Messenger RNA competition in living Xenopus oocytes

Using Xenopus oocytes in neurological disease drug discovery

Introduction: Neurological diseases present a difficult challenge in drug discovery. Many of the current treatments have limited efficiency or result in a variety of debilitating side effects. The search of new therapies is of a paramount importance, since the number of patients that require a better treatment is growing rapidly. As an in vitro model, Xenopus oocytes provide the drug developer with many distinct advantages, including size, durability, and efficiency in exogenous protein expression. However, there is an increasing need to refine the recent breakthroughs.Areas covered: This review covers the usage and recent advancements of Xenopus oocytes for drug discovery in neurological diseases from expression and functional measurement techniques to current applications in Alzheimer's disease, painful neuropathies, and amyotrophic lateral sclerosis (ALS). The existing limitations of Xenopus oocytes in drug discovery are also discussed.Expert opinion: With the rise of aging population and neurological disorders, Xenopus oocytes, will continue to play an important role in understanding the mechanism of the disease, identification and validation of novel molecular targets, and drug screening, providing high-quality data despite the technical limitations. With further advances in oocytes-related techniques toward an accurate modeling of the disease, the diagnostics and treatment of neuropathologies will be becoming increasing personalized.

Cap-independent translation initiation in Xenopus oocytes

Eukaryotic cellular mRNAs contain a cap at their 5'-ends, but some viral and cellular mRNAs bypass the cap-dependent mechanism of translation initiation in favor of internal entry of ribosomes at specific RNA sequences. Cap-dependent initiation requires intact initiation factor eIF4G (formerly eIF-4gamma, eIF-4Fgamma or p220), whereas internal initiation can proceed with eIF4G cleaved by picornaviral 2A or L proteases. Injection of recombinant coxsackievirus B4 protease 2A into Xenopus oocytes led to complete cleavage of endogenous eIF4G, but protein synthesis decreased by only 35%. Co-injection of edeine reduced synthesis by >90%, indicating that eIF4G-independent synthesis involved ongoing initiation. The spectrum of endogenous proteins synthesized was very similar in the presence or absence of intact eIF4G. Translation of exogenous rabbit globin mRNA, by contrast, was drastically inhibited by eIF4G cleavage. The N-terminal cleavage product of eIF4G (cpN), which binds eIF4E, was completely degraded within 6-12 h, while the C-terminal cleavage product (cpC), which binds to eIF3 and eIF4A, was more stable over the same period. Thus, translation initiation of most endogenous mRNAs inXenopusoocytes requires no eIF4G, or perhaps only cpC, suggesting a cap-independent mechanism.

Amylase synthesis as a simple model system for translation and hybrid arrest in Xenopus oocytes

A human alpha-amylase-encoding cDNA has been cloned in a transcription vector. When messenger RNA (mRNA) made in vitro from this construct was injected into Xenopus oocytes, amylase (AMY) activity was detected both in oocyte homogenates and in the incubation medium, indicating that the oocyte machinery correctly translated and processed the protein. Because AMY activity is easy to detect with a blue-starch assay, this expression system was used to determine the parameters of antisense oligodeoxyribonucleotide (oligo) inhibition of translation in the oocytes. Unique oligos complementary to the AMY mRNA sequence were effective in arresting translation, at approximately stoichiometric levels. Mixed oligos also inhibited translation, at levels that suggest that some mismatches may be tolerated in the formation of DNA-RNA hybrids. The AMY system provides a convenient probe of oocyte protein synthesis and processing machinery and can serve as a control substrate in investigations of other mRNAs.

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.

Eukaryotic initiation factor 4A stimulates translation in microinjected Xenopus oocytes

The injection of heterologous mRNA into fully grown Xenopus oocytes results not only in the synthesis of the heterologous protein but also in a reciprocal decrease in the synthesis of endogenous proteins. This indicates that injected and endogenous mRNAs compete for some component which is rate-limiting for translation in oocytes. We have attempted to identify this rate-limiting translational component. We find that heterologous and homologous polysomes compete with endogenous mRNAs as effectively as naked mRNA, indicating that polysomes do not contain detectable levels of the rate-limiting factor. In addition, we have used micrococcal nuclease digestion and a mRNA-specific oligonucleotide to destroy the mRNA component of polysomes. The remaining polysome factors, when injected into oocytes, failed to stimulate translation. When several eukaryotic translation initiation factors were injected into oocytes, initiation factor 4A consistently increased general oocyte protein synthesis by about twofold. It is possible that the availability of eIF-4A in oocytes is a key factor in limiting the overall rate of protein synthesis.

Multiple levels of regulation of protein synthesis at fertilization in sea urchin eggs

Fertilization of sea urchin eggs results in a large stimulation of protein synthesis. This increase in protein synthesis is mediated by the mobilization of stored maternal mRNA (mRNPs) into polysomes, but the details of the molecular mechanisms which regulate this process are not well understood. Using a sea urchin egg cell-free translation system, evidence has been obtained which indicates that the capacity to initiate protein synthesis on new mRNAs is limited. Addition of exogenous mRNAs failed to stimulate overall protein synthesis, whereas supplementing the system with a nuclease-treated reticulocyte lysate, an S-100 supernatant fraction, or purified eIF-2 stimulated nearly twofold. In addition, the levels of 43 S preinitiation complexes containing a 40 S ribosomal subunit and methionyl-tRNA were increased at pH 7.4 compared to pH 6.9, or when reticulocyte S-100 was added. However, other experiments showed clearly that mRNA availability may also regulate translation in the sea urchin egg. Sea urchin lysates only stimulated poorly the nuclease-treated reticulocyte lysate system, and the mRNPs in the sea urchin lysate did not bind to reticulocyte 43 S preinitiation complexes. Since purified sea urchin egg mRNA was active in both assays, the bulk of sea urchin mRNA must be masked in the egg, and remain masked in the in vitro assays. Thus, protein synthesis appears to be regulated at both the level of mRNA availability and the activity of components of the translational machinery.

The biosynthesis of biologically active proteins in mRNA-microinjected Xenopus oocytes

The basic properties of mRNA-injected Xenopus oocytes as a heterologous system for the production of biologically active proteins will be reviewed. The advantages and limitations involved in the use of this in ovo system will be discussed, as compared with in vitro cell-free translation systems and with in vivo microinjected mammalian cells in culture. The different assay systems that have been utilized for the identification of the biological properties of oocyte-produced proteins will be described. This section will review the determination of properties such as binding of natural ligands, like heme or alpha-bungarotoxin; immunological recognition by antibodies; subcellular compartmentalization and/or secretion; various enzymatic catalytic activities; and induction in ovo of biological activities that affect other living cells in culture, such as those of interferon and of the T-cell receptor. The limitations involved in interpretation of results obtained using mRNA-injected oocytes will be critically reviewed. Special attention will be given to the effect of oocyte proteases and of changes in the endogenous translation rate on quantitative measurements of oocyte-produced proteins. In addition, the validity of the various measurement techniques will be evaluated. The various uses of bioassays of proteins produced in mRNA-injected Xenopus oocytes throughout the last decade will be reviewed. Nuclear and cytoplasmic injections, mRNA and protein turnover measurements and abundance calculations, and the use of in ovo bioassays for molecular cloning experiments will be discussed in this section. Finally, potential future uses of the oocyte system in various fields of research, such as immunology, neurobiology, and cell biology will be suggested.

Interspersed poly(A) RNAs of amphibian oocytes are not translatable

The poly(A) RNA of the Xenopus oocytes has been shown to include both single copy and interspersed transcripts. Interspersed maternal poly(A) RNAs contain repetitive sequence elements distributed within regions transcribed from single copy sequences. When renatured these RNAs form partially double-stranded RNA networks, and as shown earlier this can be utilized for preparative separation of interspersed maternal transcripts from maternal transcripts that remain single-stranded after renaturation (Anderson et al., 1982). The translational activity of these RNA fractions was tested in vitro, in wheat germ and reticulocyte systems. While the single-stranded fractions supported protein synthesis, the interspersed oocyte RNAs displayed little translational activity. Translational activity was measured in vivo by injection into the Xenopus oocyte. Oocytes previously injected with globin mRNA were injected with increasing amounts of single-stranded, double-stranded, or denatured double-stranded RNA fractions, and the amount of globin synthesis was determined. It was found that single-stranded RNA competes with globin mRNA for the limited translational apparatus of the oocyte, as manifested by a quantitative reduction of globin synthesis. However, globin synthesis was not affected when double-stranded RNA, either in renatured or denatured form, was injected. We conclude that the interspersed RNAs are not translated within the oocyte. The amount of single and double-stranded RNAs loaded onto polysomes in the injected oocytes was also determined. Sixty seven per cent of radio-iodinated single-stranded RNA pelleted with polysomes in injected oocytes, whereas less than 20% of similarly labeled double-stranded RNA pelleted with polysomes. This value is similar to that obtained when partially hydrolyzed RNA is injected, suggesting again that essentially none of the interspersed RNA is translated in vivo. The significance of these findings in relation to translational regulation during oogenesis and early development is discussed.

Internal pH of Xenopus oocytes: a study of the mechanism and role of pH changes during meiotic maturation

The internal pH (pHi) of Xenopus laevis oocytes, as measured by the DMO method, covered a broad range of values from 7.06 +/- 0.01 to 7.93 +/- 0.01, with a mean value of 7.43 +/- 0.03. The pHi measured by DMO and microelectrodes was nearly identical in control and maturing oocytes from the same batch. The oocytes from most females elevated their pHi in response to progesterone, reaching a maximum elevation of 0.30 +/- 0.03 pH units above control values at 100% germinal vesicle breakdown (GVBD). However, some females were found to contain oocytes that already had an elevated pHi of 7.71 +/- 0.03 which did not significantly increase during maturation. Human chorionic gonadotrophin (hcG)-stimulated females had oocytes with slightly higher control pHi values than oocytes from nonstimulated females but still showed the same elevation in response to progesterone. Thus, the "stimulated" state of oocyte physiology as induced by hcG did not account for the variation in control pHi and responsiveness to progesterone. Other aspects of this variability are discussed. Elevating or lowering the external pH is shown to elevate and lower pHi, respectively, in a stable and predictable manner. Using this approach to change pHi we have found no effect of changes in pHi on the rate of protein synthesis in control and maturing oocytes. Similarly, pHi had only a slight facilitating effect on the rate of GVBD. A pH indicator gel was used to demonstrate that the pHi increase during oocyte maturation involved an acid efflux. We conclude that an elevated pHi is not necessary for oocyte maturation, yet the mechanism of the pHi elevation is discussed as a possible lead to events that are necessary.

Effects of 12-O-tetradecanoylphorbol 13-acetate on the production of mRNAs for human tissue-type plasminogen activator

The mRNA for human (tissue type) plasminogen activator from a human melanoma cell line (Bowes) was investigated in different translation systems. After translation of poly(A)-rich RNA in Xenopus oocytes a biologically active plasminogen activator was obtained. Sodium dodecyl sulphate/polyacrylamide gel electrophoresis of the secreted translation products revealed a protein band precipitable with affinospecific antibody and migrating at the same position (apparent molecular mass of approximately 70000 Da) as the native melanoma cell product. Translation in rabbit reticulocyte lysate yielded an immunoprecipitable band migrating at position corresponding to a molecular mass of 52000 Da. Addition of 12-O-tetradecanoylphorbol 13-acetate to the cell cultures resulted in increased production of plasminogen activator. Concomittantly more poly(A)-rich RNA could be extracted from the cells and this RNA was more effectively translated by oocytes into biologically active plasminogen activator. Translation of poly(A)-rich RNA from phorbol-ester-treated cells in the reticulocyte lysate system yielded the 52000-Da protein also seen with RNA from untreated cells. However, in addition a prominent protein band of apparent molecular mass of 48000 Da was detectable. Its intensity increased with increasing doses of tetradecanoylphorbol acetate. This phorbol-ester-induced protein was not precipitable with the affinospecific antibody against plasminogen activator.

Translational competition by mRNA species encoding albumin, ferritin, haemopexin and globin

Messenger RNA from rat liver was translated in a micrococcal-nuclease-treated reticulocyte lysate supplemented with liver tRNA. Synthesis of the liver proteins haemopexin, ferritin and albumin was analyzed by quantitative immunoprecipitation. The relative translation yield of these proteins changed as a function of the amount of mRNA present during protein synthesis, revealing the existence of translational competition between individual species of mRNA from the liver. The results show that the mRNA species encoding haemopexin, ferritin and albumin possess distinctly different abilities to compete for one or more critical components in translation, with competitive strength increasing in this order. Although on a weight basis total liver mRNA is apparently as effective a template for protein synthesis as is globin mRNA, the latter displays a greater resistance to inhibition of its translation by KCl. In analogy with the translation properties of alpha-globin and beta-globin mRNA [Di Segni, G., Rosen, H. and Kaempfer, R. (1979) Biochemistry, 18, 2847-2854], this finding suggests that globin mRNA possesses greater competitive strength than does total liver mRNA. Increasing amounts of globin mRNA competitively inhibit the translation of albumin and ferritin mRNA present in total liver mRNA. The competition is relieved by the addition of eukaryotic initiation factor eIF-2. Translation of ferritin mRNA responds more vigorously to relief by eIF-2 than does translation of albumin mRNA, a finding consistent with the observation that albumin mRNA competes more effectively than ferritin mRNA in translation. The results support the assumption that albumin mRNA possesses a greater affinity for eIF-2 than does ferritin mRNA.

Synthesis and secretion of ovalbumin by mouse-growing oocytes following microinjection of chick ovalbumin mRNA

Mouse-growing oocytes were injected with chick ovalbumin mRNA. The oocytes were cultured for 18 h in the presence of [3H]leucine and the labeled ovalbumin was measured by immunoprecipitation. Two types of ovalbumin were precipitated by antibody to ovalbumin; one co-migrated with authentic, glycosylated ovalbumin in an 18% polyacrylamide gel and was estimated to be 45 000 D, whereas the other migrated faster with an apparent MW of 41 500 D. Both types of ovalbumin were also detected in the culture medium. This study demonstrates that mouse-growing oocytes can translate exogenous mRNA coding for a secreted protein and secrete two forms of the product.

Mobilization of maternal mRNA in amphibian eggs with special reference to the possible role of membraneous supramolecular structures

Current knowledge of the mechanism of mobilization of maternal mRNA is summarized herein and a working hypothesis has been constructed to explain the mechanism on the assumption that the mRNA enters the cytoplasm in association with the cytoplasmic membraneous structures and is then stored in the structures until liberation and relocation at the step of oocyte maturation. An extensive turnover of poly(A) sequences as well as the occurrence of repetitive sequences in the maternal mRNA may be relevant to mRNA activation.

Structural characterization of the proteins encoded by adenovirus early region 2A

Proteins encoded by adenovirus type 2 and type 5 early region 2A isolated from infected HeLa cells were compared to translation products of E2A-specific messenger RNA in a reticulocyte cell-free system and in Xenopus oocytes. The main cell-free translation product is a 72,000 Mr polypeptide which in HeLa cells as well as in Xenopus oocytes is converted into a 75,000 Mr phosphoprotein capable of binding to single-stranded DNA. Some minor proteins are proteolytic cleavage products of the major protein. In the cell-free system, three E2A polypeptides, 32,000, 37,000 and 44,000 Mr, are translated from minor polyadenylated mRNA species that can be separated from the major mRNA. Synthesis of all E2A polypeptides in vitro is inhibited by cap-analogs. The 44,000 Mr protein is also synthesized in Xenopus oocytes. Tryptic peptide maps of [35S]methionine-labeled E2A proteins were constructed using high pressure liquid chromatography and the position of the methionyl residues within each peptide was determined by amino acid sequencing procedures. This information and the DNA sequence of the adenovirus 5 E2A gene published by Kruijer et al. (1981) were used to align the peptides and to construct a map of the E2A proteins. Our data demonstrate that the major 75,000 Mr protein is coded for by a leftward reading frame of 529 amino acid residues located between 62 and 66 map units. The data also map six sites as targets for proteolytic enzymes. The minor E2A translation products have the same carboxy terminus as the major protein. The initiation codons of the 44,000, 37,000 and 32,000 Mr polypeptides probably correspond to amino acids 170, 243 or 244 and 290 of the major protein. Some functional properties of the major E2A protein are shared by the minor proteins and thus could be mapped. Major sites of phosphorylation, the region involved in binding to single-stranded DNA and the antigenic regions recognized by immune sera are located between amino acid residues 50 to 120, 170 to 470 and 170 to 240, respectively.

Stimulation of Xenopus oocyte protein synthesis by microinjected adenovirus RNA

The ability of injected heterologous mRNAs to compete with endogenous mRNAs in Xenopus oocytes was assayed. In confirmation of previous reports, globin mRNA translation in oocytes results in a concomitant decrease in endogenous protein synthesis. In contrast, injection of adenovirus 5 mRNA into the oocyte results in a stimulation of endogenous protein synthesis. The stimulation is dose dependent and does not require nuclear transcription in the oocyte. Preliminary mapping data suggest that the stimulatory RNA is a product of one of the viral immediate early genes.

Mouse oocytes transcribe injected Xenopus 5S RNA gene

Transcripts produced after injection of the Xenopus 5S RNA gene into oocyte germinal vesicles of mice migrate electrophoretically with the 5S RNA marker, an indication that the gene is transcribed and processed with considerable accuracy. Approximately two 5S RNA molecules are transcribed per gene per hour. This system may be useful in studying DNA processing and gene regulation by the mammalian ovum and might be modified to allow permanent incorporation of specific genes into mice.

Preferential translation of mRNAs in an mRNA-dependent reticulocyte lysate

Messenger RNA competition experiments were performed in an mRNA-dependent reticulocyte lysate using three kinds of mRNA: rabbit globin mRNA, calf eye lens mRNA and RNA of turnip yellow mosaic virus. Our results indicate that at supersaturating concentrations of mRNA preferential translation of certain mRNA species can be observed. Furthermore, the pattern of mRNA selection by the translational apparatus suggests that the rate of translation of different mRNA species is limited by different components of the reticulocyte lysate. Our observations n the cell-free system are strikingly different from our previously published mRNA competition experiments in Xenopus oocytes using the same preparations of lens and globin mRNA, in which no selective translation was observed [Asselbergs et al., Eur. J. Biochem. 94, 249-254 (1979)]. The restraints on mRNA translation in vitro are apparently different from those in vivo, i.e. in oocytes.

Kinetics of synthesis and processing of precursor polypeptides of murine leukemia virus in frog oocytes following microinjection of viral RNA

Microinjection of Rauscher murine leukemia viral RNA into living oocytes from Xenopus laevis, in contrast to cell-free systems, allowed detailed studies on the processing of newly synthesized viral precursor polypeptides. The viral messenger appeared to be stable for at least 5 days. Maximal rate of translation of 70-S virion RNA was observed 10 h after injection. The predominant translation products after a 1-h labeling period were three precursor polypeptides of Mr 77000, 75000 and 65000. Following longer labeling periods the most stable precursor polypeptide of Mr 65000 was most prominent. In addition, several intermediates of Mr 35000--60000 were observed. After about 24 h, mature viral core proteins appeared. The rate of synthesis of the 75000-Mr and 77000-Mr viral proteins decreased gradually after injection, suggesting that viral core polypeptides somehow regulated processing or synthesis of the group-specific antigen precursors. A heterogeneous group of 90000--95000-Mr polypeptides seemed to be post-translationally modified products of the 75000-Mr and 77000-Mr proteins. However, in this study no envelope-related polypeptides were synthesized, when viral RNA (70-S or 35-S) was injected into the cytoplasm or the nucleus of Xenopus oocytes.

Translation of globin messenger RNA by the mouse ovum

It has been demonstrated that the Xenopus oocyte can translate rabbit haemoglobin messenger RNA (mRNA) following microinjection of the message into the cell. The Xenopus oocyte has since been shown to be capable of translating a variety of messenger RNAs from different species. This system has proved useful in un-erstanding the mechanism of message translation and has also provided information about the translation capability of the Xenopus oocyte. Several other cell types, including HeLa cells and fibroblasts, can also translate exogenous message injected into the cell. However, there have been no reports of injection of mRNA into oocytes or fertilised one-cell ova of mammalian species. Nevertheless, the latter system could be of considerable use in studying the processing of exogenous messages in a mammalian system undergoing development, as well as providing insight into the way the early embryo processes injected messages and the protein products of such messages. We report here the results of injecting message into the fertilised one-cell mouse ovum and show that both mouse and rabbit globin mRNA are translated in this system.