Transcription of muscle-specific actin genes in early Xenopus development: nuclear transplantation and cell dissociation

Evolution of Somite Compartmentalization: A View From Xenopus

Somites are transitory metameric structures at the basis of the axial organization of vertebrate musculoskeletal system. During evolution, somites appear in the chordate phylum and compartmentalize mainly into the dermomyotome, the myotome, and the sclerotome in vertebrates. In this review, we summarized the existing literature about somite compartmentalization in Xenopus and compared it with other anamniote and amniote vertebrates. We also present and discuss a model that describes the evolutionary history of somite compartmentalization from ancestral chordates to amniote vertebrates. We propose that the ancestral organization of chordate somite, subdivided into a lateral compartment of multipotent somitic cells (MSCs) and a medial primitive myotome, evolves through two major transitions. From ancestral chordates to vertebrates, the cell potency of MSCs may have evolved and gave rise to all new vertebrate compartments, i.e., the dermomyome, its hypaxial region, and the sclerotome. From anamniote to amniote vertebrates, the lateral MSC territory may expand to the whole somite at the expense of primitive myotome and may probably facilitate sclerotome formation. We propose that successive modifications of the cell potency of some type of embryonic progenitors could be one of major processes of the vertebrate evolution.

Delineating nuclear reprogramming

Nuclear reprogramming is described as a molecular switch, triggered by the conversion of one cell type to another. Several key experiments in the past century have provided insight into the field of nuclear reprogramming. Previously deemed impossible, this research area is now brimming with new findings and developments. In this review, we aim to give a historical perspective on how the notion of nuclear reprogramming was established, describing main experiments that were performed, including (1) somatic cell nuclear transfer, (2) exposure to cell extracts and cell fusion, and (3) transcription factor induced lineage switch. Ultimately, we focus on (4) transcription factor induced pluripotency, as initiated by a landmark discovery in 2006, where the process of converting somatic cells to a pluripotent state was narrowed down to four transcription factors. The conception that somatic cells possess the capacity to revert to an immature status brings about huge clinical implications including personalized therapy, drug screening and disease modeling. Although this technology has potential to revolutionize the medical field, it is still impeded by technical and biological obstacles. This review describes the effervescent changes in this field, addresses bottlenecks hindering its advancement and in conclusion, applies the latest findings to overcome these issues.

Nuclear transfer to eggs and oocytes

We review experiments in which somatic cell nuclei are transplanted singly to enucleated eggs (metaphase II) in amphibia and mammals and as multiple nuclei to the germinal vesicle of amphibian oocytes (prophase I). These experiments have shown the totipotency of some somatic cell nuclei, as well as switches in cell type and changes in gene expression. Abnormalities of nuclear transplant embryo development increase greatly as nuclei are taken from progressively more differentiated donor cells. The molecular changes that accompany the reprogramming of transplanted nuclei help to indicate the mechanisms used by eggs and oocytes to reprogram gene expression. We discuss the importance of chromosomal protein exchange, of transcription factor supply, and of chromatin access in reprogramming.

Somatic cell nuclear transfer efficiency: how can it be improved through nuclear remodeling and reprogramming?

Fertile offspring from somatic cell nuclear transfer (SCNT) is the goal of most cloning laboratories. For this process to be successful, a number of events must occur correctly. First the donor nucleus must be in a state that is amenable to remodeling and subsequent genomic reprogramming. The nucleus must be introduced into an oocyte cytoplasm that is capable of facilitating the nuclear remodeling. The oocyte must then be adequately stimulated to initiate development. Finally the resulting embryo must be cultured in an environment that is compatible with the development of that particular embryo. Much has been learned about the incredible changes that occur to a nucleus after it is placed in the cytoplasm of an oocyte. While we think that we are gaining an understanding of the reorganization that occurs to proteins in the donor nucleus, the process of cloning is still very inefficient. Below we will introduce the procedures for SCNT, discuss nuclear remodeling and reprogramming, and review techniques that may improve reprogramming. Finally we will briefly touch on other aspects of SCNT that may improve the development of cloned embryos.

Xenopus oocytes reactivate muscle gene transcription in transplanted somatic nuclei independently of myogenic factors

Transplantation into eggs or oocytes is an effective means of achieving the reprogramming of somatic cell nuclei. We ask here whether the provision of gene-specific transcription factors forms part of the mechanism by which a gene that is repressed in somatic cells is transcribed in oocytes. We find that M1 oocytes have an extremely strong transcription-inducing activity. They cause muscle genes of nuclei from non-muscle somatic cells, after injection into oocytes, to be transcribed to nearly the same extent as muscle genes in muscle cells. We show, surprisingly, that the myogenic factor MyoD and other known myogenic factors are not required to induce the transcription of muscle genes in a range of non-muscle somatic cell nuclei after transplantation to Xenopus oocytes. The overexpression of Id, a dominant-negative repressor of MyoD, prevents maternal MyoD from binding to its consensus sequences; nevertheless, muscle genes are activated in somatic nuclei to the same extent as without Id. We conclude that M1 oocytes can reprogram somatic nuclei in a different way to other experimental procedures: oocytes do not suppress the transcription of inappropriate genes and they activate a gene without the help of its known transcription factors. We suggest that these characteristics might be a special property of amphibian oocytes, and possibly of oocytes in general.

Protocadherin-18a has a role in cell adhesion, behavior and migration in zebrafish development

Protocadherin-18a (Pcdh18a) belongs to the delta 2-protocadherins, which constitute the largest subgroup within the cadherin superfamily. Here we present isolation of a full-length zebrafish cDNA that encodes a protein highly similar to human and mouse Pcdh18. Zebrafish pcdh18a is expressed in a complex and dynamic pattern in the nervous system from gastrula stages onward, with lesser expression in mesodermal derivatives. Pcdh18a-eGFP fusion protein is expressed in a punctate manner on the membranes between cells. Overexpression of pcdh18a in embryos caused cyclopia, mislocalization of hatching gland tissue, and duplication or splitting of the neural tube. Most neural markers tested were expressed in an approximately correct A-P pattern. By cell transplantation we showed that overexpression of pcdh18a causes diminished cell migration and reduced cell protrusions, resulting in a tendency of cells to stay more firmly aggregated, probably due to increased cell adhesion. In contrast, knockdown of pcdh18a by a morpholino oligonucleotide caused defects in epiboly, and led to reduced cell adhesion as shown by cell dissociation, sorting and transplantation experiments. These results suggest a role for Pcdh18a in cell adhesion, migration and behavior but not cell specification during gastrula and segmentation stages of development.

From Nuclear Transfer to Nuclear Reprogramming: The Reversal of Cell Differentiation

This is a personal historical account of events leading from the earliest success in vertebrate nuclear transfer to the current hope that nuclear reprogramming may facilitate cell replacement therapy. Early morphological evidence in Amphibia for the toti- or multipotentiality of some nuclei from differentiated cells first established the principle of the conservation of the genome during cell differentiation. Molecular markers show that many somatic cell nuclei are reprogrammed to an embryonic pattern of gene expression soon after nuclear transplantation to eggs. The germinal vesicles of oocytes in first meiotic prophase have a direct reprogramming activity on mammalian as well as amphibian nuclei and offer a route to identify nuclear reprogramming molecules. Amphibian eggs and oocytes have a truly remarkable ability to transcribe genes as DNA or nuclei, to translate mRNA, and to modify or localize proteins injected into them. The development of nuclear transplant embryos depends on the ability of cells to interpret small concentration changes of signal factors in the community effect and in morphogen gradients. Many difficulties in a career can be overcome by analyzing in increasing depth the same fundamentally interesting and important problem.

Oncogenic Met receptor induces ectopic structures in Xenopus embryos

When aberrantly expressed or activated, the Met receptor tyrosine kinase is involved in tumor invasiveness and metastasis. In this study, we have used the Xenopus embryonic system to define the role of various Met proximal-binding partners and downstream signaling pathways in regulating an induced morphogenetic event. We show that expression of an oncogenic derivative of the Met receptor (Tpr-Met) induces ectopic morphogenetic structures during Xenopus embryogenesis. Using variant forms of Tpr-Met that are engineered to recruit a specific signaling molecule of choice, we demonstrate that the sole recruitment of either the Grb2 or the Shc adaptor protein is sufficient to induce ectopic structures and anterior reduction, while the recruitment of PI-3Kinase (PI-3K) is necessary but not sufficient for this effect. In contrast, the recruitment of PLCgamma can initiate the induction, but fails to maintain or elongate supernumerary structures. Finally, evidence indicates that the Ras/Raf/MAPK pathway is necessary, but not sufficient to induce these structures. This study also emphasizes the importance of examining signaling molecules in the regulatory context that is provided by receptor/effector interactions when assessing a role in cell growth and differentiation.

Changes in the structure of nuclei after transfer to oocytes

Nuclear transfer and the potential for cloning animals have refocused attention on the oocyte. This focus is not limited to the use of the oocyte as a recipient in nuclear transfer procedures, but more broadly in terms of what factors are present in the oocyte that are responsible for establishing the developmental pattern of RNA synthesis and subsequent protein production. Deviations in the pattern of RNA synthesis can result in abortions, as well as abnormalities at birth. This paper will focus on the changes to nuclear structure that result from transfer to the cytoplasm of an oocyte, as well as some of the changes in the patterns of RNA synthesis that have been described.

Reprogramming of fibroblast nuclei after transfer into bovine oocytes

Recent landmark achievements in animal cloning have demonstrated that the events of cell differentiation can, in principle, be reversed. This reversal necessarily requires large-scale genetic reprogramming, of which little is known. In the present study we characterized the extent to which blastocyst stage-specific mRNA expression would be conserved in bovine embryos produced by nuclear transfer (NT) using fetal fibroblasts as nuclei donors (FF NT). The mRNA pool of FF NT embryos was compared with that of NT embryos reconstructed from embryonic blastomeres (Emb NT), with embryos produced under in vivo or in vitro conditions, and finally with fibroblast cells. Embryo/cell-specific mRNA pools were contrasted using differential display methodology. Random oligonucleotide primer pair combinations were used to subfractionate mRNA populations and represent individual mRNAs as copy DNA (cDNA) bands ranging in size from 100 to 800 base pairs. Regardless of whether bovine blastocysts developed in vivo or in vitro, or were derived after nuclear transplantation with embryonic blastomeres or fetal fibroblasts, their mRNA profile was highly conserved and distinct from that of fetal fibroblast cells. There was approximately 95% conservation in cDNA banding patterns between FF NT, Emb NT, and in vivo derived blastocysts, when compared with in vitro derived blastocysts. In contrast, the cDNA banding in fibroblasts was only 67% conserved with in vitro derived blastocysts (p < 0.0001), indicating that dramatic changes in gene transcription are induced by nuclear transplantation. After nuclear transplantation, gene expression in fetal fibroblasts is reprogrammed so to mimic that of preimplantation embryo development. Future characterization of these changes will be invaluable for the identification of suitable cell types to serve as nuclear donors for embryo reconstruction and provide information that can be used to improve the efficiency of cloning animals by nuclear transplantation.

RLIP mediates downstream signalling from RalB to the actin cytoskeleton during Xenopus early development

The Ras protein activates at least three different pathways during early development. Two of them regulate mesodermal gene expression and the third is thought to participate in the control of actin cytoskeleton dynamics via the Ral protein. From a yeast two-hybrid screen of a Xenopus maternal cDNA library, we identified the Xenopus orthologue of the Ral interacting protein (RLIP, RIP1 or RalBP1), a putative effector of small G protein Ral. Previously, we observed that a constitutively activated form of Ral GTPase (XralB G23V) induced bleaching of the animal hemisphere and disruption of the cortical actin cytoskeleton. To demonstrate that RLIP is the effector of RalB in early development, we show that the artificial targeting of RLIP to the membrane induces a similar phenotype to that of activated RalB. We show that overexpression of the Ral binding domain (RalBD) of XRLIP, which binds to the effector site of Ral, acts in competition with the endogenous effector of Ral and protects against the destructive effect of XralB G23V on the actin cytoskeleton. In contrast, the XRLIP has a synergistic effect on the activated form of XralB, which is dependent on the RalBD of RLIP. We provide evidence for the involvement of RLIP by way of its RalBD on the dynamics of the actin cytoskeleton and propose that signalling from Ral to RLIP is required for gastrulation.

Control of embryonicXenopusmorphogenesis by a Ral-GDS/Xral branch of the Ras signalling pathway

Ras proteins mediate biological responses through various effectors and play a key role in relaying the Fibroblast Growth Factor (FGF) mesoderm induction signal during embryogenesis of the frog, Xenopus laevis. One Ras effector pathway involves the activation of the small G protein Ral. In the present study, we have investigated the role of key components in the Ral branch of FGF and Ras signalling during early Xenopus development. Treatment of animal caps with bFGF, which converts prospective ectoderm to mesoderm, activates Xral. The Ras mutant 12V37G, which can bind to Ral-GDS but not Raf, also activates Xral as well as causing developmental defects and cortical F-actin disassembly. A similar phenotype is induced by Ral-GDS itself. FGF-induced expression of several signature mesodermal genes, by contrast, is independent of Xral signalling. This and other data suggest that the RalB branch of Ras and FGF signalling regulates the actin cytoskeleton and morphogenesis in a transcriptionally independent manner. We also find Xral to be specifically activated in the marginal zone of Xenopus embryos, and find that disruption of the Ral pathway in this region prevents closure of the blastopore during gastrulation. We conclude that Ral signalling is autonomously required by mesodermal cells to effect essential morphogenetic changes during Xenopus gastrulation.

Amphibian and mammal somatic-cell cloning: different species, common results?

Since the production of Dolly the sheep cloning methods for somatic cells have been thoroughly described and are becoming routine. However, the rate at which live clones are produced remains low in all mammalian species tested so far. Remarkably, irrespective of the cloning protocol or the donor-cell type, all clones display common abnormalities, particularly in the placenta. The process is also complicated by early mortality of somatic-cell clones and the founder mammalian clone, Dolly the sheep, died in February 2003 aged six years. Based on published data and on our own experience, our view is that mammalian somatic-cell cloning and the pioneer nuclear-transfer data from amphibians have much in common. We suggest that the only way to improve nuclear reprogramming is to modify the chromatin structure of somatic cells before nuclear transfer, to provide the oocyte with a chromosomal structure that is more compatible with the natural reprogramming machinery of the oocyte.

Nuclear reprogramming and stem cell creation

The transplantation of a somatic cell nucleus to an enucleated egg results in a major reprogramming of gene expression and switch in cell fate. We review the efficiency of nuclear reprogramming by nuclear transfer. The serial transplantation of nuclei from defective first-transfer embryos and the grafting of cells from such embryos to normal host embryos greatly increases the proportion of nuclei that can be seen to have been reprogrammed. We discuss possible reasons for the early failure of most nuclear transfers from differentiated cells and describe the potential value of growing oocytes, rather than unfertilized eggs, as a source of nuclear reprogramming molecules and for the eventual identification of these molecules. Nuclear transfer provides a possible route for the creation of stem cells from adult somatic cells.

From intestine to muscle: nuclear reprogramming through defective cloned embryos

Nuclear transplantation is one of the very few ways by which the genetic content and capacity for nuclear reprogramming can be assessed in individual cells of differentiated somatic tissues. No more than 6% of the cells of differentiated tissues have thus far been shown to have nuclei that can be reprogrammed to elicit the formation of unrelated cell types. In Amphibia, about 25% of such nuclear transfers form morphologically abnormal partial blastulae that die within 24 h. We have investigated the genetic content and capacity for reprogramming of those nuclei that generate partial blastulae, using as donors the intestinal epithelium cells of feeding Xenopus larvae. We have analyzed single nuclear transplant embryos obtained directly from intestinal tissue, thereby avoiding any genetic or epigenetic changes that might accumulate during cell culture. The expression of the intestine-specific gene intestinal fatty acid binding protein is extinguished by at least 10(4) times, within a few hours of nuclear transplantation. At the same time several genes that are normally expressed only in early embryos are very strongly activated in nuclear transplant embryos, but to an unregulated extent. Remarkably, cells from intestine-derived partial blastulae, when grafted to normal host embryos, contribute to several host tissues and participate in the normal 100-fold increase in axial muscle over several months. Thus, cells of defective cloned embryos unable to survive for more than 1 day can be reprogrammed to participate in new directions of differentiation and to maintain indefinite growth, despite the abnormal expression of early genes.

Characterization of Xenopus RalB and its involvement in F-actin control during early development

We describe the characterization and a functional analysis in Xenopus development of RalB, a small G protein. RalB RNA and protein are detectable during oogenesis and early development, but the gene is expressed only weakly in adult tissues. The RalB transcripts are processed by poly(A) extension during oocyte maturation and up to the gastrulation stage. Microinjection of wild-type or mutant RalB RNAs was performed in fertilized eggs in order to gain insight into the function of RalB during development. We show that during cleavage stages the activated GTP form of RalB specifically induces a cortical reaction that affects the localization of pigment granules. The use of different drugs suggests that this reaction is dependent on the outer cortical actin array. The relation between F-actin and RalB was shown by confocal analysis. Injection of mRNAs encoding the mutated activated form of RalB leads, at dependent doses, to a blocking of gastrulation or defects in closing of neural folding structures. In contrast, the inactivated form blocks only the closing of neural tube. Altogether, these observations suggest that RalB is part of a regulatory pathway that may affect the blastomere cytoskeleton and take part in early development.

Karyoplast-cytoplast volume ratio in bovine nuclear transfer embryos: effect on developmental potential

To evaluate the effect of karyoplast-cytoplast ratio on the development of nuclear transfer embryos, karyoplasts from day 4, day 5, and day 6 embryos were transferred to oocytes enucleated with different volumes of cytoplasm: Type 1, removal of a small volume of cytoplasm equivalent to the first polar body, Type 2, removal of a volume of cytoplasm approximately equal to the volume of the respective karyoplast, and Type 3, removal of half of the oocyte volume. In addition, the effect of experimental reduction of karyoplast cytoplasm was investigated in day 4 and day 5 karyoplasts. Intact day 4 karyoplasts fused to Type 3 cytoplasts did not support development to blastocysts, whereas these karyoplasts yielded blastocysts in combination with Type 1 (7%) and Type 2 cytoplasts (12%). After experimental reduction of cytoplasmic volume in day 4 karyoplasts, blastocysts (10%) were also obtained after fusion with Type 3 cytoplasts, probably due to reduction of cytoplasmic chimerism. With day 5 karyoplasts, blastocyst rate was higher in combination with Type 2 (34%) than with Type 1 (19%) and Type 3 cytoplasts (16%; P < 0.05). The use of day 6 intact karyoplasts resulted in a significantly (P < 0.05) higher proportion of blastocysts when fused with Type 2 (38%) or Type 1 cytoplasts (34%) than with Type 3 cytoplasts (16%). These results suggest that enucleation of oocytes with a volume similar to that of the respective karyoplast creates better conditions for cell cycle interactions with all types of karyoplasts than enucleation with minimal or large volume of cytoplasm.

Cell lineage and commitment in early amphibian development

It is clear by the neurula stage of amphibian development that the three primary cell types or germ layers can be recognized. However, little is known about when and how individual cells of the blastula and gastrula become committed to these germ layers. The laser dye tetramethyl rhodamine isothiocyanate is proving a useful cell marker for studies of cell commitment. When labelled cells from the animal or vegetal pole ofXenopus laevisearly embryos are transplanted into host blastocoels, their progeny become specified to different germ layers and differentiate in accordance with surrounding cells. By using this method we have shown that animal and vegetal pole cells are pluripotent at the early blastula stage, while vegetal pole cells are committed to form endoderm by the early gastrula stage of development. One population of cells, the primordial germ cells, which arise from the vegetal part of the early embryo, are shown still to be pluripotent at the swimming tadpole stage.

The Xenopus T-box gene, Antipodean, encodes a vegetally localised maternal mRNA and can trigger mesoderm formation

We have used differential display to identify genes inducible by activin and isolated a novel member of the T-box gene family that includes the Xenopus genes Xbrachyury and Eomesodermin. Here we show that this novel gene is unique within the T-box family because it is maternally expressed at a high level. Furthermore, it belongs to a rare class of maternal mRNAs in Xenopus that are localised to the vegetal hemisphere of the egg and we have therefore named it Antipodean. We show here that low amounts of Antipodean injected into ectoderm (animal cap cells) strongly induce pan mesodermal genes such as Xbrachyury and ventral mesodermal genes such as Xwnt-8. Overexpression of Antipodean generates mesoderm of ventral character, and induces muscle only weakly. This property is consistent with the observed late zygotic Antipodean mRNA expression in the posterior paraxial mesoderm and ventral blastopore, and its exclusion from the most dorsal mesodermal structure, the notochord. Antipodean is induced by several molecules of the TGF-beta class, but in contrast to Xbrachyury, not by bFGF. This result suggests that the expression of these T-box genes may be under the control of different regulatory pathways. Finally, we demonstrate that Antipodean and Eomesodermin induce each other and both are able to induce Xbrachyury. The early zygotic expression of Antipodean is not induced by Xbrachyury, though later it is to some extent. Considering its maternal content, Antipodean could initiate a cascade of T-box gene activations. The expression of these genes may, in turn, sustain each other's expression to define and maintain the mesoderm identity in Xenopus.

Quantitative control of gene expression by nucleocytoplasmic interactions in early mouse embryos: consequence for reprogrammation by nuclear transfer

HSP 70.1 is one of the first genes to be expressed in the mouse embryo at the time of zygotic genome activation. We studied the regulation of this gene, using a transgene associating HSP 70.1 promoter and the firefly luciferase reporter gene, which allows the precise quantification of HSP 70.1 level of expression on individual embryos. In the present work, we show first that the level of HSP 70.1 expression at the two-cell stage is significantly higher (around two-fold) in embryos whose maternal cytoplasm is from C3H strain than with BALB/c strain. We verified that this difference is not an artefact of the use of transgenic embryos, of the time of first cleavage, or of in vitro culture. This regulation of HSP 70.1 level of expression is controlled by strain-specific maternal modifiers and is independent of replication, syngamy, and mitosis. Following nuclear transfer, reactivation of HSP 70.1 is also subjected to the same epigenetic influence. Only the strain-of-origin of the recipient cytoplast modulates the level of HSP 70.1 reprogrammation; the origin of donor nucleus is not significant, demonstrating the reversibility of this strain effect. These results point out the importance of the quality of recipient cytoplast in the intensity of gene reprogrammation, which may be of importance for nuclear transfer efficiency.

Involvement of differential gene expression and mRNA stability in the developmental regulation of the hsp 30 gene family in heat-shocked Xenopus laevis embryos

Four complete hsp 30 genes have been isolated from Xenopus laevis: hsp 30A, hsp 30B (a pseudogene), hsp 30C, and hsp 30D. The hsp 30A and hsp 30C genes are first heat inducible at the early tailbud stage, as determined by RNase protection and RT-PCR assays. In this study, we determined by RT-PCR that the hsp 30D gene was first heat inducible (33 degrees C for 1 h) at the mid-tailbud stage, approximately 1 day later in development than hsp 30A and hsp 30C. Furthermore, using Northern blot analysis, we detected the presence of very low levels of hsp 30 mRNA at the heat-shocked late blastula stage. The relative levels of these pre-tailbud (PTB) hsp 30 mRNAs increased at the gastrula and neurula stage followed by a dramatic enhancement in heat shocked tailbud and tadpole stage embryos (50- to 100- fold relative to late blastula). Interestingly, treatment of blastula or gastrula embryos at high temperatures (37 degrees C for 1 h) or with the protein synthesis inhibitor, cycloheximide, followed by heat shock, led to enhanced accumulation of the pre-tailbud (PTB) hsp 30 mRNAs. hsp 70, hsp 87, and actin messages were not stabilized at high temperatures or by cycloheximide treatment. Finally, hsp 30D mRNA was not detected by RT-PCR analysis of cycloheximide-treated, heat-shocked blastula stage embryos, confirming that it is not a member of the PTB hsp 30 mRNAs. This study indicates that differential gene expression and mRNA stability are involved in the regulation of hsp 30 gene expression during early Xenopus laevis development.

Cell cycle remodeling requires cell-cell interactions in developing Xenopus embryos

Flow cytometric analysis has revealed a general remodeling of the cell cycle in developing Xenopus embryos. During early gastrulation the cell cycle is dominated by S phase, with 82% of all interphase nuclei in the S phase fraction. As development proceeds over the next 60 hours, a gradual decline in S phase cells is proportional to an increase in the number of cells in G1. By the late tailbud stage, 85% of all nuclei are found in the G1 fraction, approximating the cell cycle profile of adult somatic cells. Cell cycle remodeling occurs on schedule even in embryos that have been dissociated into a loose mound of cells which remain in close proximity to one another. However, cells that have been widely separated by manual dispersion do not undergo remodeling. These dispersed cells maintain an S phase-dominated cell cycle and continue to show patterns of blastula and gastrula gene expression at least 30 hours beyond gastrulation. We conclude that cell cycle remodeling occurs in the absence of an intact embryo but requires the inductive influences associated with a community of cells.

Slow emergence of a multithreshold response to activin requires cell-contact-dependent sharpening but not prepattern

The growth factor activin elicits mesodermal fates when applied to prospective ectodermal cells of the Xenopus blastula stage embryo. Previous experiments with dissociated cells showed that there are at least five different responses separated by closely spaced, sharp dose thresholds. Here we investigate this multithreshold activin response further using probes for genes expressed at early gastrula stages, namely Xbra, goosecoid, noggin, Xwnt-8 and Mix.1. We show that initial dose-response profiles are broad and smooth in contrast to the later threshold-bound patterns. For Xbra, goosecoid and noggin, the later expression ranges are subsets of earlier ones. Unexpectedly, Xwnt-8 is initially induced at high doses only, but later appears only in cells that have received a low dose of activin. Keeping the cells dissociated after activin treatment, rather than allowing them to reaggregate, prevents sustained expression of Xbra and Xwnt-8 but allows that of goosecoid and noggin. However, cell contact is required for sharpening the dose-response threshold of goosecoid. Finally, we show that a previously reported dorsoventral prepattern in the animal cap is also cell-contact dependent and it is not required for the multi-threshold response to activin.

A role for cytoplasmic determinants in mesoderm patterning: cell-autonomous activation of the goosecoid and Xwnt-8 genes along the dorsoventral axis of early Xenopus embryos

Although an induction event is required for the formation of mesoderm in Xenopus embryos, it is not clear that this induction is wholly sufficient to give rise to a correctly patterned mesodermal layer. We have studied the expression of the two genes, goosecoid and Xwnt-8, in Xenopus gastrulae in which cell-cell communication, and therefore mesoderm induction, has been prevented by frequent cell dispersion. Although neither the early panmesodermal marker Xbra nor the muscle-specific alpha-actin gene were activated under these conditions, goosecoid and Xwnt-8 were activated in cells of dorsal and ventrolateral origin respectively, thus correctly reflecting their distribution during normal development. We also show that the spatial pattern of expression of these two genes along the animal-vegetal axis is similar in normal and in dissociated early gastrulae: goosecoid is mainly expressed in future mesoderm while the domain of expression of Xwnt-8 spans the mesoderm-endoderm boundary. These results show that, during the blastula and early gastrula stages, gene activation can be controlled cell-autonomously along both the animal-vegetal and dorsoventral embryo axes. This suggests that the inheritance of localised maternal cytoplasmic determinants is a key event for the patterning of mesoderm. We present a modified model of mesoderm formation in which the different mesoderm cell types are produced as a result of cooperation between induction-dependent and induction-independent immediate-early genes.

Stage-specific appearance of the mouse antigen TEC-3 in normal and nuclear transfer bovine embryos: re-expression after nuclear transfer

Bovine embryos, recovered from the uterus in vivo or derived from in vitro matured and in vitro fertilized oocytes, were investigated for the presence of the developmentally regulated mouse antigen TEC-3 by indirect immunofluorescence. During preimplantation embryo development TEC-3 is expressed on bovine morulae and blastocysts. It is absent from unfertilized and fertilized oocytes, and from all stages before the 32-cell stage. The finding that TEC-3 is not expressed before the onset of embryonic transcription, which occurs at the eight-cell stage in the bovine, but only when the embryonic genome is active, makes it a potential marker for studying nuclear reprogramming after nuclear transfer. Nuclear transfer embryos were made by electrical fusion of blastomeres from morulae derived in vivo with enucleated metaphase II oocytes. Indirect immunofluorescence with the TEC-03 antibody showed that the TEC-3 antigen, present on blastomeres of the morula stage embryo, disappeared after fusion and was expressed again when the nuclear transfer embryos developed to the morula and blastocyst stage. These data suggest that the bovine embryonic nucleus may be able to revert to the equivalent of an earlier developmental stage when transferred to ooplasm, and is then capable of following the normal developmental program.

The community effect, dorsalization and mesoderm induction

The community effect is an interaction among muscle progenitor cells of amphibian gastrula, and is necessary for the initiation of muscle-specific gene expression. Dorsalization provides a signal that can convert ventral mesoderm cells to a muscle fate. Neither process involves mesoderm-inducing molecules. We suggest that the developmental significance of the community effect is to generate homogeneous but clearly demarcated groups of cells from progenitor cells arranged in a continuous gradient of developmental potential.

Multiple cell interactions are required for fate specification during male spicule development in Caenorhabditis elegans

The B blast cell divides postembryonically in C. elegans males to produce 47 progeny that include all of the cells of the copulatory spicules. During the early development of the B lineage, the anterior daughter of B, B.a, generates eight cells. These cells migrate to form four pairs of cells that flank the developing cloaca (ventral, dorsal, and two identical lateral pairs). For each pair, the more anterior cell produces a distinct lineage ('anterior fate') from the posterior cell ('posterior fate'). For the ventral and dorsal pairs, either cell can migrate to the anterior position and produce the anterior lineage, and the other cell migrates posterior and produces the posterior lineage (Sulston and Horvitz, 1977, Dev. Biol. 56, 110–156). The migration is variable, although the resultant fate pattern is invariant. In the two lateral pairs, both the migration and fate pattern are invariant. Using a laser microbeam to selectively ablate neighboring cells we have found that the cells of the lateral pair also respond to positional cues. For all four pairs other male-specific blast cells provide extracellular cues. In general, F and U promote anterior fates, Y promotes some posterior fates, and the B.a progeny promote posterior fates. Several of these cues are redundant. By ablating combinations of cells we have deduced how these signals may act in concert to specify the fates of the B.a progeny. We propose that fate specification in these pairs depends on three general classes of extracellular cues: positional cues, modulators of positional cues, and lateral signals. The B lineage thus provides an opportunity to study with single cell resolution the integration of multiple intercellular signals.

Ectoderm promotes differentiation of the presumptive ventral mesoderm in early gastrulae of Bufo arenarum

Cultures of both isolated and conjugated explants from early gastrulae of Bufo arenarum were prepared for a study of the development of ventral mesoderm. Only combinations including components of the deep ventral marginal zone and the animal pole successfully differentiated into blood cells (erythrocytes). Histological studies indicated that, while prospective mesodermal cells constituted the only source of such cells, prospective ectodermal cells provided the necessary stimulus for this kind of differentiation. Differentiated cultures, in which the tracer of cell-lineage fluorescein dextran amine was used to label these components, confirmed the above conclusions. These findings are discussed in the context of current concepts about the formation of mesoderm.

The Xenopus platelet-derived growth factor alpha receptor: cDNA cloning and demonstration that mesoderm induction establishes the lineage-specific pattern of ligand and receptor gene expression

We have cloned the Xenopus PDGF alpha receptor cDNA and have used this clone, along with cDNA encoding PDGF A, to examine their expression pattern in Xenopus embryos and to determine the factors responsible for lineage specificity. Recombinant Xenopus alpha receptor expressed in COS cells exhibits PDGF-A-dependent tyrosine kinase activity. We find that receptor mRNA is present in cultured marginal zone tissue explants and in animal cap tissue induced to form mesoderm either by grafting to vegetal tissue or by treatment with recombinant activin A. In contrast, PDGF A mRNA is expressed in cultured, untreated animal cap tissue and is suppressed by mesoderm induction. These results suggest that ectodermally produced PDGF A may act on the mesoderm during gastrulation and that mesoderm induction establishes the tissue pattern of ligand and receptor expression.

Responses of embryonic Xenopus cells to activin and FGF are separated by multiple dose thresholds and correspond to distinct axes of the mesoderm

The potent mesoderm-inducing factors activin and FGF are present as maternally synthesized proteins in embryos of X. laevis. We show that activin can act on explanted blastomeres to induce at least five different cell states ranging from posterolateral mesoderm to dorsoanterior organizer mesoderm. Each state is induced in a narrow dose range bounded by sharp thresholds. By contrast, FGF induces only posterolateral markers and does so over relatively broad dose ranges. FGF can modulate the actions of activin, potentiating them and broadening the threshold-bounded dose windows. Our results indicate that orthogonal gradients of activin and FGF would be sufficient to specify the main elements of the body plan.

Genomic potential in mammals

Embryos of amphibians, fish, sheep, cattle, swine and rabbits have been multiplied by nuclear transfer. Successful nuclear transfer in these species has been accomplished by transfer of a blastomere from a late stage embryo into an enucleated oocyte or egg with large scale multiplication achieved by serial repetition of the procedure using blastomeres from nuclear transfer embryos. This allows the production of clonal lines, which when appropriately selected for performance in a given trait, can be reproduced to capture in the offspring expression of both additive and nonadditive inheritance. The efficiency of producing offspring from nuclear transfer is low in mammals in both frequency of morula or blastocyst produced and maintenance of pregnancy after embryo transfer. In domestic animals the largest number of offspring from one embryo has been eight calves. Embryos as late as the 64-cell stage in cattle and 120-cell blastocyst in sheep have been used successfully as donors of blastomeres. Recloning has also been done in cattle. Potentially, nuclear transfer provides a mechanism for multiplication and production testing of clonal lines, a method for rapid genetic improvement and a means for rapid propagation of a selected genotype.

Degradation of a developmentally regulated mRNA in Xenopus embryos is controlled by the 3' region and requires the translation of another maternal mRNA

By injecting the appropriately constructed plasmids into one-cell Xenopus embryos, we determined that the 3' region of the maternal Xenopus Eg2 mRNA confers instability on the chimeric mRNA transcribed from these plasmids. This instability, like that of the maternal Eg2 transcript, was abolished by treatment of the embryos with cycloheximide. Analysis of the polysome distribution of the maternal Eg2 mRNA in cycloheximide-treated and untreated embryos showed that Eg2 mRNA was released from polysomes after fertilization and that the stabilization caused by cycloheximide treatment was not due to a reloading of ribosomes onto the mRNA. Insertion of a stable hairpin loop (delta G = -50 kcal/mol) 5' to the reporter gene in the injected plasmid caused a 10- to 20-fold decrease in translation from the transcribed mRNAs. This decrease in translation did not abolish the instability conferred by the 3' Eg2 region. Therefore, the degradation of these chimeric mRNAs in Xenopus embryos requires the translation of another maternal mRNA coding for a trans-acting factor involved in mRNA degradation. Further restriction of the 3' Eg2 region, placed 3' to the reporter gene, showed that a cis-acting instability-conferring sequence is contained in a 497-nucleotide fragment.

The cell cycle dependence of the secretory pathway in developing Xenopus laevis

The cell cycle during the cleavage period of the amphibian Xenopus laevis is about 30 min long and oscillates between equal periods of mitosis and interphase. At the midblastula transition (MBT) the length of interphase begins to elongate and brings about corresponding changes in the activities of cell cycle-dependent processes. In this study protein secretion and Golgi processing during embryonic Xenopus development were examined. The elongation of interphase, either during normal development or experimentally induced, resulted in an increase in the secretion of both endogenous and exogenous proteins. Secretion was found to increase linearly with the increase in interphase length, indicating that the rate of secretion was constant and was regulated by the length of interphase. M-phase arrest in embryos and oocytes produced an inhibition of protein secretion that was reversible if the cell cycle was returned to interphase. This M-phase block of the secretory pathway was found to take place between the trans Golgi compartment and the plasma membrane. The developmental increase in the function of this pathway after the MBT may affect the expression of surface and secreted proteins important for the cell-cell interactions necessary for subsequent development through gastrulation.

Degradation of a developmentally regulated mRNA in Xenopus embryos is controlled by the 3' region and requires the translation of another maternal mRNA

By injecting the appropriately constructed plasmids into one-cell Xenopus embryos, we determined that the 3' region of the maternal Xenopus Eg2 mRNA confers instability on the chimeric mRNA transcribed from these plasmids. This instability, like that of the maternal Eg2 transcript, was abolished by treatment of the embryos with cycloheximide. Analysis of the polysome distribution of the maternal Eg2 mRNA in cycloheximide-treated and untreated embryos showed that Eg2 mRNA was released from polysomes after fertilization and that the stabilization caused by cycloheximide treatment was not due to a reloading of ribosomes onto the mRNA. Insertion of a stable hairpin loop (delta G = -50 kcal/mol) 5' to the reporter gene in the injected plasmid caused a 10- to 20-fold decrease in translation from the transcribed mRNAs. This decrease in translation did not abolish the instability conferred by the 3' Eg2 region. Therefore, the degradation of these chimeric mRNAs in Xenopus embryos requires the translation of another maternal mRNA coding for a trans-acting factor involved in mRNA degradation. Further restriction of the 3' Eg2 region, placed 3' to the reporter gene, showed that a cis-acting instability-conferring sequence is contained in a 497-nucleotide fragment.

Morphogenetic and molecular correlates of teratogenesis in the amphibian embryo

In attempting to develop a system to study molecular mechanisms of teratogenesis, examination of the effects of a teratogen (dimethyl sulfoxide) on both molecular and morphological aspects of embryonic development in the amphibian Xenopus laevis has been conducted. Characteristic morphological effects, which occur during the period from 7 to 16 hours after fertilization (i.e., gastrulation) are noted. Delays in gastrulation are accompanied by changes in the regulation of transcription of several genes known to be active during gastrulation in normal development. Later morphological effects are also observed, and these probably arise as a consequence of the changes occurring during gastrulation. Thus, molecular responses to a teratogen have been detected, and a correlation between molecular and morphological responses to a teratogen is observed. These findings represent the first demonstration of the effects of a teratogen on the transcription of specific genes, and invite speculation that one or more molecular events mediate teratogenesis. They further suggest that the amphibian system may be useful for studying early molecular responses to teratogens.

Expression of a mRNA related to c-rel and dorsal in early Xenopus laevis embryos

We describe a Xenopus mRNA, Xrel1, that is related to the avian protooncogene c-rel, the embryonic pattern gene dorsal of Drosophila, and the mammalian transcription factor NK-kappa B/KBF1. The sequence of Xrel1 is homologous to the other rel-related proteins in the large amino-terminal region that defines this class of transcriptional regulators, but the carboxyl-terminal part of the protein is quite different. Xrel1 mRNA is present throughout oogenesis and during early embryogenesis at 4 x 10(5) transcripts per oocyte or embryo. Xrel1 transcripts are present in all of the dissected parts of early embryos that we have examined. They are enriched in the animal hemisphere compared to the vegetal hemisphere of oocytes and blastulae.

Endocrinization of the early embryo: an emerging role for hormones and hormone-like factors

In the last decade, we witnessed the extension of endocrinologically based concepts and molecules to many other arenas of intercellular communication, e.g. immunology, hematology and cancer biology. At the start of the new decade we are witnessing the beginning of a similar transformation in our understanding of early embryogenesis, i.e. that hormones, growth factors and other hormone-like agents and their receptors, familiar to us in other contexts, may be the long-sought mediators of many key events in early embryogenesis. Why these agents were overlooked before and how they have started to emerge is the theme of this essay. The title 'Endocrinization of the Early Embryo' refers to both the biological and intellectual developments.

Nuclear transplantation in the pig embryo: nuclear swelling

The transfer of nuclei from cleavage stage embryos to enucleated activated meiotic metaphase II oocytes results in a reprogramming of the transferred nucleus such that it behaves as a zygotic nucleus. One estimator of nuclear reprogramming is nuclear swelling after nuclear transfer. The diameter of nuclei after nuclear transfer was not found to be dependent upon the amount of cytoplasm transferred with the donor cell or the amount of cytoplasm in the recipient cell. Nuclei from 4-, 8-, and 16-cell stage embryos swelled to a similar diameter after nuclear transfer (26.9, 27.3, and 27.2 microns, respectively) and this was significantly different from the diameter of contemporary donor embryos (18.3, 14.3, and 13.0 microns, respectively). This is a swelling of 47, 91, and 109%, respectively. Since the degree of nuclear swelling does not appear to be related to cytoplasmic volume it is concluded that the components mediating nuclear swelling are not in a limiting supply.

Modulation of neural commitment by changes in target cell contacts in Pleurodeles waltl

In amphibian development, neural structures arise from the presumptive ectoderm at the gastrula stage by an inductive interaction with the chordamesoderm. It has been previously reported that early gastrula presumptive ectoderm can be neuralized when it is dissociated into single cells. A similar result is reported here with regard to Pleurodeles waltl presumptive ectoderm. Using this experimental model system we demonstrate: first, that neuronal and glial lineages can be specified from the presumptive ectoderm without any intervention of the natural inducing tissue; and second, that whereas rupture of cell-cell contacts evoked neural induction, dissociation immediately followed by reaggregation reduces the neuralizing response, pointing toward an active role played by cell-cell contacts of presumptive ectodermal cells in the modulation of neural commitment.

Stability of maternal mRNA in Xenopus embryos: role of transcription and translation

The first 12 cell divisions of Xenopus laevis embryos do not require gene transcription. This means that the regulation of gene expression during this period is controlled at post transcriptional levels and makes Xenopus early development a potentially interesting biological system with which to study the mechanisms involved. We describe here the stability characteristics of several maternal Xenopus mRNAs which are deadenylated soon after fertilisation (J. Paris and M. Philippe, Dev. Biol., in press). We show that these mRNAs were only degraded in the embryo after the midblastula transition (MBT), when gene transcription was initiated. The kinetics with which the deadenylated maternal mRNAs decreased in the post-MBT embryos showed sequence specificity. The degradation of these mRNAs after the MBT was inhibited by cycloheximide but was not affected by dactinomycin. Therefore, the destabilization of these mRNAs does not appear to be initiated by new embryonic gene transcripts. Sequence comparisons of the 3' untranslated region of these mRNAs identified several motifs which may be involved in the posttranscriptional control of these gene products.

Stability of maternal mRNA in Xenopus embryos: role of transcription and translation

The first 12 cell divisions of Xenopus laevis embryos do not require gene transcription. This means that the regulation of gene expression during this period is controlled at post transcriptional levels and makes Xenopus early development a potentially interesting biological system with which to study the mechanisms involved. We describe here the stability characteristics of several maternal Xenopus mRNAs which are deadenylated soon after fertilisation (J. Paris and M. Philippe, Dev. Biol., in press). We show that these mRNAs were only degraded in the embryo after the midblastula transition (MBT), when gene transcription was initiated. The kinetics with which the deadenylated maternal mRNAs decreased in the post-MBT embryos showed sequence specificity. The degradation of these mRNAs after the MBT was inhibited by cycloheximide but was not affected by dactinomycin. Therefore, the destabilization of these mRNAs does not appear to be initiated by new embryonic gene transcripts. Sequence comparisons of the 3' untranslated region of these mRNAs identified several motifs which may be involved in the posttranscriptional control of these gene products.

Transcription of endogenous and injected cytoskeletal actin genes during early embryonic development in Xenopus laevis

The transcriptional regulation of a cytoskeletal actin gene during Xenopus laevis embryonic development has been investigated. New transcripts of this gene begin to accumulate at approximately the mid-gastrula stage, between 12 and 16 hours after fertilization, replenishing maternal supplies of this transcript. To study the molecular processes which act to determine the timing of transcriptional activation of this gene, a gene-injection assay was devised, utilizing a cloned copy of the gene which has been marked by a small DNA insertion. Accurately-initiated transcripts of the injected gene accumulate in concert with those of the endogenous gene, showing that injected genes can undergo developmental regulation. As little as 485 nucleotides of upstream sequence is sufficient for proper temporal control of activation of an injected gene. The results presented here demonstrate the feasibility of a microinjection assay for the identification of regulatory gene sequences and transacting regulatory factors in amphibian embryos. Such an assay will be useful in achieving an understanding of general transcriptional control mechanisms acting in early development, and should also provide a means to study certain aspects of long-standing developmental problems, such as cytoplasmic localization and embryonic induction.