Zebrafish admp is required to restrict the size of the organizer and to promote posterior and ventral development

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Abstract
Bone morphogenetic proteins (Bmps) and their roles during early dorsoventral patterning of the vertebrate embryo are well understood. The role and regulation of a more distant member of this family, the anti-dorsalizing morphogenetic protein (Admp), however, are less clear. Here, we report the isolation and characterization of zebrafish admp. Unlike other bmps, admp is exclusively expressed on the dorsal side. Expression starts at blastula stages in the region of the organizer, giving rise to anterior neuroectoderm and axial mesoderm. During the course of gastrulation, both the neuroectodermal and the mesodermal admp transcripts vanish in an anterior-posterior wave. The maintenance of admp expression is positively influenced by Nodal signaling and by Bozozok (Boz), an organizer-promoting homeodomain protein acting as a repressor of early bmp2b expression. Despite the positive effect of boz on admp expression, Boz and Admp have rather opposite effects on zebrafish patterning, as revealed in gain- and loss-of-function experiments. Upon overexpression, admp has Bmp-like activities causing a smaller organizer and enhanced ventral specification, very similar to the phenotype caused by the loss of boz function in mutant embryos. Antisense-based admp knockdown, on the other side, leads to an enlarged organizer and impaired ventral and posterior development, as observed in embryos after boz overexpression. This finding indicates that admp is required for the development of embryonic structures normally suppressed by organizer activities. The seeming discrepancy between the regulative and functional relationship of boz and admp is discussed, and models are proposed according to which Admp might be part of a negative feedback loop to pattern and confine the organizer region.