IMD/ADM2 operates as a secretory factor that controls cumulus-oocyte complexes (COCs) conformation for oocytes in vitro maturation

Abstract

During in vitro maturation (IVM), the compact structure of cumulus-oocyte complexes (COCs) is vital for oocyte competence acquisition. Intermedin/Adrenomedullin-2 (IMD/ADM2) binds to the receptor RAMP (1, 2, or 3):CLR. Recently, it was demonstrated that IMD/ADM2 stimulates oocyte competence and improves bovine embryo quality. Therefore, this study aimed to examine the IMD/ADM2 as a secretory factor controlling COCs conformation for oocyte maturation. The results showed that traditional M-CDM medium induced in COCs the Imd/Adm2 gene expression during IVM and produced IMD/ADM2 peptide secretion. Furthermore, after IVM, in the oocytes, the expression of ramps (1, 2, or 3) and clr was demolished, and RAMPs and CLR proteins were decreased, with a negative Pearson correlation. These results suggest that RAMPs and CLR are synthesized and stored during oocyte maturation. Supplementing the M-CDM with α-RAMP1 or α-IMD/ADM2 antibodies elicits a negative effect (P < 0.05) in COCs compaction. Blocking the IMD/ADM2 signaling pathway with any α-RAMPs or α-CLR antibodies produces a similar lower yield of oocytes in metaphase II (P > 0.05) but was lower than control culture medium (P < 0.05). In conclusion, when COCs are cultured with M-CDM, the IMD/ADM2 becomes expressed and secreted. In turn, it acts as a ligand preferentially to RAMP1:CLR or RAMP3:CLR, present in cumulus cells and oocytes. Sequentially, COCs compact structure is conformed to promote an adequate bidirectional communication that conduces the oocytes' maturation.