Three sets of actin filaments in sensory cells of the inner ear. Identification and functional orientation determined by gel electrophoresis, immunofluorescence and electron microscopy

Stereocilia Rootlets: Actin-Based Structures That Are Essential for Structural Stability of the Hair Bundle

Sensory hair cells of the inner ear rely on the hair bundle, a cluster of actin-filled stereocilia, to transduce auditory and vestibular stimuli into electrical impulses. Because they are long and thin projections, stereocilia are most prone to damage at the point where they insert into the hair cell’s soma. Moreover, this is the site of stereocilia pivoting, the mechanical movement that induces transduction, which additionally weakens this area mechanically. To bolster this fragile area, hair cells construct a dense core called the rootlet at the base of each stereocilium, which extends down into the actin meshwork of the cuticular plate and firmly anchors the stereocilium. Rootlets are constructed with tightly packed actin filaments that extend from stereocilia actin filaments which are wrapped with TRIOBP; in addition, many other proteins contribute to the rootlet and its associated structures. Rootlets allow stereocilia to sustain innumerable deflections over their lifetimes and exemplify the unique manner in which sensory hair cells exploit actin and its associated proteins to carry out the function of mechanotransduction.

FCHSD1 and FCHSD2 are expressed in hair cell stereocilia and cuticular plate and regulate actin polymerization in vitro

Mammalian FCHSD1 and FCHSD2 are homologous proteins containing an amino-terminal F-BAR domain and two SH3 domains near their carboxyl-termini. We report here that FCHSD1 and FCHSD2 are expressed in mouse cochlear sensory hair cells. FCHSD1 mainly localizes to the cuticular plate, whereas FCHSD2 mainly localizes along the stereocilia in a punctuate pattern. Nervous Wreck (Nwk), the Drosophila ortholog of FCHSD1 and FCHSD2, has been shown to bind Wsp and play an important role in F-actin assembly. We show that, like its Drosophila counterpart, FCHSD2 interacts with WASP and N-WASP, the mammalian orthologs of Drosophila Wsp, and stimulates F-actin assembly in vitro. In contrast, FCHSD1 doesn’t bind WASP or N-WASP, and can’t stimulate F-actin assembly when tested in vitro. We found, however, that FCHSD1 binds via its F-BAR domain to the SH3 domain of Sorting Nexin 9 (SNX9), a well characterized BAR protein that has been shown to promote WASP-Arp2/3-dependent F-actin polymerization. FCHSD1 greatly enhances SNX9’s WASP-Arp2/3-dependent F-actin polymerization activity. In hair cells, SNX9 was detected in the cuticular plate, where it colocalizes with FCHSD1. Our results suggest that FCHSD1 and FCHSD2 could modulate F-actin assembly or maintenance in hair cell stereocilia and cuticular plate.

Proteomic analysis of the organ of corti using nanoscale liquid chromatography coupled with tandem mass spectrometry

The organ of Corti (OC) in the cochlea plays an essential role in auditory signal transduction in the inner ear. For its minute size and trace amount of proteins, the identification of the molecules in pathophysiologic processes in the bone-encapsulated OC requires both delicate separation and a highly sensitive analytical tool. Previously, we reported the development of a high resolution metal-free nanoscale liquid chromatography system for highly sensitive phosphoproteomic analysis. Here this system was coupled with a LTQ-Orbitrap XL mass spectrometer to investigate the OC proteome from normal hearing FVB/N male mice. A total of 628 proteins were identified from six replicates of single LC-MS/MS analysis, with a false discovery rate of 1% using the decoy database approach by the OMSSA search engine. This is currently the largest proteome dataset for the OC. A total of 11 proteins, including cochlin, myosin VI, and myosin IX, were identified that when defective are associated with hearing impairment or loss. This study demonstrated the effectiveness of our nanoLC-MS/MS platform for sensitive identification of hearing loss-associated proteins from minute amount of tissue samples.

Cochlear outer hair cell motility

Normal hearing depends on sound amplification within the mammalian cochlea. The amplification, without which the auditory system is effectively deaf, can be traced to the correct functioning of a group of motile sensory hair cells, the outer hair cells of the cochlea. Acting like motor cells, outer hair cells produce forces that are driven by graded changes in membrane potential. The forces depend on the presence of a motor protein in the lateral membrane of the cells. This protein, known as prestin, is a member of a transporter superfamily SLC26. The functional and structural properties of prestin are described in this review. Whether outer hair cell motility might account for sound amplification at all frequencies is also a critical question and is reviewed here.

Differential distribution of beta- and gamma-actin in guinea-pig cochlear sensory and supporting cells

Sensory and supporting cells of the mammalian organ of Corti have cytoskeletons containing beta- and gamma-actin isoforms which have been described as having differing intracellular distributions in chick cochlear hair cells. Here, we have used post-embedding immunogold labelling for beta- and gamma-actin to investigate semiquantitatively how they are distributed in the guinea-pig cochlea and to compare different frequency locations. Amounts of beta-actin decrease and gamma-actin increase in the order, outer pillar cells, inner pillar cells, Deiters' cells and hair cells. There is also more beta-actin and less gamma-actin in outer pillar cells in higher than lower frequency regions. In hair cells, beta-actin is present in the cuticular plate but is more concentrated in the stereocilia, especially in the rootlets and towards the periphery of their shafts; labelling densities for gamma-actin differ less between these locations and it is the predominant isoform of the hair-cell lateral wall. Alignments of immunogold particles suggest beta-actin and gamma-actin form homomeric filaments. These data confirm differential distribution of these actin isoforms in the mammalian cochlea and reveal systematic differences between sensory and supporting cells. Increased expression of beta-actin in outer pillar cells towards the cochlear base may contribute to the greater stiffness of this region.

Genetics of sensory mechanotransduction

The molecular mechanisms for the transduction of light and chemical signals in animals are fairly well understood. In contrast, the processes by which the senses of touch, balance, hearing, and proprioception are transduced are still largely unknown. Biochemical approaches to identify transduction components are difficult to use with mechanosensory systems, but genetic approaches are proving more successful. Genetic research in several organisms has demonstrated the importance of cytoskeletal, extracellular, and membrane components for sensory mechanotransduction. In particular, researchers have identified channel proteins in the DEG/ENaC and TRP families that are necessary for signaling in a variety of mechanosensory cells. Proof that these proteins are components of the transduction channel, however, is incomplete.

Structure and innervation of the cochlea

The role of the cochlea is to transduce complex sound waves into electrical neural activity in the auditory nerve. Hair cells of the organ of Corti are the sensory cells of hearing. The inner hair cells perform the transduction and initiate the depolarization of the spiral ganglion neurons. The outer hair cells are accessory sensory cells that enhance the sensitivity and selectivity of the cochlea. Neural feedback loops that bring efferent signals to the outer hair cells assist in sharpening and amplifying the signals. The stria vascularis generates the endocochlear potential and maintains the ionic composition of the endolymph, the fluid in which the apical surface of the hair cells is bathed. The mechanical characteristics of the basilar membrane and its related structures further enhance the frequency selectivity of the auditory transduction mechanism. The tectorial membrane is an extracellular matrix, which provides mass loading on top of the organ of Corti, facilitating deflection of the stereocilia. This review deals with the structure of the normal mature mammalian cochlea and includes recent data on the molecular organization of the main cell types within the cochlea.

The Identification and Localization of the Guanine Nucleotide Binding Protein G0 in the Auditory System

The identification of guanine nucleotide binding proteins (G proteins) in guinea-pig tissues was assessed by the adenosine diphosphate-ribosylation of the alpha subunit by Bordetella pertussis toxin using [alpha32P]nicotinamide adenine dinucleotide as the substrate followed by sodium dodecyl sulphate - polyacrylamide gel electrophoresis and autoradiography. Three tissues (inferior colliculus, neuroblastoma cells, and the organ of Corti) contained G0alpha (39 kD), as well as Gi2alpha (40 kD) and Gi1alpha and/or Gi3alpha (41 kD). The stria vascularis and the VIIIth nerve contained mainly Gi2alpha, Gi1alpha and/or Gi3alpha, but G0alpha was barely detectable. A purified preparation of outer hair cells from the organ of Corti contained all three pertussis toxin substrates including G0alpha, with the Gi2alpha (40 kD) subunit being the most prominent. The immunocytochemical localization of the G0alpha subunit was determined by light microscopy after incubating isolated outer hair cells, Hensen cells and the stria vascularis with affinity-purified anti-G0alpha antibodies. In hair cells a positive reaction was observed along the plasma membrane and around the perimeter of the cuticular plate (zona adherens). Positive reaction was also observed within the infracuticular network extending from the cuticular plate towards the nucleus in outer hair cells. Finally, the base of the outer hair cells also contained G0alpha. However, it is likely that the G0alpha that is present in this cell region is not within the hair cell itself, but rather in nerve terminals which remained attached during dissection.

A chemical-genetic strategy implicates myosin-1c in adaptation by hair cells

Myosin-1c (also known as myosin-Ibeta) has been proposed to mediate the slow component of adaptation by hair cells, the sensory cells of the inner ear. To test this hypothesis, we mutated tyrosine-61 of myosin-1c to glycine, conferring susceptibility to inhibition by N(6)-modified ADP analogs. We expressed the mutant myosin-1c in utricular hair cells of transgenic mice, delivered an ADP analog through a whole-cell recording pipette, and found that the analog rapidly blocked adaptation to positive and negative deflections in transgenic cells but not in wild-type cells. The speed and specificity of inhibition suggests that myosin-1c participates in adaptation in hair cells.

Survival of bundleless hair cells and subsequent bundle replacement in the bullfrog's saccule

Our senses of hearing and balance depend upon hair cells, the sensory receptors of the inner ear. Millions of people suffer from hearing and balance deficits caused by damage to hair cells as a result of exposure to noise, aminoglycoside antibiotics, and antitumor drugs. In some species such damage can be reversed through the production of new cells. This proliferative response is limited in mammals but it has been hypothesized that damaged hair cells might survive and undergo intracellular repair. We examined the fate of bullfrog saccular hair cells after exposure to a low dose of the aminoglycoside antibiotic gentamicin to determine whether hair cells could survive such treatment and subsequently be repaired. In organ cultures of the bullfrog saccule a combination of time-lapse video microscopy, two-photon microscopy, electron microscopy, and immunocytochemistry showed that hair cells can lose their hair bundle and survive as bundleless cells for at least 1 week. Time-lapse and electron microscopy revealed stages in the separation of the bundle from the cell body. Scanning electron microscopy (SEM) of cultures fixed 2, 4, and 7 days after antibiotic treatment showed that numerous new hair bundles were produced between 4 and 7 days of culture. Further examination revealed hair cells with small repaired hair bundles alongside damaged remnants of larger surviving bundles. The results indicate that sensory hair cells can undergo intracellular self-repair in the absence of mitosis, offering new possibilities for functional hair cell recovery and an explanation for non-proliferative recovery.

Molecular basis of mechanosensory transduction

Mechanotransduction - a cell's conversion of a mechanical stimulus into an electrical signal - reveals vital features of an organism's environment. From hair cells and skin mechanoreceptors in vertebrates, to bristle receptors in flies and touch receptors in worms, mechanically sensitive cells are essential in the life of an organism. The scarcity of these cells and the uniqueness of their transduction mechanisms have conspired to slow molecular characterization of the ensembles that carry out mechanotransduction. But recent progress in both invertebrates and vertebrates is beginning to reveal the identities of proteins essential for transduction.

Two mechanisms for transducer adaptation in vertebrate hair cells

Deflection of the hair bundle atop a sensory hair cell modulates the open probability of mechanosensitive ion channels. In response to sustained deflections, hair cells adapt. Two fundamentally distinct models have been proposed to explain transducer adaptation. Both models support the notion that channel open probability is modulated by calcium that enters via the transduction channels. Both also suggest that the primary effect of adaptation is to shift the deflection-response [I(X)] relationship in the direction of the applied stimulus, thus maintaining hair bundle sensitivity. The models differ in several respects. They operate on different time scales: the faster on the order of a few milliseconds or less and the slower on the order of 10 ms or more. The model proposed to explain fast adaptation suggests that calcium enters and binds at or near the transduction channels to stabilize a closed conformation. The model proposed to explain the slower adaptation suggests that adaptation is mediated by an active, force-generating process that regulates the effective stimulus applied to the transduction channels. Here we discuss the evidence in support of each model and consider the possibility that both may function to varying degrees in hair cells of different species and sensory organs.

Adaptation in hair cells

Hair cells adapt to sustained deflections of the hair bundle via Ca(2+)-dependent negative feedback on the open probability of the mechanosensitive transduction channels. A model posits that adaptation relieves the input to the transduction channels--force applied by elastic tip links between stereocilia--by repositioning the insertions of the links in the stereocilium. The tip link insertion and transduction channel are dragged by myosins moving on the stereocilium's actin core. This model accounts for many aspects of adaptation in hair cells of the frog saccule, where adaptation time constants are tens of milliseconds. Adaptation in hair cells of the turtle cochlea is much faster, possibly reflecting a more direct mechanism such as Ca2+ binding to the transduction channel. Adaptation mechanisms attenuate the transduction current at low frequencies and may be tuned to different corner frequencies according to the stimulus demands of the inner ear organ. Other sites of adaptation in the inner ear include accessory structures, voltage-dependent properties of hair cells, and afferent transmitter release. A remaining challenge is to understand how these processes work together to shape the output of the inner ear to natural stimuli.

Application of a platinum replica method to the study of the cytoskeleton of isolated hair cells, supporting cells and whole mounts of the organ of Corti

We adapted a method of platinum replica to study the cytoskeleton of isolated cells of the guinea pig organ of Corti. This technique combined high image resolution with the ability to visualize the three-dimensional organization of the cytoskeleton of a whole cell. The procedure includes: isolation of hair cells and supporting cells using collagenase digestion, attachment of the cells to a coverslip, detergent extraction, chemical fixation, critical point drying, platinum/carbon coating, and transmission electron microscopy analysis. By using the method of platinum replica, we confirmed the existence of structural domains in the cortical lattice of outer hair cells. Based on the analysis of the partly destroyed cortical lattice, we propose that circumferential filaments are underlined with a thin flexible network. In addition, we established that the base of each stereocilium had a cone-like expansion of actin filaments and was surrounded by a thin bundle of filaments. We also produced replicas of the protrusion of the cuticular plate into the cytoplasm (infracuticular network) and the reticular lamina cytoskeleton. Our data indicated that the platinum replica method is useful for studying structural interactions among different cytoskeletal elements in the reticular lamina, as well as the cortex of outer hair cells and the cytoskeleton of supporting cells.

Localization of myosin-Ibeta near both ends of tip links in frog saccular hair cells

Current evidence suggests that the adaptation motor of mechanoelectrical transduction in vertebrate hair cells is myosin-Ibeta. Previously, confocal and electron microscopy of bullfrog saccular hair cells using an anti-myosin-Ibeta antibody labeled the tips of stereocilia. We have now done quantitative immunoelectron microscopy to test whether myosin-Ibeta is enriched at or near the side plaques of tip links, the proposed sites of adaptation, using hair bundles that were serially sectioned parallel to the macular surface. The highest particle density occurred at stereocilia bases, close to the cuticular plate. Also, stereocilia of differing lengths had approximately the same number of total particles, suggesting equal targeting of myosin-Ibeta to all stereocilia. Finally, particles tended to clump in clusters of two to five particles in the distal two-thirds of stereocilia, suggesting a tendency for self-assembly of myosin-Ibeta. As expected from fluorescence microscopy, particle density was high in the distal 1 micrometer of stereocilia. If myosin-Ibeta is the adaptation motor, a difference should exist in particle density between regions containing the side plaque and those excluding it. Averaging of particle distributions revealed two regions with approximately twice the average density: at the upper ends of tip links in a 700-nm-long region centered approximately 100 nm above the side plaque, and at the lower ends of tip links within the tip plaques. Controls demonstrated no such increase. The shortest stereocilia, which lack side plaques, showed no concentration rise on their sides. Thus, the specific localization of myosin-Ibeta at both ends of tip links supports its role as the adaptation motor.

Unconventional myosins in inner-ear sensory epithelia

To understand how cells differentially use the dozens of myosin isozymes present in each genome, we examined the distribution of four unconventional myosin isozymes in the inner ear, a tissue that is particularly reliant on actin-rich structures and unconventional myosin isozymes. Of the four isozymes, each from a different class, three are expressed in the hair cells of amphibia and mammals. In stereocilia, constructed of cross-linked F-actin filaments, myosin-Ibeta is found mostly near stereociliary tips, myosin-VI is largely absent, and myosin-VIIa colocalizes with crosslinks that connect adjacent stereocilia. In the cuticular plate, a meshwork of actin filaments, myosin-Ibeta is excluded, myosin-VI is concentrated, and modest amounts of myosin-VIIa are present. These three myosin isozymes are excluded from other actin-rich domains, including the circumferential actin belt and the cortical actin network. A member of a fourth class, myosin-V, is not expressed in hair cells but is present at high levels in afferent nerve cells that innervate hair cells. Substantial amounts of myosins-Ibeta, -VI, and -VIIa are located in a pericuticular necklace that is largely free of F-actin, squeezed between (but not associated with) actin of the cuticular plate and the circumferential belt. Our localization results suggest specific functions for three hair-cell myosin isozymes. As suggested previously, myosin-Ibeta probably plays a role in adaptation; concentration of myosin-VI in cuticular plates and association with stereociliary rootlets suggest that this isozyme participates in rigidly anchoring stereocilia; and finally, colocalization with cross-links between adjacent stereocilia indicates that myosin-VIIa is required for the structural integrity of hair bundles.

Compartmentalized vesicular traffic around the hair cell cuticular plate

Through thin-section and freeze-fracture electron microscopy, we identify structural correlates of an intense vesicular traffic in a narrow band of cytoplasm around the cuticular plate of the bullfrog vestibular hair cells. Myriads of coated and uncoated vesicles associated with longitudinally oriented microtubules populate the narrow cytoplasmic region between the cuticular plate and the actin network of the apical junctional belt. If microtubules in the sensory hair cells, like those in axons, are pathways for organelle transport, then the characteristic distribution of microtubules around the cuticular plate represents transport pathways across the apical region of the hair cells. This compartmentalized membrane traffic system appears to support an intense vesicular release and uptake along a band of apical plasma membrane near the cell border. Functions of this transport system may include membrane recycling as well as exocytotic and endocytotic exchange between the hair cell cytoplasm and the endolymphatic compartment.

Sorting of actin isoforms in chicken auditory hair cells

Most nonmuscle cells of higher vertebrates contain two different actin isoforms, beta- and gamma-cytoplasmic actin. The beta-isoform is with few exceptions the predominant isoform in nonmuscle cells and tissues. Perturbation of the beta:gamma ratio has been shown to affect the organization of bundled actin filaments indicating that the beta- and gamma-genes encode functionally distinct cytoarchitectural information. In the present study we localized by immunostaining beta- and gamma-actin in chicken auditory hair cells. These highly specialized cells serve as model system for studying certain developmental and structural aspects of a complex actin filament system with high architectural precision. We show that gamma-actin is the predominant actin isoform in auditory hair cells with an apparent beta:gamma ratio of approximately 1:2. gamma-Actin is not sorted and occurs in all three actin assemblies of the hair border, i.e. the cores of sensory hairs (stereocilia), the subjacent gel-like actin filament meshwork (cuticular plate) and the zonula adherens ring. In contrast to gamma-actin, the beta-isoform is specifically sorted to the actin filament core bundle of stereocilia that is extensively crosslinked by fimbrin. In view of recent studies showing that L-plastin, the leukocyte homolog of fimbrin, has a higher binding affinity for beta-actin than for gamma-actin, a mechanism is proposed for how hair cells might restrict formation of actin filament bundles to a single cellular site (i.e. the stereocilia). The limited level of expression of beta-actin in hair cells may help to prevent ectopic bundle formation in other cellular compartments.

The molecules of mechanosensation

Mechanosensation, the transduction of mechanical forces into a cellular electrochemical signal, enables living organisms to detect touch; vibrations, such as sound; accelerations, including gravity; body movements; and changes in cellular volume and shape. Ion channels directly activated by mechanical tension are thought to mediate mechanosensation in many systems. Only one channel has been cloned that is unequivocably mechanically gated: the MscL channel in bacteria. Genetic screens for touch-insensitive nematodes or flies promise to identify the proteins that constitute a mechanosensory apparatus in eukaryotes. In Caenorhabditis elegans, the mec genes thus identified encode molecules for a candidate structure, which includes a "degenerin" channel tethered to specialized extracellular and intracellular structural proteins. In hair cells of the inner ear, evidence suggests that an extracellular tip link pulls on a channel, which attached intracellularly to actin via a tension-regulating myosin 1beta. The channel and the tip link have not been cloned. Because degenerins and MscL homologs have not been found outside of nematodes and prokaryotes, respectively, and because intracellular and extracellular accessory structures apparently differ among organs and species, it may be that mechanosensory channel complexes evolved multiple times.

The incorporation and turnover of radiolabelled amino acids in developing stereocilia of the chick cochlea

Hair cell stereocilia are composed of packed actin filaments, oriented such that the preferred end for the addition of actin monomers is at the tips of the sterocilia. It has therefore been suggested that when stereocilia grow, they do so from their tips (Tilney and DeRosier, 1986, Dev. Biol. 116, 119-129). In order to test the hypothesis, radiolabelled amino acids were applied to the air-sac of chicken eggs at day 17 of incubation, i.e., at the beginning of a phase in which the stereocilia have achieved their mature width, but are growing rapidly in length. Incorporation of radiolabel was studied autoradiographically, followed by image analysis and averaging grain counts over many hair cells. In contrast to the position expected from the above hypothesis, there was no sign of preferential incorporation of label in the upper part of the stereociliary bundle. The greatest density of labelling was found in the lower part of the bundle, while the upper part of the bundle was under-represented in the autoradiographic averages. The turnover time (to fail to 1/e) was significantly greater in the bundle (16 days) than in the cuticular plate or in the rest of the cell (9 days). The results (i) give no support for the hypothesis that stereocilia grow from the tips, and (ii) suggest that during development at least some components of the stereocilia turn over with a relatively short time course.

Anatomical basis of a congenital hearing impairment: basilar papilla dysplasia in the Belgian Waterslager canary

Recent investigations into the anatomy of the inner ear of Belgian Waterslager canaries (BWC) have demonstrated myriad malformations associated with dysgenesis of the pars inferior of the otocyst. In those studies, the surface anatomy of BWC's basilar papilla and sacculus was examined utilizing scanning electron microscopy. In the present investigation, we utilized both light microscopy and transmission electron microscopy to describe the cross sectional anatomical pathology of the BWC's basilar papilla. Examination of the BWC's organ of Corti revealed numerous dysmorphologies: 1) hair cells from the tall hair cell region appeared broad and stunted, with deformed cuticular plates, abnormal stereocilia, and recognizable microvilli; 2) quantitative analysis of these hair cells revealed disproportionately large nuclei and abnormally short stereocilia; 3) hair cells from the short hair cell region of the papilla were absent, replaced by a population of large cells with electron-lucent cytoplasm; and 4) the tectorial membrane in the BWC papilla was narrow, covering only the area where the deformed tall hair cells were found. The malformations appeared to be more severe at the apex and midsection of the basilar papilla than at the base. These observations allow us to suggest a hypothesis to account for the distinct anatomofunctional hearing deficit observed in these birds. In addition, they further support our hypothesis that the inner ear of BWC is afflicted by a disorder similar to Scheibe's dysplasia, the most common inner ear defect associated with congenital hearing loss in humans.

The kinocilium of auditory hair cells and evidence for its morphogenetic role during the regeneration of stereocilia and cuticular plates

Auditory hair cells that survive mechanical injury in culture begin their recovery by reforming the kinocilium. This study is based on cultures of the organ of Corti of newborn mice and two control animals. The axonemal patterns were examined in 165 kinocilia in cross-section. In the immature and regenerating kinocilium, one of the normally peripheral doublets is frequently located inward, forming the modified 8 + 1 (double) form; the distribution of the remaining microtubules is irregular. As the cell matures, the 9 + 0 form predominates. Overall, 34-61% of auditory kinocilia consist of 9 + 0 microtubules. The 9 + 2 (single) form, previously thought to characterize the organelle, occurs only in about 3-14%, whereas the remaining population comprises the modified 8 + 1 (double) form. Normally, the kinocilium lasts only about 10 postnatal days; however, post-traumatic hair cells reform their kinocilia regardless of age. Concomitant with the regrowth of the kinocilium, the basal body and its cilium take a central location in the cuticular plate, stereocilia regrow, and the cytoplasmic area adjacent to the basal body displays pericentriolar fibrous densities, growth vesicles, and microtubules, all surrounded by actin filaments. Pericentriolar bodies nucleate microtubules. Involvement of microtubules is seen in the alignment of actin filaments and in the formation of the filamentous matrix of the cuticular plate. We propose that reformation of the kinocilium in recovering post-traumatic hair cells indicates the possible role of its basal body in the morphogenesis and differentiation of cuticular plates and stereocilia.

Fimbrin expression in the developing rat cochlea

The expression of fimbrin in the developing rat cochlea was analyzed using an immunohistochemical technique with fimbrin antibody. The cochlea displayed temporal and lateral-longitudinal gradients for fimbrin expression during development. Fimbrin immunoreactivity first appeared in the inner hair cell stereocilia of the basal turn on the first gestational day studied (day 18). At birth, both inner (IHC) and outer hair cell (OHC) stereocilia of the basal turn showed positive labeling with fimbrin antibody. The progression of appearance was always from IHCs to OHCs and fimbrin immunostaining appeared in the apical hair cells by postnatal day 6. Immunostaining was restricted to stereocilia and the cuticular plate, and no immunoreactivity was observed in neighboring structures of the epithelium. Double labeling using both fimbrin antibody and phalloidin binding revealed similar chronological expression from the earliest stage studied. Increasing fimbrin immunoreactivity was observed in hair cells until late postnatal and adult stages. This study suggests that fimbrin is expressed with F-actin during development and fimbrin together with actin may constitute the two basic molecules that participate in stereocilia formation. We speculate that fimbrin may help maintain the parallel growth of actin filaments within the stereocilia. These data additionally support previous findings that hair cell maturation occurs from the base to the apex and from IHCs to OHCs.

Cytoskeletal organization of the vestibular sensory epithelia

The cytoskeletal organization of the guinea pig vestibular sensory epithelium was investigated by employing the saponin perfusion method and scanning electron microscopy. The skeletal framework of a a cell was found to consist of actin, intermediate filaments and microtubules. The membrane-bound organelles such as nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, etc. were also well documented. This made it possible to investigate the three-dimensional organization of cytoskeletons as well as their complex interaction with various membrane-bound organelles. The intermediate filaments were demonstrated not only in the supporting cells but also in the sensory cells. They are usually seen surrounding the nucleus and extending through the cytoplasm which connects the nuclear membrane to the plasma membrane, cuticular plate, reticular lamina or other cyto-organelles. The intermediate filaments are also closely related to the desmosomes. These findings suggest that the intermediate filaments provide mechanical support to the cell and its nucleus. In the sensory cells, microtubules are found mainly in the supranuclear portion of the cells, running parallel to the main axis of the cell body, which is directly connected to the cuticular plate. These findings suggest that the microtubules provide the cell with mechanical support and may be closely related to the sensory cell transduction system. Inside the supporting cells, the microtubules are closely related to the secretory granules, Golgi apparatus and intermediate filaments, which supports the idea that the microtubules may control the distribution of intermediate filaments, and also play an important role for transport of the secretory granules.

The changing microtubule arrangements in developing hair cells of the chick cochlea

It has been suggested that microtubules in auditory hair cells might be involved in directing the morphological and hence functional polarisation of the sensory hair bundles. The distribution of microtubules was studied in hair cells of the chick cochlea, during the developmental stages when the stereocilia and cuticular plate were being formed. Cochleae were immunofluorescently labelled with antibodies to tubulin at specific stages in development, and hair cell ultrastructure was observed by electron microscopy. We found that the microtubule array changed from a simple symmetrical apical plate with a central kinocilium before the cuticular plate forms, to a ring with the kinocilium to one side when the cuticular plate begins to form, through to a cup-like arrangement below the cuticular plate once the plate has formed. In the earliest stages, no asymmetries were observed in the distribution of the microtubules, suggesting that structures other than the microtubules set up the functional polarisation of the stereociliary bundle.

Neurofilament proteins form an annular superstructure in guinea-pig type I vestibular hair cells

Neurofilaments, the neuron-specific intermediate filaments, are composed of three immunochemically distinct subunits: NF-L, NF-M and NF-H that can be either phosphorylated or unphosphorylated. In mammals, the distribution of these subunits has been described in vestibular ganglion neurons, but there are no reports on the presence of neurofilaments in vestibular hair cells. We investigated, by immunocytochemistry, neurofilaments in vestibular hair cells from rat and guinea-pig using antibodies against the three subunits and to dephosphorylated NF-H (clone SMI 32, recognizes also NF-M on immunoblots), on Vibratome sections of the vestibular end-organs and on isolated hair cells. Various immunostaining protocols were used, as appropriate for the method of observation: laser scanning confocal microscopy (immunofluorescence) and transmission electron microscopy (immunoperoxidase, pre-embedding technique). In rat and guinea-pig cristae and utricles, neurofilament immunoreactivity was observed in axons inside and below the sensory epithelia. In guinea-pig, in addition to this staining, intensely immunoreactive annular structures were found in the basal regions of hair cells. These rings were detected with anti-NF-L, -NF-M and -dephosphorylated NF-H/M antibodies, but not with anti-phosphorylation-independent NF-H. Ring-containing hair cells were present in all regions of the sensory epithelia but were more abundant in the peripheral areas. All levels of observation (Vibratome and thin sections, and isolated hair cells) showed that only the guinea-pig type I hair cells contained a neurofilament ring. High-resolution observations showed that the ring was located below the nucleus, often close to smooth endoplasmic reticulum and the cell membrane.

Structural basis for mechanical transduction in the frog vestibular sensory apparatus: II. The role of microtubules in the organization of the cuticular plate

The actin matrix of the cuticular plate, which supports the sensory stereocilia bundle, is coupled to the axial cytoskeleton of the hair cell through a well defined microtubule columnar framework. A collection of axial microtubules in a columnar organization penetrate deep into the dense actin matrix of the cuticular plate. Each microtubule displays at the end a 300-500 nm long fuzzy cap that enmeshes with the actin matrix of the cuticular plate. The microtubule associated proteins MAP-1A and MAP-1B were localized by confocal immunofluorescence to the point of microtubule insertion in the cuticular plate. These proteins are likely components of the microtubule capping structure and may mediate the interaction of the microtubules with the actin matrix. The structural interaction of the microtubules with the cuticular plate provides important mechanical coupling of the transduction apparatus to the axial cytoskeleton of the hair cell.

Cytoskeletal organization of the vestibular supporting cells. Saponin perfusion method for observing intracellular structures by scanning electron microscopy

The cytoskeletal organization of the guinea pig vestibular supporting cells was investigated employing the saponin perfusion method using scanning electron microscopy. The skeletal framework of a cell was composed of actin, intermediate filaments and microtubules. The membrane bound organelles such as nucleus mitochondria, endoplasmic reticulum, Golgi apparatus, etc. were also well documented. This made it possible to investigate the three-dimensional organization of cytoskeletons as well as their complex interaction with various membrane bound organelles. The intermediate filaments were usually seen surrounding the nucleus and extending through the cytoplasm which connected the nuclear membrane to the plasma membrane, reticular lamina or other cytoorganelles. The intermediate filaments also closely related to the desmosomes. These findings suggest that the intermediate filaments provide mechanical support to the cell and its nucleus. The microtubules were closely related to the secretory granules, Golgi apparatus and intermediate filaments, which supports the idea that the microtubules may control the distribution of intermediate filaments, and also play an important role for the transport of the secretory granules.

Cytoskeletal organization of the vestibular sensory epithelia: saponin perfusion method for observing intracellular structures by scanning electron microscopy

The cytoskeletal organization of the guinea pig vestibular sensory epithelial cells were investigated by the use of saponin perfusion method using scanning electron microscopy. The skeletal framework of a cell is composed of thin (actin or intermediate filaments) and thick filaments (microtubules). The membrane bound organelles such as nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, etc. were also well demonstrated. This made it possible to investigate the three-dimensional structures of cytoskeletons as well as their complex interactions with various membranes bound organelles. It is therefore suspected that this technique may provide us further information about distribution, topographic relationships, and the functional role of cytoskeletons.

Identification of a 120 kd hair-bundle myosin located near stereociliary tips

By adapting to sustained stimuli, hair cells of the internal ear maintain their optimal sensitivity to minute displacements. Biophysical experiments have suggested that adaptation is mediated by a molecular motor, most likely a member of the myosin family. To provide direct evidence for the presence of myosin isozymes in hair bundles, we used photoaffinity labeling with vanadate-trapped uridine and adenine nucleotides to identify proteins of 120, 160, and 230 kd in a preparation of hair bundles purified from the bullfrog's sacculus. The photoaffinity labeling properties of these proteins, particularly the 120 kd protein, resembled those of other well-characterized myosins. A 120 kd hair-bundle protein was also recognized by a monoclonal antibody directed against a vertebrate myosin I isozyme. Immunofluorescence microscopy localized this protein near the beveled top edge of the hair bundle, the site of mechanoelectrical transduction and adaptation.

Ontogenesis of F-actin in hair cells

This report describes the ontogenesis of cochlear stereocilia using scanning electron microscopy for analysis of cilia appearance, and fluorescence microscopy of phalloidin, a label for F-actin, to determine the maturation of the cilia framework. Surface and frozen-sectioned preparations of the otic capsule were obtained from several stages of rat pup development beginning at the 16th gestational day and at various stages until adulthood. In the earliest stage investigated, strong fluorescence labeling was visible on the apical part of Kölliker's organ, revealing a reticular outline of cell junctions. Hair cells started to differentiate at the 18th day of gestation from cells within the primordial receptor area. Phalloidin labeling revealed a sequential appearance of F-actin as the hair cells differentiated from the cells with the Kölliker's organ. The differentiation of receptor cells occurred first with the appearance of a junctional complex between the hair cell and the surrounding cells. Then a cuticular plate appeared followed by the progressive emergence of stereocilia. The F-actin labeling also revealed a progressive differentiation of receptor cells from the cochlear base to its apex. There was also an inner to outer hair cell developmental gradient of label. Inner hair cells developed stereocilia before outer hair cells. The third row of outer hair cells was the last to acquire stereocilia. The adult pattern of stereocilia was reached around the 6th postnatal day. We conclude that the appearance of actin filaments in developing receptor cells and the emergence of stereocilia can be regraded as markers for correlating function and other structural differentiation.

Fodrin immunocytochemical localization in the striated organelles of the rat vestibular hair cells

The immunocytochemical distribution of a spectrin-related protein, fodrin was studied at the electron microscopic level in the rat vestibular hair cells. As previously demonstrated [Scarfone et al., Neurosci. Lett. 93, 13-18, 1988], an intense immunoreactivity was found in the cuticular plates. We demonstrate furthermore, here, for the first time the association of fodrin immunoreactivity with the striated infracuticular structures called striated organelles (SO). Fodrin was found in striated structures clearly identified as SO in both Type I and Type II hair cells. SO were labelled regardless of their location, subcuticular or associated with the plasma membrane of the cells. We suggest that fodrin, as in the cuticular plate, could participate to the Ca2+ dependent cross-linking of the actin filaments of the striated organelles and could play a role in their interaction with the submembraneous cytoskeleton.

Monoclonal antibodies to inner ear antigens: II. Antigens expressed in sensory cell stereocilia

To develop biological reagents for investigating structure-function relationships in the organ of Corti, we have raised monoclonal antibodies, (MAb) to inner ear tissues. Our first series of antibodies prepared after intrasplenic immunization of mice with guinea pig tissues, identified antigens restricted to supporting cell structures, but no hair cell specific antibodies were developed [Zajic et al., Hear. Res. 52, 59-72, 1991]. In this report we describe the isolation, binding specificity and initial characterization of the stereocilia-binding monoclonal antibodies, KHRI-4, and KHRI-5. Mice were immunized with avian, amphibian and mammalian sensory hair cell-containing tissues and antibodies were screened for selective binding to cochlear extracts in ELISA. In the inner ear, KHRI-4 and KHRI-5 bind specifically to stereocilia in both avian and mammalian cochlear and vestibular tissue preparations using immunofluorescence and immunoperoxidase assays. In other tissues only certain cells of mesothelial origin, such as smooth muscle in gut and the arteriolar vasculature, were stained by KHRI-4 indicating that the antigenic structure defined by this antibody has limited distribution. KHRI-5 binding could be detected in other tissues only at high antibody concentrations suggesting that the gene product identified by this antibody is also weakly expressed in other cell lineages. Western blot analysis showed that KHRI-4 and -5 detect different protein complexes. KHRI-4 identifies an antigenic structure common to gut, cochlea, vestibular tissue and cultured fibroblasts consisting of a approximately 195 and a 230 kDa heterodimer designated p195/230. KHRI-5 binds to a prominent approximately 200-210 kDa band in Western blots of cochlear tissues, gut and fibroblasts. In immunoprecipitation experiments, KHRI-5 precipitated three proteins of Mr approximately 200-210, 230 and 260 kDa indicating that the approximately 200-210 kDa protein carrying the epitope for this antibody is a member of a heterotrimer complex. Our results show that these protein complexes are structural components of stereocilia and that the same proteins are arrayed in conjunction with the actin stress fibers of cultured mesothelial cells. Thus, they are likely to be important for maintaining the actin structure of stereocilia essential to transduction in sensory hair cells.

Protein phosphorylation in the organ of Corti: differential regulation by second messengers between base and apex

Major aspects of cellular physiology are regulated by the phosphorylation state of proteins through the action of protein kinases and protein phosphatases. Phosphorylation of proteins by endogenous protein kinase activity was assayed in homogenates from guinea pig inner ear tissues with [gamma-32P] ATP. Phosphoproteins showed distinct distributions in organ of Corti, lateral wall and spiral ganglion. In the organ of Corti, several protein kinase activities were distinguished by their activation by appropriate agonists: protein kinase C, calmodulin-dependent protein kinases and cyclic nucleotide-dependent protein kinases. Twelve putative substrates for these kinases were identified in organ of Corti on the basis of increased 32P-incorporation with addition of lipids, calmodulin, and cyclic nucleotides, respectively. In addition, differences in phosphorylation were observed between the base and apex of the organ of Corti. 32P-incorporation into proteins of molecular weights between 45 and 100 kDa was significantly higher in apical tissue than in tissue from the base. In contrast, phosphate incorporation into proteins of around 29 kDa was much lower in apical tissues than in basal tissues. Furthermore, labeling of both the high and low molecular weight proteins from the apex but not the base markedly increased in response to calcium. These data indicate the presence of differential modes of regulation that may underlie structural and functional gradients along the sensory epithelium.

Immuno-electron microscopic study of keratin distribution in the cochlea using monoclonal antibody

Keratin distribution in the cochlea has been studied immuno-electron microscopically by both pre-embedding and postembedding methods. Keratin immunoreactivity in the guinea pig cochlea was clearly demonstrated in Hensen's cells, the reticular lamina surrounding both outer and inner hair cells, outer and inner pillar cells, Claudius' cells, inner and external sulcus cells, interdental cells of the spiral limbus, Reissner's membrane, border cells, inner phalangeal cells, Deiters' cells, and spiral prominence cells. Keratin expression at the ultrastructural level showed a nonfilamentous keratin system in the cytoplasm of cochlear supporting cells.

Organization of microtubules in cochlear hair cells

The organization of microtubules in hair cells of the guinea-pig cochlea has been investigated using transmission electron microscopy and correlated with the location of tubulin-associated immunofluorescence in surface preparations of the organ of Corti. Results from both techniques reveal consistent distributions of microtubules in inner and outer hair cells. In the inner hair cells, microtubules are most concentrated in the apex. Reconstruction from serial sections shows three main groups: firstly, in channels through the cuticular plate and in a discontinuous belt around its upper perimeter; secondly, forming a ring inside a rim extending down from the lower perimeter of the plate; and thirdly, in a meshwork underlying the main body of the plate. In the cell body, microtubules line the inner face of the subsurface cistern and extend longitudinally through a tubulo-vesicular track between the apex and base. In outer hair cells, the pattern of microtubules associated with the cuticular plate is similar, although there are fewer present than in inner hair cells. In outer hair cells from the apex of the cochlea, microtubules occur around an infracuticular protrusion of cuticular plate material. In the cell body, many more microtubules occur in the region below the nucleus compared with inner hair cells. The possible functions of microtubules in hair cells are discussed by comparison with those found in other systems. These include morphogenesis and maintenance of cell shape; intracellular transport, e.g., of neurotransmitter vesicles; providing a possible substrate for motility; mechanical support of structures associated with sensory transduction.

Immunocytochemical localization of myosin, tropomyosin and actin in vestibular hair cells of human fetuses and cats

Semithin sections (1 micron) of human fetuses and young cat vestibular epithelium embedded in Epon were reacted with primary antibodies raised against myosin, tropomyosin, and actin. The results were very similar in the two species. Myosin and tropomyosin were colocalized and strictly limited to an apical superficial corona arranged in a punctiform pattern. The stereocilia and cuticular plate were unreactive to myosin and tropomyosin antibodies. Actin antibodies stained the stereocilia, cuticular plate, and a marginal ring surrounding and underlying the cuticular plate. At this level, myosin and tropomyosin were not detected. This result suggests that the circumferential actin ring has a structural role. Colocalization of myosin, tropomyosin, and actin in a superficial area around the stereocilia bundle and along the apical surface of the hair cell imply that active processes could exist at the apex of certain sensory vestibular hair cells. The specificity of myosin antibodies and their cross-reactivity with different types of myosin are discussed.

Alpha-fodrin (brain spectrin) immunocytochemical localization in rat vestibular hair cells

The presence of a spectrin-related protein in rat vestibular sensory receptors was demonstrated by immunocytochemistry and immunoblotting using affinity purified anti alpha-fodrin antibodies. Intense immunoreactivity was found in the apical pole of sensory hair cells where it seems to be concentrated in the cuticular plate. In contrast, alpha-fodrin immunoreactivity was absent from the stereocilia. We suggest that a spectrin-related protein participates in the organization of the cuticular plate of vestibular hair cells by cross-linking actin filaments as well as by anchoring the cuticular plate to the apical cell membrane.

Volume and length changes in outer hair cells of the guinea pig after potassium-induced shortening

Measurements of volume and length were made on isolated guinea pig cochlear outer hair cells (OHCs) in an attempt to establish whether OHC shortening was accompanied by changes in cell volume. A sustained shortening in response to an isotonic 100 mM KCl solution was found to be accompanied by a significant increase in OHC volume. The application of hypotonic solutions had a similar effect. When testing solutions with unchanged or reduced [K+] [Cl-] products in order to avoid loading the cells with chloride ions and producing concomitant water influx, the most frequent response was a rapid shortening followed by an elongation beyond the original cell length. These findings of a sustained and a spontaneously reversible shortening suggest that the potassium-induced response may consist of two components: a rapid one, which may be reversible in the presence of the stimulus, and a second, slower, component resulting in sustained shortening.

Species differences in the distribution of infracuticular F-actin in outer hair cells of the cochlea

The distribution of filamentous actin (F-actin) in outer hair cells has been examined in several mammalian species using tetramethylrhodamine phalloidin, a specific marker for F-actin. The stereocilia and cuticular plates of the OHC in all species examined (pigmented guinea pig, hooded rat, chinchilla and squirrel monkey) contained F-actin; however, an infracuticular network of F-actin was present in OHC of the apical turns of the guinea pig cochlea but could not be identified in any other species examined.

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).