Evidence for calcium-binding proteins and calcium-dependent regulatory proteins in sensory cells of the organ of Corti

Expression and localization of diacylglycerol kinase ζ in guinea pig cochlea and its functional implication under noise-exposure stress conditions

Cochlear hair cells are essential for the mechanotransduction of hearing. Sensorineural hearing loss can be irreversible because hair cells have a minimal ability to repair or regenerate themselves once damaged. In order to develop therapeutic interventions to prevent hair cell loss, it is necessary to understand the signaling pathway operating in cochlear hair cells and its alteration upon damage. Diacylglycerol kinase (DGK) regulates intracellular signal transduction through phosphorylation of lipidic second messenger diacylglycerol. We have previously reported characteristic expression and localization patterns of DGKs in various organs under pathophysiological conditions. Nevertheless, little is known about morphological and functional aspects of this enzyme family in the cochlea. First RT-PCR analysis reveals predominant mRNA expression of DGKα, DGKε and DGKζ. Immunohistochemical analysis shows that DGKζ localizes to the nuclei of inner hair cells (IHCs), outer hair cells (OHCs), supporting cells and spiral ganglion neurons in guinea pig cochlea under normal conditions. It is well known that loud noise exposure induces cochlear damage, thereby resulting in hair cell loss. In particular, OHCs are highly vulnerable to noise exposure than IHCs. We found that after 1 week of noise exposure DGKζ translocates from the nucleus to the cytoplasm in damage-sensitive OHCs and gradually disappears thereafter. In sharp contrast, DGKζ remains to the nucleus in damage-resistant IHCs. These results suggest that DGKζ cytoplasmic translocation is well correlated with cellular damage under noise-exposure stress conditions and is involved in delayed cell death in cochlear outer hair cells.

Functional regulation of the SLC26-family protein prestin by calcium/calmodulin

The solute carrier gene family 26 (SLC26) encodes membrane proteins with diverse physiological roles but with the common feature of halide involvement. Here, we present bioinformatic and biochemical evidence that SLC26 proteins have intrinsically disordered regions (IDRs) in their C-terminal domains and that these regions contain calmodulin (CaM) binding sites. The veracity of these predictions and the functional consequences of CaM binding were examined in prestin, SLC26A5, as a model for the SLC26 family and as one of the most investigated and best understood members. We found that CaM binds directly to the IDR in the C-terminal domain of prestin in a calcium-obligate manner. Using both isolated murine outer hair cells (OHCs) and a heterologous expression system, we also found that this calcium-obligate CaM binding shifts the operating point of the protein to more hyperpolarized potentials with consequent alteration of the function of the prestin. Because calcium is the main intracellular second messenger used by the efferent medial olivocochlear (MOC) pathway of the auditory system and CaM is abundant in OHCs, the CaM-prestin interaction may be involved in the MOC-mediated modulation of cochlear amplification. However, this regulatory mechanism is not likely to be restricted to cochlear OHCs, in light of both clear bioinformatic evidence and the fact that calcium and CaM are ubiquitous intracellular second messengers used by virtually all cell types. Hence, the calcium/CaM-dependent regulatory mechanism described herein is likely applicable to most, if not all, SLC26 paralogs.

Examination of calcium-binding protein expression in the inner ear of wild-type, heterozygous and homozygous pituitary adenylate cyclase-activating polypeptide (PACAP)-knockout mice in kanamycin-induced ototoxicity

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide with diverse biological effects. It also occurs and exerts protective effects in sensory organs; however, little is known about its effects in the auditory system. Recently, we have shown that PACAP protects cochlear cells against oxidative-stress-induced apoptosis and homozygous PACAP-deficient animals show stronger expression of Ca(2+)-binding proteins in the hair cells of the inner ear, but there are no data about the consequences of the lack of endogenous PACAP in different ototoxic insults such as aminoglycoside-induced toxicity. In this study, we examined the effect of kanamycin treatment on Ca(2+)-binding protein expression in hair cells of wild-type, heterozygous and homozygous PACAP-deficient mice. We treated 5-day-old mice with kanamycin, and 2 days later, we examined the Ca(2+)-binding protein expression of the hair cells with immunohistochemistry. We found stronger expression of Ca(2+)-binding proteins in the hair cells of control heterozygous and homozygous PACAP-deficient mice compared with wild-type animals. Kanamycin induced a significant increase in Ca(2+)-binding protein expression in wild-type and heterozygous PACAP-deficient mice, but the baseline higher expression in homozygous PACAP-deficient mice did not show further changes after the treatment. Elevated endolymphatic Ca(2+) is deleterious for the cochlear function, against which the high concentration of Ca(2+)-buffers in hair cells may protect. Meanwhile, the increased immunoreactivity of Ca(2+)-binding proteins in the absence of PACAP provide further evidence for the important protective role of PACAP in ototoxicity, but further investigations are necessary to examine the exact role of endogenous PACAP in ototoxic insults.

Auditory and vestibular hair cell stereocilia: relationship between functionality and inner ear disease

The stereocilia of the inner ear are unique cellular structures which correlate anatomically with distinct cochlear functions, including mechanoelectrical transduction, cochlear amplification, adaptation, frequency selectivity and tuning. Their function is impaired by inner ear stressors, by various types of hereditary deafness, syndromic hearing loss and inner ear disease (e.g. Ménière's disease). The anatomical and physiological characteristics of stereocilia are discussed in relation to inner ear malfunctions.

Oncomodulin identifies different hair cell types in the mammalian inner ear

The tight regulation of Ca(2+) is essential for inner ear function, and yet the role of Ca(2+) binding proteins (CaBPs) remains elusive. By using immunofluorescence and reverse transcriptase-polymerase chain reaction (RT-PCR), we investigated the expression of oncomodulin (Ocm), a member of the parvalbumin family, relative to other EF-hand CaBPs in cochlear and vestibular organs in the mouse. In the mouse cochlea, Ocm is found only in outer hair cells and is localized preferentially to the basolateral outer hair cell membrane and to the base of the hair bundle. Developmentally, Ocm immunoreactivity begins as early as postnatal day (P) 2 and shows preferential localization to the basolateral membrane and hair bundle after P8. Unlike the cochlea, Ocm expression is substantially reduced in vestibular tissues at older adult ages. In vestibular organs, Ocm is found in type I striolar or central hair cells, and has a more diffuse subcellular localization throughout the hair cell body. Additionally, Ocm immunoreactivity in vestibular hair cells is present as early as E18 and is not obviously affected by mutations that cause a disruption of hair bundle polarity. We also find Ocm expression in striolar hair cells across mammalian species. These data suggest that Ocm may have distinct functional roles in cochlear and vestibular hair cells.

An adenylyl cyclase signaling pathway predicts direct dopaminergic input to vestibular hair cells

Adenylyl cyclase (AC) signaling pathways have been identified in a model hair cell preparation from the trout saccule, for which the hair cell is the only intact cell type. The use of degenerate primers targeting cDNA sequence conserved across AC isoforms, and reverse transcription-polymerase chain reaction (RT-PCR), coupled with cloning of amplification products, indicated expression of AC9, AC7 and AC5/6, with cloning efficiencies of 11:5:2. AC9 and AC5/6 are inhibited by Ca(2+), the former in conjunction with calcineurin, and message for calcineurin has also been identified in the trout saccular hair cell layer. AC7 is independent of Ca(2+). Given the lack of detection of calcium/calmodulin-activated isoforms previously suggested to mediate AC activation in the absence of Gαs in mammalian cochlear hair cells, the issue of hair-cell Gαs mRNA expression was re-examined in the teleost vestibular hair cell model. Two full-length coding sequences were obtained for Gαs/olf in the vestibular type II-like hair cells of the trout saccule. Two messages for Gαi have also been detected in the hair cell layer, one with homology to Gαi1 and the second with homology to Gαi3 of higher vertebrates. Both Gαs/olf protein and Gαi1/Gαi3 protein were immunolocalized to stereocilia and to the base of the hair cell, the latter consistent with sites of efferent input. Although a signaling event coupling to Gαs/olf and Gαi1/Gαi3 in the stereocilia is currently unknown, signaling with Gαs/olf, Gαi3, and AC5/6 at the base of the hair cell would be consistent with transduction pathways activated by dopaminergic efferent input. mRNA for dopamine receptors D1A4 and five forms of dopamine D2 were found to be expressed in the teleost saccular hair cell layer, representing information on vestibular hair cell expression not directly available for higher vertebrates. Dopamine D1A receptor would couple to Gαolf and activation of AC5/6. Co-expression with dopamine D2 receptor, which itself couples to Gαi3 and AC5/6, will down-modulate levels of cAMP, thus fine-tuning and gradating the hair-cell response to dopamine D1A. As predicted by the trout saccular hair cell model, evidence has been obtained for the first time that hair cells of mammalian otolithic vestibular end organs (rat/mouse saccule/utricle) express dopamine D1A and D2L receptors, and each receptor co-localizes with AC5/6, with a marked presence of all three proteins in subcuticular regions of type I vestibular hair cells. A putative efferent, presynaptic source of dopamine was identified in tyrosine hydroxylase-positive nerve fibers which passed from underlying connective tissue to the sensory epithelia, ending on type I and type II vestibular hair cells and on afferent calyces.

Genome-wide analysis of binding sites and direct target genes of the orphan nuclear receptor NR2F1/COUP-TFI

Background

Identification of bona fide direct nuclear receptor gene targets has been challenging but essential for understanding regulation of organismal physiological processes.

Results

We describe a methodology to identify transcription factor binding sites and target genes in vivo by intersecting microarray data, computational binding site queries, and evolutionary conservation. We provide detailed experimental validation of each step and, as a proof of principle, utilize the methodology to identify novel direct targets of the orphan nuclear receptor NR2F1 (COUP-TFI). The first step involved validation of microarray gene expression profiles obtained from wild-type and COUP-TFI^−/− inner ear tissues. Secondly, we developed a bioinformatic tool to search for COUP-TFI DNA binding sites in genomes, using a classification-type Hidden Markov Model trained with 49 published COUP-TF response elements. We next obtained a ranked list of candidate in vivo direct COUP-TFI targets by integrating the microarray and bioinformatics analyses according to the degree of binding site evolutionary conservation and microarray statistical significance. Lastly, as proof-of-concept, 5 specific genes were validated for direct regulation. For example, the fatty acid binding protein 7 (Fabp7) gene is a direct COUP-TFI target in vivo because: i) we identified 2 conserved COUP-TFI binding sites in the Fabp7 promoter; ii) Fapb7 transcript and protein levels are significantly reduced in COUP-TFI^−/− tissues and in MEFs; iii) chromatin immunoprecipitation demonstrates that COUP-TFI is recruited to the Fabp7 promoter in vitro and in vivo and iv) it is associated with active chromatin having increased H3K9 acetylation and enrichment for CBP and SRC-1 binding in the newborn brain.

Conclusion

We have developed and validated a methodology to identify in vivo direct nuclear receptor target genes. This bioinformatics tool can be modified to scan for response elements of transcription factors, cis-regulatory modules, or any flexible DNA pattern.

Gene expression profiling analysis of the inner ear

Recent developments in molecular genetics, including progress in the human genome project, have allowed identification of genes at an unprecedented rate. To date gene expression profiling studies have focused on identifying transcripts that are specifically or preferentially enriched within the inner ear on the assumption that they are more likely to be important for auditory and vestibular function. It is now apparent that some genes preferentially expressed in the cochleo-vestibular system are not crucial for hearing or balance or their functions are compensated for by other genes. In addition, transcripts expressed at low abundance in the inner ear are generally under-represented in gene profiling studies. In this review, we highlight the limitations of current gene expression profiling strategies as a discovery tool for genes involved in cochleo-vestibular development and function. We argue that expression profiling based on hierarchical clustering of transcripts by gene ontology, combined with tissue enrichment data, is more effective for inner ear gene discovery. This approach also provides a framework to assist and direct the functional characterization of gene products.

Nuclear factor kappaB deficiency is associated with auditory nerve degeneration and increased noise-induced hearing loss

Degeneration of the spiral ganglion neurons (SGNs) of the auditory nerve occurs with age and in response to acoustic injury. Histopathological observations suggest that the neural degeneration often begins with an excitotoxic process affecting the afferent dendrites under the inner hair cells (IHCs), however, little is known about the sequence of cellular or molecular events mediating this excitotoxicity. Nuclear factor kappaB (NFkappaB) is a transcription factor involved in regulating inflammatory responses and apoptosis in many cell types. NFkappaB is also associated with intracellular calcium regulation, an important factor in neuronal excitotoxicity. Here, we provide evidence that NFkappaB can play a central role in the degeneration of SGNs. Mice lacking the p50 subunit of NFkappaB (p50(-/-) mice) showed an accelerated hearing loss with age that was highly associated with an exacerbated excitotoxic-like damage in afferent dendrites under IHCs and an accelerated loss of SGNs. Also, as evidenced by immunostaining intensity, calcium-buffering proteins were significantly elevated in SGNs of the p50(-/-) mice. Finally, the knock-out mice exhibited an increased sensitivity to low-level noise exposure. The accelerated hearing loss and neural degeneration with age in the p50(-/-) mice occurred in the absence of concomitant hair cell loss and decline of the endocochlear potential. These results indicate that NFkappaB activity plays an important role in protecting the primary auditory neurons from excitotoxic damage and age-related degeneration. A possible mechanism underlying this protection is that the NFkappaB activity may help to maintain calcium homeostasis in SGNs.

Cellular localization of voltage-gated calcium channels and synaptic vesicle-associated proteins in the guinea pig cochlea

The cellular localization of voltage-gated calcium channels (VGCCs) and synaptic vesicle-associated proteins, SV2, synapsin I, and vesicle-associated membrane protein (VAMP) (synaptobrevin), was investigated in the guinea pig cochlea using immunocytochemistry and confocal laser scanning microscopy. Reactivity, in guinea pig, of antibodies to the alpha1 subunits of L-type, alpha1C [Cav1.2] and alpha 1D [Cav1.3]; P/Q-type, alpha1A [Cav2.1]; and R-type, a1E [Cav2.3] high voltage-activated calcium channels, was determined by Western blotting and immunolabeling of cerebellum. In the cochlea the sensory inner hair cells of the organ of Corti displayed strong intracellular staining, predominantly localized to their basolateral poles, with an antibody directed against the alpha1C subunit. Some alpha1C labeling was also observed in the inner pillar cells, in cell bodies of afferent neurons in the spiral ganglion, and in the inferior region of the spiral ligament. The supporting pillar cells were strongly immunoreactive throughout for alpha1D, but no alpha1D labeling of the inner hair cells was seen. The alpha1A subunit showed a cytoplasmic distribution in all three rows of outer hair cells. alpha1E labeling localized to the outer hair cells, predominantly in the subcuticular plate region, and also to nerve fiber bundles beneath these hair cells. Strong immunoreactivity was consistently seen with antibodies directed against SV2 and synapsin I in neuronal structures surrounding the basolateral surfaces of both the inner and outer hair cells but was absent from the sensory cells themselves. VAMP labeling was found throughout the cytoplasm of the inner hair cells and in neuronal structures beneath the hair cells. These results reveal a differential distribution of VGCC-types in the sensory and nonsensory elements of the guinea pig cochlea, with the inner hair cells expressing alpha1C L-type channels and VAMP but not synapsin I or SV2.

The mechanism of pneumolysin-induced cochlear hair cell death in the rat

Streptoccocus pneumoniae infection can result in local and systemic diseases such as otitis media, pneumonia and meningitis. Sensorineural hearing loss associated with this infection is mediated by the release of an exotoxin, pneumolysin. The goal of the present study was to characterize the mechanisms of pneumolysin toxicity in cochlear hair cells in vitro. Pneumolysin induced severe damage in cochlear hair cells, ranging from stereocilia disorganization to total cell loss. Surprisingly, pneumolysin-induced cell death preferentially targeted inner hair cells. Pneumolysin triggered in vitro cell death by an influx of calcium. Extracellular calcium appeared to enter the cell through a pore formed by the toxin. Buffering intracellular calcium with BAPTA improved hair cell survival. The mitochondrial apoptotic pathway involved in pneumolysin-induced cell death was demonstrated by the use of bongkrekic acid. Binding of pneumolysin to the hair cell plasma membrane was required to induce cell death. Increasing external calcium reduced cell toxicity by preventing the binding of pneumolysin to hair cell membranes. These results showed the significant role of calcium both in triggering pneumolysin-induced hair cell apoptosis and in preventing the toxin from binding to its cellular target.

Localization of calbindin D-28K in the otoconia of lizard Podarcis sicula

The membranous labyrinth of lizard Podarcis sicula contains calcite and aragonite crystals. Saccule, utricle and lagena contain calcite crystals while aragonite crystals are present only in the saccule where they are very abundant. We have recently demonstrated the presence of calbindin D-28K in the organic matrix of lizard P. sicula otoconia. In order to define its localization, since calbindin modulates cellular Ca2+ level, otoconia from utricle and lagena were collected separately from those from saccule and then otoconial proteins were extracted. Immunoblot assay on proteins extracted from the otoconia and confocal laser scanning microscope analyses of otoconia using monoclonal anti-calbindin D-28K antibodies indicated that calbindin D-28K is a protein typical of aragonite crystals.

The significance of the calcium signal in the outer hair cells and its possible role in tinnitus of cochlear origin

Finely tuned changes in intracellular Ca(2+) concentration modulate a variety of cellular functions in eukaryotic cells. The cytosolic Ca(2+) concentration is also tightly controlled in the outer hair cells (OHCs), the highly specialized receptor and effector cells in the mammalian auditory epithelium, which are responsible for high sensitivity and sharp frequency discrimination in hearing. OHCs possess a complex system of transporters, pumps, exchangers, channels and binding proteins to develop and to halt the regulatory Ca(2+) signal. The crucial role of elevated intracellular Ca(2+) concentration in OHCs is to increase the efficacy of the electromechanical (electromotile) feedback via remodeling of the cortical cytoskeleton. Anomalies in the Ca(2+) signaling pathway may lead to hypersensitivity of the cochlear amplifier and subsequently trigger tinnitus of cochlear origin. This review describes the dynamics of Ca(2+) signaling in the OHCs and a model that may convey a putative mechanism of development of subjective idiopathic cochlear tinnitus.

A cochlear cell line as an in vitro system for drug ototoxicity screening

Aminoglycoside antibiotics, loop diuretics, antineoplastic agents and other commonly used pharmacological drugs are ototoxic. Understanding of the cellular and molecular mechanisms underlying drug ototoxicity, however, has been hampered by the limited availability of inner ear tissues and drug side effects on laboratory animals. Immortalized cell lines derived from the auditory sensory organ, sensitive to ototoxic drugs and growing in environments that can be systematically manipulated, would facilitate the research directed at elucidating these mechanisms. Such immortalized cell lines could also be used to discover novel therapeutic agents for preventing drug-induced sensorineural hearing loss. Here, we report a conditionally immortalized organ of Corti-derived epithelial cell line, which shows evidence of activation of apoptosis when exposed to known ototoxic drugs. This cell line may be an excellent in vitro system to investigate the cellular and molecular mechanisms involved in ototoxicity and for screening of the potential ototoxicity or otoprotective properties of new pharmacological drugs.

The distribution of calcium buffering proteins in the turtle cochlea

Hair cells of the inner ear contain high concentrations of calcium-binding proteins that limit calcium signals and prevent cross talk between different signaling pathways during auditory transduction. Using light microscope immunofluorescence and post-embedding immunogold labeling in the electron microscope, we characterized the distribution of three calcium-buffering proteins in the turtle cochlea. Both calbindin-D28k and parvalbumin-beta were confined to hair cells in which they showed a similar distribution, whereas calretinin was present mainly in hair-cell nuclei but also occurred in supporting cells and nerve fibers. The hair-cell concentration of calbindin-D28k but not of parvalbumin-beta increased from the low- to high-frequency end of the cochlea. Calibration against standards containing known amounts of calcium-buffering protein processed in the same fluid drop as the cochlear sections gave cytoplasmic concentrations of calbindin-D28k as 0.13-0.63 mm and parvalbumin-beta as approximately 0.25 mm, but calretinin was an order of magnitude less. Total amount of Ca 2+-binding sites on the proteins is at least 1.0 mm in low-frequency hair cells and 3.0 mm in high-frequency cells. Reverse transcription-PCR showed that mRNA for all three proteins was expressed in turtle hair cells. We suggest that calbindin-D28k and parvalbumin-beta may serve as endogenous mobile calcium buffers, but the predominantly nuclear location of calretinin argues for another role in calcium signaling. The results support conclusions from electrophysiological measurements that millimolar concentrations of endogenous calcium buffers are present in turtle hair cells. Parvalbumin-beta was also found in both inner and outer hair cells of the guinea pig cochlea.

Changes in cytochemistry of sensory and nonsensory cells in gentamicin-treated cochleas

Effects of a single local dose of gentamicin upon sensory and nonsensory cells throughout the cochlea were assessed by changes in immunostaining patterns for a broad array of functionally important proteins. Cytochemical changes in hair cells, spiral ganglion cells, and cells of the stria vascularis, spiral ligament, and spiral limbus were found beginning 4 days post administration. The extent of changes in immunostaining varied with survival time and with cell type and was not always commensurate with the degree to which individual cell types accumulated gentamicin. Outer hair cells, types I and II fibrocytes of the spiral ligament, and fibrocytes in the spiral limbus showed marked decreases in immunostaining for a number of constituents. In contrast, inner hair cells, type III fibrocytes and root cells of the spiral ligament, cells of the stria vascularis, and interdental cells in the spiral limbus showed less dramatic decreases, and in some cases they showed increases in immunostaining. Results indicate that, in addition to damaging sensory cells, local application of gentamicin results in widespread and disparate disruptions of a variety of cochlear cell types. Only in the case of ganglion cells was it apparent that the changes in nonsensory cells were secondary to loss or damage of hair cells. These results indicate that malfunction of the ear following gentamicin treatment is widespread and far more complex than simple loss of sensory elements. The results have implications for efforts directed toward detecting, preventing, and treating toxic effects of aminoglycosides upon the inner ear.

Calbindin D28K is a component of the organic matrix of lizard Podarcis sicula otoconia

The factors controlling otoconia growth are not well known but it seems that the type of proteins contained in the otoconia regulates the initiation and/or the subsequent rates of crystal growth determining the morphology and the size of the final crystal. In order to clarify the mechanism of otoconia formation and their turnover, major proteins contained in the otoconia from the maculae of the saccule, utricle and lagena of inner ear of lizard Podarcis sicula were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Coomassie staining of SDS-PAGE resulted in a major broad band of 15 kDa and four other bands of 21, 28, 45 and 97 kDa. The proteins of 15, 21, 28 and 45 kDa were separated by high-pressure liquid chromatography on a C-4-reverse-phase column and the incubation of blots with monoclonal anti-Calbindin D28K antibodies indicated that the band of 28 kDa was Calbindin D28K, a calcium-binding protein.

Expression of mRNA encoding extracellular matrix glycoproteins SPARC and SC1 is temporally and spatially regulated in the developing cochlea of the rat inner ear

SPARC is a multifunctional extracellular matrix (ECM) glycoprotein that shares partial sequence homology with SC1/hevin. These ECM molecules exhibit calcium-binding properties and modulate cellular interactions. This study examines the expression of SC1 and SPARC mRNA in the developing cochlea of the rat inner ear prior to and after the onset of hearing. At all ages examined, SC1 mRNA is highly expressed in neurons of the spiral ganglion. In contrast, SPARC transcripts are not detected in the spiral ganglion but are enriched in the temporal bone and cartilaginous otic capsule surrounding the cochlea. Both SC1 and SPARC mRNA are expressed in connective tissue elements involved in maintaining ionic homeostasis of cochlear fluids. SC1 mRNA is localized to type III fibrocytes of the spiral ligament (slg) and marginal cells of the stria vascularis, while SPARC mRNA is apparent in the spiral limbus and type I fibrocytes of the slg. At postnatal day 10, SPARC mRNA shows a dramatic change in expression. High levels of SPARC transcripts are induced in Deiters cells (dc) of the organ of Corti. Interestingly, this induction of SPARC mRNA correlates with the onset of hearing. This suggests that SPARC may play a role in calcium regulation in dc when functional maturation of the cochlea is attained and rapid changes in calcium levels are required.

Ultrastructural localization of calmodulin in gerbil cochlea by immunogold electron microscopy

Localization of calmodulin, a calcium binding protein, was identified in adult gerbil cochleas using paraffin section immunohistochemistry and immunogold electron microscopy with monoclonal antibody against bovine calmodulin. Immunoreactive calmodulin was abundant in inner hair cells (IHCs), outer hair cells (OHCs) and Boettcher cells of the cochleas. Other cell types containing calmodulin were marginal cells and basal cells of the stria vascularis, fibrocytes in the spiral ligament, spiral ganglion neurons and vascular smooth muscle cells. Immunogold labeling for calmodulin was observed in cuticular plate, stereocilia, and within cytoplasm of IHCs and OHCs. In OHCs the labeling was mostly observed in the region underlying lateral wall corresponding to subsurface cisterna. In IHCs the staining was diffuse in the cytoplasm and denser than that in OHCs. Boettcher cells showed dense staining along the microvillous projections facing to the intercellular spaces between Boettcher cells and Claudius cells and between the neighboring Boettcher cells. These distributions of calmodulin in the hair cells consist with the assumption that IHCs act as a true neurotransducer and OHCs as an active bi-directional mechanotransducer. The rich presence of calmodulin in Boettcher cells suggests that the cells may involve in mediating Ca(2+) regulation and play a distinctive active role in ion transport.

Calcification processes in the chick otoconia and calcium binding proteins: patterns of tetracycline incorporation and calbindin-D28K distribution

In order to clarify the otoconia formation and turnover, tetracycline, an antibiotic that precipitates at calcifying fronts and serves as a fluorescent marker, was injected into eggs at different stages of chick embryonic development, as well as into postnatal chicken and into adult animals. The changes in the intensity, location patterns and time course of fluorescent labelling in each examined stage in the otolithic organs was studied. The presence and distribution of calbindin (CB)-D28K, one of the calcium-binding proteins constantly found in the mammalian and chicken cochlea and also in otolithic membrane of some adult mammals, was studied. Results in embryonal stages, postnatal and adult animals allow us to postulate that otoliths are mainly produced during the embryonal phase, but they may also be produced throughout the whole life span. Results also indicate that otoconia are dynamic structures which undergo turnover. The correspondence between the patterns of CB-D28K immunoreactivity and tetracycline fluorescence may indicate that CB-D28K participates in the formation of otoconia.

Simultaneous monitoring of slow cell motility and calcium signals of the guinea pig outer hair cells

'Slow' motility (shape changes over seconds to minutes) of the mammalian cochlear outer hair cell (OHC) could play a protection role from intense sound pressure and is associated with elevation of the cytosolic free Ca(2+) concentration ([Ca(2+)](i)). In the present work, a new approach was elaborated using fluorescent imaging for continuous monitoring of both [Ca(2+)](i) changes and slow motility of OHCs employing the Ca(2+) fluorescent indicator Fura-2. Whole OHC fluorescence and that of cell segments were analyzed to discriminate between fluorescence changes caused by [Ca(2+)](i) rise and those related to change of the cell shape. The reliability of the method was examined by simultaneous monitoring of [Ca(2+)](i) and OHC length changes induced by change of buffer osmolarity or by increase of KCl concentration. The method revealed that the time course of [Ca(2+)](i) increase and rate of cell shortening often do not coincide. It was also observed that [Ca(2+)](i) increased in 70 mM KCl more slowly than the rate of KCl delivery to OHCs. The comparison of the time courses of [Ca(2+)](i) elevation, induced by increase of K(+)/Na(+) ratio and by substitution of Na(+) with N-methyl-D-glucamine(+), indicated that the relatively slow kinetics of [Ca(2+)](i) increase in the OHC is partially attributed to regulation of Ca(2+) homeostasis by the Na(+)/Ca(2+) exchanger.

Immunolocalization of calbindin D28k and calretinin in the dog cochlea during postnatal development

The calbindin (CB) and the calretinin (CR) immunoreactivities were studied in the dog cochlea during its postnatal maturation from birth to the 33rd postnatal day. At birth, CB was expressed in the Kölliker's organ, in the immature inner (IHC) and outer hair cells (OHC), in neurons of the spiral ganglion, and in nerve fibers running in the basilar membrane of the apical turn. During the cochlear maturation, non-sensorineuronal structures, such as the Kölliker's organ, the rods of Corti, and the inner sulcus cells, displayed a transient CB-staining. In the adult-like dog cochlea, CB was found in the cytoplasm, the cuticular plate, and the stereocilia of the IHC and OHC. All the neurons of the spiral ganglion and some nerves fibers in the modulius were CB-positive. At birth, CR exhibited a neuronal distribution: about 75% of the spiral ganglion neurons, some nerve fibers in the modulius and nerve fibers running in the basilar membrane were CR-labeled. During the postnatal maturation, a CR-immunostaining appeared around the IHC body and CR was expressed transiently in the OHC. In the adult-like dog cochlea, a CR-positive network surrounded the unlabeled IHC. The neuronal CR-labeling remained unchanged from birth.

Acquired resistance to acoustic trauma by sound conditioning is primarily mediated by changes restricted to the cochlea, not by systemic responses

Hearing loss caused by intense sound exposure can be significantly reduced by pre-exposing subjects to moderate-level acoustic stimuli. This phenomenon occurs in a variety of mammals. We investigated whether sound conditioning provides acquired resistance to acoustic trauma through local mechanisms selectively in the conditioned ears or if systemic mechanisms are involved that would yield contralateral protection in unconditioned ears. Guinea pigs (group I) in which one external ear canal was occluded were exposed to conditioning sound (2-20 kHz, 85 dB SPL, 5 h/day, 10 days). After removing the occlusion, the animals were then subjected bilaterally to intense noise (2-20 kHz, 110 dB SPL, 5 h) 5 days after the last conditioning exposure. Animals without ear canal occlusion were also exposed to the intense sound without conditioning (group II) or following the same conditioning exposure (group III). Intense sound exposure caused significantly greater permanent ABR threshold shifts at all frequencies tested (4, 8, 12, 16 and 20 kHz) in group II than in group III. In group I, the occluded ears showed significantly greater threshold shifts at all frequencies compared to the unoccluded ears. The threshold shifts in the occluded ears in group I were identical to those observed in group II; and the shifts in unoccluded ears in group I were identical to those in group III. Protective effects provided by sound conditioning were almost the same in group III and in the unoccluded ears in group I. The extent of hair cell damage supported the physiological findings. These results indicate that acquired resistance to acoustic trauma provided by sound conditioning is restricted to the cochlea exposed to conditioning sound, suggesting that conditioning protection is mediated primarily by the changes that occur locally within the conditioned cochlea. This animal model, with unilateral external ear canal occlusion during sound conditioning, is useful for studies of the mechanisms of conditioning protection.

Expression pattern of mammalian cochlea outer hair cell (OHC) mRNA: screening of a rat OHC cDNA library

The aim of this study was to characterize the mRNA content of mammalian cochlear outer hair cells (OHCs) and to search for specific genes possibly involved in their unique properties. Indeed, OHCs, which feature high-frequency electromotility, are responsible for the exquisite sensitivity and frequency selectivity of the cochlea. Damage to these cells, which occurs in various conditions, causes a reduction in the cochlear sensitivity by about 50 dB and the alteration of frequency discrimination. Total RNA was extracted from about 2000 mechanically dissociated OHCs, and a polymerase chain reaction (PCR) amplified cDNA library was constructed. The presence of the alpha-9 acetylcholine receptor subunit, preferentially expressed in OHCs, was found by direct PCR amplification of the library. A systematic sequencing of 218 clones showed 78% known genes, 11% EST-related sequences, and 11% unknown genes. The known-gene group was characterized by two main features: a large proportion (55%) of mitochondrial transcripts and an abundance in calcium-binding proteins, such as calmodulin and calbindin, for which expression has already been demonstrated in OHCs. Another protein, the oncomodulin recently shown to be OHC specific, was also found, and its mRNA expression was confirmed by in situ hybridization. Among the 24 unknown genes, 7 were expressed in a restricted pattern, including one expressed in cochlea and spleen and, to a lesser extent, in lungs.

Immunohistochemical localization of Ca2+/Calmodulin-dependent protein kinase IV in outer hair cells

A smooth membrane system consisting of subsurface cisternae (SSC) underlies the lateral plasmalemma of auditory outer hair cells (OHCs). The SSC contain Ca-ATPase and are regarded as an intracellular Ca2+ reservoir like the sarcoplasmic reticulum of myocytes. Recently, it has been demonstrated that Ca-ATPase activity in sarcoplasmic reticulum is regulated by Ca2+/calmodulin-dependent protein kinases (CaM kinases). Here we investigated the presence of CaM kinases in OHCs and their possible association with the SSC. Inner ears collected from adult gerbils and from neonates at 2-day intervals between 0 and 20 days after birth were immunostained with antibodies specific for different CaM kinases. A polyclonal antiserum against CaM kinase IV yielded a strong immunostaining reaction along the lateral wall of OHCs. The staining appeared after the tenth postnatal day and continued into adulthood. No other site in the inner ear, including cochlear inner hair cells and vestibular hair cells, was reactive. The kinase's apparent association with the SSC strongly supports its involvement in intracellular Ca2+ homeostasis and suggests a role in regulating the OHCs' slow motile responses.

Appearance and distribution of two Ca2+-binding proteins during development of the cochlea in the musk shrew

In the developing cochlea of the musk shrew, Suncus murinus, the localization of two Ca2+-binding protein, calbindin and calmodulin, which are thought to play different roles in the nervous system, was examined during gestational and postpartum periods. Calbindin is thought to play a Ca2+ buffering role, while calmodulin activates other proteins. Cochleae from the musk shrews sacrificed from gestational day (GD) 15 to postnatal day (PP) 9 and as adults, were immunohistochemically analyzed. The localization and order of appearance of calmodulin in sensorineural elements were similar to those of calbindin, except for timing of appearance. Calmodulin-staining was recognized first in the spiral ganglion neurons on GD21, followed by the inner hair cells (IHCs) on GD23 and outer hair cells (OHCs) on GD26, while calbindin immunoreactivity in the spiral ganglion neurons on GD19, the IHCs on GD21 and the OHCs on GD23. In hair cells, during development, immunostaining of calbindin and calmodulin was initially seen in the cytoplasm, followed by the cuticular plate. Cytoplasmic staining then decreased in mature hair cells. Non-sensorineural components also showed positivity for both calbindin and calmodulin. The lateral wall of the cochlear duct was positive for calbindin, while the stria vascularis was positive for calmodulin. Immunoreactivity for calbindin was present earlier than that of calmodulin in sensorineural elements, suggesting that in the developing cochlea, calbindin and calmodulin have different functions and that Ca2+ buffering capacity, which is regulated by Ca2+ buffer proteins, such as calbindin, may be required before trigger proteins, such as calmodulin, function.

Evidence for differential regulation of calcium by outer versus inner hair cells: plasma membrane Ca-ATPase gene expression

The expression of mRNA encoding plasma membrane calcium ATPase (PMCA) subunit isoforms (1-4) and splice variants was examined in the adult and developing rat cochlea by PCR and in situ hybridization. High levels of PMCA mRNA expression were observed in the neurons of the spiral ganglion, and in hair cells. Spiral ganglion neurons expressed PMCA 1-3 beginning in embryonic development, reaching high levels shortly after birth, and continuing into adulthood. Inner hair cells expressed PMCA 1 at moderate levels from birth to the time of onset of cochlear function on postnatal day 12, and strongly from then until adulthood. Outer hair cells expressed PMCA 2 at high levels from shortly after birth through adulthood. The data suggest that the calcium clearance requirements of inner and outer hair cells are distinct. PMCA 2 is the isoform with the highest affinity for calmodulin, and has also been associated with high levels of inositol triphosphate. Its presence in outer hair cells suggests that regulation of the enzyme by calmodulin may be particularly important for this hair cell type. It further suggests that inositol phosphate may play a unique role in the outer hair cell.

Acoustic overstimulation increases outer hair cell Ca2+ concentrations and causes dynamic contractions of the hearing organ

The dynamic responses of the hearing organ to acoustic overstimulation were investigated using the guinea pig isolated temporal bone preparation. The organ was loaded with the fluorescent Ca2+ indicator Fluo-3, and the cochlear electric responses to low-level tones were recorded through a microelectrode in the scala media. After overstimulation, the amplitude of the cochlear potentials decreased significantly. In some cases, rapid recovery was seen with the potentials returning to their initial amplitude. In 12 of 14 cases in which overstimulation gave a decrease in the cochlear responses, significant elevations of the cytoplasmic [Ca2+] in the outer hair cells were seen. [Ca2+] increases appeared immediately after terminating the overstimulation, with partial recovery taking place in the ensuing 30 min in some preparations. Such [Ca2+] changes were not seen in preparations that were stimulated at levels that did not cause an amplitude change in the cochlear potentials. The overstimulation also gave rise to a contraction, evident as a decrease of the width of the organ of Corti. The average contraction in 10 preparations was 9 microm (SE 2 microm). Partial or complete recovery was seen within 30-45 min after the overstimulation. The [Ca2+] changes and the contraction are likely to produce major functional alterations and consequently are suggested to be a factor contributing strongly to the loss of function seen after exposure to loud sounds.

Effects of protein kinase and phosphatase inhibitors on slow shortening of guinea pig cochlear outer hair cells

The intracellular mechanisms of slow shortening in isolated guinea pig cochlear outer hair cells were investigated using inhibitors and/or an activator of protein kinases and protein phosphatases. The slow shortening was induced by tetanic electrical field stimulation, and changes in the cell length, volume and intracellular Cl- concentration were microscopically monitored using a chloride-sensitive fluorescent dye. The slow shortening was inhibited by a calmodulin inhibitor, W-7, and a calcium calmodulin-dependent protein kinase II (CaMKII) inhibitor, KN-62. The inhibition by W-7 or KN-62, was abolished by the supplemented conductance of K+ with valinomycin. Among the protein phosphatase inhibitors tested, a type 1 and 2A protein phosphatase inhibitor, calyculin A, inhibited the slow shortening. The inhibition by calyculin A was abolished by the increased Cl- permeability, but neither by the increased K+ conductance with valinomycin nor by the increased Ca2+ conductance with A23187. A protein serine/threonine phosphatase activator, N-acetylsphingosine, inhibited the shortening, which was abolished by either valinomycin or a type 2A protein phosphatase inhibitor, okadaic acid, but not by calyculin A. These findings suggest the following signaling mechanisms in the slow shortening of outer hair cells; the K+ channel opening is facilitated through protein phosphorylation by CaMKII and suppressed via okadaic acid-sensitive dephosphorylation, and the Cl- channel opening depends on calyculin A-sensitive protein phosphatase activity.

The topology of perceptive functions as a corollary of the theorem of existence in closed spaces

The capability for a system of perceiving both outer objects and an inner self are two fundamental features of abstract mathematical objects endowed with the properties of topologically closed sets. Such structures exist upon intersection of topological spaces owning different dimensions. Then, the theorem of Jordan-Veblen provides their capability of being observable, while the theorems of Brouwer and of Banach-Caccioppoli provide two kinds of fixed points which account for the properties of so-called right and left brain functions. Fixed points account for the biological 'self', and the system provides theoretical justification for the existence of brain structure/function relationships, including memory, emotion, and respective characteristics of right and left hemispheres. Hence, an abstract topological reasoning based on set properties, provides evidence that the observer's function directly infers from the phenomenon of existence and that it belongs to the same mathematical system as the property of being observable. Order relations are raised from equivalence relations by Poincaré groups, upon mappings on the sets of functions and related homotopic transformations in sequences of intersections. Therefore, time is a construction of abstract brain functions, and a living organism just fills the system with appropriate molecular structures.

Hair cell differentiation in chick cochlear epithelium after aminoglycoside toxicity: in vivo and in vitro observations

Inner ear epithelia of mature birds regenerate hair cells after ototoxic or acoustic insult. The lack of markers that selectively label cells in regenerating epithelia and of culture systems composed primarily of progenitor cells has hampered the identification of cellular and molecular interactions that regulate hair cell regeneration. In control basilar papillae, we identified two markers that selectively label hair cells (calmodulin and TUJ1 beta tubulin antibodies) and one marker unique for support cells (cytokeratin antibodies). Examination of regenerating epithelia demonstrated that calmodulin and beta tubulin are also expressed in early differentiating hair cells, and cytokeratins are retained in proliferative support cells. Enzymatic and mechanical methods were used to isolate sensory epithelia from mature chick basilar papillae, and epithelia were cultured in different conditions. In control cultures, hair cells are morphologically stable for up to 6 d, because calmodulin immunoreactivity and phalloidin labeling of filamentous actin are retained. The addition of an ototoxic antibiotic to cultures, however, causes complete hair cell loss by 2 d in vitro and generates cultures composed of calmodulin-negative, cytokeratin-positive support cells. These cells are highly proliferative for the first 2-7 d after plating, but stop dividing by 9 d. Calmodulin- or TUJ1-positive cells reemerge in cultures treated with antibiotic for 5 d and maintained for an additional 5 d without antibiotic. A subset of calmodulin-positive cells was also labeled with BrdU when it was continuously present in cultures, suggesting that some cells generated in culture begin to differentiate into hair cells.

Identification and cloning of site C splice variants of plasma membrane Ca-ATPase in the gerbil cochlea

Plasma membrane Ca-ATPase (PMCA) gene products were identified in the gerbil cochlea by reverse-transcription polymerase chain reaction (RT-PCR). Cochlear cDNA was amplified using PMCA isoform-specific primers from splice site C, the calmodulin binding domain. PCR products were cloned and sequenced. The putative housekeeping PMCA genes, 1b and 4b, as expected, were present in the gerbil cochlea and shared 98.6 and 100% amino acid homology with published rat sequences, at splice site C, respectively. PMCA2b, 3a and 3b splice variants also were detected in cochlear cDNAs and shared 95, 94.3 and 98% amino acid homology with their rat counterparts. PMCA isoforms 2 and 3 have been shown to occur in highly specialized tissues, such as muscle and brain, that require finely tuned regulation of intracellular free Ca2+ levels. The presence of several isoforms and splice variants of PMCA in the cochlea most probably reflects their differential expression among the several cell types that have been shown to contain immunoreactive PMCA. This suggests that the cochlea has developed complex mechanisms that finely tune intracellular Ca2+ levels in different highly specialized cell types.

Immunolocalization of peptide 19 and other calcium-binding proteins in the guinea pig cochlea

Calcium ions are known to play critical roles in a variety of cochlear functions. The distributions of a number of calcium binding proteins that regulate calcium ion levels within the cochlea have previously been described. In this report we extend and refine previous reports of the distribution of immunostaining for calmodulin, calbindin, and calretinin and show for the first time the distribution for peptide 19. There were longitudinal and radial gradients of immunostaining for peptide 19 within outer hair cells that appeared to match previously described gradients of efferent innervation of these cells. Gradients of immunostaining for calbindin within outer hair cells were in the opposite directions, which suggests that levels of this protein are correlated with afferent innervation density and perhaps the abundance of subsurface cisternae. No gradients were seen in the distributions of cells stained for calmodulin and calretinin, which included sensory cells and supporting cells respectively. All ganglion cells were stained for calmodulin but the other proteins appeared to be present in limited ganglion cell subpopulations. In addition to staining of sensorineural elements, antisera to all compounds but peptide 19 showed immunostaining of cells within the lateral wall and the spiral limbus. The results suggest that the proteins under study are involved in a wide variety of calcium-regulated functions within the cochlea. Knowledge of the unique distribution of each of the compounds should facilitate further studies of their roles in cochlear function.

Acute mechanical overstimulation of isolated outer hair cells causes changes in intracellular calcium levels without shape changes

Impaired auditory function following acoustic overstimulation, or noise, is mainly reported to be accompanied by cellular changes such as damage to the sensory hair bundles, but changes in the cell bodies of the outer hair cells have also been described. To investigate more closely the immediate cellular responses to overstimulation, isolated guinea pig outer hair cells were subjected to a 200 Hz oscillating water jet producing intense mechanical stimulation. The water jet was aimed at the cell body of the isolated outer hair cell. Cell shape changes were studied using video microscopy, and intracellular calcium concentration changes were monitored by means of the fluorescent calcium indicator Fluo-3. Cells exposed to a high-intensity stimulus showed surprisingly small light-microscopical alterations. The cytoplasmic calcium concentration increased in most cells, although some cells appeared very resistant to the mechanical stress. No correlation could be found be tween the calcium concentration changes and the cell length. The changes in calcium concentration reported here are suggested to be involved in the long-term pathogenesis of noise-induced hair cell damage.

Expression of plasma membrane Ca-ATPase in the adult and developing gerbil cochlea

The distribution of the plasma membrane Ca-ATPase (PMCA) was mapped in the adult and developing gerbil cochlea by immunostaining with a monoclonal antibody against the human erythrocyte PMCA. In the mature cochlea, intense immunoreactivity was present at the surface of stereocilia of both inner (IHC) and outer (OHC) hair cells. The basolateral plasma membrane of IHCs but not OHCs stained strongly whereas that of strial marginal cells and the epithelial cell layer of Reissner's membrane showed only weak reactivity. Nerve terminals underlying IHCs were also selectively stained. At birth, strong to moderate reactivity for PMCA was present in the basolateral plasma membrane of IHCs and OHCs, strial marginal cells, and epithelial cells lining the scala media surface of Reissner's membrane and in the neurolemma of spiral ganglion cells. Immunostaining in the basolateral plasmalemma of OHCs, strial marginal cells, and epithelial cells lining Reissner's membrane remained strong to moderate up to 14 days after birth when it diminished or disappeared entirely, suggesting a developmental role for PMCA activity in these sites. Expression of PMCA at the surface of IHC and OHC stereocilia was first observed at 10 days after birth and staining reached adult levels by 14 days after birth. The abundance of PMCA in the stereociliary plasma membrane of mature hair cells supports the suggested involvement of Ca2+ in regulating transduction and adaptation mechanisms.

Cytoskeletal and calcium-binding proteins in the mammalian organ of Corti: cell type-specific proteins displaying longitudinal and radial gradients

Whole mounts and tissue sections of the organ of Corti from two representative mammalian species, the Mongolian gerbil (Meriones unguiculatus) and the guinea pig (Cavea porcellus) were probed with antibodies to cytoskeletal and calcium-binding proteins (actin, tubulin, including post-translational modifications, spectrin, fimbrin, calmodulin, parvalbumin, calbindin, S-100 and calretinin). All of the proteins tested were expressed in both species. New findings include the following. Actin is present in large accumulations in cell bodies of the Deiters cells under the outer hair cells (OHC), as well as in the filament networks previously described. These accumulations are more prominent in the apical turns. Tubulin is present in sensory cells in the tyrosinated (more dynamic) form, while tubulin in the supporting cells is post-translationally modified, indicating greater stability. Fimbrin, present in the stereocilia of both IHCs and OHCs, is similar to the isoform of fimbrin found in the epithelial cells of the intestine (fimbrin-I), which implies that actin bundling by fimbrin is reduced in the presence of increased calcium. Parvalbumin appears to be an IHC-specific calcium-binding protein in the gerbil as well as in the guinea pig; labeling displays a longitudinal gradient, with hair cells at the apex staining intensely and hair cells at the base staining weakly. Calbindin displays a similar longitudinal gradient, with staining intense in the IHCs and OHCs at the apex and weak to absent in the base. In the middle turns of the guinea pig cochlea, OHCs in the first row near the pillar cells lose immunoreactivity to calbindin before those in the second and third rows. Calmodulin is found throughout the whole cochlea in the IHCs and OHCs in the stereocilia, cuticular plate, and cell body. Calretinin is present in IHCs and Deiters cells in both species, as well as the tectal cell (modified Hensen cell) in the gerbil. S-100 is a supporting cell-specific calcium-binding protein which has not been localized in the sensory cells of these two species. The supporting cells containing S-100 include the inner border, inner phalangeal, pillar, Deiters, tectal (in gerbil) and Hensen cells, where labeling displays a longitudinal gradient decreasing in intensity towards the apex (opposite to what has been seen with labeling for other proteins in the cochlea).

Post-translational modifications of tubulin suggest that dynamic microtubules are present in sensory cells and stable microtubules are present in supporting cells of the mammalian cochlea

Post-translational modifications to tubulin in the sensory and supporting cells of the cochlea were studied using antibodies specific to the tyrosinated, detyrosinated, acetylated and polyglutamylated isoforms. In the sensory cells, microtubules which label intensely with antibodies to tyrosinated tubulin are found in networks within the cytoplasm. Microtubules which label with antibodies to detyrosinated tubulin and polyglutamylated tubulin, but not acetylated tubulin, form a small component of the microtubules found in the cytoplasm only in the region below the cuticular plate. Microtubules in the supporting cells (inner and outer pillar cells and Deiters cells) are arranged in bundles and contain little tyrosinated tubulin. They are composed instead of predominantly post-translationally modified isoforms which include detyrosinated, acetylated and polyglutamylated tubulin. The findings suggest that microtubules in the sensory cells form dynamic structures, since microtubules that undergo cyclic polymerization and depolymerization predominantly contain tubulin that has not yet had its carboxy-terminal tyrosine residue removed. The presence of microtubules in the supporting cells in which the tubulin has been polymerized into microtubules long enough to be post-translationally modified, provides evidence that these microtubules are stable, long-lived and could contribute to the structural support of the sensory organ of Corti.

Calcium-binding proteins in organ of Corti and basilar papilla: CBP-15, an unidentified calcium-binding protein of the inner ear

In a previous paper (Thalmann et al., 1993) we reported that the amino acid sequence of OCP2, a low molecular weight acidic protein present in extremely high concentrations in the organ of Corti and absent in the basilar papilla, exhibits a rudimentary EF-hand--a potential calcium-binding domain. The present study was undertaken to determine whether OCP2 binds 45-calcium under non-denaturing conditions following separation by isoelectric focusing and transblotting. The same criterion was used to determine whether the EF-hands of several other calcium-binding proteins (CBP) are functional in organ of Corti and basilar papilla. OCP2 exhibited no 45-calcium binding. Calmodulin, present in the organ of Corti in extremely high concentrations and lower in basilar papilla, showed strong 45-calcium binding in both structures. While calbindin represents a major protein in basilar papilla and binds 45-calcium, this protein is a minor component in the organ of Corti; whether it binds 45-calcium remains to be decided. By extending the pI range in the acidic region of isoelectric focusing, a 15 kDa, highly acidic (pI approximately 3.1) protein was revealed that constitutes a major protein in the organ of Corti; the protein was not detectable in the basilar papilla, spiral ligament/stria vascularis complex and numerous other organs tested. It remains to be resolved whether this protein represents an isoform of parvalbumin or a novel CBP. The differential make-up of CBPs between the mammalian organ of Corti and the avian basilar papilla is discussed.

Dystrophin expression in the hair cells of the cochlea

Dystrophin is normally expressed in a number of tissues including muscle, brain and the outer plexiform layer of the retina. In Duchenne and Becker muscular dystrophy abnormal or deficient dystrophin expression leads to muscle degeneration and has been implicated in mental retardation and a form of night blindness. We have examined the expression of dystrophin immunoreactivity in cochlear tissues of normal guinea-pig and mouse, and whether expression is perturbed in the cochlea of the dystrophic MDX mouse. A single band of approximately 427 kDa, corresponding to a full-length isoform of dystrophin was detected in guinea-pig and normal mouse but was absent from the MDX mouse. Cochleae from guinea-pig, normal and MDX mouse also showed a second dystrophin isoform of 116 kDa molecular weight with the C-terminal specific antibody. Immunostained guinea pig cochlear half turns were examined by laser scanning confocal microscopy. Dystrophin was localized in both inner and outer hair cells with staining patterns which were qualitatively similar with both antibodies. In the outer hair cells labelling of the lateral wall was especially distinctive. The synaptic region of both hair cell types was also strongly labelled.

Immunological identification of candidate proteins involved in regulating active shape changes of outer hair cells

By employing immunological methods, it has been demonstrated that myosin, myosin light chain (MLC) and myosin light chain kinase (MLCK) proteins in outer hair cells (OHC) are immunologically different from isoforms in platelets, smooth muscle and heart muscle, and are probably more related to isoforms found in red blood cells (RBC). Moreover, proteins related to band 3 protein (b3p) and protein 4.1 (p 4.1), ankyrin as well as fodrin and spectrin, but not glycophorin, have been identified in isolated OHCs. Both OHCs and RBC differ from other motile non-muscle cells in their lack of smooth muscle isoforms of actin, their common high levels of spectrin-, ankyrin- and band 3-like proteins, as well as the expression of the 80 kDa protein 4.1 isoform. The data support the notion that motility of OHC may be based upon regulation of the b3p/p 4.1/ankyrin complex, and thus may be reminiscent to the active shape changes in RBC.

Endolymph calcium increases with time after surgical induction of hydrops in guinea-pigs

The ionized Ca2+ concentration in cochlear endolymph is normally extremely low. Previous studies have shown that endolymph Ca2+ levels become elevated when measured at long intervals after endolymphatic hydrops is surgically induced. The present study was designed to investigate how rapidly endolymph Ca2+ increases following endolymphatic duct ablation. Hydropic animals were tested at either 4 days, 4 weeks, 8 weeks or 16 weeks after surgery. In each animal endolymph Ca2+ and endocochlear potentials were measured in all four cochlear turns using double-barreled Ca(2+)-sensitive electrodes. Cochlear sensitivity was assessed using compound action potential thresholds. Our results confirm that hydropic animals show an elevation of endolymph Ca2+ and a reduction of EP which is initially small, but becomes more pronounced at longer times after surgery. At 16 weeks endolymph Ca2+ was increased by an average factor of 20 in the basal turn and 7.5 in the fourth turn. These findings suggest that endolymph Ca2+ changes may not be the primary factor responsible for hydrops generation, but probably contribute to cochlear dysfunction in later phases of hydrops. For some experimental groups, the elevation of AP threshold was more closely correlated with endolymph Ca2+ level than it was with endolymph volume. Endolymph Ca2+ changes must therefore be considered in order to account for dysfunction in the hydropic cochlea.

Expression of actin isoforms in the guinea pig organ of Corti: muscle isoforms are not detected

The specificity of antibodies to actin was assayed by use of immunoblots and histological sections of control tissues enriched for each of six different isoforms. On immunoblots, all antibodies stained at most one band of protein in most of the control materials, with a molecular weight of approximately 43 kDa. Their pattern of staining of muscle and nonmuscle tissues indicated their isoform specificity. On tissue sections, immunocytochemical staining demonstrated cellular and subcellular localization of the different isoforms. Once characterized with regard to specificity, these antibodies were used to probe actin in the guinea pig organ of Corti. None of the four muscle isoforms of actin were found in either immunoblots or tissue sections of the organ of Corti. Both beta- and gamma-cytoplasmic isoforms of actin were present in hair cells and supporting cells. This leaves open to investigation the role which cytoplasmic actins play in these cells of the organ of Corti.