Biology subjects

Daldello, E. M.

Publications and source records attributed to Daldello, E. M..

9 recordsLinked to original sources

bioRxiv2026

Cyclin B phosphorylation by PKA safeguards the G2-arrest in vertebrate oocytes

Entry into meiotic M-phase in vertebrate oocytes requires activation of Cdk1-Cyclin B, which is restrained by the cAMP-PKA signalling during prophase arrest. Although Cdc25C and ARPP19 are established PKA substrates in this context, they do not fully explain how PKA represses Cdk1 activation. Here we used Turbo-ID approach using catalytic PKA (PKAc) as the bait to identify substrates in Xenopus laevis oocytes. Unexpectedly, this screen identified Cyclin B2, the regulatory subunit of Cdk1, as a PKA-proximal protein. Although Cyclin B2 does not stably associate with PKAc, it harbors a conserved PKA motif around Serine 271 within Cyclin Box 2. We show that PKAc phosphorylates Cyclin B2 at S271 both in vivo and in vitro. Functionally, a phosphomimic Cyclin B2 mutant at this site fails to induce meiotic maturation and Cdk1 activation. S271 phosphorylation neither alters Cyclin B2 stability nor its binding to Cdk1 in vivo. S271 phosphorylation does not impair Cdk1 activity toward a single-site substrate, such as PP1, but slows Cyclin-B-dependent multisite phosphorylation of Cdc25C, a key regulator of the Cdk1 activation network. These findings identify Cyclin B as a direct and conserved PKA target, revealing the mechanistic link between PKA activity and Cdk1-Cyclin B repression that maintains oocyte prophase arrest.

cell biology↗

bioRxiv2024

Decoding protein phosphorylation during oocyte meiotic divisions using phosphoproteomics

Oocyte meiotic divisions represent a critical process in sexual reproduction, as a diploid non-dividing oocyte is transformed into a haploid fertilizable egg, as a prelude for the subsequent embryonic divisions and differentiation. Although cell differentiation and proliferation are governed by transcription, oocyte maturation and early embryonic divisions depend entirely on changes in protein abundance and post- translational modifications. Here, we analyze the abundance and phosphorylation of proteins during Xenopus oocyte meiotic maturation. We reveal significant shifts in protein stability, related to spindle assembly, DNA replication and RNA-binding. Our analysis pinpoints broad changes in phosphorylation correlating with key cytological meiotic milestones, noteworthy changes in membrane trafficking, nuclear envelope disassembly and modifications in microtubule dynamics. Additionally, specific phosphorylation events target regulators of protein translation, Cdk1 and the Mos/MAPK pathway, thereby providing insight into the dynamics of Cdk1 activity, as related to the meiotic cell cycle. This study sheds light on the orchestration of protein dynamics and phosphorylation events during oocyte meiotic divisions, providing a rich resource for understanding the molecular pathways orchestrating meiotic progression in the frog, and most likely applicable to other vertebrate species.

developmental biology↗

bioRxiv2024

Multiple intersecting pathways are involved in the phosphorylation of CPEB1 to activate translation during mouse oocyte meiosis

The RNA-binding protein cytoplasmic polyadenylation element binding 1 (CPEB1) plays a fundamental role in the regulation of mRNA translation in oocytes. However, the nature of protein kinase cascades modulating the activity of CPEB1 is still a matter of controversy. Using genetic and pharmacological tools and detailed time courses, here we have reevaluated the relationship between CPEB1 phosphorylation and the activation of translation during mouse oocyte maturation. We show that both the CDK1/MAPK and AURKA/PLK1 pathways converge on the phosphorylation of CPEB1 during prometaphase. Only inactivation of the CDK1/MAPK pathway disrupts translation, while inactivation of either pathway leads to CPEB1 stabilization. However, stabilization of CPEB1 induced by inactivation of the AURKA/PLK1 does not affect translation, indicating that destabilization/degradation can be dissociated from translational activation. The accumulation of the endogenous CCNB1 protein closely recapitulates the translation data. These findings support the overarching hypothesis that the activation of translation in prometaphase in mouse oocytes relies on a CDK1-dependent CPEB1 phosphorylation, and this translational activation precedes CPEB1 destabilization.

developmental biology↗

bioRxiv2023

Regulation of Oocyte Meiotic Maturation: Unraveling the Interplay between PKA Inhibition and Cdk1 Activation

Oocyte meiosis is arrested at the first prophase stage, until hormonal stimulation triggers progression into the meiotic divisions. This process, called meiotic maturation, depends on extensive post-transcriptional events. In all vertebrates, two bottleneck events orchestrate meiosis resumption: first, the inhibition of PKA and, second, the activation of Cdk1, the master regulator of eukaryotic cell division. However, the molecular events occurring between these two steps are almost unknown. To address this issue, we took advantage of a Cdk1 inhibitor to identify the early events that depend on PKA downregulation and occur independently of Cdk1 activity. Unexpectedly, we show that accumulation of Cyclin B1 and Mos, the kinase responsible for MAPK activation in oocytes, are regulated in an opposing manner by a two-step mechanism. PKA downregulation induces first the accumulation of Cyclin B1 without any increase of its translation, independently of Cdk1 activation. Subsequently, the rate of Cyclin B1 translation increases in response to Cdk1 activation. In contrast, Mos translation begins downstream PKA inhibition, but the protein does not accumulate until Cdk1 is activated. These intertwined regulations create the positive feedback loops required for the full activation of Cdk1. Additionally, we show that two consecutive waves of translation occur during the G2-M transition, the first induced by PKA inhibition and the second by Cdk1 activation. Finally, we demonstrate that Arpp19, the only known early substrate of PKA in Xenopus oocytes, is not involved in the control of these early events. This study reveals that PKA downregulation promotes multiple molecular pathways that converge on the activation of Cdk1 to induce the G2/M transition in vertebrate oocytes.

cell biology↗

bioRxiv2023

Cross-species analysis of ARPP19 phosphorylation during oocyte meiotic maturation charts the emergence of a new cAMP-dependent role in vertebrates

In many animal species, elevated cAMP-PKA signaling initiates oocyte meiotic maturation upon hormonal stimulation, whereas in vertebrates, it acts as a negative regulator of this process. To address this "cAMP paradox", we have focused on ARPP19 proteins. Dephosphorylation of Xenopus ARPP19 on a specific PKA site has been identified as a key step in initiating oocyte maturation. We first tracked evolution of the ARPP19 PKA phosphorylation site, revealing that it appeared early during the emergence of metazoans. This contrasts with strong conservation across eukaryotes of a phosphorylation site for the kinase Gwl in ARPP19 proteins, able to transform them into potent PP2A-B55 inhibitors and thus promote M-phase entry. We then compared the phosphorylation and function of Xenopus ARPP19 with its orthologue from the jellyfish Clytia, a model species showing cAMP-induced oocyte maturation. We confirmed that Clytia ARPP19 is phosphorylated on the conserved Gwl site in vitro as well as in maturing Xenopus and Clytia oocytes, behaving as a PP2A inhibitor and contributing to Cdk1 activation. However, Gwl-phosphorylated ARPP19 was unable to initiate oocyte maturation in Clytia, suggesting the presence of additional locks released by hormonal stimulation. Clytia ARPP19 was in vitro phosphorylated by PKA uniquely on the predicted site, but it was a much poorer substrate of PKA and of its antagonizing phosphatase, PP2A-B55{delta}, than the Xenopus protein. Correspondingly, PKA-phosphomimetic Clytia ARPP19 had a much weaker inhibitory activity on meiosis resumption in Xenopus oocytes than its Xenopus counterpart. Hence, poor recognition of Clytia ARPP19 by PKA and the absence of its targets in Clytia oocytes account for the cAMP paradox. This cross-species study of ARPP19 illustrates how initiation of oocyte maturation has complexified during animal evolution, and provides further insight into its biochemical regulation.

evolutionary biology↗

bioRxiv2019

The M-phase regulatory phosphatase PP2A-B55δ opposes protein kinase A on Arpp19 to initiate meiotic division

Oocytes are held in meiotic prophase for prolonged periods until hormonal signals trigger meiotic divisions. Key players of M-phase entry are the opposing Cdk1 kinase and PP2A-B55{delta} phosphatase. In Xenopus, the protein Arpp19, phosphorylated at serine 67 by Greatwall, plays an essential role in inhibiting PP2A-B55{delta}, promoting Cdk1 activation. Furthermore Arpp19 has an earlier role in maintaining the prophase arrest through a second serine (S109) phosphorylated by PKA. Prophase release, induced by progesterone, relies on Arpp19 dephosphorylation at S109, owing to an unknown phosphatase. Here we identified this phosphatase as PP2A-B55{delta}. In prophase, PKA and PP2A-B55{delta} are simultaneously active, suggesting the presence of other important targets for both enzymes. The drop in PKA activity induced by progesterone decreases S109 phosphorylation, unlocking the prophase block. Hence, PP2A-B55{delta} acts critically on Arpp19 on two distinct sites, opposing PKA and Greatwall to orchestrate the prophase release and M-phase entry.

cell biology↗

bioRxiv2019

Genome-wide analysis reveals a switch in the translational program upon oocyte meiotic resumption

During oocyte maturation, changes in gene expression depend exclusively on translation and degradation of maternal mRNAs rather than transcription. Execution of this translation program is essential for assembling the molecular machinery required for meiotic progression, fertilization, and embryo development. With the present study, we used a RiboTag/RNA-Seq approach to explore the timing of maternal mRNA translation in quiescent oocytes as well as in oocytes progressing through the first meiotic division. This genome-wide analysis reveals a global switch in maternal mRNA translation coinciding with oocyte re-entry into the meiotic cell cycle. Messenger RNAs whose translation is highly active in quiescent oocytes invariably become repressed during meiotic re-entry, whereas transcripts repressed in quiescent oocytes become activated. Experimentally, we have defined the exact timing of the switch, the repressive function of CPE elements, and identified a novel role for CPEB1 in maintaining constitutive translation of a large group of maternal mRNAs during maturation.

developmental biology↗

bioRxiv2019

The RNA binding protein DAZL functions as repressor and activator of maternal mRNA translation during oocyte maturation

Deleted in azoospermia like (DAZL) is an RNA-binding protein playing critical function during gamete development. In fully-grown oocytes, DAZL protein is detected in prophase and levels increase four to five fold during reentry into the meiotic cell cycle. Here, we have investigated the functional significance of this DAZL accumulation in maturing oocytes. Oocyte depletion of DAZL prevents progression to MII. This maturation block is associated with widespread disruption in the pattern of maternal transcripts loading on ribosomes and their translation measured using a RiboTag IP/RNASeq or qPCR strategy. In addition to decreased ribosome loading of a subset of transcripts, we found that DAZL depletion causes also translational activation of distinct subset of mRNAs. DAZL binds to mRNAs whose translation is both repressed and activated during oocyte maturation. Unexpectedly, DAZL depletion also causes increased ribosome loading of a subset of mRNAs in quiescent GV-arrested oocytes. This dual role of repression and activation is recapitulated by using YFP reporters including the 3UTR of DAZL targets. Injection of recombinant DAZL protein in DAZL-depleted oocytes rescues the translation of these targets as well as maturation to MII. Mutagenesis of putative DAZL-binding sites in these candidate mRNAs mimics the effect of DAZL depletion. These findings demonstrate that DAZL regulates translation of maternal mRNAs in mature oocytes, functioning both as translational repressor and activator.

developmental biology↗

bioRxiv2018

Cyclin B2 is required for progression through meiosis in mouse oocytes

Cyclins associate with CDK1 to generate the M-phase-promoting factor (MPF) essential for progression through mitosis and meiosis. Previous studies concluded that CCNB2 is dispensable for cell cycle progression. Given our findings of high translation rates of CcnB2 mRNA in prophase-arrested oocytes, we have reevaluated its role during meiosis. CcnB2-/- oocytes undergo delayed germinal vesicle breakdown followed by a defective M-phase due to reduced pre-MPF activity. This disrupted maturation is associated with compromised CcnB1 and Mos mRNA translation and delayed spindle assembly. Given these defects, a significant population of oocytes fail to complete meiosis I because SAC remains activated and APC function is inhibited. In vivo, CCNB2 depletion leads to decreased oocyte developmental competence, compromised fecundity, and premature ovarian failure. These findings demonstrate that CCNB2 is required to assemble sufficient pre-MPF for timely meiosis reentry and progression. Although endogenous cyclins cannot compensate, overexpression of CCNB1 rescues the meiotic phenotypes, demonstrating similar molecular properties but divergent modes of regulation of these cyclins.

developmental biology↗