Gut specific expression using mammalian promoters in transgenic Xenopus laevis

Noggin proteins are multifunctional extracellular regulators of cell signalling

Noggin is an extracellular cysteine knot protein that plays a crucial role in vertebrate dorsoventral patterning. Noggin binds and inhibits the activity of bone morphogenetic proteins via a conserved N-terminal clip domain. Noncanonical orthologs of Noggin that lack a clip domain (“Noggin-like” proteins) are encoded in many arthropod genomes and are thought to have evolved into receptor tyrosine kinase ligands that promote Torso/receptor tyrosine kinase signaling rather than inhibiting bone morphogenic protein signaling. Here, we examined the molecular function of noggin/noggin-like genes (ApNL1 and ApNL2) from the arthropod pea aphid using the dorso-ventral patterning of Xenopus and the terminal patterning system of Drosophila to identify whether these proteins function as bone morphogenic protein or receptor tyrosine kinase signaling regulators. Our findings reveal that ApNL1 from the pea aphid can regulate both bone morphogenic protein and receptor tyrosine kinase signaling pathways, and unexpectedly, that the clip domain is not essential for its antagonism of bone morphogenic protein signaling. Our findings indicate that ancestral noggin/noggin-like genes were multifunctional regulators of signaling that have specialized to regulate multiple cell signaling pathways during the evolution of animals.

Modeling endoderm development and disease in Xenopus

The endoderm is the innermost germ layer that forms the linings of the respiratory and gastrointestinal tracts, and their associated organs, during embryonic development. Xenopus embryology experiments have provided fundamental insights into how the endoderm develops in vertebrates, including the critical role of TGFβ-signaling in endoderm induction,elucidating the gene regulatory networks controlling germ layer development and the key molecular mechanisms regulating endoderm patterning and morphogenesis. With new genetic, genomic, and imaging approaches, Xenopus is now routinely used to model human disease, discover mechanisms underlying endoderm organogenesis, and inform differentiation protocols for pluripotent stem cell differentiation and regenerative medicine applications. In this chapter, we review historical and current discoveries of endoderm development in Xenopus, then provide examples of modeling human disease and congenital defects of endoderm-derived organs using Xenopus.

Targeted integration of genes in Xenopus tropicalis

With the successful establishment of both targeted gene disruption and integration methods in the true diploid frog Xenopus tropicalis, this excellent vertebrate genetic model now is making a unique contribution to modelling human diseases. Here, we summarize our efforts on establishing homologous recombination-mediated targeted integration in Xenopus tropicalis, the usefulness, and limitation of targeted integration via the homology-independent strategy, and future directions on how to further improve targeted gene integration in Xenopus tropicalis.

Zebrafish transgenic constructs label specific neurons in Xenopus laevis spinal cord and identify frog V0v spinal neurons

A correctly functioning spinal cord is crucial for locomotion and communication between body and brain but there are fundamental gaps in our knowledge of how spinal neuronal circuitry is established and functions. To understand the genetic program that regulates specification and functions of this circuitry, we need to connect neuronal molecular phenotypes with physiological analyses. Studies using Xenopus laevis tadpoles have increased our understanding of spinal cord neuronal physiology and function, particularly in locomotor circuitry. However, the X. laevis tetraploid genome and long generation time make it difficult to investigate how neurons are specified. The opacity of X. laevis embryos also makes it hard to connect functional classes of neurons and the genes that they express. We demonstrate here that Tol2 transgenic constructs using zebrafish enhancers that drive expression in specific zebrafish spinal neurons label equivalent neurons in X. laevis and that the incorporation of a Gal4:UAS amplification cassette enables cells to be observed in live X. laevis tadpoles. This technique should enable the molecular phenotypes, morphologies and physiologies of distinct X. laevis spinal neurons to be examined together in vivo. We have used an islet1 enhancer to label Rohon-Beard sensory neurons and evx enhancers to identify V0v neurons, for the first time, in X. laevis spinal cord. Our work demonstrates the homology of spinal cord circuitry in zebrafish and X. laevis, suggesting that future work could combine their relative strengths to elucidate a more complete picture of how vertebrate spinal cord neurons are specified, and function to generate behavior. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1007-1020, 2017.

Development of Xenopus resource centers: the National Xenopus Resource and the European Xenopus Resource Center

Xenopus is an essential vertebrate model system for biomedical research that has contributed to important discoveries in many disciplines, including cell biology, molecular biology, physiology, developmental biology, and neurobiology. However, unlike other model systems no central repository/stock center for Xenopus had been established until recently. Similar to mouse, zebrafish, and fly communities, which have established stock centers, Xenopus researchers need to maintain and distribute rapidly growing numbers of inbred, mutant, and transgenic frog strains, along with DNA and protein resources, and individual laboratories struggle to accomplish this efficiently. In the last 5 years, two resource centers were founded to address this need: the European Xenopus Resource Center (EXRC) at the University of Portsmouth in England, and the National Xenopus Resource (NXR) at the Marine Biological Laboratory in Woods Hole, MA. These two centers work together to provide resources and support to the Xenopus research community. The EXRC and NXR serve as stock centers and acquire, produce, maintain and distribute mutant, inbred and transgenic Xenopus laevis and Xenopus tropicalis lines. Independently, the EXRC is a repository for Xenopus cDNAs, fosmids, and antibodies; it also provides oocytes and wild-type frogs within the United Kingdom. The NXR will complement these services by providing research training and promoting intellectual interchange through hosting mini-courses and workshops and offering space for researchers to perform short-term projects at the Marine Biological Laboratory. Together the EXRC and NXR will enable researchers to improve productivity by providing resources and expertise to all levels, from graduate students to experienced PIs. These two centers will also enable investigators that use other animal systems to take advantage of Xenopus' unique experimental features to complement their studies.

pTransgenesis: a cross-species, modular transgenesis resource

As studies aim increasingly to understand key, evolutionarily conserved properties of biological systems, the ability to move transgenesis experiments efficiently between organisms becomes essential. DNA constructions used in transgenesis usually contain four elements, including sequences that facilitate transgene genome integration, a selectable marker and promoter elements driving a coding gene. Linking these four elements in a DNA construction, however, can be a rate-limiting step in the design and creation of transgenic organisms. In order to expedite the construction process and to facilitate cross-species collaborations, we have incorporated the four common elements of transgenesis into a modular, recombination-based cloning system called pTransgenesis. Within this framework, we created a library of useful coding sequences, such as various fluorescent protein, Gal4, Cre-recombinase and dominant-negative receptor constructs, which are designed to be coupled to modular, species-compatible selectable markers, promoters and transgenesis facilitation sequences. Using pTransgenesis in Xenopus, we demonstrate Gal4-UAS binary expression, Cre-loxP-mediated fate-mapping and the establishment of novel, tissue-specific transgenic lines. Importantly, we show that the pTransgenesis resource is also compatible with transgenesis in Drosophila, zebrafish and mammalian cell models. Thus, the pTransgenesis resource fosters a cross-model standardization of commonly used transgenesis elements, streamlines DNA construct creation and facilitates collaboration between researchers working on different model organisms.

Xenopus as a model to study alternative splicing in vivo

An increasing number of genes are being identified for which the corresponding mRNAs contain different combinations of the encoded exons. This highly regulated exon choice, or alternative splicing, is often tissue-specific and potentially could differentially affect cellular functions. Alternative splicing is therefore not only a means to increase the coding capacity of the genome, but also to regulate gene expression during differentiation or development. To both evaluate the importance for cellular functions and define the regulatory pathways of alternative splicing, it is necessary to progress from the in vitro or ex vivo experimental models actually used towards in vivo whole-animal studies. We present here the amphibian, Xenopus, as an experimental model highly amenable for such studies. The various experimental approaches that can be used with Xenopus oocytes and embryos to characterize regulatory sequence elements and factors are presented and the advantages and drawbacks of these approaches are discussed. Finally, the real possibilities for large-scale identification of mRNAs containing alternatively spliced exons, the tissue-specific patterns of exon usage and the way in which these patterns are modified by perturbing the relative amount of splicing factors are discussed.

A simple method of transgenesis using I-SceI meganuclease in Xenopus

Here we present a protocol for generating transgenic embryos in Xenopus using I-SceI meganuclease. This method relies on integration of DNA constructs, containing one or two I-SceI meganuclease sites. It is a simpler method than the REMI method of transgenesis, and it is ideally suited for generating transgenic lines in Xenopus laevis and Xenopus tropicalis. In addition to it being simpler than the REMI method, this protocol also results in single copy integration events rather than tandem concatemers. Although the protocol we describe is for X. tropicalis, the method can also be used to generate transgenic lines in X. laevis. We also describe a convenient method for designing and generating complex constructs for transgenesis, named pTransgenesis, based on the Multisite Gateway(®) cloning, which include I-SceI sites and Tol2 elements to facilitate genome integration.

Transdifferentiation of tadpole pancreatic acinar cells to duct cells mediated by Notch and stromelysin-3

The tadpole pancreas has differentiated acinar cells but an underdeveloped ductal system. At the climax of metamorphosis thyroid hormone (TH) induces the tadpole acinar cells to dedifferentiate to a progenitor state. After metamorphosis is complete the exocrine pancreas redifferentiates in the growing frog forming a typical vertebrate pancreas including a complex ductal system. A micro array analysis found that TH up regulates stromelysin 3 (ST3, matrix metalloproteinase 11) in the exocrine pancreas at metamorphic climax. Transgenic tadpoles were prepared with an elastase promoter driving either the ST3 gene or the constitutively active form of Notch (IC). Expression of the transgenes was controlled by the tetracycline system. A few days after either of these transgenes is activated by doxycycline the pancreatic acinar cells turn into duct-like cells. This transdetermination occurs without cell division since both acinar and ductal markers can be visualized transiently in the same cell. We propose that remodeling of the tadpole acinar cells is initiated when ST3 is up regulated by TH. Stromelysin-3 then cleaves and activates Notch.

A conserved MRF4 promoter drives transgenic expression in Xenopus embryonic somites and adult muscle

The muscle regulatory factor MRF4 is expressed in both embryonic and adult vertebrate skeletal muscle cells. In mammals the MRF4 gene has a complex cis-regulatory structure, with many kilobases (kb) of upstream sequence required for embryonic expression in transgenic mice. Here, initial functional comparison between Xenopus and mammalian MRF4 genes revealed that 610 base pairs (bp) of the XMRF4a proximal promoter drove substantial transgenic expression in X. laevis myogenic cells, from somites of neurula embryos through adult myofibers, and as little as 180 bp gave detectable expression. Over 300 bp of XMRF4a proximal promoter sequence is highly conserved among three X. laevis and X. tropicalis MRF4 genes, but only about 150 bp shows significant identity to mammalian MRF4 genes. This most-conserved XMRF4a region contains a putative MEF2 binding site essential for expression both in transgenic embryos and in transfected mouse muscle cells. A rat MRF4 minimal promoter including the conserved region also was active in transgenic X. laevis embryos, demonstrating a striking difference between the mouse and Xenopus transgenic systems. The longest XMRF4a promoter construct tested, with 9.5 kb of 5'-flanking sequence, produced significantly greater expression in transfected mouse cells than did promoters 4.3-kb or shorter, suggesting that the intervening region contains an enhancer, although no increased expression was evident when this region was included in transgenic X. laevis embryos. Further identification and analysis of Xenopus MRF4 transcriptional control elements will offer insights into the evolution of this gene and of the myogenic gene regulatory network.

Beyond early development: Xenopus as an emerging model for the study of regenerative mechanisms

While Xenopus is a well-known model system for early vertebrate development, in recent years, it has also emerged as a leading model for regeneration research. As an anuran amphibian, Xenopus laevis can regenerate the larval tail and limb by means of the formation of a proliferating blastema, the lens of the eye by transdifferentiation of nearby tissues, and also exhibits a partial regeneration of the postmetamorphic froglet forelimb. With the availability of inducible transgenic techniques for Xenopus, recent experiments are beginning to address the functional role of genes in the process of regeneration. The use of soluble inhibitors has also been very successful in this model. Using the more traditional advantages of Xenopus, others are providing important lineage data on the origin of the cells that make up the tissues of the regenerate. Finally, transcriptome analyses of regenerating tissues seek to identify the genes and cellular processes that enable successful regeneration. Developmental Dynamics 238:1226-1248, 2009. (c) 2009 Wiley-Liss, Inc.

Use of adenovirus for ectopic gene expression in Xenopus

We show that replication defective adenovirus can be used for localized overexpression of a chosen gene in Xenopus tadpoles. Xenopus contains two homologs of the Coxsackie and Adenovirus Receptor (xCAR1 and 2), both of which can confer sensitivity for adenovirus infection. xCAR1 mRNA is present from the late gastrula stage and xCAR2 throughout development, both being widely expressed in the embryo and tadpole. Consistent with the expression of the receptors, adenovirus will infect a wide range of Xenopus tissues cultured in vitro. It will also infect early embryos when injected into the blastocoel or archenteron cavities. Furthermore, adenovirus can be delivered by localized injection to tadpoles and will infect a patch of cells around the injection site. The expression of green fluorescent protein in infected cells persists for several weeks. This new gene delivery method complements the others that are already available. Developmental Dynamics 238:1412-1421, 2009. (c) 2009 Wiley-Liss, Inc.

The tetraspanin Tm4sf3 is localized to the ventral pancreas and regulates fusion of the dorsal and ventral pancreatic buds

During embryogenesis, the pancreas develops from separate dorsal and ventral buds, which fuse to form the mature pancreas. Little is known about the functional differences between these two buds or the relative contribution of cells derived from each region to the pancreas after fusion. To follow the fate of dorsal or ventral bud derived cells in the pancreas after fusion, we produced chimeric Elas-GFP transgenic/wild-type embryos in which either dorsal or ventral pancreatic bud cells expressed GFP. We found that ventral pancreatic cells migrate extensively into the dorsal pancreas after fusion, whereas the converse does not occur. Moreover, we found that annular pancreatic tissue is composed exclusively of ventral pancreas-derived cells. To identify ventral pancreas-specific genes that may play a role in pancreatic bud fusion, we isolated individual dorsal and ventral pancreatic buds, prior to fusion, from NF38/39 Xenopus laevis tadpoles and compared their gene expression profiles (NF refers to the specific stage of Xenopus development). As a result of this screen, we have identified several new ventral pancreas-specific genes, all of which are expressed in the same location within the ventral pancreas at the junction where the two ventral pancreatic buds fuse. Morpholino-mediated knockdown of one of these ventral-specific genes, transmembrane 4 superfamily member 3 (tm4sf3), inhibited dorsal-ventral pancreatic bud fusion, as well as acinar cell differentiation. Conversely, overexpression of tm4sf3 promoted development of annular pancreas. Our results are the first to define molecular and behavioral differences between the dorsal and ventral pancreas, and suggest an unexpected role for the ventral pancreas in pancreatic bud fusion.

Cell-cell interactions during remodeling of the intestine at metamorphosis in Xenopus laevis

Amphibian metamorphosis is accompanied by extensive intestinal remodeling. This process, mediated by thyroid hormone (TH) and its nuclear receptors, affects every cell type. Gut remodeling in Xenopus laevis involves epithelial and mesenchymal proliferation, smooth muscle thickening, neuronal aggregation, formation of intestinal folds, and shortening of its length by 75%. Transgenic tadpoles expressing a dominant negative TH receptor (TRDN) controlled by epithelial-, fibroblast-, and muscle-specific gene promoters were studied. TRDN expression in the epithelium caused abnormal development of virtually all cell types, with froglet guts displaying reduced intestinal folds, thin muscle and mesenchyme, absence of neurons, and reduced cell proliferation. TRDN expression in fibroblasts caused abnormal epithelia and mesenchyme development, and expression in muscle produced fewer enteric neurons and a reduced inter-muscular space. Gut shortening was inhibited only when TRDN was expressed in fibroblasts. Gut remodeling results from both cell-autonomous and cell-cell interactions.

Heat-shock inducible Cre strains to study organogenesis in transgenic Xenopus laevis

The frog Xenopus is a well established vertebrate model to study the processes involved in embryogenesis and organogenesis, as it can be manipulated easily with a whole series of methods. We have expanded these approaches by establishing two transgenic Xenopus strains that allow specific interference with the activity of defined genes using a heat-shock inducible Cre recombinase that can induce upon heat-shock expression of a reporter gene in crossings to a corresponding reporter strain. We have applied this binary technique of gene interference in Xenopus development to overexpress the mutated HNF1 beta transcription factor at distinct developmental stages. Induction of HNF1 beta P328L329del by heat-shock at the gastrula stage resulted in a dramatic phenotype including malformation of the pronephros, gut, stomach, abnormal tail development and massive edemas indicative for kidney dysfunction. Thus, we have established the first binary inducible gene expression system in Xenopus laevis that can be used to study organogenesis.

Generation of transgenic frogs

The possibility of generating transgenic animals is of obvious advantage for the analysis of gene function in development and disease. One of the established vertebrate model systems in developmental biology is the amphibian Xenopus laevis. Different techniques have been successfully applied to create Xenopus transgenics; in this chapter, the so-called meganuclease method is described. This technique is not only technically simple, but also comparably efficient and applicable to both Xenopus laevis and Xenopus tropicalis. The commercially available endonuclease I-SceI (meganuclease) mediates the integration of foreign DNA into the frog genome after coinjection into fertilized eggs. Tissue-specific gene expression, as well as germline transmission, has been observed.

Remodeling the exocrine pancreas at metamorphosis in Xenopus laevis

At metamorphosis the Xenopus laevis tadpole exocrine pancreas remodels in two stages. At the climax of metamorphosis thyroid hormone (TH) induces dedifferentiation of the entire exocrine pancreas to a progenitor state. The organ shrinks to 20% of its size, and approximately 40% of its cells die. The acinar cells lose their zymogen granules and approximately 75% of their RNA. The mRNAs that encode exocrine-specific proteins (including the transcription factor Ptf1a) undergo almost complete extinction at climax, whereas PDX-1, Notch-1, and Hes-1, genes implicated in differentiation of the progenitor cells, are activated. At the end of spontaneous metamorphosis when the endogenous TH has reached a low level, the pancreas begins to redifferentiate. Exogenous TH induces the dedifferentiation phase but not the redifferentation phase. The tadpole pancreas lacks the mature ductal system that is found in adult vertebrate pancreases, including the frog. Exocrine pancreases of transgenic tadpoles expressing a dominant negative form of the TH receptor controlled by the elastase promoter are resistant to TH. They do not shrink when subjected to TH. Their acinar cells do not dedifferentiate at climax, nor do they down-regulate exocrine-specific genes or activate Notch-1 and Hes-1. Even 2 months after metamorphosis these frogs have not developed a mature ductal system and the acinar cells are abnormally arranged. The TH-dependent dedifferentiation of the tadpole acinar cells at climax is a necessary step in the formation of a mature frog pancreas.

Transdifferentiation in developmental biology, disease, and in therapy

Transdifferentiation (or metaplasia) refers to the conversion of one cell type to another. Because transdifferentiation normally occurs between cells that arise from the same region of the embryo, understanding the molecular and cellular events in cell type transformations may help to explain the mechanisms underlying normal development. Here we review examples of transdifferentiation in nature focusing on the possible role of cell type switching in metamorphosis and regeneration. We also examine transdifferentiation in mammals in relation to disease and the use of transdifferentiated cells in cellular therapy.

Differential ability of Ptf1a and Ptf1a-VP16 to convert stomach, duodenum and liver to pancreas

Determining the functional attributes of pancreatic transcription factors is essential to understand how the pancreas is specified distinct from other endodermal organs, such as liver, stomach and duodenum, and to direct the differentiation of other cell types into pancreas. Previously, we demonstrated that Pdx1-VP16 was sufficient to convert liver to pancreas. In this paper, we characterize the functional ability of another pancreatic transcription factor, Ptf1a, in promoting ectopic pancreatic fates at early stages throughout the endoderm and later during organogenesis. Using the transthyretin promoter to drive expression in the early liver region/bud of transgenic Xenopus tadpoles, we find that Ptf1a-VP16 is able to convert liver to pancreas. Overexpression of the unmodified Ptf1a on the other hand has no effect in liver but is able to convert stomach and duodenum to pancreas. When overexpressed at earlier embryonic stages throughout the endoderm, Ptf1a activity is similarly limited, whereas Ptf1a-VP16 has increased activity. Interestingly, in all instances we find that Ptf1a-VP16 is only capable of promoting acinar cell fates, whereas Ptf1a promotes both acinar and endocrine fates. Lastly, we demonstrate that, similar to mouse and zebrafish, Xenopus Ptf1a is essential for the initial specification of both endocrine and exocrine cells during normal pancreas development.

Xenopus as a model system for vertebrate heart development

The African clawed frog, Xenopus laevis, is a valuable model system for studies of vertebrate heart development. In the following review, we describe a range of embryological and molecular methodologies that are used in Xenopus research and discuss key discoveries relating to heart development that have been made using this model system. We also discuss how the sequence of the Xenopus tropicalis genome provides a valuable tool for identification of orthologous genes and for identification of evolutionarily conserved promoter elements. Finally, both forward and reverse genetic approaches are currently being applied to Xenopus for the study of vertebrate heart development.

Germ layers to organs: Using Xenopus to study “later” development

The amphibian embryo is a highly successful model system with great promise for organogenesis research. Since the late 1800s, amphibians have been employed to understand vertebrate development and since the 1950s, the African clawed frog Xenopus laevis has been the amphibian of choice. In the past two decades, Xenopus has led the way forward in, among other things, identifying transcription factors, gene regulatory networks and inter- and intracellular signaling pathways that control early development (from fertilization through gastrulation and neurulation). Perhaps the best measure of how successful Xenopus has been as a model for early mammalian development is the observation that much of the knowledge gleaned from Xenopus studies has subsequently directly translated to discoveries of similar mechanisms operating in mouse development. Despite this great success in early development, research on organogenesis in Xenopus has lagged behind the mouse. However, recent technical advances now make Xenopus amenable for studies on later development, including organogenesis. Here, we discuss why Xenopus is well suited for such research and, we believe, permits addressing questions that have been difficult to approach using other model systems. We also highlight how Xenopus researchers have already begun studying a number of major organs, pancreas, liver, kidney and heart, and suggest how Xenopus might contribute more to these areas in the near future.

Functional conserved elements mediate intestinal-type fatty acid binding protein (I-FABP) expression in the gut epithelia of zebrafish larvae

Intestinal-type fatty acid binding protein (I-FABP) plays an important role in the intracellular binding and trafficking of long chain fatty acids in the intestine. The aim of this study, therefore, was to elucidate the regulation and spatiotemporal expression of the I-FABP gene during zebrafish larval development. We performed in vivo reporter-gene analysis in zebrafish by using a transient and transgenic approach. Green fluorescent protein-reporter analyses revealed that the proximal 192-bp region of the I-FABP promoter is sufficient to direct intestine-specific expression during zebrafish larval development. Functional dissection of a 41-bp region within this 192-bp promoter revealed that one C/EBP and two GATA-like binding sites, along with a novel 15-bp element within it are required for I-FABP gene expression in vivo. In addition, the six consensus sites (CCACATCAGCATGAA) in the 15-bp element are critical for I-FABP gene regulation in the zebrafish gut epithelia. Comparison analyses of the orthologous 15-bp element from mammalian I-FABP genes suggests that these mammalian elements are functionally equivalent to the zebrafish 15 element. These results provide the first in vivo evidence that these binding sites (C/EBP and GATA) and the novel 15-bp element contribute to intestine-specific gene expression and that they are functionally conserved across vertebrate evolution.

Xenopus: a prince among models for pronephric kidney development

Recent advances in techniques that are available to study the molecular development of the frog Xenopus make developmental studies using this amphibian amenable to experimentation. This review outlines some of the attractive features of this model organism and describes how these techniques can be and are being used in studies on the organogenesis of the larval amphibian kidney, the pronephros. The roles of micromanipulation, grafting, and in vitro culturing of animal caps are discussed as tools in the analysis of kidney development and as a source of tissue for subtractive hybridization strategies. The importance of expression cloning and functional analysis of newly identified pronephros-specific genes are also described. Finally, transgenesis and electroporation are discussed as potentially new methods of gene delivery to the pronephros. These techniques can be used to help identify the gene networks that control organogenesis of this larval kidney form, which will undoubtedly have applicability to higher vertebrate kidney development.

I-SceI meganuclease-mediated transgenesis inXenopus

Several experimental approaches have been described to generate transgenic frogs. Here, we report on the application of a novel method in Xenopus, making use of I-SceI meganuclease. The characteristic feature of this endonuclease is that it has an extended recognition site of 18 bp, which is expected to exist only once in 7 x 10(10) bp of random DNA sequences. Various reporter constructs flanked by two I-SceI recognition sites were injected together with the I-SceI meganuclease into one-cell stage Xenopus embryos. We observed an overall transgenesis frequency of 10% or more under optimized condition. The injected genes were integrated into the genome and transmitted to F1 offspring. Southern blot analysis showed that between one and eight copies of the transgene were integrated. Meganuclease-aided transgenesis, thus, provides a simple and highly efficient tool for transgenesis in Xenopus.

The mouse muscle creatine kinase promoter faithfully drives reporter gene expression in transgenicXenopus laevis

Developing Xenopus laevis experience two periods of muscle differentiation, once during embryogenesis and again at metamorphosis. During metamorphosis, thyroid hormone induces both muscle growth in the limbs and muscle death in the tail. In mammals, the muscle creatine kinase (MCK) gene is activated during the differentiation from myoblasts to myocytes and has served as both a marker for muscle development and to drive transgene expression in transgenic mice. Transcriptional control elements are generally highly conserved throughout evolution, potentially allowing mouse promoter use in transgenic X. laevis. This paper compares endogenous X. laevis MCK gene expression and the mouse MCK (mMCK) promoter driving a green fluorescent protein reporter in transgenic X. laevis. The mMCK promoter demonstrated strong skeletal muscle-specific transgene expression in both the juvenile tadpole and adult frog. Therefore, our results clearly demonstrate the functional conservation of regulatory sequences in vertebrate muscle gene promoters and illustrate the utility of using X. laevis transgenesis for detailed comparative study of mammalian promoter activity in vivo.

Zebrafish intestinal fatty acid binding protein (I-FABP) gene promoter drives gut-specific expression in stable transgenic fish

Mammalian intestinal fatty acid-binding protein (I-FABP) is a small cytosolic protein and is thought to play a crucial role of intracellular fatty acid trafficking and metabolism in gut. To establish an in vivo system for investigating its tissue-specific regulation during zebrafish intestinal development, we isolated 5'-flanking sequences of the zebrafish L-FABP gene and used a transgenic strategy to generate gut-specific transgenic zebrafish with green/red fluorescent intestine. The 4.5-kb 5'-flanking sequence of zebrafish I-FABP gene was sufficient to direct fluorescent expression in intestinal tube, first observed in 3 dpf embryos and then continuously to the adult stage. This pattern of transgenic expression is consistent with the expression pattern of the endogenous gene. In all five transgenic lines 45-52% of the F2 inheritance rates were consistent with the ratio of Mendelian segregation. These fish can also provide a valuable resource of labeled adult intestinal cells for in vivo or in vitro studies. Finally, it is possible to establish an in vivo system using these fish for screening genes required for gut development. genesis 38:26-31, 2004.

Intron disruption of the annexin IV gene reveals novel transcripts

Annexin IV (AIV), a Ca2+-dependent membrane-binding protein, is expressed in many epithelia. Annexin IV modifies membrane bilayers by increasing rigidity, reducing water and H+ permeability, promoting vesicle aggregation, and regulating ion conductances, all in a Ca2+-dependent manner. We have characterized a mouse in which a gene trap has been inserted into the first intron of annexin IV. Processing of the primary transcript is disrupted. Northern blot and immunoblot data indicated that annexin IV expression was eliminated in many but not all tissues. Immunohistochemical analysis, however, demonstrated that annexin IV expression was eliminated in some cell types, but was unaltered in others. 5'-Rapid amplification of cDNA ends analysis of intestinal and kidney RNA revealed three transcripts, AIVa, AIVb, and AIVc. AIVa is widely distributed. AIVb is expressed only in the digestive tract. AIVc expression is very restricted. A selected number of epithelial cells of unique morphology demonstrate high concentrations. All three transcripts produce an identical annexin IV protein. The different tissue and cell-specific expression profiles of the three transcripts suggest that regulation of both the annexin IV gene expression and the cellular role of the protein are complex. The AIVa-/- mouse may become a valuable model to further study transcription and the physiological role of annexin IV.

Comparative functional analysis of rat TGF-beta1 and Xenopus laevis TGF-beta5 promoters suggest differential regulations

We have carried out a comparative functional analysis of the rat TGF-beta1 and Xenopus laevis TGF-beta5 promoters across several mammalian and amphibian cell lines. Progressive deletion constructs of both the promoters have been made using a PCR based approach and the basal promoter activities studied in Xenopus tadpole cell line (XTC), Xenopus adult kidney fibroblast cell line (A6), human hepatoma cell line (HepG2), normal rat kidney cell line (NRK), and Chinese hamster ovary cell line (CHO). Data suggests that the basal promoter activity of TGF-beta1 is low as compared to TGF-beta5 promoter in XTC cells but comparable in A6 cells, while TGF-beta5 promoter shows nearly negligible activity as compared to TGF-beta5 promoter in all the tested mammalian cell lines. Moreover, TGF-beta5 promoter is found to be repressed in XTC cells on treatment with TGF-beta5 protein. Thus, the regulation of TGF-beta1 and TGF-beta5 promoters is distinct in amphibian and mammalian species. We therefore suggest that contrary to the suggested functional equivalence of TGF-beta1 and TGF-beta5 proteins, TGF-beta1 and TGF-beta5 genes have distinct functions in their respective species.

Metal ion-responsive transgenic Xenopus laevis as an environmental monitoring animal

We generated germ line-transgenic Xenopus laevis that monitors environmental heavy metal ions. Sperm nuclei were transduced with cDNA of enhanced green fluorescent protein (EGFP) driven by murine metallothionein-1 gene promoters and were microinjected into unfertilized eggs. The eggs developed to sexually matured adults. The transgenic tadpoles at the premetamorphic stage were reared in water containing Zn(2+) and Cd(2+) separately at the concentrations of 0.38-1.52 and 0.09-0.44 μM, respectively. These animals responded to Zn(2+) at as low as 0.38 μM and Cd(2+) at as low as 0.44 μM. The level of EGFP fluorescence emitted by tadpoles increased as the concentration increased up to 1.52 μM and the exposure time prolonged up to 120 h. The fluorescent response was much more sensitive to Cd(2+) than to Zn(2+). We concluded that these transgenic tadpoles are useful as an animal indicator of environmental heavy metal ions.

Experimental conversion of liver to pancreas

BACKGROUND

The liver and the pancreas arise from adjacent regions of endoderm in embryonic development. Pdx1 is a key transcription factor that is essential for the development of the pancreas and is not expressed in the liver. The aim of this study was to determine whether a gene overexpression protocol based on Pdx1 would be able to cause conversion of liver to pancreas.

RESULTS

We show that a modified form of Pdx1, carrying the VP16 transcriptional activation domain, can cause conversion of liver to pancreas, both in vivo and in vitro. Transgenic Xenopus tadpoles carrying the construct TTR-Xlhbox8-VP16:Elas-GFP were prepared. Xlhbox8 is the Xenopus homolog of Pdx1, the TTR (transthyretin) promoter directs expression to the liver, and the GFP is under the control of an elastase promoter and provides a real-time visible marker of pancreatic differentiation. In the transgenic tadpoles, part or all of the liver is converted to pancreas, containing both exocrine and endocrine cells, while liver differentiation products are lost from the regions converted to pancreas. The timing of events is such that the liver is differentiating by the time Xlhbox8-VP16 is expressed, so we consider this a transdifferentiation event rather than a reprogramming of embryonic development. Furthermore, this same construct will bring about transdifferentiation of human hepatocytes in culture, with formation of both exocrine and endocrine cells.

CONCLUSIONS

We consider that the conversion of liver to pancreas could be the basis of a new type of therapy for insulin-dependent diabetes. Although expression of the transgene is transient, once the ectopic pancreas is established, it persists thereafter.

Expression of FTZ-F1alpha in transgenic Xenopus embryos and oocytes

Fushi tarazu transcription factor-1 (FTZ-F1) was originally found as a regulator of fushi tarazu gene expression in Drosophila. The frog homologue (FTZ-F1alpha) and the 3.5 kb 5'-flanking region of the FTZ-F1alpha gene have been cloned, and it has been shown by reverse transcription-polymerase chain reaction that FTZ-F1alpha expression begins in embryos at stage 11 and becomes stronger after that. By in situ hybridization analysis, the FTZ-F1alpha mRNA was also found in immature frog oocytes. In this study, immunohistology revealed that the product of FTZ-F1alpha was localized in the cytoplasm of the immature oocyte. To analyze the promoter activity of the Rana rugosa FTZ-F1alpha gene, transgenic Xenopus were produced carrying the fusion construct, consisting of truncated 5'-flanking regions (3.0, 1.8 and 0.3 kb) of the FTZ-F1alpha gene and the green fluorescent protein (GFP) open reading frame. The 0.3 kb 5'-flanking region could drive GFP expression in Xenopus embryos at stage 20 and in immature oocytes in the ovary 2 months after metamorphosis. Gel mobility shift assay was used to test whether proteins in extracts from Xenopus embryos and ovaries bound to the 0.3 kb DNA. The extract from embryos at stage 11 formed one retarded band. The extract from ovaries formed a different retarded band. The results, taken together, indicate that production of transgenic Xenopus is very useful for the analysis of the promoter activity of genes in amphibians. The results also suggest that at least two proteins (one in the embryo and the other in the ovary of 2-month-old postmetamorphosing Xenopus) bind the 0.3 kb 5'-flanking region of the FTZ-F1alpha gene. These proteins may be involved in the regulation of FTZ-F1alpha gene expression in amphibians.

Expression of amylase and other pancreatic genes in Xenopus

To better understand the relationship between the endocrine and exocrine cell types in the Xenopus pancreas, we have cloned the Xenopus amylase cDNA and compared its expression profile with that of four other pancreatic markers: insulin, glucagon, elastase and trypsinogen. Our results demonstrate that the first pancreatic marker to be expressed is insulin, exclusively in the dorsal pancreas. These insulin-expressing cells form small groups which resemble islets, but no insulin is detected in the ventral pancreas until stage 47. In contrast, the exocrine markers, amylase, elastase and trypsinogen are first expressed only in the ventral pancreas beginning at stage 41; by stage 45 their expression extends into the dorsal pancreas. Glucagon, on the other hand, is not expressed in the pancreas until stage 45. In the endocrine cell clusters we do not find glucagon-expressing cells surrounding insulin-expressing cells, either in the tadpole or in the mature frog pancreas.

Endoderm specification and differentiation in Xenopus embryos

It is known from work with amniote embryos that regional specification of the gut requires cell-cell signalling between the mesoderm and the endoderm. In recent years, much of the interest in Xenopus endoderm development has focused on events that occur before gastrulation and this work has led to a different model whereby regional specification of the endoderm is autonomous. In this paper, we examine the specification and differentiation of the endoderm in Xenopus using neurula and tail-bud-stage embryos and we show that the current hypothesis of stable autonomous regional specification is not correct. When the endoderm is isolated alone from neurula and tail bud stages, it remains fully viable but will not express markers of regional specification or differentiation. If mesoderm is present, regional markers are expressed. If recombinations are made between mesoderm and endoderm, then the endodermal markers expressed have the regional character of the mesoderm. Previous results with vegetal explants had shown that endodermal differentiation occurs cell-autonomously, in the absence of mesoderm. We have repeated these experiments and have found that the explants do in fact show some expression of mesoderm markers associated with lateral plate derivatives. We believe that the formation of mesoderm cells by the vegetal explants accounts for the apparent autonomous development of the endoderm. Since the fate map of the Xenopus gut shows that the mesoderm and endoderm of each level do not come together until tail bud stages, we conclude that stable regional specification of the endoderm must occur quite late, and as a result of inductive signals from the mesoderm.

An amphibian with ambition: a new role for Xenopus in the 21st century

Much of our knowledge about the mechanisms of vertebrate early development comes from studies using Xenopus laevis. The recent development of a remarkably efficient method for generating transgenic embryos is now allowing study of late development and organogenesis in Xenopus embryos. Possibilities are also emerging for genomic studies using the closely related diploid frog Xenopus tropicalis.

Molecular basis of transdifferentiation of pancreas to liver

The appearance of hepatic foci in the pancreas has been described in animal experiments and in human pathology. Here we show that pancreatic cells can be converted into hepatocytes by treatment with a synthetic glucocorticoid, dexamethasone. This occurs both in a pancreatic cell line, AR42J-B13, and in organ cultures of pancreatic buds from mouse embryos. We have established several features of the mechanism behind this transdifferentiation. We show that a proportion of the hepatocytes arises directly from differentiated exocrine-like cells, with no intervening cell division. This conversion is associated with induction of the transcription factor C/EBPbeta and the activation of differentiated hepatic products. Transfection of C/EBPbeta into the cells can provoke transdifferentiation; conversely, a dominant-negative form of C/EBPbeta can inhibit the process. These results indicate that C/EBPbeta is a key component that distinguishes the liver and pancreatic programmes of differentiation.