Fgf9 signaling regulates inner ear morphogenesis through epithelial–mesenchymal interactions

Pax2/8 proteins coordinate sequential induction of otic and epibranchial placodes through differential regulation of foxi1, sox3 and fgf24

Vertebrate cranial placodes contribute vitally to development of sensory structures of the head. Amongst posterior placodes, the otic placode forms the inner ear whereas nearby epibranchial placodes produce sensory ganglia within branchial clefts. Though diverse in fate, these placodes show striking similarities in their early regulation. In zebrafish, both are initiated by localized Fgf signaling plus the ubiquitous competence factor Foxi1, and both express pax8 and sox3 in response. It has been suggested that Fgf initially induces a common otic/epibranchial field, which later subdivides in response to other signals. However, we find that otic and epibranchial placodes form 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 works with related genes pax2a/pax2b to downregulate otic expression of foxi1, a necessary step for further otic development. Additionally, pax2/8 activate otic expression of fgf24, which induces epibranchial expression of sox3. Knockdown of fgf24 or sox3 causes severe epibranchial deficiencies but has little effect on otic development. These findings clarify the roles of pax8 and sox3 and support a model whereby the otic placode forms first and induces epibranchial placodes through an Fgf-relay.

Vertebrate Lrig3-ErbB interactions occur in vitro but are unlikely to play a role in Lrig3-dependent inner ear morphogenesis

Background

The Lrig genes encode a family of transmembrane proteins that have been implicated in tumorigenesis, psoriasis, neural crest development, and complex tissue morphogenesis. Whether these diverse phenotypes reflect a single underlying cellular mechanism is not known. However, Lrig proteins contain evolutionarily conserved ectodomains harboring both leucine-rich repeats and immunoglobulin domains, suggesting an ability to bind to common partners. Previous studies revealed that Lrig1 binds to and inhibits members of the ErbB family of receptor tyrosine kinases by inducing receptor internalization and degradation. In addition, other receptor tyrosine kinase binding partners have been identified for both Lrig1 and Lrig3, leaving open the question of whether defective ErbB signaling is responsible for the observed mouse phenotypes.

Methodology/Principal Findings

Here, we report that Lrig3, like Lrig1, is able to interact with ErbB receptors in vitro. We examined the in vivo significance of these interactions in the inner ear, where Lrig3 controls semicircular canal formation by determining the timing and extent of Netrin1 expression in the otic vesicle epithelium. We find that ErbB2 and ErbB3 are present in the early otic epithelium, and that Lrig3 acts cell-autonomously here, as would be predicted if Lrig3 regulates ErbB2/B3 activity. However, inhibition of ErbB activation in the chick otic vesicle has no detectable effect on Netrin gene expression or canal morphogenesis.

Conclusions/Significance

Our results suggest that although both Lrig1 and Lrig3 can interact with ErbB receptors in vitro, modulation of Neuregulin signaling is unlikely to contribute to Lrig3-dependent processes of inner ear morphogenesis. These results highlight the similar binding properties of Lrig1 and Lrig3 and underscore the need to determine how these two family members bind to and regulate different receptors to affect diverse aspects of cell behavior in vivo.

Fgf16(IRESCre) mice: a tool to inactivate genes expressed in inner ear cristae and spiral prominence epithelium

Fibroblast growth factors play important roles in inner ear development. Previous studies showed that mouse Fgf16 is expressed asymmetrically during the otic cup and vesicle stages of development, suggesting roles in regulating or responding to anteroposterior axial cues. Here, we studied otic Fgf16 expression throughout embryonic development and found transcripts in the developing cristae and in a few cells in the lateral wall of the cochlear duct. To determine the otic function of Fgf16 and to follow the fate of Fgf16-expressing cells, we generated an Fgf16(IRESCre) allele. We show that Fgf16 does not have a unique role in inner ear development and that the Fgf16 lineage is found throughout the three cristae, in portions of the semicircular canal ducts, and in the cochlear spiral prominence epithelial cells. This strain will be useful for gene ablations in these tissues.

Homodimerization controls the fibroblast growth factor 9 subfamily's receptor binding and heparan sulfate-dependent diffusion in the extracellular matrix

Uncontrolled fibroblast growth factor (FGF) signaling can lead to human diseases, necessitating multiple layers of self-regulatory control mechanisms to keep its activity in check. Herein, we demonstrate that FGF9 and FGF20 ligands undergo a reversible homodimerization, occluding their key receptor binding sites. To test the role of dimerization in ligand autoinhibition, we introduced structure-based mutations into the dimer interfaces of FGF9 and FGF20. The mutations weakened the ability of the ligands to dimerize, effectively increasing the concentrations of monomeric ligands capable of binding and activating their cognate FGF receptor in vitro and in living cells. Interestingly, the monomeric ligands exhibit reduced heparin binding, resulting in their increased radii of heparan sulfate-dependent diffusion and biologic action, as evidenced by the wider dilation area of ex vivo lung cultures in response to implanted mutant FGF9-loaded beads. Hence, our data demonstrate that homodimerization autoregulates FGF9 and FGF20's receptor binding and concentration gradients in the extracellular matrix. Our study is the first to implicate ligand dimerization as an autoregulatory mechanism for growth factor bioactivity and sets the stage for engineering modified FGF9 subfamily ligands, with desired activity for use in both basic and translational research.

Tbx1 and Brn4 regulate retinoic acid metabolic genes during cochlear morphogenesis

BACKGROUND

In vertebrates, the inner ear is comprised of the cochlea and vestibular system, which develop from the otic vesicle. This process is regulated via inductive interactions from surrounding tissues. Tbx1, the gene responsible for velo-cardio-facial syndrome/DiGeorge syndrome in humans, is required for ear development in mice. Tbx1 is expressed in the otic epithelium and adjacent periotic mesenchyme (POM), and both of these domains are required for inner ear formation. To study the function of Tbx1 in the POM, we have conditionally inactivated Tbx1 in the mesoderm while keeping expression in the otic vesicle intact.

RESULTS

Conditional mutants (TCre-KO) displayed malformed inner ears, including a hypoplastic otic vesicle and a severely shortened cochlear duct, indicating that Tbx1 expression in the POM is necessary for proper inner ear formation. Expression of the mesenchyme marker Brn4 was also lost in the TCre-KO. Brn4-;Tbx1+/-embryos displayed defects in growth of the distal cochlea. To identify a potential signal from the POM to the otic epithelium, expression of retinoic acid (RA) catabolizing genes was examined in both mutants. Cyp26a1 expression was altered in the TCre-KO, while Cyp26c1 showed reduced expression in both TCre-KO and Brn4-;Tbx1+/- embryos.

CONCLUSION

These results indicate that Tbx1 expression in the POM regulates cochlear outgrowth potentially via control of local retinoic acid activity.

Cross-repressive interactions between Lrig3 and netrin 1 shape the architecture of the inner ear

The sense of balance depends on the intricate architecture of the inner ear, which contains three semicircular canals used to detect motion of the head in space. Changes in the shape of even one canal cause drastic behavioral deficits, highlighting the need to understand the cellular and molecular events that ensure perfect formation of this precise structure. During development, the canals are sculpted from pouches that grow out of a simple ball of epithelium, the otic vesicle. A key event is the fusion of two opposing epithelial walls in the center of each pouch, thereby creating a hollow canal. During the course of a gene trap mutagenesis screen to find new genes required for canal morphogenesis, we discovered that the Ig superfamily protein Lrig3 is necessary for lateral canal development. We show that this phenotype is due to ectopic expression of the axon guidance molecule netrin 1 (Ntn1), which regulates basal lamina integrity in the fusion plate. Through a series of genetic experiments, we show that mutually antagonistic interactions between Lrig3 and Ntn1 create complementary expression domains that define the future shape of the lateral canal. Remarkably, removal of one copy of Ntn1 from Lrig3 mutants rescues both the circling behavior and the canal malformation. Thus, the Lrig3/Ntn1 feedback loop dictates when and where basement membrane breakdown occurs during canal development, revealing a new mechanism of complex tissue morphogenesis.

Lmx1a is required for segregation of sensory epithelia and normal ear histogenesis and morphogenesis

At embryonic day 8.5, the LIM-homeodomain factor Lmx1a is expressed throughout the otic placode but becomes developmentally restricted to non-sensory epithelia of the ear (endolymphatic duct, ductus reuniens, cochlea lateral wall). We confirm here that the ears of newborn dreher (Lmx1a (dr)) mutants are dysmorphic. Hair cell markers such as Atoh1 and Myo7 reveal, for the first time, that newborn Lmx1a mutants have only three sensory epithelia: two enlarged canal cristae and one fused epithelium comprising an amalgamation of the cochlea, saccule, and utricle (a "cochlear-gravistatic" endorgan). The enlarged anterior canal crista develops by fusion of horizontal and anterior crista, whereas the posterior crista fuses with an enlarged papilla neglecta that may extend into the cochlear lateral wall. In the fused endorgan, the cochlear region is distinguished from the vestibular region by markers such as Gata3, the presence of a tectorial membrane, and cochlea-specific innervation. The cochlea-like apex displays minor disorganization of the hair and supporting cells. This contrasts with the basal half of the cochlear region, which shows a vestibular epithelium-like organization of hair cells and supporting cells. The dismorphic features of the cochlea are also reflected in altered gene expression patterns. Fgf8 expression expands from inner hair cells in the apex to most hair cells in the base. Two supporting cell marker proteins, Sox2 and Prox1, also differ in their cellular distribution between the base and the apex. Sox2 expression expands in mutant canal cristae prior to their enlargement and fusion and displays a more diffuse and widespread expression in the base of the cochlear region, whereas Prox1 is not detected in the base. These changes in Sox2 and Prox1 expression suggest that Lmx1a expression restricts and sharpens Sox2 expression, thereby defining non-sensory and sensory epithelium. The adult Lmx1a mutant organ of Corti shows a loss of cochlear hair cells, suggesting that the long-term maintenance of hair cells is also disrupted in these mutants.

Hearing loss in a mouse model of Muenke syndrome

The heterozygous Pro250Arg substitution mutation in fibroblast growth factor receptor 3 (FGFR3), which increases ligand-dependent signalling, is the most common genetic cause of craniosynostosis in humans and defines Muenke syndrome. Since FGF signalling plays dosage-sensitive roles in the differentiation of the auditory sensory epithelium, we evaluated hearing in a large group of Muenke syndrome subjects, as well as in the corresponding mouse model (Fgfr3^P244R). The Muenke syndrome cohort showed significant, but incompletely penetrant, predominantly low-frequency sensorineural hearing loss, and the Fgfr3^P244R mice showed dominant, fully penetrant hearing loss that was more severe than that in Muenke syndrome individuals, but had the same pattern of relative high-frequency sparing. The mouse hearing loss correlated with an alteration in the fate of supporting cells (Deiters'-to-pillar cells) along the entire length of the cochlear duct, with the most extreme abnormalities found at the apical or low-frequency end. In addition, there was excess outer hair cell development in the apical region. We conclude that low-frequency sensorineural hearing loss is a characteristic feature of Muenke syndrome and that the genetically equivalent mouse provides an excellent model that could be useful in testing hearing loss therapies aimed at manipulating the levels of FGF signalling in the inner ear.

Expression of ERK signaling inhibitors Dusp6, Dusp7, and Dusp9 during mouse ear development

The levels of fibroblast growth factor (FGF) signaling play important roles in coordinating development of the mouse inner, middle, and outer ears. Extracellular signal-regulated kinases (ERKs) are among the effectors that transduce the FGF signal to the nucleus and other cellular compartments. Attenuation of ERK activity by dephosphorylation is necessary to modulate the magnitude and duration of the FGF signal. Recently, we showed that inactivation of the ERK phosphatase, dual specificity phosphatase 6 (DUSP6), causes partially penetrant postnatal lethality, hearing loss and skeletal malformations. To determine whether other Dusps may function redundantly with Dusp6 during otic development, we surveyed the expression domains of the three ERK-specific DUSP transcripts, Dusp6, Dusp7, and Dusp9, in the embryonic mouse ear. We show that each is expressed in partially overlapping patterns that correspond to regions of active FGF signaling, suggesting combinatorial roles in negative regulation of this pathway during ear development.

The molecular biology of ear development - "Twenty years are nothing"

Views of classical biological problems changed dramatically with the rise of molecular biology as a common framework. It was indeed the new language of life sciences. Molecular biology increasingly moved us towards a unified view of developmental genetics as ideas and techniques were imported to vertebrates from other biological systems where genetics was in a more advanced state. The ultimate advance has been the ability to actually perform genetic manipulations in vertebrate organisms that were almost unthinkable before. During the last two decades these technical advances entered into and affected the research on ear development. These events are still very recent and have been with us for no longer than two decades, which is the reason for the title of this article. This new scenario forms the basis of the current and productive work of many laboratories, and this is what this Special Issue of The International Journal of Developmental Biology wants to show, presenting a snapshot of insights at the beginning of the 21st Century. In this article, we give an overview of the topics that are addressed in this Ear Development Special Issue, and also we take the opportunity to informally dig into the genealogy of some of those topics, trying to link the current work with some classical work of the past.

Cooperative function of Tbx1 and Brn4 in the periotic mesenchyme is necessary for cochlea formation

The T-box transcription factor TBX1 has been identified as the major gene responsible for the etiology of velocardiofacial syndrome/DiGeorge syndrome (VCFS/DGS). Conductive hearing loss occurs in a majority of patients with this syndrome, while sensorineural deafness has also been reported in some cases. Mutations in POU3F4/BRN4, a POU domain transcription factor, cause DFN3, an X-linked nonsyndromic form of deafness characterized by mixed conductive and sensorineural hearing loss. Inactivation of the murine orthologues of these genes causes similar defects to those seen in humans and has provided excellent models for the study of inner ear development. Tbx1 and Brn4 are expressed in the mesenchymal cells surrounding the otic vesicle and have been shown to play roles in cochlear outgrowth. Furthermore, expression of Brn4 is reduced in Tbx1 null mutants, suggesting a possible genetic interaction between these genes. To test whether Tbx1 and Brn4 function in a common pathway, mice mutant for both genes were generated and analyzed for inner ear defects. Brn4-;Tbx1+/- mutants displayed a significant reduction in the number of turns of the cochlea compared to Brn4- or Tbx1+/- mice. In addition, Brn4-;Tbx1+/- mice displayed structural defects in the apical cochlea indicative of Mondini dysplasia found in patients with either VCFS/DGS or DFN3. These data establish a genetic interaction between Tbx1 and Brn4 relevant to human disease and indicate a function of these genes in signaling from the periotic mesenchyme to the otic vesicle to direct proper coiling of the cochlear duct.

Fgf3 is required for dorsal patterning and morphogenesis of the inner ear epithelium

The inner ear, which contains sensory organs specialized for hearing and balance, develops from an ectodermal placode that invaginates lateral to hindbrain rhombomeres (r) 5-6 to form the otic vesicle. Under the influence of signals from intra- and extraotic sources, the vesicle is molecularly patterned and undergoes morphogenesis and cell-type differentiation to acquire its distinct functional compartments. We show in mouse that Fgf3, which is expressed in the hindbrain from otic induction through endolymphatic duct outgrowth, and in the prospective neurosensory domain of the otic epithelium as morphogenesis initiates, is required for both auditory and vestibular function. We provide new morphologic data on otic dysmorphogenesis in Fgf3 mutants, which show a range of malformations similar to those of Mafb (Kreisler), Hoxa1 and Gbx2 mutants, the most common phenotype being failure of endolymphatic duct and common crus formation, accompanied by epithelial dilatation and reduced cochlear coiling. The malformations have close parallels with those seen in hearing-impaired patients. The morphologic data, together with an analysis of changes in the molecular patterning of Fgf3 mutant otic vesicles, and comparisons with other mutations affecting otic morphogenesis, allow placement of Fgf3 between hindbrain-expressed Hoxa1 and Mafb, and otic vesicle-expressed Gbx2, in the genetic cascade initiated by WNT signaling that leads to dorsal otic patterning and endolymphatic duct formation. Finally, we show that Fgf3 prevents ventral expansion of r5-6 neurectodermal Wnt3a, serving to focus inductive WNT signals on the dorsal otic vesicle and highlighting a new example of cross-talk between the two signaling systems.

In vivo genetic ablation of the periotic mesoderm affects cell proliferation survival and differentiation in the cochlea

Tbx1 is required for ear development in humans and mice. Gene manipulation in the mouse has discovered multiple consequences of loss of function on early development of the inner ear, some of which are attributable to a cell autonomous role in maintaining cell proliferation of epithelial progenitors of the cochlear and vestibular apparata. However, ablation of the mesodermal domain of the gene also results in severe but more restricted abnormalities. Here we show that Tbx1 has a dynamic expression during late development of the ear, in particular, is expressed in the sensory epithelium of the vestibular organs but not of the cochlea. Vice versa, it is expressed in the condensed mesenchyme that surrounds the cochlea but not in the one that surrounds the vestibule. Loss of Tbx1 in the mesoderm disrupts this peri-cochlear capsule by strongly reducing the proliferation of mesenchymal cells. The organogenesis of the cochlea, which normally occurs inside the capsule, was dramatically affected in terms of growth of the organ, as well as proliferation, differentiation and survival of its epithelial cells. This model provides a striking demonstration of the essential role played by the periotic mesenchyme in the organogenesis of the cochlea.

Regulation of FGF10 by POU transcription factor Brn3a in the developing trigeminal ganglion

The POU-domain transcription factor Brn3a is expressed in specific neurons of the caudal CNS and peripheral sensory nervous system. The sensory neurons of mice lacking Brn3a exhibit marked defects in axon growth and extensive apoptosis in late gestation. Here we show that expression of the developmental regulator FGF10 is approximately 35-fold increased in the developing trigeminal ganglia of Brn3a-null mice. In order to determine whether FGF10 regulates other changes in gene expression observed in Brn3a knock-out ganglia, we have used a sensory-specific enhancer to over-express FGF10 in transgenic mice. Microarray analysis of trigeminal ganglia from individual transgenic founders effectively excludes the cell-autonomous activity of FGF10 as a mechanism for mediating the downstream effects of the loss of Brn3a, probably because developing sensory neurons lack the appropriate type of FGF receptor.

Receptor specificity of the fibroblast growth factor family. The complete mammalian FGF family

In mammals, fibroblast growth factors (FGFs) are encoded by 22 genes. FGFs bind and activate alternatively spliced forms of four tyrosine kinase FGF receptors (FGFRs 1-4). The spatial and temporal expression patterns of FGFs and FGFRs and the ability of specific ligand-receptor pairs to actively signal are important factors regulating FGF activity in a variety of biological processes. FGF signaling activity is regulated by the binding specificity of ligands and receptors and is modulated by extrinsic cofactors such as heparan sulfate proteoglycans. In previous studies, we have engineered BaF3 cell lines to express the seven principal FGFRs and used these cell lines to determine the receptor binding specificity of FGFs 1-9 by using relative mitogenic activity as the readout. Here we have extended these semiquantitative studies to assess the receptor binding specificity of the remaining FGFs 10-23. This study completes the mitogenesis-based comparison of receptor specificity of the entire FGF family under standard conditions and should help in interpreting and predicting in vivo biological activity.

Cells, molecules and morphogenesis: the making of the vertebrate ear

The development and evolution of mechanosensory cells and the vertebrate ear is reviewed with an emphasis on delineating the cellular, molecular and developmental basis of these changes. Outgroup comparisons suggests that mechanosensory cells are ancient features of multicellular organisms. Molecular evidence suggests that key genes involved in mechanosensory cell function and development are also conserved among metazoans. The divergent morphology of mechanosensory cells across phyla is interpreted here as 'deep molecular homology' that was in parallel shaped into different forms in each lineage. The vertebrate mechanosensory hair cell and its associated neuron are interpreted as uniquely derived features of vertebrates. It is proposed that the vertebrate otic placode presents a unique embryonic adaptation in which the diffusely distributed ancestral mechanosensory cells became concentrated to generate a large neurosensory precursor population. Morphogenesis of the inner ear is reviewed and shown to depend on genes expressed in and around the hindbrain that interact with the otic placode to define boundaries and polarities. These patterning genes affect downstream genes needed to maintain proliferation and to execute ear morphogenesis. We propose that fibroblast growth factors (FGFs) and their receptors (FGFRs) are a crucial central node to translate patterning into the complex morphology of the vertebrate ear. Unfortunately, the FGF and FGFR genes have not been fully analyzed in the many mutants with morphogenetic ear defects described thus far. Likewise, little information exists on the ear histogenesis and neurogenesis in many mutants. Nevertheless, a molecular mechanism is now emerging for the formation of the horizontal canal, an evolutionary novelty of the gnathostome ear. The existing general module mediating vertical canal growth and morphogenesis was modified by two sets of new genes: one set responsible for horizontal canal morphogenesis and another set for neurosensory formation of the horizontal crista and associated sensory neurons. The dramatic progress in deciphering the molecular basis of ear morphogenesis offers grounds for optimism for translational research toward intervention in human morphogenetic defects of the ear.

The role of Paraxial Protocadherin in Xenopus otic placode development

Vertebrate inner ear develops from its rudiment, otic placode, which later forms otic vesicle and gives rise to tissues comprising the entire inner ear. Although several signaling molecules have been identified as candidates responsible for inner ear specification and patterning, many details remain elusive. Here, we report that Paraxial Protocadherin (PAPC) is required for otic vesicle formation in Xenopus embryos. PAPC is expressed strictly in presumptive otic placode and later in otic vesicle during inner ear morphogenesis. Knockdown of PAPC by dominant-negative PAPC results in the failure of otic vesicle formation and the loss of early inner ear markers Sox9 and Tbx2, suggesting the requirement of PAPC in the early stage of otic vesicle development. However, PAPC alone is not sufficient to induce otic placode formation.

The retinoblastoma gene pathway regulates the postmitotic state of hair cells of the mouse inner ear

Precursors of cochlear and vestibular hair cells of the inner ear exit the cell cycle at midgestation. Hair cells are mitotically quiescent during late-embryonic differentiation stages and postnatally. We show here that the retinoblastoma gene Rb and the encoded protein pRb are expressed in differentiating and mature hair cells. In addition to Rb, the cyclin dependent kinase inhibitor (CKI) p21 is expressed in developing hair cells, suggesting that p21 is an upstream effector of pRb activity. p21 apparently cooperates with other CKIs, as p21-null mice exhibited an unaltered inner ear phenotype. By contrast, Rb inactivation led to aberrant hair cell proliferation, as analysed at birth in a loss-of-function/transgenic mouse model. Supernumerary hair cells expressed various cell type-specific differentiation markers, including components of stereocilia. The extent of alterations in stereociliary bundle morphology ranged from near-normal to severe disorganization. Apoptosis contributed to the mutant phenotype, but did not compensate for the production of supernumerary hair cells, resulting in hyperplastic sensory epithelia. The Rb-null-mediated proliferation led to a distinct pathological phenotype, including multinucleated and enlarged hair cells, and infiltration of hair cells into the mesenchyme. Our findings demonstrate that the pRb pathway is required for hair cell quiescence and that manipulation of the cell cycle machinery disrupts the coordinated development within the inner ear sensory epithelia.

The mammalian target of rapamycin-p70 ribosomal S6 kinase but not phosphatidylinositol 3-kinase-Akt signaling is responsible for fibroblast growth factor-9-induced cell proliferation

Fibroblast growth factor-9 (FGF9) is a potent mitogen that stimulates normal and cancer cell proliferation though the signaling mechanism is not fully understood. In this study, we aimed to unravel the signaling cascades mediate FGF9 actions in human uterine endometrial stromal cell. Our results demonstrate that the mitogenic effect of FGF9 is transduced via two parallel but additive signaling pathways involving mammalian target of rapamycin (mTOR) and extracellular signal-regulated kinase. Activation of mTOR by FGF9 induces p70 ribosomal S6 kinase (S6K1) phosphorylation, cyclin expression, and cell proliferation, which are independent of phosphatidylinositol 3-kinase and Akt. Coimmunoprecipitation analysis demonstrates that mTOR physically associates with S6K1 upon FGF9 treatment, whereas ablation of mTOR activity using RNA interference or pharmacological inhibitor blocks S6K1 phosphorylation and cell proliferation induced by FGF9. Further study demonstrates that activation of mTOR is regulated by a phospholipase Cgamma-controlled calcium signaling pathway. These studies provide evidence to demonstrate, for the first time, that a novel signaling cascade involving phospholipase Cgamma, calcium, mTOR, and S6K1 is activated by FGF9 in a receptor-specific manner.

Identification ofcis-element regulating expression of the mouseFgf10 gene during inner ear development

Fibroblast growth factor (FGF) signaling is crucial for the induction and growth of the ear, a sensory organ that involves intimate tissue interactions. Here, we report the abnormality of Fgf10 null ear and the identification of a cis-regulatory element directing otic expression of Fgf10. In Fgf10 null inner ears, we found that the initial development of semicircular, vestibular, and cochlear divisions is roughly normal, after which there are abnormalities of semicircular canal/cristae and vestibular development. The mutant semicircular disks remain without canal formation by the perinatal stage. To elucidate regulation of the Fgf10 expression during inner ear development, we isolated a 6.6-kb fragment of its 5'-upstream region and examined its transcriptional activity with transgenic mice, using a lacZ-reporter system. From comparison of the mouse sequences of the 6.6-kb fragment with corresponding sequences of the human and chicken Fgf10, we identified a 0.4-kb enhancer sequence that drives Fgf10 expression in the developing inner ear. The enhancer sequences have motifs for many homeodomain-containing proteins (e.g., Prx, Hox, Nkx), in addition to POU-domain factors (e.g., Brn3), zinc-finger transcription factors (e.g., GATA-binding factors), TCF/LEF-1, and a SMAD-interacting protein. Thus, FGF10 signaling is dispensable for specification of otic compartment identity but is required for hollowing the semicircular disk. Furthermore, the analysis of a putative inner ear enhancer of Fgf10 has disclosed a complicated regulation of Fgf10 during inner ear development by numerous transcription factors and signaling pathways.