The role of the homeodomain protein Bozozok in zebrafish axis formation.

The zebrafish bozozok (boz) gene encoding a homeodomain protein (also named Dharma/Nieuwkoid) is required during blastula stages for the formation of a complete Spemann-Mangold gastrula organizer and subsequent development of axial mesoderm and anterior neural structures. Expression of bozin the dorsal yolk syncytial layer (YSL) and overlying marginal blastomeres is activated by beta-catenin. Bozozok itself acts as a transcriptional repressor, and promotes organizer formation by directly inhibiting expression of the bmp2b (swirl) gene and by negatively regulating Wnt signaling by an unknown mechanism. boz cooperates with the Nodal-related secreted factors, Cyclops and Squint, in organizer formation. The incomplete organizer in boz mutants is deficient in expression of a number of factors such as Chordin that antagonize Bone morphogenetic proteins (Bmps), and Dickkopf 1, a Wnt antagonist. Conversely, the dorsal blastoderm of boz mutants exhibits ectopic expression of genes normally excluded from the dorsal midline such as wnt8 or tbx6. boz specifies the formation of anterior neuroectoderm by regulating Bmp and Wnt pathways in a fashion consistent with Nieuwkoop's two-step neural patterning model. boz promotes neural induction by limiting the anti-neuralizing activity of Bmp morphogens. In addition, by negative regulation of Wnt signaling, boz limits posteriorization of neuroectoderm. bozozok chordino double mutants exhibit a synergistic loss of head and trunk. This synthetic phenotype is due to dramatically increased Bmp signaling and consequent massive accumulation of cells in the tailbud at the expense of dorso-anterior structures. Therefore, boz and din act in overlapping pathways that provide the main mechanism to limit Bmp signaling in the zebrafish gastrula and allow for head and trunk development. Notably, Bozozok appears to function by repressing transcription of target genes such as swr (bmp2b) gene, and as such is the earliest acting repressor that the nascent dorsal axis is using to antagonize ventral influences.