In the United States, nearly 2.6 million couples have been treated for infertility with about 40 percent of those cases thought to be due to male infertility. Now, a study published in the August 22 issue of Cell identifies a new protein that is required for a sperm to bind to an egg during the process of fertilization. This research provides important new insight into the molecular mechanisms that are involved in the initial events of sperm-egg association and may shed light on what underlies some instances of male infertility.
There are a variety of factors that can lead to male infertility, including defects in sperm motility and insufficient sperm production. However, in many cases, the sperm of infertile men appear to be completely normal. Barry D. Shur, PhD, professor and chair of cell biology at Emory University School of Medicine, and postdoctoral fellow Michael A. Ensslin, PhD, took a close look at the specific molecules and events required for a sperm to recognize an egg so that fertilization can take place. The researchers identified a protein on the surface of mouse sperm that is required for the sperm to successfully bind to the outside of the egg. The protein, called SED1, binds specifically to unfertilized eggs and no longer recognizes an egg after it has been fertilized. When examined in the laboratory, sperm without SED1 were unable to bind to an egg. Further, mice without SED1 had greatly reduced fertility even though the sperm appeared normal in number and motility.
These results strongly suggest that SED1 plays a critical role in the initial association between sperm and egg. "An understanding of the molecular mechanisms underlying sperm-egg binding may give insight into the basis for at least some percentage of male infertility," explains Dr. Shur. "In addition, this work is of interest because the composition of SED1 is similar to other types of cell-cell binding proteins, which have not previously been implicated in sperm-egg binding."
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