Glucose and urea metabolic enzymes are differentially phosphorylated during freezing, anoxia, and dehydration exposures in a freeze tolerant frog

Abstract

Vertebrate freeze tolerance requires multiple adaptations underpinned by specialized biochemistry. Freezing of extracellular water leads to intracellular dehydration as pure water is incorporated into growing ice crystals and also results in the cessation of blood supply to tissues, creating an anoxic cellular environment. Hence, the freeze tolerant wood frog, Rana sylvatica, must endure both dehydration and anoxia stresses in addition to freezing. The metabolic responses to freezing, dehydration and anoxia involve both protein/enzyme adaptations and the production of metabolites with metabolic or osmotic functions, particularly glucose and urea. The present study uses a phosphoproteome analysis to examine the differential phosphorylation of metabolic enzymes involved in the production of these two metabolites in liver in response to freezing, anoxia, or dehydration exposures. Our results show stress-specific responses in the abundance of phosphopeptides retrieved from nine glycolytic enzymes and three urea cycle enzymes in liver of wood frogs exposed to 24 h freezing, 24 h anoxia, or dehydration to 40% of total body water loss, as compared with 5 °C acclimated controls. Data show changes in the abundance of phosphopeptides belonging to glycogen phosphorylase (GP) and phosphofructokinase 2 (PFK2) that were consistent with differential phosphorylation control of glycogenolysis and a metabolic block at PFK1 that can facilitate glucose synthesis as the cryoprotectant during freezing. Anoxia-exposed animals showed similar changes in GP phosphorylation but no changes to PFK2; changes that would facilitate mobilization of glycogen as a fermentative fuel for anaerobic glycolysis. Urea is commonly produced as a compatible osmolyte in response to amphibian dehydration. Selected urea cycle enzymes showed small changes in phosphopeptide abundance in response to dehydration, but during freezing differential phosphorylation occurred that may facilitate this ATP expensive process when energy resources are sparse. These results add to the growing body of literature demonstrating the importance and efficiency of reversible protein phosphorylation as a regulatory mechanism allowing animals to rapidly respond to environmental stress.