Characterization and expression of a Xenopus ras during oogenesis and development

Mechanisms Regulating Oocyte Meiotic Resumption: Roles of Mitogen-Activated Protein Kinase

Oocyte meiotic maturation is one of the important physiological requirements for species survival. However, little is known about the detailed events occurring during this process. A number of studies have demonstrated that MAPK plays a pivotal role in the regulation of meiotic cell cycle progression in oocytes, but controversial findings have been reported in both lower vertebrates and mammals. In this review, we summarized the roles of MAPK cascade and related signal pathways in oocyte meiotic reinitiation in both lower vertebrates and mammals. We also tried to reconcile the paradoxical results and highlight the new findings concerning the function of MAPK in both oocytes and the surrounding follicular somatic cells. The unresolved questions and future research directions regarding the role of MAPK in meiotic resumption are addressed.

Fertilization signalling and protein-tyrosine kinases

Fertilization is initiated by species-specific gamete cell recognition, i.e. sperm-egg interaction, followed by a rapid and sustained activation of multiple cellular and biochemical events, collectively called 'egg activation', which is indispensable for successful formation of zygotic nucleus and later embryogenesis. It is well known that sperm-induced egg activation is mediated by a transient release of calcium ions that originates from the sperm entry point and propagates through the entire egg cytoplasm. It is unclear, however, what kind of upstream events prelude to the calcium transient after sperm-egg interaction. Recently, much attention has been paid to the role of protein-tyrosine phosphorylation in egg activation process by a number of studies on some well-established model organisms. These includes marine invertebrates, frogs, and mammals. In this review, we will summarize the recent findings that begin to uncover a 'missing link' between sperm-egg interaction and egg activation with emphasis on the role of egg protein-tyrosine kinases (PTKs) in Xenopus egg fertilization.

Analysis of farnesyl transferase activity during hormone-induced maturation of Xenopus laevis oocytes

Preincubation of Xenopus laevis oocytes with insulin or insulin-like growth factor 1 (IGF-1) resulted in inhibition of farnesyl transferase (FTase) activity measured both in vivo (after microinjection of tritiated farnesyl pyrophosphate and Ras-CVIM into oocytes) and in extracts using a filtration assay. FTase activity measured in oocyte extracts was inhibited 55% after a 20 min treatment of oocytes with 1 microM insulin or 10 nM IGF-1. The apparent IC(50) for inhibition of oocyte FTase by IGF-1 is 0.3 nM. The observed decrease in FTase activity was apparently not due to translocation of enzyme from cytosol to membrane, since activities measured both in soluble extracts and resuspended crude pellets displayed comparable levels of inhibition following hormone treatment. Using a hexapeptide (TKCVIM) as substrate, FTase activity was also inhibited 65% when oocytes were pretreated with 10 nM IGF-1. Two FTase inhibitors [(alpha-hydroxyfarnesyl) phosphonic acid (HFPA) and chaetomellic acid A (CA)] effectively inhibited Xenopus oocyte FTase by 80-90% when added to assay mixtures (IC(50) values of 338 +/- 96 nM HFPA and 232 +/- 80 nM CA) or after incubation of oocytes in drug before preparation of soluble extracts for assay (IC(50) values of 7 +/- 6 nM HFPA and 328 +/- 128 nM CA). The farnesyl transferase inhibitors were observed to slow the time course of oocyte maturation but did not block the IGF-1-induced maturation response. J. Exp. Zool. 286:193-203, 2000.

Aldosterone action: induction of p21(ras) and fra-2 and transcription-independent decrease in myc, jun, and fos

Adrenal steroids induce an increase in transcellular Na+ reabsorption across Xenopus laevis A6 cell epithelia that requires the action of transcriptionally regulated gene products. In a previous study we identified K-ras2 as an aldosterone-upregulated mRNA in A6 epithelia. Here, we show that in vivo injection of aldosterone in Xenopus (2.5 h) increases K-ras2 mRNA specifically in the kidney (2.5-fold) and that in A6 epithelia aldosterone (2.5 h) increases Ras protein synthesis ( approximately 6-fold). Xl-ras, another ras mRNA expressed at a low level in A6 cells, was also induced (2-fold). Aldosterone was shown to regulate the mRNA levels of several transcription factors as well. After 2 h of aldosterone treatment, fra-2 mRNA was upregulated by 130%, whereas c-myc, c-jun, c-fos, and glucocorticoid receptor mRNAs were downregulated by 23-43%. After 16 h, c-fos and GR mRNAs were further decreased, whereas levels of fra-2, c-jun, and c-myc began to return to control levels. Interestingly, the downregulation of the protooncogene mRNAs was independent of transcription. These results support the view that aldosterone exerts complex pleiotropic transcriptional and nontranscriptional actions that involve the regulation of signaling cascade elements (i.e., K-Ras2) as well as that of transcription factors.

Inhibition of small G proteins by clostridium sordellii lethal toxin activates cdc2 and MAP kinase in Xenopus oocytes

The lethal toxin (LT) from Clostridium sordellii is a glucosyltransferase that modifies and inhibits small G proteins of the Ras family, Ras and Rap, as well as Rac proteins. LT induces cdc2 kinase activation and germinal vesicle breakdown (GVBD) when microinjected into full-grown Xenopus oocytes. Toxin B from Clostridium difficile, that glucosylates and inactivates Rac proteins, does not induce cdc2 activation, indicating that proteins of the Ras family, Ras and/or Rap, negatively regulate cdc2 kinase activation in Xenopus oocyte. In oocyte extracts, LT catalyzes the incorporation of [14C]glucose into a group of proteins of 23 kDa and into one protein of 27 kDa. The 23-kDa proteins are recognized by anti-Rap1 and anti-Rap2 antibodies, whereas the 27-kDa protein is recognized by several anti-Ras antibodies and probably corresponds to K-Ras. Microinjection of LT into oocytes together with UDP-[14C]glucose results in a glucosylation pattern similar to the in vitro glucosylation, indicating that the 23- and 27-kDa proteins are in vivo substrates of LT. In vivo time-course analysis reveals that the 27-kDa protein glucosylation is completed within 2 h, well before cdc2 kinase activation, whereas the 23-kDa proteins are partially glucosylated at GVBD. This observation suggests that the 27-kDa Ras protein could be the in vivo target of LT allowing cdc2 kinase activation. Interestingly, inactivation of Ras proteins does not prevent the phosphorylation of c-Raf1 and the activation of MAP kinase that occurs normally around GVBD.

Gene expression during amphibian limb regeneration

Limb regeneration in adult urodeles is an important phenomenon that poses fundamental questions both in biology and in medicine. In this review, we focus on recent advances in the characterization of the regeneration blastema at cellular and molecular levels and on the current understanding of the molecular basis of limb regeneration and its relationship to development. In particular, we discuss (i) the spatiotemporal distribution of genes and gene products in the mesenchyme and wound epidermis of the regenerating limb, (ii) how growth is controlled in the regeneration blastema, and (iii) molecules that are likely to be involved in patterning the regenerating limb such as homeobox genes and retinoids.

Expression of the subunits of protein kinase CK2 during oogenesis in Xenopus laevis

Northern-blot analysis of RNAs from different tissues demonstrated that the mRNA for the protein kinase CK2 alpha subunit is very abundant in the ovary of Xenopus laevis. The competitive reverse-PCR technique has been used to quantitate the mRNA for both CK2 alpha and CK2 beta subunits during oogenesis. The results obtained using eight different animals consistently show an increment of 2-3-fold in the mRNA for both subunits in vitellogenic oocytes (stages II-VI). Each stage-VI oocyte contains approximately 5 x 10(-7) molecules CK2 alpha mRNA and 1 x 10(-7) molecules CK2 beta mRNA. These amounts are considerably higher than many other mRNAs analyzed in these cells. Activity measurements of CK2 using casein or a specific model peptide revealed increments of about 10-12-fold during oogenesis, and also indicated that the amount of enzyme in the nucleus accounted for 15-30% of the total enzyme in the oocyte at all stages. Western-blot analysis of CK2 alpha indicated that the amount of this protein subunit also increased during oogenesis in a parallel fashion with the increment of enzymic activity.

Effect of synthetic peptides representing the hypervariable region of p21ras on Xenopus laevis oocyte maturation

The carboxy-terminal hypervariable regions of p21ras proteins have been highly conserved throughout evolution but no function has been assigned to them yet. This region has been suggested as a possible candidate for receptor recognition. We have tested the possibility of this region being involved in p21ras biological function. Synthetic peptides corresponding to the hypervariable domains of p21N-ras and p21K(B)-ras were microinjected into Xenopus oocytes to assess their effect on oocyte maturation. The K(B)-ras peptide inhibited insulin-dependent but not progesterone-dependent maturation, in contrast with the N-ras peptide which did not inhibit maturation significantly. A control peptide, with the same amino acid composition as the K(B)-ras peptide but with a scrambled sequence, and poly(D,L-lysine) were inactive. Pentalysine had partial activity which may be due to its mimicking the lysine-rich stretch of the K(B)-ras sequence. The data support the hypothesis that the K(B)-ras gene product specifically is involved in transducing the insulin and/or insulin-like growth factor 1 signal.

Differential accumulation of mRNA and interspersed RNA during Xenopus oogenesis and embryogenesis

Xenopus oocyte cytoplasmic poly(A)+ RNA has been shown to include two major complex classes: mRNA and interspersed RNA. The former is defined by its translatability, while the latter consists of non-translatable repeat-containing transcripts with unknown functions. In this study we compared the accumulation patterns of total mRNA and a subfamily of interspersed RNA, the XR family (McGrew & Richter, 1989, Dev. Biol. 134, 267-70). The results showed that the XR interspersed RNA level continued to increase throughout oogenesis, while the total mRNA level reached a peak at late stage II and then decreased as much as 40% between stage II and stage VI of oogenesis. In addition we have found that, like mRNA, only about half of the non-translatable XR interspersed RNA underwent deadenylation at oocyte maturation. This result suggested that about half of the interspersed RNA, like certain mRNAs, also contains the U-rich element to protect it from the automatic deadenylation, implying the poly(A) tail of interspersed RNA may play a role during early development.

Saccharomyces cerevisiae cdc15 mutants arrested at a late stage in anaphase are rescued by Xenopus cDNAs encoding N-ras or a protein with beta-transducin repeats

We have constructed a Xenopus oocyte cDNA library in a Saccharomyces cerevisiae expression vector and used this library to isolate genes that can function in yeast cells to suppress the temperature sensitive [corrected] defect of the cdc15 mutation. Two maternally expressed Xenopus cDNAs which fulfill these conditions have been isolated. One of these clones encodes Xenopus N-ras. In contrast to the yeast RAS genes, Xenopus N-ras rescues the cdc15 mutation. Moreover, overexpression of Xenopus N-ras in S. cerevisiae does not activate the RAS-cyclic AMP (cAMP) pathway; rather, it results in decreased levels of intracellular cAMP in both mutant cdc15 and wild-type cells. Furthermore, we show that lowering cAMP levels is sufficient to allow cells with a nonfunctional Cdc15 protein to complete the mitotic cycle. These results suggest that a key step of the cell cycle is dependent upon a phosphorylation event catalyzed by cAMP-dependent protein kinase. The second clone, beta TrCP (beta-transducin repeat-containing protein), encodes a protein of 518 amino acids that shows significant homology to the beta subunits of G proteins in its C-terminal half. In this region, beta Trcp is composed of seven beta-transducin repeats. beta TrCP is not a functional homolog of S. cerevisiae CDC20, a cell cycle gene that also contains beta-transducin repeats and suppresses the cdc15 mutation.

Saccharomyces cerevisiae cdc15 mutants arrested at a late stage in anaphase are rescued by Xenopus cDNAs encoding N-ras or a protein with beta-transducin repeats

We have constructed a Xenopus oocyte cDNA library in a Saccharomyces cerevisiae expression vector and used this library to isolate genes that can function in yeast cells to suppress the temperature sensitive [corrected] defect of the cdc15 mutation. Two maternally expressed Xenopus cDNAs which fulfill these conditions have been isolated. One of these clones encodes Xenopus N-ras. In contrast to the yeast RAS genes, Xenopus N-ras rescues the cdc15 mutation. Moreover, overexpression of Xenopus N-ras in S. cerevisiae does not activate the RAS-cyclic AMP (cAMP) pathway; rather, it results in decreased levels of intracellular cAMP in both mutant cdc15 and wild-type cells. Furthermore, we show that lowering cAMP levels is sufficient to allow cells with a nonfunctional Cdc15 protein to complete the mitotic cycle. These results suggest that a key step of the cell cycle is dependent upon a phosphorylation event catalyzed by cAMP-dependent protein kinase. The second clone, beta TrCP (beta-transducin repeat-containing protein), encodes a protein of 518 amino acids that shows significant homology to the beta subunits of G proteins in its C-terminal half. In this region, beta Trcp is composed of seven beta-transducin repeats. beta TrCP is not a functional homolog of S. cerevisiae CDC20, a cell cycle gene that also contains beta-transducin repeats and suppresses the cdc15 mutation.

Xenopus laevis oocyte G alpha subunits mRNAs. Detection and quantitation during oogenesis and early embryogenesis by competitive reverse PCR

The expression of mRNAs coding for different Xenopus laevis oocyte G alpha subunits was analyzed by the PCR technique. Using the nucleotide sequences of five previously cloned cDNAs for oocyte G alpha subunits [FEBS Lett. 244, 188-192, 1989; FEBS Lett. 268, 27-31, 1990] and the highly sensitive reverse PCR reaction we found that G alpha o, G alpha i-1, G alpha i-3 and G alpha s species are present in oocyte stage VI, G alpha o mRNA being the most abundant transcript. G alpha o mRNA was further quantitated through oogenesis, unfertilized eggs and early embryogenesis stages by a competitive PCR reaction using an 'in vitro' deleted G alpha o mRNA as the internal standard. Using this approach we found that Xenopus G alpha o mRNA levels were constant during oogenesis and unfertilized eggs at a concentration of 3.5 pg of mRNA/stage (5 x 10(5) molecules) and diminish gradually during early embryogenesis, reaching a level of 0.3 pg in the gastrula stage. These findings show that oocyte G alpha o, and perhaps the rest of the alpha subunits, are expressed as maternal mRNAs and could play an important role in signal transduction at the beginning of oocyte cell differentiation.

Localization of ras proto-oncogene expression during development in Xenopus laevis

The expression of the ras protooncogene was investigated in Xenopus laevis, throughout development, by in situ hybridization using a 35S-labelled antisense RNA probe. During oogenesis, the ras RNA was strongly expressed in the cytoplasm of previtellogenic oocytes and further diluted between yolk platelets; no specific localization of transcripts was observed. The signal density was particularly weak over embryo sections until the tailbud stage. On the other hand, a high level of ras RNA expression was detected on sections through the young tadpoles. An intense labelling was observed in several areas, including the branchial apparatus, gut, somites, nervous system, and lens. It is noteworthy that the heterogeneity of labelling increases as tadpoles grow older. Together, these results are discussed in relation to cellular events appearing throughout the early development.

Isolation and developmental expression of an oogenesis-specific Xenopus cDNA clone

We have characterized a cDNA clone designated XSYO, complementary to a transcript that is highly expressed during Xenopus oogenesis. XSYO is expressed as a maternal mRNA in oocytes and early embryos at a level up to 3 × 10⁸ copies per mature oocyte. This level is 100-fold higher than the concentration of an average maternal RNA in the oocyte and close to the value found for the stockpiled maternal histone mRNA. This level remains constant thoughout the first rapid cleavage stages until the midblastula transition (MBT). After this stage, the XSYO RNA is degraded within 2 h and, after the blastula stage, a very weak expression was detected. This gene is not expressed in Xenopus proliferative somatic cultured cells, suggesting that it is not a simple housekeeping gene. The presence of a potential metal-binding domain in the XSYO sequence suggests that this gene might code for a protein involved in nucleic acid binding or gene regulation specific to oogenesis or early development.

Analysis of the p21 ras system during development of meiotic competence in Xenopus laevis oocytes

The ability of Xenopus oocytes to undergo insulin- or insulin-like growth factor 1-induced meiotic maturation develops during oogenesis, with cells 1.0 mm in diameter or larger responding in a size-dependent manner. Since insulin-induced oocyte maturation was shown previously to be p21 ras-dependent, experiments were performed to test whether a deficiency in the p21 ras system might account for meiotic incompetence in small oocytes (less than or equal to 0.9 mm diameter). Both small and large oocytes were found to contain comparable levels of membrane-associated p21, as determined by protein immunoblotting. Treatment of both small and large oocytes with 2 microM insulin for 2 hr increased endogenous levels of membrane-associated p21 by approximately 70%. Stimulation of microinjected p21-membrane association by insulin was observed to be both time- and concentration-dependent in large oocytes with an EC50 of 50 nM. In addition, comparable levels of GTPase activating protein were measured in extracts prepared from oocytes ranging from 0.8 to 1.3 mm in diameter. Therefore, the p21 system is apparently not limiting during oogenesis, and expression of some other cellular component must account for development of meiotic competence in Xenopus oocytes.

Evolutionary grouping of the RAS-protein family

Over 50 proteins related to the mammalian H-, K-, and N-RAS GTP binding and hydrolyzing proteins are known. These relatively low molecular weight proteins are usually grouped into four subfamilies, termed true RAS, RAS-like, RHO, and RAB/YPT, based on the presence of shared amino acid sequence motifs in addition to those involved in guanine nucleotide binding. Here, we apply parsimony analysis to the overall amino acid sequences of these proteins to infer possible phylogenetic relationships among them.

Expression of ras-like proteins in embryonic and adult cells of Xenopus laevis

A polyclonal antibody raised against v-Ha-ras p21 was purified and its specificity was checked on Ha-ras transformed cell lines. It was used to immunoprecipitate p21 from different Xenopus laevis cell types: brain cells, blood cells, and embryonic material. By one-dimensional Western blot analysis, we show that ras p21 is synthesized very early in oogenesis and accumulates throughout vitellogenesis. The ras p21 content, estimated to be 1.1 ng in the full-grown oocyte, remains constant during oocyte maturation and egg cleavage. Increase in the amount of ras p21 occurs at the beginning of neurulation. Two-dimensional Western blot patterns reveal the presence of multiple molecular forms of p21 in all Xenopus cell types studied. The numerous resolved polypeptides were ascribed to the expression of at least two different ras genes. Furthermore, specific charge modifications of the ras polypeptides are observed in brain, blood, and embryonic cells. During oogenesis and early embryonic development, differences in two-dimensional patterns mainly concern variations in the relative amounts of the different polypeptides. The results are discussed in relation to the well documented synthesis activities of the growing oocyte and of the early developing embryo.

Expression of a mRNA related to c-rel and dorsal in early Xenopus laevis embryos

We describe a Xenopus mRNA, Xrel1, that is related to the avian protooncogene c-rel, the embryonic pattern gene dorsal of Drosophila, and the mammalian transcription factor NK-kappa B/KBF1. The sequence of Xrel1 is homologous to the other rel-related proteins in the large amino-terminal region that defines this class of transcriptional regulators, but the carboxyl-terminal part of the protein is quite different. Xrel1 mRNA is present throughout oogenesis and during early embryogenesis at 4 x 10(5) transcripts per oocyte or embryo. Xrel1 transcripts are present in all of the dissected parts of early embryos that we have examined. They are enriched in the animal hemisphere compared to the vegetal hemisphere of oocytes and blastulae.