Working in close quarters: biparental meiosis in the oocyte.

In vitro fertilization (IVF) methods involve fertilizing haploid oocytes arrested in meiosis II with haploid sperm. An experimental IVF method had been developed in mice involving injection of diploid sperm nuclei into equally diploid oocytes (biparental meiosis) to increase the chance of reproduction in cases where haploid sperm cannot be obtained. However, this method had been shown to be highly error prone. In this issue of EMBO Reports, Ogonuki et al show that reducing ooplasm volume by half reduces the segregation errors and increases the likelihood of producing viable offsprings in mice (Ogonuki et al, 2022).

Enhanced identification of endocrine disruptors through integration of science-based regulatory practices and innovative methodologies: The MERLON Project.

The prevalence of hormone-related health issues caused by exposure to endocrine disrupting chemicals (EDCs) is a significant, and increasing, societal challenge. Declining fertility rates together with rising incidence rates of reproductive disorders and other endocrine-related diseases underscores the urgency in taking more action. Addressing the growing threat of EDCs in our environment demands robust and reliable test methods to assess a broad variety of endpoints relevant for endocrine disruption. EDCs also require effective regulatory frameworks, especially as the current move towards greater reliance on non-animal methods in chemical testing puts to test the current paradigm for EDC identification, which requires that an adverse effect is observed in an intact organism. Although great advances have been made in the field of predictive toxicology, disruption to the endocrine system and subsequent adverse health effects may prove particularly difficult to predict without traditional animal models. The MERLON project seeks to expedite progress by integrating multispecies molecular research, new approach methodologies (NAMs), human clinical epidemiology, and systems biology to furnish mechanistic insights and explore ways forward for NAM-based identification of EDCs. The focus is on sexual development and function, from foetal sex differentiation of the reproductive system through mini-puberty and puberty to sexual maturity. The project aims are geared towards closing existing knowledge gaps in understanding the effects of EDCs on human health to ultimately support effective regulation of EDCs in the European Union and beyond.

Cyclin B3 implements timely vertebrate oocyte arrest for fertilization.

To ensure successful offspring ploidy, vertebrate oocytes must halt the cell cycle in meiosis II until sperm entry. Emi2 is essential to keep oocytes arrested until fertilization. However, how this arrest is implemented exclusively in meiosis II and not prematurely in meiosis I has until now remained enigmatic. Using mouse and frog oocytes, we show here that cyclin B3, an understudied B-type cyclin, is essential to keep Emi2 levels low in meiosis I. Direct phosphorylation of Emi2 at an evolutionarily highly conserved site by Cdk1/cyclin B3 targets Emi2 for degradation. In contrast, Cdk1/cyclin B1 is inefficient in Emi2 phosphorylation, and this provides a molecular explanation for the requirement of different B-type cyclins for oocyte maturation. Cyclin B3 degradation at exit from meiosis I enables Emi2 accumulation and thus timely arrest in meiosis II. Our findings illuminate the evolutionarily conserved mechanisms that control oocyte arrest for fertilization at the correct cell-cycle stage, which is essential for embryo viability.

Cycling through mammalian meiosis: B-type cyclins in oocytes.

B-type cyclins in association with Cdk1 mediate key steps of mitosis and meiosis, by phosphorylating a plethora of substrates. Progression through the meiotic cell cycle requires the execution of two cell divisions named meiosis I and II without intervening S-phase, to obtain haploid gametes. These two divisions are highly asymmetric in the large oocyte. Chromosome segregation in meiosis I and sister chromatid segregation in meiosis II requires the sharp, switch-like inactivation of Cdk1 activity, which is brought about by degradation of B-type cyclins and counteracting phosphatases. Importantly and contrary to mitosis, inactivation of Cdk1 must not allow S-phase to take place at exit from meiosis I. Here, we describe recent studies on the regulation of translation and degradation of B-type cyclins in mouse oocytes, and how far their roles are redundant or specific, with a special focus on the recently discovered oocyte-specific role of cyclin B3.

Cyclin B3 promotes anaphase I onset in oocyte meiosis.

Meiosis poses unique challenges because two rounds of chromosome segregation must be executed without intervening DNA replication. Mammalian cells express numerous temporally regulated cyclins, but how these proteins collaborate to control meiosis remains poorly understood. Here, we show that female mice genetically ablated for cyclin B3 are viable-indicating that the protein is dispensable for mitotic divisions-but are sterile. Mutant oocytes appear normal until metaphase I but then display a highly penetrant failure to transition to anaphase I. They arrest with hallmarks of defective anaphase-promoting complex/cyclosome (APC/C) activation, including no separase activity, high CDK1 activity, and high cyclin B1 and securin levels. Partial APC/C activation occurs, however, as exogenously expressed APC/C substrates can be degraded. Cyclin B3 forms active kinase complexes with CDK1, and meiotic progression requires cyclin B3-associated kinase activity. Cyclin B3 homologues from frog, zebrafish, and fruit fly rescue meiotic progression in cyclin B3-deficient mouse oocytes, indicating conservation of the biochemical properties and possibly cellular functions of this germline-critical cyclin.

Detection of Separase Activity Using a Cleavage Sensor in Live Mouse Oocytes.

Separase proteolytically removes cohesin complexes from sister chromatid arms in meiosis I, which is essential for chromosome segregation. Regulation of separase activity is essential for proper cell cycle progression and correct chromosome segregation. Onset of endogenous separase activity has not yet been observed in live oocytes.We describe here a method for detecting separase activity in mouse oocytes in vivo. This method utilizes a previously described cleavage sensor made up of H2B-mCherry fused with Scc1(107-268 aa)-YFP. The cleavage sensor is loaded on the chromosomes through its H2B-tag, and the signal from both mCherry and YFP is visible. Upon separase activation the Scc1 fragment is cleaved and YFP dissociates from the chromosomes. The change in the ratio between mCherry and YFP fluorescence intensity is a readout of separase activity.