Eye Development Genes Identified

UNIVERSITY OF UTAH MEDIA RELEASE

The study in the journal Science is embargoed for release at noon MST Thurs. April 19, 2001

Contacts:For the Science study on zebrafish eye-brain wiring:--Cornelia Fricke, neurobiologist -- (801) 585-1702, home (801) 474-2242--Chi-Bin Chien, neurobiologist -- (801) 585-1701, home (801) 359-4136 (Note: Chien will be in Salt Lake City only through April 16 and again starting April 24. During April 18-22, Chien may be reached in London at (44) 20-7233-2919 or by e-mail at [email protected])For the Proceedings of the National Academy of Sciences study on tadpole eyes:--Monica Vetter, neurobiologist -- (801) 581-4984, home (801) 581-1758, [email protected]For University of Utah public relations:--Lee Siegel, science writer -- (801) 581-8993, cell (801) 244-5399, [email protected]

Utah Scientists Identify Eye Development GenesStudies Aided by Three-Eyed Tadpoles, Fish with Incorrectly Wired Brains

April 19, 2001 -- University of Utah researchers have identified two genes necessary for eyes to take shape and get wired to the brain in developing embryos.

When one gene was knocked out of action in young zebrafish, nerve fibers growing out of their eyes zigzagged through the brain or otherwise went astray. The nerves failed to connect to the proper parts of the brain, neurobiologists Chi-Bin Chien, Cornelia Fricke and colleagues report in the April 20 issue of the journal Science.

In a second study, other Utah neurobiologists discovered a control gene that turns on other genes responsible for eye development. When frog eggs were given too much of the control gene, tadpoles developed three eyes. When the gene was deactivated, the tadpoles either developed a single eye or a puny second eye.

The tadpole study was published in the March 27 issue of Proceedings of the National Academy of Sciences by assistant professor Monica Vetter, graduate student Jennifer Rasmussen and colleagues. Researchers involved in both studies are from the University of Utah School of Medicine's neurobiology and anatomy department.

Because basic genes like those for eye development are quite similar among animals, the researchers believe the genes identified in their studies also help guide the formation of eyes and eye-brain connections in people.

"A lot of the same factors that regulate eye development in humans also regulate eye development in lower vertebrates like frogs and also in insects like fruit flies," Vetter said.

Chien said the gene identified in his study "is crucial for helping the eyes connect to the correct parts of the brain in a developing fish. We expect the same gene also will be crucial in other vertebrates like mice and people."

Fricke conducted the study for her Ph.D. thesis under the supervision of Chien, an assistant professor. They were aided by Utah doctoral student Jeong-Soo Lee and by two researchers at the Max Planck Institute for Developmental Biology in Germany.

"We're trying to understand how the brain wires up, how it makes the right connections between nerve cells so it functions -- in particular, how do nerve fibers from the eyes go to the right place in the brain so we can see," Chien said.

As an embryo develops, nerve fibers called axons "navigate" their way to the parts of the brain that process visual information. The study's German co-authors discovered mutant zebrafish with eyes not properly wired to the brain. They blamed a gene named "astray" without knowing what it was. Fricke, Chien and Lee identified the actual gene, and showed it was the same gene that had been named "roundabout" or "robo" in fruit flies because mutant forms of the gene made normally straight nerve fibers become circular in the insects.

When the scientists bred zebrafish with mutant astray genes, nerve fibers from the eyes failed to grow toward the correct parts of the brain. Instead, they went astray, growing toward the front or back of the brain, or zigzagging back and forth across the midline of the brain instead of properly linking the right eye with the left half of the brain and the left eye with the right half. In some cases, the nerve fibers failed to bundle like a cable, Chien said.

Without a normal astray gene, "it's like the axons [nerve fibers] are blind and can't read road signs" pointing them to the correct parts of the brain, he added.

Fricke, Chien and colleagues were able to watch how the mutant astray gene affected eye-brain nerve connections by putting fluorescent dye in the eyes of five-day-old zebrafish.

"The dye travels along nerve fibers and illuminates nerve fibers" when viewed through a microscope, Chien said.

The researchers transplanted tiny eyes between zebrafish embryos to reveal the astray gene's role in greater detail. Each eye is only one 250th of an inch wide. The eye transplants were "done under a microscope," Chien said. "I have good hands."

When eyes with normal astray genes were transplanted into fish with mutant astray genes, the eyes connected properly to the brain. But when mutant eyes were transplanted into normal fish, nerve fibers went haywire. That indicates the normal astray gene makes "receptors" on the growing nerve fibers. The receptors help sense chemical signals to guide the nerves to the visual parts of the brain.

"We were trying to ask, are the axons blind or are there no road signs for them to see?" Chien said. "This tells us indeed the axons are blind when they are mutant."

The fish with improperly wired eye-brain connections "can see at least a little bit" because their eyes follow moving patterns, Chien said. "They're clearly not blind, but they could be legally blind. You wouldn't want these fish driving your car."

In the second study, Vetter and Rasmussen collaborated with associate professor Mahendra Rao at the University of Utah and with University of Pennsylvania scientists.

Vetter said researchers already knew that certain genes produce proteins needed for eyes to develop normally in embryos. One such eye development gene is named Pax6. Earlier research elsewhere found that eyes develop in odd places when Pax6 is activated in tissues that normally do not produce eyes. Fruit flies with inappropriately activated Pax6 genes "get eyes at the end of antennas, eyes on the legs, eyes on the wings," Vetter said.

Rasmussen and Vetter were studying a gene named Xfz3 in frog embryos to learn how the "frizzled" family of genes influences nervous system development. The genes have that name because when some of them mutate, fruit flies end up with frizzled bristles on their bodies instead of straight bristles. Xfz3, or Xenopus frizzled 3, is the third frizzled gene in the frog genus Xenopus.

The neurobiologists collected unfertilized eggs from female South African clawed frogs, and then fertilized them in petri dishes with sperm from male frogs. As the eggs started to divide into multiple cells and become embryos, the researchers injected extra amounts of the genetic blueprint for the Xfz3 gene.

Instead of seeing nervous system effects, "we started seeing extra eyes" in some of the embryos as they developed into tadpoles, the larval stage of frogs, Vetter said. The third eye usually was located on the back of a tadpole's head. Vetter said she doubts such third eyes actually work. They were smaller and less developed than the two normal eyes.

Third eyes also appear in tadpoles when the fertilized eggs are injected with genetic material from the Pax6 gene. So the discovery suggested the Xfz3 gene produces a protein that helps activate the Pax6 gene to promote eye development, Vetter said. That was confirmed when the study also found extra Pax6 where tadpoles developed third eyes.

Next, Rasmussen and Vetter genetically manipulated frog embryos so the Xfz3 gene could not receive chemical orders that normally would tell it to activate Pax6 and other eye development genes. The tadpoles had either only one eye, or an underdeveloped second eye.

Vetter said the research eventually might have medical benefits because there are rare human disorders in which embryos fail to develop eyes properly. Her Pennsylvania collaborators have shown the Xfz3 gene also is involved in development of the neural crest, the part of an embryo's nervous system that gives rise to arm and leg nerves.

How Xfz3 works is important "not just for eye development, but for how you put together a normal, functioning nervous system," Vetter said. Her new study reveals some of the signals from genes that help the embryonic nervous system form eyes and other parts.

"You start with a one-cell embryo and need to develop all the organs from that cell," Rasmussen said. "We are looking at how the eyes develop."

Learning how that happens someday might help researchers grow retinas or other eye parts in the laboratory to restore vision to people with certain eye diseases, Rasmussen speculated.

This news release and photos to accompany it may be found at:http://www.utah.edu/unews/041901_eyes.html(Note: There is an underline between the date and eyes.html in the address above.)

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