FGF20-FGFR1 signaling through MAPK and PI3K controls sensory progenitor differentiation in the organ of Corti

BACKGROUND

Fibroblast Growth Factor 20 (FGF20)-FGF receptor 1 (FGFR1) signaling is essential for cochlear hair cell (HC) and supporting cell (SC) differentiation. In other organ systems, FGFR1 signals through several intracellular pathways including MAPK (ERK), PI3K, phospholipase C ɣ (PLCɣ), and p38. Previous studies implicated MAPK and PI3K pathways in HC and SC development. We hypothesized that one or both would be important downstream mediators of FGF20-FGFR1 signaling for HC differentiation.

RESULTS

By inhibiting pathways downstream of FGFR1 in cochlea explant cultures, we established that both MAPK and PI3K pathways are required for HC differentiation while PLCɣ and p38 pathways are not. Examining the canonical PI3K pathway, we found that while AKT is necessary for HC differentiation, it is not sufficient to rescue the Fgf20-/- phenotype. To determine whether PI3K functions downstream of FGF20, we inhibited Phosphatase and Tensin Homolog (PTEN) in Fgf20-/- explants. Overactivation of PI3K resulted in a partial rescue of the Fgf20-/- phenotype, demonstrating a requirement for PI3K downstream of FGF20. Consistent with a requirement for the MAPK pathway for FGF20-regulated HC differentiation, we show that treating Fgf20-/- explants with FGF9 increased levels of dpERK.

CONCLUSIONS

Together, these data provide evidence that both MAPK and PI3K are important downstream mediators of FGF20-FGFR1 signaling during HC and SC differentiation.

Insights into the Biology of Hearing and Deafness Revealed by Single-Cell RNA Sequencing

Single-cell RNA sequencing is a powerful tool by which to characterize the transcriptional profile of low-abundance cell types, but its application to the inner ear has been hampered by the bony labyrinth, tissue sparsity, and difficulty dissociating the ultra-rare cells of the membranous cochlea. Herein, we present a method to isolate individual inner hair cells (IHCs), outer hair cells (OHCs), and Deiters' cells (DCs) from the murine cochlea at any post-natal time point. We harvested more than 200 murine IHCs, OHCs, and DCs from post-natal days 15 (p15) to 228 (p228) and leveraged both short- and long-read single-cell RNA sequencing to profile transcript abundance and structure. Our results provide insights into the expression profiles of these cells and document an unappreciated complexity in isoform variety in deafness-associated genes. This refined view of transcription in the organ of Corti improves our understanding of the biology of hearing and deafness.

Morphological Immaturity of the Neonatal Organ of Corti and Associated Structures in Humans

Although anatomical development of the cochlear duct is thought to be complete by term birth, human newborns continue to show postnatal immaturities in functional measures such as otoacoustic emissions (OAEs). Some of these OAE immaturities are no doubt influenced by incomplete maturation of the external and middle ears in infants; however, the observed prolongation of distortion-product OAE phase-gradient delays in newborns cannot readily be explained by conductive factors. This functional immaturity suggests that the human cochlea at birth may lack fully adult-like traveling-wave motion. In this study, we analyzed temporal-bone sections at the light microscopic level in newborns and adults to quantify dimensions and geometry of cochlear structures thought to influence the mechanical response of the cochlea. Contrary to common belief, results show multiple morphological immaturities along the length of the newborn spiral, suggesting that important refinements in the size and shape of the sensory epithelium and associated structures continue after birth. Specifically, immaturities of the newborn basilar membrane and organ of Corti are consistent with a more compliant and less massive cochlear partition, which could produce longer DPOAE delays and a shifted frequency-place map in the neonatal ear.

Understanding Molecular Evolution and Development of the Organ of Corti Can Provide Clues for Hearing Restoration

The mammalian hearing organ is a stereotyped cellular assembly with orderly innervation: two types of spiral ganglion neurons (SGNs) innervate two types of differentially distributed hair cells (HCs). HCs and SGNs evolved from single neurosensory cells through gene multiplication and diversification. Independent regulation of HCs and neuronal differentiation through expression of basic helix-loop-helix transcription factors (bHLH TFs: Atoh1, Neurog1, Neurod1) led to the evolution of vestibular HC assembly and their unique type of innervation. In ancestral mammals, a vestibular organ was transformed into the organ of Corti (OC) containing a single row of inner HC (IHC), three rows of outer HCs (OHCs), several unique supporting cell types, and a peculiar innervation distribution. Restoring the OC following long-term hearing loss is complicated by the fact that the entire organ is replaced by a flat epithelium and requires reconstructing the organ from uniform undifferentiated cell types, recapitulating both evolution and development. Finding the right sequence of gene activation during development that is useful for regeneration could benefit from an understanding of the OC evolution. Toward this end, we report on Foxg1 and Lmx1a mutants that radically alter the OC cell assembly and its innervation when mutated and may have driven the evolutionary reorganization of the basilar papilla into an OC in ancestral Therapsids. Furthermore, genetically manipulating the level of bHLH TFs changes HC type and distribution and allows inference how transformation of HCs might have happened evolutionarily. We report on how bHLH TFs regulate OHC/IHC and how misexpression (Atoh1-Cre; Atoh1f/kiNeurog1) alters HC fate and supporting cell development. Using mice with altered HC types and distribution, we demonstrate innervation changes driven by HC patterning. Using these insights, we speculate on necessary steps needed to convert a random mixture of post-mitotic precursors into the orderly OC through spatially and temporally regulated critical bHLH genes in the context of other TFs to restore normal innervation patterns.

Six1 is essential for differentiation and patterning of the mammalian auditory sensory epithelium

The organ of Corti in the cochlea is a two-cell layered epithelium: one cell layer of mechanosensory hair cells that align into one row of inner and three rows of outer hair cells interdigitated with one cell layer of underlying supporting cells along the entire length of the cochlear spiral. These two types of epithelial cells are derived from common precursors in the four- to five-cell layered primordium and acquire functionally important shapes during terminal differentiation through the thinning process and convergent extension. Here, we have examined the role of Six1 in the establishment of the auditory sensory epithelium. Our data show that prior to terminal differentiation of the precursor cells, deletion of Six1 leads to formation of only a few hair cells and defective patterning of the sensory epithelium. Previous studies have suggested that downregulation of Sox2 expression in differentiating hair cells must occur after Atoh1 mRNA activation in order to allow Atoh1 protein accumulation due to antagonistic effects between Atoh1 and Sox2. Our analysis indicates that downregulation of Sox2 in the differentiating hair cells depends on Six1 activity. Furthermore, we found that Six1 is required for the maintenance of Fgf8 expression and dynamic distribution of N-cadherin and E-cadherin in the organ of Corti during differentiation. Together, our analyses uncover essential roles of Six1 in hair cell differentiation and formation of the organ of Corti in the mammalian cochlea.

Maturation of NaV and KV Channel Topographies in the Auditory Nerve Spike Initiator before and after Developmental Onset of Hearing Function

Auditory nerve excitation and thus hearing depend on spike-generating ion channels and their placement along the axons of auditory nerve fibers (ANFs). The developmental expression patterns and native axonal locations of voltage-gated ion channels in ANFs are unknown. Therefore, we examined the development of heminodes and nodes of Ranvier in the peripheral axons of type I ANFs in the rat cochlea with immunohistochemistry and confocal microscopy. Nodal structures presumably supporting presensory spiking formed between postnatal days 5 (P5) and P7, including Ankyrin-G, NaV1.6, and Caspr. These immature nodal structures lacked low-voltage-activated KV1.1 which was not enriched at juxtaparanodes until approximately P13, concurrent with the developmental onset of acoustic hearing function. Anatomical alignment of ANF spike-initiating heminodes relative to excitatory input from inner hair cell (IHC) ribbon synapses continued until approximately P30. High-voltage-activated KV3.1b and KV2.2 were expressed in mutually exclusive domains: KV3.1b was strictly localized to nodes and heminodes, whereas KV2.2 expression began at the juxtaparanodes and continued centrally along the first internode. At spike-initiating heminodes in the distal osseous spiral lamina, NaV1.1 partly overlapped NaV1.6 and ankyrin-G. ANFs displayed KV7.2 and KV7.3 at heminodes, nodes, internodes, and the unmyelinated synaptic terminal segments beneath IHCs in the organ of Corti. In response to sound, spikes are initiated at the heminode, which is tightly coupled to the IHC ribbon synapse ∼20-40 μm away. These results show that maturation of nodal alignment and ion channel content may underlie postnatal improvements of ANF excitability and discharge synchrony.

SIGNIFICANCE STATEMENT

Acoustic and electrical hearing depends on rapid, reliable, and precise spike generation in auditory nerve fibers. A limitation of current models and therapies is a lack of information on the identities and topographies of underlying ion channels. We report the developmental profile of the auditory nerve spike generator with a focus on NaV1.1, NaV1.6, KV1.1, KV2.2, KV3.1b, KV7.2, and KV7.3 in relation to the scaffold ankyrin-G. Molecular anatomy of the spike generator matures in the weeks after developmental onset of hearing function. Subcellular positioning of voltage-gated ion channels will enable multicompartmental modeling of auditory nerve responses elicited by afferent chemical neurotransmission from hair cells and modulated by efferent neurotransmitters or evoked by extracellular field stimulation from a cochlear implant.

Age-Related Hearing Loss and Degeneration of Cochlear Hair Cells in Mice Lacking Thyroid Hormone Receptor β1

A key function of the thyroid hormone receptor β (Thrb) gene is in the development of auditory function. However, the roles of the 2 receptor isoforms, TRβ1 and TRβ2, expressed by the Thrb gene are unclear, and it is unknown whether these isoforms promote the maintenance as well as development of hearing. We investigated the function of TRβ1 in mice with a Thrb(b1) reporter allele that expresses β-galactosidase instead of TRβ1. In the immature cochlea, β-galactosidase was detected in the greater epithelial ridge, sensory hair cells, spiral ligament, and spiral ganglion and in adulthood, at low levels in the hair cells, support cells and root cells of the outer sulcus. Although deletion of all TRβ isoforms causes severe, early-onset deafness, deletion of TRβ1 or TRβ2 individually caused no obvious hearing loss in juvenile mice. However, over subsequent months, TRβ1 deficiency resulted in progressive loss of hearing and loss of hair cells. TRβ1-deficient mice had minimal changes in serum thyroid hormone and thyrotropin levels, indicating that hormonal imbalances were unlikely to cause hearing loss. The results suggest mutually shared roles for TRβ1 and TRβ2 in cochlear development and an unexpected requirement for TRβ1 in the maintenance of hearing in adulthood.

Developmental expression of inositol 1, 4, 5-trisphosphate receptor in the post-natal rat cochlea

Inositol 1, 4, 5-trisphosphate receptor (IP3R) has been established to be essential for hearing. However, the expression of IP3R in the cochlea in the period of auditory development remains unknown. We investigated the expression of IP3R in the developing rat cochlea using immunohistochemistry and real-time reverse transcription polymerase chain reaction (RT-PCR). We observed its presence in the developing rat cochlea, and changes in IP3R protein expressions from the early post-natal period to adult. At birth (post-natal day 0, P0), IP3R expression was only found in Hensen's cell. IP3R immunoreactivity first appeared in the sensory hair cells in the organ of Corti at P2. This localization was confirmed by means of double-labeling experiments with Myosin VIIA, a marker for cochlear hair cells. Colocalization of IP3R and Myosin VIIA from P2 to the second post-natal week suggested early expression of IP3R in developing inner and outer hair cells. Claudius' cells near the spiral ligament were labelled for IP3R from P8 onwards. Transient IP3R expression was observed in the stria vascularis in early post-natal rat from P4 to P8. Spiral ganglion neurons also exhibited weaker IP3R fluorescence signals during post-natal development. The results of RT-PCR demonstrated that all three IP3R isoforms (IP3R1, IP3R2, and IP3R3) were present in rat cochlea during four different developmental stages of cochlea, from P0 to P28. Present immunohistochemical evidence for both change and maintenance of expression of IP3R during post-natal development of the rat cochlea indicated the possible involvement of IP3R-mediated calcium signaling in cochlear development.

HMGA2, the architectural transcription factor high mobility group, is expressed in the developing and mature mouse cochlea

Hmga2 protein belongs to the non-histone chromosomal high-mobility group (HMG) protein family. HMG proteins have been shown to function as architectural transcription regulators, facilitating enhanceosome formation on a variety of mammalian promoters. Hmga2 are expressed at high levels in embryonic and transformed cells. Terminally differentiated cells, however, have been reported to express only minimal, if any, Hmga2. Our previous affymetrix array data showed that Hmga2 is expressed in the developing and adult mammalian cochleas. However, the spatio-temporal expression pattern of Hmga2 in the murine cochlea remained unknown. In this study, we report the expression of Hmga2 in developing and adult cochleas using immunohistochemistry and quantitative real time PCR analysis. Immunolabeling of Hmga2 in the embryonic, postnatal, and mature cochleas showed broad Hmga2 expression in embryonic cochlea (E14.5) at the level of the developing organ of Corti in differentiating hair cells, supporting cells, in addition to immature cells in the GER and LER areas. By postnatal stage (P0–P3), Hmga2 is predominantly expressed in the hair and supporting cells, in addition to cells in the LER area. By P12, Hmga2 immunolabeling is confined to the hair cells and supporting cells. In the adult ear, Hmga2 expression is maintained in the hair and supporting cell subtypes (i.e. Deiters’ cells, Hensen cells, pillar cells, inner phalangeal and border cells) in the cochlear epithelium. Using quantitative real time PCR, we found a decrease in transcript level for Hmga2 comparable to other known inner ear developmental genes (Sox2, Atoh1, Jagged1 and Hes5) in the cochlear epithelium of the adult relative to postnatal ears. These data provide for the first time the tissue-specific expression and transcription level of Hmga2 during inner ear development and suggest its potential dual role in early differentiation and maintenance of both hair and supporting cell phenotypes.

Evidence for a partial epithelial-mesenchymal transition in postnatal stages of rat auditory organ morphogenesis

The epithelial-mesenchymal transition (EMT) plays a crucial role in the differentiation of many tissues and organs. So far, an EMT was not detected in the development of the auditory organ. To determine whether an EMT may play a role in the morphogenesis of the auditory organ, we studied the spatial localization of several EMT markers, the cell-cell adhesion molecules and intermediate filament cytoskeletal proteins, in epithelium of the dorsal cochlea during development of the rat Corti organ from E18 (18th embryonic day) until P25 (25th postnatal day). We examined by confocal microscopy immunolabelings on cryosections of whole cochleae with antibodies anti-cytokeratins as well as with antibodies anti-vimentin, anti-E-cadherin and anti-β-catenin. Our results showed a partial loss of E-cadherin and β-catenin and a temporary appearance of vimentin in pillar cells and Deiters between P8 and P10. These observations suggest that a partial EMT might be involved in the remodelling of the Corti organ during the postnatal stages of development in rat.

A novel Atoh1 "self-terminating" mouse model reveals the necessity of proper Atoh1 level and duration for hair cell differentiation and viability

Atonal homolog1 (Atoh1) is a bHLH transcription factor essential for inner ear hair cell differentiation. Targeted expression of Atoh1 at various stages in development can result in hair cell differentiation in the ear. However, the level and duration of Atoh1 expression required for proper hair cell differentiation and maintenance remain unknown. We generated an Atoh1 conditional knockout (CKO) mouse line using Tg(Atoh1-cre), in which the cre expression is driven by an Atoh1 enhancer element that is regulated by Atoh1 protein to “self-terminate” its expression. The mutant mice show transient, limited expression of Atoh1 in all hair cells in the ear. In the organ of Corti, reduction and delayed deletion of Atoh1 result in progressive loss of almost all the inner hair cells and the majority of the outer hair cells within three weeks after birth. The remaining cells express hair cell marker Myo7a and attract nerve fibers, but do not differentiate normal stereocilia bundles. Some Myo7a-positive cells persist in the cochlea into adult stages in the position of outer hair cells, flanked by a single row of pillar cells and two to three rows of disorganized Deiters cells. Gene expression analyses of Atoh1, Barhl1 and Pou4f3, genes required for survival and maturation of hair cells, reveal earlier and higher expression levels in the inner compared to the outer hair cells. Our data show that Atoh1 is crucial for hair cell mechanotransduction development, viability, and maintenance and also suggest that Atoh1 expression level and duration may play a role in inner vs. outer hair cell development. These genetically engineered Atoh1 CKO mice provide a novel model for establishing critical conditions needed to regenerate viable and functional hair cells with Atoh1 therapy.

Decrease of prestin expression by increased potassium concentration in organotypic cultures of the organ of Corti of newborn rats

Prestin is the motor protein of the outer hair cells of the organ of Corti and a key factor in ensuring a high sensitivity level of mammalian hearing. In the present study, we examined the effects of increased extracellular potassium (K(+)) concentration on the expression of prestin mRNA and the transcription factors Gata-3 and Carf in the organotypic culture of the organ of Corti of newborn rats. Mannitol and NaCl were used to analyze possible effects of hyperosmotic stress or ion-specific changes, respectively. An increase in prestin expression by a factor of 1.5-2.0 was seen in cultures grown in the presence of 5mM K(+). Potassium concentration of 35 and 55 mM induced a parallel decrease in prestin and Carf expression, but Gata-3 expression increased. Mannitol had no effect on gene expression whereas increased NaCl concentrations decreased prestin, but not Carf expression. The data suggest that chronic depolarization might decrease the prestin expression and possibly contribute to hearing loss and tinnitus.

β-actin and γ-actin are each dispensable for auditory hair cell development but required for Stereocilia maintenance

Hair cell stereocilia structure depends on actin filaments composed of cytoplasmic β-actin and γ-actin isoforms. Mutations in either gene can lead to progressive hearing loss in humans. Since β-actin and γ-actin isoforms are 99% identical at the protein level, it is unclear whether each isoform has distinct cellular roles. Here, we compared the functions of β-actin and γ-actin in stereocilia formation and maintenance by generating mice conditionally knocked out for Actb or Actg1 in hair cells. We found that, although cytoplasmic actin is necessary, neither β-actin nor γ-actin is required for normal stereocilia development or auditory function in young animals. However, aging mice with β-actin– or γ-actin–deficient hair cells develop different patterns of progressive hearing loss and distinct pathogenic changes in stereocilia morphology, despite colocalization of the actin isoforms. These results demonstrate overlapping developmental roles but unique post-developmental functions for β-actin and γ-actin in maintaining hair cell stereocilia.

Genetic mutations that cause progressive hearing loss offer insight into the cellular processes that are required to maintain auditory function. In humans, several such deafness-causing mutations have been identified in the gene encoding γ-actin. This protein, together with the closely-related β-actin protein, comprise the primary structural elements of stereocilia, which are specialized structures on sensory cells in the inner ear that convert mechanical sound energy into neural signals. β-actin and γ-actin are 99% identical, but their slight differences have been exactly conserved through evolution from birds to mammals suggesting that each protein may have important and distinct functions. We tested this idea by knocking out each gene in mouse auditory hair cells. Although the isoforms are completely redundant during stereocilia development, β-actin and γ-actin have different functions during stereocilia maintenance. Both knockout mice had normal hearing at young ages, but developed specific types of progressive hearing loss and stereocilia pathology that differed depending on which isoform was lost. These separate maintenance pathways are likely important for maintaining auditory function during aging and may contribute to future understanding of common forms of age-related hearing loss in humans.

Sonic hedgehog (SHH) promotes the differentiation of mouse cochlear neural progenitors via the Math1-Brn3.1 signaling pathway in vitro

Sonic hedgehog (SHH) is essential for the development of the cochlear duct that harbors the organ of Corti. However, little is known about the molecular signaling pathway through which SHH promotes the development of the organ of Corti, especially cochlear sensory epithelial cells. In this study, we demonstrated that SHH contributes to the differentiation of cochlear neural progenitors (CNPs), which are derived from the postnatal day 1 organ of Corti in mice. Addition of SHH to CNPs increased the formation of epithelial cell islands, simultaneously activated the expression of Math1 that is a transcription factor for the initial differentiation of auditory hair cells. The increased expression of Math1 then regulated the promoter activity of Brn3.1, another transcription factor that controls the further differentiation and survival of auditory hair cells. Taken together, our data suggest that SHH plays an important role in the promotion of auditory hair cell differentiation via the Math1-Brn3.1 signaling pathway.

Postnatal development of the organ of Corti in dominant-negative Gjb2 transgenic mice

Hereditary hearing loss is one of the most prevalent inherited human birth defects, affecting one in 2000. A strikingly high proportion (50%) of congenital bilateral nonsyndromic sensorineural deafness cases have been linked to mutations in the GJB2 coding for the connexin26. It has been hypothesized that gap junctions in the cochlea, especially connexin26, provide an intercellular passage by which K(+) are transported to maintain high levels of the endocochlear potential essential for sensory hair cell excitation. We previously reported the generation of a mouse model carrying human connexin26 with R75W mutation (R75W+ mice). The present study attempted to evaluate postnatal development of the organ of Corti in the R75W+ mice. R75W+ mice have never shown auditory brainstem response waveforms throughout postnatal development, indicating the disturbance of auditory organ development. Histological observations at postnatal days (P) 5-14 were characterized by i) absence of tunnel of Corti, Nuel's space, or spaces surrounding the outer hair cells, ii) significantly small numbers of microtubules in inner pillar cells, iii) shortening of height of the organ of Corti, and iv) increase of the cross-sectional area of the cells of the organ of Corti. Thus, morphological observations confirmed that a dominant-negative Gjb2 mutation showed incomplete development of the cochlear supporting cells. On the other hand, the development of the sensory hair cells, at least from P5 to P12, was not affected. The present study suggests that Gjb2 is indispensable in the postnatal development of the organ of Corti and normal hearing.

Dietary thyroid hormone replacement ameliorates hearing deficits in hypothyroid mice

Thyroid hormone (TH) insufficiency causes variable hearing impairment and mental deficiency in humans. Rodents lacking TH have congenital hearing deficiency that has been attributed to physiologic, morphologic, and developmental abnormalities of the auditory system. We examined four genetically defined strains of hypothyroid mice for development of hearing and response to TH replacement initiated during late gestation and continued through six weeks of age. Auditory brain stem response studies showed variable hearing impairment in homozygous mutants of each strain at three weeks of age relative to normal littermates. Mutants from three of the strains still had hearing deficiencies at six weeks of age. TH-enriched diet significantly improved hearing in three-week-old mutants of each strain relative to untreated mutants. Differences in the level of hearing impairment between the Prop1df and Pit1dw mutants, which have defects in the same developmental pathway, were determined to be due to genetic background modifier genes. Further physiologic and morphologic studies in the Cgatm1Sac strain indicated that poor hearing was due to cochlear defects. We conclude that TH supplement administered during the critical period of hearing development in mice can prevent deafness associated with congenital hypothyroidism of heterogeneous genetic etiology.

Expression of apoptosis-related genes in the organ of Corti, modiolus and stria vascularis of newborn rats

Cell death in the inner ear tissues is an important mechanism leading to hearing impairment. Here, using microarrays and real-time RT-PCR we analyzed expression of selected apoptosis-related genes in rat's inner ear. We determined the gene expression in tissues freshly isolated from neonatal rats (3-5 days old) and compared it to that of explants cultured for 24 h under normoxic or hypoxic conditions. For the analyses, we used pooled samples of the organ of Corti (OC), modiolus (MOD) and stria vascularis (SV), respectively. We observed region-specific changes in gene expression between the fresh tissues and the normoxic culture. In the OC, expression of the proapoptotic genes caspase-2, caspase-3, caspase-6 and calpain-1 was downregulated. In the MOD, the antioxidative defense SOD-2 and SOD-3 were upregulated. In the SV, caspase-2, caspase-6, calpain-1 and SOD-3 were downregulated and SOD-2 upregulated. We speculate that these changes could reflect survival shift in transcriptome of inner ear explants tissues under in vitro conditions. With the exception of SOD-2, hypoxic culture conditions induced the same changes in gene expression as the normoxic conditions indicating that culture preparation is likely the dominating factor, which modifies the gene expression pattern. We conclude that various culture conditions induce different expression pattern of apoptosis-related genes in the organotypic cochlear cultures, as compared to fresh tissues. This transcriptional pattern may reflect the survival ability of specific tissues and could become a tempting target for a pharmacological intervention in inner ear diseases.

Delayed Formation of Actin Filaments in the Outer Pillar Head Plate of VASP–/– Mice

Pillar cells with their rich network of tubulin and actin filaments have been reported to contribute to the rigidity of the organ of Corti. As the earliest expression of the actin filament enhancer vasodilator-stimulated phosphoprotein (VASP) in the outer pillar head plate has been found to be associated with the onset of hearing, we tested hearing development in VASP-/- compared to wild-type mice. Performing measurements of auditory brainstem responses on postnatal days (P) P14 and P21, we detected statistically significantly higher thresholds in VASP-/- compared to wild-type mice at P14, but no hearing differences at P21. Staining for prestin and synaptophysin at P12 in morphologically regularly developed cochleae of VASP-/- mice provided an immature prestin protein pattern but no evidence of developmental delay in hair cell innervations. Regularly intense staining of actin filaments in the outer pillar head plate was found only in wild-type but not in VASP-/- mice at P14. At P21, intensive actin filament staining was also observed in the outer pillar head plates of VASP-/- mice. The delayed hearing development in VASP-/- mice is supposed to be caused by a delayed formation of actin filaments in the outer pillar head plate indicating the importance of appropriate pillar cell stiffness in cochlear mechanics.

Planar cell polarity signaling in vertebrates

Planar cell polarity (PCP) refers to the polarization of a field of cells within the plane of a cell sheet. This form of polarization is required for diverse cellular processes in vertebrates, including convergent extension (CE), the establishment of PCP in epithelial tissues and ciliogenesis. Perhaps the most distinct example of vertebrate PCP is the uniform orientation of stereociliary bundles at the apices of sensory hair cells in the mammalian auditory sensory organ. The establishment of PCP in the mammalian cochlea occurs concurrently with CE in this ciliated epithelium, therefore linking three cellular processes regulated by the vertebrate PCP pathway in the same tissue and emerging as a model system for dissecting PCP signaling. This review summarizes the morphogenesis of this model system to assist the interpretation of the emerging data and proposes molecular mechanisms underlying PCP signaling in vertebrates.

MicroRNA gene expression in the mouse inner ear

MicroRNAs (miRNAs) are small non-coding RNAs that function through the RNA interference (RNAi) pathway and post-transcriptionally regulate gene expression in eukaryotic organisms. While miRNAs are known to affect cellular proliferation, differentiation, and morphological development, neither their expression nor roles in mammalian inner ear development have been characterized. We have investigated the extent of miRNA expression at various time points throughout maturation of the postnatal mouse inner ear by microarray analysis. Approximately one third of known miRNAs are detected in the inner ear, and their expression persists to adulthood. Expression of such miRNAs is validated by quantitative PCR and northern blot analysis. Further analysis by in situ hybridization demonstrates that certain miRNAs exhibit cell-specific expression patterns in the mouse inner ear. Notably, we demonstrate that miRNAs previously associated with mechanosensory cells in zebrafish are also expressed in hair cells of the auditory and vestibular endorgans. Our results demonstrate that miRNA expression is abundant in the mammalian inner ear and that certain miRNAs are evolutionarily associated with mechanosensory cell development and/or function. The data suggest that miRNAs contribute substantially to genetic programs intrinsic to development and function of the mammalian inner ear and that specific miRNAs might influence formation of sensory epithelia from the primitive otic neuroepithelium.

Aroclor 1254 impairs the hearing ability of Xenopus laevis

In this study we assessed the effects of chronic, dietary exposure of Aroclor 1254 (A1254) on the hearing of Xenopus frogs. We used the auditory brainstem response (ABR) to assay changes in hearing physiology; ABR thresholds, as well as latency-intensity and amplitude-intensity profiles of the initial positive (P1) and negative (N1) peaks were measured. Two groups of animals that received 50 ppm and 100 ppm of A1254 in their diet from 5 days post-fertilization through metamorphosis were compared to a control group that received untreated chow. The results showed significant threshold elevations in the 3-4 kHz range and significantly delayed peak latencies and reduced amplitudes at these frequencies in A1254 treated animals as compared to control animals. These findings indicate that A1254 selectively damages the high-frequency sensorineural hearing system associated with the basilar papilla of frogs. This preferential damage may be related to inherent differences in the vulnerability of the basilar versus amphibian papilla in the frog. The overall results of this study are also consistent with the reported A1254-induced auditory deficits in mammals indicating that the basilar papilla of the Xenopus frog may serve as an effective model for studying the effects of A1254 on the auditory system.

Compartmentalized and signal-selective gap junctional coupling in the hearing cochlea

Gap junctional intercellular communication (GJIC) plays a major role in cochlear function. Recent evidence suggests that connexin 26 (Cx26) and Cx30 are the major constituent proteins of cochlear gap junction channels, possibly in a unique heteromeric configuration. We investigated the functional and structural properties of native cochlear gap junctions in rats, from birth to the onset of hearing [postnatal day 12 (P12)]. Confocal immunofluorescence revealed increasing Cx26 and Cx30 expression from P0 to P12. Functional GJIC was assessed by coinjection of Lucifer yellow (LY) and Neurobiotin (NBN) during whole-cell recordings in cochlear slices. At P0, there was restricted dye transfer between supporting cells around outer hair cells. Transfer was more extensive between supporting cells around inner hair cells. At P8, there was extensive transfer of both dyes between all supporting cell types. By P12, LY no longer transferred between the supporting cells immediately adjacent to hair cells but still transferred between more peripheral cells. NBN transferred freely, but it did not transfer between inner and outer pillar cells. Freeze fracture further demonstrated decreasing GJIC between inner and outer pillar cells around the onset of hearing. These data are supportive of the appearance of signal-selective gap junctions around the onset of hearing, with specific properties required to support auditory function. Furthermore, they suggest that separate medial and lateral buffering compartments exist in the hearing cochlea, which are individually dedicated to the homeostasis of inner hair cells and outer hair cells.

Balanced levels of Espin are critical for stereociliary growth and length maintenance

Hearing and balance depend on microvilli-like actin-based projections of sensory hair cells called stereocilia. Their sensitivity to mechanical displacements on the nanometer scale requires a highly organized hair bundle in which the physical dimension of each stereocilium is tightly controlled. The length and diameter of each stereocilium are established during hair bundle maturation and maintained by life-long continuing dynamic regulation. Here, we studied the role of the actin-bundling protein Espin in stereociliary growth by examining the hair cell stereocilia of Espin-deficient jerker mice (Espn(je)), and the effects of transiently overexpressing Espin in the neuroepithelial cells of the organ of Corti cultures. Using fluorescence scanning confocal and electron microscopy, we found that a lack of Espin results in inhibition of stereociliary growth followed by progressive degeneration of the hair bundle. In contrast, overexpression of Espin induced lengthening of stereocilia and microvilli that mirrored the elongation of the actin filament bundle at their core. Interestingly, Espin deficiency also appeared to influence the localization of Myosin XVa, an unconventional myosin that is normally present at the stereocilia tip at levels proportional to stereocilia length. These results indicate that Espin is important for the growth and maintenance of the actin-based protrusions of inner ear neuroepithelial cells.

Expression of prestin mRNA in the organotypic culture of rat cochlea

To quantitate in absolute terms the prestin mRNA levels in the explant culture of rat cochlea, we used competitive RT-PCR with a synthetic internal cRNA standard. Prestin gene expression was found at levels of 100 fg specific mRNA/microg total RNA on postnatal day 3, which corresponds to about 300 copies per outer hair cell (OHC) and is indicative of an intermediate level of expression. Two days of culturing resulted in an increase of prestin mRNA levels and in the formation of an apical-basal gradient (p<0.001). To elucidate the variations the prestin mRNA levels undergo as a result of damage to the organ of Corti, we exposed the explant cultures to ischemia and hypoxia. While total RNA was observed to remain unchanged, the numbers of OHCs and the prestin mRNA levels were found to decrease by about 20% and 35%, respectively, compared to normoxia. In conclusion, we showed that the prestin mRNA levels during in vitro development increase and form an apical-basal gradient within 2 days in culture, similar to the postnatal in vivo development. Hypoxia and ischemia result in a decrease of the prestin mRNA level in parallel with OHC loss. The prestin mRNA level can therefore be used as marker of damage to or loss of OHCs.

Biotinidase reveals the morphogenetic sequence in cochlea and cochlear nucleus of mice

Hearing loss affects children with biotinidase deficiency, an inherited metabolic disorder in the recycling of biotin. The deficit appears shortly after birth during development of the auditory system. Using a mouse model, we sought to discover where and when biotinidase is expressed in the normal development of the cochlea and cochlear nucleus. In the process, we reconstructed the normal morphogenetic sequences of the constituent cells. Immunolabeling for biotinidase was localized to neurons and other cells of the adult and immature mouse, including the embryonic precursors of these regions dating from the stage of the otocyst. Its distribution was compared to the particular morphological changes occurring at each developmental stage. Biotinidase was localized in cells and their processes at the critical stages in their proliferation, migration, structural differentiation, and innervation, covering the entire span of their development. The prevalence of immunostaining peaked in the adult animal, including hair cells and ganglion cells of the cochlea and neurons of the cochlear nucleus. The findings suggest that biotinidase plays a role in the normal development of the auditory system. Besides the pattern of localization of biotinidase, this study provides the first systematic account of each developmental stage in a mammalian auditory system.

Differentiation of an auditory neuronal cell line suitable for cell transplantation

The auditory neuroblast cell line US/VOT-N33 (N33), which is conditionally immortal, was studied as an in vitro model for the differentiation of spiral ganglion neurons (SGNs) and as a candidate for cell transplantation in rodents. It expresses numerous molecular markers characteristic of auditory neuroblasts, including the transcription factors GATA3, NeuroD, Brn3a and Islet1, as well as the neuronal cytoskeletal protein beta3-tubulin. It displays active migratory behaviour in vitro and in vivo. In the presence of the fibroblast growth factors FGF1 or FGF2 it differentiates bipolar morphologies similar to those of native SGNs. In coculture with neonatal cochlear tissue it is repelled from epithelial surfaces but not from native SGNs, alongside which it extends parallel neuronal processes. When injected into the retina in vivo, EGFP-labelled N33 cells were traced for 1-2 weeks and migrated rapidly within the subretinal space. Cells that found their way into the retinal ganglion cell layer extended multiple processes but did not express beta3-tubulin. The ability of N33 to migrate, to differentiate, to localize with native SGNs in vitro and to survive in vivo suggests that they provide an effective model for SGN differentiation and for cell transplantation into the ear.

Sprouty2, a mouse deafness gene, regulates cell fate decisions in the auditory sensory epithelium by antagonizing FGF signaling

The auditory sensory epithelium (organ of Corti), where sound waves are converted to electrical signals, comprises a highly ordered array of sensory receptor (hair) cells and nonsensory supporting cells. Here, we report that Sprouty2, which encodes a negative regulator of signaling via receptor tyrosine kinases, is required for normal hearing in mice, and that lack of SPRY2 results in dramatic perturbations in organ of Corti cytoarchitecture: instead of two pillar cells, there are three, resulting in the formation of an ectopic tunnel of Corti. We demonstrate that these effects are due to a postnatal cell fate transformation of a Deiters' cell into a pillar cell. Both this cell fate change and hearing loss can be partially rescued by reducing Fgf8 gene dosage in Spry2 null mutant mice. Our results provide evidence that antagonism of FGF signaling by SPRY2 is essential for establishing the cytoarchitecture of the organ of Corti and for hearing.

Extrasynaptic localization of inactivating calcium-activated potassium channels in mouse inner hair cells

Auditory hair cells from nonmammalian vertebrates are electrically tuned to specific sound frequencies primarily by the interactions of voltage-gated calcium channels and calcium-activated potassium (BK) channels colocalized at synaptic active zones. Mammalian inner hair cells are not electrically tuned and, yet, BK channels are also thought to reside at active zones. Using patch-clamp recordings and immunofluorescence, we characterized BK channel expression in mouse inner hair cells. Unexpectedly, these channels have inactivating currents and are clustered near the apex of the cell away from synaptic sites near the base. These results indicate a novel function of BK channels in mammalian inner hair cells and provide a framework for future research.

Age effects and size effects in the ears of gekkonomorph lizards: inner ear

Audiograms have indicated greater auditory sensitivity in larger than in smaller geckos; part of this difference, interspecifically and intraspecifically, is explained by middle-ear proportions. To investigate the contribution of the inner ear to the variation in sensitivity, we examined it in museum specimens representing 11 species and three subfamilies. We measured papilla basilaris length, and, when intact, the saccular otoconial mass. Papilla length approximated 1% of rostrum-anus length in large geckos but 2% in small geckos; in some species some inter-aural difference was indicated. Over the lumped material, relative papilla length varied as a function of body length, with highly significant correlation. Similar relations prevailed within each subfamily. However, intraspecifically the correlation of papilla basilaris length with animal size was usually nonsignificant. Hair cell populations assessed from SEM photographs were larger in the larger species but intraspecifically did not relate to an individual's size. Hence interspecifically, the dependence of auditory sensitivity on animal size seems supported by inner-ear differences but intraspecifically this relation derives only from the middle ear. Otoconial mass, as measured by its volume, was correlated with animal length both interspecifically and intraspecifically.

Gene expression profile of the mouse organ of Corti at the onset of hearing

We describe the generation of an expressed sequence tag (EST) database of the mouse organ of Corti (OC). Over 20,000 independent clones were isolated, analyzed, and grouped into 8690 unique gene clusters. A large pool of novel genes unique to the OC was identified. Sequence alignments frequently revealed alternatively spliced forms of known genes potentially relevant in the OC function. We have also electronically mapped a subset of OC mouse ESTs to several syntenic regions associated with human autosomal and recessive deafness, which may prove useful for the identification of new positional candidates for these human diseases. The EST dataset is available as an interactive Web-based public database at. This resource provides both a view of the profile of gene expression in the OC at the onset of hearing and a tool to identify novel genes of importance in hearing.

Localization and developmental expression of BK channels in mammalian cochlear hair cells

The expression of Slo channels (alpha subunits of BK channels) was investigated in the developing mouse cochlea using a polyclonal antibody against the C-terminal part of the protein (residues 1098-1196). The first BK channel immunoreactivity was observed in the cochlea at E18, where it was localized within the cytoplasm of cells lining the area of the organ of Corti and the spiral ganglion. There was an increase of immunoreactivity in all cells bordering the scala media (supporting and hair cells of the organ of Corti, the stria vascularis and the Reissner's membrane) in the following stages (postnatal day [P] 0 and P6). From P12 to adult, a strong membranous labeling, increasing with age, appeared in inner hair cells. The distribution of BK channels was mainly observed as dense elongated plaques localized in the lateral membrane below the cuticular plate. In addition, a more discrete immunolabeling for BK channels, as punctuated dots, was observed in the synaptic area of inner hair cells. This dual localization of BK channels within inner hair cells was confirmed by a different technique using a fluorescently labeled high-affinity ligand of these channels: IbTX-D19C-Alexa488. We demonstrated under patch clamp experiments that this fluorescent toxin conserved its native property, i.e. to reversibly inhibit BK currents in isolated inner hair cells. The fluorescent toxin, both in living or fixed tissues, also showed a preferential binding to mature inner hair cells with a similar subcellular distribution described above using immunocytochemical technique. Overall, our present results confirm the appearance of membranous BK channels around P12 in mouse inner hair cells, an age at which the auditory system becomes functional. The expression of BK channels in mature inner hair cells, near the site of mechanical-transduction, might serve to limit receptor potential attenuation due to the space constant, and thus permitting these sensory cells to function as fast and sensitive transducers.

A transiently expressed SK current sustains and modulates action potential activity in immature mouse inner hair cells

From just after birth, mouse inner hair cells (IHCs) expressed a Ca(2+)-activated K(+) current that was reduced by intracellular BAPTA at concentrations >or= 1 mM. The block of this current by nifedipine suggests the direct involvement of Ca(v)1.3 Ca(2+) channels in its activation. On the basis of its high sensitivity to apamin (K(D) 360 pM) it was identified as a small-conductance Ca(2+)-activated K(+) current (SK), probably SK2. A similar current was also found in outer hair cells (OHCs) from the beginning of the second postnatal week. In both cell types the appearance of the SK current coincided with their becoming responsive to acetylcholine (ACh), the main efferent neurotransmitter in the cochlea. The effect of ACh on IHCs was abolished when they were simultaneously superfused with strychnine, consistent with the presence of nicotinic ACh receptors (nAChRs). Extracellular Ca(2+) either potentiated or blocked the nAChR current depending on its concentration, as previously reported for the recombinant alpha9alpha10 nAChR. Outward currents activated by ACh were reduced by blocking the SK current with apamin or by preventing SK current activation with intracellular BAPTA (>or= 10 mM). The endogenous mobile Ca(2+) buffer concentration was estimated to be equivalent to about 1 mM BAPTA, suggesting that in physiological conditions the SK channel is significantly activated by Ca(2+) influx through both Ca(v)1.3 Ca(2+) channels and alpha9alpha10 nAChRs. Current clamp experiments showed that in IHCs the SK current is required for sustaining a train of action potentials and also modulates their frequency when activated by ACh.

Developmental expression of two-pore domain K+ channels, TASK-1 and TREK-1, in the rat cochlea

Developmental expression of two-pore domain potassium (2P K) channels, TASK-1 and TREK-1, was investigated in the rat cochlea at onset of hearing and after maturity using RT-PCR and immunocytochemistry. TASK-1 and TREK-1 mRNAs were detected by RT-PCR at postnatal day (P) 9-12. TASK-1 like immunoreactivity (LIR) in the P13 cochlea was observed in Deiters', pillar, Claudius' and outer sulcus cells, spiral limbus fibrocytes, and neuroglia. At P13, TREK-1-LIR was more wide-spread, and included sensory and supporting cells of the organ of Corti, spiral ganglion, stria vascularis, Reissner's membrane, inner and outer sulcus cells, connective and support tissues surrounding modiolus. By P105 the pattern of TASK-1- and TREK-1-LIR became limited to a subset of the above structures, suggesting developmental regulation. During postnatal development, TASK-1 may be important in the onset (around P11) and maturation (by P22) of endocochlear potential and hearing. The distribution of TASK-1 and TREK-1 suggest a role in K cycling and homeostasis. As TASK-1 and TREK-1 are inhibited by local anesthetics at doses used to treat tinnitus, 2P K channels may also be important in cochlear dysfunction.

Cell type-specific reduction of beta tubulin isotypes synthesized in the developing gerbil organ of Corti

There are seven isotypic forms of the microtubule protein beta tubulin in mammals, but not all isotypes are synthesized in every cell type. In the adult organ of Corti, each of the five major cell types synthesizes a different subset of isotypes. Inner hair cells synthesize only betaI and betaII tubulin, while outer hair cells make betaI and betaIV tubulin. Only betaII and betaIV tubulin are found in inner and outer pillar cells, while betaI, betaII, and betaIV tubulin are present in Deiters cells, and betaI, betaII and betaIII tubulin are found in organ of Corti dendrites. During post-natal organ of Corti development in the gerbil, microtubules are elaborated in an orderly temporal sequence beginning with hair cells, followed by pillar cells and Deiters cells. Using beta tubulin isotype-specific antibodies, we show that, in the gerbil cochlea, the same three isotypes are present in each cell type at birth, and that a cell type-specific reduction in the isotypes synthesized occurs in hair cells and pillar cells at an unusually late stage in development. No beta tubulin isotypes were detected in mature afferent dendrites, but we show that this is because few microtubules are present in mature dendrites. In addition, we show that primary cilia in inner hair cells, a feature of early development, persist much later than previously reported. The findings represent the first description of developmental cell type-specific reductions in tubulin isotypes in any system.

Electron microscopy of degenerative changes in the chick basilar papilla after gentamicin exposure

We present a sequential study of the substructural alterations in the chick basilar papilla at the earliest signs of hair cell degeneration. Three-day posthatch chicks received a single injection of gentamicin (300 mg/kg) and were killed at 6, 8, 12, 15, 18, 21, and 24 hours after the injection. The basilar papillae were studied by conventional transmission electron microscopy. Examination was limited to the basal region, where all hair cells are eliminated by this treatment. As early as 8 hours and clearly by 12 hours, altered fine structure was seen in hair cells. Changes included rounding and swelling of the hair cells, condensation of nuclear chromatin, dissolution of ribosomes, dilatation of the mitochondria, and accumulation of inclusion bodies and lysosomes. By 15-18 hours, lysosomes increased and became denser, afferent terminals appeared swollen, and the first cell extrusion was seen. Efferents were unaffected, and supporting cells, though having inclusion bodies now, retained normal intercellular junctions. By 21-24 hours, large regions of complete hair cell loss were composed of expanded supporting cell processes with normal-appearing intercellular junctions and portions of extruded hair cells, partially attached to the supporting cell surface. These observations demonstrate that auditory hair cells undergo a rapid and controlled process of hair cell extrusion that allows preservation of the reticular lamina and minimal contamination of surrounding structures by intracytoplasmic contents of the damaged hair cells.

The inhibition of cyclin-dependent kinases induces differentiation of supernumerary hair cells and Deiters' cells in the developing organ of Corti

In the embryonic day 19 organs of Corti, we showed that roscovitine, a chemical inhibitor of cyclin-dependent kinases (CDKs), significantly increased the number of hair cells (HCs) and corresponding supporting cells (SCs) by triggering differentiation of precursor cells without interacting with cell proliferation. The effect of roscovitine was mimicked by other CDK1, 2, 5, and 7 inhibitors but not by CDK4/6 and mitogen-activated protein kinase pathway antagonists. Immunohistochemical analysis indicated that roscovitine-specific intracellular targets, CDK1, 2, 5, and 7, were expressed in the organ of Corti and especially in Hensen's cells. Affinity chromatography studies showed a tight correlation between the protein levels of CDK1/2 and 5 and the rate of roscovitine-induced supernumerary cells in the organ of Corti. In addition, we demonstrated that basal CDK activity was higher and more roscovitine-sensitive at developmental stages that are selectively permissive for the emergence of supernumerary cells. These results suggest that CDKs are involved in the normal development of the organ of Corti and that, at least in E19 embryos, inhibition of CDKs is sufficient to trigger the differentiation of HCs and corresponding SCs, presumably from the Hensen's cell progenitors and/or from progenitors located in the greater epithelial ridge area.

Localization of the calcium channel subunits Cav1.2 (alpha1C) and Cav2.3 (alpha1E) in the mouse organ of Corti

Voltage-activated Ca2+ channels play an important role in synaptic transmission, signal processing and development. The immunohistochemical localization of Cav1.2 (alpha1C) and Cav2.3 (alpha1E) Ca2+ channels was studied in the developing and adult mouse organ of Corti using subunit-specific antibodies and fluorescent secondary antibodies with cochlear cryosections. Cav1.2 immunoreactivity has been detected from postnatal day 14 (P14) onwards at the synapses between cholinergic medial efferents and outer hair cells as revealed by co-staining with anti-synaptophysin and anti-choline acetyltransferase. Most likely the Cav1.2 immunoreactivity was located presynaptically at the site of contact of the efferent bouton with the outer hair cell which suggests a role for class C L-type Ca2+ channels in synaptic transmission of the medial efferent system. The localization of the second Ca2+ channel tested, Cav2.3, showed a pronounced change during cochlear development. From P2 until P10, Cav2.3 immunoreactivity was found in the outer spiral bundle followed by the inner spiral bundle, efferent endings and by medial efferent fibers. Around P14, Cav2.3 immunoreactivity disappeared from these structures and from P19 onwards it was observed in the basal poles of the outer hair cell membranes.

Distribution of glycoconjugates during cochlea development in mice: light microscopic lectin study

During development, different epithelial cells in the mouse cochlea express different cell surface glycoconjugates, which may reflect membrane specialization. Some of the lectins tested in this study (SBA, succ-WGA, and PSA) labeled the sensory cells of the cochlea around birth. Other lectins (WGA, Con A, RCA-II, and PHA-E) labeled surfaces of the sensory cells, particularly the stereocilia, from early stages of development (gestation day (GD) 16) through 21 days after birth. These may be adhesion molecules needed to attach the newly forming tectorial membrane (TM) to the stereocilia. Lectin staining of the developing TM revealed that the substructures of the TM are biochemically distinct. Lectin staining also showed the temporal sequence of the expression of cytoplasmic glycoconjugates of the cochlear epithelium during development. Biochemical changes during development are probably the result of different cells being involved in the production of glycoconjugates, and may have functional significance, specifically with regard to the expression of adhesion and/or signaling molecules.

Olivocochlear efferent innervation of the organ of corti in hypothyroid rats

Congenital hypothyroidism induces developmental abnormalities in the auditory receptor, causing deafness due to a poor development of the outer hair cells (OHCs) and a lack of synaptogenesis between these cells and the olivocochlear axons. This efferent innervation is formed by two separate systems: the lateral system, which originates in the lateral superior olive (LSO) and reaches the inner hair cells; and the medial system, which originates in the ventral nucleus of the trapezoid body (VNTB) and innervates the OHCs. A previous study carried out in our laboratory showed that in congenitally hypothyroid animals, the neurons which give rise to the efferent system are normal in number and distribution, although smaller in size. The aim of the present work was to study the efferent fibers in the auditory receptor of hypothyroid animals, by means of stereotaxic injections of biotinylated dextran amine in the nuclei that give rise to the olivocochlear system: LSO and VNTB. In hypothyroid animals, injections in LSO gave rise to lateral olivocochlear fibers lacking their characteristic dense terminal arbors, while injections in the VNTB-labeled fibers terminating in the spiral bundle region, far from the OHCs with which they normally contact. In the latter case, only a small percentage of labeled fibers reached the OHCs area, giving off only two radial branches maximum. Because the number of neurons which develop into the efferent innervation was normal in hypothyroid animals, we conclude that medial fibers may contact a new target.

Comparative expression patterns of T-, N-, E-cadherins, beta-catenin, and polysialic acid neural cell adhesion molecule in rat cochlea during development: implications for the nature of Kölliker's organ

We investigated the expression patterns of several cell adhesion molecules (CAMs) during rat cochlea ontogeny, from embryo day 16 to adulthood, with the use of immunohistochemistry: neural cadherin (N-cad) and polysialic acid neural CAM (PSA-NCAM) as two different neural CAM paradigms; epithelial cadherin (E-cad), which was restricted to the epitheloid phenotype; and the cytoplasmic domain-free truncated-cadherin (T-cad). We made the following observations. (1) T-cad was present in all types of fibrocyte and in subdomains within the pillar cells. (2) E- and N-cad were expressed with mutually exclusive patterns and did not overlap with T-cad. All cochlear epithelial cells, including the sensory outer hair cells (OHCs), were E-cad-positive, except for the negative inner hair cells (IHCs) and the nonsensory Kölliker's organ domain close to the IHCs. N-cad expression appeared first in the developing IHCs and then in the neighboring Kölliker's organ in an increasingly mediolateral gradient in opposition to the E-cad gradient. The OHCs, which are never N-cad positive, intensively expressed E-cad, as did the Hensen cells at the beginning of their differentiation. (3) The cadherin-linked molecule beta-catenin, absent in fibrocytes, was detected in all epithelial cell membranes and was prominent in the E-cad-rich modiolar extremity of Kölliker's organ. (4) Gradual PSA-NCAM expression was observed in the lateral portion of Kölliker's organ, and the intense PSA-NCAM expression was seen surrounding the IHCs. As development proceeded, PSA-NCAM immunoreactivity progressively became restricted to the basal poles of the IHCs, where it remained in the adult rat cochlea, suggesting a synaptic plasticity. Synaptic plasticity in rat cochlea and hypotheses about T-cad functions and neosensory features of the Kölliker's organ are discussed.

Hypothyroidism prevents developmental neuronal loss during auditory organ development

The deficit of thyroid hormone leads to several structural and physiological modifications in the auditory receptor: the outer hair cells present an immature morphology, abnormal persistence of the afferent dendrites and incomplete development of the efferent terminals. The aim of this work was to perform a quantitative and morphometric study of the spiral ganglion neurons in control and hypothyroid animals. The cochleae from both experimental groups were processed in order to obtain plastic sections. In control animals the size of the neurons increased throughout development and was larger in the basal than in the apical portion of the cochlea. In hypothyroid animals, the cell death that takes place normally during development did not occur, and there was no differentiation into types I and II neurons. The size of the neurons also increased with development in treated animals, but they were smaller than in control animals, and in this case the neurons in the apex were larger than in the base. This study shows that hypothyroidism alters the normal development of the spiral ganglion neurons.

Identification and characterization of a novel mouse plexin, plexin-A4

Plexins belonging to the plexin-A subfamily form complexes with neuropilins and propagate signals of class 3 semaphorins into neurons, even though they do not directly bind the semaphorins. In this study, we identified a new member of the plexin-A subfamily in the mice, plexin-A4, and showed that it was expressed in the developing nervous system with a pattern different to that of other members of the plexin-A subfamily (plexin-A1, plexin-A2 and plexin-A3). COS-7 cells coexpressing plexin-A4 with neuropilin-1 were induced to contract by Sema3A, a member of the class 3 semaphorin. Ectopic expression of plexin-A4 in mitral cells that are originally insensitive to Sema3A resulted in the collapse of growth cones in the presence of Sema3A. These results suggest that plexin-A4 plays a role in the propagation of Sema3A activities.

Differentiation of spinous synapses in the mouse organ of corti

The inner hair cells, the primary auditory receptors, are perceived only as a means for transfer of sound signals via the auditory nerve to the central nervous system. During initial synaptogenesis, they receive relatively few and mainly somatic synapses. However, around the onset of hearing (10-14 postnatal days in the mouse), a complex network of local spinous synapses differentiates, involving inner hair cells, their afferent dendrites, and lateral olivocochlear terminals. Inner hair cell spines participate in triadic synapses between olivocochlear terminals and afferent dendrites. Triadic synapses have not yet been confirmed in the adult. Synaptic spines of afferent dendrites form axodendritic synapses with olivocochlear terminals and somatodendritic synapses with inner hair cells. The latter are of two types: ribbon-dendritic spines and stout dendritic spines surrounded only by a crown of synaptic vesicles. Formation of spinous afferent synapses results from sprouting of dendritic filopodia that intussuscept inner hair cell cytoplasm. This process continues in the adult, indicating ongoing synaptogenesis. Spinous processes of olivocochlear synaptic terminals contact adjacent afferent dendrites, thus integrating their connectivity. They develop about 14 postnatal days, but their presence in the adult has yet to be confirmed. Differentiation of spinous synapses in the organ of Corti results in a total increase of synaptic contacts and in a complexity of synaptic arrangements and connectivity. We propose that spinous synapses provide the morphological substrate for local processing of initial auditory signals within the cochlea.

Postnatal refinement of auditory nerve projections to the cochlear nucleus in cats

Studies of visual system development have suggested that competition driven by activity is essential for refinement of initial topographically diffuse neuronal projections into their precise adult patterns. This has led to the assertion that this process may shape development of topographic connections throughout the nervous system. Because the cat auditory system is very immature at birth, with auditory nerve neurons initially exhibiting very low or no spontaneous activity, we hypothesized that the auditory nerve fibers might initially form topographically broad projections within the cochlear nuclei (CN), which later would become topographically precise at the time when adult-like frequency selectivity develops. In this study, we made restricted injections of Neurobiotin, which labeled small sectors (300-500 microm) of the cochlear spiral ganglion, to study the projections of auditory nerve fibers representing a narrow band of frequencies. Results showed that projections from the basal cochlea to the CN are tonotopically organized in neonates, many days before the onset of functional hearing and even prior to the development of spontaneous activity in the auditory nerve. However, results also demonstrated that significant refinement of the topographic specificity of the primary afferent axons of the auditory nerve occurs in late gestation or early postnatal development. Projections to all three subdivisions of the CN exhibit clear tonotopic organization at or before birth, but the topographic restriction of fibers into frequency band laminae is significantly less precise in perinatal kittens than in adult cats. Two injections spaced > or = 2 mm apart in the cochlea resulted in labeled bands of projecting axons in the anteroventral CN that were 53% broader than would be expected if they were proportional to those in adults, and the two projections were incompletely segregated in the youngest animals studied. Posteroventral CN (PVCN) projections (normalized for CN size) were 36% broader in neonates than in adults, and projections from double injections in the youngest subjects were nearly fused in the PVCN. Projections to the dorsal division of the CN were 32% broader in neonates than in adults when normalized, but the dorsal CN projections were always discrete, even at the earliest ages studied.

OCP2 immunoreactivity in the human fetal cochlea at weeks 11, 17, 20, and 28, and the human adult cochlea

The two most abundant proteins of the organ of Corti, OCP1 and OCP2, are acidic, cytosolic, low molecular weight proteins diffusely distributed within the cytoplasm of supporting cells. A recent study by Henzl et al. (2001) found first, that these two proteins co-localize with connexin 26 along the epithelial gap junction system and second, that OCP2 could participate with OCP1 in an organ of Corti-specific SCF complex (Skp1, cul1in, and Fbp), a ubiquitin ligase complex. Previous study has also implicated OCP2 in the recycling and regulation of intracellular K(+) efflux as well as pH homeostatic mechanisms. In the present study, we document the emergence and distribution features of OCP2 through various stages (weeks 11-28) of gestation in human fetal cochleae. Four fetal cochleae, the cochleae of a normal hearing human adult and a mature rat for positive control were fixed in 4% formalin within 2 h post mortem. Immunohistochemical studies were performed using a rabbit polyclonal antibody raised against a synthetic peptide corresponding to amino acids 3-16. Specimens were mounted in paraffin sections. Results show that OCP2 immunoreactivity is evident at a prenatal age of 11 weeks, peaks in expression at the onset of cochlear function at 20 weeks and achieves adult-like patterns of distribution just prior to histological maturation at 28 weeks. Though this protein could be associated with the development, maturation, and electrochemical maintenance of the cochlear gap junction system, the nature of this protein's function in the developing and mature human cochlea remains unclear.

Transitional expression of OX-2 and GAP-43 glycoproteins in developing rat cochlear nerve fibers

The OX-2 and GAP-43 glycoproteins are two proteins involved in neuronal cell-to-cell interaction and/or growing of dendrites and axons. Therefore, for the auditory receptor the expression of these proteins could provide information on the afferent and efferent nerve fiber organization. The expression and distribution of OX-2 and GAP-43 were analyzed during the auditory receptor development and maturation (from embryonic day E13 to postnatal day P22). Both glycoproteins were early recognized in the cochleae of E13 rats. Then, they slowly but progressively disappeared, being absent when the animals reached the P22 postnatal day. At E13, a weak OX-2 expression was restricted to the perikaryon of the spiral ganglion neurons, while in the same period a strong GAP-43 immunostaining was found in both the neuronal perikaryon and the neurites. During the rat embryonic period (E13 to birth) the expression of both glycoproteins appeared progressively restricted to the neurites. During the rat postnatal period (P0 to P22), OX-2 and GAP-43 exhibited a dissimilar distribution pattern. The OX-2 glycoprotein appeared in the afferent, efferent and fibers of the auditory nerve, while the GAP-43 glycoprotein only appeared in the efferent nerve fibers. Present data suggest that OX-2 and GAP-43 could act as two complementary glycoproteins during the development, organization, and maturation of the cochlear nerve fibers. While both glycoproteins could participate in axonal growing and orientation, OX-2 could also be involved in a similar process for auditory dendrites.

Development of acetylcholine receptors in cultured outer hair cells

Efferents, originating in the superior olivary complex, preferentially synapse with cochlear outer hair cells (OHCs), with acetylcholine (ACh) as their primary neurotransmitter. The OHC ACh receptors (AChRs), which have unusual pharmacology, have been cloned and identified as a new subunit (alpha9) of the nicotinic AChR family. The expression of alpha9 AChRs is first detected before birth and peaks between 6 and 10 days after birth (DAB) in developing mice and rats, while functional maturation of the receptor, as determined by measuring the ACh-induced currents, takes place between 6 and 12 DAB. In this study we attempted to examine the development of AChRs in OHCs grown in explanted cultures, deprived of efferent innervation. ACh-induced currents were used as an assay. Reverse transcription-PCR analysis was also performed to detect the expression of alpha9 subunit from cultured OHCs. PCR study indicates that mRNA of the alpha9 subunit was expressed in primary cochlear cultures, similar to that seen in the cochleae of developing animals. Measurement of whole-cell currents showed that ACh-induced outward current was first detected around 5 days in a fraction of cultured OHCs. The number of responsive cells increased between 5 and 12 days in culture. The size of ACh-induced currents also increased during this period. These results suggest that the development of AChRs in cultured OHCs is not affected by removal of efferent innervation.