Proteasome dynamics during maturation and fertilization of human oocytes

Abstract

Proteasomes has been involved in the inactivation of the MPF (Metaphase Promoting Factor) and therefore in the progression from MI (Metaphase I) to MII (Metaphase II). Proteasomal inhibitors block fertilization. To better understand the function of proteasomes during oocyte meiosis and fertilization, we analyzed the distribution of proteasomes and poly-ubiquitinated proteins during both events after incubation with proteasome inhibitors MG132 and Velcade™. Descriptive and functional basic study. Human oocytes at germinal vesicle stage (GVs) were incubated during 17 h in maturation media supplemented with 10 mM MG132 (n = 57) or 100 nM Velcade™ (n = 61) at 37°C and 5% of CO2. To study the effect of proteasome inhibition during fertilization, a total of 29 supernumerary discarded normal fertilized oocytes (2 pronuclei, 2PNs) were incubated in the presence of the mentioned inhibitors. Antibodies against ab proteasomes subunits and FK1 (polyubiquitins proteins) were used for immunofluorescent labeling. Images were acquired by Confocal Microscopy and edited with Adobe Photoshop 7.0. Proteasome inhibition during in vitro maturation, arrested oocytes at different stages (MG132 and Velcade™ respectively): 14.0% and 16.4% at GV, 84.2% and 82.0% at GVBD, and 1.8% and 1.6% at MII (45% and 61% controls, (P<0.001)). The majority of GVBDs after MG132 were arrested at prometaphase stage after MG132 and at MI after Velcade™. Accumulations of poly-ubiquitin proteins in the cytoplasm were observed in all cases. After incubation of 2PNs with MG132 and Velcade™, 100% and 42.9% of them arrested at the syngamy stage respectively. In all cases, zygotes arrested with proteasome inhibitors show poly-ubiquitin proteins distributed in the cytoplasm and close to the nucleoli inside 2PNs. As expected, proteasomes were concentrated inside the 2PNs and closed to both PNs. Our results indicate that inhibition of proteasomes arrests cell cycle preventing in vitro maturation of human oocytes. This demonstrates the importance of proteasomes during the organization of the first metaphase plate suggesting a new role of the ubiquitin-proteasomes pathway in mammalian fertilization. Our data shows that proteasomal activity is essential for the decrease in MPF activity and completion of the first meiotic division in humans.