Live imaging the phagocytic activity of inner ear supporting cells in response to hair cell death

Modulating the Unfolded Protein Response with ISRIB Mitigates Cisplatin Ototoxicity

Cisplatin is a commonly used chemotherapy agent with a nearly universal side effect of sensorineural hearing loss. The cellular mechanisms underlying cisplatin ototoxicity are poorly understood. Efforts in drug development to prevent or reverse cisplatin ototoxicity have largely focused on pathways of oxidative stress and apoptosis. An effective treatment for cisplatin ototoxicity, sodium thiosulfate (STS), while beneficial when used in standard risk hepatoblastoma, is associated with reduced survival in disseminated pediatric malignancies, highlighting the need for more specific drugs without potential tumor protective effects. The unfolded protein response (UPR) and endoplasmic reticulum (ER) stress pathways have been shown to be involved in the pathogenesis of noise-induced hearing loss and cochlear synaptopathy in vivo, and these pathways have been implicated broadly in cisplatin cytotoxicity. This study sought to determine whether the UPR can be targeted to prevent cisplatin ototoxicity. Neonatal cochlear cultures and HEK cells were exposed to cisplatin and UPR-modulating drugs, and UPR marker gene expression and cell death measured. Treatment with ISRIB, a drug that activates eif2B and downregulates the pro-apoptotic PERK/CHOP pathway of the UPR, was tested in an in vivo mouse model of cisplatin ototoxicity and well as a head and neck squamous cell carcinoma (HNSCC) cell-based assay of cisplatin cytotoxicity. Cisplatin exhibited a biphasic, non-linear dose-response of cell death and apoptosis that correlated with different patterns of UPR marker gene expression in HEK cells and cochlear cultures. ISRIB treatment protected against cisplatin-induced hearing loss and hair-cell death, but did not impact the cytotoxic effects of cisplatin on HNSCC cell viability, unlike STS. These findings demonstrate that targeting the pro-apoptotic PERK/CHOP pathway with ISRIB can mitigate cisplatin ototoxicity without reducing anti-cancer cell effects, suggesting that this may be a viable strategy for drug development.

Characteristics of spatial protein expression in the mouse cochlear sensory epithelia: Implications for age-related hearing loss

Hair cells in the cochlear sensory epithelia serve as mechanosensory receptors, converting sound into neuronal signals. The basal sensory epithelia are responsible for transducing high-frequency sounds, while the apex handles low-frequency sounds. Age-related hearing loss predominantly affects hearing at high frequencies and is indicative of damage to the basal sensory epithelia. However, the precise mechanism underlying this site-selective injury remains unclear. In this study, we employed a microscale proteomics approach to examine and compare protein expression in different regions of the cochlear sensory epithelia (upper half and lower half) in 1.5-month-old (normal hearing) and 6-month-old (severe high-frequency hearing loss without hair cell loss) C57BL/6J mice. A total of 2,386 proteins were detected, and no significant differences in protein expression were detected in the upper half of the cochlear sensory epithelia between the two age groups. The expression of 20 proteins in the lower half of the cochlear sensory epithelia significantly differed between the two age groups (e.g., MATN1, MATN4, and AQP1). Moreover, there were 311 and 226 differentially expressed proteins between the upper and lower halves of the cochlear sensory epithelia in 1.5-month-old and 6-month-old mice, respectively. The expression levels of selected proteins were validated by Western blotting. These findings suggest that the spatial differences in protein expression within the cochlear sensory epithelia may play a role in determining the susceptibility of cells at different sites of the cochlea to age-related damage.

Mechanisms and Impact of Aminoglycoside-Induced Vestibular Deficits

PURPOSE

Acquired vestibulotoxicity from hospital-prescribed medications such as aminoglycoside antibiotics affects as many as 40,000 people each year in North America. However, there are no current federally approved drugs to prevent or treat the debilitating and permanent loss of vestibular function caused by bactericidal aminoglycoside antibiotics. This review will cover our current understanding of the impact of, and mechanisms underlying, aminoglycoside-induced vestibulotoxicity and highlight the gaps in our knowledge that remain.

CONCLUSIONS

Aminoglycoside-induced vestibular deficits have long-term impacts on patients across the lifespan. Additionally, the prevalence of aminoglycoside-induced vestibulotoxicity appears to be greater than cochleotoxicity. Thus, monitoring for vestibulotoxicity should be independent of auditory monitoring and encompass patients of all ages from young children to older adults before, during, and after aminoglycoside therapy.

Age-related changes in P2Y receptor signalling in mouse cochlear supporting cells

Our sense of hearing depends on the function of a specialised class of sensory cells, the hair cells, which are found in the organ of Corti of the mammalian cochlea. The unique physiological environment in which these cells operate is maintained by a syncitium of non-sensory supporting cells, which are crucial for regulating cochlear physiology and metabolic homeostasis. Despite their importance for cochlear function, the role of these supporting cells in age-related hearing loss, the most common sensory deficit in the elderly, is poorly understood. Here, we investigated the age-related changes in the expression and function of metabotropic purinergic receptors (P2Y1 , P2Y2 and P2Y4 ) in the supporting cells of the cochlear apical coil. Purinergic signalling in supporting cells is crucial during the development of the organ of Corti and purinergic receptors are known to undergo changes in expression during ageing in several tissues. Immunolabelling and Ca2+ imaging experiments revealed a downregulation of P2Y receptor expression and a decrease of purinergic-mediated calcium responses after early postnatal stages in the supporting cells. An upregulation of P2Y receptor expression was observed in the aged cochlea when compared to 1 month-old adults. The aged mice also had significantly larger calcium responses and displayed calcium oscillations during prolonged agonist applications. We conclude that supporting cells in the aged cochlea upregulate P2Y2 and P2Y4 receptors and display purinergic-induced Ca2+ responses that mimic those observed during pre-hearing stages of development, possibly aimed at limiting or preventing further damage to the sensory epithelium. KEY POINTS: Age-related hearing loss is associated with lower hearing sensitivity and decreased ability to understand speech. We investigated age-related changes in the expression and function of metabotropic purinergic (P2Y) receptors in cochlear non-sensory supporting cells of mice displaying early-onset (C57BL/6N) and late-onset (C3H/HeJ) hearing loss. The expression of P2Y1 , P2Y2 and P2Y4 receptors in the supporting cells decreased during cochlear maturation, but that of P2Y2 and P2Y4 was upregulated in the aged cochlea. P2Y2 and P2Y4 receptors were primarily responsible for the ATP-induced Ca2+ responses in the supporting cells. The degree of purinergic expression upregulation in aged supporting cells mirrored hearing loss progression in the different mouse strains. We propose that the upregulation of purinergic-mediated signalling in the aged cochlea is subsequent to age-related changes in the hair cells and may act as a protective mechanism to limit or to avoid further damage to the sensory epithelium.

Sox2 is required in supporting cells for normal levels of vestibular hair cell regeneration in adult mice

Sox2 is a transcription factor that is necessary in the mammalian inner ear for development of sensory hair cells and supporting cells. Sox2 is expressed in supporting cells of adult mammals, but its function in this context is poorly understood. Given its role in the developing inner ear, we hypothesized that Sox2 is required in vestibular supporting cells for regeneration of type II hair cells after damage. Using adult mice, we deleted Sox2 from Sox9-CreER-expressing supporting cells prior to diphtheria toxin-mediated hair cell destruction and used fate-mapping to assess regeneration. In utricles of control mice with normal Sox2 expression, supporting cells regenerated nearly 200 hair cells by 3 weeks post-damage, which doubled by 12 weeks. In contrast, mice with Sox2 deletion from supporting cells had approximately 20 fate-mapped hair cells at 3 weeks post-damage, and this number did not change significantly by 12 weeks, indicating regeneration was dramatically curtailed. We made similar observations for saccules and ampullae. We found no evidence that supporting cells lacking Sox2 had altered cellular density, morphology, or ultrastructure. However, some Sox2-negative supporting cell nuclei appeared to migrate apically but did not turn on hair cell markers, and type I hair cell survival was higher. Sox2 heterozygotes also had reduced regeneration in utricles, but more hair cells were replaced than mice with Sox2 deletion. Our study determined that Sox2 is required in supporting cells for normal levels of vestibular hair cell regeneration but found no other major requirements for Sox2 in adult supporting cells.

Expression of Calbindin-D28K in the Developing and Adult Mouse Cochlea

Herein, we aimed to use double-labeling immunofluorescence to describe the expression pattern of Calbindin-D28K (CaBP28K) in the mouse cochlea from late embryonic (E) stages to the adulthood. CaBP28K was expressed in the inner hair cells (IHCs) and the greater epithelial ridge (GER) at E17. In addition, its expression was observed in the interdental cells. On postnatal day 1 (P1), CaBP28K immunoreactivity was observed in the IHCs and outer hair cells (OHCs) and was also specifically expressed in the nucleus and the cytoplasm of spiral ganglion neurons (SGNs). At P8, CaBP28K labeling disappeared from the interdental cells, and the CaBP28K-positive domain within the GER shifted from the entire cytoplasm to only the apical and basal regions. At P14, CaBP28K immunoreactivity was lost from the GER; however, its expression in the IHCs and OHCs, as well as the SGNs, persisted into adulthood. The identification of CaBP28K in the hair cells (HCs) and cuticular plates, as well as SGNs, was confirmed by its colocalization with several markers for Sox2, Myosin VIIa, Phalloidin, and Tuj1. We also detected colocalization with calmodulin in the cytoplasm of both HCs and SGNs. Western blot revealed an increase in CaBP28K postnatal expression in the mouse cochlea.

The Otoprotective Effect of Ear Cryotherapy: Systematic Review and Future Perspectives

This systematic review investigates ear cooling and cryotherapy in the prevention and treatment of inner ear damage and disease, within the context of animal models and clinical studies. A literature search was carried out in the databases Pubmed and Cochrane Library. Ten studies were identified concerning the otoprotective properties of cryotherapy. Nine of these were rodent in vivo studies (mice, rats, gerbils, guinea pigs). One study involved human subjects and investigated cryotherapy in idiopathic sensorineural hearing loss. The studies were heterogeneous in their goals, methods, and the models used. Disorder models included ischemia and noise damage, ototoxicity (cisplatin and aminoglycoside), and CI-electrode insertion. All ten studies demonstrated significant cryotherapeutic otoprotection for their respective endpoints. No study revealed or expressly investigated otodestructive effects. While limited in number, all of the studies within the scope of the review demonstrated some degree of cryotherapeutic, otoprotective effect. These promising results support the conducting of further work to explore and refine the clinical applicability and impact of cryotherpeutics in otolaryngology.

Supporting Cells and Their Potential Roles in Cisplatin-Induced Ototoxicity

Cisplatin is a known ototoxic chemotherapy drug, causing irreversible hearing loss. Evidence has shown that cisplatin causes inner ear damage as a result of adduct formation, a proinflammatory environment and the generation of reactive oxygen species within the inner ear. The main cochlear targets for cisplatin are commonly known to be the outer hair cells, the stria vascularis and the spiral ganglion neurons. Further evidence has shown that certain transporters can mediate cisplatin influx into the inner ear cells including organic cation transporter 2 (OCT2) and the copper transporter Ctr1. However, the expression profiles for these transporters within inner ear cells are not consistent in the literature, and expression of OCT2 and Ctr1 has also been observed in supporting cells. Organ of Corti supporting cells are essential for hair cell activity and survival. Special interest has been devoted to gap junction expression by these cells as certain mutations have been linked to hearing loss. Interestingly, cisplatin appears to affect connexin expression in the inner ear. While investigations regarding cisplatin-induced hearing loss have been focused mainly on the known targets previously mentioned, the role of supporting cells for cisplatin-induced ototoxicity has been overlooked. In this mini review, we discuss the implications of supporting cells expressing OCT2 and Ctr1 as well as the potential role of gap junctions in cisplatin-induced cytotoxicity.

Semi-automated Quantification of Hair Cells in the Mature Mouse Utricle

The mouse utricle model system is the best-characterized ex vivo preparation for studies of mature mammalian hair cells (HCs). Despite the many advantages of this model system, efficient and reliable quantification of HCs from cultured utricles has been a persistent challenge with this model system. Utricular HCs are commonly quantified by counting immunolabeled HCs in regions of interest (ROIs) placed over an image of the utricle. Our data indicate that the accuracy of HC counts obtained using this method can be impacted by variability in HC density across different regions of the utricle. In addition, the commonly used HC marker myosin 7a results in a diffuse cytoplasmic stain that is not conducive to automated quantification and must be quantified manually, a labor-intensive task. Furthermore, myosin 7a immunoreactivity is retained in dead HCs, resulting in inaccurate quantification of live HCs using this marker. Here we have developed a method for semi-automated quantification of surviving HCs that combines immunoreactivity for the HC-specific transcription factor Pou4f3 with labeling of activated caspase 3/7 (AC3/7) to detect apoptotic HCs. The discrete nuclear Pou4f3 signal allowed us to utilize the binary or threshold function within ImageJ to automate HC quantification. To further streamline this process, we created an ImageJ macro that automates the process from raw image loading to a final quantified image that can be immediately evaluated for accuracy. Within this quantified image, the user can manually correct the quantification via an image overlay indicating the counted HC nuclei. Pou4f3-positive HCs that also express AC3/7 are subtracted to yield accurate counts of surviving HCs. Overall, we present a semi-automated method that is faster than manual HC quantification and identifies surviving HCs with high accuracy.

Programmed Cell Death Recruits Macrophages Into the Developing Mouse Cochlea

Programmed cell death (PCD) plays a critical role in the development and maturation of the cochlea. Significant remodeling occurs among cells of the greater epithelial ridge (GER) of Kölliker’s organ, leading to tissue regression and formation of the inner sulcus. In mice, this event normally occurs between postnatal days 5–15 (P5-15) and is regulated by thyroid hormone (T3). During this developmental time period, the cochlea also contains a large population of macrophages. Macrophages are frequently involved in the phagocytic clearance of dead cells, both during development and after injury, but the role of macrophages in the developing cochlea is unknown. This study examined the link between developmental cell death in the GER and the recruitment of macrophages into this region. Cell death in the basal GER begins at P5 and enhanced numbers of macrophages were observed at P7. This pattern of macrophage recruitment was unchanged in mice that were genetically deficient for CX3CR1, the receptor for fractalkine (a known macrophage chemoattractant). We found that injection of T3 at P0 and P1 caused GER cell death to begin at P3, and this premature PCD was accompanied by earlier recruitment of macrophages. We further found that depletion of macrophages from the developing cochlea (using CX3CR1^DTR/+ mice and treatment with the CSF1R antagonist BLZ945) had no effect on the pattern of GER regression. Together, these findings suggest that macrophages are recruited into the GER region after initiation of developmental PCD, but that they are not essential for GER regression during cochlear remodeling.

Actin Crosslinking Family Protein 7 Deficiency Does Not Impair Hearing in Young Mice

To enable hearing, the sensory hair cell contains specialized subcellular structures at its apical region, including the actin-rich cuticular plate and circumferential band. ACF7 (actin crosslinking family protein 7), encoded by the gene Macf1 (microtubule and actin crosslinking factor 1), is a large cytoskeletal crosslinking protein that interacts with microtubules and filamentous actin to shape cells. ACF7 localizes to the cuticular plate and the circumferential band in the hair cells of vertebrates. The compelling expression pattern of ACF7 in hair cells, combined with conserved roles of this protein in the cytoskeleton of various cell types in invertebrates and vertebrates, led to the hypothesis that ACF7 performs a key function in the subcellular architecture of hair cells. To test the hypothesis, we conditionally target Macf1 in the inner ears of mice. Surprisingly, our data show that in young, but mature, conditional knockout mice cochlear hair cell survival, planar cell polarity, organization of the hair cells within the organ of Corti, and capacity to hear are not significantly impacted. Overall, these results fail to support the hypothesis that ACF7 is an essential hair cell protein in young mice, and the purpose of ACF7 expression in the hair cell remains to be understood.

Mechanisms of Aminoglycoside- and Cisplatin-Induced Ototoxicity

Purpose This review article summarizes our current understanding of the mechanisms underlying acquired hearing loss from hospital-prescribed medications that affects as many as 1 million people each year in Western Europe and North America. Yet, there are currently no federally approved drugs to prevent or treat the debilitating and permanent hearing loss caused by the life-saving platinum-based anticancer drugs or the bactericidal aminoglycoside antibiotics. Hearing loss has long-term impacts on quality-of-life measures, especially in young children and older adults. This review article also highlights some of the current knowledge gaps regarding iatrogenic causes of hearing loss. Conclusion Further research is urgently needed to further refine clinical practice and better ameliorate iatrogenic drug-induced hearing loss.

Development of Tinnitus and Hyperacusis in a Mouse Model of Tobramycin Cochleotoxicity

Aminoglycosides (AG) antibiotics are a common treatment for recurrent infections in cystic fibrosis (CF) patients. AGs are highly ototoxic, resulting in a range of auditory dysfunctions. It was recently shown that the acoustic startle reflex (ASR) can assess behavioral evidence of hyperacusis and tinnitus in an amikacin cochleotoxicity mouse model. The goal of this study was to establish if tobramycin treatment led to similar changes in ASR behavior and to establish whether ebselen can prevent the development of these maladaptive neuroplastic symptoms. CBA/Ca mice were divided into three groups: Group 1 served as a control and did not receive tobramycin or ebselen, Group 2 received tobramycin (200 mg/kg/s.c.) and the vehicle (DMSO/saline/i.p.) daily for 14 continuous days, and Group 3 received the same dose/schedule of tobramycin as Group 2 and ebselen at (20 mg/kg/i.p.). Auditory brainstem response (ABR) and ASR hearing assessments were collected at baseline and 2, 6, 10, 14, and 18 weeks from the start of treatment. ASR tests included input/output (I/O) functions which assess general hearing and hyperacusis, and Gap-induced prepulse inhibition of the acoustic startle (GPIAS) to assess tinnitus. At 18 weeks, histologic analysis showed predominantly normal appearing hair cells and spiral ganglion neuron (SGN) synapses. Following 14 days of tobramycin injections, 16 kHz thresholds increased from baseline and fluctuated over the 18-week recovery period. I/O functions revealed exaggerated startle response magnitudes in 50% of mice over the same period. Gap detection deficits, representing behavioral evidence of tinnitus, were observed in a smaller subset (36%) of animals. Interestingly, increases in ABR wave III/wave I amplitude ratios were observed. These tobramycin data corroborate previous findings that AGs can result in hearing dysfunctions. We show that a 14-day course of tobramycin treatment can cause similar levels of hearing loss and tinnitus, when compared to a 14-day course of amikacin, but less hyperacusis. Evidence suggests that tinnitus and hyperacusis might be common side effects of AG antibiotics.

Engulfed by Glia: Glial Pruning in Development, Function, and Injury across Species

Phagocytic activity of glial cells is essential for proper nervous system sculpting, maintenance of circuitry, and long-term brain health. Glial engulfment of apoptotic cells and superfluous connections ensures that neuronal connections are appropriately refined, while clearance of damaged projections and neurotoxic proteins in the mature brain protects against inflammatory insults. Comparative work across species and cell types in recent years highlights the striking conservation of pathways that govern glial engulfment. Many signaling cascades used during developmental pruning are re-employed in the mature brain to "fine tune" synaptic architecture and even clear neuronal debris following traumatic events. Moreover, the neuron-glia signaling events required to trigger and perform phagocytic responses are impressively conserved between invertebrates and vertebrates. This review offers a compare-and-contrast portrayal of recent findings that underscore the value of investigating glial engulfment mechanisms in a wide range of species and contexts.

Local Macrophage-Related Immune Response Is Involved in Cochlear Epithelial Damage in Distinct Gjb2-Related Hereditary Deafness Models

The macrophage-related immune response is an important component of the cochlear response to different exogenous stresses, including noise, ototoxic antibiotics, toxins, or viral infection. However, the role of the immune response in hereditary deafness caused by genetic mutations is rarely explored. GJB2, encoding connexin 26 (Cx26), is the most common deafness gene of hereditary deafness. In this study, two distinct Cx26-null mouse models were established to investigate the types and underlying mechanisms of immune responses. In a systemic Cx26-null model, macrophage recruitment was observed, associated with extensive cell degeneration of the cochlear epithelium. In a targeted-cell Cx26-null model, knockout of Cx26 was restricted to specific supporting cells (SCs), which led to preferential loss of local outer hair cells (OHCs). This local OHC loss can also induce a macrophage-related immune response. Common inflammatory factors, including TNF-α, IL-1β, Icam-1, Mif, Cx3cr1, Tlr4, Ccl2, and Ccr2, did not change significantly, while mRNA of Cx3cl1 was upregulated. Quantitative immunofluorescence showed that the protein expression of CX3CL1 in Deiters cells, a type of SC coupled with OHCs, increased significantly after OHC death. OHC loss caused the secondary death of spiral ganglion neurons (SGNs), while the remaining SGNs expressed high levels of CX3CL1 with infiltrated macrophages. Taken together, our results indicate that CX3CL1 signaling regulates macrophage recruitment and that enhancement of macrophage antigen-presenting function is associated with cell degeneration in Cx26-null mice.

Macrophages Respond Rapidly to Ototoxic Injury of Lateral Line Hair Cells but Are Not Required for Hair Cell Regeneration

The sensory organs of the inner ear contain resident populations of macrophages, which are recruited to sites of cellular injury. Such macrophages are known to phagocytose the debris of dying cells but the full role of macrophages in otic pathology is not understood. Lateral line neuromasts of zebrafish contain hair cells that are nearly identical to those in the inner ear, and the optical clarity of larval zebrafish permits direct imaging of cellular interactions. In this study, we used larval zebrafish to characterize the response of macrophages to ototoxic injury of lateral line hair cells. Macrophages migrated into neuromasts within 20 min of exposure to the ototoxic antibiotic neomycin. The number of macrophages in the near vicinity of injured neuromasts was similar to that observed near uninjured neuromasts, suggesting that this early inflammatory response was mediated by "local" macrophages. Upon entering injured neuromasts, macrophages actively phagocytosed hair cell debris. The injury-evoked migration of macrophages was significantly reduced by inhibition of Src-family kinases. Using chemical-genetic ablation of macrophages before the ototoxic injury, we also examined whether macrophages were essential for the initiation of hair cell regeneration. Results revealed only minor differences in hair cell recovery in macrophage-depleted vs. control fish, suggesting that macrophages are not essential for the regeneration of lateral line hair cells.

Scar Formation and Debris Elimination during Hair Cell Degeneration in the Adult DTR Mouse

The auditory sensory epithelium of the mammalian inner ear is a highly organized structure that contains sensory hair cells (HCs) and non-sensory supporting cells (SCs). Following the partial loss of HCs after cochlear insults such as overstimulation or ototoxic drugs, SCs seal the luminal epithelial surface (reticular lamina) and reorganize its cellular pattern. Here we investigated the changes in the sensory epithelium following a rapid and severe cochlear insult in the diphtheria toxin receptor (DTR) mouse, where diphtheria toxin (DT) injection leads to a HC-specific lesion resulting in a complete HC loss. We found that DT-induced selective HC ablation could lead to a pattern of scar formation and apical cell-cell adherens and tight junction reorganization similar to that occurring after other types of cochlear insult. Prestin, an outer HC-specific protein, was present in amorphous clumps at the sites where SCs had expanded to fill the spaces vacated by the dead HCs for up to 2 months after the DT induced lesion. Many of the prestin clumps appeared to occupy spaces within SCs, suggesting that SCs participate in the removal process of HC corpses in the DTR deafness model. Prestin clumps could be seen in different areas all along the length of the SCs, and appeared to be inside the SCs as well as in the inter-cellular spaces between SCs. The findings suggest that HC elimination in the DTR deafness model follows a mechanism similar to that in overstimulation or ototoxicity models, making the DTR mouse useful for understanding the process underlying HC elimination and the role of SCs in this process.

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.

Cell-Specific Transcriptional Responses to Heat Shock in the Mouse Utricle Epithelium

Sensory epithelia of the inner ear contain mechanosensory hair cells (HCs) and glia-like supporting cells (SCs), both of which are required for hearing and balance functions. Each of these cell types has unique responses to ototoxic and cytoprotective stimuli. Non-lethal heat stress in the mammalian utricle induces heat shock proteins (HSPs) and protects against ototoxic drug-induced hair cell death. Induction of HSPs in the utricle demonstrates cell-type specificity at the protein level, with HSP70 induction occurring primarily in SCs, while HSP32 (also known as heme oxygenase 1, HMOX1) is induced primarily in resident macrophages. Neither of these HSPs are robustly induced in HCs, suggesting that HCs may have little capacity for induction of stress-induced protective responses. To determine the transcriptional responses to heat shock of these different cell types, we performed cell-type-specific transcriptional profiling using the RiboTag method, which allows for immunoprecipitation (IP) of actively translating mRNAs from specific cell types. RNA-Seq differential gene expression analyses demonstrated that the RiboTag method identified known cell type-specific markers as well as new markers for HCs and SCs. Gene expression differences suggest that HCs and SCs exhibit differential transcriptional heat shock responses. The chaperonin family member Cct8 was significantly enriched only in heat-shocked HCs, while Hspa1l (HSP70 family), and Hspb1 and Cryab (HSP27 and HSP20 families, respectively) were enriched only in SCs. Together our data indicate that HCs exhibit a limited but unique heat shock response, and SCs exhibit a broader and more robust transcriptional response to protective heat stress.

Exosomes mediate sensory hair cell protection in the inner ear

Hair cells, the mechanosensory receptors of the inner ear, are responsible for hearing and balance. Hair cell death and consequent hearing loss are common results of treatment with ototoxic drugs, including the widely used aminoglycoside antibiotics. Induction of heat shock proteins (HSPs) confers protection against aminoglycoside-induced hair cell death via paracrine signaling that requires extracellular heat shock 70-kDa protein (HSP70). We investigated the mechanisms underlying this non-cell-autonomous protective signaling in the inner ear. In response to heat stress, inner ear tissue releases exosomes that carry HSP70 in addition to canonical exosome markers and other proteins. Isolated exosomes from heat-shocked utricles were sufficient to improve survival of hair cells exposed to the aminoglycoside antibiotic neomycin, whereas inhibition or depletion of exosomes from the extracellular environment abolished the protective effect of heat shock. Hair cell-specific expression of the known HSP70 receptor TLR4 was required for the protective effect of exosomes, and exosomal HSP70 interacted with TLR4 on hair cells. Our results indicate that exosomes are a previously undescribed mechanism of intercellular communication in the inner ear that can mediate nonautonomous hair cell survival. Exosomes may hold potential as nanocarriers for delivery of therapeutics against hearing loss.

Defining the Inflammatory Microenvironment in the Human Cochlea by Perilymph Analysis: Toward Liquid Biopsy of the Cochlea

The molecular pathomechanisms in the majority of patients suffering from acute or progressive sensorineural hearing loss cannot be determined yet. The size and the complex architecture of the cochlea make biopsy and in-depth histological analyses impossible without severe damage of the organ. Thus, histopathology correlated to inner disease is only possible after death. The establishment of a technique for perilymph sampling during cochlear implantation may enable a liquid biopsy and characterization of the cochlear microenvironment. Inflammatory processes may not only participate in disease onset and progression in the inner ear, but may also control performance of the implant. However, little is known about cytokines and chemokines in the human inner ear as predictive markers for cochlear implant performance. First attempts to use multiplex protein arrays for inflammatory markers were successful for the identification of cytokines, chemokines, and endothelial markers present in the human perilymph. Moreover, unsupervised cluster and principal component analyses were used to group patients by lead cytokines and to correlate certain proteins to clinical data. Endothelial and epithelial factors were detected at higher concentrations than typical pro-inflammatory cytokines such as TNF-a or IL-6. Significant differences in VEGF family members have been observed comparing patients with deafness to patients with residual hearing with significantly reduced VEGF-D levels in patients with deafness. In addition, there is a trend toward higher IGFBP-1 levels in these patients. Hence, endothelial and epithelial factors in combination with cytokines may present robust biomarker candidates and will be investigated in future studies in more detail. Thus, multiplex protein arrays are feasible in very small perilymph samples allowing a qualitative and quantitative analysis of inflammatory markers. More results are required to advance this method for elucidating the development and course of specific inner ear diseases or for perioperative characterization of cochlear implant patients.

Interactions between Macrophages and the Sensory Cells of the Inner Ear

Macrophages are present in most somatic tissues, where they detect and attack invading pathogens. Macrophages also participate in many nonimmune functions, particularly those related to tissue maintenance and injury response. The sensory organs of the inner ear contain resident populations of macrophages, and additional macrophages enter the ear after acoustic trauma or ototoxicity. As expected, such macrophages participate in the clearance of cellular debris. However, otic macrophages can also influence the long-term survival of both hair cells and afferent neurons after injury. The signals that recruit macrophages into the injured ear, as well as the precise contributions of macrophages to inner ear pathology, remain to be determined.

Intercellular Ca2+ signalling in the adult mouse cochlea

KEY POINTS

Intercellular Ca²⁺ waves are increases in cytoplasmic Ca²⁺ levels that propagate between cells. Periodic Ca²⁺ waves have been linked to gene regulation and are thought to play a crucial role in the development of our hearing epithelium, the organ of Corti and the acquisition of hearing. We observed regular periodic intercellular Ca²⁺ waves in supporting cells of an ex vivo preparation of the adult mouse organ of Corti, and these waves were found to propagate independently of extracellular ATP and were inhibited by the gap junction blockers 1-octanol and carbenoxolone. Our results establish that the existence of periodic Ca²⁺ waves in the organ of Corti is not restricted to the prehearing period.

ABSTRACT

We have investigated wave-like cytoplasmic calcium (Ca²⁺ ) signalling in an ex vivo preparation of the adult mouse organ of Corti. Two types of intercellular Ca²⁺ waves that differ in propagation distance and speed were observed. One type was observed to travel up to 100 μm with an average velocity of 7 μm/s. Such waves were initiated by local tissue damage in the outer hair cell region. The propagation distance was decreased when the purinergic receptor antagonists pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS; 50 μm) or suramin (150 μm) were added to the extracellular buffer. Immunocytochemical analysis and experiments with calcium indicator dyes showed that both P2X and P2Y receptors were present in supporting cells. A second class of waves identified to travel longitudinally along the organ of Corti propagated at a lower velocity of 1-3 μm/s. These 'slow' Ca²⁺ waves were particularly evident in the inner sulcus and Deiters' cells. They travelled for distances of up to 500 μm. The slow Ca²⁺ signalling varied periodically (approximately one wave every 10 min) and was maintained for more than 3 h. The slow waves were not affected by apyrase, or by the P2 receptor agonists suramin (150 μm) or PPADS (50 μm) but were blocked by the connexin channel blockers octanol (1 mm) and carbenoxolone (100 μm). It is proposed that the observed Ca²⁺ waves might be a physiological response to a change in extracellular environment and may be involved in critical gene regulation activities in the supporting cells of the cochlea.

Screening Mammalian Cochlear Hair Cells to Identify Critical Processes in Aminoglycoside-Mediated Damage

There is considerable interest in discovering drugs with the potential to protect inner ear hair cells (HCs) from damage. One means of discovery is to screen compound libraries. Excellent screening protocols have been developed employing cell lines derived from the cochlea and zebrafish larvae. However, these do not address the differentiated mammalian hair cell. We have developed a screening method employing micro-explants of the mammalian organ of Corti (oC) to identify compounds with the ability to influence aminoglycoside-induced HC loss. The assay is based on short segments of the neonatal mouse oC, containing ~80 HCs which selectively express green fluorescent protein (GFP). This allows the screening of hundreds of potential protectants in an assay that includes both inner and outer HCs. This review article describes various screening methods, including the micro-explant assay. In addition, two micro-explant screening studies in which antioxidant and kinase inhibitor libraries were evaluated are reviewed. The results from these screens are related to current models of HC damage and protection.

Macrophages in the Human Cochlea: Saviors or Predators-A Study Using Super-Resolution Immunohistochemistry

The human inner ear, which is segregated by a blood/labyrinth barrier, contains resident macrophages [CD163, ionized calcium-binding adaptor molecule 1 (IBA1)-, and CD68-positive cells] within the connective tissue, neurons, and supporting cells. In the lateral wall of the cochlea, these cells frequently lie close to blood vessels as perivascular macrophages. Macrophages are also shown to be recruited from blood-borne monocytes to damaged and dying hair cells induced by noise, ototoxic drugs, aging, and diphtheria toxin-induced hair cell degeneration. Precise monitoring may be crucial to avoid self-targeting. Macrophage biology has recently shown that populations of resident tissue macrophages may be fundamentally different from circulating macrophages. We removed uniquely preserved human cochleae during surgery for treating petroclival meningioma compressing the brain stem, after ethical consent. Molecular and cellular characterization using immunofluorescence with antibodies against IBA1, TUJ1, CX3CL1, and type IV collagen, and super-resolution structured illumination microscopy (SR-SIM) were made together with transmission electron microscopy. The super-resolution microscopy disclosed remarkable phenotypic variants of IBA1 cells closely associated with the spiral ganglion cells. Monitoring cells adhered to neurons with “synapse-like” specializations and protrusions. Active macrophages migrated occasionally nearby damaged hair cells. Results suggest that the human auditory nerve is under the surveillance and possible neurotrophic stimulation of a well-developed resident macrophage system. It may be alleviated by the non-myelinated nerve soma partly explaining why, in contrary to most mammals, the human’s auditory nerve is conserved following deafferentiation. It makes cochlear implantation possible, for the advantage of the profoundly deaf. The IBA1 cells may serve additional purposes such as immune modulation, waste disposal, and nerve regeneration. Their role in future stem cell-based therapy needs further exploration.

Aminoglycoside-Induced Cochleotoxicity: A Review

Aminoglycoside antibiotics are used as prophylaxis, or urgent treatment, for many life-threatening bacterial infections, including tuberculosis, sepsis, respiratory infections in cystic fibrosis, complex urinary tract infections and endocarditis. Although aminoglycosides are clinically-essential antibiotics, the mechanisms underlying their selective toxicity to the kidney and inner ear continue to be unraveled despite more than 70 years of investigation. The following mechanisms each contribute to aminoglycoside-induced toxicity after systemic administration: (1) drug trafficking across endothelial and epithelial barrier layers; (2) sensory cell uptake of these drugs; and (3) disruption of intracellular physiological pathways. Specific factors can increase the risk of drug-induced toxicity, including sustained exposure to higher levels of ambient sound, and selected therapeutic agents such as loop diuretics and glycopeptides. Serious bacterial infections (requiring life-saving aminoglycoside treatment) induce systemic inflammatory responses that also potentiate the degree of ototoxicity and permanent hearing loss. We discuss prospective clinical strategies to protect auditory and vestibular function from aminoglycoside ototoxicity, including reduced cochlear or sensory cell uptake of aminoglycosides, and otoprotection by ameliorating intracellular cytotoxicity.

Protein Synthesis Inhibition and Activation of the c-Jun N-Terminal Kinase Are Potential Contributors to Cisplatin Ototoxicity

Cisplatin has been regarded as an effective and versatile chemotherapeutic agent for nearly 40 years. Though the associated dose-dependent ototoxicity is known, the cellular mechanisms by which cochleovestibular hair cell death occur are not well understood. We have previously shown that aminoglycoside ototoxicity is mediated in part by cytosolic protein synthesis inhibition. Despite a lack of molecular similarity, aminoglycosides were shown to elicit similar stress pathways to cisplatin. We therefore reasoned that there may be some role of protein synthesis inhibition in cisplatin ototoxicity. Employing a modification of the bioorthogonal noncanonical amino acid tagging (BONCAT) method, we evaluated the effects of cisplatin on cellular protein synthesis. We show that cisplatin inhibits cellular protein synthesis in organ of Corti explant cultures. Similar to what was found after gentamicin exposure, cisplatin activates both the c-Jun N-terminal kinase (JNK) and mammalian target of rapamycin (mTOR) pathways. In contrast to aminoglycosides, cisplatin also inhibits protein synthesis in all cochlear cell types. We further demonstrate that the multikinase inhibitor sorafenib completely prevents JNK activation, while providing only moderate hair cell protection. Simultaneous stimulation of cellular protein synthesis by insulin, however, significantly improved hair cell survival in culture. The presented data provides evidence for a potential role of protein synthesis inhibition in cisplatin-mediated ototoxicity.

Magnetic Nanoparticle Mediated Steroid Delivery Mitigates Cisplatin Induced Hearing Loss

Cisplatin (cis-diamminedichloroplatinum) is widely used as a chemotherapeutic drug for genitourinary, breast, lung and head and neck cancers. Though effective in inducing apoptosis in cancer cells, cisplatin treatment causes severe hearing loss among patients. Steroids have been shown to mitigate cisplatin-induced hearing loss. However, steroids may interfere with the anti-cancer properties of cisplatin if administered systemically, or are rapidly cleared from the middle and inner ear and hence lack effectiveness when administered intra-tympanically. In this work, we deliver prednisolone-loaded nanoparticles magnetically to the cochlea of cisplatin-treated mice. This magnetic delivery method substantially reduced hearing loss in treated animals at high frequency compared to control animals or animals that received intra-tympanic methylprednisolone. The method also protected the outer hair cells from cisplatin-mediated ototoxicity.

Non-autonomous Cellular Responses to Ototoxic Drug-Induced Stress and Death

The first major recognition of drug-induced hearing loss can be traced back more than seven decades to the development of streptomycin as an antimicrobial agent. Since then at least 130 therapeutic drugs have been recognized as having ototoxic side-effects. Two important classes of ototoxic drugs are the aminoglycoside antibiotics and the platinum-based antineoplastic agents. These drugs save the lives of millions of people worldwide, but they also cause irreparable hearing loss. In the inner ear, sensory hair cells (HCs) and spiral ganglion neurons (SGNs) are important cellular targets of these drugs, and most mechanistic studies have focused on the cell-autonomous responses of these cell types in response to ototoxic stress. Despite several decades of studies on ototoxicity, important unanswered questions remain, including the cellular and molecular mechanisms that determine whether HCs and SGNs will live or die when confronted with ototoxic challenge. Emerging evidence indicates that other cell types in the inner ear can act as mediators of survival or death of sensory cells and SGNs. For example, glia-like supporting cells (SCs) can promote survival of both HCs and SGNs. Alternatively, SCs can act to promote HC death and inhibit neural fiber expansion. Similarly, tissue resident macrophages activate either pro-survival or pro-death signaling that can influence HC survival after exposure to ototoxic agents. Together these data indicate that autonomous responses that occur within a stressed HC or SGN are not the only (and possibly not the primary) determinants of whether the stressed cell ultimately lives or dies. Instead non-cell-autonomous responses are emerging as significant determinants of HC and SGN survival vs. death in the face of ototoxic stress. The goal of this review is to summarize the current evidence on non-cell-autonomous responses to ototoxic stress and to discuss ways in which this knowledge may advance the development of therapies to reduce hearing loss caused by these drugs.

Musashi-1 is the candidate of the regulator of hair cell progenitors during inner ear regeneration

BACKGROUND

Hair cell loss in the cochlea is caused by ototoxic drugs, aging, and environmental stresses and could potentially lead to devastating pathophysiological effects. In adult mammals, hair cell loss is irreversible and may result in hearing and balance deficits. In contrast, nonmammalian vertebrates, including birds, can regenerate hair cells through differentiation of supporting cells and restore inner ear function, suggesting that hair cell progenitors are present in the population of supporting cells.

RESULTS

In the present study, we aimed to identify novel genes related to regeneration in the chicken utricle by gene expression profiling of supporting cell and hair cell populations obtained by laser capture microdissection. The volcano plot identified 408 differentially expressed genes (twofold change, p = 0.05, Benjamini-Hochberg multiple testing correction), 175 of which were well annotated. Among these genes, we focused on Musashi-1 (MSI1), a marker of neural stem cells involved in Notch signaling, and the downstream genes in the Notch pathway. Higher expression of these genes in supporting cells compared with that in hair cells was confirmed by quantitative reverse transcription polymerase chain reaction. Immunohistochemistry analysis demonstrated that MSI1 was mainly localized at the basal side of the supporting cell layer in normal chick utricles. During the regeneration period following aminoglycoside antibiotic-induced damage of chicken utricles, the expression levels of MSI1, hairy and enhancer of split-5, and cyclin D1 were increased, and BrdU labeling indicated that cell proliferation was enhanced.

CONCLUSIONS

The findings of this study suggested that MSI1 played an important role in the proliferation of supporting cells in the inner ear during normal and damaged conditions and could be a potential therapeutic target in the treatment of vestibular defects.

Mutations in Cockayne Syndrome-Associated Genes (Csa and Csb) Predispose to Cisplatin-Induced Hearing Loss in Mice

Cisplatin is a common and effective chemotherapeutic agent, yet it often causes permanent hearing loss as a result of sensory hair cell death. The causes of sensitivity to DNA-damaging agents in nondividing cell populations, such as cochlear hair and supporting cells, are poorly understood, as are the specific DNA repair pathways that protect these cells. Nucleotide excision repair (NER) is a conserved and versatile DNA repair pathway for many DNA-distorting lesions, including cisplatin-DNA adducts. Progressive sensorineural hearing loss is observed in a subset of NER-associated DNA repair disorders including Cockayne syndrome and some forms of xeroderma pigmentosum. We investigated whether either of the two overlapping branches that encompass NER, transcription-coupled repair or global genome repair, which are implicated in Cockayne syndrome and xeroderma pigmentosum group C, respectively, modulates cisplatin-induced hearing loss and cell death in the organ of Corti, the auditory sensory epithelium of mammals. We report that cochlear hair cells and supporting cells in transcription-coupled repair-deficient Cockayne syndrome group A (Csa(-/-)) and group B (Csb(-/-)) mice are hypersensitive to cisplatin, in contrast to global genome repair-deficient Xpc(-/-) mice, both in vitro and in vivo We show that sensory hair cells in Csa(-/-) and Csb(-/-) mice fail to remove cisplatin-DNA adducts efficiently in vitro; and unlike Xpc(-/-) mice, Csa(-/-) and Csb(-/-) mice lose hearing and manifest outer hair cell degeneration after systemic cisplatin treatment. Our results demonstrate that Csa and Csb deficiencies predispose to cisplatin-induced hearing loss and hair/supporting cell damage in the mammalian organ of Corti, and emphasize the importance of transcription-coupled DNA repair in the protection against cisplatin ototoxicity.

SIGNIFICANCE STATEMENT

The utility of cisplatin in chemotherapy remains limited due to serious side effects, including sensorineural hearing loss. We show that mouse models of Cockayne syndrome, a progeroid disorder resulting from a defect in the transcription-coupled DNA repair (TCR) branch of nucleotide excision repair, are hypersensitive to cisplatin-induced hearing loss and sensory hair cell death in the organ of Corti, the mammalian auditory sensory epithelium. Our work indicates that Csa and Csb, two genes involved in TCR, are preferentially required to protect against cisplatin ototoxicity, relative to global genome repair-specific elements of nucleotide excision repair, and suggests that TCR is a major force maintaining DNA integrity in the cochlea. The Cockayne syndrome mice thus represent a model for testing the contribution of DNA repair mechanisms to cisplatin ototoxicity.

Activation of CHK1 in Supporting Cells Indirectly Promotes Hair Cell Survival

The sensory hair cells of the inner ear are exquisitely sensitive to ototoxic insults. Loss of hair cells after exposure to ototoxic agents causes hearing loss. Chemotherapeutic agents such as cisplatin causes hair cell loss. Cisplatin forms DNA mono-adducts as well as intra- and inter-strand DNA crosslinks. DNA cisplatin adducts are repaired through the DNA damage response. The decision between cell survival and cell death following DNA damage rests on factors that are involved in determining damage tolerance, cell survival and apoptosis. Cisplatin damage on hair cells has been the main focus of many ototoxic studies, yet the effect of cisplatin on supporting cells has been largely ignored. In this study, the effects of DNA damage response in cochlear supporting cells were interrogated. Supporting cells play a major role in the development, maintenance and oto-protection of hair cells. Loss of supporting cells may indirectly affect hair cell survival or maintenance. Activation of the Phosphoinositide 3-Kinase (PI3K) signaling was previously shown to promote hair cell survival. To test whether activating PI3K signaling promotes supporting cell survival after cisplatin damage, cochlear explants from the neural subset (NS) Cre Pten conditional knockout mice were employed. Deletion of Phosphatase and Tensin Homolog (PTEN) activates PI3K signaling in multiple cell types within the cochlea. Supporting cells lacking PTEN showed increased cell survival after cisplatin damage. Supporting cells lacking PTEN also showed increased phosphorylation of Checkpoint Kinase 1 (CHK1) levels after cisplatin damage. Nearest neighbor analysis showed increased numbers of supporting cells with activated PI3K signaling in close proximity to surviving hair cells in cisplatin damaged cochleae. We propose that increased PI3K signaling promotes supporting cell survival through phosphorylation of CHK1 and increased survival of supporting cells indirectly increases hair cell survival after cisplatin damage.

Two cell populations participate in clearance of damaged hair cells from the sensory epithelia of the inner ear

The cochlea and the vestibular organs are populated by resident macrophages, but their role in inner ear maintenance and pathology is not entirely clear. Resident macrophages in other organs are responsible for phagocytosis of injured or infected cells, and it is likely that macrophages in the inner ear serve a similar role. Hair cell injury causes macrophages to accumulate within proximity of damaged regions of the inner ear, either by exiting the vasculature and entering the labyrinth or by the resident macrophages reorganizing themselves through local movement to the areas of injury. Direct evidence for macrophage engulfment of apoptotic hair cells has been observed in several conditions. Here, we review evidence for phagocytosis of damaged hair cells in the sensory epithelium by tissue macrophages in the published literature and in some new experiments that are presented here as original work. Several studies also suggest that macrophages are not the only phaogocytic cells in the inner ear, but that supporting cells of the sensory epithelium also play an important role in debris clearance. We describe the various ways in which the sensory epithelia of the inner ear are adapted to eliminate damaged and dying cells. A collaborative effort between resident and migratory macrophages as well as neighboring supporting cells results in the rapid and efficient clearance of cellular debris, even in cases where hair cell loss is rapid and complete.

Supporting cells remove and replace sensory receptor hair cells in a balance organ of adult mice

Vestibular hair cells in the inner ear encode head movements and mediate the sense of balance. These cells undergo cell death and replacement (turnover) throughout life in non-mammalian vertebrates. However, there is no definitive evidence that this process occurs in mammals. We used fate-mapping and other methods to demonstrate that utricular type II vestibular hair cells undergo turnover in adult mice under normal conditions. We found that supporting cells phagocytose both type I and II hair cells. Plp1-CreER^T2-expressing supporting cells replace type II hair cells. Type I hair cells are not restored by Plp1-CreER^T2-expressing supporting cells or by Atoh1-CreER^TM-expressing type II hair cells. Destruction of hair cells causes supporting cells to generate 6 times as many type II hair cells compared to normal conditions. These findings expand our understanding of sensorineural plasticity in adult vestibular organs and further elucidate the roles that supporting cells serve during homeostasis and after injury.

Live-cell imaging: new avenues to investigate retinal regeneration

Sensing and responding to our environment requires functional neurons that act in concert. Neuronal cell loss resulting from degenerative diseases cannot be replaced in humans, causing a functional impairment to integrate and/or respond to sensory cues. In contrast, zebrafish (Danio rerio) possess an endogenous capacity to regenerate lost neurons. Here, we will focus on the processes that lead to neuronal regeneration in the zebrafish retina. Dying retinal neurons release a damage signal, tumor necrosis factor α, which induces the resident radial glia, the Müller glia, to reprogram and re-enter the cell cycle. The Müller glia divide asymmetrically to produce a Müller glia that exits the cell cycle and a neuronal progenitor cell. The arising neuronal progenitor cells undergo several rounds of cell divisions before they migrate to the site of damage to differentiate into the neuronal cell types that were lost. Molecular and immunohistochemical studies have predominantly provided insight into the mechanisms that regulate retinal regeneration. However, many processes during retinal regeneration are dynamic and require live-cell imaging to fully discern the underlying mechanisms. Recently, a multiphoton imaging approach of adult zebrafish retinal cultures was developed. We will discuss the use of live-cell imaging, the currently available tools and those that need to be developed to advance our knowledge on major open questions in the field of retinal regeneration.

Styrene enhances the noise induced oxidative stress in the cochlea and affects differently mechanosensory and supporting cells

Experimental and human investigations have raised the level of concern about the potential ototoxicity of organic solvents and their interaction with noise. The main objective of this study was to characterize the effects of the combined noise and styrene exposure on hearing focusing on the mechanism of damage on the sensorineural cells and supporting cells of the organ of Corti and neurons of the ganglion of Corti. The impact of single and combined exposures on hearing was evaluated by auditory functional testing and histological analyses of cochlear specimens. The mechanism of damage was studied by analyzing superoxide anion and lipid peroxidation expression and by computational analyses of immunofluorescence data to evaluate and compare the oxidative stress pattern in outer hair cells versus the supporting epithelial cells of the organ of Corti. The oxidative stress hypothesis was further analyzed by evaluating the protective effect of a Coenzyme Q₁₀ analogue, the water soluble Q_ter, molecule known to have protective antioxidant properties against noise induced hearing loss and by the analysis of the expression of the endogenous defense enzymes. This study provides evidence of a reciprocal noise-styrene synergism based on a redox imbalance mechanism affecting, although with a different intensity of damage, the outer hair cell (OHC) sensory epithelium. Moreover, these two damaging agents address preferentially different cochlear targets: noise mainly the sensory epithelium, styrene the supporting epithelial cells. Namely, the increase pattern of lipid peroxidation in the organ of Corti matched the cell damage distribution, involving predominantly OHC layer in noise exposed cochleae and both OHC and Deiters' cell layers in the styrene or combined exposed cochleae. The antioxidant treatment reduced the lipid peroxidation increase, potentiated the endogenous antioxidant defense system at OHC level in both exposures but it failed to ameliorate the oxidative imbalance and cell death of Deiters' cells in the styrene and combined exposures. Current antioxidant therapeutic approaches to preventing sensory loss focus on hair cells alone. It remains to be seen whether targeting supporting cells, in addition to hair cells, might be an effective approach to protecting exposed subjects.

Macrophage recruitment and epithelial repair following hair cell injury in the mouse utricle

The sensory organs of the inner ear possess resident populations of macrophages, but the function of those cells is poorly understood. In many tissues, macrophages participate in the removal of cellular debris after injury and can also promote tissue repair. The present study examined injury-evoked macrophage activity in the mouse utricle. Experiments used transgenic mice in which the gene for the human diphtheria toxin receptor (huDTR) was inserted under regulation of the Pou4f3 promoter. Hair cells in such mice can be selectively lesioned by systemic treatment with diphtheria toxin (DT). In order to visualize macrophages, Pou4f3-huDTR mice were crossed with a second transgenic line, in which one or both copies of the gene for the fractalkine receptor CX3CR1 were replaced with a gene for GFP. Such mice expressed GFP in all macrophages, and mice that were CX3CR1(GFP/GFP) lacked the necessary receptor for fractalkine signaling. Treatment with DT resulted in the death of ∼70% of utricular hair cells within 7 days, which was accompanied by increased numbers of macrophages within the utricular sensory epithelium. Many of these macrophages appeared to be actively engulfing hair cell debris, indicating that macrophages participate in the process of 'corpse removal' in the mammalian vestibular organs. However, we observed no apparent differences in injury-evoked macrophage numbers in the utricles of CX3CR1(+/GFP) mice vs. CX3CR1(GFP/GFP) mice, suggesting that fractalkine signaling is not necessary for macrophage recruitment in these sensory organs. Finally, we found that repair of sensory epithelia at short times after DT-induced hair cell lesions was mediated by relatively thin cables of F-actin. After 56 days recovery, however, all cell-cell junctions were characterized by very thick actin cables.