The role of tolloid/mini fin in dorsoventral pattern formation of the zebrafish embryo

A highly conserved TGF-P signaling pathway is involved in the establishment of the dorsoventral axis of the vertebrate embryo. Specifically, Bone Morp hogenetic Proteins (Bmps) pattern ventral tissues of the embryo while inhib itors of Bmps, such as Chordin, Noggin and Follistatin, are implicated in d orsal mesodermal and neural development. We investigated the role of Tolloi d, a metalloprotease that can cleave Chordin and increase Bmp activity, in patterning the dorsoventral axis of the zebrafish embryo. Injection of toll oid mRNA into six dorsalized mutants rescued only one of these mutants, min i fin. Through chromosomal mapping, linkage and cDNA sequence analysis of s everal mini fin alleles, we demonstrate that mini fin encodes the tolloid g ene. Characterization of the mini fun mutant phenotype reveals that Mini fi n/Tolloid activity is required for patterning ventral tissues of the tail: the ventral fin, and the ventroposterior somites and vasculature, Gene expr ession studies show that mfn mutants exhibit reduced expression of ventrall y restricted markers at the end of gastrulation, suggesting that the loss o f ventral tail tissues is caused by a dorsalization occurring at the end of gastrulation, Based on the mini fin mutant phenotype and the expression of tolloid, we propose that Mini fin/Tolloid modifes the Bmp activity gradien t at the end of gastrulation, when the ventralmost marginal cells of the em bryo are in close proximity to the dorsal Chordin-expressing cells. At this time, unimpeded Chordin may diffuse to the most ventral marginal regions a nd inhibit high Bmp activity levels. In the presence of Mini fin/Tolloid, h owever, Chordin activity would be negatively modulated through proteolytic cleavage, thereby increasing Bmp signaling activity. This extracellular mec hanism is amplified by an autoregulatory loop for bmp gene expression.