Transcription factor regulation in mammalian hibernation: the thirteen-lined ground squirrel as a model.
The thirteen-lined ground squirrel (Spermophilus tridecemlineatus) is one of a number of mammalian species that hibernate over the winter months. Hibernation is characterized by long periods of deep torpor that are interspersed with periodic arousals. This thesis investigated three transcription factor pathways and their regulation and roles in hibernation using tools including immunoblotting of protein and phospho-protein levels, analysis of nuclear versus cytoplasmic distribution, binding to DNA, PCR to quantify mRNA transcripts, and gene sequencing. The carbohydrate response element-binding protein (ChREBP) regulates genes that are responsible for converting excess dietary carbohydrate (glucose) into triglycerides. During torpor ChREBP protein was decreased in kidney and liver and transcripts of two downstream genes (Fasn, PK-L) were differentially expressed; this was consistent with carbohydrate sparing during torpor. Growth and differentiation processes are also minimized during hibernation and ETS transcription factors are important regulators of these. Although levels of ETS increased in most organs of hibernators, ETS binding activity to DNA was greatly reduced indicating a growth suppressive function during torpor. Upstream effectors of ETS (epidermal growth factor receptor, protein kinases C and D) were also evaluated. To assess transcriptional state during hibernation, the Rb-E2F pathway was investigated. Retinoblastoma protein (Rb) associates with E2F1 and then recruits co-factors to the promoter region of target genes to suppress transcription. Rb and E2F1 proteins were elevated during hibernation and the co-factors HDAC and SUV39H1 showed parallel increases in the organs tested; oppositely, transcription of downstream genes (cMyc, Bcl2) was suppressed. These data confirm a reduced transcriptional state during torpor.