Cloning and genetic mapping of zebrafish BMP-2

The origin of bmp16, a novel Bmp2/4 relative, retained in teleost fish genomes

Background

Whole genome sequences have allowed us to have an overview of the evolution of gene repertoires. The target of the present study, the TGFβ superfamily, contains many genes involved in vertebrate development, and provides an ideal system to explore the relationships between evolution of gene repertoires and that of developmental programs.

Results

As a result of a bioinformatic survey of sequenced vertebrate genomes, we identified an uncharacterized member of the TGFβ superfamily, designated bmp16, which is confined to teleost fish species. Our molecular phylogenetic study revealed a high affinity of bmp16 to the Bmp2/4 subfamily. Importantly, further analyses based on the maximum-likelihood method unambiguously ruled out the possibility that this teleost-specific gene is a product of teleost-specific genome duplication. This suggests that the absence of a bmp16 ortholog in tetrapods is due to a secondary loss. In situ hybridization showed embryonic expression of the zebrafish bmp16 in the developing swim bladder, heart, tail bud, and ectoderm of pectoral and median fin folds in pharyngula stages, as well as gut-associated expression in 5-day embryos.

Conclusion

Comparisons of expression patterns revealed (1) the redundancy of bmp16 expression with its homologs in presumably plesiomorphic expression domains, such as the fin fold, heart, and tail bud, which might have permitted its loss in the tetrapod lineage, and (2) the loss of craniofacial expression and gain of swim bladder expression of bmp16 after the gene duplication between Bmp2, -4 and -16. Our findings highlight the importance of documenting secondary changes of gene repertoires and expression patterns in other gene families.

The basic helix-loop-helix olig3 establishes the neural plate boundary of the trunk and is necessary for development of the dorsal spinal cord

olig genes encode a previously unrecognized group of vertebrate-specific basic helix-loop-helix transcription factors. As shown in mice, chickens, and zebrafish, two members of this group, olig1 and olig2, are involved in the differentiation of motoneurons and oligodendrocytes, but nothing is known about the role of the third member, olig3. Here, we show that olig3 plays an essential role in the establishment of the neural crest-lateral neural plate boundary. In zebrafish embryos, morpholino-induced olig3 inactivation dramatically increases the number of neural crest cells, but lateral neural plate fates (interneurons and astrocytes) are missing. Zebrafish swirl mutants that have impaired bone morphogenetic protein signaling and lack neural crest cells display an expanded olig3 expression domain. Moreover, olig3 is up-regulated in mindbomb mutants lacking the neural crest because of an impaired notch signaling, and olig3 repression in such mutants rescues the neural crest. In addition, olig3 regulates ngn1 and deltaA expression in interneuron precursors. Our results indicate that olig3 has an essential proneural activity in the dorsal spinal cord and cooperates with the Delta/Notch regulatory loop to establish the boundary between the neural crest and the lateral neural plate. Thus, a proper regulation of the olig gene family is essential for the formation of three cell types (oligodendrocytes, astrocytes, and neural crest) that are unique to vertebrates.

Modeling human hematopoietic and cardiovascular diseases in zebrafish

Zebrafish have emerged as a useful vertebrate model system in which unbiased large-scale screens have revealed hundreds of mutations affecting vertebrate development. Many zebrafish mutants closely resemble known human disorders, thus providing intriguing prospects for uncovering the genetic basis of human diseases and for the development of pharmacologic agents that inhibit or correct the progression of developmental disorders. The rapid pace of advances in genomic sequencing and map construction, in addition to morpholino targeting and transgenic techniques, have facilitated the identification and analysis of genes associated with zebrafish mutants, thus promoting the development of zebrafish as a model for human disorders. This review aims to illustrate how the zebrafish has been used to identify unknown genes, to assign function to known genes, and to delineate genetic pathways, all contributing valuable leads toward understanding human pathophysiology.

Bone morphogenetic proteins negatively control oligodendrocyte precursor specification in the chick spinal cord

In the vertebrate spinal cord, oligodendrocytes originate from a restricted region of the ventral neuroepithelium. This ventral localisation of oligodendrocyte precursors (OLPs) depends on the inductive influence of sonic hedgehog (Shh) secreted by ventral midline cells. We have investigated whether the ventral restriction of OLP specification might also depend on inhibiting signals mediated by bone morphogenetic proteins (BMPs). BMPs invariably and markedly inhibited oligodendrocyte development in ventral neural tissue both in vitro and in vivo. Conversely, in vivo ablation of the dorsal most part of the chick spinal cord or inactivation of BMP signalling using grafts of noggin-producing cells promoted the appearance of neuroepithelial OLPs dorsal to their normal domain of emergence, showing that endogenous BMPs contribute to the inhibition of oligodendrocyte development in the spinal cord. BMPs were able to oppose the Shh-mediated induction of OLPs in spinal cord neuroepithelial explants dissected before oligodendrocyte induction,suggesting that BMPs may repress OLP specification by interfering with Shh signalling in vivo. Strikingly, among the transcription factors involved in OLP specification, BMP treatment strongly inhibited the expression of Olig2 but not of Nkx2.2, suggesting that BMP-mediated inhibition of oligodendrogenesis is controlled through the repression of the former transcription factor. Altogether, our data show that oligodendrogenesis is not only regulated by ventral inductive signals such as Shh, but also by dorsal inhibiting signals including BMP factors. They suggest that the dorsoventral position of OLPs depends on a tightly regulated balance between Shh and BMP activities.

Determination of the embryonic inner ear

Inner ear induction, like induction of other tissues examined in recent years, is likely to be comprised of several stages. The process begins during gastrulation when the ectoderm is competent to respond to induction. It appears that a signal from the endomesoderm underlying the otic area during gastrulation initiates induction complemented by a signal from presumptive neural tissue. By the neural plate stage, a region of ectoderm outside the neural plate is "biased" toward ear formation; this process may be part of a more general "placodal" bias shared by several sensory tissues. Induction continues during neurulation when a signal from neural tissue (possibly augmented by mesoderm underlying the otic area) results in ectoderm committed to otic vesicle formation at the time of neural tube closure. Studies on several gene families implicate them in the ear determination process. Fibroblast Growth Factor (FGF) family members are clearly involved in induction: FGFs are appropriately expressed for such a role, and have been shown to be essential for inner ear development. FGFs also have inductive activity, although it is not clear if they are sufficient for ear induction. Activation of transcription factors in the otic ectoderm, for example, by Pax gene family members, provides evidence for important changes in the responding ectoderm beginning during gastrulation and continuing through specification at the end of neurulation, although few functional tests have defined the role of these genes in determination. The challenge remains to merge embryologic data with gene function studies to develop a clear model for the molecular basis of inner ear induction.

Zebrafish SPI-1 (PU.1) marks a site of myeloid development independent of primitive erythropoiesis: implications for axial patterning

The mammalian transcription factor SPI-1 (synonyms: SPI1, PU.1, or Sfpi1) plays a critical role in myeloid development. To examine early myeloid commitment in the zebrafish embryo, we isolated a gene from zebrafish that is a SPI-1 orthologue on the basis of homology and phylogenetic considerations. The zebrafish spi1 (pu1) gene was first expressed at 12 h postfertilization in rostral lateral plate mesoderm (LPM), anatomically isolated from erythroid development in caudal lateral plate mesoderm. Fate-mapping traced rostral LPM cells from the region of initial spi1 expression to a myeloid fate. spi1 expression was lost in the bloodless mutant cloche, but rostral spi1 expression and myeloid development were preserved in the mutant spadetail, despite its complete erythropoietic failure. This dissociation of myeloid and erythroid development was further explored in studies of embryos overexpressing BMP-4, or chordin, in bmp-deficient swirl and snailhouse mutants, and chordin-deficient chordino mutants. These studies demonstrate that, in zebrafish, spi1 marks a rostral population of LPM cells committed to a myeloid fate anatomically separated from and developmentally independent of erythroid commitment in the caudal LPM. Such complete anatomical and developmental dissociation of two hematopoietic lineages adds an interesting complexity to the understanding of vertebrate hematopoietic development and presents significant implications for the mechanisms regulating axial patterning.

Expression of BMP signalling pathway members in the developing zebrafish inner ear and lateral line

In this paper we describe the mRNA expression patterns of members of the bone morphogenetic protein (BMP) signalling pathway in the developing zebrafish ear. bmp2b, 4, and 7 are expressed in discrete areas of otic epithelium, some of which correspond to sensory patches. bmp2b and 4 mark the developing cristae before and during the appearance of differentiated hair cells. bmp4 is also expressed in a dorsal, non-sensory region of the ear. Expression of bmps in cristae is conserved between zebrafish, chick, and mouse, but there are also notable differences in ear expression patterns between these species. Of five zebrafish BMP antagonists, only one (follistatin) shows significant expression in the otic epithelium. The type I receptor bmpr-IB shows localised expression in the ear epithelium. Mediators of BMP signalling, smad1 and smad5, are expressed in statoacoustic and lateral line ganglia; smad5 is also expressed at low levels throughout the ear epithelium. An inhibitory smad, smad6, is expressed laterally in the ear epithelium. Lateral line primordia and neuromasts also express bmp2b, 4, follistatin, smad1, and smad5. The conservation of bmp expression in cristae among different species adds weight to the growing evidence that BMPs are required for the development of the vertebrate ear.

Vnd/nkx, ind/gsh, and msh/msx: conserved regulators of dorsoventral neural patterning?

Expression of vnd in ventral, ind in intermediate, and msh in dorsal columns of fly neurectoderm, and of homologous gene families in corresponding domains of vertebrate neurectoderm, suggests that elements of dorsoventral neural patterning have been evolutionarily conserved. However, upstream signaling pathways regulating this columnar gene expression pattern appear to have diverged significantly throughout evolution. In addition, while recent loss-of-function studies in flies and mice indicate that these three genes may have a conserved role in regional specification, there is no obvious conservation of the particular cell fates deriving from corresponding domains. The three-column expression pattern may thus represent a developmental mechanism that is more resistant to evolutionary changes than genetic events upstream or downstream of it.

Neuropeptide Y receptor subtype with unique properties cloned in the zebrafish: the zYa receptor

Neuropeptide Y (NPY) belongs to a family of structurally related neuroendocrine peptides for which five different G-protein-coupled receptor subtypes have been cloned in mammals. To identify additional subtypes we have performed PCR with degenerate primers in different species. We describe here the cloning and pharmacological profile of a unique NPY receptor subtype in the zebrafish that has tentatively been called the zYa receptor. It has 46-50% amino acid identity to the mammalian Y1, Y4 and y6 receptors and the previously cloned zebrafish receptors zYb and zYc, and only about 27% to Y2 and Y5. The zYa receptor binds NPY and PYY from mammals as well as zebrafish with high affinities and has a K(d) of 28 pM for porcine (125)I-PYY. It has a unique binding profile displaying some features in common with each of the mammalian Y1, Y2 and Y5 receptors. In a microphysiometer assay the receptor responds with extracellular acidification. Chromosomal mapping in the zebrafish genome of zYa, zYb and zYc receptor genes indicates a possible orthologous relationship between zYc and mammalian y6, but identifies no obvious mammalian ortholog for zYa (zYb is a recent copy of zYc in the fish lineage). These results imply that previous studies of NPY in fishes, which have striven to interpret the effects within the framework of mammalian Y1, Y2, and Y5 receptors, need to be reevaluated. Thus, the sequence comparisons, pharmacological properties, and chromosomal localization suggest that the zYa receptor is a novel NPY receptor subtype which is likely to be present also in mammals.