Seasonally hibernating phenotype assessed through transcript screening

Abstract

Hibernation is a seasonally entrained and profound phenotypic transition to conserve energy in winter. It involves significant biochemical reprogramming, although our understanding of the underpinning molecular events is fragmentary and selective. We have conducted a large-scale gene expression screen of the golden-mantled ground squirrel, Spermophilus lateralis, to identify transcriptional responses associated specifically with the summer-winter transition and the torpid-arousal transition in winter. We used 112 cDNA microarrays comprising 12,288 probes that cover at least 5,109 genes. In liver, the profiles of torpid and active states in the winter were almost identical, although we identified 102 cDNAs that were differentially expressed between winter and summer, 90% of which were downregulated in the winter states. By contrast, in cardiac tissue, 59 and 115 cDNAs were elevated in interbout arousal and torpor, respectively, relative to the summer active condition, but only 7 were common to both winter states, and during arousal none was downregulated. In brain, 78 cDNAs were found to change in winter, 44 of which were upregulated. Thus transcriptional changes associated with hibernation are qualitatively modest and, since these changes are generally less than twofold, also quantitatively modest. Unbiased Gene Ontology profiling of the transcripts suggests a winter switch to ␤-oxidation of lipids in liver and heart, a reduction in metabolism of toxic compounds and the urea cycle in liver, and downregulated electron transport in the brain. We identified just one strongly winter-induced transcript common to all tissues, namely an RNA-binding protein, RBM3. This analysis clearly differentiates responses of the principal tissues, identifies a large number of new genes undergoing regulation, and broadens our understanding of affected cellular processes that, in part, account for the winter-adaptive hibernating phenotype. hibernation; torpor; microarray HIBERNATION IS AN ADAPTIVE phenotype used by a wide range of mammalian species to conserve energy during the inclement months of winter. Seasonal hibernators, such as the goldenmantled ground squirrel, Spermophilus lateralis, cycle annually between a period of growth, fattening, and reproduction in summer and a period of minimal activity and fasting in winter.

Elevated expression of protein biosynthesis genes in liver and muscle of hibernating black bears (Ursus americanus)

“…Thus, transcriptional changes associated with winter hibernation of black bears are qualitatively and quantitatively modest. This finding is generally consistent with results of the genomicscale study on differential gene expression in ground squirrels (42). However, our study reveals two major differences between the changes in hibernating transcriptomes of black bears and ground squirrel hibernators.…”

“…However, our study reveals two major differences between the changes in hibernating transcriptomes of black bears and ground squirrel hibernators. The prominent feature of the transcriptional changes in small hibernators is a reduction in transcript level; 90% of differentially expressed genes were downregulated in liver of ground squirrels during winter (42). In contrast, the majority of differentially expressed genes (67%) were upregulated in livers of hibernating black bears.…”

“…This is a notable observation because Rbm3 is the only gene consistently upregulated across different tissues in hibernating ground squirrels (42,43,44). It was suggested that this coldinduced RNA binding protein may facilitate the processing of RNA molecules in the cold and protect mRNA transcripts in hibernating ground squirrels (42,44).…”

“…In contrast to single-gene expression analysis, the genome-wide approach also allows for identification of coordinated transcriptional changes in functional groups of regulatory genes within metabolic and signaling pathways. Recent studies of differential gene expression at the genomic scale on several species of hibernating small mammals have detected expression changes for hundreds of genes when comparing animals sampled in different stages during hibernation and nonhibernating periods (5,42,43,44). Coordinated transcriptional changes associated with hibernation have identified groups of co-regulated genes involved in carbohydrate, fatty acid and protein metabolism, detoxification, and molecular transport.…”

“…We want to identify changes in functional groups of co-regulated genes to obtain insight into the biological significance of transcriptional changes associated with the hibernating phenotype in black bears. To reveal general and species-specific trends in transcriptional changes during hibernation, we compare differentially expressed genes detected in hibernating bears to transcriptional changes that have been recently reported for small mammalian hibernators (42)(43)(44).…”