A developmental timer that regulates apoptosis at the onset of gastrulation

Xenopus laevis (Daudin, 1802) as a Model Organism for Bioscience: A Historic Review and Perspective

In vitro systems have been mainly promoted by authorities to sustain research by following the 3Rs principle, but continuously increasing amounts of evidence point out that in vivo experimentation is also of extreme relevance. Xenopus laevis, an anuran amphibian, is a significant model organism in the study of evolutionary developmental biology, toxicology, ethology, neurobiology, endocrinology, immunology and tumor biology; thanks to the recent development of genome editing, it has also acquired a relevant position in the field of genetics. For these reasons, X. laevis appears to be a powerful and alternative model to the zebrafish for environmental and biomedical studies. Its life cycle, as well as the possibility to obtain gametes from adults during the whole year and embryos by in vitro fertilization, allows experimental studies of several biological endpoints, such as gametogenesis, embryogenesis, larval growth, metamorphosis and, of course, the young and adult stages. Moreover, with respect to alternative invertebrate and even vertebrate animal models, the X. laevis genome displays a higher degree of similarity with that of mammals. Here, we have reviewed the main available literature on the use of X. laevis in the biosciences and, inspired by Feymann's revised view, "Plenty of room for biology at the bottom", suggest that X. laevis is a very useful model for all possible studies.

Activation of P53 pathway contributes to Xenopus hybrid inviability

Hybrid incompatibility as a kind of reproductive isolation contributes to speciation. The nucleocytoplasmic incompatibility between Xenopus tropicalis eggs and Xenopus laevis sperm (te×ls) leads to specific loss of paternal chromosomes 3L and 4L. The hybrids die before gastrulation, of which the lethal causes remain largely unclear. Here, we show that the activation of the tumor suppressor protein P53 at late blastula stage contributes to this early lethality. We find that in stage 9 embryos, P53-binding motif is the most enriched one in the up-regulated Assay for Transposase-Accessible Chromatin with high-throughput sequencing (ATAC-seq) peaks between te×ls and wild-type X. tropicalis controls, which correlates with an abrupt stabilization of P53 protein in te×ls hybrids at stage 9. Inhibition of P53 activity via either tp53 knockout or overexpression of a dominant-negative P53 mutant or Murine double minute 2 proto-oncogene (Mdm2), a negative regulator of P53, by mRNA injection can rescue the te×ls early lethality. Our results suggest a causal function of P53 on hybrid lethality prior to gastrulation.

Translesion DNA synthesis-driven mutagenesis in very early embryogenesis of fast cleaving embryos

In early embryogenesis of fast cleaving embryos, DNA synthesis is short and surveillance mechanisms preserving genome integrity are inefficient, implying the possible generation of mutations. We have analyzed mutagenesis in Xenopus laevis and Drosophila melanogaster early embryos. We report the occurrence of a high mutation rate in Xenopus and show that it is dependent upon the translesion DNA synthesis (TLS) master regulator Rad18. Unexpectedly, we observed a homology-directed repair contribution of Rad18 in reducing the mutation load. Genetic invalidation of TLS in the pre-blastoderm Drosophila embryo resulted in reduction of both the hatching rate and single-nucleotide variations on pericentromeric heterochromatin in adult flies. Altogether, these findings indicate that during very early Xenopus and Drosophila embryos TLS strongly contributes to the high mutation rate. This may constitute a previously unforeseen source of genetic diversity contributing to the polymorphisms of each individual with implications for genome evolution and species adaptation.

BPA and BPS affect the expression of anti-Mullerian hormone (AMH) and its receptor during bovine oocyte maturation and early embryo development

BACKGROUND

Exposure to endocrine-disrupting chemicals, such as Bisphenol A (BPA) and Bisphenol S (BPS), is widespread and has negative implications on embryonic development. Preliminary evidence revealed that in women undergoing IVF treatment, urinary BPA levels were associated with low serum anti-Mullerian hormone, however a definitive relationship between the two has not yet been characterized.

METHODS

This study aimed to evaluate BPA and BPS effects on in vitro oocyte maturation and early preimplantation embryo development through i) analysis of anti-Mullerian hormone (AMH) and anti-Mullerian hormone receptor II (AMHRII), ii) investigation of developmental parameters, such as cleavage, blastocyst rates and developmental arrest, iii) detection of apoptosis and iv) assessment of possible sex ratio skew. An in vitro bovine model was used as a translational model for human early embryonic development. We first assessed AMH and AMHRII levels after bisphenol exposure during oocyte maturation. Zygotes were also analyzed during cleavage and blastocysts stages. Techniques used include in vitro fertilization, quantitative polymerase chain reaction (qPCR), western blotting, TUNEL and immunofluorescence.

RESULTS

Our findings show that BPA significantly decreased cleavage (p < 0.001), blastocyst (p < 0.005) and overall developmental rates as well as significantly increased embryonic arrest at the 2-4 cell stage (p < 0.05). Additionally, both BPA and BPS significantly increased DNA fragmentation in 2-4 cells, 8-16 cells and blastocyst embryos (p < 0.05). Furthermore, BPA and BPS alter AMH and AMHRII at the mRNA and protein level in both oocytes and blastocysts. BPA, but not BPS, also significantly skews sex ratios towards female blastocysts (p < 0.05).

CONCLUSION

This study shows that BPA affects AMH and AMHRII expression during oocyte maturation and that BPS exerts its effects to a greater extent after fertilization and therefore may not be a safer alternative to BPA. Our data lay the foundation for future functional studies, such as receptor kinetics, downstream effectors, and promoter activation/inhibition to prove a functional relationship between bisphenols and the AMH signalling system.

X-linked α-thalassemia with mental retardation is downstream of protein kinase A in the meiotic cell cycle signaling cascade in Xenopus oocytes and is dynamically regulated in response to DNA damage†

X-linked α-thalassemia with mental retardation (ATRX) is a chromatin remodeling protein that belongs to the SWItch/sucrose non-fermentable (SWI2/SNF2) family of helicase/ATPases. During meiosis, ATRX is necessary for heterochromatin formation and maintenance of chromosome stability in order to ensure proper assembly of the metaphase II spindle. Previously, we established ATRX as a novel progesterone regulated protein during bovine meiotic maturation, in addition to being dynamically regulated in response to DNA damage in oocytes. In the present study, we utilize the Xenopus laevis model system to further elucidate the signaling pathways regulating ATRX expression within the oocyte. Here, we present an analysis of endogenous ATRX protein expression during oogenesis, oocyte meiotic maturation, and early embryonic development. ATRX expression is dynamically regulated as evidenced by loss of the protein in metaphase II of meiosis. The downstream activation of meiosis via protein kinase A inhibition resulted in a similar decrease in ATRX protein expression. We demonstrate that the ATRX protein is detected in ubiquitin immuno-precipitates from germinal vesicle oocyte extracts and experimentally demonstrate that proteosomal degradation is responsible for the decreased expression of ATRX during meiosis. ATRX expression is significantly increased in response to gamma-irradiation induced DNA damage in oocytes and embryos. This increased expression is independent of p53 protein expression in apoptotic embryos, as determined by the expression of active caspase-3. Thus, regulation of ATRX protein expression impacts on G2-M progression and ultimately has consequences for cell survival.

Stage-Specific Effects of Ionizing Radiation during Early Development

Early embryonic cells are sensitive to genotoxic stressors such as ionizing radiation. However, sensitivity to these stressors varies depending on the embryonic stage. Recently, the sensitivity and response to ionizing radiation were found to differ during the preimplantation period. The cellular and molecular mechanisms underlying the change during this period are beginning to be elucidated. In this review, we focus on the changes in radio-sensitivity and responses to ionizing radiation during the early developmental stages of the preimplantation (before gastrulation) period in mammals, Xenopus, and fish. Furthermore, we discuss the underlying cellular and molecular mechanisms and the similarities and differences between species.

Mcl1 protein levels and Caspase-7 executioner protease control axial organizer cells survival

BACKGROUND

Organizing centers are groups of specialized cells that secrete morphogens, thereby influencing development of their neighboring territories. Apoptosis is a form of programmed cell death reported to limit the size of organizers. Little is known about the identity of intracellular signals driving organizer cell death. Here we investigated in Xenopus the role of both the anti-apoptotic protein Myeloid-cell-leukemia 1 (Mcl1) and the cysteine proteases Caspase-3 and Caspase-7 in formation of the axial organizing center-the notochord-that derives from the Spemann organizer, and participates in the induction and patterning of the neuroepithelium.

RESULTS

We confirm a role for apoptosis in establishing the axial organizer in early neurula. We show that the expression pattern of mcl1 is coherent with a role for this gene in early notochord development. Using loss of function approaches, we demonstrate that Mcl1 depletion decreases neuroepithelium width and increases notochord cells apoptosis, a process that relies on Caspase-7, and not on Caspase-3, activity. Our data provide evidence that Mcl1 protein levels physiologically control notochord cells' survival and that Caspase-7 is the executioner protease in this developmental process.

CONCLUSIONS

Our study reveals new functions for Mcl1 and Caspase-7 in formation of the axial signalling center.

Preserving Genome Integrity During the Early Embryonic DNA Replication Cycles

During the very early stages of embryonic development chromosome replication occurs under rather challenging conditions, including a very short cell cycle, absence of transcription, a relaxed DNA damage response and, in certain animal species, a highly contracted S-phase. This raises the puzzling question of how the genome can be faithfully replicated in such a peculiar metabolic context. Recent studies have provided new insights into this issue, and unveiled that embryos are prone to accumulate genetic and genomic alterations, most likely due to restricted cellular functions, in particular reduced DNA synthesis quality control. These findings may explain the low rate of successful development in mammals and the occurrence of diseases, such as abnormal developmental features and cancer. In this review, we will discuss recent findings in this field and put forward perspectives to further study this fascinating question.

Barhl2 maintains T-cell factors as repressors, and thereby switches off the Wnt/β-Catenin response driving Spemann organizer formation

A hallmark of Wnt/β-Catenin signaling is the extreme diversity of its transcriptional response, which varies depending on cell and developmental context. What controls this diversity is poorly understood. In all cases, the switch from transcriptional repression to activation depends on a nuclear increase in β-Catenin, which detaches the transcription factor T-cell Factor-7 like 1 (Tcf7l1) bound to Groucho (Gro) transcriptional co-repressors from its DNA binding sites and transiently converts Tcf7/Lymphoid enhancer binding factor 1 (Lef1) into a transcriptional activator. One of the earliest and evolutionarily conserved functions of Wnt/β-Catenin signaling is the induction of the blastopore lip organizer. Here, we demonstrate that the evolutionarily conserved BarH-like homeobox-2 (Barhl2) protein stabilizes the Tcf7l1-Gro complex and maintains repressed expression of Tcf target genes by a mechanism that depends on histone deacetylase 1 (Hdac-1) activity. In this way, Barhl2 switches off the Wnt/β-Catenin-dependent early transcriptional response, thereby limiting the formation of the organizer in time and/or space. This study reveals a novel nuclear inhibitory mechanism of Wnt/Tcf signaling that switches off organizer fate determination.

Paternal chromosome loss and metabolic crisis contribute to hybrid inviability in Xenopus

Hybridization of eggs and sperm from closely related species can give rise to genetic diversity, or can lead to embryo inviability owing to incompatibility. Although central to evolution, the cellular and molecular mechanisms underlying post-zygotic barriers that drive reproductive isolation and speciation remain largely unknown. Species of the African clawed frog Xenopus provide an ideal system to study hybridization and genome evolution. Xenopus laevis is an allotetraploid with 36 chromosomes that arose through interspecific hybridization of diploid progenitors, whereas Xenopus tropicalis is a diploid with 20 chromosomes that diverged from a common ancestor approximately 48 million years ago. Differences in genome size between the two species are accompanied by organism size differences, and size scaling of the egg and subcellular structures such as nuclei and spindles formed in egg extracts. Nevertheless, early development transcriptional programs, gene expression patterns, and protein sequences are generally conserved. Whereas the hybrid produced when X. laevis eggs are fertilized by X. tropicalis sperm is viable, the reverse hybrid dies before gastrulation. Here we apply cell biological tools and high-throughput methods to study the mechanisms underlying hybrid inviability. We reveal that two specific X. laevis chromosomes are incompatible with the X. tropicalis cytoplasm and are mis-segregated during mitosis, leading to unbalanced gene expression at the maternal to zygotic transition, followed by cell-autonomous catastrophic embryo death. These results reveal a cellular mechanism underlying hybrid incompatibility that is driven by genome evolution and contributes to the process by which biological populations become distinct species.

Vestigial-like 3 is a novel Ets1 interacting partner and regulates trigeminal nerve formation and cranial neural crest migration

Drosophila Vestigial is the founding member of a protein family containing a highly conserved domain, called Tondu, which mediates their interaction with members of the TEAD family of transcription factors (Scalloped in Drosophila). In Drosophila, the Vestigial/Scalloped complex controls wing development by regulating the expression of target genes through binding to MCAT sequences. In vertebrates, there are four Vestigial-like genes, the functions of which are still not well understood. Here, we describe the regulation and function of vestigial-like 3 (vgll3) during Xenopus early development. A combination of signals, including FGF8, Wnt8a, Hoxa2, Hoxb2 and retinoic acid, limits vgll3 expression to hindbrain rhombomere 2. We show that vgll3 regulates trigeminal placode and nerve formation and is required for normal neural crest development by affecting their migration and adhesion properties. At the molecular level, vgll3 is a potent activator of pax3, zic1, Wnt and FGF, which are important for brain patterning and neural crest cell formation. Vgll3 interacts in the embryo with Tead proteins but unexpectedly with Ets1, with which it is able to stimulate a MCAT driven luciferase reporter gene. Our findings highlight a critical function for vgll3 in vertebrate early development.

Chk1 Inhibition of the Replication Factor Drf1 Guarantees Cell-Cycle Elongation at the Xenopus laevis Mid-blastula Transition

The early cell divisions of many metazoan embryos are rapid and occur in the near absence of transcription. At the mid-blastula transition (MBT), the cell cycle elongates and several processes become established including the onset of bulk transcription and cell-cycle checkpoints. How these events are timed and coordinated is poorly understood. Here we show in Xenopus laevis that developmental activation of the checkpoint kinase Chk1 at the MBT results in the SCF^β-TRCP-dependent degradation of a limiting replication initiation factor Drf1. Inhibition of Drf1 is the primary mechanism by which Chk1 blocks cell-cycle progression in the early embryo and is an essential function of Chk1 at the blastula-to-gastrula stage of development. This study defines the downregulation of Drf1 as an important mechanism to coordinate the lengthening of the cell cycle and subsequent developmental processes.

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Activation of Chk1 at the Xenopus MBT results in the degradation of Drf1

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Drf1 degradation is SCF^β-TRCP dependent

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Chk1 blocks the cell cycle in the early embryo through inhibition of Drf1

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Inhibition of Drf1 is an essential function of Chk1 during gastrulation

The maternal control in the embryonic development of zebrafish

The maternal control directing the very first hours of life is of pivotal importance for ensuring proper development to the growing embryo. Thanks to the finely regulated inheritance of maternal factors including mRNAs and proteins produced during oogenesis and stored into the mature oocyte, the embryo is sustained throughout the so-called maternal-to-zygotic transition, a period in development characterized by a species-specific length in time, during which critical biological changes regarding cell cycle and zygotic transcriptional activation occur. In order not to provoke any kind of persistent damage, the process must be delicately balanced. Surprisingly, our knowledge as to the possible effects of beneficial bacteria regarding the modulation of the quality and/or quantity of both maternally-supplied and zygotically-transcribed mRNAs, is very limited. To date, only one group has investigated the consequences of the parentally-supplied Lactobacillus rhamnosus on the storage of mRNAs into mature oocytes, leading to an altered maternal control process in the F1 generation. Particular attention was called on the monitoring of several biomarkers involved in autophagy, apoptosis and axis patterning, while data on miRNA generation and pluripotency maintenance are herein presented for the first time, and can assist in laying the ground for further investigations in this field. In this review, the reader is supplied with the current knowledge on the above-mentioned biological process, first by drawing the general background and then by emphasizing the most important findings that have highlighted their focal role in normal animal development.

Regulation of DNA Replication in Early Embryonic Cleavages

Early embryonic cleavages are characterized by short and highly synchronous cell cycles made of alternating S- and M-phases with virtually absent gap phases. In this contracted cell cycle, the duration of DNA synthesis can be extraordinarily short. Depending on the organism, the whole genome of an embryo is replicated at a speed that is between 20 to 60 times faster than that of a somatic cell. Because transcription in the early embryo is repressed, DNA synthesis relies on a large stockpile of maternally supplied proteins stored in the egg representing most, if not all, cellular genes. In addition, in early embryonic cell cycles, both replication and DNA damage checkpoints are inefficient. In this article, we will review current knowledge on how DNA synthesis is regulated in early embryos and discuss possible consequences of replicating chromosomes with little or no quality control.

Efficient delivery of DNA into bovine preimplantation embryos by multiwall carbon nanotubes

The pellucid zone (PZ) is a protective embryonic cells barrier against chemical, physical or biological substances. This put, usual transfection methods are not efficient for mammal oocytes and embryos as they are exclusively for somatic cells. Carbon nanotubes have emerged as a new method for gene delivery, and they can be an alternative for embryos transfection, however its ability to cross the PZ and mediated gene transfer is unknown. Our data confirm that multiwall carbon nanotubes (MWNTs) can cross the PZ and delivery of pDNA into in vitro-fertilized bovine embryos. The degeneration rate and the expression of genes associated to cell viability were not affected in embryos exposed to MWNTs. Those embryos, however, had lower cell number and higher apoptotic cell index, but this did not impair the embryonic development. This study shows the potential utility of the MWNT for the development of new method for delivery of DNA into bovine embryos.

Regulation of zygotic genome activation and DNA damage checkpoint acquisition at the mid-blastula transition

Following fertilization, oviparous embryos undergo rapid, mostly transcriptionally silent cleavage divisions until the mid-blastula transition (MBT), when large-scale developmental changes occur, including zygotic genome activation (ZGA) and cell cycle remodeling, via lengthening and checkpoint acquisition. Despite their concomitant appearance, whether these changes are co-regulated is unclear. Three models have been proposed to account for the timing of (ZGA). One model implicates a threshold nuclear to cytoplasmic (N:C) ratio, another stresses the importance cell cycle elongation, while the third model invokes a timer mechanism. We show that precocious Chk1 activity in pre-MBT zebrafish embryos elongates cleavage cycles, thereby slowing the increase in the N:C ratio. We find that cell cycle elongation does not lead to transcriptional activation. Rather, ZGA slows in parallel with the N:C ratio. We show further that the DNA damage checkpoint program is maternally supplied and independent of ZGA. Although pre-MBT embryos detect damage and activate Chk2 after induction of DNA double-strand breaks, the Chk1 arm of the DNA damage response is not activated, and the checkpoint is nonfunctional. Our results are consistent with the N:C ratio model for ZGA. Moreover, the ability of precocious Chk1 activity to delay pre-MBT cell cycles indicate that lack of Chk1 activity limits checkpoint function during cleavage cycles. We propose that Chk1 gain-of-function at the MBT underlies cell cycle remodeling, whereas ZGA is regulated independently by the N:C ratio.

Pulling short DNA molecules having defects on different locations

We present a study on the role of defects on the stability of short DNA molecules. We consider short DNA molecules (16 base pairs) and investigate the thermal as well as mechanical denaturation of these molecules in the presence of defects that occur anywhere in the molecule. For the investigation, we consider four different kinds of chains. Not only are the ratios of AT to GC different in these molecules but also the distributions of AT and GC along the molecule are different. With suitable modifications in the statistical model to show the defect in a pair, we investigate the denaturation of short DNA molecules in thermal as well as constant force ensembles. In the force ensemble, we pulled the DNA molecule from each end (keeping other end free) and observed some interesting features of opening of the molecule in the presence of defects in the molecule. We calculate the probability of opening of the DNA molecule in the constant force ensemble to explain the opening of base pairs and hence the denaturation of molecules in the presence of defects.

RAD18 Is a Maternal Limiting Factor Silencing the UV-Dependent DNA Damage Checkpoint in Xenopus Embryos

In early embryos, the DNA damage checkpoint is silent until the midblastula transition (MBT) because of maternal limiting factors of unknown identity. Here we identify the RAD18 ubiquitin ligase as one such factor in Xenopus. We show, in vitro and in vivo, that inactivation of RAD18 function leads to DNA damage-dependent checkpoint activation, monitored by CHK1 phosphorylation. Moreover, we show that the abundance of both RAD18 and PCNA monoubiquitylated (mUb) are developmentally regulated. Increased DNA abundance limits the availability of RAD18 close to the MBT, thereby reducing PCNA(mUb) and inducing checkpoint derepression. Furthermore, we show that this embryonic-like regulation can be reactivated in somatic mammalian cells by ectopic RAD18 expression, therefore conferring resistance to DNA damage. Finally, we find high RAD18 expression in cancer stem cells highly resistant to DNA damage. Together, these data propose RAD18 as a critical embryonic checkpoint-inhibiting factor and suggest that RAD18 deregulation may have unexpected oncogenic potential.

FoxA4 favours notochord formation by inhibiting contiguous mesodermal fates and restricts anterior neural development in Xenopus embryos

In vertebrates, the embryonic dorsal midline is a crucial signalling centre that patterns the surrounding tissues during development. Members of the FoxA subfamily of transcription factors are expressed in the structures that compose this centre. Foxa2 is essential for dorsal midline development in mammals, since knock-out mouse embryos lack a definitive node, notochord and floor plate. The related gene foxA4 is only present in amphibians. Expression begins in the blastula -chordin and -noggin expressing centre (BCNE) and is later restricted to the dorsal midline derivatives of the Spemann's organiser. It was suggested that the early functions of mammalian foxa2 are carried out by foxA4 in frogs, but functional experiments were needed to test this hypothesis. Here, we show that some important dorsal midline functions of mammalian foxa2 are exerted by foxA4 in Xenopus. We provide new evidence that the latter prevents the respecification of dorsal midline precursors towards contiguous fates, inhibiting prechordal and paraxial mesoderm development in favour of the notochord. In addition, we show that foxA4 is required for the correct regionalisation and maintenance of the central nervous system. FoxA4 participates in constraining the prospective rostral forebrain territory during neural specification and is necessary for the correct segregation of the most anterior ectodermal derivatives, such as the cement gland and the pituitary anlagen. Moreover, the early expression of foxA4 in the BCNE (which contains precursors of the whole forebrain and most of the midbrain and hindbrain) is directly required to restrict anterior neural development.

Heterochrony and developmental timing mechanisms: changing ontogenies in evolution

Heterochrony, or a change in developmental timing, is an important mechanism of evolutionary change. Historically the concept of heterochrony has focused alternatively on changes in size and shape or changes in developmental sequence, but most have focused on the pattern of change. Few studies have examined changes in the mechanisms that embryos use to actually measure time during development. Recently, evolutionary studies focused on changes in distinct timekeeping mechanisms have appeared, and this review examines two such case studies: the evolution of increased segment number in snakes and the extreme rostral to caudal gradient of developmental maturation in marsupials. In both examples, heterochronic modifications of the somite clock have been important drivers of evolutionary change.

Developmental time rather than local environment regulates the schedule of epithelial polarization in the zebrafish neural rod

Background

Morphogenesis requires developmental processes to occur both at the right time and in the right place. During neural tube formation in the zebrafish embryo, the generation of the apical specializations of the lumen must occur in the center of the neural rod after the neural cells have undergone convergence, invagination and interdigitation across the midline. How this coordination is achieved is uncertain. One possibility is that environmental signaling at the midline of the neural rod controls the schedule of apical polarization. Alternatively, polarization could be regulated by a timing mechanism and then independent morphogenetic processes ensure the cells are in the correct spatial location.

Results

Ectopic transplantation demonstrates the local environment of the neural midline is not required for neural cell polarization. Neural cells can self-organize into epithelial cysts in ectopic locations in the embryo and also in three-dimensional gel cultures. Heterochronic transplants demonstrate that the schedule of polarization and the specialized cell divisions characteristic of the neural rod are more strongly regulated by time than local environmental signals. The cells’ schedule for polarization is set prior to gastrulation, is stable through several rounds of cell division and appears independent of the morphogenetic movements of gastrulation and neurulation.

Conclusions

Time rather than local environment regulates the schedule of epithelial polarization in zebrafish neural rod.

An intact brachyury function is necessary to prevent spurious axial development in Xenopus laevis

We have previously shown that the member of the HES family hairy2 induces the ectopic expression of dorsal markers when it is overexpressed in the ventral side of Xenopus embryos. Intriguingly, hairy2 represses the mesoderm transcription factor brachyury (bra) throughout its domain in the marginal zone. Here we show that in early gastrula, bra and hairy2 are expressed in complementary domains. Overexpression of bra repressed hairy2. Interference of bra function with a dominant-negative construct expanded the hairy2 domain and, like hairy2 overexpression, promoted ectopic expression of dorsal axial markers in the ventral side and induced secondary axes without head and notochord. Hairy2 depletion rescued the ectopic dorsal development induced by interference of bra function. We concluded that an intact bra function is necessary to exclude hairy2 expression from the non-organiser field, to impede the ectopic specification of dorsal axial fates and the appearance of incomplete secondary axes. This evidence supports a previously unrecognised role for bra in maintaining the dorsal fates inhibited in the ventral marginal zone, preventing the appearance of trunk duplications.

Comparative functional analysis of ZFP36 genes during Xenopus development

ZFP36 constitutes a small family of RNA binding proteins (formerly known as the TIS11 family) that target mRNA and promote their degradation. In mammals, ZFP36 proteins are encoded by four genes and, although they show similar activities in a cellular RNA destabilization assay, there is still a limited knowledge of their mRNA targets and it is not known whether or not they have redundant functions. In the present work, we have used the Xenopus embryo, a model system allowing gain- and loss-of-function studies, to investigate, whether individual ZFP36 proteins had distinct or redundant functions. We show that overexpression of individual amphibian zfp36 proteins leads to embryos having the same defects, with alteration in somites segmentation and pronephros formation. In these embryos, members of the Notch signalling pathway such as hairy2a or esr5 mRNA are down-regulated, suggesting common targets for the different proteins. We also show that mouse Zfp36 protein overexpression gives the same phenotype, indicating an evolutionary conserved property among ZFP36 vertebrate proteins. Morpholino oligonucleotide-induced loss-of-function leads to defects in pronephros formation, reduction in tubule size and duct coiling alterations for both zfp36 and zfp36l1, indicating no functional redundancy between these two genes. Given the conservation in gene structure and function between the amphibian and mammalian proteins and the conserved mechanisms for pronephros development, our study highlights a potential and hitherto unreported role of ZFP36 gene in kidney morphogenesis.

A developmental requirement for HIRA-dependent H3.3 deposition revealed at gastrulation in Xenopus

Discovering how histone variants that mark distinct chromatin regions affect a developmental program is a major challenge in the epigenetics field. To assess the importance of the H3.3 histone variant and its dedicated histone chaperone HIRA, we used an established developmental model, Xenopus laevis. After the early rapid divisions exploiting a large maternal pool of both replicative H3.2 and replacement H3.3, H3.3 transcripts show a distinct peak of expression at gastrulation. Depletion of both H3.2 and H3.3 leads to an early gastrulation arrest. However, with only H3.3 depletion, defects occur at late gastrulation, impairing further development. Providing exogenous H3.3 mRNAs, but not replicative H3.2 mRNAs, rescues these defects. Notably, downregulation of the H3.3 histone chaperone HIRA similarly impairs late gastrulation, and we find a global defect in H3.3 incorporation into chromatin comparable to H3.3 depletion. We discuss how specific HIRA-dependent H3.3 deposition is required for chromatin dynamics during gastrulation.

Xenopus Nanos1 is required to prevent endoderm gene expression and apoptosis in primordial germ cells

Nanos is expressed in multipotent cells, stem cells and primordial germ cells (PGCs) of organisms as diverse as jellyfish and humans. It functions together with Pumilio to translationally repress targeted mRNAs. Here we show by loss-of-function experiments that Xenopus Nanos1 is required to preserve PGC fate. Morpholino knockdown of maternal Nanos1 resulted in a striking decrease in PGCs and a loss of germ cells from the gonads. Lineage tracing and TUNEL staining reveal that Nanos1-deficient PGCs fail to migrate out of the endoderm. They appear to undergo apoptosis rather than convert to normal endoderm. Whereas normal PGCs do not become transcriptionally active until neurula, Nanos1-depleted PGCs prematurely exhibit a hyperphosphorylated RNA polymerase II C-terminal domain at the midblastula transition. Furthermore, they inappropriately express somatic genes characteristic of endoderm regulated by maternal VegT, including Xsox17α, Bix4, Mixer, GATA4 and Edd. We further demonstrate that Pumilio specifically binds VegT RNA in vitro and represses, along with Nanos1, VegT translation within PGCs. Repressed VegT RNA in wild-type PGCs is significantly less stable than VegT in Nanos1-depleted PGCs. Our data indicate that maternal VegT RNA is an authentic target of Nanos1/Pumilio translational repression. We propose that Nanos1 functions to translationally repress RNAs that normally specify endoderm and promote apoptosis, thus preserving the germline.

Unfertilized frog eggs die by apoptosis following meiotic exit

Background

A characteristic feature of frog reproduction is external fertilization accomplished outside the female's body. Mature fertilization-competent frog eggs are arrested at the meiotic metaphase II with high activity of the key meiotic regulators, maturation promoting factor (MPF) and cytostatic factor (CSF), awaiting fertilization. If the eggs are not fertilized within several hours of ovulation, they deteriorate and ultimately die by as yet unknown mechanism.

Results

Here, we report that the vast majority of naturally laid unfertilized eggs of the African clawed frog Xenopus laevis spontaneously exit metaphase arrest under various environmental conditions and degrade by a well-defined apoptotic process within 48 hours after ovulation. The main features of this process include cytochrome c release, caspase activation, ATP depletion, increase of ADP/ATP ratio, apoptotic nuclear morphology, progressive intracellular acidification, and egg swelling. Meiotic exit seems to be a prerequisite for execution of the apoptotic program, since (i) it precedes apoptosis, (ii) apoptotic events cannot be observed in the eggs maintaining high activity of MPF and CSF, and (iii) apoptosis in unfertilized frog eggs is accelerated upon early meiotic exit. The apoptotic features cannot be observed in the immature prophase-arrested oocytes, however, the maturation-inducing hormone progesterone renders oocytes susceptible to apoptosis.

Conclusions

The study reveals that naturally laid intact frog eggs die by apoptosis if they are not fertilized. A maternal apoptotic program is evoked in frog oocytes upon maturation and executed after meiotic exit in unfertilized eggs. The meiotic exit is required for execution of the apoptotic program in eggs. The emerging anti-apoptotic role of meiotic metaphase arrest needs further investigation.

Loss of Xenopus tropicalis EMSY causes impairment of gastrulation and upregulation of p53

EMSY interacts directly with BRCA2 and links the BRCA2 pathway to sporadic breast and ovarian cancer. It also interacts with BS69 and HP1b, both of which are involved in chromatin remodelling, and with NIF-1 and DBC-1 in the regulation of nuclear receptor-mediated transcription. Here we investigate the function of EMSY during amphibian development, and in doing so provide the first loss-of-function analysis of this protein. Injection of Xenopus tropicalis embryos with antisense morpholino oligonucleotides targeting XtEMSY impairs gastrulation movements, disrupts dorsal structures, and kills embryos by tailbud stages. Consistent with these observations, regional markers such as Xbra, Chd, Gsc, Shh, Sox3 and Sox17 are downregulated. In contrast to these regional markers, expression of p53 is upregulated in such embryos, and at later stages Bax expression is elevated and apoptotic cells can be detected. Our results demonstrate that EMSY has an essential role in development and they provide an in vivo loss-of-function model that might be used to explore the biochemical functions of this protein in more detail.

Eukaryotic initiation factor 6 (eif6) overexpression affects eye development in Xenopus laevis

The translation initiation factor eif6 has been implicated as a regulator of ribosome assembly, selective mRNA translation and apoptosis. Many of these activities depend upon the phosphorylation of eif6 serine 235 by PKC. Previous data showed that eif6 binds to the 60S ribosomal subunit when unphosphorylated, inhibiting assembly with the 40S subunit. Phosphorylation of Ser235 releases eif6 from the 60S subunit and allows assembly. eif6 acts as an anti-apoptotic factor via regulation of the bcl2/bax balance and acts selectively upstream of bcl2. This activity also depends upon phosphorylation of eif6 Ser235. One of the consequences of eif6 overexpression in Xenopus embryos is aberrant eye development. Here we evaluate the eye phenotype and show that it is transient. We show that the whole eye, particularly the retina layers, of the embryos injected with eif6-encoding mRNA recover by stage 42. Embryos over-expressing eif6 have normal expression of anterior- and brain-specific markers, indicating that outside the eye field, other neural regions appear unaffected by the eif6 injection. No eye defect was detected when morpholinos were used to reduce eif6 protein synthesis. We tested how two known pathways of eif6 function with respect to alteration of eye development. We found that injection of bcl2 did not produce the eye phenotype and eif6-bax co-injection did not rescue the eye defect, suggesting that the eye phenotype is not bearing on the anti-apoptotic role played by eif6 is not linked to its role as an anti-apoptotic factor. We also determined that PKC-dependant phosphorylation of Ser235 in eif6 is not required to produce defective eye development. These results indicate that the aberrant eye phenotype, produced by eif6 overexpression, is not directly linked to the PKC-regulated effects of eif6 on translation and ribosomal subunit interaction or on eif6 anti-apoptotic properties.

PP2A:B56{epsilon}, a substrate of caspase-3, regulates p53-dependent and p53-independent apoptosis during development

Protein phosphatase 2A (PP2A) is one of the most abundantly expressed serine/threonine protein phosphatases. A large body of evidence suggests that PP2A is a tumor suppressor and plays critical roles in regulating apoptosis. PP2A is a heterotrimeric protein complex. Its substrate specificity, localization, and activity are regulated by regulatory subunits of PP2A. A recent study has demonstrated that single nucleotide polymorphism in B56ε (PPP2R5E), a B56 family regulatory subunit of PP2A, is associated with human soft tissue sarcoma. This raises the possibility that B56ε is involved in tumorigenesis and plays important roles in regulating apoptosis. However, this hypothesis has not been tested experimentally. Our previous studies revealed that B56ε regulates a number of developmental signaling pathways during early embryonic patterning. Here we report novel functions of B56ε in regulating apoptosis. We provide evidence that B56ε has both anti- and pro-apoptotic functions. B56ε suppresses p53-independent apoptosis during neural development, but triggers p53-dependent apoptosis. Mechanistically, B56ε regulates the p53-dependent apoptotic pathway solely through controlling the stability of p53 protein. In addition to its function in regulating apoptosis, we show that B56ε undergoes proteolytic cleavage. The cleavage of B56ε is mediated by caspase-3 and occurs on the carboxyl side of an evolutionarily conserved N-terminal "DKXD" motif. These results demonstrate that B56ε, a substrate of caspase-3, is an essential regulator of apoptosis. So far, we have identified an alternative translation isoform and a caspase cleavage product of B56ε. The significance of post-transcriptional regulation of B56ε is discussed.

In X. laevis embryos high levels of the anti-apoptotic factor p27BBP/eIF6 are stage-dependently found in BrdU and TUNEL-reactive territories

p27BBP/eIF6 (β4 binding protein/eukaryotic initiation factor 6) is a highly conserved protein necessary for cell life. In adult eIF6 mice, a 50% decrease in the protein levels in all tissues is accompanied by a reduction in cell proliferation only in the liver, fat cells and cultured fibroblasts. During X. laevis embryogenesis expression of p27BBP/eIF6 is abundant in high proliferative territories. However, in Xenopus cell proliferation appears unaffected following p27BBP/eIF6 over-expression or down-regulation. Indeed, p27BBP/eIF6 is an anti-apoptotic factor acting upstream of Bcl2 that reduces endogenous apoptosis. We studied p27BBP/eIF6 protein localization in wild type embryos and compared it to proliferation and apoptosis. At the beginning of embryogenesis, high levels of p27BBP/eIF6, proliferation and apoptosis overlap. In later development stages high proliferation levels are present in the same regions where higher p27BBP/eIF6 expression is observed, while apoptosis does not appear specifically concentrated in the same sites. The higher presence of p27BBP/eIF6 would appear related to an increased need of apoptosis control in the regions where cell death is essential for normal development.

Defects and DNA replication

We introduce a rate-equation formalism to study DNA replication kinetics in the presence of defects resulting from DNA damage and find a crossover between two regimes: a normal regime, where the influence of defects is local, and an initiation-limited regime. In the latter, defects have a global impact on replication, whose progress is set by the rate at which origins of replication are activated, or initiated. Normal, healthy cells have defect densities in the normal regime. Our model can explain an observed correlation between interorigin separation and rate of DNA replication.

Identification, characterization, and effects of Xenopus laevis PNAS-4 gene on embryonic development

Apoptosis plays an important role in embryonic development. PNAS-4 has been demonstrated to induce apoptosis in several cancer cells. In this study, we cloned Xenopus laevis PNAS-4 (xPNAS-4), which is homologous to the human PNAS-4 gene. Bioinformatics analysis for PNAS-4 indicated that xPNAS-4 shared 87.6% identity with human PNAS-4 and 85.5% with mouse PNAS-4. The phylogenetic tree of PNAS-4 protein was also summarized. An analysis of cellular localization using an EGFP-fused protein demonstrated that xPNAS-4 was localized in the perinuclear region of the cytoplasm. RT-PCR analysis revealed that xPNAS-4, as a maternally expressed gene, was present in all stages of early embryo development. Whole-mount in situ hybridization showed that xPNAS-4 was mainly expressed in ectoderm and mesoderm. Furthermore, microinjection of xPNAS-4 mRNA in vivo caused developmental defects manifesting as a small eye phenotype in the Xenopous embryos, and as a small eye or one-eye phenotype in developing zebrafish embryos. In addition, embryos microinjected with xPNAS-4 antisense morpholino oligonucleotides (MOs) exhibited a failure of head development and shortened axis.

A non-enzymatic function of 17beta-hydroxysteroid dehydrogenase type 10 is required for mitochondrial integrity and cell survival

Deficiency of the mitochondrial enzyme 2-methyl-3-hydroxybutyryl-CoA dehydrogenase involved in isoleucine metabolism causes an organic aciduria with atypical neurodegenerative course. The disease-causing gene is HSD17B10 and encodes 17β-hydroxysteroid dehydrogenase type 10 (HSD10), a protein also implicated in the pathogenesis of Alzheimer's disease. Here we show that clinical symptoms in patients are not correlated with residual enzymatic activity of mutated HSD10. Loss-of-function and rescue experiments in Xenopus embryos and cells derived from conditional Hsd17b10^−/− mice demonstrate that a property of HSD10 independent of its enzymatic activity is essential for structural and functional integrity of mitochondria. Impairment of this function in neural cells causes apoptotic cell death whilst the enzymatic activity of HSD10 is not required for cell survival. This finding indicates that the symptoms in patients with mutations in the HSD17B10 gene are unrelated to accumulation of toxic metabolites in the isoleucine pathway and, rather, related to defects in general mitochondrial function. Therefore alternative therapeutic approaches to an isoleucine-restricted diet are required.

GEMC1 is a TopBP1-interacting protein required for chromosomal DNA replication

Many factors required for chromosomal DNA replication have been identified in unicellular eukaryotes. However, DNA replication in complex multicellular organisms is poorly understood. Here, we report the identification of GEMC1, a novel vertebrate protein required for chromosomal DNA replication. GEMC1 is highly conserved in vertebrates and is preferentially expressed in proliferating cells. Using Xenopus egg extract we show that Xenopus GEMC1 (xGEMC1) binds to checkpoint and replication factor TopBP1, which promotes xGEMC1 binding to chromatin during pre-replication complex (pre-RC) formation. We demonstrate that xGEMC1 directly interacts with replication factors such as Cdc45 and Cdk2-CyclinE by which it is heavily phosphorylated. Phosphorylated xGEMC1 stimulates initiation of DNA replication whereas depletion of xGEMC1 prevents DNA replication onset due to impairment of Cdc45 loading onto chromatin. Likewise, inhibition of GEMC1 expression by morpholino and siRNA oligos prevents DNA replication in embryonic and somatic vertebrate cells. These data suggest that GEMC1 promotes initiation of chromosomal DNA replication in higher eukaryotes by mediating TopBP1 and Cdk2 dependent recruitment of Cdc45 onto replication origins.

Shorter exposures to harder X-rays trigger early apoptotic events in Xenopus laevis embryos

Background

A long-standing conventional view of radiation-induced apoptosis is that increased exposure results in augmented apoptosis in a biological system, with a threshold below which radiation doses do not cause any significant increase in cell death. The consequences of this belief impact the extent to which malignant diseases and non-malignant conditions are therapeutically treated and how radiation is used in combination with other therapies. Our research challenges the current dogma of dose-dependent induction of apoptosis and establishes a new parallel paradigm to the photoelectric effect in biological systems.

Methodology/Principal Findings

We explored how the energy of individual X-ray photons and exposure time, both factors that determine the total dose, influence the occurrence of cell death in early Xenopus embryo. Three different experimental scenarios were analyzed and morphological and biochemical hallmarks of apoptosis were evaluated. Initially, we examined cell death events in embryos exposed to increasing incident energies when the exposure time was preset. Then, we evaluated the embryo's response when the exposure time was augmented while the energy value remained constant. Lastly, we studied the incidence of apoptosis in embryos exposed to an equal total dose of radiation that resulted from increasing the incoming energy while lowering the exposure time.

Conclusions/Significance

Overall, our data establish that the energy of the incident photon is a major contributor to the outcome of the biological system. In particular, for embryos exposed under identical conditions and delivered the same absorbed dose of radiation, the response is significantly increased when shorter bursts of more energetic photons are used. These results suggest that biological organisms display properties similar to the photoelectric effect in physical systems and provide new insights into how radiation-mediated apoptosis should be understood and utilized for therapeutic purposes.

The F-box protein Cdc4/Fbxw7 is a novel regulator of neural crest development in Xenopus laevis

Background

The neural crest is a unique population of cells that arise in the vertebrate ectoderm at the neural plate border after which they migrate extensively throughout the embryo, giving rise to a wide range of derivatives. A number of proteins involved in neural crest development have dynamic expression patterns, and it is becoming clear that ubiquitin-mediated protein degradation is partly responsible for this.

Results

Here we demonstrate a novel role for the F-box protein Cdc4/Fbxw7 in neural crest development. Two isoforms of Xenopus laevis Cdc4 were identified, and designated xCdc4α and xCdc4β. These are highly conserved with vertebrate Cdc4 orthologs, and the Xenopus proteins are functionally equivalent in terms of their ability to degrade Cyclin E, an established vertebrate Cdc4 target. Blocking xCdc4 function specifically inhibited neural crest development at an early stage, prior to expression of c-Myc, Snail2 and Snail.

Conclusions

We demonstrate that Cdc4, an ubiquitin E3 ligase subunit previously identified as targeting primarily cell cycle regulators for proteolysis, has additional roles in control of formation of the neural crest. Hence, we identify Cdc4 as a protein with separable but complementary functions in control of cell proliferation and differentiation.

MBD4 and MLH1 are required for apoptotic induction in xDNMT1-depleted embryos

Loss of the of the maintenance methyltransferase xDNMT1 during Xenopus development results in premature transcription and activation of a p53-dependent apoptotic program that accounts for embryo lethality. Here, we show that activation of the apoptotic response is signalled through the methyl-CpG binding protein xMBD4 and the mismatch repair pathway protein xMLH1. Depletion of xMBD4 or xMLH1 increases the survival rate of xDNMT1-depleted embryos, whereas overexpression of these proteins in embryos induces programmed cell death at the onset of gastrulation. MBD4 interacts directly with both DNMT1 and MLH1, leading to recruitment of the latter to heterochromatic sites that are coincident with DNMT1 localisation. Time-lapse microscopy of micro-irradiated mammalian cells shows that MLH1/MBD4 (like DNMT1) can accumulate at DNA damage sites. We propose that xMBD4/xMLH1 participates in a novel G2 checkpoint that is responsive to xDNMT1p levels in developing embryos and cells.

Ectopic expression of prelamin A in early Xenopus embryos induces apoptosis

Lamin proteins are components of metazoan cell nuclei. During evolution, two classes of lamin proteins evolved, A- and B-type lamins. B-type lamins are expressed in nearly all cell types and in all developmental stages and are thought to be indispensable for cellular survival. In contrast, A-type lamins have a more restricted expression pattern. They are expressed in differentiated cells and appear late in embryogenesis. In the earliest steps of mammalian development, A-type lamins are present in oocytes, pronuclei and during the first cleavage stages of the developing embryo. But latest after the 16-cell stage, A-type lamin proteins are not any longer detectable in embryonic cells. Amphibian oocytes and early embryos do not express lamin A. Moreover, extracts of Xenopus oocytes and eggs have the ability to selectively remove A-type lamins from somatic nuclei. This observation and the restricted expression pattern suggest that the presence of lamin A might interfere with developmental processes in the early phase of embryogenesis. To test this, we ectopically expressed lamin A during early embryonic development of Xenopus laevis by microinjection of synthetic mRNA. Here, we show that introducing mature lamin A does not interfere with normal development. However, expression of prelamin A or lamin A variants that cannot be fully processed cause severe disturbances and lead to apoptosis during gastrulation. The toxic effect is due to lack of the conversion of prenylated prelamin A to its mature form. Remarkably, even a cytoplasmic prelamin A variant that is excluded from the nucleus drives embryos into apoptosis.

microRNA-24a is required to repress apoptosis in the developing neural retina

Programmed cell death is important for the proper development of the retina, and microRNAs (miRNAs) may be critical for its regulation. Here, we report that miR-24a is expressed in the neural retina and is required for correct eye morphogenesis in Xenopus. Inhibition of miR-24a during development causes a reduction in eye size due to a significant increase in apoptosis in the retina, whereas overexpression of miR-24a is sufficient to prevent apoptosis. We show that miR-24a negatively regulates the proapoptotic factors caspase9 and apaf1, demonstrating a role for miRNAs in the regulation of apoptosis during normal development.

c-Jun N-terminal kinase 1 phosphorylates Myt1 to prevent UVA-induced skin cancer

The c-Jun N-terminal kinase (JNK) signaling pathway is known to mediate both survival and apoptosis of tumor cells. Although JNK1 and JNK2 have been shown to differentially regulate the development of skin cancer, the underlying mechanistic basis remains unclear. Here, we demonstrate that JNK1, but not JNK2, interacts with and phosphorylates Myt1 ex vivo and in vitro. UVA induces substantial apoptosis in JNK wild-type (JNK(+/+)) or JNK2-deficient (JNK2(-/-)) mouse embryonic fibroblasts but has no effect on JNK1-deficient (JNK1(-/-)) cells. In addition, UVA-induced caspase-3 cleavage and DNA fragmentation were suppressed by the knockdown of human Myt1 in skin cancer cells. JNK1 deficiency results in suppressed Myt1 phosphorylation and caspase-3 cleavage in skin exposed to UVA irradiation. In contrast, the absence of JNK2 induces Myt1 phosphorylation and caspase-3 cleavage in skin exposed to UVA. The overexpression of JNK1 with Myt1 promotes cellular apoptosis during the early embryonic development of Xenopus laevis, whereas the presence of JNK2 reduces the phenotype of Myt1-induced apoptotic cell death. Most importantly, JNK1(-/-) mice developed more UVA-induced papillomas than either JNK(+/+) or JNK2(-/-) mice, which was associated with suppressed Myt1 phosphorylation and decreased caspase-3 cleavage. Taken together, these data provide mechanistic insights into the distinct roles of the different JNK isoforms, specifically suggesting that the JNK1-mediated phosphorylation of Myt1 plays an important role in UVA-induced apoptosis and the prevention of skin carcinogenesis.

Phosphorylation of p53 is regulated by TPX2-Aurora A in xenopus oocytes

p53 is an important tumor suppressor regulating the cell cycle at multiple stages in higher vertebrates. The p53 gene is frequently deleted or mutated in human cancers, resulting in loss of p53 activity. This leads to centrosome amplification, aneuploidy, and tumorigenesis, three phenotypes also observed after overexpression of the oncogenic kinase Aurora A. Accordingly, recent studies have focused on the relationship between these two proteins. p53 and Aurora A have been reported to interact in mammalian cells, but the function of this interaction remains unclear. We recently reported that Xenopus p53 can inhibit Aurora A activity in vitro but only in the absence of TPX2. Here we investigate the interplay between Xenopus Aurora A, TPX2, and p53 and show that newly synthesized TPX2 is required for nearly all Aurora A activation and for full p53 synthesis and phosphorylation in vivo during oocyte maturation. In vitro, phosphorylation mediated by Aurora A targets serines 129 and 190 within the DNA binding domain of p53. Glutathione S-transferase pull-down studies indicate that the interaction occurs via the p53 transactivation domain and the Aurora A catalytic domain around the T-loop. Our studies suggest that targeting of TPX2 might be an effective strategy for specifically inhibiting the phosphorylation of Aurora A substrates, including p53.

Expression of exogenous mRNA in Xenopus laevis embryos for the study of cell cycle regulation

The microinjection of mRNA that is transcribed and capped in vitro into fertilized eggs and embryos of Xenopus laevis provides a powerful means for discovering the function of proteins during early development. Proteins may be overexpressed for a gain-of-function effect or exogenous protein function may be compromised by the microinjection of mRNA encoding "dominant-negative" proteins. This methodology is particularly suited for the investigation of the regulation of the cell cycle, checkpoints, and apoptosis in early development.