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

有毒金属离子是一类重要的环境污染物，对陆地和水生生态系统都能造成严重的环境破坏。了解有机体如何对金属污染作出反应，以及耐受或敏感的分子机制至关重要。镉是一种重要的环境污染物，（对绝大多数生物来说）在正常的生物化学中不起作用，因此仅仅是一种有毒金属。对镉的一种近乎普遍的反应是，许多生物体上调了一类被称为金属硫蛋白的富含硫的小蛋白质（硫基团与金属离子结合）。然而，另一种生化反应是植物螯合蛋白的产生——这些分子在化学上与金属硫蛋白相似，但更小。植物螯合蛋白最初被认为是植物和一些真菌所特有的，但现在在一种动物物种——居住在土壤中的线虫（线虫）秀丽隐杆线虫中发现了它。这些基因在线虫对有毒金属离子的反应能力中显然是极其重要的：敲除线虫体内植物螯合素合成基因意味着它对镉过敏，但相比之下，敲除金属硫蛋白基因对其应对镉毒性的能力只有非常小的影响。显然，在这种情况下，如果我们不研究植物螯合素的反应，我们就不能指望正确地理解镉的毒性。我们要问的问题是——有多少其他动物（除了蛔虫）会产生植物螯合素，它们会使用它们来应对镉中毒吗？基因序列数据表明，植物螯合素基因可能在蚯蚓和海胆等不同的动物身上发现，但这并不能证实这些动物仍然可以制造植物螯合素，更不用说它们是否对镉敏感了。（在不同的物种中，许多基因失去或改变了它们的功能。）因此，我们将使用化学分析直接检测植物螯合素-从而绝对确认其功能作用。我们将选择一些不同的水生和陆生无脊椎动物物种（包括蚯蚓、淡水甲壳类动物、软体动物和陆生节肢动物），并将它们暴露于镉中以获得剂量-反应曲线。然后，我们将分析未暴露和镉处理的样品，从而看看(a)我们是否可以在这些非常不同的物种中检测到植物螯合素，(b)如果可以，它们是否会因镉而增加。这个项目将增加我们对一种基本生物化学机制的基本科学理解，这种机制目前在动物身上几乎完全未知。此外，如果植物螯合素被发现并被证明具有金属响应性，它将为监测有毒金属对无脊椎动物的环境污染影响提供一套潜在的新的分子生物标志物。

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.