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

Tsukushi is essential for the formation of the posterior semicircular canal that detects gait performance

Tsukushi is a small, leucine-rich repeat proteoglycan that interacts with and regulates essential cellular signaling cascades in the chick retina and murine subventricular zone, hippocampus, dermal hair follicles, and the cochlea. However, its function in the vestibules of the inner ear remains unknown. Here, we investigated the function of Tsukushi in the vestibules and found that Tsukushi deficiency in mice resulted in defects in posterior semicircular canal formation in the vestibules, but did not lead to vestibular hair cell loss. Furthermore, Tsukushi accumulated in the non-prosensory and prosensory regions during the embryonic and postnatal developmental stages. The downregulation of Tsukushi altered the expression of key genes driving vestibule differentiation in the non-prosensory regions. Our results indicate that Tsukushi interacts with Wnt2b, bone morphogenetic protein 4, fibroblast growth factor 10, and netrin 1, thereby controlling semicircular canal formation. Therefore, Tsukushi may be an essential component of the molecular pathways regulating vestibular development.

Genetic interactions support an inhibitory relationship between bone morphogenetic protein 2 and netrin 1 during semicircular canal formation

The semicircular canals of the mammalian inner ear are derived from epithelial pouches in which epithelial cells in the central region of each pouch undergo resorption, leaving behind the region at the rim to form a tube-shaped canal. Lack of proliferation at the rim and/or over-clearing of epithelial cells in the center of the pouch can obliterate canal formation. Otic-specific knockout of bone morphogenetic protein 2 (Bmp2) results in absence of all three semicircular canals; however, the common crus and ampullae housing the sensory tissue (crista) are intact. The lack of Bmp2 causes Ntn1 (which encodes netrin 1), which is required for canal resorption, to be ectopically expressed at the canal rim. Ectopic Ntn1 results in reduction of Dlx5 and Lmo4, which are required for rim formation. These phenotypes can be partially rescued by removing one allele of Ntn1 in the Bmp2 mutants, indicating that Bmp2 normally negatively regulates Ntn1 for canal formation. Additionally, non-resorption of the canal pouch in Ntn1^−/− mutants is partially rescued by removing one allele of Bmp2. Thus, reciprocal inhibition between Bmp2 and netrin 1 is involved in canal formation of the vestibule.

Summary:Bmp2-conditional mutant analyses support the hypothesis that presumptive crista induces canal genesis zone in the canal pouch to express Bmp2, which promotes canal formation by restricting Ntn1 expression to the resorption domain.

Relative Preservation of Superior Semicircular Canal Architecture in CHARGE Syndrome

OBJECTIVES

(1) Describe common patterns of semicircular canal (SCC) anomalies in CHARGE syndrome (CS) and (2) recognize that in CS, the architecture of the superior SCC may be relatively preserved.

STUDY DESIGN

This is a retrospective review of temporal bone imaging studies.

SETTING

Quaternary care center.

SUBJECTS AND METHODS

A sample of 37 patients with CS. All subjects met clinical diagnostic criteria for CS. The presence/absence of anomalies of the middle ear, mastoid, temporal bone venous anatomy, inner ear, and internal auditory canal was recorded. Anomalies of each SCC were considered separately and by severity (normal, dysplasia, aplasia).

RESULTS

Thirty-seven subjects (74 temporal bones) were reviewed. Thirty-four (92.0%) patients demonstrated bilateral SCC anomalies. Three (8.0%) had normal SCCs. In patients with SCC anomalies, all canals demonstrated bilateral abnormalities. Thirty-two (86.5%) patients had bilateral horizontal SCC aplasia. These 32 patients also demonstrated posterior SCC aplasia in at least 1 ear. Of 74 temporal bones, 37 (50.0%) had superior SCC dysplasia. All dysplastic superior SCCs showed preservation of the anterior limb. Complete superior SCC aplasia was found in 28 (37.8%) temporal bones.

CONCLUSION

SCC anomalies occur with high frequency in CS. Complete absence of the horizontal and posterior canals is typical and usually bilateral. By contrast, the superior SCC often demonstrates relative preservation of the anterior limb.

Characterization of cis-regulatory elements for Fgf10 expression in the chick embryo

BACKGROUND

Fgf10 is expressed in various tissues and organs, such as the limb bud, heart, inner ear, and head mesenchyme. Previous studies identified Fgf10 enhancers for the inner ear and heart. However, Fgf10 enhancers for other tissues have not been identified.

RESULTS

By using primary culture chick embryo lateral plate mesoderm cells, we compared activities of deletion constructs of the Fgf10 promoter region, cloned into a promoter-less luciferase reporter vector. We identified a 0.34-kb proximal promoter that can activate luciferase expression. Then, we cloned 11 evolutionarily conserved sequences located within or outside of the Fgf10 gene into the 0.34-kb promoter-luciferase vector, and tested their activities in vitro using primary cultured cells. Two sequences showed the highest activities. By using the Tol2 system and electroporation into chick embryos, activities of the 0.34-kb promoter with and without the two sequences were tested in vivo. No activities were detected in limb buds. However, the 0.34-kb promoter exhibited activities in the dorsal midline of the brain, while Fgf10 is detected in broader region in the brain. The two noncoding sequences negatively acted on the 0.34-kb promoter in the brain.

CONCLUSIONS

The proximal 0.34-kb promoter has activities to drive expression in restricted areas of the brain. Developmental Dynamics 247:1253-1263, 2018. © 2018 Wiley Periodicals, Inc.

A spontaneous mouse deletion in Mctp1 uncovers a long-range cis-regulatory region crucial for NR2F1 function during inner ear development

Hundreds of thousands of cis-regulatory DNA sequences are predicted in vertebrate genomes, but unlike genes themselves, few have been characterized at the functional level or even unambiguously paired with a target gene. Here we serendipitously identified and started investigating the first reported long-range regulatory region for the Nr2f1 (Coup-TFI) transcription factor gene. NR2F1 is temporally and spatially regulated during development and required for patterning and regionalization in the nervous system, including sensory hair cell organization in the auditory epithelium of the cochlea. Analyzing the deaf wanderer (dwnd) spontaneous mouse mutation, we traced back the cause of its associated circling behavior to a 53 kb deletion removing five exons and adjacent intronic regions of the poorly characterized Mctp1 gene. Interestingly, loss of Mctp1 function cannot account for the hearing loss, inner ear dysmorphology and sensory hair cell disorganization observed in dwnd mutants. Instead, we found that the Mctp1dwnd deletion affects the Nr2f1 gene located 1.4 Mb away, downregulating transcription and protein expression in the embryonic cochlea. Remarkably, the Mctp1dwnd allele failed to complement a targeted inactivation allele of Nr2f1, and transheterozygotes or Mctp1dwnd homozygotes exhibit the same morphological defects observed in inner ears of Nr2f1 mutants without sharing their early life lethality. Defects include improper separation of the utricle and saccule in the vestibule not described previously, which can explain the circling behavior that first brought the spontaneous mutation to attention. By contrast, mice homozygous for a targeted inactivation of Mctp1 have normal hearing and inner ear structures. We conclude that the 53 kb Mctp1dwnd deletion encompasses a long-range cis-regulatory region essential for proper Nr2f1 expression in the embryonic inner ear, providing a first opportunity to investigate Nr2f1 function in postnatal inner ears. This work adds to the short list of long-range regulatory regions characterized as essential to drive expression of key developmental control genes.

FGF signaling: diverse roles during cochlear development

Mammalian inner ear comprises of six sensory organs; cochlea, utricle, saccule, and three semicircular canals. The cochlea contains sensory epithelium known as the organ of Corti which senses sound through mechanosensory hair cells. Mammalian inner ear undergoes series of morphogenesis during development beginning thickening of ectoderm nearby hindbrain. These events require tight regulation of multiple signaling cascades including FGF, Wnt, Notch and Bmp signaling. In this review, we will discuss the role of newly emerging signaling, FGF signaling, for its roles required for cochlear development. [BMB Reports 2017; 50(10): 487-495].

Fgf3 and Fgf16 expression patterns define spatial and temporal domains in the developing chick inner ear

The inner ear is a morphologically complex sensory structure with auditory and vestibular functions. The developing otic epithelium gives rise to neurosensory and non-sensory elements of the adult membranous labyrinth. Extrinsic and intrinsic signals manage the patterning and cell specification of the developing otic epithelium by establishing lineage-restricted compartments defined in turn by differential expression of regulatory genes. FGF3 and FGF16 are excellent candidates to govern these developmental events. Using the chick inner ear, we show that Fgf3 expression is present in the borders of all developing cristae. Strong Fgf16 expression was detected in a portion of the developing vertical and horizontal pouches, whereas the cristae show weaker or undetected Fgf16 expression at different developmental stages. Concerning the rest of the vestibular sensory elements, both the utricular and saccular maculae were Fgf3 positive. Interestingly, strong Fgf16 expression delimited these Fgf16-negative sensory patches. The Fgf3-negative macula neglecta and the Fgf3-positive macula lagena were included within weakly Fgf16-expressing areas. Therefore, different FGF-mediated mechanisms might regulate the specification of the anterior (utricular and saccular) and posterior (neglecta and lagena) maculae. In the developing cochlear duct, dynamic Fgf3 and Fgf16 expression suggests their cooperation in the early specification and later cell differentiation in the hearing system. The requirement of Fgf3 and Fgf16 genes in endolymphatic apparatus development and neurogenesis are discussed. Based on these observations, FGF3 and FGF16 seem to be key signaling pathways that control the inner ear plan by defining epithelial identities within the developing otic epithelium.

Fgf10 is required for specification of non-sensory regions of the cochlear epithelium

The vertebrate inner ear is a morphologically complex sensory organ comprised of two compartments, the dorsal vestibular apparatus and the ventral cochlear duct, required for motion and sound detection, respectively. Fgf10, in addition to Fgf3, is necessary for the earliest stage of otic placode induction, but continued expression of Fgf10 in the developing otic epithelium, including the prosensory domain and later in Kolliker׳s organ, suggests additional roles for this gene during morphogenesis of the labyrinth. While loss of Fgf10 was implicated previously in semicircular canal agenesis, we show that Fgf10(-/+) embryos also exhibit a reduction or absence of the posterior semicircular canal, revealing a dosage-sensitive requirement for FGF10 in vestibular development. In addition, we show that Fgf10(-/-) embryos have previously unappreciated defects of cochlear morphogenesis, including a somewhat shortened duct, and, surprisingly, a substantially narrower duct. The mutant cochlear epithelium lacks Reissner׳s membrane and a large portion of the outer sulcus-two non-contiguous, non-sensory domains. Marker gene analyses revealed effects on Reissner׳s membrane as early as E12.5-E13.5 and on the outer sulcus by E15.5, stages when Fgf10 is expressed in close proximity to Fgfr2b, but these effects were not accompanied by changes in epithelial cell proliferation or death. These data indicate a dual role for Fgf10 in cochlear development: to regulate outgrowth of the duct and subsequently as a bidirectional signal that sequentially specifies Reissner׳s membrane and outer sulcus non-sensory domains. These findings may help to explain the hearing loss sometimes observed in LADD syndrome subjects with FGF10 mutations.

Genetic rescue of Muenke syndrome model hearing loss reveals prolonged FGF-dependent plasticity in cochlear supporting cell fates

Muenke syndrome model mice (Fgfr3^P244R/+) have hearing loss associated with a supporting cell fate transformation of two Deiters' cells to two pillar cells. Mansour et al. find that the hearing loss is not rescued by reducing the FGFR3c ligand FGF8, but reducing FGF10 is sufficient to rescue cochlear form and function. The P244R mutation caused FGFR3b and FGFR3c to acquire responsiveness to FGF10. Fgf10 heterozygosity allows a gradual reversion of fate-switched cells toward the normal phenotype.

The stereotyped arrangement of cochlear sensory and supporting cells is critical for auditory function. Our previous studies showed that Muenke syndrome model mice (Fgfr3^P244R/+) have hearing loss associated with a supporting cell fate transformation of two Deiters' cells to two pillar cells. We investigated the developmental origins of this transformation and found that two prospective Deiters' cells switch to an outer pillar cell-like fate sequentially between embryonic day 17.5 (E17.5) and postnatal day 3 (P3). Unexpectedly, the Fgfr3^P244R/+ hearing loss and supporting cell fate transformation are not rescued by genetically reducing fibroblast growth factor 8 (FGF8), the FGF receptor 3c (FGFR3c) ligand required for pillar cell differentiation. Rather, reducing FGF10, which normally activates FGFR2b or FGFR1b, is sufficient for rescue of cochlear form and function. Accordingly, we found that the P244R mutation changes the specificity of FGFR3b and FGFR3c such that both acquire responsiveness to FGF10. Moreover, Fgf10 heterozygosity does not block the Fgfr3^P244R/+ supporting cell fate transformation but instead allows a gradual reversion of fate-switched cells toward the normal phenotype between P5 and at least P14. This study indicates that Deiters' and pillar cells can reversibly switch fates in an FGF-dependent manner over a prolonged period of time. This property might be exploited for the regulation of sensory cell regeneration from support cells.

Fgf10 expression patterns in the developing chick inner ear

The inner ear is a complex three-dimensional sensorial structure with auditory and vestibular functions. It originates from the otic placode, which invaginates, forming the otic vesicle; the latter gives rise to neurosensory and nonsensory elements of the adult membranous labyrinth. A hypothesis based on descriptive and experimental evidence suggests that the acquisition of discrete sensory patches during evolution of this primordium may be related to subdivision of an early pansensory domain. In order to gain insight into this developmental mechanism, we carried out a detailed analysis of the spatial and temporal expression pattern of the gene Fgf10, by comparing different markers of otic patterning and hair cell differentiation. Fgf10 expression labels a sensory-competent domain included in a Serrate-positive territory from which most of the sensory epithelia arise. Our data show that Fgf10 transcripts are present initially in a narrow ventromedial band of the rudimentary otocyst, extending between its rostral and caudal poles. During development, this Fgf10-expressing area splits repetitively into several separate subareas, creating six of the eight sensory organs present in birds. Only the lateral crista and the macula neglecta were initially Fgf10 negative, although they activated Fgf10 expression after their specification as sensory elements. These results allowed us to determine a timetable of sensory specification in the developing chick inner ear. The comparison of the expression pattern of Fgf10 with those of other markers of sensory differentiation contributes to our understanding of the mechanism by which vertebrate inner ear prosensory domains have arisen during evolution.

Decoupling the function of Hox and Shh in developing limb reveals multiple inputs of Hox genes on limb growth

Limb development relies on an exquisite coordination between growth and patterning, but the underlying mechanisms remain elusive. Anterior-posterior and proximal-distal specification initiates in early limb bud concomitantly with the proliferative expansion of limb cells. Previous studies have shown that limb bud growth initially relies on fibroblast growth factors (FGFs) produced in the apical ectodermal ridge (AER-FGFs), the maintenance of which relies on a positive-feedback loop involving sonic hedgehog (Shh) and the BMP antagonist gremlin 1 (Grem1). The positive cross-regulation between Shh and the HoxA and HoxD clustered genes identified an indirect effect of Hox genes on the maintenance of AER-FGFs but the respective function of Shh and Hox genes in this process remains unknown. Here, by uncoupling Hox and Shh function, we show that HoxA and HoxD genes are required for proper AER-FGFs expression, independently of their function in controlling Shh expression. In addition, we provide evidence that the Hox-dependent control of AER-FGF expression is achieved through the regulation of key mesenchymal signals, namely Grem1 and Fgf10, ensuring proper epithelial-mesenchymal interactions. Notably, HoxA and HoxD genes contribute to both the initial activation of Grem1 and the subsequent anterior expansion of its expression domain. We propose that the intricate interactions between Hox genes and the FGF and Shh signaling pathways act as a molecular network that ensures proper limb bud growth and patterning, probably contributing to the coordination of these two processes.

ISL1 directly regulates FGF10 transcription during human cardiac outflow formation

The LIM homeodomain gene Islet-1 (ISL1) encodes a transcription factor that has been associated with the multipotency of human cardiac progenitors, and in mice enables the correct deployment of second heart field (SHF) cells to become the myocardium of atria, right ventricle and outflow tract. Other markers have been identified that characterize subdomains of the SHF, such as the fibroblast growth factor Fgf10 in its anterior region. While functional evidence of its essential contribution has been demonstrated in many vertebrate species, SHF expression of Isl1 has been shown in only some models. We examined the relationship between human ISL1 and FGF10 within the embryonic time window during which the linear heart tube remodels into four chambers. ISL1 transcription demarcated an anatomical region supporting the conserved existence of a SHF in humans, and transcription factors of the GATA family were co-expressed therein. In conjunction, we identified a novel enhancer containing a highly conserved ISL1 consensus binding site within the FGF10 first intron. ChIP and EMSA demonstrated its direct occupation by ISL1. Transcription mediated by ISL1 from this FGF10 intronic element was enhanced by the presence of GATA4 and TBX20 cardiac transcription factors. Finally, transgenic mice confirmed that endogenous factors bound the human FGF10 intronic enhancer to drive reporter expression in the developing cardiac outflow tract. These findings highlight the interest of examining developmental regulatory networks directly in human tissues, when possible, to assess candidate non-coding regions that may be responsible for congenital malformations.

Inner ear dysfunction in caspase-3 deficient mice

BACKGROUND

Caspase-3 is one of the most downstream enzymes activated in the apoptotic pathway. In caspase-3 deficient mice, loss of cochlear hair cells and spiral ganglion cells coincide closely with hearing loss. In contrast with the auditory system, details of the vestibular phenotype have not been characterized. Here we report the vestibular phenotype and inner ear anatomy in the caspase-3 deficient (Casp3(-/-)) mouse strain.

RESULTS

Average ABR thresholds of Casp3(-/-) mice were significantly elevated (P < 0.05) compared to Casp3(+/-) mice and Casp3(+/+) mice at 3 months of age. In DPOAE testing, distortion product 2F1-F2 was significantly decreased (P < 0.05) in Casp3(-/-) mice, whereas Casp3(+/-) and Casp3(+/+) mice showed normal and comparable values to each other. Casp3(-/-) mice were hyperactive and exhibited circling behavior when excited. In lateral canal VOR testing, Casp3(-/-) mice had minimal response to any of the stimuli tested, whereas Casp3(+/-) mice had an intermediate response compared to Casp3(+/+) mice. Inner ear anatomical and histological analysis revealed gross hypomorphism of the vestibular organs, in which the main site was the anterior semicircular canal. Hair cell numbers in the anterior- and lateral crista, and utricle were significantly smaller in Casp3(-/-) mice whereas the Casp3(+/-) and Casp3(+/+) mice had normal hair cell numbers.

CONCLUSIONS

These results indicate that caspase-3 is essential for correct functioning of the cochlea as well as normal development and function of the vestibule.

Retinoid signaling in inner ear development: A "Goldilocks" phenomenon

Retinoic acid (RA) is a biologically active derivative of vitamin A that is indispensable for inner ear development. The normal function of RA is achieved only at optimal homeostatic concentrations, with an excess or deficiency in RA leading to inner ear dysmorphogenesis. We present an overview of the role of RA in the developing mammalian inner ear, discussing both how and when RA may act to critically control a program of inner ear development. Molecular mechanisms of otic teratogenicity involving two members of the fibroblast growth factor family, FGF3 and FGF10, and their downstream targets, Dlx5 and Dlx6, are examined under conditions of both RA excess and deficiency. We term the effect of too little or too much RA on FGF/Dlx signaling a Goldilocks phenomenon. We demonstrate that in each case (RA excess, RA deficiency), RA can directly affect FGF3/FGF10 signaling within the otic epithelium, leading to downregulated expression of these essential signaling molecules, which in turn, leads to diminution in Dlx5/Dlx6 expression. Non-cell autonomous affects of the otic epithelium subsequently occur, altering transforming growth factor-beta (TGFβ) expression in the neighboring periotic mesenchyme and serving as a putative explanation for RA-mediated otic capsule defects. We conclude that RA coordinates inner ear morphogenesis by controlling an FGF/Dlx signaling cascade, whose perturbation by deviations in local retinoid concentrations can lead to inner ear dysmorphogenesis.

BMP/SMAD signaling regulates the cell behaviors that drive the initial dorsal-specific regional morphogenesis of the otocyst

During development of the otocyst, regional morphogenesis establishes a dorsal vestibular chamber and a ventral auditory chamber, which collectively constitute the membranous labyrinth of the inner ear. We identified the earliest morphogenetic event heralding the formation of the vestibular chamber, a rapid thinning and expansion of the dorsolateral wall of the otocyst, and showed that this process is generated by changes in otocyst cell shape from columnar to squamous, as opposed to changes in other cell behaviors, such as localized changes in cell proliferation or cell death. Moreover, we showed that thinning and expansion of the dorsolateral otocyst is regulated by BMP/SMAD signaling, which is both sufficient and necessary for localized thinning and expansion. Finally, we showed that BMP/SMAD signaling causes fragmentation of E-cadherin in the dorsolateral otocyst, occurring concomitantly with cell shape change, suggesting that BMP/SMAD signaling regulates cell-cell adhesion during the initial morphogenesis of the otocyst epithelium. Collectively, our results show that BMP signaling via SMADs regulates the cell behaviors that drive the initial dorsal-specific morphogenesis of the otocyst, providing new information about how regional morphogenesis of a complex organ rudiment, the developing membranous labyrinth, is initiated.

Understanding distal transcriptional regulation from sequence, expression and interactome perspectives

Gene regulation in eukaryotes involves a complex interplay between the proximal promoter and distal genomic elements (such as enhancers) which work in concert to drive precise spatio-temporal gene expression. The experimental localization and characterization of gene regulatory elements is a very complex and resource-intensive process. The computational identification of regulatory regions that confer spatiotemporally specific tissue-restricted expression of a gene is thus an important challenge for computational biology. One of the most popular strategies for enhancer localization from DNA sequence is the use of conservation-based prefiltering and more recently, the use of canonical (transcription factor motifs) or de novo tissue-specific sequence motifs. However, there is an ongoing effort in the computational biology community to further improve the fidelity of enhancer predictions from sequence data by integrating other, complementary genomic modalities. In this work, we propose a framework that complements existing methodologies for prospective enhancer identification. The methods in this work are derived from two key insights: (i) that chromatin modification signatures can discriminate proximal and distally located regulatory regions and (ii) the notion of promoter-enhancer cross-talk (as assayed in 3C/5C experiments) might have implications in the search for regulatory sequences that co-operate with the promoter to yield tissue-restricted, gene-specific expression.

FGF signaling regulates otic placode induction and refinement by controlling both ectodermal target genes and hindbrain Wnt8a

The inner ear epithelium, with its complex array of sensory, non-sensory, and neuronal cell types necessary for hearing and balance, is derived from a thickened patch of head ectoderm called the otic placode. Mouse embryos lacking both Fgf3 and Fgf10 fail to initiate inner ear development because appropriate patterns of gene expression fail to be specified within the pre-otic field. To understand the transcriptional "blueprint" initiating inner ear development, we used microarray analysis to identify prospective placode genes that were differentially expressed in control and Fgf3(-)(/)(-);Fgf10(-)(/)(-) embryos. Several genes in the down-regulated class, including Hmx3, Hmx2, Foxg1, Sox9, Has2, and Slc26a9 were validated by in situ hybridization. We also assayed candidate target genes suggested by other studies of otic induction. Two placode markers, Fgf4 and Foxi3, were down-regulated in Fgf3(-)(/)(-);Fgf10(-)(/)(-) embryos, whereas Foxi2, a cranial epidermis marker, was expanded in double mutants, similar to its behavior when WNT responses are blocked in the otic placode. Assays of hindbrain Wnt genes revealed that only Wnt8a was reduced or absent in FGF-deficient embryos, and that even some Fgf3(-)(/)(-);Fgf10(-)(/+) and Fgf3(-)(/)(-) embryos failed to express Wnt8a, suggesting a key role for Fgf3, and a secondary role for Fgf10, in Wnt8a expression. Chick explant assays showed that FGF3 or FGF4, but not FGF10, were sufficient to induce Wnt8a. Collectively, our results suggest that Wnt8a provides the link between FGF-induced formation of the pre-otic field and restriction of the otic placode to ectoderm adjacent to the hindbrain.

Genomic analysis of the function of the transcription factor gata3 during development of the mammalian inner ear

We have studied the function of the zinc finger transcription factor gata3 in auditory system development by analysing temporal profiles of gene expression during differentiation of conditionally immortal cell lines derived to model specific auditory cell types and developmental stages. We tested and applied a novel probabilistic method called the gamma Model for Oligonucleotide Signals to analyse hybridization signals from Affymetrix oligonucleotide arrays. Expression levels estimated by this method correlated closely (p<0.0001) across a 10-fold range with those measured by quantitative RT-PCR for a sample of 61 different genes. In an unbiased list of 26 genes whose temporal profiles clustered most closely with that of gata3 in all cell lines, 10 were linked to Insulin-like Growth Factor signalling, including the serine/threonine kinase Akt/PKB. Knock-down of gata3 in vitro was associated with a decrease in expression of genes linked to IGF-signalling, including IGF1, IGF2 and several IGF-binding proteins. It also led to a small decrease in protein levels of the serine-threonine kinase Akt2/PKBbeta, a dramatic increase in Akt1/PKBalpha protein and relocation of Akt1/PKBalpha from the nucleus to the cytoplasm. The cyclin-dependent kinase inhibitor p27(kip1), a known target of PKB/Akt, simultaneously decreased. In heterozygous gata3 null mice the expression of gata3 correlated with high levels of activated Akt/PKB. This functional relationship could explain the diverse function of gata3 during development, the hearing loss associated with gata3 heterozygous null mice and the broader symptoms of human patients with Hearing-Deafness-Renal anomaly syndrome.

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.

Syndromic congenital sensorineural deafness, microtia and microdontia resulting from a novel homoallelic mutation in fibroblast growth factor 3 (FGF3)

We identified a homozygous missense mutation (c.196G-->T) in fibroblast growth factor 3 (FGF3) in 21 affected individuals from a large extended consanguineous Saudi family, phenotypically characterized by autosomal recessive syndromic congenital sensorineural deafness, microtia and microdontia. All affected family members are descendents of a common ancestor who had lived six generations ago in a geographically isolated small village. This is the second report of FGF3 involvement in syndromic deafness in humans, and independently confirms the gene's positive role in inner ear development. The c.196G-->T mutation results in substitution of glycine by cysteine at amino acid 66 (p.G66C). This residue is conserved in several species and across 18 FGF family members. Conserved glycine/proline residues are central to the 'beta-trefoil fold' characteristic of the secondary structure of FGF family proteins and substitution of these residues is likely to disrupt structure and consequently function.

Retinoic acid-induced inner ear teratogenesis caused by defective Fgf3/Fgf10-dependent Dlx5 signaling

BACKGROUND

Retinoic acid (RA) is essential for inner ear development. However, exposure to excess RA at a critical period leads to inner ear defects. These defects are associated with disruption in epithelial-mesenchymal interactions.

METHODS

This study investigates the role of Dlx5 in the epithelial-mesenchymal interactions that guide otic capsule chondrogenesis, as well as the effect of excess in utero RA exposure on Dlx5 expression in the developing mouse inner ear. Control of Dlx5 by Fgf3 and Fgf10 under excess RA conditions is investigated by examining the developmental window during which Fgf3 and Fgf10 are altered by in utero RA exposure and by testing the ability of Fgf3 and Fgf10 to mitigate the reduction in chondrogenesis and Dlx5 expression mediated by RA in high-density cultures of periotic mesenchyme containing otic epithelium, a model of epithelial-mesenchymal interactions in which chondrogenic differentiation of periotic mesenchyme ensues in response to induction by otic epithelium.

RESULTS

Dlx5 deletion alters expression of TGFbeta(1), important for otic capsule chondrogenesis, in the developing inner ear and compromises the ability of cultured periotic mesenchyme containing otic epithelium, harvested from Dlx5 null embryos, to differentiate into cartilage when compared with control cultures. Downregulation in Dlx5 ensues as a consequence of in utero RA exposure in association with inner ear dysmorphogenesis. This change in Dlx5 is noted at embryonic day 10.5 (E10.5), but not at E9.5, suggesting that Dlx5 is not a direct RA target. Before Dlx5 downregulation, Fgf3 and Fgf10 expression is modified in the inner ear by excess RA, with the ability of exogenous Fgf3 and Fgf10 to rescue chondrogenesis and Dlx5 expression in RA-treated cultures of periotic mesenchyme containing otic epithelium supporting these fibroblast growth factors (FGFs) as intermediary genes by which RA mediates its effects.

CONCLUSIONS

Disruption in an Fgf3, -10/Dlx5 signaling cascade is operant in molecular mechanisms of inner ear teratogenesis by excess RA.

MI. Characterization of the mid-foregut transcriptome identifies genes regulated during lung bud induction

To identify genes expressed during initiation of lung organogenesis, we generated transcriptional profiles of the prospective lung region of the mouse foregut (mid-foregut) microdissected from embryos at three developmental stages between embryonic day 8.5 (E8.5) and E9.5. This period spans from lung specification of foregut cells to the emergence of the primary lung buds. We identified a number of known and novel genes that are temporally regulated as the lung bud forms. Genes that regulate transcription, including DNA binding factors, co-factors, and chromatin remodeling genes, are the main functional groups that change during lung bud formation. Members of key developmental transcription and growth factor families, not previously described to participate in lung organogenesis, are expressed in the mid-foregut during lung bud induction. These studies also show early expression in the mid-foregut of genes that participate in later stages of lung development. This characterization of the mid-foregut transcriptome provides new insights into molecular events leading to lung organogenesis.

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.

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.

Wnt/beta-catenin signaling promotes expansion of Isl-1-positive cardiac progenitor cells through regulation of FGF signaling

The anterior heart field (AHF), which contributes to the outflow tract and right ventricle of the heart, is defined in part by expression of the LIM homeobox transcription factor Isl-1. The importance of Isl-1-positive cells in cardiac development and homeostasis is underscored by the finding that these cells are required for cardiac development and act as cardiac stem/progenitor cells within the postnatal heart. However, the molecular pathways regulating these cells' expansion and differentiation are poorly understood. We show that Isl-1-positive AHF progenitor cells in mice were responsive to Wnt/beta-catenin signaling, and these responsive cells contributed to the outflow tract and right ventricle of the heart. Loss of Wnt/beta-catenin signaling in the AHF caused defective outflow tract and right ventricular development with a decrease in Isl-1-positive progenitors and loss of FGF signaling. Conversely, Wnt gain of function in these cells led to expansion of Isl-1-positive progenitors with a concomitant increase in FGF signaling through activation of a specific set of FGF ligands including FGF3, FGF10, FGF16, and FGF20. These data reveal what we believe to be a novel Wnt-FGF signaling axis required for expansion of Isl-1-positive AHF progenitors and suggest future therapies to increase the number and function of these cells for cardiac regeneration.

Differential requirements for FGF3, FGF8 and FGF10 during inner ear development

FGF signaling is required during multiple stages of inner ear development in many different vertebrates, where it is involved in induction of the otic placode, in formation and morphogenesis of the otic vesicle as well as for cellular differentiation within the sensory epithelia. In this study we have looked to define the redundant and conserved roles of FGF3, FGF8 and FGF10 during the development of the murine and avian inner ear. In the mouse, hindbrain-derived FGF10 ectopically induces FGF8 and rescues otic vesicle formation in Fgf3 and Fgf10 homozygous double mutants. Conditional inactivation of Fgf8 after induction of the placode does not interfere with otic vesicle formation and morphogenesis but affects cellular differentiation in the inner ear. In contrast, inactivation of Fgf8 during induction of the placode in a homozygous Fgf3 null background leads to a reduced size otic vesicle or the complete absence of otic tissue. This latter phenotype is more severe than the one observed in mutants carrying null mutations for both Fgf3 and Fgf10 that develop microvesicles. However, FGF3 and FGF10 are redundantly required for morphogenesis of the otic vesicle and the formation of semicircular ducts. In the chicken embryo, misexpression of Fgf3 in the hindbrain induces ectopic otic vesicles in vivo. On the other hand, Fgf3 expression in the hindbrain or pharyngeal endoderm is required for formation of the otic vesicle from the otic placode. Together these results provide important insights into how the spatial and temporal expression of various FGFs controls different steps of inner ear formation during vertebrate development.

Role of Gbx2 and Otx2 in the formation of cochlear ganglion and endolymphatic duct

The boundary of gene expression of transcription factors often plays a role in making a signaling center in development. In the otic vesicle, Gbx2 is expressed in the dorso-medial region including the endolymphatic duct, and Otx2 in the ventral region. Fgf10 is expressed between their expression boundaries, and the cochleovestibular ganglion develops close to the medial side of the Fgf10 expressing domain. Similar expression patterns are observed in the central nervous system, where Otx2 and Gbx2 expression abut at the mid-hindbrain boundary, and the repressive interaction between Otx2 and Gbx2 defines the mid-hindbrain boundary. These analogous expression patterns raise a question about the role of the interaction between Gbx2 and Otx2 in the otic vesicle. To address this, we misexpressed Gbx2 and Otx2 to the otic epithelium. Ectopic Gbx2 expression could repress Otx2 expression and vice versa. In addition, Fgf10 expression was repressed and cochlear ganglion formation was interfered with. Moreover, endolymphatic duct was severely hypomorphic in the Otx2 misexpressing embryos. These results suggest that the interaction between Gbx2 and Otx2 in developing inner ear defines Fgf10 expression domain to induce the cochlear ganglion. It is also suggested that Gbx2 expression is important for the formation of the endolymphatic duct.

Molecular evolution of the vertebrate mechanosensory cell and ear

The molecular basis of mechanosensation, mechanosensory cell development and mechanosensory organ development is reviewed with an emphasis on its evolution. In contrast to eye evolution and development, which apparently modified a genetic program through intercalation of genes between the master control genes on the top (Pax6, Eya1, Six1) of the hierarchy and the structural genes (rhodopsin) at the bottom, the as yet molecularly unknown mechanosensory channel precludes such a firm conclusion for mechanosensors. However, recent years have seen the identification of several structural genes which are involved in mechanosensory tethering and several transcription factors controlling mechanosensory cell and organ development; these warrant the interpretation of available data in very much the same fashion as for eye evolution: molecular homology combined with potential morphological parallelism. This assertion of molecular homology is strongly supported by recent findings of a highly conserved set of microRNAs that appear to be associated with mechanosensory cell development across phyla. The conservation of transcription factors and their regulators fits very well to the known or presumed mechanosensory specializations which can be mostly grouped as variations of a common cellular theme. Given the widespread distribution of the molecular ability to form mechanosensory cells, it comes as no surprise that structurally different mechanosensory organs evolved in different phyla, presenting a variation of a common theme specified by a conserved set of transcription factors in their cellular development. Within vertebrates and arthropods, some mechanosensory organs evolved into auditory organs, greatly increasing sensitivity to sound through modifications of accessory structures to direct sound to the specific sensory epithelia. However, while great attention has been paid to the evolution of these accessory structures in vertebrate fossils, comparatively less attention has been spent on the evolution of the inner ear and the central auditory system. Recent advances in our molecular understanding of ear and brain development provide novel avenues to this neglected aspect of auditory neurosensory evolution.

The molecular basis of neurosensory cell formation in ear development: a blueprint for hair cell and sensory neuron regeneration?

The inner ear of mammals uses neurosensory cells derived from the embryonic ear for mechanoelectric transduction of vestibular and auditory stimuli (the hair cells) and conducts this information to the brain via sensory neurons. As with most other neurons of mammals, lost hair cells and sensory neurons are not spontaneously replaced and result instead in age-dependent progressive hearing loss. We review the molecular basis of neurosensory development in the mouse ear to provide a blueprint for possible enhancement of therapeutically useful transformation of stem cells into lost neurosensory cells. We identify several readily available adult sources of stem cells that express, like the ectoderm-derived ear, genes known to be essential for ear development. Use of these stem cells combined with molecular insights into neurosensory cell specification and proliferation regulation of the ear, might allow for neurosensory regeneration of mammalian ears in the near future.

CisView: a browser and database of cis-regulatory modules predicted in the mouse genome

To facilitate the analysis of gene regulatory regions of the mouse genome, we developed a CisView (http://lgsun.grc.nia.nih.gov/cisview), a browser and database of genome-wide potential transcription factor binding sites (TFBSs) that were identified using 134 position-weight matrices and 219 sequence patterns from various sources and were presented with the information about sequence conservation, neighboring genes and their structures, GO annotations, protein domains, DNA repeats and CpG islands. Analysis of the distribution of TFBSs revealed that many TFBSs (N = 145) were over-represented near transcription start sites. We also identified potential cis-regulatory modules (CRMs) defined as clusters of conserved TFBSs in the entire mouse genome. Out of 739 074 CRMs, 157 442 had a significantly higher regulatory potential score than semi-random sequences generated with a 3rd-order Markov process. The CisView browser provides a user-friendly computer environment for studying transcription regulation on a whole-genome scale and can also be used for interpreting microarray experiments and identifying putative targets of transcription factors.

Foxg1 is required for morphogenesis and histogenesis of the mammalian inner ear

The forkhead genes are involved in patterning, morphogenesis, cell fate determination, and proliferation. Several Fox genes (Foxi1, Foxg1) are expressed in the developing otocyst of both zebrafish and mammals. We show that Foxg1 is expressed in most cell types of the inner ear of the adult mouse and that Foxg1 mutants have both morphological and histological defects in the inner ear. These mice have a shortened cochlea with multiple rows of hair cells and supporting cells. Additionally, they demonstrate striking abnormalities in cochlear and vestibular innervation, including loss of all crista neurons and numerous fibers that overshoot the organ of Corti. Closer examination shows that some anterior crista fibers exist in late embryos. Tracing these fibers shows that they do not project to the brain but, instead, to the cochlea. Finally, these mice completely lack a horizontal crista, although a horizontal canal forms but comes off the anterior ampulla. Anterior and posterior cristae, ampullae, and canals are reduced to varying degrees, particularly in combination with Fgf10 heterozygosity. Compounding Fgf10 heterozygotic effects suggest an additive effect of Fgf10 on Foxg1, possibly mediated through bone morphogenetic protein regulation. We show that sensory epithelia formation and canal development are linked in the anterior and posterior canal systems. Much of the Foxg1 phenotype can be explained by the participation of the protein binding domain in the delta/notch/hes signaling pathway. Additional Foxg1 effects may be mediated by the forkhead DNA binding domain.

Gli3-mediated somitic Fgf10 expression gradients are required for the induction and patterning of mammary epithelium along the embryonic axes

Little is known about the regulation of cell fate decisions that lead to the formation of five pairs of mammary placodes in the surface ectoderm of the mouse embryo. We have previously shown that fibroblast growth factor 10 (FGF10) is required for the formation of mammary placodes 1, 2, 3 and 5. Here, we have found that Fgf10 is expressed only in the somites underlying placodes 2 and 3, in gradients across and within these somites. To test whether somitic FGF10 is required for the formation of these two placodes, we analyzed a number of mutants with different perturbations of somitic Fgf10 gradients for the presence of WNT signals and ectodermal multilayering, markers for mammary line and placode formation. The mammary line is displaced dorsally, and formation of placode 3 is impaired in Pax3ILZ/ILZ mutants, which do not form ventral somitic buds. Mammary line formation is impaired and placode 3 is absent in Gli3Xt-J/Xt-J and hypomorphic Fgf10 mutants, in which the somitic Fgf10 gradient is shortened dorsally and less overall Fgf10 is expressed, respectively. Recombinant FGF10 rescued mammogenesis in Fgf10(-/-) and Gli3Xt-J/Xt-J flanks. We correlate increasing levels of somitic FGF10 with progressive maturation of the surface ectoderm, and show that full expression of somitic Fgf10, co-regulated by GLI3, is required for the anteroposterior pattern in which the flank ectoderm acquires a mammary epithelial identity. We propose that the intra-somitic Fgf10 gradient, together with ventral elongation of the somites, determines the correct dorsoventral position of mammary epithelium along the flank.

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.

Gata3 is required for early morphogenesis and Fgf10 expression during otic development

Inner ear develops from an induced surface ectoderm placode that invaginates and closes to form the otic vesicle, which then undergoes a complex morphogenetic process to form the membranous labyrinth. Inner ear morphogenesis is severely affected in Gata3 deficient mouse embryos, but the onset and basis of the phenotype has not been known. We show here that Gata3 deficiency leads to severe and unique abnormalities during otic placode invagination. The invagination problems are accompanied often by the formation of a morphological boundary between the dorsal and ventral otic cup and by the precocious appearance of dorsal endolymphatic characteristics. In addition, the endolymphatic domain often detaches from the rest of the otic epithelium during epithelial closure. The expression of several cell adhesion mediating genes is altered in Gata3 deficient ears suggesting that Gata3 controls adhesion and morphogenetic movements in early otic epithelium. Inactivation of Gata3 leads also to a loss of Fgf10 expression in otic epithelium and auditory ganglion demonstrating that Gata3 is an important regulator of Fgf-signalling during otic development.

Cessation of Fgf10 signaling, resulting in a defective dental epithelial stem cell compartment, leads to the transition from crown to root formation

Mouse, rat and human molars begin to form root after the completion of crown formation. In these teeth, fibroblast growth factor (Fgf) 10 disappears in the transitional stage from crown formation to root. By contrast, rodent incisors and vole molars demonstrate continuous growth, owing to the formation and maintenance of a stem cell compartment by the constant expression of Fgf10. To clarify the relationship between root formation and disappearance of Fgf10, we carried out two experiments for the loss and gain of Fgf10 function. First, we examined postnatal growth in the incisors of Fgf10-deficient mice, which have the defect of a dental epithelial stem cell compartment referred to as ;apical bud', after implantation under the kidney capsule. The growth at the labial side in the mutant mice mimics the development of limited-growth teeth. 5'-Bromo-2'-deoxyuridine (BrdU) labeling and cytokeratin (CK) 14 and Notch2 immunostaining suggested that the inhibition of inner enamel epithelium growth and the more-active proliferation of the outer enamel epithelium and/or stellate reticulum result in Hertwig's epithelial root sheath formation. Second, we examined the effects of Fgf10 overexpression in the transitional stage of molar germs, which led to the formation of apical bud involving in the inhibition of HERS formation. Taken together, these results suggest that the disappearance of Fgf10 signaling leads to the transition from crown to root formation, owing to the loss of a dental epithelial stem cell compartment.

Comparative analysis ofGata3 andGata2 expression during chicken inner ear development

The inner ear is a complex sensory organ with hearing and balance functions. Gata3 and Gata2 are expressed in the inner ear, and to gain more insight into their roles in otic development, we made a detailed expression analysis in chicken embryos. At early stages, their expression was highly overlapping. At later stages, Gata2 expression became prominent in vestibular and cochlear nonsensory epithelia. In contrast to Gata2, Gata3 was mainly expressed in the developing sensory epithelia, reflecting the importance of this factor in the sensory-neural development of the inner ear. While the later expression patterns of both Gata3 and Gata2 were highly conserved between chicken and mouse, important differences were observed especially with Gata3 during early otic development, providing indications of divergent molecular control during placode invagination in mice and chickens. We also found indications that the regulatory hierarchy observed in mouse, where Gata3 is upstream of Gata2 and Fgf10, could be conserved in chicken.

FGF signalling controls expression of vomeronasal receptors during embryogenesis

Fibroblast growth factors (FGFs) have been shown to control formation and differentiation of multiple organ systems in the developing vertebrate embryo. The analysis of differential gene expression during embryogenesis is, therefore, a potent tool to identify novel target genes regulated by FGF signalling. Here, we have applied microarray analysis to identify differentially regulated genes in FGF mutant mouse embryos. Surprisingly, transcripts corresponding to vomeronasal receptors (VRs), which so far have been only detected in the vomeronasal organ (VNO), were found to be downregulated in FGF mutant embryos. VR expression was detected in the developing olfactory pit and the anlage of the VNO. Interestingly, several FGFs can be detected in the developing olfactory pit during mouse embryogenesis [Bachler, M., Neubuser, A. 2001. Expression of members of the Fgf family and their receptors during midfacial development. Mech. Dev. 100, 313-316]. FGF signalling may thus control expression of VRs in the olfactory pit and formation of the VNO. Moreover, VR expression was detected in unexpected locations within the developing embryo including retina, dorsal root ganglia and neural tube. The relevance of VR expression in these structures and for formation of the VNO is discussed.