Cryoprotective effects of thermal hysteresis protein on survivorship of frozen gut cells from the freeze-tolerant centipedeLithobius forficatus

The properties, biotechnologies, and applications of antifreeze proteins

By natural selection, organisms evolve different solutions to cope with extremely cold weather. The emergence of an antifreeze protein gene is one of the most momentous solutions. Antifreeze proteins possess an importantly functional ability for organisms to survive in cold environments and are widely found in various cold-tolerant species. In this review, we summarize the origin of antifreeze proteins, describe the diversity of their species-specific properties and functions, and highlight the related biotechnology on the basis of both laboratory tests and bioinformatics analysis. The most recent advances in the applications of antifreeze proteins are also discussed. We expect that this systematic review will contribute to the comprehensive knowledge of antifreeze proteins to readers.

TMT-Based Quantitative Proteomic Profiling of Overwintering Lissorhoptrus oryzophilus

Adaptations to low temperature play a critical role in restricting the geographical distribution of insects. Decreasing day lengths and temperatures trigger seasonal cold adaptations in insects. These adaptions include changes in expression at the miRNA, mRNA and protein levels. The rice water weevil (RWW), Lissorhoptrus oryzophilus, introduced from the Mississippi River, is a globally invasive pest of wetland rice that can survive at the northern border of China. To investigate the changes in expression at the protein level in overwintering female RWW adults, 6-plex tandem mass tags (TMTs) were used in overwintering and summer adults. By using a proteome database available for Curculionidae, 1077 proteins were quantified, 183 of which differed significantly between the overwintering and summer samples. To further understand these differentially expressed proteins (DEPs), bioinformatics analyses such as gene ontology (GO) enrichment analyses were performed. DEPs associated with the terms binding, structural molecule activity, catalytic activity, multicellular organismal process, extracellular region, chitin binding, metabolic process, intracellular part and organic cyclic compound binding were altered by selection during winter. The changes in the expression of these proteins suggest that the proteins are important for RWW survival in winter.

Laboratory acclimation to autumn-like conditions induces freeze tolerance in the spring field cricket Gryllus veletis (Orthoptera: Gryllidae)

Many temperate insects encounter temperatures low enough to freeze their body fluids. Remarkably, some insects are freeze-tolerant, surviving this internal ice formation. However, the mechanisms underlying freeze tolerance are not well-understood, in part due to a lack of tractable model organisms. We describe a novel laboratory model to study insect freeze tolerance, the spring field cricket Gryllus veletis (Orthopera: Gryllidae). Following acclimation to six weeks of decreasing temperature and photoperiod, G. veletis become freeze-tolerant, similar to those exposed to natural autumn conditions in London, Ontario, Canada. Acclimated crickets suppress their metabolic rate by c. 33%, and survive freezing for up to one week at -8 °C, and to temperatures as low as -12 °C. Freeze-tolerant G. veletis protect fat body cells from freeze injury in vivo, and fat body tissue from freeze-tolerant cricket survives brief freeze treatments when frozen ex vivo. Freeze-tolerant crickets freeze at c. -6 °C, which may be initiated by accumulation of ice-nucleating agents in hemolymph or gut tissue. Although we hypothesize that control of ice formation facilitates freeze tolerance, initiating ice formation at high subzero temperatures does not confer freeze tolerance on freeze-intolerant nymphs. Acclimation increases hemolymph osmolality from c. 400 to c. 650 mOsm, which may facilitate freeze tolerance by reducing ice content. Hemolymph ion concentrations do not change with acclimation, and we therefore predict that freeze-tolerant G. veletis elevate hemolymph osmolality by accumulating other molecules. Gryllus veletis is easily reared and manipulated in a controlled laboratory environment, and is therefore a suitable candidate for further investigating the mechanisms underlying freeze tolerance.

Marine Antifreeze Proteins: Structure, Function, and Application to Cryopreservation as a Potential Cryoprotectant

Antifreeze proteins (AFPs) are biological antifreezes with unique properties, including thermal hysteresis(TH),ice recrystallization inhibition(IRI),and interaction with membranes and/or membrane proteins. These properties have been utilized in the preservation of biological samples at low temperatures. Here, we review the structure and function of marine-derived AFPs, including moderately active fish AFPs and hyperactive polar AFPs. We also survey previous and current reports of cryopreservation using AFPs. Cryopreserved biological samples are relatively diverse ranging from diatoms and reproductive cells to embryos and organs. Cryopreserved biological samples mainly originate from mammals. Most cryopreservation trials using marine-derived AFPs have demonstrated that addition of AFPs can improve post-thaw viability regardless of freezing method (slow-freezing or vitrification), storage temperature, and types of biological sample type.

Animal ice-binding (antifreeze) proteins and glycolipids: an overview with emphasis on physiological function

Ice-binding proteins (IBPs) assist in subzero tolerance of multiple cold-tolerant organisms: animals, plants, fungi, bacteria etc. IBPs include: (1) antifreeze proteins (AFPs) with high thermal hysteresis antifreeze activity; (2) low thermal hysteresis IBPs; and (3) ice-nucleating proteins (INPs). Several structurally different IBPs have evolved, even within related taxa. Proteins that produce thermal hysteresis inhibit freezing by a non-colligative mechanism, whereby they adsorb onto ice crystals or ice-nucleating surfaces and prevent further growth. This lowers the so-called hysteretic freezing point below the normal equilibrium freezing/melting point, producing a difference between the two, termed thermal hysteresis. True AFPs with high thermal hysteresis are found in freeze-avoiding animals (those that must prevent freezing, as they die if frozen) especially marine fish, insects and other terrestrial arthropods where they function to prevent freezing at temperatures below those commonly experienced by the organism. Low thermal hysteresis IBPs are found in freeze-tolerant organisms (those able to survive extracellular freezing), and function to inhibit recrystallization – a potentially damaging process whereby larger ice crystals grow at the expense of smaller ones – and in some cases, prevent lethal propagation of extracellular ice into the cytoplasm. Ice-nucleator proteins inhibit supercooling and induce freezing in the extracellular fluid at high subzero temperatures in many freeze-tolerant species, thereby allowing them to control the location and temperature of ice nucleation, and the rate of ice growth. Numerous nuances to these functions have evolved. Antifreeze glycolipids with significant thermal hysteresis activity were recently identified in insects, frogs and plants.

Recombinant Dendroides canadensis antifreeze proteins as potential ingredients in cryopreservation solutions

Expanding cryopreservation methods to include a wider range of cell types, such as those sensitive to freezing, is needed for maintaining the viability of cell-based regenerative medicine products. Conventional cryopreservation protocols, which include use of cryoprotectants such as dimethylsulfoxide (Me2SO), have not prevented ice-induced damage to cell and tissue matrices during freezing. A family of antifreeze proteins (AFPs) produced in the larvae of the beetle, Dendroides canadensis allow this insect to survive subzero temperatures as low as -26°C. This study is an assessment of the effect of the four hemolymph D. canadensis AFPs (DAFPs) on the supercooling (nucleating) temperature, ice structure patterns and viability of the A10 cell line derived from the thoracic aorta of embryonic rat. Cryoprotectant solution cocktails containing combinations of DAFPs in concentrations ranging from 0 to 3mg/mL in Unisol base mixed with 1M Me2SO were first evaluated by cryomicroscopy. Combining multiple DAFPs demonstrated significant supercooling point depressing activity (∼9°C) when compared to single DAFPs and/or conventional 1M Me2SO control solutions. Concentrations of DAFPs as low as 1 μg/mL were sufficient to trigger this effect. In addition, significantly improved A10 smooth muscle cell viability was observed in cryopreservation experiments with low DAFP-6 and DAFP-2 concentrations in combination with Me2SO. No significant improvement in viability was observed with either DAFP-1 or DAFP-4. Low and effective DAFP concentrations are advantageous because they minimize concerns regarding cell cytotoxicity and manufacturing cost. These findings support the potential of incorporating DAFPs in solutions used to cryopreserve cells and tissues.

Antifreeze peptides and glycopeptides, and their derivatives: potential uses in biotechnology

Antifreeze proteins (AFPs) and glycoproteins (AFGPs), collectively called AF(G)Ps, constitute a diverse class of proteins found in various Arctic and Antarctic fish, as well as in amphibians, plants, and insects. These compounds possess the ability to inhibit the formation of ice and are therefore essential to the survival of many marine teleost fishes that routinely encounter sub-zero temperatures. Owing to this property, AF(G)Ps have potential applications in many areas such as storage of cells or tissues at low temperature, ice slurries for refrigeration systems, and food storage. In contrast to AFGPs, which are composed of repeated tripeptide units (Ala-Ala-Thr)_n with minor sequence variations, AFPs possess very different primary, secondary, and tertiary structures. The isolation and purification of AFGPs is laborious, costly, and often results in mixtures, making characterization difficult. Recent structural investigations into the mechanism by which linear and cyclic AFGPs inhibit ice crystallization have led to significant progress toward the synthesis and assessment of several synthetic mimics of AFGPs. This review article will summarize synthetic AFGP mimics as well as current challenges in designing compounds capable of mimicking AFGPs. It will also cover our recent efforts in exploring whether peptoid mimics can serve as structural and functional mimics of native AFGPs.

Expression of two self-enhancing antifreeze proteins from the beetle Dendroides canadensis in Drosophila melanogaster

Antifreeze proteins (AFPs) lower the freezing point of water without affecting the melting point. This difference between melting point and freezing point has been termed thermal hysteresis. Antifreeze protein genes, dafp-1 and/or dafp-4, from the freeze-avoiding insect, Dendroides canadensis, were transferred to Drosophila melanogaster via P-element-mediated transformation. The Northern and Western blots showed expression of DAFP(s) at both transcript and protein levels. The highest thermal hysteresis activity of 6.78+/-0.12 degrees C was detected in 5-day adult flies containing two copies of each of the dafp-1 and dafp-4 genes, while flies with two copies of either dafp-1 or dafp-4 had less activity, 5.52 and 3.24 degrees C, respectively (measured by nanoliter osmometer). This suggests synergistic enhancement of thermal hysteresis activity between DAFP-1 and DAFP-4 in transgenic D. melanogaster containing both DAFPs. Supercooling points without ice in contact with the insects were lowered in all 5 transgenic lines compared with controls, however, when ice was in contact with the flies, supercooling points were lowered only in the heterozygous <DAFP-1>+<DAFP-4> transgenic line. Also, transgenic D. melanogaster exhibited higher survivorship compared with controls when placed at low non-freezing temperatures (0 and 4 degrees C), however, DAFP-1 and DAFP-4 did not display any synergistic enhancement in these non-freezing survival experiments.

A nonprotein thermal hysteresis-producing xylomannan antifreeze in the freeze-tolerant Alaskan beetle Upis ceramboides

Thermal hysteresis (TH), a difference between the melting and freezing points of a solution that is indicative of the presence of large-molecular-mass antifreezes (e.g., antifreeze proteins), has been described in animals, plants, bacteria, and fungi. Although all previously described TH-producing biomolecules are proteins, most thermal hysteresis factors (THFs) have not yet been structurally characterized, and none have been characterized from a freeze-tolerant animal. We isolated a highly active THF from the freeze-tolerant beetle, Upis ceramboides, by means of ice affinity. Amino acid chromatographic analysis, polyacrylamide gel electrophoresis, UV-Vis spectrophotometry, and NMR spectroscopy indicated that the THF contained little or no protein, yet it produced 3.7 +/- 0.3 degrees C of TH at 5 mg/ml, comparable to that of the most active insect antifreeze proteins. Compositional and structural analyses indicated that this antifreeze contains a beta-mannopyranosyl-(1-->4) beta-xylopyranose backbone and a fatty acid component, although the lipid may not be covalently linked to the saccharide. Consistent with the proposed structure, treatment with endo-beta-(1-->4)xylanase ablated TH activity. This xylomannan is the first TH-producing antifreeze isolated from a freeze-tolerant animal and the first in a new class of highly active THFs that contain little or no protein.

Freeze tolerance in an arctic Alaska stonefly

Most aquatic insects do not survive subzero temperatures and, for those that do, the physiology has not been well characterized. Nemoura arctica is a species of stonefly widely distributed throughout arctic and subarctic Alaska. We collected nymphs from the headwaters of the Chandalar River, where we recorded streambed temperatures as low as –12.7°C in midwinter. When in contact with ice, autumn-collected N. arctica cool to –1.5±0.4°C before freezing, but individuals survived temperatures as low as –15°C, making this the first described species of freeze-tolerant stonefly. N. arctica clearly survive freezing in nature, as winter-collected nymphs encased in ice demonstrated high survivorship when thawed. In the laboratory, 87% of N. arcticanymphs frozen to –15°C for 2.5 weeks survived and, within one month of thawing, 95% of the last-instar nymphs emerged. N. arctica produce both glycerol and ice-binding factors (e.g. antifreeze protein) in response to low temperature. Hemolymph glycerol concentrations increased from 3 mmol l–1 to 930±114 mmol l–1 when temperatures were decreased from 4°C to –8°C, and N. arctica continued to produce glycerol even while frozen. Although the hemolymph of individual cold-acclimated nymphs occasionally exhibited more than a degree of thermal hysteresis, typically the hemolymph exhibited only hexagonal crystal growth, indicating a low concentration of ice-binding factor. Hemolymph of nymphs acclimated to subzero temperatures had recrystallization inhibition. These results demonstrate that, in the face of freezing conditions, N. arctica exhibit overwintering adaptations similar to those of terrestrial insects.

Ice-active proteins and cryoprotectants from the New Zealand alpine cockroach, Celatoblatta quinquemaculata

Celatoblatta quinquemaculata is moderately freezing tolerant. We have investigated low and high molecular weight compounds that may be associated with its survival. Glycerol and trehalose were identified as potential cryoprotectants, with trehalose at the higher concentration. Trehalose was at its highest concentration in late autumn, during the periods sampled. Water contents declined with time and were significantly lower in late autumn than in late summer. No thermal hysteresis activity was detected in haemolymph or in extracts of the head, muscles and the fat body. Extracts of the Malpighian tubules showed an hexagonal crystal growth form, as did those of the gut tissue and gut contents. The gut tissue had high levels of thermal hysteresis (approximately 2 degrees C) and the gut contents somewhat lower levels (approximately 0.6 degrees C). Recrystallization inhibition activity mirrored that of thermal hysteresis, with activity absent in the haemolymph or fat body cells but present in the gut tissues and contents. Activity was reduced by heating and was associated with a molecule >14kDa in size. These findings suggest an antifreeze protein is involved. In fed animals, ice nucleation is likely to start in the gut. Gut cells have a much greater resistance to freezing than do fat body or Malpighian tubule cells. The antifreeze protein may enable this tissue to survive freezing stress by inhibiting recrystallization.

Expression of a beetle, Dendroides canadensis, antifreeze protein in Drosophila melanogaster

Antifreeze protein 1 (DAFP-1), from the beetle Dendroides canadensis, was expressed in Drosophila melanogaster. Mean thermal hysteresis values (the difference between freezing and melting points), indicative of antifreeze protein activity, in the hemolymph of transgenic flies were found to be as high as 6.23+/-0.10 degrees C (using the nanoliter osmometer). Direct comparisons of the capillary and nanoliter osmometer techniques for measuring THA were made, illustrating the much higher values obtained by the latter. Transgenic Drosophila had supercooling points, both in contact with ice and not, that were slightly, but significantly, lower than wild-type controls (1.5-2.0 degrees C and 2.0-4.0 degrees C, respectively). The results indicate functionality of DAFP-1 in Drosophila melanogaster (the ability of DAFP-1 to inhibit both inoculative freezing across the cuticle and freezing initiated by endogenous ice nucleators). The much larger effects of DAFPs in inhibiting inoculative freezing and ice nucleation in Dendroides canadensis relative to the transgenic Drosophila may partially result from the lower DAFP concentrations and activities in Drosophila, however the absence of multiple types of DAFPs and absence of tissue specific expression may also contribute. Transgenic Drosophila were also able to live significantly longer than controls at 0 degrees C and 4 degrees C, indicating that DAFP-1 is able to increase cold tolerance at above freezing temperatures.

Analysis of antifreeze proteins within spruce budworm sister species

Spruce budworm (Choristoneura) species survive sub-zero winter temperatures by producing antifreeze proteins (AFPs) encoded by a multigene family of short and long isoforms. We report in this study the first analysis of antifreeze proteins from related Choristoneura sister species. The additional thirty amino acid insert found in the longer AFP isoforms maintains the proteins beta-helix and original fifteen amino acid (Thr-X-Thr) repeat motif. Analysis of the beta-helix region shows more divergent residues surround the conserved Thr residues. Maintaining the beta-helix structure and conserved Thr residues appear to be paramount for AFP function and surviving sub-zero winter temperatures. Two other species within the same lepidopteran clade, Ditrysia, do not appear to contain any AFP-like sequences.

Antifreeze proteins in Alaskan insects and spiders

Prior to this study, antifreeze proteins (AFPs) had not been identified in terrestrial arthropods from the Arctic or anywhere in Alaska. The hemolymph of 75 species of insects and six spiders from interior and arctic Alaska were screened for thermal hysteresis (a difference between the freezing and melting points), characteristic of the presence of AFPs. Eighteen species of insects and three spiders were shown to have AFPs. Ten of the insects with AFPs were beetles including the first species from the families Chrysomelidae, Pythidae, Silphidae and Carabidae. In addition, the first Neuropteran to have AFPs was identified, the lacewing Hemerobius simulans together with the second and third Diptera (the first Tipulids) and the second and third Hemiptera, the stinkbug Elasmostethus interstinctus (the first Pentatomid), and the water strider Limnoporus dissortis (the first Gerrid). Prior to this study, 33 species of insects and three spiders had been reported to have AFPs. Most AFP-producing terrestrial arthropods are freeze avoiding, and the AFPs function to prevent freezing. However, some of the AFP- producing insects identified in this study are known to be freeze tolerant (able to survive freezing) to very low temperatures (-40 to -70 degrees C).

Intracellular freezing and survival in the freeze tolerant alpine cockroach Celatoblatta quinquemaculata

The alpine cockroach Celatoblatta quinquemaculata is common at altitudes of around 1500 m on the Rock and Pillar range of Central Otago, New Zealand where it experiences freezing conditions in the winter. The cockroach is freeze tolerant, but only to c. -9 degrees C. The cause of death at temperatures below this is unknown but likely to be due to osmotic damage to cells (shrinkage). This study compared the effect of different ice nucleation temperatures (-2 and -4 degrees C) on the viability of three types of cockroach tissue (midgut, Malpighian tubules and fat body cells) and cooling to three different temperatures (-5, -8, -12 degrees C). Two types of observations were made (i) cryomicroscope observations of ice formation and cell shrinkage (ii) cell integrity (viability) using vital stains. Cell viability decreased with lower treatment temperatures but ice nucleation temperature had no significant effect. Cryomicroscope observations showed that ice spread through tissue faster at -4 than -2 degrees C and that intracellular freezing only occurred when nucleated at -4 degrees C. From temperature records during cooling, it was observed that when freezing occurred, latent heat immediately increased the insect's body temperature close to its melting point (c. -0.3 degrees C). This "rebound" temperature was independent of nucleation temperature. Some tissues were more vulnerable to damage than others. As the gut is thought to be the site of freezing, it is significant that this tissue was the most robust. The ecological importance of the effect of nucleation temperature on survival of whole animals under field conditions is discussed.

Ice binding, recrystallization inhibition, and cryoprotective properties of ice-active substances associated with Antarctic sea ice diatoms

Extracellular macromolecules associated with Antarctic sea ice diatoms were previously shown to have ice-binding activities. The function of these ice-active substances (IASs) has not been identified. Here we show that two of the IASs have a strong ability to inhibit the recrystallization of ice, possibly signifying a cryoprotectant function. To test this possibility, two species of marine diatom (one Antarctic and one temperate) were subjected to a single freeze-thaw cycle (approximately 20h at -4 or -5 degrees C) in the presence or absence of IAS. Viability, based on a double staining technique, was 15-29% higher in the presence of IAS. Etching of single crystal ice hemispheres grown from dilute IAS solutions indicated that the IASs bind to specific faces of ice and are incorporated into the ice lattice. Together, these results suggest that the IASs acts as a cryoprotectant, probably through some ice-binding mechanism.

The role of endogenous antifreeze protein enhancers in the hemolymph thermal hysteresis activity of the beetle Dendroides canadensis

Antifreeze proteins (AFPs) lower the freezing point of water by a non-colligative mechanism, but do not lower the melting point, therefore producing a difference between the freezing and melting points termed thermal hysteresis. Thermal hysteresis activity (THA) of AFPs from overwintering larvae of the beetle Dendroides canadensis is dependent upon AFP concentration and the presence of enhancers of THA which may be either other proteins or low molecular mass enhancers. The purpose of this study was to determine the relative contributions of endogenous enhancers in winter D. canadensis hemolymph.Winter hemolymph collected over four successive winters (1997-1998 to 2000-2001) was tested. The first three of these winters were the warmest on record in this area, while December of the final year was the coldest on record. Protein and low molecular mass enhancers raised hemolymph THA 60-97% and 35-55%, respectively, based on hemolymph with peak THA for each year collected over the four successive winters. However, the hemolymph AFPs were not maximally enhanced since addition of the potent enhancer citrate (at non-physiologically high levels) resulted in large increases in THA. (13)NMR showed that glycerol was the only low molecular mass solute present in sufficiently high concentrations in the hemolymph to function as an enhancer. Maximum THA appears to be approximately 8.5 degrees C.

Antifreeze and ice nucleator proteins in terrestrial arthropods

Terrestrial arthropods survive subzero temperatures by becoming either freeze tolerant (survive body fluid freezing) or freeze avoiding (prevent body fluid freezing). Protein ice nucleators (PINs), which limit supercooling and induce freezing, and antifreeze proteins (AFPs), which function to prevent freezing, can have roles in both freeze tolerance and avoidance. Many freeze-tolerant insects produce hemolymph PINs, which induce freezing at high subzero temperatures thereby inhibiting lethal intracellular freezing. Some freeze-tolerant species have AFPs that function as cryoprotectants to prevent freeze damage. Although the mechanism of this cryoprotection is not known, it may involve recrystallization inhibition and perhaps stabilization of the cell membrane. Freeze-avoiding species must prevent inoculative freezing initiated by external ice across the cuticle and extend supercooling abilities. Some insects remove PINs in the winter to promote supercooling, whereas others have selected against surfaces with ice-nucleating abilities on an evolutionary time scale. However, many freeze-avoiding species do have proteins with ice-nucleating activity, and these proteins must be masked in winter. In the beetle Dendroides canadensis, AFPs in the hemolymph and gut inhibit ice nucleators. Also, hemolymph AFPs and those associated with the layer of epidermal cells under the cuticle inhibit inoculative freezing. Two different insect AFPs have been characterized. One type from the beetles D. canadensis and Tenebrio molitor consists of 12- and 13-mer repeating units with disulfide bridges occurring at least every six residues. The spruce budworm AFP lacks regular repeat units. Both have much higher activities than any known AFPs.

Thermal hysteresis proteins

Extreme environments present a wealth of biochemical adaptations. Thermal hysteresis proteins (THPs) have been found in vertebrates, invertebrates, plants, bacteria and fungi and are able to depress the freezing point of water (in the presence of ice crystals) in a non-colligative manner by binding to the surface of nascent ice crystals. The THPs comprise a disparate group of proteins with a variety of tertiary structures and often no common sequence similarities or structural motifs. Different THPs bind to different faces of the ice crystal, and no single mechanism has been proposed to account for THP ice binding affinity and specificity. Experimentally THPs have been used in the cryopreservation of tissues and cells and to induce cold tolerance in freeze susceptible organisms. THPs represent a remarkable example of parallel and convergent evolution with different proteins being adapted for an anti-freeze role.

Ice nucleation and antinucleation in nature

Plants and ectothermic animals use a variety of substances and mechanisms to survive exposure to subfreezing temperatures. Proteinaceous ice nucleators trigger freezing at high subzero temperatures, either to provide cold protection from released heat of fusion or to establish a protective extracellular freezing in freeze-tolerant species. Freeze-avoiding species increase their supercooling potential by removing ice nucleators and accumulating polyols. Terrestrial invertebrates and polar marine fish stabilize their supercooled state by means of noncolligatively acting antifreeze proteins. Some organisms also depress their body fluid melting point to ambient temperature by evaporation and/or solute accumulation.

Isolation and characterization of cDNA clones encoding antifreeze proteins of the pyrochroid beetle Dendroides canadensis

Temporal differences in the expression of Dendroides canadensis antifreeze protein (DAFP) are indicated from seasonal comparison of dafp-1 transcript level, thermal hysteresis activity and temperature changes. DAFP-1 transcript abundance correlates with the thermal hysteresis activity level in late fall/early winter and appears to follow overall seasonal temperature changes with peak transcript levels occurring in December. A cDNA library created from December larvae yielded clones encoding a set of novel putative DAFPs. Some of the cDNA clones isolated display significant divergence at the primary amino acid level, yet, maintain conservation of key residues that are presumably important for structure and function of antifreeze proteins in this cold-hardy organism. Seasonal analysis of two dafps (dafp-1 and dafp-7) revealed differences on the transcriptional level, suggesting that DAFPs may serve somewhat different functions.

Mapping of disulfide bridges in antifreeze proteins from overwintering larvae of the beetle Dendroides canadensis

Antifreeze proteins (AFPs) have been identified in certain high-latitude marine fish, insects and other terrestrial arthropods, and plants. Despite considerable structural variation, the mechanisms of their noncolligative antifreeze activity are probably quite similar. AFPs hydrogen bond onto the surface of potential seed ice crystals at preferred growth sites, thereby preventing growth of the crystals. AFPs from overwintering larvae of the beetle Dendroidescanadensis are among the most active AFPs. These 8.7-kDa proteins consist of seven 12- or 13-mer repeating units. Their most striking feature is the location of cysteines every six residues throughout their length. Consequently, identification of the disulfide linkages of these cysteines is essential to understanding the structure of these AFPs. This study demonstrated that all of the 16 Cys residues in the Dendroides AFPs are disulfide bridged. All of the seven 12- or 13-mer repeats have internal disulfide bridges, and in all but the first repeat the Cys residues at positions 1 and 7 of the repeats are linked. In repeat 1 the Cys at position 1 is linked to the Cys at position 10, rather than the Cys at position 7 as in the other repeats, and the Cys at position 7 of the first repeat is linked to a Cys at position 4 of the second repeat. The disulfide bridges probably function to position the hydrophilic side chains of serine and threonine residues so that they hydrogen bond with ice.

Cold hardiness, supercooling ability and causes of low-temperature mortality in the soft tick, Argas reflexus, and the hard tick, Ixodes ricinus (Acari: Ixodoidea) from Central Europe

Seasonal supercooling points (SCPs=temperature of crystallization) and cold hardiness were investigated in the indigenous hard tick, I. ricinus, and in A. reflexus, a soft tick introduced to Central Europe from the South. Both species proved to be freeze-susceptible as well as highly susceptible to inoculative freezing. None of the postembryonic developmental stages of either species showed any distinct seasonal pattern of SCP. Unexpectedly, the introduced A. reflexus exhibited a distinctly higher degree of cold hardiness in terms of lower lethal temperature (LT(50): 24h exposure) as well as lethal time (T(50): time of survival at -10.1 degrees C) than I. ricinus. Engorged I. ricinus larvae as well as engorged summer acclimatized A. reflexus larvae showed some mortality at temperatures well above the SCP. This mortality was generally expressed as a failure of the following stage to eclose properly. A 10-day cold acclimation at +3 degrees C eliminated that kind of mortality in summer acclimatized A. reflexus larvae, but not in I. ricinus larvae. It was frequently observed that freezing of ticks resulted-possibly via leakage from the midgut-in a subsequent reddish brown discoloration of the ticks after thawing. Taking into account that discoloration was an indication of previous freezing, it was concluded, that after long-term exposure (for >/=30 days) at -10.1 degrees C, a temperature well above the SCP, some tick mortality could be observed that was not caused by previous freezing. Weighing experiments clearly demonstrated, that the level of dehydration was not critical for survival of A. reflexus during long-term cold exposure, even at low RH. This indicates, that cold-related factors other than freezing and dehydration were detrimental to this species.

The supercooling ability of ticks (Acari, Ixodoidea)

The supercooling capacity of nine laboratory-held species of ticks originating from different geographical areas, as well as five field-collected species from Germany, was investigated. All but one tick species showed mean supercooling points between about -17 and -23 degrees C, suggesting that the capacity to supercool to temperatures of < or = -17 degrees C might be an inherent property of many tick species unrelated to their geographic origin. Photoperiod did not influence the mean supercooling point in any of the species and there was also no distinct seasonal pattern of supercooling in seasonally acclimatized Dermacentor marginatus. Thus, the supercooling ability was independent of the presence/absence of diapause. The finding of thermal hysteresis in D. marginatus hemolymph raises the question of whether or not anti-freeze proteins are involved in the supercooling capacity of that species. An interspecies comparison revealed a weak negative correlation between relative water content and supercooling point of the ticks and an even weaker correlation between body mass or body water mass and the supercooling point. Since the ticks exhibited low supercooling points both before and shortly after feeding, the blood used as food should lack potent ice nucleators.