Freeze Tolerance and Freeze Avoidance in Ectotherms

Engineered Compounds to Control Ice Nucleation and Recrystallization

One of the greatest concerns in the subzero storage of cells, tissues, and organs is the ability to control the nucleation or recrystallization of ice. In nature, evidence of these processes, which aid in sustaining internal temperatures below the physiologic freezing point for extended periods of time, is apparent in freeze-avoidant and freeze-tolerant organisms. After decades of studying these proteins, we now have easily accessible compounds and materials capable of recapitulating the mechanisms seen in nature for biopreser-vation applications. The output from this burgeoning area of research can interact synergistically with other novel developments in the field of cryobiology, making it an opportune time for a review on this topic.

Role of the Insect Neuroendocrine System in the Response to Cold Stress

Insects are the largest group of animals. They are capable of surviving in virtually all environments from arid deserts to the freezing permafrost of polar regions. This success is due to their great capacity to tolerate a range of environmental stresses, such as low temperature. Cold/freezing stress affects many physiological processes in insects, causing changes in main metabolic pathways, cellular dehydration, loss of neuromuscular function, and imbalance in water and ion homeostasis. The neuroendocrine system and its related signaling mediators, such as neuropeptides and biogenic amines, play central roles in the regulation of the various physiological and behavioral processes of insects and hence can also potentially impact thermal tolerance. In response to cold stress, various chemical signals are released either via direct intercellular contact or systemically. These are signals which regulate osmoregulation - capability peptides (CAPA), inotocin (ITC)-like peptides, ion transport peptide (ITP), diuretic hormones and calcitonin (CAL), substances related to the general response to various stress factors - tachykinin-related peptides (TRPs) or peptides responsible for the mobilization of body reserves. All these processes are potentially important in cold tolerance mechanisms. This review summarizes the current knowledge on the involvement of the neuroendocrine system in the cold stress response and the possible contributions of various signaling molecules in this process.

Hepatic transcriptome of the freeze-tolerant Cope's gray treefrog, Dryophytes chrysoscelis: responses to cold acclimation and freezing

Background

Cope’s gray treefrog, Dryophytes chrysoscelis, withstands the physiological challenges of corporeal freezing, partly by accumulating cryoprotective compounds of hepatic origin, including glycerol, urea, and glucose. We hypothesized that expression of genes related to cryoprotectant mobilization and stress tolerance would be differentially regulated in response to cold. Using high-throughput RNA sequencing (RNA-Seq), a hepatic transcriptome was generated for D. chrysoscelis, and gene expression was compared among frogs that were warm-acclimated, cold-acclimated, and frozen.

Results

A total of 159,556 transcripts were generated; 39% showed homology with known transcripts, and 34% of all transcripts were annotated. Gene-level analyses identified 34,936 genes, 85% of which were annotated. Cold acclimation induced differential expression both of genes and non-coding transcripts; freezing induced few additional changes. Transcript-level analysis followed by gene-level aggregation revealed 3582 differentially expressed genes, whereas analysis at the gene level revealed 1324 differentially regulated genes. Approximately 3.6% of differentially expressed sequences were non-coding and of no identifiable homology. Expression of several genes associated with cryoprotectant accumulation was altered during cold acclimation. Of note, glycerol kinase expression decreased with cold exposure, possibly promoting accumulation of glycerol, whereas glucose export was transcriptionally promoted by upregulation of glucose-6-phosphatase and downregulation of genes of various glycolytic enzymes. Several genes related to heat shock protein response, DNA repair, and the ubiquitin proteasome pathway were upregulated in cold and frozen frogs, whereas genes involved in responses to oxidative stress and anoxia, both potential sources of cellular damage during freezing, were downregulated or unchanged.

Conclusion

Our study is the first to report transcriptomic responses to low temperature exposure in a freeze-tolerant vertebrate. The hepatic transcriptome of Dryophytes chrysoscelis is responsive to cold and freezing. Transcriptomic regulation of genes related to particular pathways, such as glycerol biosynthesis, were not all regulated in parallel. The physiological demands associated with cold and freezing, as well as the transcriptomic responses observed in this study, are shared with several organisms that face similar ecophysiological challenges, suggesting common regulatory mechanisms. The role of transcriptional regulation relative to other cellular processes, and of non-coding transcripts as elements of those responses, deserve further study.

Thermal stress causes DNA damage and mortality in a tropical insect

Cold tolerance is considered an important factor determining geographic distribution of insects. We've previously shown that despite tropical origin, cockroach Gromphadorinha coquereliana is capable of surviving exposures to cold. However, freezing tolerance of this species had not yet been examined. Low temperature is known to alter membranes integrity in insects but whether chilling or freezing compromises DNA integrity remains a matter of speculation. In the present study, we subjected the G. coquereliana adults to freezing to determine their supercooling point (SCP) and evaluated whether the cockroaches were capable of surviving partial and complete freezing. Next, we conducted single cell gel electrophoresis assay (SCGE) to determine whether heat, cold and freezing altered haemocytes DNA integrity. The SCP of this species was high and around -4.76°C, which is within typical range of freezing-tolerant species. Most cockroaches survived one day after partial ice formation (20% mortality), but died progressively in the next few days after cold stress (70% mortality after 4 days). One day after complete freezing, most insects died (70% mortality), and after 4 days, 90% of them had succumbed. The SCGE assays showed substantial level of DNA damage in haemocytes. When cockroaches were heat-stressed, the level of DNA damage was similar to that observed in the freezing treatment; though all heat-stressed insects survived. The study shows that G. coquereliana can surprisingly be considered as moderately freezing-tolerant species, and for first time that extreme low temperature stress can affect DNA integrity, suggesting that this cockroach may possess an efficient DNA repair system.

Urea and plasma ice-nucleating proteins promoted the modest freeze tolerance in Pleske's high altitude frog Nanorana pleskei

The frog Nanorana pleskei (Dicroglossidae) is indigenous to the Qinghai-Tibetan Plateau. To identify its strategies in coping with the cold climate, we measured the hibernacula microhabitat temperature during winter. We also examined the freezing-induced and seasonal variation of several putative cryoprotectants in the heart, liver, brain, kidney and muscle, as well as ice-nucleating protein in plasma. Our results showed that N. pleskei survived exposure to temperatures as low as - 2.5 ± 0.40 °C during hibernation, which was lower than the body fluid freezing point (- 0.43 ± 0.01 °C). Experimental freezing results indicated that four of six specimens could survive 12 h of freezing at - 2 °C with 27.5 ± 2.5% of total body water as ice. Concomitantly, the water contents of all examined organs decreased after being frozen for 24 h at - 2 °C. The levels of urea in heart significantly increased from 71.05 ± 7.19 to 104.59 ± 10.11 µmol g^-1, and in muscle increased from 72.23 ± 3.40 to 102.42 ± 6.24 µmol g^-1 when exposed to freezing; other cryoprotectants (glucose, glycerol, and lactate) showed no significant increase in all examined tissues. In addition, urea levels were significantly higher in fall-collected frogs than summer-collected frogs in the tissues of heart, brain, kidney, and muscle. The results of differential scanning calorimetry indicated that the ice-nucleating protein was present only in cold-acclimated and fall-collected frogs' plasma. We concluded that the urea serves as a primary cryoprotectant and accumulates in anticipation of freezing in N. pleskei, coupling with the seasonal production of plasma ice-nucleating protein.

Freezing or supercooling: how does an aquatic subterranean crustacean survive exposures at subzero temperatures?

Crystallization temperature (Tc), resistance to inoculative freezing (IF), ice contents, bound water, protein and glycogen body contents were measured in the aquatic subterranean crustacean Niphargus rhenorhodanensis and in the morphologically close surface-dwelling aquatic crustacean Gammarus fossarum, both acclimated at 12°C, 3°C and -2°C. Cold acclimation induced an increase in the Tc values in both species but no survival was observed after thawing. However, after inoculation at high sub-zero temperatures, cold-acclimated N. rhenorhodanensis survived whereas all others, including the 3°C and -2°C acclimated G. fossarumdied. In its aquatic environment, N. rhenorhodanensis is likely to encounter inoculative freezing before reaching the Tc and IF tolerance appears as a highly adaptive trait in this species. Bound water and glycogen were found to increase in the 3°C and -2°C acclimated N. rhenorhodanensis, whereas no variation was observed in G. fossarum. Considering the hydrophilic properties of glycogen, such a rise may be correlated with the increased bound water measured in cold-acclimated N. rhenorhodanensis, and may be linked to the survival of this species when it was inoculated. The ecological significance of the survival of the aquatic subterranean crustacean to inoculative freezing is paradoxical, as temperature is currently highly buffered in its habitat. However, we assume that past geographical distribution and resulting life history traits of N. rhenorhodanensis are key parameters in the current cold-hardiness of the species.

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

1. Winter survival for numerous cold-blooded animals includes freeze tolerance: the ability to endure the conversion of as much as 65% of total body water into extracellular ice. Selected molecular adaptations underlying freeze tolerance (e.g. cryoprotectants, ice nucleating proteins) have been widely studied, but the full range of metabolic adjustments needed for freeze endurance remains unknown. 2. Recent studies using gene screening techniques are providing a different approach to the search for biochemical responses that support freezing survival by identifying genes and proteins that are up-regulated by freezing or thawing in freeze-tolerant amphibians and reptiles. 3. Screening of a cDNA library from wood frog liver revealed the freeze-induced up-regulation of genes coding for the alpha- and gamma-subunits of fibrinogen (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 responsive to two of the component stresses of freezing (dehydration and anoxia), indicating 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 appears 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 ischaemia resistance and studies with freeze-tolerant turtles have shown that other genes encoding proteins involved in mitochondrial energetics (NADH-ubiquinone oxido-reductase 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 tolerance as a natural phenomenon and also highlight new key areas that can be targeted by applied interventions for the optimization of medical cryopreservation techniques for cells, tissues and organs.

Biochemical adaptation for cold hardiness in insects

Specific biochemical adaptations permit winter survival at subzero temperatures by both freeze-tolerant and freeze-avoiding insects. Common to both survival strategies is the accumulation of high concentrations of polyols, providing deep supercooling point depression for freeze-avoiding forms and regulating cell volume reduction during extracellular freezing in freeze-tolerant insects. Studies in my laboratory have elucidated the molecular mechanisms (temperature effects on enzyme properties, allosteric regulation, reversible protein phosphorylation) that control the massive conversion of glycogen to polyols and, in some species, regulate the differential synthesis of dual polyols. New studies have highlighted the importance of aerobic ATP production for glycerol biosynthesis, suggested the importance of microcompartmentation for optimal conversion efficiency, documented seasonal changes in the capacity for polyol synthesis versus reconversion to glycogen and analysed the role of protein phosphorylation in enzyme regulation during polyol synthesis.