Cell-cycle control during meiotic maturation

A conserved germline-specific Dsn1 alternative splice isoform supports oocyte and embryo development

The chromosome segregation and cell division programs associated with somatic mitosis and germline meiosis display dramatic differences such as kinetochore orientation, cohesin removal, or the presence of a gap phase.1,2,3,4,5,6 These changes in chromosome segregation require alterations to the established cell division machinery.5,6 It remains unclear what aspects of kinetochore function and its regulatory control differ between the mitotic and meiotic cell divisions to rewire these core processes. Alternative RNA splicing can generate distinct protein isoforms to allow for the differential control of cell processes across cell types. However, alternative splice isoforms that differentially modulate distinct cell division programs have remained elusive. Here, we demonstrate that mammalian germ cells express an alternative mRNA splice isoform for the kinetochore component, DSN1, a subunit of the MIS12 complex that links the centromeres to spindle microtubules during chromosome segregation. This germline DSN1 isoform bypasses the requirement for Aurora kinase phosphorylation for its centromere localization due to the absence of a key regulatory region allowing DSN1 to display persistent centromere localization. Expression of the germline DSN1 isoform in somatic cells results in constitutive kinetochore localization, chromosome segregation errors, and growth defects, providing an explanation for its tight cell-type-specific expression. Reciprocally, precisely eliminating expression of the germline-specific DSN1 splice isoform in mouse models disrupts oocyte maturation and early embryonic divisions coupled with a reduction in fertility. Together, this work identifies a germline-specific splice isoform for a chromosome segregation component and implicates its role in mammalian fertility.

Cytokine-Supplemented Maturation Medium Enhances Cytoplasmic and Nuclear Maturation in Bovine Oocytes

Bovine in vitro oocyte maturation (IVM) is an easy way to obtain oocytes for subsequent assisted reproductive techniques but is inefficient compared to in vivo maturation. Supplementation of three cytokines, fibroblast growth factor 2 (FGF2), leukemia inhibitory factor (LIF), and insulin-like growth factor 1 (IGF1), or FLI, has increased oocyte maturation and embryo development in multiple species, but studies have not explored the oocyte differences caused by FLI IVM supplementation. This study aimed to assess important nuclear and cytoplasmic maturation events in high-quality oocytes. FLI-supplemented oocytes had a decreased GV (3.0% vs. 13.7%, p < 0.01) and increased telophase I incidence (34.6% vs. 17.6%, p < 0.05) after IVM, increased normal meiotic spindles (68.8% vs. 50.0%, p < 0.001), and an increased nuclear maturation rate (75.1% vs. 66.8%, p < 0.001). Moreover, in metaphase II oocytes, the percentage of FLI-treated oocytes with a diffuse mitochondrial distribution was higher (87.7% vs. 77.5%, p < 0.05) and with a cortical mitochondrial distribution was lower (11.6% vs. 17.4%, p < 0.05). Additionally, FLI-supplemented oocytes had more pattern I cortical granules (21.3% vs. 14.4%, p < 0.05). These data suggest that FLI supplementation in bovine in vitro maturation medium coordinates nuclear and cytoplasmic maturation to produce higher-quality oocytes.

Multiple intersecting pathways are involved in CPEB1 phosphorylation and regulation of translation during mouse oocyte meiosis

The RNA-binding protein cytoplasmic polyadenylation element binding 1 (CPEB1) plays a fundamental role in regulating mRNA translation in oocytes. However, the specifics of how and which protein kinase cascades modulate CPEB1 activity are still controversial. Using genetic and pharmacological tools, and detailed time courses, we have re-evaluated the relationship between CPEB1 phosphorylation and translation activation during mouse oocyte maturation. We show that both the CDK1/MAPK and AURKA/PLK1 pathways converge on CPEB1 phosphorylation during prometaphase of meiosis I. Only inactivation of the CDK1/MAPK pathway disrupts translation, whereas inactivation of either pathway alone leads to CPEB1 stabilization. However, CPEB1 stabilization induced by inactivation of the AURKA/PLK1 pathway does not affect translation, indicating that destabilization and/or degradation is not linked to translational activation. The accumulation of endogenous CCNB1 protein closely recapitulates the translation data that use an exogenous template. These findings support the overarching hypothesis that the activation of translation during prometaphase in mouse oocytes relies on a CDK1/MAPK-dependent CPEB1 phosphorylation, and that translational activation precedes CPEB1 destabilization.

A conserved germline-specific Dsn1 alternative splice isoform supports oocyte and embryo development

Alternative mRNA splicing can generate distinct protein isoforms to allow for the differential control of cell processes across cell types. However, alternative splice isoforms that differentially modulate distinct cell division programs have remained elusive. Here, we demonstrate that mammalian germ cells express an alternate mRNA splice isoform for the kinetochore component, DSN1, a subunit of the MIS12 complex that links the centromeres to spindle microtubules during chromosome segregation. This germline DSN1 isoform bypasses the requirement for Aurora kinase phosphorylation for its centromere localization due to the absence of a key regulatory region allowing DSN1 to display persistent centromere localization. Expression of the germline DSN1 isoform in somatic cells results in constitutive kinetochore localization, chromosome segregation errors, and growth defects, providing an explanation for its tight cell type-specific expression. Reciprocally, precisely eliminating expression of the germline DSN1 splice isoform in mouse models disrupts oocyte maturation and early embryonic divisions coupled with a reduction in fertility. Together, this work identifies a germline-specific splice isoform for a chromosome segregation component and implicates its role in mammalian fertility.

Variable parallelism in the genomic basis of age at maturity across spatial scales in Atlantic Salmon

Complex traits often exhibit complex underlying genetic architectures resulting from a combination of evolution from standing variation, hard and soft sweeps, and alleles of varying effect size. Increasingly, studies implicate both large-effect loci and polygenic patterns underpinning adaptation, but the extent that common genetic architectures are utilized during repeated adaptation is not well understood. Sea age or age at maturation represents a significant life history trait in Atlantic Salmon (Salmo salar), the genetic basis of which has been studied extensively in European Atlantic populations, with repeated identification of large-effect loci. However, the genetic basis of sea age within North American Atlantic Salmon populations remains unclear, as does the potential for a parallel trans-Atlantic genomic basis to sea age. Here, we used a large single-nucleotide polymorphism (SNP) array and low-coverage whole-genome resequencing to explore the genomic basis of sea age variation in North American Atlantic Salmon. We found significant associations at the gene and SNP level with a large-effect locus (vgll3) previously identified in European populations, indicating genetic parallelism, but found that this pattern varied based on both sex and geographic region. We also identified nonrepeated sets of highly predictive loci associated with sea age among populations and sexes within North America, indicating polygenicity and low rates of genomic parallelism. Despite low genome-wide parallelism, we uncovered a set of conserved molecular pathways associated with sea age that were consistently enriched among comparisons, including calcium signaling, MapK signaling, focal adhesion, and phosphatidylinositol signaling. Together, our results indicate parallelism of the molecular basis of sea age in North American Atlantic Salmon across large-effect genes and molecular pathways despite population-specific patterns of polygenicity. These findings reveal roles for both contingency and repeated adaptation at the molecular level in the evolution of life history variation.

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.

Embryonic Development of Parthenogenetic and Sexual Eggs in Lower Termites

Worldwide, termites are one of few social insects. In this research, the stages of embryonic development in the parthenogenetic and sexual eggs of Reticulitermes aculabialis and R. flaviceps were observed and described. In R. flaviceps, the egg development of the FF and FM groups happened during the early phases of development, whereas in R. aculabialis, this appeared mainly during the late phase of development. The variance in the number of micropyles between the R. flaviceps FF colony type and the R. aculabialis FF colony type was statistically significant. Five stages of egg development were found in both types of R. aculabialis but only the sexual eggs of R. flaviceps. In R. flaviceps, 86% of the parthenogenetic eggs stopped growing during the blastoderm development, with the yolk cell assembling frequently in the center of the egg. According to the results of the single-cell transcriptome sequencing, we investigated the egg-to-larval expression level of genes (pka, map2k1, mapk1/3, hgk, mkp, and pax6) and indicated that the levels of essential gene expression in RaFF were considerably higher than in RfFF (p < 0.05). We also discovered that the oocyte cleavage rate in the FF colony type was considerably lower in R. flaviceps compared to R. aculabialis, which gave rise to a smaller number of mature oocytes in R. flaviceps. During ovulation in both species, oocytes underwent activation and one or two cleavage events, but the development of unfertilized eggs ceased in R. flaviceps. It was shown that termite oocyte and embryonic development were heavily influenced by genes with significant expressions. Results from the databases KEGG, COG, and GO unigenes revealed the control of numerous biological processes. This study is the first to complete a database of parthenogenetic and sexual eggs of R. flaviceps and R. aculabialis.

Epitalon protects against post-ovulatory aging-related damage of mouse oocytes in vitro

The developmental potential of oocytes decreases with time after ovulation in vivo or in vitro. Epitalon is a synthetic short peptide made of four amino acids (alanine, glutamic acid, aspartic acid, and glycine), based on a natural peptide called epithalamion extracted from the pineal gland. It is a potent antioxidant, comparable to melatonin, that may confer longevity benefits. The current study aims to test the protective effects of Epitalon on the quality of post-ovulatory aging oocytes. Epitalon at 0.1mM was added to the culture medium, and the quality of oocytes was evaluated at 6h, 12h, and 24h of culture. We found that 0.1mM Epitalon reduced intracellular reactive oxygen species. Epitalon treatment significantly decreased frequency of spindle defects and abnormal distribution of cortical granules during aging for 12h and 24h, while increased mitochondrial membrane potential and DNA copy number of mitochondria, thus decreasing apoptosis of oocytes by 24h of in vitro aging. Our results suggest that Epitalon can delay the aging process of oocytes in vitro via modulating mitochondrial activity and ROS levels.

Separase cleaves the kinetochore protein Meikin at the meiosis I/II transition

To generate haploid gametes, germ cells undergo two consecutive meiotic divisions requiring key changes to the cell division machinery. Here, we demonstrate that the protease separase rewires key cell division processes at the meiosis I/II transition by cleaving the meiosis-specific protein Meikin. Separase proteolysis does not inactivate Meikin but instead alters its function to create a distinct activity state. Full-length Meikin and the C-terminal Meikin separase cleavage product both localize to kinetochores, bind to Plk1 kinase, and promote Rec8 cleavage, but our results reveal distinct roles for these proteins in controlling meiosis. Mutations that prevent Meikin cleavage or that conditionally inactivate Meikin at anaphase I result in defective meiosis II chromosome alignment in mouse oocytes. Finally, as oocytes exit meiosis, C-Meikin is eliminated by APC/C-mediated degradation prior to the first mitotic division. Thus, multiple regulatory events irreversibly modulate Meikin activity during successive meiotic divisions to rewire the cell division machinery at two distinct transitions.

Nuclear and cytoplasmic quality of oocytes derived from serum-free culture of secondary follicles in vitro

In vitro culture of follicles is a promising technology to generate large quantities of mature oocytes and it could offer a novel option of assisted reproductive technologies. Here we described a 2-dimensional follicular serum-free culture system with 3-dimensional effect that can make secondary follicles develop into antral follicles (78.52%), generating developmentally mature oocytes in vitro (66.45%). The oocytes in this serum-free system completed the first meiosis; spindle assembly and chromosome congression in most oocytes matured from follicular culture were normal. However, these oocytes showed significantly lower activation and embryonic development rates, and their ability to produce Ca²⁺ oscillations was also lower in response to parthenogenetic activation, after which a 2-cell embryonic developmental block occurred. Oocytes matured from follicular culture displayed increased abnormal mitochondrial distribution and increased reactive oxygen species levels when compared to in vivo matured oocytes. These data are important for understanding the reasons for reduced developmental potential of oocytes matured from follicular culture, and for further improving the cultivation system.

"Immunolocalization and effect of low concentrations of Insulin like growth factor-1 (IGF-1) in the canine ovary"

Insulin like growth factor‐1 (IGF‐1) plays an important role in the regulation of ovarian function. Despite its extensive study in several species, there is a paucity of information about IGF‐1`s function and localization in the canine ovary. The aim of the present study was to assess the effect of IGF‐1 on oocyte nuclear maturation and to immunolocalize the IGF‐1 and its receptor (IGF‐1R) in the ovary. Cumulus‐oocyte complexes (COCs) were obtained from 34 bitches. The COCs from each bitch were incubated in TCM 199‐HEPES in the absence (n = 199) or presence (n = 204) of 100 ng/ml IGF‐1 for 96 hr at 38ºC in 5% CO₂, stained and evaluated for nuclear maturation by fluorescence microscopy. The results showed that the addition of IGF‐1 did not have an effect (p ˃ 0.05) on the nuclear maturation under these conditions. The immunohistochemical study revealed nuclear and cytoplasmic staining for IGF‐1 and IGF‐1R, respectively. Both were localized in all ovarian structures including the corpus luteum, but not in the granulosa cells from primordial follicles. In addition, IGF‐1 was not localized in the oocytes in tertiary follicles. The results obtained show the presence of IGF‐1 through the stages of follicular growth and in the corpus luteum of the canine ovary. However, its role on oocyte nuclear maturation could not be demonstrated.

N-acetyl-L-cysteine (NAC) delays post-ovulatory oocyte aging in mouse

The quality of post-ovulatory oocytes decreases with aging. In this study, we aimed to investigate the effects of N-acetyl-L-cysteine (NAC), a broadly used antioxidant, on oocyte quality in mouse post-ovulatory oocyte aging in vitro. NAC at 0.6mM concentration was added to culture medium (M2), and the quality of oocytes was analyzed at 6h, 12h, 18h and 24h of culture. We found that the frequency of spindle defects decreased in NAC-treated oocytes compared to those without NAC treatment. NAC treatment significantly decreased abnormal distribution of cortical granules (CGs) in oocytes during aging for 18h and 24h. Decreased intracellular reactive oxygen species (ROS) was also observed. Increased intracellular ATP levels and decreased abnormal distribution of mitochondria could be observed with NAC supplementation during post-ovulatory oocyte aging in vitro. These results indicate that NAC will maintain the quality of oocytes, and delay post-ovulatory oocyte aging as studied in the mouse.

SGK phosphorylates Cdc25 and Myt1 to trigger cyclin B-Cdk1 activation at the meiotic G2/M transition

The kinase cyclin B-Cdk1 complex is a master regulator of M-phase in both mitosis and meiosis. At the G2/M transition, cyclin B-Cdk1 activation is initiated by a trigger that reverses the balance of activities between Cdc25 and Wee1/Myt1 and is further accelerated by autoregulatory loops. In somatic cell mitosis, this trigger was recently proposed to be the cyclin A-Cdk1/Plk1 axis. However, in the oocyte meiotic G2/M transition, in which hormonal stimuli induce cyclin B-Cdk1 activation, cyclin A-Cdk1 is nonessential and hence the trigger remains elusive. Here, we show that SGK directly phosphorylates Cdc25 and Myt1 to trigger cyclin B-Cdk1 activation in starfish oocytes. Upon hormonal stimulation of the meiotic G2/M transition, SGK is activated by cooperation between the Gβγ-PI3K pathway and an unidentified pathway downstream of Gβγ, called the atypical Gβγ pathway. These findings identify the trigger in oocyte meiosis and provide insights into the role and activation of SGK.

Serotonergic Mechanisms of Oocyte Germinal Vesicle Breakdown in the Mud Crab, Scylla paramamosain

The mechanism of serotonin (5-HT)-induced oocyte germinal vesicle breakdown (GVBD) in the mud crab, Scylla paramamosain, was investigated in this study. Histological staining showed that there were two meiotic arrests in oocyte, appearing at prophase I and metaphase I. This result indicated that meiosis I arrest at prophase I in S. paramamosain was similar to that of vertebrates, but meiosis II arrest at metaphase I was different from that of vertebrates. Resumption of oocytes arrest at meiosis prophase I could be induced by 5-HT rapidly within 5 min in S. paramamosain. We obtained the sequence of the 5-HT receptor type 1A (5-HTR_1A) from the NCBI database, and found that 5-HTR_1A was expressed in oocytes and follicle cells. In addition, we found that an agonist 8-OH-DPAT which binds 5-HTR_1A induced GVBD and an antagonist WAY100635 which inhibited 5-HT induced GVBD in S. paramamosain. This result showed that 5-HTR_1A mediated the regulation of oocyte GVBD by 5-HT. To explore the functional mechanism of 5-HT in inducing oocyte GVBD, forskolin, a cAMP agonist was used. Results showed that, forskolin significantly blocked 5-HT-induced GVBD, and there was a negative correlation between GVBD rate and cAMP level. Our data indicate that there are two meiotic arrests in S. paramamosain, and the resumption of prophase I arrest can be induced by 5-HT, which binds to 5-HTR_1A, and this process is mediated by cAMP, which acts as negative regulator via cAMP signaling pathway.

Melatonin Abrogates the Anti-Developmental Effect of the AKT Inhibitor SH6 in Bovine Oocytes and Embryos

Melatonin, a nighttime-secreted antioxidant hormone produced by the pineal gland, and AKT, a serine/threonine-specific protein kinase, have been identified as regulators for several cellular processes essential for reproduction. The current study aimed to investigate the potential interplay between melatonin and AKT in bovine oocytes in the context of embryo development. Results showed that the inclusion of SH6, a specific AKT inhibitor, during in vitro maturation (IVM) significantly reduced oocyte maturation, cumulus cell expansion, cleavage, and blastocyst development that were rescued upon addition of melatonin. Oocytes treated with SH6 in the presence of melatonin showed lower levels of reactive oxygen species (ROS) and blastocysts developed exhibited low apoptosis while the mitochondrial profile was significantly improved compared to the SH6-treated group. The RT-qPCR results showed up-regulation of the mRNA of maturation-, mitochondrial-, and cumulus expansion-related genes including GDF-9, BMP-15, MARF1, ATPase, ATP5F1E, POLG2, HAS2, TNFAIP6, and PTGS2 and down-regulation of Bcl-2 associated X apoptosis regulator (BAX), caspase 3, and p21 involved in apoptosis and cell cycle arrest in melatonin-SH6 co-treated group compared to SH6 sole treatment. The immunofluorescence showed high levels of caspase 3 and caspase 9, and low AKT phosphorylation in the SH6-treated group compared to the control and melatonin-SH6 co-treatment. Taken together, our results showed the importance of both melatonin and AKT for overall embryonic developmental processes and, for the first time, we report that melatonin could neutralize the deleterious consequences of AKT inhibition, suggesting a potential role in regulation of AKT signaling in bovine oocytes.

Cyclin B3 is required for metaphase to anaphase transition in oocyte meiosis I

Meiosis with a single round of DNA replication and two successive rounds of chromosome segregation requires specific cyclins associated with cyclin-dependent kinases (CDKs) to ensure its fidelity. But how cyclins control the distinctive meiosis is still largely unknown. In this study, we explored the role of cyclin B3 in female meiosis by generating Ccnb3 mutant mice via CRISPR/Cas9. Ccnb3 mutant oocytes characteristically arrested at metaphase I (MetI) with normal spindle assembly and lacked enough anaphase-promoting complex/cyclosome (APC/C) activity, which is spindle assembly checkpoint (SAC) independent, to initiate anaphase I (AnaI). Securin siRNA or CDK1 inhibitor supplements rescued the MetI arrest. Furthermore, CCNB3 directly interacts with CDK1 to exert kinase function. Besides, the MetI arrest oocytes had normal development after intracytoplasmic sperm injection (ICSI) or parthenogenetic activation (PA), along with releasing the sister chromatids, which implies that Ccnb3 exclusively functioned in meiosis I, rather than meiosis II. Our study sheds light on the specific cell cycle control of cyclins in meiosis.

Specialization of CDK1 and cyclin B paralog functions in a coenocystic mode of oogenic meiosis

Oogenesis in the urochordate, Oikopleura dioica, occurs in a large coenocyst in which vitellogenesis precedes oocyte selection in order to adapt oocyte production to nutrient conditions. The animal has expanded Cyclin-Dependant Kinase 1 (CDK1) and Cyclin B paralog complements, with several expressed during oogenesis. Here, we addressed functional redundancy and specialization of CDK1 and cyclin B paralogs during oogenesis and early embryogenesis through spatiotemporal analyses and knockdown assays. CDK1a translocated from organizing centres (OCs) to selected meiotic nuclei at the beginning of the P4 phase of oogenesis, and its knockdown impaired vitellogenesis, nurse nuclear dumping, and entry of nurse nuclei into apoptosis. CDK1d-Cyclin Ba translocated from OCs to selected meiotic nuclei in P4, drove meiosis resumption and promoted nuclear envelope breakdown (NEBD). CDK1d-Cyclin Ba was also involved in histone H3S28 phosphorylation on centromeres and meiotic spindle assembly through regulating Aurora B localization to centromeres during prometaphase I. In other studied species, Cyclin B3 commonly promotes anaphase entry, but we found O. dioica Cyclin B3a to be non-essential for anaphase entry during oogenic meiosis. Instead, Cyclin B3a contributed to meiotic spindle assembly though its loss could be compensated by Cyclin Ba.

Retention of Core Meiotic Genes Across Diverse Hymenoptera

The cellular mechanisms of meiosis are critical for proper gamete formation in sexual organisms. Functional studies in model organisms have identified genes essential for meiosis, yet the extent to which this core meiotic machinery is conserved across non-model systems is not fully understood. Moreover, it is unclear whether deviation from canonical modes of sexual reproduction is accompanied by modifications in the genetic components involved in meiosis. We used a robust approach to identify and catalogue meiosis genes in Hymenoptera, an insect order typically characterized by haplodiploid reproduction. Using newly available genome data, we searched for 43 genes involved in meiosis in 18 diverse hymenopterans. Seven of eight genes with roles specific to meiosis were found across a majority of surveyed species, suggesting the preservation of core meiotic machinery in haplodiploid hymenopterans. Phylogenomic analyses of the inventory of meiosis genes and the identification of shared gene duplications and losses provided support for the grouping of species within Proctotrupomorpha, Ichneumonomorpha, and Aculeata clades, along with a paraphyletic Symphyta. The conservation of meiosis genes across Hymenoptera provides a framework for studying transitions between reproductive modes in this insect group.

Evolutionary mysteries in meiosis

Meiosis is a key event of sexual life cycles in eukaryotes. Its mechanistic details have been uncovered in several model organisms, and most of its essential features have received various and often contradictory evolutionary interpretations. In this perspective, we present an overview of these often 'weird' features. We discuss the origin of meiosis (origin of ploidy reduction and recombination, two-step meiosis), its secondary modifications (in polyploids or asexuals, inverted meiosis), its importance in punctuating life cycles (meiotic arrests, epigenetic resetting, meiotic asymmetry, meiotic fairness) and features associated with recombination (disjunction constraints, heterochiasmy, crossover interference and hotspots). We present the various evolutionary scenarios and selective pressures that have been proposed to account for these features, and we highlight that their evolutionary significance often remains largely mysterious. Resolving these mysteries will likely provide decisive steps towards understanding why sex and recombination are found in the majority of eukaryotes.This article is part of the themed issue 'Weird sex: the underappreciated diversity of sexual reproduction'.

Potential role of polyunsaturated fatty acids, with particular regard to the signaling pathways of arachidonic acid and its derivatives in the process of maturation of the oocytes: Contemporary review

Oocyte meiotic maturation is one of the significant physiological requirements for ovulation and fertility. It is believed that Cyclic Adenosine Monophosphate, protein kinase A and protein kinase C pathways along with eicosanoids, particularly prostaglandin E_2, and steroids are the key factors regulating mammalian oocyte maturation. The aim of the current study was to highlight the molecular events triggered by arachidonic acid during oocyte meiotic arrest and resumption at the time of gonadotrophin surge. It should be noted that arachidonic acid release is tightly regulated by Follicle-stimulating and Luteinizing hormones during oocyte development.

The BTG4 and CAF1 complex prevents the spontaneous activation of eggs by deadenylating maternal mRNAs

Once every menstrual cycle, eggs are ovulated into the oviduct where they await fertilization. The ovulated eggs are arrested in metaphase of the second meiotic division, and only complete meiosis upon fertilization. It is crucial that the maintenance of metaphase arrest is tightly controlled, because the spontaneous activation of the egg would preclude the development of a viable embryo (Zhang et al. 2015 J. Genet. Genomics 42, 477-485. (doi:10.1016/j.jgg.2015.07.004); Combelles et al. 2011 Hum. Reprod. 26, 545-552. (doi:10.1093/humrep/deq363); Escrich et al. 2011 J. Assist. Reprod. Genet. 28, 111-117. (doi:10.1007/s10815-010-9493-5)). However, the mechanisms that control the meiotic arrest in mammalian eggs are only poorly understood. Here, we report that a complex of BTG4 and CAF1 safeguards metaphase II arrest in mammalian eggs by deadenylating maternal mRNAs. As a follow-up of our recent high content RNAi screen for meiotic genes (Pfender et al. 2015 Nature 524, 239-242. (doi:10.1038/nature14568)), we identified Btg4 as an essential regulator of metaphase II arrest. Btg4-depleted eggs progress into anaphase II spontaneously before fertilization. BTG4 prevents the progression into anaphase by ensuring that the anaphase-promoting complex/cyclosome (APC/C) is completely inhibited during the arrest. The inhibition of the APC/C relies on EMI2 (Tang et al. 2010 Mol. Biol. Cell 21, 2589-2597. (doi:10.1091/mbc.E09-08-0708); Ohe et al. 2010 Mol. Biol. Cell 21, 905-913. (doi:10.1091/mbc.E09-11-0974)), whose expression is perturbed in the absence of BTG4. BTG4 controls protein expression during metaphase II arrest by forming a complex with the CAF1 deadenylase and we hypothesize that this complex is recruited to the mRNA via interactions between BTG4 and poly(A)-binding proteins. The BTG4-CAF1 complex drives the shortening of the poly(A) tails of a large number of transcripts at the MI-MII transition, and this wave of deadenylation is essential for the arrest in metaphase II. These findings establish a BTG4-dependent pathway for controlling poly(A) tail length during meiosis and identify an unexpected role for mRNA deadenylation in preventing the spontaneous activation of eggs.

Extracellular microRNAs in follicular fluid and their potential association with oocyte fertilization and embryo quality: an exploratory study

PURPOSE

The purpose of this study is to determine the profile of extracellular microRNAs (exmiRNAs) in follicular fluid (FF) and explore their association with fertilization potential and embryo quality.

METHODS

We collected FF from single follicles containing mature oocytes from 40 women undergoing IVF and we screened for the expression of 754 exmiRNAs in FF using the TaqMan OpenArray® qPCR platform. To determine the association of exmiRNAs and IVF outcomes, we compared their expression levels in FF samples that differ by fertilization status (normally, abnormally, and failed to fertilize) and embryo quality (top vs. non-top).

RESULTS

We detected 207 exmiRNAs, of which miR-30d-5p, miR-320b, miR-10b-3p, miR-1291, and miR-720 were most prevalent. We identified four exmiRNAs with significant fold change (FC) when FF that contained normally fertilized was compared to failed to fertilize oocytes [miR-202-5p (FC = 1.82, p = 0.01), miR-206 (FC = 2.09, p = 0.04), miR-16-1-3p (FC = 1.88, p = 0.05), and miR-1244 (FC = 2.72, p = 0.05)]. We also found four exmiRNAs to be significantly differentially expressed in FF that yielded top quality versus non-top quality embryos [(miR-766-3p (FC = 1.95, p = 0.01), miR-663b (FC = 0.18, p = 0.02), miR-132-3p (FC = 2.45, p = 0.05), and miR-16-5p (FC = 3.80, p = 0.05)]. In-silico analysis revealed that several of these exmiRNAs are involved in pathways implicated in reproductive system diseases, organismal abnormalities, and organ development.

CONCLUSIONS

Our findings suggest that exmiRNAs in the follicular fluid can lead to downstream events that will affect fertilization and day 3 embryo morphology. We encourage further observational and experimental studies to confirm our findings and to determine the role of exmiRNAs in human reproduction.

MAPK/ERK activity is required for the successful progression of mitosis in sea urchin embryos

Using sea urchin embryos, we demonstrate that the MEK/MAPK/ERK cascade is essential for the proper progression of the cell cycle. Activation of a limited fraction of MAPK/ERK is required between S-phase and M-phase. Neither DNA replication nor CDK1 activation are impacted by the inhibition of this small active MAPK/ERK fraction. Nonetheless, the chromatin and spindle organisations are profoundly altered. Early morphological disorders induced by the absence of MAPK/ERK activation are correlated with an important inhibition of global protein synthesis and modification in the cyclin B accumulation profile. After appearance of morphological disorders, there is an increase in the level of the inhibitor of protein synthesis, 4E-BP, and, ultimately, an activation of the spindle checkpoint. Altogether, our results suggest that MAPK/ERK activity is required for the synthesis of (a) protein(s) implicated in an early step of chromatin /microtubule attachment. If this MAPK/ERK-dependent step is not achieved, the cell activates a new checkpoint mechanism, involving the reappearance of 4E-BP that maintains a low level of protein translation, thus saving cellular energy.

Ovarian Transcriptome Analysis of Vitellogenic and Non-Vitellogenic Female Banana Shrimp (Fenneropenaeus merguiensis)

The banana shrimp (Fenneropenaeus merguiensis) is one of the most commercially important penaeid species in the world. Its numbers are declining in the wild, leading to a loss of broodstock for farmers of the shrimp and a need for more successful breeding programs. However, the molecular mechanism of the genes involved in this shrimp’s ovarian maturation is still unclear. Consequently, we compared transcriptomic profiles of ovarian tissue from females in both the vitellogenic stage and the non-vitellogenic stage. Using RNA-Seq technology to prepare the transcriptome libraries, a total of 12,187,412 and 11,694,326 sequencing reads were acquired from the non-vitellogenic and vitellogenic stages respectively. The analysis of the differentially expressed genes identified 1,025 which were significantly differentially expressed between the two stages, of which 694 were up-regulated and 331 down-regulated. Four genes putatively involved in the ovarian maturation pathway were chosen for validation by quantitative real-time PCR (RT-qPCR). The data from this study provided information about gene expression in ovarian tissue of the banana shrimp which could be useful for a better understanding of the regulation of this species’ reproductive cycle.

Role of Cyclic Nucleotide Phosphodiesterases During Meiotic Resumption From Diplotene Arrest in Mammalian Oocytes

Cyclic nucleotide phosphodiesterases (PDEs) are group of enzymes that hydrolyze cyclic nucleotides in wide variety of cell types including encircling granulosa cells as well as associated oocytes. One group of PDEs are located in encircling granulosa cells and another group get expressed in the oocyte, while few other PDEs are expressed in both compartments. The PDE1A, PDE4D, PDE5A, PDE8A, and PDE8B are granulosa cell specific PDEs that hydrolyze adenosine 3',5'-cyclic monophosphate (cAMP) as well as guanosine 3',5'-cyclic monophosphate (cGMP) with different affinities. PDE3A, PDE8A as well as PDE9A are expressed in oocyte and specifically responsible for the cyclic nucleotide hydrolysis in the oocyte itself. Few other PDEs such as PDE7B, PDE10A, and PDE11A are either detected in granulosa cells or oocytes. Activation of these PDEs either in encircling granulosa cells or in oocyte directly or indirectly reduces intraoocyte cAMP level. Reduction of intraoocyte cAMP level modulates phosphorylation status of cyclin-dependent kinase 1 (Cdk1) and triggers cyclin B1 degradation that destabilizes maturation promoting factor (MPF) and/or increases Cdk1 activity. The destabilized MPF and/or increased Cdk1 activity leads to resumption of meiosis, which initiates the achievement of meiotic competency in preovulatory follicles of several mammalian species. Use of specific PDEs inhibitors block cyclic nucleotides hydrolysis that results in increase of intraoocyte cyclic nucleotides level, which leads to maintenance of meiotic arrest at diplotene stage in vivo as well as in vitro. Thus, cyclic nucleotide PDEs play important role in the achievement of meiotic competency by reducing intraoocyte cyclic nucleotides level in mammalian oocytes. J. Cell. Biochem. 118: 446-452, 2017. © 2016 Wiley Periodicals, Inc.

Aberrant Meiotic Prophase I Leads to Genic Male Sterility in the Novel TE5A Mutant of Brassica napus

Genic male sterility (GMS) has already been extensively utilized for hybrid rapeseed production. TE5A is a novel thermo-sensitive dominant GMS line in Brassica napus, however, its mechanisms of GMS remain largely unclear. Histological and Transmission electron microscopy (TEM) analyses of anthers showed that the male gamete development of TE5A was arrested at meiosis prophase I. EdU uptake of S-phase meiocytes revealed that the TE5A mutant could accomplish DNA replication, however, chromosomal and fluorescence in situ hybridization (FISH) analyses of TE5A showed that homologous chromosomes could not pair, synapse, condense and form bivalents. We then analyzed the transcriptome differences between young floral buds of sterile plants and its near-isogenic fertile plants through RNA-Seq. A total of 3,841 differentially expressed genes (DEGs) were obtained, some of which were associated with homologous chromosome behavior and cell cycle control during meiosis. Dynamic expression changes of selected candidate DEGs were then analyzed at different anther developmental stages. The present study not only demonstrated that the TE5A mutant had defects in meiotic prophase I via detailed cytological analysis, but also provided a global insight into GMS-associated DEGs and elucidated the mechanisms of GMS in TE5A through RNA-Seq.

Two new competing pathways establish the threshold for cyclin-B-Cdk1 activation at the meiotic G2/M transition

Extracellular ligands control biological phenomena. Cells distinguish physiological stimuli from weak noise stimuli by establishing a ligand-concentration threshold. Hormonal control of the meiotic G2/M transition in oocytes is essential for reproduction. However, the mechanism for threshold establishment is unclear. In starfish oocytes, maturation-inducing hormones activate the PI3K–Akt pathway through the G_βγ complex of heterotrimeric G-proteins. Akt directly phosphorylates both Cdc25 phosphatase and Myt1 kinase, resulting in activation of cyclin-B–Cdk1, which then induces meiotic G2/M transition. Here, we show that cyclin-B–Cdk1 is partially activated after subthreshold hormonal stimuli, but this triggers negative feedback, resulting in dephosphorylation of Akt sites on Cdc25 and Myt1, thereby canceling the signal. We also identified phosphatase activity towards Akt substrates that exists independent of stimuli. In contrast to these negative regulatory activities, an atypical G_βγ-dependent pathway enhances PI3K–Akt-dependent phosphorylation. Based on these findings, we propose a model for threshold establishment in which hormonal dose-dependent competition between these new pathways establishes a threshold; the atypical G_βγ-pathway becomes predominant over Cdk-dependent negative feedback when the stimulus exceeds this threshold. Our findings provide a regulatory connection between cell cycle and signal transduction machineries.

Summary: Ligand–dose thresholds control ligand-dependent responses. To establish the hormonal threshold for driving meiosis, a stimulus-dependent positive regulatory pathway competes against negative feedback from cell cycle machinery.

Functional specialization of chordate CDK1 paralogs during oogenic meiosis

Cyclin-dependent kinases (CDKs) are central regulators of eukaryotic cell cycle progression. In contrast to interphase CDKs, the mitotic phase CDK1 is the only CDK capable of driving the entire cell cycle and it can do so from yeast to mammals. Interestingly, plants and the marine chordate, Oikopleura dioica, possess paralogs of the highly conserved CDK1 regulator. However, whereas in plants the 2 CDK1 paralogs replace interphase CDK functions, O. dioica has a full complement of interphase CDKs in addition to its 5 odCDK1 paralogs. Here we show specific sub-functionalization of odCDK1 paralogs during oogenesis. Differential spatiotemporal dynamics of the odCDK1a, d and e paralogs and the meiotic polo-like kinase 1 (Plk1) and aurora kinase determine the subset of meiotic nuclei in prophase I arrest that will seed growing oocytes and complete meiosis. Whereas we find odCDK1e to be non-essential, knockdown of the odCDK1a paralog resulted in the spawning of non-viable oocytes of reduced size. Knockdown of odCDK1d also resulted in the spawning of non-viable oocytes. In this case, the oocytes were of normal size, but were unable to extrude polar bodies upon exposure to sperm, because they were unable to resume meiosis from prophase I arrest, a classical function of the sole CDK1 during meiosis in other organisms. Thus, we reveal specific sub-functionalization of CDK1 paralogs, during the meiotic oogenic program.

Entry into mitosis: a solution to the decades-long enigma of MPF

Maturation or M phase-promoting factor (MPF) is the universal inducer of M phase common to eukaryotic cells. MPF was originally defined as a transferable activity that can induce the G2/M phase transition in recipient cells. Today, however, MPF is assumed to describe an activity that exhibits its effect in donor cells, and furthermore, MPF is consistently equated with the kinase cyclin B-Cdk1. In some conditions, however, MPF, as originally defined, is undetectable even though cyclin B-Cdk1 is fully active. For over three decades, this inconsistency has remained a long-standing puzzle. The enigma is now resolved through the elucidation that MPF, defined as an activity that exhibits its effect in recipient cells, consists of at least two separate kinases, cyclin B-Cdk1 and Greatwall (Gwl). Involvement of Gwl in MPF can be explained by its contribution to the autoregulatory activation of cyclin B-Cdk1 and by its stabilization of phosphorylations on cyclin B-Cdk1 substrates, both of which are essential when MPF induces the G2/M phase transition in recipient cells. To accomplish these tasks, Gwl helps cyclin B-Cdk1 by suppressing protein phosphatase 2A (PP2A)-B55 that counteracts cyclin B-Cdk1. MPF, as originally defined, is thus not synonymous with cyclin B-Cdk1, but is instead a system consisting of both cyclin B-Cdk1 that directs mitotic entry and Gwl that suppresses the anti-cyclin B-Cdk1 phosphatase. The current view that MPF is a synonym for cyclin B-Cdk1 in donor cells is thus imprecise; instead, MPF is best regarded as the entire pathway involved in the autoregulatory activation of cyclin B-Cdk1, with specifics depending on the experimental system.

Disrupting Cyclin Dependent Kinase 1 in Spermatocytes Causes Late Meiotic Arrest and Infertility in Mice

While cyclin dependent kinase 1 (CDK1) has a critical role in controlling resumption of meiosis in oocytes, its role has not been investigated directly in spermatocytes. Unique aspects of male meiosis led us to hypothesize that its role is different in male meiosis than in female meiosis. We generated a conditional knockout (cKO) of the Cdk1 gene in mouse spermatocytes to test this hypothesis. We found that CDK1-null spermatocytes undergo synapsis, chiasmata formation, and desynapsis as is seen in oocytes. Additionally, CDK1-null spermatocytes relocalize SYCP3 to centromeric foci, express H3pSer10, and initiate chromosome condensation. However, CDK1-null spermatocytes fail to form condensed bivalent chromosomes in prophase of meiosis I and instead are arrested at prometaphase. Thus, CDK1 has an essential role in male meiosis that is consistent with what is known about the role of CDK1 in female meiosis, where it is required for formation of condensed bivalent metaphase chromosomes and progression to the first meiotic division. We found that cKO spermatocytes formed fully condensed bivalent chromosomes in the presence of okadaic acid, suggesting that cKO chromosomes are competent to condense, although they do not do so in vivo. Additionally, arrested cKO spermatocytes exhibited irregular cell shape, irregular large nuclei, and large distinctive nucleoli. These cells persist in the seminiferous epithelium through the next seminiferous epithelial cycle with a lack of stage XII checkpoint-associated cell death. This indicates that CDK1 is required upstream of a checkpoint-associated cell death as well as meiotic metaphase progression in mouse spermatocytes.

Selective Regulation of Oocyte Meiotic Events Enhances Progress in Fertility Preservation Methods

Following early embryonic germ cell migration, oocytes are surrounded by somatic cells and remain arrested at diplotene stage until luteinizing hormone (LH) surge. Strict regulation of both meiotic arrest and meiotic resumption during dormant stage are critical for future fertility. Inter-cellular signaling system between the somatic compartment and oocyte regulates these meiotic events and determines the follicle quality. As well as the collected number of eggs, their qualities are also important for in vitro fertilization (IVF) outcome. In spontaneous and IVF cycles, germinal vesicle (GV)–stage oocytes, premature GV breakdown, and persistence of first meiotic arrest limit the reproductive performance. Likewise, both women with premature ovarian aging and young cancer women are undergoing chemoradiotherapy under the risk of follicle loss because of unregulated meiotic events. Understanding of oocyte meiotic events is therefore critical for the prevention of functional ovarian reserve. High levels of cyclic guanosine monophophate (cGMP), cyclic adenosine monophophate (cAMP) and low phosphodiesterase (PDE) 3A enzyme activity inside the oocyte are responsible for maintaining of meiotic arrest before the LH surge. cGMP is produced in the somatic compartment, and natriuretic peptide precursor C (Nppc) and natriuretic peptide receptor 2 (Npr2) regulate its production. cGMP diffuses into the oocyte and reduces the PDE3A activity, which inhibits the conversion of cAMP to the 5′AMP, and cAMP levels are enhanced. In addition, oocyte itself has the ability to produce cAMP. Taken together, accumulation of cAMP inside the oocyte induces protein kinase activity, which leads to the inhibition of maturation-promoting factor and meiotic arrest also continues. By stimulating the expression of epidermal growth factor, LH inhibits the Nppc/Npr2 system, blocks cGMP synthesis, and initiates meiotic resumption. Oocytes lacking the functional of this pathway may lead to persistence of the GV oocyte, which reduces the number of good quality eggs. Selective regulation of somatic cell signals and oocyte meiotic events enhance progress in fertility preservation methods, which may give us the opportunity to prevent follicle loss in prematurely aging women and young women with cancer are undergoing chemoradiotherapy.

IGFs mediate the action of LH on oocyte maturation in zebrafish

LH signaling is required for oocyte maturation in fish and other vertebrates. However, the downstream factors mediating LH signaling are largely unexplored in fish. In this study, we investigated whether IGFs could mediate LH action on oocyte maturation in zebrafish. Our results show that all igfs, including igf1, igf2a, igf2b, and igf3, are dynamically expressed during folliculogenesis, with the expression of igf3 reaching its maximal level in full grown stage follicles. The expression of igfs is regulated by LH through a cAMP pathway in intact follicles as well as in primary cultured follicular cells, with igf3 expression being the most sensitive to human chorionic gonadotropin (hCG) treatment. Moreover, recombinant zebrafish IGF-2a, IGF-2b, and IGF-3 proteins significantly enhanced oocyte maturation via IGF-1 receptors (IGF-1rs), with IGF-3 exhibiting the most potent stimulatory action on oocyte maturation. Furthermore, we have demonstrated that IGF-3 or hCG treatment could stimulate IGF-1rs phosphorylation, and hCG-induced oocyte maturation could be attenuated by IGF-1r inhibitors as well as by an anti-IGF-3 antiserum in vitro and in vivo, indicating that the IGF system especially IGF-3 plays a crucial role in mediating LH action on oocyte maturation. In addition, igf3 expression is significantly attenuated in LH β-subunit (lhb) mutant zebrafish and treatment with recombinant IGF-3 could partially rescue the oocyte maturation defects of the lhb mutants in vitro and in vivo. Collectively, our results clearly demonstrated that IGFs, particularly the gonad-specific IGF-3, act as important mediators of LH action on oocyte maturation in zebrafish.

An RNAi-based suppressor screen identifies interactors of the Myt1 ortholog of Caenorhabditis elegans

Oocyte maturation in all species is controlled by a protein complex termed the maturation promoting factor (MPF). MPF comprises a cyclin-dependent kinase (CDK) and its partner cyclin, and it is regulated by dueling regulatory phosphorylation events on the CDK. In Caenorhabditis elegans, the Wee1/Myt1 ortholog WEE-1.3 provides the inhibitory phosphorylations on CDK-1 that keep MPF inactive and halt meiosis. Prior work has shown that depletion of WEE-1.3 in C. elegans results in precocious oocyte maturation in vivo and a highly penetrant infertility phenotype. This study sought to further define the precocious maturation phenotype and to identify novel interactors with WEE-1.3. We found that WEE-1.3 is expressed throughout the germline and in developing embryos in a perinuclear pattern, and demonstrated that oocytes in WEE-1.3–depleted germlines have begun to transcribe embryonic genes and exhibit inappropriate expression of proteins normally restricted to fertilized eggs. In addition, we performed an RNAi suppressor screen of the infertile phenotype to identify novel factors that, when co-depleted with WEE-1.3, restore fertility to these animals. We screened ∼1900 essential genes by RNAi feeding and identified 44 (∼2% of the tested genes) that are suppressors of the WEE-1.3 depletion phenotype. The suppressors include many previously unidentified players in the meiotic cell cycle and represent a pool of potential WEE-1.3 interacting proteins that function during C. elegans oocyte maturation and zygotic development.

Dynamics of Male and Female Chromatin during Karyogamy in Rice Zygotes

In angiosperms, the conversion of an egg cell into a zygote involves two sequential gametic processes: plasmogamy, the fusion of the plasma membranes of male and female gametes, and karyogamy, the fusion of the gametic nuclei. In this study, the nuclei and nuclear membranes of rice (Oryza sativa) gametes were fluorescently labeled using histones 2B-green fluorescent protein/red fluorescent protein and Sad1/UNC-84-domain protein2-green fluorescent protein, respectively, which were heterologously expressed. These gametes were fused in vitro to produce zygotes, and the nuclei and nuclear membranes in the zygotes were observed during karyogamy. The results indicated that the sperm nucleus migrates adjacent to the egg nucleus 5 to 10 min after plasmogamy via an actin cytoskelton, and the egg chromatin then appears to move unidirectionally into the sperm nucleus through a possible nuclear connection. The enlargement of the sperm nucleus accompanies this possible chromatin remodeling. Then, 30 to 70 min after fusion, the sperm chromatin begins to decondense with the completion of karyogamy. Based on these observations, the development of early rice zygotes from plasmogamy to karyogamy was divided into eight stages, and using reverse transcription PCR analyses, paternal and de novo synthesized transcripts were separately detected in zygotes at early and late karyogamy stages, respectively.

Possible role of p38 MAPK-MNK1-EMI2 cascade in metaphase-II arrest of mouse oocytes

The Mos-MAPK signaling pathway involving the Mos-MEK1/2-ERK1/2-RSK1/2/3 or MSK1-EMI2 cascade is directly linked to metaphase-II arrest of vertebrate oocytes. In this study, we examined whether p38, a member of the MAPK subfamily, is regulated under the control of Mos and contributes to metaphase-II arrest in the mouse oocyte. Morpholino oligonucleotide-mediated depletion of Mos revealed a remarkable decrease in phosphorylation of p38. Simultaneous treatment of oocytes with two chemical inhibitors of p38 and MEK1/2 induced both release from metaphase II and degradation of cyclin B1, whereas the treatment with each of these two inhibitors had little effect. Moreover, phosphorylation of EMI2 was dramatically abolished by addition of the two inhibitors. Indeed, MNK1, a kinase downstream of p38, exhibited the ability to phosphorylate EMI2. These results suggest that in addition to the Mos-MEK1/2 pathway, the Mos-mediated p38 pathway may be implicated in metaphase-II arrest.

Proteomic analysis of ovarian proteins and characterization of thymosin-β and RAC-GTPase activating protein 1 of the giant tiger shrimp Penaeus monodon

Cellular proteomics of total proteins in ovaries of domesticated and wild giant tiger shrimp (Penaeus monodon) were examined using GeLC-MS/MS. In total, 1638 proteins matched those previously deposited in databases and 1253 (76.50%) of these significantly matched known proteins. Several reproduction-related proteins (e.g. Cdc2, Cyclin B, Cdc25, 14-3-3, thymosin-β and Rac-GTPase activating protein 1) were identified. In addition, the full-length cDNA of P. monodon thymosin-β (PmTmsb; 1084 bp with an ORF of 387 bp and 128 deduced aa) and Rac-GTPase activating protein 1 (PmRacgap1; an ORF of 1881 bp and 626 deduced aa) were further characterized. PmTmsb was constitutively expressed in all tissues. In contrast, PmRacgap1 was more abundantly expressed in gonads than in several non-reproductive tissues (e.g. subcuticular epithelium, hepatopancreas, intestine, pleopods, stomach and thoracic ganglion). The expression levels of PmTmsb and PmRacgap1 in ovaries of wild adult broodstock were significantly greater than those in ovaries of juveniles (P<0.05). However, their expression levels did not vary significantly during ovarian development stages in intact broodstock. However, eyestalk ablation resulted in a significant reduction in PmTmsb expression at stages I and III ovaries (P<0.05), although it did not affect PmRacgap1 transcription significantly at these stages. On the other hand, use of polyclonal antibodies derived from recombinant PmTmsb and PmRacgap1 revealed that levels of both proteins decreased at the late stage (IV) of ovarian development. Our results suggested that PmTmsb and PmRacgap1 may act as negative effectors during ovarian development in P. monodon.

Cloning, expression and characterization of a gene encoding mitogen activated protein kinase 2 (MPK2) from Tetrahymena thermophila

Environmental effects and mitogens determine cell phenotype in eukaryotes mainly through MAPK pathways. However, MAPK signaling pathways in T. thermophila have not been studied comprehensively. This study aims to express recombinant MPK2, a MAPK from T. thermophila, in E. coli to characterize its kinase activity. MPK2 was cloned by RT-PCR using degenerate oligonucleotide primers and RACE method. The full-length cDNA of the MPK2 gene is 1705bp that includes 1281bp ORF coding for a putative protein of 426 amino acids having a mass of 50.2kDa. The putative MPK2 protein contains all eleven conserved subdomains that are characteristics of serine/threonine protein kinases, and a TDY motif, which is a putative dual phosphorylation site common in Protista. MPK2 displays highest 48% overall identity to human ERK5 (MAPK7). The expression vector pGEX4T-1-MPK2 was constructed by inserting the coding region of MPK2 cDNA into pGEX4T-1 after introducing the nine point mutations, and then transformed into E. coli BL21(DE3). Autophosphorylation of 76kDa GST-MPK2 at tyrosine residues was confirmed not only by Western blot using anti-phosphotyrosine monoclonal antibody but also by in vitro kinase assay. GST-MPK2 was also able to phosphorylate the artificial substrate myelin basic protein. This study concludes that the free-living unicellular protist T. thermophila MPK2 has commonly conserved MAPK enzyme features, possibly involved in the regulation of cell survival responding to abiotic or biotic stressors, and the production and movement of haploid gametic nuclei between pairs during conjugation.

Cyclin B-Cdk1 inhibits protein phosphatase PP2A-B55 via a Greatwall kinase-independent mechanism

The role of Greatwall kinase in autoregulatory activation of cyclin B–Cdk1 at M phase onset can be bypassed by cyclin B–Cdk1–mediated direct phosphorylation of Arpp19, leading to PP2A-B55 inhibition.

Entry into M phase is governed by cyclin B–Cdk1, which undergoes both an initial activation and subsequent autoregulatory activation. A key part of the autoregulatory activation is the cyclin B–Cdk1–dependent inhibition of the protein phosphatase 2A (PP2A)–B55, which antagonizes cyclin B–Cdk1. Greatwall kinase (Gwl) is believed to be essential for the autoregulatory activation because Gwl is activated downstream of cyclin B–Cdk1 to phosphorylate and activate α-endosulfine (Ensa)/Arpp19, an inhibitor of PP2A-B55. However, cyclin B–Cdk1 becomes fully activated in some conditions lacking Gwl, yet how this is accomplished remains unclear. We show here that cyclin B–Cdk1 can directly phosphorylate Arpp19 on a different conserved site, resulting in inhibition of PP2A-B55. Importantly, this novel bypass is sufficient for cyclin B–Cdk1 autoregulatory activation. Gwl-dependent phosphorylation of Arpp19 is nonetheless necessary for downstream mitotic progression because chromosomes fail to segregate properly in the absence of Gwl. Such a biphasic regulation of Arpp19 results in different levels of PP2A-B55 inhibition and hence might govern its different cellular roles.

Release from meiotic arrest in ascidian eggs requires the activity of two phosphatases but not CaMKII

The fertilising sperm triggers a transient Ca(2+) increase that releases eggs from cell cycle arrest in the vast majority of animal eggs. In vertebrate eggs, Erp1, an APC/C(cdc20) inhibitor, links release from metaphase II arrest with the Ca(2+) transient and its degradation is triggered by the Ca(2+)-induced activation of CaMKII. By contrast, many invertebrate groups have mature eggs that arrest at metaphase I, and these species do not possess the CaMKII target Erp1 in their genomes. As a consequence, it is unknown exactly how cell cycle arrest at metaphase I is achieved and how the fertilisation Ca(2+) transient overcomes the arrest in the vast majority of animal species. Using live-cell imaging with a novel cyclin reporter to study cell cycle arrest and its release in urochordate ascidians, the closest living invertebrate group to the vertebrates, we have identified a new signalling pathway for cell cycle resumption in which CaMKII plays no part. Instead, we find that the Ca(2+)-activated phosphatase calcineurin (CN) is required for egg activation. Moreover, we demonstrate that parthenogenetic activation of metaphase I-arrested eggs by MEK inhibition, independent of a Ca(2+) increase, requires the activity of a second egg phosphatase: PP2A. Furthermore, PP2A activity, together with CN, is required for normal egg activation during fertilisation. As ascidians are a sister group of the vertebrates, we discuss these findings in relation to cell cycle arrest and egg activation in chordates.

Immunoblotting analyses of changes in protein phosphorylations during oocyte maturation in marine nemertean worms

Immunoblotting analyses combined with phospho-specific antibodies can provide a powerful means for assessing protein activity states in various cellular extracts. This chapter describes a traditional, film-based immunoblotting method for monitoring the phosphorylation status of proteins in marine nemertean oocytes undergoing maturation. Similarly, with minor modifications, the protocol could potentially be applied to a wider variety of cellular processes and extract types that might be analyzed in other investigations of marine invertebrate development.

Why do primordial germ cells migrate through an embryo and what does it mean for biological evolution?

An explanation of the role of primordial germ cell (PGC) migration during embryogenesis is proposed. According to the hypothesis, various PGCs during their migrations through an early embryo are contacting with anlagen of organs and acquiring nonidentical organ specificities. An individual PGC gets such an organ specificity, which corresponds to specificity of the first anlage with which this PGC has the first contact. As a result, the cellular descendants of PGCs (oocytes or spermatocytes) will express nonidentical organ-specific receptors, hence becoming functionally heterogeneous. Therefore, each clone of germ cells becomes capable of recognizing specifically the molecular signals that correspond only to "its" organ of the body. Such signals are produced by the body's organ when it functions in an extreme mode. Signals from the "exercising" organ of the body are delivered to the gonad only via the brain retransmitter, which is composed of neurons grouped as virtual organs of a homunculus. Homunculi are so-called somatotopic maps of the skeletomotor and other parts of the body represented in the brain. Signals, as complexes of regulatory RNAs and proteins, are transported from the "exercising" organ of the body to the corresponding virtual organ of the homunculus where they are processed and then forwarded to the gonad. The organ-specific signal will be selectively recognized by certain gametocytes according to their organ specificity, and then it will initiate the directed epimutation in the gametocyte genome. The nonrandomness of the gene order in chromosomes, that is the synteny and genetic map, is controlled by the so-called creatron that consolidates the soma and germline into a united system, providing the possibility of evolutionary responses of an organism to environmental influences.

DDAH1 deficiency attenuates endothelial cell cycle progression and angiogenesis

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide (NO) synthase (NOS). ADMA is eliminated largely by the action of dimethylarginine dimethylaminohydrolase1 (DDAH1). Decreased DDAH activity is found in several pathological conditions and is associated with increased risk of vascular disease. Overexpression of DDAH1 has been shown to augment endothelial proliferation and angiogenesis. To better understand the mechanism by which DDAH1 influences endothelial proliferation, this study examined the effect of DDAH1 deficiency on cell cycle progression and the expression of some cell cycle master regulatory proteins. DDAH1 KO decreased in vivo Matrigel angiogenesis and depressed endothelial repair in a mouse model of carotid artery wire injury. DDAH1 deficiency decreased VEGF expression in HUVEC and increased NF1 expression in both HUVEC and DDAH1 KO mice. The expression of active Ras could overcome the decreased VEGF expression caused by the DDAH1 depletion. The addition of VEGF and knockdown NF1 could both restore proliferation in cells with DDAH1 depletion. Flow cytometry analysis revealed that DDAH1 sRNAi knockdown in HUVEC caused G1 and G2/M arrest that was associated with decreased expression of CDC2, CDC25C, cyclin D1 and cyclin E. MEF cells from DDAH1 KO mice also demonstrated G2/M arrest that was associated with decreased cyclin D1 expression and Akt activity. Our findings indicate that DDAH1 exerts effects on cyclin D1 and cyclin E expression through multiple mechanisms, including VEGF, the NO/cGMP/PKG pathway, the Ras/PI3K/Akt pathway, and NF1 expression. Loss of DDAH1 effects on these pathways results in impaired endothelial cell proliferation and decreased angiogenesis. The findings provide background information that may be useful in the development of therapeutic strategies to manipulate DDAH1 expression in cardiovascular diseases or tumor angiogenesis.

The Mos-MAPK pathway regulates Diaphanous-related formin activity to drive cleavage furrow closure during polar body extrusion in starfish oocytes

Maintenance of spindle attachment to the cortex and formation of the cleavage furrow around the protruded spindle are essential for polar body extrusion (PBE) during meiotic maturation of oocytes. Although spindle movement to the cortex has been well-studied, how the spindle is maintained at the cortex during PBE is unknown. Here, we show that activation of Diaphanous-related formin mediated by mitogen-activated protein kinase (MAPK) is required for tight spindle attachment to the cortex and cleavage furrow closure during PBE in starfish (Asterina pectinifera) oocytes. A. pectinifera Diaphanous-related formin (ApDia) had a distinct localization in immature oocytes and was localized to the cleavage furrow during PBE. Inhibition of the Mos-MAPK pathway or the actin nucleating activity of formin homology 2 domain prevented cleavage furrow closure and resulted in PBE failure. In MEK/MAPK-inhibited oocytes, activation of ApDia by relief of its intramolecular inhibition restored PBE. In summary, this study elucidates a link between the Mos-MAPK pathway and Diaphanous-related formins, that is responsible for maintaining tight spindle attachment to the cortex and cleavage furrow closure during PBE.

Thioredoxin-interacting protein regulates glucose metabolism and affects cytoplasmic streaming in mouse oocytes

Thioredoxin-interacting protein (Txnip) regulates intracellular redox state and prompts oxidative stress by binding to and inhibiting Thioredoxin (Trx). In addition, via a Trx-independent mechanism, Txnip regulates glucose metabolism and thus maintains intracellular glucose levels. Previously, we found Txnip mRNA highly expressed in immature germinal vesicle (GV) oocytes, but currently there is no report describing the role of Txnip in oocytes. Therefore, we conducted the present study to determine the function of Txnip in mouse oocytes' maturation and meiosis by using RNA interference (RNAi) method. Upon specific depletion of Txnip, 79.5% of oocytes were arrested at metaphase I (MI) stage. Time-lapse video microscopy analysis revealed that the formation of granules in the oocyte cytoplasm increased concurrent with retarded cytoplasmic streaming after Txnip RNAi treatment. Txnip RNAi-treated oocytes had upregulated glucose uptake and lactate production. To confirm the supposition that mechanism responsible for these observed phenomena involves increased lactate in oocytes, we cultured oocytes in high lactate medium and observed the same increased granule formation and retarded cytoplasmic streaming as found by Txnip RNAi. The MI-arrested oocytes exhibited scattered microtubules and aggregated chromosomes indicating that actin networking was disturbed by Txnip RNAi. Therefore, we conclude that Txnip is a critical regulator of glucose metabolism in oocytes and is involved in maintaining cytoplasmic streaming in mouse oocytes.