Isolation of ferret protein promising for cancer, reproductive studies

June 24, 2003

CHAMPAIGN, Ill. -- Biologists studying early pregnancy in ferrets have isolated a protein vital to embryonic implantation. The discovery at the University of Illinois at Urbana-Champaign eventually could enhance assisted-reproductive efforts in many threatened species.

In addition to its implications for reproduction, the discovery opens a window to study numerous cancerous tumors that secrete the same protein, said Janice M. Bahr, a professor of reproductive physiology in the department of animal sciences at Illinois.

The findings appear in a study published online this week by the Proceedings of the National Academy of Sciences. The study will appear later in a print issue of PNAS.

The protein -- glucose-6-phosphate isomerase (GPI) -- already is widely known as a highly conserved enzyme occurring in intracellular metabolism, converting sugars in glycolysis in many organisms and humans. It had been considered "an unlikely candidate for an implantation protein," the researchers said.

"In the domestic ferrets that we studied, we found a unique role for this enzyme as a secreted protein that is essential in the reproductive process," Bahr said. "Interestingly, the same ability to secrete this protein is found in many types of metastatic tumors, suggesting that tumor cells have co-opted this process. The secretion of GPI allows the tumors to find and lock onto receptors to invade healthy tissues."

The ability of tumors to spread is similar to the invasive process of implantation, suggesting a potential mechanism for GPI during implantation, noted Bahr and colleague Laura Clamon Schulz, a doctoral student at Illinois during the study.

In domestic ferrets, GPI triggers a signal that allows a fertilized embryo to implant successfully into the wall of the uterus.

Implantation occurs 12 days after mating when the trophoblast invades the uterus to establish a fetal-maternal connection to allow nutrients into the fetus. If ovulation occurs without fertilization, ferrets experience a pseudopregnancy.

Implantation of the embryo is the first step in the establishment of pregnancy in mammals that have placentas. Failure to implant successfully is a leading cause of infertility in humans and other animals, Bahr said.

The activity of GPI in domestic ferrets may well apply to many other mustelid carnivores, such as the black-footed ferret, an endangered animal in North America, and mink, skunks and badgers. The protein also may be crucial to the implantation process in other carnivores, including seals, bears, pandas, sea lions and walruses.

The discovery suggests that GPI supplementation could encourage successful pregnancies in threatened or endangered species, especially when a fertility problem is due more to a failure in the mother than to the embryo, Bahr and Schulz said.

"The GPI work is basic science, but in order to do any sort of assisted reproduction, or to understand any reproductive problems in captive species, we need to understand normal reproduction in these animals," said Schulz, who now is doing postdoctoral work at Boston University. "There are major gaps in our understanding of reproduction in carnivores. There isn't even a pregnancy test available for carnivores."

Bahr and Schulz utilized advanced molecular analysis, including chromatofocusing chromatography, gel electrophoresis, mass spectrophotometry and reverse-transciptase polymerase chain reaction, to isolate and identify GPI. Once they isolated it, they tested its presence and effect during early pregnancy by measuring GPI in serum, passive immunization of pregnant ferrets and in vitro. Immunizing female ferrets against the protein severely limited the number of embryos that were able to implant in the uterus.

The research shows that GPI is crucial in the implantation process in the ferret. It also helps to better understand pregnancy in carnivores, which do not follow the typical laboratory rodent model, the researchers said.
The National Science Foundation funded the research.

University of Illinois at Urbana-Champaign

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