Endocytosis in the mouse oocyte and its contribution to cAMP signaling during meiotic arrest

Mammalian oocytes are arrested at prophase I of meiosis until a preovulatory surge of LH stimulates them to resume meiosis. Prior to the LH surge, high levels of cAMP within the oocyte maintain meiotic arrest; this cAMP is generated in the oocyte through the activity of the constitutively active, G(s)-coupled receptor, G-protein-coupled receptor 3 (GPR3) or GPR12. Activated GPRs are typically targeted for desensitization through receptor-mediated endocytosis, but a continuously high level of cAMP is needed for meiotic arrest. The aim of this study was to examine whether receptor-mediated endocytosis occurs in the mouse oocyte and whether this could affect the maintenance of meiotic arrest. We found that constitutive endocytosis occurs in the mouse oocyte. Inhibitors of receptor-mediated endocytosis, monodansylcadaverine and dynasore, inhibited the formation of early endosomes and completely inhibited spontaneous meiotic resumption. A red fluorescent protein-tagged GPR3 localized in the plasma membrane and within early endosomes in the oocyte, demonstrating that GPR3 is endocytosed. However, overexpression of G-protein receptor kinase 2 and β-arrestin-2 had only a modest effect on stimulating meiotic resumption, suggesting that these proteins do not play a major role in GPR3 endocytosis. Inhibition of endocytosis elevated cAMP levels within oocytes, suggesting that there is an accumulation of GPR3 at the plasma membrane. These results show that endocytosis occurs in the oocyte, leading to a decrease in cAMP production, and suggest that there is a balance between cAMP production and degradation in the arrested oocyte that maintains cAMP levels at an appropriate level during the maintenance of meiotic arrest.