MAPK interacts with XGef and is required for CPEB activation during meiosis in Xenopus oocytes

Oocyte quality and maternal control of development

The oocyte is a unique and highly specialized cell responsible for creating, activating, and controlling the embryonic genome, as well as supporting basic processes such as cellular homeostasis, metabolism, and cell cycle progression in the early embryo. During oogenesis, the oocyte accumulates a myriad of factors to execute these processes. Oogenesis is critically dependent upon correct oocyte-follicle cell interactions. Disruptions in oogenesis through environmental factors and changes in maternal health and physiology can compromise oocyte quality, leading to arrested development, reduced fertility, and epigenetic defects that affect long-term health of the offspring. Our expanding understanding of the molecular determinants of oocyte quality and how these determinants can be disrupted has revealed exciting new insights into the role of oocyte functions in development and evolution.

Ca2+ homeostasis regulates Xenopus oocyte maturation

In contrast to the well-defined role of Ca2+ signals during mitosis, the contribution of Ca2+ signaling to meiosis progression is controversial, despite several decades of investigating the role of Ca2+ and its effectors in vertebrate oocyte maturation. We have previously shown that during Xenopus oocyte maturation, Ca2+ signals are dispensable for entry into meiosis and for germinal vesicle breakdown. However, normal Ca2+ homeostasis is essential for completion of meiosis I and extrusion of the first polar body. In this study, we test the contribution of several downstream effectors in mediating the Ca2+ effects during oocyte maturation. We show that calmodulin and calcium-calmodulin-dependent protein kinase II (CAMK2) are not critical downstream Ca2+ effectors during meiotic maturation. In contrast, accumulation of Aurora kinase A (AURKA) protein is disrupted in cells deprived of Ca2+ signals. Since AURKA is required for bipolar spindle formation, failure to accumulate AURKA may contribute to the defective spindle phenotype following Ca2+ deprivation. These findings argue that Ca2+ homeostasis is important in establishing the oocyte's competence to undergo maturation in preparation for fertilization and embryonic development.

RINGO/cdk1 and CPEB mediate poly(A) tail stabilization and translational regulation by ePAB

One activity that controls mRNA translation in vertebrate oocytes, embryos, and neurons is cytoplasmic polyadenylation. In Xenopus oocytes, where much of the biochemistry of this process has been elucidated, nuclear pre-mRNAs containing a cytoplasmic polyadenylation element (CPE) in their 3' untranslated regions (UTRs) have long poly(A) tails; once the RNAs are spliced and transported to the cytoplasm, the tails are shortened. Following the resumption of meiosis, the poly(A) tails are lengthened and translation ensues. CPEB is a sequence-specific RNA-binding protein that coordinates these events and does so by binding to the CPE as well as several factors including Gld2, a poly(A) polymerase, and PARN [poly(A)-specific ribonuclease], a deadenylase. Here, we show that ePAB, embryonic poly(A)-binding protein, transiently associates with the polyadenylation complex; it initially interacts with CPEB, but after polyadenylation, it binds the poly(A) tail. ePAB dissociation from CPEB is regulated by RINGO (Rapid Inducer of G(2)/M progression in Oocytes), a cyclin B1-like cofactor that activates cdk1, a protein kinase that phosphorylates CPEB. Subsequent ePAB binding to the poly(A) tail is necessary to protect the homopolymer from degradation by deadenylating enzymes. Poly(A)-bound ePAB also interacts with eIF4G, which instigates translation initiation of CPEB-bound mRNAs.

Mechanism of degradation of CPEB during Xenopus oocyte maturation

CPEB, a cytoplasmic polyadenylation element-binding protein, plays an important role in translational control of maternal mRNAs in early animal development. During Xenopus oocyte maturation, CPEB undergoes a Cdc2-mediated phosphorylation- and ubiquitin-dependent degradation that is required for proper entry into meiosis II. However, the precise mechanism of CPEB degradation, including the identity of the responsible E3 ubiquitin ligase, is not known. Here, we show that the SCF(beta-TrCP) E3 ubiquitin ligase complex targets CPEB for degradation during Xenopus oocyte maturation. beta-TrCP, the F-box protein of SCF(beta-TrCP), specifically binds to a sequence (190)TSGFSS(195) (termed here the TSG motif) of CPEB, thereby targeting CPEB for degradation. beta-TrCP binding depends on phosphorylation of Thr-190, Ser-191, and Ser-195 in the TSG motif. Among these residues, Ser-191 is phosphorylated by the Polo-like kinase Plx1, which binds CPEB at a specific Thr-125 residue prephosphorylated by Cdc2. Finally, Cdc2-mediated phosphorylation of other multiple Ser residues, previously implicated in CPEB degradation, is required for both Thr-125 phosphorylation and beta-TrCP binding, presumably causing conformational changes of CPEB. We propose that Cdc2 and Plx1 sequentially phosphorylate CPEB and target it for SCF(beta-TrCP)-dependent degradation in Xenopus oocytes. We suggest that many other proteins carrying the TSG-like motif may be targeted by SCF(beta-TrCP).

Regulation of the oocyte-to-zygote transition

Oocytes, the female germ cells, contain all the messenger RNAs necessary to start a new life but typically wait until fertilization to begin development. The transition from oocyte to fertilized egg (zygote) involves many changes, including protein synthesis, protein and RNA degradation, and organelle remodeling. These changes occur concurrently with the meiotic divisions that produce the haploid maternal genome. Accumulating evidence indicates that the cell-cycle regulators that control the meiotic divisions also regulate the many changes that accompany the oocyte-to-zygote transition. We suggest that the meiotic machinery functions as an internal pacemaker that propels oocytes toward embryogenesis.