Do phytochelatins play a fundamental role in invertebrate responses to toxic metals?

Lead Research Organisation: Imperial College London
Department Name: Dept of Surgery and Cancer

Abstract

Toxic metal ions are an important class of environmental pollutants, and can cause severe environmental damage, for both terrestrial and aquatic ecosystems. Iit is of utmost importance to understand how organisms respond to metal pollution, and the molecular mechanisms underlying either tolerance or sensitivity. Cadmium is an important environmental pollutant that (for the vast majority of organisms) does not play a role in normal biochemistry, and is thus solely a toxic metal. One near-universal response to cadmium is that many organisms upregulate a class of small, sulphur-rich proteins called metallothioneins (the sulphur groups bind to metal ions). However, an alternative biochemical response is production of phytochelatins - these molecules are chemically similar to metallothioneins, but are smaller. Phytochelatins were originally thought to be unique to plants and some fungi, but have now been found in an animal species, the soil-dwelling roundworm (nematode) Caenorhabditis elegans. These are clearly extremely important in the worm's ability to respond to toxic metal ions: knocking out the genes for phytochelatin synthesis in C. elegans means that it becomes hypersensitive to cadmium, but knocking out the genes for metallothioneins has, in comparison, only a very small effect on its ability to cope with cadmium toxicity. Clearly, in this case, we could not hope to properly understand cadmium toxicity if we did not study phytochelatin responses. We will ask the question - how many other animals (in addition to roundworms) make phytochelatins, and do they use them in response to cadmium toxicity? Genetic sequence data implies that phytochelatin genes may be found in animals as different as earthworms and sea urchins - but this does not confirm that the animals can still even make phytochelatins, let alone whether they are cadmium-responsive. (Many genes lose or change their function in different species.) Hence, we will use chemical analysis to detect phytochelatins directly - thus absolutely confirming their functional role. We will select a number of different aquatic and terrestrial invertebrate species (to include an earthworm; freshwater crustaceans; molluscs; and a terrestrial arthropod), and expose them to cadmium to obtain dose-response curves. We will then analyse both unexposed and cadmium-treated samples, and thus see if (a) we can detect phytochelatins in these very different species, and (b) if so, if they are increased in response to cadmium. This project will increase our basic scientific understanding of a fundamental biochemical mechanism which is currently almost completely unknown in animals. In addition, if phytochelatins are found and turn out to be metal-responsive, it will offer a potential new set of molecular biomarkers for monitoring the effects of environmental contamination by toxic metals in invertebrates.

Publications

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Description Phytochelatins (PCs) are small molecules that help to deal with toxic metals. They were originally thought to be produced only by plants, but it is now realised that they are found in at least some animal species. Our project had the broad aim of determining if PCs are more widely found in the animal kingdom than previously known, and if they are responsive to the toxic metal ion and common pollutant cadmium.



Overall, we successfully confirmed that phytochelatins are produced in response to cadmium in at least one new animal group: earthworms, phylum Annelida. Previously, PCs had only been identified in one animal (Caenorhabditis elegans, phylum Nematoda), and so this project has established that this is likely to be a general and highly conserved biological mechanism for dealing with toxic metal pollution. This will have important consequences for researchers wishing to understand biological responses to toxic metal ions, and might also provide a potential future biomarker, e.g. for environmental monitoring purposes.

We also observed exciting hints that PCs may be cadmium-responsive in other animal species, such as sea squirts (phylum Chordata) - although the much lower concentration levels compared to earthworms, and possible contamination from food sources, mean that this cannot yet be considered definite. This comparative lack of response in the other animal phyla compared to earthworms should not be considered evidence of absence of PC biosynthesis - it is still very possible that another experimental condition (e.g. different levels or time of cadmium exposure, or exposure to different toxic elements such as copper or arsenic) might have resulted in a PC response.