Dynamo, a new zebrafish DVR member of the TGF-beta superfamily is expressed in the posterior neural tube and is up-regulated by Sonic hedgehog

Regulation of zebrafish melanocyte development by ligand-dependent BMP signaling

Preventing terminal differentiation is important in the development and progression of many cancers including melanoma. Recent identification of the BMP ligand GDF6 as a novel melanoma oncogene showed GDF6-activated BMP signaling suppresses differentiation of melanoma cells. Previous studies have identified roles for GDF6 orthologs during early embryonic and neural crest development, but have not identified direct regulation of melanocyte development by GDF6. Here, we investigate the BMP ligand gdf6a, a zebrafish ortholog of human GDF6, during the development of melanocytes from the neural crest. We establish that the loss of gdf6a or inhibition of BMP signaling during neural crest development disrupts normal pigment cell development, leading to an increase in the number of melanocytes and a corresponding decrease in iridophores, another neural crest-derived pigment cell type in zebrafish. This shift occurs as pigment cells arise from the neural crest and depends on mitfa, an ortholog of MITF, a key regulator of melanocyte development that is also targeted by oncogenic BMP signaling. Together, these results indicate that the oncogenic role ligand-dependent BMP signaling plays in suppressing differentiation in melanoma is a reiteration of its physiological roles during melanocyte development.

Ligand-activated BMP signaling inhibits cell differentiation and death to promote melanoma

Oncogenomic studies indicate that copy number variation (CNV) alters genes involved in tumor progression; however, identification of specific driver genes affected by CNV has been difficult, as these rearrangements are often contained in large chromosomal intervals among several bystander genes. Here, we addressed this problem and identified a CNV-targeted oncogene by performing comparative oncogenomics of human and zebrafish melanomas. We determined that the gene encoding growth differentiation factor 6 (GDF6), which is the ligand for the BMP family, is recurrently amplified and transcriptionally upregulated in melanoma. GDF6-induced BMP signaling maintained a trunk neural crest gene signature in melanomas. Additionally, GDF6 repressed the melanocyte differentiation gene MITF and the proapoptotic factor SOX9, thereby preventing differentiation, inhibiting cell death, and promoting tumor growth. GDF6 was specifically expressed in melanomas but not melanocytes. Moreover, GDF6 expression levels in melanomas were inversely correlated with patient survival. Our study has identified a fundamental role for GDF6 and BMP signaling in governing an embryonic cell gene signature to promote melanoma progression, thus providing potential opportunities for targeted therapy to treat GDF6-positive cancers.

Unveiling the bmp13 enigma: redundant morphogen or crucial regulator?

Bone morphogenetic proteins are a diverse group of morphogens with influences not only on bone tissue, as the nomenclature suggests, but on multiple tissues in the body and often at crucial and influential periods in development.

The purpose of this review is to identify and discuss current knowledge of one vertebrate BMP, Bone Morphogenetic Protein 13 (BMP13), from a variety of research fields, in order to clarify BMP13's functional contribution to developing and maintaining healthy tissues, and to identify potential future research directions for this intriguing morphogen. BMP13 is highly evolutionarily conserved (active domain >95%) across diverse species from Zebrafish to humans, suggesting a crucial function. In addition, mutations in BMP13 have recently been associated with Klippel-Feil Syndrome, causative of numerous skeletal and developmental defects including spinal disc fusion. The specific nature of BMP13's crucial function is, however, not yet known.

The literature for BMP13 is focused largely on its activity in the healing of tendon-like tissues, or in comparisons with other BMP family molecules for whom a clear function in embryo development or osteogenic differentiation has been identified. There is a paucity of detailed information regarding BMP13 protein activity, structure or protein processing. Whilst some activity in the stimulation of osteogenic or cartilaginous gene expression has been reported, and BMP13 expression is found in post natal cartilage and tendon tissues, there appears to be a redundancy of function in the BMP family, with several members capable of stimulating similar tissue responses. This review aims to summarise the known or potential role(s) for BMP13 in a variety of biological systems.

Heterotypy in the N-terminal region of growth/differentiation factor 5 (GDF5) mature protein during teleost evolution

Heterotypy is now recognized as a generative force in the formation of new proteins through modification of existing proteins. We report that heterotypy in the N-terminal region of the mature growth/differentiation factor 5 (GDF5) protein occurred during evolution of teleosts. N-terminal length variation of GDF5 was found among teleost interfamilies and interorders but not within teleost families or among tetrapods. We further show that increase of proline and glutamine to the N-terminal region of mature GDF5 occurred in Eurypterygii, the higher lineage of teleosts. Because the basic amino acids, believed to control diffusion, are conserved in this region across all species examined, we suggest that the N-terminal elongation of the mature GDF5 protein during evolution has altered the protein diffusion in Eurypterygii, leading to high concentrations of the protein in the joint of the pharyngeal skeleton, the location of cartilage formation during development.

Commercially available recombinant sonic hedgehog up-regulates Ptc and modulates the cytokine and chemokine expression of human macrophages: an effect mediated by endotoxin contamination?

The Sonic hedgehog (Shh) signalling pathway plays an important role in developmental patterning and proliferation. Recent evidence suggests that Shh also plays a role in the development of the immune system. Here, we demonstrate that components of the Shh signalling pathway are expressed in human macrophages and that the receptor for Shh, Ptc, is up-regulated by a commercially available recombinant preparation of Shh (CArShh). Further, we report that the addition of CArShh up-regulates the production of IL-6, IL-8, MCP-1, IP-10, MIG and RANTES by macrophages, an effect enhanced by the presence of fetal calf serum in the culture medium. In contrast, TGF-beta, TNF-alpha, IL-1b, IL-12 and IL-10 production were not modulated by CArShh and VEGF was minimally up-regulated even in the presence of serum. The up-regulation of these cytokines and chemokines was abrogated by CD14 inhibition and polymixin B, but not reliably inhibited by the specific Shh pathway inhibitor cyclopamine. These results suggest that, although components of the Shh signalling pathway are expressed in macrophages, the modulation of macrophage cytokine and chemokine effector function seen in response to commercially available rShh results from low levels of endotoxin contained within the CArShh preparations employed to explore the effects of Shh in vitro.

In ovo electroporation of Crim1 in the developing chick spinal cord

The novel mammalian gene Crim1 encodes a transmembrane bound protein with similarity to the secreted bone morphogenetic protein (BMP) antagonists, vertebrate Chordin, and its Drosophila homologue short gastrulation. Crim1 is expressed in the neural tube in mouse in a restricted pattern, but its function in central nervous system development is largely unknown. We isolated the chicken Crim1 orthologue and analyzed its expression in the developing neural tube. Chicken CRIM1 shares strong homology to human/mouse CRIM1 and C. elegans CRIM1-like proteins. Crim1 is expressed in a similar but not identical pattern to that in the developing spinal cord of mouse, including the notochord, floor plate, motor neurons, and the roof plate. Unlike follistatin, a secreted inhibitor of BMPs, in ovo electroporation of CRIM1, as a full-length transmembrane bound or secreted ectodomain was not sufficient to disrupt early patterning of the neural tube. However, ectodomain CRIM1 overexpression leads to an approximate 50% decrease in populations of specific ventral neuronal populations, including ISL-1(+) motor neurons, CHX-10(+) V1, and EN-1(+) V2 interneurons.

Radar is required for the establishment of vascular integrity in the zebrafish

The precise assembly of an integrated network of blood vessels is essential for the survival of vertebrate embryos. However, the processes by which primitive endothelial cells form mature vessels capable of supplying oxygen and nutrients to developing tissues remain incompletely understood. Here, we propose a role for Radar, one of the zebrafish orthologues of gdf6, in establishing integrity of the trunk vasculature in zebrafish embryos. We show that radar expression is appropriately placed, both spatially and temporally, to perform such a role. Transcripts for radar are detected in the hypochord and the primitive gut endoderm. These tissues intimately flank developing axial vessels in the trunk and have been previously implicated in the regulation of vascular development. Morpholino-based targeted gene knock-down has generated a Radar-specific loss-of-function zebrafish model. These embryos display normal initiation of vascular patterning and commencement of circulation. However, by day 2 of development, the integrity of the axial vasculature is compromised with hemorrhages and circulation short-circuits throughout the developing trunk. We show that this aberrant vascular development is specific to a reduction of the radar gene product. These results suggest that Radar is involved in a signaling pathway required for establishing the integrity of the axial vessels during zebrafish development.

Isolation and characterization of myostatin complementary deoxyribonucleic acid clones from two commercially important fish: Oreochromis mossambicus and Morone chrysops

In mammals, skeletal muscle mass is negatively regulated by a muscle-derived growth/differentiating factor named myostatin (MSTN) that belongs to the transforming growth factor-beta superfamily. Although putative MSTN homologs have been identified from several vertebrates, nonmammalian orthologs remained poorly defined. Thus, we isolated and characterized MSTN complementary DNA clones from the skeletal muscle of the tilapia Oreochromis mossambicus and the white bass Morone chrysops. The nucleic and amino acid sequences from both fish species are highly homologous to the previously identified mammalian and avian orthologs, and both possess conserved cysteine residues and putative RXXR proteolytic processing sites that are common to all transforming growth factor-beta family members. Western blotting of conditioned medium from human embryonal kidney (HEK293) cells overexpressing a His-tagged tilapia MSTN indicates that the secreted fish protein is processed in a manner similar to mouse MSTN. However, in contrast to mice, MSTN expression in tilapia is not limited to skeletal muscle as it occurs in many tissues. Furthermore, the timing of MSTN expression in developing tilapia larvae coincides with myogenesis. These results suggest that the biological actions of MSTN in the tilapia and possibly in other fishes may not be limited to myocyte growth repression, but may additionally influence different cell types and organ systems.

The ventralizing activity of Radar, a maternally expressed bone morphogenetic protein, reveals complex bone morphogenetic protein interactions controlling dorso-ventral patterning in zebrafish

In Xenopus and zebrafish, BMP2, 4 and 7 have been implicated, after the onset of zygotic expression, in inducing and maintaining ventro-lateral cell fate during early development. We provide evidence here that a maternally expressed bone morphogenetic protein (BMP), Radar, may control early ventral specification in zebrafish. We show that Radar ventralizes zebrafish embryos and induces the early expression of bmp2b and bmp4. The analysis of Radar overexpression in both swirl/bmp2b mutants and embryos expressing truncated BMP receptors shows that Radar-induced ventralization is dependent on functional BMP2/4 pathways, and may initially rely on an Alk6-related signaling pathway. Finally, we show that while radar-injected swirl embryos still exhibit a strongly dorsalized phenotype, the overexpression of Radar into swirl/bmp2b mutant embryos restores ventral marker expression, including bmp4 expression. Our results suggest that a complex regulation of different BMP pathways controls dorso-ventral (DV) patterning from early cleavage stages until somitogenesis.

Regulation of the neural patterning activity of sonic hedgehog by secreted BMP inhibitors expressed by notochord and somites

The secretion of Sonic hedgehog (Shh) from the notochord and floor plate appears to generate a ventral-to-dorsal gradient of Shh activity that directs progenitor cell identity and neuronal fate in the ventral neural tube. In principle, the establishment of this Shh activity gradient could be achieved through the graded distribution of the Shh protein itself, or could depend on additional cell surface or secreted proteins that modify the response of neural cells to Shh. Cells of the neural plate differentiate from a region of the ectoderm that has recently expressed high levels of BMPs, raising the possibility that prospective ventral neural cells are exposed to residual levels of BMP activity. We have examined whether modulation of the level of BMP signaling regulates neural cell responses to Shh, and thus might contribute to the patterning of cell types in the ventral neural tube. Using an in vitro assay of neural cell differentiation we show that BMP signaling markedly alters neural cell responses to Shh signals, eliciting a ventral-to-dorsal switch in progenitor cell identity and neuronal fate. BMP signaling is regulated by secreted inhibitory factors, including noggin and follistatin, both of which are expressed in or adjacent to the neural plate. Conversely, follistatin but not noggin produces a dorsal-to-ventral switch in progenitor cell identity and neuronal fate in response to Shh both in vitro and in vivo. These results suggest that the specification of ventral neural cell types depends on the integration of Shh and BMP signaling activities. The net level of BMP signaling within neural tissue may be regulated by follistatin and perhaps other BMP inhibitors secreted by mesodermal cell types that flank the ventral neural tube.

Gdf16, a novel member of the growth/differentiation factor subgroup of the TGF-beta superfamily, is expressed in the hindbrain and epibranchial placodes

We have isolated and characterized the developmental expression of Xenopus gdf16, a novel member of the growth/differentiation factor (gdf) gene family. The gdf16 gene encodes a pre-proprotein of 413 amino acids and a mature peptide of 122 amino acids. Gdf16 is most closely related to the zebrafish genes dynamo and radar, but exhibits a completely different expression pattern. Gene expression is detected at early tailbud (stage 25) in the first two epibranchial placodes and in a hindbrain-specific domain. As development proceeds, the gene is expressed in all the epibranchial placodes, the hindbrain, and the diencephalon.

Development of specific connectivity between premotor neurons and motoneurons in the brain stem and spinal cord

Astounding progress has been made during the past decade in understanding the general principles governing the development of the nervous system. An area of prime physiological interest that is being elucidated is how the neural circuitry that governs movement is established. The concerted application of molecular biological, anatomical, and electrophysiological techniques to this problem is yielding gratifying insight into how motoneuron, interneuron, and sensory neuron identities are determined, how these different neuron types establish specific axonal projections, and how they recognize and synapse upon each other in patterns that enable the nervous system to exercise precise control over skeletal musculature. This review is an attempt to convey to the physiologist some of the exciting discoveries that have been made, within a context that is intended to link molecular mechanism to behavioral realization. The focus is restricted to the development of monosynaptic connections onto skeletal motoneurons. Principal topics include the inductive mechanisms that pattern the placement and differentiation of motoneurons, Ia sensory afferents, and premotor interneurons; the molecular guidance mechanisms that pattern the projection of premotor axons in the brain stem and spinal cord; and the precision with which initial synaptic connections onto motoneurons are established, with emphasis on the relative roles played by cellular recognition versus electrical activity. It is hoped that this review will provide a guide to understanding both the existing literature and the advances that await this rapidly developing topic.

The BMP-related protein radar: a maintenance factor for dorsal neuroectoderm cells?

We have previously cloned several members of the TGF-beta superfamily of growth factors in zebrafish, one of which, Radar, belongs to the Dpp-Vg1-related (DVR) subgroup, with highest homology to GDF6. The pattern of expression of Radar suggested a possible involvement in several induction steps during embryogenesis including in the dorsal neural tube, red blood cells, the dorsal fin and the retina. We have analyzed the pattern of expression of Radar in comparison with that of a marker of dorsal neural tube structures, msxC and show that Radar and msxC are expressed in similar and/or adjacent tissues throughout embryogenesis. In order to demonstrate a functional relationship between these two proteins, we have generated a full-length cDNA for Radar and shown that Radar overexpression by DNA injection maintains expression of msxC in tissues where it is normally expressed then turned off, in particular in the dorsal neurectoderm. Study of the phenotype of a mutant carrying a deletion of Radar shows a loss of identity and death of the cells of the dorsal neural tube. Taken together these results suggest that Radar could be involved in maintaining the identity of cells of the dorsal-most neural tube and of at least a subset of neural crest cells.

Isolation of Zebrafish gdf7 and Comparative Genetic Mapping of Genes Belonging to the Growth/Differentiation Factor 5, 6, 7 Subgroup of the TGF-β Superfamily

The Growth/differentiation factor (Gdf)5, 6, 7 genes form a closely related subgroup belonging to the TGF-β superfamily. In zebrafish, there are three genes that belong to the Gdf5, 6, 7subgroup that have been named radar, dynamo, andcontact. The genes radar and dynamo both encode proteins most similar to mouse GDF6. The orthologous identity of these genes on the basis of amino acid similarities has not been clear. We have identified gdf7, a fourth zebrafish gene belonging to the Gdf5, 6, 7 subgroup. To assign correct orthologies and to investigate the evolutionary relationships of the human, mouse, and zebrafish Gdf5, 6, 7subgroup, we have compared genetic map positions of the zebrafish and mammalian genes. We have mapped zebrafish gdf7 to linkage group (LG) 17, contact to LG9, GDF6 to human chromosome (Hsa) 8 and GDF7 to Hsa2p. The radar anddynamo genes have been localized previously to LG16 and LG19, respectively. A comparison of syntenies shared among human, mouse, and zebrafish genomes indicates that gdf7 is the ortholog of mammalian GDF7/Gdf7. LG16 shares syntenic relationships with mouse chromosome (Mmu) 4, including Gdf6. Portions of LG16 and LG19 appear to be duplicate chromosomes, thus suggesting thatradar and dynamo are both orthologs of Gdf6. Finally, the mapping data is consistent with contact being the zebrafish ortholog of mammalian GDF5/Gdf5.

[The sequence data described in this paper have been submitted to the GenBank data library under accession numbers AF113022 and AF113023.]

Expression of contact, a new zebrafish DVR member, marks mesenchymal cell lineages in the developing pectoral fins and head and is regulated by retinoic acid

Contact, a new zebrafish transforming growth factor-beta (TGF-beta) member is most closely related to mouse GDF5 and to human CDMP-1 responsible, when mutated, for limb brachypodism phenotype and Hunter-Thompson syndrome, respectively. Contact exhibits a dynamic spatial expression pattern in the pharyngeal arches and the pectoral fin buds that much prefigures cartilage formation. Within the fin buds, contact expression is detected in the proximal mesenchyme from which the endoskeleton will develop. Exogeneously applied retinoic acid (RA) induces duplication of the pectoral fin rudiment in zebrafish embryos as well as contact expression along the proximal margin of the fin mesenchyme showing that both endoskeleton and exoskeleton can be duplicated.