FLP and Cre recombinase function in Xenopus embryos

5' Transgenes drive leaky expression of 3' transgenes in Cre-inducible bi-cistronic vectors

Molecular cloning techniques enabling contemporaneous expression of two or more protein-coding sequences provide an invaluable tool for understanding the molecular regulation of cellular functions. The Cre-lox system is used for inducing the expression of recombinant proteins encoded within a bi-/poly-cistronic cassette. However, leak expression of transgenes is often observed in the absence of Cre recombinase activity, compromising the utility of this approach. To investigate the mechanism of leak expression, we generated Cre-inducible bi-cistronic vectors to monitor the expression of transgenes positioned either 5' or 3' of a 2A peptide or internal ribosomal entry site (IRES) sequence. Cells transfected with these bi-cistronic vectors exhibited Cre-independent leak expression specifically of transgenes positioned 3' of the 2A peptide or IRES sequence. Similarly, AAV-FLEX vectors encoding bi-cistronic cassettes or fusion proteins revealed the selective Cre-independent leak expression of transgenes positioned at the 3' end of the open reading frame. Our data demonstrate that 5' transgenes confer promoter-like activity that drives the expression of 3' transgenes. An additional lox-STOP-lox cassette between the 2A sequence and 3' transgene dramatically decreased Cre-independent transgene expression. Our findings highlight the need for appropriate experimental controls when using Cre-inducible bi-/poly-cistronic constructs and inform improved design of vectors for more tightly regulated inducible transgene expression.

Bidirectional multiciliated cell extrusion is controlled by Notch-driven basal extrusion and Piezo1-driven apical extrusion

Xenopus embryos are covered with a complex epithelium containing numerous multiciliated cells (MCCs). During late-stage development, there is a dramatic remodeling of the epithelium that involves the complete loss of MCCs. Cell extrusion is a well-characterized process for driving cell loss while maintaining epithelial barrier function. Normal cell extrusion is typically unidirectional, whereas bidirectional extrusion is often associated with disease (e.g. cancer). We describe two distinct mechanisms for MCC extrusion, a basal extrusion driven by Notch signaling and an apical extrusion driven by Piezo1. Early in the process there is a strong bias towards basal extrusion, but as development continues there is a shift towards apical extrusion. Importantly, response to the Notch signal is age dependent and governed by the maintenance of the MCC transcriptional program such that extension of this program is protective against cell loss. In contrast, later apical extrusion is regulated by Piezo1, such that premature activation of Piezo1 leads to early extrusion while blocking Piezo1 leads to MCC maintenance. Distinct mechanisms for MCC loss underlie the importance of their removal during epithelial remodeling.

Bidirectional multiciliated cell extrusion is controlled by Notch driven basal extrusion and Piezo 1 driven apical extrusion

Xenopus embryos are covered with a complex epithelium containing numerous multiciliated cells (MCCs). During late stage development there is a dramatic remodeling of the epithelium that involves the complete loss of MCCs. Cell extrusion is a well-characterized process for driving cell loss while maintaining epithelial barrier function. Normal cell extrusion is typically unidirectional whereas bidirectional extrusion is often associated with disease (e.g. cancer). We describe two distinct mechanisms for MCC extrusion, a basal extrusion driven by Notch signaling and an apical extrusion driven by Piezo1. Early in the process there is a strong bias towards basal extrusion, but as development continues there is a shift towards apical extrusion. Importantly, receptivity to the Notch signal is age-dependent and governed by the maintenance of the MCC transcriptional program such that extension of this program is protective against cell loss. In contrast, later apical extrusion is regulated by Piezo 1 such that premature activation of Piezo 1 leads to early extrusion while blocking Piezo 1 leads to MCC maintenance. Distinct mechansms for MCC loss underlie the importance of their removal during epithelial remodeling.

Summay Statement

Cell extrusion typically occurs unidirectionally. We have identified a single population of multiciliated cells that extrudes bidirectionally: Notch-driven basal extrusion and Piezo 1-mediated apical extrusion.

Inducible and tissue-specific cell labeling in Cre-ERT2 transgenic Xenopus lines

Investigating cell lineage requires genetic tools that label cells in a temporal and tissue‐specific manner. The bacteriophage‐derived Cre‐ER^T2/loxP system has been developed as a genetic tool for lineage tracing in many organisms. We recently reported a stable transgenic Xenopus line with a Cre‐ER^T2/loxP system driven by the mouse Prrx1 (mPrrx1) enhancer to trace limb fibroblasts during the regeneration process (Prrx1:CreER line). Here we describe the detailed technological development and characterization of such line. Transgenic lines carrying a CAG promoter‐driven Cre‐ER^T2/loxP system showed conditional labeling of muscle, epidermal, and interstitial cells in both the tadpole tail and the froglet leg upon 4‐hydroxytamoxifen (4OHT) treatment. We further improved the labeling efficiency in the Prrx1:CreER lines from 12.0% to 32.9% using the optimized 4OHT treatment regime. Careful histological examination showed that Prrx1:CreER lines also sparsely labeled cells in the brain, spinal cord, head dermis, and fibroblasts in the tail. This work provides the first demonstration of conditional, tissue‐specific cell labeling with the Cre‐ER^T2/loxP system in stable transgenic Xenopus lines.

Simple embryo injection of long single-stranded donor templates with the CRISPR/Cas9 system leads to homology-directed repair in Xenopus tropicalis and Xenopus laevis

We report model experiments in which simple microinjection of fertilized eggs has been used to effectively perform homology-directed repair (HDR)-mediated gene editing in the two Xenopus species used most frequently for research: X. tropicalis and X. laevis. We have used long single-stranded DNAs having phosphorothioate modifications as donor templates for HDR at targeted genomic sites using the Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated protein 9 (CRISPR/Cas9) system. First, X. tropicalis tyr mutant (i.e., albino) embryos were successfully rescued: partially pigmented tadpoles were seen in up to 35% of injected embryos, demonstrating the potential for efficient insertion of targeted point mutations. Second, in order to demonstrate the ability to tag genes with fluorescent proteins (FPs), we targeted the melanocyte-specific gene slc45a2.L of X. laevis to label it with the Superfolder green FP (sfGFP), seeing mosaic expression of sfGFP in melanophores in up to 20% of injected tadpoles. Tadpoles generated by these two approaches were raised to sexual maturity, and shown to successfully transmit HDR constructs through the germline with precise targeting and seamless recombination. F1 embryos showed rescue of the tyr mutation (X. tropicalis) and tagging in the appropriate pigment cell-specific manner of slc45a2.L with sfGFP (X. laevis).

The Cre/Lox System to Assess the Development of the Mouse Brain

Cre-mediated recombination has become a powerful tool to confine gene deletions (conditional knockouts) or overexpression of genes (conditional knockin/overexpression). By spatiotemporal restriction of genetic manipulations, major problems of classical knockouts such as embryonic lethality or pleiotropy can be circumvented. Furthermore, Cre-mediated recombination has broad applications in the analysis of the cellular behavior of subpopulations and cell types as well as for genetic fate mapping. This chapter gives an overview about applications for the Cre/LoxP system and their execution.

History of genome editing in yeast

For thousands of years humans have used the budding yeast Saccharomyces cerevisiae for the production of bread and alcohol; however, in the last 30-40 years our understanding of the yeast biology has dramatically increased, enabling us to modify its genome. Although S. cerevisiae has been the main focus of many research groups, other non-conventional yeasts have also been studied and exploited for biotechnological purposes. Our experiments and knowledge have evolved from recombination to high-throughput PCR-based transformations to highly accurate CRISPR methods in order to alter yeast traits for either research or industrial purposes. Since the release of the genome sequence of S. cerevisiae in 1996, the precise and targeted genome editing has increased significantly. In this 'Budding topic' we discuss the significant developments of genome editing in yeast, mainly focusing on Cre-loxP mediated recombination, delitto perfetto and CRISPR/Cas.

Efficient Genome Manipulation by Variants of Site-Specific Recombinases R and TD

Genome engineering benefits from the availability of DNA modifying enzymes that have different target specificities and have optimized performance in different cell types. This variety of site-specific enzymes can be used to develop complex genome engineering applications at multiple loci. Although eight yeast site-specific tyrosine recombinases are known, only Flp is actively used in genome engineering. To expand the pool of the yeast site-specific tyrosine recombinases capable of mediating genome manipulations in mammalian cells, we engineered and analyzed variants of two tyrosine recombinases: R and TD. The activity of the evolved variants, unlike the activity of the native R and TD recombinases, is suitable for genome engineering in Escherichia coli and mammalian cells. Unexpectedly, we found that R recombinase benefits from the shortening of its C-terminus. We also found that the activity of wild-type R can be modulated by its non-consensus "head" sequence but this modulation became not apparent in the evolved R variants. The engineered recombinase variants were found to be active in all recombination reactions tested: excision, integration, and dual recombinase-mediated cassette exchange. The analysis of the latter reaction catalyzed by the R/TD recombinase pair shows that the condition supporting the most efficient replacement reaction favors efficient TD-mediated integration reaction while favoring efficient R-mediated integration and deletion reactions.

The Cre/Lox system to assess the development of the mouse brain

Cre-mediated recombination has become a powerful tool to confine gene deletions (conditional knockouts) or overexpression of genes (conditional knockin/overexpression). By spatiotemporal restriction of genetic manipulations, major problems of classical knockouts such as embryonic lethality can be circumvented. Furthermore Cre-mediated recombination has broad applicability in the analysis of the cellular behavior of subpopulations and cell types as well as for genetic fate mapping. This chapter will give an overview about applications for the Cre/LoxP system and their execution.

Mutants of Cre recombinase with improved accuracy

Despite rapid advances in genome engineering technologies, inserting genes into precise locations in the human genome remains an outstanding problem. It has been suggested that site-specific recombinases can be adapted towards use as transgene delivery vectors. The specificity of recombinases can be altered either with directed evolution or via fusions to modular DNA-binding domains. Unfortunately, both wild-type and altered variants often have detectable activities at off-target sites. Here we use bacterial selections to identify mutations in the dimerization surface of Cre recombinase (R32V, R32M and 303GVSdup) that improve the accuracy of recombination. The mutants are functional in bacteria, in human cells and in vitro (except for 303GVSdup, which we did not purify), and have improved selectivity against both model off-target sites and the entire E. coli genome. We propose that destabilizing binding cooperativity may be a general strategy for improving the accuracy of dimeric DNA-binding proteins.

Heat-shock mediated overexpression of HNF1β mutations has differential effects on gene expression in the Xenopus pronephric kidney

The transcription factor HNF1B, encoded by the TCF2 gene, plays an important role in the organogenesis of vertebrates. In humans, heterozygous mutations of HNF1B are associated with several diseases, such as pancreatic β-cell dysfunction leading to maturity-onset diabetes of the young (MODY5), defective kidney development, disturbed liver function, pancreas atrophy, and malformations of the genital tract. The African claw frog Xenopus laevis is an excellent model to study the processes involved in embryogenesis and organogenesis, as it can be manipulated easily with a series of methods. In the present study, we overexpressed HNF1β mutants in the developing Xenopus embryo to assess their roles during organogenesis, particularly in the developing pronephric kidney. Towards this goal, we developed a heat-shock inducible binary Cre/loxP system with activator and effector strains. Heat-shock activation of the mutant HNF1B variants P328L329del and A263insGG resulted in malformations of various organs and the affected larvae developed large edemas. Defects in the pronephros were primarily confined to malformed proximal tubules. Furthermore, the expression of the proximal tubule marker genes tmem27 and slc3a1, both involved in amino acid transport, was affected. Both P328L329del and A263insGG downregulated expression of slc3a1. In addition, P328L329del reduced tmem27 expression while A263insGG overexpression decreased expression of the chloride channel clcnk and the transcription factor pax2. Overexpression of two mutant HNF1B derivatives resulted in distinct phenotypes reflected by either a reduction or an enlargement of pronephros size. The expression of selected pronephric marker genes was differentially affected upon overexpression of HNF1B mutations. Based on our findings, we postulate that HNF1B mutations influence gene regulation upon overexpression in specific and distinct manners. Furthermore, our study demonstrates that the newly established Cre/loxP system for Xenopus embryos is an attractive alternative to examine the gene regulatory potential of transcription factors in developing pronephric kidney as exemplified here for HNF1B.

[Progress in Cre/lox site-specific recombination system in higher eukaryotes]

Cre/lox system derived from P1 bacteriaphage can quickly and effectively achieve gene insertion, deletion, replacement, and inversion by means of site-specific recombination. As one of the most important tools for gene targeting at present, Cre/lox system has been widely used in Arabidopsis thaliana, Oryza sativa L., Mus musculus, Drosophila melanogaster, Danio rerio, and other higher eukaryotic organisms. This review roundly described the basic profile of Cre/lox system, and its application in higher eukaryotes. In addition, we also discussed the main problems and developmental trend of the Cre/lox system in this review, which can be a good reference for using Cre/lox system to realize the gene manipulations of the different high eukaryotic organisms.

Site-specific transgenesis in Xenopus

Transgenesis is an essential, powerful tool for investigating gene function and the activities of enhancers, promoters, and transcription factors in the chromatin environment. In Xenopus, current methods generate germ-line transgenics by random insertion, often resulting in mosaicism, position-dependent variations in expression, and lab-to-lab differences in efficiency. We have developed and tested a Xenopus FLP-FRT recombinase-mediated transgenesis (X-FRMT) method. We demonstrate transgenesis of Xenopus laevis by FLP-catalyzed recombination of donor plasmid cassettes into F(1) tadpoles with host cassette transgenes. X-FRMT provides a new method for generating transgenic Xenopus. Once Xenopus lines harboring single host cassettes are generated, X-FRMT should allow for the targeting of transgenes to well-characterized integration site(s), requiring no more special reagents or training than that already common to most Xenopus labs.

Nucleic-acid based gene therapeutics: delivery challenges and modular design of nonviral gene carriers and expression cassettes to overcome intracellular barriers for sustained targeted expression

The delivery of nucleic acid molecules into cells to alter physiological functions at the genetic level is a powerful approach to treat a wide range of inherited and acquired disorders. Biocompatible materials such as cationic polymers, lipids, and peptides are being explored as safer alternatives to viral gene carriers. However, the comparatively low efficiency of nonviral carriers currently hampers their translation into clinical settings. Controlling the size and stability of carrier/nucleic acid complexes is one of the primary hurdles as the physicochemical properties of the complexes can define the uptake pathways, which dictate intracellular routing, endosomal processing, and nucleocytoplasmic transport. In addition to nuclear import, subnuclear trafficking, posttranscriptional events, and immune responses can further limit transfection efficiency. Chemical moieties, reactive linkers or signal peptide have been conjugated to carriers to prevent aggregation, induce membrane destabilization and localize to subcellular compartments. Genetic elements can be inserted into the expression cassette to facilitate nuclear targeting, delimit expression to targeted tissue, and modulate transgene expression. The modular option afforded by both gene carriers and expression cassettes provides a two-tier multicomponent delivery system that can be optimized for targeted gene delivery in a variety of settings.

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.

Flp and Cre expressed from Flp-2A-Cre and Flp-IRES-Cre transcription units mediate the highest level of dual recombinase-mediated cassette exchange

Recombinase-mediated cassette exchange (RMCE) is a powerful tool for unidirectional integration of DNA fragments of interest into a pre-determined genome locale. In this report, we examined how the efficiency of dual RMCE catalyzed by Flp and Cre depends on the nature of transcription units that express the recombinases. The following recombinase transcription units were analyzed: (i) Flp and Cre genes expressed as individual transcription units located on different vectors, (ii) Flp and Cre genes expressed as individual transcription units located on the same vector, (iii) Flp and Cre genes expressed from a single promoter and separated by internal ribosome entry sequence and (iv) Flp and Cre coding sequences separated by the 2A peptide and expressed as a single gene. We found that the highest level of dual RMCE (35-45% of the transfected cells) can be achieved when Flp and Cre recombinases are expressed as Flp-2A-Cre and Flp-IRES-Cre transcription units. In contrast, the lowest level of dual RMCE (∼1% of the transfected cells) is achieved when Flp and Cre are expressed as individual transcription units. The analysis shows that it is the relative Flp-to-Cre ratio that critically affects the efficiency of dual RMCE. Our results will be helpful for maximizing the efficiency of dual RMCE aimed to engineer and re-engineer genomes.

Double conditional human embryonic kidney cell line based on FLP and ΦC31 mediated transgene integration

BACKGROUND

FLP recombinase mediated integration into a pre-integrated FRT site is routinely used to generate highly reproducible stable transgenic cell lines. In this study, we broaden the system of site specific integration by introducing ΦC31 integrase mediated integration into attP sites.

RESULTS

We generated a HEK293 host cell line with a single copy FRT as well as an attP site allowing site specific integration of two distinct transgenes. To achieve conditional control, we used the tetracycline and Shld1 inducible systems. By introducing fluorescent reporters we show that integration and induction of two transgenes are completely independent. We applied this new technique to investigate the effect of HNF4α on proliferation of HEK293 cells by introducing HNF4α into each integration site. We obtained in two independent cell lines highly reproducible results that prove the usefulness of this novel HEK-attP/FRT cell line.

CONCLUSIONS

In this study we have established and applied a HEK-attP/FRT cell line that allows site specific integration of two conditional transgenes using the FLP recombinase as well as the ΦC31 integrase.

FLPe functions in zebrafish embryos

To assay the efficiency of the FLP/FRT site-specific recombination system in Danio rerio, a construct consisting of a muscle-specific promoter driving EGFP flanked by FRT sites was developed. FLPe capped RNA was microinjected into transgenic single cell stage zebrafish embryos obtained by crossing hemizygous transgenic males with wild-type females. By 48 h post fertilization (hpf), the proportion of embryos displaying green fluorescence following FLPe RNA microinjection was significantly lower (7.7%; P < 0.001) than would be expected from a cross in the absence of the recombinase (50%). Embryos that retained fluorescence displayed marked mosaicism. Inheritance of the excised transgene in non-fluorescent, transgenic embryos was verified by PCR analysis and FLPe-mediated recombination was confirmed by DNA sequencing. Sperm derived from confirmed transgenic males in these experiments was used to fertilize wild-type eggs to determine whether germline excision of the transgene had occurred. Clutches sired by FLPe-microinjected males contained 0-4% fluorescent embryos. Transgenic males that were phenotypically wild-type produced no fluorescent progeny, demonstrating complete excision of the transgene from their germline. FLPe microinjected males that retained some fluorescent muscle expression produced a small proportion of fluorescent offspring, suggesting that in mosaic males not all germline cells had undergone FLPe-mediated transgene excision. Our results show that FLPe, which is derived from Saccharomyces cerevisiae, is an efficient recombinase in zebrafish maintained at 28.5°C.

Eye field specification in Xenopus laevis

Vertebrate eyes begin as a small patch of cells at the most anterior end of the early brain called the eye field. If these cells are removed from an amphibian embryo, the eyes do not form. If the eye field is transplanted to another location on the embryo or cultured in a dish, it forms eyes. These simple cut and paste experiments were performed at the beginning of the last century and helped to define the embryonic origin of the vertebrate eye. The genes necessary for eye field specification and eventual eye formation, by contrast, have only recently been identified. These genes and the molecular mechanisms regulating the initial formation of the Xenopus laevis eye field are the subjects of this review.

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.

Red fluorescent Xenopus laevis: a new tool for grafting analysis

BACKGROUND

Fluorescent proteins such as the green fluorescent protein (GFP) have widely been used in transgenic animals as reporter genes. Their use in transgenic Xenopus tadpoles is especially of interest, because large numbers of living animals can easily be screened. To track more than one event in the same animal, fluorescent markers that clearly differ in their emission spectrum are needed.

RESULTS

We established the transgenic Xenopus laevis strain tom3 that expresses ubiquitously red fluorescence from the tdTomato gene through all larval stages and in the adult animal. This new tool was applied to track transplanted blastemas obtained after tail amputation. The blastema can regenerate ectopic tails marked by red fluorescence in the host animal. Surprisingly, we also found contribution of the host animal to form the regenerate.

CONCLUSION

We have established a useful new tool to label grafts in Xenopus transplantation experiments.

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.

Transgenesis procedures in Xenopus

Stable integration of foreign DNA into the frog genome has been the purpose of several studies aimed at generating transgenic animals or producing mutations of endogenous genes. Inserting DNA into a host genome can be achieved in a number of ways. In Xenopus, different strategies have been developed which exhibit specific molecular and technical features. Although several of these technologies were also applied in various model organizms, the attributes of each method have rarely been experimentally compared. Investigators are thus confronted with a difficult choice to discriminate which method would be best suited for their applications. To gain better understanding, a transgenesis workshop was organized by the X-omics consortium. Three procedures were assessed side-by-side, and the results obtained are used to illustrate this review. In addition, a number of reagents and tools have been set up for the purpose of gene expression and functional gene analyses. This not only improves the status of Xenopus as a powerful model for developmental studies, but also renders it suitable for sophisticated genetic approaches. Twenty years after the first reported transgenic Xenopus, we review the state of the art of transgenic research, focusing on the new perspectives in performing genetic studies in this species.

Highly-restricted, cell-specific expression of the simian CMV-IE promoter in transgenic zebrafish with age and after heat shock

Promoters with high levels of ubiquitous expression are of significant utility in the production of transgenic animals and cell lines. One such promoter is derived from the human cytomegalovirus immediate early (CMV-IE) gene. We sought to ascertain if the simian CMV-IE promoter (sCMV), used extensively in non-mammalian vertebrate research, also directs intense, widespread expression when stably introduced into zebrafish. Analysis of sCMV-driven expression revealed a temporal and spatial pattern not predicted by studies using the hCMV promoter in other transgenic animals or by observations of early F0 embryos expressing injected sCMV-reporter plasmids. Unexpectedly, in transgenic fish produced by both integration of linearized plasmid or Tol2-mediated transgenesis, sCMV promoter expression was generally observed in a small population of cells in telencephalon and spinal cord between days 2 and 7, and was thereafter confined to discrete regions of CNS that included the olfactory bulb, retina, cerebellum, spinal cord, and lateral line. In skeletal muscle, intense transgene expression was not observed until well into adulthood (>2-3 months post-fertilization). One final unexpected characteristic of the sCMV promoter in stable transgenic fish was tissue-specific responsiveness of the promoter to heat shock at both embryonic and adult stages. These data suggest that, in the context of stable transgenesis, the simian CMV-IE gene promoter responds differently to intracellular regulatory forces than other characterized CMV promoters.

Tissue-specific transcription factor HNF4alpha inhibits cell proliferation and induces apoptosis in the pancreatic INS-1 beta-cell line

Hepatocyte nuclear factor 4alpha (HNF4alpha) is a tissue-specific transcription factor expressed in many cell types, including pancreatic beta-cells. Mutations in the HNF4alpha gene in humans give rise to maturity-onset diabetes of the young (MODY1) characterized by defective insulin secretion by beta-cells. To elucidate the mechanism underlying this disease, we introduced the splice form HNF4alpha2 or HNF4alpha8 into the rat beta-cell line INS-1. Upon tetracycline-induced expression, both HNF4alpha isoforms caused distinct changes in cell morphology and a massive loss of cell numbers that was correlated with reduced proliferation and induced apoptosis. This differential activity was reflected in oligonucleotide microarray analysis that identified more genes affected by HNF4alpha2 compared to HNF4alpha8, and suggests that both isoforms regulate largely the same set of genes, with HNF4alpha2 being a stronger transactivator. We verified the induction of selected transcripts by real-time RT-PCR, including KAI1 and AIF, both known to have apoptotic potential. By establishing cell lines with inducible expression of these target genes, we deduce that both factors are insufficient to induce apoptosis. We propose that the anti-proliferative and apoptotic properties of HNF4alpha may be an essential feature impaired in MODY1 and possibly also in type 2 diabetes.

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.

Use of a ROSA26:GFP transgenic line for long-term Xenopus fate-mapping studies

Widespread and persistent marker expression is a prerequisite for many transgenic applications, including chimeric transplantation studies. Although existing transgenic tools for the clawed frog, Xenopus laevis, offer a number of promoters that drive widespread expression during embryonic stages, obtaining transgene expression through metamorphosis and into differentiated adult tissues has been difficult to achieve with this species. Here we report the application of the murine ROSA26 promoter in Xenopus. GFP is expressed in every transgenic tissue and cell type examined at post-metamorphic stages. Furthermore, transgenic ROSA26:GFP frogs develop normally, with no apparent differences in growth or morphology relative to wild-type frogs. ROSA26 transgenes may be used as a reliable marker for embryonic fate-mapping of adult structures in Xenopus laevis. Utility of this transgenic line is illustrated by its use in a chimeric grafting study that demonstrates the derivation of the adult bony jaw from embryonic cranial neural crest.

Transgenic Xenopus laevis strain expressing cre recombinase in muscle cells

For reproducible analyses of gene function in Xenopus, the use of transgenic strains is a promising approach but has limitations when investigating factors interfering with development. Therefore, inducible systems are attractive alternatives, and a binary system based on recombinases is a most versatile approach. We have shown previously that Cre and FLP recombinases are active in Xenopus laevis and can induce a silent reporter gene in a corresponding reporter strain. Here, we describe the establishment of the transgenic Xenopus laevis strain A7 expressing Cre recombinase under the control of the muscle-specific cardiac actin promoter. Upon crossing to several distinct reporter strains, A7 is able to induce EYFP, DsRed2, or LacZ reporter genes in a muscle-specific manner. This first Cre-expressing strain allows conditional activation of any gene of interest in muscle cells and, thus, opens up the use of recombinases as a new experimental strategy in Xenopus.

Cre-mediated site-specific recombination in zebrafish embryos

Cre-mediated site-specific recombination has become an invaluable tool for manipulation of the murine genome. The ability to conditionally activate gene expression or to generate chromosomal alterations with this same tool would greatly enhance zebrafish genetics. This study demonstrates that the HSP70 promoter can be used to inducibly control expression of an enhanced green fluorescent protein (EGFP) -Cre fusion protein. The EGFP-Cre fusion protein is capable of promoting recombination between lox sites in injected plasmids or in stably inherited transgenes as early as 2 hr post-heat shock induction. Finally, the levels of Cre expression achieved in a transgenic fish line carrying the HSP70-EGFP-cre transgene are compatible with viability and both male and female transgenic fish are fertile subsequent to induction of EGFP-Cre expression. Hence, our data suggests that Cre-mediated recombination is a viable means of manipulating gene expression in zebrafish.

HNF4alpha reduces proliferation of kidney cells and affects genes deregulated in renal cell carcinoma

Hepatocyte nuclear factor 4alpha (HNF4alpha) is a tissue-specific transcription factor known to regulate a large number of genes in hepatocytes and pancreatic beta cells. Although HNF4alpha is highly expressed in some sections of the kidney, little is known about its role in this organ and about HNF4alpha-regulated genes in the kidney cells. The abundance and activity of HNF4alpha are frequently reduced in renal cell carcinoma (RCC) indicating some tumor suppressing function of HNF4alpha in renal cells. To determine the potential role of HNF4alpha in RCC, we used Flp recombinase-mediated gene integration to generate human embryonic kidney cells (HEK293) that conditionally express wild-type or mutated HNF4alpha. Expression of wild-type HNF4alpha but not of the mutants led to reduction of proliferation and alterations of cell morphology. These effects were reversible and induced at physiological concentrations of HNF4alpha. Using gene expression profiling by microarrays, we determined genes regulated by HNF4alpha. Interestingly, many of the genes regulated by HNF4alpha have been shown to be deregulated in RCC microarray studies. These genes (ACY1, WT1, SELENBP1, COBL, EFHD1, AGXT2L1, ALDH5A1, THEM2, ABCB1, FLJ14146, CSPG2, TRIM9 and HEY1) are good candidates for genes whose activity is changed upon the decrease of HNF4alpha in RCC.

Identification of target genes of the transcription factor HNF1beta and HNF1alpha in a human embryonic kidney cell line

Hepatocyte nuclear factor 1beta (HNF1beta, TCF2) is a tissue-specific transcription factor whose mutation in humans leads to renal cysts, genital malformations, pancreas atrophy and maturity onset diabetes of the young (MODY5). Furthermore, HNF1beta overexpression has been observed in clear cell cancer of the ovary. To identify potential HNF1beta target genes whose activity may be deregulated in human patients, we established a human embryonic kidney cell line (HEK293) expressing HNF1beta conditionally. Using Flp recombinase, we introduced wild type or mutated HNF1beta at a defined chromosomal position allowing a most reproducible induction of the HNF1beta derivatives upon tetracycline addition. By oligonucleotide microarrays we identified 25 HNF1beta-regulated genes. By an identical approach, we identified that the related transcription factor HNF1alpha (TCF1) affects only nine genes in HEK293 cells and thus is a less efficient factor in these kidney cells. The HNF1beta target genes dipeptidyl peptidase 4 (DPP4), angiotensin converting enzyme 2 (ACE2) and osteopontin (SPP1) are most likely direct target genes, as they contain functional HNF1 binding sites in their promoter region. Since nine of the potential HNF1beta target genes are deregulated in clear cell carcinoma of the ovary, we propose that HNF1beta overexpression in the ovarian cancer participates in the altered expression pattern.

Cre/lox-regulated transgenic zebrafish model with conditional myc-induced T cell acute lymphoblastic leukemia

We have created a stable transgenic rag2-EGFP-mMyc zebrafish line that develops GFP-labeled T cell acute lymphoblastic leukemia (T-ALL), allowing visualization of the onset and spread of this disease. Here, we show that leukemias from this transgenic line are highly penetrant and render animals moribund by 80.7 +/- 17.6 days of life (+/-1 SD, range = 50-158 days). These T cell leukemias are clonally aneuploid, can be transplanted into irradiated recipient fish, and express the zebrafish orthologues of the human T-ALL oncogenes tal1/scl and lmo2, thus providing an animal model for the most prevalent molecular subgroup of human T-ALL. Because T-ALL develops very rapidly in rag2-EGFP-mMyc transgenic fish (in which "mMyc" represents mouse c-Myc), this line can only be maintained by in vitro fertilization. Thus, we have created a conditional transgene in which the EGFP-mMyc oncogene is preceded by a loxed dsRED2 gene and have generated stable rag2-loxP-dsRED2-loxP-EGFP-mMyc transgenic zebrafish lines, which have red fluorescent thymocytes and do not develop leukemia. Transgenic progeny from one of these lines can be induced to develop T-ALL by injecting Cre RNA into one-cell-stage embryos, demonstrating the utility of the Cre/lox system in the zebrafish and providing an essential step in preparing this model for chemical and genetic screens designed to identify modifiers of Myc-induced T-ALL.

Messenger RNA electroporation is highly efficient in mouse embryonic stem cells: successful FLPe- and Cre-mediated recombination

Development of efficient short-term gene transfer technologies for embryonic stem (ES) cells is urgently needed for various existing and new ES cell-based research strategies. In this study, we present a highly efficient, nonviral non-DNA technology for genetic loading of mouse ES cells based on electroporation of defined mRNA. Here, we show that mouse ES cells can be efficiently loaded with mRNA encoding a green fluorescent reporter protein, resulting in a level of at least 90% of transgene expression without loss of cell viability and phenotype. To show that transgenes, introduced by mRNA electroporation, exert a specific cellular function in transfected cells, we electroporated stably transfected ES cell lines with mRNA encoding FLPe or Cre recombinase proteins in order to excise an FRT- or LoxP-flanked reporter gene. The results, as determined by the disappearance and/or appearance of a fluorescent reporter gene expression, show that FLPe and Cre recombinase proteins, introduced by mRNA electroporation, efficiently exert their function without influence on further culture of undifferentiated ES cell populations and their ability to differentiate towards a specific lineage.

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.

Genetic manipulation of circadian rhythms in Xenopus

Xenopus laevis retina is an important experimental model system for the study of circadian oscillator mechanisms, as light input pathways, central oscillator mechanisms, and multiple output pathways are all contained within this tissue. These retinas continue to exhibit robust circadian rhythms even after being maintained in culture for many days. The usefulness of this system has been improved even further by the development of a technique for simple genetic manipulation of these animals, which is complemented by expanded genomics resources (Xenopus genome project, microarray, etc.). By taking advantage of the transgenic technique in Xenopus described in this article, many types of analysis can be done on the primary transgenic animals within a couple of weeks after transgenesis. The availability of many cell-type-specific promoters and well-characterized cell types within the Xenopus retina provides the advantage of cell-specific modification of clock function using this method; in other words, contributions of different cell types within the circadian system can be analyzed independently by "molecular dissociation" of these cells. This article describes both how this transgenic technique is useful and various considerations that should be taken into account when these types of experiments are planned and interpreted. Application of these new techniques to studies of clock function provide an opportunity to rapidly assess gene expression and?or function in the context of the intact retina.

Pattern of genes influenced by conditional expression of the transcription factors HNF6, HNF4alpha and HNF1beta in a pancreatic beta-cell line

Using the rat insulinoma cell line INS-1 we generated beta-cell clones that are most efficient for gene transfer, as they contain an FRT site for Flp recombinase-mediated, site-directed integration of a single copy transgene. Therefore, the gene-of-interest can be introduced by DNA transfection without the need to select individual cell clones. Additionally, the clones contain the tetracycline repressor allowing tetracycline induction of the transgene. By oligonucleotide microarray we define the beta-cell specific phenotype of the Flp-In T-REx cell clones. Using a clone expressing the HNF6, HNF4alpha and HNF1beta transcription factors at a limited level, we introduced the expression vectors encoding these factors. We show efficient tetracycline induction of these transcription factors by western blots and immunocytochemistry. Microarrays reveal that these three factors affect a similar number of genes with only few genes regulated in common. Statistical analysis reveals that the three transcription factors affect genes categorized to different biological processes. Furthermore, we document the usefulness of these Flp-In T-REx cells for the functional analysis of mutated HNF1beta transcription factors found in human MODY5 patients. We show that the expression of the mutant P328L329del and A263insGG affects only very few transcripts and these are predominantly distinct from those induced by wild-type HNF1beta.

Tagging muscle cell lineages in development and tail regeneration using Cre recombinase in transgenic Xenopus

The use of Cre and FLP recombinases to analyze embryogenesis and organogenesis in Xenopus has not been applied so far. We report on the generation of transgenic Xenopus animals containing a Cre-activated reporter gene cassette expressing blue fluorescent protein that can be switched over to yellow fluorescent protein expression upon Cre-mediated recombination. By injecting Cre mRNA into the two-cell stage embryo we show that Cre-mediated activation of the yellow fluorescent protein gene occurs. In addition, we observe upon injection an extinction of blue fluorescence in animals expressing the transgene and the induction of blue fluorescence in larvae containing a silent reporter gene. By crossing the reporter strains with animals expressing a muscle-specific Cre transgene we obtained an efficient and specific recombination of the reporter gene that leads to yellow fluorescence in myotomes and myofibrils of the developing larvae. Removal of the tail tips of these larvae allows the continuous recording of muscle cell differentiation in the regenerating tail. We detect a dramatic increase in transgene expression at the site of tissue removal in the tail stump. In the regenerated tail, yellow fluorescence is restricted to the myotomes thus excluding transdifferentiation of muscle cells.

Xenopus, the next generation: X. tropicalis genetics and genomics

A small, fast-breeding, diploid relative of the frog Xenopus laevis, Xenopus tropicalis, has recently been adopted for research in developmental genetics and functional genomics. X. tropicalis shares advantages of X. laevis as a classic embryologic system, but its simpler genome and shorter generation time make it more convenient for multigenerational genetic, genomic, and transgenic approaches. Its embryos closely resemble those of X. laevis, except for their smaller size, and assays and molecular probes developed in X. laevis can be readily adapted for use in X. tropicalis. Genomic manipulation techniques such as gynogenesis facilitate genetic screens, because they permit the identification of recessive phenotypes after only one generation. Stable transgenic lines can be used both as in vivo reporters to streamline a variety of embryologic and molecular assays, or to experimentally manipulate gene expression through the use of binary constructs such as the GAL4/UAS system. Several mutations have been identified in wild-caught animals and during the course of generating inbred lines. A variety of strategies are discussed for conducting and managing genetic screens, obtaining mutations in specific sequences, achieving homologous recombination, and in developing and taking advantage of the genomic resources for Xenopus tropicalis.

Construction of a mariner-based transposon for epitope-tagging and genomic targeting

Mobile genetic elements are often employed for constructing gene fusions or to perform mutagenesis. mariner transposons are well-suited to such applications because of their low site specificity, in vitro activity, and exceptionally broad host range. This report describes a mariner-based method for rapidly creating a large number of insertion mutants that can be converted to in-frame epitope fusions in a single step. First, a mariner-based vector is used to deliver a FLP recombinase substrate randomly into a target molecule. Expression of the FLP recombinase is then induced to catalyse the excision of sequences flanked by FLP recombinase target recognition sites, leaving behind a triple-FLAG epitope. The reversibility of the excision event provides opportunities for using genomic targeting methods easily to create transcriptional or translational fusions to genes of interest.

Genetic manipulation of mouse embryonic stem cells by mutant lambda integrase

Mutant lambda integrases catalyze site-specific recombination reactions inside mammalian cells. Here we demonstrate that the integrase system can be used to eliminate resistance marker genes from the genome of mouse embryonic stem cells. So-called integrative and excisive recombination pathways led to the precise deletion of the neomycin gene, which was inserted together with a flanking pair of directly repeated recombination sites into the ROSA26 locus by standard targeting techniques. The excision of the resistance gene led to the expression of enhanced green fluorescence protein, which served as a means to sort out cells that had undergone site-specific recombination. Southern analysis and DNA sequencing confirmed that strand exchange reactions had occurred in the genome as expected. Hence, the integrase system may be used in conjunction with other site-specific recombinases as a tool in genome manipulation protocols.

HST-1/FGF-4 gene activation induces spermatogenesis and prevents adriamycin-induced testicular toxicity

We previously demonstrated expression of the HST-1/FGF-4 gene in the testis of normal adult animals, which suggests its possible role in spermatogenesis. For an understanding of its functional significance in the testis, conditional transgene expression was used. Precise genetic switches can be efficiently generated in a straightforward manner using adenovirus-carrying Cre recombinase, which means our new strategies promise to contribute substantially to a better and prompt understanding of the functions of genes in vivo by controlling the expression of any gene to any organ at any desired time. Our new method demonstrated for the first time that the specific gain of function of the HST-1/FGF-4 gene in the testis resulted in markedly enhanced spermatogenesis. To further investigate the function and therapeutic potency of HST-1/FGF-4, transgenic mice with enhanced HST-1/FGF-4 expression in the testis were exposed to adriamycin (ADR), an anticancer drug causing severe testicular toxicity. Degree of damage to spermatogenesis was assessed by sperm count, testicular weight, histology, and DNA ploidy. Induced expression of HST-1/FGF-4 markedly enhanced the recovery of ADR-induced testicular damage. Furthermore, adenoviruses carrying the HST-1/FGF-4 gene ameliorated testicular toxicity of ADR. These results with new adenovirus-mediated Cre/lox conditional mice indicated that HST-1/FGF-4 could be an important factor for spermatogenesis, presenting a new paradigm to treat impaired fertility.

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.