Natural freezing survival by wood frogs (Rana sylvatica) involves multiple organ-specific changes in gene expression. Screening of a cDNA library made from brain of frozen frogs revealed freeze-responsive up-regulation of the glycolytic enzyme, phosphoglycerate kinase 1 (PGK1). Northern blots showed an approximately two-fold increase in pgk1 transcripts in brain of frozen frogs whereas PGK1 protein levels rose by three- to five-fold within 4-8 hr of freezing. Freezing also elevated pgk1 transcripts in liver but not in skin. Both transcript and protein levels also rose in response to two of the component stresses of freezing (anoxia and dehydration) with a particularly pronounced (11-fold) increase in PGK1 protein in brain in response to anoxia. Amino acid sequence analysis showed 92.5% identity between wood frog and Xenopus laevis PGK1 and 86-88% identity with the zebrafish, chicken, and human protein. Four unique amino acid substitutions in the wood frog protein could be important in maintaining the functional conformation of the wood frog protein at low body temperatures. Elevated amounts of PGK1, one of the ATP-generating reactions of glycolysis, in wood frog brain during freezing would enhance the glycolytic capacity of the organ and support the maintenance of cellular energetics under the ischemic conditions of the frozen state.

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Journal Journal of Experimental Zoology Part A: Ecological Genetics and Physiology
Wu, S. (Shaobo), Storey, J, & Storey, K. (2009). Phosphoglycerate kinase 1 expression responds to freezing, anoxia, and dehydration stresses in the freeze tolerant wood frog, Rana sylvatica. Journal of Experimental Zoology Part A: Ecological Genetics and Physiology, 311(1), 57–67. doi:10.1002/jez.495