Ultrastructure indicative of ion transport in tectal, Deiters, and tunnel cells: differences between gerbil and chinchilla basal and apical cochlea. (1/53)

Ultrastructural examination revealed an epithelium of about five tectal cells (TCs) roofing the outer tunnel (OT) in the mid to upper, but not the basal, region of gerbil and chinchilla cochlea. Structures in TCs that are apparently specialized for retrieval of K(+) released into tunnel fluid from outer hair cells (OHCs) include surface fimbriae in the gerbil and canalicular reticulum in the chinchilla. A tunnel roof of organelle-rich TCs appeared to be better equipped for ion resorption than a roof composed of organelle-poor Hensen cells (HCs). Fimbriae, filopodia, and the cell body of TCs descended to contact the third Deiters cell (DC3) in the gerbil, and the hypertrophied DC3 phalanx rose to contact TCs in the chinchilla, which suggests a solute exchange between TCs and DCs. Previously unrecognized structures that are speculated to provide ATP ligand for cochlear purinoreceptors occurred in the chinchilla DC and gerbil TC. The observation of a microtubule stalk in DCs indicated that they also function in cochlear mechanics. A newly delineated lateral tunnel cell (LTC) intervened between the DC3 and HC in both species. The apicomedial plasmalemma of all DCs fitted closely to the base of OHCs and enveloped afferent nerves. The morphologic specializations reported here provide further support for the proposed transcellular lateral flow route for K(+) currents generated by sound exposure and neural activity. The previously demonstrated expansion of Boettcher cells, outer sulcus cell roots, type Il and IV fibrocytes, and apical microvilli on HCs and Claudius cells (CCs) in the base of the cochlea is postulated here to mediate a basal parallel current that could supply the increased K(+) transport required for the basally elevated electric potential (EP).  (+info)

Expression of glutamate transporter GLAST in the developing mouse cochlea. (2/53)

The immunohistochemical localization of glutamate transporter GLAST in the developing mouse cochlea was studied at different ages between 0 and 30 days after birth (DAB). In the adult mouse cochlea, intense GLAST-like immunoreactivity was found in the supporting cells adjacent to the inner hair cells of the organ of Corti, the type II and suprastrial fibrocytes of the cochlear lateral wall, the fibrocytes of the spiral limbus and the satellite cells surrounding the spiral ganglion cells. At 0 DAB, weak GLAST-like immunoreactivity was found in the supporting cells around the immature inner hair cells. Immature fibrocytes in the cochlea were also positively immunostained. At 3 DAB, weak immunostaining of GLAST appeared in the immature satellite cells in the spiral ganglion. The GLAST-like immunoreactivity in the supporting cells around the inner hair cells, in the fiborocytes in the spiral ligament and the spiral limbus and in the satellite cells in the spiral ganglion increased progressively during the second postnatal week, and reached the adult level at 15 DAB. This time course correlates with the electrophysiological onset and maturation of the mouse auditory function, which is mediated by glutamatergic neurotransmission. These results suggest that the expression of GLAST may be needed for the efficient removal and metabolism of the released glutamate in the cochlea and may play important roles in the onset and maturation of the auditory system.  (+info)

Cell type-specific reduction of beta tubulin isotypes synthesized in the developing gerbil organ of Corti. (3/53)

There are seven isotypic forms of the microtubule protein beta tubulin in mammals, but not all isotypes are synthesized in every cell type. In the adult organ of Corti, each of the five major cell types synthesizes a different subset of isotypes. Inner hair cells synthesize only betaI and betaII tubulin, while outer hair cells make betaI and betaIV tubulin. Only betaII and betaIV tubulin are found in inner and outer pillar cells, while betaI, betaII, and betaIV tubulin are present in Deiters cells, and betaI, betaII and betaIII tubulin are found in organ of Corti dendrites. During post-natal organ of Corti development in the gerbil, microtubules are elaborated in an orderly temporal sequence beginning with hair cells, followed by pillar cells and Deiters cells. Using beta tubulin isotype-specific antibodies, we show that, in the gerbil cochlea, the same three isotypes are present in each cell type at birth, and that a cell type-specific reduction in the isotypes synthesized occurs in hair cells and pillar cells at an unusually late stage in development. No beta tubulin isotypes were detected in mature afferent dendrites, but we show that this is because few microtubules are present in mature dendrites. In addition, we show that primary cilia in inner hair cells, a feature of early development, persist much later than previously reported. The findings represent the first description of developmental cell type-specific reductions in tubulin isotypes in any system.  (+info)

Identification and characterization of choline transporter-like protein 2, an inner ear glycoprotein of 68 and 72 kDa that is the target of antibody-induced hearing loss. (4/53)

The Kresge Hearing Research Institute-3 (KHRI-3) antibody binds to a guinea pig inner ear supporting cell antigen (IESCA) and causes hearing loss. To gain insight into the mechanism of antibody-induced hearing loss, we used antibody immunoaffinity purification to isolate the IESCA, which was then sequenced by mass spectroscopy, revealing 10 guinea pig peptides identical to sequences in human choline transporter-like protein 2 (CTL2). Full-length CTL2 cDNA sequenced from guinea pig inner ear has 85.9% identity with the human cDNA. Consistent with its expression on the surface of supporting cells in the inner ear, CTL2 contains 10 predicted membrane-spanning regions with multiple N-glycosylation sites. The 68 and 72 kDa molecular forms of inner ear CTL2 are distinguished by sialic acid modification of the carbohydrate. The KHRI-3 antibody binds to an N-linked carbohydrate on CTL2 and presumably damages the organ of Corti by blocking the transporter function of this molecule. CTL2 mRNA and protein are abundantly expressed in human inner ear. Sera from patients with autoimmune hearing loss bind to guinea pig inner ear with the same pattern as CTL2 antibodies. Thus, CTL2 is a possible target of autoimmune hearing loss in humans.  (+info)

Survival of adult spiral ganglion neurons requires erbB receptor signaling in the inner ear. (5/53)

Degeneration of cochlear sensory neurons is an important cause of hearing loss, but the mechanisms that maintain the survival of adult cochlear sensory neurons are not clearly defined. We now provide evidence implicating the neuregulin (NRG)-erbB receptor signaling pathway in this process. We found that NRG1 is expressed by spiral ganglion neurons (SGNs), whereas erbB2 and erbB3 are expressed by supporting cells of the organ of Corti, suggesting that these molecules mediate interactions between these cells. Transgenic mice in which erbB signaling in adult supporting cells is disrupted by expression of a dominant-negative erbB receptor show severe hearing loss and 80% postnatal loss of type-I SGNs without concomitant loss of the sensory cells that they contact. Quantitative RT-PCR analysis of neurotrophic factor expression shows a specific downregulation in expression of neurotrophin-3 (NT3) in the transgenic cochleas before the onset of neuronal death. Because NT3 is critical for survival of type I SGNs during development, these results suggest that it plays similar roles in the adult. Together, the data indicate that adult cochlear supporting cells provide critical trophic support to the neurons, that survival of postnatal cochlear sensory neurons depends on reciprocal interactions between neurons and supporting cells, and that these interactions are mediated by NRG and neurotrophins.  (+info)

Electrical coupling in sustentacular cells of the mouse olfactory epithelium. (6/53)

Sustentacular cells (SCs) line the apical surface of the olfactory epithelium (OE) and provide trophic, metabolic, and mechanical support for olfactory receptor neurons. Morphological studies have suggested that SCs possess gap junctions, although physiological evidence for gap junctional communication in mammalian SCs is lacking. In the present study we investigated whether coupling exists between SCs situated in tissue slices of OE from neonatal (P0-P4) mice. Using whole cell and cell-attached patch recordings from SCs, we demonstrate that SCs are electrically coupled by junctional resistances on the order of 300 M(omega). Under whole cell recording conditions, Alexa 488 added to the pipette solution failed to reveal dye coupling between SCs. Electrical coupling was deduced from the biexponential decay of capacitive currents recorded from SCs and from the bell-shaped voltage dependency of a P2Y-receptor-activated current, both of which were abolished by 18beta-glycyrrhetinic acid (20-50 microM), a blocker of gap junctions. These data provide strong evidence for functional coupling between SCs, the physiological importance of which is discussed.  (+info)

Influence of supporting cells on neuronal degeneration after hair cell loss. (7/53)

In sensorineural hearing loss, hair cell loss is often followed by loss of cochlear nerve fibers, which can continue for years after the insult. The degree and time course of neuronal loss varies, but the reasons for this variation are unclear. The present study addresses this issue with a quantitative analysis of hair cell, supporting cell, and neuronal survival in animals with long-term survival of up to 5.5 years from two types of drug-induced hair cell loss: aminoglycoside antibiotics and platinum-containing chemotherapeutics. To complement the analysis of the effects of organ of Corti damage on neuronal survival, cases of primary neuronal degeneration, via auditory nerve section, are also assessed. Analysis shows that (1) long-term neuronal survival is enhanced when supporting cells in the inner hair cell (IHC) area remain intact; (2) after hair cell loss, the time course of neuronal loss is slower in the apex than in the base; (3) primary loss of cochlear nerve fibers does not lead to secondary degeneration of sensory cells or supporting cells in the organ of Corti; and (4) after auditory nerve section, there can be a massive reinnervation of the IHC region, especially in the apex. Results are consistent with the idea that supporting cells participate in the regulation of neuronal survival and neuronal sprouting in the organ of Corti.  (+info)

Distinct and gradient distributions of connexin26 and connexin30 in the cochlear sensory epithelium of guinea pigs. (8/53)

Connexin26 (Cx26) and Cx30 are predominant isoforms of gap junction channels in the cochlea and play a critical role in hearing. In this study, the cellular distributions of Cx26 and Cx30 in the cochlear sensory epithelium of guinea pigs were examined by immunofluorescent staining and confocal microscopy in whole mounts of the cochlear sensory epithelium and dissociated cell preparations. The expression of Cx26 and Cx30 demonstrated a longitudinal gradient distribution in the epithelium and was reduced threefold from the cochlear apex to base. The reduction was more pronounced in the Deiters cells and pillar cells than in the Hensen cells. Cx26 was expressed in all types of supporting cells, but little Cx30 labeling was seen in the Hensen cells. Cx26 expression in the Hensen cells was concentrated mainly in the second and third rows, forming a distinct band along the sensory epithelium at its outer region. In the dissociated Deiters cells and pillar cells, Cx30 showed dense labeling at the cell bodies and processes in the reticular lamina. Cx26 labeling largely overlapped that of Cx30 in these regions. Cx26 and Cx30 were also coexpressed in the gap junctional plaques between Claudius cells. Neither Cx26 nor Cx30 labeling was seen in the hair cells and spiral ganglion neurons. These observations demonstrate that Cx26 and Cx30 have a longitudinal gradient distribution and distinct cellular expression in the auditory sensory epithelium. This further supports our previous reports that Cx26 and Cx30 can solely and concertedly perform different functions in the cochlea.  (+info)