Follistatin and Noggin are excluded from the zeabrafish organizer

The patterning activity of the Spemann organizer in early amphibian embryos has been characterized by a number of organizer-specific secreted proteins including Chordin, Noggin, and Follistatin, which all share the same induc tive properties. They can neuralize ectoderm and dorsalize ventral mesoderm by blocking the ventralizing signals Bmp2 and Bmp4. In the zebrafish, null mutations in the chordin gene, named chordino, lead to a severe reduction of organizer activity, indicating that Chordino is an essential, but not th e only, inductive signal generated by the zebrafish organizer. A second gen e required for zebrafish organizer function is mercedes, but the molecular nature of its product is not known as yet. To investigate whether and how F ollistatin and Noggin are involved in dorsoventral (D-V) patterning of the zebrafish embryo, we have now isolated and characterized their zebrafish ho mologues. Overexpression studies demonstrate that both proteins have the sa me dorsalizing properties as their Xenopus homologues. However, unlike the Xenopus genes, zebrafish follistatin and noggin are not expressed in the or ganizer region, nor are they linked to the mercedes mutation. Expression of both genes starts at midgastrula stages. While no patterned noggin express ion was detectable by in situ hybridization during gastrulation stages, lat er expression is confined to presumptive cartilage cells in the branchial a rches and the neurocranium and to proximal regions of the pectoral fin buds , follistatin transcripts in gastrulating embryos are confined to anterior paraxial regions, which give rise to head mesoderm and the first five somit es. The dorsolateral extent of this expression domain is regulated by Bmp2b , Chordino, and Follistatin itself. In addition, transient expression was o bserved in a subset of cells in the posterior notochord anlage. Later, foll istatin is expressed in brain, eyes, and somites. Comparison of the spatiot emporal expression pattern of follistatin and noggin with those of bmp2b an d bmp4 and overexpression studies suggest that Noggin and Follistatin may f unction as Bmp antagonists in later processes of zebrafish development, inc luding late phases of D-V patterning, to refine the early pattern set up by the interaction of Chordino and Bmp2/4. It thus appears that many, but not all, aspects of early dorsoventral patterning are shared among different v ertebrate species. (C) 1998 Academic Press.