Translational control in oocyte development

Abstract

Translational control of specific mRNAs is a widespread mechanism of gene regulation, and it is especially important in pattern formation in the oocytes of organisms in which the embryonic axes are established maternally. Drosophila and Xenopus have been especially valuable in elucidating the relevant molecular mechanisms. Here, we comprehensively review what is known about translational control in these two systems, focusing on examples that illustrate key concepts that have emerged. We focus on protein-mediated translational control, rather than regulation mediated by small RNAs, as the former appears to be predominant in controlling these developmental events. Mechanisms that modulate the ability of the specific mRNAs to be recruited to the ribosome, that regulate polyadenylation of specific mRNAs, or that control the association of particular mRNAs into translationally inert ribonucleoprotein complexes will all be discussed.

Figure 1.

Translational repression by eIF4E binding proteins. (A) An eIF4E binding protein (4E-BP) competes with eIF4G (4G) for interaction with the cap-binding protein eIF4E (4E). In general eIF4G has a higher affinity for eIF4E than does the regulatory eIF4E binding protein and an active cap-binding complex including eIF4A (4A) can assemble, but that equilibrium is reversed if the regulatory protein is recruited to a target mRNA by an RNA binding protein (RBP) that binds to the 3′ UTR. Then eIF4E is sequestered away from the cap-binding complex and translation is repressed. 4E-BPs that operate in this way include Drosophila Cup and Xenopus Maskin. RBPs that recruit these 4E-BPs to specific mRNAs include Drosophila Bru, Drosophila Smg, and Xenopus CPEB. (B) Translational repression by 4E homology proteins. 4E homology protein (4E-HP) can bind the 5′ cap structure of the mRNA but not eIF4G. It competes with eIF4E for cap binding and represses translation of mRNAs with which it is associated. The affinity of 4E-HP for the 5′ cap structure is less than that of eIF4E, but that equilibrium is reversed if 4E-HP is recruited to target mRNAs by RBPs that bind the 3′ UTR. In Drosophila, 4E-HP can be recruited to target mRNAs by Bicoid or by the Pumilio/Nanos/Brain Tumor complex, whereas in mammalian cells 4E-HP can be recruited by Prep1.

Figure 2.

Proposed mechanism for Vas-mediated translational activation. eIF5B (5B) is required for recruitment of the large ribosomal subunit (60S) to the initiation complex, so that the elongation phase of translation can proceed. When this process is blocked, translation is stalled with the small ribosomal subunit and initiator tRNA-Met (43S) stalled at the start codon (AUG). Vas binds a sequence element in the 3′ UTR and eIF5B (5B), thus promoting the recruitment of the 60S subunit and activating translation. Regulation of an mRNA expressed in erythroid cells, r15-LOX, at the level of subunit joining involves a 3′ UTR binding repressor that inhibits eIF5B recruitment (Ostareck et al. 2003). It is unknown whether Vas-mediated activation involves displacement of such a repressor.

Figure 3.

Cytoplasmic polyadenylation. In immature oocytes, certain maternal mRNAs contain a CPE (general structure of UUUUUAU) that is bound by CPEB, and the polyadenylation hexanucleotide AAUAAA that is bound by CPSF (for simplicity, only the 3′ UTR is depicted in the figure). CPEB is also bound by Gld2, a poly(A) polymerase; PARN, a deadenylating enzyme; and ePAB, a poly(A) binding protein. The entire complex is assembled on symplekin, a scaffold protein. When exported from the nucleus, the CPE-containing RNA has a long poly(A) tail. Once assembled with CPEB and the other factors noted above, PARN shortens the poly(A) tail to ∼20–40 nucleotides. Gld2 is also activated at this time and catalyzes poly(A) addition, but because PARN is more active, the poly(A) tail is maintained in a shortened state. Progesterone secretion from follicle cells sets off a signaling cascade in the oocyte that results in activation of the kinase Aurora A, which phosphorylates CPEB on a single site (serine 174). This event causes the expulsion of PARN from the ribonucleoprotein (RNP) complex, resulting in default Gld2-catalyzed polyadenylation. Next, the kinase cdk1 is activated, which phosphorylates CPEB on six additional sites; these events cause ePAB to dissociate from CPEB and bind the newly elongated poly(A) tail. ePAB not only protects the poly(A) tail from hydrolysis by exonucleases, but it binds the initiation factor eIF4G and helps stimulate translation.