Expression of SNAP23 and Rab3A in mouse oocytes and fertilized eggs and their role in cortical granules exocytosis
The proteins and molecular machinery mediating the release of cortical granule (CG) contents from fertilized embryos is not completely understood. The process of vesicle fusion involves linking chaperones prior to vesicle to membrane contact. Rab3A, a member of a low-molecular weight GTP-binding protein superfamily has been detected in mouse embryos from the unfertilized meiotic II stage to the 2-cell. It is believed to positively regulate the final step of CG exocytosis by binding to Rabphillin, calcium ions (Ca2+), and phospholipids. SNAP23 a member of soluble NSF [N-ethylmaleimidesensitive factor] attachment protein receptors (SNAREs) binds together with parts of the Rab3A-rabphilin3A complex and is believed to be involved in the Ca2+-dependent exocytosis of non-neuronal systems. In this study we observed the mRNA expression for SNAP23 and Rab3A in pre-Meiotic I, post-Meiotic I unfertilized eggs (pre-MI UFE and post-MI UFE), and fertilized eggs (FE) utilizing RT-PCR. The products were analyzed in 2% agarose gel stained with ethidium bromide. Density analysis using a globin external standard showed that the levels of mRNA transcripts declined from the UFE to the FE in both genes, SNAP23 and Rab3A. Immunofluorescence was used for the detection and localization of Rab3A protein within the pre-MI and post-MI UFE and FE mouse egg. Eggs were stained with anti-Rab3A primary antibody and lens culinaris agglutinin (LCA) conjugated to FITC. Rab3A showed punctate staining in pre- and post-MI UFEs on small vesicles assumed to be CGs and in FEs on vesicles of a larger size. Uniform cytoplasmic expression was also seen, throughout the cells cortical and subcortical regions in each stage (pre- and post-MI UFEs and FEs), but with decreasing intensity as the eggs matured. This cytoplasmic stain may represent inactive Rab3A in the cytosol. The LCA stain showed punctate expression of cortical granules with localization within the cortical region and the plasma membrane. The addition of information on SNAP23 and Rab3A will aid in the process of studying CG exocytosis as well as in understanding the temporal and spatial development pathways involved in stimulating the cortical reaction.