245 ANALYSIS OF CORTICAL GRANULE EXUDATE OBTAINED BY CHEMICAL OOCYTE ACTIVATION IN PIGS

Abstract

Cortical granules (CG) are clue organelles in the mammalian oocyte because once released, their content modifies the zona pellucida (ZP) and oolema, thus preventing polyspermy. However, research on putative CG proteins has progressed slowly because of the picogram amount of proteins contained in CG. Isolation and identification of CG contents in porcine oocytes would help to elucidate the molecular mechanism involved in blocking polyspermic fertilization. Our objective was to study the contents of CG from in vitro-matured (IVM) porcine oocytes, and to achieve this objective, CG exudate was collected after its release from chemically activated oocytes. Oocytes were subjected to IVM in porcine oocyte medium supplemented with 50 μM β-mercaptoethanol for 44 h. After the IVM period, the ZP was removed by protease treatment (0.5% pronase in PBS), and the ZP-free oocytes were activated with calcium ionophore A23187 (6.5 μM, 2 min) in a medium consisting of 114.06 mM NaCl, 3.20 mM KCl, 0.50 mM MgCl₂·6H₂O, 10.00 mM sodium lactate, 0.35 mM NaH₂PO₄, 5.00 mM glucose, 25.07 mM NaHCO₃, and 8.00 mM calcium lactate·5H₂O. After activation, oocytes were transferred to fresh medium without calcium ionophore and kept for 30 min to allow release of the CG content. After this time, medium containing the CG exudate was collected, as well as the activated oocytes, and both samples were stored at –80°C until analysis. Samples were thawed and the CG proteins were concentrated by centrifugation in 10-kDa centrifugal devices (Microcon, Millipore, Billerica, MA) following the manufacturer’s instructions. The CG exudates from activated oocytes (n = 300) and activated oocytes (n = 125) were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions. In brief, 4% stacking and 12% separating gel was used and run using 25 mM Tris–0.2 M glycine buffer, pH 8.6, containing 0.1% SDS for 1.5 h at 150 V and room temperature. After electrophoresis, the gel was silver stained. Thirteen strong bands were identified in the CG exudate lane, with an approximate molecular mass from approximately 45 to 105 kDa. However, the lane for activated oocytes showed faint protein bands. The presence of well-defined bands in the CG exudate lane might correspond to different CG-derived proteins. These preliminary results show a new approach for studying CG content. Further proteomic analysis of the bands will help to describe specific proteins contained in these organelles, shedding light on the role of the cortical reaction in pigs.