The Role of Fgf and Its Downstream Effectors in Otic and Epibranchial Development in Zebrafish

In vertebrates, the otic placode forms inner ear and epibranchial placodes produce sensory ganglia within branchial clefts. Fibroblast growth factor (FGF) family of protein ligands from the surrounding tissues are responsible for otic and epibranchial placode induction. Members of pax2/5/8 family of transcription factors function as mediators during otic induction. To understand the temporal and spatial requirements of Fgf and their interaction with pax2/8 for otic induction, we used heat shock inducible transgenic lines of zebrafish to misexpress fgf3/8 and pax2a/8 under the control of hsp70 promoter. Loss of function studies were done to examine the functions of pax2/8 genes in regulating otic and epibranchial development. We show that global transient activation of hs:fgf3 or hs:fgf8 at mid-late gastrula stages (7-8 hpf) severely impairs otic induction, in part by disrupting formation of the principal signaling centers in the hindbrain. Additionally, mosaic studies show that high-level misexpression blocks otic fate cell-autonomously, whereas low to moderate levels promote otic development. At later stages high-level Fgf misexpression, both globally and locally does not inhibit otic fate, but rather causes a dramatic expansion of endogenous otic domains. Misexpression of hs:pax2a or hs:pax8 also expands endogenous otic domains but is not sufficient to bypass the requirement for Fgf signaling. Co-misexpression of Fgf with pax2a or pax8 leads to production of ectopic otic tissue in a broad range of cranial ectoderm. These data show that changes in timing, distribution and level of Fgf signaling and its downstream effectors influences otic induction. We show that otic and epibranchial placodes are induced at different times and by distinct mechanisms. Initially, Fgf from surrounding tissues induces otic expression of pax8 and sox3, which cooperate synergistically to establish otic fate. Subsequently, pax8 along with pax2a/pax2b downregulate foxi1 expression in otic cells, which is necessary for further otic development. Additionally, pax2/8 activate otic expression of fgf24, which induces epibranchial expression of sox3. Blocking functions of fgf24 or sox3 causes severe epibranchial deficiencies but has little effect on otic development. These results support the model whereby the otic placode forms first and induces epibranchial placodes through pax2/8-dependent Fgf24 signaling.