oa Early effect of progesterone on GPRx endocytosis in xenopus oocytes

Xenopus oocytes are naturally arrested at prophase of meiosis I for sustained periods of time before resuming meiosis in a process called maturation that prepares the egg for fertilization. GPRx is a G-Protein coupled Receptor (GPCR) expressed in Xenopus oocytes that sustains its meiotic arrest by increasing intracellular levels of cAMP via GaSß? proteins. Progesterone (PG) is the widely used hormone to release Xenopus oocytes meiotic arrest by reducing cAMP levels via its non-classical membrane PG receptor. However and to date the exact mechanism by which PG decreases cAMP levels and releases the meiotic arrest is still unclear. In this study we aimed to determine if PG treatment can promote GPRx desensitization and endocytosis leading to Xenopus oocyte maturation. As previously shown, overexpression of GPRx-GFP in Xenopus oocyte completely blocks PG-induced maturation. We defined GPRx subcellular distribution (~80% at the cell membrane), confirmed its presence in early endosomes and showed that its membrane localization is crucial for its meiotic arrest. Although early data showed a dip in cAMP levels within 15 sec after PG treatment, we found that no less than 30 minutes treatment with PG was needed to fully induce oocyte maturation compared to overnight treatment with PG, and was accompanied with a dip in cAMP levels and a significant shift of endogenous and overexpressed GPRx-GFP distribution from the membrane toward the inside of the oocyte. PG may induce GPRx endocytosis by phosphorylating the Serines/Threonine (S-TSS) domain at GPRx C-terminal end, since mutating these residues in the S-TSS motif completely blocked the effectiveness of PG in inducing GPRx endocytosis. Moreover, dose response experiments using GPRx wt or GPRx S-TSS mutant showed that PG induce maturation more efficiently in the presence of low intermediate concentration of GPRx wt in comparison to GPRx S-TSS mutant. Our data argue that PG releases Xenopus meiotic arrest by inducing GPRx desensitization and endocytosis.