sprouty4 acts in vivo as a feedback-induced antagonist of FGF signaling inzebrafish

In looking for novel factors involved in the regulation of the fibroblast g rowth factor (FGF) signaling pathway, we have isolated a zebrafish sprouty4 gene, based on its extensive similarities with the expression patterns of both fgf8 and fgf3, Through gain- and loss-of-function experiments, we demo nstrate that Fgf8 and Fgf3 act in vivo to induce the expression of Spry4, w hich in turn can inhibit activity of these growth factors. When overexpress ed at low doses, Spry4 induces loss of cerebellum and reduction in size of the otic vesicle, thereby mimicking the fgf8/acerebellar mutant phenotype, Injections of high doses of Spry4 cause ventralization of the embryo, an op posite phenotype to the dorsalisation induced by overexpression of Fgf8 or Fgf3, Conversely we have shown that inhibition of Spry4 function through in jection of antisense morpholino oligonucleotide leads to a weak dorsalizati on of the embryo, the phenotype expected for an upregulation of Fgf8 or Fgf 3 signaling pathway. Finally, we show that Spry4 interferes with FGF signal ing downstream of the FGF receptor 1 (FGFR1), In addition, our analysis rev eals that signaling through FGFR1/Ras/mitogen-activated protein kinase path way is involved, not in mesoderm induction, but in the control of the dorso ventral patterning via the regulation of bone morphogenetic protein (BMP) e xpression.