Zebrafish embryos make good research models because it is possible to observe them from the moment of fertilization, and researchers can precisely regulate the nutrients they receive largely free from maternal influence. Pictured: A normal zebrafish embryo (above) and a Calamity mutant (below).

_{by David F. Salisbury}

A humble aquarium fish may hold the key to finding therapies capable of preventing the structural birth defects that account for one out of three infant deaths in the United States today.

That is one implication of a new study published online Aug. 8 in the journal Cell Metabolism. The paper describes a number of striking parallels between a rare but fatal human birth defect called Menkes disease and a lethal mutation in the small tropical zebrafish, which has become an important animal model for studying early development.

Zebrafish are easy and inexpensive to raise and lay eggs that are transparent and develop outside the mother’s body. Much of the zebra-fish genome has been sequenced, allowing researchers to identify human versions of zebrafish genes and vice versa. These qualities make the zebrafish exceptionally handy for studying the complex relationship between genes and nutrition during development, a puzzle that has stood in the way of developing effective treatments for birth defects.

“This (proves the) concept that we can use the zebrafish to finally understand the role that maternal nutrition plays in causing structural birth defects and develop new treatments that can prevent them,” said the paper’s co-author Jonathan Gitlin, the Helene B. Robertson Professor of Pediatrics at Washington University in St. Louis.

Collaborating in the study were Professor of Biological Sciences Lilianna Solnica-Krezel, postdoctoral fellow Thomas P. Wilm and graduate student Chunyue Yin from Vanderbilt, along with Associate Professor of Genetics Stephen L. Johnson and graduate student Bryce A. Mendelsohn from Washington University.

For many years, scientists have relied on the mouse and the frog Xenopus laevis as research models for vertebrates (animals with backbones), but zebrafish have an important advantage for studying early development: It is possible for researchers to watch the changes that take place in their embryos from the moment of fertilization. The small fish have another advantage for studying the role of nutrition – scientists can precisely regulate the nutrients that the embryo receives largely free from maternal influence.

Key to the research was the discovery of a mutation in the zebrafish that disrupts the distribution of the critical nutrient copper within the fish cells and causes defects that are remarkably similar to those observed in children suffering from Menkes disease. Menkes is caused by disruptions in the way copper is distributed in the body. Gitlin was attempting to duplicate this by exposing zebrafish embryos to a chemical agent that disrupts copper metabolism.

Working separately, Solnica-Krezel’s lab found a mutation that they named “Calamity” because it impairs so many aspects of normal development that the embryo falls apart in two days. Solnica-Krezel heard a talk that Gitlin gave at a scientific meeting and was struck by the similarity between his results and those produced by Calamity. She approached him, and they exchanged information and decided to collaborate.

Working together, the researchers were able to demonstrate that the molecular machinery that handles copper in zebrafish cells is basically the same as that in human cells. They are so similar, in fact, that an injection of the human version of the enzyme that is disrupted by the Calamity mutation is capable of rescuing the defective embryos, allowing them to develop nearly normally.

The researchers also were able to explain why attempts to treat Menkes disease with copper supplements have not worked very well. They established that the condition was not caused by lack of copper intake, but due to a defect in a key protein that transports copper into the cell and delivers it to the location where it can be made into a number of enzymes that are critical for proper cell functioning.

The research also suggests an approach to developing new therapies for Menkes: testing hundreds or thousands of compounds to see if any can restore proper copper-handling in Calamity mutants. If such a drug can be found, it would be a strong candidate for treating the disorder in human embryos.

“The zebrafish is the first animal model that allows us to watch the process of early vertebrate development and manipulate it. There is no reason why the same approach that we have used with copper cannot work for other nutrients as well,” Gitlin said.

The research was funded by grants from the National Institutes of Health.

For a multimedia version of the story, visit Exploration – Vanderbilt’s online research magazine – at www.exploration.vanderbilt.edu.

_{Posted 08/14/06}