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Electric fish may be key to better understanding muscles and nerves

By studying a small fish that generates a weak electrical current, a New Mexico State University biologist hopes to learn more about the signals nerves send to muscles, eventually helping us better understand diseases like spinal muscle dystrophies and Lou Gehrig's disease.



Graciela Unguez, an assistant professor of biology at New Mexico State University, uses a small fish that generates a weak electrical field to study how the nervous system influences changes in skeletal muscle cells. (NMSU photo by Darren Phillips)

m feeder in South American rivers, the knife fish is eight to 12 inches long, with a blunt nose and small eyes almost the same color as its gray skin. Its only hints of color are the thin yellow streaks that run down each side of its body from just behind the gills to the tip of its tail. The streaks start and end at about the same points as an organ that generates an electrical field. Other organs in the fish's body can sense electricity in its surroundings, said biology Assistant Prof. Graciela Unguez.

Knife fish's ability to use electricity to communicate and explore their world the way whales and dolphins use sonar has long fascinated biologists, but what fascinates Unguez is not the way they use the organs, but the development of the organs themselves, she said.


"The electrical organ comes from skeletal muscle. When they're born knife fish don't have an electrical organ. During development, they first develop skeletal muscle cells, then some of the muscle cells convert to cells of the electrical organ. That's a profound change. It's as if the muscles in your legs changed to become electricity-generating organs," she said.

That profound change gives Unguez a sharp dividing line around which to study the development of different cell types. "You could study cell changes in a number of contexts, but few have this kind of contrast," she said.

Fortunately, knife fish, like some types of lizards and frogs, can regenerate a limb if it is lost to a predator or a falling rock. In an adult the whole process of cell development is replicated each time it regrows its tail. By sedating a fish, then snipping its tail, Unguez can monitor the replication right to the point where the muscle cells turn into electricity-generating cells. At that point, she takes another small sample from the tail and compares the ribonucleic acid (RNA) from the muscle cells with that in the cells of the electrical organ. RNA is genetic material present in all the body's cells, but some genes will be expressed only in muscle cells, while some will be expressed only in electrical cells -- and that, said Unguez, is precisely the point.

"That's my immediate question -- what's happening at the cellular level; what genes are being turned on and what genes are being turned off?" she said.

More specifically, Unguez is interested in how input from the nervous system leads to changes in the genes' expression.

"The nervous system plays a very important part in the conversion from muscles cells to electrical cells," she said. "Normally nerve cells come in contact with the muscle cells. If I remove a part of the spinal cord from a knife fish, so that there is no contact with the tail, the muscle cells don't convert to electrical cells. In an adult, if I remove part of the spinal cord, in four to five weeks we see the electrical organ cells converting into muscles cells again," she said.

The observations from her experiments could have far-reaching implications, she added.

"It's obvious that input from the nervous system is a key player in the maintenance of muscle identity, but the mechanism for that is something we're still trying to figure out. We still don't know a lot about how signals from the nervous system cause some genes to be expressed and others not to be expressed," she said.

"Once we know what the target genes are, we can ask how to activate that pathway to keep that muscle healthy," she added.

This could be helpful in diseases like amyotrophic lateral sclerosis -- Lou Gehrig's disease -- and some types of dystrophy, where input from the nervous system is not normal, causing harm to the muscles, she said. But the implications of understanding how genes in the developing body are switched on and off could reach even farther, she said.

"All of us, all living entities, start as a single cell. Following thousands, actually millions, of cell divisions we become different, complex, entities. How do we get so many different types of cells? Why and how do so many different cell types develop, and once developed, how are they maintained? How are they induced to express only a tiny fraction of the genetic material that is in each and every cell in the body?" she asked.

Once we understand that process, Unguez said, we might be able to answer such questions as, "If I lose one cell, how do I induce other cells to become the cell type that I lost?"

Photo is available at http://ucommphoto.nmsu.edu/newsphoto/unguez_graciela.jpg.
CUTLINE: Graciela Unguez, an assistant professor of biology at New Mexico State University, uses a small fish that generates a weak electrical field to study how the nervous system influences changes in skeletal muscle cells. (NMSU photo by Darren Phillips)

Jack King
April 29, 2002