Role of Gbx2 and Otx2 in the formation of cochlear ganglion and endolymphatic duct

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

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

Regulation of otocyst patterning by Tbx2 and Tbx3 is required for inner ear morphogenesis in the mouse

All epithelial components of the inner ear, including sensory hair cells and innervating afferent neurons, arise by patterning and differentiation of epithelial progenitors residing in a simple sphere, the otocyst. Here, we identify the transcriptional repressors TBX2 and TBX3 as novel regulators of these processes in the mouse. Ablation of Tbx2 from the otocyst led to cochlear hypoplasia, whereas loss of Tbx3 was associated with vestibular malformations. The loss of function of both genes (Tbx2/3cDKO) prevented inner ear morphogenesis at midgestation, resulting in indiscernible cochlear and vestibular structures at birth. Morphogenetic impairment occurred concomitantly with increased apoptosis in ventral and lateral regions of Tbx2/3cDKO otocysts around E10.5. Expression analyses revealed partly disturbed regionalisation, and a posterior-ventral expansion of the neurogenic domain in Tbx2/3cDKO otocysts at this stage. We provide evidence that repression of FGF signalling by TBX2 is important to restrict neurogenesis to the anterior-ventral otocyst and implicate another T-box factor, TBX1, as a crucial mediator in this regulatory network.

Time-lapse imaging system with shell-less culture chamber

We have developed a system for imaging whole chick embryos from embryonic day 1.5 (E1.5) to E4.5. Our system consists of a custom-made culture chamber, the top and bottom of which were heated and the inside was humidified. The system also has a fixed stage uplight fluorescent microscope, and long-working distance objective lenses were adopted. The albumen removed-yolk with the embryo in the dish was put in the chamber. It is of importance that we adopted long working distance lenses because the working distance of conventional objective lenses is too short for observation of the embryo in a humidified chamber. The objective lens we adopted has sufficient resolution to detect fluorescent protein expression at the single-cell level. Transparent glass heater set on the top of the chamber helps to reduce dew condensation; the bottom heater keeps the temperature inside the chamber, and the water bath surrounding the egg maintains humidity. This system was used to detect fluorescent protein expressing cells in embryos. We could successfully trace those cells for 17 h in vivo. In conclusion, this system is useful for time-lapse analysis of fluorescent protein expression and distribution for a longer period of time.

Inner ear hair cell-like cells from human embryonic stem cells

In mammals, the permanence of many forms of hearing loss is the result of the inner ear's inability to replace lost sensory hair cells. Here, we apply a differentiation strategy to guide human embryonic stem cells (hESCs) into cells of the otic lineage using chemically defined attached-substrate conditions. The generation of human otic progenitor cells was dependent on fibroblast growth factor (FGF) signaling, and protracted culture led to the upregulation of markers indicative of differentiated inner ear sensory epithelia. Using a transgenic ESC reporter line based on a murine Atoh1 enhancer, we show that differentiated hair cell-like cells express multiple hair cell markers simultaneously. Hair cell-like cells displayed protrusions reminiscent of stereociliary bundles, but failed to fully mature into cells with typical hair cell cytoarchitecture. We conclude that optimized defined conditions can be used in vitro to attain otic progenitor specification and sensory cell differentiation.

Fgf10 expression patterns in the developing chick inner ear

The inner ear is a complex three-dimensional sensorial structure with auditory and vestibular functions. It originates from the otic placode, which invaginates, forming the otic vesicle; the latter gives rise to neurosensory and nonsensory elements of the adult membranous labyrinth. A hypothesis based on descriptive and experimental evidence suggests that the acquisition of discrete sensory patches during evolution of this primordium may be related to subdivision of an early pansensory domain. In order to gain insight into this developmental mechanism, we carried out a detailed analysis of the spatial and temporal expression pattern of the gene Fgf10, by comparing different markers of otic patterning and hair cell differentiation. Fgf10 expression labels a sensory-competent domain included in a Serrate-positive territory from which most of the sensory epithelia arise. Our data show that Fgf10 transcripts are present initially in a narrow ventromedial band of the rudimentary otocyst, extending between its rostral and caudal poles. During development, this Fgf10-expressing area splits repetitively into several separate subareas, creating six of the eight sensory organs present in birds. Only the lateral crista and the macula neglecta were initially Fgf10 negative, although they activated Fgf10 expression after their specification as sensory elements. These results allowed us to determine a timetable of sensory specification in the developing chick inner ear. The comparison of the expression pattern of Fgf10 with those of other markers of sensory differentiation contributes to our understanding of the mechanism by which vertebrate inner ear prosensory domains have arisen during evolution.

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

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

Conditional deletion of N-Myc disrupts neurosensory and non-sensory development of the ear

Ear development requires interactions of transcription factors for proliferation and differentiation. The proto-oncogene N-Myc is a member of the Myc family that regulates proliferation. To investigate the function of N-Myc, we conditionally knocked out N-Myc in the ear using Tg(Pax2-Cre) and Foxg1(KiCre). N-Myc CKOs had reduced growth of the ear, abnormal morphology including fused sensory epithelia, disrupted histology, and disorganized neuronal innervation. Using Thin-Sheet Laser Imaging Microscopy (TSLIM), 3D reconstruction and quantification of the cochlea revealed a greater than 50% size reduction. Immunochemistry and in situ hybridization showed a gravistatic organ-cochlear fusion and a "circularized" apex with no clear inner and outer hair cells. Furthermore, the abnormally developed cochlea had cross innervation from the vestibular ganglion near the basal tip. These findings are put in the context of the possible functional relationship of N-Myc with a number of other cell proliferative and fate determining genes during ear development.

A DNA variant within the MYO7A promoter regulates YY1 transcription factor binding and gene expression serving as a potential dominant DFNA11 auditory genetic modifier

Mutations within MYO7A can lead to recessive and dominant forms of inherited hearing loss. We previously identified a large pedigree (referred to as the HL2 family) with hearing loss that first impacts the low and mid frequencies segregating a dominant MYO7A mutation in exon 17 at DNA residue G2164C. The MYO7A(G2164C) mutation predicts a nonconservative glycine-to-arginine (G722R) amino acid substitution at a highly conserved glycine residue. The degree of low and mid frequency hearing loss varies markedly in the family, suggesting the presence of a genetic modifier that either rescues or exacerbates the primary MYO7A(G2164C) mutation. Here we describe a single nucleotide polymorphism (SNP) T/C at position -4128 in the wild-type MYO7A promoter allele that sorts with the degree of hearing loss severity in the pedigree. Electrophoretic mobility shift assay analysis indicates that the SNP differentially regulates the binding of the YY1 transcription factor with the T(-4128) allele creating an YY1 binding site. Immunocytochemistry demonstrates that Yy1 is expressed in hair cell nuclei within the cochlea. Given that Myo7a is also expressed in cochlear hair cells, Yy1 shows the appropriate localization to regulate Myo7a transcription within the inner ear. YY1 appears to be acting as a transcriptional repressor as the MYO7A promoter allele containing the T(-4128) SNP drives 41 and 46% less reporter gene expression compared with the C(-4128) SNP in the ARPE-19 and HeLa cell lines, respectively. The T(-4128) SNP may be contributing to the severe hearing loss phenotype in the HL2 pedigree by reducing expression of the wild-type MYO7A allele.

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

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

Identification of differentially expressed genes in early inner ear development

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

Epithelial-mesenchymal transition as a possible mechanism of semicircular canal morphogenesis in chick inner ear

Semicircular canals are sensory organs for balance, consisting of fluid-filled tubules that are arranged perpendicularly to each other in inner ear. The precise mechanism of the morphogenesis of this unique organ is still under investigation. Semicircular canals arise from the flattened pouch of epithelium. The centers of two apposing wall of the pouch approach each other and form a fusion plate. The clearing of the fusion plate makes a hole and leaves the remaining tissue as semicircular canals. Three mechanisms have been proposed for this clearing: programmed cell death, epithelial-mesenchymal transition, and retraction of the cells in the fusion plate to surrounding semicircular canals. Previous studies have revealed programmed cell death in the fusion plate, although other two hypotheses were not disproved. Here we examined the contribution of epithelial-mesenchymal transition and epithelial retraction to the morphogenesis of semicircular canals. We analyzed immunohistochemically the structural change in the epithelium of the developing fusion plate using molecular markers, basal lamina component laminin, cytoskeletal F-actin, and cellular junctional marker beta-catenin. Our observation revealed that fusion plate epithelium lost its apico-basal polarity and intermingled with facing fusion plate cells, associated with the disruption of basal lamina. Moreover, there were several cells with mesenchymal appearance adjacent to the torn basal lamina. We also found the merging of apposing basal laminae at the border between forming canal and breaking fusion plate. These observations suggest that the epithelial-mesenchymal transition, rather than the epithelial retraction, may be responsible for clearing fusion plate cells.

LIF promotes neurogenesis and maintains neural precursors in cell populations derived from spiral ganglion stem cells

Background

Stem cells with the ability to form clonal floating colonies (spheres) were recently isolated from the neonatal murine spiral ganglion. To further examine the features of inner ear-derived neural stem cells and their derivatives, we investigated the effects of leukemia inhibitory factor (LIF), a neurokine that has been shown to promote self-renewal of other neural stem cells and to affect neural and glial cell differentiation.

Results

LIF-treatment led to a dose-dependent increase of the number of neurons and glial cells in cultures of sphere-derived cells. Based on the detection of developmental and progenitor cell markers that are maintained in LIF-treated cultures and the increase of cycling nestin-positive progenitors, we propose that LIF maintains a pool of neural progenitor cells. We further provide evidence that LIF increases the number of nestin-positive progenitor cells directly in a cell cycle-independent fashion, which we interpret as an acceleration of neurogenesis in sphere-derived progenitors. This effect is further enhanced by an anti-apoptotic action of LIF. Finally, LIF and the neurotrophins BDNF and NT3 additively promote survival of stem cell-derived neurons.

Conclusion

Our results implicate LIF as a powerful tool to control neural differentiation and maintenance of stem cell-derived murine spiral ganglion neuron precursors. This finding could be relevant in cell replacement studies with animal models featuring spiral ganglion neuron degeneration. The additive effect of the combination of LIF and BDNF/NT3 on stem cell-derived neuronal survival is similar to their effect on primary spiral ganglion neurons, which puts forward spiral ganglion-derived neurospheres as an in vitro model system to study aspects of auditory neuron development.

Role of Bone morphogenetic protein 4 in zebrafish semicircular canal development

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