Compensatory endocytosis occurs after cortical granule exocytosis in mouse eggs

Compensatory endocytosis (CE) is one of the primary mechanisms through which cells maintain their surface area after exocytosis. Considering that in eggs massive exocytosis of cortical granules (CG) takes place after fertilization, the aim of this study was to evaluate the occurrence of CE following cortical exocytosis in mouse eggs. For this purpose, we developed a pulse-chase assay to detect CG membrane internalization. Results showed internalized labeling in SrCl₂-activated and fertilized eggs when chasing at 37°C, but not at a nonpermissive temperature (4°C). The use of kinase and calcineurin inhibitors led us to conclude that this internal labeling corresponded to CE. Further experiments showed that CE in mouse eggs is dependent on actin dynamics and dynamin activity, and could be associated with a transient exposure of phosphatidylserine. Finally, CE was impaired in A23187 ionophore-activated eggs, highlighting once again the mechanistic differences between the activation methods. Altogether, these results demonstrate for the first time that egg activation triggers CE in mouse eggs after exocytosis of CG, probably as a plasma membrane homeostasis mechanism.