Widespread changes in the translation and adenylation of maternal messenger RNAs following fertilization of Spisula oocytes

We have reported previously that sequence-specific adenylations and deadenylations accompany changes in the translation of maternal mRNA following fertilization of Spisula oocytes (E.T. Rosenthal, T.R. Tansey, and J.V. Ruderman, 1983, J. Mol. Biol. 166, 309-327). The data presented here confirm and extend those observations. We have identified four classes of maternal mRNA with respect to translation: Class 1-not translated in oocytes and translated at very high efficiency immediately after fertilization, Class 2-not translated in oocytes and partially utilized for translation following fertilization, Class 3-translated in oocytes and not translated in embryos, and Class 4-not translated either before or after fertilization. There is an excellent, although not perfect, correlation between the translation of an mRNA and its polyadenylation status. The poly(A) tails of all the mRNAs which are translated in oocytes and untranslated in embryos are shortened at fertilization, and the poly(A) tails of those mRNAs which are untranslated in oocytes and translated in embryos are lengthened at fertilization. These adenylations and deadenylations occur simultaneously during the first 20 min following fertilization.     

Dissociation of mRNA cytoplasmic polyadenylation from translational activation by structural modification of the 5'-UTR.

During early metazoan development, certain maternal mRNAs are translationally activated by elongation of their poly(A) tails. Bicoid ( bcd ) mRNA is a Drosophila maternal mRNA that is translationally activated by cytoplasmic polyadenylation during the first hour after egg deposition. The sequences necessary and sufficient to promote its poly(A) elongation, and hence translation, are contained within its 3'-untranslated region (UTR). The mechanism by which poly(A) elongation at the 3'-end affects translational initiation at the 5'-end remains unknown. To investigate this question, we have analyzed a bicoid mRNA whose 5'-UTR contains a short antisense sequence directed against a portion of the coding region. This mutated RNA is efficiently translated in vitro. After injection into Drosophila embryos, this RNA is stable and polyadenylated, but inefficiently translated. These experiments show that structural modification of the 5'-end of an mRNA can perturb the translational activation normally conferred by polyadenylation in vivo.     

Translational control during early development

Early development in many animals is programmed by maternally inherited messenger RNAs. Many of these mRNAs are translationally dormant in immature oocytes, but are recruited onto polysomes during meiotic maturation, fertilization, or early embryogenesis. In contrast, other mRNAs that are translated in oocytes are released from polysomes during these later stages of development. Recent studies have begun to define the cis and trans elements that regulate both translational repression and translational induction of maternal mRNA. The inhibition of translation of some mRNAs during early development is controlled by discrete sequences residing in the 3' and 5' untranslated regions, respectively. The translation of other RNAs is due to polyadenylation which, at least in oocytes of the frog Xenopus laevis, is regulated by a U-rich cytoplasmic polyadenylation element (CPE). Although similar, the CPE sequences of various mRNAs are sufficiently different to be bound by different proteins. Two of these proteins and their interactions are described here.     

Multiple sequence elements and a maternal mRNA product control cdk2 RNA polyadenylation and translation during early Xenopus development.

Cytoplasmic poly(A) elongation is one mechanism that regulates translational recruitment of maternal mRNA in early development. In Xenopus laevis, poly(A) elongation is controlled by two cis elements in the 3' untranslated regions of responsive mRNAs: the hexanucleotide AAUAAA and a U-rich structure with the general sequence UUUUUAAU, which is referred to as the cytoplasmic polyadenylation element (CPE). B4 RNA, which contains these sequences, is polyadenylated during oocyte maturation and maintains a poly(A) tail in early embryos. However, cdk2 RNA, which also contains these sequences, is polyadenylated during maturation but deadenylated after fertilization. This suggests that cis-acting elements in cdk2 RNA signal the removal of the poly(A) tail at this time. By using poly(A) RNA-injected eggs, we showed that two elements which reside 5' of the CPE and 3' of the hexanucleotide act synergistically to promote embryonic deadenylation of this RNA. When an identical RNA lacking a poly(A) tail was injected, these sequences also prevented poly(A) addition. When fused to CAT RNA, the cdk2 3' untranslated region, which contains these elements, as well as the CPE and the hexanucleotide, promoted poly(A) addition and enhanced chloramphenicol acetyltransferase activity during maturation, as well as repression of these events after fertilization. Incubation of fertilized eggs with cycloheximide prevented the embryonic inhibition of cdk2 RNA polyadenylation but did not affect the robust polyadenylation of B4 RNA. This suggests that a maternal mRNA, whose translation occurs only after fertilization, is necessary for the cdk2 deadenylation or inhibition of RNA polyadenylation. This was further suggested when poly(A)+ RNA isolated from two-cell embryos was injected into oocytes that were then allowed to mature. Such oocytes became deficient for cdk2 RNA polyadenylation but remained proficient for B4 RNA polyadenylation. These data show that CPE function is developmentally regulated by multiple sequences and factors.     

Patterns of maternal messenger RNA accumulation and adenylation during oogenesis in Urechis caupo

We have investigated the accumulation and adenylation of the maternal mRNA during oogenesis in the oocytes of the marine worm Urechis caupo. The analysis, using in vitro translation and cDNA probes to assay for specific mRNAs, demonstrates that different maternal mRNAs accumulate with different patterns during oogenesis. One class of maternal mRNAs accumulates throughout oogenesis and remains at a steady level in the full-grown oocyte. These mRNAs do not have a poly(A) tail long enough to mediate binding to oligo(dT)-cellulose in oocytes, but are rapidly adenylated immediately following fertilization. The other maternal mRNAs accumulate in growing oocytes as poly(A)+ RNA and undergo some deadenylation in full-grown oocytes and embryos. Some of these mRNAs attain their highest concentration fairly early in oogenesis, while others continue to accumulate during later stages. Many of the mRNAs that accumulate as poly(A)+ RNA in growing oocytes diminish dramatically in concentration in full-grown oocytes.     

Regulated polyadenylation of clam maternal mRNAs in vitro

During meiotic maturation of Spisula oocytes, maternal mRNAs undergo changes in translation and in the length of their poly(A) tails. In general, those mRNAs that are translationally activated, i.e., unmasked become polyadenylated, while deactivated mRNAs lose their poly(A) tails. The activated class of mRNAs encode ribonucleotide reductase, cyclins A and B and histone H3, while the proteins that stop being made include tubulin and actin. Previously, we demonstrated that mRNA-specific unmasking can be brought about in vitro by preventing the interaction of protein(s) with central portions of the 3′ noncoding regions (masking regions) of ribonucle-otide reductase and cyclin A mRNAs. In this report, we show that clam egg extracts are capable of sequence-specific polyadenylation of added RNAs since the 3′ untranslated regions (UTRs) of ribonu-cleotide reductase and histone H3 mRNAs are polyadenylated, while that of actin mRNA is not. In contrast, oocyte extracts, as in vivo, are essentially devoid of polyadenylation activity. We present an initial characterisation of the cis-acting sequences in the 3′ UTR of ribonucleotide reductase mRNA required for polyadenylation. The results suggest that the sequences for cytoplasmic polyadenylation are more complex and extensive than those determined in vertebrates and that they may partly overlap with the masking regions. © 1993 Wiley-Liss, Inc.     

Sequence-specific adenylations and deadenylations accompany changes in the translation of maternal messenger RNA after fertilization of Spisula oocytes

A dramatic change in the pattern of protein synthesis occurs within ten minutes after fertilization of Spisula oocytes. This change is regulated entirely at the translational level. We have used DNA clones complementary to five translationally regulated messenger RNAs to follow shifts in mRNA utilization at fertilization and to characterize alterations in mRNA structure that accompany switches in translational activity in vivo. Four of the mRNAs studied are translationally inactive in the oocyte. After fertilization two of these mRNAs are completely recruited onto polysomes, and two are partially recruited. All four of these mRNAs have very short poly(A) tracts in the oocyte; after fertilization the poly(A) tails lengthen considerably. In contrast, a fifth mRNA, that encoding alpha-tubulin mRNA, is translated very efficiently in the oocyte and is rapidly lost from polysomes after fertilization. Essentially all alpha-tubulin mRNA in the oocyte is poly(A)+ and a large portion of this mRNA undergoes complete deadenylation after fertilization. These results reveal a striking relationship between changes in adenylation and translational activity in vivo. This correlation is not perfect, however. Evidence for and against a direct role for polyadenylation in regulating these translational changes is discussed. Changes in poly(A) tails are the only alterations in mRNA sizes that we have been able to detect. This indicates that, at least for the mRNAs studied here, translational activation is not due to extensive processing of larger translationally incompetent precursors. We have also isolated several complementary DNA clones to RNAs encoded by the mitochondrial genome. Surprisingly, the poly(A) tracts of at least two of the mitochondrial RNAs also lengthen in response to fertilization.     

A novel method for constructing murine cDNA library enriched with maternal mRNAs exhibiting de novo independent post-fertilization polyadenylation

Recently, mouse maternal mRNAs such as SSEC-D, Spin, beta-catenin, Ptp4a1, and Maid have been found to exhibit de novo independent polyadenylation after fertilization. To obtain an overall picture of post-fertilization polyadenylation events, we developed a novel method for constructing murine fertilized egg cDNA library enriched with cDNAs exhibiting de novo independent polyadenylation. As a pilot study, we isolated at least four new maternal mRNAs exhibiting extension of poly(A) tail in fertilized 1-cell eggs. Moreover, various types of polyadenylation of maternal RNAs were observed at this stage, suggesting the presence of novel mechanisms for regulating the length of poly(A) tails of maternal mRNA. This is the first report of successful construction of a cDNA library enriched with newly polyadenylated maternal mRNAs derived from post-fertilized mouse eggs. This cDNA library will be useful for molecular analysis of the mechanisms underlying post-fertilization polyadenylation of mammalian maternal RNAs.     

Breaking the code of polyadenylation-induced translation

The translation of many maternal mRNAs is regulated by dynamic changes in poly(A) tail length. During maturation of Xenopus oocytes, polyadenylation is mediated by three different cis elements in the 3' untranslated region (UTR) of maternal mRNAs. In this issue, Piqué et al. (2008) explore the interplay of these elements to elucidate a combinatorial code that predicts the timing of polyadenylation and translation of maternal mRNAs.     

Poly(A) polymerase and the regulation of cytoplasmic polyadenylation

Translational activation in oocytes and embryos is often regulated via increases in poly(A) length. Cleavage and polyadenylation specificity factor (CPSF), cytoplasmic polyadenylation element binding protein (CPEB), and poly(A) polymerase (PAP) have each been implicated in cytoplasmic polyadenylation in Xenopus laevis oocytes. Cytoplasmic polyadenylation activity first appears in vertebrate oocytes during meiotic maturation. Data presented here shows that complexes containing both CPSF and CPEB are present in extracts of X. laevis oocytes prepared before or after meiotic maturation. Assessment of a variety of RNA sequences as polyadenylation substrates indicates that the sequence specificity of polyadenylation in egg extracts is comparable to that observed with highly purified mammalian CPSF and recombinant PAP. The two in vitro systems exhibit a sequence specificity that is similar, but not identical, to that observed in vivo, as assessed by injection of the same RNAs into the oocyte. These findings imply that CPSFs intrinsic RNA sequence preferences are sufficient to account for the specificity of cytoplasmic polyadenylation of some mRNAs. We discuss the hypothesis that CPSF is required for all polyadenylation reactions, but that the polyadenylation of some mRNAs may require additional factors such as CPEB. To test the consequences of PAP binding to mRNAs in vivo, PAP was tethered to a reporter mRNA in resting oocytes using MS2 coat protein. Tethered PAP catalyzed polyadenylation and stimulated translation approximately 40-fold; stimulation was exclusively cis-acting, but was independent of a CPE and AAUAAA. Both polyadenylation and translational stimulation required PAPs catalytic core, but did not require the putative CPSF interaction domain of PAP. These results demonstrate that premature recruitment of PAP can cause precocious polyadenylation and translational stimulation in the resting oocyte, and can be interpreted to suggest that the role of other factors is to deliver PAP to the mRNA.     

Poly(A) metabolism and polysomal recruitment of maternal mRNAs during early Xenopus development

Up to the midblastula stage, cell division in Xenopus is directed by maternal mRNAs and proteins that are inherited by the fertilized egg. We have isolated cDNA clones for mRNAs that undergo either adenylation or deadenylation during this developmental period. Coincidental with this poly(A) metabolism are changes in polysomal localization. The relationship between polyadenylation and translation is discussed.     

Modifications of the 5′ Cap of mRNAs during Xenopus Oocyte Maturation: Independence from Changes in Poly(A) Length and Impact on Translation

The translation of specific maternal mRNAs is regulated during early development. For some mRNAs, an increase in translational activity is correlated with cytoplasmic extension of their poly(A) tails; for others, translational inactivation is correlated with removal of their poly(A) tails. Recent results in several systems suggest that events at the 3′ end of the mRNA can affect the state of the 5′ cap structure, m⁷G(5′)ppp(5′)G. We focus here on the potential role of cap modifications on translation during early development and on the question of whether any such modifications are dependent on cytoplasmic poly(A) addition or removal. To do so, we injected synthetic RNAs into Xenopus oocytes and examined their cap structures and translational activities during meiotic maturation. We draw four main conclusions. First, the activity of a cytoplasmic guanine-7-methyltransferase increases during oocyte maturation and stimulates translation of an injected mRNA bearing a nonmethylated GpppG cap. The importance of the cap for translation in oocytes is corroborated by the sensitivity of protein synthesis to cap analogs and by the inefficient translation of mRNAs bearing nonphysiologically capped 5′ termini. Second, deadenylation during oocyte maturation does not cause decapping, in contrast to deadenylation-triggered decapping in Saccharomyces cerevisiae. Third, the poly(A) tail and the N-7 methyl group of the cap stimulate translation synergistically during oocyte maturation. Fourth, cap ribose methylation of certain mRNAs is very inefficient and is not required for their translational recruitment by poly(A). These results demonstrate that polyadenylation can cause translational recruitment independent of ribose methylation. We propose that polyadenylation enhances translation through at least two mechanisms that are distinguished by their dependence on ribose modification.     

The 36-kilodalton embryonic-type cytoplasmic polyadenylation element-binding protein in Xenopus laevis is ElrA, a member of the ELAV family of RNA-binding proteins.

The translational activation of several maternal mRNAs in Xenopus laevis is dependent on cytoplasmic poly(A) elongation. Messages harboring the UUUUUAU-type cytoplasmic polyadenylation element (CPE) in their 3' untranslated regions (UTRs) undergo polyadenylation and translation during oocyte maturation. This CPE is bound by the protein CPEB, which is essential for polyadenylation. mRNAs that have the poly(U)12-27 embryonic-type CPE (eCPE) in their 3' UTRs undergo polyadenylation and translation during the early cleavage and blastula stages. A 36-kDa eCPE-binding protein in oocytes and embryos has been identified by UV cross-linking. We now report that this 36-kDa protein is ElrA, a member of the ELAV family of RNA-binding proteins. The proteins are identical in size, antibody directed against ElrA immunoprecipitates the 36-kDa protein, and the two proteins have the same RNA binding specificity in vitro. C12 and activin receptor mRNAs, both of which contain eCPEs, are detected in immunoprecipitated ElrA-mRNP complexes from eggs and embryos. In addition, this in vivo interaction requires the eCPE. Although a number of experiments failed to define a role for ElrA in cytoplasmic polyadenylation, the expression of a dominant negative ElrA protein in embryos results in an exogastrulation phenotype. The possible functions of ElrA in gastrulation are discussed.