Developmentally regulated activation of apoptosis early in Xenopus gastrulation results in cyclin A degradation during interphase of the cell cycle

Multiple Cdk1 inhibitory kinases regulate the cell cycle during development

The Wee kinases block entry into mitosis by phosphorylating and inhibiting the activity of the mitotic cyclin-dependent kinase, Cdk1. We have found that the various Xenopus Wee kinases have unique temporal and spatial patterns of expression during development. In addition, we have isolated and characterized a new Wee1-like kinase, Xenopus Wee2. By both in vivo and in vitro tests, Xenopus Wee2 functions as a Wee1-like kinase. The previously isolated Wee1-like kinase, Xenopus Wee1, is expressed only as maternal gene product. In contrast, Xenopus Wee2 is predominantly a zygotic gene product, while the third Wee kinase, Xenopus Myt1, is both a maternal and zygotic gene product. Concurrent with the changing levels of these Cdk inhibitory kinases, the pattern of embryonic cell division becomes asynchronous and spatially restricted in the Xenopus embryo. Interestingly, once zygotic transcription begins, Xenopus Wee2 is expressed in regions of the embryo that are devoid of mitotic cells, such as the involuting mesoderm. In contrast, Xenopus Myt1 is expressed in regions of the embryo that have high levels of proliferation, such as the developing neural tissues. The existence of multiple Wee kinases may help explain how distinct patterns of cell division arise and are regulated during development.

Molecular cloning and characterization of dullard: a novel gene required for neural development

In a screen for genes expressed in neural tissues and pronephroi, we isolated a novel gene, named dullard. Dullard protein contains the C-terminal conserved domain of NLI-IF (Nuclear LIM Interactor-Interacting Factor), a protein whose function is not yet characterized. Dullard mRNA was maternally derived and localized to the animal hemisphere. At neurula stages, the expression was in neural regions and subsequently localized to neural tissues, branchial arches, and pronephroi. Using antisense morpholino oligonucleotide-mediated inhibition, we showed that dullard was required for neural development. The translational knock-down of dullard resulted in failure of neural tube development and the embryos consequently showed a reduction of head development. Expression of neural marker genes in dullard-inhibited embryos was also suppressed. These results suggest that dullard is necessary for neural development.

A maternal Smad protein regulates early embryonic apoptosis in Xenopus laevis

We identified cDNAs encoding the Xenopus Smad proteins most closely related to mammalian Smad8, and we present a functional analysis of this activity (also referred to recently as xSmad11). Misexpression experiments indicate that xSmad8(11) regulates pathways distinct from those regulated by the closely related xSmad1. Embryos that develop from eggs depleted of xSmad8(11) mRNA fail to gastrulate; instead, at the time of gastrulation, they initiate a widespread program of apoptosis, via a CPP32/caspase 3 pathway. Embryos that avoid this fate display gastrulation defects. Activation of apoptosis is rescued by expression of xSmad8(11) but not xSmad1. Our results demonstrate an embryonic requirement for Smad8(11) activity and show that a maternally derived Smad signaling pathway is required for gastrulation and for mediating a cell survival program during early embryogenesis. We suggest that xSmad8(11) functions as part of a maternally derived mechanism shown previously by others to monitor Xenopus early embryonic cell cycles.

Apoptosis in mammalian preimplantation embryos: regulation by survival factors

The formation of a developmentally competent mammalian blastocyst requires the transition from a unicellular state, the fertilized zygote, to a differentiated multicellular structure. In common with other developing organisms, generation of the required cell population involves the processes of cell division, differentiation and cell death, all of which can be regulated by peptide growth factors. Cell death in the preimplantation embryo occurs by apoptosis and, by analogy with other systems, may serve to eliminate unwanted cells during the critical developmental transitions that take place during this period. Cells may be eliminated because they are abnormal or possess defects, including damaged DNA or chromosomal abnormalities. At the early cleavage stages, apoptosis may be associated with activation of the embryonic genome and may contribute to the blastomere fragmentation commonly observed in human IVF embryos. The major wave of apoptosis occurs in a number of species in the inner cell mass of the blastocyst, as identified using nuclear labelling including terminal transferase-mediated dUTP nick end labelling (TUNEL) and fluorescence and confocal microscopy. Apoptosis may protect the integrity and cellular composition of the inner cell mass, by eliminating damaged cells or possibly those with an inappropriate phenotype. Preimplantation embryos express genes involved in the regulation and execution of apoptosis and their cells can undergo this default pathway in the absence of exogenous survival signals. Evidence is now accumulating from several species that apoptosis in the embryo is regulated by soluble peptide growth factors acting as survival factors in an autocrine or paracrine manner. To date, these include transforming growth factor alpha and members of the insulin-like growth factor family. Apoptosis may also be affected by environmental factors, including culture conditions and the composition of media. The regulation of apoptosis in the preimplantation embryo is likely to be of critical importance for both embryo viability and for later development, since the cells of the inner cell mass give rise to the fetus and carry the germ line.

Post-cytochrome C protection from apoptosis conferred by a MAPK pathway in Xenopus egg extracts

In response to many different apoptotic stimuli, cytochrome c is released from the intermembrane space of the mitochondria into the cytoplasm, where it serves as a cofactor in the activation of procaspase 9. Inhibition of this process can occur either by preventing cytochrome c release or by blocking caspase activation or activity. Experiments involving in vitro reconstitution of apoptosis in cell-free extracts of Xenopus laevis eggs have suggested that extracts arrested in interphase are susceptible to an endogenous apoptotic program leading to caspase activation, whereas extracts arrested in meiotic metaphase are not. We report here that Mos/MEK/MAPK pathways active in M phase-arrested eggs are responsible for rendering them refractory to apoptosis. Interestingly, M phase-arrested extracts are competent to release cytochrome c, yet still do not activate caspases. Concomitantly, we have also demonstrated that recombinant Mos, MEK, and ERK are sufficient to block cytochrome c-dependent caspase activation in purified Xenopus cytosol, which lacks both transcription and translation. These data indicate that the MAP kinase pathway can target and inhibit post-cytochrome c release apoptotic events in the absence of new mRNA/protein synthesis and that this biochemical pathway is responsible for the apoptotic inhibition observed in meiotic X. laevis egg extracts.

Frizzled receptors activate a novel JNK-dependent pathway that may lead to apoptosis

Extracellular Wnt ligands and their receptors of the Frizzled family control cell fate, proliferation, and polarity during metazoan development. Frizzled signaling modulates target gene expression through a beta-catenin-dependent pathway, functions to establish planar cell polarity in Drosophila epithelia, and activates convergent extension movements and intracellular Ca(2+) signaling in frog and fish embryos. Here, we report that a Frizzled receptor, Xenopus Frizzled 8 (Xfz8), activates c-Jun N-terminal kinases (JNK) and triggers rapid apoptotic cell death in gastrulating Xenopus embryos. This activity of Xfz8 required the cytoplasmic tail of the receptor and was blocked by a dominant inhibitor of JNK. Moreover, the cytoplasmic tail of Xfz8 targeted to the membrane was sufficient for activation of JNK and apoptosis. The apoptotic signaling was shared by a specific subset of Frizzled receptors, was inhibited by Wnt5a, and occurred in a Dishevelled- and T cell factor (TCF)-independent manner. Thus, our experiments identify a novel Frizzled-dependent signaling pathway, which involves JNK and differs from the beta-catenin-dependent and convergent extension pathways.

Postmeiotic unfertilized starfish eggs die by apoptosis

Fertilization of starfish eggs during meiosis results in rapid progression to embryogenesis as soon as meiosis II is completed. Unfertilized eggs complete meiosis and arrest in postmeiotic interphase for an, until now, indeterminate time. If they remain unfertilized, the mature postmeiotic eggs ultimately die. The aim of this study is to characterize the mechanism of death in postmeiotic unfertilized starfish eggs. We report that, in two species of starfish, in the absence of fertilization, postmeiotic interphase arrest persists for 16-20 h, after which time the cells synchronously and rapidly die. Dying eggs extrude membrane blebs, undergo cytoplasmic contraction and darkening, and fragment into vesicles in a manner reminiscent of apoptotic cells. The DNA of dying eggs is condensed, fragmented, and labeled by the TUNEL assay. Taken together, these data suggest that the default fate of postmeiotic starfish eggs, like their mammalian counterparts, is death by apoptosis. We further report that the onset and execution of apoptosis in this system is dependent on ongoing protein synthesis and is inhibited by a rise in intracellular Ca(2+), an essential component of the fertilization signaling pathway. We propose starfish eggs as a useful model to study developmentally regulated apoptosis.

Tumor-specific proteolytic processing of cyclin E generates hyperactive lower-molecular-weight forms

Cyclin E is a G(1) cyclin essential for S-phase entry and has a profound role in oncogenesis. Previously this laboratory found that cyclin E is overexpressed and present in lower-molecular-weight (LMW) isoforms in breast cancer cells and tumor tissues compared to normal cells and tissues. Such alteration of cyclin E is linked to poor patient outcome. Here we report that the LMW forms of cyclin E are hyperactive biochemically and they can more readily induce G(1)-to-S progression in transfected normal cells than the full-length form of the protein can. Through biochemical and mutational analyses we have identified two proteolytically sensitive sites in the amino terminus of human cyclin E that are cleaved to generate the LMW isoforms found in tumor cells. Not only are the LMW forms of cyclin E functional, as they phosphorylate substrates such as histone H1 and GST-Rb, but also their activities are higher than the full-length cyclin E. These nuclear localized LMW forms of cyclin E are also biologically functional, as their overexpression in normal cells increases the ability of these cells to enter S and G(2)/M. Lastly, we show that cyclin E is selectively cleaved in vitro by the elastase class of serine proteases to generate LMW forms similar to those observed in tumor cells. These studies suggest that the defective entry into and exit from S phase by tumor cells is in part due to the proteolytic processing of cyclin E, which generates hyperactive LMW isoforms whose activities have been modified from that of the full-length protein.

Activation of the maternally preset program of apoptosis by microinjection of 5-aza-2'-deoxycytidine and 5-methyl-2'-deoxycytidine-5'-triphosphate in Xenopus laevis embryos

The present study examines the effects on embryogenesis of microinjecting Xenopus laevis fertilized eggs with 5-aza-2'-deoxycytidine (5-Aza-CdR), which induces hypomethylation of DNA, and 5-methyl-2'- deoxycytidine-5'-triphosphate (5-methyl-dCTP), which induces hypermethylation of DNA. Embryos injected with either one of these analogs cleaved normally until the mid-blastula stage, but underwent massive cell dissociation and stopped development at the early gastrula stage. Dissociated cells that appeared here were positive by terminal deoxyribonucleotidyl transferase-mediated deoxyuridine triphosphate-digoxigenin nick end-labeling and contained fragmented nuclei with condensed chromatin. The DNA from these cells formed a "ladder" on electrophoresis. Furthermore, the induction of cell dissociation by 5-Aza-CdR and 5-methyl-dCTP was postponed by 2-3 h by co-injection of Bcl-2 mRNA and the normal metabolite (CdR and dCTP, respectively). Using a specific antibody against 5-methyl-cytosine, we confirmed that 5-Aza-CdR induces hypomethylation, whereas 5-methyl-dCTP induces hypermethylation in X. laevis embryos before the onset of cell dissociation. Incorporation of radioactive precursors revealed that synthesis of DNA, and also RNA, is inhibited significantly in both 5-Aza-CdR-injected and 5-methyl-dCTP-injected embryos. These results show that 5-Aza-CdR and 5-methyl-dCTP are incorporated into DNA and induce apoptosis, probably through alteration of DNA methylation coupled with inhibition of DNA replication and/or transcription.

Cleavage of cyclin A at R70/R71 by the bacterial protease OmpT

Previous work has shown that cyclin A can be cleaved at Arg-70/Arg-71 by a proteolytic activity present in an in vitro-coupled transcription/translation system by using rabbit reticulocyte lysate programmed by plasmid DNA encoding p27(KIP1), a cyclin-dependent kinase inhibitor, but not by plasmid DNAs encoding other cyclin-dependent kinases inhibitors. Here we report that cyclin A is also cleaved by translation product programmed by plasmid DNA encoding cyclin B. Several findings indicate that the cleavage activity in this assay is provided by the bacterial protease OmpT, which cofractionates with cyclin B and p27(KIP1) plasmid DNAs and is thus carried over into the coupled in vitro transcription/translation reactions. (i) Cleavage activity appeared even when transcription or translation of the cyclin B or p27(KIP1) was blocked. (ii) Activity resembling OmpT, a serine protease that cleaves between dibasic residues, routinely copurifies with p27(KIP1) and cyclin B plasmid DNAs. (iii) Both cyclin A cleavage activity and OmpT activity are heat stable, resistant to denaturation, and inhibited by Zn(2+), Cu(2+), or benzamidine. (iv) Cyclin A cleavage activity is detected when using lysates or DNAs prepared from Escherichia coli strains that contained OmpT but not with strains lacking OmpT. (v) Purified OmpT enzyme itself cleaves cyclin A at R70/R71. These data indicate that OmpT can be present in certain DNA preparations obtained by using standard plasmid purification protocols, and its presence can potentially affect the outcome and interpretation of studies carried out using in vitro-translated proteins.

c-Jun N-terminal kinase activation in Xenopus laevis eggs and embryos. A possible non-genomic role for the JNK signaling pathway

The c-Jun N-terminal kinases (JNKs) are members of the mitogen-activated protein kinase family that play critical roles in stress responses and apoptosis. We have discovered that JNK is present in Xenopus oocytes, an experimental system that offers a variety of powerful experimental approaches to questions of protein function and regulation. Like ERK2/p42 MAPK, JNK is activated just prior to germinal vesicle breakdown during Xenopus oocyte maturation and remains active throughout meiosis I and II. However, unlike p42 MAPK, which is inactivated about 30 min after eggs are fertilized or parthenogenetically activated, JNK stays constitutively active until the early gastrula stage of embryogenesis. These findings suggest that the JNK pathway may play a role in oocyte maturation and embryogenesis. JNK was activated by microinjection of Mos, by activation of an estrogen-inducible form of Raf, and by a constitutively active MEK-1 (MEK R4F), indicating that the p42 MAPK cascade can trigger JNK activation. However, the MEK inhibitor U0126 blocked progesterone-induced p42 MAPK activation but not progesterone-induced JNK activation. Thus, progesterone can stimulate JNK activation both through the MEK/p42 MAPK pathway and through MEK/p42 MAPK-independent pathways. Many of the key substrates of JNKs identified to date are transcriptional regulators. However, since transcription is not required for germinal vesicle breakdown in progesterone-treated oocytes or for the early embryonic cell cycles, our findings suggest that in these contexts the JNK pathway exerts nongenomic effects.

Wee1-regulated apoptosis mediated by the crk adaptor protein in Xenopus egg extracts

Many of the biochemical reactions of apoptotic cell death, including mitochondrial cytochrome c release and caspase activation, can be reconstituted in cell-free extracts derived from Xenopus eggs. In addition, because caspase activation does not occur until the egg extract has been incubated for several hours on the bench, upstream signaling processes occurring before full apoptosis are rendered accessible to biochemical manipulation. We reported previously that the adaptor protein Crk is required for apoptotic signaling in egg extracts (Evans, E.K., W. Lu, S.L. Strum, B.J. Mayer, and S. Kornbluth. 1997. EMBO (Eur. Mol. Biol. Organ.) J. 16:230-241). Moreover, we demonstrated that removal of Crk Src homology (SH)2 or SH3 interactors from the extracts prevented apoptosis. We now report the finding that the relevant Crk SH2-interacting protein, important for apoptotic signaling in the extract, is the well-known cell cycle regulator, Wee1. We have demonstrated a specific interaction between tyrosine-phosphorylated Wee1 and the Crk SH2 domain and have shown that recombinant Wee1 can restore apoptosis to an extract depleted of SH2 interactors. Moreover, exogenous Wee1 accelerated apoptosis in egg extracts, and this acceleration was largely dependent on the presence of endogenous Crk protein. As other Cdk inhibitors, such as roscovitine and Myt1, did not act like Wee1 to accelerate apoptosis, we propose that Wee1-Crk complexes signal in a novel apoptotic pathway, which may be unrelated to Wee1's role as a cell cycle regulator.

Use of large-scale expression cloning screens in the Xenopus laevis tadpole to identify gene function

We have conducted an expression cloning screen of approximately 50, 000 cDNAs from a tadpole stage Xenopus laevis cDNA library to functionally identify genes affecting a wide range of cellular and developmental processes. Fifty-seven cDNAs were isolated for their ability to alter gross tadpole morphology or the expression patterns of tissue-specific markers. Thirty-seven of the cDNAs have not been previously described for Xenopus, and 15 of these show little or no similarity to sequences in the NCBI database. The screen and the identified genes are presented in this paper to demonstrate the power, ease, speed, and flexibility of expression cloning in the X. laevis embryo. Future screens such as this one can be done on a larger scale and will complement the sequence-based screens and genome-sequencing projects which are producing a large body of novel genes without ascribed functions.

Heterotopic expression of the Xl-Fli transcription factor during Xenopus embryogenesis: modification of cell adhesion and engagement in the apoptotic pathway

In the Xenopus laevis embryo, the overexpression of the Xl-FLI protein, a transcription factor of the ETS family, provokes severe developmental anomalies, which affect anteroposterior and dorsoventral polarities, optic cup formation, head cartilage morphogenesis, and erythrocyte differentiation. It has been proposed that these effects could be correlated to modifications of cell adhesion properties and/or to an increased engagement of cells in the apoptotic pathway during early development (Remy et al., Int. J. Dev. Biol. 40, 577-589, 1996). To address these questions, we have first analyzed the behavior of cells overexpressing the protein in both aggregation and adhesion assays. We observe perturbations of cell-cell interactions as well as perturbations of cell adhesion and spreading on fibronectin and extracellular matrix (ECM). Second, we have analyzed apoptosis of cells overexpressing the Xl-FLI protein, by testing DNA fragmentation, caspase-3 activity and by performing TUNEL assay. We show that Xl-Fli overexpression results in the appearance of hallmarks of apoptosis, including exclusion of cells from the interior of the embryo, internucleosomal fragmentation of DNA and dose-dependent induction of caspase-3, resulting in the hydrolysis of poly(ADP-ribose) polymerase. In addition, a dominant-negative mutation of BMPs receptors decreases the effects of Xl-Fli overexpression, suggesting that a modification of the BMP signalling could be responsible for increased apoptosis. The latter appears to affect predominantly ventral and ventrolateral regions of the embryo.

Induction of mitosis-mediated apoptosis by sodium arsenite in HeLa S3 cells

Arsenic has been used effectively as a chemotherapeutic drug for the treatment of acute promyelocytic leukemia patients. Numerous studies have demonstrated that arsenic induces apoptosis in various cell types. In the present study, we showed that approximately 35% of arsenite-treated HeLa S3 cells arrested in mitosis. After release from arsenite treatment, more than 80% of arsenite-arrested mitotic cells subsequently underwent apoptosis, as indicated by anachronistic nuclear envelope reformation, DNA ladder occurrence, chromatin condensation, and activation of caspases 3 and 9. In exploring how these cells entered apoptosis mechanistically, we found an inverse correlation between mitotic indexes and apoptotic frequencies. As shown by using Percoll density gradient fractionation and flow cytometric analysis, the mitosis-mediated apoptosis induced by arsenite was accompanied by delayed cyclin B degradation and altered mitotic exit. Furthermore, treatment of arsenite-arrested mitotic cells with staurosporine or 2-aminopurine resulted in a rapid degradation of cyclin B, moved these cells forward to interphase without cell division, and abrogated apoptosis. These results suggest that apoptosis occurs in arsenite-arrested mitotic cells that exit mitosis abnormally.

Dissection of the XChk1 signaling pathway in Xenopus laevis embryos

Checkpoint pathways inhibit cyclin-dependent kinases (Cdks) to arrest cell cycles when DNA is damaged or unreplicated. Early embryonic cell cycles of Xenopus laevis lack these checkpoints. Completion of 12 divisions marks the midblastula transition (MBT), when the cell cycle lengthens, acquiring gap phases and checkpoints of a somatic cell cycle. Although Xenopus embryos lack checkpoints prior to the MBT, checkpoints are observed in cell-free egg extracts supplemented with sperm nuclei. These checkpoints depend upon the Xenopus Chk1 (XChk1)-signaling pathway. To understand why Xenopus embryos lack checkpoints, xchk1 was cloned, and its expression was examined and manipulated in Xenopus embryos. Although XChk1 mRNA is degraded at the MBT, XChk1 protein persists throughout development, including pre-MBT cell cycles that lack checkpoints. However, when DNA replication is blocked, XChk1 is activated only after stage 7, two cell cycles prior to the MBT. Likewise, DNA damage activates XChk1 only after the MBT. Furthermore, overexpression of XChk1 in Xenopus embryos creates a checkpoint in which cell division arrests, and both Cdc2 and Cdk2 are phosphorylated on tyrosine 15 and inhibited in catalytic activity. These data indicate that XChk1 signaling is intact but blocked upstream of XChk1 until the MBT.

Isolation and characterization of three novel serine protease genes from Xenopus laevis

Three novel cDNAs encoding serine proteases, that may play a role in early vertebrate development, have been identified from Xenopus laevis. These Xenopus cDNAs encode trypsin-like serine proteases and are designated Xenopus embryonic serine protease (Xesp)-1, Xesp-2, and XMT-SP1, a homolog of human MT-SP1. Xesp-1 is likely to be a secreted protein that functions in the extracellular space. Xesp-2 and XMP-SP1 are likely to be type II membrane proteases with multidomain structures. Xesp-2 has eight low density lipoprotein receptor (LDLR) domains and one scavenger receptor cysteine-rich (SRCR) domain, and XMT-SP1 has four LDLR domains and two CUB domains. The temporal expressions of these serine protease genes show distinct and characteristic patterns during embryogenesis, and they are differently distributed in adult tissues. Overexpression of Xesp-1 caused no significant defect in embryonic development, but overexpression of Xesp-2 or XMT-SP1 caused defective gastrulation or apoptosis, respectively. These results suggest that these proteases may play important roles during early Xenopus development, such as regulation of cell movement in gastrulae.

Endogenous patterns of TGFbeta superfamily signaling during early Xenopus development

Transforming growth factor beta (TGFbeta) superfamily signaling has been implicated in patterning of the early Xenopus embryo. Upon ligand stimulation, TGFbeta receptors phosphorylate Smad proteins at carboxy-terminal SS(V/M)S consensus motifs. Smads 1/5/8, activated by bone morphogenetic protein (BMP) signaling, induce ventral mesoderm whereas Smad2, activated by activin-like ligands, induces dorsal mesoderm. Although ectopic expression studies are consistent with roles for TGFbeta signals in early Xenopus embryogenesis, when and where BMP and activin-like signaling pathways are active endogenously has not been directly examined. In this study, we investigate the temporal and spatial activation of TGFbeta superfamily signaling in early Xenopus development by using antibodies specific for the type I receptor-phosphorylated forms of Smad1/5/8 and Smad2. We find that Smad1/5/8 and two distinct isoforms of Smad2, full-length Smad2 and Smad2(delta)exon3, are phosphorylated in early embryos. Both Smad1/5/8 and Smad2/Smad2(delta)exon3 are activated after, but not before, the mid-blastula transition (MBT). Endogenous activation of Smad2/Smad2(delta)exon3 requires zygotic transcription, while Smad1/5/8 activation at MBT appears to involve transcription-independent regulation. We also find that the competence of embryonic cells to respond to TGF(delta) superfamily ligands is temporally regulated and may be a determinant of early patterning. Levels of phospho-Smad1/5/8 and of phospho-Smad2/Smad2(delta)exon3 are asymmetrically distributed across both the animal-vegetal and dorsoventral axes. The timing of the development of these asymmetries differs for phospho-Smad1/5/8 and for phospho-Smad2/Smad2(delta)exon3, and the spatial distribution of phosphorylation of each Smad changes dramatically as gastrulation begins. We discuss the implications of our results for endogenous functions of BMP and activin-like signals as candidate morphogens regulating primary germ layer formation and dorsoventral patterning of the early Xenopus embryo.

Distinct effects of XBF-1 in regulating the cell cycle inhibitor p27(XIC1) and imparting a neural fate

XBF-1 is an anterior neural plate-specific, winged helix transcription factor that affects neural development in a concentration-dependent manner. A high concentration of XBF-1 results in suppression of endogenous neuronal differentiation and an expansion of undifferentiated neuroectoderm. Here we investigate the mechanism by which this expansion is achieved. Our findings suggest that XBF-1 converts ectoderm to a neural fate and it does so independently of any effects on the mesoderm. In addition, we show that a high dose of XBF-1 promotes the proliferation of neuroectodermal cells while a low dose inhibits ectodermal proliferation. Thus, the neural expansion observed after high dose XBF-1 misexpression is due both to an increase in the number of ectodermal cells devoted to a neural fate and an increase in their proliferation. We show that the effect on cell proliferation is likely to be mediated by p27(XIC1), a cyclin-dependent kinase (cdk) inhibitor. We show that p27(XIC1) is expressed in a spatially restricted pattern in the embryo, including the anterior neural plate, and when misexpressed it is sufficient to block the cell cycle in vivo. We find that p27(XIC1)is transcriptionally regulated by XBF-1 in a dose-dependent manner such that it is suppressed or ectopically induced by a high or low dose of XBF-1, respectively. However, while a low dose of XBF-1 induces ectopic p27(XIC1)and ectopic neurons, misexpression of p27(XIC1)does not induce ectopic neurons, suggesting that the effects of XBF-1 on cell fate and cell proliferation are distinct. Finally, we show that p27(XIC1)is suppressed by XBF-1 in the absence of protein synthesis, suggesting that at least one component of p27(XIC1)regulation by XBF-1 may be direct. Thus, XBF-1 is a neural-specific transcription factor that can independently affect both the cell fate choice and the proliferative status of the cells in which it is expressed.

Developmental regulation of induced and programmed cell death in Xenopus embryos

We have analyzed the role of cell death during early Xenopus development and have identified two distinct types of cell death programs during the period between fertilization and the tadpole stage. One is a maternal cell death program that is activated at the onset of gastrulation following damage to the pre-midblastula transition embryo, resulting in the death of non-viable cells. The activation of this cell death program at a specific time during development is a maternally programmed event under the control of a developmental timer set at fertilization, and does not depend on the type of stress applied, on cell cycle progression, or de novo protein synthesis. Subsequently, a second program corresponding to programmed cell death is initiated as part of the normal development of the embryo. Programmed cell death starts at the onset of gastrulation and we have analyzed its spatio-temporal patterns by a whole-mount in situ DNA end labeling technique (the TUNEL protocol).

Endoderm patterning by the notochord: development of the hypochord in Xenopus

The patterning and differentiation of the vertebrate endoderm requires signaling from adjacent tissues. In this report, we demonstrate that signals from the notochord are critical for the development of the hypochord, which is a transient, endodermally derived structure that lies immediately ventral to the notochord in the amphibian and fish embryo. It appears likely that the hypochord is required for the formation of the dorsal aorta in these organisms. We show that removal of the notochord during early neurulation leads to the complete failure of hypochord development and to the elimination of expression of the hypochord marker, VEGF. Removal of the notochord during late neurulation, however, does not interfere with hypochord formation. These results suggest that signals arising in the notochord instruct cells in the underlying endoderm to take on a hypochord fate during early neural stages, and that the hypochord does not depend on further notochord signals for maintenance. In reciprocal experiments, when the endoderm receives excess notochord signaling, a significantly enlarged hypochord develops. Overall, these results demonstrate that, in addition to patterning neural and mesodermal tissues, the notochord plays an important role in patterning of the endoderm.

Egg timers: how is developmental time measured in the early vertebrate embryo?

Eggs and early embryos appear to be programmed to undertake particular developmental decisions at characteristic times, although precisely how these decisions are timed is unknown. We discuss the possible roles and interactions during early vertebrate development of two broad categories of timers: 1) those that involve cyclic or sequential mechanisms, referred to as clocks; and 2) those that require an increase or decrease in some factor to a threshold level for progression of time, referred to as hourglass timers. It is concluded that both clock-like timers linked to various features of the cell cycle and hourglass timers are involved in early developmental timing. The possible involvement of elements of circadian clock timers is also considered. BioEssays 22:57-63, 2000.

Translation of maternal TATA-binding protein mRNA potentiates basal but not activated transcription in Xenopus embryos at the midblastula transition

Early embryonic development in Xenopus laevis is characterized by transcriptional repression which is relieved at the midblastula stage (MBT). Here we show that the relative abundance of TATA-binding protein (TBP) increases robustly at the MBT and that the mechanism underlying this increase is translation of maternally stored TBP RNA. We show that TBP is rate-limiting in egg extract under conditions that titrate nucleosome assembly. Precocious translation of TBP mRNA in Xenopus embryos facilitates transcription before the MBT, without requiring TBP to be prebound to the promoter before injection. This effect is transient in the absence of chromatin titration and is sustained when chromatin is titrated. These data show that translational regulation of TBP RNA contributes to limitations on the transcriptional capacity before the MBT. Second, we examined the ability of trans-acting factors to contribute to promoter activity before the MBT. Deletion of cis-acting elements does not affect histone H2B transcription in egg extract, a finding indicative of limited trans-activation. Moreover, in the context of the intact promoter, neither the transcriptional activator Oct-1, nor TBP, nor TFIID enable transcriptional activation in vitro. HeLa cell extract, however, reconstitutes activated transcription in mixed extracts. These data suggest a deficiency in egg extract cofactors required for activated transcription. We show that the capacity for activated H2B transcription is gradually acquired at the early gastrula transition. This transition occurs well after the blastula stage when the basal transcription machinery can first be complemented with TBP.

Regulated formation of extrachromosomal circular DNA molecules during development in Xenopus laevis

Extrachromosomal circular DNA molecules of chromosomal origin have been detected in many organisms and are thought to reflect genomic plasticity in eukaryotic cells. Here we report a developmentally regulated formation of extrachromosomal circular DNA that occurs de novo in preblastula Xenopus embryos. This specific DNA population is not detected in the male or female germ cells and is dramatically reduced in later developmental stages and in adult tissues. The activity responsible for the de novo production of extrachromosomal circles is maternally inherited, is stored in the unfertilized egg, and requires genomic DNA as a template. The formation of circular molecules does not require genomic DNA replication but both processes can occur simultaneously in the early development. The production of extrachromosomal circular DNA does not proceed at random since multimers of the tandemly repeated sequence satellite 1 were over-represented in the circle population, while other sequences (such as ribosomal DNA and JCC31 repeated sequence) were not detected. This phenomenon reveals an unexpected plasticity of the embryonic genome which is restricted to the early developmental stage.

Developmental activation of the capability to undergo checkpoint-induced apoptosis in the early zebrafish embryo

In this study, we demonstrate the developmental activation, in the zebrafish embryo, of a surveillance mechanism which triggers apoptosis to remove damaged cells. We determine the time course of activation of this mechanism by exposing embryos to camptothecin, an agent which specifically inhibits topoisomerase I within the DNA replication complex and which, as a consequence of this inhibition, also produces strand breaks in the genomic DNA. In response to an early (pre-gastrula) treatment with camptothecin, apoptosis is induced at a time corresponding approximately to mid-gastrula stage in controls. This apoptotic response to a block of DNA replication can also be induced by early (pre-MBT) treatment with the DNA synthesis inhibitors hydroxyurea and aphidicolin. After camptothecin treatment, a high proportion of cells in two of the embryo's three mitotic domains (the enveloping and deep cell layers), but not in the remaining domain (the yolk syncytial layer), undergoes apoptosis in a cell-autonomous fashion. The first step in this response is an arrest of the proliferation of all deep- and enveloping-layer cells. These cells continue to increase in nuclear volume and to synthesize DNA. Eventually they become apoptotic, by a stereotypic pathway which involves cell membrane blebbing, "margination" and fragmentation of nuclei, and cleavage of the genomic DNA to produce a nucleosomal ladder. Fragmentation of nuclei can be blocked by the caspase-1,4,5 inhibitor Ac-YVAD-CHO, but not by the caspase-2,3,7[, 1] inhibitor Ac-DEVD-CHO. This suggests a functional requirement for caspase-4 or caspase-5 in the apoptotic response to camptothecin. Recently, Xenopus has been shown to display a developmental activation of the capability for stress- or damaged-induced apoptosis at early gastrula stage. En masse, our experiments suggest that the apoptotic responses in zebrafish and Xenopus are fundamentally similar. Thus, as for mammals, embryos of the lower vertebrates exhibit the activation of surveillance mechanisms, early in development, to produce the selective apoptosis of damaged cells.

Involvement of germ cell apoptosis in the induction of testicular toxicity following hydroxyurea treatment

The present study investigated the occurrence of apoptotic cell death in the mouse testis at various intervals following the administration of hydroxyurea (HU). The presence of apoptosis was assessed by the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) method and by DNA fragmentation assay using ligation-mediated polymerase chain reaction. Both the incidence of apoptotic cells and the level of DNA fragmentation in the testis increased depending on the HU dose, and they were most apparent at the highest dose (400 mg/kg). The incidence of apoptotic cells in the HU-treated group increased continuously and peaked at 12 h, but then decreased gradually, reaching control levels by 48 h. After HU treatment, TUNEL-positive apoptotic cells increased in the seminiferous epithelium of the tubules, and affected cells were found synchronously in the tubules of animals treated with HU. Spermatogonia and spermatocytes were found to be affected selectively. TUNEL-positive cells were found to be stage-specific and were primarily in stage IV-VI tubules. It has been shown that in vivo HU exposure induced testicular germ cell apoptosis dose dependently in a time- and stage-specific manner, and damaged cells appeared to be eliminated by phagocytosis by neighboring cells. Apoptosis of damaged testicular germ cells is apparently a common response to various testicular toxicants therefore protecting the next generations of germ cells from the damaged cell population.

The cyclin-dependent kinase inhibitor p27(Kip1) induces N-terminal proteolytic cleavage of cyclin A

Progression through the cell cycle is regulated in part by the sequential activation and inactivation of cyclin-dependent kinases (CDKs). Many signals arrest the cell cycle through inhibition of CDKs by CDK inhibitors (CKIs). p27(Kip1) (p27) was first identified as a CKI that binds and inhibits cyclin A/CDK2 and cyclin E/CDK2 complexes in G1. Here we report that p27 has an additional property, the ability to induce a proteolytic activity that cleaves cyclin A, yielding a truncated cyclin A lacking the mitotic destruction box. Other CKIs (p15(Ink4b), p16(Ink4a), p21(Cip1), and p57(Kip2)) do not induce cleavage of cyclin A; other cyclins (cyclin B, D1, and E) are not cleaved by the p27-induced protease activity. The C-terminal half of p27, which is dispensable for its kinase inhibitory activity, is required to induce cleavage. Mechanistically, p27 does not appear to cause cleavage through direct interaction with cyclin/CDK complexes. Instead, it activates a latent protease that, once activated, does not require the continuing presence of p27. Mutation of cyclin A at R70 or R71, residues at or very close to the cleavage site, blocks cleavage. Noncleavable mutants are still recognized by the anaphase-promoting complex/cyclosome pathway responsible for ubiquitin-dependent proteolysis of mitotic cyclins, indicating that the p27-induced cleavage of cyclin A is part of a separate pathway. We refer to this protease as Tsap (pTwenty-seven- activated protease).

Expression and regulation of genes associated with cell death during murine preimplantation embryo development

The newly fertilized preimplantation embryo depends entirely on maternal mRNAs and proteins deposited and stored in the oocyte prior to its ovulation. If the oocyte is not sufficiently equipped with maternally stored products, or if zygotic gene expression does not commence at the correct time, the embryo will die. One of the major abnormalities observed during early development is cellular fragmentation. We showed previously that cellular fragmentation in human embryos can be attributed to programmed cell death (PCD). Here, we demonstrate that the PCD that occurs during the 1-cell stage of mouse embryogenesis is likely to be regulated by many cell death genes either maternally inherited or transcribed from the embryonic genome. We have demonstrated for the first time the temporal expression patterns of nine cell death regulatory genes, and our preliminary experiments show that the expression of these genes is altered in embryos undergoing fragmentation. The expression of genes involved in cell death (MA-3, p53, Bad, and Bcl-xS) seems to be elevated, whereas the expression of genes involved in cell survival (Bcl-2) is reduced. We propose that PCD may occur by default in embryos that fail to execute essential developmental events during the first cell cycle.

Programmed cell death during Xenopus development: a spatio-temporal analysis

Programmed cell death (PCD) is an integral part of many developmental processes. In vertebrates little is yet known on the patterns of PCD and its role during the early phases of development, when embryonic tissue layers migrate and pattern formation takes place. We describe the spatio-temporal patterns of cell death during early Xenopus development, from fertilization to the tadpole stage (stage 35/36). Cell death was analyzed by a whole-mount in situ DNA end-labeling technique (the TUNEL protocol), as well as by serial sections of paraffin-embedded TUNEL-stained embryos. The first cell death was detected during gastrulation, and as development progressed followed highly dynamic and reproducible patterns, strongly suggesting it is an important component of development at these stages. The detection of PCD during neural induction, neural plate patterning, and later during the development of the nervous system highlights the role of PCD throughout neurogenesis. Additionally, high levels of cell death were detected in the developing tail and sensory organs. This is the first detailed description of PCD throughout early development of a vertebrate, and provides the basis for further studies on its role in the patterning and morphogenesis of the embryo.

Clocks regulating developmental processes

Developmental clocks are hypothetical embryonic time-measuring devices--some are run by oscillators, whereas others depend on rate-limiting processes. Their existence has been deduced from recent studies of the timing of the midblastula transition, the opening of the Hox cluster during organogenesis, and oligodendrocyte progenitor differentiation; however, the mechanisms underlying their function remain largely unknown.

Are caspases involved in the death of cells with a transcriptionally inactive nucleus? Sperm and chicken erythrocytes

We show that mouse sperm die spontaneously within 1–2 days in culture and that treatment with either staurosporine (STS) and cycloheximide (CHX) or a peptide caspase inhibitor does not accelerate or delay the cell death. Chicken erythrocytes, by contrast, are induced to die by either serum deprivation or treatment with STS and CHX, and embryonic erythrocytes are more sensitive than adult erythrocytes to both treatments. Although these erythrocyte deaths display a number of features that are characteristic of apoptosis, they are not blocked, or even delayed, by peptide caspase inhibitors, and most of the cells die without apparently activating caspases. A small proportion of the dying erythrocytes do activate caspase-3, but even these cells, which seem to be the least mature erythrocytes, die just as quickly in the presence of caspase inhibitors. Our findings raise the possibility that both mouse sperm and chicken erythrocytes have a death programme that may not depend on caspases and that chicken erythrocytes lose caspases as they mature. Chicken erythrocytes may provide a useful ‘stripped down’ cell system to try to identify the protein components of such a death programme, which may serve to back-up the conventional caspase-dependent suicide mechanism in many cell types.

The caspase-RB connection in cell death

Execution of the cell-death programme requires the activation of a family of cysteine proteases known as caspases. Specific cellular proteins are cleaved by caspases during apoptosis, including the retinoblastoma tumour-suppressor protein (RB1). A caspase-resistant RB1 can attenuate the death response to tumour necrosis factor alpha. The cleavage of RB1 during cell death, together with the increased cell death during embryonic development of Rb-knockout mice, suggests that RB1 degradation contributes to the activation of the cell-death pathway.

Loss of cell adhesion in Xenopus laevis embryos mediated by the cytoplasmic domain of XLerk, an erythropoietin-producing hepatocellular ligand

The erythropoietin-producing hepatocellular (Eph) family of ligands and receptors has been implicated in the control of axon guidance and the segmental restriction of cells during embryonic development. In this report, we show that ectopic expression of XLerk, a Xenopus homologue of the murine Lerk-2 (ephrin-B1) transmembrane ligand, causes dissociation of Xenopus embryonic blastomeres by the mid-blastula transition. Moreover, a mutant that lacks the extracellular receptor binding domain can induce this phenotype. The carboxyl-terminal 19 amino acids of the cytoplasmic domain of XLerk are necessary but not sufficient to induce cellular dissociation. Basic fibroblast growth factor, but not activin, can rescue both the loss of cell adhesion and mesoderm induction in ectodermal explants expressing XLerk. Collectively, these results show that the cytoplasmic domain of XLerk has a signaling function that is important for cell adhesion, and fibroblast growth factor signaling modulates this function.

A developmental timer that regulates apoptosis at the onset of gastrulation

Recent work identified an apoptotic program in gastrulation stage Xenopus embryos (Anderson, J.A., Lewellyn, A.L., Maller, J.L., 1997. Mol. Biol. Cell 8, 1195-1206; Stack, J.H., Newport, J.W., 1997. Development 124, 3185-3195). Here, we characterize in detail this maternal cell death program, which is set up at fertilization and abruptly activated at the onset of gastrulation, following DNA damage or treatment of embryos with inhibitors of transcription, translation, or replication, between the time of fertilization and the midblastula transition (MBT). This apoptotic pathway is activated under tightly regulated developmental control(s): if the same treatments are applied after the MBT the apoptotic response is abrogated. Embryos displayed many characteristic apoptotic features, including DNA fragmentation, caspase activation, and embryonic death was blocked in vivo by the ectopic expression of Bcl-2, or injection of the caspase-3 inhibitor z-DEVD-fmk. The precise timing and the execution of this maternal cell death program is set at fertilization and does not depend on the type of stress applied, on cell cycle progression, or on de novo protein synthesis. This maternal developmental program might palliate the lack of cell cycle checkpoints in the pre-MBT embryo.