Pronounced infracuticular endocytosis in mammalian outer hair cells

mTORC2 regulates auditory hair cell structure and function

mTOR broadly controls cell growth, but little is known about the role of mTOR complex 2 (mTORC2) in the inner ear. To investigate the role of mTORC2 in sensory hair cells (HCs), we generated HC-specific Rictor knockout (HC-RicKO) mice. HC-RicKO mice exhibited early-onset, progressive, and profound hearing loss. Increased DPOAE thresholds indicated outer HC dysfunction. HCs are lost, but this occurs after hearing loss. Ultrastructural analysis revealed stunted and absent stereocilia in outer HCs. In inner HCs, the number of synapses was significantly decreased and the remaining synapses displayed a disrupted actin cytoskeleton and disorganized Ca2+ channels. Thus, the mTORC2 signaling pathway plays an important role in regulating auditory HC structure and function via regulation of the actin cytoskeleton. These results provide molecular insights on a central regulator of cochlear HCs and thus hearing.

Vesicle traffic in the outer hair cell

The plasma-membrane marker FM1-43 was employed to reveal the relative significance of different types of endocytic and transcytic mechanisms in outer hair cells (OHCs) of the guinea-pig cochlea. A double-barrel local perfusion system was used to label independently the apical or synaptic pole of the isolated OHC to study mechanisms of vesicle uptake at the poles and of vesicle trafficking along and across the cell. Treatment with an inhibitor of macropino- and phagocytosis, phenylarsine oxide, or of clathrin-mediated endocytic activity, concanavalin A, significantly reduced the dye uptake at both the apical and the synaptic poles, indicating the presence of both clathrin-independent and clathrin-mediated processes at both poles. However, measurement of uptake speed in the presence of the inhibitors suggested that clathrin-independent processes contribute more extensively to endocytosis at the basal pole than the apical pole. Treatment with an inhibitor of myosin VI, 2,4,6-triiodophenol, significantly delayed both the apicobasal and the basoapical fluorescence signals. However, treatment with an inhibitor of kinesin, monastrol, or of dynein, ciliobrevin D, significantly delayed the signals only in the basoapical direction. The myosinVI inhibitor, but neither the kinesin nor dynein inhibitors, significantly delayed the signals to the subsurface cisternae. That is, myosin VI carries vesicles in both longitudinal directions as well as radially to the subsurface cisternae, whereas kinesin and dynein participate primarily in basoapical trafficking. This fundamental information is essential for elucidating recycling mechanisms of specific proteins involved in establishing, controlling and maintaining the electromechanical action of OHCs and, therefore, is vital for understanding auditory perception.

Cnr2 Is Important for Ribbon Synapse Maturation and Function in Hair Cells and Photoreceptors

The role of the cannabinoid receptor 2 (CNR2) is still poorly described in sensory epithelia. We found strong cnr2 expression in hair cells (HCs) of the inner ear and the lateral line (LL), a superficial sensory structure in fish. Next, we demonstrated that sensory synapses in HCs were severely perturbed in larvae lacking cnr2. Appearance and distribution of presynaptic ribbons and calcium channels (Ca_v1.3) were profoundly altered in mutant animals. Clustering of membrane-associated guanylate kinase (MAGUK) in post-synaptic densities (PSDs) was also heavily affected, suggesting a role for cnr2 for maintaining the sensory synapse. Furthermore, vesicular trafficking in HCs was strongly perturbed suggesting a retrograde action of the endocannabinoid system (ECs) via cnr2 that was modulating HC mechanotransduction. We found similar perturbations in retinal ribbon synapses. Finally, we showed that larval swimming behaviors after sound and light stimulations were significantly different in mutant animals. Thus, we propose that cnr2 is critical for the processing of sensory information in the developing larva.

Altered Outer Hair Cell Mitochondrial and Subsurface Cisternae Connectomics Are Candidate Mechanisms for Hearing Loss in Mice

Organelle crosstalk is vital for cellular functions. The propinquity of mitochondria, ER, and plasma membrane promote regulation of multiple functions, which include intracellular Ca2+ flux, and cellular biogenesis. Although the purposes of apposing mitochondria and ER have been described, an understanding of altered organelle connectomics related to disease states is emerging. Since inner ear outer hair cell (OHC) degeneration is a common trait of age-related hearing loss, the objective of this study was to investigate whether the structural and functional coupling of mitochondria with subsurface cisternae (SSC) was affected by aging. We applied functional and structural probes to equal numbers of male and female mice with a hearing phenotype akin to human aging. We discovered the polarization of cristae and crista junctions in mitochondria tethered to the SSC in OHCs. Aging was associated with SSC stress and decoupling of mitochondria with the SSC, mitochondrial fission/fusion imbalance, a remarkable reduction in mitochondrial and cytoplasmic Ca2+ levels, reduced K+-induced Ca2+ uptake, and marked plasticity of cristae membranes. A model of structure-based ATP production predicts profound energy stress in older OHCs. This report provides data suggesting that altered membrane organelle connectomics may result in progressive hearing loss.

Different patterns of endocytosis in cochlear inner and outer hair cells of mice

Precise and efficient endocytosis is critical for sustained neurotransmission during continuous neuronal activity. Endocytosis is a prerequisite for maintaining the auditory function. However, the differences between the patterns of endocytosis in cochlear inner hair cells (IHCs) and outer hair cells (OHCs) remain unclear. Both IHCs and OHCs were obtained from adult C57 mice. Patterns of endocytosis in cells were estimated by analyzing the uptake of FM1-43, a fluorescent. The observations were made using live confocal imaging, fluorescence intensities were calculated statistically. Results revealed the details about following phenomenon, i) sites of entry: the FM1-43 dye was found to enter IHC at the apical area initially, the additional sites of entry were then found at basolateral membrane of the cells, The entry of the dye into OHCs initially appeared to be occurring around whole apical membranes area, which then diffused towards the other membrane surface of the cells, ii) capacity of endocytosis: fluorescence intensity in IHCs showed significantly higher than that of OHCs (P<0.01). We have found different patterns of endocytosis between IHCs and OHCs, this indicated functional distinctions between them. Moreover, FM1-43 dye can be potentially used as an indicator of the functional loss or repair of cochlear hair cells.

Different rates of endocytic activity and vesicle transport from the apical and synaptic poles of the outer hair cell

Background

Intense endocytic activity at the apex of outer hair cells (OHCs)—the electromechanical cells of the cochlea—has been demonstrated using the vital plasma-membrane marker FM1-43 and confocal laser-scanning microscopy. Vesicular traffic toward the cell nucleus to distinct locations of the endoplasmic reticulum has also been shown.

Objective

The current study characterizes the dynamics of endocytic activity, as well as apicobasal and basoapical trafficking, using a local perfusion technique that we recently developed and published to visualize bidirectional trafficking in isolated bipolar cells.

Materials and methods

The fluorescent plasma-membrane markers FM1-43 (10 µM) and FM4-64 (10 µM), together with a fluid-phase marker, Lucifer yellow (50 µM), were used to label endocytosed vesicles in isolated OHCs of the guinea pig cochlea. Targets of endocytosed vesicles were examined with a fluorescent marker of subsurface cisternae, DiOC₆ (0.87 µM). Single- and two-photon confocal laser-scanning microscopy was used to visualize labeled vesicles.

Results

The plasma-membrane markers presented more intense vesicle internalization at the synaptic pole than at the apical pole of the OHC. Intracellular basoapical vesicle trafficking was faster than apicobasal trafficking. Vesicles endocytosed at the synaptic pole were transcytosed to the endoplasmic reticulum system. An intracellular Lucifer yellow signal was not detected.

Conclusion

The larger endocytic fluorescent signals in the synaptic pole and the faster basoapical trafficking imply that membrane internalization and vesicle trafficking are more efficient at the synaptic pole than at the apical pole of the OHC.

Quantitative Analysis of Aquaporin Expression Levels during the Development and Maturation of the Inner Ear

Aquaporins (AQPs) are a family of small membrane proteins that transport water molecules across the plasma membrane along the osmotic gradient. Mammals express 13 subtypes of AQPs, including the recently reported "subcellular AQPs", AQP11 and 12. Each organ expresses specific subsets of AQP subtypes, and in the inner ear, AQPs are essential for the establishment and maintenance of two distinct fluids, endolymph and perilymph. To evaluate the contribution of AQPs during the establishment of inner ear function, we used quantitative reverse transcription polymerase chain reaction to quantify the expression levels of all known AQPs during the entire development and maturation of the inner ear. Using systematic and longitudinal quantification, we found that AQP11 was majorly and constantly expressed in the inner ear, and that the expression levels of several AQPs follow characteristic longitudinal patterns: increasing (Aqp0, 1, and 9), decreasing (Aqp6, 8, and 12), and peak of expression on E18 (Aqp2, 5, and 7). In particular, the expression level of Aqp9 increased by 70-fold during P3-P21. We also performed in situ hybridization of Aqp11, and determined the unique localization of Aqp11 in the outer hair cells. Immunohistochemistry of AQP9 revealed its localization in the supporting cells inside the organ of Corti, and in the root cells. The emergence of AQP9 expression in these cells was during P3-P21, which was coincident with the marked increase of its expression level. Combining these quantification and localization data, we discuss the possible contributions of these AQPs to inner ear function.

Extrinsic sound stimulations and development of periphery auditory synapses

The development of auditory synapses is a key process for the maturation of hearing function. However, it is still on debate regarding whether the development of auditory synapses is dominated by acquired sound stimulations. In this review, we summarize relevant publications in recent decades to address this issue. Most reported data suggest that extrinsic sound stimulations do affect, but not govern the development of periphery auditory synapses. Overall, periphery auditory synapses develop and mature according to its intrinsic mechanism to build up the synaptic connections between sensory neurons and/or interneurons.

Solute Carrier Family 26 Member a2 (slc26a2) Regulates Otic Development and Hair Cell Survival in Zebrafish

Hearing loss is one of the most prevalent human birth defects. Genetic factors contribute to the pathogenesis of deafness. It is estimated that one-third of deafness genes have already been identified. The current work is an attempt to find novel genes relevant to hearing loss using guilt-by-profiling and guilt-by-association bioinformatics analyses of approximately 80 known non-syndromic hereditary hearing loss (NSHL) genes. Among the 300 newly identified candidate deafness genes, slc26a2 were selected for functional studies in zebrafish. The slc26a2 gene was knocked down using an antisense morpholino (MO), and significant defects were observed in otolith patterns, semicircular canal morphology, and lateral neuromast distributions in morphants. Loss-of-function defects are caused primarily by apoptosis, and morphants are insensitive to sound stimulation and imbalanced swimming behaviours. Morphant defects were found to be partially rescued by co-injection of human SLC26A2 mRNA. All the results suggest that bioinformatics is capable of predicting new deafness genes and this showed slc26a2 is to be a critical otic gene whose dysfunction may induce hearing impairment.

Intracellular Delivery of Short Interfering RNA in Rat Organ of Corti Using a Cell-penetrating Peptide PepFect6

RNA interference (RNAi) using short interfering RNA (siRNA) is an attractive therapeutic approach for treatment of dominant-negative mutations. Some rare missense dominant-negative mutations lead to congenital-hearing impairments. A variety of viral vectors have been tested with variable efficacy for modulating gene expression in inner ear. However, there is concern regarding their safety for clinical use. Here, we report a novel cell-penetrating peptide (CPP)-based nonviral approach for delivering siRNA into inner ear tissue using organotypic cultures as model system. PepFect6 (PF6), a variant of stearyl-TP10, was specially designed for improved delivery of siRNA by facilitating endosomal release. We show that PF6 was internalized by all cells without inducing cytotoxicity in cochlear cultures. PF6/siRNA nanoparticles lead to knockdown of target genes, a housekeeping gene and supporting cell-specific connexin 26. Interestingly, application of PF6/connexin 26 siRNA exhibited knockdown of both connexin 26 and 30 mRNA and their absence led to impaired intercellular communication as demonstrated by reduced transfer of calcein among the PF6/connexin 26-siRNA–treated cells. Thus, we conclude that PF6 is an efficient nonviral vector for delivery of siRNA, which can be applied as a tool for the development of siRNA-based therapeutic applications for hearing impairments.

Cell-cell junctions: a target of acoustic overstimulation in the sensory epithelium of the cochlea

Background

Exposure to intense noise causes the excessive movement of the organ of Corti, stretching the organ and compromising sensory cell functions. We recently revealed changes in the transcriptional expression of multiple adhesion-related genes during the acute phases of cochlear damage, suggesting that the disruption of cell-cell junctions is an early event in the process of cochlear pathogenesis. However, the functional state of cell junctions in the sensory epithelium is not clear. Here, we employed graded dextran-FITC, a macromolecule tracer that is impermeable to the organ of Corti under physiological conditions, to evaluate the barrier function of cell junctions in normal and noise-traumatized cochlear sensory epithelia.

Results

Exposure to an impulse noise of 155 dB (peak sound pressure level) caused a site-specific disruption in the intercellular junctions within the sensory epithelium of the chinchilla cochlea. The most vulnerable sites were the junctions among the Hensen cells and between the Hensen and Deiters cells within the outer zone of the sensory epithelium. The junction clefts that formed in the reticular lamina were permeable to 40 and 500 but not 2,000 kDa dextran-FITC macromolecules. Moreover, this study showed that the interruption of junction integrity occurred in the reticular lamina and also in the basilar membrane, a site that had been considered to be resistant to acoustic injury. Finally, our study revealed a general spatial correlation between the site of sensory cell damage and the site of junction disruption. However, the two events lacked a strict one-to-one correlation, suggesting that the disruption of cell-cell junctions is a contributing, but not the sole, factor for initiating acute sensory cell death.

Conclusions

Impulse noise causes the functional disruption of intercellular junctions in the sensory epithelium of the chinchilla cochlea. This disruption occurs at an early phase of cochlear damage. Understanding the role of this disruption in cochlear pathogenesis will require future study.

Onset of cholinergic efferent synaptic function in sensory hair cells of the rat cochlea

In the developing mammalian cochlea, the sensory hair cells receive efferent innervation originating in the superior olivary complex. This input is mediated by α9/α10 nicotinic acetylcholine receptors (nAChRs) and is inhibitory due to the subsequent activation of calcium-dependent SK2 potassium channels. We examined the acquisition of this cholinergic efferent input using whole-cell voltage-clamp recordings from inner hair cells (IHCs) in acutely excised apical turns of the rat cochlea from embryonic day 21 to postnatal day 8 (P8). Responses to 1 mm acetylcholine (ACh) were detected from P0 on in almost every IHC. The ACh-activated current amplitude increased with age and demonstrated the same pharmacology as α9-containing nAChRs. Interestingly, at P0, the ACh response was not coupled to SK2 channels, so that the initial cholinergic response was excitatory and could trigger action potentials in IHCs. Coupling to SK current was detected earliest at P1 in a subset of IHCs and by P3 in every IHC studied. Clustered nAChRs and SK2 channels were found on IHCs from P1 on using Alexa Fluor 488 conjugated α-bungarotoxin and SK2 immunohistochemistry. The number of nAChRs clusters increased with age to 16 per IHC at P8. Cholinergic efferent synaptic currents first appeared in a subset of IHCs at P1 and by P3 in every IHC studied, contemporaneously with ACh-evoked SK currents, suggesting that SK2 channels may be necessary at onset of synaptic function. An analogous pattern of development was observed for the efferent synapses that form later (P6-P8) on outer hair cells in the basal cochlea.

The styryl dye FM1-43 suppresses odorant responses in a subset of olfactory neurons by blocking cyclic nucleotide-gated (CNG) channels

Many olfactory receptor neurons use a cAMP-dependent transduction mechanism to transduce odorants into depolarizations. This signaling cascade is characterized by a sequence of two currents: a cation current through cyclic nucleotide-gated channels followed by a chloride current through calcium-activated chloride channels. To date, it is not possible to interfere with these generator channels under physiological conditions with potent and specific blockers. In this study we identified the styryl dye FM1-43 as a potent blocker of native olfactory cyclic nucleotide-gated channels. Furthermore, we characterized this substance to stain olfactory receptor neurons that are endowed with cAMP-dependent transduction. This allows optical differentiation and pharmacological interference with olfactory receptor neurons at the level of the signal transduction.

Evidence-based modification of intratympanic gentamicin injections in patients with intractable vertigo

OBJECTIVES

To compare the cochlear distribution of low-dose fluorescent gentamicin after intra-tympanic administration in guinea pig (GPs) with clinical data of low dose intra-tympanic gentamicin in patients with intractable vertigo.

MATERIALS AND METHODS

Purified gentamicin-Texas Red (GTTR) was injected intratympanically into GPs and the cochlear distribution and time course of GTTR fluorescence in outer hair cells (OHCs) was determined using confocal microscopy.

RESULTS

GTTR was rapidly taken up by OHCs, particularly in the subcuticular zone. GTTR was distributed in the cochlea in a decreasing baso-apical gradient, and was retained within OHCs without significant decrease in fluorescence until 4 weeks after injection.

CONCLUSION

OHCs rapidly take up GTTR after intra-tympanic administration with slow clearance.

CLINICAL APPLICATION

A modified low-dose titration intratympanic approach was applied to patients with intractable Ménière's Disease (MD) based on our animal data and the clinical outcome was followed. After the modified intratympanic injections for MD patients, vertigo control was achieved in 89% patients, with hearing deterioration identified in 16% patients. The 3-week interval titration injection technique thereby had a relatively high vertigo control rate with a low risk of hearing loss, and is a viable alternative to other intratympanic injection protocols.

P2X antagonists inhibit styryl dye entry into hair cells

The styryl pyridinium dyes, FM1-43 and AM1-43, are fluorescent molecules that can permeate the mechanotransduction channels of hair cells, the sensory receptors of the inner ear. When these dyes are applied to hair cells, they enter the cytoplasm rapidly, resulting in a readily detectable intracellular fluorescence that is often used as a molecular indication of mechanotransduction channel activity. However, such dyes can also permeate the ATP receptor, P2X(2). Therefore, we explored the contribution of P2X receptors to the loading of hair cells with AM1-43. The chick inner ear was found to express P2X receptors and to release ATP, similar to the inner ear of mammals, allowing for the endogenous stimulation of P2X receptors. The involvement of these receptors was evaluated pharmacologically, by exposing the sensory epithelium of the chick inner ear to 5 microM AM1-43 under different experimental conditions and measuring the fluorescence in hair cells after fixation of the tissue. Pre-exposure of the tissue to 5 mM EGTA for 15 min, which should eliminate most of the gating "tip links" of the mechanotransduction channels, deceased fluorescence by only 44%. In contrast, P2X receptor antagonists (pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid [PPADS], suramin, 2',3'-O-(2,4,6-trinitrophenyl) ATP [TNP-ATP], and d-tubocurarine) had greater effects on dye loading. PPADS, suramin, and TNP-ATP all decreased intracellular AM1-43 fluorescence in hair cells by at least 69% when applied at a concentration of 100 microM. The difference between d-tubocurarine-treated and control fluorescence was statistically insignificant when d-tubocurarine was applied at a concentration that blocks the mechanotransduction channel (200 microM). At a concentration that also blocks P2X(2) receptors (2 mM), d-tubocurarine decreased dye loading by 72%. From these experiments, it appears that AM1-43 can enter hair cells through endogenously activated P2X receptors. Thus, the contribution of P2X receptors to dye entry should be considered when using styryl pyridinium dyes to detect hair cell mechanotransduction channel activity, especially in the absence of explicit mechanical stimulation of stereocilia.

Lipid lateral mobility in cochlear outer hair cells: regional differences and regulation by cholesterol

The outer hair cell (OHC) lateral plasma membrane houses the transmembrane protein prestin, a necessary component of the yet unknown molecular mechanism(s) underlying electromotility and the exquisite sensitivity and frequency selectivity of mammalian hearing. The importance of the plasma membrane environment in modulating OHC electromotility has been substantiated by recent studies demonstrating that membrane cholesterol alters prestin activity in a manner consistent with cholesterol-induced changes in auditory function. Cholesterol is known to affect membrane material properties, and measurements of lipid lateral mobility provide a method to asses these changes in living OHCs. Using fluorescence recovery after photobleaching (FRAP), we characterized regional differences in the lateral diffusion of the lipid analog di-8-ANEPPS in OHCs and investigated whether lipid mobility, which reflects membrane fluidity, is sensitive to membrane cholesterol. FRAP experiments revealed quantitative differences in lipid lateral mobility among the apical, lateral, and basal regions of the OHC and demonstrated that diffusion in individual regions is uniquely sensitive to cholesterol manipulations. Interestingly, in the lateral region, both cholesterol depletion and loading significantly reduced the effective diffusion coefficient from control values. Thus, the fluidity of the OHC lateral plasma membrane is regulated by cholesterol levels in a non-monotonic manner, suggesting that the overall material properties of the lateral plasma membrane are optimally tuned for OHC function in the native state. These results support the idea that the cholesterol-dependent regulation of prestin function and electromotility correlates with changes in the properties of the lipid environment that surrounds and supports prestin.

Detection without deflection? A hypothesis for direct sensing of sound pressure by hair cells

It is widely thought that organisms detect sound by sensing the deflection of hair-like projections, the stereocilia, at the apex of hair cells. In the case of mammals, the standard interpretation is that hair cells in the cochlea respond to deflection of stereocilia induced by motion generated by a hydrodynamic travelling wave. But in the light of persistent anomalies, an alternative hypothesis seems to have some merit: that sensing cells (in particular the outer hair cells) may, at least at low intensities, be reacting to a different stimulus - the rapid pressure wave that sweeps through the cochlear fluids at the speed of sound in water. This would explain why fast responses are sometimes seen before the peak of the travelling wave. Yet how could cells directly sense fluid pressure? Here, a model is constructed of the outer hair cell as a pressure vessel able to sense pressure variations across its cuticular pore, and this 'fontanelle' model, based on the sensing action of the basal body at this compliant spot, could explain the observed anomalies. Moreover, the fontanelle model can be applied to a wide range of other organisms, suggesting that direct pressure detection is a general mode of sensing complementary to stereociliar displacement.

Membrane disruption: an early event of hair cell apoptosis induced by exposure to intense noise

Membrane leakage has been found in hair cells undergoing apoptosis following exposure to intense noise. However, it is not known whether this membrane damage is the consequence of apoptotic degeneration or direct mechanical stress. The current study was designed to investigate whether membrane damage occurred before the onset of apoptosis and to determine the level of the membrane damage. Chinchillas were exposed to an impulse noise at 155 dB peak SPL. The noise-induced membrane damage was assessed functionally, using membrane tracers with graded molecular sizes (propidium iodide and FITC-dextrans with molecular sizes of 3, 40, 500, and 2000 kDa), and morphologically, using DiO staining and semithin sections. The study revealed two major findings. First, exposure to intense noise caused a rapid increase in membrane permeability, and the onset of membrane leakage preceded the manifestation of nuclear condensation. This indicates that the early membrane damage observed in apoptosis is the direct consequence of mechanical stress. Second, the level of membrane damage was severe, allowing the entry of 3 kDa and 40 kDa FITC-dextrans, but the membrane was not completely broken down, as evidenced by the preservation of the ability to exclude 500 kDa and 2000 kDa FITC-dextran molecules and the maintenance of the cell boundary. Notably, despite the membrane damage, hair cells continue to undergo the apoptotic process, leading to the generation of a type of apoptosis with early membrane damage.

Membrane traffic in outer hair cells of the adult mammalian cochlea

Outer hair cells (OHCs), the sensory-motor cells responsible for the extraordinary frequency selectivity and dynamic range of the cochlea, rapidly endocytose membrane and protein at their apical surface. Endocytosis and transcytosis in isolated OHCs from the mature guinea-pig cochlea were investigated using the amphipathic membrane probe FM1-43. We observed membrane transport from the apical surface to both the basolateral wall and the subnuclear pole. By double-labelling with DiOC6, a stain for endoplasmic reticulum, and aspiration of the plasma membrane, we showed that the basolateral target was the subsurface cisternae. The fluorescent signal was about three times weaker at the basal than at the apical pole. The speed of vesicle transport to the subnuclear pole was approximately 0.4 microm/s. Changing extracellular Ca2+ concentration from 25 microM to 2 mM accelerated rapid endocytosis. Extracellular application of BAPTA-AM (25 microM), an intracellular Ca2+ chelator, and TFP (20 microM), a specific inhibitor of calmodulin, reduced endocytic activity, as did depolarization of the whole cell. The presence of extracellular Cd2+ (200 microM), a Ca2+-channel blocker, had no effect on the voltage dependence of endocytosis at the apical pole, and inhibited the voltage dependence at the subnuclear pole. These results suggest that rapid endocytosis is a Ca2+/calmodulin-dependent process, with extracellular Ca2+ entering through voltage-gated Ca2+ channels at the basal pole. The two distinct destinations of endocytosed membrane are consistent with the functional polarization of the OHC, with the basolateral wall being dedicated to electromechanical transduction and the subnuclear pole being dedicated to electrochemical transduction processes.

Affects of Aging on Receptive Fields in Rat Primary Auditory Cortex Layer V Neurons

Advanced age is commonly associated with progressive cochlear pathology and central auditory deficits, collectively known as presbycusis. The present study examined central correlates of presbycusis by measuring response properties of primary auditory cortex (AI) layer V neurons in the Fischer Brown Norway rat model. Layer V neurons represent the major output of AI to other cortical and subcortical regions (primarily the inferior colliculus). In vivo single-unit extracellular recordings were obtained from 114 neurons in aged animals (29–33 mo) and compared with 105 layer V neurons in young-adult rats (4–6 mo). Three consecutive repetitions of a pure-tone receptive field map were run for each neuron. Age was associated with fewer neurons exhibiting classic V/U-shaped receptive fields and a greater percentage of neurons with more Complex receptive fields. Receptive fields from neurons in aged rats were also less reliable on successive repetitions of the same stimulus set. Aging was also associated with less firing during the stimulus in V/U-shaped receptive field neurons and more firing during the stimulus in Complex neurons, which were generally associated with inhibited firing in young controls. Finally, neurons in aged rats with Complex receptive fields were more easily driven by current pulses delivered to the soma. Collectively, these findings provide support for the notion that age is associated with diminished signal-to-noise coding by AI layer V neurons and are consistent with other research suggesting that GABAergic neurotransmission in AI may be compromised by aging.

Tonic mechanosensitivity of outer hair cells after loss of tip links

Tip links - the extracellular connectors between the distal ends of adjacent stereocilia - are essential for the fast mechanical gating of hair-cell transducer channels. Transduction in the absence of tip links was investigated for outer hair cells of the adult guinea-pig cochlea by patch-clamp recordings of the whole-cell current during mechanical stimulation of the hair bundle. Loss of tip links induced by application of BAPTA led to permanently opened transducer channels, as evidenced by a constant inward current, loss of response to sinusoidal mechanical deflection of the hair bundle and block by the open-channel blocker dihydrostreptomycin (100 microM). Step deflection of the hair bundle (200-500 nm) in the inhibitory direction exponentially reduced this current to a constant value with time constant, tau(on), of the order of seconds. The current returned exponentially to the pre-stimulus level with time-constant, tau(off), also of the order of seconds. tau(on) was dependent on the inter-stimulus interval, Deltat, such that reducing this interval below about 40 s resulted in an exponentially faster response. tau(off) was independent of Deltat. Application of the calcium ionophore, ionomycin (10 microM), showed that tau(on) became independent of Deltat after saturating elevation of the intracellular Ca(2+) concentration. Flash-photolytic release of intracellular caged calcium (25-microM NP-EGTA/AM) showed that tau(on) is dependent on intracellular Ca(2+) concentration. These experiments imply an intracellular, calcium-dependent gating mechanism for hair-cell transducer channels.

Cochlear function in Prestin knockout mice

Gross-potential recordings in mice lacking the Prestin gene indicate that compound action potential (CAP) thresholds are shifted by approximately 45 dB at 5 kHz and by approximately 60 dB at 33 kHz. However, in order to conclude that outer hair cell (OHC) electromotility is associated with the cochlear amplifier, frequency selectivity must be evaluated and the integrity of the OHC's forward transducer ascertained. The present report demonstrates no frequency selectivity in CAP tuning curves recorded in homozygotes. In addition, CAP input-output functions indicate that responses in knockout mice approach those in controls at high levels where the amplifier has little influence. Although the cochlear microphonic in knockout mice remains approximately 12 dB below that in wild-type mice even at the highest levels, this deficit is thought to reflect hair cell losses in mice lacking prestin. A change in OHC forward transduction is not implied because knockout mice display non-linear responses similar to those in controls. For example, homozygotes exhibit a bipolar summating potential (SP) with positive responses at high frequencies; negative responses at low frequencies. Measurement of intermodulation distortion also shows that the cubic difference tone, 2f(1)-f(2), is approximately 20 dB down from the primaries in both homozygotes and their controls. Because OHCs are the sole generators of the negative SP and because 2f(1)-f(2) is also thought to originate in OHC transduction, these data support the idea that forward transduction is not degraded in OHCs lacking prestin. Finally, application of AM1-43, which initially enters hair cells through their transducer channels, produces fluorescence in wild-type and knockout mice indicating transducer channel activity in both inner and outer hair cells.

Apical endocytosis in outer hair cells of the mammalian cochlea

Outer hair cells (OHCs), the sensory-motor cells of the mammalian cochlea, contain an endocytic tubulovesicular compartment below their apical stereocilia. We have used two-photon imaging of FM1-43 in the intact epithelium to show that these cells take up membrane in a Ca(2+)-dependent manner from a distinct apical site. The uptake rate was 0.8 microm(2)/s and internalized membrane was trafficked rapidly to a compartment along the lateral wall and distinct intracellular compartments. Double labelling with FM1-43 and DiOC(6), an endoplasmic reticulum (ER) marker, showed that these compartments are part of the tubulovesicular endoplasmic reticulum of OHCs. Labelling with a lysosomal marker showed that OHC lysosomes are restricted to the apex. Using the protein marker wheat germ agglutinin (WGA-FITC) we demonstrate that apical protein internalization and trafficking is about eight times slower than membrane internalization. Using double labelling with FM1-43 and WGA-FITC, we show that membrane and protein internalization are apically colocalized but that patterns of protein and membrane traffic differ. Protein was targeted only to the most apical third of the lateral wall. In control conditions, OHCs displayed only weak WGA-FITC surface labelling at the site of endocytosis. Lowering the rate of apical endocytosis increased this surface signal. The results suggest that OHCs endocytose membrane and membrane proteins with a high turnover rate and that these cells may use apical endocytosis to sort proteins via an indirect pathway to the lateral membrane.

The mechanical properties of chick (Gallus domesticus) sensory hair bundles: relative contributions of structures sensitive to calcium chelation and subtilisin treatment

Up to four link types are found between the stereocilia of chick vestibular hair bundles: tip links, horizontal top connectors, shaft connectors and ankle links. A fifth type, the kinocilial link, couples the hair bundle to the kinocilium. Brownian-motion microinterferometry was used to study the mechanical properties of the hair bundle and investigate changes caused by removing different links with the calcium chelator BAPTA or the protease subtilisin. Immunofluorescence with an antibody to the hair-cell antigen (HCA) and electron microscopy were used to verify destruction of the links. The root mean square displacement and the corresponding absolute stiffness of untreated hair bundles were 4.3 nm and 0.9 mN m(-1), respectively. The ratio of Brownian-motion spectra before and after treatment was calculated and processed using a single oscillator model to obtain relative stiffness. Treatment with BAPTA, which cleaves tip, kinocilial and ankle links, reduces hair-bundle stiffness by 43%, whilst subtilisin treatment, which breaks ankle links and shaft connectors, reduces stiffness by 48%. No changes were detected in viscous damping following either treatment. The time course of the subtilisin-induced stiffness change was close to that of HCA loss, but not to the disappearance of the ankle links, suggesting that shaft connectors make a more significant contribution to hair-bundle stiffness. Sequential treatments of the hair bundles with BAPTA and subtilisin show that the effects are additive. The implication of complete additivity is that structures resistant to both agents (e.g. top connectors and stereocilia pivots) are responsible for approximately 9% of the overall bundle stiffness.

Lighting up the senses: FM1-43 loading of sensory cells through nonselective ion channels

We describe a novel mechanism for vital fluorescent dye entry into sensory cells and neurons: permeation through ion channels. In addition to the slow conventional uptake of styryl dyes by endocytosis, small styryl dyes such as FM1-43 rapidly and specifically label hair cells in the inner ear by entering through open mechanotransduction channels. This labeling can be blocked by pharmacological or mechanical closing of the channels. This phenomenon is not limited to hair cell transduction channels, because human embryonic kidney 293T cells expressing the vanilloid receptor (TRPV1) or a purinergic receptor (P2X2) rapidly take up FM1-43 when those receptor channels are opened and not when they are pharmacologically blocked. This channel permeation mechanism can also be used to label many sensory cell types in vivo. A single subcutaneous injection of FM1-43 (3 mg/kg body weight) in mice brightly labels hair cells, Merkel cells, muscle spindles, taste buds, enteric neurons, and primary sensory neurons within the cranial and dorsal root ganglia, persisting for several weeks. The pattern of labeling is specific; nonsensory cells and neurons remain unlabeled. The labeling of the sensory neurons requires dye entry through the sensory terminal, consistent with permeation through the sensory channels. This suggests that organic cationic dyes are able to pass through a number of different sensory channels. The bright and specific labeling with styryl dyes provides a novel way to study sensory cells and neurons in vivo and in vitro, and it offers new opportunities for visually assaying sensory channel function.

Distribution of gentamicin in the guinea pig inner ear after local or systemic application

Uptake and retention of gentamicin by cells in the guinea pig inner ear after a single peritoneal injection or local application on the round window were investigated using immunocytochemistry to localize the drug. The cells that accumulated the drug under the two conditions were the same, but staining for the drug was more intense and was often accompanied by widespread cochlear degeneration following local application. Soon after drug administration by either route, there was diffuse staining for the drug throughout all tissue within the labyrinth, including bone. At later times when distinct cell staining became evident, virtually all cell types were found to be positive, with several cell types staining more darkly for the drug than hair cells, indicating that hair cells were not the most avid in accumulating gentamicin. The infracuticular portion of auditory and vestibular hair cells as well as type III fibrocytes of the spiral ligament were positively stained in almost all cases and these sites were found to be positive for as long as six months post administration. In animals with loss of the organ of Corti, there was unusually intense staining for gentamicin in root cells of the spiral ligament, in marginal cells of the stria vascularis, and in cells of the spiral limbus. Dark staining of surviving cells in cases with overt tissue destruction suggests that variability in the extent of damage caused by the drug was determined more by the degree of its local uptake than by differences in animals' capacities to metabolize the drug systemically. The present results show that gentamicin may damage or destroy all cochlear cells following a single round window application. The findings broaden the scope of our knowledge of cochlear gentamicin uptake and damage and have implications for treatment of patients with vestibular disorders by infusion of aminoglycosides into the middle ear, as well as implications for prospects of rehabilitating patients that have been deafened by aminoglycosides.

Ocsyn, a novel syntaxin-interacting protein enriched in the subapical region of inner hair cells

Sensory (hair) cells of the inner ear contain two specialized areas of membrane delivery. The first, located at the cell base, is the afferent synapse where rapid delivery of synaptic vesicles is required to convey information about auditory signals with exceedingly high temporal precision. The second area is at the apex. To accommodate the continuous movement of stereocilia and facilitate their repair, recycling of membrane components is required. Intense vesicular traffic is restricted to a narrow band of cytoplasm around the cuticular plate, which anchors stereocilia. Our previous analyses showed that SNARE proteins (syntaxin 1A/SNAP25/VAMP1) are concentrated at both poles of hair cells, consistent with their involvement in membrane delivery at both locations. To investigate further the molecules involved in membrane delivery at these two sites, we constructed a two-hybrid library of the organ of Corti and probed it with syntaxin 1A. Here we report the cloning of a novel syntaxin-binding protein that is concentrated in a previously uncharacterized organelle at the apex of inner hair cells.

Fm1-43 reveals membrane recycling in adult inner hair cells of the mammalian cochlea

Neural transmission of complex sounds demands fast and sustained rates of synaptic release from the primary cochlear receptors, the inner hair cells (IHCs). The cells therefore require efficient membrane recycling. Using two-photon imaging of the membrane marker FM1-43 in the intact sensory epithelium within the cochlear bone of the adult guinea pig, we show that IHCs possess fast calcium-dependent membrane uptake at their apical pole. FM1-43 did not permeate through the stereocilial mechanotransducer channel because uptake kinetics were neither changed by the blockers dihydrostreptomycin and d-tubocurarine nor by treatment of the apical membrane with BAPTA, known to disrupt mechanotransduction. Moreover, the fluid phase marker Lucifer Yellow produced a similar labeling pattern to FM1-43, consistent with FM1-43 uptake via endocytosis. We estimate the membrane retrieval rate at approximately 0.5% of the surface area of the cell per second. Labeled membrane was rapidly transported to the base of IHCs by kinesin-dependent trafficking and accumulated in structures that resembled synaptic release sites. Using confocal imaging of FM1-43 in excised strips of the organ of Corti, we show that the time constants of fluorescence decay at the basolateral pole of IHCs and apical endocytosis were increased after depolarization of IHCs with 40 mm potassium, a stimulus that triggers calcium influx and increases synaptic release. Blocking calcium channels with either cadmium or nimodipine during depolarization abolished the rate increase of apical endocytosis. We suggest that IHCs use fast calcium-dependent apical endocytosis for activity-associated replenishment of synaptic membrane.