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

Optimized inner ear organoids for efficient hair cell generation and ototoxicity response modeling

Hair cells in the mammalian cochlea are highly vulnerable to damage from drug toxicity, noise exposure, aging, and genetic mutations, with no capacity for regeneration. Progress in hair cell protection research has been limited by the scarcity of cochlear tissue and suitable in vitro models. Here, we present a novel one-step, self-organizing inner ear organoid system optimized with small molecules, which bypasses the need for multi-step expansion and forced differentiation protocols. This approach efficiently generates hair cells and supporting cells that recapitulate the molecular, cellular, and structural characteristics of the inner ear. Single-cell RNA sequencing revealed the diversity and fidelity of cell populations within the organoids. Utilizing this platform, we validated the protective effects of candidate compounds against hair cell damage, highlighting its potential as a powerful tool for drug discovery and mechanistic studies of hair cell protection.

A cochlear progenitor pool influences patterning of the mammalian sensory epithelium via MYBL2

During embryonic development, Wnt signaling influences both proliferation and sensory formation in the cochlea. How this dual nature of Wnt signaling is coordinated is unknown. In this study, we define a novel role for a Wnt-regulated gene, Mybl2, which was already known to be important for proliferation, in determining the size and patterning of the sensory epithelium in the murine cochlea. Using a quantitative spatial analysis approach and analyzing Mybl2 loss-of-function, we show that Mybl2 promoted proliferation in the inner sulcus domain but limited the size of the sensory domain by influencing their adjoining boundary position via Jag1 regulation during development. Mybl2 loss-of-function simultaneously decreased proliferation in the inner sulcus and increased the size of the sensory domain, resulting in a wider sensory epithelium with ectopic inner hair cell formation during late embryonic stages. These data suggest that progenitor cells in the inner sulcus determine boundary formation and pattern the sensory epithelium via MYBL2.

Loss of noggin1, a classic embryonic inducer gene, in elasmobranchs

Secreted proteins of the Noggin family serve as pivotal regulators of early development and cell differentiation in all multicellular animals, including vertebrates. Noggin1 was identified first among all Noggins. Moreover, it was described as the first known embryonic inducer specifically secreted by the Spemann organizer and capable of inducing a secondary body axis when expressed ectopically. In the classical default model of neural induction, Noggin1 is presented as an antagonist of BMP signalling, playing a role as a neural inducer. Additionally, Noggin1 is involved in the dorsalization of embryonic mesoderm and later controls the differentiation of various tissues, including muscles, bones, and neural crest derivatives. Hitherto, noggin1 was found in all studied vertebrates. Here, we report the loss of noggin1 in elasmobranchs (sharks, rays and skates), which is a unique case among vertebrates. noggin2 and noggin4 retained in this group and studied in the embryos of the grey bamboo shark Chiloscyllium griseum revealed similarities in expression patterns and functional properties with their orthologues described in other vertebrates. The loss of noggin1 in elasmobranchs may be associated with histological features of the formation of their unique internal cartilaginous skeleton, although additional research is required to establish functional connections between these events.

Sensory progenitors influence patterning of the mammalian auditory sensory epithelium

During embryonic development Wnt signaling has been shown to influence proliferation and sensory formation in the cochlea. How the dual nature of Wnt signaling is coordinated is unknown. In this study, we define a novel role for a Wnt regulated gene, Mybl2, which was already known to be important for proliferation, in influencing patterning and determining the size of the sensory epithelium in the murine cochlea. Using a quantitative spatial analysis approach and analyzing Mybl2 loss-of-function cochleas, we show that Mybl2 simultaneously specifies the progenitor niche and the size of the sensory domain, and influences the positioning of the medial sensory domain boundary via Jag1 regulation during the mid-gestational stages. Mybl2 conditional knockout resulted in a decrease of proliferation within the progenitor niche. During the late embryonic stages, conditional knockout of Mybl2 produced a wider sensory epithelium across the radial axis with an increase in ectopic inner hair cell formation. These data suggest that Mybl2 -positive progenitors play a role in boundary formation and patterning the sensory epithelium.

Summary Statement

Mybl2 is a Wnt-regulated gene encoding a transcription factor that is expressed in the cochlear progenitor niche and influences the boundary formation between the niche and the sensory domain during mid-cochlear developmental stages, thereby impacting the size of the sensory epithelium.

Cochlear Development; New Tools and Approaches

The sensory epithelium of the mammalian cochlea, the organ of Corti, is comprised of at least seven unique cell types including two functionally distinct types of mechanosensory hair cells. All of the cell types within the organ of Corti are believed to develop from a population of precursor cells referred to as prosensory cells. Results from previous studies have begun to identify the developmental processes, lineage restrictions and signaling networks that mediate the specification of many of these cell types, however, the small size of the organ and the limited number of each cell type has hampered progress. Recent technical advances, in particular relating to the ability to capture and characterize gene expression at the single cell level, have opened new avenues for understanding cellular specification in the organ of Corti. This review will cover our current understanding of cellular specification in the cochlea, discuss the most commonly used methods for single cell RNA sequencing and describe how results from a recent study using single cell sequencing provided new insights regarding cellular specification.

Follistatin promotes LIN28B-mediated supporting cell reprogramming and hair cell regeneration in the murine cochlea

Hair cell (HC) loss within the inner ear cochlea is a leading cause for deafness in humans. Before the onset of hearing, immature supporting cells (SCs) in neonatal mice have some limited capacity for HC regeneration. Here, we show that in organoid culture, transient activation of the progenitor-specific RNA binding protein LIN28B and Activin antagonist follistatin (FST) enhances regenerative competence of maturing/mature cochlear SCs by reprogramming them into progenitor-like cells. Transcriptome profiling and mechanistic studies reveal that LIN28B drives SC reprogramming, while FST is required to counterbalance hyperactivation of transforming growth factor-β-type signaling by LIN28B. Last, we show that LIN28B and FST coactivation enhances spontaneous cochlear HC regeneration in neonatal mice and that LIN28B may be part of an endogenous repair mechanism that primes SCs for HC regeneration. These findings indicate that SC dedifferentiation is critical for HC regeneration and identify LIN28B and FST as main regulators.

A Model of Waardenburg Syndrome Using Patient-Derived iPSCs With a SOX10 Mutation Displays Compromised Maturation and Function of the Neural Crest That Involves Inner Ear Development

Waardenburg syndrome (WS) is an autosomal dominant inherited disorder that is characterized by sensorineural hearing loss and abnormal pigmentation. SOX10 is one of its main pathogenicity genes. The generation of patient-specific induced pluripotent stem cells (iPSCs) is an efficient means to investigate the mechanisms of inherited human disease. In our work, we set up an iPSC line derived from a WS patient with SOX10 mutation and differentiated into neural crest cells (NCCs), a key cell type involved in inner ear development. Compared with control-derived iPSCs, the SOX10 mutant iPSCs showed significantly decreased efficiency of development and differentiation potential at the stage of NCCs. After that, we carried out high-throughput RNA-seq and evaluated the transcriptional misregulation at every stage. Transcriptome analysis of differentiated NCCs showed widespread gene expression alterations, and the differentially expressed genes (DEGs) were enriched in gene ontology terms of neuron migration, skeletal system development, and multicellular organism development, indicating that SOX10 has a pivotal part in the differentiation of NCCs. It's worth noting that, a significant enrichment among the nominal DEGs for genes implicated in inner ear development was found, as well as several genes connected to the inner ear morphogenesis. Based on the protein-protein interaction network, we chose four candidate genes that could be regulated by SOX10 in inner ear development, namely, BMP2, LGR5, GBX2, and GATA3. In conclusion, SOX10 deficiency in this WS subject had a significant impact on the gene expression patterns throughout NCC development in the iPSC model. The DEGs most significantly enriched in inner ear development and morphogenesis may assist in identifying the underlying basis for the inner ear malformation in subjects with WS.

Characterization of the development of the mouse cochlear epithelium at the single cell level

Mammalian hearing requires the development of the organ of Corti, a sensory epithelium comprising unique cell types. The limited number of each of these cell types, combined with their close proximity, has prevented characterization of individual cell types and/or their developmental progression. To examine cochlear development more closely, we transcriptionally profile approximately 30,000 isolated mouse cochlear cells collected at four developmental time points. Here we report on the analysis of those cells including the identification of both known and unknown cell types. Trajectory analysis for OHCs indicates four phases of gene expression while fate mapping of progenitor cells suggests that OHCs and their surrounding supporting cells arise from a distinct (lateral) progenitor pool. Tgfβr1 is identified as being expressed in lateral progenitor cells and a Tgfβr1 antagonist inhibits OHC development. These results provide insights regarding cochlear development and demonstrate the potential value and application of this data set.

How the development of the cochlear epithelium is regulated is unclear. Here, the authors use single cell RNAseq analysis to provide insight into the transcriptional changes arising during development of the murine cochlear inner and outer hair cells.

Idgenes are required for morphogenesis and cellular patterning in the developing mammalian cochlea

Inhibitor of differentiation and DNA-binding (Id) proteins, Id1 to Id4, function in the regulation of cellular proliferation and differentiation. Id proteins have been shown to interact with bHLH proteins and other proteins involved in regulating cellular proliferation and differentiation, suggesting a widespread regulatory function. Id1-3 are known to be expressed in the prosensory domain of developing cochlea. However, the roles of Id genes in cochlear development are not fully elucidated. The deficiency of any of the Id1-3 genes individually has little effect on the cochlear development, and therefore the functional redundancy among these genes have been presumed to explain the absence of phenotype. Here, we show that conditional knockout of Id1/2/3 genes (Id TKO) causes major defects in morphogenesis and cellular patterning in the development of mammalian cochlea. Id TKO cochlea was 82% shorter than control, and both decreased proliferation and increased cell death caused the hypomorph. Sox2-positive prosensory domain was formed in Id TKO cochlea, but the formation of the medial-lateral (central-peripheral) axis was disturbed; the boundary between the medial and lateral compartments in the prosensory domain was partially doubled; the number of inner hair cells per unit length increased, and the number of outer hair cells decreased. Furthermore, the lateral non-sensory compartment expressing Bmp4 and Lmo3 was missing. Thus, the patterning of the lateral epithelium was more affected than the medial epithelium. These results suggested that Id genes are crucial for morphogenesis of the cochlea duct and patterning of the lateral epithelium in the developing cochlea. Further analyses by quantitative RT-PCR and immunostaining using cochlear explants with a Bmp pathway inhibitor revealed that the Bmp-Id pathway originates from the lateral non-sensory compartment and promotes outer hair cell differentiation.

GSK3 regulates hair cell fate in the developing mammalian cochlea

The development of asymmetric patterns along biologically relevant axes is a hallmark of many vertebrate organs or structures. One example is the sensory epithelium of the mammalian auditory system. Two distinct types of mechanosensory hair cells (inner and outer) and at least six types of associated supporting cells are precisely and asymmetrically arrayed along the radial (medial-lateral) axis of the cochlear spiral. Immunolabeling of developing cochleae indicates differential expression of Glycogen synthase kinase 3β (GSK3β) along the same axis. To determine whether GSK3β plays a role in specification of cell fates along the medial-lateral axis, GSK3 activity was blocked pharmacologically in cochlear explants. Results indicate significant changes in both the number of hair cells and in the specification of hair cell phenotypes. The overall number of inner hair cells increased as a result of both a shift in the medial boundary between sensory and non-sensory regions of the cochlea and a change in the specification of inner and outer hair cell phenotypes. Previous studies have inhibited GSK3 as a method to examine effects of canonical Wnt signaling. However, quantification of changes in Wnt pathway target genes in GSK3-inhibited cochleae, and treatment with more specific Wnt agonists, indicated that the Wnt pathway is not activated. Instead, expression of Bmp4 in a population of GSK3β-expressing cells was shown to be down-regulated. Finally, addition of BMP4 to GSK3-inhibited cochleae achieved a partial rescue of the hair cell phenotype. These results demonstrate a role for GSK3β in the specification of cellular identities along the medial-lateral axis of the cochlea and provide evidence for a positive role for GSK3β in the expression of Bmp4.

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.

Spatial and temporal inhibition of FGFR2b ligands reveals continuous requirements and novel targets in mouse inner ear morphogenesis

Morphogenesis of the inner ear epithelium requires coordinated deployment of several signaling pathways, and disruptions cause abnormalities of hearing and/or balance. The FGFR2b ligands FGF3 and FGF10 are expressed throughout otic development and are required individually for normal morphogenesis, but their prior and redundant roles in otic placode induction complicates investigation of subsequent combinatorial functions in morphogenesis. To interrogate these roles and identify new effectors of FGF3 and FGF10 signaling at the earliest stages of otic morphogenesis, we used conditional gene ablation after otic placode induction, and temporal inhibition of signaling with a secreted, dominant-negative FGFR2b ectodomain. We show that both ligands are required continuously after otocyst formation for maintenance of otic neuroblasts and for patterning and proliferation of the epithelium, leading to normal morphogenesis of both the cochlear and vestibular domains. Furthermore, the first genome-wide identification of proximal targets of FGFR2b signaling in the early otocyst reveals novel candidate genes for inner ear development and function.

Characterization of spatial and temporal development of Type I and Type II hair cells in the mouse utricle using new cell-type-specific markers

The utricle of the inner ear, a vestibular sensory structure that mediates perception of linear acceleration, is comprised of two morphologically and physiologically distinct types of mechanosensory hair cells, referred to as Type Is and Type IIs. While these cell types are easily discriminated in an adult utricle, understanding their development has been hampered by a lack of molecular markers that can be used to identify each cell type prior to maturity. Therefore, we collected single hair cells at three different ages and used single cell RNAseq to characterize the transcriptomes of those cells. Analysis of differential gene expression identified Spp1 as a specific marker for Type I hair cells and Mapt and Anxa4 as specific markers for Type II hair cells. Antibody labeling confirmed the specificity of these markers which were then used to examine the temporal and spatial development of utricular hair cells. While Type I hair cells develop in a gradient that extends across the utricle from posterior-medial to anterior-lateral, Type II hair cells initially develop in the central striolar region and then extend uniformly towards the periphery. Finally, by combining these markers with genetic fate mapping, we demonstrate that over 98% of all Type I hair cells develop prior to birth while over 98% of Type II hair cells develop post-natally. These results are consistent with previous findings suggesting that Type I hair cells develop first and refute the hypothesis that Type II hair cells represent a transitional form between immature and Type I hair cells.

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.

Gfi1Cre mice have early onset progressive hearing loss and induce recombination in numerous inner ear non-hair cells

Studies of developmental and functional biology largely rely on conditional expression of genes in a cell type-specific manner. Therefore, the importance of specificity and lack of inherent phenotypes for Cre-driver animals cannot be overemphasized. The Gfi1^Cre mouse is commonly used for conditional hair cell-specific gene deletion/reporter gene activation in the inner ear. Here, using immunofluorescence and flow cytometry, we show that the Gfi1^Cre mice produce a pattern of recombination that is not strictly limited to hair cells within the inner ear. We observe a broad expression of Cre recombinase in the Gfi1^Cre mouse neonatal inner ear, primarily in inner ear resident macrophages, which outnumber the hair cells. We further show that heterozygous Gfi1^Cre mice exhibit an early onset progressive hearing loss as compared with their wild-type littermates. Importantly, vestibular function remains intact in heterozygotes up to 10 months, the latest time point tested. Finally, we detect minor, but statistically significant, changes in expression of hair cell-enriched transcripts in the Gfi1^Cre heterozygous mice cochleae compared with their wild-type littermate controls. Given the broad use of the Gfi1^Cre mice, both for gene deletion and reporter gene activation, these data are significant and necessary for proper planning and interpretation of experiments.

Notch-Wnt-Bmp crosstalk regulates radial patterning in the mouse cochlea in a spatiotemporal manner

The sensory cells of the mammalian organ of Corti assume a precise mosaic arrangement during embryonic development. Manipulation of Wnt signaling can modulate the proliferation of cochlear progenitors, but whether Wnts are responsible for patterning compartments, or specific hair cells within them, is unclear. To address how the precise timing of Wnt signaling impacts patterning across the radial axis, mouse cochlear cultures were initiated at embryonic day 12.5 and subjected to pharmacological treatments at different stages. Early changes in major patterning genes were assessed to understand the mechanisms underlying alterations of compartments. Results show that Wnt activation can promote medial cell fates by regulating medially expressed Notch genes in a spatiotemporal manner. Wnts can also suppress lateral cell fates by antagonizing Bmp4 expression. Perturbation of the Notch and Bmp pathways revealed which secondary effects were linked to these pathways. Importantly, these effects on cochlear development are dependent on the timing of drug delivery. In conclusion, Wnt signaling in the cochlea influences patterning through complex crosstalk with the Notch and Bmp pathways at several stages of embryonic development.

Gene-expression analysis of hair cell regeneration in the zebrafish lateral line

Deafness caused by the terminal loss of inner ear hair cells is one of the most common sensory diseases. However, nonmammalian animals (e.g., birds, amphibians, and fish) regenerate damaged hair cells. To understand better the reasons underpinning such disparities in regeneration among vertebrates, we set out to define at high resolution the changes in gene expression associated with the regeneration of hair cells in the zebrafish lateral line. We performed RNA-Seq analyses on regenerating support cells purified by FACS. The resulting expression data were subjected to pathway enrichment analyses, and the differentially expressed genes were validated in vivo via whole-mount in situ hybridizations. We discovered that cell cycle regulators are expressed hours before the activation of Wnt/β-catenin signaling following hair cell death. We propose that Wnt/β-catenin signaling is not involved in regulating the onset of proliferation but governs proliferation at later stages of regeneration. In addition, and in marked contrast to mammals, our data clearly indicate that the Notch pathway is significantly down-regulated shortly after injury, thus uncovering a key difference between the zebrafish and mammalian responses to hair cell injury. Taken together, our findings lay the foundation for identifying differences in signaling pathway regulation that could be exploited as potential therapeutic targets to promote either sensory epithelium or hair cell regeneration in mammals.

Secreted factor R-Spondin 2 is involved in refinement of patterning of the mammalian cochlea

BACKGROUND

In the cochlea, patterning of the organ of Corti is tightly regulated to produce a single row of sound-detecting inner hair cells and three rows of outer hair cells, which amplify and refine the signal. The recently identified R-Spondin family of signaling molecules usually act as co-activators of Wnt signaling; it is thought that they regulate turnover of Wnt receptors at the membrane. We sought to test whether R-Spondins function in the developing cochlea.

RESULTS

Expression analysis of all four members of the R-Spondin family showed that only R-Spondin2 (Rspo2) is expressed in the cochlea during development of the sensory epithelium. Examination of an Rspo2(-/-) mouse showed that loss of Rspo2 results in an additional single row of outer hair cells and disruption of peripheral innervation pattern. Addition of Rspo2 recombinant protein to organotypic cochlear cultures resulted in a small but significant decrease in the number of outer hair cells.

CONCLUSIONS

Rspo2 is required to limit the number of outer hair cells to three rows and for optimal arrangement of peripheral nerve fibers. The Rspo2 gain- and loss-of-function studies show that in the ear, Rspo2 function is not consistent with its assigned role as a Wnt potentiator.

The genetics of hair cell development and regeneration

Sensory hair cells are exquisitely sensitive vertebrate mechanoreceptors that mediate the senses of hearing and balance. Understanding the factors that regulate the development of these cells is important, not only to increase our understanding of ear development and its functional physiology but also to shed light on how these cells may be replaced therapeutically. In this review, we describe the signals and molecular mechanisms that initiate hair cell development in vertebrates, with particular emphasis on the transcription factor Atoh1, which is both necessary and sufficient for hair cell development. We then discuss recent findings on how microRNAs may modulate the formation and maturation of hair cells. Last, we review recent work on how hair cells are regenerated in many vertebrate groups and the factors that conspire to prevent this regeneration in mammals.

A systematic survey of carbonic anhydrase mRNA expression during mammalian inner ear development

BACKGROUND

Carbonic anhydrases (CAs), which catalyze CO(2) hydration to bicarbonate and protons, have been suggested to regulate potassium homeostasis and endocochlear potential in the mammalian cochlea. Sixteen mammalian CA isozymes are currently known. To understand the specific roles of CA isozymes in the inner ear, a systematic survey was conducted to reveal temporal and spatial expression patterns of all 16 CA isozymes during inner ear development.

RESULTS

Our quantitative reverse transcriptase-polymerase chain reaction results showed that different tissues express unique combinations of CA isozymes. During inner ear development, transcripts of four cytosolic isozymes (Car1, Car2, Car3, and Car13), two membrane-bound isozymes (Car12 and Car14), and two CA-related proteins (Car8 and Car11) were expressed at higher levels than other isozymes. Spatial expression patterns of these isozymes within developing inner ears were determined by in situ hybridization. Each isozyme showed a unique expression pattern during development. For example, Car12 and Car13 expression closely overlapped with Pendrin, an anion exchanger, while Car2 overlapped with Na-K-ATPase in type II and IV otic fibrocytes, suggesting functional relationships in the inner ear.

CONCLUSIONS

The temporal and spatial expression patterns of each CA isozyme suggest unique and differential roles in inner ear development and function.

Conditional gene expression in the mouse inner ear using Cre-loxP

In recent years, there has been significant progress in the use of Cre-loxP technology for conditional gene expression in the inner ear. Here, we introduce the basic concepts of this powerful technology, emphasizing the differences between Cre and CreER. We describe the creation and Cre expression pattern of each Cre and CreER mouse line that has been reported to have expression in auditory and vestibular organs. We compare the Cre expression patterns between Atoh1-CreER(TM) and Atoh1-CreER(T2) and report a new line, Fgfr3-iCreER(T2), which displays inducible Cre activity in cochlear supporting cells. We also explain how results can vary when transgenic vs. knock-in Cre/CreER alleles are used to alter gene expression. We discuss practical issues that arise when using the Cre-loxP system, such as the use of proper controls, Cre efficiency, reporter expression efficiency, and Cre leakiness. Finally, we introduce other methods for conditional gene expression, including Flp recombinase and the tetracycline-inducible system, which can be combined with Cre-loxP mouse models to investigate conditional expression of more than one gene.

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.

Intrinsic regenerative potential of murine cochlear supporting cells

The lack of cochlear regenerative potential is the main cause for the permanence of hearing loss. Albeit quiescent in vivo, dissociated non-sensory cells from the neonatal cochlea proliferate and show ability to generate hair cell-like cells in vitro. Only a few non-sensory cell-derived colonies, however, give rise to hair cell-like cells, suggesting that sensory progenitor cells are a subpopulation of proliferating non-sensory cells. Here we purify from the neonatal mouse cochlea four different non-sensory cell populations by fluorescence-activated cell sorting (FACS). All four populations displayed proliferative potential, but only lesser epithelial ridge and supporting cells robustly gave rise to hair cell marker-positive cells. These results suggest that cochlear supporting cells and cells of the lesser epithelial ridge show robust potential to de-differentiate into prosensory cells that proliferate and undergo differentiation in similar fashion to native prosensory cells of the developing inner ear.

Dissecting the molecular basis of organ of Corti development: Where are we now?

This review summarizes recent progress in our understanding of the molecular basis of cochlear duct growth, specification of the organ of Corti, and differentiation of the different types of hair cells. Studies of multiple mutations suggest that developing hair cells are involved in stretching the organ of Corti through convergent extension movements. However, Atoh1 null mutants have only undifferentiated and dying organ of Corti precursors but show a near normal extension of the cochlear duct, implying that organ of Corti precursor cells can equally drive this process. Some factors influence cochlear duct growth by regulating the cell cycle and proliferation. Shortened cell cycle and premature cell cycle exit can lead to a shorter organ of Corti with multiple rows of hair cells (e.g., Foxg1 null mice). Other genes affect the initial formation of a cochlear duct with or without affecting the organ of Corti. Such observations are consistent with evolutionary data that suggest some developmental uncoupling of cochlear duct from organ of Corti formation. Positioning the organ of Corti requires multiple genes expressed in the organ of Corti and the flanking region. Several candidate factors have emerged but how they cooperate to specify the organ of Corti and the topology of hair cells remains unclear. Atoh1 is required for differentiation of all hair cells, but regulation of inner versus outer hair cell differentiation is still unidentified. In summary, the emerging molecular complexity of organ of Corti development demands further study before a rational approach towards regeneration of unique types of hair cells in specific positions is possible.

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.

Conditional deletion of Atoh1 using Pax2-Cre results in viable mice without differentiated cochlear hair cells that have lost most of the organ of Corti

Atonal homolog1 (Atoh1, formerly Math1) is a crucial bHLH transcription factor for inner ear hair cell differentiation. Its absence in embryos results in complete absence of mature hair cells at birth and its misexpression can generate extra hair cells. Thus Atoh1 may be both necessary and sufficient for hair cell differentiation in the ear. Atoh1 null mice die at birth and have some undifferentiated cells in sensory epithelia carrying Atoh1 markers. The fate of these undifferentiated cells in neonates is unknown due to lethality. We use Tg(Pax2-Cre) to delete floxed Atoh1 in the inner ear. This generates viable conditional knockout (CKO) mice for studying the postnatal development of the inner ear without differentiated hair cells. Using in situ hybridization we find that Tg(Pax2-Cre) recombines the floxed Atoh1 prior to detectable Atoh1 expression. Only the posterior canal crista has Atoh1 expressing hair cells due to incomplete recombination. Most of the organ of Corti cells are lost in CKO mice via late embryonic cell death. Marker genes indicate that the organ of Corti is reduced to two rows of cells wedged between flanking markers of the organ of Corti (Fgf10 and Bmp4). These two rows of cells (instead of five rows of supporting cells) are positive for Prox1 in neonates. By postnatal day 14 (P14), the remaining cells of the organ of Corti are transformed into a flat epithelium with no distinction of any specific cell type. However, some of the remaining organ of Corti cells express Myo7a at late postnatal stages and are innervated by remaining afferent fibers. Initial growth of afferents and efferents in embryos shows no difference between control mice and Tg(Pax2-Cre)::Atoh1 CKO mice. Most afferents and efferents are lost in the CKO mutant before birth, except for the apex and few fibers in the base. Afferents focus their projections on patches that express the prosensory specifying gene, Sox2. This pattern of innervation by sensory neurons is maintained at least until P14, but fibers target the few Myo7a positive cells found in later stages.

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.