A direct role for Fgf but not Wnt in otic placode induction

Abstract

Induction of the otic placode, which gives rise to all tissues comprising the inner ear, is a fundamental aspect of vertebrate development. A number of studies indicate that fibroblast growth factor (Fgf), especially Fgf3, is necessary and sufficient for otic induction. However, an alternative model proposes that Fgf must cooperate with Wnt8 to induce otic differentiation. Using a genetic approach in zebrafish, we tested the roles of Fgf3, Fgf8 and Wnt8. We demonstrate that localized misexpression of either Fgf3 or Fgf8 is sufficient to induce ectopic otic placodes and vesicles, even in embryos lacking Wnt8. Wnt8 is expressed in the hindbrain around the time of otic induction, but loss of Wnt8 merely delays expression of preotic markers and otic vesicles form eventually. The delay in otic induction correlates closely with delayed expression of fgf3 and fgf8 in the hindbrain. Localized misexpression of Wnt8 is insufficient to induce ectopic otic tissue. By contrast, global misexpression of Wnt8 causes development of supernumerary placodes/vesicles, but this reflects posteriorization of the neural plate and consequent expansion of the hindbrain expression domains of Fgf3 and Fgf8. Embryos that misexpress Wnt8 globally but are depleted for Fgf3 and Fgf8 produce no otic tissue. Finally, cells in the preotic ectoderm express Fgf(but not Wnt) reporter genes. Thus, preotic cells respond directly to Fgf but not Wnt8. We propose that Wnt8 serves to regulate timely expression of Fgf3 and Fgf8 in the hindbrain, and that Fgf from the hindbrain then acts directly on preplacodal cells to induce otic differentiation.

Genetic interactions underlying otic placode induction and formation

“…Full-length cDNA for fgf3 and fgf8 were cloned into pCS2ϩ vector and injected into wild-type embryos at the eight-cell stage (fgf3, 0.03 ng/ embryo; fgf8, 0.06 ng/embryo; Phillips et al, 2003).…”

Groucho corepressor proteins regulate otic vesicle outgrowth

“…In zebrafish, wnt signalling has been proposed to play a role in otic placode induction, by regulating expression of Fgf3 and Fgf8 in the hindbrain. Furthermore, injection of Wnt antagonists (GSK3b; Saint-Germain et al, 2004) and Wnt8 morpholino oligos (Phillips et al, 2004) resulted in otic vesicles of reduced size. Therefore, Tcf/Lef proteins play a positive role in the proposed pathway, the phenotypic consequences for Tle/Aes misexpression nevertheless would be contradictory to our results.…”

“…The best candidates for otic-inducing factors are members of the fibroblast growth factor (Fgf) family of peptide ligands. In particular, Fgf8 and Fgf3 are sufficient to ectopically induce otic vesicles (Lombardo et al, 1998;Vendrell et al, 2000;Bajoghli et al, 2004;Phillips et al, 2004). In zebrafish acerebellar mutants, which contain a disrupted Fgf8 gene, the otic vesicles show a reduced size (Whitfield et al, 1996;Reifers et al, 1998).…”

“…In chick, an ortolog of the Wnt8 gene (Wnt8c) is expressed in the hindbrain between the two prospective otic anlagen and human Wnt8 can induce a variety of otic markers in chick embryos (Ladher et al, 2000). In zebrafish, the indirect function of Wnt8 in this process has been revealed; disruption of the gene leads to a delay in preotic expression of Fgf8 and Fgf3, thus, indirectly causing otic vesicles of reduced size (Phillips et al, 2004). The Sox gene family also plays an important role in the development of the vertebrate ear (de Martino et al, 2000;Groves and Bronner-Fraser, 2000;Koster et al, 2000;Chiang et al, 2001;Dutton et al, 2001;SaintGermain et al, 2004).…”

Loss of Fgfr3 leads to excess hair cell development in the mouse organ of Corti

“…At early stages of embryogenesis, Fgfr1, Fgfr2, and several FGFs, including FGF3, FGF8, FGF10, and FGF19, are critical for either the induction of the otic vesicle or the initial development of the sensory epithelium (Mansour et al, 1993;Ladher et al, 2000Ladher et al, , 2005Pirvola et al, 2000Pirvola et al, , 2002Phillips et al, 2001Phillips et al, , 2004Pauley et al, 2003;. At intermediate stages of development, FGF signaling is also important; tissuespecific deletion of Fgfr1 results in severe defects in the development of the auditory sensory epithelium apparently due to a severe reduction in the precursor cells that give rise to the organ of Corti.…”