Characterization of inner ear sensory hair cells after rapid-freezing and freeze-substitution

3D Ultrastructure of the Cochlear Outer Hair Cell Lateral Wall Revealed By Electron Tomography

Outer Hair Cells (OHCs) in the mammalian cochlea display a unique type of voltage-induced mechanical movement termed electromotility, which amplifies auditory signals and contributes to the sensitivity and frequency selectivity of mammalian hearing. Electromotility occurs in the OHC lateral wall, but it is not fully understood how the supramolecular architecture of the lateral wall enables this unique form of cellular motility. Employing electron tomography of high-pressure frozen and freeze-substituted OHCs, we visualized the 3D structure and organization of the membrane and cytoskeletal components of the OHC lateral wall. The subsurface cisterna (SSC) is a highly prominent feature, and we report that the SSC membranes and lumen possess hexagonally ordered arrays of particles. We also find the SSC is tightly connected to adjacent actin filaments by short filamentous protein connections. Pillar proteins that join the plasma membrane to the cytoskeleton appear as variable structures considerably thinner than actin filaments and significantly more flexible than actin-SSC links. The structurally rich organization and rigidity of the SSC coupled with apparently weaker mechanical connections between the plasma membrane (PM) and cytoskeleton reveal that the membrane-cytoskeletal architecture of the OHC lateral wall is more complex than previously appreciated. These observations are important for our understanding of OHC mechanics and need to be considered in computational models of OHC electromotility that incorporate subcellular features.

Hypotonic swelling of salicylate-treated cochlear outer hair cells

The outer hair cell (OHC) is a hydrostat with a low hydraulic conductivity of Pf=3x10(-4) cm/s across the plasma membrane (PM) and subsurface cisterna that make up the OHC's lateral wall. The SSC is structurally and functionally a transport barrier in normal cells that is known to be disrupted by salicylate. The effect of sodium salicylate on Pf is determined from osmotic experiments in which isolated, control and salicylate-treated OHCs were exposed to hypotonic solutions in a constant flow chamber. The value of Pf=3.5+/-0.5x10(-4) cm/s (mean+/-s.e.m., n=34) for salicylate-treated OHCs was not significantly different from Pf=2.4+/-0.3x10(-4) cm/s (mean+/-s.e.m., n=31) for untreated OHCs (p=.3302). Thus Pf is determined by the PM and is unaffected by salicylate treatment. The ratio of longitudinal strain to radial strain epsilonz/epsilonc=-0.76 for salicylate-treated OHCs was significantly smaller (p=.0143) from -0.72 for untreated OHCs, and is also independent of the magnitude of the applied osmotic challenge. Salicylate-treated OHCs took longer to attain a steady-state volume which is larger than that for untreated OHCs and increased in volume by 8-15% prior to hypotonic perfusion unlike sodium alpha-ketoglutarate-treated OHCs. It is suggested that depolymerization of cytoskeletal proteins and/or glycogen may be responsible for the large volume increase in salicylate-treated OHCs as well as the different responses to different modes of application of the hypotonic solution.

Electromechanical models of the outer hair cell composite membrane

The outer hair cell (OHC) is an extremely specialized cell and its proper functioning is essential for normal mammalian hearing. This article reviews recent developments in theoretical modeling that have increased our knowledge of the operation of this fascinating cell. The earliest models aimed at capturing experimental observations on voltage-induced cellular length changes and capacitance were based on isotropic elasticity and a two-state Boltzmann function. Recent advances in modeling based on the thermodynamics of orthotropic electroelastic materials better capture the cell's voltage-dependent stiffness, capacitance, interaction with its environment and ability to generate force at high frequencies. While complete models are crucial, simpler continuum models can be derived that retain fidelity over small changes in transmembrane voltage and strains occurring in vivo. By its function in the cochlea, the OHC behaves like a piezoelectric-like actuator, and the main cellular features can be described by piezoelectric models. However, a finer characterization of the cell's composite wall requires understanding the local mechanical and electrical fields. One of the key questions is the relative contribution of the in-plane and bending modes of electromechanical strains and forces (moments). The latter mode is associated with the flexoelectric effect in curved membranes. New data, including a novel experiment with tethers pulled from the cell membrane, can help in estimating the role of different modes of electromechanical coupling. Despite considerable progress, many problems still confound modelers. Thus, this article will conclude with a discussion of unanswered questions and highlight directions for future research.

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.

Micropipette aspiration on the outer hair cell lateral wall

The mechanical properties of the lateral wall of the guinea pig cochlear outer hair cell were studied using the micropipette aspiration technique. A fire-polished micropipette with an inner diameter of approximately 4 microm was brought into contact with the lateral wall and negative pressure was applied. The resulting deformation of the lateral wall was recorded on videotape and subjected to morphometric analysis. The relation between the length of the aspirated portion of the cell and aspiration pressure is characterized by the stiffness parameter, K(s) = 1.07 +/- 0.24 (SD) dyn/cm (n = 14). Values of K(s) do not correlate with the original cell length, which ranges from 29 to 74 microm. Theoretical analysis based on elastic shell theory applied to the experimental data yields an estimate of the effective elastic shear modulus, mu = 15.4 +/- 3.3 dyn/cm. These data were obtained at subcritical aspiration pressures, typically less than 10 cm H2O. After reaching a critical (vesiculation) pressure, the cytoplasmic membrane appeared to separate from the underlying structures, a vesicle with a length of 10-20 microm was formed, and the cytoplasmic membrane resealed. This vesiculation process was repeated until a cell-specific limit was reached and no more vesicles were formed. Over 20 vesicles were formed from the longest cells in the experiment.

Ultrastructure of quick-frozen and freeze-substituted chick osteoclasts

For comparison with chemically fixed osteoclasts, we prepared chick osteoclasts by quick freezing followed by freeze-substitution. In spite of technical difficulties this demonstrated that osteoclasts can be satisfactorily frozen in situ by the metal contact method. Ultrastructural differences were revealed between conventional fixation and quick freezing. Compared with conventional fixation, the quick freezing method appeared to improve preservation: (1) a discrete trilaminar plasma membrane and other intracellular membranes showed a smooth profile without undulation or rupture; (2) cytoskeletal components appeared to be clearer, straighter, and more numerous; (3) the interior of the ruffled finger contained interconnected lattice structures whereas highly organised microfilaments were seen in the clear zone; (4) well developed tubulovesicular structures (TVSs) that branched or anastomosed with each other were revealed in the cytoplasm; (5) the contents of intracellular membrane systems including the nuclear envelope, endoplasmic reticulum, and Golgi complex were stained to a various extent; (6) vesicles and vacuoles were much smaller, round and well-defined with electron-dense contents; (7) crystalline structures were seen at the extracellular channels of the ruffled border, in the lumen of TVSs, and in vesicles; (8) in some instances mitochondrial granules were visible; (9) within the resorptive lacuna, osteoclasts adhered to the degraded bone matrix without any intervening empty space.

Effects of endolymphatic and perilymphatic application of salicylate in the pigeon. II: Fine structure of auditory hair cells

Large doses of salicylate are known to cause reversible ototoxic effects including fine structural alterations of the auditory hair cells in mammals. To investigate possible fine structural correlates of salicylate effects on pigeon auditory hair cells, the basilar papillae following perilymphatic or endolymphatic application of salicylate were fixed and processed for transmission electron microscopy. The pigeon auditory hair cells possessed organelles typically described in avians. A single or multi-layered array of cisternae along the cytoplasmic side of the lateral plasma membrane, i.e. subsurface cisternae that are characteristic for mammalian outer hair cells, was not seen. The most prominent fine structural alterations of hair cells after salicylate application were an increase in the luminal width of smooth and rough endoplasmic reticulum as well as the frequent occurrence of prominent single-membrane-bound vesicles filled with electron-dense bodies. Based on the assumption that subsurface cisternae represent a specialized form of endoplasmic reticulum, the present findings indicate that the structural correlates of salicylate toxicity are similar in mammalian and avian auditory hair cells.

Characterization of the outer hair cell's lateral wall membranes

We examined the properties of outer hair cell (OHC) lateral wall membranes by application of 2 fluorescent membrane probes. The markers, C6-NBD-Ceramide and DiOC6, have been used in other cell types to label Golgi apparatus and endoplasmic reticulum, respectively. In living isolated OHCs NBD-Ceramide demonstrated uninterrupted fluorescence along the OHC lateral wall, while DiOC6 labeling proved punctate and notably less uniform in this region. In aldehyde-fixed isolated OHCs both probes exhibited distinct, continuous lateral wall fluorescence. Fixed preparations of the organ of Corti labeled with each probe demonstrated diffuse fluorescence throughout the inner hair cell cytoplasm unlike the uniform, circumferential lateral wall fluorescence seen in OHCs. OHCs exposed to salicylate following NBD-Ceramide labeling displayed patchy, less distinct labeling along the OHC lateral wall. The thickness of lateral wall fluorescence in salicylate exposed cells was 49% greater than control OHCs. We interpreted the salicylate induced change in lateral wall labeling as a fluorescent representation of previously described ultrastructural dilatation and vesiculation of the subsurface cisternae. The distribution of these 2 fluorescent probes along OHC lateral wall membranes suggests that the OHC's subsurface cisternae are neither Golgi nor ER, but share characteristics of both.