Myostatin update

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Abundance of myostatin in infected swine may result in reduced muscle

mass

http://www.medicalnewstoday.com/medicalnews.php?newsid=16523 18 Nov 2004

A study looking at chronic infectious respiratory diseases that affect

most swine during their critical growing stage has shed new light on the

reasons for restricted weight gain and reduced muscle mass.

In the November issue of the Journal of Nutrition, scientists at the

University of Illinois at Urbana-Champaign report that the production of

inflammatory cytokines by immune cells appears to be responsible for

declines of both protein accretion and weight gain in swine infected

with Porcine Reproductive and Respiratory Syndrome Virus (PRRSV).

The study also suggests that myostatin, a protein that limits muscle

growth, is overproduced during infection, thereby reducing the growth of

skeletal muscle, said Rodney W. , a professor in the department

of animal sciences and the interdisciplinary Division of Nutritional

Sciences.

and colleagues isolated pigs in disease-containment chambers and

exposed different experimental groups to the bacterium Mycoplasma

hyopneumoniae and/or PRRSV.

Almost all U.S. swine are exposed to the bacterium in production

facilities, while about 60 percent are exposed to PRRSV. These pathogens

open the way for other infectious agents. During the pivotal growing

stage, pigs are at the most risk and suffer from cough, fevers and

depressed appetite. Reduced market weight or increased time for pigs to

reach a desired market weight can be a substantial cost to producers.

Infection from the bacterium alone did not reduce weight gain compared

with the control group during the four-week-long experiment, but it did

lead to the development of lesions that affected 8 percent of the total

lung area in infected pigs. The finding was similar to earlier work in

's laboratory. However, the introduction of PRRSV caused damage

to the lungs from the bacterium to jump to 40 percent.

" One thing the virus does is suppress the immune system, " said.

" When PRRSV and mycoplasma are together, the PRRSV-induced

immunosuppression allows the mycoplasma to spread unchecked. It really

takes over the lungs. "

PRRSV infection alone resulted in a daily weight gain of just 50 percent

of that of the control animals (300 grams per day compared with 600

grams per day) and substantially less protein accretion. The drop in

growth began three days after exposure to PRRSV and continued for the

remaining two weeks of the trial.

PRRSV infects macrophages, a type of white blood cell that attacks

pathogens. The virus is spread from the lungs as the macrophages migrate

to other tissues. Before infected macrophages die from the virus, they

produce inflammatory cytokines, hormone-like molecules that enable the

immune system to influence other parts of the body. One part affected is

the brain, which is why animals have reduced appetite when they are

sick.

" The cytokine molecules are the key, because they are the messengers

used by the immune system to alter other systems that are relevant to

growth, " said.

At the suggestion of co-author Jeffery Escobar, a former doctoral

student now with the USDA/ARS Children's Research Center at the Baylor

College of Medicine in Houston, the researchers examined myostatin gene

expression in the infected pigs.

Myostatin's role in muscle development is becoming clear, said.

Mice with the myostatin gene deleted become muscle-bound, and a

defective myostatin gene has been linked to double muscling in cattle

and to abnormally large muscles in a German child.

's team found a substantial increase in the amount of myostatin

mRNA in the muscles of infected pigs. " We have shown, using an

infectious disease model where animals grow slowly, that there is an

increase in muscle myostatin mRNA. "

In addition to and Escobar, co-authors were G. Van

Alstine of the Animal Disease and Diagnostic Laboratory at Purdue

University and H. Baker, a professor of animal sciences at

Illinois. The Illinois Council for Food and Agricultural Research funded

the study.