Antifreeze proteins differentially affect model membranes during freezing

Cryopreservation of assay-ready hepatocyte monolayers by chemically-induced ice nucleation: preservation of hepatic function and hepatotoxicity screening capabilities

Cell culture plays a critical role in biomedical discovery and drug development. Primary hepatocytes and hepatocyte-derived cell lines are especially important cellular models for drug discovery and development. To enable high-throughput screening and ensure consistent cell phenotypes, there is a need for practical and efficient cryopreservation methods for hepatocyte-derived cell lines and primary hepatocytes in an assay-ready format. Cryopreservation of cells as adherent monolayers in 96-well plates presents unique challenges due to low volumes being susceptible to supercooling, leading to low recovery and well-to-well variation. Primary cell cryopreservation is also particularly challenging due to the loss of cell viability and function. In this study, we demonstrate the use of soluble ice nucleator materials (IN) to cryopreserve a hepatic-derived cell line (HepG2) and primary mouse hepatocytes, as adherent monolayers. HepG2 cell recovery was near 100% and ∼75% of primary hepatocytes were recovered 24 hours post-thaw compared to just 10% and 50% with standard 10% DMSO, respectively. Post-thaw assessment showed that cryopreserved HepG2 cells retain membrane integrity, metabolic activity, proliferative capacity and differentiated hepatic functions including urea secretion, cytochrome P450 levels and lipid droplet accumulation. Cryopreserved primary hepatocytes exhibited reduced hepatic functions compared to fresh hepatocytes, but functional levels were similar to commercial suspension-cryopreserved hepatocytes, with the added benefit of being stored in an assay-ready format. In addition, normal cuboidal morphology and minimal membrane damage were observed 24 hours post-thaw. Cryopreserved HepG2 and mouse hepatocytes treated with a panel of pharmaceutically active compounds produced near-identical dose-response curves and EC50 values compared to fresh hepatocytes, confirming the utility of cryopreserved bankable cells in drug metabolism and hepatotoxicity studies. Cryopreserved adherent HepG2 cells and primary hepatocytes in 96 well plates can significantly reduce the time and resource burden associated with routine cell culture and increases the efficiency and productivity of high-throughput drug screening assays.

Synthesis of Poly(2-(methylsulfinyl)ethyl methacrylate) via Oxidation of Poly(2-(methylthio)ethyl methacrylate): Evaluation of the Sulfoxide Side Chain on Cryopreservation

Conventional cryopreservation solutions rely on the addition of organic solvents such as DMSO or glycerol, but these do not give full recovery for all cell types, and innovative cryoprotectants may address damage pathways which these solvents do not protect against. Macromolecular cryoprotectants are emerging, but there is a need to understand their structure-property relationships and mechanisms of action. Here we synthesized and investigated the cryoprotective behavior of sulfoxide (i.e., "DMSO-like") side-chain polymers, which have been reported to be cryoprotective using poly(ethylene glycol)-based polymers. We also wanted to determine if the polarized sulfoxide bond (S⁺O^- character) introduces cryoprotective effects, as this has been seen for mixed-charge cryoprotective polyampholytes, whose mechanism of action is not yet understood. Poly(2-(methylsulfinyl)ethyl methacrylate) was synthesized by RAFT polymerization of 2-(methylthio)ethyl methacrylate and subsequent oxidation to sulfoxide. A corresponding N-oxide polymer was also prepared and characterized: (poly(2-(dimethylamineoxide)ethyl methacrylate). Ice recrystallization inhibition assays and differential scanning calorimetry analysis show that the sulfoxide side chains do not modulate the frozen components during cryopreservation. In cytotoxicity assays, it was found that long-term (24 h) exposure of the polymers was not tolerated by cells, but shorter (30 min) incubation times, which are relevant for cryopreservation, were tolerated. It was also observed that overoxidation to the sulfone significantly increased the cytotoxicity, and hence, these materials require a precision oxidation step to be deployed. In suspension cell cryopreservation investigations, the polysulfoxides did not increase cell recovery 24 h post-thaw. These results show that unlike hydrophilic backboned polysulfides, which can aid cryopreservation, the installation of the sulfoxide group onto a polymer does not necessarily bring cryoprotective properties, highlighting the challenges of developing and discovering macromolecular cryoprotectants.

Assay-ready Cryopreserved Cell Monolayers Enabled by Macromolecular Cryoprotectants

Cell monolayers underpin the discovery and screening of new drugs and allow for fundamental studies of cell biology and disease. However, current cryopreservation technologies do not allow cells to be stored frozen while attached to tissue culture plastic. Hence, cells must be thawed from suspension, cultured for several days or weeks, and finally transferred into multiwell plates for the desired application. This inefficient process consumes significant time handling cells, rather than conducting biomedical research or other value-adding activities. Here, we demonstrate that a synthetic macromolecular cryoprotectant enables the routine, reproducible, and robust cryopreservation of biomedically important cell monolayers, within industry-standard tissue culture multiwell plates. The cells are simply thawed with media and placed in an incubator ready to use within 24 h. Post-thaw cell recovery values were >80% across three cell lines with low well-to-well variance. The cryopreserved cells retained healthy morphology, membrane integrity, proliferative capacity, and metabolic activity; showed marginal increases in apoptotic cells; and responded well to a toxicological challenge using doxorubicin. These discoveries confirm that the cells are “assay-ready” 24 h after thaw. Overall, we show that macromolecular cryoprotectants can address a long-standing cryobiological challenge and offers the potential to transform routine cell culture for biomedical discovery.

Structural diversity of marine anti-freezing proteins, properties and potential applications: a review

Cold-adapted organisms, such as fishes, insects, plants and bacteria produce a group of proteins known as antifreeze proteins (AFPs). The specific functions of AFPs, including thermal hysteresis (TH), ice recrystallization inhibition (IRI), dynamic ice shaping (DIS) and interaction with membranes, attracted significant interest for their incorporation into commercial products. AFPs represent their effects by lowering the water freezing point as well as preventing the growth of ice crystals and recrystallization during frozen storage. The potential of AFPs to modify ice growth results in ice crystal stabilizing over a defined temperature range and inhibiting ice recrystallization, which could minimize drip loss during thawing, improve the quality and increase the shelf-life of frozen products. Most cryopreservation studies using marine-derived AFPs have shown that the addition of AFPs can increase post-thaw viability. Nevertheless, the reduced availability of bulk proteins and the need of biotechnological techniques for industrial production, limit the possible usage in foods. Despite all these drawbacks, relatively small concentrations are enough to show activity, which suggests AFPs as potential food additives in the future. The present work aims to review the results of numerous investigations on marine-derived AFPs and discuss their structure, function, physicochemical properties, purification and potential applications.

Combinatorial Biomaterials Discovery Strategy to Identify New Macromolecular Cryoprotectants

Cryoprotective agents (CPAs) are typically solvents or small molecules, but there is a need for innovative CPAs to reduce toxicity and increase cell yield, for the banking and transport of cells. Here we use a photochemical high-throughput discovery platform to identify macromolecular cryoprotectants, as rational design approaches are currently limited by the lack of structure-property relationships. Using liquid handling systems, 120 unique polyampholytes were synthesized using photopolymerization with RAFT agents. Cryopreservation screening identified "hit" polymers and nonlinear trends between composition and function, highlighting the requirement for screening, with polymer aggregation being a key factor. The most active polymers reduced the volume of dimethyl sulfoxide (DMSO) required to cryopreserve a nucleated cell line, demonstrating the potential of this approach to identify materials for cell storage and transport.

Extracellular Antifreeze Protein Significantly Enhances the Cryopreservation of Cell Monolayers

The cryopreservation of cells underpins many areas of biotechnology, healthcare, and fundamental science by enabling the banking and distribution of cells. Cryoprotectants are essential to prevent cold-induced damage. Here, we demonstrate that extracellular localization of antifreeze proteins can significantly enhance post-thaw recovery of mammalian cell monolayers cryopreserved using dimethyl sulfoxide, whereas they show less benefit in suspension cryopreservation. A type III antifreeze protein (AFPIII) was used as the macromolecular ice recrystallization inhibitor and its intra/extracellular locations were controlled by using Pep-1, a cell-penetrating peptide. Flow cytometry and confocal microscopy confirmed successful delivery of AFPIII. The presence of extracellular AFPIII dramatically increased post-thaw recovery in a challenging 2-D cell monolayer system using just 0.8 mg·mL-1, from 25% to over 60%, whereas intracellularly delivered AFPIII showed less benefit. Interestingly, the antifreeze protein was less effective when used in suspension cryopreservation of the same cells, suggesting that the cryopreservation format is also crucial. These observations show that, in the discovery of macromolecular cryoprotectants, intracellular delivery of ice recrystallization inhibitors may not be a significant requirement under "slow freezing" conditions, which will help guide the design of new biomaterials, in particular, for cell storage.

Mimicking the Ice Recrystallization Activity of Biological Antifreezes. When is a New Polymer "Active"?

Antifreeze proteins and ice-binding proteins have been discovered in a diverse range of extremophiles and have the ability to modulate the growth and formation of ice crystals. Considering the importance of cryoscience across transport, biomedicine, and climate science, there is significant interest in developing synthetic macromolecular mimics of antifreeze proteins, in particular to reproduce their property of ice recrystallization inhibition (IRI). This activity is a continuum rather than an "on/off" property and there may be multiple molecular mechanisms which give rise to differences in this observable property; the limiting concentrations for ice growth vary by more than a thousand between an antifreeze glycoprotein and poly(vinyl alcohol), for example. The aim of this article is to provide a concise comparison of a range of natural and synthetic materials that are known to have IRI, thus providing a guide to see if a new synthetic mimic is active or not, including emerging materials which are comparatively weak compared to antifreeze proteins, but may have technological importance. The link between activity and the mechanisms involving either ice binding or amphiphilicity is discussed and known materials assigned into classes based on this.

Improvement in Ovarian Tissue Quality with Supplementation of Antifreeze Protein during Warming of Vitrified Mouse Ovarian Tissue

Ice easily recrystallizes during warming after vitrification, and antifreeze protein (AFP) can inhibit the re-crystallization. However, no study has evaluated the effect of AFP treatment only thereon during warming. This study sought to compare AFP treatment protocols: a conventional protocol with AFP treatment during vitrification and first-step warming and a new protocol with AFP treatment during the first-step warming only. According to the protocols, 10 mg/mL of LeIBP (a type of AFP) was used. Five-week-old B6D2F1 mouse ovaries were randomly divided into a vitrified-warmed control and two experimental groups, one treated with the conventional AFP treatment protocol (LeIBP-all) and the other with the new AFP treatment protocol (LeIBP-w). For evaluation, ratios of ovarian follicle integrity, apoptosis, and DNA double-strand (DDS) damage/repairing were analyzed. The LeIBP-treated groups showed significantly higher intact follicle ratios than the control, and the results were similar between the LeIBP-treated groups. Apoptotic follicle ratios were significantly lower in both LeIBP-treated groups than the control, and the results were not significantly different between the LeIBP-treated groups. With regard to DDS damage/repairing follicle ratio, significantly lower ratios were recorded in both LeIBP-treated groups, compared to the control, and the results were similar between the LeIBP-treated groups. This study demonstrated that both protocols with LeIBP had a beneficial effect on maintaining follicle integrity and preventing follicle apoptosis and DDS damage. Moreover, the new protocol showed similar results to the conventional protocol. This new protocol could optimize the mouse ovary vitrification-warming procedure using AFP, while minimizing the treatment steps.

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.

Ice-binding proteins confer freezing tolerance in transgenic Arabidopsis thaliana

Lolium perenne is a freeze-tolerant perennial ryegrass capable of withstanding temperatures below -13 °C. Ice-binding proteins (IBPs) presumably help prevent damage associated with freezing by restricting the growth of ice crystals in the apoplast. We have investigated the expression, localization and in planta freezing protection capabilities of two L. perenne IBP isoforms, LpIRI2 and LpIRI3, as well as a processed IBP (LpAFP). One of these isoforms, LpIRI2, lacks a conventional signal peptide and was assumed to be a pseudogene. Nevertheless, both LpIRI2 and LpIRI3 transcripts were up-regulated following cold acclimation. LpIRI2 also demonstrated ice-binding activity when produced recombinantly in Escherichia coli. Both the LpIRI3 and LpIRI2 isoforms appeared to accumulate in the apoplast of transgenic Arabidopsis thaliana plants. In contrast, the fully processed isoform, LpAFP, remained intracellular. Transgenic plants expressing either LpIRI2 or LpIRI3 showed reduced ion leakage (12%-39%) after low-temperature treatments, and significantly improved freezing survival, while transgenic LpAFP-expressing lines did not confer substantial subzero protection. Freeze protection was further enhanced by with the introduction of more than one IBP isoform; ion leakage was reduced 26%-35% and 10% of plants survived temperatures as low as -8 °C. Our results demonstrate that apoplastic expression of multiple L. perenne IBP isoforms shows promise for providing protection to crops susceptible to freeze-induced damage.

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.

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 Ixodes scapularis antifreeze glycoprotein enhances cold tolerance in Drosophila melanogaster

Drosophila melanogaster experience cold shock injury and die when exposed to low non-freezing temperatures. In this study, we generated transgenic D. melanogaster that express putative Ixodes scapularis antifreeze glycoprotein (IAFGP) and show that the presence of IAFGP increases the ability of flies to survive in the cold. Male and female adult iafgp-expressing D. melanogaster exhibited higher survival rates compared with controls when placed at non-freezing temperatures. Increased hatching rates were evident in embryos expressing IAFGP when exposed to the cold. The TUNEL assay showed that flight muscles from iafgp-expressing female adult flies exhibited less apoptotic damage upon exposure to non-freezing temperatures in comparison to control flies. Collectively, these data suggest that expression of iafgp increases cold tolerance in flies by preventing apoptosis. This study defines a molecular basis for the role of an antifreeze protein in cryoprotection of flies.

Transcriptional profiling of stage specific gene expression in the parasitic ciliate Ichthyophthirius multifiliis

The parasitic ciliate, Ichthyophthirius multifiliis (Ich), is among the most important protozoan pathogens of freshwater fish. Ichthyophthirius cannot be grown in cell culture, and the development of effective prophylactic and therapeutic treatments has been hampered by a lack of information regarding genes involved in virulence, differentiation and growth. To help address this issue, we have generated EST libraries from the two major stages of the parasite life cycle that infect and develop within host tissues. A total of 25,084 ESTs were generated from non-normalized libraries prepared from polyA+ RNA of infective theronts and host-associated trophonts, respectively. Cluster analysis identified 5311 unique transcripts (UniScripts), of which 2091 were contigs and 3220 singletons. Extrapolation of the data based on rates of EST discovery suggests that more than half the expected protein-coding genes of I. multifiliis are represented in this data. BLASTX comparisons against GenBank nr, UniProtKB (SwissProt and TrEMBL), as well as Tetrahymena thermophila, Plasmodium falciparum, and Paramecium tetraurelia protein databases produced 3694 significant (E-value ≤1e(-10)) hits, of which 1178 were annotated using gene ontology (GO) analysis. A high proportion of UniScripts (63%) showed similarity to other ciliate proteins. When combined with expression profiling data, GO ontology analysis of Biological Process, Cellular Component, and Molecular Function revealed interesting differences in gene families expressed in the two stages. Indeed, the most abundant transcripts were highly stage-specific and coincided with the metabolic activities associated with each stage. This work provides an effective genomics resource to further our understanding of Ichthyophthirius biology, and lays the groundwork for the identification of potential drug targets and vaccines candidates for the control of this devastating fish pathogen.

Transcriptional effects of dietary exposure of oil-contaminated Calanus finmarchicus in Atlantic herring (Clupea harengus)

Suppression subtractive hybridization (SSH) cDNA library construction and characterization was used to identify differentially regulated transcripts from oil exposure in liver of male Atlantic herring (Clupea harengus) fed a diet containing 900 mg crude oil/kg for 2 mo. In total, 439 expressed sequence tags (EST) were sequenced, 223 from the forward subtracted library (enriched for genes putatively upregulated by oil exposure) and 216 from the reverse subtracted library (enriched for genes putatively downregulated by oil exposure). Follow-up reverse-transcription (RT) quantitative polymerase chain reaction (qPCR) analyses of gene transcription were conducted on additional herring exposed to food containing 9 (low), 90 (medium), and 900 (high) mg crude oil/kg feed for 2 mo. Chronic exposure of Atlantic herring to an oil-contaminated diet mediated upregulation of transcripts encoding antifreeze proteins, proteins in the classical complement pathway (innate immunity), and iron-metabolism proteins. Gene ontology (GO) analysis showed that "cellular response to stress," "regulation to biological quality," "response to abiotic stimuli," and "temperature homeostasis" were the most affected go at the biological processes level, and "carbohydrate binding," "water binding," and "ion binding" at the molecular function level. Of the genes examined with RT-qPCR, CYP1A, antifreeze protein, retinol binding protein 1, deleted in malignant brain tumor 1, and ovary-specific C1q-like factor demonstrated a significant upregulation. Myeloid protein 1, microfibrillar-associated protein 4, WAP65, and pentraxin were downregulated in liver of fish from the high exposure group. In conclusion, this study suggests that 2 mo of oil exposure affected genes encoding proteins involved in temperature homeostasis and possible membrane stability in addition to immune-responsive proteins in Atlantic herring.

Influence of sulfation on anti-myocardial ischemic activity of Ophiopogon japonicus polysaccharide

Ophiopogon japonicus polysaccharide (FOJ-5) from Radix ophiopogonis has shown anti-myocardial ischemic action in vitro and in vivo in our previous studies. In order to clarify the influence of chemical modifications on the action, a series of sulfated FOJ-5 (FOJ-5-S) with different substitution degrees were prepared and the anti-myocardial ischemic action of the natural FOJ-5 and the FOJ-5-S were studied in vitro and in vivo. Langendorff isolated rat hearts and acute myocardial ischemic rats induced by isoprenaline were employed as myocardial ischemic models in our experiments. The amplitude and frequency of cardiac contraction, coronary blood flow at different time points after ischemia/reperfusion were measured in vitro. The ST segment shift in electrocardiogram and lactate dehydrogenase level in blood plasma were observed on the in vivo model. The results indicated that FOJ-5 and FOJ-5-S had the anti-myocardial ischemic action compared with non-treated vehicle groups. Furthermore, it was found that FOJ-5-S had significant action on the in vivo model compared with FOJ-5 (P < 0.05). And the obtained results from the further study also indicated that only when the degree of substitution was in a certain range, the FOJ-5-S had excellent anti-myocardial ischemic activity.

Protection of liposomes against fusion during drying by oligosaccharides is not predicted by the calorimetric glass transition temperatures of the dry sugars

Sugars play an important role in the desiccation tolerance of most anhydrobiotic organisms. It has been shown in previous studies that different structural families of oligosaccharides have different efficacies to interact with phospholipid headgroups and protect membranes from solute leakage during drying. Here, we have compared three families of linear oligosaccharides (fructans (inulins), malto-oligosaccharides, manno-oligosaccharides) for their chain-length dependent protection of egg phosphatidylcholine liposomes against membrane fusion. We found increased protection with chain length up to a degree of polymerization (DP) of 5 for malto-oligosaccharides, and a decrease for inulins and manno-oligosaccharides. Differential scanning calorimetry measurements showed that for all sugars the glass transition temperature (T_g) increased with DP, although to different degrees for the different oligosaccharide families. Higher T_g values resulted in reduced membrane fusion only for malto-oligosaccharides below DP5. Contrary to expectation, for inulins, manno-oligosaccharides and malto-oligosaccharides of a DP above five, fusion increased with increasing T_g, indicating that other physical parameters are more important in determining the ability of different sugars to protect membranes against fusion during drying. Further research will be necessary to experimentally define such parameters.

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.

Liposomal nanocapsules in food science and agriculture

Liposomes, spherical bilayer vesicles from dispersion of polar lipids in aqueous solvents, have been widely studied for their ability to act as drug delivery vehicles by shielding reactive or sensitive compounds prior to release. Liposome entrapment has been shown to stabilize encapsulated, bioactive materials against a range of environmental and chemical changes, including enzymatic and chemical modification, as well as buffering against extreme pH, temperature, and ionic strength changes. Liposomes have been especially useful to researchers in studies of various physiological processes as models of biological membranes in both eukaryotes and prokaryotes. Industrial applications include encapsulation of pharmaceuticals and therapeutics, cosmetics, anti-cancer and gene therapy drugs. In the food industry, liposomes have been used to deliver food flavors and nutrients and more recently have been investigated for their ability to incorporate food antimicrobials that could aid in the protection of food products against growth of spoilage and pathogenic microorganisms. In this review we briefly introduce key physicochemical properties of liposomes and review competing methods for liposome production. A survey of non-agricultural and food applications of liposomes are given. Finally, a detailed up-to-date summary of the emerging usage of liposomes in the food industry as delivery vehicles of nutrients, nutraceuticals, food additives, and food antimicrobials is provided.

Characterization of antifreeze protein gene expression in summer spruce budworm larvae

Not surprisingly, in the spruce budworm, Choristoneura fumiferana, antifreeze protein (AFP) gene expression is most abundant in the second instar, overwintering stage. However, low level RNA and protein expression was also found in the sixth instar larvae, a summer stage. In situ hybridization further confirmed the presence of AFP mRNA in sixth instar midgut tissues. Sequencing of cDNAs corresponding to "summer-expressed" transcripts revealed an isoform that was not apparent in a cDNA library made to second instar larvae. Although similar to AFP cDNAs obtained from overwintering larvae, this AFP-like isoform (CfAFP6) has two Cys substitutions. Since AFPs from this species fold into a beta-helix that is stabilized by disulfide bonds, it was of interest to determine if this summer-expressed isoform had AFP activity. No thermal hysteresis activity was found when CfAFP6 was cloned and expressed in E. coli, even after in vitro denaturation and refolding. As well, there was no activity detected when the sequence of a known, active isoform was changed to mimic the Cys substitutions in CfAFP6. Since CfAFP6 does not appear to contribute to freeze resistance, its apparent absence in the overwintering second instar should not in itself be considered curious.

Hyperactive spruce budworm antifreeze protein expression in transgenic Drosophila does not confer cold shock tolerance

Drosophila melanogaster, a freeze intolerant and cold shock sensitive insect, was transformed with the hyperactive insect antifreeze protein gene (AFP) from the spruce budworm, Choristoneura fumiferana. Transformation P-element constructs (pCasper) were made with CfAFP 337 isoform DNA using a strong constitutive promoter, Actin 5c. This is the first report of insect AFP used to transform another insect. Properly folded active insect AFP was only detected when signal sequences were used to target proteins to the endoplasmic reticulum for secretion into the hemolymph. The 18 residue Drosophila binding protein signal sequence (BiP) constructs resulted in transformed fly lines with significantly higher AFP expression in hemolymph than when the native C. fumiferana AFP signal sequence was used. The resultant transgene fly lines have the highest levels of thermal hysteresis, 0.8 degrees C, seen for any engineered Drosophila. Despite the high level of expression, even higher than some overwintering fish with natural levels of endogenous AFP, the transformants did not display any cold shock resistance compared to controls or low AFP expressing lines. These results indicate that insect AFP alone cannot protect Drosophila from cold shock and may not be useful for Drosophila cryopreservation.

'Antifreeze' glycoproteins from polar fish

Antifreeze glycoproteins (AFGPs) constitute the major fraction of protein in the blood serum of Antarctic notothenioids and Arctic cod. Each AFGP consists of a varying number of repeating units of (Ala-Ala-Thr)n, with minor sequence variations, and the disaccharide beta-D-galactosyl-(1-->3)-alpha-N-acetyl-D-galactosamine joined as a glycoside to the hydroxyl oxygen of the Thr residues. These compounds allow the fish to survive in subzero ice-laden polar oceans by kinetically depressing the temperature at which ice grows in a noncolligative manner. In contrast to the more widely studied antifreeze proteins, little is known about the mechanism of ice growth inhibition by AFGPs, and there is no definitive model that explains their properties. This review summarizes the structural and physical properties of AFGPs and advances in the last decade that now provide opportunities for further research in this field. High field NMR spectroscopy and molecular dynamics studies have shown that AFGPs are largely unstructured in aqueous solution. While standard carbohydrate degradation studies confirm the requirement of some of the sugar hydroxyls for antifreeze activity, the importance of following structural elements has not been established: (a) the number of hydroxyls required, (b) the stereochemistry of the sugar hydroxyls (i.e. the requirement of galactose as the sugar), (c) the acetamido group on the first galactose sugar, (d) the stereochemistry of the beta-glycosidic linkage between the two sugars and the alpha-glycosidic linkage to Thr, (e) the requirement of a disaccharide for activity, and (f) the Ala and Thr residues in the polypeptide backbone. The recent successful synthesis of small AFGPs using solution methods and solid-phase chemistry provides the opportunity to perform key structure-activity studies that would clarify the important residues and functional groups required for activity. Genetic studies have shown that the AFGPs present in the two geographically and phylogenetically distinct Antarctic notothenioids and Arctic cod have evolved independently, in a rare example of convergent molecular evolution. The AFGPs exhibit concentration dependent thermal hysteresis with maximum hysteresis (1.2 degrees C at 40 mg x mL-1) observed with the higher molecular mass glycoproteins. The ability to modify the rate and shape of crystal growth and protect cellular membranes during lipid-phase transitions have resulted in identification of a number of potential applications of AFGPs as food additives, and in the cryopreservation and hypothermal storage of cells and tissues.

Lipid unsaturation determines the interaction of AFP type I with model membranes during thermotropic phase transitions

We have previously shown that antifreeze protein (AFP) type I from winter flounder interacts with the acyl chains of lipids in model membranes containing a mixture of dimyristoylphosphatidylcholine (DMPC) and the plant thylakoid lipid digalactosyldiacylglycerol (DGDG), most likely through hydrophobic interactions. By contrast, in studies with pure phospholipid membranes, no such interaction was seen. DGDG is a highly unsaturated lipid, which renders these studies quite different from the previous studies of AFP-membrane interaction where the lipids were saturated or trans-unsaturated. Therefore, it seemed possible that either the digalactose headgroups or the unsaturated DGDG acyl chains, or both, may be important for interactions of membranes with AFP type I. To distinguish between these possibilities, we catalytically hydrogenated the DGDG to obtain a galactolipid with completely saturated fatty acyl chains. The results with the hydrogenated DGDG were strikingly different from those obtained previously with the unsaturated DGDG; the clear binding of AFPs to the bilayer appeared to be lost. Nevertheless, the temperature-dependent folding of AFP type I was inhibited in the presence of liposomes containing either the unsaturated or the hydrogenated DGDG. The results indicate that the liposomes and protein still interact, even following hydrogenation of the acyl chains, perhaps at the membrane-solution interface.

Plants in a cold climate

Plants are able to survive prolonged exposure to sub-zero temperatures; this ability is enhanced by pre-exposure to low, but above-zero temperatures. This process, known as cold acclimation, is briefly reviewed from the perception of cold, through transduction of the low-temperature signal to functional analysis of cold-induced gene products. The stresses that freezing of apoplastic water imposes on plant cells is considered and what is understood about the mechanisms that plants use to combat those stresses discussed, with particular emphasis on the role of the extracellular matrix.

Temperature-induced extended helix/random coil transitions in a group 1 late embryogenesis-abundant protein from soybean

Group 1 late embryogenesis-abundant (LEA) proteins are a subset of hydrophilins that are postulated to play important roles in protecting plant macromolecules from damage during freezing, desiccation, or osmotic stress. To better understand the putative functional roles of group 1 LEA proteins, we analyzed the structure of a group 1 LEA protein from soybean (Glycine max). Differential scanning calorimetry of the purified, recombinant protein demonstrated that the protein assumed a largely unstructured state in solution. In the presence of trifluoroethanol (50% [w/v]), the protein acquired a 30% alpha-helical content, indicating that the polypeptide is highly restricted to adopt alpha-helical structures. In the presence of sodium dodecyl sulfate (1% [w/v]), 8% of the polypeptide chain adopted an alpha-helical structure. However, incubation with phospholipids showed no effect on the protein structure. Ultraviolet absorption and circular dichroism spectroscopy revealed that the protein existed in equilibrium between two conformational states. Ultraviolet absorption spectroscopy studies also showed that the protein became more hydrated upon heating. Furthermore, circular dichroism spectral measurements indicated that a minimum of 14% of amino acid residues existed in a solvent-exposed, left-handed extended helical or poly (L-proline)-type (PII) conformation at 20 degrees C with the remainder of the protein being unstructured. The content of PII-like structure increased as temperature was lowered. We hypothesize that by favoring the adoption of PII structure, instead of the formation of alpha-helical or beta-sheet structures, group 1 LEA proteins retain a high content of surface area available for interaction with the solvent. This feature could constitute the basis of a potential role of LEA proteins in preventing freezing, desiccation, or osmotic stress damage.

A mechanism for stabilization of membranes at low temperatures by an antifreeze protein

Polar fish, cold hardy plants, and overwintering insects produce antifreeze proteins (AFPs), which lower the freezing point of solutions noncolligatively and inhibit ice crystal growth. Fish AFPs have been shown to stabilize membranes and cells in vitro during hypothermic storage, probably by interacting with the plasma membrane, but the mechanism of this stabilization has not been clear. We show here that during chilling to nonfreezing temperatures the alpha-helical AFP type I from polar fish inhibits leakage across model membranes containing an unsaturated chloroplast galactolipid. The mechanism involves binding of the AFP to the bilayer, which increases the phase transition temperature of the membranes and alters the molecular packing of the acyl chains. We suggest that this change in acyl chain packing results in the reduced membrane permeability. The data suggest a hydrophobic interaction between the peptide and the bilayer. Further, we suggest that the expression of AFP type I in transgenic plants may be significant for thermal adaptation of chilling-sensitive plants.