The missing piece to understanding mammalian conception has finally been uncovered. Last week, Gavin Wright, a researcher at the Wellcome Trust Sanger Institute, and his team reported the discovery of the protein that facilitates sperm-egg binding. In an article published in Nature, the British scientists describe the identification of Juno, a protein located on the surface of murine eggs.
Since 2005, scientists have known that Izumo1, which is a protein located on the surface of sperm, allows sperm to bind to an egg. However, the complementary receptor protein for Izumo1 was unknown. This missing knowledge left a huge gap in scientific understanding of how sperm-egg binding conferred species specificity onto fertilization.
Why did the identification of Juno take upwards of a decade? Scientists had difficulty finding the Izumo1 complementary protein because the initial interactions are relatively weak. The weak bonding made observation and isolation extremely difficult. To identify Juno, Wright and his team developed a method to cluster Izumo1 proteins in cell culture. In smaller scales of sperm-egg interaction, the bonding of surface proteins are challenging to detect. With Wright and his team’s method, however, the higher magnitude of interactions become stabilized and are easy to detect. After the sperm proteins were clustered, the researchers could analyze the bound proteins.
As expected, the team also found in an experiment that eggs that lack the expression of Juno protein are infertile and do not attract sperm. Furthermore, after Izumo1-Juno interactions, the fertilized egg will rapidly shed Juno to ensure that each egg will only bind to one sperm. After merely thirty to forty-five minutes, the egg surface did not contain Juno. This protects the egg from polyspermy, a condition that could lead to genetic chaos. Antibodies that block the Izumo1 and Juno interaction also inhibited fertilization of the egg.
The two romantically involved proteins have yet to unravel the entirety of the sperm-egg relationship. For example, the underlying mechanisms needed to fuse the two gametes together is still unclear. Some scientists postulate that other proteins are involved to guide Izumo1 and Juno to each other. Interestingly, because of their weak bonding, researchers suggest that the local clustering of Juno in the cell membrane is essential for efficient binding of Izumo1.
Though the experiment only used murine sperm and egg, the researchers suggest that the results can be applied to several other mammalian species, including humans. This breakthrough, which provides a fuller explanation of the process of conception, can give rise to more targeted research for new forms of birth control or infertility treatments. As the climax of the fertilization process is illuminated, scientists can finally begin developments for drugs that target these love proteins.
