Seasonal variation and freezing response of glucose transporter 2 in liver of the wood frog: implications for geographic variation in freeze tolerance

Subarctic populations of the wood frog Rana sylvatica survive freezing to temperatures at least 10–13°C below those of more southerly conspecifics. This profound freeze tolerance is due in part to an enhanced glucosic cryoprotectant system that requires rapid mobilization of glucose from hepatocytes during the early hours of freezing. To determine if glucose transporter 2 (GLUT2) in the liver contributes to geographic variation in freeze tolerance, we examined changes in the protein’s abundance seasonally and in response to experimental freezing in frogs from Interior Alaska and southern Ohio, USA. Using immunoblotting techniques, we found that GLUT2 abundance increased in preparation for winter in both populations, but tests with Ohioan frogs showed that that altered temperature alone does not cause these seasonal changes. In Ohioan frogs, transporter expression apparently was regulated transcriptionally, as mRNA levels, assessed using quantitative real-time polymerase chain reaction, changed in accordance with protein abundance. However, this pattern was not observed in Alaskan frogs, suggesting that other mechanisms of regulation are important in this phenotype. Overall, GLUT2 abundance was constitutively greater and more responsive to freezing in Alaskan R. sylvatica, suggesting that GLUT2 contributes substantively to the extreme freeze tolerance of subarctic wood frogs.