Rbpj regulates development of prosensory cells in the mammalian inner ear

EBF1 Limits the Numbers of Cochlear Hair and Supporting Cells and Forms the Scala Tympani and Spiral Limbus during Inner Ear Development

Early B-cell factor 1 (EBF1) is a basic helix-loop-helix transcription factor essential for the differentiation of various tissues. Our single-cell RNA sequencing data suggest that Ebf1 is expressed in the sensory epithelium of the mouse inner ear. Here, we found that the murine Ebf1 gene and its protein are expressed in the prosensory domain of the inner ear, medial region of the cochlear duct floor, otic mesenchyme, and cochleovestibular ganglion. Ebf1 deletion in mice results in incomplete formation of the spiral limbus and scala tympani, increased number of cells in the organ of Corti and Kölliker's organ, and aberrant course of the spiral ganglion axons. Ebf1 deletion in the mouse cochlear epithelia caused the proliferation of SOX2-positive cochlear cells at E13.5, indicating that EBF1 suppresses the proliferation of the prosensory domain and cells of Kölliker's organ to facilitate the development of appropriate numbers of hair and supporting cells. Furthermore, mice with deletion of cochlear epithelium-specific Ebf1 showed poor postnatal hearing function. Our results suggest that Ebf1 is essential for normal auditory function in mammals.

Mapping of Notch signaling in the developing organ of Corti in common marmosets

Introduction

The well-regulated development of the sensory epithelium is essential for hearing. This process involves the specification of a pro-sensory epithelium containing common progenitors that differentiate into hair and supporting cells. Notch signaling is one of the most critical pathways during these processes, and its modification is thought to be a feasible approach for treating hearing loss. Despite interspecies differences between rodents and primates or humans, most of our current knowledge regarding cochlear development has been obtained from rodent models.

Methods

We therefore examined and mapped the expression patterns of Notch signal components in the developing cochlea of the common marmoset (Callithrix jacchus), a small monkey species native to the New World, a primate model animal.

Results

In contrast to the preserved expression patterns of the Notch signaling components in the hair cell differentiation between primates and rodents, we unveiled relatively large interspecies differences during the maturation of supporting cells.

Discussion

This improved knowledge of Notch signaling during primate cochlear development will facilitate the development of future regenerative therapies.

Comparative assessment of Fgf's diverse roles in inner ear development: A zebrafish perspective

Progress in understanding mechanisms of inner ear development has been remarkably rapid in recent years. The research community has benefited from the availability of several diverse model organisms, including zebrafish, chick, and mouse. The complexity of the inner ear has proven to be a challenge, and the complexity of the mammalian cochlea in particular has been the subject of intense scrutiny. Zebrafish lack a cochlea and exhibit a number of other differences from amniote species, hence they are sometimes seen as less relevant for inner ear studies. However, accumulating evidence shows that underlying cellular and molecular mechanisms are often highly conserved. As a case in point, consideration of the diverse functions of Fgf and its downstream effectors reveals many similarities between vertebrate species, allowing meaningful comparisons the can benefit the entire research community. In this review, I will discuss mechanisms by which Fgf controls key events in early otic development in zebrafish and provide direct comparisons with chick and mouse.

Embryonic Origins of Virus-Induced Hearing Loss: Overview of Molecular Etiology

Hearing loss, one of the most prevalent chronic health conditions, affects around half a billion people worldwide, including 34 million children. The World Health Organization estimates that the prevalence of disabling hearing loss will increase to over 900 million people by 2050. Many cases of congenital hearing loss are triggered by viral infections during different stages of pregnancy. However, the molecular mechanisms by which viruses induce hearing loss are not sufficiently explored, especially cases that are of embryonic origins. The present review first describes the cellular and molecular characteristics of the auditory system development at early stages of embryogenesis. These developmental hallmarks, which initiate upon axial specification of the otic placode as the primary root of the inner ear morphogenesis, involve the stage-specific regulation of several molecules and pathways, such as retinoic acid signaling, Sonic hedgehog, and Wnt. Different RNA and DNA viruses contributing to congenital and acquired hearing loss are then discussed in terms of their potential effects on the expression of molecules that control the formation of the auditory and vestibular compartments following otic vesicle differentiation. Among these viruses, cytomegalovirus and herpes simplex virus appear to have the most effect upon initial molecular determinants of inner ear development. Moreover, of the molecules governing the inner ear development at initial stages, SOX2, FGFR3, and CDKN1B are more affected by viruses causing either congenital or acquired hearing loss. Abnormalities in the function or expression of these molecules influence processes like cochlear development and production of inner ear hair and supporting cells. Nevertheless, because most of such virus-host interactions were studied in unrelated tissues, further validations are needed to confirm whether these viruses can mediate the same effects in physiologically relevant models simulating otic vesicle specification and growth.

The Notch Ligand Jagged1 Is Required for the Formation, Maintenance, and Survival of Hensen's Cells in the Mouse Cochlea

During cochlear development, the Notch ligand JAGGED 1 (JAG1) plays an important role in the specification of the prosensory region, which gives rise to sound-sensing hair cells and neighboring supporting cells (SCs). While JAG1's expression is maintained in SCs through adulthood, the function of JAG1 in SC development is unknown. Here, we demonstrate that JAG1 is essential for the formation and maintenance of Hensen's cells, a highly specialized SC subtype located at the edge of the auditory epithelium. Using Sox2 ^CreERT2/+::Jag1^loxP/loxP mice of both genders, we show that Jag1 deletion at the onset of differentiation, at embryonic day 14.5, disrupted Hensen's cell formation. Similar loss of Hensen's cells was observed when Jag1 was deleted after Hensen's cell formation at postnatal day (P) 0/P1 and fate-mapping analysis revealed that in the absence of Jag1, some Hensen's cells die, but others convert into neighboring Claudius cells. In support of a role for JAG1 in cell survival, genes involved in mitochondrial function and protein synthesis were downregulated in the sensory epithelium of P0 cochlea lacking Jag1 Finally, using Fgfr3-iCreER^T2 ::Jag1^loxP/loxP mice to delete Jag1 at P0, we observed a similar loss of Hensen's cells and found that adult Jag1 mutant mice have hearing deficits at the low-frequency range.SIGNIFICANCE STATEMENT Hensen's cells play an essential role in the development and homeostasis of the cochlea. Defects in the biophysical or functional properties of Hensen's cells have been linked to auditory dysfunction and hearing loss. Despite their importance, surprisingly little is known about the molecular mechanisms that guide their development. Morphologic and fate-mapping analyses in our study revealed that, in the absence of the Notch ligand JAGGED1, Hensen's cells died or converted into Claudius cells, which are specialized epithelium-like cells outside the sensory epithelium. Confirming a link between JAGGED1 and cell survival, transcriptional profiling showed that JAGGED1 maintains genes critical for mitochondrial function and tissue homeostasis. Finally, auditory phenotyping revealed that JAGGED1's function in supporting cells is necessary for low-frequency hearing.

Hear, Hear for Notch: Control of Cell Fates in the Inner Ear by Notch Signaling

The vertebrate inner ear is responsible for detecting sound, gravity, and head motion. These mechanical forces are detected by mechanosensitive hair cells, arranged in a series of sensory patches in the vestibular and cochlear regions of the ear. Hair cells form synapses with neurons of the VIIIth cranial ganglion, which convey sound and balance information to the brain. They are surrounded by supporting cells, which nourish and protect the hair cells, and which can serve as a source of stem cells to regenerate hair cells after damage in non-mammalian vertebrates. The Notch signaling pathway plays many roles in the development of the inner ear, from the earliest formation of future inner ear ectoderm on the side of the embryonic head, to regulating the production of supporting cells, hair cells, and the neurons that innervate them. Notch signaling is re-deployed in non-mammalian vertebrates during hair cell regeneration, and attempts have been made to manipulate the Notch pathway to promote hair cell regeneration in mammals. In this review, we summarize the different modes of Notch signaling in inner ear development and regeneration, and describe how they interact with other signaling pathways to orchestrate the fine-grained cellular patterns of the ear.

Sox2 and FGF20 interact to regulate organ of Corti hair cell and supporting cell development in a spatially-graded manner

The mouse organ of Corti, housed inside the cochlea, contains hair cells and supporting cells that transduce sound into electrical signals. These cells develop in two main steps: progenitor specification followed by differentiation. Fibroblast Growth Factor (FGF) signaling is important in this developmental pathway, as deletion of FGF receptor 1 (Fgfr1) or its ligand, Fgf20, leads to the loss of hair cells and supporting cells from the organ of Corti. However, whether FGF20-FGFR1 signaling is required during specification or differentiation, and how it interacts with the transcription factor Sox2, also important for hair cell and supporting cell development, has been a topic of debate. Here, we show that while FGF20-FGFR1 signaling functions during progenitor differentiation, FGFR1 has an FGF20-independent, Sox2-dependent role in specification. We also show that a combination of reduction in Sox2 expression and Fgf20 deletion recapitulates the Fgfr1-deletion phenotype. Furthermore, we uncovered a strong genetic interaction between Sox2 and Fgf20, especially in regulating the development of hair cells and supporting cells towards the basal end and the outer compartment of the cochlea. To explain this genetic interaction and its effects on the basal end of the cochlea, we provide evidence that decreased Sox2 expression delays specification, which begins at the apex of the cochlea and progresses towards the base, while Fgf20-deletion results in premature onset of differentiation, which begins near the base of the cochlea and progresses towards the apex. Thereby, Sox2 and Fgf20 interact to ensure that specification occurs before differentiation towards the cochlear base. These findings reveal an intricate developmental program regulating organ of Corti development along the basal-apical axis of the cochlea.

Open chromatin dynamics in prosensory cells of the embryonic mouse cochlea

Hearing loss is often due to the absence or the degeneration of hair cells in the cochlea. Understanding the mechanisms regulating the generation of hair cells may therefore lead to better treatments for hearing disorders. To elucidate the transcriptional control mechanisms specifying the progenitor cells (i.e. prosensory cells) that generate the hair cells and support cells critical for hearing function, we compared chromatin accessibility using ATAC-seq in sorted prosensory cells (Sox2-EGFP⁺) and surrounding cells (Sox2-EGFP⁻) from E12, E14.5 and E16 cochlear ducts. In Sox2-EGFP⁺, we find greater accessibility in and near genes restricted in expression to the prosensory region of the cochlear duct including Sox2, Isl1, Eya1 and Pou4f3. Furthermore, we find significant enrichment for the consensus binding sites of Sox2, Six1 and Gata3—transcription factors required for prosensory development—in the open chromatin regions. Over 2,200 regions displayed differential accessibility with developmental time in Sox2-EGFP⁺ cells, with most changes in the E12-14.5 window. Open chromatin regions detected in Sox2-EGFP⁺ cells map to over 48,000 orthologous regions in the human genome that include regions in genes linked to deafness. Our results reveal a dynamic landscape of open chromatin in prosensory cells with potential implications for cochlear development and disease.

Clonal Expansion of Lgr5-Positive Cells from Mammalian Cochlea and High-Purity Generation of Sensory Hair Cells

Death of cochlear hair cells, which do not regenerate, is a cause of hearing loss in a high percentage of the population. Currently, no approach exists to obtain large numbers of cochlear hair cells. Here, using a small-molecule approach, we show significant expansion (>2,000-fold) of cochlear supporting cells expressing and maintaining Lgr5, an epithelial stem cell marker, in response to stimulation of Wnt signaling by a GSK3β inhibitor and transcriptional activation by a histone deacetylase inhibitor. The Lgr5-expressing cells differentiate into hair cells in high yield. From a single mouse cochlea, we obtained over 11,500 hair cells, compared to less than 200 in the absence of induction. The newly generated hair cells have bundles and molecular machinery for transduction, synapse formation, and specialized hair cell activity. Targeting supporting cells capable of proliferation and cochlear hair cell replacement could lead to the discovery of hearing loss treatments.

Notch Signaling in Development, Tissue Homeostasis, and Disease

Notch signaling is an evolutionarily highly conserved signaling mechanism, but in contrast to signaling pathways such as Wnt, Sonic Hedgehog, and BMP/TGF-β, Notch signaling occurs via cell-cell communication, where transmembrane ligands on one cell activate transmembrane receptors on a juxtaposed cell. Originally discovered through mutations in Drosophila more than 100 yr ago, and with the first Notch gene cloned more than 30 yr ago, we are still gaining new insights into the broad effects of Notch signaling in organisms across the metazoan spectrum and its requirement for normal development of most organs in the body. In this review, we provide an overview of the Notch signaling mechanism at the molecular level and discuss how the pathway, which is architecturally quite simple, is able to engage in the control of cell fates in a broad variety of cell types. We discuss the current understanding of how Notch signaling can become derailed, either by direct mutations or by aberrant regulation, and the expanding spectrum of diseases and cancers that is a consequence of Notch dysregulation. Finally, we explore the emerging field of Notch in the control of tissue homeostasis, with examples from skin, liver, lung, intestine, and the vasculature.

Septin7 regulates inner ear formation at an early developmental stage

Septins are guanosine triphosphate-binding proteins that are evolutionally conserved in all eukaryotes other than plants. They function as multimeric complexes that interact with membrane lipids, actomyosin, and microtubules. Based on these interactions, septins play essential roles in the morphogenesis and physiological functions of many mammalian cell types including the regulation of microtubule stability, vesicle trafficking, cortical rigidity, planar cell polarity, and apoptosis. The inner ear, which perceives auditory and equilibrium sensation with highly differentiated hair cells, has a complicated gross morphology. Furthermore, its development including morphogenesis is dependent on various molecular mechanisms, such as apoptosis, convergent extension, and cell fate determination. To determine the roles of septins in the development of the inner ear, we specifically deleted Septin7 (Sept7), the non-redundant subunit in the canonical septin complex, in the inner ear at different times during development. Foxg1Cre-mediated deletion of Sept7, which achieved the complete knockout of Sept7 within the inner ear at E9.5, caused cystic malformation of inner ears and a reduced numbers of sensory epithelial cells despite the existence of mature hair cells. Excessive apoptosis was observed at E10.5,E11.5 and E12.5 in all inner ear epithelial cells and at E10.5 and E11.5 in prosensory epithelial cells of the inner ears of Foxg1Cre;Septin7^{floxed/floxed} mice. In contrast with apoptosis, cell proliferation in the inner ear did not significantly change between control and mutant mice. Deletion of Sept7 within the cochlea at a later stage (around E15.5) with Emx2Cre did not result in any apparent morphological anomalies observed in Foxg1Cre;Septin7^{floxed/floxed} mice. These results suggest that SEPT7 regulates gross morphogenesis of the inner ear and maintains the size of the inner ear sensory epithelial area and exerts its effects at an early developmental stage of the inner ear.

Destabilization of Atoh1 by E3 Ubiquitin Ligase Huwe1 and Casein Kinase 1 Is Essential for Normal Sensory Hair Cell Development

Proneural basic helix-loop-helix transcription factor, Atoh1, plays a key role in the development of sensory hair cells. We show here that the level of Atoh1 must be accurately controlled by degradation of the protein in addition to the regulation of Atoh1 gene expression to achieve normal cellular patterning during development of the cochlear sensory epithelium. The stability of Atoh1 was regulated by the ubiquitin proteasome system through the action of Huwe1, a HECT-domain, E3 ubiquitin ligase. An interaction between Huwe1 and Atoh1 could be visualized by a proximity ligation assay and was confirmed by co-immunoprecipitation and mass spectrometry. Transfer of a lysine 48-linked polyubiquitin chain to Atoh1 by Huwe1 could be demonstrated both in intact cells and in a cell-free system, and proteasome inhibition or Huwe1 silencing increased Atoh1 levels. The interaction with Huwe1 and polyubiquitylation were blocked by disruption of casein kinase 1 (CK1) activity, and mass spectrometry and mutational analysis identified serine 334 as an important phosphorylation site for Atoh1 ubiquitylation and subsequent degradation. Phosphorylation by CK1 thus targeted the protein for degradation. Development of an extra row of inner hair cells in the cochlea and an approximate doubling in the number of afferent synapses was observed after embryonic or early postnatal deletion of Huwe1 in cochlear-supporting cells, and hair cells died in the early postnatal period when Huwe1 was knocked out in the developing cochlea. These data indicate that the regulation of Atoh1 by the ubiquitin proteasome pathway is necessary for hair cell fate determination and survival.

Selection of cell fate in the organ of Corti involves the integration of Hes/Hey signaling at the Atoh1 promoter

Determination of cell fate within the prosensory domain of the developing cochlear duct relies on the temporal and spatial regulation of the bHLH transcription factor Atoh1. Auditory hair cells and supporting cells arise in a wave of differentiation that patterns them into discrete rows mediated by Notch-dependent lateral inhibition. However, the mechanism responsible for selecting sensory cells from within the prosensory competence domain remains poorly understood. We show in mice that rather than being upregulated in rows of cells, Atoh1 is subject to transcriptional activation in groups of prosensory cells, and that highly conserved sites for Hes/Hey repressor binding in the Atoh1 promoter are needed to select the hair cell and supporting cell fate. During perinatal supporting cell transdifferentiation, which is a model of hair cell regeneration, we show that derepression is sufficient to induce Atoh1 expression, suggesting a mechanism for priming the 3' Atoh1 autoregulatory enhancer needed for hair cell expression.

Nonlinear Network Reconstruction from Gene Expression Data Using Marginal Dependencies Measured by DCOL

Reconstruction of networks from high-throughput expression data is an important tool to identify new regulatory relations. Given that nonlinear and complex relations exist between biological units, methods that can utilize nonlinear dependencies may yield insights that are not provided by methods using linear associations alone. We have previously developed a distance to measure predictive nonlinear relations, the Distance based on Conditional Ordered List (DCOL), which is sensitive and computationally efficient on large matrices. In this study, we explore the utility of DCOL in the reconstruction of networks, by combining it with local false discovery rate (lfdr)–based inference. We demonstrate in simulations that the new method named nlnet is effective in recovering hidden nonlinear modules. We also demonstrate its utility using a single cell RNA seq dataset. The method is available as an R package at https://cran.r-project.org/web/packages/nlnet.

Dual role for Sox2 in specification of sensory competence and regulation of Atoh1 function

The formation of inner ear sensory epithelia is believed to occur in two steps, initial specification of sensory competent (prosensory) regions followed by determination of specific cell-types, such as hair cells (HCs) and supporting cells. However, studies in which the HC determination factor Atoh1 was ectopically expressed in nonprosensory regions indicated that expression of Atoh1 alone is sufficient to induce HC formation suggesting that prosensory formation may not be a prerequisite for HC development. To test this hypothesis, interactions between Sox2 and Atoh1, which are required for prosensory and HC formation respectively, were examined. Forced expression of Atoh1 in nonprosensory cells resulted in transient expression of Sox2 prior to HC formation, suggesting that expression of Sox2 is required for formation of ectopic HCs. Moreover, Atoh1 overexpression failed to induce HC formation in Sox2 mutants, confirming that Sox2 is required for prosensory competence. To determine whether expression of Sox2 alone is sufficient to induce prosensory identity, Sox2 was transiently activated in a manner that mimicked endogenous expression. Following transient Sox2 activation, nonprosensory cells developed as HCs, a result that was never observed in response to persistent expression of Sox2. These results, suggest a dual role for Sox2 in inner ear formation. Initially, Sox2 is required to specify prosensory competence, but subsequent down-regulation of Sox2 must occur to allow Atoh1 expression, most likely through a direct interaction with the Atoh1 promoter. These results implicate Sox2-mediated changes in prosensory cells as an essential step in their ability to develop as HCs. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 3-13, 2017.

Where hearing starts: the development of the mammalian cochlea

The mammalian cochlea is a remarkable sensory organ, capable of perceiving sound over a range of 10(12) in pressure, and discriminating both infrasonic and ultrasonic frequencies in different species. The sensory hair cells of the mammalian cochlea are exquisitely sensitive, responding to atomic-level deflections at speeds on the order of tens of microseconds. The number and placement of hair cells are precisely determined during inner ear development, and a large number of developmental processes sculpt the shape, size and morphology of these cells along the length of the cochlear duct to make them optimally responsive to different sound frequencies. In this review, we briefly discuss the evolutionary origins of the mammalian cochlea, and then describe the successive developmental processes that lead to its induction, cell cycle exit, cellular patterning and the establishment of topologically distinct frequency responses along its length.

Canonical Notch signaling plays an instructive role in auditory supporting cell development

The auditory sensory epithelium, composed of mechano-sensory hair cells (HCs) and highly specialized glial-like supporting cells (SCs), is critical for our ability to detect sound. SCs provide structural and functional support to HCs and play an essential role in cochlear development, homeostasis and repair. Despite their importance, however, surprisingly little is known about the molecular mechanisms guiding SC differentiation. Here, we provide evidence that in addition to its well-characterized inhibitory function, canonical Notch signaling plays a positive, instructive role in the differentiation of SCs. Using γ-secretase inhibitor DAPT to acutely block canonical Notch signaling, we identified a cohort of Notch-regulated SC-specific genes, with diverse functions in cell signaling, cell differentiation, neuronal innervation and synaptogenesis. We validated the newly identified Notch-regulated genes in vivo using genetic gain (Emx2^Cre/+; Rosa26^N1ICD/+) and loss-of-function approaches (Emx2^Cre/+; Rosa26^DnMAML1/+). Furthermore, we demonstrate that Notch over-activation in the differentiating murine cochlea (Emx2^Cre/+; Rosa26^N1ICD/+) actively promotes a SC-specific gene expression program. Finally, we show that outer SCs –so called Deiters’ cells are selectively lost by prolonged reduction (Emx2^Cre/+; Rosa26^DnMAML1/+/+) or abolishment of canonical Notch signaling (Fgfr3-iCreER; Rbpj^−/Δ), indicating a critical role for Notch signaling in Deiters’ cell development.

The Wnt and Notch signalling pathways in the developing cochlea: Formation of hair cells and induction of regenerative potential

The Wnt and Notch signalling pathways control proliferation, specification, and cell fate choices during embryonic development and in adult life. Hence, there is much interest in both signalling pathways in the context of stem cell biology and tissue regeneration. In the developing ear, the Wnt and Notch signalling pathways specify otic cells and refine the ventral boundary of the otic placode. Since both signalling pathways control events essential for the formation of sensory cells, such as proliferation and hair cell differentiation, these pathways could hold promise for the regeneration of hair cells in adult mammalian cochlea. Indeed, modulating either the Wnt or Notch pathways can trigger the regenerative potential of supporting cells. In the neonatal mouse cochlea, Notch-mediated regeneration of hair cells partially depends on Wnt signalling, which implies an interaction between the pathways. This review presents how the Wnt and Notch signalling pathways regulate the formation of sensory hair cells and how modulating their activity induces regenerative potential in the mammalian cochlea.

Single-cell RNA-Seq resolves cellular complexity in sensory organs from the neonatal inner ear

In the inner ear, cochlear and vestibular sensory epithelia utilize grossly similar cell types to transduce different stimuli: sound and acceleration. Each individual sensory epithelium is composed of highly heterogeneous populations of cells based on physiological and anatomical criteria. However, limited numbers of each cell type have impeded transcriptional characterization. Here we generated transcriptomes for 301 single cells from the utricular and cochlear sensory epithelia of newborn mice to circumvent this challenge. Cluster analysis indicates distinct profiles for each of the major sensory epithelial cell types, as well as less-distinct sub-populations. Asynchrony within utricles allows reconstruction of the temporal progression of cell-type-specific differentiation and suggests possible plasticity among cells at the sensory–nonsensory boundary. Comparisons of cell types from utricles and cochleae demonstrate divergence between auditory and vestibular cells, despite a common origin. These results provide significant insights into the developmental processes that form unique inner ear cell types.

Heterogeneous sensory epithelia of the inner ear are difficult to study owing to the few cells that can be isolated. Here the authors provide insight into the developmental processes underlying the formation of these cells by single-cell RNA-Seq.

Sensory hair cell development and regeneration: similarities and differences

Sensory hair cells are mechanoreceptors of the auditory and vestibular systems and are crucial for hearing and balance. In adult mammals, auditory hair cells are unable to regenerate, and damage to these cells results in permanent hearing loss. By contrast, hair cells in the chick cochlea and the zebrafish lateral line are able to regenerate, prompting studies into the signaling pathways, morphogen gradients and transcription factors that regulate hair cell development and regeneration in various species. Here, we review these findings and discuss how various signaling pathways and factors function to modulate sensory hair cell development and regeneration. By comparing and contrasting development and regeneration, we also highlight the utility and limitations of using defined developmental cues to drive mammalian hair cell regeneration.

Numb is not a critical regulator of Notch-mediated cell fate decisions in the developing chick inner ear

The Notch signaling pathway controls differentiation of hair cells and supporting cells in the vertebrate inner ear. Here, we have investigated whether Numb, a known regulator of Notch activity in Drosophila, is involved in this process in the embryonic chick. The chicken homolog of Numb is expressed throughout the otocyst at early stages of development and is concentrated at the basal pole of the cells. It is asymmetrically allocated at some cell divisions, as in Drosophila, suggesting that it could act as a determinant inherited by one of the two daughter cells and favoring adoption of a hair-cell fate. To test the implication of Numb in hair cell fate decisions and the regulation of Notch signaling, we used different methods to overexpress Numb at different stages of inner ear development. We found that sustained or late Numb overexpression does not promote hair cell differentiation, and Numb does not prevent the reception of Notch signaling. Surprisingly, none of the Numb-overexpressing cells differentiated into hair cells, suggesting that high levels of Numb protein could interfere with intracellular processes essential for hair cell survival. However, when Numb was overexpressed early and more transiently during ear development, no effect on hair cell formation was seen. These results suggest that in the inner ear at least, Numb does not significantly repress Notch activity and that its asymmetric distribution in dividing precursor cells does not govern the choice between hair cell and supporting cell fates.

Changes in the regulation of the Notch signaling pathway are temporally correlated with regenerative failure in the mouse cochlea

Sensorineural hearing loss is most commonly caused by the death of hair cells in the organ of Corti, and once lost, mammalian hair cells do not regenerate. In contrast, other vertebrates such as birds can regenerate hair cells by stimulating division and differentiation of neighboring supporting cells. We currently know little of the genetic networks which become active in supporting cells when hair cells die and that are activated in experimental models of hair cell regeneration. Several studies have shown that neonatal mammalian cochlear supporting cells are able to trans-differentiate into hair cells when cultured in conditions in which the Notch signaling pathway is blocked. We now show that the ability of cochlear supporting cells to trans-differentiate declines precipitously after birth, such that supporting cells from six-day-old mouse cochlea are entirely unresponsive to a blockade of the Notch pathway. We show that this trend is seen regardless of whether the Notch pathway is blocked with gamma secretase inhibitors, or by antibodies against the Notch1 receptor, suggesting that the action of gamma secretase inhibitors on neonatal supporting cells is likely to be by inhibiting Notch receptor cleavage. The loss of responsiveness to inhibition of the Notch pathway in the first postnatal week is due in part to a down-regulation of Notch receptors and ligands, and we show that this down-regulation persists in the adult animal, even under conditions of noise damage. Our data suggest that the Notch pathway is used to establish the repeating pattern of hair cells and supporting cells in the organ of Corti, but is not required to maintain this cellular mosaic once the production of hair cells and supporting cells is completed. Our results have implications for the proposed used of Notch pathway inhibitors in hearing restoration therapies.

The cochlear sensory epithelium derives from Wnt responsive cells in the dorsomedial otic cup

Wnt1 and Wnt3a secreted from the dorsal neural tube were previously shown to regulate a gene expression program in the dorsal otic vesicle that is necessary for vestibular morphogenesis (Riccomagno et al., 2005. Genes Dev. 19, 1612-1623). Unexpectedly, Wnt1(-/-); Wnt3a(-/-) embryos also displayed a pronounced defect in the outgrowth of the ventrally derived cochlear duct. To determine how Wnt signaling in the dorsal otocyst contributes to cochlear development we performed a series of genetic fate mapping experiments using two independent Wnt responsive driver strains (TopCreER and Gbx2(CreER)) that when crossed to inducible responder lines (Rosa(lacZ) or Rosa(zsGreen)) permanently labeled dorsomedial otic progenitors and their derivatives. Tamoxifen time course experiments revealed that most vestibular structures showed some degree of labeling when recombination was induced between E7.75 and E12.5, consistent with continuous Wnt signaling activity in this tissue. Remarkably, a population of Wnt responsive cells in the dorsal otocyst was also found to contribute to the sensory epithelium of the cochlear duct, including auditory hair and support cells. Similar results were observed with both TopCreER and Gbx2(CreER) strains. The ventral displacement of Wnt responsive cells followed a spatiotemporal sequence that initiated in the anterior otic cup at, or immediately prior to, the 17-somite stage (E9) and then spread progressively to the posterior pole of the otic vesicle by the 25-somite stage (E9.5). These lineage-tracing experiments identify the earliest known origin of auditory sensory progenitors within a population of Wnt responsive cells in the dorsomedial otic cup.

Manipulating cell fate in the cochlea: a feasible therapy for hearing loss

Mammalian auditory hair cells do not spontaneously regenerate, unlike hair cells in lower vertebrates, including fish and birds. In mammals, hearing loss due to the loss of hair cells is permanent and intractable. Recent studies in the mouse have demonstrated spontaneous hair cell regeneration during a short postnatal period, but this regenerative capacity is lost in the adult cochlea. Reduced regeneration coincides with a transition that results in a decreased pool of progenitor cells in the cochlear sensory epithelium. Here, we review the signaling cascades involved in hair cell formation and morphogenesis of the organ of Corti in developing mammals, the changing status of progenitor cells in the cochlea, and the regeneration of auditory hair cells in adult mammals.

Jag1b is essential for patterning inner ear sensory cristae by regulating anterior morphogenetic tissue separation and preventing posterior cell death

The sensory patches of the vertebrate inner ear, which contain hair cells and supporting cells, are essential for hearing and balance functions. How the stereotypically organized sensory patches are formed remains to be determined. In this study, we isolated a zebrafish mutant in which the jag1b gene is disrupted by an EGFP insertion. Loss of Jag1b causes cell death in the developing posterior crista and results in downregulation of fgf10a in the posterior prosensory cells. Inhibition of FGFR activity in wild-type embryos also causes loss of the posterior crista, suggesting that Fgf10a mediates Jag1b activity. By contrast, in the anterior prosensory domain, Jag1b regulates separation of a single morphogenetic field into anterior and lateral cristae by flattening cells destined to form a nonsensory epithelium between the two cristae. MAPK activation in the nonsensory epithelium precursors is required for the separation. In the jag1b mutant, MAPK activation and cell flattening are extended to anterior crista primordia, causing loss of anterior crista. More importantly, inhibition of MAPK activity, which blocks the differentiation of nonsensory epithelial cells, generated a fused large crista and extra hair cells. Thus, Jag1b uses two distinct mechanisms to form three sensory cristae in zebrafish.

Hey1 and Hey2 control the spatial and temporal pattern of mammalian auditory hair cell differentiation downstream of Hedgehog signaling

Mechano-sensory hair cells (HCs), housed in the inner ear cochlea, are critical for the perception of sound. In the mammalian cochlea, differentiation of HCs occurs in a striking basal-to-apical and medial-to-lateral gradient, which is thought to ensure correct patterning and proper function of the auditory sensory epithelium. Recent studies have revealed that Hedgehog signaling opposes HC differentiation and is critical for the establishment of the graded pattern of auditory HC differentiation. However, how Hedgehog signaling interferes with HC differentiation is unknown. Here, we provide evidence that in the murine cochlea, Hey1 and Hey2 control the spatiotemporal pattern of HC differentiation downstream of Hedgehog signaling. It has been recently shown that HEY1 and HEY2, two highly redundant HES-related transcriptional repressors, are highly expressed in supporting cell (SC) and HC progenitors (prosensory cells), but their prosensory function remained untested. Using a conditional double knock-out strategy, we demonstrate that prosensory cells form and proliferate properly in the absence of Hey1 and Hey2 but differentiate prematurely because of precocious upregulation of the pro-HC factor Atoh1. Moreover, we demonstrate that prosensory-specific expression of Hey1 and Hey2 and its subsequent graded downregulation is controlled by Hedgehog signaling in a largely FGFR-dependent manner. In summary, our study reveals a critical role for Hey1 and Hey2 in prosensory cell maintenance and identifies Hedgehog signaling as a novel upstream regulator of their prosensory function in the mammalian cochlea. The regulatory mechanism described here might be a broadly applied mechanism for controlling progenitor behavior in the central and peripheral nervous system.

Sensory hair cell regeneration in the zebrafish lateral line

BACKGROUND

Damage or destruction of sensory hair cells in the inner ear leads to hearing or balance deficits that can be debilitating, especially in older adults. Unfortunately, the damage is permanent, as regeneration of the inner ear sensory epithelia does not occur in mammals.

RESULTS

Zebrafish and other non-mammalian vertebrates have the remarkable ability to regenerate sensory hair cells and understanding the molecular and cellular basis for this regenerative ability will hopefully aid us in designing therapies to induce regeneration in mammals. Zebrafish not only possess hair cells in the ear but also in the sensory lateral line system. Hair cells in both organs are functionally analogous to hair cells in the inner ear of mammals. The lateral line is a mechanosensory system found in most aquatic vertebrates that detects water motion and aids in predator avoidance, prey capture, schooling, and mating. Although hair cell regeneration occurs in both the ear and lateral line, most research to date has focused on the lateral line due to its relatively simple structure and accessibility.

CONCLUSIONS

Here we review the recent discoveries made during the characterization of hair cell regeneration in zebrafish.

Segregating neural and mechanosensory fates in the developing ear: patterning, signaling, and transcriptional control

The vertebrate inner ear is composed of multiple sensory receptor epithelia, each of which is specialized for detection of sound, gravity, or angular acceleration. Each receptor epithelium contains mechanosensitive hair cells, which are connected to the brainstem by bipolar sensory neurons. Hair cells and their associated neurons are derived from the embryonic rudiment of the inner ear epithelium, but the precise spatial and temporal patterns of their generation, as well as the signals that coordinate these events, have only recently begun to be understood. Gene expression, lineage tracing, and mutant analyses suggest that both neurons and hair cells are generated from a common domain of neural and sensory competence in the embryonic inner ear rudiment. Members of the Shh, Wnt, and FGF families, together with retinoic acid signals, regulate transcription factor genes within the inner ear rudiment to establish the axial identity of the ear and regionalize neurogenic activity. Close-range signaling, such as that of the Notch pathway, specifies the fate of sensory regions and individual cell types. We also describe positive and negative interactions between basic helix-loop-helix and SoxB family transcription factors that specify either neuronal or sensory fates in a context-dependent manner. Finally, we review recent work on inner ear development in zebrafish, which demonstrates that the relative timing of neurogenesis and sensory epithelial formation is not phylogenetically constrained.

Therapeutic potential of a gamma-secretase inhibitor for hearing restoration in a guinea pig model with noise-induced hearing loss

Background

Notch signaling plays a crucial role in the fate determination of cochlear progenitor cells, hair cells, and supporting cells in the developing cochlea. Recent studies have demonstrated the temporal activation of Notch signaling in damaged mature cochleae, and have demonstrated the induction of new hair cells by pharmacologically inhibiting Notch signaling. The present study aimed to illustrate the feasibility of pharmacologically inhibiting Notch signaling by using a gamma-secretase inhibitor for treating sensorineural hearing loss.

Results

The effect of the sustained local delivery of MDL28170, a gamma-secretase inhibitor, on hearing and hair cell induction was tested in a guinea pig model with noise-induced hearing loss. MDL28170 was directly delivered into the cochlear fluids via a micro-osmotic pump. Drug application was initiated 7 days after noise exposure. Measurements of auditory brainstem responses revealed better hearing in the MDL28170-treated animals than in the vehicle controls. Histological analysis demonstrated a higher number of outer hair cells in the MDL28170-treated cochleae than the vehicle-treated cochleae.

Conclusion

These findings strongly suggest that local sustained delivery of a gamma-secretase inhibitor into the cochlea could be a novel strategy for treating acute hearing loss that is refractory to conventional treatment.

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.

FGFR1-Frs2/3 signalling maintains sensory progenitors during inner ear hair cell formation

Inner ear mechanosensory hair cells transduce sound and balance information. Auditory hair cells emerge from a Sox2-positive sensory patch in the inner ear epithelium, which is progressively restricted during development. This restriction depends on the action of signaling molecules. Fibroblast growth factor (FGF) signalling is important during sensory specification: attenuation of Fgfr1 disrupts cochlear hair cell formation; however, the underlying mechanisms remain unknown. Here we report that in the absence of FGFR1 signaling, the expression of Sox2 within the sensory patch is not maintained. Despite the down-regulation of the prosensory domain markers, p27^Kip1, Hey2, and Hes5, progenitors can still exit the cell cycle to form the zone of non-proliferating cells (ZNPC), however the number of cells that form sensory cells is reduced. Analysis of a mutant Fgfr1 allele, unable to bind to the adaptor protein, Frs2/3, indicates that Sox2 maintenance can be regulated by MAP kinase. We suggest that FGF signaling, through the activation of MAP kinase, is necessary for the maintenance of sensory progenitors and commits precursors to sensory cell differentiation in the mammalian cochlea.

The ability of our brain to perceive sound depends on its conversion into electrical impulses within the cochlea of the inner ear. The cochlea has dedicated specialized cells, called inner ear hair cells, which register sound energy. Environmental effects, genetic disorders or just the passage of time can damage these cells, and the damage impairs our ability to hear. If we could understand how these cells develop, we might be able to exploit this knowledge to generate new hair cells. In this study we address an old problem: how do signals from the fibroblast growth factor (FGF) family control hair cell number? We used mice in which one of the receptors for FGF (Fgfr1) is mutated and found that the expression of a stem cell protein, Sox2 is not maintained. Sox2 generally acts to keep precursors in the cochlea in a pre-hair cell state. However, in mutant mice Sox2 expression is transient, diminishing the ability of precursors to commit to a hair cell fate. These findings suggest that it may be possible to amplify the number of hair cell progenitors in culture by tuning FGF activity, providing a route to replace damaged inner ear hair cells.

In vivo visualization of Notch1 proteolysis reveals the heterogeneity of Notch1 signaling activity in the mouse cochlea

Mechanosensory hair cells (HCs) and surrounding supporting cells (SCs) in the mouse cochlea are important for hearing and are derived from the same prosensory progenitors. Notch1 signaling plays dual but contrasting and age-dependent roles in mouse cochlear development: early lateral induction and subsequent lateral inhibition. However, it has been difficult to directly visualize mouse cochlear cells experiencing various levels of Notch1 activity at single cell resolution. Here, we characterized two knock-in mouse lines, Notch1^{Cre (Low)/+} and Notch1^{Cre (High)/+}, with different Cre recombinase activities, that can detect Notch1 receptor proteolysis or Notch1 activity at high and low thresholds, respectively. Using both lines together with a highly sensitive Cre reporter line, we showed that Notch1 activity is nearly undetectable during lateral induction but increases to medium and high levels during lateral inhibition. Furthermore, we found that within the neonatal organ of Corti, the vast majority of cells that experience Notch1 activity were SCs not HCs, suggesting that HCs kept undetectable Notch1 activity during the entire lineage development. Furthermore, among SC subtypes, ∼85–99% of Deiters’ and outer pillar cells but only ∼19–38% of inner pillar cells experience medium and high levels of Notch1 activity. Our results demonstrate that Notch1 activity is highly heterogeneous: 1) between lateral induction and inhibition; 2) between HC and SC lineages; 3) among different SC subtypes; 4) among different cells within each SC subtype. Such heterogeneity should elucidate how the development of the cochclear sensory epithelium is precisely controlled and how HC regeneration can be best achieved in postnatal cochleae.

Coupling the cell cycle to development and regeneration of the inner ear

Cell cycle exit and acquirement of a postmitotic state is essential for the proper development of organs. In the present review, we examine the role of the cell cycle control in the sensory epithelia of the mammalian inner ear. We describe the roles of the core cell cycle regulators in the proliferation of prosensory cells and in the initiation and maintenance of terminal mitosis of the sensory epithelia. We also discuss how other intracellular signalling may influence the cell cycle. Finally, we address the question of whether manipulations of the cell cycle may have the potential to create replacement cells for the damaged inner sensory epithelia.

Notch signaling during cell fate determination in the inner ear

In the inner ear, Notch signaling has been proposed to specify the sensory regions, as well as regulate the differentiation of hair cells and supporting cell within those regions. In addition, Notch plays an important role in otic neurogenesis, by determining which cells differentiate as neurons, sensory cells and non-sensory cells. Here, I review the evidence for the complex and myriad roles Notch participates in during inner ear development. A particular challenge for those studying ear development and Notch is to decipher how activation of a single pathway can lead to different outcomes within the ear, which may include changes in the intrinsic properties of the cell, Notch modulation, and potential non-canonical pathways.

The Atoh1-lineage gives rise to hair cells and supporting cells within the mammalian cochlea

The organ of Corti, located within the mammalian cochlea, contains a precise mosaic of hair cells (HC) and supporting cells (SC), the patterning of which is critical for auditory function. Progenitors of HCs and SCs are found in the same post-mitotic region of the cochlear duct during early stages of cochlear development, and both HCs and SCs are absent in mice lacking the transcription factor Atoh1. Based on existing data, Atoh1 is thought to be the earliest determinant of HC fate, and to have a cell-autonomous role in HC differentiation, but the lineage of Atoh1-positive cells within the cochlear duct remains unclear. To address this issue, we used an inducible Atoh1(Cre⁎PR) allele to permanently mark Atoh1-expressing cells at different developmental time points. We found that up to 30% of cells from the Atoh1-lineage develop as SCs, and that the number of Atoh1-positive SCs decreases both spatially and temporally in a pattern consistent with ongoing commitment. Modulation of Notch signaling, necessary for formation of the HC-SC mosaic, changes the percentage of cells from the Atoh1-lineage that develop as either HCs or SCs. The HC-SC ratio is also affected by morphogenesis of the cochlea, as inhibiting the outgrowth of the cochlear duct increases the number of Atoh1-lineage cells that develop as SCs. Our results demonstrate that the Atoh1-lineage is established early in cochlear development, but also show that expression of Atoh1 does not absolutely result in commitment to a HC fate.

A dual function for canonical Wnt/β-catenin signaling in the developing mammalian cochlea

The canonical Wnt/β-catenin signaling pathway is known to play crucial roles in organogenesis by regulating both proliferation and differentiation. In the inner ear, this pathway has been shown to regulate the size of the otic placode from which the cochlea will arise; however, direct activity of canonical Wnt signaling as well as its function during cochlear mechanosensory hair cell development had yet to be identified. Using TCF/Lef:H2B-GFP reporter mice and transfection of an independent TCF/Lef reporter construct, we describe the pattern of canonical Wnt activity in the developing mouse cochlea. We show that prior to terminal mitosis, canonical Wnt activity is high in early prosensory cells from which hair cells and support cells will differentiate, and activity becomes reduced as development progresses. Using an in vitro model we demonstrate that Wnt/β-catenin signaling regulates both proliferation and hair cell differentiation within the developing cochlear duct. Inhibition of Wnt/β-catenin signaling blocks proliferation during early mitotic phases of development and inhibits hair cell formation in the differentiating organ of Corti. Conversely, activation increases the number of hair cells that differentiate and induces proliferation in prosensory cells, causing an expansion of the Sox2-positive prosensory domain. We further demonstrate that the induced proliferation of Sox2-positive cells may be mediated by the cell cycle regulator cyclin D1. Lastly, we provide evidence that the mitotic Sox2-positive cells are competent to differentiate into hair cells. Combined, our data suggest that Wnt/β-catenin signaling has a dual function in cochlear development, regulating both proliferation and hair cell differentiation.

Patterning and cell fate in the inner ear: a case for Notch in the chicken embryo

The development of the inner ear provides a beautiful example of one basic problem in development, that is, to understand how different cell types are generated at specific times and domains throughout embryonic life. The functional unit of the inner ear consists of hair cells, supporting cells and neurons, all deriving from progenitor cells located in the neurosensory competent domain of the otic placode. Throughout development, the otic placode resolves into the complex inner ear labyrinth, which holds the auditory and vestibular sensory organs that are innervated in a highly specific manner. How does the early competent domain of the otic placode give rise to the diverse specialized cell types of the different sensory organs of the inner ear? We review here our current understanding on the role of Notch signaling in coupling patterning and cell fate determination during inner ear development, with a particular emphasis on contributions from the chicken embryo as a model organism. We discuss further the question of how these two processes rely on two modes of operation of the Notch signaling pathway named lateral induction and lateral inhibition.

Otospheres derived from neonatal mouse cochleae retain the progenitor cell phenotype after ex vivo expansions

Because of their limited regenerative potential, cochlear hair cell loss is one of the major causes of permanent hearing loss in mammals. However, recent studies have shown that postnatal cochlear epithelia retain the progenitor cells that form otospheres. Otospheres are capable of self-renewing and differentiating into inner ear cell lineages, thereby suggesting a promising source for hair cell regeneration. We investigated retention of the progenitor cell phenotype in otospheres after ex vivo expansion, which is crucial for transplantation approaches. Reverse transcriptase-polymerase chain reaction and immunocytochemical analyses showed that otospheres derived from neonatal mice retained expression of stem and cochlear cell markers. After in vitro differentiation, otosphere-consisting cells differentiated into hair cell phenotypes after ex vivo expansion. However, the capacity of otospheres for self-renewal weakened with subsequent generations of ex vivo expansion. Our results indicate that ex vivo expanded-otospheres are useful experimental tools for studying hair cell regeneration in transplantation approaches and that the mechanisms for retention of the progenitor cell phenotype in otospheres should be investigated.

Notch prosensory effects in the Mammalian cochlea are partially mediated by Fgf20

Hearing loss is becoming an increasingly prevalent problem affecting more than 250 million people worldwide. During development, fibroblast growth factors (FGFs) are required for inner ear development as well as hair cell formation in the mammalian cochlea and thus make attractive therapeutic candidates for the regeneration of sensory cells. Previous findings showed that Fgfr1 conditional knock out mice exhibited hair cell and support cell formation defects. Immunoblocking with Fgf20 antibody in vitro produced a similar phenotype. While hair cell differentiation in mice starts at embryonic day (E)14.5, beginning with the inner hair cells, Fgf20 expression precedes hair cell differentiation at E13.5 in the cochlea. This suggests a potential role for Fgf20 in priming the sensory epithelium for hair cell formation. Treatment of explants with a gamma-secretase inhibitor, DAPT, decreased Fgf20 mRNA, suggesting that Notch is upstream of Fgf20. Notch signaling also plays an early role in prosensory formation during cochlear development. In this report we show that during development, Notch-mediated regulation of prosensory formation in the cochlea occurs via Fgf20. Addition of exogenous FGF20 compensated for the block in Notch signaling and rescued Sox2, a prosensory marker, and Gfi1, an early hair cell marker in explant cultures. We hypothesized that Fgf20 plays a role in specification, amplification, or maintenance of Sox2 expression in prosensory progenitors of the developing mammalian cochlea.

Postnatal development, maturation and aging in the mouse cochlea and their effects on hair cell regeneration

The organ of Corti in the mammalian inner ear is comprised of mechanosensory hair cells (HCs) and nonsensory supporting cells (SCs), both of which are believed to be terminally post-mitotic beyond late embryonic ages. Consequently, regeneration of HCs and SCs does not occur naturally in the adult mammalian cochlea, though recent evidence suggests that these cells may not be completely or irreversibly quiescent at earlier postnatal ages. Furthermore, regenerative processes can be induced by genetic and pharmacological manipulations, but, more and more reports suggest that regenerative potential declines as the organ of Corti continues to age. In numerous mammalian systems, such effects of aging on regenerative potential are well established. However, in the cochlea, the problem of regeneration has not been traditionally viewed as one of aging. This is an important consideration as current models are unable to elicit widespread regeneration or full recovery of function at adult ages yet regenerative therapies will need to be developed specifically for adult populations. Still, the advent of gene targeting and other genetic manipulations has established mice as critically important models for the study of cochlear development and HC regeneration and suggests that auditory HC regeneration in adult mammals may indeed be possible. Thus, this review will focus on the pursuit of regeneration in the postnatal and adult mouse cochlea and highlight processes that occur during postnatal development, maturation, and aging that could contribute to an age-related decline in regenerative potential. Second, we will draw upon the wealth of knowledge pertaining to age related senescence in tissues outside of the ear to synthesize new insights and potentially guide future research aimed at promoting HC regeneration in the adult cochlea.

Transcription factors that control inner ear development and their potential for transdifferentiation and reprogramming

Transcription factors (TFs) participate during various processes throughout inner ear development such as induction, morphogenesis and determination of cell fate and differentiation. The analysis of mouse mutants has been essential to define the requirement of different members of TF families during these processes. Next to their roles during normal development TFs have also been tested for their capacity to induce differentiation or reprogram cells upon misexpression. Recently the capacity of TFs to transdifferentiate easily accessible cells such as fibroblasts to highly specialized cell types has opened a new pathway for regenerative therapies. In this review the influence of TFs acting during different phases and processes of inner ear development will be summarized. A special focus will be given to TFs with a potential to reprogram or transdifferentiate cells to sensory cell types of the inner ear such as hair cells or neurons and thus may form part of future protocols directed to generate replacement cells in a clinical context.

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.

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.

Fgf signaling regulates development and transdifferentiation of hair cells and supporting cells in the basilar papilla

The avian basilar papilla (BP) is a likely homolog of the auditory sensory epithelium of the mammalian cochlea, the organ of Corti. During mammalian development Fibroblast growth factor receptor-3 (Fgfr3) is known to regulate the differentiation of auditory mechanosensory hair cells (HCs) and supporting cells (SCs), both of which are required for sound detection. Fgfr3 is expressed in developing progenitor cells (PCs) and SCs of both the BP and the organ of Corti; however its role in BP development is unknown. Here we utilized an in vitro whole organ embryonic culture system to examine the role of Fgf signaling in the developing avian cochlea. SU5402 (an antagonist of Fgf signaling) was applied to developing BP cultures at different stages to assay the role of Fgf signaling during HC formation. Similar to the observed effects of inhibition of Fgfr3 in the mammalian cochlea, Fgfr inhibition in the developing BP increased the number of HCs that formed. This increase was not associated with increased proliferation, suggesting that inhibition of the Fgf pathway leads to the direct conversion of PCs or supporting cells into HCs, a process known as transdifferentiation. This also implies that Fgf signaling is required to prevent the conversion of PCs and SCs into HCs. The ability of Fgf signaling to inhibit transdifferentiation suggests that its down-regulation may be essential for the initial steps of HC formation, as well as for the maintenance of SC phenotypes.