Hair cell differentiation in chick cochlear epithelium after aminoglycoside toxicity: in vivo and in vitro observations

A Novel in vitro Model Delineating Hair Cell Regeneration and Neural Reinnervation in Adult Mouse Cochlea

The study of an adult mammalian auditory system, such as regeneration, has been hampered by the lack of an in vitro system in which hypotheses can be tested efficiently. This is primarily due to the fact that the adult inner ear is encased in the toughest bone of the body, whereas its removal leads to the death of the sensory epithelium in culture. We hypothesized that we could take advantage of the integral cochlear structure to maintain the overall inner ear architecture and improve sensory epithelium survival in culture. We showed that by culturing adult mouse cochlea with the (surrounding) bone intact, the supporting cells (SCs) survived and almost all hair cells (HCs) degenerated. To evaluate the utility of the explant culture system, we demonstrated that the overexpression of Atoh1, an HC fate-determining factor, is sufficient to induce transdifferentiation of adult SCs to HC-like cells (HCLCs). Transdifferentiation-derived HCLCs resemble developmentally young HCs and are able to attract adult ganglion neurites. Furthermore, using a damage model, we showed that degenerated adult ganglions respond to regenerated HCLCs by directional neurite outgrowth that leads to HCLC-neuron contacts, strongly supporting the intrinsic properties of the HCLCs in establishing HCLC-neuron connections. The adult whole cochlear explant culture is suitable for diverse studies of the adult inner ear including regeneration, HC-neuron pathways, and inner ear drug screening.

Development of the hair cells of the human cochlea: A scanning electron microscopic study

Introduction

In the mammalian auditory system, the cochlea is the first to attain structural and functional maturity. Although ultrastructural details of the developing cochlea of lower animals have been elucidated in the last few decades, comprehensive studies on human cochlea are lacking.

Materials and Methods

In the present investigation we studied the development and maturation of the hair cells of ten human fetal cochlea from gestational weeks (GW) 12 to 37 by scanning electron microscopy.

Result

We observed undifferentiated hair cells possessing numerous surface projections and long kinocilium during GW 14. At GW16, the primitive hair cells were arranged in one inner and four outer rows and had globular apices indicating the initiation of stereocilia formation. By GW 22, the globular apices were replaced by linear stereocilia and occasional kinocillia. Mature hair cells with sterocilia were observed in the basal turn at 30th week of gestation. At GW 37, the stereocilia were arranged in a typical "V" shaped pattern at the middle and apical coil, while the stereocilia of the basal turn were shorter in length resembling the adult cochlea. The inner hair cells were long and slender while outer hair cells were pear shaped, kinocilium were absent and the tunnel of Corti were well formed.

Conclusion

It is concluded that in human, the morphological maturation of the hair cells starts in the basal turn around GW 22 and continues till 37th week in the apical turn indicating that early maturation of the cochlea may have a role on development of the higher auditory pathway connections.

Insights into the molecular mechanisms regulating mammalian hair cell regeneration

PURPOSE OF REVIEW

To give an overview of recent advances in mammalian auditory hair cell regeneration.

RECENT FINDINGS

Supporting cells act as progenitors to regenerate hair cells in the prehearing mammalian cochlea but not in the mature cochlea. To overcome this developmental obstacle, manipulation of multiple genes and intracellular pathways has been investigated, which has obtained promising data. This review focuses on recent advances in auditory hair cell regeneration, including synergic gene regulation associated with Atoh1 and Notch signaling, epigenetics, and functional recovery of regenerated hair cells. Co-manipulation of genes critical for hair cell development and cell cycle re-entry, including Atoh1, Isl1, Pou4f3, Gata3, Gfi1, P27kip1, RB, Myc, and Notch-signaling genes, has generated hair cell-like cells in the adult cochlea both in vitro and in vivo. The epigenetic mechanism has been studied in hair cell development and regeneration. Regeneration of hair cell function has a very limited progress, which lacks in-vitro and in-vivo electrophysiology data.

SUMMARY

Regeneration of adult auditory hair cells remains a major challenge. Manipulation of multiple genes and pathways together with epigenetic regulation might potentially regenerate functional hair cells in the adult mammalian cochlea.

Transcriptomic characterization of dying hair cells in the avian cochlea

Sensory hair cells are prone to apoptosis caused by various drugs including aminoglycoside antibiotics. In mammals, this vulnerability results in permanent hearing loss because lost hair cells are not regenerated. Conversely, hair cells regenerate in birds, making the avian inner ear an exquisite model for studying ototoxicity and regeneration. Here, we use single-cell RNA sequencing and trajectory analysis on control and dying hair cells after aminoglycoside treatment. Interestingly, the two major subtypes of avian cochlear hair cells, tall and short hair cells, respond differently. Dying short hair cells show a noticeable transient upregulation of many more genes than tall hair cells. The most prominent gene group identified is associated with potassium ion conductances, suggesting distinct physiological differences. Moreover, the dynamic characterization of >15,000 genes expressed in tall and short avian hair cells during their apoptotic demise comprises a resource for further investigations toward mammalian hair cell protection and hair cell regeneration.

Signaling pathways (Notch, Wnt, Bmp and Fgf) have additive effects on hair cell regeneration in the chick basilar papilla after streptomycin injury in vitro: Additive effects of signaling pathways on hair cell regeneration

Hair cells can be regenerated after damage by transdifferentiation in which a supporting cell directly differentiates into a hair cell without mitosis. However, such regeneration is at the cost of exhausting the support cells in the mammalian mature cochlea. Thus, more effective methods should be found to promote mitotic regeneration but partially preserve support cells after damage. To address the issue, we first injured hair cells in the chick basilar papillae (BP) by treatment with streptomycin in vitro. We then compared the mitotic regeneration on the neural side in the middle part of BP after treatment with a pharmacological inhibitor or agonist of the Notch (DAPT), Wnt (LiCl), Bmp (Noggin) or Fgf (SU5402) signaling pathway, with that after treatment with combinations of two or three inhibitors or agonist of these pathways. Our results indicate that treatments with a single inhibitor or agonist of the Notch, Wnt, Bmp or Fgf signaling pathway could significantly increase mitotic regeneration as well as direct transdifferentiation. The results also show that hair cells (Myosin 7a+), support cells (Sox2+) and mitotically regenerated hair cells (Myosin 7a+/Sox2+/BrdU+) increased significantly on the neural side in the middle part of BP after two or three combinations of the inhibition of Notch, Bmp or Fgf signaling pathway or the activation of Wnt signaling pathway, besides the reported coregulatory effects of Notch and Wnt signaling. The study of the effects of systematic combinations of pathway modulators provided more insight into hair cell regeneration from mitosis.

Initiation of Supporting Cell Activation for Hair Cell Regeneration in the Avian Auditory Epithelium: An Explant Culture Model

Sensorineural hearing loss is a common disability often caused by the loss of sensory hair cells in the cochlea. Hair cell (HCs) regeneration has long been the main target for the development of novel therapeutics for sensorineural hearing loss. In the mammalian cochlea, hair cell regeneration is limited, but the auditory epithelia of non-mammalian organisms retain the capacity for hair cell regeneration. In the avian basilar papilla (BP), supporting cells (SCs), which give rise to regenerated hair cells, are usually quiescent. Hair cell loss induces both direct transdifferentiation and mitotic division of supporting cells. Here, we established an explant culture model for hair cell regeneration in chick basilar papillae and validated it for investigating the initial phase of hair cell regeneration. The histological assessment demonstrated hair cell regeneration via direct transdifferentiation of supporting cells. Labeling with 5-ethynyl-2′-deoxyuridine (EdU) revealed the occurrence of mitotic division in the supporting cells at specific locations in the basilar papillae, while no EdU labeling was observed in newly generated hair cells. RNA sequencing indicated alterations in known signaling pathways associated with hair cell regeneration, consistent with previous findings. Also, unbiased analyses of RNA sequencing data revealed novel genes and signaling pathways that may be related to the induction of supporting cell activation in the chick basilar papillae. These results indicate the advantages of our explant culture model of the chick basilar papillae for exploring the molecular mechanisms of hair cell regeneration.

Perspectives on Human Hearing Loss, Cochlear Regeneration, and the Potential for Hearing Restoration Therapies

Most adults who acquire hearing loss find it to be a disability that is poorly corrected by current prosthetics. This gap drives current research in cochlear mechanosensory hair cell regeneration and in hearing restoration. Birds and fish can spontaneously regenerate lost hair cells through a process that has become better defined in the last few years. Findings from these studies have informed new research on hair cell regeneration in the mammalian cochlea. Hair cell regeneration is one part of the greater problem of hearing restoration, as hearing loss can stem from a myriad of causes. This review discusses these issues and recent findings, and places them in the greater social context of need and community.

Vascular endothelial growth factor is required for regeneration of auditory hair cells in the avian inner ear

Hair cells in the auditory organ of the vertebrate inner ear are the sensory receptors that convert acoustic stimuli into electrical signals that are conveyed along the auditory nerve to the brainstem. Hair cells are highly susceptible to ototoxic drugs, infection, and acoustic trauma, which can cause cellular degeneration. In mammals, hair cells that are lost after damage are not replaced, leading to permanent hearing impairments. By contrast, supporting cells in birds and other non-mammalian vertebrates regenerate hair cells after damage, which restores hearing function. The cellular mechanisms that regulate hair cell regeneration are not well understood. We investigated the role of vascular endothelial growth factor (VEGF) during regeneration of auditory hair cells in chickens after ototoxic injury. Using RNA-Seq, immunolabeling, and in situ hybridization, we found that VEGFA, VEGFC, VEGFR1, VEGFR2, and VEGFR3 were expressed in the auditory epithelium, with VEGFA expressed in hair cells and VEGFR1 and VEGFR2 expressed in supporting cells. Using organotypic cultures of the chicken cochlear duct, we found that blocking VEGF receptor activity during hair cell injury reduced supporting cell proliferation as well as the numbers of regenerated hair cells. By contrast, addition of recombinant human VEGFA to organ cultures caused an increase in both supporting cell division and hair cell regeneration. VEGF's effects on supporting cells were preserved in isolated supporting cell cultures, indicating that VEGF can act directly upon supporting cells. These observations demonstrate a heretofore uncharacterized function for VEGF signaling as a critical positive regulator of hair cell regeneration in the avian inner ear.

Vitamin C alleviates ototoxic effect caused by coadministration of amikacin and furosemide

BACKGROUND

Drug-induced ototoxicity is still a main clinical problem in otolaryngology. It is widely known that aminoglycoside antibiotics combined with loop diuretics significantly contribute to permanent ototoxicity. The aim of this study was to find out whether ascorbic acid (vitamin C) is able to reverse or alleviate ototoxicity evoked by systemic (ip) administration of combination of amikacin and furosemide in experimental male albino Swiss mice.

METHODS

Ototoxic combination of amikacin and furosemide was isobolographically evaluated based on the hearing threshold decreasing doses by 20% and 50% (TDD₂₀ and TDD₅₀), respectively. Linear regression analysis was used to determine the TDD₂₀ and TDD₅₀ values for amikacin, furosemide, vitamin C administered alone and in combination (at the fixed-ratio of 1:1).

RESULTS

Vitamin C (in a dose of 500 mg/kg, ip) alleviated the impairment in hearing threshold evoked by combined ip administration of amikacin and furosemide (at the fixed-ratio of 1:1) in mice by reducing TDD₅₀ values from 49.82 to 21.56 (p <  0.01). In contrast, vitamin C (500 mg/kg, ip) had no significant effect on TDD₂₀ values for the combination of amikacin and furosemide at the fixed-ratio of 1:1.

CONCLUSIONS

Vitamin C administered together with ototoxic drug combination of amikacin and furosemide reduced ototoxicity evoked by this two-drug combination in the experimental mice.

Study of the Mechanisms by Which Aminoglycoside Damage Is Prevented in Chick Embryonic Hair Cells

A major side effect of aminoglycoside antibiotics is mammalian hair cell death. It is thus intriguing that embryonic chick hair cells treated with aminoglycosides at embryonic day (E) 12 are insensitive to ototoxicity. To exclude some unknown factors in vivo that might be involved in preventing aminoglycoside damage to embryonic hair cells, we first cultured chick embryonic basilar papilla (BP) with an aminoglycoside antibiotic in vitro. The results indicated that the hair cells were almost intact at E12 and E14 and were only moderately damaged in most parts of the BP at E16 and E18. Generally, hair cells residing in the approximate and abneural regions were more susceptible to streptomycin damage. After incubation with gentamicin-conjugated Texas Red (GTTR), which is typically used to trace the entry route of aminoglycosides, GTTR fluorescence was not remarkable in hair cells at E12, was weak at E14, but was relatively strong in the proximal part of BP at E18. This result indicates that the amounts of GTTR that entered the hair cells are related to the degrees of aminoglycoside damage. The study further showed that the fluorescence intensity of GTTR decreased to a low level at E14 to E18 after disruption of mechanotransduction machinery, suggesting that the aminoglycoside entry into hair cells was mainly through mechanotransduction channels. In addition, most of the entered GTTR was not found to be colocalized with mitochondria even at E18. This finding provides another reason to explain why embryonic chick hair cells are insensitive to aminoglycoside damage.

Bone morphogenetic protein 4 antagonizes hair cell regeneration in the avian auditory epithelium

Permanent hearing loss is often a result of damage to cochlear hair cells, which mammals are unable to regenerate. Non-mammalian vertebrates such as birds replace damaged hair cells and restore hearing function, but mechanisms controlling regeneration are not understood. The secreted protein bone morphogenetic protein 4 (BMP4) regulates inner ear morphogenesis and hair cell development. To investigate mechanisms controlling hair cell regeneration in birds, we examined expression and function of BMP4 in the auditory epithelia (basilar papillae) of chickens of either sex after hair cell destruction by ototoxic antibiotics. In mature basilar papillae, BMP4 mRNA is highly expressed in hair cells, but not in hair cell progenitors (supporting cells). Supporting cells transcribe genes encoding receptors for BMP4 (BMPR1A, BMPR1B, and BMPR2) and effectors of BMP4 signaling (ID transcription factors). Following hair cell destruction, BMP4 transcripts are lost from the sensory epithelium. Using organotypic cultures, we demonstrate that treatments with BMP4 during hair cell destruction prevent supporting cells from upregulating expression of the pro-hair cell transcription factor ATOH1, entering the cell cycle, and fully transdifferentiating into hair cells, but they do not induce cell death. By contrast, noggin, a BMP4 inhibitor, increases numbers of regenerated hair cells. These findings demonstrate that BMP4 antagonizes hair cell regeneration in the chicken basilar papilla, at least in part by preventing accumulation of ATOH1 in hair cell precursors.

Interplay of proliferation and proapoptotic and antiapoptotic factors is revealed in the early human inner ear development

HYPOTHESIS

Spatiotemporal interplay of factors controlling proliferation, differentiation and apoptosis within the developing human inner ear is essential for labyrinth morphogenesis and development of vestibular and cochlear functions.

BACKGROUND

Studies on the early human inner ear development are scarce and insufficient.

METHODS

The immunolocalization of Ki-67, Bcl-2, caspase-3, and IGF-1 was analyzed in 6 human inner ears, 5 to 10 gestational weeks old. Statistical data were analyzed using the Kruskal-Wallis test.

RESULTS

During the analyzed period, the otocyst has transformed into cochlear duct and saccule ventrally and semicircular canals and utricle dorsally. Initial differentiation of sensorineural fields characterized organ of Corti, maculae, and cristae ampullares. Intense (50%) and evenly distributed proliferation Ki-67 in the otocyst decreased to 24% to 30% and became spatially restricted within the membranous labyrinth epithelium. Simultaneously, expression of antiapoptotic Bcl-2 protein increased in sensorineural fields of organ of Corti, macula, and crista ampullaris. Throughout the investigated period, apoptotic caspase-3 positive cells were mainly distributed at the luminal and basal surfaces of labyrinth epithelium. An inhibitor of apoptosis IGF-1 co-expressed with Bcl-2 and increased in the sensorineural fields with advancing development.

CONCLUSION

The described expression pattern indicates roles for cell proliferation in the growth of the inner ear and Bcl-2 in differentiation of sensorineural fields and protection from apoptosis. Both IGF-1-and caspase-3-mediated apoptosis seem to contribute to proper morphogenesis, differentiation, and innervations of sensorineural fields within the cochlea, semicircular canals, saccule, and utricle. Alterations in spatiotemporal interplay of investigated factors might lead to disturbances of vestibular and cochlear function.

A brief history of hair cell regeneration research and speculations on the future

Millions of people worldwide suffer from hearing and balance disorders caused by loss of the sensory hair cells that convert sound vibrations and head movements into electrical signals that are conveyed to the brain. In mammals, the great majority of hair cells are produced during embryogenesis. Hair cells that are lost after birth are virtually irreplaceable, leading to permanent disability. Other vertebrates, such as fish and amphibians, produce hair cells throughout life. However, hair cell replacement after damage to the mature inner ear was either not investigated or assumed to be impossible until studies in the late 1980s proved this to be false. Adult birds were shown to regenerate lost hair cells in the auditory sensory epithelium after noise- and ototoxic drug-induced damage. Since then, the field of hair cell regeneration has continued to investigate the capacity of the auditory and vestibular epithelia in vertebrates (fishes, birds, reptiles, and mammals) to regenerate hair cells and to recover function, the molecular mechanisms governing these regenerative capabilities, and the prospect of designing biologically-based treatments for hearing loss and balance disorders. Here, we review the major findings of the field during the past 25 years and speculate how future inner ear repair may one day be achieved.

Depletion of resident macrophages does not alter sensory regeneration in the avian cochlea

Macrophages are the primary effector cells of the innate immune system and are also activated in response to tissue injury. The avian cochlea contains a population of resident macrophages, but the precise function of those cells is not known. The present study characterized the behavior of cochlear macrophages after aminoglycoside ototoxicity and also examined the possible role of macrophages in sensory regeneration. We found that the undamaged chick cochlea contains a large resting population of macrophages that reside in the hyaline cell region, immediately outside the abneural (inferior) border of the sensory epithelium. Following ototoxic injury, macrophages appear to migrate out of the hyaline cell region and towards the basilar membrane, congregating immediately below the lesioned sensory epithelium. In order to determine whether recruited macrophages contribute to the regeneration of sensory receptors, we quantified supporting cell proliferation and hair cell recovery after the elimination of most resident macrophages via application of liposomally-encapsulated clodronate. Examination of macrophage-depleted specimens at two days following ototoxic injury revealed no deficits in hair cell clearance, when compared to normal controls. In addition, we found that elimination of macrophages did not affect either regenerative proliferation of supporting cells or the production of replacement hair cells. However, we did find that macrophage-depleted cochleae contained reduced numbers of proliferative mesothelial cells below the basilar membrane. Our data suggest that macrophages are not required for normal debris clearance and regeneration, but that they may play a role in the maintenance of the basilar membrane.

Transcriptomic analysis of the zebrafish inner ear points to growth hormone mediated regeneration following acoustic trauma

Background

Unlike mammals, teleost fishes are capable of regenerating sensory inner ear hair cells that have been lost following acoustic or ototoxic trauma. Previous work indicated that immediately following sound exposure, zebrafish saccules exhibit significant hair cell loss that recovers to pre-treatment levels within 14 days. Following acoustic trauma in the zebrafish inner ear, we used microarray analysis to identify genes involved in inner ear repair following acoustic exposure. Additionally, we investigated the effect of growth hormone (GH) on cell proliferation in control zebrafish utricles and saccules, since GH was significantly up-regulated following acoustic trauma.

Results

Microarray analysis, validated with the aid of quantitative real-time PCR, revealed several genes that were highly regulated during the process of regeneration in the zebrafish inner ear. Genes that had fold changes of ≥ 1.4 and P -values ≤ 0.05 were considered significantly regulated and were used for subsequent analysis. Categories of biological function that were significantly regulated included cancer, cellular growth and proliferation, and inflammation. Of particular significance, a greater than 64-fold increase in growth hormone (gh1) transcripts occurred, peaking at 2 days post-sound exposure (dpse) and decreasing to approximately 5.5-fold by 4 dpse. Pathway Analysis software was used to reveal networks of regulated genes and showed how GH affected these networks. Subsequent experiments showed that intraperitoneal injection of salmon growth hormone significantly increased cell proliferation in the zebrafish inner ear. Many other gene transcripts were also differentially regulated, including heavy and light chain myosin transcripts, both of which were down-regulated following sound exposure, and major histocompatability class I and II genes, several of which were significantly regulated on 2 dpse.

Conclusions

Transcripts for GH, MHC Class I and II genes, and heavy- and light-chain myosins, as well as many others genes, were differentially regulated in the zebrafish inner ear following overexposure to sound. GH injection increased cell proliferation in the inner ear of non-sound-exposed zebrafish, suggesting that GH could play an important role in sensory hair cell regeneration in the teleost ear.

Development of tonotopy in the auditory periphery

Acoustic frequency analysis plays an essential role in sound perception, communication and behavior. The auditory systems of most vertebrates that perceive sounds in air are organized based on the separation of complex sounds into component frequencies. This process begins at the level of the auditory sensory epithelium where specific frequencies are distributed along the tonotopic axis of the mammalian cochlea or the avian/reptilian basilar papilla (BP). Mechanical and electrical mechanisms mediate this process, but the relative contribution of each mechanism differs between species. Developmentally, structural and physiological specializations related to the formation of a tonotopic axis form gradually over an extended period of time. While some aspects of tonotopy are evident at early stages of auditory development, mature frequency discrimination is typically not achieved until after the onset of hearing. Despite the importance of tonotopic organization, the factors that specify unique positional identities along the cochlea or basilar papilla are unknown. However, recent studies of developing systems, including the inner ear provide some clues regarding the signalling pathways that may be instructive for the formation of a tonotopic axis.

Comparative analysis of combination kanamycin-furosemide versus kanamycin alone in the mouse cochlea

Combinations of aminoglycosides and loop diuretics have been known to have a synergistic effect in ototoxic injury. Because murine hair cells are relatively resistant to ototoxicity compared to those of other mammals, investigators have turned to combination therapies to create ototoxic lesions in the mouse inner ear. In this paper, we perform a systematic comparison of hearing thresholds, hair cell damage and monocyte migration into the mouse cochlea after kanamycin versus combined kanamycin/furosemide and explore the pathophysiology of enhanced hair cell loss in aminoglycoside ototoxicity in the presence of loop diuretic. Combined kanamycin-furosemide resulted in elevation of threshold not only in the high frequencies, but across all frequencies with more extensive loss of outer hair cells when compared to kanamycin alone. The stria vascularis was severely atrophied and stellate cells in the spiral limbus were missing in kanamycin-furosemide exposed mice while these changes were not observed in mice receiving kanamycin alone. Monocytes and macrophages were recruited in large numbers to the spiral ligament and spiral ganglion in these mice. Combination therapy resulted in a greater number of macrophages in total, and many more macrophages were present further apically when compared to mice given kanamycin alone. Combined kanamycin-furosemide provides an effective method of addressing the relative resistance to ototoxicity that is observed in most mouse strains. As the mouse becomes increasingly more common in studies of hearing loss, and combination therapies gain popularity, recognition of the overall effects of combined aminoglycoside-loop diuretic therapy will be critical to interpretation of the interventions that follow.

Supporting cell division is not required for regeneration of auditory hair cells after ototoxic injury in vitro

In chickens, nonsensory supporting cells divide and regenerate auditory hair cells after injury. Anatomical evidence suggests that supporting cells can also transdifferentiate into hair cells without dividing. In this study, we characterized an organ culture model to study auditory hair cell regeneration, and we used these cultures to test if direct transdifferentiation alone can lead to significant hair cell regeneration. Control cultures (organs from posthatch chickens maintained without streptomycin) showed complete hair cell loss in the proximal (high-frequency) region by 5 days. In contrast, a 2-day treatment with streptomycin induced loss of hair cells from all regions by 3 days. Hair cell regeneration proceeded in culture, with the time course of supporting cell division and hair cell differentiation generally resembling in vivo patterns. The degree of supporting cell division depended upon the presence of streptomycin, the epithelial region, the type of culture media, and serum concentration. On average, 87% of the regenerated hair cells lacked the cell division marker BrdU despite its continuous presence, suggesting that most hair cells were regenerated via direct transdifferentiation. Addition of the DNA polymerase inhibitor aphidicolin to culture media prevented supporting cell division, but numerous hair cells were regenerated nonetheless. These hair cells showed signs of functional maturation, including stereociliary bundles and rapid uptake of FM1-43. These observations demonstrate that direct transdifferentiation is a significant mechanism of hair cell regeneration in the chicken auditory after streptomycin damage in vitro.

Feathers and fins: non-mammalian models for hair cell regeneration

Death of mechanosensory cells in the inner ear results in two profound disabilities: hearing loss and balance disorders. Although mammals lack the capacity to regenerate hair cells, recent studies in mice and other rodents have offered valuable insight into strategies for stimulating hair cell regeneration in mammals. Investigations of model organisms that retain the ability to form new hair cells after embryogenesis, such as fish and birds, are equally important and have provided clues as to the cellular and molecular mechanisms that may block hair cell regeneration in mammals. Here, we summarize studies on hair cell regeneration in the chicken and the zebrafish, discuss specific advantages of each model, and propose future directions for the use of non-mammalian models in understanding hair cell regeneration.

Notch regulation of progenitor cell behavior in quiescent and regenerating auditory epithelium of mature birds

Unlike mammals, birds regenerate auditory hair cells (HCs) after injury. During regeneration, mature non-sensory supporting cells (SCs) leave quiescence and convert into HCs, through non-mitotic or mitotic mechanisms. During embryogenesis, Notch ligands from nascent HCs exert lateral inhibition, restricting HC production. Here, we examined whether Notch signaling (1) is needed in mature birds to maintain the HC/SC pattern in the undamaged auditory epithelium or (2) governs SC behavior once HCs are injured. We show that Notch pathway genes are transcribed in the mature undamaged epithelium, and after HC injury, their transcription is upregulated in the region of highest mitotic activity. In vitro treatment with DAPT, an inhibitor of Notch activity, had no effect on SCs in the undamaged epithelium. Following HC damage, DAPT had no direct effect on SC division. However, after damage, DAPT caused excessive regeneration of HCs at the expense of SCs, through both mitotic and non-mitotic mechanisms. Conversely, overexpression of activated Notch in SCs after damage caused them to maintain their phenotype and inhibited HC regeneration. Therefore, signaling through Notch is not required for SC quiescence in the healthy epithelium or to initiate HC regeneration after damage. Rather, Notch prevents SCs from regenerating excessive HCs after damage.

Peptide- and collagen-based hydrogel substrates for in vitro culture of chick cochleae

The overall goal of this work is to improve the culture of the auditory organ of birds for the dual use of developing a hair cell regeneration model and charting a pathway to the eventual replacement of the hearing organ. In doing so, we develop a protocol for removing the auditory organ from its basement membrane in the inner ear, attach the organ to a series of artificial basement membranes, and conduct qualitative and quantitative analysis of how cell morphology, viability and function change with time. Native matrix cultures, where the epithelium was floating in media with the basement membrane and accessory structures attached, were used as a basis of comparison. PuraMatrix, collagen I, collagen I/chondroitin-sulfate and Matrigel were chosen to encompass a diverse range of mechanical properties and macromolecule moieties. Surprisingly, we find that PuraMatrix outperformed the other matrices as a scaffold for sensory organ culture. PuraMatrix a self-assembled peptide hydrogel, is a biochemically specific culture substrate that contains none of the extracellular matrix (ECM) molecules and growth factors contained in the inner ear's basement membrane. Rheological measurements reveal that PuraMatrix may be a closer approximation to the stiffness of the soft tissue supporting the auditory organ. Cell density on the PuraMatrix substrate is comparable to that of the native matrix cultures, despite the absence of the basement membrane and accessory structures. Further studies show that PuraMatrix supports the culture of functional hair cells over a 72 h period, with a significant increase in the number of functional hair cells in comparison to the organ cultured without a matrix. This is the first example of adhesion of the adult auditory epithelium to a biomaterial for an extended period of time. With further optimization, this system will enable the performance of many novel biophysical and pharmacological studies involving hair cells and supporting cells.

Differential expression of Sox2 and Sox3 in neuronal and sensory progenitors of the developing inner ear of the chick

The generation of the mechanosensory elements of the inner ear during development proceeds in a precise temporal and spatial pattern. First, neurosensory precursors form sensory neurons. Then, prosensory patches emerge and give rise to hair and supporting cells. Hair cells are innervated by cochleovestibular neurons that convey sound and balance information to the brain. SOX2 is an HMG transcription factor characteristic of the stem-cell genetic network responsible for progenitor self-renewal and commitment, and its loss of function generates defects in ear sensory epithelia. The present study shows that SOX2 protein is expressed in a spatially and temporally restricted manner throughout development of the chick inner ear. SOX2 is first expressed in the neurogenic region that gives rise to sensory neurons. SOX2 is then restricted to the prosensory patches in E4 and E5 embryos, as revealed by double and parallel labelling with SOX2 and Tuj1, MyoVIIa, or Islet1. Proliferating cell nuclear antigen labelling showed that SOX2 is expressed in proliferating cells during those stages. By E5, SOX2 is also expressed in the Schwann cells of the cochleovestibular ganglion, but not in the otic neurons. At E8 and E17, beyond stages of sensory cell specification, SOX2 is transiently expressed in hair cells, but its level remains high in supporting cells. SOX3 is concomitantly expressed with SOX2 in the neurogenic domain of the otic cup, but not in prosensory patches. Our data are consistent with a role for SOX2 in specifying a population of otic progenitors committed to a neural fate, giving rise to neurons and hair cells.

Closure of supporting cell scar formations requires dynamic actin mechanisms

In many vertebrate inner ear sensory epithelia, dying sensory hair cells are extruded, and the apices of surrounding supporting cells converge to re-seal the epithelial barrier between the electrochemically-distinct endolymph and perilymph. These cellular mechanisms remain poorly understood. Dynamic microtubular mechanisms have been proposed for hair cell extrusion; while contractile actomyosin-based mechanisms are required for cellular extrusion and closure in epithelial monolayers. The hypothesis that cytoskeletal mechanisms are required for hair cell extrusion and supporting cell scar formation was tested using bullfrog saccules incubated with gentamicin (6h), and allowed to recover (18h). Explants were then fixed, labeled for actin and cytokeratins, and viewed with confocal microscopy. To block dynamic cytoskeletal processes, disruption agents for microtubules (colchicine, paclitaxel) myosin (Y-27632, ML-9) or actin (cytochalasin D, latrunculin A) were added during treatment and recovery. Microtubule disruption agents had no effect on hair cell extrusion or supporting cell scar formation. Myosin disruption agents appeared to slow down scar formation but not hair cell extrusion. Actin disruption agents blocked scar formation, and largely prevented hair cell extrusion. These data suggest that actin-based cytoskeletal processes are required for hair cell extrusion and supporting cell scar formation in bullfrog saccules.

Atoh1 expression defines activated progenitors and differentiating hair cells during avian hair cell regeneration

In the avian inner ear, nonsensory supporting cells give rise to new sensory hair cells through two distinct processes: mitosis and direct transdifferentiation. Regulation of supporting cell behavior and cell fate specification during avian hair cell regeneration is poorly characterized. Expression of Atoh1, a proneural transcription factor necessary and sufficient for developmental hair cell specification, was examined using immunofluorescence in quiescent and regenerating hair cell epithelia of mature chickens. In untreated birds, Atoh1 protein was not detected in the auditory epithelium, which is quiescent. In contrast, numerous Atoh1-positive nuclei were seen in the utricular macula, which undergoes continual hair cell turnover. Atoh1-positive nuclei emerged in the auditory epithelium by 15 hr post-ototoxin administration, before overt hair cell damage and supporting cell re-entry into the cell cycle. Subsequently, Atoh1 labeling was seen in 15% of dividing supporting cells. During cell division, Atoh1 was distributed symmetrically to daughter cells, but Atoh1 levels were dramatically regulated shortly thereafter. After cellular differentiation, Atoh1 labeling was confined to hair cells regenerated through either mitosis or direct transdifferentiation. However, Atoh1 expression in dividing progenitors did not necessarily predict hair cell fate specification in daughter cells. Finally, predominant modes of hair cell regeneration varied significantly across the radial axis of the auditory epithelium, with mitosis most frequent neurally and direct transdifferentiation most frequent abneurally. These observations suggest a role for Atoh1 in re-specifying supporting cells and in biasing postmitotic cells toward the hair cell fate during hair cell regeneration in the mature chicken ear.

Anatomical and functional recovery of the goldfish (Carassius auratus) ear following noise exposure

Fishes can regenerate lateral line and inner ear sensory hair cells that have been lost following exposure to ototoxic antibiotics. However, regenerative capabilities following noise exposure have not been explored in fish. Moreover, nothing is known about the functional relationship between hair cell damage and hearing loss, or the time course of morphological versus functional recovery in fishes. This study examines the relationship between hair cell damage and physiological changes in auditory responses following noise exposure in the goldfish (Carassius auratus). Goldfish were exposed to white noise (170 dB re. 1 muPa RMS) for 48 h and monitored for 8 days after exposure. Auditory thresholds were determined using the auditory evoked potential technique, and morphological hair cell damage was analyzed using phalloidin and DAPI labeling to visualize hair cell bundles and nuclei. A TUNEL assay was used to identify apoptotic cells. Following noise exposure, goldfish exhibited a significant temporary threshold shift (TTS; ranging from 13 to 20 dB) at all frequencies tested (from 0.2-2 kHz). By 7 days post-exposure, goldfish hearing recovered significantly (mean TTS<4 dB). Increased apoptotic activity was observed in the saccules and lagenae between 0 and 2 days post-exposure. Immediately after noise exposure, the central and caudal regions of saccules exhibited significant loss of hair bundles. Hair bundle density in the central saccule recovered by the end of the experiment (8 days post-exposure) while bundle density in the caudal saccule did not return to control levels in this time frame. These data demonstrate that goldfish inner ear epithelia show damage following noise exposure and that they are capable of significant regenerative responses similar to those seen following ototoxic drug treatment. Interestingly, functional recovery preceded morphological recovery in the goldfish saccule, suggesting that only a subset of hair cells are necessary for normal auditory responses, at least to the extent that hearing was measured in this study.

The use of zebrafish for assessing ototoxic and otoprotective agents

Zebrafish and other fish exhibit hair cells in the lateral-line neuromasts which are structurally and functionally similar to mammalian inner ear hair cells. To facilitate drug screening for ototoxic or otoprotective agents, we report a straightforward, quantitative in vivo assay to determine potential ototoxicity of drug candidates and to screen otoprotective agents in zebrafish larva. In this study, a fluorescent vital dye, DASPEI (2-(4-(dimethylamino)styryl)-N-ethylpyridinium iodide), was used to stain zebrafish hair cells in vivo and morphometric analysis was performed to quantify fluorescence intensity and convert images to numerical endpoints. Various therapeutics, including gentamicin, cisplatin, vinblastine sulfate, quinine, and neomycin, which cause ototoxicity in humans, also resulted in hair cell loss in zebrafish. In addition, protection against cisplatin-induced ototoxicity was observed in zebrafish larva co-treated with cisplatin and different antioxidants including, glutathione (GSH), allopurinol (ALO), N-acetyl l-cysteine (l-NAC), 2-oxothiazolidine-4-carboxylate (OTC) and d-methionine (d-MET). Our data indicate that results of ototoxicity and otoprotection in zebrafish correlated with results in humans, supporting use of zebrafish for preliminary drug screening.

Critical signaling events during the aminoglycoside-induced death of sensory hair cells in vitro

Sensory hair cells undergo apoptosis following exposure to aminoglycoside antibiotics. In neurons, apoptosis is associated with a transient increase in intracellular Ca2+, phosphorylation of the transcription factor c-Jun, and the release of cytochrome c from mitochondria into the cytosol, which along with other cofactors results in the activation of caspases. To examine the possible role of these events in the survival and death of the sensory receptors of the inner ear, we examined the effects of neomycin treatment on cytoplasmic calcium, activation of c-Jun-N-Terminal kinases (JNKs), cytochrome c release, and caspase-3 activation in cultured vestibular hair cells. Increased numbers of phospho-c-Jun-labeled hair cells (a downstream indicator of JNK activation) were observed at 3-12 h after neomycin treatment, whereas increased numbers of cells with cytoplasmic cytochrome c were observed at 12-18 h following the onset of neomycin treatment. This was followed by an increase in the number of cells that contained activated caspase-3 and displayed pyknotic nuclei. Treatment with the general caspase inhibitor BAF did not affect the release of cytochrome c and the number of p-c-Jun-labeled cells, but reduced the number of cells with activated caspase-3 and pyknotic nuclei. In contrast, treatment with CEP-11004, an indirect inhibitor of the JNK signaling pathway, promoted hair cell survival following neomycin treatment and reduced the number of cells with phosphorylated JNK and c-Jun, cytoplasmic cytochrome c, and activated caspase 3. These results suggest that JNK activation occurs upstream of the release of cytochrome c and that cytochrome c release precedes caspase activation. Cytochrome c release and JNK activation were also preceded by large changes in cytoplasmic calcium. Cytoplasmic calcium increases may be causally related to the release of cytochrome c, and may also be a potential pathway for activation of JNK in hair cells.

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.

Characterization of leukocyte subtypes in chicken inner ear sensory epithelia

Human hearing and balance require intact inner ear sensory hair cells, which transduce mechanical stimuli into electrical signals that are transmitted to the brain. Loss of hair cells after birth in mammals is irreversible, whereas birds are able to regenerate hair cells after insult and demonstrate ongoing hair cell production in the vestibular epithelia. Leukocytes reside in undamaged sensory epithelia of the avian inner ear and increase in number after trauma, prior to the proliferation of hair cell progenitors. It has been hypothesized that leukocyte-produced growth factors or cytokines may be involved in triggering hair cell regeneration. Little is known about the specific leukocyte subtypes present in avian ear. Immunohistochemistry with a panel of monoclonal antibodies to chicken leukocytes was used to identify leukocyte subtypes in normal posthatch chicken ear sensory epithelia. The responsiveness of the leukocytes to aminoglycoside-induced damage was also observed. Based on immunocytochemical and morphological criteria, we quantified leukocyte subtypes in normal and drug-damaged auditory and vestibular sensory epithelia. Data indicate that lymphocytes (B and T cells) do not reside in normal or drug-damaged ear sensory epithelia at 1-3 days post insult but are present in adjacent nonsensory tissues. The most common leukocytes in inner ear sensory epithelia are ramified cells of the myeloid lineage. Many of these are MHC class II positive, and a small percentage are mature tissue macrophages. An absence of leukocytes in lesioned areas of the auditory sensory epithelium suggests they may not play a critical role in triggering hair cell regeneration.

cProx1 immunoreactivity distinguishes progenitor cells and predicts hair cell fate during avian hair cell regeneration

In birds, mature sensory hair cells are regenerated continually in vestibular epithelia and after damage in the auditory basilar papilla. Molecular mechanisms governing the cellular processes associated with hair cell regeneration are poorly understood. Transcription factors are critical regulators of cell proliferation and differentiation in developing tissues. We examined immunoreactivity for cProx1 during both ongoing and damage-induced hair cell regeneration in chickens. Homologues of this divergent homeobox transcription factor are required for cell cycle withdrawal and differentiation in several vertebrate and invertebrate tissues. In the mitotically quiescent basilar papilla, a population of resting progenitor cells (supporting cells) shows faint nuclear immunoreactivity for cProx1. When auditory hair cell regeneration is triggered by experimental damage, nuclear cProx1 immunolabel is highly elevated in approximately 50% of dividing progenitor cells. Shortly after cytokinesis, all sibling pairs show symmetric patterns of nuclear cProx1 labeling, but pairs with asymmetric labeling emerge shortly thereafter. Strongly immunoreactive cells acquire the hair cell fate, whereas cells with low nuclear immunoreactivity differentiate as supporting cells. By contrast, cProx1 is not detected in any dividing progenitor cells during ongoing regeneration in the utricle. However, nuclear cProx1 immunoreactivity becomes asymmetric in postmitotic sibling cells, and as in the basilar papilla, cells with elevated cProx1 label differentiate as hair cells. In conclusion, cProx1 immunolabeling varies across sensory epithelial progenitors and distinguishes early differentiating hair cells from supporting cells. cProx1 may regulate the proliferative or differentiative capacities of progenitor cells and specify hair cell fate in postmitotic cells during avian hair cell regeneration.

Ultrastructural analysis of [3H]thymidine-labeled cells in the rat utricular macula

Ototoxic drugs stimulate cell proliferation in adult rat vestibular sensory epithelia, as does the infusion of transforming growth factor alpha (TGFalpha) plus insulin. We sought to determine whether new hair cells can be regenerated by means of a mitotic pathway. Previously, studies have shown that the nuclei of some newly generated cells are located in the lumenal half of the sensory epithelium, suggesting that some may be newly generated sensory hair cells. The aim of this study was to examine the ultrastructural characteristics of newly proliferated cells after TGFalpha stimulation and/or aminoglycoside damage in the utricular sensory epithelium of the adult rat. The cell proliferation marker tritiated-thymidine was infused, with or without TGFalpha plus insulin, into the inner ears of normal or aminoglycoside-damaged rats for 3 or 7 days by means of osmotic pumps. Autoradiographic techniques and light microscopy were used to identify cells synthesizing DNA. Sections with labeled cells were re-embedded, processed for transmission electron microscopy, and the ultrastructural characteristics of the labeled cells were examined. The following five classes of tritiated-thymidine labeled cells were identified in the sensory epithelium: (1) labeled cells with synaptic specializations that appeared to be newly generated hair cells, (2) labeled supporting cells, (3) labeled leukocytes, (4) labeled cells that we have classified as "active cells" in that they are relatively nondescript but contain massive numbers of polyribosomes, and (5) labeled degenerating hair cells. These findings suggest that new hair cells can be generated in situ by means of a mitotic mechanism in the vestibular sensory epithelium of adult mammals.

Hair cell development in vivo and in vitro: analysis by using a monoclonal antibody specific to hair cells in the chick inner ear

The purpose of this study was to establish a hair cell-specific marker and a convenient explant culture system for developing chick otocysts to facilitate in vivo and in vitro studies focusing on hair cell genesis in the inner ear. To achieve this, a hair cell-specific monoclonal antibody, 2A7, was generated by immunizing chick inner ear tissues to a mouse. Through the use of immunofluorescence and immunoelectron microscopy, it was shown that 2A7 immunoreactivity (2A7-IR) was primarily restricted to the apical region of inner ear hair cells, including stereocilia, kinocilia, apical membrane amongst the extending cilia, and superficial layer of the cuticular plate. Although the 2A7 antibody immunolabeled basically all of the hair cells in the posthatch chick inner ear, two different patterns of 2A7-IR were observed; hair cells located in the striolar region of the utricular macula, which consist of two distinct cell types identifiable on the basis of the type of nerve ending, Type I and II hair cells, showed labeling restricted to the basal end of the hair bundles. On the other hand, hair cells in the extrastriolar region, which are exclusively of Type II, showed labeling extending over virtually the entire length of the bundles. These findings raised the possibility that chick vestibular Type II hair cells, characterized by their bouton-type afferent nerve endings, can be divided into two subpopulations. Analysis of developing inner ear by using the 2A7 antibody revealed that this antibody also recognizes newly differentiated immature hair cells. Thus, the 2A7 antibody is able to recognize both immature and mature hair cells in vivo. The developmental potential of embryonic otocysts in vitro was then assessed by using explant cultures as a model. In this study, conventional otocyst explant cultures were modified by placing the tissues on floating polycarbonate filters on culture media, thereby allowing the easy manipulation of explants. In these cultures, 2A7-positive hair cells were differentiated from dividing precursor cells in vitro on the same schedule as in vivo. Furthermore, it was found that hair cells with both types of 2A7-IR were generated in culture as in vivo, indicating that a maturational process of hair cells also occurred. All these results as presented here suggest that the 2A7 monoclonal antibody as a hair cell-specific marker together with the culture system could be a potential tool in analysis of mechanisms underlying hair cell development.

Inhibition of caspases prevents ototoxic and ongoing hair cell death

Sensory hair cells die after acoustic trauma or ototoxic insults, but the signal transduction pathways that mediate hair cell death are not known. Here we identify several important signaling events that regulate the death of vestibular hair cells. Chick utricles were cultured in media supplemented with the ototoxic antibiotic neomycin and selected pharmacological agents that influence signaling molecules in cell death pathways. Hair cells that were treated with neomycin exhibited classically defined apoptotic morphologies such as condensed nuclei and fragmented DNA. Inhibition of protein synthesis (via treatment with cycloheximide) increased hair cell survival after treatment with neomycin, suggesting that hair cell death requires de novo protein synthesis. Finally, the inhibition of caspases promoted hair cell survival after neomycin treatment. Sensory hair cells in avian vestibular organs also undergo continual cell death and replacement throughout mature life. It is unclear whether the loss of hair cells stimulates the proliferation of supporting cells or whether the production of new cells triggers the death of hair cells. We examined the effects of caspase inhibition on spontaneous hair cell death in the chick utricle. Caspase inhibitors reduced the amount of ongoing hair cell death and ongoing supporting cell proliferation in a dose-dependent manner. In isolated sensory epithelia, however, caspase inhibitors did not affect supporting cell proliferation directly. Our data indicate that ongoing hair cell death stimulates supporting cell proliferation in the mature utricle.

FGFR3 expression during development and regeneration of the chick inner ear sensory epithelia

Several studies suggest fibroblast growth factor receptor 3 (FGFR3) plays a role in the development of the auditory epithelium in mammals. We undertook a study of FGFR3 in the developing and mature chicken inner ear and during regeneration of this epithelium to determine whether FGFR3 shows a similar pattern of expression in birds. FGFR3 mRNA is highly expressed in most support cells in the mature chick basilar papilla but not in vestibular organs of the chick. The gene is expressed early in the development of the basilar papilla. Gentamicin treatment sufficient to destroy hair cells in the basilar papilla causes a rapid, transient downregulation of FGFR3 mRNA in the region of damage. In the initial stages of hair cell regeneration, the support cells that reenter the mitotic cycle in the basilar papilla do not express detectable levels of FGFR3 mRNA. However, once the hair cells have regenerated in this region, the levels of FGFR3 mRNA and protein expression rapidly return to approximate those in the undamaged epithelium. These results indicate that FGFR3 expression changes after drug-induced hair cell damage to the basilar papilla in an opposite way to that found in the mammalian cochlea and may be involved in regulating the proliferation of support cells.

Ongoing cell death and immune influences on regeneration in the vestibular sensory organs

Hair cells in the vestibular organs of birds have a relatively short life span. Mature hair cells appear to die spontaneously and are then quickly replaced by new hair cells that arise from the division of epithelial supporting cells. A similar regenerative mechanism also results in hair cell replacement after ototoxic damage. The cellular basis of hair cell turnover in the avian ear is not understood. We are investigating the signaling pathways that lead to hair cell death and the relationship between ongoing cell death and cell production. In addition, work from our lab and others has demonstrated that the avian inner ear contains a resident population of macrophages and that enhanced numbers of macrophages are recruited to sites of hair cells lesions. Those observations suggest that macrophages and their secretory products (cytokines) may be involved in hair cell regeneration. Consistent with that suggestion, we have found that treatment with the anti-inflammatory drug dexamethasone reduces regenerative cell proliferation in the avian ear, and that certain macrophage-secreted cytokines can influence the proliferation of vestibular supporting cells and the survival of statoacoustic neurons. Those results suggest a role for the immune system in the process of sensory regeneration in the inner ear.

Return of potassium ion channels in regenerated hair cells: possible pathways and the role of intracellular calcium signaling

Recent electrophysiological studies in pigeon have demonstrated that potassium channels are completely functional in regenerated type II hair cells at 21 days post-treatment (PT) with ototoxic doses of streptomycin. The currents return in the order they appear during development. The mixture of ionic currents in a regenerated type II hair cell in a particular region of the neuroepithelium is the same as in its ancestor in that region. The return of currents in regenerated type I hair cells is more complicated. The dominant conductance gKI is not present until after 70 days PT. Before 70 days, the ionic currents in type I hair cells resemble those of regenerated type II hair cells, suggesting that the ionic currents in type II hair cells might be precursors of the ionic currents in regenerated type I hair cells. New data show that at one year PT, the kinetics and drug sensitivity of the dominant K+ conductance in type I hair cells are identical to gKI. Supporting cells, believed to be the precursors of regenerated type II hair cells, have effectively no voltage-gated outward potassium channels, suggesting that regenerated type II hair cells must develop these channels de novo. The next step is to understand the mechanisms by which the potassium channel protein is synthesized, migrates through the cytosol, and is inserted into the plasmalemma of regenerating hair cells. These mechanisms are unknown. We propose that intracellular calcium is involved in this process, as well as in the differentiation, proliferation, and gene regulation of precursor cells fated to become hair cells.

Hair cell recovery in mitotically blocked cultures of the bullfrog saccule

Hair cells in many nonmammalian vertebrates are regenerated by the mitotic division of supporting cell progenitors and the differentiation of the resulting progeny into new hair cells and supporting cells. Recent studies have shown that nonmitotic hair cell recovery after aminoglycoside-induced damage can also occur in the vestibular organs. Using hair cell and supporting cell immunocytochemical markers, we have used confocal and electron microscopy to examine the fate of damaged hair cells and the origin of immature hair cells after gentamicin treatment in mitotically blocked cultures of the bullfrog saccule. Extruding and fragmenting hair cells, which undergo apoptotic cell death, are replaced by scar formations. After losing their bundles, sublethally damaged hair cells remain in the sensory epithelium for prolonged periods, acquiring supporting cell-like morphology and immunoreactivity. These modes of damage appear to be mutually exclusive, implying that sublethally damaged hair cells repair their bundles. Transitional cells, coexpressing hair cell and supporting cell markers, are seen near scar formations created by the expansion of neighboring supporting cells. Most of these cells have morphology and immunoreactivity similar to that of sublethally damaged hair cells. Ultrastructural analysis also reveals that most immature hair cells had autophagic vacuoles, implying that they originated from damaged hair cells rather than supporting cells. Some transitional cells are supporting cells participating in scar formations. Supporting cells also decrease in number during hair cell recovery, supporting the conclusion that some supporting cells undergo phenotypic conversion into hair cells without an intervening mitotic event.

Cellular studies of auditory hair cell regeneration in birds

A decade ago it was discovered that mature birds are able to regenerate hair cells, the receptors for auditory perception. This surprising finding generated hope in the field of auditory neuroscience that new hair cells someday may be coaxed to form in another class of warm-blooded vertebrates, mammals. We have made considerable progress toward understanding some cellular and molecular events that lead to hair cell regeneration in birds. This review discusses our current understanding of avian hair cell regeneration, with some comparisons to other vertebrate classes and other regenerative systems.

Differential expression of beta tubulin isotypes in the adult gerbil cochlea

Tubulin, the principal component of microtubules, exists as two polypeptides, termed alpha and beta. Seven isotypes of beta tubulin are known to exist in mammals. The distributions of four beta tubulin isotypes, beta(I), beta(II), beta(III), and beta(IV), have been examined in the adult cochlea by indirect immunofluorescence using isotype-specific antibodies. In the organ of Corti, outer hair cells contained only beta(I) and beta(IV), while inner hair cells contained only beta(I) and beta(II). Inner and outer pillar cells contained beta(II) and beta(IV), but Deiters cells contained those isotypes plus beta(I). Fine fibers in the inner spiral bundle, tunnel crossing fibers, and outer spiral fibers, probably efferent in character, contained beta(I), beta(II), and beta(III), but not beta(IV). In the spiral ganglion, the somas and axons of neurons contained all four isotypes, and the myelination of ganglion cells also contained beta(I). Fibers of the intraganglionic spiral bundle contained beta(I), beta(II), and beta(III). No antibody labeled the dendritic processes of spiral ganglion neurons. The differences in isotype distribution in organ of Corti and neurons described here are consistent with and support the multi-tubulin hypothesis, which states that tubulin isotypes are expressed specifically in different cell types and may therefore have different functions.

The reinnervation of regenerated hair cells in the basilar papilla of chicks after kanamycin ototoxicity

When newly hatched chicks were injected with kanamycin on 8 successive days, the hair cells were destroyed completely in the area 0.4 to 0.8 mm from the proximal end of the basilar papilla. At this time, and 1 and 7 days following the completion of 10 days of injections, the nerve fibres in the basilar papilla of chicks show no sign of injury. On the first day following 10 days kanamycin administration the regenerated hair cells obtained not only afferent innervation, but also efferent innervation. At 15 days following drug cessation, afferent innervation of the regenerated hair cells was already similar to the controls, and the thresholds measured at this time were significantly better than those at 1 and 7 days. The chalice efferent terminals did not appear until the 60th day of drug cessation. Efferent innervation of the regenerated hair cells also approached maturity at this time. Compared with the half month after the completion of drug injection, the hearing of birds had no evident increase. It was obvious that afferent innervation of the regenerated hair cells was related more to the recovery of hearing than efferent innervation. The regenerated hair cells beginning reinnervation early and maturing were important factors influencing the recovery of the birds' hearing function after kanamycin ototoxicity.

Basic fibroblast growth factor inhibits cell proliferation in cultured avian inner ear sensory epithelia

Postembryonic production of inner ear hair cells occurs after insult in nonmammalian vertebrates. Recent studies suggest that the fibroblast family of growth factors may play a role in stimulating cell proliferation in mature inner ear sensory epithelium. Effects of acidic fibroblast growth factor (FGF-1) and basic fibroblast growth factor (FGF-2) were tested on progenitor cell division in cultured auditory and vestibular sensory epithelia taken from posthatch chickens. The effects of heparin, a glycosaminoglycan that often potentiates the effects of the FGFs, were also assessed. Tritiated-thymidine autoradiographic techniques and 5-bromo-2;-deoxyuridine (BrdU) immunocytochemistry were used to identify cells synthesizing DNA. The terminal deoxynucleotidyl transferase (TdT)-mediated deoxyuridine triphosphate (dUTP)-biotin nick-end-label (TUNEL) method was used to identify apoptotic cells. TUNEL and overall counts of sensory epithelial cell density were used to assess possible cytotoxic effects of the growth factors. FGF-2 inhibited DNA synthesis in vestibular and auditory sensory epithelia and was not cytotoxic at the concentrations employed. FGF-1 did not significantly alter sensory epithelial cell proliferation. Heparin by itself inhibited DNA synthesis in the vestibular sensory epithelia and failed to potentiate the effects of FGF-1 or FGF-2. Heparin was not cytotoxic at the concentrations employed. Results presented here suggest that FGF-2 may be involved in inhibiting cell proliferation or stimulating precursor cell differentiation in avian inner ear sensory epithelia.

Effect of violation of the labyrinth on the sensory epithelium in the chick cochlea

Models in which a single large systemic dose of gentamicin is used to cause near-synchronous hair cell (HC) loss in the basal end of the chick cochlea have proven increasingly useful in the study of HC regeneration. We quantified the amount of HC death, as a percentage of the length of the basilar papilla, following single doses of 200 mg/kg and 300 mg/kg of gentamicin in 23-day-old chicks. Following 200 mg/kg of gentamicin, there was total HC loss in the basal 18.0% of the sensory epithelium and partial HC loss in the basal 26.3%. Following 300 mg/kg of gentamicin, there was total HC loss in the basal 30.5% of the epithelium and partial HC loss in the basal 40.9%. The second goal of this study was to determine whether cannula implantation in the inner ear, and infusion of bromodeoxyuridine causes HC damage. We found that creation of a fistula in the labyrinth is not associated with HC damage, but that cannula implantation can cause HC death, and can also cause potentiation of gentamicin-induced HC death. Revision of the cannula and surgical technique to ensure minimal penetration into the labyrinth almost entirely eliminated these effects. We conclude that surgical technique is critical in experimental models in which the labyrinth is violated.

Temporal, spatial, and morphologic features of hair cell regeneration in the avian basilar papilla

Hair cell-selective antibodies were used in combination with the nucleotide bromode-oxyuridine (BrdU) to examine the temporal, spatial, and morphologic progression of auditory hair cell regeneration in chicks after a single gentamicin injection. New hair cells are first identifiable with an antibody to class III beta (beta) tubulin (TuJ1) by 14 hours after BrdU incorporation, but progenitor cells in S phase and M phase are TuJ1-negative. TuJ1 labeling reveals that new hair cells are first detected at 3 days after gentamicin, in the base, and the emergence and maturation of regenerating hair cells spreads apically over time. Differentiation of regenerating hair cells consists of a progressive series of morphologic changes. During early differentiation (14 hours to 1 day after BrdU), regenerating hair cells are round or fusiform and remain near the lumen, where they are generated. During intermediate differentiation (2-4 days after BrdU), regenerating hair cells resemble support cells; their somata are elongated, their nuclei are in the support cell layer, and they appear to contact both the lumenal surface and the basal lamina. The 275-kDa hair cell antigen is first expressed in regenerating hair cells during this period. During late differentiation (7 days after BrdU and later), TuJ1-positive cells acquire the globose shape of mature hair cells. Labeling with antibodies to hair cell antigen, calmodulin, and ribosomal RNA confirms this morphologic progression. Examination of sister cells born at 3 days post-gentamicin reveals that there is equal likelihood that they will assume the hair cell or support cell fate (i.e., both asymmetric and symmetric differentiation occur).

Dynamic studies of ototoxicity in mature avian auditory epithelium

Hearing loss induced by ototoxicity is a worldwide problem despite the development of newer antibiotics and chemotherapy agents. The cellular mechanisms responsible for aminoglycoside-induced hearing loss are still poorly understood. We have developed two different methods of studying the dynamic cellular and subcellular changes in the chick auditory sensory epithelium that occur during hair cell death. The first study was performed in mature chicks after a single, high dose injection of gentamicin, which results in the rapid loss of all hair cells in the basal third of the cochlea. Chicks were sacrificed at discrete time points after drug treatment, and transmission electron microscopy was performed to study the ultrastructural changes in basal hair cells during the course of cell death. We noted various changes in the cell morphology including accumulation of cytoplasmic inclusion bodies, dispersion of the cytoplasmic polyribosomes, mitochondrial swelling, and cellular extrusion by 24 h after injection. The next two studies were performed using tissue cultures from mature avian auditory sensory epithelium. Cultured cells were labeled using vital fluorescent markers, and levels of intracellular calcium and reactive oxygen species within hair cells were studied following aminoglycoside exposure. We identified a dose-dependent increase in the levels of intracellular calcium, which was blocked by an inhibitor of voltage-gated calcium channels. We also found that levels of reactive oxygen species in hair cells greatly increased after exposure to gentamicin, and this response was blocked by two different antioxidants. These studies serve to identify key cellular and molecular changes in hair cells in response to ototoxic drugs. Further study of these processes may lead to a better understanding of how ototoxicity is induced and to potential preventative interventions.