Isolation of novel murine maternal mRNAs regulated by cytoplasmic polyadenylation

Maid: a maternally transcribed novel gene encoding a potential negative regulator of bHLH proteins in the mouse egg and zygote

We isolated an abundant novel cDNA SSEC-8 from a subtraction cDNA library enriched for maternal transcripts that are still present in the mouse 2 cell stage embryo. This gene is evolutionarily conserved and maps to the distal region of mouse chromosome 2. The deduced polypeptide sequence of the encoded protein contains a conserved helix-loop-helix (HLH) motif without a basic DNA binding domain, suggesting that it functions as a negative regulator of basic (b) HLH transcription factors. Gel mobility shift assays show that in vitro translated protein prevents the E12/MyoD bHLH dimer from binding to DNA. Also, transient overexpression of this protein in C2C12 cells reduced the transcription of a CAT-reporter regulated by an E12/MyoD driven enhancer. The 3'-UTR contains consensus sequences of cytoplasmic polyadenylation elements (CPE's), and the length of its poly (A) tail changes during oocyte maturation, indicating that its expression is controlled by timely activation of translation. This new gene, Maid, models the translational and transcriptional regulation of gene expression during the transition from gamete to embryo.

New member of the Snf1/AMPK kinase family, Melk, is expressed in the mouse egg and preimplantation embryo

The initial phase of mammalian preimplantation development is directed by stored maternal mRNAs and their encoded proteins, yet most of the molecules controlling this process have not been described. We have used differential display analysis of cDNA libraries prepared from unfertilized eggs and preimplantation embryos to isolate three maternal cDNAs that represent novel genes exhibiting different patterns of expression during this developmental period. One of these, Melk, encodes a protein with a kinase catalytic domain and a leucine zipper motif, a new member of the Snf1/AMPK family of kinases. This gene product may play a role in the signal transduction events in the egg and early embryo.

Messenger RNA deadenylylation precedes decapping in mammalian cells

In yeast, the major mRNA degradation pathway is initiated by poly(A) tail shortening that triggers mRNA decapping. The mRNA is then degraded by 5'-to-3' exonucleolysis. In mammalian cells, even though poly(A) tail shortening also precedes mRNA degradation, the degradation pathway has not been elucidated. We have used a reverse transcription-PCR approach that relies on mRNA circularization to measure the poly(A) tail length of four mammalian mRNAs. This approach allows for the simultaneous analysis of the 5' and 3' ends of the same mRNA molecule. For all four mRNAs analyzed, this strategy permitted us to demonstrate the existence of small amounts of decapped mRNA species which have a shorter poly(A) tail than their capped counterparts. Kinetic analysis of one of these mRNAs indicates that the decapped species with a short poly(A) tail are mRNA degradation products. Therefore, our results indicate that decapping is preceded by a shortening of the poly(A) tail in mammalian cells, as it is in yeast, suggesting that this mRNA degradation pathway is conserved throughout eukaryotic evolution.

Function of untranslated regions in the mouse spermatogenesis-specific gene Tcp10 evaluated in transgenic mice

The mouse Tcp10 genes are transcribed exclusively in male germ cells and display multiple 5' and 3' untranslated variations generated by alternative splicing and polyadenylation signal usage. To investigate the possible role of untranslated sequences in the regulation of these genes, chimeric expression constructs with or without endogenous 5' and 3' untranslated sequences were generated and used to make transgenic mice. Analysis of these animals showed that the untranslated sequences have no effect on the transcription or translation of an attached lacZ reporter gene, thereby implying these sequences are dispensible. However, the endogenous pattern of polyadenylation site usage was altered when Tcp10 3' untranslated sequences were linked to lacZ, indicating that internal coding sequence can influence recognition of polyadenylation signals in testis. The characteristics of alternative splicing and polyadenylation signal variability reflects a common theme of promiscuity in testicular gene expression.

Shortening of poly (A) tail of glucose transporter--one mRNA in experimental diabetes mellitus

To determine the molecular mechanisms of diabetes-related changes in the expression of GLUT-1 in cerebral tissue, streptozotocin-induced diabetic rats and vehicle injected controls were studied after 4 weeks of diabetes. The GLUT-1 mass in cerebral microvessels was reduced in diabetic rats by approximately 38% (P < 0.01). The GLUT-1 concentration in insulin-treated diabetic group was not significantly different from controls. The GLUT-1 mRNA content of cerebral tissue in diabetic rats (0.064 +/- 0.007) was significantly reduced compared to control rats (0.122 +/- 0.011) or insulin-treated diabetic rats (0.122 +/- 0.015) P < 0.01. The in vitro translation of GLUT-1 mRNA of diabetic rats (0.793 +/- 0.047 arbitrary units) was also significantly lower than that in control rats (1.403 +/- 0.153) P < 0.01 or insulin-treated diabetic rats. (1.124 +/- 0.083) P < 0.01. These changes occurred in asssociation with a reduction in poly (A) tail length of GLUT-1 mRNA which decreased from a control value of 200-350 nt to only 50-100 nt in diabetic rats. Shortening of poly (A) tail of mRNAs is a novel mechanism of diabetes-related changes in the expression of specific genes which are regulated at a translational level.

In vivo antisense oligodeoxynucleotide mapping reveals masked regulatory elements in an mRNA dormant in mouse oocytes

In mouse oocytes, tissue-type plasminogen activator (tPA) mRNA is under translational control. The newly transcribed mRNA undergoes deadenylation and translational silencing in growing oocytes, while readenylation and translation occur during meiotic maturation. To localize regulatory elements controlling tPA mRNA expression, we identified regions of the endogenous transcript protected from hybridization with injected antisense oligodeoxynucleotides. Most of the targeted sequences in either the 5' untranslated region (5'UTR), coding region, or 3'UTR were accessible to hybridization, as revealed by inhibition of tPA synthesis and by RNase protection. Two protected regions were identified in the 3'UTR of tPA mRNA in primary oocytes: the adenylation control element (ACE) and the AAUAAA polyadenylation signal. These sequences were previously shown to be involved in the translational control of injected reporter transcripts. During the first hour of meiotic maturation, part of the ACE and the AAUAAA hexanucleotide became accessible to hybridization, suggesting a partial unmasking of the 3'UTR of this mRNA before it becomes translationally competent. Our results demonstrate that in vivo antisense oligodeoxynucleotide mapping can reveal the dynamics of regulatory features of a native mRNA in the context of the intact cell. They suggest that specific regions in the 3'UTR of tPA mRNA function as cis-acting masking determinants involved in the silencing of tPA mRNA in primary oocytes.

Oocyte selection of mutations affecting cytoplasmic polyadenylation of maternal mRNAs

Translational activation by cytoplasmic polyadenylation is a conserved mechanism in metazoan early development. In Xenopus and mouse, the regulatory sequences that control this process during oocyte meiotic maturation have been identified in the 3' untranslated region (3'-UTR) of a class of maternal messenger RNAs (mRNAs). In this report, we have investigated sequences controlling cytoplasmic polyadenylation of a mouse maternal mRNA. Pools of RNAs, transcribed from DNA randomly mutated by a PCR-based method, were micro-injected into the cytoplasm of mouse primary oocytes to allow in vivo selection of inefficiently polyadenylated transcripts. After oocyte maturation, the nonelongated RNAs were gel-isolated, and single base substitutions that alter poly(A) addition were identified. Analysis of these mutant RNAs identified single nucleotides that influence efficiency of cytoplasmic polyadenylation during mouse oocyte maturation. In addition, this strategy should facilitate identification of yet unknown sequence elements responsible for basic biological mechanisms during and after early development.

Chromatin modifications during oogenesis in the mouse: removal of somatic subtypes of histone H1 from oocyte chromatin occurs post-natally through a post-transcriptional mechanism

We examined the distribution of the somatic subtypes of histone H1 and the variant subtype, H1(0), and their encoding mRNAs during oogenesis and early embryogenesis in the mouse. As detected using immunocytochemistry, somatic H1 was present in the nuclei of oocytes of 18-day embryos. Following birth, however, somatic H1 became less abundant in both growing and non-growing oocytes, beginning as early as 4 days of age in the growing oocytes, and was scarcely detectable by 19 days. Together with previous results, this defines a period of time when somatic H1 is depleted in oocytes, namely, from shortly after birth when the oocytes are at prophase I until the 4-cell stage following fertilization. At the stages when somatic H1 was undetectable, oocyte nuclei could be stained using an antibody raised against histone H1(0), which suggests that this may be a major linker histone in these cells. In contrast to the post-natal loss of somatic H1 protein, mRNAs encoding four (H1a, H1b, H1d, H1e) of the five somatic subtypes were present, as detected using RT-PCR in growing oocytes of 9-day pups, and all five subtypes including H1c were present in fully grown oocytes of adults. All five subtypes were also present in embryos, both before and after activation of the embryonic genome. mRNA encoding H1(0) was also detected in oocytes and early embryos. Whole-mount in situ hybridization using cloned H1c and H1e cDNAs revealed that the mRNAs were present in the cytoplasm of oocytes and 1-cell embryos, in contrast to the sea urchin early embryo where they are sequestered in the cell nucleus. We suggest that, as in many somatic cell types, the chromatin of mouse oocytes becomes depleted of somatic H1 and relatively enriched in histone H1(0) postnatally, and that somatic H1 is reassembled onto chromatin in cleavage-stage embryos. The post-natal loss of somatic H1 appears to be regulated post-transcriptionally by a mechanism not involving nuclear localization.

Spindlin, a major maternal transcript expressed in the mouse during the transition from oocyte to embryo

Timely translation of maternal transcripts and post-translational modification of their gene products control the initial development of preimplantation-stage embryos. We have isolated and characterized a gene encoding a stage-specific embryonic protein. This novel gene, spindlin (Spin), is an abundant maternal transcript present in the unfertilized egg and 2-cell, but not 8-cell, stage embryo. Spin exhibits high homology to a multicopy gene, Y-linked spermiogenesis-specific transcript (Ssty), and together they form a new gene family expressed during gametogenesis. We find that spindlin associates with the meiotic spindle and is modified by phosphorylation in a cell-cycle-dependent fashion. Furthermore, it comigrates with the previously described 30x10(3) Mr metaphase complex which is posttranslationally modified during the first mitotic cell cycle. Our data suggest that spindlin plays a role in cell-cycle regulation during the transition from gamete to embryo.

Mouse cytoplasmic polyadenylylation element binding protein: an evolutionarily conserved protein that interacts with the cytoplasmic polyadenylylation elements of c-mos mRNA

Cytoplasmic polyadenylylation is an essential process that controls the translation of maternal mRNAs during early development and depends on two cis elements in the 3' untranslated region: the polyadenylylation hexanucleotide AAUAAA and a U-rich cytoplasmic polyadenylylation element (CPE). In searching for factors that could mediate cytoplasmic polyadenylylation of mouse c-mos mRNA, which encodes a serine/threonine kinase necessary for oocyte maturation, we have isolated the mouse homolog of CPEB, a protein that binds to the CPEs of a number of mRNAs in Xenopus oocytes and is required for their polyadenylylation. Mouse CPEB (mCPEB) is a 62-kDa protein that binds to the CPEs of c-mos mRNA. mCPEB mRNA is present in the ovary, testis, and kidney; within the ovary, this RNA is restricted to oocytes. mCPEB shows 80% overall identity with its Xenopus counterpart, with a higher homology in the carboxyl-terminal portion, which contains two RNA recognition motifs and a cysteine/histidine repeat. Proteins from arthropods and nematodes are also similar to this region, suggesting an ancient and widely used mechanism to control polyadenylylation and translation.

Masking and unmasking maternal mRNA. The role of polyadenylation, transcription, splicing, and nuclear history

We establish that masked mRNAs synthesized from exogenous plasmid templates microinjected into the nuclei of Xenopus oocytes are translationally activated (unmasked) on oocyte maturation concomitant with polyadenylation. Synthetic mRNA injected into the cytoplasm of the oocyte is translated over an order of magnitude more efficiently than is the cognate mRNA synthesized in vivo. Both mRNA synthesized in vivo and mRNA microinjected into the oocyte cytoplasm require a cytoplasmic polyadenylation element in the 3'-untranslated region to activate translation on maturation. Although polyadenylation upon oocyte maturation can relieve the translational repression of mRNA synthesized in vivo, the excision of an intron within the nucleus does not relieve repression. We suggest that the translational repression coupled to the transcription process will more effectively repress inappropriate gene expression in the oocyte and offer the potential to achieve a wider range of gene regulation.

Temporal patterns of gene expression of G1-S cyclins and cdks during the first and second mitotic cell cycles in mouse embryos

Cell-cycle progression in somatic cells is regulated by a family of cyclins and cyclindependent kinases (cdks) that form specific complexes as a function of cell-cycle progression. However, the transcript abundance of G1-S cyclins and cdks during the meiotic and mitotic cell cycles of mammalian embryos has not been previously reported. Using a reverse transcription-polymerase chain reaction (PCR) assay that detects changes in either mRNA abundance or polyadenylation state, we examined the relative levels of gene expression for the G1-S cyclins and cdks, as well as for p21, p27, and the retinoblastoma (Rb) gene in mouse oocytes, metaphase II-arrested eggs, and 1-2-cell embryos. The PCR products for cyclins D1, D3, and A, as well as cdk4, p21, and Rb, displayed similar levels in meiotically incompetent and competent oocytes, as well as in metaphase II-arrested eggs. The levels of PCR products for cyclin D2, p27, and two forms of cdk2 were similar in meiotically incompetent and competent oocytes but decreased during oocyte maturation. Finally, the level of PCR products for cyclin E and cdk2 gradually decreased during the progression from meiotically incompetent oocytes to metaphase II-arrested eggs. When the levels of PCR products for the G1-S regulatory genes were evaluated during the first and second mitotic cell cycles, four main patterns were found: 1) steady levels for cyclin A; 2) steady levels followed by a 2-3-fold increase during the G2 phase of the second mitotic cell cycle for cyclins D1, E, cdk2, and p21; 3) a transient increase during the S and/or G2 phases of the first mitotic cell cycle for p27, cyclin D3, and the two forms of cdk2; and 4) higher levels during the first cell cycle and then a decrease with lower levels during the second mitotic cell cycle for cyclin D2 and Rb. cdk4 expression displayed a combination of patterns 2 and 3. The increase in the amount of PCR product for the cdk4 gene during the first mitotic cell cycle was due to polyadenylation, whereas the increase in the amount of PCR product for cdk4, cdk2, and cyclins D1 and E in the second mitotic cell cycle was a product of activation of the embryonic genome.

Mechanisms of translational control in early development

Oocytes accumulate a dowry of maternal mRNAs in preparation for embryogenesis. These maternal transcripts are kept dormant until late oogenesis or early embryogenesis when their translation is activated. In recent years, three types of translational control acting on maternal mRNAs have emerged: translational activation by cytoplasmic polyadenylation, translational activation by RNA localization, and regulated translational repression. In each case, translational control depends on the binding of trans-acting factors to sequences in the 3' untranslated region (3'UTR). Identification of these trans-acting factors is beginning to shed light on the molecular mechanisms that mediate translational control.

Evolutionary conservation of sequence elements controlling cytoplasmic polyadenylylation

Cytoplasmic polyadenylylation is an evolutionarily conserved mechanism involved in the translational activation of a set of maternal messenger RNAs (mRNAs) during early development. In this report, we show by interspecies injections that Xenopus and mouse use the same regulatory sequences to control cytoplasmic poly(A) addition during meiotic maturation. Similarly, Xenopus and Drosophila embryos exploit functionally conserved signals to regulate polyadenylylation during early post-fertilization development. These experiments demonstrate that the sequence elements that govern cytoplasmic polyadenylylation, and hence one form of translational activation, function across species. We infer that the requisite regulatory sequence elements, and likely the trans-acting components with which they interact, have been conserved since the divergence of vertebrates and arthropods.

Transcription of c-mos protooncogene in the pig involves both tissue-specific promoters and alternative polyadenylation sites

The function of the c-mos gene has been intensively studied, but its role in the mammal is still a subject for debate. For this reason, and because the gene is regulated posttranscriptionally, further study of the gene from other mammalian species is timely. The pig c-mos gene has been cloned, and the genomic sequence is presented here. The gene has no introns and shows close similarity to human and monkey genes, with striking sequence similarities in both the 5' and 3' flanking regions. The significance of this similarity in the context of gene regulation is discussed. c-mos expression was found to be restricted to gonadal tissues in the pig. The major start sites for transcription initiation in ovary and testis were identified by primer extension and found to be distinct, as in the mouse. Within the ovary, expression is confined to oocytes. Messenger RNA is synthesized in growing oocytes, and remains stable during oocyte maturation, but begins to be degraded in electrically stimulated eggs. Unexpectedly, RNase protection assays revealed that the 3' ends of transcripts in the pig ovary are heterogeneous, and this, together with the identification of three distinct cDNA clones, shows that multiple polyadenylation sites are used. The significance of these transcripts in terms of translational control is discussed.

G protein gene expression during mouse oocyte growth and maturation, and preimplantation embryo development

Fertilization in mammals initiates "egg activation," a series of events leading to embryo development. The signal transduction events that occur as a result of sperm-egg interactions and that initiate egg activation may be analogous to a ligand-receptor-effector pathway, but the details of this signaling pathway are poorly understood. Several lines of evidence support a role for guanine nucleotide-binding regulatory proteins (G proteins) in mammalian egg activation. Prior to initiating studies to examine further the role of specific G proteins in sperm-induced mouse egg activation, we needed to define the complement of G proteins expressed in the egg. Using a reverse transcription-polymerase chain reaction (RT-PCR) assay, the relative levels of mRNAs encoding specific G protein alpha, beta, and gamma subunits were determined in meiotically incompetent oocytes, fully-grown competent oocytes, metaphase II-arrested eggs, one-, two-, and eight-cell embryos, and blastocysts. mRNA transcripts representing all of the heterotrimeric G protein families were present at all of the stages examined, and all underwent significant changes in their patterns of expression. The following heterotrimeric G protein mRNA transcripts were present in oocytes, eggs, or preimplantation embryos: G alpha q family (q, 11, and 14), G alpha 12 family (12 and 13), G alpha i family (i1, i2, i3, t2, z, and s), beta subunits 1, 2, 4, and 5, and gamma subunits 2, 3, 5, and 7. A recently described large molecular weight G protein, G alpha h (Nakaoka et al., 1994: Science 264:1593-1596), was also present, G alpha 15, G alpha t1, G alpha olf, G alpha oA, G beta 3, G gamma 1, and G gamma 8 mRNA transcripts were not detected using this method. The most common pattern of expression observed was a maturation-associated decrease followed by an increase after the two-cell stage. Some transcripts, however, were expressed at low levels until the eight-cell to blastocyst stages, whereas others were expressed at high levels in the oocyte but following maturation declined and remained at a low level throughout preimplantation development.

Mutations that perturb poly(A)-dependent maternal mRNA activation block the initiation of development

Translational recruitment of maternal mRNAs is an essential process in early metazoan development. To identify genes required for this regulatory pathway, we have examined a collection of Drosophila female-sterile mutants for defects in translation of maternal mRNAs. This strategy has revealed that maternal-effect mutations in the cortex and grauzone genes impair translational activation and cytoplasmic polyadenylation of bicoid and Toll mRNAs. Cortex embryos contain a bicoid mRNA indistinguishable in amount, localization, and structure from that in wild-type embryos. However, the bicoid mRNA in cortex embryos contains a shorter than normal polyadenosine (poly(A)) tail. Injection of polyadenylated bicoid mRNA into cortex embryos allows translation demonstrating that insufficient polyadenylation prevents endogenous bicoid mRNA translation. In contrast nanos mRNA, which is activated by a poly(A)-independent mechanism, is translated in cortex embryos, indicating that the block in maternal mRNA activation is specific to a class of mRNAs. Cortex embryos are fertilized, but arrest at the onset of embryogenesis. Characterization of grauzone mutations indicates that the phenotype of these embryos is similar to cortex. These results identify a fundamental pathway that serves a vital role in the initiation of development.

Analysis of G protein alpha subunit mRNA abundance in preimplantation mouse embryos using a rapid, quantitative RT-PCR approach

We have developed a novel reverse transcription-polymerase chain reaction (RT-PCR)-based approach for systematically quantifying in a single experiment the abundances of many different mRNAs in preimplantation mouse embryos. With this approach, the entire mRNA population from a small number of embryos is amplified while preserving the relative abundance of each mRNA in the cDNA population. The cDNA is analyzed by quantitative hybridization to radiolabeled probes. The approach is very sensitive and provides reliable, quantitative data regarding changes in mRNA abundance. A major advantage of this method is that estimates of mRNA copy number can be obtained and compared between different mRNAs. With this approach, we analyzed the patterns of expression of nine G protein alpha subunit mRNAs (G alpha s, G alpha i, G alpha q, G alpha o, and G alpha 11-15) in oocytes, eggs, and preimplantation embryos from fertilization to the blastocyst stage. Six alpha subunit mRNAs were expressed at significant levels, all of which underwent significant temporal alterations in expression. The mRNAs encoding some alpha subunit types were expressed predominantly in the egg and 1-cell embryo, underwent sharp reductions during the 2-cell stage, and were re-expressed between the 8-cell and blastocyst stages. One alpha subunit mRNA increased in abundance at the early blastocyst stage. The possible significance of these alterations in G protein mRNA abundance to embryonic development is discussed.

The embryonic RNA helicase gene (ERH): a new member of the DEAD box family of RNA helicases

DEAD box proteins share several highly conserved motifs including the characteristic Asp-Glu-Ala-Asp (D-E-A-D in the amino acid single-letter code) motif and have established or putative ATP-dependent RNA helicase activity. These proteins are implicated in a range of cellular processes that involve regulation of RNA function, including translation initiation, RNA splicing and ribosome assembly. Here we describe the isolation and characterization of an embryonic RNA helicase gene, ERH, which maps to mouse chromosome 1 and encodes a new member of the DEAD box family of proteins. The predicted ERH protein shows high sequence similarity to the testes-specific mouse PL10 and to the maternally acting Xenopus An3 helicase proteins. The ERH expression profile is similar, to that of An3, which localizes to the animal hemisphere of oocytes and is abundantly expressed in the embryo. ERH is expressed in oocytes and is a ubiquitous mRNA in the 9 days-post-conception embryo, and at later stages of development shows a more restricted pattern of expression in brain and kidney. The similarities in sequence and in expression profile suggest that ERH is the murine equivalent of the Xenopus An3 gene, and we propose that ERH plays a role in translational activation of mRNA in the oocyte and early embryo.

Temporal regulation of the Xenopus FGF receptor in development: a translation inhibitory element in the 3′ untranslated region

Early frog embryogenesis depends on a maternal pool of mRNA to execute critical intercellular signalling events. FGF receptor-1, which is required for normal development, is stored as a stable, untranslated maternal mRNA transcript in the fully grown immature oocyte, but is translationally activated at meiotic maturation. We have identified a short cis-acting element in the FGF receptor 3′ untranslated region that inhibits translation of synthetic mRNA. This inhibitory element is sufficient to inhibit translation of heterologous reporter mRNA in the immature oocyte without changing RNA stability. Deletion of the poly(A) tract or polyadenylation signal sequences does not affect translational inhibition by this element. At meiotic maturation, we observe the reversal of translational repression mediated by the inhibitory element, mimicking that seen with endogenous maternal FGF receptor mRNA at meiosis. In addition, the activation of synthetic transcripts at maturation does not appear to require poly(A) lengthening. We also show that an oocyte cytoplasmic protein specifically binds the 3′ inhibitory element, suggesting that translational repression of Xenopus FGF receptor-1 maternal mRNA in the oocytes is mediated by RNA-protein interactions. These data describe a mechanism of translational control that appears to be independent of poly(A) changes.

Cloning and characterization of a Xenopus poly(A) polymerase

During oocyte maturation and early embryogenesis in Xenopus laevis, the translation of several mRNAs is regulated by cytoplasmic poly(A) elongation, a reaction catalyzed by poly(A) polymerase (PAP). We have cloned, sequenced, and examined several biochemical properties of a Xenopus PAP. This protein is 87% identical to the amino-terminal portion of bovine PAP, which catalyzes the nuclear polyadenylation reaction, but lacks a large region of the corresponding carboxy terminus, which contains the nuclear localization signal. When injected into oocytes, the Xenopus PAP remains concentrated in the cytoplasm, suggesting that it is a specifically cytoplasmic enzyme. Oocytes contain several PAP mRNA-related transcripts, and the levels of at least the one encoding the putative cytoplasmic enzyme are relatively constant in oocytes and early embryos but decline after blastulation. When expressed in bacteria and purified by affinity and MonoQ-Sepharose chromatography, the protein has enzymatic activity and adds poly(A) to a model substrate. Importantly, affinity-purified antibodies directed against Xenopus PAP inhibit cytoplasmic polyadenylation in egg extracts. These data suggest that the PAP described here could participate in cytoplasmic polyadenylation during Xenopus oocyte maturation.

Inhibition of zona pellucida gene expression by antisense oligonucleotides injected into mouse oocytes

During murine oogenesis, the zona pellucida proteins (ZP1, ZP2, and ZP3) are synthesized and secreted to form an extracellular matrix that surrounds the oocyte and mediates specific biological functions essential to mammalian fertilization and early development. To investigate the relationship among the zona proteins during zona matrix assembly, we have undertaken to inhibit de novo biosynthesis of specific zona proteins with antisense oligonucleotides complementary to the 5'-ends of ZP2 (nucleotide position 19-42) and ZP3 (nucleotide 21-44) mRNAs. When injected into the cytoplasm of growing mouse oocytes, the antisense oligonucleotides targeted specific zona mRNAs for degradation, as confirmed by a RNase protection assay. Individual zona pellucida protein synthesis was followed by immunoprecipitation with ZP2- and ZP3-specific monoclonal antibodies. New zona protein synthesis from the targeted mRNA was abolished, but nontargeted zona protein continued to be synthesized. Interestingly, abolishment of either ZP2 or ZP3 protein synthesis prevented the incorporation of the other protein into the extracellular zona matrix. These results suggest that ZP2 and ZP3 proteins are independent of each other in their biosynthesis but are dependent upon each other for their incorporation into the zona pellucida matrix. This study provides an experimental system in which destruction of a targeted mRNA generates a transient loss-of-expression phenotype during mouse oocyte growth.

Coordinate initiation of Drosophila development by regulated polyadenylation of maternal messenger RNAs

Pattern formation in Drosophila depends initially on the translational activation of maternal messenger RNAs (mRNAs) whose protein products determine cell fate. Three mRNAs that dictate anterior, dorsoventral, and terminal specification--bicoid, Toll, and torso, respectively--showed increases in polyadenylate [poly(A)] tail length concomitant with translation. In contrast, posteriorly localized nanos mRNA, although also translationally activated, was not regulated by poly(A) status. These results implicate at least two mechanisms of mRNA activation in flies. Studies with bicoid mRNA showed that cytoplasmic polyadenylation is necessary for translation, establishing this pathway as essential for embryogenesis. Combined, these experiments identify a regulatory pathway that can coordinate initiation of maternal pattern formation systems in Drosophila.

Further analysis of cytoplasmic polyadenylation in Xenopus embryos and identification of embryonic cytoplasmic polyadenylation element-binding proteins

Early development in Xenopus laevis is programmed in part by maternally inherited mRNAs that are synthesized and stored in the growing oocyte. During oocyte maturation, several of these messages are translationally activated by poly(A) elongation, which in turn is regulated by two cis elements in the 3' untranslated region, the hexanucleotide AAUAAA and a cytoplasmic polyadenylation element (CPE) consisting of UUUUUAU or similar sequence. In the early embryo, a different set of maternal mRNAs is translationally activated. We have shown previously that one of these, C12, requires a CPE consisting of at least 12 uridine residues, in addition to the hexanucleotide, for its cytoplasmic polyadenylation and subsequent translation (R. Simon, J.-P. Tassan, and J.D. Richter, Genes Dev. 6:2580-2591, 1992). To assess whether this embryonic CPE functions in other maternal mRNAs, we have chosen Cl1 RNA, which is known to be polyadenylated during early embryogenesis (J. Paris, B. Osborne, A. Couturier, R. LeGuellec, and M. Philippe, Gene 72:169-176, 1988). Wild-type as well as mutated versions of Cl1 RNA were injected into fertilized eggs and were analyzed for cytoplasmic polyadenylation at times up to the gastrula stage. This RNA also required a poly(U) CPE for cytoplasmic polyadenylation in embryos, but in this case the CPE consisted of 18 uridine residues. In addition, the timing and extent of cytoplasmic poly(A) elongation during early embryogenesis were dependent upon the distance between the CPE and the hexanucleotide. Further, as was the case with Cl2 RNA, Cl1 RNA contains a large masking element that prevents premature cytoplasmic polyadenylation during oocyte maturation. To examine the factors that may be involved in the cytoplasmic polyadenylation of both C12 and C11 RNAs, we performed UV cross-linking experiments in egg extracts. Two proteins with sizes of ~36 and ~45 kDa interacted specifically with the CPEs of both RNAs, although they bound preferentially to the C12 CPE. The role that these proteins might play in cytoplasmic polyadenylation is discussed.

Further analysis of cytoplasmic polyadenylation in Xenopus embryos and identification of embryonic cytoplasmic polyadenylation element-binding proteins

Early development in Xenopus laevis is programmed in part by maternally inherited mRNAs that are synthesized and stored in the growing oocyte. During oocyte maturation, several of these messages are translationally activated by poly(A) elongation, which in turn is regulated by two cis elements in the 3' untranslated region, the hexanucleotide AAUAAA and a cytoplasmic polyadenylation element (CPE) consisting of UUUUUAU or similar sequence. In the early embryo, a different set of maternal mRNAs is translationally activated. We have shown previously that one of these, C12, requires a CPE consisting of at least 12 uridine residues, in addition to the hexanucleotide, for its cytoplasmic polyadenylation and subsequent translation (R. Simon, J.-P. Tassan, and J.D. Richter, Genes Dev. 6:2580-2591, 1992). To assess whether this embryonic CPE functions in other maternal mRNAs, we have chosen Cl1 RNA, which is known to be polyadenylated during early embryogenesis (J. Paris, B. Osborne, A. Couturier, R. LeGuellec, and M. Philippe, Gene 72:169-176, 1988). Wild-type as well as mutated versions of Cl1 RNA were injected into fertilized eggs and were analyzed for cytoplasmic polyadenylation at times up to the gastrula stage. This RNA also required a poly(U) CPE for cytoplasmic polyadenylation in embryos, but in this case the CPE consisted of 18 uridine residues. In addition, the timing and extent of cytoplasmic poly(A) elongation during early embryogenesis were dependent upon the distance between the CPE and the hexanucleotide. Further, as was the case with Cl2 RNA, Cl1 RNA contains a large masking element that prevents premature cytoplasmic polyadenylation during oocyte maturation. To examine the factors that may be involved in the cytoplasmic polyadenylation of both C12 and C11 RNAs, we performed UV cross-linking experiments in egg extracts. Two proteins with sizes of ~36 and ~45 kDa interacted specifically with the CPEs of both RNAs, although they bound preferentially to the C12 CPE. The role that these proteins might play in cytoplasmic polyadenylation is discussed.

S. pombe mei2+ encodes an RNA-binding protein essential for premeiotic DNA synthesis and meiosis I, which cooperates with a novel RNA species meiRNA

The molecular controls over meiosis are poorly understood compared with those over mitosis. Here, we show that S. pombe mei2, which is essential for the initiation of premeiotic DNA synthesis, encodes an RNA-binding protein. A temperature-sensitive mei2 mutant performs premeiotic DNA synthesis but does not undergo meiotic divisions, suggesting that Mei2 is required also for meiosis I. A novel, polyadenylated RNA species (meiRNA), which suppresses this temperature-sensitive defect if overexpressed, specifically binds to Mei2 both in vivo and in vitro. Cells without meiRNA perform premeiotic DNA synthesis but cannot undergo meiosis I. Mutations that apparently block the RNA binding ability of Mei2 inhibit premeiotic DNA synthesis. Mei2 is thus likely to couple with another RNA species to promote premeiotic DNA synthesis.

Expression patterns of novel genes during mouse preimplantation embryogenesis

Little is known about the repertoire of genes expressed following zygotic gene activation, which occurs during the two-cell stage in the mouse. As an initial attempt to isolate novel genes, we used previously prepared two-cell and two-cell subtraction cDNA libraries (Rothstein et al., Genes Dev 6:1190-1201, 1992) to isolate a panel of seven cDNA clones. Three cDNAs had no match in the current DNA sequence data banks and three others revealed sequence homology to portions of sequences in the data banks. One cDNA was 90% homologous to the ras-related gene Krev/rap 1A. The temporal patterns of expression of these genes during oocyte maturation and preimplantation development were analyzed by a reverse transcription-polymerase chain reaction (RT-PCR) assay developed to measure relative levels of mRNAs. Three distinct temporal patterns of expression, designated Classes 1-3, were found. The two Class 1 genes displayed an actin-like pattern, with a gradual decline in expression during oocyte maturation and through the two-cell stage, followed by increases at the eight-cell and/or blastocyst stages. The four genes in Class 2 were expressed at relatively high levels during oocyte maturation and through the one-cell stage and then declined abruptly between the one- and two-cell stages; an increase then occurred at the eight-cell and/or blastocyst stages. The expression of the gene in Class 3 declined during oocyte maturation, but then showed a transient increase at the one-cell stage, with only a very slight increase in synthesis at either the eight-cell or blastocyst stage.

Poly(A) and translation: development control

The stage-specific translational control of maternal mRNAs is determined by their differential polyadenylation and deadenylation. In the past year, a growing number of cis-acting elements that both positively and negatively regulate polyadenylation and deadenylation have been delineated. Considerable progress has been made on the biochemical characterization and regulation of trans-acting polyadenylation and deadenylation factors. This review summarizes these advances and their relevance to the roles of polyadenylation and deadenylation in translational control.

Poly(A) tail length of a heat shock protein RNA is increased by severe heat stress, but intron splicing is unaffected

The small heat shock proteins (sHSPs) are induced in all eukaryotes in response to high temperature stress, but are most abundant among members of the plant kingdom where they accumulate in multiple subcellular compartments. We have analyzed the expression of the chloroplast-localized sHSP from Arabidopsis thaliana, HSP21, and characterized the structure of the gene encoding this protein to facilitate future genetic studies on the function of HSP21 in the heat shock response. HSP21 is encoded in Arabidopsis by a single gene whose coding region is interrupted by a single intron. Previous studies have shown that intron processing is disrupted by severe, abrupt heat stress but is protected by pretreatments that induce thermotolerance. The processing of the HSP21 transcript was investigated in response to an abrupt heat stress regime and a gradual heat stress regime, the latter of which is known to confer thermotolerance in plants. Under abrupt stress conditions the HSP21 transcript is somewhat longer than under gradual heat stress conditions. However, the molecular basis for the size difference is not impaired intron splicing, but rather a difference in the length of the poly(A) tail depending on the heat stress regime. The results suggest that an increase in poly(A) tail length may be a generalized response to severe, abrupt heat stress and that poly(A) tail metabolism may be one of numerous cellular processes normally protected in thermotolerant cells from the otherwise damaging effects of high temperature stress.

Site-specific RNA cleavage generates the 3' end of a poxvirus late mRNA

The cowpox virus late mRNAs encoding the major protein of the A-type inclusions have 3' ends corresponding to a single site in the DNA template. The DNA sequence of the Alu I-Xba I fragment at this position encodes an RNA cis-acting signal, designated the AX element, which directs this RNA 3' end formation. In cells infected with vaccinia virus the AX element functions independently of either the nature of the promoter element or the RNA polymerase responsible for generating the primary RNA. At late times during virus replication, vaccinia virus induces or activates a site-specific endoribonuclease that cleaves primary RNAs within the AX element. The 3' end produced by RNA cleavage is then polyadenylylated to form the 3' end of the mature mRNA. Therefore, the poxviruses employ at least two mechanisms of RNA 3' end formation during the viral replication cycle. One mechanism, which is operative at early times in viral replication, involves the termination of transcription [Rohrmann, G., Yuen, L. & Moss, B. (1986) Cell 46, 1029-1035]. A second mechanism, which is operative at late times during viral replication, involves the site-specific cleavage of primary RNAs.