The chicken c-erbA proto-oncogene is preferentially expressed in erythrocytic cells during late stages of differentiation.

We analyzed the expression of the c-erbA proto-oncogene in different tissues of chicken embryos. c-erbA transcripts were found at low levels in the lung, kidney, liver, and heart and in high amounts in embryonic blood cells. Nuclease mapping assays proved that these transcripts were true c-erbA transcripts. In situ hybridization on fractionated embryonic blood cells showed that c-erbA transcripts were predominantly found in erythroblasts, particularly during the final step of differentiation. Life span analysis of c-erbA mRNAs revealed their relative instability, demonstrating that the high level of c-erbA transcripts in embryonic erythroblasts was not the result of passive accumulation. These results suggest that the c-erbA genes play some role in erythrocyte differentiation.

Expression pattern of the frizzled 7 gene during zebrafish embryonic development.

We have identified a novel frizzled gene in zebrafish, (Danio rerio): frizzled 7, highly related to mouse, chick and Xenopus fz7. No maternal expression was detected. Zygotic transcription starts at the end of gastrulation anteriorly in the presumptive neurectoderm and in the presomitic mesoderm. During somitogenesis expression is detected in developing central nervous system, including forebrain, midbrain, hindbrain and spinal cord, the lateral mesoderm and the anterior part of the forming somites. The strong sequence conservation of fz7 to the mouse, chick and Xenopus counterparts is also true for their expression pattern.

Expression pattern of the Sox31 gene during zebrafish embryonic development.

We have identified a novel Sox gene in zebrafish (Danio rerio), Sox31, closely related to mammalian group B Sox genes. The gene is maternally expressed. Zygotic transcription starts at gastrulation, in the presumptive neuroectoderm. Later, expression is restricted to the developing central nervous system, including forebrain, midbrain, hindbrain and spinal cord.

An activated form of type I serine/threonine kinase receptor TARAM-A reveals a specific signalling pathway involved in fish head organiser formation.

The role of Transforming Growth Factor beta (TGF-beta)-related molecules in axis formation and mesoderm patterning in vertebrates has been extensively documented, but the identity and mechanisms of action of the endogenous molecules remained uncertain. In this study, we isolate a novel serine/threonine kinase type I receptor, TARAM-A, expressed during early zebrafish embryogenesis first ubiquitously and then restricted to dorsal mesoderm during gastrulation. A constitutive form of the receptor is able to induce the most anterior dorsal mesoderm rapidly and to confer an anterior organizing activity. By contrast, the wild-type form is only able to induce a local expansion of the dorsal mesoderm. Thus an activated form of TARAM-A is sufficient to induce dorsoanterior structures and TARAM-A may be activated by dorsally localized signals. Our data suggest the existence in fish of a specific TGF-beta-related pathway for anterior dorsal mesoderm induction, possibly mediated by TARAM-A and activated at the late blastula stage by localized dorsal determinant.

Coordinate expression of the murine Hox-5 complex homoeobox-containing genes during limb pattern formation.

The homoebox-containing genes of the Hox-5 complex are expressed in different but overlapping domains in limbs during murine development. The more 5' the position of these genes in the complex, the later and more distal is their expression. Antero-posterior differences are also observed. A model is proposed that accounts for the establishment of these expression domains in relation to the existence of a morphogen released by the zone of polarizing activity. Comparison of these observations with the expression patterns of the genes of Hox complexes in the early embryo suggests that similar molecular mechanisms are involved in the positional signalling along the axes of both the embryonic trunk and the fetal limbs.

Comparison of mouse and human HOX-4 complexes defines conserved sequences involved in the regulation of Hox-4.4.

We have cloned and sequenced, in both mouse and human, regions of the HOX-4 complex which contain two Abd-B like genes, Hox-4.4 and Hox-4.5 (HOX4C and HOX4D in human, respectively). The high degree of conservation between the homeoprotein sequences extends to non-coding areas, which suggests that the mechanisms of regulation have been conserved. We show that the Hox-4.5/Hox-4.4 intergenic region can be broadly subdivided into three domains based on DNA conservation between rodents and primates. The presence of all these domains in association with sequences located 3' to the transcription termination site are required to mimick the spatial regulation of Hox-4.4 in transgenic mouse embryos. Several highly conserved short sequences located in this region were studied in gel retardation assays for their binding to potential regulatory factors. One such factor is detected in embryonal carcinoma cells but absent from other differentiated cell lines. This specific binding activity is down regulated upon retinoic acid treatment.

Murine genes related to the Drosophila AbdB homeotic genes are sequentially expressed during development of the posterior part of the body.

The cloning, characterization and developmental expression patterns of two novel murine Hox genes, Hox-4.6 and Hox-4.7, are reported. Structural data allow us to classify the four Hox-4 genes located in the most upstream (5') position in the HOX-4 complex as members of a large family of homeogenes related to the Drosophila homeotic gene Abdominal B (AbdB). It therefore appears that these vertebrate genes are derived from a selective amplification of an ancestral gene which gave rise, during evolution, to the most posterior of the insect homeotic genes so far described. In agreement with the structural colinearity, these genes have very posteriorly restricted expression profiles. In addition, their developmental expression is temporally regulated according to a cranio-caudal sequence which parallels the physical ordering of these genes along the chromosome. We discuss the phylogenetic alternative in the evolution of genetic complexity by amplifying either genes or regulatory sequences, as exemplified by this system in the mouse and Drosophila. Furthermore, the possible role of 'temporal colinearity' in the ontogeny of all coelomic (metamerized) metazoans showing a temporal anteroposterior morphogenetic progression is addressed.

HOX4 genes encode transcription factors with potential auto- and cross-regulatory capacities.

We have looked for the binding of several HOX4 complex homeoproteins in the genomic region containing the HOX4C promoter, between the human HOX4C and HOX4D genes. The HOX4C, HOX4D and Hox-4.3 homeoproteins bind to a phylogenetically highly conserved DNA fragment, which is located in the proximal part of this intergenic region and contains multiple binding sites for these HOX4 proteins. Using cotransfection experiments, we show that this endogenous DNA sequence can mediate transactivation by the HOX4D and HOX4C proteins and that this effect requires the presence of TAAT-related binding sites. The Hox-4.3 protein, in contrast, is unable to activate and can repress the activation observed with the two other proteins. These results show that the HOX4D and HOX4C genes are genuine sequence-specific transcription factors and suggest that, as in Drosophila, cross-regulatory interactions between these genes might be essential for their proper expression.

Primary structure and embryonic expression pattern of the mouse Hox-4.3 homeobox gene.

We report the cloning, genomic localization, primary structure and developmental expression pattern of the novel mouse Hox-4.3 gene. This gene is located within the HOX-4(5) complex, at a position which classifies it as a member of the Hox-3.1 and -2.4 subfamily, the DNA and predicted protein sequences further confirmed this classification. Hox-4.3 has a primary structure characteristic of a Hox gene but, in addition, contains several monotonic stretches of amino acids, one of the 'paired'-like type. As expected from its presence and position within the complex. Hox-4.3 is developmentally expressed in structures of either mesodermal or neurectodermal origin located or derived from below a precise craniocaudal level. However, a very important offset between anteroposterior boundaries within neuroectoderm versus mesoderm derivatives is observed. Like other genes of the HOX-4(5) complex, Hox-4.3 is expressed in developing limbs and gonads, suggesting that 'cluster specificity' could be a feature of the HOX network.

Inhibitory interactions controlling organizer activity in fish.

An experimental system allowing the observation of 2 active organizing centers during zebrafish development is described. It was achieved by injection into a single marginal cell at the 16-cell stage of TARAM-A-D mRNA. TARAM-A-D was previously described as the mutated constitutive form of a type I receptor for transforming growth factor beta. In 80% of the injected embryos, 2 distinct organizers were observed at the onset of gastrulation. At the end of gastrulation, these embryos showed duplicated axial structures. Nevertheless, only 25% of the injected embryos displayed a recognizable axis duplication after 1 day of development. This paradox is taken as an evidence for suppressive effects exerted in a reciprocal manner when more than 1 organizing center is present.

The zebrafish bozozok locus encodes Dharma, a homeodomain protein essential for induction of gastrula organizer and dorsoanterior embryonic structures.

The dorsal gastrula organizer plays a fundamental role in establishment of the vertebrate axis. We demonstrate that the zebrafish bozozok (boz) locus is required at the blastula stages for formation of the embryonic shield, the equivalent of the gastrula organizer and expression of multiple organizer-specific genes. Furthermore, boz is essential for specification of dorsoanterior embryonic structures, including notochord, prechordal mesendoderm, floor plate and forebrain. We report that boz mutations disrupt the homeobox gene dharma. Overexpression of boz in the extraembryonic yolk syncytial layer of boz mutant embryos is sufficient for normal development of the overlying blastoderm, revealing an involvement of extraembryonic structures in anterior patterning in fish similarly to murine embryos. Epistatic analyses indicate that boz acts downstream of beta-catenin and upstream to TGF-beta signaling or in a parallel pathway. These studies provide genetic evidence for an essential function of a homeodomain protein in beta-catenin-mediated induction of the dorsal gastrula organizer and place boz at the top of a hierarchy of zygotic genes specifying the dorsal midline of a vertebrate embryo.