Xl-fli, the Xenopus homologue of the fli-1 gene, is expressed during embryogenesis in a restricted pattern evocative of neural crest cell distribution

Hypomorphic zebrafish models mimic the musculoskeletal phenotype of β4GalT7-deficient Ehlers-Danlos syndrome

β4GalT7 is a transmembrane Golgi enzyme, encoded by B4GALT7, that plays a pivotal role in the proteoglycan linker region formation during proteoglycan biosynthesis. Defects in this enzyme give rise to a rare autosomal recessive form of Ehlers-Danlos syndrome (EDS), currently known as 'spondylodysplastic EDS (spEDS-B4GALT7)'. This EDS subtype is mainly characterized by short stature, hypotonia and skeletal abnormalities, thereby illustrating its pleiotropic importance during human development. Insights into the pathogenic mechanisms underlying this disabling disease are very limited, in part due to the lack of a relevant in vivo model. As the majority of mutations identified in patients with spEDS-B4GALT7 are hypomorphic, we generated zebrafish models with partial loss of B4galt7 function, including different knockdown (morphant) and mosaic knockout (crispant) b4galt7 zebrafish models and studied the morphologic, functional and molecular aspects in embryonic and larval stages. Morphant and crispant zebrafish show highly similar morphological abnormalities in early development including a small, round head, bowed pectoral fins, short body-axis and mild developmental delay. Several craniofacial cartilage and bone structures are absent or strongly misshapen. In addition, the total amount of sulfated glycosaminoglycans is significantly diminished and particularly heparan and chondroitin sulfate proteoglycan levels are greatly reduced. We also show impaired cartilage patterning and loss of chondrocyte organization in a cartilage-specific Tg(Col2a1aBAC:mcherry) zebrafish reporter line. The occurrence of the same abnormalities in the different models confirms these are specifically caused by B4galt7 deficiency. A disturbed actin pattern, along with a lack of muscle tone, was only noted in morphants in which translation of b4galt7 was blocked. In conclusion, we generated the first viable animal models for spEDS-B4GALT7, and show that in early development the human spEDS-B4GALT7 phenotype is faithfully mimicked in these zebrafish models. Our findings underscore a key role for β4GalT7 in early development of cartilage, bone and muscle. These models will lead to a better understanding of spEDS-B4GALT7 and can be used in future efforts focusing on therapeutic applications.

The ets transcription factor Fli-1 in development, cancer and disease

Friend Leukemia Virus Induced erythroleukemia-1 (Fli-1), an ETS transcription factor, was isolated a quarter century ago through a retrovirus mutagenesis screen. Fli-1 has since been recognized to play critical roles in normal development and homeostasis. For example, it transcriptionally regulates genes that drive normal hematopoiesis and vasculogenesis. Indeed, Fli-1 is one of 10 key regulators of hematopoietic stem/progenitor cell maintenance and differentiation. Aberrant expression of Fli-1 also underlies a number of virally induced leukemias, including Friend virus-induced erythroleukemia and various types of human cancers, and it is the target of chromosomal translocations in childhood Ewing’s sarcoma. Abnormal expression of Fli-1 is important in the aetiology of auto-immune diseases such as Systemic Lupus Erythematosus (SLE) and Systemic Sclerosis (SSc). These studies establish Fli-1 as a strong candidate for drug development. Despite difficulties in targeting transcription factors, recent studies identified small molecule inhibitors for Fli-1. Here we review past and ongoing research on Fli-1 with emphasis on its mechanistic function in autoimmune disease and malignant transformation. The significance of identifying Fli-1 inhibitors and their clinical applications for treatment of disease and cancer with deregulated Fli-1 expression are discussed.

Regulation of endothelial cell development by ETS transcription factors

The ETS family of transcription factors plays an essential role in controlling endothelial gene expression. Multiple members of the ETS family are expressed in the developing endothelium and evidence suggests that the proteins function, to some extent, redundantly. However, recent studies have demonstrated a crucial non-redundant role for ETV2, as a primary player in specification and differentiation of the endothelial lineage. Here, we review the contribution of ETS factors, and their partner proteins, to the regulation of embryonic vascular development.

Progress in the molecular biology of ewing tumors

Purpose/results/discussion. Rearrangement of the EWS gene with an ETS oncogene by chromosomal translocation is a hallmark of the Ewing family of tumors (EFT). Detectability, incidence, tumor specificity and variability of this aberration have been matters of intense investigation in recent years. A number of related alterations have also been found in other malignancies. The common consequence of these gene rearrangements is the generation of an aberrant transcription factor. In EFT, the ETS partner is responsible for target recognition. However, synergistic and possibly tissue-restricted transcription factors interacting with either the EWS or the ETS portion may influence target selection. Minimal domains of both fusion partners were defined that have proved necessary for the in vitro transformation of murine fibroblasts. These functional studies suggest a role for aberrant transcriptional regulation of transforming target genes by the chimeric transcription factors. Also, fusion of the two unrelated protein domains may affect overall protein conformation and consequently DNA binding specificity. Recent evidence suggests that EWS, when fused to a transcription factor, interacts with different partners than germ-line EWS. Variability in EWS–ETS gene fusions has recently been demonstrated to correlate with clinical outcome. This finding may reflect functional differences of the individual chimeric transcription factors. Alternatively, type and availability of specific recombinases at different time-points of stem cell development or in different stem cell lineages may determine fusion type. Studies on EFT cell lines using EWS–ETS antagonists do suggest a rate-limiting essential role for the gene rearrangement in the self-renewal capacity of EFT cells. The presence of additional aberrations varying in number and type that may account for immortalization and full transformation is postulated. Knowledge about such secondary alterations may provide valuable prognostic markers that could be used for treatment stratification.

FAF1, a gene that is disrupted in cleft palate and has conserved function in zebrafish

Cranial neural crest (CNC) is a multipotent migratory cell population that gives rise to most of the craniofacial bones. An intricate network mediates CNC formation, epithelial-mesenchymal transition, migration along distinct paths, and differentiation. Errors in these processes lead to craniofacial abnormalities, including cleft lip and palate. Clefts are the most common congenital craniofacial defects. Patients have complications with feeding, speech, hearing, and dental and psychological development. Affected by both genetic predisposition and environmental factors, the complex etiology of clefts remains largely unknown. Here we show that Fas-associated factor-1 (FAF1) is disrupted and that its expression is decreased in a Pierre Robin family with an inherited translocation. Furthermore, the locus is strongly associated with cleft palate and shows an increased relative risk. Expression studies show that faf1 is highly expressed in zebrafish cartilages during embryogenesis. Knockdown of zebrafish faf1 leads to pharyngeal cartilage defects and jaw abnormality as a result of a failure of CNC to differentiate into and express cartilage-specific markers, such as sox9a and col2a1. Administration of faf1 mRNA rescues this phenotype. Our findings therefore identify FAF1 as a regulator of CNC differentiation and show that it predisposes humans to cleft palate and is necessary for lower jaw development in zebrafish.

Small molecule screen for compounds that affect vascular development in the zebrafish retina

Blood vessel formation in the vertebrate eye is a precisely regulated process. In the human retina, both an excess and a deficiency of blood vessels may lead to a loss of vision. To gain insight into the molecular basis of vessel formation in the vertebrate retina and to develop pharmacological means of manipulating this process in a living organism, we further characterized the embryonic zebrafish eye vasculature, and performed a small molecule screen for compounds that affect blood vessel morphogenesis. The screening of approximately 2000 compounds revealed four small molecules that at specific concentrations affect retinal vessel morphology but do not produce obvious changes in trunk vessels, or in the neuronal architecture of the retina. Of these, two induce a pronounced widening of vessel diameter without a substantial loss of vessel number, one compound produces a loss of retinal blood vessels accompanied by a mild increase of their diameter, and finally one other generates a severe loss of retinal vessels. This work demonstrates the utility of zebrafish as a screening tool for small molecules that affect eye vasculature and presents several compounds of potential therapeutic importance.

Kruppel-like factor 2 cooperates with the ETS family protein ERG to activate Flk1 expression during vascular development

The VEGF receptor, FLK1, is essential for differentiation of the endothelial lineage and for embryonic vascular development. Using comparative genomics, we have identified conserved ETS and Krüppel-like factor (KLF) binding sites within the Flk1 enhancer. In transgenic studies, mutation of either site results in dramatic reduction of Flk1 reporter expression. Overexpression of KLF2 or the ETS transcription factor ERG is sufficient to induce ectopic Flk1 expression in the Xenopus embryo. Inhibition of KLF2 function in the Xenopus embryo results in a dramatic reduction in Flk1 transcript levels. Furthermore, we show that KLF2 and ERG associate in a physical complex and that the two proteins synergistically activate transcription of Flk1. Since the ETS and KLF protein families have independently been recognized as important regulators of endothelial gene expression, cooperation between the two families has broad implications for gene regulation during development, normal physiology and vascular disease.

Induction of Cells Expressing Vascular Endothelium Markers from Undifferentiated Xenopus Presumptive Ectoderm by Co-treatment with Activin and Angiopoietin-2

Activin is a potent inducer of mesoderm in amphibian embryos. We previously reported that low concentrations of activin could induce the formation of blood cells from Xenopus explants (animal caps). Both hematopoietic and vascular endothelial cell lineages are believed to share a common precursor, termed hemangioblasts. In this study, we tried to induce differentiation of vascular endothelial cells in aggregates derived from Xenopus animal caps. Aggregates formed from cells that were co-treated with activin and angiopoietin-2 expressed the vascular endothelial markers, X-msr, Xtie2 and Xegfl7. However, none of these aggregates expressed the hematopoietic marker genes, globin alpha T3, alpha T5, alpha A or GATA-1. We used microarray analysis to compare the gene expression profiles of aggregates treated with activin alone or with activin and angiopoietin. The combination, but not activin alone, induced expression of vascular-related genes such as Xl-fli and VEGF. These results demonstrate that treatment of dissociated animal cap cells with activin and angiopoietin-2 can induce differentiation of endothelial cells, and provides a promising model system for the in vitro study of blood vessel induction in vertebrates.

The developmental expression of cell cycle regulators in Xenopus laevis

Several cyclins and cdks have been cloned in Xenopus, but their developmental expression has not been thoroughly examined. We have analyzed the temporal and spatial expression of cdk1, cdk2, cdk4 and cyclins D1, D2, E, A1, A2 and B1 by in situ hybridization. The transcripts of these cyclins and cdks exhibit striking tissue-restricted expression patterns very early in development that cannot be strictly correlated with proliferation. While the cdks and their activating cyclins are expressed in somewhat overlapping patterns, they are not precisely coincident. Additionally, maternal and zygotic cyclin forms demonstrate markedly different expression patterns.

In vivo imaging of embryonic vascular development using transgenic zebrafish

In this study we describe a model system that allows continuous in vivo observation of the vertebrate embryonic vasculature. We find that the zebrafish fli1 promoter is able to drive expression of enhanced green fluorescent protein (EGFP) in all blood vessels throughout embryogenesis. We demonstrate the utility of vascular-specific transgenic zebrafish in conjunction with time-lapse multiphoton laser scanning microscopy by directly observing angiogenesis within the brain of developing embryos. Our images reveal that blood vessels undergoing active angiogenic growth display extensive filopodial activity and pathfinding behavior similar to that of neuronal growth cones. We further show, using the zebrafish mindbomb mutant as an example, that the expression of EGFP within developing blood vessels permits detailed analysis of vascular defects associated with genetic mutations. Thus, these transgenic lines allow detailed analysis of both wild type and mutant embryonic vasculature and, together with the ability to perform large scale forward-genetic screens in zebrafish, will facilitate identification of new mutants affecting vascular development.

Expression and evolution studies of ets genes in a primitive coelomate, the polychaete annelid, Hediste (Nereis) diversicolor

The Ets family includes numerous proteins with a highly conserved DNA-binding domain of 85 amino acids named the ETS domain. Phylogenetic analyses from ETS domains revealed that this family could be divided into 13 groups, among them are ETS and ERG. The ets genes are present in the Metazoan kingdom and we have previously characterized the Nd ets and Nd erg genes in the polychaete annelid Hediste diversicolor. Here, we isolated a fragment encoding the ETS domain from Nd Ets, by genomic library screening. By Northern blot analysis, we showed that this gene was transcribed as one major mRNA of 2.6 kb and one minor mRNA of 3.2 kb. By in situ hybridization, we observed that Nd ets was expressed in the intestine and oocytes and that Nd erg was expressed in cellular clumps present in the coelomic cavity, in an area of proliferating cells situated between the last metamere and the pygidium. Finally, we showed that Nd erg shared the expression pattern of Nd ets in oocytes. Molecular modeling studies have revealed that the spatial structure of ETS domain of Nd Ets and Nd Erg was conserved, in comparison to the murine Ets-1 and human Fli-1 proteins, respectively.

High-dose chemoradiotherapy (HDC) in the Ewing family of tumors (EFT)

EFT is defined by the expression of ews/ets fusion genes. The type of the fusion transcript impacts on the clinical biology. EFT requires risk adapted treatment. A risk-adapted treatment is determined by tumor localisation, tumor stage and volume. For metastatic and relapsed disease the pattern of spread and the time of relapse are the determinants of risk stratification. Staging of Ewing tumors has been considerably improved by magnetic resonance imaging and modern isotope scanning techniques. However, the determination of the extent of the metastatic spread in particular number of involved bones remains an unresolved issue. The prognosis for high-risk Ewing tumors has been improved by multimodal and high-dose radio/chemotherapy (HDC). The concepts for high-dose therapy in Ewing tumors are based on dose response and dose intensity relationships. In single agent HDC most experience exists with Melphalan. Several chemotherapeutic agents have been used in combination HDC with or without TBI such as Adriamycin, BCNU, Busulphan, Carboplatin, Cyclophosphamide, Etoposide, Melphalan, Thiotepa Procarbazin and Vincristine. To date, superiority of any high-dose chemotherapy regimen has not been established. However, the clinical biology, the pattern of spread and the time of relapse determine the prognosis of patient who are eligible for HDC. In particular, patients with multifocal bone or bone marrow metastases have a poorer prognosis than patients with lung metastases. In addition, patients with a relapse within 24 months have a poorer prognosis than patients with a relapse later than 24 months after diagnosis. This review will analyze the results of single- and multi-agent chemotherapy with respect to agent combination, dose and risk stratum of patient population. Future therapeutic modalities for the treatment of EFT might encompass immunotherapeutic and genetic strategies including allogeneic stem cell transplantation.

Tissue-specific expression of the Ets gene Xsap-1 during Xenopus laevis development

We report the cloning of Xenopus laevis Xsap-1 cDNA, encoding a member of the ternary complex factor subfamily of ETS transcription factors. The expression pattern of Xsap-1 was examined during Xenopus embryogenesis using whole-mount in situ hybridization. Spatial expression of Xsap-1 mRNA is first detected at the animal pole at the mid-blastula stage. During neurulation Xsap-1 is expressed in cells participating in neural tube formation, in the sensorial layer of the epidermal ectoderm, and in an anterior region of the ventral mesoderm. Later, Xsap-1 expression is observed in the eye, ear vesicle, branchial arches, heart, pronephros, in the somites, and the developing nervous system, such as fore-, mid-, and hindbrain as well as in the cranial ganglion X.

Biology of EWS/ETS fusions in Ewing's family tumors

Tumor-associated chromosomal translocations lead to the formation of chimeric fusions between the EWS gene and one of five different ETS transcription factors in Ewing's family tumors (EFTs). The resultant EWS/ETS proteins promote oncogenesis in a dominant fashion in model systems and are necessary for continued growth of EFT cell lines. EWS belongs to a family of genes that encode proteins that may serve as adapters between the RNA polymerase II complex and RNA splicing factors. EWS/ETS fusions have biochemical characteristics of aberrant transcription factors and appear to promote abnormal cellular growth by transcriptionally modulating a network of target genes. Early evidence suggests that EWS/ETS proteins may also impact gene expression through alteration in RNA processing. Elucidation of EWS/ETS target gene networks in the context of other signaling pathways will hopefully lead to biology based therapeutic strategies for EFT.

The Ets-transcription factor family in embryonic development: lessons from the amphibian and bird

This chapter reviews the expression and role of Ets-genes during embryogenesis of amphibians and birds. In addition to overlapping expression domains, some of them exhibit cell type-specific expression. Many of them are expressed in migratory cells: neural crest, endothelial, and pronephric duct cells for instance. They are also transcribed in embryonic areas affected by epithelio-mesenchymal transitions. Both processes involve modifications of cellular adhesion. Ets-family genes appear to coordinate changes in the expression of adhesion molecules and degradation of the extracellular matrix upon regulation of matrix metalloproteinases and their specific inhibitors. These functions are essential for physiological processes like tissue remodelling during embryogenesis or wound healing. Unfortunately they also play a harmful role in metastasis. Recent studies in the nervous system showed that Ets-genes contribute to the establishment of a cellular identity. This identity could rely on definite cell-surface determinants, among which cadherins could play an important role. In addition to cell-type specific expression, other factors contribute to the specificity of function of Ets-genes. These genes have a broad specificity of recognition of target sequences in gene promoters, insufficient for accurate control of gene expression. A fine tuning could arise from combinatorial interactions with other Ets- or accessory proteins.

Genetic analysis of ETS genes in C. elegans

The recent completion of the Caenorhabditis elegans genome has revealed that this nematode worm has 10 members of the ETS gene family. Isolation and analysis of C. elegans mutants and subsequent screens to identify interacting genes can proceed very quickly in this model organism. Molecular genetic analysis of the receptor tyrosine kinase-Ras-MAP kinase signaling pathway in C. elegans identified the ETS family transcription factor Lin-1 as a nuclear effector of this evolutionarily conserved signal transduction pathway. Here we review classical genetic approaches used to discover the role of Lin-1 in the Ras-MAP kinase signaling pathway and describe new technologies that can be applied to the analyses of signaling pathways and transcription factor regulatory networks in C. elegans.

Heterotopic expression of the Xl-Fli transcription factor during Xenopus embryogenesis: modification of cell adhesion and engagement in the apoptotic pathway

In the Xenopus laevis embryo, the overexpression of the Xl-FLI protein, a transcription factor of the ETS family, provokes severe developmental anomalies, which affect anteroposterior and dorsoventral polarities, optic cup formation, head cartilage morphogenesis, and erythrocyte differentiation. It has been proposed that these effects could be correlated to modifications of cell adhesion properties and/or to an increased engagement of cells in the apoptotic pathway during early development (Remy et al., Int. J. Dev. Biol. 40, 577-589, 1996). To address these questions, we have first analyzed the behavior of cells overexpressing the protein in both aggregation and adhesion assays. We observe perturbations of cell-cell interactions as well as perturbations of cell adhesion and spreading on fibronectin and extracellular matrix (ECM). Second, we have analyzed apoptosis of cells overexpressing the Xl-FLI protein, by testing DNA fragmentation, caspase-3 activity and by performing TUNEL assay. We show that Xl-Fli overexpression results in the appearance of hallmarks of apoptosis, including exclusion of cells from the interior of the embryo, internucleosomal fragmentation of DNA and dose-dependent induction of caspase-3, resulting in the hydrolysis of poly(ADP-ribose) polymerase. In addition, a dominant-negative mutation of BMPs receptors decreases the effects of Xl-Fli overexpression, suggesting that a modification of the BMP signalling could be responsible for increased apoptosis. The latter appears to affect predominantly ventral and ventrolateral regions of the embryo.

Insights into early vasculogenesis revealed by expression of the ETS-domain transcription factor Fli-1 in wild-type and mutant zebrafish embryos

Fli-1 is an ETS-domain transcription factor whose locus is disrupted in Ewing's Sarcoma and F-MuLV induced erythroleukaemia. To gain a better understanding of its normal function, we have isolated the zebrafish homologue. Similarities with other vertebrates, in the amino acid sequence and DNA binding properties of Fli-1 from zebrafish, suggest that its function has been conserved during vertebrate evolution. The initial expression of zebrafish fli-1 in the posterior lateral mesoderm overlaps with that of gata2 in a potential haemangioblast population which likely contains precursors of blood and endothelium. Subsequently, fli-1 and gata2 expression patterns diverge, with separate fli-1 and gata2 expression domains arising in the developing vasculature and in sites of blood formation respectively. Elsewhere in the embryo, fli-1 is expressed in sites of vasculogenesis. The expression of fli-1 was investigated in a number of zebrafish mutants, which affect the circulatory system. In cloche, endothelium is absent and blood is drastically reduced. In contrast to the blood and endothelial markers that have been studied previously, fli-1 expression was initiated normally in cloche embryos, indicating that induction of fli-1 is one of the earliest indicators of haemangioblast formation. Furthermore, although fli-1 expression in the trunk was not maintained, the normal expression pattern in the anterior half of the embryo was retained. These anterior cells did not, however, condense to form blood vessels. These data indicate that cloche has previously unsuspected roles at multiple stages in the formation of the vasculature. Analysis of fli-1 expression in midline patterning mutants floating head and squint, confirms a requirement for the notochord in the formation of the dorsal-aorta. The formation of endothelium in one-eyed pinhead, cyclops and squint embryos indicates a novel role for the endoderm in the formation of the axial vein. The phenotype of sonic-you mutants implies a likely role for Sonic Hedgehog in mediating these processes.

Xl erg: expression pattern and overexpression during development plead for a role in endothelial cell differentiation

The ets gene family encodes transcription factors related to the proto-oncogene c-ets-1 and involved in cell proliferation, differentiation, and oncogenic transformation. We have characterized the Xenopus homologue of the human erg gene, an ets-related-gene, and its expression has been examined throughout early embryonic development. Xl erg encodes at least two proteins, resulting from alternative splicing events. The transcripts are restricted to the forming endocardium, the endothelial cells of the blood vessels and to the neural crest-derived mesenchyme cells of the pharyngial arches. When Xl ERG is expressed ectopically in Xenopus embryos by microinjection of synthetic mRNA, multiple developmental defects are observed. Dorsally injected embryos have their AP axis shortened and present severe defects in eye and somite morphogenesis. Ventrally injected embryos show a posteriorization of the cells having received the message together with ectopic endothelial cell differentiation as revealed by the accumulation of X-msr transcripts. In both cases, accumulation of erythrocytes in structures not connected with the blood circulatory system can be observed. Our data suggest that Xl erg may be involved in cell motility and in the development of the circulatory system.

Relation of neurological marker expression and EWS gene fusion types in MIC2/CD99-positive tumors of the Ewing family

The Ewing family of tumors (EFT) is characterized by high MIC2/CD99 expression and specific EWS/ETS gene rearrangements, resulting in different chimeric transcripts. Further division into peripheral primitive neuroectodermal tumors and Ewing's sarcoma is still debated and, in the absence of distinct morphological parameters, has been based on the reactivity with neuroglial markers (NgM). We investigated 44 EFT in terms of a possible correlation between the type of EWS chimeric transcripts and reactivity with the following NgM: polyclonal and monoclonal neuron-specific enolase (NSE), S-100, chromogranin A, synaptophysin, Leu-7, glial fibrillary acid protein, and neurofilament. EWS/Fli1 fusion type 1 was detected in 30 of 44 and type 2 in 11 of 44 tumors. Three tumors, presenting with an uncommon morphology, carried rare chimeric transcripts. Our results indicate an association of lack of NgM staining with type 1 EWS/Fli1 translocations, found in 16 of 18 tumors with no NgM expression as detectable by any of the antibodies we applied. Using the monoclonal NSE antibody, 21 of 26 tumors without NgM staining expressed type 1 EWS/FLI1chimeric RNA, whereas in the groups with 1 or more and 2 or more NgM, only 9 of 17 and 1 of 5 tumors, respectively, carried type 1 EWS/Fli1 fusion transcripts. Despite this association of increased NgM expression with a non-type 1 EWS/Fli1 gene fusion, a strict correlation between the extent of NgM expression and certain EWS fusion types was not evident. This fortifies the concept to consider EFT as a spectrum of tumors and suggests the type of EWS fusion transcripts as one, but not the only parameter influencing the extent of differentiation.

The Ewing family of tumors and the search for the Achilles' heel

The ideal cancer therapy would accomodate the specific biology of a tumor and be based upon understanding the mechanisms of malignancy. This vision has been the driving force in cancer research. However, the story of success in clinical cancer management is a story of empirie largely independent from progress in basic research. For the Ewing family of tumors (EFT) comprising Ewing's sarcoma and peripheral primitive neuroectodermal tumor, significant insights into the molecular basis have appeared recently. Some of last year's discoveries may have taken us closer to the identification of the Achilles' heel in this disease. The first clue has been obtained to the mechanism of chromosomal translocation, which constitutes a rate-limiting step in EFT pathogenesis. Also, researchers have progressed in understanding the control of EFT cell proliferation, differentiation, and death. A major role in these processes has been attributed to the EWS-ets gene rearrangement. Specific growth factor circuits appear to be involved in deregulated tumor cell growth. By analogy to heterologous cellular systems, it is possible to postulate an important functional role for CD99(MIC2) as it contributes to the malignant phenotype of EFT. In vitro, as well as the first in vivo, experimental evidence suggests that tumor cell expansion and spread can be counteracted by breaking these physiological pathways. Still, we are far from a tailored biological therapy of EFT. Before this goal may be achieved, we must seek further improvements and diversification of today's standard and intensified treatment regimens.

Development of neural crest in Xenopus

The neural crest is a unique cell population among embryonic cell types, displaying properties of both ectodermal and mesodermal lineages. Most of the recent studies examining the neural crest have been performed in avian embryos. Only in the first half of this century were amphibians extensively used. We first summarize this important older source of information, reviewing studies made since the turn of the century. Due to the increasingly detailed in cellular and molecular knowledge of the early development of Xenopus laevis, the remainder of the review focuses on this species. We describe the route of migration and fate of the neural crest and propose a new model of neural crest induction in which prospective cells are induced independently of the neural plate by a double gradient of a morphogen that patterns the entire ectoderm. This model is also discussed in a more general context in connection with the dorsoventral patterning of the neural tube. Finally, we discuss some ideas concerning neural crest evolution and propose a novel hypothesis about its phylogenetic origin.

The Xenopus Ets transcription factor XER81 is a target of the FGF signaling pathway

We report the cloning of a cDNA encoding a Xenopus laevis Ets-type transcription factor. This new Xenopus gene belongs to the PEA3 subfamily of Ets proteins and shows the highest degree of sequence similarity to the mouse and human ER81 genes. The Xenopus ER81 gene (XER81) is transcribed in the embryo after mid blastula transition (MBT) and three transcripts of 3, 4 and 6 kb are detected throughout embryogenesis. XER81 mRNA is localized in the animal pole of the late blastula stage and higher levels of XER81 transcripts are detected in the marginal zone at the onset of gastrulation. In later embryogenesis XER81 transcripts are found in neural crest cells, eyes, otic vesicles and pronephros. The transcription of XER81 can be stimulated by bFGF and eFGF in animal and vegetal cap explants. Expression of the dominant negative FGF receptor mutant in animal caps and embryos blocks XER81 transcription, arguing that the expression of this Ets gene requires active FGF signaling. The spatial overlap of eFGF and XER81 expression domains supports the idea that XER81 transcription could be a marker for regions with active FGF signaling in the embryo.

Characterization of the Ets-type protein ER81 in Xenopus embryos

A function for FGF-type peptide growth factors has been implied for early mesodermal patterning events in Xenopus laevis. FGF signalling operates via the MAP kinase cascade that can directly activate the transcription of organizer-expressed genes, such as Xbra and Xegr-1. We have recently provided evidence for a critical role of Ets-type transcription factors in FGF mediated Xegr-1 transcription activation. Here, we report on the identification of the Xenopus Ets-type protein ER81 that is expressed in a pattern overlapping with the ones of Xegr-1 and Xbra during gastrulation. Microinjection in XER81 encoding mRNA into ventral blastomeres of Xenopus embryos results in the induction of ectopic, tail-like protrusions, whereas dorsal overexpression results in disturbed eye development. In the animal cap assay, ectopic expression of XER81 is found to interfere with activin mediated induction of Xegr-1 and gsc, but not with the Xbra response to activin.

The cloche and spadetail genes differentially affect hematopoiesis and vasculogenesis

In vertebrates, hematopoietic and vascular progenitors develop from ventral mesoderm. The first primitive wave of hematopoiesis yields embryonic red blood cells, whereas progenitor cells of subsequent definitive waves form all hematopoietic cell lineages. In this report we examine the development of hematopoietic and vasculogenic cells in normal zebrafish and characterize defects in cloche and spadetail mutant embryos. The zebrafish homologs of lmo2, c-myb, fli1, flk1, and flt4 have been cloned and characterized in this study. Expression of these genes identifies embryonic regions that contain hematopoietic and vascular progenitor cells. The expression of c-myb also identifies definitive hematopoietic cells in the ventral wall of the dorsal aorta. Analysis of b316 mutant embryos that carry a deletion of the c-myb gene demonstrates that c-myb is not required for primitive erythropoiesis in zebrafish even though it is expressed in these cells. Both cloche and spadetail mutant embryos have defects in primitive hematopoiesis and definitive hematopoiesis. The cloche mutants also have significant decreases in vascular gene expression, whereas spadetail mutants expressed normal levels of these genes. These studies demonstrate that the molecular mechanisms that regulate hematopoiesis and vasculogenesis have been conserved throughout vertebrate evolution and the clo and spt genes are key regulators of these programs.

Mesodermal expression of the chicken erg gene associated with precartilaginous condensation and cartilage differentiation

The ets gene superfamily encodes a class of transcription factors that bind to a purine rich sequence through a 85 amino-acid ETS domain. Among them, the human erg gene has been found to be involved in Ewing's sarcoma, primitive neurectodermal tumour of childhood and acute myeloid leukaemia. Nevertheless, little is known about human erg expression. Northern blot analyses have shown a human erg expression restricted to few cell lines and thymus, but the status concerning expression during development remains unknown probably because no homologue of this gene has yet been isolated and studied in other vertebrates. We thus choose to clone the chicken erg gene (ck-erg) and to study its expression during chicken development. We obtained a bona fide clone of ck-erg and defined the transcriptional modulating properties of its product. The ck-Erg protein acts as a transcriptional activator through a conventional consensus ETS binding site. Northern blot studies on various chicken tissues, in situ analyses and comparison with the well-characterised c-ets-1 expression show that ck-erg is expressed in mesoderm- and, to a lesser extent, in ectoderm-derived tissues. During chicken development, two salient features could be observed. From stage E1 to E3.5, ck-erg expression was widely distributed in mesodermal derivatives and neural crest, resembling c-ets-1 expression. However, by E6, the expression of ck-erg exhibited, unlike c-ets-1, a drastically new and strong signal in precartilaginous condensation zones and cartilaginous skeletal primordia. These stages are the first steps of bone formation during skeletal elaboration. Our results show for the first time a possible specific involvement of ck-erg in cartilage morphogenesis.