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Monday, Sep 11, 2006
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Zebrafish studies may unlock secrets to repairing nerves

By Katharine Ott

Milwaukee Journal Sentinel

(MCT)

MILWAUKEE - Current research on fish at the Great Lakes WATER Institute may someday help create new treatments for spinal cord injuries and degenerative diseases such as glaucoma.

These irreversible conditions damage nerve connections in the central nervous system. Humans cannot repair the broken connections because, in general, they do not regenerate their central nervous system.

Ava Udvadia, a biology professor at University of Wisconsin-Milwaukee and a scientist at the WATER Institute, researches the central nervous system of the zebrafish - a small aquarium fish that can regenerate its central nervous system.

She studies the genes responsible for turning on growth in the central nervous system of zebrafish.

Once these genes are identified, it may be possible to express them artificially in humans and stimulate regeneration.

The central nervous system is made up of the brain and spinal cord.

Cells in the central nervous system, called neurons, transmit information between the brain and other parts of the body. These neurons are connected to each other by axons.

During development of the central nervous system, the axon grows out of the main part of the neuron to connect with other cells. When the neuron cell reaches maturity, the axon stops growing.

In humans, axons will never grow again once development of the central nervous system is complete.

But zebrafish can turn axon growth back on to repair damage.

In order to understand how that happens, Udvadia's lab studies axon growth during development of the central nervous system, when the fish is between 16 hours and three days old, and regenerative axon growth following an injury to the fish's retina.

They look for similarities and differences in the genes during these two processes.

"We want to determine what turns on and what turns off the capacity for axon growth, then we can go back and try to figure out what is missing in neurons that are not regenerating," said Udvadia.

Udvadia's lab has two projects under way.

The first examines a gene that is known already to be important in axon growth; the second identifies more genes essential to axon growth.

GAP-43 is an important gene in axon growth.

To see when this gene is expressed, or turned on, Udvadia connects GAP-43 to a "reporter gene" that contains green fluorescent protein.

When axon growth takes place, GAP-43 is expressed and the reporter gene causes the neuron to glow green.

Researchers can see the green cells through a microscope.

In the first project, scientists make a series of mutations to the GAP-43 gene. These mutations change the sequence, or instruction manual of the gene, altering when, where and how much of the gene is expressed.

Scientists compare axon growth in zebrafish with different mutations of GAP-43 by looking at the presence or absence of green fluorescent protein.

For instance, a zebrafish with a certain mutation of GAP-43 may exhibit axon growth during development but not during regeneration.

These trials will help scientists understand exactly how GAP-43 regulates axon growth.

In the second project, un-mutated GAP-43 is attached to green fluorescent protein. When axon growth occurs, researchers remove the green cells and look at what genes are expressed in the neuron at that particular time.

Udvadia's lab has found 60 genes expressed during axon growth.

To test which genes are essential to axon growth, researchers either over-express or knock down the gene, meaning they increase or decrease the amount of that particular gene in the cell, and record how this affects axon growth.

Zebrafish are an ideal model organism for this research, allowing for a large amount of trials at a relatively low cost because they reproduce rapidly, develop quickly and are inexpensive to store.

Although they seem very different from humans, genetically the two species are similar.

"The genes that regulate (axon growth) are evolutionarily conserved from worms and fruit flies all the way up to humans," said Mary Halloran, a neuroscientist at the University of Wisconsin-Madison who studies central nervous system development in zebrafish. "Fish have the same kind of axon pathways, the same kind of neurons and the same molecules and genes to regulate their growth as higher mammals and humans."

Pate Skene, a neurobiologist at Duke University, studies axon growth in mice, a species much closer to humans than zebrafish in the evolutionary chain and one that also does not regenerate its central nervous system.

He used conclusions from Udvadia's research as a guide in his own studies.

"We have every reason to believe that the lessons Ava learns from fish are good guides to mammals ... including the mammals we care most about - us," Skene said.

---

İ 2006, Milwaukee Journal Sentinel.

Visit JSOnline, the Journal Sentinel's World Wide Web site, at http://www.jsonline.com

Distributed by McClatchy-Tribune Information Services.

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