By Dean Haycock
Special To BioWorld Today
Drawings depicting the chillingly humorous adventures and macabre outlook of members of the Addams Family appeared in The New Yorker magazine from 1935 to 1989. They led the family to fame in print, television and film. Cartoonist Charles Addams believed that, if one of his cartoons needed a caption — no matter how witty — it represented a type of failure.
There is family with a similar name that is of great interest to biologists. Researchers would love to write detailed captions describing the activities of members of the ADAM family, a group of cell surface proteins with roles in cell adhesion, proteolysis or both.
The Addams family had some influence on the ADAM family name, which stands for "A Disintegrin And Metalloprotein."
"That was, I think, our intended joke," said Diana Myles, professor of molecular and cell biology at the University of California at Davis (UCD).
TNF-alpha converting enzyme, which activates TNF-alpha, a molecule important in mediating inflammatory responses, is an example of an ADAM protein. Nearly 30 proteins in the family have been identified so far, some widespread in distribution, others confined to specific tissues.
The first ADAM family member to be described, fertilin, is found only in testes.
"Very few molecules on either gamete [sperm or egg] surface have been identified and shown to have a functional role in fertilization," Myles told BioWorld Today.
Fertilin, one of the few, is located on the surface membrane of mammalian sperm where, since the 1980s, it has been suspected of involvement in membrane-membrane interactions between sperm and egg. According to findings described in "Fertilization Defects in Sperm from Mice Lacking Fertilin beta," a report in the Sept. 18 issue of Science, fertilin does a lot more than expected.
Noting that fertilin is a composed of alpha and beta sub-units which each contain the structural elements that characterize members of the ADAM family, Myles and her co-authors from UCD and the National Institute of Environmental Health Sciences, in Research Triangle Park, N.C., prepared a strain of mice that lacked the fertilin beta gene. These knockout mice appear and act normal in every way. Their sperm appear normal under the microscope but functionally they are significantly different.
"When we remove fertilin, sperm don't bind very well to the egg membrane and they don't fuse with it very well," Myles said.
Bigger surprises came when first author and UCD postdoctoral fellow Chunghee Cho and colleagues tested other aspects of sperm function. They found indications that, in addition to playing an important role in fusion of egg and sperm, fertilin plays a role in the migration of sperm through the oviduct, and affects its ability to penetrate the protective layer around mammalian egg cells called the zona pellucida.
In vitro, this meant that sperm lacking fertilin beta showed decreased ability to bind to and fertilize eggs. In vivo, it meant that sperm lacking fertilin beta did not migrate from the uterus to the fallopian tubes, where eggs are fertilized. The fertility of the knockout males was reduced by 98 percent. Females of the knockout strain, however, showed normal fertility when mated with normal males.
Barry Shur, chairman of cell biology at Emory University, in Atlanta, said what makes the work "very satisfying and very important is that the males are basically sterile, and that the sperm do not fuse or bind apparently with the egg plasma membrane. If that is all that they reported, it would be a straightforward, very nice confirmation of the importance of fertilin sperm-egg attractions and fertilization. What makes the paper more intriguing is that the sperm shows many more additional abnormalities than would be anticipated from the original work."
The research thus identifies a sperm protein that appears to play a highly influential role in multiple steps involved in the fertilization of mammalian eggs.
ADAM proteins have two potential functional domains. One is the disintegrin domain. It is similar to a family of snake venom peptides which prevent platelets from binding in snake-bite victims.
Miles said that region of the protein can potentially be involved in binding. "That is the particular region of fertilin beta that we think is functional in binding the sperm membrane to the egg membrane," she said.
The other region is the metalloprotease region, which would be expected to prepare other proteins to assume full activity.
"We think that either this protein is directly involved in these additional steps of fertilization or, we'd guess more likely, that perhaps this protein in that metalloprotease region may be important in actually affecting other proteins on the surfaces of cells," Myles said.
The researchers are following this lead up to see if other proteins are affected by the lack of fertilin in the sperm. The research is part of the efforts at the Contraceptive Center at UCD to identify proteins involved in fertilization.
This approach will involve the search for other proteins affected by the lack of fertilin on sperm, presumably due to the lack of metalloprotease activity.
"They then offer potential targets of drugs or they can be used as immunogens to try to develop contraceptive vaccines," Myles said. "We are very excited because we think we may have a path to identify even more [proteins involved in fertilization]." *