Temporal control of rRNA synthesis in cleavage-arrested embryos of Xenopus laevis

Nuclear size scaling during Xenopus early development contributes to midblastula transition timing

Early Xenopus laevis embryogenesis is a robust system for investigating mechanisms of developmental timing. After a series of rapid cell divisions with concomitant reductions in cell size, the first major developmental transition is the midblastula transition (MBT), when zygotic transcription begins and cell cycles elongate. Whereas the maintenance of a constant nuclear-to-cytoplasmic (N/C) volume ratio is a conserved cellular property, it has long been recognized that the N/C volume ratio changes dramatically during early Xenopus development. We investigated how changes in nuclear size and the N/C volume ratio during early development contribute to the regulation of MBT timing. Whereas previous studies suggested a role for the N/C volume ratio in MBT timing, none directly tested the effects of altering nuclear size. In this study, we first quantify blastomere and nuclear sizes in X. laevis embryos, demonstrating that the N/C volume ratio increases prior to the MBT. We then manipulate nuclear volume in embryos by microinjecting different nuclear scaling factors, including import proteins, lamins, and reticulons. Using this approach, we show that increasing the N/C volume ratio in pre-MBT embryos leads to premature activation of zygotic gene transcription and early onset of longer cell cycles. Conversely, decreasing the N/C volume ratio delays zygotic transcription and leads to additional rapid cell divisions. Whereas the DNA-to-cytoplasmic ratio has been implicated in MBT timing, our data show that nuclear size also contributes to the regulation of MBT timing, demonstrating the functional significance of nuclear size during development.

Timing system for the start of gastrulation in the Xenopus embryo

This study examined which component of the egg, the nucleus or cytoplasm, is involved in the timing of the start of gastrulation in the Xenopus embryo, and when it starts to measure time. First, nuclei of cells of 256-cell stage embryos were transplanted to enucleated eggs 60 min after activation. These eggs showed first cleavage 20-30 min later than control eggs fertilized at the same time as the activation of recipient eggs, and started gastrulation 25-35 min later than control embryos (depending on the delay in the first cleavage). Second, eggs whose nuclei were temporarily isolated by the extrusion of the portion containing the nucleus out of the fertilization envelope showed first cleavage 60-90 min later than sibling control eggs, because of delayed introduction of the nucleus from the extruded portion. They started gastrulation 60-90 min later than sibling control embryos (depending on the delay in the first cleavage). The portion inside the envelope underwent two to three rounds of oscillation in cell cycle relevant activities before the first cleavage, while the portion outside underwent the same rounds of cleavage as the inside portion. From the present and previous results it is concluded that the putative timing system for the start of gastrulation in the Xenopus embryo, whether it consists of a single or of multiple clocks, starts measuring time at or around the first cleavage, and that the presence of both the nucleus and the cytoplasm in the same cell and occurrence of mitosis and/or cleavage there are indispensable for the timing system to work, although the role of the cytoplasm is superior to that of the nucleus.

Involvement of a urethane-sensitive system in timing the onset of gastrulation in Xenopus laevis embryos

This paper describes success in delaying the onset of gastrulation in Xenopus laevis embryos without damage to their subsequent development by temporarily arresting cleavage with urethane. Exposure of X. laevis embryos to 150 mM urethane before gastrulation resulted in cleavage arrest and its removal led to cleavage resumption. During cleavage arrest, cyclic activities including nuclear replication and the M-phase-promoting factor cycle continued, although their duration was lengthened to nearly 1.8-fold that of the controls. Because of a 30-min time lag from removal of urethane to resumption of cleavage, as well as the retardation of cyclic activities during cleavage arrest, the development of embryos after a 60-min exposure to urethane lagged two cell cycles behind that of control embryos. Here, the two cell cycle delay is equivalent to 50 min at 22-23 degrees C. The start of gastrulation in exposed embryos was accordingly delayed about 50 min, although the delay in mid-blastula transition was as little as 20-25 min. Consistent results were obtained in embryos exposed to urethane for 90 or 120 min and those exposed to procaine or NH4Cl for 60 min. Although these results imply that delay in the start of gastrulation in exposed embryos is ascribed simply to delay in their development raised by cleavage arrest, at the same time they suggest that the onset of gastrulation is timed by systems sensitive to urethane, procaine and NH4Cl in X. laevis embryos.

Inhibition of transcription of class II, but not class III and I, genes in Xenopus postblastular embryos overexpressed with the TBP-binding protein, Dr1 (NC2beta)

Dr1 (NC2beta) is known to effectively repress transcription of class II genes, and much less effectively class III genes, but not class I genes through its binding to the TATA-binding protein (TBP), which is the major component of the basal transcription factor for polymerases II, III, and I. Previously, we isolated Xenopus Dr1 cDNA, and demonstrated that its mRNA is transcribed in oocytes and is inherited into early embryos, but its level decreases in later stages. Here, we overexpressed Xenopus Dr1 in Xenopus embryos and, found that the overexpression significantly reduces the levels of poly(A), cytoskeletal actin and histone H4 mRNAs, and the labeling of heterogeneous mRNA-like RNA in postblastular embryos, without affecting tRNA and rRNA syntheses. These results indicate that the overexpressed Dr1 specifically down-regulates the transcription of class II, but not class III and I, genes, and suggest that Dr1 plays an important role in the control of transcription in Xenopus embryogenesis.

Developmental changes in RNA polymerase I and TATA box-binding protein during early Xenopus embryogenesis

Xenopus early embryos are transcriptionally quiescent until the midblastula transition (MBT). We have examined the question of whether the absence of rRNA synthesis is related to a deficiency in the RNA polymerase I (pol I) transcription machinery. Previously we have demonstrated that the maternally provided pol I transcription factor UBF already binds to the inactive rRNA genes of pre-MBT embryos (P. Bell et al., 1997, J. Cell Sci. 110, 2053-2063). Here we have analyzed the fate of pol I and the TATA box-binding protein (TBP) through immunofluorescence and immunoblotting experiments. Pol I stockpiled in the egg is taken up by in vitro assembled pronuclei and concentrated into numerous distinct nuclear domains. Comparable storage sites of template-free pol I are also seen in nuclei of blastula to neurula stage embryos. In contrast, the amount of TBP is relatively low in oocytes and eggs but increases dramatically during the cleavage stages. Most of the newly synthesized TBP colocalizes with the stored form of pol I in the extranucleolar domains of blastula/gastrula embryos. The amount of TBP per embryo reaches peak values at the blastula/gastrula stage and then rapidly declines to normal somatic levels. The positive correlation of maximal TBP levels with the timing of the MBT suggests that overproduction of TBP is required for the formation of productive transcription complexes.

Early transcription in Caenorhabditis elegans embryos

We have analysed early transcription in devitellinized, cultured embryos of the nematode Caenorhabditis elegans by two methods: measurement of [32P]UTP uptake into TCA-precipitable material and autoradiographic detection of [3H]UTP labelling both in the presence and absence of alpha-amanitin. RNA synthesis was first detected at the 8- to 12-cell stage, and alpha-amanitin sensitivity also appeared at this time, during the cleavages establishing the major founder cell lineages. The requirements for maternally supplied versus embryonically produced gene products in early embryogenesis were examined in the same culture system by observing the effects of alpha-amanitin on cell division and the early stereotyped lineage patterns. In the presence of high levels of alpha-amanitin added at varying times from two cells onward, cell division continued until approximately the 100-cell stage and then stopped during a single round of cell division. The characteristic unequal early cleavages, orientation of cleavage planes and lineage-specific timing of early divisions were unaffected by alpha-amanitin in embryos up to 87 cells. These results indicate that embryonic transcription starts well before gastrulation in C. elegans embryos, but that although embryonic transcripts may have important early functions, maternal products can support at least the mechanics of the first 6 to 7 cell cycles.

Control of 4-8S RNA transcription at the midblastula transition in Xenopus laevis embryos

Transcription of Xenopus laevis U1 snRNA genes is subject to a precise program with respect both to the timing of activation at the midblastula transition (MBT) and to the relative levels of the two embryonic U1 RNAs (xU1b1 and b2) that are made. Here, we demonstrate that exogenous xU1b genes injected into developing X. laevis embryos come under the same controls as the endogenous genes. Injected U1 genes, unlike exogenous RNA polymerase III genes, remain quiescent until MBT and their activation at MBT requires protein synthesis during the early cleavage stages. Significantly, the onset of 4-8S RNA transcription occurs at the normal time, even when the DNA content of the embryo has been increased by injection of exogenous DNA or reduced through cleavage arrest, indicating that transcriptional activation at MBT is independent of the ratio of DNA (nucleus) to cytoplasm. In cleavage-arrested (coenocytic) embryos, the reduced level of DNA at MBT results both in a decrease in snRNA and tRNA synthesis (reflecting the lower gene dosage) and in a prolonged synthesis of large amounts of unusual RNA polymerase III transcripts, OAX RNAs. In normally cleaving embryos, small amounts of these unstable OAX RNAs (encoded by satellite I DNA) are synthesized only briefly at MBT. Our demonstration that RNA and DNA metabolism is aberrant in cleavage-arrested embryos requires reevaluation of previous experiments on transcriptional activation that utilized such coenocytic embryos.

Changes in the patterns of RNA synthesis in early embryogenesis of Xenopus laevis

We analyzed the accumulation of newly-synthesized heterogeneous mRNA-like RNA, 4 S RNA, 5 S RNA, snRNAs and rRNA before and after the midblastula transition (MBT) in Xenopus laevis embryogenesis. Based on the kinetics of the labeling, we concluded that the pattern of RNA synthesis in Xenopus embryogenesis changes following at least three characteristically different phases. The first phase is the pre-MBT stage, which is characterized by the synthesis of heterogeneous mRNA-like RNA, accompanied by the synthesis of small amounts of 4 S RNA, 5 S RNA and snRNAs. The second phase is the MBT stage which is characterized by a large activation (about 50-fold increase on a per cell basis) of 4 S RNA synthesis. The third phase is the post-MBT stage which is characterized by the commencement and increase in rRNA synthesis. We assume that RNA polymerases II, III and I are activated in this order in early Xenopus embryogenesis.

Reconstitution of endoplasmic reticulum in rapidly dividing cells of early Xenopus embryos

The cytology of early blastomeres of Xenopus laevis embryos was examined. Particular attention was given to the organization of the nuclear envelope of karyomeres (chromosome vesicles) and the endoplasmic reticulum (ER) at different stages in early cleavage cycles of frog development. Nuclear envelope formation was observed to occur rapidly around individual chromosomes during early anaphase, and karyomeres fused subsequently to yield the final nucleus during telophase. Endoplasmic reticulum in the perinuclear cytoplasm was observed to be vesicular during metaphase and cisternal in form during telophase. Following microinjection of rat liver rough microsomes into early blastomeres, heterologous ER components were identified by electron microscope immunocytochemistry. The foreign ER was observed as large, reconstituted cisternae at stages in the cell cycle when the nuclear envelope was intact. Therefore, transplanted ER maintained the capacity to reconstitute in the cytoplasm of a rapidly dividing cell. In an attempt to better assess ER structure at the metaphase stage of the cell cycle, we next slowed down the division process by treating Xenopus embryos with anti-microtubule agents. Treatment with critical concentrations of colchicine, nocodazole, or vinblastine led to cleavage arrest but not to inhibition of the nuclear cycle. Following such treatment, homologous ER was observed in a vesicular form at all stages of the nuclear cycle. Heterologous ER, however, identified by immunocytochemistry in microinjected cells treated with nocodazole, displayed both vesicular and cisternal forms. We conclude that microinjected ER membranes exhibit cell-cycle-specific behavior, which is different from that of the host cell ER.

Inhibition of RNA synthesis in embryo of Xenopus laevis by protease inhibitor

We have studied the role of proteases during the development of Xenopus laevis embryos with the aid of protease inhibitors. The activity of proteases was found to be only minimal in the unfertilized egg and during the initiation of development, but activity began to increase at the morula stage. When the activity of proteases was inhibited by antipain, an inhibitor of endopeptidase activity, RNA synthesis in the embryo was inhibited. To examine the relationship between the inhibitory effect of antipain on protease activity and its effect on RNA synthesis, antipain was reduced with NaBH4 to inactivate its protease inhibitory activity. The reduced antipain did not inhibit RNA synthesis in the embryo. Antipain effectively inhibited synthesis of both rRNA and poly(A)+RNA but not 4S RNA. We therefore suggest that protease activity plays an important role in the initiation and/or continuation of RNA synthesis.

Synthesis of heterogeneous mRNA-like RNA and low-molecular-weight RNA before the midblastula transition in embryos of Xenopus laevis

It has been proposed and is now widely accepted that in Xenopus laevis embryogenesis RNA synthesis starts only at and after 12 rounds of cleavage, at the time of the midblastula transition (MBT). In this report, however, we provide evidence that RNA synthesis takes place prior to the MBT stage in normally developing Xenopus embryos. In the present experiments, we cultured fertilized eggs in 80 mM phosphate buffer and loosened the adhesion between blastomeres, so that [3H]uridine could be incorporated into blastomeres from the surrounding medium. By this method and also by microinjection of [3H]GTP, we found that embryos synthesize heterogeneous, nonribosomal, high-molecular-weight RNAs and a relatively small amount of low-molecular-weight RNA as early as the sixth cleavage. RNAs synthesized were not of mitochondrial origin, and the synthesis was sensitive to actinomycin D and alpha-amanitin. From these results we conclude that mRNA-like RNA and low-molecular-weight RNA start to be synthesized during the cleavage stage.