05 - Heavy Metal: Effects of Environmental Toxins on C. elegans
Background Information Environmental Toxicology and Bioassays
Environmental toxicology is the study of the effects of harmful chemicals on human health and the environment. This field rocketed into the public consciousness in 1962 with Rachel Carson’s book “Silent Spring” which looked at the effects of dichlorodiphenyltrichloroethane (DTT) on humans and wildlife. Today, scientists from a range of backgrounds work to determine where a toxicant will end up in an ecosystem and how long it will remain. Using a series of bioassays – tests that measure the potency of a substance by observing its effects on molecules, cells, tissues or organisms – toxicologists also predict safe exposure limits that will keep the environ- ment and us healthy.
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Predictive toxicol- ogy focuses on accurately forecasting the interac- tion between a toxicant and different biological systems and can be gener- ally divided between in vitro and in vivo studies. In
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Figure 1: C. elegans anatomy
vitro, Latin for “in the glass”, involves exposing molecules, cells, tissues and certain microorgan- ism to both a standard neutral solution and to a concentration gradient of a toxicant. These targets are then monitored to see whether or when certain adverse effects — called endpoints — occur. For example, common endpoints in cell based in vitro studies are membrane damage, nuclear condensation, and apoptosis. In vitro tests are easy to carry out in large replicates, less expensive, and allow researchers to focus on specific biological processes. Alternatively, the potency of a toxicant can be tested by observing how it affects the behavior, growth, reproduction, or lifespan of popular animal models. This is in vivo or “within the living” testing. These tests account for the fact that individuals have developed protective mechanisms to reduce the damaging effects of certain toxins, but that these mechanisms mainly manifest at the organism level. For example, when an animal is exposed to a high metal dose, several systems often work together to expel the absorbed molecules, decrease environmental uptake, and activate general stress responses. In addition, in vivo studies can highlight emergent health threats that may have been missed during studies that only focus on a certain cell type or a single organ. However, in vivo studies are also slow, costly, and raise both legal and moral is- sues related to animal rights. Bioassays using Caenorhabditis elegans (see-no-rab-DITE-iss el-leh-GANS) offer many of the conveniences of in vitro testing with the fuller picture provided by in vivo testing. Because C. elegans is a popular model organism in other research fields like development, neurology, and genetics there is a wealth of scientific data that can guide and enhance basic toxicology tests. Today, these organisms are increasingly being used to test the safety of potential pollutants. Why Study C. elegans A model organism is any plant, animal or microorganism that allows us to study fundamental questions in biology that may be hard to study directly in complex organisms like humans. In the 1970s, Dr. Sydney Brenner established the nematode C. elegans as a model organism because they have a simple genome, a fast generation time, and are easy and inexpensive to
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