Molecular biology of freezing tolerance

Winter survival for many kinds of animals involves freeze tolerance, the ability to endure the conversion of about 65% of total body water into extracellular ice and the consequences that freezing imposes including interruption of vital processes (e.g., heartbeat and breathing), cell shrinkage, elevated osmolality, anoxia/ischemia, and potential physical damage from ice. Freeze-tolerant animals include various terrestrially hibernating amphibians and reptiles, many species of insects, and numerous other invertebrates inhabiting both terrestrial and intertidal environments. Well-known strategies of freezing survival include accumulation of low molecular mass carbohydrate cryoprotectants (e.g., glycerol), use of ice nucleating agents/proteins for controlled triggering of ice growth and of antifreeze proteins that inhibit ice recrystallization, and good tolerance of anoxia and dehydration. The present article focuses on more recent advances in our knowledge of the genes and proteins that support freeze tolerance and the metabolic regulatory mechanisms involved. Important roles have been identified for aquaporins and transmembrane channels that move cryoprotectants, heat shock proteins and other chaperones, antioxidant defenses, and metabolic rate depression. Genome and proteome screening has revealed many new potential targets that respond to freezing, in particular implicating cytoskeleton remodeling as a necessary facet of low temperature and/or cell volume adaptation. Key regulatory mechanisms include reversible phosphorylation control of metabolic enzymes and microRNA control of gene transcript expression. These help to remodel metabolism to preserve core functions while suppressing energy expensive metabolic activities such as the cell cycle. All of these advances are providing a much more complete picture of life in the frozen state.