Progression of mouse oocytes from metaphase I to metaphase II is inhibited by fusion with G2 cells

We show that in contrast to metaphase II oocytes, metaphase I oocytes cannot be activated by fusion with the zygote. Fusion of metaphase I oocytes with G2 zygotes was followed by premature chromosome condensation, with 60% of the hybrids becoming arrested at metaphase I, the remainder progressing and arresting at metaphase II. Hybrids of metaphase I oocytes and M-phase zygotes underwent accelerated maturation, but all arrested at metaphase II. In both cases the arrest could be overcome by treatment with the parthenogenetic activators ethanol and cycloheximide. We discuss these findings in relation to the possibility that the metaphase I oocyte contains cytostatic factor activity that is activated by its zygotic partner. Alternatively, the G2 zygote may provide an inhibitor of anaphase, normally never present in the metaphase I oocyte and which is absent from the M-phase zygote.

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.

Polarity of the mouse embryo is anticipated before implantation

In most species, the polarity of an embryo underlies the future body plan and is determined from that of the zygote. However, mammals are thought to be an exception to this; in the mouse, polarity is generally thought to develop significantly later, only after implantation. It has not been possible, however, to relate the polarity of the preimplantation mouse embryo to that of the later conceptus due to the lack of markers that endure long enough to follow lineages through implantation. To test whether early developmental events could provide cues that predict the axes of the postimplantation embryo, we have used the strategy of injecting mRNA encoding an enduring marker to trace the progeny of inner cell mass cells into the postimplantation visceral endoderm. This tissue, although it has an extraembryonic fate, plays a role in axis determination in adjacent embryonic tissue. We found that visceral endoderm cells that originated near the polar body (a marker of the blastocyst axis of symmetry) generally became distal as the egg cylinder formed, while those that originated opposite the polar body tended to become proximal. It follows that, in normal development, bilateral symmetry of the mouse blastocyst anticipates the polarity of the later conceptus. Moreover, our results show that transformation of the blastocyst axis of symmetry into the axes of the postimplantation conceptus involves asymmetric visceral endoderm cell movement. Therefore, even if the definitive axes of the mouse embryo become irreversibly established only after implantation, this polarity can be traced back to events before implantation.

Mouse polo-like kinase 1 associates with the acentriolar spindle poles, meiotic chromosomes and spindle midzone during oocyte maturation

We have examined the dynamics of the localisation of the polo-like kinase 1 (Plk1) during maturation of the mouse oocyte. Levels of Plk1 protein increase following germinal vesicle breakdown, at which time the enzyme begins to accumulate at discrete positions on the condensing chromosomes and, subsequently, at the poles of the meiotic spindle, which moves towards the cortex of the egg. Interestingly, at metaphase in both meiotic divisions, Plk1 shows a punctate localisation along the broad spindle poles. Moreover, the punctate distribution of Plk1 on the meiotic chromosomes appears at early anaphase to correspond to the centromeric regions. The protein relocates to the spindle midzone during late anaphase and then associates with the midbody at telophase. We have confirmed the specific pattern of immuno-localisation seen in fixed preparations by observing the distribution of Plk1 tagged with green fluorescent protein in living oocytes. We discuss the localisation of the enzyme in light of the structure of the spindle poles, which are known to lack centrioles, and the highly asymmetric nature of the meiotic divisions.

Fertile offspring derived from mammalian eggs lacking either animal or vegetal poles

In all animals so far tested, removing either pole of the undivided egg prevents normal development: embryos may arrest early, lack organs, or the adults may be sterile. These experiments have shown that spatial patterning of the egg is of utmost importance for subsequent development. However, the significance of spatial patterning in mammalian eggs is still controversial. To test the importance of egg polarity in the mouse a substantial amount of material either from the animal (polar body-associated) or the vegetal (opposite) pole of the fertilised egg was removed. One pole of the egg was cut away manually with a glass needle and the eggs were allowed to develop in vitro. Both kinds of surgical operation permit the development of blastocysts, which, after transfer to the uteri of pseudo-pregnant foster mothers, can produce viable offspring. Furthermore, these develop into fertile adult mice. I conclude that mouse eggs have no essential components that are localised uniquely to the animal or the vegetal pole and, therefore, do not rely for their axial development on maternal determinants that are so localised in the fertilised egg. Thus the mammalian egg appears to be very unusual in the animal kingdom in that it establishes the embryonic axes after the zygote has begun development.

Following cell fate in the living mouse embryo

It has been difficult to follow many of the dramatic changes in cell fate and cell migration during mouse development. This is because there has been no enduring marker that would allow cells to be recognised in the living embryo. We believe that we have overcome this problem by developing a novel form of green fluorescent protein, named MmGFP, that proves to be easily visible and non toxic to mouse cells and does not perturb embryogenesis. We show that synthetic mRNA encoding MmGFP can be injected into blastomeres to follow the fate of their progeny during preimplantation development. We have made a stable embryonic stem cell line that expresses MmGFP and introduced these fluorescent cells into mouse embryos. For the first time, we have been able to follow the fate of embryonic stem cells in living embryos and to observe directly the contribution of these cells to distinct lineages of the postimplantation embryo. This approach should lead to a more complete description of the dynamics of cell fate in the mouse.

Protein phosphatases control MAP kinase activation and microtubule organization during rat oocyte maturation

Mitogen-activated protein kinase (MAP) is involved in many signal transduction pathways and is activated during meiotic maturation in various species. In this study, we used the rat oocyte to identify some of the control mechanisms involved in MAP kinase activation which is triggered at resumption of meiosis. We examined the respective contribution of this kinase and maturation promoting factor (MPF), or cdc2 kinase, in the regulation of microtubule behavior and in the reorganization of chromatin during meiotic maturation. We found that the resumption of meiotic division in rat oocytes coincided with the activation of MPF and was followed 3 h later by the activation of MAP kinase. The activation of the two kinases also occurred in oocytes undergoing maturation in the presence of the protein phosphatase inhibitor okadaic acid (OA). However, the activation of cdc2 kinase was only partial, whereas activation of MAP kinase was accelerated and began 1 h after the resumption of meiosis, i.e. 2 h earlier than in control oocytes. We also showed that protein synthesis was required to activate MAP kinase, but not cdc2 kinase. However, once MAP kinase was activated, ongoing protein synthesis was not necessary to maintain its activity. These results suggest that a negative regulation of MAP kinase slows down its activation at the resumption of meiosis, mediated through the level of phosphatase activity. Moreover, MAP kinase activation requires protein synthesis, even upon phosphatase inactivation by OA, suggesting also the existence of a positive control pathway. We observed that during the first meiotic M-phase, the spindle did not form immediately after cdc2 kinase activation, but that its formation coincided with the appearance of MAP kinase activity. However, earlier activation of MAP kinase by treatment with OA did not lead to premature spindle formation, but instead a large aster formed consisting of long microtubules radiating from the condensed chromatin. In OA-treated oocytes, spindles did not form and an interphase network of microtubule developed with time. Thus, MAP kinase is unable to substitute for MPF under these conditions, its activity alone being insufficient to maintain the progression through meiotic maturation.

An indelible lineage marker for Xenopus using a mutated green fluorescent protein

We describe the use of a DNA construct (named GFP.RN3) encoding green fluorescent protein as a lineage marker for Xenopus embryos. This offers the following advantages over other lineage markers so far used in Xenopus. When injected as synthetic mRNA, its protein emits intense fluorescence in living embryos. It is non-toxic, and the fluorescence does not bleach when viewed under 480 nm light. It is surprisingly stable, being strongly visible up to the feeding tadpole stage (5 days), and in some tissues for several weeks after mRNA injection. We also describe a construct that encodes a blue fluorescent protein. We exemplify the use of this GFP.RN3 construct for marking the lineage of individual blastomeres at the 32- to 64-cell stage, and as a marker for single transplanted blastula cells. Both procedures have revealed that the descendants of one embryonic cell can contribute single muscle cells to nearly all segmental myotomes rather than predominantly to any one myotome. An independent aim of our work has been to follow the fate of cells in which an early regulatory gene has been temporarily overexpressed. For this purpose, we co-injected GFP.RN3 mRNA and mRNA for the early Xenopus gene Eomes, and found that a high concentration of Eomes results in ectopic muscle gene activation in only the injected cells. This marker may therefore be of general value in providing long term identification of those cells in which an early gene with ephemeral expression has been overexpressed.

Cytostatic factor inactivation is induced by a calcium-dependent mechanism present until the second cell cycle in fertilized but not in parthenogenetically activated mouse eggs

Cytostatic factor (CSF) is an activity responsible for the metaphase II arrest in vertebrate oocytes. This activity maintains a high level of maturation promoting factor (MPF) in the oocyte and both activities are destroyed after fertilization or parthenogenetic activation. To study some of the characteristics of the mechanism involved in MPF and CSF destruction, we constructed hybrid cells between metaphase II arrested oocytes and early embryos obtained after fertilization or artificial activation. We found that the behavior of hybrid cells differed depending upon the type of oocyte activation. Initially, the reaction of both types of hybrid cells was similar, the nuclear envelope broke down and chromatin condensation was induced. However, while metaphase II oocytes fused with parthenogenetic eggs remained arrested in M-phase, the oocytes fused with fertilized eggs underwent activation and passed into interphase. This ability of fertilized eggs to induce oocyte activation was still present at the beginning, but not at the end of the second embryonic cell cycle. Oocyte activation induced by fusion with a fertilized egg could be prevented when calcium was chelated by BAPTA. Thus, element(s) of the mechanism involved in calcium release triggered by a sperm component at fertilization remain(s) active until the second cell cycle and is (are) inactivated before the end of the 2-cell stage.

Activation of embryonic genes during preimplantation rat development

Expression of the embryonic genome has been examined during preimplantation rat development. Proteins synthesized at different stages of embryogenesis were labelled with [35S]methionine and then separated by one-dimensional gel electrophoresis. A major transformation in the pattern of protein synthesis has been observed between the two- and the four-cell stages of embryonic development. Also the culture of embryos with an inhibitor of transcription (alpha-amanitin) has shown that the first alpha-amanitin-sensitive events take place during the late two-cell stage. However, inhibition of transcription does not arrest the embryo development up to the four-cell stage. Taken together, the results indicate that in rats the initiation of embryonic gene activation occurs at the late two-cell stage. However, the first two cleavage divisions can occur in the absence of transcription.

Okadaic acid affects spindle organization in metaphase II-arrested rat oocytes

We used okadaic acid (OA), a potent phosphatase inhibitor, to study the role of phosphatases in the organization of the spindle microtubules in metaphase II-arrested rat oocytes. OA induced a dramatic elongation of the spindle and disorganization of the metaphase plate. Biochemical analysis revealed some dramatic changes after OA treatment: increased histone H1 kinase activity, a burst of protein phosphorylation, and inhibition of protein synthesis. The latter effect alone cannot be responsible for the changes in the spindle structure since inhibition of protein synthesis with puromycin had no such effect. To identify the targets of OA among the proteins associated with the spindle, meiotic spindles of OA-treated and control oocytes were purified. After OA treatment some proteins gained and others lost their association with the spindle. A few proteins that lost their affinity for the spindle also underwent hyperphosphorylation after OA treatment. These results suggest that hyperphosphorylation of some proteins associated with the spindle alters their affinity to microtubules and thus stabilizes them.

Cytoskeletal organization of rat oocytes during metaphase II arrest and following abortive activation: a study by confocal laser scanning microscopy

In metaphase II arrested rat oocytes (M II), microtubules were found in the taper-shaped meiotic spindle and in the cytoplasm as asters and free microtubules. Whereas spindle microtubules were acetylated, those located in the cytoplasm were not. Cytoplasmic microtubules were also labile as assessed by mild cooling. In contrast to mouse oocytes, rat microtubule organizing centers (MTOCs) did not react with MPM-2 antibody by immunofluorescence despite the fact that this antibody reacts with several proteins as shown by immunoblot. However, cytoplasmic MTOCs in M II-arrested rat oocytes could be detected by their nucleating capacity in the presence of taxol, a drug that induced the formation of numerous cytoplasmic asters. In addition, taxol caused a change in the spindle shape and the formation of astral microtubules at the spindle poles. Meiotic spindles (as well as chromosomes devoid of microtubules after nocodazole-treatment) were overlaid by an actin-rich domain. Spontaneous abortive activation led to the extrusion of the second polar body followed by another metaphase arrest--metaphase III; however, normal spindles did not form and dispersed chromosomes surrounded by microtubules were observed. Electron microscopic studies confirmed these observations and revealed that the kinetochores, are located deep within the chromosomes in contrast to mouse kinetochores, and this might be responsible for the absence of a metaphase III spindle in the rat oocyte. Induced activation caused transition to interphase with the formation of a characteristic microtubule network. This study shows that there are several significant differences in the cytoskeletal organization of rat and mouse oocytes.

Full activation of the rat oocyte by protein synthesis inhibition requires protein phosphatase activity

The rat oocyte provides an interesting system in which to dissect the control mechanisms involved in the transition between a meiotic M phase and a mitotic interphase. In this study, we show that in rat oocytes activated parthenogenetically by puromycin, okadaic acid (a potent inhibitor of protein phosphatases 1 and 2A) induced an increase in histone H1 kinase activity suggesting that MPF was reactivated. However, the inhibition of phosphatases 1 and 2A shortly after second polar body extrusion did not allow the formation of a metaphase-like spindle, although microtubule polymerization was not inhibited. Instead, the chromatin remained condensed as a single mass and a large aster formed around it.

Spontaneous and induced activation of rat oocytes

Ovulated rat oocytes undergo spontaneous activation during in vitro culture. After extrusion of the second polar body, they do not enter interphase but are arrested again in next metaphase-like stage (M III arrest). The present study demonstrates that puromycin and chloral hydrate can trigger transition to interphase of metaphase II and spontaneously (incompletely) activated rat oocytes. The response of oocytes to these activators depends on their stage at the time of application of a stimulus. Metaphase II oocytes enter interphase at 86.8% when treated with puromycin and in 28.7% after chloral hydrate activation. Oocytes activated with chloral hydrate at the time of spontaneously induced anaphase II enter interphase at 64.8%, but after reaching the stage of telophase II their capability to shift to interphase is again low (28.8%). Finally, M III oocytes cannot be forced to enter interphase by either chloral hydrate or puromycin treatment. This study shows that resumption of the second meiotic division and transition to interphase--the two processes that normally occur in succession as a response to oocyte activatin--can be experimentally separated.