It's been over a year and a half since the landmark issue of the journal Development described nearly 2,000 new mutants of zebrafish (Danio rerio), the newest animal model of vertebrate development in a menagerie of such species. Now, developmental biologists all over the world are earnestly studying these mutants -- and adding new ones to the list -- to determine what role they play in complex biochemical pathways. As part of this effort, researchers at the University of Pennsylvania Medical Center have identified new mutants important in the first steps that control the dorsoventral -- back to abdomen -- arrangement of embryonic tissues, a process called patterning.
These genes guide embryonic cells to become the belly or the back of the developing fish, with mutations resulting in abnormally shaped embryos. Because many genetic guidance systems are generic, or conserved, across species -- from invertebrates to vertebrates -- understanding the biochemical nature of these abnormalities could point to similar anomalies that cause birth defects in humans and might shed light on what happens in human embryos in the mysterious first days after conception.
Mary C. Mullins, PhD, assistant professor of cell and development biology and one of the key authors in the 1996 zebrafish issue, studies dorsoventral patterning determined by the bone morphogenetic protein (bmp) signaling pathway, which has already been implicated in dorsoventral patterning in frogs. In humans, this pathway is involved in bone formation.
"Working with zebrafish has allowed us to study the in vivo function of this pathway, which isn't possible with frogs and mice," says Mullins. "The zebrafish mutants don't die as early as in the mouse, so we can study dorsoventral patterning in the zebrafish, which so far has not been possible in the mouse. This is important because this pathway sets up the body's future shape and organization."
The Penn team found three mutants -- swirl, somitabun and snailhouse -- which are critical in determining abdominal regions of the embryo. Mullins and lead authors Vu H. Nguyen and Bettina Schmid, both of whom are doctoral students in Mullins' lab, postulate that the growth of the neural crest -- a neural tissue found at the back-belly boundary in normal embryos -- is determined by a gradient of bmp activity from the back to the front of the embryo. This activity is modulated by other proteins produced in the abdominal region that differentially repress bmp and subsequently effect its actions on the three genes. This modulating gradient is what sets up the normal patterning of neural tissue growth in the back region of the embryo and other tissue types in the abdominal region.
"In the swirl mutant, all abdominal tissue is absent, but the surprise came when we started to look at the dorsal neural tissue and found that most of it expanded and took over the ventral side," says Mullins. "The swirl embryos contain rings of hind brain tissue growing around the circumference instead of just running down the back. The embryos have spiraled neural tissue--hence the mutants' names." Interestingly, the somitabun mutant (named after a popular swirled cinnamon pastry) shows an expanded growth of the neural crest, whereas in the swirl mutant it's absent. The swirl and somitabun genes have a strong, but opposite, effect on the development of the neural crest, whereas snailhouse plays a smaller role.
Previous work postulated that the neural crest was specified later in development. The Mullins team found that this important event happens much earlier, about five hours earlier in the development of zebrafish. Time-wise, this corresponds in mammals to a day to a couple of days earlier, a stage sometimes crucial to catching a rare glimpse of the earliest stages of vertebrate development.
This work is described in a July issue of Developmental Biology and funded by the National Institutes of Health and the Howard Hughes Medical Institute.
Editor's Note: Dr. Mullins can be reached at 215-898-2644 or mullins@mail.med.upenn.edu. Photos comparing wild-type and mutant embryos are available.
The University of Pennsylvania Medical Center's sponsored research ranks third in the United States, based on grant support from the National Institutes of Health, the primary funder of biomedical research in the nation. In federal fiscal year 1997, the medical center received $175 million. News releases from the medical center are available to reporters by direct E-mail, fax, or U.S. mail, upon request. They are also posted to the center's website (http://www.med.upenn.edu) and EurekAlert! (http://www.eurekalert.org), a resource sponsored by the American Association for the Advancement of Science.
Journal
Developmental Biology