Molecular Physiology of Freeze Tolerance in Vertebrates

Abstract

Freeze tolerance is an amazing winter survival strategy used by various amphibians and reptiles living in seasonally cold environments. These animals may spend weeks or months with up to ∼65% of their total body water frozen as extracellular ice and no physiological vital signs, and yet after thawing they return to normal life within a few hours. Two main principles of animal freeze tolerance have received much attention: the production of high concentrations of organic osmolytes (glucose, glycerol, urea among amphibians) that protect the intracellular environment, and the control of ice within the body (the first putative ice-binding protein in a frog was recently identified), but many other strategies of biochemical adaptation also contribute to freezing survival. Discussed herein are recent advances in our understanding of amphibian and reptile freeze tolerance with a focus on cell preservation strategies (chaperones, antioxidants, damage defense mechanisms), membrane transporters for water and cryoprotectants, energy metabolism, gene/protein adaptations, and the regulatory control of freeze-responsive hypometabolism at multiple levels (epigenetic regulation of DNA, microRNA action, cell signaling and transcription factor regulation, cell cycle control, and anti-apoptosis). All are providing a much more complete picture of life in the frozen state.

The cryoprotectant system of Cope’s gray treefrog, Dryophytes chrysoscelis: responses to cold acclimation, freezing, and thawing

“…The decrease in hepatic glucose concentration in thawed frogs suggests glucose is being converted back into hepatic glycogen (Costanzo and Lee 2013; Storey and Storey 2017). Elevated insulin levels during thawing possibly facilitate hepatic glycogenesis (Hemmings and Storey 1996), but likely contribute to the elevated glycerol levels observed in liver and plasma of thawed frogs, as this hormone inhibits both hepatic gluconeogenesis (Edgerton et al 2009), and aquaglyceroporin expression in hepatic tissue (Lebeck 2014).…”

“…One of the strategies that contributes to freezing survival is the accumulation of cryoprotectants: low molecular weight compounds that colligatively reduce ice formation, may serve as energetic substrates, and possibly stabilize proteins and membranes (Costanzo and Lee 2013; Storey and Storey 2017). …”

Advances in Comparative Immunology

“…Immune competence declined in two species of freshwater fish of the Iberian Peninsula (Jesus et al 2017). Hibernation and other survival strategies by poikilothermic animals (Storey and Storey 2017) depend heavily on ambient temperatures, which are likely to increase by global warming.…”

In defense of proteins: Chaperones respond to freezing, anoxia, or dehydration stress in tissues of freeze tolerant wood frogs

“…These North American frogs range from the eastern deciduous forests of the Appalachian region right across the northern boreal forests from Labrador to Alaska. Spending the winter on the forest floor, they freeze for weeks or months at a time showing no vital signs and with 65–70% of their body water converted into extracellular ice (Larson et al, ; Storey & Storey, ). Yet, come early spring, they thaw, regain all physiological functions, and move to ephemeral snow‐melt ponds on the forest floor for a quick few nights of chorusing and breeding before dispersing again into the forest.…”