Remodeling of Gamete Membrane during Mammalian Fertilization

Abstract

Fertilization is a central event in the life of sexual organisms. It guarantees the survival of species. Close to the oocyte, the spermatozoon still has to interact with the oolemma. For the mammals, the fertilization process is almost doomed to fail without the GPI-AP Juno on the oocyte and the Immunoglobulin-Super Family protein Izumo on the spermatozoon. Nevertheless, the central question about how membranes merge is none answered. Our study aims a better understanding of the membrane dynamics triggered by the adhesion of a sperm on the oocyte membrane leading to gamete fusion.To do so, we developed two novel approaches. The first one consists in generating a biomimetic pure lipids sperm membrane to measure the fusogenic capacity of the sperm's fatty acids. We found that Nature's wisdom minimizes the spermatozoon's Energy requirement to fuse with standard plasma membrane, compared to other kind of cell lipids composition. Then, incorporating Izumo, we noticed an accommodating oligomerization of this essential protein correlated with the type of lipids. For the second approach, we developed a microfluidic tool to induce a physiological gamete interaction, controlling the time and area of the gamete encounter. This original device offers, in live and full resolution, unique point of view to study live membrane remodeling. We could demonstrate the simultaneity of membrane fusion and spermatozoon internalization as well as a particular way of swimming that seems to be common to sperm which manage to fertilize. These new biophysical tools provide an insight of the dynamics of gamete interplay. They set in motion the usually static biological description of this interaction.