Temporally Coordinated Cell Cycle- and Clock-Dependent Mechanisms Control the Maternal-Embryonic Transition During Early Embryogenesis.

Abstract

Early embryonic development is directed by mRNAs and proteins that accumulate in the oocyte before fertilization. Degradation of these factors permits the embryonic genome to assume control of development, a process that is frequently termed the maternal-embryonic transition. Although the degradation of maternal mRNAs has been extensively studied, relatively little is known of the mechanisms that eliminate maternally encoded protein. The stem-loop-binding protein (SLBP) regulates the stability and translation of most histone mRNAs. SLBP is expressed during S-phase in somatic cells and is degraded at G2 by a mechanism that requires its phosphorylation by cyclin-dependent kinase (cdk). In contrast, SLBP accumulates in growing oocytes and during meiotic maturation. This maternal stock is inherited by the newly fertilized embryo and must be degraded to enable the cell cycle-regulated pattern of somatic cells to be established. We report here that the quantity of embryonic SLBP remains little changed between fertilization and late G2 of the 2-cell stage, and then declines by about 75% between 44 and 52 hr post-hCG, prior to entry into M-phase. When embryos were prevented from reaching late G2, by inhibition of either DNA replication or transcription, or when cdk activity was inhibited, SLBP degradation was partially inhibited. Nonetheless, a substantial fraction of the SLBP was degraded under these conditions. Inhibition of transcription and cdk activity together did not further inhibit degradation, indicating that they participated in the same pathway. These results indicated that (i) progression to late G2 and cdk activity are required for efficient degradation of SLBP and (ii) SLBP can be degraded independently of these events. To investigate the G2- and cdk-independent SLBP degradation, we examined parthenogenetic embryos that, as compared to fertilized embryos, were developmentally delayed 6 hours later relative to the time of hCG injection. SLBP became partially degraded between 44 and 52 hr post-hCG in the parthenotes, even though they had not yet reached late G2. This suggests that SLBP is also degraded by a mechanism that becomes active 44-52 hr after the initiation of meiotic maturation. We propose that degradation of SLBP is regulated by two mechanisms: one that becomes active at late G2, requires cdk-dependent phosphorylation, and likely corresponds to the mechanism in somatic cells; and a second that is independent of progression to G2 and SLBP phosphorylation, and is governed by a clock linked to maturation. The temporally co-ordinated activity of these two mechanisms in normal embryos ensures the rapid and timely degradation of the large maternal stock of SLBP.