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

Myc and Fgf Are Required for Zebrafish Neuromast Hair Cell Regeneration

Unlike mammals, the non-mammalian vertebrate inner ear can regenerate the sensory cells, hair cells, either spontaneously or through induction after hair cell loss, leading to hearing recovery. The mechanisms underlying the regeneration are poorly understood. By microarray analysis on a chick model, we show that chick hair cell regeneration involves the activation of proliferation genes and downregulation of differentiation genes. Both MYC and FGF are activated in chick hair cell regeneration. Using a zebrafish lateral line neuromast hair cell regeneration model, we show that the specific inhibition of Myc or Fgf suppresses hair cell regeneration, demonstrating that both pathways are essential to the process. Rapid upregulation of Myc and delayed Fgf activation during regeneration suggest a role of Myc in proliferation and Fgf in differentiation. The dorsal-ventral pattern of fgfr1a in the neuromasts overlaps with the distribution of hair cell precursors. By laser ablation, we show that the fgfr1a-positive supporting cells are likely the hair cell precursors that directly give rise to new hair cells; whereas the anterior-posterior fgfr1a-negative supporting cells have heightened proliferation capacity, likely to serve as more primitive progenitor cells to replenish lost precursors after hair cell loss. Thus fgfr1a is likely to mark compartmentalized supporting cell subtypes with different capacities in renewal proliferation and hair cell regeneration. Manipulation of c-MYC and FGF pathways could be explored for mammalian hair cell regeneration.

Hair cell fate decisions in cochlear development and regeneration

The discovery of avian cochlear hair cell regeneration in the late 1980s and the concurrent development of new techniques in molecular and developmental biology generated a renewed interest in understanding the genetic mechanisms that regulate hair cell development in the embryonic avian and mammalian cochlea and regeneration in the mature avian cochlea. Research from many labs has demonstrated that the development of the inner ear utilizes a complex series of genetic signals and pathways to generate the endorgans, specify cell identities, and establish innervation patterns found in the inner ear. Recent studies have shown that the Notch signaling pathway, the Atoh1/Hes signaling cascade, the stem cell marker Sox2, and some of the unconventional myosin motor proteins are utilized to regulate distinct steps in inner ear development. While many of the individual genes involved in these pathways have been identified from studies of mutant and knockout mouse cochleae, the interplay of all these signals into a single systemic program that directs this process needs to be explored. We need to know not only what genes are involved, but understand how their gene products interact with one another in a structural and temporal framework to guide hair cell and supporting cell differentiation and maturation.

5-Ethynyl-2'-deoxyuridine labeling detects proliferating cells in the regenerating avian cochlea

OBJECTIVES/HYPOTHESIS

The avian cochlea regenerates hair cells following aminoglycoside treatment through supporting cell proliferation. Immunocytochemical labeling of 5-bromo-2'-deoxyuridine (BrdU), a thymidine analog, is a popular nonradioactive marker for identifying cells in the DNA synthesis (S phase) of the cell cycle. However, it requires harsh treatments to denature double-stranded DNA for the antibody to bind BrdU. We explored a new method using 5-ethynyl-2'-deoxyuridine (EdU) as a thymidine analog and a nonantibody azide/alkyne reaction between EdU and the fluorescent probe. We propose that EdU is as effective as BrdU, but without the requirement for harsh denaturation or the use of antibodies for detection.

STUDY DESIGN

Two-week-old chicks received a single gentamicin injection followed by a single EdU injection 72 hours later. Cochleae were extracted 4-8 hours later, fixed, and processed for fluorescent detection of EdU.

METHODS

Cochleae were processed for detection of incorporated EdU using the Click-iT Imaging Kit (Invitrogen/Molecular Probes, Carlsbad, CA) and colabeled with Sox2, myosin VI, or myosin VIIa antibodies. Whole-mount cochlear preparations were examined with confocal microscopy.

RESULTS

Supporting cells incorporated EdU into their newly synthesized DNA during the 4-8 hours following the EdU injection and were readily detected with little background signal. The intensity and quantity of cells labeled were similar to or better than that seen for BrdU.

CONCLUSIONS

The EdU method is as effective as BrdU, without requiring harsh denaturation or secondary antibodies to identify proliferating cells. Thus, the nonantibody EdU system allows more flexibility by enabling colabeling with multiple antibodies to other cellular proteins involved in regeneration.

Comparison of activated caspase detection methods in the gentamicin-treated chick cochlea

Aminoglycoside antibiotics induce caspase-dependent apoptotic death in cochlear hair cells. Apoptosis, a regulated form of cell death, can be induced by many stressors, which activate signaling pathways that result in the controlled dismantling of the affected cell. The caspase family of proteases is activated in the apoptotic signaling pathway and is responsible for cellular destruction. The initiator caspase-9 and the effector caspase-3 are both activated in chick cochlear hair cells following aminoglycoside exposure. We have analyzed caspase activation in the avian cochlea during gentamicin-induced hair cell death to compare two different methods of caspase detection: caspase antibodies and CaspaTag kits. Caspase antibodies bind to the cleaved activated form of caspase-9 or caspase-3 in specific locations in fixed tissue. CaspaTag is a fluorescent inhibitor that binds to a reactive cysteine residue on the large subunit of the caspase heterodimer in unfixed tissue. To induce cochlear hair cell loss, 1-2 week-old chickens received a single injection of gentamicin (300 mg/kg). Chicks were sacrificed 24, 30, 42, 48, 72, or 96 h after injection. Cochleae were dissected and labeled for activated caspase-9 or caspase-3 using either caspase-directed antibodies or CaspaTag kits. Ears were co-labeled with either phalloidin or myosin VI to visualize hair cells and to determine the progression of cochlear damage. The timing of caspase activation was similar for both assays; however, caspase-9 and caspase-3 antibodies labeled only those cells currently undergoing apoptotic cell death. Conversely, CaspaTag-labeled all the cells that have undergone apoptotic cell death and ejection from the sensory epithelium, in addition to those that are currently in the cell death process. This makes CaspaTag ideal for showing an overall pattern or level of cell death over a period of time, while caspase antibodies provide a snapshot of cell death at a specific time point.

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.

Differential expression of unconventional myosins in apoptotic and regenerating chick hair cells confirms two regeneration mechanisms

Hair cells of the inner ear are damaged by intense noise, aging, and aminoglycoside antibiotics. Gentamicin causes oxidative damage to hair cells, inducing apoptosis. In mammals, hair cell loss results in a permanent deficit in hearing and balance. In contrast, avians can regenerate lost hair cells to restore auditory and vestibular function. This study examined the changes of myosin VI and myosin VIIa, two unconventional myosins that are critical for normal hair cell formation and function, during hair cell death and regeneration. During the late stages of apoptosis, damaged hair cells are ejected from the sensory epithelium. There was a 4-5-fold increase in the labeling intensity of both myosins and a redistribution of myosin VI into the stereocilia bundle, concurrent with ejection. Two separate mechanisms were observed during hair cell regeneration. Proliferating supporting cells began DNA synthesis 60 hours after gentamicin treatment and peaked at 72 hours postgentamicin treatment. Some of these mitotically produced cells began to differentiate into hair cells at 108 hours after gentamicin (36 hours after bromodeoxyuridine (BrdU) administration), as demonstrated by the colabeling of myosin VI and BrdU. Myosin VIIa was not expressed in the new hair cells until 120 hours after gentamicin. Moreover, a population of supporting cells expressed myosin VI at 78 hours after gentamicin treatment and myosin VIIa at 90 hours. These cells did not label for BrdU and differentiated far too early to be of mitotic origin, suggesting they arose by direct transdifferentiation of supporting cells into hair cells.

Ephrin A2 may play a role in axon guidance during hair cell regeneration

OBJECTIVE

Hair cell regeneration in the avian cochlea is accompanied by frequency specific reinnervation and recovery of physiologic function. The molecular cues that guide ganglion cells to tonotopically appropriate new hair cells have not been identified. We investigated the potential of ephrin A2 in this process.

STUDY DESIGN

Ephrin A2 expression was characterized in acoustic ganglion cells of normal and gentamicin-treated early post hatch chicks.

METHODS

Ephrin A2 expression was determined by Western analysis of total protein isolated from acoustic ganglia in normal animals. Protein localization was characterized by fluorescence immunohistochemistry in sections of acoustic ganglia of normal and gentamicin treated animals. Patterns of ephrin A2 expression in acoustic ganglia were determined and quantified during hair cell regeneration.

RESULTS

Ephrin A2 expression was found in acoustic ganglia by Western analysis. Localization of this protein by immunofluorescence revealed its presence in acoustic ganglion cells in normal chicks. After gentamicin treatment, ephrin A2 expression was lost in a subset of acoustic ganglion cells. The spatial and temporal pattern of ephrin A2 loss coincides with the pattern of hair cell loss and regeneration.

CONCLUSIONS

The changes in ephrin A2 immunoreactivity in acoustic ganglion cells during cochlear damage and regeneration suggests that ephrin A2 may be involved in the guidance of ganglion cells to tonotopically appropriate hair cell targets during regeneration. Ephrin A2 in hair cell regeneration.

Direct transdifferentiation gives rise to the earliest new hair cells in regenerating avian auditory epithelium

The avian auditory epithelium is capable of complete regeneration after hair cell (HC) loss. Most new HCs arise via cell division, but approximately one-third of new HCs arise via direct transdifferentiation (DT), in which supporting cells (SCs) alter their phenotype without dividing. In this study, we used synchronous, gentamicin-induced near-total HC loss in the basal end of the epithelium and continuous infusion of the cell division marker bromodeoxyuridine (BrdU) to identify the origin of each individual regenerating HC. Early new HCs were identified by immunolabeling for the HC-specific marker myosin-VIIa, and mitotic cells with BrdU immunolabeling. The first new HCs arising via DT appear 72-96 hr after gentamicin, 24-48 hr earlier than the first new mitotic HCs. After Day 6, however, most new HCs are mitotic. The "intermediate" morphology that has been suggested to be characteristic of DT is seen in HCs arising via both pathways. These findings suggest that DT is a simpler, more rapid process that produces the first new HCs, and that mitotic regeneration is somewhat slower but ultimately produces most new HCs. The identical morphology of regenerating HCs from both pathways suggests that once HC fate is established, all new HCs follow similar cellular processes during differentiation and reorganization into the regenerated epithelium.

Perilymphatic application of alpha-melanocyte stimulating hormone ameliorates hearing loss caused by systemic administration of cisplatin

It has previously been demonstrated that ototoxicity induced by systemic administration of cisplatin is reduced by concomitant systemic administration of alpha-melanocyte stimulating hormone (alpha-MSH). In this study we investigated the effects of cochlear, perilymphatic application of alpha-MSH during intraperitoneal administration of cisplatin. Guinea pigs, implanted with a round-window electrode, allowing daily monitoring of the compound action potential (CAP), and also implanted with a mini-osmotic pump, pumping at a rate of 0.25 microl/h either physiological saline or alpha-MSH solution (0.02, 2, and 20 microg/ml), were treated daily with a bolus injection of cisplatin (2 mg/kg) until the electrocochleogram showed a persistent decrease in CAP amplitude (> or = 40 dB threshold shift at 8 kHz). Then, cisplatin treatment was stopped, but intracochlear perfusion of alpha-MSH or physiological saline was continued for 10 days to evaluate possible effects of alpha-MSH on the expected recovery. On day 10, the animals were killed and the cochleas were fixed and processed for histological analysis. All groups required 6-7 days of cisplatin to reach the criterion CAP threshold shift. Ten days after cessation of the cisplatin treatment, recovery of the CAP was observed in all groups and at all frequencies, although it was more pronounced at the lower frequencies. With respect to recovery, small statistically significant differences were found between the saline and the alpha-MSH co-treated groups. Histological results showed significantly less outer hair cell (OHC) loss in the group co-treated with 2 microg/ml alpha-MSH as compared to the group co-treated with saline. Since alpha-MSH was directly delivered to the cochlea, the ameliorating effect of alpha-MSH on OHC survival is likely to involve a cochlear target.

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.

Progression of hair cell ejection and molecular markers of apoptosis in the avian cochlea following gentamicin treatment

Aminoglycoside treatment induces caspase-dependent apoptotic death in inner ear sensory hair cells. The timing of apoptotic signaling in sensory hair cells following systemic aminoglycoside treatment has not been characterized in vivo. We administered a single subcutaneous injection of the aminoglycoside gentamicin (300 mg/kg) to 12-16-day-old chicks and used immunocytochemical techniques to document the following responses in affected hair cells: T-cell restricted intracellular antigen-related protein (TIAR) translocation from the nucleus to the cytoplasm, cytochrome c release from the mitochondria, caspase-3 activation, nuclear condensation, and an orderly progression of hair cell ejection from the proximal end of the basilar papilla. Hair cells in the proximal tip exhibited TIAR translocation from the nucleus and aggregation into punctate granules in the cytoplasm 12 hours after injection and the response progressed distally. Cytochrome c release from the mitochondria into the cytoplasm and caspase-3 activation were observed in affected hair cells immediately prior to and during ejection. Hair cell ejection occurred between 30 and 54 hours after injection, beginning in the proximal tip and progressing distally. Nuclear condensation accompanied ejection while the loss of: 1) membrane integrity; 2) phalloidin labeling of F-actin; and 3) TO-PRO-1 labeling of nuclear contents occurred within 48 hours following ejection. Our results present a timeline of aminoglycoside-induced inner ear sensory hair cell apoptotic death that includes an 18-hour window between the initial apoptotic response and the later stages of programmed death signaling that accompany ejection and a gradual breakdown of hair cells following ejection.

Caspase inhibitors promote vestibular hair cell survival and function after aminoglycoside treatment in vivo

The sensory hair cells of the inner ear undergo apoptosis after acoustic trauma or aminoglycoside antibiotic treatment, causing permanent auditory and vestibular deficits in humans. Previous studies have demonstrated a role for caspase activation in hair cell death and ototoxic injury that can be reduced by concurrent treatment with caspase inhibitors in vitro. In this study, we examined the protective effects of caspase inhibition on hair cell death in vivo after systemic injections of aminoglycosides. In one series of experiments, chickens were implanted with osmotic pumps that administrated the pan-caspase inhibitor z-Val-Ala-Asp(Ome)-fluoromethylketone (zVAD) into inner ear fluids. One day after the surgery, the animals received a 5 d course of treatment with streptomycin, a vestibulotoxic aminoglycoside. Direct infusion of zVAD into the vestibule significantly increased hair cell survival after streptomycin treatment. A second series of experiments determined whether rescued hair cells could function as sensory receptors. Animals treated with streptomycin displayed vestibular system impairment as measured by a greatly reduced vestibulo-ocular response (VOR). In contrast, animals that received concurrent systemic administration of zVAD with streptomycin had both significantly greater hair cell survival and significantly increased VOR responses, as compared with animals treated with streptomycin alone. These findings suggest that inhibiting the activation of caspases promotes the survival of hair cells and protects against vestibular function deficits after aminoglycoside treatment.

REST mRNA expression in normal and regenerating avian auditory epithelium

Hair cells (HCs) and supporting cells (SCs) in the auditory epithelium initially arise from a sheet of undifferentiated cells. Although much has been learned about the initial steps leading to the fate determination of HCs and SCs, respectively, little is known about what molecular events 'finalize' cell fate determination. We investigated the role of repressor element-1 (RE-1) silencing transcription factor (REST), whose inactivation is known to be a requirement for a cell to assume a neuronal identity. Here we show by in situ hybridization (ISH) that SCs express REST messenger RNA (mRNA) but sensory HCs lack detectable expression. Using a more sensitive reverse transcription-polymerase chain reaction assay, however, we detected the presence of a neuron-specific splice variant in the epithelium, suggesting that HCs express REST mRNA at levels too low to be detectable by ISH. In regenerating auditory epithelium, we found that REST mRNA was expressed and upregulated in all remaining cells in the damaged region of the epithelium, consistent with its expression pattern during development prior to neurogenesis. Surprisingly, REST mRNA was also upregulated in SCs in the apical, undamaged region of the epithelium, and readily detectable by ISH in the HCs in this region. This finding suggests that the grossly undamaged region of the epithelium is in fact biochemically altered towards a 'less developed' state. Our results indicate that REST inactivation is an important step in finalizing HC fate in the chick inner ear.