Environmental estrogens affect early developmental activity in zebrafish

New research to be presented at the 2012 International Zebrafish Development and Genetics Conference, held June 20-24 in Madison, Wisconsin, reveals that these substances may act even earlier than previously realized, at the very beginning stages of embryonic development.

Estrogenic compounds in the environment are both naturally occurring, such as in food plants, and synthetic, such as bisphenol A (BPA), used in making hard plastic bottles, like baby bottles and metal-based food and beverage cans, including those for baby formula. They are known to affect development of the sex organs, but not much is known about other effects, including any at beginning embryonic development. “The timing of exposure is critical. Evidence from animals suggests that there are critical periods of development when endocrine disruptors could be more deleterious than exposure during adulthood,” says Daniel Gorelick, Ph.D., a postdoctoral fellow at the Carnegie Institution for Science.

Working with Professor Marnie Halpern, Ph.D., Dr. Gorelick discovered a new and unexpected activity of estrogenic compounds during early stages of embryonic development. He will present his findings at the conference on Sunday, June 24.

The researchers used zebrafish, which offer several advantages for studying this question. “People have used fish as environmental sentinels for aquatic pollution for decades,” Dr. Gorelick says. Most studies, however, have been limited to fairly crude effects such as death or large-scale changes in single genes.

The researchers took advantage of the genetic tools available for zebrafish to study where and when estrogen receptors are active throughout the body. They genetically developed fish whose cells make a green fluorescent protein when their estrogen receptors are activated and looked at the fish early in development, during formation of the major tissues and organ systems, including the heart, gut, and central nervous system. Because zebrafish embryos are optically transparent during early development, the researchers were able to see individual estrogen-responsive cells in living, growing embryos.

“We found some things that were expected, which was estrogen receptor activity in the liver and parts of the brain known to be estrogen-responsive,” Dr. Gorelick says. “The big surprise was finding it in the heart, and specifically in heart valves, which to my knowledge had not been known to be sensitive to estrogens.”

In fact, the heart appears to be even more sensitive than other organs to some estrogenic compounds, particularly genistein (a common dietary estrogen found in plants) and BPA.

That finding prompted the researchers to look for possible effects of environmental estrogens. In collaboration with the Fish Health Branch of the U.S. Geological Survey and the University of Maryland School of Medicine, they collected concentrated water samples from in and around the Chesapeake Bay and found that these water samples also activate the zebrafish estrogen receptors, with especially high activation in the heart valves.

Researchers don't yet know what role estrogen sensitivity in the heart may play, nor how the fish's development may be affected by such early exposure to estrogenic compounds. As with many signaling molecules, it's likely that both the timing and the amount of exposure are critical.

“They can respond to estrogens in the lab, but also estrogens in the environment in samples from local rivers and streams,” Dr.Gorelick says. “They're everywhere and they're unavoidable, but it's the dose that makes the poison.”

Dr. Gorelick and his colleagues are now working to identify specific compounds from the water samples that activate the receptors, as well as to learn what physiological role estrogen receptor activity plays in heart development and function.

ABOUT THE INTERNATIONAL CONFERENCE ON ZEBRAFISH GENETICS AND DEVELOPMENT: The zebrafish emerged as a major model system in 1994 with the first international conference at Cold Spring Harbor with 350 participants. This year the zebrafish community celebrates its 10th biennial international conference with more than 900 participants in Madison, WI. Studies using the zebrafish as a model system have allowed us to understand the genetic control of early development that underlie many human diseases. For more information about the conference, see http://www.zebrafishgenetics.org/

ABOUT GSA: Founded in 1931, the Genetics Society of America (GSA) is the professional membership organization for scientific researchers, educators, bioengineers, bioinformaticians and others interested in the field of genetics. Its nearly 5,000 members work to advance knowledge in the basic mechanisms of inheritance, from the molecular to the population level. GSA is dedicated to promoting research in genetics and to facilitating communication among geneticists worldwide through its conferences, including the biennial conference on Model Organisms to Human Biology, an interdisciplinary meeting on current and cutting edge topics in genetics research, as well as annual and biennial meetings that focus on the genetics of particular organisms, including C. elegans, Drosophila, fungi, mice, yeast, and zebrafish. GSA publishes Genetics, a leading journal in the field and an online, open-access journal, G3: Genes|Genomes|Genetics. For more information about GSA, please visit www.genetics-gsa.org. Also follow GSA on Facebook at facebook.com/GeneticsGSA and on Twitter @GeneticsGSA.