Longitudinal and radial differences in the subsurface cisternal system in the gerbil cochlea

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.

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.

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.

Cytologic evidence for mechanisms of K+ transport and genesis of Hensen bodies and subsurface cisternae in outer hair cells

A system commonly termed the tubulocisternal endoplasmic reticulum (TCER), but designated here the canalicular reticulum (CR), occurs selectively in ion-transporting epithelia, in which it is interpreted as facilitating the transcellular diffusion of ions. Mechanoelectrical transduction in the cochlear outer hair cells (OHCs) depends on the apical influx and the subsequent basolateral efflux of K+. Cytologic structures that possibly mediate K+ transport in gerbil OHCs were investigated here.

METHODS

Cochleas were fixed primarily with glutaraldehyde and secondarily with reagents for demonstrating TCER or were fixed with a ferrocyanide-osmium tetroxide solution to preserve intracellular membranes. The distribution of membranous structures retained with these techniques was examined by using electron microscopy.

RESULTS

Secondary fixation with osmium tetroxide-ferrocyanide permitted ultrastructural demonstration in OHCs of increased numbers of Hensen bodies and newly detected membranous systems, including CR, linear cisternae, small clusters of cytosolic vesicles and complexes of canaliculi, segmented cisternae, and mitochondria. CR filling an apical stratum beside and below the cuticular plate and contacting laterally the uppermost subsurface cisternae (SSC) was situated to sequester and transport the apical K+ influx that attends the acoustically generated receptor potential and the silent current. The close association of CR with numerous, highly developed Golgi bodies exclusively in the apex of the cell suggested genesis of CR from Golgi cisternae. Nonbranching, linear cisternae occupied a lower cell stratum and spread from CR laterally to a more inferior region of the SSC. Small clusters of vesicles in the central cytosol resembled Hensen bodies in their envelopment by branching canaliculi and segmented cisternae in close association with mitochondria. Viewing the vesicles in Hensen bodies and the small clusters as functioning like most other cytoplasmic vesicles in transport of cell membrane permitted the interpretation that these vesicles move nascent membrane from the canalicular-mitochondrial complex to the SSC. Other small clusters of vesicles contacted the innermost layer of the SSC, often at cisterna-depleted foci in which the vesicles appeared to either replenish the SSC or arise in the course of its turnover. Proximity of multivesicular bodies and lysosomes to small vesicle clusters in foci of depleted SSC implicated the lysosomes in digesting vesicles released from the SCC. Populations of unique, large, lysosome-like bodies and of small, dense bodies in the upper cytosol of OHCs appeared to be involved in different catabolic pathways mediating the turnover ofSSC, CR, and other structures.

CONCLUSIONS

Cochlear OHCs contain previously unrecognized membranous organelles that facilitate ion transport and presumably contribute thereby to mechanoelectrical transduction. Vesicles in small clusters and Hensen bodies arise from complexes of canaliculi, cisternae, and mitochondria and contribute membrane to the genesis of the SSC.

Development of intracellular Ca-ATPase in the gerbil outer hair cell lateral wall

The development of Ca-ATPase immunoreactivity in gerbil outer hair cells (OHCs), assayed by immunofluorescence and postembedding immunocytochemistry, is reported here. In the adult, a linear array of label is seen inside the lateral plasma membrane. The ultrastructural distribution of Ca-ATPase near the OHC lateral plasma membrane was examined using immunogold cytochemistry and showed this calcium pumping enzyme to be present throughout the subsurface cisternal complex (SSC), especially near the innermost layers. During development, Ca-ATPase immunoreactivity appeared in patches near the lateral plasma membrane of some OHCs of the third row by 12 days after birth (DAB). By 15-16 DAB, punctate immunoreactivity was detected in the second and first rows. At 20 DAB, immunostaining near OHC lateral plasma membrane was increased, but was less continuous than OHC staining in the adult cochlea. The appearance of Ca-ATPase in OHCs coincides with the onset of auditory function and isolated OHC motility in the gerbil. The ultrastructural demonstration of abundant sites of calcium pumps in the SSC supports a role for this structure in the intracellular storage of calcium. These findings suggest a possible role of Ca-ATPase and the SSC in the regulation of slow motility of OHCs which has been reported to depend on intracellular calcium concentration.