The zebrafish is a sleek aquarium fish with black and white stripes running horizontally along its sides. It looks at home in any home or office fish tank.
Several technical web sites are devoted to it. But the sites are not maintained by hobbyists. They are maintained by professional research scientists who use Brachydanio rerio, as the zebrafish is referred to in Latin, as a model system for studying vertebrate developmental biology.
The usefulness of this unconventional lab animal was reaffirmed with the publication of a letter in the December issue of Nature Genetics, "Patterning the zebrafish axial skeleton requires early chordin function," by Shannon Fisher and Marnie Halpern. The two researchers at the Carnegie Institution of Washington in Baltimore showed the fish appears to be an excellent model for studying the development of the vertebrate skeleton.
"It is basically a good laboratory animal," Fisher told BioWorld Today. "It is easy to raise in large numbers with a small amount of space and money, relatively speaking, compared, for example, to keeping a large mouse colony. A lot of the basic processes during development, and even in adult physiology, are similar to what goes on in mammals. It is also possible to do large-scale screens for mutants in a way that you just can't do with mice or other popular model systems."
Fisher and Halpern showed mutant zebrafish lacking a polypeptide called chordin develop skeletal defects. Chordin, along with another secreted polypeptide called noggin, regulate key proteins that in turn control the pattern of skeletal formation. These crucial proteins are called bone morphogenetic proteins (BMPs). They stimulate cells that deposit components of bone to build the skeleton. The chordin-less fish show both mis-expression BMP genes and abnormal bones in their fins and hind region.
"Chordin and noggin both are known to interact with the BMPs," Fisher explained. "However, BMPs also are important in a lot of other developmental processes, including very early development, so their role is not confined to the skeleton. It was thought originally for a long time that noggin and chordin primarily had a role interacting with the BMPs early on and that probably other molecules were doing similar things at later stages of development, such as during skeletal development. The BMPs have been around for a while and were known to be important. The fact that chordin is important was appreciated a little more recently."
The skeleton develops fairly late in the zebrafish so it wasn't clear when chordin was having its most important effect on the skeleton. "It wasn't known if it was something that was going on very early or if it was something that was occurring at one month, when you can actually see the skeleton," Fisher said. "We took these chordin-deficient embryos and rescued their early development by supplying them with in vitro transcribed chordin RNA. So we can put back the RNA in the early embryo but the RNA that we put in doesn't stick around very long. It is only there for about the first 10 hours of development. Then it is degraded and gone.
"The embryos are mutant for the gene so they can't make any more [chordin] RNA at later stages," Fisher continued. "However, those embryos [that received RNA injections] still develop normal skeletons. We could show that just by replacing chordin within the first 10 hours of development, we could completely rescue this skeleton that doesn't develop for another several weeks. I think the most important point of our paper was showing that connection between an early signal and later development."
Chordin Appears To Be Key Signaling Factor
As a result of these findings, chordin emerges as a key signaling factor in the development of the vertebrate skeleton. Fisher noted, "We are coming to appreciate in many areas, the role that early patterning signals play in shaping organ systems that develop later."
Rik Derynck, a professor at the University of California in San Francisco wrote a News & Views article titled, "Skeletal development in the zebrafish," in the same issue of Nature Genetics. Fisher and Halpern's work, he wrote, underscores "the advantages of using genetic screens of zebrafish to identify regulators of skeletal formation in vertebrates."
Fisher is trying to develop the zebrafish as a good model to study the skeleton and all aspects of its development. She has been using X-rays of adult zebrafish to screen for new mutations. "You can get a lot of detail," she said. "The zebrafish skeleton is a good system to look for mutations. It is somewhat less complicated than the skeletons of tetrapods like mice. I think you can maybe see a change or alteration in the pattern a little more easily. You can also get most of the information from a single view of the X-ray. You can take one picture and see everything. That would be pretty hard to do in a mouse."
The findings described by the Carnegie Institution researchers could have implications for human diseases and for projects designed to correct or stimulate bone growth. "There are a large number of human skeletal dysplasias and disorders whose genes have not been identified. I think in some ways it's easier to find disease genes in zebrafish," Fisher said. "There are a lot of tools that make it easy to do that. It may be possible in some cases to find a gene in zebrafish that causes some problems in the skeleton and then go back to the human and look for the same gene."
Zebrafish have another potentially valuable feature that people unfamiliar with them may not know. "You can cut the fins off of these fish and they grow back perfectly normally, including the bones," she said. "I think that could prove to be a really interesting model system for looking at repair and regeneration. I think that process is going to recapitulate a lot of what goes on in mammals when you break a bone and have to repair it."