Morphogenesis of the inner ear epithelium requires coordinated deployment of several signaling pathways and
disruptions cause abnormalities of hearing and/or balance. With the advent of cochlear implantation to treat
hearing loss even in cases of inner ear malformation, it is critical to understand exactly how such malformations
affect the auditory ganglia and innervation. Also, in light of the intense focus on in vitro generation of inner ear
cell types for transplantation and in vivo manipulation of developmental signaling molecules to promote
differentiation of various inner ear cells for hearing restoration, elucidating the roles and regulation of such
signals and their effectors governing otic differentiation and morphogenesis are necessary to advance treatment.
The genes encoding FGF3 and FGF10, ligands that signal through FGFR2b and FGFR1b, are expressed
dynamically throughout otic development in both epithelial and ganglion domains. Studies conducted by the
Mansour Lab of both conventional Fgf3 and Fgf10 conditional knockout mice and those expressing a
doxycycline-inducible ligand trap (dnFGFR2b) that rapidly inhibits signaling through both FGFR1b and
FGFR2b, showed that Fgf3 and Fgf10 are not required in the placode lineage for otocyst formation, but are
required subsequently for otocyst patterning, neuroblast maintenance, epithelial proliferation and both
vestibular and cochlear morphogenesis. Furthermore, the first genome wide analyses of otocyst mRNA revealed
FGFR2b/1b signaling targets that define novel candidates for genes involved in otic morphogenesis and function.
This proposal has two Aims addressing the hypotheses that 1) FGFR2b/1b signaling is required continuously
for both otic neuroblast specification and maintenance, and that at later stages, mesenchymal signaling, as well
as that in the epithelial and ganglion domains, is required for cochlear epithelial differentiation and ganglion
maintenance and 2) FGFR2b/1b downstream target genes mediate some or all of the effects of FGFR2b/1b
signaling on otic morphogenesis and gangliogenesis. To determine the early role of FGFR2b/1b signaling in otic
ganglion formation and its later role in epithelial differentiation and ganglion maintenance, DOX-induced
ubiquitous and CRE-limited expression of dnFGFR2b will be employed and morphology and molecular markers
of otic patterning, proliferation and survival in both tissues will be assessed. To determine the roles of
downstream targets of FGFR2b/1b signaling, two genes encoding transcription factors that are activated by
FGFR2b/1b signaling and one gene encoding a BMP signaling regulator that is repressed by FGFR2b/1b
signaling will be studied. Otic conditional mutants will be generated for each gene, and their morphologic and
functional development will be assessed. In addition, the extent to which the BMP regulator contributes to the
dnFGFR2b phenotypes and the effects of overexpressing the BMP regulator will be assessed.
The results will contribute new knowledge that will facilitate future efforts to manipulate the FGF signaling
system for hearing restoration.