Chordin and dickkopf-1b are essential for the formation of head structures through activation of the FGF signaling pathway in zebrafish.

The ability of the Spemann organizer to induce dorsal axis formation is dependent on downstream factors of the maternal Wnt/β-catenin signaling pathway. The fibroblast growth factor (FGF) signaling pathway has been identified as one of the downstream components of the maternal Wnt/β-catenin signaling pathway. The ability of the FGF signaling pathway to induce the formation of a dorsal axis with a complete head structure requires chordin (chd) expression; however, the molecular mechanisms involved in this developmental process, due to activation of FGF signaling, remain unclear.

Correct anteroposterior patterning of the zebrafish neurectoderm in the absence of the early dorsal organizer.

Background: The embryonic organizer (i.e., Spemann organizer) has a pivotal role in the establishment of the dorsoventral (DV) axis through the coordination of BMP signaling.

Induction and patterning of trunk and tail neural ectoderm by the homeobox gene eve1 in zebrafish embryos.

In vertebrates, Evx homeodomain transcription factor-encoding genes are expressed in the posterior region during embryonic development, and overexpression experiments have revealed roles in tail development in fish and frogs. We analyzed the molecular mechanisms of posterior neural development and axis formation regulated by eve1. We show that eve1 is involved in establishing trunk and tail neural ectoderm by two independent mechanisms: First, eve1 posteriorizes neural ectoderm via induction of aldh1a2, which encodes an enzyme that synthesizes retinoic acid; second, eve1 is involved in neural induction in the posterior ectoderm by attenuating BMP expression.

Molecular dissection of Otx1 functional domains in the zebrafish embryo.

Otx proteins are involved in the induction of neurectoderm patterning and morphogenetic movements, leading to the formation of the vertebrate central nervous system. Despite lack of homology of sequence outside the homeodomain, a large body of evidence has shown that the Otx/Otd class of proteins has similar functions in many animal phyla. Thus, characterization of functional domains in proteins of this family would help in understanding how this functional equivalence operates.

The terminal region of beta-catenin promotes stability by shielding the Armadillo repeats from the axin-scaffold destruction complex.

Post-translational stabilization of beta-catenin is a key step in Wnt signaling, but the features of beta-catenin required for stabilization are incompletely understood. We show that forms of beta-catenin lacking the unstructured C-terminal domain (CTD) show faster turnover than full-length or minimally truncated beta-catenins. Mutants that exhibit faster turnover show enhanced association with axin in co-transfected cells, and excess CTD polypeptide can compete binding of the beta-catenin armadillo (arm) repeat domain to axin in vitro, indicating that the CTD may restrict beta-catenin binding to the axin-scaffold complex.

Mechanism of development of ionocytes rich in vacuolar-type H(+)-ATPase in the skin of zebrafish larvae.

Mitochondrion-rich cells (MRCs), or ionocytes, play a central role in aquatic species, maintaining body fluid ionic homeostasis by actively taking up or excreting ions. Since their first description in 1932 in eel gills, extensive morphological and physiological analyses have yielded important insights into ionocyte structure and function, but understanding the developmental pathway specifying these cells remains an ongoing challenge. We previously succeeded in identifying a key transcription factor, Foxi3a, in zebrafish larvae by database mining.

Chordin expression, mediated by Nodal and FGF signaling, is restricted by redundant function of two beta-catenins in the zebrafish embryo.

Using embryos transgenic for the TOP-GFP reporter, we show that the two zebrafish beta-catenins have different roles in the organizer and germ-ring regions of the embryo. beta-Catenin-activated transcription in the prospective organizer region specifically requires beta-catenin-2, whereas the ventrolateral domain of activated transcription is abolished only when both beta-catenins are inhibited. chordin expression during zebrafish gastrulation has been previously shown in both axial and paraxial domains, but is excluded from ventrolateral domains.

FGF signaling is required for {beta}-catenin-mediated induction of the zebrafish organizer.

We have used the maternal effect mutant ichabod, which is deficient in maternal beta-catenin signaling, to test for the epistatic relationship between beta-catenin activation, FGF signaling and bozozok, squint and chordin expression. Injection of beta-catenin RNA into ichabod embryos can completely rescue normal development. By contrast, when FGF signaling is inhibited, beta-catenin did not induce goosecoid and chordin, repress bmp4 expression or induce a dorsal axis.

Essential and opposing roles of zebrafish beta-catenins in the formation of dorsal axial structures and neurectoderm.

In Xenopus, Wnt signals and their transcriptional effector beta-catenin are required for the development of dorsal axial structures. In zebrafish, previous loss-of-function studies have not identified an essential role for beta-catenin in dorsal axis formation, but the maternal-effect mutation ichabod disrupts beta-catenin accumulation in dorsal nuclei and leads to a reduction of dorsoanterior derivatives. We have identified and characterized a second zebrafish beta-catenin gene, beta-catenin-2, located on a different linkage group from the previously studied beta-catenin-1, but situated close to the ichabod mutation on LG19.

The zebrafish dorsal axis is apparent at the four-cell stage.

A central question in the development of multicellular organisms pertains to the timing and mechanisms of specification of the embryonic axes. In many organisms, specification of the dorsoventral axis requires signalling by proteins of the Transforming growth factor-beta and Wnt families. Here we show that maternal transcripts of the zebrafish Nodal-related morphogen, Squint (Sqt), can localize to two blastomeres at the four-cell stage and predict the dorsal axis.

The zebrafish dog-eared mutation disrupts eya1, a gene required for cell survival and differentiation in the inner ear and lateral line.

To understand the molecular basis of sensory organ development and disease, we have cloned and characterized the zebrafish mutation dog-eared (dog) that is defective in formation of the inner ear and lateral line sensory systems. The dog locus encodes the eyes absent-1 (eya1) gene and single point mutations were found in three independent dog alleles, each prematurely truncating the expressed protein within the Eya domain. Moreover, morpholino-mediated knockdown of eya1 gene function phenocopies the dog-eared mutation.

Bone morphogenetic protein-4 expression characterizes inductive boundaries in organs of developing zebrafish.

 We have cloned and examined the expression pattern of zebrafish bone morphogenetic protein-4 (BMP4) as a start to evaluating signals which might participate in the fashioning of organ systems in this genetically tractable species. The predicted sequence of the mature zebrafish protein is more than 75% identical to that of other vertebrates and 66% identical to Drosophila decapentaplegic (Dpp). As in other species, BMP4 is expressed ventrally during gastrulation, but the zebrafish is unusual in having an additional dorsal domain of expression.