From Here to Affinity: Researchers Aiding in Protection of Birds By Streamlining Method Used to Identify Contaminant Susceptibility

By Jeffrey M. Brodeur, Woods Hole Sea Grant

Lots of folks have pondered the chicken and the egg. For some Sea Grant-funded researchers attempting to understand the impact of environmental contaminants on aquatic birds, the question first revolved around the chicken and the tern.

Several years ago biologists Sibel Karchner and Mark Hahn of the Woods Hole Oceanographic Institution wanted to know why domestic chickens were so sensitive to dioxins - toxic compounds formed by industrial processes involving chlorine - while terns were not.

Their research revealed that it all came down to three amino acids in a protein called the aryl hydrocarbon receptor, or AHR, that holds a total of 858 amino acids. There is a location within the AHR where the dioxin binds, but when two of the three amino acids differ, as was the case in terns, the binding shape changes so the dioxin can't fit as well.

In chickens, however, dioxin fits the shape of the protein like a glove and sets off the harmful effects of the pollutant, a process that alters the function and activity of other genes in the animal's cells.

Now, with additional Woods Hole Sea Grant support, Karchner, a research specialist, and Hahn, a senior scientist, are working on an easier way to measure that binding affinity of the receptors for dioxins so that they can apply the approach to a much wider variety of species.

Sibel Karchner, Mark Hahn and Diana Franks at work in their lab in Woods Hole.

Hahn says that during their work with chickens and terns, the binding assays were much more drawn out. Karchner had to first clone the full cDNA sequence, encoding the full-length protein, and then had to express that protein in a test tube before research associate Diana Franks could complete the binding assays.

But their discovery that only a small part of the protein is responsible for the differences in abilities of the chicken and tern receptors to bind dioxin was liberating.

Because less of the sequence needs to be analyzed, bird species not previously analyzed can be sampled and their affinity for interaction can be determined much more easily, he says.

"The piece we think we would need to determine the binding affinity is only about 170 [amino acids long] or so. A fifth of the whole sequence would be enough to get the information," Hahn says. "It would just be a much more straightforward way to get the answer we're looking for."

Their hope, in part, is that the work will allow wildlife managers and regulatory agencies to better understand the impact of contaminants and make informed decisions to protect bird species, such as the common eider and double-crested cormorant.

"The fact that there are species differences to sensitivity to the contaminants means that the risk assessors don't really know, if they're looking at a specific site or ecosystem, which species are the ones that they should be regulating to protect," Hahn says. "If we can give them some tools for understanding that better or for doing some reasonably straightforward measurements that would help them identify the species that they should be concerned about, that would be helpful."

Karchner and Hahn are working once more with longtime collaborator Sean Kennedy, a wildlife toxicologist with Environment Canada and the University of Ottawa.

Kennedy seized on the "compelling story" in the chicken-tern comparison, and is helping expand the number of species whose affinities are determined, Hahn says. Other species in which the sensitivity values have been tested are the turkey, wood duck, and American kestrel.

Dioxins have been a concern, from an environmental prospective, for more than 40 years. The chemical TCDD that was present in the herbicide Agent Orange widely used during the Vietnam war is one of the most widely know examples of the contaminant.

"It also was [a] by-product of the bleaching of pulp by paper mills and in the incineration of municipal or medical waste," Hahn says. "It's the most potent, most toxic synthetic chemical that we know of, so it takes very, very tiny amounts to cause defects in various animals."

It's also very persistent.

"Once it's out there, it's not easily gotten rid of," Hahn says. "There are places, [such as] the Great Lakes, that became contaminated in the 1950s and 1960s that still have those compounds today."

Helping identify the affinities and aiding in the management of wildlife is rewarding, Hahn says, but he is also intrigued by just the pure science of the issue.

"It's a really interesting basic research question," he says. "To try and get into the nitty-gritty of how protein structure determines its function, and how that translates up from a cellular level to the whole animal and may influence the sensitivity of the whole animal to something in its environment is very intellectually satisfying."