Localization of proteins associated with the outer hair cell plasma membrane in the gerbil cochlea

Anion exchanger 1b in stereocilia is required for the functioning of mechanotransducer channels in lateral-line hair cells of zebrafish

The anion exchanger (AE) plays critical roles in physiological processes including CO2 transport and volume regulation in erythrocytes and acid-base regulation in renal tubules. Although expression of the AE in inner-ear hair cells was reported, its specific localization and function are still unclear. Using in situ hybridization, we found that the AE1b transcript is expressed in lateral-line hair cells of zebrafish larvae. An immunohistochemical analysis with a zebrafish-specific antibody localized AE1b to stereocilia of hair cells, and the expression was eliminated by morpholino knockdown of AE1b. A non-invasive, scanning ion-selective electrode technique was applied to analyze mechanotransducer (MET) channel-mediated Ca2+ influx at stereocilia of hair cells of intact fish. Ca2+ influx was effectively suppressed by AE1b morpholino knockdown and inhibitor (DIDS) treatment. Elevating external Ca2+ (0.2 to 2 mM) neutralized the inhibition of DIDS. Taken together, this study provides solid evidence to show that AE1b in stereocilia is required for the proper functioning of MET channels.

Generation of somatic electromechanical force by outer hair cells may be influenced by prestin-CASK interaction at the basal junction with the Deiter's cell

The motor protein, prestin, situated in the basolateral plasma membrane of cochlear outer hair cells (OHCs), underlies the generation of somatic, voltage-driven mechanical force, the basis for the exquisite sensitivity, frequency selectivity and dynamic range of mammalian hearing. The molecular and structural basis of the ontogenetic development of this electromechanical force has remained elusive. The present study demonstrates that this force is significantly reduced when the immature subcellular distribution of prestin found along the entire plasma membrane persists into maturity, as has been described in previous studies under hypothyroidism. This observation suggests that cochlear amplification is critically dependent on the surface expression and distribution of prestin. Searching for proteins involved in organizing the subcellular localization of prestin to the basolateral plasma membrane, we identified cochlear expression of a novel truncated prestin splice isoform named prestin 9b (Slc26A5d) that contains a putative PDZ domain-binding motif. Using prestin 9b as the bait in a yeast two-hybrid assay, we identified a calcium/calmodulin-dependent serine protein kinase (CASK) as an interaction partner of prestin. Co-immunoprecipitation assays showed that CASK and prestin 9b can interact with full-length prestin. CASK was co-localized with prestin in a membrane domain where prestin-expressing OHC membrane abuts prestin-free OHC membrane, but was absent from this area for thyroid hormone deficiency. These findings suggest that CASK and the truncated prestin splice isoform contribute to confinement of prestin to the basolateral region of the plasma membrane. By means of such an interaction, the basal junction region between the OHC and its Deiter's cell may contribute to efficient generation of somatic electromechanical force.

Three-dimensional structure of outer hair cell pillars

CONCLUSIONS. Electron tomography was used to generate three-dimensional reconstructions of the pillars that connect the cell membrane with the cytoskeleton of the outer hair cell. Results are consistent with the hypothesis that pillars are important for mechanically linking the membrane with the cytoskeleton.

OBJECTIVE

To make a qualitative assessment of the morphology of the sub-membrane pillars of cochlear outer hair cells.

MATERIALS AND METHODS

Guinea pig cochleae were fixed and prepared for electron microscopy using protocols described previously. Sections were imaged on an electron microscope equipped with a goniometer. The specimens were tilted through a range of 120°, and an image was acquired at each tilt angle. Filtered back-projection was used to generate three-dimensional reconstructions.

RESULTS

Twelve individual pillars were successfully reconstructed. Pillars often connect to the cell membrane through a thin segment, and to the cytoskeleton through a forking structure that may form a central cavity.

alphaII-betaV spectrin bridges the plasma membrane and cortical lattice in the lateral wall of the auditory outer hair cells

The sensitivity and frequency selectivity of the mammalian cochlea involves a mechanical amplification process called electromotility, which requires prestin-dependent length changes of the outer hair cell (OHC) lateral wall in response to changes in membrane electric potential. The cortical lattice, the highly organized cytoskeleton underlying the OHC lateral plasma membrane, is made up of F-actin and spectrin. Here, we show that alphaII and two of the five beta-spectrin subunits, betaII and betaV, are present in OHCs. betaII spectrin is restricted to the cuticular plate, a dense apical network of actin filaments, whereas betaV spectrin is concentrated at the cortical lattice. Moreover, we show that alphaII-betaV spectrin directly interacts with F-actin and band 4.1, two components of the OHC cortical lattice. betaV spectrin is progressively recruited into the cortical lattice between postnatal day 2 (P2) and P10 in the mouse, in parallel with prestin membrane insertion, which itself parallels the maturation of cell electromotility. Although betaV spectrin does not directly interact with prestin, we found that addition of lysates derived from mature auditory organs, but not from the brain or liver, enables betaV spectrin-prestin interaction. Using this assay, betaV spectrin, via its PH domain, indirectly interacts with the C-terminal cytodomain of prestin. We conclude that the cortical network involved in the sound-induced electromotility of OHCs contains alphaII-betaV spectrin, and not the conventional alphaII-betaII spectrin.

Lateral wall protein content mediates alterations in cochlear outer hair cell mechanics before and after hearing onset

Specialized outer hair cells (OHCs) housed within the mammalian cochlea exhibit active, nonlinear, mechanical responses to auditory stimulation termed electromotility. The extraordinary frequency resolution capacity of the cochlea requires an exquisitely equilibrated mechanical system of sensory and supporting cells. OHC electromotile length change, stiffness, and force generation are responsible for a 100-fold increase in hearing sensitivity by augmenting vibrational input to non-motile sensory inner hair cells. Characterization of OHC mechanics is crucial for understanding and ultimately preventing permanent functional deficits due to overstimulation or as a consequence of various cochlear pathologies. The OHCs' major structural assembly is a highly-specialized lateral wall. The lateral wall consists of three structures; a plasma membrane highly-enriched with the motor-protein prestin, an actin-spectrin cortical lattice, and one or more layers of subsurface cisternae. Technical difficulties in independently manipulating each lateral wall constituent have constrained previous attempts to analyze the determinants of OHCs' mechanical properties. Temporal separations in the accumulation of each lateral wall constituent during postnatal development permit associations between lateral wall structure and OHC mechanics. We compared developing and adult gerbil OHC axial stiffness using calibrated glass fibers. Alterations in each lateral wall component and OHC stiffness were correlated as a function of age. Reduced F-actin labeling was correlated with reduced OHC stiffness before hearing onset. Prestin incorporation into the PM was correlated with increased OHC stiffness at hearing onset. Our data indicate lateral wall F-actin and prestin are the primary determinants of OHC mechanical properties before and after hearing onset, respectively.

Imaging of the cortical cytoskeleton of guinea pig outer hair cells using atomic force microscopy

Mammalian outer hair cells (OHCs) are known to respond to acoustical stimulation with elongation and contraction of the cells' cylindrical soma in vivo, and this motility is related to both the protein motors distributed along the OHC plasma membrane and the cytoskeleton beneath it. Therefore, the cytoskeleton seems to play an important role in the motility of the OHC. Recently, an atomic force microscope (AFM) was used to investigate the OHC cytoskeleton under physiological conditions. However, details were not made clear in that study. In this study, the ultrastructure of the cytoskeleton of fixed OHCs of guinea pigs, which were extracted with Triton X-100, was investigated using the AFM. As a result, the cortical cytoskeleton, which is formed by discrete oriented domains, was imaged, and circumferential filaments and cross-links were observed within the domain. Morphological change of the cytoskeleton of the OHC induced by diamide treatment was then examined using the AFM, and reduction of cross-links was observed. The examination indicates that the cortical cytoskeleton comprises circumferential actin filaments and spectrin cross-links.

Characterization of novel and identified genes in guinea pig organ of corti

A number of proteins are expressed in the organ of Corti and are considered to be responsible for hearing. However, most of them have not been identified. Therefore, to achieve a better understanding of the genetic factors influencing these traits, the first step is to characterize the genes expressed in the organ of Corti. In the present study, a cDNA library was constructed from the guinea pig organ of Corti. After sequencing isolated clones, 196 expressed sequence tags (ESTs) were identified with FASTA analysis: 65 ESTs showed significant sequence homology to previously identified genes in guinea pig, human or other species, and 131 ESTs showed no significant matches to sequences already present in the DNA database DDBJ/GenBank/EMBL. A variety of matching sequences, some of which were known to be cochlea-specific, were found through FASTA analysis of the 65 clones. RT-PCR with a panel of 10 different tissue mRNA revealed the restricted expression of 13 unknown clones. The results of our analysis allowed the establishment of a list of genes expressed in the guinea pig organ of Corti.

Single-cell RT-PCR demonstrates expression of voltage-dependent chloride channels (ClC-1, ClC-2 and ClC-3) in outer hair cells of rat cochlea

We investigated whether voltage-dependent chloride channels (ClC-1, ClC-2 and ClC-3) are expressed in outer hair cells (OHCs) of rat cochlea using a single-cell reverse transcription-polymerase chain reaction (RT-PCR) technique. The OHCs were isolated from rat cochlea and the cytoplasm of each OHC was suctioned into a glass pipette containing RT-PCR reaction buffer with RNase inhibitor. RT-PCR revealed the presence of transcripts of ClC-1, ClC-2 and ClC-3, which were verified by DNA sequencing. The possible roles of these chloride channels in OHCs are discussed.

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.

The guinea pig cochlear AE2 anion exchanger: cDNA cloning and in situ localization within the cochlea

This study has characterized the repertoire of the anion exchanger (AE) family members expressed within the guinea pig organ of Corti, the auditory neuroepithelia. Both AE2 and AE3 cDNAs were present, but AE1 cDNA was not detected. The more abundant AE2 was sequenced and its expression characterized in the cochlea. The 3888 base pairs (bp) AE2 sequence, compiled from multiple clones, includes 150 bp of upstream non-coding sequence and 3717 bp of open reading frame encoding a protein of 1238 amino acids. Immunoblot of cochlear homogenate revealed a single AE2-immunoreactive band of Mr 180 kDa. In situ hybridization and immunohistochemical analysis localized AE2 expression to several tissues and cell types within the guinea pig inner ear, including superior half of the spiral ligament and within the interdental cells lining the spiral limbus. However, AE2 was not clearly detected in the outer hair cells (OHC) of the organ of Corti by either immunohistochemistry or in situ hybridization. The results of these studies imply a physiologic role of AE2 in the cochlear homeostasis, but do not support its role as a potential 'motor protein' in mediating the in vitro-observed voltage-gated, ATP-independent OHC motility.