Ectopic noggin blocks sensory and nonsensory organ morphogenesis in the chicken inner ear

The multifaceted links between hearing loss and chronic kidney disease

Hearing loss affects nearly 1.6 billion people and is the third-leading cause of disability worldwide. Chronic kidney disease (CKD) is also a common condition that is associated with adverse clinical outcomes and high health-care costs. From a developmental perspective, the structures responsible for hearing have a common morphogenetic origin with the kidney, and genetic abnormalities that cause familial forms of hearing loss can also lead to kidney disease. On a cellular level, normal kidney and cochlea function both depend on cilial activities at the apical surface, and kidney tubular cells and sensory epithelial cells of the inner ear use similar transport mechanisms to modify luminal fluid. The two organs also share the same collagen IV basement membrane network. Thus, strong developmental and physiological links exist between hearing and kidney function. These theoretical considerations are supported by epidemiological data demonstrating that CKD is associated with a graded and independent excess risk of sensorineural hearing loss. In addition to developmental and physiological links between kidney and cochlear function, hearing loss in patients with CKD may be driven by specific medications or treatments, including haemodialysis. The associations between these two common conditions are not commonly appreciated, yet have important implications for research and clinical practice.

Signaling pathways (Notch, Wnt, Bmp and Fgf) have additive effects on hair cell regeneration in the chick basilar papilla after streptomycin injury in vitro: Additive effects of signaling pathways on hair cell regeneration

Hair cells can be regenerated after damage by transdifferentiation in which a supporting cell directly differentiates into a hair cell without mitosis. However, such regeneration is at the cost of exhausting the support cells in the mammalian mature cochlea. Thus, more effective methods should be found to promote mitotic regeneration but partially preserve support cells after damage. To address the issue, we first injured hair cells in the chick basilar papillae (BP) by treatment with streptomycin in vitro. We then compared the mitotic regeneration on the neural side in the middle part of BP after treatment with a pharmacological inhibitor or agonist of the Notch (DAPT), Wnt (LiCl), Bmp (Noggin) or Fgf (SU5402) signaling pathway, with that after treatment with combinations of two or three inhibitors or agonist of these pathways. Our results indicate that treatments with a single inhibitor or agonist of the Notch, Wnt, Bmp or Fgf signaling pathway could significantly increase mitotic regeneration as well as direct transdifferentiation. The results also show that hair cells (Myosin 7a+), support cells (Sox2+) and mitotically regenerated hair cells (Myosin 7a+/Sox2+/BrdU+) increased significantly on the neural side in the middle part of BP after two or three combinations of the inhibition of Notch, Bmp or Fgf signaling pathway or the activation of Wnt signaling pathway, besides the reported coregulatory effects of Notch and Wnt signaling. The study of the effects of systematic combinations of pathway modulators provided more insight into hair cell regeneration from mitosis.

Building inner ears: recent advances and future challenges for in vitro organoid systems

While inner ear disorders are common, our ability to intervene and recover their sensory function is limited. In vitro models of the inner ear, like the organoid system, could aid in identifying new regenerative drugs and gene therapies. Here, we provide a perspective on the status of in vitro inner ear models and guidance on how to improve their applicability in translational research. We highlight the generation of inner ear cell types from pluripotent stem cells as a particularly promising focus of research. Several exciting recent studies have shown how the developmental signaling cues of embryonic and fetal development can be mimicked to differentiate stem cells into “inner ear organoids” containing otic progenitor cells, hair cells, and neurons. However, current differentiation protocols and our knowledge of embryonic and fetal inner ear development in general, have a bias toward the sensory epithelia of the inner ear. We propose that a more holistic view is needed to better model the inner ear in vitro. Moving forward, attention should be made to the broader diversity of neuroglial and mesenchymal cell types of the inner ear, and how they interact in space or time during development. With improved control of epithelial, neuroglial, and mesenchymal cell fate specification, inner ear organoids would have the ability to truly recapitulate neurosensory function and dysfunction. We conclude by discussing how single-cell atlases of the developing inner ear and technical innovations will be critical tools to advance inner ear organoid platforms for future pre-clinical applications.

Transcriptomic analysis of mouse cochleae suffering from gentamicin damage reveals the signalling pathways involved in hair cell regeneration

There is a strong capacity for hair cell regeneration after damage in the inner ear of non-mammals. However, mammalian hair cells are substantially unable to regenerate. To obtain insights into the mechanism of this difference, we analyzed the transcriptomic changes in the mouse cochleae suffered from gentamicin damage and compared them with those in the chick cochleae suffered from the same damage. The results indicated that 2,230 genes had significantly differential expression between the gentamicin- and saline-treated mouse cochleae. Some of the differentially expressed genes were grouped into 265 signaling pathways, including the Notch, Wnt (Wingless and INT-1), Bmp (bone morphogenetic protein), FGF (fibroblast growth factor) and Shh (sonic hedgehog) pathways. Using pharmacological inhibitors or agonists of these pathways, the effects of these pathways on hair cell regeneration were further studied. The results indicated that Bmp alone and its coregulation with the Notch or Wnt signaling pathways increased the numbers of generated cells from transdifferentiation or proliferation in the mouse cochlea after damage, in addition to the reported coregulation of Notch and Wnt. Thus, this work indicates a new signaling pathway (Bmp) and its synergetic coregulation in mammalian hair cell regeneration, providing potential therapeutic targets to increase mammalian hair cell regeneration.

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.

Developmental abnormalities of the otic capsule and inner ear following application of prolyl-hydroxylase inhibitors in chick embryos

BACKGROUND

Naturally hypoxic conditions in amniote embryos play important roles in normal development. We previously showed that a hypoxic condition is required to produce a sufficient amount of neural crest cells (NCCs) during embryogenesis and that promoting a hypoxic response by prolyl-hydroxylase (PHD) inhibitors increases NCCs. Given that PHD inhibitors are considered as a potential treatment for anemia and ischemic diseases, we investigated the phenotypic effect of PHD inhibitors on embryonic development.

METHODS

Chick embryos were administered with PHD inhibitors prior to the induction of NCCs on day 1.5. Three main events relating to hypoxia, NCCs induction, vasculogenesis and chondrogenesis, were examined.

RESULTS

PHD inhibitors caused an increase of Sox10-positive NCCs in vivo. Vasculogenesis was promoted temporarily, although rapid vasculogenesis diminished the effect by day 5 in cephalic and pharyngeal regions. Studies on chondrogenesis at day 7 showed advanced development of the otic capsule, a cartilaginous structure encapsulating the inner ear. Analysis by X-ray micro-computed-tomography (μCT) revealed smaller otic capsule, suggesting premature differentiation. This in turn, deformed the developing semicircular canals within it. Other skeletal structures such as the palate and jaw were unaffected. The localized effect on the otic capsule was considered a result of the multiple effects from the hypoxic responses, increased NCCs and promoted chondrogenesis.

CONCLUSION

Given the wide range of clinical applications being considered for PHD inhibitors, this study provides crucial information to caution and guide use of PHD inhibitors when treating women of childbearing age.

Bone morphogenetic proteins and inner ear development

Bone morphogenetic proteins (BMPs) are the largest subfamily of the transforming growth factor-β superfamily, and they play important roles in the development of numerous organs, including the inner ear. The inner ear is a relatively small organ but has a highly complex structure and is involved in both hearing and balance. Here, we discuss BMPs and BMP signaling pathways and then focus on the role of BMP signal pathway regulation in the development of the inner ear and the implications this has for the treatment of human hearing loss and balance dysfunction.

BMP regulates regional gene expression in the dorsal otocyst through canonical and non-canonical intracellular pathways

The inner ear consists of two otocyst-derived, structurally and functionally distinct components: the dorsal vestibular and ventral auditory compartments. BMP signaling is required to form the vestibular compartment, but how it complements other required signaling molecules and acts intracellularly is unknown. Using spatially and temporally controlled delivery of signaling pathway regulators to developing chick otocysts, we show that BMP signaling regulates the expression of Dlx5 and Hmx3, both of which encode transcription factors essential for vestibular formation. However, although BMP regulates Dlx5 through the canonical SMAD pathway, surprisingly, it regulates Hmx3 through a non-canonical pathway involving both an increase in cAMP-dependent protein kinase A activity and the GLI3R to GLI3A ratio. Thus, both canonical and non-canonical BMP signaling establish the precise spatiotemporal expression of Dlx5 and Hmx3 during dorsal vestibular development. The identification of the non-canonical pathway suggests an intersection point between BMP and SHH signaling, which is required for ventral auditory development.

Gene expression profiling of rubella virus infected primary endothelial cells of fetal and adult origin

BACKGROUND

Rubella virus (RV) infection is usually a mild illness in children and adults. However, maternal infection during the first trimester of pregnancy can lead to congenital rubella syndrome (CRS) in the infant. Fetuses with CRS show damage to the endothelium of the heart and blood vessels; thus, it has been speculated that the clinical manifestations associated with CRS may be a result of endothelial cells persistently infected with RV. Here, we compared the effects of RV infection on gene expression in primary endothelial cells of fetal (HUVEC) and of adult (HSaVEC) origin by transcriptional profiling.

RESULTS

More than 75 % of the genes differentially regulated following RV infection were identical in both cell types. Gene Ontology (GO) analysis of these commonly regulated genes showed an enrichment of terms involved in cytokine production and cytokine regulation. Increased accumulation of inflammatory cytokines following RV infection was verified by protein microarray. Interestingly, the chemokine CCL14, which is implicated in supporting embryo implantation at the fetal-maternal interface, was down-regulated following RV infection only in HUVEC. Most noticeably, when analyzing the uniquely regulated transcripts for each cell type, GO term-based cluster analysis of the down-regulated genes of HUVEC revealed an enrichment of the GO terms "sensory organ development", "ear development" and "eye development".

CONCLUSION

Since impairment in vision and hearing are the most prominent clinical manifestations observed in CRS patients, the here detected down-regulated genes involved in the development of sensory organs sheds light on the molecular mechanisms that may contribute to the teratogenic effect of RV.

Expression of the Olig gene family in the developing mouse inner ear

Transcription factors are believed to play key roles in determining cell fate in inner ear development. Olig genes, which are basic helix-loop-helix transcription factors, have been reported to play important roles in the development of the central nervous system. However, members of this family have not previously been implicated in inner ear development, despite the similarity between otocyst and neural tube development. Olig1 begins to be expressed at the ventral domain of the otocyst at embryonic day (E) 9.5, and Olig1 expression in the epithelium of the developing inner ear persists to E15.5. Olig2 expression is localized to the cochleovestibular ganglia from E12.5 through E14.5. Olig3 has a diffuse expression pattern in the developing inner ear from E12.5 through the postnatal stage. Furthermore, at early stages of inner ear development, the Olig1 expression domain overlaps a region that is positive for Sox2 and Jagged1. This observation indicates that Olig1 may play an important role in the specification of the prosensory domain in the developing inner ear. As Olig genes are expressed in the mouse developing inner ear in a temporospatially distinct fashion, they may play substantial roles in the regulation of mammalian inner ear development.

Tfap2a promotes specification and maturation of neurons in the inner ear through modulation of Bmp, Fgf and notch signaling

Neurons of the statoacoustic ganglion (SAG) transmit auditory and vestibular information from the inner ear to the hindbrain. SAG neuroblasts originate in the floor of the otic vesicle. New neuroblasts soon delaminate and migrate towards the hindbrain while continuing to proliferate, a phase known as transit amplification. SAG cells eventually come to rest between the ear and hindbrain before terminally differentiating. Regulation of these events is only partially understood. Fgf initiates neuroblast specification within the ear. Subsequently, Fgf secreted by mature SAG neurons exceeds a maximum threshold, serving to terminate specification and delay maturation of transit-amplifying cells. Notch signaling also limits SAG development, but how it is coordinated with Fgf is unknown. Here we show that transcription factor Tfap2a coordinates multiple signaling pathways to promote neurogenesis in the zebrafish inner ear. In both zebrafish and chick, Tfap2a is expressed in a ventrolateral domain of the otic vesicle that includes neurogenic precursors. Functional studies were conducted in zebrafish. Loss of Tfap2a elevated Fgf and Notch signaling, thereby inhibiting SAG specification and slowing maturation of transit-amplifying cells. Conversely, overexpression of Tfap2a inhibited Fgf and Notch signaling, leading to excess and accelerated SAG production. However, most SAG neurons produced by Tfap2a overexpression died soon after maturation. Directly blocking either Fgf or Notch caused less dramatic acceleration of SAG development without neuronal death, whereas blocking both pathways mimicked all observed effects of Tfap2a overexpression, including apoptosis of mature neurons. Analysis of genetic mosaics showed that Tfap2a acts non-autonomously to inhibit Fgf. This led to the discovery that Tfap2a activates expression of Bmp7a, which in turn inhibits both Fgf and Notch signaling. Blocking Bmp signaling reversed the effects of overexpressing Tfap2a. Together, these data support a model in which Tfap2a, acting through Bmp7a, modulates Fgf and Notch signaling to control the duration, amount and speed of SAG neural development.

Neurons of the statoacoustic ganglion (SAG) transmit impulses from the inner ear necessary for hearing and balance. SAG cells exhibit a complex pattern of development, regulation of which remains poorly understood. Here we show that transcription factor Tfap2a coordinates multiple cell signaling pathways needed to regulate the quantity and pace of SAG neuron production. SAG progenitors originate within the developing inner ear and then migrate out of the ear towards the hindbrain before forming mature neurons. We showed previously that Fgf initiates formation of SAG progenitors in the inner ear, but rising levels of Fgf signaling eventually terminate this process. Elevated Fgf also stimulates proliferation of SAG progenitors outside the ear and delays their maturation. Notch signaling is also known to limit SAG development. Tfap2a governs the strength of Fgf and Notch signaling by activating expression of Bmp7a, which inhibits Fgf and Notch. Together these signals stabilize the pool of SAG progenitors outside the ear by equalizing rates of maturation and proliferation. This balance is critical for sustained accumulation of SAG neurons during larval growth as well as regeneration following neural damage. These findings could inform development of stem cell therapies to correct auditory neuropathies in humans.

Signaling regulating inner ear development: cell fate determination, patterning, morphogenesis, and defects

The membranous labyrinth of the inner ear is a highly complex organ that detects sound and balance. Developmental defects in the inner ear cause congenital hearing loss and balance disorders. The membranous labyrinth consists of three semicircular ducts, the utricle, saccule, and endolymphatic ducts, and the cochlear duct. These complex structures develop from the simple otic placode, which is established in the cranial ectoderm adjacent to the neural crest at the level of the hindbrain at the early neurula stage. During development, the otic placode invaginates to form the otic vesicle, which subsequently gives rise to neurons for the vestibulocochlear ganglion, the non-sensory and sensory epithelia of the membranous labyrinth that includes three ampullary crests, two maculae, and the organ of Corti. Combined paracrine and autocrine signals including fibroblast growth factor, Wnt, retinoic acid, hedgehog, and bone morphogenetic protein regulate fate determination, axis formation, and morphogenesis in the developing inner ear. Juxtacrine signals mediated by Notch pathways play a role in establishing the sensory epithelium, which consists of mechanosensory hair cells and supporting cells. The highly differentiated organ of Corti, which consists of uniformly oriented inner/outer hair cells and specific supporting cells, develops during fetal development. Developmental alterations/arrest causes congenital malformations in the inner ear in a spatiotemporal-restricted manner. A clearer understanding of the mechanisms underlying inner ear development is important not only for the management of patients with congenital inner ear malformations, but also for the development of regenerative therapy for impaired function.

Reconstruction of the mouse otocyst and early neuroblast lineage at single-cell resolution

The otocyst harbors progenitors for most cell types of the mature inner ear. Developmental lineage analyses and gene expression studies suggest that distinct progenitor populations are compartmentalized to discrete axial domains in the early otocyst. Here, we conducted highly parallel quantitative RT-PCR measurements on 382 individual cells from the developing otocyst and neuroblast lineages to assay 96 genes representing established otic markers, signaling-pathway-associated transcripts, and novel otic-specific genes. By applying multivariate cluster, principal component, and network analyses to the data matrix, we were able to readily distinguish the delaminating neuroblasts and to describe progressive states of gene expression in this population at single-cell resolution. It further established a three-dimensional model of the otocyst in which each individual cell can be precisely mapped into spatial expression domains. Our bioinformatic modeling revealed spatial dynamics of different signaling pathways active during early neuroblast development and prosensory domain specification.

Molecular mechanisms of inner ear development

The inner ear is a structurally complex vertebrate organ built to encode sound, motion, and orientation in space. Given its complexity, it is not surprising that inner ear dysfunction is a relatively common consequence of human genetic mutation. Studies in model organisms suggest that many genes currently known to be associated with human hearing impairment are active during embryogenesis. Hence, the study of inner ear development provides a rich context for understanding the functions of genes implicated in hearing loss. This chapter focuses on molecular mechanisms of inner ear development derived from studies of model organisms.

Delayed fusion and altered gene expression contribute to semicircular canal defects in Chd7 deficient mice

Proper morphogenesis of inner ear semicircular canals requires precise regulation of cellular proliferation, epithelial-to-mesenchymal transition, and fusion of epithelial plates. Epigenetic regulation of these processes is not well understood, but is likely to involve chromatin remodeling enzymes. CHD7 is a chromodomain-containing, ATP dependent helicase protein that is highly expressed in the developing ear and is required for semicircular canal development in both humans and mice. Here we report that mice with heterozygous loss of Chd7 function exhibit delayed semicircular canal genesis, delayed Netrin1 expression and disrupted expression of genes that are critical for semicircular canal formation (Bmp2, Bmp4, Msx1 and Fgf10). Complete loss of Chd7 results in aplasia of the semicircular canals and sensory vestibular organs, with reduced or absent expression of Otx1, Hmx3, Jagged1, Lmo4, Msx1 and Sox2. Our results suggest that Chd7 may have critical selector gene functions during inner ear morphogenesis. Detailed analysis of the epigenetic modifications underlying these gene expression changes should provide insights into semicircular canal development and help in the design of therapies for individuals with inner ear malformations.

Mutation of Rubie, a novel long non-coding RNA located upstream of Bmp4, causes vestibular malformation in mice

Background

The vestibular apparatus of the vertebrate inner ear uses three fluid-filled semicircular canals to sense angular acceleration of the head. Malformation of these canals disrupts the sense of balance and frequently causes circling behavior in mice. The Epistatic circler (Ecl) is a complex mutant derived from wildtype SWR/J and C57L/J mice. Ecl circling has been shown to result from the epistatic interaction of an SWR-derived locus on chromosome 14 and a C57L-derived locus on chromosome 4, but the causative genes have not been previously identified.

Methodology/Principal Findings

We developed a mouse chromosome substitution strain (CSS-14) that carries an SWR/J chromosome 14 on a C57BL/10J genetic background and, like Ecl, exhibits circling behavior due to lateral semicircular canal malformation. We utilized CSS-14 to identify the chromosome 14 Ecl gene by positional cloning. Our candidate interval is located upstream of bone morphogenetic protein 4 (Bmp4) and contains an inner ear-specific, long non-coding RNA that we have designated Rubie (RNA upstream of Bmp4 expressed in inner ear). Rubie is spliced and polyadenylated, and is expressed in developing semicircular canals. However, we discovered that the SWR/J allele of Rubie is disrupted by an intronic endogenous retrovirus that causes aberrant splicing and premature polyadenylation of the transcript. Rubie lies in the conserved gene desert upstream of Bmp4, within a region previously shown to be important for inner ear expression of Bmp4. We found that the expression patterns of Bmp4 and Rubie are nearly identical in developing inner ears.

Conclusions/Significance

Based on these results and previous studies showing that Bmp4 is essential for proper vestibular development, we propose that Rubie is the gene mutated in Ecl mice, that it is involved in regulating inner ear expression of Bmp4, and that aberrant Bmp4 expression contributes to the Ecl phenotype.

Meis gene expression patterns in the developing chicken inner ear

We are interested in stable gene network activities operating sequentially during inner ear specification. The implementation of this patterning process is a key event in the generation of functional subdivisions of the otic vesicle during early embryonic development. The vertebrate inner ear is a complex sensory structure that is a good model system for characterization of developmental mechanisms controlling patterning and specification. Meis genes, belonging to the TALE family, encode homodomain-containing transcription factors remarkably conserved during evolution, which play a role in normal and neoplastic development. To gain understanding of the possible role of homeobox Meis genes in the developing chick inner ear, we comprehensively analyzed their spatiotemporal expression patterns from early otic specification stages onwards. In the invaginating otic placode, Meis1/2 transcripts were observed in the borders of the otic cup, being absent in the portion of otic epithelium closest to the hindbrain. As development proceeds, Meis1 and Meis2 expressions became restricted to the dorsomedial otic epithelium. Both genes were strongly expressed in the entire presumptive domain of the semicircular canals, and more weakly in all associated cristae. The endolymphatic apparatus was labeled in part by Meis1/2. Meis1 was also expressed in the lateral wall of the growing cochlear duct, while Meis2 expression was detected in a few cells of the developing acoustic-vestibular ganglion. Our results suggest a possible role of Meis assigning regional identity in the morphogenesis, patterning, and specification of the developing inner ear.

BMP signaling is necessary for patterning the sensory and nonsensory regions of the developing mammalian cochlea

The mammalian inner ear detects sound with the organ of Corti, an intricately patterned region of the cochlea in which one row of inner hair cells and three rows of outer hair cells are surrounded by specialized supporting cells. The organ of Corti derives from a prosensory domain that runs the length of the cochlear duct and is bounded by two nonsensory domains, Kölliker's organ on the neural side and the outer sulcus on the abneural side. Although much progress has been made in identifying the signals regulating organ of Corti induction and differentiation, less is known about the mechanisms that establish sensory and nonsensory territories in the cochlear duct. Here, we show that a gradient of bone morphogenetic protein (BMP) signaling is established in the abneural-neural axis of the cochlea. Analysis of compound mutants of Alk3/6 type I BMP receptors shows that BMP signaling is necessary for specification of the prosensory domain destined to form the organ of Corti. Reduction of BMP signaling in Alk3/6 compound mutants eliminates both the future outer sulcus and the prosensory domain, with all cells expressing markers of Kölliker's organ. BMP4 upregulates markers of the future outer sulcus and downregulates marker genes of Kölliker's organ in cochlear organ cultures in a dose-dependent manner. Our results suggest BMP signaling is required for patterning sensory and nonsensory tissue in the mammalian cochlea.

WFIKKN1 and WFIKKN2 bind growth factors TGFβ1, BMP2 and BMP4 but do not inhibit their signalling activity

WFIKKN1 and WFIKKN2 are large extracellular multidomain proteins consisting of a WAP domain, a follistatin domain, an immunoglobulin domain, two Kunitz-type protease inhibitor domains and an NTR domain. Recent experiments have shown that both proteins have high affinity for growth and differentiation factor (GDF)8 and GDF11. Here we study the interaction of WFIKKN proteins with several additional representatives of the transforming growth factor (TGF)β family using SPR measurements. Analyses of SPR sensorgrams suggested that, in addition to GDF8 and GDF11, both WFIKKN proteins bind TGFβ1, bone morphogenetic protein (BMP)2 and BMP4 with relatively high affinity (K(d) ∼ 10(-6) m). To assess the biological significance of these interactions we studied the effect of WFIKKN proteins on the activity of GDF8, GDF11, TGFβ1, BMP2 and BMP4 using reporter assays. These studies revealed that WFIKKN1 and WFIKKN2 inhibited the biological activity of GDF8 and GDF11 in the nanomolar range, whereas they did not inhibit the activities of TGFβ1, BMP2 and BMP4 even in the micromolar range. Our data indicate that WFIKKN proteins are antagonists of GDF8 and GDF11, but in the case of TGFβ1, BMP2 and BMP4 they function as growth factor binding proteins. It is suggested that the physical association of WFIKKN proteins with these growth factors may localize their action and thus help to establish growth factor gradients in the extracellular space.

Id gene regulation and function in the prosensory domains of the chicken inner ear: a link between Bmp signaling and Atoh1

Bone morphogenetic proteins (Bmps) regulate the expression of the proneural gene Atoh1 and the generation of hair cells in the developing inner ear. The present work explored the role of Inhibitor of Differentiation genes (Id1-3) in this process. The results show that Id genes are expressed in the prosensory domains of the otic vesicle, along with Bmp4 and Bmp7. Those domains exhibit high levels of the phosphorylated form of Bmp-responding R-Smads (P-Smad1,5,8), and of Bmp-dependent Smad transcriptional activity as shown by the BRE-tk-EGFP reporter. Increased Bmp signaling induces the expression of Id1-3 along with the inhibition of Atoh1. Conversely, the Bmp antagonist Noggin or the Bmp-receptor inhibitor Dorsomorphin elicit opposite effects, indicating that Bmp signaling is necessary for Id expression and Atoh1 regulation in the otocyst. The forced expression of Id3 is sufficient to reduce Atoh1 expression and to prevent the expression of hair cell differentiation markers. Together, these results suggest that Ids are part of the machinery that mediates the regulation of hair cell differentiation exerted by Bmps. In agreement with that, during hair cell differentiation Bmp4 expression, P-Smad1,5,8 levels and Id expression are downregulated from hair cells. However, Ids are also downregulated from the supporting cells which contrarily to hair cells exhibit high levels of Bmp4 expression, P-Smad1,5,8, and BRE-tk-EGFP activity, suggesting that in these cells Ids escape from Bmp/Smad signaling. The differential regulation of Ids in time and space may underlie the multiple functions of Bmp signaling during sensory organ development.

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.

A symphony of inner ear developmental control genes

The inner ear is one of the most complex and detailed organs in the vertebrate body and provides us with the priceless ability to hear and perceive linear and angular acceleration (hence maintain balance). The development and morphogenesis of the inner ear from an ectodermal thickening into distinct auditory and vestibular components depends upon precise temporally and spatially coordinated gene expression patterns and well orchestrated signaling cascades within the otic vesicle and upon cellular movements and interactions with surrounding tissues. Gene loss of function analysis in mice has identified homeobox genes along with other transcription and secreted factors as crucial regulators of inner ear morphogenesis and development. While otic induction seems dependent upon fibroblast growth factors, morphogenesis of the otic vesicle into the distinct vestibular and auditory components appears to be clearly dependent upon the activities of a number of homeobox transcription factors. The Pax2 paired-homeobox gene is crucial for the specification of the ventral otic vesicle derived auditory structures and the Dlx5 and Dlx6 homeobox genes play a major role in specification of the dorsally derived vestibular structures. Some Micro RNAs have also been recently identified which play a crucial role in the inner ear formation.

Differences in gene expression between the otic capsule and other bones

Our long term goal is to understand the molecular pathology of otosclerosis and to develop better forms of therapy. Toward this goal, the current study focused on characterizing the molecular factors responsible for the unique biological features of the otic capsule: its minimal rate of remodeling, and lack of healing capacity when fractured. We compared expression levels of 62 genes involved in bone metabolism between the adult murine otic capsule and the tibia and parietal bones; the latter exemplify bones formed by endochondral and intramembranous ossification, respectively. Gene expression levels were measured using real-time quantitative RT-PCR and analyzed using tools of bioinformatics. Expression patterns of key genes were verified with in situ hybridization. The molecular profile of the otic capsule was distinctly different from that of the tibia and parietal bone. Genes found to be most characteristic of the otic capsule were: osteoprotegerin (opg), bone morphogenetic protein receptor 1b (bmpr1b) and bone morphogenetic protein 3 (bmp3). Expression levels were high for opg and bmpr1b, and minimal for bmp3 within the otic capsule. We concluded that opg and bmpr1b likely play important roles in inhibition of remodeling within the otic capsule.

Role of bone morphogenetic proteins on cochlear hair cell formation: analyses of Noggin and Bmp2 mutant mice

The mammalian organ of Corti of the inner ear is a highly sophisticated sensory end organ responsible for detecting sound. Noggin is a secreted glycoprotein, which antagonizes bone morphogenetic proteins 2 and 4 (Bmp2 and Bmp4). The lack of this antagonist causes increased rows of inner and outer hair cells in the organ of Corti. In mice, Bmp2 is expressed transiently in nascent cochlear hair cells. To investigate whether Noggin normally modulates the levels of Bmp2 for hair cell formation, we deleted Bmp2 in the cochlear hair cells using two cre strains, Foxg1(cre/+) and Gfi1(cre/+). Bmp2 conditional knockout cochleae generated using these two cre strains show normal hair cells. Furthermore, Gfi1(cre/+);Bmp2(lox/-) mice are viable and have largely normal hearing. The combined results of Noggin and Bmp2 mutants suggest that Noggin is likely to regulate other Bmps in the cochlea such as Bmp4.

Genetic modification of the inner ear lateral semicircular canal phenotype of the Bmp4 haplo-insufficient mouse

In the mouse, development of the lateral semicircular canal of the inner ear is sensitive to Bmp4 heterozygosity. In the C57BL6 background 30% of the heterozygotes display circling behavior, 66% have a specific defect in the vestibular part of the inner ear, namely the constriction, interruption or absence of the lateral semicircular canal. Only mice having both ears affected display circling behavior. In the (C57BL6xCBA)N1 background, the penetrance of the canal phenotype is greatly reduced, and bilateral lateral canal defect is not sufficient to induce circling. We found association of the canal phenotype with the genotype of markers on chromosome 14 and 4, co-localizing with Ecs and Eclb identified in the Ecl mouse with similar lateral canal defects. Candidate genes to contain the causal mutation are Bmp4 on chromosome 14, and Rere on chromosome 4.

Chondrocyte-specific Smad4 gene conditional knockout results in hearing loss and inner ear malformation in mice

Smad4 is the central intracellular mediator of transforming growth factor-beta (TGF-beta) signaling, which plays crucial roles in tissue regeneration, cell differentiation, embryonic development, and regulation of the immune system. Conventional Smad4 gene knockout results in embryonic lethality, precluding its use in studies of the role of Smad4 in inner ear development. We used chondrocyte-specific Smad4 knockout mice (Smad4Co/Co) to investigate the function of Smad4 in inner ear development. Smad4Co/Co mice were characterized by a smaller cochlear volume, bone malformation, and abnormalities of the osseous spiral lamina and basilar membrane. The development of the hair cells was also abnormal, as evidenced by the disorganized stereocilia and reduced density of the neuronal processes beneath the hair cells. Auditory function tests revealed the homozygous Smad4Co/Co mice suffered from severe sensorineural hearing loss. Our results suggest that Smad4 is required for inner ear development and normal auditory function in mammals.

Requirement for Lmo4 in the vestibular morphogenesis of mouse inner ear

During development, compartmentalization of an early embryonic structure produces blocks of cells with distinct properties and developmental potentials. The auditory and vestibular components of vertebrate inner ears are derived from defined compartments within the otocyst during embryogenesis. The vestibular apparatus, including three semicircular canals, saccule, utricle, and their associated sensory organs, detects angular and linear acceleration of the head and relays the information through vestibular neurons to vestibular nuclei in the brainstem. How the early developmental events manifest vestibular structures at the molecular level is largely unknown. Here, we show that LMO4, a LIM-domain-only transcriptional regulator, is required for the formation of semicircular canals and their associated sensory cristae. Targeted disruption of Lmo4 resulted in the dysmorphogenesis of the vestibule and in the absence of three semicircular canals, anterior and posterior cristae. In Lmo4-null otocysts, canal outpouches failed to form and cell proliferation was reduced in the dorsolateral region. Expression analysis of the known otic markers showed that Lmo4 is essential for the normal expression of Bmp4, Fgf10, Msx1, Isl1, Gata3, and Dlx5 in the dorsolateral domain of the otocyst, whereas the initial compartmentalization of the otocyst remains unaffected. Our results demonstrate that Lmo4 controls the development of the dorsolateral otocyst into semicircular canals and cristae through two distinct mechanisms: regulating the expression of otic specific genes and stimulating the proliferation of the dorsolateral part of the otocyst.

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.

A late role for bmp2b in the morphogenesis of semicircular canal ducts in the zebrafish inner ear

BACKGROUND

The Bone Morphogenetic Protein (BMP) genes bmp2 and bmp4 are expressed in highly conserved patterns in the developing vertebrate inner ear. It has, however, proved difficult to elucidate the function of BMPs during ear development as mutations in these genes cause early embryonic lethality. Previous studies using conditional approaches in mouse and chicken have shown that Bmp4 has a role in semicircular canal and crista development, but there is currently no direct evidence for the role of Bmp2 in the developing inner ear.

METHODOLOGY/PRINCIPAL FINDINGS

We have used an RNA rescue strategy to test the role of bmp2b in the zebrafish inner ear directly. Injection of bmp2b or smad5 mRNA into homozygous mutant swirl (bmp2b(-/-)) embryos rescues the early patterning defects in these mutants and the fish survive to adulthood. As injected RNA will only last, at most, for the first few days of embryogenesis, all later development occurs in the absence of bmp2b function. Although rescued swirl adult fish are viable, they have balance defects suggestive of vestibular dysfunction. Analysis of the inner ears of these fish reveals a total absence of semicircular canal ducts, structures involved in the detection of angular motion. All other regions of the ear, including the ampullae and cristae, are present and appear normal. Early stages of otic development in rescued swirl embryos are also normal.

CONCLUSIONS/SIGNIFICANCE

Our findings demonstrate a critical late role for bmp2b in the morphogenesis of semicircular canals in the zebrafish inner ear. This is the first demonstration of a developmental role for any gene during post-embryonic stages of otic morphogenesis in the zebrafish. Despite differences in the early stages of semicircular canal formation between zebrafish and amniotes, the role of Bmp2 in semicircular canal duct outgrowth is likely to be conserved between different vertebrate species.

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.

Bmp4 is essential for the formation of the vestibular apparatus that detects angular head movements

Angular head movements in vertebrates are detected by the three semicircular canals of the inner ear and their associated sensory tissues, the cristae. Bone morphogenetic protein 4 (Bmp4), a member of the Transforming growth factor family (TGF-β), is conservatively expressed in the developing cristae in several species, including zebrafish, frog, chicken, and mouse. Using mouse models in which Bmp4 is conditionally deleted within the inner ear, as well as chicken models in which Bmp signaling is knocked down specifically in the cristae, we show that Bmp4 is essential for the formation of all three cristae and their associated canals. Our results indicate that Bmp4 does not mediate the formation of sensory hair and supporting cells within the cristae by directly regulating genes required for prosensory development in the inner ear such as Serrate1 (Jagged1 in mouse), Fgf10, and Sox2. Instead, Bmp4 most likely mediates crista formation by regulating Lmo4 and Msx1 in the sensory region and Gata3, p75Ngfr, and Lmo4 in the non-sensory region of the crista, the septum cruciatum. In the canals, Bmp2 and Dlx5 are regulated by Bmp4, either directly or indirectly. Mechanisms involved in the formation of sensory organs of the vertebrate inner ear are thought to be analogous to those regulating sensory bristle formation in Drosophila. Our results suggest that, in comparison to sensory bristles, crista formation within the inner ear requires an additional step of sensory and non-sensory fate specification.

Disruption of the sense of balance is highly debilitating, causing vertigo and nausea. Maintenance of proper balance requires sensory inputs from many body parts, including the inner ears and the eyes. Within the inner ear, the vestibular apparatus plays a key role in the sense of balance and is responsible for detecting head orientation and movements. The portion of the vestibular apparatus that detects angular head movements consists of three fluid-filled, semicircular canals oriented at right angles to each other. At one end of each canal is an enlargement that houses the sensory tissue, crista ampullaris, consisting of sensory hair cells and supporting cells. Bone morphogenetic protein 4 (Bmp4), a secreted signaling molecule, is expressed in these sensory regions during development. However, the lack of Bmp4 in mice affects the formation of not only the sensory regions but also their associated canals. These results demonstrate for the first time that a single gene, Bmp4, is required for the formation of the entire sensory apparatus for detecting angular head movements.

Spalt4 mediates invagination and otic placode gene expression in cranial ectoderm

Vertebrate placodes are regions of thickened head ectoderm that contribute to paired sensory organs and cranial ganglia. We demonstrate that the transcription factor Spalt4 (also known as Sall4) is broadly expressed in chick preplacodal epiblast and later resolves to otic, lens and olfactory placodes. Ectopic expression of Spalt4 by electroporation is sufficient to induce invagination of non-placodal head ectoderm and prevent neurogenic placodes from contributing to cranial ganglia. Conversely, loss of Spalt4 function in the otic placode results in abnormal otic vesicle development. Intriguingly, Spalt4 appears to initiate a placode program appropriate for the axial level but is not involved in later development of specific placode fates. Fgfs can regulate Spalt4, since implantation of Fgf2 beads into the area opaca induces its expression. The results suggest that Spalt4 is involved in early stages of placode development, initiating cranial ectodermal invagination and region-specific gene regulatory networks.

Role of Bone morphogenetic protein 4 in zebrafish semicircular canal development

Bone morphogenetic proteins (BMPs) are known to play roles in inner ear development of higher vertebrates. In zebrafish, there are several reports showing that members of the BMP family are expressed in the otic vesicle. We have isolated a novel zebrafish mutant gallery, which affects the development of the semicircular canal. Gallery merely forms the lateral and the immature anterior protrusion, and does not form posterior and ventral protrusions. We found that the expression of bmp2b and bmp4, both expressed in the normal optic vesicle at the protrusion stage, are extremely upregulated in the otic vesicle of gallery. To elucidate the role of BMPs in the development of the inner ear of zebrafish, we have applied excess BMP to the wild-type otic vesicle. The formation of protrusions was severely affected, and in some cases, they were completely lost in BMP4-treated embryos. Furthermore, the protrusions in gallery treated with Noggin were partially rescued. These data indicate that BMP4 plays an important role in the development of protrusions to form semicircular canals.

Role of hindbrain in inner ear morphogenesis: analysis of Noggin knockout mice

Signaling from rhombomeres 5 and 6 of the hindbrain is thought to be important for inner ear patterning. In Noggin -/- embryos, the gross anatomy of the inner ear is distorted and malformed, with cochlear duct outgrowth and coiling most affected. We attributed these defects to a caudal shift of the rhombomeres caused by the shortened body axis and the kink in the neural tube. To test the hypothesis that a caudal shift of the rhombomeres affects inner ear development, we surgically generated chicken embryos in which rhombomeres 5 and 6 were similarly shifted relative to the position of the inner ears, as in Noggin mutants. All chicken embryos with shifted rhombomeres showed defects in cochlear duct formation indicating that signaling from rhombomeres 5 and 6 is important for cochlear duct patterning in both chicken and mice. In addition, the size of the otic capsule is increased in Noggin -/- mutants, which most likely is due to unopposed BMP signaling for chondrogenesis in the peri-otic mesenchyme.

Notch signalling is needed to maintain, but not to initiate, the formation of prosensory patches in the chick inner ear

Notch signalling is well-known to mediate lateral inhibition in inner ear sensory patches, so as to generate a balanced mixture of sensory hair cells and supporting cells. Recently, however, we have found that ectopic Notch activity at an early stage can induce the formation of ectopic sensory patches. This suggests that Notch activity may have two different functions in normal ear development, acting first to promote the formation of the prosensory patches, and then later to regulate hair-cell production within the patches. The Notch ligand Serrate1 (Jag1 in mouse and humans) is expressed in the patches from an early stage and may provide Notch activation during the prosensory phase. Here, we test whether Notch signalling is actually required for prosensory patch development. When we block Notch activation in the chick embryo using the gamma-secretase inhibitor DAPT, we see a complete loss of prosensory epithelial cells in the anterior otocyst, where they are diverted into a neuroblast fate via failure of Delta1-dependent lateral inhibition. The cells of the posterior prosensory patch remain epithelial, but expression of Sox2 and Bmp4 is drastically reduced. Expression of Serrate1 here is initially almost normal, but subsequently regresses. The patches of sensory hair cells that eventually develop are few and small. We suggest that, in normal development, factors other than Notch activity initiate Serrate1 expression. Serrate1, by activating Notch, then drives the expression of Sox2 and Bmp4, as well as expression of the Serrate1 gene itself. The positive feedback maintains Notch activation and thereby preserves and perhaps extends the prosensory state, leading eventually to the development of normal sensory patches.

Studies of Otic Capsule Morphology and Gene Expression in the Mov13 Mouse – An Animal Model of Type I Osteogenesis Imperfecta

Type I osteogenesis imperfecta (OI) is a disorder of skeletal bones characterized by bone fragility and blue sclera, which can result from mutations in genes encoding for type I collagen--the COL1A1 and COL1A2 genes. Fifty percent of patients with type I OI develop hearing loss and associated histopathological changes in the otic capsule that are indistinguishable from otosclerosis, a major cause of acquired hearing loss. In an attempt to elucidate molecular and cellular mechanisms of hearing loss in type I OI, we have studied the Mov13 mouse, which has served as an animal model of type I OI by virtue of exhibiting variable transcriptional block of the COL1A1 gene. We studied the morphometry of the Mov13 otic capsule and compared expression levels of 60 genes in the otic capsule with those in the tibia and parietal bone of the Mov13 and wild-type mice. The degree of transcriptional block of the COL1A1 gene and its downstream effects differed significantly between the bones examined. We found that expression levels of bone morphogenetic protein 3 and nuclear factor kappa-B1 best distinguished Mov13 otic capsule from wild-type otic capsule, and that osteoprotegerin, caspase recruitment domain containing protein 1, and partitioning defective protein 3 best distinguished Mov13 otic capsule from Mov13 tibia and parietal bone. Although the Mov13 mouse did not demonstrate evidence of active abnormal otic capsule remodeling as seen in type I OI and otosclerosis, studying gene expression in the Mov13 mouse has provided evidence that osteocytes of the otic capsule differ from osteocytes in other bones.

Dan is required for normal morphogenesis and patterning in the developing chick inner ear

During vertebrate inner ear development, compartmentalization of the auditory and vestibular apparatuses along two axes depends on the patterning of transcription factors expressed in a region-specific manner. Although most of the patterning is regulated by extrinsic signals, it is not known how Nkx5.1 and Msx1 are patterned. We focus on Dan, the founding member of the Cerberus/Dan gene family that encodes BMP antagonists, and describe its function in morphogenesis and patterning. First, we confirmed that Dan is expressed in the dorso-medial region of the otic vesicle that corresponds to the presumptive endolymphatic duct and sac (ed/es). Second, we used siRNA knockdown to demonstrate that depletion of Dan induced both a severe reduction in the size of the ed/es and moderate deformities of the semicircular canals and cochlear duct. Depletion of Dan also caused suppression of Nkx5.1 in the dorso-lateral region, suppression of Msx1 in the dorso-medial region, and ectopic induction of Nkx5.1 and Msx1 in the ventro-medial region. Most of these phenotypes also appeared following misexpression of the constitutively active form of BMP receptor type Ib. Thus, Dan is required for the normal morphogenesis of the inner ear and, by inhibiting BMP signaling, for the patterning of the transcription factors Nkx5.1 and Msx1.

Regulation of cell fate in the sensory epithelia of the inner ear

The sensory epithelia of the inner ear contain mechanosensory hair cells and non-sensory supporting cells. Both classes of cell are heterogeneous, with phenotypes varying both between and within epithelia. The specification of individual cells as distinct types of hair cell or supporting cell is regulated through intra- and extracellular signalling pathways that have been poorly understood. However, new methodologies have resulted in significant steps forward in our understanding of the molecular pathways that direct cells towards these cell fates.

Shaping the mammalian auditory sensory organ by the planar cell polarity pathway

The human ear is capable of processing sound with a remarkable resolution over a wide range of intensity and frequency. This ability depends largely on the extraordinary feats of the hearing organ, the organ of Corti and its sensory hair cells. The organ of Corti consists of precisely patterned rows of sensory hair cells and supporting cells along the length of the snail-shaped cochlear duct. On the apical surface of each hair cell, several rows of actin-containing protrusions, known as stereocilia, form a "V"-shaped staircase. The vertices of all the "V"-shaped stereocilia point away from the center of the cochlea. The uniform orientation of stereocilia in the organ of Corti manifests a distinctive form of polarity known as planar cell polarity (PCP). Functionally, the direction of stereociliary bundle deflection controls the mechanical channels located in the stereocilia for auditory transduction. In addition, hair cells are tonotopically organized along the length of the cochlea. Thus, the uniform orientation of stereociliary bundles along the length of the cochlea is critical for effective mechanotransduction and for frequency selection. Here we summarize the morphological and molecular events that bestow the structural characteristics of the mammalian hearing organ, the growth of the snail-shaped cochlear duct and the establishment of PCP in the organ of Corti. The PCP of the sensory organs in the vestibule of the inner ear will also be described briefly.

The role of bone morphogenetic protein 4 in inner ear development and function

Bone Morphogenetic Protein 4 (BMP4) is a member of the TGF-beta superfamily and is known to be important for the normal development of many tissues and organs, including the inner ear. Bmp4 homozygous null mice die as embryos, but Bmp4 heterozygous null (Bmp4(+/-)) mice are viable and some adults exhibit a circling phenotype, suggestive of an inner ear defect. To understand the role of BMP4 in inner ear development and function, we have begun to study C57BL/6 Bmp4(+/-) mice. Quantitative testing of the vestibulo-collic reflex, which helps maintain head stability, demonstrated that Bmp4(+/-) mice that exhibit circling behavior have a poor response in the yaw axis, consistent with semicircular canal dysfunction. Although the hair cells of the ampullae were grossly normal, the stereocilia were greatly reduced in number. Auditory brainstem responses showed that Bmp4(+/-) mice have elevated hearing thresholds and immunohistochemical staining demonstrated decreased numbers of neuronal processes in the organ of Corti. Thus Bmp4(+/-) mice have structural and functional deficits in the inner ear.

Dlx5 and Dlx6 homeobox genes are required for specification of the mammalian vestibular apparatus

The mammalian inner ear is a complex organ that develops from a surface ectoderm into distinct auditory and vestibular components. Congenital malformation of these two components resulting from single or multiple gene defects is a common clinical occurrence and is observed in patients with split hand/split foot malformation, a malformation which is phenocopied by Dlx5/6 null mice. Analysis of mice lacking Dlx5 and Dlx6 homeobox genes identified their restricted and combined expression in the otic epithelium as a crucial regulator of vestibular cell fates. Otic induction initiates without incident in Dlx5/6(-/-) embryos, but dorsal otic derivatives including the semicircular ducts, utricle, saccule, and endolymphatic duct fail to form. Dlx5 and Dlx6 seem to influence vestibular cell fates by restricting Pax2 and activating Gbx2 and Bmp4 expression domains. Given their proximity to the disease locus and the observed phenotype in Dlx5/6 null mice, Dlx5/6 are likely candidates to mediate the inner ear defects observed in patients with split hand/split foot malformation.

Eya1 regulates the growth of otic epithelium and interacts with Pax2 during the development of all sensory areas in the inner ear

Members of the Eyes absent (Eya) gene family are important for auditory system development. While mutations in human EYA4 cause late-onset deafness at the DFNA10 locus, mutations in human EYA1 cause branchio-oto-renal (BOR) syndrome. Inactivation of Eya1 in mice causes an early arrest of the inner ear development at the otocyst stage. To better understand the role of Eya1 in inner ear development, we analyzed the cellular and molecular basis of the early defect observed in the Eya1 mutant embryos. We report here that Eya1-/- otic epithelium shows reduced cell proliferation from E8.5 and increased cell apoptosis from E9.0, thus providing insights into the cellular basis of inner ear defect which occurred in the absence of Eya1. Previous studies have suggested that Pax, Eya and Six genes function in a parallel or independent pathway during inner ear development. However, it remains unknown whether Pax genes interact with Eya1 or Six1 during inner ear morphogenesis. To further evaluate whether Pax genes function in the Eya1-Six1 pathway or whether they interact with Eya1 or Six1 during inner ear morphogenesis, we have analyzed the expression pattern of Eya1, Pax2 and Pax8 on adjacent sections of otic epithelium from E8.5 to 9.5 by in situ hybridization and the inner ear gross structures of Pax2, Eya1 and Six1 compound mutants at E17.5 by latex paintfilling. Our data strongly suggest that Pax2 interacts with Eya1 during inner ear morphogenesis, and this interaction is critical for the development of all sensory areas in the inner ear. Furthermore, otic marker analysis in both Eya1-/- and Pax2-/- embryos indicates that Eya1 but not Pax2 regulates the establishment of regional specification of the otic vesicle. Together, these results show that, while Eya1 exerts an early function essential for normal growth and patterning of the otic epithelium, it also functionally synergizes with Pax2 during the morphogenesis of all sensory areas of mammalian inner ear.

BMP-signaling regulates the generation of hair-cells

Bone morphogenetic proteins (BMPs) are diffusible molecules involved in a variety of cellular interactions during development. Bmp4 expression accompanies the development of the ear sensory organs during patterning and specification of sensory cell fates, yet there is no understanding of the role of BMP4 in this process. The present work was aimed at exploring the effects of BMP-signaling on the development of hair-cells. For this purpose, we studied gene expression, cell proliferation and cell death in isolated chick otic vesicles that were grown in vitro in the presence of recombinant BMP4 or the BMP-inhibitor Noggin. Cath1 was used as a marker for hair-cell specification. BMP4 reduced the number of Cath1-cells and, conversely, Noggin increased the size of the sensory patches and the number of Cath1-positive cells. The effect of BMP4 was irreversible and occurred before hair-cell specification. Lfng and Fgf10 were expressed in the prosensory domain before Cath1, and their expression was expanded by Noggin. At these stages, modifications of BMP activity did not respecify non-sensory epithelium of the otic vesicle. The expression of Bmp4 at sensory patches was suppressed by BMP4 and induced by Noggin suggesting an autoregulatory loop. Analysis of BrdU incorporation during 6 and 18 h indicated that the effects of BMP4 were due to its ability to reduce the number of actively proliferating progenitors and inhibit cell fate specification. BMP4 induced cell death within the prosensory domain of the otic vesicle, along with the expression of Msx1, but not Msx2. On the contrary, BMP-inhibition with Noggin favored hair-cell specification without changes in the overall cell proliferation. We propose that about the stage of terminal division, the balance between BMP and BMP-inhibitory signals regulates survival and specification of hair-cell precursors, the final number of sensory hair-cells being limited by excess levels of BMPs. The final size of sensory patches would hence depend on the balance between BMP4 and opposing signals.

Retinoid signaling in inner ear development

The inner ear originates from an embryonic ectodermal placode and rapidly develops into a three-dimensional structure (the otocyst) through complex molecular and cellular interactions. Many genes and their products are involved in inner ear induction, organogenesis, and cell differentiation. Retinoic acid (RA) is an endogenous signaling molecule that may play a role during different phases of inner ear development, as shown from pathological observations. To gain insight into the function of RA during inner ear development, we have investigated the spatio-temporal expression patterns of major components of RA signaling pathway, including cellular retinoic acid binding proteins (CRABPs), cellular retinoid binding proteins (CRBPs), retinaldehyde dehydrogenases (RALDHs), catabolic enzymes (CYP26s), and nuclear receptors (RARs). Although the CrbpI, CrabpI, and -II genes are specifically expressed in the inner ear throughout development, loss-of-function studies have revealed that these proteins are dispensable for inner development and function. Several Raldh and Cyp26 gene transcripts are expressed at embryological day (E) 9.0-9.5 in the otocyst and show mainly complementary distributions in the otic epithelium and mesenchyme during following stages. From Western blot, RT-PCR, and in situ hybridization analysis, there is a low expression of Raldhs in the early otocyst at E9, while Cyp26s are strongly expressed. During the following days, there is an up-regulation of Raldhs and a down-regulation for Cyp26s. Specific RA receptor (Rar and Rxr) genes are expressed in the otocyst and during further development of the inner ear. At the otocyst stage, most of the components of the retinoid pathway are present, suggesting that the embryonic inner ear might act as an autocrine system, which is able to synthesize and metabolize RA necessary for its development. We propose a model in which two RA-dependent pathways may control inner ear ontogenesis: one indirect with RA from somitic mesoderm acting to regulate gene expression within the hindbrain neuroepithelium, and another with RA acting directly on the otocyst. Current evidence suggests that RA may regulate several genes involved in mesenchyme-epithelial interactions, thereby controlling inner ear morphogenesis. Our investigations suggest that RA signaling is a critical component not only of embryonic development, but also of postnatal maintenance of the inner ear.

Dlx gene expression during chick inner ear development

Members of the Dlx gene family play essential roles in the development of the zebrafish and mouse inner ear, but little is known regarding Dlx genes and avian inner ear development. We have examined the inner ear expression patterns of Dlx1, Dlx2, Dlx3, Dlx5, and Dlx6 during the first 7 days of chicken embryonic development. Dlx1 and Dlx2 expression was seen only in nonneuronal cells of the cochleovestibular ganglion and nerves from stage 21 to stage 32. Dlx3 marks the otic placode beginning at stage 9 and becomes limited to epithelium adjacent to the hindbrain as invagination of the placode begins. Dlx3 expression then resolves to the dorsal otocyst and gradually becomes limited to the endolymphatic sac by stage 30. Dlx5 and Dlx6 expression in the developing inner ear is first seen at stages 12 and 13, respectively, in the rim of the otic pit, before spreading throughout the dorsal otocyst. As morphogenesis proceeds, Dlx5 and Dlx6 expression is seen throughout the forming semicircular canals and endolymphatic structures. During later stages, both genes are seen to mark the distal surface of the forming canals and display expression complementary to that of BMP4 in the vestibular sensory regions. Dlx5 expression is also seen in the lagena macula and the cochlear and vestibular nerves by stage 30. These findings suggest important roles for Dlx genes in the vestibular and neural development of the avian inner ear.

BMP4 signaling is involved in the generation of inner ear sensory epithelia

Background

The robust expression of BMP4 in the incipient sensory organs of the inner ear suggests possible roles for this signaling protein during induction and development of auditory and vestibular sensory epithelia. Homozygous BMP4-/- animals die before the inner ear's sensory organs develop, which precludes determining the role of BMP4 in these organs with simple gene knockout experiments.

Results

Here we use a chicken otocyst culture system to perform quantitative studies on the development of inner ear cell types and show that hair cell and supporting cell generation is remarkably reduced when BMP signaling is blocked, either with its antagonist noggin or by using soluble BMP receptors. Conversely, we observed an increase in the number of hair cells when cultured otocysts were treated with exogenous BMP4. BMP4 treatment additionally prompted down-regulation of Pax-2 protein in proliferating sensory epithelial progenitors, leading to reduced progenitor cell proliferation.

Conclusion

Our results implicate BMP4 in two events during chicken inner ear sensory epithelium formation: first, in inducing the switch from proliferative sensory epithelium progenitors to differentiating epithelial cells and secondly, in promoting the differentiation of hair cells within the developing sensory epithelia.

Wnt-dependent regulation of inner ear morphogenesis is balanced by the opposing and supporting roles of Shh

The inner ear is partitioned along its dorsal/ventral axis into vestibular and auditory organs, respectively. Gene expression studies suggest that this subdivision occurs within the otic vesicle, the tissue from which all inner ear structures are derived. While the specification of ventral otic fates is dependent on Shh secreted from the notochord, the nature of the signal responsible for dorsal otic development has not been described. In this study, we demonstrate that Wnt signaling is active in dorsal regions of the otic vesicle, where it functions to regulate the expression of genes (Dlx5/6 and Gbx2) necessary for vestibular morphogenesis. We further show that the source of Wnt impacting on dorsal otic development emanates from the dorsal hindbrain, and identify Wnt1 and Wnt3a as the specific ligands required for this function. The restriction of Wnt target genes to the dorsal otocyst is also influenced by Shh. Thus, a balance between Wnt and Shh signaling activities is key in distinguishing between vestibular and auditory cell types.