Involvement of Mos-MEK-MAPK pathway in cytostatic factor (CSF) arrest in eggs of the parthenogenetic insect, Athalia rosae

Identification and Characterization of C-Mos in Pearl Mussel Hyriopsis cumingii and Its Role in Gonadal Development

C-Mos, a proto-oncogene, regulates oocyte maturation by activating the classical MAPK pathway in cells. To examine the function of C-Mos in Hyriopsis cumingii, C-Mos was identified in this study. The full-length cDNA of C-Mos was 2213 bp, including 144 bp in the 5' UTR, 923 bp in 3' the UTR, and 1146 bp in the open reading frame (ORF) region. During early gonad development, the expression of C-Mos from 4 to 6 months of age in H. cumingii was significantly higher than that in other months, with the highest expression in 6-month-old H. cumingii, suggesting that C-Mos may be involved in early gonadal development in H. cumingii. Clear hybridization signals were found by in situ hybridization in the oocytes, oocyte nucleus and oogonium, and a small number of hybridization signals were found in the follicular wall of the male gonads. In addition, the C-Mos RNA interference (RNAi) assay results showed that the knockdown of C-Mos caused a down-regulation of ERK and P90rsk. In summary, these results indicate that C-Mos has a crucial part to play in gonadal development in H. cumingii.

Egg Development After In Vitro Insemination in Japanese Quail (Coturnix japonica)

In vitro fertilization has been widely used to produce offspring in several mammalian species. We previously successfully produced Japanese quail chicks using intracytoplasmic sperm injection (ICSI), whereas in vitro insemination was not successful. This may be due to the difficulties associated with mimicking the sperm-egg fusion process and subsequent events in physiological polyspermic fertilization in vitro. In the present study, we observed egg development after in vitro insemination and investigated the inactivation of metaphase-promoting factor (MPF) and cytostatic factor (CSF), which are downstream of the Ca²⁺ signaling pathway in the egg, due to fertilizing sperm. We found a sperm number-dependent increase in hole formation caused by sperm penetration of the perivitelline membrane, the extracellular coat surrounding the egg. Egg development was observed following in vitro insemination; however, the developmental rate and stages after 24-h culture were inferior to those of ICSI eggs, even when insemination was performed with a high number of sperm (2 × 10⁴). We also noted the downregulation of inositol 1,4,5-trisphosphate receptor-1, ryanodine receptor-3, cyclin B1, and c-MOS, which are important regulatory components of MPF and CSF in the egg, which was dependent on the number of sperm used for insemination. However, the decreases observed in these components did not reach the levels observed in the ICSI eggs. Collectively, the present results suggest that a sperm number higher than 2 × 10⁴ is required for the progression of the Ca²⁺ signaling pathway, which initiates subsequent egg development in Japanese quail.

Comparative transcriptomics of the irradiated melon fly (Zeugodacus cucurbitae) reveal key developmental genes

Irradiation can be used as an insect pest management technique to reduce post-harvest yield losses. It causes major physiological changes, impairing insect development and leading to mortality. This technique is used to control the melon fly Zeugodacus cucurbitae, a major pest of Cucurbitaceae in Asia. Here, we applied irradiation to melon fly eggs, and the larvae emerged from irradiated eggs were used to conduct comparative transcriptomics and thereby identify key genes involved in the development and survival. We found 561 upregulated and 532 downregulated genes in irradiated flies compared to non-irradiated flies. We also observed abnormal small-body phenotypes in irradiated flies. By screening the 532 downregulated genes, we selected eight candidate genes putatively involved in development based in described functions in public databases and in the literature. We first established the expression profile of each candidate gene. Using RNA interference (RNAi), we individually knocked down each gene in third instar larvae and measured the effects on development. The knockdown of ImpE2 ecdysone-inducible gene controlling life stage transitions-led to major body size reductions in both pupae and adults. The knockdown of the tyrosine-protein kinase-like tok (Tpk-tok) caused severe body damage to larvae, characterized by swollen and black body parts. Adults subject to knockdown of the eclosion hormone (Eh_1) failed to shed their old cuticle which remained attached to their bodies. However, no obvious developmental defects were observed following the knockdown of the heat shock protein 67B1-like (Hsp67), the insulin receptor (Insr), the serine/threonine-protein kinase Nek4 (Nek4), the tyrosine-protein kinase transmembrane receptor Ror (Ror_1) and the probable insulin-like peptide 1 (Insp_1). We argue that irradiation can be successfully used not only as a pest management technique but also for the screening of essential developmental genes in insects via comparative transcriptomics. Our results demonstrate that ImpE2 and Eh_1 are essential for the development of melon fly and could therefore be promising candidates for the development of RNAi-based pest control strategies.

MAPK Signaling Pathway Is Essential for Female Reproductive Regulation in the Cabbage Beetle, Colaphellus bowringi

The mitogen-activated protein kinase (MAPK) signaling pathway is a well-conserved intracellular signal transduction pathway, and has important roles in mammalian reproduction. However, it is unknown whether MAPK also regulates insect reproductive mechanisms. Therefore, we investigated the role of the MAPK signaling pathway in ovarian growth and oviposition in the cabbage beetle Colaphellus bowringi, an economically important pest of Cruciferous vegetables. As an initial step, 14 genes from the extracellular regulated protein kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 MAPK (P38) cascades were knocked down using RNA interference (RNAi). The results revealed that RNAi knockdown of MAPK-ERK kinase (MEK), ERK, Kinase suppressor of RAS 2 (KSR2), and P38 induced ovarian development stagnation, low fecundity, and decreased longevity, which indicate that ERK and P38 signaling pathways are important for female C. bowringi survival and reproduction. The potential regulatory role of ERK and P38 pathways in the female reproductive process was investigated using quantitative real-time PCR. We found that ERK pathway possibly regulated ecdysone biosynthesis and P38 pathway possibly involved in the germline stem cell (GSC) development and differentiation. Our findings demonstrated the importance of the MAPK signaling pathway in the female reproduction of insects, and further enhanced the molecular mechanism of female reproductive regulation in insects.

Observing silkworm embryos at the fertilization stage using a tissue clearing reagent

There are various reports on embryogenesis in silkworm, Bombyx mori, a model organism for lepidopteran insects. New tissue observation methods have been developed with the development of biological science. Applying these methods to silkworm eggs makes it possible to capture morphological and histological features that have not been observed until now. Tissue transparency technology is a method of observation that has advanced remarkably recently. This study emphasized the CUBIC (Clear, Unobstructed Brain/Body Imaging Cocktails and Computational analysis) method. The tissue clearing reagent used in CUBIC is water-soluble, containing urea and amino alcohol, which easily and effectively clears animal tissue by immersing the sample in the reagent. This study showed that silkworm eggs are made transparent using the modified CUBIC method at the fertilization stage. Furthermore, we observed the behavior of egg nucleus, polar body nuclei, and sperm nuclei at the fertilization stage.

Protein phosphatase 2A is essential to maintain meiotic arrest, and to prevent Ca2+ burst at spawning and eventual parthenogenesis in the larvacean Oikopleura dioica

Unfertilized eggs of most animals are arrested at a certain point in the meiotic cell cycles. Reinitiation of meiosis and the start of embryogenesis are triggered by fertilization. This arrest is essential for preventing parthenogenetic activation and for promoting proper initiation of development by fertilization. In the larvacean Oikopleura dioica, which is a simple model organism for studies of chordate development, the unfertilized egg is arrested at metaphase of meiosis I. We show here that protein phosphatase 2A (PP2A) is essential for maintenance of meiotic arrest after spawning of oocytes. Knockdown (KD) of the maternal PP2A catalytic subunit, which was found in functional screening of maternal factors, caused unfertilized eggs to spontaneously release polar bodies after spawning, and then start pseudo-cleavages without fertilization, namely, parthenogenesis. Parthenogenetic embryos failed to undergo proper mitosis and cytokinesis because of lack of a centrosome, which is to be brought into the egg by a sperm. Activation of the KD oocytes was triggered by possible rise of ambient and intracellular pH upon their release from the gonad into seawater at spawning. Live recording of intracellular calcium level of the KD oocytes indicated that the pH rise caused an aberrant Ca²⁺ burst, which mimicked the Ca²⁺ burst that occurs at fertilization. Then, the aberrant Ca²⁺ burst triggered meiosis resumption through Calcium/calmodulin-dependent protein kinase (CaMK II). Therefore, PP2A is essential for maintenance of meiotic arrest and prevention of parthenogenesis by suppressing the aberrant Ca²⁺ burst at spawning.

Knockout of a transgene by transcription activator-like effector nucleases (TALENs) in the sawfly, Athalia rosae (Hymenoptera) and the ladybird beetle, Harmonia axyridis (Coleoptera)

Transcription activator-like effector nucleases (TALENs) are efficient tools for targeted genome editing and have been utilized in a number of insects. Here, we demonstrate the gene disruption (knockout) caused by TALENs targeting a transgene, 3xP3-driven enhanced green fluorescence protein (EGFP), that is integrated in the genome of two species, the sawfly Athalia rosae (Hymenoptera) and the ladybird beetle Harmonia axyridis (Coleoptera). Messenger RNAs of TALENs targeting the sequences adjacent to the chromophore region were microinjected into the eggs/embryos of each species. In At. rosae, when microinjection was performed at the posterior end of eggs, 15% of G(0) individuals showed a somatic mosaic phenotype for eye EGFP fluorescence. Three-quarters of the somatic mosaics produced EGFP-negative G(1) progeny. When eggs were injected at the anterior end, 63% of the G(0) individuals showed somatic mosaicism, and 17% of them produced EGFP-negative G(1) progeny. In H. axyridis, 25% of posterior-injected and 8% of anterior-injected G(0) individuals produced EGFP-negative G(1) progeny. In both species, the EGFP-negative progeny retained the EGFP gene, and various deletions were detected in the target sequences, indicating that gene disruption was successfully induced. Finally, for both species, 18-21% of G(0) founders produced gene knockout progeny sufficient for establishing knockout strains.

Males are here to stay: fertilization enhances viable egg production by clonal queens of the little fire ant (Wasmannia auropunctata)

Evolution of reproduction strategies is affected by both phylogenetic and physiological constraints. Although clonality may benefit females, it may not be selected if a male contribution is necessary to start egg laying and embryo development. In little fire ant, Wasmannia auropunctata, sexual populations employ a typical Hymenopteran system of reproduction. In clonal populations, however, queens and males are produced with only maternal and paternal genomes, respectively, whereas sterile workers are produced sexually. Although this system requires both sexes for worker production, previous work has shown that workers may also be produced clonally by the queens. If so, why are males maintained in this species? Our data suggest that fertilization is necessary to increase the hatching rate of eggs. Although clonal queens can indeed produce both workers and queens without mating, the hatching rate is far below the level necessary to maintain functional colonies. On the other hand, virgin queens from populations exhibiting the original Hymenopteran reproduction system also show low hatching rates, but produce only haploid male eggs. Reasons for the existence of W. auropunctata males have been disputed. However, our data suggest that physiological constraints, such as the requirement for insemination, must be considered in regard to evolution of reproduction systems, in addition to ecological data and theoretical considerations of fitness.

The boule gene is essential for spermatogenesis of haploid insect male

boule (bol), a member of the Deleted in Azoospermia (DAZ) gene family plays an important role in meiosis (reductional maturation divisions) in a spermatogenesis-specific manner in animals by regulating translation of the downstream cell division cycle 25 (cdc25) phosphatase mRNA. Orthologues of bol are conserved among animals and found in the genomes of hymenopteran insects, in which the general mode of reproduction is haplodiploidy: female is diploid and male is haploid. In this mode of reproduction, haploid males produce haploid sperm through non-reductional maturation divisions. The question thus arises of whether the bol gene actually functions during spermatogenesis in these haploid males. In this study, we identified two transcriptional isoforms of bol orthologue (Ar bol and Ar bol-2), and one cdc25 orthologue (Ar cdc25) in the hymenopteran sawfly, Athalia rosae. Ar bol was expressed exclusively in the testis when maturation divisions occurred, while Ar bol-2 was expressed ubiquitously. Knockdown of all bol transcripts (both Ar bol and Ar bol-2) resulted in a lack of mature sperm, whereas males with sole knockdown of Ar bol-2 were able to produce a small number of mature sperm. The cell cycle was arrested before maturation divisions in the testis in which all bol transcripts were knocked down, as revealed by flow cytometry. Although no mature sperm was produced, sperm elongation was partially observed when Ar cdc25 alone was knocked down. These results indicate that Ar bol is essential for the entry and progression of maturation divisions and sperm differentiation in haploid males.

Two p90 ribosomal S6 kinase isoforms are involved in the regulation of mitotic and meiotic arrest in Artemia

There are multiple isoforms of p90 ribosomal S6 kinase (RSK), which regulate diverse cellular functions such as cell growth, proliferation, maturation, and motility. However, the relationship between the structures and functions of RSK isoforms remains undetermined. Artemia is a useful model in which to study cell cycle arrest because these animals undergo prolonged diapauses, a state of obligate dormancy. A novel RSK isoform was identified in Artemia, which was termed Ar-Rsk2. This isoform was compared with an RSK isoform that we previously identified in Artemia, termed Ar-Rsk1. Ar-Rsk2 has an ERK-docking motif, whereas Ar-Rsk1 does not. Western blot analysis revealed that Ar-Rsk1 was activated by phosphorylation, which blocked meiosis in oocytes. Knockdown of Ar-Rsk1 reduced the level of phosphorylated cdc2 and thereby suppressed cytostatic factor activity. This indicates that Ar-Rsk1 regulates the cytostatic factor in meiosis. Expression of Ar-Rsk2 was down-regulated in Artemia cysts in which mitosis was arrested. Knockdown of Ar-Rsk2 resulted in decreased levels of cyclin D3 and phosphorylated histone H3, and the production of pseudo-diapause cysts. This indicates that Ar-Rsk2 regulates mitotic arrest. PLK and ERK RNAi showed that Ar-Rsk2, but not Ar-Rsk1, could be activated by PLK-ERK in Artemia. This is the first study to report that RSK isoforms with and without an ERK-docking motif regulate mitosis and meiosis, respectively. This study provides insight into the relationship between the structures and functions of RSK isoforms.

Arrest at metaphase of meiosis I in starfish oocytes in the ovary is maintained by high CO2 and low O2 concentrations in extracellular fluid

During the spawning process in starfish, oocytes are arrested at metaphase of meiosis I (MI) within the ovary, and reinitiate meiosis only after they have been released into the seawater. However, this arrest does not occur if the ovary is removed from the animal. As the pH of the coelomic fluid is buffered by CO2/H(+)/HCO3(-), we investigated the involvement of gas concentrations in MI arrest. In vivo, the CO2 level in the coelomic fluid was high (∼1.5% vs. 0.04% in air) and the O2 level was low (0.1-1.0% vs. ∼20% in air). When these gas conditions were reproduced in isolated coelomic fluid or seawater, ovarian oocytes arrested at MI, just as in vivo. Isolated oocytes from the ovary required the similar high CO2 and low O2 level to remain arrested in MI and had an intracellular pH of ∼6.9. Intracellular pH increased to ∼7.3 when oocytes were transferred to seawater equilibrated with air, a condition that mimics that of spawning. We used ammonium acetate to clamp intracellular pH at different levels and found that MI arrest occurred when intracellular pH was ∼6.9. Our results support the idea that high CO2 and low O2 in the ovarian environment lead to low intracellular pH and MI arrest, while spawning into the seawater with low CO2 and high O2 results in high intracellular pH and release from MI arrest. The biological significance of MI arrest is that oocytes are spawned into seawater at the optimal physiological state of MI when the least polyspermy occurs.

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.

Visualization and live imaging analysis of a mosquito saliva protein in host animal skin using a transgenic mosquito with a secreted luciferase reporter system

Mosquitoes inject saliva into a vertebrate host during blood feeding. The analysis of mosquito saliva in host skin is important for the elucidation of the inflammatory responses to mosquito bites, the development of antithrombotic drugs, and the transmission-blocking of vector-borne diseases. We produced transgenic Anopheles stephensi mosquitoes expressing the secretory luciferase protein (MetLuc) fused to a saliva protein (AAPP) in the salivary glands. The transgene product (AAPP-MetLuc) of transgenic mosquitoes exhibited both luciferase activity as a MetLuc and binding activity to collagen as an AAPP. The detection of luminescence in the skin of mice bitten by transgenic mosquitoes showed that AAPP-MetLuc was injected into the skin as a component of saliva via blood feeding. AAPP-MetLuc remained at the mosquito bite site in host skin with luciferase activity for at least 4 h after blood feeding. AAPP was also suspected of remaining at the site of injury caused by the mosquito bite and blocking platelet aggregation by binding to collagen. These results demonstrated the establishment of visualization and time-lapse analysis of mosquito saliva in living vertebrate host skin. This technique may facilitate the analysis of mosquito saliva after its injection into host skin, and the development of new drugs and disease control strategies.

Meeting report – Oocyte maturation and fertilization: lessons from canonical and emerging models

The EMBO workshop ‘Oocyte maturation and fertilization: lessons from canonical and emerging models’ was held at the Oceanologic Observatory of Banyuls in France in June 2013 and was organized by Anne-Marie Geneviere, Olivier Haccard, Peter Lenart and Alex McDougall. A total of 78 participants shared their research on germline formation, oocyte development, sperm, fertilization and early development. Here, we report the highlights of this meeting.

Appearance of differentiated cells derived from polar body nuclei in the silkworm, Bombyx mori

In Bombyx mori, polar body nuclei are observed until 9 h after egg lying, however, the fate of polar body nuclei remains unclear. To examine the fate of polar body nuclei, we employed a mutation of serosal cell pigmentation, pink-eyed white egg (pe). The heterozygous pe/+^pe females produced black serosal cells in white eggs, while pe/pe females did not produce black serosal cells in white eggs. These results suggest that the appearance of black serosal cells in white eggs depends on the genotype (pe/+^pe) of the mother. Because the polar body nuclei had +^pe genes in the white eggs laid by a pe/+^pe female, polar body nuclei participate in development and differentiate into functional cell (serosal cells). Analyses of serosal cells pigmentation indicated that ~30% of the eggs contained polar-body-nucleus-derived cells. These results demonstrate that polar-body-nucleus-derived cells appeared at a high frequency under natural conditions. Approximately 80% of polar-body-nucleus-derived cells appeared near the anterior pole and the dorsal side, which is opposite to where embryogenesis occurs. The number of cells derived from the polar body nuclei was very low. Approximately 26% of these eggs contained only one black serosal cell. PCR-based analysis revealed that the polar-body-nucleus-derived cells disappeared in late embryonic stages (stage 25). Overall, polar-body-nuclei-derived cells were unlikely to contribute to embryos.

Artificial activation of mature unfertilized eggs in the malaria vector mosquito, Anopheles stephensi (Diptera, Culicidae)

In the past decade, many transgenic lines of mosquitoes have been generated and analyzed, whereas the maintenance of a large number of transgenic lines requires a great deal of effort and cost. In vitro fertilization by an injection of cryopreserved sperm into eggs has been proven to be effective for the maintenance of strains in mammals. The technique of artificial egg activation is a prerequisite for the establishment of in vitro fertilization by sperm injection. We demonstrated that artificial egg activation is feasible in the malaria vector mosquito, Anopheles stephensi (Diptera, Culicidae). Nearly 100% of eggs dissected from virgin females immersed in distilled water darkened, similar to normally oviposited fertilized eggs. It was revealed by the cytological examination of chromosomes that meiotic arrest was relieved in these eggs approximately 20 min after incubation in water. Biochemical examinations revealed that MAPK (mitogen-activated protein kinase)/ERK (extracellular signal-regulated protein kinase) and MEK (MAPK/ERK kinase) were dephosphorylated similar to that in fertilized eggs. These results indicate that dissected unfertilized eggs were activated in distilled water and started development. Injection of distilled water into body cavity of the virgin blood-fed females also induced activation of a portion of eggs in the ovaries. The technique of artificial egg activation is expected to contribute to the success of in vitro fertilization in A. stephensi.

Evolution of the acquisition of fertilization competence and polyspermy blocks during meiotic maturation

In many animals, fully grown oocytes are arrested at prophase of meiosis I. Before or after ovulation/spawning, a secondary arrest occurs at metaphase of meiosis I or II (MI or II, respectively). MI arrest in the ovary is released after spawning, and is followed by fertilization, whereas MI and MII arrest after ovulation are released by fertilization. Insemination of isolated oocytes from the ovaries at an inappropriate time increases the rate of polyspermy, indicating that ovaries provide the proper environment for acquisition of the polyspermy blocks and the development of competence to be fertilized normally. Due to MI arrest in the ovaries or MI/MII arrest after ovulation/spawning, the fertilizable period can be elongated. Thus, MI and MII arrest may play a role in maintaining the cell-cycle phases to enable normal fertilization. Here, the evolution of fertilization timing is discussed.

Molecular cloning and expression analysis of the Mitogen-activating protein kinase 1 (MAPK1) gene and protein during ovarian development of the giant tiger shrimp Penaeus monodon

Isolation and characterization of genes and/or proteins differentially expressed in ovaries are necessary for understanding ovarian development in the giant tiger shrimp (Penaeus monodon). In this study, the full-length cDNA of P. monodon mitogen-activating protein kinase 1 (PmMAPK1) was characterized. PmMAPK1 was 1,398 bp in length containing an open reading frame of 1,098 bp that corresponded to a polypeptide of 365 amino acids. PmMAPK1 was more abundantly expressed in ovaries than in testes of P. monodon. Quantitative real-time PCR revealed differential expression levels of PmMAPK1 mRNA during ovarian development of intact broodstock, where it peaked in early cortical rod (stage III) ovaries (P < 0.05) and slightly decreased afterwards (P > 0.05). Likewise, the expression level of PmMAPK1 in early cortical rod and mature (IV) ovaries was significantly greater than that in previtellogenic (I) and vitellogenic (II) ovaries of eyestalk-ablated broodstock (P < 0.05). The PmMAPK1 transcript was localized in ooplasm of previtellogenic oocytes. In intact broodstock, the expression of the PmMAPK1 protein was clearly increased from previtellogenic ovaries in subsequent stages of ovarian development (P < 0.05). In contrast, the level of ovarian PmMAPK1 protein was comparable during oogenesis in eyestalk-ablated broodstock (P > 0.05). The PmMAPK1 protein was localized in ooplasm of previtellogenic and vitellogenic oocytes. It was also detected around the nuclear membrane of early cortical rod oocytes in both intact and eyestalk-ablated broodstock. Results indicated that PmMAPK1 gene products seem to play functional roles in the development and maturation of oocytes/ovaries in P. monodon.

Mos limits the number of meiotic divisions in urochordate eggs

Mos kinase is a universal mediator of oocyte meiotic maturation and is produced during oogenesis and destroyed after fertilization. The hallmark of maternal meiosis is that two successive M phases (meiosis I and II) drive two rounds of asymmetric cell division (ACD). However, how the egg limits the number of meioses to just two, thereby preventing gross aneuploidy, is poorly characterized. Here, in urochordate eggs, we show that loss of Mos/MAPK activity is necessary to prevent entry into meiosis III. Remarkably, maintaining the Mos/MAPK pathway active after fertilization at near physiological levels induces additional rounds of meiotic M phase (meiosis III, IV and V). During these additional rounds of meiosis, the spindle is positioned asymmetrically resulting in further rounds of ACD. In addition, inhibiting meiotic exit with Mos prevents pronuclear formation, cyclin A accumulation and maintains sperm-triggered Ca2+ oscillations, all of which are hallmarks of the meiotic cell cycle in ascidians. It will be interesting to determine whether Mos availability in mammals can also control the number of meioses as it does in the urochordates. Our results demonstrate the power of urochordate eggs as a model to dissect the egg-to-embryo transition.

Mos in the oocyte: how to use MAPK independently of growth factors and transcription to control meiotic divisions

In many cell types, the mitogen-activated protein kinase (MAPK) also named extracellular signal-regulated kinase (ERK) is activated in response to a variety of extracellular growth factor-receptor interactions and leads to the transcriptional activation of immediate early genes, hereby influencing a number of tissue-specific biological activities, as cell proliferation, survival and differentiation. In one specific cell type however, the female germ cell, MAPK does not follow this canonical scheme. In oocytes, MAPK is activated independently of growth factors and tyrosine kinase receptors, acts independently of transcriptional regulation, plays a crucial role in controlling meiotic divisions, and is under the control of a peculiar upstream regulator, the kinase Mos. Mos was originally identified as the transforming gene of Moloney murine sarcoma virus and its cellular homologue was the first proto-oncogene to be molecularly cloned. What could be the specific roles of Mos that render it necessary for meiosis? Which unique functions could explain the evolutionary cost to have selected one gene to only serve for few hours in one very specific cell type? This review discusses the original features of MAPK activation by Mos and the roles of this module in oocytes.

Regulation of intracellular pH by p90Rsk-dependent activation of an Na(+)/H(+) exchanger in starfish oocytes

Starfish oocytes arrest at metaphase of the first meiotic division (MI arrest) in the ovary and resume meiosis after spawning into seawater. MI arrest is maintained by lower intracellular pH (pH(i)) and release from arrest by cellular alkalization. To elucidate pH(i) regulation in oocytes, we cloned the starfish (Asterina pectinifera) Na(+)/H(+) exchanger 3 (ApNHE3) expressed in the plasma membrane of oocytes. The cytoplasmic domain of ApNHE3 contains p90 ribosomal S6 kinase (p90Rsk) phosphorylation sites, and injection of a constitutively active p90Rsk and the upstream regulator Mos to immature oocytes, stimulated an increase in pH(i). This increase was blocked by 5-(N-ethyl-N-isopropyl)-amiloride, a NHE inhibitor, and SL0101, a specific Rsk inhibitor. The MAPK kinase (MEK) inhibitor U0126 blocked the Mos-induced, but not the p90Rsk-induced, pH(i) increase, suggesting that the Mos-MEK-MAPK-p90Rsk pathway promotes ApNHE3 activation. In a cell-free extract, the Mos-MEK-MAPK-p90Rsk pathway phosphorylates ApNHE3 at Ser-590, -606, and -673. When p90Rsk-dependent ApNHE3 phosphorylation was blocked by a dominant-negative C-terminal fragment, or neutralizing antibody, the p90Rsk-induced pH(i) increase was suppressed in immature oocytes. However, ApNHE3 is up-regulated via the upstream phosphatidylinositol 3-kinase pathway before MAPK activation and the active state is maintained until spawning, suggesting that the p90Rsk-dependent ApNHE3 phosphorylation is unlikely to be the primary regulatory mechanism involved in MI arrest exit. After meiosis is completed, unfertilized eggs maintain their elevated pH(i) ( approximately 7.4) until the onset of apoptosis. We suggest that the p90Rsk/ApNHE3-dependent elevation of pH(i) increases fertilization success by delaying apoptosis initiation.

Conserved functions for Mos in eumetazoan oocyte maturation revealed by studies in a cnidarian

The kinase Mos, which activates intracellularly the MAP kinase pathway, is a key regulator of animal oocyte meiotic maturation. In vertebrate and echinoderm models, Mos RNA translation upon oocyte hormonal stimulation mediates "cytostatic" arrest of the egg after meiosis, as well as diverse earlier events [1-5]. Our phylogenetic survey has revealed that MOS genes are conserved in cnidarians and ctenophores, but not found outside the metazoa or in sponges. We demonstrated MAP kinase-mediated cytostatic activity for Mos orthologs from Pleurobrachia (ctenophore) and Clytia (cnidarian) by RNA injection into Xenopus blastomeres. Analyses of endogenous Mos in Clytia with morpholino antisense oligonucleotides and pharmacological inhibition demonstrated that Mos/MAP kinase function in postmeiotic arrest is conserved. They also revealed additional roles in spindle formation and positioning, strongly reminiscent of observations in starfish, mouse, and Xenopus. Unusually, cnidarians were found to possess multiple Mos paralogs. In Clytia, one of two maternally expressed paralogs accounted for the majority MAP kinase activation during maturation, whereas the other may be subject to differential translational regulation and have additional roles. Our findings indicate that Mos appeared early during animal evolution as an oocyte-expressed kinase and functioned ancestrally in regulating core specializations of female meiosis.

Start of the embryonic cell cycle is dually locked in unfertilized starfish eggs

A key event in the oocyte-to-embryo transition is the start of the embryonic mitotic cell cycle. Prior to this start, the cell cycle in oocytes is generally arrested at a particular stage during meiosis, and the meiotic arrest is released by fertilization. However, it remains unclear how release from the meiotic arrest is implicated in the start of the embryonic cell cycle. To elucidate this link, we have used starfish eggs, in which G1 phase arrest occurs after completion of meiosis if the mature oocytes are not fertilized, and fertilization simply directs the start of the embryonic cell cycle. The starfish G1 arrest is known to rely on the Mos-MAPK-Rsk (p90 ribosomal S6 kinase) pathway, and inactivation of Rsk induces S phase in the absence of fertilization. However, here we show that this S phase is not followed by M phase when MAPK remains active, owing to poly(A)-independent repression of cyclin A and B synthesis. By contrast, inactivation of MAPK alone induces M phase, even when S phase is inhibited by constitutively active Rsk. Thus, there is a divergence of separate pathways downstream of MAPK that together block the start of the embryonic mitotic cycle. One is the previously known Rsk-dependent pathway that prevents S phase, and the other is a novel pathway that is not mediated by Rsk and that leads to prevention of the first mitotic M phase through suppression of protein synthesis of M phase cyclins. Release from such a 'dual-lock' by fertilization results in the start of the embryonic cell cycle.