Carb-Loading: Freeze-Induced Activation of the Glucose-Responsive ChREBP Transcriptional Network in Wood Frogs.

The freeze-tolerant wood frog, Rana sylvatica, is one of few vertebrate organisms that can tolerate freezing, with up to 70% of its total body water being converted into extracellular ice. Physiologically, wood frogs show no signs of muscle movement, breathing, heartbeat, or brain activity for several weeks or months at a time but emerge unharmed upon thawing. Given that wood frogs rely mainly on carbohydrate metabolism during freezing, the involvement of the carbohydrate-responsive element-binding protein (ChREBP) in response to freezing is of interest. In liver tissue, protein and transcript levels of ChREBP increased by 1.4±0.09-fold and 1.9±0.26-fold, respectively, and nuclear distribution and DNA-binding activity rose by 2.0±0.08-fold and 1.5±0.08-fold, respectively. This enhanced transcriptional activity of ChREBP was corroborated by increased transcript expression of select downstream genes of fatty acid synthase, pyruvate kinase, and stearoyl-CoA desaturase in liver tissue. While liver tissue displayed ChREBP activation, in muscle tissue, ChREBP protein levels, DNA-binding activity, and downstream gene targets were generally found to decrease or to remain unchanged during freezing. Overall, our results demonstrate that ChREBP regulates metabolism in a tissue-dependent manner during freezing, when its activity is required for liver tissue but not for skeletal muscle tissue in wood frogs.