Sylvain Demuynck is fascinated by the enormous variety of invertebrate life that populates our planet. He always has been, but at the moment he's focusing on just one: the annelid worm Nereis diversicolor. Found living in brackish coastal waters, the worm's habitats can be close to ex-industrial sites that are often polluted by heavy metals such as cadmium. Unfortunately cadmium is not an element required by animals; in fact it can compete with other essential metals such as zinc or iron, causing serious metabolic problems for organisms exposed to high doses. Fortunately, some estuarine species have overcome the dangers posed by heavy metals by mopping them up with proteins known as metallothioneins. But Nereis has no such defences. So just how does our wriggly friend survive on contaminated coasts?Puzzled by this conundrum, Demuynck and his colleagues have gone some way towards unravelling the mystery of this invertebrate's toxic tolerance().
A major entry route for heavy metals is via the gut, so Demuynck and his team examined the intestines of cadmium-exposed worms with chromatography to look for proteins that might be binding the element. They found cadmium bound to a protein called MPII. Demuynck already knew that MPII is very similar to a Nereis protein called myohemerythrin, which usually binds iron and carries oxygen round the worm's primitive blood system. Could the mysterious protein be involved in decontaminating the heavy metal?
The team began analysing the worms' intestines with an anti-MPII antibody,and found many MPII-positive cells in the lining of the gut; an unusual location for a blood-related molecule. Demuynck's suspicions that this protein might play a role in removing cadmium were confirmed when he found an increase in MPII protein levels when the worms were exposed to the metal. In addition the MPII cells changed in morphology, suggesting protein production was being stepped up in response to cadmium poisoning.
Curious to find how levels of the decontaminating protein are regulated,the team looked at the levels of mRNA in Nereis's gut cells and found that mRNA didn't rise in response to cadmium exposure; regulation of the protein must occur at the post-transcriptional level. This could either be achieved by an increase in protein translation from an existing pool of mRNA or by increasing the stability of MPII, but which mechanism the invertebrate uses is unclear. In addition, the precise function of the protein in these worms remains a mystery. Demuynck suspects that different forms of the protein perform distinct functions; perhaps one form carries oxygen in the blood,while another removes cadmium.
Digging by hand and losing your boots in the mud of a smelly estuary might not sound like everyone's idea of a great science project, but Demuynck loves it, and hopes that by trawling through stinking mud for worms, he will eventually work out how these toxins are taken up and how they affect the animals that make their homes there. But while he is confident that the mystery of Nereis' toxic tolerance may be a step closer to being solved, Demuynck adds a note of caution that the levels and mixtures of pollutants accumulating on our industrial coastlines is becoming worryingly high, not just for fauna, but for humans too.
