Essential role of Bmp7 (snailhouse) and its prodomain in dorsoventral patterning of the zebrafish embryo

Bone morphogenetic proteins (Bmps) are signaling molecules that have been implicated in a variety of inductive processes. We report here that zebrafish Bmp7 is disrupted in snailhouse (snh) mutants. The allele snh(st1) is a translocation deleting the bmp7 gene, while snh(ty68) displays a Val->Gly exhange in a conserved motif of the Bmp7 prodomain. The snh(ty68) mutation is temperature-sensitive, leading to severalfold reduced activity of mutant Bmp7 at 28 degrees C and non-detectable activity at 33 degrees C. This prodomain lesion affects secretion and/or stability of secreted mature Bmp7 after processing has occurred. Both snh(st1) and snh(ty68) mutant zebrafish embryos are strongly dorsalized, indicating that bmp7 is required for the specification of ventral cell fates during early dorsoventral patterning. At higher temperature, the phenotype of snh(ty68) mutant embryos is identical to that caused by the amorphic bmp2b mutation swirl swr(ta72) and similar to that caused by the smad5 mutation somitabun sbn(dtc24). mRNA injection studies and double mutant analyses indicate that Bmp2b and Bmp7 closely cooperate and that Bmp2b/Bmp7 signaling is transduced by Smad5 and antagonized by Chordino.