to seasonal changes in neuronal plasticity and function of the hypothalamus, priming the brain in anticipation of shifting physiological demands upon hibernation termination.NEW & NOTEWORTHY We examine how the transcriptome of hypothalamic micropunches changes across the hibernation season. Our analyses uncover sex-specific changes to regulatory processes associated with hormone transport and neurogenesis. Genes linked to these processes and regulators are strongly localized to third ventricle tanycytes, consistent with the key role these cells play in regulating seasonal physiological changes. Our study supports that using sex as a biological variable is essential for understanding the mechanisms underlying seasonal life-history transitions.