Morphology of inner hair cell stereocilia in C57BL/6J mice as studied by scanning electron microscopy

Mechanotransduction in mammalian sensory hair cells

In the inner ear, the auditory and vestibular systems detect and translate sensory information regarding sound and balance. The sensory cells that transform mechanical input into an electrical signal in these systems are called hair cells. A specialized organelle on the apical surface of hair cells called the hair bundle detects mechanical signals. Displacement of the hair bundle causes mechanotransduction channels to open. The morphology and organization of the hair bundle, as well as the properties and characteristics of the mechanotransduction process, differ between the different hair cell types in the auditory and vestibular systems. These differences likely contribute to maximizing the transduction of specific signals in each system. This review will discuss the molecules essential for mechanotransduction and the properties of the mechanotransduction process, focusing our attention on recent data and differences between the auditory and vestibular systems.

Dimensions of a Living Cochlear Hair Bundle

The hair bundle is the mechanosensory organelle of hair cells that detects mechanical stimuli caused by sounds, head motions, and fluid flows. Each hair bundle is an assembly of cellular-protrusions called stereocilia, which differ in height to form a staircase. Stereocilia have different heights, widths, and separations in different species, sensory organs, positions within an organ, hair-cell types, and even within a single hair bundle. The dimensions of the stereociliary assembly dictate how the hair bundle responds to stimuli. These hair-bundle properties have been measured previously only to a limited degree. In particular, mammalian data are either incomplete, lack control for age or position within an organ, or have artifacts owing to fixation or dehydration. Here, we provide a complete set of measurements for postnatal day (P) 11 C57BL/6J mouse apical inner hair cells (IHCs) obtained from living tissue, tissue mildly-fixed for fluorescent imaging, or tissue strongly fixed and dehydrated for scanning electronic microscopy (SEM). We found that hair bundles mildly-fixed for fluorescence had the same dimensions as living hair bundles, whereas SEM-prepared hair bundles shrank uniformly in stereociliary heights, widths, and separations. By determining the shrinkage factors, we imputed live dimensions from SEM that were too small to observe optically. Accordingly, we created the first complete blueprint of a living IHC hair bundle. We show that SEM-prepared measurements strongly affect calculations of a bundle’s mechanical properties – overestimating stereociliary deflection stiffness and underestimating the fluid coupling between stereocilia. The methods of measurement, the data, and the consequences we describe illustrate the high levels of accuracy and precision required to understand hair-bundle mechanotransduction.

Organotypic Culture of Neonatal Murine Inner Ear Explants

The inner ear is a complex organ containing highly specialised cell types and structures that are critical for sensing sound and movement. In vivo, the inner ear is difficult to study due to the osseous nature of the otic capsule and its encapsulation within an intricate bony labyrinth. As such, mammalian inner ear explants are an invaluable tool for the study and manipulation of the complex intercellular connections, structures, and cell types within this specialised organ. The greatest strength of this technique is that the complete organ of Corti, or peripheral vestibular organs including hair cells, supporting cells and accompanying neurons, is maintained in its in situ form. The greatest weakness of in vitro hair cell preparations is the short time frame in which the explanted tissue remains viable. Yet, cochlear explants have proven to be an excellent experimental model for understanding the fundamental aspects of auditory biology, substantiated by their use for over 40 years. In this protocol, we present a modernised inner ear explant technique that employs organotypic cell culture inserts and serum free media. This approach decreases the likelihood of explant damage by eliminating the need for adhesive substances. Serum free media also restricts excessive cellular outgrowth and inter-experimental variability, both of which are side effects of exogenous serum addition to cell cultures. The protocol described can be applied to culture both cochlear and vestibular explants from various mammals. Example outcomes are demonstrated by immunohistochemistry, hair cell quantification, and electrophysiological recordings to validate the versatility and viability of the protocol.

Overexpression of X-Linked Inhibitor of Apoptotic Protein (XIAP) reduces age-related neuronal degeneration in the mouse cochlea

Previously, we showed that age-related hearing loss (AHL) was delayed in C57BL6 mice overexpressing X-Linked Inhibitor of Apoptotic Protein (XIAP), and the delayed AHL was associated with attenuated hair cell (HC) loss in XIAP-overexpressing mice. Similar to other reports, the HC loss in aged mice was restricted to the basal turn in this previous study, and occurred slightly at the apical end of the cochlea, showing considerably less spread than the frequency region of hearing loss. In the present study, we examined whether and how AHL is related to the degeneration of neuronal innervation of the cochlea and if the overexpression of XIAP exerts a protective effect against age-related degeneration in both afferent and efferent cochlear neurites. In contrast to HC loss, degeneration of both afferent and efferent neurites spread to the middle turns of the cochlea. Moreover, XIAP-overexpressing mice lost fewer HC afferent dendrites and efferent axons, as well as fewer spiral ganglion neurons (SGNs) between 3– 14 months of age in comparison to wild-type littermates. The results suggest that age-related degeneration of cochlear neurites may be independent of HC loss. Further, the inhibition of apoptosis by XIAP appears to reduce degeneration of both afferent and efferent cochlear neurites.

Progressive hearing loss and gradual deterioration of sensory hair bundles in the ears of mice lacking the actin-binding protein Eps8L2

Mechanotransduction in the mammalian auditory system depends on mechanosensitive channels in the hair bundles that project from the apical surface of the sensory hair cells. Individual stereocilia within each bundle contain a core of tightly packed actin filaments, whose length is dynamically regulated during development and in the adult. We show that the actin-binding protein epidermal growth factor receptor pathway substrate 8 (Eps8)L2, a member of the Eps8-like protein family, is a newly identified hair bundle protein that is localized at the tips of stereocilia of both cochlear and vestibular hair cells. It has a spatiotemporal expression pattern that complements that of Eps8. In the cochlea, whereas Eps8 is essential for the initial elongation of stereocilia, Eps8L2 is required for their maintenance in adult hair cells. In the absence of both proteins, the ordered staircase structure of the hair bundle in the cochlea decays. In contrast to the early profound hearing loss associated with an absence of Eps8, Eps8L2 null-mutant mice exhibit a late-onset, progressive hearing loss that is directly linked to a gradual deterioration in hair bundle morphology. We conclude that Eps8L2 is required for the long-term maintenance of the staircase structure and mechanosensory function of auditory hair bundles. It complements the developmental role of Eps8 and is a candidate gene for progressive age-related hearing loss.

Hair cell bundles: flexoelectric motors of the inner ear

Microvilli (stereocilia) projecting from the apex of hair cells in the inner ear are actively motile structures that feed energy into the vibration of the inner ear and enhance sensitivity to sound. The biophysical mechanism underlying the hair bundle motor is unknown. In this study, we examined a membrane flexoelectric origin for active movements in stereocilia and conclude that it is likely to be an important contributor to mechanical power output by hair bundles. We formulated a realistic biophysical model of stereocilia incorporating stereocilia dimensions, the known flexoelectric coefficient of lipid membranes, mechanical compliance, and fluid drag. Electrical power enters the stereocilia through displacement sensitive ion channels and, due to the small diameter of stereocilia, is converted to useful mechanical power output by flexoelectricity. This motor augments molecular motors associated with the mechanosensitive apparatus itself that have been described previously. The model reveals stereocilia to be highly efficient and fast flexoelectric motors that capture the energy in the extracellular electro-chemical potential of the inner ear to generate mechanical power output. The power analysis provides an explanation for the correlation between stereocilia height and the tonotopic organization of hearing organs. Further, results suggest that flexoelectricity may be essential to the exquisite sensitivity and frequency selectivity of non-mammalian hearing organs at high auditory frequencies, and may contribute to the “cochlear amplifier” in mammals.

The Mass1frings mutation underlies early onset hearing impairment in BUB/BnJ mice, a model for the auditory pathology of Usher syndrome IIC

The human ortholog of the gene responsible for audiogenic seizure susceptibility in Frings and BUB/BnJ mice (mouse gene symbol Mass1) recently was shown to underlie Usher syndrome type IIC (USH2C). Here we report that the Mass1frings mutation is responsible for the early onset hearing impairment of BUB/BnJ mice. We found highly significant linkage of Mass1 with ABR threshold variation among mice from two backcrosses involving BUB/BnJ mice with mice of strains CAST/EiJ and MOLD/RkJ. We also show an additive effect of the Cdh23 locus in modulating the progression of hearing loss in backcross mice. Together, these two loci account for more than 70% of the total ABR threshold variation among the backcross mice at all ages. The modifying effect of the strain-specific Cdh23ahl variant may account for the hearing and audiogenic seizure differences observed between Frings and BUB/BnJ mice, which share the Mass1frings mutation. During postnatal cochlear development in BUB/BnJ mice, stereocilia bundles develop abnormally and remain immature and splayed into adulthood, corresponding with the early onset hearing impairment associated with Mass1frings. Progressive base-apex hair cell degeneration occurs at older ages, corresponding with the age-related hearing loss associated with Cdh23ahl. The molecular basis and pathophysiology of hearing loss suggest BUB/BnJ and Frings mice as models to study cellular and molecular mechanisms underlying USH2C auditory pathology.

Elongation of hair cell stereocilia is defective in the mouse mutant whirler

The recessive mouse mutant whirler (wi) shows no response to sound and exhibits circling and head-tossing behaviour, indicative of both auditory and vestibular dysfunction. The wi mutation maps genetically to mouse chromosome 4. We examined the organ of Corti of whirler mutants to explore the possibility that the wi mutation affects sensory hair cells. Scanning electron microscopy (SEM) reveals that the specialised microvilli (stereocilia) that are projected by the sensory hair cells and are vital for sound transduction are abnormal in wi homozygotes. Specifically, wi homozygous inner hair cell (IHC) stereocilia are approximately half the length of equivalent stereocilia in heterozygous littermates. They are arranged normally into ranks, but the gradation in height and width of stereocilia in adjacent ranks is less prominent in wi homozygotes. Analysis of IHC stereocilia during the course of their development shows that, by embryonic day 18.5, mutant stereocilia are already significantly shorter than those in controls. Mutant stereocilia elongate at a normal rate, at least until postnatal day 1, but prematurely stop elongating between postnatal days 1 and 4. Stereocilia length then decreases. At postnatal day 15, outer hair cell (OHC) stereocilia in wi homozygotes appear short and are arranged in a rounded, "U" shape rather than the normal "W" or "V" shape. Eventually, both IHCs and OHCs degenerate. We show that the whirler locus encodes a protein(s) required for the elongation and maintenance of IHC and OHC stereocilia.

Hair bundle profiles along the chick basilar papilla

Cochlear hair cells play a central role in the transduction of sound into neural output. Anatomical descriptions of these cells, and their protruding hair bundles, are of fundamental interest since hair cell transduction is dependent on hair bundle micromechanics and hair bundle micromechanics depends on hair bundle morphology. In this paper, we describe quantitatively changes in the staircase profile of the hair bundle along the apical portion of the chick's basilar papilla. Images of hair cells from 8 discretely dissected segments of the apical 3rd of the basilar papilla were archived, and the profile contour outlined by the tips of the stereocilia was digitised and curves were fitted by linear and power equations. The hair bundles of tall hair cells exhibited both linear and curvilinear profiles, which were equally distributed along the papilla. All short hair cells in our sample had straight contours. The differences in hair bundle shape among the tall hair cells may lead to differential susceptibility to injury and some variance in the current-displacement transduction curves due to differences in the translation of forces throughout the hair bundle.

Postnatal development of the hamster cochlea. II. Growth and differentiation of stereocilia bundles

The postnatal development of stereocilia was studied in the Syrian golden hamster. The purpose was to describe the morphological changes underlying the differentiation of stereocilia bundles and to define the time course of their growth in different regions of the cochlea. Differentiation of the hair bundle occurred by progressive changes in stereocilia number, dimensions, and spatial relationships. The overall transformation of the bundle is interpreted as a four-stage process involving the initial production of stereocilia (stage I), differentiation into tall and short populations (stage II), formation of distinct ranks (stage III), and resorption of excess stereocilia (stage IV). The orientation and arrangement of stereocilia during stage II began to occur before the tectorial membrane grew over the hair cell field. Growth in the dimensions of stereocilia occurred continuously throughout these four stages with increases in length and width occurring simultaneously. The time frame of the growth process depended both on location along the organ of Corti and on the type of hair cell. Hair bundles in the basal turn began growing and reached maturity a few days earlier than those in the apical turn. Stereocilia of outer hair cells matured earlier than those of inner hair cells. Outer hair cell stereocilia reached their adult lengths by 14 days after birth, those of inner hair cells between 16 and 18 days after birth. A kinocilium was present on almost all hair cells on the day of birth, but most were eliminated by 14 days after birth. Tip links were observed as early as 4 days after birth, and their growth appeared to be synchronous with the growth of stereocilia. The spatial gradient of stereocilia length, which increased toward the apex in the adult, was nearly the reverse of that seen at birth. The gradient for inner hair cells was associated with a gradient in the rate of stereocilia growth. The data further expand the foundation for interpreting mechanisms underlying the morphogenesis of stereocilia bundles in mammals and for understanding structure-function relationships during development.

The actin filament content of hair cells of the bird cochlea is nearly constant even though the length, width, and number of stereocilia vary depending on the hair cell location

By direct counts off scanning electron micrographs, we determined the number of stereocilia per hair cell of the chicken cochlea as a function of the position of the hair cell on the cochlea. Micrographs of thin cross sections of stereociliary bundles located at known positions on the cochlea were enlarged and the total number of actin filaments per stereocilium was counted and recorded. By comparing the counts of filament number with measurements of actin filament bundle width of the same stereocilium, we were able to relate actin filament bundle width to filament number with an error margin (r2) of 16%. Combining this data with data already published or in the process of publication from our laboratory on the length and width of stereocilia, we were able to calculate the total length of actin filaments present in stereociliary bundles of hair cells located at a variety of positions on the cochlea. We found that stereociliary bundles of hair cells contain 80,000-98,000 micron of actin filament, i.e., the concentration of actin is constant in all hair cells with a range of values that is less than our error in measurement and/or biological variation, the greatest variation being in relating the diameters of the stereocilia to filament number. We also calculated the membrane surface needed to cover the stereocilia of hair cells located throughout the cochlea. The values (172-192 micron 2) are also constant. The implications of our observation that the total amount of actin is constant even though the length, width, and number of stereocilia per hair cell vary are discussed.

Electrophysiology of the auditory system

This review has attempted to summarise the properties of electro physiological responses in the auditory system. The treatment was broad and consequently somewhat sketchy. For a more detailed recent treatment of the physiology of the auditory system the reader is referred to Pickles (1982), Møller (1983), or Altschuller et al (1986). The data on acoustic injury have been reviewed recently by Schmiedt (1984). Discussions of a number of topics such as development, hair cell function and speech encoding are found in Berlin (1984).

The distribution of hair cell bundle lengths and orientations suggests an unexpected pattern of hair cell stimulation in the chick cochlea

A detailed analysis of the morphological polarity of the hair cell bundles on the chick cochlea was carried out. Although the pattern is identical from cochlea to cochlea, the morphological polarity of the bundles varies at different positions on the cochlea. More specifically, the hair cell bundles located immediately adjacent to the inferior and superior edges are oriented with their morphological polarity perpendicular to the margins. As we move across the cochlea (transect it), there is a gradual rotation in the polarity of the bundles so that in the center of the cochlea the hair cells are oriented at an angle to those at the edges. As we continue to the superior edge the polarity gradually rotates back again. The amount of rotation depends on the position of the transect such that at the extreme proximal end there is little rotation, while at the distal end the rotation is up to 90 degrees. The rotation is always in the same direction with the tallest rows of stereocilia nearest the distal end of the cochlea. Measurements of the length of the longest stereocilia in the hair cell bundles revealed that not only are the bundles systematically longer from the proximal to distal end of the cochlea, but also the hair cells on the superior edge are significantly longer than those on the inferior edge at the same distance from one end of the cochlea. If we draw on micrographs of the cochlea contour lines through hair cells whose stereocilia are the same height, these lines coincide with the morphological polarity of the hair cells included in these contours. Furthermore analysis of damage to the cochlea induced by pure tones of high intensity also roughly follows the same contour lines. We conclude that unlike what has been thought, the stimulation of hair cells by pure tones may not occur in a strictly transverse pattern, but instead may follow the oblique contours demonstrated here.

Recovery of threshold shift in hair-cell stereocilia following exposure to intense stimulation

The isolated cochlear coil preparation was used to study changes in hair-cell stereocilia stiffness before and after overstimulation. Results were obtained from inner and outer hair cells in the apical and middle turns of the guinea pig cochlea. The stereocilia bundles were stimulated with an oscillating water jet and their movements were identified with stroboscopic illumination in a differential interference contrast microscope. The intensity of the water jet could be varied in decibel steps and the attenuation needed to achieve a 'visual detection level' threshold of movement was the criterion response throughout. Pre-exposure thresholds were sampled, the stereocilia bundle was overstimulated, and thresholds were measured during a recovery interval. Sensory hair bundles on all hair-cell rows showed a loss in stiffness following overstimulation which was proportional to the impedance of the stereocilia bundle. During recovery, stiffness increased and often showed a return to the pre-exposure threshold level. The results demonstrated that the loss of sensory hair stiffness following overstimulation recovered with the passage of time. The magnitude of the initial threshold shift, however, was related to the exposure conditions, cochlear location, and the impedance of the sensory hair bundle. The rate of recovery appeared to be independent of cochlear location, hair-cell row, or exposure conditions.

Single-neuron labeling and chronic cochlear pathology. IV. Stereocilia damage and alterations in rate- and phase-level functions

The rate and phase of auditory-nerve response to tone bursts were studied as a function of stimulus level in normal and acoustically traumatized animals. The rate- and phase-level functions of normal auditory-nerve fibers are often separable into a low-intensity component (component I) and high-intensity component (component II), as defined by a dip in the rate function and a simultaneous abrupt shift in the phase function at stimulus levels near 90 dB SPL [10,12,9]. Baseline data are established by defining the relation between stimulus frequency and the characteristic frequency and spontaneous discharge rate of a fiber normally required for the appearance of these two components in the response. Abnormalities of the level functions are shown to occur in acoustically traumatized ears. Noise-induced threshold shift is often characterized by selective attenuation of component I. In some instances, it appears that component I has been eliminated, leaving a response which is identical in threshold, phase and maximum discharge rate to a normal component II. Results of single-unit labeling in such a case suggest that the selective attenuation of component I is associated with selective loss of the tallest row of stereocilia on the inner hair cells (IHCs). It is suggested that component I is normally generated through an interaction between the outer hair cells and the tall row of IHC stereocilia, while component II requires only the shorter row of IHC stereocilia.

Some electrical circuit properties of the organ of Corti. I. Analysis without reactive elements

A simplified network model of the organ of Corti is analyzed with the assumption of parametric excitation via resistance changes in the hair cells' apical membrane. Pertinent network variables (intracellular resting and receptor potentials, cellular input resistance, extracellular potentials) depend on the ratios of basal (perilymphatic face) and apical (endolymphatic face) receptor cell resistances, denoted as shape factors. In the Appendix two methods are suggested for the computation of shape factors; both are based on the geometrical properties of hair cells. Various electrical quantities computed on the basis of shape factors are consistent with recent recordings from third turn inner and outer hair cells (Dallos et al. (1982): Science 218, 582-584). The model provides a plausible explanation for the experimentally observed discrepancy between inner and outer hair cell resting and receptor potentials. One potentially significant result of the analysis is the demonstration that since shape factors for outer hair cells are probably longitudinally graded, so must be all cellular electrical characteristics. Another interesting finding is that electrical interaction among neighboring hair cells is unlikely. A large-signal analysis of the circuit demonstrates that even in the absence of a non-linear input, the parametrically excited circuit itself generates pronounced distortion. The most significant consequence of this nonlinearity is a response asymmetry in which the depolarizing phase is greater than the hyperpolarizing one. Thus the circuit nonlinearity may, at least in part, account for the large positive d.c. response seen in both types of receptor cell (Dallos et al. (1982): Science 218, 582-584; Russell and Sellick (1978): J. Physiol. Lond. 284, 261-290).