1. Researchers discovered a folate-receptor protein on the surface of mouse eggs (Juno) that is responsible for binding sperm during fertilization.
2. After binding the sperm membrane protein, Juno is sequestered to extracellular vesicles, preventing polyspermy.
Study Rundown: For the first time, researchers have identified an egg cell-surface folate receptor protein, Folr4 (Juno) that binds to sperm. Previous research has identified the sperm surface protein (Izumo1) that allows for egg-sperm fusion, but this is the first research study to identify the receptor on the egg surface that binds Izumo1 and is involved in the acrosomal reaction. Discovery of the Juno-Izumo interaction marks the first identified protein pair necessary for gamete recognition in any organism. Further research to clarify additional necessary steps to allow for oocyte fertilization are merited. This newly discovered protein could serve as a target for future translational interventions, including infertility treatments.
In-Depth [basic science study]: Using a novel technique of clustering proteins to account for weak binding interactions, researchers used Izumo1, a sperm cell-surface protein, as a binding probe to identify a reciprocal receptor on the surface of the mouse oocyte. The receptor protein was identified as folate receptor 4 (Folr4) and the receptor-protein interaction was conserved in multiple mammalian oocytes, including humans.
Using anti-Juno monoclonal antibodies in IVF assays, researchers demonstrated that blocking the Juno binding site prevented fertilization. Additionally, mice without the Juno gene were infertile. Immunogold electron microscopy demonstrated that 30-45 minutes after fertilization Juno was not detectable on the surface of the oocyte, but was present in extracellular vesicles.
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