Living in the cold: Freeze-induced gene responses in freeze-tolerant vertebrates

1. Winter survival for numerous cold-blooded animals includes freeze tolera nce: the ability to endure the conversion of as much as 65% of total body w ater into extracellular ice. Selected molecular adaptations underlying free ze tolerance(e.g. cryoprotectants, ice nucleating proteins) have been widel y studied, but the full range of metabolic adjustments needed for freeze en durance remains unknown. 2. Recent studies using gene screening techniques are providing a different approach to the search for biochemical responses that support freezing sur vival by identifying genes and proteins that are up-regulated by freezing o r thawing in freeze-tolerant amphibians and reptiles. 3. Screening of a cDNA library from wood frog liver revealed the freeze-ind uced up-regulation of genes coding for the alpha- and gamma-subunits of fib rinogen (a plasma clotting protein), the mitochondrial ADP/ATP translocase and a novel 10 kDa protein containing a nuclear exporting sequence. 4. Northern blotting revealed that these genes were differentially responsi ve to two of the component stresses of freezing (dehydration and anoxia), i ndicating that different genes are induced by signals radiating either from cell volume change or oxygen deprivation during freezing. 5. Freeze up-regulation of fibrinogen synthesis in liver and other organs a ppears to be a damage repair response that anticipates a need for enhanced plasma clotting capacity to deal with ice crystal damage to capillary beds. 6. Up-regulation of ADP/ATP translocase in frog liver is linked with ischae mia resistance and studies with freeze-tolerant turtles have shown that oth er genes encoding proteins involved in mitochondrial energetics (NADH-ubiqu inone oxidoreductase subunit 5, cytochrome C oxidase subunit 1) are also up -regulated by both anoxia and freezing exposures. 7. These studies are making major advances in our understanding of freeze t olerance as a natural phenomenon and also highlight new key areas that can be targeted by applied interventions for the optimization of medical cryopr eservation techniques for cells, tissues and organs.