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

Ephrin-B2 governs morphogenesis of endolymphatic sac and duct epithelia in the mouse inner ear

Control over ionic composition and volume of the inner ear luminal fluid endolymph is essential for normal hearing and balance. Mice deficient in either the EphB2 receptor tyrosine kinase or the cognate transmembrane ligand ephrin-B2 (Efnb2) exhibit background strain-specific vestibular-behavioral dysfunction and signs of abnormal endolymph homeostasis. Using various loss-of-function mouse models, we found that Efnb2 is required for growth and morphogenesis of the embryonic endolymphatic epithelium, a precursor of the endolymphatic sac (ES) and duct (ED), which mediate endolymph homeostasis. Conditional inactivation of Efnb2 in early-stage embryonic ear tissues disrupted cell proliferation, cell survival, and epithelial folding at the origin of the endolymphatic epithelium. This correlated with apparent absence of an ED, mis-localization of ES ion transport cells relative to inner ear sensory organs, dysplasia of the endolymph fluid space, and abnormally formed otoconia (extracellular calcite-protein composites) at later stages of embryonic development. A comparison of Efnb2 and Notch signaling-deficient mutant phenotypes indicated that these two signaling systems have distinct and non-overlapping roles in ES/ED development. Homozygous deletion of the Efnb2 C-terminus caused abnormalities similar to those found in the conditional Efnb2 null homozygote. Analyses of fetal Efnb2 C-terminus deletion heterozygotes found mis-localized ES ion transport cells only in the genetic background exhibiting vestibular dysfunction. We propose that developmental dysplasias described here are a gene dose-sensitive cause of the vestibular dysfunction observed in EphB-Efnb2 signaling-deficient mice.

Embryological manipulations in the developing Xenopus inner ear reveal an intrinsic role for Wnt signaling in dorsal-ventral patterning

BACKGROUND

The inner ear develops from an ectodermal thickening known as the otic placode into a complex structure that is asymmetrical along both the anterior-posterior (A-P) and dorsal-ventral (D-V) axes. Embryological manipulations in Xenopus allow us to test regenerative potential along specific axes and timing of axis determination. We explore the role of Wnt signaling with gain and loss of function experiments.

RESULTS

In contrast to A or P half ablations, D or V half ablations almost never result in mirror duplications or normal ears. Instead there is a loss of structures, especially those associated with the ablated region. Rotation experiments inverting the D-V axis reveal that it is determined by stage 24-26 which is just before expression of the dorsal otic marker Wnt3a. Conditional blocking of canonical Wnt signaling results in reductions in the number of sensory organs and semicircular canals which could be placed in one of three categories, the most common phenotypes being similar to those seen after dorsal ablations.

CONCLUSIONS

There is less regenerative potential along the D-V axis. Wnt3a protein alone is sufficient to rescue the severe loss of inner ear structures resulting from dorsal but not ventral half ablations.

Correct timing of proliferation and differentiation is necessary for normal inner ear development and auditory hair cell viability

BACKGROUND

Hearing restoration through hair cell regeneration will require revealing the dynamic interactions between proliferation and differentiation during development to avoid the limited viability of regenerated hair cells. Pax2-Cre N-Myc conditional knockout (CKO) mice highlighted the need of N-Myc for proper neurosensory development and possible redundancy with L-Myc. The late-onset hair cell death in the absence of early N-Myc expression could be due to mis-regulation of genes necessary for neurosensory formation and maintenance, such as Neurod1, Atoh1, Pou4f3, and Barhl1.

RESULTS

Pax2-Cre N-Myc L-Myc double CKO mice show that proliferation and differentiation are linked together through Myc and in the absence of both Mycs, altered proliferation and differentiation result in morphologically abnormal ears. In particular, the organ of Corti apex is re-patterned into a vestibular-like organization and the base is truncated and fused with the saccule.

CONCLUSIONS

These data indicate that therapeutic approaches to restore hair cells must take into account a dynamic interaction of proliferation and differentiation regulation of basic Helix-Loop-Helix transcription factors in attempts to stably replace lost cochlear hair cells. In addition, our data indicate that Myc is an integral component of the evolutionary transformation process that resulted in the organ of Corti development.

Spatiotemporal expression of Zic genes during vertebrate inner ear development

BACKGROUND

Inner ear development involves signaling from surrounding tissues, including the adjacent hindbrain, periotic mesenchyme, and notochord. These signals include SHH, FGFs, BMPs, and WNTs from the hindbrain and SHH from the notochord. Zic genes, which are expressed in the dorsal neural tube and act during neural development, have been implicated as effectors of these pathways. This report examines whether Zic genes' involvement in inner ear development is a tenable hypothesis based on their expression patterns.

RESULTS

In the developing inner ear of both the chick and mouse, all of the Zic genes were expressed in the dorsal neural tube and variably in the periotic mesenchyme, but expression of the Zic genes in the otic epithelium was not found. The onset of expression differed among the Zic genes; within any given region surrounding the otic epithelium, multiple Zic genes were expressed in the same place at the same time.

CONCLUSIONS

Zic gene expression in the region of the developing inner ear is similar between mouse and chick. Zic expression domains overlap with sites of WNT and SHH signaling during otocyst patterning, suggesting a role for Zic genes in modulating signaling from these pathways.

Inner ear progenitor cells can be generated in vitro from human bone marrow mesenchymal stem cells

AIM

Mouse mesenchymal stem cells (MSCs) can generate sensory neurons and produce inner ear hair cell-like cells. An equivalent source from humans is highly desirable, given their potential application in patient-specific regenerative therapies for deafness. In this study, we explored the ability of human MSCs (hMSCs) to differentiate into otic lineages.

MATERIALS & METHODS

hMSCs were exposed to culture media conditioned by human fetal auditory stem cells.

RESULTS

Conditioned media induced the expression of otic progenitor markers PAX8, PAX2, GATA3 and SOX2. After 4 weeks, cells coexpressed ATOH1, MYO7A and POU4F3 (indicators of hair cell lineage) or neuronal markers NEUROG1, POU4F1 and NEFH. Inhibition of WNT signaling prevented differentiation into otic progenitors, while WNT activation partially phenocopied results seen with the conditioned media.

CONCLUSION

This study demonstrates that hMSCs can be driven to express key genes found in the otic lineages and thereby promotes their status as candidates for regenerative therapies for deafness.

Progression of neurogenesis in the inner ear requires inhibition of Sox2 transcription by neurogenin1 and neurod1

Sox2 is required for proper neuronal formation in the CNS, but the molecular mechanisms involved are not well characterized. Here, we addressed the role of Sox2 in neurogenesis of the developing chicken inner ear. Overexpressing Sox2 from a constitutive (β-actin) promoter induces the expression of the proneural gene, Neurogenin1 (Ngn1); however, the expression of a downstream target of Ngn1, Neurod1, is unchanged. As a result, there is a reduction of neural precursors to delaminate and populate the developing cochleo-vestibular ganglion. In contrast, overexpression of either Ngn1 or Neurod1 is sufficient to promote the neural fate in this system. These results suggest that high levels of Sox2 inhibit progression of neurogenesis in the developing inner ear. Furthermore, we provide evidence that Ngn1 and Neurod1 inhibit Sox2 transcription through a phylogenetically conserved Sox2 enhancer to mediate neurogenesis. We propose that Sox2 confers neural competency by promoting Ngn1 expression, and that negative feedback inhibition of Sox2 by Ngn1 is an essential step in the progression from neural precursor to nascent neuron.

Wnt signaling during cochlear development

Wnt signaling is a hallmark of all embryonic development with multiple roles at multiple developmental time points. Wnt signaling is also important in the development of several organs, one of which is the inner ear, where it participates in otic specification, the formation of vestibular structures, and the development of the cochlea. In particular, we focus on Wnt signaling in the auditory organ, the cochlea. Attempting to dissect the multiple Wnt signaling pathways in the mammalian cochlea is a challenging task due to limited expression data, particularly at proliferating stages. To offer predictions about Wnt activity, we compare cochlear development with that of other biological systems such as Xenopus retina, brain, cancer cells and osteoblasts. Wnts are likely to regulate development through crosstalk with other signaling pathways, particularly Notch and FGF, leading to changes in the expression of Sox2 and proneural (pro-hair cell) genes. In this review we have consolidated the known signaling pathways in the cochlea with known developmental roles of Wnts from other systems to generate a potential timeline of cochlear development.

Sox10-dependent neural crest origin of olfactory microvillous neurons in zebrafish

The sense of smell in vertebrates is detected by specialized sensory neurons derived from the peripheral nervous system. Classically, it has been presumed that the olfactory placode forms all olfactory sensory neurons. In contrast, we show that the cranial neural crest is the primary source of microvillous sensory neurons within the olfactory epithelium of zebrafish embryos. Using photoconversion-based fate mapping and live cell tracking coupled with laser ablation, we followed neural crest precursors as they migrated from the neural tube to the nasal cavity. A subset that coexpressed Sox10 protein and a neurogenin1 reporter ingressed into the olfactory epithelium and differentiated into microvillous sensory neurons. Timed loss-of-function analysis revealed a critical role for Sox10 in microvillous neurogenesis. Taken together, these findings directly demonstrate a heretofore unknown contribution of the cranial neural crest to olfactory sensory neurons in zebrafish and provide important insights into the assembly of the nascent olfactory system.

DOI:http://dx.doi.org/10.7554/eLife.00336.001

Neurons have crucial roles in both the peripheral and central nervous systems. The role of the neurons in the sensory organs (the eyes, ears, and nose) is to sense stimuli—including light, sound, and odor—and transmit this sensory information to the neurons of the central nervous system for processing. The first step in sensing an odor relies on the peripheral nerves of the olfactory epithelium. This tissue, which lines the inside of the nasal cavity, includes two main types of olfactory sensory neurons: ciliated sensory neurons that detect volatile or easily evaporated substances and microvillous sensory neurons that detect pheromones, nucleotides, and/or amino acids.

During vertebrate embryogenesis, an embryo develops three distinct germ layers, the ectoderm, mesoderm, and endoderm, each of which gives rise to the different tissues of the body. The ectoderm has three parts—the external ectoderm, the neural crest, and the neural tube—and together they give rise to the neurons of the peripheral and central nervous systems. The neurons within the eye and ear are known to originate from a thickened portion of the ectoderm, and it has been proposed that olfactory neurons develop in a similar manner.

Now, Saxena et al. show that, unlike what happens in the eye and ear, some olfactory sensory neurons originate from the neural crest. By studying the development of the olfactory system in zebrafish, Saxena et al. discovered that microvillous neurons, but not ciliated neurons, develop from neural crest cells, and that the transcription factor Sox10 is critical for the development of microvillous neurons. By establishing that neural crest cells are involved in the development of a substantial proportion of olfactory sensory neurons, this work sets the stage for future studies of olfactory nerve growth and regeneration. It may also assist researchers working on anosmia (the inability to smell).

DOI:http://dx.doi.org/10.7554/eLife.00336.002

Divergent roles for Wnt/β-catenin signaling in epithelial maintenance and breakdown during semicircular canal formation

The morphogenetic program that shapes the three semicircular canals (SSCs) must be executed with extreme precision to satisfy their complex vestibular function. The SSCs emerge from epithelial outgrowths of the dorsal otocyst, the central regions of which fuse and resorb to leave three fluid-filled canals. The Wnt/β-catenin signaling pathway is active at multiple stages of otic development, including during vestibular morphogenesis. How Wnt/β-catenin functionally integrates with other signaling pathways to sculpt the SSCs and their sensory patches is unknown. We used a genetic strategy to spatiotemporally modulate canonical Wnt signaling activity during SSC development in mice. Our findings demonstrate that Wnt/β-catenin signaling functions in a multifaceted manner during SSC formation. In the early phase, Wnt/β-catenin signaling is required to preserve the epithelial integrity of the vertical canal pouch perimeter (presumptive anterior and posterior SSCs) by establishing a sensory-dependent signaling relay that maintains expression of Dlx5 and opposes expression of the fusion plate marker netrin 1. Without this Wnt signaling activity the sensory to non-sensory signaling cascade fails to be activated, resulting in loss of vestibular hair and support cells and the anterior and posterior SSCs. In the later phase, Wnt/β-catenin signaling becomes restricted to the fusion plate where it facilitates the timely resorption of this tissue. Mosaic recombination of β-catenin in small clusters of canal pouch cells prevents their resorption, causing instead the formation of ectopic SSCs. Together, these disparate functions of the Wnt/β-catenin pathway in epithelial maintenance and resorption help regulate the size, shape and number of SSCs.

A historical to present-day account of efforts to answer the question: "what puts the brakes on mammalian hair cell regeneration?"

Hearing and balance deficits often affect humans and other mammals permanently, because their ears stop producing hair cells within a few days after birth. But production occurs throughout life in the ears of sharks, bony fish, amphibians, reptiles, and birds allowing them to replace lost hair cells and quickly recover after temporarily experiencing the kinds of sensory deficits that are irreversible for mammals. Since the mid 1970s, researchers have been asking what puts the brakes on hair cell regeneration in mammals. Here we evaluate the headway that has been made and assess current evidence for alternative mechanistic hypotheses that have been proposed to account for the limits to hair cell regeneration in mammals.

Roles of Wnt8a during formation and patterning of the mouse inner ear

Fgf and Wnt signalling have been shown to be required for formation of the otic placode in vertebrates. Whereas several Fgfs including Fgf3, Fgf8 and Fgf10 have been shown to participate during early placode induction, Wnt signalling is required for specification and maintenance of the otic placode, and dorsal patterning of the otic vesicle. However, the requirement for specific members of the Wnt gene family for otic placode and vesicle formation and their potential interaction with Fgf signalling has been poorly defined. Due to its spatiotemporal expression during placode formation in the hindbrain Wnt8a has been postulated as a potential candidate for its specification. Here we have examined the role of Wnt8a during formation of the otic placode and vesicle in mouse embryos. Wnt8a expression depends on the presence of Fgf3 indicating a serial regulation between Fgf and Wnt signalling during otic placode induction and specification. Wnt8a by itself however is neither essential for placode specification nor redundantly required together with Fgfs for otic placode and vesicle formation. Interestingly however, Wnt8a and Fgf3 are redundantly required for expression of Fgf15 in the hindbrain indicating additional reciprocal interactions between Fgf and Wnt signalling. Further reduction of Wnt signalling by the inactivation of Wnt1 in a Wnt8a mutant background revealed a redundant requirement for both genes during morphogenesis of the dorsal portion of the otic vesicle.

Elongation factor 1 alpha1 and genes associated with Usher syndromes are downstream targets of GBX2

Gbx2 encodes a DNA-binding transcription factor that plays pivotal roles during embryogenesis. Gain-and loss-of-function studies in several vertebrate species have demonstrated a requirement for Gbx2 in development of the anterior hindbrain, spinal cord, inner ear, heart, and neural crest cells. However, the target genes through which GBX2 exerts its effects remain obscure. Using chromatin immunoprecipitation coupled with direct sequencing (ChIP-Seq) analysis in a human prostate cancer cell line, we identified cis-regulatory elements bound by GBX2 to provide insight into its direct downstream targets. The analysis revealed more than 286 highly significant candidate target genes, falling into various functional groups, of which 51% are expressed in the nervous system. Several of the top candidate genes include EEF1A1, ROBO1, PLXNA4, SLIT3, NRP1, and NOTCH2, as well as genes associated with the Usher syndrome, PCDH15 and USH2A, and are plausible candidates contributing to the developmental defects in Gbx2(-/-) mice. We show through gel shift analyses that sequences within the promoter or introns of EEF1A1, ROBO1, PCDH15, USH2A and NOTCH2, are directly bound by GBX2. Consistent with these in vitro results, analyses of Gbx2(-/-) embryos indicate that Gbx2 function is required for migration of Robo1-expressing neural crest cells out of the hindbrain. Furthermore, we show that GBX2 activates transcriptional activity through the promoter of EEF1A1, suggesting that GBX2 could also regulate gene expression indirectly via EEF1A. Taken together, our studies show that GBX2 plays a dynamic role in development and diseases.

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.

Visualization and exploration of Tcf/Lef function using a highly responsive Wnt/β-catenin signaling-reporter transgenic zebrafish

Evolutionarily conserved Tcf/Lef transcription factors (Lef1, Tcf7, Tcf7l1, and Tcf7l2) mediate gene expression regulated by Wnt/β-catenin signaling, which has multiple roles in early embryogenesis, organogenesis, adult tissue homeostasis, and tissue regeneration. However, the spatiotemporal dynamics of Tcf/Lef activity during these events remain poorly understood. We generated stable transgenic zebrafish lines carrying a new Wnt/β-catenin signaling reporter, Tcf/Lef-miniP:dGFP. The reporter revealed the transcriptional activities of four Tcf/Lef members controlled by Wnt/β-catenin signaling, which were expressed in known Wnt/β-catenin signaling-active sites during embryogenesis, organ development and growth, and tissue regeneration. We used the transgenic lines to demonstrate the contribution of Tcf/Lef-mediated Wnt/β-catenin signaling to the development of the anterior lateral line, dorsal and secondary posterior lateral lines, and gill filaments. Thus, these reporter lines are highly useful tools for studying Tcf/Lef-mediated Wnt/β-catenin signaling-dependent processes.

Members of the BMP, Shh, and FGF morphogen families promote chicken statoacoustic ganglion neurite outgrowth and neuron survival in vitro

Mechanosensory hair cells of the chicken inner ear are innervated by the peripheral processes of statoacoustic ganglion (SAG) neurons. Members of several morphogen families are expressed within and surrounding the chick inner ear during stages of SAG axon outgrowth and pathfinding. On the basis of their localized expression patterns, we hypothesized that bone morphogenetic proteins (BMPs), fibroblast growth factors (FGFs), and sonic hedgehog (Shh) may function as guidance cues for growing axons and/or may function as trophic factors once axons have reached their targets. To test this hypothesis, three-dimensional collagen cultures were used to grow Embryonic Day 4 (E4) chick SAG explants for 24 h in the presence of purified proteins or beads soaked in proteins. The density of neurite outgrowth was quantified to determine effects on neurite outgrowth. Explants displayed enhanced neurite outgrowth when cultured in the presence of purified BMP4, BMP7, a low concentration of Shh, FGF8, FGF10, or FGF19. In contrast, SAG neurons appeared unresponsive to FGF2. Collagen gel cultures were labeled with terminal dUTP nick-end labeling and immunostained with anti-phosphohistone H3 to determine effects on neuron survival and proliferation, respectively. Treatments that increased neurite outgrowth also yielded significantly fewer apoptotic cells, with no effect on cell proliferation. When presented as focal sources, BMP4, Shh, and FGFs -8, -10, and -19 promoted asymmetric outgrowth from the ganglion in the direction of the beads. BMP7-soaked beads did not induce this response. These results suggest that a subset of morphogens enhance both survival and axon outgrowth of otic neurons.

Understanding the evolution and development of neurosensory transcription factors of the ear to enhance therapeutic translation

Reconstructing a functional organ of Corti is the ultimate target towards curing hearing loss. Despite the impressive technical gains made over the last few years, many complications remain ahead for the two main restoration avenues: in vitro transformation of pluripotent cells into hair cell-like cells and adenovirus-mediated gene therapy. Most notably, both approaches require a more complete understanding of the molecular networks that ensure specific cell types form in the correct places to allow proper function of the restored organ of Corti. Important to this understanding are the basic helix-loop-helix (bHLH) transcription factors (TFs) that are highly diverse and serve to increase functional complexity but their evolutionary implementation in the inner ear neurosensory development is less conspicuous. To this end, we review the evolutionary and developmentally dynamic interactions of the three bHLH TFs that have been identified as the main players in neurosensory evolution and development, Neurog1, Neurod1 and Atoh1. These three TFs belong to the neurogenin/atonal family and evolved from a molecular precursor that likely regulated single sensory cell development in the ectoderm of metazoan ancestors but are now also expressed in other parts of the body, including the brain. They interact extensively via intracellular and intercellular cross-regulation to establish the two main neurosensory cell types of the ear, the hair cells and sensory neurons. Furthermore, the level and duration of their expression affect the specification of hair cell subtypes (inner hair cells vs. outer hair cells). We propose that appropriate manipulation of these TFs through their characterized binding sites may offer a solution by itself, or in conjunction with the two other approaches currently pursued by others, to restore the organ of Corti.

Prevention of premature fusion of calvarial suture in GLI-Kruppel family member 3 (Gli3)-deficient mice by removing one allele of Runt-related transcription factor 2 (Runx2)

Mutations in the gene encoding the zinc finger transcription factor GLI3 (GLI-Kruppel family member 3) have been identified in patients with Grieg cephalopolysyndactyly syndrome in which premature fusion of calvarial suture (craniosynostosis) is an infrequent but important feature. Here, we show that Gli3 acts as a repressor in the developing murine calvaria and that Dlx5, Runx2 type II isoform (Runx2-II), and Bmp2 are expressed ectopically in the calvarial mesenchyme, which results in aberrant osteoblastic differentiation in Gli3-deficient mouse (Gli3(Xt-J/Xt-J)) and resulted in craniosynostosis. At the same time, enhanced activation of phospho-Smad1/5/8 (pSmad1/5/8), which is a downstream mediator of canonical Bmp signaling, was observed in Gli3(Xt-J/Xt-J) embryonic calvaria. Therefore, we generated Gli3;Runx2 compound mutant mice to study the effects of decreasing Runx2 dosage in a Gli3(Xt-J/Xt-J) background. Gli3(Xt-J/Xt-J) Runx2(+/-) mice have neither craniosynostosis nor additional ossification centers in interfrontal suture and displayed a normalization of Dlx5, Runx2-II, and pSmad1/5/8 expression as well as sutural mesenchymal cell proliferation. These findings suggest a novel role for Gli3 in regulating calvarial suture development by controlling canonical Bmp-Smad signaling, which integrates a Dlx5/Runx2-II cascade. We propose that targeting Runx2 might provide an attractive way of preventing craniosynostosis in patients.

In vivo Wnt signaling tracing through a transgenic biosensor fish reveals novel activity domains

The creation of molecular tools able to unravel in vivo spatiotemporal activation of specific cell signaling events during cell migration, differentiation and morphogenesis is of great relevance to developmental cell biology. Here, we describe the generation, validation and applications of two transgenic reporter lines for Wnt/β-catenin signaling, named TCFsiam, and show that they are reliable and sensitive Wnt biosensors for in vivo studies. We demonstrate that these lines sensitively detect Wnt/β-catenin pathway activity in several cellular contexts, from sensory organs to cardiac valve patterning. We provide evidence that Wnt/β-catenin activity is involved in the formation and maintenance of the zebrafish CNS blood vessel network, on which sox10 neural crest-derived cells migrate and proliferate. We finally show that these transgenic lines allow for screening of Wnt signaling modifying compounds, tissue regeneration assessment as well as evaluation of potential Wnt/β-catenin genetic modulators.

Overactivation of Notch1 signaling induces ectopic hair cells in the mouse inner ear in an age-dependent manner

Background

During mouse inner ear development, Notch1 signaling first specifies sensory progenitors, and subsequently controls progenitors to further differentiate into either hair cells (HCs) or supporting cells (SCs). Overactivation of NICD (Notch1 intracellular domain) at early embryonic stages leads to ectopic HC formation. However, it remains unclear whether such an effect can be elicited at later embryonic or postnatal stages, which has important implications in mouse HC regeneration by reactivation of Notch1 signaling.

Methodology/Principal Findings

We performed comprehensive in vivo inducible overactivation of NICD at various developmental stages. In CAG^CreER+; Rosa26-NICD^loxp/+ mice, tamoxifen treatment at embryonic day 10.5 (E10.5) generated ectopic HCs in the non-sensory regions in both utricle and cochlea, whereas ectopic HCs only appeared in the utricle when tamoxifen was given at E13. When tamoxifen was injected at postnatal day 0 (P0) and P1, no ectopic HCs were observed in either utricle or cochlea. Interestingly, Notch1 signaling induced new HCs in a non-cell-autonomous manner, because the new HCs did not express NICD. Adjacent to the new HCs were cells expressing the SC marker Sox10 (either NICD+ or NICD-negative).

Conclusions/Significance

Our data demonstrate that the developmental stage determines responsiveness of embryonic otic precursors and neonatal non-sensory epithelial cells to NICD overactivation, and that Notch 1 signaling in the wild type, postnatal inner ear is not sufficient for generating new HCs. Thus, our genetic mouse model is suitable to test additional pathways that could synergistically interact with Notch1 pathway to produce HCs at postnatal ages.

Shaping sound in space: the regulation of inner ear patterning

The inner ear is one of the most morphologically elaborate tissues in vertebrates, containing a group of mechanosensitive sensory organs that mediate hearing and balance. These organs are arranged precisely in space and contain intricately patterned sensory epithelia. Here, we review recent studies of inner ear development and patterning which reveal that multiple stages of ear development - ranging from its early induction from the embryonic ectoderm to the establishment of the three cardinal axes and the fine-grained arrangement of sensory cells - are orchestrated by gradients of signaling molecules.

Dual embryonic origin of the mammalian otic vesicle forming the inner ear

The inner ear and cochleovestibular ganglion (CVG) derive from a specialized region of head ectoderm termed the otic placode. During embryogenesis, the otic placode invaginates into the head to form the otic vesicle (OV), the primordium of the inner ear and CVG. Non-autonomous cell signaling from the hindbrain to the OV is required for inner ear morphogenesis and neurogenesis. In this study, we show that neuroepithelial cells (NECs), including neural crest cells (NCCs), can contribute directly to the OV from the neural tube. Using Wnt1-Cre, Pax3(Cre/+) and Hoxb1(Cre/+) mice to label and fate map cranial NEC lineages, we have demonstrated that cells from the neural tube incorporate into the otic epithelium after otic placode induction has occurred. Pax3(Cre/+) labeled a more extensive population of NEC derivatives in the OV than did Wnt1-Cre. NEC derivatives constitute a significant population of the OV and, moreover, are regionalized specifically to proneurosensory domains. Descendents of Pax3(Cre/+) and Wnt1-Cre labeled cells are localized within sensory epithelia of the saccule, utricle and cochlea throughout development and into adulthood, where they differentiate into hair cells and supporting cells. Some NEC derivatives give rise to neuroblasts in the OV and CVG, in addition to their known contribution to glial cells. This study defines a dual cellular origin of the inner ear from sensory placode ectoderm and NECs, and changes the current paradigm of inner ear neurosensory development.

Redundant functions of Rac GTPases in inner ear morphogenesis

Development of the mammalian inner ear requires coordination of cell proliferation, cell fate determination and morphogenetic movements. While significant progress has been made in identifying developmental signals required for inner ear formation, less is known about how distinct signals are coordinated by their downstream mediators. Members of the Rac family of small GTPases are known regulators of cytoskeletal remodeling and numerous other cellular processes. However, the function of Rac GTPases in otic development is largely unexplored. Here, we show that Rac1 and Rac3 redundantly regulate many aspects of inner ear morphogenesis. While no morphological defects were observed in Rac3(-/-) mice, Rac1(CKO); Rac3(-/-) double mutants displayed enhanced vestibular and cochlear malformations compared to Rac1(CKO) single mutants. Moreover, in Rac1(CKO); Rac3(-/-) mutants, we observed compromised E-cadherin-mediated cell adhesion, reduced cell proliferation and increased cell death in the early developing otocyst, leading to a decreased size and malformation of the membranous labyrinth. Finally, cochlear extension was severely disrupted in Rac1(CKO); Rac3(-/-) mutants, accompanied by a loss of epithelial cohesion and formation of ectopic sensory patches underneath the cochlear duct. The compartmentalized expression of otic patterning genes within the Rac1(CKO); Rac3(-/-) mutant otocyst was largely normal, however, indicating that Rac proteins regulate inner ear morphogenesis without affecting cell fate specification. Taken together, our results reveal an essential role for Rac GTPases in coordinating cell adhesion, cell proliferation, cell death and cell movements during otic development.

Restriction of Wnt signaling in the dorsal otocyst determines semicircular canal formation in the mouse embryo

The mouse inner ear develops from a simple epithelial pouch, the otocyst, with the dorsal and ventral portions giving rise to the vestibule and cochlea, respectively. The otocyst undergoes a morphological change to generate flattened saclike structures, known as outpocketings, in the dorsal and lateral regions. The semicircular canals of the vestibule form from the periphery of the outpocketings, with the central region (the fusion plate) undergoing de-epithelialization and disappearing. However, little is known of the mechanism that orchestrates formation of the semicircular canals. We now show that the area of canonical Wnt signaling changes dynamically in the dorsal otocyst during its morphogenesis. The genes for several Wnt ligands were found to be expressed in the dorsal otocyst according to specific patterns, whereas those for secreted inhibitors of Wnt ligands were expressed exclusively in the ventral otocyst. With the use of whole-embryo culture in combination with potent modulators of canonical Wnt signaling, we found that forced persistence of such signaling resulted in impaired formation both of the lateral outpocketing and of the fusion plates of the dorsal outpocketing. Canonical Wnt signaling was found to suppress Netrin1 expression and to preserve the integrity of the outpocketing epithelium. In addition, inhibition of canonical Wnt signaling reduced the size of the otocyst, likely through suppression of cell proliferation and promotion of apoptosis. Our stage-specific functional analysis suggests that strict regulation of canonical Wnt signaling in the dorsal otocyst orchestrates the process of semicircular canal formation.

Otic ablation of smoothened reveals direct and indirect requirements for Hedgehog signaling in inner ear development

In mouse embryos lacking sonic hedgehog (Shh), dorsoventral polarity within the otic vesicle is disrupted. Consequently, ventral otic derivatives, including the cochlear duct and saccule, fail to form, and dorsal otic derivatives, including the semicircular canals, endolymphatic duct and utricle, are malformed or absent. Since inner ear patterning and morphogenesis are heavily dependent on extracellular signals derived from tissues that are also compromised by the loss of Shh, the extent to which Shh signaling acts directly on the inner ear for its development is unclear. To address this question, we generated embryos in which smoothened (Smo), an essential transducer of Hedgehog (Hh) signaling, was conditionally inactivated in the otic epithelium (Smo(ecko)). Ventral otic derivatives failed to form in Smo(ecko) embryos, whereas vestibular structures developed properly. Consistent with these findings, we demonstrate that ventral, but not dorsal, otic identity is directly dependent on Hh. The role of Hh in cochlear-vestibular ganglion (cvg) formation is more complex, as both direct and indirect signaling mechanisms are implicated. Our data suggest that the loss of cvg neurons in Shh(-/-) animals is due, in part, to an increase in Wnt responsiveness in the otic vesicle, resulting in the ectopic expression of Tbx1 in the neurogenic domain and subsequent repression of Ngn1 transcription. A mitogenic role for Shh in cvg progenitor proliferation was also revealed in our analysis of Smo(ecko) embryos. Taken together, these data contribute to a better understanding of the intrinsic and extrinsic signaling properties of Shh during inner ear development.

The molecular basis of making spiral ganglion neurons and connecting them to hair cells of the organ of Corti

The bipolar spiral ganglion neurons apparently delaminate from the growing cochlear duct and migrate to Rosenthal's canal. They project radial fibers to innervate the organ of Corti (type I neurons to inner hair cells, type II neurons to outer hair cells) and also project tonotopically to the cochlear nuclei. The early differentiation of these neurons requires transcription factors to regulate migration, pathfinding and survival. Neurog1 null mice lack formation of neurons. Neurod1 null mice show massive neuronal death combined with aberrant central and peripheral projections. Prox1 protein is necessary for proper type II neuron process navigation, which is also affected by the neurotrophins Bdnf and Ntf3. Neurotrophin null mutants show specific patterns of neuronal loss along the cochlea but remaining neurons compensate by expanding their target area. All neurotrophin mutants have reduced radial fiber growth proportional to the degree of loss of neurotrophin alleles. This suggests a simple dose response effect of neurotrophin concentration. Keeping overall concentration constant, but misexpressing one neurotrophin under regulatory control of another one results in exuberant fiber growth not only of vestibular fibers to the cochlea but also of spiral ganglion neurons to outer hair cells suggesting different effectiveness of neurotrophins for spiral ganglion neurite growth. Finally, we report here for the first time that losing all neurons in double null mutants affects extension of the cochlear duct and leads to formation of extra rows of outer hair cells in the apex, possibly by disrupting the interaction of the spiral ganglion with the elongating cochlea.

Pronephric tubulogenesis requires Daam1-mediated planar cell polarity signaling

Canonical β-catenin-mediated Wnt signaling is essential for the induction of nephron development. Noncanonical Wnt/planar cell polarity (PCP) pathways contribute to processes such as cell polarization and cytoskeletal modulation in several tissues. Although PCP components likely establish the plane of polarization in kidney tubulogenesis, whether PCP effectors directly modulate the actin cytoskeleton in tubulogenesis is unknown. Here, we investigated the roles of Wnt PCP components in cytoskeletal assembly during kidney tubule morphogenesis in Xenopus laevis and zebrafish. We found that during tubulogenesis, the developing pronephric anlagen expresses Daam1 and its interacting Rho-GEF (WGEF), which compose one PCP/noncanonical Wnt pathway branch. Knockdown of Daam1 resulted in reduced expression of late pronephric epithelial markers with no apparent effect upon early markers of patterning and determination. Inhibiting various points in the Daam1 signaling pathway significantly reduced pronephric tubulogenesis. These data indicate that pronephric tubulogenesis requires the Daam1/WGEF/Rho PCP pathway.

Sox2 and Fgf interact with Atoh1 to promote sensory competence throughout the zebrafish inner ear

Atoh1 is required for differentiation of sensory hair cells in the vertebrate inner ear. Moreover, misexpression of Atoh1 is sufficient to establish ectopic sensory epithelia, making Atoh1 a good candidate for gene therapy to restore hearing. However, competence to form sensory epithelia appears to be limited to discrete regions of the inner ear. To better understand the developmental factors influencing sensory-competence, we examined the effects of misexpressing atoh1a in zebrafish embryos under various developmental conditions. Activation of a heat shock-inducible transgene, hs:atoh1a, resulted in ectopic expression of early markers of sensory development within 2h, and mature hair cells marked by brn3c:GFP began to accumulate 9h after heat shock. The ability of atoh1a to induce ectopic sensory epithelia was maximal when activated during placodal or early otic vesicle stages but declined rapidly thereafter. At no stage was atoh1a sufficient to induce sensory development in dorsal or lateral non-sensory regions of the otic vesicle. However, co-misexpression of atoh1a with fgf3, fgf8 or sox2, genes normally acting in the same gene network with atoh1a, stimulated sensory development in all regions of the otic vesicle. Thus, expression of fgf3, fgf8 or sox2 strongly enhances competence to respond to Atoh1.

Regeneration of Hair Cells: Making Sense of All the Noise

Hearing loss affects hundreds of millions of people worldwide by dampening or cutting off their auditory connection to the world. Current treatments for sensorineural hearing loss (SNHL) with cochlear implants are not perfect, leaving regenerative medicine as the logical avenue to a perfect cure. Multiple routes to regeneration of damaged hair cells have been proposed and are actively pursued. Each route not only requires a keen understanding of the molecular basis of ear development but also faces the practical limitations of stem cell regulation in the delicate inner ear where topology of cell distribution is essential. Improvements in our molecular understanding of the minimal essential genes necessary for hair cell formation and recent advances in stem cell manipulation, such as seen with inducible pluripotent stem cells (iPSCs) and epidermal neural crest stem cells (EPI-NCSCs), have opened new possibilities to advance research in translational stem cell therapies for individuals with hearing loss. Despite this, more detailed network maps of gene expression are needed, including an appreciation for the roles of microRNAs (miRs), key regulators of transcriptional gene networks. To harness the true potential of stem cells for hair cell regeneration, basic science and clinical medicine must work together to expedite the transition from bench to bedside by elucidating the full mechanisms of inner ear hair cell development, including a focus on the role of miRs, and adapting this knowledge safely and efficiently to stem cell technologies.

Dynamic expression of Lgr5, a Wnt target gene, in the developing and mature mouse cochlea

The Wnt signaling pathway is a recurring theme in tissue development and homeostasis. Its specific roles during inner ear development are just emerging, but few studies have characterized Wnt target genes. Lgr5, a member of the G protein-coupled receptor family, is a Wnt target in the gastrointestinal and integumentary systems. Although its function is unknown, its deficiency leads to perinatal lethality due to gastrointestinal distension. In this study, we used a knock-in reporter mouse to examine the spatiotemporal expression of Lgr5 in the cochlear duct during embryonic and postnatal periods. In the embryonic day 15.5 (E15.5) cochlear duct, Lgr5-EGFP is expressed in the floor epithelium and overlapped with the prosensory markers Sox2, Jagged1, and p27(Kip1). Nascent hair cells and supporting cells in the apical turn of the E18.5 cochlear duct express Lgr5-EGFP, which becomes downregulated in hair cells and subsets of supporting cells in more mature stages. In situ hybridization experiments validated the reporter expression, which gradually decreases until the second postnatal week. Only the third row of Deiters' cells expresses Lgr5-EGFP in the mature organ of Corti. Normal cochlear development was observed in Lgr5(EGFP/EGFP) and Lgr5(EGFP/+) mice, which exhibited normal auditory thresholds. The expression pattern of Lgr5 contrasts with another Wnt target gene, Axin2, a feedback inhibitor of the Wnt pathway. Robust Axin2 expression was found in cells surrounding the embryonic cochlear duct and becomes restricted to tympanic border cells below the basilar membrane in the postnatal cochlea. Both Lgr5 and Axin2 act as Wnt targets in the cochlea because purified Wnt3a promoted and Wnt antagonist suppressed their expression. Their differential expression among cell populations highlights the dynamic but complex distribution of Wnt-activated cells in and around the embryonic and postnatal cochlea.

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.

A sensitive and bright single-cell resolution live imaging reporter of Wnt/ß-catenin signaling in the mouse

BACKGROUND

Understanding the dynamic cellular behaviors and underlying molecular mechanisms that drive morphogenesis is an ongoing challenge in biology. Live imaging provides the necessary methodology to unravel the synergistic and stereotypical cell and molecular events that shape the embryo. Genetically-encoded reporters represent an essential tool for live imaging. Reporter strains can be engineered by placing cis-regulatory elements of interest to direct the expression of a desired reporter gene. In the case of canonical Wnt signaling, also referred to as Wnt/β-catenin signaling, since the downstream transcriptional response is well understood, reporters can be designed that reflect sites of active Wnt signaling, as opposed to sites of gene transcription, as is the case with many fluorescent reporters. However, even though several transgenic Wnt/β-catenin reporter strains have been generated, to date, none provides the single-cell resolution favored for live imaging studies.

RESULTS

We have placed six copies of a TCF/Lef responsive element and an hsp68 minimal promoter in front of a fluorescent protein fusion comprising human histone H2B to GFP and used it to generate a strain of mice that would report Wnt/β-catenin signaling activity. Characterization of developmental and adult stages of the resulting TCF/Lef:H2B-GFP strain revealed discrete and specific expression of the transgene at previously characterized sites of Wnt/β-catenin signaling. In support of the increased sensitivity of the TCF/Lef:H2B-GFP reporter, additional sites of Wnt/β-catenin signaling not documented with other reporters but identified through genetic and embryological analysis were observed. Furthermore, the sub-cellular localization of the reporter minimized reporter perdurance, and allowed visualization and tracking of individual cells within a cohort, so facilitating the detailed analysis of cell behaviors and signaling activity during morphogenesis.

CONCLUSION

By combining the Wnt activity read-out efficiency of multimerized TCF/Lef DNA binding sites, together with the high-resolution imaging afforded by subcellularly-localized fluorescent fusion proteins such as H2B-GFP, we have created a mouse transgenic line that faithfully recapitulates Wnt signaling activity at single-cell resolution. The TCF/Lef:H2B-GFP reporter represents a unique tool for live imaging the in vivo processes triggered by Wnt/β-catenin signaling, and thus should help the formulation of a high-resolution understanding of the serial events that define the morphogenetic process regulated by this signaling pathway.

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.

The ATP-dependent chromatin remodeling enzyme CHD7 regulates pro-neural gene expression and neurogenesis in the inner ear

Inner ear neurogenesis is positively regulated by the pro-neural bHLH transcription factors Ngn1 and NeuroD, but the factors that act upstream of this regulation are not well understood. Recent evidence in mouse and Drosophila suggests that neural development depends on proper chromatin remodeling, both for maintenance of neural stem cells and for proper neuronal differentiation. Here, we show that CHD7, an ATP-dependent chromatin remodeling enzyme mutated in human CHARGE syndrome, is necessary for proliferation of inner ear neuroblasts and inner ear morphogenesis. Conditional deletion of Chd7 in the developing otocyst using Foxg1-Cre resulted in cochlear hypoplasia and complete absence of the semicircular canals and cristae. Conditional knockout and null otocysts also had reductions in vestibulo-cochlear ganglion size and neuron number in combination with reduced expression of Ngn1, Otx2 and Fgf10, concurrent with expansion of the neural fate suppressor Tbx1 and reduced cellular proliferation. Heterozygosity for Chd7 mutations had no major effects on expression of otic patterning genes or on cell survival, but resulted in decreased proliferation within the neurogenic domain. These data indicate that epigenetic regulation of gene expression by CHD7 must be tightly coordinated for proper development of inner ear neuroblasts.

Dynamic expression of a LEF-EGFP Wnt reporter in mouse development and cancer

We have characterized a transgenic mouse line in which enhanced green fluorescent protein (EGFP) is expressed under the control of multimerized LEF-1 responsive elements. In embryos, EGFP was detected in known sites of Wnt activation, including the primitive streak, mesoderm, neural tube, somites, heart, limb buds, mammary placodes, and whisker follicles. In vitro cultured transgenic embryonic fibroblasts upregulated EGFP expression in response to activation of Wnt signaling by GSK3beta inhibition. Mammary tumor cell lines derived from female LEF-EGFP transgenic mice treated with the carcinogen 7, 12-dimethylbenz[a]anthracene (DMBA) also express EGFP. Thus, this transgenic line is useful for ex vivo and in vitro studies of Wnt signaling in development and cancer.

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.

Expression of Gpr177, a Wnt trafficking regulator, in mouse embryogenesis

Wls/Evi/Srt encoding a multipass transmembrane protein has been identified as a regulator for proper sorting and secretion of Wnt in flies. We have previously demonstrated that Gpr177 is the mouse ortholog required for axis determination. Gpr177 is a transcriptional target of Wnt that is activated to assist its subcellular distribution in a feedback regulatory loop. We, therefore, proposed that reciprocal regulation of Wnt and Gpr177 is essential for the Wnt-dependent developmental and pathogenic processes. Here, we examine the expression pattern of Gpr177 in mouse development. Gpr177 is expressed in a variety of tissues and cell types during organogenesis. Furthermore, Gpr177 is a glycoprotein primarily accumulating in the Golgi apparatus in signal-producing cells. The glycosylation of Gpr177 is necessary for proper transportation in the secretory pathway. Our findings suggest that the Gpr177-mediated regulation of Wnt is crucial for organogenesis in health and disease.

Dynamic expression pattern of Sonic hedgehog in developing cochlear spiral ganglion neurons

Sonic hedgehog (Shh) signaling plays important roles in the formation of the auditory epithelium. However, little is known about the detailed expression pattern of Shh and the cell sources from which Shh is secreted. By analyzing Shh(CreEGFP/+) mice, we found that Shh was first expressed in all cochlear spiral ganglion neurons by embryonic day 13.5, after which its expression gradually decreased from base to apex. By postnatal day 0, it was not detected in any spiral ganglion neurons. Genetic cell fate mapping results also confirmed that Shh was exclusively expressed in all spiral ganglion neurons and not in surrounding glia cells. The basal-to-apical wave of Shh decline strongly resembles that of hair cell differentiation, supporting the idea that Shh signaling inhibits hair cell differentiation. Furthermore, this Shh(CreEGFP/+) mouse is a useful Cre line in which to delete floxed genes specifically in spiral ganglion neurons of the developing cochlea.

Canonical Wnt signaling modulates Tbx1, Eya1, and Six1 expression, restricting neurogenesis in the otic vesicle

To understand the mechanism by which canonical Wnt signaling sets boundaries for pattern formation in the otic vesicle (OV), we examined Tbx1 and Eya1-Six1 downstream of activated beta-catenin. Tbx1, the gene for velo-cardio-facial syndrome/DiGeorge syndrome (VCFS/DGS), is essential for inner ear development where it promotes Bmp4 and Otx1 expression and restricts neurogenesis. Using floxed beta-catenin gain-of-function (GOF) and loss-of-function (LOF) alleles, we found Tbx1 expression was down-regulated and maintained/enhanced in the two mouse mutants, respectively. Bmp4 was ectopically expressed and Otx1 was lost in beta-catenin GOF mutants. Normally, inactivation of Tbx1 causes expanded neurogenesis, but expression of NeuroD was down-regulated in beta-catenin GOF mutants. To explain this paradox, Eya1 and Six1, genes for branchio-oto-renal (BOR) syndrome were down-regulated in the OV of beta-catenin GOF mutants independently of Tbx1. Overall, this work helps explain the mechanism by which Wnt signaling modulates transcription factors required for neurogenesis and patterning of the OV.

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.

Repression of Hedgehog signalling is required for the acquisition of dorsolateral cell fates in the zebrafish otic vesicle

In zebrafish, Hedgehog (Hh) signalling from ventral midline structures is necessary and sufficient to specify posterior otic identity. Loss of Hh signalling gives rise to mirror symmetric ears with double anterior character, whereas severe upregulation of Hh signalling leads to double posterior ears. By contrast, in mouse and chick, Hh is predominantly required for dorsoventral otic patterning. Whereas a loss of Hh function in zebrafish does not affect dorsoventral and mediolateral otic patterning, we now show that a gain of Hh signalling activity causes ventromedial otic territories to expand at the expense of dorsolateral domains. In a panel of lines carrying mutations in Hh inhibitor genes, Hh pathway activity is increased throughout the embryo, and dorsolateral otic structures are lost or reduced. Even a modest increase in Hh signalling has consequences for patterning the ear. In ptc1(-/-) and ptc2(-/-) mutant embryos, in which Hh signalling is maximal throughout the embryo, the inner ear is severely ventralised and medialised, in addition to displaying the previously reported double posterior character. Transplantation experiments suggest that the effects of the loss of Hh pathway inhibition on the ear are mediated directly. These new data suggest that Hh signalling must be kept tightly repressed for the correct acquisition of dorsolateral cell fates in the zebrafish otic vesicle, revealing distinct similarities between the roles of Hh signalling in zebrafish and amniote inner ear patterning.

Mapping of Wnt, frizzled, and Wnt inhibitor gene expression domains in the avian otic primordium

Wnt signaling activates at least three different pathways involved in development and disease. Interactions of secreted ligands and inhibitors with cell-surface receptors result in the activation or regulation of particular downstream intracellular cascades. During the developmental stages of otic vesicle closure and beginning morphogenesis, the forming inner ear transcribes a plethora of Wnt-related genes. We report expression of 23 genes out of 25 tested in situ hybridization probes on tissue serial sections. Sensory primordia and Frizzled gene expression share domains, with Fzd1 being a continuous marker. Prospective nonsensory domains express Wnts, whose transcripts mainly flank prosensory regions. Finally, Wnt inhibitor domains are superimposed over both prosensory and nonsensory otic regions. Three Wnt antagonists, Dkk1, SFRP2, and Frzb are prominent. Their gene expression patterns partly overlap and change over time, which adds to the diversity of molecular microenvironments. Strikingly, prosensory domains express Wnts transiently. This includes: 1) the prosensory otic region of high proliferation, neuroblast delamination, and programmed cell death at stage 20/21 (Wnt3, -5b, -7b, -8b, -9a, and -11); and 2) sensory primordia at stage 25 (Wnt7a and Wnt9a). In summary, robust Wnt-related gene expression shows both spatial and temporal tuning during inner ear development as the otic vesicle initiates morphogenesis and prosensory cell fate determination.

Making senses development of vertebrate cranial placodes

Cranial placodes (which include the adenohypophyseal, olfactory, lens, otic, lateral line, profundal/trigeminal, and epibranchial placodes) give rise to many sense organs and ganglia of the vertebrate head. Recent evidence suggests that all cranial placodes may be developmentally related structures, which originate from a common panplacodal primordium at neural plate stages and use similar regulatory mechanisms to control developmental processes shared between different placodes such as neurogenesis and morphogenetic movements. After providing a brief overview of placodal diversity, the present review summarizes current evidence for the existence of a panplacodal primordium and discusses the central role of transcription factors Six1 and Eya1 in the regulation of processes shared between different placodes. Upstream signaling events and transcription factors involved in early embryonic induction and specification of the panplacodal primordium are discussed next. I then review how individual placodes arise from the panplacodal primordium and present a model of multistep placode induction. Finally, I briefly summarize recent advances concerning how placodal neurons and sensory cells are specified, and how morphogenesis of placodes (including delamination and migration of placode-derived cells and invagination) is controlled.

Wnt to build a tube: contributions of Wnt signaling to epithelial tubulogenesis

Epithelial tubes are crucial to the function of organ systems including the cardiovascular system, pulmonary system, gastrointestinal tract, reproductive organ systems, excretory system, and auditory system. Using a variety of animal model systems, recent studies have substantiated the role of Wnt signaling via the canonical/beta-catenin-mediated trajectory, the non-canonical Wnt trajectories, or both, in forming epithelial tubular tissues. This review focuses on the involvement of the Wnt pathways in the induction, specification, proliferation, and morphogenesis involved in tubulogenesis within tissues including the lungs, kidneys, ears, mammary glands, gut, and heart. The ultimate goal is to describe the developmental processes forming the various tubulogenic organ systems to determine the relationships between these processes.

Identification of differentially expressed genes in early inner ear development

To understand the etiology of congenital hearing loss, a comprehensive understanding of the molecular genetic mechanisms underlying normal ear development is required. We are identifying genes involved in otogenesis, with the longer term goal of studying their mechanisms of action, leading to inner ear induction and patterning. Using Agilent microarrays, we compared the differential expression of a test domain (which consisted of the pre-otic placodal ectoderm with the adjacent hindbrain ectoderm and the underlying mesendodermal tissues) with a rostral control domain (which included tissue that is competent, but not specified, to express inner ear markers in explant assays). We identified 1261 transcripts differentially expressed between the two domains at a 2-fold or greater change: 463 were upregulated and 798 were downregulated in the test domain. We validated the differential expression of several signaling molecules and transcription factors identified in this array using in situ hybridization. Furthermore, the expression patterns of the validated group of genes from the test domain were explored in detail to determine how the timing of their expression relates to specific events of otic induction and development. In conclusion, we identified a number of novel candidate genes for otic placode induction.

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.

Distinct contributions from the hindbrain and mesenchyme to inner ear morphogenesis

A mature inner ear is a complex structure consisting of vestibular and auditory components. Microsurgical ablations, rotations, and translocations were performed in ovo to identify the tissues that control inner ear morphogenesis. We show that mesenchyme/ectoderm adjacent to the developing ear specifically governs the shape of vestibular components - the semicircular canals and ampullae - by conferring anteroposterior axial information to these structures. In contrast, removal of individual hindbrain rhombomeres adjacent to the developing ear preferentially affects the growth and morphogenesis of the auditory subdivision, the cochlear duct, or basilar papilla. Removal of rhombomere 5 affects cochlear duct growth, while rhombomere 6 removal affects cochlear growth and morphogenesis. Rotating rhombomeres 5 and 6 along the anteroposterior axis also impacts cochlear duct morphogenesis but has little effect on the vestibular components. Our studies indicate that discrete tissues, acting at a distance, control the morphogenesis of distinct elements of the inner ear. These results provide a basis for identifying factors that are essential to vestibular and auditory development in vertebrates.

Beta-catenin up-regulates Atoh1 expression in neural progenitor cells by interaction with an Atoh1 3' enhancer

Atoh1, a basic helix-loop-helix transcription factor, plays a critical role in the differentiation of several epithelial and neural cell types. We found that beta-catenin, the key mediator of the canonical Wnt pathway, increased expression of Atoh1 in mouse neuroblastoma cells and neural progenitor cells, and baseline Atoh1 expression was decreased by siRNA directed at beta-catenin. The up-regulation of Atoh1 was caused by an interaction of beta-catenin with the Atoh1 enhancer that could be demonstrated by chromatin immunoprecipitation. We found that two putative Tcf-Lef sites in the 3' enhancer of the Atoh1 gene displayed an affinity for beta-catenin and were critical for the activation of Atoh1 transcription because mutation of either site decreased expression of a reporter gene downstream of the enhancer. Tcf-Lef co-activators were found in the complex that bound to these sites in the DNA together with beta-catenin. Inhibition of Notch signaling, which has previously been shown to induce bHLH transcription factor expression, increased beta-catenin expression in progenitor cells of the nervous system. Because this could be a mechanism for up-regulation of Atoh1 after inhibition of Notch, we tested whether siRNA to beta-catenin prevented the increase in Atoh1 and found that beta-catenin expression was required for increased expression of Atoh1 after Notch inhibition.