Danio Is Important Model For Muscle Growth

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Giant Danio Can Keep Growing - Paired With Zebrafish, Danio Is

Important Model For Muscle Growth

http://www.medicalnewstoday.com/medicalnews.php?newsid=57558

Two fish that share much in common genetically appear to have

markedly different abilities to grow, a finding that could provide a

new way to research such disparate areas as muscle wasting disease

and fish farming, a new study shows.

The study in the November issue of the American Journal of

Physiology-Regulatory, Integrative and Comparative Physiology, finds

that the giant danio, unlike its cousin the zebrafish, appears to

have the ability to recruit new muscle throughout its life. Humans

have the same ability before birth, but mostly lose it after birth.

Because the zebrafish and giant danio are closely related, and the

zebrafish's genome has already been mapped, scientists hope they can

more easily identify the genetic keys to the difference in growth

potential between them. According to co-author Peggy R. Biga, " I

don't think there will be a major genomic difference between them. I

believe it will be easy to define the difference. "

The study " Zebrafish and giant danio as models for muscle growth:

Determinate versus indeterminate growth as determined by

morphometric analysis, " was carried out by Biga and Frederick W.

Goetz, both of the University of Wisconsin-Milwaukee and the Great

Lakes Wisconsin Aquatic Technology and Environmental Research

(WATER) Institute, Milwaukee. The American Physiological Society

published the study.

Zebrafish versus Giant Danio

Zebrafish and giant danios, members of the minnow family, are easy

to raise and are popular inhabitants of home aquariums. They are

native to the warm waters around India and some other areas of

southern Asia. Although both are small, the danio is a giant among

minnows, growing to a maximum of six inches. The zebrafish, in

contrast, grows to about two inches.

The zebrafish has been a staple of genetic research because it is

easy to raise and maintain. But the fish is too tiny for

physiological research. So the researchers were interested in

finding a close zebrafish relative to allow them to do physiological

research while drawing on the knowledge base of the zebrafish genome.

The researchers saw the giant danio as the physiological model and

began a series of experiments on differences in muscle growth

between the fish. In this two-phased experiment, they discovered

that the giant danio can keep growing even into adulthood, but the

zebrafish stops growing. The giant danio can get bigger because it

exhibits a type of muscle growth that is different from the

zebrafish.

" One of these species either lost or gained the ability to

continuously grow, but most likely, the zebrafish lost it " Biga

explained. Because genes are the key, the zebrafish's mapped genome

will be invaluable in finding what is different about the danio's

genetic pathways that allow it to keep growing.

Study: Phase I - First four weeks of life

In the first phase of the study, the researchers tracked the size of

the fish from the time they hatched through the first four weeks of

their larval stage. They found that the two species were the same

size at hatch. By the end of week one, the giant danio was

significantly larger and remained so. This result was not

surprising, since the giant danio is so much larger at adulthood,

about twice as large. But the study was the first to report the

early larval growth of these species, the authors said.

The researchers also documented muscle changes over the first four

weeks in the fleshy portion of the fish's midsection, known as the

myotome. They measured growth in length and width and tracked the

number and size of muscle fibers, and found that the fish employ

different methods of growth.

" Muscle growth in vertebrates is defined as being either determinate

or indeterminate, " the authors wrote. " Animals such as mammals

exhibit determinate growth, in which there is a finite size. In

contrast, many fish species growth is indeterminate, in which there

is no fixed size and some growth may continue throughout the life of

the fish. "

Indeterminate growers increase muscle mass both by recruiting new

muscle fibers (hyperplasia) and by increasing the size of the

existing muscle fibers (hypertrophy). Many fish species exhibit

indeterminate (hyperplastic) growth.

In this first phase of the study, the researchers found that

hyperplasia accounts for 67 percent of the giant danio's muscle

growth, compared to the zebrafish in which hyperplasia accounts for

only 47 percent of the growth.

Study: Phase II - Adult growth

In the study's second phase, the researchers applied growth hormone

once every three weeks for 17 weeks to adults of both species. The

adult danio continued to grow during the 17-week trial and analysis

of muscle fibers showed hyperplastic muscle growth.

Adult zebrafish did not exhibit increased growth or hyperplasia in

response to growth hormone, suggesting that zebrafish reach a growth

plateau similar to mammals and hence exhibit determinate growth. The

zebrafish exhibited little hyperplastic growth after the juvenile

phase.

Next step

This model can be used to investigate muscle wasting diseases such

as muscular dystrophy. Other studies have already shown that a

condition similar to muscular dystrophy can be experimentally

induced in zebrafish. One of the next steps for the researchers is

to induce the condition in the giant danio.

" We anticipate that the giant danio will be able to handle the

muscular dystrophy better because of their ability to increase

muscle, " Biga said. What makes this intriguing is that human embryos

exhibit hyperplasia, but then lose that ability after birth, with

one exception. When humans injure a muscle, the muscle sends a

signal to special cells attached to the muscle fibers, telling these

cells to grow and join to adjacent muscle fiber to repair the injury.

There is also an application to fish farming. " If we understood how

fish grow, we could figure out strategies for enhancing the growth

of farmed fish such as Atlantic salmon and decrease the time to get

to market size. " The more we understand about growth, the more

likely we can come up with acceptable ways to enhance it, " she said.

Funding

This project was supported by National Research Initiative

competitive grants from the U.S. Department of Agriculture's

ative State Research, Education, and Extension Service to

Peggy R. Biga and Frederick W. Goetz and by the University of

Wisconsin-Milwaukee Great Lakes WATER Institute.