Mouse Mos protooncogene product is present and functions during oogenesis

Role of zinc in female reproduction

Zinc is a critical component in a number of conserved processes that regulate female germ cell growth, fertility, and pregnancy. During follicle development, a sufficient intracellular concentration of zinc in the oocyte maintains meiotic arrest at prophase I until the germ cell is ready to undergo maturation. An adequate supply of zinc is necessary for the oocyte to form a fertilization-competent egg as dietary zinc deficiency or chelation of zinc disrupts maturation and reduces the oocyte quality. Following sperm fusion to the egg to initiate the acrosomal reaction, a quick release of zinc, known as the zinc spark, induces egg activation in addition to facilitating zona pellucida hardening and reducing sperm motility to prevent polyspermy. Symmetric division, proliferation, and differentiation of the preimplantation embryo rely on zinc availability, both during the oocyte development and post-fertilization. Further, the fetal contribution to the placenta, fetal limb growth, and neural tube development are hindered in females challenged with zinc deficiency during pregnancy. In this review, we discuss the role of zinc in germ cell development, fertilization, and pregnancy with a focus on recent studies in mammalian females. We further detail the fundamental zinc-mediated reproductive processes that have only been explored in non-mammalian species and speculate on the role of zinc in similar mechanisms of female mammals. The evidence collected over the last decade highlights the necessity of zinc for normal fertility and healthy pregnancy outcomes, which suggests zinc supplementation should be considered for reproductive age women at risk of zinc deficiency.

Regulation of Cell Division

The challenging task of mitotic cell divisions is to generate two genetically identical daughter cells from a single precursor cell. To accomplish this task, a complex regulatory network evolved, which ensures that all events critical for the duplication of cellular contents and their subsequent segregation occur in the correct order, at specific intervals and with the highest possible fidelity. Transitions between cell cycle stages are triggered by changes in the phosphorylation state and levels of components of the cell cycle machinery. Entry into S-phase and M-phase are mediated by cyclin-dependent kinases (Cdks), serine-threonine kinases that require a regulatory cyclin subunit for their activity. Resetting the system to the interphase state is mediated by protein phosphatases (PPs) that counteract Cdks by dephosphorylating their substrates. To avoid futile cycles of phosphorylation and dephosphorylation, Cdks and PPs must be regulated in a manner such that their activities are mutually exclusive.

Participation of D-serine in the development and reproduction of the silkworm Bombyx mori

The silkworm Bombyx mori contains high concentrations of free D-serine, an optical isomer of L-serine. To elucidate its function, we first investigated the localization of D-serine in various organs of silkworm larvae, pupae, and adult moths. Using immunohistochemical analysis with an anti-D-serine antibody, we found D-serine in the microvilli of midgut goblet and cylindrical cells and in peripheral matrix components of testicular and ovarian cells. By spectrophotometric analysis, D-serine was also found in the hemolymph and fat body. D-Alanine was not detected in the various organs by immunohistochemistry. Serine racemase, which catalyzes the inter-conversion of L- and D-serine, was found to co-localize with D-serine, and D-serine production from L-serine by intrinsic serine racemase was suggested. O-Phospho-L-serine is an inhibitor of serine racemase, and it was administered to the larvae to reduce the D-serine level. This reagent decreased the midgut caspase-3 level and caused a delay in spermatogenesis and oogenesis. The reagent also decreased mature sperm and egg numbers, suggesting D-serine participation in these processes. D-Serine administration induced an increase in pyruvate levels in testis, midgut, and fat body, indicating conversion of D-serine to pyruvate. On the basis of these results, together with our previous investigation of ATP biosynthesis in testis, we consider the possible involvement of D-serine in ATP synthesis for metamorphosis and reproduction.

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.

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.

Zinc maintains prophase I arrest in mouse oocytes through regulation of the MOS-MAPK pathway

Meiosis in mammalian females is marked by two arrest points, at prophase I and metaphase II, which must be tightly regulated in order to produce a haploid gamete at the time of fertilization. The transition metal zinc has emerged as a necessary and dynamic regulator of the establishment, maintenance, and exit from metaphase II arrest, but the roles of zinc during prophase I arrest are largely unknown. In this study, we investigate the mechanisms of zinc regulation during the first meiotic arrest. Disrupting zinc availability in the prophase I arrested oocyte by treatment with the heavy metal chelator N,N,N',N'-tetrakis-(2-pyridylmethyl)-ethylenediamine (TPEN) causes meiotic resumption even in the presence of pharmacological inhibitors of meiosis. We further show that the MOS-MAPK pathway mediates zinc-dependent prophase I arrest, as the pathway prematurely activates during TPEN-induced meiotic resumption. Conversely, inhibition of the MOS-MAPK pathway maintains prophase I arrest. While prolonged zinc insufficiency ultimately results in telophase I arrest, early and transient exposure of oocytes to TPEN is sufficient to induce meiotic resumption and bypass the telophase I block, allowing the formation of developmentally competent eggs upon parthenogenetic activation. These results establish zinc as a crucial regulator of meiosis throughout the entirety of oocyte maturation, including the maintenance of and release from the first and second meiotic arrest points.

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.

Zinc requirement during meiosis I-meiosis II transition in mouse oocytes is independent of the MOS-MAPK pathway

Zinc is essential for many biological processes, including proper functioning of gametes. We recently reported that zinc levels rise by over 50% during oocyte maturation and that attenuation of zinc availability during this period could be achieved using the membrane-permeable heavy metal chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN). This zinc insufficiency resulted in formation of large polar bodies, failure to establish metaphase II arrest, and impaired establishment of cortical polarity. As these phenotypes resemble those of MOS null oocytes, we examined the impact of zinc insufficiency on the MOS-MAPK pathway. Reduced levels of both MOS protein and phosphorylation of MAP2K1/2 are observed in zinc-insufficient oocytes; however, these differences appear only after completion of the first meiotic division. In addition, activation of the downstream effector of the MOS pathway, MAPK3/1, is not affected by zinc insufficiency, and reduced MOS levels are observed only with the presence of TPEN after the first polar body extrusion. These data are inconsistent with the hypothesis that reduced MOS mediates the observed phenotype. Finally, MOS overexpression does not rescue the phenotype of zinc-insufficient oocytes, confirming that the observed disruption of asymmetric division and spindle abnormalities cannot be attributed to impaired MOS signaling. Zinc-insufficient oocytes do not increase maturation promoting factor (MPF) activity following the first meiotic division, and increasing MPF activity through expression of nondegradable cyclin B1 partially rescues the ability of zinc-insufficient oocytes to enter metaphase II. Although we have shown that zinc has a novel role in the meiotic cell cycle, it is not mediated through the MOS-MAPK pathway.

Cytostatic factor proteins are present in male meiotic cells and beta-nerve growth factor increases mos levels in rat late spermatocytes

BACKGROUND

In co-cultures of pachytene spermatocytes with Sertoli cells, beta-NGF regulates the second meiotic division by blocking secondary spermatocytes in metaphase (metaphase II), and thereby lowers round spermatid formation. In vertebrates, mature oocytes are arrested at metaphase II until fertilization, because of the presence of cytostatic factor (CSF) in their cytoplasm. By analogy, we hypothesized the presence of CSF in male germ cells.

METHODOLOGY/PRINCIPAL FINDINGS

We show here, that Mos, Emi2, cyclin E and Cdk2, the four proteins of CSF, and their respective mRNAs, are present in male rat meiotic cells; this was assessed by using Western blotting, immunocytochemistry and reverse transcriptase PCR. We measured the relative cellular levels of Mos, Emi2, Cyclin E and Cdk2 in the meiotic cells by flow cytometry and found that the four proteins increased throughout the first meiotic prophase, reaching their highest levels in middle to late pachytene spermatocytes, then decreased following the meiotic divisions. In co-cultures of pachytene spermatocytes with Sertoli cells, beta-NGF increased the number of metaphases II, while enhancing Mos and Emi2 levels in middle to late pachytene spermatocytes, pachytene spermatocytes in division and secondary spermatocytes.

CONCLUSION/SIGNIFICANCE

Our results suggest that CSF is not restricted to the oocyte. In addition, they reinforce the view that NGF, by enhancing Mos in late spermatocytes, is one of the intra-testicular factors which adjusts the number of round spermatids that can be supported by Sertoli cells.

Oogenesis: making the mos of meiosis

Meiosis is an ancient type of cell division whose advent allowed the evolution of sexual reproduction. The evolutionary history of the specialization that allowed gamete production to emerge from a simple reduction division has been unclear. New data now suggest that the molecular mechanisms involved in animal oocyte specialization may have origins that predate the emergence of bilaterian animals.

Proteasomal activity is required to initiate and to sustain translational activation of messenger RNA encoding the stem-loop-binding protein during meiotic maturation in mice

Developmentally regulated translation plays a key role in controlling gene expression during oogenesis. In particular, numerous mRNA species are translationally repressed in growing oocytes and become translationally activated during meiotic maturation. While many studies have focused on a U-rich sequence, termed the cytoplasmic polyadenylation element (CPE), located in the 3'-untranslated region (UTR) and the CPE-binding protein (CPEB) 1, multiple mechanisms likely contribute to translational control in oocytes. The stem-loop-binding protein (SLBP) is expressed in growing oocytes, where it is required for the accumulation of nonpolyadenylated histone mRNAs, and then accumulates substantially during meiotic maturation. We report that, in immature oocytes, Slbp mRNA carries a short poly(A) tail, and is weakly translated, and that a CPE-like sequence in the 3'-UTR is required to maintain this low activity. During maturation, Slbp mRNA becomes polyadenylated and translationally activated. Unexpectedly, proteasomal activity is required both to initiate and to sustain translational activation. This proteasomal activity is not required for the polyadenylation of Slbp mRNA during early maturation; however, it is required for a subsequent deadenylation of the mRNA that occurs during late maturation. Moreover, although CPEB1 is degraded during maturation, inhibiting its degradation by blocking mitogen-activated protein kinase 1/3 activity does not prevent the accumulation of SLBP, indicating that CPEB1 is not the protein whose degradation is required for translational activation of Slbp mRNA. These results identify a new role for proteasomal activity in initiating and sustaining translational activation during meiotic maturation.

Genomic RNA profiling and the programme controlling preimplantation mammalian development

Preimplantation development shifts from a maternal to embryonic programme rapidly after fertilization. Although the majority of oogenetic products are lost during the maternal to embryonic transition (MET), several do survive this interval to contribute directly to supporting preimplantation development. Embryonic genome activation (EGA) is characterized by the transient expression of several genes that are necessary for MET, and while EGA represents the first major wave of gene expression, a second mid-preimplantation wave of transcription that supports development to the blastocyst stage has been discovered. The application of genomic approaches has greatly assisted in the discovery of stage specific gene expression patterns and the challenge now is to largely define gene function and regulation during preimplantation development. The basic mechanisms controlling compaction, lineage specification and blastocyst formation are defined. The requirement for embryo culture has revealed plasticity in the developmental programme that may exceed the adaptive capacity of the embryo and has fostered important research directions aimed at alleviating culture-induced changes in embryonic programming. New levels of regulation are emerging and greater insight into the roles played by RNA-binding proteins and miRNAs is required. All of this research is relevant due to the necessity to produce healthy preimplantation embryos for embryo transfer, to ensure that assisted reproductive technologies are applied in the most efficient and safest way possible.

Oocyte quality and maternal control of development

The oocyte is a unique and highly specialized cell responsible for creating, activating, and controlling the embryonic genome, as well as supporting basic processes such as cellular homeostasis, metabolism, and cell cycle progression in the early embryo. During oogenesis, the oocyte accumulates a myriad of factors to execute these processes. Oogenesis is critically dependent upon correct oocyte-follicle cell interactions. Disruptions in oogenesis through environmental factors and changes in maternal health and physiology can compromise oocyte quality, leading to arrested development, reduced fertility, and epigenetic defects that affect long-term health of the offspring. Our expanding understanding of the molecular determinants of oocyte quality and how these determinants can be disrupted has revealed exciting new insights into the role of oocyte functions in development and evolution.

mos proto-oncogene function

Maturation promoting factor (MPF) is a cytoplasmic activity that causes oocytes arrested in prophase to resume meiosis. An inactive form of MPF termed pre-MPF exists in fully grown oocytes. In Xenopus oocytes, progesterone induces maturation and pre-MPF activation. These early maturation events require protein synthesis. We have shown that p39mos synthesis is rapidly induced in progesterone-treated Xenopus oocytes during the protein synthesis sensitive period and prior to activation of pre-MPF or germinal vesicle breakdown (GVBD). p39mos may qualify, therefore, as an 'initiator' of maturation. Mouse oocytes undergoing meiotic maturation also express p39mos. Microinjection of antisense mos oligodeoxynucleotides into fully grown mouse and Xenopus oocytes results in the block of meiotic maturation. In Xenopus, antisense-injected oocytes not only lack p39mos, but also lack MPF and fail to undergo GVBD. In the mouse, the microinjected oocytes progress through GVBD, but fail to produce the first polar body; cytogenetic analysis shows they are arrested at the bivalent chromosome stage of metaphase I. This and additional studies with Xenopus oocytes indicate that p39mos is also required throughout maturation. We have shown that p39mos is indistinguishable from the protein product constitutively expressed in NIH/3T3 cells transformed with activated c-mos. It is likely that its activity as a transforming gene may be due to activation of pre-MPF activities in interphase in the somatic cell cycle.

Mos and the mitogen-activated protein kinase do not show cytostatic factor activity in early mouse embryos

Mos and the mitogen-activated protein kinase (MAPK) cascade have been established as crucial regulators of second meiotic metaphase arrest, the so-called CSF arrest, in mammalian oocytes. They are also thought to play a role in regulating mitotic metaphase arrest of early mammalian embryos. In the present study, we examined whether mitotic arrest is induced in early mouse embryos by activation of extracellular signal-regulated kinases (ERKs), which are major MAPKs in mouse eggs, and their substrate, p90Ribosomal S6 kinase (RSK), as reported in Xenopus embryos. Wild-type Mos (wt-Mos), degradation-resistant Mos mutant (P2G-Mos) or constitutive active mutant of MAPK/ERK kinase, MEK (SDSE-MEK), was expressed in early mouse embryos by injecting the respective expression vectors into the pronucleus of fertilized eggs, and the developmental rates were then examined up to 72 h after insemination. Expression of P2G-Mos and SDSE-MEK succeeded in activating ERKs and RSK in developing mouse embryos, while wt-Mos failed to activate them in spite of expression of mos mRNA, indicating that the wt-Mos protein is unstable in early mouse embryos. Although the activated levels of ERKs and RSK in the vector-injected embryos were comparable to those of meiotically arrested mouse oocytes, their developmental rates were identical to those of the control embryos. These results suggest that activation of MAPK and RSK does not induce mitotic arrest in early mouse embryos. The present study indicates that there are large physiological differences between early mouse embryos and mouse oocytes and that CSF arrest of mouse eggs in mitosis should be discussed separately from that in meiosis.

H1foo is indispensable for meiotic maturation of the mouse oocyte

Oocyte-specific linker histone H1foo is localized in the oocyte nucleus, either diffusely or bound to chromatin, during the processes of meiotic maturation and fertilization. This expression pattern suggests that H1foo plays a key role in the control of gene expression and chromatin modification during oogenesis and early embryogenesis. To reveal the function of H1foo, we microinjected antisense morpholino oligonucleotides (MO) against H1foo into mouse germinal-vesicle stage oocytes. The rate of in vitro maturation of the antisense MO group was significantly lower than that of the control group. Eggs that failed to extrude a first polar body following injection of antisense MO arrested at metaphase I. Additionally, co-injection of in vitro synthesized H1foo mRNA along with antisense MO successfully rescued expression of H1foo and improved the in vitro maturation rate. There was no difference in the rate of parthenogenesis between the antisense MO and control groups. These results indicate that H1foo is essential for maturation of germinal vesicle-stage oocytes.

How eggs arrest at metaphase II: MPF stabilisation plus APC/C inhibition equals Cytostatic Factor

Oocytes from higher chordates, including man and nearly all mammals, arrest at metaphase of the second meiotic division before fertilization. This arrest is due to an activity that has been termed 'Cytostatic Factor'. Cytostatic Factor maintains arrest through preventing loss in Maturation-Promoting Factor (MPF; CDK1/cyclin B). Physiologically, Cytostatic Factor – induced metaphase arrest is only broken by a Ca²⁺ rise initiated by the fertilizing sperm and results in degradation of cyclin B, the regulatory subunit of MPF through the Anaphase-Promoting Complex/Cyclosome (APC/C). Arrest at metaphase II may therefore be viewed as being maintained by inhibition of the APC/C, and Cytostatic Factor as being one or more pathways, one of which inhibits the APC/C, consorting in the preservation of MPF activity.

Many studies over several years have implicated the c-Mos/MEK/MAPK pathway in the metaphase arrest of the two most widely studied vertebrates, frog and mouse. Murine downstream components of this cascade are not known but in frog involve members of the spindle assembly checkpoint, which act to inhibit the APC/C. Interesting these downstream components appear not to be involved in the arrest of mouse eggs, suggesting a lack of conservation with respect to c-Mos targets. However, the recent discovery of Emi2 as an egg specific APC/C inhibitor whose degradation is Ca²⁺ dependent has greatly increased our understanding of MetII arrest. Emi2 is involved in both the establishment and maintenance of metaphase II arrest in frog and mouse suggesting a conservation of metaphase II arrest. Its identity as the physiologically relevant APC/C inhibitor involved in Cytostatic Factor arrest prompted us to re-evaluate the role of the c-Mos pathway in metaphase II arrest.

This review presents a model of Cytostatic Factor arrest, which is primarily induced by Emi2 mediated APC/C inhibition but which also requires the c-Mos pathway to set MPF levels within physiological limits, not too high to induce an arrest that cannot be broken, or too low to induce parthenogenesis.

Murine homologues of the Drosophila gustavus gene are expressed in ovarian granulosa cells

Mammalian homologues of genes that control oogenesis in other organisms may play similar roles in mammalian ovarian development. In Drosophila melanogaster, GUSTAVUS (GUS) protein physically interacts with and is necessary for the proper posterior localization of VASA protein, and thus is required for specification of germ cells. We identified two mouse genes, SSB-1 and SSB-4 (SPRY domain SOCS box protein), whose protein products share 75% identity and are each approximately 70% identical to Drosophila GUS. Both SSB-1 and SSB-4 mRNA were detectable in mouse ovaries by Northern blotting of total and poly(A) + RNA, but were expressed in few other tissues. SSB-1 was detectable in testes, although the 3'-untranslated region of the mRNA was considerably shorter than the ovarian mRNA. In situ hybridization and RT-PCR analysis of ovaries revealed that both genes were expressed in granulosa cells at all stages of follicular development. In contrast, expression was barely detectable in in oocytes. Immunoblotting analysis revealed that SSB-1 protein was present in follicles at different stages of growth, and immunocytochemistry confirmed that SSB-1 and SSB-4 were detectable in granulosa cells of primary and subsequent stage follicles and that they were present in both mural and cumulus granulosa cells of antral follicles. These results establish that GUS-related proteins, which in Drosophila are restricted to the germ cells, are in the mouse instead expressed in the granulosa cells and are present throughout folliculogenesis. Based on their tissue-restricted pattern of expression and apparent abundance in granulosa cells, we propose that SSB-1 and SSB-4 play key roles in regulating granulosa cell physiology.

In vitro germline potential of stem cells derived from fetal porcine skin

Two of the unanswered questions in mammalian developmental biology are when and where the fate of the germ cell is specified. Here, we report that stem cells isolated from the skin of porcine fetuses have the intrinsic ability to differentiate into oocyte-like cells. When differentiation was induced, a subpopulation of these cells expressed markers such as Oct4, Growth differentiation factor 9b (GDF9b), the Deleted in Azoospermia-like (DAZL) gene and Vasa - all consistent with germ-cell formation. On further differentiation, these cells formed follicle-like aggregates that secreted oestradiol and progesterone and responded to gonadotropin stimulation. Some of these aggregates extruded large oocyte-like cells that expressed oocyte markers, such as zona pellucida, and the meiosis marker, synaptonemal complex protein 3 (SCP3). Some of these oocyte-like cells spontaneously developed into parthenogenetic embryo-like structures. The ability to generate oocyte-like cells from skin-derived cells may offer new possibilities for tissue therapy and provide a new in vitro model to study germ-cell formation and oogenesis.

Mechanisms and chemical induction of aneuploidy in rodent germ cells

The objective of this review is to suggest that the advances being made in our understanding of the molecular events surrounding chromosome segregation in non-mammalian and somatic cell models be considered when designing experiments for studying aneuploidy in mammalian germ cells. Accurate chromosome segregation requires the temporal control and unique interactions among a vast array of proteins and cellular organelles. Abnormal function and temporal disarray among these, and others to be identified, biochemical reactions and cellular organelles have the potential for predisposing cells to aneuploidy. Although numerous studies have demonstrated that certain chemicals (mainly those that alter microtubule function) can induce aneuploidy in mammalian germ cells, it seems relevant to point out that such data can be influenced by gender, meiotic stage, and time of cell-fixation post-treatment. Additionally, a consensus has not been reached regarding which of several germ cell aneuploidy assays most accurately reflects the human condition. More recent studies have shown that certain kinase, phosphatase, proteasome, and topoisomerase inhibitors can also induce aneuploidy in rodent germ cells. We suggest that molecular approaches be prudently incorporated into mammalian germ cell aneuploidy research in order to eventually understand the causes and mechanisms of human aneuploidy. Such an enormous undertaking would benefit from collaboration among scientists representing several disciplines.

Xenopus embryonic poly(A) binding protein 2 (ePABP2) defines a new family of cytoplasmic poly(A) binding proteins expressed during the early stages of vertebrate development

We describe a new RNA binding protein from Xenopus we have named ePABP2 (embryonic poly(A) binding protein type II). Based on amino acid similarity, ePABP2 is closely related to the ubiquitously expressed nuclear PABP2 protein that directs the elongation of mRNA poly(A) tails during pre-mRNA processing. However, in contrast to known PABP2 proteins, Xenopus ePABP2 is a cytoplasmic protein that is predominantly expressed during the early stages of Xenopus development and in adult ovarian tissue. Biochemical experiments indicate ePABP2 binds poly(A) with specificity and that this binding requires the RRM domain. Mouse and human ePABP2 proteins were also identified and mouse ePABP2 expression is also confined to the earliest stages of mouse development and adult ovarian tissue. We propose that Xenopus ePABP2 is the founding member of a new class of poly(A) binding proteins expressed in vertebrate embryos. Possible roles for this protein in regulating mRNA function in early vertebrate development are discussed.

Fertilization and InsP3-induced Ca2+ release stimulate a persistent increase in the rate of degradation of cyclin B1 specifically in mature mouse oocytes

Vertebrate oocytes proceed through meiosis I before undergoing a cytostatic factor (CSF)-mediated arrest at metaphase of meiosis II. Exit from MII arrest is stimulated by a sperm-induced increase in intracellular Ca2+. This increase in Ca2+ results in the destruction of cyclin B1, the regulatory subunit of cdk1 that leads to inactivation of maturation promoting factor (MPF) and egg activation. Progression through meiosis I also involves cyclin B1 destruction, but it is not known whether Ca2+ can activate the destruction machinery during MI. We have investigated Ca2+ -induced cyclin destruction in MI and MII by using a cyclin B1-GFP fusion protein and measurement of intracellular Ca2+. We find no evidence for a role for Ca2+ in MI since oocytes progress through MI in the absence of detectable Ca2+ transients. Furthermore, Ca2+ increases induced by photorelease of InsP3 stimulate a persistent destruction of cyclin B1-GFP in MII but not MI stage oocytes. In addition to a steady decrease in cyclin B1-GFP fluorescence, the increase in Ca2+ stimulated a transient decrease in fluorescence in both MI and MII stage oocytes. Similar transient decreases in fluorescence imposed on a more persistent fluorescence decrease were detected in cyclin-GFP-injected eggs undergoing fertilization-induced Ca2+ oscillations. The transient decreases in fluorescence were not a result of cyclin B1 destruction since transients persisted in the presence of a proteasome inhibitor and were detected in controls injected with eGFP and in untreated oocytes. We conclude that increases in cytosolic Ca2+ induce transient changes in autofluorescence and that the pattern of cyclin B1 degradation at fertilization is not stepwise but exponential. Furthermore, this Ca2+ -induced increase in degradation of cyclin B1 requires factors specific to mature oocytes, and that to overcome arrest at MII, Ca2+ acts to release the CSF-mediated brake on cyclin B1 destruction.

A binding site for germ cell nuclear factor within c-mos regulatory sequences

The proto-oncogene c-mos is specifically expressed in male and female germ cells. Previous studies have shown that the orphan nuclear receptor chicken ovalbumin upstream promoter transcription factor (COUP-TF) contributes to the repression of c-mos in somatic cells by binding to an inverted hexamer repeat within the c-mos regulatory region. In the present study, we demonstrate that another nuclear receptor superfamily member, germ cell nuclear factor (GCNF), binds to a sequence overlapping the c-mos COUP-TF binding site. Electrophoretic mobility shift assays with recombinant GCNF and both wild-type and mutant c-mos oligonucleotides demonstrated the binding of GCNF to an extended half site, CCAAGTTCA, which overlaps the first hexamer of the COUP-TF binding site. Transient transfection assays in NIH 3T3 cells further demonstrated that GCNF fused to a VP16 activation domain stimulated transcription from reporter constructs containing the c-mos GCNF binding site. Since GCNF is expressed in male and female germ cells at the same stages of development at which c-mos is transcribed, these results suggest that GCNF may serve as a regulator of c-mos transcription. Mol. Reprod. Dev. 67: 55-64, 2004.

Okadaic acid, an inhibitor of protein phosphatase 1 and 2A, induces premature separation of sister chromatids during meiosis I and aneuploidy in mouse oocytes in vitro

Recent advances in understanding some of the molecular aspects of chromosome segregation during mitosis and meiosis provide a background for investigating potential mechanisms of aneuploidy in mammalian germ cells. Numerous protein kinases and phosphatases have important functions during mitosis and meiosis. Alterations in these enzyme activities may upset the normal temporal sequence of biochemical reactions and cellular organelle modifications required for orderly chromosome segregation. Protein phosphatases 1 (PP1) and 2A (PP2A) play integral roles in regulating oocyte maturation (OM) and the metaphase-anaphase transitions. Mouse oocytes were transiently exposed in vitro to different dosages (0, 0.01, 0.1, or 1.0 microg/ml) of the PP1 and PP2A phosphatase inhibitor okadaic acid (OA) during meiosis I and oocytes were cytogenetically analyzed. Significant (p < 0.05) OA dose-response increases in the frequencies of metaphase I (MI) arrested oocytes, MI oocytes with 80 chromatids instead of the normal 20 tetrads, and anaphase I telophase I (AI-TI) oocytes with two groups of an unequal number of chromatids were found. Analysis of MII oocytes revealed significant (p < 0.05) increases in the frequencies of premature sister chromatid separation, single-unpaired chromatids, and hyperploidy. Besides showing that OA is aneugenic, these data suggest that OA-induced protein phosphatase inhibition upsets the normal kinase-phosphatase equilibrium during mouse OM, resulting in precocious removal of cohesion proteins from chromosomes.

12-O-Tetradecanoylphorbol-13-acetate and UV Radiation-induced Nucleoside Diphosphate Protein Kinase B Mediates Neoplastic Transformation of Epidermal Cells

The molecular changes associated with early skin carcinogenesis are largely unknown. We have previously identified 11 genes whose expression was up- or down-regulated by 12-O-tetradecanoylphorbol-13-acetate (TPA) in mouse skin keratinocyte progenitor cells (Wei, S.-J., Trempus, C. S., Cannon, R. E., Bortner, C. D., and Tennant, R. W. (2003) J. Biol. Chem. 278, 1758-1768). Here, we show an induction of a nucleoside diphosphate protein kinase B (NDPK-B) gene in response to TPA or UV radiation (UVR). TPA or UVR significantly induced the expression of NDPK-B both in vivo hyperplastic mouse skin and in vitro mouse JB6 Cl 41-5a epidermal cells. Indeed, this gene was also up-regulated in TPA or UVR-mediated skin tumors including papillomas, spindle cell tumors, and squamous cell carcinomas, relative to adjacent normal skins. Functional studies by constitutive expression of nm23-M2/NDPK-B in TPA susceptible JB6 Cl 41-5a and TPA-resistant JB6 Cl 30-7b preneoplastic epidermal cell lines showed a remarkable gene dosage-dependent increase in foci-forming activity, as well as an enhancement in the efficiency of neoplastic transformation of these cells in soft agar but no effect on proliferation in monolayer cultures. Interestingly, stable transfection of the nm23-M2/NDPK-B del-RGD or G106A mutant gene in JB6 Cl 41-5a cells selectively abrogated NDPK-B-induced cellular transformation, implicating a possible Arg105-Gly106-Asp107 regulatory role in early skin carcinogenesis.

Involvement of mitogen-activated protein kinase cascade during oocyte maturation and fertilization in mammals

Mitogen-activated protein kinase (MAPK) is a family of Ser/Thr protein kinases that are widely distributed in eukaryotic cells. Studies in the last decade revealed that MAPK cascade plays pivotal roles in regulating the meiotic cell cycle progression of oocytes. In mammalian species, activation of MAPK in cumulus cells is necessary for gonadotropin-induced meiotic resumption of oocytes, while MAPK activation is not required for spontaneous meiotic resumption. After germinal vesicle breakdown (GVBD), MAPK is involved in the regulation of microtubule organization and meiotic spindle assembly. The activation of this kinase is essential for the maintenance of metaphase II arrest, while its inactivation is a prerequisite for pronuclear formation after fertilization or parthenogenetic activation. MAPK cascade interacts extensively with other protein kinases such as maturation-promoting factor, protein kinase A, protein kinase C, and calmodulin-dependent protein kinase II, as well as with protein phosphatases in oocyte meiotic cell cycle regulation. The cross talk between MAPK cascade and other protein kinases is discussed. The review also addresses unsolved problems and discusses future directions.

Maturation-promoting factor governs mitogen-activated protein kinase activation and interphase suppression during meiosis of rat oocytes

Meiosis is a particular example of a cell cycle, characterized by two successive divisions without an intervening interphase. Resumption of meiosis in oocytes is associated with activation of maturation-promoting factor (MPF) and mitogen-activated protein kinase (MAPK). The activity of MPF declines during the transition between the two meiotic divisions, whereas the activity of MAPK is sustained. Attempts to disclose the interplay between these key regulators of meiosis in both amphibian and mammalian oocytes generated contradictory results. Furthermore, the enzyme that governs the suppression of interphase in mammals is still unidentified. To our knowledge, we provide herein the first demonstration in a mammalian system that inhibition of MPF at reinitiation of meiosis abrogated Mos expression and MAPK activation. We also show that oocytes, in which reactivation of MPF at completion of the first telophase was prevented, exhibited an interphase nucleus with decondensed chromosomes. Inhibition of MAPK did not interfere with the progression to the second meiotic metaphase but, rather, resulted in parthenogenic activation. We conclude that in rat oocytes, MPF regulates MAPK activation and its timely reactivation prevents the oocytes from entering interphase.

Identification of Dss1 as a 12-O-tetradecanoylphorbol-13-acetate-responsive gene expressed in keratinocyte progenitor cells, with possible involvement in early skin tumorigenesis

This study identifies genes expressed early in 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced skin carcinogenesis in genetically initiated Tg.AC v-Ha-ras transgenic mice. Keratinocyte progenitor cells from TPA-treated Tg.AC mice were isolated with fluorescence-activated cell sorting and expression was analyzed using cDNA microarray technology. Eleven genes were identified whose expression changed significantly in response to carcinogen treatment. Deleted in split hand/split foot 1 (Dss1) is a gene associated with a heterogeneous limb developmental disorder called split hand/split foot malformation. cDNA microarray expression analysis showed that the mouse homologue of Dss1 is induced by TPA. Dss1 overexpression was detected by Northern blot analysis in early TPA-treated hyperplastic skins and in JB6 Cl 41-5a epidermal cells. Interestingly, Dss1 expression was also shown to be elevated in skin papillomas relative to normal skins, and further increased in squamous cell malignancies. Functional studies by ectopically constitutive expression of Dss1 in JB6 Cl 41-5a preneoplastic cells strongly increased focus formation and proliferation of these cells and enhanced efficiency of neoplastic transformation of the cells in soft agar. These results strongly suggest that Dss1 is a TPA-inducible gene that may play an important role in the early stages of skin carcinogenesis.

Stem-loop binding protein accumulates during oocyte maturation and is not cell-cycle-regulated in the early mouse embryo

The stem-loop binding protein (SLBP) binds to the 3' end of histone mRNA and participates in 3'-processing of the newly synthesized transcripts, which protects them from degradation, and probably also promotes their translation. In proliferating cells, translation of SLBP mRNA begins at G1/S and the protein is degraded following DNA replication. These post-transcriptional mechanisms closely couple SLBP expression to S-phase of the cell cycle, and play a key role in restricting synthesis of replication-dependent histones to S-phase. In contrast to somatic cells, replication-dependent histone mRNAs accumulate and are translated independently of DNA replication in oocytes and early embryos. We report here that SLBP expression and activity also differ in mouse oocytes and early embryos compared with somatic cells. SLBP is present in oocytes that are arrested at prophase of G2/M, where it is concentrated in the nucleus. Upon entry into M-phase of meiotic maturation, SLBP begins to accumulate rapidly, reaching a very high level in mature oocytes arrested at metaphase II. Following fertilization, SLBP remains abundant in the nucleus and the cytoplasm throughout the first cell cycle, including both G1 and G2 phases. It declines during the second and third cell cycles, reaching a relatively low level by the late 4-cell stage. SLBP can bind the histone mRNA-stem-loop at all stages of the cell cycle in oocytes and early embryos, and it is the only stem-loop binding activity detectable in these cells. We also report that SLBP becomes phosphorylated rapidly following entry into M-phase of meiotic maturation through a mechanism that is sensitive to roscovitine, an inhibitor of cyclin-dependent kinases. SLBP is rapidly dephosphorylated following fertilization or parthenogenetic activation, and becomes newly phosphorylated at M-phase of mitosis. Phosphorylation does not affect its stem-loop binding activity. These results establish that, in contrast to Xenopus, mouse oocytes and embryos contain a single SLBP. Expression of SLBP is uncoupled from S-phase in oocytes and early embryos, which indicates that the mechanisms that impose cell-cycle-regulated expression of SLBP in somatic cells do not operate in oocytes or during the first embryonic cell cycle. This distinctive pattern of SLBP expression may be required for accumulation of histone proteins required for sperm chromatin remodelling and assembly of newly synthesized embryonic DNA into chromatin.

MG-132, an inhibitor of proteasomes and calpains, induced inhibition of oocyte maturation and aneuploidy in mouse oocytes

BACKGROUND: Although chromosome missegregation during oocyte maturation (OM) is a significant contributor to human morbidity and mortality, very little is known about the causes and mechanisms of aneuploidy. Several investigators have proposed that temporal perturbations during OM predispose oocytes to aberrant chromosome segregation. One approach for testing this proposal is to temporarily inhibit the activity of protein proteolysis during OM. We used the reversible proteasome inhibitor MG-132 to transiently perturb the temporal sequence of events during OM and subsequently analyzed mouse metaphase II (MII) for cytogenetic abnormalities. The transient inhibition of proteasome activity by MG-132 resulted in elevated levels of oocytes containing extra chromatids and chromosomes. RESULTS: The transient inhibition of proteasome-mediated proteolysis during OM by MG-132 resulted in dose-response delays during OM and elevated levels of aneuploid MII oocytes. Oocytes exposed in vitro to MG-132 exhibited greater delays during metaphase I (MI) as demonstrated by significantly (p < 0.01) higher levels of MI arrested oocytes and lower frequencies of premature sister chromatid separation in MII oocytes. Furthermore, the proportions of MII oocytes containing single chromatids and extra chromosomes significantly (p < 0.01) increased with MG-132 dosage. CONCLUSIONS: These data suggest that the MG-132-induced transient delay of proteasomal activity during mouse OM in vitro predisposed oocytes to abnormal chromosome segregation. Although these findings support a relationship between disturbed proteasomal activity and chromosome segregation, considerable additional data are needed to further investigate the roles of proteasome-mediated proteolysis and other potential molecular mechanisms on chromosome segregation during OM.

Mos is not required for the initiation of meiotic maturation in Xenopus oocytes

In Xenopus oocytes, the c-mos proto-oncogene product has been proposed to act downstream of progesterone to control the entry into meiosis I, the transition from meiosis I to meiosis II, which is characterized by the absence of S phase, and the metaphase II arrest seen prior to fertilization. Here, we report that inhibition of Mos synthesis by morpholino antisense oligonucleotides does not prevent the progesterone-induced initiation of Xenopus oocyte meiotic maturation, as previously thought. Mos-depleted oocytes complete meiosis I but fail to arrest at metaphase II, entering a series of embryonic-like cell cycles accompanied by oscillations of Cdc2 activity and DNA replication. We propose that the unique and conserved role of Mos is to prevent mitotic cell cycles of the female gamete until the fertilization in Xenopus, starfish and mouse oocytes.

Identification of protein tyrosine phosphatase 1B and casein as substrates for 124-v-Mos

BACKGROUND

The mos proto-oncogene encodes a cytoplasmic serine/threonine-specific protein kinase with crucial function during meiotic cell division in vertebrates. Based on oncogenic amino acid substitutions the viral derivative, 124-v-Mos, displays constitutive protein kinase activity and functions independent of unknown upstream effectors of mos protein kinase. We have utilized this property of 124-v-Mos and screened for novel mos substrates in immunocomplex kinase assays in vitro.

RESULTS

We generated recombinant 124-v-Mos using the baculovirus expression system in Spodoptera frugiperda cells and demonstrated constitutive kinase activity by the ability of 124-v-Mos to auto-phosphorylate and to phosphorylate vimentin, a known substrate of c-Mos. Using this approach we analyzed a panel of acidic and basic substrates in immunocomplex protein kinase assays and identified novel in vitro substrates for 124-v-Mos, the protein tyrosine phosphatase 1B (PTP1B), alpha-casein and beta-casein. We controlled mos-specific phosphorylation of PTP1B and casein in comparative assays using a synthetic kinase-inactive 124-v-Mos mutant and further, tryptic digests of mos-phosphorylated beta-casein identified a phosphopeptide specifically targeted by wild-type 124-v-Mos. Two-dimensional phosphoamino acid analyses showed that 124-v-mos targets serine and threonine residues for phosphorylation in casein at a 1:1 ratio but auto-phosphorylation occurs predominantly on serine residues.

CONCLUSION

The mos substrates identified in this study represent a basis to approach the identification of the mos-consensus phosphorylation motif, important for the development of specific inhibitors of the Mos protein kinase.

Microinjection of antisense c-mos oligonucleotides prevents the progression of meiosis in human and hamster oocytes

OBJECTIVE

To investigate the role of c-mos proto-oncogene in the progression of meiosis in human and hamster oocytes.

DESIGN

Controlled basic research study.

SETTING

Assisted reproduction units at medical institutions.

PATIENT(S)

Consenting in vitro fertilization patients.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Maturation to metaphase II (MII) 24 hours following microinjection of prophase I (PI) hamster oocytes with antisense (AS) and sense (S) c-mos oligonucleotides. Control oocytes (C) injected with medium or left uninjected (UI). In human oocytes, maturation to metaphase II was also measured except culture was extended to 48 hours and the sense group was omitted.

RESULT(S)

The percentage of hamster oocytes reaching metaphase II after 24 hours was as follows: 1.5% (1 of 65) for the antisense group; 63.1% (41 of 65) in the sense group; 66.1% (41 of 62) in the control group; and 69.3% (52 of 75) in the uninjected group. The percentage of human oocytes at metaphase II was 33.3% (4 of 12) in the antisense group, 83.3% (10 of 12) in the control group, and 82.8% (24 of 29) in the uninjected group.

CONCLUSION(S)

These results demonstrate that injection of c-mos antisense oligonucleotide significantly inhibits the progression of meiosis in hamster (P=.0001) and human (P=.05) oocytes. Thus, c-mos proto-oncogene may be one of the critical regulators of meiosis in these two species.

The maternal legacy to the embryo: cytoplasmic components and their effects on early development

RNA molecules and proteins are accumulated in the oocyte cytoplasm during its growth phase and are used to sustain the early phases of embryonic development before embryo DNA transcription begins. This makes the oocyte a very special cell, quite different from somatic cells where RNA and proteins usually undergo a rapid turnover. To enable the storage and timely use of such stored molecules, various mechanisms are effective in the oocyte and are gradually being elucidated. Our understanding of such mechanisms is important for constantly improving therapy for human and animal reproductive disorders as well as for understanding the process of nuclear reprogramming during cloning procedure or stem cell generation. This review focuses on the various aspects of these regulatory processes in an attempt to give an overview of the present knowledge on post-transcriptional and post-translational mechanisms taking place during oocyte maturation and early development. Mechanisms such as cytoplasmic regulation of the poly(A) tail, RNA localization and protein phosphorylation are described in some detail. Because most data are available from lower species these are presented together with appropriate reference to the mammalian oocyte when data are known, or when important differences have been described.

Selective reduction of dormant maternal mRNAs in mouse oocytes by RNA interference

Specific mRNA degradation mediated by double-stranded RNA (dsRNA), which is termed RNA interference (RNAi), is a useful tool with which to study gene function in several systems. We report here that in mouse oocytes, RNAi provides a suitable and robust approach to study the function of dormant maternal mRNAs. Mos (originally known as c-mos) and tissue plasminogen activator (tPA, Plat) mRNAs are dormant maternal mRNAs that are recruited during oocyte maturation; translation of Mos mRNA results in the activation of MAP kinase. dsRNA directed towards Mos or Plat mRNAs in mouse oocytes effectively results in the specific reduction of the targeted mRNA in both a time- and concentration-dependent manner. Moreover, dsRNA is more potent than either sense or antisense RNAs. Targeting the Mos mRNA results in inhibiting the appearance of MAP kinase activity and can result in parthenogenetic activation. Mos dsRNA, therefore, faithfully phenocopies the Mos null mutant. Targeting the Plat mRNA with Plat dsRNA results in inhibiting production of tPA activity. Finally, effective reduction of the Mos and Plat mRNA is observed with stoichiometric amounts of Mos and Plat dsRNA, respectively.

Specific interference with gene function by double-stranded RNA in early mouse development

The use of double-stranded (ds) RNA is a powerful way of interfering with gene expression in a range of organisms, but doubts have been raised about whether it could be successful in mammals. Here, we show that dsRNA is effective as a specific inhibitor of the function of three genes in the mouse, namely maternally expressed c-mos in the oocyte and zygotically expressed E-cadherin or a GFP transgene in the preimplantation embryo. The phenotypes observed are the same as those reported for null mutants of the endogenous genes. These findings offer the opportunity to study development and gene regulation in normal and diseased cells.

Analysis of the mechanism(s) of metaphase I-arrest in strain LT mouse oocytes: delay in the acquisition of competence to undergo the metaphase I/anaphase transition

Fully grown oocytes of most laboratory mice progress without interruption from the germinal vesicle (GV) stage to metaphase II, where meiosis is arrested until fertilization. In contrast, many oocytes of strain LT mice arrest precociously at metaphase I and often undergo subsequent spontaneous parthenogenetic activation. Cytostatic factor (CSF), which prevents the degradation of cyclin B and maintains high maturation-promoting factor (MPF) activity, is required for maintenance of metaphase I-arrest in LT oocytes, similar to its requirement for maintaining metaphase II-arrest in normal oocytes. However, CSF does not instigate metaphase I-arrest since a temporary metaphase I-arrest occurs in MOS-null LT oocytes. This paper addresses the mechanism(s) that may instigate metaphase I-arrest and tests the hypothesis that there may be one or more defects in LT oocytes that delay their acquisition of competence to trigger the cascade of processes that normally drive entry into and progression through anaphase I. To test this hypothesis, MPF activity was artificially abrogated by treating oocytes with a general protein kinase inhibitor, 6-DMAP, at various times during the progression of meiosis I. This allowed a comparison of the time at which LT and normal oocytes become competent to undergo the metaphase I/anaphase transition even if oocytes were arrested at metaphase I when 6-DMAP-treatment was begun. There were no differences between LT and control oocytes in the kinetics of MPF suppression by 6-DMAP. However, it was found that LT oocytes do not acquire competence to undergo the metaphase I/anaphase transition in response to 6-DMAP until 50-60 min after normal oocytes. A similar delay was observed in strain CX8-4 oocytes, which also have a high incidence of metaphase I-arrest, but not in strain CX8-11 oocytes, which exhibit a low incidence of metaphase I-arrest. MOS-null LT oocytes also exhibit a delay in acquisition of competence to undergo the metaphase I/anaphase transition. Thus, a delay in competence to undergo the metaphase I/anaphase transition in response to 6-DMAP-treatment correlates with metaphase I-arrest. It is therefore hypothesized that the observed delay in acquisition of competence to enter anaphase I may instigate the sustained metaphase I-arrest in LT oocytes by allowing CSF activity to rise to a level that prevents cyclin B degradation and maintains high MPF activity before anaphase can be initiated by normal triggering mechanisms.

Expression and phosphorylation of mitogen-activated protein kinases during spermatogenesis and epididymal sperm maturation in mice

The expression and phosphorylation/dephosphorylation of mitogen-activated protein (MAP) kinases during mouse spermatogenesis and epididymal sperm maturation have been investigated by immunoblotting and immunohistochemical staining with commercially available anti-ERK2 and anti-Active MAPK antibodies. Two forms of MAP kinases, p42ERK2 and p44ERK1, were expressed in a similar amount in spermatogenic cells at different stages. ERK1 and ERK2 were phosphorylated (activated) in early spermatogenic cells from primitive spermatogonia to zygotene primary spermatocytes, while only a small quantity of phosphorylated MAP kinases could be detected in pachytene primary spermatocytes and spermatids. MAP kinase activity in primative spermatogonia and preleptotene primary spermatocytes was the highest among spermatogenic cells. ERK1 and ERK2 were also present in epididymal spermatozoa, and their phosphorylation was increased while spermatozoa pass through epididymis and vas deferens for maturation. It would appear that MAP kinase activation may contribute to the mitotic proliferation of primative spermatogonia, an early phase of spermatogenic meiosis, and, later, sperm motility acquirement.

eIF4E expression in tumors: its possible role in progression of malignancies

A central issue in the study of neoplastic transformation is to understand how proto-oncogene products deregulate normal processes of cell growth and differentiation: an intrinsic aspect of this is to probe the sequence of events leading to altered expression of proto-oncogenes. In the past few years, studies aimed at understanding the regulation and function of protein synthesis initiation factors, eIF4E initially, culminated in the unexpected finding that a moderate overexpression of this factor results in dramatic phenotypic changes, including rapid proliferation and malignant transformation. Conversely, the tumorigenic properties of cancer cells can be strongly inhibited by antisense-RNA against eIF4E, or overexpression of the inhibitory proteins: 4E-BPs. Furthermore, eIF4E is elevated in carcinomas of the breast, head and neck (HNSCC) and prostate, but not in typical benign lesions. This is a strong indication that elevated eIF4E expression may mark a critical transition in cancer progression. Establishing a greater protein synthesis output may be a necessary step for cancer cells in order to sustain their rapid proliferation. However, analysis of cells transformed by eIF4E revealed that the synthesis of only a few proteins was greatly enhanced, while synthesis of most was minimally increased. One possible explanation is that eIF4E causes these effects by specifically increasing the translational efficiency of several oncogene transcripts, leading to overexpression of their products. The feasibility of this hypothesis was confirmed experimentally with the identification of several important products that are specifically upregulated in eIF4E-overexpressing cells. These include: c-Myc, cyclin DI and ODC, which control cycle progression and tumorigenesis; basic fibroblast growth factor (FGF-2) and vascular endothelial growth factor (VEGF), which are powerful promoters of cell growth and angiogenesis. A deeper understanding of the mRNAs that are strongly dependent on excess eIF4E/F for efficient translation will eventually result in fuller understanding of the fundamental role of translational control in different pathophysiological conditions, including malignancy.

SPIN, a substrate in the MAP kinase pathway in mouse oocytes

The newly cloned gene Spin encodes a 30-kDa protein, a well-defined abundant molecule found in mouse oocytes and early embryos. This protein SPIN undergoes metaphase-specific phosphorylation and binds to the spindle. To understand the role of SPIN in oocyte meiosis, oocytes were treated with drugs that affect the cell cycle by activating or inactivating specific kinases. The posttranslational modification of SPIN in the treated oocytes was then investigated by one- and two-dimensional gel electrophoresis. Modification of SPIN is inhibited by treatment with 6-dimethylaminopurine (DMAP), suggesting that SPIN is phosphorylated by a serine-threonine kinase. Furthermore, SPIN from cycloheximide-treated oocytes that lack detectable MAP kinase activity is only partially phosphorylated, indicating that SPIN may be phosphorylated by the MOS/MAP kinase pathway. To confirm this observation, SPIN was analyzed in Mos-null mutant mice lacking MAP kinase activity. Normal posttranslational modification of SPIN did not occur in Mos-null mutant oocytes. In addition, there is reduced association of SPIN with the metaphase I spindle in Mos-null mutant oocytes, as determined by immunohistochemical analysis. These findings suggest that SPIN is a substrate in the MOS/ MAP kinase pathway and further that this phosphorylation of SPIN may be essential for its interaction with the spindle.

Suppression of fibroblast growth factors 1 and 2 by antisense oligonucleotides in embryonic chick retinal cells in vitro inhibits neuronal differentiation and survival

As retinal histogenesis proceeds there is a pronounced increase in the expression of fibroblast growth factor (FGF), reaching its maximum in the mature retina and largely in terminal differentiated retinal neurons. Recent in vivo evidence suggests that exogenous FGF functions as a differentiation and survival factor for a wide variety of cell types including CNS neurons and that endogenous FGF may perform similar functions. We have examined the consequences of selectively and independently inhibiting FGF1 or FGF2 expression using antisense oligonucleotides in embryonic chick retinal cells, differentiating in vitro. Whether FGF1 or FGF2 expression was inhibited the results were the same: a marked reduction in neuronal photoreceptor cells differentiation, an increase in programmed cell death, but no effects on cell proliferation. Even although these two related factors promote the same final effect on retinal cells, namely, neuronal differentiation and survival, their normal combined activities or levels appear to be important in achieving this effect. Stimulation with either exogenous FGF1 or FGF2 served to increase endogenous levels of both FGF1 and FGF2 and reversed the effects of antisense blockade of either FGF1 or FGF2. Our data suggest that although other sources of FGF exist within the eye, the function of endogenous FGF in differentiating retinal neurons may be to stimulate their differentiation and promote their survival.

Components of the signaling pathway linking the 1-methyladenine receptor to MPF activation and maturation in starfish oocytes

Starfish oocytes are arrested at the G2/M-phase border of meiosis I. Exposure to their natural mitogen, 1-methyladenine (1-MA), leads to the activation of MPF and MAP kinase, resumption of the meiotic cell cycle, and fertilization competency. The 1-MA receptor has not yet been identified, but it is known to be linked functionally to a pertussis toxin-sensitive G-protein. G beta gamma appears to be the major effector of the 1-MA receptor, since injection of G beta gamma, but not activated G alpha i, leads to the activation of MPF, entry into meiosis, and oocyte maturation. The components that connect G beta gamma to MPF and MAP kinase activation in oocytes are unknown. In mammalian cells, a novel phosphatidylinositol 3-kinase, PI-3 kinase-gamma, links G beta gamma to the MAP kinase activation pathway. Here we show that PI-3 kinase is required for starfish oocyte maturation. LY294002 and wortmannin, inhibitors of PI-3 kinase, block MPF and MAP kinase activation and entry into meiosis. Inhibition by LY294002 is reversible and limited to the hormone-dependent period. Neither inhibitor, however, blocks the earliest hormone-induced event, formation of actin spikes at the cell membrane. By contrast, pertussis toxin blocks both actin spiking and later events, arguing that PI-3 kinase functions downstream of G beta gamma. Finally, we show that unlike the well-studied case in Xenopus oocytes, where MAP kinase is an essential component of the MPF activation pathway, MAP kinase is not required for either MPF activation or subsequent oocyte maturation in starfish. Instead, its major role appears to be suppression of DNA synthesis in unfertilized, haploid eggs.

The oocyte metaphase arrest

Usually, oocyte meiosis reinitiation appears as a two step process during which release from the prophase block is followed by a second arrest in metaphase I or II. In this review, we will examine the mechanisms required to maintain the metaphase arrest and stabilize MPF activity at this stage. Then, we will analyse the processes required to exit from the metaphase block. These may drive the cells forward to the metaphase-anaphase transition, as a result of fertilization, activation or protein synthesis inhibition. Instead, inhibiting protein phosphorylation drives the oocyte back to interphase. All these treatments result in derepression of DNA synthesis.

Analysis of the mechanism(s) of metaphase I arrest in strain LT mouse oocytes: participation of MOS

Oocytes of almost all vertebrates become arrested at metaphase II to await fertilization. Arrest is achieved with the participation of a protein complex known as cytostatic factor (CSF) that stabilizes histone H1 kinase activity. MOS and mitogen-activated protein kinase (MAPK) are important components of CSF. Strain LT/Sv mice, and strains related to LT/Sv, produce a high percentage of atypical oocytes that are arrested at metaphase I when normal oocytes have progressed to metaphase II. The potential role of MOS in metaphase I arrest was investigated using strain LT/Sv and LT-related recombinant inbred strains, LTXBO and CX8-4. MOS and MAPK are produced and functional in maturing LT oocytes. Two experimental paradigms were used to reduce or delete MOS in LT oocytes and assess effects on metaphase I arrest. First, sense and antisense Mos oligonucleotides were microinjected into metaphase I-arrested oocytes. Antisense, but not sense, Mos oligonucleotides promoted the activation of metaphase I-arrested oocytes. Second, mice carrying a Mos null mutation were crossed with LT mice, the null mutation was backcrossed three times to LT mice, and Mos(+/−) N3 mice were intercrossed to produce Mos(−/−), Mos(+/−) and Mos(+/+) N3F1 mice. Oocytes of all three Mos genotypes of N3F1 mice sustained meiotic arrest for 17 hours indicating that metaphase I arrest is not initiated by a MOS-dependent mechanism. However, unlike Mos(+/+) and Mos(+/−) CX8-4 N3F1 oocytes, metaphase I arrest of Mos(−/−) CX8-4 N3F1 oocytes was not sustained after 17 hours and became reversed gradually. These results, like the antisense Mos oligonucleotide microinjection experiments, suggest that MOS participates in sustaining metaphase I arrest in LT oocytes.

CENP-E is an essential kinetochore motor in maturing oocytes and is masked during mos-dependent, cell cycle arrest at metaphase II

CENP-E, a kinesin-like protein that is known to associate with kinetochores during all phases of mitotic chromosome movement, is shown here to be a component of meiotic kinetochores as well. CENP-E is detected at kinetochores during metaphase I in both mice and frogs, and, as in mitosis, is relocalized to the midbody during telophase. CENP-E function is essential for meiosis I because injection of an antibody to CENP-E into mouse oocytes in prophase completely prevented progression of those oocytes past metaphase I. Beyond this, CENP-E is modified or masked during the natural, Mos-dependent, cell cycle arrest that occurs at metaphase II, although it is readily detectable at the kinetochores in metaphase II oocytes derived from mos-deficient (MOS-/-) mice that fail to arrest at metaphase II. This must reflect a masking of some CENP-E epitopes, not the absence of CENP-E, in meiosis II because a different polyclonal antibody raised to the tail of CENP-E detects CENP-E at kinetochores of metaphase II-arrested eggs and because CENP-E reappears in telophase of mouse oocytes activated in the absence of protein synthesis.

Delta-1 is a regulator of neurogenesis in the vertebrate retina

In the retina, cell fate determination is thought to be regulated by a series of local cell-cell interactions. Evidence suggests that retinal precursors utilize Notch-mediated intercellular signaling to regulate their fates. However, the identity of the endogenous ligand and its role in the Notch-signaling pathway is not well understood. We have identified C-Delta-1 as the putative endogenous ligand for Notch, in the developing chick retina. C-Delta-1 is coexpressed spatially and temporally with C-Notch-1 and their expression is associated with the temporal aspects of cell birth in the developing retina. This suggests that Delta-Notch signaling is utilized to maintain progenitors in an uncommitted state and that a subtle fluctuation in this signaling helps to sort out competent cells during successive cell-fate determination. We have tested the latter possibility in the specification of the ganglion cells. In early stages of retinal development when ganglion cells are the predominant cells born, decreasing C-Delta-1 expression with antisense oligonucleotides increases the proportion of RA4 antigen-expressing ganglion cells which are recruited predominantly in the periphery. Conversely, use of exogenous Drosophila Delta leads to a decrease in the RA4 antigen-expressing ganglion cells. Our results suggest that C-Delta-1 activation of the Notch pathway regulates the specification of retinal neurons in general and of ganglion cells in particular.