A mutation in the Lunatic fringe gene suppresses the effects of a Jagged2 mutation on inner hair cell development in the cochlea

Lrrn1 Regulates Medial Boundary Formation in the Developing Mouse Organ of Corti

One of the most striking aspects of the sensory epithelium of the mammalian cochlea, the organ of Corti (OC), is the presence of precise boundaries between sensory and nonsensory cells at its medial and lateral edges. A particular example of this precision is the single row of inner hair cells (IHCs) and associated supporting cells along the medial (neural) boundary. Despite the regularity of this boundary, the developmental processes and genetic factors that contribute to its specification are poorly understood. In this study we demonstrate that Leucine Rich Repeat Neuronal 1 (Lrrn1), which codes for a single-pass, transmembrane protein, is expressed before the development of the mouse organ of Corti in the row of cells that will form its medial border. Deletion of Lrrn1 in mice of mixed sex leads to disruptions in boundary formation that manifest as ectopic inner hair cells and supporting cells. Genetic and pharmacological manipulations demonstrate that Lrrn1 interacts with the Notch signaling pathway and strongly suggest that Lrrn1 normally acts to enhance Notch signaling across the medial boundary. This interaction is required to promote formation of the row of inner hair cells and suppress the conversion of adjacent nonsensory cells into hair cells and supporting cells. These results identify Lrrn1 as an important regulator of boundary formation and cellular patterning during development of the organ of Corti.SIGNIFICANCE STATEMENT Patterning of the developing mammalian cochlea into distinct sensory and nonsensory regions and the specification of multiple different cell fates within those regions are critical for proper auditory function. Here, we report that the transmembrane protein Leucine Rich Repeat Neuronal 1 (LRRN1) is expressed along the sharp medial boundary between the single row of mechanosensory inner hair cells (IHCs) and adjacent nonsensory cells. Formation of this boundary is mediated in part by Notch signaling, and loss of Lrrn1 leads to disruptions in boundary formation similar to those caused by a reduction in Notch activity, suggesting that LRRN1 likely acts to enhance Notch signaling. Greater understanding of sensory/nonsensory cell fate decisions in the cochlea will help inform the development of regenerative strategies aimed at restoring auditory function.

Lfng and Dll3 cooperate to modulate protein interactions in cis and coordinate oscillatory Notch pathway activation in the segmentation clock

In mammalian development, oscillatory activation of Notch signaling is required for segmentation clock function during somitogenesis. Notch activity oscillations are synchronized between neighboring cells in the presomitic mesoderm (PSM) and have a period that matches the rate of somite formation. Normal clock function requires cyclic expression of the Lunatic fringe (LFNG) glycosyltransferase, as well as expression of the inhibitory Notch ligand Delta-like 3 (DLL3). How these factors coordinate Notch activation in the clock is not well understood. Recent evidence suggests that LFNG can act in a signal-sending cell to influence Notch activity in the clock, raising the possibility that in this context, glycosylation of Notch pathway proteins by LFNG may affect ligand activity. Here we dissect the genetic interactions of Lfng and Dll3 specifically in the segmentation clock and observe distinctions in the skeletal and clock phenotypes of mutant embryos showing that paradoxically, loss of Dll3 is associated with strong reductions in Notch activity in the caudal PSM. The patterns of Notch activity in the PSM suggest that the loss of Dll3 is epistatic to the loss of Lfng in the segmentation clock, and we present direct evidence for the modification of several DLL1 and DLL3 EGF-repeats by LFNG. We further demonstrate that DLL3 expression in cells co-expressing DLL1 and NOTCH1 can potentiate a cell's signal-sending activity and that this effect is modulated by LFNG, suggesting a mechanism for coordinated regulation of oscillatory Notch activation in the clock by glycosylation and cis-inhibition.

Promotion of In Vitro Hair Cell-like Cell Differentiation from Human Embryonic Stem Cells through the Regulation of Notch Signaling

The Notch signaling pathway plays an important role in otic neurogenesis by regulating the differentiation of inner ear hair cells and supporting cells. Notch-regulated differentiation is required for the regeneration of hair cells in the inner ear. The temporal expression pattern of Notch ligands and receptors during in vitro hair cell-like cell differentiation from human embryonic stem cells (hESCs) was detected by quantitative reverse transcription-polymerase chain reaction (qRT-PCR). Subsequently, pAJ-U6-shRNA-CMV-Puro/GFP recombinant lentiviral vectors encoding short hairpin RNAs were used to silence JAG-1, JAG-2, and DLL-1, according to the temporal expression pattern of Notch ligands. Then, the effect of each ligand on the in vitro differentiation of hair cells was examined by RT-PCR, immunofluorescence, and scanning electron microscopy (SEM). The results showed that the individual deletion of JAG-2 or DLL-1 had no significant effect on the differentiation of hair cell-like cells. However, the simultaneous inhibition of both DLL-1 and JAG-2 increased the number of hair cell-like cells and decreased the number of supporting cells. JAG-2 and DLL-1 may have a synergistic role in in vitro hair cell differentiation.

Greater epithelial ridge cells are the principal organoid-forming progenitors of the mouse cochlea

In mammals, hearing loss is irreversible due to the lack of regenerative potential of non-sensory cochlear cells. Neonatal cochlear cells, however, can grow into organoids that harbor sensory epithelial cells, including hair cells and supporting cells. Here, we purify different cochlear cell types from neonatal mice, validate the composition of the different groups with single-cell RNA sequencing (RNA-seq), and assess the various groups' potential to grow into inner ear organoids. We find that the greater epithelial ridge (GER), a transient cell population that disappears during post-natal cochlear maturation, harbors the most potent organoid-forming cells. We identified three distinct GER cell groups that correlate with a specific spatial distribution of marker genes. Organoid formation was synergistically enhanced when the cells were cultured at increasing density. This effect is not due to diffusible signals but requires direct cell-to-cell contact. Our findings improve the development of cell-based assays to study culture-generated inner ear cell types.

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.

Development of the cochlea

The cochlea, a coiled structure located in the ventral region of the inner ear, acts as the primary structure for the perception of sound. Along the length of the cochlear spiral is the organ of Corti, a highly derived and rigorously patterned sensory epithelium that acts to convert auditory stimuli into neural impulses. The development of the organ of Corti requires a series of inductive events that specify unique cellular characteristics and axial identities along its three major axes. Here, we review recent studies of the cellular and molecular processes regulating several aspects of cochlear development, such as axial patterning, cochlear outgrowth and cellular differentiation. We highlight how the precise coordination of multiple signaling pathways is required for the successful formation of a complete organ of Corti.

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.

Making sense out of missense mutations: Mechanistic dissection of Notch receptors through structure-function studies in Drosophila

Notch signaling is involved in the development of almost all organ systems and is required post-developmentally to modulate tissue homeostasis. Rare variants in Notch signaling pathway genes are found in patients with rare Mendelian disorders, while unique or recurrent somatic mutations in a similar set of genes are identified in cancer. The human genome contains four genes that encode Notch receptors, NOTCH1-4, all of which are linked to genetic diseases and cancer. Although some mutations have been classified as clear loss- or gain-of-function alleles based on cellular or rodent based assay systems, the functional consequence of many variants/mutations in human Notch receptors remain unknown. In this review, I will first provide an overview of the domain structure of Notch receptors and discuss how each module is known to regulate Notch signaling activity in vivo using the Drosophila Notch receptor as an example. Next, I will introduce some interesting mutant alleles that have been isolated in the fly Notch gene over the past > 100 years of research and discuss how studies of these mutations have facilitated the understanding of Notch biology. By identifying unique alleles of the fly Notch gene through forward genetic screens, mapping their molecular lesions and characterizing their phenotypes in depth, one can begin to unravel new mechanistic insights into how different domains of Notch fine-tune signaling output. Such information can be useful in deciphering the functional consequences of rare variants/mutations in human Notch receptors, which in turn can influence disease management and therapy.

Age-related transcriptome changes in Sox2+ supporting cells in the mouse cochlea

Background

Inner ear supporting cells (SCs) in the neonatal mouse cochlea are a potential source for hair cell (HC) regeneration, but several studies have shown that the regeneration ability of SCs decreases dramatically as mice age and that lost HCs cannot be regenerated in adult mice. To better understand how SCs might be better used to regenerate HCs, it is important to understand how the gene expression profile changes in SCs at different ages.

Methods

Here, we used Sox2^GFP/+ mice to isolate the Sox2+ SCs at postnatal day (P)3, P7, P14, and P30 via flow cytometry. Next, we used RNA-seq to determine the transcriptome expression profiles of P3, P7, P14, and P30 SCs. To further analyze the relationships between these age-related and differentially expressed genes in Sox2+ SCs, we performed gene ontology (GO) analysis.

Results

Consistent with previous reports, we also found that the proliferation and HC regeneration ability of isolated Sox2+ SCs significantly decreased as mice aged. We identified numerous genes that are enriched and differentially expressed in Sox2+ SCs at four different postnatal ages, including cell cycle genes, signaling pathway genes, and transcription factors that might be involved in regulating the proliferation and HC differentiation ability of SCs. We thus present a set of genes that might regulate the proliferation and HC regeneration ability of SCs, and these might serve as potential new therapeutic targets for HC regeneration.

Conclusions

In our research, we found several genes that might play an important role in regulating the proliferation and HC regeneration ability of SCs. These datasets are expected to serve as a resource to provide potential new therapeutic targets for regulating the ability of SCs to regenerate HCs in postnatal mammals.

Notch polymorphisms associated with sensitivity of noise induced hearing loss among Chinese textile factory workers

BACKGROUND

Noise induced hearing loss (NIHL) is a polygenic disease involving both genetic and environmental factors, and is one of the most important occupational health hazards worldwide. To date, the influence of Notch1 variants on the risk to develop NIHL has not been illuminated. This study was conducted to explore the effects of Notch1 polymorphisms on individual susceptibility to NIHL.

METHODS

A total of 2689 industrial workers from one textile factory in east China were recruited to participate in the current study. Venous blood was collected, basic clinical data was obtained by questionnaires and pure-tone audiometry (PTA) tests were conducted by specialist physicians. Next we performed genotyping of three selected SNPs (rs3124594, rs3124599 and rs3124603) in the Notch1 gene in 535 NIHL patients and 535 controls. Subsequently, the main effects of the genotypes and their interactions were evaluated.

RESULTS

Our results revealed that individuals with a GG of rs3124594, TT of rs3124603 (OR = 4.70 and 1.59 respectively) and the haplotype AAC (rs3124594-rs3124599-rs3124603) (OR = 14.95) were associated with an increased risk of NIHL in our study cohort. Stratified analysis showed that an increased NIHL risk was found in individuals exposed to work related noise for ≤16 years that also had the rs3124594 GG or rs3124603 CT/TT genotype with an OR of 4.20 and 1.73 respectively. Multifactor dimensionality reduction analysis indicated that rs3124594, rs3124599 and rs3124603 interacted with each other and were related to an increased risk to develop NIHL (OR = 3.60).

CONCLUSIONS

The genetic polymorphisms rs3124594 and rs3124603 within the Notch1 gene are associated with an increased risk of NIHL in a Chinese population and could potentially be used as biomarkers for NIHL in noise exposed workers.

FGF-induced Pea3 transcription factors program the genetic landscape for cell fate determination

FGF signaling is a potent inducer of lacrimal gland development in the eye, capable of transforming the corneal epithelium into glandular tissues. Here, we show that genetic ablation of the Pea3 family of transcription factors not only disrupted the ductal elongation and branching of the lacrimal gland, but also biased the lacrimal gland epithelium toward an epidermal cell fate. Analysis of high-throughput gene expression and chromatin immunoprecipitation data revealed that the Pea3 genes directly control both the positive and negative feedback loops of FGF signaling. Importantly, Pea3 genes are also required to suppress aberrant Notch signaling which, if gone unchecked, can compromise lacrimal gland development by preventing the expression of both Sox and Six family genes. These results demonstrate that Pea3 genes are key FGF early response transcriptional factors, programing the genetic landscape for cell fate determination.

FGF signaling regulates cell fate decision by inducing genome-wide changes in gene expression. We identified Pea3 family transcription factors as the key effectors of FGF signaling in reprograming the epithelia transcriptome. Pea3 factors control both the feedback and feedforward circuities of FGF signaling in lacrimal gland development. They also activate specific expression of Six and Sox family genes and suppress aberrant activation of Notch signaling. In the absence of Pea3 genes, the lacrimal gland progenitors become epidermal-like in their gene expression patterns. The study of Pea3 function resolves the long standing conundrum of how FGF induces the lacrimal gland fate, providing direction for regenerating the lacrimal gland to treat dry eye diseases.

Redundancy and cooperation in Notch intercellular signaling

During metazoan development, Notch signaling drives spatially coordinated differentiation by establishing communication between adjacent cells. This occurs through either lateral inhibition, in which adjacent cells acquire distinct fates, or lateral induction, in which all cells become equivalent. Notch signaling is commonly activated by several distinct ligands, each of which drives signaling with a different efficiency upon binding to the Notch receptor of adjacent cells. Moreover, these ligands can also be distinctly regulated by Notch signaling. Under such complex circumstances, the overall spatial coordination becomes elusive. Here, we address this issue through both mathematical and computational analyses. Our results show that when two ligands have distinct efficiencies and compete for the same Notch receptor, they cooperate to drive new signaling states, thereby conferring additional robustness and evolvability to Notch signaling. Counterintuitively, whereas antagonistically regulated ligands cooperate to drive and enhance the response that is expected from the more efficient ligand, equivalently regulated ligands coordinate emergent spatial responses that are dependent on both ligands. Our study highlights the importance of ligand efficiency in multi-ligand scenarios, and can explain previously reported complex phenotypes.

Putative binding sites for mir-125 family miRNAs in the mouse Lfng 3'UTR affect transcript expression in the segmentation clock, but mir-125a-5p is dispensable for normal somitogenesis

BACKGROUND

In vertebrate embryos, a "segmentation clock" times somitogenesis. Clock-linked genes, including Lunatic fringe (Lfng), exhibit cyclic expression in the presomitic mesoderm (PSM), with a period matching the rate of somite formation. The clock period varies widely across species, but the mechanisms that underlie this variability are not clear. The half-lives of clock components are proposed to influence the rate of clock oscillations, and are tightly regulated in the PSM. Interactions between Lfng and mir-125a-5p in the embryonic chicken PSM promote Lfng transcript instability, but the conservation of this mechanism in other vertebrates has not been tested. Here, we examine whether this interaction affects clock activity in a mammalian species.

RESULTS

Mutation of mir-125 binding sites in the Lfng 3'UTR leads to persistent, nonoscillatory reporter transcript expression in the caudal-most mouse PSM, although dynamic transcript expression recovers in the central PSM. Despite this, expression of endogenous mir-125a-5p is dispensable for mouse somitogenesis.

CONCLUSIONS

These results suggest that mir-125a sites in the Lfng 3' untranslated region influence transcript turnover in both mouse and chicken embryos, and support the existence of position-dependent regulatory mechanisms in the PSM. They further suggest the existence of compensatory mechanisms that can rescue the loss of mir-125a-5p in mice. Developmental Dynamics 246:740-748, 2017. © 2017 Wiley Periodicals, Inc.

Do as I say, Not(ch) as I do: Lateral control of cell fate

Breaking symmetry in populations of uniform cells, to induce adoption of an alternative cell fate, is an essential developmental mechanism. Similarly, domain and boundary establishment are crucial steps to forming organs during development. Notch signaling is a pathway ideally suited to mediating precise patterning cues, as both receptors and ligands are membrane-bound and can thus act as a precise switch to toggle cell fates on or off. Fine-tuning of signaling by positive or negative feedback mechanisms dictate whether signaling results in lateral induction or lateral inhibition, respectively, allowing Notch to either induce entire regions of cell specification, or dictate binary fate choices. Furthermore, pathway activity is modulated by Fringe modification of receptors or ligands, co-expression of receptors with ligands, mode of ligand presentation, and cell surface area in contact. In this review, we describe how Notch signaling is fine-tuned to mediate lateral induction or lateral inhibition cues, and discuss examples from C.elegans, D. melanogaster and M. musculus. Identifying the cellular machinery dictating the choice between lateral induction and lateral inhibition highlights the versatility of the Notch signaling pathway in development.

Fine-tuning of Notch signaling sets the boundary of the organ of Corti and establishes sensory cell fates

The signals that induce the organ of Corti and define its boundaries in the cochlea are poorly understood. We show that two Notch modifiers, Lfng and Mfng, are transiently expressed precisely at the neural boundary of the organ of Corti. Cre-Lox fate mapping shows this region gives rise to inner hair cells and their associated inner phalangeal cells. Mutation of Lfng and Mfng disrupts this boundary, producing unexpected duplications of inner hair cells and inner phalangeal cells. This phenotype is mimicked by other mouse mutants or pharmacological treatments that lower but not abolish Notch signaling. However, strong disruption of Notch signaling causes a very different result, generating many ectopic hair cells at the expense of inner phalangeal cells. Our results show that Notch signaling is finely calibrated in the cochlea to produce precisely tuned levels of signaling that first set the boundary of the organ of Corti and later regulate hair cell development.

DOI:http://dx.doi.org/10.7554/eLife.19921.001

Differential and Cooperative Cell Adhesion Regulates Cellular Pattern in Sensory Epithelia

Animal tissues are composed of multiple cell types arranged in complex and elaborate patterns. In sensory epithelia, including the auditory epithelium and olfactory epithelium, different types of cells are arranged in unique mosaic patterns. These mosaic patterns are evolutionarily conserved, and are thought to be important for hearing and olfaction. Recent progress has provided accumulating evidence that the cellular pattern formation in epithelia involves cell rearrangements, movements, and shape changes. These morphogenetic processes are largely mediated by intercellular adhesion systems. Differential adhesion and cortical tension have been proposed to promote cell rearrangements. Many different types of cells in tissues express various types of cell adhesion molecules. Although cooperative mechanisms between multiple adhesive systems are likely to contribute to the production of complex cell patterns, our current understanding of the cooperative roles between multiple adhesion systems is insufficient to entirely explain the complex mechanisms underlying cellular patterning. Recent studies have revealed that nectins, in cooperation with cadherins, are crucial for the mosaic cellular patterning in sensory organs. The nectin and cadherin systems are interacted with one another, and these interactions provide cells with differential adhesive affinities for complex cellular pattern formations in sensory epithelia, which cannot be achieved by a single mechanism.

A central to peripheral progression of cell cycle exit and hair cell differentiation in the developing mouse cristae

The inner ear contains six distinct sensory organs that each maintains some ability to regenerate hair cells into adulthood. In the postnatal cochlea, there appears to be a relationship between the developmental maturity of a region and its ability to regenerate as postnatal regeneration largely occurs in the apical turn, which is the last region to differentiate and mature during development. In the mature cristae there are also regional differences in regenerative ability, which led us to hypothesize that there may be a general relationship between the relative maturity of a region and the regenerative competence of that region in all of the inner ear sensory organs. By analyzing adult mouse cristae labeled embryonically with BrdU, we found that hair cell birth starts in the central region and progresses to the periphery with age. Since the peripheral region of the adult cristae also maintains active Notch signaling and some regenerative competence, these results are consistent with the hypothesis that the last regions to develop retain some of their regenerative ability into adulthood. Further, by analyzing embryonic day 14.5 inner ears we provide evidence for a wave of hair cell birth along the longitudinal axis of the cristae from the central regions to the outer edges. Together with the data from the adult inner ears labeled with BrdU as embryos, these results suggest that hair cell differentiation closely follows cell cycle exit in the cristae, unlike in the cochlea where they are uncoupled.

The intracellular domains of Notch1 and Notch2 are functionally equivalent during development and carcinogenesis

Although Notch1 and Notch2 are closely related paralogs and function through the same canonical signaling pathway, they contribute to different outcomes in some cell and disease contexts. To understand the basis for these differences, we examined in detail mice in which the Notch intracellular domains (N1ICD and N2ICD) were swapped. Our data indicate that strength (defined here as the ultimate number of intracellular domain molecules reaching the nucleus, integrating ligand-mediated release and nuclear translocation) and duration (half-life of NICD-RBPjk-MAML-DNA complexes, integrating cooperativity and stability dependent on shared sequence elements) are the factors that underlie many of the differences between Notch1 and Notch2 in all the contexts we examined, including T-cell development, skin differentiation and carcinogenesis, the inner ear, the lung and the retina. We were able to show that phenotypes in the heart, endothelium, and marginal zone B cells are attributed to haploinsufficiency but not to intracellular domain composition. Tissue-specific differences in NICD stability were most likely caused by alternative scissile bond choices by tissue-specific γ-secretase complexes following the intracellular domain swap. Reinterpretation of clinical findings based on our analyses suggests that differences in outcome segregating with Notch1 or Notch2 are likely to reflect outcomes dependent on the overall strength of Notch signals.

Quantitative High-Resolution Cellular Map of the Organ of Corti

The organ of Corti harbors highly specialized sensory hair cells and surrounding supporting cells that are essential for the sense of hearing. Here, we report a single cell gene expression data analysis and visualization strategy that allows for the construction of a quantitative spatial map of the neonatal organ of Corti along its major anatomical axes. The map displays gene expression levels of 192 genes for all organ of Corti cell types ordered along the apex-to-base axis of the cochlea. Statistical interrogation of cell-type-specific gene expression patterns along the longitudinal gradient revealed features of apical supporting cells indicative of a propensity for proliferative hair cell regeneration. This includes reduced expression of Notch effectors, receptivity for canonical Wnt signaling, and prominent expression of early cell-cycle genes. Cochlear hair cells displayed expression gradients of genes indicative of cellular differentiation and the establishment of the tonotopic axis.

The candidate splicing factor Sfswap regulates growth and patterning of inner ear sensory organs

The Notch signaling pathway is thought to regulate multiple stages of inner ear development. Mutations in the Notch signaling pathway cause disruptions in the number and arrangement of hair cells and supporting cells in sensory regions of the ear. In this study we identify an insertional mutation in the mouse Sfswap gene, a putative splicing factor, that results in mice with vestibular and cochlear defects that are consistent with disrupted Notch signaling. Homozygous Sfswap mutants display hyperactivity and circling behavior consistent with vestibular defects, and significantly impaired hearing. The cochlea of newborn Sfswap mutant mice shows a significant reduction in outer hair cells and supporting cells and ectopic inner hair cells. This phenotype most closely resembles that seen in hypomorphic alleles of the Notch ligand Jagged1 (Jag1). We show that Jag1; Sfswap compound mutants have inner ear defects that are more severe than expected from simple additive effects of the single mutants, indicating a genetic interaction between Sfswap and Jag1. In addition, expression of genes involved in Notch signaling in the inner ear are reduced in Sfswap mutants. There is increased interest in how splicing affects inner ear development and function. Our work is one of the first studies to suggest that a putative splicing factor has specific effects on Notch signaling pathway members and inner ear development.

Notch signaling limits supporting cell plasticity in the hair cell-damaged early postnatal murine cochlea

In mammals, auditory hair cells are generated only during embryonic development and loss or damage to hair cells is permanent. However, in non-mammalian vertebrate species, such as birds, neighboring glia-like supporting cells regenerate auditory hair cells by both mitotic and non-mitotic mechanisms. Based on work in intact cochlear tissue, it is thought that Notch signaling might restrict supporting cell plasticity in the mammalian cochlea. However, it is unresolved how Notch signaling functions in the hair cell-damaged cochlea and the molecular and cellular changes induced in supporting cells in response to hair cell trauma are poorly understood. Here we show that gentamicin-induced hair cell loss in early postnatal mouse cochlear tissue induces rapid morphological changes in supporting cells, which facilitate the sealing of gaps left by dying hair cells. Moreover, we provide evidence that Notch signaling is active in the hair cell damaged cochlea and identify Hes1, Hey1, Hey2, HeyL, and Sox2 as targets and potential Notch effectors of this hair cell-independent mechanism of Notch signaling. Using Cre/loxP based labeling system we demonstrate that inhibition of Notch signaling with a γ- secretase inhibitor (GSI) results in the trans-differentiation of supporting cells into hair cell-like cells. Moreover, we show that these hair cell-like cells, generated by supporting cells have molecular, cellular, and basic electrophysiological properties similar to immature hair cells rather than supporting cells. Lastly, we show that the vast majority of these newly generated hair cell-like cells express the outer hair cell specific motor protein prestin.

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.

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.

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.

Differential requirements for Wnt and Notch signaling in hematopoietic versus thymic niches

All blood cells are derived from multipotent stem cells, the so-called hematopoietic stem cells (HSCs), that in adults reside in the bone marrow. Most types of blood cells also develop there, with the notable exception of T lymphocytes that develop in the thymus. For both HSCs and developing T cells, interactions with the surrounding microenvironment are critical in regulating maintenance, differentiation, apoptosis, and proliferation. Such specialized regulatory microenvironments are referred to as niches and provide both soluble factors as well as cell-cell interactions between niche component cells and blood cells. Two pathways that are critical for early T cell development in the thymic niche are Wnt and Notch signaling. These signals also play important but controversial roles in the HSC niche. Here, we review the differences and similarities between the thymic and hematopoietic niches, with particular focus on Wnt and Notch signals, as well as the latest insights into regulation of these developmentally important pathways.

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.

In vivo Notch reactivation in differentiating cochlear hair cells induces Sox2 and Prox1 expression but does not disrupt hair cell maturation

BACKGROUND

Notch signaling is active in mouse cochlear prosensory progenitors but declines in differentiating sensory hair cells (HCs). Overactivation of the Notch1 intracellular domain (NICD) in progenitors blocks HC fate commitment and/or differentiation. However, it is not known whether reactivation of NICD in differentiating HCs also interrupts their developmental program and reactivates its downstream targets.

RESULTS

By analyzing Atoh1(CreER+) ; Rosa26-NICD(loxp/+) or Atoh1(CreER+) ; Rosa26-NICD(loxp/+) ; RBP-J(loxp/loxp) mice, we demonstrated that ectopic NICD in differentiating HCs caused reactivation of Sox2 and Prox1 in an RBP-J-dependent manner. Interestingly, Prox1 reactivation was exclusive to outer HCs (OHCs). In addition, lineage tracing analysis of Prox1(CreER/+) ; Rosa26-EYFP(loxp/+) and Prox1(CreEGFP/+) ; Rosa26-EYFP(loxp/+) mice showed that nearly all HCs experiencing Prox1 expression were OHCs. Surprisingly, these HCs still matured normally with expression of prestin, wild-type-like morphology, and uptake of FM4-64FX dye at adult ages.

CONCLUSIONS

Our results suggest that the developmental program of cochlear differentiating HCs is refractory to Notch reactivation and that Notch is an upstream regulator of Sox2 and Prox1 in cochlear development. In addition, our results support that Sox2 and Prox1 should not be the main blockers for terminal differentiation of HCs newly regenerated from postnatal cochlear SCs that still maintain Sox2 and Prox1 expression.

Notch signaling: simplicity in design, versatility in function

Notch signaling is evolutionarily conserved and operates in many cell types and at various stages during development. Notch signaling must therefore be able to generate appropriate signaling outputs in a variety of cellular contexts. This need for versatility in Notch signaling is in apparent contrast to the simple molecular design of the core pathway. Here, we review recent studies in nematodes, Drosophila and vertebrate systems that begin to shed light on how versatility in Notch signaling output is generated, how signal strength is modulated, and how cross-talk between the Notch pathway and other intracellular signaling systems, such as the Wnt, hypoxia and BMP pathways, contributes to signaling diversity.

Jagged 1 regulates the restriction of Sox2 expression in the developing chicken inner ear: a mechanism for sensory organ specification

Hair cells of the inner ear sensory organs originate from progenitor cells located at specific domains of the otic vesicle: the prosensory patches. Notch signalling is necessary for sensory development and loss of function of the Notch ligand jagged 1 (Jag1, also known as serrate 1) results in impaired sensory organs. However, the underlying mechanism of Notch function is unknown. Our results show that in the chicken otic vesicle, the Sox2 expression domain initially contains the nascent patches of Jag1 expression but, later on, Sox2 is only maintained in the Jag1-positive domains. Ectopic human JAG1 (hJag1) is able to induce Sox2 expression and enlarged sensory organs. The competence to respond to hJag1, however, is confined to the regions that expressed Sox2 early in development, suggesting that hJag1 maintains Sox2 expression rather than inducing it de novo. The effect is non-cell-autonomous and requires Notch signalling. hJag1 activates Notch, induces Hes/Hey genes and endogenous Jag1 in a non-cell-autonomous manner, which is consistent with lateral induction. The effects of hJag1 are mimicked by Jag2 but not by Dl1. Sox2 is sufficient to activate the Atoh1 enhancer and to ectopically induce sensory cell fate outside neurosensory-competent domains. We suggest that the prosensory function of Jag1 resides in its ability to generate discrete domains of Notch activity that maintain Sox2 expression within restricted areas of an extended neurosensory-competent domain. This provides a mechanism to couple patterning and cell fate specification during the development of sensory organs.

Specification of cell fate in the mammalian cochlea

Mammalian auditory sensation is mediated by the organ of Corti, a specialized sensory epithelium found in the cochlea of the inner ear. Proper auditory function requires that the many different cell types found in the sensory epithelium be precisely ordered within an exquisitely patterned cellular mosaic. The development of this mosaic depends on a series of cell fate decisions that transform the initially nearly uniform cochlear epithelium into the complex structure of the mature organ of Corti. The prosensory domain, which contains the progenitors of both the mechanosensory hair cells and their associated supporting cells, first becomes distinct from both the neural and the nonsensory domains. Further cell fate decisions subdivide prosensory cells into populations of inner and outer hair cells, and several different types of supporting cells. A number of different signaling pathways and transcription factors are known to be necessary for these developmental processes; in this review, we will summarize these results with an emphasis on recent findings.

Hes5 expression in the postnatal and adult mouse inner ear and the drug-damaged cochlea

The Notch signaling pathway is known to have multiple roles during development of the inner ear. Notch signaling activates transcription of Hes5, a homologue of Drosophila hairy and enhancer of split, which encodes a basic helix-loop-helix transcriptional repressor. Previous studies have shown that Hes5 is expressed in the cochlea during embryonic development, and loss of Hes5 leads to overproduction of auditory and vestibular hair cells. However, due to technical limitations and inconsistency between previous reports, the precise spatial and temporal pattern of Hes5 expression in the postnatal and adult inner ear has remained unclear. In this study, we use Hes5-GFP transgenic mice and in situ hybridization to report the expression pattern of Hes5 in the inner ear. We find that Hes5 is expressed in the developing auditory epithelium of the cochlea beginning at embryonic day 14.5 (E14.5), becomes restricted to a particular subset of cochlear supporting cells, is downregulated in the postnatal cochlea, and is not present in adults. In the vestibular system, we detect Hes5 in developing supporting cells as early as E12.5 and find that Hes5 expression is maintained in some adult vestibular supporting cells. In order to determine the effect of hair cell damage on Notch signaling in the cochlea, we damaged cochlear hair cells of adult Hes5-GFP mice in vivo using injection of kanamycin and furosemide. Although outer hair cells were killed in treated animals and supporting cells were still present after damage, supporting cells did not upregulate Hes5-GFP in the damaged cochlea. Therefore, absence of Notch-Hes5 signaling in the normal and damaged adult cochlea is correlated with lack of regeneration potential, while its presence in the neonatal cochlea and adult vestibular epithelia is associated with greater capacity for plasticity or regeneration in these tissues; which suggests that this pathway may be involved in regulating regenerative potential.

Building the world's best hearing aid; regulation of cell fate in the cochlea

In mammals, auditory perception is initially mediated through sensory cells located in a rigorously patterned mosaic of unique cell types located within the coiled cochlea. Identification of the factors that direct multipotent progenitor cells to develop as each of these specialized cell types has the potential to enhance our understanding of the development of the auditory system and to identify potential targets for regenerative therapies. Recent results have identified specific signaling molecules and pathways, including Notch, Hedgehog, Sox2 and Fgfs, that guide progenitor cells to develop first as a sensory precursor, referred to as a prosensory cell, and subsequently as one of the specialized cell types within the sensory mosaic.

Early identification of inner pillar cells during rat cochlear development

Although the structure of the auditory organ in mature mammals, the organ of Corti, is clearly established, its development is far from being elucidated. Here, we examine its spatio-temporal development in rats from embryonic day 16 (E16) to E19 by using cytochemical and immunocytochemical methods at the light- and electron-microscope levels. We demonstrate that the organ of Corti develops from a non-proliferating cell zone that is located in the junctional region between two edges of the dorsal epithelium of the cochlear duct. We also reveal that the first cells to develop in this zone are the inner pillar cells, a particular type of non-sensory supporting cell, which arise in the base of the cochlear duct at the boundary between the two ridges at E16. Cell differentiation in this prosensory region continues according to a base-to-apex gradient; the inner hair cells appear in the greater epithelial ridge at E17 and the outer hair cells in the lesser epithelial ridge at E18. At E19, the various cell types of the organ of Corti are in place. Finally, we show that unlike the development of all the supporting cell types of the organ of Corti, the development of inner pillar cells within the prosensory domain seems not to involve Notch1 activation. These results highlight the central role that the inner pillar cells probably play in the development of the organ of Corti.

Progress in the understanding of the genetic etiology of vertebral segmentation disorders in humans

Vertebral malformations contribute substantially to the pathophysiology of kyphosis and scoliosis, common health problems associated with back and neck pain, disability, cosmetic disfigurement, and functional distress. This review explores (1) recent advances in the understanding of the molecular embryology underlying vertebral development and relevance to elucidation of etiologies of several known human vertebral malformation syndromes; (2) outcomes of molecular studies elucidating genetic contributions to congenital and sporadic vertebral malformation; and (3) complex interrelationships between genetic and environmental factors that contribute to the pathogenesis of isolated syndromic and nonsyndromic congenital vertebral malformation. Discussion includes exploration of the importance of establishing improved classification systems for vertebral malformation, future directions in molecular and genetic research approaches to vertebral malformation, and translational value of research efforts to clinical management and genetic counseling of affected individuals and their families.

Bile duct proliferation in Jag1/fringe heterozygous mice identifies candidate modifiers of the Alagille syndrome hepatic phenotype

Alagille syndrome (AGS) is a heterogeneous developmental disorder associated with bile duct paucity and various organ anomalies. The syndrome is caused by mutations in JAG1, which encodes a ligand in the Notch signaling pathway, in the majority of cases and mutations in the NOTCH2 receptor gene in less than 1% of patients. Although a wide array of JAG1 mutations have been identified in the AGS population, these mutational variants have not accounted for the wide phenotypic variability observed in patients with this syndrome. The Fringe genes encode glycosyltransferases, which modify Notch and alter ligand-receptor affinity. In this study, we analyzed double heterozygous mouse models to examine the Fringe genes as potential modifiers of the Notch-mediated hepatic phenotype observed in AGS. We generated mice that were haploinsufficient for both Jag1 and one of three paralogous Fringe genes: Lunatic (Lfng), Radical (Rfng), and Manic (Mfng). Adult Jag1(+/-)Lfng(+/-) and Jag1(+/-)Rfng(+/-) mouse livers exhibited widespread bile duct proliferation beginning at 5 weeks of age and persisting up to 1 year. The Jag1(+/-)Mfng(+/-) livers showed a subtle, yet significant increase in bile duct numbers and bile duct to portal tract ratios. These abnormalities were not observed in the newborn period. Despite the portal tract expansion by bile ducts, fibrosis was not increased and epithelial to mesenchymal transition was not shown in the affected portal tracts.

CONCLUSION

Mice heterozygous for mutations in Jag1 and the Fringe genes display striking bile duct proliferation, which is not apparent at birth. These findings suggest that the Fringe genes may regulate postnatal bile duct growth and remodeling, and serve as candidate modifiers of the hepatic phenotype in AGS.

Eya1 gene dosage critically affects the development of sensory epithelia in the mammalian inner ear

Haploinsufficiency of the transcription co-activator EYA1 causes branchio-oto-renal syndrome, congenital birth defects that account for as many as 2% of profoundly deaf children; however, the underlying cause for its dosage requirement and its specific role in sensory cell development of the inner ear are unknown. Here, an allelic series of Eya1 were generated to study the basis of Eya1 dosage requirements for sensory organ development. Our results show different threshold requirements for the level of Eya1 in different regions of the inner ear. Short and disorganized hair cell sterocilia was observed in wild-type/null heterozygous or hypomorphic/hypomorphic homozygous cochleae. Patterning and gene-marker analyses indicate that in Eya1 hypomorphic/null heterozygous mice, a reduction of Eya1 expression to 21% of normal level causes an absence of cochlear and vestibular sensory formation. Eya1 is initially expressed in the progenitors throughout the epithelium of all six sensory regions, and later on during sensory cell differentiation, its expression becomes restricted to the differentiating hair cells. We provide genetic evidence that Eya1 activity, in a concentration-dependent manner, plays a key role in the regulation of genes known to be important for sensory development. Furthermore, we show that Eya1 co-localizes with Sox2 in the sensory progenitors and both proteins physically interact. Together, our results indicate that Eya1 appears to be upstream of very early events during the sensory organ development, hair cell differentiation and inner-ear patterning. These results also provide a molecular mechanism for understanding how hypomorphic levels of EYA1 cause inner-ear defects in humans.

In vivo consequences of deleting EGF repeats 8-12 including the ligand binding domain of mouse Notch1

BACKGROUND

Notch signaling is highly conserved in the metazoa and is critical for many cell fate decisions. Notch activation occurs following ligand binding to Notch extracellular domain. In vitro binding assays have identified epidermal growth factor (EGF) repeats 11 and 12 as the ligand binding domain of Drosophila Notch. Here we show that an internal deletion in mouse Notch1 of EGF repeats 8-12, including the putative ligand binding domain (lbd), is an inactivating mutation in vivo. We also show that maternal and zygotic Notch1(lbd/lbd) mutant embryos develop through gastrulation to mid-gestation.

RESULTS

Notch1(lbd/lbd) embryos died at mid-gestation with a phenotype indistinguishable from Notch1 null mutants. In embryonic stem (ES) cells, Notch1(lbd) was expressed on the cell surface at levels equivalent to wild type Notch1, but Delta1 binding was reduced to the same level as in Notch1 null cells. In an ES cell co-culture assay, Notch signaling induced by Jagged1 or Delta1 was reduced to a similar level in Notch1(lbd) and Notch1 null cells. However, the Notch1(lbd/lbd) allele was expressed similarly to wild type Notch1 in Notch1(lbd/lbd) ES cells and embryos at E8.75, indicating that Notch1 signaling is not essential for the Notch1 gene to be expressed. In addition, maternal and zygotic Notch1 mutant blastocysts developed through gastrulation.

CONCLUSION

Mouse Notch1 lacking the ligand binding domain is expressed at the cell surface but does not signal in response to the canonical Notch ligands Delta1 and Jagged1. Homozygous Notch1(lbd/lbd) mutant embryos die at approximately E10 similar to Notch1 null embryos. While Notch1 is expressed in oocytes and blastocysts, Notch1 signaling via canonical ligands is dispensable during oogenesis, blastogenesis, implantation and gastrulation.

Dll3 is expressed in developing hair cells in the mammalian cochlea

Notch mediates the process of lateral inhibition that controls the production of hair cells in the inner ear. Hair cells are known to express Notch ligands Dll1 and Jag2, which signal through Notch1 in adjacent supporting cells. However, recent genetic and pharmacological studies indicate that the level of Notch-mediated lateral inhibition is greater than can be accounted for by Dll1 and Jag2. Here, we report that another Notch ligand, Dll3, is expressed in developing hair cells, in a pattern that overlaps that of Dll1 and Jag2. We analyzed the cochleae of Dll3(pu) mutant mice, but did not detect any abnormalities. However, earlier studies have demonstrated that there is functional redundancy among Notch ligands in cochlear development and loss of one ligand can be at least partially compensated for by another. Thus Dll3 may play a role in lateral inhibition similar to that of Dll1 and Jag2.

Monitoring Notch1 activity in development: evidence for a feedback regulatory loop

Signaling through Notch receptors, which regulate cell fate decisions and embryonic patterning, requires ligand-induced receptor cleavage to generate the signaling active Notch intracellular domain (NICD). Here, we show an analysis at specific developmental stages of the distribution of active mouse Notch1. We use an antibody that recognizes N1ICD, and a highly sensitive staining technique. The earliest N1ICD expression was observed in the mesoderm and developing heart, where we detected expression in nascent endocardium, presumptive cardiac valves, and ventricular and atrial endocardium. During segmentation, N1ICD was restricted to the presomitic mesoderm. N1ICD expression was also evident in arterial endothelium, and in kidney and endodermal derivatives such as pancreas and thymus. Ectodermal N1ICD expression was found in central nervous system and sensory placodes. We found that Notch1 transcription and activity was severely reduced in zebrafish and mouse Notch pathway mutants, suggesting that vertebrate Notch1 expression is regulated by a positive feedback loop.

Basic helix-loop-helix gene Hes6 delineates the sensory hair cell lineage in the inner ear

The basic helix-loop-helix (bHLH) gene Hes6 is known to promote neural differentiation in vitro. Here, we report the expression and functional studies of Hes6 in the inner ear. The expression of Hes6 appears to be parallel to that of Math1 (also known as Atoh1), a bHLH gene necessary and sufficient for hair cell differentiation. Hes6 is expressed initially in the presumptive hair cell precursors in the cochlea. Subsequently, the expression of Hes6 is restricted to morphologically differentiated hair cells. Similarly, the expression of Hes6 in the vestibule is in the hair cell lineage. Hes6 is dispensable for hair cell differentiation, and its expression in inner ear hair cells is abolished in the Math1-null animals. Furthermore, the introduction of Hes6 into the cochlea in vitro is not sufficient to promote sensory or neuronal differentiation. Therefore, Hes6 is downstream of Math1 and its expression in the inner ear delineates the sensory lineage. However, the role of Hes6 in the inner ear remains elusive.

Strategies to regenerate hair cells: identification of progenitors and critical genes

Deafness commonly results from a lesion of the sensory cells and/or of the neurons of the auditory part of the inner ear. There are currently no treatments designed to halt or reverse the progression of hearing loss. A key goal in developing therapy for sensorineural deafness is the identification of strategies to replace lost hair cells. In amphibians and birds, a spontaneous post-injury regeneration of all inner ear sensory hair cells occurs. In contrast, in the mammalian cochlea, hair cells are only produced during embryogenesis. Many studies have been carried out in order to demonstrate the persistence of endogenous progenitors. The present review is first focused on the occurrence of spontaneous supernumerary hair cells and on nestin positive precursors found in the organ of Corti. A second approach to regenerating hair cells would be to find genes essential for their differentiation. This review will also focus on critical genes for embryonic hair cell formation such as the cell cycle related proteins, the Atoh1 gene and the Notch signaling pathway. Understanding mechanisms that underlie hair cell production is an essential prerequisite to defining therapeutic strategies to regenerate hair cells in the mature inner ear.

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

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

Multiple roles of Notch signaling in cochlear development

Notch signaling inhibits hair cell differentiation, based on studies on mice deficient in Notch signaling-related genes and its downstream genes. However, the precise mechanisms of this inhibition are unknown because it is difficult to control the timing and duration of the suppression of Notch signaling. Here, we developed a novel in vitro culture and analysis method for mouse fetal cochleae and examined the roles of Notch signaling by its reversible inhibition through the use of Notch signaling inhibitors of gamma-secretase and TNF-alpha-converting enzyme. Notch inhibition with Notch signaling inhibitor treatment increases the number of cochlear hair cells, as observed in gene deletion experiments. We elucidated that this increase is regulated by the dichotomy between hair cells and supporting cells from common progenitors. We also propose other roles of Notch signaling in cochlear development. First, Notch signaling arrests the cell cycle of the cochlear epithelium containing putative hair cells and supporting cell progenitors because Notch inhibition with inhibitor treatment increases the number of 5-bromo-2'-deoxyuridine (BrdU)-positive cells that can differentiate into hair cells or supporting cells. Second, Notch signaling is required for the induction of Prox1-positive supporting cells. Third, Notch signaling is required for the maintenance of supporting cells.