Expression, regulation and function of the stem-loop binding protein during mammalian oogenesis

Although mRNAs encoding the histone proteins are among the most abundant mRNAs in mammalian oocytes, the mechanism regulating their translation in these cells has not been identified. Most histone mRNAs are not polyadenylated but instead carry in their 3'-utr a highly conserved stem-loop structure. In somatic cells, the stem-loop binding protein (SLBP) is expressed during S-phase of the cell cycle and associates with the stem-loop of histone mRNAs promoting their processing and translation and thereby coordinating their expression to DNA replication. As histone mRNAs are abundant in immature oocytes which are in G2 of the cell cycle and in ovulated or mature oocytes which are in M-phase, I examined the expression and the regulation of histone mRNAs in immature and maturing mouse oocytes. First, I described SLBP expression during oogenesis and pre-implantation embryonic development. I showed that SLBP is present at low levels in the nucleus of the immature oocyte and accumulates significantly during maturation of the oocyte. At both stages, SLBP is the only stem-loop binding activity present. I showed that SLBP meiotic accumulation correlates with the adenylation of SLBP mRNA and is mediated by the presence of a cytoplasmic polyadenylation element in SLBP 3'-utr. Also, I demonstrated that histones are synthesized in the immature and mature oocyte and that the translation of a reporter mRNA bearing the histone 3'-utr increases dramatically during oocyte maturation consistent with the accumulation of SLBP. I specifically blocked SLBP accumulation using RNA interference and observed that both translation of the reporter mRNA and endogenous histone synthesis are significantly reduced. Moreover, SLBP-depleted eggs display a significant decrease in pronuclear size and in the total amount of histones detectable on their chromatin. Finally, I also showed that elevating the amount of SLBP in immature (G2) oocytes is sufficient to increase translati