Strategies for exploration of freeze responsive gene expression: advances in vertebrate freeze tolerance.

Winter survival for many cold-blooded species involves freeze tolerance, the capacity to endure the freezing of a high percentage of total body water as extracellular ice. The wood frog (Rana sylvatica) is the primary model animal used for studies of vertebrate freeze tolerance and current studies in my lab are focused on the freeze-induced changes in gene expression that support freezing survival. Using cDNA library screening, we have documented the freeze-induced up-regulation of a number of genes in wood frogs including both identifiable genes (fibrinogen, ATP/ADP translocase, and mitochondrial inorganic phosphate carrier) and novel proteins (FR10, FR47, and Li16). All three novel proteins share in common the presence of hydrophobic regions that may indicate that they have an association with membranes, but apart from that each shows unique tissue distribution patterns, stimulation by different signal transduction pathways and responses to two of the component stresses of freezing, anoxia, and dehydration. The new application of cDNA array screening technology is opening up a whole new world of possibilities in the search for molecular mechanisms that underlie freezing survival. Array screening of hearts from control versus frozen frogs hints at the up-regulation of adenosine receptor signaling for the possible mediation of metabolic rate suppression, hypoxia inducible factor mediated adjustments of anaerobic metabolism, natriuretic peptide regulation of fluid dynamics, enhanced glucose transporter capacity for cryoprotectant accumulation, defenses against the accumulation of advanced glycation end products, and improved antioxidant defenses as novel parts of natural freeze tolerance that remain to be explored.