Sprouty proteins and RTK pathway antagonism in the lateral line primordium

The lateral line primodrium migrates caudally along the trunk of the embryo, periodically generating and depositing epithelialized rosettes which will go on to form the mature neuromasts of the lateral line. Within the migrating posterior lateral line primordium (pLLp), fibroblast growth factor (FGF) signals are thought to induce both the specification of a sensory hair cell by driving expression of the proneural gene atoh1a as well as the morphogenesis of a cellular rosette surrounding this hair cell precursor. The pLLp must therefore coordinate two complex morphogenetic processes: cell migration and rosette morphogenesis. In the pLLp, FGF signaling also drives the expression of sprouty genes, which typically act as feedback inhibitors of FGF signaling. We show that in the primordium, loss of sprouty function leads to a surprising decrease in expression of the FGF target genes pea3 and atoh1a, suggesting that sproutys are not acting as feedback inhibitors of FGF‐dependent gene expression. Instead, they may act to antagonize alternate pathways downstream of RTK signaling which promote migratory behavior. In this manner, by driving sprouty expression, FGF signaling both promotes the formation of rosettes and prevents the migratory behavior which predominates at the leading edge and which may interfere with formation of stable epithelial structures.