The deaf jerker mouse has a mutation in the gene encoding the espin actin-bundling proteins of hair cell stereocilia and lacks espins

The espins are actin-bundling proteins of brush border microvilli and Sertoli cell-spermatid junctions. We have determined that espins are also present in hair cell stereocilia and have uncovered a connection between the espin gene and jerker, a recessive mutation that causes hair cell degeneration, deafness, and vestibular dysfunction. The espin gene maps to the same region of mouse chromosome 4 as jerker. The tissues of jerker mice do not accumulate espin proteins but contain normal levels of espin mRNAs. The espin gene of jerker mice has a frameshift mutation that affects the espin C-terminal actin-bundling module. These data suggest that jerker mice are, in effect, espin null and that the jerker phenotype results from a mutation in the espin gene.

In Search of the Hair-Cell Gating Spring

Mechanotransduction in vertebrate hair cells involves a biophysically defined elastic element (the "gating spring") that pulls on the transduction channels. The tip link, a fine filament made of cadherin 23 linking adjacent stereocilia in hair-cell bundles, has been suggested to be the gating spring. However, TRP channels that mediate mechanotransduction in Drosophila, zebrafish, and mice often have cytoplasmic domains containing a large number of ankyrin repeats that are also candidates for the gating spring. We have explored the elastic properties of cadherin and ankyrin repeats through molecular dynamics simulations using crystallographic structures of proteins with one cadherin repeat or 4 and 12 ankyrin repeats, and using models of 17 and 24 ankyrin repeats. The extension and stiffness of large ankyrin-repeat structures were found to match those predicted by the gating-spring model. Our results suggest that ankyrin repeats of TRPA1 and TRPN1 channels serve as the gating spring for mechanotransduction.

Olivocochlear innervation in the mouse: immunocytochemical maps, crossed versus uncrossed contributions, and transmitter colocalization

To further understand the roles and origins of gamma-aminobutyric acid (GABA) and calcitonin gene-related peptide (CGRP) in the efferent innervation of the cochlea, we first produced in the mouse an immunocytochemical map of the efferent terminals that contain acetylcholine (ACh), CGRP, and GABA. Olivocochlear (OC) terminals in inner and outer hair cell (IHC and OHC) regions were analyzed quantitatively along the cochlear spiral via light-microscopic observation of cochlear wholemounts immunostained with antibodies to glutamic acid decarboxylase (GAD), vesicular acetylcholine transporter (VAT), or the peptide CGRP. Further immunochemical characterization was performed in mice with chronic OC transection at the floor of the fourth ventricle to distinguish crossed from uncrossed contributions and, indirectly, the contributions of lateral versus medial components of the OC system. The results in mouse showed that (1) there are prominent GABAergic, cholinergic, and CGRPergic innervations in the OHC and IHC regions, (2) GABA and CGRP are extensively colocalized with ACh in all OC terminals in the IHC and OHC areas, (3) the longitudinal gradient of OC innervation peaks roughly at the 10-kHz region in the OHC area and is more uniform along the cochlear spiral in the IHC area, (4) in contrast to other mammalian species there is no radial gradient of OC innervation of the OHCs, and (5) all OHC efferent terminals arise from the medial OC system and terminals in the IHC area arise from the lateral OC system.

The concentrations of calcium buffering proteins in mammalian cochlear hair cells

Calcium buffers are important for shaping and localizing cytoplasmic Ca2+ transients in neurons. We measured the concentrations of the four main calcium-buffering proteins (calbindin-D28k, calretinin, parvalbumin-alpha, and parvalbumin-beta) in rat cochlear hair cells in which Ca2+ signaling is a central element of fast transduction and synaptic transmission. The proteins were quantified by calibrating immunogold tissue counts against gels containing known amounts of each protein, and the method was verified by application to Purkinje cells in which independent estimates exist for some of the protein concentrations. The results showed that, in animals with fully developed hearing, inner hair cells had 110 of the proteinaceous calcium buffer of outer hair cells in which the cell body contained parvalbumin-beta (oncomodulin) and calbindin-D28k at levels equivalent to 5 mm calcium-binding sites. Both proteins were partially excluded from the hair bundles, which may permit fast unbuffered Ca2+ regulation of the mechanotransducer channels. The sum of the calcium buffer concentrations decreased in inner hair cells and increased in outer hair cells as the cells developed their adult properties during cochlear maturation. The results suggest that Ca2+ has distinct roles in the two types of hair cell, reflecting their different functions in auditory transduction. Ca2+ is used in inner hair cells primarily for fast phase-locked synaptic transmission, whereas Ca2+ may be involved in regulating the motor capability underlying cochlear amplification of the outer hair cell. The high concentration of calcium buffer in outer hair cells, similar only to skeletal muscle, may protect against deleterious consequences of Ca2+ loading after acoustic overstimulation.

SNARE complex at the ribbon synapses of cochlear hair cells: analysis of synaptic vesicle- and synaptic membrane-associated proteins

Neurotransmitters are released via exocytosis of synaptic vesicles involving a fusion complex consisting of a set of highly conserved proteins, which form a multiprotein complex resulting in the docking of synaptic vesicles at the site of release. There are three major differences between cochlear hair cell synapses and CNS synapses: (i) hair cells have a specialized structure, the synaptic ribbon, to which synaptic vesicles are attached; (ii) hair cells can maintain high and sustained release of neurotransmitter; and (iii) hair cells lack synaptophysin and synapsin. These differences suggest that an unconventional mechanism of neurotransmitter release may be involved at ribbon synapses. In this study we used different and complementary approaches to determine whether or not ribbon-containing hair cells of the cochlea express any component of the core fusion complex found in conventional synapses. Syntaxin 1, the synaptic membrane synaptosome-associated protein (SNAP)-25 and vesicle-associated membrane protein (VAMP or synaptobrevin) were found to be present in the organ of Corti of both rat and guinea-pig, as shown by reverse transcription polymerase chain reaction and Western blotting. In situ hybridization and immunocytochemistry showed mRNA and protein expression, respectively, in both inner and outer hair cells. Synaptotagmins I and II, generally considered to play major roles in neurotransmitter release at central synapses, were not detected in the organ of Corti.

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.

Reorganization of cytoskeletal and junctional proteins during cochlear hair cell degeneration

Experiments were carried out to elucidate changes in cytoskeletal elements and intercellular junctions in the organ of Corti, when hair cells degenerate and phalangeal scars form. Hair cell damage was induced by exposing guinea pigs to high intensity noise. The spatial and temporal changes in the organization of microfilaments, intermediate filaments, and tight junction-specific proteins were investigated using scanning and transmission electron microscopy and histochemistry. The results show that microfilaments, cytokeratins, adherens junctions, and tight junctions rearrange their distribution in damaged areas. From the temporal sequence of these changes it appears that phalangeal scars develop simultaneous with hair cell degeneration, and that the integrity of the luminal membranes in the organ of Corti is not interrupted. Each scar is formed by two supporting cells which expand and invade the sub-apical region of the dying hair cell. This region becomes cytokeratin-positive. The two supporting cells meet at the mid-line of the scar, where a new junctional complex is formed. The junctional complex consists of tight junction and adherens-type junction, but desmosomes are absent.

Myosin VIIa as a common component of cilia and microvilli

The distribution of myosin VIIa, which is defective or absent in Usher syndrome 1B, was studied in a variety of tissues by immunomicroscopy. The primary aim was to determine whether this putative actin-based mechanoenzyme is a common component of cilia. Previously, it has been proposed that defective ciliary function might be the basis of some forms of Usher syndrome. Myosin VIIa was detected in cilia from cochlear hair cells, olfactory neurons, kidney distal tubules, and lung bronchi. It was also found to cofractionate with the axonemal fraction of retinal photoreceptor cells. Immunolabeling appeared most concentrated in the periphery of the transition zone of the cilia. This general presence of a myosin in cilia is surprising, given that cilia are dominated by microtubules, and not actin filaments. In addition to cilia, myosin VIIa was also found in actin-rich microvilli of different types of cell. We conclude that myosin VIIa is a common component of cilia and microvilli.

Endocytosis of aminoglycoside antibiotics in sensory hair cells

Immuno-gold electron microscopy was used to assess the uptake pathways of aminoglycoside antibiotic kanamycin (KM) in sensory hair cells. Accumulation of gold particles was evident on the plasma membrane as well as in large smooth vesicles beneath the apical surfaces of hair cells 12 h after a systemic administration of KM. Immuno-gold was exclusively localized in the vesicles 27 h post-injection. Cationic ferritin, a membrane-bound insoluble marker, was colocalized with KM in the vesicle structures after their simultaneous in vitro application. These results strongly suggest that KM is taken up into sensory hair cells via receptor-mediated endocytosis at their apical surfaces. In addition, the profound time lag between KM uptake and hair cell death suggests involvement of targeting mechanisms in cytotoxic signalling pathways of the drugs.

Molecular organization of a type of peripheral glutamate synapse: the afferent synapses of hair cells in the inner ear

The synapses between sensory cells in the inner ear and the afferent dendrites of ganglion cells are well suited to investigations of fundamental mechanisms of fast synaptic signalling. The presynaptic elements can be isolated for electrophysiological and functional studies while the synapses can be easily recognized in the electron microscope due to their distinct morphological features. This allows for a broader range of correlative functional and structural analyses than can be applied to synapses in the central nervous system (CNS). As in most fast excitatory synapses in the CNS the transmitter in the afferent hair cell synapses appears to be glutamate or a closely related compound. Recent studies have revealed many of the key molecular players at this type of synapse and how they are spatially and functionally coupled. By use of high resolution immunogold cytochemistry it has been shown that AMPA glutamate receptors are specifically expressed in the postsynaptic specialization of afferent hair cell synapses (except at those established by outer hair cells in the organ of Corti) and that their density varies as a function of the distance from the release sites (demonstrated for the afferent contacts of inner hair cells). The glutamate transporter GLAST is localized in supporting cell membranes and concentrated in those membrane domains that face the synaptic regions. Glutamine synthetase and phosphate-activated glutaminase--which are responsible for the interconversion of glutamate and glutamine--are selectively localized in non-neuronal and neuronal elements, respectively. Taken together with quantitative immunogold data on the cellular compartmentation of glutamate and glutamine the above findings suggest that the sensory epithelia in the inner ear sustain a cycling of glutamate carbon skeletons. In this process, the supporting cells may carry out functions analogous to those of glial cells in the CNS. Functional and morphological analyses of the presynaptic membrane indicate that L-type Ca(2+)-channels and Ca(2+)-activated K(+)-channels are colocalized and clustered at the active zone. Influx through the L-type channels triggers synaptic release and their close spatial association with Ca(2+)-activated K(+)-channels appears to be critical for frequency tuning. The focal expression of different Ca(2+)-channels combined with a high intracellular buffering capacity permits several Ca(2+)-signalling pathways to operate in parallel without undue interference. The molecular organization of the afferent hair cell synapses reflects the functional demand for speed and precision and attests to the ability of the pre- and postsynaptic elements to target and anchor key proteins at specific membrane domains.

High-purity isolation of bullfrog hair bundles and subcellular and topological localization of constituent proteins

The small number of hair cells in auditory and vestibular organs severely impedes the biochemical characterization of the proteins involved in mechano-electrical transduction. By developing an efficient and clean "twist-off" method of hair bundle isolation, and by devising a sensitive, nonradioactive method to detect minute quantities of protein, we have partially overcome this limitation and have extensively classified the proteins of the bundles. To isolate hair bundles, we glue the saccular macula of the bullfrog to a glass coverslip, expose the tissue to a molten agarose solution, and allow the agarose to solidify to a firm gel. By rotating the gel disk with respect to the fixed macula, we isolate the hair bundles by shearing them at their mechanically weak bases. The plasma membranes of at least 80% of the stereocilia reseal. To visualize the proteins of the hair bundle, we covalently label them with biotin, separate them by SDS-PAGE, and transfer them to a charged nylon membrane. We can detect less than 500 fg of protein by probing the membrane with streptavidin-alkaline phosphatase and detecting the chemiluminescent product from the hydrolysis of the substrate 3-(4-methoxyspiro-(1,2-dioxetane-3,2'-tricyclo-[3.3.1. 1(3.7)]decan)-4-yl) phenyl phosphate (AMPPD). These techniques reveal a distinct constellation of proteins in and associated with hair bundles. Several proteins, such as calmodulin, calbindin, actin, tubulin, and fimbrin, have previously been described. A second class of proteins in the preparation appears to be derived from extracellular sources. Finally, several heretofore undescribed bundle proteins are identified and characterized by their membrane topology, subcellular localization, and glycosidase and protease sensitivities.

Morphology and cell type heterogeneities of the inner ear epithelia in adult and juvenile zebrafish (Danio rerio)

Although the zebrafish has become an important model for genetic analysis of the vertebrate auditory system, a comprehensive description of the zebrafish ear has been provided for embryonic and larval development only (Haddon and Lewis [1996] J. Comp. Neurol. 365:113). Here we describe the development of sensory maculae in juvenile fish and the morphology of the adult zebrafish ear. This description was obtained via three-dimensional reconstruction of serial sections and confocal microscopy of immunolabeled preparations and includes the Weberian ossicles and fluid spaces. Phalloidin staining, which labels actin filaments of stereocilia, was used to delineate the sensory epithelia, to visualize the distribution of hair cells, to estimate their density in different areas of the maculae, and to perform hair cell counts. Morphology of ciliary bundles in different regions of the lagena, saccule, utricle, macula neglecta, and cristae was characterized with an anti-acetylated tubulin antibody and by phalloidin staining. We have identified two antibodies characterized by region-specific staining patterns in the inner ear epithelia. Zn-1 antibody staining largely correlates with the presence of short-bundle hair cells in the peripheral regions of sensory epithelia. Zn-4 antibody, on the other hand, labels a zone of epithelial cells surrounding the sensory maculae. These analyses extend previous observations of cell-type heterogeneity in both sensory and nonsensory epithelia of the fish ear.

Hair cell recovery in mitotically blocked cultures of the bullfrog saccule

Hair cells in many nonmammalian vertebrates are regenerated by the mitotic division of supporting cell progenitors and the differentiation of the resulting progeny into new hair cells and supporting cells. Recent studies have shown that nonmitotic hair cell recovery after aminoglycoside-induced damage can also occur in the vestibular organs. Using hair cell and supporting cell immunocytochemical markers, we have used confocal and electron microscopy to examine the fate of damaged hair cells and the origin of immature hair cells after gentamicin treatment in mitotically blocked cultures of the bullfrog saccule. Extruding and fragmenting hair cells, which undergo apoptotic cell death, are replaced by scar formations. After losing their bundles, sublethally damaged hair cells remain in the sensory epithelium for prolonged periods, acquiring supporting cell-like morphology and immunoreactivity. These modes of damage appear to be mutually exclusive, implying that sublethally damaged hair cells repair their bundles. Transitional cells, coexpressing hair cell and supporting cell markers, are seen near scar formations created by the expansion of neighboring supporting cells. Most of these cells have morphology and immunoreactivity similar to that of sublethally damaged hair cells. Ultrastructural analysis also reveals that most immature hair cells had autophagic vacuoles, implying that they originated from damaged hair cells rather than supporting cells. Some transitional cells are supporting cells participating in scar formations. Supporting cells also decrease in number during hair cell recovery, supporting the conclusion that some supporting cells undergo phenotypic conversion into hair cells without an intervening mitotic event.

Appearance and distribution of the 275 kD hair-cell antigen during development of the avian inner ear

The 275 kD hair-cell antigen (HCA) is a protein that was originally identified using immunological techniques in the inner ears of early hatchling and adult chickens. The HCA is specifically associated with the apical surface of sensory hair cells; in the vestibular system the antigen is distributed over the entire stereocilia bundle, but in the auditory system it only extends a short distance up the shafts of the stereocilia. The objectives of this study were to ascertain when the HCA is first expressed during inner ear development, to compare the temporal and spatial patterns of HCA expression with those of neurite ingrowth, and to determine how the distribution of the antigen observed in the auditory system arises during development. Serial sections of otocysts from embryonic day (ED) 4 to ED7.5 (stages 24 to 32) were stained with a monoclonal antibody to the HCA and polyclonal antibodies to the neuron-glial cell adhesion molecule in order to analyse patterns of HCA expression and neurite ingrowth. Nerve fibres are first observed in the anterior pole of the otocyst at ED4.5 (stage 24), and in the evaginating basilar papilla by ED5 (stage 26). The HCA first appears within the vestibular system in the anterior pole of the otocyst at ED5 (stage 26), and within the auditory system in the distal end of the basilar papilla at ED6.5 (stage 29). Serial section analysis indicates that expression of the HCA is always limited to areas of the epithelium where nerve fibres are found, although the delay between the onset of innervation and the onset of HCA expression varies from one region of the otocyst to another. The growth of stereocilia bundles in the auditory system was studied from ED10 to 2 days after hatching in sections from the medial to distal regions of the basilar papilla double labelled with rhodamine phalloidin and monoclonal anti-HCA. At ED12 the stereocilia bundles are 1.7 microns high and the staining observed with both phalloidin and the antibody extend to the same maximum height above the apical surface of the hair cell. The maximum height that anti-HCA staining extends up the stereocilia bundle remains almost constant between ED12 and postnatal day 2, but between ED15 and ED18 the stereocilia bundle grows rapidly in height, with a membrane domain lacking the HCA forming at the distal ends of the stereocilia.(ABSTRACT TRUNCATED AT 400 WORDS)

Espins are multifunctional actin cytoskeletal regulatory proteins in the microvilli of chemosensory and mechanosensory cells

Espins are associated with the parallel actin bundles of hair cell stereocilia and are the target of mutations that cause deafness and vestibular dysfunction in mice and humans. Here, we report that espins are also concentrated in the microvilli of a number of other sensory cells: vomeronasal organ sensory neurons, solitary chemoreceptor cells, taste cells, and Merkel cells. Moreover, we show that hair cells and these other sensory cells contain novel espin isoforms that arise from a different transcriptional start site and differ significantly from other espin isoforms in their complement of ligand-binding activities and their effects on actin polymerization. The novel espin isoforms of sensory cells bundled actin filaments with high affinity in a Ca(2+)-resistant manner, bound actin monomer via a WASP (Wiskott-Aldrich syndrome protein) homology 2 domain, bound profilin via a single proline-rich peptide, and caused a dramatic elongation of microvillus-type parallel actin bundles in transfected epithelial cells. In addition, the novel espin isoforms of sensory cells differed from other espin isoforms in that they potently inhibited actin polymerization in vitro, did not bind the Src homology 3 domain of the adapter protein insulin receptor substrate p53, and did not bind the acidic, signaling phospholipid phosphatidylinositol 4,5-bisphosphate. Thus, the espins constitute a family of multifunctional actin cytoskeletal regulatory proteins with the potential to differentially influence the organization, dimensions, dynamics, and signaling capabilities of the actin filament-rich, microvillus-type specializations that mediate sensory transduction in various mechanosensory and chemosensory cells.

Distribution of the 275 kD hair cell antigen and cell surface specialisations on auditory and vestibular hair bundles in the chicken inner ear

The 275 kD hair cell antigen (HCA) is a protein that is specifically associated with the apical surface of sensory hair cells in the chick inner ear. A comparative study of the vestibular and auditory organs of the inner ear, using both wholemounts and cryosections double labelled for the HCA and F-actin, reveals that two distinct types of hair cells can be distinguished on the basis of antibody staining in each of the vestibular epithelia. One type of hair cell has the HCA restricted to the base of the stereocilia bundle and is found in the striolae of the maculae and in a large, centrally located region of each ampulla. The other type of hair cell is found in the extrastriolar regions of the maculae and the peripheral regions of the ampullae and has the HCA distributed over the entire surface of the stereocilia bundle. In the basilar papilla, the auditory epithelium of the chick inner ear, the HCA is, as in the striolar regions of the maculae, restricted to the base of the hair bundles. In all sensory epithelia the HCA is also present on the apical, nonstereociliary surface of the hair cells. Ultrastructural examination of the basilar papilla and the striolar and the extrastriolar regions of the lagenar macula after staining with ruthenium red and tannic acid shows that there are four morphologically different types of interstereociliary connectors (oblique tip connectors, horizontal tip connectors, shaft connectors and basal connectors) associated with the hair bundles. Oblique tip connectors and basal connectors are found on hair cells from all regions and have a similar distribution. Horizontal tip connectors are seen only on hair cells in the basilar papilla and the striolar region of the lagenar macula. Shaft connectors extend all the way to the tips of extrastriolar hair cell bundles, but extend only a short way up the bundles of hair cells in the basilar papilla and striolar region of the lagenar macula. Immunogold labelling confirms the results obtained with immunofluorescence microscopy and demonstrates that the distribution of the HCA on the surface of adjacent stereocilia correlates closely with that of the shaft connectors; i.e., immunostaining is observed up to the tips of the extrastriolar hair cell bundles, but is restricted to the lower regions of hair cell bundles in the striolar region and basilar papilla.(ABSTRACT TRUNCATED AT 400 WORDS)

Calmodulin and calmodulin-binding proteins in hair bundles

Calcium ion plays an important role in the hair cell's mechanoelectrical transduction process; in particular, Ca2+ controls adaptation to protracted mechanical stimuli. Because calmodulin is a ubiquitous intracellular receptor for Ca2+ and has been shown to accumulate at the tips of stereocilia, we determined its concentration and identified the proteins with which it interacts in the hair bundle. By performing quantitative immunoblot analysis on isolated bundles, we ascertained that the average concentration of calmodulin within each stereocilium is approximately 70 microM. Extraction experiments disclosed that, in the presence of 20 microM Ca2+, 50% of the calmodulin is bound to detergent-soluble receptors. To distinguish these receptors, we developed an assay that utilizes calmodulin crosslinked to alkaline phosphatase. This technique is approximately 100-fold more sensitive than calmodulin-binding assays that employ 125I- or biotin-labeled calmodulin. When used with chemiluminescence detection in a blot-overlay assay, the calmodulin-alkaline phosphatase conjugate identified hair-bundle proteins of molecular masses 25, 35, 145, 175, 240, and 350 kDa. We examined the subcellular distribution of these receptors; all but the 240-kDa molecule are soluble in a nonionic detergent. The relatively high concentration of calmodulin and the presence of several calmodulin-binding proteins provide evidence for a role of calmodulin in hair bundles.

Radixin is a constituent of stereocilia in hair cells

Proteins of the ezrin-radixin-moesin family are ubiquitous constituents of the submembrane cortex, especially in epithelial cells. Earlier biochemical results suggested that a protein of this family occurs in the hair bundle, the cluster of actin-filled stereocilia that serves as the mechanoreceptive organelle of each hair cell in the inner ear. We prepared antipeptide antisera directed against chicken radixin and ezrin and demonstrated their specificity and absence of crossreactivity. When used in immunocytochemical studies of isolated hair cells, anti-radixin produced an intense band of labeling at the bases of hair bundles from the chicken, frog, mouse, and zebrafish. Electron microscopic immunocytochemistry disclosed that radixin labeling commenced in the stereociliary taper, peaked in the lower stereociliary shaft, and declined progressively toward the hair bundle's top. Labeling with anti-ezrin produced no signal in hair bundles. Radixin is thus a prominent constituent of stereocilia, where it may participate in anchoring the "pointed" ends of actin filaments to the membrane.

Cryoembedding and sectioning of cochleas for immunocytochemistry and in situ hybridization

Current emphasis on biochemical and molecular aspects of cochlear anatomy underscores the necessity for high quality cryostat sections of the inner ear. The large volume of fluid space within the cochlea makes cryoembedding and sectioning of the organ more problematic than that of other, more homogeneous tissues. Our method for cryoembedding of cochleas for immunocytochemistry and in situ hybridization uses slow infiltration with increasing concentrations of sucrose followed by degassed embedding medium before final orientation and freezing. This method permits high quality cryosections to be cut which preserve overall structure and cellular resolution.

Scaffolding proteins organize multimolecular protein complexes for sensory signal transduction

Scaffolding proteins composed of protein-protein interaction domains have emerged as organizers of multiprotein complexes in diverse cellular compartments, including neuronal synapses, cell-cell junctions of epithelial cells, and the stimulus perceiving structures of sensory neurons. This review focuses on the INAD-assembled signalling complex of Drosophila photoreceptors, which organizes key components of the phototransduction cascade into a multiprotein signal transduction unit. The structure, the physiological consequences, and the assembly and targeting of the members of the INAD signalling complex will be described. In addition, the existence of signalling complexes in vertebrate photoreceptors, olfactory neurons and mechanosensitive hair cells will be discussed.

Transcript profiling of functionally related groups of genes during conditional differentiation of a mammalian cochlear hair cell line

We have used Affymetrix high-density gene arrays to generate a temporal profile of gene expression during differentiation of UB/OC-1, a conditionally immortal cell line derived from the mouse cochlea. Gene expression was assessed daily for 14 days under differentiating conditions. The experiment was replicated in two separate populations of cells. Profiles for selected genes were correlated with those obtained by RT-PCR, TaqMan analysis, immunoblotting, and immunofluorescence. The results suggest that UB/OC-1 is derived from a population of nonsensory epithelial cells in the greater epithelial ridge that have the potential to differentiate into a hair-cell-like phenotype, without the intervention of Math1. Elements of the Notch signaling cascade were identified, including the receptor Notch3, with a transient up-regulation that suggests a role in hair cell differentiation. Several genes showed a profile similar to Notch3, including the transcriptional co-repressor Groucho1. UB/OC-1 also expressed Me1, a putative partner of Math1 that may confer competence to differentiate into hair cells. Cluster analysis revealed expression profiles for neural guidance genes associated with Gata3. The temporal dimension of this analysis provides a powerful tool to study genetic mechanisms that underlie the conversion of nonsensory epithelial cells into hair cells.

Effects of free radicals on the intracellular calcium concentration in the isolated outer hair cell of the guinea pig cochlea

The cytosolic free calcium concentration ([Ca2+]i) isolated from the cochlear outer hair cell (OHC) of the guinea pig was measured using microfluorimetric imaging technique and the effects of free radicals were investigated. Hypoxanthine (HX) plus xanthine oxidase (XO) induced a rise in [Ca2+]i in the presence of external Ca2+. Elimination of external Ca2+ (pCa = 7) did not show an increase in [Ca2+i, indicating that the increased [Ca2+]i is dependent on external Ca2+. The elevation of [Ca2+]i induced by HX-XO was reduced by addition of superoxide dismutase or nifedipine but not by addition of catalase. A single admission of HX or XO failed to affect [Ca2+]i. These findings suggest that superoxide anion generated in the OHC increases the Ca2+ influx across the membrane, presumably leading to some pathological changes in the acoustic transduction by modulating the OHC motility.

Expression of neurotrophin receptor trkB in rat cochlear hair cells at time of rearrangement of innervation

The spatio-temporal distribution of the high-affinity neurotrophin receptor trkB was monitored during postnatal development of the rat cochlea. In addition to expression in presumptive afferent type I collaterals, afferent type II fibers, and efferent fibers, trkB immunoreactivity also transiently appeared in the sensory hair cells themselves, from postnatal days 5-9 in the basal turn, and from postnatal days 9-13 in the apical turn. A comparison of trkB with p75(NGFR), which is expressed in afferent and efferent fibers, and GAP-43 and synaptophysin, which are expressed in efferent fibers, revealed a time/space correlation of trkB receptor expression in hair cells with the rearrangement of their innervation. Co-expression of the neurotrophin receptor and its ligand has been proposed to be functionally involved in regulating the survival of neurons independent of target-derived neurotrophin factor. Thus, the presence of trkB in target hair cells, suggests that auto/paracrine mechanisms play a role during this critical period of rearrangement of neural connections.

Variations in the ensemble of potassium currents underlying resonance in turtle hair cells

1. Potassium currents were characterized in turtle cochlear hair cells by whole-cell voltage clamp during superfusion with the potassium channel antagonists, tetraethylammonium (TEA) and 4-aminopyridine (4-AP). The estimated resonant frequency, f0, was inferred from tau, the time constant of deactivation of outward current upon repolarization to -50 mV, according to the empirical relation, f0 = k1 tau-1/2 + k2. 2. Dose-response relations for TEA and 4-AP were obtained by exposing single cells to ten concentrations exponentially distributed over four orders of magnitude. Potassium current in cells tuned to low frequencies was carried by a single class of channels with an apparent affinity constant, K1, for TEA of 35.9 mM. Half-blocking concentrations of 4-AP were correlated with the time constant of deactivation and varied between 26.2 and 102 microM. In cells tuned to higher frequencies, K+ current was carried by a single class of channels with high affinity for TEA (K1 = 0.215 mM) and low affinity for 4-AP (K1 = 12.3 mM). This pharmacological profile suggests that K+ current in low frequency cells is purely voltage gated and in high frequency cells, it is gated by both Ca2+ and voltage. 3. For each current type, the voltage dependence of activation was determined from tail current amplitude at -50 mV. The purely voltage-gated current, IK(V), was found to increase e-fold in 4.0 +/- 0.3 mV (n = 3) in low frequency cells exposed to TEA (25 mM). The Ca(2+)- and voltage-gated current, IK(Ca), was more steeply voltage dependent, increasing e-fold in 1.9 mV (n = 2) in high frequency cells exposed to 4-AP (0.8 mM). 4. IK(V) was found to inactivate slowly during prolonged voltage steps (approximately 10 s). Steady-state inactivation increased with depolarization from -70 mV and was incomplete such that on average IK(v) did not fall below approximately 0.39 of its maximum value. 5. Superfusion of 4-AP (0.8 mM) reversibly depolarized a low frequency cell and eliminated steady voltage oscillations, while TEA (6 mM) had no effect. In a high frequency cell, voltage oscillations were abolished by TEA, but not by 4-AP. 6. The differential pharmacology of IK(V) and IK(Ca) was used to measure their contribution to K+ current in cells tuned to different frequencies. Both currents exhibited a frequency-dependent increase in maximum conductance. IK(V) accounted for nearly all K+ current in cells tuned to less than 60 Hz, while IK(Ca) was the dominant current in higher frequency cells. 7. Mapping resonant frequency onto epithelial position suggests an exponential relation between K+ current size and position. IK(V) appeared to be limited to the apical or low frequency portion of the basilar papilla and coincided with maximal expression of a K(+)-selective inward rectifier, IK(IR). This finding is consistent with the notion that low frequency resonance is produced by interaction of IK(V) and IK(IR) with the voltage-gated Ca2+ current, ICa, and the cell's capacitance. The ionic events underlying higher frequency resonance are dominated by the action of IK(Ca) and ICa and include a contribution from IK(IR).

Two types of calcium channels in bullfrog saccular hair cells

Ca2+ channels were studied in cell-attached recordings from the basolateral membrane of the bullfrog saccular hair cells with the EPC-9 patch-clamp system. Pipettes contained 110 mM Ba2+ and the membrane potential was zeroed with isotonic potassium aspartate. Data acquisition and analysis were performed using E9SCREEN and M2LAB software. L-type channel was distinguished by a single-channel conductance of 26 pS, activation range between -10 and +40 mV and intense activity even at a holding potential of -40 mV. The L-type channel showed characteristic bursts of brief openings (mode 1) interrupted occasionally by longer openings (mode 2). Bay K 8644 promoted the mode 2 activity and nifedipine inhibited L-type channel activity. Another type of calcium channels, 20 pS channel, was detected by -50 to +10 mV depolarizing steps from a holding potential of -40 or -80 mV. This channel was insensitive to dihydropyridines and resembled the N-type channel.