By David F. Salisbury
Dec. 18, 2000

It only takes two genes to make the difference between an ordinary embryo and one that develops without a head.

At least, that is the case in the zebrafish, a team of geneticists from Vanderbilt University report in the Dec. 15 issue of the journal Genes & Development. According to the paper’s senior author, Lilianna Solnica-Krezel,something similar is likely to hold true for mammals and humans. “One of these genes has an equivalent in human development; the other one we’re not sure of,” says the assistant professor of biological sciences at Vanderbilt.

This is the latest in a series of recent studies that have begun to unravel the mysteries of development at the molecular level. For hundreds of years scientists have wondered how an early embryo that is made up of identical, undifferentiated cells, can develop into nerves, muscles, lungs and other organs. Answers to this question may lead to new treatments for birth defects and other illnesses caused by defective development.

The picture that is emerging is one of elegant simplicity. The cells in the embryo secrete a protein called bone morphogenetic protein, or BMP. A structure, called the (Spemann) gastrula organizer, forms on the egg’s surface in what will become the dorsal, or backside of the animal. The organizer becomes a source of “negative regulators” of BMP—proteins that reduce its production or function. This interaction produces variations in the concentration of BMP in different parts of the embryo, providing instructions that effectively determine top and bottom, left and right, front and back. The cells then use this information to begin differentiating into various types of tissue and to move to appropriate locations within the developing embryo.

“Where BMP concentrations are lowest, cells develop into nervous tissue and backbone and where they are highest cells tend to become skin, blood and tail,” Solnica-Krezel observes.

(In 1992, Brigid L. M. Hogan, the Hortense B. Ingram Chair in Molecular Oncology at Vanderbilt, and Christopher V.E. Wright, professor of cell biology, were among the first to suggest such a critical role for BMP.)

In order to study the interaction between BMP and its antagonists, Solnica-Krezel’s researchers combined mutations that disabled two genes known to produce proteins that interfere with BMP activity in individual zebrafish embryos. When they did so they found that the embryos developed not only without heads but also without trunks. Instead most of the cells formed an enlarged tail.

Furthermore, the head and trunk magically reappear when BMP and the two genes are all inactivated. According to Solnica-Krezel, “This indicates that the main function of the two genes is to cooperate in limiting BMP function to allow for head and trunk formation.”

Previous studies had suggested that a number of genes were required, so the researchers were surprised that so few genes had such a dramatic impact. “It’s really quite amazing that it only takes three genes talking to one another to specify for head, trunk and tail,” Solnica-Krezel says.

In addition to BMP, the two genes involved are named chordino and bozozok. They were found as part of large-scale genetic screens carried out in Boston by Wolfgang Driever at Massachusetts General Hospital with Solnica-Krezel’s participation and in Tubingen, Germany by Nobel laureate Christiane Nüsslein-Volhard, who directs the Max Planck Institute for Developmental Biology. Chordino is a BMP antagonist that was first discovered in the frog Xenopus. Bozozok is a protein called a transcription factor that plays a role in the regulation of the production of proteins from DNA templates.

The Vanderbilt researchers also discovered that bozozok—named for Japanese motorcycle thugs—plays a key role in the development of the gastrula organizer and in regulating the production of chordino. So they decided to see what happens when they put both genes out of commission at the same time.

Co-authors on the paper are Christopher V.E. Wright, professor of cell biology; post doctoral fellows Encina M. Gonzalez and Jacek Topczewsi; and doctoral students Kimberley Fekany-Lee, Amanda Carmany-Rampey and Caroline Erter.

The research was supported by grants from the National Institutes of Health and the March of Dimes Birth Defects Foundation.