Specification of cell fate in the mammalian cochlea

Pcolce2 overexpression promotes supporting cell reprogramming in the neonatal mouse cochlea

Hair cell (HC) damage is a leading cause of sensorineural hearing loss, and in mammals supporting cells (SCs) are unable to divide and regenerate HCs after birth spontaneously. Procollagen C-endopeptidase enhancer 2 (Pcolce2), which encodes a glycoprotein that acts as a functional procollagen C protease enhancer, was screened as a candidate regulator of SC plasticity in our previous study. In the current study, we used adeno-associated virus (AAV)-ie (a newly developed adeno-associated virus that targets SCs) to overexpress Pcolce2 in SCs. AAV-Pcolce2 facilitated SC re-entry into the cell cycle both in cultured cochlear organoids and in the postnatal cochlea. In the neomycin-damaged model, regenerated HCs were detected after overexpression of Pcolce2, and these were derived from SCs that had re-entered the cell cycle. These findings reveal that Pcolce2 may serve as a therapeutic target for the regeneration of HCs to treat hearing loss.

GIPC3 couples to MYO6 and PDZ domain proteins and shapes the hair cell apical region

GIPC3 has been implicated in auditory function. Initially localized to the cytoplasm of inner and outer hair cells of the cochlea, GIPC3 increasingly concentrated in cuticular plates and at cell junctions during postnatal development. Early postnatal Gipc3 ^KO/KO mice had mostly normal mechanotransduction currents, but had no auditory brainstem response at one month of age. Cuticular plates of Gipc3 ^KO/KO hair cells did not flatten during development as did those of controls; moreover, hair bundles were squeezed along the cochlear axis in mutant hair cells. Junctions between inner hair cells and adjacent inner phalangeal cells were also severely disrupted in Gipc3 ^KO/KO cochleas. GIPC3 bound directly to MYO6, and the loss of MYO6 led to altered distribution of GIPC3. Immunoaffinity purification of GIPC3 from chicken inner ear extracts identified co-precipitating proteins associated with adherens junctions, intermediate filament networks, and the cuticular plate. Several of immunoprecipitated proteins contained GIPC-family consensus PDZ binding motifs (PBMs), including MYO18A, which binds directly to the PDZ domain of GIPC3. We propose that GIPC3 and MYO6 couple to PBMs of cytoskeletal and cell-junction proteins to shape the cuticular plate.

Summary statement

The PDZ-domain protein GIPC3 couples the molecular motors MYO6 and MYO18A to actin cytoskeleton structures in hair cells. GIPC3 is necessary for shaping the hair cell’s cuticular plate and hence the arrangement of the stereocilia in the hair bundle.

Csf1 Signaling Regulates Maintenance of Resident Macrophages and Bone Formation in the Mouse Cochlea

In the mammalian cochlea, resident macrophages settle in the spiral ligament, spiral ganglion, and stria vascularis, even at the steady state. Resident macrophages in the cochlea are believed to maintain homeostasis in the inner ear and become active, as part of the front line defense, following inner ear damage. However, the exact roles of cochlear resident macrophages require further clarification. Colony stimulating factor-1 (Csf1) signaling regulates survival, proliferation, and differentiation of resident macrophages and appears to be essential for resident macrophages in the inner ear. To examine the roles of Csf1 signaling in auditory function, we examined the ossicles and inner ear of homozygous Csf1 mutant (Csf1 ^op/op ) mice. The ossicles including the incus and stapes of Csf1 ^op/op mice macroscopically demonstrated bone thickening, and the otic capsules of the inner ear were also thick and opaque. Histological analyses demonstrated that the otic capsules in Csf1 ^op/op mice were thickened and showed spongy bone degeneration. Measurements of the auditory brainstem response revealed significant elevation of thresholds in 4-week old Csf1 ^op/op mice compared with wild-type littermates, indicating that Csf1 ^op/op mice demonstrate hearing loss due to, at least in part, deformity of the ossicles and bone capsule of the inner ear. Furthermore, Csf1 ^op/op mice are deficient in the number of resident macrophages in the spiral ligament and stria vascularis, but not in the spiral ganglion. These data provide evidence that Csf1 signaling is important not only for bone formation in the inner ear, but also for the maintenance of resident macrophages in the spiral ligament and stria vascularis in the adult mouse 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.

GSK3 regulates hair cell fate in the developing mammalian cochlea

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

Stem-cell therapy for hearing loss: are we there yet?

Introduction

Mammalian hair cells and auditory neurons do not show regenerative capacity. Hence, damage to these cell types is permanent and leads to hearing loss. However, there is no treatment that re-establishes auditory function. Regenerative therapies using stem cells represent a promising alternative.

Objective

This article aims to review the current literature about the main types of stem cells with potential for application in cell therapy for sensorineural hearing loss, the most relevant experiments already performed in animals, as well as the advances that have been recently made in the field.

Methods

Research included the databases PubMed/MEDLINE, Web of Science, Science Direct and SciELO, as well as gray literature. Search strategy included the following main terms: “stem cells”, “hair cells” and “auditory neurons”. Additionally, the main terms were combined with the following secondary terms: “mesenchymal”, “iPS”, “inner ear”, “auditory”. The research was conducted independently by three researchers.

Results

Differentiation of stem cells into hair cells and auditory neurons has a high success rate, reaching up to 82% for the first and 100% for the latter. Remarkably, these differentiated cells are able to interact with hair cells and auditory neurons of cochlear explants through formation of new synapses. When transplanted into the cochlea of animals with hearing loss, auditory restoration has been documented to date only in deafferented animals.

Conclusion

Advances have been more prominent in cases of auditory neuropathy, since partial improvement of auditory nerve conditions through cell-based therapy may increase the number of patients who can successfully receive cochlear implants.

FGF signaling: diverse roles during cochlear development

Mammalian inner ear comprises of six sensory organs; cochlea, utricle, saccule, and three semicircular canals. The cochlea contains sensory epithelium known as the organ of Corti which senses sound through mechanosensory hair cells. Mammalian inner ear undergoes series of morphogenesis during development beginning thickening of ectoderm nearby hindbrain. These events require tight regulation of multiple signaling cascades including FGF, Wnt, Notch and Bmp signaling. In this review, we will discuss the role of newly emerging signaling, FGF signaling, for its roles required for cochlear development. [BMB Reports 2017; 50(10): 487-495].

Differential effects of pannexins on noise-induced hearing loss

Hearing loss, including noise-induced hearing loss, is highly prevalent and severely hinders an individual's quality of life, yet many of the mechanisms that cause hearing loss are unknown. The pannexin (Panx) channel proteins, Panx1 and Panx3, are regionally expressed in many cell types along the auditory pathway, and mice lacking Panx1 in specific cells of the inner ear exhibit hearing loss, suggesting a vital role for Panxs in hearing. We proposed that Panx1 and/or Panx3 null mice would exhibit severe hearing loss and increased susceptibility to noise-induced hearing loss. Using the auditory brainstem response, we surprisingly found that Panx1^-/- and Panx3^-/- mice did not harbor hearing or cochlear nerve deficits. Furthermore, while Panx1^-/- mice displayed no protection against loud noise-induced hearing loss, Panx3^-/- mice exhibited enhanced 16- and 24-kHz hearing recovery 7 days after a loud noise exposure (NE; 12 kHz tone, 115 dB sound pressure level, 1 h). Interestingly, Cx26, Cx30, Cx43, and Panx2 were up-regulated in Panx3^-/- mice compared with wild-type and/or Panx1^-/- mice, and assessment of the auditory tract revealed morphological changes in the middle ear bones of Panx3^-/- mice. It is unclear if these changes alone are sufficient to provide protection against loud noise-induced hearing loss. Contrary to what we expected, these data suggest that Panx1 and Panx3 are not essential for baseline hearing in mice tested, but the therapeutic targeting of Panx3 may prove protective against mid-high-frequency hearing loss caused by loud NE.

Histone demethylase KDM4B regulates otic vesicle invagination via epigenetic control of Dlx3 expression

In vertebrate embryos, the histone demethylase KDM4B affects otic placode proliferation, intercellular adhesion, and invagination by directly regulating Dlx3 expression.

In vertebrates, the inner ear arises from the otic placode, a thickened swathe of ectoderm that invaginates to form the otic vesicle. We report that histone demethylase KDM4B is dynamically expressed during early stages of chick inner ear formation. A loss of KDM4B results in defective invagination and striking morphological changes in the otic epithelium, characterized by abnormal localization of adhesion and cytoskeletal molecules and reduced expression of several inner ear markers, including Dlx3. In vivo chromatin immunoprecipitation reveals direct and dynamic occupancy of KDM4B and its target, H3K9me3, at regulatory regions of the Dlx3 locus. Accordingly, coelectroporations of DLX3 or KDM4B encoding constructs, but not a catalytically dead mutant of KDM4B, rescue the ear invagination phenotype caused by KDM4B knockdown. Moreover, a loss of DLX3 phenocopies a loss of KDM4B. Collectively, our findings suggest that KDM4B play a critical role during inner ear invagination via modulating histone methylation of the direct target Dlx3.

Nonviral Reprogramming of Human Wharton's Jelly Cells Reveals Differences Between ATOH1 Homologues

The transcription factor atonal homolog 1 (ATOH1) has multiple homologues that are functionally conserved across species and is responsible for the generation of sensory hair cells. To evaluate potential functional differences between homologues, human and mouse ATOH1 (HATH1 and MATH-1, respectively) were nonvirally delivered to human Wharton's jelly cells (hWJCs) for the first time. Delivery of HATH1 to hWJCs demonstrated superior expression of inner ear hair cell markers and characteristics than delivery of MATH-1. Inhibition of HES1 and HES5 signaling further increased the atonal effect. Transfection of hWJCs with HATH1 DNA, HES1 siRNA, and HES5 siRNA displayed positive identification of key hair cell and support cell markers found in the cochlea, as well as a variety of cell shapes, sizes, and features not native to hair cells, suggesting the need for further examination of other cell types induced by HATH1 expression. In the first side-by-side evaluation of HATH1 and MATH-1 in human cells, substantial differences were observed, suggesting that the two atonal homologues may not be interchangeable in human cells, and artificial expression of HATH1 in hWJCs requires further study. In the future, this line of research may lead to engineered systems that would allow for evaluation of drug ototoxicity or potentially even direct therapeutic use.

Signaling regulating inner ear development: cell fate determination, patterning, morphogenesis, and defects

The membranous labyrinth of the inner ear is a highly complex organ that detects sound and balance. Developmental defects in the inner ear cause congenital hearing loss and balance disorders. The membranous labyrinth consists of three semicircular ducts, the utricle, saccule, and endolymphatic ducts, and the cochlear duct. These complex structures develop from the simple otic placode, which is established in the cranial ectoderm adjacent to the neural crest at the level of the hindbrain at the early neurula stage. During development, the otic placode invaginates to form the otic vesicle, which subsequently gives rise to neurons for the vestibulocochlear ganglion, the non-sensory and sensory epithelia of the membranous labyrinth that includes three ampullary crests, two maculae, and the organ of Corti. Combined paracrine and autocrine signals including fibroblast growth factor, Wnt, retinoic acid, hedgehog, and bone morphogenetic protein regulate fate determination, axis formation, and morphogenesis in the developing inner ear. Juxtacrine signals mediated by Notch pathways play a role in establishing the sensory epithelium, which consists of mechanosensory hair cells and supporting cells. The highly differentiated organ of Corti, which consists of uniformly oriented inner/outer hair cells and specific supporting cells, develops during fetal development. Developmental alterations/arrest causes congenital malformations in the inner ear in a spatiotemporal-restricted manner. A clearer understanding of the mechanisms underlying inner ear development is important not only for the management of patients with congenital inner ear malformations, but also for the development of regenerative therapy for impaired function.

The evolution of the vertebrate cerebellum: absence of a proliferative external granule layer in a non-teleost ray-finned fish

The cerebellum represents one of the most morphologically variable structures in the vertebrate brain. To shed light on its evolutionary history, we have examined the molecular anatomy and proliferation of the developing cerebellum of the North American paddlefish, Polyodon spathula. Absence of an external proliferative cerebellar layer and the restriction of Atonal1 expression to the rhombic lip and valvular primordium demonstrate that transit amplification in a cerebellar external germinal layer, a prominent feature of amniote cerebellum development, is absent in paddlefish. Furthermore, expression of Sonic hedgehog, which drives secondary proliferation in the mouse cerebellum, is absent from the paddlefish cerebellum. These data are consistent with what has been observed in zebrafish and suggest that the transit amplification seen in the amniote cerebellum was either lost very early in the ray-finned fish lineage or evolved in the lobe-finned fish lineage. We also suggest that the Atoh1-positive proliferative valvular primordium may represent a synapomorphy (shared derived character) of ray-finned fishes. The topology of valvular primordium development in paddlefish differs significantly from that of zebrafish and correlates with the adult cerebellar form. The distribution of proliferative granule cell precursors in different vertebrate taxa is thus the likely determining factor in cerebellar morphological diversity.

Disrupted bone remodeling leads to cochlear overgrowth and hearing loss in a mouse model of fibrous dysplasia

Normal hearing requires exquisite cooperation between bony and sensorineural structures within the cochlea. For example, the inner ear secretes proteins such as osteoprotegrin (OPG) that can prevent cochlear bone remodeling. Accordingly, diseases that affect bone regulation can also result in hearing loss. Patients with fibrous dysplasia develop trabecular bone overgrowth resulting in hearing loss if the lesions affect the temporal bones. Unfortunately, the mechanisms responsible for this hearing loss, which could be sensorineural and/or conductive, remain unclear. In this study, we used a unique transgenic mouse model of increased G_s G-protein coupled receptor (GPCR) signaling induced by expression of an engineered receptor, Rs1, in osteoblastic cells. These ColI(2.3)⁺/Rs1⁺ mice showed dramatic bone lesions that histologically and radiologically resembled fibrous dysplasia. We found that ColI(2.3)⁺/Rs1⁺ mice showed progressive and severe conductive hearing loss. Ossicular chain impingement increased with the size and number of dysplastic lesions. While sensorineural structures were unaffected, ColI(2.3)⁺/Rs1⁺ cochleae had abnormally high osteoclast activity, together with elevated tartrate resistant acid phosphatase (TRAP) activity and receptor activator of nuclear factor kappa-B ligand (Rankl) mRNA expression. ColI(2.3)⁺/Rs1⁺ cochleae also showed decreased expression of Sclerostin (Sost), an antagonist of the Wnt signaling pathway that normally increases bone formation. The osteocyte canalicular networks of ColI(2.3)⁺/Rs1⁺ cochleae were disrupted and showed abnormal osteocyte morphology. The osteocytes in the ColI(2.3)⁺/Rs1⁺ cochleae showed increased expression of matrix metalloproteinase 13 (MMP-13) and TRAP, both of which can support osteocyte-mediated peri-lacunar remodeling. Thus, while the ossicular chain impingement is sufficient to account for the progressive hearing loss in fibrous dysplasia, the deregulation of bone remodeling extends to the cochlea as well. Our findings suggest that factors regulating bone remodeling, including peri-lacunar remodeling by osteocytes, may be useful targets for treating the bony overgrowths and hearing changes of fibrous dysplasia and other bony pathologies.

The potential of stem cells for the restoration of auditory function in humans

Hearing loss is one of the most common disabilities, affecting approximately 10% of the population. Hair cells and spiral ganglion neurons are usually damaged in most cases of hearing loss. Currently, there is virtually no biological approach to replace damaged hearing cells. Recent developments in stem cell technology provide new opportunities for the treatment of deafness. Two major strategies have been investigated: differentiation of endogenous stem cells into new hair cells; and introduction of exogenous cells into the inner ear to substitute injured hearing neurons. Although there is still a learning curve in stem cell-based replacement, the probability exists to utilize personalized stem cells to eventually provide a novel intervention for patients with deafness in future clinical research trials.

Round window membrane intracochlear drug delivery enhanced by induced advection

Delivery of therapeutic compounds to the inner ear via absorption through the round window membrane (RWM) has advantages over direct intracochlear infusions; specifically, minimizing impact upon functional hearing measures. However, previous reports show that significant basal-to-apical concentration gradients occur, with the potential to impact treatment efficacy. Here we present a new approach to inner ear drug delivery with induced advection aiding distribution of compounds throughout the inner ear in the murine cochlea. Polyimide microtubing was placed near the RWM niche through a bullaostomy into the middle ear cavity allowing directed delivery of compounds to the RWM. We hypothesized that a posterior semicircular canalostomy would induce apical flow from the patent cochlear aqueduct to the canalostomy due to influx of cerebral spinal fluid. To test this hypothesis, young adult CBA/CaJ mice were divided into two groups: bullaostomy approach only (BA) and bullaostomy+canalostomy (B+C). Cochlear function was evaluated by distortion product otoacoustic emission (DPOAE) and auditory brainstem response (ABR) thresholds during and after middle ear infusion of salicylate in artificial perilymph (AP), applied near the RWM. The mice recovered for 1week, and were re-tested. The results demonstrate there was no significant impact on auditory function utilizing the RWM surgical procedure with or without the canalostomy, and DPOAE thresholds were elevated reversibly during the salicylate infusion. Comparing the threshold shifts for both methods, the B+C approach had more of a physiological effect than the BA approach, including at lower frequencies representing more apical cochlear locations. Unlike mouse cochleostomies, there was no deleterious auditory functional impact after 1week recovery from surgery. The B+C approach had more drug efficacy at lower frequencies, underscoring potential benefits for more precise control of delivery of inner ear therapeutic compounds.

Hedgehog signaling regulates prosensory cell properties during the basal-to-apical wave of hair cell differentiation in the mammalian cochlea

Mechanosensory hair cells and supporting cells develop from common precursors located in the prosensory domain of the developing cochlear epithelium. Prosensory cell differentiation into hair cells or supporting cells proceeds from the basal to the apical region of the cochleae, but the mechanism and significance of this basal-to-apical wave of differentiation remain to be elucidated. Here, we investigated the role of Hedgehog (Hh) signaling in cochlear development by examining the effects of up- and downregulation of Hh signaling in vivo. The Hh effector smoothened (Smo) was genetically activated or inactivated specifically in the developing cochlear epithelium after prosensory domain formation. Cochleae expressing a constitutively active allele of Smo showed only one row of inner hair cells with no outer hair cells (OHCs); abnormal undifferentiated prosensory-like cells were present in the lateral compartment instead of OHCs and their adjacent supporting cells. This suggests that Hh signaling inhibits prosensory cell differentiation into hair cells or supporting cells and maintains their properties as prosensory cells. Conversely, in cochlea with the Smo conditional knockout (Smo CKO), hair cell differentiation was preferentially accelerated in the apical region. Smo CKO mice survived after birth, and exhibited hair cell disarrangement in the apical region, a decrease in hair cell number, and hearing impairment. These results indicate that Hh signaling delays hair cell and supporting cell differentiation in the apical region, which forms the basal-to-apical wave of development, and is required for the proper differentiation, arrangement and survival of hair cells and for hearing ability.

Induction of inner ear hair cell-like cells from Math1-transfected mouse ES cells

Math1, a basic helix-loop-helix transcription factor homolog of the Drosophila atonal gene, is considered to be a key factor for induction of sensory hair cells (HCs) during development of the organ of Corti or cochlea. Although embryonic stem (ES) cells are able to produce HC-like cells, the role of Math1 in induction of those cells has not been thoroughly elucidated. In the present study, we introduced Math1 into ES cells in order to achieve efficient generation of HC-like cells. ES cells carrying Tet-inducible Math1, Math1-ES cells, were generated using a Tet-On gene expression system. Embryoid bodies (EBs) formed in the absence of doxycycline (Dox) for 4 days were allowed to grow for an additional 14 days in the dishes in the presence of 400 μg/ml of Dox. At the end of those 14-day cultures, approximately 10% of the cells in EB outgrowths expressed the HC-related markers myosin6, myosin7a, calretinin, α9AchR, and Brn3c (also known as Pou4f3) and showed formation of stereocilia-like structures, whereas few cells in EB outgrowths grown without Dox showed those markers. Reporter assays of Math1-ES cells using a Brn3c-promoter plasmid demonstrated positive regulation of Brn3c by Math1. Furthermore, such HC-related marker-positive cells derived from Math1-ES cells were found to be incorporated in the developing inner ear after transplantation into chick embryos. Math1-ES cells are considered to be an efficient source of ES-derived HC-like cells, and Math1 may be an important factor for induction of HC-like cells from differentiating ES cells.

Making connections in the inner ear: recent insights into the development of spiral ganglion neurons and their connectivity with sensory hair cells

In mammals, auditory information is processed by the hair cells (HCs) located in the cochlea and then rapidly transmitted to the CNS via a specialized cluster of bipolar afferent connections known as the spiral ganglion neurons (SGNs). Although many anatomical aspects of SGNs are well described, the molecular and cellular mechanisms underlying their genesis, how they are precisely arranged along the cochlear duct, and the guidance mechanisms that promote the innervation of their hair cell targets are only now being understood. Building upon foundational studies of neurogenesis and neurotrophins, we review here new concepts and technologies that are helping to enrich our understanding of the development of the nervous system within the inner ear.

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.

An in vitro mouse model of congenital cytomegalovirus-induced pathogenesis of the inner ear cochlea

Congenital human cytomegalovirus (CMV) infection is the leading nongenetic etiology of sensorineural hearing loss (SNHL) at birth and prelingual SNHL not expressed at birth. The paucity of temporal bone autopsy specimens from infants with congenital CMV infection has hindered the critical correlation of histopathology with pathogenesis. Here, we present an in vitro embryonic mouse model of CMV-infected cochleas that mimics the human sites of viral infection and associated pathology. There is a striking dysplasia/hyperplasia in mouse CMV-infected cochlear epithelium and mesenchyme, including organ of Corti hair and supporting cells and stria vascularis. This is concomitant with significant dysregulation of p19, p21, p27, and Pcna gene expression, as well as proliferating cell nuclear antigen (PCNA) protein expression. Other pathologies similar to those arising from known deafness gene mutations include downregulation of KCNQ1 protein expression in the stria vascularis, as well as hypoplastic and dysmorphic melanocytes. Thus, this model provides a relevant and reliable platform within which the detailed cell and molecular biology of CMV-induced deafness may be studied.

Early development of the vertebrate inner ear

This is a review of the biological processes and the main signaling pathways required to generate the different otic cell types, with particular emphasis on the actions of insulin-like growth factor I. The sensory organs responsible of hearing and balance have a common embryonic origin in the otic placode. Lineages of neural, sensory, and support cells are generated from common otic neuroepithelial progenitors. The sequential generation of the cell types that will form the adult inner ear requires the coordination of cell proliferation with cell differentiation programs, the strict regulation of cell survival, and the metabolic homeostasis of otic precursors. A network of intracellular signals operates to coordinate the transcriptional response to the extracellular input. Understanding the molecular clues that direct otic development is fundamental for the design of novel treatments for the protection and repair of hearing loss and balance disorders.

A mutation in the Srrm4 gene causes alternative splicing defects and deafness in the Bronx waltzer mouse

Sensory hair cells are essential for hearing and balance. Their development from epithelial precursors has been extensively characterized with respect to transcriptional regulation, but not in terms of posttranscriptional influences. Here we report on the identification and functional characterization of an alternative-splicing regulator whose inactivation is responsible for defective hair-cell development, deafness, and impaired balance in the spontaneous mutant Bronx waltzer (bv) mouse. We used positional cloning and transgenic rescue to locate the bv mutation to the splicing factor-encoding gene Ser/Arg repetitive matrix 4 (Srrm4). Transcriptome-wide analysis of pre–mRNA splicing in the sensory patches of embryonic inner ears revealed that specific alternative exons were skipped at abnormally high rates in the bv mice. Minigene experiments in a heterologous expression system confirmed that these skipped exons require Srrm4 for inclusion into the mature mRNA. Sequence analysis and mutagenesis experiments showed that the affected transcripts share a novel motif that is necessary for the Srrm4-dependent alternative splicing. Functional annotations and protein–protein interaction data indicated that the encoded proteins cluster in the secretion and neurotransmission pathways. In addition, the splicing of a few transcriptional regulators was found to be Srrm4 dependent, and several of the genes known to be targeted by these regulators were expressed at reduced levels in the bv mice. Although Srrm4 expression was detected in neural tissues as well as hair cells, analyses of the bv mouse cerebellum and neocortex failed to detect splicing defects. Our data suggest that Srrm4 function is critical in the hearing and balance organs, but not in all neural tissues. Srrm4 is the first alternative-splicing regulator to be associated with hearing, and the analysis of bv mice provides exon-level insights into hair-cell development.

The identification of novel deafness-causing mutations has been instrumental in revealing unexpected mechanisms that are required for development of the sound- and gravity-sensing hair cells of the inner ear. The Bronx waltzer (bv) mouse is characterized by defects in hair-cell development, as well as by deafness and impaired balance. Here, we report on our identification of a mutation in the Ser/Arg repetitive matrix 4 (Srrm4) gene as the source of these defects. The encoded protein, Srrm4, belongs to a family of RNA splicing factors that regulate the inclusion of certain genetic information (i.e. alternative exons) into the transcribed RNA. We analyzed the molecular function of Srrm4 by comparing the exon composition of RNAs in the inner ear of bv and control mice. This approach revealed that, in the bv mice, specific alternative exons were omitted from protein-encoding RNAs. The affected transcripts shared two features: they contained a short sequence motif that was required for Srrm4-dependent splicing, and they encoded proteins that were related predominantly to secretion and neurotransmission. In addition, RNAs of a few gene expression regulators contained Srrm4-regulated exons. Our data suggest that Srrm4-dependent alternative splicing has a profound effect on the developmental program of hair cells.

Electrosensory ampullary organs are derived from lateral line placodes in cartilaginous fishes

Ampullary organ electroreceptors excited by weak cathodal electric fields are used for hunting by both cartilaginous and non-teleost bony fishes. Despite similarities of neurophysiology and innervation, their embryonic origins remain controversial: bony fish ampullary organs are derived from lateral line placodes, whereas a neural crest origin has been proposed for cartilaginous fish electroreceptors. This calls into question the homology of electroreceptors and ampullary organs in the two lineages of jawed vertebrates. Here, we test the hypothesis that lateral line placodes form electroreceptors in cartilaginous fishes by undertaking the first long-term in vivo fate-mapping study in any cartilaginous fish. Using DiI tracing for up to 70 days in the little skate, Leucoraja erinacea, we show that lateral line placodes form both ampullary electroreceptors and mechanosensory neuromasts. These data confirm the homology of electroreceptors and ampullary organs in cartilaginous and non-teleost bony fishes, and indicate that jawed vertebrates primitively possessed a lateral line placode-derived system of electrosensory ampullary organs and mechanosensory neuromasts.

Otic mesenchyme cells regulate spiral ganglion axon fasciculation through a Pou3f4/EphA4 signaling pathway

Peripheral axons from auditory spiral ganglion neurons (SGNs) form an elaborate series of radially and spirally oriented projections that interpret complex aspects of the auditory environment. However, the developmental processes that shape these axon tracts are largely unknown. Radial bundles are comprised of dense SGN fascicles that project through otic mesenchyme to form synapses within the cochlea. Here, we show that radial bundle fasciculation and synapse formation are disrupted when Pou3f4 (DFNX2) is deleted from otic mesenchyme. Further, we demonstrate that Pou3f4 binds to and directly regulates expression of Epha4, Epha4⁻/⁻ mice present similar SGN defects, and exogenous EphA4 promotes SGN fasciculation in the absence of Pou3f4. Finally, Efnb2 deletion in SGNs leads to similar fasciculation defects, suggesting that ephrin-B2/EphA4 interactions are critical during this process. These results indicate a model whereby Pou3f4 in the otic mesenchyme establishes an Eph/ephrin-mediated fasciculation signal that promotes inner radial bundle formation.

Insulin-like growth factor signaling regulates the timing of sensory cell differentiation in the mouse cochlea

The mammalian auditory sensory epithelium, the organ of Corti, is a highly ordered cellular structure that comprises two types of auditory hair cells and several types of nonsensory supporting cells. During embryogenesis, a stereotyped sequence of cellular and molecular events is required for its development. These processes are assumed to be regulated by multiple growth and transcription factors. However, the majority of these factors have not been identified. One potential regulator of cochlear development is the insulin-like growth factor (IGF) signaling family. To examine the roles of the IGF pathway in inner ear formation, cochleae from Igf1r mutant mice were analyzed. Deletion of Igf1r leads to several changes in inner ear development including a shortened cochlear duct, a decrease in the total number of cochlear hair cells, and defects in the formation of the semicircular canals. In addition, maturation of the cochlear sensory epithelium was delayed at the transition point between cellular proliferation and differentiation. To determine the molecular basis for these defects, inhibition of IGF signaling was replicated pharmacologically in vitro. Results indicated that IGF signaling regulates cochlear length and hair cell number as well as Atoh1 expression through the phosphatidylinositol 3-kinase/Akt signaling pathway. These results demonstrate novel roles for IGF signaling in inner ear development including regulation of vestibular formation, length of the cochlear duct, and the number of cochlear hair cells. The results also provide new insights regarding the pathological processes that underlie auditory defects in the absence of IGF signaling in both humans and mice.

Planar cell polarity in the inner ear

The inner ears of vertebrates represent one of the most striking examples of planar cell polarity (PCP). Populations of directionally sensitive mechanosensory hair cells develop actin-based stereociliary bundles that are uniformly oriented. Analysis of perturbations in bundle polarity in mice with mutations in Vangl2 formed the basis for the initial demonstration of conservation of the PCP signaling pathway in vertebrates. Subsequent studies have demonstrated roles for other "core" PCP genes, such as Frizzled, Disheveled, and Celsr, and for identifying novel PCP molecules such as Scribble and Ptk7. In addition, the demonstration of hearing deficits in humans with mutations in cilia genes combined with analysis of PCP defects in mice with ciliary deletion has implicated the cilia as an important modulator of hair cell polarization. Finally, the presence of shortened cochleae in many PCP mouse mutants has revealed an additional role for the PCP pathway in the development of the auditory system.

Sensory epithelial cells acquire features of prosensory cells via epithelial to mesenchymal transition

Epithelial to mesenchymal transition (EMT) plays a critical role during normal development and in adult tissue repair. It is known that immortalized epithelial cells can undergo an EMT and become cancer stem cells, and that epithelial cells from mouse pancreatic islet and avian inner ear can acquire mesenchymal traits in vitro via EMT. However, it is unclear whether epithelial cells from mammalian sensory system can undergo an EMT and obtain features of stem/progenitor cells. In this study, we used mouse utricle sensory epithelial cells (MUCs) as a mammalian cell model to address this issue. When cultured on 2-dimensional substrates, dissociated MUCs gradually lost their columnar shape and started to expand on the substrate with downregulation of expression of epithelial junction markers and upregulation of genes and proteins that are widely shown in mesenchymal cells. Moreover, MUCs expressed genes and proteins that are usually presented in prosensory epithelial cells and stem cells. These MUCs showed potential to differentiate into epithelial cells via a reverse EMT when they were forced to suspend in culture medium. Our findings reveal that sensory epithelial cells from mammalian tissue can undergo an EMT to become cells expressing features of stem cells that can be induced to become epithelial cells via a reverse EMT. The outcomes of this study may provide a novel approach to generate epithelial progenitors for use in cell replacement therapy to treat a number of human diseases, such as hearing loss and vision loss.

Biotechnology in the treatment of sensorineural hearing loss: foundations and future of hair cell regeneration

PURPOSE

To provide an overview of the methodologies involved in the field of hair cell regeneration. First, the author provides a tutorial on the biotechnological foundations of this field to assist the reader in the comprehension and interpretation of the research involved in hair cell regeneration. Next, the author presents a review of stem cell and gene therapy and provides a critical appraisal of their application to hair cell regeneration. The methodologies used in these approaches are highlighted.

METHOD

The author conducted a narrative review of the fields of cellular, molecular, and developmental biology, tissue engineering, and stem cell and gene therapy using the PubMed database.

RESULTS

The use of biotechnological approaches to the treatment of hearing loss--approaches such as stem cell and gene therapy-has led to new methods of regenerating cochlear hair cells in mammals.

CONCLUSIONS

Incredible strides have been made in assembling important pieces of the puzzle that comprise hair cell regeneration. However, mammalian hair cell regeneration using stem cell and gene therapy are years--if not decades--away from being clinically feasible. If the goals of the biological approaches are met, these therapies may represent future treatments for hearing loss.

Regulated reprogramming in the regeneration of sensory receptor cells

Vision, olfaction, hearing, and balance are mediated by receptors that reside in specialized sensory epithelial organs. Age-related degeneration of the photoreceptors in the retina and the hair cells in the cochlea, caused by macular degeneration and sensorineural hearing loss, respectively, affect a growing number of individuals. Although sensory receptor cells in the mammalian retina and inner ear show only limited or no regeneration, in many nonmammalian vertebrates, these sensory epithelia show remarkable regenerative potential. We summarize the current state of knowledge of regeneration in the specialized sense organs in both nonmammalian vertebrates and mammals and discuss possible areas where new advances in regenerative medicine might provide approaches to successfully stimulate sensory receptor cell regeneration. The field of regenerative medicine is still in its infancy, but new approaches using stem cells and reprogramming suggest ways in which the potential for regeneration may be restored in individuals suffering from sensory loss.

TAK1 expression in the cochlea: a specific marker for adult supporting cells

Transforming growth factor-β-activated kinase-1 (TAK1) is a mitogen activated protein kinase kinase kinase that is involved in diverse biological roles across species. Functioning downstream of TGF-β and BMP signaling, TAK1 mediates the activation of the c-Jun N-terminal kinase signaling pathway, serves as the target of pro-inflammatory cytokines, such as TNF-α, mediates NF-κβ activation, and plays a role in Wnt/Fz signaling in mesenchymal stem cells. Expression of TAK1 in the cochlea has not been defined. Data mining of previously published murine cochlear gene expression databases indicated that TAK1, along with TAK1 interacting proteins 1 (TAB1), and 2 (TAB2), is expressed in the developing and adult cochlea. The expression of TAK1 in the developing cochlea was confirmed using RT-PCR and immunohistochemistry. Immunolabeling of TAK1 in embryonic, neonatal, and mature cochleas via DAB chromogenic and fluorescent immunohistochemistry indicated that TAK1 is broadly expressed in both the developing otocyst and periotic mesenchyme at E12.5 but becomes more restricted to specific types of supporting cells as the organ of Corti matures. By P1, TAK1 immunolabeling is found in cells of the stria vascularis, hair cells, supporting cells, and Kölliker's organ. By P16, TAK1 labeling is limited to cochlear supporting cells. In the adult cochlea, TAK1 immunostaining is only present in the cytoplasm of Deiters' cells, pillar cells, inner phalangeal cells, and inner border cells, with no expression in any other cochlear cell types. While the role of TAK1 in the inner ear is unclear, TAK1 expression may be used as a novel marker for specific sub-populations of supporting cells.

Cooperative functions of Hes/Hey genes in auditory hair cell and supporting cell development

Notch-mediated lateral inhibition has been reported to regulate auditory hair cell and supporting cell development from common precursors. While the Notch effector genes Hes1, Hes5 and Hey1 are expressed in the developing cochlea, inactivation of either of them causes only mild abnormality, suggesting their functional redundancy. To explore the roles of Hes/Hey genes in cochlear development, we examined compound heterozygous or homozygous mutant mice that lacked Hes1, Hes5 and Hey1 alleles. We found that a reduction in Hes/Hey gene dosage led to graded increase of hair cell formation. However, if at least one allele of Hes1, Hes5 or Hey1 was intact, excessive hair cells were accompanied by overproduction of supporting cells, suggesting that the hair cell increase does not occur at the expense of supporting cells, and that each Hes/Hey gene functions to induce supporting cells. By contrast, when all alleles of Hes1, Hes5 and Hey1 were inactivated, the number of hair cells increased more drastically, whereas that of supporting cells was unchanged compared with control, suggesting that supporting cell formation was balanced by their overproduction and fate conversion into hair cells. The increase of the cell numbers seemed to occur after the prosensory domain formation in the mutants because the proliferation state and the size of the prosensory domain were not affected. Thus, Hes1, Hes5 and Hey1 cooperatively inhibit hair cell formation, and one allele of Hes1, Hes5 or Hey1 is sufficient for supporting cell production probably by lateral inhibition in the sensory epithelium. Strikingly, Hes/Hey mutations lead to disorganized cell alignment and polarity and to hearing loss despite hair cell overproduction. These results suggest that Hes/Hey gene dosage is essential not only for generation of appropriate numbers of hair cells and supporting cells by controlling cell proliferation and lateral inhibition but also for the hearing ability by regulating the cell alignment and polarity.

Connecting the ear to the brain: Molecular mechanisms of auditory circuit assembly

Our sense of hearing depends on precisely organized circuits that allow us to sense, perceive, and respond to complex sounds in our environment, from music and language to simple warning signals. Auditory processing begins in the cochlea of the inner ear, where sounds are detected by sensory hair cells and then transmitted to the central nervous system by spiral ganglion neurons, which faithfully preserve the frequency, intensity, and timing of each stimulus. During the assembly of auditory circuits, spiral ganglion neurons establish precise connections that link hair cells in the cochlea to target neurons in the auditory brainstem, develop specific firing properties, and elaborate unusual synapses both in the periphery and in the CNS. Understanding how spiral ganglion neurons acquire these unique properties is a key goal in auditory neuroscience, as these neurons represent the sole input of auditory information to the brain. In addition, the best currently available treatment for many forms of deafness is the cochlear implant, which compensates for lost hair cell function by directly stimulating the auditory nerve. Historically, studies of the auditory system have lagged behind other sensory systems due to the small size and inaccessibility of the inner ear. With the advent of new molecular genetic tools, this gap is narrowing. Here, we summarize recent insights into the cellular and molecular cues that guide the development of spiral ganglion neurons, from their origin in the proneurosensory domain of the otic vesicle to the formation of specialized synapses that ensure rapid and reliable transmission of sound information from the ear to the brain.

Retention of progenitor cell phenotype in otospheres from guinea pig and mouse cochlea

BACKGROUND

Culturing otospheres from dissociated organ of Corti is an appropriate starting point aiming at the development of cell therapy for hair cell loss. Although guinea pigs have been widely used as an excellent experimental model for studying the biology of the inner ear, the mouse cochlea has been more suitable for yielding otospheres in vitro. The aim of this study was to compare conditions and outcomes of otosphere suspension cultures from dissociated organ of Corti of either mouse or guinea pig at postnatal day three (P3), and to evaluate the guinea pig as a potential cochlea donor for preclinical cell therapy.

METHODS

Organs of Corti were surgically isolated from P3 guinea pig or mouse cochlea, dissociated and cultivated under non-adherent conditions. Cultures were maintained in serum-free DMEM:F12 medium, supplemented with epidermal growth factor (EGF) plus either basic fibroblast growth factor (bFGF) or transforming growth factor alpha (TGFα). Immunofluorescence assays were conducted for phenotype characterization.

RESULTS

The TGFα group presented a number of spheres significantly higher than the bFGF group. Although mouse cultures yielded more cells per sphere than guinea pig cultures, sox2 and nestin distributed similarly in otosphere cells from both organisms. We present evidence that otospheres retain properties of inner ear progenitor cells such as self-renewal, proliferation, and differentiation into hair cells or supporting cells.

CONCLUSIONS

Dissociated guinea pig cochlea produced otospheres in vitro, expressing sox2 and nestin similarly to mouse otospheres. Our data is supporting evidence for the presence of inner ear progenitor cells in the postnatal guinea pig. However, there is limited viability for these cells in neonatal guinea pig cochlea when compared to the differentiation potential observed for the mouse organ of Corti at the same developmental stage.