Repeated freeze-thaw cycles in freeze-tolerant treefrogs: novel interindividual variation of integrative biochemical, cellular, and organismal responses

The freeze-tolerant anuran Dryophytes chrysoscelis, Cope's gray treefrog, mobilizes a complex cryoprotectant system that includes glycerol, glucose, and urea to minimize damage induced by freezing and thawing of up to 65% of body water. In this species' eastern Northern American temperate habitat, oscillations of temperature above and below freezing are common; however, the effects of repeated freezing and thawing in this species are unstudied. The biochemical and physiological effects of repeated freeze-thaw cycles were therefore evaluated and compared with cold acclimation and single freeze-thaw episodes. Glycerol was elevated in plasma, liver, and skeletal muscle of both singly and repeatedly frozen and thawed animals compared with cold-acclimated frogs. In contrast, urea was unchanged by freezing and thawing, whereas glucose was elevated in singly frozen and thawed animals but was reduced toward cold acclimation levels after repeated bouts of freezing. Overall, the cryoprotectant system was maintained, but not further elevated, in all tissues assayed in repeatedly frozen and thawed animals. For repeated freeze-thaw only, hepatic glycogen was depleted and plasma hemoglobin, indicative of erythrocyte hemolysis, increased. Postfreeze recovery of locomotor function, including limb and whole body movement, was delayed with repeated freeze-thaw and was associated with glycerol accumulation and glycogen depletion. Individuals that resumed locomotor function more quickly also accumulated greater cryoinjury. Integrated analyses of cryoprotectant and cryoinjury accumulation suggest that winter survival of D. chrysoscelis may be vulnerable to climate change, limited by carbohydrate stores, cellular repair mechanisms, and plasticity of the cryoprotectant system.

Freeze tolerance in neotropical frogs: an intrageneric comparison using Pristimantis species of high elevation and medium elevation

Paramos are high-elevation tropical Andean ecosystems above the tree line that display variable temperature and frequent freezing spells. Because a significant anuran community lives in this environment, physiological protection against freezing must characterise individuals in this community. Antifreeze protection has been studied in amphibians from other communities, and it is likely that Paramo anurans rely on the same underlying molecules that convey such protection to Nearctic species. However, given the pervasive presence of freezing spells in the Paramos year-round, the processes of activating protection mechanisms may differ from that of seasonal counterparts. Accordingly, this study investigated cryoprotection strategies in high-elevation tropical frogs, using as a model the terrestrial and nocturnal genus Pristimantis, specifically P. bogotensis, P. elegans and P. nervicus from Paramos, and the warm ecosystem counterparts P. insignitus, P. megalops and P. sanctaemartae. We focused on freeze tolerance and its relationship with glucose accumulation and ice formation. Under field conditions, the highest elevation P. nervicus exhibited higher glucose concentration at dawn compared to noon (1.7 ± 0.6 mmol/L versus 3.5 ± 1.32 mmol/L). Under experimental thermal freeze exposure for 2 hours between −2 and −4 ºC, the glucose concentration of the three Paramo species increased but physiological diversity was evident (P. nervicus 126%; P. bogotensis 100%; and P. elegans 55%). During this test, body ice formation was assessed calorimetrically. The species with the highest body ice formation was P. bogotensis (17% ± 5.37; maximum value: 63%; n = 8), followed by P. nervicus (5% ± 3.27; maximum value: 11%; n = 5) and P. elegans (0.34% ± 0.09; maximum value: 1%; n = 4). The study shows physiological diversity both within a genus and across the amphibian community around the freezing contour. Overall, Paramo species differ in freezing physiology from their low-elevation counterparts. Thus, climate shifts increasing freezing spells may affect the structure of communities in this zone.

Regulation of antioxidant systems in response to anoxia and reoxygenation in Rana sylvatica

The wood frog (Rana sylvatica) is a remarkable species. These frogs can endure prolonged oxygen deprivation as well as dehydration to ~60% of total body water lost and, combining these two abilities, they survive whole body freezing for weeks at a time during the winter. Episodes of anoxia/reoxygenation or freeze/thaw can trigger elevated production of reactive oxygen species (ROS) causing cellular damage, especially when oxygen is reintroduced during reoxygenation or thawing. To mitigate ROS damage, stress-responsive transcription factors such as the Octamer Binding Transcription factor (OCT4) and Nuclear factor (erythroid-derived 2)-like 2 transcription factor (Nrf2) were postulated to be involved in enhancing pro-survival pathways and antioxidant defenses. The present study used immunoblotting to analyze OCT4 and Nrf2 responses (and downstream factors under their control) to 24 h anoxia and 4 h reoxygenation in liver and skeletal muscle of wood frogs, with an emphasis on antioxidant systems. Surprisingly, no change was observed in relative total protein expression of either of the two transcription factors in liver. Furthermore, a significant decrease in total protein levels of OCT4 and Nrf2 occurred in skeletal muscle after 4 h recovery. However, essential cofactors of OCT4 and Nrf2 were significantly upregulated during anoxia and/or recovery. Downstream targets of the Nrf2-ARE pathway were evaluated, including glutathione-S-transferases (GSTs) and aldo-keto reductases (AKRs). Significant increases in GSTT1 and GSTP1 were observed in liver and muscle whereas AKRs showed a tissue specific response to both anoxia and recovery from anoxia. This study demonstrates activation of antioxidants as a cell protective mechanism against generation of reactive oxygen species during anoxia in wood frogs.

Overwintering adaptations and extreme freeze tolerance in a subarctic population of the wood frog, Rana sylvatica

The terrestrially hibernating wood frog (Rana sylvatica) is well-known for its iconic freeze tolerance, an overwintering adaptation that has received considerable investigation over the past 35 years. Virtually, all of this research has concerned frogs indigenous to the temperate regions of its broad range within North America. However, recent investigations have shown that frogs of subarctic populations are extremely cold hardy, being capable of surviving freezing for longer periods and at much lower temperatures as compared to conspecifics from temperate regions. Their exceptional freeze tolerance is partly supported by an enhanced cryoprotectant system that uses very high levels of urea and glucose to limit ice formation, regulate metabolism, and protect macromolecules and cellular structures from freezing/thawing stresses. In the weeks before they begin hibernating, northern frogs undertake radical physiological transitions, such as depletion of fat stores and catabolism of muscle protein, that prime the cryoprotectant system by accruing urea and stockpiling glycogen from which glucose is mobilized during freezing. Concentrations of cryoprotectants ultimately achieved in Alaskan frogs when freezing occurs vary among tissues but generally are higher than those of frogs inhabiting milder climates. This review summarizes the molecular, biochemical, and physiological adaptations permitting this northern phenotype to survive the long and harsh winters of the region.

Daily thermal fluctuations to a range of subzero temperatures enhance cold hardiness of winter-acclimated turtles

Although seasonal increases in cold hardiness are well documented for temperate and polar ectotherms, relatively little is known about supplemental increases in cold hardiness during winter. Because many animals are exposed to considerable thermal variation in winter, they may benefit from a quick enhancement of cold tolerance prior to extreme low temperature. Hatchling painted turtles (Chrysemys picta) overwintering in their natal nests experience substantial thermal variation in winter, and recently, it was found that brief subzero chilling of winter-acclimated hatchlings decreases subsequent chilling-induced mortality, increases blood concentrations of glucose and lactate, and protects the brain from cryoinjury. Here, we further characterize that phenomenon, termed 'cold conditioning', by exposing winter-acclimated hatchling turtles to -3.5, -7.0, or -10.5 °C gradually or repeatedly via daily thermal fluctuations over the course of 5 days and assessing their survival of a subsequent cold shock to a discriminating temperature of -12.7 °C. To better understand the physiological response to cold conditioning, we measured changes in glucose and lactate concentrations in the liver, blood, and brain. Cold conditioning significantly increased cold-shock survival, from 9% in reference turtles up to 74% in cold-conditioned turtles, and ecologically relevant daily thermal fluctuations were at least as effective at conferring cryoprotection as was gradual cold conditioning. Cold conditioning increased glucose concentrations, up to 25 μmol g^-1, and lactate concentrations, up to 30 μmol g^-1, in the liver, blood, and brain. Turtles that were cold conditioned with daily thermal fluctuations accumulated more glucose in the liver, blood, and brain, and had lower brain lactate, than those gradually cold conditioned. Given the thermal variation to which hatchling painted turtles are exposed in winter, we suggest that the supplemental protection conferred by cold conditioning, especially that induced by daily thermal fluctuations, may be important for their overwinter survival. Investigation into the duration of the cold-conditioning induced protection and its occurrence in natural field conditions is needed to better understand its ecological significance. We suggest that future work exploring the underlying mechanisms of cold conditioning should focus on non-colligative effects of glucose, expression of small Hsps, changes in membrane structure, and ion homeostasis.