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The role of maternal effect genes, Gas6 and Sebox
The conserved The oocytes acquire competence to undergo complex processes, oocyte growth and oocyte maturation, and the capacity for fertilization and preimplantational embryo development, by accumulating RNAs and proteins in the ooplasm. Therefore, the identification of the genes expressed in the oocyte and its functional analysis will provide valuable resources to study molecular regulatory mechanism of oocyte maturation, fertilization and early embryogenesis. To better understand these mechanisms, a decade ago, we identified a list of differentially expressed genes between GV and MII oocytes using annealing control primer (ACP)-PCR technology. Among these genes, we selected two genes, Gas6 (growth arrest-specific 6), and Sebox (skin-embryobrain- oocyte homeobox) that expressed significantly higher levels in GV than MII and analyzed its functions by using RNA interference (RNAi). Unexpectedly, and fortunately, it turned out that both of genes are new candidate of maternal effect genes (MEGs) that is important for fertilization and/or early embryogenesis but not crucial for oocyte meiotic maturation. In particular, Gas6 is essential in maintaining the proper mitochondrial function, and biosynthesis of heparan sulfate and glutathione, which are required for normal sperm chromatin decondensation, pronuclear formation, and mainly for the sufficient cytoplasmic maturation of oocytes. We suggest that the correction in the Gas6 signaling network in oocytes may improve the embryonic developmental capacity caused by deterioration of the mitochondrial functions and/or contents during oocyte maturation. Meanwhile, Sebox is crucial for zygotic genome activation (ZGA) required for subsequent embryonic development beyond the 2-cell stage by coordinating the expression of other maternal factors, such as c-mos, Gdf9, Ube2a and Wee1. In conclusion, the observed failure of fertilization after Gas6 RNAi and the embryonic development at the 2-cell stage after Sebox RNAi was similar to the loss-of-function of the previously well-known MEGs. Based on these findings, we added Gas6 and Sebox as new mammalian MEGs. Findings of our research would broaden our knowledge regarding MEGs and a field of maternal programming in oocytes.
