Date: 2009-01-12
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COLLEGE STATION – A Texas AgriLife Research scientist whose work helped discover that sheep need retroviruses for reproduction will appear on National Geographic Explorer. Dr. Tom Spencer and other scientists from the Laboratory of Uterine Biology and Pregnancy at Texas A&M University will be featured as part of The Virus Hunters, scheduled to air Jan. 13 at 9 p.m., Jan. 17 at 7 p.m. and Jan. 20 at 5 p.m. More information about the program can be found at http://channel.nationalgeographic.com/series/explorer/3828/Overview.
(Media-Newswire.com) - COLLEGE STATION – A Texas AgriLife Research scientist whose work helped discover that sheep need retroviruses for reproduction will appear on National Geographic Explorer.
Dr. Tom Spencer and other scientists from the Laboratory of Uterine Biology and Pregnancy at Texas A&M University will be featured as part of The Virus Hunters, scheduled to air Jan. 13 at 9 p.m., Jan. 17 at 7 p.m. and Jan. 20 at 5 p.m. More information about the program can be found at http://channel.nationalgeographic.com/series/explorer/3828/Overview.
The work highlights the discovery that naturally occurring endogenous retroviruses are required for pregnancy in sheep. The television program will feature various segments shot on location at Texas A&M, including Spencer and other researchers performing surgery and working with animals.
Spencer, an AgriLife Research Faculty Fellow and an associate professor in the department of animal science at Texas A&M, worked with Dr. Massimo Palmarini, a virologist at The University of Glasgow Veterinary School, to discover that endogenous retroviruses are critical during the early phase of pregnancy in sheep when the placenta first begins to develop.
The findings were first published in 2006 by the National Academy of Sciences.
“What we discovered was that if you inhibit production of a particular sheep endogenous retrovirus, it compromises early pregnancy,” he said. “The National Geographic program explains how we went about discovering this.”
Retroviruses are best known for their ability to cause diseases. HIV is one class of retrovirus found in humans. During the evolution of mammals, some retroviruses infected the cells of the ovary and testis that have genetic material passed to offspring. These are known as endogenous retroviruses and are present in the genome of all mammals, Spencer said, including humans.
“Retrovirus genes are inherited and expressed like normal genes once they enter the germline,” he said. “During evolution these retroviruses infected certain individuals and are now transmitted from generation to generation.”
Endogenous retroviruses are in general, harmless and contain mutations that prevent production of infectious retroviruses. However, several endogenous retroviruses appear to provide protection from infection and are involved in reproduction, Spencer said.
“For instance, the exogenous Jaagsiekte Sheep Retrovirus or JSRV, causes lung tumors in sheep and led to the death of Dolly, the world's first mammal cloned from an adult cell,” he said.
The idea that endogenous retroviruses are important for reproduction in mammals has been around for about 30 years, Spencer said. Studies in cultured cells have shown that a protein of a human endogenous retrovirus might have a role in development of the human placenta. The team blocked expression of the envelope, enJSRVs, using morpholino antisense oligonucleotides, which inhibit translation of specific messenger RNA.
“When production of the envelope protein was blocked in the early placenta, the growth of the placenta was reduced and a certain cell type, termed giant binucleate cells, did not develop,” Spencer said.
The result was that embryos could not implant and the sheep miscarried, Spencer said. Miscarriage is a serious medical problem for all mammals, including humans.
"Our research supports the idea that endogenous retroviruses shaped the evolution of the placenta in mammals and then became indispensable for pregnancy, and thus may be why they are expressed in the placenta of many mammals," said Palmarini.
"The enJSRVs arose from ancient infections of small ruminants during their evolution," Palmarini said. "This infection was beneficial to the host and was then positively selected for during evolution. In other words, animals with enJSRVs were better equipped than those without. Therefore, enJSRVs became a permanent part of the sheep genome and, in these days, sheep can't do without them."
The research team is trying to determine exactly how enJSRVs function in development of the sheep placenta and to understand how enJSRVs were involved in evolution of sheep by studying them in populations across the world. Knowledge of enJSRVs function can be used to develop tools to prevent infection by the exogenous and pathogenic form of the retrovirus, according to the researchers. Their results should have implications for both animal production and human health, they say.
The research activities have been funded by the Wellcome Trust in the United Kingdom and the National Institutes of Health in the U.S.