Poly(A) elongation during Xenopus oocyte maturation is required for translational recruitment and is mediated by a short sequence element

Xenopus oocytes contain several mRNAs that are mobilized into polysomes only at the completion of meiosis (maturation) or at specific times following fertilization. To investigate the mechanisms that control translation during early development, we have focused on an mRNA, termed G10, that is recruited for translation during oocyte maturation. Coincident with its translation, the poly(A) tail of this message is elongated from approximately 90 to 200 adenylate residues. To identify the cis sequence that is required for this cytoplasmic adenylation and recruitment, we have synthesized wild-type and deletion mutant G10 mRNAs with SP6 polymerase. When injected into oocytes that subsequently were induced to mature with progesterone, wild-type G10 mRNA, but not mutant transcripts lacking a 50-base sequence in the 3'-untranslated region, was polyadenylated and recruited for translation. The 50-base sequence was sufficient to confer polyadenylation and translation when fused to globin mRNA, which does not normally undergo these processes during oocyte maturation. Further mutational analysis of this region revealed that a U-rich sequence 5' to the AAUAAA hexanucleotide nuclear polyadenylation signal, as well as the hexanucleotide itself, were both required for polyadenylation and translation. The 50-base cis element directs polyadenylation, but not translation per se, as a transcript that terminates with 3'-deoxyadenosine (cordycepin) is not recruited for translation. The available data suggest that the dynamic process of polyadenylation, and not the length of the poly(A) tail, is required for translational recruitment during oocyte maturation.

Expression of a histone H1-like protein is restricted to early Xenopus development

Genes whose expression is restricted to oogenesis and early development may have important functions in these processes. Northern analysis showed that Xenopus B4 mRNA is expressed in oogenesis and embryogenesis through to the neurula stage. Immunocytochemistry with anti-B4 antibodies showed that B4 protein is only detectable in preneurula stages; it is localized to nuclei and is associated with metaphase chromosomes. Immunoblotting revealed approximately constant levels of B4 protein per embryo for the first 2 days of development. Thus, as the number of nuclei increases during early development, the amount of B4 protein per nucleus is diluted out. Sequencing of two B4 cDNA clones revealed that the predicted B4 translation product is a 29-kD protein with 29% identity with histone H1, distributed over the entire length of its sequence. The B4 protein also has certain other H1 protein characteristics--a tripartite structure consisting of a mainly hydrophobic central domain flanked by an amino-terminal segment and a long hydrophilic carboxyterminal tail containing a tandemly repeated amino acid motif. However, in contrast to histone H1 mRNA, B4 mRNA has a classic polyadenylation signal, is polyadenylated, and lacks the histone H1 3' noncoding consensus sequence involved in RNA processing.

Multiple ubiquitin mRNAs during Xenopus laevis development contain tandem repeats of the 76 amino acid coding sequence

A cDNA clone, pXlgC20, was isolated from a library constructed from poly(A)+ RNA from stage 10 X. laevis gastrulae. This sequence hybridizes with up to nine different RNA species ranging in size from 1600 to 3500 nucleotides, regularly spaced at intervals of about 230 nucleotides. Clone pXlgC20 contains two complete repeats of a 228 bp sequence as well as part of a third repeat, all adjacent and in the same orientation. One possible translational reading frame in pXlgC20 completely spans the repeat sequences, coding for a protein composed of tandem 76 amino acid units. The amino acid sequence of each unit completely matches that of human ubiquitin. Ubiquitin is translated in the form of a multimeric precursor molecule containing several units. We show that genomic DNA fragments exist that contain at least 12 of these units in tandem and propose that the different mRNA size classes vary in their number of ubiquitin coding sequences.

The maternal histone H1 variant, H1M (B4 protein), is the predominant H1 histone in Xenopus pregastrula embryos

The Xenopus laevis H1M protein (formerly called B4) is a maternally inherited histone H1 subtype restricted in its expression to early development. Levels of H1M, as well as of the somatic histones H1A, H1B, and H1C, were determined during early embryogenesis using subtype-specific anti-peptide antisera. H1M accumulates late in oogenesis to a titer of approximately 1 ng/unfertilized egg. Following fertilization, H1M persists at slowly decreasing titers for 3 days of development. In contrast, somatic H1 histones are virtually absent from eggs and cleavage-stage embryos (< 80 pg/egg for H1A, < 2 pg/egg for H1B and H1C). H1M thus represents the predominant histone H1 variant in embryos until the beginning of gastrulation, when the amount of newly synthesized H1A increases beyond the 1 ng level. By in situ immunofluorescence, H1M is detected in association with egg and embryonic chromosomes. When expressed at high levels by transient transfection in a Xenopus cell line, H1M competes for chromatin binding with resident H1A. High-level expression of either H1M or H1A causes aberrant chromatin condensation in the transfected cells.

Functions of maternal mRNA in early development

In this review, the types of mRNAs found in oocytes and eggs of several animal species, particularly Drosophila, marine invertebrates, frogs, and mice, are described. The roles that proteins derived from these mRNAs play in early development are discussed, and connections between maternally inherited information and embryonic pattern are sought. Comparisons between genetically identified maternally expressed genes in Drosophila and maternal mRNAs biochemically characterized in other species are made when possible. Regulation of the meiotic and early embryonic cell cycles is reviewed, and translational control of maternal mRNA following maturation and/or fertilization is discussed with regard to specific mRNAs.

Destruction of a translationally controlled mRNA in Xenopus oocytes delays progesterone-induced maturation

The maternal mRNA D7 is a moderately abundant transcript in Xenopus laevis whose expression is highest in, and perhaps restricted to, oogenesis and early embryogenesis. The nucleotide sequence of cloned D7 cDNA was determined and shown to have the capacity to code for a 31-kD protein. This amino acid sequence was searched against a protein data base, and no homologous proteins were found. Antibodies directed against D7 recognize in Xenopus embryos a soluble, cytoplasmic protein with an apparent molecular weight on SDS gels of 36,000. The D7 protein is absent from oocytes and first begins to accumulate during oocyte maturation. Its levels are highest during the first day of embryonic development and then decrease; D7 protein was not detected in adult tissues. D7 mRNA was selectively destroyed by injection into oocytes of antisense oligodeoxynucleotides. Analysis of injected oocytes by Northern and Western blotting showed site-specific cleavage and subsequent degradation of the D7 mRNA and the failure of the D7 protein to accumulate during progesterone-induced maturation. The loss of D7 protein affects the maturation process itself, significantly delaying the time course of germinal vesicle breakdown. Thus, D7 is a newly described protein involved in oocyte maturation.

Enolase isoenzymes in adult and developing Xenopus laevis and characterization of a cloned enolase sequence

As part of a study of glycolysis during early development we have examined the pattern of expression of enolase isoenzymes in Xenopus laevis. In addition, the nucleotide sequence of a cDNA clone coding for the complete amino acid sequence of one enolase gene (ENO1) in X. laevis was determined. X. laevis ENO1 shows highest homology to mammalian non-neuronal enolase. Analysis of enolase isoenzymes in X. laevis by non-denaturing electrophoresis on cellulose acetate strips revealed five isoenzymes. One form was present in all tissues tested, two additional forms were expressed in oocytes, embryos, adult liver and adult brain, and two further forms were restricted to larval and adult muscle. Since enolase is a dimer, three different monomers (gene products) could account for the observed number of isoenzymes. This pattern of enolase isoenzyme expression in X. laevis differs from that of birds and mammals. In birds and mammals the most acidic form is neuron-specific and there is only one major isoenzyme expressed in the liver. RNAase protection experiments showed the presence of ENO1 mRNA in oocytes, liver and muscle, suggesting that it codes for a non-tissue-restricted isoenzyme. ENO1 mRNA concentrations are high in early oocytes, decrease during oogenesis and decrease further after fertilization. Enolase protein, however, is maintained at high concentrations throughout this period.

Metabolic regulation during early frog development: glycogenic flux in Xenopus oocytes, eggs, and embryos

32P-labeled glucose 6-phosphate and phosphoenolpyruvate were injected into oocytes, fertilized eggs, and early embryos of Xenopus laevis, and the 32P label was followed into glycolytic enzymes and acid-soluble metabolites. The kinetics of labeling of phosphoglucomutase and phosphoglyceromutase and the formation of specific metabolites were used to measure carbon flux through glycolytic intermediates in these cells. In full-grown stage VI oocytes, fertilized eggs, and cells of cleaving embryos, carbon metabolism is in the glycogenic direction. Glycolytic intermediates injected into these cells were metabolized into UDP-glucose and then presumably into glycogen. Carbon flow between phosphoenolpyruvate and glucose 6-phosphate does not utilize fructose 1,6-bisphosphatase; rather, it may depend largely on enzymes of the pentose phosphate pathway. Maturation and fertilization of the oocyte did not result in a change in the qualitative pattern of metabolites formed. Pyruvate kinase, although abundant in oocytes and embryos, is essentially inactive in these cells. Pyruvate kinase also appears to be inactive in small previtellogenic stage II oocytes; however, in these cells injected glycolytic intermediates were not metabolized to UDP-glucose.

Nonspecific effects of oligodeoxynucleotide injection in Xenopus oocytes: a reevaluation of previous D7 mRNA ablation experiments

Microinjection of oligodeoxynucleotides (ODNs) complementary to cellular mRNAs has been advanced as an experimental approach to degrade target mRNAs in vivo and thereby obtain information as to the function of their cognate proteins. It is shown here that ODNs can induce a variety of aberrations in cell metabolism and structure when injected into Xenopus oocytes. Examination of histological sections of ODN-injected oocytes revealed the frequent abnormal accumulation of heavily staining basophilic material in the area of the germinal vesicle (gv). Ultrastructural analysis detected further abnormalities including blebbing of the plasma membrane, anomalous cytoskeletal structures, hyperorganised annulate lamellae, hyperinvagination of the gv, and formation of irregular nucleoli within the gv. Analysis of newly synthesised proteins by [35S]methionine radiolabelling of oocytes demonstrated that ODN injection can trigger a general decrease in both label uptake and protein synthesis. Qualitative effects on protein synthesis could also be observed, particularly a decrease in synthesis of high molecular weight proteins. The severity of ODN-induced effects is dose-dependent and highly variable from ODN to ODN. The previously reported delay in progesterone-induced maturation observed in oocytes depleted of the maternal mRNA D7 by ODN-directed degradation (Smith R. C., Dworkin M. B. and Dworkin-Rastl E. (1988) Genes and Devpt. 2, 1296–1306) is most likely a result of nonspecific ODN effects in the oocyte. Oocytes injected with effective antisense D7 ODNs that do not display detectable side effects matured with normal kinetics.

Carbon metabolism in early amphibian embryos

Xenopus embryos undergoing cleavage utilize amino acids as their main carbon source for metabolism. Glycolysis (from stored glycogen) begins near the onset of gastrulation. Thus, a major transition in the metabolism of the early embryo occurs before morphological differentiation. The enzymology that supports the carbon metabolism of the cleaving amphibian embryo resembles that of many mammalian tumor cells.

MOlecular cloning of human alpha and beta interferon genes from Namalwa cells

A cDNA library constructed from poly(A)+RNA of Sendai virus-induced human lymphoblastoid (Namalwa) cells was screened with a synthetic oligonucleotide specific for interferon genes. Recombinant plasmids containing sequences derived from alpha and beta interferon (IFN) mRNAs were obtained. The clones were characterized by RNA transfer hybridization, translation of hybrid-isolated RNA, and DNA sequencing. One alpha interferon (IFN-alpha) clone obtained is a variant differing from previously described clones in the location of its polyadenylation site.

Construction of expression plasmids producing high levels of human leukocyte-type interferon in Escherichia coli

An expression plasmid was constructed, consisting of the promoter/operator region of the tryptophan operon from Serratia marcescens and a synthetic ribosome-binding site ligated into pBR322. Leukocyte-type interferon gene fragments (IFN-alpha A and IFN-alpha C) isolated from a cDNA library from human lymphoblastoid (Namalwa) cells were inserted into the unique HindIII site of the expression plasmid, and the resulting recombinant plasmids directed the synthesis of up to 5 X 10(5) units of A-type preinterferon, 2 X 10(7) units of A-type mature interferon and 8 X 10(5) units of C-type mature interferon per liter culture.

Regulation of carbon flux from amino acids into sugar phosphates in Xenopus embryos

Xenopus laevis oocytes and embryos are glycogenic cells, metabolizing sugar phosphates into glycogen. These cells have very low pyruvate kinase activity in vivo and, consequently, make little pyruvate and lactate through glycolysis. Nevertheless, oocytes and embryos do contain significant pyruvate and lactate levels. To determine the source of carbon for sugar phosphates and pyruvate, 14C-labeled intermediary metabolites were injected into fertilized eggs and their metabolism examined by thin-layer chromatography. Alanine, pyruvate, and lactate form a pool of carbon that fluxes into sugar phosphates. Cytosolic (nonmitochondrial) aspartate, oxaloacetate, and malate form a pool of carbon which is largely blocked in the short-term from entering the smaller alanine/pyruvate/lactate pool. The data indicate that the major source of carbon for sugar phosphates in fertilized eggs and rapidly cleaving embryos is the alanine/pyruvate/lactate pool. Pyruvate from this pool is converted in the mitochondria to phosphoenolpyruvate, which in turn is metabolized outside the mitochondria to sugar phosphates. A key enzyme in regulating flux from amino acid carbon to pyruvate is malic enzyme. Three malic enzyme isozymes, one soluble and two mitochondrial, were partially isolated and kinetically characterized from total ovarian tissue. Full-grown oocytes and eggs, however, have very low soluble malic enzyme activity, which results in the separation of the cytosolic aspartate/oxaloacetate/malate and alanine/pyruvate/lactate pools.

The accumulation of prominent tadpole mRNAs occurs at the beginning of neurulation in Xenopus laevis embryos

Cloned cDNA probes have been used to measure the sizes and titers of transcripts in total RNA preparations during early development in Xenopus laevis. Of more than 20 different sequences derived from abundant and moderately abundant RNA which were present in full-grown oocytes and persisted during early development, the transcript sizes of all but 3 of these sequences were invariant. Two transcripts were of a higher molecular weight in oocytes than in embryos, but their titers in oocytes were less than 5% their titers in embryos and thus these larger maternal transcripts do not significantly contribute to embryonic, polysomal mRNA. The oocyte transcripts and the embryonic transcripts of one of these sequences are transcribed from different though cross-hybridizing genes. Cellular titers of a number of RNA sequences have also been studied and show that increases in the cellular titers of several poly(A)+RNA species are the result of de novo transcription and not simply polyadenylation. A number of sequences abundant in tadpole RNA but absent or very rare in eggs have also been examined. All of these sequences first appear in development in substantial titers in the late gastrula or early neurula, 12-15 hr after fertilization. Many other sequences already present in eggs which persist during development show an increase in titer 12-15 hr after fertilization. These data suggest that this late gastrula transcriptional event may be a major transition of gene expression that accompanies the cellular differentiation and morphogenesis that begin at this developmental time.

Glycogen breakdown in cleaving Xenopus embryos is limited by ADP

Xenopus eggs contain large stores of glycogen, but this glycogen is not glycolytically processed during cleavage. The Embden-Meyerhof pathway is inhibited by the absence of pyruvate kinase activity in vivo, and lactate and pyruvate are present at relatively low levels. In the late blastula, just preceding gastrulation, lactate levels increase, indicating the onset of glycogen breakdown and glycolytic flux. Glycolysis from microinjected [14C]glucose-6-phosphate could be transiently activated, however, by the coinjection of ADP into fertilized eggs, and constitutively activated by the injection of the ATPase potato apyrase, indicating the presence of all enzymes necessary for glycolytic activity. The isozyme profiles of pyruvate kinase and malic enzyme, two enzymes involved in carbon metabolism during cleavage or in the subsequent activation of glycogen breakdown, do not change between the egg and gastrula stages. These data suggest that the activation of glycogen breakdown and glycolysis in the late blastula is probably not a result of new gene activity but may be the metabolic consequence of increased free ADP that is then able to support the pyruvate kinase reaction.

Constancy of DNA organization of polymorphic and nonpolymorphic genes during development in Xenopus

A study was undertaken to test for the occurrence of DNA rearrangements or amplifications during embryonic development in Xenopus laevis. DNA isolated from testes and liver was digested with four restriction enzymes, separated on agarose gels, transferred to nitrocellulose, and hybridized with over 50 cloned cDNA probes generated from embryonic poly (A)+ RNA. No qualitative or quantitative differences were detectable in the DNA hybridization patterns of testes and liver DNA, suggesting that, at least during liver development, selective amplifications or rearrangements occur rarely if at all. In the course of this investigation a wide range of restriction-site polymorphisms for different genes was observed. While some genes showed little polymorphism among different animals, several genes showed considerable polymorphism, involving changes in several restriction enzyme sites. These complex polymorphisms could be the result of gene rearrangements that occur occasionally during the course of sexual reproduction rather than during development.

Metabolic regulation during early frog development: flow of glycolytic carbon into phospholipids in Xenopus oocytes and fertilized eggs

32P-labeled glucose 6-phosphate, [32P]phosphoenolpyruvate, and [gamma-32P]ATP were injected into oocytes and fertilized eggs of Xenopus laevis, and the incorporation of the 32P label was followed into phospholipids. Several classes of phospholipids incorporated 32P label from the injected glycolytic intermediates, including lysophosphatidic acid, phosphatidic acid, phosphatidylinositol, and phosphatidylinositol phosphates, inferring de novo synthesis of these lipids from dihydroxyacetone phosphate or glycerol 3-phosphate. Injection of [gamma-32P]ATP into oocytes and fertilized eggs led to labeling of phosphatidylinositol phosphate and phosphatidylinositol bisphosphate, indicating an active phosphatidylinositol cycle in resting oocytes and fertilized eggs. Maturation and fertilization of the oocyte led to a qualitative change in phosphatidylinositol metabolism, increased labeling of phosphatidylinositol phosphate compared to phosphatidylinositol bisphosphate (either from glycerol 3-phosphate or from ATP). This change occurs late in the maturation process, and the new pattern of phosphatidylinositol metabolism is maintained during the rapid cleavage stages of early embryogenesis.

Tumorigenic Xenopus cells express several maternal and early embryonic mRNAs

Recombinant cDNA libraries were constructed from poly(A)+ RNA isolated from different stages of oogenesis and embryogenesis from the clawed toad Xenopus laevis. Hybridization analyses were used to describe the accumulation of specific RNAs represented by these cDNA clones in oocytes, embryos, adult liver, a cell line derived from Xenopus borealis embryos (Xb693), and a tumorigenic substrain of that cell line (Xb693T). It was found that from 550 cDNA clones analysed, six sequences accumulate to higher titers in poly(A)+ RNA isolated from the tumorigenic cell line compared with the non-tumorigenic cell line. All six sequences were expressed at high levels during oogenesis, and the titers of three of these sequences decreased considerably during oogenesis. DNA sequencing of these three sequences followed by a computer search of protein data banks has identified them as coding for the glycolytic enzyme enolase, the ATP-ADP carrier protein, and a-tubulin.

Pyruvate kinase isozymes in oocytes and embryos from the frog Xenopus laevis

1. The kinetic characteristics of pyruvate kinase isozymes from oocytes, embryos, liver and skeletal muscle from the clawed frog Xenopus laevis were measured in cell extracts. 2. The muscle and liver isozymes display Michaelis-Menten kinetics with Kms for phosphoenolpyruvate (PEP) of 0.02 and 0.05 mM, respectively. 3. Pyruvate kinase from oocytes and embryos displays cooperative kinetics for PEP with a Km of about 0.15 mM; the kinetics become hyperbolic and the Km for PEP is reduced to 0.05 mM in the presence of microM concentrations of fructose-1,6-bisphosphate. 4. These data serve to characterize pyruvate kinase activity in oocytes and embryos and the kinetics are compared to mammalian pyruvate kinase isozymes.

Synthesis and modification of D7 protein during Xenopus oocyte maturation

The Xenopus maternal mRNA D7 is translationally repressed during oogenesis, only becoming recruited into polysomes during oocyte maturation, with D7 protein being detectable for the first time prior to germinal vesicle breakdown (GVBD). The synthesis of D7 protein was found to be induced by a variety of maturation-promoting agents including cyclin, c-mos and crude preparations of MPF. D7 protein induced by all these agents is post-translationally modified and exists as a number of variants of differing molecular weight. In contrast to endogenous D7 mRNA, D7 RNA injected into the stage VI oocyte is efficiently translated, resulting in the accumulation of predominantly unmodified D7 polypeptides, which become increasingly modified during oocyte maturation to produce a pattern of polypeptides similar to those derived from endogenous D7 mRNA. Thus, the system that results in the post-translational modification of the D7 protein is itself activated during oocyte maturation. The nature of the protein modification is not known but does not appear to be phosphorylation. The translation of exogenous D7 RNA in the stage VI oocyte does not lead to translational derepression of endogenous D7 mRNA.

The maternal gene product D7 is not required for early Xenopus development

The Xenopus D7 gene codes for a novel protein whose expression is restricted to early development. D7 protein is synthesized for the first time during oocyte maturation (1988, Genes Dev. 2, 1296-1306). Injection of D7 RNA into the full-grown oocyte and its subsequent translation into D7 protein neither induced oocyte maturation nor affected the kinetics of hormone-induced maturation. Overexpression of D7 protein by 20-fold in the early Xenopus embryo by injection of D7 RNA into fertilized eggs did not affect subsequent development. Oocytes specifically lacking D7 mRNA were generated by oligodeoxynucleotide-mediated RNA destruction within the oocyte. Unfertilized eggs generated from such oocytes lacked detectable D7 protein, but nevertheless could be activated and fertilized. Embryos generated from such eggs, estimated to contain less than 5% of wildtype levels of D7 protein, developed normally up to the tailbud stage. Thus the D7 protein, the product of a maternal mRNA that is under strict translational repression in oocytes, appears not to be required for oocyte maturation, activation, fertilization or early embryonic development in Xenopus.

Localization of specific mRNA sequences in Xenopus laevis embryos by in situ hybridization

In situ hybridization of cloned cDNA probes to frozen sections of Xenopus laevis stage-42 tadpoles has been used to determine the tissue localization of several mRNAs. Nine out of sixteen probes tested hybridized to most or all tadpole tissues; seven probes exhibited tissue-specific hybridization. The non-tissue-specific sequences hybridized to RNA species that are also present in maternal RNA while the tissue-specific sequences hybridized to embryonic RNA species induced after gastrulation and undetectable in maternal RNA. Tissue-specific hybridization was observed with muscle (five clones), epidermis (one clone), and the nervous system (one clone). All muscle-specific sequences hybridized to somites and lateral plate muscles, but they differed in their hybridization to heart muscle.

Metabolic regulation during early frog development. Identification of proteins labeled by 32P-glycolytic intermediates

When 32P-labeled phosphoenolpyruvate is injected into Xenopus laevis oocytes, a 50-60-kDa protein of subunit size Mr 29,000 is rapidly labeled, followed by a second (monomeric) protein of 66 kDa concomitant with the loss of label from the first protein. We have identified these proteins as, respectively, the glycolytic enzymes phosphoglyceromutase and phosphoglucomutase. The phosphoglyceromutase is labeled at a histidine and the phosphoglucomutase at a serine, presumably at their active sites during the gluconeogenic transformation of phosphoenolpyruvate into glycogen. The transfer of the 32P label from phosphoenolpyruvate to these two enzymes also occurs in in vitro lysates made from full-grown Xenopus oocytes, eggs, or early embryos, but with a slower time course. Lysates prepared from leg muscle show labeling of the phosphoglyceromutase, but not the phosphoglucomutase, when incubated with [32P]phosphoenolpyruvate. This last result is expected in tissues showing metabolic flux largely in the glycolytic direction. The data indicate that in full-grown oocytes and embryos metabolic flux occurs largely in the gluconeogenic direction.