If the worm turns blue, the river's polluted
Ruth McKernan looks at the development and use of `biological litmus tests'
Tuesday 04 April 1995
A group in Nottingham is using a modified strain of worm that is normally colourless but can turn blue when exposed to organic chemicals in water and soil. Meanwhile, in the US, genetically modified flies form the basis of a new laboratory test for detecting teratogenic drugs such as thalidomide, potentially reducing the need to experiment on pregnant rats and mice.
Both groups are exploiting aphenomenon that occurs in animal cells when subject to stress. When exposed to sudden rapid increases in temperature, toxic chemicals or radioactivity, all cells try to protect their cellular components from breaking down. Heat or radiation will kill animal cells in much the same way that they melt jelly or scramble eggs. Cells react by producing a large family of molecules, some of which act like scaffolding to support key enzymes and structural components of the cell, maintaining their integrity and keeping them intact.
Such molecules are nicknamed "molecular chaperones''. The supporting molecules are present not only at times of stress but also under normal conditions during development, when they nurture the correct formation of cell components.
The production of stress proteins was first observed 30 years ago by the Italian biologist FM Ritossa. He saw segments of DNA on chromosomes from flies swell up when exposed to high temperature. Ten years later scientists deduced that the "puffed-up" DNA signified active genes producing protective molecules which they called "heat shock protein''. They now know such genes can be activated by a range of stressful stimuli, not just heat, and that many behave as molecular chaperones.
Stress proteins are difficult to measure in the laboratory, so scientists have linked this stress response to an enzyme that is easy to measure. This biochemical trick is achieved by stitching together the gene for one heat shock protein with the gene for the enzyme galactosidase, which can convert a colourless sugar to a strong, royal blue product, easily detectable in minute amounts. The more toxic the chemical, the bluer the worms.
The modified worms used by Dr de Pomerai, a lecturer in life science at the University of Nottingham, produce the stress protein and the "reporter" enzyme when exposed to pollution.Aquatic pollution is usually detected by measuring the mortality rate of the microscopic plankton, Daphnia or the effects on growth of invertebrates and fish. In some countries, analysts put baby trout in the water and return a few days later to count the number of dead ones.
Water pollution is difficult to measure because it can be transient whereas soil pollution tends to be longer lasting. According to Dr de Pomerai, the blue worm system can be applied to both types of pollution. "We have looked at organic pollutants such as phenols, some common insecticides, fungicides and at a whole range of heavy metals and they all work.''
Dr de Pomerai can pick up changes in response to heavy metals over seven hours, much quicker than the current plankton-based tests which take between 24 and 72 hours. His next step is to test worms in samples of naturally polluted water. He estimates that it will take a further year of analysis to be confident that the test is viable.
In Canada, scientists are closer to a commercially available test. Dr Peter Candido, at the University of British Columbia, uses the same soil-dwelling worm, C. Elegans, and the same reporter enzyme, but it is linked to a more sensitive stress protein gene. The genetically modified organism has now been patented by the biotechnology company Stressgen.
The biological litmus test isalso being developed to detect whether new drugs have harmful side-effects. Researchers usually test new medicines onpregnant rats or mice and look for malformations in the foetuses or born offspring. The number of laboratory animals could be reduced by a test-tube system developed by Richard Voellmy, professor of biochemistry at the University of Miami Medical School. "Our method could limit the number of animals used," he claims. "It will not replace animal studies altogether, but it could be easily used in the early stages of drug development.''
Professor Voellmy's test uses the same type of reporter system as Dr de Pomerai's pollution detector. The reporter package (the hybrid gene that regulates synthesis of both the stress protein and the enzyme galactosidase) is put into the fly's genetic material permanently so that it is inherited. Genetically modified embryos just two hours old are collected and added to the test compounds. Teratogenic chemicals turn the cells blue. The test works for ethanol, thalidomide and steroids, according to Professor Voellmy. The system has been patented by the University of Miami.
According to Jeanie McGuire, in the university's technology transfer office, several drug companies have shown interest in the test, but the US Food and Drug Administration, America's regulatory body for new drugs, requires teratogenic drugs to be appropriately tested on animals. "There is no pressure on companies to use the test. It is ahead of its time,'' she says.
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