Abstract
The wood frog, Rana sylvatica, is the best-studied of a small group of amphibian species that survive whole body freezing during the winter months. These frogs endure the freezing of 65–70% of their total body water in extracellular ice masses. They have implemented multiple adaptations that manage ice formation, deal with freeze-induced ischemia/reperfusion stress, limit cell volume reduction with the production of small molecule cryoprotectants (glucose, urea) and adjust a wide variety of metabolic pathways for prolonged life in a frozen state. All organs, tissues, cells and intracellular organelles are affected by freeze/thaw and its consequences. This article explores mitochondria in the frozen frog with a focus on both the consequences of freezing (e.g., anoxia/ischemia, cell volume reduction) and mitigating defenses (e.g., antioxidants, chaperone proteins, upregulation of mitochondria-encoded genes, enzyme regulation, etc.) in order to identify adaptive strategies that defend and adapt mitochondria in animals that can be frozen for six months or more every year. A particular focus is placed on freeze-responsive genes in wood frogs that are encoded on the mitochondrial genome including ATP6/8, ND4 and 16S RNA. These were strongly up-regulated during whole body freezing (24 h at −2.5 °C) in the liver and brain but showed opposing responses to two component stresses: strong upregulation in response to anoxia but no response to dehydration stress. This indicates that freeze-responsive upregulation of mitochondria-encoded genes is triggered by declining oxygen and likely has an adaptive function in supporting cellular energetics under indeterminate lengths of whole body freezing.
Highlights
Every species on Earth is endowed with a set of biochemical and physiological strategies that can optimize their survival and reproductive capacity in the face of biotic and abiotic stresses
The action of another transcription factor involved in antioxidant defense, Nrf2, responded to whole body freezing in hatchling painted turtles (Chrysemys picta marginata)
These results indicate that urea cycle activity is maintained and differentially regulated during freezing and that carbamoyl phosphate synthetase 1 (CPS1) activation during freezing has an important contribution to make to cryoprotection by increasing the osmotic resistance against cell water loss into extracellular ice
Summary
Every species on Earth is endowed with a set of biochemical and physiological strategies that can optimize their survival and reproductive capacity in the face of biotic and abiotic stresses. One of the most amazing survival strategies in the animal kingdom is freeze tolerance, the capacity of numerous invertebrate and a few vertebrate species to withstand ice formation within extracellular fluid spaces of their bodies while resisting intracellular freezing. This is a crucial adaptation for winter survival of many terrestrial ectotherms living in seasonally cold climates [1,2]. Antioxidants 2021, 10, 543 due to increased reactive oxygen species (ROS) generation can be high during thawing when breathing and heartbeat resume and tissues undergo reperfusion and renewed oxygen uptake [4,5]. The present article explores the role of antioxidants and the adaptive responses of mitochondria to whole body freezing, summarizing research carried out on the primary vertebrate model of freeze tolerance, the wood frog, Rana sylvatica
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
