Obtaining Xenopus Eggs and Embryos

Collecting eggs from adult Xenopus laevis and Xenopus tropicalis to raise healthy embryos and tadpoles is relatively simple but requires careful handling of the frog. Eggs can be fertilized through natural matings or by in vitro fertilization and examined visually. Here we review how eggs are obtained and how to recognize healthy eggs that will develop into high-quality embryos.

Xenopus Husbandry

Adult frogs that are well-cared-for will give high-quality eggs and embryos for use in every Xenopus protocol. Thoughtful frog husbandry is thus pivotal to successful research using these organisms. Protocols for successfully raising tadpoles, establishing and maintaining water quality, and detecting specific pathogens are key to maintaining healthy frog populations.

Brain Ventricular System and Cerebrospinal Fluid Development and Function: Light at the End of the Tube: A Primer with Latest Insights

The brain ventricular system is a series of connected cavities, filled with cerebrospinal fluid (CSF), that forms within the vertebrate central nervous system (CNS). The hollow neural tube is a hallmark of the chordate CNS, and a closed neural tube is essential for normal development. Development and function of the ventricular system is examined, emphasizing three interdigitating components that form a functional system: ventricle walls, CSF fluid properties, and activity of CSF constituent factors. The cellular lining of the ventricle both can produce and is responsive to CSF. Fluid properties and conserved CSF components contribute to normal CNS development. Anomalies of the CSF/ventricular system serve as diagnostics and may cause CNS disorders, further highlighting their importance. This review focuses on the evolution and development of the brain ventricular system, associated function, and connected pathologies. It is geared as an introduction for scholars with little background in the field.

Validation of Protein Knockout in Mutant Zebrafish Lines Using In Vitro Translation Assays

Advances in genome-editing technology have made creation of zebrafish mutant lines accessible to the community. Experimental validation of protein knockout is a critical step in verifying null mutants, but this can be a difficult task. Absence of protein can be confirmed by Western blotting; however, this approach requires target-specific antibodies that are generally not available for zebrafish proteins. We address this issue using in vitro translation assays, a fast and standard procedure that can be easily implemented.

Isolation of DNA from red blood cells in Xenopus

The amphibian Xenopus laevis is an important model organism that is particularly valuable for studies of early vertebrate development. Genomic DNA constitutes the total genetic information of an organism and it is used for Southern blotting, for determining gene structure, and for detecting the presence or absence of genes of interest. Genomic DNA can be extracted from Xenopus red blood cells, which are unlike the mammalian equivalent in that they contain nuclei. This article describes a protocol for the isolation of genomic DNA from frog red blood cells.

Calibration of the injection volume for microinjection of Xenopus oocytes and embryos

Microinjection of Xenopus oocytes or embryos with messenger RNA (mRNA) or DNA is a powerful technique for studying development. Before microinjection can be performed, the injection volume must be calibrated carefully. This protocol describes the calibration procedure for a pressure injector.

Microinjection of Xenopus oocytes

Xenopus oocytes can be injected with messenger RNA (mRNA) or DNA constructs into the cytoplasm or nucleus, respectively. The cytoplasm can withstand the introduction of up to 50 nL of injected material, and the nucleus can tolerate up to 20 nL. This protocol describes microinjection of both defolliculated and folliculated Xenopus oocytes. Oocytes remain in good condition for a relatively long time, and injections can be continued, if necessary, over a period of days.

Isolation of Xenopus oocytes

Xenopus oocytes are obtained from sexually mature females by surgically removing parts of the ovary. The operation is not fatal and can be performed on an anesthetized frog several times during its lifetime. However, a recovery period of 2 wk is recommended between operations. A careful record of all operations performed, including details of oocyte quality, should be kept. A frog that produces one good batch of oocytes; e.g., those that translate injected messenger RNAs (mRNAs) efficiently, should be recorded and used again, because oocyte quality is generally frog-dependent.

Defolliculation of Xenopus oocytes

It is possible to microinject Xenopus oocytes that are still contained within their ovarian follicles, but most researchers find it more convenient to work with defolliculated oocytes. Defolliculation can be carried out enzymatically by treatment with collagenase, or it can be performed manually. Enzymatic treatment is recommended for preparation of large numbers of oocytes (more than 1000); however, this treatment causes complications because it often damages the quality of the oocytes. The manual procedure, which requires some practice, is recommended for experiments requiring only a few hundred oocytes. This protocol details both the enzymatic and manual procedures for defolliculation of Xenopus oocytes.

Microinjection of Xenopus embryos

This article describes the factors that should be considered when microinjecting Xenopus oocytes. Forward planning is particularly important for embryo injection because the frogs lay eggs only for a limited time, and once the eggs are fertilized there is a very short period during which the embryos are suitable for injection.

Microinjection of RNA and preparation of secreted proteins from Xenopus oocytes

Microinjection of Xenopus oocytes or embryos with messenger RNA (mRNA) is a powerful technique for studying development, including protein expression during development. This protocol describes a method for the preparation of secreted (35)S-labeled proteins following mRNA microinjection into Xenopus oocytes.

The Wnt antagonists Frzb-1 and Crescent locally regulate basement membrane dissolution in the developing primary mouth

The primary mouth forms from ectoderm and endoderm at the extreme anterior of the embryo, a conserved mesoderm-free region. In Xenopus, a very early step in primary mouth formation is loss of the basement membrane between the ectoderm and endoderm. In an unbiased microarray screen, we defined genes encoding the sFRPs Frzb-1 and Crescent as transiently and locally expressed in the primary mouth anlage. Using antisense oligonucleotides and ;face transplants', we show that frzb-1 and crescent expression is specifically required in the primary mouth region at the time this organ begins to form. Several assays indicate that Frzb-1 and Crescent modulate primary mouth formation by suppressing Wnt signaling, which is likely to be mediated by beta-catenin. First, a similar phenotype (no primary mouth) is seen after loss of Frzb-1/Crescent function to that seen after temporally and spatially restricted overexpression of Wnt-8. Second, overexpression of either Frzb-1 or Dkk-1 results in an enlarged primary mouth anlage. Third, overexpression of Dkk-1 can restore a primary mouth to embryos in which Frzb-1/Crescent expression has been inhibited. We show that Frzb-1/Crescent function locally promotes basement membrane dissolution in the primary mouth primordium. Consistently, Frzb-1 overexpression decreases RNA levels of the essential basement membrane genes fibronectin and laminin, whereas Wnt-8 overexpression increases the levels of these RNAs. These data are the first to connect Wnt signaling and basement membrane integrity during primary mouth development, and suggest a general paradigm for the regulation of basement membrane remodeling.

Baskets for in situ hybridization and immunohistochemistry

INTRODUCTIONFor large in situ hybridization and immunohistochemistry experiments, changing solutions in vials becomes tedious, and baskets should be used. Commercially available baskets, such as 15-mm Netwell baskets (Corning), fit into 12-well tissue culture plates. Although they are expensive, they can, with extensive washing, be reused. However, these baskets are not readily adaptable to large-scale use, and their relatively shallow wells make cross-contamination between wells a real danger. This protocol describes a procedure for making homemade baskets with plastic microcentrifuge tubes and nylon mesh that are more adaptable to large-scale experiments. The baskets are narrow and deep and thus can be placed in a rack at high density. To change solutions, individual tubes or whole racks of tubes can be moved from one bath of solution to another. Because many tubes can be manipulated simultaneously, there is an enormous saving in work. It is also quite difficult to lose embryos in baskets; embryos in vials stand a good chance of being sucked into the aspirator during solution changes and lost.

Synthesis and purification of digoxigenin-labeled RNA probes for in situ hybridization

INTRODUCTIONIn situ hybridization is the most versatile method for determining when and where embryonic transcripts are expressed. Although a detailed analysis of gene expression often requires analysis of sectioned material, the whole-mount approach is invariably the first method used to localize gene expression. Whole-mount in situ hybridization has also become an invaluable tool for analyzing experimentally manipulated embryos and explants. The information gained using this technique is similar to that obtained from immunohistochemistry and includes not only the location of expression but a semiquantitative estimate of the relative levels of gene expression in different parts of the embryo. Because most genes are first analyzed as genomic or cDNA clones, it is straightforward to derive specific probes from these sequences. In this protocol, standard RNA synthesis using a bacteriophage polymerase is carried out incorporating a digoxigenin-substituted ribonucleotide, dig-UTP. In vitro transcription in the direction opposite to in vivo transcription of the mRNA makes an antisense RNA that is complementary to the mRNA. The ratio of dig-UTP to UTP has been optimized to give efficient synthesis and efficient detection of the probe. The three commercially available bacteriophage polymerases (SP6, T7, and T3) all incorporate the substituted nucleotide with equivalent efficiencies, and thus there is a wide choice of plasmid vectors to prepare the template.

Xenopus laevis Keller Explants

INTRODUCTIONThe basic Keller explant is a rectangle of dorsal mesendoderm and ectoderm from an early-gastrula-stage Xenopus laevis embryo. It is ~60° to 90° wide, extending from the bottle cells to the animal pole. This protocol describes how to dissect, assemble, and cultivate Keller explants. The purpose of Keller explants was initially to allow observation of gastrulation movements, particularly convergent extension, in culture. This is difficult to do when explants curl up, but in Keller sandwiches, the explants are cultured flat, either as a single sheet (open-face explant) or more frequently as two sheets sandwiched together with their inner surfaces apposed (closed sandwich). Explants are cultured beneath a coverslip fragment or a glass bridge resting on silicone vacuum grease until the desired stage, usually during or after neurulation. Instead of involuting beneath the ectoderm, mesoderm elongates in a plane with adjacent ectoderm. Explants are made at the onset of gastrulation before significant vertical juxtaposition of ectoderm and mesoderm has occurred.

Xenopus laevis Einstecks

INTRODUCTION"Einstecks" refers to a procedure for placing a piece of tissue into the blastocoel of an early gastrula, in order to assess the inductive potential of the introduced tissue. The foreign tissue adheres to surrounding tissue and becomes incorporated into the host embryo. This simple transplant procedure, which is described here, has been used to assess which types of axial tissue can be induced by different regions of the mesendoderm or ectoderm, and how forced expression of genes or treatment with various factors can alter this potential.

Ectodermal (Animal Cap) Layer Separations in Xenopus laevis

INTRODUCTIONIn Xenopus laevis, the blastula animal cap comprises two morphologically distinct cell layers, an outer monolayer termed the epithelial layer, which consists of tightly adherent pigmented cells, and an inner layer several cells thick termed the sensorial layer, which consists of loosely adherent cells. It is possible to isolate cell layers and to test their developmental potential and response to induction. This protocol describes how to separate the cell layers most easily, that is, by dissecting an intact Xenopus embryo.

Dissociation and Reaggregation of Xenopus laevis Animal Caps

INTRODUCTIONPreparations of single cells from Xenopus laevis animal caps are useful for assaying the activity of inducing molecules. These dissociated cells can be exposed to more uniform concentrations of inducing factors than the multilayered intact cap. Single cells derived from caps are also used to analyze the role of cell-to-cell contact. This protocol describes the preparation of cells from Xenopus for use in such assays. Animal caps require divalent cations for their integrity and thus can be dissociated by exposure to medium lacking Ca(++) and Mg(++) ions. With this treatment, the cells comprising the inner layers of the cap dissociate within ~20 minutes. Cells can be reaggregated after this treatment and should survive well. The outer layer of cells is more recalcitrant to dissociation and requires a more severe treatment.

Xenopus laevis Animal Cap/Vegetal Endoderm Conjugates

INTRODUCTIONIn Xenopus laevis, mesoderm can be induced in animal cap cells by contact with vegetal cells (presumptive endoderm). This juxtaposition of tissues was the first indication that the process of mesoderm induction could be separated from neural induction. This protocol describes how to set up a conjugate using Xenopus animal cap and vegetal tissue. In theory, a single embryo can provide both animal and vegetal cells; however, in practice, this is difficult to achieve.

Dissection of Tightly Adhering Xenopus laevis Tissues by Trypsin Treatment

INTRODUCTIONIn older Xenopus laevis embryos (late gastrula and beyond), tissues begin to stick to one another and cannot be peeled apart. For assays requiring isolated tissues, it is necessary to separate such tissues enzymatically, which can be readily accomplished by mild trypsin treatment. Embryos are treated singly, or in small numbers, to avoid possible toxic effects of excessive trypsin digestion. This protocol describes a method for the separation of neural tissue from a neural-plate-stage embryo, although the technique can be adapted to many different tissue types. Enzymatically dissected tissue can be used in quantitative gene expression assays or in specification or induction assays.

Cortical Isolation from Xenopus laevis Oocytes and Eggs

INTRODUCTIONIn Xenopus laevis, the cortex is the layer of gelatinous cytoplasm that lies just below the plasma membrane of the egg. Rotation of the cortex relative to the deeper cytoplasm soon after fertilization is intimately linked to normal dorsal axis specification. The cortex can be dissected from the egg to analyze its composition and activity or to clone associated RNAs. This protocol describes a procedure for isolating the vegetal cortex of the fertilized egg.

Microdissection: Explant and Transplant Assays in Xenopus laevis

INTRODUCTIONWhen analyzing any developmental process, important questions are those relating to the formation of different cell types. What makes a cell decide to become a particular cell type? What cell interactions are involved in this decision? The "commitment" of cells to a particular lineage is also called "specification" or "determination." Analysis of the cell interactions, or inductions, required for tissue-type determination is a critical step in the identification of the genes that control the process. Lineage commitment and inductive interactions can be assessed by explant or transplant assays with embryos from Xenopus laevis.

Animal Cap Isolation from Xenopus laevis

INTRODUCTIONThis protocol presents a method for isolating Xenopus laevis animal cap cells, which are cells situated around the animal (pigmented) pole of a blastula or very early gastrula-stage embryo. This tissue is fated to become cement gland/neurectoderm on the dorsal side of the embryo and epidermis on the ventral side. Animal caps are composed of pluripotent cells that can be induced to form endodermal, mesodermal, or ectodermal cell types, and can therefore serve as a useful substrate to assess the activity of various inducing factors. Caps from embryos that have been injected with expression constructs can also be removed and analyzed to assess the activity of various genes. In addition, caps can be used in conjugation (induction) assays.

Transplantation of Xenopus laevis Lens Ectoderm

INTRODUCTIONIn Xenopus laevis, transplanting a piece of tissue from one site to another (other than into the blastocoel) is a useful way of examining the sequence of inductions required to produce a particular organ. For example, the young primordium of a particular organ can be transplanted from a young individual into an older embryo to determine whether the older embryo still has the capacity to induce the tissue to form the organ. Although this capacity can also be assayed using explants, the complex surroundings of the host embryo cannot be reproduced in an explant. This protocol describes how to transplant a piece of gastrula-stage animal cap ectoderm into the presumptive lens of a neural-plate-stage host embryo. Note that the technique can be adapted for use in other regions of the embryo. Care must be taken to inflict as little damage on the embryo as possible during this procedure. Well-treated embryos recover rapidly, whereas embryos with extensive injuries do not.

Handling Xenopus laevis Adults

INTRODUCTIONThis protocol describes the proper technique for handling adult Xenopus animals.

Isolating Xenopus laevis Testes

INTRODUCTIONThis protocol describes a method of isolating Xenopus laevis testes for use in in vitro fertilization. The testes from one male Xenopus contain sufficient sperm to fertilize several thousand eggs.

Dejellying Xenopus laevis Embryos

INTRODUCTIONThis protocol presents a method for preparing Xenopus embryos for manipulation. Embryos are surrounded by a series of thick, protective jelly membranes. Removal of these membranes is the first step in most micromanipulation procedures. These membranes must be completely removed for embryo dissection. For microinjection, membranes can be completely or partially removed.

Removing the Vitelline Membrane from Xenopus laevis Embryos

INTRODUCTIONThis protocol presents a method for the removal of the vitelline membrane from Xenopus embryos, which is essential for embryo dissection experiments. Removal of the vitelline membrane is a technique that becomes easier with practice. The easiest embryos on which to learn this technique are those with a large space between the membrane and the embryo, that is, late neurula stages.

Embryo dissection and micromanipulation tools

INTRODUCTIONThis article describes the creation and maintenance of tools for use in dissection and micromanipulation of embryos. All tools must be kept clean and rinsed with 70% ethanol to keep them sterile.

Xenopus laevis In Vitro Fertilization and Natural Mating Methods

INTRODUCTIONThis protocol describes in vitro fertilization and natural mating methods for Xenopus laevis.

Housing and Feeding of Xenopus laevis

INTRODUCTIONGood animal husbandry is vital for maintaining a healthy frog population. This requires some effort but is generally rewarded by high-quality egg and embryo production. A healthy frog is placid, with moderately slimy skin and a nice pear shape. Jumpy frogs, frogs with dry or excessively slimy skin, bloated frogs, and frogs that look gray and thin or reddish are not healthy and should not be used for egg collection, as this would lead to further deterioration of the animals' condition, and the resulting eggs would be generally unsuitable for experimental purposes. This article describes proper husbandry techniques for Xenopus laevis in the laboratory.

Xenopus laevis Egg Collection

INTRODUCTIONThis protocol describes procedures for collecting Xenopus laevis eggs from females in which ovulation has been induced.

Inducing Ovulation in Xenopus laevis

INTRODUCTIONThis protocol describes a method for inducing ovulation in Xenopus laevis for the purpose of in vitro fertilization. Ovulation is induced by injection of human chorionic gonadotropin (hCG) into the dorsal lymph sac of a female frog. Females can lay many hundreds of eggs. However, because the actual number of eggs laid and the efficiency of fertilization are unpredictable, ovulation should be induced in more than one female, even when only a few eggs are required.

Zebrafish promoter microarrays identify actively transcribed embryonic genes

The development and verification of a genomic microarray for ChIP-chip analysis of zebrafish genes is described.

We have designed a zebrafish genomic microarray to identify DNA-protein interactions in the proximal promoter regions of over 11,000 zebrafish genes. Using these microarrays, together with chromatin immunoprecipitation with an antibody directed against tri-methylated lysine 4 of Histone H3, we demonstrate the feasibility of this method in zebrafish. This approach will allow investigators to determine the genomic binding locations of DNA interacting proteins during development and expedite the assembly of the genetic networks that regulate embryogenesis.

The zic1 gene is an activator of Wnt signaling

The zic1 gene plays an important role in early patterning of the Xenopus neurectoderm. While Zic1 does not act as a neural inducer, it synergizes with the neural inducing factor Noggin to activate expression of posterior neural genes, including the midbrain/hindbrain boundary marker engrailed-2. Since the Drosophila homologue of zic1, odd-paired (opa), regulates expression of the wingless and engrailed genes and since Wnt proteins posteriorize neural tissue in Xenopus, we asked whether Xenopus Zic1 acted through the Wnt pathway. Using Wnt signaling inhibitors, we demonstrate that an active Wnt pathway is required for activation of en-2 expression by zic1. Consistent with this result, Zic1 induces expression of several wnt genes, including wnt1, wnt4 and wnt8b. wnt1 gene expression activates expression of engrailed in various organisms, including Xenopus, as demonstrated here. Together, our data suggest that zic1 is an upstream regulator of several wnt genes and that the regulatory relationships between opa, wingless and engrailed seen in Drosophila are also present in vertebrates.

Identification of a BMP inhibitor-responsive promoter module required for expression of the early neural gene zic1

Expression of the transcription factor zic1 at the onset of gastrulation is one of the earliest molecular indicators of neural fate determination in Xenopus. Inhibition of bone morphogenetic protein (BMP) signaling is critical for activation of zic1 expression and fundamental for establishing neural identity in both vertebrates and invertebrates. The mechanism by which interruption of BMP signaling activates neural-specific gene expression is not understood. Here, we report identification of a 215 bp genomic module that is both necessary and sufficient to activate Xenopus zic1 transcription upon interruption of BMP signaling. Transgenic analyses demonstrate that this BMP inhibitory response module (BIRM) is required for expression in the whole embryo. Multiple consensus binding sites for specific transcription factor families within the BIRM are required for its activity and some of these regions are phylogenetically conserved between orthologous vertebrate zic1 genes. These data suggest that interruption of BMP signaling facilitates neural determination via a complex mechanism, involving multiple regulatory factors that cooperate to control zic1 expression.

Identification and characterisation of the posteriorly-expressed Xenopus neurotrophin receptor homolog genes fullback and fullback-like

We have identified fullback and fullback-like, two Xenopus laevis neurotrophin receptor homolog (NRH1) genes. The sequences of Fullback and Fullback-like are very similar to that of the neurotrophin receptor p75NTR, in both their extracellular and their intracellular domains. As their names imply, fullback and fullback-like are expressed in essentially identical patterns in the posterior of the embryo from the early gastrula stage onward. At tailbud and tadpole stages transcripts are also present in dorsal somites and the head, in addition to the growing tailbud. This expression pattern differs from that of p75NTR, suggesting that fullback and fullback-like have different functions from p75NTR during early development.

Can zebrafish be used as a model to study the neurodevelopmental causes of autism?

The zebrafish has proven to be an excellent model for analyzing issues of vertebrate development. In this review we ask whether the zebrafish is a viable model for analyzing the neurodevelopmental causes of autism. In developing an answer to this question three topics are considered. First, the general attributes of zebrafish as a model are discussed, including low cost maintenance, rapid life cycle and the multitude of techniques available. These techniques include large-scale genetic screens, targeted loss and gain of function methods, and embryological assays. Second, we consider the conservation of zebrafish and mammalian brain development, structure and function. Third, we discuss the impressive use of zebrafish as a model for human disease, and suggest several strategies by which zebrafish could be used to dissect the genetic basis for autism. We conclude that the zebrafish system could be used to make important contributions to understanding autistic disorders.

What's your position? the Xenopus cement gland as a paradigm of regional specification

The correct positioning of organs during embryonic development requires multiple cues. The Xenopus cement gland is a mucus-secreting epithelium that is a simple model for organogenesis, allowing detailed analysis of this complex process. The cement gland forms at a conserved anterior position, where embryonic ectoderm and endoderm touch. In all deuterostomes, this region will form the stomodeum (primitive mouth) and, in some aquatic larva, will also form a cement gland. In recent years, a model has been put forward suggesting that an intermediate level of BMP signaling in the ectoderm leads to cement gland formation. We propose an alternative model whereby, during gastrulation, the cement gland (CG) is positioned by the overlap of three domains, corresponding to anterodorsal identity (AD), ventrolateral identity (VL), and ectodermal outer layer identity (EO), defining the equation (AD + VL + EO = CG). Anterodorsal identity requires a contribution by the transcription factor Otx2 while ventrolateral identity requires the BMP4 signaling pathway. These postional cues are integrated to activate cement gland differentiation. This integration appears to require intermediate steps, including expression of pitx genes, and members of the ATF/CREB and Ets transcription factor families.

Direct and indirect regulation of derrière, a Xenopus mesoderm-inducing factor, by VegT

One candidate for an endogenous mesoderm-inducing factor in Xenopus is derrière, a member of the TGFβ family closely related to Vg1. In this paper we first show that derrière is able to exert long-range effects in the early Xenopus embryo, reinforcing the view that it functions as a secreted factor required for proper formation of posterior structures. Analysis of the derrière promoter shows that expression of the gene is controlled through a complex inductive network involving VegT and TGFβ-related molecules and also, perhaps, FGF family members. The work confirms that derrière plays an important role in mesoderm formation and it illustrates the complex regulation to which inducing factors are subject.

Cement gland-specific activation of the Xag1 promoter is regulated by co-operation of putative Ets and ATF/CREB transcription factors

The cement gland marks the extreme anterior ectoderm of the Xenopus embryo, and is determined through the overlap of several positional domains. In order to understand how these positional cues activate cement gland differentiation, the promoter of Xag1, a marker of cement gland differentiation, was analyzed. Previous studies have shown that Xag1 expression can be activated by the anterior-specific transcription factor Otx2, but that this activation is indirect. 102 bp of upstream genomic Xag1 sequence restricts reporter gene expression specifically to the cement gland. Within this region, putative binding sites for Ets and ATF/CREB transcription factors are both necessary and sufficient to drive cement gland-specific expression, and cooperate to do so. Furthermore, while the putative ATF/CREB factor is activated by Otx2, a factor acting through the putative Ets-binding site is not. These results suggest that Ets-like and ATF/CREB-like family members play a role in regulating Xag1 expression in the cement gland, through integration of Otx2 dependent and independent pathways.

derriere: a TGF-beta family member required for posterior development in Xenopus

TGF-beta signaling plays a key role in induction of the Xenopus mesoderm and endoderm. Using a yeast-based selection scheme, we isolated derriere, a novel TGF-beta family member that is closely related to Vg1 and that is required for normal mesodermal patterning, particularly in posterior regions of the embryo. Unlike Vg1, derriere is expressed zygotically, with RNA localized to the future endoderm and mesoderm by late blastula, and to the posterior mesoderm by mid-gastrula. The derriere expression pattern appears to be identical to the zygotic expression domain of VegT (Xombi, Brat, Antipodean), and can be activated by VegT as well as fibroblast growth factor (FGF). In turn, derriere activates expression of itself, VegT and eFGF, suggesting that a regulatory loop exists between these genes. derriere is a potent mesoderm and endoderm inducer, acting in a dose-dependent fashion. When misexpressed ventrally, derriere induces a secondary axis lacking a head, an effect that is due to dorsalization of the ventral marginal zone. When misexpressed dorsally, derriere suppresses head formation. derriere can also posteriorize neurectoderm, but appears to do so indirectly. Together, these data suggest that derriere expression is compatible only with posterior fates. In order to assess the in vivo function of derriere, we constructed a dominant interfering Derriere protein (Cm-Derriere), which preferentially blocks Derriere activity relative to that of other TGFbeta family members. Cm-derriere expression in embryos leads to posterior truncation, including defects in blastopore lip formation, gastrulation and neural tube closure. Normal expression of anterior and hindbrain markers is observed; however, paraxial mesodermal gene expression is ablated. This phenotype can be rescued by wild-type derriere and by VegT. Our findings indicate that derriere plays a crucial role in mesodermal patterning and development of posterior regions in Xenopus.

Subtractive cloning: past, present, and future

Subtractive cloning is a powerful technique for isolating genes expressed or present in one cell population but not in another. This method and a related one termed positive selection have their origins in nucleic acid reassociation techniques. We discuss the history of subtractive techniques, and fundamental information about the nucleic acid composition of cells that came out of reassociation analyses. We then explore current techniques for subtractive cloning and positive selection, discussing the merits of each. These techniques include cDNA library-based techniques and PCR-based techniques. Finally, we briefly discuss the future of subtractive cloning and new approaches that may augment or supersede current methods.

Efficient hormone-inducible protein function in Xenopus laevis

A major problem in analyzing gene function during Xenopus development has been the inability to induce gene expression in a temporally controlled manner. We have attempted to solve this problem with a system of hormone-activated protein function, using the myogenic gene MyoD as a paradigm. We show that microinjection of RNA for MyoD fused to the ligand-binding domain of either the estrogen or glucocorticoid receptor results in hormone-dependent activation of MyoD function, as assayed by ectopic induction of muscle-specific actin mRNA. Induction is tightly regulated in both isolated animal caps and intact embryos, with ectopic muscle-specific actin expression inducible after 2 hr of hormone treatment. Higher levels of MyoD-receptor fusion proteins that native MyoD protein are present in embryos, apparently a result of increased fusion protein stability. This is the first demonstration that hormone-inducible fusion proteins can work effectively in a complex embryo.

Regulation of the Xenopus labial homeodomain genes, HoxA1 and HoxD1: activation by retinoids and peptide growth factors

Vertebrate homologues of Drosophila labial are likely to play key roles in anteroposterior axis formation. However, little is known about the regulation of these genes during vertebrate development. Here we examine the expression and regulation of the Xenopus labial homeodomain genes, HoxA1 and HoxD1. HoxA1 was expressed around the dorsoventral circumference of the trunk in neurula embryos, with later expression in spinal cord, midbrain, hindbrain, and endolymphatic duct. By mid gastrula, HoxD1 was predominantly expressed in dorsolateral and ventral ectoderm, with a gap in expression at the dorsal midline. By neurula, ventral expression had declined with most expression restricted to dorsolateral mesoderm and ectoderm. Retinoic acid strongly induced HoxA1 and HoxD1 throughout the ectoderm and mesendoderm of gastrula stages, while in older embryos retinoids induced ectopic expression of these genes in more limited regions. Induction by retinoids was independent of protein synthesis. Surprisingly, HoxA1 was expressed at high levels in isolated animal caps in the absence of retinoic acid. The peptide growth factors bFGF and activin A strongly induced expression of HoxD1, but not HoxA1, in animal caps; however, RNA accumulated only many hours after the application of these factors. Overexpression of thyroid hormone receptor (c-erbA) prevented induction of HoxD1 by retinoic acid in animal caps. c-erbA also ablated expression of HoxD1 in whole embryos, suggesting a role for endogenous retinoids in the regulation of HoxD1 expression. Dominant interfering activin and FGF receptors prevented expression of HoxD1 in vivo, implicating these factors in the normal induction of HoxD1. Our data indicate that induction of labial-like homeodomain genes is complex and may require many factors.

The frog prince-ss: a molecular formula for dorsoventral patterning in Xenopus

Collapse

Retinoic acid perturbs the expression of Xhox.lab genes and alters mesodermal determination in Xenopus laevis

Retinoic acid (RA) treatment of Xenopus laevis embryos leads to anterior truncation of the body axis (Durston et al. 1989; Sive et al. 1990). These initial studies suggested that RA may play a role in the patterning of the primary body axis. At least one target of RA was shown previously to be dorsal ectoderm. In this report we extend this observation and also ask whether RA alters the determination and inducing capacity of mesodermal tissue. To facilitate this analysis we isolated the homeo-domain-containing genes Xhox.lab1 and Xhox.lab2. These genes were expressed in both ectoderm and mesoderm during the RA-sensitive period and were strongly induced by RA in both germ layers. In particular, anterior regions expressed low levels of Xhox.lab RNAs in untreated embryos but showed increased expression after RA application. We show further that although RA-treated embryos contained anterior-inducing mesoderm, the amount of this activity appeared to be lower than that of controls. Additionally, we document that RA suppressed lateral (muscle) and ventral (blood) mesoderm differentiation. The data indicate that RA alters mesodermal determination and causes axial perturbation both by depressing the ability of dorsal mesoderm to induce anterior structures and by altering the response of dorsal ectoderm to induction. These analyses suggest that Xhox.lab genes may be responsible, in part, for mediating the RA effect.