Biochemical and physiological aspects of anhydrobiosis

Water content, transition temperature and fragility influence protection and anhydrobiotic capacity

Water is essential for metabolism and all life processes. Despite this, many organisms distributed across the kingdoms of life survive near-complete desiccation or anhydrobiosis. Increased intracellular viscosity, leading to the formation of a vitrified state is necessary, but not sufficient, for survival while dry. What properties of a vitrified system make it desiccation-tolerant or -sensitive are unknown. We have analyzed 18 different in vitro vitrified systems, composed of one of three protective disaccharides (trehalose, sucrose, or maltose) and glycerol, quantifying their enzyme-protective capacity and their material properties in a dry state. Protection conferred by mixtures containing maltose correlates strongly with increased water content, increased glass-transition temperature, and reduced glass former fragility, while the protection of glasses formed with sucrose correlates with increased glass transition temperature and the protection conferred by trehalose glasses correlates with reduced glass former fragility. Thus, in vitro different vitrified sugars confer protection through distinct material properties. Next, we examined the material properties of a dry desiccation tolerant and intolerant life stage from three different organisms. The dried desiccation tolerant life stage of all organisms had an increased glass transition temperature and reduced glass former fragility relative to its dried desiccation intolerant life stage. These results suggest in nature organismal desiccation tolerance relies on a combination of various material properties. This study advances our understanding of how protective and non-protective glasses differ in terms of material properties that promote anhydrobiosis. This knowledge presents avenues to develop novel stabilization technologies for pharmaceuticals that currently rely on the cold-chain.

Statement of significance

For the past three decades the anhydrobiosis field has lived with a paradox, while vitrification is necessary for survival in the dry state, it is not sufficient. Understanding what property(s) distinguishes a desiccation tolerant from an intolerant vitrified system and how anhydrobiotic organisms survive drying is one of the enduring mysteries of organismal physiology. Here we show in vitro the enzyme-protective capacity of different vitrifying sugars can be correlated with distinct material properties. However, in vivo, diverse desiccation tolerant organisms appear to combine these material properties to promote their survival in a dry state.

Water content, transition temperature and fragility influence protection and anhydrobiotic capacity

Water is essential for metabolism and all life processes. Despite this, many organisms distributed across the kingdoms of life survive near-complete desiccation or anhydrobiosis (Greek for "life without water"). Increased intracellular viscosity, leading to the formation of a vitrified state is necessary, but not sufficient, for survival while dry. What properties of a vitrified system make it desiccation-tolerant or -sensitive are unknown. We have analyzed 18 different in vitro vitrified systems, composed of one of three protective disaccharides (trehalose, sucrose, or maltose) and varying amounts of glycerol, quantifying their enzyme-protective capacity and their material properties in a dry state. We find that protection conferred by mixtures containing maltose correlates strongly with increased water content, increased glass-transition temperature, and reduced glass former fragility, while the protection of glasses formed with sucrose correlates with increased glass transition temperature and the protection conferred by trehalose glasses correlates with reduced glass former fragility. Thus, in vitro different vitrified sugars confer protection through distinct material properties. Extending on this, we have examined the material properties of a dry desiccation tolerant and intolerant life stage from three different organisms. In all cases, the dried desiccation tolerant life stage of an organism had an increased glass transition temperature relative to its dried desiccation intolerant life stage, and this trend is also seen in all three organisms when considering reduced glass former fragility. These results suggest that while drying of different protective sugars in vitro results in vitrified systems with distinct material properties that correlate with their enzyme-protective capacity, in nature organismal desiccation tolerance relies on a combination of these properties. This study advances our understanding of how protective and non-protective glasses differ in terms of material properties that promote anhydrobiosis. This knowledge presents avenues to develop novel stabilization technologies for pharmaceuticals that currently rely on the cold-chain.

Carbonylation accumulation of the Hypsibius exemplaris anhydrobiote reveals age-associated marks

Together with nematodes and rotifers, tardigrade belong to micrometazoans that can cope with environmental extremes such as UV and solar radiations, dehydration, supercooling or overheating. Tardigrade can resist the harshest conditions by turning to cryptobiosis, an anhydrobiotic state that results from almost complete dehydration and is characterized by an ametabolic status. Although reports have challenged the molecular basis of the mechanisms underlying genomic injury resistance, little is yet known regarding the possible involvement of other tardigrade macromolecules in injury during a stress experience. In this report, we show that the tardigrade Hypsibius exemplaris can accumulate molecular damages by means of in situ detection of carbonyls. Furthermore, we demonstrate that living tardigrade can accumulate carbonylation. Finally, we reveal that anhydrobiotic tardigrade can be constitutively affected by carbonylation that marks aging in other metazoans.

Preservation of membranes in anhydrobiotic organisms: the role of trehalose

Trehalose is a nonreducing disaccharide of glucose commonly found at high concentrations in anhydrobiotic organisms. In the presence of trehalose, dry dipalmitoyl phosphatidylcholine (DPPC) had a transition temperature similar to that of the fully hydrated lipid, whereas DPPC dried without trehalose had a transition temperature about 30 degrees Kelvin higher. Results obtained with infrared spectroscopy indicate that trehalose and DPPC interact by hydrogen bonding between the OH groups in the carbohydrate and the polar head groups of DPPC. These and previous results show that this hydrogen bonding alters the spacing of the polar head groups and may thereby decrease van der Waals interactions in the hydrocarbon chains of the DPPC. This interaction between trehalose and DPPC is specific to trehalose. Hence this specificity may be an important factor in the ability of this molecule to stabilize dry membranes in anhydrobiotic organisms.

DNA damage in storage cells of anhydrobiotic tardigrades

In order to recover without any apparent damage, tardigrades have evolved effective adaptations to preserve the integrity of cells and tissues in the anhydrobiotic state. Despite those adaptations and the fact that the process of biological ageing comes to a stop during anhydrobiosis, the time animals can persist in this state is limited; after exceedingly long anhydrobiotic periods tardigrades fail to recover. Using the single cell gel electrophoresis (comet assay) technique to study the effect of anhydrobiosis on the integrity of deoxyribonucleic acid, we showed that the DNA in storage cells of the tardigrade Milnesium tardigradum was well protected during transition from the active into the anhydrobiotic state. Specimens of M. tardigradum that had been desiccated for two days had only accumulated minor DNA damage (2.09 +/- 1.98% DNA in tail, compared to 0.44 +/- 0.74% DNA in tail for the negative control with active, hydrated animals). Yet the longer the anhydrobiotic phase lasted, the more damage was inflicted on the DNA. After six weeks in anhydrobiosis, 13.63 +/- 6.41% of DNA was found in the comet tail. After ten months, 23.66 +/- 7.56% of DNA was detected in the comet tail. The cause for this deterioration is unknown, but oxidative processes mediated by reactive oxygen species are a possible explanation.

Oxidative stress and its effects during dehydration

Water is usually thought to be required for the living state, but several organisms are capable of surviving complete dehydration (anhydrobiotes). Elucidation of the mechanisms of tolerance against dehydration may lead to development of new methods for preserving biological materials that do not normally support drying, which is of enormous practical importance in industry, in clinical medicine as well as in agriculture. One of the molecular mechanisms of damage leading to death in desiccation-sensitive cells upon drying is free-radical attack to phospholipids, DNA and proteins. This review aims to summarize the strategies used by anhydrobiotes to cope with the danger of oxygen toxicity and to present our recent results about the importance of some antioxidant defense systems in the dehydration tolerance of Saccharomyces cerevisiae, a usual model in the study of stress response.

Non-equivalence of D- and L-trehalose in stabilising alkaline phosphatase against freeze-drying and thermal stress. Is chiral recognition involved?

Comparison of the ability of the enantiomeric forms of trehalose to stabilise alkaline phosphatase towards dehydration and heat showed that natural D-trehalose is superior to L-trehalose, although both disaccharides provide some protection for the enzyme. The result of this novel experiment suggests a chiral differentiation between carbohydrate and protein and thus lends support for the water replacement hypothesis of solute-based stabilisation of biomolecules, but the non-crystallinity and the physical form of the L-isomer may also be a key factor.

Synthesis of L-trehalose and observations on isomer and by-product formation

With a view to gaining evidence on the mechanism by which D-trehalose is able to stabilise biomolecules towards dehydration (anhydrobiosis) and heat, L-trehalose has been prepared in order to allow comparative studies to be made. Little change can be induced in the ratio of the alpha,alpha-, alpha,beta-, beta,beta-1,1'-stereoisomers of the disaccharide formed from 2,3,4,6-tetra-O-benzyl-L-glucose by using different reaction procedures and by varying the reaction conditions. Benzyl 2,3,4,6-tetra-O-benzyl alpha- and beta-L-glucopyranoside are by-products in the trimethylsilyl trifluoromethanesulphonate mediated formation of the 1,1'-linked disaccharides.

Experimentally induced anhydrobiosis in the tardigrade Richtersius coronifer: phenotypic factors affecting survival

The ability of some animal taxa (e.g., nematodes, rotifers, and tardigrades) to enter an ametabolic (cryptobiotic) state is well known. Nevertheless, the phenotypic factors affecting successful anhydrobiosis have rarely been investigated. We report a laboratory study on the effects of body size, reproductive condition, and energetic condition on anhydrobiotic survival in a population of the eutardigrade Richtersius coronifer. Body size and energetic condition interacted in affecting the probability of survival, while reproductive condition had no effect. Large tardigrades had a lower probability of survival than medium-sized tardigrades and showed a positive response in survival to energetic condition. This suggests that energy constrained the possibility for large tardigrades to enter and to leave anhydrobiosis. As a possible alternative explanation for low survival in the largest specimens we discuss the expression of senescence. In line with the view that processes related to anhydrobiosis are connected with energetic costs we documented a decrease in the size of storage cells over a period of anhydrobiosis, showing for the first time that energy is consumed in the process of anhydrobiosis in tardigrades.

Alterations in the levels of glycogen and glycogen synthase transcripts during desiccation in the insect-killing nematode Steinernema feltiae IS-6

The ability to withstand desiccation by entering anhydrobiosis is important for the survival of many nematode species. We are interested in the metabolic changes that occur during dehydration in the semiarid strain IS-6 of the insect parasitic nematode Steinernema feltiae. These changes may enable IS6 to be more tolerant to desiccation than temperate strains. We identified genes of IS-6 that exhibit changes in transcript levels during dehydration. These included glycogen synthase (Sf-gsy-1), which is the rate-limiting enzyme in the synthesis of glycogen, which is likely to play a role in desiccation survival. We established the changes in the steady state level of Sf-gsy-1 transcripts upon dehydration and determined the biochemical changes in the level of its product, glycogen, during the dehydration and rehydration of nematodes. Our results suggest a shift from glycogen to trehalose synthesis during dehydration, which is regulated at least in part by suppression of glycogen synthase transcription.

The thermodynamic mechanism of protein stabilization by trehalose

The stabilization of ribonuclease A by alpha-alpha-trehalose was studied by preferential interaction and thermal unfolding. The protein is stabilized by trehalose at pH 2.8 and pH 5.5. Wyman linkage analysis showed increased exclusion of trehalose from the protein domain on denaturation. Preferential interaction measurements were carried out at 52 degrees C at pH 5.5 and 2.8, where the protein is native and unfolded, respectively, and at 20 degrees C where the protein is native at both pH values. At the low temperature, the interaction was preferential exclusion. At 52 degrees C the interaction was that of preferential binding, greater to the native than the unfolded protein, the increment on denaturation being identical to that deduced from the Wyman analysis. The stabilizing effect of trehalose can be fully accounted by the change in transfer free energy on unfolding. The temperature dependence of the preferential interactions of 0.5 M trehalose with ribonuclease A showed that it is the smaller preferential binding to the unfolded protein than to the native one which gives rise to the stabilization. A thermodynamic analysis of the data led to approximate values of the transfer enthalpies and transfer entropies for the trehalose-ribonuclease A system.

Metabolic responses to anabiosis in the fourth stage juveniles of Ditylenchus dipsaci (Nematoda)

The fourth stage juveniles of the stem nematode Ditylenchus dipsaci can lose almost all of their body water and survive in an anabiotic state for long periods of time. Desiccation of the juveniles does not seem to result in any appreciable denaturation of the metabolic enzymes. Comparison of the metabolite profiles of active and anabiotic juveniles shows a decrease in the content of the glycolytic intermediates in the anabiotic stages, but little change in the tricarboxylic acid cycle intermediates. The adenylate charge is greatly reduced in the anabiotic state, but ATP is still present in measurable amounts. The dry anabiotic juveniles of D. dipsaci are permeable to water and hydrate rapidly, although there is a lag phase of 2-3 h after hydration before spontaneous activity starts. Metabolism, as judged by heat output, oxygen uptake or 14 CO 2 production from labelled substrate begins immediately after hydration. The metab­olite levels also recover quickly; the ATP levels, however, do not return to normal for several hours and there is evidence that for some time after hydration the mitochondria are essentially uncoupled. During the dehydration-hydration cycle of the juveniles, membrane function is disrupted and the lag phase following hydration may result from the time required for the re-establishment of ionic and metabolic gradients within the tissues.

Protein reaction kinetics in a room-temperature glass

Protein reaction kinetics in aqueous solution at room temperature are often simplified by the thermal averaging of conformational substates. These substates exhibit widely varying reaction rates that are usually exposed by trapping in a glass at low temperature. Here, it is shown that the solvent viscosity, rather than the low temperature, is primarily responsible for the trapping. This was demonstrated by placement of myoglobin in a glass at room temperature and subsequent observation of inhomogeneous reaction kinetics. The high solvent viscosity slowed the rate of crossing the energy barriers that separated the substates and also suppressed any change in the average protein conformation after ligand dissociation.

Degradation of functional integrity during long-term storage of a freeze-dried biological membrane

Trehalose, and to some extent a few other carbohydrates, is capable of stabilizing the structure and function of isolated biological membranes during lyophilization. In this paper the results of investigations into the long-term stability of the lyophilized membrane-carbohydrate mixtures were reported. The effects of varying water content, oxygen level, and light on the rates of oxidation, browning, and degradation of biological activity were reported. The efficiency with which three carbohydrates stabilized membrane structure was also reported, with glucose shown to be less efficient than maltose or trehalose. Increased water content accelerated loss of biological activity, possibly because, under the same conditions, nonenzymatic browning and photooxidation were accelerated also. Glucose-containing samples were especially unstable at elevated humidities. Efficiency of preservation could be maximized by storage under conditions of low oxygen, low humidity, and dark, and by the inclusion of high levels of trehalose.

Infrared spectroscopic studies on interactions of water and carbohydrates with a biological membrane

Infrared spectroscopy was used to investigate the changes in bands assigned to phospholipids and proteins in dehydrated and rehydrated sarcoplasmic reticulum. The changes in CH2 and CH3 stretching bands, amide bands, and phosphate stretching bands are similar to shifts in frequency seen for those bands in phospholipid and protein preparations during thermotropic phase transitions and hydration. IR studies on dry trehalose-sarcoplasmic reticulum mixtures show similar results; with increasing trehalose concentration in the dry mixtures, amide and phosphate bands shift to frequencies characteristic of hydrated samples. Changes in bands assigned to OH deformations in the trehalose suggest that the interaction between the carbohydrate and membrane is by means of hydrogen bonding between these -OH groups and membrane components.