Hydrogen peroxide toxicity on auditory cells: An in vitro study

In recent decades, interest has increased in the role of reactive oxygen species (ROS) in health and disease. The ROS are key causative factors in several hearing loss pathologies including ototoxicity, noise trauma, cochlear ageing and ischemic injury. In order to investigate ROS effects on inner ear cells and counteract them, we developed an in vitro model of oxidative stress by exposing the inner ear cell line OC-k3 to hydrogen peroxide (H₂O₂) at concentrations able to affect in vivo cellular components but allowing cell survival. The treatment with high concentrations (20 and 30 μM) resulted in reduction of cell viability, activation of apoptosis/necrosis and alteration of morphology, cell cycle progression and antioxidant defences. The ROS effects in inner ear cells are difficult to assess in vivo. Organocultures may provide preservation of tissue architecture but involve ethical issues and can be used only for a limited time. An in vitro model that could be commercially available and easy to handle is necessary to investigate inner ear oxidative stress and the ways to counteract it. The OC-k3 line is a suitable in vitro model to study ROS effects on inner ear cells because the observed cell alterations and damages were similar to those reported in studies investigating ROS effects of ototoxic drugs, noise trauma and cochlear ageing.

Chimera states and frequency clustering in systems of coupled inner-ear hair cells

Coupled hair cells of the auditory and vestibular systems perform the crucial task of converting the energy of sound waves and ground-borne vibrations into ionic currents. We mechanically couple groups of living, active hair cells with artificial membranes, thus mimicking in vitro the coupled dynamical system. We identify chimera states and frequency clustering in the dynamics of these coupled nonlinear, autonomous oscillators. We find that these dynamical states can be reproduced by our numerical model with heterogeneity of the parameters. Furthermore, we find that this model is most sensitive to external signals when poised at the onset of synchronization, where chimera and cluster states are likely to form. We, therefore, propose that the partial synchronization in our experimental system is a manifestation of a system poised at the verge of synchronization with optimal sensitivity.

Progenitor Cells from the Adult Human Inner Ear

Loss of inner ear hair cells leads to incurable balance and hearing disorders because these sensory cells do not effectively regenerate in humans. A potential starting point for therapy would be the stimulation of quiescent progenitor cells within the damaged inner ear. Inner ear progenitor/stem cells, which have been described in rodent inner ears, would be principal candidates for such an approach. Despite the identification of progenitor cell populations in the human fetal cochlea and in the adult human spiral ganglion, no proliferative cell populations with the capacity to generate hair cells have been reported in vestibular and cochlear tissues of adult humans. The present study aimed at filling this gap by isolating colony-forming progenitor cells from surgery- and autopsy-derived adult human temporal bones in order to generate inner ear cell types in vitro. Sphere-forming and mitogen-responding progenitor cells were isolated from vestibular and cochlear tissues. Clonal spheres grown from adult human utricle and cochlear duct were propagated for a limited number of generations. When differentiated in absence of mitogens, the utricle-derived spheres robustly gave rise to hair cell-like cells, as well as to cells expressing supporting cell-, neuron-, and glial markers, indicating that the adult human utricle harbors multipotent progenitor cells. Spheres derived from the adult human cochlear duct did not give rise to hair cell-like or neuronal cell types, which is an indication that human cochlear cells have limited proliferative potential but lack the ability to differentiate into major inner ear cell types. Anat Rec, 303:461-470, 2020. © 2019 The Authors. The Anatomical Record published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.

Early ear neuronal development, but not olfactory or lens development, can proceed without SOX2

SOX2 is essential for maintaining neurosensory stem cell properties, although its involvement in the early neurosensory development of cranial placodes remains unclear. To address this, we used Foxg1-Cre to conditionally delete Sox2 during eye, ear, and olfactory placode development. Foxg1-Cre mediated early deletion of Sox2 eradicates all olfactory placode development, and disrupts retinal development and invagination of the lens placode. In contrast to the lens and olfactory placodes, the ear placode invaginates and delaminates NEUROD1 positive neurons. Furthermore, we show that SOX2 is not necessary for early ear neurogenesis, since the early inner ear ganglion is formed with near normal central projections to the hindbrain and peripheral projections to the undifferentiated sensory epithelia of E11.5-12.5 ears. However, later stages of ear neurosensory development, in particular, the late forming auditory system, critically depend on the presence of SOX2. Our data establish distinct differences for SOX2 requirements among placodal sensory organs with similarities between olfactory and lens but not ear placode development, consistent with the unique neurosensory development and molecular properties of the ear.

Renewed proliferation in adult mouse cochlea and regeneration of hair cells

The adult mammalian inner ear lacks the capacity to divide or regenerate. Damage to inner ear generally leads to permanent hearing loss in humans. Here, we present that reprogramming of the adult inner ear induces renewed proliferation and regeneration of inner ear cell types. Co-activation of cell cycle activator Myc and inner ear progenitor gene Notch1 induces robust proliferation of diverse adult cochlear sensory epithelial cell types. Transient MYC and NOTCH activities enable adult supporting cells to respond to transcription factor Atoh1 and efficiently transdifferentiate into hair cell-like cells. Furthermore, we uncover that mTOR pathway participates in MYC/NOTCH-mediated proliferation and regeneration. These regenerated hair cell-like cells take up the styryl dye FM1-43 and are likely to form connections with adult spiral ganglion neurons, supporting that Myc and Notch1 co-activation is sufficient to reprogram fully mature supporting cells to proliferate and regenerate hair cell-like cells in adult mammalian auditory organs.

Six1 and Irx1 have reciprocal interactions during cranial placode and otic vesicle formation

The specialized sensory organs of the vertebrate head are derived from thickened patches of cells in the ectoderm called cranial sensory placodes. The developmental program that generates these placodes and the genes that are expressed during the process have been studied extensively in a number of animals, yet very little is known about how these genes regulate one another. We previously found via a microarray screen that Six1, a known transcriptional regulator of cranial placode fate, up-regulates Irx1 in ectodermal explants. In this study, we investigated the transcriptional relationship between Six1 and Irx1 and found that they reciprocally regulate each other throughout cranial placode and otic vesicle formation. Although Irx1 expression precedes that of Six1 in the neural border zone, its continued and appropriately patterned expression in the pre-placodal region (PPR) and otic vesicle requires Six1. At early PPR stages, Six1 expands the Irx1 domain, but this activity subsides over time and changes to a predominantly repressive effect. Likewise, Irx1 initially expands Six1 expression in the PPR, but later represses it. We also found that Irx1 and Sox11, a known direct target of Six1, reciprocally affect each other. This work demonstrates that the interactions between Six1 and Irx1 are continuous during PPR and placode development and their transcriptional effects on one another change over developmental time.

Ascorbic acid prevents chloroquine-induced toxicity in inner glial cells

Ototoxicity is a collateral effect of prolonged treatment with chloroquine which is a widely utilized as an anti-lupus and anti-malarial drug. Glial cells of inner ear are responsible for maintenance of neuronal cells homeostasis in auditory system. In the current study we have evaluated chloroquine-induced toxicity and protective effect of ascorbic acid treatment on Schwann glial cell cultures of inner ear. Glial cells were cultured from organ of Corti of mice cochlear structure. Purity of Schwann glial cell was confirmed by S100 protein staining. Cell viability was evaluated in control and cultures treated with different concentrations of chloroquine. Glutamate uptake and ROS production were measured by HPLC and DCFH-DA probe fluorescence, respectively. Results have shown that chloroquine treatment evoked concentration and time -dependent toxicity (LC50 = 70 μM) as well as significant decrease on glutamate uptake and high production of ROS in glial cell cultures. Co-treatment with ascorbic acid has prevented both chloroquine-induced ROS production and chloroquine toxicity on glial cell cultures. This pre-clinical study is the first one to demonstrate chloroquine-induced ROS production by glial cells of inner ear as well as the protective effect exerted by ascorbic acid on these cells.

DNA methylation dynamics during embryonic development and postnatal maturation of the mouse auditory sensory epithelium

The inner ear is a complex structure responsible for hearing and balance, and organ pathology is associated with deafness and balance disorders. To evaluate the role of epigenomic dynamics, we performed whole genome bisulfite sequencing at key time points during the development and maturation of the mouse inner ear sensory epithelium (SE). Our single-nucleotide resolution maps revealed variations in both general characteristics and dynamics of DNA methylation over time. This allowed us to predict the location of non-coding regulatory regions and to identify several novel candidate regulatory factors, such as Bach2, that connect stage-specific regulatory elements to molecular features that drive the development and maturation of the SE. Constructing in silico regulatory networks around sites of differential methylation enabled us to link key inner ear regulators, such as Atoh1 and Stat3, to pathways responsible for cell lineage determination and maturation, such as the Notch pathway. We also discovered that a putative enhancer, defined as a low methylated region (LMR), can upregulate the GJB6 gene and a neighboring non-coding RNA. The study of inner ear SE methylomes revealed novel regulatory regions in the hearing organ, which may improve diagnostic capabilities, and has the potential to guide the development of therapeutics for hearing loss by providing multiple intervention points for manipulation of the auditory system.

Inner ear exosomes and their potential use as biomarkers

Exosomes are nanovesicles involved in intercellular communications. They are released by a variety of cell types; however, their presence in the inner ear has not been described in the literature. The aims of this study were to determine if exosomes are present in the inner ear and, if present, characterize the changes in their protein content in response to ototoxic stress. In this laboratory investigation, inner ear explants of 5-day-old Wistar rats were cultured and treated with either cisplatin or gentamicin. Hair cell damage was assessed by confocal microscopy. Exosomes were isolated using ExoQuick, serial centrifugation, and mini-column methods. Confirmation and characterization of exosomes was carried out using transmission electron microscopy (TEM), ZetaView, BCA protein analysis, and proteomics. Vesicles with a typical size distribution for exosomes were observed using TEM and ZetaView. Proteomic analysis detected typical exosome markers and markers for the organ of Corti. There was a statistically significant reduction in the exosome protein level and number of particles per cubic centimeter when the samples were exposed to ototoxic stress. Proteomic analysis also detected clear differences in protein expression when ototoxic medications were introduced. Significant changes in the proteomes of the exosomes were previously described in the context of hearing loss and ototoxic treatment. This is the first report describing exosomes derived from the inner ear. These findings may present an opportunity to conduct further studies with the hope of using exosomes as a biomarker to monitor inner ear function in the future.

Differential regulation of NMDA receptor-expressing neurons in the rat hippocampus and striatum following bilateral vestibular loss demonstrated using flow cytometry

There is substantial evidence that loss of vestibular function impairs spatial learning and memory related to hippocampal (HPC) function, as well as increasing evidence that striatal (Str) plasticity is also implicated. Since the N-methyl-d-aspartate (NMDA) subtype of glutamate receptor is considered essential to spatial memory, previous studies have investigated whether the expression of HPC NMDA receptors changes following vestibular loss; however, the results have been contradictory. Here we used a novel flow cytometric method to quantify the number of neurons expressing NMDA receptors in the HPC and Str following bilateral vestibular loss (BVL) in rats. At 7 and 30 days post-op., there was a significant increase in the number of HPC neurons expressing NMDA receptors in the BVL animals, compared to sham controls (P ≤ 0.004 and P ≤ 0.0001, respectively). By contrast, in the Str, at 7 days there was a significant reduction in the number of neurons expressing NMDA receptors in the BVL group (P ≤ 0.05); however, this difference had disappeared by 30 days post-op. These results suggest that BVL causes differential changes in the number of neurons expressing NMDA receptors in the HPC and Str, which may be related to its long-term impairment of spatial memory.

Modeling human early otic sensory cell development with induced pluripotent stem cells

The inner ear represents a promising system to develop cell-based therapies from human induced pluripotent stem cells (hiPSCs). In the developing ear, Notch signaling plays multiple roles in otic region specification and for cell fate determination. Optimizing hiPSC induction for the generation of appropriate numbers of otic progenitors and derivatives, such as hair cells, may provide an unlimited supply of cells for research and cell-based therapy. In this study, we used monolayer cultures, otic-inducing agents, Notch modulation, and marker expression to track early and otic sensory lineages during hiPSC differentiation. Otic/placodal progenitors were derived from hiPSC cultures in medium supplemented with FGF3/FGF10 for 13 days. These progenitor cells were then treated for 7 days with retinoic acid (RA) and epidermal growth factor (EGF) or a Notch inhibitor. The differentiated cultures were analyzed in parallel by qPCR and immunocytochemistry. After the 13 day induction, hiPSC-derived cells displayed an upregulated expression of a panel of otic/placodal markers. Strikingly, a subset of these induced progenitor cells displayed key-otic sensory markers, the percentage of which was increased in cultures under Notch inhibition as compared to RA/EGF-treated cultures. Our results show that modulating Notch pathway during in vitro differentiation of hiPSC-derived otic/placodal progenitors is a valuable strategy to promote the expression of human otic sensory lineage genes.

Three-dimensional Organotypic Cultures of Vestibular and Auditory Sensory Organs

The sensory organs of the inner ear are challenging to study in mammals due to their inaccessibility to experimental manipulation and optical observation. Moreover, although existing culture techniques allow biochemical perturbations, these methods do not provide a means to study the effects of mechanical force and tissue stiffness during development of the inner ear sensory organs. Here we describe a method for three-dimensional organotypic culture of the intact murine utricle and cochlea that overcomes these limitations. The technique for adjustment of a three-dimensional matrix stiffness described here permits manipulation of the elastic force opposing tissue growth. This method can therefore be used to study the role of mechanical forces during inner ear development. Additionally, the cultures permit virus-mediated gene delivery, which can be used for gain- and loss-of-function experiments. This culture method preserves innate hair cells and supporting cells and serves as a potentially superior alternative to the traditional two-dimensional culture of vestibular and auditory sensory organs.

Creating a stem cell niche in the inner ear using self-assembling peptide amphiphiles

The use of human embryonic stem cells (hESCs) for regeneration of the spiral ganglion will require techniques for promoting otic neuronal progenitor (ONP) differentiation, anchoring of cells to anatomically appropriate and specific niches, and long-term cell survival after transplantation. In this study, we used self-assembling peptide amphiphile (PA) molecules that display an IKVAV epitope (IKVAV-PA) to create a niche for hESC-derived ONPs that supported neuronal differentiation and survival both in vitro and in vivo after transplantation into rodent inner ears. A feature of the IKVAV-PA gel is its ability to form organized nanofibers that promote directed neurite growth. Culture of hESC-derived ONPs in IKVAV-PA gels did not alter cell proliferation or viability. However, the presence of IKVAV-PA gels increased the number of cells expressing the neuronal marker beta-III tubulin and improved neurite extension. The self-assembly properties of the IKVAV-PA gel allowed it to be injected as a liquid into the inner ear to create a biophysical niche for transplanted cells after gelation in vivo. Injection of ONPs combined with IKVAV-PA into the modiolus of X-SCID rats increased survival and localization of the cells around the injection site compared to controls. Human cadaveric temporal bone studies demonstrated the technical feasibility of a transmastoid surgical approach for clinical intracochlear injection of the IKVAV-PA/ONP combination. Combining stem cell transplantation with injection of self-assembling PA gels to create a supportive niche may improve clinical approaches to spiral ganglion regeneration.

Microarray analyses of otospheres derived from the cochlea in the inner ear identify putative transcription factors that regulate the characteristics of otospheres

Various tissues possess tissue-specific stem/progenitor cells, including the inner ears. Stem/progenitor cells of the inner ear can be isolated as so-called otospheres from differentiated cells using a sphere forming assay. Although recent studies have demonstrated the characteristics of otospheres to some extent, most of the features of these cells are unknown. In this report, we describe the findings of transcriptome analyses with a cDNA microarray of otospheres derived from the cochleae of the inner ears of neonatal mice in order to clarify the gene expression profile of otic stem/progenitor cells. There were common transcription factors between otospheres and embryonic stem cells, which were supposed to be due to the stemness of otospheres. In comparison with the cochlear sensory epithelium, the otospheres shared characteristics with the cochlea, although several transcription factors specific for otospheres were identified. These transcription factors are expected to be essential for maintaining the characteristics of otospheres, and appear to be candidate genes that promote the direct conversion of cells into otic stem/progenitor cells.

Pharmacogenetic variants in TPMT alter cellular responses to cisplatin in inner ear cell lines

Cisplatin is a highly-effective and widely-used chemotherapeutic agent that causes ototoxicity in many patients. Pharmacogenomic studies of key genes controlling drug biotransformation identified variants in thiopurine methyltransferase (TPMT) as predictors of cisplatin-induced ototoxicity, although the mechanistic basis of this interaction has not been reported. Expression constructs of TPMT*3A, *3B and *3C variants were generated and monitored in cultured cells. Cellular TPMT*3A levels were detected at >20-fold lower amounts than the wild type confirming the unstable nature of this variant. The expression of wild type TPMT (TPMT*1) in two murine ear cell lines, HEI-OC1 and UB/OC-1, significantly mitigated their susceptibility to cisplatin toxicity. Cisplatin treatment induced Tlr4 gene expression in HEI-OC1 cells and this response was blunted by the expression of wild type TPMT but not TPMT*3A. In line with the significant mitigation of TPMT*1-expressing cells to cisplatin cytotoxicity, these findings demonstrate a drug-gene interaction between increased TPMT activity and decreased susceptibility to cisplatin-induced toxicity of inner ear cells.

Cochlear Cytokine Gene Expression in Murine Chronic Otitis Media

OBJECTIVE: To investigate chronic otitis media (COM) induction of cochlear cytokine genes.

STUDY DESIGN: RNA from cochleas of five C3H/HeJ mice with and without COM was isolated for cytokine expression in gene arrays. Immunohistochemistry was performed for the protein products of up-regulated genes to confirm their expression in cochlear tissues.

RESULTS: Cochleas from COM mice showed increased expression of 29 genes (>2X normal) and decreased expression of 19 genes (<0.5X normal). Cytokines expressed were largely those related to inflammation and tissue remodeling. Cochlear immuno-histochemistry confirmed the presence of numerous cytokines, as well as NF- kB, a major inflammatory transcription factor that drives cytokine expression.

CONCLUSION: COM causes elevated levels of cochlear cytokine mRNA, which demonstrates that inner ear tissues are capable of NF- kB activation and cytokine production. This may be another mechanism of otitis media-induced cochlear cytotoxicity in addition to that caused by migration of inflammatory cytokines from the middle ear.

SIGNIFICANCE: Cochlear tissues are capable of mounting an immunological response to middle ear inflammatory stimuli.

Comparative Auditory Neuroscience: Understanding the Evolution and Function of Ears

Comparative auditory studies make it possible both to understand the origins of modern ears and the factors underlying the similarities and differences in their performance. After all lineages of land vertebrates had independently evolved tympanic middle ears in the early Mesozoic era, the subsequent tens of millions of years led to the hearing organ of lizards, birds, and mammals becoming larger and their upper frequency limits higher. In extant species, lizard papillae remained relatively small (<2 mm), but avian papillae attained a maximum length of 11 mm, with the highest frequencies in both groups near 12 kHz. Hearing-organ sizes in modern mammals vary more than tenfold, up to >70 mm (made possible by coiling), as do their upper frequency limits (from 12 to >200 kHz). The auditory organs of the three amniote groups differ characteristically in their cellular structure, but their hearing sensitivity and frequency selectivity within their respective hearing ranges hardly differ. In the immediate primate ancestors of humans, the cochlea became larger and lowered its upper frequency limit. Modern humans show an unusual trend in frequency selectivity as a function of frequency. It is conceivable that the frequency selectivity patterns in humans were influenced in their evolution by the development of speech.

Spontaneous Activity of Cochlear Hair Cells Triggered by Fluid Secretion Mechanism in Adjacent Support Cells

Spontaneous electrical activity of neurons in developing sensory systems promotes their maturation and proper connectivity. In the auditory system, spontaneous activity of cochlear inner hair cells (IHCs) is initiated by the release of ATP from glia-like inner supporting cells (ISCs), facilitating maturation of central pathways before hearing onset. Here, we find that ATP stimulates purinergic autoreceptors in ISCs, triggering Cl(-) efflux and osmotic cell shrinkage by opening TMEM16A Ca(2+)-activated Cl(-) channels. Release of Cl(-) from ISCs also forces K(+) efflux, causing transient depolarization of IHCs near ATP release sites. Genetic deletion of TMEM16A markedly reduces the spontaneous activity of IHCs and spiral ganglion neurons in the developing cochlea and prevents ATP-dependent shrinkage of supporting cells. These results indicate that supporting cells in the developing cochlea have adapted a pathway used for fluid secretion in other organs to induce periodic excitation of hair cells.

Reconstruction of the mouse otocyst and early neuroblast lineage at single-cell resolution

The otocyst harbors progenitors for most cell types of the mature inner ear. Developmental lineage analyses and gene expression studies suggest that distinct progenitor populations are compartmentalized to discrete axial domains in the early otocyst. Here, we conducted highly parallel quantitative RT-PCR measurements on 382 individual cells from the developing otocyst and neuroblast lineages to assay 96 genes representing established otic markers, signaling-pathway-associated transcripts, and novel otic-specific genes. By applying multivariate cluster, principal component, and network analyses to the data matrix, we were able to readily distinguish the delaminating neuroblasts and to describe progressive states of gene expression in this population at single-cell resolution. It further established a three-dimensional model of the otocyst in which each individual cell can be precisely mapped into spatial expression domains. Our bioinformatic modeling revealed spatial dynamics of different signaling pathways active during early neuroblast development and prosensory domain specification.

Thrombospondins 1 and 2 are important for afferent synapse formation and function in the inner ear

Thrombospondins (TSPs) constitute a family of secreted extracellular matrix proteins that have been shown to be involved in the formation of synapses in the central nervous system. In this study, we show that TSP1 and TSP2 are expressed in the cochlea, and offer the first description of their putative roles in afferent synapse development and function in the inner ear. We examined mice with deletions of TSP1, TSP2 and both (TSP1/TSP2) for inner ear development and function. Immunostaining for synaptic markers indicated a significant decrease in the number of formed afferent synapses in the cochleae of TSP2 and TSP1/TSP2 knockout (KO) mice at postnatal day (P)29. In functional studies, TSP2 and TSP1/TSP2 KO mice showed elevated auditory brainstem response (ABR) thresholds as compared with wild-type littermates, starting at P15, with the most severe phenotype being seen for TSP1/TSP2 KO mice. TSP1/TSP2 KO mice also showed reduced wave I amplitudes of ABRs and vestibular evoked potentials, suggesting synaptic dysfunction in both the auditory and vestibular systems. Whereas ABR thresholds in TSP1 KO mice were relatively unaffected at early ages, TSP1/TSP2 KO mice showed the most severe phenotype among all of the genotypes tested, suggesting functional redundancy between the two genes. On the basis of the above results, we propose that TSPs play an important role in afferent synapse development and function of the inner ear.

Localization of sirtuins in the mouse inner ear

CONCLUSION

It is suggested that SIRT1 and 3, and probably SIRT4 and 5, play an important role in the neuroprotection of the inner ear. SIRT2 may be related to neuroprotection and myelin sheath formation, while SIRT6 seems to have a significant role in maintaining the energy balance by metabolic regulation.

OBJECTIVE

To analyze the expression of sirtuins (SIRT1-7) in the normal mouse inner ear.

METHODS

CBA/J mice were used for this study. The localization of SIRT1-7 in the inner ear, i.e. cochlea, vestibular end organs, and endolymphatic sac, was investigated using real-time PCR and immunohistochemistry.

RESULTS

We found high levels of mRNA of all seven sirtuins in the inner ear. In the immunohistochemical study, SIRT1-7 were abundant in many inner ear structures, i.e. stria vascularis, inner and outer hair cells, spiral ganglion cells, vestibular sensory and ganglion cells, vestibular dark and transitional cells, and the endolymphatic sac.

Molecular mechanisms and potentials for differentiating inner ear stem cells into sensory hair cells

In mammals, hair cells may be damaged or lost due to genetic mutation, infectious disease, chemical ototoxicity, noise and other factors, causing permanent sensorineural deafness. Regeneration of hair cells is a basic pre-requisite for recovery of hearing in deaf animals. The inner ear stem cells in the organ of Corti and vestibular utricle are the most ideal precursors for regeneration of inner ear hair cells. This review highlights some recent findings concerning the proliferation and differentiation of inner ear stem cells. The differentiation of inner ear stem cells into hair cells involves a series of signaling pathways and regulatory factors. This paper offers a comprehensive analysis of the related studies.

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

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

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

A coregulatory network of NR2F1 and microRNA-140

Background

Both nuclear receptor subfamily 2 group F member 1 (NR2F1) and microRNAs (miRNAs) have been shown to play critical roles in the developing and functional inner ear. Based on previous studies suggesting interplay between NR2F1 and miRNAs, we investigated the coregulation between NR2F1 and miRNAs to better understand the regulatory mechanisms of inner ear development and functional maturation.

Results

Using a bioinformatic approach, we identified 11 potential miRNAs that might coregulate target genes with NR2F1 and analyzed their targets and potential roles in physiology and disease. We selected 6 miRNAs to analyze using quantitative real-time (qRT) -PCR and found that miR-140 is significantly down-regulated by 4.5-fold (P=0.004) in the inner ear of NR2F1 knockout (Nr2f1^–/–) mice compared to wild-type littermates but is unchanged in the brain. Based on this, we performed chromatin-immunoprecipitation followed by qRT-PCR and confirmed that NR2F1 directly binds and regulates both miR-140 and Klf9invivo. Furthermore, we performed luciferase reporter assay and showed that miR-140 mimic directly regulates KLF9-3’UTR, thereby establishing and validating an example coregulatory network involving NR2F1, miR-140, and Klf9.

Conclusions

We have described and experimentally validated a novel tissue-dependent coregulatory network for NR2F1, miR-140, and Klf9 in the inner ear and we propose the existence of many such coregulatory networks important for both inner ear development and function.

Conditional deletion of pten leads to defects in nerve innervation and neuronal survival in inner ear development

All cellular phenomena and developmental events, including inner ear development, are modulated through harmonized signaling networks. Phosphatase and tensin homolog deleted on chromosome 10 (PTEN), a tumor suppressor, is a major signaling component involved in cross talk with key regulators of development; i.e., Wnt, Notch, and bone morphogenetic proteins. Although Pten function has been studied in various systems, its role in inner ear development is poorly understood. Here, we used inner ear-specific Pten conditional knockout mice and examined the characteristics of the inner ear. In a detailed analysis of the phenotype, reduced cochlear turning and widened epithelia were observed. Phalloidin staining of sensory epithelium revealed that hair cell patterns were disturbed; i.e., additional rows of hair cells were discovered. The neural abnormality revealed a reduction in and disorganization of nerve fibers, including apoptosis at the neural precursor stage. Pten deficiency induced increased phosphorylation of Akt at Ser473. The elevation of inhibitory glycogen synthase kinase 3β Ser9 phosphorylation (pGSK3β) was sustained until the neuronal differentiation stage at embryonic day 14.5, instead of pGSK3β downregulation. This is the first report on the influence of Pten/Akt/GSK3β signaling on the development of spiral ganglia. These results suggest that Pten is required for the maintenance of neuroblast number, neural precursors, and differentiation in the inner ear.

Swept field laser confocal microscopy for enhanced spatial and temporal resolution in live-cell imaging

Confocal fluorescence microscopy is a broadly used imaging technique that enhances the signal-to-noise ratio by removing out of focal plane fluorescence. Confocal microscopes come with a variety of modifications depending on the particular experimental goals. Microscopes, illumination pathways, and light collection were originally focused upon obtaining the highest resolution image possible, typically on fixed tissue. More recently, live-cell confocal imaging has gained importance. Since measured signals are often rapid or transient, thus requiring higher sampling rates, specializations are included to enhance spatial and temporal resolution while maintaining tissue viability. Thus, a balance between image quality, temporal resolution, and tissue viability is needed. A subtype of confocal imaging, termed swept field confocal (SFC) microscopy, can image live cells at high rates while maintaining confocality. SFC systems can use a pinhole array to obtain high spatial resolution, similar to spinning disc systems. In addition, SFC imaging can achieve faster rates by using a slit to sweep the light across the entire image plane, thus requiring a single scan to generate an image. Coupled to a high-speed charge-coupled device camera and a laser illumination source, images can be obtained at greater than 1,000 frames per second while maintaining confocality.

Overactivation of Notch1 signaling induces ectopic hair cells in the mouse inner ear in an age-dependent manner

Background

During mouse inner ear development, Notch1 signaling first specifies sensory progenitors, and subsequently controls progenitors to further differentiate into either hair cells (HCs) or supporting cells (SCs). Overactivation of NICD (Notch1 intracellular domain) at early embryonic stages leads to ectopic HC formation. However, it remains unclear whether such an effect can be elicited at later embryonic or postnatal stages, which has important implications in mouse HC regeneration by reactivation of Notch1 signaling.

Methodology/Principal Findings

We performed comprehensive in vivo inducible overactivation of NICD at various developmental stages. In CAG^CreER+; Rosa26-NICD^loxp/+ mice, tamoxifen treatment at embryonic day 10.5 (E10.5) generated ectopic HCs in the non-sensory regions in both utricle and cochlea, whereas ectopic HCs only appeared in the utricle when tamoxifen was given at E13. When tamoxifen was injected at postnatal day 0 (P0) and P1, no ectopic HCs were observed in either utricle or cochlea. Interestingly, Notch1 signaling induced new HCs in a non-cell-autonomous manner, because the new HCs did not express NICD. Adjacent to the new HCs were cells expressing the SC marker Sox10 (either NICD+ or NICD-negative).

Conclusions/Significance

Our data demonstrate that the developmental stage determines responsiveness of embryonic otic precursors and neonatal non-sensory epithelial cells to NICD overactivation, and that Notch 1 signaling in the wild type, postnatal inner ear is not sufficient for generating new HCs. Thus, our genetic mouse model is suitable to test additional pathways that could synergistically interact with Notch1 pathway to produce HCs at postnatal ages.

Tol2-mediated gene transfer and in ovo electroporation of the otic placode: a powerful and versatile approach for investigating embryonic development and regeneration of the chicken inner ear

The vertebrate inner ear is composed of several specialized epithelia containing mechanosensory "hair" cells, sensitive to sound and head movements. In mammals, the loss of hair cells for example during aging or after noise trauma is irreversible and results in permanent sensory deficits. By contrast, avian, fish, and amphibians can efficiently regenerate lost hair cells following trauma. The chicken inner ear is a classic model system to investigate the cellular and molecular mechanisms of inner ear development and regeneration, yet it suffered until recently from a relative lack of flexible tools for genetic studies. With the introduction of in ovo electroporation and of Tol2 transposon vectors for gene transfer in avian cells, the field of experimental possibilities has now expanded significantly in this model. Here we provide a general protocol for in ovo electroporation of the chicken otic placode and illustrate how this approach, combined with Tol2 vectors, can be used to drive long-term and inducible gene expression in the embryonic chicken inner ear. This method will be particularly useful to investigate the function of candidate genes regulating progenitor cell behavior and sensory cell differentiation in the inner ear.

Laser capture microdissection for protein and NanoString RNA analysis

Laser capture microdissection (LCM) allows the precise procurement of enriched cell populations from a heterogeneous tissue, or live cell culture, under direct microscopic visualization. Histologically enriched cell populations can be procured by harvesting cells of interest directly or isolating specific cells by ablating unwanted cells. The basic components of laser microdissection technology are (a) visualization of cells via light microscopy, (b) transfer of laser energy to a thermolabile polymer with either the formation of a polymer-cell composite (capture method) or transfer of laser energy via an ultraviolet laser to photovolatize a region of tissue (cutting method), and (c) removal of cells of interest from the heterogeneous tissue section. The capture and cutting methods (instruments) for laser microdissection differ in the manner by which cells of interest are removed from the heterogeneous sample. Laser energy in the capture method is infrared (810 nm), while in the cutting mode the laser is ultraviolet (355 nm). Infrared lasers melt a thermolabile polymer that adheres to the cells of interest, whereas ultraviolet lasers ablate cells for either removal of unwanted cells or excision of a defined area of cells. LCM technology is applicable to an array of applications including mass spectrometry, DNA genotyping and loss-of-heterozygosity analysis, RNA transcript profiling, cDNA library generation, proteomics discovery, and signal kinase pathway profiling. This chapter describes LCM using an Arcturus(XT) instrument for downstream protein sample analysis and using an mmi CellCut Plus® instrument for RNA analysis via NanoString technology.

Differentiation of the lateral compartment of the cochlea requires a temporally restricted FGF20 signal

A large proportion of age-related hearing loss is caused by loss or damage to outer hair cells in the organ of Corti. The organ of Corti is the mechanosensory transducing apparatus in the inner ear and is composed of inner hair cells, outer hair cells, and highly specialized supporting cells. The mechanisms that regulate differentiation of inner and outer hair cells are not known. Here we report that fibroblast growth factor 20 (FGF20) is required for differentiation of cells in the lateral cochlear compartment (outer hair and supporting cells) within the organ of Corti during a specific developmental time. In the absence of FGF20, mice are deaf and lateral compartment cells remain undifferentiated, postmitotic, and unresponsive to Notch-dependent lateral inhibition. These studies identify developmentally distinct medial (inner hair and supporting cells) and lateral compartments in the developing organ of Corti. The viability and hearing loss in Fgf20 knockout mice suggest that FGF20 may also be a deafness-associated gene in humans.

Activation of apoptotic pathways in the absence of cell death in an inner-ear immortomouse cell line

Aminoglycoside antibiotics and cisplatin (CDDP) are the major ototoxins of clinical medicine due to their capacity to cause significant and permanent hearing loss by targeting the mammalian sensory cells. Understanding the pathogenesis of damage is the first step in designing effective prevention of drug-induced hearing loss. In-vitro systems greatly enhance the efficiency of biochemical and molecular investigations through ease of access and manipulation. HEI-OC1, an inner ear cell line derived from the immortomouse, expresses markers for auditory sensory cells and, therefore, is a potential tool to study the ototoxic mechanisms of drugs like aminoglycoside antibiotics and CDDP. HEI-OC1 cells (and also HeLa cells) efficiently take up fluorescently tagged gentamicin and respond to drug treatment with changes in cell death and survival signaling pathways. Within hours, the c-Jun N-terminal kinase pathway and the transcription factor AP-1 were activated and at later times, the "executioner caspase", caspase-3. These responses were robust and elicited by both gentamicin and kanamycin. However, despite the initiation of apoptotic pathways and transient changes in nuclear morphology, cell death was not observed following aminoglycoside treatment, while administration of CDDP led to significant cell death as determined by flow cytometric measurements; β-galactosidase analysis ruled out senescence in gentamicin-treated cells. The ability to withstand treatment with aminoglycosides but not with CDDP suggests that this cell line might be helpful in providing some insight into the differential actions of the two ototoxic drugs.

Mature mice lacking Rbl2/p130 gene have supernumerary inner ear hair cells and supporting cells

Adult mammalian auditory hair cells (HCs) and their associated supporting cells (SCs) do not proliferate, and HC death leads to irreversible neurosensory hearing loss and balance impairment. In nonmammalian vertebrates, loss of HCs induces mitotic proliferation of adjacent nonsensory SCs and/or direct SC transdifferentiation to generate replacement cells. This results in the structural and functional recovery of the nonmammalian sensory systems. Potential replacement of mammalian auditory HCs, either by transplanting cells or by transforming existing cells through molecular therapy, has long been proposed. However, HC replacement strategies with clear therapeutic potential remain elusive. The retinoblastoma (pRB) family of cell cycle regulators, Rb1, Rbl1 (p107), and Rbl2 (p130), regulate the G(1)- to S-phase transition in proliferating cells. In the inner ear, the biochemical and molecular pathways involving pRBs, particularly p107 and p130, are relatively unexplored and their therapeutic suitability is yet to be determined. In this study, we analyzed the cochleae of adult p130 knock-out (p130(-/-)) mice and showed that lack of the p130 gene results in extra rows of HCs and SCs in the more apical regions of the cochlea. No evidence of transdifferentiation of these supernumerary SCs into HCs was observed in the p130(-/-) mouse. Nevertheless, unscheduled proliferation of SCs in the adult p130(-/-) cochlea coupled to downregulation of bona fide cell cycle inhibitors provides a mechanistic basis for the role of p130 as a regulator of SC and HC mitotic quiescence in the more apical regions of the cochlea. Interestingly, p130(-/-) mice exhibited nearly normal peripheral auditory sensitivity.

Interactions between intrinsic membrane and emerging network properties determine signal processing in central vestibular neurons

Head/body motion-related sensory signals are transformed in second-order vestibular neurons (2°VN) into commands for appropriate motor reactions that stabilize gaze and posture during locomotion. In all vertebrates, these neurons form functional subgroups with different membrane properties and response dynamics, compatible with the necessity to process a wide range of motion-related sensory signals. In frog, 2°VN subdivide into two well-defined populations with distinctly different intrinsic membrane properties, discharge dynamics and synaptic response characteristics. Tonic 2°VN form low-pass filters with membrane properties that cause synaptic amplification, whereas phasic 2°VN form band-pass filters that cause shunting of repetitive inputs. The different, yet complementary, filter properties render tonic neurons suitable for integration and phasic neurons for differentiation and event detection. Specific insertion of phasic 2°VN into local vestibular networks of inhibitory interneurons reinforces the functional consequences of the intrinsic membrane properties of this particular cell type with respect to the processing of afferent sensory signals. Thus, the combination of matching intrinsic cellular and emerging network properties generates sets of neuronal elements that form adjustable, frequency-tuned filter components for separate transformation of the various dynamic aspects of head motion-related signals. The overall frequency tuning of central vestibular neurons differs between vertebrates along with variations in species-specific locomotor dynamics, thereby illustrating an ecophysiological plasticity of the involved neuronal elements. Moreover, separation into multiple, dynamically different subtypes at any neuronal level along the vestibulo-motor reflex pathways suggests an organization of head motion-related sensory-motor transformation in parallel, frequency-tuned channels.

Direct delivery of MIF morpholinos into the zebrafish otocyst by injection and electroporation affects inner ear development

In recent years, electroporation has become a popular technique for in vivo transfection of DNA, RNA, and morpholinos into various tissues, including the eye, brain, and somites of zebrafish. The advantage of electroporation over other methods of genetic manipulation is that specific tissues can be targeted, both spatially and temporally, for the introduction of macromolecules by the application of electrical current. Here we describe the use of electroporation for transfecting mif and mif-like morpholinos into the tissues of the developing inner ear of the zebrafish. In past studies, mif morpholino injected into embryos at the 1- to 8-cell stage resulted in widespread morphological changes in the nervous system and eye, as well as the ear. By targeting the tissues of the inner ear at later stages in development, we can determine the primary effects of MIF in the developing inner ear, as opposed to secondary effects that may result from the influence of other tissues. By using phalloidin and acetylated tubulin staining to study the morphology of neurons, neuronal processes, and hair cells associated with the posterior macula, we were able to assess the efficacy of electroporation as a method for targeted transfection in the zebrafish inner ear. The otic vesicles of 24hpf embryos were injected with morpholinos and electroporated and were then compared to embryos that had received no treatment or had been only injected or electroporated. Embryos that were injected and electroporated showed a decrease in hair cell numbers, decreased innervation by the statoacoustic ganglion (SAG) and fewer SAG neurons compared with control groups. Our results showed that direct delivery of morpholinos into otocysts at later stages avoids the non-specific nervous system and neural crest effects of morpholinos delivered at the 1-8 cell stage. It also allows examination of effects that are directed to the inner ear and not secondary effects on the ear from primary effects on the brain, neural crest or periotic mesenchyme.

Visualization and analysis of superparamagnetic iron oxide nanoparticles in the inner ear by light microscopy and energy filtered TEM

Nanoparticles as potential carriers for local drug transfer are an alternative to systemic drug delivery into the inner ear. We report on the first in vitro tests of a new ferrogel consisting of superparamagnetic iron oxide nanoparticles (SPIONs) and a Pluronic(®) F127 (PF127) copolymer. Pluronic copolymers possess a unique viscosity-adjustable property that makes PF127 gels easy to handle compared to conventional cross-linked hydrogels. This ferrogel was successfully tested in cadaver human temporal bones as well as in organotypic explant cultures of mouse inner ears. SPIONs were identified by light microscopy and localized with different imaging modes in energy-filtered transmission electron microscopy. Our approach shows a promising possibility to use iron oxide nanoparticles, which are suitable for visualization and characterization at both the light- and electron-microscopic levels.

FROM THE CLINICAL EDITOR

The authors report the first in vitro tests of a new ferrogel consisting of superparamagnetic iron oxide nanoparticles (SPIONs) and a Pluronic® F127 (PF127) copolymer for drug delivery in the inner ear, demonstrasting a promising possibility to use iron oxide nanoparticles, which are suitable for visualization and characterization at both the light- and electron-microscopic levels.

Dual contribution to amplification in the mammalian inner ear

The inner ear achieves a wide dynamic range of responsiveness by mechanically amplifying weak sounds. The enormous mechanical gain reported for the mammalian cochlea, which exceeds a factor of 4000, poses a challenge for theory. Here we show how such a large gain can result from an interaction between amplification by low-gain hair bundles and a pressure wave: hair bundles can amplify both their displacement per locally applied pressure and the pressure wave itself. A recently proposed ratchet mechanism, in which hair-bundle forces do not feed back on the pressure wave, delineates the two effects. Our analytical calculations with a WKB approximation agree with numerical solutions.

[Inner ears (auris interna) of mammals in ontogenesis]

In both phylogenesis and ontogenesis, the inner ear is formed first as a core, which, phylogenetically, is the most ancient part of the peripheral region of the auditory system. Along with the development of the inner ear, other links of different evolutionary ages (the auricle is a phylogenetically young link) begin to form in the peripheral auditory system.

Emx2 and early hair cell development in the mouse inner ear

Emx2 is a homeodomain protein that plays a critical role in inner ear development. Homozygous null mice die at birth with a range of defects in the CNS, renal system and skeleton. The cochlea is shorter than normal with about 60% fewer auditory hair cells. It appears to lack outer hair cells and some supporting cells are either absent or fail to differentiate. Many of the hair cells differentiate in pairs and although their hair bundles develop normally their planar cell polarity is compromised. Measurements of cell polarity suggest that classic planar cell polarity molecules are not directly influenced by Emx2 and that polarity is compromised by developmental defects in the sensory precursor population or by defects in epithelial cues for cell alignment. Planar cell polarity is normal in the vestibular epithelia although polarity reversal across the striola is absent in both the utricular and saccular maculae. In contrast, cochlear hair cell polarity is disorganized. The expression domain for Bmp4 is expanded and Fgfr1 and Prox1 are expressed in fewer cells in the cochlear sensory epithelium of Emx2 null mice. We conclude that Emx2 regulates early developmental events that balance cell proliferation and differentiation in the sensory precursor population.

Expression of transient receptor potential channel mucolipin (TRPML) and polycystine (TRPP) in the mouse inner ear

CONCLUSIONS

TRPML3 may play distinct roles in the inner ear, such as stereociliar organization, sensory cell transduction, and inner ear fluid homeostasis, and TRPP3 may be important for fluid homeostasis in the inner ear.

OBJECTIVE

To study the expression of TRPML1-3 and TRPP2, 3, and 5 in the mouse inner ear.

MATERIALS AND METHODS

Localization of TRPML1-3 and TRPP2, 3, and 5 in the inner ear of CBA/J mice was investigated by immunohistochemistry.

RESULTS

TRPML1-3 immunoreactivity was evident in the stria vascularis, spiral prominence, and spiral ligament. TRPML immunoreactivity was also observed in outer and inner hair cells, supporting cells, and spiral ganglion cells. The vestibular end organs, vestibular sensory cells, dark cells, and ganglion cells all showed immunoreactivity to TRPML. TRPP2 immunoreactivity was evident in the outer lining of the lateral wall of the cochlea, spiral ganglion cells, vestibular sensory cells, and ganglion cells. TRPP3 immunoreactivity was present in stria vascularis, spiral ganglion cells, vestibular sensory cells, dark cells, and ganglion cells. Faint TRPP5 immunoreactivity was observed in the spiral ganglion cells and vestibular ganglion cells.

Effects of age and sex on the expression of estrogen receptor alpha and beta in the mouse inner ear

CONCLUSION

Estrogen receptor (ER) alpha and beta were expressed in the inner ear, and expression decreased with increasing age. ERalpha may alter cochlear and vestibular sensory transduction, and ERbeta may have a neuroprotective function in the inner ear.

OBJECTIVE

Expression of ERalpha and ERbeta in the mouse inner ear and its alterations with sex and aging were analyzed.

MATERIALS AND METHODS

Male and female CBA/J mice aged 8 weeks and 24 months were used. The localization and the intensity of ERalpha and ERbeta immunoreactivity in the inner ear of young and old mice of both sexes were investigated by immunohistochemistry.

RESULTS

ERalpha and ERbeta were co-expressed in the inner ear, i.e. in the nuclei of stria vascularis, outer and inner hair cells, spiral ganglion cells and vestibular ganglion cells, vestibular dark cells and endolymphatic sac. Strial marginal cells, outer hair cells and type II ganglion cells showed less expression of ERalpha. No gender- or age-related difference was noted in the expression pattern of ERalpha or ERbeta, but fluorescence intensity of ERalpha was stronger in young female mice than in young male mice. In contrast, ERbeta revealed no significant difference. In the old mice, fluorescence intensities of both ERalpha and ERbeta were significantly decreased in both sexes.

Adding insult to injury: cochlear nerve degeneration after "temporary" noise-induced hearing loss

Overexposure to intense sound can cause temporary or permanent hearing loss. Postexposure recovery of threshold sensitivity has been assumed to indicate reversal of damage to delicate mechano-sensory and neural structures of the inner ear and no persistent or delayed consequences for auditory function. Here, we show, using cochlear functional assays and confocal imaging of the inner ear in mouse, that acoustic overexposures causing moderate, but completely reversible, threshold elevation leave cochlear sensory cells intact, but cause acute loss of afferent nerve terminals and delayed degeneration of the cochlear nerve. Results suggest that noise-induced damage to the ear has progressive consequences that are considerably more widespread than are revealed by conventional threshold testing. This primary neurodegeneration should add to difficulties hearing in noisy environments, and could contribute to tinnitus, hyperacusis, and other perceptual anomalies commonly associated with inner ear damage.

[Protection of cochlear function from aminoglycosides ototoxicity by manganese superoxide dismutase gene in aging rat]

OBJECTIVE

determine the feasibility of manganese superoxide dismutase (MnSOD) gene therapy for protecting the cochlear function against aminoglycoside-induced oxidative stress in aging rats.

METHODS

The aging model of SD rats were obtained with 8 weeks daily of D-gal (150 mg/kg per day) hypodermic injection. In the 9th week, amikacin (500 mg/kg per day) were injected intramuscularly into some aging SD rats. The viral particles of recombinant adeno-associated viral vector II/MnSOD (6 microl, 5 x 10(11) vector genomes/ml) were injected into the perilymph through the round window membrane (RWM). The feasibility of MnSOD gene therapy against aminoglycoside-induced oxidative stress in aging rats was evaluated with the methods of caspase-3 protein analysis, apoptosis detection with immunohistochemical, the detection of MnSOD concentration, stretched preparation of basilar membrane and evaluation of hearing threshold with ABR-click.

RESULTS

Compared with the control group, the concentration of MnSOD of cochlear tissue was increased (P < 0.05), and the active fragment expression of caspase-3, the numbers of apoptosis bodies and the hearing threshold were decreased (P < 0.05).

CONCLUSIONS

MnSOD could play a partly role to treat cochlear aminoglycoside-induced oxidative damage in aging rats.

Hair cell bundles: flexoelectric motors of the inner ear

Microvilli (stereocilia) projecting from the apex of hair cells in the inner ear are actively motile structures that feed energy into the vibration of the inner ear and enhance sensitivity to sound. The biophysical mechanism underlying the hair bundle motor is unknown. In this study, we examined a membrane flexoelectric origin for active movements in stereocilia and conclude that it is likely to be an important contributor to mechanical power output by hair bundles. We formulated a realistic biophysical model of stereocilia incorporating stereocilia dimensions, the known flexoelectric coefficient of lipid membranes, mechanical compliance, and fluid drag. Electrical power enters the stereocilia through displacement sensitive ion channels and, due to the small diameter of stereocilia, is converted to useful mechanical power output by flexoelectricity. This motor augments molecular motors associated with the mechanosensitive apparatus itself that have been described previously. The model reveals stereocilia to be highly efficient and fast flexoelectric motors that capture the energy in the extracellular electro-chemical potential of the inner ear to generate mechanical power output. The power analysis provides an explanation for the correlation between stereocilia height and the tonotopic organization of hearing organs. Further, results suggest that flexoelectricity may be essential to the exquisite sensitivity and frequency selectivity of non-mammalian hearing organs at high auditory frequencies, and may contribute to the “cochlear amplifier” in mammals.

Residual microRNA expression dictates the extent of inner ear development in conditional Dicer knockout mice

Inner ear development requires coordinated transformation of a uniform sheet of cells into a labyrinth with multiple cell types. While numerous regulatory proteins have been shown to play critical roles in this process, the regulatory functions of microRNAs (miRNAs) have not been explored. To demonstrate the importance of miRNAs in inner ear development, we generated conditional Dicer knockout mice by the expression of Cre recombinase in the otic placode at E8.5. Otocyst-derived ganglia exhibit rapid neuron-specific miR-124 depletion by E11.5, degeneration by E12.5, and profound defects in subsequent sensory epithelial innervations by E17.5. However, the small and malformed inner ear at E17.5 exhibits residual and graded hair cell-specific miR-183 expression in the three remaining sensory epithelia (posterior crista, utricle, and cochlea) that closely corresponds to the degree of hair cell and sensory epithelium differentiation, and Fgf10 expression required for morphohistogenesis. The highest miR-183 expression is observed in near-normal hair cells of the posterior crista, whereas the reduced utricular macula demonstrates weak miR-183 expression and develops presumptive hair cells with numerous disorganized microvilli instead of ordered stereocilia. The correlation of differential and delayed depletion of mature miRNAs with the derailment of inner ear development demonstrates that miRNAs are crucial for inner ear neurosensory development and neurosensory-dependent morphogenesis.

The forkhead transcription factor Foxi1 is a master regulator of vacuolar H-ATPase proton pump subunits in the inner ear, kidney and epididymis

The vacuolar H(+)-ATPase dependent transport of protons across cytoplasmic membranes in FORE (forkhead related) cells of endolymphatic epithelium in the inner ear, intercalated cells of collecting ducts in the kidney and in narrow and clear cells of epididymis require expression of several subunits that assemble into a functional multimeric proton pump. We demonstrate that expression of four such subunits A1, B1, E2 and a4 all co-localize with the forkhead transcription factor Foxi1 in a subset of epithelial cells at these three locations. In cells, of such epithelia, that lack Foxi1 we fail to identify any expression of A1, B1, E2 and a4 demonstrating an important role for the transcription factor Foxi1 in regulating subunit availability. Promoter reporter experiments, electrophoretic mobility shift assays (EMSA) and site directed mutagenesis demonstrate that a Foxi1 expression vector can trans-activate an a4-promoter reporter construct in a dose dependent manner. Furthermore, we demonstrate using chromatin immunoprecipitation (ChIP) assays that Foxi1-dependent activation to a large extent depends on cis-elements at position -561/-547 in the a4 promoter. Thus, we provide evidence that Foxi1 is necessary for expression of at least four subunits in three different epithelia and most likely is a major determinant for proper assembly of a functional vacuolar H(+)-ATPase complex at these locations.

Expression of erythropoietin and angiogenic growth factors following inner ear injury of newborn rats

Recently we have demonstrated that recombinant human erythropoietin (EPO) protects neurosensory hair cells in the organotypic culture of the organ of Corti by reducing apoptosis and necrosis. In the present study, we tested the hypothesis that EPO may be involved in reparative angiogenesis. We analyzed in parallel the endogenous erythropoietin (Epo) mRNA expression and that of Epo receptor (Epor) and of genes associated with angiogenesis in the organ of Corti, the modiolus and the stria vascularis using real time reverse transcription polymerase chain reaction and microarray. We compared the expression levels of freshly prepared tissue (control) and tissue cultured for 24 h under normoxia or hypoxia. The basal expression of Epo- and Epor mRNA in controls of all regions was very low. However, after 24 h in culture, a 20-100 fold increase of these two transcripts was measured. In culture, the vascular endothelial growth factor and the Cxcr4 (the receptor for the stromal cell-derived factor-1, Sdf-1) mRNA levels, were found to be increased and the Sdf-1 mRNA level to be decreased. Changes in mRNA expression occurred in all pathways activated in non-erythroid cells by the application of EPO (phosphoinositide 3-kinase/serine-threonine protein kinase B, Janus-type protein tyrosine kinase 2/signal transducer and activator of transcription 3, and the mitogen activated protein kinase). These data suggest that the neuroprotective effect of EPO may include at least two molecular events, the decrease of hair cell death rate and the induction of angiogenic genes.

ATP release through connexin hemichannels and gap junction transfer of second messengers propagate Ca2+ signals across the inner ear

Extracellular ATP controls various signaling systems including propagation of intercellular Ca(2+) signals (ICS). Connexin hemichannels, P2x7 receptors (P2x7Rs), pannexin channels, anion channels, vesicles, and transporters are putative conduits for ATP release, but their involvement in ICS remains controversial. We investigated ICS in cochlear organotypic cultures, in which ATP acts as an IP(3)-generating agonist and evokes Ca(2+) responses that have been linked to noise-induced hearing loss and development of hair cell-afferent synapses. Focal delivery of ATP or photostimulation with caged IP(3) elicited Ca(2+) responses that spread radially to several orders of unstimulated cells. Furthermore, we recorded robust Ca(2+) signals from an ATP biosensor apposed to supporting cells outside the photostimulated area in WT cultures. ICS propagated normally in cultures lacking either P2x7R or pannexin-1 (Px1), as well as in WT cultures exposed to blockers of anion channels. By contrast, Ca(2+) responses failed to propagate in cultures with defective expression of connexin 26 (Cx26) or Cx30. A companion paper demonstrates that, if expression of either Cx26 or Cx30 is blocked, expression of the other is markedly down-regulated in the outer sulcus. Lanthanum, a connexin hemichannel blocker that does not affect gap junction (GJ) channels when applied extracellularly, limited the propagation of Ca(2+) responses to cells adjacent to the photostimulated area. Our results demonstrate that these connexins play a dual crucial role in inner ear Ca(2+) signaling: as hemichannels, they promote ATP release, sustaining long-range ICS propagation; as GJ channels, they allow diffusion of Ca(2+)-mobilizing second messengers across coupled cells.