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    Role of the JUNO oocyte protein in the mechanism of block to polyspermy in mammals


    Sophie Favier

     

    Friday, March 20th 2020 at 2:30 pm

    Ecole Normale Supérieure

    Conf IV room

    24 rue Lhomond, Paris 5ème





    Supervisors: Ahmed Ziyyat et Christine Gourier

    Team (Institut Cochin): From Gametes to Birth: Genomics, Epigenetics and Physiopathology of Reproduction

    Department: Development, Reproduction and Cancer (DRC)

    Abstract:

    Zona pellucida(ZP) is a barrier that the sperm has to pass through in order to reach the perivitelline space (EPV), which contains the oocyte. Fusion of gametes leads to the formation of a zygote, which will become a new being, unless polyspermy happens. This phenomenon constitutes a fertilization abnormality, harmful to development. Mechanisms implemented by the zygote to prevent other fusion are partially understood. The ZP rigidifies to prevent other sperm from entering the perivitelline space. However, this block is only effective after one hour of fusion, period during which other sperm may also reach the zygote. This suggests that there should be another mechanism to counter fusion of other sperm. By conducting a statistical study in mice, I have proven the existence of this second mechanism and its physiological aspect. I then sought out to characterize it with the following working hypothesis: after initial fusion, the release of JUNO, an oocyte protein essential for fertilization via its binding with its spermatic ligand IZUMO1, into the EPV alters the adhesive and fusogenic capacities of any sperm entering the EPV. By combining molecular biology, quantitative imaging, micromanipulation and laser ablation I studied the impact of the JUNO protein on the fusion capacity of the sperm by two complementary approaches, one using sperm directly recovered in the EPV of zygotes, and two using sperm artificially put in presence of JUNO. In parallel, in order to kinetically characterize the release of JUNO and its attachment to sperm, entering the EPV, I undertook to genetically modify JUNO expressed by the oocyte, so as to make it fluorescent without altering its function.