Maturation of Xenopus laevis oocyte by progesterone requires poly(A) tail elongation of mRNA

Meiotic Cell Cycle Progression in Mouse Oocytes: Role of Cyclins

All eukaryotic cells, including oocytes, utilize an engine called cyclin-dependent kinase (Cdk) to drive the cell cycle. Cdks are activated by a co-factor called cyclin, which regulates their activity. The key Cdk-cyclin complex that regulates the oocyte cell cycle is known as Cdk1-cyclin B1. Recent studies have elucidated the roles of other cyclins, such as B2, B3, A2, and O, in oocyte cell cycle regulation. This review aims to discuss the recently discovered roles of various cyclins in mouse oocyte cell cycle regulation in accordance with the sequential progression of the cell cycle. In addition, this review addresses the translation and degradation of cyclins to modulate the activity of Cdks. Overall, the literature indicates that each cyclin performs unique and redundant functions at various stages of the cell cycle, while their expression and degradation are tightly regulated. Taken together, this review provides new insights into the regulatory role and function of cyclins in oocyte cell cycle progression.

A Systematic Review of the Biological Effects of Cordycepin

We conducted a systematic review of the literature on the effects of cordycepin on cell survival and proliferation, inflammation, signal transduction and animal models. A total of 1204 publications on cordycepin were found by the cut-off date of 1 February 2021. After application of the exclusion criteria, 791 papers remained. These were read and data on the chosen subjects were extracted. We found 192 papers on the effects of cordycepin on cell survival and proliferation and calculated a median inhibitory concentration (IC50) of 135 µM. Cordycepin consistently repressed cell migration (26 papers) and cellular inflammation (53 papers). Evaluation of 76 papers on signal transduction indicated consistently reduced PI3K/mTOR/AKT and ERK signalling and activation of AMPK. In contrast, the effects of cordycepin on the p38 and Jun kinases were variable, as were the effects on cell cycle arrest (53 papers), suggesting these are cell-specific responses. The examination of 150 animal studies indicated that purified cordycepin has many potential therapeutic effects, including the reduction of tumour growth (37 papers), repression of pain and inflammation (9 papers), protecting brain function (11 papers), improvement of respiratory and cardiac conditions (8 and 19 papers) and amelioration of metabolic disorders (8 papers). Nearly all these data are consistent with cordycepin mediating its therapeutic effects through activating AMPK, inhibiting PI3K/mTOR/AKT and repressing the inflammatory response. We conclude that cordycepin has excellent potential as a lead for drug development, especially for age-related diseases. In addition, we discuss the remaining issues around the mechanism of action, toxicity and biodistribution of cordycepin.

Translational Control of Xenopus Oocyte Meiosis: Toward the Genomic Era

The study of oocytes has made enormous contributions to the understanding of the G2/M transition. The complementarity of investigations carried out on various model organisms has led to the identification of the M-phase promoting factor (MPF) and to unravel the basis of cell cycle regulation. Thanks to the power of biochemical approaches offered by frog oocytes, this model has allowed to identify the core signaling components involved in the regulation of M-phase. A central emerging layer of regulation of cell division regards protein translation. Oocytes are a unique model to tackle this question as they accumulate large quantities of dormant mRNAs to be used during meiosis resumption and progression, as well as the cell divisions during early embryogenesis. Since these events occur in the absence of transcription, they require cascades of successive unmasking, translation, and discarding of these mRNAs, implying a fine regulation of the timing of specific translation. In the last years, the Xenopus genome has been sequenced and annotated, enabling the development of omics techniques in this model and starting its transition into the genomic era. This review has critically described how the different phases of meiosis are orchestrated by changes in gene expression. The physiological states of the oocyte have been described together with the molecular mechanisms that control the critical transitions during meiosis progression, highlighting the connection between translation control and meiosis dynamics.

Genome-wide analysis identifies cis-acting elements regulating mRNA polyadenylation and translation during vertebrate oocyte maturation

Most cells change patterns of gene expression through transcriptional regulation. In contrast, oocytes are transcriptionally silent and regulate mRNA poly(A) tail length to control protein production. However, the genome-wide relationship of poly(A) tail changes to mRNA translation during vertebrate oocyte maturation is not known. We used Tail-seq and polyribosome analysis to measure poly(A) tail and translational changes during oocyte maturation in Xenopus laevis We identified large-scale poly(A) and translational changes during oocyte maturation, with poly(A) tail length changes preceding translational changes. Proteins important for completion of the meiotic divisions and early development exhibited increased polyadenylation and translation during oocyte maturation. A family of U-rich sequence elements was enriched near the polyadenylation signal of polyadenylated and translationally activated mRNAs. We propose that changes in mRNA polyadenylation are a conserved mechanism regulating protein expression during vertebrate oocyte maturation and that these changes are controlled by a spatial code of cis-acting sequence elements.

The Translation of Cyclin B1 and B2 is Differentially Regulated during Mouse Oocyte Reentry into the Meiotic Cell Cycle

Control of protein turnover is critical for meiotic progression. Using RiboTag immunoprecipitation, RNA binding protein immunoprecipitation, and luciferase reporter assay, we investigated how rates of mRNA translation, protein synthesis and degradation contribute to the steady state level of Cyclin B1 and B2 in mouse oocytes. Ribosome loading onto Ccnb1 and Mos mRNAs increases during cell cycle reentry, well after germinal vesicle breakdown (GVBD). This is followed by the translation of reporters containing 3′ untranslated region of Mos or Ccnb1 and the accumulation of Mos and Cyclin B1 proteins. Conversely, ribosome loading onto Ccnb2 mRNA and Cyclin B2 protein level undergo minimal changes during meiotic reentry. Degradation rates of Cyclin B1 or B2 protein at the GV stage are comparable. The translational activation of Mos and Ccnb1, but not Ccnb2, mRNAs is dependent on the RNA binding protein CPEB1. Inhibition of Cdk1 activity, but not Aurora A kinase activity, prevents the translation of Mos or Ccnb1 reporters, suggesting that MPF is required for their translation in mouse oocytes. Conversely, Ccnb2 translation is insensitive to Cdk1 inhibition. Thus, the poised state that allows rapid meiotic reentry in mouse GV oocytes may be determined by the differential translational control of two Cyclins.

Drosha protein levels are translationally regulated during Xenopus oocyte maturation

MicroRNAs (miRNAs) are ∼21-nucleotide-long, single-stranded noncoding RNAs that regulate gene expression. Biogenesis of miRNAs is mediated by the two RNase III-like enzymes, Drosha and Dicer. Here we study miRNA biogenesis during maturation of Xenopus oocytes to eggs using microinjection of pri-miRNAs. We show that processing of exogenous and endogenous primary miRNAs (pri-miRNAs) is strongly enhanced upon maturation of oocytes to eggs. Overexpression of cloned Xenopus Drosha in oocytes, however, boosts pri-miRNA processing dramatically, indicating that Drosha is a rate-limiting factor in Xenopus oocytes. This developmental regulation of Drosha is controlled by poly(A) length addition to the Drosha mRNA, which boosts translation upon transition from oocytes to eggs. Processing of pri-miRNAs by Drosha and Dicer has been shown to be affected by adenosine-to-inosine deamination-type RNA editing. Using activated Xenopus eggs for microinjection experiments, we demonstrate that RNA editing can reduce pri-miRNA processing in vivo. This processing block is determined by the structural but not sequence changes introduced by RNA editing.

Developmental timing of mRNA translation--integration of distinct regulatory elements

Targeted mRNA translation is emerging as a critical mechanism to control gene expression during developmental processes. Exciting new findings have revealed a critical role for regulatory elements within the mRNA untranslated regions to direct the timing of mRNA translation. Regulatory elements can be targeted by sequence-specific binding proteins to direct either repression or activation of mRNA translation in response to developmental signals. As new regulatory elements continue to be identified it has become clear that targeted mRNAs can contain multiple regulatory elements, directing apparently contradictory translational patterns. How is this complex regulatory input integrated? In this review, we focus on a new challenge area-how sequence-specific RNA binding proteins respond to developmental signals and functionally integrate to regulate the extent and timing of target mRNA translation. We discuss current understanding with a particular emphasis on the control of cell cycle progression that is mediated through a complex interplay of distinct mRNA regulatory elements during Xenopus oocyte maturation.

Cytoplasmic polyadenylation and cytoplasmic polyadenylation element-dependent mRNA regulation are involved in Xenopus retinal axon development

BACKGROUND

Translation in axons is required for growth cone chemotropic responses to many guidance cues. Although locally synthesized proteins are beginning to be identified, how specific mRNAs are selected for translation remains unclear. Control of poly(A) tail length by cytoplasmic polyadenylation element (CPE) binding protein 1 (CPEB1) is a conserved mechanism for mRNA-specific translational regulation that could be involved in regulating translation in axons.

RESULTS

We show that cytoplasmic polyadenylation is required in Xenopus retinal ganglion cell (RGC) growth cones for translation-dependent, but not translation-independent, chemotropic responses in vitro, and that inhibition of CPE binding through dominant-negative interference severely reduces axon outgrowth in vivo. CPEB1 mRNA transcripts are present at low levels in RGCs but, surprisingly, CPEB1 protein was not detected in eye or brain tissue, and CPEB1 loss-of-function does not affect chemotropic responses or pathfinding in vivo. UV cross-linking experiments suggest that CPE-binding proteins other than CPEB1 in the retina regulate retinal axon development.

CONCLUSION

These results indicate that cytoplasmic polyadenylation and CPE-mediated translational regulation are involved in retinal axon development, but that CPEB1 may not be the key regulator of polyadenylation in the developing retina.

Effects of polyadenylation inhibition on meiosis progression in relation to the polyadenylation status of cyclins A2 and B1 during in vitro maturation of bovine oocytes

The control of protein synthesis during maturation in oocytes is mainly exerted through cytoplasmic polyadenylation of stored mRNAs. We first analyzed the polyadenylation status of cyclins A2 and B1 during in vitro maturation (IVM) of bovine oocytes, using Rapid Amplification of cDNA Ends-Polyadenylation Technique (RACE-PAT). An inconstant elongation of the poly(A) tail was observed for cyclin A2 transcripts after maturation, while a constant lengthening was observed for cyclin B1, occurring during the first 12 hr of incubation. We then evaluated the effects of the polyadenylation inhibitor 3'-deoxyadenosine (3'-dA), on polyadenylation and nuclear maturation. The presence of 0.02 mM 3'-dA during the whole incubation period or from 6 hr after its beginning completely prevented meiosis progression in 100% of the oocytes. Polyadenylation of cyclin B1 was also completely prevented when 3'-dA was added at 0 hr, and greatly reduced when added at 6 hr. When 3'-dA was added at 12 hr, around metaphase I (MI), 46.9% of the oocytes have reached metaphase II (MII, vs. 78.8% in the control group) at 24 hr. The use of the same concentration of 3'-deoxyguanosine (3'-dG), that impairs transcription but not polyadenylation, did not affect cyclins polyadenylation, nor nuclear maturation, whatever was the timing of addition. These results suggest that the polyadenylation of cyclin B1 could be related to the first peak of activity of MPF, occurring around MI (10-12 hr after the onset of the maturation period). They also show that, in our culture conditions, inhibition of polyadenylation prevents meiosis progression, especially up to the MI stage, while inhibition of transcription does not.

Large T antigen on the simian virus 40 origin of replication: a 3D snapshot prior to DNA replication

Large T antigen is the replicative helicase of simian virus 40. Its specific binding to the origin of replication and oligomerization into a double hexamer distorts and unwinds dsDNA. In viral replication, T antigen acts as a functional homolog of the eukaryotic minichromosome maintenance factor MCM. T antigen is also an oncoprotein involved in transformation through interaction with p53 and pRb. We obtained the three-dimensional structure of the full-length T antigen double hexamer assembled at its origin of replication by cryoelectron microscopy and single-particle reconstruction techniques. The double hexamer shows different degrees of bending along the DNA axis. The two hexamers are differentiated entities rotated relative to each other. Isolated strands of density, putatively assigned to ssDNA, protrude from the hexamer-hexamer junction mainly at two opposite sites. The structure of the T antigen at the origin of replication can be understood as a snapshot of the dynamic events leading to DNA unwinding. Based on these results a model for the initiation of simian virus 40 DNA replication is proposed.

Gene transcription and regulation of oocyte maturation

The developmental potential of an embryo is dependent on the developmental potential of the oocyte from which it originates. The process of oocyte maturation is critical for the efficient application of biotechnologies such as in vitro embryo production and mammalian cloning. However, the overall efficiency of in vitro maturation remains low because oocytes matured in vitro have a lower developmental competence than oocytes matured in vivo. Furthermore, oocytes that have been exposed to gonadotropins have greater developmental competence than oocytes matured in the absence of gonadotropins. By understanding the molecular mechanisms underlying gonadotropin-induced maturation, improvement in oocyte maturation technologies may be expected as procedures to manipulate specific factors involved in signalling for resumption of meiosis are identified. The present review will focus on transcriptional mechanisms underlying the maturation of mammalian oocytes in vitro, as well as on the acquisition of oocyte developmental competence. In addition, a working model for the transcriptional control of mammalian oocyte maturation is proposed.

In vitro maturation of bovine oocytes requires polyadenylation of mRNAs coding proteins for chromatin condensation, spindle assembly, MPF and MAP kinase activation

In the oocyte mRNA molecules are stored in order to be used during the maturation process when transcription is silenced. Translational activation of stored mRNA templates commonly is correlated with their cytoplasmic polyadenylation. In the present study, the effects of cordycepin, a potent polyadenylation inhibitor, on the in vitro maturation MPF and MAP kinase activation of bovine cumulus oocyte complexes (COCs) were examined. The presence of cordycepin (5 microg/ml) during the whole culture period (24 h) prevents chromatin condensation and germinal vesicle breakdown (GVBD) as well as MPF and MAP kinase activation. When COCs were first cultivated in inhibitor-free medium for 6 or 9 h and subsequently transferred to cordycepin supplemented medium for further 18 or 15 h neither MPF nor MAP kinase became activated and 86 and 85%, respectively, of the oocytes underwent GVBD but failed to form a spindle and hypercondensed their chromatin. Extending the first culture period in inhibitor-free medium to 12 or 15 h before transferring the COCs to cordycepin supplemented medium for a further 12 or 9 h allowed 48 and 79%, respectively of oocytes to reach the metaphase 2 (M 2) stage. From these data, it is concluded that mRNA molecules coding for proteins required for chromatin condensation and GVBD become polyadenylated during the first 6 h following the onset of culture whereas mRNA molecules coding for proteins required for spindle assembly of metaphase 1 (M 1) and MPF and MAP kinase activation become polyadenylated between 9 and 12 h following initiation of culture.

Posttranscriptional regulation of cyclin A1 and cyclin A2 during mouse oocyte meiotic maturation and preimplantation development

A shift from a meiotic cell cycle to a mitotic cell cycle occurs following fertilization. The molecular basis for this transition, however, is poorly understood. Although cyclin A1 is proposed to regulate M phase in the meiotic cell cycle, and cyclin A2 is proposed to regulate S and M phases in the mitotic cell cycle, little is known about changes in the expression levels of cyclin A1 and A2 during meiotic and mitotic cell cycles in mammalian oocytes. We report that the mRNA levels of both cyclins A1 and A2 decrease during oocyte maturation. The amount of cyclin A1 mRNA then increases between the one-cell and blastocyst stages, whereas that of cyclin A2 remains relatively constant. The amount of cyclin A1 protein declines during maturation and is not readily detected from the two-cell to the blastocyst stage. In contrast, cyclin A2 is not readily detected in the oocyte and metaphase II-arrested egg but is detected following fertilization and throughout the subsequent stages of preimplantation development. The appearance of cyclin A2 protein following fertilization positively correlates with an increase in the size of the mRNA. This increase, as well as the increase in the amount of cyclin A2 protein, is prevented by 3'-deoxyadenosine (3'-dA), an inhibitor of polyadenylation. Consistent with a role for cyclin A2 in regulating the G1/S transition, 3'-dA also inhibits DNA replication in treated one-cell embryos. These results suggest that regulation of expression of cyclins A1 and A2 is under posttranscriptional regulation and that the observed changes in their expression may be involved in the transformation of a meiotic cell cycle to a mitotic cell cycle following fertilization.

Regulation of the mRNAs encoding proteins of the BMP signaling pathway during the maternal stages of Xenopus development

Activation of the Xenopus bone morphogenetic protein (BMP) pathway is coincident with the onset of zygotic transcription but requires maternal signaling proteins. The mechanisms controlling the translation of mRNAs that encode proteins of the BMP pathway were investigated by using polysome association as an assay for translational activity. Our results indicate that five different mRNAs encoding proteins of the BMP pathway were translationally regulated during Xenopus development. These mRNAs were either not associated or inefficiently associated with polysomes in oocytes, and each was recruited to polysomes at a different developmental stage. The Smad1 and ALK-2 mRNAs were recruited to polysomes during oocyte maturation, whereas the BMP-7 and XSTK9 mRNAs were recruited during the early stages of embryogenesis. The ALK-3 mRNA was not efficiently associated with polysomes during any maternal stage of development and was efficiently recruited to polysomes only after the onset of zygotic transcription. In general, for all stages except oocytes, polysome recruitment was associated with the presence of a 3' poly(A) tail. However, there was not an obvious correlation between the absolute length of poly(A) and the efficiency of polysome recruitment, indicating that the relationship between poly(A) tail length and translation during early frog embryogenesis is complex. We further focused on the BMP-7 mRNA and demonstrated that sequence elements within the 3'UTR were necessary for recruitment of the BMP-7 mRNA to polysomes and sufficient to direct the addition of poly(A) and activate translation of a reporter during embryogenesis. Interestingly, the BMP-7 mRNA lacks the previously defined eCPE sequences proposed to direct poly(A) addition and translational activation during embryogenesis. The implications of our findings for translational regulation of maternal mRNAs during embryogenesis and for the activation of the BMP pathway are discussed.

Translational recruitment of Xenopus maternal mRNAs in response to poly(A) elongation requires initiation factor eIF4G-1

Xenopus oocytes accumulate maternal mRNAs which are then recruited to ribosomes during meiotic cell cycle progression in response to progesterone and coincident with poly(A) elongation. Prior to stimulation, most protein synthesis ( approximately 70%) does not require intact translation factor eIF4G (B. D. Keiper and R. E. Rhoads, 1997, Nucleic Acids Res. 25, 395-402). In the present study we have addressed the requirement of eIF4G in the recruitment of mRNAs during meiosis. Cleavage of eIF4G by coxsackievirus protease 2A inhibited progesterone-induced meiotic progression in 88% of the oocytes; prevented the recruitment of maternal mRNAs encoding cyclin B1, c-Mos, D7, and B9; and disrupted the association of eIF4G with poly(A)-binding protein. Poly(A) elongation, however, was not inhibited by eIF4G cleavage. Injection of MPF restored meiotic cell cycle progression to >60% of the oocytes but not the recruitment of cyclin B1 or B9 mRNA. Previously recruited maternal mRNAs were removed from polyribosomes following subsequent cleavage of eIF4G, indicating that eIF4G is required both to recruit and also to maintain maternal mRNAs on polyribosomes. The expression of a cleavage-resistant variant of human eIF4G-1 (G486E) significantly restored the ability to synthesize c-Mos in response to progesterone and to translate exogenous beta-globin mRNA, indicating that the inhibition by protease 2A is due to cleavage of eIF4G alone. These results indicate that intact eIF4G is required for the poly(A)-dependent recruitment of several maternal mRNAs (cyclin B1, c-Mos, D7, and B9) during meiotic cell cycle progression but not for the synthesis of most proteins.

Meiotic maturation in Xenopus requires polyadenylation of multiple mRNAs

Cytoplasmic polyadenylation of specific mRNAs commonly is correlated with their translational activation during development. Here, we focus on links between cytoplasmic polyadenylation, translational activation and the control of meiotic maturation in Xenopus oocytes. We manipulate endogenous c-mos mRNA, which encodes a protein kinase that regulates meiotic maturation. We determined that translational activation of endogenous c-mos mRNA requires a long poly(A) tail per se, rather than the process of polyadenylation. For this, we injected 'prosthetic' poly(A)_synthetic poly(A) tails designed to attach by base pairing to endogenous c-mos mRNA that has had its own polyadenylation signals removed. This prosthetic poly(A) tail activates c-mos translation and restores meiotic maturation in response to progesterone. Thus the role of polyadenylation in activating c-mos mRNA differs from its role in activating certain other mRNAs, for which the act of polyadenylation is required. In the absence of progesterone, prosthetic poly(A) does not stimulate c-mos expression, implying that progesterone acts at additional steps to elevate c-Mos protein. By using a general inhibitor of polyadenylation together with prosthetic poly(A), we demonstrate that these additional steps include polyadenylation of at least one other mRNA, in addition to that of c-mos mRNA. These other mRNAs, encoding regulators of meiotic maturation, act upstream of c-Mos in the meiotic maturation pathway.

The deadenylation conferred by the 3' untranslated region of a developmentally controlled mRNA in Xenopus embryos is switched to polyadenylation by deletion of a short sequence element

The maternal Xenopus Eg mRNAs are adenylated and translated in the mature oocyte and then, after fertilization, are deadenylated and released from polysomes. Therefore, after fertilization, a change occurs in the cellular mechanisms that control mRNA adenylation. In the study reported here, we show that the 3' untranslated region of Eg2 mRNA contains a cis-acting element that is required for the deadenylation of chimeric RNAs after fertilization. This cis-acting element is contained within a single 17-nucleotide portion of the Eg2 mRNA. Disruption of this deadenylation element allows adenylation of the chimeric transcripts in the embryo. Therefore, this cis-acting element is part of the sequence information required for the developmental switch from adenylation to deadenylation of the maternal Eg2 mRNA in Xenopus embryos.

The deadenylation conferred by the 3' untranslated region of a developmentally controlled mRNA in Xenopus embryos is switched to polyadenylation by deletion of a short sequence element

The maternal Xenopus Eg mRNAs are adenylated and translated in the mature oocyte and then, after fertilization, are deadenylated and released from polysomes. Therefore, after fertilization, a change occurs in the cellular mechanisms that control mRNA adenylation. In the study reported here, we show that the 3' untranslated region of Eg2 mRNA contains a cis-acting element that is required for the deadenylation of chimeric RNAs after fertilization. This cis-acting element is contained within a single 17-nucleotide portion of the Eg2 mRNA. Disruption of this deadenylation element allows adenylation of the chimeric transcripts in the embryo. Therefore, this cis-acting element is part of the sequence information required for the developmental switch from adenylation to deadenylation of the maternal Eg2 mRNA in Xenopus embryos.