Anhydrobiosis and humans

Although the idea of putting humans into suspended animation is likely to remain in the realms of science fiction, at least for some time to come, anhydrobiotic organisms affect human welfare and research on anhydrobiosis has produced, or has the potential to produce, technologies of use to us. The ability of many microorganisms to survive anhydrobiotically means they can be dispersed through the air. This results in the contamination and spoilage of food and in the spread of disease. Allergies (hayfever) result from the inhalation of airborne pollen and algae. The seeds of many weed plants are also dispersed by air. Plant-parasitic nematodes cause crop failure and reduced yields.

Their anhydrobiotic stages enable them to be dispersed in dry soil, seeds and plant debris and their resistance limits our ability to control these pests. The infective larvae of many animal parasites are also capable of anhydrobiosis.

On the positive side, the natural abilities of microorganisms and seeds to survive anhydrobiosis enable us to store them for long periods of time. The discovery of the properties of trehalose in stabilising membranes and proteins has led to its use in preserving a wide range of biological products. It has been successfully used to air dry and store antibodies, enzymes and blood coagulation factors. It may also be useful for preserving pharmaceutical products, vaccines and the systems which deliver these to their target sites; liposomes, artificial membrane spheres which are used as drug delivery systems, can be dried and stored with the aid of trehalose. The taste and properties of foods change upon air drying. If dried with trehalose, however, blended fresh eggs, fruit purées, herbs and even fruit slices retain more of the properties of the fresh product than if they are dried without trehalose. Attempts have been made to use trehalose to induce anhydrobiosis in cells, organs and even whole organisms which do not normally have this ability. Simply adding trehalose has had little success since it does not easily pass through cell membranes. The trehalose needs to be inside the cells to have any protective effect and, to preserve a membrane, it needs to attach to both of its sides. This could be achieved by providing cells with the ability to synthesise trehalose using the techniques of genetic engineering. Tobacco plants which have had the enzyme trehalose synthase from yeast inserted into their cells show an increased resistance to drought, by the development of drought avoidance mechanisms.

Drawing on tardigrades for their inspiration, Kunihiro Seki and his colleagues at Kanagawa University in Japan have flushed trehalose through rat hearts before desiccating them by packing them in silica gel which absorbs water. They then perfused the hearts with perfluoro-carbon, a biologically inert compound, and stored them at 4°C in airtight jars. After 10 days, the team were able to revive the hearts and get them beating again. The reasons for their success are a bit perplexing, since the trehalose is unlikely to have penetrated the cells of the heart. However, if the integrity of hearts, and other organs, can be maintained during long-term storage, there are obvious implications for transplant surgery.

We would like to be able to improve our methods of storing biological materials for a variety of medical, commercial and conservation applications. If we can understand how anhydrobiotic organisms survive in their remarkable way, we can apply this knowledge to the storage of cells, organs and organisms which do not naturally possess this ability.

has the problem of anhydrobiosis been solved? Although we have some important clues as to how organisms survive anhydrobiosis, it is clear that we do not, as yet, fully understand the phenomenon. The problem is indeed intimidating, as Michael Potts of the Virginia Polytechnic Institute and State University was led to comment with respect to bacteria: 'the prospects for explaining how a single cell with a complement of some 3 000 proteins can have the bulk of its water removed, remain desiccated for perhaps tens of years . . . and then resume coordinated metabolic activities within seconds of rehy-dration is daunting at best!' Explaining the survival of the much more complex plants and animals is even more difficult.

There is clearly an association between trehalose or sucrose and anhydrobiosis. However, there are many examples where trehalose or sucrose are produced and yet the organism does not survive anhydrobi-osis. While trehalose or sucrose appears to be necessary for anhydrobiosis, it does not appear to be in itself sufficient. The challenge is now to understand what other mechanisms are involved. Although trehalose can stabilise proteins and membranes in the desiccated cell, we need to investigate how anhydrobiotic organisms solve the higher order problems of metabolic and physiological integration during desiccation and rehydration. There are many problems to be solved before we fully understand the phenomenon of life without water.

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