Cooperative roles of Bozozok/Dharma and Nodal-related proteins in the formation of the dorsal organizer in zebrafish

Abstract

In vertebrates, specification of the dorso-ventral axis requires Wnt signaling, which leads to formation of the Nieuwkoop center and the Spemann organizer (dorsal organizer), through the nuclear accumulation of beta-catenin. Zebrafish bozozok/dharma (boz) and squint (sqt), which encode a homeodomain protein and a Nodal-related protein, respectively, are required for the formation of the dorsal organizer. The zygotic expression of boz and sqt in the dorsal blastoderm and dorsal yolk syncytial layer (YSL) was dependent on the maternally derived Wnt signal, and their expression at the late blastula and early gastrula stages was dependent on the zygotic expression of their own genes. The dorsal organizer genes, goosecoid (gsc) and chordin (din), were ectopically expressed in wild-type embryos injected with boz or sqt RNA. The expression of gsc strictly depended on both boz and sqt while the expression of din strongly depended on boz but only partially depended on sqt and cyclops (cyc, another nodal-related gene). Overexpression of boz in embryos defective in Nodal signaling elicited the ectopic expression of din but not gsc and resulted in dorsalization, implying that boz could induce part of the organizer, independent of the Nodal proteins. Furthermore, boz; sqt and boz;cyc double mutants displayed a severely ventralized phenotype with anterior truncation, compared with the single mutants, and boz;sqt;cyc triple mutant embryos exhibited an even more severe phenotype, lacking the anterior neuroectoderm and notochord, suggesting that Boz/Dharma and the Nodal-related proteins cooperatively regulate the formation of the dorsal organizer.

Regulation of the early expression patterns of the zebrafish Dishevelled-interacting proteins Dapper1 and Dapper2

“…Boz and Nodal are both required for dorsal determination, with boz;cyc;sqt mutants being severely ventroposteriorized, as revealed by loss of early dorsal marker genes (Shimizu et al, 2000;Sirotkin et al, 2000). Some dorsal genes require the individual contribution of either Boz or Nodals, whereas others, such as gsc and chordin, require both (Sirotkin et al, 2000).…”

“…To test this possibility, I injected embryos from boz heterozygote intercrosses with antivin (atv), a potent inhibitor of Nodal signaling that phenocopies MZoep mutants (and cyc;sqt mutants, which are equivalent; Thisse and Thisse, 1999). I expected that all resulting embryos would have the MZoep (cyc;sqt) phenotype (because they all receive Antivin) and one fourth have the additional boz phenotype (i.e., phenocopy boz;MZoep or its equivalent boz;cyc;sqt; Shimizu et al, 2000;Sirotkin et al, 2000). Embryo phenotypes conformed to the expected phenotypes at 24hrs with 73% MZoep and 27% boz;MZoep (n ϭ 15; not shown; Sirotkin et al, 2000).…”

FGF signaling is required for β-catenin-mediated induction of the zebrafish organizer

“…sqt (Feldman et al, 1998;Erter et al, 1998;Rebagliati et al, 1998) and boz (Yamanaka et al, 1998;Koos and Ho, 1998;Fekany et al, 1999) act in parallel downstream of ␤-catenin (Shimizu et al, 2000;Kelly et al, 2000) to establish the organizer (Sirotkin et al, 2000;Shimizu et al, 2000). As embryos bred from homozygous ichabod mothers fail to form a Nieuwkoop center equivalent region (Kelly et al, 2000), they can be used to determine which factors and signaling pathway components are downstream of ␤-catenin, and which aspects of embryonic patterning are dependent on this pathway (Kelly et al, 2000;Tsang et al, 2004;Kudoh et al, 2004;Gore et al, 2005).…”

Tumor Suppressor Lzap Suppresses Wnt/β-Catenin Signaling to Promote Zebrafish Embryonic Ventral Cell Fates via the Suppression of Inhibitory Phosphorylation of Glycogen Synthase Kinase 3

“…In zebrafish embryos, wnt3a, which is co-expressed with wnt8a at the gastrula margin (4), is also able to activate the maternal Wnt/␤-catenin pathway (3). Downstream targets of ␤-catenin include bozozok/dharma and fibroblast growth factors (fgfs), which may act in parallel to activate transcripts involved in dorsal cell fate specification (5)(6)(7). For example, Chordin is a BMP 5 antagonist involved in the establishment of the ventral to dorsal BMP signaling gradient (8).…”

“…Downstream targets of ␤-catenin include bozozok/dharma and fibroblast growth factors (fgfs), which may act in parallel to activate transcripts involved in dorsal cell fate specification (5)(6)(7). For example, Chordin is a BMP 5 antagonist involved in the establishment of the ventral to dorsal BMP signaling gradient (8). However, the mechanisms that restrict ␤-catenin nuclear localization to the future dorsal side have not been fully addressed.…”